X-ray absorption spectroscopy studies on the carbothermal reduction reaction products of 3 mol% yttria-stabilized zirconia

Abstract

Extended X-ray absorption spectroscopy (EXAFS) at the Zr K edge has been used to determine changes in various bond lengths in 3 mol% yttria-stabilized zirconia (YSZ) during zirconium carbide (ZrC) formation. The principal objective of this study was to determine if ZrC formation at the YSZ/carbon interface alters the zirconia structure. A mixed-phase sample (YSZ and graphite) was carbothermally reduced to form ZrC. X-ray diffraction phase quantification by Rietveld analysis confirmed the formation of $50% ZrC in the analyzed sample volume. EXAFS data of ZrC and YSZ powders and a sintered YSZ pellet ($96.7% density) were used as standards to compare with the carbothermally reduced sample. Ab inito calculations using these spectra quantified various Zr-O, Zr-C and Zr-Zr bond distances in the system. Best fit results revealed Zr-O I (tetragonal), Zr-O (monoclinic), Zr-Zr (tetragonal) and Zr-Zr (monoclinic) bond length values of 2.10, 2.25, 3.65 and 3.52 Å , respectively, in the YSZ powder, Zr-O I (tetragonal) and Zr-Zr (tetragonal) bond length values of 2.12 and 3.62 Å , respectively, in the sintered pellet, and Zr-C and Zr-Zr bond lengths of 2.32 and 3.33 Å , respectively, in the ZrC powder. Similar fitting procedures were carried out on the carbothermally reduced pellet, with measured Zr-O, Zr-Zr (of YSZ), Zr-C and Zr-Zr (of ZrC) bond lengths of 2.13, 3.62, 2.36 and 3.33 Å , respectively. These bond lengths indicate that the formation of ZrC in the YSZ matrix does not influence the local structure when compared to pure standards. Therefore, carbothermal reduction does not induce any apparent strain or thermally induced effects on the first and second coordination shells of Zr as measured by the X-ray absorption spectra of the carbothermally reduced sample. Interestingly, the results indicated that sintering of the YSZ powder into pellets did not result in any significant change in the Zr-O and Zr-Zr distances for tetragonal YSZ.