Dongxin He

Tao Zhang

Fansong Meng

Qingquan Li

Hongshun Liu

Gilbert Teyssedre

Space Charge Behaviours in Cable Insulation under a DC-Superimposed Pulsed Electric Field

When HVDC cables are subjected to a DC-superimposed pulsed electric stress, the pulsed voltage applied facilitates electrical tree generation and breakdown; however, the mechanism involved remains unclear. To study the deterioration mechanism of cable insulation under a pulsed electric field, an experiment on the dynamic characteristics of space charge under a DC-superimposed pulsed electric field was conducted. For this experiment, a pulse time trigger control circuit was developed to accurately measure the change law of the space charge in cross-linked polyethylene samples at the rising and falling edges of the pulsed electric field. The experimental results showed an unusual change law: the space charge density increases with falling voltage amplitude and decreases with rising voltage amplitude. The sudden change in the electric field breaks the balance of the forces acting on the space charge, leading to injection, extraction, and migration. The energy released during these dynamic processes may destroy the microstructure of the insulation material and contributes to the growth of the electrical tree. This study provides an insight on the initiation mechanism of electrical tree in HVDC cables and proposes a strategy to suppress electrical tree initiation.

Introduction

Cross-linked polyethylene (XLPE) cables are widely used in HVDC transmission systems owing to their excellent electrical and mechanical properties. The operational reliability of XLPE cables is vital for the safety and stability of HVDC power systems [START_REF] Liu | Model to estimate the trapping parameters of cross-linked polyethylene cable peelings of different service years and their relationships with dc breakdown strengths[END_REF]. Any deterioration of the XLPE cables can compromise the safe operation of the entire transmission system. Therefore, it is important to analyse the cause of cable insulation failure and propose solutions to improve the reliability and safety of cable operation. This can help improve power safety and reduce maintenance costs. Space charge is a vital factor leading to the dielectric aging and breakdown of DC cables [START_REF] Zhai | Space charge suppression of polyethylene induced by blending with ethylene-butyl acrylate copolymer[END_REF][START_REF] He | The Influence mechanism of semiconductive material on space charge accumulation in HVDC cable accessory[END_REF]. Electrical tree development is considered the main cause of cable aging, leading to breakdown. The initiation and growth of the electrical tree may be driven by the behaviour of space charges [START_REF] Zheng | Space charge controlled electric field preceding inception of electric tree in XLPE at AC voltage[END_REF][START_REF] Wu | Effect of surface modification of electrodes on charge injection and dielectric characteristics of propylene carbonate[END_REF]. The space charges accumulated inside an insulation material modify the electric field distribution, and the injection, extraction, trapping, and detrapping processes of the space charges are accompanied by energy transfer and release, which may destroy the microstructure of the cable material [START_REF] Li | Novel HVDC spacers by adaptively controlling surface charges -Part II: Experiment[END_REF].

An HVDC cable in operation is affected by lightning overvoltage, switching surges, and commutation failure, leading to various shapes of pulsed electric fields superimposed on the nominal DC voltage [START_REF] Du | Electrical tree characteristics in silicone rubber under repetitive pulse voltage[END_REF]. Recent studies suggest that the pulsed electric field has an important effect on the electrical tree initiation and breakdown of DC cables. When a DC voltage is applied to the cable insulation, the electrical tree cannot easily initiate and grow, despite the considerable accumulation of the space charge [START_REF] Liu | Growth and partial discharge characteristics of DC electrical trees in cross-linked polyethylene[END_REF]. However, under a DC-superimposed pulsed electric field, the onset voltage for electrical tree initiation is lower than that under a DC electric field alone, thus facilitating electrical tree propagation [START_REF] Han | Electrical tree in HTV silicone rubber with temperature gradient under repetitive pulse voltage[END_REF][START_REF] Hammarström | Partial discharge characteristics of electrical treeing in XLPE insulation exposed to voltages of different rise times[END_REF]. Moreover, the electrical tree grows with the variation in the voltage amplitude but does not grow at constant voltage [START_REF] Zhang | Evidence of strong correlation between space-charge buildup and breakdown in cable insulation[END_REF]. This experimental phenomenon was attributed to the dynamic behaviour of the space charges, i.e., the space charges are trapped and detrapped under the impact of pulsed voltage while the energy released during these processes degrades the microstructure of the insulator [START_REF] Zhang | Electrical treeing behaviors in silicone rubber under impulse voltage considering high temperature[END_REF], leading to breakdown.

The generation of the electrical tree is pronounced when a cable insulation is grounded after DC polarization [START_REF] Liu | Electrical tree initiation in XLPE cable insulation under constant DC, grounded DC, and at elevated temperature[END_REF]. This is attributed to the effect of the fast-varying voltage applied to the space charges when grounding [START_REF] Wang | Effect of temperature on space charge detrapping and periodic grounded DC tree in crosslinked polyethylene[END_REF]. Thus, it can be concluded that the dynamic behaviour of the space charges under pulsed voltage plays an important role in the initiation of the electrical tree and degradation of the cable insulation. To investigate the deterioration mechanism, it is important to study the space charge behaviour under a DCsuperimposed pulsed electric field [START_REF] Du | Effects of DC and pulse voltage combination on surface charge dynamic behaviors of epoxy resin[END_REF].

Few studies have been conducted on the charge characteristics under pulsed voltage. The dynamic behaviours of surface charges exhibit a close relationship with flashover [START_REF] Du | Effect of pulse duration on surface charge of direct-fluorinated polyimide films[END_REF][START_REF] Wang | Surface potential and breakdown characteristics of epoxy/AlN nanocomposites under DC and pulse voltages[END_REF]; however, the surface charges cannot be measured using the surface potential method at the same time as under pulsed voltage application. The pulsed electroacoustic (PEA) method can be used to synchronously measure the space charge in the process of stress application; the space charge from the slope with a rise time in the millisecond time range has been studied [START_REF] Fukuma | Millisecond timerange analysis of space-charge distribution and electroluminescence in insulating polymers under transient electric stress[END_REF][START_REF] Hozumi | Behaviour of space charge correlated with electroluminescence in cross-linked polyethylene[END_REF][START_REF] Liu | Progress of research into the polyimide insulation failure mechanism under square impulse stress[END_REF]. Some researchers investigated the space charge accumulation characteristics under a square-wave voltage with different parameters (polarity, frequency, duty ratio) [START_REF] Wang | Space charge accumulation property in XLPE under applied voltage of square wave[END_REF]. According to the analysis of research on electrical tree, the space charge behaviour at the edge of the pulse voltage may adversely affect cable insulation. However, recent studies have not focused on the dynamic characteristics of space charge at the rising and falling edges.

Measuring the space charge dynamic characteristics under a DC-superimposed pulsed electric field requires a precise pulsed-voltage-controlling technology. In this paper, the PEA method was improved and the space charge was successfully measured online by matching the charge measurement time with the pulse edge. Through the improved platform, the space charge behaviour under a DC superimposed pulsed electric field, especially at the rising and falling edges of the voltage, was observed and analysed. This study provides valuable insight on the initiation mechanism of electrical tree in HVDC cables and proposes a strategy to suppress electrical tree initiation.

Experiment

Experimental Platform

Based on the measurement principle of the PEA method, an experimental platform for testing the space charge was built. Fig. 1 shows a schematic of the space charge test system. It is composed of charge test unit, two pulsed power generators (one is square-wave voltage -power generator 1 for material stressing, the other is pulsed voltage power generator 2 for space charge probing by PEA), trigger control circuit, coupling circuit, oscilloscope and computer. The black arrow lines show the flow of high pulsed voltage, while the blue arrow lines show the flow of low voltage signal.

The key objective of this experiment was to accurately match the space charge test pulse voltage with the rising and falling edges of all the test cycles. The pulse voltage trigger time control method was used to design a "trigger control circuit", which outputted a trigger pulse to measure the space charge at a specified time, thereby realizing a flexible control of the space charge test time under a periodic electric field. The trigger control circuit transforms the polarized square-wave signal from power generator 1 into tens of short continuous square-wave signals, which are synchronous with different phases of the polarized voltage, such as the rising and falling edge, and the specific time in the high-voltage and low-voltage level. The power generator 2 that is used to probe space charge is triggered by the short square-wave signal from trigger control circuit to output continuous pulsed voltage with 5ns rising edge. The polarized square-wave voltage from power generator 1 and the synchronous pulsed voltage from power generator 2 are applied on the test sample simultaneously through coupling circuit, thus space charge distribution and dynamics can be measured under polarized square-wave voltage. The charge measurement signal is acquired by oscilloscope and transmitted to the computer, and the raw signal can be calibrated and recovered to obtain charge profiles by waveform processing program. Fig. 2 shows a schematic of the trigger control circuit, which is composed of isolation transformer, filter, zerocrossing comparator, phase adjuster, signal converter and optocoupler. Its working principle is as follows: the high pulsed voltage produced by the Polarized square-wave voltage power generator 1 is converted to a low-voltage pulse signal by an attenuator. The square-wave signal is synchronized with the pulse voltage by subjecting the signal to filtering, zero-crossing comparator circuit, and time-delay processing. The converter then generates a fixed number of short square-wave signals (the number is 59 in this research) to trigger the Pulse voltage power generator 2. A train of voltage pulses is then transmitted to the PEA cell for charge detection. Through calibration, the matching accuracy of the trigger control circuit can be at the nanosecond level.

Fig. 2. Schematic of the trigger control circuit.

Experimental Preparation

To study the characteristics of the space charge in XLPE cables, the XLPE samples were made into sheets. The samples used in these experiments were cut from a 10 kV XLPE-insulated cable with a size of 5×5 cm² and a thickness of 240 µm. The cutting method is schematized in Fig. 3. A section of the cable was installed in a rotary machine and cut by a lathe whose knife was moving forward in a tangential direction. The width and flatness of the peelings were optimized by adjusting the rotation speed of the cable section and the moving speed of the lathe knife.

Fig. 3. Cutting method scheme of cable insulation.

As electrical tree initiation is sensitive to the polarity of the DC field, we employed the easiest method to initiate the electrical trees, i.e., by applying a negative voltage [START_REF] Liu | Electrical tree initiation characteristics under DC-impulse composite voltages in XLPE[END_REF]. The power supply led to the superposition of the DC voltage level and the square-wave voltage with a low level of -3.6 kV and a high level of -7.2 kV as the polarization voltages, at a frequency of 50 Hz. In this experiment, the falling edge represents the change from the high level of -7.2 kV to the low level of -3.6 kV, and the rising edge represents the change from -3.6 to -7.2 kV. Both the rising and falling edges of the square wave were set to 50 µs. Fig. 4 shows the voltage waveform. The electric field strengths corresponding to the low and high levels are -15 and -30 kV/mm, respectively, and the polarization time is 2 h.

Fig. 4. Waveform of DC-superimposed square signal. (In red color: trigger for pulsed voltage generation).

The pulsed voltage used to measure the space charge is accurately matched with the polarized square-wave voltage, to compare the change in the space charge before and after the edges. A fixed number of pulse sequences are generated by the pulse trigger circuit in each test period. The interval for probe pulse generation is 334 µs; this corresponds to 59 test points in one period of 20 ms. As shown in Fig. 4, the test points 1 to 30 are at low-voltage levels of the stress waveform, while the rest 31-59 are at high-voltage levels.

To obtain an appropriate signal-to-noise ratio, the acoustic signals are averaged over 166 periods of the square voltage, yielding a total measurement time of 3.32 s. Fig. 5 shows the matching result of the probe pulsed voltage and square stress voltage. The time corresponding to the rising and falling edges is 50 µs, and the matching accuracy is at the nanosecond level, which is sufficient for the accuracy of the measurement. The most important objective was to measure the space charge immediately after the rising and falling edges, such that the dynamic features of the space charge at the rising and falling edges can be observed. By comparing the variation of the space charge distribution before and after the edges, we can analyse the mechanism of space charge motion and then summarize its behavioural characteristics at the rising and falling edges. Moreover, the accumulation and attenuation characteristics of the space charge can be obtained from the measurements taken at high and low voltage levels at different times.

Experiment Results

Space Charge Accumulation

The space charge measured using the PEA method, like with any other perturbation method, is the net charge present locally, and may lead to the superposition of the electronic and ionic carriers with different charge polarities. Fig. 6 shows the waveform of space charge at the initial time on an XLPE sample that was never subjected to an electric field before. In order to improve the signal to noise ratio, signals in 166 continuous periods of square-wave voltage are acquired, and the original signals are averaged in 166 times to get smooth waveform. After that the raw signal is calibrated and recovered to obtain charge profiles by waveform processing program, in which the overshoot and the attenuation of the signal caused by the system are recovered and Gaussian filter is used to make the waveform clean. As shown in Fig. 6, only capacitive charges are detected at the interface with the electrode, with no space charge inside the XLPE. Moreover, the space charge peak amplitude is proportional to the voltage (x2 between low and high voltage levels). Fig. 7 illustrates the process of space charge evolution under a low-voltage level with the electric field of -15 kV/mm and a high-voltage level with the electric field of -30 kV/mm in the XLPE samples in a stressing time of 2 h. The experimental results show that as the time of applied polarization voltage increases, the space charges with a polarity opposite to those on the interface charges at the electrode are gradually accumulated at the two ends of the XLPE sample. It needs to be mentioned that the electric field of -15 kV/mm does not reach the threshold electric field to generate space charges, as scarce charges accumulated inside the sample when the sample was subjected to DC -15 kV/mm stress. Thus the space charges detected in the lower stress level of the square-wave voltage come from the charges generated in the higher level of -30 kV/mm and also during the rising and falling edges, and the generated charges experience few dissipation in the low level.

Because the applied electric field is not very high, the space charge injected from the electrode is less than that generated by the internal impurity ionization. As a result, the heteropolar space charge is mainly composed of positive and negative ions that come from impurity ionization. In the process of XLPE cross-linking, dicumyl peroxide, a type of cross-linking agent, decomposes and produces various cross-linking by-products such as methane, cumyl alcohol, acetophenone, and methyl styrene [START_REF] Meunier | Molecular modeling of electron traps in polymer insulators: Chemical defects and impurities[END_REF][START_REF] He | Space charge behavior in XLPE cable insulation under ac stress and its relation to thermoelectrical aging[END_REF]. Cross-linking byproducts are reported to dissociate under an electric field, and the greater the electric field strength, the greater the charge dissociation. Furthermore, the positive ions migrate to the cathode, and the negative ions migrate to the anode. Traps are generated because of the chemical and structural defects in the XLPE insulation material, mostly in the amorphous zone. These traps will trap the ions dissociated by the cross-linking by-products. As the energy level of the traps increases, it becomes more difficult for the trapped space charge to escape. With time, a large number of heteropolar space charges accumulate near the interfaces of the XLPE.

Change in Heteropolar Space Charge Density at Rising and Falling Edges

In order to further research and understand the impact of pulsed voltage on space charges, the dynamic behaviour of charges at the rising and falling edges needs to be investigated. The duration time of the rising and falling edges are both set at 50 µs, which is much shorter than the time interval of charge measurement (334 µs); thus, it is not possible to measure charge distribution for more than 2 times while in the rising or falling process. In this paper, a simple method to reflect the charge dynamic behaviour is proposed, in which the charge movement is deduced by comparing the space charge distribution before and after the rising and falling edge. As Fig. 4 shows, the charge distribution profiles are tested both before the rising and falling edge and after the rising and falling edge by matching the charge measurement time using the trigger control circuit. As the space charges may be in transient state after the edges, space charge distribution should be measured immediately after the rising and falling edge (the delay time is set less than 10 ns); and the time interval is mostly located in the period before the edge, because the charges are in steady state when the voltage is constant. Thus, the variation of charge distribution between before and after the rising and falling edges can reflect the dynamic behaviour of space charges at the edges. Fig. 8 shows the change in the space charge density at the rising and falling edges when the sample was stressed under square-wave voltage for 1 h. It can be seen that the space charge density at the electrode varies with the voltage amplitude. It increases with rising voltage and decreases with falling voltage. This phenomenon is similar to the capacitive charges at the initial time in Fig. 6. What should be focused on is the heteropolar charges inside the samples. Interestingly, the changing trend of the space charge inside the XLPE is in contrast to that of the voltage amplitude. When the voltage amplitude decreases, the density of the space charge increases, whereas the density decreases when the voltage amplitude increases, as indicated by the arrows in Fig. 8. Fig. 9 shows the change in the space charge density before and after the rising and falling edges when the sample was stressed under square-wave voltage for 2 h. The change trend is the same as in Fig. 8. The charge distribution at the other test time, such as 15 min, 30 min, 90 min, all show the same trend. Moreover, the space charges were measured under other voltages, for example, with the lowvoltage level of -4 kV and high-voltage level of -8 kV, also showed the same changing law. The additional experimental data is not presented in the paper for concision. Besides the change in the space charge densities at the rising and falling edges, whether the space charge changes during the constant voltage also needs to be evaluated. Fig. 10 shows the change in the heteropolar space charge density under low-stress level at 2 h. The test positions are 1, 6, 12, 18, 24, and 30. The graph shows that the space charge density barely changes. This means that the space charges have very low mobility when under constant electric field, thus the variation can scarcely be recognized with one period. This phenomenon is consistent with a previous report [START_REF] Hozumi | Behaviour of space charge correlated with electroluminescence in cross-linked polyethylene[END_REF], which also showed that the space charge has a low accumulation speed when the voltage is constant.

Discussion

Dynamic Mechanism of Space Charge at the Rising and Falling Edges

Schottky injection and the theory of hopping conductivity are generally used to explain the migration of space charges. However, in these processes, the rate of space charge change is low, and it takes a long time to complete the migration and accumulation of space charges. In this experiment, the time corresponding to the rising and falling edges is in the microseconds range. Therefore, these theories cannot be used to explain the experimental results.

According to the research theory of gas discharge, the forces acting on the space charge can be considered to explain for charge dynamics under the pulsed electric field. In the study of plasma, the gas discharge under the pulsed electric field shows the characteristics of stronger energy and more intense discharge, compared with that under the steady-state electric field, in the same way as solid insulation material. The relevant mechanism has been studied and some consensus has been reached [START_REF] Kasashima | Impulsive force phenomenon of electric field stress causing serious particle contamination in plasma etching equipment[END_REF][START_REF] Zhang | Modeling study on the effects of pulse rise rate in atmospheric pulsed discharges[END_REF]. The studies indicate that the rapid change in an electric field accounts for the effect of the forces acting on charges intensified for gas discharge, and the pulsed electric field can work as an impulsive force [START_REF] Kasashima | Impulsive force phenomenon of electric field stress causing serious particle contamination in plasma etching equipment[END_REF]. Under the microsecond-level rising and falling time of the pulsed electric field, this effect of the changing electric field force is more significant. Thus, we can analyze this phenomenon from the perspective of the changing electric field force, which means that the sudden change in the electric field may result in the unbalanced stress, which results in the transient movement of charges.

To clarify how the changing electric field impacts the space charge behaviour, the forces acting on the space charge need to be analysed firstly. The space charges are subjected to multiple forces inside the XLPE. To simplify the analysis, the forces are classified into two categories: electric field force and "material stress." The former constitutes the force the space charge is subjected to under the electric field. The "material stress" includes the repulsive and attractive forces between the charges, the force of the material itself acting on the space charge, and other forces. The resultant of these forces is the "material stress."

Under the polarization voltage, in addition to the significant accumulation of the heteropolar space charges in the XLPE, there exists some homopolar charges injected from the electrode, though the amount of heteropolar space charges is apparently more than that of homopolar space charges. The width of the interface charge peaks is approximately 30 µm with the current experimental resolution, so the homopolar space charge cannot be observed in the profiles. Therefore, the net space charge observed on the profiles is heteropolar. For example, near the cathode, there exists a large amount of positive charges attracted from the inside and possibly a small amount of negative charges injected from the cathode; however, the measured charge density is the net charge, whose amplitude is the positive charge density minus the negative charge density. Fig. 11 shows the space charge distribution and the illustration of the forces acting on the space charge in the constant-voltage stage, i. e., in the high-voltage level and the low-voltage level. The material stress is opposite to the electric field force, and the forces acting on the space charge is balanced; thus the space charge keeps stationary. When the rising or falling edges comes, the balance between the electric field force and the material force is broken. The change in the force magnitude can be clearly seen in Fig. 12. The electric field force diminishes rapidly at the falling edge, while the material stress remains stable as the relative position between the charges does not change quickly. Therefore, the material stress is greater than the -------------- ---

+ + + + + + + + + + + + + + + + + - + E Material stress
Electric field force electrical field force. This leads positive charge migration into the XLPE near the cathode, and the negative charge is drawn off from the surface of the XLPE. Therefore, the positive charge density near the cathode increases at the rising edge. At the rising edge, the electric field force is enhanced while the material stress does not change immediately. Thus, near the cathode, the positive charge is drawn off, and the negative charge migrates into the XLPE. Subsequently, the positive charge density shows a decreasing trend. The same analysis method can also be applied to explain the change in the space charge density near the anode. Thus, voltage steps would unbalance the electrostatic forces, provoking a transient variation in the heterocharges by ionic transport, when the field strength suddenly varies.

From Charge Dynamics to Treeing Initiation

The most interesting experimental observation is the unusual phenomenon wherein the space charge density exhibits an opposite trend to the voltage amplitude. This may be closely related to the electrical tree initiation. The effect of the pulsed voltage will lead to the migration, injection, and extraction of space charges at the rising and falling edges, and charge migration involves trapping and detrapping processes. All these charge dynamic behaviours are accompanied with the release of energy [START_REF] Dong | Finite-element analysis for surface discharge on polyimide insulation in air at atmospheric pressure under pulsed electrical stress[END_REF]. The dissipated energy may break the chemical bonds of the polymer and alter the microstructure, as hypothesized in electrical aging concepts, following a variety of possible mechanisms. The damage to the macromolecules produces a large number of free radicals and small molecular products. The free radicals generated have a catalytic effect on the degradation of the polymer, and in presence of residual molecular oxygen dissolved in the cable, it will lead to a wider range of degradation and eventually form a lowdensity zone. The electrons injected from the electrode ionize the molecular chains by collision in the low-density region [START_REF] Matsui | Space charge behavior in low-density polyethylene at pre-breakdown[END_REF]. A part of the energy released is converted to electromagnetic radiation, and a part of the energy is used to break more polymer molecules. This forms a hollow electrical tree channel, from which partial discharges can ultimately be triggered and accelerate the growth of the electrical tree. Finally, it will result in polymer breakdown. Fig. 13 illustrates the prospected effect of space charge behaviour on the electrical tree.

The electrons injected by field emission contribute to the electrical tree initiation if their energy exceeds a certain critical threshold [START_REF] Tanaka | Effects of charge injection and extraction on tree initiation in polyethylene[END_REF]; the greater the total energy, the greater the damage to the dielectric. The detrapping of the space charges will result in insulation material depolarization, releasing a large amount of energy, like an avalanche, and causing electric field redistribution. Because of the pulsed nature of the electric field, the rates of injection, extraction, trapping, and detrapping of the space charge are reinforced, and the rates of energy conversion and release are also intensified, thus facilitating electrical tree generation as compared to that under a DC electric field alone.

Fig. 13. Schematic of space charge behaviour.

Comparing Fig. 9 and Fig. 10, charge mobility between the rising and falling edges is much larger than that at the constant-voltage stage. This phenomenon can be explained by force analysis. When the applied voltage is constant, space charge locates in stationary state, i.e., the electric field force and the material stress are almost the same; therefore, the charge mobility is very low. While at the rising and falling stages, the stress balance is broken and significant difference appears between the electric field force and the material stress, which leads to quite large charge motion, corresponding to the dynamic behaviours analysed above.

Our experiment suggests dynamic behaviours of the space charges in XLPE at the rising and falling edges. However, current knowledge on the behaviour of space charges inside insulating materials is limited, and the influence of the parameters of the pulse voltage on the characteristics of the space charge is unclear, such as the slope of the rising and falling edges, and polarity of the impulse voltage. Hence, in future study, it is necessary to thoroughly explore the influence of space charge on the breakdown of insulation.

Conclusion

In this study, an experiment was conducted to investigate the dynamic behaviour of space charges in cable insulation when subjected to a pulsed electric field. The experimental results showed that at the rising and falling edges, the changing trend of the space charge inside the XLPE sample is opposite to the trend of the applied voltage. That is, when the voltage amplitude decreases, the space charge density increases, whereas the space charge density decreases with increasing voltage amplitude.

The effect of forces is used to explain this phenomenon. In the time interval under constant voltage, the two kinds of forces acting on the space charge, electric filed force and material stress, are balanced. At the rising and falling edges, this balance is broken, because the electric field force suddenly changes, while the material stress remains relatively stable, leading to the movement of space charges, i.e., injection, extraction, trapping, and detrapping. This repeated dynamic behaviour can lead to the breaking of the chemical bonds on the polymer molecules, thereby inducing electrical tree initiation. Therefore, studying the dynamic behaviour of space charges under DCsuperimposed pulsed electric field is important to reveal the breakdown mechanism of HVDC cable insulation under overvoltage conditions.

Fig. 1 .

 1 Fig. 1. Schematic of space charge measurement system.

Fig. 5 .

 5 Fig. 5. Matching relationship between space charge test time and polarization pulsed voltage.

Fig. 6 .

 6 Fig. 6. Waveform of space charge at initial time.

Fig. 7 .

 7 Fig. 7. Accumulation trend of space charge with squarewave voltage application for up to 2 h.

Fig. 8 .

 8 Fig. 8. Change in the space charge density at the rising and falling edges at 1 h.

Fig. 9 .

 9 Fig. 9. Change in the space charge density at the rising and falling edges at 2 h.

Fig. 10 .

 10 Fig. 10. Change in space charge at a low-voltage level of -15 kV/mm applied for 2 h.

Fig. 11 .

 11 Fig. 11. Illustration of the forces acting on the space charge in the constant-stage stage.

  (a) Change in the forces at the falling edge (from -30 to -15 kV/mm).(b) Change in the forces at the rising edge (from -15 to -30 kV/mm).

Fig. 12 .

 12 Fig. 12. Schematic of the force change near the cathode at the falling and rising edges.

Acknowledgments

The research work of this paper was financed by the National Natural Science Foundation of China (Grant No.51907105) and the Shandong Provincial Natural Science Foundation (Grant No. ZR2019QEE013). I would like to express my heartfelt thanks to them.


Maxime Roger

Kassem Amro

Joëlle Rault-Berthelot

Mathias Quiot

Arie Van Der Lee

Cyril Poriel

Sébastien Richeter

Sébastien Clément

Philippe Gerbier

Synthesis, photophysical and electropolymerization properties of thiophene-substituted 2,3-diphenylbuta-1,3-dienes

Introduction

Luminogens with aggregation-induced emission (AIE) features (AIEgens) have been the subject of a large number of studies over the past two decades and continue to draw considerable attention due to their fascinating photophysical properties. 1,2 Indeed, contrary to conventional organic fluorophores containing large planar aromatic rings whose fluorescence is drastically decreased or quenched in the aggregated or solid state due to aggregation-caused quenching (ACQ) phenomenon, AIE fluorophores exhibit strong emission in the solid state or upon aggregation due to the restriction of intramolecular motions (RIM) which shut down the non-radiative relaxation pathway. 3,4 In addition to this high solid-state emission, AIEgens are featured with large Stokes shift, high photostability, and low background noise in dilute solution. 1,5 As such, the AIE effect has enabled to take advantage of the aggregation process and has triggered to new developments in an array of fields including bioimaging, [6][7][8][9] therapy, [10][11][12] chemosensing, [13][14][15][16] optoelectronics [17][18] and stimuli-responsive systems. [19][20][21][22][23] Due to this dynamic research, a wide variety of AIEgens has been developed such as tetraphenylethylene (TPE) [24][25][26] , silole 27,28 , quinoline malononitrile (QM) 29,30 , triarylamine [31][32][33][START_REF] Hua | Properties of Triarylamine Derivatives with AIE and Large Two-Photon Absorbing Cross-Sections[END_REF] , cyanostilbene [START_REF] Yamamoto | [END_REF]36 or 9,10distyrylanthracene (DSA) [START_REF] Xu | Aggregation-Induced Emission of 9, 10-Distyrylanthracene Derivatives and Their Applications[END_REF][START_REF] Zhang | [END_REF] derivatives.

Buta-1,3-diene unit, which is prevalent in many biologically active natural compounds and pharmarceuticals, [39][40][41][42][43][44][45] has recently emerged as an interesting conjugated building block for the design of AIE or aggregation-enhanced emission (AEE) luminophores, allowing to extend π-conjugation of TPE 46 . In this respect, multiphenyl-substituted 1,3-butadienes (MPBs) have recently received increasing attention 47,48 . Noteworthy examples of this class of AIE (AEE) luminophores are 1,1,4,4-tetraphenylbuta-1,3-diene (TPB), a well-known highly efficient blue-emitting material [49][50][51][52][53] and 1,1,2,3,4,4-hexaphenylbuta-1,3-diene (HPB), which possess a helical structure like polyacetylene. [54][55][56] Such MPBs exhibit multiple rotors in their structure and adopt twisted conformations allowing to avoid the presence of - stacking interactions and thus, to minimize the possible formation of excimers. After the formation of the aggregates, the rotations are restricted leading to high emission efficiency. To extend the scope of the applications of these AIE (AEE) luminophores, synthetic efforts have been pursued to modify the nature of the aromatic groups and the substituent on the aromatic groups exploiting a wide range of synthetic strategies such as dienyl rearrangement 57 , alkaline desilylation 58 , allene isomerization 59,60 , aryne dimerization 46,[61][62][63][64] and Suzuki coupling reactions between phenylboronic acid or its derivatives and bromo-substituted 1,4-diphenyl-1,3-butadienes. 54,64,65 Despite these successful strategies, the development of new buta-1,3-dienebased building blocks whose optical properties (absorption/emission) can be tuned through judicious choice of the substituent linked to the 1,3-butadiene unit is still required. In this respect, we describe herein the synthesis of 2,3-diphenylbuta-1,3dienes (DPBs) bearing thienyl (T-DPB), bis-thienyl (BT-DPB) and EDOT (EDOT-DPB) units (see chemical structures in Scheme 2) through the desilylation of the corresponding silole derivatives (see chemical structures in Scheme 1). The presence of the bulky phenyl substituents at the 2,3-position helps to suppress potential intramolecular interactions which may improve their fluorescent properties, while the substituents attached at the 1,4-position enables increasing the conjugation length. 50,58,66 The choice of thienyl susbtituents was motivated by: i) their electron-rich character leading in general to significant bathochromic shifts in the absorption and emission spectra 67-69, and ii) their ability to be polymerized allowing further modification of the optical characteristics of buta-1,3-diene through main-chain conjugation [70][71][72] . As such, these building blocks are both interesting in the form of small molecules and polymers and required further investigations. The study of the optical properties of these DPBs reveals an AIE or AEE behaviour with a strong increase of the fluorescence when aggregated. The electrochemical properties of these new materials were also evaluated revealing the formation of low band gap polymers through electropolymerization processes.

Results and discussion

Synthesis and crystal structure of DPB derivatives

DPBs bearing external thiophene (T), bithiophene (BT) or ethylenedioxythiophene (EDOT) units were synthesized in a two-step strategy. First, sil-T and sil-BT were prepared following reported literature procedure by using the method previously described by Tamao et al. (Scheme 1). 58 Namely, silacyclopentadiene Sil-ZnCl was prepared in situ through the intramolecular reductive cyclization from dimethylbis(phenylethynyl)silane followed by the reaction with dichloro(N,N,N',N'tetramethylethylenediamine)zinc (ZnCl 2 -TMEDA). Then, a Negishi coupling between this organozinc species and a 2-bromothienyl derivative was performed using PdCl 2 (PPh 3 ) 2 as catalyst. Applying these conditions to sil-EDOT did not result in the formation of the desired compound. Alternatively, sil-EDOT was prepared by a Stille cross-coupling reaction between 1,1-dimethyl-2,5-dibromo-3,4-diphenylsilole (sil-Br) and 2-stannyl-EDOT in THF in a 40% yield (Scheme 1) 73 . sil-T and sil-BT can also be synthesized exploiting this strategy in 50% and 44% yields, respectively (Scheme 1).

Scheme 1. Synthetic routes to silole derivatives.

The structures of sil-T, sil-BT and sil-EDOT were confirmed by multinuclear spectroscopy ( 1 H, 13 C{ 1 H} and 29 Si{ 1 H}) and mass spectrometry. The data obtained for sil-T and sil-BT are in good agreement with those previously published in the literature. 58 For sil-EDOT, a multiplet at 4.20 ppm corresponding to the CH 2 O group of EDOT as well as a singlet assigned to the methyl group linked to the silicon at 0.62 ppm are noted in the 1 H NMR spectrum. A singlet at 10.8 ppm is also observed in the 29 Si{ 1 H} NMR spectrum corresponding to the silicon atom of the silole core.

Single crystals were also obtained by slow evaporation of a solution of sil-EDOT in CH 2 Cl 2 /n-hexane. As observed for related siloles derivatives, 74,76 the crystal structure of sil-EDOT (Fig. 1 and Fig. S24 in the Supporting Information) indicates that the 2,5thiophene rings have anti-coplanar arrangements to the central silole ring; the twisted angle between the two thiophene mean planes and the silole mean plane is only 1.96°. This coplanar arrangement is favoured by the presence of Si---O interactions since the intramolecular distance between the Si and O atoms of the silole and the EDOT rings of 3.04 Å is markedly smaller than the sum of the van der Waals radii (3.62 Å). Strong Si---O interactions with short Si---O interatomic distance of ∼1.9-2.4 Å are well known in the literature. [77][78][79][START_REF] Kost | The Chemistry of Organic Silicon Compounds[END_REF] The distance of 3.04 Å observed in sil-EDOT suggests that the Si---O interactions are weak. This was confirmed by using 29 Si CP/MAS NMR spectroscopy by comparing the 29 Si chemical shifts of sil-EDOT and sil-T, where no Si---O interactions are present in the solid state. Indeed, the downfield 29 Si chemical shift of sil-EDOT compared to sil-T (10.2 ppm vs. 7.9 ppm) clearly indicates that this interaction is very weak (Fig. S23 in the Supporting Information).

The rather strong S-π interactions 83 (d S-π = 3.24 Å) between the thiophene moieties and the adjacent phenyl rings likely explains the quite uncommon nearly perpendicular arrangement of these groups in respect to the silole mean plane (torsion angle = 88.4°). Desilylation reactions of 3,4-diphenylsiloles are known to afford the corresponding DPBs in good yields. 58,74 Following this strategy, DPBs were obtained in 60-87% yields by the treatment of 1,1-dimethylsiloles with n Bu 4 NF in THF under reflux (Scheme 2).

The desilylation reaction was confirmed by 1 H NMR spectroscopy with the disappearance of the singlet around 0.7 ppm assigned to the Si-CH 3 group and the appearance of a singlet at 6.3-6.4 ppm corresponding to the ethylenic protons. These compounds were also characterized by mass spectrometry.

Optical properties

The optical properties of DPBs were investigated by UV-Visible absorption and emission spectroscopy and the corresponding data are summarized in Table 1. All the DPBs exhibit absorption bands in the range between 340 and 460 nm with a more or less pronounced vibronic structure (Fig. 3). 48,58 The introduction of a stronger donating thiophene group > EDOT-DPB (2.94 eV) > BT-DPB (2.56 eV) (Table 1). close to those experimentally determined by UV-Visible absorption spectroscopy (see Table 1). This indicates that the conformation adopted by the butadienes in solution is well reproduced by the molecular optimization. The electrodeposition process is observed when reaching potential more anodic than the third oxidation process (see Fig. S36 in the Supporting Information) as classically observed for other molecular systems. 91,92 The deposits obtained from BT-DPB are also electroactives with p-and n-doping processes, the latter being also less intense than the pdoping process but relatively more intense compared to the one of poly(T-DPB) (Fig. 10b).

The onset potential of the p-doping process is recorded at 0.68 V and presents two successive oxidation waves with maxima at 1.18 and 1.87 V. In reduction, the n-doping process starts at -0.94 V and presents a reversible n-doping process with a maximum in reduction at -1.39 V. The E gelec of poly(BT-DPB) is therefore calculated at 1.62 eV slightly larger than the one of poly(T-DPB) (1.54 eV).

Finally, for EDOT-DPB, the first oxidation process with the maximum at 0.63 V is irreversible (Fig. S37 in the Supporting Information). However, upon cycling up to 0.9 V, an electrodeposition process is observed. This process being also more intense reaching anodic values increasing from 0.9 to 2.0 V (Fig. S38 in the Supporting Information). As a deposit is formed on the working electrode since the onset potential of the second oxidation wave (1.4

V), it is difficult to clearly ascribe the nature of the different oxidation processes occurring at more anodic potentials (1.58, 1.82, 2.17 V) as the different waves may be due to the oxidation of EDOT-DPB or of its derived polymer. The poly(EDOT-DPB) deposits also present p-and n-doping processes (Fig. 10c) with the n-doping process being less intense than the pdoping one but more intense relatively to the n-doping processes of the previous polymers

(see above n-doping processes of poly(T-DPB) and poly(BT-DPB)). Poly(EDOT-DPB) presents

a p-doping process with an onset potential at 0.34 V and a maximum at 0.76 V and a ndoping process with an onset potential at -1.1 V and a maximum at -1.4 V.

As observed in Fig. 10 observed for other deposits with n-doping either less intense 93,94 or more intense 95,96 than the p-doping process and may be due to the nature of the polymers but also, to the composition of the electrolytic medium in which the polymer is studied. The difference between the HOMO (-4.74 eV) and LUMO (-3. What is the electropolymerization process? Due to the presence of thienyl, bithienyl

or EDOT units at the external position of the diphenylbuta-1,3-diene core, the coupling between two monomer units should involve these thienyl, bithienyl or EDOT units. Such coupling classically occurs between the carbon atoms in α position of the sulfur of the thienyl 97,98 or the EDOT groups. [99][100][101][102][103] Depending on the studied molecule, on the potential reached during the electrodeposition process and on the number of scans, the insoluble deposit possesses its own electrochemical behaviour. A possible structure of the polymers is proposed in insets of Fig. 10.

Electrodepositions were also performed along oxidation at fixed potentials on classical platinum disk and also on transparent glasses coated by ITO (indium-tin oxide) in order to record the absorption spectra of the deposits under their neutral (undoped) or pdoped state. Although appealing to go more in-depth in the knowledge of the electronic properties of the deposits, these studies are difficult to be completed. In fact, if the deposit is too thick, it is difficult to be totally reduced and on the other hand, too thin deposits may be more soluble and thus, may be dissolved when rinsing the modified ITO electrode between the electrochemical cell and the UV-visible spectrophotometer tank or may also be scratched from the ITO surface upon rinsing.

The UV-Visible absorption spectra of the three deposits under their neutral or pdoped states are presented in Figure 11. In Figure 11a, the p-doped poly(T-DPB) exhibits a large absorption band centered around 400 nm and a conduction band centered at 1050 nm (black line). Under its undoped neutral state (red line), the polymer absorbs between 300 and 700 nm with an onset absorption wavelength around 630 nm which expresses in a 1.96

Techniques

The NMR spectra were recorded on a BRUKER Avance III -500 MHz or a BRUKER Avance

Conclusions

In summary, we have prepared DPBs containing electropolymerizable thienyl,

Conflicts of interest

There are no conflicts to declare

Figure 1 .

 1 Figure 1. X-ray single-crystal structure of sil-EDOT showing the intramolecular Si-O and S-π interactions.

Scheme 2 . 84 Figure 2 .

 2842 Scheme 2. Desilylation reaction for the synthesis of butadiene chromophores.

(

  EDOT) and a bisthienyl unit with a longer conjugation length results in an increase in the absorption maxima compared to T-DPB, following the order: 434 nm (BT-DPB) > 383 nm (EDOT-DPB) > 362 nm (T-DPB). The optical bandgaps (Eg) were also estimated from the onset wavelength of the UV-Vis absorption spectra, which are found in the order of T-DPB(3.11 eV)

Figure 3 .

 3 Figure 3. UV-Visible absorption spectra of DPBs in THF.

Figure 4 .

 4 Figure 4. Energy and shape of the HOMO (bottom) and LUMO (top)orbitals of optimized structures of T-DPB, BT-DPB and EDOT-DPB, as calculated at the B3LYP/6-31+G* level.

Figure 5 .

 5 Figure 5. Emission spectra of T-DPB ( exc = 360 nm, 2.2 x 10 -6 M, black), BT-DPB ( exc = 445 nm, 2.5 x 10 -5 M, red) and EDOT-DPB ( exc = 380 nm, 4 x 10 -5 M, blue) in THF.

Figure 6 . 6 , 82 Figure 7 .

 66827 Figure 6. Changes of fluorescence intensity of DPBs (1 mM) in THF under the continuous irradiation from a 9 W UV lamp.

Figure 9 .

 9 Figure 9. Normalized cyclic voltammograms of T-DPB, BT-DPB and EDOT-DPB, CH 2 Cl 2 + Bu 4 NPF 6 0.2 M, 100 mV.s-1, Pt disk working electrode. The CVs are normalized at the first respective a) reduction and b) oxidation waves.

  ).Looking more deeply to the successive oxidation waves of each compounds shows the different electrochemical behaviour of the three molecules (see detailed CVs of each compounds in SI). For T-DPB, the first oxidation is irreversible (Fig.S33in the Supporting Information) whatever the sweep-rate. When cycling at more positive values, i.e. higher than 1.75 V (the onset potential of the second oxidation wave with a maximum at 1.81 V), an electrodeposition process is observed along recurrent scans by the appearance and the growth of a new reversible redox process at potential less anodic than the first oxidation of T-DPB) and by the covering of the electrode surface by an insoluble deposit (Fig.S34in the Supporting Information). The presence of such deposit on the working electrode surface since the second oxidation of T-DPB renders difficult to fully detail the electrochemical process occurring at higher potential values (1.93 V) because this third oxidation process may come from the oxidation of T-DPB as well as from the oxidation of its derived polymer.The electrodeposition process is more and more intense, increasing the anodic potential values from 1.75 to 2.0 V. The insoluble deposits covering the electrode surface after such recurrent sweeps are electroactives both in oxidation and in reduction, the n-doping process being however, largely less intense than the p-doping process (Fig.10a). The p-doping process present a reversible wave with a maximum at 1.06 V and an onset oxidation value of 0.73 V (HOMO of poly(T-DPB): -5.13 eV). In reduction, the onset potential is of -0.81 eV showing a LUMO of -3.59 eV. The electrochemical bandgap E gelec of poly(T-DPB) is therefore of 1.54 eV. For BT-DPB, the two first waves are reversible (see Fig. S35 in the Supporting Information) expressing a higher stability of the radical cation (BT-DPB .+ ) and of the bis-radical cation or dication (BT-DPB 2+ ) due to the extended conjugation of this compound compared to T-DPB.

Figure 10 .

 10 Figure 10. Cyclic voltammograms of three deposits obtained from the oxidation of T-DPB, BT-DPB and EDOT-DPB, CH 2 Cl 2 + Bu 4 NPF 6 0.2 M, 100 mV.s -1 , Pt disk working electrode. In black: the p-doping

  , the n-doping processes of the three deposits are not similar both in term of onset potential values (LUMO) and of intensity of the electrochemical processes. Poly(T-DPB) possesses the lowest LUMO (-3.59 eV) followed by poly(BT-DPB) (-3.46 eV) and the poly(EDOT-DPB) (-3.30 eV). The LUMO levels have decreased from the monomer to the polymer of around 1 eV (1.27 eV for T-DPB, 0.87 eV for BT-DPB and 1.02 eV for EDOT-DPB). In term of intensity, the less intense n-doping process (in comparison with the p-doped process) is observed for poly(T-DPB) whereas the n-doping is more intense for poly(BT-DPB) and poly(EDOT-DPB). This variation in intensity may be related to the difficulty of the rather bulky tetrabutylammonium cation to diffuse in the different polymer bulks to insure electroneutrality of the n-doped polymer. This may be in relation with the different structure in term of density or porosity of the three polymers. It should be pointed out that such difference in intensity between the p-and n-doping processes has been previously

  30 eV) levels, calculated from the onset potential values, indicates that poly(EDOT-DPB) presents a bandgap of 1.44 eV, smallest compared to poly(T-DPB): 1.54 eV and poly(BT-DPB): 1.62 eV.

eV optical bandgap contracted of 1

 1 eV compared to the optical bandgap of its monomer T-DPB (2.96 eV). This bandgap contraction shows a significant extension of conjugation in the deposit. The conduction band centered at 1050 nm indicates a conduction band of less than 1 eV (ca: 0.88 eV by extrapolation of the onset wavelength of the conduction band (1404 nm).

Figure 11 .

 11 Figure 11. Absorption spectra of the polymers obtained on ITO surface by anodic oxidation of T-DPB, BT-DPB and EDOT-DPB. The spectra are recorded with the neutral (undoped) or p-doped states of the deposits.

III - 600 3 . 3 .

 60033 MHz. The chemical shifts were referred to the solvent peak, δ = 5.32 ppm and δ = 54.0 ppm for CD 2 Cl 2 , δ = 7.26 ppm and δ = 77.16 ppm for CDCl3 or δ = 2.50 ppm and δ = 39.52 ppm for dmso-d6 for 1 H and13 C{ 1 H} NMR spectra, respectively. Solid-state29 Si CP/MAS NMR spectra were recorded on a Varian VNMRS 300 MHz spectrometer at resonance frequency of 59.60 MHz for29 Si using the cross-polarization (CP), magic-angle spinning (MAS), and a high-power 1 H decoupling. The powder samples were placed in a pencil-type zirconia rotor of 3.2 mm diameter. The spectra were obtained at a spinning rate of 6 kHz (4 μs 90° pulses), a 5-ms CP pulse, and a recycle delay of 20 s. The Si signal of tetramethylsilane (TMS) at 0 ppm was used as the reference of29 Si chemical shift. The mass spectra were recorded on a Synapt G2-S (Waters) with an ASAP ionization source and a Q-TOF analyser. The UV-Visible absorption spectra were recorded at 25°C on a JASCO V-650 spectrophotometer in 10 mm quartz cells (Hellma). The extinction coefficient were determined by preparing solutions of butadiene derivatives at different concentration in THF. The concentration range was chosen to remain in the linear range of the Beer-Lambert relationship (A ca. 0.2-0.8). The onset wavelength of the absorption spectra was determined by the intersection of the straight line fitted to the right hand side of the maximum peak baseline of the absorption spectra 106,107 (FigureS27in the Supporting Information). The emission spectra were recorded at 25°C on a fluorescence spectrophotometer (FS920, Edinburgh Instrument), equipped with a calibrated photomultiplier in a Peltier (air cooled) housing (R928P, Hamamatsu), with a 450W continuous Xenon arc lamp as the excitation source for steady-state photoluminescence measurement using a 10 mm quartz cells (Hellma) with excitation path length. All electrochemical experiments were performed under an argon atmosphere, using a Pt disk electrode (diameter1 mm), the counter electrode was a vitreous carbon rod and the reference electrode was a silver wire in a 0.1 M AgNO 3 solution in CH 3 CN. Ferrocene was added to the electrolyte solution at the end of a series of experiments. The ferrocene/ferrocenium (Fc/Fc + ) couple served as the internal standard. The three electrode cell was connected to a PAR Model 273 potentiostat/galvanostat (PAR, EG&G, USA) monitored with the ECHEM Software. Activated Al 2 O 3 was added to the electrolytic solution to remove excess moisture. All potentials are referred to the SCE electrode that was calibrated at -0.405 V vs. the Fc/Fc + system. Synthetic procedures General procedure for the preparation of silole derivatives by Stille crooscoupling. 2,5-dibromo-1,1-dimethyl-3,4-diphenylsilole (200 mg, 0.5 mmol, 1eq), tris(dibenzylideneacetone)palladium (0)-chloroform adduct (24.8 mg, 25 µmol, 5 mol%), triphenylphosphine (12.5 mg, 50 µmol, 10 mol%) were dissolved in anhydrous THF into a two-neck round-bottom flask under argon atmosphere. The stannyl derivative was added into the solution, then the mixture was stirred and heated under reflux for 15 h. The mixture was concentrated and purified by flash chromatography using cyclohexane / DCM as eluent (100/0 -90/10

  bisthienyl and EDOT units through desilylation reactions of the corresponding 3,4diphenylsiloles derivatives. These three DPBs possess typical AIE/AEE characteristics with, as expected, an increase of the fluorescence when aggregated. Specifically, T-DPB and EDOT-DPB probably with stronger aromatic C-H/ interactions are found to be good AIEgens while BT-DPB exhibited AEE behaviour. The electrochemical properties of these DPBs also revealed for all these compounds the occurrence of electropolymerization process leading to electrogenerated polymers with low bandgaps (1.54 eV for poly(T-DPB), 1.62 eV for poly(BT-DPB) and 1.44 eV for poly(EDOT-DPB)). After p-doping process, UV-Visible absorption studies suggest that poly(T-DPB) and poly(EDOT-DPB) exhibit a lower conduction band than poly(BT-DPB).

  IEM -UMR 5635, ENSCM, CNRS, Université de Montpellier, Montpellier, France † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: Characterization and crystal data for Sil-EDOT and DPBs derivatives. See DOI: 10.1039/x0xx00000x

	Roger, Sébastien Richeter, a Kassem Amro, a Sébastien Clément * a Joëlle Rault-Berthelot, a and Philippe Gerbier * a Mathias Quiot, a	a Arie Van der Lee,	a Cyril Poriel,	a
	a.	ICGM, Univ. Montpellier, CNRS, ENSCM, Montpellier, France. E-mail:
	sebastien.clement1@umontpellier.fr; philippe.gerbier@umontpellier.fr
	b.	Univ Rennes, CNRS, ISCR-UMR CNRS 6226, F-35000 Rennes, France.
	c.	Institut Européen des Membranes,

Table 1 .

 1 Optical characteristics of DPBs.

		T-DPB	BT-DPB	EDOT-DPB
	 abs (nm) (ε (L.mol -1. cm -1 ))  em (nm) Ф THF (%) Ф agg (%) a Optical Eg (eV) Calculated E g (eV) b	346 (27 700) 362 (38 800) 382 (29 500) 452 1 16 3.11 3.20	408 (39 700) 434 (53 700) 460 (39 100) 529 2 4 2.56 2.52	365 (36 100) 383 (56 700) 405 (49 500) 466 1 11 2.94 3.04

a aggregation with a H 2 O:THF (8:2) mixture; b Energy band gap between HOMO-LUMO.

  The increase of E gelec from BT-DPB (2.47 eV) to EDOT-DPB (2.67 eV) and T-DPB (2.96 eV) is in accordance with the one obtained from optical data (2.56, 2.94 and 3.11 eV, see table 1) and from theoretical calculations (2.52, 3.04 and 3.20 eV, see table 1

  ). .95-6.91 (m, 4H), 6.82 (d,3 J H-H = 3.9 Hz, 2H), 0.62 (s, 6H, CH 3 ) ppm.13 C{1 H} NMR (125 MHz, CD 2 Cl 2 , δ): 153.9, 142.6, 139.4, 138.0, 137.4, 132.4, 130.0, 129.1, 128.6, 128.3, 127.9, 127.9, 124.8, 123.8, 123.5, -1.6 (Si-CH 3 ) ppm.29 Si{ 1 H} NMR (99.5 MHz, CD 2 Cl 2 , δ): 10.8 ppm. HR-MS (ASAP + ) m/z calc. for C 34 H 27 S 4 Si + [Sil-BT + H] + 591.0765; found, MHz, CD 2 Cl 2 , δ): 10.8 ppm. HR-MS (ASAP + ) m/z calc. for C 30 H 27 O 4 S 2 Si + [Sil-EDOT + H] + 542.1042; found, 542.1050.General procedure for the preparation of DPB derivatives. Silole (0.55 mmol) was dissolved in THF (20 mL) into a two-neck round-bottom flask. A solution of ntetrabutylammonium fluoride (5.5 mL, 1M in THF, 5.5 mmol, 10 eq.) was added. Then, the mixture was stirred and heated under reflux for 4 h. After cooling at room temperature, the mixture was quenched with water (100 mL). The reaction mixture was extracted with CH 2 Cl 2 (2 x 50 mL). Then, the combined organic phases were dried with MgSO 4 and evaporated under vacuum. The residue was recrystallized from CH 2 Cl 2 / n-hexane mixture.T-DPB: pale yellow crystals. Yield: 178 mg (0.48 mmol, 87%).1 H NMR (600 MHz, CDCl 3 , δ): 7.59 -7.50 (m, 6H), 7.39 -7.33 (m, 4H), 6.98 (d, 2H, 3 J H-H = 5.1 Hz), 6.77 (dd, 2H, 3 J H-H = 5.1, 3.7 Hz), 6.62 (dd, 2H, 3 J H-H = 3.7 Hz, 1 Hz), 6.36 (s, 2H, =CH) ppm;13 C{ 1 H} NMR (150 MHz, CDCl 3 , δ): 142.8, 141.6, 138.5, 130.7, 129.7, 129.2, 128.3, 127.0, 126.2, 125.0 ppm. HR-MS (ASAP + ) m/z: calc for C 24 H 19 S 2 + [T-DPB+ H] + 370.0850; found, 370.0853; UV-Vis (THF) λmax (ε, L.mol -1 .cm -1 ) = 346 (27 700), 362 (38 800), 382 (29 500) nm. BT-DPB: orange crystals. Yield: 209 mg (0.39 mmol, 71%). 1 H NMR (500 MHz, CDCl 3 , δ): 7.60-7.52 (m, 6H), 7.38-7.33 (m, 4H), 7.12-7.09 (dd, 2H, 3 J H-H = 2, 2.2 Hz), 6,93-6,89 (m, 4H), 6.89-6.83 (d, 2H, 3 J H-H = 4 Hz), 6.56-6.52 (d, 2H, 3 J H-H = 4 Hz), 6.29 (s, 2H, =CH) ppm. 13 C{ 1 H} NMR (125 MHz, CDCl 3 , δ): 142.9, 140.8, 138.5, 138.2, 137.7, 130.7, 130.6, 129.8, 128.5, 127.9, 125.1, 124.4, 123.5, 123.1 ppm. HR-MS (ASAP + ) m/z: calc for C 32 H 23 S 4 + [BT-DPB + H] + , 534.0604; found, 534.0599. UV-Vis (THF) λmax (ε, L.mol - 1 .cm -1 ) = 408 (39 700), 434 (53 400), 460 (39 100) nm. EDOT-DPB: orange powder. Yield: 161 mg (0.33 mmol, 60%). 1 H NMR (600 MHz, DMSO-d6, δ): 7.57-7.48 (m, 6H), 7.27 (d, 4H, 3 J H-H = 7.1 Hz), 6.29 (s, 2H), 6.17 (s, 2H), 4.05 (q, 8H, 3 J H-H = 5.2 Hz, CH 2 ) ppm. 13 C{ 1 H} NMR (150 MHz, DMSO-d6, δ): 140.8, 140.2, 140.1, 138.0, 130.3, 129.5, 128.4, 118.8, 115.3, 101.4, 64.6, 64.2 ppm. HR-MS (ASAP+) m/z: calc. for C 28 H 23 O 4 S 2 + [EDOT-DPB + H] + 486.0960; found, 486.0967. UV-Vis (THF) λ (ε, L.mol -1 .cm -1 ) = 365 (36 100), 383 (56 700), 405 (49 500) nm.

	Sil-T: yellow powder. Yield 106 mg (0.25 mmol, 50%). 1 H NMR (500 MHz, CD 2 Cl 2 , δ): 7.22-7.17 (m, 6H), 7.04-7.00 (m, 6H), 6.92-6.87 (m, 4H), 0.69 (s, 6H, CH 3 ) ppm. 13 C{ 1 H} NMR (125 MHz, CD 2 Cl 2 , δ): 153.0, 143.2, 139.8, 132.2, 130.1, 129.0, 127.6, 127.5, 126.7, 126.2, -1.8 (Si-CH3) ppm. 29 Si{ 1 H} NMR (99.5 MHz, CD 2 Cl 2 , δ): 9.5 ppm. HR-MS Sil-EDOT: orange-yellow powder. Yield: 108 mg (0.2 mmol, 40%). 1 H NMR (500 MHz, CD 2 Cl 2 , δ): 7.16-7.12 (m, 6H), 6.97-6.95 (m, 4H), 5.98 (s, 2H), 4.22-4.18 (m, 8H, CH 2 ), 0.62 (s, 6H, CH 3 ) ppm. C{ 1 H} NMR (125 MHz, CD 2 Cl 2 , δ): 150.9, 141.3, 140.3, 140.0, 130.8, 129.6, 128.7, 127.5, 119.9, 100.1, 65.1, 64.0, -3.3 (Si-CH3) ppm. 29 Si{ 1 H} (ASAP 591.0766. NMR (99.5

+ ) m/z calc. for C 26 H 23 S 2 Si + [Sil-T + H] + 427.1010; found, 427.1014. Sil-BT: red powder. Yield 128 mg (0.22 mmol, 44%). 1 H NMR (500 MHz, CD 2 Cl 2 , δ): 7.26-7.20 (m, 6H), 7.17-7.16 (m, 2H), 7.06-7.03 (m, 4H), 6.97 (d, 3 J H-H = 3.9 Hz, 2H), 6

Acknowledgements

The authors thank the Université de Montpellier for financial support.


J Martin

P Haraux

V Ntomprougkidis

S Migot

S Bruyère

G Henrion

Characterization of metal oxide micro/nanoparticles elaborated by plasma electrolytic oxidation of aluminium and zirconium alloys

Keywords: Plasma electrolytic oxidation (PEO), Micro-arcs oxidation (MAO), Aluminium, Zirconium, Metal oxide particles, Oxide coatings

 PEO process produces polycrystalline metal oxide particles (Al2O3 and ZrO2)  Particles exhibit a hollow spherical morphology with size between 0.01-10 µm  Morphology of the particles is correlated to the PEO "pancake" structure  A formation mechanism of particles during PEO is proposed Graphical

Introduction

Plasma electrolytic oxidation (PEO), also referred to as micro-arc oxidation (MAO), is an anodic plasma electrolysis process that allows growing protective ceramic-like oxide coatings on valve metals such as aluminium, titanium, magnesium and zirconium alloys [START_REF] Gh | Plasma electrolytic oxidation of magnesium and its alloys: Mechanism, properties and applications[END_REF][START_REF] Cheng | Key factors determining the development of two morphologies of plasma electrolytic coatings on an Al-Cu-Li alloy in aluminate electrolytes[END_REF][START_REF] Cheng | Wear resistant coatings formed on Zircaloy-2 by plasma electrolytic oxidation in sodium aluminate electrolytes[END_REF][START_REF] Mathis | Electrochemical behaviour of titanium in KOH at high potential[END_REF]. PEO process gains a growing interest in various industrial domains (transport, energy, medicine) to replace conventional chromic acid anodizing (CAA) or hard acid anodizing (HAA) [START_REF] Yerokhin | Plasma electrolysis for surface engineering[END_REF][START_REF] Gupta | Electrolytic plasma technology: Science and engineering -An overview[END_REF].

Indeed, PEO results in improved wear and corrosion resistance together with enhanced thermal stability and biocompatibility properties [START_REF] Gupta | Electrolytic plasma technology: Science and engineering -An overview[END_REF][START_REF] Curran | Thermo-physical properties of plasma electrolytic oxide coatings on aluminium[END_REF][START_REF] Santos-Coquillat | Bioactive multi-elemental PEO-coatings on titanium for dental implant applications[END_REF]. PEO also uses low concentrated alkaline electrolytes depleted in toxic compounds, which makes it an environmentally friendly process.

Achieved at a voltage higher than the dielectric breakdown voltages of the growing oxide layer, PEO coatings are formed under an anodic sparking regime characterized by the ignition of tiny and short-lived micro-discharges that ignite randomly over the processed surface. This sparking regime gradually leads to the conversion of the processed metal into a crystalline oxide layer [START_REF] Matykina | Real-time imaging of coating growth during plasma electrolytic oxidation of titanium[END_REF][START_REF] Monfort | Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte[END_REF][START_REF] Dehnavi | Phase transformation in plasma electrolytic oxidation on 6061 aluminium alloy[END_REF][START_REF] Martin | Formation of a mestastable nanostructured mullite during Plasma Electrolytic Oxidation of aluminium in "soft" regime condition[END_REF]. The formation mechanisms of the protective PEO coatings remain complex due to the combination of thermal-, electro-, and plasma-assisted chemical reactions that simultaneously occur in a low affected volume (tens of µm 3 ) [START_REF] Yerokhin | Discharge characterization in plasma electrolytic oxidation of aluminium[END_REF][START_REF] Hussein | Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process[END_REF][START_REF] Jovović | Spectroscopic study of plasma during electrolytic oxidation of magnesium-and aluminium-alloy[END_REF][START_REF] Stojadinović | Investigation of plasma electrolytic oxidation on valve metals by means of molecular spectroscopya review[END_REF][START_REF] Clyne | A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals[END_REF].

Among the inherent aspects of the PEO process, it is frequently observed that the PEO process generates residual particles that are usually found at the bottom of the electrolytic cell.

The particles produced during the PEO process have been very little studied. Zhan et al. [START_REF] Zhan | The synthesis of micro and nano WO3 powders under the sparks of plasma electrolytic oxidation of Al in a tungstate electrolyte[END_REF] reported the synthesis of WO3 particles during PEO of Al in a tungstate electrolyte. They showed that particles exhibit an irregular shape with a size distribution ranging from 100 nm to 1 µm. Authors explained their formation by the decomposition of the electrolyte species within the micro-discharge plasma. Surprisingly, they did not evidence the simultaneous formation of Al2O3 particles. In addition, the synthesis of particles via cathodic plasma electrolysis processes is widely documented in the literature [START_REF] Allagui | Gas film formation time and gas film lifetime during electrochemical discharge phenomenon[END_REF][START_REF] Gromov | Cathode plasma electrolysis in diluted potassium hydroxide solutions : Particles formation and energetic estimation[END_REF][START_REF] Abdul Kareem | Glow discharge plasma electrolysis for nanoparticles synthesis[END_REF][START_REF] Toriyabe | Controlled formation of metallic nanoballs during plasma electrolysis[END_REF][START_REF] Liu | One-step preparation of TiO2 particles with controllable phase and morphology by plasma electrolysis[END_REF][START_REF] Wu | Titanium dioxide nanospheres with wide spectral absorption prepared by low-voltage plasma electrolysis[END_REF][START_REF] Zhang | Plasma-electrolysis synthesis of TiO2 nano/microspheres with optical absorption extended into the infra-red region[END_REF][START_REF] Julaihi | formation of stainless steel nanoballs via submerged glow-discharge plasma and their microstructural analysis with evaluation of photocatalytic activity[END_REF][START_REF] Liu | Peanut shaped titanium oxide microparticles achieved by cathode plasma electrolysis and their electrorheological characteristics[END_REF][START_REF] Wu | Cathodic plasma electrolysis for preparation of diamond-like carbon particles in glycerol solution[END_REF][START_REF] Allagui | Synthesis of Ni and Pt nanomaterials by cathodic contact glow discharge electrolysis in acidic and alkaline media[END_REF]. During cathodic plasma electrolysis, a thin gaseous envelope (mainly containing hydrogen) forms around the cathode; the latter is therefore separated from the surrounding electrolyte. When the applied voltage is high enough to induce the dielectric breakdown of the gaseous envelope, a quite stable glow-discharge plasma sheath is created and covers the cathode [START_REF] Allagui | Gas film formation time and gas film lifetime during electrochemical discharge phenomenon[END_REF]. Within this cathodic plasma discharge regime, the processed metal rapidly erodes and melted particles are released from the cathode into the water-based electrolyte [START_REF] Gromov | Cathode plasma electrolysis in diluted potassium hydroxide solutions : Particles formation and energetic estimation[END_REF][START_REF] Abdul Kareem | Glow discharge plasma electrolysis for nanoparticles synthesis[END_REF]. When particles enter into the surrounding cold electrolyte, they usually solidify into spherical nanoballs with diameter ranging from 10 nm to 10 µm, depending on the operating material and the electrolyte chemistry [START_REF] Toriyabe | Controlled formation of metallic nanoballs during plasma electrolysis[END_REF][START_REF] Liu | One-step preparation of TiO2 particles with controllable phase and morphology by plasma electrolysis[END_REF]. When subsequently exposed to air, the surface of some particles gets oxidized, thus forming core-shell structures with attractive new properties like photocatalytic effects for instance [START_REF] Wu | Titanium dioxide nanospheres with wide spectral absorption prepared by low-voltage plasma electrolysis[END_REF][START_REF] Zhang | Plasma-electrolysis synthesis of TiO2 nano/microspheres with optical absorption extended into the infra-red region[END_REF][START_REF] Julaihi | formation of stainless steel nanoballs via submerged glow-discharge plasma and their microstructural analysis with evaluation of photocatalytic activity[END_REF][START_REF] Liu | Peanut shaped titanium oxide microparticles achieved by cathode plasma electrolysis and their electrorheological characteristics[END_REF]. Various metal or metal oxide nanoparticles have been successfully produced by cathodic plasma electrolysis including steel, copper titanium, zinc, silver, nickel, gold and platinum [START_REF] Abdul Kareem | Glow discharge plasma electrolysis for nanoparticles synthesis[END_REF][START_REF] Wu | Cathodic plasma electrolysis for preparation of diamond-like carbon particles in glycerol solution[END_REF][START_REF] Allagui | Synthesis of Ni and Pt nanomaterials by cathodic contact glow discharge electrolysis in acidic and alkaline media[END_REF].

However, the mechanisms that govern the cathodic plasma electrolysis strongly differ from those encountered in PEO during which localized micro-discharges ignite at the anode due to the dielectric breakdown of the growing oxide layer [START_REF] Yerokhin | Plasma electrolysis for surface engineering[END_REF][START_REF] Yerokhin | Discharge characterization in plasma electrolytic oxidation of aluminium[END_REF][START_REF] Clyne | A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals[END_REF]. Consequently, it is reasonable to assume that particles produced with the PEO process would exhibit microstructural differences compared to those collected during cathodic plasma electrolysis. This motivated the present study that consists in deeply investigating the microstructure of the particles formed during the PEO process. Comparatively, the microstructure of the PEO oxide layer grown within the same conditions are also characterized and compared to the produced particles.

Finally, on the basis of the experimental results, a descriptive mechanism of the formation of particles during anodic plasma electrolysis is discussed.

Experimental procedure 2.1 Materials

The PEO processed materials consisted of three commercial metallic alloys, a low-alloyed aluminium (Al-1050 grade), a copper-rich aluminium alloy (Al-2024 grade) and a low-alloyed zirconium (Zr-M5 grade). Table 1 gives the chemical composition of these three alloys. The Al-1050 and the Al-2024 aluminium samples had a rectangular shape of 45 mm  25 mm  5 mm in size while the Zr-M5 zirconium samples exhibited a tubular shape of 30 mm in height, 15 mm in external diameter and 1 mm in thickness. Dimensions of all samples were chosen to keep the same surface area (0.28 dm 2 ), and therefore the same current density applied to the PEO-processed samples.

PEO processing conditions

The samples were PEO-processed in 4 L beaker containing a low-concentrated solution of potassium hydroxide ([KOH] = 1 g.L -1  0.018 mol.L -1 ) diluted in deionised water. In order to prevent the electrolyte from any pollution, and to avoid the dissemination of the produced particles in the cooling circuit, the electrolyte was cooled using a double wall system. As described in Fig. 1a, the beaker containing the electrolyte is located into a larger tank filled with a stream of cold water. A magnetic stirrer at the bottom of the beaker ensures the homogenization of the electrolyte temperature. Within these cooling conditions, the electrolyte temperature was kept below 60 °C. For each PEO treatment, the processed sample was systematically located at 90 mm apart from the counter-electrodes (two titanium plates of 200 mm  200 mm  1 mm in size). PEO treatments were carried out using a pulsed bipolar current generator within the so-called "soft" regime electrical conditions [START_REF] Jaspard-Mecuson | Tailored aluminium oxide layers by bipolar current adjustment in the Plasma Electrolytic Oxidation (PEO) process[END_REF][START_REF] Rogov | The role of cathodic current in Plasma Electrolytic Oxidation of aluminium: phenomenological concepts of the "soft sparking" mode[END_REF][START_REF] Tsai | Probe the micro arc softening phenomenon with pulse transient analysis in plasma electrolytic oxidation[END_REF][START_REF] Martin | Delay in micro-discharges appearance during PEO of Al: Evidence of a mechanism of charge accumulation at the electrolyte /oxide interface[END_REF]. Within these electrical conditions, the ratio of anodic to cathodic charge quantity (RCQ = Qp/Qn) was set at 0.9. The current pulse frequency (F), the anodic current amplitude (Ia) and the anodic current density (ja) were set at 100 Hz, 10 A and 36 A.dm -2 , respectively. The duration of all the PEO treatments was fixed at 30 min. It is worth mentioning that, although PEO treatments were performed in electrical conditions that would allow the "soft" regime to occur at about 50 min., the transition from the "arc" to the "soft" growth regime did not appear due to a too short processing duration [START_REF] Martin | Effects of electrical parameters on plasma electrolytic oxidation of aluminium[END_REF].

Particles collection and ex situ characterization

As illustrated in Fig. 1b, the electrolyte containing suspended particles was then filtered using the Büchner filtering method. The filter paper used (Whatman  Cyclopore polycarbonate membrane) allowed retaining particles with size larger than 100 nm.

Additionally, a TEM grid that consists of a 300 mesh nickel grid covered with a carbon film was also placed onto the surface of the filter paper. After filtering, the filter paper and the TEM grid were dried in an oven at 60 °C for 12 h.

Pictures of the electrolyte and the filter paper were taken before and after filtering. The particles remaining onto both the filter paper and on the TEM grid were observed using SEM and TEM techniques. Different magnifications were used in order to estimate the size distribution of the produced particles. The chemical composition and the elemental distribution of the formed particles were determined by EDX and STEM/EDX analyses. The crystallographic phase composition of the particles was investigated by XRD measurements operated in Bragg-Brentano geometry with a step size of 0.005° and a scan range from 30 to 60°. XRD measurements were performed on the surface of the filter paper using a Cu-K1 radiation at wavelength  = 0.1542 nm. Observation of the high-resolution TEM images combined with the analysis of the corresponding FFT patterns also led us to determine the phase composition of the produced particles. Additionally, FIB milling technique (Focused Ion Beam using a Ga + beam) was also used to cut the produced particles in half and to characterize their internal morphology. Finally, for comparison reasons, PEO coatings grown on the different substrates were also characterized by SEM, EDX and XRD techniques.

Results

Morphology of the particles

Fig. 2 shows the visual aspect of the electrolytes and the filter papers before and after the filtering step. Whatever the processed substrate, the electrolytes are colored before filtering while they are transparent after filtering. This suggests the presence of particles. The visual aspect of the filter papers after filtering also confirms the presence of particles in the electrolyte after the PEO treatment. PEO processing of the copper-alloyed aluminum (Al-2024) results in a brown coloured filter. In contrast, the filter exhibits a whitish appearance in the case of the low-alloyed aluminum (Al-1050). For the low-alloyed zirconium, the filter paper appears pinkish-brown. Fig. 3 shows the SEM micrographs of the filter paper before and after the filtering step in the case of the PEO process of the copper-alloyed aluminium (Al-2024). Before filtering, no particle is observed at the surface of the filter paper (Fig. 3a) while spherical particles with different sizes are observed after filtering (Fig. 3b,c,d). Fig. 4 shows the size distribution of the spherical particles observed for all the processed substrates. Whatever the processed substrate, particles exhibit quite the same size distribution, in the range of 0.1 -10 µm, with a mean size of  4 µm and the most probable size of 3 µm. Additionally, high magnification SEM micrograph in Fig. 3d show that particles are not perfectly spherical since some cracks and holes are visible over their surface. It is also worth mentioning that similar observations were done in the case of the PEO processing of Al-1050 and Zr-M5 samples. Some of these particles were cut in half using FIB milling technique. Whatever the PEOprocessed material, SEM micrographs in Fig. 5 show that particles exhibit an internal cavity that, for certain particles, opens outwards. Interestingly, the internal and the external morphology of the particles remain the same irrespective of the processed alloy. It suggests that the formation of particles follows an inherent mechanism of the PEO process and is independent of the processed metallic alloy.

Chemical composition of the particles

The elemental composition of the produced particles was determined by using EDX method.

The EDX spectra given in Fig. 6 were recorded on the particles as indicated in Fig. 5 2). The particles produced with the zirconium alloy are rich in Zr and O and their atomic concentration is close to the stoichiometry of zirconia ZrO2 (Spot 7 in Table 2). Additionally, in the specific case of the copper-alloyed aluminium (Al-2024), bright-field micrographs evidence the presence of nanometre-size needles surrounding the Al, O-rich spherical particle. The EDX element quantification performed on these needles (Spot 5 in Table 2) reveals the presence of Cu and O in large amount suggesting that they consist of copper oxide. In contrast, no similar needle was detected in the case of the low-alloyed aluminium substrate (Al-1050). Finally, detection of carbon residues is associated to the carbon film deposited on the TEM grid while titanium traces may come from the gradual erosion of the titanium counter-electrodes during the PEO treatments.

Crystallographic phase composition of the particles

Fig. 8 shows the XRD patterns of the PEO processed alloys (Al-2024, Al-1050 and Zr-M5), the resulting PEO oxide coatings and the filter papers (after filtering the electrolyte). First, XRD patterns of the PEO layers show the presence of Al and Zr peaks in the case of the PEO process of the aluminium alloys (Al-1050 and Al-2024) and the zirconium alloy (Zr-M5), respectively.

They originate from the metallic substrate lying beneath the oxide layer due to the X-ray penetration depth that exceeds the overall thickness of the formed PEO coating. Second, and as usually encountered for the PEO process of aluminium [START_REF] Martin | Formation of a mestastable nanostructured mullite during Plasma Electrolytic Oxidation of aluminium in "soft" regime condition[END_REF] and zirconium alloys [START_REF] Cheng | Wear resistant coatings formed on Zircaloy-2 by plasma electrolytic oxidation in sodium aluminate electrolytes[END_REF], the XRD patterns reveal the presence of two types of alumina (face-centred cubic and rhombohedral -Al2O3) through the PEO coatings grown on the two aluminium alloys and one type of zirconia (monoclinic ZrO2) through the PEO layer produced on the zirconium alloy. Third, indexing the XRD patterns recorded on the filter papers only evidences the presence of -,-Al2O3 and ZrO2 for the PEO process of the aluminium and the zirconium alloys, respectively. In the specific case of the copper-alloyed aluminium (Al-2024), the presence of copper oxide CuO is observed.

As a consequence, XRD measurements performed on the filter papers confirm the fact that the spherical particles consist of crystallized alumina or zirconia, depending on the metallic substrate, while the needle-shaped nanostructures are composed of crystallized copper oxide.

The presence of CuO in the case of the copper-alloyed aluminium explains the brown-coloured electrolyte as seen in Fig. 2. The partial dissolution in the alkaline electrolyte of the Al2Cu intermetallic compounds contained in the Al-2024 substrate can explain the formation of CuO. This results in the formation of Cu(OH)2 precipitates that subsequently experiences a plasmaassisted thermal decomposition into CuO [START_REF] Li | Fabrication of CuO nanofibers via the plasma decomposition of Cu(OH)2[END_REF][START_REF] Chen | Large-scale synthesis of CuO shuttle-like crystals via a convenient hydrothermal decomposition route[END_REF].

These results were doubly checked by performing deeper TEM observations as shown in Fig. 9. Bright-field TEM micrographs show that nanometre-sized spherical particles are also produced. High-resolution TEM micrographs confirm the crystalline state of the produced particles. Moreover, indexing the associated FFT patterns confirm the presence of -Al2O3 in the spherical particles produced during PEO of the Al-2024 substrate. The composition of the surrounding needle shaped particles is indeed CuO. Finally, spherical particles produced from the Zr-M5 substrate are crystallized under the m-ZrO2 phase.

Discussion

The discussion hereafter focuses on the formation of metal oxide particles during the PEO process. Particularly, a descriptive mechanism is proposed to explain their specific morphology i.e. a hollow spherical shape with cracks at their surface; this morphology is independent of the nature of the processed material. The proposed mechanism is mainly based on a close comparison between the morphology of these particles and the morphology of the corresponding elaborated PEO coatings. Fig. 10 shows the cross-sectional and the top-surface SEM micrographs of the PEO coating elaborated on the copper-alloyed aluminium (Al-2024). The observed morphology is characteristic of PEO coatings elaborated under the "arc" sparking regime (before the appearance of the "soft" regime) with the presence of large open discharge channels forming "pancake"-like structures over the top-most surface. Moreover, the associated XRD patterns evidence the presence of two crystalline polymorphs of alumina i.e. and -Al2O3. As a comparison, it was previously established that particles produced from the Al2024 alloy also consist of these two crystalline polymorphs (Fig. 8 and9). More interestingly, a close examination of the top-surface of the coating reveals the presence of spherical particles (Fig. 10). The latter is as attached at the top-surface of the coating, particularly near the "pancake" structure. Their spherical shape is the same as that of the particles observed on the filter paper. Fig. 10 also shows that the size of the attached particle is of the same order of magnitude as the diameter of the cavity of the surrounding "pancake", in the micrometre range. Finally, the EDX spectrum recorded on this attached particle shows that it consists mainly of Al and O suggesting the presence of crystalline alumina. Thus, close similarities are drawn between the microstructure of the produced particles and the grown PEO coating, mainly the typical morphology of the "pancake" structure. As depicted in Fig. 11, these similarities allow proposing a formation mechanism of metal oxide particles inherent to the PEO process.

As described previously with SEM observations, Fig. 11a presents the typical "pancake" structure that develops during the PEO within the "arc" regime. Nominé et al. have demonstrated that micro-discharges appear in "cascade" through the discharge channel of a "pancake" structure [START_REF] Nominé | High speed video evidence for localised discharge cascades during plasma electrolytic oxidation[END_REF][START_REF] Troughton | Effect of individual discharge cascades on the microstructure of plasma electrolytic oxidation coatings[END_REF]. From optical emission spectroscopy measurements, Martin et al.

recently estimated the gas temperature in the micro-discharge at about 3200 K [START_REF] Martin | Formation of a mestastable nanostructured mullite during Plasma Electrolytic Oxidation of aluminium in "soft" regime condition[END_REF]. At this temperature, the pre-formed oxide layer is locally and rapidly melted at a heating rate of ~10 8 K.s -1 (Fig. 11b). Troughton et al. have also reported the development of a pressure shockwave that radially propagates towards outside [START_REF] Troughton | Synchronised electrical monitoring and high-speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation[END_REF]. It results in the vaporisation of the surrounding electrolyte and the expansion of a gas bubble. By using fast video imaging to follow the dynamic of bubbles created by discharge in dielectric liquids and, by applying the Rayleigh-Plesset's cavitation model, Hamdan et al. estimated the initial pressure inside the bubble at several hundreds of bars [START_REF] Hamdan | Dynamics of bubbles created by plasma in heptane for micro-gap conditions[END_REF][START_REF] Hamdan | Time-resolved imaging of nanosecond-pulsed micro-discharges in heptane[END_REF]. More recently, by investigating individual discharges during plasma electrolytic oxidation, Throughton et al. estimated the pressure in the gas bubble at about 2.5 bars [START_REF] Troughton | Synchronised electrical monitoring and high-speed video of bubble growth associated with individual discharges during plasma electrolytic oxidation[END_REF]. After the extinction of the micro-discharges, whose life-time is about 10 µs [START_REF] Matykina | Real-time imaging of coating growth during plasma electrolytic oxidation of titanium[END_REF][START_REF] Clyne | A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals[END_REF][START_REF] Martin | Delay in micro-discharges appearance during PEO of Al: Evidence of a mechanism of charge accumulation at the electrolyte /oxide interface[END_REF][START_REF] Martin | Effects of electrical parameters on plasma electrolytic oxidation of aluminium[END_REF][START_REF] Nominé | High speed video evidence for localised discharge cascades during plasma electrolytic oxidation[END_REF], the gas bubble continues its expansion through the cavity of the "pancake" structure as illustrated in Fig. 11c. Because of the pressure gradient between the pancake cavity filled with a gas phase (P > Patm) and the electrolyte (Patm), the molten part of the oxide is pushed out of the cavity of the "pancake" structure towards the electrolyte. The molten oxide can only leave through the narrow orifice located at the top of the "pancake" structure and gradually inflates similarly to a rubber balloon (Fig. 11d). In contact with the cooled electrolyte, the molten oxide is rapidly cooled at a cooling rate of ~ 10 7 K.s -1 [START_REF] Martin | Formation of a mestastable nanostructured mullite during Plasma Electrolytic Oxidation of aluminium in "soft" regime condition[END_REF]. Under such a severe cooling condition, the oxide is quenched keeping a hollow spherical shape. Cracks appear at its surface due to the volume retraction of the oxide during solidification (Fig. 11e).The solidified particle may then either remain attached at the pancake or be ejected into the electrolyte if the electrolyte turbulence is sufficiently strong or if other micro-discharges ignite in the "pancake" cavity (Fig. 11f).

Conclusions

The formation of particles during the PEO processing of aluminium and zirconium alloys was investigated. The microstructure of the particles was characterized at different scales, mainly by using transmission electron microscopy. Whatever the processed material, results

show that particles exhibit the same morphology i.e. a hollow spherical shape with diameters in the range of 10 nm to 10 µm. They consist of polycrystalline metallic oxides i.e and alumina for the PEO of aluminium alloys and monoclinic zirconia for the zirconium alloy.

Microstructure of the produced particles was then compared to the microstructure of the PEO coatings grown on the metallic substrates. A correlation was drawn between the microstructure of the particles and the typical "pancake"-like structure of the PEO coating. On this basis, a descriptive mechanism of the formation of these particles was suggested as follows: Ignition of a micro-discharge inside the "pancake" cavity induces the propagation of a pressure shockwave that pushes the molten oxide through the exit orifice of the pancake. Like a rubber balloon, the molten oxide forms a hollow sphere and is rapidly solidified by the surrounding electrolyte and ejected towards the electrolyte. "Investissements d'avenir" operated by the French National Research Agency (ANR) and referenced to as ANR-11-LABX-0008-01 ('LabEx DAMAS').

 The authors would like to acknowledge contributions of the following:

-P. Boulet and the competence cluster on X-ray diffraction (CC-X) at Institut Jean Lamour for providing advices in XRD measurements and analyses.

-S. Mathieu, E. Etienne and the competence cluster on electron microscopy (CC 3M) at Institut Jean Lamour for providing advices in SEM observations and EDX analyses. c) The molten oxide is pushed towards the electrolyte under a pressure gradient between the "pancake" cavity (P > Patm) and the electrolyte (Patm) and inflates through the open hole of the "pancake" structure. A spherical particle is formed and rapidly quenched by the surrounding cooled electrolyte. d) Solidification cracks appear due to volume retraction and the solidified particle is ejected into the electrolyte.

List of tables

  . EDX spectra recorded on the particles produced with Al-2024 and Al-1050 show that they exclusively contain Al and O elements suggesting thereby the formation of alumina particles. The particles produced with the Zr-M5 are exclusively composed of Zr and O elements suggesting in this case the formation of zirconia particles. These first elemental analyses were confirmed by the element distributions and quantifications done with STEM/EDX analyses as shown in Fig. 7 and in Table 2, respectively. Spherical particles formed during the PEO process of the two aluminium alloys (Al-1050 and Al-2024) are mainly composed of Al and O. Their atomic concentration is close to the stoichiometry of alumina Al2O3 (Spot 4 and spot 6 in Table

Fig. 1 :

 1 Fig. 1: Schematic view of the experimental procedure that allows a) producing particles using the PEO process (picture of a 2024 aluminium alloy during PEO at 15 min processing time) and b) collecting particles using the Büchner filtering method (picture of the filter paper and the TEM grid after filtering).

Figure 2 :

 2 Figure 2: Visual aspect of the electrolytes and the filter papers (before and after filtering) for the PEO process of Al-2024, Al-1050 and Zr-M5 alloys.

Figure 3 :

 3 Figure 3: SEM micrographs of the filter paper recorded at different magnifications a) before filtering and b), c), d) after filtering in the case of the PEO process of an Al-2024 aluminium sample.

Figure 4 :

 4 Figure 4: Size distribution of the spherical particles observed on the filter paper by SEM and TEM after the PEO process of Al-1050, Al-2024 and Zr-M5 alloys.

Figure 5 :

 5 Figure 5: SEM micrographs of the particles (before and after FIB milling) produced during the PEO process of Al-1050, Al-2024 and Zr-M5 alloys. Coloured spots indicate locations of EDX measurements shown in Fig. 6.

Figure 6 :

 6 Figure 6: EDX spectra recorded on the particles produced during the PEO process of Al-1050, Al-2024 and Zr-M5 alloys. Location of spots 1, 2 and 3 are given in SEM micrographs in Fig.

Figure 7 :

 7 Figure 7: Bright-field STEM (BF-STEM) micrographs recorded on the particles produced during the PEO process of Al-1050, Al-2024 and Zr-M5 alloys and the associated STEM/EDX elemental map distributions.

Figure 8 :

 8 Figure 8: XRD patterns of the metallic substrate, the PEO oxide coating and the filter paper (after filtering the electrolyte) in the case of a) Al-1050, b) Al-2024 and c) Zr-M5 alloys.

Figure 9 :

 9 Figure 9: Bright-field TEM micrographs, high-resolution TEM micrographs and the associated FFT patterns recorded on the particles produced during the PEO process of Al-2024 and Zr-M5 alloys.

Figure 10 :

 10 Figure 10: Cross-sectional and top-view SEM micrographs of the PEO coating elaborated on the copper-alloyed 2024 aluminium. XRD pattern of the elaborated PEO coating and EDX spectrum specifically recorded on a spherical particle attached on the top-surface of the PEO coating.

Figure 11 :

 11 Figure 11: Schematic description of the formation mechanism of the metal oxide particles during the PEO process of aluminium and zirconium alloys. a) "Pancake" structure before the appearance of a micro-discharge. b) Ignition of a micro-discharge inside the "pancake" cavity resulting in the melting of the pre-existing oxide and the propagation of a pressure shockwave.

Table 1 :

 1 Chemical compositions of a 1050 aluminium alloy (noted Al-1050), a 2024 aluminium alloy (noted Al-2024) and a M5 zirconium alloy (noted Zr-M5).

	Elements Fe	Si	Zn Cu, Mg, Mn, Ti	Al
	Al-1050
	in wt.% 0.40 0.25 0.07	<0.05		Balance
	Elements	Cu		Mg	Mn	Si, Fe, Zn, Cr, Ti	Al
	Al-2024
	in wt.% 3.8 -4.9 1.2 -1.8 0.3 -0.9		<0.5	Balance
	Elements Nb O Fe, S	Zr
	Zr-M5
	in wt.%	1 0.14 <0.05 Balance

Table 2 :

 2 Elemental composition of the particles produced during the PEO process of Al-1050, Al-2024 and Zr-M5 alloys.

		Al	Zr	O	Cu	Ti	C
	Spot 4 * (in at.%)	38.4	-	58.4	3.2	-	-
	Spot 5 (in at.%)	1.1	-	52.3	43.5	-	3.1
	Spot 6 (in at.%)	41.3	-	57.7	-	1	-
	Spot 7 (in at.%)			65.7	-	-	-
	* From STEM/EDX analysis performed in regions reported in Fig.7.

Acknowledgments

 This work was supported by the French Government through the program

Data availability

The data that support the findings of this study are available upon reasonable request addressed to the corresponding author.

All the authors analysed the results and reviewed the manuscript.

Additional information

The authors declare no competing financial interests.