Combustion synthesis of ultrafine tungsten carbide powder
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The synthesis of ultrafine tungsten carbide (WC) powder has been investigated from a WO3 + Mg + C mixture via combustion technique. The values of combustion parameters were estimated over the Mg concentration range 3 to 16 mol. Fast increasing tendency of the WC/W2C phase ratio from Mg concentration has been found in the final products. Phase pure WC was prepared with more than 10 mol Mg, and a small amount of ammonium carbonate (or urea) was blended with the WO3 + C mixture. The effects of the combustion conditions on product morphology and composition were evaluated using scanning electron microscopy and x-ray diffraction analysis. The results of the investigation indicate that carbon-containing compounds significantly enhance the combustion synthesis process; leading to higher conversion efficiencies and phase pure WC formation at 1500–1550 °C. The crystalline particles of WC showed a narrow distribution in particle size, with a mean diameter around 200 nm. The results are discussed in the context of gas-phase and solid-phase transport models.
I. INTRODUCTION
Ultrafine tungsten carbide (WC) powders have attracted considerable attention in the past few years because of their unique characteristics, which cannot be obtained in conventional powders. In this respect, the synthesis and structural characterization of such ultrafine powders are important fields of research. To meet the challenge of improving the properties of powders by reducing their grain size, the study of synthesizing nanostructured WC powders has been the subject of much attention during recent years.1–4 L. Gao and B.H. Kear investigated the thermochemical synthesis of nanophase WC powders.1,2 WC with a particle size of 30 nm was produced by low temperature carburization of nanophase W powder, utilizing a controlled carbon activity gas phase environment. Mechanical manufacturing methods have also been used to mix tungsten powder with carbon black, and then to grind them together in a high-energy ball miller.5,6 Metallic tungsten and carbon powders have been simultaneously crushed and reacted at low temperatures to form fine WC powders. Ultrafine (12 nm) cubic WC1–X powders can be produced using an ion-arc method in which tungsten and graphite electrodes react with one another,7 but the hexagonal WC phase was not
found when using this method. Electric discharge machining (EDM) is used to cut and remove by arc erosion all kinds of electro-conductive materials, including WC. During EDM processing, the melted workpiece reacts with an electrode and is rapidly solidified in dielectric liquid to form powder debris.8 The essential part of the above mentioned technique is mainly realized on a laboratory scale, producing small quantities of product, which is not suitable for the demands of industry. Therefore, the development of efficient and large scale technology for producing large quantities of ultrafine WC powder is important today. The combustion synthesis is attractive because it enables the rapid synthesis of many inorganic powders, including WC.
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