Doctoral Dissertations

Keywords and Phrases

Electrical conductivity; Lorenz number; Thermal conductivity; Ultra-high temperature ceramics; ZrB2

Abstract

"The research presented in this dissertation is focused on the thermal conductivity (k) of ZrB2 ceramics. The goal was to develop a better understanding of how various solid solutions and second phases affect the thermal and electrical transport in ZrB2, with a focus on the effect of C, W, and ZrC. The first study showed C additions improved densification and it was proposed that the reduction of boria was the impetus for this result. Boron carbide was formed by the reaction of excess C with reduced B and its formation was mitigated by the addition of ZrH2. This allowed the ZrB2-C binary system to be evaluated for study two. Study two showed the k of ZrB2 is reduced by C in solid solution and as a second phase due to the decrease in the electron contribution to thermal conductivity. Conductivities of 99 (25⁰C) and 76 W/m·K (2000⁰C) were obtained for the most pure ZrB2 (0.026 wt% C in solution and 0.2 vol% zirconia) produced in this study, which are the highest reported values for ZrB2 processed using commercial powders since 1980.

The third study evaluated the electrical resistivity of ZrB2 up to 1860⁰C using the van der Pauw technique. Separate linear regimes were observed below and above 950⁰C, whereas, previous studies assumed a linear relation. Finally the effect of ZrC on the (Zr,W)B2 solid solution was evaluated in study four. The formation of (Zr,W)C initially increased k, but further ZrC additions resulted in decreased thermal conductivities.

In the end, this research provides both: (1) usable information for the design of future ultra-high temperature ceramic systems; and (2) fundamental research that lays the groundwork for future studies aimed at understanding thermal transport in diboride based materials"--Abstract, page iv.

Advisor(s)

Hilmas, Greg

Committee Member(s)

Fahrenholtz, William
Huebner, Wayne
Smith, Jeffrey D.
Waddill, George Daniel

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Ceramic Engineering

Sponsor(s)

National Science Foundation (U.S.)

Publisher

Missouri University of Science and Technology

Publication Date

Fall 2014

Journal article titles appearing in thesis/dissertation

  • Effect of carbon and oxygen on the densification and microstructure of hot pressed zirconium diboride
  • Effect of carbon on the thermal and electrical transport properties of zirconium diboride
  • High temperature electrical testing of zirconium diboride
  • Thermal conductivity modifications to (Zr,W)B2 by ZrC additions

Pagination

xv, 238 pages

Note about bibliography

Includes bibliographical references (pages 226-237).

Rights

© 2014 Gregory John Kenneth Harrington, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Library of Congress Subject Headings

Zirconium compounds
Ceramic materials -- Thermal conductivity
Materials at high temperatures
Thermal stresses

Thesis Number

T 10609

Electronic OCLC #

902731589

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