Event Details
In-situ Studies of Crystal Chemistry and Structure using High Temperature X-ray and Neutron Diffraction
Presenter: E.A. Payzant - Oak Ridge National Laboratory, Oak Ridge, TN
Supervisor:
Date: Mon, April 8, 2002
Time: 11:00:00 - 12:00:00
Place: EOW 430
ABSTRACT
Abstract:
High temperature X-ray powder diffraction (HTXRD) and neutron powder diffraction (HTNPD) are powerful characterization tools for in-situ characterization of such phenomena as reaction kinetics, phase transformations, grain growth, lattice expansion, and evolution of microstresses in polycrystalline materials. Experimental facilities for controlling and changing the sample environment at high temperature extend the utility of these methods for studying the effects of real-world oxidizing, inert, humid, corrosive, or reducing conditions on materials processing. Some recent HTXRD and HTNPD investigations will be highlighted in this presentation.
The thermal expansion coefficient is an important property of any engineering material. In composites and structures, differences in CTE can sometimes be useful but can lead to deformation or even failure. In non-cubic crystal structures the coefficient of thermal expansion (CTE) can be highly anisotropic and, in polycrystals, highly anisotropic thermal expansion can cause high degrees of strain or microcracking. Ternary molybdenum silicides were studied using HTXRD to experimentally confirm a reduction in CTE anisotropy predicted from theory.
In the past decade new ceramic oxides have been discovered which have negative thermal expansion, and which are stable over a considerable temperature range. Such oxides may be used to fabricate bulk materials with precisely engineered thermal expansion coefficients. Time-resolved HTXRD was employed to study the formation and decomposition of cubic ZrMo2O8 as a function of time and temperature in order to establish a "useful lifetime" of the material as a function of temperature.
Solid state electrolytes based on alkali earth doped bismuth oxide are more conductive than yttria stabilized zirconia (YSZ) and lanthanum gallate, and have potential applications in solid oxide fuel cells and oxygen separation membranes. Both HTXRD and HTNPD were employed to study the system Bi2O3-CaO, which provided a detailed picture of the crystal structure and the distribution of cations, anions, and vacancies on the available sites as a function of composition and temperature.