|dc.description.abstract||Ferrite have found a variety of uses in electronic and
communication engineering. Mn-Zn ferrites (Mn1-XZnxFe2O4 ) are widely used as filter core materials over a range of frequencies
varying from several hundred Hz to several MHz. There
are many other applications such as in television receivers as
deflection yokes and E.H.T. cores etc.
The development of a ferrite suitable for a particular
application is an interesting scientific problem and technological
challenge. The properties of ferrites are determined by
a number of intrinsic properties and their interaction with
the ceramic microstructure.
Impurities, present in or added to the raw materials
used for processing ferrites, play an important role in determining
the properties of the ferrites. The cost of ferrites
is very much related to the purity level of the raw materials
used. It is, therefore, both scientifically and economically
important that the behaviour of ferrites is studied with
additions of controlled amounts of impurities commonly present
in the raw materials cheaply available.
Silica (Si0 2) is commonly found in the raw materials
and is also, to some extent, contributed by atmospheric dust.
In order to determine the tolerance of SiO2 as an impurity in
the raw materials and during processing, a systematic study
has been carried out to investigate the effect of SiO2 addition
in the raw materials. The effect of controlled additions
of oxides of germanium and tin to the raw materials has also
been studied since Si, Ge and Sn are all elements of the IV
group in the periodic table.
The studies have been presented in five chapters.
Chapter I gives the background essential for the present
study. This chapter deals with the important properties
of ferrites. The properties of ferrites can be classified in
two categories. Firstly intrinsic properties i.e. those properties
which are decided by the basic composition of the
ferrite. Secondly extrinsic properties i.e. those properties
which depend upon the microstructure and processing parameters.
The literature presently available with special reference to
impurities and their effect on the magnetic properties has
also been discussed.
The effect of impurities on the magnetic properties of
ferrites depends upon whether they go in solid solution with
the ferrite or stay insoluble. The nature of the three
impurities - SiO2 , GeO2 and SnO2 in the ferrite has been
studied in chapter II by employing scanning electron microscopy,
X-ray and Auger microprobe and X-ray diffraction
(lattice parameter measurement) techniques. Both. Si0 2 and
Ge20 have been found to have a limited solubility in the
ferrite and tend to segregate at the grain boundaries, Si02
has been found to form a compound at the grain boundaries.
Si-rich inclusions have also been detected in the grains at
larger concentrations of Si02 (1.28 mol%) . Ge-enrichment
at the grain boundaries has been observed with the help of
Auger electron spectroscopy.
No detectable segregation of Sn0 2 , was observed with
the help of XMA even for as high a concentration as 5.70 mol%.
This may be due to a high solid solubility of Sn0 2 in the
ferrite. Ferrite lattice has been found to expand on additions
of Sn0 2 up to a level of 5.70 mol%, the highest concentration
The third chapter deals with the effect of these impurities
-- SiO2 , GeO2 and SnO2 doped in various amounts on
the microstructure of the Mn-Zn ferrite. It has been discussed
that the impurities present could affect the microstructure in
a number of ways. Impurities present in solid solution could
give rise to an impurity drag effect which impedes boundary
motion. The insoluble impurities would disturb the course of
normal grain growth during sintering more drastically. It has
been discussed that small concentrations of the insoluble
impurity such that the impurity remains as a dispersed phase
result in abnormal or discontinuous grain growth. At higher
concentrations, the impurity phase would exist as plates or
films on the grain boundaries altering the kinetics for
Microstructure studies of SiO2 and GeO2 doped Mn-Zn
ferrites show that these impurities lower the sintering
temperature thereby enhancing the rate of grain growth.
Both of these impurities give rise to discontinuous grain
growth. Giant grains with almost entire porosity being intragranular are formed at a SiO2 content of 0.08 mol% and
at a Ge0 2 content of 1.28 mol%. From these observations,
it is inferred that up to these levels they exist as dispersed
phase therefore giving rise to discontinuous grain growth. At a level higher than 0.64 mol% SiO2 and 3.82 mol% GeO2 somewhat regular grain structure reappears through with much higher intragranular porosity as compared to undoped ferrite. It is discussed that at these levels, the impurities are present as second phase film around the grains suppressing the abnormal grain control.
regular grain structure reappears though with much higher
intragranular porosity as compared to undoped ferrite, It is
discussed that at these levels, the impurities are present as
second phase film around the grains suppressing the abnormal
Sn02 additions are not found to affect the microstructure
even up to a level of 5.70 mol%. These results are in agreement with the findings in the first chapter that SnO2
goes in solid solution with the ferrite.
In chapter IV, the effect of these impurities on the magnetic property - initial permeability, u1, and resistivity
of the Mn-Zn ferrite has been studied. It is known that an
increase in density increases the saturation magnetization,
Ms, and hence the initial permeability. It has also been
discussed that in samples containing intragranular porosity,
an increase in pore to pore distance, D, increases the span
of domain walls and hence the ui.
In the case of SiO2 as dopant, it ha s been observed
that ui increases up to a Si0 2 concentration of 0.04 mol%.
This is in confirmity with the increase in the product Mg.D
in this range. At a Si0 2 concentration of 0.08 mol%, even
though Ms increases, /ui decreases on account of a decrease
in Dp 'because here the microstructure shows large intra--_
granular porosity. Beyond a silica concentration of C.32
mol%, although the product Ms Dp does not decrease, the ui
is found to decrease because of the formation of a nonmagnetic layer at the grain boundaries and precipitation inside
In the case of Ge0 2, similar effects are observed though
the peak in ui occurs at a content of 0.64 mol%.
Sn0 2 additions are not found to affect ui appresiably.
A study of the temperature variation of ,u i shows' that
in the case of SnO2, there is a shift in the secondary maximum.
peak (SMP) indicating a solubility of SnO2 in the ferrite.
Such an observation is not appreciable in Si0 2 and GeO2 . Thei-T curves further show a flatness at higher impurity concentrations
in all the three cases presumably due to wall discontinuities
at the grain boundaries.
A study of disaccommodation with various concentrations
of these impurities shows that the disaccommodation decreases
with impurity concentrations, the maximum change being in SnO2.
The results have been discussed in terms of the solubility of
Sn4+ ions and their tendency to localize Fe 2+ ions,
Variations in the resistivity with temperature for the
three impurities indicate that the predominant conduction
mechanism is the electron hopping from Fe 3+ to Fe 2+ and that
Sn4+ - Fe 2+ pairs dissociate at higher temperatures.
Chapter V deals with the effect of these impurities on
the core losses of the Mn-Zn ferrite. The core losses have
been studied at a flux density of 0.2 wb/m
(2000 gauss) and
up to a frequency of 15.75 KHz.
At 15.75 KHz, the core losses decrease up to a silica
content of 0.04 moi corresponding to an increase in. yu i . At
higher silica contents the core losses are always higher going
through a peak at 0.08 mol%. The peak becomes more predominant
as frequency increases.
Similar effects have been observed in the case of Ge02
additions though the peak in the core losses occurs at 1.28
mol%. This peak is also found to be more predominant at
Sn02 additions are n ot found to affect the core losses
The hysteresis loops under similar conditions have also
been studied. The measurements of loop areas suggest that the
core losses measured are essentially the hysteresis losses.
Core losses/frequency therefore represent the loop area.
At low impurity concentrations (up to 0.04 mol% Si02
and 0.64 mol% Ge0 2 ), core losses decrease mainly due to a
decrease in H e which has been found to follow the relationship
Ho :.DP ' S . At a Si0 2 concentration of 0.08 mol% and a Ge02
concentration of 1,28 mol%, an increase in loop area with
frequency is observed. These very samples also exhibit giant
grains with large trapped porosity. These effects have been
attributed to the relaxation of amplitude permeability at high
field strengths. At higher field strengths, closure domains
are formed at intragranular pores enabling the domain walls
to become detached from the pores. This process is a comparatively
slow process and therefore the amplitude permeability
is subjected to a strong relaxation even at frequencies
of 10 to 50 KHz,
At further higher Si0 2 and Ge0 0 contents the core
losses/frequency are always high but are independent of
frequency. The coercive force, H o is found to be much higher
than that given by the relationship Here Dp0"5. It is discussed
that this increase in Nc has been brought in by the presence
of inclusions inside the grains. The Ares ence of the grain
boundary phase around the grains is also responsible for
increasing the losses since it gives rise to wall discontinuities
resulting in the demagnetizing effects. This phase
also puts the grains under stress while cooling further
deteriorating the core losses.
Finally, the major conclusions drawn from the entire
study are listed under the 'CONCLUSIONS'.||en_US