Catalytic growth of metallic tungsten whiskers based on the vapor–solid–solid mechanism

Abstract

Metallic W whiskers with tip diameters of 50-250 nm and lengths of 2-4 µm have been successfully synthesized in large quantities using Co-Ni alloyed catalysts. The relatively low growth temperature of 850 °C and the large catalyst size (over 100 nm) suggest that the growth of the W whiskers must be governed by the vapor-solid-solid mechanism. Our results show that the vapor-solid-solid model is suitable not only for the growth of nano-scaled whiskers with diameters below 100 nm, but also for submicro-scaled whiskers with diameters well above 100 nm. This technique has great potential to synthesize well controlled metallic whiskers.

Concurrent Growth of Kirkendall Pores and Vapor–Solid–Solid Protuberances on Ni Wires During Mo Vapor-Phase Deposition

“…Additional features, such as faceted tips and tapered sides, also support the VSS growth mode, as emphasized by previous studies. [23][24][25]36,37] Although mentioned by other researchers, the possibility concerning the size effect on the melting temperature depression is not applicable to our case because all protuberances have tip sizes far exceeding the size threshold (<100 nm). [19,20,25] Further compelling evidence for the operation of VSS is the presence of surface terraces and steps in sizeable quantities.…”

“…[19,21,[23][24][25][35][36][37][38] Since the eutectic temperature of Ni-Mo binary system is 1323°C, [33] markedly above the processing temperature of 1273 K (1000°C), and since there are no other substances in liquid form available that could function as effective catalyst, the VLS option can be eliminated, leaving VSS as the active mechanism for the growth of protuberance. Additional features, such as faceted tips and tapered sides, also support the VSS growth mode, as emphasized by previous studies.…”

“…The formation of intermetallic phases, according to prior investigations, could significantly accelerate the VSS process. [21][22][23][24][25]38] Additionally, Kirkendall pores, which reduce the interdiffusion fluxes of Mo and Ni, could enhance the outward growth of Mo-rich protuberances.…”

“…Researchers have recently demonstrated that nanowires and/or whiskers may grow below the eutectic temperature without the presence of liquid phases in a vaporsolid-solid (VSS) mechanism which is explained by the size-dependent eutectic temperature depression hypothesis [19,20] or by the formation of catalyzing intermetallic phases. [21][22][23][24][25] However, prevailing studies on both VLS and VSS are mostly addressing nanowire and/or whiskers growth at the sub-micron and nanometric scale. We are not aware of reports on VLS or VSS features with supra-micron scales, as described in the present study.…”

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.…”