Investigation of the kinetics of reduction of nickel tungstate by hydrogen
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I.
INTRODUCTION
THE application of tungsten-reinforced nickel as a structural material for application under extreme conditions, as in the case of turbo-jet engines, LIJ necessitates an examination of the various methods for producing these alloys. In synthesizing nickel alloys with tungsten fibers by the reduction of oxides, it is important to have a knowledge of the reduction kinetics. A thermodynamic investigation of the ternary system Ni-W-O has recently been carried out in the present laboratory.~2~ The results of this study indicate that the only ternary oxide that exists in this system is NiWO4. Woo and Roy t3~have identified this compound as the major oxide formed in the oxidation of nickel-tungsten composites. A similar observation has also been made by Kvernes and Kofstad.t4] A possible route to the production of the alloy coatings is to reduce NiWO4 oxide layers over suitable substrate using a reductant like hydrogen or carbon monoxide. Studies of the kinetics of reduction of NiWO4 are also quite relevant in the production of complex carbides which are of importance in the hard metal industry. In this connection, an earlier study on the influence of nickel on the hydrogen reduction of WO3tSj shows that nickel metal is formed in the first stage of reduction and tungsten whiskers are formed in the second stage. The study of the reduction of nickel tungstate is of interest even from a theoretical viewpoint. The Department of Theoretical Metallurgy, Royal Institute of Technology,t2~ is seriously involved in a project on the study of the reduction mechanisms of complex oxides. Toward this aim, experimental and theoretical studies have been carried out earlier on the kinetics of hydrogen reduction of the oxides of molybdenumt61 and tungsten.J7] The reduction kinetics of nickel oxide by H 2 is currently being investigated. It would be interesting to compare S. SRIDHAR, formerly with Royal Institute of Technology, Stockholm, Sweden, is Special Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. DU SICHEN, Research Associate, and S. SEETHARAMAN, Professor, are with the Division of Theoretical Metallurgy, Department of Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted July 29, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
the reduction kinetics of the complex oxide NiO-WO3 with that of the corresponding simple oxides under identical experimental conditions. The present investigation seeks to develop an understanding of the reduction kinetics of nickel tungstate by hydrogen in the temperature range 891 to 1141 K. The studies have been carried out using a thermogravimetric method. II.
EXPERIMENTAL
A. Materials
NiWO4 powder (99 pct purity) was supplied by Johnson and Matthey Ltd., United Kingdom, and was calcined at 1023 K for 24 hours. The samples were preserved in a desiccator before being used in the experiments. The argon gas used during the prereduction period of the thermogravimetric experiments
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