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UV Raman Spectroscopic Studies On The Phase Structure And Phase Transformation Of Zirconia And Doped Zirconia

Posted on:2003-03-18Degree:DoctorType:Dissertation
Country:ChinaCandidate:M J LiFull Text:PDF
GTID:1101360122975116Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
The phase structure of zirconia is important for many catalytic reactions and materials. The bulk properties of zirconia and doped zirconia have been extensively studied. However, the structure and phase in the surface region of zirconia are not well understood. Many chemical and physical properties, especially catalytic properties of metal oxides largely depend on the phase structure in the surface region. To get further insight into the mechanism of the phase transformation of zirconia-based materials and the relationship between the surface properties and the surface phase structure, we have extensively investigated the phase transformation and surface properties of both zirconia and different stabilizers (including Y2O3 and La2O3) -doped zirconia by using UV Raman spectroscopy, XRD, XPS and IR techniques.Studies on the phase transformation of zirconia sample caclined at different temperatures using UV Raman Spectroscopy, visible Raman spectroscopy and XRD have demonstrated that UV Raman spectroscopy is a surface-sensitive tool at least for ZrO2. UV Raman spectra of ZrO2 clearly indicate that the phase transformation of ZrO2 from tetragonal to monoclinic phase takes place initially at the surface region of ZrO2 particle, then progresses into the bulk. The calcination of samples in inert and reductive gases delays the formation of crystal phase, and benefits the stabilization of the tetragonal phase. But the tetragonal phase in the surface region transforms easily to monoclinic phase when the samples are exposed to air. It is possible that oxygen vacancies formed in the surface region in the absence of oxygen can prevent the transformation from the tetragonal to the monoclinic phase. When the sample is exposed to air, the elimination of these vacancies by the incorporation of oxygen facilitates the transformation from the tetragonal to the monoclinic phase.For yttrium oxide and lanthanum oxide doped ZrO2, the results obtained from UV Raman spectroscopy, visible Raman spectroscopy and XRD indicate that the phase transformation is slowed down and the tetragonal phase can be stabilized in thebulk. However the tetragonal phase in the surface region is difficult to stabilize, particularly when the content of stabilizers is low. The surface structural tension and the enrichment of the ZrO2 component in the surface region of ZrO2-Y2O3 and ZrO2-La2O3 might be the reasons for the striking difference between the phase change in the surface region and the bulk. When the content of stabilizers is high enough in the surface region, the formation of the tetragonal phase or the transition state from tetragonal to cubic phase in the surface region can significantly prevent the phase transition from developing into bulk. The tetragonal phase is stabilized more effectively by lanthanum oxide than yttrium oxide.Based on the well characterized surface phase of ZrO2 by UV Raman spectroscopy, the IR studies on the adsportion of different probe molecules (CO, H2 and CO2) on ZrO2 with different surface phases suggest that the characteristics of the cus Zr4+ and Zr4+-O2- pair are different for both the tetragonal and monoclinic zirconia surfaces, indicating that the surfaces of the two zirconia phases possess different structures. Under the same preparation conditions, the tetragonal and monoclinic zirconia have different sets of OH groups, which indicates that the surface chemical properties are associated with the phase structure in the surface region.
Keywords/Search Tags:Zirconia, doped-zirconia, phase transformation, UV Raman spectroscopy, XRD, IR
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