• Korean Journal of Materials Research
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• v.13 no.3
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• pp.169-173
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• 2003

W-Cu composite powders can be prepared by mechanochemical process, where the $WO_3$-CuO composite powders were mechanically synthesized from the elemental oxide powders and subsequently reduced to W-Cu composite powders. In the present work, reduction behavior of$ WO_3$-CuO composite powders that were synthesized at different milling time was examined in terms of hygrometric analysis. In case of $WO_3$-CuO ball-milled for 20 h, the reaction temperature of CuO\longrightarrowCu became lower than in case of 1 h. Also, the reaction of $WO_3$\longrightarrow$WO_{2.9-2.72}$ and $WO_{2.9-2.72}$ \longrightarrow$WO_2$were shifted to lower temperatures and the peaks were changed to much sharper shape. While the reaction of $WO_2$\longrightarrowW in case of ball-milling for 20 h started at lower temperature, the peak temperature was the same as in 1 h ball-milling. The reduced W particle size was somewhat finer fer 20 h ball-milling. It was considered that the refinement of oxide particles caused by ball-milling process leads to such a change in the reduction behavior.

• Korean Journal of Materials Research
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• v.27 no.10
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• pp.513-517
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• 2017

An optimum route to fabricate a hybrid-structured W powder composed of nano and micro size powders was investigated. The mixture of nano and micro W powders was prepared by a ball milling and hydrogen reduction process for $WO_3$ and W powders. Microstructural observation for the ball-milled powder mixtures revealed that the nano-sized $WO_3$ particles were homogeneously distributed on the surface of large W powders. The reduction behavior of $WO_3$ powder was analyzed by a temperature programmed reduction method with different heating rates in Ar-10% $H_2$ atmosphere. The activation energies for the reduction of $WO_3$, estimated by the slope of the Kissinger plot from the amount of reaction peak shift with heating rates, were measured as 117.4 kJ/mol and 94.6 kJ/mol depending on reduction steps from $WO_3$ to $WO_2$ and from $WO_2$ to W, respectively. SEM and XRD analysis for the hydrogen-reduced powder mixture showed that the nano-sized W particles were well distributed on the surface of the micro-sized W powders.

• Journal of Powder Materials
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• v.13 no.3 s.56
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• pp.205-210
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• 2006

The effect of Cu content on hydrogen reduction behavior of ball-milled $WO_3$-CuO nanocomposite powders was investigated. Hydrogen reduction behavior and reduction percent(${\alpha}$) of nanopowders were characterized by thermogravimetry (TG) and hygrometry measurements. Activation energy for hydrogen reduction of $WO_3$ nanopowders with different Cu content was calculated at each heating rate and reduction percent(${\alpha}$). The activation energy for reduction of $WO_3$ obtained in this study existed in the ranging from 129 to 139 kJ/mol, which was in accordance with the activation energy for $WO_3$ powder reduction of conventional micron-sized.

• Journal of Powder Materials
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• v.10 no.3
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• pp.186-189
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• 2003

A chemical vapor condensation (CVC) process using the pyrolysis of metal-organic precursors was applied to produce the nanosized $WO_3$ powders. Morphology and phase changes of the synthesized $WO_3$ powder as a function of CVC parameters were investigated by XRD, BET and TEM. The agglomerated nanosized monoclinic $WO_3$ powders with nearly spherical shape and 10-38 nm in mean diameter could be obtained. Conditions to produce the $WO_3$ nanopowders are presented in this paper.

• Journal of Powder Materials
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• v.9 no.2
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• pp.89-95
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• 2002

Spherical fine powders of tungsten oxide powders were prepared by the emulsion evaporation method. The characteristics of the powders prepared were examined by means of TGA, X-ray diffraction, SEM and image analysis. The emulsions were prepared by fast mixing of aqueous phase containing tugsten and the organic phase which composed of kerosene, surfactant, and paraffin oil. Precursors were made by evaporating the emulsionin the kerosene bath at $160^{\circ}C$, and then calcined at $650^{\circ}C$ in order to produce tungsten oxide powders. The average particle size of the tungsten oxide powders was $0.5\mutextrm{m}$ and their shapes were spherical at the both case of w/o and o/w type emulsions. As the HLB value of the surfactant increased and the concentration of tungsten ions decreased the mean particle siqe of tungsten oxide powders decreased whereas agglomerationsize increased. The optimum concentration of Span 80 was 8 percent by volume, and the optimum stirring speed in the emulsion formation was 5000 rpm in order to obtain fine and well dispersed $WO_3$ powders.

• Korean Journal of Materials Research
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• v.21 no.6
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• pp.299-302
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• 2011

[ $WO_3$ ]powders were ball-milled with an alumina ball for 0-72 hours. $In_2O_3$ doped $WO_3$ was prepared by soaking ball-milled $WO_3$ in an $InCl_3$ solution. The mixed powder was annealed at $700^{\circ}C$ for 30 min in an air atmosphere. A paste for screen-printing the thick film was prepared by mixing the $WO_3$:In2O3 powders with ${\alpha}$-terpinol and glycerol. $In_2O_3$ doped $WO_3$ thick films were fabricated into a gas sensor by a screen-printing method on alumina substrates. The structural properties of the $WO_3$:$InO_3$ thick films were a monoclinic phase with a (002) dominant orientation. The particle size of the $WO_3$:$InO_3$ decreased with the ball-milling time. The sensing characteristics of the $In_2O_3$ doped $WO_3$ were investigated by measuring the electrical resistance of each sensor in the test-box. The highest sensitivity to 5 ppm $CH_4$ gas and 5 ppm $CH_3CH_2CH_3$ gas was observed in the ball-milled $WO_3$:$InO_3$ gas sensors at 48 hours. The response time of $WO_3$:$In_2O_3$ gas sensors was 7 seconds and recovery time was 9 seconds for the methane gas.

• Journal of the Korean Ceramic Society
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• v.39 no.3
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• pp.321-326
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• 2002

ZnWO$_4$ nano-powders were successfully prepared by polymerized complex method using zinc nitrate and tungstic acid as starting materials. In order to investigate the thermal decomposition and crystallization process, the polymeric precursors were heat-treated at temperatures from 300 to 600$^{\circ}$C for 3 h, and the heat-treated powders were characterized by XRD and FTIR. The surface morphology of the heat-treated powders were observed using SEM and TEM. The crystallite size was measured by X-ray analysis. Crystallization of the ZnWO$_4$ powders were detected at 400$^{\circ}$C and entirely completed at a temperature of 600$^{\circ}$C. The particles heat-treated 400 and 500$^{\circ}$C showed primarily co-mixed morphology with spherical and silkworm-like forms, while the particles heat-treated at 600$^{\circ}$C showed more homogeneous morphology. The average crystalline size were 19.9∼24.nm showing an ordinary tendency to increase with the temperatures from 400 to 600$^{\circ}$C.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.24 no.5
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• pp.213-218
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• 2014

An investigation of the influence of $WO_3$ addition with different precursors and preparation methods on the phase formation and selective catalytic reduction (SCR) efficiency of anatase-$TiO_2$ powders has been carried out. An anatase-$TiO_2$ synthesized by precipitation process was used as a catalyst support. For $WO_3(10wt%)/TiO_2$, the W loading to the $TiO_2$ support led to the lower in anatase to rutile transition temperature to ${\sim}900^{\circ}C$ from $1200^{\circ}C$ of the $TiO_2$ support alone. In the case of $WO_3(10wt%)/TiO_2$ SCR powders obtained from a wet process with ammonium meta-tungstate (AMT) precursor, the highest $NO_X$ conversion efficiency was achieved at $450^{\circ}C$ remaining high efficiency at $500^{\circ}C$, while the same composition prepared from a dry process with $WO_3$ addition showed the lowered efficiency with temperature after reaching the efficiency maximum at $350^{\circ}C$. The same tendency has been found that the $V_2O_5(5wt%)-WO_3(10wt%)/TiO_2$ SCR powders obtained from the wet process with AMT precursor has shown the superior $NO_X$ conversion efficiency over 90 % in a wider temperature range of $300{\sim}500^{\circ}C$.

• Korean Journal of Materials Research
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• v.12 no.12
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• pp.930-934
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• 2002

Nanosized $WO_3$ powders were synthesized by the sol-precipitation process using $WCl_{6}$ as the starting material, ethanol as a solvent and $NH_4$OH solution as a precipitant, followed by a washing-drying treatment and calcination. The effects on the powder crystallinity and microstructure of calcination temperature were investigated with XRD and FE-SEM. The $WO_3$ powders calcined at $500^{\circ}C$ and $700^{\circ}C$ showed good crystallinity and their mean particle size was 30nm and 70nm, respectively. These powders were used for the preparation of pastes which were printed as thick films on alumina substrates with comb-type Pt electrodes. The particle size strongly influenced the $NO_2$ gas sensing property of the thick films. A significant reduction in the $NO_2$ sensitivity was observed for the film prepared from larger particle size, having thus a larger grain size. For the film having a smaller grain size, on the other hand, the higher $NO_2$ sensitivity was observed and the sensitivity increased with $NO_2$ concentration.

• Journal of Powder Materials
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• v.24 no.5
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• pp.384-388
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• 2017

The effect of the mixing method on the characteristics of hybrid-structure W powder with nano and micro sizes is investigated. Fine $WO_3$ powders with sizes of ${\sim}0.6{\mu}m$, prepared by ball milling for 10 h, are mixed with pure W powder with sizes of $12{\mu}m$ by various mixing process. In the case of simple mixing with ball-milled $WO_3$ and micro sized W powders, $WO_3$ particles are locally present in the form of agglomerates in the surface of large W powders, but in the case of ball milling, a relatively uniform distribution of $WO_3$ particles is exhibited. The microstructural observation reveals that the ball milled $WO_3$ powder, heat-treated at $750^{\circ}C$ for 1 h in a hydrogen atmosphere, is fine W particles of ~200 nm or less. The powder mixture prepared by simple mixing and hydrogen reduction exhibits the formation of coarse W particles with agglomeration of the micro sized W powder on the surface. Conversely, in the powder mixture fabricated by ball milling and hydrogen reduction, a uniform distribution of fine W particles forming nano-micro sized hybrid structure is observed.