Mechanism of the Single-Crystal Tungsten Whiskers Growth in the

J. Phys. Chem. C 2008, 112, 18455–18458

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Mechanism of the Single-Crystal Tungsten Whiskers Growth in the Process of the NiWO4 Reduction by CO Nataliya F. Karpovich, Nataliya V. Lebukhova,* Victor G. Zavodinsky, and Konstantin S. Makarevich Institute for Materials Science of the Russian Academy of Sciences, Tikhookeanskaya 153, KhabaroVsk, 680042 Russia ReceiVed: June 19, 2008; ReVised Manuscript ReceiVed: September 19, 2008

Experimental evidence is presented that the growth of tungsten whiskers in the process of the NiWO4 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 NiWO4-Na2WO4 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 NiWO4 increases with addition of CuO and Na2WO4 which are capable to form easy-melting phases in the reactive system. Introduction In the past few decades, there has been considerable research and technological interest in new commercially viable processes for the growth of whiskers of metals and their compounds and in the underlying growth mechanisms.1-3 Particular attention has been paid to the preparation of single crystals no larger than 1 µm in cross-section size, as they possess the greatest perfection of structure and unique high mechanical strength, chemical stability, and unusual electrophysical characteristics. Some of the transition metals nanowires can find potential applications in gas sensors and as electron sources in electron microscopes. Even though many studies were concerned with the growth of whiskers of various materials, processes for the preparation of hard-melting metal whiskers have been described in very few reports. There is limited information regarding the growth of tungsten whiskers, predominantly dendritic, through hightemperature halide vapor transport,1 and tungsten oxide whiskers in the presence of halides of other metals (Ni, Pt, Pd) in hydrogen atmosphere.4 The vapor-liquid-solid (VLS) method is very effective in preparing of nanoscale single crystals, but according to Givargizov1 it is rather difficult to use it for the growing of hard-melting metals whiskers because of the large difference between the formation temperatures of stable liquid phases and the decomposition temperatures of gaseous compounds of the metals. Recently, there are several reports about fabrication methods of tungsten nanowires with using chemical vapor deposition process and catalytic metal systems. The condensation of tungsten oxide above decomposition temperature (∼1450 °C) leads to nucleation and growth of tungsten metal nanowires with a thickness of 40-70 nm.5 Crystalline tungsten nanowires were prepared from the directionally solidified NiAl-W alloys cooling starting from 1700 °C.6 The growth of tungsten nanowires at 850 °C had been found as result of the thermal treatment of tungsten oxide films in H2 gas,7 and also has been induced by using of pure Ni or (Fe-Ni) alloy * To whom correspondence should be addressed. Phone:(4212)226598. Fax: (4212)226598. E-mail: [email protected].

catalysts.8,9 According to Wang S. et al.,8,9 the formation of Ni and (Fe-Ni)-catalyzed W nanowires should be controlled by the vapor-solid-solid mechanism, rather than the traditional vapor-liquid-solid mechanism, because the growth temperature is significantly less than the lowest eutectic temperature of the Fe-Ni-W ternary system (1455 °C) and of the Ni-W binary system (1495 °C). Our investigations have shown10 that tungsten whiskers can be grown by reduction of NiWO4 at 950 °C in the flow of CO. The tungsten crystals orientated in the κ growth direction have hexagonal pencil-like cross sections, with lengths of 10-50 µm and thicknesses of 0.2-0.3 µm. Growth of whiskers was accompanied by the formation of liquid phases, and this fact allows us to assume realization of the VLS mechanism. The purpose of this work is to investigate the conditions and mechanisms of the tungsten whiskers growth. Experimental Methods Nickel tungstate, NiWO4, was prepared by grinding a stoichiometric mixture of WO3 and NiO in an agate mortar with followed annealing at 600 °C for 6 h and then additionally at 800 °C for 12 h. The tungstates of molar parities NiWO4/ Na2WO4 ) 1:0.15, 1:0.03 and nickel tungstate with oxides of molar parities NiWO4/MO ) 1:0.15 (M ) Cu, Ba) were mixed and chafed in the agate mortar. The particle size distribution was evaluated using Analyzette 22 Comfort laser analyzer. Most of the particles of the resultant nickel tungstate powder were 14-20 µm in diameter. Samples up to 500 mg in weight were reduced in alundum boats mounted in an SNOL 0.2/1250 tubular furnace, with a programmed heating rate of 15 °C/min. The samples were heated to the work temperature in the argon atmosphere, and then CO gas was introduced into the furnace at the flow rate of 15 L/h for 2 h. The particle size, morphology, and element composition of reduction products were determined by scanning electron microscopy (SEM) using the LEO-1420 instrument equipped with a Rontec energy-dispersive X-ray spectrometer. The

10.1021/jp805430b CCC: $40.75  2008 American Chemical Society Published on Web 10/31/2008

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Karpovich et al.

Figure 1. XRD patterns of samples obtained by reducing NiWO4 in flowing CO at different temperatures.

element composition was also refined using the JEOL 35-SDS wavelength-dispersive spectrometer. The crystal structure of the whiskers was determined using the UMV-100K transmission electron microscope. The phase composition of reaction products was determined by X-ray diffraction (XRD) using the DRON-7 diffractometer with CuKR radiation. The phases were identified using JCPDS data. The intermetallic compounds NiW and Ni4W were identified using the interplanar spacing reported by Bashev et al.11 and Pavlova,12 respectively. Results and Discussion The growth of W crystals during the reduction of nickel tungstate has been observed starting from 880 °C, and the number of crystals increased markedly when the temperature was raised to 1000 °C. Figure 1 displays the XRD patterns of samples obtained by reducing NiWO4 in the CO flow for 1 h at 800, 850, 900, and 950 °C. None of the diffraction patterns show reflections from nickel tungstate, indicating that the metals in the reduced and intermediate oxide phases are fully separated as a result of a selection process. At temperatures below 900 °C, the metals are allocated from NiWO4 with significantly different rates. Starting from 800 °C, we observe the formation of a nickel-based solid solution evidenced by changes of the angle of the peak position conforming to the lattice parameter of Ni from 2θ ) 44.505° to 2θ ) 44.03° corresponds to the incorporation of up to 10 at. % W.13 The formation of the intermediate oxide WO2 under these conditions is quite consistent with the phase formation sequence in the CO reduction of WO3, which was studied in detail by Venables and Brown: 14

650°C

750°C

800°C

900°C

WO3 98 WO2,9 98 WO2,72 98 W,WO2 98 W,WC

(1) Raising the reduction temperature to 850 °C decreases the WO2 content in the samples and, accordingly, increases the content of tungsten reduced to the metallic state. According to the phase diagram of the Ni-W system,15 this may lead to the formation of tungsten-enriched Ni-based solid solutions or

intermetallic compounds. The XRD patterns of the samples reduced at 850 °C show a broad complicated diffraction peak in the angular range of 2θ ) 43-44°. Its major components, together with other diffraction peaks, are attributable both to Ni-based solid solutions containing more than 10 atomic percent of W and to the intermetallic phase Ni4W. Reduction at 900 °C leads to the formation of the more tungsten-rich phase NiW and R-W. Raising the reduction temperature to 950 °C considerably increases the intensity of the diffraction peaks corresponding to R-W. In both cases, the presence of the intermediate oxide WO2.72 attests to changes in the mechanism of the process, which is no longer consistent with the above scheme of the CO reduction of WO3. According to the phase diagram of the Ni-W system,15 the Ni4W phase is formed by a peritectoid reaction between Ni and W at 970 °C, and the NiW phase, found in alloys prepared by the diffusion couple method, is formed at 1060 °C. At the same time, Bashev et al. 11 reported the NiW formation in the temperature range of 600-900 °C during the crystallization of thin amorphous films produced by ion-plasma sputtering of Ni and W in various ratios. In the amorphous films containing more than 50 at. % W, the initial stages of heating led to nucleation of a phase with the intended composition, while rise of the heat-treatment temperature to 890 °C resulted in the formation of two phases: namely, W and NiW. It seems likely that it is this mechanism, rather than diffusion saturation of the metals, which underlies phase formation during the coreduction of W and Ni in the crystal lattice of nickel tungstate. The composition of the phases forming in the equilibrium state is then governed by the relative contents of reduced metals in the reaction zone. In our experiments, rapid growth of tungsten whiskers was observed only at 950 °C. At the same time, the samples, reduced in the flow at 900 °C, have contained elongated tungsten crystals of up to 10 µm of length. This gives some grounds to believe that, during further reduction, the tungsten single crystals appearing at temperatures above 900 °C in the two-phase region R-W + NiW may serve as whisker growth surfaces. The R-W + NiW phase assemblages were observed primarily in regions where liquid zones were formed, eliminating kinetic limitations on the CO reduction of tungsten oxides, and where Ni and W were allocated from the crystal lattice of nickel tungstate with comparable rates.

NiWO4 Reduction by CO

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Figure 3. Electron micrographsy of the whiskers grown through the reduction of mixture of NiWO4/Na2WO4 (1:0.03 molar parities) in flowing CO.

Figure 2. Electron micrographys (at different magnifications) of whiskers grown through the reduction of NiWO4 in flowing CO. Inset: selected area electron diffraction pattern.

The morphology of the whiskers prepared at 950 °C is illustrated in Figure 2. There are the both isolated and agglomerated crystals containing several (as a rule five-ten) whiskers. According to the XRD patterns WO2.72 content in the sample increases at the reduction temperature of 950 °C. It is known14,16 that that the whiskers tungsten oxides WOx are condensed usually in form nanotubes with a hollow inside, although they look very similar to the tungsten whiskers, obtained in our experiments. The electron diffraction pattern (in the inset to Figure 2) corresponds to the cubic structure of R-W, with a lattice parameter of 0.316 nm. The elemental local analysis of the isolated tungsten whiskers indicates that the W concentration is equal to 99.75-99.85 wt % with the rest of Ni. In agglomerated crystals the Ni concentration is of 3 wt percent, and the Na concentration is of 1.02 wt %. According to phase-diagram data, the NiWO4 melts at 1420 °C, and the NiWO4 + WO3 eutectic melts at 1250 °C. The chemicals reagent-compositions used in this study contained up to 0.1 wt % of alkali metals as impurities. The segregation of mobile sodium ions in tungstate particles during heating may create the local zones with a sodium concentration enough to form thin layers of the NiWO4-Na2WO4 solid dissolutions, which melt at low temperatures because of the low melting point of Na2WO4 (698 °C). It is well-known1 that the formation of easy-melting-point eutectics needed for whiskers growth can be ensured by introducing some elements into the starting mixture.1 The additives of sodium tungstate and oxides of bivalent metals capable to form tungstates MWO4 and eutectics MWO4 + WO3

have been used for preparation of the reaction mixes with NiWO4. As shown above, intensive growth of whiskers of tungsten at the reduction of NiWO4 has been registered at the interval of 890-950 °C. Therefore the oxides for a forming the tungstates, which have melting point below 890 °C (Na2WO4), at 930 °C (CuWO4), and also in the field of temperatures above 950 °C (BaWO4) have been chosen. The reduction of the reaction mixes by CO gas has been fulfilled at 950 °C. Whereupon the samples has been analyzed by X-ray diffraction in order to verify any possible reaction. Some alloyed phases of light-yellow color with a metallic luster were found in the products of reduction mixes NiWO4 with Na2WO4 (1:0.15 molar parities); they were identified as tungsten bronzes of variable composition NaxWO3. The addition small quantity of Na2WO4 (1:0.03 molar parities) promoted full reduction of system to NiW and R-W. The morphology of prepared tungsten crystals of 10-20 µm length is illustrated in Figure 3. The local element analysis of fine-grained phases with grains of the 1-2 µm cross section indicates both W and NiW phases (the ratio Ni/W is close to equimolar). It is possible to conclude that the additive of sodium tungstate promoted the reduction of the significant part of the W in the granular form at earlier stages of the process which precede growth of the tungsten whiskers. The XRD analysis has shown that the solid phase reactions between NiWO4 and oxides BaO and CuO during heating up to 950 °C of the mixes result in formation of corresponding tungstate BaWO4, and CuWO4, which cannot be reduced by CO. In case of the addition BaO, the reductions are accompanied by the formation of NiW phase and R-W. Comparing the intensity of growth and the sizes, the tungsten whiskers of 10-50 µm length and of 0.2-0.3 µm thickness are similar to the crystals formed at the reduction of NiWO4 without the additives of other oxides. It must be noted that there are a large quantity of the melted particles in the products of reduction and even the molten phase on the surface of the whiskers (Figure 4). According to phase-diagram data,17 the melting point of the BaWO4 + WO3 (75 mol %) eutectic is equal to 935 °C. The element analysis of the melted areas has shown that their composition is close to Ba0.25WO4. In accordance with the XRD analysis, the addition of CuO leads to the R-W and Ni metal phases as the products of reduction. The tungsten whiskers of the 20-30 µm length are visible in the electron microscope picture (Figure 5), and there are the agglomerated crystals

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Figure 4. Electron micrographs of the whisker grown through the reduction of mixture of NiWO4/BaO (1:0.15 molar parities) in flowing CO.

Karpovich et al. of R-W, NiW, and WO2.72. Raising the reduction temperature up to 950 °C considerably increases the intensity of the diffraction peaks corresponding to R-W. The rates of the allocation of Ni and W from nickel tungstate become the comparable in the regions of the melting; it leads to the formation of the Ni-W phase with a very high content of W. The tungsten single crystals appearing within the two-phase R-W + NiW system can serve as the nucleuses for the oriented whiskers growth during further reduction of the tungstate system. The electron diffraction the tungsten whiskers corresponds to the cubic structure of R-W, with a lattice parameter of 0.316 nm. A liquid phase appears at 940 °C owing to the melting of NiWO4-Na2WO4 eutectic regions which seem to be a result of the segregation of sodium impurities present in the reagents used. We have experimentally shown, that the reactivity of NiWO4 increases due to addition of tungstate Na2WO4 or oxide CuO which are capable to formation of easy-melting phases in the reacting system. The reaction the NiWO4-Na2WO4 mixture with CO results in the complete reduction of NiWO4 and to the formation of significant part of W in the granular form. The addition of CuO leads to the formation of the large agglomerated crystals containing of 20-50 tungsten whiskers. Acknowledgments. . This work was supported by the Russian Foundation for Basic Research, Grant No. 08-02-98500. The authors thank S. A. Pyachin, V. I. Palazhchenko, and M.A. Pugachevsky for the help to this work. References and Notes

Figure 5. Electron micrographs of the whiskers grown through the reduction of mixture of NiWO4/CuWO4 (1:0.15 molar parities) in flowing CO.

containing a large quantity (from 20 to 50) of the face-sharing whiskers. The element composition of fine-grained phases with the 1-2 µm cross-section diameter corresponds to the solid solution (82-85 wt % of Ni, the rest is Cu). Conclusions The XRD data indicate that the phase composition of samples obtained by reduction NiWO4 in the flow of CO at 800, 850, 900, and 950 °C is governed by the relative contents of reduced metals in the reaction zone. Reduction at 800 °C leads to the predominant formation of WO2 and a nickel-based solid solution containing up to 10 at. % of tungsten. The raising of reduction temperature to 850 °C decreases the WO2 content in the samples and leads to the formation of the more tungsten-rich nickelbased solid solutions. The samples reduced at 900 °C consist

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