Preparation and Characterization of Hexagonal W-type Barium Ferrite ...

Progr Colloid Polym Sci (2008) 135: 38–43 DOI 10.1007/2882_2008_099 © Springer-Verlag Berlin Heidelberg 2008 Published online: 9 September 2008

László Trif Gyula Tolnai István Sajó Erika Kálmán

László Trif (u) · Gyula Tolnai · Erika Kálmán Institute of Surface Chemistry and Catalysis, Chemical Research Center of the Hungarian Academy of Sciences, Pusztaszeri street 59–67, 1025 Budapest, Hungary e-mail: [email protected] István Sajó Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, Pusztaszeri street 59–67, 1025 Budapest, Hungary

NANOPARTICLE S , COLLOID SY STEMS

Preparation and Characterization of Hexagonal W-type Barium Ferrite Nanoparticles

Abstract New chemical synthesis procedure for preparation of nickel-zinc doped W-type hexagonal, nickel-zinc doped barium ferrite nanoparticles has been developed, using the nitrate-citrate sol–gel auto-combustion method (NCSAM). The crystalline phase attributes, microstructure, morphology, thermal behavior of the as-burnt phase and the sintered powders were characterized using XRD, SEM, TG-DTA, FT-IR measurements. The pure W-type ferrite phase is formed during

Introduction Hexagonal ferrites are a wide family of ferromagnetic oxides, with peculiar and useful properties. The crystal structure of the different known types of hexagonal ferrites (M, W, X, Y, Z, and U) is very complex and can be considered as a superposition of R and S blocks along the hexagonal c axis, RSR∗ S∗ for M-type and RSSR∗ S∗ S∗ for W-type, where R is a three-oxygen-layer block with composition BaFe6 O11 , S (spinel block) is a two-oxygen-layer block with composition Fe6 O8 , and the asterisk means that the corresponding block has been turned 180◦ around the hexagonal axis [1, 2]. Among the hexagonal ferrites W-type compounds with a general formula AMe2 Fe16 O27 (A = alkali earth metals, usually Ba, Sr, Ca, and Me = dblock metals, Zn, Co, Ni, etc.) have created interest due to the main magnetic parameter values, that are more suitable for microwave applications than those of other hexagonal ferrites [3]. A variety of techniques have been developed to synthesize W-type hexaferrites. The most used method for

4 h annealing at a temperature of 1200 ◦ C.

Keywords Ceramics · Hexaferrite · Nanoparticle · Self-combustion · Sol–gel

the preparation of barium ferrite in industrial and technical application is the classical ceramic method, which results in a material that is inhomogeneous at a microscopic scale [4]. The conventional mechanical grinding [5] and glass crystallization method [6] for W-type hexaferrites preparation have disadvantages such as time consuming and introducing impurities into material composition. Furthermore, the high calcination temperature (≥ 1300 ◦ C) results in the formation of coarse aggregation and the vaporization of some volatile components. Nowadays, new wet chemical methods have been developed for the preparation of nanograined hexagonal W-type ferrites. Various advantages such as low processing cost, energy efficiency and high production rate [7] are among the benefits of the sol– gel combustion synthesis method, which has been applied to the preparation of various high performance materials including ceramics, intermetallics, and composites [8, 9]. In this paper, sol–gel auto-combustion method has been used to synthesize nanocrystalline, nickel and zinc doped W-hexaferrite. The synthesis process and characterizations of nanocrystalline ferrite is reported.


Experimental Materials Nickel-zinc W-type barium hexaferrite powders were prepared by nitrate-citrate sol–gel auto-combustion method (NCSAM). The starting materials were Ba(NO3 )2 (≥ 99%), Ni(NO3 )2 · 6H2 O (≥ 97%), Zn(NO3 )2 · 6H2 O (≥ 99%), Fe(NO3 )3 · 9H2 O (≥ 97%), citric acid anhydrous (≥ 99.5%) and ammonia (∼ 25%), all of analytical purity. All the reagents were purchased from Fluka, and were used without any further purification. Preparation The preparation process can be described as follows. The stoichiometric amounts of barium, nickel, zinc nitrates and iron nitrates were dissolved together in distilled water, under continuous stirring during 30 min. The calculated amount of citric acid was poured into this mixture, changing the colour of the solution from orange to brownishyellow. The metal nitrates to citric acid molar ratio were chosen to be 1 : 2. After a subsequent 30 min of stirring, the pH was adjusted to the value of 2, by adding concentrated ammonia solution, followed by 6 h of continuous stirring at room temperature. The dark green coloured clear sol was heated on a hot plate at circa 110 ◦ C under continuous stirring, until the 3/4 volume of the water evaporated, and suddenly the gelation occurred, resulting a viscous dark green gel, which was further dried in a drying chamber at 140 ◦ C. During this process, the gel burnt in a self-propagating combustion manner (Fig. 1) to form a dark-grey colored fluffy, loose powder, which was heattreated in air atmosphere at various temperatures between 900–1200 ◦ C.

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in the range of 2θ = 10–70◦ , with a scanning speed of 0.04◦ min−1 on a Philips PW 105 diffractometer, operated at 40 kV, 35 mA and using Cu K α radiation. The average crystallite size in the sample was calculated using the Scherrer equation: D=

Kλ , h 1/2 cos Θ

(1)

where D = average size of the crystallites, K = Scherrer ˚ h 1/2 = constant, λ = wavelength of radiation (1.54186 A), peak width at half height and Θ corresponds to the peak position. Elemental Analysis by Inductively Coupled Plasma Spectroscopy (ICP). The stoichiometry analysis of the heat treated powders was carried out using a inductively coupled plasma spectrometer. The results were almost equal with the initial, calculated composition. Transmission Electron Spectroscopy (TEM). The micrograph and the crystallite size of the calcined samples were examined using a MORGAGNI 268D TEM (100 kV; W filament, top-entry; point-resolution = 0.5 nm) transmission electron microscope. Fourier Transformed Infrared Spectroscopy (FT-IR). The infrared spectra of the samples were recorded using a THERMO Nicolet AVATAR 320 FT-IR spectrometer (laser frequency 15 798.8 cm−1 ) in the range of 4000–400 cm−1 . The number of sample and background scans was 32 each. The recorded spectra were evaluated with the spectrometer’s software (EZ OMNIC version 6.1a). Sample preparation included mixing ∼ 0.7 mg powdered sample with 250 mg dried KBr and pressing them into pellets. Scanning Electron Microscopy (SEM). Scanning electron micrographs and analysis of the morphology of the samples were carried out on a JEOL JSM-6100 scanning microscope, operated at 15 kV high voltage, with EDAX EDS detector system.

Fig. 1 a Gel and b the product after completion of the reaction

Characterization X-ray Diffraction Spectroscopy (XRD). XRD patterns of the samples treated at various temperatures were recorded

Thermogravimetry and differential thermal analysis (TGDTA). The thermal behavior of the powders was investigated with thermo gravimetric analysis (TG) and differential thermal analysis (DTA), using a Setaram Setsys 16/18 TG-DTA instrument. Experimental conditions were: flowing synthetic air atmosphere (80% N2 , 20% O2 ), in the temperature range 25–1300 ◦ C, with a heating rate of 10 K/min, using 100 µl Al2 O3 crucibles. The spectra were evaluated using the thermogravimeter’s software (SETSOFT, ver. 1.54).

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Results and Discussion The X-ray diffraction (XRD) patterns of the studied Ba(Ni0.5 Zn0.5 )2 Fe16 O27 ferrite samples heat treated at various temperatures are shown in Fig. 2. In the case of the sample annealed during 4 h at 900 ◦ C (Fig. 2a), the main constituent is BaFe12 O19 M-type hexaferrite, contaminated with magnetite and hematite. When increasing the calcination temperature, the magnetite and hematite content decreases. During 4 h annealing at 1200 ◦ C, the formation of the pure Ba(Ni0.5 Zn0.5 )2 Fe16 O27 W-type ferrite phase can be ob-

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served (Fig. 2b). The crystalline size of the W-type ferrite, calculated using Scherrer‘s formula (Eq. 1), and found to be 110 nm. It can be concluded, that the formation of W-type ferrite occurs at high temperatures through complex solid-state reactions, in which the melted, previously formed M-type ferrite dissolves the remained unreacted iron, nickel and zinc oxides, to form pure Ba(Ni0.5 Zn0.5 )2 Fe16 O27 ferrite phase. The annealing temperature and time greatly influences the amount of W-type ferrite phase. The FT-IR spectrum of an as burnt powder, and a sintered sample at 1200 ◦ C, 4 h is shown in the Fig. 3.

Fig. 2 XRD pattern of hexaferrite powders sintered (a) 4 h at 900 ◦ C and (b) 4 h at 1200 ◦ C


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Fig. 3 FT-IR spectra of the as burnt and sintered powder

Fig. 4 TG and DTA curves of the gel dried at 110 ◦ C

On the as burnt powder spectrum, the broad absorption band around 3370 cm−1 is a characteristic stretching vibration of hydroxyl group (O–H), and absorption peaks around 1600 cm−1 and 1390 cm−1 are assigned to asym-

metrical and symmetrical stretching vibration of carboxylate groups coordinated to metallic ions. Peak localized at 1059 cm−1 is assigned to symmetrical stretching vibration of C–O–C group, and the band at ∼ 1430 cm−1 is charac-

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teristic of the presence of CO3 2− anion. The sharp peak localized at 850 cm−1 is assigned to deformation vibration of C–H group. In the range 1000–100 cm−1 , the IR bands of solids are usually assigned to vibration of ions in the crystal lattice [10]. Two main broad metal-oxygen bands are seen in the IR spectra of ferrites, which can be found on both curves in the Fig. 3. The highest one usually located in the range of 600–550 cm−1 , corresponds to stretching vibrations of metal ions at the tetrahedral site, whereas the lowest, observed in the range 450–370 cm−1 is assigned to stretching vibrations in octahedral sites. It could be concluded that after completion of the self-combustion reaction, the resulting powder’s main components are metal oxides and carbonates, but a small amount of unreacted organic material also can be found. In the spectra of the annealed powder only the peaks assigned to vibration of ions in the crystal lattice can be seen, so it could be concluded, that metal oxides are exclusively present, which is also proved by the XRD measurements. The TG-DTA results are shown in the Fig. 4. Measurements were carried out on small amounts of solid samples, obtained by drying the gel at ∼ 110 ◦ C in a drying chamber, followed by pulverization in an agate mortar. Evaluating the spectra, it can be seen, that a broad endothermic peak with two small shoulders located at 130.7 and 160.9 ◦ C respectively can be found in the temperature range room temperature (25 ◦ C)– ∼ 160 ◦ C, accompanied by a mass loss of approximately 6.67%, which can be attributed to the vaporization of planar and inner water. The largest sharp exothermal peak at 208.4 ◦ C, accompanied by a drastic mass loss (∼ 66%) in the temperature range of 160–260 ◦ C, is due to the autocatalytic oxidationreduction reaction between the metal nitrates (particularly the nitrate anions) and citric acid. In this reaction citric acid acts not only as chelating agent but also as fuel. The broader exothermic peak with the maximum at 358.5 ◦ C is accompanied only by a small weight loss. This is the result of the decomposition of the remaining organic matter, but also the decomposition of metal carbonates formed during the self-combustion reaction. In the temperature region 400–1300 ◦ C no significant mass loss occurs. A SEM micrograph of a ferrite sample heat treated 4 h at 1200 ◦ C is shown in Fig. 5. The particle size estimated from the photograph is ∼ 1–3 µm, and homogenous wellformed hexagonal-shaped crystals can be seen. Increasing the temperature the grain size of the ferrite increases also, which is in accordance with the results of Kishan Reddy et al. [11]. The microstructure of the ferrite powder obtained at 1200 ◦ C is shown in Fig. 6. It was observed, that the crystallites were more or less uniform in size with an average size of 100 nm, which is in good agreement with the calculated results obtained by the XRD measurements. The selected area electron diffraction (SAED) pattern is shown in the insert of Fig. 6, corresponding to that of a W-type ferrite phase.

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Fig. 5 SEM micrograph of the hexaferrite powder sintered 4 h at 1200 ◦ C (scale bar is 2 µm)

Fig. 6 TEM micrograph of the hexaferrite powder sintered 4 h at 1200 ◦ C (scale bar is 100 nm)

The ICP spectroscopy stoichiometry analysis result of a 1200 ◦ C calcined powder is summarized in Table 1. It can be seen that the elemental composition of the ferrite does not differ from the calculated theoretical values. Table 1 Elemental composition of the 4 h 1200 ◦ C sintered ferrite powder m/m %

Theoretical composition Sintered powder

Ba

Ni

Zn

Fe

8.65 8.82

3.69 3.75

4.12 3.99

56.30 56.41


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the pure materials in order to investigate their magnetic properties.

Conclusion The sol–gel auto-combustion method is convenient for the synthesis of nano-sized nickel-zinc doped W-type barium hexaferrites. The main components of the resulting powder, after the completion of self-combustion reaction, are metal oxides and carbonates. The formation of pure W-type ferrite occurs at relatively lower temperature (1200 ◦ C) than in the case of classical ceramic synthesis route. Magnetic measurements will be carried out on

Acknowledgement The authors are grateful to the financial support of the Hungarian National Office for Research and Technology (NKTH) in the frame of the NKFP-3/A 2004 (NANOFERRIT) project. László Trif wishes to thank to Patrizio Madau (ULB, Brussels, Belgium) for the SEM measurements and for the Bay Zoltán Foundation for Applied Research, Institute for Materials Science and Technology for the TG-DTA investigations.

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4. Haneda K et al. (1974) J Am Ceram Soc 57:68 5. Abo El Ata AM et al. (1999) J Magn Magn Mater 204:36 6. Sürig C et al. (1995) J Magn Magn Mater 150:270 7. Crider JF (1982) Ceram Eng Sci Proc 3:519

8. Avakyan PB et al. (1996) Am Ceram Soc Bull 75:50 9. Munir ZA (1988) Ceram Bull 67:342 10. Brabers VAM (1969) Phys Stat Solidi 33:563 11. Kishan Reddy N et al. (2002) Mater Chem Phys 76:75–77