Arab J Sci Eng (2013) 38:3587–3592 DOI 10.1007/s13369-013-0621-2
RESEARCH ARTICLE - MECHANICAL ENGINEERING
Synthesis and Characterization of Aluminium Nanoparticles by Electric Arc Technique ˙ B. Yanık · H. A˘gustos · Y. Ipek · A. Koyun · D. Uzunsoy
Received: 22 December 2011 / Accepted: 3 July 2013 / Published online: 11 August 2013 © King Fahd University of Petroleum and Minerals 2013
Abstract In this work, a process which generates a plasma phase on metal was developed by the electric arc method. The arc process was carried out at constant current and electrode gap which was adjusted by an automatic control system. Particles formed after arc-plasma were stored in cooling liquid with the effect of gravitational force. Electric arc discharge was a successful technique to synthesize aluminium nanopowders with median particle sizes’ range from 51 to 139 nm using different processing parameters such as currents and diverse media [distilled water (DW), ethylene glycol (EG)]. The synthesized particles were characterized by several techniques like scanning electron microscopy (SEM), XRD, BET and Nano ZS in order to develop deep understanding for correlation between material properties and fabrication parameters. Keywords Aluminium nanoparticles · Electric arc discharge · Synthesis · Characterization
B. Yanık Department of Metallurgy and Materials Engineering, Yıldız Technical University, Davutpa¸sa Mah., Davutpa¸sa Caddesi, 34220 Esenler, Istanbul, Turkey H. A˘gustos Department of Mechanical Engineering, Yıldız Technical University, Barbaros Bulvarı, 34349 Yıldız, Istanbul, Turkey Y. ˙Ipek Central Laboratories, Yıldız Technical University, Barbaros Bulvarı, 34349 Yıldız, Istanbul, Turkey A. Koyun · D. Uzunsoy (B) Department of Mechatronics Engineering, Yıldız Technical University, Barbaros Bulvarı, 34349 Yıldız, Istanbul, Turkey e-mail:
[email protected]
1 Introduction Nanoparticles and ultrafine particles are a class of materials with high chemical reactivity. These materials have characteristics of high surface area and crystal structure with high dislocation density and surface area compared to micronsized particles [1]. Aluminium is highly reactive when synthesized in nanosize. These particles are used to prepare explosive mixtures for aerospace, defence industry and submarines [2,3]. Research has been especially concentrated on high-performance rocket fuels from nanosized aluminium particles [4]. The burning rate of aluminium nanoparticles (Al NPS ) is higher than that of other aluminium particles [5]. Metallic powders with sizes smaller than 1 µm have been used commonly in industry. Nanosized metallic powders are the most popular group of materials among high-technology materials and used in various applications. The most important application area of these nanopowders is communication, aerospace, automotive, electric– electronic, biology, genetical engineering and defence industry [6]. In recent years, a variety of new technologies have been developed to produce sub-micron powders such as
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wire explosion, plasma and wet chemicals. However, each of these processes has complexity and cost disadvantages, they were not used for large-scale production of nanoparticles [7]. Currently, aluminium powders are used in the metallurgical, chemical, paint and pigment industries [8]. However, ultrafine aluminium (e.g. average size < 40 µm) powders have specialist end uses in rocketry, explosives, thermal spray, powder metallurgy, etc. There is an increasing use of powder metallurgy (PM) products based on aluminium alloys in lightweight structural parts and internal combustion engine components including camshaft bearing, shock absorber pistons and brake callipers [9] driven by the need of automotive industries to reduce the weight of vehicles. To increase the mechanical performance of aluminiumbased PM parts, it is necessary to achieve a nanostructured microstructure [10]. Reactive nanoparticles as energetic materials have recently received much attention for a variety of existing and/or potential applications. Amongst those more extensively investigated are nanosized (sub-100 nm) aluminium (Al) particles. Latest development in the synthesis of Al NPS has resulted in a lot of interest due to their application in energetic formulations such as fuels, propellants and explosives [11]. Currently, nanoscale Al particles have also been studied as high-capacity hydrogen storage materials [12]. Therefore, significant effort has been made on the development of synthesis methodologies for Al Nps of desired properties. Synthesis of aluminium nanomaterials has been an area that continues to spark interest. Over the last 20 years, nanoaluminium particles have been synthesized using several emerging techniques such as ball milling, explosion of wire, laser ablation, plasma, wet chemistry and arc-discharge technique [13]. Electric arc technique (EAT) has been developed as a costeffective and flexible process to synthesize a variety of metallic nanoparticles [14]. Numerous arc-discharge research has been performed on the synthesis of metallic nanoparticles using continuous DC power supply at different processing parameters such as a range of currents (60–1,500 A) over fixed potential differences [15]. Several researchers reported that the synthesis of Al NPS using arc-discharge technique has been performed successfully [16]. Park et al. [17] showed that the smallest Al NPS obtained at approximately 19 nm in diameter and highest reactivity. Emphasis is given in this paper to the synthesis of Al Nps by electric arc-discharge technique. The influence of arc media and current on the morphology, size, chemical composition and structure of the synthesized Al Nps is also investigated in detail using several techniques such as scanning electron microscopy (SEM), TEM, XRD and TG/DTA.
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Fig. 1 Electric arc-discharge apparatus used in the experiments Table 1 Median particle size of Al Nps synthesized in DW and EG at different currents Key parameters
Values
Voltage (average value)
20 V
Current (average value)
65 A
Aluminium anode (diameter)
6,35 mm
Tungsten cathode (diameter)
3,175 mm
Discharge time
20 s
Pressure
Standard pressure (1 atm)
Volume of cooling media
550 ml
2 Experimental Figure 1 shows tailor-made electric arc-discharge system for creating Al Nps . The arc-discharge apparatus simply consisted of an open vessel, a DC power supply and servo control system. The servo control system maintained the distance between aluminium and tungsten electrodes. The discharge current and time are also monitored during the experiment. High-purity aluminium and tungsten rods used in the present work are supplied by Alfa Aesar with product code of 11493 and 10407, respectively. The important parameters used in the EAT are summarized in Table 1. The operation in EAT is performed as follows. A sufficiently large direct current density is applied between the anode and cathode, which are in contact initially. The electrodes are surrounded by a pool of media inside a container.
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Fig. 2 The optical photos of Al Nps in a DW and b EG
Two different media, distilled water (DW) and ethylene Glycol (EG), are used in the synthesis of Al Nps . An arc column is formed when two electrodes begin to separate to a given spacing. The arc discharge is initiated by slowly detaching the moveable anode from the cathode. The cathode–anode gap is controlled at approximately 1 mm to obtain a stable discharge current and average voltage of 20 V. When the electrode separation starts to increase, the voltage increases at first. This is followed by the voltage drop while bringing the electrodes close together. The voltages and currents are
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recorded when stable discharge conditions are obtained. Gas bubbles are formed in the water and ethylene glycol during the arc process due to the formation of plasma vaporization between the anode material and media. The vapour is quenched into solid powder particles with sizes below 1 µm by the surrounding media. Figure 2a, b shows the colloidal Al Nps produced using arc discharge in DW and EG media. The nanoparticles produced are characterized using a BrukerT M D8-Advance X-ray powder diffraction (XRD) using a Cu Kα radiation at a voltage of 40 kV and current of 40 mA. The scan rate of XRD is 2◦ /min and 2θ range between 30◦ and 90◦ . Particle size distributions of the synthesized Al nanoparticles are measured using Malvern InstrumentsT M Zeta Sizer Nano. The Brunauer–Emmet– Teller (BET) method is used to determine the specific surface area of the synthesized nanoparticles. In addition, particle shape and morphology were characterized by SEM of a Zeiss Evo 40 microscope at 20 kV accelerating voltage. Thermo-gravimetric and differential thermal analyses (TGDTA) were used to determine the thermal degradation and oxidation behaviour of Al Nps . This technique was carried out between room temperature and 800 ◦ C at a rate of 10 ◦ C/min under an inert (N2 ) atmosphere.
Fig. 3 Particle size distribution of Al Nps synthesized in a EG and b DW at 65 A, c EG at 90 A
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3 Results and Discussion The effect of media and applied current on the size and morphology of Al Nps was evaluated by performing electric arc discharge in EG and DW at 65–90 Amps. The powder size and distribution were strongly influenced by the applied current. Figure 3a, b shows the particle size distribution of the synthesized Al Nps in EG and DW using EAT at 65 A, respectively. It can be seen that the particle sizes range from 29 to 147 nm and the median diameter (taken as average particle diameter) is about 55 nm, being deduced from the images. EG as a media provides the most monodispersed nanoparticles as shown in Fig. 3. Another important finding of this work is that the increase in applied current results in a gradual increase in the particle size. This is believed to be due to the higher rate of vaporization of aluminium atoms leading to larger particle size. The influence of applied current on powder size and distribution of the synthesized powder
Table 2 Important parameters of EAT Applied current (A)
Media
Median particle size (d50 ) nm
65
Ethylene glycol
65
Distilled water
66
90
Ethylene glycol
84
90
Distilled water
139
51
is quite significant as shown in Fig. 3c. A high current leads to powders with wide size of distribution and large median particle size (d50 ) than those produced using lower current. Chang et al. [15] reported that high applied current leads to coarser particles due to the larger arc column. Another work by Kassaee [18] also reported that higher rate of vaporization of copper atoms at higher currents resulted in larger particle size. Table 2 summarizes a list of median particle size (d50 ) of Al Nps synthesized in EG and DW at two different currents (65 and 90 A). The size trend for Al Nps fabricated at 65 A is EG (51 nm), DW (66 nm) while, at 90 A, EG (84 nm) < DW (139 nm). These results also show that the Al Nps size is affected strongly by the medium. Another finding of this study is the influence of dispersion stability of the medium on the particle size of synthesized particles. The dispersion stability is inspected visually by the sedimentation volume. The findings in EG and DW are compatible with the previous research. The particles are well dispersed and settled separately in DW. In addition to this, EG leads to the highest purity (fairly less Al2 O3 ) and relatively smallest Al Nps . The size, colour, composition and morphology of the synthesized powder are influenced by the choice of media used in arc discharging. The previous work reported that the use of EG as a media prevented the surface from self-igniting when exposed to air. In this work, they also found that Al Nps surrounded by EG molecules resulted with the formation of a stable suspension [16]. The result of the present study agreed with the
Fig. 4 XRD spectra of the aluminium rod a before arc discharge and b after arc discharge in EG, c after arc discharge in DW at 65 A
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Fig. 5 Transmission electron micrographs (TEM) of Al Nps synthesized in a EG and b DW at 65 A
Fig. 6 TG/DTA curves of the synthesized AlNps in DW
findings in a previous work by Pranda et al. [1]. The use of EG as an arc media yielded monodispersed Al Nps with small size and spherical morphology. The XRD spectra of aluminium electrode before and after the arc discharge in EG and DW were presented in Fig. 4a–c. Regardless of the media used during the arc discharge, the highest intensity XRD peaks were the characteristics of aluminium (2θ = 38.380, 44.650, 65.130 and 78.210) corresponding to the formation of face-centred cubic (fcc) Al Nps . XRD pattern of as-supplied aluminium electrode showed characteristics of metallic Al in the spectra as shown in Fig. 4a. In the XRD pattern obtained after the arc discharge in EG, along with Al Nps less nanoalumina (γ -Al2 O3 ) impurity was also encountered as in Fig. 4b. The XRD pattern of nanoparticles produced in DW showed Al
Nps lines along with both nanoalumina (γ -Al2 O3 ) and nanoaluminium hydroxide (Al (OH)3 ) in comparable quantities in the XRD spectrum as presented in Fig. 4c. Figure 5a, b shows transmission electron micrographs of Al Nps produced by this method in EG and DW, respectively. Al Nps synthesized in EG appears to have fairly spherical morphology, relatively small size and most monodispersed distribution when compared with distilled water media. The nanosized powders synthesized in both media have high agglomeration trend due to their high specific surface energies. The specific surface area of the synthesized Al Nps in DW was found to be as 40 m2 /g. Decomposition kinetics and thermal stabilities of the synthesized Al Nps by EAT were evaluated by TG/DTA in N2 at a heating rate of 10 ◦ C/min. The TG/DTA curve showed
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that rapid weight decreases at 100 ◦ C due to the vaporization of water content and single endothermic peak at 654 ◦ C corresponding to the melting of Al Nps . The TG/DTA analysis revealed ∼7 % weight loss in the range of 50–800 ◦ C indicating thermal stability of the synthesized Al Nps . No oxidation behaviour of Al Nps is observed as shown in Fig. 6. It has been reported that single endothermic peak at 656 ◦ C in argon is attributed to the melting of Al Nps [16].
4 Conclusions This study presents a process called EAT for creating Al Nps . The results of the present work can be summarized as follows: 1. Al Nps are prepared by arc discharging with homemade apparatus at different applied currents (65–90 Amps) and arc media (DW and EG). 2. The density of applied current and media has profound impact on particle size, morphology and chemical composition of synthesized Al Nps . It is found that decrease in applied current results in a decrease in particle size. 3. EG as a media provided the highest purity, most dispersed and relatively smallest size of Al Nps . 4. The trend of Al Nps fabricated at 65 A is EG (51 nm), DW (66 nm) while, at 90 A, is EG (84 nm) < DW (139 nm). 5. XRD, TEM and TG/DTA analysis results indicated that the formation of fcc Al Nps . Acknowledgments This work was supported by the Scientific Research Project Coordination Office—Yildiz Technical University, ˙Istanbul—Turkey with a project number of 28-07-02-05.
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