Structural and Dielectric Properties of Substituted Calcium Hexaferrites

Structural and Dielectric Properties of Substituted Calcium Hexaferrites

1. N. Christy1

   1Department of Physics,

   Hislop College, Civil Lines, Nagpur 440001, Maharashtra.

2. G. Rewatkar2

2Department of Physics,

Dr. Ambedkar College, Deeksha Bhoomi, Nagpur 440010, Maharashtra

P. S. Sawadp

3Department of Applied Physics, Bapurao Deshmukh College of Engineering,

Sewagram, Maharashtra

Abstract- Calcium hexaferrites have a narrow size of particle size distribution and offers a promising material for industrial applications because of its good magnetic, electrical, mechanical and magneto optical properties along with perfect thermal and chemical stability. In this paper we have studied the structural and dielectric properties of Calcium hexaferrites substituted with Cobalt (Co2+) and Zirconium (Zr4+). The substituted Calcium hexaferrites have been synthesized by microwave assisted sol-gel auto combustion method. The synthesized samples were analysed by X-ray diffraction studies and the samples belong to Space Group P63/mmc and are single-phase magneto-plumbite (M) structure. The samples have a better signal-to-noise ratio, as the particle size was less than 50nm. Transmission electron microscopy (TEM) shows the formation of hexagonal platelets. The dielectric constant decreased with the increase in frequency, is a traditional dispersion behaviour due to the lagging of space charge carriers. The samples can be used for higher frequency applications.

Keywords-M-type hexaferrites, Sol-gel auto combustion process, Transmission electron spectroscopy, dielectric studies.

1. INTRODUCTION

   Ferrites are most widely used advanced magnetic materials. These are hard, brittle, ceramic like materials and regarded as structure sensitive materials owing to their much higher values of electrical resistances than metals. Since the special properties of nanomaterials are strongly size- dependent, the prerequisite for their practical application is gaining reliable control over their particle size and shape in the production phase.

   After the discovery of hexaferrites, it has been continuously studied by researchers and they find their applications in microwave devices, micro strip antennas, high frequency transformers, memory core, radar devices and high-density recording media. Mostly researched M-type hexagonal ferrites are BaFe12O19 and SrFe12O19. They are known for their high uniaxial magneto crystalline anisotropy with the easy axis of magnetization along the hexagonal c- axis and their chemical stability [1].

   Our earth is rich in Calcium when compared to Barium and Strontium. Calcium is cheaper than barium and strontium. M-type calcium hexaferrites is a candidate material for low cost permanent magnets.

   Calcium hexaferrites been a subject of interest due to narrow range of particle size distribution. It offers a promising material for industrial applications because of its good magnetic, electrical, mechanical and magneto-optical properties along with perfect thermal and chemical stability. The electric and magnetic properties of these hexaferrites can be tuned by making suitable metallic substitutions for iron (Fe3+) with paramagnetic and diamagnetic cations.

   Lotgering, F. K. (1980) [2], Kobayashi, Y. (2008) [3], T. Kikuchi (2011) [4]have studied the characteristics of M-type Calcium hexaferrites doped with La or La and Co by different methods of synthesis. S. V. Blazhevich (2011) [5], in his paper have investigated and aimed at preparing fine grained crystals of calcium hexaferrite. They have justified that Calcium hexaferrites can be used for medical applications as it has better biological compatibility with other base compositions than alkaline earth hexaferrites.

   Influence of the substitution of the dopants in the structure of M-type hexaferrites of Barium and Strontium have been extensively studied by several researchers continuously and systematically. Buzinaro (2016) [6] has mentioned that when the dopants are added to the Fe3+ sites there was a decrease in hematite content and also the reduction of Fe2O3 secondary phase.

   M. J. Iqbal (2010) [7] had doped Calcium with Strontium as Calcium belongs to the same group in the periodic table and has the same electronic configuration as that of Barium and Strontium. They studied the electrical properties due to the substitution of Calcium in Strontium hexaferrites. Also, Ihsan Ali (2013) [8] in his paper has mentioned that Calcium has a better control of the grain growth and improves the coercivity of the hexaferrites.

   After a detailed literature survey, only a few literatures have reported on Calcium hexaferrites as mentioned in the previous paragraph. In this paper we have synthesized Ca(Co- Zr)x Fe12-2xO19 (x=0.2 and 1.0) nanoparticles using microwave assisted sol-gel auto combustion technique and studied the effect of substitution of Cobalt and Zirconium in the Fe3+ sites

   and studied the structure and electrical properties of the Co and Zr substituted Calcium hexaferrites.

2. EXPERIMENTAL AND CHARACTERIZATION The samples were synthesized using microwave

   assisted sol-gel technique as it requires low temperature and no intermediate phases will be obtained and it will meet the stringent requirements of the fineness and morphology of the particles and optimal size. [9], [10] [11] [12] [13]. According to the composition, stoichiometric amounts of high purity AR grade (Merck) Calcium nitrate, Cobalt nitrate, Zirconyl nitrate and Ferric nitrate were dissolved one by one in about

   30 ml of deionized triple filtered water. A proportionate amount of urea was added to the mixture while stirring to be used as a reducing agent to supply requisite energy to initiate exothermic reaction amongst oxidants. All these mixtures were kept for constant stirring on a magnetic stirrer hot plate between 60oC to 70oC to avoid precipitation. The solution was in the form of a homogeneous aqueous solution and then turned into a viscous gel. The gel was placed in the microwave oven, followed by an instantaneous gel ignition. After the water evaporates from the gel, the wet gel reaches the combustion point. It starts to burn and became a solid when the temperature is about 1000oC. The combustion is not complete until all the substances were burnt off and the resulting ashes were loose and fluffy and Cobalt and Zirconium substituted Calcium hexaferrites were formed. The precursor materials were then calcined in air in a temperature programmed furnace at 800oC for 4 hours at a heating rate of 5oC/min. At this temperature, we get a pure M-type phase hexaferrite. The precursor material was ground into a fine powder using pestle and mortar. The powdered samples were pressed at 10 KN/m2 to form pellets of 6.5 mm diameter and

   ~2 to 2.5 mm thickness which were used in measurements for dielectric properties.

   The X-ray diffraction (XRD) technique was used to investigate the structure of the prepared samples using the Bruker AXS D8 Advance, Cu, Wavelength 1.5406 Ã… (SAIF, COCHIN). Electrical properties of the synthesized samples were studied using Precision Impedance Analyzer Wayne Kerr 6500B. The transmission Electron Microscope micrographs were taken by Jeol/JEM 2100(SAIF, COCHIN).

3. RESULTS AND DISCUSSION

3.1 XRD ANALYSIS

The formation of a single-phase hexagonal magnetoplumbite structure of the substituted Co2+ and Zr4+ in the M-type Calcium hexaferrites was confirmed by X-ray diffraction analysis. The X-ray diffraction patterns were indexed on the basis of magneto plumbite structure and space group P63/mmc (No.194) was retained (Figure 1). The structure of the synthesized Calcium hexaferrites substituted with Cobalt and Zirconium has been already discussed in our previous paper [14]. In our previous paper we had calculated the crystallize size using the well-known Scherrers equation. This equation uses the width of the diffraction peak to calculate the crystallite size. K. Venkateswarlu [15] have reported that the formula underestimates or provides only

lower bound to the size of the crystallite as it has not taken account of peak broadening resulting from inhomogeneous strain and instrumental effects. Also, in nanomaterials the strain affects the measurement of crystallite size correctly. So, in this paper we have calculated the crystallite size and strain in the lattice using Williamson-Hall plot to the XRD peaks. The W-H equation is given by equation (1) [16] which is from the uniform deformation model(UDM). In this model the strain is assumed to be uniform in all the crystallographic directions [17].

(1)

where, is the full width at half maximum, D is the crystallite size, is the elastic strain, is the wavelength of X-rays and is Braggs diffraction angle.

Figure 1: XRD patterns for CaFe12-2x(Co Zr)xO19 (x=0.2 and 1.0)

The W-H plots for x=0.2 and x=1.0 are shown in figure 2(a) and 2(b). The lattice strain was calculated from the ordinate intercept and the crystallite size from the slope. From table 1 the particle size was estimated to be 48.6 nm and 35.3nm for x=0.2 and x=1.0 respectively and accordingly the calculated tensile strain in the lattice has increased. The particle size slightly decreased with the substitution of Cobalt and Zirconium in the hexaferrite lattice. Also there was an increase in strain is due to the Co2+ and Zr4+ ions getting accommodated in Fe3+ sites in the Calcium hexaferrite lattice. The calculated lattice strain is a measure of the variation of the calculated lattice constants arising due to crystal imperfections such as crystal dislocation and the other sources are due to grain boundary, contact or sinter stresses and stacking faults as mentioned by Ghulam M. Mustafa [18]. Sunil Kumar in his paper has also reported similar result that when Ca2+ was added to BaFe12O19 the strain in the crystal lattice increased and particle size for different concentrations slightly decreased for increasing concentrations [19].

The synthesized samples exhibited a constant lattice parameter a and a varying c, suggesting that the change of easy magnetised c-axis is larger than the a axis. The

addition of cations have expanded the lattice due to the interaction with neighbouring atoms affecting bond length hence causing variation in the expansion of lattice [20]. Also in our synthesized material, Co2+ and Zr4+ has larger ionic radii 0.72 Ã… and 0.80 Ã… respectively when compared to ionic radii of Fe3+, which would have caused the increasing strain in the lattice without disturbing the crystal structure and confirming that the substituted cations have entered the crystal lattice. From the figure 2(a) and 2(b) the calculated value of the slope is positive suggesting the strain is tensile [21].

Table 1: Particle size, strain calculated from the Williamson Hall plots for CaFe12-2x(Co Zr)xO19.

Figure 2(a): Plot of cos Vs. 4sin for CaFe12-2x(Co Zr)xO19 (x=0.2)

3.3 Dielectric studies

Dielectric properties provide an idea about the behaviour of electrical charge carriers. It represents the inherent ability of the material to withstand an applied voltage without undergoing any structural degradation or becoming electrically conducting. The dielectric loss tangent is the amount of energy loss during each cycle. M-type hexaferrites possess low dielectric strengths because of their high resistivity. The variation of dielectric constant, tan and dielectric loss with frequency in the range 1000 Hz to 1MHz was studied.

The dielectric constant, dielectric tangent loss factor and dielectric loss were calculated using the following relations: [22]

= (2)

where is the dielectric constant, C is the capacitance of the pellet in farad, d is the thickness of the pellet in metres, A is the cross-sectional area of the flat surface of the pellet and 0 is the permittivity of free space

tan = (3)

where, tan is the dielectric tangent loss factor, is the loss angle, Rpis the equivalent parallel resistance, Cpis the equivalent parallel capacitance and f is the frequency.

= tan (4)

Where, is the dielectric loss.

The dielectric constant, dielectric tangent loss factor and dielectric loss at 1kHz and 10kHz are tabulated in table 2.

Table 2: Dielectric constant, tan , dielectric loss of Ca Fe12- 2x(Co Zr)x O19 as a function of frequency(x=0.2 and 0.8):

Figure 2(b): Plot of cos Vs. 4sin for CaFe (Co Zr O

(x=1.0)

12-2x )x 19

3.2 Transmission Electron Microscopy analysis

From the figure 3, the particles display a hexagonal nature of the synthesized materials. There were agglomeration and clusters formed and it is due to the increase in the grain size when the substitutions were increased. The crystal lattice and morphology of the Co2+and Zr4+substituted Calcium hexaferrites was not affected by the small substitutions.

Figure 3: TEM images of CaFe12-2x(Co Zr)xO19 (x=0.2,1.0)

Figure 4(a): Variation of Dielectric constant with applied frequency for Ca Fe12-2x(Co Zr)x O19(x=1.0)

Figure 4(b): Variation of Tan with applied fequency for Ca Fe12-2x(Co Zr)x O19(x=0.2)

The frequency dependence of dielectric constant for Cobalt and Zirconium substituted Calcium hexaferrite samples (x=0.2 and 0.8) were studied at room temperature in the frequency range 1000Hz to 1MHz. The samples showed a high dielectric constant at lower frequencies and there was a sharp decrease when the frequency was increased. This is the usual behaviour of ferrite materials. Many researchers while studying the dielectric properties of M-type hexaferrites have obtained similar results [23] [24] [25] [26]. Figure 4(a) and 4(b) gives the variation of dielectric constant and tan with frequency respectively. From table 2, the dielectric constant, dielectric tangent loss factor and dielectric loss for the synthesized samples was high at lower frequencies.

The dielectric behaviour of the synthesized samples is a traditional dispersion behaviour due to the lagging of the space charge carriers. It is also similar to conduction mechanism which rises from the electronic exchange between Fe3+ and Fe2+ ions [27]. Electrical polarization is due to exchange of electrons between Fe3+ and Fe2+ ions. When the frequency of the external applied is increased the exchange of electron cannot follow the alternating field thereby decreases the dielectric constant. This is because of the low relaxation time and the space charge carriers are not able to align themselves in the direction of the field at high frequencies and polarization could not achieve saturation value [28].

The observed result was similar to the studies on Zr- Cd Strontium hexaferrites investigated by M. N. Ashiq (2009) [22] and Chetna C. Chauhan [29]. They had synthesized the same material using simple heat treatment method. The observations are similar to the results we obtained i.e. at lower frequencies there were more of dielectric dispersion. According to Koops theory, the initial dielectric constant at 1000Hz is due to the high resistivity at the grains boundary and at higher frequencies it is due to the grains having small dielectric constant values due to low resistivity. The high dielectric values are due to defects in the material like voids, dislocation etc. M. N. Ashiq has mentioned that the materials with these behaviors can warrant their application at high frequencies. This is because high values of dielectric constant decrease the penetration depth of the electromagnetic waves by increasing the skin effect.

The dielectric constant of the Cobalt-Zirconium substituted Calcium hexaferrites decreases as the substitution increases. The presence of Co2+ ions gives rise to p type carriers. Their presence contributes to the net polarization in addition to the n type carriers from Fe3+ ions. But it is very much smaller as its mobility is less than the mobility of the n type carriers. So, the contribution to the polarization decreases rapidly even at low frequencies. Also, the values of dielectric constant were lower in the synthesized samples.

The dielectric constant, dielectric loss tangent decreased swiftly with the increasing frequency. As the values of dielectric constant are lower in the synthesized materials it can be used at higher frequencies. Dielectric loss is an important part of the total core loss in ferrites. For low core loss, low dielectric losses are desirable. The dielectric loss profiles are similar to those of the real part of dielectric constant.

1. CONCLUSIONS

   Cobalt and Zirconium substituted Calcium hexaferrites were synthesized by microwave assisted auto combustion sol-gel method. The synthesized samples had their particle size less than 50 nm when calculated from the W-H plots. The synthesized materials can find their use in high-density recording media, since a reasonable signal-to- noise ratio can be obtained. The dielectric constant and dielectric loss tangent decreased with increasing frequency suggesting that these nanomaterials can be used for high frequency applications.

2. REFERENCES

1. P. D. Popa, E. Rezlescu, C. Doroftei and N. Rezlescu, "Influence of Calcium on properties of Strontium and Barium ferrites for magnetic media prepared by combustion," Journal of Optoelectronics and Advanced Materials, pp. 1553-1556, 2005.

2. F. K. Lotgering and M.A.H.Huyberts, "Composition and magnetic properties of hexagonal Ca, La ferrite with magnetoplumbite structure," Solid State Communications, vol. 34, no. 1, pp. 49-50, 1980.

3. Y.Kobayashi, S.Hosokawa, E.Oda and S.Toyota, Journal of the Japan Society of Powder and Powder Metallurgy, vol. 55, 2008.

4. T. Kikuchi, T. Nakamura, T. Yamasaki, M. Nakanishi, T. Fujii, J. Takada and Y. Ikeda, IOP Conf. Series: Materials Science and Engineering ) , vol. 18, 2011.

5. S. Blazhevich, L. Olkhovik, A. Kamzin, S. Chernikov, T.G.Kuzmicheva and N.V.Tkachenko, Protection of Metals and Physical Chemistry of Surfaces, vol. 47, no. 5, pp. 638-644, 2011.

6. S. K. Chawla, R. K. R.K. Mudsainiyan, S. S. Meena and S. M. Yusuf, "Solgel synthesis, structural and magnetic properties of nanoscale M-type barium hexaferrites BaCoxZrxFe(12 2x)O19," Journal of Magnetism and Magnetic Materials, vol. 350, pp. 23- 29, 2014.

7. A. Ihsan, M. Islam, M. S. Awan, A. Mukhtar, M. N. Ashiq and S. Naseem, "Effect of Tb3+ substitution on the structural and magnetic properties of M-type hexaferrites synthesized by solgel auto-combustion technique," Journal of Alloys and Compounds, vol. 550, pp. 564-572, 2013.

8. Y. S. Hong, C. M. Ho, H. Y. Hsu and C. T. Liu, "Synthesis of nanocrystalline Ba(MnTi)xFe12 2xO19 powders by the solgel combustion method in citrate acidmetal nitrates system (x=0, 0.5, 1.0, 1.5, 2.0)," Journal of Magnetism and Magnetic Materials, vol. 279, no. 2-3, pp. 401-410, August 2004.

9. W. Zhong, W. Ding, N. Zhang, J. Hong, Q. Yan and Y. Du, "Key step in synthesis of ultrafine BaFei2019 by sol-gel technique," Journal of Magnetism and Magnetic Materials, vol. 168, pp. 196- 202, 1997.

10. N. Rezlescu, C. Doroftei, E. Rezlescu and P. Popa, "The influence of heat-treatment on microstructure and magnetic properties of rare-earth substituted SrFe12O19," Journal of Alloys and Compounds, vol. 451, pp. 492-496, 2008.

11. J. N. Christy, K. G. Rewatkar and P. Sawadh, "Structural and Magnetic Behavior of M-type Co-Zr Substituted Calcium Hexaferrites," Materials Today: Proceedings, no. 4, p. 11857 11865, 2017.

12. K. Venkateswarlu, A. Chandra Bose and N. Rameshbabu, "X-ray peak broadening studies of nanocrystalline hydroxyapatite by WilliamsonHall analysis," Physica B, vol. 405, pp. 4256-4261, 2010.

13. K. Sunil, S. Supriya, R. Pandey, L. K. Pradhan, R. K. Singh and

    M. Kar, "Effect of lattice strain on structural and magnetic properties of Ca substituted barium hexaferrite," Journal of Magnetism and Magnetic Materials , Vols. 458, , pp. 30-38 , 2018.

14. V. D. Mote, Y. Purushotham and B. N. Dole, "Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles," Journal of Theoretical and Applied Physics, vol. 6, no. 6, pp. 1-8, 2012.

15. G. M. Mustafa, S. Atiq, S. K. Abbas, S. Riaz and S. Naseem, "Tunable structural and electrical impedance properties of pyrochlores based Nd doped lanthanum zirconate nanoparticles for capacitive applications," Ceramics Internation, vol. 44, no. 2, pp. 2170-2177, 2018.

16. S. Kumar, S. Supriya, R. Pandey, L. K. Pradhan, R. K. Singh and

    M. Kar, "Effect of Lattice Strain on Structural and Magnetic Properties of Ca Substituted Barium Hexaferrite," Journal of Magnetism and Magnetic Materials, vol. 458, pp. 30-38, 2018.

17. T. Kaur, S. Kumar, B. H. Bhat and A. K. Srivastava, "Enhancement in physical properties of barium hexaferrite with substitution," Journal of Materials Reseach, vol. 30, no. 18, pp. 2753-2762,2015.

18. M. Sharma, S. C. C. Kashyap, M. C. Dimri and K. Asokan, "Enhancement of Curie temperature of barium hexaferrite by dense electronic excitations," AIP Advances, vol. 4, no. 7, 2014.

19. M. N. Ashiq, M. J. Iqbal and I. H. Gul, "Structural, magnetic and dielectric properties of ZrCd substituted strontium hexaferrite (SrFe12O19) nanoparticles," Journal of Alloys and Compounds, vol. 487, pp. 341-345, 2009.

20. A. Kumar, S. S. Yadava, P. Gautam, A. Khare and K. D. Mandal, "Magnetic and dielectric studies of barium hexaferrite (BaFe12O19) ceramic synthesized by chemical route," Journal of Electroceramics, pp. 1-10, 2018.

21. R. A. Nandotaria, R. B. Jotania, C. S. Sandhu, M. Hashim, S. S. Meena, P. Bhatt and S. E. Shirsath, "Magnetic interactions and dielectric dispersion in Mg substituted M-type Sr- Cu hexaferrite nanoparticles prepared using one step solvent free synthesis technique," Ceramics International, vol. 44, pp. 4426-4435, 2018.

22. S. Mansour, O. Hemeda, M. Abdo and W. Nada, "Improvement on the magnetic and dielectric behavior of hard/soft ferrite nanocomposites," Journal of Molecular Structure, vol. 1152, pp. 207-214, 2018.

23. R. A. Khann, S. Mir, A. M. Khan, B. Ismail and A. R. Khan, "Doping magnesium ion to tune electrical and dielectric properties of BaCo2 hexaferrites," Ceramics International, vol. 40, pp. 11205-11211, 2014.

24. S. Pervin, M. M. Rahman, F. Ahmed and M. A. Hakim, "Investigation of magnetic, dielectric and electrical properties of Ba-hexaferrites," Indian Journal of Physics, vol. 86, no. 12, pp. 1065-1072, December 2012.

25. M. Z. Ansar, S. Atiq, K. Alamgir and S. Nadeem, "Frequency and Temperature Dependent Dielectric Response of Fe3O4 Nano- crytallites," Journal of Scientific Research, vol. 6, no. 3, pp. 399- 406, 2014.

26. C. C. Chauhan, A. R. Kagdi, R. B. Jotania, A. Upadhyay, C. S. Sandhu, S. E. Shirsath and S. S. Meena, "Structural, Magnetic and Dielectric Properties of Co-Zr Substituted M-type Calcium Hexagonal Ferrite Nanoparticles in the Presence of -Fe2O3 Phase," Ceramics International, vol. 44, no. 15, pp. 17812-17823

, 2018.