Sintering is the densification of a powder-compact by solid-state diffusion. It requires moving nearly half the mass to fill the pores; hence several hours at high temperatures, typically above 0.7TM are needed. Superimposition of electrical and electromagnetic fields can hasten the process quite dramatically: for example, in flash sintering, zirconia which nominally requires several hours above 1400°C, can be sintered at furnace temperatures well below 1000°C in mere seconds. Field-assisted sintering can be an enabling technology for complex oxides which may decompose or suffer from phase transformations during conventional sintering.
The objective of this tutorial is to discuss the scientific themes that may form a common ground for understanding flash, microwave and SPS. They build upon the established theory of sintering, which explains the role of the grain size, surface energy, externally imposed stresses and the time-and-temperature dependence through the Arrhenius nature of chemical diffusion. The tutorial will seek to address how electrical fields can produce an enhancement, sometimes spectacularly so, in the rate of sintering. These mechanisms include Joule heating through I^2R and dielectric loss, the generation of defects and the effect of electrical fields on grain growth.
The practice of field-assisted sintering requires an understanding of the nuances and caveats of the experimental techniques. These are particularly important in flash sintering where the field is applied with two electrodes directly to the specimen using a conventional furnace. In this instance the furnace temperature, and the exquisite control of the voltage and current applied to the specimen, at the millisecond timescale, is needed. The understanding of how these process parameters influence the density, the grain size and phase evolution in the specimen is needed to develop the process for technology applications, such as solid-state electrolytes for lithium-ion batteries.
Recent work is emphasizing the influence of field-assisted sintering parameters on both mechanical and functional properties of ceramics. Significant discoveries are emerging which will be discussed.
This half-day tutorial will cover the following topics: (i) a brief summary of mechanisms of conventional sintering; (ii) phenomenological results from microwave, SPS and flash sintering on different materials systems; (iii) the discussion of mechanisms based upon Joule heating, grain growth and defect physics; and (iv) technological impacts, present and future, of field-assisted sintering methods. The attendees are encouraged to be familiar with the following paper as a preliminary background for the tutorial: R. Raj, M. Cologna, and J.S. Francis, "Influence of externally imposed and internally generated electrical fields on grain growth, diffusional creep, sintering and related phenomena in ceramics," J. Am. Ceram. Soc. 94, 1941–65 (2011).
3:00 pm BREAK
- Rishi Raj, University of Colorado Boulder
- Eugene Olevsky, San Diego State University