Document Sample
                                 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.

   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 [1]. 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).

  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:,
    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
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∆ B ( Tesla )

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                               Time (ns)
Figure 6: Pulse ∆B vs. time for large Ni-Zn ferrite toroid.

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