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Size-controlled synthesis of Fe–Ni alloy nanoparticles by hydrogen reduction of metal chlorides
Powder Technology 161 (2006) 196 – 201 www.elsevier.com/locate/powtec
Size-controlled synthesis of Fe–Ni alloy nanoparticles by hydrogen reduction of metal chlorides Yong Jae Suh a, Hee Dong Jang a,*, Hankwon Chang a, Won Baek Kim a, Heon Chang Kim b a
Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea b Department of Chemical Engineering, Hoseo University, Asan 336-795, South Korea Received 25 April 2005; received in revised form 18 August 2005; accepted 1 November 2005 Available online 15 December 2005
Abstract We synthesized crystalline Fe – Ni nanoparticles with various particle sizes by reducing NiCl2 and FeCl2 vapors with hydrogen simultaneously. To control the primary particle size, processing variables of evaporator temperature, reaction zone temperature, and total gas flow rate were varied. The nanoparticles were nearly spherical and formed directional linkage between them due to magnetic interaction. The XRD patterns and elemental compositions measured by EDS showed that the Fe – Ni nanoparticles were mainly composed of cubic FeNi3. With various evaporation temperatures from 800 to 900 -C, the reactant concentrations were estimated to range from 7.94 10 6 to 2.68 10 5 mol/l, which resulted in the specific surface area and Sauter diameter of the particles from 11.1 to 8.8 m2/g and from 65 to 82 nm, respectively. The geometric standard deviations of the primary particle sizes obtained from TEM micrographs ranged from 1.24 to 1.27, indicating very narrow particle size distribution. The increase in the reaction temperature from 850 to 950 -C led to the reduction of the Sauter diameter from 69 to 63 nm. As the total gas flow rate decreased from 5 to 3 l/min, the Sauter diameter increased from 56 to 69 nm. D 2005 Elsevier B.V. All rights reserved. Keywords: Synthesis; Fe – Ni; Nanoparticles; Iron chloride; Nickel chloride; Hydrogen reduction
1. Introduction Magnetic nanoparticles have important applications in catalysts, magnetic fluids, and high-density recording media. In such applications, composition of the particles is considered the key characteristic affecting their magnetic properties. Many kinds of compositions including Fe, Ni, and Co have been investigated [1–4]. To further improve the properties of metallic particles, it has been of practical interest to synthesize metallic alloy particles composed of Fe–Ni [5–7], Fe–Co [1], or Ni–Co [8]. Ohno [5] has prepared particles made up of Fe – Ni alloys, which have been widely used as magnetic materials, by evaporating starting materials in an inert gas atmosphere at temperatures over 1600 -C. The particles prepared from Fe –36 at.% Ni foil have ranged from 20 to 100 nm in diameter and mostly consisted of a single phase of ferrite or austenite. Li et al. [6] have synthesized nanoparticles of six kinds of Fe– Ni alloys by hydrogen plasma reaction. The spherical Fe –Ni nanoparti* Corresponding author. Tel.: +82 42 868 3612; fax: +82 42 861 9727. E-mail address: [email protected] (H.D. Jang). 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.11.002
cles with a mean particle size less than 35 nm have been prepared. Dong et al. [7] have synthesized Fe – Ni nanoparticles ranging from 19 to 34 nm in their mean size using the hydrogen plasma reaction method in a mixture of H2 and Ar. They have found that two phases with austenite and martensite structures coexist over a wide compositional range in the nanoparticles. In addition to the compositions, particle size is also a key factor in the applications mentioned above. For example, magnetic recording density is remarkably enhanced as the magnetic particle size decreases. However, control of the size of Fe –Ni alloy nanoparticles has not been studied systematically yet to date although many studies on the magnetic properties with wide varieties of compositions have been made as mentioned above. In this study, we synthesized crystalline FeNi3 nanoparticles by reducing NiCl2 and FeCl2 vapors with hydrogen simultaneously according to the overall reaction as follows: FeCl2 ðgÞ þ 3NiCl2 ðgÞ þ 4H2 ðgÞYFeNi3 ðsÞ þ 8HClðgÞ
ð1Þ
Concentrations of the reactants and reaction temperature were systematically manipulated to control the growth of the
Y.J. Suh et al. / Powder Technology 161 (2006) 196 – 201
particles. In addition, residence time of the reactants in the reaction zone was varied. Then, we investigated the effect of the process variables on the particle size and crystallinity by a transmission electron microscopy (TEM), a BET method, and an X-ray diffraction (XRD). To examine the compositional homogeneity of each particle, we performed an elemental analysis for individual nanoparticles and a collection of them using two energy-dispersive X-ray spectroscopy (EDS) systems. 2. Experimental A typical experimental apparatus and procedure for the preparation of nanoparticles were used as in the previous studies [4,9]. The experimental apparatus consists of a multistage aerosol reactor, a particle collector and an off-gas treatment part (Fig. 1). A multi-stage aerosol reactor is made of quartz and divided into three sections: 40 cm long evaporation section, 40 cm long preheating section, and 75 cm long reaction section. The temperature of each section was controlled separately in an electrical tubular furnace. In the evaporation section (I.D. 5 cm), two quartz boats loaded with solid phases of FeCl2 (Junsei Chemical Co., 99.5%) and NiCl2 (Showa Chemical Co., 96%) were placed separately. The feed rate in mol/min of the metal chlorides was controlled by evaporation temperature and their concentration in mol/l by the carrier gas (Ar, 99.995%) flow rate. The preheating section consists of two concentric tubes with inner diameters of 3 cm and 5 cm for inner and outer tubes, respectively. Ar gas containing metal chloride vapors flowed through the inner tube while hydrogen (H2) and additional Ar gases through the outer. Reactants that have been separately heated to the reaction temperature were mixed at the exit of the nozzle installed between preheating and reaction sections. The nozzle has inner and outer diameters of 1.0 cm and 1.4 cm, respectively, constituting concentric tubes together with the outer tube of I.D. 1.8 cm. Fe –Ni nanoparticles that have been synthesized in the reaction zone (I.D. 3 cm) are collected by a Teflon membrane filter with an average pore diameter of 20 Am. These nanoparticles were allowed to react with small amount of oxygen existing in Ar gas for about 12 h since Fe – Ni nanoparticles are easily oxidized in ambient air. This might
197
result in a thin oxide film on the particle surface, protecting the particles from oxidizing further [3,7]. Then the nanoparticles were analyzed by a TEM (Philips Co. Model CM12) for particle morphology and size distribution. The specific surface area of the particles was measured by nitrogen adsorption at 196 -C using a BET (Micrometrics Model ASAP 2400) and in turn Sauter diameter, surface area mean diameter, was calculated. And the crystalline structure was examined by an X-ray diffractometer (Rigaku Co. Model RTP 300 RC). The compositional homogeneity of each particle was estimated by an elemental analysis for individual nanoparticles and an area analysis using EDS systems equipped with a TEM (Technai G2 F30) and a scanning electron microscope (SEM, JEOL, JSM 6380LA), respectively. To synthesize nanoparticles with desired properties, processing variables of evaporator temperature (T E), reaction zone temperature (T R), and total gas flow rate ( Q) were controlled. In this type of reactor, particles are first formed by homogeneous/heterogeneous nucleation and grow by coagulation, surface reaction, and sintering between neighboring particles. Since the nucleation is largely temperature dependent, the preheating temperature may be an important factor in controlling the nucleation and consequently the particle size distribution. Thus, the preheating section was set at the same temperature as the reaction zone so that the nucleation and growth can take place under the same condition. The reaction temperature was set above 850 -C, at which the conversion of NiCl2 vapor has been estimated to be 90% in the previous study [10]. The molar ratio of reactant vapors FeCl2 to NiCl2 was kept constant at about 0.5 for all the experiments in this study. This ratio was empirically determined to supply FeCl2 vapor the same amount as NiCl2 because the vaporization rates of two chlorides are different from each other. Otsuka et al. [1] reported on the synthesis of nanoparticles composed of Ni, Co and Fe by the hydrogen reduction of respective chlorides. They have successfully synthesized Fe nanoparticles by this process in spite of relatively small equilibrium constants, probably because the reaction proceeded rapidly enough to form nanoparticles. The equilibrium constant (K p) of Fe chloride is much smaller than that of Ni chloride at reaction temperatures, e.g., log(K p) = 0.11 and 3.5 for FeCl2 and NiCl2 at 950 -C,
Fig. 1. Schematic drawing of experimental apparatus for the production of Fe – Ni nanoparticles.
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Table 1 Experimental conditions and the properties of the particles produced Run no.
A B C D E F G a b c d
Temperature (-C) T Ea
b
TR
800 850 900 800 800 800 800
900 900 900 850 950 900 900
Flow rate (l/min) c
d
Ar (in)
Ar (out)
H2 (out)
2 2 2 2 2 2 2
1 1 1 1 1 0 2
1 1 1 1 1 1 1
Reactant concentration in reactor (mol/l)
Residence time in reactor (s)
Specific surface area (m2/g)
Sauter diameter by BET (nm)
7.94 10 6 1.77 10 5 2.68 10 5 8.30 10 6 7.62 10 6 1.06 10 5 6.35 10 6
0.81 0.81 0.81 0.84 0.78 1.08 0.65
11.1 9.2 8.8 10.4 11.4 10.4 12.8
65 78 82 69 63 69 56
Evaporation temperature. Reaction zone temperature. Used as carrier gas for the reactants. Supplied with hydrogen through outer tube for varying total gas flow rate.
respectively; while the conversion of Fe and Ni chlorides were 80.9% and 90.7% at the same temperature, respectively [1]. Based on the experimental study by Otsuka et al., it was expected that Fe – Ni alloy nanoparticles could be formed by simultaneous reduction of two respective chlorides by hydrogen in gas phase. Hydrogen supply of 1.0 l/min far excessive relative to the stoichiometry was made so that the reaction rate becomes independent of H2 concentration and in turn only a function of metal chlorides and HCl concentrations [10]. The experimental conditions in this study were summarized in Table 1. 3. Results and discussion 3.1. Effects of the reactant concentration The effects of the reactant concentrations on the particle properties were investigated by varying the evaporation temperature while holding T R and Q at 900 -C and 4.0 l/ min, respectively (run no. A –C in Table 1). At the evaporation temperatures of 800, 850 and 900 -C, the reactant feed rate was measured at 1.25 10 4, 2.78 10 4 and 4.22 10 4 mol/ min, respectively, and in turn the reactant concentrations were estimated at 7.94 10 6, 1.77 10 5, and 2.68 10 5 mol/l, respectively, in the reaction zone. Fig. 2 shows representative TEM micrographs for the run no. of A – C in Table 1. The nanoparticles were nearly spherical and formed directional linkage between them due to magnetic
interaction. The particles all formed chains in spite of their relatively larger particle size than those in the work by Dong et al. [7]. Dong et al. have shown that 34 nm Fe– Ni nanoparticles did not form clear chains due to lower saturation magnetization attributed to their composition of Fe – 19.09 wt.% Ni, in which the maximum paramagnetic fraction exists. The XRD patterns of the nanoparticles produced in this work corresponded to a cubic structure of Fe –Ni alloy phase as shown in Fig. 3. All the particles produced at the experimental conditions listed in Table 1 were of the same crystalline phase. However, it is difficult to assign a precise phase from the XRD pattern since the strong reflections of cubic iron nickel (FeNi, JCPDS no. 47-1417), cubic FeNi3 (JCPDS no. 38-0419), and cubic Fe64Ni36 (JCPDS no. 47-1405) occur at almost the same 2h values of 44-, 52-, and 76- [11]. An elemental mapping by the EDS (TEM) exhibited overall uniformity of Fe and Ni distribution in the nanoparticles (not shown). Also, a line analysis showed that relative intensities of two elements were rather evenly distributed (Fig. 4). The quantitative point analysis estimated the amount of the elements to exist 26.4 at.% Fe and 73.6 at.% Ni excluding the other elements like Cu for TEM grid, C for supporting film, and oxygen. To further clarify the elemental homogeneity, an area analysis over a collection of the nanoparticles was carried out using the EDS equipped with the SEM to result in the composition of 29.2 at.% Fe and 70.8 at.% Ni. The compositional data from two EDS systems agreed well with each other, indicating a good compositional homogeneity across the Fe – Ni nanoparticles.
Fig. 2. TEM images of Fe – Ni nanoparticles prepared at different reactant concentrations: (a) 7.94 10 6, (b) 1.77 10 5, and (c) 2.68 10 5 mol/l.
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Fig. 3. X-ray diffraction patterns of Fe – Ni nanoparticles produced at various reaction temperatures, T R: (a) 850, (b) 900, and (c) 950 -C (run no. D, A, and E in Table 1).
Considering both XRD patterns and elemental compositions from the EDS, we concluded that the Fe –Ni nanoparticles were mainly composed of cubic FeNi3. Actually, crystalline states of the particles are mainly determined by the temperature profile and residence time in the reaction zone. The temperature gradient is very small in the reaction zone by setting the preheating temperature at T R as mentioned before. In addition, our system provides 30 cm of the isothermal zone as a welldefined region for the process. Thus, it can be concluded that the reaction conditions of our experiments were suitable to form the crystalline phase of Fe– Ni alloy. From the TEM micrographs Fig. 2a –c, particle sizes were measured and count median diameters (or geometric mean diameter) and geometric standard deviations, r g, of the primary particles were determined by log-normal distribution plots (Fig. 5a –c). As the reactant concentrations increased, the median diameter increased from 74 to 109 nm; while the geometric standard deviation decreased slightly from 1.27 to 1.24,
Fig. 5. Size distributions of Fe – Ni primary particles produced at various evaporation temperatures, T E: (a) 800, (b) 850, and (c) 900 -C (run no. A – C in Table 1).
Fig. 4. Elemental composition profile of Fe and Ni along the line of Fe – Ni nanoparticles taken by the EDS equipped with a TEM. Inset shows the secondary electron image of the particles under the line analysis. As-received data were smoothed in aid of comparison.
indicating narrower particle size distribution with higher concentration. Considering the concentrations, the size of Fe –Ni particles were rather larger than those of Ni particles, ranging from 47 to 106 nm, produced at NiCl2 concentrations from 3.85 10 5 to 1.54 10 3 mol/l with the same reaction temperature and total gas flow rate as in the present study [10].
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The cause of the difference is not available now and needs to be studied in further investigation. Nevertheless, a more uniform particle size distribution could be obtained than r g = 1.2 –1.5 for iron nanoparticles [2] and r g = 1.4 – 1.6 for Ni nanoparticles [1]. With an increase in the concentrations, the specific surface area was estimated by the BET method to decrease from 11.1 to 8.8 m2/g. And in turn Sauter diameters of the particles, calculated from the specific surface area, increased from 65 to 82 nm. This is attributed to the increase in the number of metallic nuclei and to the higher collision and coalescence rates due to higher mass loading of the metal chlorides [12]. 3.2. Effects of the reaction temperature The effects of the reaction temperature on the particle size were investigated while keeping the other conditions at constant values. As the reaction zone temperature increased, the reactant concentration and the residence time decreased in the reaction zone (run no. D, A, and E in Table 1), attributed to thermal expansion of the gas in the reactor despite the same total gas flow rate, 4 l/min, at the inlet of the reactor. The increase in the reaction temperature from 850 to 950 -C led to the reduction of the Sauter diameter of the particles from 69 to 63 nm. The effects of T R on the particle growth in our tubular reactor may be explained by considering two competing aspects: first, higher temperature leads to higher rate of coagulation to result in greatly enhanced opportunity for the nuclei to grow into larger particles [13]. On the other hand, the residence time of the reactants in the reaction zone decreases at higher temperature due to volume expansion of the gas, providing the generated nuclei with less chance to grow into larger particles by coagulation [14]. Meanwhile, nucleation rate remained almost the same over the whole range of the reaction temperatures in this study due to very high saturation ratio [12], the ratio of actual pressure of the vapor to vapor pressure at equilibrium, which was estimated to be over 5000. The particle growth could also be affected by the change of concentration and conversion of metal chlorides in the reaction zone; the total concentration of chlorides decreased from 8.30 10 6 to 7.62 10 6 mol/l with the reaction temperature, while the conversion of NiCl2 increased from 90% to 99% [10]. The partial concentration of Ni vapor was then estimated to be 3.74 10 6 and 3.77 10 6 mol/l at T R of 850 and 950 -C, respectively. Assuming that the Fe vapor concentration varied in a similar way, the variation of the metal vapor concentrations at various T R might not cause noticeable change in the particle growth. As a result, shorter residence time might compete higher rate of the coagulation at higher temperature, led to a small decrease in the particle size. This reduced growth of the Fe –Ni particles appeared to be opposed to the previous studies for Ni and Co nanoparticles [9,10]. Given almost the same experimental conditions, the only distinct difference was the composition of reactant vapors: NiCl2 or CoCl2 in the previous studies and both FeCl2 and NiCl2 in this work. However, the precise cause of the difference is not available now.
3.3. Effects of the residence time The total gas flow rate and in turn the residence time was controlled by varying Ar gas flow rate through the outer tube with the other conditions kept constant (run no. A, F, and G in Table 1). The residence time was calculated for 30 cm long reaction zone, which is a well-defined isothermal space. As the residence time decreased from 1.08 s to 0.65 s, the Sauter diameter of the particles decreased from 69 to 56 nm. A decrease in the residence time would reduce the chance for particles to grow, resulting in a decrease in primary particle size. The particle growth might also be suppressed at higher total flow rate due to a decrease in the reactant concentrations in the reaction zone; although the concentrations in the evaporator were kept constant by holding the carrier gas flow rate resulting in constant vaporization rates. 4. Conclusions In this study, we synthesized Fe – Ni alloy nanoparticles with the cubic structure by simultaneously reducing NiCl2 and FeCl2 vapors with hydrogen. The nanoparticles with Sauter diameter between 56 and 82 nm could be obtained by controlling the processing variables such as evaporator temperature, reaction zone temperature, and total gas flow rate. The geometric standard deviations (around 1.26) of the primary particle size demonstrated that our experimental system provided a well-defined reaction conditions for the synthesis of nanoparticles with a narrow size distribution, compared with the previous studies. To understand the reduced growth of the Fe– Ni particles with the reaction temperature opposed to the results of the previous studies for Ni or Co nanoparticles requires a further study. We are planning to study the syntheses of Fe – Ni particles with different stoichiometries of constituent metals to better understand the alloy particle growth from two sources coexisting. In addition, it is required to further reduce the particle size to find more applications. References [1] K. Otsuka, H. Yamamoto, A. Yosizawa, Preparation of Fe, Co, and Ni ultrafine particles by hydrogen reduction of chloride vapors, Jpn. J. Chem. 6 (1984) 869 – 878. [2] K.Y. Park, H.D. Jang, C.S. Choi, Generation of ultrafine iron powders by gas-phase reduction of ferrous chloride with hydrogen, J. Aerosol Sci. 22 (Suppl. 1) (1991) S113 – S116. [3] H. He, R.H. Heist, B.L. McIntyre, T.N. Blanton, Ultrafine nickel particles generated by laser-induced gas phase photonucleation, Nanostruct. Mater. 8 (1997) 879 – 888. [4] H.D. Jang, D.W. Hwang, D.P. Kim, H.C. Kim, B.Y. Lee, I.B. Jeong, Preparation of cobalt nanoparticles by hydrogen reduction of cobalt chloride in the gas phase, Mater. Res. Bull. 39 (2004) 63 – 70. [5] T. Ohno, Growth of small particles of iron-nickel alloys prepared by gasevaporation technique, Jpn. J. Appl. Phys. 32 (1993) 4648 – 4651. [6] X.G. Li, A. Chiba, S. Takahashi, Preparation and magnetic properties of ultrafine particles of Fe – Ni alloys, JMMM 170 (1997) 339 – 345. [7] X.L. Dong, Z.D. Zhang, X.G. Zhao, Y.C. Chung, S.R. Jin, W.M. Sun, The preparation and characterization of ultrafine Fe – Ni particles, J. Mater. Res. 14 (1999) 398 – 406.
Y.J. Suh et al. / Powder Technology 161 (2006) 196 – 201 [8] X.G. Li, T. Murai, A. Chiba, S. Takahashi, Particle features, oxidation behaviors and magnetic properties of ultrafine particles of Ni – Co alloy prepared by hydrogen plasma metal reaction, J. Appl. Phys. 86 (1999) 1867 – 1873. [9] H.D. Jang, D.W. Hwang, D.P. Kim, H.C. Kim, B.Y. Lee, I.B. Jeong, Preparation of cobalt nanoparticles in the gas phase (I): Kinetics of cobalt dichloride reduction, Korean J. Ind. Eng. Chem. 9 (2003) 407 – 411. [10] Y.J. Suh, H.D. Jang, H.K. Chang, D.W. Hwang, H.C. Kim, Kinetics of gas phase reduction of nickel chloride in preparation for nickel nanoparticles, Mater. Res. Bull. 40 (2005) 2100 – 2109.
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[11] ICDD PDF-2 Release 2004, JCPDS-ICDD (International Centre for Diffraction Data), 2004. [12] S.K. Friedlander, Smoke, Dust and Haze: Fundamentals of Aerosol Dynamics, 2nd edR, Oxford University Press, New York, 2000, Chaps.7, 9, 10. [13] K. Okuyama, Y. Kousaka, N. Tohge, S. Yamamoto, J.J. Wu, R.C. Flagan, J.H. Seinfeld, Production of ultrafine metal oxide aerosol particles by thermal decomposition of metal alkoxide vapors, AIChE J. 32 (1986) 2010 – 2019. [14] H.D. Jang, J. Jeong, The effect of temperature on particle size in gas-phase production of TiO2, Aerosol Sci. Technol. 23 (1995) 553 – 560.
Size-controlled synthesis of Fe–Ni alloy nanoparticles by hydrogen reduction of metal chlorides Yong Jae Suh a, Hee Dong Jang a,*, Hankwon Chang a, Won Baek Kim a, Heon Chang Kim b a
Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea b Department of Chemical Engineering, Hoseo University, Asan 336-795, South Korea Received 25 April 2005; received in revised form 18 August 2005; accepted 1 November 2005 Available online 15 December 2005
Abstract We synthesized crystalline Fe – Ni nanoparticles with various particle sizes by reducing NiCl2 and FeCl2 vapors with hydrogen simultaneously. To control the primary particle size, processing variables of evaporator temperature, reaction zone temperature, and total gas flow rate were varied. The nanoparticles were nearly spherical and formed directional linkage between them due to magnetic interaction. The XRD patterns and elemental compositions measured by EDS showed that the Fe – Ni nanoparticles were mainly composed of cubic FeNi3. With various evaporation temperatures from 800 to 900 -C, the reactant concentrations were estimated to range from 7.94 10 6 to 2.68 10 5 mol/l, which resulted in the specific surface area and Sauter diameter of the particles from 11.1 to 8.8 m2/g and from 65 to 82 nm, respectively. The geometric standard deviations of the primary particle sizes obtained from TEM micrographs ranged from 1.24 to 1.27, indicating very narrow particle size distribution. The increase in the reaction temperature from 850 to 950 -C led to the reduction of the Sauter diameter from 69 to 63 nm. As the total gas flow rate decreased from 5 to 3 l/min, the Sauter diameter increased from 56 to 69 nm. D 2005 Elsevier B.V. All rights reserved. Keywords: Synthesis; Fe – Ni; Nanoparticles; Iron chloride; Nickel chloride; Hydrogen reduction
1. Introduction Magnetic nanoparticles have important applications in catalysts, magnetic fluids, and high-density recording media. In such applications, composition of the particles is considered the key characteristic affecting their magnetic properties. Many kinds of compositions including Fe, Ni, and Co have been investigated [1–4]. To further improve the properties of metallic particles, it has been of practical interest to synthesize metallic alloy particles composed of Fe–Ni [5–7], Fe–Co [1], or Ni–Co [8]. Ohno [5] has prepared particles made up of Fe – Ni alloys, which have been widely used as magnetic materials, by evaporating starting materials in an inert gas atmosphere at temperatures over 1600 -C. The particles prepared from Fe –36 at.% Ni foil have ranged from 20 to 100 nm in diameter and mostly consisted of a single phase of ferrite or austenite. Li et al. [6] have synthesized nanoparticles of six kinds of Fe– Ni alloys by hydrogen plasma reaction. The spherical Fe –Ni nanoparti* Corresponding author. Tel.: +82 42 868 3612; fax: +82 42 861 9727. E-mail address: [email protected] (H.D. Jang). 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.11.002
cles with a mean particle size less than 35 nm have been prepared. Dong et al. [7] have synthesized Fe – Ni nanoparticles ranging from 19 to 34 nm in their mean size using the hydrogen plasma reaction method in a mixture of H2 and Ar. They have found that two phases with austenite and martensite structures coexist over a wide compositional range in the nanoparticles. In addition to the compositions, particle size is also a key factor in the applications mentioned above. For example, magnetic recording density is remarkably enhanced as the magnetic particle size decreases. However, control of the size of Fe –Ni alloy nanoparticles has not been studied systematically yet to date although many studies on the magnetic properties with wide varieties of compositions have been made as mentioned above. In this study, we synthesized crystalline FeNi3 nanoparticles by reducing NiCl2 and FeCl2 vapors with hydrogen simultaneously according to the overall reaction as follows: FeCl2 ðgÞ þ 3NiCl2 ðgÞ þ 4H2 ðgÞYFeNi3 ðsÞ þ 8HClðgÞ
ð1Þ
Concentrations of the reactants and reaction temperature were systematically manipulated to control the growth of the
Y.J. Suh et al. / Powder Technology 161 (2006) 196 – 201
particles. In addition, residence time of the reactants in the reaction zone was varied. Then, we investigated the effect of the process variables on the particle size and crystallinity by a transmission electron microscopy (TEM), a BET method, and an X-ray diffraction (XRD). To examine the compositional homogeneity of each particle, we performed an elemental analysis for individual nanoparticles and a collection of them using two energy-dispersive X-ray spectroscopy (EDS) systems. 2. Experimental A typical experimental apparatus and procedure for the preparation of nanoparticles were used as in the previous studies [4,9]. The experimental apparatus consists of a multistage aerosol reactor, a particle collector and an off-gas treatment part (Fig. 1). A multi-stage aerosol reactor is made of quartz and divided into three sections: 40 cm long evaporation section, 40 cm long preheating section, and 75 cm long reaction section. The temperature of each section was controlled separately in an electrical tubular furnace. In the evaporation section (I.D. 5 cm), two quartz boats loaded with solid phases of FeCl2 (Junsei Chemical Co., 99.5%) and NiCl2 (Showa Chemical Co., 96%) were placed separately. The feed rate in mol/min of the metal chlorides was controlled by evaporation temperature and their concentration in mol/l by the carrier gas (Ar, 99.995%) flow rate. The preheating section consists of two concentric tubes with inner diameters of 3 cm and 5 cm for inner and outer tubes, respectively. Ar gas containing metal chloride vapors flowed through the inner tube while hydrogen (H2) and additional Ar gases through the outer. Reactants that have been separately heated to the reaction temperature were mixed at the exit of the nozzle installed between preheating and reaction sections. The nozzle has inner and outer diameters of 1.0 cm and 1.4 cm, respectively, constituting concentric tubes together with the outer tube of I.D. 1.8 cm. Fe –Ni nanoparticles that have been synthesized in the reaction zone (I.D. 3 cm) are collected by a Teflon membrane filter with an average pore diameter of 20 Am. These nanoparticles were allowed to react with small amount of oxygen existing in Ar gas for about 12 h since Fe – Ni nanoparticles are easily oxidized in ambient air. This might
197
result in a thin oxide film on the particle surface, protecting the particles from oxidizing further [3,7]. Then the nanoparticles were analyzed by a TEM (Philips Co. Model CM12) for particle morphology and size distribution. The specific surface area of the particles was measured by nitrogen adsorption at 196 -C using a BET (Micrometrics Model ASAP 2400) and in turn Sauter diameter, surface area mean diameter, was calculated. And the crystalline structure was examined by an X-ray diffractometer (Rigaku Co. Model RTP 300 RC). The compositional homogeneity of each particle was estimated by an elemental analysis for individual nanoparticles and an area analysis using EDS systems equipped with a TEM (Technai G2 F30) and a scanning electron microscope (SEM, JEOL, JSM 6380LA), respectively. To synthesize nanoparticles with desired properties, processing variables of evaporator temperature (T E), reaction zone temperature (T R), and total gas flow rate ( Q) were controlled. In this type of reactor, particles are first formed by homogeneous/heterogeneous nucleation and grow by coagulation, surface reaction, and sintering between neighboring particles. Since the nucleation is largely temperature dependent, the preheating temperature may be an important factor in controlling the nucleation and consequently the particle size distribution. Thus, the preheating section was set at the same temperature as the reaction zone so that the nucleation and growth can take place under the same condition. The reaction temperature was set above 850 -C, at which the conversion of NiCl2 vapor has been estimated to be 90% in the previous study [10]. The molar ratio of reactant vapors FeCl2 to NiCl2 was kept constant at about 0.5 for all the experiments in this study. This ratio was empirically determined to supply FeCl2 vapor the same amount as NiCl2 because the vaporization rates of two chlorides are different from each other. Otsuka et al. [1] reported on the synthesis of nanoparticles composed of Ni, Co and Fe by the hydrogen reduction of respective chlorides. They have successfully synthesized Fe nanoparticles by this process in spite of relatively small equilibrium constants, probably because the reaction proceeded rapidly enough to form nanoparticles. The equilibrium constant (K p) of Fe chloride is much smaller than that of Ni chloride at reaction temperatures, e.g., log(K p) = 0.11 and 3.5 for FeCl2 and NiCl2 at 950 -C,
Fig. 1. Schematic drawing of experimental apparatus for the production of Fe – Ni nanoparticles.
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Table 1 Experimental conditions and the properties of the particles produced Run no.
A B C D E F G a b c d
Temperature (-C) T Ea
b
TR
800 850 900 800 800 800 800
900 900 900 850 950 900 900
Flow rate (l/min) c
d
Ar (in)
Ar (out)
H2 (out)
2 2 2 2 2 2 2
1 1 1 1 1 0 2
1 1 1 1 1 1 1
Reactant concentration in reactor (mol/l)
Residence time in reactor (s)
Specific surface area (m2/g)
Sauter diameter by BET (nm)
7.94 10 6 1.77 10 5 2.68 10 5 8.30 10 6 7.62 10 6 1.06 10 5 6.35 10 6
0.81 0.81 0.81 0.84 0.78 1.08 0.65
11.1 9.2 8.8 10.4 11.4 10.4 12.8
65 78 82 69 63 69 56
Evaporation temperature. Reaction zone temperature. Used as carrier gas for the reactants. Supplied with hydrogen through outer tube for varying total gas flow rate.
respectively; while the conversion of Fe and Ni chlorides were 80.9% and 90.7% at the same temperature, respectively [1]. Based on the experimental study by Otsuka et al., it was expected that Fe – Ni alloy nanoparticles could be formed by simultaneous reduction of two respective chlorides by hydrogen in gas phase. Hydrogen supply of 1.0 l/min far excessive relative to the stoichiometry was made so that the reaction rate becomes independent of H2 concentration and in turn only a function of metal chlorides and HCl concentrations [10]. The experimental conditions in this study were summarized in Table 1. 3. Results and discussion 3.1. Effects of the reactant concentration The effects of the reactant concentrations on the particle properties were investigated by varying the evaporation temperature while holding T R and Q at 900 -C and 4.0 l/ min, respectively (run no. A –C in Table 1). At the evaporation temperatures of 800, 850 and 900 -C, the reactant feed rate was measured at 1.25 10 4, 2.78 10 4 and 4.22 10 4 mol/ min, respectively, and in turn the reactant concentrations were estimated at 7.94 10 6, 1.77 10 5, and 2.68 10 5 mol/l, respectively, in the reaction zone. Fig. 2 shows representative TEM micrographs for the run no. of A – C in Table 1. The nanoparticles were nearly spherical and formed directional linkage between them due to magnetic
interaction. The particles all formed chains in spite of their relatively larger particle size than those in the work by Dong et al. [7]. Dong et al. have shown that 34 nm Fe– Ni nanoparticles did not form clear chains due to lower saturation magnetization attributed to their composition of Fe – 19.09 wt.% Ni, in which the maximum paramagnetic fraction exists. The XRD patterns of the nanoparticles produced in this work corresponded to a cubic structure of Fe –Ni alloy phase as shown in Fig. 3. All the particles produced at the experimental conditions listed in Table 1 were of the same crystalline phase. However, it is difficult to assign a precise phase from the XRD pattern since the strong reflections of cubic iron nickel (FeNi, JCPDS no. 47-1417), cubic FeNi3 (JCPDS no. 38-0419), and cubic Fe64Ni36 (JCPDS no. 47-1405) occur at almost the same 2h values of 44-, 52-, and 76- [11]. An elemental mapping by the EDS (TEM) exhibited overall uniformity of Fe and Ni distribution in the nanoparticles (not shown). Also, a line analysis showed that relative intensities of two elements were rather evenly distributed (Fig. 4). The quantitative point analysis estimated the amount of the elements to exist 26.4 at.% Fe and 73.6 at.% Ni excluding the other elements like Cu for TEM grid, C for supporting film, and oxygen. To further clarify the elemental homogeneity, an area analysis over a collection of the nanoparticles was carried out using the EDS equipped with the SEM to result in the composition of 29.2 at.% Fe and 70.8 at.% Ni. The compositional data from two EDS systems agreed well with each other, indicating a good compositional homogeneity across the Fe – Ni nanoparticles.
Fig. 2. TEM images of Fe – Ni nanoparticles prepared at different reactant concentrations: (a) 7.94 10 6, (b) 1.77 10 5, and (c) 2.68 10 5 mol/l.
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Fig. 3. X-ray diffraction patterns of Fe – Ni nanoparticles produced at various reaction temperatures, T R: (a) 850, (b) 900, and (c) 950 -C (run no. D, A, and E in Table 1).
Considering both XRD patterns and elemental compositions from the EDS, we concluded that the Fe –Ni nanoparticles were mainly composed of cubic FeNi3. Actually, crystalline states of the particles are mainly determined by the temperature profile and residence time in the reaction zone. The temperature gradient is very small in the reaction zone by setting the preheating temperature at T R as mentioned before. In addition, our system provides 30 cm of the isothermal zone as a welldefined region for the process. Thus, it can be concluded that the reaction conditions of our experiments were suitable to form the crystalline phase of Fe– Ni alloy. From the TEM micrographs Fig. 2a –c, particle sizes were measured and count median diameters (or geometric mean diameter) and geometric standard deviations, r g, of the primary particles were determined by log-normal distribution plots (Fig. 5a –c). As the reactant concentrations increased, the median diameter increased from 74 to 109 nm; while the geometric standard deviation decreased slightly from 1.27 to 1.24,
Fig. 5. Size distributions of Fe – Ni primary particles produced at various evaporation temperatures, T E: (a) 800, (b) 850, and (c) 900 -C (run no. A – C in Table 1).
Fig. 4. Elemental composition profile of Fe and Ni along the line of Fe – Ni nanoparticles taken by the EDS equipped with a TEM. Inset shows the secondary electron image of the particles under the line analysis. As-received data were smoothed in aid of comparison.
indicating narrower particle size distribution with higher concentration. Considering the concentrations, the size of Fe –Ni particles were rather larger than those of Ni particles, ranging from 47 to 106 nm, produced at NiCl2 concentrations from 3.85 10 5 to 1.54 10 3 mol/l with the same reaction temperature and total gas flow rate as in the present study [10].
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The cause of the difference is not available now and needs to be studied in further investigation. Nevertheless, a more uniform particle size distribution could be obtained than r g = 1.2 –1.5 for iron nanoparticles [2] and r g = 1.4 – 1.6 for Ni nanoparticles [1]. With an increase in the concentrations, the specific surface area was estimated by the BET method to decrease from 11.1 to 8.8 m2/g. And in turn Sauter diameters of the particles, calculated from the specific surface area, increased from 65 to 82 nm. This is attributed to the increase in the number of metallic nuclei and to the higher collision and coalescence rates due to higher mass loading of the metal chlorides [12]. 3.2. Effects of the reaction temperature The effects of the reaction temperature on the particle size were investigated while keeping the other conditions at constant values. As the reaction zone temperature increased, the reactant concentration and the residence time decreased in the reaction zone (run no. D, A, and E in Table 1), attributed to thermal expansion of the gas in the reactor despite the same total gas flow rate, 4 l/min, at the inlet of the reactor. The increase in the reaction temperature from 850 to 950 -C led to the reduction of the Sauter diameter of the particles from 69 to 63 nm. The effects of T R on the particle growth in our tubular reactor may be explained by considering two competing aspects: first, higher temperature leads to higher rate of coagulation to result in greatly enhanced opportunity for the nuclei to grow into larger particles [13]. On the other hand, the residence time of the reactants in the reaction zone decreases at higher temperature due to volume expansion of the gas, providing the generated nuclei with less chance to grow into larger particles by coagulation [14]. Meanwhile, nucleation rate remained almost the same over the whole range of the reaction temperatures in this study due to very high saturation ratio [12], the ratio of actual pressure of the vapor to vapor pressure at equilibrium, which was estimated to be over 5000. The particle growth could also be affected by the change of concentration and conversion of metal chlorides in the reaction zone; the total concentration of chlorides decreased from 8.30 10 6 to 7.62 10 6 mol/l with the reaction temperature, while the conversion of NiCl2 increased from 90% to 99% [10]. The partial concentration of Ni vapor was then estimated to be 3.74 10 6 and 3.77 10 6 mol/l at T R of 850 and 950 -C, respectively. Assuming that the Fe vapor concentration varied in a similar way, the variation of the metal vapor concentrations at various T R might not cause noticeable change in the particle growth. As a result, shorter residence time might compete higher rate of the coagulation at higher temperature, led to a small decrease in the particle size. This reduced growth of the Fe –Ni particles appeared to be opposed to the previous studies for Ni and Co nanoparticles [9,10]. Given almost the same experimental conditions, the only distinct difference was the composition of reactant vapors: NiCl2 or CoCl2 in the previous studies and both FeCl2 and NiCl2 in this work. However, the precise cause of the difference is not available now.
3.3. Effects of the residence time The total gas flow rate and in turn the residence time was controlled by varying Ar gas flow rate through the outer tube with the other conditions kept constant (run no. A, F, and G in Table 1). The residence time was calculated for 30 cm long reaction zone, which is a well-defined isothermal space. As the residence time decreased from 1.08 s to 0.65 s, the Sauter diameter of the particles decreased from 69 to 56 nm. A decrease in the residence time would reduce the chance for particles to grow, resulting in a decrease in primary particle size. The particle growth might also be suppressed at higher total flow rate due to a decrease in the reactant concentrations in the reaction zone; although the concentrations in the evaporator were kept constant by holding the carrier gas flow rate resulting in constant vaporization rates. 4. Conclusions In this study, we synthesized Fe – Ni alloy nanoparticles with the cubic structure by simultaneously reducing NiCl2 and FeCl2 vapors with hydrogen. The nanoparticles with Sauter diameter between 56 and 82 nm could be obtained by controlling the processing variables such as evaporator temperature, reaction zone temperature, and total gas flow rate. The geometric standard deviations (around 1.26) of the primary particle size demonstrated that our experimental system provided a well-defined reaction conditions for the synthesis of nanoparticles with a narrow size distribution, compared with the previous studies. To understand the reduced growth of the Fe– Ni particles with the reaction temperature opposed to the results of the previous studies for Ni or Co nanoparticles requires a further study. We are planning to study the syntheses of Fe – Ni particles with different stoichiometries of constituent metals to better understand the alloy particle growth from two sources coexisting. In addition, it is required to further reduce the particle size to find more applications. References [1] K. Otsuka, H. Yamamoto, A. Yosizawa, Preparation of Fe, Co, and Ni ultrafine particles by hydrogen reduction of chloride vapors, Jpn. J. Chem. 6 (1984) 869 – 878. [2] K.Y. Park, H.D. Jang, C.S. Choi, Generation of ultrafine iron powders by gas-phase reduction of ferrous chloride with hydrogen, J. Aerosol Sci. 22 (Suppl. 1) (1991) S113 – S116. [3] H. He, R.H. Heist, B.L. McIntyre, T.N. Blanton, Ultrafine nickel particles generated by laser-induced gas phase photonucleation, Nanostruct. Mater. 8 (1997) 879 – 888. [4] H.D. Jang, D.W. Hwang, D.P. Kim, H.C. Kim, B.Y. Lee, I.B. Jeong, Preparation of cobalt nanoparticles by hydrogen reduction of cobalt chloride in the gas phase, Mater. Res. Bull. 39 (2004) 63 – 70. [5] T. Ohno, Growth of small particles of iron-nickel alloys prepared by gasevaporation technique, Jpn. J. Appl. Phys. 32 (1993) 4648 – 4651. [6] X.G. Li, A. Chiba, S. Takahashi, Preparation and magnetic properties of ultrafine particles of Fe – Ni alloys, JMMM 170 (1997) 339 – 345. [7] X.L. Dong, Z.D. Zhang, X.G. Zhao, Y.C. Chung, S.R. Jin, W.M. Sun, The preparation and characterization of ultrafine Fe – Ni particles, J. Mater. Res. 14 (1999) 398 – 406.
Y.J. Suh et al. / Powder Technology 161 (2006) 196 – 201 [8] X.G. Li, T. Murai, A. Chiba, S. Takahashi, Particle features, oxidation behaviors and magnetic properties of ultrafine particles of Ni – Co alloy prepared by hydrogen plasma metal reaction, J. Appl. Phys. 86 (1999) 1867 – 1873. [9] H.D. Jang, D.W. Hwang, D.P. Kim, H.C. Kim, B.Y. Lee, I.B. Jeong, Preparation of cobalt nanoparticles in the gas phase (I): Kinetics of cobalt dichloride reduction, Korean J. Ind. Eng. Chem. 9 (2003) 407 – 411. [10] Y.J. Suh, H.D. Jang, H.K. Chang, D.W. Hwang, H.C. Kim, Kinetics of gas phase reduction of nickel chloride in preparation for nickel nanoparticles, Mater. Res. Bull. 40 (2005) 2100 – 2109.
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