Hesham M. Ahmed

Simultaneous reduction nitridation for the synthesis of tungsten nitrides from Ni–W–O precursors

Abstract Tungsten nitrides were synthesised from NiO–WO3 and NiWO4 precursors at 973–1273 K in a flow of H2–N2 gas mixture. The reduction–nitridation reactions were carried out isothermally in fluidised bed reactor, and the off-gas from the reactions was continuously analysed by gas chromatography. The effect of reaction temperature and precursor composition on the rate of formation of Ni–W nitrides was studied. The different phases developed during the reduction–nitridation reactions were identified by X-ray diffraction analysis technique. The morphology and the grain structure of the precursors were examined by SEM, and the elemental composition in the structure was analysed by electron dispersive spectrometry. The results showed that the reduction of Ni–W–O precursors proceeded in a stepwise manner (NiWO4→Ni–WO3→Ni–WO2→Ni–W). Tungsten nitrides (WN and WN2) were formed from the reaction of the freshly reduced W metal with N2 gas and WN was the predominant phase detected at higher temperatures. The reaction mechanisms were elucidated from the apparent activation energy values and the application of different formulations derived from the gas–solid reaction model at early and later stages of reactions. It was concluded that the interfacial chemical reaction is the rate determining step at initial stages, while a combined effect of gaseous diffusion and interfacial chemical reaction controlled the reaction at later stages. At final stages, the nitridation reactions contributed to the reaction mechanism leading to produce tungsten nitrides.

Isothermal dynamic thermal diffusivity studies of the reduction of NiO and NiWO4 precursors by hydrogen

Abstract Thermal diffusivity measurements of uniaxially cold pressed NiO and NiWO4 were carried out in a dynamic mode in order to monitor the kinetics of hydrogen reduction of the above-mentioned materials using a laser flash unit. The calculated activation energy was found to be higher than that for chemically-controlled reaction obtained earlier by thermogravimetry. The difference has been attributed to physical changes occurring along with the chemical reaction. The activation energy of sintering of the products was evaluated to be 33 and 36 kJ · mol−1 for NiO and NiWO4, respectively. Thermal conductivities were calculated taking into consideration the change in heat capacity considering the compositional and the structural changes with the progress of the reaction. The potentiality of the laser-flash method as a complementary technique to thermogravimetry in understanding the mechanism of gas – solid reactions is discussed.

Reduction-Carburization of the Oxides of Ni and W Towards the Synthesis of Ni-W-C Carbides

Ternary Ni-W-C cemented carbides were synthesized directly from mixture powder of NiO-WO3 by simultaneous reduction-carburization in mixed H2-CH4 gas environment in a thin bed reactor in the temperature range 973-1273K. The kinetics of the reaction was closely followed by monitoring the mass change using thermogravimetric method (TGA). The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. The gas mixture ratio was adjusted in such a way that the Ni-W-C formed was close to the two phase tie line. In view of the fact that each particle was in direct contact with the gas mixture, the reaction rate could be conceived as being controlled by the combined reduction-carburization reaction. From the reaction rate, the Arrhenius activation energies were evaluated. Characterization of the carbides produced was carried out by using X-ray diffraction, SEM-EDS as well as high resolution electron microscope (HREM). The grain sizes were also determined. Correlations were found between the carbide composition as well as grain size and the process parameters such as temperature of the reduction-carburization reaction as well as the composition of the gas mixture. The results are discussed in the light of the kinetics of the reduction of oxides and the thermodynamic constraints.

Dynamic Thermal Diffusivity Measurements – A Tool for Studying Gas-Solid Reactions

In the present work, the thermal diffusivity measurements of uniaxially cold pressed NiWO4 has been carried out. The measurements were performed isothermally at temperatures between 973 and 1273 K under H2 gas using the laser flash technique. The experimental thermal diffusivity values were found to increase with the reduction progress as well as with increasing temperature. The calculated activation energy was found to be higher than that for chemically controlled reaction. The difference has been attributed to factors like agglomeration of the product as well as sintering of the precursor along with the chemical reaction. In order to sort out the sintering effect on the thermal diffusivity values, complementary experiments have been done on pressed NiWO4 and Ni-W, produced by the reduction of NiWO4 at 1123K, under Argon gas. The porosity change and its effect on thermal diffusivity values have been studied.