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A case study in vapor phase synthesis of Mg–Al alloy nanoparticles by plasma arc evaporation technique
Accepted Manuscript A case study in vapor phase synthesis of Mg-Al alloy nanoparticles by plasma arc evaporation technique M. Karbalaei Akbari, R. Derakhshan, O. Mirzaee PII: DOI: Reference:
S1385-8947(14)01107-3 http://dx.doi.org/10.1016/j.cej.2014.08.053 CEJ 12560
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
17 June 2014 7 August 2014 16 August 2014
Please cite this article as: M. Karbalaei Akbari, R. Derakhshan, O. Mirzaee, A case study in vapor phase synthesis of Mg-Al alloy nanoparticles by plasma arc evaporation technique, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.08.053
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A case study in vapor phase synthesis of Mg-Al alloy nanoparticles by plasma arc evaporation technique a
b
M. Karbalaei Akbari 1, R. Derakhshan , O. Mirzaee a b
a
Faculty of Metallurgical and Materials Engineering, Semnan University, Semnan, Iran
Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran
Abstract Alloy nanoparticles in the Mg–Al system were prepared by plasma arc discharge method from the Mg x − Al ( 45% 〈 x 〈 65 wt.%) bulk alloys. Powders were produced at various applied voltages. High purity argon and helium were separately employed as cooling and carrier gases. Morphology, composition, phase structure and particle size of the products were investigated by field emission scanning electron microscopy (FESEM), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD) and dynamic light scattering (DLS) techniques. Morphological studies show the formation of spherical nanoparticles with smooth surface. Higher voltages (related to electrode gap distance) accompanied by helium as cooling gas provide finer nanoparticles. Because of difference in evaporation rates for both magnesium and aluminum in the master alloy, the compositions of synthesized powders are found to be different from those of the raw materials. Three crystalline phases including Mg, Al and
Al12 Mg17 are detected in prepared powders. Oxide phases are not detected in XRD pattern. The 1
Corresponding author. Tel.: +989111704469; fax: +981714422858; Semnan province P.C 35131-19111; Iran E-mail address: [email protected]
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DLS studies demonstrate the mean particle size of 52 nm and 160 nm related to powders synthesized under helium and argon atmosphere, respectively. Keywords: Intermetallics; Vapor deposition; Phase diagrams; Scanning electron microscopy, SEM; X-ray diffraction. 1. Introduction It is now well known that nanoparticles exhibit unique and improved physical, thermal, electrical and chemical properties different from their corresponding bulk materials due to the small size, surface, quantum size and quanta tunnel effects [1-3]. Regarding their unique properties, nanoparticles are candidates for catalysts, sintering aids, microwave absorption, magnetic recording media, magnetic fluids, fuel additive, biomarkers, biosensors and development of nanofluids. Investigation of these properties has been one of the main scientific pursuits in the recent decades. Accordingly, the preparation and characterization of metal nanopowders has become an active field [4-11]. Various techniques have been developed to prepare metal nanoparticles, such as gas-phase chemical reaction, mechanical milling, spray pyrolysis, water-heating reaction, laser ablation, flame processing, vapor deposition, microwave plasma synthesis, arc plasma discharge, sol–gel methods and electromagnetic levitation [12-19]. Among various synthetic methods, the arc plasma evaporation technique demonstrates several advantages, including high temperature processing, supersaturated vapor phase preparation and high heating and cooling rate [20-22]. However; one the main specifications of this technique is the accessibility of device and feasibility of method. In this technique the chemical composition, shape and size of nanoparticles can be easily controlled by changing the process parameters, such as applied voltage and amperage of electrical source, composition of raw materials, type and concentration of inert gas Page 2 of 31
in reaction chamber and reaction time. Successful preparation of pure metal nanopowders by this technique has been reported [23-27]. In the previous researches, considerable efforts have been directed to the formation and characterization of less reactive metal powders and their alloys by arc discharge method. It was concluded that substitutional solid solutions always form in these nanoparticles. However, while alloy nanoparticles have a great potential for technological application the mechanisms of raw material vaporization, nucleation and nanoparticle growth in binary systems, especially when two constituents have very different vapor pressures, are a topic of conversation. Formation of intermetallic-compounds has become a new appealing topic because of their attractive properties, such as good corrosion resistance, interesting thermal specifications, lightweight, excellent catalytic and magnetic properties [28, 29]. A case study in Fe-Sn binary system showed the formation of Fe-Sn nanoparticles with a core/shell structure. They characterized an intermetallic core (FeSn 2 and Fe 3Sn 2 ) covered with an oxide shell
(SnO 2 ) with 5-10 nm thickness [30]. In the other study, in the Mg-Cu synthesized nanopowders four phases, i.e. Mg, MgO and intermetallic compounds of Mg 2 Cu and MgCu2 , were detected [31]. Recently, Mg-based ultra fine particles, such as Mg, Mg–Zn, Mg–V and Mg-La-Al have been fabricated by a hydrogen plasma–metal reaction method [32-35]. Among alloying elements, Mg-Al based alloy particles are known as one of the most attractive candidates for various engineering applications, such as hydrogen storage and fuel additives in automobile industries [35]. There are various aspects needed to be investigated when researchers study the Mg-Al alloy powders and work on the fabrication of intermetallic compounds. One of them is related to the preparation of Al-Mg stoichiometric compounds by conventional melting methods because of the large difference in the vapor pressure and the melting point between Mg and the other metals [36]. Most of the works on Mg-Al alloy systems Page 3 of 31
in the literatures used ball milling/mechanical alloying technique as the preparation method. Ball milling technique seems to be an effective way to prepare nanocrystalline and non-equilibrium samples. One of the main disadvantages of ball milling method is related to the contamination of products by balls, cup, medium of milling process and air, even in very good protection conditions [37, 38]. The second issue is related to the application of Mg-Al alloy particles. To achieve the automobile application goals, many kinds of hydrogen storage materials have been explored and studied. Among these materials, Mg-based alloys are always attractive to many researchers because of the great abundance, the light weight of Mg and high hydrogen capacity of Mg hydrides. However, as hydrogen storage material Mg shows poor hydrogen sorption kinetics. The addition of Al plays a crucial role in enhancing the sorption kinetics of the Mg nanoparticles [39-41]. From another point of view, Mg-Al particles release more combustion energy compared with that of the pure magnesium. Researchers investigated the potential of Mg-Al nanoparticles as an additive in biodiesel. Mg-Al nanoparticles reduce the energy consumption and improve the thermal efficiency; the reason is that, the additive releases energy during combustion in biodiesel fuel and enhances the thermal energy of fuel. Moreover, researchers claimed that nanoparticles improve the performance of engines by means of micro explosion phenomena and hence it reduces the formation of pollutants. The combustion of mixed fuels is accompanied by explosion of nanoparticles. Due to micro explosion, the air/fuel mixing will be proper and hence it results in reduction in hydrocarbon and carbon monoxide emission [42]. Regarding the mentioned advantages, a lot of attention has been devoted to the fabrication and characterization of high purity Mg-Al nanopowders. The application of nanoparticles strongly depends on their properties, such as the chemical composition, phase structures and also related to particle size,
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shape and agglomeration. At the present study, Mg–Al alloy nanopowders were prepared from bulk alloys by arc plasma evaporation method. The particle size, morphology, phase structure and composition of nanoparticles were investigated. 2. Materials and methods In order to prepare the magnesium-aluminum alloys by casting process, pure Al and Mg ingots, with chemical composition presented in Table 1, were used. Al ingot was melted in silicon carbide crucible by using an induction furnace at 900 o C under the protective atmosphere of argon gas. Then, the furnace was turned off. Subsequently, appropriate amounts of Mg chips were introduced into the molten alloy and then covered with flux and followed by casting in steel water-cooled mould. To prevent unwanted reaction between Fe and molten alloy, the surface of steel mould was covered by a suspension of boron nitride in water. Samples were produced with various weight ratios of Mg to Al including: 45:55, 50:50, 55:45 and 65:35. After casting, alloys were grinded through 240 up to 2500 grit papers followed by polishing and etching by a 10% HF-distilled water solution and coating with gold. Scanning electron microscope (SEM, CAMSCAN-MV2300) coupled with an energy-dispersive X-ray spectrometer (EDS) was used to locally measure the chemical composition of specimens. The phase constituents were determined by X-ray diffraction (XRD) analysis using Cu − K α radiation with λ=1.54046 A o (X’ pert Philips). The chemical compositions of alloys were determined by
inductively coupled plasma- atomic emission spectroscopy (ICP-AES) technique. From master alloys, nanoparticles were synthesized by electrical arc discharge technique. The electrical instrument was a high current DC power supply facilitated with digital equipments to monitor the applied voltage and amperage. The apparatus consists of a water-cooled steel reactor chamber covered with an internal graphite layer, including anode and moveable cathode
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and cooling facilities, as shown in Fig. 1. A micrometer was used to adjust the distance between two electrodes. In the chamber, Mg-Al alloy electrode, with an average weight of 5-7 gr, was placed on a water-cooled platform that served as the anode and an upper tungsten pole was used as the cathode. The chamber was vacuumed ( 10 −3 Pa ) and then backfilled with two various inert gases, including argon and helium. The current was kept at 100 A, while the voltage varied (1224 V) with the distance between two electrodes. During the production process, regarding the variation of voltage, the distance between the electrodes was manually altered. The operation continued for 7 min. During the production process the inert gas was forced to circulate in the steel chamber and the powder collection container. The flow rate of carrier gas was 8 lit/min. The flow rate of inert gas was adjusted according to the practical observations to cause continues flow rate. Inert gas was horizontally directed to the position of interaction. The carrier gas transported the synthesized particles through a quartz tube into the lateral collector container and passed them through palm oil methyl ester (POME) and collected in the Pyrex cup. The produced suspension was dried in an oven in controlled atmosphere and consequently nanoparticles were in situ passivated in a mixture atmosphere of Ar– O 2 (approximately 1 at. % O 2 ) at room temperature for 10 h. After passivation process, the oxygen content of oven was
gradually raised and reached to the ambient oxygen which prevents them from a rapid spontaneous oxidation as exposed to air. The structural analysis of nanoparticles was carried out by X-ray diffraction (XRD) technique. The morphological observations were investigated by Field-emission scanning electron microscope (Hitachi-S4160). The chemical composition of nanopowders was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Varian)
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method. Particle size measurement was carried out by dynamic light scattering (DLS) technique (Zeta- sizer 3000 HS). 3. Results and discussion 3.1. Alloys preparation and characterization The intermetallic compounds in the alloys were identified by SEM analysis and XRD test. The SEM micrograph and corresponding EDS results of Mg-50 wt. % Al alloy are shown in Fig. 2 and Table 2, respectively. It can be seen that a relatively uniform dendritic structure has been formed in the alloy. According to the equilibrium phase diagram in Mg-Al binary system, the Al12 Mg 17 intermetallic forms at broad composition range (approximately 45 to 60.5 wt.% Mg)
[41,42]. The eutectic single-phase structure at points 1 through 3 contains about 53 at. % Mg and 46 at. % Al. According to the binary phase diagram, this section is estimated to consist of the Al12 Mg 17 intermetallic compound [45-49].
Fig. 3 shows the SEM image and the line scanning result of Mg–50 wt. % Al alloy. The line scanning shows that the Mg concentration in the interdendritic region is higher than that of the dendrite. Studying the Al12 Mg17 intermetallic compound within the eutectic structure formed 437 oC
due to L → Al12 Mg 17 + Mg(δ) eutectic transformation and comparing the range of chemical composition for the Al12 Mg17 phase with the stoichiometric composition of the
Al12 Mg17 intermetallic compound (41.38 at.% Al-58.62 at.% Mg) and considering the concentration gradient of the Al and Mg elements at the interdendritic regions, it can be concluded that the dendrites are primary Al12 Mg17 and the interdendritic regions are the Mg eutectic [50-54]. Existence of such constituents at the alloy has been proved by using X-ray diffraction (XRD), shown in Fig. 4. The diffraction peaks of Al12 Mg 17 are identified, thus
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confirming the existence of the compound in the alloy, consistent with the measured compositions presented in Table 2. Moreover, Al 3 Mg 2 intermetallic compound which has a relatively narrow chemical composition range in the Mg-Al binary phase diagram and almost forms at constant chemical composition adjacent to the Al metal was not detected in master alloys [55-57].
3.2. Nanoparticle features Fig. 5 shows typical nanopowders synthesized at a constant applied current and at different voltages. The operation voltage varied (12-24 V) with the distance between two electrodes. The size of nanopowders decreases with increasing electrode gap spacing, as shown in Fig. 5 (a) and (b), respectively. In the present study, the size of the particles, prepared at lower voltage is in the range of 150 to 300 nm. With an increase in electrode gap particle size decreased to less than 60 nm. An increase in the electrode gap spacing generates a greater length of the plasma, accompanied by a linear rise of the arc voltage and increasing energy input to the system. The expansion of the plasma causes the ‘surface’ of plasma to enlarge and radiation heat losses. Therefore, only part of additional energy input is available for heating the molten metal. Furthermore, a large plasma volume dilutes the metal vapour and reduces the metal vapour concentrations, which in turn causes a shrinking mean particle size and less agglomeration of the nanopowders [58, 59]. Fig. 6 shows the average values of Mg concentration in synthesized powders. Also, average content of Al in nanopowders are presented in Table 3. It is worth noting that the results of Table 3 are not directly related to the corresponding results of Fig. 6. Regarding the ICP-AES measurements, Mg content in nanoparticles is higher than that of the corresponding raw Page 8 of 31
materials. Moreover, as shown in Table 3, the Al concentration in nanoparticle is markedly lower than that of the corresponding bulk materials. These differences are also attributed to the differences in vapor pressure between Mg and Al. During the arc plasma synthesis, the plasma heats up anode material to above its boiling point. This leads to the evaporation of the anode material in presence a carrier and protective gas in reaction chamber. The evaporation rate is described by the equation (1) [60]: w = 1 .574 α P
M T
(1)
where α is the condensation constant of the metal, P is the evaporating pressure (Pa) of metal at temperature T (K) and M is the molar mass of metal (g mol) . The condensation constant for most crystalline metals approaches unity [61]. The Al and Mg melting points are 660.32 and 650 o
C , respectively. However, Mg vapor pressure is approximately 10 7 times higher than that of the
Al at any temperature. At the constant temperature the evaporation rate is dominated by vapor pressure of substance. The vapor pressure of Mg is much higher than that of the Al; consequently the evaporation rate of Mg is higher than that of the Al [62-67]. This difference explains the higher magnesium content in the synthesized powders compared with that of the master alloys. Similar results were also reported in other binary systems, but the differences in composition between produced nanoparticles and raw materials are much larger in the present study and it is related to a great difference in vapor pressure of constituent elements [68, 69]. Fig. 7 shows the XRD pattern of Mg-28 wt. % Al nanoparticles, prepared from Mg-50 wt.% Al master alloy. Results show that nanoparticles have multiple phases which include Mg, Al, and Al12 Mg17 . The intensity of Al peak is weaker than that of the master alloy. A high Al content in the Mg–Al particles leads to the formation of Al together with intermetallics. From
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equilibrium phase diagram, Mg-Al binary system has several intermetallic phases, including
β − Al 3 Mg 2 , R − Al58 Mg 24 and γ − Al12 Mg17 . Nevertheless, one of them is observed in the synthesized nanopowders. In order to explain the presence of stable phases in synthesized nanoparticles, the thermodynamic situation should be considered. Literature gives the Gibbs free energy (G) expressions of Al12 Mg17 and Al3 Mg 2 . The temperature dependence of the Gibbs free energies of two intermetallic compounds in Mg-Al alloy system is presented in Fig. 8 [70, 71]. It is clear that the value of the Gibbs free energy of Al3 Mg 2 is lower than that of Al12 Mg17 at lower temperatures, indicating that Al3 Mg 2 is more stable at room temperature in equilibrium thermodynamic condition. However, Al3 Mg 2 was not detected in nanopowders. It is known that stoichiometric particle compounds can not form at the very high cooling rate during the arc plasma synthesis process, whereas the non-stoichiometric compounds with a wide composition range can nucleate and grow into intermetallic compound relatively easily [72]. The Al12 Mg17 compound has an extremely broad composition range in the Mg–Al binary system, shown in the Fig. 9 [49]. This can explain the presence of Al12 Mg17 intermetallic in nanopowders. Furthermore, it indicates that the non-equilibrium thermodynamic condition is the dominant situation during formation of nanoparticles rather than the chemical composition of the alloys. From the XRD pattern, oxide phases are not detected in the high sensitive nanopowders. It is possible that the oxide phases in nanopowders are less than the minimum content required to identify by X-Ray method. Also, it can show that nanoparticles have been passivated efficiently. Furthermore, it is possible that the amorphous oxide phases are not detected by XRD test. Another proposed mechanism is related to the process of nanoparticle solidification and formation. Gas-phase nucleation is the basis for a gas-to- solid particle transportation process. In
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the present synthesis method, Al and Mg atoms are evaporated from the raw materials with the assistance of high temperature arc plasma. Due to presence of cooling gas, large thermal gradient are created between the hot arc and cooling chamber. The driving forces and nucleation energies are provided by super cooling and lead to a series of transitions, from atoms to clusters and then from clusters to nuclei. A supersaturated state is a preliminary condition for the formation of stable clusters and nuclei. Generally, two or more atoms can form a cluster by random collisions, followed by collisions with other atoms or clusters to form larger clusters. [73]. It is speculated that there are multiple stages throughout the formation of these nanoparticles as shown in Fig. 9. Regarding the boiling temperature of Al and Mg (Al: 2470 o C , Mg: 1090 o C ), Al will firstly accomplish the described transition and form the nuclei prior to Mg, meanwhile, Mg exists in the gas state and surrounds the Al nuclei, as sketched in Fig. 9 (a). Second, the Al core adsorbs Mg atoms or clusters on its surface and grows to be a nanoparticle by the coagulation process, as seen in Fig. 9 (b). Mg atoms collide with Al atoms and begin to coagulate into liquid Mg–Al clusters, Fig. 9 (c). Below the melting point of alloy, part of the Mg and Al start to nucleate homogeneously into Mg solid solution, whereas the rest of Mg and Al remain in the liquid state outside the solid Mg core, see Fig.9 (d). When the temperature decreases further, the liquid Mg–Al shell continuously nucleates into the (Mg) solid solution that contains more Al than the primary Mg solution. In the mean time, these solid– liquid core–shell particles continue to collide and grow. At the temperature of 437 o C , the remaining liquid Mg–Al shell starts the eutectic phase transformation, forming (Mg) solid solution and the intermetallic compound, Fig. 9 (e). The formation of such a core- shell structured in Mg-Al ultra fine particles has been reported by Liu et al. [74]. The literature proposed that Al12 Mg17 intermetallic shell with 2-5 nm thickness is formed around the Mg core. The broad composition range of Al12 Mg17 Page 11 of 31
enables its nucleation even at a high cooling rate. Since the intermetallic compound is more stable against oxidation than pure Mg, the Mg particle core is protected from oxidation. Although it has been reported that the addition of Al greatly reduces the particle size of pure Mg powders due to the low evaporation rate of Al and the effect of Al–Mg chemical bonding on the molten Mg–Al [74], tangible particle size variations are not observed in the present study. The chemical composition of the prepared nanoparticles is only weakly dependent on the composition of the master alloy, explaining the similar particle size distribution found in all nanopowders. Fig. 10 (a) and (b) illustrates FESEM micrographs of arc plasma synthesized powders using argon and helium as carrier gases, respectively. The powders are suspended in Palm oil methyl ester. As it can be seen, all the powders are spherical in shape. The powders synthesized by helium cooling media are much smaller than those prepared in argon media and under the same fabrication condition. Fig. 11 shows dynamic light scattering (DLS) results. The DLS results show the mean particle size of 160 and 52 nm related to powders synthesized under argon and helium atmospheres, respectively. In the previous researches, it has been found that the composition of the carrier gas has a great effect on the particles production rate in this method. According to Murphy et al. [75] the important parameters of arc influenced by the gas compositions are the heat flux density, current density and arc pressure near the anode, which can lead to an increase in anode temperature and thereby directly influencing the evaporation rate. The heat flux and current density increase in ascending order when helium, nitrogen and hydrogen are added to argon. Argon has the highest electric conductivity and the lowest heat capacity which leads to less material evaporation regarding low power arc. It was reported that the change of gas from Ar to N 2 led to an increase in temperature and voltage of the arc plasma, Page 12 of 31
increasing the input energy to the system and enhances the melting and evaporation processes, consequently resulted in large particle sizes similar to the particle growth caused by increasing the arc current. [76]. Also, it was claimed that the presence of H 2 led to enhance in evaporation rate. Consequently, this results in coarse particles size owing to increasing growth and agglomeration. This appears to be a consequence of the formation of tiny hydrogen bubbles in the molten feedstock, which impacts feedstock evaporation significantly in bi-atomic gases. This interplay was proposed for interaction between nitrogen and molten metal [76, 77]. According to ionization potential of gases, the arc efficiency value in argon shield is 22-46% and this amount in helium is 55-80 % [78]. Regarding the effects of the gas on arc specifications, Helium has lower electric conductivity and higher arc efficiency compared with that of Argon, resulting higher input energy. However, in the present study, using the helium as carrier gas resulted in lower particle size. It should be mentioned that the size of synthesized nanopowders is also influenced by the gas flow rate and thermal conductivity of carrier gas. This observation in the case of nanoparticle size can be related to greater thermal conductivity of helium (0.152 W/m/K) compared with that of the argon (0.0177 W/m/K) [79]. The mechanism of nanoparticle formation in this process includes nucleation, particle growth, particle coagulation and coalescence. Homogenous nucleation could occur by interaction between ascending metallic vapour and the carrier gas [80]. Collision between the metallic clusters and the remaining metallic vapour results in particle growth. Eventually, particle coagulation and coalescence may occur between metallic clusters. The mean particle size has an inverse relation with thermal conductivity and therefore a direct relation to the atomic mass of the carrier gas. On collision, heavier gas atoms can absorb more energy from the metallic atoms in the hot vapour phase resulting in higher growth rate of metal clusters. Heavier carrier gas molecules not only increase growth rate, but also result in
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higher collision rate which further increase growth rate. In addition, lower thermal conductivity causes the carrier gas to cool slower. Therefore, the particles synthesized under helium atmosphere should have smaller sizes than those obtained under argon atmosphere. Hence, it seems that in the present study, the impact of the thermal conductivity of carrier gas on the particle size of powders is found to be greater then the other mentioned factors. Synthesized nanoparticles were collected in palm oil methyl ester (POME) which is known as a biodiesel fuel. The specifications of POME are presented in Table 4. In liquid, the degree of dispersion stability in the medium was determined by a visual inspection of the sedimentation volume. It was observed that without any stabilizers and surfactants, nanoparticles (52 nm) created a stable suspension. Stability which is the most crucial issue can be hampered by particle aggregation. The agglomeration of nanoparticles directly affects the stability and performance of nanofluids. Aggregation of nanoparticles is due to the sum of attractive and repulsive forces between particles. Enhancement of repulsive forces over attractive forces can prevent particle aggregation and ensure stability. There are various mechanisms for nanoparticle stabilization in liquids. Steric stabilization of nanoparticles is achieved by attaching (grafting or chemisorption) macromolecules such as polymers or surfactants to the surfaces of the particles. The stabilization is due to the large adsorbents which provide steric barrier to prevent particles coming close to each other [81, 82]. In the present study, stability of particle suspension may be related to the possible role of organic agents. Oleic acid ( C18 H 34 O 2 ), classified as a monounsaturated fatty acid, is one of the constituents of POME and widely used as a surfactant in colloids synthesis. The density of an oleic acid molecule is 0.895 g/mL. It is 1.97 nm in length and 0.5 nm in width. Due to large surface areas, nanoparticles are expected to adsorb the adjacent oleic acid molecules in suspension. Different types of interactions between the carboxylate head of hydrocarbons and
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nanoparticles surface have been identified. The formation of bonds between nanoparticles and oleic acid by chemisorption of carboxylate groups of oleic acid on surface of metal and metal oxide nanoparticles has been reported. Oleic Acid molecules form a monolayer on surface of nanoparticles, called oleate layer, in which the tails of them are non-polar so cause a repulsive force between particles, as shown in Fig. 12 [83-86]. Oxidation prevention of ultra fine nanoparticles may be attributed to chemical interaction between ultra fine nanoparticles and hydrocarbon agents leading to prevention from self-ignition [87]. However, better comprehension on the interaction between Mg-Al nanoparticles and POME needs further comprehensive studies. 4. Conclusion In the present study, Mg–Al alloy nanoparticles with different chemical compositions were prepared by arc plasma evaporation technique from bulk alloys. Morphology, composition, phase structure and particle size of powders were investigated. The as-produced nanoparticles have spherical shapes and smooth surfaces structures. The average diameter of nanoparticles prepared under the atmosphere of pure helium was 52 nm, while that under the argon atmosphere was 160 nm. The size of nanoparticles is determined by thermal and physical properties of the carrier gas. In the present study, the impact of the thermal conductivity of carrier gas on the particle size of powders is found to be greater then the other factors. Particle size of powders synthesized at higher voltage (related to electrode gap distance) was smaller than that of the powders prepared at lower voltage. The composition of the prepared particles is different from that of the raw bulk materials. The composition diversity between the bulk and its’ corresponding nanoparticles is attributed to the difference in evaporation rate of Mg and Al. Tangible particle size variations are not observed in nanoparticles synthesized of various master alloys. Three crystalline phases
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including Mg, Al and Al12 Mg17 were detected in prepared powders. Presence of Al12 Mg17 intermetallic phase indicates that the non-equilibrium thermodynamic condition is dominating factor during formation of nanoparticles in arc plasma evaporation technique. Moreover, metallic oxide phases were not detected in the XRD pattern.
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Figure and Figure captions
Fig. 1. Schematic diagram of the experimental setup. Fig. 2. The SEM micrograph and corresponding EDS result of Mg-50 wt. % Al alloy. Fig. 3. Scanning electron image and concentration of Mg and Al element crosses the interdendritic region in Mg-50 wt. % Al alloy. Fig. 4. XRD characteristic of Mg-50 wt. % Al alloy. Fig. 5. FESEM micrographs of Mg- Al particles synthesized at (a) lower and (b) higher voltages based on electrode gap distance. Fig. 6. Average values of Mg concentration in nanoparticles. Fig. 7. XRD pattern of Mg-28 wt. % Al nanoparticles. Fig. 8. Gibbs free energy versus temperature for compounds of Mg-Al phase diagram [71]. Fig. 9 Mg-Al phase diagram [49] and schematic illustration of formation of nanoparticles [74]. Fig. 10. FESEM micrographs of arc plasma synthesized powders using (a) argon and (b) helium as carrier gases, suspended in POME. Fig. 11. Dynamic light scattering (DLS) results of prepared powders using argon and helium as carries gas. Fig. 12. (a) Oleic acid molecular formula and (b) proposed interaction between the carboxylate head and nanoparticles.
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Fig. 1. Schematic diagram of experimental setup.
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Fig. 2. The SEM micrograph and corresponding EDS of Mg-50 wt. % Al alloy.
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Fig. 3. Scanning electron image and concentration of Mg and Al crosses the interdendritic region in Mg-50 wt. % Al alloy.
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Fig. 4. XRD characteristic of Mg-50 wt. % Al alloy.
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Fig. 5. FESEM micrographs of Mg- Al particles synthesized at (a) lower and (b) higher voltages based on electrode gap distance.
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Fig. 6. Average values of Mg concentration in nanoparticles.
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Fig. 7. XRD pattern of Mg-28 wt. % Al nanoparticles.
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Fig. 8. Gibbs free energy versus temperature for components of Mg-Al phase diagram [71].
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Fig. 9 Mg-Al phase diagram [49] and schematic illustration of formation of nanoparticles [74].
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Fig. 10. FESEM micrographs of arc plasma synthesized powders using (a) argon and (b) helium as carrier gases, suspended in POME.
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.
Fig. 11. Dynamic light scattering (DLS) results of prepared powders using argon and helium as carries gas.
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Fig. 12. (a) Oleic acid molecular formula and (b) proposed interaction between the carboxylate head and nanoparticles.
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Table and Table Captions
Elements (wt. %)
Al
Fe
Si
Cu
Mg
Mn
Zn
Al ingot
Bal.
0.171
0.131
0.002
0027
0.009
0
Mg ingot
0
0.002
0.029
0.012
Bal.
0.017
0.093
Table 1. The Chemical composition of raw materials.
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Element (at %)
Al
Mg
Phase
Point 1
47.11
52.89
Al12 Mg17
Point 2
46.10
53.90
Al12 Mg17
Point 3
46.02
53.98
Al12 Mg17
Table 2. The EDS results of Fig. 2.
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Samples
Al concentration in Al concentration in master alloy (wt.%) nanopowders(wt.%) A 35 24 B 45 26 C 50 28 D 65 33 Table 3. Al concentration in master alloy and powders.
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Property Density
Palm Oil methyl ester
Viscosity
4.415 mm 2 .s −1
Flash Point
182 o C
Cloud Point
15.2 o C
Pour Point
15 o C
Methanol Content
≤ 0. 2 % mm−1
−3 0.878 gr.cm
Table 4. The specifications of palm oil methyl ester (POME).
.
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1- Fabrication of master Al-Mg alloys with designed chemical composition. 2- Synthesize of Mg- Al metal nanopowders and characterization of materials property. 3- Detection of Al and Mg metallic elements and Al12 Mg17 intermetallic in nanopowders. 4- Higher voltage (related to electrode gap distance) caused finer powders. 5- Helium as carries gas produced finer particles compared with the size of powders prepared by argon.
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S1385-8947(14)01107-3 http://dx.doi.org/10.1016/j.cej.2014.08.053 CEJ 12560
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
17 June 2014 7 August 2014 16 August 2014
Please cite this article as: M. Karbalaei Akbari, R. Derakhshan, O. Mirzaee, A case study in vapor phase synthesis of Mg-Al alloy nanoparticles by plasma arc evaporation technique, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.08.053
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A case study in vapor phase synthesis of Mg-Al alloy nanoparticles by plasma arc evaporation technique a
b
M. Karbalaei Akbari 1, R. Derakhshan , O. Mirzaee a b
a
Faculty of Metallurgical and Materials Engineering, Semnan University, Semnan, Iran
Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran
Abstract Alloy nanoparticles in the Mg–Al system were prepared by plasma arc discharge method from the Mg x − Al ( 45% 〈 x 〈 65 wt.%) bulk alloys. Powders were produced at various applied voltages. High purity argon and helium were separately employed as cooling and carrier gases. Morphology, composition, phase structure and particle size of the products were investigated by field emission scanning electron microscopy (FESEM), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD) and dynamic light scattering (DLS) techniques. Morphological studies show the formation of spherical nanoparticles with smooth surface. Higher voltages (related to electrode gap distance) accompanied by helium as cooling gas provide finer nanoparticles. Because of difference in evaporation rates for both magnesium and aluminum in the master alloy, the compositions of synthesized powders are found to be different from those of the raw materials. Three crystalline phases including Mg, Al and
Al12 Mg17 are detected in prepared powders. Oxide phases are not detected in XRD pattern. The 1
Corresponding author. Tel.: +989111704469; fax: +981714422858; Semnan province P.C 35131-19111; Iran E-mail address: [email protected]
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DLS studies demonstrate the mean particle size of 52 nm and 160 nm related to powders synthesized under helium and argon atmosphere, respectively. Keywords: Intermetallics; Vapor deposition; Phase diagrams; Scanning electron microscopy, SEM; X-ray diffraction. 1. Introduction It is now well known that nanoparticles exhibit unique and improved physical, thermal, electrical and chemical properties different from their corresponding bulk materials due to the small size, surface, quantum size and quanta tunnel effects [1-3]. Regarding their unique properties, nanoparticles are candidates for catalysts, sintering aids, microwave absorption, magnetic recording media, magnetic fluids, fuel additive, biomarkers, biosensors and development of nanofluids. Investigation of these properties has been one of the main scientific pursuits in the recent decades. Accordingly, the preparation and characterization of metal nanopowders has become an active field [4-11]. Various techniques have been developed to prepare metal nanoparticles, such as gas-phase chemical reaction, mechanical milling, spray pyrolysis, water-heating reaction, laser ablation, flame processing, vapor deposition, microwave plasma synthesis, arc plasma discharge, sol–gel methods and electromagnetic levitation [12-19]. Among various synthetic methods, the arc plasma evaporation technique demonstrates several advantages, including high temperature processing, supersaturated vapor phase preparation and high heating and cooling rate [20-22]. However; one the main specifications of this technique is the accessibility of device and feasibility of method. In this technique the chemical composition, shape and size of nanoparticles can be easily controlled by changing the process parameters, such as applied voltage and amperage of electrical source, composition of raw materials, type and concentration of inert gas Page 2 of 31
in reaction chamber and reaction time. Successful preparation of pure metal nanopowders by this technique has been reported [23-27]. In the previous researches, considerable efforts have been directed to the formation and characterization of less reactive metal powders and their alloys by arc discharge method. It was concluded that substitutional solid solutions always form in these nanoparticles. However, while alloy nanoparticles have a great potential for technological application the mechanisms of raw material vaporization, nucleation and nanoparticle growth in binary systems, especially when two constituents have very different vapor pressures, are a topic of conversation. Formation of intermetallic-compounds has become a new appealing topic because of their attractive properties, such as good corrosion resistance, interesting thermal specifications, lightweight, excellent catalytic and magnetic properties [28, 29]. A case study in Fe-Sn binary system showed the formation of Fe-Sn nanoparticles with a core/shell structure. They characterized an intermetallic core (FeSn 2 and Fe 3Sn 2 ) covered with an oxide shell
(SnO 2 ) with 5-10 nm thickness [30]. In the other study, in the Mg-Cu synthesized nanopowders four phases, i.e. Mg, MgO and intermetallic compounds of Mg 2 Cu and MgCu2 , were detected [31]. Recently, Mg-based ultra fine particles, such as Mg, Mg–Zn, Mg–V and Mg-La-Al have been fabricated by a hydrogen plasma–metal reaction method [32-35]. Among alloying elements, Mg-Al based alloy particles are known as one of the most attractive candidates for various engineering applications, such as hydrogen storage and fuel additives in automobile industries [35]. There are various aspects needed to be investigated when researchers study the Mg-Al alloy powders and work on the fabrication of intermetallic compounds. One of them is related to the preparation of Al-Mg stoichiometric compounds by conventional melting methods because of the large difference in the vapor pressure and the melting point between Mg and the other metals [36]. Most of the works on Mg-Al alloy systems Page 3 of 31
in the literatures used ball milling/mechanical alloying technique as the preparation method. Ball milling technique seems to be an effective way to prepare nanocrystalline and non-equilibrium samples. One of the main disadvantages of ball milling method is related to the contamination of products by balls, cup, medium of milling process and air, even in very good protection conditions [37, 38]. The second issue is related to the application of Mg-Al alloy particles. To achieve the automobile application goals, many kinds of hydrogen storage materials have been explored and studied. Among these materials, Mg-based alloys are always attractive to many researchers because of the great abundance, the light weight of Mg and high hydrogen capacity of Mg hydrides. However, as hydrogen storage material Mg shows poor hydrogen sorption kinetics. The addition of Al plays a crucial role in enhancing the sorption kinetics of the Mg nanoparticles [39-41]. From another point of view, Mg-Al particles release more combustion energy compared with that of the pure magnesium. Researchers investigated the potential of Mg-Al nanoparticles as an additive in biodiesel. Mg-Al nanoparticles reduce the energy consumption and improve the thermal efficiency; the reason is that, the additive releases energy during combustion in biodiesel fuel and enhances the thermal energy of fuel. Moreover, researchers claimed that nanoparticles improve the performance of engines by means of micro explosion phenomena and hence it reduces the formation of pollutants. The combustion of mixed fuels is accompanied by explosion of nanoparticles. Due to micro explosion, the air/fuel mixing will be proper and hence it results in reduction in hydrocarbon and carbon monoxide emission [42]. Regarding the mentioned advantages, a lot of attention has been devoted to the fabrication and characterization of high purity Mg-Al nanopowders. The application of nanoparticles strongly depends on their properties, such as the chemical composition, phase structures and also related to particle size,
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shape and agglomeration. At the present study, Mg–Al alloy nanopowders were prepared from bulk alloys by arc plasma evaporation method. The particle size, morphology, phase structure and composition of nanoparticles were investigated. 2. Materials and methods In order to prepare the magnesium-aluminum alloys by casting process, pure Al and Mg ingots, with chemical composition presented in Table 1, were used. Al ingot was melted in silicon carbide crucible by using an induction furnace at 900 o C under the protective atmosphere of argon gas. Then, the furnace was turned off. Subsequently, appropriate amounts of Mg chips were introduced into the molten alloy and then covered with flux and followed by casting in steel water-cooled mould. To prevent unwanted reaction between Fe and molten alloy, the surface of steel mould was covered by a suspension of boron nitride in water. Samples were produced with various weight ratios of Mg to Al including: 45:55, 50:50, 55:45 and 65:35. After casting, alloys were grinded through 240 up to 2500 grit papers followed by polishing and etching by a 10% HF-distilled water solution and coating with gold. Scanning electron microscope (SEM, CAMSCAN-MV2300) coupled with an energy-dispersive X-ray spectrometer (EDS) was used to locally measure the chemical composition of specimens. The phase constituents were determined by X-ray diffraction (XRD) analysis using Cu − K α radiation with λ=1.54046 A o (X’ pert Philips). The chemical compositions of alloys were determined by
inductively coupled plasma- atomic emission spectroscopy (ICP-AES) technique. From master alloys, nanoparticles were synthesized by electrical arc discharge technique. The electrical instrument was a high current DC power supply facilitated with digital equipments to monitor the applied voltage and amperage. The apparatus consists of a water-cooled steel reactor chamber covered with an internal graphite layer, including anode and moveable cathode
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and cooling facilities, as shown in Fig. 1. A micrometer was used to adjust the distance between two electrodes. In the chamber, Mg-Al alloy electrode, with an average weight of 5-7 gr, was placed on a water-cooled platform that served as the anode and an upper tungsten pole was used as the cathode. The chamber was vacuumed ( 10 −3 Pa ) and then backfilled with two various inert gases, including argon and helium. The current was kept at 100 A, while the voltage varied (1224 V) with the distance between two electrodes. During the production process, regarding the variation of voltage, the distance between the electrodes was manually altered. The operation continued for 7 min. During the production process the inert gas was forced to circulate in the steel chamber and the powder collection container. The flow rate of carrier gas was 8 lit/min. The flow rate of inert gas was adjusted according to the practical observations to cause continues flow rate. Inert gas was horizontally directed to the position of interaction. The carrier gas transported the synthesized particles through a quartz tube into the lateral collector container and passed them through palm oil methyl ester (POME) and collected in the Pyrex cup. The produced suspension was dried in an oven in controlled atmosphere and consequently nanoparticles were in situ passivated in a mixture atmosphere of Ar– O 2 (approximately 1 at. % O 2 ) at room temperature for 10 h. After passivation process, the oxygen content of oven was
gradually raised and reached to the ambient oxygen which prevents them from a rapid spontaneous oxidation as exposed to air. The structural analysis of nanoparticles was carried out by X-ray diffraction (XRD) technique. The morphological observations were investigated by Field-emission scanning electron microscope (Hitachi-S4160). The chemical composition of nanopowders was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Varian)
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method. Particle size measurement was carried out by dynamic light scattering (DLS) technique (Zeta- sizer 3000 HS). 3. Results and discussion 3.1. Alloys preparation and characterization The intermetallic compounds in the alloys were identified by SEM analysis and XRD test. The SEM micrograph and corresponding EDS results of Mg-50 wt. % Al alloy are shown in Fig. 2 and Table 2, respectively. It can be seen that a relatively uniform dendritic structure has been formed in the alloy. According to the equilibrium phase diagram in Mg-Al binary system, the Al12 Mg 17 intermetallic forms at broad composition range (approximately 45 to 60.5 wt.% Mg)
[41,42]. The eutectic single-phase structure at points 1 through 3 contains about 53 at. % Mg and 46 at. % Al. According to the binary phase diagram, this section is estimated to consist of the Al12 Mg 17 intermetallic compound [45-49].
Fig. 3 shows the SEM image and the line scanning result of Mg–50 wt. % Al alloy. The line scanning shows that the Mg concentration in the interdendritic region is higher than that of the dendrite. Studying the Al12 Mg17 intermetallic compound within the eutectic structure formed 437 oC
due to L → Al12 Mg 17 + Mg(δ) eutectic transformation and comparing the range of chemical composition for the Al12 Mg17 phase with the stoichiometric composition of the
Al12 Mg17 intermetallic compound (41.38 at.% Al-58.62 at.% Mg) and considering the concentration gradient of the Al and Mg elements at the interdendritic regions, it can be concluded that the dendrites are primary Al12 Mg17 and the interdendritic regions are the Mg eutectic [50-54]. Existence of such constituents at the alloy has been proved by using X-ray diffraction (XRD), shown in Fig. 4. The diffraction peaks of Al12 Mg 17 are identified, thus
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confirming the existence of the compound in the alloy, consistent with the measured compositions presented in Table 2. Moreover, Al 3 Mg 2 intermetallic compound which has a relatively narrow chemical composition range in the Mg-Al binary phase diagram and almost forms at constant chemical composition adjacent to the Al metal was not detected in master alloys [55-57].
3.2. Nanoparticle features Fig. 5 shows typical nanopowders synthesized at a constant applied current and at different voltages. The operation voltage varied (12-24 V) with the distance between two electrodes. The size of nanopowders decreases with increasing electrode gap spacing, as shown in Fig. 5 (a) and (b), respectively. In the present study, the size of the particles, prepared at lower voltage is in the range of 150 to 300 nm. With an increase in electrode gap particle size decreased to less than 60 nm. An increase in the electrode gap spacing generates a greater length of the plasma, accompanied by a linear rise of the arc voltage and increasing energy input to the system. The expansion of the plasma causes the ‘surface’ of plasma to enlarge and radiation heat losses. Therefore, only part of additional energy input is available for heating the molten metal. Furthermore, a large plasma volume dilutes the metal vapour and reduces the metal vapour concentrations, which in turn causes a shrinking mean particle size and less agglomeration of the nanopowders [58, 59]. Fig. 6 shows the average values of Mg concentration in synthesized powders. Also, average content of Al in nanopowders are presented in Table 3. It is worth noting that the results of Table 3 are not directly related to the corresponding results of Fig. 6. Regarding the ICP-AES measurements, Mg content in nanoparticles is higher than that of the corresponding raw Page 8 of 31
materials. Moreover, as shown in Table 3, the Al concentration in nanoparticle is markedly lower than that of the corresponding bulk materials. These differences are also attributed to the differences in vapor pressure between Mg and Al. During the arc plasma synthesis, the plasma heats up anode material to above its boiling point. This leads to the evaporation of the anode material in presence a carrier and protective gas in reaction chamber. The evaporation rate is described by the equation (1) [60]: w = 1 .574 α P
M T
(1)
where α is the condensation constant of the metal, P is the evaporating pressure (Pa) of metal at temperature T (K) and M is the molar mass of metal (g mol) . The condensation constant for most crystalline metals approaches unity [61]. The Al and Mg melting points are 660.32 and 650 o
C , respectively. However, Mg vapor pressure is approximately 10 7 times higher than that of the
Al at any temperature. At the constant temperature the evaporation rate is dominated by vapor pressure of substance. The vapor pressure of Mg is much higher than that of the Al; consequently the evaporation rate of Mg is higher than that of the Al [62-67]. This difference explains the higher magnesium content in the synthesized powders compared with that of the master alloys. Similar results were also reported in other binary systems, but the differences in composition between produced nanoparticles and raw materials are much larger in the present study and it is related to a great difference in vapor pressure of constituent elements [68, 69]. Fig. 7 shows the XRD pattern of Mg-28 wt. % Al nanoparticles, prepared from Mg-50 wt.% Al master alloy. Results show that nanoparticles have multiple phases which include Mg, Al, and Al12 Mg17 . The intensity of Al peak is weaker than that of the master alloy. A high Al content in the Mg–Al particles leads to the formation of Al together with intermetallics. From
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equilibrium phase diagram, Mg-Al binary system has several intermetallic phases, including
β − Al 3 Mg 2 , R − Al58 Mg 24 and γ − Al12 Mg17 . Nevertheless, one of them is observed in the synthesized nanopowders. In order to explain the presence of stable phases in synthesized nanoparticles, the thermodynamic situation should be considered. Literature gives the Gibbs free energy (G) expressions of Al12 Mg17 and Al3 Mg 2 . The temperature dependence of the Gibbs free energies of two intermetallic compounds in Mg-Al alloy system is presented in Fig. 8 [70, 71]. It is clear that the value of the Gibbs free energy of Al3 Mg 2 is lower than that of Al12 Mg17 at lower temperatures, indicating that Al3 Mg 2 is more stable at room temperature in equilibrium thermodynamic condition. However, Al3 Mg 2 was not detected in nanopowders. It is known that stoichiometric particle compounds can not form at the very high cooling rate during the arc plasma synthesis process, whereas the non-stoichiometric compounds with a wide composition range can nucleate and grow into intermetallic compound relatively easily [72]. The Al12 Mg17 compound has an extremely broad composition range in the Mg–Al binary system, shown in the Fig. 9 [49]. This can explain the presence of Al12 Mg17 intermetallic in nanopowders. Furthermore, it indicates that the non-equilibrium thermodynamic condition is the dominant situation during formation of nanoparticles rather than the chemical composition of the alloys. From the XRD pattern, oxide phases are not detected in the high sensitive nanopowders. It is possible that the oxide phases in nanopowders are less than the minimum content required to identify by X-Ray method. Also, it can show that nanoparticles have been passivated efficiently. Furthermore, it is possible that the amorphous oxide phases are not detected by XRD test. Another proposed mechanism is related to the process of nanoparticle solidification and formation. Gas-phase nucleation is the basis for a gas-to- solid particle transportation process. In
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the present synthesis method, Al and Mg atoms are evaporated from the raw materials with the assistance of high temperature arc plasma. Due to presence of cooling gas, large thermal gradient are created between the hot arc and cooling chamber. The driving forces and nucleation energies are provided by super cooling and lead to a series of transitions, from atoms to clusters and then from clusters to nuclei. A supersaturated state is a preliminary condition for the formation of stable clusters and nuclei. Generally, two or more atoms can form a cluster by random collisions, followed by collisions with other atoms or clusters to form larger clusters. [73]. It is speculated that there are multiple stages throughout the formation of these nanoparticles as shown in Fig. 9. Regarding the boiling temperature of Al and Mg (Al: 2470 o C , Mg: 1090 o C ), Al will firstly accomplish the described transition and form the nuclei prior to Mg, meanwhile, Mg exists in the gas state and surrounds the Al nuclei, as sketched in Fig. 9 (a). Second, the Al core adsorbs Mg atoms or clusters on its surface and grows to be a nanoparticle by the coagulation process, as seen in Fig. 9 (b). Mg atoms collide with Al atoms and begin to coagulate into liquid Mg–Al clusters, Fig. 9 (c). Below the melting point of alloy, part of the Mg and Al start to nucleate homogeneously into Mg solid solution, whereas the rest of Mg and Al remain in the liquid state outside the solid Mg core, see Fig.9 (d). When the temperature decreases further, the liquid Mg–Al shell continuously nucleates into the (Mg) solid solution that contains more Al than the primary Mg solution. In the mean time, these solid– liquid core–shell particles continue to collide and grow. At the temperature of 437 o C , the remaining liquid Mg–Al shell starts the eutectic phase transformation, forming (Mg) solid solution and the intermetallic compound, Fig. 9 (e). The formation of such a core- shell structured in Mg-Al ultra fine particles has been reported by Liu et al. [74]. The literature proposed that Al12 Mg17 intermetallic shell with 2-5 nm thickness is formed around the Mg core. The broad composition range of Al12 Mg17 Page 11 of 31
enables its nucleation even at a high cooling rate. Since the intermetallic compound is more stable against oxidation than pure Mg, the Mg particle core is protected from oxidation. Although it has been reported that the addition of Al greatly reduces the particle size of pure Mg powders due to the low evaporation rate of Al and the effect of Al–Mg chemical bonding on the molten Mg–Al [74], tangible particle size variations are not observed in the present study. The chemical composition of the prepared nanoparticles is only weakly dependent on the composition of the master alloy, explaining the similar particle size distribution found in all nanopowders. Fig. 10 (a) and (b) illustrates FESEM micrographs of arc plasma synthesized powders using argon and helium as carrier gases, respectively. The powders are suspended in Palm oil methyl ester. As it can be seen, all the powders are spherical in shape. The powders synthesized by helium cooling media are much smaller than those prepared in argon media and under the same fabrication condition. Fig. 11 shows dynamic light scattering (DLS) results. The DLS results show the mean particle size of 160 and 52 nm related to powders synthesized under argon and helium atmospheres, respectively. In the previous researches, it has been found that the composition of the carrier gas has a great effect on the particles production rate in this method. According to Murphy et al. [75] the important parameters of arc influenced by the gas compositions are the heat flux density, current density and arc pressure near the anode, which can lead to an increase in anode temperature and thereby directly influencing the evaporation rate. The heat flux and current density increase in ascending order when helium, nitrogen and hydrogen are added to argon. Argon has the highest electric conductivity and the lowest heat capacity which leads to less material evaporation regarding low power arc. It was reported that the change of gas from Ar to N 2 led to an increase in temperature and voltage of the arc plasma, Page 12 of 31
increasing the input energy to the system and enhances the melting and evaporation processes, consequently resulted in large particle sizes similar to the particle growth caused by increasing the arc current. [76]. Also, it was claimed that the presence of H 2 led to enhance in evaporation rate. Consequently, this results in coarse particles size owing to increasing growth and agglomeration. This appears to be a consequence of the formation of tiny hydrogen bubbles in the molten feedstock, which impacts feedstock evaporation significantly in bi-atomic gases. This interplay was proposed for interaction between nitrogen and molten metal [76, 77]. According to ionization potential of gases, the arc efficiency value in argon shield is 22-46% and this amount in helium is 55-80 % [78]. Regarding the effects of the gas on arc specifications, Helium has lower electric conductivity and higher arc efficiency compared with that of Argon, resulting higher input energy. However, in the present study, using the helium as carrier gas resulted in lower particle size. It should be mentioned that the size of synthesized nanopowders is also influenced by the gas flow rate and thermal conductivity of carrier gas. This observation in the case of nanoparticle size can be related to greater thermal conductivity of helium (0.152 W/m/K) compared with that of the argon (0.0177 W/m/K) [79]. The mechanism of nanoparticle formation in this process includes nucleation, particle growth, particle coagulation and coalescence. Homogenous nucleation could occur by interaction between ascending metallic vapour and the carrier gas [80]. Collision between the metallic clusters and the remaining metallic vapour results in particle growth. Eventually, particle coagulation and coalescence may occur between metallic clusters. The mean particle size has an inverse relation with thermal conductivity and therefore a direct relation to the atomic mass of the carrier gas. On collision, heavier gas atoms can absorb more energy from the metallic atoms in the hot vapour phase resulting in higher growth rate of metal clusters. Heavier carrier gas molecules not only increase growth rate, but also result in
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higher collision rate which further increase growth rate. In addition, lower thermal conductivity causes the carrier gas to cool slower. Therefore, the particles synthesized under helium atmosphere should have smaller sizes than those obtained under argon atmosphere. Hence, it seems that in the present study, the impact of the thermal conductivity of carrier gas on the particle size of powders is found to be greater then the other mentioned factors. Synthesized nanoparticles were collected in palm oil methyl ester (POME) which is known as a biodiesel fuel. The specifications of POME are presented in Table 4. In liquid, the degree of dispersion stability in the medium was determined by a visual inspection of the sedimentation volume. It was observed that without any stabilizers and surfactants, nanoparticles (52 nm) created a stable suspension. Stability which is the most crucial issue can be hampered by particle aggregation. The agglomeration of nanoparticles directly affects the stability and performance of nanofluids. Aggregation of nanoparticles is due to the sum of attractive and repulsive forces between particles. Enhancement of repulsive forces over attractive forces can prevent particle aggregation and ensure stability. There are various mechanisms for nanoparticle stabilization in liquids. Steric stabilization of nanoparticles is achieved by attaching (grafting or chemisorption) macromolecules such as polymers or surfactants to the surfaces of the particles. The stabilization is due to the large adsorbents which provide steric barrier to prevent particles coming close to each other [81, 82]. In the present study, stability of particle suspension may be related to the possible role of organic agents. Oleic acid ( C18 H 34 O 2 ), classified as a monounsaturated fatty acid, is one of the constituents of POME and widely used as a surfactant in colloids synthesis. The density of an oleic acid molecule is 0.895 g/mL. It is 1.97 nm in length and 0.5 nm in width. Due to large surface areas, nanoparticles are expected to adsorb the adjacent oleic acid molecules in suspension. Different types of interactions between the carboxylate head of hydrocarbons and
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nanoparticles surface have been identified. The formation of bonds between nanoparticles and oleic acid by chemisorption of carboxylate groups of oleic acid on surface of metal and metal oxide nanoparticles has been reported. Oleic Acid molecules form a monolayer on surface of nanoparticles, called oleate layer, in which the tails of them are non-polar so cause a repulsive force between particles, as shown in Fig. 12 [83-86]. Oxidation prevention of ultra fine nanoparticles may be attributed to chemical interaction between ultra fine nanoparticles and hydrocarbon agents leading to prevention from self-ignition [87]. However, better comprehension on the interaction between Mg-Al nanoparticles and POME needs further comprehensive studies. 4. Conclusion In the present study, Mg–Al alloy nanoparticles with different chemical compositions were prepared by arc plasma evaporation technique from bulk alloys. Morphology, composition, phase structure and particle size of powders were investigated. The as-produced nanoparticles have spherical shapes and smooth surfaces structures. The average diameter of nanoparticles prepared under the atmosphere of pure helium was 52 nm, while that under the argon atmosphere was 160 nm. The size of nanoparticles is determined by thermal and physical properties of the carrier gas. In the present study, the impact of the thermal conductivity of carrier gas on the particle size of powders is found to be greater then the other factors. Particle size of powders synthesized at higher voltage (related to electrode gap distance) was smaller than that of the powders prepared at lower voltage. The composition of the prepared particles is different from that of the raw bulk materials. The composition diversity between the bulk and its’ corresponding nanoparticles is attributed to the difference in evaporation rate of Mg and Al. Tangible particle size variations are not observed in nanoparticles synthesized of various master alloys. Three crystalline phases
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including Mg, Al and Al12 Mg17 were detected in prepared powders. Presence of Al12 Mg17 intermetallic phase indicates that the non-equilibrium thermodynamic condition is dominating factor during formation of nanoparticles in arc plasma evaporation technique. Moreover, metallic oxide phases were not detected in the XRD pattern.
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Figure and Figure captions
Fig. 1. Schematic diagram of the experimental setup. Fig. 2. The SEM micrograph and corresponding EDS result of Mg-50 wt. % Al alloy. Fig. 3. Scanning electron image and concentration of Mg and Al element crosses the interdendritic region in Mg-50 wt. % Al alloy. Fig. 4. XRD characteristic of Mg-50 wt. % Al alloy. Fig. 5. FESEM micrographs of Mg- Al particles synthesized at (a) lower and (b) higher voltages based on electrode gap distance. Fig. 6. Average values of Mg concentration in nanoparticles. Fig. 7. XRD pattern of Mg-28 wt. % Al nanoparticles. Fig. 8. Gibbs free energy versus temperature for compounds of Mg-Al phase diagram [71]. Fig. 9 Mg-Al phase diagram [49] and schematic illustration of formation of nanoparticles [74]. Fig. 10. FESEM micrographs of arc plasma synthesized powders using (a) argon and (b) helium as carrier gases, suspended in POME. Fig. 11. Dynamic light scattering (DLS) results of prepared powders using argon and helium as carries gas. Fig. 12. (a) Oleic acid molecular formula and (b) proposed interaction between the carboxylate head and nanoparticles.
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Fig. 1. Schematic diagram of experimental setup.
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Fig. 2. The SEM micrograph and corresponding EDS of Mg-50 wt. % Al alloy.
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Fig. 3. Scanning electron image and concentration of Mg and Al crosses the interdendritic region in Mg-50 wt. % Al alloy.
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Fig. 4. XRD characteristic of Mg-50 wt. % Al alloy.
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Fig. 5. FESEM micrographs of Mg- Al particles synthesized at (a) lower and (b) higher voltages based on electrode gap distance.
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Fig. 6. Average values of Mg concentration in nanoparticles.
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Fig. 7. XRD pattern of Mg-28 wt. % Al nanoparticles.
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Fig. 8. Gibbs free energy versus temperature for components of Mg-Al phase diagram [71].
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Fig. 9 Mg-Al phase diagram [49] and schematic illustration of formation of nanoparticles [74].
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Fig. 10. FESEM micrographs of arc plasma synthesized powders using (a) argon and (b) helium as carrier gases, suspended in POME.
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.
Fig. 11. Dynamic light scattering (DLS) results of prepared powders using argon and helium as carries gas.
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Fig. 12. (a) Oleic acid molecular formula and (b) proposed interaction between the carboxylate head and nanoparticles.
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Table and Table Captions
Elements (wt. %)
Al
Fe
Si
Cu
Mg
Mn
Zn
Al ingot
Bal.
0.171
0.131
0.002
0027
0.009
0
Mg ingot
0
0.002
0.029
0.012
Bal.
0.017
0.093
Table 1. The Chemical composition of raw materials.
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Element (at %)
Al
Mg
Phase
Point 1
47.11
52.89
Al12 Mg17
Point 2
46.10
53.90
Al12 Mg17
Point 3
46.02
53.98
Al12 Mg17
Table 2. The EDS results of Fig. 2.
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Samples
Al concentration in Al concentration in master alloy (wt.%) nanopowders(wt.%) A 35 24 B 45 26 C 50 28 D 65 33 Table 3. Al concentration in master alloy and powders.
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Property Density
Palm Oil methyl ester
Viscosity
4.415 mm 2 .s −1
Flash Point
182 o C
Cloud Point
15.2 o C
Pour Point
15 o C
Methanol Content
≤ 0. 2 % mm−1
−3 0.878 gr.cm
Table 4. The specifications of palm oil methyl ester (POME).
.
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1- Fabrication of master Al-Mg alloys with designed chemical composition. 2- Synthesize of Mg- Al metal nanopowders and characterization of materials property. 3- Detection of Al and Mg metallic elements and Al12 Mg17 intermetallic in nanopowders. 4- Higher voltage (related to electrode gap distance) caused finer powders. 5- Helium as carries gas produced finer particles compared with the size of powders prepared by argon.
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