Thermal Properties of Porous Mullite Ceramics Modified with Microsized ZrO(2) and WO(3)

Mullite ceramics are well known as materials with a high temperature stability, strength and creep resistance. In this research, the effect of a modification with magnesia-stabilized zirconia and yttria-stabilized zirconia, separately, as well as in a mixture with WO(3), in 1:1 and 1:2 ratios on the thermal properties of porous mullite ceramics was investigated. The porous mullite-containing ceramics were prepared by a slip casting of the concentrated slurry of raw materials with the addition of a suspension of Al paste for the pore formation due to the H(2) evolution as a result of the reaction of Al with water. The formed samples were sintered at 1600 °C and the holding time was 1 h. The materials were characterized using X-ray diffractometry, scanning electron microscopy, mercury porosimetry, the laser flash contactless method, thermal shock resistance testing and the non-destructive impulse excitation method for determining the elasticity modulus. The modification of the porous mullite ceramic with a mixture of ZrO(2) and WO(3) oxides had a positive effect by decreasing the thermal conductivity, due to the increased porosity, in comparison to the undoped samples and samples with only ZrO(2). The doubling of the WO(3) amount in the modifying oxide mixtures improved the ceramic thermal shock resistance. The porous mullite ceramics which were modified with magnesia-stabilized zirconia (2.8 mol% MgO) and WO(3) had a lower thermal conductivity and improved thermal shock resistance than the samples with yttria-stabilized zirconia (8 mol% Y(2)O(3)) and WO(3).