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I.

INTRODUCTION

IN recent years, mechanical alloying and mechanochemical processing have been shown to be effective processes for producing a variety of nanocrystalline and ultrafine metallic powders.[1–5] The principle of mechanochemical processing is based on mechanically activated solid-state chemical reactions. Of particular interest are reactions between metal oxides or chlorides and reducing agents such as Na, Mg, Ca, and Al. These displacement reactions in using active metals as reducing agents inevitably produce two solid phases, the refined metal and a by-product oxide or chloride phase. The two product phases are intimately mixed at the nanoscale as a natural consequence of the milling process. It is often necessary to remove the reaction byproducts by various postmilling operations in order to produce nanocrystalline or nanoparticulate metallic powders.[5,6,7] This has proven to be a challenging task with reactive metals.[8] This study was carried out to investigate the synthesis of single-phase nanoscaled metallic powders directly by mechanical milling via solid-state mechanochemical reactions, which produce gaseous by-products that are removed simultaneously during the reaction. Graphite was selected as a reducing agent for oxide reductions to take advantage of the gaseous products of CO2 and CO. Previous studies[9,10] have shown that the formation of nanocrystalline phases accompanies the mechanochemical reduction of CuO and Fe2O3, respectively, with carbon. In this article, results of a study of the effect of mechanochemical processing on the reduction of nickel oxide with graphite were reported. II.

EXPERIMENTAL TECHNIQUE

The materials used in this study were NiO (AJAX, LR)

H. YANG, Lecturer, and P.G. McCORMICK, Professor, are with the Department of Mechanical and Materials Engineering, University of Western Australia, Nedlands, Western Australia 6907, Australia. Manuscript submitted July 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B

and graphite (CERAC,* 99.4 pct, 150 to 420 mm). Milling *CERAC is a trademark of Cerac, Inc., Milwaukee, WI.

was carried out in a SPEX** 8000 mixer/mill at different **SPEX is a trademark of SPEX Industries, Edison, NJ.

ambient temperatures from room temperature to 623 K. Milling at elevated temperatures was achieved using two electric ring heaters attached to the outside of the vial. A thermocouple attached to the outer surface of the vial was used to control the temperature. The milling container was a hardened steel cylindrical vial with inner dimensions of f40 3 54 mm, which was sealed with a high-temperature O-ring. The vial was loaded with ten hardened steel balls of 12.7 mm in diameter and a powder charge of 8 g, giving a ball-to-powder mass ratio of 12:1. The loading of the vial and all subsequent powder handling after milling were carried out in an argon atmosphere glovebox. Milling was performed for different durations up to 24 hours. The structure of the milled powders was analyzed using a SIEMENS* D5000 X-ray diffractometer with a Cu Ka *SIEME