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Influence of Sn on the magnetic ordering of Ni–Sn alloy synthesized using chemical reduction method
Journal of Magnetism and Magnetic Materials 406 (2016) 103–109
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of Sn on the magnetic ordering of Ni–Sn alloy synthesized using chemical reduction method K. Dhanapal a, V. Narayanan b, A. Stephen a,n a b
Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600025, India Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600025, India
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 26 November 2015 Accepted 12 December 2015 Available online 18 December 2015
The Ni–Sn alloy was synthesized using borohydride assisted chemical reduction method. The composition of the synthesized alloy was determined using atomic absorption spectroscopy which revealed that the observed composition of Sn is high when compared to the initial composition. The ultrafine particles are clearly observed from field emission scanning electron microscope for all the sample. The X-ray diffraction measurement confirmed that the as-synthesized samples are of amorphous like nature while the samples annealed at 773 K showed crystalline nature. The Fourier transform infrared spectroscopy confirmed metallic bond stretching in the alloy samples. The crystallization and phase transition temperature was observed from differential scanning calorimetry. The shift in the crystallization temperature of Ni with increasing percentage of Sn was observed. The vibrating sample magnetometer was employed to understand the magnetic behavior of the Ni–Sn alloy. As-synthesized alloy samples showed paramagnetic nature while the annealed ones exhibit the soft ferromagnetic, antiferromagnetic and paramagnetic nature. The saturation magnetization value and magnetic ordering in the Ni–Sn alloys depend on the percentage of Sn present in the alloy. & 2015 Elsevier B.V. All rights reserved.
Keywords: Chemical reduction Ni–Sn alloy Phase transition Magnetic study Ferro magnet
1. Introduction The nickel based alloys find applications in various fields like, chemical, mechanical and electronic industries due to their corrosion and wear resistance. The Ni–Sn is one such kind of alloy with added applications like heat resistance, wettability, surface coating of electronic components, printed circuit board and in magnetic recording medium. The Ni–Sn alloy also have an important role in lithium ion batteries which is the most widely used power source in portable devices [1,2]. The Ni–Sn alloy forms the single phase (NiSn) at Sn-65:Ni-35 weight percentage composition. This phase is not thermodynamically stable as it does not appear in the phase diagram. The Ni–Sn alloy also have other stable intermediate phases like Ni3Sn, Ni3Sn2 and Ni3Sn4. These intermediate phases are formed only after annealing above 300 °C. The physical and chemical properties of these intermediate phases are also similar to that of single phase [3–5]. Different methods are available on the literature for the preparation of nickel based alloys, the important methods are electrodeposition, chemical reduction, sputtering etc. The chemical reduction method has advantages like producing large quantity of n
Corresponding author. E-mail address: [email protected] (A. Stephen).
http://dx.doi.org/10.1016/j.jmmm.2015.12.058 0304-8853/& 2015 Elsevier B.V. All rights reserved.
ultra-fine alloy particles. The composition of the individual elements in the alloy can be easily controlled using chemical reduction method without much complication. The alloy prepared using chemical reduction method is suitable for compaction process, ferrofluids and magnetic memory systems [6]. The sodium borohydride is chosen as a reducing agent in chemical reduction method for the reduction of metal ions among the other reducing agents like hydrazine due to the following reason: (i) 1 mol of sodium borohydride is capable of producing 8 mol of electrons for reduction. (ii) This has a redox potential of – 1.24 V (iii) it can reduce metal ions in water, organic solvents medium with acidic, neutral and alkaline condition [7]. As there are large number of reports available on the synthesis and characterization of Ni–Sn alloy, only limited number of papers are available on the magnetic property of Ni–Sn alloy. In the present study attempts were made to prepare Ni–Sn alloy with different composition of Sn as 10%, 25%, 50% and 75% with respect to Ni and to study the effect of Sn constitution in the structural and magnetic property of nickel.
2. Experiment Ni–Sn alloy was prepared using the chemicals, nickel(II) sulfate, tin(II) sulfate and sodium borohydride. All these chemicals were
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purchased from Merk India and used as received without any further treatment. In the preparation of Ni–Sn alloy, first the aqueous solution is prepared using nickel sulfate and tin sulfate salts. The molarity of tin sulfate was varied as 0.01 M, 0.025 M, 0.05 M and 0.075 M with respect to nickel sulfate so that the final molarity of the aqueous solution was 0.1 M. The capping agent poly vinyl pyrrolidone of 0.1 M was taken and dissolved in 50 ml of distilled water then added to the aqueous solution drop wise. This aqueous solution was kept under constant stirring for 30 minutes for the uniform distribution of nickel and tin ions. Then 0.5 M of sodium borohydride was taken in 50 ml of distilled water and added to the aqueous solution drop wise. When the pH of the aqueous solution reaches 10, black precipitate was observed which confirms the reduction of metal ions, then the solution was further stirred for 2 h to reduce all the metal ions present in the aqueous solution. The reduced black precipitate was collected and rinsed with distilled water for 6 times to remove the unwanted residues which may be formed while reducing metal ions and the reactants. Finally, the precipitate was washed with acetone and dried in room temperature [8]. The chemical reaction involved while reducing nickel and tin sulfate are given below.
Fig. 1. Relationship between the initial and final value of tin percentage in the alloy.
NiSO4 þ2 NaBH4-Niþ H2 þB2H6 þ Na2SO4
the dominant reduction of tin than nickel [9,10].
SnSO4 þ2 NaBH4-Sn þH2 þB2H6 þNa2SO4
3.2. Morphological nature
The Ni–Sn alloys were prepared with the following percentage of nickel sulfate and tin sulfate precursor, (i) Ni-90:Sn-10 (Sample A), (ii) Ni-75:Sn-25 (Sample B), (iii) Ni-50:Sn-50 (Sample C) and (iv) Ni-25:Sn-75 (Sample D) along with this nickel and Sn were also prepared using the same procedure discussed above. All the prepared samples except tin was annealed in nitrogen atmosphere for 3 h at 773 K. The quantitative analysis of nickel and tin in the alloy was done using atomic absorption spectroscopy (AAS – Perkin Elmer AAS 700). The surface morphology of the annealed samples were observed using field emission scanning electron microscopy (FESEM-FEI Quanta FEG 200). The structural information of both asprepared and annealed samples were studied using powder X-ray diffraction (XRD-GE-XRD 3003 TT). Differential scanning calorimetry (DSC – Perkin Elmer Optima 5300 DV) was performed in nitrogen atmosphere to investigate the crystallization processes of the amorphous as-synthesized samples. The metallic bond stretching in the annealed samples were determined using Fourier transform infrared (FTIR – Perkin Elmer Spectrum) spectroscopy. The magnetic behavior of both as-synthesized and annealed samples were analysed by utilizing vibrating sample magnetometer (VSM – Lakeshore VSM 7410).
3. Result and discussion 3.1. Compositional analysis The composition of individual constituent (nickel and tin) in the alloy is analysed using AAS. The relation between the initial composition and final composition of tin in the alloy is shown in Fig. 1 which also shows the expected composition of tin for comparison. From the figure, it is observed that in all the prepared alloy samples, reduction of tin is rich when compared to nickel and composition of tin is excess while comparing the initial composition. The excess of tin composition than the initial value in the alloy increases with increasing initial composition and reaches maximum for sample C then decreases for sample D. The reason behind this tin rich reduction is, the standard electrode potential of Ni (E0 ¼ 0.25 V) is less than the tin (E0 ¼ 0.14 V). This leads to
The surface morphology of the nickel, tin and alloy samples are observed using FESEM. The morphology of all the samples is shown in Fig. 2. The nickel shows the sheet or foam like nature while in the tin sample the particles are of uneven sized and shaped. The tin sample contains spherical, rod and agglomerated fine particles. FESEM image of sample A, shows that the fine particles are dispersed in foam with high agglomeration. The mixer of fine particles and foam are well observed in sample B, while the sample C shows the largely agglomerated fine particles. The morphology of sample D is spherical like nature with accumulation of large number of fine particles. This confirms that as the percentage of tin increases the morphology of the Ni–Sn alloy changes form foam like nature to ultrafine spherical like nature with agglomeration. 3.3. Structural analysis The crystalline nature of both as-synthesized and annealed samples are examined using XRD. The XRD is measured with CuKα of wavelength 1.5406 Å in the scan range 20° to 80° with 0.04°/Sec step. The XRD pattern of as-synthesized tin, nickel and different composition of tin–nickel samples are shown in Fig. 3. The XRD pattern of as-synthesized tin shows the diffraction peaks corresponds to the ‘d’ values 2.90 Å, 2.78 Å, 2.06 Å, 2.01 Å, 1.66 Å, 1.48 Å, 1.46 Å and 1.44 Å which corresponds to the hkl planes (200), (101), (202), (211), (301), (112), (400) and (321) for metallic tin with reference to JCPDS file number 89-2958. The tin is in tetragonal structure with space group I41/amd. The XRD pattern of as-synthesized nickel, sample B and D show diffraction peaks with lower intensity. These diffraction peaks are correspond to nickel hydroxide and tin hydroxide. The XRD pattern of annealed samples is shown in Fig. 4. The nickel sample shows the diffraction peaks with ‘d’ values 2.03 Å, 1.76 Å and 1.24 Å which corresponds to the hkl planes (111), (200) and (220) for metallic nickel phase with reference to JCPDS file number 04-0850. The nickel sample is in fcc structure with lattice parameter 3.52 Å and belongs to the space group Fm-3m. The XRD pattern of sample A also shows the nickel fcc phase with lattice parameter 3.54 Å. The increase in the lattice parameter is due to the substitution of tin atom in the place of nickel. In the case of
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Fig. 2. Morphology of Ni, Sn and Ni–Sn alloy samples.
Fig. 3. XRD pattern of as-synthesized Ni–Sn alloy samples.
sample B, Ni3Sn (JCPDS file number 65-3521) phase is observed. The lattice parameter of hexagonal Ni3Sn alloy is calculated and found to be a ¼5.29 Å and c ¼4.25 Å with space group P63/mmc. In the sample C, Ni phase and Sn phase (JCPDS file number 18-1380) are present, while Sn phase is only observed for sample D. From AAS analysis, for samples C and D tin composition is more than 60 weight percentage, at this percentage the phase diagram of Ni–Sn alloy, shows tin rich phase which matches well with the XRD pattern. In the XRD pattern of annealed samples, the metallic nickel phase decreases from nickel to sample D as observed from the decrement in the intensity of the phase and vanishes completely for the sample D. This is due to the decrease in the composition of nickel with respect to tin. The Ni3Sn phase is observed for the sample B and disappears after increasing the tin composition forming two individual metallic phases. This confirms that the
Fig. 4. XRD pattern of annealed Ni–Sn alloy samples.
phase formation in the alloy is totally depends on the composition of tin. 3.4. FTIR measurement The alloy formation in the Ni–Sn alloy was observed using FTIR spectroscopy for all the samples. The FTIR spectra of sample A, B, C and D are shown in Fig. 5. FTIR spectra shows that the major absorption peak around 1600 cm 1 is observed for all the samples, which is attributed to bending mode of OH group present in the surface of the samples. The FTIR spectra also show the absorption peak around 1400 cm 1 for all the samples. This is due to the metallic stretching between nickel and tin. The bonding between the nickel and tin is in the form of p–d hybridization according to
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respectively. This shows that the enthalpy for the crystallization of nickel is high. The enthalpy for precipitation of nickel is nearly 10 J/g [16] which is in accordance with obtained result. In the case of sample C, The peak at 336.8 °C is due to the crystallization of tin. The peaks at 361.6 °C & 419.6 °C are due to the separation of nickel from amorphous matrix and transforming to crystalline nickel, which is in accordance with the XRD results. The enthalpies for the exothermic transition are 32.5 J/g, 2.5 J/g and 4.4 J/g at 336.8 °C, 361.6 °C and 419.6 °C respectively. The enthalpy to the crystallization of tin is higher when compare to the nickel. As the crystallization of Sn is occurred first, the energy evolved during nickel crystallization is found to be lowered. The crystallization temperature of nickel shift towards high temperature with increasing percentage of tin in the alloy [17]. 3.6. Magnetic investigation
Fig. 5. FTIR spectra of annealed Ni–Sn alloy.
Gosh [11,12]. The decrease in the intensity of the peak is from the increasing the composition of tin. 3.5. Phase transition analysis The crystallization process in the as-synthesized Ni–Sn alloy during heating is measured using DSC with heating rate of 10 K/ min in nitrogen atmosphere. The DSC curve for sample A and sample C are shown in Fig. 6. The DSC curve clearly shows the appearance of two endothermic peaks and one broad exothermic peak in the sample A. Similarly, two endo and two exothermic peaks are observed in the sample C. The broad exothermic peaks in the sample A and C are deconvoluted and represented in Fig. 7. The endothermic peaks appeared at 68.2 °C and 102 °C in sample A and at 61.4 °C and 109.5 °C in sample B are due to the removal of water from hydroxide. The deconvolution of the broad exothermic peaks show three peaks in sample A and two peaks in sample C. The peak temperatures are found to be 297.3 °C, 349.5 °C and 395.9 °C for sample A. The first peak may be due to the structural relaxation then the second peaks for the separation of fcc nickel from the amorphous matrix. The peak at 395.9 °C is for the transformation of amorphous nickel to crystalline nickel [13–15]. This is evident in the XRD result observed for nickel phase in sample A. The total enthalpy evolved during these transition is found to be 48.8 J/g, they are deconvoluted as 18.3 J/g, 11.8 J/g and 18.6 J/g for the exothermic peaks 297.3 °C, 349.5 °C and 395.9 °C
The room temperature magnetic behavior of both as-synthesized and annealed Ni–Sn alloy is analysed using VSM. The hysteresis loop of as-synthesized samples are shown in Fig. 8. From the hysteresis loops, as-synthesized nickel shows soft ferromagnetic nature while the alloy samples A, B, C and D shows the paramagnetic nature. The as-prepared tin sample shows the diamagnetic nature. The hysteresis loops clearly explain that as the percentage of tin increases from 10, 25, 50 and 75 the magnetization values decreases as 0.22 emu/g, 0.20 emu/g, 0.12 emu/g, 0.07 emu/g and 0.06 emu/g and reaches minimum for sample D. In the as-synthesized samples the nickel atoms are randomly oriented due the amorphous nature. The diamagnetic tin atom may take a position in between nickel atom, this leads to the paramagnetic nature. The magnetization value decreases when the percentage of tin increases and reaches minimum for sample D in which tin concentration is maximum. The hysteresis loop of annealed Ni–Sn alloy samples and nickel are represented in Fig. 9. The shape of the loop shows that the annealed samples are in soft ferromagnetic nature except the sample C and D. The sample C and D shows the antiferromagnetic and paramagnetic nature respectively. The saturation magnetization, remanence, coercivity, Mr/Ms and hysteresis loss values are tabulated in Table 1 for nickel, sample A and B. The maximum saturation magnetization values of 21.3 emu/g for nickel decreases to 6.28 emu/g as the percentage of tin increases. This confirms that the magnetization value of Ni–Sn alloy purely depends on the percentage of tin present in it. The antiferromagnetic nature in the sample C is confirmed by using Arrott–Belov–Kouvel (BK) plot. The ABK plot of sample C is shown in Fig. 10. The ABK plot of sample A and B are also shown in
Fig. 6. DSC curves of sample A and C.
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Fig. 7. Deconvoluted DSC curves of sample A and C.
Fig. 8. The hystersis loop of as-synthesized Ni–Sn alloy.
Fig. 8 for comparison. The concave nature of ABK plot confirms the antiferromagnetic nature in sample C while convex nature in sample A and B confirms its ferromagnetic nature [18,19]. The high temperature magnetization measurement was carried out for sample C up to 400 °C at 1000 Oe of applied field. The thermosmagnetization curve of sample C is shown in Fig. 11. The antiferromagnetic to paramagnetic transition is found to be at 292 °C, which is observed from the dM/dT curve shown in the inset of Fig. 11. In the annealed samples nickel atoms are regularly arranged and giving rise to the ferromagnetic behavior due to the exchange interaction between the nickel atoms. The tin atom is substituted
Table 1 Magnetic parameter values of annealed Ni–Sn alloy. Parameter
Nickel
Sample A
Sample B
Saturation Magnetization Ms (emu/g) Remanence Mr (emu/g) Coercivity Hc (Oe) Squarness ratio Mr/Ms (no unit) Hysteresis Loss (erg/g. cycle)
21.33 5.98 230 0.28 12,693
7.02 2.61 280 0.37 8147
4.47 1.42 312 0.31 5187
in the position of nickel atom for samples A. The exchange interaction between Ni–Ni become weak due the presence of Sn (Ni–
Fig. 9. The hystersis loop of annealed Ni–Sn alloy.
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Fig. 10. ABK plot of samples A, B and C showing ferro and antiferromagnetic nature.
which tin composition is found to be high when compared to the initial composition. The morphology of the samples observed from FESEM analysis points the morphology changes from foam like nature to fine spherical like nature as the percentage of tin increases. The XRD studies confirmed the amorphous like nature in the as-synthesized samples while the samples annealed at 773 K showed crystalline nature belongs to nickel, tin and Ni3Sn phases. The FTIR spectroscopy confirmed the metallic bond stretching in the annealed Ni–Sn alloy samples. The phase transformation temperature are determined using DSC measurement which also revealed the increasing crystallization temperature with increasing percentage of tin in the alloy. The VSM measurement exhibit the soft ferromagnetic nature of the few samples. The magnetization studies also explains the dependence of magnetic ordering and magnetization values on the percentage of tin in the Ni–Sn alloy.
Fig. 11. High temperature magnetization curve of sample C.
Sn–Ni) resulting in low magnetization values [20]. When the percentage of tin increases this Ni–Ni exchange interaction is getting weaker. This reduces further magnetization in sample B. In the critical percentage of tin Ni–Sn alloy becomes antiferromagnetic nature. On further increasing tin concentration more number of Ni–Sn–Ni interaction are created which makes the alloy to have the paramagnetic nature. The coercivity value of the alloy sample increases with increasing percentage of tin while remanence value decreases. This explains that as the percentage of tin increases the more magnetic field is needed to demagnetize nickel. The Mr/Ms value are in the range of 0.2–0.4 which shows that the domains in the nickel, sample A and B are of pseudo single domain type [21,22]. The hysteresis loss decreases with increasing percentage of tin in the Ni–Sn alloy [23,24]. In the annealed sample C and D, the magnetization values are increased when compared to the as-synthesized sample. This is due to the ordering of nickel and tin while annealing in the samples. This magnetization study confirms that the nature of magnetic ordering and saturation magnetization values in the Ni–Sn alloy depends only on the percentage of tin in it.
4. Conclusion The Ni–Sn alloy was successfully synthesized using borohydride assisted chemical reduction method. The composition of individual constituent in the alloy was measured using AAS in
Acknowledgment Author KD would like to acknowledge NCNSNT, University of Madras for financial support. The authors would like to acknowledge SAIF-IITM for DSC and VSM measurement.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of Sn on the magnetic ordering of Ni–Sn alloy synthesized using chemical reduction method K. Dhanapal a, V. Narayanan b, A. Stephen a,n a b
Materials Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600025, India Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600025, India
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 26 November 2015 Accepted 12 December 2015 Available online 18 December 2015
The Ni–Sn alloy was synthesized using borohydride assisted chemical reduction method. The composition of the synthesized alloy was determined using atomic absorption spectroscopy which revealed that the observed composition of Sn is high when compared to the initial composition. The ultrafine particles are clearly observed from field emission scanning electron microscope for all the sample. The X-ray diffraction measurement confirmed that the as-synthesized samples are of amorphous like nature while the samples annealed at 773 K showed crystalline nature. The Fourier transform infrared spectroscopy confirmed metallic bond stretching in the alloy samples. The crystallization and phase transition temperature was observed from differential scanning calorimetry. The shift in the crystallization temperature of Ni with increasing percentage of Sn was observed. The vibrating sample magnetometer was employed to understand the magnetic behavior of the Ni–Sn alloy. As-synthesized alloy samples showed paramagnetic nature while the annealed ones exhibit the soft ferromagnetic, antiferromagnetic and paramagnetic nature. The saturation magnetization value and magnetic ordering in the Ni–Sn alloys depend on the percentage of Sn present in the alloy. & 2015 Elsevier B.V. All rights reserved.
Keywords: Chemical reduction Ni–Sn alloy Phase transition Magnetic study Ferro magnet
1. Introduction The nickel based alloys find applications in various fields like, chemical, mechanical and electronic industries due to their corrosion and wear resistance. The Ni–Sn is one such kind of alloy with added applications like heat resistance, wettability, surface coating of electronic components, printed circuit board and in magnetic recording medium. The Ni–Sn alloy also have an important role in lithium ion batteries which is the most widely used power source in portable devices [1,2]. The Ni–Sn alloy forms the single phase (NiSn) at Sn-65:Ni-35 weight percentage composition. This phase is not thermodynamically stable as it does not appear in the phase diagram. The Ni–Sn alloy also have other stable intermediate phases like Ni3Sn, Ni3Sn2 and Ni3Sn4. These intermediate phases are formed only after annealing above 300 °C. The physical and chemical properties of these intermediate phases are also similar to that of single phase [3–5]. Different methods are available on the literature for the preparation of nickel based alloys, the important methods are electrodeposition, chemical reduction, sputtering etc. The chemical reduction method has advantages like producing large quantity of n
Corresponding author. E-mail address: [email protected] (A. Stephen).
http://dx.doi.org/10.1016/j.jmmm.2015.12.058 0304-8853/& 2015 Elsevier B.V. All rights reserved.
ultra-fine alloy particles. The composition of the individual elements in the alloy can be easily controlled using chemical reduction method without much complication. The alloy prepared using chemical reduction method is suitable for compaction process, ferrofluids and magnetic memory systems [6]. The sodium borohydride is chosen as a reducing agent in chemical reduction method for the reduction of metal ions among the other reducing agents like hydrazine due to the following reason: (i) 1 mol of sodium borohydride is capable of producing 8 mol of electrons for reduction. (ii) This has a redox potential of – 1.24 V (iii) it can reduce metal ions in water, organic solvents medium with acidic, neutral and alkaline condition [7]. As there are large number of reports available on the synthesis and characterization of Ni–Sn alloy, only limited number of papers are available on the magnetic property of Ni–Sn alloy. In the present study attempts were made to prepare Ni–Sn alloy with different composition of Sn as 10%, 25%, 50% and 75% with respect to Ni and to study the effect of Sn constitution in the structural and magnetic property of nickel.
2. Experiment Ni–Sn alloy was prepared using the chemicals, nickel(II) sulfate, tin(II) sulfate and sodium borohydride. All these chemicals were
104
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purchased from Merk India and used as received without any further treatment. In the preparation of Ni–Sn alloy, first the aqueous solution is prepared using nickel sulfate and tin sulfate salts. The molarity of tin sulfate was varied as 0.01 M, 0.025 M, 0.05 M and 0.075 M with respect to nickel sulfate so that the final molarity of the aqueous solution was 0.1 M. The capping agent poly vinyl pyrrolidone of 0.1 M was taken and dissolved in 50 ml of distilled water then added to the aqueous solution drop wise. This aqueous solution was kept under constant stirring for 30 minutes for the uniform distribution of nickel and tin ions. Then 0.5 M of sodium borohydride was taken in 50 ml of distilled water and added to the aqueous solution drop wise. When the pH of the aqueous solution reaches 10, black precipitate was observed which confirms the reduction of metal ions, then the solution was further stirred for 2 h to reduce all the metal ions present in the aqueous solution. The reduced black precipitate was collected and rinsed with distilled water for 6 times to remove the unwanted residues which may be formed while reducing metal ions and the reactants. Finally, the precipitate was washed with acetone and dried in room temperature [8]. The chemical reaction involved while reducing nickel and tin sulfate are given below.
Fig. 1. Relationship between the initial and final value of tin percentage in the alloy.
NiSO4 þ2 NaBH4-Niþ H2 þB2H6 þ Na2SO4
the dominant reduction of tin than nickel [9,10].
SnSO4 þ2 NaBH4-Sn þH2 þB2H6 þNa2SO4
3.2. Morphological nature
The Ni–Sn alloys were prepared with the following percentage of nickel sulfate and tin sulfate precursor, (i) Ni-90:Sn-10 (Sample A), (ii) Ni-75:Sn-25 (Sample B), (iii) Ni-50:Sn-50 (Sample C) and (iv) Ni-25:Sn-75 (Sample D) along with this nickel and Sn were also prepared using the same procedure discussed above. All the prepared samples except tin was annealed in nitrogen atmosphere for 3 h at 773 K. The quantitative analysis of nickel and tin in the alloy was done using atomic absorption spectroscopy (AAS – Perkin Elmer AAS 700). The surface morphology of the annealed samples were observed using field emission scanning electron microscopy (FESEM-FEI Quanta FEG 200). The structural information of both asprepared and annealed samples were studied using powder X-ray diffraction (XRD-GE-XRD 3003 TT). Differential scanning calorimetry (DSC – Perkin Elmer Optima 5300 DV) was performed in nitrogen atmosphere to investigate the crystallization processes of the amorphous as-synthesized samples. The metallic bond stretching in the annealed samples were determined using Fourier transform infrared (FTIR – Perkin Elmer Spectrum) spectroscopy. The magnetic behavior of both as-synthesized and annealed samples were analysed by utilizing vibrating sample magnetometer (VSM – Lakeshore VSM 7410).
3. Result and discussion 3.1. Compositional analysis The composition of individual constituent (nickel and tin) in the alloy is analysed using AAS. The relation between the initial composition and final composition of tin in the alloy is shown in Fig. 1 which also shows the expected composition of tin for comparison. From the figure, it is observed that in all the prepared alloy samples, reduction of tin is rich when compared to nickel and composition of tin is excess while comparing the initial composition. The excess of tin composition than the initial value in the alloy increases with increasing initial composition and reaches maximum for sample C then decreases for sample D. The reason behind this tin rich reduction is, the standard electrode potential of Ni (E0 ¼ 0.25 V) is less than the tin (E0 ¼ 0.14 V). This leads to
The surface morphology of the nickel, tin and alloy samples are observed using FESEM. The morphology of all the samples is shown in Fig. 2. The nickel shows the sheet or foam like nature while in the tin sample the particles are of uneven sized and shaped. The tin sample contains spherical, rod and agglomerated fine particles. FESEM image of sample A, shows that the fine particles are dispersed in foam with high agglomeration. The mixer of fine particles and foam are well observed in sample B, while the sample C shows the largely agglomerated fine particles. The morphology of sample D is spherical like nature with accumulation of large number of fine particles. This confirms that as the percentage of tin increases the morphology of the Ni–Sn alloy changes form foam like nature to ultrafine spherical like nature with agglomeration. 3.3. Structural analysis The crystalline nature of both as-synthesized and annealed samples are examined using XRD. The XRD is measured with CuKα of wavelength 1.5406 Å in the scan range 20° to 80° with 0.04°/Sec step. The XRD pattern of as-synthesized tin, nickel and different composition of tin–nickel samples are shown in Fig. 3. The XRD pattern of as-synthesized tin shows the diffraction peaks corresponds to the ‘d’ values 2.90 Å, 2.78 Å, 2.06 Å, 2.01 Å, 1.66 Å, 1.48 Å, 1.46 Å and 1.44 Å which corresponds to the hkl planes (200), (101), (202), (211), (301), (112), (400) and (321) for metallic tin with reference to JCPDS file number 89-2958. The tin is in tetragonal structure with space group I41/amd. The XRD pattern of as-synthesized nickel, sample B and D show diffraction peaks with lower intensity. These diffraction peaks are correspond to nickel hydroxide and tin hydroxide. The XRD pattern of annealed samples is shown in Fig. 4. The nickel sample shows the diffraction peaks with ‘d’ values 2.03 Å, 1.76 Å and 1.24 Å which corresponds to the hkl planes (111), (200) and (220) for metallic nickel phase with reference to JCPDS file number 04-0850. The nickel sample is in fcc structure with lattice parameter 3.52 Å and belongs to the space group Fm-3m. The XRD pattern of sample A also shows the nickel fcc phase with lattice parameter 3.54 Å. The increase in the lattice parameter is due to the substitution of tin atom in the place of nickel. In the case of
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Fig. 2. Morphology of Ni, Sn and Ni–Sn alloy samples.
Fig. 3. XRD pattern of as-synthesized Ni–Sn alloy samples.
sample B, Ni3Sn (JCPDS file number 65-3521) phase is observed. The lattice parameter of hexagonal Ni3Sn alloy is calculated and found to be a ¼5.29 Å and c ¼4.25 Å with space group P63/mmc. In the sample C, Ni phase and Sn phase (JCPDS file number 18-1380) are present, while Sn phase is only observed for sample D. From AAS analysis, for samples C and D tin composition is more than 60 weight percentage, at this percentage the phase diagram of Ni–Sn alloy, shows tin rich phase which matches well with the XRD pattern. In the XRD pattern of annealed samples, the metallic nickel phase decreases from nickel to sample D as observed from the decrement in the intensity of the phase and vanishes completely for the sample D. This is due to the decrease in the composition of nickel with respect to tin. The Ni3Sn phase is observed for the sample B and disappears after increasing the tin composition forming two individual metallic phases. This confirms that the
Fig. 4. XRD pattern of annealed Ni–Sn alloy samples.
phase formation in the alloy is totally depends on the composition of tin. 3.4. FTIR measurement The alloy formation in the Ni–Sn alloy was observed using FTIR spectroscopy for all the samples. The FTIR spectra of sample A, B, C and D are shown in Fig. 5. FTIR spectra shows that the major absorption peak around 1600 cm 1 is observed for all the samples, which is attributed to bending mode of OH group present in the surface of the samples. The FTIR spectra also show the absorption peak around 1400 cm 1 for all the samples. This is due to the metallic stretching between nickel and tin. The bonding between the nickel and tin is in the form of p–d hybridization according to
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respectively. This shows that the enthalpy for the crystallization of nickel is high. The enthalpy for precipitation of nickel is nearly 10 J/g [16] which is in accordance with obtained result. In the case of sample C, The peak at 336.8 °C is due to the crystallization of tin. The peaks at 361.6 °C & 419.6 °C are due to the separation of nickel from amorphous matrix and transforming to crystalline nickel, which is in accordance with the XRD results. The enthalpies for the exothermic transition are 32.5 J/g, 2.5 J/g and 4.4 J/g at 336.8 °C, 361.6 °C and 419.6 °C respectively. The enthalpy to the crystallization of tin is higher when compare to the nickel. As the crystallization of Sn is occurred first, the energy evolved during nickel crystallization is found to be lowered. The crystallization temperature of nickel shift towards high temperature with increasing percentage of tin in the alloy [17]. 3.6. Magnetic investigation
Fig. 5. FTIR spectra of annealed Ni–Sn alloy.
Gosh [11,12]. The decrease in the intensity of the peak is from the increasing the composition of tin. 3.5. Phase transition analysis The crystallization process in the as-synthesized Ni–Sn alloy during heating is measured using DSC with heating rate of 10 K/ min in nitrogen atmosphere. The DSC curve for sample A and sample C are shown in Fig. 6. The DSC curve clearly shows the appearance of two endothermic peaks and one broad exothermic peak in the sample A. Similarly, two endo and two exothermic peaks are observed in the sample C. The broad exothermic peaks in the sample A and C are deconvoluted and represented in Fig. 7. The endothermic peaks appeared at 68.2 °C and 102 °C in sample A and at 61.4 °C and 109.5 °C in sample B are due to the removal of water from hydroxide. The deconvolution of the broad exothermic peaks show three peaks in sample A and two peaks in sample C. The peak temperatures are found to be 297.3 °C, 349.5 °C and 395.9 °C for sample A. The first peak may be due to the structural relaxation then the second peaks for the separation of fcc nickel from the amorphous matrix. The peak at 395.9 °C is for the transformation of amorphous nickel to crystalline nickel [13–15]. This is evident in the XRD result observed for nickel phase in sample A. The total enthalpy evolved during these transition is found to be 48.8 J/g, they are deconvoluted as 18.3 J/g, 11.8 J/g and 18.6 J/g for the exothermic peaks 297.3 °C, 349.5 °C and 395.9 °C
The room temperature magnetic behavior of both as-synthesized and annealed Ni–Sn alloy is analysed using VSM. The hysteresis loop of as-synthesized samples are shown in Fig. 8. From the hysteresis loops, as-synthesized nickel shows soft ferromagnetic nature while the alloy samples A, B, C and D shows the paramagnetic nature. The as-prepared tin sample shows the diamagnetic nature. The hysteresis loops clearly explain that as the percentage of tin increases from 10, 25, 50 and 75 the magnetization values decreases as 0.22 emu/g, 0.20 emu/g, 0.12 emu/g, 0.07 emu/g and 0.06 emu/g and reaches minimum for sample D. In the as-synthesized samples the nickel atoms are randomly oriented due the amorphous nature. The diamagnetic tin atom may take a position in between nickel atom, this leads to the paramagnetic nature. The magnetization value decreases when the percentage of tin increases and reaches minimum for sample D in which tin concentration is maximum. The hysteresis loop of annealed Ni–Sn alloy samples and nickel are represented in Fig. 9. The shape of the loop shows that the annealed samples are in soft ferromagnetic nature except the sample C and D. The sample C and D shows the antiferromagnetic and paramagnetic nature respectively. The saturation magnetization, remanence, coercivity, Mr/Ms and hysteresis loss values are tabulated in Table 1 for nickel, sample A and B. The maximum saturation magnetization values of 21.3 emu/g for nickel decreases to 6.28 emu/g as the percentage of tin increases. This confirms that the magnetization value of Ni–Sn alloy purely depends on the percentage of tin present in it. The antiferromagnetic nature in the sample C is confirmed by using Arrott–Belov–Kouvel (BK) plot. The ABK plot of sample C is shown in Fig. 10. The ABK plot of sample A and B are also shown in
Fig. 6. DSC curves of sample A and C.
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Fig. 7. Deconvoluted DSC curves of sample A and C.
Fig. 8. The hystersis loop of as-synthesized Ni–Sn alloy.
Fig. 8 for comparison. The concave nature of ABK plot confirms the antiferromagnetic nature in sample C while convex nature in sample A and B confirms its ferromagnetic nature [18,19]. The high temperature magnetization measurement was carried out for sample C up to 400 °C at 1000 Oe of applied field. The thermosmagnetization curve of sample C is shown in Fig. 11. The antiferromagnetic to paramagnetic transition is found to be at 292 °C, which is observed from the dM/dT curve shown in the inset of Fig. 11. In the annealed samples nickel atoms are regularly arranged and giving rise to the ferromagnetic behavior due to the exchange interaction between the nickel atoms. The tin atom is substituted
Table 1 Magnetic parameter values of annealed Ni–Sn alloy. Parameter
Nickel
Sample A
Sample B
Saturation Magnetization Ms (emu/g) Remanence Mr (emu/g) Coercivity Hc (Oe) Squarness ratio Mr/Ms (no unit) Hysteresis Loss (erg/g. cycle)
21.33 5.98 230 0.28 12,693
7.02 2.61 280 0.37 8147
4.47 1.42 312 0.31 5187
in the position of nickel atom for samples A. The exchange interaction between Ni–Ni become weak due the presence of Sn (Ni–
Fig. 9. The hystersis loop of annealed Ni–Sn alloy.
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Fig. 10. ABK plot of samples A, B and C showing ferro and antiferromagnetic nature.
which tin composition is found to be high when compared to the initial composition. The morphology of the samples observed from FESEM analysis points the morphology changes from foam like nature to fine spherical like nature as the percentage of tin increases. The XRD studies confirmed the amorphous like nature in the as-synthesized samples while the samples annealed at 773 K showed crystalline nature belongs to nickel, tin and Ni3Sn phases. The FTIR spectroscopy confirmed the metallic bond stretching in the annealed Ni–Sn alloy samples. The phase transformation temperature are determined using DSC measurement which also revealed the increasing crystallization temperature with increasing percentage of tin in the alloy. The VSM measurement exhibit the soft ferromagnetic nature of the few samples. The magnetization studies also explains the dependence of magnetic ordering and magnetization values on the percentage of tin in the Ni–Sn alloy.
Fig. 11. High temperature magnetization curve of sample C.
Sn–Ni) resulting in low magnetization values [20]. When the percentage of tin increases this Ni–Ni exchange interaction is getting weaker. This reduces further magnetization in sample B. In the critical percentage of tin Ni–Sn alloy becomes antiferromagnetic nature. On further increasing tin concentration more number of Ni–Sn–Ni interaction are created which makes the alloy to have the paramagnetic nature. The coercivity value of the alloy sample increases with increasing percentage of tin while remanence value decreases. This explains that as the percentage of tin increases the more magnetic field is needed to demagnetize nickel. The Mr/Ms value are in the range of 0.2–0.4 which shows that the domains in the nickel, sample A and B are of pseudo single domain type [21,22]. The hysteresis loss decreases with increasing percentage of tin in the Ni–Sn alloy [23,24]. In the annealed sample C and D, the magnetization values are increased when compared to the as-synthesized sample. This is due to the ordering of nickel and tin while annealing in the samples. This magnetization study confirms that the nature of magnetic ordering and saturation magnetization values in the Ni–Sn alloy depends only on the percentage of tin in it.
4. Conclusion The Ni–Sn alloy was successfully synthesized using borohydride assisted chemical reduction method. The composition of individual constituent in the alloy was measured using AAS in
Acknowledgment Author KD would like to acknowledge NCNSNT, University of Madras for financial support. The authors would like to acknowledge SAIF-IITM for DSC and VSM measurement.
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