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Synthesis of nickel zinc ferrite ceramics on enhancing gyromagnetic properties by a novel low-temperature sintering approach for LTCC applications
Journal of Alloys and Compounds 778 (2019) 8e14
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of nickel zinc ferrite ceramics on enhancing gyromagnetic properties by a novel low-temperature sintering approach for LTCC applications Yan Yang a, b, *, Jie Li b, **, Jianxiong Zhao a, Xia Chen a, Gongwen Gan b, Gang Wang b, Linfeng He a a b
College of Communication Engineering, Chengdu University of Information Technology, Chengdu 610225, China State Key Laboratory of Electronic Thin Film and Integrated Devices, 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 12 October 2018 Received in revised form 10 November 2018 Accepted 12 November 2018 Available online 13 November 2018
Saturation magnetization and ferromagnetic resonance linewidth are important parameters for microwave devices, such as circulator/isolator. Conventional sintering of ferrite ceramics is prevalently performed to promote the densification. Here, we present a new sintering approach for obtaining uniform and dense NiCuZn ferrite ceramics with enhanced gyromagnetic properties at low temperatures. The NiZn-series ferrites (Ni0.2Cu0.2Zn0.6Fe2O3 e 1.0 wt% Bi2O3þ0.5 wt% MnO2) was synthesized by solid reaction routes with new sintering approaches. The results indicate that a one-transient-step sintering process (OTSP) can significantly improve the gyromagnetic properties at low sintering temperatures by suppressing the anomalous grain growth, such as higher saturation magnetization (4pMs ¼ ~3904 Gauss) and lower ferromagnetic resonance linewidth (DH ¼ ~170 Oe). Therefore, such a methodology is quite significant due to it provides a referential experience for effective sintering route that can enable broad practical applications, such as other ferrite ceramics. © 2018 Elsevier B.V. All rights reserved.
Keywords: NiCuZn ferrite Gyromagnetic properties Low sintering temperatures
1. Introduction As a promising candidate material for a new-generation microwave integrated circuit technology, the ferrite ceramic, which is based on high resistivity and excellent magnetic properties, has been intensively investigated recently [1,2]. But the most important challenge in the future application of microwave devices is miniaturization and integration. For instance, Liao et al. studied gyromagnetic properties of compact LiZnTi ferrites based LTCC technology [3]. However, sintering temperature should be strictly controlled to lower than ~960 C (co-fired with Ag electrode) for LTCC technology [4,5]. Inevitably, a low temperature often leads to insufficient grain growth, which immensely deteriorates of gyromagnetic properties. Therefore, there is a need for a study to adjust a sintering route to control the grain growth. More recently, LiZn ferrite ceramics have attracted a lot of
* Corresponding author. College of Communication Engineering, Chengdu University of Information Technology, Chengdu 610225, China. ** Corresponding author. E-mail addresses: [email protected] (Y. Yang), [email protected] (J. Li). https://doi.org/10.1016/j.jallcom.2018.11.144 0925-8388/© 2018 Elsevier B.V. All rights reserved.
attentions in phase shifter and circulator [6e8]. Xie et al. demonstrated a low-temperature sintering mechanism for LiZnTiMn ferrite [6,7]. Zhou et al. reported the effect of NiZn ferrite nanoparticles upon the structure and gyromagnetic properties of lowtemperature processed LiZnTi ferrites [9]. However, NiCuZn ferrite ceramics with lower ferromagnetic resonance line widths for LTCC technology have shown tremendous potential for microwave ferrite devices in X band. Lately, W.A.Bayoumy et al. studied characterization and magnetic properties of NiCuZn nanocrystalline ferrite sintered at low temperatures, but it has low saturation magnetization due to the presence of nanoparticles, which can not meet the characteristics of high saturation magnetization of microwave devices [10]. Moreover, when NiCuZn ferrite sintered at low temperatures, non-uniform grain growth of NiCuZn ferrite ceramics resulted in deterioration of gyromagnetic performance [7,11]. Therefore, traditional sintering approaches have become the norm and have limited significant densification for LTCC applications. In order to successfully synthesize dense and uniform NiCuZn ferrite ceramics with enhanced gyromagnetic properties, here, we discuss novel transient sintering strategies to enhance the
Y. Yang et al. / Journal of Alloys and Compounds 778 (2019) 8e14
densification of NiCuZn ferrite ceramics and significantly improve their gyromagnetic properties by suppressing grain growth. Using transient sintering treatments, namely one-transient-step sintering process (OTSP) and two-transient-step sintering process (TTSP), respectively, can effectively control the grain growth. At the same time, to obtain excellent gyromagnetic properties (high Ms and low DH) of a series NiCuZn ferrite ceramics at low temperatures, Bi2O3 and MnO2 powders were chosen as good reaction media. Bi2O3 was an excellent addition of NiCuZn samples due to it could reduce sintering temperature and promote densification [12e14]. Similarly, MnO2 can improve 4pMs as we reported in previous work [15]. Thus, for the new transient sintering NiCuZn ferrite ceramics at low temperatures, adding an optimal volume of Bi2O3-MnO2 additive to further promote densification and enhance gyromagnetic properties. Furthermore, compared with the methods of the transient sintering treatments, we also discuss a modified OTSP and TTSP approaches (namely second one-transient-step sintering process (SOTSP) and second two-transient-step sintering process (STTSP), respectively) that clearly promote grain growth but weaken gyromagnetic properties. In a word, the results indicate that the OTSP strategy is greatly effective for ferrite ceramics with superior gyromagnetic properties by the suppressing grain growth (average grain size is less than ~1 mm). 2. Experiments 2.1. NiCuZn ferrites synthesis Ni0.2Cu0.2Zn0.6Fe2O4 ferrites were synthesized via a solid-state method. Analytically raw materials of powders (Fe2O3, ZnO and CuO, 99%; Kelong, Chengdu, China; NiO, 99%, Aladdin Chemical Reagent, China) were weighed in stoichiometric amounts. Then, the powders were mixed with deionized water for 12 h using zirconia balls. The mixed powder was dried and pre-sintered at 800 C for 3 h to obtain the spinel phase. And the pre-sintered powder with 1.0 wt% Bi2O3 (99%; Kelong, Chengdu, China) and 0.5 wt% MnO2 (98%, Aladdin Chemical Reagent, Shanghai, China) additives was wet-milled again for 12 h. The dried powers were granulated with 10 wt% polyvinyl alcohol (PVA) as a binder and pressed into 2e3 mm thick plates with ~12 MPa intensity of pressure. Therefore, the ferrite ceramics were obtained. To explore uniform and dense ferrite ceramics, new sintering methods are designed to these samples as depicted in Fig. 1. In Fig. 1(a), the samples were sintered
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by using OTSP method (every transient time are 5 min and 55min, respectively, one repetitions, the highest and lowest temperature of transient sintering are 860 C and 950 C, respectively). To further study the densification and gyromagnetic of ferrites, two-transientstep sintering process (TTSP, every transient time are 5 min and 55min, respectively, two repetitions, the highest and lowest tem perature of transient sintering are 860 C and 950 C, respectively) has been developed as seen in Fig. 1(c). Also, in order to investigate the influence of sintering behavior on grain size and densification, we extend the OTSP and TTSP sintering strategies, named SOTSP (based on OTSP method, then the samples implement second sin tering at 950 C at a rate of 2 C/min for 2 h, Fig. 1(b)) and STTSP method (based on TTSP method, then the samples implement second sintering at 950 C for 2 h, as shown in Fig. 1(d)) methods. By contrast, normal sintering strategy experiments are conducted, named normal sintering process (NSP, sintering at 900 C for 2 h) and second normal sintering process (SNSP, based on NSP method, second sintering at 950 C for 2 h). 2.2. Characterization analysis The phase structures of the samples were checked by XRD (Cu Ka radiation, Rigaku, Japan) to confirm phase formation. Microstructure images of the samples were performed on as-sintered section using a JSM-6490LV scanning electron microscope (SEM; JEOL JSM-6490). Densities of the samples sintered at different sintering strategies were determined by Archimedean method, which were calculated by using the following formulas [16]:
r¼
m0 $r0 m1 m2
(1)
where r0 is the density of distilled water, m0 is the weighing mass of samples in the air. After filling the gap in the samples with distilled water, the mass was weighed again, marked m1. Then, the samples were immersed in distilled water and their mass weighed, named m2. For the gyromagnetic properties, the ferromagnetic resonance (FMR) line width (DH) was measured in TE106 perturbation method cavity at ~ 9.5 GHz and ~11.5 GHz (i.e., X-band), respectively. The magnetic hysteresis loops were obtained using a vibrating sample magnetometer (VSM) with a ±5000 Oe direct current magnetic field.
Fig. 1. Ni0.2Cu0.2Zn0.6Fe2O4 ferrite ceramics with different sintering treatments: (a) OTSP, (b) SOTSP, (c) TTSP, (d) STTSP.
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3. Results and discussions Fig. 2 displays the phase structure of the Ni0.2Cu0.2Zn0.6Fe2O4 with 1.0 wt% Bi2O3-0.5 wt% MnO2 composite samples sintered at different sintering strategies (OTSP, TTSP, NSP, SOTSP, STTSP, SNSP). Also, details within a specific range (311) reflections are also marked as Fig. 1 for better illustration. It is observed that diffraction peaks emerged at plans (220), (311), (400), (422), (511), (440), (620) and (533), suggesting that all the patterns can be indexed as spinel crystal structure. Also, they are closely matching the crystal structure database file JCPSD #No. 48-0489. As we can see, the characteristic peak (311) shifts visibly to the left. This phenomenon can be explained by the fact that with the increase of sintering temperature, the grain growth is affected by the sintering temperature and irregular growth occurs in the (311) lattice orientation. Also, this is consistent with the variation of lattice constants listed in Table 1. Bulk density and grain size versus different sintering strategies were shown in Fig. 3. The bulk density of the samples exhibited an evident increase from 4.477 g/cm3 (sintered by NSP) to 5.409 g/cm3 (sintered by OTSP), due to the densification and uniformity by particle rearrangement [17]. Interestingly, the secondary sintering process (SNSP, SOTSP, STTSP) based on NSP,OTSP, and TTSP was beneficial to the increase of densification. This phenomenon can be attributed to sufficient grain growth (bigger average grain size) at higher temperature (enough high sintering temperature: second sintering at 950 C for 2 h) [18]. Meanwhile, the average grain size was also shown in Fig. 3 and the values were also listed in Table 1. It is observed that grains with an estimated average grain size ~0.64 mm were obtained by OTSP method, while comparatively large grains having an average size ~6.15 mm were obtained by NSP method. In short, the fine-grained NiCuZn ferrite ceramics sintered by OTSP at low temperatures exhibited smaller grain size and
Fig. 3. Bulk density and grain size versus different sintering strategies.
uniform size distribution. Fig. 4 shows the microstructure images of Ni0.2Cu0.2Zn0.6Fe2O4 with 1.0 wt% Bi2O3-0.5 wt% MnO2 composite ferrite ceramics by different sintering strategies. The results indicate that uniform and compact of NiCuZn ferrite ceramics (average grain size is ~0.64 mm) can be obtained using OTSP method via suppressing grain growth. Fig. 4(a) shows that big grains (the average grain size ~2.96 mm) and non-uniform grains are existed, indicating that the NSP sintering method limits the formation of dense ferrite ceramics. However, after the sintering method of OTSP has been implemented, great changes have taken place in the microstructure. Fig. 4(b) shows SEM images of NiCuZn ferrite ceramics via OTSP method. As seen in
Fig. 2. XRD patterns of the samples with various sintering methods and the inset shows the main peak for (311) reflections.
Table 1 Characteristic parameters of Ni0.2 Cu0.2 Zn0.6 Fe2O4 ferrite ceramics with 1.0 wt% Bi2O3-0.5 wt% MnO2 composition sintered at different sintering strategies. Sintering strategies
4pMs (Gauss)
Density (g/cm3)
Ms (emu/g)
DH (Oe @9.3 GHz)
Lattice constant (Å)
Average grain size (mm)
NSP OTSP TTSP SNSP SOTSP STTSP
3710.97 3904.27 3400.69 4387.84 4097.82 4378.72
4.477 5.409 4.606 5.155 5.304 5.262
67.51 57.55 58.86 69.87 61.56 67.04
211 170 193 270 194 206
8.4241 8.4118 8.4136 8.4345 8.4266 8.4275
2.96 ± 0.38 0.64 ± 0.08 0.89 ± 0.06 6.15 ± 0.32 2.83 ± 0.24 4.45 ± 0.15
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Fig. 4. SEM micrographs of the samples sintered at different strategies: (a) NSP, (b) OTSP, (c) TTSP, (d)SNSP, (e) SOTSP, (f) STTSP.
Fig. 4(b), the average grain size is immensely decreased to ~0.64 mm. Thus, a highly homogeneous and dense ferrite ceramics are obtained via suppressing grain growth at low temperatures. This phenomenon can be attributed that appropriate activation energy for grain diffusion and enhanced densification by the OTSP strategy [19,20]. Meanwhile, this proved that OTSP is very effective for controlling grain growth. To further analyze the influence of the transient sintering approaches on microstructure, Fig. 4(c) shows SEM images of NiCuZn ferrite ceramics via TTSP method. Compare with Fig. 4(b), the grains of samples became larger (the average grain size ~0.89 mm) due to the transient time of high temperature is longer by the TTSP strategy (Fig. 1(c)). But the densification deteriorated and the pores increased. Comparatively, Fig. 4(d), (e) and (f) show the SEM images using the sintering methods SNSP, SOTSP and STTSP, respectively. Apparently, the grain size of the samples significantly increased, but some abnormal grains appeared. The largest grain size reaches ~6.15 mm, this can be explained by sufficient sintering kinetics (enough hold time after first sintering corresponding to the GH stage of Fig. 1(b)) [3,20,21]. Taken together, these results suggest that OTSP method can be helpful to achieve uniform and dense NiCuZn ferrite ceramics by means of suppressing grain growth at low temperatures. Fig. 5 shows the distribution of grain size in NiCuZn ferrite ceramics sintered by OTSP method and TTSP method. The size distribution determined by the approximate intercept method of SEM images. As shown in Fig. 5, NiCuZn ferrite ceramics showed smaller grain size. After sintering at OTSP, most of the NiCuZn ferrite particles were ranging from 400 nm to 800 nm, and the samples sintered at TTSP were in the range of 800 nme1000 nm. In other words, the samples sintered at TTSP exhibited larger grain size, due to the longer time of high temperature in transient sintering process. Therefore, the strategy of OTSP could achieve dense and uniform ferrite ceramics with small grains.
Fig. 5. Distribution of grain size in NiCuZn ferrite ceramics sintered at (a) OTSP method and (b) TTSP method.
Fig. 6 illustrates magnetic hysteresis loops of the composite samples sintered at different strategies. Also, the variation of saturation magnetization of different sintering ferrite ceramics calculated from magnetic hysteresis loops is shown in Fig. 7(a). As seen in Figs. 6 and 7(a), each of sample has a good saturation magnetization. For non-secondary sintering ferrite ceramics (sintered by OTSP, TTSP, and NSP methods), the saturation magnetization of the samples sintered by NSP increases from 57.55 emu/g to 67.51 emu/g. Additionally, for secondary sintering ferrite ceramics (sintered by SOTSP, STTSP, and SNSP), the saturation magnetization of the samples prepared by SNSP is larger than that of SOTSP. The trend of saturation magnetization increases can be attributed that magnetic performance increased with ceramic grain growth, which well agreed with the change of microstructure as seen in Fig. 4 [8,22]. Fig. 7(b) exhibites the calculated 4pMs of the ferrite ceramics sintered at different sintering strategies based on Ms. Also, relevant magnetic parameters were listed in Table 1. Actually, as was reported by Hord W.E., 4pMs of magnetic materials is a vital parameter to evaluate the performance of gyromagnetic ferrites for microwave devices [23e26]. As shown in Fig. 7(b) and Table 1, the strategy of OTSP could significantly enhance 4pMs due to a good control of grain growth and increasement of densification. However, 4pMs of the samples sintered by TTSP showed a decrease from 3904.27 emu/g to 3400.69 emu/g, which was due to the
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Fig. 6. Magnetic hysteresis (M-H) loops of the samples with different sintering strategies: (a) NSP, (b) OTSP, (c) TTSP, (d)SNSP, (e) SOTSP, (f) STTSP.
different sintering temperature (OTSP: 950 C for 5 min, TTSP: 950 C for 10 min) and the deteriorated densification. Additionally, the 4pMs of the samples with secondary sintering ferrite ceramics increased, which was related to the higher sintering temperature promoted grain growth. In short, the results indicated that high 4pMs with small grains were obtained at low sintering temperatures with OTSP strategy. Ferromagnetic resonance (FMR) line widths of Lorentz fitting of Ni0.2Cu0.2Zn0.6Fe2O4 with 1.0 wt% Bi2O3-0.5 wt% MnO2 composite ferrite ceramics sintered at different sintering methods are shown in Fig. 8. All the experimental data are fitted well with a Lorentz distribution. Obviously, the experimental results of all samples are highly consistent with the fitting results at high frequency. The FMR line width (DH) values (@ 9.3 GHz and @11.5 GHz) of the samples based on Lorentz fit are summarized in Fig. 9. As shown in Fig. 9, the difference between the FMR value at 9.3 GHz and that at 11.5 GHz is
very small, which proves that the transient sintering method proposed in this paper is very helpful to the application of broadband microwave devices [3,27]. Additionally, results indicate that the samples via OTSP approach can effectively reduce DH values at low temperatures. Futhermore, FMR values of samples using transient sintering approaches (OTSP, TTSP, SOTSP and STTSP) are significantly lower than NSP and SNSP approaches, and the DH of the samples via OTSP strategy reaches a minimum value ~170 Oe at 9.3 GHz. The reason can be attributed that NiCuZn ferrites sintered at OTSP present relatively uniform size distribution and decreased pores (Fig. 4(b)), which effectively caused the reduction of DH [28]. Additionally, the decrease of FMR line width is much lower than that of LiZn ferrite reported in the literatures, which demonstrates that NiCuZn ferrite can be applied X-band microwave device [3,9,22,29,30]. Generally, the FMR line width (DH) can be expressed as [31]:
Y. Yang et al. / Journal of Alloys and Compounds 778 (2019) 8e14
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Fig. 7. (a) Variation of saturation magnetization (Ms) of the samples with different sintering strategies. (b) Calculated 4pMs of the ferrite ceramics sintered at different sintering strategies based on Ms.
Fig. 8. Ferromagnetic resonance (FMR) line width based on Lorentzian fitted spectra of ferrite ceramics with different sintering strategies: (a) NSP, (b) OTSP, (c) TTSP, (d)SNSP, (e) SOTSP, (f) STTSP.
H2 DH ¼ DHint þ 2:07ð a Þ þ 1:5ð4pMs ÞP 4pMs
(2)
where DHint is the intrinsic line widths, Ha is random anisotropy
field, and the last part is attributed to the porosity of grains. Therefore, the increasing of 4pMs as shown in Fig. 7(b) was also a factor to explain the decrease of DH [3,32]. In summary, a novel low-temperature transient sintering approach (OTSP) use to
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Y. Yang et al. / Journal of Alloys and Compounds 778 (2019) 8e14
Fig. 9. FMR values of different sintering samples based on Lorentzian fit.
prepare NiCuZn ferrites with reduced DH at 9.3 GHz and 11.5 GHz, which revels it is a good candidate for the wide range of X-band microwave device application. 4. Conclusion In summary, we demonstrate an effective OTSP route to achieve uniform and dense Ni0.2Cu0.2Zn0.6Fe2O4 ferrite ceramics with enhancing gyromagnetic properties via supressing grain growth. The introduction of optimized Bi2O3-MnO2 additives can significantly reduce sintering temperature and has no obvious effect on the formation of pure phase spinel phase. SEM results indicated that uniform and compact ferrite ceramics with the small grains (~0.64 mm) can be achieved by OTSP method. Meanwhile, OTSP was useful for densification of NiCuZn ferrite ceramics by controlling grain growth at low temperatures. Compared with NS and TTSP strategies, OTSP can effectively control the grain growth and suppress abnormal grain growth, which is significantly beneficial to improve 4pMs (~3904 Gauss) and reduce FMR line width (~170 Oe @9.3 GHz). Interestingly, the difference between the FMR values of 9.3 GHz and 11.5 GHz is very small, indicating that the NiCuZn ferrite prepared by transient sintering method is very suitable for broadband microwave devices. And, the synthetic NiZn-series ferrite ceramics are promising for LTCC applications. Acknowledgments The authors acknowledge financial support by the National Nature Science Foundation of China (Nos. 51602036), and by the Sichuan Science and Technology Project (Grant No. 18MZGC0025). References [1] Q. Yang, H. Zhang, Y. Liu, Q. Wen, L. Jia, Microstructure and magnetic properties of microwave sintered NiCuZn ferrite for application in LTCC devices, Mater. Lett. 79 (2012) 103e105. [2] H. Su, C. Zhao, H. Guo, Q. Wang, C. Dai, H. Zhang, Y. Li, X. Tang, Correlation between the microstructure and permeability stability of ferrite materials, Ceram. Int. 44 (2018) 2304e2310. [3] T. Zhou, H. Zhang, C. Liu, L. Jin, F. Xu, Y. Liao, N. Jia, Y. Wang, G. Gan, H. Su, Li2 O-B2O3-SiO2-CaO-Al2O3 and Bi2O3 co-doped gyromagnetic Li0.43Zn0.27Ti0.13Fe2.17O4 ferrite ceramics for LTCC technology, Ceram. Int. 42 (2016) 16198e16204. [4] J. Li, D. Wen, Q. Li, T. Qiu, G. Gan, H. Zhang, Equal permeability and permittivity in a low temperature co-fired In-doped Mg-Cd ferrite, Ceram. Int. 44 (2018) 678e682. [5] A. Hajian, M. Stoeger-Pollach, M. Schneider, D. Mueftueoglu, F.K. Crunwell, U. Schmid, Porosification behaviour of LTCC substrates with potassium hydroxide, J. Eur. Ceram. Soc. 38 (2018) 2369e2377.
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Synthesis of nickel zinc ferrite ceramics on enhancing gyromagnetic properties by a novel low-temperature sintering approach for LTCC applications Yan Yang a, b, *, Jie Li b, **, Jianxiong Zhao a, Xia Chen a, Gongwen Gan b, Gang Wang b, Linfeng He a a b
College of Communication Engineering, Chengdu University of Information Technology, Chengdu 610225, China State Key Laboratory of Electronic Thin Film and Integrated Devices, 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 12 October 2018 Received in revised form 10 November 2018 Accepted 12 November 2018 Available online 13 November 2018
Saturation magnetization and ferromagnetic resonance linewidth are important parameters for microwave devices, such as circulator/isolator. Conventional sintering of ferrite ceramics is prevalently performed to promote the densification. Here, we present a new sintering approach for obtaining uniform and dense NiCuZn ferrite ceramics with enhanced gyromagnetic properties at low temperatures. The NiZn-series ferrites (Ni0.2Cu0.2Zn0.6Fe2O3 e 1.0 wt% Bi2O3þ0.5 wt% MnO2) was synthesized by solid reaction routes with new sintering approaches. The results indicate that a one-transient-step sintering process (OTSP) can significantly improve the gyromagnetic properties at low sintering temperatures by suppressing the anomalous grain growth, such as higher saturation magnetization (4pMs ¼ ~3904 Gauss) and lower ferromagnetic resonance linewidth (DH ¼ ~170 Oe). Therefore, such a methodology is quite significant due to it provides a referential experience for effective sintering route that can enable broad practical applications, such as other ferrite ceramics. © 2018 Elsevier B.V. All rights reserved.
Keywords: NiCuZn ferrite Gyromagnetic properties Low sintering temperatures
1. Introduction As a promising candidate material for a new-generation microwave integrated circuit technology, the ferrite ceramic, which is based on high resistivity and excellent magnetic properties, has been intensively investigated recently [1,2]. But the most important challenge in the future application of microwave devices is miniaturization and integration. For instance, Liao et al. studied gyromagnetic properties of compact LiZnTi ferrites based LTCC technology [3]. However, sintering temperature should be strictly controlled to lower than ~960 C (co-fired with Ag electrode) for LTCC technology [4,5]. Inevitably, a low temperature often leads to insufficient grain growth, which immensely deteriorates of gyromagnetic properties. Therefore, there is a need for a study to adjust a sintering route to control the grain growth. More recently, LiZn ferrite ceramics have attracted a lot of
* Corresponding author. College of Communication Engineering, Chengdu University of Information Technology, Chengdu 610225, China. ** Corresponding author. E-mail addresses: [email protected] (Y. Yang), [email protected] (J. Li). https://doi.org/10.1016/j.jallcom.2018.11.144 0925-8388/© 2018 Elsevier B.V. All rights reserved.
attentions in phase shifter and circulator [6e8]. Xie et al. demonstrated a low-temperature sintering mechanism for LiZnTiMn ferrite [6,7]. Zhou et al. reported the effect of NiZn ferrite nanoparticles upon the structure and gyromagnetic properties of lowtemperature processed LiZnTi ferrites [9]. However, NiCuZn ferrite ceramics with lower ferromagnetic resonance line widths for LTCC technology have shown tremendous potential for microwave ferrite devices in X band. Lately, W.A.Bayoumy et al. studied characterization and magnetic properties of NiCuZn nanocrystalline ferrite sintered at low temperatures, but it has low saturation magnetization due to the presence of nanoparticles, which can not meet the characteristics of high saturation magnetization of microwave devices [10]. Moreover, when NiCuZn ferrite sintered at low temperatures, non-uniform grain growth of NiCuZn ferrite ceramics resulted in deterioration of gyromagnetic performance [7,11]. Therefore, traditional sintering approaches have become the norm and have limited significant densification for LTCC applications. In order to successfully synthesize dense and uniform NiCuZn ferrite ceramics with enhanced gyromagnetic properties, here, we discuss novel transient sintering strategies to enhance the
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densification of NiCuZn ferrite ceramics and significantly improve their gyromagnetic properties by suppressing grain growth. Using transient sintering treatments, namely one-transient-step sintering process (OTSP) and two-transient-step sintering process (TTSP), respectively, can effectively control the grain growth. At the same time, to obtain excellent gyromagnetic properties (high Ms and low DH) of a series NiCuZn ferrite ceramics at low temperatures, Bi2O3 and MnO2 powders were chosen as good reaction media. Bi2O3 was an excellent addition of NiCuZn samples due to it could reduce sintering temperature and promote densification [12e14]. Similarly, MnO2 can improve 4pMs as we reported in previous work [15]. Thus, for the new transient sintering NiCuZn ferrite ceramics at low temperatures, adding an optimal volume of Bi2O3-MnO2 additive to further promote densification and enhance gyromagnetic properties. Furthermore, compared with the methods of the transient sintering treatments, we also discuss a modified OTSP and TTSP approaches (namely second one-transient-step sintering process (SOTSP) and second two-transient-step sintering process (STTSP), respectively) that clearly promote grain growth but weaken gyromagnetic properties. In a word, the results indicate that the OTSP strategy is greatly effective for ferrite ceramics with superior gyromagnetic properties by the suppressing grain growth (average grain size is less than ~1 mm). 2. Experiments 2.1. NiCuZn ferrites synthesis Ni0.2Cu0.2Zn0.6Fe2O4 ferrites were synthesized via a solid-state method. Analytically raw materials of powders (Fe2O3, ZnO and CuO, 99%; Kelong, Chengdu, China; NiO, 99%, Aladdin Chemical Reagent, China) were weighed in stoichiometric amounts. Then, the powders were mixed with deionized water for 12 h using zirconia balls. The mixed powder was dried and pre-sintered at 800 C for 3 h to obtain the spinel phase. And the pre-sintered powder with 1.0 wt% Bi2O3 (99%; Kelong, Chengdu, China) and 0.5 wt% MnO2 (98%, Aladdin Chemical Reagent, Shanghai, China) additives was wet-milled again for 12 h. The dried powers were granulated with 10 wt% polyvinyl alcohol (PVA) as a binder and pressed into 2e3 mm thick plates with ~12 MPa intensity of pressure. Therefore, the ferrite ceramics were obtained. To explore uniform and dense ferrite ceramics, new sintering methods are designed to these samples as depicted in Fig. 1. In Fig. 1(a), the samples were sintered
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by using OTSP method (every transient time are 5 min and 55min, respectively, one repetitions, the highest and lowest temperature of transient sintering are 860 C and 950 C, respectively). To further study the densification and gyromagnetic of ferrites, two-transientstep sintering process (TTSP, every transient time are 5 min and 55min, respectively, two repetitions, the highest and lowest tem perature of transient sintering are 860 C and 950 C, respectively) has been developed as seen in Fig. 1(c). Also, in order to investigate the influence of sintering behavior on grain size and densification, we extend the OTSP and TTSP sintering strategies, named SOTSP (based on OTSP method, then the samples implement second sin tering at 950 C at a rate of 2 C/min for 2 h, Fig. 1(b)) and STTSP method (based on TTSP method, then the samples implement second sintering at 950 C for 2 h, as shown in Fig. 1(d)) methods. By contrast, normal sintering strategy experiments are conducted, named normal sintering process (NSP, sintering at 900 C for 2 h) and second normal sintering process (SNSP, based on NSP method, second sintering at 950 C for 2 h). 2.2. Characterization analysis The phase structures of the samples were checked by XRD (Cu Ka radiation, Rigaku, Japan) to confirm phase formation. Microstructure images of the samples were performed on as-sintered section using a JSM-6490LV scanning electron microscope (SEM; JEOL JSM-6490). Densities of the samples sintered at different sintering strategies were determined by Archimedean method, which were calculated by using the following formulas [16]:
r¼
m0 $r0 m1 m2
(1)
where r0 is the density of distilled water, m0 is the weighing mass of samples in the air. After filling the gap in the samples with distilled water, the mass was weighed again, marked m1. Then, the samples were immersed in distilled water and their mass weighed, named m2. For the gyromagnetic properties, the ferromagnetic resonance (FMR) line width (DH) was measured in TE106 perturbation method cavity at ~ 9.5 GHz and ~11.5 GHz (i.e., X-band), respectively. The magnetic hysteresis loops were obtained using a vibrating sample magnetometer (VSM) with a ±5000 Oe direct current magnetic field.
Fig. 1. Ni0.2Cu0.2Zn0.6Fe2O4 ferrite ceramics with different sintering treatments: (a) OTSP, (b) SOTSP, (c) TTSP, (d) STTSP.
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3. Results and discussions Fig. 2 displays the phase structure of the Ni0.2Cu0.2Zn0.6Fe2O4 with 1.0 wt% Bi2O3-0.5 wt% MnO2 composite samples sintered at different sintering strategies (OTSP, TTSP, NSP, SOTSP, STTSP, SNSP). Also, details within a specific range (311) reflections are also marked as Fig. 1 for better illustration. It is observed that diffraction peaks emerged at plans (220), (311), (400), (422), (511), (440), (620) and (533), suggesting that all the patterns can be indexed as spinel crystal structure. Also, they are closely matching the crystal structure database file JCPSD #No. 48-0489. As we can see, the characteristic peak (311) shifts visibly to the left. This phenomenon can be explained by the fact that with the increase of sintering temperature, the grain growth is affected by the sintering temperature and irregular growth occurs in the (311) lattice orientation. Also, this is consistent with the variation of lattice constants listed in Table 1. Bulk density and grain size versus different sintering strategies were shown in Fig. 3. The bulk density of the samples exhibited an evident increase from 4.477 g/cm3 (sintered by NSP) to 5.409 g/cm3 (sintered by OTSP), due to the densification and uniformity by particle rearrangement [17]. Interestingly, the secondary sintering process (SNSP, SOTSP, STTSP) based on NSP,OTSP, and TTSP was beneficial to the increase of densification. This phenomenon can be attributed to sufficient grain growth (bigger average grain size) at higher temperature (enough high sintering temperature: second sintering at 950 C for 2 h) [18]. Meanwhile, the average grain size was also shown in Fig. 3 and the values were also listed in Table 1. It is observed that grains with an estimated average grain size ~0.64 mm were obtained by OTSP method, while comparatively large grains having an average size ~6.15 mm were obtained by NSP method. In short, the fine-grained NiCuZn ferrite ceramics sintered by OTSP at low temperatures exhibited smaller grain size and
Fig. 3. Bulk density and grain size versus different sintering strategies.
uniform size distribution. Fig. 4 shows the microstructure images of Ni0.2Cu0.2Zn0.6Fe2O4 with 1.0 wt% Bi2O3-0.5 wt% MnO2 composite ferrite ceramics by different sintering strategies. The results indicate that uniform and compact of NiCuZn ferrite ceramics (average grain size is ~0.64 mm) can be obtained using OTSP method via suppressing grain growth. Fig. 4(a) shows that big grains (the average grain size ~2.96 mm) and non-uniform grains are existed, indicating that the NSP sintering method limits the formation of dense ferrite ceramics. However, after the sintering method of OTSP has been implemented, great changes have taken place in the microstructure. Fig. 4(b) shows SEM images of NiCuZn ferrite ceramics via OTSP method. As seen in
Fig. 2. XRD patterns of the samples with various sintering methods and the inset shows the main peak for (311) reflections.
Table 1 Characteristic parameters of Ni0.2 Cu0.2 Zn0.6 Fe2O4 ferrite ceramics with 1.0 wt% Bi2O3-0.5 wt% MnO2 composition sintered at different sintering strategies. Sintering strategies
4pMs (Gauss)
Density (g/cm3)
Ms (emu/g)
DH (Oe @9.3 GHz)
Lattice constant (Å)
Average grain size (mm)
NSP OTSP TTSP SNSP SOTSP STTSP
3710.97 3904.27 3400.69 4387.84 4097.82 4378.72
4.477 5.409 4.606 5.155 5.304 5.262
67.51 57.55 58.86 69.87 61.56 67.04
211 170 193 270 194 206
8.4241 8.4118 8.4136 8.4345 8.4266 8.4275
2.96 ± 0.38 0.64 ± 0.08 0.89 ± 0.06 6.15 ± 0.32 2.83 ± 0.24 4.45 ± 0.15
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Fig. 4. SEM micrographs of the samples sintered at different strategies: (a) NSP, (b) OTSP, (c) TTSP, (d)SNSP, (e) SOTSP, (f) STTSP.
Fig. 4(b), the average grain size is immensely decreased to ~0.64 mm. Thus, a highly homogeneous and dense ferrite ceramics are obtained via suppressing grain growth at low temperatures. This phenomenon can be attributed that appropriate activation energy for grain diffusion and enhanced densification by the OTSP strategy [19,20]. Meanwhile, this proved that OTSP is very effective for controlling grain growth. To further analyze the influence of the transient sintering approaches on microstructure, Fig. 4(c) shows SEM images of NiCuZn ferrite ceramics via TTSP method. Compare with Fig. 4(b), the grains of samples became larger (the average grain size ~0.89 mm) due to the transient time of high temperature is longer by the TTSP strategy (Fig. 1(c)). But the densification deteriorated and the pores increased. Comparatively, Fig. 4(d), (e) and (f) show the SEM images using the sintering methods SNSP, SOTSP and STTSP, respectively. Apparently, the grain size of the samples significantly increased, but some abnormal grains appeared. The largest grain size reaches ~6.15 mm, this can be explained by sufficient sintering kinetics (enough hold time after first sintering corresponding to the GH stage of Fig. 1(b)) [3,20,21]. Taken together, these results suggest that OTSP method can be helpful to achieve uniform and dense NiCuZn ferrite ceramics by means of suppressing grain growth at low temperatures. Fig. 5 shows the distribution of grain size in NiCuZn ferrite ceramics sintered by OTSP method and TTSP method. The size distribution determined by the approximate intercept method of SEM images. As shown in Fig. 5, NiCuZn ferrite ceramics showed smaller grain size. After sintering at OTSP, most of the NiCuZn ferrite particles were ranging from 400 nm to 800 nm, and the samples sintered at TTSP were in the range of 800 nme1000 nm. In other words, the samples sintered at TTSP exhibited larger grain size, due to the longer time of high temperature in transient sintering process. Therefore, the strategy of OTSP could achieve dense and uniform ferrite ceramics with small grains.
Fig. 5. Distribution of grain size in NiCuZn ferrite ceramics sintered at (a) OTSP method and (b) TTSP method.
Fig. 6 illustrates magnetic hysteresis loops of the composite samples sintered at different strategies. Also, the variation of saturation magnetization of different sintering ferrite ceramics calculated from magnetic hysteresis loops is shown in Fig. 7(a). As seen in Figs. 6 and 7(a), each of sample has a good saturation magnetization. For non-secondary sintering ferrite ceramics (sintered by OTSP, TTSP, and NSP methods), the saturation magnetization of the samples sintered by NSP increases from 57.55 emu/g to 67.51 emu/g. Additionally, for secondary sintering ferrite ceramics (sintered by SOTSP, STTSP, and SNSP), the saturation magnetization of the samples prepared by SNSP is larger than that of SOTSP. The trend of saturation magnetization increases can be attributed that magnetic performance increased with ceramic grain growth, which well agreed with the change of microstructure as seen in Fig. 4 [8,22]. Fig. 7(b) exhibites the calculated 4pMs of the ferrite ceramics sintered at different sintering strategies based on Ms. Also, relevant magnetic parameters were listed in Table 1. Actually, as was reported by Hord W.E., 4pMs of magnetic materials is a vital parameter to evaluate the performance of gyromagnetic ferrites for microwave devices [23e26]. As shown in Fig. 7(b) and Table 1, the strategy of OTSP could significantly enhance 4pMs due to a good control of grain growth and increasement of densification. However, 4pMs of the samples sintered by TTSP showed a decrease from 3904.27 emu/g to 3400.69 emu/g, which was due to the
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Fig. 6. Magnetic hysteresis (M-H) loops of the samples with different sintering strategies: (a) NSP, (b) OTSP, (c) TTSP, (d)SNSP, (e) SOTSP, (f) STTSP.
different sintering temperature (OTSP: 950 C for 5 min, TTSP: 950 C for 10 min) and the deteriorated densification. Additionally, the 4pMs of the samples with secondary sintering ferrite ceramics increased, which was related to the higher sintering temperature promoted grain growth. In short, the results indicated that high 4pMs with small grains were obtained at low sintering temperatures with OTSP strategy. Ferromagnetic resonance (FMR) line widths of Lorentz fitting of Ni0.2Cu0.2Zn0.6Fe2O4 with 1.0 wt% Bi2O3-0.5 wt% MnO2 composite ferrite ceramics sintered at different sintering methods are shown in Fig. 8. All the experimental data are fitted well with a Lorentz distribution. Obviously, the experimental results of all samples are highly consistent with the fitting results at high frequency. The FMR line width (DH) values (@ 9.3 GHz and @11.5 GHz) of the samples based on Lorentz fit are summarized in Fig. 9. As shown in Fig. 9, the difference between the FMR value at 9.3 GHz and that at 11.5 GHz is
very small, which proves that the transient sintering method proposed in this paper is very helpful to the application of broadband microwave devices [3,27]. Additionally, results indicate that the samples via OTSP approach can effectively reduce DH values at low temperatures. Futhermore, FMR values of samples using transient sintering approaches (OTSP, TTSP, SOTSP and STTSP) are significantly lower than NSP and SNSP approaches, and the DH of the samples via OTSP strategy reaches a minimum value ~170 Oe at 9.3 GHz. The reason can be attributed that NiCuZn ferrites sintered at OTSP present relatively uniform size distribution and decreased pores (Fig. 4(b)), which effectively caused the reduction of DH [28]. Additionally, the decrease of FMR line width is much lower than that of LiZn ferrite reported in the literatures, which demonstrates that NiCuZn ferrite can be applied X-band microwave device [3,9,22,29,30]. Generally, the FMR line width (DH) can be expressed as [31]:
Y. Yang et al. / Journal of Alloys and Compounds 778 (2019) 8e14
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Fig. 7. (a) Variation of saturation magnetization (Ms) of the samples with different sintering strategies. (b) Calculated 4pMs of the ferrite ceramics sintered at different sintering strategies based on Ms.
Fig. 8. Ferromagnetic resonance (FMR) line width based on Lorentzian fitted spectra of ferrite ceramics with different sintering strategies: (a) NSP, (b) OTSP, (c) TTSP, (d)SNSP, (e) SOTSP, (f) STTSP.
H2 DH ¼ DHint þ 2:07ð a Þ þ 1:5ð4pMs ÞP 4pMs
(2)
where DHint is the intrinsic line widths, Ha is random anisotropy
field, and the last part is attributed to the porosity of grains. Therefore, the increasing of 4pMs as shown in Fig. 7(b) was also a factor to explain the decrease of DH [3,32]. In summary, a novel low-temperature transient sintering approach (OTSP) use to
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Fig. 9. FMR values of different sintering samples based on Lorentzian fit.
prepare NiCuZn ferrites with reduced DH at 9.3 GHz and 11.5 GHz, which revels it is a good candidate for the wide range of X-band microwave device application. 4. Conclusion In summary, we demonstrate an effective OTSP route to achieve uniform and dense Ni0.2Cu0.2Zn0.6Fe2O4 ferrite ceramics with enhancing gyromagnetic properties via supressing grain growth. The introduction of optimized Bi2O3-MnO2 additives can significantly reduce sintering temperature and has no obvious effect on the formation of pure phase spinel phase. SEM results indicated that uniform and compact ferrite ceramics with the small grains (~0.64 mm) can be achieved by OTSP method. Meanwhile, OTSP was useful for densification of NiCuZn ferrite ceramics by controlling grain growth at low temperatures. Compared with NS and TTSP strategies, OTSP can effectively control the grain growth and suppress abnormal grain growth, which is significantly beneficial to improve 4pMs (~3904 Gauss) and reduce FMR line width (~170 Oe @9.3 GHz). Interestingly, the difference between the FMR values of 9.3 GHz and 11.5 GHz is very small, indicating that the NiCuZn ferrite prepared by transient sintering method is very suitable for broadband microwave devices. And, the synthetic NiZn-series ferrite ceramics are promising for LTCC applications. Acknowledgments The authors acknowledge financial support by the National Nature Science Foundation of China (Nos. 51602036), and by the Sichuan Science and Technology Project (Grant No. 18MZGC0025). References [1] Q. Yang, H. Zhang, Y. Liu, Q. Wen, L. Jia, Microstructure and magnetic properties of microwave sintered NiCuZn ferrite for application in LTCC devices, Mater. Lett. 79 (2012) 103e105. [2] H. Su, C. Zhao, H. Guo, Q. Wang, C. Dai, H. Zhang, Y. Li, X. Tang, Correlation between the microstructure and permeability stability of ferrite materials, Ceram. Int. 44 (2018) 2304e2310. [3] T. Zhou, H. Zhang, C. Liu, L. Jin, F. Xu, Y. Liao, N. Jia, Y. Wang, G. Gan, H. Su, Li2 O-B2O3-SiO2-CaO-Al2O3 and Bi2O3 co-doped gyromagnetic Li0.43Zn0.27Ti0.13Fe2.17O4 ferrite ceramics for LTCC technology, Ceram. Int. 42 (2016) 16198e16204. [4] J. Li, D. Wen, Q. Li, T. Qiu, G. Gan, H. Zhang, Equal permeability and permittivity in a low temperature co-fired In-doped Mg-Cd ferrite, Ceram. Int. 44 (2018) 678e682. [5] A. Hajian, M. Stoeger-Pollach, M. Schneider, D. Mueftueoglu, F.K. Crunwell, U. Schmid, Porosification behaviour of LTCC substrates with potassium hydroxide, J. Eur. Ceram. Soc. 38 (2018) 2369e2377.
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