Vladimir L Solozhenko
email: vladimir.solozhenko@univ-paris13.fr

Yann Le Godec

A Hunt for Ultrahard Materials

teaching and research institutions in France or abroad, or from public or private research centers.

low-compressible (B = 420 GPa) cubic BN-C solid solutions with stoichiometry close to BCN and statistically uniform atom distribution were synthesized by isoentropic shock compression [9].

In 2009, another ultrahard phase, diamond-like BC5, was synthesized under high pressurehigh temperature conditions [10]. This new phase corresponds to the ultimate solubility of boron in diamond and possesses Vickers hardness of 71 (8) GPa, unusually high for superhard materials fracture toughness (~10 MPa•m ½ ), and very high (up to 1900 K) thermal stability which makes it an exceptional superabrasive overcoming diamond and promising material for high-temperature electronics. The beneficial combination of electrical conductivity, band structure that is uncommon for phases due to electron deficiency of boron atoms, and high thermal stability will eventually allow the expansion of the boundaries of high-power electronics at extreme conditions.

It should be noted that diamond-like BC5 and cubic BC2N are metastable phases and can be synthesized only in the relatively narrow temperature ranges under pressures of ~20 GPa which makes their production quite a challenge. Superhard and ultrahard ternary B-C-N structures of other stoichiometries (such as BC6N [11] and BC8N of Gao et al. [12]) have been predicted from firstprinciples studies, but none of them have ever been synthesized.

The ultra-hardness can be also achieved extrinsically by creation of nanostructures and/or nanocomposites that leads to significant increase of the material hardness, mainly due to the grainboundary strengthening (Hall-Petch effect) (see Brazhkin and Solozhenko [6] and references therein).

In 2003, synthesis of nanocrystalline diamond by direct solid-phase transformation of graphite at pressures above 12 GPa and high (2600-2800 K) temperatures was reported [13]. Knoop hardness of such nanocrystalline bulks reached 140 GPa, which is significantly higher than that of single-crystal diamond. Aggregated diamond nanorods synthesized by direct transition of C60 fullerite at 20 GPa and 2500 K were claimed to be the densest and least compressible form of carbon [14]. Nanotwinned diamond with claimed Vickers hardness up to 200 GPa was recently synthesized at 20 GPa and 2200 K from onion carbon nanoparticles [15], however, such unprecedented hardness should be considered highly questionable [6]. In general, nano-polycrystalline diamond materials (irrespective of the precursor; e.g. graphite, fullerenes, carbon nanotubes, soot) are ultrahard and have significantly higher wear resistance and fracture toughness than single-crystal diamond. In contrast, Vickers hardness of highly polymerized 3D fullerites does not exceed 65 GPa, as shown by Lyapin et al. [16].

The first attempt to synthesize nanocrystalline cBN resulted in the formation of ultrahard aggregated nanocomposite of cubic and wurtzitic (wBN) polymorphs [17]. This nanostructured material has Vickers hardness above 80 GPa, but its thermal stability is relatively low due to the presence of metastable wBN. Successful synthesis of single-phase nanocrystalline cBN was performed by direct solid-state phase transformation of graphite-like BN with "ideal random layer" structure at 20 GPa and 1770 K [18]. Solozhenko et al. report the material shows very high hardness (HV = 85(3) GPa) and superior fracture toughness (KIc = 10.5 MPa•m 1/2 ) [19]; high thermal stability and oxidation resistance (up to 1500 K) have also been demonstrated [18]. Later synthesis of ultrahard nanotwinned cBN has been reported [20], however, extremely high Vickers hardness (up to 108 GPa) claimed by the authors is unjustified [21]. This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/1.5139489

Another concept for the design of novel ultrahard materials involves the formation of nanocomposites consisting of two or more nanocrystalline hard phases (e.g. TiN, VN, and W2N) imbedded into amorphous matrix (e.g. Si3N4 and BN) by chemical vapor deposition (CVD), reactive sputtering or vacuum arc evaporation combined with plasma CVD [22,23]. In particular, nanocrystalline TiN/TiSix coatings with Vickers hardness from 80 to 105 GPa have been reported [23], however, these values seem to be overestimated.

In the last two decades, along with experimental work on synthesis of ultrahard materials, theoretical methods for predicting the mechanical properties of solids have been developed very rapidly, from empirical, but physically motivated models of hardness [24,25] and fracture toughness, including very recent works by Mazhnik and Oganov [26] and Niu et al. [27], to ab-initio calculations of elastic constants by Xie et al. [28] and Ivashchenko et [29], and, finally, to computational discovery of superhard and ultrahard materials by Kvashnin et al. [30]. The proposed methods have been shown to work successfully even in the case of boron-rich solids which are characterized by an extreme complexity of the crystal structure and a large number of atoms in a unit cell (Jay et al. [31] and others [32]). Thus, the modern evolutionary algorithms and powerful computer systems can be used to predict new structures and compounds, and to search for materials with optimal mechanical properties in order to create a "treasure map" of superhard materials. At the same time, precise calculations of mechanical properties of superhard materials (hardness, in particular) often lie beyond the capabilities of the most advanced and modern techniques. Besides, not all theoretically predicted structures exist or can be synthesized. A vivid illustration is the case of hypothetical cubic form of C3N4 with bulk modulus exceeding that of diamond [5]. Despite the enormous efforts (new attempts are still being undertaken, for instance, by Gao et al. [33]), this phase has not been synthesized so far, and its expected ultrahardness has never been demonstrated.

To summarize, we should note that the search for novel ultrahard materials is just at the cutting edge of fundamental science and promises great prospects for the creation of new technologies required for emerging applications. In particular, over the past decade, nanodiamonds have progressed to fully functioning materials that can be used for a wide range of applications. Recent achievements in the field clearly indicate that synthesis of phases with hardness exceeding that of diamond is very unlikely (or even impossible [6]). Rather than harder, we have to consider the possibility to synthesize (or design) materials that are more useful that diamond, e.g. more thermally and chemically stable, and harder than cubic boron nitride.

As Guest Editors, we do hope that the Special Topic: Ultra-Hard Materials will help the readers to gain a better understanding of the subject, the current trends and problems in the field, and will inspire the readers to generate new ideas on creation and design of novel superhard and ultrahard materials. We would like to thank the authors for their contributions and editors of the Journal of Applied Physics for coordinating the Special Topic.

  ORCID IDs : Vladimir L. Solozhenko https://orcid.org/0000-0002-0881-9761 This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/1.5139489

This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/1.5139489


A Wiesner
email: awiesner@javeriana.edu.co

R Diez
email: rdiez@javeriana.edu.co

D Florez

H Piquet

System for experimental investigation of DBD excilamps in view of control and optimization of UV emission

Keywords: DBD, Power converters, Current-mode, Interleaved converters

This paper presents the design of a system aimed at characterizing a diverse set of dielectric barrier discharge UV lamps. This experimental bench allows the study of the lamps performances regarding UV emission maximization and energy conversion optimization. It includes a current-mode pulsed power supply for the lamps, the design of which is detailed. This converter presents 3 degrees of freedom (DOF) can control and adjust independently the frequency, amplitude and duration of the current pulses injected into the lamps. Theoretical modeling of the lamps, by means of equivalent electrical circuits, is used to define the most interesting operating points to be explored, as well as for the converter's design; these models are afterwards validated by experimental tests on the entire set of lamps. The operation of the bench is controlled by a supervising program: the latter manages the data acquisition and performs sweeps of the DOFs of the pulsed current. Examples of analyzes, oriented toward optimization of electric to UV energy conversion, are shown.

Introduction

Dielectric Barrier Discharge (DBD) UV lamps are environmentally friendly (mercury free) UV sources with various applications, as disinfection, microelectronics, surface treatment, and skin phototherapy. The practical usefulness and applicability of currently available DBD excilamps has been demonstrated [START_REF] García-Guerrero | Use of a COAX-DBD plasma fluidized-bed reactor for surface modification of TiO2 and potato-starch powders[END_REF][START_REF] Lomaev | Excilamps and their applications[END_REF]; today, studies oriented toward their performance improvement are in development. This can be achieved by means of the reactor design optimization (geometry, filling mixture, pressure and materials) [START_REF] Viejo | Comparison between different discharge lamp models based on lamp dynamic conductance[END_REF][START_REF] Cheng | A cold planar nitrogen-based atmospheric-pressure dielectric barrier discharge jet with enhanced UV emission and radical generation using short electrodes[END_REF][START_REF] Erofeev | Compact dielectric barrier discharge excilamps[END_REF][START_REF] Gnapowski | Changes in the power discharge in a plasma reactor using porous versus solid dielectric barriers and meshes electrodes[END_REF][START_REF] Meißer | Resonant Behaviour of Pulse Generators for the Efficient Drive of Optical Radiation Sources Based on Dielectric Barrier Discharges[END_REF], and also by choosing the proper electrical operating conditions [START_REF] Bozhko | Determination of energy of a pulsed dielectric barrier discharge and method for increasing its efficiency[END_REF][START_REF] Diez | Current-mode approach in power supplies for DBD excilamps: Review of 4 topologies[END_REF][START_REF] Florez | Optimizing the operation of DBD excilamps[END_REF][START_REF] Florez | Square-shape current-mode supply for parametric control of the DBD excilamp power[END_REF][START_REF] Guo | Analysis and design of pulse frequency modulation discontinuous-current-mode dielectric barrier corona discharge with constant applied electrode voltage[END_REF][START_REF] Huang | Statistical analysis of pulsed microdischarges and ozone generation in dielectric barrier discharges[END_REF][START_REF] Nassour | Comparative experimental analysis of ozone generation between surface and volume DBD generators[END_REF]. The approach developed in this paper is aimed at performing system level optimization, integrating in the same step DBD reactor and power supply designs.

The impact of the electrical operating point (OP) over the DBD excilamps performance has been already reported by different teams [START_REF] Akashi | Development of streamers in dielectric-barrier-discharge excimer lamp[END_REF][START_REF] Doanh | Estimation of the light output power and efficiency of a XeCl dielectric barrier discharge exciplex lamp using one-dimensional drift-diffusion model for various voltage waveforms[END_REF][START_REF] Lomaev | Excilamps efficient sources of spontaneous UV and VUV radiation[END_REF][START_REF] Lomaev | Excilamps and their applications[END_REF]. However, because the geometrical and gas mixture parameters of a DBD excilamp cannot be easily adjusted [START_REF] Díez | Predictive model of a DBD lamp for power supply design and method for the automatic identification of its parameters[END_REF] and constructed, most of these studies are developed for a fixed lamp design. The impact of the DBD lamp geometry over the system's performances has not been widely reported.

Considering that maximization and optimization of the UV production is one objective of research groups working with excimer lamps, the development of a system able to vary the electrical parameters in the discharge, and having the possibility to characterize different DBD lamps is proposed here. This test bench needs to be versatile, in order to supply different DBDs under various operative conditions; it also varies the supply's characteristic and collects the measurements, in view of performances analysis and improvement. In this document the test-bench design is detailed and a prototype, tested for a set of UV excimer lamps, is presented.

With the aim to provide deeper insight and experimental evidence about the UV production dependence on the geometry of the DBD reactor, a set of nineteen diverse DBD exciplex UV lamps of coaxial type, has been built. As shown in Fig. 1, the bulbs differ with the values of, T, the dielectric walls thickness, a the internal radius, b the external radius and l the length. Each one of the 19 bulbs presents a different set of geometrical parameters {T, a, b, l}; they all are filled with the same Xe-Cl gas mixture, under the same pressure.

The proposed study includes the design of a static converter capable of supplying these 19 different excilamps at different controlled electrical operating points. Additionally, the test-bench must be able to measure for each experiment the required variables in order to compare the performance of the lamps, in terms of the obtained UV power and also of the electrical to UV power conversion efficiency. The block diagram of the proposed system is displayed in Fig. 2 and can be divided in two main parts: the power supply and the acquisition interface.

From previous works a significant impact of the supplying conditions over the lamps performance has been evidenced. Particularly, the methodology proposed in [START_REF] Diez | Current-mode approach in power supplies for DBD excilamps: Review of 4 topologies[END_REF][START_REF] Florez | Optimizing the operation of DBD excilamps[END_REF] makes use of a square-shape current waveform for the parametric study of DBDs. By means of the lamp current waveform (i D B D ), the lamp power can be controlled with three degrees of freedom (DOFs): duty cycle (D), current intensity (I) and frequency (f ) of the current pulses, as shown in Fig. 3. Accordingly, the power electronic modules must be capable to deliver and vary these three parameters. The power supply consists of a DC adjustable current source in cascade with a full-bridge current inverter [START_REF] Diez | Current-mode approach in power supplies for DBD excilamps: Review of 4 topologies[END_REF]. A step-up transformer is used to connect the bulb to the current inverter, in order to adapt the DBD peak voltage to the rating of the semiconductors.

The dimensioning and implementation of power supply converters for the proposed study is a challenge due to the wide range of the operating conditions imposed by the set of 19 lamps. According to the dimensions of the bulbs, the power range of this supply has been chosen to be from 0 to 500 W.

The power supply design process is presented in the following sections: first, the electrical model of each lamp is theoretically obtained as a basis for the converter design. Then, using this model and taking into account the different characteristics of the 19 bulbs, the operating range of interest, concerning the properties of the current pulses, is determined. Finally, the converter design is presented, pointing out the challenges of supplying such a different set of loads at this level of power (up to 500 W).

The second part of the test-bench, the acquisition interface, is a software that stores the measurements of the DBD instrumentation. It also sends the OP request (f, D and I) to the converters via the control board (FPGA), as shown in Fig. 2. Details about this interface are given in Section 4.

DBD lamps under study

The nineteen coaxial DBD lamps, used for this work, present the configuration shown in Fig. 1. and have different geometrical parameters that are categorized as:

• External diameter: with two different values, 25 mm and 45 mm.

• Internal diameter: six different values varying from 5 mm to 30 mm in steps of 5 mm.

• Thickness of the dielectric barriers: three different values, 1 mm, 1.5 mm and 2 mm. All lamps have same inner and outer barrier thicknesses.

All the lamps have a length (l) of 600 mm. The gas mixture used to fill the lamp bulbs is Xe-Cl, producing a radiation wavelength at 308 nm.

In order to design a power supply capable of driving all the lamps under study, initially, the electrical magnitudes are calculated using the simplified electrical model of the DBD developed in [START_REF] Diez | Current mode converter for dielectric barrier discharge lamp[END_REF][START_REF] Lopez | Parameters identification and gas behavior characterization of DBD systems[END_REF][START_REF] Pipa | Experimental determination of dielectric barrier discharge capacitance[END_REF], as shown in Fig. 4.

In this model, C d is the dielectric series equivalent capacitance, C g the gas equivalent capacitance and V th the gas breakdown voltage. The values of C g and C d are estimated using the cylindrical capacitor Eqs. (1) and [START_REF] Bendali | Design methodology of an interleaved buck converter for onboard automotive application, multi-objective optimization under multi-physic constraints[END_REF]. The Paschen's law in Eq. ( 3) is used to determine the breakdown voltage V th .

C g = 2π • ε g• l ln ( b-T a+T ) ≈ K g • l ln ( b-T a+T ) (1)
C d = 2π • ε d • l ln ( a•b T 2 +a•T +b•T +a•b ) ≈ K d • l ln ( a•b T 2 +a•T +b•T +a•b ) (2)
(V th ) 1167 V-2579 V V th = C • p gas • d ln ( A• p gas •d ln ( 1+ 1 γ ) ) ≈ K th • d K ′ + ln(d) (3)
In these equations, a, b, l and T are the geometrical parameters of the lamp, previously shown in Fig. 1; A is a constant which depends on the electron kinetic temperature, C is related to A and to the effective ionization potential V * (C = A V * ), p gas is the gas pressure, d is the distance between dielectrics (gas gap), and γ is the electron emission coefficient [START_REF] Roth | Dark Electrical Discharges in Gases[END_REF].

In the approximations used in these three equations, the K g , K d , K th and K ′ terms are constants, which values are derived from measurements achieved on a previously identified bulb, having the same barriers material, gas mixture and pressure [START_REF] Díez | Predictive model of a DBD lamp for power supply design and method for the automatic identification of its parameters[END_REF].

With the geometry known for each lamp, the parameters of the electrical model are calculated, obtaining the ranges presented in Table 1.

Operating points range

Once the 19 sets of (C d , C g , V th ) parameters for the electrical model are obtained, it is possible to find out the values of the amplitude (I) of the lamp current (i D B D ) and the peak voltage for each lamp, thanks to Eqs. ( 4) and ( 5), deduced from work detailed in [START_REF] Viejo | Comparison between different discharge lamp models based on lamp dynamic conductance[END_REF].

I = P D • V th + f 4 • C g • V th D (4)
VDBD = V th + I • D 4 • f • C d (5)
P is the desired average electric power on the DBD (which maximum has been set to 500 W); D, I, and f are the three degrees of freedom of the i D B D current shape, controlled thanks to the current inverter (f, D) and the DC current source (J = I • n), n being the turns ratio of the step-up transformer. Fig. 5 presents, for the nineteen different lamps, the current pulse amplitude I, versus the f frequency for two different duty cycles (D = 5% and D = 100%) needed, according to Eq. ( 4), to obtain the maximum requested power (P = 500 W). As expected, the lower the duty cycle, the higher the I current amplitude. One can observe that the desired power is obtained, for D = 100%, with a relatively low I value (this value will define the operating conditions of the current inverter and the current source, considering that these converters control the J current in the primary side of the transformer). To avoid the oversizing of the current converters, the maximum value of I is set to 1.4 A. As a matter of fact, with this limitation, all the points associated with D = 5% will not reach 500 W. For this duty cycle, the specific power (power per unit of bulb length) will be maintained thanks to an outer metallic electrode of reduced length.

Combining Eqs. ( 4) and ( 5), it is possible to obtain Eq. ( 6), where it can be seen that the lamp peak voltage ( VDBD ) does not explicitly depend on the duty cycle D.

VDBD = V th ( 1 + C g C d ) + P f • 4 • C d • V th (6)
For the same power level P = 500 W, Fig. 6. presents the lamp peak voltage ( VDBD ) versus frequency, for the 19 lamps.

This figure shows that the requested lamp voltage in low frequencies is up to 40 kV; thus, to avoid oversizing of the step-up transformer, the minimum operating frequency has been limited to 30 kHz. The maximum operating frequency has been set to 200 kHz, considering implementation aspects, as the speed of semiconductor devices and the limitation of magnetic materials. To conclude this section, we summarize below the operating range of the parameters for the i D B D pulses:

• 0 < I < 1.4 A • 5% < D < 100% • 30 kHz < f < 200 kHz

Converter design

After finding the operating range for the 3 degrees of freedom (I, D, f ), that will be used to make the characterization of the DBDs, the static converter's design is explained in this section.

Inverter switch selection

The selection of the inverter switches is performed under the assumption that the power can be delivered to the lamp independently of the transformer turns ratio, that will be chosen later. For that purpose, the product of voltage (taken from Fig. 6.) and current (taken from Fig. 5.-D = 100%) in each lamp is obtained for the most demanding case, resulting in the decaying curve shown in Fig. 7. for a given lamp.

The obtained curve is compared to the product of the blocking voltage and the current rating of the candidate switches for the converter (flat lines in Fig. 7). The operating points that are above the intersection between the lamp and switch curves are the ones that cannot be attained by the switch, regardless of the transformer turns ratio.

The switch is chosen using a compromise between its price and the number of reachable OP. Fig. 6 will be useful later to determine the value of the minimum operating frequency for the selected semiconductor device, keeping a blocking voltage less than the one allowed in the switch specifications.

Transformer

Once the switches have been chosen, the interesting OPs from Fig. 7 are tested for different transformer turns ratios (n). The points to be discarded are the ones where the peak voltage or the current in the lamp (seen in the primary side of the transformer) exceeds the ratings of the switch. The points with power losses in the switches greater than 30 W are also removed due to thermal limitation for the available cooling system (natural convection heatsink).

Fig. 8. shows the result of this analysis and the turns ratio is selected maximizing the number of reachable points, but also assuring a wide frequency range. In this case a turn ratio of 10 is selected.

Current source

To avoid the influence of the i D B D current ripple on the DBD performance, the frequency bandwidth of the current source which supplies the current inverter should be much higher than the maximum frequency of the latter. Since the lamp current has a maximum frequency of 200 kHz it is not possible to comply with this condition.

For this reason, parallel current choppers as shown in Fig. 9, with an interleaved configuration increases the apparent frequency of the delivered current [START_REF] Bendali | Design methodology of an interleaved buck converter for onboard automotive application, multi-objective optimization under multi-physic constraints[END_REF][START_REF] Esteki | High step-down interleaved buck converter with low voltage stress[END_REF]. In addition, this parallel configuration allows to reduce the average current in each switch. The maximum DC current being J = 14 A, the average output current of both DC current sources is 7 A, seen by each of their switch.

The maximum frequency of the current sources is defined by the ratings of the semiconductor devices, switching losses and the limitation of magnetic materials. The worst case in terms of current ripple happens when the maximum frequency on the inverter and the maximum frequency on each current source are similar, as shown in Fig. 10. This figure plots the output voltage of the chopper v ch and the input voltage of the current inverter v B . L inductor which controls the current ripple at the input of the current inverter is defined by Eqs. ( 7)- [START_REF] Doanh | Estimation of the light output power and efficiency of a XeCl dielectric barrier discharge exciplex lamp using one-dimensional drift-diffusion model for various voltage waveforms[END_REF]. The current ripple on the L inductor is defined by:

∆i L = 1 L ∫ t+D ch T ch t V in dt - 1 L ∫ t+D ch T ch t v B dt (7)
∆i L = D ch L f ch ( V in -V 0 - m D ch 2 f ch ) (8)
Being,

m ≈ 2 • VDBD f n • D (9)
Assuming the approximation of the shape of the v B voltage, highlighted with the red slope (m) in Fig. 10, the current ripple is defined by:

∆i L = D ch L • f ch ( V in -v B -VDBD • f • D ch n • f ch • D ) ( 10
)
v B is a voltage that varies between -VDBD and + VDBD as shown in Fig. 10. n is the transformer's turns ratio, f and D are the frequency and duty cycle of the current inverter, respectively, and f ch and D ch are the frequency and duty cycle of the current sources. Note that this converter differs from conventional DC-DC converters, because v B presents high frequency (at inverter's frequency) bidirectional changes. This fact implies a high inductance value to maintain a low current ripple.

Current source control

The peak current mode control is chosen for the DC current sources, using the IC TPS40180. The current set point (J) is sent to this current controller from the FPGA. The current is sensed with a shunt resistor placed between the source of the low-side MOSFET and the circuit ground.

The TPS40180 controller can receive an external clock, as shown in Fig. 11, allowing the synchronization of two or more parallel current sources. The clock signals are generated by the FPGA, with a phase delay of 180 • , minimizing the current ripple.

Working range

Once defined the different devices of the system, it is important to finally find out the actually accessible OP for each lamp. To do that, a cloud of OP is plotted for each lamp, varying f in steps of 5 kHz, D in steps of 5% and I in steps of 0.2 A.

These points are discriminated with colors, as shown for a given bulb, in Fig. 12: the green cells are the most interesting OP for the experiment as their power lies between 400 W and 500 W. With the blue ones the injected power is less than 400 W.

The others are eliminated for various reasons: the orange ones do not ignite the lamp; the white ones (blank spaces over the green or blue OP) are eliminated because one of the following restrictions is exceeded:

• the peak voltage across the switches is higher than the maximum allowed value; • the power is higher than 500 W.

One should remember here that peak voltages as well as the maximum power depend on the geometrical parameters (T, a, b, l) of the bulbs; for that reason, Fig. 12 shape and size change for each lamp.

Configuration and data acquisition interface

As explained in Section 1, the test-bench comprises a set of sensors, measurement instruments and also an acquisition system (interface and storage). Electrical measurements of the excilamp voltage and current are acquired using an oscilloscope LECROY HDO4024 with voltage probes TESTEC TT-SI 9110 and current probes LECROY AP015. The lamp temperature is monitored using the Omega OS210-150-300-800 sensor. The average UV power is measured with a Gigahertz Optik P9710 optometer and the UV instantaneous response is acquired using a Thorlabs PDA-25k photodetector, connected to the oscilloscope. Finally, the radiation spectrum is registered by means of an OCEAN-Optics 2000 spectrometer. All these measurements are acquired and saved for each experiment.

Once the safe operating points range for the excilamp under study is determined, the system is configured accordingly through the user interface specially developed for the test-bench, shown in Fig. 13. This interface allows the following adjustments: the parametric sweep of the operating points (start, end and step values for each variable); the number of trials for each point and the delay between trials.

For each trial the interface saves the following measurements: the excilamp DBD average power, DBD Peak voltage, DBD frequency, DBD current intensity, DBD UV average power density, temperature and the wavelength spectrum (from 200 to 1000 nm). Also, the waveforms of voltage, current and UV are saved for each OP. Note that the system only starts the acquisition of the measurements when the system has detected that the OP introduced in the user interface is reached in the DBD excimer lamp. This means that all measurements are stable (steady state).

Several measurements are taken for the same operating point, allowing data post-processing, like the computation of error and arithmetical average. Once the averages are calculated in all measurements, these values are assigned to the corresponding OP to perform the analysis of the different DBDs. It is possible to characterize the different excilamps obtaining their electrical model (C d , C g and V th ), energy conversion performance and to investigate the impact of the lamp geometry in the aspects previously mentioned. With the data saved is also possible to accomplish a statistical analysis if required.

Simulation and experimental test

Some experimental waveforms are presented in this section; the latter are compared with the simulation results of the lamp-supply model used for the design of the test-bench.

Comparison between theoretical, simulation and experimental results

To validate the theoretical analysis of the DBD power supply, the simulation software PSIM is used. The curves displayed in Fig. 14. correspond to the OP with f = 170 kHz, D = 15%, I = 1.4 A, and the electrical model of the DBD with the following parameters C d = 204 pF, C g = 67 pF and V th = 2.43 kV.

For the same OP, Fig. 15. shows the experimental waveforms for the DBD voltage (V D B D ) and the DBD current (i D B D ).

The simulation and the experimental tests are in good agreement with the theoretical analysis, as shown in Table 2 (first column gives the theoretical values obtained with the equations used for design). Additionally, for the entire set of lamps a comparison is performed, between the theoretical values of the electrical model parameters (C d , C g and V th ), obtained by Eqs. (1) to (3), and the experimental values, identified through the Manley diagram [START_REF] Manley | The Electric Characteristics of Ozonator Discharge, Eighty-Fourth General Meeting held[END_REF].

Fig. 16, shows the comparison for the threshold voltage, V th . The lamps are labeled, in the X -axis for this figure, from L01 to L19 and are ordered by the length of the gas gap. The lamps with big gas gaps are in good agreement with the approximation given in Eq. (3). For small gas gap lamps, the approximation should be improved. Fig. 17, presents results concerning the dielectric capacitance, C d . The order in the X -axis is selected according to the geometrical expression proposed for the approximation in Eq. ( 2)

( ln ( b-T a+T ))
. This approximation has a big error for the three first lamps, which present too small difference between the external and internal electrode (gas gap of 1 mm). As the gas gap increases the approximation is better. ))

, related to bulb geometry.

Figs. [START_REF] García-Guerrero | Use of a COAX-DBD plasma fluidized-bed reactor for surface modification of TiO2 and potato-starch powders[END_REF] to 18 show that the increasing and decreasing trends are the same for the theoretical and experimental parameters of the model. However, adjustment of the approximations used for Eqs. ( 1) to (3) should be done, since there are considerable errors for several lamps. These errors could be a consequence of using, in the approximations, the same constants from a single identified model in previous work. The identification through the Manley's diagram of the different lamps should improve the approximation and are a good tool for future design.

Another comparison between theoretical and experimental results is presented in Fig. 19: the lamp peak voltage and the lamp power are both displayed. This figure shows the results for only one lamp and similar agreement are obtained for the other 18 lamps. These results confirm that the electric model and the values calculated theoretically are the right support for the converter design.

System validation

In order to validate and put forward the capabilities of the test-bench, a parametric sweep is performed for one of the lamps and the results are shown in Fig. 20. In this figure, the average UV specific power vs the electric power on the DBD is observed for different values of current intensities (I). For this experiment, 10 different measurements are taken for each operating point. Each point shown in Fig. 20 corresponds to the average of those 10 values. The entire set of operating points present a standard deviation of 2.3% concerning the value of the electric power and 4.9% for the UV measurement.

The results show that the same amount of electric power could deliver different values of UV radiation, implying that the three DOF (f, D and I) influence the energy conversion efficiency of the lamp, since the same electric power could be injected to the lamp with different combinations of these three DOF as can be seen with Eq. (4). For instance, in Fig. 20, it is possible to observe the positive impact that the high values of current intensity have over the UV radiation: the green square points are mostly above the yellow diamonds and the blue circle points.

The results obtained in Fig. 20 are the characterization of one lamp as a function of one DOF, similar figures can be obtained for different electrical parameters. For example, in Fig. 21, the current (I) is maintained at a constant level and variations of the duty cycle and the frequency (D and f ) are performed in order to point out the influence of the discharge ON-time (time during which the gas is ignited) on the UV radiation. Here, the UV radiation as a function of the electric power is plotted again, with the discharge ON-time grouped in small ranges. Here one can observe an influence of the discharge ON-time on the energy conversion efficiency.

Results shown in last figures agree with previous experimental works, where the current amplitude and the pulse duration have an influence over the lamp efficiency [START_REF] Carman | High pressure (>1 bar) dielectric barrier discharge lamps generating short pulses of high-peak power vacuum ultraviolet radiation[END_REF][START_REF] Florez | Efficiency of an exciplex DBD lamp excited under different methods[END_REF][START_REF] Oda | One-dimensional modelling of low-frequency and high-pressure Xe barrier discharges for the design of excimer lamps[END_REF]. This is only a sample of the possible analyses that can be performed using the test-bench developed in this paper, that will help the designers of DBD lamps and power supplies with the optimization of the system-level efficiency.

Conclusions

The procedure to design a characterization test-bench for DBD excimer lamps has been explained and validated with a set of 19 different lamps.

The theoretical modeling of the lamps, necessary to perform the dimensioning of the power supply, is found to be relatively accurate. The inverter switches are selected based on a good balance between cost and the number of interesting operating points that are attainable. The transformer turns ratio is also selected maximizing the interesting operating points that are attainable and the most desired frequency range.

According to the frequency range of the current pulses (f max = 200 kHz), the realization of the DC current source, which needs to present an even higher frequency is a real challenge. With the current limitations of the semiconductors, the most relevant solution is the implementation of parallel current source converters in interleaved configuration, enabling a reduction in the output current ripple and multiplying the apparent switching frequency.

The system has the capability to automatically collect data changing electrical and geometrical parameters, helping with the UV process optimization in excimer lamps.

Fig. 1 .

 1 Fig. 1. DBD lamp geometry.

Fig. 2 .

 2 Fig. 2. DBD characterization system.

Fig. 3 .

 3 Fig.3. DBD current and voltage waveforms using a square-shape current source.

Fig. 4 .

 4 Fig. 4. DBD electrical model.

Fig. 5 .

 5 Fig. 5. Current in the 19 lamps as a function of the frequency. Cases for duty cycle of 5% (top) and 100% (bottom).

Fig. 6 .

 6 Fig. 6. Lamp peak voltage as a function of the frequency.

Fig. 7 .

 7 Fig. 7. Selection of the switches via the V I product.

Fig. 8 .

 8 Fig. 8. Operating points attained vs. the frequency for different turns ratio of the transformer.

Fig. 9 .

 9 Fig. 9. DBD system with two parallel current sources.

Fig. 10 .

 10 Fig. 10. Output voltage of the chopper v ch and input voltage of the current inverter v B -case where f ch is similar than f B . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 11 .

 11 Fig. 11. Diagram of current control.

Fig. 12 .

 12 Fig. 12. Possible operating points and the ones that inject between 400 W and 500 W, for one of the lamps . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 13 .

 13 Fig. 13. LabVIEW user interface.

Fig. 14 .

 14 Fig. 14. Simulated waveforms for a specific case: V D B D (top) and i D B D (bottom).

Fig. 15 .

 15 Fig. 15. Experimental measurements for the same OP: specific case: V D B D (top) and i D B D (bottom).

Fig. 16 .

 16 Fig. 16. Comparison between experimental and theoretical values for V th .

Fig. 17 .

 17 Fig. 17. Comparison between experimental and theoretical values for C d .

Fig. 18

 18 Fig. 18 shows the comparison for the gas capacitance. In this case the lamps are ordered according to the expression proposed in Eq. (2) ( ln ( a•b T 2 +a•T +b•T +a•b

Fig. 18 .

 18 Fig. 18. Comparison between experimental and theoretical values for Cg.

Fig. 19 .

 19 Fig. 19. Comparison between peak voltage and power on DBD with them theoretical and experimental values.

Fig. 20 .

 20 Fig. 20. UV vs. P as a function of I for one lamp.

Fig. 21 .

 21 Fig. 21. UV as a function of the electric power with constant current amplitude.

Table 1

 1 DBD model parameters.

	Range

Table 2

 2 Results comparison ( f = 170 kHz, D = 15%, I = 1.4 A).

		Theoretical	Simulation	Measures
	VDB D	3.9 kV	3.4 kV	3.4 kV
	P	308 W	323 W	350 W

Acknowledgments

The authors acknowledge specially to ECOS-Nord/COLCENCIAS/ICETEX French-Colombian cooperation program, ECOS Nord Program under Project C16P01, by Colciencias, Colombia under contracts 663-2015 and 294-2016 and by Universidad Sergio Arboleda, Colombia under Project IN.BG.086.16.013 and COLCEN-CIAS/COLFUTURO because part of this work is supported by them.


Ziad Obeid

Antoine Picot

Sylvain Poignant

Jérémi Régnier

Olivier Darnis

Pascal Maussion
email: pascalmaussion@laplace.univ-tlsefr

Experimental comparison between diagnostic indicators for bearing fault detection in synchronous machine by spectral Kurtosis and energy analysis

Keywords: Bearings, Diagnostic, Bearing fault detection, Current analysis, Spectral Kurtosis I

In this paper, some indicators are developed for efficient detection of bearing defaults in high speed synchronous machines. These indicators are based on the analysis of stator current. As bearing defect signatures can be tracked through amplitude increase of some current harmonics, two specific indicators have been built based on energy considerations and on the Spectral Kurtosis analysis. These indicators are tested on a real industrial fan equipped with ceramic balls, in its environment. Several measurements for different operating points are tested to validate the approach and to its robustness during long time tests. From an experimental comparison between a healthy fan and another with damaged bearings, a frequency selection is performed to identify the frequency ranges where the energy is the most sensitive to the considered faults. This actuator is used in an air conditioning fan in aeronautic applications.

INTRODUCTION

Health monitoring has become an important industrial concern for safety and reliability, especially for aeronautic applications. It has been shown in [START_REF] Albrecht | Assessment of the Reliability of Motors in Utility Applications -Updated[END_REF] that ball bearing defects are among the elements of greatest occurrence (40% of machine failure). As bearing failures could lead to critical events such as abnormal temperature or vibration level, rotor locking, stator friction… sub-system suppliers investigate the bearing's health monitoring. Traditionally, bearing fault detection uses vibration analysis, but this solution could be expensive. The stator current analysis has been successfully investigated in the relevant literature, but mainly for induction machines. This paper deals with bearing fault detection in a PMSM for an air conditioning fan used in aeronautic.

Bearing fault detection is usually based on vibration analyses [START_REF] Zhaoxia | Motor Fault Diagnosis Based on the Vibration Signal Testing and Analysis[END_REF][3][4] [START_REF] Lim | Vibration-based fault diagnostic platform for rotary machines[END_REF] in which characteristic frequencies can point out a bearing damage in the vibration spectrum. But for cost reduction objectives, the stator current signal analysis has been successfully investigated in recent years [START_REF] Pires | Stator winding fault diagnosis in induction motors using the dq current trajectory mass center[END_REF][7] [START_REF] Trajin | Indicator for bearing fault detection in asynchronous motors using stator current spectral analysis[END_REF]. Several techniques based on the stator current spectrum analysis have been studied over time by authors. A large and interesting list of possible techniques based on stator current spectrum analysis is reviewed in [START_REF] Zhou | Stator Current-Based Bearing Fault Detection Techniques: A General Review[END_REF] [START_REF] Benbouzid | What stator current processingbased technique to use for induction motor rotor faults diagnosis?[END_REF]. These applications are mainly dedicated to the induction machines and few works deal with bearing fault detection in permanent magnet synchronous machines [11][12]. This paper investigates bearing fault detection for a high speed permanent magnet mac hine w hic h is part of an air conditioning fan used in aeronautic. The classical stator current signatures related to vibration bearing frequencies are not sufficient for such applications due to their low amplitude level and to their dependence on the operating point. The bearing fault detection technique presented in this paper includes some indicators based on a classical energetic approach and on a rather new technique, the spectral kurtosis analysis such as in [13] [14].

This work is part of the French national project PREMEP [START_REF] Immovilli | Detection of Generalized-Roughness Bearing Fault by Spectral-Kurtosis Energy of Vibration or Current Signals[END_REF] (PRojEt Moteur Electronique de Pilotage), labelled by the Aerospace Valley cluster and involving the LAPLACE and Airbus suppliers such as Technofan, Liebherr Aerospace, CIRTEM, DELTY and ADN. The objective of PREMEP is to prepare new equipment for the new aerospace power supply network (230VAC and 540HVDC). The project is funded by the French single interdepartmental fund (fonds unique interministériel), Midi Pyrénées region and Aquitaine region.

II. SYSTEM DESCRIPTION

The application system is an air conditioning fan which is used in most of the commercial aircrafts. It consists of a highspeed (140000 rpm) permanent magnet synchronous machine, cf Fig. 1, with sinusoidal back electromotive forces, fed by a PWM current source inverter operating sequentially to provide 120°square wave currents according to Fig. 2.

VI. CONCLUSION

Our work was focused on two indicators both based on selected frequencies in the current spectrum. One is based on energy considerations and the other one on the spectral kurtosis (SK) analysis. SK is a powerful statistical tool measuring the non-stationarity of the power spectral density, i.e. how the signal differs from a Gaussian distribution for different frequencies. The indicators were applied to different measurements of three stator currents and to the inverter current, on an industrial fan, operating at different speeds during long term tests. The results provide good separation for all measured signals between healthy and faulty cases.

Moreover, through an accelerating aging process, the capacity of the indicator to detect the fault occurrence before the bearing death has been demonstrated. The spectral kurtosis based indicators achieve the best discrimination between healthy and faulty cases, but energy indicators, when calculated on a selected bunch of frequencies, provide simple default indicator. A new test session will be run to test the algorithm robustness with new healthy and faulty bearings.

Fig. 1 .

 1 Fig.1. Picture of the whole fan from SAFRAN-Technofan Maximum Speed : 14 100 Rpm

Fig 21 :

 21 Fig 21 : Stator current spectral Kurtosis indicator for 10000 rpm with reference for fs+fc

1 -

 1 Speed = 10000RPM -Ind SKREF -C1 -Bande f s -f c -N = 211 -F e = 2kHz Observation NumberSKREF Indicator


Konstantina Lambrinou
email: klambrin@sckcen.be

Marc Verwerft
email: marc.verwerft@sckcen.be

Jozef Vleugels
email: jozef.vleugels@kuleuven.be

Alfons Weisenburger
email: alfons.weisenburger@kit.edu

Christophe Lorrette
email: christophe.lorrette@cea.fr

Yann De Carlan
email: yann.decarlan@cea.fr

Fabio Di Fonzo
email: fabio.difonzo@iit.it

M Barzoum

Akira Kohyama
email: kohyama.oasis@gmail.com

James Marrow
email: james.marrow@materials.ox.ac.uk

Michel Barsoum
email: barsoumw@drexel.edu

Innovative accident-tolerant fuel cladding

Keywords:

published or not. The documents may come

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I. INTRODUCTION

Safety, reliability and economic considerations have always been the driving force for the continuous improvements in nuclear fuel system performance. Today's light water reactor (LWR) fuels are based on the UO2/zirconium alloy system, which achieved full maturity and excellent performance records after several decades of research, return of experience and steady technological advancements. The tsunami following the Great East Japan Earthquake in March 2011 caused a prolonged station blackout (SBO) of the Fukushima Daichii plants, followed by core meltdown, hydrogen explosion and release of radioactive matter beyond the power plant containments. The material and immaterial consequences of this event prompted international discussions on the necessity of enhanced accident tolerance in LWRs, as it is reflected, for example, in the amended European Nuclear Safety Directive (Council Directive 2014/87/EURATOM of July 2014).

When compared with the standard UO2/zirconium alloy system currently used, nuclear fuel systems with enhanced accident tolerance (Ref. 1) can tolerate loss of active cooling in the reactor core for considerably prolonged periods during design-basis accidents (DBA) and design extension conditions (DEC) (also known as "severe accidents", SA), while maintaining or improving the fuel performance during normal operation and operational transients ("anticipated operational occurrences", AOO). Enhanced accident tolerance includes reduced steam reaction kinetics, lower hydrogen generation rate, delayed core degradation and reduction of the initial stored energy in the core, while retaining acceptable cladding and fuel thermomechanical properties, fuel-clad interactions, and fission product behaviour.

Efforts directed towards ATF development are deployed at both national and supranational levels. In February 2017, the European Commission evaluated favorably the H2020 project IL TROVATORE ("Innovative cladding materials for advanced accident-tolerant energy systems"). IL TROVATORE gathers thirty organizations from Europe, the USA and Japan with the aim of accelerating ATF cladding material development, in response to the pressing global societal and industrial demand for enhanced nuclear energy safety in the current generation LWRs. IL TROVATORE aims at optimizing and validating some of the most promising ATF cladding material concepts in industrially-relevant conditions. The present paper describes the material down-selection process and key performance indicators implemented in the IL TROVATORE project.

II. MATERIAL DOWN-SELECTION IN H2020 IL TROVATORE

II.A. Initial ATF Cladding Material Pre-selection

IL TROVATORE focuses on innovative fuel cladding material concepts with potential for enhanced accident tolerance; these materials have achieved an experimental proof-of concept and have also undergone validation in the lab, at least for some of the key fuel cladding properties. A critical pre selection of candidate materials with the potential to address the stringent requirements of the ATF cladding application has already been performed, taking into account the latest global academic and industrial achievements. The ATF cladding material concepts to be further studied in IL TROVATORE are: 1. Select SiC/SiC composite concepts produced by different processing routes and with different performance-defining fiber/matrix interfaces. Apart from their refractoriness, the great advantage of SiC/SiC composites is their damage tolerance, i.e., the fact that they exhibit pseudo ductile fracture with high strain at failure (εf > 0.5%) in contrast to the brittle fracture of monolithic SiC (εf < 0.1%). 2. Coated cladding material concepts are considered an appealing (near term) technology, since their performance will rely on the performance of the well known/commercial substrate clads that are expected to remain largely unaltered by the applied coatings. Both coatings and coating/substrate interfaces will be tailored to guarantee reliable performance in application-relevant conditions. IL TROVATORE will not evaluate all 'mature' coated ATF cladding material concepts, as some of them are currently being evaluated in other research reactors or might soon be evaluated in power reactors. 3); the middle metallic layer, which is Nb or Ta in fuel cladding concepts for Gen-IV gas-fast reactors (Gen-IV GFRs), will be tailored to meet the needs of the ATF cladding application. Cladding tubes can be produced with different fiber braiding patterns (Fig. 1e), so as to tailor the material mechanical properties.

II.A.2. GESA surface-alloyed clads

The GESA process (Ref. 4) has already been successfully applied to modify steel clads intended for use in Gen-IV leadfast reactors (Gen-IV LFRs). For example, the surface-alloyed stainless steels DIN 1.4970 and 316 (Figs. 1f-1g) have been exposed for long times to liquid lead-bismuth eutectic (LBE) at high temperature (600°C) without suffering the typical liquid metal attack, due to the in-situ formation of a protective alumina scale. Typically, the GESA process involves the deposition

Water Reactor Fuel Performance Meeting

September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju • Jeju Island, Korea by plasma spraying on the steel surface of a FeCrAl coating (20-40 µm-thick), followed by melting of the coating by intense pulsed electron beam; the restructured surface-alloyed layer acts as a 'reservoir' layer of Al atoms that enables the in-service formation of an alumina scale (Fig. 1h). This technology has been upscaled to 1 m-long tubes; its suitability for the targeted ATF application relies on the chemistry of the applied coating and the hydrothermal stability of the oxide scale that will form in service. It is believed that the latter can be improved, if necessary, by the co-deposition of the appropriate dopants during the first stage of the GESA process; dopant co-deposition is expected to improve the oxide scale hydrothermal stability, presumably by dopant grain boundary decoration during the restructuration of the surface-alloyed layer.

II.A.3. MAX phase-coated clads

The MAX phases are a fascinating class of layered solids with hybrid metallic-ceramic behavior and properties that depend on stoichiometry, which is given by the general formula Mn+1AXn, where M is an early transition metal, A is an Agroup element, X is C or N, and n typically takes the values 1, 2 or 3. MAX phases form solid solutions on the M, A and X sites; solid solutions may often have better properties than the 'parent' MAX phases (Ref. 5). For example, the thermal stability of the (Nb,Zr)4AlC3 solid solution (Ref. 6) is better than that of the starting Nb4AlC3; the thermal stability of the former is comparable to that of refractory metals W and Mo (Fig. 2f). The fact that many of the properties of the MAX phases (e.g., deformability, thermal stability, oxidation resistance, etc.) can be tailored by producing solid solutions makes them quite versatile materials; for example, (Zr,Ti)2AlC (Figs. 2b-2c) has an improved oxidation resistance compared to the 'parent' Zr2AlC (Fig. 2a).

Water Reactor Fuel Performance Meeting

September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju • Jeju Island, Korea

The MAX phases exhibit better high-temperature radiation tolerance than their respective binary carbides: an example is given in the inset of Fig. 2d, which shows a highly-damaged TiC grain next to a defect-free Ti3SiC2 grain, both irradiated at ∼735°C to 3.4 dpa. The fact that the MAX phases are capable of effective self-annihilation of radiation-induced defects at elevated temperatures (700-1100°C; Figs. 2d-2e) makes them promising coating materials for ATF clads. Producing phasepure MAX phases is challenging, as the produced MAX phase-based materials often contain non-negligible amounts of 'parasitic' phases (Figs. 2b-2c) that can lead to differential swelling-induced intergranular cracking under neutron irradiation. The aim of MAX phase research in this project is to synthesize phase-pure, compositionally-and microstructurally-tailored MAX phases that will subsequently be applied as coatings on commercial clads. MAX phase coatings have already been successfully deposited on different substrates via plasma vapor deposition techniques (Ref. 7) and cold spraying (Ref. 8); similar low-temperature coating deposition approaches will be adopted in this project, so as not to alter the substrate clad microstructure and bulk properties. The coating/substrate interfaces in the candidate MAX phase-coated clad concepts will be engineered to ensure reliable in-service performance under both nominal and transient/accidental operation conditions.

II.A.4. Oxide-coated clads

High-quality nanocrystalline alumina (Al2O3) coatings have been successfully made by pulsed laser deposition (PLD). These coatings, often vitreous in the as-deposited state, started immediately crystallizing under heavy ion irradiation (dual ion beam of 12 MeV Au 5+ and 18 MeV W 8+ ), maintaining their crystallinity up to high dpa levels (20-150 dpa; no delamination was observed up to the maximum achieved displacement dose of 450 dpa; Figs. 3b-3c). These coatings have demonstrated a remarkable resistance to mechanical (even impact) damage, as shown by the adherence of the nano-impacted coating to the substrate (Fig. 3d). TEM analysis has shown that the crack-like features in Fig. 3d are filled with vitreous matter, indicating that these coatings might have a remarkable ability to self-repair mechanical damage (Ref. 9). The in-service irradiation of impacted/damaged coatings is expected to result in crystallization of the vitreous matter filling the crack-like features, thus resulting in coatings with remarkable self-healing capacity.

Water Reactor Fuel Performance Meeting

September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju • Jeju Island, Korea If needed, the resistance of the nanocrystalline oxide coatings to hydrothermal ageing could be improved by the codeposition of select dopants that segregate preferentially at grain boundaries during radiation-enabled crystallization. The concept of improving the hydrothermal stability of oxides by grain boundary engineering has been demonstrated by the addition of various dopants to 3Y-TZP (3 mol% yttria-stabilized tetragonal polycrystalline zirconia) dental ceramics: some of the added dopants (La 3+ , Nd 3+ and Al 3+ ) effectively decorated the material grain boundaries (Fig. 3a), thus preventing the intergranular diffusion of water species that is considered responsible for hydrothermal ageing (Ref. 10). The composition of the oxide coatings will be substrate-specific (other oxides are foreseen for conventional clads, such as zircaloys, and other for innovative clads, such as SiC/SiC composites) and the substrate/coating interface in each coated system will be engineered to guarantee reliable in-service performance under both nominal and transient/accidental operation conditions.

II.A.5. ODS-FeCrAl alloy clads

Oxide-dispersed-strengthened (ODS) iron-chromium-aluminum (FeCrAl) alloys produced by mechanical alloying are very promising candidate ATF cladding materials, due to their excellent oxidation resistance in steam up to 1450°C and their appealing mechanical properties (high strength, good resistance to irradiation creep and swelling) (Ref. 11). Their oxidation resistance is attributed to the combined Al and Cr content of the FeCrAl matrix (both Al and Cr form protective oxide scales), while their mechanical performance is associated with the distribution of ultra-fine dispersoids in the FeCrAl matrix (Fig. 4a); these dispersoids are capable of pinning dislocations, resulting in high yield strengths at all temperatures. Strengthening the FeCrAl matrix by incorporating oxide dispersoids is deemed necessary for these clads, since they must be manufactured with reduced thickness (< 400 µm) to compensate for the neutronic penalty associated with the use of Fe-based alloys instead of zircaloys.

Water Reactor Fuel Performance Meeting

September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju • Jeju Island, Korea

In the past 15 years, the development of the ODS fuel clads has mainly targeted Gen-IV sodium-fast reactors (Gen-IV SFRs) and Gen-IV supercritical water reactors (Gen-IV SCWRs). The previously achieved progress in ODS fuel cladding tube fabrication (Fig. 4c) will be exploited to develop ODS-FeCrAl alloys suitable for the ATF application. At present, the international community is mainly studying ODS-FeCrAl alloys with ∼5% Al and 12-20% Cr: the highest end of the Cr range favors oxidation resistance, while 12-15% Cr is expected to limit the formation of the brittle α′-Cr-rich phase upon neutron irradiation at low temperatures. The neutron irradiation tolerance of ODS alloys appears to be very satisfactory, both in terms of microstructural stability and mechanical property retention after irradiation (Fig. 4b), which is an additional motivation for the ongoing development of various ODS-FeCrAl alloy grades.

II.A.6. Predictive modeling activities

The reliable prediction of the long-term in-service behavior of the candidate ATF cladding concepts studied in this project demands the development of robust predictive models that are based on the understanding and accurate simulation of the damage accumulating in the material's lifetime. To that end, various approaches will be tried, one of which relies on the use of the FEMME (Finite Element Microstructure Mesh-free) fracture model to simulate the thermomechanical degradation and deformation process to failure of complex fuel cladding materials (Ref. 12), such as the SiC/SiC composites studied in this project. The FEMME model uses microstructure-related input acquired by means of 3D imaging (micro-computed X-ray tomography, µXCT, combined with digital volume correlation, DVC) to simulate in-service damage evolution in a cost-and time-effective manner (Fig. 5).

II.B. ATF Cladding Material Screening before Neutron Irradiation in BR2

This section describes the ATF cladding material down-selection process in IL TROVATORE prior to the irradiation in BR2 (Belgian Reactor 2) at SCK•CEN. The BR2 irradiation campaign aims at providing validation of the best ATF cladding concepts in an industrially-relevant environment, i.e., under neutron irradiation up to a maximum displacement of 7 dpa in PWR-like water. The candidate ATF cladding material candidates and their joints/welds will be irradiated simultaneously in 2017 Water Reactor Fuel Performance Meeting September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju • Jeju Island, Korea BR2, so as to pinpoint the most suitable material(s) for the targeted application, thus expediting the transfer of the anticipated key innovation to market. The first 2 years of the project will be dedicated to the optimization of the pre-selected ATF cladding material concepts described in the previous section. The timely optimization of the candidate ATF cladding material concepts prior to the irradiation in BR2 -scheduled for two full years -is a prerequisite for the success of the project, the principal aspiration of which is to deliver proof-of-concept ATF cladding materials that have been already validated in an industrially-relevant environment. Material optimization in IL TROVATORE is expected to help establishing a culture of accelerated materials development (AMD), which relies on the continuous feedback between application-driven material design, material production and material performance assessment under application-relevant conditions. The successful implementation of the AMD approach relies on high throughput material screening to reliably assess material performance prior to its demonstration in actual industrial conditions. Examples include ion and proton irradiation to assess the radiation tolerance of innovative nuclear materials prior to neutron irradiation, and judiciously-chosen modeling approaches (atomicscale, thermodynamic, finite element).

The down-selection of the ATF candidate cladding materials to be irradiated in BR2 will rely on the outcome of a set of critical screening tests that aim at assessing: 1. Resistance to aqueous corrosion at nominal operation conditions (specific exposure conditions): materials are expected to demonstrate a step improvement in corrosion rate when compared to commercial zircaloy clads; 2. Resistance to high temperature steam oxidation (specific exposure conditions): materials are expected to demonstrate a step improvement in steam oxidation resistance when compared to commercial zircaloy clads; 3. Hermeticity of sealed tubes: it includes the permeability of the bulk material to critical elements, in particular helium and tritium, and the demonstration of suitable end-plug joining techniques (i.e., welding or similar); and 4. Acceptable irradiation tolerance: materials and their constituents must show no decomposition or unacceptable swelling upon ion/proton irradiation to dpa levels relevant for the envisaged fuel cladding application.

III. KEY PERFORMANCE INDICATORS IN H2020 IL TROVATORE

The ATF cladding material concepts considered in IL TROVATORE have already reached an adequate level of technological maturity (TRL 3 -experimental proof-of-concept demonstrated and/or TRL 4 -technology validated in the lab). Their suitability in addressing the ATF challenge will be assessed at two moments during the project lifetime: (a) at the end of the 2 nd year (month 24) and (b) at the end of the project (month 54). In month 24, the optimized ATF clads that have successfully passed the pre-screening critical tests mentioned in the previous section, thus demonstrating a minimum threshold performance, will be allowed to partake in the BR2 irradiation. In month 54, the one or more proof-of-concept ATF clads that performed satisfactorily during the BR2 irradiation (i.e., achieved TRL 5 -technology validated in an industriallyrelevant environment) will be considered fit to embark on the next step towards deployment, which will be the demonstration of fuel-cladding compatibility. The IL TROVATORE project foresees a limited number of fuel/cladding compatibility tests between the best ATF cladding material candidates and standard LWR fuels, such as UO2 and MOX (mixed oxide fuels).

IV. INVOLVED PARTIES IN H2020 IL TROVATORE

Table I presents the H2020 IL TROVATORE beneficiaries. Apart from the beneficiaries entrusted with performing the foreseen R&D tasks, three expert advisory committees were established to ensure that the IL TROVATORE scientific & technical (S&T) objectives will be reached within the project timeframe (duration: 4.5 years; kick-off date: 1 st October 2017), i.e., a scientific advisory committee (SAC), an end users group (EUG), and a standardisation advisory committee (STC).

The SAC involves prominent members of the international scientific community with recognised expertise in disciplines that relate closely to the project R&D activities. The SAC experts have the necessary qualifications/experience to peer-review the appropriateness and robustness of the research approaches adopted by the project partners, helping to ensure scientific excellence. The EUG involves industrial experts that can communicate to the project beneficiaries the stakeholder knowledge and industrial expectations, in particular relative to ATF cladding material performance requirements, thus helping the project to achieve its S&T objectives. Moreover, the EUG involves nuclear safety regulators and experts involved in the development of Gen-IV systems that envisage the use of similar materials with those studied in the project. The STC involves experts associated with standardisation bodies and, more specifically, with standardisation activities on nuclear materials and advanced ceramics similar to some of the fuel cladding material concepts studied in the IL TROVATORE project. The STC experts provide feedback on existing and/or developing standards, assess the standardisation potential of the project results,

II.A. 1 .

 1 SiC/SiC composite clads Some of the recent progress in the development of leak-tight SiC/SiC composite cladding tubes is shown in Figs. 1a-1b for the DEMO-NITE SiC/SiC clads developed by OASIS (Organization of Advanced Sustainability Initiative for Energy System/Material), MuroranIT, Japan, and in Figs. 1c-1e for the "sandwich" SiC/SiC clads developed by CEA, France. The DEMO-NITE process uses a polymer-based slurry of SiC nano-powders to infiltrate SiC fibers that have been CVD-coated with pyrolytic carbon (Ref. 2); the intermediate SiC/SiC materials (green sheets → prepreg sheets → preforms) turn into fully-densified tubes based on the transient eutectic process by means of pressure-assisted sintering methods (hot pressing, hot isostatic pressing). The CEA "sandwich" SiC cladding concept contains a thin (50-100 µm) metallic liner between two layers of SiC/SiC composites (Fig. 1c) (Ref.

Fig. 1 .

 1 Fig. 1. DEMO-NITE SiC/SiC cladding: (a) fracture surface showing fiber pull-out, fiber & matrix micro-crystallinity and perfect densification; (b) tubes. The CEA SiC "sandwich" concept: (c) tube cross-section showing (1) an inner SiC/SiC layer, (2) a middle metallic layer, and (3) an external SiC/SiC layer; (d) tube; (e) SiC/SiC layouts with different braiding patterns: (I) filament winding, (II) 2D braiding, and (III) 3D braiding. (f) GESA FeCrAlY-coated DIN 1.4970 stainless steel cladding tubes exposed at 600°C for 8500 h to liquid LBE with 10 -6 mass% oxygen. (g) GESA Al surface-alloyed 316 stainless steel exposed at 600°C for 10000 h to liquid LBE with 10 -6 mass% oxygen. (h) GESA process of surface modification by an intense pulsed electron beam: volumetric heating → formation of a melt layer → restructured surface-alloyed layer.

Fig. 2 .

 2 Fig. 2. (a) MAX phases in the Zr-Al-C system (left: Zr2AlC; right: Zr3AlC2) are the basis for the development of oxidationresistant solid solution coatings with financially-acceptable neutronic penalty. Producing phase-pure MAX phases and solid solutions thereof is often challenging: STEM images of a (Zr,Ti)2AlC MAX phase grain next to a ZrC 'impurity' grain (b) and the interface between them (c). (d) Neutron-irradiated Ti3SiC2 at ∼735°C to 3.4 dpa: large defect-denuded zones are established next to MAX phase grain boundaries (GBs), which act as potent radiation-induced defect 'sinks'; the Ti3SiC2 MAX phase grain appears less damaged than the neighboring TiC grain (inset). (e) Neutron-irradiated Ti3SiC2 at ∼1085°C to 3.4 dpa: the grains appear devoid of defects, except from few isolated dislocations (arrows). (f) Comparison of the hightemperature stability of select MAX phases (commercial Maxthal 211 ® & 312 ® produced by Sandvik, Sweden, and (Nb,Zr)4AlC3 solid solution) with that of refractory metals tungsten (W) and molybdenum (Mo).

Fig. 3 .

 3 Fig. 3. (a) Dopant-specific grain boundary decoration in 3Y-TZP (3 mol% yttria-stabilized tetragonal polycrystalline zirconia). Grain boundary decoration of 3Y-TZP ceramics by La 3+ , Nd 3+ and Al 3+ dopants increased their hydrothermal stability; grain boundary decoration -and consequently improvement of the hydrothermal stability of the doped oxidescould not be achieved by adding Sc 3+ . TEM images of heavy ion-irradiated amorphous Al2O3 coatings made by PLD show nanocrystalline domain formation -20 dpa (b) and growth -150 dpa (c). (d) TEM data of a nano-impacted, heavily ionirradiated (150 dpa) Al2O3 coating deposited by PLD: crack-like features are filled with vitreous matter as result of impactinduced local amorphisation.

Fig. 4 .

 4 Fig. 4. ODS-FeCrAl alloys: (a) TEM image a certain FeCrAl alloy with nanosized oxide dispersoids -dark particlesdispersed in the grey FeCrAl matrix. (b) Yield strength vs. total elongation for alloys (ss: stainless steels; ms: martensitic steels; SA: solution-annealed; CW: cold-worked; AQ: as-quenched; T: tempered) irradiated at 325°C in the OSIRIS research reactor up to 9 dpa; the ODS alloys (MA957 & EM10+Y2O3) exhibit appreciable residual ductility at a dose of 5 dpa. (c) Fe-14Cr ODS tube produced at CEA Saclay (outer diameter = 10.7 mm; wall thickness = 0.50 mm).

Fig. 5 .

 5 Fig. 5. (a) The mechanical damage (arrows) in a SiC/SiC composite cladding tube is revealed in the high-resolution (0.7-µm voxel size) µXCT scan of the tube cross-section; the 3D impression of the deformation field around the site of mechanical damage is given by DVC. (b) The FEMME (III) 3D simulation of thermomechanical damage evolution provides a comparable level of discretization (accuracy) as a fine finite element model (II), but at the low computational cost of a coarse finite element model (I).

ACKNOWLEDGMENTS

This project has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 740415. The authors of this article would like to gratefully acknowledge the contribution of many of their colleagues to 2017 Water Reactor Fuel Performance Meeting September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju • Jeju Island, Korea the results presented here as indicative of the state-of-the-art achievements in the development of the ATF cladding material concepts to be studied in the project. Due to the large number of involved persons, not all contributors could be named.

Water Reactor Fuel Performance Meeting

September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju • Jeju Island, Korea and offer recommendations on the appropriate standardisation strategy leading to the development of new standards (material specifications, material test methods).

V. CONCLUSIONS

The conventional "cook and look" approach cannot be adopted, if accident tolerant fuel (ATF) systems are to be implemented in the current generation of nuclear reactors. Accelerated material development strongly relies on physics-based approaches to expedite the development, demonstration and licensing processes for innovative fuel cladding materials. This need is not only reflected in the contemporary efforts to improve the nuclear energy safety via the development of ATF cladding materials, but it also regards the deployment of future generation systems (Gen IV, fusion). The envisaged developments in the IL TROVATORE project intend to validate specific candidate ATF cladding concepts in an industriallyrelevant environment, thus helping to take a critical step towards an improved nuclear energy safety worldwide.


M Freyss

M Bertolus

Mf Barthe

HAL is

Positron lifetime can be computed using electronic structure calculations in the two component density functional theory (TCDFT) [1]. Self-consistent calculations are performed to obtain the

Methods

Positron lifetimes are computed using electronic structure calculations in the two component density functional theory (TCDFT) [2]. Self-consistent calculations are performed to obtain the electron and positron densities.

In our study the relaxation effect due to the vacancy creation and the positron localized in the defect is taken into account.

Calculations are performed with the ABINIT code [3] using 96 atom supercells. The DFT+U [4] method is used to take into account the strong correlations between the 5f electrons.

Preliminary results : unexpected localization of positron in bulk actinide oxides For pure tri-valent elements, bulk lifetime within the GGGC-GGA scheme is much higher than expected.

Mixed oxides supercell are simulated using the SQS method.

La 3+ U 5+ localized positron Delocalized positron in bulk UO 2 , UCeO 2 and UPuO 2 .

In ULaO 2 and UAmO 2 , positron localized in the vicinity of La 3+ and Am 3+ respectively.

t V O -UCeO 2 =227ps t V O -ULaO 2 =202ps

For

  oxygen mono-vacancy : Coupled first-principles and experimental positron annihilation study of defects in actinide mixed oxides Gérald Jomard 1,* , Ibrahim Cheik Njifon 1 , Martin S. Talla Noutack 1 , Michel Freyss 1 , Marjorie Bertolus 1 , Marie-France Barthe 2 1 CEA, DEN, DEC, Centre de Cadarache, 13108 Saint Paul Lez Durance, France 2 CEMHTI, CNRS/UPR3079, 45071 Orléans, France * gerald.jomard@cea.fr Context  Positron Annihilation Spectroscopy (PAS) is a powerful technique to detect vacancy-type defects in materials  We demonstrated its capability to identify irradiation-induced defects in UO 2 by coupling experiments and ab initio calculations [1]  MOX fuels (UPuO 2 ) will be the fuel of Gen IV reactors  MOX fuels fabricated from spent fuels  contains few % of minor actinide elements like Am  Under irradiation, defects are created in the fuel and have strong impact on behaviour of fission products and evolution of microstructure Strong interest in characterizing vacancy defects created in actinides mixed oxides