Synthesis and characterization of pure metallic titanium nanoparticles by an electromagnetic levitation melting gas condensation method

Abstract

Pure titanium nanoparticles were synthesized by utilizing an Electromagnetic Levitation Melting Gas Condensation (ELM-GC) method. Pure bulk titanium samples were melted and evaporated by electromagnetic levitation technique in an inert gas atmosphere in a silica tube. Titanium nanoparticles were formed from ascending vapor by employing high purity argon and helium as carrier gases and cooling agents. Particle size and morphology of the produced nanoparticles were studied by Field-Emission Scanning Electron Microscopy (FE-SEM) and Dynamic Light Scattering (DLS) analysis. Results showed almost spherical nanoparticles with a narrow size distribution under both cooling atmospheres. The purity of the produced titanium nanoparticles was confirmed through powder X-ray diffraction (XRD) as well as X-ray fluorescence (XRF), and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) analysis. As to the impact of carrier gas, titanium nanoparticles synthesized under helium atmosphere had smaller particle size with narrower particle size distribution compared to those produced in argon. Average particle size of synthesized titanium nanoparticles under Ar and He atmospheres were about 42 and 31 nm, respectively.

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1. Introduction

There have been very few reports on production of titanium nanoparticles with average particle size less than 100 nm.¹ All the studies available in the literature have reported processing titanium nanoparticles encapsulated in coatings or containing significant amounts of titanium oxides.^1–6 Titanium nanoparticles have a wide range of applications including aerospace materials, biocompatible composites and metal-oxide nanocomposites, microsensors, modification of optical properties of glass window materials, optical filters, and waveguide layers.^7,8 Different methods have been employed to synthesize titanium nanopowder or powder in the past few years. For example, Dabhade et al.⁴ used high-energy attrition milling of Ti powder to produce nanocrystalline titanium powder with average particle size of 35 nm after 75 h of milling under argon atmosphere. The disadvantages of this method include impurity in products, wide particle size distribution, and the long time involved in the processing. Attar et al.² reported the production of titanium nanopowders by reacting titanium tetrachloride with sodium vapor. The resulting Ti particles were encapsulated in layers of sodium chloride, which is difficult to separate. Other methods employed for the processing of titanium nanoparticles include pulsed wire discharge,¹ bacterial assisted synthesis,³ gas phase combustion,⁵ and electronically mediated reaction (EMR).⁹

One particular problem with the previously reported methods in ref. 1–9 is that none has the aptitude to produce high purity metallic titanium powder with narrow particle size distribution.

Electromagnetic Levitation Melting Gas Condensation (ELM-GC) is a novel method for production of high purity nanoparticles. In the electromagnetic levitation melting process, a metallic sample, which is pre-positioned in an electromagnetic field generated by induction coils of suitable geometry, is melted and levitated stably.^10–21 The molten levitated sample will evaporate at a rate dependent on the temperature and vapor pressure of the metal. The vapor will then condense into metallic powder of very small size upon colliding with an inert cooling gas blew onto the surface of molten droplet. Therefore, the size of the produced particles is affected by droplet temperature and flow rate and thermal properties of the carrier gas.^10–13 This novel method has several major advantages such as containerless melting of the samples, which results in high purity products, no need for reducing atmosphere or vacuum, rapid processing, and formation of powders with narrow size distribution.^10–13

In the past few years, nanoparticles such as silver, nickel, iron, zinc, iron oxide (Fe₂O₃), and zinc oxide (ZnO) have been produced by ELM-GC method.^10–16 However, there have not been any reports on synthesis of nano-sized titanium particles by this method, which are very eager to be oxidized.

In this paper, we report the synthesis of pure metallic titanium nanoparticles by utilizing the novel ELM-GC technique. Two different inert gases were used to condense the particles and collect them in a dispersant. The particle size and morphology of the particles were characterized using DLS and FE-SEM analysis. The purity of the nanoparticles synthesized was confirmed using powder XRD, XRF, and ICP-AES. The effect of different process parameters such as carrier gas flow rate on produced nanoparticles was also investigated. Finally, the direction for future research is pointed out.

2. Experimental procedure

Electromagnetic levitation melting chamber, which consisted of a 16 mm outer diameter silica (quartz) tube with a wall thickness of 1 mm, was completely sealed from outside atmosphere by using O-rings. Argon (Ar) and helium (He) gases were used as carrier gas and cooling agent. Each gas passed through a silica gel column and then a tube furnace containing copper and aluminum chips at 550 °C prior to entering the levitation chamber, thus ensuring oxygen as well as nitrogen levels were kept to acceptable values and moisture was removed. Based on Ellingham diagram oxygen partial pressure of the gases entering the chamber was calculated to be approximately 10⁻¹⁶ atm.

Bulk titanium samples with a purity of 99.9% weighting approximately 1.20 g were used as raw materials. A 15 kW, 450 kHz RF generator manufactured by Tapka was used as power generator. Electromagnetic levitation coils were wound using annealed 4 mm OD copper tubes. Different coil designs were prepared and tested to accomplish a coil capable of sustaining a stable sample and heating it to temperatures above melting point of titanium. Schematic diagram of the experimental setup is shown in Fig. 1.

Fig. 1 Schematic diagram of the ELM-GC setup. The induction coil is coupled to a power generator and Ti sample is melted and levitated in the electromagnetic field produced by it.

Three different gas flow rates of 5, 10, and 15 liters per minute were used to investigate the effect of gas flow rate on the particle size of the produced nanoparticles. Experiments showed because of the generator's power limits, flow rates higher than 15 L min⁻¹ caused temperature of sample to drop below its melting point. Cooling gas was at room temperature at the entrance of the chamber. The temperature of molten samples was measured using a disappearing filament pyrometer. Before initiation of each run, the titanium sample was placed into the chamber from top section on an alumina holder. The holder was then lowered to the level of the coil. Generator was run at full power, the Ti sample levitated and the alumina holder was withdrawn. A view of experimental setup in running test is shown in Fig. 2. The levitation device, electromagnetic coil, and the chamber can be seen in this image. Temperature of sample was adjusted by setting generator power and the effect of droplet temperature on particle size of the produced nanoparticles was investigated. In this method the process for Ti to melt and turn into nanopowder takes only less than 2 minutes, as compared with hours for conventional methods.

Fig. 2 A view of experimental setup in which the levitation device, electromagnetic coil, and the chamber are shown during test.

The produced titanium nanoparticles were carried by the cooling gas and collected in a Drechsel bottle containing hexane as dispersant to avoid oxidation and agglomeration of products. XRD analysis of the nanopowders was performed by INEL Equinox 3000 X-ray diffraction diffractometer. Purity of the titanium nanoparticles was determined by XRF analysis using a microanalyzer XMF-104 Unisantis device and Varian Liberty 150 AX ICP-AES Spectrometer. Particle size distribution of the nanoparticles was characterized by DLS analysis using a Malvern Nano ZS (red badge) ZEN 3600. FE-SEM studies were performed by HITACHI 4160 and TESCAN VEGA.

3. Results and discussion

Phase and purity analysis

XRD pattern of titanium nanoparticles synthesized under helium atmosphere at temperature of 1700 ± 20 °C and gas flow rate of 5 liters per minute is shown in Fig. 3. The peaks correspond to pure Ti (JCPCS card # 00-044-1294) unambiguously. The high and titled background at 2-theta of less 40° is due to the polymer sample holder and the relatively small quantity of samples analyzed. It is noted that, unlike previous reports on Ti nanoparticle synthesis^1–6 that show TiO_x phases or minor Ti peaks compared to the impurity available in their products, XRD analysis didn't reveal any peaks corresponding to any kind of crystalline titanium oxides (TiO_x), which indicates the high purity of the produced titanium nanoparticles. In addition to XRD analysis, purity of the produced powder was determined by ICP-AES and XRF analysis. ICP analysis confirmed titanium nanoparticles with metal purity of 99.743%, while XRF showed titanium purity of +99.7%.

Fig. 3 XRD pattern of titanium nanoparticles synthesized under helium atmosphere at temperature of 1700 ± 20 °C and gas flow rate of 5 L min⁻¹.

Particle size distribution

Particle size distribution of the produced titanium nanoparticle is obtained using DLS analysis, and Fig. 4 shows the particle size distributions in number for nano titanium powders prepared under both Ar and He atmospheres. The mean particle diameter is 42 and 31 nm for titanium nanoparticles prepared in Ar and He, respectively. The reason for the smaller particle size in He is attributed to the greater thermal conductivity of He (0.152 W m⁻¹ K⁻¹) compared to that of Ar (0.0177 W m⁻¹ K⁻¹).^10–13 Considering the mechanism of nanoparticle formation in inert gas condensation process, interaction between ascending metallic vapor and the carrier gas result in homogenous nucleation. Particle growth occurs by collision between the metallic clusters and the remaining metallic vapor.

Fig. 4 Comparison of DLS results for particle size distribution of synthesized nanopowder under argon and helium atmospheres at constant temperature of 1700 ± 20 °C and gas flow rate of 5 L min⁻¹. The results show a narrower particle size distribution for He atmosphere compared to Ar.

In the gas condensation processes, the average particle size has a direct relation to the atomic mass of the carrier gas.¹⁰ The heavier gas atoms results in higher growth rate of metal clusters. The reason is attributed to the higher energy absorbed from metallic atoms in the hot vapor phase collisions by heavier gas atoms.^10,16 Consequently, heavier carrier gas molecules can result in higher collision efficiencies, which also increases growth rate. In the other word, collision between heavier gas atoms and Ti atoms/cluster produces higher kinetic energy for particle growth and also increases particle temperature, which results in increased growth rate and bigger particles size. On the contrary, a carrier gas with higher thermal conductivity cools faster and allows less time for particles to coalesce, thus the growth rate decreases. Therefore, helium atmosphere could produce smaller particles compared to argon atmosphere.¹⁰

Obtained results for particle size of the produced nanopowders under Ar and He atmospheres are in good agreement with Malekzadeh's¹⁰ work in production of silver nanoparticles by ELM-GC method. DLS also showed a narrower particle size distribution for particles synthesized in He atmosphere compared to Ar atmosphere, which verifies SEM outcomes (shown later in Fig. 5) and is in good agreement with the work of Malekzadeh et al.¹⁰

Fig. 5 FE-SEM images of synthesized titanium nanoparticles using (a) Ar and (b) He as cooling media at constant temperature of 1700 ± 20 °C and gas flow rate of 5 L min⁻¹.

Particle morphology

Fig. 5 exhibits FE-SEM images of the synthesized titanium nanoparticles under argon and helium atmospheres at constant temperature of 1700 ± 20 °C. Consistent with the particle size distribution analysis by DLS, titanium particles synthesized under He atmosphere appear to have smaller sizes compared to those processed in Ar at the same gas flow rate of 5 liters per minute. According to computer programmed image analysis, average particle diameter of the synthesized titanium nanoparticles under Ar and He are about 50 and 35 nm, respectively, and the standard deviation for diameter were obtained to be around 46 for particles produced under Ar atmospheres and 30 for He. SEM analysis also showed that although the nanopowders were cooled so fast and collected in hexane some particle coagulation and coalescence happen between metallic clusters, which form some agglomerated particles as it is seen in Fig. 6.^10,11,16,22

Fig. 6 SEM image of produced titanium nanoparticles under Ar atmosphere as cooling gas which represents almost spherical morphology (at temperature of 1700 ± 20 °C and gas flow rate of 5 L min⁻¹).

Fig. 7 shows FE-SEM image of titanium nanoparticles synthesized under helium atmosphere with gas flow rate of 15 liters per minute at constant temperature of 1700 ± 20 °C. The particles were agglomerated, with primary particle size of smaller than 28 nm. Whereas, DLS analysis showed that particles with average diameter as small as 21.04 nm were produced at gas flow rate of 15 liters per minute. Comparing with Fig. 5 and 6, it is observed that increasing gas flow rate from 5 to 15 liters per minute at constant temperature decreases particle size of the produced titanium nanoparticles. According to Wegner et al.²³ gas flow rate has a direct relation to cooling rate and increasing it will result in higher cooling rates. Therefore, condensation rate of vapor atoms by gas molecules increases and probability of collision of vapor atoms with each other decreases, which subsequently results in smaller particle size.²⁴

Fig. 7 FE-SEM images of titanium nanoparticles synthesized under helium atmosphere with gas flow rate of 15 L min⁻¹ at constant temperature of 1700 ± 20 °C.

Samples at higher temperatures result in higher degrees of supersaturation in the vapor.^16,24 By increasing supersaturation the chance of collision between each atom increases as the number of available atoms in vapor phase have been increased. As a result, critical nucleation radius and therefore particle size is reduced.

4. Conclusions

High purity metallic titanium nanoparticles with spherical shape and narrow size distribution were synthesized using electromagnetic levitation melting gas condensation technique using both argon and helium as the carrier gas. The mean particle size of the produced powders was in the range of ∼20–50 nm. Using helium as carrier gas results in smaller particles and narrower particle size distribution compared to argon at constant temperature and constant gas flow rate. On the other hand, increasing gas flow rate and sample temperature results in smaller particles.

Future research in this field along the directions of optimizing of ELM-GC method, studying the effect of process time on particles size distribution, and producing alloys' nanopowders by ELM-GC method will be of great interests and could be continued.

Acknowledgements

The authors would also like to thank Dr Mohammad Nusheh for his help and cooperation in this project. The authors would also like to thank Dr Zhe Cheng at Florida International University for reading the manuscript and providing suggestions.

References

1. Y. Tokoi, T. Suzuki, T. Nakayama, H. Suematsu, F. Kaneko and K. Niihara, Preparation of titanium nanopowders covered with organics by pulsed wire discharge, Scr. Mater., 2010, 63, 937–940.
2. A. Attar, M. Halali, M. Sobhani and R. Taheri Ghandehari, Synthesis of titanium nano-particles via chemical vapor condensation processing, J. Alloys Compd., 2011, 509, 5825–5828.
3. K. Prasad, A. K. Jha and A. R. Kulkarni, Lactobacillus assisted synthesis of titanium nanoparticles, Nav. Res. Logist., 2007, 2, 248–250.
4. V. V. Dabhade, T. R. Rama Mohan and P. Ramakrishnan, Nanocrystalline titanium powders by high energy attrition milling, Powder Technol., 2007, 171, 177–183.
5. D. Y. Maeng, C. K. Rhee, K. H. Kim and W. W. Kim, Synthesis and characterization of nanosized titanium particles by gas-phase combustion, Mater. Sci. Eng., A, 2004, 375–377, 609–614.
6. V. V. Dabhade, T. R. Rama Mohan and P. Ramakrishnan, Synthesis of nanosized titanium powder by high energy milling, Appl. Surf. Sci., 2001, 182, 390–393.
7. NanoAmor – Nanostructured & Amorphous Materials, Inc., http://www.nanoamor.com/inc/sdetail/24880, consulted March 2013.
8. US Research Nanomaterials, Inc., The Advanced Nanomaterials Provider, http://www.us-nano.com/inc/sdetail/175, consulted March 2013.
9. I. Park, T. Abiko and T. H. Okabe, Production of titanium powder directly from TiO₂ in CaCl₂ through an electronically mediated reaction (EMR), J. Phys. Chem. Solids, 2005, 66, 410–413.
10. M. Malekzadeh and M. Halali, Production of silver nanoparticles by electromagnetic levitation gas condensation, Chem. Eng. J., 2011, 168, 441–445.
11. M. Malekzadeh and M. Halali, Method of Producing High Purity Silver Nanoparticles, US Pat., US 2012/0060649 A1, 2012.
12. A. Kermanpur, B. Nekooei Rizi, M. Vaghayenegar and H. Ghasemi Yazdabadi, Bulk synthesis of monodisperse Fe nanoparticles by electromagnetic levitational gas condensation method, Mater. Lett., 2009, 63, 575–577.
13. M. Vaghayenegar, A. Kermanpur, M. H. Abbasi and H. Ghasemi Yazdabadi, Effects of process parameters on synthesis of Zn ultrafine/nanoparticles by electromagnetic levitational gas condensation, Adv. Powder Technol., 2010, 21, 556–563.
14. Y. R. Uhm, B. S. Han, M. K. Lee, S. J. Hong and C. K. Rhee, Synthesis and characterization of nanoparticles of ZnO by levitational gas condensation, Mater. Sci. Eng., A, 2007, 449–451, 813–816.
15. Y. R. Uhm, W. W. Kim and C. K. Rhee, Phase control and Mössbauer spectra of nano-Fe₂O₃ particles synthesized by the levitational gas condensation (LGC) method, Phys. Status Solidi A, 2004, 201(8), 1934–1937.
16. M. Pishahang, An investigation on the production of nickel nanopowders by electromagnetic levitation melting, M.Sc. thesis, Sharif University of Technology, January 2008.
17. L. Baptiste, N. V. Landschoot, G. Gleijm, J. Priede, J. S. v. Westrum, H. Velthuis and T. Y. Kim, Electromagnetic levitation: a new technology for high rate physical vapour deposition of coatings onto metallic strip, Surf. Coat. Technol., 2007, 202, 1189–1193.
18. M. G. Frohberg, Thirty years of levitation melting calorimetry – a balance, Thermochim. Acta, 1999, 337, 7–17.
19. M. Vaghayenegar, A. Kermanpur and M. H. Abbasi, Formation mechanism of ZnO nanorods produced by the electromagnetic levitational gas condensation method, Sci. Iran., Trans. F, 2011, 18(6), 1647–1651.
20. J. A. Treverton and J. L. Margrave, Thermodynamic properties by levitation calorimetry. The enthalpies of fusion and heat capacities for the liquid phases of iron, titanium, and vanadium, Rice University Publications, 1971, pp. 473–481.
21. Z. Moghimi, M. Halali and M. Nusheh, An investigation on the temperature and stability behavior in the levitation melting of nickel, Metall. Mater. Trans. B, 2006, 37, 997–1005.
22. A. Simchi, R. Ahmadi, S. M. Seyed Reihani and A. Mahdavi, Kinetics and mechanisms of nanoparticle formation and growth in vapour phase condensation process, Mater. Des., 2007, 28, 850–856.
23. K. Wegner, B. Walker, S. Tsantilis and S. E. Pratsinis, Design of metal nanoparticle synthesis by vapor flow condensation, Chem. Eng. Sci., 2002, 57, 1753–1762.
24. J. D. Gunton, Homogeneous Nucleation, J. Stat. Phys., 1999, 95, 909–923.

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