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Preparation of nickel powders from nickel salt by ultrasonic chemical reduction method
Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) © 2006 Elsevier B.V. All rights reserved
773
Preparation of nickel powders from nickel salt by ultrasonic chemical reduction method S.Y. Jeon, J.S. Yun, D.H. Son, C.H. Hwang, S.S. Hong, and S.S. Park* Division of Applied Chemical Engineering, Pukyong National University, Busan 608-739, South Korea Submicron nickel powders have been synthesized successfully from aqueous NiCla at various temperatures and times with ethanol-water solvent by using the conventional and ultrasonic chemical reduction method. The reductive condition was prepared by the dissolution of hydrazine hydrate into basic solution. The samples synthesized in various conditions were characterized by the means of an X-ray diffractometry (XRD), a scanning electron microscopy (SEM), a thermo-gravimetry (TG) and an X-ray photoelectron speetroscopy (XPS). It was found that the samples obtained by the ultrasonic method were more smoothly spherical in shape, smaller in size and narrower in particle size distribution, compared to the conventional one, 1. INTRODUCTION Ultrasonic technology recently has attracted interest as an alternative to conventional processing. It has been reported that ultrasound may enhance chemical and physical changes in a liquid medium through the generation and subsequent destruction of cavitation bubbles [1, 2]. Over the past decade, fine nickel powder have been studied extensively due to its potential applications such as conducting paints, rechargeable batteries, chemical catalysts, optoelectronics, magnetic recording media, etc. [3]. Recently, it has attracted a great deal of attention as the inexpensive internal electrode of a multiplayer ceramic capacitor (MLCC) in the electronic industry, hi the other hand, to prepare fine metal powders, ball milling, electrodeposition, thermal plasma, polyol process, chemical vapor deposition, wet chemical reduction in aqueous solution, and many other methods have been developed [4]. As the needs for the desired properties of fine metal powder and the economical aspects of process, one of candidate methods is the wet chemical reduction method [5]. In this method, the morphology of nickel powder, such as the shape, the size, and the size distribution of particles, can be easily controlled by reaction parameter, solvent composition, a nucleation agent, a reduction agent, etc. [6, 7]. In this study, the chemical reduction in aqueous solution using conventional and ultrasonic hydrothermal reduction method were conducted for the preparation of fine nickel powders from the aqueous solution of nickel salt by reducing with hydrazine. The differences in the reaction parameters and final product properties resulting from two methods were identified to find the effects of ultrasound.
* To whom correspondence should be addressed. E-mail: [email protected]
774
2. EXPERIMENTAL A commercially available ultrasonic cleaner was used for the preparation of nickel powders from nickel salt in aqueous solution. This cleaner, Model 3210 (Branson Ultrasonic Corp., CT), is normally used as a cleaning apparatus, working at a frequency of 47 kHz with the power of 130 W that consists of a stainless-steel bath of 5.17 1 capacity and has an ultrasonic transducer attached to the bottom of the bath. A liquid solution temperature in the bath can be varied from room temperature to maximum of 80 °C. Starting material was nickel chloride hexahydrate (NiCl2-6HjO). Doubly distilled water and reagent-grade ethanol were used for preparing alcohol-water solvent. Sodium hydroxide (NaOH) and sodium carboxyl methyl cellulose (Na-CMC) were used as a pH control agent and a dispersant, respectively. Hydrazine hydrate (NaHrHaO) was used as a reducing agent. All reagents were used in as-received form with no further purification. Starting solution was prepared by agitating for 15 min after adding 0.8 mol I"1 of NiCl2-6H2O and 4 g I"1 of NaCMC into ethanol-water solvent (the volume ratio of ethanol = 0.4) at room temperature. The pH of starting solution was controlled to be about 12.0 by adding NaOH. Slurry was obtained by adding slowly 2.0 of molar ratio of Na^-HzO to NiCh-fiHaO into the starting solution, agitated continuously, and then heated at various temperatures for 40 min by using a conventional and ultrasonic heating unit. The addition of N^H^-HzO turned the slurry's color to black typically within a few minutes, which indicates the nucleation of nickel particles. After mixing and heating, the slurry became supersaturated and precipitated. Precipitate was separated from mother liquor by vacuum filtration, and then washed repeatedly in distilled water until pH became 7. The washed precipitate was dried at 70 °C for 24 h in a vacuum dry oven. 3. RESULTS AND DISCUSSION To find die effect of reaction temperature and ultrasound for the preparation of nickel powders, hydrothermal reductions were performed at 60 "C, 70 °C and 80 °C for various times by using the conventional and ultrasonic hydrothermal reduction method. Table 1 shows that the induction time, when starts turning the solution's color to black, decreases with increasing the reaction temperature in both the method. The induction time in the ultrasonic method was relatively shorter, compared to the conventional one. It assumes that hydrothermal reduction is faster in the ultrasonic method than the conventional one due to the cavitation effect of ultrasound. Table 1 Comparison of induction time and the properties of samples prepared in the conventional and ultrasonic hydrothermal reduction method Reaction temp. Particle size Induction time Tap density (°C) (min) (urn) (g cm"3) 0.54 12.9 1.39 60 0.34 8,3 Conventional 70 1.35 0.32 6.0 1.34 SO 0.30 60 10.0 1.45 0.27 Ultrasound 6.0 1.40 70 0.23 3.5 80 1.38
775
From XRD analyses (Fig. 1), the crystalline peaks of nickel were only observed for all of the samples obtained for 40 min by both the method. From SEM analyses (Fig. 2), the particles in all of samples were almost spherical shape without agglomeration. The average particle sizes of the samples were ranged from 0.32 ^m to 0.54 urn in the conventional method and from 0,23 urn to 0.30 um in the ultrasonic method. The particle size distribution of the samples obtained by the ultrasonic method was much narrower than that of the samples obtained by the conventional one. Above results indicates that the average particle size of the samples obtained by the conventional method is relatively larger at the same condition than that obtained by the ultrasonic one because the spherical surface of nickel particles is much irregular and some large particles exists along with the small particles in the samples obtained by the conventional one. Also, it is known that the average particle size decreased with increasing reaction temperature in the conventional and ultrasonic method because the formation of many nuclei during the nucleation period suppresses the growth of particles.
(b)
Ni I Ni(OH)j
• Ni
a &
Conventional
Intensity
?
Conventional
UItra«onk
Ultrasonic
ULJLJ; 10
20
30
40
50
60
JL 70
80
i
10
20
2 8 (degrees)
*
i
30
40
50
60
70
m
2 0 (degrees)
Fig. 1. XRD patterns of the samples prepared at 80 °C for (a) 10 min and (b) 40 min by the conventional and ultrasonic hydrothermal reduction method (a)
(b)
1µ m
1µ m (d)
(c)
(e)
1µ m
1µ m (f)
1µ m
1µ m
Fig. 2. SEM micrographs of the samples prepared at various temperatures for 40 min by the conventional and ultrasonic hydrothermal reduction method; (a) 60 °C, (b) 70 °C, and (c) 80 °C in the conventional method and (d) 60 °C, (e) 70 °C, and (f) 80 °C in the ultrasonic method
776
From TG results, it was found that the weight loss occurs at about 300 °C, and weight gain of about 20-24 % starts at about 320 °C and then stops at about 600 °C in all of the samples. The weight loss at about 300 °C may be due to the dehydration of Ni(OH)2-xH2O or the decomposition of Ni(OH)2 into MO and the weight gain at above 320 °C may be due to the thermal oxidation of Ni into MO [7]. Even though XRD results reveal that the single phase of crystalline nickel exists only in all of the samples, the possibility of existence of M(OH)2 on the surface of samples can be checked. From XPS results, it was found that M(OH)a exists on the surface of all of the samples. It means that the weight loss at about 300 °C in TG results is caused by the dehydration of M(OH)2'xH2O or the decomposition of M(OH)a existed on the surface of all of the samples. Therefore, it is known that the surface area of the samples obtained by using both the method is not reduced perfectly to form pure nickel powders due to the rapid and strong reduction of hydrazine hydrate used as a reduction agent. As the previously shown in Table 1, the tap density of the sample obtained by using the ultrasonic method was relatively higher than that obtained by the conventional one. The reason is that the surface morphology and particle size of the sample obtained by the ultrasonic method are much smooth and small as the shown in SEM results, respectively, 4. CONCLUSIONS The spherical fine nickel powders have been prepared from aqueous NiClz and hydrazine hydrate at various temperatures with ethanol-water solvent by the conventional and ultrasonic hydrothermal reduction method. The induction time decreased with increasing the reaction temperature in both the method, but was relatively shorter in the ultrasonic method. Compared to the conventional one, the surface morphology and particle size of the sample obtained by the ultrasonic method was much smooth and regular in spherical shape and was much small, respectively. Therefore, the tap density of the sample obtained by the ultrasonic method was relatively higher than that obtained by the conventional one. ACKNOWLEDGEMENTS This research was supported by the Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce, Industry and Energy of the Korean Government and by Brain Busan 21 Project.
REFERENCES [1] K.S. Suslick, Science, 247 (1990) 1439. [2] M.A. Beckett and I. Hua, J, Phys, Ghem. A., 105 (2001) 3796. [3] S.H. Park, C.H. Kim, Y.C. Rang and Y.H. Kim, J. Mat. Sci. Lett., 22 (2003) 1537. [4] K. Yurij, F. Asuncion, R.T. Cristina, C. Juan, P. Pilar, P. Ruslan and G. Aharon, Chem. Mater., 11 (1999) 1331. [5] H.G. Zheng, J.H. Liang, J.H, Zeng and Y.T. Qian, Mat. Res. Bull., 36 (2001) 947. [6] Y.T. Moon, H.K. Park, D.K. Kim and C.H. Kim, J. Am. Ceram. Soc, 78 (1995) 2690. [7] K.H. Kim, Y.B. Lee, E.Y. Choi, H.C. Park and S.S. Park, Mat. Chem. Phys., 86 (2004) 420.
773
Preparation of nickel powders from nickel salt by ultrasonic chemical reduction method S.Y. Jeon, J.S. Yun, D.H. Son, C.H. Hwang, S.S. Hong, and S.S. Park* Division of Applied Chemical Engineering, Pukyong National University, Busan 608-739, South Korea Submicron nickel powders have been synthesized successfully from aqueous NiCla at various temperatures and times with ethanol-water solvent by using the conventional and ultrasonic chemical reduction method. The reductive condition was prepared by the dissolution of hydrazine hydrate into basic solution. The samples synthesized in various conditions were characterized by the means of an X-ray diffractometry (XRD), a scanning electron microscopy (SEM), a thermo-gravimetry (TG) and an X-ray photoelectron speetroscopy (XPS). It was found that the samples obtained by the ultrasonic method were more smoothly spherical in shape, smaller in size and narrower in particle size distribution, compared to the conventional one, 1. INTRODUCTION Ultrasonic technology recently has attracted interest as an alternative to conventional processing. It has been reported that ultrasound may enhance chemical and physical changes in a liquid medium through the generation and subsequent destruction of cavitation bubbles [1, 2]. Over the past decade, fine nickel powder have been studied extensively due to its potential applications such as conducting paints, rechargeable batteries, chemical catalysts, optoelectronics, magnetic recording media, etc. [3]. Recently, it has attracted a great deal of attention as the inexpensive internal electrode of a multiplayer ceramic capacitor (MLCC) in the electronic industry, hi the other hand, to prepare fine metal powders, ball milling, electrodeposition, thermal plasma, polyol process, chemical vapor deposition, wet chemical reduction in aqueous solution, and many other methods have been developed [4]. As the needs for the desired properties of fine metal powder and the economical aspects of process, one of candidate methods is the wet chemical reduction method [5]. In this method, the morphology of nickel powder, such as the shape, the size, and the size distribution of particles, can be easily controlled by reaction parameter, solvent composition, a nucleation agent, a reduction agent, etc. [6, 7]. In this study, the chemical reduction in aqueous solution using conventional and ultrasonic hydrothermal reduction method were conducted for the preparation of fine nickel powders from the aqueous solution of nickel salt by reducing with hydrazine. The differences in the reaction parameters and final product properties resulting from two methods were identified to find the effects of ultrasound.
* To whom correspondence should be addressed. E-mail: [email protected]
774
2. EXPERIMENTAL A commercially available ultrasonic cleaner was used for the preparation of nickel powders from nickel salt in aqueous solution. This cleaner, Model 3210 (Branson Ultrasonic Corp., CT), is normally used as a cleaning apparatus, working at a frequency of 47 kHz with the power of 130 W that consists of a stainless-steel bath of 5.17 1 capacity and has an ultrasonic transducer attached to the bottom of the bath. A liquid solution temperature in the bath can be varied from room temperature to maximum of 80 °C. Starting material was nickel chloride hexahydrate (NiCl2-6HjO). Doubly distilled water and reagent-grade ethanol were used for preparing alcohol-water solvent. Sodium hydroxide (NaOH) and sodium carboxyl methyl cellulose (Na-CMC) were used as a pH control agent and a dispersant, respectively. Hydrazine hydrate (NaHrHaO) was used as a reducing agent. All reagents were used in as-received form with no further purification. Starting solution was prepared by agitating for 15 min after adding 0.8 mol I"1 of NiCl2-6H2O and 4 g I"1 of NaCMC into ethanol-water solvent (the volume ratio of ethanol = 0.4) at room temperature. The pH of starting solution was controlled to be about 12.0 by adding NaOH. Slurry was obtained by adding slowly 2.0 of molar ratio of Na^-HzO to NiCh-fiHaO into the starting solution, agitated continuously, and then heated at various temperatures for 40 min by using a conventional and ultrasonic heating unit. The addition of N^H^-HzO turned the slurry's color to black typically within a few minutes, which indicates the nucleation of nickel particles. After mixing and heating, the slurry became supersaturated and precipitated. Precipitate was separated from mother liquor by vacuum filtration, and then washed repeatedly in distilled water until pH became 7. The washed precipitate was dried at 70 °C for 24 h in a vacuum dry oven. 3. RESULTS AND DISCUSSION To find die effect of reaction temperature and ultrasound for the preparation of nickel powders, hydrothermal reductions were performed at 60 "C, 70 °C and 80 °C for various times by using the conventional and ultrasonic hydrothermal reduction method. Table 1 shows that the induction time, when starts turning the solution's color to black, decreases with increasing the reaction temperature in both the method. The induction time in the ultrasonic method was relatively shorter, compared to the conventional one. It assumes that hydrothermal reduction is faster in the ultrasonic method than the conventional one due to the cavitation effect of ultrasound. Table 1 Comparison of induction time and the properties of samples prepared in the conventional and ultrasonic hydrothermal reduction method Reaction temp. Particle size Induction time Tap density (°C) (min) (urn) (g cm"3) 0.54 12.9 1.39 60 0.34 8,3 Conventional 70 1.35 0.32 6.0 1.34 SO 0.30 60 10.0 1.45 0.27 Ultrasound 6.0 1.40 70 0.23 3.5 80 1.38
775
From XRD analyses (Fig. 1), the crystalline peaks of nickel were only observed for all of the samples obtained for 40 min by both the method. From SEM analyses (Fig. 2), the particles in all of samples were almost spherical shape without agglomeration. The average particle sizes of the samples were ranged from 0.32 ^m to 0.54 urn in the conventional method and from 0,23 urn to 0.30 um in the ultrasonic method. The particle size distribution of the samples obtained by the ultrasonic method was much narrower than that of the samples obtained by the conventional one. Above results indicates that the average particle size of the samples obtained by the conventional method is relatively larger at the same condition than that obtained by the ultrasonic one because the spherical surface of nickel particles is much irregular and some large particles exists along with the small particles in the samples obtained by the conventional one. Also, it is known that the average particle size decreased with increasing reaction temperature in the conventional and ultrasonic method because the formation of many nuclei during the nucleation period suppresses the growth of particles.
(b)
Ni I Ni(OH)j
• Ni
a &
Conventional
Intensity
?
Conventional
UItra«onk
Ultrasonic
ULJLJ; 10
20
30
40
50
60
JL 70
80
i
10
20
2 8 (degrees)
*
i
30
40
50
60
70
m
2 0 (degrees)
Fig. 1. XRD patterns of the samples prepared at 80 °C for (a) 10 min and (b) 40 min by the conventional and ultrasonic hydrothermal reduction method (a)
(b)
1µ m
1µ m (d)
(c)
(e)
1µ m
1µ m (f)
1µ m
1µ m
Fig. 2. SEM micrographs of the samples prepared at various temperatures for 40 min by the conventional and ultrasonic hydrothermal reduction method; (a) 60 °C, (b) 70 °C, and (c) 80 °C in the conventional method and (d) 60 °C, (e) 70 °C, and (f) 80 °C in the ultrasonic method
776
From TG results, it was found that the weight loss occurs at about 300 °C, and weight gain of about 20-24 % starts at about 320 °C and then stops at about 600 °C in all of the samples. The weight loss at about 300 °C may be due to the dehydration of Ni(OH)2-xH2O or the decomposition of Ni(OH)2 into MO and the weight gain at above 320 °C may be due to the thermal oxidation of Ni into MO [7]. Even though XRD results reveal that the single phase of crystalline nickel exists only in all of the samples, the possibility of existence of M(OH)2 on the surface of samples can be checked. From XPS results, it was found that M(OH)a exists on the surface of all of the samples. It means that the weight loss at about 300 °C in TG results is caused by the dehydration of M(OH)2'xH2O or the decomposition of M(OH)a existed on the surface of all of the samples. Therefore, it is known that the surface area of the samples obtained by using both the method is not reduced perfectly to form pure nickel powders due to the rapid and strong reduction of hydrazine hydrate used as a reduction agent. As the previously shown in Table 1, the tap density of the sample obtained by using the ultrasonic method was relatively higher than that obtained by the conventional one. The reason is that the surface morphology and particle size of the sample obtained by the ultrasonic method are much smooth and small as the shown in SEM results, respectively, 4. CONCLUSIONS The spherical fine nickel powders have been prepared from aqueous NiClz and hydrazine hydrate at various temperatures with ethanol-water solvent by the conventional and ultrasonic hydrothermal reduction method. The induction time decreased with increasing the reaction temperature in both the method, but was relatively shorter in the ultrasonic method. Compared to the conventional one, the surface morphology and particle size of the sample obtained by the ultrasonic method was much smooth and regular in spherical shape and was much small, respectively. Therefore, the tap density of the sample obtained by the ultrasonic method was relatively higher than that obtained by the conventional one. ACKNOWLEDGEMENTS This research was supported by the Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce, Industry and Energy of the Korean Government and by Brain Busan 21 Project.
REFERENCES [1] K.S. Suslick, Science, 247 (1990) 1439. [2] M.A. Beckett and I. Hua, J, Phys, Ghem. A., 105 (2001) 3796. [3] S.H. Park, C.H. Kim, Y.C. Rang and Y.H. Kim, J. Mat. Sci. Lett., 22 (2003) 1537. [4] K. Yurij, F. Asuncion, R.T. Cristina, C. Juan, P. Pilar, P. Ruslan and G. Aharon, Chem. Mater., 11 (1999) 1331. [5] H.G. Zheng, J.H. Liang, J.H, Zeng and Y.T. Qian, Mat. Res. Bull., 36 (2001) 947. [6] Y.T. Moon, H.K. Park, D.K. Kim and C.H. Kim, J. Am. Ceram. Soc, 78 (1995) 2690. [7] K.H. Kim, Y.B. Lee, E.Y. Choi, H.C. Park and S.S. Park, Mat. Chem. Phys., 86 (2004) 420.