tailieunhanh - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 1 Part 9

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 1 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 192 12 Irreversible Thermodynamics and Diffusion in solid-state diffusion problems the coefficients Lj are functions of temperature and pressure but they do not depend on the gradient of the chemical potential. 2. The Onsager matrix is composed in part of diagonal terms Lii connecting each generalised force with its conjugate flux. For example a gradient of the chemical potential causes a generalised diffusion force and the associated diffusion response is determined by the material s diffusivity. Similarly an applied temperature gradient creates a generalised force associated with heat flow. In this case the amount of heat flow is determined by the thermal conductivity. The Onsager matrix also contains off-diagonal coefficients Lj. Each off-diagonal coefficient determines the influence of a generalised force on a non-conjugate flux. For example a concentration gradient of one species can give rise to a flux of another species. The electric field which exerts a force on electrons in metals to produce an electric current has a crossinfluence on the flow of heat known as the Peltier effect. Conversely the thermal force temperature gradient that normally causes heat flow also has a cross-influence on the distribution of electrons - known as the Thomson effect. The Thomson and Peltier effects combine and provide the basis for thermoelectric devices thermopiles can be used to convert heat flow into electric current in thermocouples a voltage is produced by a temperature difference. Another example is that an electronic current and the associated electron wind causes a flow of matter called electromigration see also Chap. 11 . Electromigration can be a major cause for the failure of interconnects in microelectronic devices. The Onsager matrix is symmetric provided that no magnetic field is present. The relationship Lij Lji is known as the Onsager reciprocity theorem. 3. The central idea of non-equilibrium thermodynamics is that each of the thermodynamic forces .

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