Synthesis of Co2Z Ba-ferrites

Materials Letters 59 (2005) 3959 – 3962 www.elsevier.com/locate/matlet

Synthesis of Co2Z Ba-ferrites J. Jeong a, K.W. Cho a, D.W. Hahn a, B.C. Moon b, Y.H. Han a,* a

Department of Materials Engineering, Sungkyunkwan University, Suwon 440-746, Korea b Samsung Electromechanics Co., Suwon 442-743, Korea Received 21 May 2005; accepted 16 July 2005 Available online 3 August 2005

Abstract Synthesis of Co2Z (Ba3Co2Fe24O41) Ba-ferrites were studied in terms of process conditions. Co2Z phase was synthesized by one-step calcination at 1350 -C. Single Co2Z phase was obtained through calcining powders first at 900¨1100 -C and then postcalcination at 1350 -C, and other extra phases disappeared completely. Permeabilities of the Co2Z ferrites synthesized by the two-step calcination at 900 and 1350 -C ranged between 3.5¨4.5, whereas the specimens including the intermediate products Co2Y and M phases formed by the one-step calcination at 1300 -C showed smaller permeabilities about 2 to 2.5. D 2005 Elsevier B.V. All rights reserved. Keywords: Co2Z Ba ferrite; Inductor; Two-step calcination

1. Introduction Various types of electronic devices require inductors for GHz range applications. NiCuZn ferrites have been extensively studied as a material of choice for MHz range. The resonance frequency of NiCuZn ferrites is below 500 MHz. Hexagonal Ba-ferrites are widely suggested as a ferromagnetic material for GHz range application [1 –3]. The Ba ferrites with M (BaFe12O19), Co2Z (Ba3Co2Fe24O41), Co2Y (Ba2Co2Fe12O22) and Co2W (BaCo2Fe16O27) types were developed in the 1950s by Philips researchers. [4,5]. Co2Z ferrite has been chosen as the inductor material for GHz range application because of relatively high permeability and resonance frequency [2,3,6 –8]. Co2Z ferrites exhibit a similar magnetization behavior to soft ferrites at room temperature because the magnetic vector can easily rotate within the preferred plane, although a large amount of energy is consumed to move out of this plane [9– 11]. Owing to these properties, the Co2Z ferrite is applied as an inductor material for the GHz region. However, the single phase of Co2Z ferrite is not easy to be synthesized because of its complex crystal structure. * Corresponding author. Tel.: +82 31 290 7392; fax: +82 31 290 7410. E-mail address: [email protected] (Y.H. Han). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.044

In the conventional ceramic process, Co2Z ferrite powders are formed at high temperature above 1250 -C [1,2,6,12,13]. Pullar et al. suggested that the Co2Z phase is obtained from the topotactic reaction of the two previously formed M and Co2Y phases [10,13]. Tachibana et al. reported that the high oxygen partial pressure (Po2) during the sintering process and the increase in iron content (x = 0¨0.6) of Ba3Co2 xFe24 + xO41 are effective to obtain the single Co2Z phase [12]. The single Co2Z phase was also obtained in the samples doped with Cu – Si and Mn sintered at temperature range above 1250 -C [6]. Several chemical routes such as citrate precursor, self propagating and sol – gel method have been also studied to synthesize single Co2Z ferrite powders at the low temperature [3,4,10,14]. However, it has been confirmed that the synthesis of Co2Z powders is difficult at temperature range below 1100 -C. Even though the mixing process of raw materials is carefully controlled and the calcination temperature is kept above 1100 -C, most of calcined Co2Z ferrite powders include the extra phases such as Co2W, Co2Y, and M ferrites [1 –4,10,14]. It is thus required to develop a new powder preparation technique for the single phase of Co2Z ferrite. Two-step calcination technique has been used to synthesize the oxides having a complex structure such as lead-based multi-component ferroelectrics [15]. In the two-step calci-

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(c) 1300oC

synthesized powders were examined at room temperature using a Rigaku X-ray diffractometer with Cu –Ka radiation. The specific surface area of pulverized powder was obtained by Micromeritics surface area analyzer (Gemini Ø-2375) using a Brunauer – Emmett –Teller (BET) method. Agilent impedance analyzer (4291B) and fixture (16454A) were used to measure the frequency dependence of permeability (l) and quality factor ( Q).

(b) 1100oC

3. Results and discussions

Standard Fe2O3 Standard B Standard M

(a) 900oC

20

24

28

32

36

40

44

48

2θ (Degree) Fig. 1. XRD diffractograms of powders precalcined with raw materials (Fe2O3, BaCO3) at several temperatures.

nation process, constituent raw materials except for some oxides are first calcined at a given condition and then the precalcined powder is mixed with the precluded oxides before the second calcination, to obtain the powder having the desired composition and phase. In this paper, the synthesis of Co2Z powder by two-step calcination will be presented and the related process parameters will be discussed.

2. Experimental Co2Z (Ba3Co2Fe24O41) ferrite powders were prepared from the raw materials (70.59 mol% Fe2O3, 11.76 mol% CoO, 17.65 mol% BaCO3) by conventional ceramic process (one-step calcination) and two-step calcination technique. The raw materials were mixed in a planetary mill (FritschPulverisette 5) for 30 min and then dried in an oven. The dried powder calcined at 1350 -C for 2 h. For synthesis of single phase Co2Z Ba ferrite, two-step calcination process was carried out at temperature range between 900 and 1350 -C. The raw materials (Fe2O3, BaCO3) were mixed first without Co3O4 and precalcined at 900¨1300 -C for 2 h. The precalcined powders were pulverized with Co3O4 for 3 h and then postcalcined repeatedly at 1250¨1350 -C. Finally calcined powders were milled with the additives (5.0 wt.% Bi2O3, 5.0 wt.% CuO) for low temperature sintering. After milling the finally calcined powder for 3 h, some polyvinyl alcohol (PVA) binder was added. The slurry was dried and granulated using a sieve. The granuled powder was formed into a toroidal core under uniaxial pressure 1.0 t/cm2. Binder was carefully burned out in air with slow heating rate. Toroidal cores were then sintered at 900¨930 -C for 2 h in air. The crystallographic phases of

X-ray diffractograms for the precalcined powders of Fe2O3 and BaCO3 at the temperature range from 900 to 1300 -C are shown in Fig. 1. The calcined powders included BaFe12O19 (M) and BaFe2O4 (B) phases as a major and a minor phase, respectively [5]. The intensities of both phases M and B increase with increasing the calcination temperature. At 900 -C, the hematite phase (Fe2O3) still existed as an unreacted tracer, which disappeared at 1100¨1300 -C. The phase equilibrium diagram of BaO – Fe2O3 above 800 -C demonstrates that a heterogeneous mixture of M and B phase forms in the molar ratio of Fe2O3 / BaO between 1.0 and 5.0 [16]. Vinnik also reported that this mixture could be formed at the composition, Fe2O3 / BaO = 2.0 [13]. The calcined powders in this study have been prepared by mixing 80 mol% Fe2O3 and 20 mol% BaCO3 (Fe2O3 / BaO = 4.0) as a starting ingredient. It is thus confirmed that the simultaneous appearance of M and B phase is in good agreement with the previous results [13,16]. Fig. 2 shows the X-ray diffraction patterns of powders calcined once at 1350 -C or twice at 900¨1350 -C. The intensity of Co2Z (Ba3Co2Fe24O41) peaks increases in the twice calcined powders and other phases such as M and Y completely disappear according to the calcination condition, first calcining at 900 -C and secondly at 1350 -C, as shown in Fig. 2(d). However, the powder calcined

Standard Co2Z Standard Co2Y Standard M

(d) 2 step cal.(900oC+1350oC)

(c) 2 step cal.(1100oC+1350oC)

(b) 2 step cal.(1300oC+1350oC)

(a) 1 step cal.(1350oC)

20

24

28

32

36

40

44

48

2θ (Degree) Fig. 2. Comparisons of X-ray diffraction patterns for powders synthesized with several process conditions.

J. Jeong et al. / Materials Letters 59 (2005) 3959 – 3962

16

Table 1 Phases of powders indicated by X-ray diffraction after final calcination M(BaFe12O19), Y(Co2Y: Ba2Co2Fe12O22), Z(Co2Z: Ba3Co2Fe24O41)

Two step calcination

1250 1300 1350 900 Y 1250 900 Y 1300 900 Y 1350 1100 Y 1250 1100 Y 1300 1100 Y 1350 1300 Y 1250 1300 Y 1300 1300 Y 1350

M, M, M, M, M, Z M, M, Z M, M, M,

Y Y Y, Z Y, Z Y, Z Y, Z Y, Z

o

o

o

o

o

12

o

1 step cal. (1300 C)+sintering(930 C) o

o

o

2 step cal. (900 C+1350 C)+sintering(930 C)

10 8 6 4 2

107

108

109

Frequency (Hz) Fig. 3. Frequency spectrum of real part of permeability (l r).

Co2Y and M phases during the second calcinations. The optimum precalcination temperature and the high reactivity of the precalcined powders are thus required to yield the single phase Co2Z. Permeabilities (real part) and quality ( Q)-factors of specimens sintered at 900¨930 -C are given as a function of frequency in Figs. 3 and 4, respectively. The specimens with Co2Z powder synthesized by the two-step calcination at 900 -C and 1350 -C show permeabilities between 3.5¨4.5, whereas the specimens synthesized by the one-step calcination at 1300 -C exhibit lower permeabilities l = 2¨2.5. The resonance frequency does not occur at the frequency smaller than 1 GHz. The maximum Q-factors of the samples with the two-step calcination and the one-step calcinations occur at the frequency region around 350 MHz and around 500 MHz, respectively. These results are in good agreement with the previous works, where specimens were sintered below 1000 -C with oxide additives such as Bi2O3, borosilicate glass and

28 o

o

1 step cal. (1300 C)+sintering(900 C)

24

o

o

o

2 step cal. (900 C+1350 C)+sintering(900 C) o

o

1 step cal. (1300 C)+sintering(930 C) o

o

o

2 step cal. (900 C+1350 C)+sintering(930 C)

20 16 12 8

Table 2 Specific surface areas of powders pulverized for 3 h Process conditions

Specific surface area*

900 -C 1100 -C 1300 -C One-step calcination powder 1300 -C Two-step calcination powder 900 -C Y 1350 -C

3.83 2.38 1.69 2.48 1.98

*After 3 h milling.

o

1 step cal. (1300 C)+sintering(900 C) 2 step cal. (900 C+1350 C)+sintering(900 C)

Y Y Y, Z

only once at 1350 -C comprises Co2Z, Co2Y (Ba2Co2Fe12O22) and M phases. As shown in Fig. 2 (a) and (b), the second main peak (2h = 30.40-) of Co2Z phase is stronger than the first main peak (2h = 32.68-). It could be attributed to the overlapping of Co2Z and Co2Y peaks due to the existence of extra Co2Y phase. The powder with one-step calcination also shows the overlapped peaks of Co2Z – Co2Y and M (BaFe12O19) – Co2Y phases around 30.83-/ 41.22- and 32.08-, respectively. However, it is interesting that the powders pre-calcined at 900 and 1100 -C exhibit the three strongest main peaks of Co2Z phase without overlapping, comparing with Fig. 2 (a) and (b). The phases synthesized by either one-step calcination or twostep calcination technique at the temperature range between 900 and 1350 -C are summarized in Table 1. At the one-step calcination, the appearance of Co2Z phase was confirmed with the powder calcined at 1350 -C, which still included extra phases such as M and Co2Y. In the two-step calcination, the formation of Co2Z phase was observed at a low temperature, 1250 -C. A single phase Co2Z was obtained with the powders precalcined at 900¨1100 -C and postcalcined at 1350 -C. However, when the first calcination temperature was increased to 1300 -C, the M and Co2Y type phases did not disappear even after the second calcinations at 1350 -C. It should be noted that the powder precalcined at 1300 -C yielded M and Co2Y as well as Co2Z after the second calcination at 1350 -C, whereas M and Co2Y disappeared when they were precalcined at lower temperatures. This is compatible with the data of specific surface areas shown in Table 2. There is a systematic decrease in surface area from 3.83 m2/g to 1.69 m2/g with increasing precalcination temperature. It is well known that the high calcination temperature leads to the hard agglomeration of primary particles. The strong agglomeration may bring in poor pulverization with a low specific surface area. The small surface area would limit the solid state reaction between

Precalcination powders

14

Existing phases

Permeability (µr)

One step calcination

Calcination temperature (-C)

Q-factor

Calcination method

3961

m2/g m2/g m2/g m2/g m2/g

4

107

108

Frequency (Hz) Fig. 4. Frequency spectrum of quality factor ( Q).

109

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CuO [2,3,8]. The Q-factor (2pfL S / R S) increases with increasing the inductive reactance, and thus reaches a maximum value at the frequency with minimum losses and then decreases with frequency. At the frequency around 300 MHz, the Co2Z ferrite specimens with two-step calcination showed higher Q-factors, which may be due to the contribution of higher permeabilities, compared with other specimens.

4. Conclusions The powder with one-step calcination showed the overlapped peaks of M (BaFe12O19), Co2Z (Ba3Co2Fe24O41) and Co2Y (Ba2Co2Fe12O22) phases. However, the powders with two-step calcination, which were precalcined at 900¨1100 -C and postcalcined at 1350 -C exhibited three strongest main peaks of Co2Z phase without other phases. The single Co2Z phase was obtained with the powders precalcined at 900¨1100 -C and postcalcined at 1350 -C. As the precalcined temperature was increased to 1300 -C, M and Co2Y type phases were also observed together with Co2Z phase. Permeabilities and Q-factors of the specimens with the two-step calcination show higher values, compared with specimens with the one-step calcination.

References [1] H. Marusawa, C. Kato, T. Konoike, K. Tomono, in: M. Abe, Y. Yamazaki (Eds.), Proc. of the 8th Inter. Conf. on Ferrites, The Japan Society of Powder, Kyoto, Japan, 2000, p. 942.

[2] M. Endo, A. Nakano, in: M. Abe, Y. Yamazaki (Eds.), Proc. of the 8th Inter. Conf. on Ferrites, The Japan Society of Powder, Kyoto, Japan, 2000, p. 1168. [3] X. Wang, L. Li, S. Shu, Z. Yue, Z. Ma, J. Zhou, Mater. Lett. 55 (2002) 8. [4] R.C. Pullar, S.G. Appleton, M.H. Stacey, M.D. Talor, A.K. Bhattacharya, J. Magn. Magn. Mater. 186 (1998) 313. [5] J. Smit, H.P.J. Wijn, Ferrites, Phillips Technical Library and Powder Metallurgy, Eindhoven, Netherlands, 1959. [6] N. Hiratsuka, R. Ikeda, R. Wada, K. Kakizaki, in: M. Abe, Y. Yamazaki (Eds.), Proc. of the 8th Inter. Conf. on Ferrites, The Japan Society of Powder, Kyoto, Japan, 2000, p. 939. [7] H. Zhang, J. Zhou, Y. Wang, L. Li, Z. Yue, Z. Gui, J. Electroceram. 9 (2002) 73. [8] Y. Matsuo, M. Inagaki, in: M. Abe, Y. Yamazaki (Eds.), Proc. of the 8th Inter. Conf. on Ferrites, The Japan Society of Powder, Kyoto, Japan, 2000, p. 945. [9] J.H. Hankiewicz, Z. Pajak, A.A. Murakhowski, J. Magn. Magn. Mater. 140 (1995) 2093. [10] R.C. Pullar, M.H. Stacey, M.D. Taylor, A.K. Bhattacharya, Acta Mater. 49 (2001) 4241. [11] X.H. Wang, L. Li, Z. Gui, S. Shu, J. Zhou, Mater. Chem. Phys. 77 (2002) 248. [12] T. Tachibana, K. Ohta, T. Shimada, T. Nakagawa, T.A. Yamamoto, M. Katusura, in: M. Abe, Y. Yamazaki (Eds.), Proc. of the 8th Inter. Conf. on Ferrites, The Japan Society of Powder, Kyoto, Japan, 2000, p. 888. [13] M.A. Vinnik, Russ. J. Inorg. Chem. 10 (1965) 1164. [14] H. Zhang, L. Li, J. Zhou, Z. Yue, Z. Ma, Z. Gui, J. Eur. Ceram. Soc. 21 (2001) 149. [15] M. Orita, H. Satoh, K. Aizawa, Jpn. J. Appl. Phys. 31 (1992) 3261. [16] Y. Goto, T. Takada, J. Am. Ceram. Soc. 43 (1960) 151.