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Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating
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Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating A.V. Malyshevn, E.N. Lysenko, V.A. Vlasov Tomsk Polytechnic University, Lenina Avenue 30, 634050 Tomsk, Russia Received in revised form 25 June 2015, 27 July 2015; accepted 27 July 2015
Abstract Polycrystalline zinc substituted lithium ferrites (lithium–zinc ferrites) with the chemical formula Li0.4Fe2.4Zn0.2O4 were prepared by heating the mixture using a high-energy beam of electrons accelerated to energy of 2.4 MeV. The sintering temperature and time were 1100 1C and 2 h, respectively. The microstructure of the samples was investigated by XRD and SEM analyses, and the density and porosity were determined by hydrostatic weighing. The magnetic (saturation magnetization, the Curie temperature) and dielectric (electrical conductivity, frequency dependence of the dielectric constant and dielectric loss tangent) properties for lithium–zinc ferrites were studied. The results were compared with the results obtained for the samples prepared for the samples in compact pallets form by conventional ceramic technology through heating the mixture in a resistance furnace. The XRD analysis confirmed the formation of the spinel structure of the produced ferrite samples. The results of the study show that the samples sintered using radiation-thermal heating exhibit a higher density and less porosity. These samples are characterized by lower electrical resistivity, higher dielectric losses and high saturation magnetization. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Sintering; C. Magnetic properties; C. Dielectric properties; D. Ferrites; Radiation-thermal heating
1. Introduction Lithium and lithium-substituted ferrites are an issue of permanent interest to researchers due to their high practical relevance as an inexpensive magnetic material of microwave technology, which is characterized by high values of electrical resistivity, the Curie temperature, low dielectric and magnetic losses [1–3]. It is well known that lithium ferrite provides active interaction with electromagnetic waves at the low frequency band of the microwave range. For this purpose, to increase the saturation magnetization in LiFe5O8 lithium ferrite, partial substitution with Zn2 þ ions is performed [4–6]. As a result of this substitution, the electromagnetic wave absorption increases due to a drop in electrical resistivity and increase in dielectric constant of ferrites [7–11]. The prepared materials are in agreement with the general formula Li0.5(1 x)ZnxFe2.5 0.5xO4 with a lattice parameter of E8.35 Å. However, as the zinc n
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content increases (xZ0.4), the saturation magnetization sharply decreases [12–14]. The ceramic method is currently considered to be most common to produce lithium and lithium-substituted ferrites. It involves two-stage high-temperature heating of reagent mixtures including synthesis of ferrites with the formation of single-phase ferrite compositions and further sintering at a higher temperature to produce high-density ferrite ceramics [2,3]. The main disadvantage of the ceramic method is high probability of the presence of unreacted oxides and intermediate products in the composition of the sintered ferrites. These defects, as well as the porosity of the material, create fields of elastic stresses which distort the magnetic anisotropy of ferrite and thus result in deterioration of its magnetic characteristics. To eliminate these disadvantages, high temperature and long synthesis and sintering time are used, and this leads to volatilization of Li2O from the samples, thereby deteriorating the electromagnetic properties of the fabricated lithium–zinc ferrite [15]. Therefore, the efficiency of lithium ferrite material production is being constantly improved. It includes mechanical activation of the initial
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Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
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reagents [16,17] and sol–gel and citrate-gel technologies used to produce nanostructured lithium–zinc ferrites [18–21]. The magnetic and dielectric properties of these lithium–zinc ferrites are well-known. In [22–28], it is shown that one of the effective ways to improve the homogeneity and to intensify solid–phase interactions is radiation-thermal (RT) synthesis of ferrite materials when exposed to the accelerated electron beam. It was found that radiation-thermal heating of the initial reagent mixture significantly increases the reactivity of the solid-phase system, which reduces the synthesis temperature and increases the homogeneity of the end product. According to the results of the investigation of Li0.4Fe2.4Zn0.2O4 lithium–zinc ferrite synthesized by heating of mechanically activated initial Li2CO3–ZnO–Fe2O3 reagent mixture by a high-energy electron beam with subsequent high-temperature sintering in the laboratory furnace [28], the use of the composite based on this material as an absorbing coating in the microwave frequency is considered to be promising. However, along with the studies performed in this area, the main properties of lithium–zinc ferrites with the chemical formula Li0.5(1 x)ZnxFe2.5 0.5xO4 prepared by radiation-thermal heating have not been studied sufficiently. This applies especially to their electromagnetic and dielectric properties which determine the propagation of electromagnetic waves in the material. This paper presents the results of the research in the structural and electrical properties of lithium–zinc ferrite sintered by highenergy electron beam heating. For the comparative analysis of the obtained results, ferrite sintering was carried out by two methods: conventional thermal (T) heating and radiation-thermal (RT) heating by high-energy electron beams. 2. The object of the study and experimental technique To prepare Li0.4Fe2.4Zn0.2O4 lithium–zinc ferrite, the reactants used were: Fe2O3, Li2CO3, ZnO industrial powders, which were premixed and mechanically activated in the AGO-2S planetary mill (Novic, Russia) using steel grinding jars and balls at room temperature for 60 min. The weight ratio of the material to the balls was 1:10, and the ball mill rotation speed was 2220 rpm. According to the results in [26], this mode of mechanical activation makes possible to produce lithium–zinc ferrites of high phase homogeneity. After mechanical activation, the samples were pressed into compact pallets with a diameter of 15 mm and a thickness of 2 mm using the method of single-action cold compaction. The compaction pressure was 200 MPa. Thermal synthesis of the samples was carried out in the laboratory resistance furnace at 800 1C in air for 120 min. Before sintering, the synthesized samples were milled, mixed with a binder (12% aqueous polyvinyl alcohol solution) and pressed into compact pallets. For T and RT sintering, the samples were divided in two parts. Both sintering processes were performed at a temperature of 1100 1C for 140 min. Note that higher temperatures for sintering lithium ferrites are not desirable due to the volatilization of lithium and zinc. T sintering of the samples was carried out in the laboratory high temperature resistance furnace. RT sintering was performed under the same temperature-time
Fig. 1. Structural scheme of the cell for ferrite sintering in the accelerated electron beam. Upper cover made of fireclay (1), insulator made of fireclay (2), case of the stainless steel cell (3), thermocouple junction (4), control sample (5), fireclay heat shields (6), electron beam (7), compacted samples (8), ceramic tube support (9), ground wire of the thermocouple measuring junction (10).
conditions with the pulsed electron accelerator ILU-6 [29]. The electron energy was equal to 2.4 MeV, the beam current pulse was 400 mA, the pulse duration was 500 μs and the pulse repetition frequency was 12.5–25 Hz. The average radiation dose rate in the isothermal mode was 5 kGy/s/s. Within a single pulse, the dose was equal to 800 kGy/s. The cooling rate for T and RT heating was 20 1C/min. T and RT sintered samples were 12 mm in diameter and 1.5 mm in thickness. The samples were irradiated in air in the insulated cell (Fig. 1) made of lightweight fireclay 1 with the mass thickness of the horizontal plates equal to 0.16 g/cm2. The experimental cell was size of 200 mm in length, 120 mm in width and 60 mm in height. The electron energy loss in the upper cover of that thickness did not exceed 8% and could be neglected. Sintered samples 7 inside the cell were placed on a thin plate made of 3 mm lightweight fireclay located on ceramic tube 8. The temperature was controlled with thermocouple 3 (type S), its measuring junction being located in test sample 4 placed in the immediate vicinity of the samples. The thermo-EMF values of the thermocouple were used for computer control of the thermal sintering program under varying electron pulse repetition rate. The phase composition and lattice parameters of the samples were determined by XRD analysis with the diffractometer ARL X'TRA (Switzerland). The diffraction patterns were measured with Cukα-radiation in the range of 2θ¼ 10–901 at a scanning speed of 0.021/s. Phase identification was carried out using powder database PDF-4þ of the International Center for Diffraction Data (ICDD). The experimental diffraction patterns were processed using the program PowderCell 2.4. The density and open porosity of the ceramic samples were measured by hydrostatic weighing using Shimadzu AUW 220D high-precision analytical balance. The electronic micrographs from the cleavage surface of the ceramic samples were made using the SEM Philips 515 scanning electron microscope. The saturation magnetization, Ms, was measured at room temperature with the vibrating sample magnetometer with the maximum field of 10 kOe. The Curie temperature of the samples was measured by thermomagnetometry method, which is the thermogravimetric TG/DTG analysis of samples in the magnetic field [30]. TG/
Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
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DTG measurements were performed with the STA 449C Jupiter thermal analyzer (Netzsch, Germany). The dielectric parameters: resistivity ρ, dielectric constant ε0 and dielectric loss tangent tgδ were measured by two-electrode method using the LCR-819 Meter. Silver electrodes were deposited on the sample surface (sample thickness of 0.24 mm) through thermal evaporation in vacuum; the diameter of the measuring electrode was 5 mm. The measurements were carried out in the 12–106 Hz test signal frequency range at room temperature. 3. Results and discussion The X-ray pattern for the synthesized samples is shown in Fig. 2. According to the results of the XRD analysis, all the samples were single-phase and corresponded to the chemical formula of lithium zinc ferrospinel with Li0.4Fe2.4Zn0.2O4 composition. The value of the lattice parameter was 8.357 Å, and it is close to the values corresponding to Li0.4Fe2.4Zn0.2O4 [12,21]. The ferrite crystallite size was calculated using the Williamson–Hall method, and it amounted to 103 nm. After T and RT sintering, the phase composition of lithium– zinc ferrites remained unchanged. The structural parameters for these samples are shown in Table 1. It indicates a slight increase in the lattice parameter and a decrease in the size of crystallites for samples prepared by RT sintering. Fig. 3 shows micrographs for lithium–zinc ferrites produced by T (Fig. 3(a)) and RT sintering (Fig. 3(b)). The ferrite ceramic structure is seen to be polycrystalline with well-formed grains. However, the porosity and the grain size are found to be different. The samples prepared by RT sintering have a higher grain size and lower porosity compared to the samples fabricated by T sintering. The microstructure of the RT samples is characterized
Fig. 2. X-ray diffraction pattern of the lithium zinc ferrospinel after synthesis. Symbols are the calculated curve for the reflection from Li0.4Fe2.4Zn0.2O4 and the solid line is an experimental XRD pattern.
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by the development of secondary recrystallization when the grain size is observed to be sharply different. The average grain sizes were calculated by intercept method and found to vary from 1.6 μm for T samples to 4 μm for RT samples. The data obtained by hydrostatic weighing of the samples is consistent with the results of the microstructural analysis. It was found that after RT sintering, the ceramics is denser and less porous compared to that produced by conventional thermal sintering. We analyzed the role of possible radiation mechanisms in the discovered effect in the literature. The stimulating effect of electron heating on ferrite ceramic sintering can be explained in terms of the surface-recombination mechanism of high-temperature radiationinduced mass transfer in ion structures proposed in [31]. The mechanism of the process is as follows. The regions of structural failure in heterogeneous structures (ceramics) are characterized by higher rate of nonradiative electron–hole and exciton recombination compared to that in the volume that causes local temperature gradients, defects and stresses. This process intensifies the mass transfer at the interphase boundaries, and as a result, it can accelerate ferrite ceramic sintering. According to the data in Table 1, the saturation magnetization value is similar for all the samples, and it equals to 70 emu/g. For samples of lithium–zinc ferrite prepared by T sintering, the value of electrical resistivity is high, and it is equal to 104 Ω cm. However, in samples prepared by RT sintering, ρ decreases by an order of magnitude due to the reduced porosity and increased density of the sample. According to thermogravimetric measurements in the magnetic field (Fig. 4), the Curie temperature varies within 508– 509 1C for all the samples, and these values correspond to Li0.4Fe2.4Zn0.2O4 lithium–zinc ferrite [30]. Note that the Curie temperature value is sufficiently high, and this indicates satisfactory thermal stability properties of ferrite. Fig. 5 shows the frequency dependence of the lithium–zinc ferrite dielectric characteristics. The dielectric constant is characterized by high dispersion caused by relaxation polarization in the investigated frequency range. As the frequency increases, the dielectric constant decreases for all the samples. For samples prepared by T sintering, ε0 decreases rapidly at lower frequencies and slows at high frequencies, which, according to [32], is a normal dielectric behavior. However, a sharp drop in ε0 values in the samples prepared by RT sintering is shifted to higher frequencies. Positively charged ions (Me3 þ , Me2 þ ) are always present in ferrite. Weakly bound electrons are grouped around these ions (or complexes) due to the Coulomb interaction. Thermal motion can make these electrons move from one ion to another, and Me3 þ ion, as a result of the transition of one of the weakly bound
Table 1 Structural and electromagnetic properties of lithium–zinc ferrite. Sintering type
Lattice parameter (Å)
Crystallite size (nm)
Density (g/ cm3)
Porosity (%)
Average grain size Saturation magnetization Ms ρ at T¼ 20 1С (μm) (emu/g) (Ω cm)
T RT
8.355 8.356
148 128
4.16 4.37
10.5 5.4
1.6 4
69.3 70.3
2.6 104 4.3 103
Curie temperature (1С) 509 508
Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
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Fig. 3. SEM micrographs of lithium–zinc ferrite samples prepared by T (a) and RT (b) sintering.
Fig. 4. TG and DTG curves for lithium–zinc ferrites prepared by T (a) and RT (b) sintering.
trivalent ion is equivalent to the interchange of the position of these ions. Under an external electric field, the electron transition occurs mainly along the field direction. Thus, the process of electron transitions Me3 þ þ e2Me2 þ causes polarization of the relaxors in the form of oppositely charged ion pairs. The greater number of ion vacancies and weakly bound electrons in ferrite, the greater number of electric dipoles formed and, consequently, the higher ferrite dielectric constant. As the temperature increases and/or the test signal frequency decreases, the distance which bound electrons can travel from the ions increases. As a result, the polarizability of ferrites and, therefore, the dielectric constant increase. As can be seen in Fig. 5, in all the measured frequency range, the ε0 values of the ferrites prepared by RT sintering are higher compared to the values obtained for ferrites produced by T sintering. It is obvious that the drop in resistivity in the samples prepared by RT sintering leads to an increase in dielectric losses in these ferrites, and it can be observed at frequencies up to 105 Hz. The obtained results show that lithium–zinc ferrite samples prepared by RT sintering are characterized by the properties that satisfy the requirements of their further use as an absorbing material: high specific magnetization values in combination with low resistivity values which provide high dielectric losses [28].
4. Conclusions Fig. 5. Frequency dependence of the dielectric constant (a) and the dielectric loss tangent (b) for lithium–zinc ferrites.
electrons, becomes a divalent Me2 þ ion and remains stable until some valence electron leaves it. In this case, it becomes a Me3 þ ion again. The transition of electrons from the bivalent ion to the
The comparative analysis of the properties of the samples sintered by accelerated electron beam heating with the properties of the samples prepared by conventional thermal sintering shows that RT sintering improves the structural parameters of lithium–zinc ferrite. In particular, the ferrite density increases, and its porosity decreases. These samples are characterized by lower values of electrical resistivity at relatively high saturation
Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
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magnetization values. It can be assumed that these lithium–zinc ferrites can be used to develop coatings interacting effectively with the electromagnetic wave in the microwave range, which are characterized by high saturation magnetization with large magnetic and dielectric losses. Acknowledgments This research was supported by the Ministry of Education and Science of the Russian Federation in part of the ʻʻScience’’ program. The authors express appreciation to Tomsk Center for Collective Use in Material Science (Tomsk State University) for assistance in structural studies with the scanning electron microscope SEM Philips 515. The authors express appreciation to Dr. M.V. Korobeynikov (Institute of Nuclear Physics, SB RAS, Novosibirsk) for assistance in the experiment on radiation-thermal action by high-energy electron beam. References [1] P.D. Baba, G.M. Argentina, W.E. Courtney, G.F. Dionne, D.H. Temme, Fabrication and properties of microwave lithium ferrites, IEEE Trans. Magn. 8 (1972) 83–94. [2] Diptia, Parveen Kumar, J.K. Juneja, Sangeeta Singh, K.K. Rainae, Chandra Prakash, Improved dielectric and magnetic properties in modified lithium–ferrites, Ceram. Int. 41 (2015) 3293–3297. [3] S.A. Mazen, N.I. Abu-Elsaad, Characterization and magnetic investigations of germanium-doped lithium ferrite, Ceram. Int. 40 (2014) 11229–11237. [4] G.O. White, C.E. Patton, Magnetic properties of lithium ferrite microwave materials, J. Magn. Magn. Mater. 9 (1978) 299–317, http://dx.doi.org/10.1016/0304-8853(78)90085-9. [5] M.S. Ruiz, S.E. Jacobo, Electromagnetic properties of lithium zinc ferrites doped with aluminum, Physica B 407 (2012) 3274–3277, http://dx.doi.org/10.1016/j.physb.2011.12.085. [6] B. Edelio, T. le Mercier, M. Quarton, Microstructure and physicochemical studies of pure and zinc-substituted lithium ferrites sintered above 1000 1C, J. Am. Ceram. Soc. 78 (1995) 365–368. [7] R. Raman, V.R.K. Murthy, B. Viswanathan, Microwave dielectric loss studies on lithium–zinc ferrites, J. Appl. Phys. 69 (1991) 4053–4055, http://dx.doi.org/10.1063/1.348415. [8] M. Shahjahan, N.A. Ahmed, S.N. Rahman, et al., Structural and electrical characterization of Li–Zn ferrites, Int. J. Innov. Technol. Explor. Eng. 3 (2014) 48–52. [9] E. D.e. Fazio, P.G. Bercoff, S.E. Jacobo, Electromagnetic properties of manganese–zinc ferrite with lithium substitution, J. Magn. Magn. Mat 323 (2011) 2813–2817, http://dx.doi.org/10.1016/j.jmmm.2011.06.022. [10] A.N. Yusoff, M.N. Abdullah, Microwave electromagnetic and absorption properties of some Li–Zn ferrites, J. Magn. Magn. Mater. 269 (2004) 271–280, http://dx.doi.org/10.1016/S0304-8853(03)00617-6. [11] E.N. Lysenko, A.V. Malyshev, V.A. Vlasov, Microwave properties of Li–Zn ferrite ceramics, in: Proceedings of the IEEE 15th International Conference of Young Specialists on Micro/Nanotechnologies and Electron Devices, 6882489, 2014, pp. 114–116. doi: 10.1109/ EDM.2014.6882489. [12] S.H. Gee, Y.K. Hong, M.H. Park, D.W. Erickson, P.J. Lamb, Synthesis of nanosized (Li0.5xFe0.5xZn1 x)Fe2O4 particles and magnetic properties, J. Appl. Phys. 91 (2002) 7586–7588, http://dx.doi.org/10.1063/1.1453931. [13] S. Misra, S. Ram, R.S. Shinde, Magnetic and dielectric behavior of nanostructured (Li0.5Fe0.5)(1 x) ZnxFe2O4 (x r1.0) spinel ferrites, AIP Conf. Proc. 1447 (2012) 413–414, http://dx.doi.org/10.1063/1.4710055.
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Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
CERAMICS INTERNATIONAL
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Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating A.V. Malyshevn, E.N. Lysenko, V.A. Vlasov Tomsk Polytechnic University, Lenina Avenue 30, 634050 Tomsk, Russia Received in revised form 25 June 2015, 27 July 2015; accepted 27 July 2015
Abstract Polycrystalline zinc substituted lithium ferrites (lithium–zinc ferrites) with the chemical formula Li0.4Fe2.4Zn0.2O4 were prepared by heating the mixture using a high-energy beam of electrons accelerated to energy of 2.4 MeV. The sintering temperature and time were 1100 1C and 2 h, respectively. The microstructure of the samples was investigated by XRD and SEM analyses, and the density and porosity were determined by hydrostatic weighing. The magnetic (saturation magnetization, the Curie temperature) and dielectric (electrical conductivity, frequency dependence of the dielectric constant and dielectric loss tangent) properties for lithium–zinc ferrites were studied. The results were compared with the results obtained for the samples prepared for the samples in compact pallets form by conventional ceramic technology through heating the mixture in a resistance furnace. The XRD analysis confirmed the formation of the spinel structure of the produced ferrite samples. The results of the study show that the samples sintered using radiation-thermal heating exhibit a higher density and less porosity. These samples are characterized by lower electrical resistivity, higher dielectric losses and high saturation magnetization. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Sintering; C. Magnetic properties; C. Dielectric properties; D. Ferrites; Radiation-thermal heating
1. Introduction Lithium and lithium-substituted ferrites are an issue of permanent interest to researchers due to their high practical relevance as an inexpensive magnetic material of microwave technology, which is characterized by high values of electrical resistivity, the Curie temperature, low dielectric and magnetic losses [1–3]. It is well known that lithium ferrite provides active interaction with electromagnetic waves at the low frequency band of the microwave range. For this purpose, to increase the saturation magnetization in LiFe5O8 lithium ferrite, partial substitution with Zn2 þ ions is performed [4–6]. As a result of this substitution, the electromagnetic wave absorption increases due to a drop in electrical resistivity and increase in dielectric constant of ferrites [7–11]. The prepared materials are in agreement with the general formula Li0.5(1 x)ZnxFe2.5 0.5xO4 with a lattice parameter of E8.35 Å. However, as the zinc n
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content increases (xZ0.4), the saturation magnetization sharply decreases [12–14]. The ceramic method is currently considered to be most common to produce lithium and lithium-substituted ferrites. It involves two-stage high-temperature heating of reagent mixtures including synthesis of ferrites with the formation of single-phase ferrite compositions and further sintering at a higher temperature to produce high-density ferrite ceramics [2,3]. The main disadvantage of the ceramic method is high probability of the presence of unreacted oxides and intermediate products in the composition of the sintered ferrites. These defects, as well as the porosity of the material, create fields of elastic stresses which distort the magnetic anisotropy of ferrite and thus result in deterioration of its magnetic characteristics. To eliminate these disadvantages, high temperature and long synthesis and sintering time are used, and this leads to volatilization of Li2O from the samples, thereby deteriorating the electromagnetic properties of the fabricated lithium–zinc ferrite [15]. Therefore, the efficiency of lithium ferrite material production is being constantly improved. It includes mechanical activation of the initial
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reagents [16,17] and sol–gel and citrate-gel technologies used to produce nanostructured lithium–zinc ferrites [18–21]. The magnetic and dielectric properties of these lithium–zinc ferrites are well-known. In [22–28], it is shown that one of the effective ways to improve the homogeneity and to intensify solid–phase interactions is radiation-thermal (RT) synthesis of ferrite materials when exposed to the accelerated electron beam. It was found that radiation-thermal heating of the initial reagent mixture significantly increases the reactivity of the solid-phase system, which reduces the synthesis temperature and increases the homogeneity of the end product. According to the results of the investigation of Li0.4Fe2.4Zn0.2O4 lithium–zinc ferrite synthesized by heating of mechanically activated initial Li2CO3–ZnO–Fe2O3 reagent mixture by a high-energy electron beam with subsequent high-temperature sintering in the laboratory furnace [28], the use of the composite based on this material as an absorbing coating in the microwave frequency is considered to be promising. However, along with the studies performed in this area, the main properties of lithium–zinc ferrites with the chemical formula Li0.5(1 x)ZnxFe2.5 0.5xO4 prepared by radiation-thermal heating have not been studied sufficiently. This applies especially to their electromagnetic and dielectric properties which determine the propagation of electromagnetic waves in the material. This paper presents the results of the research in the structural and electrical properties of lithium–zinc ferrite sintered by highenergy electron beam heating. For the comparative analysis of the obtained results, ferrite sintering was carried out by two methods: conventional thermal (T) heating and radiation-thermal (RT) heating by high-energy electron beams. 2. The object of the study and experimental technique To prepare Li0.4Fe2.4Zn0.2O4 lithium–zinc ferrite, the reactants used were: Fe2O3, Li2CO3, ZnO industrial powders, which were premixed and mechanically activated in the AGO-2S planetary mill (Novic, Russia) using steel grinding jars and balls at room temperature for 60 min. The weight ratio of the material to the balls was 1:10, and the ball mill rotation speed was 2220 rpm. According to the results in [26], this mode of mechanical activation makes possible to produce lithium–zinc ferrites of high phase homogeneity. After mechanical activation, the samples were pressed into compact pallets with a diameter of 15 mm and a thickness of 2 mm using the method of single-action cold compaction. The compaction pressure was 200 MPa. Thermal synthesis of the samples was carried out in the laboratory resistance furnace at 800 1C in air for 120 min. Before sintering, the synthesized samples were milled, mixed with a binder (12% aqueous polyvinyl alcohol solution) and pressed into compact pallets. For T and RT sintering, the samples were divided in two parts. Both sintering processes were performed at a temperature of 1100 1C for 140 min. Note that higher temperatures for sintering lithium ferrites are not desirable due to the volatilization of lithium and zinc. T sintering of the samples was carried out in the laboratory high temperature resistance furnace. RT sintering was performed under the same temperature-time
Fig. 1. Structural scheme of the cell for ferrite sintering in the accelerated electron beam. Upper cover made of fireclay (1), insulator made of fireclay (2), case of the stainless steel cell (3), thermocouple junction (4), control sample (5), fireclay heat shields (6), electron beam (7), compacted samples (8), ceramic tube support (9), ground wire of the thermocouple measuring junction (10).
conditions with the pulsed electron accelerator ILU-6 [29]. The electron energy was equal to 2.4 MeV, the beam current pulse was 400 mA, the pulse duration was 500 μs and the pulse repetition frequency was 12.5–25 Hz. The average radiation dose rate in the isothermal mode was 5 kGy/s/s. Within a single pulse, the dose was equal to 800 kGy/s. The cooling rate for T and RT heating was 20 1C/min. T and RT sintered samples were 12 mm in diameter and 1.5 mm in thickness. The samples were irradiated in air in the insulated cell (Fig. 1) made of lightweight fireclay 1 with the mass thickness of the horizontal plates equal to 0.16 g/cm2. The experimental cell was size of 200 mm in length, 120 mm in width and 60 mm in height. The electron energy loss in the upper cover of that thickness did not exceed 8% and could be neglected. Sintered samples 7 inside the cell were placed on a thin plate made of 3 mm lightweight fireclay located on ceramic tube 8. The temperature was controlled with thermocouple 3 (type S), its measuring junction being located in test sample 4 placed in the immediate vicinity of the samples. The thermo-EMF values of the thermocouple were used for computer control of the thermal sintering program under varying electron pulse repetition rate. The phase composition and lattice parameters of the samples were determined by XRD analysis with the diffractometer ARL X'TRA (Switzerland). The diffraction patterns were measured with Cukα-radiation in the range of 2θ¼ 10–901 at a scanning speed of 0.021/s. Phase identification was carried out using powder database PDF-4þ of the International Center for Diffraction Data (ICDD). The experimental diffraction patterns were processed using the program PowderCell 2.4. The density and open porosity of the ceramic samples were measured by hydrostatic weighing using Shimadzu AUW 220D high-precision analytical balance. The electronic micrographs from the cleavage surface of the ceramic samples were made using the SEM Philips 515 scanning electron microscope. The saturation magnetization, Ms, was measured at room temperature with the vibrating sample magnetometer with the maximum field of 10 kOe. The Curie temperature of the samples was measured by thermomagnetometry method, which is the thermogravimetric TG/DTG analysis of samples in the magnetic field [30]. TG/
Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
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DTG measurements were performed with the STA 449C Jupiter thermal analyzer (Netzsch, Germany). The dielectric parameters: resistivity ρ, dielectric constant ε0 and dielectric loss tangent tgδ were measured by two-electrode method using the LCR-819 Meter. Silver electrodes were deposited on the sample surface (sample thickness of 0.24 mm) through thermal evaporation in vacuum; the diameter of the measuring electrode was 5 mm. The measurements were carried out in the 12–106 Hz test signal frequency range at room temperature. 3. Results and discussion The X-ray pattern for the synthesized samples is shown in Fig. 2. According to the results of the XRD analysis, all the samples were single-phase and corresponded to the chemical formula of lithium zinc ferrospinel with Li0.4Fe2.4Zn0.2O4 composition. The value of the lattice parameter was 8.357 Å, and it is close to the values corresponding to Li0.4Fe2.4Zn0.2O4 [12,21]. The ferrite crystallite size was calculated using the Williamson–Hall method, and it amounted to 103 nm. After T and RT sintering, the phase composition of lithium– zinc ferrites remained unchanged. The structural parameters for these samples are shown in Table 1. It indicates a slight increase in the lattice parameter and a decrease in the size of crystallites for samples prepared by RT sintering. Fig. 3 shows micrographs for lithium–zinc ferrites produced by T (Fig. 3(a)) and RT sintering (Fig. 3(b)). The ferrite ceramic structure is seen to be polycrystalline with well-formed grains. However, the porosity and the grain size are found to be different. The samples prepared by RT sintering have a higher grain size and lower porosity compared to the samples fabricated by T sintering. The microstructure of the RT samples is characterized
Fig. 2. X-ray diffraction pattern of the lithium zinc ferrospinel after synthesis. Symbols are the calculated curve for the reflection from Li0.4Fe2.4Zn0.2O4 and the solid line is an experimental XRD pattern.
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by the development of secondary recrystallization when the grain size is observed to be sharply different. The average grain sizes were calculated by intercept method and found to vary from 1.6 μm for T samples to 4 μm for RT samples. The data obtained by hydrostatic weighing of the samples is consistent with the results of the microstructural analysis. It was found that after RT sintering, the ceramics is denser and less porous compared to that produced by conventional thermal sintering. We analyzed the role of possible radiation mechanisms in the discovered effect in the literature. The stimulating effect of electron heating on ferrite ceramic sintering can be explained in terms of the surface-recombination mechanism of high-temperature radiationinduced mass transfer in ion structures proposed in [31]. The mechanism of the process is as follows. The regions of structural failure in heterogeneous structures (ceramics) are characterized by higher rate of nonradiative electron–hole and exciton recombination compared to that in the volume that causes local temperature gradients, defects and stresses. This process intensifies the mass transfer at the interphase boundaries, and as a result, it can accelerate ferrite ceramic sintering. According to the data in Table 1, the saturation magnetization value is similar for all the samples, and it equals to 70 emu/g. For samples of lithium–zinc ferrite prepared by T sintering, the value of electrical resistivity is high, and it is equal to 104 Ω cm. However, in samples prepared by RT sintering, ρ decreases by an order of magnitude due to the reduced porosity and increased density of the sample. According to thermogravimetric measurements in the magnetic field (Fig. 4), the Curie temperature varies within 508– 509 1C for all the samples, and these values correspond to Li0.4Fe2.4Zn0.2O4 lithium–zinc ferrite [30]. Note that the Curie temperature value is sufficiently high, and this indicates satisfactory thermal stability properties of ferrite. Fig. 5 shows the frequency dependence of the lithium–zinc ferrite dielectric characteristics. The dielectric constant is characterized by high dispersion caused by relaxation polarization in the investigated frequency range. As the frequency increases, the dielectric constant decreases for all the samples. For samples prepared by T sintering, ε0 decreases rapidly at lower frequencies and slows at high frequencies, which, according to [32], is a normal dielectric behavior. However, a sharp drop in ε0 values in the samples prepared by RT sintering is shifted to higher frequencies. Positively charged ions (Me3 þ , Me2 þ ) are always present in ferrite. Weakly bound electrons are grouped around these ions (or complexes) due to the Coulomb interaction. Thermal motion can make these electrons move from one ion to another, and Me3 þ ion, as a result of the transition of one of the weakly bound
Table 1 Structural and electromagnetic properties of lithium–zinc ferrite. Sintering type
Lattice parameter (Å)
Crystallite size (nm)
Density (g/ cm3)
Porosity (%)
Average grain size Saturation magnetization Ms ρ at T¼ 20 1С (μm) (emu/g) (Ω cm)
T RT
8.355 8.356
148 128
4.16 4.37
10.5 5.4
1.6 4
69.3 70.3
2.6 104 4.3 103
Curie temperature (1С) 509 508
Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
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Fig. 3. SEM micrographs of lithium–zinc ferrite samples prepared by T (a) and RT (b) sintering.
Fig. 4. TG and DTG curves for lithium–zinc ferrites prepared by T (a) and RT (b) sintering.
trivalent ion is equivalent to the interchange of the position of these ions. Under an external electric field, the electron transition occurs mainly along the field direction. Thus, the process of electron transitions Me3 þ þ e2Me2 þ causes polarization of the relaxors in the form of oppositely charged ion pairs. The greater number of ion vacancies and weakly bound electrons in ferrite, the greater number of electric dipoles formed and, consequently, the higher ferrite dielectric constant. As the temperature increases and/or the test signal frequency decreases, the distance which bound electrons can travel from the ions increases. As a result, the polarizability of ferrites and, therefore, the dielectric constant increase. As can be seen in Fig. 5, in all the measured frequency range, the ε0 values of the ferrites prepared by RT sintering are higher compared to the values obtained for ferrites produced by T sintering. It is obvious that the drop in resistivity in the samples prepared by RT sintering leads to an increase in dielectric losses in these ferrites, and it can be observed at frequencies up to 105 Hz. The obtained results show that lithium–zinc ferrite samples prepared by RT sintering are characterized by the properties that satisfy the requirements of their further use as an absorbing material: high specific magnetization values in combination with low resistivity values which provide high dielectric losses [28].
4. Conclusions Fig. 5. Frequency dependence of the dielectric constant (a) and the dielectric loss tangent (b) for lithium–zinc ferrites.
electrons, becomes a divalent Me2 þ ion and remains stable until some valence electron leaves it. In this case, it becomes a Me3 þ ion again. The transition of electrons from the bivalent ion to the
The comparative analysis of the properties of the samples sintered by accelerated electron beam heating with the properties of the samples prepared by conventional thermal sintering shows that RT sintering improves the structural parameters of lithium–zinc ferrite. In particular, the ferrite density increases, and its porosity decreases. These samples are characterized by lower values of electrical resistivity at relatively high saturation
Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165
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magnetization values. It can be assumed that these lithium–zinc ferrites can be used to develop coatings interacting effectively with the electromagnetic wave in the microwave range, which are characterized by high saturation magnetization with large magnetic and dielectric losses. Acknowledgments This research was supported by the Ministry of Education and Science of the Russian Federation in part of the ʻʻScience’’ program. The authors express appreciation to Tomsk Center for Collective Use in Material Science (Tomsk State University) for assistance in structural studies with the scanning electron microscope SEM Philips 515. The authors express appreciation to Dr. M.V. Korobeynikov (Institute of Nuclear Physics, SB RAS, Novosibirsk) for assistance in the experiment on radiation-thermal action by high-energy electron beam. References [1] P.D. Baba, G.M. Argentina, W.E. Courtney, G.F. Dionne, D.H. Temme, Fabrication and properties of microwave lithium ferrites, IEEE Trans. Magn. 8 (1972) 83–94. [2] Diptia, Parveen Kumar, J.K. Juneja, Sangeeta Singh, K.K. Rainae, Chandra Prakash, Improved dielectric and magnetic properties in modified lithium–ferrites, Ceram. Int. 41 (2015) 3293–3297. [3] S.A. Mazen, N.I. Abu-Elsaad, Characterization and magnetic investigations of germanium-doped lithium ferrite, Ceram. Int. 40 (2014) 11229–11237. [4] G.O. White, C.E. Patton, Magnetic properties of lithium ferrite microwave materials, J. Magn. Magn. Mater. 9 (1978) 299–317, http://dx.doi.org/10.1016/0304-8853(78)90085-9. [5] M.S. Ruiz, S.E. Jacobo, Electromagnetic properties of lithium zinc ferrites doped with aluminum, Physica B 407 (2012) 3274–3277, http://dx.doi.org/10.1016/j.physb.2011.12.085. [6] B. Edelio, T. le Mercier, M. Quarton, Microstructure and physicochemical studies of pure and zinc-substituted lithium ferrites sintered above 1000 1C, J. Am. Ceram. Soc. 78 (1995) 365–368. [7] R. Raman, V.R.K. Murthy, B. Viswanathan, Microwave dielectric loss studies on lithium–zinc ferrites, J. Appl. Phys. 69 (1991) 4053–4055, http://dx.doi.org/10.1063/1.348415. [8] M. Shahjahan, N.A. Ahmed, S.N. Rahman, et al., Structural and electrical characterization of Li–Zn ferrites, Int. J. Innov. Technol. Explor. Eng. 3 (2014) 48–52. [9] E. D.e. Fazio, P.G. Bercoff, S.E. Jacobo, Electromagnetic properties of manganese–zinc ferrite with lithium substitution, J. Magn. Magn. Mat 323 (2011) 2813–2817, http://dx.doi.org/10.1016/j.jmmm.2011.06.022. [10] A.N. Yusoff, M.N. Abdullah, Microwave electromagnetic and absorption properties of some Li–Zn ferrites, J. Magn. Magn. Mater. 269 (2004) 271–280, http://dx.doi.org/10.1016/S0304-8853(03)00617-6. [11] E.N. Lysenko, A.V. Malyshev, V.A. Vlasov, Microwave properties of Li–Zn ferrite ceramics, in: Proceedings of the IEEE 15th International Conference of Young Specialists on Micro/Nanotechnologies and Electron Devices, 6882489, 2014, pp. 114–116. doi: 10.1109/ EDM.2014.6882489. [12] S.H. Gee, Y.K. Hong, M.H. Park, D.W. Erickson, P.J. Lamb, Synthesis of nanosized (Li0.5xFe0.5xZn1 x)Fe2O4 particles and magnetic properties, J. Appl. Phys. 91 (2002) 7586–7588, http://dx.doi.org/10.1063/1.1453931. [13] S. Misra, S. Ram, R.S. Shinde, Magnetic and dielectric behavior of nanostructured (Li0.5Fe0.5)(1 x) ZnxFe2O4 (x r1.0) spinel ferrites, AIP Conf. Proc. 1447 (2012) 413–414, http://dx.doi.org/10.1063/1.4710055.
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Please cite this article as: A.V. Malyshev, et al., Microstructure, electromagnetic and dielectric properties of zinc substituted lithium ferrites prepared by radiation-thermal heating, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.165