The Theoretical Analysis And Numerical Simulation On The Residual Stresses In Thermoelectric Materials Sintering By High-pressure Method

Thermoelectric material is a class of environment-friendly functional material which can directly invert heat energy into electric energy. Due to its so many advantages, such as long life-span, stable performance and so on , it means significant application prospects on energy engineering, aviation and space flight engineering, refrigeration engineering, etc.High-pressure technique is one important technical way to prepare bulk thermoelectric material, which contains merits of short synthesizing time, broad material choosing scope, etc. However, explorative researches on high-pressure sintering thermoelectric material indicate that there exist high residual stresses in the sintered product of thermoelectric material that would cause fracture in the sintered product and even crack. To investigate and expose the relation between residual stress and those as material properties and fabrication argument is the key to successfully obtain complete bulk sintered product.Using theoretical research method in conjunction with numerical simulation, the thesis preliminarily traced the residual stress problem in the sintered product during high-pressure sintering process of the thermoelectric material, and discussed the effect of grain size, sintering pressure and material's mechanical properties on residual stress. The main content and achievements are as follows.First, carried out the high-pressure sintering experiment for CoSb₃, and observed the macroscopic damage pattern of the sintered product; using SEM, examined the fracture of the sintered product, and have discovered that the residual stress in the high-pressure sintered skutterudite product is caused by spring back, and the microscopic damage pattern is that cracks mainly occur at the grain boundaries, i.e., the crack exists at the grain boundary and extends along the grain boundaries.On the basis of experimental investigation of the damage mechanism, and on the concept of two-ball sintering model proposed, constructed the linear elastic mechanical model for high-pressure sintering residual stress analysis in terms of theory on spring back of the sintered product. Compared the theoretical model solution to the finite element solution, the reliability of the model in the thesis is verified.Using the theoretical model above, studied the impact of grain dimension, sintering pressure and material mechanical properties on the residual stress. It is found that at linear elastic deformation, relative dimension of the sintering region is independent on grain size, and is proportional to the cube root of the material's elastic modulus and sintering pressure; the residual stress is independent on the grain size as well, but increases with the increasing of the sintering pressure.Considering the effect of material elastic-plastic behavior and using software ANSYS, performed residual stress problem simulation during high-pressure sintering process. Numerical simulation results indicate that, if material's elastic-plastic behavior concerned, the residual stress and the relative dimension of sintering region are irrelevant to grain size, which is consistent with the conclusion deduced from theoretical model based on linear elastic assumption; however, since near the sintering region, the material comes into plastic state during sintering course, the residual stress distribution pattern differs from linear elastic solution significantly in the same condition of grain size and sintering pressure, and the totally residual stress level is relatively lower.