**1. Introduction**

Aluminum oxide (Al2O3) commonly referred as to alumina is one of the most widely used as engineering oxide ceramics. From crystalline structure difference, there are many forms of Al2O3 (α, χ, η, δ, θ, γ and ρ), with α-Al2O3 being thermodynamically the most stable form. An example of α phase of Al2O3 is corundum or sapphire [1]. In the present chapter, α-Al2O3 is discussed and described as Al2O3. With a high melting temperature, chemical stability, Al2O3 is leading to applications as high-temperature components, catalyst substrates and biomedical implants. Al2O3 has excellent optical transparency and along with additives such as chromium and titanium, it is important as a sodium lamp (sapphire), a gem stone (sapphire and ruby) and a laser host (ruby).

Usually, Al2O3 ceramics were produced by sintering Al2O3 powder, that is, polycrystalline Al2O3. Sintered polycrystalline Al2O3 ceramics were opaque because of light scattering by closed pores and grain boundaries. In order to fabricate transparent polycrystalline Al2O3, many sintering techniques have been studied such as hot-pressing (HP), hot isostatic pressing (HIP), microwave sintering and pulsed electric current sintering (PECS).

PECS is also known as spark plasma sintering (SPS) or plasma activated sintering (PAS). The sintering technique is the latest pressure-sintering process to consolidate advanced materials such as ceramics, metallic materials, composites, polymers, semiconductors and oxide superconductors, in which the powder is heated by the application of electric current under uniaxial pressure.

PECS is a promising sintering technique for producing transparent polycrystalline Al2O3. In the present chapter, progress in PECS for transparent Al2O3 was discussed as well as other

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oxides. Fundamentals of PECS were also discussed in the present chapter in order to under‐ stand PECS for transparent Al2O3.
