DEVELOPMENT OF LARGE SIZE FERRITE TOROIDS FOR FAST MAGNETIC SWITCHING APPLICATIONS IN ACCELERATORS L. Aditya*, P. Pareek and R. S. Shinde, Ferrite Lab, Accelerator Component Engineering & Fabrication Division, Raja Ramanna Centre for Advanced Technology, Indore – 452013, India. Abstract ray Diffraction (XRD). Microstructure morphology was A magnetic circuit in fast switching magnetic device studied in the fractured surface of the sintered samples utilizes high rate of magnetization (5 Tesla / µs) which is using a Scanning Electron Microscope (SEM). associated with low coercivity, high remanence, high Orientation imaging microscopy was carried out using pulse permeability and low magnetic losses. Iron deficient Electron Back Scattered Diffraction (EBSD) of the ultra- Ni-Zn ferrite has been developed with these fast magnetic polished samples. Pulsed magnetic measurements were switching characteristics and large sized toroids (φ150 x performed in the toroid samples with a maximum applied φ110 x 15 mm) have been developed following powder pulse field of 40 Oe, using a laboratory designed pulsed metallurgical techniques for indigenous development of magnetometer. fast current transformers (FCT) and magnetic switches. Pulsed magnetic characteristics have been studied and optimized in relation to microstructure of the Ni-Zn ferrites. This paper presents material selection criteria, development of a fast switching Ni-Zn ferrite for the fabrication of a FCT and observed pulse response of large Ni-Zn ferrite toroids in FCT circuits. INTRODUCTION Increased demands for suitable materials for high speed pulsed magnetic devices attracted lot of attention in past few decades for the development of fast switching NiZn ferrites . The fast magnetic switching characteristics are associated with low coercivity, high permeability, high squareness, large flux swing, high resistivity and low loss characteristics at high frequencies. It is possible to tailor these characteristics in Nickel Zinc ferrites by suitable choice of composition, additives and process parameters. Use of V2O5 as a sintering aid in ferrite processing is reported in the literature [2-5]. But improvements of magnetic switching characteristics with V2O5 addition are not reported elsewhere. We performed a systematic study on the role of V2O5 on sinterability, microstructure evolution and magnetic switching characteristics of iron deficient NiZnCoMn spinel ferrites following standard ceramic routes and developed a fast switching ferrite for use in fast current transformer (FCT) and magnetic pulse compressor (MPC). MATERIAL DEVELOPMENT Nickel-zinc ferrites (Ni0.47Zn0.48Co0.07Mn0.05Fe1.83O4) with different V2O5 concentrations (0, 400, 800 and 1200 ppm) were synthesised following standard ceramic route. The as-pressed small torroids (φ25 x φ15 x 7 mm thick) were sintered at different sintering temperatures (TS) in the range of 1150oC - 1350oC for five hours in air Figure 1: Pulsed magnetic properties of Ni-Zn ferrites as a atmosphere using a sintering kiln. Nickel-zinc ferrite function of V2O5 concentration; a) coercivity, b) spinel phase was confirmed in the sintered samples by X- squareness and c) flux swing. __________________________________________________ * Corresponding author; E. mail: email@example.com, firstname.lastname@example.org Pulsed magnetic properties of the nickel-zinc ferrites sintered at different temperatures are presented in Fig. 1 as a function of V2O5 concentration. Fig. 1a shows the influence of V2O5 concentration on coercivity (HC). It is observed that at the range of studied sintering temperatures (1150-1350oC), HC initially decreases with increasing V2O5 concentration and minimized at 800 ppm level of V2O5. Further increase of V2O5 content leads to increase of HC. The lowest HC is achieved in the sample sintered at 1350oC. The squareness ratio, S (= Br / BS) follows exactly opposite trend, i.e. initial increase till 800 ppm V2O5 concentration and later decreases as V2O5 Figure 2: SEM images of the 800 ppm V2O5 doped Ni-Zn concentration approaches 1200 ppm (Fig.1b). The flux ferrites sintered at a) 1150°C and b) 1350°C swing, ∆B (= Bm + Br) is also maximized at 800 ppm V2O5 concentration at all sintering temperatures, as shown in Fig.1c. It is observed that the best magnetic properties are achieved at a V2O5 concentration of 800 ppm. SEM images of the 800 ppm V2O5 doped Ni-Zn ferrites sintered at 1150°C and 1350°C are shown in Fig. 2a and 2b respectively. SEM study reveals that V2O5 has no significant effect on grain size at lower sintering temperature (1150 oC). However, V2O5 plays significant role towards microstructure evolution at higher sintering temperatures (1300-1350oC). Although with increasing V2O5 content at 1350°C, a monotonous decrease in grain size is observed, the microstructure is homogeneous with almost no exaggerated grains particularly at 400 and 800 ppm of V2O5 concentration. Average grain size data were Figure 3: Grain growth and pore elimination with computed from the SEM micrographs and presented as a increasing TS of the 800 ppm V2O5 doped Ni-Zn ferrites function of sintering temperature in Fig. 3, along with respective porosity for 800 ppm V2O5 doped Ni-Zn ferrites. The porosity data were calculated based on the difference of x-ray density and bulk density. It is observed that grain growth is significant and monotonous with increasing sintering temperature. Rapid removal of porosity is accompanied with increasing bulk density at elevated sintering temperature, as observed. Monotonous increase of S and ∆B with increasing sintering temperature can be ascribed to improved densification at higher temperatures followed by decreasing porosity (Fig. 3). Monotonous decrease of coercivity with increasing sintering temperature can be ascribed to monotonous grain growth with improved densification and decreasing porosity at elevated sintering temperatures. The best magnetic properties are achieved in the 800 ppm V2O5 Figure 4: Large Ni-Zn ferrite toroids doped Ni-Zn ferrites sintered at 1350°C for 5 hours. Using these optimized process parameters, large ferrite Table 1: Magnetic properties of large ferrite torroids torroids (150 mm OD x 110 mm ID x 15 mm thick) have been fabricated, as shown in Fig. 4. Measured pulse Properties Values magnetic properties of the large toroids are given in Table Maximum flux density, Bm (Tesla) 0.44 1. Fig. 5a shows the EBSD pattern of a large Ni-Zn ferrite toroid. Corresponding grain structure, orientation Coercivity (Oe) 0.35 ± 0.03 distribution (001 Inverse pole figure) and misorientation Squareness 0.85 ± 0.02 distribution (grain average misorientations) are shown in Fig.’s 5b, 5c and 5d respectively. It is interesting to note Flux swing at 400 ns pulse (Tesla) 0.83 ± 0.02 that grain average misorientations are confined to a limit Specific resistance (ohm-cm) ~ 108 of 5° with maxima close to 1°, indicating very low defect Curie Temperature, TC ( °C ) ~ 230 concentration in the large Ni-Zn ferrite toroid. Figure 7: Pulse response (at 400 ns) of NiZnCo ferrite toroids in fast switching magnetic circuits Figure 5: Microstructure of large Ni-Zn ferrite toroid; a) EBSD pattern with corresponding b) Grain structure, c) CONCLUSIONS 001 Inverse pole figure and d) Grain average Ni-Zn ferrite material with fast magnetic switching misorientation characteristics has been developed following conventional ceramic route. Large Ni-Zn ferrite toroids (150 mm OD x PULSE MAGNETIC RESPONSE OF 110 mm ID x 15 mm thick) with reasonably high flux LARGE FERRITE TOROIDS density (Bm ∼ 44 Tesla), high flux swing (∼ 83 Tesla), high squareness (∼ 0.85) and low coercivity (0.35 Oe) A high voltage pulse testing setup has been built to have been fabricated using the optimised process study the fast magnetic switching response of the ferrite at parameters. Pulsed magnetic tests of these large ferrite high flux amplitudes (Bpeak: 200–300 mT). Measured toroids suggests that these are useful in the construction pulse response of a large toroid at 300 mT is shown in of magnetic circuits in fast switching magnetic devices Fig. 6. A pulse excitation method has been developed to like FCT’s and MPC’s. study high frequency magnetic properties of the large ferrite toroidal cores. B-H characteristics of large ferrite toroidal cores were measured by a 500 MHz digitizer. ACKNOWLEDGEMENTS Fig. 7 shows measured B-H response at 400 ns in fast The authors would like to thank Shri A. K. Jain, Head, switching magnetic circuit at CVL Lab, RRCAT. These ACEFD for his support during the work. Thanks are also test results have been directly applied to the development due to Prof. I. Samajdar, IIT Bombay for providing EBSD of pulsed magnetic devices. measurements of the samples. Technical assistance of Karan Singh, Shiv Bachan and Lalchand Ghongde is also 0.6 acknowledged. 0.5 REFERENCES 0.4  Y. Xu, S. Simizu, D. Petasis, Rita Ramachandran, S. ∆ B ( Tesla ) G. Sankar and W. E. Wallace, ICF-5, Bombay, 1989, 0.3 Advances in Ferrites, p. 611 (1989).  S. Gasiorek and J. Kulikowski, J. Magn. Magn. 0.2 Mater. 26 (1982) 295.  H. T. Kim, H. B. Im, J. Mat. Sc. 22 (1987) 1235. 0.1  O. Kimura, ICF-5, Bombay, 1989, Advances in Ferrites, p. 169 (1989). 0.0  Ram Narayan, R. B. Tripathi and B. K. Das, ICF-5, 0 20 40 60 80 100 Bombay, 1989, Advances in Ferrites, p. 267 (1989). Time (ns) Figure 6: Pulse ∆B vs. time for large Ni-Zn ferrite toroid.
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