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Latin American applied research

versão impressa ISSN 0327-0793

Lat. Am. appl. res. v.38 n.3 Bahía Blanca jul. 2008

 

Microwave absorptive behavior ofnanocrystalline Ba3Co1.3Zn0.3Cu0.4Fe24O41 powder

V. R. Caffarena1, T. Ogasawara2, J. L. Capitaneo3 and M. Silveira Pinho4

1 Petróleo Brasileiro S.A - Gas&Power - Av. Almirante Barroso, 81 - 31o andar - Centro, ZIP CODE 20031-004, Rio de Janeiro, Brazil, valeskac@petrobras.com.br, valeska@cbpf.br
2 Department of Metallurgical and Materials Engineering - COPPE/UFRJ PEMM, P.O. Box 68505, Ilha do Fundão - ZIP CODE 21945-970 - Rio de Janeiro-RJ, Brazil, Phone: 55-21-2290-1615, Fax: 55-21-2290-6626, ogasawat@metalmat.ufrj.br
3 Instituto de Macromoléculas Professora Eloisa Mano, Ilha do Fundão, CT, Bl. J, sala 205, Rio de Janeiro, RJ; ZIP Code 21945-970, P.O. Box 68525, jeff@ima.ufrj.br
4 Brazilian Navy Research Institute - IPqM , Rua Ipiru n0 2, Praia da Bica, Ilha do Governador, Rio de Janeiro, R J, Brazil ZIP CODE 21931-090, magalipinho@yahoo.com.br

Abstract — In the present work, Cu2+ and Zn2+ ions were incorporated into the structure of Ba3Co2Fe24O41, in order to obtain the Ba3Co1.3Zn0.3Cu0.4Fe24O41 structure, by the sol-gel precursor method in N2 inert atmosphere. The magnetization curves of Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite nanopowders were analyzed as a function of temperature, to evaluate the super-paramagnetic (SPM) behavior and its effect on the microwave absorption measurements. The permittivity (ε) and permeability (μ) values were obtained by using the transmission/reflection (T/R) method in a waveguide to evaluate the microwave absorption properties at S and X-Ku bands, for radar absorber material (RAM) application. The partial substitu-tion of Co for Zn and Cu ions and the superpara-magnetic contribution improved the ceramics´ magnetic permeability, resulting in a microwave absorption greater than 90.0 % (reflectivity ≤ -10 dB) at 10.0-12.5 GHz and 13.0-16.0 GHz, with 2.5 and 2.0 mm thick, respectively.

Keywords — Z-Type Hexaferrite. RAM. Magnetic Properties. Sol-Gel Synthesis.

I. INTRODUCTION

Nanoscopic magnetic systems display a large variety of interesting physical properties, forming a unique group for the study of diverse problems in solid state physics, such as superparamagnetism (Nakamura and Hankui, 2003).

In single-domain particles, the coercivity decreases with the reduction in particle size due to the effects of thermal energy randomization (Nakamura and Hankui, 2003). As the particle size decreases within the multi-domain (MD) range, the coercivity decreases down to a minimum in a critical particle size where it is no longer possible to accommodate a domain wall, and the particle can only exist as a single-domain (SD).

The change in the magnetization of MD particles requires the translation of the domain walls, a process that is energetically favorable and is observed in a relatively low magnetic field. The SD particles can only be magnetized by rotation of the magnetization, an energetically unfavorable process (Knobel, 2000)..

As it is well known, the decrease of particle size within the range of nanometric dimensions is followed by decrease in the remanence and the coercivity, which is zero in a critical size, below which the particle is called superparamagnetic (SPM).

The shape of the hysteresis curve for SPM particles is extremely thin and its behavior is very similar to that of paramagnetic materials, but the grains present high magnetic momentum.

Interestingly, when the particle size is in nanoscale, the superparamagnetic contribution may lead to a great improve in the microwave absorbing properties. If the absorber particle size is small enough and the discrete energy level spacing is in the microwave energy range, the electron can absorb the energy and it leaps from one level to another, what may lead to an increment on attenuation (Ruan et al., 2002).

Superparamagnetic (SPM) grains of MD particles are hard to discern, based on the hysteresis properties measured at room temperature, but significant changes in the hysteresis parameters, resulting from a variation in the temperature from 300 K to 4.2 K, can be useful to determine the superparamagnetic (SPM) contribution (Knobel, 2000).

In this work, the effect of temperature variation on the magnetization curves was evaluated to Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite nanometric powder, to evaluate the superparamagnetic (SPM) contribution to the microwave absorption properties.

II. MATERIALS AND METHODS

Nanosized (Zn-Co-Cu)2Z powders were synthesized by the citrate precursor method, using the reagent grade Fe(NO3)3.9H2O, Ba(NO3)2, monohydrate citric acid, Co(NO3)2.6H2O, Cu(NO3)2.3H2O and Zn(NO3)2.6H2O, in stoichiometric molar ratios to achieve the Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite. The preparation of the solutions was carried out by weighting solids and placing them into adequate vessels. Bidistilled water was added to each one, under agitation, until total dissolution of solids.

Then, the solutions have been transferred to a round bottomed flask (2,000 mL) and mixed therein. The resulting mixture was heated up to 70 ºC to complete the reaction, under reflux, in order to be made possible future additions of NH4OH, that was added drop by drop into solution, to make it neutral or slightly alkaline, for subsequent precipitation of the organo-metallic complex.

The key-metal cations reacted with citric acid, under controlled pH conditions, to produce the respective metal citrate precursor. Ethanol was then added under vigorous stirring, into reacting mixture to promote the precipitation of barium, iron, zinc, copper and cobalt citrate gel complex. After that, the remaining aqueous solution was removed by drying at 70 oC, thereby leaving the desired solid phase, at a highly viscous residue. All the synthesis process was carried out in a N2 inert atmosphere (Caffarena, 2004).

Determination of the ideal temperature for the citrate gel decomposition, as well as the whole behavior of the complex under heating, was carried out by using thermogravimetric and differential thermal analysis. Based on the results from the thermal analyses, the gel was submitted to calcination inside muffle furnace with temperature variation from 600 to 1200 ºC (Caffarena, 2004).

The calcination was performed in the following heating schedule: 2 oC/min up to 410 oC, a 410 oC plateau for 1 h, 10 oC/min up to the final calcination temperature (with a residence time at the calcination temperature of 4 h). Then, it was cooled down at 10 oC/min to room temperature.

Figure 1 shows the experimental procedure employed for the hexaferrite preparation.

The calcined product was submitted to X-ray diffraction in order to assure the formation of the magnetic crystalline phase of the Z-type barium hexaferrite, which only occurs at 950 oC.

Thermogravimetry (TGA) and differential thermal (DTA) analyses were carried out using a TA Instru-ments SDT-2960. The experiments were carried out in static air, using platinum crucibles between 20 and 1,000 oC, with a heating rate of 10 oCmin-1.


Figure 1. Diagram of the Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite preparation by the citrate precursor method.

X-ray fluorescence measurements were carried out on a Philips model PW 2400 sequential spectrometer. This quantitative method was used to determine the stoichiometry of the ferrite samples, which were analyzed in the form of fused beads, using lithium tetraborate flux.

For the X-ray diffraction analysis, the solid materials were placed on a glass sample holder and spread out to form a thin layer. A Siemens AXS D5005 diffracto-meter with a dwell time of 1 o/min, in the θ-2θ Bragg-Brentano geometry was employed.

The morphological study was performed using a Topometrix II® Atomic Force Microscopy and a ZEISS Scanning Electron Microscope model DSM 940 A, operating with accelerating voltages of 20 kV, 24 kV and 25 kV.

The magnetic hysteresis loops of Ba3Co2Fe24O41 and Ba3Co1.3Zn0.3Cu0.4Fe24O41 compositions were obtained using the vibrating sample magnetometer VSM 4,500 PAR. The effect of temperature variation on the magnetization curves of Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite nanopowder (0.0251 g) was determined by using the PPMS extraction magnetometer Model 6000, in a temperature lower than the melting temperature of helium (4.2 K).

In order to obtain the composites for the microwave absorption measurements, the obtained powders were mixed with polychloroprene (CR), resulting in the composition of 80:20 (wt. %, ferrite:polychloroprene). The processing was carried out in a Berstorff two roll mill, at room temperature, with velocities of 22 and 25 rpm (back and forward). Vulcanized samples with 8.0 x 4.0 cm and variable thickness were obtained by compression moulding in a hydraulic press at 150 °C and 6.7 MPa. The dispersion of the magnetic particles in CR was evaluated by SEM.

The microwave absorption measurements were based on the transmission/reflection method (T/R) using a rectangular waveguide as the confining support for the samples (Caffarena, 2004). Using the data obtained (ε', ε'', μ' and μ'' values) from each of the analyzed samples measured, an expected prediction of the microwave reflectivity levels for sheet absorbers was made by plotting the variations of reflection loss (dB) versus frequency (GHz), using the HP 8510 network analyzer system. The materials were analyzed in the frequencies from 2.6 to 4.0 GHz (S-band) and 8.0 to 16.0 GHz (X/Ku- bands).

III. RESULTS AND DISCUSSION

Results from X-ray fluorescence analysis (XRF) confirmed that the synthesized powders achieved the planned stoichiometry. In turn, Fig. 2 illustrates TGA and DTA curves of the gel.

The ideal temperature for the citrate gel decomposi-tion and the behavior of the complex under heating were determined by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). Based on the results of these thermal analyses, the batch of dried solid was calcined inside a muffle furnace.

Then, calcination was performed using the following heating schedule: 2 C min−1 up to 410°C, a 410 C plateau for 1 h, 10 C min−1 up to the final sintering temperature with a 4-h residence time. The material was then cooled down to room temperature at a rate of 10 C min−1.


Figure 2. TG-DTA curves for the precursor gel.

Unlike preparation of M-type BaFe12O19, the Z-type phase cannot be directly obtained, due to the complexity of its structure, which imposes progressive transforma-tion through intermediate ferrites before achieving the final required structure.

The powder was calcined at different temperatures between 800 and 1100oC and the product were subject-ted to X-ray powder diffraction in order to ensure the formation of the crystalline and magnetic phase of Z-type barium hexaferrite.

X-ray Diffraction analysis (Fig. 3) indicates that: at 900 oC, Z-type hexaferrite becomes the predominant phase; and at 950 oC, this Z-type phase was clearly the majority phase. However, according to the literature (Pullar, 1998) this material contains trace amounts of Y-type phase (Ba2Co2Fe12O22), coexisting with the Z-type.

Atomic Force Microscopy was used to characterize the powders obtained at 950 oC/4 h. It is observed that the particles are mostly uniform in size and shaped as sharp hexagonal plates with nanometric size (Fig. 4).

The effect of Zn and Cu ions partial substitution on the hysteresis curves for the Ba3Co2Fe24O41 and Ba3Co1.3Zn0.3Cu0.4Fe24O41 calcined at 950 °C is illustrated in Fig. 5.

The hysteresis curves show typical feature of magnetically soft ferrites. The saturation magnetization Ms was obtained by extrapolating M(1/H)- curves to 1/H = 0, resulting in the value of 62,34 emu/g for Ba3Co1.3Zn0.3Cu0.4Fe24O41 and 52,39 emu/g for Ba3Co2Fe24O41.

Figure 6 shows the temperature effect on the magne-tization curves of Ba3Co1.3Zn0.3Cu0.4Fe24O41 nano-powder, evidencing the magnetic saturation increase with decreasing temperatures.

Table 1 shows the magnetic saturation (Ms) values obtained for the Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite nanopowder, in the considered temperatures.


Figure 3. X-ray Diffraction pattern to the synthesized Z-type hexaferrite at 950oC.


Figure 4. AFM of the Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite nanopowder.


Figure 5. Magnetic hysteresis curves of Ba3Co2Fe24O41 and Ba3Co1.3Zn0.3Cu0.4Fe24O41 powders at room temperature (25oC).


Figure 6. Magnetization curves of Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite at different temperatures (4.2 K to 300 K).

Table 1 - Values of magnetic saturation (Ms) for the Ba3Co1.3Zn0.3Cu0.4Fe24O41 nanopowder sample, obtained by the extraction magnetometer (maximum field of 90 kOe)

The Langevin's law can be expressed, as a function of the magnetic saturation (Ms), Boltzman's constant (kB), the magnetic permeability (μ), and the absolute temperature (T), by Eq. 1:

(1)

The deviations from the Langevin's law are attributed to interactions between particles, the presence of blocked particles, or relatively high anisotropies (Pinho et al, 2002).

An ideal superparamagnetic system exhibits a strong characteristic: the magnetization curves (M) measured at different temperatures, are superposed when plotted on a graph versus H/T. However, in practice this behavior is not usually observed, due to several complicating factors such as the presence of a magnetic momentum distribution inside a sample, due to varied particle sizes.

In nanocrystalline systems the magnetic interactions affect the experimental results. In the case of non-interacting particles, in accordance to the Langevin's law, the M/Ms versus H/T hysteresis curves are expected to superpose, which does not experimentally occur unless perfect superparamagnets are involved (Bertorello et al, 2004).

The system under study (Ba3Co1.3Zn0.3Cu0.4Fe24O41) follows the Langevin's law at temperatures up to 50 K, as shown in Fig. 7. Hence, there is an expected contribution of typical superparamagnetic behavior.

The normalized hysteresis curve corresponding to the temperature of 50 K is not superposable on the others, and the presence of magnetic interactions in the system may have exerted some effects. The superpara-magnetic model itself does not usually explain the macroscopic magnetic behavior in a magnetic nanopar-ticles' system. This discrepancy can be attributed to dipolar magnetic interactions that can cause significant alterations in the system's magnetic properties.

The SEM micrography (Fig. 8) for the composition 80:20 (wt. %) of Z-type hexaferrite nanopowder mixed with polychloroprene (CR), shows good dispersion and individualization of the nanoparticles, in spite of the high weight concentration used and the tendency of these nanoparticles to form magnetic agglomerates (Kwon et al, 1994).

Figure 9 illustrates the frequency dependence of dielectric (ε''/ε') and magnetic losses (μ''/μ'), for the composites at the frequency range of 2.6-4.0 GHz and 8.0 - 16.0 GHz in Fig. 10.


Figure 7. Hysteresis cycles normalized by the magnetic saturation (Ms) as a function of H/T for the Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexaferrite.


Figure 8. Scanning electron micrograph of Ba3Co1.3Zn0.3Cu0.4 Fe24O41:CR (80:20, wt. %).


Figure 9. Frequency dependence of dielectric and magnetic losses for the composite at S-band.

As it can be seen, the dielectric losses are very small (about 0.05) and the magnetic losses are higher than the dielectric ones for all frequencies, with higher values in the frequency range of 8.0-16.0 GHz. The change of these losses with frequency, may explain the performan-ce of the absorbers, illustrated by the reflectivity curves in Fig. 11. The reflection loss (RL) was calculated by Eq. 2, for a given f (frequency) and d (thickness) of the absorber:

RL (dB) = 20 log |(Zin - 1)/ (Zin + 1) | (2)

where Zin is the normalized input impedance given by

Zin = (μ/ε)1/2 tangh [j (2 π c) (μ/ε)1/2 f d] (3)


Figure 10. Frequency dependence of dielectric and magnetic losses for the composite at X-Ku band.

where μ is the complex permeability (μ=μ'- j μ''), ε is the complex permittivity (ε = ε' - j ε'') and c is the velocity of light (Caffarena, 2004).

From Fig. 11 (b), the substitution of Co by Cu and Zn, resulted in microwave absorptions greater than 90.0% at 10.0-12.5 GHz and at 13.0-16.0 GHz for X/Ku-bands, with 2.5 and 2.0 mm thick, respectively, which can be attributed to the highest magnetic loss, caused by the increase of the saturation magnetization of this ceramic that, consequently, improved the magne-tic permeability of the composite. So, the superpara-magnetic contribution led to the improvement of micro-wave absorption. For the S-band (Fig. 11-a), the best RAM performance at 3.1 GHz was obtained with greater thickness (6.0 mm).


(a)


(b)

Figure 11. Reflectivity curves for 80:20 (weight %) Ba3Co1.3Zn0.3Cu0.4Fe24O41:CR composites for (a) S-band and (b) X-kU bands.

IV. CONCLUSIONS

The partial substitution of Co for Zn and Cu ions, resulted on an increase of 11 emu/g in the Ms, and the superparamagnetic contribution, making possible its use as a potential magnetic loss material for X/Ku bands, with microwave absorptions greater than 90.0 % (reflectivity ≤ -10 dB) at 10.0-12.5 GHz and at 13.0-16.0 GHz, with 2.5 and 2.0 mm thickness, respectively.

ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support and other forms of aid provided by Petrobras, CNPq, PEMM/COPPE, CBPF, IF/UFRJ and IPqM, which were crucial for the success of this research.

REFERENCES
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Received: December 14, 2006.
Accepted: November 8, 2007.
Recommended by Subject Editor José Pinto.

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