Nanophased CoFe2O4 prepared by combustion method

Nanophased CoFe2O4 prepared by combustion method

PERGAMON Solid State Communications 111 (1999) 287–291 Nanophased CoFe2O4 prepared by combustion method C.-H. Yan*, Z.-G. Xu, F.-X. Cheng, Z.-M. Wan...

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PERGAMON

Solid State Communications 111 (1999) 287–291

Nanophased CoFe2O4 prepared by combustion method C.-H. Yan*, Z.-G. Xu, F.-X. Cheng, Z.-M. Wang, L.-D. Sun, C.-S. Liao, J.-T. Jia State Key Laboratory of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, People’s Republic of China Received 28 January 1999; accepted 17 February 1999 by Z.Z. Gan

Abstract The combustion method has been utilized to prepare nanophased powders of cobalt spinel ferrite using glycine as fuel. Structural and magnetic properties of the products were investigated with an X-ray diffractometer, a surface analyzer, and an alternating gradient magnetometer, respectively. Cobalt spinel ferrite prepared by the present method can easily form the wellcrystallized nanoscale particles with a large specific surface area. The magnetization and coercivity show a strong dependence on the G/N (glycine to nitrates) ratio in the range from 0.2 to 1.0. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetically ordered materials; A. Nanostructures; B. Chemical synthesis; B. Nanofabrications

1. Introduction Homogeneous, highly crystalline nanophased powders are needed for the materials with a wide range of technical applications. The conventional methods for the preparation of powder materials involve some soft chemical processes such as sol– gel and precipitation methods, and the solid state reaction of finely ground powders that are heated at temperatures above at least 10008C for up to several days. The combustion method [1–3] has been introduced in some literature to speed up the synthesis of complex materials. This method is characterized by its simpler process, a significant saving in time and energy consumption over the traditional methods, and small crystallite size of the resultants, latter of which may have an important influence on the properties of * Corresponding author. Tel.: 1 86-10-6275-4179; fax: 1 8610-6275-5926. E-mail address: [email protected] (C.H. Yan)

the materials prepared. This method has been successfully employed to obtain high Tc superconductors and some perovskite-type semiconductor materials [4–7]. We here report a novel combustion route for the production of cobalt spinel ferrite, CoFe2O4, which has been regarded as one of the competitive candidates for high-density recording media because of its moderate saturation magnetization, high coercivity, mechanical hardness and chemical stability [8,9] 2. Experimental In an appropriate ratio, analytical grade nitrates Fe(NO3)3·9H2O, Co(NO3)2·6H2O and glycine were dissolved in deionized water to obtain the precursor solution. The precursor solution was concentrated in a hot porcelain crucible until excess free water evaporated, and the final spontaneous ignition occurred. Within a few seconds, the combustion reaction was completed with the resultant black, porous ash filling the container. For comparison in the present study, two other samples were also synthesized by the

0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00119-2

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(MicroMage, 2900) at room temperature and the hysteresis loops were recorded up to 20 kOe. All the magnetization values presented in this paper are in conventional cgs electromagnetic units (emu), and the antimagnetic contribution of the polymer membrane used to wrap the samples was separately measured and subtracted from the original data.

3. Results and discussion Glycine is considered to serve as fuel for the combustion reaction, being oxidized by nitrate ions. Stoichiometrically balanced [10], the exothermic reaction can be expressed as Fig. 1. Typical IR spectra for samples b and c.

well-established sol–gel method [8] and the solid state reaction method. The phases of all the products were identified by a X-ray diffractometer (XRD, Dmax-2000, Rigaku, CuKa radiation). The IR spectra were recorded using a Nicolet magna-IR 750 spectrometer with a diffusive deflection attachment. A Micrometeritics ASAP surface analyzer was used to measure the BET specific surface. The magnetic measurements were performed on an alternating gradient magnetometer

Fig. 2. X-ray diffraction patterns of powder samples a–e annealed at 4008C for 1 h.

18Fe…NO3 †3 1 9Co…NO3 †2 1 32NH2 CH2 COOH 1 54O2 ! 9CoFe2 O4 1 16N2 1 72NO2 1 34CO2 1 80H2 O The temperature reached in the combustion reaction has an important effect on the crystallite size of the powder resultants. By adjusting the glycine-to-nitrite ratio (G/N), we can control the reaction temperature, and thereby control the crystallite size of the resultants. Our results indicate that the lower G/N value leads to the smaller crystallite size. The IR spectra show a few traces of residual organic compounds and NO2 3 ions in the as-prepared samples, these samples were therefore calcinated at 4008C for 1 h to decompose the residual substances completely. For the sake of convenience, the calcinated samples were numbered from a to f relative to different precursor solutions in which the G/N values were 0.2, 0.5, 1.0, 1.5, 2.0 and 2.5, respectively. Fig. 1 shows the typical IR spectra of samples b and c. The IR spectra of samples a–f show no residual organic compounds and NO2 3 ions after calcination, and the peaks at about 700 cm 21 are assigned to the stretching vibrating mode of M–O (M ˆ Fe,Co) bands. The XRD patterns of samples a–e are shown in Fig. 2. All the diffraction peaks correspond to a spinel-type ˚ . There lattice (space group Fd3m) with a0 ˆ 8.380 A are no detectable traces of extra crystalline or amorphous phase. When the G/N ratio of the precursor solution is much lower than the stoichiometrical ratio (32/27), the diffraction peaks of the samples

C.-H. Yan et al. / Solid State Communications 111 (1999) 287–291 Table 1 Average crystallite size (nm) estimated from the FWHMs of the Xray diffraction peaks Sample no. 311

400

333

440

Average

a b c da

4 11 86 . 100

4 16 90 . 100

4 13 86 . 100

4 13 85 –

4 13 80 . 100

a The present evaluation is inappropriate for the grain size larger than 100 nm due to a large resulted deviation.

were broadened, but still can be resolved on a smooth background. The full-width at half-maximum (FWHM) of the peaks decrease with increase in the G/N ratio. The average size of the crystallites can be estimated from the FWHMs of the peaks using the Scherrer equation, and the results are listed in Table 1. The resultant nanoscaled crystallite size of samples is owing to two characteristics of the reaction system. One is that the reactants have been uniformly dispersed at an atomic or molecular level before reaction, so when ignition occurred, the nucleation process can be completed through only the rearrangement and short-distance diffusion of nearby atoms. The other is that the rate of combustion reaction is so high that enough time and energy are not provided for the long-distance diffusion of atoms and obvious growth of the crystallites, as a result of which, the initial nanophase is retained. Table 2 shows the lattice constants of CoFe2O4 obtained from the combustion, sol–gel route and solid-state reaction. There are no remarkable differences among them, which indicates that the combustion method is suitable for the preparation of the cobalt spinel ferrite. Table 3 displays the reproducibility of the present

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Table 3 Reproducibility of the combustion method Aliquot no.

˚) Lattice constant a0 (A

˚ 3) Volume (A

1 2 3 4

8.380 8.373 8.385 8.381

588.5 587.0 589.5 588.7

combustion method. All the samples were derived from the same component precursor solution with the G/N ratio of 1.0. The XRD results show no significant differences among them, which reveals the good reproducibility of this preparation method. The BET surface area plots and the isotherm adsorption/desorption curves were recorded on a surface analyzer. The results show that the specific surface area of sample b is 29 m 2/g, whereas that of the corresponding bulk sample is only 0.33 m 2/g, about 1% of sample b. Such a large surface area of sample b can be ascribed to its small crystallite size and the large number of pores among the grains. The latter is confirmed by the morphology of sample b (see Fig. 3) viewed with an Amary 1910 field emission scanning electron microscope (SEM). The dependence of the saturated magnetization (Ms) and the coercive force (Hc) of all the samples on the G/N ratio is displayed in Fig. 4. The magnetization of sample a is considerably lower than those of the others, which is due to the superparamagnetic behavior caused by its small crystallite size as revealed by XRD characterization. As previously reported [11], when the crystallite size is less than

Table 2 Lattice constants of CoFe2O4 powders obtained by different preparation methods Method of preparation

˚) Lattice constant a0 (A

˚ 3) Volume (A

Combustion a Sol–gel method Solid state

8.380 8.382 8.384

588.5 588.9 589.3

a

Average of the four aliquots listed in Table 3.

Fig. 3. SEM image of the sample prepared with the G/N ratio 0.5.

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products exceed the single domain size, consequently, Hc keeps steady, but shows a slight decreasing tendency. 4. Conclusion

Fig. 4. The variations of Hc and Ms of CoFe2O4 powder with the G/N ratio.

14 nm, CoFe2O4 will present a predominantly superparamagnetic behavior at room temperature, therefore sample a with an average crystallite size of about 4 nm displays the superpararmagnetic property, while samples c–f exhibit the similar magnetic behavior to the corresponding bulk materials for their fine crystallization and relatively large crystallite sizes. As for sample b, considering the statistical meaning of crystallite sizes estimated from the XRD data, it is understandable that the sizes of some crystallites are above 14 nm, and others below 14 nm. Thus the Ms of sample b, 34 emu/g, about half that of the corresponding bulk material (80 emu/g) [12], is reasonable. The G/N ratio dependence of Hc of samples a–f is similar to that of Ms. Since the samples were crystallized in the absence of external field, their anisotropy is mainly from the magnetocrystalline contribution, as a result, there is weaker coercive force for sample b because of its poor crystallization. Although the coercive force is closely related to microstructure as well as some other complex factors, it has been confirmed that the dimension of the magnetic crystallites exceeding the single domain critical size (70 nm for CoFe2O4) will cause the decrease of the coercive force for the appearance of multi-domain grains [13]. The crystallite size increases with the G/N ratio. When the ratio is larger than 1.0, the crystallite sizes of

The novel combustion method provides a rapid and reproducible route for the preparation of cobalt spinel ferrite using glycine as a fuel. At the high temperature reached in this reaction system, the reaction rate is no longer rate limiting and the crystallized composite oxide can be formed in quite a short time. The materials prepared by the combustion method are porous powders with a considerably large surface area. The magnetization and coercivity of the resulting ferrites show a strong dependence on the G/N ratio at the range from 0.2 to 1.0. The novel combustion processing provides a convenient and fast approach to doping with foreign ions, to modify the properties of Co ferrite. This work is now underway. Acknowledgements This project was supported by NSFC (Nos. 29701001&29525101), the Target Basic Research Project of MOST, the Training Project for Doctoral Student of MOE, and Founder Foundation of Peking University. References [1] Hlavacek, Ceram. Bull. 70 (1991) 240–243. [2] H.C. Yi, J.J. Moore, J. Mater. Sci. 25 (1990) 1159. [3] Y. Tao, G.W. Zhao, W.P. Zhang, S.D. Xia, Mater. Res. Bull. 32 (1996) 501. [4] L.A. Chick, L.P. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. [5] L.P. Pederson, G.D. Maupin, W.J. Weber, D.J. Macready, R.W. Stephens, Mater. Lett. 10 (1990) 437. [6] P.R. Bonneau, R.F. Jarvis Jr, R.B. Kaner, Nature 349 (1991) 510. [7] V.G. Milt, R. Spretz, M.A. Ulla, E.A. Lombardo, J. Mater. Sci. Lett. 14 (1995) 428. [8] F.X. Cheng, Z.Y. Peng, C.S. Liao, Z.G. Xu, S. Gao, C.H. Yan, Solid State Commun. 107 (1996) 471. [9] J.G. Lee, J.Y. Park, Y.J. Oh, C.S. Kim, J. Appl. Phys. 84 (1998) 2801. [10] S.R. Jain, K.C. Adiga, V. Pai Verneker, Combust. Flame 40 (1981) 71.

C.-H. Yan et al. / Solid State Communications 111 (1999) 287–291 [11] T. Sato, T. Iijima, M. Seki, N.J. Inagaki, J. Magn. Magn. Mater. 65 (1987) 252. [12] A. Berkowitz, W.T. Schuele, J. Appl. Phys. 30 (1959) 134S.

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