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Mixtures of elementary oxides, MgO–Al2O3, were used to fabricate transparent polycrystalline magnesium aluminate spinel specimens by means of the spark plasma sintering technique. A sintering aid, 1 wt% of LiF, was added to the mixed powder. The presence of the additive promotes the synthesis of spinel that starts at 900°C and is completed at 1100°C. The LiF additive wets spinel on its melting and promotes densification, which is completed at 1600°C. LiF vapor plays a cardinal role in eliminating residual carbon contamination and in the fully dense state, allows attaining a 78% level of optical transmittance. The optimal conditions for achieving adequate transparency were determined and the role of the LiF addition in the various stages of the process is discussed.

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The effect of TiO 2 and of Ti additions on the sintering behavior of B 4 C was studied in the 1800 ЊC to 2190 ЊC temperature range. According to thermodynamic predictions that were verified by the experimental results, TiB 2 is formed in situ in both instances. Titanium oxide is reduced by carbon that originates in B 4 C and decreases its carbon content. The activity of carbon in the boron carbide phase is reduced significantly with decreasing carbon content. By sintering at 2160 ЊC for 1 hour, a mixture of fine-sized B 4 C with 40 wt pct TiO 2 is transformed into a 95 pct dense two-phase composite that consists of substoichiometric boron carbide and TiB 2 and displays a bending strength of 420 MPa. The significantly improved sintering behavior of this composite ceramic is attributed to the enhanced mass transport processes that takes place in substoichiometric B 4 C. The low carbon activity values in substoichiometric B 4 C ensure that no deleterious carbide phases are produced by reaction with molten metals.