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Of current interest are dense polycrystalline ceramics with <100 nm grain size. The densification of such ceramics requires additional driving forces such as pressure or electricity to suppress grain growth while promoting high density. In this study, we use a ultrahigh energy polychromatic synchrotron radiation with superb temporal resolution to study the time evolution of prototype multicomponent ceramic oxide as a function of hydrostatic pressure and electric field. Firstly, we study the temperature and pressure dependence of phase evolution in 0.5MgO-0.5Y2O3 and 0.8MgO-0.2Y2O3 IR nanocomposites using a diamond anvil apparatus in conjunction with in situ synchrotron energy dispersive x-ray diffraction at 1000 oC with 5.5-7.0 GPa hydrostatic pressure . Isothermal and isobaric hold at (1273 K, 5.5-7 GPa) for 60 min, the macroscopic shrinkage due to densification is 3% by volume which endorses densification. Furthermore, volumetric expansion around 1%, on MgO site is observed due to Y2O3 dissolving in cubic MgO despite the large differences in the ionic radii of the cations during isobaric and isothermal hold. The release of pressure at room temperature preserves the MgO lattice expansion and results in a metastable composite the cubic phase of MgO, and the cubic, hexagonal and monoclinic phases of Y2O3. Aging up to 240 h did not destroy the 4-phase co-existence. A crystallographic model is proposed due to observed volumetric expansion of the MgO unit cell based on Coulomb repulsion among O-2 ions in the vicinity of Mg+2 vacancies, and misfit strain due to differences in ionic radii. Secondly, we study the densification of 8% yttria doped zirconia (8YSZ) under superimposed thermal and electric field using time-resolved in-situ high temperature EDXRD method with a polychromatic 200 keV synchrotron probe as a function of applied electric field. Nonisothermal densification occurred in the 790–930 oC range with 3 Amps maximum current draw, resulting in 95-98 % density. No local melting at particle-particle contacts was observed in pertaining electron microscopy analysis. The onset of densification scales inversely with the applied field. Densification is accompanied by transients of high current draw, anomalous anelastic volume expansion ranging from 1% to 3%. No phase transformations are observed. We attribute the reduction in densification temperature and time to ultrafast ambipolar diffusion of species arising from the superposition of mass fluxes due to Fickian diffusion, thermodiffusion (Soret effect), and electromigration, which in turn are a consequence of a superposition of chemical, temperature, and electrical potential gradients. This densification mode is named field assisted sintering or “burst-mode” due to its discontinuous nature.

ETD_6247

Ph.D.

Includes bibliographical references

by Ilyas Savkliyildiz

doi:10.7282/T3B85B00

ETD doctoral

The author owns the copyright to this work.