Preparation of nano-crystalline tungsten powders from gaseous WO2(OH)2

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 WO2(OH)2 by hydrogen was investigated. The process consists of two stages. In the first stag,e WO2(OH)2 is formed by reacting WO3 with water vapour at temperatures of 1000–1100 °C. In the second stage, WO2(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 WO2(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.

Microwave sintering of tungsten heavy alloys

To understand microwave sintering of heavy alloys with high tungsten content, 93W-4.9Ni-2.1Fe alloy was sintered using a 6 kW, 2.45 GHz microwave sintering furnace at 1783 K (1510˚ºC) and 1793 K (1520˚ºC). The alloy sintered at 1793 K (1520˚ºC) achieved full densification and had improved microstructural features, superior mechanical properties compared to 99.4% densification and relatively inferior properties obtained in the alloy sintered at 1783 K (1510˚ºC). This study also includes a comparison between microwave sintered and conventionally sintered 93W-4.9Ni-2.1Fe alloy (sintered at 1793K (1520ºC)). Contrary to the full densification and superior mechanical properties obtained in microwave sintering, conventional sintering at 1793K (1520ºC) resulted in only 99.6% densification and substantially inferior properties. Analyses of microstructure and fracture surface revealed that key microstructural parameters such as tungsten grain size, tungsten-tungsten contiguity, matrix volume fraction and also the fracture mode were significantly different between the alloys processed by the two routes. Possible reasons behind dissimilar densification, significantly different microstructures and mechanical properties obtained between these two modes of sintering, are also discussed in this study.

Fabrication and Properties of W-Cu Functionally Graded Material by Tape-Casting

In this paper, the W-Cu functionally graded material (FGM) was prepared by using the non-aqueous tape-casting technique combined with vacuum hot-pressing sintering. The graded composite material with high density, uniform transition and graded component was designed by 7 layers with the copper content range from 40 to 100 wt. %. Then the structures and properties of the composite were characterized. The scanning acoustic microscope (SAM) results for the W-Cu graded material showed that the interface between different layers was of high smoothness and parallel. The SEM-EDS results of cross section show that the W and Cu content changed gradually along the laminating direction after sintering. The equivalent electrical conductivity and the equivalent thermal conductivity of the W-Cu graded material were 0.3976×108 S/m and 323.5 W/(m·K), respectively, which were much higher than that of the W-40 wt. % Cu homogeneous composite. The Vickers hardness of the high tungsten content surface and the high copper surface were 163 HV and 80 HV, respectively, which were same with that of the homogeneous material.