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In-situ alloying on synthesis of nanosized Ni-Fe powder
NanoStmctured Materials, Vol. 9, pp. 153-156, 1997 Elsevier Scienee Lid © 1997 Acta Metallargica Inc. Printed in the USA. All rights reserved 0965-9773/97 $17.00 + .00
Pergamon
Pll S0965-9773(97)00041-X
I N - S I T U A L L O Y I N G O N S Y N T H E S I S O F N A N O S I Z E D Ni-Fe P O W D E R J. S. Lee, T. H. Kim, J. H. Yu, and S. W. Chung Department of Metallurgy and Materials Science, Hanyang University, Ansan, 425-791 KOREA Abstract -- In-situ alloying process of Ni-Fe system on synthesis of nanosized Ni-Fe powder by hydrogen reduction of ball milled NiO-Fe203 powder was investigated. It was found that the alloying of Ni-Fe during reduction of NiO-Fe203 was enhanced in the ball milled oxide mixture. Such rapid alloying process yielded the stable ?,Ni-Fe alloy aggregates consisting of 20-70 nm nanosized alloy particles. Basically, in-situ alloying by nanosized effect (chemical homogeneity with large surface area) during reduction process is believed to be responsible for the formation of nanoalloy powder. © 1997 Acta Metallurgica Inc. INTRODUCTION With the nanosized particle and the negative formation enthalpy, the spontaneous alloying of binary metal system has been reported to be possible at temperature even below 0°C (1-3). This surprising phenomenon offers a new concept for fabricating nanosized alloy powder. From the practical viewpoint, the achievement of such nanoalloy powder is of importance owing to its chemical homogeneity and large surface area that provides enormous driving force for sintering process. Recently, Lee et al.(4-6) fabricated nanocomposite metal powder by hydrogen reduction of ball milled metal oxide. The present study attempted to examine the feasibility of this process to the new concept of spontaneous alloying for nanoalloy powder synthesis. For this purpose, Ni-50wt% Fe system with a negative formation enthalpy (7) was chosen as a model system. Namely, in-situ alloying process during hydrogen reduction of ball milled NiO-Fe203 powder was investigated and discussed in terms of phase decomposition and powder characteristics.
EXPERIMENTAL The dry mixed (DM) oxide powder was prepared by blending Fe203(30txm, 99.99%) and NiO(4~tm, 99.99%) to have the alloy composition of Ni-50wt%Fe. The ball milled (BM) oxide powder was made by ball milling the DM mixture in an attritor with a speed of 350 rpm for 10 hours. The ball media and impeller were made of stainless steel and the milling agent was methyl alcohol. After ball milling, the oxide mixture was dried at 60°C for 1 hour. These two DM and BM powder mixtures were heated by raising temperature with 10°C/rain in hydrogen gas atmosphere (dew point -76°C) up to 600°C. The weight loss by decomposition during heat up was detected through the thermogravimetty (TG). The hygrometry system for measuring the content of water vapor in outlet gas was equipped with TG system in order to make both
153
154
JS LEE, TH K=u,JH Yu, ANOSW CHUNG
Figure 1. SEM morphology of NiO-Fe203 ball milled powder mixture. measurements simultaneously (6). The X-ray diffraction analysis was conducted with TG to identify the phase by the decomposition. The powder characteristics such as morphology and particle size were examined by SEM
RESULTS AND DISCUSSION
Figure 1 shows the morphology of NiO-Fe203 ball milled powder mixture having an aggregate form. As seen in the Figure, the BM aggregates consist of homogeneously mixed submicron oxide particles which approximately have the same particle sizes. In particular, the XRD results revealed no phase decomposition of NiO-Fe203 by milling (see the peak of 250°C in Figure 4). Figure 2 shows the TG curve from thermogravimetry experiment for the hydrogen reduction of Ni-Fe oxide mixtures. While the DM mixture underwent the weight loss at two different temperatures, the BM mixture lost the weight continuously between those two temperatures. The result of hygrometry study of Figure 3 corresponds with the TG curves of
1.0 '~-~0.8. "~ 0.6.
BM DM
.~0.4"
/
," 64J
~ 0.2. 0.0
0
200
10°C/min
/
400
600
Temperature(°C) Figure 2. Thermogravimetric curves of hydrogen reduction process during heat up with 10°C/min.
i:t2_ 0
200
400
600
Temperature(°C)
Figure 3. Hygrometric curves of hydrogen reduction process during heat up with 10°C/rain.
IN-SITuALLOYINGONSYNTHESISOFNANOSlZEDNi-Fe POWOER
155
DM and BM mixtures respectively and explains the weight loss by reduction process (9). As seen in the result, the DM mixture shows two large peaks separated at two different temperatures, but the BM mixture has two peaks almost overlapped seemingly into a larger one
peak. X-ray diffraction results of Figure 4 identify the phases produced by the reduction process of BM mixture in the temperature range of 250-600°C. These results with TG and hygrometry disclose the reduction of NiO at lower temperature than that of Fe2Os. We could know that two oxides were reduced simultaneously between these two temperatures and changed to the y alloy completely after heating up to 600°C. It was also found that the DM mixture underwent the formation of a limited amount of y alloy phase at 600°C. Such a different 7 formation in both samples gives an implication that the initial powder characteristics of BM mixture as homogeneously mixed submicron oxide particles play an important role for in-situ alloying of y phase in BM sample. It is well known that the reduction of Fe203 to Fe proceeds through the procedure of Fe203-Fe304-Fe (8). Moreover, the thermodynamic data (9) assure the high temperature y phase of Ni-50wt%Fe composition stable at temperature above -180°C when it is cooled relatively fast (with rate of 2-150°C/rain). Hence, it is believed that the formation of 7 phase is an evidence of in-situ alloying during the oxides reduction. Figure 5 shows the nanosized y alloy particles obtained by reducing BM powder at 600°C for 5 minutes. It is seen that the coarse y aggregate (Figure 5a) consists of 20~70 nm 7 alloy particles (Figure 5b). The diffusion depth, d=-l.2nm (d=-2~Dt) by interdiffusion between Ni 250 "C
It
A
300"C
450"C o
I 600°C
'1 20
40
i ;
6b io 60 lio 14o 20(degree)
• Fe203 • F%O4 &Ni • NiO [] F%0( (Ni,Fe)F%0 40(x'F¢ • "t-(Ni,Fo)
Figure 4. X-ray diffration patterns of hydrogen reduction process for BM powder durin~ heat uo with 10°C/min.
Figure 5. SEM micrographs of a) aggregate and b) particles of ~, alloy powder obtained from isothermal reduction of BM mixture (600°C for 5min. with 10°C/rain).
156
JS LEE,TH KJM,JH Yu, ANDSW CHUNG
and Fe is much less than the size of the "f phase alloyed particle where the interdiffusion coefficient of Ni-Fe (l~-5.14xlff:2m2/sec) at 600°C was extrapolated from high temperature (10) and the true annealing time (t) was estimated to be about 800 seconds from the 5 minutes annealing at 600°C including heat up time (11). Therefore it seems unlikely that the interditfusion between Ni and Fe predominates the complete formation of 7 in such a short time. This argument might be reasonable since the 7 phase was not produced in bulk Ni or Fe phase but in-situ produced during reduction process. Moreover, the experimental results (TG, hygrometry, XRD and microstructure) of BM powder suggest the formation of stable 7 phase more rapid than the growth of Ni or Fe bulk phase in the finely reduced particle mixture. CONCLUSION In-situ alloying process occurred in Ni-Fe system during synthesis of nanosized Ni-Fe powder by hydrogen reduction of ball milled NiO-Fe203 powder. Such rapid alloying process yielded the stable y Ni-Fe nanosized alloy powder consisting of 20--°70 nm particles. Basically, in-situ alloying process in the BM powder seems unlikely to be governed by interdiflhsion between Ni and Fe phases. The rapid alloying of BM mixture is presumed to be possible by the nanosized effect (chemical homogeneity with large surface area) during the reduction process. ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support of the Korea Ministry of Education Research Fund for Advanced Materials in 1996. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
H. Yasuda and H. Mori, Physical Review Letters 69, 3747 (1992). H. Mori, H. Yasuda and T. Kamino, Philosophical Magazine Letters 69, 279 (1994). H. Mori and H. Yasuda, J. of Microscopy 180, 33 (1995). T.H. Kim and J. S. Lee, J. of Korean Inst. Met. & Mater. 30, 203 (1992). J.S. Lee and T. H. Kim, Solid State Phenomena 25&26, 143 (1992). T.H. Kim, Dissertation, Hanyang University (1996). O. Kuhaschewski and C. B. Alcock, Metallurgical Thermochemistry, p. 408, Pergamon Press, Oxford (1980). D.R. Gaskell, Introduction to Metallurgical Thermodynamics, p. 261, McGrow-Hill Book Company, New York (1981). J.F. Albersten, J. M. Kudsen, N. O. Roy-Poulsen and L. Vistisen, Phys. Scripta 22, 171 (1980). J.I.. Goldstein, R. E. I-Ianneman, R. E. Ogilvie, Trans. Metall. Soc. AIME 233, 812 (1965). J. Philibert, Atom Movements Diffusion and Mass Transport in Solids. p. 379, translated by S. J. Rotlunan, Les Editions de Phisique, Les Ulis (1991).
Pergamon
Pll S0965-9773(97)00041-X
I N - S I T U A L L O Y I N G O N S Y N T H E S I S O F N A N O S I Z E D Ni-Fe P O W D E R J. S. Lee, T. H. Kim, J. H. Yu, and S. W. Chung Department of Metallurgy and Materials Science, Hanyang University, Ansan, 425-791 KOREA Abstract -- In-situ alloying process of Ni-Fe system on synthesis of nanosized Ni-Fe powder by hydrogen reduction of ball milled NiO-Fe203 powder was investigated. It was found that the alloying of Ni-Fe during reduction of NiO-Fe203 was enhanced in the ball milled oxide mixture. Such rapid alloying process yielded the stable ?,Ni-Fe alloy aggregates consisting of 20-70 nm nanosized alloy particles. Basically, in-situ alloying by nanosized effect (chemical homogeneity with large surface area) during reduction process is believed to be responsible for the formation of nanoalloy powder. © 1997 Acta Metallurgica Inc. INTRODUCTION With the nanosized particle and the negative formation enthalpy, the spontaneous alloying of binary metal system has been reported to be possible at temperature even below 0°C (1-3). This surprising phenomenon offers a new concept for fabricating nanosized alloy powder. From the practical viewpoint, the achievement of such nanoalloy powder is of importance owing to its chemical homogeneity and large surface area that provides enormous driving force for sintering process. Recently, Lee et al.(4-6) fabricated nanocomposite metal powder by hydrogen reduction of ball milled metal oxide. The present study attempted to examine the feasibility of this process to the new concept of spontaneous alloying for nanoalloy powder synthesis. For this purpose, Ni-50wt% Fe system with a negative formation enthalpy (7) was chosen as a model system. Namely, in-situ alloying process during hydrogen reduction of ball milled NiO-Fe203 powder was investigated and discussed in terms of phase decomposition and powder characteristics.
EXPERIMENTAL The dry mixed (DM) oxide powder was prepared by blending Fe203(30txm, 99.99%) and NiO(4~tm, 99.99%) to have the alloy composition of Ni-50wt%Fe. The ball milled (BM) oxide powder was made by ball milling the DM mixture in an attritor with a speed of 350 rpm for 10 hours. The ball media and impeller were made of stainless steel and the milling agent was methyl alcohol. After ball milling, the oxide mixture was dried at 60°C for 1 hour. These two DM and BM powder mixtures were heated by raising temperature with 10°C/rain in hydrogen gas atmosphere (dew point -76°C) up to 600°C. The weight loss by decomposition during heat up was detected through the thermogravimetty (TG). The hygrometry system for measuring the content of water vapor in outlet gas was equipped with TG system in order to make both
153
154
JS LEE, TH K=u,JH Yu, ANOSW CHUNG
Figure 1. SEM morphology of NiO-Fe203 ball milled powder mixture. measurements simultaneously (6). The X-ray diffraction analysis was conducted with TG to identify the phase by the decomposition. The powder characteristics such as morphology and particle size were examined by SEM
RESULTS AND DISCUSSION
Figure 1 shows the morphology of NiO-Fe203 ball milled powder mixture having an aggregate form. As seen in the Figure, the BM aggregates consist of homogeneously mixed submicron oxide particles which approximately have the same particle sizes. In particular, the XRD results revealed no phase decomposition of NiO-Fe203 by milling (see the peak of 250°C in Figure 4). Figure 2 shows the TG curve from thermogravimetry experiment for the hydrogen reduction of Ni-Fe oxide mixtures. While the DM mixture underwent the weight loss at two different temperatures, the BM mixture lost the weight continuously between those two temperatures. The result of hygrometry study of Figure 3 corresponds with the TG curves of
1.0 '~-~0.8. "~ 0.6.
BM DM
.~0.4"
/
," 64J
~ 0.2. 0.0
0
200
10°C/min
/
400
600
Temperature(°C) Figure 2. Thermogravimetric curves of hydrogen reduction process during heat up with 10°C/min.
i:t2_ 0
200
400
600
Temperature(°C)
Figure 3. Hygrometric curves of hydrogen reduction process during heat up with 10°C/rain.
IN-SITuALLOYINGONSYNTHESISOFNANOSlZEDNi-Fe POWOER
155
DM and BM mixtures respectively and explains the weight loss by reduction process (9). As seen in the result, the DM mixture shows two large peaks separated at two different temperatures, but the BM mixture has two peaks almost overlapped seemingly into a larger one
peak. X-ray diffraction results of Figure 4 identify the phases produced by the reduction process of BM mixture in the temperature range of 250-600°C. These results with TG and hygrometry disclose the reduction of NiO at lower temperature than that of Fe2Os. We could know that two oxides were reduced simultaneously between these two temperatures and changed to the y alloy completely after heating up to 600°C. It was also found that the DM mixture underwent the formation of a limited amount of y alloy phase at 600°C. Such a different 7 formation in both samples gives an implication that the initial powder characteristics of BM mixture as homogeneously mixed submicron oxide particles play an important role for in-situ alloying of y phase in BM sample. It is well known that the reduction of Fe203 to Fe proceeds through the procedure of Fe203-Fe304-Fe (8). Moreover, the thermodynamic data (9) assure the high temperature y phase of Ni-50wt%Fe composition stable at temperature above -180°C when it is cooled relatively fast (with rate of 2-150°C/rain). Hence, it is believed that the formation of 7 phase is an evidence of in-situ alloying during the oxides reduction. Figure 5 shows the nanosized y alloy particles obtained by reducing BM powder at 600°C for 5 minutes. It is seen that the coarse y aggregate (Figure 5a) consists of 20~70 nm 7 alloy particles (Figure 5b). The diffusion depth, d=-l.2nm (d=-2~Dt) by interdiffusion between Ni 250 "C
It
A
300"C
450"C o
I 600°C
'1 20
40
i ;
6b io 60 lio 14o 20(degree)
• Fe203 • F%O4 &Ni • NiO [] F%0( (Ni,Fe)F%0 40(x'F¢ • "t-(Ni,Fo)
Figure 4. X-ray diffration patterns of hydrogen reduction process for BM powder durin~ heat uo with 10°C/min.
Figure 5. SEM micrographs of a) aggregate and b) particles of ~, alloy powder obtained from isothermal reduction of BM mixture (600°C for 5min. with 10°C/rain).
156
JS LEE,TH KJM,JH Yu, ANDSW CHUNG
and Fe is much less than the size of the "f phase alloyed particle where the interdiffusion coefficient of Ni-Fe (l~-5.14xlff:2m2/sec) at 600°C was extrapolated from high temperature (10) and the true annealing time (t) was estimated to be about 800 seconds from the 5 minutes annealing at 600°C including heat up time (11). Therefore it seems unlikely that the interditfusion between Ni and Fe predominates the complete formation of 7 in such a short time. This argument might be reasonable since the 7 phase was not produced in bulk Ni or Fe phase but in-situ produced during reduction process. Moreover, the experimental results (TG, hygrometry, XRD and microstructure) of BM powder suggest the formation of stable 7 phase more rapid than the growth of Ni or Fe bulk phase in the finely reduced particle mixture. CONCLUSION In-situ alloying process occurred in Ni-Fe system during synthesis of nanosized Ni-Fe powder by hydrogen reduction of ball milled NiO-Fe203 powder. Such rapid alloying process yielded the stable y Ni-Fe nanosized alloy powder consisting of 20--°70 nm particles. Basically, in-situ alloying process in the BM powder seems unlikely to be governed by interdiflhsion between Ni and Fe phases. The rapid alloying of BM mixture is presumed to be possible by the nanosized effect (chemical homogeneity with large surface area) during the reduction process. ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support of the Korea Ministry of Education Research Fund for Advanced Materials in 1996. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
H. Yasuda and H. Mori, Physical Review Letters 69, 3747 (1992). H. Mori, H. Yasuda and T. Kamino, Philosophical Magazine Letters 69, 279 (1994). H. Mori and H. Yasuda, J. of Microscopy 180, 33 (1995). T.H. Kim and J. S. Lee, J. of Korean Inst. Met. & Mater. 30, 203 (1992). J.S. Lee and T. H. Kim, Solid State Phenomena 25&26, 143 (1992). T.H. Kim, Dissertation, Hanyang University (1996). O. Kuhaschewski and C. B. Alcock, Metallurgical Thermochemistry, p. 408, Pergamon Press, Oxford (1980). D.R. Gaskell, Introduction to Metallurgical Thermodynamics, p. 261, McGrow-Hill Book Company, New York (1981). J.F. Albersten, J. M. Kudsen, N. O. Roy-Poulsen and L. Vistisen, Phys. Scripta 22, 171 (1980). J.I.. Goldstein, R. E. I-Ianneman, R. E. Ogilvie, Trans. Metall. Soc. AIME 233, 812 (1965). J. Philibert, Atom Movements Diffusion and Mass Transport in Solids. p. 379, translated by S. J. Rotlunan, Les Editions de Phisique, Les Ulis (1991).