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By controlling the heating rate at o101C/min during sparkplasma-sintering (SPS) processing, transparent polycrystalline spinel with an in-line transmission of 50% and 70% in the visible-and infrared-wavelengths, respectively, can be successfully fabricated for only a 20-min soak at 13001C. The high transmission can be attained by reducing the residual porosity and pore size, which was achieved by the low-heating rate. At high heating rates, many closed pores are formed due to the high densification rate during the heating process and remain as large pores around grain junctions. At temperatures 413001C, the coalescence of the residual pores and the precipitation of second phases, which are caused by rapid grain growth, degrade the transparency. The present study demonstrates that although the high heating rates have been regarded as a primary advantage for the SPS processing, the low heating rate is highly effective in attaining a high transparency in the spinel even at low temperatures and for short sintering times.

Aiming to characterize the effect of alumina dopant on transparency, powders of yttria stabilized tetragonal zirconia doped with alumina (TZ-3Y-E) are used as starting material to fabricate transparent tetragonal ZrO2by high-pressure spark plasma sintering (HP-SPS). However, low transparency of the resultant TZ-3Y-E specimens does not suggest a beneficial effect of alumina dopant although nanometric grains and high density have been achieved. The mechanism is analyzed by comparing with the results of as-sintered yttria stabilized tetragonal zirconia without alumina dopant.

Highly transparent pure alumina with an average grain size of 200 nm was fabricated by means of high‐pressure spark plasma sintering. The alumina sintered either at 950° or at 1000°C for 10 min under an applied pressure of 500 MPa had an in‐line transmission of about 64% for a wavelength of 645 nm. The application of high pressure allowed to obtain highly transparent full dense alumina at low temperatures with no considerable grain growth.

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The optical properties and microstructure are evaluated for a transparent cubic (8 mol% yttria-stabilized) zirconia (c-YSZ) prepared by high-pressure spark plasma sintering. Oxygen vacancies and residual pores are primary defects in the present transparent c-YSZ, and the combination influence of these two defects determines the optical properties. The as-sintered transparent zirconia strongly absorbs the incident light due to the formation of high concentration of oxygen vacancies. Annealing treatment in oxidizing atmosphere can improve the transparency by reducing oxygen defects. The existence of fine pores in the present zirconia causes scattering of the incident light.