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Metastable Hafnium Oxide Polymorphs: Synthetic Strategies, Local Structure Elucidation, and Applications in Dielectric and Negative-Thermal-Expansion Composites
Abstract
The Hf—O phase diagram is characterized by multiple metastable polymorphs. The polymorphs show promise in applications such as dielectric composites, random-access memory, and solid electrolytes. Intense current interest has focused on the stabilization of orthorhombic and tetragonal phases motivated by potential applications in ferroelectrics and high-κ dielectrics, respectively. However, accessing a phase other than monoclinic HfO2 at room temperature and ambient pressure represents a significant synthetic challenge. This dissertation will examine the role of thermodynamics and kinetics in stabilizing metastable polymorphs, especially technologically relevant orthorhombic HfO2, which is of interest for applications in ferroelectric RAM.
High-κ property of HfO2 is used for the design of hybrid nanocomposite electrically insulated layers given by the inevitably low dielectric constants of most polymers. However, the polymer and nanocrystals usually have poor compatibility, leading to a low percolation threshold and poor dielectric property. In this dissertation, we have developed a methodology combining the surface functionalization and olefin metathesis reaction to fabricate polymer nanocomposite thin films with a high HfO2 loading and an active interface between HfO2 and polynorbornene, which results in an excellent compatibility and outstanding dielectric properties.
Thermal insulation is another vital topic due to a serious global energy crisis. Here, we discuss the significant loss of thermal energy during the operation of oil extraction process in steam assisted gravity drainage. The energy losses occur primarily through the surfaces of pipelines as a result of the high thermal conductivity of pipeline materials and large thermal gradients. In this dissertation, we reported an insulating coating design that comprises polybenzimidazole textured with hollow glass microbubbles to handle the heat transfer through the steel pipelines or other steel surfaces. However, the failure of coatings such as delamination is often discovered with a vigorous temperature change during operations due to the accumulation of thermal stress resulting from the mismatch in thermal expansion coefficients of the substrate and the coating. Therefore, in this dissertation, we have reported a generalizable strategy— compromising positive thermal expansion with the negative thermal expansion to achieve an zero thermal expansion, and thereby, alleviate the compressive stress of the coating.
Citation
Liu, Guan-Wen (2023). Metastable Hafnium Oxide Polymorphs: Synthetic Strategies, Local Structure Elucidation, and Applications in Dielectric and Negative-Thermal-Expansion Composites. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /198858.