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Ceramics International 41 www.elsevier.com/locate/ceramint

Microstructure, electromagnetic and dielectric properties of zinc substitutedlithium 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 2015Available online 4 August 2015

Abstract

Polycrystalline zinc substituted lithium ferrites (lithium–zinc ferrites) with the chemical formula Li0.4Fe2.4Zn0.2O4 were prepared by heating themixture 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 byhydrostatic weighing. The magnetic (saturation magnetization, the Curie temperature) and dielectric (electrical conductivity, frequencydependence of the dielectric constant and dielectric loss tangent) properties for lithium–zinc ferrites were studied. The results were comparedwith the results obtained for the samples prepared for the samples in compact pallets form by conventional ceramic technology through heatingthe mixture in a resistance furnace. The XRD analysis confirmed the formation of the spinel structure of the produced ferrite samples. The resultsof the study show that the samples sintered using radiation-thermal heating exhibit a higher density and less porosity. These samples arecharacterized 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 ofpermanent interest to researchers due to their high practicalrelevance as an inexpensive magnetic material of microwavetechnology, which is characterized by high values of electricalresistivity, the Curie temperature, low dielectric and magneticlosses [1–3]. It is well known that lithium ferrite provides activeinteraction with electromagnetic waves at the low frequencyband of the microwave range. For this purpose, to increase thesaturation magnetization in LiFe5O8 lithium ferrite, partialsubstitution with Zn2þ ions is performed [4–6]. As a result ofthis substitution, the electromagnetic wave absorption increasesdue to a drop in electrical resistivity and increase in dielectricconstant of ferrites [7–11]. The prepared materials are inagreement with the general formula Li0.5(1�x)ZnxFe2.5�0.5xO4

with a lattice parameter of E8.35 Å. However, as the zinc

10.1016/j.ceramint.2015.07.16515 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.:þ7 3822 564531.ss: malyshev@tpu.ru (A.V. Malyshev).

content increases (xZ0.4), the saturation magnetization sharplydecreases [12–14].The ceramic method is currently considered to be most

common to produce lithium and lithium-substituted ferrites. Itinvolves two-stage high-temperature heating of reagent mixturesincluding synthesis of ferrites with the formation of single-phaseferrite compositions and further sintering at a higher temperatureto produce high-density ferrite ceramics [2,3]. The maindisadvantage of the ceramic method is high probability of thepresence of unreacted oxides and intermediate products in thecomposition of the sintered ferrites. These defects, as well as theporosity of the material, create fields of elastic stresses whichdistort the magnetic anisotropy of ferrite and thus result indeterioration of its magnetic characteristics. To eliminate thesedisadvantages, high temperature and long synthesis and sinter-ing time are used, and this leads to volatilization of Li2O fromthe samples, thereby deteriorating the electromagnetic propertiesof the fabricated lithium–zinc ferrite [15]. Therefore, theefficiency of lithium ferrite material production is being con-stantly improved. It includes mechanical activation of the initial

Fig. 1. Structural scheme of the cell for ferrite sintering in the acceleratedelectron 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 measuringjunction (10).

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reagents [16,17] and sol–gel and citrate-gel technologies used toproduce nanostructured lithium–zinc ferrites [18–21]. Themagnetic and dielectric properties of these lithium–zinc ferritesare well-known.

In [22–28], it is shown that one of the effective ways toimprove the homogeneity and to intensify solid–phase inter-actions is radiation-thermal (RT) synthesis of ferrite materialswhen exposed to the accelerated electron beam. It was foundthat radiation-thermal heating of the initial reagent mixturesignificantly increases the reactivity of the solid-phase system,which reduces the synthesis temperature and increases thehomogeneity of the end product. According to the results ofthe investigation of Li0.4Fe2.4Zn0.2O4 lithium–zinc ferritesynthesized by heating of mechanically activated initialLi2CO3–ZnO–Fe2O3 reagent mixture by a high-energy elec-tron beam with subsequent high-temperature sintering in thelaboratory furnace [28], the use of the composite based on thismaterial as an absorbing coating in the microwave frequency isconsidered to be promising.

However, along with the studies performed in this area, themain properties of lithium–zinc ferrites with the chemical formulaLi0.5(1�x)ZnxFe2.5�0.5xO4 prepared by radiation-thermal heatinghave not been studied sufficiently. This applies especially to theirelectromagnetic and dielectric properties which determine thepropagation of electromagnetic waves in the material.

This paper presents the results of the research in the structuraland electrical properties of lithium–zinc ferrite sintered by high-energy electron beam heating. For the comparative analysis ofthe obtained results, ferrite sintering was carried out by twomethods: 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 reactantsused were: Fe2O3, Li2CO3, ZnO industrial powders, which were pre-mixed and mechanically activated in the AGO-2S planetary mill(Novic, Russia) using steel grinding jars and balls at roomtemperature for 60 min. The weight ratio of the material to theballs was 1:10, and the ball mill rotation speed was 2220 rpm.According to the results in [26], this mode of mechanical activationmakes possible to produce lithium–zinc ferrites of high phasehomogeneity. After mechanical activation, the samples were pressedinto compact pallets with a diameter of 15 mm and a thickness of2 mm using the method of single-action cold compaction. Thecompaction pressure was 200 MPa. Thermal synthesis of thesamples was carried out in the laboratory resistance furnace at800 1C in air for 120 min. Before sintering, the synthesized sampleswere milled, mixed with a binder (12% aqueous polyvinyl alcoholsolution) and pressed into compact pallets.

For T and RT sintering, the samples were divided in twoparts. Both sintering processes were performed at a temperatureof 1100 1C for 140 min. Note that higher temperatures forsintering lithium ferrites are not desirable due to the volatiliza-tion of lithium and zinc. T sintering of the samples was carriedout in the laboratory high temperature resistance furnace. RTsintering was performed under the same temperature-time

conditions with the pulsed electron accelerator ILU-6 [29].The electron energy was equal to 2.4 MeV, the beam currentpulse was 400 mA, the pulse duration was 500 μs and the pulserepetition frequency was 12.5–25 Hz. The average radiationdose rate in the isothermal mode was �5 kGy/s/s. Within asingle pulse, the dose was equal to 800 kGy/s. The cooling ratefor T and RT heating was 20 1C/min. T and RT sintered sampleswere 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 thehorizontal plates equal to �0.16 g/cm2. The experimental cell wassize 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 7inside the cell were placed on a thin plate made of 3 mmlightweight fireclay located on ceramic tube 8. The temperaturewas controlled with thermocouple 3 (type S), its measuringjunction being located in test sample 4 placed in the immediatevicinity of the samples. The thermo-EMF values of thethermocouple were used for computer control of the thermalsintering program under varying electron pulse repetition rate.The phase composition and lattice parameters of the samples

were determined by XRD analysis with the diffractometer ARLX'TRA (Switzerland). The diffraction patterns were measuredwith Cukα-radiation in the range of 2θ¼10–901 at a scanningspeed of 0.021/s. Phase identification was carried out usingpowder database PDF-4þ of the International Center forDiffraction Data (ICDD). The experimental diffraction patternswere processed using the program PowderCell 2.4.The density and open porosity of the ceramic samples were

measured by hydrostatic weighing using Shimadzu AUW 220Dhigh-precision analytical balance. The electronic micrographsfrom the cleavage surface of the ceramic samples were madeusing the SEM Philips 515 scanning electron microscope.The saturation magnetization, Ms, was measured at room

temperature with the vibrating sample magnetometer with themaximum field of 10 kOe.The Curie temperature of the samples was measured by

thermomagnetometry method, which is the thermogravimetricTG/DTG analysis of samples in the magnetic field [30]. TG/

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DTG measurements were performed with the STA 449CJupiter thermal analyzer (Netzsch, Germany).

The dielectric parameters: resistivity ρ, dielectric constant ε0 anddielectric loss tangent tgδ were measured by two-electrode methodusing the LCR-819 Meter. Silver electrodes were deposited on thesample surface (sample thickness of 0.24 mm) through thermalevaporation in vacuum; the diameter of the measuring electrodewas 5 mm. The measurements were carried out in the 12–106 Hztest signal frequency range at room temperature.

3. Results and discussion

The X-ray pattern for the synthesized samples is shown inFig. 2. According to the results of the XRD analysis, all thesamples were single-phase and corresponded to the chemicalformula 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 theWilliamson–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 forthese samples are shown in Table 1. It indicates a slightincrease in the lattice parameter and a decrease in the size ofcrystallites for samples prepared by RT sintering.

Fig. 3 shows micrographs for lithium–zinc ferrites produced byT (Fig. 3(a)) and RT sintering (Fig. 3(b)). The ferrite ceramicstructure 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 sizeand lower porosity compared to the samples fabricated by Tsintering. 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 andthe solid line is an experimental XRD pattern.

Table 1Structural and electromagnetic properties of lithium–zinc ferrite.

Sinteringtype

Latticeparameter (Å)

Crystallite size(nm)

Density (g/cm3)

Porosity(%)

Average(μm)

T 8.355 148 4.16 10.5 1.6RT 8.356 128 4.37 5.4 4

by the development of secondary recrystallization when the grainsize is observed to be sharply different. The average grain sizeswere calculated by intercept method and found to vary from1.6 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 wasfound that after RT sintering, the ceramics is denser and less porouscompared 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 electronheating on ferrite ceramic sintering can be explained in terms of thesurface-recombination mechanism of high-temperature radiation-induced mass transfer in ion structures proposed in [31]. Themechanism of the process is as follows. The regions of structuralfailure in heterogeneous structures (ceramics) are characterized byhigher rate of nonradiative electron–hole and exciton recombina-tion compared to that in the volume that causes local temperaturegradients, defects and stresses. This process intensifies the masstransfer at the interphase boundaries, and as a result, it canaccelerate ferrite ceramic sintering.According to the data in Table 1, the saturation magnetiza-

tion value is similar for all the samples, and it equals to�70 emu/g. For samples of lithium–zinc ferrite prepared by Tsintering, the value of electrical resistivity is high, and it isequal to 104 Ω� cm. However, in samples prepared by RTsintering, ρ decreases by an order of magnitude due to thereduced porosity and increased density of the sample.According to thermogravimetric measurements in the mag-

netic field (Fig. 4), the Curie temperature varies within 508–509 1C for all the samples, and these values correspond toLi0.4Fe2.4Zn0.2O4 lithium–zinc ferrite [30]. Note that the Curietemperature value is sufficiently high, and this indicatessatisfactory thermal stability properties of ferrite.Fig. 5 shows the frequency dependence of the lithium–zinc

ferrite dielectric characteristics. The dielectric constant ischaracterized by high dispersion caused by relaxation polar-ization 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 highfrequencies, which, according to [32], is a normal dielectricbehavior. However, a sharp drop in ε0 values in the samplesprepared by RT sintering is shifted to higher frequencies.Positively charged ions (Me3þ , Me2þ ) are always present in

ferrite. Weakly bound electrons are grouped around these ions (orcomplexes) due to the Coulomb interaction. Thermal motion canmake these electrons move from one ion to another, and Me3þ

ion, as a result of the transition of one of the weakly bound

grain size Saturation magnetization Ms

(emu/g)ρ at T¼20 1С(Ω cm)

Curie temperature(1С)

69.3 2.6� 104 50970.3 4.3� 103 508

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.

Fig. 5. Frequency dependence of the dielectric constant (a) and the dielectricloss tangent (b) for lithium–zinc ferrites.

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electrons, becomes a divalent Me2þ ion and remains stable untilsome valence electron leaves it. In this case, it becomes a Me3þ

ion again. The transition of electrons from the bivalent ion to the

trivalent ion is equivalent to the interchange of the position ofthese ions. Under an external electric field, the electron transitionoccurs mainly along the field direction. Thus, the process ofelectron transitions Me3þþe2Me2þ causes polarization of therelaxors 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 dipolesformed and, consequently, the higher ferrite dielectric constant.As the temperature increases and/or the test signal frequencydecreases, the distance which bound electrons can travel fromthe 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 highercompared to the values obtained for ferrites produced by Tsintering. It is obvious that the drop in resistivity in the samplesprepared by RT sintering leads to an increase in dielectric losses inthese 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 thatsatisfy the requirements of their further use as an absorbingmaterial: high specific magnetization values in combination withlow resistivity values which provide high dielectric losses [28].

4. Conclusions

The comparative analysis of the properties of the samplessintered by accelerated electron beam heating with the propertiesof the samples prepared by conventional thermal sinteringshows that RT sintering improves the structural parameters oflithium–zinc ferrite. In particular, the ferrite density increases,and its porosity decreases. These samples are characterized bylower values of electrical resistivity at relatively high saturation

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magnetization values. It can be assumed that these lithium–zincferrites can be used to develop coatings interacting effectivelywith the electromagnetic wave in the microwave range, whichare characterized by high saturation magnetization with largemagnetic and dielectric losses.

Acknowledgments

This research was supported by the Ministry of Education andScience of the Russian Federation in part of the ʻʻScience’’ program.

The authors express appreciation to Tomsk Center forCollective Use in Material Science (Tomsk State University)for assistance in structural studies with the scanning electronmicroscope SEM Philips 515.

The authors express appreciation to Dr. M.V. Korobeynikov(Institute of Nuclear Physics, SB RAS, Novosibirsk) forassistance in the experiment on radiation-thermal action byhigh-energy electron beam.

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