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MSE_442_Lecture_16_Oct 30 2013

Process of diusion involving atoms ions or molecules q

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Unformatted text preview: is the equilibrium constant for the reacHon q༇  In pracHce, a chemical reacHon is hardly ever used deliberately to drive the densificaHon process in advanced ceramics because microstructure control becomes extremely difficult when a chemical reacHon occurs concurrently with sintering 6 Defects in Solids q༇  The driving forces provide thermodynamic reasons for sintering q༇  The actual occurrence of sintering requires transport of mager, which in crystalline solids occurs by a process of diffusion involving atoms, ions, or molecules. q༇  Crystalline solids are not ideal in structure. At any temperature they contain various imperfecHons called defects. It is the presence of these defects that allow diffusional mass transport to take place. q༇  Most of our discussion will be confined to point defects in ionic solids (ceramics). Line defects, commonly referred to as dislocaHons, are characterized by displacements in the periodic structure of the laice in certain direcHons. They play their most important role in the plasHc deformaHon of metals. 7 Point Defects q༇  A key disHncHon between point defects in ceramics and those in metals is that in ceramics, the defects can be electrically charged q༇  Another important disHncHon between ceramic and metallic systems is that the composiHon of the ceramic may become nonstoichiometric by annealing in a suitable gaseous atmosphere (e.g., a controlled oxygen parHal pressure). q༇  Intrinsic defects...
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