Date of this Version
LOW-DIMENSIONAL MATERIALS WITH PROTECTIVE COATINGS FOR APPLICATIONS UNDER EXTREME CONDITIONS, Q. Zou, PhD Dissertation, University of Nebraska. DigitalCommons@University of Nebraska - Lincoln, 2019
Traditional ceramic-based, conductive materials used under extreme conditions are severely limited due to their conditional electrical conductivity and poor stability under harsh circumstances. Advanced composite structures based on vertically aligned carbon nanotubes (VACNTs) and high-temperature ceramics are expected to address this grand challenge, in which ceramic serves as a shielding layer protecting the VACNTs from the oxidation and erosive environment, while the VACNTs work as a conductor. However, it is still a great challenge to fabricate VACNT/ceramic composite structures due to the limited infiltration of ceramics inside the VACNT arrays. Hence developing a feasible method to infiltrate ceramics into VACNT arrays is necessary. The following research topics are covered in this dissertation including 1) Thermal CVD growth of ultralong vertically aligned CNTs, 2) CVD growth of h-BN, silicon and gallium nitride thin films 3) Infiltration of Si, GaN and BN into dense VACNT arrays, 4) Defect free coating of 2D materials by sputtering and RTP, 5) Evaluation of thermal stability enhancement of the CNT-ceramic composites.
Ultralong vertically aligned CNTs are fabricated using two different methods in this study. The mechanism and kinetics of which are discussed and compared. In addition, different level of infiltration was achieved via different CVD methods. As an example, in laser-assisted CVD, ceramics were quickly deposited at the VACNT subsurfaces/surfaces, resulting in noninfiltrated composite structures. Unlike laser-assisted CVD, thermal CVD activated the precursors inside and outside the VACNTs simultaneously, which realized uniform infiltrated VACNT/ceramic composite structures. Similar levels of infiltration were achieved in the case of boron nitride and silicon, and anisotropic conducting behavior exhibited by CNT-BN was discovered. The mechanism and kinetics of infiltration into VACNT/ceramic composites, which we attributed to the different temperature distributions and gas diffusion mechanism in VACNTs, were investigated. More importantly, the as-fabricated composite structures exhibited excellent multifunctional properties, such as excellent antioxidative ability (up to 1100 °C), high thermal stability (up to 1400 °C) and good high-velocity hot gas erosion resistance. As an extension to the CNT coating techniques, defect generation and removal of 2D materials via sputtering and RTP was studied as well.
Adviser: Yongfeng Lu