The elastic constants of UO2 have been measured as a function of pressure up to 20 kbar (2.0 GPa) at 23 °C. To within experimental uncertainty, the constants are linear functions of pressure, and no pressure-induced phase transitions occur in this 20-kbar pressure range. The elastic constant data are used to estimate the various polycrystalline elastic moduli and their pressure dependences, and the zero-pressure moduli are compared with data from polycrystalline material. Isothermal moduli are calculated from the adiabatic values. The elastic-constant data are also used to discuss the separation of the ’’pure’’ volume and temperature contributions to the (isobaric) temperature dependences of the elastic constants as well as to estimate the Grüneisen constant, Debye temperature, and equation of state of this material.
The extent of relaxation and orientation of linearly graded InxAl1-xAs (x=0.05–0.25) buffers grown on GaAs were examined using a novel x-ray diffraction reciprocal-space mapping technique (kmap). Samples were grown at temperatures ranging from 370 to 550 °C. The fractional relaxation of the buffers grown between 470 and 550 °C was essentially identical (77%) and symmetric in orthogonal 〈110〉 directions. These buffers are believed to be in equilibrium indicating that the incomplete relaxation is not a kinetic effect. The extent of relaxation was less than that expected for equilibrium relaxation in the absence of dislocation–dislocation interactions indicating that such interactions must be considered to accurately predict the extent of relaxation. The saturation of the relaxation as a function of temperature indicates that at the grading rate used (8% In/μm or 0.69% strain/μm), we are not working in a growth regime where the relaxation is nucleation limited. In addition, all the buffers are slightly tilted with respect to the GaAs substrate about [11̄0] toward the [110] direction suggesting either a bias in the dislocation types in the boule-grown GaAs, or a bias in the way in which α and β dislocations interact with unintentional substrate miscuts.
Changes of ultrasonic transit times in compression-annealed pyrolytic graphite (CAPG) were measured as a function of temperature (4–300°K at atmospheric pressure) and of pressure (0–20 kbar at 295°K). From the low-temperature results, obtained for five independent acoustic modes, the temperature variations of all five elastic constants were calculated using the thermal expansion data of Bailey and Yates. From the high-pressure data, taken for four independent modes, the changes of all five elastic constants and of the unit cell dimensions with pressure were calculated under the assumption that the small a-axis compressibility is independent of pressure. The initial pressure derivatives of C33 and C44 are, respectively, about 25 and 35% lower than those obtained by Green et al. for CAPG. The pressure dependences of the volume and c-axis compressibilities are in good agreement with measurements on natural graphite crystals by Lynch and Drickamer and by Bridgman, and in less satisfactory agreement with data of Kabalkina and Vereshchagin. The results support the contention that CAPG has bulk elastic properties very similar to those of single-crystalline graphite.
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