A study of novel silicon carbide composites: Design, fabrication, characterization and modeling.

摘要

This dissertation focuses on the design, fabrication, experimental characterization and analytical modeling of a group of novel nanostructured and microporous SiC composites. Nanostructured and microporous morphologies provide materials with the high specific strength and low thermal conductivity required for multifunctional integrated thermal structures. Thus, this family of engineered materials serves as innovative candidate materials for NASA Space Launch Initiative and hypersonic aircraft development, as well as wide spread commercial applications.; The technical objectives were to develop a novel lightweight, high specific strength and low thermal conductivity material to serve as a critical component which joins the hot external skin to the cool inside propellant tanks and structure. These objectives were met by accomplishment of the following five tasks: (1) Conceptional design; (2) Material components identification and acquirement; (3) Fabrication of test coupons; (4) Experimental characterization of mechanical and thermal properties; (5) Development of analytical models.; By varying the processing procedure and types of reinforcements, significantly reduced thermal conductivity was achieved with the unique microstructure.; Explicit models were built for prediction of effective thermal conductivity of the materials under investigation. One is a semi-empirical model from thermal resistance theory, with which thermal conductivity can be predicted efficiently once a material constant C is determined experimentally from the matrix materials. Mathematical models based on EMA (Effective Medium Approximation) were developed to predict thermal conductivity of composites with dispersion of coated spheres when coating thickness is not negligible, or with both micron-sized and nano-sized reinforcement particles. The EMA models in present work advance traditional EMA models from prediction of two-phase material systems to three-phase composites. To the knowledge of the author, mathematical model for three-phase composites has never been explicitly solved in the literature.; FEM (Finite Element Models) were constructed to systematically study the parameters affecting effective thermal conductivity. Functional dependency of effective thermal conductivity on the constituent properties and their distribution was investigated, which would offer sufficient guide lines for material optimization. The simulation of composite structure results in good correlation with experimental data and the mathematical EMA predictive models.