The fabrication of dense W composites dispersed with La2O3 and Y2O3 nanoparticles had been attempted by combining a spark plasma sintering with post hot isostatic pressing. W powders dispersed with ...La2O3 particles were prepared by ultrasonic spray pyrolysis using ammonium metatungstate hydrate and lanthanum nitrate, and then yttrium nitrate was added to finally synthesize W-La2O3-Y2O3 composite powders. TEM analysis revealed that La2O3 and Y2O3 nanoparticles were uniformly distributed in the synthesized W composite powders. The relative density of composites prepared by spark plasma sintering was found to increase with increasing amounts of oxide particles and also with post hot isostatic pressing. The densified composites containing 1 wt% each of La2O3 and Y2O3 particles showed smaller grain size and higher hardness than that containing 0.5 wt% each of oxide particles. Changes in relative density, grain size and hardness with the oxide content are discussed considering the observed microstructural features and the role of oxide particles in mass transport and grain growth.
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•W-La2O3-Y2O3 powders were prepared by ultrasonic spray pyrolysis combined with polymeric additive method and H2-reduction.•Synthesized W-La2O3-Y2O3 powder showed homogeneously distributed dispersoids.•Sintering was performed by pre-spark plasma sintering and post-hot isostatic pressing.•W-La2O3-Y2O3 exhibited near 99% relative density and superior mechanical properties.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Interface engineering of thermoelectric powder materials via atomic layer deposition (ALD) has attracted significant research interest owing to the dramatic improvement in energy conversion ...efficiency. Using ALD to uniformly coat ultrathin (a few nanometers) ZnO layers on the microscale irregular shape of bismuth telluride-based powders is a challenge. An ALD reactor that fluidizes or agitates the powders can be adapted for this purpose. In this study, two types of ALD reactors, a gas fluidization reactor and a rotary reactor, were used to coat selenium-doped bismuth telluride powders with ZnO. Uniform and conformal ZnO layers were successfully grown using both the ALD reactors. However, the crystalline structure, particle size distribution, and chemical bonding states of ZnO were affected by reactor type. Pelletization of the ALD-coated powders was performed by spark plasma sintering at a high temperature (500 °C) and pressure (60 MPa). The morphologies of the powders did not change with palletization; however, differences in the chemical states of the ZnO layers on the BTS powders were observed. It was observed that the remnant water molecules and mobile ion species might compensate for the carrier mobility in pellets made of ALD-coated powders.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We fabricated various morphological SnO2 nanofibres (NFs), which are controlled from dense to fibre-in-hollow and hollow nanostructures, by electrospinning with the introduction of camphene. To ...control the morphologies of the SnO2 NFs, the relative loading amounts of camphene to solvent were adjusted to be 0, 50, and 125wt%. As a result, the SnO2 NFs formed at 0wt% camphene showed a dense nanostructure due to the shrinkage effect caused by the thermal decomposition of the polyvinylpyrrolidones. For 50wt% camphene, the SnO2 NFs formed the fibre-in-hollow nanostructure because of the occurrence of the Kirkendal effect on the surface of the as-spun SnO2 NFs. For 125wt% camphene, the morphology exhibited a hollow nanostructure because of the occurrence of the Kirkendal effect on the entire area of the as-spun SnO2 NFs. Thus, as the loading amount of camphene was increased, the morphology of the SnO2 NFs changed from dense to fibre-in-hollow and to hollow.
•Different morphological SnO2 nanofibres (NFs) synthesized by electrospinning.•A study on formation mechanisms for different morphological properties by camphene.•Fibre-in-hollow nanostructure formed by Kirkendal effect on surface of as-spun NF.•Hollow nanostructure formed by Kirkendal effect on entire area of as-spun NF.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Oxide-dispersion-strengthened tungsten (ODS-W) is the most promising structural materials for military, aerospace and nuclear industries because of their excellent mechanical properties and stability ...at high temperature. However, conventional mechanical milling methods have reached certain limits in further improving the properties of W and its alloys. In this study, with the aim of synthesizing W based composite powder with homogeneously dispersed yttrium oxide (Y2O3) nanoparticles, a novel process methodology combining ultrasonic spray pyrolysis and subsequent hydrogen reduction has been described. Our results reveal that the W-Y2O3 composite powders consisted of multiple primary particles of approximately 30 nm with a three-dimensional network and are of high purity. TEM and XPS analysis results demonstrated that Y2O3 nanoparticles were homogeneously dispersed into the tungsten matrix. In comparison with pure W sintered body, the W-Y2O3 composite showed a refined grain structure, higher relative density and Vickers hardness, and the combination of an intergranular and a transgranular fracture mode and these were mainly attributed to the homogeneous dispersion of Y2O3 nanoparticles. Our work shows that ultrasonic spray pyrolysis and subsequent hydrogen reduction is a promising method for preparing high quality ODS-W with high density, fine grains and uniformly dispersed strengthening phase nanoparticles.
•A simple and novel process for ODS-W is demonstrated.•The process is based on an ultrasonic spray pyrolysis and subsequent hydrogen reduction.•W-Y2O3 composite powders consisted of multiple primary particles of approximately 30 nm with a three-dimensional network.•W-Y2O3 composite showed the combination of an intergranular and a transgranular fracture mode.•W-Y2O3 composite showed higher relative density and Vickers hardness compared to pure W.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The fabrication of dense tungsten was attempted with the combination process of presintering by spark plasma sintering and final hot isostatic pressing. The specimens fabricated by spark plasma ...sintering (1500–1600 °C) of commercial powder exhibited a relative density above 90.4%. In the case of the post hot isostatic pressing at 1650 °C for 2 h, improved relative density in the range of 96.4% to 97.2% was obtained. X-ray diffractometry and X-ray photoelectron spectroscopy analysis indicated that the sintered specimen was composed mainly of tungsten, and the trace amounts of metallic tungsten are bound with oxygen due to the surface oxidation. Strength measurement was conducted by biaxial bending test using a piston-on-three balls. Maximum flexural strength of 761 MPa were obtained for tungsten consolidated by a combined process of spark plasma sintering and hot isostatic pressing. The strength increase is explained by the increase in relative density rather than the effect of particle size.
•The dense W was fabricated by a combined process of spark plasma sintering and hot isostatic pressing.•The specimen fabricated by post hot isostatic pressing showed a typical intragranular fracture and a reduced porosity.•Maximum relative density and flexural strength of 97.2% and 761 MPa were obtained for consolidated W.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The characterization and understanding of the hydrogen reduction and sintering behavior of powder mixtures prepared from WO3 and WO3-NiO have been investigated. The nano-sized W and W-Ni powders were ...prepared by ball milling and hydrogen reduction of oxide powders. The reduction behavior is analyzed by temperature-programmed reduction method with different heating rates in Ar-10% H2 atmosphere. X-ray diffractometry analysis revealed that the oxide powders are changed to W and W-Ni powders with an average particle size of about 100 nm by hydrogen reduction at 800 °C for 1 h. The hydrogen reduction kinetics was evaluated by the amount of peak shift with heating rates. The activation energies for the reduction of pure WO3 and WO3-NiO, estimated by the slope of the Kissinger plot, were measured as 87.4–117.4 kJ/mol depending on reduction steps. The consolidated W-Ni by spark plasma sintering has relatively dense and large grains with neck growth by enhanced mass transport due to the addition of Ni. These results are help to optimize the powder synthesis process and to understand the hydrogen reduction behavior and Ni addition effect related to microstructure of powders and sintered bodies.
•Microstructure and hydrogen reduction behavior of WO3 and WO3-NiO powders were investigated.•The oxide powders by the hydrogen reduction at 800 °C showed W and W-Ni phase with a particle size of about 100 nm.•The activation energies for the reduction of oxide powders were measured as 87.4–117.4 kJ/mol depending on reduction steps.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The fabrication of W composite with homogeneous dispersion of Y
2
O
3
and La
2
O
3
nanoparticles was attempted with the combination process of ultrasonic spray pyrolysis, polymeric additive solution ...route and spark plasma sintering processes. The Y
2
O
3
dispersed W powders were prepared by ultrasonic spray pyrolysis using ammonium metatungstate hydrate and yttrium nitrate. The composite powders with homogeneously coated La
2
O
3
on their surfaces were synthesised by calcination of a precursor solution containing lanthanum nitrate and polyvinyl alcohol. Microstructural analysis revealed that composite powders can be successfully produced by the ultrasonic spray pyrolysis and polymeric additive solution route. The composite powder was consolidated by spark plasma sintering yielding a sound microstructure and high hardness. The fine grain size and increased density in composite were mainly attributed to the dispersion of oxide particles and to the enhanced sinterability by the formation of liquid-like phase. The increased hardness was discussed by the observed microstructural features.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The effect of powder characteristics on the sintering behaviour and microstructure of densified W by spark plasma sintering and post-hot isostatic pressing was investigated. W powder was prepared by ...ultrasonic spray pyrolysis of a W precursor and hydrogen reduction, and a commercially available powder was used for comparison. The synthesised W powders were present in the form of large agglomerates composed of nano-sized particles. The densified specimens prepared by ultrasonic spray pyrolysis showed lower relative density and larger grain size than when commercial powder was sintered under the same conditions. Microstructural observation for the specimen sintered using the powder of ultrasonic spray pyrolysis revealed that the relatively large pores are present along the grain boundaries. The observed microstructural features were discussed by sintering behaviour based on the effect of intra-agglomerate and inter-agglomerate pores in the initial powder on densification and grain growth.
The characteristics and hydrogen reduction behavior of W-0.8 wt% Ni-0.4 wt% Cu powder synthesized from WO3-NiOCuO composite powder have been investigated. The metal oxide powder mixtures were ...prepared using a ball milling process. XRD analysis and HR-TEM revealed that the oxide powders are changed to W and CuNi alloy powders with an average particle size of about 100 nm by hydrogen reduction. To understand the reduction behavior of oxide powders, TG analysis was performed, and the reduction kinetics was evaluated by the amount of peak shift with heating rates. The activation energies for the reduction of WO3-NiO-CuO, estimated by the slope of the Kissinger plot, were measured as 60.6-114.4 kJ/mol depending on reduction steps.
