Microwave-hydrothermal processing of metal powders
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Novel microwave-hydrothermal processing has been developed by us recently for the synthesis of a wide variety of ceramic powders. Herein, we report the use of microwave-hydrothermal processing to synthesize several metal powders such as Cu, Ni, Co, and Ag by reducing their corresponding metal salts or hydroxides with ethylene glycol. Metal powders have been produced extremely rapidly a (few minutes) by microwave catalysis. The kinetics of metal powder synthesis have been increased by at least an order of magnitude by microwave-hydrothermal processing compared to the conventional refluxing process in ethylene glycol at about 195 °C.
I. INTRODUCTION Ceramic and metal powders that have numerous technological applications have been prepared by various techniques, including the hydrothermal method. The hydrothermal method is a truly low temperature process and is environmentally benign because the reactions are carried out under closed system conditions. The conventional hydrothermal method has been used in (a) synthesis of ceramic powders,1"6 (b) growth of single crystals,7 (c) processing of cement blocks,8 (d) synthesis of hydroxylated and hydrated phases, 910 (e) sintering of ceramics,11 (f) formation of carbon coatings,12 and (g) destruction of organic pollutants.13 There have been recent innovations of the hydrothermal conditions to prepare powders and films14"16 which has been named (i) the hydrothermal-electrochemical method and (ii) the use of microwave field by us for the first time to synthesize a wide variety of ceramic powders 1722 which has been named the microwave-hydrothermal method. The main advantages of the microwave-hydrothermal method over conventional hydrothermal methods in powder synthesis are (i) the kinetics of reaction are increased by one to two orders of magnitude, 17 ' 22 (ii) novel phases are formed, 1720 and (iii) the initial heating is rapid which can lead to energy savings.22 In addition to the synthesis of ceramic oxide powders, the microwave-hydrothermal method has been used to prepare metal-intercalated clay nanocomposites19 and metal powders.21 Preliminary results have demonstrated the synthesis of metal powders21 using the so-called polyol process23"25 under the new microwave-hydrothermal conditions. There is a great deal of interest in metal powders for industrial applications such as powder metallurgy, conductive inks and pastes for electronic materials26"28 in thick film technology, and for the preparation of conductive paints. a)
Also with the Department of Agronomy. J. Mater. Res., Vol. 10, No. 7, Jul 1995 http://journals.cambridge.org
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Therefore, the objective of this study was to determine the usefulness of the microwave-hydrothermal process for the preparation of metal powders such as Cu, Ni, Ag, and Co. II. EXPERIMENTAL A. Materials Following the conventional polyol process,23"25 metal compounds of Cu(OH)2, Cu(NO 3 ) 2 , Ni(OH) 2 , NiSO 4 , Co(NO 3 ) 2 , Co(OH) 2 , AgNO 3 , and ethylene glycol were used in metal powder preparation. One gram of nit
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