Abstract

 Elaboration of transparent polycrystalline ceramics is an important technological challenge because single crystals exhibit a high transparency but a low mechanical resistance. Polycrystalline ceramics have good mechanical properties. Nevertheless, in order to obtain a good transparency, the following objectives must be fulfilled:

- very low porosity (less than 0.05%) with a nanometer pore size

- for the non-cubic (birefringent) polycrystalline ceramics, the grain size must be lower than about 0.5 mm for visible light

 - ceramics must be single phase materials and cannot be sintered with a liquid phase.

 The tranparency needs to use very fine powders and specific sintering techniques. The nanopowders must be pure without the presence of strong agglomerates and can be doped in order to prevent grain growth. The most used sintering technique is the pressureless sintering followed by a hot isostatic pressing (HIP).

 A new interesting technique can be also utilized: the spark plasma sintering (SPS).

 The case of polycrystalline alumina will be particularly examined.

Commercial a-alumina nanopowder (BMA15, Baikowski Chimie, France) with a purity of 99.99%, an average particule size of about 170 nm and a surface area of 14 m2/g and doped a-alumina nanopowder with 50, 150 and 300 ppm of MgO were used.

 The powders were sintered using a SPS equipment FCT HPD21 (Germany) in a graphite mold. Different temperature cycles (heating rates and sintering temperatures) have been studied.

Microstructure was observed on the fracture surface. The in-line transmission was measured with a spectrophotometer Jasco V670 as a function of the wavelength in the range 200-2500 nm.

 The results are discussed as a function of the different cycles of temperature used during SPS and the effect of MgO doping is presented.

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