Atomistic Structure of Sodium and Calcium Silicate Intergranular Films in Alumina
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Atomistic structure of sodium and calcium silicate intergranular films in alumina David A. Litton and Stephen H. Garofalini Department of Ceramic and Materials Engineering, Interfacial Molecular Science Lab, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, New Jersey 08854 (Received 5 March 1998; accepted 21 October 1998)
Sodium silicate intergranular films (IGF) in contact with the [0001] basal plane of a-alumina were studied using the molecular dynamics computer simulation technique. The results were compared to previous simulations of calcium silicate and sol-gel silica IGF’s in contact with alumina. An ordered, cagelike structure was observed at the interface. Sodium ions segregated to the cages at the interfaces. Calcium and hydrogen ions were also observed to segregate to the cages in the previous simulations. The modifier ions were surrounded by more oxygen ions in the cages at the interface than in the bulk of the IGF. This explains the segregation of modifiers at the interface. Interface energy decreased as the sodium content of the IGF increased. Interface energy decreased faster as a function of Na2 O content than as a function of CaO content. However, interface energy decreased slower as a function of Na1 content than as a function of Ca21 content.
I. INTRODUCTION
Polycrystalline aluminas have widespread applications as optical, structural, and insulating materials.1,2 With the use of proper additives, high purity alumina products can be sintered until nearly pore-free, and thus translucent and strong. The Lucalox process uses MgO as an additive to produce envelopes for sodium vapor lamps.3 In the absence of these additives, commercial alumina powders undergo anisotropic and often abnormal grain growth during sintering.3–25 Anisotropic grain growth refers to growth of grains with a large aspect ratio as opposed to equiaxed grains. This occurs when certain crystallographic planes grow faster than others do. Higher grain growth rates for certain planes can be due to relatively low interfacial energies for those planes providing a higher driving force for growth or lower activation energies for growth. Abnormal grain growth refers to the presence of very large grains growing in a matrix of grains orders of magnitude finer. This occurs when the growth rates of certain grains are orders of magnitude larger than the growth rates of other grains. Under these conditions, grain boundary mobility often exceeds pore mobility, resulting in trapped pores in the final product. Abnormal grain growth is thus detrimental to the optical and mechanical properties of sintered materials. Impurities such as Si, Ca, and Na are present in commercial alumina powders unless strict measures are taken to avoid them.4 Since these elements have a very low solubility limit in alumina, they tend to segregate to intergranular regions to form (intergranular) glassy films (IGF) during sintering.3–10,12,14–18,20–24,26–31 The interface energies of alumina in contact
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