The possibility of using the chemical precipitation method of up-cycled ammonium paratungstate (APT) was studied and compared with the thermal decomposition method. WO3 particles were synthesized by chemical precipitation method using a 1: 2 weight ratio of APT: Di-water. For thermal decomposition, APT powder was heated for 4h at 600 oC in air atmosphere. The reaction products were characterized by X-ray diffraction (XRD), X-ray fluorescence spectrometer (XRF), particle size analyzer (PSA), and field emission-scanning electron microscopy (FE-SEM). Thermogravimetric analysis (TGA) of the upcycled APT allowed for the identification of the sequence of decomposition and reduction reactions that occurred during the heat treatment. TGA data indicated a total weight loss of 10.78% with the reactions completed in 658 oC. The XRD results showed that APT completely decomposed to WO3 by thermal decomposition and chemical precipitation. The particle size of the synthesized WO3 powders by thermal decomposition with 2 h of planetary milling was around 2 μm. During the chemical precipitation process, the particle size of the synthesized WO3 powders showed a round-shape with ~0.6 μm size.

The mullite-zirconia composites were prepared by the pressureless sintering with addition of 10~20 vol% ZrO(TZ3Y) in the fused mullite and sol-gel mullite matrix. The densification rate of sol-gel mullite was higher than that of fused mullite, and the addition of ZrO(TZ3Y) was effective on the densification of fused mullite. The enhancement of densification and anisotropic growth of mullite in ZrOadded specimen can be explained by the solid solution effect of ion in mullite. Both mechanical strength and fracture toughness of mullite-zirconia composite were enhanced compared to those of mullite. The enhancement of mechanical properties is attributed to the hinderance of grain growth and the combined toughening effects of tetra-mono phase transformation and crack deflection due to the residual stress between mullite/ZrO.