- Email: [email protected]
Cu alloying
Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 653–655
VUV reflectance spectroscopy study of Fe / Cu alloying a, b R.N. Suave *, A.R.B. de Castro
´ ´ Grupo IUV, Laboratorio Nacional de Luz Sıncrotron , 13083 -360, Campinas, SP, Brazil a ´ ´ , ES, Brazil Santo, 29060 -900, Vitoria DFIS, Universidade Federal do Espırito b DFESCM, Universidade Estadual de Campinas, 13083 -970, Campinas, SP, Brazil
Abstract We present here the results of vacuum ultraviolet (VUV) reflectance measurements performed on Fe 30 Cu 70 alloyed by high-energy milling. The results show that true alloying occurs at the atomic level, and a change in the Fermi level of the system accompanying the alloying progress is observed. This is in reasonable agreement with theoretical calculations. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fermi level; VUV synchrotron radiation; Reflectance spectroscopy; Metallic alloy; Ball milling
There has been a great deal of work performed with the technique of ball milling (BM) to synthesize nanocrystalline metastable alloys in the last ten years. BM is a high-energy process that forces solidstate reactions among the particles of the elemental powder mixture, so extending the metastable phases A x B1002x of immiscible elements A and B over the whole composition range 0#x#100 atom-%. This method can also be applied to grind a powder of crystalline compounds, which can stabilize the disordered or nanocrystal structure through the defects and / or vacancies created by the severe mechanical work [1]. Binary systems characterized by a large positive heat of mixing, such as A–Cu [2–4], where A5Fe, Ag, V, W and Ta, or Fe–B, where B5Cu and Ag [4–7], have been prepared by BM. The formation of supersaturated solid solutions beyond the solubility *Corresponding author. Tel.: 155-27-335-2482; fax: 155-27335-2823. E-mail address: [email protected] (R.N. Suave)
limit was observed, in clear contrast with what is expected from the equilibrium phase diagrams. Particularly interesting is the Fe–Cu system, which exhibits a maximum solubility of 1.8 atom-% for Cu in a matrix of bcc–Fe and of 4.8 atom-% for Fe in a matrix of fcc–Cu [8]. The solid-state reaction that occurs between particles of Fe and Cu in the beginning of the mechanical alloying process is not yet well understood. The aim of the present work is to highlight the alloying process with measurements related to the Fermi level of the alloy. This can be done with VUV spectroscopy. This technique is sensitive to surfaces and to grain interfaces. We have prepared a sequence of samples labeled Fe 30 Cu 70 2t according to the various milling times, with t56, 24, 34, 48 and 72 h. The initial sample was a mixture of high-purity (better than 99.9%) elemental powders, with the desired atomic composition. The sample was sealed in a vial made of hardened stainless steel, containing a massive cylindrical milling tool with a typical cylinder-to-powder
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00330-2
654
R.N. Suave, A.R.B. de Castro / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 653 – 655
weight ratio of 8:1. The vial was then clamped and put to vibrate at room temperature (RT) in a commercial table-top machine. All manipulation of samples before and during milling was carried out under a high-purity argon atmosphere. To collect samples at the preset times, the vial was opened inside a glove chamber filled with argon gas. Collected samples were kept in a liquid nitrogen dewar before the intended experiments were performed, in order to avoid oxidation and / or a possible RT annealing. ¨ RT X-ray diffraction (XRD) and Mossbauer spectroscopy (MS) were used in these samples to monitor alloy formation [9]. It was shown that the intensity of the Bragg peak related to the [110] direction of bcc–Fe is continuously decreasing and disappears after 24 h of milling. There is no observable shift in this peak position, but the [111] fcc–Cu Bragg peak shifts towards lower angles and persists after longer milling times. These results indicate a possible formation of a solid solution, with Fe randomly substituting Cu in the fcc dilated matrix. The fcc grain size, as derived from XRD data, suffers a strong reduction during the first hours of milling, finally stabilizing at about 15 nm in accordance with previous reports [10,11]. A possible phase transformation, from bcc a-Fe to fcc g-Fe, is not seen in the MS measurements. The samples were glued with double-sided tape to a sample-holder in a sample chamber, where the vacuum level was about 10 28 Torr. Reflectance spectra in the soft X-ray region, 45–100 eV, were obtained for the various samples in the Toroidal
Grating Monochromator (TGM) beamline of ´ ´ Laboratorio Nacional de Luz Sıncrotron (LNLS). Light impinged the samples at approximately normal incidence, where the reflectivity intensity is proportional to the sample coefficient for X-ray absorption. The spectra were obtained with monochromatic Xray photons from a 600 grooves / mm toroidal grating, and detected with a channeltron operating at 22000 V in the photon-counting mode. Each spectrum shown in Fig. 1(a) represents a statistical mean of other spectra taken under approximately the same experimental conditions. The experimental spectra show lines corresponding to electric-dipole transitions M 2,3 →M 4,5 for Fe, and transitions M 2,3 →valence band (VB) for Cu [marked peaks at Fig. 1(a)]. Also, peaks associated with the corresponding transitions for Fe and Cu oxides appear in the spectra. The peaks related to Cu have their positions changed as alloying progresses, so a variation in the corresponding transition energy is observed. The observed change in transition energy is attributed to a change in the Fermi level of the system. The variation attributed to the Fermi energy is shown in Fig. 1(b). The change in Fermi energy indicates that true alloying between Fe and Cu is under way, forced by the severe cold work, and we see that mechanical alloying approaches equilibrium after 48 h of milling. An analogous conclusion is obtained ¨ from the XRD data and from Mossbauer spectroscopy experiments performed on these samples, which has shown that Fe atoms are already dissolved in the Cu matrix even at the beginning of the milling
Fig. 1. (a) VUV reflectance spectra for Fe 30 Cu 70 at various milling times. (b) Corresponding energy variation of the transition from the Cu M 2,3 edge to the valence band.
R.N. Suave, A.R.B. de Castro / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 653 – 655
655
process [9]. Also, this is confirmed by extended X-ray absorption fine-structure measurements, which will be published elsewhere. It is believed that the variation of the Fermi energy is due to a small charge transfer from the 4s–4p orbital of iron to the 3d band of copper, which increases the Fermi level eF of the resulting alloy. We know that the density of charge carriers, n, is proportional to e 3F / 2 for a free-electron system [12], so
data analysis, and to A.S. Pinto (DFA / UNICAMP) for help on sample preparation. R.N. Suave would like to express his gratitude to Conselho Nacional de ´ ´ Desenvolvimento Cientıfico e Tecnologico (CNPq) for a post-doctoral research grant, and to Coor˜ de Aperfeic¸oamento do Pessoal de Ensino denac¸ao Superior (CAPES) for financial support.
Dn 3 De ] 5 ] ]F . n 2 eF
References
As eF 57.0 eV for copper, we get a transfer of 0.17 e / atom from iron to copper for the observed final variation DeF 50.8 eV in the transition energy. This small charge transfer can be related to the metastable character of this alloy. The constituents begin to segregate at temperatures above 5008C. The increase in Fermi level as BM promotes the mechanical alloying is in accordance with previous theoretical calculations [13,14]. Guenzburger and Ellis [14] have studied the electronic structure and magnetic properties of an fcc–iron particle dissolved in a copper matrix, represented by the embedded cluster Fe 14 atoms Cu 48 atoms , which has a lower composition (x(23 atom-%) compared to the nominal composition of our samples. By using a local-density discrete variational calculation, these authors have found an atomic charge transfer of 0.1 e / atom from Fe to Cu, which is in satisfactory agreement with the result predicted by the free-electron model from our experimental results.
Acknowledgements The authors are grateful to D.W.L. Monteiro and to Dr. C. Larica (DFIS / UFES) for the XRD and MS
[1] A.W. Weeber, H. Bakker, Physica B 153 (1988) 93. [2] K. Uenishi, K.F. Kobayashi, S. Nasu, H. Hatano, K.N. Ishihara, P.H. Shingu, Z. Metallk 83 (1992) 132. ´ T. Benameur, Phys. Rev. Lett. 68 [3] A.R. Yavari, P.J. Desre, (1992) 2235. [4] A.H. Clauer, J.H. de Barbadillo (Eds.), Solid State Powder Processing, The Minerals, Metals and Materials Society, Cincinnati, OH, 1990, p. 21. [5] T. Fukunaga, K. Nakamura, K. Suzuki, U. Mizutani, J. Non-Cryst. Solids 117–118 (1990) 700. [6] E. Gaffet, C. Louison, M. Harmelin, F. Faudet, Mater. Sci. Eng. A 134 (1991) 1410. [7] T. Fukunaga, M. Mori, K. Inou, U. Mizutani, Mater. Sci. Eng. A 134 (1991) 863. [8] O. Kubaschewski (Ed.), Iron — Binary Phase Diagrams, Springer-Verlag, Bonn, 1982, pp. 35–37. [9] D.W.L. Monteiro, C. Larica, E. Nunes Filho, E.C. Passamani, K.M.B. Alves, Hyperfine Interaction 3 (1998) 17. [10] J. Eckert, J.C. Holzer, C.E. Krill III, W.L. Johnson, J. Appl. Phys. 73 (1993) 2794. [11] E. Ma, M. Atzmon, F.E. Pinkerton, J. Appl. Phys. 74 (1993) 955. [12] C. Kittel (Ed.), Introduction To Solid State Physics, Wiley and Sons, New York, 1996, pp. 146–151. [13] M.A. Bowen, J.D. Dow, Phys. Rev. B 32 (1985) 5119. [14] D. Guenzburger, D.E. Ellis, Phys. Rev. B 52 (1995) 13390.
VUV reflectance spectroscopy study of Fe / Cu alloying a, b R.N. Suave *, A.R.B. de Castro
´ ´ Grupo IUV, Laboratorio Nacional de Luz Sıncrotron , 13083 -360, Campinas, SP, Brazil a ´ ´ , ES, Brazil Santo, 29060 -900, Vitoria DFIS, Universidade Federal do Espırito b DFESCM, Universidade Estadual de Campinas, 13083 -970, Campinas, SP, Brazil
Abstract We present here the results of vacuum ultraviolet (VUV) reflectance measurements performed on Fe 30 Cu 70 alloyed by high-energy milling. The results show that true alloying occurs at the atomic level, and a change in the Fermi level of the system accompanying the alloying progress is observed. This is in reasonable agreement with theoretical calculations. 1999 Elsevier Science B.V. All rights reserved. Keywords: Fermi level; VUV synchrotron radiation; Reflectance spectroscopy; Metallic alloy; Ball milling
There has been a great deal of work performed with the technique of ball milling (BM) to synthesize nanocrystalline metastable alloys in the last ten years. BM is a high-energy process that forces solidstate reactions among the particles of the elemental powder mixture, so extending the metastable phases A x B1002x of immiscible elements A and B over the whole composition range 0#x#100 atom-%. This method can also be applied to grind a powder of crystalline compounds, which can stabilize the disordered or nanocrystal structure through the defects and / or vacancies created by the severe mechanical work [1]. Binary systems characterized by a large positive heat of mixing, such as A–Cu [2–4], where A5Fe, Ag, V, W and Ta, or Fe–B, where B5Cu and Ag [4–7], have been prepared by BM. The formation of supersaturated solid solutions beyond the solubility *Corresponding author. Tel.: 155-27-335-2482; fax: 155-27335-2823. E-mail address: [email protected] (R.N. Suave)
limit was observed, in clear contrast with what is expected from the equilibrium phase diagrams. Particularly interesting is the Fe–Cu system, which exhibits a maximum solubility of 1.8 atom-% for Cu in a matrix of bcc–Fe and of 4.8 atom-% for Fe in a matrix of fcc–Cu [8]. The solid-state reaction that occurs between particles of Fe and Cu in the beginning of the mechanical alloying process is not yet well understood. The aim of the present work is to highlight the alloying process with measurements related to the Fermi level of the alloy. This can be done with VUV spectroscopy. This technique is sensitive to surfaces and to grain interfaces. We have prepared a sequence of samples labeled Fe 30 Cu 70 2t according to the various milling times, with t56, 24, 34, 48 and 72 h. The initial sample was a mixture of high-purity (better than 99.9%) elemental powders, with the desired atomic composition. The sample was sealed in a vial made of hardened stainless steel, containing a massive cylindrical milling tool with a typical cylinder-to-powder
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00330-2
654
R.N. Suave, A.R.B. de Castro / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 653 – 655
weight ratio of 8:1. The vial was then clamped and put to vibrate at room temperature (RT) in a commercial table-top machine. All manipulation of samples before and during milling was carried out under a high-purity argon atmosphere. To collect samples at the preset times, the vial was opened inside a glove chamber filled with argon gas. Collected samples were kept in a liquid nitrogen dewar before the intended experiments were performed, in order to avoid oxidation and / or a possible RT annealing. ¨ RT X-ray diffraction (XRD) and Mossbauer spectroscopy (MS) were used in these samples to monitor alloy formation [9]. It was shown that the intensity of the Bragg peak related to the [110] direction of bcc–Fe is continuously decreasing and disappears after 24 h of milling. There is no observable shift in this peak position, but the [111] fcc–Cu Bragg peak shifts towards lower angles and persists after longer milling times. These results indicate a possible formation of a solid solution, with Fe randomly substituting Cu in the fcc dilated matrix. The fcc grain size, as derived from XRD data, suffers a strong reduction during the first hours of milling, finally stabilizing at about 15 nm in accordance with previous reports [10,11]. A possible phase transformation, from bcc a-Fe to fcc g-Fe, is not seen in the MS measurements. The samples were glued with double-sided tape to a sample-holder in a sample chamber, where the vacuum level was about 10 28 Torr. Reflectance spectra in the soft X-ray region, 45–100 eV, were obtained for the various samples in the Toroidal
Grating Monochromator (TGM) beamline of ´ ´ Laboratorio Nacional de Luz Sıncrotron (LNLS). Light impinged the samples at approximately normal incidence, where the reflectivity intensity is proportional to the sample coefficient for X-ray absorption. The spectra were obtained with monochromatic Xray photons from a 600 grooves / mm toroidal grating, and detected with a channeltron operating at 22000 V in the photon-counting mode. Each spectrum shown in Fig. 1(a) represents a statistical mean of other spectra taken under approximately the same experimental conditions. The experimental spectra show lines corresponding to electric-dipole transitions M 2,3 →M 4,5 for Fe, and transitions M 2,3 →valence band (VB) for Cu [marked peaks at Fig. 1(a)]. Also, peaks associated with the corresponding transitions for Fe and Cu oxides appear in the spectra. The peaks related to Cu have their positions changed as alloying progresses, so a variation in the corresponding transition energy is observed. The observed change in transition energy is attributed to a change in the Fermi level of the system. The variation attributed to the Fermi energy is shown in Fig. 1(b). The change in Fermi energy indicates that true alloying between Fe and Cu is under way, forced by the severe cold work, and we see that mechanical alloying approaches equilibrium after 48 h of milling. An analogous conclusion is obtained ¨ from the XRD data and from Mossbauer spectroscopy experiments performed on these samples, which has shown that Fe atoms are already dissolved in the Cu matrix even at the beginning of the milling
Fig. 1. (a) VUV reflectance spectra for Fe 30 Cu 70 at various milling times. (b) Corresponding energy variation of the transition from the Cu M 2,3 edge to the valence band.
R.N. Suave, A.R.B. de Castro / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 653 – 655
655
process [9]. Also, this is confirmed by extended X-ray absorption fine-structure measurements, which will be published elsewhere. It is believed that the variation of the Fermi energy is due to a small charge transfer from the 4s–4p orbital of iron to the 3d band of copper, which increases the Fermi level eF of the resulting alloy. We know that the density of charge carriers, n, is proportional to e 3F / 2 for a free-electron system [12], so
data analysis, and to A.S. Pinto (DFA / UNICAMP) for help on sample preparation. R.N. Suave would like to express his gratitude to Conselho Nacional de ´ ´ Desenvolvimento Cientıfico e Tecnologico (CNPq) for a post-doctoral research grant, and to Coor˜ de Aperfeic¸oamento do Pessoal de Ensino denac¸ao Superior (CAPES) for financial support.
Dn 3 De ] 5 ] ]F . n 2 eF
References
As eF 57.0 eV for copper, we get a transfer of 0.17 e / atom from iron to copper for the observed final variation DeF 50.8 eV in the transition energy. This small charge transfer can be related to the metastable character of this alloy. The constituents begin to segregate at temperatures above 5008C. The increase in Fermi level as BM promotes the mechanical alloying is in accordance with previous theoretical calculations [13,14]. Guenzburger and Ellis [14] have studied the electronic structure and magnetic properties of an fcc–iron particle dissolved in a copper matrix, represented by the embedded cluster Fe 14 atoms Cu 48 atoms , which has a lower composition (x(23 atom-%) compared to the nominal composition of our samples. By using a local-density discrete variational calculation, these authors have found an atomic charge transfer of 0.1 e / atom from Fe to Cu, which is in satisfactory agreement with the result predicted by the free-electron model from our experimental results.
Acknowledgements The authors are grateful to D.W.L. Monteiro and to Dr. C. Larica (DFIS / UFES) for the XRD and MS
[1] A.W. Weeber, H. Bakker, Physica B 153 (1988) 93. [2] K. Uenishi, K.F. Kobayashi, S. Nasu, H. Hatano, K.N. Ishihara, P.H. Shingu, Z. Metallk 83 (1992) 132. ´ T. Benameur, Phys. Rev. Lett. 68 [3] A.R. Yavari, P.J. Desre, (1992) 2235. [4] A.H. Clauer, J.H. de Barbadillo (Eds.), Solid State Powder Processing, The Minerals, Metals and Materials Society, Cincinnati, OH, 1990, p. 21. [5] T. Fukunaga, K. Nakamura, K. Suzuki, U. Mizutani, J. Non-Cryst. Solids 117–118 (1990) 700. [6] E. Gaffet, C. Louison, M. Harmelin, F. Faudet, Mater. Sci. Eng. A 134 (1991) 1410. [7] T. Fukunaga, M. Mori, K. Inou, U. Mizutani, Mater. Sci. Eng. A 134 (1991) 863. [8] O. Kubaschewski (Ed.), Iron — Binary Phase Diagrams, Springer-Verlag, Bonn, 1982, pp. 35–37. [9] D.W.L. Monteiro, C. Larica, E. Nunes Filho, E.C. Passamani, K.M.B. Alves, Hyperfine Interaction 3 (1998) 17. [10] J. Eckert, J.C. Holzer, C.E. Krill III, W.L. Johnson, J. Appl. Phys. 73 (1993) 2794. [11] E. Ma, M. Atzmon, F.E. Pinkerton, J. Appl. Phys. 74 (1993) 955. [12] C. Kittel (Ed.), Introduction To Solid State Physics, Wiley and Sons, New York, 1996, pp. 146–151. [13] M.A. Bowen, J.D. Dow, Phys. Rev. B 32 (1985) 5119. [14] D. Guenzburger, D.E. Ellis, Phys. Rev. B 52 (1995) 13390.