Nanocrystalline Ceramics Close to Stoichiometric MgAl2O4—Overview and Comparison of Traditional Manufacturing Processes with Crystallization Processes at High Pressure

Three processes for the production of ceramics close to stoichiometric MgAl2O4 are benchmarked against each other. The traditional ceramic route is based on mostly crystalline starting powder, which is converted into ceramic via shaping and heat treatment (IKTS). The other two processes are based on glasses. Partial or complete crystallization without pressure (ISC) or complete crystallization with pressure (CAU) leads to (glass) ceramics. Spinel powder is mixed with various dopants (BaO, TiO2, CaO and SrO), with the aim to reduce the grain size (IKTS). The doping results in a second, partly interfering phase, and the transmission decreases strongly due to absorption with increasing content of the added oxide. For the glass route without pressure (ISC), it is shown that a network-forming oxide (B2O3, TiO2) is needed to produce the glasses. Compared to the starting glasses, the resultant glass ceramics suffer loss of transparency due to crystallization. Using the levitation furnace, it is possible to produce amorphous glass beads from MgAl2O4 enriched with 25 wt% SiO2 without a container. The nanocrystalline ceramics synthesized from these glasses and the ISC glasses via the high-pressure route (CAU) are moderately transparent to translucent.