بخشی از مقاله
Abstract
Nanocrystalline tungsten carbide - WC - was synthesized via a simple route by the reaction of tungsten hexachloride with metallic sodium and pitch in an organic solvent at an autoclave reactor. The reaction was performed in a thermal system with temperature range of 600-900 °C, and the effect of reaction temperature on the formation of different phases was determined. The effect of other parameters on particle size such as diluting inert phase was investigated by addition of fine NaCl powder. X-ray powder diffraction pattern indicated that the sample produced in 900 °C was pure hexagonal WC phase. Scanning electron microscopy image showed that this procedure can make WC with size distribution near to 30 nm.
INTRODUCTION
transition metal Carbides, have a number of valuable properties, which make them the most promising materials for use in various new fields of technology - Koc & Kodambaka, 2000 - . WC ceramic has a wide interesting properties such as high melting point - 2800 °C - , high degree of hardness - Hv = 22GPa - , high modulus of elasticity - 696 GPa - , high fracture toughness - 28MPa m1/2 - , and good wear resistance over a wide range of temperatures - Scussel, 1992 - . It is widely used in cutting tools, abrasives, coating industrials with the advantage of immense hardness and chemical stability at high temperature. These mechanical properties are further enhanced with reduced WC grain size - Z.Lin, L.Wang, J.Zhang, H.K.Mao, & Y.Zhao, 2009 - . WC has been shown to have platinum - Pt - -like behavior for the chemisorption's of hydrogen and oxygen, and its applicability as an alternative electro-catalyst of Pt has been demonstrated - Levy & Boudart, 1973 - . Recently, WC has been considerably studied in hydrogenation, dehydrogenation, isomerization, and fuel cells because of its Pt-like catalytic activities, excellent stability, and CO poisoning tolerance - C.Guordano, C. Erpen, WT Yao, & Antonietti, 2008 - . The low electrical resistivity combined with chemical and thermal stability makes tungsten carbide an attractive thin film diffusion barrier in microelectronics industrials - Ghaisas, 1991 - . Presently, tungsten carbide powders are produced by direct carburization of tungsten powder. This process is an expensive process. It involves the production of pure, fine W powder in the first step. The second step is the carburization of W by carbon. The reduction conditions have a great influence on the characteristics of the metal powder and of the carbides produced - Koc & Kodambaka, 2000 - . In recent years, efforts are being made to synthesize nanosize ultrafine tungsten carbide particles at lower temperatures. because decreased grain size to nanometer scale results in enhancement in strength of materials - Kumar, Singh, & Pandey, 2010 - .
Kim synthesized nanostructured tungsten carbide powders by the chemical vapor condensation - CVC - process using tungsten hexacarbonyl - W - CO - 6 - precursor - Kim & Kim, 2004 - . The loose agglomerated WC1 x powders, which had a rounded cubic shape, were obtained by carburization with carbon from the dissociation of CO gas in the temperature range of 600-800 C. grain size was decreased from 53 to 28 nm with increasing reaction temperature. Ma et al. reported a new method that Nanocrystalline tungsten carbide - WC - was synthesized via a simple route by the reaction of metallic magnesium with sodium carbonate and tungsten hexachloride in an autoclave at 600 °C - Ma & Du, 2008 - . Cao and Kear investigated the thermo-chemical synthesis of nanophase WC powders - Cao & Kear, 1997 - . Nanophase WC with the particle sizes of 30 nm were produced by carburization of nanophase W powder. In this study, we report, for the first time, synthesis of tungsten semi carbide - W2C - and ultrafine tungsten mono carbide - WC - via a simple route by the reaction of tungsten hexachloride with metallic sodium and pitch in an organic solvent at moderate temperature range - 650-950 °C - .
Experimental
The starting materials consist of tungsten hexachloride - Merck, 99.0% - as a tungsten precursor, metallic sodium - Merck 98% - industrial pitch - 98% - as carbon precursor and industrial xylene - 98% - as solvent. sodium was selected as reducing agent. In a typical experiment 2.0 g tungsten hexachloride was dissolved in 25 ml xylene with magnetic stirring. The mixture was poured into a stainless steel autoclave and heated on a heater. Then 0.1 g pitch and 0.69 g metallic sodium was added into the mixture. When the solvent was completely evaporated, the sample was placed in an electrical furnace and heated under argon atmosphere to 650 °C with a heating rate of 10 °C/min for 5 hours. After a given reaction time, the furnace was cooled to room temperature. The obtained black product was washed several times with absolute ethanol, dilute HCl solution and distilled water to remove the impurities. Finally, the product was washed three times with absolute ethanol to remove water. The final product was dried at 100 °C for 2 h to make black WC nano powder. Effect of reaction temperature in formation of different phases In order to investigate the role of reaction temperature effect on synthesis of WC nanoparticles, the autoclave was charged with the same proportion of WCl6, Na, pitch and xylene - 2g: 0.69g: 0.1 g: 25ml - and heated for a 5 h at different temperature with a constant heating rate of 10 °C/ min as given in Table 1 - S1 to S5 - . Table 1-Details of reaction parameters
Samples no. Reaction Temperature - °C -
S1 600
S2 650
S3 750
S4 850
S5 900
Effect of adding an inert salt on WC particle size
In order to investigate the effect of adding inert salts on WC particle size, reaction was done with addition of different molar ratios of fine NaCl powder - Stoichiometric ratio in equation - .
