Investigation on the micro-structure and magnetic properties of LMZBS-Bi2O3 doped low-temperature sintered NiCuZn ferrites

NiCuZn ferrites were widely used in communication and electronic information fields because of the high frequency, high impedance, wide band and low power loss. In this work, it reports that the (Ni0.2Cu0.2Zn0.6O)1.03(Fe2O3)0.97 ferrites were synthesized based on the solid-state reaction method and LMZBS (Li2CO3-MgO-ZnO-B2O3-SiO2)-Bi2O3 were also used as the dopant to regulate the micro-structure and magnetic properties. The crystal phase and micro-structure of the LMZBS-Bi2O3 doped NiCuZn ferrites were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The magnetic properties including saturation magnetization (Ms), coercive force (Hc), Q-factor and permeability as well as the power loss (Pcv) were investigated and analyzed in detail. The results showed that LMZBS (0.2wt%)-Bi2O3 (0.3wt%) doped NiCuZn ferrites exhibited a uniform and dense microstructure, and possessed the highest Ms, Q factor and magnetic permeability as well as the lowest Hc and Pcv. Overall, LMZBS-Bi2O3 doped NiCuZn ferrites possessed the outstanding micro-structure and magnetic properties, which make it promising for a series of applications.


Introduction
Soft magnetic ferrites are widely used as basic support materials in communication and electronic information industries [1][2][3][4][5]. In recent years, with the rapid development of communication and electronic information technology, electronic equipment and instruments continue to miniaturize, integrate and high-frequency development, which not only increases the demand for high-performance soft magnetic ferrite materials, but also requires soft magnetic ferrite materials to develop in the direction of high frequency, high permeability and low power loss [6][7][8].
NiCuZn ferrite has good characteristics of high frequency, high impedance, wide band and low power loss. It is widely used in communication and electronic information fields. NiCuZn ferrite is an important kind of soft magnetic ferrite formed by introducing CuO into NiZn ferrite. Due to the introduction of Cu ions, the lattice distortion and ion occupation in NiCuZn ferrite can be changed, resulting in the change of permeability and saturation magnetic induction intensity of NiCuZn ferrite. At the same time, compared with NiZn ferrites, NiCuZn ferrite has lower sintering temperature and cost. Therefore, the application of NiCuZn ferrite is expanding, and the research on NiCuZn ferrite has become a hot spot in the field of soft magnetic ferrite materials [4,[8][9][10][11][12][13].
Sintering NiCuZn ferrites at low temperatures is of great significance for the miniaturization and integration of magnetic devices and has attracted extensive attention. Low temperature sintering refers to the co-firing of silver electrodes and ceramics, so the sintering temperature must be lower than the temperature of 961°C of melting point of Ag [14][15][16][17][18]. In recent decades, the preparation of ferrites by sintering at low temperature has received extensive attention. For example, Xu et al studied the effect of the addition of Nb 2 O 5 and glass sintering additive on the growth mechanisms and gyromagnetic properties of the ferrite at low sintering temperature [19]. Jeong et al studied the addition of Bi 2 O 3 nanoparticles on the sintering temperature, micro-structure, magnetic and permeability performances of NiCuZn for use in ferrite core applications [20]. Wang studied the effect of V 2 O 5 on the crystal structure, microstructure and magnetic properties of NiCuZn ferrite at lower sintering temperature [21]. These related researches show that the addition of flux could reduce the sintering temperature of NiCuZn ferrites. Although there are many reports about the fabrication of NiCuZn ferrite at low sintering temperature, there are few reports about Li 2 CO 3 -MgO-ZnO-B 2 O 3 -SiO 2 (LMZBS) glass and Bi 2 O 3 mixtures on the effect of sintered NiCuZn ferrites at low temperature.
In this work, the NiCuZn ferrites were synthesized based on the solid-state reaction method and LMZBS-Bi 2 O 3 was also used as the dopant to regulate the micro-structure and magnetic properties. The crystal phase and micro-structure of the LMZBS-Bi 2 O 3 doped NiCuZn ferrites were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The magnetic properties including saturation magnetization (Ms), coercive force (Hc), Q-factor and permeability as well as the power loss (Pcv) were investigated and compared in detail to analyze the corresponding underlying mechanisms.  figure 1 gives the preparation process of LMZBS-Bi 2 O 3 doped NiCuZn ferrites. The powder mixtures were added to deionized water, and then the mixture slurry was milled for 24 h at a running speed of 220 rpm. Next, the mixture powders were dried and calcined at 800°C for 5 h. Then, the dopants including LMZBS and Bi 2 O 3 were added to the mixture powers. After that, the mixture powders were milled again at a running speed of 220 rpm for another 24 h. These mixtures were granulated with 10 wt% of polyvinyl alcohol (PVA) and then were pressed into toroidal samples under a pressure of 10 MPa for 15 s. Finally, LMZBS-Bi 2 O 3 doped NiCuZn ferrites were obtained after a sintering at 910°C for 3 h.

Characterization of LMZBS-Bi 2 O 3 doped NiCuZn ferrites
An x-ray diffraction (Miniflex 600, XRD, Japan) with Cu Kα radiation was employed to examine the phase compositions of as-prepared NiCuZn samples. The 2 theta ranged from 10 to 70 degree and the scan step was 0.02 degree. A scanning electron microscopy (EVO-18, Zeiss, Germany) was used to obtain the images of the NiCuZn ferrites microstructure. An Iwatsu BH analyzer (SY8232) was employed to characterize the saturation magnetization (Ms), coercive force (Hc) and power loss (Pcv). An RF impedance analyzer (E4991B, Agilent) was employed to determine the Q-factor and permeability. All of the above characterizations were conducted at room temperature.  Figure 2 presents the variations in Ms and Hc as LMZBS concentration increased from 0.05 wt% to 0.4 wt%. It can be seen that Ms initially increased and then decreased as the LMZBS concentration increased. The maximum value of Ms was obtained by the 0.2 wt% LMZBS doped NiCuZn ferrite. As a contrast, Hc showed a completely opposite trend to Ms. With the increase in the concentration of LMZBS, the Hc first decreased and then increased. When the concentration of LMZBS was also 0.2 wt%, the as-prepared NiCuZn sample exhibited the lowest Hc. The related researches have reported that LMZBS with proper concentration as dopant could promote NiCuZn to form a compact crystal structure with uniform grain size [22]. Therefore, 0.2 wt% LMZBS doped NiCuZn exhibited relatively good magnetic performances. Herein, in order to further investigate the influence of LMZBS-Bi 2 O 3 on the micro-structure and magnetic performances of NiCuZn ferrites, the LMZBS concentration was set as 0.2 wt% in the following experiments. Figure 3 gives the x-ray diffraction patterns of the LMZBS-Bi 2 O 3 doped NiCuZn. It was evident that all the XRD patterns of the NiCuZn ferrites were indexed to a cubic lattice and there were no other impurity peaks appearing in the XRD patterns, indicating that all the as-prepared NiCuZn samples had no heterozygous phases and there was only existing a single-crystalline phase structure [22][23][24]. Moreover, there was also no obvious shift in the peak position of XRD pattern, suggesting no substitution taken place in the process of the solid-phase reaction. These XRD results implied that LMZBS-Bi 2 O 3 doped NiCuZn ferrites were successfully synthesized during sintering at the temperature of 910°C.  In order to probe the influence of LMZBS-Bi 2 O 3 on the micro-structure of the as-prepared NiCuZn ferrites, figure 4 gives the SEM pictures of the cross-sectional NiCuZn doped with 0.2 wt% LMZBS and different contents of Bi 2 O 3 at sintering temperature of 910°C. In general, it is hard to provide enough energy to promote the growth of ferrite crystals at low sintering temperatures [22]. However, observing the images shown in figure 4, all the NiCuZn ferrites were successfully fabricated, which indicated that LMZBS made a contribution to reduce the sintering temperature. As the Bi 2 O 3 content increased from 0.1 wt% to 0.3 wt%, the crystal grain had obvious grain boundaries. The grain size and the degree of densification was gradually raised. When the Bi 2 O 3 content was 0.3 wt%, the NiCuZn sample exhibited a relatively uniform grain size and a relatively dense micro-structure. When the content of Bi 2 O 3 was 0.4 wt%, although the grain size continued to increase, the uniformity and compactness of samples decreased obviously. With the content of Bi 2 O 3 increasing further to 0.5 wt% and 0.6 wt%, a dual micro-structure with small-sized and large-sized grains were generated because of the melting aid of Bi 2 O 3 . The previous researches showed that constituent atom diffusion significantly affected the growth of pure NiCuZn ferrite grain, resulting in the lowest grain growth rate and the highest activation energy [23,25]. When doping with Bi 2 O 3 , the densification and growth of the grain was changed to solutionreprecipitation process. As a result, the densification and size of the grain increased with the reducing activation energy [25]. Meanwhile, referring to the results shown in the studies of M Drofenik and M L S Teo [25,26], there was a critical thickness for the liquid phase layer. At this time, the solution-reprecipitation persisted during the process of the reaction to promote the abnormal grain growth, resulting in the appearance of the bimodal microstructure [26]. In this work, the critical value of the Bi 2 O 3 content may be 0.5wt%. Consequently, the 0.2 wt% LMZBS-0.5 wt% Bi 2 O 3 doped NiCuZn ferrites exhibited a nonuniform and bimodal microstructure. Aa a result, the optimal content of dopant in NiCuZn ferrite was about 0.2 wt% LMZBS and 0.3 wt% Bi 2 O 3 .

Results and discussion
Based on the Archimedes method, the bulk density was measured and calculated by the equation (1)   Here, the ρ o and m o is the density of deionized water and the mass of NiCuZn in air, respectively. The m 1 and m 2 is the mass of the NiCuZn filled with deionized water and immersed in deionized water, respectively. As shown in figure 5, it was obvious that the bulk density of the as-prepared NiCuZn ferrites increased first and then decreased as the addition content of Bi 2 O 3 increased. This result suggested that NiCuZn ferrites had a good crystallization and densification during the preparation [22][23][24]. In particular, when the x value was 0.3, the LMZBS-Bi 2 O 3 significantly promoted the densification and growth of the grain of the NiCuZn ferrites at the low sintering temperature of 910°C. Therefore, NiCuZn ferrite doped with 0.2 wt% LMZBS and 0.3 wt% Bi 2 O 3 exhibited the biggest bulk density among all the tested samples. Figure 6 presents the magnetic hysteresis loops (M-H) and variation of Ms and Hc of the NiCuZn samples doped with different concentrations of LMZBS-Bi 2 O 3 . All NiCuZn samples showed typical hysteresis behavior under an external magnetic field. It also can be seen from the figure 6(b) that the Ms increased rapidly and then decreased with the increasing addition of LMZBS-Bi 2 O 3 . It has been reported that Ms was closely related to the composition and density [23,27]. Here, all the NiCuZn ferrites had the same composition, therefore, the value of Ms was dependent on the density. A dense microstructure could promote NiCuZn ferrite to exhibit outstanding magnetic performances [27]. As a result, Ms and density exhibited the same variation trend. The maximum value of Ms was obtained by the 0.2 wt% LMZBS-0.3 wt% Bi 2 O 3 doped as-prepared NiCuZn ferrite. Meanwhile, comparing figures 5 and 6(b), it also could find that when the content of Bi 2 O 3 exceed 0.4 wt%, the value of Ms decreased faster than the density, which may be attributed to the negative influence of the excessive nonmagnetic Bi 2 O 3 on the magnetic moment of magnetic ferrite grains. It also can be found that the variation of Hc was exactly opposite of the trend of Ms. The value of Hc was significantly affected by the microstructure and density [28]. The denser and more uniform of the grain size is, the lower Hc is. The 0.2 wt% LMZBS-0.3 wt% Bi 2 O 3 doped NiCuZn samples had a relatively dense microstructure and uniform grain size, thereby possessing the lowest Hc among all the tested samples.   As for all the samples, the value of the Pcv decreased first and then increased with the increasing addition of LMZBS-Bi 2 O 3 . Meanwhile, it was also found that the Pcv increased with the raising value of frequency or Bm. In general, as shown by equation (2), Pcv was consisted of three parts including hysteresis loss (Ph), eddy current loss (Pe) and residual loss (Pr) [23].
It has been reported that when f was relatively low, Pcv was mainly dependent by Ph, and Ph is proportional to 1/u i 3 [23,31,32]. Therefore, combining the results shown in figure 9, the Pcv first decreased and then increased, and the NiCuZn doped with 0.2 wt% LMZBS-0.3 wt% Bi 2 O 3 obtained the minimum value. The pores of high size grain did not easily block the domain all movement and thus the grain boundaries became a major factor in domain-wall pining. The NiCuZn ferrites with a high average size possibly resulted in low Ph and Pcv    values [23,32]. Therefore, 0.2wt% LMZBS-0.3wt% Bi 2 O 3 doped NiCuZn ferrite presented the lowest Pcv due to the high magnetic properties under relatively high excitation conditions of 25 and 50 mT. When f reached a relatively high value of 500 KHz, Pcv was mainly determined by Pe and Pr. It has been reported that Ph and Pe is proportional to f and f 2 , respectively, and Pr is also roughly proportional to f [23,33]. As a result, all the samples exhibited a relatively high Pcv value under the frequency of 500 KHz. NiCuZn samples with dense microstructure and high density could exhibit the low Pe and Pr [27][28][29]. Therefore, combining the results shown in figures 4 and 5, the 0.2 wt% LMZBS-0.3 wt% Bi 2 O 3 doped NiCuZn ferrite exhibited the lowest Pcv under different induction conditions and different frequencies. Figure 9 gives the permeability spectra of the as-prepared NiCuZn samples doped with different concentrations of LMZBS-Bi 2 O 3 . As the contents of LMZBS-Bi 2 O 3 increased from 0.1 wt % to 0.6 wt%, the measured permeability (u′) increased first and then decreased, indicating that the addition of LMZBS-Bi 2 O 3 changed the u′. When the x = 0.3, the NiCuZn sample exhibited the biggest permeability. It also can be observed that all the samples had a low u″ value over a wide frequency band. The addition of LMZBS-Bi 2 O 3 maded a greater influence on u′ than u″. It is well known that the change of permeability is caused by two kinds of magnetization mechanisms: spin rotation and magnetic domain wall motion [24,34,35]. When the NiCuZn was doped with 0.2 wt% LMZBS and 0.3 wt% Bi 2 O 3 , the highest permeability was obtained. It is reported that the permeability of NiCuZn ferrite is strongly affected by the grain size and density [36]. Each unit volume of crystal contains more grain boundaries, which makes NiCuZn ferrite possess smaller grain size [21]. More grain boundaries will hinder the movement process and lead to a reversible domain displacement. The addition of LMZBS-Bi 2 O 3 reduces the grain boundary per unit volume through capillary force, which promotes to increase the grain size and density and improve the homogeneity of NiCuZn ferrite. As a consequence, with the contents of LMZBS-Bi 2 O 3 increasing, the permeability increased first and the NiCuZn doped with 0.2 wt% LMZBS and 0.3 wt% Bi 2 O 3 obtained the highest permeability. However, when the content of Bi 2 O 3 exceeded 0.3 wt%, the permeability of samples decreased, which may be attributed to that the addition of excessive LMZBS-Bi 2 O 3 make a negative effect on the uniformity and integrity of samples grains. These molten glasses are non-magnetic. When they are added excessively, their diffusion in the ferrite weakens the magnetism of the ferrite to a certain extent, leading to the reduction of the magnetic permeability [37][38][39].
Herein, in order to further evaluate the performances of LMZBS-Bi 2 O 3 doped NiCuZn ferrite, a series of NiCuZn ferrites doped by other kinds of dopants including V 2 O 3 , LMZBS, Bi 2 O 3 -Nb 2 O 5 and BBSZ-Nb 2 O 5 were prepared as the previous researches reported [21,22,24,29]. Their magnetic properties were measured under the same condition and table 1 listed the results. It can be observed that all the samples exhibited a high density (>5.0 g cm −3 ), implying all the dopants could promote NiCuZn ferrite to form a dense microstructure. NiCuZn ferrites doped with V 2 O 3 , Bi 2 O 3 -Nb 2 O 5 or LMZBS-Bi 2 O 3 also had a high Ms and low Hc, which may be attributed to their higher density (about 5.1 g cm −3 ) than others. LMZBS(0.2wt%)-Bi 2 O 3 (0.3wt%) doped NiCuZn synthesized in this work exhibited the highest Ms and permeability among all the samples. Based on the above analysis shown in figures 4-9, the reason may be lie in that appropriate amount of LMZBS-Bi 2 O 3 as dopant could promote NiCuZn to possess a uniform grain size and dense micro-structure [27,36].

Conclusions
In this work, LMZBS-Bi 2 O 3 doped NiCuZn ferrites were successfully synthesized by the conventional solid-state reaction method at the low sintering temperature of 910°C. The crystal phase, micro-structure and magnetic properties including saturation magnetization (Ms), coercive force (Hc), Q-factor and permeability as well as the power loss (Pcv) were investigated and analyzed in detail. The XRD and SEM analysis showed that the 0.2 wt% LMZBS-0.3 wt% Bi 2 O 3 doped NiCuZn ferrite had a single-crystalline phase and possessed a dense microstructure. The series of magnetic properties tests showed that as the concentrations of LMZBS-Bi 2 O 3 increased, the density, Ms, Q-factor and magnetic permeability increased first and then decreased, while the Hc and Pcv exhibited the opposite trends of change. Among all the as-prepared samples, the 0.2 wt% LMZBS-0.3wt% Bi 2 O 3 doped NiCuZn ferrite had the highest Ms, Q factor and magnetic permeability as well as the lowest Hc and Pcv.