Influence of Sn-substitution on temperature dependence and magnetic disaccommodation of manganese–zinc ferrites

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 2121–2124

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Influence of Sn-substitution on temperature dependence and magnetic disaccommodation of manganese–zinc ferrites Haining Ji , Zhongwen Lan, Zhong Yu, Ke Sun, Lezhong Li State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China

a r t i c l e in fo

abstract

Article history: Received 28 August 2008 Received in revised form 7 December 2008 Available online 21 January 2009

In this paper, the effects of Sn-substitution on temperature dependence and magnetic disaccommodation of manganese–zinc ferrites were investigated. Toroidal cores were prepared by the conventional ceramic process and sintered at 1360 1C for 4 h in atmosphere controlled by using the equation for equilibrium oxygen partial pressure. The experimental results show that the substitution of Sn4+ in manganese–zinc ferrites can influence the thermal stability and disaccommodation remarkably. Secondly, the temperature dependence of the initial permeability mi and disaccommodation of Sn-substitution manganese–zinc ferrites have an internal relationship. The experimental results are explained and compared with those of Ti-substitution manganese–zinc ferrite. & 2009 Elsevier B.V. All rights reserved.

Keywords: Manganese–zinc ferrite Sn Temperature dependence Disaccommodation spectra

1. Introduction MnZn ferrites are widely used as core materials for transformers in switching mode power supplies (SMPS) and DC/DC converters. However, the performances of manganese–zinc ferrites are affected by temperature, magnetic shock, etc. With the development of electronic technology, the requirement for manganese–zinc ferrites becomes more and more strict. Besides high performance, high stability is also needed. Properties of MnZn ferrites are decided by the main compositions, the types and weights of dopants, the characteristics of powder and the sintering condition. Many investigators indicate that using some ions substitution in the main composition can improve the thermal stability, such as Ti4+, Ni2+, Sn4+, etc. [1–7], while Sn4+ can also play a positive role in magnetic disaccommodation of ferrite [8]. Therefore, this paper focuses on the effects of Sn-substitution on temperature dependence and disaccommodation spectra of manganese–zinc ferrites. The relationship between temperature dependence and disaccommodation spectra of manganese–zinc ferrite is also discussed.

2. Experimental procedure 2.1. Sample preparation The Mn0.698xZn0.236SnxFe2.066O4 spinels (x ¼ 0.000, 0.002, 0.004, and 0.006) were synthesized via a conventional ceramic processing using high-purity Fe2O3, ZnO, MnO, and SnO2 powders.  Corresponding author. Tel.: +86 28 83208437; fax: +86 28 83201673.

E-mail address: [email protected] (H. Ji). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.01.001

The compositions were milled in a planetary mill with deionized water for 2 h. After drying, the mixture of oxide powder was homogenized and calcined at 960 1C for 2.5 h in air. Then the calcinated products were each milled in a planetary mill again. To reduce the loss of samples, CaCO3, V2O5, and Nb2O5were added. Following the addition of polyvinyl alcohols (PVA), the milled powders were granulated and formed into toroidal-shaped samples, which were sintered in a computer-driven furnace at 1360 1C for 4 h with 6% oxygen and then cooled at equilibrium conditions in a N2/O2 atmosphere. 2.2. Measurement technique The initial permeability mi, the total power loss Pcv , the hysteresis loss Ph and the eddy current loss Pe of samples were measured by a B-H analyzer (IWATSU, SY-8232). Magnetic disaccommodation measurements were carried out with an automated system based on an LCR bridge in the 25 1CoTo220 1C temperature range. The disaccommodation curves were defined as DAð%Þ ¼

mðt 1 ; TÞ  mðt2 ; TÞ  100 mðt 1 ; TÞ

(1)

where t1 ¼ 60 s and t2 ¼ 600 s.

3. Results and discussion 3.1. Influence of SnO2 on initial permeability Fig. 1 shows the results of the investigation of temperature dependence of the initial permeability mi. There are two

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Fig. 2. K1–T curve of Fe2+ compensation.

Fig. 1. Initial permeability vs. temperature of Sn-substituted MnZn ferrites.

maximum peaks in the mi–T curve. The temperatures of the first maximum peak and the secondary maximum peak are approximately 210 and 100 1C, respectively. The secondary maximum peak is moved to a lower temperature with the increase in Sn content, while the temperature of the first maximum peak shifts little with the increase in Sn content. As we all know, initial permeability of ferrite obeys [9]   3ls s 1=3 d K1 þ (2) mi / M2s b 2 d where Ms is the saturation magnetization, K1 is magnetocrystalline anisotropy constant, ls is magnetostriction coefficient, s is the inner stress and b is the volume concentration of impurities , d is thickness of the domain wall, and d is diameter of impurities. To obtain high initial permeability, we have to reduce the magnetocrystalline anisotropy constant K1 and magnetostriction coefficient ls, increase the saturation magnetization Ms, which are all related to temperature. Because the magnetocrystalline anisotropy constant K1 is changed far more quickly than the square of the saturation magnetization Ms while the temperature varied, and that Sn4+ ion has a strong tendency to occupy B sites in the ferrite lattice, though it has been found to be occupying A sites as well under certain conditions, such situation is not expected to alter Ms significantly for small substitution [10]. The magnetocrystalline anisotropy constant K1 is the main factor affecting temperature dependence. Sn4+ has a strong octahedral-site preference. It substitutes the Fe 3+ and at the same time generates Fe2+ ions, in order to maintain electrical neutrality, i.e., 2Fe3+-Sn4++Fe2+. It is well known that K1 of MnZn ferrite is negative, while K1 of Fe2+ is positive. Fig. 2 shows the K1–T curves of manganese–zinc ferrites matrix and Fe2+. At the same time, their compensation curves are expressed by broken lines. Fig. 2 reveals that the compensation curve of Fe2+ is negative below the compensation node and positive upon the compensation node. Thus, Sn-substitution makes a positive contribution to K1 for the composite ferrites with excess Fe2+, reduces the sum of K1 and shifts the compensation node of K1 to a lower temperature, which moves the secondary maximum peak of the mi–T curve to lower temperatures. This result is consistent with the experimental results of Ti-substitution [11].

3.2. Influence of SnO2 on power loss As is known, power loss is the sum of contributions of hysteresis loss Ph, eddy current loss Pe, and residual loss Pr, but

Fig. 3. Power loss vs. temperature of Sn-substituted MnZn ferrites.

at low frequencies Pr can be neglected. So the total power loss Pcv can be expressed as PcvEPh+Pe. The hysteresis loss Ph is inversely proportional to the cubic of initial permeability, namely, Php1/m3i . The eddy current loss Pe is inversely proportional to the electrical resistivity and proportional to the square of average grain size, i.e., Pepd2/r. Total power loss Pcv at 200 mT and 100 kHz were measured and the results are shown in Fig. 3 as a function of temperature. There is a deep valley that can be observed around 100 1C for all samples. With the increase in Sn content, the minimum temperature of the Pcv–T curve shifts to low temperature. Figs. 4 and 5 show the results of the temperature dependence of the hysteresis loss Ph and eddy current loss Pe, respectively. With the increase in Sn content, the variety of the Ph–T curve is in agreement with the Pcv–T curve. Pe is lower and changes little at lower temperature for all samples, then increases with higher temperature. With the increase in Sn content, eddy current loss Pe decreases slightly under lower temperature. However, compared with the case when Sn is absent, it increases much faster when the temperature is high. The behavior of the hysteresis loss Ph as a function of temperature, at constant frequency and magnetic field strength, is actually determined by the temperature behavior of the initial permeability mi, which in turn is determined by the magnetocrystalline anisotropy K1 [12]. As is mentioned in the previous discussion, Sn4+ dissolves into the spinel lattice and forms

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temperature dependence of power loss can be obtained by changing the content of SnO2. Manganese–zinc ferrite with lowest total power loss Pcv and the compensate temperature T0 at about 80 1C is achieved when x is equal to 0.004.

3.3. Influence of SnO2 on Disaccommodation Spectra

Fig. 4. Hysteresis loss Ph vs. temperature of Sn-substituted MnZn ferrites.

Fig. 5. Eddy current loss Pe vs. temperature of Sn-substituted MnZn ferrites.

Fe2+–Sn4+ pairs. As a result, the magnetocrystalline anisotropy constant K1 is compensated, the secondary maximum peak of the mi–T curve is shifted to lower temperature, and the initial permeability mi is effectively increased below the compensate temperature T0, while it is decreased above the compensate temperature T0. So, the hysteresis loss Ph decreases with the increase in Sn content below the compensate temperature T0, and increases above the compensate temperature T0. The Sn4+ ion, like the Ti4+ ion, has the property of localizing a 2+ Fe ion. The Fe2+–Sn4+ pairs do not participate in the hopping mechanism and the eddy current loss Pe of the samples decreases with the increase in Sn content. However, since the ionic radius of Sn4+ is 0.071 nm, larger than that of Ti4+, which is 0.069 nm, the Fe2+–Sn4+pairs are not as stable as Fe2+–Ti4+ pairs. So, the Fe2+ ions in Fe2+–Sn4+ couples are less tightly bound. At higher temperatures these pairs become weak and tend to disassociate [13]. Therefore, the eddy current loss Pe of the sample with a higher content of Sn will increase steeply at higher temperature. Thus the total power loss Pcv under the compensate temperature T0 decreases, and on the contrary, the total power loss Pcv beyond the compensate temperature T0 increases. Sn-substitution in MnZn ferrites can make the compensation temperature T0 shift to a lower temperature, and MnZn ferrites with improved

Fig. 6 shows the disaccommodation spectra of MnZn ferrites with the composition of Mn0.698xZn0.236SnxFe2.066O4 (x ¼ 0.000, 0.002, 0.004, and 0.006). As can be noted, the addition of Sn4+ ions changes drastically the disaccommodation spectra, even for the lowest amount of this substitution. Two disaccommodation processes correspond to two peaks, respectively. Peak II appears at about 140 1C for all samples, whereas peak III occurs at 5 1C approximately. With increase in Sn content, peak III shifts to lower temperatures, and its intensity increases. This phenomenon is consistent with the experimental results of Ti-substitution [14,15]. As for peak II, its temperature changes little, which is in full accord with the results of Ti-substitution. However, its intensity is not suppressed with further addition of Ti4+, as suggested by Willey [14]. In this case, the intensity of peak II increases when the substitution of Sn4+ is increased. The disaccommodation spectra of the Sn-substituted MnZn ferrites depend on vacancy, Fe2+ concentration and Sn4+ concentration. The intensity of peak III is proportional to Fe2+ concentration. So, it is proportional to Sn4+ concentration. For peak II, Knowles [15] has postulated that the decrease in intensity of peak II of Ti-substituted MnZn ferrites disaccommodation spectra is a consequence of the existence of the strongly bound Fe2+–Tn4+ pairs. However, the bound Fe2+–Sn4+ pairs in Sn-substituted MnZn ferrites are not as steady as the Fe2+–Tn4+ pairs. It will become weak and tend to disassociate at higher temperatures [13]. Therefore, the intensity of peak II will augment with increase in Sn content. A comparison of Fig. 6 and Fig. 1 shows that peak II corresponds to the valley between two peaks of the mi–T curve. The lower the peak value of disaccommodation, the flatter the mi–T curve between the two peaks. From the experimental facts, it is assumed that the vacancy and Fe2+ concentration influence both disaccommodation and the thermal stability of the mi–T curve between the two peaks. With SnO2 substituted in the main composition of MnZn ferrites, the thermal stability of the mi–T curve between the two peaks decreases, while the peak value of disaccommodation increases.

Fig. 6. Influence of SnO2 content on disaccommodation spectra of MnZn ferrites.

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4. Conclusions

References

The substitution of Sn4+ in manganese–zinc ferrites can influence the temperature dependence of initial permeability mi, total power loss Pcv, and disaccommodation remarkably. Firstly, Sn-substitution can enhance the intensity of disaccommodation peak III and II, and deteriorate the thermal stability of the mi–T curve between the first and the secondary maximum peak. Secondly, Sn-substitution can shift the secondary maximum peak of the mi–T curve to a lower temperature, and simultaneously increase the initial permeability mi and decrease the total power loss Pcv below the compensate temperature T0. Especially, manganese–zinc ferrite with lowest total power loss Pcv and the compensate temperature T0 at about 80 1C is achieved when x is equal to 0.004.

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Acknowledgment The authors are grateful for the Fund of UESTC for Distinguished Young Scholars (No. JX0616).