Sintering characteristics of LiZn ferrites fabricated by a sol–gel process

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 52–55

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Sintering characteristics of LiZn ferrites fabricated by a sol–gel process X.N. Jiang , Z.W. Lan, Z. Yu, P.Y. Liu, D.Z. Chen, C.Y. Liu State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China

a r t i c l e in f o

a b s t r a c t

Article history: Received 28 September 2007 Received in revised form 14 July 2008 Available online 23 August 2008

Ultra-fine powders of LiZn ferrites with composition of Li0.4Zn0.2Fe2.4O4 were synthesized by a sol–gel process. X-ray diffraction pattern reflected that the synthesized powders were single spinel structure, with crystallite size of about 35 nm. Then the powders were granulated, pressed and sintered at different temperatures. The sintered samples were investigated by means of characterizing microstructures and magnetic properties by scanning electron microscope and B–H analyzer, respectively. The results indicate that when compared with a traditional ceramic process, the sol–gel process could slightly bring down the sintering temperature of LiZn ferrite whereas the microstructures are not homogeneous in the sintered samples. The sintering mechanisms of LiZn ferrites sintered at 1360 1C were studied, which reveal that during sintering, solid mass transfer is dominant in the LiZn ferrites fabricated by a traditional ceramic process while in the gel-derived ferrites, gas mass transfer is dominant. & 2008 Elsevier B.V. All rights reserved.

Keywords: LiZn ferrite Sol–gel Sintering temperature Gas mass transfer

1. Introduction Lithium and substituted lithium ferrites are promising for microwave device applications, for example, phase shifters. Compared with MgMn and Ni ferrites, lithium ferrites are less sensitive to stress and have higher Curie temperature when they are used for microwave devices. In addition, lithium ferrites do not contain costly ingredients [1]. In order to tailor the material to specific microwave requirements, compositions must be fabricated with room temperature magnetization from 200 to 5000 Gs [2]. Zn substitution is usually adopted because it is effective in adjusting the molecular magnetization moment [3] and reducing the magnetocrystalline anisotropy constant. Moreover, Zn can markedly improve the crystal grains growth and densification [1]. Therefore, Zn-substituted lithium (LiZn) ferrites are widely used in microwave applications. Compared with the traditional ceramic process, the sol–gel process is more helpful to achieve thin or thick film materials and adapt to the tendency of miniaturization and complanation [4]. Furthermore, sintering temperature of ferrites can be reduced besides low-melting additives (e.g., Bi2O3 and V2O5) because the gel-derived powders have high activity [5]. Ultra-fine ferrite powders fabricated by the sol–gel process have been investigated in recent years. Lee [6] fabricated z-type hexaferrites with ultra-fine powders at 850–930 1C. Yu et al. [7] prepared MnZn ferrites and studied the effect of pH on magnetic characteristics of

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E-mail addresses: [email protected], [email protected] (X.N. Jiang). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.07.044

ultra-fine powders. Yue et al. [5] prepared LiTiZn ferrites at 950 1C. Dey et al. [8] investigated lithium ferrite particles, of which most grains are 10–19 nm in diameter. The effects of pH values, compositions and reaction temperatures on the powder characteristics of LiZn ferrites have been investigated by many experts. In this paper, sintering characteristics and mechanisms of LiZn ferrites are investigated.

2. Experimental LiZn ferrite powders with composition of Li0.4Zn0.2Fe2.4O4 were prepared by the sol–gel processing route (designated as G, i.e. gel-derived route), and the mixed oxide method (designated as T, i.e. traditional ceramic process), respectively. For sol–gel processing, analar grade LiNO3, Fe(NO3)3  9H2O, Zn(NO3)2  6H2O and C6H8O7  H2O in required proportions were dissolved in de-ionized water and mixed together, and the citric acid was then added as a gelling agent. The pH value of the final solution was adjusted to 7 by some ammonia. Sol could be achieved by agitating the solution for 6 h at 60 1C. Dried gel was achieved after sol was oven-dried for 24 h at 120 1C. Ultra-fine powders of LiZn ferrites were achieved by an auto-combustion process. For the traditional ceramic process, analar grade Li2CO3, Fe2O3, ZnO in required proportions were mixed by wet balling milling for 2 h. The powders were calcined at 800 1C for 2 h after drying. Then the above two resultant powders were de-agglomerated by wet ball milling for 3 h. After drying at 95 1C and the powders were granulated with polyvinyl alcohol (PVA) and pressed into toroids (18 mm outside diameter, 8 mm inner

ARTICLE IN PRESS X.N. Jiang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 52–55

D ¼ 0:89l=ðB  B0 Þ cos y

(1)

120 Ο

Intensity (a. u.)

diameter and 4 mm height) at 70 MPa, then the samples were sintered for 2.5 h at different temperatures in air. Magnetic properties of the ferrites were measured by a B–H analyzer SY-8232, and the microstructures were observed using a S-530 type scanning electron microscope (SEM). Phase compositions of the powders were analyzed using a Philips PW 1729 type X-ray diffraction (XRD) with Cu Ka radiation. Average grain size of the auto-combustion powders was determined by Scherer equation which is expressed as [9]

53

Ο Li0.4Zn0.2Fe2.4O4

80

Ο

Ο

Ο

40 Ο

where D is the crystallite size in nm, l the radiation wavelength (0.1542 nm for Cu Ka), B the halfwidth, B0 the corrected value of halfwidth, and y the diffraction peak angle.

Ο

Ο

0 3. Results and discussion

20

Relationship of saturation magnetization Ms and saturation magnetic flux density Bs of ferrites can be expressed as Bs ¼ m0 ðH þ Ms Þ

sol-gel process 400

traditional ceramic process

350 Bs (mT)

40

50 2θ (degree)

60

70

Fig. 2. XRD pattern of auto-combustion powders.

(2)

where m0 is the space permeability and H the testing magnetic field (H ¼ 1194 A m1). Fig. 1 shows the variation of Bs (a) and Hc (b) with sintering temperature. Sintered at 980 1C, Bs of LiZn ferrites fabricated by the sol–gel process (designated as Bs,G) reaches 170 mT, while that of LiZn ferrites fabricated by the traditional ceramic process

300 250 200 150 1000

1100

1200

1300

1400

Sintering temperature (°C)

800

sol-gel process

700

traditional ceramic process

600 Hc (A⋅m-1)

30

500 400 300 200 100 1000

1100

1200

1300

1400

Sintering temperature (°C) Fig. 1. The variation of Bs (a) and Hc (b) with sintering temperature.

Table 1 The densities of sintered LiZn ferrites (g cm3) Sintering temperature (1C) Sol–gel process Traditional ceramic process

980 4.2 

1060 4.3 4.3

1160 4.3 4.7

1260 4.3 4.8

1360 4.3 4.8

(designated as Bs,T) is only 13.5 mT which is not exhibited in the figure. This indicates that the solid-phase reaction for both ferrites is incomplete because of low sintering temperature. In addition, it reveals that sintering temperature can be reduced by the sol–gel process. Average grain size of the gel-derived powders is in nanometer scale, which makes the gel-derived powders have higher activities and can be sintered at lower temperature as compared with the calcined powders which are of micron dimension. Crystallite size of the gel-derived powders was calculated by means of XRD. Fig. 2 shows the XRD pattern of auto-combustion powders. It exhibits that the composition of the powders is pure Li0.4Zn0.2 Fe2.4O4. Crystal size of the auto-combustion powders is calculated by the main peak of the XRD pattern by Scherer equation, which is about 35 nm, ensuring that powders have high surface ratio area and high activity which contributing to reduce sintering temperatures [10]. With the increase in sintering temperature, Bs of the ferrites prepared by the two different routes increase initially and reach their maxima at about 1120 and 1160 1C, respectively, then decrease with higher sintering temperature. Furthermore, Bs,G is lower than Bs,T. This can be explained by densification and Li volatilization. Lithium ferrites are difficult to densify sufficiently [1], so granulating and pressing gel-derived LiZn ferrites are more difficult because of the high surface ratio area. For gel-derived powders, an effective way to improve its density is granulation and press in a negative pressure condition [11]. Table 1 shows the densities of sintered LiZn ferrites fabricated by the two routes. At the same sintering temperature, the densities of gel-derived sintered samples are lower, and sintering temperature almost has no effect on the densities of gel-derived samples. While densities of the samples derived from the traditional ceramic process increase obviously from 1060 to 1160 1C, and then become almost changeless with further increasing sintering temperature. The relationship between Bs and densities d of ferrites can be expressed as Bs ¼ m0 ðH þ dNM=mÞ

(3)

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X.N. Jiang et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 52–55

where d is the density of LiZn ferrites, N the Avogadro constant, M the molecular magnetic moment of Li0.4Zn0.2Fe2.4O4, and m the molecular weight of Li0.4Zn0.2Fe2.4O4. Formula (3) reveals that Bs improves with increasing d. But if lithium volatilize heavily, Bs is determined by d and M. Bs decreases with the decrease in sintering temperature because Li volatilization induces the reduction of M. At about 1160 1C, Bs reaches the maximum and microstructures of the samples are fine. Above 1160 1C, Bs decreases with the increase in sintering temperature though densities of LiZn ferrites increase because of the aggravating Li volatilization. Fig. 1(b) shows that with the increase in sintering temperature, coercivity Hc decreases rapidly when sintering temperature is lower than 1160 1C, and then decreases slowly to stabilization. In addition, Hc of gel-derived ferrites is higher than that of samples fabricated by the traditional ceramic process. These indicate that there are more pores inside gel-derived ferrites and the grains are uneven below 1160 1C because Hc is related to the anisotropy field, porosity and grain size [1]. With the increase in sintering temperature, Hc decreases because the grain size increases and the porosity decreases. Fig. 3 shows SEM micrographs of samples sintered at (a) 1060 1C for sol–gel process, (b) 1060 1C for traditional ceramic process, (c) 1360 1C for sol–gel process and (d) 1360 1C for traditional ceramic process. In Fig. 3(a) and (b), the porosities are 12.7% and 12.1%, respectively. Furthermore, the crystal grains in Fig. 3(a) are not homogeneous because the powders in the flans reunite (see Fig. 4). The reunited powders grow up to big crystal grains (see Fig. 3(a)). In Fig. 3(b), the crystal size is more homogeneous except for many pores because the crystallite size of the calcined powders is several microns and the activity of the powders is lower. Sintered at 1160 1C, Hc of gel-derived samples (designated as Hc,G) reaches 332 A m1, whereas that of samples fabricated by a traditional ceramic process (designated as Hc,T) is only 104 A m1. This can be explained by microstructures. Hc is proportional to

domain wall energy in activation volume [12] and Hc is high when the microstructures of sintered ferrites are inhomogeneous. Microstructures of samples fabricated by the traditional ceramic process are more uniform than that of gel-derived sintered magnets, leading to the lower Hc. According to sintering dynamics, the crystal grain growth depends on grain boundaries migrating and larger crystal grains swallowing the small ones [13]. During the growth the more different in size of crystal grains, the more beneficial for larger crystal grains to swallow smaller ones, so some pores inside the sintered ferrites are eliminated and the crystal growth improves. Gas mass transfer is mainly achieved by the gas phase among crystallites in sintered ferrites and solid mass transfer is realized by interactions among surfaces, interfaces or diffusions in sintered samples [14]. At 1360 1C, sintering mechanisms of samples prepared by the two routes are different. This can be interpreted

Fig. 4. SEM micrograph of the flan fabricated by a sol–gel process.

Fig. 3. SEM micrographs of samples sintered at (a) 1060 1C for sol–gel process, (b) 1060 1C for traditional ceramic process, (c) 1360 1C for sol–gel process and (d) 1360 1C for traditional ceramic process.

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by the different microstructures. Microstructures of the ferrites sintered at 1360 1C are shown in Fig. 3(c) and (d). In Fig. 3(c), there are many pores in the cores. Furthermore, the crystals are not homogeneous. This phenomenon is similar to that of Fig. 3(a), but the crystal size in Fig. 3(c) is 6 times as that in Fig. 3(a) due to higher sintering temperature. It indicates that it is difficult for crystals growth in gel-derived samples, so the pores become changeless though the crystal grain size increases. In Fig. 3(d), almost all crystal grains become one gigantic crystal and the crystal boundaries almost disappear. There are some pores inside the gigantic crystal grains. It shows that solid mass transfer is yet dominant in ferrites fabricated by a traditional ceramic process sintered at 1360 1C. The surroundings inside gel-derived LiZn ferrites are closed relatively. Many Li+ volatilize into pores and then congeal in the sintered samples. The repeated evaporating and congealing phenomenon inside the sintered ferrites is called a gas mass transfer process, in which the shape of pores changes but it is difficult for pores to vent out from the sintered ferrites. Sintered at 1360 1C, gas mass transfer is dominant for gel-derived ferrites, so lithium volatilization is restrained and pores inside the ferrites cannot be removed. This is the reason why the densities of gel-derived ferrites cannot be improved and crystal grains grow difficultly, which gives rise to the decrease of Bs and increase of Hc accordingly.

4. Conclusions It is difficult to densify gel-derived nanometer powders because of high surface ratio areas and reactive activities. Sintering temperature of gel-derived ferrites decrease slightly, because the activity of gel-derived powders was reduced after ball

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milling and granulation. Sintered at above 1060 1C, Bs,G is lower than Bs,T and Hc,G is higher than Hc,T. Pure LiZn ferrites prepared by the two routes can be sintered at about 1160 1C to achieve high Bs and low Hc. Sintered at 1360 1C, gas mass transfer is dominant for gel-derived ferrites while for LiZn ferrites prepared by the traditional ceramic process solid mass transfer is dominant.

Acknowledgement The authors are grateful for the financial support from the department of defense of China (no. 51307060305). References [1] P.D. BaBa, IEEE Trans. Magn. MAG-8 (1972) 83. [2] G.M. Argentina, P.D. Baba, IEEE Trans. Microwave Theory Tech. MTT-22 (1974) 652. [3] M. Nogues, J.L. Dormann, M. Perrin, et al., IEEE Trans. Magn. MAG-15 (1979) 1729. [4] M. Pardavi-Horvath, J. Magn. Magn. Mater. 215–216 (2000) 171. [5] Z.X. Yue, J. Zhou, X.H. Wang, et al., J. Eur. Ceram. Soc. 23 (2003) 189. [6] J.G. Lee, J. Mater. Sci. 33 (1998) 3965. [7] Z. Yu, Z.W. Lan, J.M. Wang, et al., Gongneng Cailiao S1 (2000) 34. [8] S. Dey, A. Roy, D. Das, et al., J. Magn. Magn. Mater. 270 (2004) 224. [9] H.P. Klug, L.E. Alexander, J. Appl. Crystallogr. 8 (1975) 573. [10] L.D. Zhang, J.M. Mou, Nanometer Materials and Nanometer Structure, House of Scientific, Beijing, 2001. [11] Z.K. Zhang, Z.L. Cui, Nanometer Technology and Nanometer Materials, House of Defense Industry, Beijing, 2001. [12] D. Givord, P. Tenaud, T. Viadieu, IEEE Trans. Magn. 24 (1988) 1921. [13] G.W. Cui, Defects Diffusion and Sintering, House of Tsinghua University, Beijing, 1990. [14] B.R. Li, Principle of Electronic Ceramics Process, Huazhong Institute of Technology, Wuhan, 1986.