Effect of treatment conditions on structure and magnetodielectric properties of barium hexaferrites

Journal Pre-proofs Effect of treatment conditions on structure and magnetodielectric properties of barium hexaferrites D.A. Vinnik, F.V. Podgornov, N.S. Zabeivorota, E.A. Trofimov, V.E. Zhivulin, A.S. Chernukha, M.V. Gavrilyak, S.A. Gudkova, D.A. Zherebtsov, A.V. Ryabov, S.V. Trukhanov, T.I. Zubar, L.V. Panina, S.V. Podgornaya, M.V. Zdorovets, A.V. Trukhanov PII: DOI: Reference:

S0304-8853(19)33445-6 https://doi.org/10.1016/j.jmmm.2019.166190 MAGMA 166190

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Journal of Magnetism and Magnetic Materials

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2 October 2019 18 November 2019 20 November 2019

Please cite this article as: D.A. Vinnik, F.V. Podgornov, N.S. Zabeivorota, E.A. Trofimov, V.E. Zhivulin, A.S. Chernukha, M.V. Gavrilyak, S.A. Gudkova, D.A. Zherebtsov, A.V. Ryabov, S.V. Trukhanov, T.I. Zubar, L.V. Panina, S.V. Podgornaya, M.V. Zdorovets, A.V. Trukhanov, Effect of treatment conditions on structure and magnetodielectric properties of barium hexaferrites, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.166190

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Effect of treatment conditions on structure and magnetodielectric properties of barium hexaferrites D.A. Vinnik1, F.V. Podgornov1, N.S. Zabeivorota1, E.A. Trofimov1, V.E. Zhivulin1, A.S. Chernukha1, M.V. Gavrilyak1, S.A. Gudkova1,2, D.A. Zherebtsov1, A.V. Ryabov1, S.V. Trukhanov1, 3, 4, *, T.I. Zubar4, L.V. Panina3, 5, S.V. Podgornaya3, M.V. Zdorovets6, A.V. Trukhanov1, 3, 4 1South 2Moscow

Ural State University, 454080, Chelyabinsk, Lenin’s prospect 76, Russia

Institute of Physics and Technology (State University), 141700, Dolgoprudny, Institutskiy per., Russia

3National

University of Science and Technology “MISiS”, 119049, Moscow, Leninskii av. 4, Russia

4Scientific

Practical Materials Research Centre of National Academy of Sciences of Belarus, 220072,

Minsk, P. Brovki str. 19, Belarus, *corresponding author e-mail: [email protected] 5Institute

of Physics, Mathematics & IT, Immanuel Kant Baltic Federal University, 236041 Kaliningrad, A. Nevskogo 14, Russia 6L.N.

Gumilyov Eurasian National University, Nur-Sultan, Kazakhstan

Abstract BaFe12O19 were synthesized using usual ceramic technology in two stages. The structure and dielectric properties of the obtained samples after different processing conditions were the subject of study. At the final stage, the samples were thoroughly reground and pressed into pellets with different pressures. The unit cell parameters of the obtained samples were determined by X-ray diffraction method. The density of the sample pellets as a function of the applied pressure and sintering temperature was collected. The density of the initial sample pellets monotonously depends on the applied pressure. The density of the final sample pellets has a maximum at 1.055 GPa, after which a slight decrease was observed. Complex impedance spectra of all the obtained samples were measured in the frequency range of 10-1 - 106 Hz. Different treatment conditions result in two ranges of impedance spectrum. The impedance decreases with increasing processing pressure up to 1.055 GPa, above which, it starts gradually decrease. Complex spectra of impedance were fitted with a complex nonlinear least squares method based on the Levenberg-Marquardt minimization algorithm. Keywords: Ceramic technology; treatment conditions; X-ray diffraction; barium haxaferrite; impedance. 1

1. Introduction Complex oxides of transition metals attract the close attention of many researchers working in the field of materials science [1-3]. For a long time, hexferrites have been the subject of close attention because of their important role in the production of magnetic materials. [4-6]. M-type hexagonal ferrites with general formula (Sr, Ba, Pb)Fe12O19 first obtained in 1950’s are actively used not only as permanent magnets [7-9], but also in microwave technology [10-12]. Due to their useful properties such as high chemical and temperature stability, high resistivity, large magnetocrystalline uniaxial anisotropy, high coercivity and saturation magnetization, hexaferrites are an excellent candidate for being used in microwave devices in the frequency range from 1 to 100 GHz [13-17]. Hexaferrites are also used for high density memory storage applications, magnetic composites and in medicine [18-20]. It is should be noted that hexaferrites are also physiologically harmless. It was reported that large spontaneous polarization at ambient temperature was recently discovered in doped barium hexaferrites. The magnetoelectric coefficient of hexaferrites fabricated obtained using the improved method is more advanced than that of a well-studied compound such as BiFeO3 bismuth orthoferrite [21-25]. Magnetic properties of hexaferrites significantly depend on the processing conditions [26]. For hexaferrite synthesis can be used sol-gel autocombination [27], mechanical activation [28], polymer precursor [29], solid state reaction [30], mechanical alloying [31], sputtering deposition [32], flux growth [33], floating zone [34] methods. Their description can be found in a recent review [35]. Magnetic and dielectric properties of hexaferrites are defined by the chemical composition, stoichiometry, average crystallite size, structure distortions and etc. These parameters largely depend on the processing conditions during the synthesis. Compression is an important process step in the preparation of hexaferrite samples. Pressure controls the magnetic and dielectric properties of the samples through a change in their density, porosity, average crystallite size and their dispersion. So, all magnetic and dielectric quantities such as the saturation magnetization, coercive force, anisotropy field, magnetic permeability, ferromagnetic resonance line width, depend on the sample density. An important but poorly studied factor presented in this paper is the influence of the pressure value applied to barium hexaferrite powders on the magnetic parameters of the obtained sintered material. 2. Experimental part Iron oxide Fe2O3 and barium carbonate BaCO3 with a purity of 99.5 % were used as initial components with batch composition of 82.92 wt. % Fe2O3 and 17.08 wt. % BaCO3. Samples were calcinated in platinum crucibles in a horizontal SiC tube furnace with maximum temperature 1450 °С. Both metal components were mixed and grinded in an agate mortar during 1 hour. At the first stage the loose powder was calcinated at 1100, 1250, 1300 or 1350 °C for 180 min to determine the optimal temperature regime. At the second stage the resulted powder was thoroughly reground and pressed into pellets with the various pressures for varying their influence on the material structure and properties. The composition of sintered samples was investigated using an energy dispersive spectrometer Oxford INCA X-max 80 attached to a scanning electron microscope Jeol JSM7001F. The chemical composition was calculated as the average of ten measurements on 1x1.5 mm areas of the sintered and reground pellets. 2

The X-ray powder diffraction analysis was performed on a powder diffractometer Rigaku Ultima IV in the 2θ angular range from 10 to 90 deg with the speed of 2°θ/min with filtered CuKα λ = 1.541874 Å radiation. For these measurements the samples were thoroughly powdered and placed on a single-crystalline silicon holder. The Rietveld analysis and the FullProf software program have been also used [5]. All the unit cell parameters presented in the study have been obtained using Rietveld fitting. The obtained samples were investigated by an AC impedance spectroscopy technique. This method is a promising tool for probing electrical properties associated with crystal interiors, grain boundaries and surfaces. This, in particular, is the case when the results of various methods of presenting data are compared since often, one method alone, such as using complex impedance plane plots may give information that is incomplete. It is possible to characterize both bulk phenomena and grain boundary using combined impedance, electric modulus and conductivity spectroscopy. More detailes on this is given in [36]. 3. Results and discussion 3.1. Samples prepared from not pressed powders The synthesis of submicron particulate materials with a specific particle shape is a rather difficult task. The reason is that practically used powders are usually multi-component. Therefore, as a result of synthesis experiments, the multiphase composition of the final product is due to the incompleteness of diffusion processes occurring in low-temperature region [37]. Significant rise in synthesis temperature results in increase of the particle size [38, 39]. Another important characteristic is that synthesis of multicomponent powders needs a high degree of homogeneity of the initial component distribution during all process stages [40]. Sintering of a well ground stoichiometric mixture of Fe2O3 and BaCO3 as the loose powder at 1100, 1250, 1300, 1350 °C allows one to determine completeness of the synthesis. The results of PXRD analysis are presented in Table 1. From the data obtained, it is clear that BaCO3 reacts fast with iron oxide forming BaFe2O4. No residual BaCO3 was detected even after 1100 °C, despite the relatively large grain size of barium carbonate compared to Fe2O3. After that quick step, Fe2O3 slowly reacts with BaFe2O4, finally forming BaFe12O19. The amounts of impurity phases decline gradually with rising the calcinations temperature and reaches zero only after 180 min at 1350 °C. Increasing the temperature to 1350 °C also increases the grain size. The Fe/Ba ratio of obtained BaFe12O19 (Table 2) is equal to stoichiometry within the error limits. Together with a temperature search, the effect of exposure time at 1350 °C on the phase composition was investigated. Sintering was performed for 30, 60, 120, 150 and 180 min. It was found that after 30 mins exposure, the sample has extra 10 % BaFe2O4 and 5 % Fe2O3 phases, and after 60 min the diffractogram presents a single phase BaFe12O19 (Fig. 1). Thus, for the synthesis of powder barium hexaferrite from Fe2O3 and BaCO3 the optimum temperature and time were determined as 1350 °C and 60 mins. The samples which were prepared under ambient pressure conditions are marked as series № 1. 3.2 Samples prepared from pressed pellets Fig. 1 presents the powder XRD pattern of barium hexaferrite prepared by sintering of the powders at 1350 °C and 1.055 GPa compaction pressure during 60 min. The peak positions of the obtained sample coincide with the theoretical pattern. However, due to the sample texture the peak intensities are different. The peaks with h=k=0 like to (006) at 22.99°deg and (008) at 30.82 deg have 3

higher intensity. The cell parameters of the samples prepared at different conditions and literature data [41, 42] are presented in Table 3. To optimize the parameters of the sintering process, a series of pellets was prepared under different pressure. Firstly, Fe2O3 and BaCO3 were mixed and ground in the agate mortar for 60 min. After that non-compacted blend was calcinated at 1100 °C for 1224 min in a porcelain crucible to obtain carbonate-free material [43]. Resulting powder was grinded again and divided into identical portions of 0.3 g that were pressed with varying load into 5 mm diameter pellets and sintered at 1350 °C for 60 and 600 min (Table 4). Before and after sintering the size and the weight of pellets were measured with an accuracy of 0.01 mm and 0.0001 g, and then the apparent density was calculated [44]. After sintering for 60 min and density measurements all pellets were returned into the furnace for the next 360 min. The results of measurements of the magnetic properties of the samples after 600 min sintering are shown in Table 4. Supposing full occupation of all the atomic positions in stoichiometric BaFe12O19 with an average unit cell volume 698.31 ų its X-ray density should be equal 5.290 g/cm3. Apparently, all samples have a large porosity, visible also on SEM images of fractured pellets [45-47]. Calculated porosity of the sample with maximum density of 4.471 g/cm3 is equal to 15.5 %, comparing to 20.4 % for 4.213 g/cm3. Fig. 2 presents the density of pellets before and after sintering as a function of the applied pressure. As it could be expected during sintering the density of the samples vastly increased. The density of “green” pellets before sintering monotonously rises with the applied pressure, whereas the density after sintering has a weak maximum at 1.055 GPa, after which there is a slight decrease. An increase in the sintering time from 60 up to 600 min increases the density of the samples pressed at load smaller than 1.055 GPa. For the samples pressed at 1.055-1.451 GPa extension of sintering time didn’t help to increase the density. The higher pressure and longer time are economically unprofitable. The samples which were prepared from pressed powders are also marked as series № 2. Some magnetic data on the obtained samples could be found in [43]. 3.3 Dielectric properties of samples The complex impedance spectra (𝑍 = 𝑍′ ― 𝑖𝑍") of the samples confined between ITO electrodes were measured in the spectral range from 10-1 Hz to 106 Hz with a Novocontrol Beta System impedance spectrometer (Fig. 3). The amplitude of the probing oscillating voltage was set to 0.5 V. All measurements were carried out at room temperature (T  298 K) [48]. The experimentally measured complex impedance spectra as well as the Nyquist diagrams are presented in Figs. 4 a-f. As one can see from the data shown in Figs. 4 a-f, the pressure treatment of the samples in both series results in two pressure ranges of impedance spectra behavior [49]. Upon increase of the pressure from 0.494 GPa up to 1.055 GPa, one can clearly observe the decrease in real and imaginary parts of impedance as well as simultaneous increase in the characteristic relaxation frequency [50]. When pressure exceeds 1.055 GPa, these parameters starts gradually decrease. It means that the trend is reversing. Moreover, the treatment leads to the broadening of the relaxation peak which is the consequence of the distinguishable increase in the dispersion of relaxation times. Additionally, Nyquist diagrams of the samples for both series, which can be seen in Figs. 4 c, f, reveals non-Debye nature of the above-mentioned relaxation processes. Moreover, as follows from Nyquist diagrams, one can distinguish that the samples of the series № 1 have two relaxation frequencies. However, for the samples of the series № 2, the existence of two relaxation processes is less obvious because of their overlapping. To characterize these processes and investigate impact of pressure on the material parameters as well as the structural transformation of the samples, we accepted a porous model of our samples, 4

according to which the sample is a two-phase system consisting of barium hexaferrite and air–filled pores. The equivalent electric circuit of such heterogeneous system can be represented as the series of two RC circuits, which may be visible in Fig. 3 e. The real (𝑍′) and imaginary (𝑍") parts of the samples impedance can be written as: 𝑅1

𝑅2

𝑍′ = (1 + (2𝜋𝑓𝑅 𝐶 )2) + (1 + (2𝜋𝑓𝑅 𝐶 )2) 1 1

2 2

2𝜋𝑓𝑅21𝐶1

2𝜋𝑓𝑅22𝐶2

𝑍" = (1 + (2𝜋𝑓𝑅 𝐶 )2) + (1 + (2𝜋𝑓𝑅 𝐶 )2) 1 1

2 2

(1)

(2)

To retrieve such parameters as R1, R2, C1 and C2 of the equivalent circuit, the experimental complex spectra of impedance (Figs. 4 a, b, d, e) were fitted with formulae using a complex nonlinear least squares (CNLS) method based on the Levenberg-Marquardt minimization algorithm. The fitted values of the equivalent circuit parameters such as the R1, R2, C1, C2 are plotted as a function of pressure for both series of the samples in Figs. 5 a-d. For a physical interpretation of these plots, we assumed that BaFe12O19 grains are incompressible and their parameters should slightly depend on pressure. As follows from this assumptions and experimental data one can conclude that R1 and C1 correspond to the grains whereas R2 and C2 could be assigned to the air-filled pores. Upon increase in pressure, the parameters R2 and C2 strongly change their values. The increase in R2 and decrease in C2 could be explained by the cracks in pellets appeared due to high pressure. Conclusion The BaFe12O19 hexaferrites have been obtained with two stages ceramic technology. Different treatment conditions such as synthesis temperature, synthesis time, compaction pressure were the subject of study. Effect of the different pressure and temperature exposure time values on the structure and magnetodielectric properties has been investigated. It was found that after 60 min of annealing in air at 1350 °С optimal material has been obtained and the diffractogram presents single phase of BaFe12O19. All the unit cell parameters presented in the study have been obtained using Rietveld fitting. The unit cell parameters of the samples prepared at different conditions have been presented in comparison with literature data. Density of pellets before and after sintering as a function of the applied pressure is presented. Sintering pressure changed from 0.494 up to 1.451 GPa. The density of initial pellets before sintering monotonously rises with the applied pressure, whereas the density after sintering has a weak maximum at 1.055 GPa, after which there is a slight decrease. The complex impedance spectra of the samples have been measured in the spectral range from 101 Hz up to 106 Hz. Pressure effect on the samples in both series results in two pressure ranges of impedance spectra behavior. Upon increase of the pressure from 0.494 GPa up to 1.055 GPa, the decrease in real and imaginary parts of impedance is observed as well as simultaneous increase in the characteristic relaxation frequency. When pressure exceeds 1.055 GPa, these parameters starts gradually decrease. Nyquist diagrams of samples from both series reveals non-Debye nature of the above-mentioned relaxation processes. The samples of the series № 1 have two relaxation frequencies. For the series № 2, the existence of two relaxation processes is less obvious because of their overlapping. Experimental complex spectra of impedance have been fitted with formula using the complex nonlinear least squares method based on the Levenberg-Marquardt minimization 5

algorithm. Upon increase in pressure, the parameters of resistivity and capacitance of the air-filled pores strongly change their values. Acknowledgements The work was partly supported by Act 211 Government of the Russian Federation, contract No 02.A03.21.0011. This work was also partially supported by the Ministry of Education and Science of the Russian Federation. Government task in SUSU 4.1346.2017/4.6 and 10.9639.2017/8.9 and the framework of Increase Competitiveness Program of MISiS P 02-2017-2-4 and by the Russian Foundation for Basic Research (No 19-53-04010) and Belarussian Foundation for Basic Research, Belarus (No F19RM-011).

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10

Table 1. Phase composition of the samples, sintered for 180 min at different temperature, (mol.%). № T, °C BaFe2O4 Fe2O3 BaFe12O19 1 1100 40 32 28 2 1250 52 32 16 3 1300 84 11 5 4 1350 100 -

Table 2. Elemental composition of sintered BaFe12O19 after 1350 °С during 60 min, at. %. O Fe Ba Fe/Ba 59.31±0.21 37.65±0.13 3.03±0.05 12.43±0.08

Table 3. Unit cell parameters of the BaFe12O19 samples prepared at different treatment conditions. Temperature, Calcination a c V [Å] [Å] [ų] °C time, min 1250 1300 1350 1350 1350 1350 1350 1350

180 180 15 30 60 90 120 180

5.896(13) 5.8946(3) 5.8948(4) 5.8933(6) 5.8930(5) 5.8965(7) 5.8960(4) 5.8939(6) 5.8929(4) 5.893

[38] [39]

23.212(8) 23.204(13) 23.219(2) 23.204(2) 23.197(16) 23.208(2) 23.208(13) 23.202(14) 23.194(2) 23.194

698.90(3) 698.23(5) 698.72(7) 697.93(9) 697.65(8) 698.81(11) 698.67(7) 697.99(9) 697.54(6) 697.51

Table 4. The BaFe12O19 pellet density as a function of pressure before and after sintering at 1350 °C and apparent density of the samples after 600 min at 1350 °C.

N

Pressure, GPa

1 2 3 4 5 6 7 8

0.494 0.659 0.791 0.923 1.055 1.187 1.319 1.451

Apparent density, g/cm3 As pressed

3.418 3.525 3.589 3.659 3.710 3.781 3.825 3.865

11

Sintered for 60 min

Sintered for 600 min

4.213 4.305 4.398 4.418 4.471 4.448 4.455 4.447

4.365 4.374 4.434 4.465 4.468 4.440 4.427 4.420

Figure captions Fig. 1. Powder XRD pattern of the BaFe12O19 barium hexaferrite after 1350 °C annealing and 1.055 GPa compaction pressure during the 60 min. Experimental data are fitted by Rietveld method in Fullprof. Fig. 2. The BaFe12O19 pellet density before and after sintering at 1350 °C. Fig. 3. Surface morphology of BaFe12O19 pellets a) after compaction pressure of 0.494 GPa and b) 1.055 GPa followed by calcination at 1350 °C for 600 min, c) after compaction pressure of 0.659 GPa and d) 1.451 GPa followed by calcinationat 1350 °C for 600 min; (e) equivalent electric circuit of palletes. Fig. 4. Experimentally measured real and imaginary parts of the impedance spectra as well as Nyquist diagrams of the investigated samples: for the series № 1 (a) - real part of impedance spectra, (b) - imaginary part of impedance spectra, (c) - Nyquist diagram and for the series № 2 (d) - real part of impedance spectra, (e) - imaginary part of impedance spectra, (f) Nyquist diagram. All measurements are realized at room T  298 K temperature. Fig. 5. Fitted values of the equivalent circuit’s components: for the series № 1 - capacitances C1 and C2 (a), resistances R1 and R2 (b); for the series № 2 - capacitances C1 and C2 (c), resistances R1 and R2 (d). All measurements are realized at room T  298 K temperature.

12

BaFe 12O 19

I ( arb. un. )

20

10

0 20

30

40

2  ( deg )

Fig. 1

13

50

60

4,5 600 min

3

D ( g/sm )

180 min 60 min

4,0 30 min

3,5 0 min

0,4

0,8

1,2

P ( GPa ) Fig. 2

14

1,6

a

b

e

c

d

Fig. 3

15

2.0x105

-Z'', Ohm

1.5x105

1.0x105

f

Series 1 0.4943 x 109 Pa 0.6595 x 109 Pa 0.7914 x 109 Pa 0.9533 x 109 Pa 1.0550 x 109 Pa 1.1870 x 109 Pa 1.3190 x 109 Pa 1.4510 x 109 Pa

5.0x104

0.0 0.0

Series 2 0.4943 x109 Pa 0.6595 x109 Pa 0.7914 x109 Pa 0.9533 x109 Pa 1.0550 x109 Pa 1.1870 x109 Pa 1.3190 x109 Pa 1.4510 x109 Pa

6.0x105

-Z'', Ohm

c

4.0x105

2.0x105

0.0 0.0

5.0x104 1.0x105 1.5x105 2.0x105

Z', Ohm

2.0x105

4.0x105

Z',Ohm

Fig. 4

16

6.0x105

Series 2

4.0x10-11

4.0x10-11

0.0

2x104

R1,Ohm

1.0x105

Series 2

5x105

5x105

4x105

R1 R2 4x105

3x105

3x105

2x105

2x105

1x105

1x105

Fig. 5

17

Pressure, x109 Pa

1.451

1.319

1.187

1.055

0.9533

Pressure, x109 Pa

0.7914

0

0.6595

1.451

1.319

1.187

1.055

0.9533

0.7914

0.6595

0.4943

0.0

0.4943

5.0x104

1x104

0

R2,Ohm

1.5x105

R2,Ohm

R1 R2

3x104

0

d

2.0x105

4x104

R1,Ohm

Pressure, x109 Pa

Series 1

5x104

0.0

1.451

0

1.319

8.0x10-11

1.187

8.0x10-11

1.055

1.2x10-10

0.9533

1.2x10-10

0.7914

1.6x10-10

Pressure, x109 Pa

b

2.0x10-10

C1 C2 1.6x10-10

C2,F

2.0x10-10

0.6595

0.7914

0.4943

C1,F

1x10-10

C2,F

2.0x10-12

1.451

2x10-10

1.319

4.0x10-12

1.187

3x10-10

1.055

6.0x10-12

0.9533

8.0x10-12

C1 C2 4x10-10

0.0

c

5x10-10

0.4943

Series 1

1.0x10-11

0.6595

C1,F

a

BaFe12O19 hexaferrites were obtained using two stages ceramic technology. Different treatment conditions were the subject of study. Density of pellets as a function of the applied pressure is presented. Complex impedance spectra were measured in spectral range of 10-1 - 106 Hz.

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Manuscript « Effect of treatment conditions on structure and magnetodielectric properties of barium hexaferrites » by authors D.A. Vinnik, F.V. Podgornov, N.S. Zabeivorota, E.A. Trofimov, V.E. Zhivulin, A.S. Chernukha, M.V. Gavrilyak, S.A. Gudkova, D.A. Zherebtsov, A.V. Ryabov, S.V. Trukhanov, T.I. Zubar, L.V. Panina, S.V. Podgornaya, M.V. Zdorovets, A.V. Trukhanov reports the effect of different treatment conditions such as synthesis temperature, synthesis time, compaction pressure on structure and magnetodielectric properties of BaFe12O19 barium hexaferrites which have been synthesized with “two stages” ceramic technology. At final stage the samples have been thoroughly reground and pressed into pellets with different pressures to change in their structure and magnetodielectric properties. It has been also especially investigated the effect of exposure time. The density of initial pellets before sintering monotonously rises with the applied pressure in range of 0.494 - 1.451 GPa, whereas the density of final pellets has a weak maximum at 1.055 GPa, after which there is a slight decrease. Upon increase of the pressure up to 1.055 GPa, the decrease in real and imaginary parts of impedance is observed as well as simultaneous increase in the characteristic relaxation frequency for both series. Experimental complex spectra of impedance have been fitted with formula using the complex nonlinear least squares method based on the Levenberg-Marquardt minimization algorithm.

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