Mechanism of the Single-Crystal Tungsten Whiskers Growth in the Process of the NiWO₄ Reduction by CO

Abstract

Experimental evidence is presented that the growth of tungsten whiskers in the process of the NiWO 4 reduction by carbon oxide occurs in accordance with the vapor-liquid-solid mechanism. A liquid phase appears at 940 °C owing to the melting of the NiWO 4 -Na 2 WO 4 eutectic regions which are perhaps results of segregation of sodium impurities included in the reagents used. The presence of the liquid phase ensures for both Ni and W the comparable rates of allocation from the nickel tungstate crystal lattice leading to the formation of the R-W + NiW two-phase regions. The oriented growth of tungsten whiskers during the further reduction of the melt tungstate zones is associated with the W accumulation on the growth surfaces of the W single crystals in the two-phase regions. The result demonstrates that the reactivity of NiWO 4 increases with addition of CuO and Na 2 WO 4 which are capable to form easy-melting phases in the reactive system.

Growth of tungsten whiskers oriented in <111> direction

“…The tungsten single crystals appearing within the two-phase α-W+NiW (or Ni 4 W) system at heat-treatment temperature above 850 °C can serve as the nucleuses for the oriented whiskers growth during further reduction of the tungstate system. As it follows from results of the article [4] and our researches [6,7] the different crystal orientation and form of the W crystals can be prepared at various conditions of NiWO 4 reduction. It is possible to assume, that the forming crystalline structure of the Ni-W metallic phases (Ni-W, Ni 4 W, NiW) has the dominant significance at the nucleation of the W crystals oriented in the <111> or <100> directions.…”

“…There are few reports devoted to fabrication of tungsten whiskers using chemical vapor deposition process and catalytic metal systems [1][2][3][4][5][6][7]. Special attention has been paid to the structure perfection and growth mechanisms of tungsten nanowires because they demonstrate unusually high mechanical strength, chemical stability and unique electrophysical characteristics.…”

“…Our previous investigations have shown [6,7] that tungsten whiskers can be grown by reduction of NiWO 4 in the flow of CO gas. The grown tungsten crystals had hexagonal pencil-like cross sections and were probably orientated in the <111> direction.…”

Low-temperature growth of tetragonal tungsten nanowire arrays on tungsten substrate using Ni solid catalysts

“…Very recently, W nanowires and the oxide nanowires, with well-defined singlecrystalline structure and unique geometry, have attracted intensive attention due to their great potential applications as field emitters and strength agents [2][3][4][5][6][7][8][9][10][11][12][13]. So far, various methods have been developed to fabricate W nanowires, such as direct CVD [2][3][4][5], metal-catalyzed CVD [14][15][16][17][18], electron beam induced deposition [19][20][21] and self-organization methods [22,23]. In terms of feasibility, metal-catalyzed CVD method has been developed to be one of the most successful methods for controlling synthesis of semiconductor nanowires [24][25][26].…”

Large-scale synthesis of tungsten single-crystal microtubes via vapor-deposition process

“…W whiskers with a single-crystalline structure are well known for their outstanding mechanical properties (tensile strength: $28 GPa and Young's modulus: $ 410 GPa) [29][30][31] and field emission properties [29,32]. To date, a number of techniques for fabricating W whiskers have been developed [32][33][34][35][36][37][38][39][40], among which the hightemperature chemical vapor-deposition (HT-CVD) method [34][35][36] has been demonstrated as a promising approach with great potential for the large-scale synthesis of W whiskers. In HT-CVD method, a high temperature (typically above 1500 1C) is required in order to generate gaseous tungsten oxides (WO x ), which possess a relatively low vapor pressure.…”