The Scaled-Up Synthesis of Nanostructured Ultra-High-Temperature Ceramics and Resistance Sintering of Tantalum Carbide Nanopowders and Composites
Date
2013-02
Authors
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Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
Ultra-high temperature ceramics (UHTCs) are a unique class of materials with the
potential to withstand harsh environments due to covalent bonding, which gives these
materials high melting temperatures, although decomposition temperatures should also be
considered. For example, the melting temperature of TaC is near 4000 K, but may
vaporize at lower temperratures. The high melting temperatures also make them difficult
to process without high pressures and temperatures and to achieve dense ceramics with a
nanostructure. Such materials however are appealing for aerospace technologies. The
ability to generate high density compacts and maintain a nanostructure could allow for
unprecedented control and improvement to the mechanical properties.
The goal of this work is to develop processes for the synthesis and consolidation
of nanostructured UHTCs. A self-propagating solvothermal synthesis technique for
making UHTC nanopowders is presented. The technique is fast, scalable, and requires
minimal external energy input. Synthesis of transition metal boride, carbide, and nitride
powders is demonstrated. TaC is synthesized using a range of synthesis conditions and
characterized to determine the fundamental mechanisms controlling the nanopowder
characteristics. Discussion on purification of the powders is also presented.
The sintering of TaC nanopowders produced by the solvothermal synthesis
method is performed by resistance sintering. The effects of temperature, heating rate, and
dwell time on densification and grain growth is presented. Adequate powder processing,
carbon content, volatilization, and additives are found to be critical factors affecting the
densification, microstructure, and grain growth.
The optimal range of carbon addition for minimizing oxygen content is
determined. WC and ZrC are evaluated as additives for reducing grain growth of TaC.
Secondary phases and/or solid solutions are capable of suppressing grain growth. A
unified approach to solid solution chemistries to control the densification, microstructure,
and properties of UHTCs in general is presented. This work has important consequences
on advancing the properties of UHTCs.
Description
Advisory committee members: Linda Jones, William Carty, Geoffrey Bowers. Dissertation completed in partial fulfillment of the requirements for the degree of PHD in Ceramics at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis