Mullite from clay-reactive alumina for insulating substrate application

Clay-based mullite was prepared for insulating substrate applications. The clay and reactive alumina are the starting materials for the stoichiometric mullite prepared by reaction sintering at 1600 degreesC/3 h. MgO and boehmite were added as sintering aid to improve the density and decrease the sintering temperature. The properties of the sintered bodies, such as thermal, electrical and mechanical proper-ties of the sintered samples were studied. The boehmite (5.0 wt.%) substituted for reactive alumina mixture showed higher density (96% theoretical density). Clay and reactive alumina completely reacted at 1600 degreesC and formed mullite phase. Samples with 3.0 wt.% MgO addition showed the presence of secondary phases such as alpha-alumina and spinel. The mullite crystals formed are needle-shaped with rectangular faces. The MgO (3.0 wt.%) addition slightly decreased the thermal expansion. Dielectric constant values are almost same for the samples with boehmite or MgO addition. (C) 2003 Elsevier B.V. All rights reserved.