Kinetic Studies on Hydrogen Reduction of MoO 3 and Morphological Analysis of Reduced Mo Powder
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MOLYBDENUM, a group VI B refractory metal, has a body-centered-cubic crystal structure and possesses good high-temperature strength, creep resistance, low coefficient of thermal expansion, and high thermal conductivity.[1] The high strength of the molybdenum base alloys such as Mo-14Re, Mo-44.5Re,[2] and TZM[2,3] at elevated temperatures makes them suitable candidates for the structures of high-temperature nuclear reactors. Powder metallurgy (PM) is the preferred technique for preparing molybdenum alloys.[1] Molybdenum metal powder of suitable morphology, size, and purity is of immense importance for alloy preparation by PM.[4] Reduction of MoO3 by hydrogen is one of the methods used to obtain metallic molybdenum of high purity. The reduction takes place in two stages: first, MoO3 is converted to MoO2; and, second, MoO2 is converted to Mo.[5] Formation of an intermediate oxide Mo4O11 during the first stage has also been reported in the literature.[6,7] The mechanism of the reaction is described best by the classical gas-solid reaction model, namely, the shrinking core model (SCM),[8] which describes a shrinking core of unreacted solid, surrounded by a growing layer of product during the reaction of a solid particle with gas. The crackling core model (CCM)[8] assumes that the transformation takes place in two steps: the first step is the development of a system S. MAJUMDAR and I.G. SHARMA, Scientists, are with the Materials Processing Division, Bhabha Atomic Research Centre, Mumbai-400085, India. Contact e-mail: [email protected] I. SAMAJDAR, Professor, and P. BHARGAVA, Associate Professor, are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India. Manuscript submitted August 7, 2007. Article published online May 30, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
of cracks and fissures from the surface to the center, resulting in a grainy material; and the second step comprises the grains of the now porous layer subsequently reacting via the SCM to the final product.[9] The kinetics of the reduction of MoO3 to MoO2 was investigated by many authors.[10] A sigmoidal[11] shape of the a-t (degree of reduction vs time) curve, indicating a maximum of the reaction rate, has been observed by many researchers. The linear dependence of avs time has also been reported.[10] Orehotsky and Kaczenski[12] reported a linear dependence of weight loss with reduction time for the reduction of MoO2 to Mo in a static bed during the initial period when the interface (MoO2/Mo) is within 1 cm of the top of the bed. When the moving interface is more than 1 cm below the top of the bed, the weight loss was not linear and a kinetic model was developed for this stage. The stepwise differential isothermal analysis technique was used by Gasik et al. to study hydrogen reduction of MoO3 mixed with 30 pct Fe.[13] Criado and Maqueda have reported on the sample-controlled thermal analysis technique used for kinetic study of solid-state reactions.[14] In the present
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