A large-scale fabrication of flower-like submicrometer-sized tungsten whiskers via metal catalysis
© Ma et al.; licensee Springer. 2012
Received: 20 May 2012
Accepted: 10 June 2012
Published: 21 June 2012
Tungsten powder mixed with an appropriate amount of nickel and iron powders is used as raw material to fabricate large-scale tungsten whisker-like structure. The morphology, microstructure and composition of the whisker-like tungsten are observed and tested by scanning electron microscope and FESEM, transmission electron microscopy, X-ray spectroscopy, and X-ray diffraction, respectively. The main component of the tungsten whisker-like structure is tungsten, which has the axial growth along the <100 > direction with large aspect ratio and possesses flower-like structure. Large-scale submicrometer-sized whisker-like tungsten was fabricated via vapor phase deposition approach with the aid of metal catalysts at 800°C by holding for 6 h in the appropriate atmosphere. The growth procedure of flower-like tungsten whisker is probably based on the vapor–liquid–solid mechanism at beginning of the formation of tungsten nuclei, then vapor-solid mechanism is dominant.
One-dimensional nanomaterials of tungsten have excellent performance of chemical, physical, electrical, and mechanical properties , so metallic tungsten one-dimensional nanomaterials have attracted considerable attention as promising materials for field emitters [2–4], displays , sensors [6, 7], etc. Many approaches were used to prepare tungsten one-dimensional nanomaterials, such as metal catalysis induction method [8, 9], vapor deposition [10, 11], template  or substrate , etching [14, 15], sputtering  or electron beam-induced deposition [3, 16], etc. However, organizing these one-dimensional nanomaterials into highly ordered arrays can be extremely challenging. The key problem in the preparation of tungsten one-dimensional nanomaterials is to control their morphology, size and uniformity effectively at the same time. So, many growth mechanisms had been proposed according to the experiment results to control the growth of tungsten one-dimensional nanomaterials accurately.
It is crucial to understand the tungsten one-dimensional nanomaterial (e.g., nanowires, nanofibers, nanorods, nanoneedles) growth mechanism for the growth control of tungsten nanomaterials accurately. Because of its high melting point, low atomic diffusion rate and other special properties, there are both similarities and differences between the growth mechanism of tungsten nanomaterials and general inorganic nanomaterials. For this reason, the researchers had proposed a number of growth models which could be used for reference, for example, VS model [17, 18], vapor–liquid–solid (VLS) model [19–22], VSS model [8, 23], etc. The growth mechanism is different with distinct preparation conditions.
In this research, a novel route with low cost was used, and large-scale flower-like submicrometer-sized tungsten whiskers were prepared by the vapor phase deposition method with the aid of metal catalysts. According to the experiment results, the growth model of tungsten whisker-like structure was proposed.
The raw materials are tungsten powders (purity, 99.5 wt.%; particle size, 1 to 3 μm), carbonyl iron powders (purity, 99.5 wt.%; particle size, 3 to 5 μm) and carbonyl nickel powders (purity, 99.5 wt.%; particle size, 3 to 5 μm). The mixed powders (weight ratio of W:Ni:Fe equals 93:4.9:2.1) were obtained by wet milling for 2 h, then the mixed powders with a weight of 2.0 g were reacted by holding for 6 h in a horizontal tube furnace at 800°C. The atmosphere was a mixed gas of N2, H2 and a little water vapor. The flow rates of N2 and H2 were 0.3 and 0.03 L/min, respectively. The water vapor was brought into the system by the water bath of H2, and the water bath temperature was 80°C. The as-products were characterized by X-ray diffraction (XRD), scanning electron microscopy and FESEM, energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), respectively.
Results and discussion
Characteristics of the submicrometer-sized tungsten whiskers
where is the Gibbs free energy diversification of the reduction system; is the Gibbs free energy at the standard state; R is constant, equal to 8.314 J·K−1·mol−1; T is reaction temperature; Q is pressure ratio of the materials at actual conditions; Kp is equilibrium constant; , and are the partial pressures of H2O, H2 and WO2(OH)2, respectively.
The oxidation of W by water vapor and the reduction of WO3 by H2 are reversible processes. The direction of reaction can be changed by adjusting the partial pressure ratio of [H2O]/[H2] and further affects the growing process of tungsten whisker-like structure. For the presence of volatile compounds of WO2(OH)2 in the chemical reaction, the reaction atmosphere has a significant impact on the final product. The premise of ensuring , increasing partial pressure ratio of water vapor and hydrogen, is conductive to the generation of many volatile compounds of WO2(OH)2,which is the origin of fabrication of tungsten whiskers by vapor phase transmission and reduction of hydrogen.
Due to the appearance of local high-temperature area, Ni and W can come into little solid solution (Ni4W). Otherwise, because of a considerable solid solution of W in Ni (about 16 at%) at 950°C and the very low solid solubility of Ni in W (about 0.05at%) , W atoms may dissolve into Ni particles to form solid Ni-W alloyed particles, and it is easy to reach saturation. W atoms diffuse through the solid Ni-W particle to form the nucleus of pure W nanocrystal. The subsequent W atoms integrate directly with the preformed W <100 > layer and result in the formation of a protrudent W <100 > whisker-like structure.
According to above description, the growth procedure of flower-like tungsten whiskers is probably based on the VLS mechanism at beginning of the formation of tungsten nuclei, that is, the VLS stage is from Figure8a-d. After that, vapor-solid mechanism is dominant (Figure8e,f).
In summary, a novel route with low-cost to large-scale flower-like submicrometer-sized tungsten whiskers were fabricated by the vapor phase deposition method with the aid of metal catalysts. A large-scale submicrometer-sized tungsten whisker-like structure had the axial growth along the <100 > direction with large aspect ratio and possessed flower-like structure grown from the aggregation formed by sintering of W powder mixed with Ni powder. The growth procedure of flower-like tungsten whiskers is probably based on the VLS mechanism at beginning of the formation of tungsten nuclei. After that, vapor-solid mechanism is dominant.
The authors thank the National Natural Science Foundation of China (grant no. 50774098) and Creative Research Group of National Natural Science Foundation of China (grant no. 50721003) for the financial support.
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