AbstractThis investigation employs Anomalous Small Angle X-ray Scattering (ASAXS), and further expands the technique for conducting time resolved experiments within synchrotron facilities. ASAXS utilises the absorption effects of a given element as photon energies approach an absorption edge, and is used to extract partial scattering functions of the individual species within a composite material.
In situ ASAXS is used to explore and understand the complex reactions in sol gel processing. The gelation process for zirconia, yttria stabilised zirconia (YSZ), YSZ and zirconia in silica, and zinc-silica systems are observed using in situ ASAXS across the zirconia and zinc absorption edges respectively. A new technique of high temperature time resolved double ASAXS, which explores two absorption edges in a single experiment, has also been employed to investigate phase changes during the sintering of YSZ and YSZ in silica.
A computational model for ASAXS is also proposed, which can be used as a tool for data analysis. It is shown that monochromator resolution induces an uncertainty into the correct values of the coefficients - used to extract scattering information from individual species within a composite material. The model suggests methods to reduce this uncertainty in order to converge on the correct solution. New techniques for future time resolved ASAXS experiments are also presented.
Time resolved ASAXS of the gelation process reveals contrast in the resonant term for sol-gels containing zirconia. The profile of which is independent of the relative concentrations of yttria and silica. It is proposed that the zirconia is being fully integrated into the gel network to form zirconia-silica chains. In comparison, the zinc systems did not reveal this effect, and instead display contrast in both the resonant and cross terms, implying a gel network is forming at the exclusion of zinc.
High temperature in situ double ASAXS of the YSZ materials reveals the nucleation of nano zirconia at temperatures close to 400ºC. The nano crystals grow to eventually incorporate yttria, preserving the zirconia in the cubic and tetragonal phases. Inhomogeneities are revealed during crystal growth; caused by the mixed phase states of zirconia and YSZ present in the material. It is also shown that the presence of a silica matrix delays the growth of these crystals until a temperature of 780 ºC is reached. Samples with high concentrations of yttria also reveal an additional feature beyond 900ºC indicating a saturation point for cubic YSZ formation.
|Date of Award||18 Nov 2010|
|Sponsors||Engineering and Physical Sciences Research Council|
|Supervisor||Rudolf Winter (Supervisor) & Xing Li (Supervisor)|