Publication Date:
2017-06-13
Description:
The evolution of metric parameters of 2:1 and 3:2 mullites have been measured between 4 K and 1320 K using neutron and X-ray powder diffraction. Negative thermal expansion was observed at low temperature for the a -cell parameter and consequently for the cell-volume, which is more pronounced for 2:1 mullite than those for 3:2 mullite. Each parameter is simulated using Grüneisen first-order approximation for the zero pressure equation of state at 0 K, where the vibrational energy was calculated using microscopic approach. While the b - and c -cell parameters require only one Debye term, a second Debye spectrum with negative Grüneisen parameter was required to fit the a -cell parameter as well as the cell volume. At 4 K, 300 K and 1320 K the model respectively calculates the volume thermal expansion coefficients of 0.09x10 -6 K −1 , 9x10 -6 K −1 and 17.3x10 -6 K −1 for 2:1 mullite, and 0.09x10 -6 K −1 , 8.7x10 -6 K −1 and 17.3x10 -6 K −1 for 3:2 mullite. Temperature-dependent Raman spectra and phonon density of states hint for the possible microscopic sources of the cell contraction at low temperature. A simple polynomial approach is presented to calculate the elastic stiffness coefficients of the 3:2 mullite, which are not available from experiments. This article is protected by copyright. All rights reserved.
Print ISSN:
0002-7820
Electronic ISSN:
1551-2916
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
