The development of ceramics and composites from geopolymer precursors is leading to exciting new avenues in ceramic synthesis and composite applications. In this work, we developed, synthesized, and characterized materials derived from geopolymers for use in extreme environments.In the first section, a method of engineering tectosilicate material systems with tailorable thermal expansion was developed with the goal of designing new candidate materials for environmental barrier applications. The materials were studied with in situ high temperature synchrotron X-ray diffraction to explore the atomic mechanisms responsible for the thermal evolution of these compounds. The framework distortions identified were then systematically manipulated to give tailored thermal expansion values. Several material candidates were identified for environmental barrier coating applications.Rule of mixtures models were used to predict unit cell volumes and thermal expansion coefficients for compounds of intermediate composition. The error in such predictions did not exceed 0.19% (4.98 Å3) in unit cell volume and 0.132 10-5/°C (9.13%) in thermal expansion coefficients.In the second section, geopolymer matrix composites were developed to improve the toughness over unreinforced geopolymers. The ultimate flexure strengths in 3-point bending of fiber-reinforced and minibar-reinforced composite systems were 20% and 59% higher than that of the pure potassium geopolymer, respectively; ultimate compressive strengths were 41% and 36% lower; and fracture toughness was 936% and 4243% higher, as measured by the single-edge notched beam method.
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