Ni–Zn nanoferrite for radar-absorbing material

Abstract Nanoparticles of nickel–zinc ferrite have been prepared by using the citrate precursor method. According to scanning electron microscopy (SEM), the particle size is nanometric for the powder calcined at 350 °C/3.5 h. The phase formation has been studied by applying different calcining atmospheres, such as air and argon. Pure Ni–Zn ferrite has been observed when calcined in argon at the temperature of 350 °C. Hysteresis analyses have been done with magnetization of 53.01 emu/g at 350 °C and obtaining 84.62 emu/g at 1100 °C due to an optimization of domains formation at high temperature. Measures of reflectivity of Ni–Zn ferrite/epoxy composite have been obtained below 21% at 350 °C and above 96% at 1100 °C with a coercive field of 26.61 Oe. Low value of coercive field increased the mobilization of domains wall and increased the radiation absorption.


Introduction
Nanoferrites based on Ni-Zn are of great importance from the point of view of scientific and industrial applications [1]. They have been used as high-frequency ferrites for transformers cores, rod antennas, radio frequency coil and more recently as radar-absorbing materials (RAM) [2]. The usual process of preparing ferrite materials is the conventional solid solution method, which starts from metallic oxides. The use of heating for long time caused evaporation of some of the constituents, changed the initial stoichiometry and increased the particle size. The use of wet chemical methods led to obtaining a high homogeneity of phases, fine particle size and promoted high magnetic characteristics [3][4][5].
RAM have the property to change electromagnetic radiation energy by thermal energy. These materials have been used in several areas, such as telecommunication in cellular telephones and reception/transmission antennas. Studies done by Paulo et al. [6] developed a RAM based on Ni-Zn ferrite into a polychloroprene matrix. The composite showed high RAM performance with a microwave absorption that was greater than 96.9% (À15 dB). Dias et al. [7] used commercial ferrites based on Ni-Zn and Mn-Zn for radar-absorbing materials. The best values of radiation attenuation ($70%) are obtained when the polyurethanic formulations contain, simultaneously, carbon black and ferrites particles.
This work aimed to synthesize a nanoferrite based on Ni-Zn (spinel phase) by using the citrate precursor method. The powder was mixed with epoxy resin and analyzed by applying the principle of RAM. The analysis was done by the wave-guide method, considering the absorbing frequency range from 8 to 12 GHz. In this case, the ferrite is the absorption center of the incident radiation and the epoxy resin, with its electrical conductivity, favors energy loss and heat.

Experimental procedure
The materials used in the synthesis of Ni 0. 5  were weighed according to the required stoichiometric portion. Three solutions were prepared using small portions of distilled water, citric acid and nitrate. The solutions were heated at 70 1C during 2 h, for homogenization. The metallic citrates were mixed and homogenized during 2 h forming a complex stoichiometric resin. The polymeric resin was calcined from 350 to 1100 1C and analyzed by thermo gravimetric analysis (TGA), infrared spectroscopy (IR) and X-ray diffraction pattern (XRD). The Ni-Zn ferrite was mixed with epoxy in a radio of 40/60 wt% and the reflectivity was analyzed. The TGA was performed in the powder calcined at 350 1C/3.5 h in a thermo balance (Perkin-Elmer model TGA-7 HT) with the heating temperature up to 1200 1C and a heating rate of 50 1C/min with nitrogen flux. The infrared equipment was a BOMEN model ABB, MB 104 series with transmission from 4000 to 400 cm À1 using pellets of KBr with a resolution of 4 cm À1 . Analysis by XRD was done by X-ray diffraction SHIMATZU XRD 6000 with CuKa radiation of 1.5418 Å , tension of 30 KV and current of 20 mA. The refinement method of Rietveld was applied with the MAUD program (Materials Analysis Using Diffraction) 2.044 version. The powder was analyzed by scanning electron microscopy, MEV by using an XL 30 ESEM model of Philips with a voltage acceleration of 20 kV. The electromagnetic characterization was performed using a direction coupler model X752C (Hewlett-Packard), a synthetized seeper model 83,752A (Agilent) and a spectrum analyzer model 70,000 (Hewlett-Packard).
Reflectivity measures were carried out using the wave-guide method [8]. The powder was mixed with epoxy resin and analyzed in a spectrum analyzer model 70,000 (Hewlett-Packard). Fig. 1 shows the XRD pattern of calcined powder in ambient atmosphere ranging from 350 1C/3.5 h to 1100 1C/3 h with an occurrence of Ni-Zn ferrite formation up to 500 1C/3 h and Ni-Zn ferrite and hematite phase above 600 1C/3 h (Table 1). Hematite phase is basically antiferrimagnetic and alters the powder's magnetic characteristics.

Results and discussion
To obtain only one phase, the powder was calcined from 1000 to 1100 1C in argon atmosphere and the phases analyzed by the Rietveld method (Fig. 2) and shows the Ni-Zn ferrite phase without formation of the hematite phase. According to Table 2  These results indicated that the stoichiometry and powder homogeneity were maintained with only one formed phase.
The powder analysis by scanning electron microscopy (SEM) (Fig. 3) shows an average particle size from 20 to   100 nm at 350 1C, 0.5 mm in the powder calcined at 1000 1C and 1.0-1.5 mm at 1100 1C. The range of particle size at 1000 and 1100 1C obtained by SEM is close to those results obtained by the Rietveld method (Table 3). The grain size could indicate that the domain volume is limited in the powder calcined at 1000 1C (Fig. 3a). Grain sizes above 1 mm (Fig. 3b) facilitate an increase of domain wall motion during the absorption radiation. The calculated Dcritical is 770.20 nm and the particle size of powder calcined at 1000 1C/3 h obtained by the Rietveld method is 429.00 nm. The critical diameter of particles, in this case, is above the average particle size of powder, indicating that monodomains were formed with a coercive field of 34.76 Oe. The sample calcined at 1100 1C presents a particle size of 979.12 nm with an indication that multidomain is present with a coercive field of 26.61 Oe. Fig. 4 shows a decrease of coercive field with an increase of particle diameter. A high value of magnetization was obtained at 350 1C despite the disturbed structure of surface of the powder that reduces the magnetization. The raise in calcining temperature increases the particle size and decreases the coercive field   with an indication that multidomains were formed into the powder calcined from 1000 to 1100 1C. The multidomains formation led to coercive field decrease, causing an increase of mobility of domains wall. Fig. 5 shows the magnetic hysteresis of powder calcined from 350 1C/3.5 h to 1000 1C/3 h. The saturation magnetization of powder calcined at 350 1C in ambient atmosphere is 53.01 emu/g and the powder calcined at 1000 and 1100 1C obtained 88.31 and 84.62 emu/g, respectively, when calcined in argon. The low magnetization in 350 1C is due to the absence of domains with very small particle size (20 nm) below critical diameter (22.50 nm). Both powders calcined at 1000 and 1100 1C present high magnetization; however, the magnetization is more effective in the powder calcined at 1000 1C due to the bulk of structure. The level of magnetization indicated high magnetic performance of powder with an optimization of domain size resulting in a high magnetic moment into the crystal. The powder calcined at 1000 1C in ambient atmosphere obtained 69.25 emu/g below those obtained for the powder calcined in argon (88.31 emu/g) due to the hematite phase formation (nonmagnetic phase) that decreases the magnetization of the material.

ARTICLE IN PRESS
Analysis of powder reflectivity (absorption radiation) calcined at 350 1C/3 h, (Fig. 6) shows attenuation below 1 dB, presenting a reflectivity of o21%. The magnetic performance of powder presents low saturation magnetization (53.01 emu/g) when compared with those obtained at temperatures from 1000 to 1100 1C. The powder calcined at 350 1C did form an antiferrimagnetic hematite phase that degraded the magnetic characteristics of the material. Antiparalell spins needs a high level of energy for change direction into the atoms. These characteristics increase the coercive field of the ferrite phase due to the minor mobility    of domain walls into the structure. The average attenuation of powder calcined at 1000 1C (Fig. 7) presents À1 dB of attenuation with a reflectivity of 21%. The powder calcined at 1100 1C (Fig. 8) shows À3.2 dB in 8GHz and À15 dB in 12 GHz, presenting a reflectivity of 96.6% (À15 dB). Dias et al. [7] developed a work based on polyurethanic coating containing carbon and Ni-Zn ferrite particles and obtained an absorption radiation of 65% (À4 dB) in the range 8-12 GHz. The coercive field of the calcined material at 350 1C was 47.45 Oe and decreases to 34.76 Oe at 1000 1C and 26.61 Oe at 1100 1C. These indicated that at 1100 1C the domain walls mobility is more intense. The increase of magnetization due to the domain walls mobility caused by the low coercive field needs energy in the electronic structure, favors the radiation absorption considering that the absorption alters the spin state (magnetization) and facilitates the heat dissipation into the material. The absorption of microwave by the domains is related to domain wall mobility. Low coercive field values led to domain walls mobilization that absorbs the magnetic wave and changes it into heat. The minor reflectivity obtained in a sample calcined at 1000 1C is due to the higher coercive field related to the powder calcined at 1100 1C.

Conclusion
The calcinations of powder in argon atmosphere form only one phase of Ni-Zn ferrite. According to the Rietveld method, the synthesis process is efficient to maintain the stoichiometry and analysis by SEM observed nanometric particles at low calcining temperature. The magnetization increases with the raise of calcining temperature with substantial magnetization at 350 1C and multidomains formation when calcined at 1000 and 1100 1C in argon. The results of powder reflectivity are similar to those of magnetization measures showing reflectivity below 21% at 350 1C, 21% at 1000 1C and 96.6% at 1100 1C. These results indicate that the material calcined at 1100 1C could be used for RAM.