LaNbO₄-Addition On The Fracture Toughness And Ionic Conductivity Of The NiO/YSZ Cermet

The anode-support planar SOFC is an important developing direction in SOFC. The Ni/YSZ cermet is the most used as anode, which is achieved via the NiO/YSZ cermet at room temperature after reduction at high temperature.The toughness of the porous NiO/YSZ cermet is not enough to bear the pressure brought in the assembly of SOFCs. The breakage of the NiO/YSZ cermet is one of the important invalidation in SOFCs. The aim of the paper is to improve the toughness the NiO/YSZ cermet by addition of the thire stuff without influencing the electronic conductivity, and to create qualification for the super thin SOFC.Lanthanum orthoniobate (LaNbO4), is a ferroelastic phase of monoclinic structure at room temperature, is heavily domained in two orientations. Under an external stress, the domains switch. Upon removal of external stress, the domains switch backward. It is expected that such a phase may be used to toughen ceramics. In this paper, LaNbO4 has been used as the third phase to enhance the toughness of cermet. The main content includes the following:1) Synthesis and characterization of the LaNbO4 power2) The mechanical properties of NiO/YSZ/LaNbO4 composite3) The phase transformation and its influence on the electrical donductivity of the NiO/YSZ/LaNbO4 compositeThe decisive step in chemical precipitation of LaNbO4 preparation is to adjust the pH value of the solvent into 9~10, by which purified LaNbO4 crystal with 1~1.5μm particle size was prepared. The LaNbO4-addition results in the phase transformation of ZrO2 from cubic to monoclinic in YSZ. The KIC of LaNbO4/NiO/YSZ cermet varies with the fraction of LaNbO4 in NiO/YSZ/LaNbO4 composition: The KIC of LaNbO4/NiO/YSZ cermet decrease with the growing of LaNbO4 fraction ranging 0~10%wt, while increase with the growing of LaNbO4 fraction ranging 10~20%wt. Monoclinic ZrO2 is a fragile inpurity phase which influences the microstructure of LaNbO4/NiO/YSZ cermet. Only when the fraction of LaNbO4 increased up to 10%wt, can the thoughness improving effect of LaNbO4 balance the influence of monoclinic ZrO2, after that the toughness improving effect of LaNbO4 plays a dominant role in the mechanic properties of the material. The partial phase transformation of ZrO2 in YSZ from cubic phase to monoclinic phase also result in the decrease of ionic conductivity of the material.