tailieunhanh - Kinetics of Materials - R. Balluff S. Allen W. Carter (Wiley 2005) WW Part 4

Tham khảo tài liệu 'kinetics of materials - r. balluff s. allen w. carter (wiley 2005) ww part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | CHAPTER 5 SOLUTIONS TO THE DIFFUSION EQUATION In Chapter 4 we described many of the general features of the diffusion equation and several methods of solving it when D varies in different ways. We now address in more detail methods to solve the diffusion equation for a variety of initial and boundary conditions when D is constant and therefore has the relatively simple form of Eq. that is Ễ dt This equation is a second-order linear partial-differential equation with a rich mathematical literature 1 . For a large class of initial and boundary conditions the solution has theorems of uniqueness and existence as well as theorems for its maximum and minimum Many texts such as Crank s treatise on diffusion 2 contain solutions in terms of simple functions for a variety of conditions indeed the number of worked problems is enormous. As demonstrated in Section the differential equation for the diffusion of heat by thermal conduction has the same form as the mass diffusion equation with the concentration replaced by the temperature and the mass diffusivity replaced by the thermal diffusivity K. Solutions to many heat-flow 1If the diffusivity is imaginary the diffusion equation has the same form as the time-dependent Schrodinger s equation at zero potential. Also Eq. implies that the velocity of the diffusant can be infinite. Schrodinger s equation violates this relativistic principle. Kinetics of Materials. By Robert w. Balluffi Samuel M. Allen and w. Craig Carter. 99 Copyright 2005 John Wiley Sons Inc. 100 CHAPTER 5 SOLUTIONS TO THE DIFFUSION EQUATION boundary-value problems can therefore be adopted as solutions to corresponding mass diffusion For problems with relatively simple boundary and initial conditions solutions can probably be found in a library. However it can be difficult to find a closed-form solution for problems with highly specific and complicated boundary conditions. In such cases numerical methods could be employed. For .

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