Mullite crystallization kinetics of lanthanum doped sol–gel derived precursors

Kinetics of mullite crystallization of lanthanum-doped gels was studied under isothermal and non-isothermal conditions by using differential thermal analysis (DTA). The precursors for gel preparations are tetraethoxysilane, aluminum nitrate nonahydrate and lanthanum nitrate hexahydrate dissolved in ethanol. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to identify the phase composition of samples. Prior to crystallization, the phase separation in two microphases occurs in the gels. The first microphase was alumina-rich and the second silica- and lanthanum-rich. The crystallization kinetics of mullite was influenced by the phase separation, which became more pronounced with an increasing amount of lanthanum. The phase separation reduced the incubation time and increased the Avrami exponent. The values of activation energy for mullitization slightly decreased with the increasing amount of lanthanum, due to the change in composition of the microphases from which mullite crystallized.