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Physical and electromagnetic properties of nanosized Gd substituted Mg–Mn ferrites by solution combustion method
Physica B 461 (2015) 134–139
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Physical and electromagnetic properties of nanosized Gd substituted Mg–Mn ferrites by solution combustion method Nilar Lwin a,n, Ahmad Fauzi M.N.b, Srimala Sreekantan b, Radzali Othman c a
Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia c Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, 76100 Durian Tunggal, Malacca, Malaysia b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 July 2014 Received in revised form 9 December 2014 Accepted 5 January 2015 Available online 6 January 2015
Nanosized powders of Gd substituted Mg–Mn ferrites synthesized by solution combustion method using high purity metal nitrates are presented. These powders were calcined, compacted and sintered at 1250 °C. The powders were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The effect of Gd substitution on phase formation, microstructure and bulk density was also studied. Gd2O3 facilitates the formation of a secondary phase on the grain boundary which suppresses abnormal grain growth. The bulk density was found to decrease from 4.26 to 3.38 g/cm3 with an increase of Gd substitution, but the electrical resistivity was increased. Ferrite with a low dielectric constant in the range of 6–12 was observed and there was no maximum dielectric loss in the frequency range measured to 1 GHz. A decrease in saturation magnetization was also observed by a small fraction of Gd substitution. Correlation between magnetic properties and physical properties were discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ferrite Solution combustion Microstructure Rare earths Density/electromagnetic properties
1. Introduction Magnesium‐Manganese ferrites are of great importance from the point of view of their industrial applications such as for high quality filters, rod antennas, radio frequency circuits, transformer cores, due to their inherent high resistivity, low dielectric losses and chemical stability [1]. The substitution of rare-earth ions in the host spinel lattice structure can control the electric and magnetic properties such as resistivity, dielectric constant, saturation magnetization, etc. It is known that rare-earth oxides are good electrical insulators and have high resistivity which is about 106 Ω cm at room temperature. The influence of rare-earth oxides on the properties of different ferrites had been investigated by many researchers. The investigations showed an improved densification in Mg ferrite and an increase in resistivity by Tb substitution [2]. La and Y incorporation in MgCu ferrite indicated decreased resistivity and saturation magnetization in this ferrite. However, from the literature, Gd substitutions revealed improved electrical and dielectric properties in Mg ferrite [3]. Similarly, these Gd substitutions may improve the electromagnetic properties in Mg–Mn ferrite and hence further research is still required to investigate the physical and electromagnetic properties in Mg–Mn ferrites. n
Corresponding author. E-mail address: [email protected] (N.-n. Lwin).
http://dx.doi.org/10.1016/j.physb.2015.01.001 0921-4526/& 2015 Elsevier B.V. All rights reserved.
On the other hand, many methods have been reported for the preparation of nano-sized soft ferrites, which includes coprecipitation [4], hydrothermal [5], mechanically alloying, microemulsion and solution combustion method. In the present study, an attempt was made to investigate the physical and electromagnetic properties of Gd substituted Mg–Mn ferrites processed by a solution combustion method.
2. Experimental Nanocrystalline Gd substituted Mg–Mn ferrite powders were prepared by a solution combustion method (SCM). Fe(NO3)2 9H2O (Aldrich, 99.8%), Mg(NO3)2 6H2O (Sigma-Aldrich, 99%), Mn (NO3)2 6H2O, Gd(NO3)2 6H2O and citric acid (Sigma-Aldrich, 99.5%) were used as sources of metal ions and chelating-fuel agent. Their aqueous solutions were mixed together into a Pyrex beaker with 1:1 M ratio of metal to citric acid. The clear orange brown solution thus obtained had a pH o1. The dark brown solution was allowed to evaporate on a hot plate maintaining the solution temperature at 80 °C. Upon the formation of a dense sticky gel, the temperature was increased to 120 °C for the condensation process. The temperature was then increased rapidly and when it reached to ∼200 °C, large amount of gases (CO2, H2O, N2) were liberated and a dark brown ferrite powder was produced through the combustion process. The as burnt ash was calcined at 500 °C for 5 h to obtain better crystallization and a homogeneous
N. Lwin et al. / Physica B 461 (2015) 134–139
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cation distribution in the spinel. As the auto combustion is so rapid that the cations may not be well distributed in ferrite lattice of as burnt ashes. The calcined ferrite power was granulated using an agate mortar and was uniaxially pressed at a pressure of 100 MPa to form pellet specimens. The specimens were sintered at 1250 °C in air atmosphere.
3. Results and discussions 3.1. X ray analysis The XRD patterns of as burnt ferrite powders are shown in Fig. 1. The figure shows that the ferrites formed are in a crystalline state and contain cubic spinel phase. The broad peak in XRD patterns indicates fine crystallite size of the ferrite particles. The crystallite size of the as burnt ferrite powders was calculated using Scherrer's equation (t = 0.9λ /B cos θ , where t¼the crystallite size λ ¼the wavelength of the radiation, θ ¼the Bragg's angle, B ¼ the full width at half maximum) and it was in the range of 6–12 nm. This observation reveals that the nanocrystalline Gd doped Mg– Mn ferrite powders can be synthesized directly by the auto combustion of nitrate-citrate gels. The calcined and sintered Gd substituted Mg–Mn ferrites were also characterized by XRD. As predicted, the crystallite size increases with successive heat treatment (calcination and sintering) of the ferrites. Figs. 2 and 3 show the diffraction patterns of calcined and sintered ferrites for the different compositions. The formation of a second phase in the sintered Mg–Mn ferrite upon Gd substitution is identified by XRD analysis and it can be seen in Fig. 3. All the samples were found to match well with a cubic spinel structure. It was observed that Gadolinium iron oxide, GdFeO3 phase [ICDD 00-047-0067] is detected in all Gd substituted ferrites. The peak intensity of GdFeO3 increases with increasing Gd content. This could be attributed to the ionic radius of Gd3 þ (0.094 nm) which is larger than that of Fe3 þ ions (0.064 nm), and therefore, the amount of Fe3 þ ions substituted by Gd3 þ have a limit. Thus, remnant Gd3 þ ions will aggregate on the grain boundaries forming the GdFeO3 phase during the sintering process. GdFeO3 phase in the samples could help to improve the resistivity of ferrite since it behaves as an insulating layer at the grain boundary. 3.2. TEM microstructural analysis Fig. 4 shows the TEM images which represents the general
Fig. 2. XRD spectra of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite calcined powders (500 °C) of (a) x ¼0.0, (b) x¼ 0.025, (c) x ¼ 0.050, and (d) x¼ 0.075.
Fig. 3. XRD spectra of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite sintered pellets (1250 °C) of (a) x ¼0.0, (b) x¼ 0.025, (c) x ¼ 0.050, and (d) x¼ 0.075.
images of calcined powders for Mg0.9Mn0.1GdxFe1.8 xO4 ferrites. It is found that the shape of the particle is regular and is exhibited as an agglomerated monophase polycrystalline powder, and the particle sizes of the samples are homogeneously distributed, being in the range of 7–13 nm. It is observed that the particle sizes of the samples are consistent with the average crystallite size determined by the XRD data which is about 9–15 nm. 3.3. Densification and SEM observation
Fig. 1. XRD spectra of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite as burnt powder of (a) x¼ 0.0, (b) x¼ 0.025, (c) x ¼0.050, and (d) x¼ 0.075.
Fig. 5 exhibits the bulk density and porosity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites with different Gd contents. The density of the sintered specimen decreases upon Gd substitution, indicating a reduced densification with the substitution. This decrease would probably be due to an increased intragranular porosity resulting from discontinuous grain growth as described by Burke [6]. The morphological changes with an increasing substitution of Gd in Mg–Mn ferrites observed with FESEM are shown in Fig. 6. It is distinctly observed that the presence of Gd2O3 affect drastically the microstructure of Mg–Mn ferrite. The microstructural observation shows a decrease in the average grain size and an increase in porosity. The values of grain size data are given in Table 1. This decrease in average grain size can be explained as
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Fig. 4. TEM images of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite (a) x ¼0.0, (b) x ¼0.025, (c) x¼ 0.050 and (d) x ¼0.075
phase being the small whitish round grains at the grain boundary/ junction. 3.4. Magnetic properties
Fig. 5. Variation of density and porosity of sintered Mg0.9Mn0.1GdxFe1.8 xO4 ferrites with different Gd content (x¼ 0.0, 0.025, 0.050, 0.075).
follows. As the ionic radius of Gd is 0.938 Å, being larger than that of Fe3 þ ions 0.64 Å, hence when some Fe3 þ ions at the ferrite lattice site were substituted by some Gd3 þ ions, the lattice parameter will be changed due to the compression of the spinel lattice [7]. However, the variations of lattice parameters will lead to lattice strains which will produce the internal stress [8]. Such stresses could hinder the growth of grains and hence the grain sizes of the samples substituted with Gd ions become smaller than that of Mg–Mn ferrite nanocrystals. In this study, it is observed that the larger amount of Gd substitutions, the smaller are the grain sizes formed. The grain size is found to decrease from about 2–0.7 μm, i.e. about 60% reduction in size when Gd was added from x¼ 0 to 0.075. Specimen which was not substituted by Gd (Fig. 6a) shows the presence of a monophasic and homogeneous microstructure with an average grain size of ∼2 mm. However, Gd-substituted specimens (Fig. 6b–d) show a biphasic microstructure, the second
Fig. 7 presents the magnetic hysteresis loop of different Mg0.9Mn0.1GdxFe1.8 xO4 ferrites. The magnetization was obtained at room temperature under a magnetic field of 7 10 kOe field. The magnetic properties such as saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc) and magnetic moment of unit cell values of the different compositions with increasing Gd substitution are tabulated in Table 2. The saturation magnetization of substituted ferrites was lower than the unsubstituted ferrite and this might be due to the decrease in densification as discussed earlier. Consequently, the decrease of saturation magnetization with increasing Gd3 þ ions can be attributed to the replacement of Fe3 þ by non-magnetic Gd3 þ ions in B sites which leads to a decrease of magnetization of B-site and hence saturation magnetization, Ms, would decrease [9]. The coercivity of the samples increases with the Gd content. Generally, it can be mentioned that in this study, the formation of the secondary phase at the grain boundaries may affect the homogeneous composition and microstructure, and induce some distortion in the internal grain region which lead to larger internal stress. Al-Hilla et al. [10] had worked on gadolinium substituted Li–Ni ferrite. They observed that there was a limited solubility of Gd ion in the spinel structure of Li–Ni ferrite and Gd3 þ ions would precipitate as a secondary phase of GdFeO3 at the grain boundaries, which may inhibit the grain growth in the samples. In addition, it is known that the grain boundary increases with decreasing crystallite size. In this work, the occupation of the cations in the tetrahedral A site and octahedral B site were also calculated from the observed magnetic moment per unit cell and presented in Table 3. It can be observed that the decrease in magnetization is reflected from the ratio of cation occupation in the spinel lattice. The decrease in the fraction of Fe3 þ ion in B sites is responsible for the decrease in
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Fig. 6. SEM images of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite (a) x¼ 0.0, (b) x ¼0.025, (c) x¼ 0.050 and (d) x ¼ 0.075
Table 1 Bulk density, relative density and porosity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites by SCM. Composition (x) Bulk density (g/cm3) Relative density (%)
Apparent porosity (%)
x ¼ 0.0 x ¼ 0.025 x ¼ 0.050 x ¼ 0.075
2.56 8.43 14.39 15.11
4.26 4.13 3.93 3.89
96.01 94.61 91.59 90.98
Average grain size (mm) 2.01 1.26 0.98 0.79
Table 2 The value of Ms, Mr, Hc and nB for the Mg0.9Mn0.1GdxFe1.8 xO4 ferrites prepared by SCM with different Fe content. Compositions
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
nB (mB)
x ¼0.0 x ¼0.025 x ¼0.050 x ¼0.075
38.41 30.01 27.31 25.77
0.98 2.82 4.66 5.28
20.25 22.05 23.46 24.22
1.32 1.05 0.98 0.91
Table 3 Expected cation distribution in Mg0.9Mn0.1GdxFe1.8 xO4 ferrites prepared by SCM with different Fe content. Compositions (x)
Occupation of cations A site
x ¼0.0 x ¼0.025 x ¼0.050 x ¼0.075
2þ Mg0.188 þ Mg20.160 þ Mg20.155 þ Mg20.157
B site 3þ Fe0.812 þ Fe30.840 þ Fe30.845 þ Fe30.843
2þ Mg0.712 þ Mg20.740 þ Mg20.745 þ Mg20.743
2þ Mn0.1 Mn20.1þ Mn20.1þ Mn20.1þ
3þ Fe0.988 þ þ Gd30.025 Fe30.910 þ þ Gd30.05 Fe30.858 þ þ Gd30.075 Fe30.807
the domain wall motions, thus the coercivity of the samples increases with Gd content. This phenomenon is also observed by many researchers [12,13]. 3.5. Electrical properties
Fig. 7. The magnetization curves of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites with different compositions measured by VSM 7 10 kOe at room temperature
saturation magnetization [11]. In this study, Mg0.9Mn0.1GdxFe1.8 xO4 ferrites show a decrease in crystallite size with increasing Gd content. The area of disordered arrangement for atoms on grain boundaries may hinder
Dielectric constant as a function of compositions for Mg0.9Mn0.1GdxFe1.8 xO4 ferrites compact by SCM at a sintered temperature of 1250 °C is shown in Fig. 8. It is observed that all the samples show a dependence on the amount of Gd content, i.e the dielectric constant decreases with increasing amount of Gd. The observed higher values of εʹ with x ¼0.0 (Gd un-substituted) sample might be due to the presence of a higher number of Fe2 þ formed at sintering temperature of 1250 °C. However, there is a further observation in the insert of Fig. 8 where all the samples do not show significant frequency-dependent phenomenon i.e the
138
Fig. 8. Variation of dielectric constant Mg0.9Mn0.1GdxFe1.8 xO4 ferrite sintered at 1250 °C.
N. Lwin et al. / Physica B 461 (2015) 134–139
with
composition
for
permittivity maintains almost a constant value within 1 MHz– 1 GHz. At higher frequency, the εʹ values are usually found to be invariable with frequency. This is due to the fact that the electronic movements are not able to follow the rapid switching of the AC field at higher frequency. Dielectric losses as a function of frequency for Mg0.9Mn0.1GdxFe1.8 xO4 ferrites prepared by SCM at different compositions are shown in Fig. 9. It is found that the dielectric loss tangent decreases with increasing Gd content. The increase in RE oxide's resistivity may give rise to the reduction in tan δ, and this loss factor can be explained with the electrical resistivity (tan δ ¼1/ρ2πfεʹ) [14]. This formula shows that an increase in resistivity (ρ) will bring a reduction in tan δ at a given frequency. The lower values of dielectric loss obtained in this study can be attributed to the curtailing (reduction) of the Fe2 þ ions (Fe2 þ 2Fe3 þ þe ) in the system. It was found that there is no maximum dielectric loss within the measured frequency range up to 1 GHz due to excellent compositional control obtained in this solution combustion method. It is expected that the compositions are well controlled at the atomic scale by this method. This finding is in agreement with the study of the compositional stoichiometry on dielectric loss by Brockman [15]. They have reported the dielectric properties of stoichiometric Ni–Zn ferrites. The low dielectric constant (∼15) and low loss values were observed with careful control of stoichiometry of the compositions in their study. Resistivity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites sintered at 1250 °C for different compositions is represented as a function of
Fig. 10. Variation of resistivity with frequency for Mg0.9Mn0.1GdxFe1.8 xO4 ferrite sintered at 1250 °C
Table 4 Resistivity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite prepared by SCM sintered at 1250 °C for the different Gd content. Composition
Resistivity (106 Ω-cm) at 1 MHz
x¼ 0.0 x¼ 0.025 x¼ 0.050 x¼ 0.075
0.38 17.09 19.50 25.74
frequency in Fig. 10 and the values measured at 1 MHz are listed in Table 4. Generally, high values of resistivity are observed in this study. The resistivity increases with Gd substitution. The conduction in ferrites arises mainly from the electron hopping between Fe2 þ and Fe3 þ ions that were located at the octahedral sites of the spinel ferrite structure. Sample x ¼0.075 shows the highest resistivity among all compositions, which may be attributed the presence of lower amount of Fe2 þ ions in the ferrites. In fact, there are less amount of Fe due to the substitution of Gd, which is good insulator in the compositions and thus higher resistivity in the Gd substituted samples. In this study, Gd substituted compositions were deficient in Fe content and the crystallization of GdFeO3 phase impeded the reduction of Fe3 þ ions inside the grains. Hence, the formation of Fe2 þ ions were expected to be at minimum in Gd substituted ferrites. This might be the reason for increased resistivity in these compositions. High resistivities are desirable to reduce eddy current losses encountered at high frequencies. Thus, the ferrites prepared in the present study are expected to be worthy for high frequency applications.
4. Conclusions
Fig. 9. Variation of Loss tangent with frequency for Mg0.9Mn0.1 GdxFe1.8 xO4 ferrite sintered at 1250 °C
The effect of Gd substitutions on physical and electromagnetic properties of Mg–Mn ferrites has been investigated. In this study, the substitution of Gd for Fe in (Mg0.9Mn0.1GdxFe1.8 xO4) ferrites indicated single phase cubic spinel structure along with the formation of the secondary phase GdFeO3. Bulk density and grain size of the ferrites decreased with increasing Gd substitution. The AC resistivities of substituted ferrites also increased from 0.38 106 to 25 106 Ω-cm with the increase of Gd content which can be explained in terms of grain growth and microstructure of respective samples. This is due to the formation of secondary phases at the grain boundary that inhibit the grain growth and increase the
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resistivity. The dielectric properties can be understood similarly to the AC resistivities. A slight solubility of Gd3 þ ions in the spinel matrix results a decrease of saturation magnetization. The increase in coercivity with increase in substitutions of Gd for Fe in the spinel structure was observed. The physical and electromagnetic properties of Mg–Mn ferrites are strongly affected by substitution of different Gd content.
Acknowledgment The authors are grateful to the Universiti Sains Malaysia for providing financial assistance under USM fellowship scheme to conduct this research. The authors also would like to acknowledge the “e‐Science Fund” (Grant no. 6013343) and MOSTI (03-01-05-SF 0375) for financial support.
References [1] B.M. Kunal, H.V. Nimish, K.L. Vinay, K.P. Tushar, J. Adv. Microsc. Res. 7 (2012)
139
40–43. [2] M.A. Khan, M.U. Islam, M. Ishaque, I.Z. Rahman, Ceram. Int. 37 (2011) 2519–2526. [3] J. Chand, M. Singh, J. Alloys Compd. 486 (2009) (2009) 376–379. [4] M. Naeem, N. Abbas Shah, I. Hussain Gul, A. Maqsood, J. Alloys Compd. 487 (2009) 739–743. [5] X. Li, G. Wang, J.Magn. Magn. Mater. 321 (2009) 1276–1279. [6] J.E. Burke, W.D. Kingery (Eds.), Ceramic Fabrication Processes, Wiley, New York, 1958, p. 28. [7] N. Rezlescu, E. Rezlescu, C. Pasnicu, M.L. Craus, J. Magn. Magn. Mater. 136 (1994) 319–326. [8] H. Yang, L. Zhao, X. Yang, L. Shen, L. Yu, W. Sun, Y. Yan, W. Wang, S. Feng, J. Magn. Magn. Mater. 271 (2004) 230–236. [9] S.E. Jacobo, S. Duhalde, H.R. Bertorello, J. Magn. Magn. Mater. 272 (2004) 2253–2254. [10] M.F. Al-Hilli, S. Li, K.S. Kassim, Mater. Chem. Phys. 128 (2011) 127–132. [11] M.U. Rana, M. Islam, T. Abbas, Mater. Lett. 41 (1999) 52. [12] L. Zhao, H. Yang, L. Yu, Y. Cui, W. San, Y. Yan, S. Feng, Phys. Status Solidi A 201 (2004) 3121–3128. [13] S.U. Romero, O.P. Pérez, G. Gutiérrez, J. Appl. Phys. (2010), http://dx.doi.org/ 10.1063/1.3338847, 09A508–09A508-3. [14] J. Sun, J. Li, G. Sun, J. Magn. Magn. Mater. 250 (2002) 20–24. [15] F.G. Brockman, R.P. White, J. Am. Ceram. Soc. 54 (1971) 183–187.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Physical and electromagnetic properties of nanosized Gd substituted Mg–Mn ferrites by solution combustion method Nilar Lwin a,n, Ahmad Fauzi M.N.b, Srimala Sreekantan b, Radzali Othman c a
Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia c Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, 76100 Durian Tunggal, Malacca, Malaysia b
art ic l e i nf o
a b s t r a c t
Article history: Received 21 July 2014 Received in revised form 9 December 2014 Accepted 5 January 2015 Available online 6 January 2015
Nanosized powders of Gd substituted Mg–Mn ferrites synthesized by solution combustion method using high purity metal nitrates are presented. These powders were calcined, compacted and sintered at 1250 °C. The powders were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The effect of Gd substitution on phase formation, microstructure and bulk density was also studied. Gd2O3 facilitates the formation of a secondary phase on the grain boundary which suppresses abnormal grain growth. The bulk density was found to decrease from 4.26 to 3.38 g/cm3 with an increase of Gd substitution, but the electrical resistivity was increased. Ferrite with a low dielectric constant in the range of 6–12 was observed and there was no maximum dielectric loss in the frequency range measured to 1 GHz. A decrease in saturation magnetization was also observed by a small fraction of Gd substitution. Correlation between magnetic properties and physical properties were discussed. & 2015 Elsevier B.V. All rights reserved.
Keywords: Ferrite Solution combustion Microstructure Rare earths Density/electromagnetic properties
1. Introduction Magnesium‐Manganese ferrites are of great importance from the point of view of their industrial applications such as for high quality filters, rod antennas, radio frequency circuits, transformer cores, due to their inherent high resistivity, low dielectric losses and chemical stability [1]. The substitution of rare-earth ions in the host spinel lattice structure can control the electric and magnetic properties such as resistivity, dielectric constant, saturation magnetization, etc. It is known that rare-earth oxides are good electrical insulators and have high resistivity which is about 106 Ω cm at room temperature. The influence of rare-earth oxides on the properties of different ferrites had been investigated by many researchers. The investigations showed an improved densification in Mg ferrite and an increase in resistivity by Tb substitution [2]. La and Y incorporation in MgCu ferrite indicated decreased resistivity and saturation magnetization in this ferrite. However, from the literature, Gd substitutions revealed improved electrical and dielectric properties in Mg ferrite [3]. Similarly, these Gd substitutions may improve the electromagnetic properties in Mg–Mn ferrite and hence further research is still required to investigate the physical and electromagnetic properties in Mg–Mn ferrites. n
Corresponding author. E-mail address: [email protected] (N.-n. Lwin).
http://dx.doi.org/10.1016/j.physb.2015.01.001 0921-4526/& 2015 Elsevier B.V. All rights reserved.
On the other hand, many methods have been reported for the preparation of nano-sized soft ferrites, which includes coprecipitation [4], hydrothermal [5], mechanically alloying, microemulsion and solution combustion method. In the present study, an attempt was made to investigate the physical and electromagnetic properties of Gd substituted Mg–Mn ferrites processed by a solution combustion method.
2. Experimental Nanocrystalline Gd substituted Mg–Mn ferrite powders were prepared by a solution combustion method (SCM). Fe(NO3)2 9H2O (Aldrich, 99.8%), Mg(NO3)2 6H2O (Sigma-Aldrich, 99%), Mn (NO3)2 6H2O, Gd(NO3)2 6H2O and citric acid (Sigma-Aldrich, 99.5%) were used as sources of metal ions and chelating-fuel agent. Their aqueous solutions were mixed together into a Pyrex beaker with 1:1 M ratio of metal to citric acid. The clear orange brown solution thus obtained had a pH o1. The dark brown solution was allowed to evaporate on a hot plate maintaining the solution temperature at 80 °C. Upon the formation of a dense sticky gel, the temperature was increased to 120 °C for the condensation process. The temperature was then increased rapidly and when it reached to ∼200 °C, large amount of gases (CO2, H2O, N2) were liberated and a dark brown ferrite powder was produced through the combustion process. The as burnt ash was calcined at 500 °C for 5 h to obtain better crystallization and a homogeneous
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cation distribution in the spinel. As the auto combustion is so rapid that the cations may not be well distributed in ferrite lattice of as burnt ashes. The calcined ferrite power was granulated using an agate mortar and was uniaxially pressed at a pressure of 100 MPa to form pellet specimens. The specimens were sintered at 1250 °C in air atmosphere.
3. Results and discussions 3.1. X ray analysis The XRD patterns of as burnt ferrite powders are shown in Fig. 1. The figure shows that the ferrites formed are in a crystalline state and contain cubic spinel phase. The broad peak in XRD patterns indicates fine crystallite size of the ferrite particles. The crystallite size of the as burnt ferrite powders was calculated using Scherrer's equation (t = 0.9λ /B cos θ , where t¼the crystallite size λ ¼the wavelength of the radiation, θ ¼the Bragg's angle, B ¼ the full width at half maximum) and it was in the range of 6–12 nm. This observation reveals that the nanocrystalline Gd doped Mg– Mn ferrite powders can be synthesized directly by the auto combustion of nitrate-citrate gels. The calcined and sintered Gd substituted Mg–Mn ferrites were also characterized by XRD. As predicted, the crystallite size increases with successive heat treatment (calcination and sintering) of the ferrites. Figs. 2 and 3 show the diffraction patterns of calcined and sintered ferrites for the different compositions. The formation of a second phase in the sintered Mg–Mn ferrite upon Gd substitution is identified by XRD analysis and it can be seen in Fig. 3. All the samples were found to match well with a cubic spinel structure. It was observed that Gadolinium iron oxide, GdFeO3 phase [ICDD 00-047-0067] is detected in all Gd substituted ferrites. The peak intensity of GdFeO3 increases with increasing Gd content. This could be attributed to the ionic radius of Gd3 þ (0.094 nm) which is larger than that of Fe3 þ ions (0.064 nm), and therefore, the amount of Fe3 þ ions substituted by Gd3 þ have a limit. Thus, remnant Gd3 þ ions will aggregate on the grain boundaries forming the GdFeO3 phase during the sintering process. GdFeO3 phase in the samples could help to improve the resistivity of ferrite since it behaves as an insulating layer at the grain boundary. 3.2. TEM microstructural analysis Fig. 4 shows the TEM images which represents the general
Fig. 2. XRD spectra of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite calcined powders (500 °C) of (a) x ¼0.0, (b) x¼ 0.025, (c) x ¼ 0.050, and (d) x¼ 0.075.
Fig. 3. XRD spectra of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite sintered pellets (1250 °C) of (a) x ¼0.0, (b) x¼ 0.025, (c) x ¼ 0.050, and (d) x¼ 0.075.
images of calcined powders for Mg0.9Mn0.1GdxFe1.8 xO4 ferrites. It is found that the shape of the particle is regular and is exhibited as an agglomerated monophase polycrystalline powder, and the particle sizes of the samples are homogeneously distributed, being in the range of 7–13 nm. It is observed that the particle sizes of the samples are consistent with the average crystallite size determined by the XRD data which is about 9–15 nm. 3.3. Densification and SEM observation
Fig. 1. XRD spectra of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite as burnt powder of (a) x¼ 0.0, (b) x¼ 0.025, (c) x ¼0.050, and (d) x¼ 0.075.
Fig. 5 exhibits the bulk density and porosity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites with different Gd contents. The density of the sintered specimen decreases upon Gd substitution, indicating a reduced densification with the substitution. This decrease would probably be due to an increased intragranular porosity resulting from discontinuous grain growth as described by Burke [6]. The morphological changes with an increasing substitution of Gd in Mg–Mn ferrites observed with FESEM are shown in Fig. 6. It is distinctly observed that the presence of Gd2O3 affect drastically the microstructure of Mg–Mn ferrite. The microstructural observation shows a decrease in the average grain size and an increase in porosity. The values of grain size data are given in Table 1. This decrease in average grain size can be explained as
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Fig. 4. TEM images of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite (a) x ¼0.0, (b) x ¼0.025, (c) x¼ 0.050 and (d) x ¼0.075
phase being the small whitish round grains at the grain boundary/ junction. 3.4. Magnetic properties
Fig. 5. Variation of density and porosity of sintered Mg0.9Mn0.1GdxFe1.8 xO4 ferrites with different Gd content (x¼ 0.0, 0.025, 0.050, 0.075).
follows. As the ionic radius of Gd is 0.938 Å, being larger than that of Fe3 þ ions 0.64 Å, hence when some Fe3 þ ions at the ferrite lattice site were substituted by some Gd3 þ ions, the lattice parameter will be changed due to the compression of the spinel lattice [7]. However, the variations of lattice parameters will lead to lattice strains which will produce the internal stress [8]. Such stresses could hinder the growth of grains and hence the grain sizes of the samples substituted with Gd ions become smaller than that of Mg–Mn ferrite nanocrystals. In this study, it is observed that the larger amount of Gd substitutions, the smaller are the grain sizes formed. The grain size is found to decrease from about 2–0.7 μm, i.e. about 60% reduction in size when Gd was added from x¼ 0 to 0.075. Specimen which was not substituted by Gd (Fig. 6a) shows the presence of a monophasic and homogeneous microstructure with an average grain size of ∼2 mm. However, Gd-substituted specimens (Fig. 6b–d) show a biphasic microstructure, the second
Fig. 7 presents the magnetic hysteresis loop of different Mg0.9Mn0.1GdxFe1.8 xO4 ferrites. The magnetization was obtained at room temperature under a magnetic field of 7 10 kOe field. The magnetic properties such as saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc) and magnetic moment of unit cell values of the different compositions with increasing Gd substitution are tabulated in Table 2. The saturation magnetization of substituted ferrites was lower than the unsubstituted ferrite and this might be due to the decrease in densification as discussed earlier. Consequently, the decrease of saturation magnetization with increasing Gd3 þ ions can be attributed to the replacement of Fe3 þ by non-magnetic Gd3 þ ions in B sites which leads to a decrease of magnetization of B-site and hence saturation magnetization, Ms, would decrease [9]. The coercivity of the samples increases with the Gd content. Generally, it can be mentioned that in this study, the formation of the secondary phase at the grain boundaries may affect the homogeneous composition and microstructure, and induce some distortion in the internal grain region which lead to larger internal stress. Al-Hilla et al. [10] had worked on gadolinium substituted Li–Ni ferrite. They observed that there was a limited solubility of Gd ion in the spinel structure of Li–Ni ferrite and Gd3 þ ions would precipitate as a secondary phase of GdFeO3 at the grain boundaries, which may inhibit the grain growth in the samples. In addition, it is known that the grain boundary increases with decreasing crystallite size. In this work, the occupation of the cations in the tetrahedral A site and octahedral B site were also calculated from the observed magnetic moment per unit cell and presented in Table 3. It can be observed that the decrease in magnetization is reflected from the ratio of cation occupation in the spinel lattice. The decrease in the fraction of Fe3 þ ion in B sites is responsible for the decrease in
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Fig. 6. SEM images of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite (a) x¼ 0.0, (b) x ¼0.025, (c) x¼ 0.050 and (d) x ¼ 0.075
Table 1 Bulk density, relative density and porosity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites by SCM. Composition (x) Bulk density (g/cm3) Relative density (%)
Apparent porosity (%)
x ¼ 0.0 x ¼ 0.025 x ¼ 0.050 x ¼ 0.075
2.56 8.43 14.39 15.11
4.26 4.13 3.93 3.89
96.01 94.61 91.59 90.98
Average grain size (mm) 2.01 1.26 0.98 0.79
Table 2 The value of Ms, Mr, Hc and nB for the Mg0.9Mn0.1GdxFe1.8 xO4 ferrites prepared by SCM with different Fe content. Compositions
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
nB (mB)
x ¼0.0 x ¼0.025 x ¼0.050 x ¼0.075
38.41 30.01 27.31 25.77
0.98 2.82 4.66 5.28
20.25 22.05 23.46 24.22
1.32 1.05 0.98 0.91
Table 3 Expected cation distribution in Mg0.9Mn0.1GdxFe1.8 xO4 ferrites prepared by SCM with different Fe content. Compositions (x)
Occupation of cations A site
x ¼0.0 x ¼0.025 x ¼0.050 x ¼0.075
2þ Mg0.188 þ Mg20.160 þ Mg20.155 þ Mg20.157
B site 3þ Fe0.812 þ Fe30.840 þ Fe30.845 þ Fe30.843
2þ Mg0.712 þ Mg20.740 þ Mg20.745 þ Mg20.743
2þ Mn0.1 Mn20.1þ Mn20.1þ Mn20.1þ
3þ Fe0.988 þ þ Gd30.025 Fe30.910 þ þ Gd30.05 Fe30.858 þ þ Gd30.075 Fe30.807
the domain wall motions, thus the coercivity of the samples increases with Gd content. This phenomenon is also observed by many researchers [12,13]. 3.5. Electrical properties
Fig. 7. The magnetization curves of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites with different compositions measured by VSM 7 10 kOe at room temperature
saturation magnetization [11]. In this study, Mg0.9Mn0.1GdxFe1.8 xO4 ferrites show a decrease in crystallite size with increasing Gd content. The area of disordered arrangement for atoms on grain boundaries may hinder
Dielectric constant as a function of compositions for Mg0.9Mn0.1GdxFe1.8 xO4 ferrites compact by SCM at a sintered temperature of 1250 °C is shown in Fig. 8. It is observed that all the samples show a dependence on the amount of Gd content, i.e the dielectric constant decreases with increasing amount of Gd. The observed higher values of εʹ with x ¼0.0 (Gd un-substituted) sample might be due to the presence of a higher number of Fe2 þ formed at sintering temperature of 1250 °C. However, there is a further observation in the insert of Fig. 8 where all the samples do not show significant frequency-dependent phenomenon i.e the
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Fig. 8. Variation of dielectric constant Mg0.9Mn0.1GdxFe1.8 xO4 ferrite sintered at 1250 °C.
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with
composition
for
permittivity maintains almost a constant value within 1 MHz– 1 GHz. At higher frequency, the εʹ values are usually found to be invariable with frequency. This is due to the fact that the electronic movements are not able to follow the rapid switching of the AC field at higher frequency. Dielectric losses as a function of frequency for Mg0.9Mn0.1GdxFe1.8 xO4 ferrites prepared by SCM at different compositions are shown in Fig. 9. It is found that the dielectric loss tangent decreases with increasing Gd content. The increase in RE oxide's resistivity may give rise to the reduction in tan δ, and this loss factor can be explained with the electrical resistivity (tan δ ¼1/ρ2πfεʹ) [14]. This formula shows that an increase in resistivity (ρ) will bring a reduction in tan δ at a given frequency. The lower values of dielectric loss obtained in this study can be attributed to the curtailing (reduction) of the Fe2 þ ions (Fe2 þ 2Fe3 þ þe ) in the system. It was found that there is no maximum dielectric loss within the measured frequency range up to 1 GHz due to excellent compositional control obtained in this solution combustion method. It is expected that the compositions are well controlled at the atomic scale by this method. This finding is in agreement with the study of the compositional stoichiometry on dielectric loss by Brockman [15]. They have reported the dielectric properties of stoichiometric Ni–Zn ferrites. The low dielectric constant (∼15) and low loss values were observed with careful control of stoichiometry of the compositions in their study. Resistivity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrites sintered at 1250 °C for different compositions is represented as a function of
Fig. 10. Variation of resistivity with frequency for Mg0.9Mn0.1GdxFe1.8 xO4 ferrite sintered at 1250 °C
Table 4 Resistivity of Mg0.9Mn0.1GdxFe1.8 xO4 ferrite prepared by SCM sintered at 1250 °C for the different Gd content. Composition
Resistivity (106 Ω-cm) at 1 MHz
x¼ 0.0 x¼ 0.025 x¼ 0.050 x¼ 0.075
0.38 17.09 19.50 25.74
frequency in Fig. 10 and the values measured at 1 MHz are listed in Table 4. Generally, high values of resistivity are observed in this study. The resistivity increases with Gd substitution. The conduction in ferrites arises mainly from the electron hopping between Fe2 þ and Fe3 þ ions that were located at the octahedral sites of the spinel ferrite structure. Sample x ¼0.075 shows the highest resistivity among all compositions, which may be attributed the presence of lower amount of Fe2 þ ions in the ferrites. In fact, there are less amount of Fe due to the substitution of Gd, which is good insulator in the compositions and thus higher resistivity in the Gd substituted samples. In this study, Gd substituted compositions were deficient in Fe content and the crystallization of GdFeO3 phase impeded the reduction of Fe3 þ ions inside the grains. Hence, the formation of Fe2 þ ions were expected to be at minimum in Gd substituted ferrites. This might be the reason for increased resistivity in these compositions. High resistivities are desirable to reduce eddy current losses encountered at high frequencies. Thus, the ferrites prepared in the present study are expected to be worthy for high frequency applications.
4. Conclusions
Fig. 9. Variation of Loss tangent with frequency for Mg0.9Mn0.1 GdxFe1.8 xO4 ferrite sintered at 1250 °C
The effect of Gd substitutions on physical and electromagnetic properties of Mg–Mn ferrites has been investigated. In this study, the substitution of Gd for Fe in (Mg0.9Mn0.1GdxFe1.8 xO4) ferrites indicated single phase cubic spinel structure along with the formation of the secondary phase GdFeO3. Bulk density and grain size of the ferrites decreased with increasing Gd substitution. The AC resistivities of substituted ferrites also increased from 0.38 106 to 25 106 Ω-cm with the increase of Gd content which can be explained in terms of grain growth and microstructure of respective samples. This is due to the formation of secondary phases at the grain boundary that inhibit the grain growth and increase the
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resistivity. The dielectric properties can be understood similarly to the AC resistivities. A slight solubility of Gd3 þ ions in the spinel matrix results a decrease of saturation magnetization. The increase in coercivity with increase in substitutions of Gd for Fe in the spinel structure was observed. The physical and electromagnetic properties of Mg–Mn ferrites are strongly affected by substitution of different Gd content.
Acknowledgment The authors are grateful to the Universiti Sains Malaysia for providing financial assistance under USM fellowship scheme to conduct this research. The authors also would like to acknowledge the “e‐Science Fund” (Grant no. 6013343) and MOSTI (03-01-05-SF 0375) for financial support.
References [1] B.M. Kunal, H.V. Nimish, K.L. Vinay, K.P. Tushar, J. Adv. Microsc. Res. 7 (2012)
139
40–43. [2] M.A. Khan, M.U. Islam, M. Ishaque, I.Z. Rahman, Ceram. Int. 37 (2011) 2519–2526. [3] J. Chand, M. Singh, J. Alloys Compd. 486 (2009) (2009) 376–379. [4] M. Naeem, N. Abbas Shah, I. Hussain Gul, A. Maqsood, J. Alloys Compd. 487 (2009) 739–743. [5] X. Li, G. Wang, J.Magn. Magn. Mater. 321 (2009) 1276–1279. [6] J.E. Burke, W.D. Kingery (Eds.), Ceramic Fabrication Processes, Wiley, New York, 1958, p. 28. [7] N. Rezlescu, E. Rezlescu, C. Pasnicu, M.L. Craus, J. Magn. Magn. Mater. 136 (1994) 319–326. [8] H. Yang, L. Zhao, X. Yang, L. Shen, L. Yu, W. Sun, Y. Yan, W. Wang, S. Feng, J. Magn. Magn. Mater. 271 (2004) 230–236. [9] S.E. Jacobo, S. Duhalde, H.R. Bertorello, J. Magn. Magn. Mater. 272 (2004) 2253–2254. [10] M.F. Al-Hilli, S. Li, K.S. Kassim, Mater. Chem. Phys. 128 (2011) 127–132. [11] M.U. Rana, M. Islam, T. Abbas, Mater. Lett. 41 (1999) 52. [12] L. Zhao, H. Yang, L. Yu, Y. Cui, W. San, Y. Yan, S. Feng, Phys. Status Solidi A 201 (2004) 3121–3128. [13] S.U. Romero, O.P. Pérez, G. Gutiérrez, J. Appl. Phys. (2010), http://dx.doi.org/ 10.1063/1.3338847, 09A508–09A508-3. [14] J. Sun, J. Li, G. Sun, J. Magn. Magn. Mater. 250 (2002) 20–24. [15] F.G. Brockman, R.P. White, J. Am. Ceram. Soc. 54 (1971) 183–187.