Degree Type

Dissertation

Date of Award

1988

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

Mufit Akinc

Second Advisor

D. R. Wilder

Abstract

Several commercial alumino-silicate minerals doped with small amounts of light rare-earth oxides were tested for resistance to molten aluminum alloy attack. Although undoped compositions were severely attacked by aluminum alloy 7075, the addition of rare-earth oxides up to 15 percent by weight virtually eliminated the alloy attack. These rare-earth containing compositions were also resistant to the attack of Na[subscript]2CO[subscript]3 containing alloy. The dopants CeO[subscript]2, La[subscript]2O[subscript]3 and bastnasite, a rare-earth concentrate, were all effective in rendering the alumino-silicate minerals resistant to alloy attack;The addition of rare-earth oxide to bauxite was shown to alter its microstructure, forming a network of uniformly shaped corundum crystals interlinked with cerium containing strands. This unique microstructure was found to be primarily responsible for the enhanced attack resistance of the doped alumino-silicates. Several other possible mechanisms of protection were not supported by the experimental findings. Among these were enhanced sintering, the excess oxygen content of CeO[subscript]2 dopant and higher thermodynamic stability of the doped refractory compositions;The lattice energies of crystals were calculated from a computer summation of the Coulombic interaction energies. After evaluating this technique using several simple crystal systems, the lattice energies of the light rare-earth (La through Eu) pyrosilicates, RE[subscript]2Si[subscript]2O[subscript]7, were calculated. The lattice energies calculated for simple crystal systems were within 10% of the experimentally measured values. The enthalpies of formation for the rare-earth pyrosilicate were estimated from the calculated lattice energies by applying the Born-Haber cycle.

DOI

https://doi.org/10.31274/rtd-180813-11316

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Michael Hughes O'Brien

Language

en

Proquest ID

AAI8825428

File Format

application/pdf

File Size

143 pages

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