Lower WC grain sizes in the nanometer range have positive effects on the properties of hardmetals(e.g.,hardness),but the established production processes of WC are limited to grain sizes of about 150 nm.To produce WC ...Lower WC grain sizes in the nanometer range have positive effects on the properties of hardmetals(e.g.,hardness),but the established production processes of WC are limited to grain sizes of about 150 nm.To produce WC powder with grain sizes in the lower nanometer range,an alternative WC production process based on the chemical vapor transport(CVT) reaction of WO_(3) and H_(2)O forming gaseous WO_(2)(OH)_(2) at about 1100 ℃,followed by a carburation reaction with H_(2)/CH_(4)-gas mixtures was investigated.The influences of different process parameters such as furnace temperature,humidity and gas flows were investigated to improve the process.With the right set of parameters the produced powder consisted mainly of agglomerated WC grains with a size of about 5 nm.Beside the common hexagonal WC phase,the cubic WC1-xphase was stabilized due to the small crystallite sizes.In addition,a thin layer of amorphous carbon was present on the powder surface due to the catalytic methane decomposition on the WC surface.The amount of oxidic and metallic residues in the product powder was minimized with the parameter optimization and the powder yield was increased up to about 50%.With further optimization of the process parameters and usage of improved flow breakers,the purity and yield of the product powder can be further improved.Since an application in the hardmetal section is not realistic at the moment,applications in the catalysis sector could be considered due to the small grain size and good catalytic activity of the cubic WC1-xphase.展开更多
The industrial production of tungsten powder is carried out by the reduction of tungsten oxide powder via hydrogen.In this process,the size of the W particles is limited to particle sizes larger than 100 nm.To get bel...The industrial production of tungsten powder is carried out by the reduction of tungsten oxide powder via hydrogen.In this process,the size of the W particles is limited to particle sizes larger than 100 nm.To get below this limit,alternative processes are needed.In the current work,the possibility of preparing W powder below 100 nm via a vapour phase reduction of volatile WO_(2)(OH)_(2)by hydrogen was investigated.The process consists of two stages.In the first stag,e WO_(2)(OH)_(2)is formed by reacting WO3 with water vapour at temperatures of 1000–1100°C.In the second stage,WO_(2)(OH)_(2)is reduced by hydrogen at about 1000°C to form metallic tungsten.The influence of process parameters such as furnace temperature,humidity and gas flow on the WO_(2)(OH)_(2)evaporation and formation of tungsten powder was investigated.The characterization of the resulting powders was performed by X-ray diffraction(XRD),field emission scanning electron microscopy(FE-SEM)and transmission electron microscopy(TEM).By optimization of the reaction conditions,powder with a metallic tungsten content of about 70 at%besides tungsten oxides was produced with metal particle sizes down to 5 nm.Further optimization should lead to a high tungsten content and a high product yield.Due to the small particle size,applications in catalysis might be possible,although an industrial realization of the process seems unrealistic at moment.展开更多
文摘Lower WC grain sizes in the nanometer range have positive effects on the properties of hardmetals(e.g.,hardness),but the established production processes of WC are limited to grain sizes of about 150 nm.To produce WC powder with grain sizes in the lower nanometer range,an alternative WC production process based on the chemical vapor transport(CVT) reaction of WO_(3) and H_(2)O forming gaseous WO_(2)(OH)_(2) at about 1100 ℃,followed by a carburation reaction with H_(2)/CH_(4)-gas mixtures was investigated.The influences of different process parameters such as furnace temperature,humidity and gas flows were investigated to improve the process.With the right set of parameters the produced powder consisted mainly of agglomerated WC grains with a size of about 5 nm.Beside the common hexagonal WC phase,the cubic WC1-xphase was stabilized due to the small crystallite sizes.In addition,a thin layer of amorphous carbon was present on the powder surface due to the catalytic methane decomposition on the WC surface.The amount of oxidic and metallic residues in the product powder was minimized with the parameter optimization and the powder yield was increased up to about 50%.With further optimization of the process parameters and usage of improved flow breakers,the purity and yield of the product powder can be further improved.Since an application in the hardmetal section is not realistic at the moment,applications in the catalysis sector could be considered due to the small grain size and good catalytic activity of the cubic WC1-xphase.
文摘The industrial production of tungsten powder is carried out by the reduction of tungsten oxide powder via hydrogen.In this process,the size of the W particles is limited to particle sizes larger than 100 nm.To get below this limit,alternative processes are needed.In the current work,the possibility of preparing W powder below 100 nm via a vapour phase reduction of volatile WO_(2)(OH)_(2)by hydrogen was investigated.The process consists of two stages.In the first stag,e WO_(2)(OH)_(2)is formed by reacting WO3 with water vapour at temperatures of 1000–1100°C.In the second stage,WO_(2)(OH)_(2)is reduced by hydrogen at about 1000°C to form metallic tungsten.The influence of process parameters such as furnace temperature,humidity and gas flow on the WO_(2)(OH)_(2)evaporation and formation of tungsten powder was investigated.The characterization of the resulting powders was performed by X-ray diffraction(XRD),field emission scanning electron microscopy(FE-SEM)and transmission electron microscopy(TEM).By optimization of the reaction conditions,powder with a metallic tungsten content of about 70 at%besides tungsten oxides was produced with metal particle sizes down to 5 nm.Further optimization should lead to a high tungsten content and a high product yield.Due to the small particle size,applications in catalysis might be possible,although an industrial realization of the process seems unrealistic at moment.
