粉末冶金教程.ppt

Basic concepts All powder processing starts with a powder, therefore, one must understand the nature of powder to understand the process Particle size and distribution Particle shape and its variation with particle size Surface area Interparticle friction Flow and packing Internal structure, and Composition, homogeneity, and contamination Part 2：粉末性能表征 Shape and size SEMs of metal:a)tellurium, accicular, b ) iron alloyargon atomization,sphere c)tungsten, polygonal aggregatesgas reduced, d)tin,are atomized, rounded and ligamental, e) iron alloy, centrifugally atomized spherical, f)tin,flake Part 2：粉末性能表征 g)stainless steel,water atomized, round and irregular,h) palladium, electrolytic, sponge, i) nickel, cabopnyl decomposition, porous and cubic,j) iron-based metallic glass, crushen riboon, angular plates, k)titanium,sodium reduced and milled, irregular, l)niobium hydride, milled, Part 2：粉末性能表征 Number of particles in one gram sample of monosized powders of Al, Fe, and W Many modern analytic instruments require sample in one gram, thought we often produce products in kilograms or in tons. Assuming a spherical shape, particle population in one gram depends on size and material density. Particle diameter μm Density Al-2.7 Density Fe-7.86 Density W-19.3 0.01 1.0 1000 7.0 x 1017 7.0 x 1011 7.0 x102 2.4 x 1017 2.4 x 1011 2.4x102 9.9 x 1017 9.9 x 1011 9.9 x102 Part 2：粉末性能表征 Most particles are cohesive because of their small size. Adsorbed moisture causes particle agglomeration, they are not easily dispersed, through van der Waals force are small. Particle held together by stron bonds,can not be easily dispersed. Part 2：粉末性能表征 Particle have both internal energy and surface energy: ET( totalenergy)=γA(relatedsurface)+ ξV(related volume) ET/V= ξ+6 γ /D Because the energy per particle scale inversely with the particle size, the driving force for agglomeration becomes large as the particle size decrease.