**3.3 State of the art of the materials investigated**

A large number of sintered materials have been tested until now among which we can cite: metals and alloys, intermetallics, carbides, borides, nitrides, silicides, oxides, sialon, ceramic–metal and ceramic–intermetallic composites, ceramic–ceramic composites, hydroxyapatite-based materials, chalcogenides, polymer-based materials, functional graded materials, systems for joining, graphite/carbon-based materials including composites containing carbon-nanotubes, multi-materials including solid state batteries (Aboulaich et al., 2011) and other systems. A review of the materials tested can be found elsewhere Orru et al., 2009). The highlights are that higher densities can be achieved at lower sintering temperature in shorter sintering time and that smaller grain sizes ceramics are typically obtained with a consequent direct effect on nearly all properties investigated on the fabricated dense materials.

In the literature, the sintering of glasses by SPS mostly concerns conductor metallic glasses (Li et al., 2009; Shin et al., 2010 & Choi et al. 2007). These glasses which composition can include several chemical elements among Zr, Ti, Al, Cu, Pd, Mg, Pt, Ca, Fe, Ni, Co and sometimes rare earth are largely studied for their good mechanical properties. In this case, the SPS technique is very interesting since the heating rates are fast enough to avoid the crystallization phenomenon of these unstable glasses. These amorphous materials are most often obtained by mechanical alloying, melt-spinning or atomization. Recently, Nowak *et al.* (Nowak et al., 2011) and Perriere *et al.* (Perriere et al., 2011) studied the phenomenon during SPS treatment at the origin of the microstructure and densification of glasses. The devitrification in metallic glasses during SPS treatment has also been studied in order to obtain composite materials or glass-ceramics (Li et al., 2010). Except this study on metallic glasses, we can cite the research activity of Mayerhofer *et al.* on the sintering of amorphous silica nano-particles by SPS (Mayerhofer et al., 2008). The sintering of chalcogenide glasses and the *in situ* synthesis of glass-ceramics by SPS has been little studied to the best of our knowledge. We can identify a study on oxide glasses from the SiO2-Al2O3-Li2O:Er3+ system (Riello et al., 2006) and the ones we propose in this chapter on the GeS2-Sb2S3-CsCl and GeSe2-Ga2Se3 glassy systems (Hubert et al., submitted & Delaizir et al., 2010). These last studies clearly show the advantage of SPS for the shaping of glass and the gain of time

A Novel Approach to Develop Chalcogenide Glasses and

**4.2 Glasses of investigation and results** 

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 293

So far two compositions have been tested: 62.5GeS2-12.5Sb2S3-25Cscl and 80GeSe2-20Ga2Se3 (mol %). However this technique is believed to be suitable for all glass compositions to get amorphous bulks as well as glass-ceramics. These compositions were chosen for their

The glass composition 62.5GeS2-12.5Sb2S3-25Cscl has been obtained through a melt-quenching technique and the resulting bulk has been grinded and sieved (45μm) while the glass composition 80GeSe2-20Ga2Se3 has been obtained through an 80h mechanical milling process from high purity metallic elements Ge, Ga and Se (Fig. 3a). The size distribution, measured using laser diffusion technique shows low mean particle size (D50 = 3.72 μm). The thermal properties of these glasses are summarized in Table 2. Tc1 corresponds to the crystallization of CsCl and GeGa4Se8 (or Ga2Se3) species respectively for the 62.5GeS2-12.5Sb2S3-25Cscl and

The powders were then inserted into a graphite die (Ø8, 20 or 35mm) with inner tantalum foil to prevent the glass from carbon contamination (Fig. 10). It is noteworthy that the glass powder is yellow due to the presence of sulphur in the composition. The corresponding sintered glass is red since the optical band gap is in the 600-700nm region. According to the glass composition, different SPS parameters were tested. The typical load ranges from 50 to 100MPa and the optimized sintering temperatures were found to be respectively 290°C and 390°C for the 62.5GeS2-12.5Sb2S3-25Cscl and 80GeSe2-20Ga2Se3 glass compositions. The dwell times at the sintering temperature range from 2 minutes to get amorphous bulk to 90

Fig. 10. Photograph of the processed glass sample for the composition 62.5GeS2-12.5Sb2S3-

Tg (°C) Tc1 (°C) Tc2 (°C)

potential applications (Huber et al., submitted & Delaizir et al., 2010).

80GeSe2-20Ga2Se3 compositions. Tc2 corresponds to the crystallization of GeSe2.

62.5GeS2-12.5Sb2S3-25Cscl 260 380 - 80GeSe2-20Ga2Se3 347 449 470

Table 2. Thermal properties of two chalcogenide glasses.

minutes to get glass-ceramics as shown by XRD patterns (Fig. 11).

25Cscl.

concerning the synthesis of glass-ceramics in comparison with a conventional technique in a conventional furnace.

Recent results also suggest that the Spark Plasma Sintering is a new technique to achieve very fast solid state chemistry (Galy et al., 2008). This technique appears as a new synthesis technique which permits to decrease both the temperature and time reaction while mastering the particle size. Even though all the mechanisms are not well understood, it is generally agreed that an accelerated diffusion process due to the electrical discharge is at the origin of the fast reactivity by SPS (Galy et al., 2008).

The SPS or PCES still remains controversial with plasma formation or not, removal of oxides (breakdown of oxides films) and adsorbed gases from the particle surfaces with a resulting cleaning effect, high localized temperature at the contact area between particles, enhanced diffusion of materials at forming particle necks (Orru et al., 2009) since no evidence has been proven. Thermal gradients are also very discussed especially for large diameters. However research on SPS mechanisms to answer to these questions as well as modeling is of growing and fundamental interests.
