SURFACE NI2+ DIFFUSION IN SOL-GEL-DERIVED TETRAGONAL AND MONOCLINIC ZRO2 MATRICES

Tetragonal and monoclinic ZrO2 gel matrices have been synthesized from the zirconium n-propoxide-acetylacetonewater-isopropanol system. Surface Ni2+ ion diffusion in these sol-gel-derived tetragonal and monoclinic zirconia matrices have been studied with Fourier transform infrared spectroscopy, differential thermal analysis and X-ray photoelectron spectroscopy (XPS). It is found that the Ni2+ metal ion diffuses into the ZrO2 continuously over the temperature range of 400-600 degrees C for both tetragonal and monoclinic ZrO2 matrices, and later forms a thermodynamically stable Ni2+/ZrO2 solid solution at the elevated calcination temperatures. Higher Ni2+ surface contents in tetragonal ZrO2 are measured by XPS, indicating a larger specific surface area for tetragonal gel matrix. In addition to the +2 oxidation state, +3 for Ni is also detected in Ni/ZrO2 systems. The atomic ratio of Ni2+/Ni3+ varies and peaks at 700 degrees C in the tetragonal ZrO2. However, in the monoclinic case, the ratio remains constant throughout the course of experiments. Diffusion thermodynamic quantities are investigated and the diffusion activation energy determined for Ni2+ ion in ZrO2 matrices is 0.40 eV. The oxidation states of surface metals (Ni and Zr) and crystallinity of the system during the phase transformation are also addressed.