Letter pubs.acs.org/NanoLett
Formation of Metallic Glass Nanowires by Gas Atomization Koji S. Nakayama,*,† Yoshihiko Yokoyama,‡ Takeshi Wada,‡ Na Chen,† and Akihisa Inoue‡ †
World Premier Institution-Advanced Institute for Materials Research, and ‡Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ABSTRACT: Gas atomization which is a conventional technique in powder metallurgy is adapted for the formation of metallic glass nanowires. This approach is able to produce a large quantity of nanowires with diameters in the 50−2000 nm range. Experiments performed with different conditions and alloy compositions confirm that the key mechanism of the nanowire formation is the spinnability which increases exponentially when the melt stream is supercooled from the liquid state. KEYWORDS: Nanowire, metallic glass, gas atomization, spinnability
T
MG), which has much lower thermoplastic formability and narrow ΔTx (52 K).19 Figure 1 illustrates a typical gas atomizer composed of a quartz crucible and an atomizing die. About 40 g in weight of a metallic glass or a master alloy was heated by the induction heating coil. The melt stream was forced out by the differential pressure of 0.3 Kgf/cm2 through the crucible nozzle that had the diameter of 6 mm into the gas expansion zone. The gas nozzle diameter was 0.4 mm and 18 nozzles were circumferentially arranged with the inclined angle of 22.5°. The kinetic energy transferred from a high velocity jet-gas to the melt stream causes the fragmentation into a variety of shapes such as flakes, ellipsoids, ligaments, and droplets down to submicrometer sizes.17,20 The shapes and sizes can be varied by the acceleration force of the jet-gas pressure and the viscosity of a molten alloy. In this study, we changed the Ar gas pressure (PAr) and the induction heating temperature (Tin) to optimize the nanowire formation. The temperatures were monitored by an optical pyrometer with the emissivity of 0.35. The measured temperatures consist with the melting point (Tm) of each alloy and the pyrometer accuracy should be ±10 K. The sample was once heated above Tm and then the molten alloy was supercooled at Tin = 1103 K (cf. Tm = 1148 K), followed by the gas atomization with PAr = 105 Kgf/cm2. A lump of the nanowires with the size of about a centimeter is shown in Figure 2a. Figure 2b shows a scanning electron microscopy (SEM) image of the Zr-MG nanowires, forming a high density nanofiber. Each nanowire exhibits atomically smooth surfaces having the diameters as small as 50 nm, as shown in Figure 2c. In this experiment, the averaged diameter was about 480 nm and the distribution of the diameters is shown in Figure 2d. The analysis of electron induced X-ray fluorescence shows that they contain the original precursor compositions and a small amount of oxygen. A large quantity of
he superior functionalities in metallic glass such as ultrahigh mechanical strength, chemical activity, thermoplastic formability, and soft-magnetism are attractive for nanoscale devices and architectures.1,2 Recent attention has been focused on metallic glass micronano structures because they are promising candidates for applications in micronano electromechanical system devices,3,4 heterogeneous catalysts,5,6 data storage media,7,8 and magnetic sensors.9,10 Various nanostructures can be fabricated by top-down methods such as imprinting, lithography/dry etching, and injection molding which are powerful but costly micronano fabrication tools perfected in industry. A few other successful fabrication methods are the shear fracture,11,12 which leads to the nanowire formation during plastic deformation in the shear bands, and the fiber drawing method,13,14 which is based on superplastic deformation in the supercooled liquid region (ΔTx). The drawn amorphous nanowires could have a length up to a centimeter,13 which is far beyond the growth length of conventional crystalline nanowires because of the grain size limitations.15,16 However, these production methods still do not achieve low cost and high throughput. Further, a fundamental understanding of the formation mechanism is necessary for the achievement of high-yield productivity and reproducibility. In this paper, we report a new approach to the formation of the metallic glass nanowires that exploit a conventional gas atomization process.17 The advantage of this method is massive production established in powder metallurgy and it leads to the nanowire formation in large quantity and high density. The key to create the nanowires is the supercooling that yields a high viscosity in the melt steam and allows ligament elongation. The results follow the spinnability rule where the viscosity below the melting point plays an important role in determining the aspect ratio of the nanostructures. We used Zr65Cu18Ni7Al10 metallic glass (Zr-MG) which is known to have high glass forming ability, ultrahigh mechanical strength (∼1.6 GPa), and wide ΔTx (170 K).18 To compare the nanowire formability, we used Fe76Si9.6B8.4P6 metallic glass (Fe© 2012 American Chemical Society
Received: January 30, 2012 Revised: March 30, 2012 Published: April 11, 2012 2404
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the XRD patterns which consist of broad halo peaks only. The DSC result as shown in Figure 2f indicates the glass transition temperature (Tg) at 649 K and the crystallization temperature (Tx) at 764 K, concluding that the obtained nanowires maintain the glassy phases. The atomization processes involve many practical operating parameters such as metal stream pressure, nozzle geometry, atomizing gas pressure, alloy type, and stream viscosity. The question is raised about what is the key role to control the nanowire formability. For the conventional gas atomization, sufficient superheating above Tm is effective to create droplets because of spheroidization associated with low viscosity. In fact, when we increased Tin by 50 K (Tin = 1153 K) without supercooling, the doubled droplet density, namely 1.4 × 105 cm−2, was obtained. The result is consistent with the spinnability concept21 which is empirically derived as L = DVη(T )/γ(T )
(1)
where L is the spinnability length, D is the wire diameter, V is the spinning velocity, η(T) is the viscosity, and γ(T) is the surface tension. Generally, γ(T) is expressed by Eötvös' law γ(T) = kγV−2/3 m (TC − T), where TC is the critical temperature that satisfies γ(TC) = 0, Vm is the volume, and kγ is Eötvös′ constant, indicating that γ(T) has a linear relation with temperature.22 In contrast, η(T) of metallic glasses is explained by Vogel−Fulcher−Tammann (VFT) relation η(T) = η0 exp{D*T0/(T − T0)}, where η0 is the viscosity at infinite temperature, D* is a measurement of the kinetic fragility, and T0 is the VFT temperature.23 In this case, η(T) exponentially increases with decreasing temperature. Now, the aspect ratio L/ D can be derived by substituting these relations into eq 1 as
Figure 1. Illustration of a conventional gas atomizer. The sample was once heated above Tm and then the molten alloy was supercooled, followed by the gas atomization. The aerodynamic force in the gas expansion zone causes the fragmentation of the melt stream into nanowires and droplets.
the nanowires enables us to easily handle by tweezers and to carry out conventional X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements. Figure 2e shows
Figure 2. (a) Lump of nanowires with the size of about a centimeter. (b) SEM image shows the high density of Zr-MG nanowires, forming nanofiber configuration. (c) Enlarged SEM image shows the nanowires having 50, 270, and 1043 nm diameters. (d) The distribution of Zr-MG nanowire diameters. (e) XRD patterns of the nanofiber show a broad halo peak only. (f) DSC measurements show a clear glass transition peak at 649 K, indicating that the obtained nanowires are in the glassy state. Heating rate was 40 K/min. 2405
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significantly increased and a few nanowires are recognized, as shown in Figure 4b. The advantage of gas atomization is an instantaneous process under aerodynamic drag forces where the gas flow in the expansion zone can easily reach a sonic speed,29 and the extremely high cooling rate that avoids the crystallization of metallic glasses. To examine the effect of inlet gas pressures that are proportional to jet-gas velocities, Fe-MG was atomized under different gas pressures. The nanowire density monotonically increases with increasing gas pressure, as shown in Figure 4c. The result is also consistent with eq 2, assuming that the jetgas velocity equals the spinning velocity V in eq 2. We have shown a new approach to the formation of metallic glass nanowires. Although the exact fragmentation processes of droplets and wires at nanoscale require computational fluid dynamics calculations,30,31 the growth of nanostructures can be expressed by the spinnability which is a macroscopic approach. Furthermore, the simple calculation in Figure 3 predicts that Ptbased metallic glass could form high aspect ratio nanostructures. High surface areas of such noble-metal-based nanowires hold considerable technological promise as electrodes and heterogeneous catalysts for fuel-cell applications.32,33
Vη0 exp{D*T0/(T − T0)} k γV m−2/3(TC − T )
(2)
Figure 3 shows the temperature-dependent aspect ratio where the temperatures are normalized by each melting point.
Figure 3. Temperature dependent spinnability for considered materials obtained at V = 340 m/s. For Zr 65 Al10 Ni 10 Cu 15 , Pd43Ni10Cu27P20, Pt57.5Cu15Mo14Er2C15B6, and Au49Ag5.5Pd2.3Cu26.9Si16.3, we used the parameters of η(T) in ref 26 and γ = 1 N/m. For pure Zr, Fe, Pd, and Pt, the parameters in refs 22 and 28 were used. For SiO2, the parameters of η(T) in ref 23 and those of γ(T) in ref 27 were used. The experimental aspect ratios obtained for Zr-MG were plotted.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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For Fe-MG and Zr50Cu40Al10, the parameters of kγV−2/3 m , TC, η0, D*, and T0 were obtained by the electrostatic levitation method. 24,25 For Zr 65 Al 10 Ni 10 Cu 15 , Pd 43 Ni 10 Cu 27 P 20 , Pt57.5Cu15Mo14Er2C15B6, and Au49Ag5.5Pd2.3Cu26.9Si16.3, we used the parameters of η(T) reported in ref 26 and supposed the case γ = 1 N/m. For comparison, SiO2 and pure metals of Zr, Fe, Pd, and Pt are also plotted above Tm.22,23,27,28 The aspect ratio is about 100 at Tm and increases exponentially with decreasing temperature (Tm/T > 1) for Zr-based metallic glasses. The experimental aspect ratios of Zr-MG nanowires are plotted in Figure 3, showing a good agreement with the calculated values of the Zr-based metallic glasses. In contrast, the aspect ratio of Fe-MG and the pure metals becomes unity at Tm. To confirm this, Fe-MG was atomized under the conditions of Tin ≈ 1300 K (cf. Tm = 1298 K) and PAr = 95 Kgf/cm2. The SEM image in Figure 4a indicates that the number of droplets
ACKNOWLEDGMENTS We thank M. W. Chen for stimulating discussion and Tohoku University IMR for the used of electron microscopy. This work was supported by MEXT Grant-in-Aid for Exploratory Research #23656416, JST Research Seeds Quest Program, Yazaki Memorial Foundation, and WPI Fusion Research Program.
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REFERENCES
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Figure 4. (a) SEM image of Fe-MG droplets and nanowires produced at Tin ≈ 1300 K. The number of droplets increases significantly compared with these of Zr-MG. (b) Enlarged SEM image of a Fe-MG nanowire, having 113 nm in diameter. (c) Inlet gas pressure dependence on Fe-MG nanowire density. The nanowire density increases with increasing the inlet gas pressure. 2406
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