Bentonite-derived high-temperature structural materials
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This thesis provides new information that is relevant to the science and applications of hot-pressed bentonite and hot-pressed organobentonite, which are emerging high-temperature structural materials. The hot pressing involves no binder. The hardness, coefficient of friction, wear resistance and scratch resistance are greater for hot-pressed bentonite than hot-pressed organobentonite. This means that the resistance to strain-induced damage is superior for hot-pressed bentonite. Hot-pressed organobentonite exhibits a degree of lubricity. The modulus is higher for hot-pressed organobentonite than hot-pressed bentonite. The energy dissipation, deformability and degree of reversibility of the deformation are similar for hot-pressed bentonite and hot-pressed organobentonite. The values of the modulus and hardness of hot-pressed bentonite and hot-pressed organobentonite are lower than those of alumina, but are higher than those of polycrystalline graphite. The energy dissipation and deformability of hot-pressed bentonite or hot-pressed organobentonite are higher than those of alumina, but are lower than those of polycrystalline graphite. The values of the coefficient of friction of hot-pressed bentonite and hot-pressed organobentonite are higher than those of Inconel and stainless steel, and are much higher than that of polycrystalline graphite. The wear resistance of hot-pressed bentonite is similar to that of Inconel and stainless steel. The wear resistance of hot-pressed organobentonite is inferior to these, but is superior to that of polycrystalline graphite. The temperature rise upon friction/wear for hot-pressed bentonite and hot-pressed organobentonite is lower than that of Inconel, but is similar to those of stainless steel and is higher than that of polycrystalline graphite. The through-thickness relative dielectric constant is essentially equal (9) for hot-pressed bentonite and hot-pressed organobentonite. Both through-thickness and in-plane resistivities are higher for hot-pressed bentonite than hot-pressed organobentonite, due to the carbon in the latter. The through-thickness resistivity decreases with increasing frequency for both materials, particularly at frequencies below 300 Hz. The through-thickness resistivity is higher than the in-plane resistivity for both materials, such that the anisotropy is about 20 for both. The hot pressing increases the resistivity of bentonite, but decreases the resistivity of organobentonite. It increases the relative dielectric constant from 6.5 to 9.1 for bentonite and from 2.5 to 8.7 for organobentonite. These effects are attributed to the ceramic phase transformations during hot pressing and the carbon formation in case of organobentonite. The heating of bentonite particles up to 800°C increases the resistivity from 2x10 5 to 1x10 8 ohm.cm and decreases the relative dielectric constant from 41 to 6.5, due to water loss. The volume fraction of ceramic (non-organic component) in organobentonite is 0.43. The relative dielectric constant of the organic component in organobentonite is 1.73. The density of the organic component in organobentonite is 1.3 g/cm 3 .