Effect of LaPO4 on thermal expansion coefficient and fracture toughness of La2Ce2O7 ceramics

Abstract

Lanthanum-cerium oxide has attracted much attention due to its low thermal conductivity, low sintering rate and stable chemical properties. However, its application in the field of high-temperature thermal insulation is seriously affected by its thermal shrinkage effect in the low-temperature region (200–400°C) and poor fracture toughness. In this work, lanthanum phosphate with weak interfacial bonding and layered structure was introduced into lanthanum-cerium oxide as the second phase to prepare lanthanum-cerium oxide ceramics with different lanthanum phosphate additions. The effects of lanthanum phosphate addition contents on the phase composition, microstructure, thermal expansion coefficient and fracture toughness of lanthanum-cerium oxide ceramics were investigated. It was demonstrated that the thermal shrinkage effect of lanthanum-cerium oxide in the low-temperature region (200–400°C) could be totally suppressed by adding lanthanum phosphate as the second phase with a loose layered structure and a weak interface with the matrix that prevented the transverse motion of the M-O bond (M = metal atoms). The system's fracture toughness doubles when 30 % lanthanum phosphate was added to the matrix. This was primarily because the lanthanum phosphate's uniform distribution throughout the matrix creates a pinning effect that encourages crack propagation along grain boundaries. Therefore, the use of weakly interfacial bonded layered compounds can optimize the fracture toughness of ceramics as well as the thermal shrinkage effect, which is an important reference for the improvement of the properties of other ceramic material systems.