Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Rishi Raj

Second Advisor

Scott Bunch

Third Advisor

Yifu Ding

Fourth Advisor

Brian Gorman

Fifth Advisor

Todd Murray

Abstract

A new method for the sintering of ceramics will be presented in detail. This method called Flash-Sintering was first reported in 2010 by Cologna et al. In Flash-Sintering an electric field is applied across a "green" sample with a pair of electrodes and the sintering is measured as a function of the field and temperature. The electric field is shown to remarkably enhance densification. Both the sintering time to achieve near full density and the temperature required are reduced substantially. These changes allow for sintering of 3m% yttria stabilized zirconia at furnace temperatures below 850°C in a matter of seconds. The objective of this dissertation is to understand the phenomenological behavior of flash-sintering. This new method is a highly non-linear event which occurs at a particular temperature for a given applied field and sintering is accompanied by an abrupt rise in the conductivity. The development of relationships between the electrical control parameters, the sintering behavior, and the evolution of the microstructure are the principal themes of this doctoral research. The present work covers the following topics: (i) The influence of uniaxial pressure applied in combination with electrical field on sintering and superplastic deformation, which show an equivalence between mechanical and electrical driving forces, (ii) A shift of the flash to a higher temperature with increasing particle size of the ceramic powders, (iii) The influence of the electric field on the incubation time for the onset of the flash in experiments carried out at isothermal furnace temperatures, and the effect of the current density immediately following the flash on densification, (iv) The relationship between electrical parameters on microstructure (grain size) evolution, (v) A comparison with the microstructure and mechanical strength of specimens prepared by conventional sintering, and (vi) Measurement of luminescence spectra, which lies in the visible range, that accompanies flash sintering. The unusually low processing temperatures and short sintering times portend a potential for a new era in ceramic manufacturing. The energy savings can be substantial, and the tooling costs can be significantly lower. The short sintering times offer the possibility of continuous rather than batch processing of ceramics. These ideas outside the entire scope of this thesis, however, they point toward the potential for broad impact of this research.

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