**3.1 Outline**

The Pulsed Current Electrical Sintering (PCES) also known as Spark Plasma Sintering (SPS), Field Assisted Sintering Technique (FAST) or Electric Current Activated Sintering (ECAS) is a powerful technique for powder consolidation. This technology started in the late 1920s

A Novel Approach to Develop Chalcogenide Glasses and

nitride (BN) layer as graphite barrier diffusion.

fabricated dense materials.

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

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 291

This technique is also very interesting as it permits to sinter and shape in a single step a wide range of materials into conventional (cylinder, etc) and non conventional shapes (ball, etc) (Hungria et al., 2009). The diameter, depending on the machine and the sintering conditions, can range from 8mm to 100mm or up. It should be however mentioned that in PCES processes the problem of adequate electrical conductance of the powders and the achievement of homogenous temperature distribution is particularly acute, especially for large diameter samples. In fact, current and consequent temperature distributions within the sample are very sensitive to the homogeneity of density distribution. Moreover, large density spatial variation, especially at the beginning of current flow, may result in high local overheating or even melting (Orru et al., 2009). That is why, some researchers focused now on modeling the thermal gradient during experiment. Then, some graphite pollution can occur during SPS experiment that is detrimental for optical applications for example (Bernard-Granger et al., 2009). This can be solved by using inner tantalum (Ta) foil or boron

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

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

when a sintering process using electrical energizing was introduced in the USA. In the meantime, in Japan, an ongoing research on the process of pulsed current activated sintering intensified and was patented in the 1960s [Inoue, 1962 & Inoue, 1966). This method seems to be characterized by technical and economical advantages over conventional sintering methods such as Hot Uniaxial Pressing (HUP), Hot Isostatic Pressing (HIP), etc. Indeed, the fast heating rate, the short holding times, the absence of sintering aids, the lower sintering temperatures, the improved properties of the process ceramic in comparison with other sintering methods, no need of cold compaction make the PCES a very competitive technique for industrial applications and for the field of materials research. As a proof, the number of research papers is exponential since the late 1990s when research lab worldwide started to be equipped with this technology [Orru et al., 2009]. Moreover, with the fast heating rates and short holding times, the processing of nano-powders was then possible to keep the nanostructure of the ceramics.

#### **3.2 Process description**

The description of the process is illustrated in Fig. 9. DC pulses allow the conducting die (graphite, WC, stainless steel) to heat by Joule effect while a uniaxial load can be applied. The PCES can therefore be seen as similar as Hot Uniaxial Pressing (HUP) technique with faster heating rate up to 1000°C/min. However, specific experimental sets-up have been designed in order to apply isostatic (Saito & Sawaoka, 1973) or quasi-isostatic (Song et al., 2004) pressure to the sample to be consolidated. The sintered powder can be either conductive or insulating. Depending on the electrical properties of the powder, the heating mechanisms are different (Anselmi-Tamburini et al., 2005). In the case of insulating powder, the heating mostly occurs through the container (die) while conducting powders are heated by Joule effect and by heat transfer from the container and electrodes. If the powder is conductive, an insulating die can be used. The mechanical load applied depends on the nature of the container. The load is usually limited to 120MPa in the case of graphite die but can be much more in the case of WC die (800MPa). However, graphite die can be heated up to 2000°C while WC can be heated up to 900°C depending on the applied load. Different working atmosphere can be applied during experiments: vacuum, neutral, reducing, etc.

Fig. 9. Process description of PCES technique (Hungria et al., 2009).

when a sintering process using electrical energizing was introduced in the USA. In the meantime, in Japan, an ongoing research on the process of pulsed current activated sintering intensified and was patented in the 1960s [Inoue, 1962 & Inoue, 1966). This method seems to be characterized by technical and economical advantages over conventional sintering methods such as Hot Uniaxial Pressing (HUP), Hot Isostatic Pressing (HIP), etc. Indeed, the fast heating rate, the short holding times, the absence of sintering aids, the lower sintering temperatures, the improved properties of the process ceramic in comparison with other sintering methods, no need of cold compaction make the PCES a very competitive technique for industrial applications and for the field of materials research. As a proof, the number of research papers is exponential since the late 1990s when research lab worldwide started to be equipped with this technology [Orru et al., 2009]. Moreover, with the fast heating rates and short holding times, the processing of nano-powders was then possible to keep the

The description of the process is illustrated in Fig. 9. DC pulses allow the conducting die (graphite, WC, stainless steel) to heat by Joule effect while a uniaxial load can be applied. The PCES can therefore be seen as similar as Hot Uniaxial Pressing (HUP) technique with faster heating rate up to 1000°C/min. However, specific experimental sets-up have been designed in order to apply isostatic (Saito & Sawaoka, 1973) or quasi-isostatic (Song et al., 2004) pressure to the sample to be consolidated. The sintered powder can be either conductive or insulating. Depending on the electrical properties of the powder, the heating mechanisms are different (Anselmi-Tamburini et al., 2005). In the case of insulating powder, the heating mostly occurs through the container (die) while conducting powders are heated by Joule effect and by heat transfer from the container and electrodes. If the powder is conductive, an insulating die can be used. The mechanical load applied depends on the nature of the container. The load is usually limited to 120MPa in the case of graphite die but can be much more in the case of WC die (800MPa). However, graphite die can be heated up to 2000°C while WC can be heated up to 900°C depending on the applied load. Different working atmosphere can be applied during experiments: vacuum, neutral, reducing, etc.

Fig. 9. Process description of PCES technique (Hungria et al., 2009).

nanostructure of the ceramics.

**3.2 Process description** 

This technique is also very interesting as it permits to sinter and shape in a single step a wide range of materials into conventional (cylinder, etc) and non conventional shapes (ball, etc) (Hungria et al., 2009). The diameter, depending on the machine and the sintering conditions, can range from 8mm to 100mm or up. It should be however mentioned that in PCES processes the problem of adequate electrical conductance of the powders and the achievement of homogenous temperature distribution is particularly acute, especially for large diameter samples. In fact, current and consequent temperature distributions within the sample are very sensitive to the homogeneity of density distribution. Moreover, large density spatial variation, especially at the beginning of current flow, may result in high local overheating or even melting (Orru et al., 2009). That is why, some researchers focused now on modeling the thermal gradient during experiment. Then, some graphite pollution can occur during SPS experiment that is detrimental for optical applications for example (Bernard-Granger et al., 2009). This can be solved by using inner tantalum (Ta) foil or boron nitride (BN) layer as graphite barrier diffusion.
