Understanding the phase transformation mechanism of metastable amorphous boron nitride
Description
TitleUnderstanding the phase transformation mechanism of metastable amorphous boron nitride
Date Created2018
Other Date2018-10 (degree)
Extent1 online resource (171 pages: illustrations)
DescriptionBoron nitride (BN) is an attractive ceramic material existing in various polymorphic structures such as low-density amorphous, turbostratic, explosive, hexagonal, rhombohedral phases, and high-density wurtzite and cubic phases. Among these, hexagonal (h-BN) and cubic (c-BN) forms of BN have been widely used for various industrial applications thanks to their superior properties. For instance, h-BN possesses high thermal conductivity, high oxidation resistance, and low friction coefficient in air. On the other hand, c-BN is a super hard structural ceramic with a hardness value of ~ 45-70 GPa, second only to diamond. Its superior chemical stability and oxidation resistance, as compared to diamond, makes it the material of choice in cutting and grinding tools, where diamond fails.
The chemical and structural characteristics of h-BN or c-BN strongly depend on the starting material used for their synthesis. Among the starting materials, BCNO compounds are the most common and widely used precursors for h-BN and c-BN synthesis. However, there is lack of understanding on how the chemical and microstructural characteristics of the BCNO precursors impact the formation of BNs and the chemical and microstructural characteristics of the BN formed.
To this end, first BCNO compounds with various chemical/structural characteristics have been synthesized by systematically varying starting composition (boric acid (H3BO3) - melamine (C3H6N6) ratio) and synthesis temperature. Synthesized BCNOs were then subjected to post heat or heat-pressure treatments under equilibrium or non-equilibrium environments to study the relationship between the chemical and microstructural characteristics of the BCNO compounds and their impacts on the formation and structural ordering of h-BN and c-BN. It was found that higher H3BO3 content in the starting composition promotes h-BN formation and its structural ordering. In addition, post heat and heat-pressure treatments also confirmed the above phenomena. The possible mechanisms leading to these phenomena are discussed within the framework of thermodynamic and kinetic principles.
It was also found that plasma spraying metastable BCNO compounds results in c-BN formation without applying external pressure, which, typically higher than 6 GPa, is essential for c-BN formation. Moreover, formation of metastable explosive (e-BN) and wurtzite (w-BN) phases of BN was achieved, for the first time, by emulsion detonation synthesis (EDS) method at the pressures of 5 and 7 GPa, respectively. The results suggest that EDS method has a potential to synthesis metastable and high-density phases of BN.
NotePh.D.
NoteIncludes bibliographical references
Noteby Metin Örnek
Genretheses, ETD doctoral
Languageeng
CollectionSchool of Graduate Studies Electronic Theses and Dissertations
Organization NameRutgers, The State University of New Jersey
RightsThe author owns the copyright to this work.
Statistical Profile