The Formation of Icositetrahedron Molybdenum Powder and the Characteristics of Sintering Performance
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NTRODUCTION
MOLYBDENUM powder is the main source of Mo metal products, which are used widely in the aerospace industry, high-temperature furnaces, glass melting, sputtered layers, electronic elements, laser mirrors, and so on because of their unique properties, including hightemperature strength, mechanical stability, and superior conductivity of heat and electricity. Optimizing the material function is an ongoing topic, especially in the age of limited resources and energy crises. Materials with the same composition but different morphologies always show great differences in performance for different patterns of crystal packing. The regular polyhedral crystalline powder is an important source for some excellent capabilities for the low surface energy compared with amorphous or mealy powder. The commercial molybdenum metal powder is spherical or near spherical. The facet morphology of molybdenum was mentioned and named as Icositetrahedron[1] Mo powder. The polyhedral Mo powder was also obtained by a reduction of MoO2 powder doped with La (NO3)3 solutions.[2] Based on the feasibility of polyhedral Mo powder, the subsequent process feature of this material was explored. Inspired by the development of pore-free ceramics through powder sintering,[3] the high-density molybdenum material using polyhedral powder was expected with its superior dispersion, low surface oxygen content, and regular crystal boundary. The production of Molybdenum powder is a process of hydrogen reduction of MoO3 or ammonium XIANQIN WANG, Senior Engineer, JUNHUAI LIU, Senior Engineer, FEI ZHUANG, Engineer, HU ZHAO, Engineer, and LI JING, Engineer, are with the JinDuiCheng Molybdenum Group Technical Center, Xi’an 710077, P.R. China. Contact e-mail: [email protected] Manuscript submitted July 24, 2009. Article published online July 27, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
molybdate with two stages based on the following reactions: MoO3 þ H2 ðgÞ ! MoO2 þ H2 OðgÞ þ heat MoO2 þ H2 ðgÞ ! Mo þ H2 OðgÞ heat The first stage reduction usually is carried out in the temperature range of 623 K to 873 K (350 °C to 600 °C), and mixed process gas or H2 with a dew point of 278 K to 283 K (5 °C to 10 °C) is used to avoid the runaway reaction. In the second stage reduction, the temperature range of 1073 K to 1373 K (800 °C to 1100 °C) and pure hydrogen are adopted for the endothermic reaction. By controlling combinedly the temperature, H2 flow rate, feeding rate, and other conventional parameters, near spherical powder is easy to obtain. To gain the polyhedral crystalline powder, the procedural principle of MoO2 and Mo particle forming should be known. The relationship between intermediate MoO3-X and final Mo powder were discussed according to the thermodynamic data in the first stage, and the results showed a proper amount of intermediate remarkable enhanced production of metallic powder bearing a fine particle size distribution.[4] The experiments of MoO3 fed with a different particle size indicated that MoO3-X appeared only when certain large MoO3 were r
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