All ETDs from UAB

Advisory Committee Chair

Yogesh Vohra

Advisory Committee Members

Aaron Catledge

Cheng-Chien Chen

Guoyin Shen

Vinoy Thomas

Document Type

Dissertation

Date of Award

2023

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

The high-entropy materials containing five or more constituent elements are of current interest because of enhancement in physical, mechanical, and thermal properties that are not achievable by simple elements or binary alloys. In this thesis, high pressure and high temperature synthesis studies have been carried out on High-Entropy Borides(HEB) like (Hf0.2, Mo0.2, Nb0.2, Ta0.2, Zr0.2)B2 , (Hf0.2, Mo0.2, Nb0.2, Ta0.2, Ti0.2)B2, and (Hf0.2, Ti0.2, Zr0.2, Ta0.2, Mo0.2)B2, at the Advanced Photon Source(APS) High Pressure Collaborative Access Team (HPCAT) Facility. These High Entropy materials are synthesized under high pressure and high temperature starting from a precursor powder composed of desired transition metal oxides and a source of boron. Once an AlB2 type hexagonal phase emerges it is then studied to higher pressure and temperature to establish its thermal equation of state. These studies, reaching up to 9.5GPa and 2000◦C, demonstrates these materials thermal stability along with their high hardness, with thermal EOS giving a bulk modulus, its temperature derivative, and thermal expansion coefficients. Post synthesis, these materials are then used in Diamond Anvil Cell (DAC) experiments. My static compression experiments in DAC established stability of hexagonal AlB2 phase to 220 GPa and a volume compression of 30%. A high shear strength to shear modulus ratio of 8% was measured for HEB at 65GPa in a radial DAC. In-situ Energy Dispersive X-ray Diffraction allowed for the observation of various High Entropy Boride compositions transitioning from an oxide precursor into AlB2 type hexagonal phases, with a pure boron powder boro-thermal reduction chemistry producing the purest samples. This data was used to refine chemical reactions used in the oxide precursor to reduce/remove contamination found in previous methods. Kinetic studies done utilizing the EDXD, constrained unknow parameters such as time and temperature needed for a full conversion. With these parameters bulk HEB material were synthesized for oxidization studies. Oxidization studies were carried out to 1300 ◦C in a thermogravimetric analysis, showing better oxidization resistance than both TiB2 and ZrB2. All these studies done on, (Hf0.2, Mo0.2, Nb0.2, Ta0.2, Zr0.2)B2 , (Hf0.2, Mo0.2, Nb0.2, Ta0.2, Ti0.2)B2, and (Hf0.2, Ti0.2, Zr0.2, Ta0.2, Mo0.2)B2, materials have demonstrated the potential use of HEB material in extreme conditions experienced by hypersonic vehicles on reentry in the Earth’s atmosphere.

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