Effect of CaCO 3 and V 2 O 5 on the microstructure and magnetic property of MnZn ferrites.

Manganese-zinc (MnZn) ferrites have been widely applied in convertors and switching mode power supplies, on account of its high saturation magnetic induction $B_{\mathrm{s}}$, initial permeability μ i and low core losses $P_{\mathrm{L}}$ [1- 3]. With the development of miniaturization and integration of electronic devices, the application frequency of MnZn ferrite core has been raised from tens of kilohertz to several megahertz (2∼4MHz). Thus, MnZn ferrites with low core losses especially at high frequency (∼3MHz) are urgently demanded. Aiming at this goal, many efforts have been done to investigate the factors that may influence core losses, such as the main compositions, fabrication processes of powders and additives [4- 6]. K. Praveena [4] et al investigated the effect of Zn2+ content on the core losses for Mn 1-x Zn x Fe 2 O 4 and found that optimized Zn2+ content could reduce the core losses from 358kW/m3 $( x=0)$ to 163kW/m3$( x=0.9)$ at 1MHz. Haining Ji [5] et al studied the effect of second milling time on the loss characteristics of MnZn ferrites. The results showed that when the second milling time is 2h, the core losses achieved its minimum value of 430kW/m3 at 100kHz and 200mT. A low-loss MnZn ferrite material DMR508, sintered in precisely controlled oxygen partial, has been developed through optimized additives of CaO, SiO 2 , Nb 2 O 5 , TiO 2 and ZrO 2 [6]. The core losses of this material were only 200kW/m3 at 3MHz and 10mT. To sum up, it is a feasible way to unify these factors harmoniously for the realization of even lower losses at 3MHz. In this work, Mn 0.67 Zn 0.21 Fe 2.12 O 4 ferrites doped with CaCO 3 (0∼0.3wt%) and V 2 O 5 (0∼0.03wt%) have been prepared by solid-state reaction method, and ultra-low core losses at 3MHz 10mT and 30mT were achieved through the combinative doping of CaCO 3 and V 2 O 5 , as well as other additives (TiO 2 , Co 2 O 3 and SnO 2 ). The effect of CaCO 3 and V 2 O 5 on the microstructure and electromagnetic properties of MnZn ferrites have been investigated in detail, such as initial permeability μ i , saturation magnetic induction $B_{\mathrm{s}}$, coercivity $H_{\mathrm{c}}$ and resistivity ρ. In addition, based on core losses separation method, the core losses mechanism has been discussed. Core losses are divided into three parts: hysteresis loss $P_{\mathrm{h}}$, eddy current loss $P_{\mathrm{e}}$ and residual loss $P_{\mathrm{r}}$ through following equation: where $K_{\mathrm{h}}$ and $K_{\mathrm{e}}$ are constants, $B$ is the magnetic flux density, $f$ is the frequency and ρ is the resistivity. In view of the relationship curve between $P_{\mathrm{L}}/ f$ and $f$, $P_{\mathrm{h}}/ f$, $P_{\mathrm{e}}/ f$ and $P_{\mathrm{r}}/ f$ could be obtained from the interception part, slope of linear part and nonlinear part, respectively. The SEM micrographs of MnZn ferrites are shown in Fig. 1. Remarkably, the average grain size decreased with the increase of CaCO 3 content via the refinement grain growth as shown in Fig. 1(a)-(b). Besides, small and even grains were obtained with 0.1wt% CaCO 3 and 0.01wt% V 2 O 5 co-doping in Fig. 1(c). Fig. 2(a) shows the $P_{\mathrm{L}}$ variation of MnZn ferrites with different CaCO 3 contents at room temperature. With the increase of CaCO 3 content, the core losses at 3MHz 10mT and 30mT showed a tendency of decreasing then increasing with the minimum of 56kW/m3 and 859kW/m3 for 0.1wt% CaCO 3 doping. Furthermore, on the basis of 0.1wt% CaCO 3 doping, the core losses were furtherly reduced with 0.01wt% V 2 O 5 doping. The core losses at 3MHz 10mT and 30mT were only 33kW/m3 and 598kW/m3, respectively, as shown in Fig. 2(b). The excellent properties of MnZn ferrite core with ultra-low core losses make it a candidate for the application of high frequency of 3MHz.