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Correlation of milling time on formation of TiCoSb phase by mechanical alloying
Journal of Alloys and Compounds 462 (2008) 267–270
Correlation of milling time on formation of TiCoSb phase by mechanical alloying P. Amornpitoksuk a,∗ , S. Suwanboon b a b
Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Materials Science Program, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Received 3 July 2007; received in revised form 3 August 2007; accepted 3 August 2007 Available online 9 August 2007
Abstract Nanocrystalline TiCoSb powder was synthesized by mechanical alloying from elemental Ti, Co and Sb powders. The samples milled at different times were characterized by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The synthesized powder shows a half-Heusler phase in MgAgAs structure after 6 h of milling. The crystallite sizes decrease with increasing milling time. The minimum crystallite size is about 31.7 nm after 40 h of milling. The magnetic properties of this sample give evidence of the existence of ferromagnetic clusters as a secondary phase. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Mechanical alloying; Scanning electron microscopy
1. Introduction Mechanical alloying (MA) has shown to be a powerful technique to produce amorphous materials and metastable or unstable phases under highly non-equilibrium conditions with grain sizes in the nanometer range. The nanostructured materials have a significant fraction of atoms residing in lattice defects and grain boundaries which might offer different physical properties from the bulk materials. MA has several intrinsic advantages, such as relatively inexpensive equipment, reduction of processing steps and possibility of scaling up. During MA, the particles are subjected to severe mechanical deformation and are repeatedly deformed, cold welded and fractured. However, the physical mechanisms involved are not yet clearly understood. TiCoSb half-Heusler phase has been found in the MgAgAs structure-type exhibiting the semiconducting character with a wide band gap of about 0.95 eV [1]. This compound exhibit large n-type Seebeck coefficients and high resistivities that reached to 500 V/K at 300 K and 1500 /cm at 4.2 K [2], respectively. Normally, this compound must be synthesized by thermal techniques (melting or arc melting + annealing step) but in this work nanocrystalline TiCoSb powder had been synthesized via ∗
Corresponding author. E-mail address: [email protected] (P. Amornpitoksuk).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.009
MA and the effect of milling time on the phase transformation had been studied. Moreover, the magnetic properties have been investigated. 2. Experimental MA was performed in a planetary mill with stainless steel vial and balls. The Ti, Co and Sb powders were introduced into the vial under an argon atmosphere so as to prevent oxidation during milling. The ball-to-powder weight ratio was 10:1. The milling was operated at 450 rpm and the milling was stopped every 30 min in order to prevent rapid engine wear. After selected time (1, 5, 10, 20, 30, 40 and 60 h), the milling was stopped to collect small amounts of the gained sample. The structural evolution during MA was investigated by X-ray diffractometer (XRD) using Cu K␣ radiation. The morphological and elemental analyses of the grained sample were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The magnetization curve as a function between 2 and 300 K at various applied magnetic field between 0 and 5 T and the susceptibility curve in a magnetic field of 1 T were measured using the SQUID technique. The crystallite size and lattice strain of TiCoSb powders after mechanical alloying can be estimated by Williamson-Hall method [3]. This method is based on the broadening of the diffraction lines due to the strain and crystallite size: B cos θ =
0.9λ + η sin θ d
where B is the line width in radians at half maximum intensity, λ the wavelength of X-ray used, d the crystallite size, θ the Bragg angle and η is the strain. When Bcos θ is plotted against sin θ, a straight line is obtained with the slope as η and the intercept as 0.9λ/d.
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Fig. 1. XRD patterns of TiCoSb powders after milling for 1, 4, 6, 10, 30, 40 and 60 h, respectively.
Fig. 3. XRD patterns of TiCoSb powders at 60 h (a) before and (b) after annealing at 888 ◦ C.
XRD patterns of the samples prepared from of elemental Ti, Co and Sb powders that were milled according to the selected times, are presented in Fig. 1. It is obvious that the starting materials were still observed in XRD pattern after they were milled for 4 h. The TiCoSb powder completely formed without any starting materials left when milling for 6 h and the X-ray analysis of this sample confirmed that it presented of a single phase crystalliz¯ ing on MgAgAs structure-type (space group F 43m). The XRD patterns also show that the milling processes cause a broadening and reduction in the intensities of the peaks, suggesting that the average crystallite size is diminished with increasing the milling time. The crystallite size and lattice strain of TiCoSb powder calculated from Williamson-Hall method [3] are shown in Fig. 2. The crystallite size of TiCoSb powders decreases from the initial size of about 84.8 nm to ∼33 nm after milling for 30 h, and the average crystallite size of this sample is constant. If the milling time is more than 30 h, the internal lattice strain of TiCoSb pow-
der increases rapidly with the refinement of the crystallite size. Based on the XRD information, when milling for 40 h, the lattice parameter of this sample is a = 0.5887(9) nm. This value is the same value as reporting in other publications [4–6]. This phase was analyzed by EDS and it comprises 32.95 at.% of Ti, 32.70 at.% of Co and 34.35 at.% of Sb. We found that the secondary phase occurred again at the milling time beyond 60 h as clearly seen in Fig. 1. So, this sample was annealed at 888 ◦ C for obtaining a good crystallinity and being easy to identify the secondary phase. The XRD pattern of annealed sample at 60 h of milling shows a mixture of TiCoSb and CoSb phases as shown in Fig. 3. The lattice parameters of CoSb phase are a = 0.3892(2) nm and c = 0.5186(2) nm which is very close to the stochiometric compound of CoSb [7] so we presume that the Ti does not substitute or insert in CoSb lattice. We furthermore observe these two phases in back-scattering electron image as shown in Fig. 4. Although Ti is neither detected with XRD nor EDS, in principle of the mass balance, we believe that the TiCoSb compound maybe decomposed to Ti and CoSb when milling for a long time.
Fig. 2. Variation of crystallite size (d) and lattice strain (η) at different milling times.
Fig. 4. Back scattering electron image of TiCoSb powder after milling for 60 h.
3. Results and discussion
P. Amornpitoksuk, S. Suwanboon / Journal of Alloys and Compounds 462 (2008) 267–270
269
Fig. 5. Morphology of TiCoSb powders milling for (a) 6 h, (b) 20 h and (c) 40 h.
The morphology of TiCoSb powder after milling for 6, 20 and 40 h is shown in Fig. 5. The particle size of the sample decreases with increasing milling time because the samples are crushed and flattened by the collision between the powder and the milling media and the average particle size is about 2.5 m at 40 h. The crystallite size obtained by the indirect diffraction peak broadening studies and the direct microscopic technique may not always match exactly because microscopic examination normally gives the particle size, whereas the diffraction technique gives the crystallite size [3]. The magnetic susceptibility (χ) versus temperature of TiCoSb powder at 40 h of milling depicts in Fig. 6. In an
applied magnetic field of 1 T at room temperature, the specific susceptibility of this compound is 6.31 × 10−5 emu/g which close to the reported value of annealed TiCoSb [4]. For this compound, the annealed sample seems to behave as a magnetically ordered substance whereas the as-cast sample showed Pauli paramagnetic behavior [8,9]. The specific susceptibility value of annealed sample is nearly five times higher than as-cast sample due to a small amount precipitate of a ferromagnetic phase which cannot detect by XRD [8,9]. The existence of the ferromagnetic phase was confirmed by the saturation of magnetization curves at low magnetic field as seen in Fig. 6.
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formation of TiCoSb half-Heusler phase. The minimum crystallite size of about 31.7 nm was obtained when milling for 40 h. We found that this sample shows the ferromagnetic phase as the secondary phases that precipitated after the heat treatment like annealed TiCoSb prepared by classical method. At 60 h of milling, the TiCoSb maybe decomposes to Ti and CoSb. References
Fig. 6. Magnetic susceptibility at H = 1 T and magnetization curves (inside) at selected temperature of TiCoSb after milling for 40 h.
4. Conclusion High energy mechanical alloying of the mixed Ti, Co and Sb powders under argon atmosphere resulted in the
[1] J. Tobola, J. Pierre, S. Kaprzko, R.V. Skolozdra, M.A. Kouacou, J. Phys. Condens. Matter 10 (1998) 1013–1032. [2] Y. Xia, V. Ponnambalam, S. Bhattacharya, A.L. Pope, S.J. Poon, T.M. Tritt, J. Phys. Condens. Matter 13 (2001) 1–13. [3] C. Surynarayana, Prog. Mater. Sci. 46 (2001) 1–184. [4] Y. Stadnyk, Y. Gorelenko, A. Tkachuk, A. Goryn, V. Davydov, O. Bodak, J. Alloys Compd. 329 (2001) 37–41. [5] P. Webster, K.R.A. Ziebeck, J. Phys. Chem. Solids 34 (1973) 1647. [6] C.B.-H. Evers, C.G. Richter, K. Hartjes, W. Jeitschko, J. Alloys Compd. 252 (1997) 93. [7] W.B. Person, A. Kjekshus, Prog. Solid State Chem. 1 (1964) 83. [8] M. Terada, K. Endo, Y. Fujita, R. Kimura, J. Phys. Soc. Jpn. 32 (1972) 91. [9] Y. Stadnyk, L. Romaka, A. Hotyn, A. Tkachnk, Y. Gorelenko, P. Rogl, J. Alloys Compd. 387 (2005) 251.
Correlation of milling time on formation of TiCoSb phase by mechanical alloying P. Amornpitoksuk a,∗ , S. Suwanboon b a b
Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Materials Science Program, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Received 3 July 2007; received in revised form 3 August 2007; accepted 3 August 2007 Available online 9 August 2007
Abstract Nanocrystalline TiCoSb powder was synthesized by mechanical alloying from elemental Ti, Co and Sb powders. The samples milled at different times were characterized by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The synthesized powder shows a half-Heusler phase in MgAgAs structure after 6 h of milling. The crystallite sizes decrease with increasing milling time. The minimum crystallite size is about 31.7 nm after 40 h of milling. The magnetic properties of this sample give evidence of the existence of ferromagnetic clusters as a secondary phase. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Mechanical alloying; Scanning electron microscopy
1. Introduction Mechanical alloying (MA) has shown to be a powerful technique to produce amorphous materials and metastable or unstable phases under highly non-equilibrium conditions with grain sizes in the nanometer range. The nanostructured materials have a significant fraction of atoms residing in lattice defects and grain boundaries which might offer different physical properties from the bulk materials. MA has several intrinsic advantages, such as relatively inexpensive equipment, reduction of processing steps and possibility of scaling up. During MA, the particles are subjected to severe mechanical deformation and are repeatedly deformed, cold welded and fractured. However, the physical mechanisms involved are not yet clearly understood. TiCoSb half-Heusler phase has been found in the MgAgAs structure-type exhibiting the semiconducting character with a wide band gap of about 0.95 eV [1]. This compound exhibit large n-type Seebeck coefficients and high resistivities that reached to 500 V/K at 300 K and 1500 /cm at 4.2 K [2], respectively. Normally, this compound must be synthesized by thermal techniques (melting or arc melting + annealing step) but in this work nanocrystalline TiCoSb powder had been synthesized via ∗
Corresponding author. E-mail address: [email protected] (P. Amornpitoksuk).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.009
MA and the effect of milling time on the phase transformation had been studied. Moreover, the magnetic properties have been investigated. 2. Experimental MA was performed in a planetary mill with stainless steel vial and balls. The Ti, Co and Sb powders were introduced into the vial under an argon atmosphere so as to prevent oxidation during milling. The ball-to-powder weight ratio was 10:1. The milling was operated at 450 rpm and the milling was stopped every 30 min in order to prevent rapid engine wear. After selected time (1, 5, 10, 20, 30, 40 and 60 h), the milling was stopped to collect small amounts of the gained sample. The structural evolution during MA was investigated by X-ray diffractometer (XRD) using Cu K␣ radiation. The morphological and elemental analyses of the grained sample were examined using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The magnetization curve as a function between 2 and 300 K at various applied magnetic field between 0 and 5 T and the susceptibility curve in a magnetic field of 1 T were measured using the SQUID technique. The crystallite size and lattice strain of TiCoSb powders after mechanical alloying can be estimated by Williamson-Hall method [3]. This method is based on the broadening of the diffraction lines due to the strain and crystallite size: B cos θ =
0.9λ + η sin θ d
where B is the line width in radians at half maximum intensity, λ the wavelength of X-ray used, d the crystallite size, θ the Bragg angle and η is the strain. When Bcos θ is plotted against sin θ, a straight line is obtained with the slope as η and the intercept as 0.9λ/d.
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Fig. 1. XRD patterns of TiCoSb powders after milling for 1, 4, 6, 10, 30, 40 and 60 h, respectively.
Fig. 3. XRD patterns of TiCoSb powders at 60 h (a) before and (b) after annealing at 888 ◦ C.
XRD patterns of the samples prepared from of elemental Ti, Co and Sb powders that were milled according to the selected times, are presented in Fig. 1. It is obvious that the starting materials were still observed in XRD pattern after they were milled for 4 h. The TiCoSb powder completely formed without any starting materials left when milling for 6 h and the X-ray analysis of this sample confirmed that it presented of a single phase crystalliz¯ ing on MgAgAs structure-type (space group F 43m). The XRD patterns also show that the milling processes cause a broadening and reduction in the intensities of the peaks, suggesting that the average crystallite size is diminished with increasing the milling time. The crystallite size and lattice strain of TiCoSb powder calculated from Williamson-Hall method [3] are shown in Fig. 2. The crystallite size of TiCoSb powders decreases from the initial size of about 84.8 nm to ∼33 nm after milling for 30 h, and the average crystallite size of this sample is constant. If the milling time is more than 30 h, the internal lattice strain of TiCoSb pow-
der increases rapidly with the refinement of the crystallite size. Based on the XRD information, when milling for 40 h, the lattice parameter of this sample is a = 0.5887(9) nm. This value is the same value as reporting in other publications [4–6]. This phase was analyzed by EDS and it comprises 32.95 at.% of Ti, 32.70 at.% of Co and 34.35 at.% of Sb. We found that the secondary phase occurred again at the milling time beyond 60 h as clearly seen in Fig. 1. So, this sample was annealed at 888 ◦ C for obtaining a good crystallinity and being easy to identify the secondary phase. The XRD pattern of annealed sample at 60 h of milling shows a mixture of TiCoSb and CoSb phases as shown in Fig. 3. The lattice parameters of CoSb phase are a = 0.3892(2) nm and c = 0.5186(2) nm which is very close to the stochiometric compound of CoSb [7] so we presume that the Ti does not substitute or insert in CoSb lattice. We furthermore observe these two phases in back-scattering electron image as shown in Fig. 4. Although Ti is neither detected with XRD nor EDS, in principle of the mass balance, we believe that the TiCoSb compound maybe decomposed to Ti and CoSb when milling for a long time.
Fig. 2. Variation of crystallite size (d) and lattice strain (η) at different milling times.
Fig. 4. Back scattering electron image of TiCoSb powder after milling for 60 h.
3. Results and discussion
P. Amornpitoksuk, S. Suwanboon / Journal of Alloys and Compounds 462 (2008) 267–270
269
Fig. 5. Morphology of TiCoSb powders milling for (a) 6 h, (b) 20 h and (c) 40 h.
The morphology of TiCoSb powder after milling for 6, 20 and 40 h is shown in Fig. 5. The particle size of the sample decreases with increasing milling time because the samples are crushed and flattened by the collision between the powder and the milling media and the average particle size is about 2.5 m at 40 h. The crystallite size obtained by the indirect diffraction peak broadening studies and the direct microscopic technique may not always match exactly because microscopic examination normally gives the particle size, whereas the diffraction technique gives the crystallite size [3]. The magnetic susceptibility (χ) versus temperature of TiCoSb powder at 40 h of milling depicts in Fig. 6. In an
applied magnetic field of 1 T at room temperature, the specific susceptibility of this compound is 6.31 × 10−5 emu/g which close to the reported value of annealed TiCoSb [4]. For this compound, the annealed sample seems to behave as a magnetically ordered substance whereas the as-cast sample showed Pauli paramagnetic behavior [8,9]. The specific susceptibility value of annealed sample is nearly five times higher than as-cast sample due to a small amount precipitate of a ferromagnetic phase which cannot detect by XRD [8,9]. The existence of the ferromagnetic phase was confirmed by the saturation of magnetization curves at low magnetic field as seen in Fig. 6.
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P. Amornpitoksuk, S. Suwanboon / Journal of Alloys and Compounds 462 (2008) 267–270
formation of TiCoSb half-Heusler phase. The minimum crystallite size of about 31.7 nm was obtained when milling for 40 h. We found that this sample shows the ferromagnetic phase as the secondary phases that precipitated after the heat treatment like annealed TiCoSb prepared by classical method. At 60 h of milling, the TiCoSb maybe decomposes to Ti and CoSb. References
Fig. 6. Magnetic susceptibility at H = 1 T and magnetization curves (inside) at selected temperature of TiCoSb after milling for 40 h.
4. Conclusion High energy mechanical alloying of the mixed Ti, Co and Sb powders under argon atmosphere resulted in the
[1] J. Tobola, J. Pierre, S. Kaprzko, R.V. Skolozdra, M.A. Kouacou, J. Phys. Condens. Matter 10 (1998) 1013–1032. [2] Y. Xia, V. Ponnambalam, S. Bhattacharya, A.L. Pope, S.J. Poon, T.M. Tritt, J. Phys. Condens. Matter 13 (2001) 1–13. [3] C. Surynarayana, Prog. Mater. Sci. 46 (2001) 1–184. [4] Y. Stadnyk, Y. Gorelenko, A. Tkachuk, A. Goryn, V. Davydov, O. Bodak, J. Alloys Compd. 329 (2001) 37–41. [5] P. Webster, K.R.A. Ziebeck, J. Phys. Chem. Solids 34 (1973) 1647. [6] C.B.-H. Evers, C.G. Richter, K. Hartjes, W. Jeitschko, J. Alloys Compd. 252 (1997) 93. [7] W.B. Person, A. Kjekshus, Prog. Solid State Chem. 1 (1964) 83. [8] M. Terada, K. Endo, Y. Fujita, R. Kimura, J. Phys. Soc. Jpn. 32 (1972) 91. [9] Y. Stadnyk, L. Romaka, A. Hotyn, A. Tkachnk, Y. Gorelenko, P. Rogl, J. Alloys Compd. 387 (2005) 251.