M L Bouchetou

J Poirier

L Arbelaez Morales

T Chotard

O R Joubert

M Weissenbacher

Synthesis of an innovative zirconia-mullite raw material sintered from andalusite and zircon precursors and an evaluation of its corrosion and thermal shock performance

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

Introduction

Zirconia mullite composites are commonly used as a raw material in refractory products due to their excellent properties. Zirconia has a high refractoriness (Tmelting = 2715 °C) and has an exceptional corrosion resistance against slags [1]. Many authors have studied zirconia mullite composites [START_REF] Boch | Preparation and properties of reaction-sintered mullite-ZrO2 ceramics[END_REF][START_REF] Cambier | Formation of microstructural defects in mullite-zirconia and mullite-alumina-zirconia composites obtained by reactionsintering of mixed powders[END_REF][START_REF] Di Rupo | Solid state reactions in the ZrO2-SiO2-αAl2O3 system[END_REF] and have observed high toughness, high strength and an interesting thermal shock resistance [START_REF] Rendtorff | Mullite-zirconia-zircon composites: properties and thermal shock resistance[END_REF][START_REF] Rendtorff | Thermal shock behavior of dense mullitezirconia composites obtained by two processing routes[END_REF][START_REF] Hamidouche | Thermomechanical behaviour of mullite-zirconia composite[END_REF][START_REF] Rendtorff | Effect of the addition of mullite-zirconia to the thermal shock behavior of zircon materials[END_REF]. Because of the properties imparted by zirconia grains, they are thought responsible for the enhanced mechanical properties. These composites are used in the production of iron, steel, non-ferrous metals, cement and glass, as well as for hazardous waste incineration.

However, in many cases, the desired operating life is not achieved when using zirconia mullite composites owing to their low thermal shock and corrosion resistances. In particular, the performance of conventional zirconia mullite refractories are insufficient in terms of the service life needs when used as a replacement for alumina chromium oxide refractories that 2 give rise to severe environmental problems [START_REF] Chan | Effect of Cr2O3 on slag resistance of Al2O3-SiO2 refractories[END_REF]. Furthermore, there is still a demand in the refractory industry for improved and environmentally safe mullite/zirconia raw material for refractory products.

The classical way to generate mullite/zirconia composites is by the reaction of alumina with zircon. This reaction is carried out at very high temperatures due to the thermodynamic stability of zircon. Commercial electro-fused zirconia mullite is produced through a fusion process by melting a mixture of ZrSiO4 and Al2O3 in an electric arc furnace. Commercial sintered zirconia mullite raw material is produced through a sintering process at very high temperatures.

Another possible source of mullite is andalusite. Andalusite is a well-known alumina-silicate mineral employed in the refractory industry to provide high thermal shock resistance and strong mechanical properties. Majidian et al. [START_REF] Majidian | Phase evolution, microstructure and mechanical properties of alumina-mullite-zirconia composite prepared by Iranian andalusite[END_REF] determined that the addition of andalusite in an alumina-mullite-zirconia composite improves properties such as thermal shock resistance.

The addition of well-dispersed zirconia grains also improves the corrosion resistance against slags and molten glasses [START_REF] Biswas | Comparative study of zirconia-mullite and alumina-zirconia composites[END_REF].

The purpose of this paper is to present a cost-cutting and innovative raw material, formed at a lower temperature, with an improved corrosion and thermal shock resistance. This sintered zirconia mullite refractory composite is developed from andalusite (SiO2.Al2O3), alumina Al2O3 and ZrSiO4 raw materials. This paper is composed of three main parts. In the first part, the high-temperature transformations of andalusite and zircon as precursors of mullite and zirconia are studied. In the second part, the synthesis of a sintered zirconia mullite raw material and the reactions that occur during the sintering are described. In the third part, the properties of the sintered zirconia mullite composite are evaluated. The effect of corrosion by soda-lime glass and Al2O3-CaO slags on the microstructural transformation of the zirconia mullite composite is investigated. Finally, the characterization of the thermomechanical behaviour of the composite is conducted by studying the thermal shock behavior evaluated using non-destructive ultrasonic methods (pulse echography and acoustic emission) to follow the microstructural changes at high temperature.

Materials and methods

Raw materials and synthesis

The raw materials used as the precursors for the synthesis of the sintered zirconia mullite composite are fine powders of micronized KF andalusite (Imerys), zircon (Ceradel) and CT3000SG alumina (Alcoa). Their compositions and grain sizes are shown in Table 1.

These powders were mixed in water and the samples shaped by slip casting in a plaster mould. After drying, the samples were heated at 1600 °C in air, for ten hours, at the heating rate of 900 °C/h. Micronised

Characterization methods

Crystalline phases in the fired samples were determined by XRD analysis on ground powders (-325 mesh) analysed in a Bragg-Brentano diffractometer (D8 Advance, Bruker) using CuKα radiation. Quantitative analyses were performed using the Rietveld method. The microstructures of polished and carbon coated samples were examined with an optical microscope and a scanning electron microscope. Bulk density and apparent porosity were measured using the standard water immersion method (ISO 5017).

The corrosion resistance against molten alumina-lime slag and against soda-lime glass was evaluated. The glass and slag compositions are given in Table 2. The corrosion test was conducted in alumina crucibles filled with grains of the zirconia mullite samples and covered with the pressed fine powder of the slag or glass corrosive agent (Fig. 1). After corrosion, the depth of diffusion of the calcium in the grains was measured for every sample with a scanning electron microscope.

Fig. 1. Corrosion test and determination of Ca diffusion

The zirconia-mullite material, sintered at 1600 °C from andalusite, alumina and zircon powders, was first thermally shocked by heating the samples to 1200 °C for two hours and subsequently cooling to room temperature. The heating rate and the cooling rate, measured with thermocouples, were 300 °C/h and 220 °C/min. The dimensions of samples were: 10 mm thickness, 15 mm width and 40 mm length.

The effect of the thermal shock on residual mechanical performances (evolution of elastic properties) was evaluated using a classical ultrasonic pulse echography technique in transmission mode.

No standard test can provide reliable results because of the great variety of microstructures to be tested and due to the necessary optimisation of the testing parameters (transmission or reflection mode, working frequency).

The ultrasonic wave propagation velocity was measured using a Sofranel EPOCH system and M106 (2 MHz) sensors. The reflection method could not be applied here due to the significant acoustic absorbance of the composite. The evolution of the ultrasonic pression wave velocity, which depends on several parameters (mineralogical phases, porosity, crack networks, and chemical composition), was studied to estimate the resistance of the material after experiencing several repeated thermal shocks (a maximum of 5). It is important to note that the variation in the elastic properties (the modulus of elasticity E, the Coulomb's modulus G and the Poison's ratio ν) are not given in this publication, but the evolution of the porosity and relative density of the material will be presented and discussed in a future work. In the present paper, the analysis of the wave velocity variation is semi-quantitative.

The thermomechanical behaviour of the sintered zirconia/mullite refractory composite was also studied using the acoustic emission (AE) technique at high temperature. The samples were prepared with standard machining tools and had dimensions of 25 mm × 4 mm × 4 mm.

The AE technique is devoted to the in-situ investigation and characterization of microstructure evolution when submitted to stress (mechanical, thermal, etc.). In the case of solid materials, microstructural changes such as dislocation slip, plastic deformation of grains, grain boundary sliding, and crack formation and propagation are sources of elastic waves [START_REF] Bianchi | Wavelet packet transform for detection of single events in acoustic emission signals[END_REF][START_REF] Maia | A new approach for detection of wear mechanisms and determination of tool life in turning using acoustic emission tribol[END_REF][START_REF] Vinogradov | On the nature of acoustic emission and internal friction during cyclic deformation of metals[END_REF]. Reference [START_REF] Patapy | Characterization by acoustic emission pattern recognition of microstructure evolution in a fused-cast refractory during high temperature cycling[END_REF] described the experimental procedure applied in this research and for different types of material. The rapid releases of energy phenomena are detected and recorded in real-time and through established chronology of events (occurrence of signals coming with time from the tested material), phenomena which become very helpful to characterize and identify the involved mechanisms (Fig. 2). A PAC Micro-80 SN AB21

wideband sensor (200 to 900 kHz-Mistras Group, Inc., USA) was used to perform this test.

The threshold was set at 40 dB to avoid any background noise. The acoustic signals were

acquired by an AE Express-8 system and the data were processed using the NOESIS software.

Synthesis of the sintered zirconia mullite raw material

Commercial fused and sintered zirconia mullite refractory raw materials, industrially produced from zircon (ZrSiO4) and alumina (Al2O3), are generated at very high temperatures with a high production cost. For example, the chemical compositions and the microstructures of two commercial zirconia mullite raw materials (Z 72 Symulox® -Nabeltec and FZM -Imerys) are given Table 3, and their microstructures are shown in the Fig. 3 and Fig. Andalusite is an interesting precursor of mullite. Because of its microstructure, the use of mullitized andalusite in refractory materials contributes to improvement in the thermal shock resistance. From an economic point of view, an interesting source of zirconia is zircon. Zircon transforms into zirconia and silica, and andalusite leads to mullite and silica. The excess silica in andalusite after forming mullite can interact with the added alumina to form additional mullite. The raw materials, whose chemical compositions are indicated in Table 1, are mixed with the following molar ratio: Andalusite (0.43 mole) + Zircon (0.43 mole) + Alumina (0.94 mole)

Mullite (0,44 mole) + Zirconia (0.43 mole).

A sintered zirconia mullite raw material was developed from zircon (ZrSiO4), alumina (Al2O3) and andalusite (SiO2.Al2O3). The composition of the grains is 78.1wt.% mullite, 21.6

wt.% zirconia and 0.3 wt.% impurities. The details of the manufacturing are presented in section 3.4.

The main challenge was lowering the sintering temperature. The temperature of decomposition of mullite and zircon had to be compatible with the sintering furnaces of the refractory industry (approximately 1600 °C). The andalusite and zircon (as precursors of mullite and zirconia) transformations at a temperature lower than 1600 °C are described below.

Andalusite used as a precursor of mullite

Andalusite, a natural mineral from low-grade metamorphic rock, is available as granular particles up to 8 mm in diameter, which are composed of single crystals or fragments of single crystals. Andalusite consists of Al2SiO5 crystals and some mineral impurities such as quartz, biotite, pyrite, and albite.

By heating above 1280 °C, andalusite yields mullite and silica [START_REF] Schneider | Kinetics and Mechanism of the Solid State High Temperature Transformation of Andalusite (Al2SiO5) into 3/2 Mullite (3Al2O3•2SiO2) and Silica (SiO2)[END_REF]. Mullitization of andalusite yields a composite made of mullite and silica-rich glass produced according to the reaction:

3 (Al2O3 SiO2) 3Al2O3 2SiO2 + SiO2
Approximately 83 wt.% of the heated product becomes mullite above 1280 °C, and the other 17 wt.% is a silica-rich glass. A fraction of the silica-rich liquid is expelled from the initial andalusite grain [START_REF] Jp | Mullitization of andalusite in refractory bricks[END_REF][START_REF] Jp | Mullitization of andalusite in bricks and castables[END_REF]. The main part of the silica-rich liquid was trapped in the initial grain and formed a capillary network of interconnected tubes with a diameter in the micrometre range and was elongated along the c axis common to the neoformed mullite and the parent andalusite crystal (Fig. 5.). Mullitization proceeds as a pseudo morphosis, retaining the shape of the initial andalusite grain. Mullite is topotactically oriented relative to the andalusite host crystal [START_REF] Schneider | Kinetics and Mechanism of the Solid State High Temperature Transformation of Andalusite (Al2SiO5) into 3/2 Mullite (3Al2O3•2SiO2) and Silica (SiO2)[END_REF][START_REF] Pannhorts | The high-temperature transformation of andalusite (Al2SiO5) into 3/2 mullite (3Al2O32SiO2) and vitreous silica (SiO2)[END_REF][START_REF] Hülsmann | The transformation of andalusite to mullite and silica : Part I. Transformation mechanism in [001]A direction[END_REF][START_REF] Hülsmann | The transformation of andalusite to mullite and silica: Part II[END_REF].

Despite the presence of this glassy phase, the refractory products developed with andalusite show excellent creep resistance and thermal shock resistance. This is due to the specific microstructure of the mullitized andalusite, as shown in Fig. 5.

J. Poirier et al. have studied the thermal shock behaviour of andalusite and mullitized crystals using ultrasonic methods [START_REF] Poirier | Andalusite : an attractive raw material for its excellent thermal shock resistance[END_REF]. The polished grains, size up to 8 mm, were submitted to successive thermal shocks. They were heated (at a heating rate of 300 °C/h) at 1200 °C for two hours, then quenched at room temperature. Scanning electron microscopy (SEM)

micrographs revealed that at high temperature, by capillary action, the silica-rich vitreous phase filled the network of cracks created by the previous thermal shock. This melting phase, arising owing to alkalis and iron contained in the minor mineral inclusions of andalusite (quartz, phyllosilicates, and ilmenite), prevents the existing cracks and stops the new cracks that will be generated by the next thermal shock [START_REF] Poirier | Andalusite : an attractive raw material for its excellent thermal shock resistance[END_REF].

During thermal treatment and thermal shock, the behaviour of the mullitized andalusite crystals depends on the microstructure of the mullite-glass composite. The mullitized andalusite crystals exhibited a behaviour typical of composite materials; the microcracks were deflected and stopped in glass zones. Each additional heating/cooling cycle (heating rate: 300 °C/h, cooling rate: 220 °C/min) led to the repairing of the existing cracks (Fig. 6).

As shown in the paper [START_REF] Poirier | Andalusite : an attractive raw material for its excellent thermal shock resistance[END_REF], this microstructure evolution led to excellent thermal shock resistance compared to that of monolithic monocrystalline mullite. Andalusite-based refractories are well-suited for thermal cycling use. These results offer interesting possibilities for the development of innovative refractories using andalusite raw materials with an optimized microstructure. (b) After a second heating (at 1200 °C) and a second thermal shock, the existing crack is partially repaired by the heating [START_REF] Bouchetou | Mullite grown from fired andalusite grains: the role of impurities and of the high temperature on the kinetics of mullitization and consequences on thermal shocks resistance[END_REF].

Backscattered electron SEM micrographs of a polished section (parallel to the (001) plane of the initial andalusite)

To increase the amount of mullite and to decrease the amount of the glassy phase in the composite, which contains approximately 83 wt.% of mullite and 17 wt.% of silica-rich glass, alumina powder was mixed with andalusite grains.

During sintering, the free silica-rich liquid expelled out of the grains reacted with alumina to create a secondary mullite according to the reaction:

3Al2O3 + 2SiO2 3Al2O3 2SiO2
The secondary mullite precipitates from a transient liquid phase containing silica and alumina.

This secondary mullite may act as a ceramic binder between the primary mullite composite grains. After two hours at 1600 °C, the obtained material was composed of the mullite phase (94 wt.%) and residual glass (6 wt.%) [START_REF] Bouchetou | Production of high mullite material from natural andalusite: influence of grain size[END_REF]. These reactions led to the production of a sintered material with a high mullite content. The batch formula, mixing, shaping and sintering procedures are detailed in 3.4 paragraph.

Zircon used as a precursor of zirconia

Zircon (ZrSiO4) is a naturally occurring zirconium silicate mineral, a very chemically and thermally stable compound. At very high temperatures, zircon decomposes to ZrO2 and SiO2:

ZrSiO4 → ZrO2 + SiO2
Many authors have studied the transformation of zircon into zirconia. According to the ZrO2-SiO2 phase equilibrium diagram proposed by Kaiser et al. (Fig. 7) [START_REF] Kaiser | Thermal stability of zircon (ZrSiO4)[END_REF], the decomposition of pure zircon begins at 1673 °C. However, the particle size, and the nature and amount of impurities influence the temperature and the rate of zircon decomposition. Kaiser et al. [START_REF] Kaiser | Thermal stability of zircon (ZrSiO4)[END_REF] have studied the thermal stability of ZrSiO4 based on annealing experiments with two different zircon powders of very close grains size and known impurities: a natural zircon powder (98% ZrSiO4 purity and particles smaller than 3 µm) and a synthetic powder (100% ZrSiO4 purity and particles smaller than 4 µm). The decomposition starts between 1450 °C (for 98%

ZrSiO4 purity) and 1600 °C (for 100% ZrSiO4 purity). The decomposition is rather low at temperatures below 1650 °C (degree of ZrSiO4 decomposition <5%) and is high at temperatures above 1700 °C. Zircon decomposes by a solid-state reaction releasing silica.

Effect of the addition of andalusite on the temperature and the kinetics of zircon decomposition

Since impurities strongly influence the decomposition temperature of zircon into zirconia, it is of interest to add andalusite to the zircon powder. Indeed, at high temperature, the andalusite transforms into mullite and silica-rich glass containing impurities.

In the publication [START_REF] Bouchetou | Mullite grown from fired andalusite grains: the role of impurities and of the high temperature on the kinetics of mullitization and consequences on thermal shocks resistance[END_REF], M.L. 1, were mixed (wt.% ratio 1/1). The samples (diameter 1cm -thickness 5 mm) were pressed, dried (at 150 °C for 0.5 h) and heated for an hour at 1600 °C (heating rate of 900 °C/h and free cooling). The SEM micrographs show the very interesting influence of the andalusite grains. The transformation of zircon into zirconia only initiated close to the andalusite grains (Fig. 9 and Fig. 10). Note that the thickness of the zirconia layer in Fig. 10 is smaller than the layer in Fig. 9. This difference is explained by the local variations of silica rich glass composition that may be attributed to the initial distribution of mineral impurities in andalusite grains.

To study the influence of temperature on the zircon decomposition, samples made of 28.47 wt.% fine KF andalusite (>10 µm), 32.1 wt.% fine zircon and 39.47 wt.% CT3000 alumina were mixed. These samples (1cm diameter -5 mm thickness) were dried, pressed under a pressure application of 5 tons (62,4 kN/cm 2 ), then heated at 1600 °C at temperatures ranging from 1300-1600 °C for 1 hour (heating rate of 900 °C/h and free cooling).

After heating, the phases were analysed by X-ray diffraction (XRD) (Table 4). Samples heated for an hour at 1600 °C were found to have andalusite transformed into mullite, and a fraction of zircon was decomposed into zirconia (Fig. 11). Furthermore, while some authors [START_REF] Anseau | Somme studies on the thermal stability of zircon[END_REF][START_REF] Klute | Phasenbeziehungen im System Al2O3-Cr2O3-ZrO2-SiO2 unter besonderer Berücksichtigung des korundhaltigen Bereiches[END_REF] have shown that at 1600 °C the degree of zircon decomposition is very low. Anseau et al. [START_REF] Anseau | Somme studies on the thermal stability of zircon[END_REF] studied the thermal decomposition of natural Australian zircon sand. Its chemical composition (wt.%) was: 99.13% ZrO2, 0.32 % Al2O3, 0.25 % TiO2, 0.15 wt.% Fe2O3). The samples were heated at 300 °C/min to temperatures between 1400 and 1690 °C for 8 hours. The degree of zircon decomposition was measured by the ZrO2/ZrSiO4 ratio as recorded by XRD as well as by chemical analysis of the amount of free SiO2. The decomposition starts between 1525 and 1550 °C at a relatively low velocity, but by approximately 1650 °C, the reaction accelerates significantly.

Klute [START_REF] Klute | Phasenbeziehungen im System Al2O3-Cr2O3-ZrO2-SiO2 unter besonderer Berücksichtigung des korundhaltigen Bereiches[END_REF] synthetized high-purity zircon powder from precipitation and thermally decomposition at 1050 °C of ethyl silicate and zirconyl sulfate, dispersed in an aqueous solution. These powders were heated for 1-2 days in air at various temperatures, quenched and characterised by XRD. The binary eutectic between ZrO2 and SiO2 was determined at 1699±3 °C. The addition of andalusite leads to a complete decomposition of zircon after 10 hours at 1600 °C.

The decomposition of zircon leads to zirconia and a liquid phase. Furthermore, in this liquid phase, K2O and Na2O close to the zirconia grains are detected by EDS analysis.

Majidian et al. [START_REF] Majidian | Phase evolution, microstructure and mechanical properties of alumina-mullite-zirconia composite prepared by Iranian andalusite[END_REF] had studied the phase evolution and the microstructures of aluminamullite-zirconia composites mixed using Iranian andalusite and have shown similar results.

The suspensions were prepared by mixing alumina and zircon (85:15 wt. % ratio) with 0,2,5 and 10 wt. % of andalusite powder, then dried and granulated. The green samples were sintered at 1500, 1600 and 1650 °C.

H. Pooladvand et al [START_REF] Pooladvand | Effects of alumina and zirconia addition on transformation of andalusite to mullite[END_REF] had investigated the effects of alumina and zirconia addition on transformation of andalusite to mullite. Alumina -zirconia -mullite composites were prepared by adding zirconia to andalusite with a mass ration of 0.24:0.78 and a mass ratio of 0.28 alumina/0.72 andalusite was chosen to form secondary mullite during andalusite transformation.

It has been found that andalusite powder can reduce the decomposition temperature of zircon to 1600 °C [START_REF] Majidian | Phase evolution, microstructure and mechanical properties of alumina-mullite-zirconia composite prepared by Iranian andalusite[END_REF][START_REF] Pooladvand | Effects of alumina and zirconia addition on transformation of andalusite to mullite[END_REF] and accelerate the sintering of alumina-zircon, which leads to an increase of the mullite amount and improves the thermal shock resistance of the alumina-mullitezirconia composite.

The presence of 28.47 wt.% andalusite enables the complete decomposition of 32.1 wt.% zircon, with andalusite impurities forming a liquid phase that promotes diffusion. The microstructure contains mullite, zirconia grains and an amorphous phase homogeneously distributed in the mullitized andalusite.

The introduction of low-melting additives such as alkali in the mix is a complementary approach to enhancing the zircon decomposition. The influence of the Na2O and K2O alkalis was studied by introducing a small quantity of Na2CO3 and K2CO3 in the mixture of andalusite KF (28.47 wt.%), zircon (32.1 wt.%) and alumina CT3000 (39.47 wt.%). Thirty samples were made with contents of K2O and Na2O ranging from 0.5-2.0 wt. % under the same pressing and heating conditions as the previous samples. After heating at 1600 °C, the amount of zirconia and zircon was determined by XRD analysis (Fig. 12). without the additional alkalis, all the zircon was transformed into zirconia.

Shaping and sintering of the zirconia-mullite raw material

Test samples (10x15x40 mm size) were prepared from andalusite, alumina and zircon powders, shaped by slip casting in a plaster mould, and after drying, were sintered at 1600 °C in air. The batch formula, mixing, casting and sintering procedure are detailed in Fig. 13.

Fig. 13-Manufacturing process of zirconia-mullite raw material

During sintering, several reactions occurred at high temperature (Fig. 14). The andalusite transformed into a mullite and a silica-rich glass. This reaction started at approximately 1300 °C. Zircon decomposed to yield zirconia and silica. The silica rich glass formed reacted with the added alumina to crystallize and form secondary mullite.

Raw materials

At high temperature

Material after cooling

consisting essentially of homogeneously distributed fine zirconia particles in a mullite rich glassy matrix. The average particle diameter of the zirconia particles was less than 5 μm (Fig. 15). The crystallization of mullite from the silica and the added alumina allowed the densification of the material. The open porosity after 10 hours at 1600 °C was very low (<3%). The use of zircon associated with andalusite and alumina formed a sintered zirconia mullite composite at 1600 °C.

Properties of the sintered zirconia-mullite raw material

In use, refractory materials are subject not only to corrosion by glass or slag but also to thermal shock stresses. The corrosion and thermal shock resistances of this synthesized zirconia mullite raw material were evaluated and compared to other commercial raw materials.

Corrosion resistance

Zirconia in mullite and alumina grains improves the corrosion resistance because zirconia has a lower solubility than mullite and alumina in soda-lime glass and slag. Mullite and alumina are dissolved in the glassy phase, and thus, random needle-like corundum or mullite crystals are formed and begin to grow in the corroded area, during cooling [START_REF] Aksel | The microstructural features of an alumina-mullite-zirconia refractory material corroded by molten slag[END_REF][START_REF] Asokan | Microstructural features of fusion cast Al2O3-ZrO2-SiO2 refractories[END_REF][START_REF] Prigent | Andalusite: an amazing refractory raw material with excellent corrosion resistance to sodium vapours[END_REF]. Askel [START_REF] Aksel | The microstructural features of an alumina-mullite-zirconia refractory material corroded by molten slag[END_REF] has observed that zirconia grains in alumina mullite zirconia refractories served as an effective corrosion barrier in the corroded zone. Zirconia creates a stable interface and increases the corrosion resistance.

The corrosion resistance of the sintered zirconia mullite grains against molten corrosive agents was tested. Two different corrosive agents were used: a soda-lime glass (71.6 wt.% SiO2-14.2% Na2O-9.5%CaO-4.7% MgO) provided by Ceradel and an alumina-lime slag (50 wt.% Al2O3-50 wt.% CaO) made in our laboratory by melting an alumina (50 wt.%) -lime (50 wt.%) mixture at 1600 °C, for 5 hours. Alumina crucibles were filled with the zirconiamullite grains (7 gr) on which was placed a pressed powder of slag or glass (0.5 gr). The crucibles were heated in an electric furnace for 10 minutes at 1550 °C using a heating rate of 300 °C/h. After air quenching, the samples were cut, polished, coated with carbon and observed with a scanning electron microscope. To estimate the corrosion, the depth of diffusion of the calcium in the grains was measured for every sample.

The results were compared with those obtained for a commercially available fused zirconia mullite material (FZM-Imerys) produced through a fusion process by melting a mixture of zircon and alumina, and a commercially available sintered zirconia mullite material (Z 72 Symulox®-Nabaltec), formed by a stoichiometric sintered mullite and microcrystalline baddeleyite zirconia. The mineralogical compositions of the initial grains (no corroded), determined by XRD analysis using the Rietveld method, are presented in Fig. 16.

Fig. 16. Initial mineralogical compositions of the grains (synthetized sintered zirconia-mullite, FZM commercial fused zirconia mullite, Z72 commercial sintered zirconia mullite)

Impregnation depth measurements of calcium (see SEM micrographs, Fig. 17 and Fig. 18) and the histogram (Fig. 19) showed two different behaviours. The fused zirconia mullite grains were completely impregnated with calcium, while the sintered zirconia mullite and the commercial sintered zirconia mullite were only partially impregnated. As shown by Askel [START_REF] Aksel | The microstructural features of an alumina-mullite-zirconia refractory material corroded by molten slag[END_REF], needle-like corundum crystals form in the glassy phase of the corroded area.

The chemical composition of the glassy phase (composed of Al2O3, SiO2, CaO, Na2O, MgO, ZrO2 oxides) close to the impregnated grain was determined by EDS analyses and the distribution of Ca element was determined by mapping. The viscosity of the glassy phase at high temperature was calculated using Urbain's model [32]. The results show that, at high temperatures, the viscosity of the glassy phase of the sintered zirconia mullite was always higher than the viscosity of the glassy phases of commercial FZM fused zirconia mullite and Z 72 sintered zirconia mullite (Fig. 20. ).

The sintered zirconia mullite and the commercial zirconia mullite showed similar impregnation behaviours at the beginning of the corrosion by glass or slag. Nevertheless, the viscosity of the liquid phase at high temperature (between 1200 and 1600 °C) remained higher in the case of the sintered zirconia mullite composite synthesized using zircon and andalusite.

The fused zirconia mullite product presented a lower corrosion resistance: the depth of penetration of the calcium was higher and the liquid phase (composed of Al2O3, SiO2, CaO, Na2O, MgO, ZrO2 oxides) was substantially less viscous at high temperature (between 1200 and 1600 °C). . Viscosity, calculated with Urbain's model [START_REF] Urbain | Viscosity of Silica Melts[END_REF], of the liquid phase close to the impregnated grains

Thermal shock resistance and thermomechanical behaviour

The thermal shock resistance of the zirconia mullite composite was evaluated by heating sintered samples (size 10x15x40 mm) in air to 1200 °C for two hours and cooling to room temperature. The heating rate and the cooling rate were 300 °C/h and 220 °C/min, respectively. Ultrasonic pulse echography in transmission mode was used to determine the evolution of the propagation wave velocity within the sample and to evaluate the effect on the thermal shock resistance. Fig. 21 shows the experimental procedure of the successive thermal shocks. The results were compared to the results obtained for dense alumina (purity 98,5 wt.%) and cordierite under the same thermal shock conditions, as shown in Fig. 22.

Corrosion with slag Corrosion with glass

Fig. 22. Evolution of the relative ultrasonic wave velocity in sintered zirconia/mullite composites versus the number of thermal shocks (1200 °C-20 °C.) compared with that of other refractory materials

The observed evolution of the relative velocity for the composite (slight increase and then stabilization) is quite unusual for such a material and indicates that the thermal shock resistance of the material is improved. However, if the tendency is clearly established, the magnitude of the variation has to be tempered by the value of the relative porosity of the sample, which could vary after several thermal shocks and is a conjugation of a damage mechanism (cracking) and repairing process. A liquid phase formed at 1200 °C has repaired the existing cracks generated by the previous thermal shock. The evolution of the same parameter plotted for other materials such as cordierite and alumina underlines a significantly different thermal shock behaviour with a fall observed starting from the first thermal shock.

Rendtorff et al. [START_REF] Rendtorff | Mullite-zirconia-zircon composites: properties and thermal shock resistance[END_REF] studied the thermal shock behaviour of various mullite-zirconia/zircon composites. These composites were prepared by slip casting using mullite-zirconia electrofused grains and 15-25-35-45 wt.% of zircon and were sintered at temperatures between 1300 and 1600 °C for 2 hours. The authors report that the thermal shock produces a decrease in the elastic properties mostly due to the onset of a microcracking network induced by the thermal expansion mismatch between phases. After two thermal cycles, Young's modulus remains constant, showing that the composite exhibits a high resistance to crack propagation.

Other authors [START_REF] Boch | Preparation and properties of reaction-sintered mullite-ZrO2 ceramics[END_REF][START_REF] Cambier | Formation of microstructural defects in mullite-zirconia and mullite-alumina-zirconia composites obtained by reactionsintering of mixed powders[END_REF][START_REF] Hamidouche | Thermomechanical behaviour of mullite-zirconia composite[END_REF] have shown that the incorporation of fine zirconia grains in a mullite matrix results in a significant toughening. Different mechanisms such as stress-induced transformation, micro-cracking and crack deflection are involved in this toughening. The thermomechanical behaviour of this type of material is sensitive to the phase transformation of zirconia and depends on different parameters such as matrix stiffness, zirconia grain size and chemical composition [START_REF] Hamidouche | Thermomechanical behaviour of mullite-zirconia composite[END_REF][START_REF] Claussen | Mechanical properties of sintered, in situ-reacted mullite-zirconia composites[END_REF].

Claussen and Jahne [START_REF] Claussen | Mechanical properties of sintered, in situ-reacted mullite-zirconia composites[END_REF] have studies the mechanical properties of sintered, in situ-reacted mullite-zirconia composites. Mixtures of zircon and Al2O3, in ratios according to the reaction 2ZrSiO4 + 3Al2O3 3Al2O3.2SiO2 + 2ZrO2, were attrition-milled in water for 6 hours. The samples were isostatically pressed at 600 MPa and sintered in air at less than the reaction temperature (~1450 °C). Claussen and Jahne have shown that during the sintering of zirconia mullite composites, two types of zirconia grains, which are not stabilized, can be observed.

They found that the major fraction consists of monoclinic zirconia grains, and the other fraction contains very small tetragonal zirconia grains. Microcracks are formed during cooling through the zirconia tetragonal-monoclinic transformation that occurs at 500-750 °C [START_REF] Hamidouche | Thermomechanical behaviour of mullite-zirconia composite[END_REF][START_REF] Claussen | Mechanical properties of sintered, in situ-reacted mullite-zirconia composites[END_REF] as noted by cracks often observed around the monoclinic zirconia grains. During heating, the monoclinic-tetragonal transformation occurs at 970-1100 °C. The induced volume expansion creates stress in the surrounding mullite matrix.

In our case, the obtained results seem to indicate a different behaviour with a global improvement in the thermal shock resistance of the composite often noted with the first temperature cycle.

It is likely that the presence of the zirconia grains improved the shock resistance owing to the deviation of cracks by zirconia grains. The results obtained from acoustic emission tests at high temperature allow us to better understand the mechanisms that occur in the microstructure when the temperature increases as well as the possible damage that may be generated. The results of the tests performed up to 1300 °C (5 °C/s for heating and cooling stages) on samples without thermal shock and after three thermal shock cycles are presented in Fig. 23. These graphs present the evolution of the cumulated signals (measurements, i.e. "hits" which are standard AE denomination [START_REF] Bianchi | Wavelet packet transform for detection of single events in acoustic emission signals[END_REF][START_REF] Maia | A new approach for detection of wear mechanisms and determination of tool life in turning using acoustic emission tribol[END_REF][START_REF] Vinogradov | On the nature of acoustic emission and internal friction during cyclic deformation of metals[END_REF][START_REF] Patapy | Characterization by acoustic emission pattern recognition of microstructure evolution in a fused-cast refractory during high temperature cycling[END_REF]) versus temperature during three successive thermal cycles. Two samples are compared, one without any applied thermal shock and one tested after three thermal shocks. The following three observations can be made:

1. The recorded AE activity occurs mainly during cooling for both samples (could be an indication of the onset of a common mechanism). The sudden rise in the cumulated hits is systematically noticed during the cooling stage at temperatures ranging from 800 °C to 600 °C. This observation will be discussed next.

2. It is apparent that the global amount of recorded hits is higher for the non-shocked (NS) sample (over 1230) than that for the 3-times shocked (3TS) one (approximately 1100)

3. Concerning this rapid augmentation of the number of cumulated hits, one can note that the evolution of the slope, cycle after cycle, is different for the non-shocked sample and the 3TS one. Indeed, the values of the slope seem to be rather equivalent for each cycle followed by non-AE activity for a range of temperatures (from 660 °C to room temperature) for the NS sample. Although this assumption could be considered for the first thermal cycle of the 3-times shocked sample, one can notice a very different evolution of the slope for cycles 2 and 3. For cycle 3, the AE activity appears to be continuous from 800 °C to room temperature. This evolution tends to demonstrate a different damage behaviour exhibited by the 3-times shocked sample compared to that of the non-shocked one.

A hypothesis is proposed to explain these different observations. As mentioned above, the transformation of tetragonal zirconia into monoclinic is generated during cooling at temperatures between 500 °C and 750 °C. In this range, a very high percentage of AE activity occurs. A previous study carried out on refractories with a very high content of zirconia [START_REF] Patapy | Characterization by acoustic emission pattern recognition of microstructure evolution in a fused-cast refractory during high temperature cycling[END_REF] has shown that this AE activity could be related in the early stage to the movement of the crystallographic variants present in the phases at the time of transformation. Patapy et al. [START_REF] Patapy | Characterization by acoustic emission pattern recognition of microstructure evolution in a fused-cast refractory during high temperature cycling[END_REF] also showed that the transformation of zirconia from monoclinic to tetragonal phase (or the reverse), which has a volume expansion (or shrinkage) for specific temperatures, tends to generate significant cracking in the glassy phase at the interface with the dendrites of ZrO2.

This explanation can satisfy the previous observations 1 and 2, assuming that for both samples, the early beginning of AE activity is a consequence of the motion of the variants inducing an internal state of stress in the material, leading to the microcracking of the glassy phase and self-producing a large and sudden release of energy.

Moreover, Ewais et al. [START_REF] Ewais | Tailoring of functionally graded zirconia-mullite/alumina ceramics[END_REF] have observed in sintered zirconia mullite alumina composites that the cracks generated from the thermal shock test do not propagate and are stopped by zirconia grains and some pores. Rendtorff et al. [START_REF] Rendtorff | Mullite-zirconia-zircon composites: properties and thermal shock resistance[END_REF] studied the thermal shock behaviour of a dense zirconia mullite composite, finding a decrease in the elastic modulus because of the creation of microcracks. After two thermal cycles, Young's modulus remains constant, indicating that the composite exhibited a high resistance to crack propagation. The dispersion of zirconia grains in the mullite matrix improves the thermomechanical properties because the zirconia transformation induces microcrack formations and residual stresses. The small cracks resulting from the transformation of zirconia interact with the thermal shocks cracks; therefore, the thermal shock resistance can be explained by three phenomena:

-the deviation of thermal shock cracks by the zirconia grains -the formation of microcracks around the zirconia grains -the residual stresses

In the innovative zirconia-mullite raw material, sintered from an andalusite/alumina/zircon mix, most zirconia grains are monoclinic. High-temperature X-rays analysis shows that the transformation of the monoclinic ZrO2 phase (baddeleyite) into the tetragonal phase with increasing temperature is a reversible transformation (Fig. 24). The mineralogy is similar before and after the thermal shock. Therefore, as shown in the SEM micrographs of zirconia grains (Fig. 25 a-initial state, b-after the first thermal shock), this transformation does not create modifications of the microstructure in and around the zirconia grains.

X X - 950°C X X - 1170°C X 1200°C (t=0) X 1200°C (t=2h) X 20°C X X -
phenomenon that would explain the slight increase in the ultrasonic velocity after the first thermal shock. This behaviour of the sintered zirconia mullite composite is caused by the presence of mullitized andalusite. Indeed, at high temperature, mullite can crystallize from the residual glassy phase, and this liquid glassy phase can repair the microcrack network. This hypothesis confirms observations 2 and 3 made for the 3-times shocked sample. Fig. 26 shows that the defect present in the initial state was partially repaired after the first thermal shock. Microcracks were deflected and stopped in the glassy zones, mullite had crystallized from the liquid phase and formed bridges which connect the edges of the cracks. This liquid phase is highly advantageous in terms of thermal shock resistance.

Conclusion

An innovative zirconia mullite raw material was developed by a sintering process of andalusite, zircon and alumina fine powders. This sintered zirconia mullite raw material is composed of mullite (2SiO2.3Al2O3, 74 wt.%) zirconia (ZrO2, 19 wt.%) and silica-rich glass (7 wt.%). The microstructure consists of fine zirconia particles in a mullite matrix and a residual glassy phase.

The material shows a very interesting behaviour when it is subjected to thermal shock.

Surprisingly, during the first thermal shock, the material appears to strengthen and is not damaged during the subsequent thermal cycling. This specific behaviour is obtained owing to the presence of the residual glassy phase and the microstructure of the mullitized andalusite.

Furthermore, this material has good resistance to corrosion by alumina-lime slag or soda-lime glass. It is impregnated less by slag and glass than fused zirconia mullite. Moreover, the liquid phase is more viscous at high temperatures than either the fused zirconia mullite or the commercial sintered product.

Combining mullite and zirconia into a composite enhances the thermal shock resistance and the corrosion resistance against an alumina-lime slag and a soda-lime-silica glass. The combination of several mechanisms leads to the improved properties. The role of andalusite is predominant:

-It is the main source of mullite.

-The alkali in the andalusite accelerate the zircon decomposition -A specific microstructure is obtained by decomposition to mullite and a glassy phase.

At high temperature, this silica-rich glassy phase is viscous.

During thermal shock, the glassy phase could deflect and stop microcracks, and during further heating treatment, the liquid glassy phase repairs the cracks.

Zirconia grains also play an important role. The transformation of zirconia at high temperature creates microcracks and residual stresses that can deflect thermal shock cracks.

Furthermore, since zirconia has a low solubility in glass and slag, zirconia grains could make a corrosion barrier in the impregnated zone.

The use of andalusite as a source of mullite is highly beneficial. Andalusite causes the decomposition of zircon at lower temperatures than 1600 °C and creates a specific microstructure of a mullite matrix with a residual glassy phase. During thermal shock, some microcracks are created, and during further heating treatment, the liquid glassy phase repairs the cracks. The transformation of zirconia creates microcracks and residual stresses that can deflect the thermal shock cracks.

All the hypothesis proposed to explain the occurrence of the microcracking mechanisms and the repairing process acting after several thermal shock cycles are validated by rigorous microstructural observations closely coupled to the acoustic emission technique, which allows a better understanding of the chronology of damage within the material.

This sintered zirconia mullite material appears to be promising for use as a raw material for refractory manufacture that will be subjected to thermal shocks and corrosion by slag or glass.

This product could be an interesting alternative to replace alumina-chromium refractories, which have high costs and an unfavourably environmental impact.

Fig. 3 .Fig. 4 .

 34 Fig.3. Microstructure of fused zirconia mullite grains, FZM -Imerys (backscattered electron SEM micrograph) (Z: zirconia, M: mullite, and V: glassy phase)

Fig. 5 .

 5 Fig. 5. Polished section of an andalusite grain transformed into a mullite-glass composite by heating to 1700 °C (backscattered electron SEM micrograph)

Fig. 6 .

 6 Fig. 6. Evolution of microcracks during heating/cooling cycle

Fig. 7 .Figure 8

 78 Fig. 7. ZrO2-SiO2 phase equilibrium diagram[START_REF] Kaiser | Thermal stability of zircon (ZrSiO4)[END_REF]

Fig. 8 .

 8 Fig. 8. Degree of zircon decomposition [25]: data of Anseau et al. ( ) (purity = 99,28% ZrSiO4) [26] and Klute ( ) (purity = 100% ZrSiO4) [27].

Fig. 9 .Fig. 10 .

 910 Fig. 9. Polished section of coarse andalusite grains surrounded by fine zircon and heated for 1 hour at 1600 °C (optical micrograph)

Fig. 12 .

 12 Fig. 12. Influence of alkali additives on the amount of zircon and zirconia after 1 hour (a), 10 hours (b) and 24 hours (c) at 1600 °C

Fig

  Fig. 14. Phases formed by sintering a mixture of andalusite, corundum, and zircon fired at 1600 °C in air for 10 hours

Fig. 15 .

 15 Fig.15. Microstructure of the sintered zirconia mullite composite (backscattered electron SEM micrograph)

Fig. 17 .Fig. 18 .Fig. 19 .

 171819 Fig.17. Microstructures of samples corroded by an alumina-lime-slag (backscattered electron SEM micrographs)

Fig. 21 .

 21 Fig. 21. Experimental procedure applied for thermal shock study on sintered samples

Fig. 23 .

 23 Fig. 23. Cumulated acoustic emission (AE) hits versus temperature for three consecutive thermal cycles: a) sample not thermally shocked (NS) and b) sample after three thermal shocks (3TS).

Fig. 24 .Fig. 25 .

 2425 Fig. 24. Crystalline phases at 20 °C and at 1200 °C (XRD analysis)

Fig. 26 .

 26 Fig. 26. Detail of the crack repairing process (During the heating, the cracks generated by the prior thermal shock are partially repaired by a crystallized mullite bridge and by the glassy phase as shown in this figure)

Table 1 .

 1 Chemical composition and particle size of raw materials

		KF andalusite	CT3000SG alumina	Zircon
		(Imerys)	(Alcoa Chemie GmbH)	(Ceradel)
	Chemical composition (weight %)
	Al2O3	61.1	99.8	0.5
	SiO2	37.8	0.03	34.1
	ZrO2 + HfO2			64.4
	CaO	0.04
	MgO	0.04	0.08
	TiO2	0.12
	Fe2O3	0.44		0.07
	Na2O	0.04	0.08
	K2O	0.2
	Particle size
	D50 (µm)	5 µm	0.8	1.3

Table 2 .

 2 Composition of the corrosive agents (glass and slag)

	Weight %	Al2O3	SiO2	Na2O	CaO	MgO
	Soda-lime glass	0	71.6	14.2	9.5	4.7
	Al203-CaO slag	50	0	0	50	0

Table 3 .

 3 Chemical composition of two commercial zirconia mullite raw materials

  Bouchetou et al. have studied the role of impurities on the mullitization of fired andalusite grains. The impurities are concentrated in the silica rich glass

	which contains potassium (K2O≈1-3 wt.%), iron oxides (Fe2O3≈1-3 wt.%) and other
	elements; with amounts below 1 wt.%. This low-melting glass shifts the initial dissociation of
	zircon to lower temperatures. 1-2 mm KF andalusite grains (Imerys) and fine (<10 µm) zircon
	powder (Ceradel), whose chemical compositions are given in Table

Table 4 .

 4 Crystallized phases formed after firing 28.47 wt.% KF andalusite, 32.1 wt.% zircon and 39.47 wt.% CT3000 alumina samples at temperatures ranging from 1300-1600 °C for 1 hour Fig.11. Amount of zircon and zirconia (XRD with the Rietveld method) after a heating at 1600 °C in air, with different hold times: 1,10 and 24 hours. The initial mixture was 28.47 wt.% KF andalusite, 32.1 wt.% zircon and 39.47 wt.% CT 3000 alumina.

	Temperature	Andalusite	Zircon	Mullite	Baddeleyite
	(°C)	SiO 2 .Al 2 O 3	ZrSiO 4	2SiO 2 .3Al 2 O 3	ZrO 2
	1300°C	++	+++	Traces	-
	1400°C	Traces	+++	++	-
	1500°C	-	+++	++	-
	1600°C	-	++	+++	+++
		+++ : major phase		++ : minor phase

Andalusite SiO 2 .Al 2 O 3 Primary Mullite + liquid phase 2SiO 2 .3Al 2 O 3 SiO 2 + impurities Corundum Al 2 O 3 Corundum Al 2 O 3 Primary Mullite 2SiO 2 .3Al 2 O 3 « Secondary »Mullite 2SiO 2 .3Al 2 O 3 Sintered Zirconia mullite composite Zircon ZrSiO 4 Liquid phase + Zirconia SiO 2 + impurities ZrO 2 Zirconia ZrO 2 Glass SiO 2 + impurities

	. 14. Phases formed by sintering a mixture of andalusite, corundum, and zircon fired at
	1600 °C in air for 10 hours

After 10 hours at 1600 °C, the sintered zirconia mullite raw material contained 74 wt.% mullite (2SiO2.3Al2O3), 19 wt.% zirconia (ZrO2) and 7 wt.% glassy phases (determined by XRD analysis with the Rietveld method). The sintered composite had a microstructure

ZrO 2 baddeleyite mullite mullite ZrO 2 tetra 20°C 1200°C ZrO 2 baddeleyite Mullite ZrO 2 Baddeleyite (monoclinic) ZrO 2 tetragonal 20°C

	25.6	26	27	28	29	30	31	32	33


I El Hajj

L Speyer

S Cahen

L Herbuvaux

P Lagrange

G Medjahdi

C Hérold

Intercalation of barium into graphite by molten salts method: synthesis of massive samples for crystal structure determination of BaC6

A solid-liquid method is used to synthesize the first stage bulk BaC6 graphite intercalation compound (GIC). This way of synthesis allows the preparation of bulk and massive samples useful for crystal structure determination. Through an eutectic medium LiCl-KCl, and by submerging a graphite platelet in barium-molten solution at 450°C for several days, the bulk BaC6 compound is obtained and X-ray diffraction investigations allowed a complete structural study. The collected data certify that BaC6 crystallizes in the hexagonal P63/mmc space group with the following parameters a = b = 430.6 pm and c = 1058.2 pm.

Introduction

Graphite intercalation compounds (GIC) are an interesting class of materials for various theoretical and experimental studies, especially the first stage ones where all the van der Waals gaps of pristine graphite are occupied. In the last decades, these compounds have triggered the scientific community, especially due to their superconducting properties at low temperature.

Numerous GIC with different intercalated species have been synthesized and reported in the literature. The donor type compounds involving alkali metals have firstly been described in 1926 [START_REF] Fredenhagen | Die bindung von kalium durch kohlenstoff[END_REF]. A well-known example is the case of lithium intercalated into graphite [START_REF] Guérard | Intercalation of lithium into graphite and other carbons[END_REF], now used in the lithium-ion battery technology. Since, numerous donor-type GIC associating graphite with different metals have been investigated. Hence, some research has been focused on alkaline-earth metals and the understanding of their reactivity towards graphite. 1 sebastien.cahen@univ-lorrain.fr Institut Jean Lamour -UMR 7198 CNRS-Université de Lorraine Campus ARTEM -2 Allée André Guinier B.P. 50840 F54011 Nancy Cedex Several attempts have been made in hopes of intercalating alkaline-earth metals into graphite in pure and bulk sample form. Guérard et al. [START_REF] Guérard | Synthèse directe de composés d'insertion du baryum dans le graphite[END_REF] have been the first ones to note the obtaining of BaC6 samples using solid-vapor method. Nonetheless, the reaction remains incomplete except with micrometric single crystal graphite samples. In the 2000s, several studies have been conducted in order to obtain superconducting BaC6 samples using the same solid-vapor route while modifying the experimental conditions. These results seem inconsistent, leading to the synthesis of a so-called homogeneous BaC6 even if the presence of residual pristine graphite remains. It should be highlighted that only little crystallographic data have been provided to sustain these declarations [START_REF] Kim | Superconductivity in heavy alkaline-earth intercalated graphite[END_REF][START_REF] Heguri | Synthesis and physical properties of alkaline earth metal graphite compounds[END_REF][START_REF] Heguri | Superconductivity in the graphite intercalation compound BaC6[END_REF].

Based on the work of Pruvost et al. concerning the use of Li-based molten alloy for intercalation reactions into graphite [START_REF] Pruvost | On the great difficulty of intercalating lithium with a second element into graphite[END_REF], Emery et al. studied the intercalation of some alkalineearth metals and have elaborated a bulk first stage calcium-graphite intercalation compound using the same approach [START_REF] Emery | Structural study and crystal chemistry of the first stage calcium graphite intercalation compound[END_REF]. Then, in the Li-Ba-graphite system, the same authors have succeeded in obtaining a bulk first stage BaC6 binary compound [START_REF] Emery | The synthesis of binary metal-graphite intercalation compounds using molten lithium alloys[END_REF]. However, complementary studies are mandatory about this compound.

In the recent years [START_REF] Hajj | Crystal structure of first stage strontium-graphite intercalation compound[END_REF][START_REF] Fauchard | An efficient medium to intercalate metals into graphite: LiCl-KCl molten salts[END_REF][START_REF] Bolmont | LiCl-KCl eutectic molten salt as an original and efficient medium to intercalate metals into graphite: Case of europium[END_REF], a novel method has been developed to obtain bulk and pure binary samples. On referring to the procedure proposed by Hagiwara et al. [START_REF] Hagiwara | Graphite intercalation compounds of lanthanide metals prepared in molten chlorides[END_REF], consisting in using LiCl-KCl molten salts as a liquid medium to intercalate lanthanide into graphite, a tremendous success in producing fully intercalated first stage compounds such as EuC6 and SrC6 has been evidenced [START_REF] Hajj | Crystal structure of first stage strontium-graphite intercalation compound[END_REF], [START_REF] Bolmont | LiCl-KCl eutectic molten salt as an original and efficient medium to intercalate metals into graphite: Case of europium[END_REF].

Thus, the purpose of this paper is to synthesize a bulk and homogeneous first stage BaC6 sample using the molten salts solid-liquid method. Then, thanks to the so-obtained massive sample starting from pristine pyrolytic graphite, a full structural study performed by means of X-ray diffraction techniques supported by complementary Raman spectroscopy analysis is presented, in order to definitively solve the crystal structure of this compound.

Experimental

Synthesis

The experiments are conducted in a glove box under purified argon due to the sensitivity of the reagents and products towards air and moisture. First, lithium and potassium chlorides (99%), used as solvent, have been individually outgassed for at least 24 h at 240°C under secondary vacuum (10 -6 mbar). The eutectic mixture is prepared in the glove box by mixing 58.2 and 41.8 mol % of LiCl and KCl respectively [START_REF] Basin | The LiCl-KCl binary system[END_REF]. A total mass of 4 g of this blend is then introduced in a stainless-steel reactor and heated up at 450°C to ensure its melting. The suitable amount of barium (mBa about 200 mg; 99.9%, Sigma-Aldrich) for this quantity of molten salt is then added, which is around 2 at.%, meaning that the used metal is in excess compared to graphite (mgraphite about 10 mg). Subsequently, the solution is manually stirred for at least 15 minutes in the glove box. Once the metal is dissolved, a single platelet of pyrolytic graphite is immersed in the reactive liquid. The reactor is tightly closed with a Swagelok® plug and transferred in a metal enclosure under argon in order to safely perform the heat treatment outside the glove box. The whole device is heated up in a furnace and the reaction is performed for 12 days at 450°C. During this procedure, the oven undergoes regular swaying to guarantee the homogenization of the reactive medium. When the time span of the reaction is up, the sample is removed from the molten mixture inside the glove box, then scrapped and finally placed in a glass capillary sealed under argon, for X-ray characterizations.

X-ray experiments

The characterization methods are focused on XRD measurements, mainly performed with a Brüker D8 Advance diffractometer (λMoKα1 = 70.926 pm) using a (θ, 2θ) configuration and enhanced with a Soller slit (2.5°). GIC prepared from extremely anisotropic pyrolytic graphite platelets are distinguished by a single-crystal behavior along the c ⃗ axis and considered as powder in the perpendicular (ab) plane. Hence, by changing the sample orientation on the diffractometer, the 00l and hk0 reflections can be separately recorded. The corresponding diffractograms allow the determination of a and b parameters of the elementary unit cell as well as the repeat distance IC, the interplanar distance di and the stage s.

The quantitative analysis of 00l reflections allows the calculation of the experimental c ⃗ axis electronic density profile and hence contributes to the resolution of the stacking sequence of the atomic planes in this direction.

Elsewhere, a Brüker Kappa APEX duo diffractometer is used in order to measure the rotating crystal diffraction pattern equipped with a two-dimensional CCD APEX II detector and an X-ray micro-source (diameter spot 100 mm; λMoKα1 = 70.936 pm). A sample with small dimensions (hundreds of microns) is transferred into a Lindemann capillary (diameter = 0.5 mm), then placed on the Kappa goniometer and exposed to the X-ray source.

Thanks to this method, all reflections can be easily identified and the three-dimensional unit cell can be determined.

Raman analysis

Raman spectroscopy is mainly used in order to confirm the structure of the sample. It is carried out by using Jobin Yvon Horiba LabRAM HR spectrometer equipped with a He/Ne gas laser source and operating with a wavelength of 632.8 nm. The sample is cleaved in order to get a clean surface. Then, it is introduced in a quartz cell which is sealed under low helium partial pressure. The laser beam is then focalized on a selected area of the sample before recording the spectra.

Results and discussion

The obtained compound exhibits a shiny silver color related to the first stage binary barium-graphite intercalation compound. The X-ray diffraction pattern shown Figure 1 indicates individualized and intense 00l peaks that can be assigned to BaC6. Except a minor amount of chloride salts on the surface, the diffractogram reveals the absence of any reflection due to residual metal, graphite, carbide or other intercalation compounds. This result confirms the synthesis of pure and bulk first stage BaC6 compound.

The repeat distance IC extracted from this diffractogram is equal to 529.1 pm, slightly different from the one reported by Guérard et al. [START_REF] Guérard | Synthèse directe de composés d'insertion du baryum dans le graphite[END_REF] who mentioned an IC value of 525 pm. This tiny difference may be due to the involved way of synthesis involving different diffusion mechanism of barium atoms between graphene planes considering Ba atoms are intercalated using solidgas or solid-liquid intercalation reactions. In the former, Ba atoms directly diffuse into graphite; in the latter, first lithium is intercalated into graphite, then substituted with barium. It is clear that this inter-diffusion between Li and Ba lead to some more textural defaults, increasing the IC value.

The Fourier transform of the 00l structure factors, determined thanks to their corresponding experimental intensities, allows to plot the experimental electronic density profile which is in excellent agreement with a calculated one considering a three-layered model consisting in a monoatomic layer of barium between two graphene sheets (Figure 2). The structure factors of the corresponding experimental and calculated data are listed in Table 1. The residual factor evaluated by least square method is RF = 4.4% after thermal agitation correction (Debye-Waller coefficient B = 1.2 Ų) confirming the significant agreement between the experimental and calculated data. Thus, this modeling confirms the preparation of a bulk BaC6. The only divergent value that could be pointed out in Table 1 is the structure factor referring to the 002 reflection. The calculated factor (29.7) is slightly different from the experimental one (20.1). This discrepancy can be explained by the X-ray absorption phenomenon that occurs for low diffraction angle either due to the heavy element (i.e. barium) contained in the sample and due to the pyrex glass capillary.

A typical Raman spectrum recorded on the BaC6 sample is presented Figure 3. Three particular zones are studied and show the presence of the Cz peak at 481 cm -1 , a broad Cxy band located around 1500 cm -1 and the absence of the 2D band of pristine graphite which should be observed around 2700 cm -1 . These three features are specific to stage-1 binary compounds [START_REF] Dresselhaus | Lattice vibrations on graphite and intercalation compounds of graphite[END_REF][START_REF] Nemanich | Raman scattering from intercalated donor compounds of graphite[END_REF][START_REF] Hlinka | Lattice dynamics of CaC6 by Raman spectroscopy[END_REF][START_REF] Dean | Nonadiabatic phonons within the doped graphene layers of XC6 compounds[END_REF] and support the X-ray diffraction observations. More specifically, the Cz peak corresponds to an out-of-plane vibration mode of carbon atoms in stage-1 GIC [START_REF] Dresselhaus | Lattice vibrations on graphite and intercalation compounds of graphite[END_REF][START_REF] Hlinka | Lattice dynamics of CaC6 by Raman spectroscopy[END_REF][START_REF] Dean | Nonadiabatic phonons within the doped graphene layers of XC6 compounds[END_REF],

and its position depends of the nature of intercalant. The value of 481 cm -1 obtained after curve fitting with a Lorentzian peak is in excellent accordance with the one reported by Dean et al.

for BaC6 sample [START_REF] Dean | Nonadiabatic phonons within the doped graphene layers of XC6 compounds[END_REF]. The broad and asymmetric Cxy band is the result of the E2g vibration mode which corresponds to an in-plane bond-stretching mode. In stage-1 GIC, this mode is broadened and shifted in comparison with the G band of pristine graphite due to the presence of the intercalate [START_REF] Hlinka | Lattice dynamics of CaC6 by Raman spectroscopy[END_REF][START_REF] Dean | Nonadiabatic phonons within the doped graphene layers of XC6 compounds[END_REF]. Then, the whole results extracted from Raman analysis confirm the BaC6 nature of the sample together with its bulk intercalation.

The investigation of the binary compound is pursued with the study of its hk0 and its hkl reflections in order to identify the unit cell parameters. The analysis of hk0 diffraction pattern (Figure 4) shows relevant peaks corresponding to the BaC6 compound accompanied with some very weak reflections related to LiCl or KCl. Previously, we mentioned that 00l reflections revealed that no more graphite was persisting in the final sample. Here, it appears that the position of some hk0 reflections of BaC6 are equivalent with those of pristine graphite. Moreover, all reflections can be indexed considering the hexal unit cell of BaC6. For instance, d110 of BaC6 matches with d100 of graphite, or d300 of BaC6 is equivalent to d110 of graphite. So, this X-ray pattern displays the commensurability of the intercalated metallic sheet in respect with the graphene network.

Lagrange et al. [START_REF] Lagrange | Exhaustive inventory of 2D unit cells commensurate with honeycomb graphene structure[END_REF] ensured a detailed mathematical approach about exhaustive commensurability versus graphene to sustain the compatibility between the two-dimensional lattice of graphite and the intercalated sheets. Therefore, every GIC exhibiting commensurability owns planar unit cell parameters obtained by combination of the graphite ones, with ag the lattice parameter of graphite equals to 246.1 pm. In this work, the a and b parameters, calculated from the hk0 pattern, are equivalent to a g √3, hence a value of 430.6 pm.

The 2D unit cell of the BaC6 compound (Figure 5) exhibits a 30° rotation regarding the graphitic one, allowing to express the lattice by (√3×√3) R30°.

Figure 1 .

 1 Figure 1. 00l X-ray diffraction pattern (λMoKα1 = 70.936 pm) of the first stage BaC6 compound (IC = 529.1 pm). ICCD numbers = LiCl: 00-004-0664; KCl: 00-041-1476 A picture of the bulk sample is attached to the diffractogram.

Figure 2 .

 2 Figure 2. 1D c ⃗ axis electronic density profiles of the BaC6 compound (calculated and experimental are drawn in blue and green respectively).

Figure 3 .

 3 Figure 3. Raman spectrum (λ = 632.8 nm) of the BaC6 compound: a) Cz band b) Cxy region c) graphite 2D band region.

Figure 4 .

 4 Figure 4. hk0 X-ray diffraction pattern (λMoKα1 = 70.926 pm) of the first stage BaC6 compound (IC = 529.1 pm)

Figure 5 .

 5 Figure 5. 2D unit cells of graphene ( ) and BaC6 compound ( ) rotated by 30°. -Ba and  -Ba correspond to Ba atoms belonging to different van der Waals gaps.

Figure 6 .

 6 Figure 6. Rotating crystal pattern obtained by X-ray diffraction (λMoKα1 = 70.926 pm) of the BaC6 compound.

Table 1 .

 1 Experimental and calculated structure factors of the 00l reflections used for the calculation of the electronic density profile of BaC6.

	00l	 (°, λMoKα1)	d (pm)	IC (pm) F00l exp.	F00l calc.
	002	7.68	529.3	529.3	20.1	29.7
	004	15.40	265.5	531.0	100.0	100.0
	006	23.20	176.3	528.9	35.2	35.0
	008	31.30	132.2	528.8	72.7	69.8
	0010	39.80	105.7	528.5	30.5	30.5
	0012	47.45	88.1	528.6	54.8	55.8

Acknowledgments

The authors want to thank Sébastien Hupont, Hervé Rinnert and the Optics-Laser Competence Center of the Institut Jean Lamour for formation session on Raman device and fruitful discussions.

Funding source

The authors claim that no funding sources are involved in this research article.

intensities, especially the highly intense 004. The absence of 00l reflections for l an odd number confirms that these reflections obey to diffraction conditions l = 2n (n ≠ 0). Another interesting observation regarding the odd l numbers is their existence in the hkl strata (101 for example) implying that the c parameter is twice higher than the repeat distance IC, thus its value is 1058.2 pm. This also indicates the stacking arrangement along the c ⃗ axis, AαAβAα… (A = graphene layer;  and  = barium layers).

All these information prove that BaC6 crystallizes in the hexagonal system with a P63/mmc space group, with lattice parameters: a = b = 430.6 pm and c = 1058.2 pm.

Wyckoff's atomic positions are subsequently pointed out, and the representation of the BaC6 compound is given in Figure 7. The structure factors and intensities of the hkl reflections can subsequently be calculated after the determination of the lattice parameters. The corresponding data are given in Table 2. This table reports data in good agreement with the aforementioned analysis. Among the 00l reflections, 004 is the most intense one, which is well suited with the 00l diffractogram (Figure 1) and the data collected in Table 1. Likewise, the intensities of the odd reflections along the c ⃗ axis such as 001, 003… are zero, which is in agreement with the information recovered from the rotating crystal pattern (Figure 6). The intensities of the hk0 reflections are weaker than the 00l ones, as already observed. Finally, the 101 reflection is the most intense one, as reported the literature [START_REF] Heguri | Synthesis and physical properties of alkaline earth metal graphite compounds[END_REF].

Conclusion

For the first time, a massive bulk and fully intercalated binary BaC6 compound has been synthesized thanks to a solid-liquid method involving a LiCl-KCl eutectic molten salt as a reaction medium, which has already been highlighted for the successful elaboration of several other bulk binary compounds. Using such a massive sample, an extensive structural study becomes possible. Various X-ray diffraction measurements and complementary Raman spectroscopy analysis allowed an extended structural study of the as-obtained BaC6 sample.

This study confirms that BaC6 crystallizes in a hexagonal unit cell with the P63/mmc space group with a = b = 430.6 pm and c = 2IC = 1058.2 pm.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.