**4.2 Glasses of investigation and results**

292 Sintering of Ceramics – New Emerging Techniques

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

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

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

**4. The Pulsed Current Electrical Sintering (PCES) technique applied to the** 

As previously mentioned, the common way to prepare chalcogenide glasses is the meltquenching technique in silica tube under vacuum. Due to the low thermal conductivity of silica during quenching, this leads to heterogeneous glass sample when the composition is unstable (Tc-Tg<100°C). Usually the inner part of the glass is crystallized and the outer part amorphous. Therefore, this limits the preparation of glass samples to small diameters to ensure homogeneity of the glass. Glass-ceramics are obtained by heating a glass bulk with an appropriate thermal treatment in a furnace. According to targeted crystal fraction and crystals size, this can take up to hundreds of hours. It is well known that grained glass samples devitrify more easily than glass bulk samples and upon a process of amorphous state sintering and simultaneous or subsequent crystallization, glass-ceramic materials can

Grained glass samples can be either obtained by mechanical milling or by grinding and sieving pre-existing glass bulk synthesized by the conventional melt-quenching technique. The phenomenon at the origin of easier devitrification is mostly due to pre-existing structural defects and new surfaces that act as nucleation sites in powders obtained from mechanical milling process or grinding. Therefore, it should be possible to change the

Moreover, the SPS or PCES technique has fast heating rates that prevent the glass powder from uncontrollable ceramization. By combining glass powder obtained from mechanical milling or grinding of pre-existing glass bulk and SPS technique, the idea is to develop an easy way to produce glass/glass-ceramics bulks with large diameters even for unstable

kinetic process in comparison with devitrification of common glass bulks.

**preparation of chalcogenide glasses and glass-ceramics** 

conventional furnace.

and fundamental interests.

be obtained (Gutzow et al., 1998).

compositions that are of great interest for optics.

**4.1 Principle** 

origin of the fast reactivity by SPS (Galy et al., 2008).

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

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


Table 2. Thermal properties of two chalcogenide glasses.

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

Fig. 10. Photograph of the processed glass sample for the composition 62.5GeS2-12.5Sb2S3- 25Cscl.

A Novel Approach to Develop Chalcogenide Glasses and

chalcogenide glasses.

Transmission (%)

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 295

a slight diffusion of C graphite (papyex) or tantalum used as inner part of the die can also interfere (Fig. 12c). These all parameters can be improved by working under controlled atmosphere from the beginning to the end of the experiments and by pre-compacting the powder at room temperature. The sintering of large diameter is challenging in SPS experiment due to thermal gradient between the center and the periphery of the pellet. A micro-diffraction experiment has been carried out to investigate the amorphous character of the glass bulk along the diameter. Results show that no crystallization peaks are observed in the 36mm diameter glass and the glass is still amorphous (Fig. 12d). For recall, this glass composition doesn't exceed a diameter of 9mm using the conventional way of synthesis of

(a) (b)

Glass

Base 2 min

60 min 30 min 15 min

(c) (d)

Similar results are obtained with the 62.5GeS2-12.5Sb2S3-25Cscl glass composition that has been grinded from a bulk obtained by melt-quenching and sieved (Fig. 13). It is noteworthy that better optical transmissions are obtained with this technique in comparison with the sintering of powder obtained by mechanical milling. This supposes that some oxidation may occur during the long time mechanical milling process leading to lower optical properties.

Fig. 12. (a) Photograph of the SPS processed 80GeSe2-20Ga2Se3 glasses (9, 20, 36mm diameter), (b) IR picture of the corresponding glasses observed with a 8-12µm thermal camera, (c) IR spectra of the base glass synthesized in sealed silica tube, finely grinded and sintered by SPS (390°C, 50MPa, 2'), glass and glass-ceramics made by mechanical milling and SPS at 390°C for 2, 15, 30 and 60 min under 50MPa, (d) X-Ray micro-diffraction patterns

of the 36mm diameter sample from the center to the periphery.

0 5 10 15 20

Wavelength (µm)

Fig. 11. X-ray diffraction patterns of (a) glass–ceramics obtained from the 62.5GeS2–12.5Sb2S3–25CsCl-based glass powder for different SPS treatment times at 290°C under 100 MPa, (b) glass–ceramics obtained from the 80GeSe2-20Ga2Se3 glass powder for different SPS treatment times at 390°C under 50 MPa, (1) initial 80GeSe2-20Ga2Se3 powder, (2) 2'SPS treatment time, (3) 30' SPS treatment time, (4) 60' SPS treatment time, (5) crystalline Ga2Se3, (6) crystalline GeSe2.

The transmission of the processed glassy samples whatever their dimensions is good in the mid infrared range as observable in the picture taken by a thermal camera working in the third atmospheric window from 8 to 12µm (Figs. 12a and 12b). The absorption observed in this picture is firstly due to reflection on both faces because of the high refractive index of the 80GeSe2-20Ga2Se3 glass composition (n~2.41). Secondly, the presence of oxygen leading to the formation of Ge-O bonds inside the bulk glass induces phonon absorption and finally

(a)

(b)

(1) (2) (3) (4)

(5) (6)

290°C under 100 MPa, (b) glass–ceramics obtained from the 80GeSe2-20Ga2Se3 glass powder

The transmission of the processed glassy samples whatever their dimensions is good in the mid infrared range as observable in the picture taken by a thermal camera working in the third atmospheric window from 8 to 12µm (Figs. 12a and 12b). The absorption observed in this picture is firstly due to reflection on both faces because of the high refractive index of the 80GeSe2-20Ga2Se3 glass composition (n~2.41). Secondly, the presence of oxygen leading to the formation of Ge-O bonds inside the bulk glass induces phonon absorption and finally

Fig. 11. X-ray diffraction patterns of (a) glass–ceramics obtained from the

(5) crystalline Ga2Se3, (6) crystalline GeSe2.

62.5GeS2–12.5Sb2S3–25CsCl-based glass powder for different SPS treatment times at

for different SPS treatment times at 390°C under 50 MPa, (1) initial 80GeSe2-20Ga2Se3 powder, (2) 2'SPS treatment time, (3) 30' SPS treatment time, (4) 60' SPS treatment time, a slight diffusion of C graphite (papyex) or tantalum used as inner part of the die can also interfere (Fig. 12c). These all parameters can be improved by working under controlled atmosphere from the beginning to the end of the experiments and by pre-compacting the powder at room temperature. The sintering of large diameter is challenging in SPS experiment due to thermal gradient between the center and the periphery of the pellet. A micro-diffraction experiment has been carried out to investigate the amorphous character of the glass bulk along the diameter. Results show that no crystallization peaks are observed in the 36mm diameter glass and the glass is still amorphous (Fig. 12d). For recall, this glass composition doesn't exceed a diameter of 9mm using the conventional way of synthesis of chalcogenide glasses.

Fig. 12. (a) Photograph of the SPS processed 80GeSe2-20Ga2Se3 glasses (9, 20, 36mm diameter), (b) IR picture of the corresponding glasses observed with a 8-12µm thermal camera, (c) IR spectra of the base glass synthesized in sealed silica tube, finely grinded and sintered by SPS (390°C, 50MPa, 2'), glass and glass-ceramics made by mechanical milling and SPS at 390°C for 2, 15, 30 and 60 min under 50MPa, (d) X-Ray micro-diffraction patterns of the 36mm diameter sample from the center to the periphery.

Similar results are obtained with the 62.5GeS2-12.5Sb2S3-25Cscl glass composition that has been grinded from a bulk obtained by melt-quenching and sieved (Fig. 13). It is noteworthy that better optical transmissions are obtained with this technique in comparison with the sintering of powder obtained by mechanical milling. This supposes that some oxidation may occur during the long time mechanical milling process leading to lower optical properties.

A Novel Approach to Develop Chalcogenide Glasses and

confirmed by Perriere et al. who suggest that (Perriere et al., 2011)

� ��������

considering an equivalent electrical outline of the SPS apparatus. Therefore,

������� <sup>=</sup> ��������������

respectively areas of the die or the powder crossed by the current.

heating rate.

treatment (Perriere et al., 2011).

relationship (Murray et al., 1954):

��

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 297

Due to the insulating property of the glass composition (Giridhar et al., 1990 & Raspopova et al., 1980), it is thus believed that heating mostly occurs through heating of carbon die. This is

> <sup>=</sup> � ����

������� = �������� <sup>×</sup> �

powder and the die, Iapplied the macroscopic current intensity applied to the system,

�������

With ipowder and idie the respectively electrical current passing through the chalcogenide glass

with ρdie and ρpowder the respectively electrical resistivity of the die and the powder, ldie and lpowder the length of the die and the powder crossed by the current, Sdie and Spowder the

Considering an electrical conductivity σpowder=1×10-5 S.m-1 at 390°C (Giridhar et al., 1990), i.e ρpowder= 1×105 Ω.m and ρdie= 20×10-6 Ω.m, ldie=30mm, lpowder=1.5mm, Ødie=25mm and Øpowder=8mm, we end with ������� → 0. The macroscopic current is therefore mostly driven by the graphite die. In the case of the 80GeSe2-20Ga2Se3 amorphous powder, the SPS experiment can thus be seen as a Hot Uniaxial Pressing (HUP) experiment with faster

The sintering of glass occurs by viscous flow, to reduce surface energy of a porous compact, through neck growth and densification involving deformation of initial particles (Rahamn, 2008). Different mechanisms leading to the production of glass-ceramics have been hypothesized. The first one would be the densification of glassy powder through viscous sintering followed by subsequent devitrification of the matrix. The second mechanism would imply densification and concurrent gradual crystallization of the matrix through the growth of neck between glassy particles. For one given glass composition, the corresponding crystalline phase has a considerably higher viscosity than the amorphous phase, so the sintering of polycrystalline material is more difficult than the amorphous phase. This suggests that the first mechanism described above, i.e achieving full density prior to any significant crystallization should be favored. However, in the specific case of metallic glasses, it is reported that concurrent mechanisms are involved during SPS

Sintering of powdered glasses based on a viscous flow mechanism has been intensively studied by Frenkel, Scherer or Kingery and Berg (Frenkel, 1945; Kingery, 1955 & Scherer, 1977). Especially Murray et al. described the densification process of amorphous powder under load. The theoretical model based on viscous flow mechanism obeys the following

�� � = �����(� � �) or ln(���) <sup>=</sup> ���

<sup>+</sup> � �������

> ��������� ����

(5), ���� <sup>=</sup> ��������

����

(3)

(4)

(6)

��� � + ln�(� � ��) (7)

Fig. 13. (a) Transmission of different glass–ceramics obtained from the 62.5GeS2–12.5Sb2S3– 25CsCl-based glass powder for SPS treatment times of 2 min, 30 min, 60 min, 90 min, and 120 min at 290°C under 100 MPa (thickness 1 mm) (b) Visual aspect of glass–ceramics samples for SPS treatment times of (a) 10 min, (b) 30 min, (c) 60 min, (d) 90 min, and (e) 120 min at 290°C under 100 MPa, (c) Observation of glass–ceramics under scanning electronic microscope for SPS treatment times of 2 min, 10 min, 30 min, 60 min, 90 min, and 120 min at 290°C under 100 MPa.

#### **4.3 Mechanisms of sintering**

A specific study on the mechanisms of sintering of chalcogenide glassy powder by SPS has been investigated on the special composition 80GeSe2-20Ga2Se3.

As previously mentioned, the Spark Plasma Sintering (SPS) technique is a non-conventional sintering technique based on the use of pulsed current. It is well known that the current distributions in the SPS die and through the sample is different in the case of a conductor and an insulator material (Anselmi-Tamburini et al., 2005). In the case of a non-electrically conducting sample such as alumina or the 80GeSe2-20Ga2Se3 glassy powder in our case, no Joule heating is expected through the sample and the heating of the sample is mostly due to heating of the die. This is in contrast with the case of conducting materials such as metals sample, where Joule heating starts immediately through the sample.

(a) (c) Fig. 13. (a) Transmission of different glass–ceramics obtained from the 62.5GeS2–12.5Sb2S3– 25CsCl-based glass powder for SPS treatment times of 2 min, 30 min, 60 min, 90 min, and 120 min at 290°C under 100 MPa (thickness 1 mm) (b) Visual aspect of glass–ceramics samples for SPS treatment times of (a) 10 min, (b) 30 min, (c) 60 min, (d) 90 min, and (e) 120 min at 290°C under 100 MPa, (c) Observation of glass–ceramics under scanning electronic microscope for SPS treatment times of 2 min, 10 min, 30 min, 60 min, 90 min, and

A specific study on the mechanisms of sintering of chalcogenide glassy powder by SPS has

As previously mentioned, the Spark Plasma Sintering (SPS) technique is a non-conventional sintering technique based on the use of pulsed current. It is well known that the current distributions in the SPS die and through the sample is different in the case of a conductor and an insulator material (Anselmi-Tamburini et al., 2005). In the case of a non-electrically conducting sample such as alumina or the 80GeSe2-20Ga2Se3 glassy powder in our case, no Joule heating is expected through the sample and the heating of the sample is mostly due to heating of the die. This is in contrast with the case of conducting materials such as metals

been investigated on the special composition 80GeSe2-20Ga2Se3.

sample, where Joule heating starts immediately through the sample.

120 min at 290°C under 100 MPa.

**4.3 Mechanisms of sintering** 

(b)

Due to the insulating property of the glass composition (Giridhar et al., 1990 & Raspopova et al., 1980), it is thus believed that heating mostly occurs through heating of carbon die. This is confirmed by Perriere et al. who suggest that (Perriere et al., 2011)

$$\frac{1}{l\_{Iapptted}} = \frac{1}{l\_{date}} + \frac{1}{l\_{powerder}}\tag{3}$$

considering an equivalent electrical outline of the SPS apparatus. Therefore,

$$I\_{power} = I\_{applied} \times \frac{1}{1 + \frac{R\_{power}}{R\_{dle}}} \tag{4}$$

With ipowder and idie the respectively electrical current passing through the chalcogenide glass powder and the die, Iapplied the macroscopic current intensity applied to the system,

$$R\_{power} = \frac{\rho\_{power \times l\_{power}}}{s\_{power}} \quad \text{ (5)} \; R\_{dle} = \frac{\rho\_{dle \times l\_{dle}}}{s\_{dle}} \tag{6}$$

with ρdie and ρpowder the respectively electrical resistivity of the die and the powder, ldie and lpowder the length of the die and the powder crossed by the current, Sdie and Spowder the respectively areas of the die or the powder crossed by the current.

Considering an electrical conductivity σpowder=1×10-5 S.m-1 at 390°C (Giridhar et al., 1990), i.e ρpowder= 1×105 Ω.m and ρdie= 20×10-6 Ω.m, ldie=30mm, lpowder=1.5mm, Ødie=25mm and Øpowder=8mm, we end with ������� → 0. The macroscopic current is therefore mostly driven by the graphite die. In the case of the 80GeSe2-20Ga2Se3 amorphous powder, the SPS experiment can thus be seen as a Hot Uniaxial Pressing (HUP) experiment with faster heating rate.

The sintering of glass occurs by viscous flow, to reduce surface energy of a porous compact, through neck growth and densification involving deformation of initial particles (Rahamn, 2008). Different mechanisms leading to the production of glass-ceramics have been hypothesized. The first one would be the densification of glassy powder through viscous sintering followed by subsequent devitrification of the matrix. The second mechanism would imply densification and concurrent gradual crystallization of the matrix through the growth of neck between glassy particles. For one given glass composition, the corresponding crystalline phase has a considerably higher viscosity than the amorphous phase, so the sintering of polycrystalline material is more difficult than the amorphous phase. This suggests that the first mechanism described above, i.e achieving full density prior to any significant crystallization should be favored. However, in the specific case of metallic glasses, it is reported that concurrent mechanisms are involved during SPS treatment (Perriere et al., 2011).

Sintering of powdered glasses based on a viscous flow mechanism has been intensively studied by Frenkel, Scherer or Kingery and Berg (Frenkel, 1945; Kingery, 1955 & Scherer, 1977). Especially Murray et al. described the densification process of amorphous powder under load. The theoretical model based on viscous flow mechanism obeys the following relationship (Murray et al., 1954):

$$
\left. \frac{dD}{dt} \right\rangle\_{\rm dt} = 3P/4\eta (1 - D) \quad \text{or} \quad \ln(1 - D) = \,^{-3P}\!/\_{4\eta \, t} + \ln(1 - D\_l) \tag{7}
$$

A Novel Approach to Develop Chalcogenide Glasses and

**1μm**

(a) (b) (c)

(d) (e) (f)

(g)

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 299

Fig. 15. FEG-SEM photographs of 80GeSe2-20Ga2Se3 SPS glass compacts sintered at different dwell temperatures (50MPa, 2 minutes dwell time), (a) 250°C (b) 280°C (c) 310°C (d) 330°C

(a)

(e) 350°C (f) 370°C (g) 390°C (same scale for all the photographs).

Where D is the relative density (D=ρ/ρs, ρ: the measured density and ρs, the theoretical density), Di the initial relative density, η the viscosity and t, the time. The slope of the ln (1- D) versus time t can be used to calculate the viscosity at one given temperature.

A formal sintering analysis, in order to help formulate hypotheses concerning the mechanism(s) controlling densification and devitrification has been performed. The influence of the dwell temperature (250°C, 280°C, 310°C, 330°C, 350°C, 370°C and 390°C) as well as the dwell time (2, 15 and 60 min) were studied while keeping the load constant (50MPa). These temperatures were chosen according to the sintering curve (Fig. 14) obtained from an SPS experiment (SPS parameters: 390°C, 50MPa, 2 minutes dwell time) where the full densification of glass is achieved without crystallization (Dglass=4.39). The heating rate is 100°C/min until 200°C and is then decreased to 30°C/min until the set temperature, i.e 390°C. The sintering of the glass is a two-step process. At 270°C, the shrinkage starts intensively. Viscous flow between particles leads to the formation of a neck. The growth of the necks occurs until 350°C which corresponds to the Tg of the glass (347°C). Then the mechanism of densification slows down and restarts around 360°C with lower amplitude. This second mechanism is believed to correspond to the closure of residual porosity measured from the Archimedean technique which is less than 10% at this stage.

Fig. 14. Shrinkage rate (dz/dt) and load applied as a function of temperature during SPS experiment.

Fig. 15 shows micrographs obtained on the fracture surface of powder compacts heat treated at different temperatures from 250°C to 390°C for 2 minutes dwell time (load of 50MPa). At 290°C, far below the glass transition temperature Tg, the particles begin to soften and fuse together but a lot of inter-granular porosity is still present (27%). At 390°C, particles are totally fused together and no porosity is observed.

The relative densities, D, according to the dwell temperature and dwell time are reported in Figs. 16a and 16b. At 250°C, the compactness is only 70%. The residual porosity decreases gradually until 390°C where the full densification is obtained.

The influence of the dwell time at one given temperature was also studied. For all the dwell temperatures, the compactness regularly increases upon the increase in dwell time, for example from 90% (370°C, 2 min) to 96% (370°C, 30 min).

Where D is the relative density (D=ρ/ρs, ρ: the measured density and ρs, the theoretical density), Di the initial relative density, η the viscosity and t, the time. The slope of the ln (1-

A formal sintering analysis, in order to help formulate hypotheses concerning the mechanism(s) controlling densification and devitrification has been performed. The influence of the dwell temperature (250°C, 280°C, 310°C, 330°C, 350°C, 370°C and 390°C) as well as the dwell time (2, 15 and 60 min) were studied while keeping the load constant (50MPa). These temperatures were chosen according to the sintering curve (Fig. 14) obtained from an SPS experiment (SPS parameters: 390°C, 50MPa, 2 minutes dwell time) where the full densification of glass is achieved without crystallization (Dglass=4.39). The heating rate is 100°C/min until 200°C and is then decreased to 30°C/min until the set temperature, i.e 390°C. The sintering of the glass is a two-step process. At 270°C, the shrinkage starts intensively. Viscous flow between particles leads to the formation of a neck. The growth of the necks occurs until 350°C which corresponds to the Tg of the glass (347°C). Then the mechanism of densification slows down and restarts around 360°C with lower amplitude. This second mechanism is believed to correspond to the closure of residual porosity

D) versus time t can be used to calculate the viscosity at one given temperature.

measured from the Archimedean technique which is less than 10% at this stage.

Fig. 14. Shrinkage rate (dz/dt) and load applied as a function of temperature during SPS

**Temperature (°C)**

0 100 200 300 400

Fig. 15 shows micrographs obtained on the fracture surface of powder compacts heat treated at different temperatures from 250°C to 390°C for 2 minutes dwell time (load of 50MPa). At 290°C, far below the glass transition temperature Tg, the particles begin to soften and fuse together but a lot of inter-granular porosity is still present (27%). At 390°C, particles are

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007

**dz/dt**

The relative densities, D, according to the dwell temperature and dwell time are reported in Figs. 16a and 16b. At 250°C, the compactness is only 70%. The residual porosity decreases

The influence of the dwell time at one given temperature was also studied. For all the dwell temperatures, the compactness regularly increases upon the increase in dwell time, for

experiment.

totally fused together and no porosity is observed.

**Load (MPa)**

gradually until 390°C where the full densification is obtained.

example from 90% (370°C, 2 min) to 96% (370°C, 30 min).

Fig. 15. FEG-SEM photographs of 80GeSe2-20Ga2Se3 SPS glass compacts sintered at different dwell temperatures (50MPa, 2 minutes dwell time), (a) 250°C (b) 280°C (c) 310°C (d) 330°C (e) 350°C (f) 370°C (g) 390°C (same scale for all the photographs).

A Novel Approach to Develop Chalcogenide Glasses and

temperature and Eη is the activation energy of viscous flow.

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 301

where η is the viscosity, η0 the pre-exponential factor, R the universal gas constant, T the

Fig. 17. XRD patterns of (a) 80GeSe2-20Ga2Se3 initial powder, (b) SPS glass sample (370°C, 50MPa, 15') (c) SPS glass sample (370°C, 50MPa, 30') (d) SPS glass sample (390°C, 50MPa, 2')

(b) (c) (d) (e) (f)

(a)

Fig. 18. FEG-SEM photographs of 80GeSe2-20Ga2Se3 SPS samples sintered at 390°C under 50MPa for different dwell times (a) 2 minutes, (b) 15 minutes, (c) 30 minutes, (d) TEM observation of glass-ceramic (c) obtained for SPS treatment time of 30 min (390°C, 50MPa)

**20 nm** 

showing Ga2Se3 crystals.

(e) SPS glass sample (390°C, 50MPa, 15') (f) SPS glass sample (390°C, 50MPa, 30').

(a) (b)

(c) (d)

 **1μm** 

(c)

Fig. 16. (a) Influence of the dwell temperature on the relative density (50MPa, 2minutes dwell time), (b) Influence of the dwell time on the relative density for several dwell temperatures, (c) Plots of the ln (1-D) vs t curve for two dwell temperatures.

At 390°C for 2 minutes dwell time, the almost full densification of the glass is reached and the glass remains amorphous as shown by XRD patterns (Fig. 17). At this temperature only, the increase of dwell time induces partial devitrification of the glass as shown in Figs. 17 and 18. The crystals composition is Ga2Se3 (or GaGe4Se8) (Roze et al., 2008). Thus, there is clear evidence of densification mechanism prior to devitrification.

From eq. (7), we can deduce the viscosity for one given temperature (Fig. 16c). For temperatures close to the glass transition temperature, Tg, or higher temperatures, calculations give η(350°C)~2×1011 Pa.s and η(370°C)~9×1010 Pa.s (Fig. 16c). Considering a Newtonian behavior in this range of temperature (Roze et al., 2011), the dependence of its viscosity as a function of temperature can be described by a simple Arrhenius equation:

$$
\eta = \eta\_0 e^{E\_\eta}/\_{RT} \tag{8}
$$

(b)

(c)

At 390°C for 2 minutes dwell time, the almost full densification of the glass is reached and the glass remains amorphous as shown by XRD patterns (Fig. 17). At this temperature only, the increase of dwell time induces partial devitrification of the glass as shown in Figs. 17 and 18. The crystals composition is Ga2Se3 (or GaGe4Se8) (Roze et al., 2008). Thus, there is clear

From eq. (7), we can deduce the viscosity for one given temperature (Fig. 16c). For temperatures close to the glass transition temperature, Tg, or higher temperatures, calculations give η(350°C)~2×1011 Pa.s and η(370°C)~9×1010 Pa.s (Fig. 16c). Considering a Newtonian behavior in this range of temperature (Roze et al., 2011), the dependence of its viscosity as a function of temperature can be described by a simple Arrhenius equation:

݁ߟൌߟ

ாആ ோ்

ൗ (8)

Fig. 16. (a) Influence of the dwell temperature on the relative density (50MPa, 2minutes dwell time), (b) Influence of the dwell time on the relative density for several dwell temperatures, (c) Plots of the ln (1-D) vs t curve for two dwell temperatures.

evidence of densification mechanism prior to devitrification.

where η is the viscosity, η0 the pre-exponential factor, R the universal gas constant, T the temperature and Eη is the activation energy of viscous flow.

Fig. 17. XRD patterns of (a) 80GeSe2-20Ga2Se3 initial powder, (b) SPS glass sample (370°C, 50MPa, 15') (c) SPS glass sample (370°C, 50MPa, 30') (d) SPS glass sample (390°C, 50MPa, 2') (e) SPS glass sample (390°C, 50MPa, 15') (f) SPS glass sample (390°C, 50MPa, 30').

Fig. 18. FEG-SEM photographs of 80GeSe2-20Ga2Se3 SPS samples sintered at 390°C under 50MPa for different dwell times (a) 2 minutes, (b) 15 minutes, (c) 30 minutes, (d) TEM observation of glass-ceramic (c) obtained for SPS treatment time of 30 min (390°C, 50MPa) showing Ga2Se3 crystals.

A Novel Approach to Develop Chalcogenide Glasses and

optics, broadening the target market to a more general public.

viscosity between Tg and Tc.

pp. 139–148

pp. 129-132

337-351

Vol 7, pp. 1103-1112

**6. References** 

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 303

enough for applications without crystallization and inhomogeneity concerns. It offers the possibility to make glasses or glass-ceramics of desired shape and size with sintering slightly higher than the glass transition temperature in few minutes. Indeed, the fast heating rates of PCES allow getting amorphous bulks in few minutes and partial devitrification starts after tens of minutes. Thus, this new process would lead to lower manufacturing costs of new chalcogenide glasses and glass-ceramics which could be adapted to numerous setups such as thermal imaging, infrared laser sources and detectors, thermoelectric devices, etc. Ultimately, this new synthetic route will lead to larger scale production of infrared

The mechanisms of sintering of one given chalcogenide glass composition that has an electrical insulating property have been investigated. Results show that heating of the glass powder occurs mainly through the heating of the die due to the electrical insulating property of the glassy powder. It is also shown that densification of the glass powder occurs prior to the devitrification of the glass. The model of Murray et al. has been successfully applied to our glass and allows the determination of viscosity and activation energy for

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Amorphous Alloys and Amorphous-crystalline Composites Produced by

Fe bulk metallic glasses produced by mechanical alloying and spark-plasma

We can thus deduce the activation energy of viscosity between Tg and Tc1 from the curve ln η=f(1/T) which is found to be equal to ~500±100kJ.mol-1 (Fig. 19). This value has been compared to literature on 80GeSe2-20Ga2Se3 synthesized by the common melt-quenching technique instead of mechanical alloying (Roze et al., 2011). From the data viscosity, similar activation energy is found.

Fig. 19. Plots of the curve ln η= f(1/T) in the case of our study (a) and in [63] (b).

#### **5. Conclusion**

This chapter provides an overview of a new technique to synthesize chalcogenide glasses and glass-ceramics. This technique combines either the synthesis of glass powder by mechanical milling or the grinding and sieving of pre-existing bulk obtained by the conventional melt-quenching technique and the PCES technique (or SPS). Many chalcogenide glasses presenting real potential applications were set aside since the common synthesis in sealed silica tubes under vacuum did not permit to produce samples big enough for applications without crystallization and inhomogeneity concerns. It offers the possibility to make glasses or glass-ceramics of desired shape and size with sintering slightly higher than the glass transition temperature in few minutes. Indeed, the fast heating rates of PCES allow getting amorphous bulks in few minutes and partial devitrification starts after tens of minutes. Thus, this new process would lead to lower manufacturing costs of new chalcogenide glasses and glass-ceramics which could be adapted to numerous setups such as thermal imaging, infrared laser sources and detectors, thermoelectric devices, etc. Ultimately, this new synthetic route will lead to larger scale production of infrared optics, broadening the target market to a more general public.

The mechanisms of sintering of one given chalcogenide glass composition that has an electrical insulating property have been investigated. Results show that heating of the glass powder occurs mainly through the heating of the die due to the electrical insulating property of the glassy powder. It is also shown that densification of the glass powder occurs prior to the devitrification of the glass. The model of Murray et al. has been successfully applied to our glass and allows the determination of viscosity and activation energy for viscosity between Tg and Tc.
