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Ultra-low core losses at high frequencies and temperatures in MnZn ferrites with nano-BaTiO3 additives
Journal of Alloys and Compounds 821 (2020) 153573
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Ultra-low core losses at high frequencies and temperatures in MnZn ferrites with nano-BaTiO3 additives Guohua Wu *, Zhong Yu , Ke Sun , Rongdi Guo , Hanyu Zhang , Xiaona Jiang , Chuanjian Wu , Zhongwen Lan School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 610054, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2019 Received in revised form 19 December 2019 Accepted 26 December 2019 Available online 27 December 2019
The miniaturization and integration of power conversion devices and components, such as converters and switching power supplies, require ferrite cores with high operating frequencies and temperatures. Hence, the frequency and temperature dependence of core losses is a bottleneck restricting the development of power conversion devices. Herein, nanoparticles of a dielectric materialdbarium titanate (BTO)dare adopted to suppress high-frequency and high-temperature core losses in MnZn ferrites. Using a loss separation method, the effects of BTO on core losses and their temperature dependence are investigated. Residual loss at 25 C and eddy current loss at high temperatures were significantly suppressed by the addition of BTO. Remarkably, adding 0.04 wt% BTO nanoparticles resulted in MnZn ferrites with ultra-low core losses of 302 kW/m3 (25 C) and 890 kW/m3 (100 C) at 3 MHz and 30 mT. These superior MnZn ferrites with ultra-low core losses have potential applications in power conversion devices with high operating frequencies and temperatures. This study also provides a novel approach for suppressing core losses. © 2019 Elsevier B.V. All rights reserved.
Keywords: MnZn ferrites BTO nanoparticles additives Microstructure Core losses Temperature dependence
1. Introduction Power conversion devices with high transfer efficiency, such as AC-DC converters, DC-DC converters, and switching mode power supplies, do not function well without high-performance core materials. Due to their high initial permeability, saturation magnetic induction, and resistivity, and low core losses, MnZn ferrites have become an excellent option for core materials [1e5]. Currently, the operating frequency of MnZn ferrites has increased to the megahertz range to meet the miniaturization and integration requirements of electronic devices. Unfortunately, high losses in the magnetic core induced by high frequencies decrease the transfer efficiency of power conversion devices [6e8]. Additionally, due to the high power density of electronic devices, high operating temperatures (usually 80e100 C) can deteriorate the performance of ferrite cores [9e11]. Therefore, reducing core losses at high frequencies and temperatures is vital for the realization of highefficiency conversion devices. Theoretically, the core losses in MnZn ferrites can be
* Corresponding author. E-mail address: [email protected] (G. Wu). https://doi.org/10.1016/j.jallcom.2019.153573 0925-8388/© 2019 Elsevier B.V. All rights reserved.
decomposed into hysteresis loss (Ph), eddy current loss (Pe), and residual loss (Pr), according to the respective physical mechanism [12e14]. Scholars have reported that the Pe induced by eddy currents in an alternating magnetic field, and the Pr induced by resonance, become significant at high frequencies and temperatures [15e17]. Accordingly, suppressing core losses at high frequencies and temperatures requires the reduction of eddy current and residual losses. On the one hand, regular oxides such as CaCO3, SiO2, ZrO2, HfO2, Ta2O5, TiO2, and SnO2 can be added to modify the resistivity and eddy current loss of a material [18e22]. On the other hand, low sintering temperatures and fine grains have been reported to benefit the reduction of residual loss [23e25]. Recently, in addition to regular oxide additives, other additives have been explored, such as dielectric materials with particular properties. BaTiO3 (BTO) is a widely used dielectric material with a high dielectric permittivity [26e30]. Adding 0.08 wt% BTO nanoparticles into MnZn ferrites has been reported to reduce the core losses occurring at 500 kHz, 30 mT, and room temperature (25 C) from 283.4 to 82.7 kW/m3, which is attributed to a decrease in eddy current loss [31]. However, the effect of BTO nanoparticles remains uncertain with regard to core losses at high frequencies (2e4 MHz) and high temperatures, where the constitution of core losses is more complicated than that at low frequencies. Moreover, the
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physical mechanism by which BTO influences core losses and temperature dependence has not been revealed. In this paper, to suppress core losses at high frequencies and temperatures, we utilize BTO nanoparticles as additives to MnZn ferrites. Most importantly, the effects of BTO on core losses and temperature dependence are investigated in detail using the loss separation method. Adjusting the content of BTO at 3 MHz and 30 mT significantly suppressed core losses at both low and high temperatures due to reductions in eddy current and residual losses. This work reveals a feasible method to suppress the core losses in power ferrites at high frequencies and temperatures, by the addition of dielectric materials with special properties. 2. Materials and methods MnZn ferrites with a major composition of Mn0.67Zn0.21Fe2.12O4 were fabricated using a solid-state reaction method. Highly pure oxides of Fe2O3, Mn3O4, and ZnO were weighed precisely, milled in a planetary mill for 2 h with deionized water, and then calcined at 900 C for 2 h. Various contents of BTO nanoparticles (0.00e0.14 wt %) were introduced into the calcined powder. Other additives with fixed contents were also added to improve the electromagnetic properties of the MnZn ferrites: 0.30 wt% Co2O3, 0.10 wt% CaCO3, 0.01 wt% V2O5, 0.20 wt% TiO2, and 0.06 wt% SnO2. The mixtures were milled in a planetary mill for 4 h. Subsequently, the mixtures were granulated with 13 wt% poly-vinyl alcohol and compacted into toroidal-shaped cores (18 mm 8 mm 5 mm) and discshaped samples (18 mm 3 mm) under a 6 MPa pressure. The impacted samples were sintered at 1150 C for 6 h at a 5% partial oxygen pressure and cooled in a carefully controlled N2/O2 atmosphere. X-ray diffraction (XRD, MAXima XRD-7000) was performed to characterize the crystalline phase of the prepared samples. The microstructure of the fracture surface was observed using a scanning electron microscope (SEM, JEOL JSM-6490LV), and the average grain size was determined from the SEM images through the interception method. Core losses, PL, were tested through a B-H analyzer (Iwatsu SY-8232) from 25 to 120 C. The inductance of the toroidal core samples and the AC resistance of silver-plated disk samples were tested using a digital electric bridge (Tonghui TH2829). Initial permeability and AC resistivity were calculated according to the inductance and resistance, respectively. The impedance of the disk samples was measured using an LCR bridge meter (WK 6500P). Transmission electron microscopy (TEM, JEM 2100, 200 kV) was utilized to perform energy dispersive X-ray analysis (EDX). 3. Results and discussion 3.1. Microstructure and crystalline phase Fig. 1(a)e(h) present micrographs of samples with different BTO contents. Fig. 1(i) presents the measured average grain size, D. The addition of BTO can refine the grains when the content of BTO is less than 0.08 wt%. During the sintering process, the existence of BTO nanoparticles along the grain boundary hinders grain growth, because the melting point of BTO (over 1600 C) is far higher than the sintering temperature of MnZn ferrites (1150 C) [32e34]. Moreover, for the 0.02e0.08 wt% BTO samples, the standard deviation of the grain size remained almost unchanged, which indicates that the homogeneity of the grains deteriorated on the premise of decreased grain size. However, when the content of BTO exceeded 0.08 wt%, BTO nanoparticles formed impurities and slightly increased grain growth, causing an increase in grain size. The XRD patterns for MnZn ferrite samples with different BTO
contents are shown in Fig. 1(j). As the figure shows, all the samples exhibited a pure cubic spinel ferrite phase, which matches well with JCPDS card no. 74e2401. Even though BTO is a typical perovskite structure, the content of BTO in the MnZn ferrites was small; thus, the addition of BTO had no influence on the crystalline phase. 3.2. Influence of BTO on core losses at room temperature (25 C) Fig. 2 presents the variation in PL with BTO content at 3 MHz, 30 mT, and 25 C. As the BTO content increased, core losses initially decreased and subsequently increased, with a clear minimum of 302 kW/m3 at 0.04 wt% BTO content. Loss separation was performed to analyze the effect of BTO addition on core losses. The core losses of MnZn ferrites were decomposed into hysteresis loss (Ph), eddy current loss (Pe), and residual loss (Pr). The expression of PL is as follows [16,17,21]:
. PL ¼ Ph þ Pe þ Pr ¼ Kh B3 f þ Ke B2 f 2 D2 r þ Pr
(1)
where Kh and Ke represent constants, f is the frequency, B is the magnetic induction, and r is the AC resistivity at the measured frequency. By dividing PL by f in Equation (1) and plotting PL/f ~ f, each component of the core losses can be determined. The intercept (KhB3) represents Ph/f, the slope (KeB2fD2/r) represents Pe/f, and the rest represents Pr/f. The separation results are shown in Figs. 3e5. Fig. 3 presents the hysteresis loss, Ph, for the MnZn ferrite samples with different BTO contents. As the BTO content increased, Ph initially increased and subsequently decreased. Induced by irreversible domain wall movements, Ph was related to the initial permeability mi (Ph f 1/m3i ) [35]. The variation in the initial permeability, initially decreasing and subsequently increasing, was the opposite of that of Ph, as shown in Fig. 3. Initial permeability relates to domain wall movement and domain rotation, which constitute the magnetization process [36e38]. As the BTO content increased (for <0.08 wt%), the rapid decrease in grain size directly reduced the number of domain walls. Moreover, the addition of a nonmagnetic phase (BTO) hindered the magnetization process. Both factors resulted in a decrease of mi and an increase of Ph. When the BTO content was greater than 0.08 wt%, increasing the grain size enhanced domain wall movement. As a result, the initial permeability increased and the hysteresis loss decreased. Fig. 4(a) presents the variation in Pe with BTO content. As the BTO content increased, Pe exhibited a tendency to increase and subsequently decrease. Eddy current loss originates from the eddy current that emerges inside the material at high frequency, and is related to the AC resistivity, r. As shown in Fig. 4(b), the addition of BTO caused the resistivity of the MnZn ferrites to initially decrease and subsequently increase. Determining the unambiguous distribution of BTO in MnZn ferrites was necessary to illustrate the effect of BTO on resistivity. Fig. 4(c) and (d) present the TEM and EDX analyses results for the 0.14 wt% BTO sample. In the TEM micrograph, the G region is the grain, the GB region is the grain boundary, and the TP region is the triple point region. The element Ba was not detected in all regions due to weak signal intensity and the small content of BTO compared with the matrix MnZn ferrites. However, the element Ti was detected in both the GB and TP regions, but not in the G region. The distribution of Ti indirectly proved that BTO was distributed at the grain boundary, even though extra TiO2 was present in the additives. BTO has been confirmed to possess high resistivity above 1010 U m [39]. Essentially, the distribution of BTO nanoparticles at the grain boundary is supposed to improve the resistivity of the MnZn ferrite material. However, the increase in BTO content
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Fig. 1. SEM images of samples with (a) 0.00 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt%, (e) 0.08 wt%, (f) 0.10 wt%, (g) 0.12 wt%, and (h) 0.14 wt% BTO. (i) Average grain size of MnZn ferrite samples with different BTO contents. (j) XRD patterns of MnZn ferrite samples with different BTO contents.
Fig. 2. Variation in core losses with BTO content at 3 MHz, 30 mT, and 25 C.
(<0.08 wt%) led to a decrease in resistivity. This should be attributed to the deterioration of the microstructure induced by the addition of BTO, as shown in Fig. 1. Scholars have reported that nonhomogeneous microstructures are often accompanied by lattice imperfections, which lower the resistivity [40,41]. When the BTO content increased further, the deterioration of the microstructure ceased and the role of BTO in improving the resistivity of the GB became apparent, leading to an increase in resistivity. Unfortunately, the variation in resistivity cannot adequately explain the eddy current loss, because Pe is also related to grain size according to Equation (1). Thus, D2/r was calculated as presented in Fig. 4(b). As the BTO content increased, the value of D2/r initially increased and subsequently decreased, which was in accordance with the variation in Pe.
Fig. 3. Variation in Ph and mi with BTO content.
Due to the resonant phenomenon occurring in the highfrequency range, Pr is related to the resonant frequency, fr. Fig. 5(a) represents the variation in both residual loss and resonant frequency. The addition of BTO into MnZn ferrites caused an initial decrease and subsequent increase trend in the residual loss. Further, fr, which is extracted from the peak of the imaginary part of the complex permeability m00 , exhibited a trend opposite to that of Pr. It is confirmed that fine grains are beneficial in suppressing the resonance and Pr [21,23]. Thus, the evolution of the microstructure was responsible for the variations in fr and Pr. Compared with the 0.00 wt% BTO sample, the sample with the proper content of BTO nanoparticles had the lowest core losses at 3 MHz, 30 mT, and 25 C, indicating a remarkable effect of dielectric materials on reducing the high-frequency core losses of MnZn ferrites. The proportion of each loss is presented in Fig. 6. Despite
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Fig. 4. (a) Variation in eddy current loss Pe with BTO content. (b) Variation in AC resistivity r at 3 MHz and D2/r with BTO content. (c) TEM image of MnZn ferrite samples with 0.14 wt% BTO. (d) EDX analysis in the G, GB, and TP regions for the 0.14 wt% BTO sample.
Fig. 5. (a) Variation in residual loss Pr and resonant frequency fr. (b) Frequency spectra of the imaginary part m00 of complex permeability.
the increase in Ph and Pe, the suppression of core losses by adding 0.04 wt% BTO nanoparticles was mainly attributed to the refinement of grains and, thereby, the decrease in residual loss. 3.3. Influence of BTO on core losses temperature dependence
Fig. 6. Percentage of each type of loss for samples at different BTO contents.
Apart from the core losses at room temperature discussed above, temperature dependence is also a key performance parameter of MnZn power ferrites, owing to the high operating temperatures of ferrite cores. Fig. 7(a) presents the variation in core losses with temperature at 3 MHz and 30 mT for two representative samples (0.00 and 0.04 wt% BTO). As temperature increased, the core losses monotonically increased in the two investigated samples. However, the rates of increase in the core losses were different, especially in the high-temperature range. The 0.04 wt% BTO sample exhibited a smaller rate of increase in core losses, and had lower core losses at a high temperature (100 C, 890 kW/m3). Loss separation was carried out at a temperature range of 25e120 C to investigate the effect of BTO on the temperature dependence of the core losses.
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Fig. 7. Variation in (a) core losses, (b) hysteresis loss and initial permeability, (c) eddy current loss, and (d) residual loss with measurement temperature.
Fig. 7(b)e(d) demonstrate the temperature dependence of each component of loss. As temperature increased, hysteresis loss slightly decreased and played a minor role in core losses throughout the temperature range. Both eddy current and residual losses monotonically increased and contributed a large part of the core losses. No visible difference was observed between the 0.00 and 0.04 wt% BTO samples with regard to the variations in hysteresis and residual losses. As shown in Fig. 7(b), the slight decrease in hysteresis loss was due to the increase in initial permeability with increasing temperature. It has previously been confirmed that the dramatic increase in residual loss is induced by the inevitable reduction of the resonant frequency at high temperatures [16]. However, the two samples exhibited a great difference in their eddy current loss trends. As shown in Fig. 7(c), when the temperature increased, the eddy current loss increased sharply in the 0.00 wt% BTO sample, while the rate of increase was slightly lower in the 0.04 wt% BTO sample. The AC resistivity at 3 MHz was measured from 25 to 120 C to illustrate differences in the temperature dependence of eddy current loss, as presented in Fig. 8. The AC resistivity of both samples decreased as temperature increased, indicating a semiconducting characteristic of MnZn ferrites [12]. However, the resistivity of the 0.00 wt% BTO sample (denoted as r0.00) rapidly decreased, while that of the 0.04 wt% BTO sample (denoted as r0.04) exhibited a gradual descending trend, as temperature increased. Remarkably, r0.00 was much higher than r0.04 at low temperatures; however, at 120 C, r0.00 became lower than r0.04. It is assumed that if the temperature exceeded 120 C, the difference between r0.00 and r0.04 would be greater. This phenomenon adequately demonstrates the difference in the temperature dependence of the eddy current loss in the two samples. Furthermore, a complex impedance plot at different temperatures was constructed (Fig. 9(a) and (b)). Based on Koops’ model, the resistances of the grains (Rg) and grain boundary (Rgb) can be obtained from the impedance plot [42,43]. The variations in Rg and
Rgb with temperature are presented in Fig. 9(c) and (d). Both Rg and Rgb decreased with increasing temperature, but the rates of decrease in Rgb were different in the two samples. As temperature increased, Rgb for the 0.04 wt% BTO sample decreased gradually, compared with the rapid descent in the 0.00 wt% BTO sample. A similar trend between resistivity (Fig. 8) and grain boundary resistance (Fig. 9(d)) indicated that the grain boundary resistance was responsible for the temperature dependence of resistivity. Essentially, BTO ceramic has a positive temperature coefficient (PTC), which means that the resistivity of the BTO ceramic increases with temperature [44e46]. Therefore, the addition of 0.04 wt% BTO nanoparticles to the MnZn ferrite material can suppress the rates of
Fig. 8. Variations in AC resistivity with temperature at 3 MHz for the 0.00 and 0.04 wt % BTO samples.
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Fig. 9. Impedance plots of the (a) 0.00 wt% and (b) 0.04 wt% BTO sample. Variation in the (c) resistance of the grain Rg and (d) resistance of the grain boundary Rgb with temperature.
Fig. 10. Percentage of each component of loss in samples with (a) 0.00 wt% and (b) 0.04 wt% BTO.
decrease in grain boundary resistance and resistivity of the material. Fig. 10 presents the percentage of each component of loss at different temperatures for the two samples. The percentage of each loss component for the 0.00 wt% BTO sample remained almost unchanged and residual loss was the greatest loss component throughout the temperature range. When 0.04 wt% BTO was added, eddy current loss contributed a large component at room temperature (25 C). As temperature increased, the proportion of Pe gradually decreased and that of Pr increased. Therefore, by adding 0.04 wt% BTO into MnZn ferrites, core losses at high temperatures can be reduced via increased resistivity, which suppresses Pe. 4. Conclusions This work proposes an effective approach for suppressing core losses in MnZn ferrites at high frequencies and temperatures by adding a dielectric materialdBTO nanoparticles. The results
indicate that with increasing BTO content, core losses at 3 MHz, 30 mT, and 25 C initially decrease and subsequently increase, reaching a minimum of 302 kW/m3 at 0.04 wt% BTO content. At room temperature (25 C), the suppression of core losses is mainly attributed to the refinement of grain size, resulting in a reduction of residual loss. With regard to temperature dependence, the core losses of the 0.04 wt% BTO sample are significantly suppressed between 25 and 120 C compared with those of the 0.00 wt% BTO sample. Particularly, at 3 MHz, 30 mT, and 100 C, the core losses of the 0.04 wt% BTO sample are only 890 kW/m3. When 0.04 wt% BTO is added, suppression of core losses at high temperature is caused by the restraining of resistivity, which reduces eddy current loss. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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CRediT authorship contribution statement Guohua Wu: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. Zhong Yu: Conceptualization, Methodology, Project administration. Ke Sun: Visualization, Project administration, Funding acquisition. Rongdi Guo: Software, Validation, Visualization. Hanyu Zhang: Investigation, Resources. Xiaona Jiang: Formal analysis, Supervision. Chuanjian Wu: Supervision. Zhongwen Lan: Funding acquisition, Supervision. Acknowledgements This work is supported by the National Natural Science Foundation of China under Grant No. 51472045 and 51772046, and the Fundamental Research Funds for the Central Universities ZYGX2018J038, ZYGX2018J040, and ZYGX2018J042. References [1] S. Yuan, Y. Huang, J.F. Zhou, Q. Xu, C.Y. Song, P. Thompson, Magnetic field energy harvesting under overhead power lines, IEEE Trans. Power Electron. 30 (2015) 6191e6202. [2] M.A. Ahmed, K.E. Rady, M.S. Shams, Enhancement of electric and magnetic properties of Mn-Zn ferrite by Ni-Ti ions substitution, J. Alloy. Comp. 622 (2015) 269e275. € pfer, A. Angermann, Complex additive systems for Mn-Zn ferrites with [3] J. To low power loss, J. Appl. Phys. 117 (2015), 17A504. [4] D. Liu, X.P. Chen, Y. Ying, L. Zhang, W.C. Li, L.Q. Jiang, S.L. Che, MnZn power ferrite with high Bs and low core loss, Ceram. Int. 42 (2016) 9152e9156. [5] X. Yang, Z. Zhou, T. Nan, Y. Gao, G.M. Yang, M. Liu, N.X. Sun, Recent advances in multiferroic oxide heterostructures and devices, J. Mater. Chem. C 4 (2016) 234e243. [6] F. Fiorillo, C. Beatrice, O. Bottauscio, A. Manzin, Approach to magnetic losses and their frequency dependence in Mn-Zn ferrites, Appl. Phys. Lett. 89 (2006), 122513. [7] C. Beatrice, O. Bottauscio, M. Chiampi, F. Fiorillo, A. Manzin, Magnetic loss analysis in Mn-Zn ferrite cores, J. Magn. Magn. Mater. 304 (2006) e743ee745. k, V. Tsakaloudi, C. Ragusa, F. Fiorillo, L. Martino, V. Zaspalis, [8] C. Beatrice, S. Doba Magnetic loss, permeability, and anisotropy compensation in CoO-doped MnZn ferrites, AIP Adv. 8 (2017), 047803. [9] X. She, A.Q. Huang, R. Burgos, Review of solid-state transformer technologies and their application in power distribution systems, IEEE Trans. Power Electron. 1 (2013) 186e198. [10] G.D. Capua, N. Femia, A nover method to predict the real operation of ferrite inductors with moderate saturation in switching power supply applications, IEEE Trans. Power Electron. 31 (2016) 2456e2464. [11] Z.G. Dang, J.A. Qahouq, Evaluation of high-current toroid power inductor with NdFeB magnet for DC-DC power converters, IEEE Trans. Ind. Electron. 62 (2015) 6868e6876. [12] R.D. Guo, X.F. Yang, K. Sun, K.W. Li, Z. Yu, G.H. Wu, X.F. Zhang, X.N. Jiang, Z.W. Lan, Temperature characteristics of core losses for HfO2 doped manganese-zinc ferrites, J. Magn. Magn. Mater. 491 (2019), 165554. [13] K. Sun, G.H. Wu, B. Wang, Q.Y. Zhong, Y. Yang, Z. Yu, C.J. Wu, P.W. Wei, X.N. Jiang, Z.W. Lan, Cation distribution and magnetic property of Ti/Snsubstituted manganese-zinc ferrites, J. Alloy. Comp. 650 (2015) 363e369. [14] L.Z. Li, Z.W. Lan, Z. Yu, K. Sun, Z.Y. Xu, Effect of Co-substitution on wide temperature ranging characteristic of electromagnetic properties in MnZn ferrites, J. Alloy. Comp. 476 (2009) 755e759. si [15] M. Drofenik, A. Znidar c, I. Zajc, Highly resistive grain boundaries in doped MnZn ferrites for high frequency power supplies, J. Appl. Phys. 82 (1997) 333e340. [16] K. Sun, Z.W. Lan, Z. Yu, Z.Y. Xu, X.N. Jiang, Z. H Wang, Z. Liu, M. Luo, Temperature and frequency characteristics of low-loss MnZn ferrite in a wide temperature range, J. Appl. Phys. 109 (2011), 106103. [17] K. Sun, Z.W. Lan, Z. Yu, L.Z. Li, X.L. Nie, Z.Y. Xu, Analysis of losses in NiO doped MnZn ferrites, J. Alloy. Comp. 468 (2009) 315e320. [18] S.F. Wang, Y.J. Chiang, Y.F. Hsu, C.H. Chen, Effects of additives on the loss characteristics of Mn-Zn ferrite, J. Magn. Magn. Mater. 365 (2014) 119e125. [19] B. Sun, F.G. Chen, D. Xie, W.D. Yang, H.Q. Shen, A large domain wall pinning effect on the magnetic properties of ZrO2 added Mn-Zn ferrites, Ceram. Int. 40 (2014) 6351e6354.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Ultra-low core losses at high frequencies and temperatures in MnZn ferrites with nano-BaTiO3 additives Guohua Wu *, Zhong Yu , Ke Sun , Rongdi Guo , Hanyu Zhang , Xiaona Jiang , Chuanjian Wu , Zhongwen Lan School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 610054, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2019 Received in revised form 19 December 2019 Accepted 26 December 2019 Available online 27 December 2019
The miniaturization and integration of power conversion devices and components, such as converters and switching power supplies, require ferrite cores with high operating frequencies and temperatures. Hence, the frequency and temperature dependence of core losses is a bottleneck restricting the development of power conversion devices. Herein, nanoparticles of a dielectric materialdbarium titanate (BTO)dare adopted to suppress high-frequency and high-temperature core losses in MnZn ferrites. Using a loss separation method, the effects of BTO on core losses and their temperature dependence are investigated. Residual loss at 25 C and eddy current loss at high temperatures were significantly suppressed by the addition of BTO. Remarkably, adding 0.04 wt% BTO nanoparticles resulted in MnZn ferrites with ultra-low core losses of 302 kW/m3 (25 C) and 890 kW/m3 (100 C) at 3 MHz and 30 mT. These superior MnZn ferrites with ultra-low core losses have potential applications in power conversion devices with high operating frequencies and temperatures. This study also provides a novel approach for suppressing core losses. © 2019 Elsevier B.V. All rights reserved.
Keywords: MnZn ferrites BTO nanoparticles additives Microstructure Core losses Temperature dependence
1. Introduction Power conversion devices with high transfer efficiency, such as AC-DC converters, DC-DC converters, and switching mode power supplies, do not function well without high-performance core materials. Due to their high initial permeability, saturation magnetic induction, and resistivity, and low core losses, MnZn ferrites have become an excellent option for core materials [1e5]. Currently, the operating frequency of MnZn ferrites has increased to the megahertz range to meet the miniaturization and integration requirements of electronic devices. Unfortunately, high losses in the magnetic core induced by high frequencies decrease the transfer efficiency of power conversion devices [6e8]. Additionally, due to the high power density of electronic devices, high operating temperatures (usually 80e100 C) can deteriorate the performance of ferrite cores [9e11]. Therefore, reducing core losses at high frequencies and temperatures is vital for the realization of highefficiency conversion devices. Theoretically, the core losses in MnZn ferrites can be
* Corresponding author. E-mail address: [email protected] (G. Wu). https://doi.org/10.1016/j.jallcom.2019.153573 0925-8388/© 2019 Elsevier B.V. All rights reserved.
decomposed into hysteresis loss (Ph), eddy current loss (Pe), and residual loss (Pr), according to the respective physical mechanism [12e14]. Scholars have reported that the Pe induced by eddy currents in an alternating magnetic field, and the Pr induced by resonance, become significant at high frequencies and temperatures [15e17]. Accordingly, suppressing core losses at high frequencies and temperatures requires the reduction of eddy current and residual losses. On the one hand, regular oxides such as CaCO3, SiO2, ZrO2, HfO2, Ta2O5, TiO2, and SnO2 can be added to modify the resistivity and eddy current loss of a material [18e22]. On the other hand, low sintering temperatures and fine grains have been reported to benefit the reduction of residual loss [23e25]. Recently, in addition to regular oxide additives, other additives have been explored, such as dielectric materials with particular properties. BaTiO3 (BTO) is a widely used dielectric material with a high dielectric permittivity [26e30]. Adding 0.08 wt% BTO nanoparticles into MnZn ferrites has been reported to reduce the core losses occurring at 500 kHz, 30 mT, and room temperature (25 C) from 283.4 to 82.7 kW/m3, which is attributed to a decrease in eddy current loss [31]. However, the effect of BTO nanoparticles remains uncertain with regard to core losses at high frequencies (2e4 MHz) and high temperatures, where the constitution of core losses is more complicated than that at low frequencies. Moreover, the
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physical mechanism by which BTO influences core losses and temperature dependence has not been revealed. In this paper, to suppress core losses at high frequencies and temperatures, we utilize BTO nanoparticles as additives to MnZn ferrites. Most importantly, the effects of BTO on core losses and temperature dependence are investigated in detail using the loss separation method. Adjusting the content of BTO at 3 MHz and 30 mT significantly suppressed core losses at both low and high temperatures due to reductions in eddy current and residual losses. This work reveals a feasible method to suppress the core losses in power ferrites at high frequencies and temperatures, by the addition of dielectric materials with special properties. 2. Materials and methods MnZn ferrites with a major composition of Mn0.67Zn0.21Fe2.12O4 were fabricated using a solid-state reaction method. Highly pure oxides of Fe2O3, Mn3O4, and ZnO were weighed precisely, milled in a planetary mill for 2 h with deionized water, and then calcined at 900 C for 2 h. Various contents of BTO nanoparticles (0.00e0.14 wt %) were introduced into the calcined powder. Other additives with fixed contents were also added to improve the electromagnetic properties of the MnZn ferrites: 0.30 wt% Co2O3, 0.10 wt% CaCO3, 0.01 wt% V2O5, 0.20 wt% TiO2, and 0.06 wt% SnO2. The mixtures were milled in a planetary mill for 4 h. Subsequently, the mixtures were granulated with 13 wt% poly-vinyl alcohol and compacted into toroidal-shaped cores (18 mm 8 mm 5 mm) and discshaped samples (18 mm 3 mm) under a 6 MPa pressure. The impacted samples were sintered at 1150 C for 6 h at a 5% partial oxygen pressure and cooled in a carefully controlled N2/O2 atmosphere. X-ray diffraction (XRD, MAXima XRD-7000) was performed to characterize the crystalline phase of the prepared samples. The microstructure of the fracture surface was observed using a scanning electron microscope (SEM, JEOL JSM-6490LV), and the average grain size was determined from the SEM images through the interception method. Core losses, PL, were tested through a B-H analyzer (Iwatsu SY-8232) from 25 to 120 C. The inductance of the toroidal core samples and the AC resistance of silver-plated disk samples were tested using a digital electric bridge (Tonghui TH2829). Initial permeability and AC resistivity were calculated according to the inductance and resistance, respectively. The impedance of the disk samples was measured using an LCR bridge meter (WK 6500P). Transmission electron microscopy (TEM, JEM 2100, 200 kV) was utilized to perform energy dispersive X-ray analysis (EDX). 3. Results and discussion 3.1. Microstructure and crystalline phase Fig. 1(a)e(h) present micrographs of samples with different BTO contents. Fig. 1(i) presents the measured average grain size, D. The addition of BTO can refine the grains when the content of BTO is less than 0.08 wt%. During the sintering process, the existence of BTO nanoparticles along the grain boundary hinders grain growth, because the melting point of BTO (over 1600 C) is far higher than the sintering temperature of MnZn ferrites (1150 C) [32e34]. Moreover, for the 0.02e0.08 wt% BTO samples, the standard deviation of the grain size remained almost unchanged, which indicates that the homogeneity of the grains deteriorated on the premise of decreased grain size. However, when the content of BTO exceeded 0.08 wt%, BTO nanoparticles formed impurities and slightly increased grain growth, causing an increase in grain size. The XRD patterns for MnZn ferrite samples with different BTO
contents are shown in Fig. 1(j). As the figure shows, all the samples exhibited a pure cubic spinel ferrite phase, which matches well with JCPDS card no. 74e2401. Even though BTO is a typical perovskite structure, the content of BTO in the MnZn ferrites was small; thus, the addition of BTO had no influence on the crystalline phase. 3.2. Influence of BTO on core losses at room temperature (25 C) Fig. 2 presents the variation in PL with BTO content at 3 MHz, 30 mT, and 25 C. As the BTO content increased, core losses initially decreased and subsequently increased, with a clear minimum of 302 kW/m3 at 0.04 wt% BTO content. Loss separation was performed to analyze the effect of BTO addition on core losses. The core losses of MnZn ferrites were decomposed into hysteresis loss (Ph), eddy current loss (Pe), and residual loss (Pr). The expression of PL is as follows [16,17,21]:
. PL ¼ Ph þ Pe þ Pr ¼ Kh B3 f þ Ke B2 f 2 D2 r þ Pr
(1)
where Kh and Ke represent constants, f is the frequency, B is the magnetic induction, and r is the AC resistivity at the measured frequency. By dividing PL by f in Equation (1) and plotting PL/f ~ f, each component of the core losses can be determined. The intercept (KhB3) represents Ph/f, the slope (KeB2fD2/r) represents Pe/f, and the rest represents Pr/f. The separation results are shown in Figs. 3e5. Fig. 3 presents the hysteresis loss, Ph, for the MnZn ferrite samples with different BTO contents. As the BTO content increased, Ph initially increased and subsequently decreased. Induced by irreversible domain wall movements, Ph was related to the initial permeability mi (Ph f 1/m3i ) [35]. The variation in the initial permeability, initially decreasing and subsequently increasing, was the opposite of that of Ph, as shown in Fig. 3. Initial permeability relates to domain wall movement and domain rotation, which constitute the magnetization process [36e38]. As the BTO content increased (for <0.08 wt%), the rapid decrease in grain size directly reduced the number of domain walls. Moreover, the addition of a nonmagnetic phase (BTO) hindered the magnetization process. Both factors resulted in a decrease of mi and an increase of Ph. When the BTO content was greater than 0.08 wt%, increasing the grain size enhanced domain wall movement. As a result, the initial permeability increased and the hysteresis loss decreased. Fig. 4(a) presents the variation in Pe with BTO content. As the BTO content increased, Pe exhibited a tendency to increase and subsequently decrease. Eddy current loss originates from the eddy current that emerges inside the material at high frequency, and is related to the AC resistivity, r. As shown in Fig. 4(b), the addition of BTO caused the resistivity of the MnZn ferrites to initially decrease and subsequently increase. Determining the unambiguous distribution of BTO in MnZn ferrites was necessary to illustrate the effect of BTO on resistivity. Fig. 4(c) and (d) present the TEM and EDX analyses results for the 0.14 wt% BTO sample. In the TEM micrograph, the G region is the grain, the GB region is the grain boundary, and the TP region is the triple point region. The element Ba was not detected in all regions due to weak signal intensity and the small content of BTO compared with the matrix MnZn ferrites. However, the element Ti was detected in both the GB and TP regions, but not in the G region. The distribution of Ti indirectly proved that BTO was distributed at the grain boundary, even though extra TiO2 was present in the additives. BTO has been confirmed to possess high resistivity above 1010 U m [39]. Essentially, the distribution of BTO nanoparticles at the grain boundary is supposed to improve the resistivity of the MnZn ferrite material. However, the increase in BTO content
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Fig. 1. SEM images of samples with (a) 0.00 wt%, (b) 0.02 wt%, (c) 0.04 wt%, (d) 0.06 wt%, (e) 0.08 wt%, (f) 0.10 wt%, (g) 0.12 wt%, and (h) 0.14 wt% BTO. (i) Average grain size of MnZn ferrite samples with different BTO contents. (j) XRD patterns of MnZn ferrite samples with different BTO contents.
Fig. 2. Variation in core losses with BTO content at 3 MHz, 30 mT, and 25 C.
(<0.08 wt%) led to a decrease in resistivity. This should be attributed to the deterioration of the microstructure induced by the addition of BTO, as shown in Fig. 1. Scholars have reported that nonhomogeneous microstructures are often accompanied by lattice imperfections, which lower the resistivity [40,41]. When the BTO content increased further, the deterioration of the microstructure ceased and the role of BTO in improving the resistivity of the GB became apparent, leading to an increase in resistivity. Unfortunately, the variation in resistivity cannot adequately explain the eddy current loss, because Pe is also related to grain size according to Equation (1). Thus, D2/r was calculated as presented in Fig. 4(b). As the BTO content increased, the value of D2/r initially increased and subsequently decreased, which was in accordance with the variation in Pe.
Fig. 3. Variation in Ph and mi with BTO content.
Due to the resonant phenomenon occurring in the highfrequency range, Pr is related to the resonant frequency, fr. Fig. 5(a) represents the variation in both residual loss and resonant frequency. The addition of BTO into MnZn ferrites caused an initial decrease and subsequent increase trend in the residual loss. Further, fr, which is extracted from the peak of the imaginary part of the complex permeability m00 , exhibited a trend opposite to that of Pr. It is confirmed that fine grains are beneficial in suppressing the resonance and Pr [21,23]. Thus, the evolution of the microstructure was responsible for the variations in fr and Pr. Compared with the 0.00 wt% BTO sample, the sample with the proper content of BTO nanoparticles had the lowest core losses at 3 MHz, 30 mT, and 25 C, indicating a remarkable effect of dielectric materials on reducing the high-frequency core losses of MnZn ferrites. The proportion of each loss is presented in Fig. 6. Despite
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Fig. 4. (a) Variation in eddy current loss Pe with BTO content. (b) Variation in AC resistivity r at 3 MHz and D2/r with BTO content. (c) TEM image of MnZn ferrite samples with 0.14 wt% BTO. (d) EDX analysis in the G, GB, and TP regions for the 0.14 wt% BTO sample.
Fig. 5. (a) Variation in residual loss Pr and resonant frequency fr. (b) Frequency spectra of the imaginary part m00 of complex permeability.
the increase in Ph and Pe, the suppression of core losses by adding 0.04 wt% BTO nanoparticles was mainly attributed to the refinement of grains and, thereby, the decrease in residual loss. 3.3. Influence of BTO on core losses temperature dependence
Fig. 6. Percentage of each type of loss for samples at different BTO contents.
Apart from the core losses at room temperature discussed above, temperature dependence is also a key performance parameter of MnZn power ferrites, owing to the high operating temperatures of ferrite cores. Fig. 7(a) presents the variation in core losses with temperature at 3 MHz and 30 mT for two representative samples (0.00 and 0.04 wt% BTO). As temperature increased, the core losses monotonically increased in the two investigated samples. However, the rates of increase in the core losses were different, especially in the high-temperature range. The 0.04 wt% BTO sample exhibited a smaller rate of increase in core losses, and had lower core losses at a high temperature (100 C, 890 kW/m3). Loss separation was carried out at a temperature range of 25e120 C to investigate the effect of BTO on the temperature dependence of the core losses.
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Fig. 7. Variation in (a) core losses, (b) hysteresis loss and initial permeability, (c) eddy current loss, and (d) residual loss with measurement temperature.
Fig. 7(b)e(d) demonstrate the temperature dependence of each component of loss. As temperature increased, hysteresis loss slightly decreased and played a minor role in core losses throughout the temperature range. Both eddy current and residual losses monotonically increased and contributed a large part of the core losses. No visible difference was observed between the 0.00 and 0.04 wt% BTO samples with regard to the variations in hysteresis and residual losses. As shown in Fig. 7(b), the slight decrease in hysteresis loss was due to the increase in initial permeability with increasing temperature. It has previously been confirmed that the dramatic increase in residual loss is induced by the inevitable reduction of the resonant frequency at high temperatures [16]. However, the two samples exhibited a great difference in their eddy current loss trends. As shown in Fig. 7(c), when the temperature increased, the eddy current loss increased sharply in the 0.00 wt% BTO sample, while the rate of increase was slightly lower in the 0.04 wt% BTO sample. The AC resistivity at 3 MHz was measured from 25 to 120 C to illustrate differences in the temperature dependence of eddy current loss, as presented in Fig. 8. The AC resistivity of both samples decreased as temperature increased, indicating a semiconducting characteristic of MnZn ferrites [12]. However, the resistivity of the 0.00 wt% BTO sample (denoted as r0.00) rapidly decreased, while that of the 0.04 wt% BTO sample (denoted as r0.04) exhibited a gradual descending trend, as temperature increased. Remarkably, r0.00 was much higher than r0.04 at low temperatures; however, at 120 C, r0.00 became lower than r0.04. It is assumed that if the temperature exceeded 120 C, the difference between r0.00 and r0.04 would be greater. This phenomenon adequately demonstrates the difference in the temperature dependence of the eddy current loss in the two samples. Furthermore, a complex impedance plot at different temperatures was constructed (Fig. 9(a) and (b)). Based on Koops’ model, the resistances of the grains (Rg) and grain boundary (Rgb) can be obtained from the impedance plot [42,43]. The variations in Rg and
Rgb with temperature are presented in Fig. 9(c) and (d). Both Rg and Rgb decreased with increasing temperature, but the rates of decrease in Rgb were different in the two samples. As temperature increased, Rgb for the 0.04 wt% BTO sample decreased gradually, compared with the rapid descent in the 0.00 wt% BTO sample. A similar trend between resistivity (Fig. 8) and grain boundary resistance (Fig. 9(d)) indicated that the grain boundary resistance was responsible for the temperature dependence of resistivity. Essentially, BTO ceramic has a positive temperature coefficient (PTC), which means that the resistivity of the BTO ceramic increases with temperature [44e46]. Therefore, the addition of 0.04 wt% BTO nanoparticles to the MnZn ferrite material can suppress the rates of
Fig. 8. Variations in AC resistivity with temperature at 3 MHz for the 0.00 and 0.04 wt % BTO samples.
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Fig. 9. Impedance plots of the (a) 0.00 wt% and (b) 0.04 wt% BTO sample. Variation in the (c) resistance of the grain Rg and (d) resistance of the grain boundary Rgb with temperature.
Fig. 10. Percentage of each component of loss in samples with (a) 0.00 wt% and (b) 0.04 wt% BTO.
decrease in grain boundary resistance and resistivity of the material. Fig. 10 presents the percentage of each component of loss at different temperatures for the two samples. The percentage of each loss component for the 0.00 wt% BTO sample remained almost unchanged and residual loss was the greatest loss component throughout the temperature range. When 0.04 wt% BTO was added, eddy current loss contributed a large component at room temperature (25 C). As temperature increased, the proportion of Pe gradually decreased and that of Pr increased. Therefore, by adding 0.04 wt% BTO into MnZn ferrites, core losses at high temperatures can be reduced via increased resistivity, which suppresses Pe. 4. Conclusions This work proposes an effective approach for suppressing core losses in MnZn ferrites at high frequencies and temperatures by adding a dielectric materialdBTO nanoparticles. The results
indicate that with increasing BTO content, core losses at 3 MHz, 30 mT, and 25 C initially decrease and subsequently increase, reaching a minimum of 302 kW/m3 at 0.04 wt% BTO content. At room temperature (25 C), the suppression of core losses is mainly attributed to the refinement of grain size, resulting in a reduction of residual loss. With regard to temperature dependence, the core losses of the 0.04 wt% BTO sample are significantly suppressed between 25 and 120 C compared with those of the 0.00 wt% BTO sample. Particularly, at 3 MHz, 30 mT, and 100 C, the core losses of the 0.04 wt% BTO sample are only 890 kW/m3. When 0.04 wt% BTO is added, suppression of core losses at high temperature is caused by the restraining of resistivity, which reduces eddy current loss. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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CRediT authorship contribution statement Guohua Wu: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing. Zhong Yu: Conceptualization, Methodology, Project administration. Ke Sun: Visualization, Project administration, Funding acquisition. Rongdi Guo: Software, Validation, Visualization. Hanyu Zhang: Investigation, Resources. Xiaona Jiang: Formal analysis, Supervision. Chuanjian Wu: Supervision. Zhongwen Lan: Funding acquisition, Supervision. Acknowledgements This work is supported by the National Natural Science Foundation of China under Grant No. 51472045 and 51772046, and the Fundamental Research Funds for the Central Universities ZYGX2018J038, ZYGX2018J040, and ZYGX2018J042. References [1] S. Yuan, Y. Huang, J.F. Zhou, Q. Xu, C.Y. Song, P. Thompson, Magnetic field energy harvesting under overhead power lines, IEEE Trans. Power Electron. 30 (2015) 6191e6202. [2] M.A. Ahmed, K.E. Rady, M.S. Shams, Enhancement of electric and magnetic properties of Mn-Zn ferrite by Ni-Ti ions substitution, J. Alloy. Comp. 622 (2015) 269e275. € pfer, A. Angermann, Complex additive systems for Mn-Zn ferrites with [3] J. To low power loss, J. Appl. Phys. 117 (2015), 17A504. [4] D. Liu, X.P. Chen, Y. Ying, L. Zhang, W.C. Li, L.Q. Jiang, S.L. Che, MnZn power ferrite with high Bs and low core loss, Ceram. Int. 42 (2016) 9152e9156. [5] X. Yang, Z. Zhou, T. Nan, Y. Gao, G.M. Yang, M. Liu, N.X. Sun, Recent advances in multiferroic oxide heterostructures and devices, J. Mater. Chem. C 4 (2016) 234e243. [6] F. Fiorillo, C. Beatrice, O. Bottauscio, A. Manzin, Approach to magnetic losses and their frequency dependence in Mn-Zn ferrites, Appl. Phys. Lett. 89 (2006), 122513. [7] C. Beatrice, O. Bottauscio, M. Chiampi, F. Fiorillo, A. Manzin, Magnetic loss analysis in Mn-Zn ferrite cores, J. Magn. Magn. Mater. 304 (2006) e743ee745. k, V. Tsakaloudi, C. Ragusa, F. Fiorillo, L. Martino, V. Zaspalis, [8] C. Beatrice, S. Doba Magnetic loss, permeability, and anisotropy compensation in CoO-doped MnZn ferrites, AIP Adv. 8 (2017), 047803. [9] X. She, A.Q. Huang, R. Burgos, Review of solid-state transformer technologies and their application in power distribution systems, IEEE Trans. Power Electron. 1 (2013) 186e198. [10] G.D. Capua, N. Femia, A nover method to predict the real operation of ferrite inductors with moderate saturation in switching power supply applications, IEEE Trans. Power Electron. 31 (2016) 2456e2464. [11] Z.G. Dang, J.A. Qahouq, Evaluation of high-current toroid power inductor with NdFeB magnet for DC-DC power converters, IEEE Trans. Ind. Electron. 62 (2015) 6868e6876. [12] R.D. Guo, X.F. Yang, K. Sun, K.W. Li, Z. Yu, G.H. Wu, X.F. Zhang, X.N. Jiang, Z.W. Lan, Temperature characteristics of core losses for HfO2 doped manganese-zinc ferrites, J. Magn. Magn. Mater. 491 (2019), 165554. [13] K. Sun, G.H. Wu, B. Wang, Q.Y. Zhong, Y. Yang, Z. Yu, C.J. Wu, P.W. Wei, X.N. Jiang, Z.W. Lan, Cation distribution and magnetic property of Ti/Snsubstituted manganese-zinc ferrites, J. Alloy. Comp. 650 (2015) 363e369. [14] L.Z. Li, Z.W. Lan, Z. Yu, K. Sun, Z.Y. Xu, Effect of Co-substitution on wide temperature ranging characteristic of electromagnetic properties in MnZn ferrites, J. Alloy. Comp. 476 (2009) 755e759. si [15] M. Drofenik, A. Znidar c, I. Zajc, Highly resistive grain boundaries in doped MnZn ferrites for high frequency power supplies, J. Appl. Phys. 82 (1997) 333e340. [16] K. Sun, Z.W. Lan, Z. Yu, Z.Y. Xu, X.N. Jiang, Z. H Wang, Z. Liu, M. Luo, Temperature and frequency characteristics of low-loss MnZn ferrite in a wide temperature range, J. Appl. Phys. 109 (2011), 106103. [17] K. Sun, Z.W. Lan, Z. Yu, L.Z. Li, X.L. Nie, Z.Y. Xu, Analysis of losses in NiO doped MnZn ferrites, J. Alloy. Comp. 468 (2009) 315e320. [18] S.F. Wang, Y.J. Chiang, Y.F. Hsu, C.H. Chen, Effects of additives on the loss characteristics of Mn-Zn ferrite, J. Magn. Magn. Mater. 365 (2014) 119e125. [19] B. Sun, F.G. Chen, D. Xie, W.D. Yang, H.Q. Shen, A large domain wall pinning effect on the magnetic properties of ZrO2 added Mn-Zn ferrites, Ceram. Int. 40 (2014) 6351e6354.
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