Using Lattice Energies to Model the Physical/Chemical Behavior of a Doped Refractory Oxide
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Introduction Burned periclase brick became a commonly used refractory material during the 1940s and early 1950s in the steelmaking industry. Unfortunately, periclase brick easily reacts with water or water vapor and results in dimensional instability, i.e., a volume expansion. This may lead to the mechanical failure of any article made from it. Considérable research has been performed in the past 30 years to suppress the hydration susceptibility of magnesia refractory. Boron has been found to be extremely effective in improving the hydration résistance of magnesia. 1 ' 2 It can be added to magnesite, brucite, light calcined magnesia or it can be deposited on post dead burned magnesia. However, the use of boron decreases the hot loading bearing properties of the magnesia and the dissolution of the boron into certain grades of steel may adversely affect their mechanical properties. Moreover, the addition of boron compounds requires a high-temperature calcination, normally higher than 1600°C, which has been proven uneconomical. Other dopants, incorporated either on the surface or in the bulk, hâve been reported to hâve various effects on the hydration susceptibility.3 The ultimate
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Figure 1. MgO supercell, consisting ofa 2 x 2 x 2 array ofMgO unit cells, in which the defects in this study were distributed. Large open circles indicate the oxygen positions and smaller circles indicate the Mg positions.
goal of the work reported hère is to détermine if there is a corrélation between the hydration susceptibility of MgO having various cation substitutions for Mg and the énergies of the resulting lattices. If a corrélation can be found then it should be possible to sug-
gest a cation or mixture of cations that will enhance the desired physical and chemical properties. The first lattice energy calculation for silicate material was reported by Hylleraas4 for /3-quartz. He determined the Madelung constant of the hexagonal crystal /3-quartz as a function of two parameters. Since then, much work has concentrated on mineralogically important silicates, a wide range of oxide and halide crystals including the alkali and alkaline earth halides and transition métal.5 Using lattice energy calculations, a variety of ordering or substitution schemes can be modeled to détermine the most likely cause of the positional disorder or to ascertain whether shortrange ordering of certain atoms is energetically favorable. Giese6 used lattice energy calculations to investigate shortrange ordering of Ta and Nb in columbite. Brown and Fenn7 and Post and Burnham8'9 calculated the lattice energy to détermine the Na positions in albite. Chamberlain et al. 10 suggested that short-range ordering of Cl" and CO2," in scapolite is energetically favorable. Cohen and Burnham11 worked on ordering (Al, Si) in pyroxenes. In récent years, Post and Burnham modeled tunnelcation displacements in hollandites using structure energy calculation and the authors also compared relative stabilities of Ti0 2 polymorphs and modeled the quartz, forsterite, and diopside
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