Sintering Theory Practice Randall M German Free PATCHED Download E Book
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The present book views fine grain as a primary sintering process similar to the conventional solid state sintering processes. This is because the compacted powders are not in solution and because the shear rates at the powder-liquid interface are high. It is well established that the non-Newtonian rheological response of the liquid is a source of progressive coalescence and repulsive forces at the powder-liquid interface. In fine grain sintering, the coalescence of the powder grains is arrested and the powder undergoes incremental flow. The evolution of both the packing and the structure of the powder depend on the state of the liquid, which is a function of the process. The function of the process, in turn, is a function of temperature. A key difference between fine grain sintering and coarse grain sintering is the flow of liquid at the powders. In coarse grain sintering, we have a concentrated particle liquid. In fine grain sintering, there is a free particle liquid. The free particle liquid is not a true liquid, but it is a continuum of a complex fluid [ 104 ]. The state of the liquid during sintering is complicated by the coupled effects of temperature and particle size on the chemical reaction. At first glance, this explanation seems difficult because the chemical reaction is straightforward, but it is a complicated process. The first step is diffusion, which is slow because only the very reactants are mobile and fast because the diffusion coefficients for metals are small. Many species are formed or consumed in each chemical reaction. These species are mobile, and they can react with other mobile species. The particles are segregated into regions that are rich or poor in species. Over time, this segregation changes because the particle will have a different chemical environment. A critical aspect of chemistry is that it is entropic in nature. If a reaction is slow, it is because there is a large entropic penalty for the particle to get from one state to another. At the same time, the rate of a reaction is proportional to the surface area of the particles. Thus, the rate is slow when the particle surface is large, and it is fast when the particle surface is small. If the particle size is large, the surface area is large. 3d9ccd7d82