- Email: [email protected]
Synthesis of Cu-Al2O3 nano composite powder
Scripta mater. 44 (2001) 2137–2140 www.elsevier.com/locate/scriptamat
SYNTHESIS OF Cu-Al2O3 NANO COMPOSITE POWDER D.W. Lee, G.H. Ha and B.K. Kim Korea Institute of Machinery and Materials, 66, Sangnam-Dong, Changwon, Kyungnam, South Korea (Received August 21, 2000) (Accepted in revised form December 14, 2000) Abstract—To improve electrical and mechanical properties of electrodes for contact welding, the Cu-Al2O3 nano composite powders were developed by thermo-chemical process. This method uses water-soluble copper and aluminium nitrates as the starting material and consists of four steps: the preparation of nitrates water solution, spray drying of this solution to prepare initial powder with subsequent salt decomposition heat treatment in air atmosphere to prepare the oxide powder containing CuO and Al2O3 and a final hydrogen reduction of copper oxide to copper. Suggested method has shown a good result to achieve the agglomerated copper powder with about 20nm size of aluminium oxide clusters and a homogenous distribution of oxide particles. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
Keywords: Nanostructure; Powder processing; Copper; Composites; Oxide dispersion strengthening
Introduction Dispersion strengthened Cu-Al2O3 composite materials are extensively used as materials for products, which require high-strength and electrical properties, such as electrode materials for lead wires, relay blades, contact supports and electrode materials for spot welding. Such materials also have superior strength at a high temperature and wear-resistance for electrical discharge. The main requirement for structure of dispersion-strengthened materials is a homogenous distribution and small size of oxide particles in the copper matrix. Melting and casting techniques usually can’t manufacture such composites because of the high interfacial energy between the molten metal and oxide. Same materials should be produced by the powder metallurgy methods. The first step of these methods is the production of composite powder, followed by the extrusion process to get a full density of material. Conventional method for producing of metal-oxide composite powder is the internal oxidation method. It has been regarded as the most suitable process for the synthesis of Cu-Al2O3 system for the production of qualitative electrode materials (1,2). The size of Al2O3 particle in the Cu-based composite powder produced by means of this process is from 10nm to 100nm (3). One of main defects of internal oxidation method is the non-homogeneous distribution of oxide particles, which negatively influences on the mechanical and electrical characteristics of Cu-Al2O3 composite material. Mechanical alloying and rapid solidification methods had been suggested to solve this problem (4). In this study, the thermo-chemical process to produce the Cu-Al2O3 nano composite powder with a homogenous distribution of oxide particles is introduced. All steps of present processes had been investigated in detail to optimize the production of the final composite powders. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00764-3
2138
NANO Cu-Al2O3 COMPOSITE
Vol. 44, Nos. 8/9
Figure 1. X-ray diffraction results of precursor powder after spray drying (a) and oxide powder after heating of precursor powder with the rate of 6°C/min up to 400°C (b).
Experimental The suggested method for copper-alumina composite powder production is comprised in follows: 1. Process of preparing a water solution which contains the water-soluble salts of Cu-Nitrate (Cu(NO3)23H2O) and Al-Nitrate (Al(NO3)39H2O). A total salt concentration in water was 50wt.%. The weight ratio of solved salts was set according to the composition of the final product. 2. Process of producing precursor powder by the spray drying from water-soluble salts containing Cu and Al. An effect of the process parameters on properties of a spray-dried powder was discussed in detail in previous work (5). 3. Process of heat treatment of powder after spray drying allowing to remove moisture and volatile components contained in the precursor power, and to form the Cu and Al oxide particles. The heat treatment for salt decomposition was carried out in a laboratory furnace in air atmosphere. 4. Process of reduction of the copper oxide to pure copper. Reduction heat treatment was carried out at 150°C and 200°C for 0.5 and 1 hour in hydrogen atmosphere. The microstructures and phase composition of powders produced in each step were characterized by high-resolution scanning electron microscopy and X-ray diffraction methods.
Figure 2. The results of X-ray diffraction analysis of reduced powders, depending on the reduction time and temperatures. a: reduction at 150°C for 1 hr, b: 200°C for 30 min, and c: 200°C for 1hr.
Vol. 44, Nos. 8/9
NANO Cu-Al2O3 COMPOSITE
2139
Figure 3. SEM micrograph of powder after the hydrogen reduction at 200°C for 30 min.
Results and Discussion After spray drying of initial salt solution the precursor powder consisted of spherical shape particles with the size of about 20 –50 m and amorphous structure (Fig. 1(a)). Every particle is comprised in a homogeneous molecular mixture of Cu and Al-based salts and, probably, moisture, which can be easily absorbed into spray-dried powder. To define the temperature of precursor powder heat treatment, the weight loss of spray-dried powder during the heating (TGA profile) was measured. TGA curve showed that the weight drastically decreased at 100°C and 250°C. There was no any weight change at the temperatures upper than 300°C, and after heat treatment the full weight loss of powder was about 30%. The result of X-ray diffraction investigation of spray-dried powder after heat treatment is presented in the Fig. 1(b). It shows that only reflects from copper oxide (CuO) are observed. The absence of Al2O3 peaks is due to the small quantity of this phase in present powder (less that 1%). After the heat treatment of powder in air atmosphere, the powder consisting of CuO and Al2O3 was reduced at 150°C for one hour, 200°C for 30 minutes, and 200°C for one hour in the hydrogen atmosphere. The results of X-ray diffraction analysis of powder after reduction are shown in Fig. 2(a-c) correspondingly. Fig. 2 shows that after reduction at 150°C the oxide powder is not completely reduced yet. After the reduction at the 200°C for 30 min and 1 hour there are no diffraction reflects from CuO phase and diffraction pattern looks like the same. So, the reduction at 200°C for 30 minutes (minimal temperature and time of reduction) was defined to be optimal for the purposes herein. Fig. 3 shows the micrograph (x 1,000) of the reduced powder after the heat-treatment in the hydrogen atmosphere at the
Figure 4. X-ray diffraction pattern of the Al2O3 phase extracted from the powder reduced at 200°C for 30min after the heat-treatment for 30min at the temperature 550°C (a), 700°C (b) and 850°C (c).
2140
NANO Cu-Al2O3 COMPOSITE
Vol. 44, Nos. 8/9
Figure 5. SEM micrograph of ␥Al2O3 particles extracted from the powder reduced at 200°C for 30min after the heat-treatment of oxide powder at 850°C for 30min.
optimal temperature and time of 200°C for 30 min. Particles after reduction have dendrite structure and the average size of about 30m. The structure of aluminium oxide was formed during the heat treatment of spray-dried powder in air atmosphere. To define the aluminium oxide condition in the final powder after reduction the copper matrix was dissolved in nitric acid solution and the phase composition and the shape of extracted particles were investigated. After above-mentioned heat treatment of oxide particles (400°C, about 3h) the oxide sediment extracted on the filter was amorphous. In subsequent experiments the oxide powders were prepared at the variable temperatures of heat treatment. X-ray diffraction results of oxide sediment are presented in the Fig. 4(a-c). After the heat treatment of oxide powder at the temperature of 850°C for 30min very clear peaks from ␥Al2O3 phase appeared. Fig. 5 shows the size and shape of ␥Al2O3 particles extracted from powder reduced at 200°C for 30min after heat treatment of oxide powder at 850°C for 30min. Finally the optimum condition of powder preparation was set as follows: heat treatment of spray-dried powder in air atmosphere at 850°C for 30 min to prepare oxide powder consisting of CuO and ␥Al2O3 particles with hydrogen reduction of copper oxide at 200°C for 30 min. Conclusion Cu-␥Al2O3 nano composite powders may be successfully synthesized by the thermo-chemical method. In final powder, the ␥Al2O3 particle sizes were near 20 nm, and alumina was uniformly distributed inside the matrix of copper aggregates. References 1. 2. 3. 4. 5.
A. V. Nadkarni, G. Burnie, and E. Klar, U.S. Patent 3,779,714 (1973). J. D. Troxell, P/M Conference in Aerospace Technologies, p. 4, Tampa, FL (1991). N. Komatsu and N. J. Grant, Trans AIME. 224, 705 (1962). H. Hujimaki, T. Takkai, and M. Kiushi, J. Jpn. Soc. Powder Powder Metall. 43(3), 377 (1996). D. W. Lee, G. G. Lee, B. K. Lee, and G. H. Ha, J. Korean Powder Metall. Inst. 5(1), 28 (1997).
SYNTHESIS OF Cu-Al2O3 NANO COMPOSITE POWDER D.W. Lee, G.H. Ha and B.K. Kim Korea Institute of Machinery and Materials, 66, Sangnam-Dong, Changwon, Kyungnam, South Korea (Received August 21, 2000) (Accepted in revised form December 14, 2000) Abstract—To improve electrical and mechanical properties of electrodes for contact welding, the Cu-Al2O3 nano composite powders were developed by thermo-chemical process. This method uses water-soluble copper and aluminium nitrates as the starting material and consists of four steps: the preparation of nitrates water solution, spray drying of this solution to prepare initial powder with subsequent salt decomposition heat treatment in air atmosphere to prepare the oxide powder containing CuO and Al2O3 and a final hydrogen reduction of copper oxide to copper. Suggested method has shown a good result to achieve the agglomerated copper powder with about 20nm size of aluminium oxide clusters and a homogenous distribution of oxide particles. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
Keywords: Nanostructure; Powder processing; Copper; Composites; Oxide dispersion strengthening
Introduction Dispersion strengthened Cu-Al2O3 composite materials are extensively used as materials for products, which require high-strength and electrical properties, such as electrode materials for lead wires, relay blades, contact supports and electrode materials for spot welding. Such materials also have superior strength at a high temperature and wear-resistance for electrical discharge. The main requirement for structure of dispersion-strengthened materials is a homogenous distribution and small size of oxide particles in the copper matrix. Melting and casting techniques usually can’t manufacture such composites because of the high interfacial energy between the molten metal and oxide. Same materials should be produced by the powder metallurgy methods. The first step of these methods is the production of composite powder, followed by the extrusion process to get a full density of material. Conventional method for producing of metal-oxide composite powder is the internal oxidation method. It has been regarded as the most suitable process for the synthesis of Cu-Al2O3 system for the production of qualitative electrode materials (1,2). The size of Al2O3 particle in the Cu-based composite powder produced by means of this process is from 10nm to 100nm (3). One of main defects of internal oxidation method is the non-homogeneous distribution of oxide particles, which negatively influences on the mechanical and electrical characteristics of Cu-Al2O3 composite material. Mechanical alloying and rapid solidification methods had been suggested to solve this problem (4). In this study, the thermo-chemical process to produce the Cu-Al2O3 nano composite powder with a homogenous distribution of oxide particles is introduced. All steps of present processes had been investigated in detail to optimize the production of the final composite powders. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00764-3
2138
NANO Cu-Al2O3 COMPOSITE
Vol. 44, Nos. 8/9
Figure 1. X-ray diffraction results of precursor powder after spray drying (a) and oxide powder after heating of precursor powder with the rate of 6°C/min up to 400°C (b).
Experimental The suggested method for copper-alumina composite powder production is comprised in follows: 1. Process of preparing a water solution which contains the water-soluble salts of Cu-Nitrate (Cu(NO3)23H2O) and Al-Nitrate (Al(NO3)39H2O). A total salt concentration in water was 50wt.%. The weight ratio of solved salts was set according to the composition of the final product. 2. Process of producing precursor powder by the spray drying from water-soluble salts containing Cu and Al. An effect of the process parameters on properties of a spray-dried powder was discussed in detail in previous work (5). 3. Process of heat treatment of powder after spray drying allowing to remove moisture and volatile components contained in the precursor power, and to form the Cu and Al oxide particles. The heat treatment for salt decomposition was carried out in a laboratory furnace in air atmosphere. 4. Process of reduction of the copper oxide to pure copper. Reduction heat treatment was carried out at 150°C and 200°C for 0.5 and 1 hour in hydrogen atmosphere. The microstructures and phase composition of powders produced in each step were characterized by high-resolution scanning electron microscopy and X-ray diffraction methods.
Figure 2. The results of X-ray diffraction analysis of reduced powders, depending on the reduction time and temperatures. a: reduction at 150°C for 1 hr, b: 200°C for 30 min, and c: 200°C for 1hr.
Vol. 44, Nos. 8/9
NANO Cu-Al2O3 COMPOSITE
2139
Figure 3. SEM micrograph of powder after the hydrogen reduction at 200°C for 30 min.
Results and Discussion After spray drying of initial salt solution the precursor powder consisted of spherical shape particles with the size of about 20 –50 m and amorphous structure (Fig. 1(a)). Every particle is comprised in a homogeneous molecular mixture of Cu and Al-based salts and, probably, moisture, which can be easily absorbed into spray-dried powder. To define the temperature of precursor powder heat treatment, the weight loss of spray-dried powder during the heating (TGA profile) was measured. TGA curve showed that the weight drastically decreased at 100°C and 250°C. There was no any weight change at the temperatures upper than 300°C, and after heat treatment the full weight loss of powder was about 30%. The result of X-ray diffraction investigation of spray-dried powder after heat treatment is presented in the Fig. 1(b). It shows that only reflects from copper oxide (CuO) are observed. The absence of Al2O3 peaks is due to the small quantity of this phase in present powder (less that 1%). After the heat treatment of powder in air atmosphere, the powder consisting of CuO and Al2O3 was reduced at 150°C for one hour, 200°C for 30 minutes, and 200°C for one hour in the hydrogen atmosphere. The results of X-ray diffraction analysis of powder after reduction are shown in Fig. 2(a-c) correspondingly. Fig. 2 shows that after reduction at 150°C the oxide powder is not completely reduced yet. After the reduction at the 200°C for 30 min and 1 hour there are no diffraction reflects from CuO phase and diffraction pattern looks like the same. So, the reduction at 200°C for 30 minutes (minimal temperature and time of reduction) was defined to be optimal for the purposes herein. Fig. 3 shows the micrograph (x 1,000) of the reduced powder after the heat-treatment in the hydrogen atmosphere at the
Figure 4. X-ray diffraction pattern of the Al2O3 phase extracted from the powder reduced at 200°C for 30min after the heat-treatment for 30min at the temperature 550°C (a), 700°C (b) and 850°C (c).
2140
NANO Cu-Al2O3 COMPOSITE
Vol. 44, Nos. 8/9
Figure 5. SEM micrograph of ␥Al2O3 particles extracted from the powder reduced at 200°C for 30min after the heat-treatment of oxide powder at 850°C for 30min.
optimal temperature and time of 200°C for 30 min. Particles after reduction have dendrite structure and the average size of about 30m. The structure of aluminium oxide was formed during the heat treatment of spray-dried powder in air atmosphere. To define the aluminium oxide condition in the final powder after reduction the copper matrix was dissolved in nitric acid solution and the phase composition and the shape of extracted particles were investigated. After above-mentioned heat treatment of oxide particles (400°C, about 3h) the oxide sediment extracted on the filter was amorphous. In subsequent experiments the oxide powders were prepared at the variable temperatures of heat treatment. X-ray diffraction results of oxide sediment are presented in the Fig. 4(a-c). After the heat treatment of oxide powder at the temperature of 850°C for 30min very clear peaks from ␥Al2O3 phase appeared. Fig. 5 shows the size and shape of ␥Al2O3 particles extracted from powder reduced at 200°C for 30min after heat treatment of oxide powder at 850°C for 30min. Finally the optimum condition of powder preparation was set as follows: heat treatment of spray-dried powder in air atmosphere at 850°C for 30 min to prepare oxide powder consisting of CuO and ␥Al2O3 particles with hydrogen reduction of copper oxide at 200°C for 30 min. Conclusion Cu-␥Al2O3 nano composite powders may be successfully synthesized by the thermo-chemical method. In final powder, the ␥Al2O3 particle sizes were near 20 nm, and alumina was uniformly distributed inside the matrix of copper aggregates. References 1. 2. 3. 4. 5.
A. V. Nadkarni, G. Burnie, and E. Klar, U.S. Patent 3,779,714 (1973). J. D. Troxell, P/M Conference in Aerospace Technologies, p. 4, Tampa, FL (1991). N. Komatsu and N. J. Grant, Trans AIME. 224, 705 (1962). H. Hujimaki, T. Takkai, and M. Kiushi, J. Jpn. Soc. Powder Powder Metall. 43(3), 377 (1996). D. W. Lee, G. G. Lee, B. K. Lee, and G. H. Ha, J. Korean Powder Metall. Inst. 5(1), 28 (1997).