Fabrication of Transparent Spinel Ceramics by Reactive Hot-pressing

Abstract

Transparent magnesia-alumina spinel ceramics were prepared from equimolar mixture of the oxides by hot-pressing. Formation of spinel from the oxide mixture accompanied with a volume expansion of about 7.9%. The expansion which could be suppressed with applying pressure was utilized to promote densification of the spinel ceramics. Results were as follows;
(1) Applying pressure higher than 330kg/cm² at 950°C, densification of the compact was promoted through suppressing the volume expansion with the spinel formation.
(2) Hot-pressing conditions, e.g. temperature, pressure, soaking time and stepwise heating schedule had marked effects on densification and microstructure of the compact. However, soaking time longer than 1h had not marked effect on the densification.
(3) To obtain the dense spinel ceramics, the most favorable oxide mixture was the one prepared by calcination of Mg(OH)₂ at 900°C for 2h and Al(OH)₃ at 1400°C for 2h.
(4) Using step heating hot-pressing technique (770kg/cm², 1300°C and 1350°C for 0.5h and 1400°C for 1h), transparent spinel ceramics having nearly theoretical density and grain size of 1 to 2μm were obtained. Their optical transmittance was 35 to 75% in the visible range and 75 to 85% in the IR range.