The image on the left is 3Y-TZP sintered at 1,500°C. The image on the right is of 3Y-TZP, which had a 60 Hertz AC electric field applied to it followed by sintering at 1,250°C. Credit: Hans Conrad, NC State University.
Last year we reported on a number of papers that were published in 2010 on electric field assisted sintering, including a few by Hans Conrad at North Carolina State University. According to press releases, the application of AC or DC fields during sintering was effective in reducing processing temperatures necessary for elimination of porosity, while simultaneously reducing grain growth. The effect is athermal, i.e., essentially independent of heating rate and sintering temperature.
Two mechanisms for the effect have been proposed to explain the suppression of grain growth. One possibility is the grain boundary energy (the driving force) is reduced because of an interaction of the electric field with the space charge. The other is that there may be Joule heating at the grain boundaries.
A Rapid Communication (doi: 10:111/j.1551-2916.2011.04823.x) in the August 2011 Journal of The American Ceramic Society by Hans Conrad presents an analytical approach to testing the feasibility of the grain boundary energy reduction mechanism.
Yttria-stabilized zirconia (3Y-TZP) samples were sintered under electric field at temperatures between 800°C and 1500°C. (Process parameters are detailed in referenced papers). Grain size was measured using SEM.
Starting with the equation for grain growth, which relates change in grain size over a time interval to an activation energy term and to the driving force. Field-adjusted values are substituted in for the variables. The mathematics eventually leads to a value for the space charge potential, which the paper states “is in reasonable accord with predictions, calculations, and measurements of the potential in oxides.”
The authors conclude that the “agreement of the calculated values of the space charge potential and the grain boundary energy with theoretical considerations and actual measurements provides support [for the theory] that the mechanism for the retardation of grain growth” is related to an electric field-induced reduction of the grain boundary energy through an interaction between the applied field and the space charge.