hexaferrites - BUET Central Library

The increase in lattice parameters with increasing Zn content can be explained on the basis of ion radii. The bulk density of various polycrystalline Ba2Ni2−xZnxFe12O22 increases with increasing sintering temperature up to an optimum temperature (1200oC) above which it decreases.

LIST OF TABLES

K1 Magnetocrystalline anisotropy Lo Inductance of sample without winding Ls Inductance of sample with winding M Magnetization.

INTRODUCTION

• Introduction
• Brief Review of the Previous Work
• Objectives of the Present Work
• Summary of the Thesis

The electromagnetic and microwave absorption properties of Z-type Ba-ferrite/polymer composites were investigated [15]. 9]studied “Thermal characterization of intermediate products of the synthesis of Zn-substituted Y-barium hexaferrite.

LITERATURE REVIEW

Overview of the materials

Magnetic ordering

2.1(e) shows that in the paramagnetic region the variation of the inverse sensitivity with temperature of a ferrite material is decidedly nonlinear. Temperature dependence of the inverse sensitivity for: (a) a diamagnetic material; (b) a paramagnetic material showing Curie's law behavior; (c) a ferromagnetic material exhibiting spontaneous magnetization for TTC; (d) an antiferromagnetic material; (e) a ferrimagnetic material exhibiting a net spontaneous magnetization for TTC.

Hexaferrites

Y-type hexaferrites

Using the same description as for the M and Y hexaferrites crystal structure, that of the U hexaferrites can be described as. Based on this structure, cation distribution of Y-type hexaferrites is given in Table 2.2.

Exchange interactions in ferrites

It appears that, for the overall energy of the system to be a minimum, the moments of the manganese ions on either side of the oxygen ion must be antiparallel. The oxygen ion now has a moment of 1µB and if there is negative interaction between the oxygen ion and the right-hand manganese ion, the moments of the manganese ions will again be antiparallel.

Fig. 2.5. Illustrating superexchange in MnO.

Neel theory of ferrimagnetism

In the case of ferrites, the coupling is of the indirect type involving overlap of oxygen wave functions with those of the neighboring cations. The essential point is that when an oxygen p-orbital overlaps with a cation d-orbital, one of the p-electrons can be accepted by the cations. When one of the transition metal cations is brought close, the O2-, partial electron overlap (between a 3d electron of the cation and a 2p electron forms the O2-) can only occur for antiparallel spins, because electrons with the same spin are repelled .

Spins on one sublattice are affected by exchange forces due to spins on the other sublattice as well as due to other spins on the same sublattice.. and MrB . are the magnetizations of the two sublattices and λ are the Weiss constants. Since the interaction between the sublattices is antiferromagnetic, λAB must be negative, and λAA and λBB can be either negative or positive, depending on the crystal structure and the nature of the interacting atoms.

Fig. 2.7. The temperature dependence of the inverse susceptibility for ferrimagnets [15]

Microstructure

In the cases of no AB interaction, antiferromagnetic ordering can be expected either in the A or in the B sublattice. A small capping agent can drastically change the nature and concentration of defects in the matrix, affecting grain boundary movement, pore mobility, and pore removal [ 15 , 19 ]. If it is not soluble at the sintering temperature, the dopant becomes a second phase that usually precipitates to the grain boundary.

An undesirable effect in ceramic samples is the formation of excessive or discontinuous grain growth, which is characterized by the excessive growth of some grains at the expense of small, adjacent grains, Fig. Discontinuous growth is believed to result from one or more of the following: powder mixtures with impurities; a very large initial particle size distribution; sintering at excessive temperatures; in ferrites containing Zn and/or Mn, a low O2 partial pressure in the sintering atmosphere.

Fig. 2.9. Porosity character: (a) intergranular, (b) intragranular [15].

Theories of permeability

• Mechanisms of permeability
• Wall Permeability
• Rotational Permeability
• Frequency dependent Permeability Curve

Curves showing the variation of both µ/ and µ// with frequency are called the magnetic spectrum or the permeability spectrum of the material [11]. The permeability of a ferrimagnetic substance is the combined effect of wall permeability and rotational permeability mechanisms. The equilibrium positions of the walls are due to interactions with magnetization in adjacent domains and the influence of pores; crystal boundaries and chemical inhomogeneities favoring certain wall positions.

The wall permeability mechanism arises from the displacement of domain walls in small fields. The direction of M can be found by minimizing the magnetic energy E as a function of the orientation.

Fig. 2.11. Schematic magnetization curve showing the important parameter: initial permeability, µ i (the slope of the curve at low fields) and the main magnetization mechanism in each magnetization range [15]

Magnetization Mechanism

• Concept of Magnetic Domain and Domain Wall (Weiss Domain Structure)
• The dynamic behaviour of Domains
• Bulk Material Magnetization
• The Magnetization Curve

The actual thickness of the domain wall is determined by the counterbalance of the exchange energy and the anisotropy energy. This will cause the center of the domain wall to move towards the domain opposite the field. The possible steps to complete the orientation of the domains or the magnetization of the material are also shown in Fig.

In the second phase magnetization curve, if the field is increased, the intensity of the magnetization increases more drastically, this is called the irreversible magnetization range. Shin J.Y., and Oh, J.H., "The microwave absorbing and resonance Ba3Co1.3Zn0.3Cu0.4Fe24O41 hexagonal ferrite microwave absorbers", J.

Fig. 2.16. Possible domain structures showing progressively low energy .Each part is representing a cross-section of a ferromagnetic single crystal [15, 23]

Introduction

Conventional Solid State Reaction Method

This chapter discusses the basic experimental methods and techniques for measuring network parameters, frequency-dependent AC permittivity, and DC magnetization of a ferrite sample. Pellets or toroidal samples are prepared from these calcined powders using a die and hydrostatic or isostatic pressure.

Details of Calcining, Pressing and Sintering

A small particle size of the reactant powders provides a high contact surface area for initiation of the solid state reaction; diffusion paths are shortened, leading to a more efficient completion of the reaction. A narrow size distribution of spherical particles as well as a dispersed state is important for compaction of the powder during grain-body formation. The binder facilitates the flow of the particles during compaction and increases the bonding between the particles, presumably by forming bonds of the particle-binder-particle type.

The driving force for sintering is the reduction in the free surface energy of the powder. The mobility of the atoms or ions is greatly enhanced by the presence of lattice defects.

Fig. 3.1. Flow chart of the stages in preparation of spinel ferrite.

Preparation of the Present Samples

In the initial phase, neighboring particles form a neck by surface diffusion and apparently also at high temperatures by an evaporation-condensation mechanism. To reduce and ultimately eliminate pore volume, a net transport of materials into the pores by volume diffusion is required. The sintering mechanism involves the creation of free spaces on the curved surfaces of the pores, their transport through the grains and their absorption at the grain boundaries, which play the role of a sink.

Since grain boundaries are sinks for vacancies, grain growth tends to decrease the pore elimination rate due to increasing the distance between pores and grain boundaries, and reducing the total grain boundary surface area. In the final stage, grain growth increases significantly and the remaining pores can be isolated.

X-ray Diffraction

Porosity was calculated from the ratio{100(dth−dexp)/dth}%, where dexp is the bulk density measured by the formula.

Microstructural Investigation

Complex Permeability Measurement

• Techniques for the Permeability Measurement
• Frequency Characteristics Measurement

DC magnetization measurement

12] Hussaain, S., Maqsood, A., “Influence of sintering time on structural, magnetic and electric properties of Si-Ca added Sr-hexa ferrites”, Vol.-316, pp.

RESULTS AND DISCUSSION

X-ray diffraction

Density and porosity

This is because the thermal energy during the sintering process generates a force that causes the grain boundaries to grow over the pores, reducing the pore volume and making the material denser. It is known that the porosity of ceramic samples arises from two sources: intragranular porosity and intergranular porosity [4]. At higher sintering temperatures, the density decreases because the intragranular porosity increases due to discontinuous grain growth.

When the grain growth rate is too high, pores can be left behind by the rapidly moving grain boundaries, resulting in pores that become trapped within the grains. Continuous grain growth increases with temperature and therefore contributes to a decrease in bulk density.

Fig. 4.5. Variation of experimental density and porosity with Zn content ‘x’ of various polycrystalline

Microstructure

When the porosity is reduced to such a value that secondary grain growth can occur, extensive grain growth can occur if the sintering temperature is too high for a particular composition. As a result, many pores are isolated from the grain boundaries, and the diffusion distance between the pores and the grain boundary becomes large. The grain growth behavior reflects the competition between the driving force to move the grain boundaries and the restraining force exerted by the pores [7].

When the driving force of the grain boundary in each grain is homogeneous, the sintered body achieves uniform grain size distribution; In contrast, discontinuous grain growth occurs when this driving force is inhomogeneous. The discontinuous growth of grain increases with temperature, hindering the migration from the pore to the grain boundary and thereby contributing to the reduction of sintered density.

Fig. 4.7. The optical micrographs of various polycrystalline Ba 2 Ni 2 − x Zn x Fe 12 O 22 sintered at 1200ºC for 5h in air

Complex initial permeability

Higher sintering temperatures result in increased sample density, which facilitates spin movement, as the number of pores that impede wall movement is reduced. Increasing the sintering temperature also results in a decrease in internal stresses, which reduce the hindrance to the movement of the domain walls resulting in an increase in the value of μi/[10] Similar results were observed for all the samples studied [11 ] . The permeability increases with increasing Zn content, reaches a maximum value, and then decreases with further increasing Zn content.

Real and imaginary part of the initial permeability for different Ba2Ni2−xZnxFe12O22, sintered at 1150 °C for 5 hours in air. Real and imaginary part of the initial permeability for different Ba2Ni2−xZnxFe12O22 sintered at 1200 °C for 5 h in air.

Fig. 4.8. Real and imaginary part of initial permeability for various Ba 2 Ni 2 − x Zn x Fe 12 O 22 sintered at 1150°C for 5h in air

Loss factor

Relative quality factor

DC magnetization

The variation of the saturation magnetization of the compositions with the Zn content is depicted in Fig. 4.20. It is clearly indicated that the saturation magnetization of the compositions increases with increasing Zn content up to a certain level (x = 0.1) and then decreases with further increase in Zn content. On the other hand, nonmagnetic ion substitution for magnetic ion will weaken the superexchange interaction and thus lead to the reduction of the Curie temperature Tc.

Therefore, at room temperature, the saturation magnetization Ms will decrease faster for the samples with higher Zn concentration due to the effect of thermal agitation. M., “Improvement of the magnetic properties of Mn-Ni-Zn ferrite by the non-magnetic Al3+ ion substitution”, J.

Fig. 4.19. The magnetization as a function of applied magnetic field at 300 K for various polycrystalline Ba 2 Ni 2- 2-x Zn x Fe 12 O 22 sintered at 1200 o C

CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK

Conclusions

From the loss factor, the relative quality factor (or Q-factor) is calculated for all the compositions sintered at and 1250 oC. For inductors used in filter applications, the quality factor is often used as a measure of performance. Ba −x x compositions increase with increasing Zn content up to x = 0.1 and then decrease with further increase in Zn content with little inconsistency.

This can be explained by the fact that the diamagnetic cation Zn2+ occupies the spin-down and tetrahedral regions of the T- and S-blocks. The decrease of the saturation magnetization above x = 0.1 can also be explained by the local rotation with a magnetic component along the hexagonal axis c.

Suggestions for future work