The Role of Composition in Ultrafine-Grain Ceramics
The importance of composition and impurities to the behavior of ultrafine-grain ceramics is discussed with emphasis on: (1) significance of large grain boundary area per unit volume; (2) nature and distribution of impurities; (3) source and elimination of impurities; and (4) analysis of impurities.
The importance of large grain boundary area per unit volume in ultrafine-grain ceramics is considered in light of our present lack of data and models for the extent, structure, and composition of any grain boundaries in ceramics. However, much evidence exists suggesting that these boundaries occupy a large volume and behave markedly differently than the bulk lattice, which would have a substantial effect in ultrafine-grain materials.
The nature and distribution of impurities in such ceramics is discussed, emphasizing the importance of anions and their especial importance in fine-grain material. These anions are expected to have large and varied effects, as indicated by theory and by meager experimental data, especially with reference to densification during fabrication. The sources of impurities in ultrafine-grain ceramics, primarily anions, are discussed in relation to the surfaces of the fine powders from which these ceramics are produced. Possible means of purification of such powders before and during further processing are presented, again with emphasis on the form and location of the impurities in the powders.
Analytical procedures for the determination of the quantity and distribution of impurities in such ceramics are discussed, along with the reasons for the long neglect of anion impurities, emphasizing the previous lack of suitable analytical techniques. Consideration is given to the success of mass spectroscopy, and its associated sensitivity, sample preparation, and handling problems.
KeywordsImpurity Distribution Boundary Volume Beryllium Oxide Polycrystalline Ceramic Chemical Analysis Technique
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