Magnetic Properties and Mechanical Strength of MnZn Ferrite
Y. Matsuo, K. Ono, T. Hashimoto, and F. Nakao
Abstract-- From MnZn ferrite ("6H20" of FDK production), cores having
grain size of about 8¡Á10 ¦Ìm were produced by controlling the calcination
temperature, after-calcination milling time, and sintering conditions. The
ferrite powder production conditions and core microstructure were examined to
determine their effects on the magnetic properties and mechanical strength of
MnZn ferrite cores. A tendency of core loss to decline with a decrease in grain
size was confirmed. Strength increased also with a decrease in grain size. Thus,
a relation similar to that between strength and fracture toughness in the
Equation of Griffith-Irwin was noted. By these findings, it has become possible
to produce MnZn ferrite cores with lower loss and greater strength.
Index Terms-- MnZn ferrite, sintering conditions, magnetic properties,
mechanical strength.
I. INTRODUCTION
THE use of MnZn ferrite is spreading
from switching power supplies to inductive charging systems for electric
vehicles and automated guided vehicles. These inductive charging systems
incorporate a connector whose contact transformer has its primary and secondary
sides divided in the middle[1]-[3]. The circuit composition of the inductive
charging systems adopts a high-frequency switching method of 100 to 370 kHz[4].
Inductive charging systems require lower loss cores for minimizing heat
generation during battery charging[1]. They also require cores with a high
saturation magnetic flux density to match large current inputs. Moreover, they
require cores with greater mechanical strength since electric vehicles are often
driven on rough unpaved roads. For these reasons, MnZn ferrite ("6H20" of FDK
make) production conditions and core microstructure were examined to determine
their effects on the magnetic properties and strength values of MnZn ferrite
cores.
II. SAMPLE PREPARATION AND MEASUREMENT
The samples used in this experiment were prepared by the conventional powder
metallurgical processes summarized in Fig. 1. ZnO, MnO and Fe2O3 were weighed
to the basic formula, and were wet-mixed in a ball mill. Three groups of
the mixture, after drying, were calcined at 800¡î, 900¡î and 1000¡î, respectively,
in the atmosphere. Then, they were wet-milled in a ball mill under the
conditions shown in Table. 1. The resultant ferrite powder, following the
addition of PVA, was granulated. The resultant granules were pressed of
PVA, was granulated. The resultant granules were pressed into bar and toroidal
shapes. Then the bars and toroids were sintered at peak temperatures between
1250 to 1310¡î in an oxygen concentration controlled atmosphere. The sintered
samples had the shapes and dimensions shown in Fig. 2. The maximum fracture
load of bars was measured under the conditions shown in Fig. 3. Their 3-point
bending strength values were calculated, by using Equation (1)/ Fig. 3,
from the numerical values of their dimensions, maximum fracture load, and
distances between supports. The core loss of toroids (sized 25 mm in outer
diameter, 15 mm in inner diameter, 5 mm in thickness) was measured with
a B-H analyzer SY-8232 of Iwatsu make at ambient temperatures raised from
room temperature to 120¡î. The microstructure of toroidal cores was observed
by SEM micrography, and their density by the Archimedean method. Samples
of calcinated powder were set aside for measurement. The crystalline phase
of this calcinated powder was observed by the powder X-ray diffraction
method using Multiflex¡¿XRD (Cu K¦Á, 40 kV, 30 mA) of Rigaku make. For thermal
shrinkage measurement, calcinated powder was pressed into ¦Õ4¡ß4 mm shapes
of uniform density, and their shrinkages were measured with TMA-8310 equipment
of Rigaku make as the applied temperature was raised from 600 to 1400¡î
at a rate of 10¡î/min.
III. RESULTS AND DISCUSSION
3.1 Properties of Calcinated
Powders
At the beginning of the sample preparation process, the weighed raw materials
were wet-mixed before calcination. A part of this mixed powder was taken
as samples for the examination of their X-ray diffraction patterns at measured
temperatures between 25¡î and 1200¡î. It was found that, as the temperature
was raised, the peaks of Fe2O3 and ZnO diminished while the spinel phase
peaks became more prominent as shown in Fig. 4. At temperatures above 1000¡î,
the spinel phase peaks were extremely predominant. The remainder of the
above mixed powder was divided into three groups and were calcinated at
800¡î, 900¡î and 1000¡î, respectively. Then, the calcinated powder of each
group was milled in a ball mill for 3, 5 and 7 hours, respectively. This
milled powder was examined and found to have the relation between its particle
size and milling time as shown in Fig. 5. At all the milling periods, the
particle size increased with a rise in calcination temperature. This was
attributed to the increase of spinel phase by a rise in calcination temperature,
as shown in Fig. 4.
In other words, because spinel phase causes particles to melt together, it becomes more difficult to mill calcinated powder into small particles. Fig. 6 presents the TMA curves of powder samples calcinated at various temperatures. The temperature at which shrinkage starts was found to go up with an increase in calcination temperature. The amount of shrinkage increased with a drop in calcination temperature. This was attributed to a decrease in particle as shown in Fig. 5. Smaller particle size means a greater number of contact points among the particles so that spinel phase formation is promoted thus resulting in greater shrinkage. Milled powder, after calcinating, was granulated and then pressed into toroidal shapes which were sintered at a peak temperature between 1250¡î and 1310¡î. The grain size of these sintered cores was measured to be between 8¦Ìm and 11¦Ìm as shown in Fig. 7. The relation between their density and grain size is presented in Fig. 8. The density ranged 4.82 ¡Þ 0.04 g/cm3 and indicated a tendency to increase with an increase in grain size.
3.2 Magnetic
Properties
Fig. 9 shows the core loss values obtained from sintered toroids in relation
to their grain size and calcination temperature. Core loss tended to go
up with an increase in grain size. At this measurement frequency level,
hysteresis loss (Phv) and eddy current loss (Pev) were dominant; therefore,
both of these losses had to be reduced in order to lower the core loss.
Fig. 10 gives the results of loss separation in toroidal cores calcinated
at 900¡î. Both hysteresis and eddy current losses declined as grain size
became smaller, with the former leveling off while the latter indicating
a possibility of further decline. This was attributed to an increase in
alternating current resistance as smaller grain size works to suppress
eddy current. Fig. 11 shows the relation between the core loss and measuring
temperature of sintered toroids which calcination temperature was 900¡î.
It was found that the core loss (100 kHz, 200 mT, 80¡î) could be reduced
by about 30% if grain size was made smaller.
3.3 Mechanical
Strength
Fig. 12 shows the relation between the 3-point bending strength and
grain size of bar cores. The strength tended to decline with an increase in
grain size, thus indicating a relation given by the Equation of Griffith-Irwin
(Fig. 13). High strength values were obtained from cores (calcinated at 900¡î)
having a grain size of about 8 and 9 ¦Ìm. Fig. 14 presents the SEM photographs of
fractured surfaces of bar cores calcinated at 900¡î.
The photographs indicate that high strength cores (b) and (c), as compared
to low strength cores (a), had smaller grain size and fewer pores. As indicated
by the Equation of Griffith-Irwin, porosity is one essential factor for
sharply reducing core strength. To obtain strong cores, therefore, it is
important to select the production conditions which will minimize pores,
crack, defect.
IV. CONCLUSIONS
By regulating the calcination temperature, milling time and sintering temperature, we produced MnZn ferrite core samples having grain size of around 8 to 10 ¦Ìm. Then, relation between core loss, 3-point bending strength, and milling time was investigated. The conclusions were:
1. The core loss was reduced by 30% under the conditions of longer after-calcination
milling time and smaller grain size.
2. The bending strength was confirmed to increase under the condition of
small grain size. As a result, strength behavior similar to the one expressed
by the Equation of Griffith-Irwin was noted.
3. The lowest core loss and highest strength were obtained under the conditions of 900¡î calcination, 1280¡î sintering, and 9 ¦Ìm grain size. On the basis of the above findings, we plan to develop MnZn ferrite cores of even lower loss and higher strength for use in the inductive charging systems for electric vehicles and automated guided vehicles.
REFERENCES
[1] Y. Matsuo, S. Otobe, F. Nakao, H. Sakamoto, "Development of a
Ferrite Material for Inductive Chargers" , EVS-16 (16th International
Electric Vehicle Symposium), Beijing, 1999.
[2] H. Sakamoto, K. Harada, S. Washimiya, K. Takehara, Y. Matsuo and F.Nakao,
"Large air gap coupler for inductive charger" , IEEE Trans Magn.,vol.
35, no.5, pp.3526-3528, 1999
[3] Y. Matsuo, K.Ono, M.Kondoh, Y. Matsuo, "Die
mechanisms affection the forming of super-large ferrite cores, FP-06, Digests of
the 2000 Intermag-Conrerence (Toronto).
[4] D. Ouwerkerk, T. Sekimori, H. Satoh, "New Inductive Charging Approach" , JEVA Electric Vehicle Forum, pp.181-187, 1998. |
