M Godfroy

A Russel

F Mercier

M Granier

T Jarrosson

C Niebel

F Serein Spirau

Romain Viennois

Mickael Beaudhuin
email: mickael.beaudhuin@umontpellier.fr

F Serein Spirau

Nanocrystalline chromium disilicide synthesized by a fast Chlorine-Transfer-Reaction

Keywords: Nanocrystalline materials, Microstructure, Metallurgy, Thermodynamics and kinetics of processes in materials

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Chromium disilicide is studied since several years for its thermoelectric properties [START_REF] Zhou | Determinatin of transport properties in chromium disilicide nanowires via combined thermoelectric and structural characterizations[END_REF] and for electrical transportation [START_REF] Szczech | Single-crystal semiconducting chromium disilicide nanowires synthesized via chemical vapor transport[END_REF][START_REF] Seo | Synthesis and electrical properties of single crystalline CrSi2 nanowires[END_REF]. It is a p-type semiconductor with a bandgap of about 0.35 eV, its power factor, (with the Seebeck coefficient and the electrical conductivity), is similar to the best industrial thermoelectric (TE) materials [START_REF] Rowe | Thermoelectric Handbook -Macro to Nano[END_REF].

However its figure of merit is 0.25 [START_REF] Rowe | Thermoelectric Handbook -Macro to Nano[END_REF], while it is about 1 for the best industrial TE materials. This low value is mainly related to its relatively high thermal conductivity which is about 10 W/mK at 300 K. Different routes have been investigated in the past years to decrease : doping with heavy elements, insertion of point defects or even by decreasing the dimensionality. This last point could be promising, however additional efforts are still needed in the synthesis of nanomaterials. In this study we will focus on the synthesis of CrSi2 in the shape of nanocrystals. Nanostructuration of chromium disilicide has been investigated in the past few years by top-down approach such as 2 mechanical alloying/milling [START_REF] Karuppaiah | Investigation on the thermoelectric properties of nanostructured Cr1-xTixSi2[END_REF][START_REF] Hermet | Revisited phonon assignment and electro-mechanical properties of chromium disilicide[END_REF][START_REF] Khalil | A modeling approach for new CrSi 2 based alloys: Application to metastable Cr1-xZrxSi2 as a potential thermoelectric material[END_REF] and by bottom-up approach by Physical Vapor Transportation (PVT) which enables the growth of CrSi2 nanostructures in the shape of nanowires or 2D nanostructured objects [START_REF] Zhou | Determinatin of transport properties in chromium disilicide nanowires via combined thermoelectric and structural characterizations[END_REF][START_REF] Szczech | Single-crystal semiconducting chromium disilicide nanowires synthesized via chemical vapor transport[END_REF][START_REF] Seo | Synthesis and electrical properties of single crystalline CrSi2 nanowires[END_REF][START_REF] Yu | An in-situ chloridegenerated route to various one-dimensional nanostructures of chromium-based compounds[END_REF][START_REF] Zhang | Morphology-controlled growth of chromium silicide nanostructures and their field emission properties[END_REF][START_REF] Schmitt | Synthesis and applications of metal silicide nanowires[END_REF][START_REF] Hou | Room-temperature ferromagnetism in CrSi2 (core)/SiO2 (shell) semiconducting nanocables[END_REF].

In this study we will focus on the synthesis of nanostructured CrSi2 by Chlorine-Transfer-Reaction (CTR) by adapting the protocol described by Trentler et al. [START_REF] Trentler | Preparation of nanocrystalline molybdenum disilicide (MoSi2) by a chlorine-transfer reaction[END_REF] on MoSi2 with the support of thermodynamic calculations. We also report the effect of an inert diluent into the reaction mixture to control the size of the aggregates and thereby enable the formation of nanocrystalline powders. Laplace algorithm). SEM analysis is conducted using a Zeiss Ultra55.

Materials and methods

CrCl3

Thermodynamic analysis of the gaseous phase chemistry is made using Factsage thermodynamic software. The thermodynamic database SGTE is completed with the thermodynamic data of the gaseous compounds in the Si-Cl-H system [START_REF] Allendorf | Theoretical study of thermochemistry of molecules in the silicon-carbon-chlorine-hydrogn system[END_REF] and the thermodynamic data of the stoichiometric compound CrSi [START_REF] Du | Experimental reinvestigation of the CrSi-Si partial system and update of the thermodynamic description of the entire Cr-Si system[END_REF].

Results and discussion

From the thermodynamic calculations, the heating of CrCl3(s) in an atmosphere of argon mainly leads to the formation of CrCl3(g) between 500°C and 1000°C. In lower amount, CrCl4(g), CrCl2(g), Cl(g) and Cl2(g) are also produced (Figure 1a). Then, CrCl3(g) reacts with

Si(s) to form solid chromium silicides and gaseous species. Thermodynamic calculations show that the nature of solid silicides is strongly influenced by the initial CrCl3(s):Si(s) ratio (not given here). For the experimental ratio of 1:2.75, calculations indicate the formation of solid CrSi2 at more than 99 mol% (the other solid is Cr5Si3) between 500°C and 1000°C. The gaseous phase is mainly composed of SiCl4(g) together with a small amount of Si2Cl6(g), SiCl3(g) and CrCl3(g), the latter probably coming from the resublimation of CrSi2(s) at high temperatures (Figure 1b). The Chlorine-Transfer Reaction can therefore be summarized by the equation:

( ) + 2.75 ( ) → ( ) + 0.75 "( )
Experimentally, dark gray powders have been obtained in the reaction zone together with the formation of SiCl4, condensed in the collection zone of the tube. From Figure 2, Rietveld analysis shows the presence of three solids: unreacted silicon and two silicides, CrSi2 and Cr5Si3. The quantity of CrSi2 increases from 52 to >94 wt. % for respectively 600 °C and 800°C, while on the same temperature range, the quantity of impurities, Si and Cr5Si3, decrease respectively from 20 to <1 wt. % and from 30 to <5 wt. %. These amounts are below the sensibility limit of XRD and agree with the thermodynamic results which indicate the formation of 99 wt. % pure CrSi2 from 500°C to 1000°C. The presence of Cr5Si3 and Si in the experiments at 600°C and 700°C can be explained by the local inhomogeneity of composition due to the remaining CrCl3(s) in the mixture at these temperatures. Below 750°C, thermodynamic calculations show that 66% of CrCl3 stay in its solid form while above 750°C, all the CrCl3(s) is converted into chromium chloride gaseous species (Figure 1a). Therefore, a compromise is found experimentally at 800°C

to ensure an almost complete reaction between CrCl3 and Si without favoring a grain coarsening mechanism at higher temperatures. From Rietveld analysis, the peaks are thin and shows that CrSi2 is well crystallized at 800°C. The CrSi2 powder obtained consists of aggregates with a mean size of about 2 µm, as measured by DLS and SEM, a value similar to the initial silicon powder (Table 1). Rietveld's refinement followed by a microstructural analysis (Figure 3) shows that the aggregated are composed of crystallites with sizes ranging from 50 nm to 200 nm.

We have also investigated the role of LiCl and NaCl on the synthesis of chromium silicides by the Chlorine-Transfer Reaction. The Rietveld analysis show that CrSi2 is obtained with a purity >94 wt. % with Si and Cr5Si3 as the main impurities. In Figure 3, it is observed that the use of NaCl and LiCl lead to refined microstructures. Additionally, we observe a strong influence of the diluent on the average Particle Size Distribution (# $ %%%%%% ) either with LiCl or NaCl diluent. The # $ %%%%%% falls to respectively 970 and 670 nm in volume for 10 mol.% NaCl and 10 mol.% LiCl, it is slightly lower in number with respectively 420 and 440 nm. We also have observed that after 1h decantation, the # $ %%%%%% in volume decreases to about 410 nm which would permit to prepare nanometric powder in two steps. The possible reason could be the formation of a liquid phase assisting the synthesis of CrSi2 that could modify the growth mechanisms from vaporsolid mechanisms (without diluent) to vapor-liquid-solid mechanisms when a diluent is added (NaCl or LiCl). Indeed, LiCl is already liquid from ~605°C and the binary NaCl-CrCl3 presents a liquid region for the conditions investigated [START_REF] Cook | The systems NaCl-CrCl3 and KCl-CrCl3[END_REF]. The refined microstructure obtained with LiCl diluent compared to NaCl diluent confirm this assumption.

Conclusion

We have demonstrated that CrSi2 nanopowders with crystallite size about 50 to 200 nm and mean volume particle size distribution down to about 670 nm in volume and about 420 nm in number can be obtained through Chlorine-Transfer-Reaction followed by sonication. It has been demonstrated that the use of diluent is needed and permit to decrease the PSD of CrSi2 by a factor ~4. NaCl, e-f) 5% LiCl, g-h) magnification of typical aggregates.

Captions

  100mm length each): a reaction zone which is located into the furnace and a by-product collection zone which is located outside of the furnace. The reaction is conducted in a tubular furnace possessing a 30 mm temperature homogeneous zone. The quartz vessel is connected to an argon gas flow (2L/min) equipped with a bubbler to prevent a pressure build up and to catch SiCl4 gaseous species which could leave the by-product collection zone. The mixture is heated up to 600°C, 700°C or 800°C with a ramp rate of 500°C/h and maintained to this temperature for 5 minutes. The reaction tube is quenched under air by removing the fused silica tube from the furnace in order to prevent grain coarsening and sintering. The mixture is cleaned 5 times with deionized water and centrifugated. The powders are then dispersed in absolute ethanol with an ultrasonic bath (90 minutes, 25 kHz) and extracted after centrifugation. The samples are characterized by powder X-Ray Diffraction (Philips X'PERT). Rietveld refinement with Fullprof software is used for structural analysis of each sample[START_REF] Rodriguez-Carvajal | FULLPROF: a program for Rietveld refinement and pattern matching analysis[END_REF]. Dynamic Light Scattering (DLS) measurements are performed with a Vasco Particle Size Analyzer DL135 (Cordouan Technologies, = 657 , autocorrelation function modelled by Pade

Figure 1 :

 1 Figure 1: Calculated partial pressures of Clx and CrClx gaseous species as a function of the temperature for the initial mixtures: a) 1 CrCl3(s) + 10 Ar(g). b) 1 CrCl3(s) + 2.75 Si(s)+10 Ar(g). Total pressure: 1 atm.

Figure 2 :

 2 Figure 2: Observed (black), calculated (red) and difference (blue) XRD profiles for the Rietveld refinement of CrSi2 after Chlorine-Transfer-Reaction (800°C, Ar, 5 min, no diluent). The Bragg peaks of the most intense peak are also given. The amount (w%) of the phases at different temperature is given in the inset.

Figure 3 :

 3 Figure 3: SEM Secondary-Electron images of CrSi2 aggregates: a-b) no diluent, c-d) 5%

Table 1 :

 1 # $ %%%%%% in volume (number) of CrSi2 aggregates after sonication in function of LiCl and NaCl dopant ratio.

Table 1 Diluent (molar ratio)

 1

		No
			0.5 LiCl	5 LiCl	10 LiCl	0.5 NaCl	5 NaCl	10 NaCl
		diluent
	(nm)	2100	1340	1040	670	1700	1010	970
	volume	(1750)	(1250)	(870)	(440)	(880)	(940)	(420)
	(number)

Acknowledgment

This work was supported by the CheMISyst labex ANR-10-LABX-05-01 and University of Montpellier. The authors would also like to thank Jean-Olivier Durand (ICGM, France) for his help with DLS.