Thermal expansion

About: Thermal expansion is a research topic. Over the lifetime, 21040 publications have been published within this topic receiving 349407 citations. The topic is also known as: heat expansion.

The role of hydrogen bonding in the thermal expansion and dehydration of brushite, di-calcium phosphate dihydrate

Abstract: The unit-cell and atomic parameters of perdeuterated brushite have been extracted from Rietveld analysis of neutron powder diffraction data within the temperature range 4.2 to 470 K. The thermal expansion of brushite is anisotropic, with the largest expansion along the b axis due principally to the effect of the O(1)···D(4) and O(3)···D(2) hydrogen bonds. Expansion along the c axis, influenced by the Ow1···D(5) interwater hydrogen bond, is also large. The high temperature limits for the expansion coefficients for the unit-cell edges a, b and c are 9.7(5) × 10−6, 3.82(9) × 10−5 and 5.54(5) × 10−5 K−1, respectively, and for the cell volume it is 9.7(1) × 10−5 K−1. The β angle displays oscillatory variation, and empirical data analysis results in α β = 1.28(3) × 10−6sin(0.0105 T) K−1, within this temperature range. The evolution of the thermal expansion tensor of brushite has been calculated between 50 ≤T≤ 400 K. At 300 K the magnitudes of the principal axes are α11 = 50(6) × 10−6 K−1, α22 = 26.7(7) × 10−6 K−1 and α33 = 7.0(5) × 10−6 K−1. The intermediate axis, α22, is parallel to b, and using IRE convention for the tensor orthonormal basis, the axes α11 and α33 have directions equal to (−0.228, 0, −0.974) and (−0.974, 0, 0.228) respectively. Under the conditions of these experiments, the onset of dehydration occurred at temperatures above 400 K. Bond valence analysis combined with assessments of the thermal evolution of the bonding within brushite suggests that dehydration is precipitated through instabilities in the chemical environment of the second water molecule.

A kinetic study of the quartz-cristobalite phase transition

Abstract: Cristobalite is a common silica polymorph in ceramics, as it can crystallize in SiO 2 -rich systems during high temperature processes. Its occurrence in final traditional ceramic bodies remarkably affects their thermal expansion, thus playing an important role in the shrinkage upon cooling. The quartz–cristobalite transformation kinetics is investigated by in-situ isothermal X-ray powder diffraction experiments and then correlated to the average particle size (〈 d 〉) of the starting quartz using a model here developed. An Avrami-like rate equation, i.e. α ( t ) = 1 − exp(− k × t ) n , in which the n -term is assumed to account for the dependence on the average particle size, has provided the best fitting of theoretical to experimental data, yielding activation energy values that range from 181 to 234 kJ mol −1 , and exponential n -coefficients from 0.9 to 1.5. Ex-situ observations have demonstrated that the formation of cristobalite from quartz after 50 min, 2, 4 and 6 h at 1200 and 1300 °C, exhibits a remarkable dependence on 〈 d 〉 of quartz, showing comparable behaviours in the case of 〈 d 〉 equal to 15.8 and 28.4 μm, but significant differences for 〈 d 〉 of 4.1 μm. The formation of cristobalite is boosted remarkably at temperature higher than 1200 °C, with an increase by weight even of 500%, with respect to its content at lower temperature. The method of sample preparation (dry powder, wet powder and tablet of compressed dry powder) seems to influence the results only at temperature > 1200 °C and in the case of fine powder.

Thermal expansion coefficients of high‐Tc superconductor substrate NdGaO3 single crystal

Abstract: We report the temperature dependence of lattice constants for a NdGaO3 single crystal grown by the Czochralski method and thermal expansion coefficients which were obtained for the first time. Lattice mismatch between the NdGaO3 substrate and YBa2Cu3Ox superconducting film is 0.27% at elevated temperatures and the appropriate expansivities are similar. The crystalline quality of deposited YBa2Cu3Ox films on NdGaO3 by the laser ablation method was extremely good. NdGaO3 is, therefore, an excellent candidate for epitaxial high‐Tc superconducting films.