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Sintering, Microstructure Development and Functional Properties of Ceramics

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Ceramics are typically produced by powder processes such as green forming and sintering rather than melt processes like many metals and polymers. In powder processes, ceramic powders are formed into a shape and heated to temperatures between 50 and 80% of the melting point of the material. At these high temperatures, mass transport is thermally activated and ions migrate to the spaces between the powder particles driven by the reduction of surface energy and surface curvature. This causes the piece to transform from a compact of individual powder grains to a single cohesive material. The result is a polycrystalline material with grain sizes and chemical compositions controlled by the starting powder, green forming method, and sintering conditions. The final combination of grain size, grain shape, porosity, and phase composition is called “microstructure.” The keys to sintering are controlling the shrinkage so that the part maintains the general shape produced during green forming while achieving the desired microstructure.

Microstructure development during sintering

(Top) Plotting the grain size against the density helps to understand how to tailor sintering conditions to achieve fine-grain-size, fully dense ceramics.
1. Initial random close particle packing.
2. At an intermediate stage of sintering, continuous pore channels limit grain growth.
3. At 92% density, pore channels pinch off to form isolated pores.
4. Grain growth increases rapidly in the fully dense areas, while sintering continues to eliminate the final pores. Controlling the slope during the final stage of densification is the key to obtaining nanograin-size fully dense ceramics.
5. Colorized scanning electron micrograph of a dense ceramic. Figure reprinted from 1

A fundamental tenet of materials science is that a material’s properties are controlled by both its composition and microstructure. In our laboratory, we focus on using green forming and sintering, including new sintering technologies like Field Assisted Sintering, to produce microstructures that maximize functional properties for focused applications. A typical example is the well known Hall-Petch relationship between grain size and mechanical strength where reducing the average grain size in a material leads to increased strength and hardness.

Another example of the links between microstructure and properties comes from mixed ionic conductors for oxygen separation. In these materials, both oxygen ions and electrons must transport across a dense membrane. In most cases, grain boundaries act as fast diffusion paths for oxygen ions, but they also block conduction of electrons. Therefore, engineering microstructures that maximize transport of both charged species is a nontrivial problem currently being pursued by our lab. 

Green forming is also critical to achieving the desired microstructures and properties in advanced engineering ceramics. The ice-templating team in the LSFC has developed novel methods of producing original porous ceramics, and we are working with them to understand the interactions between green forming and sintering to produce optimized microstructures.

1. Messing, G.L. and Stevenson, A.J., Toward pore-free ceramics, Science, 322, 383 (2008)