Anisotropic Thermal Expansion in CaAl 4 O 7
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Crystals of CaAl4O7 (space group C2/c) have been examined by high-temperature powder x-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 1473 K. The lattice deformation induced by thermal expansion has been investigated by matrix algebra analysis to determine the directions and magnitudes of the principal distortions. During heating from 298 K, two of the three principal distortions, 1 and 2, invariably showed a positive thermal expansion, the magnitudes of which at 1473 K are respectively approximately 1.0% and approximately 0.8%. The direction of 1 was nearly parallel to [304¯], and the 2 axis invariably coincides with the crystallographic b axis. On the other hand, the distortion 3 showed a negative thermal expansion below approximately 1173 K; the maximum contraction of approximately 0.2% occurred at approximately 673 K nearly along [207]. During further heating to 1473 K, 3 showed, to the contrary, a positive expansion of approximately 0.2% nearly along [50, 16]. The negative thermal expansion along the 3 axis substantially accounts for the mean linear thermal expansion being very low at temperatures below approximately 473 K.
I. INTRODUCTION
Thermal expansions of ceramics have been categorized by Roy et al.1 for convenience into three broad groups: very low expansion (␣m < 2 × 10−6), low expansion (2 × 10−6 < ␣m < 8 × 10−6), and high expansion (␣m > 8 × 10−6), where ␣m (K−1) is the mean linear thermal expansion coefficient. For the CaAl4O7 crystal, the temperature dependence of ␣m has been measured using a dilatometer over the ranges from approximately 293 up to 1173 K.2,3 The crystal showed a low thermal expansion behavior of ␣m 艋4.1 × 10−6. This material therefore has bright prospects as a new non-silicate refractory.2– 4 When a crystal is uniformly heated, it homogeneously deforms and as a result expands uniformly. Because of anisotropy of the crystals except for the cubic system, the thermal deformation depends on direction. If the initial crystal has a spherical shape, the resulting crystal after heating will be ellipsoidal. The deformation has been described by the linear thermal expansion coefficients, ␣i (i ⳱ 1, 2, and 3), along the mutually perpendicular principal axes of the ellipsoid.5,6 With crystals belonging to the tetragonal, trigonal, hexagonal, and orthorhombic systems, the principal axes are exactly parallel to the crystallographic axes. However, with crystals belonging to the monoclinic system, one of the principal axes of deformation is parallel to the diad axis (b axis for second setting), and the two others do not necessarily coincide with the crystallographic axes. 1050
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J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 16 Mar 2015
In this study, the thermal expansion of CaAl4O7 monoclinic crystals has been followed in situ by hightemperature x-ray diffractometry to determine their accurate cell dimensions. The temperature dependence of the principal distortions as well as ␣i has been evaluated by matrix
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