Processing of nanocrystalline ceramics from ultrafine particles

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Abstract
The recent interest in the properties of ceramics with grain sizes less than 100 nm has created a need for processing routes with which to manufacture such ceramics. This article briefly reviews the variety of techniques currently used to manufacture ultrafine starting powders, compact the powders, and sinter them into bulk nanocrystalline ceramics. The unique challenges associated with processing such fine structures are discussed together with each technique. Major obstacles have included the difficulty (now largely overcome) of producing sufficient quantities of ultrafine powders; the strong tendency of nanocrystalline powders to agglomerate; the difficulty of manufacturing homogeneous, stress free compacts from ultrafine powders; and the ever present obstacle of unwanted grain growth during sintering. Despite numerous technical difficulties, researchers have managed to develop techniques for processing ultrafine powders into nanocrystalline ceramics which are both fully dense (or near fully dense) and still retain a grain size less than 100 nm. Pressureless sintering, sinter-forging, hot pressing, and hot isostatic pressing, when carried out under the correct conditions, have all been shown to be capable of producing nanocrystalline ceramics. Microwave sintering, rapid rate sintering, plasma activated sintering, and shock compaction techniques have produced near-nanocrystalline ceramics. At the root of these accomplishments is an improved understanding of, and exploitation of, the microstructural events which occur during traditional powder processing procedures.