Synthesis & Fundamental Formation Mechanism Study of High Temperature & Ultrahigh Temperature Ceramics

Authors

Paniz ForoughiFollow

Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Major/Program

Materials Science and Engineering

First Advisor's Name

Dr. Zhe Cheng

First Advisor's Committee Title

Committee Chair

Second Advisor's Name

Dr. Arvind Agarwal

Second Advisor's Committee Title

Committee member

Third Advisor's Name

Dr. Surendra K. Saxena

Third Advisor's Committee Title

Committee member

Fourth Advisor's Name

Dr. Yu Zhong

Fourth Advisor's Committee Title

Committee member

Fifth Advisor's Name

Dr. Wenzhi Li

Fifth Advisor's Committee Title

Committee member

Keywords

UHTCs, borides, carbides, nanocrystalline, solid solution

Date of Defense

4-10-2018

Abstract

Borides and carbides of tantalum and hafnium are of great interest due to their ultrahigh temperature applications. Properties of these ceramics including oxidation resistance and mechanical properties might be further improved through solid solution/composite formation. Synthesis of single-phase Ta_xHf_1-xC and Ta_xHf_1-xB₂ solid solution powders including nanopowders via carbothermal reduction (CTR) is complicated due to noticeable difference in reactivity of parent oxides with carbon, and also the low solubility of those oxides in each other. Moreover, for TaC-HfC system the solid solution may go through phase separation due to the presence of a miscibility gap at temperatures below 887°C.In this study, a method of low-cost aqueous solution processing followed by CTR was used to synthesize Ta_xHf_1-xC and Ta_xHf_1-xB₂ solid solution powders. In fact, method was first used to synthesize boron carbide (B₄C) powders as it paves the way for a detailed study on the synthesis of Ta_xHf_1-xC and Ta_xHf_1-xB₂ solid solutions powders considering the fact that B₄C contains both carbon and boron in its structure. Particular emphasis was given to investigate the influences of starting compositions and processing conditions on phase separation during the formation of both carbide and boride phase(s). It was found that individual TaC-HfC and TaB₂-HfB₂ phases always form quickly but separately during the CTR process (e.g., at 1600 °C within a few minutes). Those carbides and borides remain phase-separated unless heated to much higher temperatures for long time due to the slow inter-diffusion between them. It was also found that for Ta_xHf_1-xC applying a DC electric field through the use of spark plasma sintering (SPS) system significantly accelerates the inter-diffusion of Ta and Hf leading to formation of a single-phase Ta_xHf_1-xC solid solution at 1600 °C for 15 minutes. On the other hand, for borides alkali metal reduction reaction (AMR) method appears to be an excellent alternative to CTR-based method for formation of a single-phase
Ta_xHf_1-xB₂ solid solution. In this method, chlorides of tantalum and hafnium are directly reduced using sodium borohydride (NaBH₄) giving rise to formation of a single-phase Ta_0.5Hf_0.5B₂ solid solution nanopowders in one step at much lower temperatures (e.g., 700 °C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR-based process.

Identifier

FIDC006906

ORCID

https://orcid.org/0000-0003-1521-8005

Recommended Citation

Foroughi, Paniz, "Synthesis & Fundamental Formation Mechanism Study of High Temperature & Ultrahigh Temperature Ceramics" (2018). FIU Electronic Theses and Dissertations. 3730.
https://digitalcommons.fiu.edu/etd/3730