The effect of dopants SiO2, GeO2, and SnO2, on the microstructure and magnetic properties of Mn-Zn ferrites
MetadataShow full item record
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 (ii) 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 studied. 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 growth. 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, (iii) 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 grain growth. 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 P' 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 P 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 the grains. 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 2 (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 (v) 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 higher frequencies. Sn02 additions are n ot found to affect the core losses appreciably. 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 (vi) 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'.
The following license files are associated with this item: