Sintering Properties Of Mo-25Cu Nanoscale Powders Prepared By Microwave-assisted Wet-Chemical Method

Mo-Cu composites have been applied in many kinds of electronic packaging materials, aeronautics and some advanced fields owing to their excellent characteristics such as low thermal expansion coefficient (CTE), high thermal conductivity and good high-temperature resistance.In the work, synthesis of Mo-Cu composite powders by wet-chemical methods and the sintering process were investigated through X-ray diffraction (XRD), differential scanning calorimetry and thermal gravimetric analysis (DTA-TG), scanning electron microcopy (SEM) and transmission electron microcopy (TEM). The influence of organic additive on the microstructures and sizes of the Mo-Cu powders was studied. The reduction mechanism was discussed from the view of thermodynamics and kinetics. Furthermore, the influence of different sintering techniques including solid phase sintering, liquid phase sintering and activated sintering on the properties of Mo-Cu sintered parts was researched.The precursors, including (NH4)2Cu4(NH3)3Mo5O20and CUMoO4N0.42H126·2.4H2O, can be prepared by drying the mixed solution of ammonium paramolybdate, copper nitrate and excess ammonia. CuMoO4N0.42H1.26·2.4H2O is an unstable phase which can be decomposed into Cu3Mo2O9when calcined at360℃. Then MoO2is easily obtained by reducing Cu3Mo2O9at200℃. Due to the self-catalysis of MoO2, Mo-Cu composite powders with the average grain size of80nm can be obtained at low temperature(650℃).The resultant Mo-Cu nanocomposite powders were compacted in a steel die under different pressures to produce the green parts which were sintered at different temperatures. It has been found that the relative density of97.77%is achieved by compacting the Mo-25Cu powders at300MPa and sintering at1200℃for1.5h in hydrogen atmosphere. Under the best sintering conditions, the hardness、electrical conductivity、 thermal conductivity and coefficient of thermal expansion of the sintered products are218.7HV、23.85MS·m-1、172.57W·m-1·K-1、7.9×10-6K-1, respectively.The activated sintering was performed through adding different contents of Ag element. The activator gives the highest increase in sintered density(99.2%) when the content of Ag is1.5wt.%. Unlike the traditional activators of Fe, Co and Ni, the element Ag has a positive effect on the electrical conductivity and thermal conductivity of the sintered parts. In conclusion, the comprehensive performance of the Mo-Cu alloys was improved a lot with the addition of Ag. The hardness、 electrical conductivity、thermal conductivity and coefficient of thermal expansion are224.4HV,27.82MS·m-1、185.43W·m-1·K-1、8.1×10-6K-1, respectively. In this thesis, there are41figures,10tables and110references.