PASCal: a principal axis strain calculator for thermal expansion and compressibility determination

This article describes a web-based tool (PASCal; principal axis strain calculator; http://pascal.chem.ox.ac.uk) designed to simplify the determination of principal coefficients of thermal expansion and compressibilities from variable-temperature and variable-pressure lattice parameter data. In a series of three case studies,PASCalis used to reanalyse previously published lattice parameter data and show that additional scientific insight is obtainable in each case. First, the two-dimensional metal–organic framework [Cu2(OH)(C8H3O7S)(H2O)]·2H2O is found to exhibit the strongest area negative thermal expansion (NTE) effect yet observed; second, the widely used explosive HMX exhibits much stronger mechanical anisotropy than had previously been anticipated, including uniaxial NTE driven by thermal changes in molecular conformation; and third, the high-pressure form of the mineral malayaite is shown to exhibit a strong negative linear compressibility effect that arises from correlated tilting of SnO6and SiO4coordination polyhedra.

Ionic Size Limits for A Ions in Brannerite (ATi2O6) and Pyrochlore (CaATi2O7) Titanate Structures ( A = tetravalent rare earths and actinides)

ABSTRACTThe lower limit of the size of the octahedral A4+ ion in the ATi2O6 brannerite structure is just smaller than that of Ce/Pu. Attempts to expand the A ion size beyond that of Th by (a) substituting a Ba ion plus two U5+ ions for three A ions or (b) substituting one Ba plus one hexavalent ion for two A ions did not succeed. Ge, Sn and Zr substitutions in the Ti site of ThTi2O6 do not exceed 0.2 formula unit in ceramic preparations. These and other coupled substitutions in the B site of ThTi2O6 showed that the average B site size could tolerate deviations of < 1%. Ce4+ is unusually stabilised in air atmospheres at temperatures close to the melting point of 1400°C in the A site of brannerite. Lattice parameter data on different endmember ATi2O6 brannerites are given. The lower and upper size limits for the eightfold A ions in the pyrochlore structure are around 0.100 and 0.117 nm respectively. A BaUTi2O7 stoichiometry did not produce a pyrochlore structure, and when fired in either argon or air yielded a mixture of BaUTiO6, whose structure is still uncertain, plus brannerite and rutile.

Thermal Expansion Of GaN And Ain

The temperature dependence of the thermal expansion and the bulk modulus are critical for predicting the residual stress distribution in epitaxial films and provides information relevant for interatomic potentials and equations of state. The thermal expansions of aluminum nitride (AIN) and gallium nitride (GaN) are calculated with two models that employ the limited elastic and lattice parameter data. These semiempirical models allow prediction of the thermal expansions to higher temperatures. Calculated results are compared with experimental data.

Preparation Techniques For Structural Characterization of Powdered and Composite Materials

ABSTRACTStandard preparation techniques such as ultramicrotomy and ion-milling are essential for obtaining accurate and detailed information about the structural and/or chemical properties of various categories of materials. The choice of a formulated matrix media has improved the preparation of thin sections of various powders ranging in degree of hardness from zinc oxide to tungsten carbide. This medium has shown superior performance when utilized in ion-milling powders and composites of high hardness such as boron carbide. This support vehicle has facilitated analysis of various geometric structures including hollow ceramic spheres. The homogeneity of chemically derived dielectric powders has been investigated by studying crystallite lattice parameter data derived from analysis of thin sections.