Evolution of molecular structure during the sol-gel processing of lithium niobate and the development of microstructure in alkoxide-derived thin layers
Eichorst, Dennis John
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https://hdl.handle.net/2142/23102
Description
Title
Evolution of molecular structure during the sol-gel processing of lithium niobate and the development of microstructure in alkoxide-derived thin layers
Author(s)
Eichorst, Dennis John
Issue Date
1991
Doctoral Committee Chair(s)
Payne, David A.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Engineering, Materials Science
Language
eng
Abstract
Lithium niobate powders and thin-layers were prepared by sol-gel processing. Lithium niobium ethoxide and lithium niobium methoxyethoxide bimetallic alkoxides were used for both gel and thin-layer formation. Advantages of sol-gel processing were demonstrated by synthesis of LiNb(OCH$\sb2$CH$\sb3)\sb6$crystals having the proper cation ratio for stoichiometric LiNbO$\sb3$. The alkoxide deposition method also allowed for the formation of layers of uniform quality at 650$\sp\circ$C, thereby, avoiding lithium oxide volatility.
"Lithium and niobium alkoxides reacted immediately to form bimetallic alkoxides. The structure of LiNb(OCH$\sb2 $CH$ \sb3)\sb6$ consisted of niobium ethoxide octahedra linked by lithium. Studies of hydrolysis indicated a possible gelation mechanisms of cross-linking of individual lithium niobium ethoxide ""polymers"". Raman studies indicated the precursor alkoxide may be maintained during formation of an amorphous oxide, and considerable long range order developed prior to crystallization. Crystallization was followed by x-ray diffraction, infrared and Raman spectroscopies. At approximately 500$\sp\circ$C, the material crystallized directly into the R3c phase of lithium niobate. Heat-treatment above 650$\sp\circ$C gave changes associated with lithia volatility."
Lithium and niobium methoxyethoxides also reacted to produce a bimetallic alkoxide. However, lithium niobium methoxyethoxide could not be crystallized from the parent alcohol. Raman studies indicated significantly less long-range order in the methoxyethoxide specimens after similar heat-treatments. The difference in long-range order was attributed to the similarity in structure of lithium niobium ethoxide with lithium niobate. During heat-treatment, structural relaxation may provide for long range order and hence crystal nuclei. Lithium niobium methoxyethoxide, on the other hand, may require greater structural rearrangement prior to crystallization.
Ceramic microstructures depended on solution formulation in addition to heat-treatment conditions. Generally, rapid heating gave dense layers and a larger grain size, than by heating at 10$\sp\circ$C/min. Water and ammonium hydroxide additions aided densification at lower heating rates. The results demonstrate the important relationships between solution chemistry, oligomeric structures, and final microstructures.
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