Novel Process for Solid State Reduction of Metal Oxides and Hydroxides
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ODUCTION
THIS report is on a new chemistry/process of direct reduction of metal oxides to metal, a topic of research and development dating to the dawn of the Iron Age. This research is a natural segue from the recent demonstration that zero valent metal particles and alloys, at the sub-micron scale and nanoscale, can be produced by heating physical mixtures of urea and metal nitrates under an inert gas such as nitrogen or argon. In the earlier study, reductive expansion synthesis (RES) was employed to make graphite-coated nickel, iron, and iron-nickel alloy particles from physical mixtures of urea with nitrates by rapid, short duration heating under flows of inert gas.[1] The temperatures required for creation of pure metals or even bulk alloys in relatively large quantities were far below the melting temperatures of the metals/alloys [maximum temperature approximately 1025 K (752 C)]. In contrast, heating the nitrates ‘‘neat’’ under the same conditions produced only metal oxide particles. The formation of metal oxide from nitrate decomposition, even under an CLAUDIA LUHRS, Associate Professor, and MARGARET KANE, Student, are with the Department of Aerospace and Mechanical Engineering, Naval Postgraduate School, Monterey CA 93943. ZAYD LESEMAN, Associate Professor, is with the Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM 87131. JONATHAN PHILLIPS, Research Professor, is with the Physics Department, Naval Postgraduate School, Monterey CA 93943. Contact e-mail: [email protected] Manuscript submitted April 12, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
inert gas flow, is not a surprising result as it is totally consistent with years of study of this process, generally known as ‘‘combustion synthesis’’.[2–7] In all cases in the first RES study, the particles were primarily sub-micron in scale (520 K METALLURGICAL AND MATERIALS TRANSACTIONS B
(247 C), but will not form from CO alone even at very elevated temperatures.[49,50] Thus, over Ni(OH)2, CO is not converted to CO2 by an oxidation/reduction and consequently is present in sufficient quantity to disproportionate and form CO2, leaving carbon on the particle surface.
IV.
SUMMARY
Metal oxides and hydroxides can be rapidly converted to the metallic state using the RES process; however, it is notable that the products of RES synthesis are a function of the identity of the precursor, the flow gas rate, the ratio of urea to metal, and the maximum temperature achieved during the process. Some of these impacts are quite dramatic. For example, if NiO is employed as the precursor, the final product is a nearly entirely micron scale Ni metal particle. In contrast, if Ni(OH)2 is the metal precursor and the final high temperature is greater than 873 K (600 C), the product is a graphite-coated Ni sub-micron scale metal particle. All of the data are consistent with the earlier postulated RES mechanism model: Initially, the metal
precursor decomposes (except in the case of a thermally stable precursor such as NiO), and then the products of th
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