CBE30361 Lecture 25: Materials lecture 25

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The different forms have very different properties because they have different structures. Diamond cubic crystal structure (similar to zinc blende) Each carbon atom is covalently bonded to 4 others in a tetrahedron. Small crystals -- used to cut/grind other materials. Each atom is covalently bonded to three others in the layer. Weak van der waal"s bonding between the layers. Planes slide easily over one another -- good lubricant. Vacancies exist in ceramics for both cations and anions. Normally not observed for anions because anions are large relative to the interstitial sites. A paired set of cation and anion vacancies. Electroneutrality (charge balance) must be maintained when impurities are present. In highly ionic solids, dislocation motion is difficult. Resistance to motion of ions of like charge past one another. Fluxural tests -- measurement of elastic modulus, e. The results from a three point bend test appear similar to those from a tensile test.

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