SYNTHESIS OF ULTRAFINE NICKEL-IRON POWDER BY THE MAGNESIUM REDUCTION IN GAS PHASE

Transcription

1 2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM) International Journal of Modern Physics: Conference Series Vol. 5 (2012) World Scientific Publishing Company DOI: /S SYNTHESIS OF ULTRAFINE NICKEL-IRON POWDER BY THE MAGNESIUM REDUCTION IN GAS PHASE SEYED YOUSEF TABATABAEI Department of Mining and Metallurgy, Amirkabir University of Technology Tehran, , Iran y.tabatabaei@aut.ac.ir ALI BATENI Department of Mining and Metallurgy, Amirkabir University of Technology Tehran, , Iran a.bateni@aut.ac.ir SADEGH FIROOZI a Department of Mining and Metallurgy, Amirkabir University of Technology Tehran, , Iran s.firoozi@aut.ac.ir Ultra fine metallic powders have an increasing application in magnetic materials, catalysts and chemical and metallic industries. In this research, magnesium-thermic reduction in gas phase was used to synthesize an ultra-fine nickel-iron intermetallic compound. Synthesis of nickel-iron submicron powder by metal chlorides and magnesium vapors was studied. Evaporation rate of the chlorides and magnesium in the range of 800 to 920 degrees Celsius was measured in order to control the composition of the resulting compound. The simultaneous reduction of the chlorides was done in a tubular furnace under argon atmosphere at 920 degrees Celsius. Chemical analysis of the resulting compound and the X ray diffraction analysis identified the formation of Ni 3Fe compound. The particle size of the compound was measured 60 to 230 nano-meters by scanning electron microscopy. In addition, the particles were spherical and homogeneous. The product may be used for production of magnetic and catalytic applications. Keywords: Synthesis; Ultra-fine particles; Reduction in gas phase; Ni 3Fe; Magnesium reduction. 1. Introduction Application of ultrafine powder has increased in recent decades. Amongst these, magnetic ultrafine materials have attracted attention because of their excellent physical, catalytic, and magnetic properties. These materials have a wide range of application a Sadegh Firoozi, Assistant professor, is with the Department of Mining and Metallurgy, Amirkabir University of Technology, Hafez Ave., Tehran, , Iran 111

2 112 S. Y. Tabatabaei, A. Bateni & S. Firoozi including high density magnetic storage devices and recording media, magnetic fluids, magnetic sensors and catalysts. 1 Iron nickel alloys are important soft magnetic materials with a wide range of application. 2 When the ratio of iron to nickel is near 1 to 3, an intermetallic compound Ni 3 Fe forms that presents interesting properties as a soft magnetic material. 3 In such applications, composition of the particles and the size of particles are considered the key characteristic affecting their magnetic properties. 4-5 With a decrease in magnetic particle size properties such as magnetic recording density, the noise suppression, and the material lifetime are remarkably improved. 6 Different methods are reported for producing iron-nickel ultrafine particles or nanoparticles such as mechanical attrition, hydrothermal reduction, reduction in aqueous solution, sol gel combined H 2 reduction at high temperatures and chemical vapor synthesis (CVS). 3-4, 6 In CVS method, vapor phase precursors are brought into a hot-wall reactor under conditions that favors nucleation of particles in the vapor phase rather than deposition of a film on the wall. 7 The solid metallic products nucleate homogeneously throughout the hot region of furnace and start to grow or coagulate. 8 Since the particles quickly enter the cold region of the furnace, the particle growth ceases and as the result ultra-fine or nano-size grains are produced. 9 In CVS process, particle size and morphology are controlled mainly by the residence time in reaction zone, reaction 7, 10 temperature, reactants concentration and type and mixing with carrier gas. CVS method is suitable for the compounds with high vapor pressure. Many metallic chlorides have relatively high vapor pressure and hence it is possible to perform the synthesis at moderate temperatures. 7 Most CVS processes use hydrogen as the reducing agent. 4-5,11-13 Very few researches have been reported for production of ultra-fine metallic particles by CVS method using magnesium or other reactive metal vapor. 6-7 Sohn and Paldey used the vapor phase reduction to prepare nickel aluminides and titanium aluminides ultra-fine intermetallic compounds using chloride precursor and magnesium vapor as the reducing agent. In this study, production of nickel-iron ultra-fine intermetallic powder by CVS method using metallic magnesium as the reducing agent was studied. 2. Experimental Thermodynamic equilibrium calculation was performed to establish the feasibility of the process. For this purpose, thermodynamic system of Fe-Ni-Mg-Cl-Ar was examined by FactSage thermochemical software. 16 The simultaneous reactions of NiCl 2 (g) and FeCl 2 (g) with magnesium vapor in argon atmosphere were examined at 920 C. The production of nickel-iron particles were performed in a horizontal quartz reactor. The experimental apparatus consists of a furnace reactor, quartz tube, a particle collector, gas flow meter and inlet and outlet gas treatment. Figure 1 shows a schematic drawing of the setup.

3 Synthesis of Ultrafine Nickel-Iron Powder by the Magnesium Reduction in Gas Phase 113 Chlorides Mg Quartz reaction tube Flow-meter and moisture absorbing Argon Resistance Furnace Particle collector Fig. 1. Schematic drawing of experimental setup for the production of Fe Ni ultrafine particles. The furnace was a tubular horizontal resistance type. The reaction chamber was a quartz tube of 5 cm diameter and 100 cm length with sealed joints for inlet and outlet gas. Argon gas was passed over moisture absorbing agent and hot copper wire in order to reduce moisture and oxygen in the reactor. FeCl 2.2H 2 O (Merck, 99.98%) and NiCl 2.6H 2 O (Merck, 99.98%) were dehydrated before each experiment at 300 C and were immediately placed in separate alumina boats. Magnesium (pure commercial grade) was placed in a graphite crucible and was placed at a distance from the chloride boats. Experiments were conducted three times to ensure the repeatability of the results. About two grams of dehydrated chlorides of nickel and iron were separately placed in alumina boat and three grams of magnesium was placed in the graphite crucibles and they were all located in the cold section of the furnace. After flushing the reactor with argon, temperature was increased to the desired temperature and thereafter samples were transferred into the hot zone of the furnace and were kept there for 20 minutes. Argon flow rate as the carrier gas was kept at 0.6 lit/min throughout the experiment. A particle collector was placed outside the furnace tube as shown in the Figure 1 in order to gather the solid particles. Solid particles were washed with methanol to remove the chlorides. It is important to perform the synthesis under controlled evaporation rate of chlorides and magnesium to prepare Ni 3 Fe compound. Hence, prior to the synthesis experiments, the rate of evaporation of the chlorides were measured between 800 C to 900 C. The experiments were conducted similar to the synthesis experiments but with no magnesium as the reducing agents. In each temperature the weight loss was measured for 10 and 20 minutes. Similarly, the rate of evaporation of magnesium was measured at 920 C. Finally, X-ray diffraction analysis (XRD) was used to identify the phases present in the sample. Also, scanning electron microscopy (SEM) equipped with wavelengthdispersive spectrometer (WDS) was used to determine the chemical composition, particle dimensions and morphology of the particles. For chemical analysis with WDS, five random regions in sample were chosen and elemental analysis was performed for Ni, Fe and Mg.

4 114 S. Y. Tabatabaei, A. Bateni & S. Firoozi 3. Results and Discussion The evaporation rate of the chlorides at 870 C and magnesium at 920 C are shown in Table 1. Table 1. The evaporation rate of the reactant Reactant Temperature ( C) Evaporation rate (mol/s) 10-6 NiCl FeCl Mg The results of the thermodynamic equilibrium calculation using FactSage thermochemical software are shown in Table 2. The input mass of the reactants were the actual measured evaporated mass of the reactants in a constant time interval. As the result, the mass of the nickel chloride, iron chloride and magnesium in the reaction zone was assumed to be 15, 9 and 25 moles, respectively. Based on the flow rate of the argon gas, 300 moles of argon was added as the carrier gas. The reaction products at 920 C are shown in Table 2. The thermodynamic calculation showed that at 920 C, 18 moles of an FCC nickel-iron metallic phase with two to one molar ratio of nickel to iron was formed. Upon cooling to room temperature, the FCC metallic phase transforms to five moles of Ni 3 Fe intermetallic compound and four moles of BCC iron. Table 2. Thermodynamic prediction of the products of iron and nickel chloride reaction with magnesium at 920 C Phases (moles) Gas (307) FCC metallic phase (23) Liquid salt phase (25) Phase analysis (mol fraction) Ar (0.98) MgCl 2 (0.02) Ni (0.63) Fe (0.37) MgCl 2 (1) The large negative value of the Gibbs energy of the reduction reactions with magnesium (Reaction 1) shows the feasibility of producing intermetallic phase Ni 3 Fe. Magnesium being a very reactive element has a very large negative Gibbs energy of the reaction with log(k) = Comparing this reaction with the Ni 3 Fe synthesis with hydrogen (Reaction 2) used by Suh et al. 5 with log(k) = 12.8, it appears that there is a bigger tendency for reaction with magnesium and it is more likely that reaction moves toward completion. FeCl 2 (g) + 3NiCl 2 (g) + 4Mg(g) 3Ni.1Fe(FCC) + 4MgCl 2 (l) G(920 C) = kj (1) FeCl 2 (g) + 3NiCl 2 (g) + 4H 2 (g) 3Ni.1Fe(FCC) + 8HCl(g) G(920 C) = -291 kj (2) The SEM image of the washed product with magnesium is shown in Figure 2. The image showed that the particles were uniform, spherical and with a relatively narrow size distribution. The measured size particle for the sample was in the range of 60nm to 230

5 Synthesis of Ultrafine Nickel-Iron Powder by the Magnesium Reduction in Gas Phase 115 nm. An elemental mapping by WDS (Figure 3) showed a uniform distribution of nickel and iron. The result of the quantitative elemental analysis by WDS was 70.1% Ni, 27.4% Fe and 2.5%Mg. This measurement is in good agreement with the thermodynamic prediction of the process (Table 2). Fig. 2. SEM secondary electron image of the Ni 3Fe ultrafine particles. XRD pattern of the sample is shown in Figure 4. The main peaks were related to intermetallic compound Ni 3 Fe and the minor peaks were magnesium oxide. XRD results of the synthesis products as well as the uniform distribution of nickel and iron showed by WDS confirmed the production of this intermetallic compound. XRD and WDS analysis measured that a small amount of unreacted magnesium metal entered the product. That was expected, because based on the measured evaporation rate, magnesium was slightly higher than stoichiometric amount needed in Reaction 2. Because of magnesium high affinity for oxygen, magnesium was oxidized into MgO, which was observed in the XRD pattern of the product. Sun and Paldey measured 0.5 to 2 mol percent magnesium in synthesis of aluminum titanium intermetallic compound with magnesium. It is possible to lower the magnesium impurity by increasing the ratio of chlorides to magnesium vapor and saturation of the reaction zone with chloride vapors.

6 116 S. Y. Tabatabaei, A. Bateni & S. Firoozi Fig. 3. Top right) SEM secondary electron image, top left) WDS elemental mapping of Mg, bottom right) WDS elemental mapping of Ni, and bottom left) WDS elemental mapping of Fe in the Ni3Fe ultrafine particles. Fig. 4. XRD pattern of the synthesized products with magnesium.

7 Synthesis of Ultrafine Nickel-Iron Powder by the Magnesium Reduction in Gas Phase 117 Based on the carrier gas flow rate ( Nm 3 /s) and ten centimeters length of the furnace hot zone, the residence time of the gases in the reaction zone is estimated to be about 4.5 s. Considering the relatively low residence time of the reactants in the reaction zone, particles did not have enough time to grow and as a result ultrafine particles were produced. The residence time measured by Suh et al. 5 for synthesis of Ni-Fe alloy synthesized with hydrogen gas as the reducing agent was in the range of 0.65 to 1.06 s. The average diameter of the particles reported by them decreased from 69 to 65nm with decreasing the residence time. It is expected that by reducing the residence time by changing the shape of reactor or by increasing the gas flow rate, particle size would substantially decrease. The residence time of the products in the reaction zone is an important variable that determines the product size; other variables also affect the final product size. The reaction temperature in this research (920 C) is within the range studied by Suh et al. 5 (900 to 950 C); hence reaction temperature may not be used for a comparison. Another variable that needs to be mentioned is the effect of the salt formation in the reaction zone. Table 2 shows that one of the synthesis products with magnesium was liquid MgCl 2 ; however with hydrogen (Reaction 2), HCl gas was formed. Once a liquid salt covers the particles in the reaction zone, it may act as an inhibitor for the particle growth. Swihart 17 suggested that the salt encapsulation can prevent particle agglomeration in CVS. 4. Conclusions Thermodynamic calculations predicted the formation Ni 3 Fe intermetallic compound by reduction of iron and nickel chlorides with magnesium in the experimental conditions. By measuring the evaporation rate of the reactants, the evaporation temperature for the reactants were determined to be 870 C for chlorides and 920 C for magnesium, in order to prepare Ni 3 Fe compound. Nickel-Iron ultrafine intermetallic compound were synthesized by simultaneous reduction of nickel and iron chloride with magnesium in a tubular furnace in argon atmosphere. The CVS method with magnesium, because of its higher reactivity than hydrogen and formation of liquid salt may be used as an alternative method to hydrogen for producing ultrafine intermetallic or alloy compounds. References 1. X. M. Zhou and X. W. Wei, Cryst. Growth Des. 9, 7-12 (2009). 2. H. Wang, J. Li, X. Kou and L. Zhang, J. Cryst. Growth 310, (2008). 3. X. Lu, G. Liang, and Y. Zhang, Mater. Sci. Eng., B139, (2007). 4. Y. J. Suh, H. D. Jang, H. Chang, W. B. Kim, and H. C. Kim, Powder Technol. 161, (2006). 5. Y. J. Suh, H. D. Jang, H. K. Chang, D. W. Hwang, and H. C. Kim, Mater. Res. Bull. 40, (2005). 6. Q. Liao, R. Tannenbaum, and Z. L. Wang, J. Phys. Chem. B 110, (2006). 7. H. Hahn, Acta Metall. 9, 3-12 (1997).

8 118 S. Y. Tabatabaei, A. Bateni & S. Firoozi 8. S. E. Pratsinis, and S. Vemury, Powder Technol. 88, (1996). 9. F. Einar Kruis, H. Fissan, and A. Peled, J. Aerosol Sci. 29, (1998). 10. A. Gutsch, M. Kramer, G. Michael, H. Mühlenweg, M. Pridohl and G. Zimmermann, KONA (2002). 11. K.Y. Park, H.D. Jang, and C.S. Choi, J. Aerosol Sci. 22, S113-S116 (1991). 12. H. Y. Lee and S. G. Kim, Powder Technol. 152, (2005). 13. H. D. Jang, D. W. Hwang, D. P. Kim, H. C. Kim, B. Y. Leeand I. B. Jeong, Mater. Res. Bull. 39, (2004). 14. H. Y. Sohn, and S. Paldey, Metall. Mater. Trans. B 29B, (1998). 15. H. Y. Sohn, and S. Paldey, Metall. Mater. Trans. B 29B, (1998). 16. C. W. Bale, P. Chartrand, S. A. Degterov, G. Eriksson, K. Hack, R. B. Mafoud, J. Melançon, A. D. Pelton, and S. Petersen, Calphad, 26, (2002). 17. M. T. Swihart, Curr. Opin. Colloid Interface Sci. 8, (2003).