Multi-functional Ceramic Composites of Boron Nitride
Graduate Student: Celia Chari
Hall-effect thrusters used in electric propulsion systems have historically required a dielectric such as hexagonal boron nitride (h-BN) that has good thermal emissivity, is chemically inert and is resistant to thermal shock. However, the brittle nature of bulk BN poses challenges under dynamic loads, such as the launch environment, where the stiff material is subject to vibration and strain that can lead to catastrophic failures. Graphite is a less expensive and more compliant (less stiff) alternative to bulk BN, with a greater resistance to thermal shock as well as excellent thermal emissivity. This project is a collaboration with investigators at NASA’s Jet Propulsion Laboratory to advance a new concept for inclusion of a dielectric through use of BN layers on graphite. In this way, the most desired properties for thrusters can be combined into a single composite material.
Hexagonal BN has a similar structure to graphite, with the two materials having comparable lattice parameters and coefficients of thermal expansion. Nevertheless, the two materials have vastly different electrical and oxidative properties. For example, h-BN is commonly used for its insulating properties and its resistance to oxidation up to 900 °C. In contrast, graphite is used for its conductive properties and superior processability, yet it is susceptible to oxidation at temperatures close to 600 °C. Despite their essential differences, it is possible to couple h-BN and graphite to form functional systems that can combine the distinct properties of each material. Such a layered system can be examined not only as an insulator-conductor pair, but also as an oxidation-resistant coated substrate, allowing for its multifunctionality.
The h-BN dielectric is grown on the surface of graphite through the high-temperature carbothermal reduction of boric acid in nitrogen (taking place at 1600 °C), where the surface of graphite is replaced by a layer of h-BN. This results in a layered system that is oxidation-resistant and insulating on the surface due to the h-BN, and compliant as well as conductive in the substrate due to the structural body of graphite. We are particularly interested in studying the interface of the h-BN/graphite system to assess the chemical and mechanical adhesion of the coatings. Our studies use chemical characterization methods (e.g. X-ray diffraction, energy dispersive X-ray spectroscopy), imaging methods (e.g. optical microscope, scanning electron microscope, transmission electron microscope) and electrical and mechanical tests (e.g. resistivity measurements, fracture tests, thermal shock tests).
Additionally, we are interested in the environmental degradation of the composite in space, and in coupling the material with oxides to make novel self-healing ceramics. The rising popularity of h-BN in aerospace and automobile applications has heightened the demand to study how its volatile and highly reactive oxidation product (B2O3) affects the chemical and mechanical properties of ceramic systems. The oxidation products of SiC-BN ceramic systems are known to react to form a borosilicate glass that, due to the fluidity of the B2O3 liquid oxide phase, is capable of consolidating microcracks within the composite material, displaying healing-like behavior. This project seeks to explore the expanded role of B2O3 in the healing of ceramics by investigating BN composites.
SEM micrograph of h-BN layer on graphite substrate