Keywords: Tin Monoxide, Ceramics, Spark Plasma Sintering, Metastable Materials, Low Temperature Densification

The thermodynamic stability limits of Sn(II) under ambient conditions imposes constraints on the densification of divalent tin based oxides. In the case of tin monoxide (SnO), a low temperature (≤ 300 °C) electric field assisted processing route (Cool-SPS) affords densification up to 90% of theoretical density. This is demonstrated for both conventional SnO and reactive tin(II) oxyhydroxide [Sn6O4(OH)4] precursor powders. The choice of starting precursor impacts both the optimized processing parameters and the resulting ceramic microstructure. Characterization of phase content and stability has been performed on both the precursor powders and resulting ceramics. Preliminary electrochemical property measurements are presented and their connection to observed microstructure and choice of initial precursor is discussed.

Introduction

From the perspective of both applied materials science and fundamental chemistry, divalent tin monoxide, also known as stannous oxide, is an intriguing compound with an unconventional history. Initial reports of synthetic SnO from the mid 1800's [START_REF]Preparation of protoxide of tin[END_REF][START_REF] Ditte | Action des dissolutions acides sur le protoxyde d'étain[END_REF] were later revisited in greater detail [START_REF] Kwestroo | Preparation of three modifications of pure tin (II) oxide[END_REF]. Similarly, the first descriptions of a two-dimensional layered tetragonal (P4/nmm space group) structure for α-SnO by Pauling et al. [START_REF] Moore | The Crystal Structures of the Tetragonal Monoxides of Lead, Tin, Palladium, and Platinum[END_REF] were later refined with Rietveld analysis of neutron powder diffraction data [START_REF] Pannetier | Tin(II) oxide: structure refinement and thermal expansion[END_REF] revealing how the sterically active lone pair Sn 2+ cations form weak van der Waals bonds between sheets of covalently bonded SnO4 pyramidal units. Unexpectedly, SnO was subsequently discovered as a naturally occurring mineral (with the assigned name Romarchite) forming via surface corrosion processes on Sn-rich metal alloys uncovered during archeological studies of sunken shipwreck sites, such as the pirate Blackbeard's ship Queen Anne's Revenge [START_REF] Dunkle | Romarchite, Hydroromarchite And Abhurite Formed During The Corrosion Of Pewter Artifacts From The Queen Anne's Revenge (1718)[END_REF].

Recently this compound has received increased scrutiny for its prospects as a more environmentally friendly functional metal oxide in electronic and optoelectronic technological applications. Undoped, SnO is a p-type semiconducting metal oxide [START_REF] Ogo | Tin monoxide as an s-orbitalbased p-type oxide semiconductor: Electronic structures and TFT application[END_REF]. Multiple studies have examined its use in gas sensors, transparent conducting oxides, and thin film transistors [START_REF] Suman | Comparative gas sensor response of SnO2, SnO and Sn3O4 nanobelts to NO2 and potential interferents[END_REF][START_REF] Saji | P-type SnO thin films and SnO/ZnO heterostructures for all-oxide electronic and optoelectronic device applications[END_REF][START_REF] Okamura | Solution-processed oxide semiconductor SnO in p-channel thin-film transistors[END_REF]. Due to a relativity large interlayer spacing, SnO has also been considered as intercalation type electrode material for lithium and sodium-based energy storage applications [START_REF] Ning | Syntheses, Characterizations, and Applications in Lithium Ion Batteries of Hierarchical SnO Nanocrystals[END_REF][START_REF] Jaśkaniec | Solvent engineered synthesis of layered SnO for high-performance anodes[END_REF]. This two-dimensional structure type may also be associated with ferromagnetism, as recently predicted by first principles calculations for hole-doped SnO [START_REF] Seixas | Multiferroic Two-Dimensional Materials[END_REF][START_REF] Houssa | Ferromagnetism in two-dimensional hole-doped SnO[END_REF]. In addition, theoretical and experimental studies have also investigated polycrystalline SnO as a potential thermoelectric material due to its relatively high Hall mobility [START_REF] Miller | SnO as a potential oxide thermoelectric candidate[END_REF]. Nevertheless, despite these intriguing properties and applications, processing of tin monoxide remains challenging. Metastable in its divalent oxidation state, blue-black Sn 2+ O undergoes disproportionation between 200 -400 °C by a decomposition reaction to Sn 4+ O2 and metallic Sn via an intermediary phase of Sn3O4. The precise decomposition temperatures vary with sample morphology, dwell time and atmospheric oxygen content [START_REF] Moreno | Kinetic study of the disproportionation of tin monoxide[END_REF][START_REF] Giefers | Kinetics of the disproportionation of SnO[END_REF][START_REF] Campo | Thermal behaviour of romarchite phase SnO in different atmospheres: a hypothesis about the phase transformation[END_REF]. Conventional high-temperature sintering routes therefore do not present feasible paths towards single phase dense SnO ceramics; instead, novel routes compatible with the low thermal stability limits of the compound need to be considered. Several low temperature sintering approaches have been developed in recent years [START_REF] Biesuz | What's new in ceramics sintering? A short report on the latest trends and future prospects[END_REF], which includes Room Temperature Fabrication [START_REF] Kähäri | Dielectric Properties of Lithium Molybdate Ceramic Fabricated at Room Temperature[END_REF], Cold Sintering Processes [START_REF] Guo | Cold Sintering: A Paradigm Shift for Processing and Integration of Ceramics[END_REF], Hydrothermal Sintering [START_REF] Ndayishimiye | Roadmap for densification in cold sintering: Chemical pathways[END_REF], and Cool-SPS [START_REF] Herisson De Beauvoir | Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials[END_REF]. These developing approaches can offer additional advantages in terms of energy efficiency, integrated co-sintering possibilities, and physical property tuning by control of nanocrystalline structures [START_REF] Sohrabi Baba | Contrasting energy efficiency in various ceramic sintering processes[END_REF][START_REF] Elissalde | Single-step sintering of zirconia ceramics using hydroxide precursors and Spark Plasma Sintering below 400 °C[END_REF]. For SnO, the effectiveness of a low temperature solvent assisted Cold Sintering Process (CSP) strategy was recently demonstrated [START_REF] Bang | Densification of thermodynamically unstable tin monoxide using cold sintering process[END_REF]. In this CSP approach, relative densities of 80% -90% are achieved in under 2 hours with uniaxial pressure at temperatures ≤ 265 °C. Vibratory ball milling of the initial commercial SnO precursor powders led to higher pellet densities after CSP processing due to decreased particle size and increased reactivity, although X-ray diffraction revealed the presence of SnO2 above 200 °C.

In the current work presented here, we consider an alternative approach towards dense SnO ceramics by use of a Cool-SPS (SPS: Spark Plasma Sintering) method. SPS is a pressure assisted sintering technique using pulsed electric currents for heating. Initially targeted at rapid high temperature processing of metal and refractory materials, SPS has been increasing examined as a versatile tool for the sintering and solid-state synthesis applicable to a broad class of materials [START_REF] Franceschin | Sintering and Reactive Sintering by Spark Plasma Sintering (SPS)[END_REF]. Under lower processing temperatures (below 600 °C, within the pressure range 40 -900 MPa) the Cool-SPS approach has recently been shown to be effective for the sintering of diverse thermodynamically unstable compounds; including molecular materials and oxides exploiting reactive precursors that convert into the desired phase upon processing [START_REF] Herisson De Beauvoir | Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials[END_REF][START_REF] Herisson De Beauvoir | Cool-Spark plasma sintering: An opportunity for the development of molecular ceramics[END_REF][START_REF] Hérisson De Beauvoir | Densification of MnSO4 ceramics by Cool-SPS: Evidences for a complex sintering mechanism and magnetoelectric coupling[END_REF]. Given the limited literature concerning SPS processing of SnO [START_REF] Kuwahara | Synthesis of High-Density Bulk Tin Monoxide and Its Thermoelectric Properties[END_REF], in the current study reported here we further examine the advantages of a Cool-SPS type approach as applied to tin monoxide and consider the role of starting precursors in enhancing reactivity during processing. Sn6O4(OH)4 is explored as a reactive precursor and is compared with conventional tin(II) oxide powders. It is shown that with careful control of the processing conditions, highly dense SnO ceramics can be obtained from very different starting precursors, but with final microstructure and physical properties linked to the nature of the initial powders.

Experimental 2.1. Powder preparation

Synthesis of a tin(II) oxyhydroxide precursor [Sn6O4(OH)4] was performed by a precipitation method as described previously [START_REF] Donaldson | Hydrous Tin(II) Oxide[END_REF][START_REF] Kitabayashi | Thermal Decomposition of Tin(II) Oxyhydroxide and Subsequent Oxidation in Air: Kinetic Deconvolution of Overlapping Heterogeneous Processes[END_REF], with an optimization of several synthesis details in the current study. Starting with 100 ml of a 0.5 M acetic acid aqueous solution, 9.48 grams of SnCl2•2H2O (Alfa Aesar 98%) was completely dissolved at 50°C after constant stirring for 15 minutes. The resulting solution which is cloudy due to SnOCl2 salt formation [START_REF] Wiberg | Inorganic chemistry[END_REF] was centrifuged to separate a transparent acidic Sn 2+ solution. Subsequently, a basic solution of ammonium hydroxide (1.13 M) was added dropwise to the acidic Sn 2+ solution with periodic stirring until the pH level of 4 was reached, leading to the precipitation of a pale-yellow powder. This precipitate was separated by centrifuging and repeated washings with deionized water until of pH value of 7 was reached in order to eliminate residual chlorides followed by drying at 50°C overnight. It is noted that precise synthetic reproducibility of tin(II) oxyhydroxide proved challenging. Further details and opportunities for optimizing the synthesis of this precursor will be discussed in a future publication. In order to make a comparison with the tin(II) oxyhydroxide precursor, a second precursor of conventional tin(II) oxide was prepared from commercial SnO powder (Alfa Aesar 99.9%) after ball milling (FRITSCH planetary micro mill) for two hours in ethanol with 10 mm agate grinding beads in an agate container, followed by drying under air at 80°C for 1 hour.

SPS Processing

SPS Sintering was performed using Dr. Sinter Lab model SPS-515S with 30kW generator coupled with 50 kN press under dynamic vacuum atmosphere. The precursor powders were loaded into a tungsten carbide (WC) mould assembly (inner ϕ =10.4 mm with 0.2 mm thick single carbon foils between the powders and inner surface of the mould. Thin carbon and tantalum metal discs were positioned between the precursor powder and WC pistons (top and bottom, along the uniaxial pressure direction) of the mould assembly to reduce reactivity with the sample. Subsequently the WC mould assembly was placed into the SPS chamber and subjected to a computer-controlled cycle of pressure and temperature. For this work, pressure was systematically applied first (ramp rate of 3.8 kN/min), followed by the desired heating cycle (typically using thermal ramp rates of 25 °C/min, dwell times of 5 mins, and quench cooling). During the entirety of each SPS experiment, the applied piston force and thermocouple temperature of the WC mould assembly was monitored and logged, in addition to other key parameters such as chamber gas pressure (measurement resolution of 0.1 Pa), power (voltage, current, with 0.1 V and 1 A precision, respectively), and relative piston displacement (within the minimum displacement resolution of 0.01 mm).

Material Characterizations

X-ray diffraction (XRD) was performed on precursor powders and ceramics using a Bragg-Brentano geometry PANalytical X'Pert MPD Pro diffractometer with a Cu Kα source. Measurements on both powders and ceramics pellets were performed with a 2θ step size of 0.016° over the 2θ range 10 -80°. Analysis of XRD data was performed with the TOPAS software package using a Fundamental Parameters (FP) approach for the instrumental contribution. Starting crystallographic models were obtained from the ICSD database [START_REF] Zagorac | Recent developments in the Inorganic Crystal Structure Database: theoretical crystal structure data and related features[END_REF] for Sn6O4(OH)4 (ICSD Nº 203206), SnO (ICSD Nº 16481) and SnO2 (ICSD Nº 183984). Refined terms included: unit cell parameters, background polynomial coefficients, Sn site atomic displacement parameter, and microstructure peak shape profile terms. For some XRD data, in particular for measurements of SPS processed samples, anisotropic strain and March-Dollase type preferred orientation terms were required. 119 Sn Mössbauer measurements were performed with a constant acceleration Halder-type spectrometer operating in transmission geometry with a room temperature 119m Sn (CaSnO3) source (370 MBq). The Mössbauer spectra of thin absorbers containing about 10 mg.cm -2 of Sn ( 119 Sn natural abundance 8.59%) were recorded at room temperature and the refinement of the Mössbauer hyperfine parameters (δ isomer shift, Δ quadrupole splitting, Γ Lorentzian line width and relative areas) was performed using both homemade programs and the WinNormos software (Wissenschaftliche Elektronik GmbH) [START_REF] Brand | WinNormos Mössbauer Fitting Program[END_REF]. The 119 Sn isomer shifts are referenced to CaSnO3 at room temperature. Due to nonequivalent f (Lamb-Mössbauer) factors for Sn 2+ and Sn 4+ at ambient temperature, content of the former is likely underestimated in quantitative analysis performed for this study.

Sample morphology of powders and fracture ceramic surfaces were examined by scanning electronic microscopy using both a Zeiss EVO 50 SEM and a high resolution JEOL JSM-6700F FEG-SEM in secondary scattering mode. Fourier transform infrared spectroscopy (FTIR, BRUKER Equinox 55) were performed (reflection geometry) to identify functional groups in starting precursor powders. Thermogravimetric analysis (TGA, TA-Q50) of precursor powders were performed under flowing air with 5°C/min temperature ramps. Geometric density values of obtained ceramics were determined by a classical geometrical approach, considering the measured mass, thickness and diameter of the pellets and theoretical crystallographic SnO density of 6.45 g/cm 3 . Uncertainty values for the geometric densities are derived from deviations within multiple sampling of thickness and diameter values for a single pellet. The greatest source of measurement error was in the thickness of the samples, resulting in an estimated uncertainty of 2-3% even after pellets were polished with SiC abrasive papers prior to density measurement. Electrical resistivity measurements were performed with a homemade four-point probe setup, using tungsten carbide needles (ϕ = 100µm) with 1 mm spacing. A DC current source (0.1 mA) used by the instrument ADRET 103A while the temperature was regulated by a Eurotherm 902 (PT100 sensor). Electrochemical Impedance Spectroscopy (EIS) measurements were performed at room temperature with Autolab potentiostat/galvanostat (PSTAT302N) with FRA32M module for a frequency range of 10Hz-1MHz.

Results and Discussion

Concerning the Precursor Powders

The tin(II) oxyhydroxide precursor was synthesised as described in section 2.1, with an end product appeared to be pale yellow in color after final drying. Initial characterization of the powder by XRD and FTIR suggested a largely phase pure material consistent with previous investigations on the composition Sn6O4(OH)4 [START_REF] Abrahams | Tin(II) Oxyhydroxide by X-ray Powder Diffraction[END_REF][START_REF] Deng | Combined X-ray Diffraction and Diffuse Reflectance Analysis of Nanocrystalline Mixed Sn(II) and Sn(IV) Oxide Powders[END_REF]. The main plot in Fig. 1a shows the single-phase Rietveld fit, using the tetragonal P4 # 21c space group with refined values for lattice [a = 7.952(1) Å, c = 9.139(1) Å] and crystallite size (giving a volume weighted integral breadth estimate of < 200 nm). This agrees well with the reported structure of Abrahams et al. [START_REF] Abrahams | Tin(II) Oxyhydroxide by X-ray Powder Diffraction[END_REF], which is the original source for the often cited Sn6O4(OH)4 structural database entries in both the ICSD (Nº 203206) and JCPDS (Nºs 84-2157 and 46-1486). The inset of Fig. 1a shows the FTIR spectrum, with strong but broad absorption peaks around 3400 cm -1 and 550 cm -1 , which can be associated with hydroxide O-H group stretching and Sn-O bond stretching, respectively. A small absorption feature near 1600 cm -1 may be associated with minor residual H2O molecules [START_REF] Kitabayashi | Thermal Decomposition of Tin(II) Oxyhydroxide and Subsequent Oxidation in Air: Kinetic Deconvolution of Overlapping Heterogeneous Processes[END_REF]. SEM imaging (Fig. 2a) of the powder reveals loosely agglomerated clusters of very fine sub 100 nm sized particles. TGA analysis of the tin(II) oxyhydroxide precursor under air shows a curve (Fig. 3, red line) which is also in good agreement with previous reports [START_REF] Khanderi | Hydrolysis of bis(dimethylamido)tin to tin (II) oxyhydroxide and its selective transformation into tin (II) or tin (IV) oxide[END_REF]. An initial weight loss of ≈ 2.5% below 200 ºC is observed and is attributed to the release of hydroxide groups to form SnO (vs. an expected theoretical weight loss for Sn6O4(OH)4 à SnO of 4.3%). Above 200 ºC, further heating under air results in weight gain associated with the oxidation of SnO à SnO2. The weight gain increases sharply with temperature, before abruptly flattening above 380 ºC, at which point the oxidation of Sn 2+ to Sn 4+ may be largely complete. However, the observed experimental gain between 200 ºC and 400 ºC of ≈ 5 % is less than the expected theoretical weight gain of 11.9% for SnO à SnO2. Further analysis of the reactive tin(II) oxyhydroxide precursor powder by 119 Sn Mössbauer helps understand these discrepancies in observed TGA weight gain/loss. A typical fit of the experimentally measured spectra (Fig. 4a) shows the expected doublet with isomer shift characteristic (δ ≈ 3.0 mm/s) of divalent Sn 2+ (blue shaded area), but also the significant signature of a singlet doublet with near-zero isomer shift characteristic of Sn 4+ (green shaded area). Semi-quantitative analysis estimates the Sn 4+ fraction is below 40%. Subsequent reanalysis by powder XRD with an internal standard for quantification of non-crystalline content, indicates that amorphous phase(s) make up approximately 30% mass fraction of the sample, which is assumed to incorporate the Sn 4+ species observed by Mössbauer analysis. This apparent reduction in divalent tin containing phases participating in decomposition/oxidation processes is qualitatively consistent with the smaller than theoretically expected mass changes measured in TGA.

Despite repeated attempts to optimize the synthesis conditions for tin(II) oxyhydroxide, it was not possible to fully eliminate this amorphous content. It is speculated that some level of undetected amorphous Sn 4+ material was also present in Sn6O4(OH)4 powders examined by multiple previous literature reports [START_REF] Ning | Syntheses, Characterizations, and Applications in Lithium Ion Batteries of Hierarchical SnO Nanocrystals[END_REF][START_REF] Kitabayashi | Thermal Decomposition of Tin(II) Oxyhydroxide and Subsequent Oxidation in Air: Kinetic Deconvolution of Overlapping Heterogeneous Processes[END_REF]. Enhancement of synthesis parameters will be investigated further in future publications. Nevertheless, as discussed below, the tin(II) oxyhydroxide powder obtained in the current study was sufficient to demonstrate the unique possibility of reactive precursors as a processing route for Sn 2+ containing ceramic oxides by Cool-SPS.

The reactive nature of the powders encompasses both a small particle size distribution and its propensity for reactivity at low temperatures. For the later aspect, TGA analysis was used to assess and compare with conventional tin(II) oxide precursor powders. As shown in Fig. 3 (black line), the as-received commercial SnO powder remains relatively inert under air up to 350 ºC, with < 1% weight gain from oxidation even at 400 ºC. However, the decreased agglomerate sizes resulting from only 2 hours of ball milling lowers the onset temperature of decomposition/oxidation processes, with a clear mass increase starting from 300 ºC (blue line, Fig. 3). SEM images after ball milling (Fig. 2c) show an inhomogeneous distribution of particle sizes. These results agree well with the previous report of Bang et al., in which a further reduction in decomposition temperature is observed after 24 hours of SnO ball milling [START_REF] Bang | Densification of thermodynamically unstable tin monoxide using cold sintering process[END_REF]. However the tin(II) oxyhydroxide of the current study extends this trend even farther, with a lower onset temperature and larger relative weight gains after the release of hydroxide groups from 200 ºC. Note that phase purity of the precursor SnO powder appears unaffected by ball milling, as assessed by both a single-phase Rietveld fit of XRD data [Fig. 1c, tetragonal space group P4/nmm with refined lattice parameters of a = 3.802(1) Å, c = 4.841(1) Å, in excellent accord with ICSD database values] and clean divalent Sn doublet observed by Mössbauer analysis (Fig. 4c).

Processing Parameters

Using low temperature Cool-SPS processing conditions to target dense SnO ceramics, a study was made comparing the impact and efficiency of the different precursor powders. For both the reactive tin(II) oxyhydroxide and conventional SnO powders, a wide range of different pressure and temperature parameters were explored in order to establish optimized processing conditions for each. As discussed earlier in section 2.2., pressure was systematically applied first during SPS processing, followed by a programed thermal ramp at constant pressure which lead to dwell period (typically ≈ 5 minutes) at constant pressure and temperature. The processing cycle was completed by a rapid cooling in < 5 minutes to ambient temperature, and subsequent decompression, in order to extract the sample from the SPS mould assembly for characterization.

Real-time monitoring of several SPS parameters during processing provides indirect insight into the sequence of reaction mechanisms occurring under different conditions. This is illustrated in Fig. 5a and Fig. 5b, which plots the evolution of two parameters; Piston Displacement and Chamber Pressure, left and right axes respectively, during processing. Piston Displacement (units of mm) provides information about sample compaction or densification; although impossible to separate from the contributions of the mould and spacerslarger values of displacement can be associated with decreasing sample thickness (as the upper plate in the SPS chamber is fixed). Chamber Pressure (units of Pa) is qualitatively connected to sample outgassing events, such as the release of hydroxide groups. Higher chamber pressure values indicate larger outgassing volumes, but without any specific information on the chemical nature of these species. This is illustrated in Fig. 5a, where these parameters are plotted for a typical SPS processing sequence of conventional ball-milled SnO precursor powders. Starting from the left side of the plot, the initial ambient temperature sample compaction with increasing load is observed. Next, the sample temperature is increased while maintaining the constant target force (400 MPa in this example). With temperature increase, a small raise in chamber pressure (likely chemisorbed water) is noted after 100 ºC, but little further sample densification is observed suggesting densification is mainly dependent on pressure-driven mechanisms. In the third sequence step, a 5-minute dwell at constant temperature and compression (300 ºC and 400 MPa), the minor outgassing events subside. Outgassing continues to decrease during the last quench cooling and de-compression steps, with an initial contraction of the sample dimensions upon cooling before a final expansion as the piston pressure is released. By contrast, monitored parameters from processing of reactive tin(II) oxyhydroxide precursors (Fig. 5b) reveal different mechanisms in play especially during heating. While the initial sample compaction at ambient temperature is similar to conventional SnO (up to 300 MPa), significant outgassing and densification events are observed during the heating step. Based on the crystallographic unit cell volume per Sn cation in the formula units of the starting precursors (≈ 70 Å 3 vs. ≈ 95 Å 3 for SnO and Sn6O4(OH)4, respectively), a certain greater compaction is expected during the transformation of the reactive precursor to an SnO ceramic. Starting from 100 ºC, an abrupt sample compaction is presumably connected to the almost simultaneous onset of a significant outgassing event that continues beyond ≈ 160 ºC. It is assumed this is associated with the initial decomposition of tin(II) oxyhydroxide and the release of OH groups. A later increase in outgassing just prior to the constant temperature plateau of 270 ºC is also associated with increased sample compaction. It is noted that Khanderi et al also observed multiple discrete events in their thermogravimetry coupled IR analysis of Sn6O4(OH)4 transformations on heating under a nitrogen atmosphere [START_REF] Khanderi | Hydrolysis of bis(dimethylamido)tin to tin (II) oxyhydroxide and its selective transformation into tin (II) or tin (IV) oxide[END_REF]. However, without additional in situ information concerning the chemical nature of the released species or the structural evolution of the sample during SPS processing, it is difficult to definitively assign specific reaction pathways with events inferred from the SPS monitoring. Subsequently, on cooling and de-compression, the trends echo that seen in Fig. 5a, hinting at the fact that the final processed samples from both processing routes are similar. However, it is clear that the chemical reaction paths and compaction mechanisms followed to arrive at a dense SnO ceramic varies significantly during low temperature SPS processing depending on the choice of initial starting precursor powders.

It is noted that direct comparisons with complementary TGA data under air on the same precursor powders is complicated by unique heating mechanism and sample environment of the SPS mould and the dynamic vacuum atmosphere of the SPS chamber. Moreover, the combination of carbon foils and a dynamic vacuum creates reducing conditions in the SPS chamber. Thus, some speculation on chemical reaction routes is unavoidable in the absence of direct chemical information on the outgassing species; future SPS experiments incorporating residual gas analysis are planned.

Sintering and Phase Stability

Numerous experiments on both conventional and reactive precursor powders were examined in the current study in order to understand and optimize the processing sequences for each precursor. In particular, these results have been compared with recent work on SnO densification by a CSP approach [START_REF] Bang | Densification of thermodynamically unstable tin monoxide using cold sintering process[END_REF]. Table 1 lists the experimental SPS temperature and pressure conditions, and resulting geometric pellet density for selected attempts. The attempts performed at 265 ºC and 350 MPa (using commercial and ball-milled SnO precursor powders, respectively) were selected for direct comparison with the optimized CSP parameters recently reported by Bang et al. For both CSP and SPS, ball milling of the commercial SnO precursor consistently appears to promote greater densification. However, initial targeting of the same plateau parameters (265 ºC and 350 MPa) in the current SPS study resulting in pellet densities (approximately 72 and 78%, for commercial and ball-milled powders, respectively) notably lower than what was reported by CSP (84 and 89%, respectively) under the same nominal temperature and physical pressure conditions [START_REF] Bang | Densification of thermodynamically unstable tin monoxide using cold sintering process[END_REF]. Two important experimental differences may help explain these results. Firstly; in the CSP approach, the total processing time (ramp and dwell) is much longer; more than 100 minutes for the reported CSP work on SnO compared with under 30 minutes for SPS in the current study. Secondly, and perhaps more importantly, the CSP study of Bang et. al made use of additional transient liquids to promote dissolution/precipitation processes. Additional experiments to explore SPS processing parameters for ball milled commercial SnO powders have demonstrated that improved ceramic densities of approximately 90% (with a measurement uncertainly of 3%) can be achieved at slightly higher temperature and pressure values (Table 1, line 3). In the current study, optimal conditions were found when using at least 400 MPa of compression pressure at a dwell temperature of approximately 300 ºC. Similar ceramic SnO densities of 85 -90 % were reported by Kuwahara et al. after SPS processing at 325 ºC and 500 MPa [START_REF] Kuwahara | Synthesis of High-Density Bulk Tin Monoxide and Its Thermoelectric Properties[END_REF]. Refinement of XRD data (Fig. 1d) from the resulting pellet shows a near single phase SnO sample (P4/nmm with a = 3.801(1) Å, c = 4.839(1) Å). A faint peak in the background attributed to a minor SnO2 contamination is marked by asterisk. This is in good agreement with Mössbauer analysis (Fig. 4d) showing the expected dominant doublet peak for divalent Sn 2+ (blue shaded area, ≳ 90%) with a minor divalent Sn 2+ (blue shaded area, ≲ 10%). In general, lower values of pressure and temperature produced pellets with inferior geometric density values. Greater pressure values (> 400 MPa) did not improve final pellet densities, and instead decreased the likelihood of successfully extracting intact samples from the WC mould. The upper temperature limit for majority SnO phase stability under SPS processing parameters explored in this study varied inversely with pressure, decreasing from approximately 500 ºC at 50 MPa to approximately 300 ºC at 600 MPa, similar to the trend noted by Kuwahara et al. [START_REF] Kuwahara | Synthesis of High-Density Bulk Tin Monoxide and Its Thermoelectric Properties[END_REF]. Experiments performed with processing parameters beyond these thermodynamic limits consistently exhibited evidence of significant decomposition / disproportion of SnO towards SnO2 and metallic Sn.

A similar exploration of processing parameters was undertaken for the reactive tin(II) oxyhydroxide precursor. Consistent with the TGA results (Fig. 3), these precursor powders showed greater thermal reactivity, in particular at lower compression pressures. The upper temperature limit for phase stability was also reduced; evidence for significant SnO2 and/or metallic Sn formation started from 350 ºC even at lower pressures of 50 MPa. But most significantly, it was demonstrated that tin(II) oxyhydroxides precursors can be used to realize robust SnO ceramics with geometric densities and phase purities equivalent to that obtained with conventional SnO precursors. Optimal processing conditions are found when using 300 MPa compression pressure and a dwell temperature of approximately 270 ºC (Table 1, line 4). Resulting XRD data and Mössbauer data are very similar to those observed for samples derived from conventional SnO precursors. XRD refinements (Fig. 1b) show a SnO phase (P4/nmm with a = 3.803(1) Å, c = 4.836(1) Å), again with minor SnO2 contamination. Mössbauer analysis (Fig. 4b) reveal almost identical spectra (with ≈ 90/10% factions for Sn 2+ and Sn 4+ , respectively). Critically, it is noted that within the optimized processing limits for the tin(II) oxyhydroxide precursor, the reducing environment promotes conversion of Sn 4+ to divalent Sn; while limited tetravalent tin is formed during processing of the conventional SnO precursor. This is revealed by the Mössbauer analysis, where the fraction of Sn 4+ is diminished following processing (Figs. 4a, 4b), while Sn 4+ , absent from the initial SnO conventional precursor, is observed in the ceramics (Figs 4c,4d). With the tin(II) oxyhydroxide precursor, no evidence of improved densities was found for greater compression pressures up to 500 MPa. Dwell temperatures down to even 200 ºC resulted in ceramic densities of at least 85%. Cohesive samples were difficult to obtain outside the window of these optimized processing parameters. Ensuring that the largest outgassing event preceded the dwell plateau temperature during the processing sequence (Fig. 5b) was critical in achieving the best SnO phase purity of the resulting ceramic.

Taken together, these results clearly demonstrate that densification of Sn(II)-based oxides by SPS can offer certain advantages in overall processing time and phase stability when compared to other approaches. Moreover, through a Cool-SPS type processing strategy and the use of tin(II) oxyhydroxide precursors, it is possible to achieve ceramics with equivalent phase purity and geometric density at reduced temperature and applied pressure processing conditions. With further optimization of tin(II) oxyhydroxide precursors, it is expected that SnO ceramics with pure divalent tin content can be achieved in view of the active reducing tendencies of the optimized processing conditions. Our results also reveal that the choice of precursor strongly influences the resulting microstructure of the dense SnO ceramics. The comparison of SEM micrographs from ball-milled SnO precursor powders (Fig. 2c) with the resulting dense pellet (Fig. 2d) shows the expected increase in densification but reveals that the initial inhomogeneous distribution of particle sizes is partially maintained after processing. However; SEM examination of the ceramic fracture surface obtained from the tin(II) oxyhydroxide precursor (Fig. 2b) displays a very different dense, homogeneous and fine grained microstructure. High resolution FEG-SEM imaging shows a compact ceramic with apparent grain sizes on the order of ≈ 100 nm. The use of anisotropic peak shape profiles in the fitting of XRD data prevents an independent analysis of crystallite sizes in the ceramic derived from the reactive precursor. However, the clear qualitative differences in grain size and densification morphology after SPS processing of the two precursors are shown below to strongly impact physical properties of the resulting SnO ceramics.

Transport Characterizations

Bulk transport properties of dense SnO ceramics were characterized by variable temperature DC conductivity and room temperature AC impedance spectroscopy measurements. These results are shown in Fig. 6. Significant differences in the conductivity values and temperature behavior of DC conductivity are found between densified ceramics obtained from conventional SnO precursors and those derived from reactive tin(II) oxyhydroxide precursors (Fig. 5a). For the sample originating from conventional SnO powders, a modest increase in conductivity with temperature over the range 25 -220 ºC is observed, as expected for a p-type semiconductor [START_REF] Ogo | Tin monoxide as an s-orbitalbased p-type oxide semiconductor: Electronic structures and TFT application[END_REF]. The measured room temperature DC conductivity of 0.06 S/cm is well within the range of experimentally reported values for equivalently dense SnO pellets of 0.1 S/m measured after SPS processing [START_REF] Kuwahara | Synthesis of High-Density Bulk Tin Monoxide and Its Thermoelectric Properties[END_REF] and 0.01 S/cm recently reported for SnO pellets processed by CSP [START_REF] Bang | Densification of thermodynamically unstable tin monoxide using cold sintering process[END_REF]; minor differences in sample processing or measurement details might explain this range of DC conductivity values. In contrast, tin(II) oxyhydroxide precursors produced ceramics with DC conductivities near 1x10 -3 S/cm; roughly two orders of magnitude lower than that measured on ceramics derived from the conventional SnO precursor. Moreover, a non-linear thermal dependence of conductivity is observed. It is proposed that these contrasting properties are linked with the dissimilarities in final pellet microstructures between the two processing routes. For the reactive oxyhydroxide route samples, overall sample conductivity is likely reduced by more substantial grain boundary effects accompanying the fine grain sizes (see SEM micrographs, Fig. 2b), or by disordered interface layers between grains. The thermal dependence may be correlated with greater interfacial contributions that vary with temperature under non-reducing atmospheres. A preliminary evaluation of electrochemical conduction mechanisms was performed with ambient temperature AC electrochemical impedance spectroscopy (EIS). Plotted as Nyquist type plots, the EIS results are consistent with the DC conductivity data, in that the conventional SnO precursors pellets (Fig. 5b) exhibit much lower bulk sample impedance values than what is measured on ceramics derived from reactive oxyhydroxide precursors (Fig. 5c). In the low frequency DC limit, the extrapolated x-axis intercept Z' values are 75 Ω and 2400 Ω, respectively. Within the limits of the used frequency ranges, an analysis attempt to extract capacitance values for different features of the Nyquist plots was performed with the equation ωmaxRC = 1 [START_REF] Irvine | Electroceramics: Characterization by Impedance Spectroscopy[END_REF]. For conventional SnO precursor derived ceramics (Fig. 5b), this gives a single feature with C = 1x10 -8 F; while for ceramics derived from tin (II) oxyhydroxide precursors (Fig. 5c) two features with C = 2x10 -8 F and with C = 5x10 -7 F are estimated. The smaller near-equivalent capacitive features may represent the similar bulk phenomenon, with the second larger capacitive feature in the latter providing further evidence of additional grain boundary effects or phase segregation in sintered SnO ceramics derived from the oxyhydroxide precursors. Further investigation, in particular variable temperature EIS beyond the scope of this study, is needed to fully elucidate the electrochemical and microstructural characteristics of SPS sintered SnO ceramics.

Conclusions

In the pursuit of achieving dense SnO ceramics, an efficient approach has been successfully demonstrated, affording rapid densification up to approximately 90% of theoretical density. This approach offers flexibility in the choice of starting precursor chemistry, working well with both conventional ball-milled SnO and reactive tin(II) oxyhydroxide precursor powders, although care must be taken to track and optimize processing parameters in each case. Extensive characterization of samples before and after processing by XRD, FTIR, TGA, and 119 Sn-Mössbauer spectroscopy provides details concerning phase content and Sn oxidization state. Interestingly, under appropriate temperature and pressure conditions, the Cool-SPS route actively promotes stability of the desired Sn 2+ species. It is also found that the nature of the starting precursor strongly impacts the final sintered SnO microstructure, which subsequently affects the bulk electrochemical properties. With a strategic selection of precursors, it is possible to achieve significant reductions in overall sample conductivity, which can present opportunities for exploiting the possible ferroelectric response of lone-pair Sn 2+ cations.

Although only the simple case of SnO is explored in the current work, the rapid processing times, chemical flexibility and resulting tuning of structure/property relationships offered by a low temperature Cool-SPS approach likely offers interesting avenues of future exploration for more complex Sn(II) based oxide systems.

Fig. 1 .

 1 Fig. 1. Rietveld refinement fit profiles of powder XRD data measured on (a) starting tin(II) oxyhydroxide precursor powders and (b) the resulting SPS ceramic, followed by (c) the initial ball-milled SnO precursor and (d) corresponding ceramic after SPS processing. Each main plot displays Yobs (black points), Ycalc (red line), a fit difference curve (blue line), and vertical phase tickmarks. Asterisks mark the position of minor SnO2 phase peaks. The inset of Fig. 1(a) displays the FTIR spectra for tin(II) oxyhydroxide precursor.

Fig. 2 .

 2 Fig. 2. SEM micrographs at top of initial (a) tin(II) oxyhydroxide precursor powders and (b) a fracture surface of the resulting ceramic. Inset of image 2(b) shows the same sample surface at higher magnification. In parallel, at bottom, micrographs of (c) conventional ball-milled SnO powders and (d) a fracture surface of the ceramic after processing.

Fig. 3 .

 3 Fig. 3. Comparison of TGA measurements performed on heating under air (5 °C /min) for different precursor powders. Experimental measurement uncertain are smaller than the plotted symbols.

Fig. 4 .

 4 Fig. 4. Room temperature 119 Sn Mössbauer spectra for the (a) tin(II) oxyhydroxide precursor and (b) resulting sample after SPS processing; similarly (c) for the conventional ball-milled SnO powder and (d) corresponding sample after SPS processing. Green and blue shaded areas of the spectra indicate the fitted fractions for Sn 2+ and Sn 4+ environments, respectively.

Fig. 5 .

 5 Fig. 5. Schematic plots showing the typical evolution of two SPS monitoring parameters discussed in the main text over the entire processing sequence for (a) conventional SnO and (b) reactive tin(II) oxyhydroxide precursor powders. Sample compaction is directly related to Piston Displacement (red lines, left axis), while decomposition and sample outgassing events are inferred from Chamber pressure values (blue lines, right axis). Experimental measurement uncertain values are smaller than the plotted line widths.

Fig. 6 .

 6 Fig. 6. Variable temperature DC conductivity measurements of SnO pellets (a) obtained after processing from conventional and reactive precursor powders (blue and red points, respectively). Ambient temperature Nyquist type plots for frequency dependent EIS analysis of samples obtained from (b) conventional SnO and (c) reactive tin(II) oxyhydroxide precursor powders.

Table 1

 1 Summary of measured geometric ceramic densities (% of theoretical density, with measurement uncertainties in brackets) and corresponding precursor and SPS processing conditions for select SnO samples discuss in the main text.

			Pressure	Dwell	Ceramic
	Precursor Powder	Temp. (ºC)	(MPa)	Time	Density
	SnO (commercial)	265	350	5 min	72(2) %
	SnO (Ball Milled)	265	350	5 min	78(2) %
	SnO (Ball Milled)	300	400	5 min	91(3) %
	Tin(II) Oxyhydroxide	270	300	5 min	90(3) %

Acknowledgements

We thank the Université de Bordeaux and MESRI for PhD fellowship funding (SBB) and a 6 months sabbatical to develop Cool-SPS, following the "délégation" previously awarded by CNRS (MJ). We greatly appreciate the technical assistance and discussions concerning sample characterization provided by colleagues at both the ICMCB (Laetitia ETIENNE, Fabrice MAUVY, Sonia BUFFIÈRE, Rodolphe DECOURT, and Stéphane RELEXANS) and PLACAMAT (UMS 3626, Philippe LEGROS).

Impact of reactive precursors on the sintering of tin monoxide

Subhransu Subhasis Bhoi a , Mathieu Duttine a , U-Chan Chung a , Michaël Josse a , and Matthew R. Suchomel a *

Declaration of Competing Interest

The authors report no declaration of interest.


Anthony Maldonado

Arnaud Lemière

Frédéric Désévédavy

Jean-Charles C Jules

Wagner Correr

Yannick Ledemi

Younès Messaddeq

Clément Strutynski

Sylvain Danto

Thierry Cardinal

F Smektala

Elaboration of multimaterials optical fibers combining tellurite glass and metal for electro-optical applications

Keywords: Hybrid multimaterials optical fibers, electro-optical functionalities, tellurite glasses

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

INTRODUCTION

Multimaterial systems are of great interest in research and industry due to the range of possible perspectives and applications in fields such as medicine, environment or aeronautics. Indeed, the combination of different materials with various properties allows the development of more efficient and compact multifunctional components. Among these systems, the multimaterial fibers are very interesting for the possibility to obtain complex geometry at microscale but also long lengths and a better compacity. These fibers are obtained by the drawing of a macroscopic preform while maintaining the initial geometry. With the significant progress made in fiber drawing processes due to the development of optical fibers for telecommunications, the photonics sector is particularly interested in multimaterial glass/metal fibers in order to combine the remarkable properties of glasses (transparency and light guidance) with those of metal (electrical conduction) to combine an electrical pulse transport function with an optical signal. The idea is to generate electro-optic effects to polarize the medium and create an index variation that would allow the possibility to change the amplitude, phase or path of an optical wave. In the case of a mono-material fiber, the drawing parameters are determined by the viscosity of the material at the drawing temperature. This manufacturing process is ideally suited to amorphous materials such as polymers or glasses which have a wide range of viscosity and softening temperature. Crystalline materials such as metals or semiconductors cannot be drawn separately because of the sudden drop in viscosity above their melting temperature characteristic of a phase transition. In a multimaterial approach, by confining and controlling the crystalline materials flow in an amorphous support compatible with the drawing, it is possible to design fibers with metal if this one has a melting temperature lower than the drawing temperature [START_REF] Schmidt | Hybrid Optical Fibers -An Innovative Platform for In-Fiber Photonics Devices[END_REF][START_REF] Tao | Multimaterial fibers: a new concept in infrared fiber optics[END_REF][START_REF] Tao | Multimaterial Fibers[END_REF].

Several glass/metal fibers have already been reported in the literature. These vary according to their geometry, manufacturing techniques and selected materials. Many techniques have been used to insert metal electrodes into a fiber. The most commonly reported method is the pre-drawing of the vitreous support by holding the drilled holes under pressure and then, post-drawing, the liquid metal injection [START_REF] Fokine | Integrated fiber Mach-Zehnder interferometer for electro-optic switching[END_REF][START_REF] Myrén | Wide wedge-shaped depletion region in thermally poled fiber with alloy electrode[END_REF]. However, co-drawing of the two materials with the insertion of the metal before drawing allows longer lengths of fibers with electrodes to be obtained [START_REF] Lee | 200-m optical fiber with an integrated electrode and its poling[END_REF][START_REF] Strutynski | Tellurite-based core-clad dual electrodes composite fibers[END_REF]. In addition, the main glasses used are silicates whose optical transparency is limited to the visible and near infrared, with weak nonlinear optical properties and strong diffusion of the metal within it altering the optical properties.

Therefore, in this work, we report the development of TeO2-based glass/metal fibers with much stronger nonlinear optical properties, optical transparency further in the mid infrared and less metal scattering in order to combine an electrical pulse transport function with a broadband optical signal in the mid infrared.

We describe the manufacturing process optimization of these TeO2-based glass/metal fibers with the enhancement of glass/metal interface quality and mechanical behavior of these fibers by changing the TeO2-based composition (TeO2-ZnO-Na2O [TZN] in TeO2-ZnO-La2O3 [TZL]) with a higher glass transition temperature and better mechanical properties. Previous fibers [START_REF] Fokine | Integrated fiber Mach-Zehnder interferometer for electro-optic switching[END_REF] with TZN exhibited bubbles at the glass/metal interface and the appearance of defects along the electrodes as a result of irregular metal flow during drawing. In addition, the fibers were brittle making them difficult to use. Our new TZL fibers overcome these drawbacks.

EXPERIMENTAL SECTION 2.1 Bulk glasses preparation

TZL glasses were prepared at free atmosphere by the conventional melt-quenching technique from mixed precursors. Batches of 10g weight using commercial raw materials TeO2 (Fox Chemicals, 99%), ZnO (Alfa Aesar, 99,99%), La2O3 (Alfa Aesar, 99,99%) and Na2CO3 (Alfa Aesar, 99,5%) were loaded in a platinum crucible, then melted at 850°C for 1h in an electric furnace, poured into a brass mold at Tg-30°C and finally annealed at Tg-10°C for 4h to remove any residual internal stress induced by the rapid quench of the glass melt (Figure 1). For the two TZN compositions (80TeO2 -10ZnO-10Na2O and 70TeO2 -25ZnO-05Na2O in %mol), there is an additional decarbonation step at 650°C for 30min to remove any residual CO2 gas from Na2CO3 precursor. After synthesis, the obtained glass plates of 15 x 10 x 5 mm 3 dimensions were finally polished for the investigations.

Core-cladding preform preparation and hybrid glass-metal optical fiber drawing

To produce the step-index tellurite glass fibers, the method consists in the combination of two successive steps: Built-in-Casting (BiC) and Rod-in-Tube (RiT) techniques. Firstly, a large-core step-index preform from two different glasses (core and clad) is prepared by BiC as described in Figure 2 [START_REF] Strutynski | Fabrication and characterization of step-index tellurite fibers with varying numerical aperture for near-and mid-infrared nonlinear optics[END_REF]. Then, the drawing of this preform allows to obtain 800µm thin rods. Finally, a clad preform is mechanically drilled and by RiT the thin clad/core rod is inserted in order to get a small core fiber. During this insertion step, we drilled two more holes on each side of the core to insert two Au80Sn20 alloy wires. We have chosen this alloy for its excellent electrical conduction and its Tm below the drawing temperature allowing the co-drawing. The preform drawing is allowed by using a dedicated 3-meters high optical fiber drawing tower. The preform was slowly fed into the furnace and the temperature was gradually increased up to ~400°C. The glass was brought to its softening temperature regime while the drawing parameters were continuously monitored to produce the targeted fiber diameter. For the optimization of tellurite glass/metal fibers, we decided to change the vitreous matrix by looking for a new TeO2based composition with a higher Tg and higher mechanical properties to facilitate its handling. For this, we turned to TZL compositions which combine these two factors according to Rhonehouse's works [START_REF] Rhonehouse | Low-loss UV to mid IR optical tellurium oxide glass and fiber for linear, non-linear and active devices[END_REF][START_REF] Rhonehouse | Low loss, wide transparency, robust tellurite glass fibers for mid-IR (2-5µm) applications[END_REF]. In order to find suitable core and clad compositions for the conception of our multimaterial fibers, we explored the TZL ternary diagram [START_REF] Maldonado | La2O3-tellurite glass system investigation for optical fibers manufacturing[END_REF] to identify which compositions were vitrified and what the boundaries were (Figure 4). After that, thermal analysis was performed by differential thermal analysis (DSC 2920 TA Instruments) on 20mg samples. Glass transition temperatures were measured as the inflection point of the endotherm at a heating rate of 10°C/min and gathered into a ternary diagram (Figure 4) to show the diversity of Tg that can be obtained with this TZL system creating an impressive range of possibilities and allowing their use in different applications. To select the appropriate compositions, other measurements were performed on each bulk composition (thermal expansion coefficient by dilatometry, transmission by UV-visible and FTIR spectrophotometer, loss by cut-back technique, refractive index at 1550nm using a homemade prism (TiO2) coupler refractometer, density, Raman analysis and Vickers hardness with a microdurometer). These investigations are detailed and described more extensively in [START_REF] Maldonado | La2O3-tellurite glass system investigation for optical fibers manufacturing[END_REF].

Another study was carried out in parallel consisting of incorporating Na2O in small quantities into the selected compositions in order to add Na + ions which are migratory species [START_REF] Maldonado | La2O3-tellurite glass system investigation for optical fibers manufacturing[END_REF]. This aspect can be very interesting for the future works with the possibility of making thermal poling and second harmonic generation on these multimaterial glass/metal fibers. -Cladding composition: TZLN 68.6-24.5-4.9-2 with small amount of Na2O for the above reasons These three compositions were chosen in order to test different fiber geometries (classic step-index and step-index with a W-profile). Their thermal and optical properties are gathered in Table 1. Thermally, these compositions have the advantage of having close fiber drawing temperatures, close thermal expansion coefficients avoiding any fracture and ensuring good cohesion of the assembly and finally a thermal stability (ΔT) higher than 100°C limiting possible surface crystallization. From an optical point of view, refractive index is adapted to ensure the confinement of the light in the core of the fiber, the bulk transmission reaches 6.4µm (Figure 5) and minimum losses of 0.8dB/m are obtained at 1.4µm. In addition to thermal and optical measurements, impedance spectroscopy investigations were carried out in order to evaluate the ionic conductivity of the selected compositions and to determine their activation energy (Ea). These two parameters will help us to better understand possible phenomena during future thermal poling and second harmonic generation works. These measurements will be unveiled in a forthcoming publication.

Table 1 : Thermal and optical properties of TZL and TZLN selected compositions

Hybrid glass/metal fiber characterization

The switch to TZL compositions allows to carry out an annealing before drawing at higher temperature. This annealing improves the adhesion between glass and metal and thus the quality of the interface between the two materials. This change in the manufacturing process results in a more regular metal flow during the drawing process. This reduces defects at the glass/metal interface and irregularities in electrodes diameter. The fibers were observed with a Scanning Electron Microscope (SEM) to check the quality of the interface between the two materials (Figure 6 to the left). These pictures show a suitable quality without defects unlike the TZN-based fibers which presented bubbles (Figure 6 to the right). An additional Energy Dispersive Spectroscopy (EDS) analysis was also performed along the section of the fiber (red line) to detect stoichiometric deviation or inter-diffusion between the glass and electrodes.

Then, to complete the information collected at the SEM on the quality of the glass/metal interface, Atomic Force Microscopy (AFM) was employed to observe the topography of the surface and the phase with a Veeco and phase contrast microscope (Figure 7). The AFM confirms the improvement in the quality of the glass/metal interface. Nevertheless, slight metallic deposits from the electrode can bee seen on the glass. Subsequently, it will be necessary to evaluate the diffusion of the metal in the glass to quantify how it alters the optical properties of the fiber.

In order to verify the continuity of the electrode, electrical measurements were performed by applying silver lacquer or an InGa mixture to the ends of several fiber lengths. Thus, the continuity of the Au80Sn20 electrodes along the fiber could be checked with a multimeter by detecting a resistance. Initially, we were able to demonstrate that we obtain several tens of meters of fibers with embedded electrodes. Then, by measuring the resistance of several fiber lengths with various electrode diameters, an average resistivity of 2.53 x 10 -7 Ω.m was determined. This value is very closed to that given by the supplier of Au80Sn20 raw wire (1.68 x 10 -7 Ω.m). C. Strutynski [START_REF] Strutynski | Tellurite-based core-clad dual electrodes composite fibers[END_REF] had noticed a discrepancy in these resistance values and proposed an explanation with a possible contamination of the metallic electrode or a generation of grain boundaries within the metal during drawing. By optimizing the glass/metal interface, we have overcome this discrepancy.

Finally, these fibers are mechanically more robust and easier to handle. Mechanical tests (tensile strengths and critical bending radius) on these fibers are underway to show the mechanical reinforcement with the transition to TZL compositions. These tests will also be unveiled in an upcoming publication [START_REF] Maldonado | La2O3-tellurite glass system investigation for optical fibers manufacturing[END_REF]. Computer-assisted finite element simulations with the LISA software were also performed by applying a voltage of 100V to the two electrodes (Figure 8). The objective was to understand how the electric potential and the electric field behaved over the fiber cross-section and to evaluate the order of magnitude of the electric field at the optical core. Indeed, the electrodes must be close enough to maximize the electric field between them but far enough from the core to avoid inducing additional optical losses. It is therefore important to find a compromise between the electrode-toelectrode distance, the electrode-to-core distance and the glass breakdown voltage. The connection of the electrodes to a closed circuit is a crucial and essential parameter for the use of these hybrid fibers. The first method is to connect the electrodes to the fiber ends through a micro-station equipped with tungsten tips connected to a DC voltage generator. The tips are affixed to the fiber electrodes using a microscope. The drawback of this technique is the possible contact loss of the first electrode when connecting the second one. This technique is used to study the electro-optical Kerr effect and calculate the Kerr constant (setup detailed in Figure 9). Measurements are actually under way.

CONCLUSION AND FUTURE WORKS

We have therefore reported here on the optimization of the manufacturing of tellurite-based dual-electrodes core-clad composite fibers. The engineering of these multimaterial glass/metal fibers is based on the drilling of a cladding preform and the insertion of a core rod and two Au80Sn20 metallic wires. This alloy has been chosen to be thermally compatible with the selected glassy matrix in order to have a co-drawing with the glass which supports the flow of metal during the drawing process. By proceeding in this way, we were able to obtain long lengths of optical fibers with continuous embedded metallic electrodes. The transition from TZN to TZL made it possible to carry out annealing before drawing, thus reducing the appearance of defects and irregularities at the glass/metal interface along the fiber as well as possible contaminations that would disturb the optical and electrical properties of the composite fiber. Furthermore, this transition allows also to improve its mechanical properties and its handling for the future works. The optical properties of the fibers can be further improved by the purification of the glasses used by preparing them in controlled atmosphere (glovebox) with higher purity precursors in order to limit parasitic absorptions from OH species and transition metals.

In the future works, the technique to connect the electrodes to a closed circuit has to be enhanced and developed in order to study different nonlinear optical effects like Kerr static effect and measure the Kerr coefficient of these hybrid fibers by controlling the refractive index variation into the fiber thanks to an external electrical excitation.

Potential applications of these hybrid tellurite glass/metal fibers could be electro-endoscopes for medical treatment or frequency doubling and electro-optical modulators for telecommunications.

Figure 1 :

 1 Figure 1 : Picture of TZL bulk

Figure 2 :

 2 Figure 2 : Core-cladding preform preparation by Built-in-Casting technique

Figure 3 :

 3 Figure 3 : Hybrid glass-metal preform made by Rod-in-Tube before drawing and view of fiber cross-section

Figure 4 :

 4 Figure 4 : TZL ternary diagrams: composition influence on the structure and the glass transition temperature

(Figure 5 :

 5 Figure 5 : TZL transmission curve and loss for a step-index fiber with TZL 70-25-05 as core and TZL 65-30-05 as cladding. Pictureshowing the output beam intensity distribution using a microscope objective and a CCD camera

Figure 6 :Figure 7 :

 67 Figure 6 : SEM pictures of hybrid glass/metal optical fibers (TZL to the left and TZN to the right) and EDS measurement on the cross section (red line)

Figure 8 :

 8 Figure 8 : Application of a 100V-voltage to the two electrodes by computer simulation with the LISA software to see the electric potential and electric field magnitude behavior on the fiber cross-section 3.4 Electrode connectivity to a closed circuit to study Static Kerr effect

Figure 9 :

 9 Figure 9 : Setup for measuring the static Kerr effect by connecting the electrodes with a tungsten-tipped micro-station at the fiber end. (Mirror: M1, M2 and M3; Polarizer: P1 and P2; Lens: L1 and L2)

ACKNOWLEDGEMENTS

This research is the result of a collaboration between the Laboratoire Interdisciplinaire Carnot de Bourgogne in Dijon in France supported by the French National Research Agency (ANR-17-CE08-0042-04, PROTEus project) and the Centre d'Optique, Photonique et Laser in Quebec in Canada supported by the Canadian Excellence Research Chair Program (CERC) in Photonics Innovation and the Natural Sciences and Engineering Research Council of Canada (NSERC). We are also grateful to acknowledge the Council of the French Region Bourgogne Franche-Comté and the European program FEDER. Finally, this work has been supported by the EIPHI Graduate School (contract ANR-17-EURE-0002).