**2.2.1.2 Mechanical milling**

High energy ball milling process has been demonstrated to be a promising method for the preparation of amorphous powder with fine microstructure. The amorphization reaction during mechanical milling is usually attributed to microstructural breakdown followed by the interdiffusion of elements (Johnson, 1986) or mechanically driven atomic mixing (Lund et al., 2004 & Delogu et al., 2005) among previously formed nanocrystalline multilayers. In mechanical milling experiments, the kinetic process of the amorphization reactions usually proceeds slowly and therefore a glass forming composition is determined only after milling for an extended time (usually >100 h) (Eckert et al., 1997, Schurak et al., 1999 & Choi et al., 2006) (Fig. 3). The ability to synthesize a dense material from amorphous powders obtained by mechanical-alloying has recently been demonstrated for many systems regarding metallic glasses (Yan et al., 2008, Patil et al., 2005 & Kim et al., 2005). Also, amorphous powders in the systems Li2S-P2S5 or AgI-As2Se3 have been synthesized using the same method for solid electrolyte applications (Hayashi et al., 2001, Sekine et al., 2007 & Trevey et al., 2009). Few papers report the amorphization of metallic elements used in the manufacture of IR glasses; they are limited to the study of Ge-Se powder but none of these papers are relevant for the production of optical devices (Shirakawa et al., 2001 & Machado et al., 2005).

A Novel Approach to Develop Chalcogenide Glasses and

Temperature

Rate

Teq

where E is the activation energy and k0, a frequency factor.

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 285

���� × exp (��

Without nucleation, crystal growth cannot happen and without growth, no crystals can appear. These processes must take place within a certain range of temperatures which is critical for the devitrification (Fig. 4b). Above the melting temperature, Tm, the liquid constitutes the stable phase. When the liquid cools down, the crystal growth can theoretically happen between T2 and T4. However, the initial nuclei needed before the crystal growth can theoretically happen between T1 and T2. The critical range is therefore

(a)

Time

(b)

Temperature

 Nucleation only Nucleation and growth simultaneously

Growth only

Fig. 4. (a) Example of TTT curve, (b) Nucleation and growth phenomena as a function of

temperature.

between T2 and T3 (in the case of non addition of nucleating agents in the glass).

**50% 90%**

**0% transformed**

��) (2)

Fig. 3. (a) Evolution of the 80 GeSe2 - 20 Ga2Se3 powder color with the milling duration (b) XRD patterns of the corresponding powders obtained at different milling durations showing the progressive amorphization of the powder.

#### **2.2.2 Glass-ceramics**

Glass-ceramics synthesis, or the devitrification process of the glassy matrix, implies a twostep procedure: nucleation and crystal growth.

Nucleation represents the first step of glass devitrification. It consists in inducing germinate from which the growth can start. It is based on kinetic parameters. Nucleation may be homogeneous or heterogeneous. In homogeneous nucleation, the first tiny seeds are of the same constitution as the crystals which grow upon them, whereas, in the case of heterogeneous nucleation, the nuclei can be quite different chemically from the crystals which are deposited. Some substances enabling or hastening bulk nucleation can be added to glass composition; they are termed nucleating agent. We can distinguish two types of nucleating agents. Metallic nucleating agent such as Au, Cu, Pt, etc, are added to the glass in very small amounts (0.01 to 1% mass). The mechanisms of the effect of these nucleation agents in increasing the nucleation rate of the principal crystalline phase is quite complex but is based on heterogeneous nucleation. A second group of nucleating agents including TiO2, ZrO2, SnO2, P2O5 (in the case of oxide glass-ceramics) or metallic sulphide can be added in greater amounts (mostly up to 20%) in order to get oxide glass-ceramics. They are part of the oxide glassy composition and they are found to be effective nucleating agents in specific initiation of bulk nucleation.

The so called TTT curve (Time, Transformation rate, Temperature) can predict the time needed to crystallize a fraction of glass at one given temperature. The advantage of the TTT curve (Fig. 4a) lies in the fact that it permits the determination of a critical point for which the time needed for crystallization is minimal and the temperature for instability is maximal. Avrami's equation permits to build TTT curve (Avrami, 1939).

$$\pounds = 1 - e^{-f} \quad \text{with } f = (kt)^n \tag{1}$$

Where n is the Avrami exponent. This equation is valid if the nucleation is monotonous.

It is assumed that k, the rate constant, varies with time according to the Arrhenius law:

(a) (b)

Glass-ceramics synthesis, or the devitrification process of the glassy matrix, implies a two-

Nucleation represents the first step of glass devitrification. It consists in inducing germinate from which the growth can start. It is based on kinetic parameters. Nucleation may be homogeneous or heterogeneous. In homogeneous nucleation, the first tiny seeds are of the same constitution as the crystals which grow upon them, whereas, in the case of heterogeneous nucleation, the nuclei can be quite different chemically from the crystals which are deposited. Some substances enabling or hastening bulk nucleation can be added to glass composition; they are termed nucleating agent. We can distinguish two types of nucleating agents. Metallic nucleating agent such as Au, Cu, Pt, etc, are added to the glass in very small amounts (0.01 to 1% mass). The mechanisms of the effect of these nucleation agents in increasing the nucleation rate of the principal crystalline phase is quite complex but is based on heterogeneous nucleation. A second group of nucleating agents including TiO2, ZrO2, SnO2, P2O5 (in the case of oxide glass-ceramics) or metallic sulphide can be added in greater amounts (mostly up to 20%) in order to get oxide glass-ceramics. They are part of the oxide glassy composition and they are found to be effective nucleating agents in

The so called TTT curve (Time, Transformation rate, Temperature) can predict the time needed to crystallize a fraction of glass at one given temperature. The advantage of the TTT curve (Fig. 4a) lies in the fact that it permits the determination of a critical point for which the time needed for crystallization is minimal and the temperature for instability is maximal.

Where n is the Avrami exponent. This equation is valid if the nucleation is monotonous. It is assumed that k, the rate constant, varies with time according to the Arrhenius law:

ݔൌͳെ݁ି with ݂ ൌ ሺ݇ݐሻ (1)

Fig. 3. (a) Evolution of the 80 GeSe2 - 20 Ga2Se3 powder color with the milling duration (b) XRD patterns of the corresponding powders obtained at different milling durations

showing the progressive amorphization of the powder.

step procedure: nucleation and crystal growth.

specific initiation of bulk nucleation.

Avrami's equation permits to build TTT curve (Avrami, 1939).

**2.2.2 Glass-ceramics** 

$$k = k\_0 \times \exp(\overline{\underline{\phantom{e}}}\_{RT}^{\overline{\underline{\phantom{e}}}}) \tag{2}$$

where E is the activation energy and k0, a frequency factor.

Without nucleation, crystal growth cannot happen and without growth, no crystals can appear. These processes must take place within a certain range of temperatures which is critical for the devitrification (Fig. 4b). Above the melting temperature, Tm, the liquid constitutes the stable phase. When the liquid cools down, the crystal growth can theoretically happen between T2 and T4. However, the initial nuclei needed before the crystal growth can theoretically happen between T1 and T2. The critical range is therefore between T2 and T3 (in the case of non addition of nucleating agents in the glass).

Fig. 4. (a) Example of TTT curve, (b) Nucleation and growth phenomena as a function of temperature.

A Novel Approach to Develop Chalcogenide Glasses and

devitrification do not present any crystallization peak.

Tc and Tg and better is the stability against devitrification.

optical window is limited by the multi-phonon absorption.

localized states participate in the absorption process.

Fig. 6. Typical DSC curve for a glass.

 **exo** 

**2.3.2 Optical properties** 

**2.3 Physical properties 2.3.1 Thermal properties** 

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 287

Thermal characteristics of a glass, such as glass transition temperature Tg and crystallization temperature Tc, are determined using Differential Scanning Calorimetry (DSC). Fig. 6 represents the thermogram heat flow versus temperature for one given glass undergoing crystallization phenomenon (exothermic peak). Glasses which are stable against

The glass transition temperature, Tg, is the main characteristic of a glass. Before Tg, the viscosity is infinite (solid state), at Tg, the viscosity is equal to 1013 poises (1012 Pa.s) and after Tg, the viscosity decreases as the temperature increases, therefore, the material can be easily shaped. The crystallization phenomenon is characterized by the rearrangement of atoms in organized lattice due to the change of viscosity. Crystallization is at the origin of the loss of the viscoplastic properties as well as the optical transparency. The stability against devitrification is associated with the difference Tc-Tg. The higher is the difference between

Optical transmission of glasses is characterized by its optical window. At shorter wavelengths, the band gap limits the optical window while at longer wavelengths the

The band gap results from electronic transition inside the glass. Photons with sufficient energy are absorbed by exciting electrons across the forbidden band-gap. The electrons are excited from the top of the valence band to the bottom of the conduction band. In glasses, additional states exist just above the valence band and just below the conduction band. These states are present because the disorder creates localized electronic states. These

The common treatment to obtain glass-ceramics is an appropriate thermal treatment from a base glass. The thermal treatment used in industry to get this material is a "two plateaus" treatment. It consists of heating the glassy matrix (base glass) at a temperature above the glass transition temperature, Tg in order to induce nuclei in the glass. The temperature is then increased to a second plateau to induce the growth of these nuclei. A second technique consists in a single plateau. The glass is heated at a temperature above Tg but below the crystallization temperature, Tc. This technique allows the nucleation phenomenon and avoids excessive growth.

Oxide glass-ceramics are by far the most studied glass-ceramics. They have been widely investigated since 1950 and the research associated to this area is now slowing down. Today, research is more focused on the nucleation and growth phenomena to have a better understanding. However chalcogenide glass-ceramics still remain of great interest because of their transparency in the infrared range associated to better mechanical properties. As previously mentioned, potential applications are infrared lenses for thermal camera.

Chalcogenide glass-ceramics transparent in the range 8-12 µm were first synthesized in 1973 by Mecholsky in the system 0.3 PbSe-0.7 Ge1.5As0.5Se3 with a 60% crystalline fraction. He showed that the glass-ceramic modulus of rupture was increased to as much as twice that of the base glass and the Vickers hardness increased by 30% (Mecholsky et al., 1976).

Other researchers worked on systems such as As-Ge-Se-Sn (Cheng, 1982), Ga-Ge-Sb-Se (Ma et al., 2003) or Ge-Te-Se (Song et al., 1997) but the reproducibility of the glass-ceramics synthesis remained difficult.

First chalco-halide glass-ceramics, transparent in the far infrared (10 µm) was obtained in 2003 within the system GeS2-Sb2S3-CsCl in the "Glass and Ceramic" laboratory in Rennes (France) (Zhang et al., 2004) (Fig. 5). The simultaneous presence of ionic and covalent compounds prevent from the rapid and uncontrollable crystallization. Three years later, glass-ceramics transparent until 14µm, covering the second and third atmospheric windows entirely, were synthesized in the system GeS2-Ga2Se3-CsCl (Calvez et al., 2007).

Fig. 5. Glass composition 62.5GeS2-12.5Sb2S3-25CsCl heated at 290°C for different crystallization times (a) No thermal treatment (b) 7h (c) 73h and (d) 144h.
