**2.3.2 Optical properties**

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 optical window is limited by the multi-phonon absorption.

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 localized states participate in the absorption process.

A Novel Approach to Develop Chalcogenide Glasses and

Fig. 8. Young's modulus for different materials (Strnad, 1986).

against weathering and a wide range of chemicals.

telluride based-glass composition (Elliott, 1991).

**3. The Pulsed Current Electrical Sintering (PCES) technique** 

telluride based-glass composition.

10 2 -10 -9 Ω -1 .cm -1

**3.1 Outline** 

Glass-Ceramics by Pulsed Current Electrical Sintering (PCES) 289

The increase of mechanical properties is maybe the most important advantage of glassceramics over glasses but glass-ceramics present other great advantages. Because of their adjustable coefficient of thermal expansion, glass-ceramics are resistant to thermal shock and permit the sealing to a variety of metals (mostly for oxide glass-ceramics). Then depending on the crystal size, glass-ceramics can be totally transparent or opaque. Oxide glass-ceramics are also used in electronic for their wide range of dielectric constants and are chosen instead of glasses for their lower dielectric losses. They are also corrosion resistant

Electrical properties of chalcogenide glasses depend on their chemical composition. They can be seen as semiconductors because they possess a band gap energy (~2 eV) characteristic of semiconductor materials (l-3 eV). This band-gap (Eg) depends on the glass composition and is smaller for tellurium based-glass composition (Eg(S)>Eg(Se)>Eg(Te)) (Table 1). The electrical conductivity of semi-conductors at room temperature is in the range

glass compositions are very low. They can therefore be seen as insulator in contrary to

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

 Eg (eV) σ (Ω-1.cm-1) As2S3 2.12 10-17 As2Se3 1.53 10-12 As2Te3 0.3 10-4 Table 1. Comparison of band-gap and electrical conductivity of sulphide, selenide and

(Kittel, 1998). The electrical conductivities of sulfur and selenium based-

In chalcogenide glasses, absorption phenomena are due to the excitation of non bonding electrons of chalcogenide chemical element: S, Se or Te. Because non bonding electrons of selenium (4s2p4) or tellurium (5s2p4) are higher in energy than bonding electrons of sulphur (3s2p4), they are more excitable. Therefore, the band gap shifts from visible with sulphurbased glass to near infrared for selenium or tellurium-based glass.

The multi-phonon absorption at longer wavelengths deals with interaction between light and vibration modes of the chemical bonds inside the glass. The phonon energy, E, is directly linked to the atoms weight and is inversely proportional (Kittel, 1998). The large atomic mass of chalcogenide elements causes the phonon vibrations to have low energies. Materials with high phonon energies have multi-phonon absorption in the mid- and nearinfrared region. This is especially true for materials with lightweight, strongly bounded atoms such as silica glasses and it limits their usefulness for infrared applications. Chalcogenide glasses typically have optical windows that extend into the far infrared beyond 12 μm (Klocek et al., 1987).

The transmission spectra (Fig. 7) illustrate the multi-phonon absorption for different types of glass (oxide, fluoride and chalcogenide glasses with S, Se and Te). The molecular weight (MW) is in the order: MW(O)<MW(F)<MW(S)<MW(Se)<MW(Te), therefore the multiphonon absorption for chalcogenide glasses is shifted toward longer wavelengths compared to oxide or fluoride based-glass compositions.

Fig. 7. Optical window of different types of glass.

#### **2.3.3 Other properties**

General observation regarding glass-ceramics compared to the derivative base glass shows an increase of toughness, stiffness and hardness as well as an interruption of the crack propagation due to crystals (Choi et al., 2003) (Fig. 8). Mechanical properties of glassceramics are influenced by several factors such as particle size and volume fraction of crystalline phase (which can be up to 90%), interfacial bond strength, differences in elastic modulus and thermal expansion between the glassy matrix and the crystals, etc.

In chalcogenide glasses, absorption phenomena are due to the excitation of non bonding electrons of chalcogenide chemical element: S, Se or Te. Because non bonding electrons of selenium (4s2p4) or tellurium (5s2p4) are higher in energy than bonding electrons of sulphur (3s2p4), they are more excitable. Therefore, the band gap shifts from visible with sulphur-

The multi-phonon absorption at longer wavelengths deals with interaction between light and vibration modes of the chemical bonds inside the glass. The phonon energy, E, is directly linked to the atoms weight and is inversely proportional (Kittel, 1998). The large atomic mass of chalcogenide elements causes the phonon vibrations to have low energies. Materials with high phonon energies have multi-phonon absorption in the mid- and nearinfrared region. This is especially true for materials with lightweight, strongly bounded atoms such as silica glasses and it limits their usefulness for infrared applications. Chalcogenide glasses typically have optical windows that extend into the far infrared

The transmission spectra (Fig. 7) illustrate the multi-phonon absorption for different types of glass (oxide, fluoride and chalcogenide glasses with S, Se and Te). The molecular weight (MW) is in the order: MW(O)<MW(F)<MW(S)<MW(Se)<MW(Te), therefore the multiphonon absorption for chalcogenide glasses is shifted toward longer wavelengths compared

General observation regarding glass-ceramics compared to the derivative base glass shows an increase of toughness, stiffness and hardness as well as an interruption of the crack propagation due to crystals (Choi et al., 2003) (Fig. 8). Mechanical properties of glassceramics are influenced by several factors such as particle size and volume fraction of crystalline phase (which can be up to 90%), interfacial bond strength, differences in elastic

Wavelength (μm)

Fluoride Sulphide Selenide

Telluride

modulus and thermal expansion between the glassy matrix and the crystals, etc.

based glass to near infrared for selenium or tellurium-based glass.

beyond 12 μm (Klocek et al., 1987).

to oxide or fluoride based-glass compositions.

Oxide

Fig. 7. Optical window of different types of glass.

**2.3.3 Other properties** 

% Transmission

Fig. 8. Young's modulus for different materials (Strnad, 1986).

The increase of mechanical properties is maybe the most important advantage of glassceramics over glasses but glass-ceramics present other great advantages. Because of their adjustable coefficient of thermal expansion, glass-ceramics are resistant to thermal shock and permit the sealing to a variety of metals (mostly for oxide glass-ceramics). Then depending on the crystal size, glass-ceramics can be totally transparent or opaque. Oxide glass-ceramics are also used in electronic for their wide range of dielectric constants and are chosen instead of glasses for their lower dielectric losses. They are also corrosion resistant against weathering and a wide range of chemicals.

Electrical properties of chalcogenide glasses depend on their chemical composition. They can be seen as semiconductors because they possess a band gap energy (~2 eV) characteristic of semiconductor materials (l-3 eV). This band-gap (Eg) depends on the glass composition and is smaller for tellurium based-glass composition (Eg(S)>Eg(Se)>Eg(Te)) (Table 1). The electrical conductivity of semi-conductors at room temperature is in the range 10 2 -10 -9 Ω -1 .cm -1 (Kittel, 1998). The electrical conductivities of sulfur and selenium basedglass compositions are very low. They can therefore be seen as insulator in contrary to telluride based-glass composition.


Table 1. Comparison of band-gap and electrical conductivity of sulphide, selenide and telluride based-glass composition (Elliott, 1991).
