The formation of phases with low or negative linear thermal expansion coefficient in porous mullite ceramics

Mullite ceramic has a low linear thermal expansion coefficient compared to other ceramics, such as alumina or zirconia ceramics.Applying porous mullite ceramics as modern refractory materials requires an extra reduction of linear thermal expansion.The main purposes of the investigation are the modification of porous mullite ceramic with WO 3 and differently stabilized ZrO 2 as well as in situ formation of crystalline phases with low or negative linear thermal expansion in such ceramic.Modified porous mullite ceramics were formed by slip casting the concentrated slurry of raw materials and were sintered at 1600 °C for 1 hour.Porosity of mullite ceramics obtained due to hydrogen gas evolution as a result of the reaction between the used aluminium paste and water.Using yttria stabilized zirconia (YSZ, 8 mol% Y 2 O 3 ), magnesia stabilized zirconia (MSZ, 2.8 mol% MgO), WO 3 , α-and γ-Al 2 O 3 , amorphous SiO 2 and kaolin allowed sintering the mullite ceramics with additional in situ formed crystalline phase of ZrSiO 4 and Al 2 (WO 4 ) 3 , which decreased the linear thermal expansion of certain porous mullite ceramics' samples.Used synthesis conditions allow to achieve stability of ZrSiO 4 and Al 2 (WO 4 ) 3 phases .The differently stabilized zirconia additive had influence on the formation of ZrSiO 4 .Doubling the WO 3 content in the mixture of components increased the formation of Al 2 (WO 4 ) 3 with a negative linear thermal expansion coefficient.