Ceramics Project - Tom

8/7/2019 Ceramics Project - Tom

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MECH 423 Engineering Ceramics

Term Project

University of Victoria

Dr. H.W. King

April 8, 2010

Author: Tom BurdynyStudent

Number:

V00163499

CERAMIC MAGNETSFOR TRANSFORMERAND INDUCTORCORE APPLICATIONS

DEPARTMENTOF MECHANICAL ENGINEERING


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Ceramic Magnets for Transformer andInductor Core Applications

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EXECUTIVE SUMMARY


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TABLEOF CONTENTS

Executive Summary.............................................................................................................i

Table of Contents................................................................................................................ii

List of Figures.....................................................................................................................ii

List of Tables.....................................................................................................................iii

1 Introduction......................................................................................................................1

1.1 Project Aim................................................................................................................1

1.2 Background of Ceramic Magnets..............................................................................2

1.2.1 Hysteresis Loop...................................................................................................3

1.2.2 General Types and Compositions.......................................................................4

2 Ceramics as Magnetic Cores.............................................................................................5

2.1 Composition and Crystal Structure............................................................................6

2.2 Properties...................................................................................................................7

2.2.1 Magnetic Properties............................................................................................7

2.2.2 Mechanical and Thermal Properties.................................................................10

2.2.3 Effect of Temperature on Properties.................................................................11

2.3 Manufacturing..........................................................................................................13

2.4 Applications.............................................................................................................15

2.5 Limitations...............................................................................................................18

3 Conclusions and Recommendations...............................................................................19

4 References.......................................................................................................................21

LISTOF FIGURES

Figure 1: Hysteresis loop for a spinel type ferrite [1]..........................................................3

Figure 2: Examples of some ferrite cores [2].......................................................................6

Figure 3: Part of a spinel ferrite unit cell [4]........................................................................7

Figure 4: Effect of temperature on the magnetization of a magnet...................................12

Figure 5: Saturation inductance with minute changes in composition [6].........................13

Figure 6: Graph of the core loss (hysteresis + eddy currents) vs B for ferrites [9]...........18


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LISTOF TABLES

Table 1: Magnetic properties of ceramic and metal magnets used in magnetic cores [5]...7

Table 2: Mechanical and thermal properties of ceramic magnets [5]................................10

Table 3: Change in the initial permeability of Mn-Zn ferrites over temperature [5].........11


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1 INTRODUCTION

Magnetism is a material property exhibited by all materials in one form or another.

Some metals and ceramics however, display this phenomenon to a much greater extent.

For ceramic magnets, also known as ferrites, this enhanced interaction with a magnetic

field provides the opportunity for practical applications including permanent magnets,

transformer and inductor cores, telecommunications, and transducers. By altering the

composition and manufacturing process of these ceramics, the magnetic, mechanical and

thermal properties can be designed to meet specific applications and improve the

performance of systems that currently utilize metal magnets. This can lead to new

technical innovations and reduced operating costs.

1.1 PROJECT AIM

The primary focus of this report will be the investigation of ceramic magnets used in

transformer and inductor cores. Transformer and inductor cores provide a unique

scenario because both metal and ceramic magnets are used in industry applications.

Specific emphasis will be placed on situations where ceramic magnets are used in place

of their metal alternates. For example, while metal alloys and powdered cores are used

for applications up to about 20 kHz, ferrite cores are preferred in the 100 kHz to 1 MHz

region; this is based upon the ferrites containing more favourable conditions such as

lower core loss and eddy currents in that region. Some of the factors which determine the


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useful applications of each material type are cost, size, energy losses, strength and

magnetic properties.

The crystal structure of the ferrite magnets will be examined in addition to the

magnetic, mechanical and thermal properties for a select few compositions relative to

magnetic cores. These properties are also compared to their metal alternatives. With this

understanding of the materials, their applications in industry will be discussed based upon

the advantages and disadvantages of these properties. Manufacturing processes and

limitations of the ferrites will also be described in the report to give perspective of the

currents industry standards and areas for improvement.

1.2 BACKGROUNDOF CERAMIC MAGNETS

Magnetism in ceramic magnets occurs when different types of paramagnetic ions are

present in the crystal structure. This random array of magnetic moments within the

structure initially results in a net magnetization of zero unless an externally applied

magnetic field forces the ions to align. Application of this field results in the formation

of an inductance in the material. By altering the magnetic field strength from positive to

negative a ferrimagnetic hysteresis loop, which is fundamental in the eventual application

of the ceramic, can be obtained. Properties of these ceramics are primarily decided by

their compositions which can be grouped into three different categories.


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1.2.1 HYSTERESIS LOOP

The prime factor which determines the general application of a ceramic magnet is the

shape of the hysteresis loop that is formed when the inductance of a magnet is plotted

against a varying magnetic field. Magnetic inductance is a measure of the stored energy,

or the concentration of a magnetic field, within the magnet. Figure 1 below demonstrates

a possible hysteresis loop for a ferrite.

Figure 1: Hysteresis loop for a spinel type ferrite [1]

In a hysteresis loop there are three important values which effect the possible

applications of the ceramic: saturation flux density, remnant magnetization and

coercivity. As seen in Figure 1, after an initial magnetic field has been applied, the

material fully aligns and a saturation inductance is created. This saturation inductance,

Bsat, is a measure of the maximum amount of energy the magnet can store. A material

with a higher Bsat could then be physically smaller than one with a lower Bsat while still

producing the same power for a given field strength. After removal of this field a certain

amount of remnant magnetic inductance, Br, remains in the ceramic. This is due to the


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net overall polarized grains within the crystal structure that did not switch back to their

original magnetic orientation. The coercivity of a ceramic, Hc, identifies the amount of

magnetic strength that must be applied to reduce the magnetization in the ceramic to zero.

Hysteresis largely determines the application of a given ceramic magnet. Magnets

with a high Br and high Hc are used as permanent magnets because magnetization remains

high after the field is removed and it is difficult to demagnetize. Ceramics with a large Br

value but a low Hc are useful in transformer applications as cycling entails only a small

magnetic field resulting in low losses. Other applications, such as magnetic memories,

show abrupt changes in inductance at Hc giving instantaneous pole change.

1.2.2 GENERAL TYPESAND COMPOSITIONS

Ceramic magnets are classified in terms of their composition. Three types of ferrites

which exist are spinels, magnetoplumbites (hexagonal) and magnetic garnets (rare earth

garnets). The spinel ferrite obtained its name from its spinel structure and is composed of

an iron (III) oxide (Fe3+2O2-3) and a metal oxide (M2+O2-) where the metal has a valence of

two; this metal can be Fe2+, Mn2+ or Co2+ among others. Hexagonal ferrites, as the name

implies, contain a hexagonal crystal structure consisting of a hexagon of 6(Fe2O3)

molecules interlinked with a metal oxide. Possible oxides include BaO, SrO or Y2O3.

Rare earth garnets obtain their magnetization properties from the large magnetic moments

of Gd3+ and Y3+ which form the ceramics Gd3Fe5O12 and Y3Fe5O12, respectively.


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As discussed in Section 1.1, transformer and inductor cores will be the primary report

basis. Therefore the spinel structure which is utilized in these cores will be the ferrites of

interest from this point on.

2 CERAMICSAS MAGNETIC CORES

Spinel ferrites are the ceramic magnets of choice in magnetic core applications due

largely to the low core losses associated with their use. These magnets are termed to be

soft as their magnetization is easily changed by small magnitude magnetic fields as

opposed to hard magnets which require large magnetic fields to demagnetize. This

property permits faster and less energy intensive cycling from positive to negative

inductance. They also contain large saturation magnetizations which allows for smaller

cores than other types of ceramic magnets. Figure 2 shows some examples of ferrite

cores.


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Figure 2: Examples of some ferrite cores [2]

2.1 COMPOSITIONAND CRYSTAL STRUCTURE

The crystal structure of magnetic cores is that of the spinel structure, A2+B3+2O2-4,

where an iron (III) oxide is paired with a metal oxide of valence two for the metal. For

transformer and inductor core applications the metal in the metal oxide is either a

manganese-zinc or nickel-zinc combination [3].

In the spinel structure there are eight units per cell. This gives a total of 32 oxygen

ions, 16 iron (III) ions and 8 additional metal ions. Within this unit cell there are 16

octahedral and 8 tetrahedral sites; an octahedral site is surrounded by six oxygen ions

while a tetrahedral site is surrounded by four oxygen ions. Half of the octahedral sites

and all of the tetrahedral sites are occupied by the iron (III) ions. Since the octahedral

sites are oriented antiparallel (opposite magnetic directions) to the tetrahedral sites the net

magnetism from the Fe3+

ions is zero. Thus the net ferrimagnetism from the material is

due purely to the additional metal ions in the octahedral sites. Figure 3 below shows the

position of the octahedral and tetrahedral sites within the spinel unit cell.


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Figure 3: Part of a spinel ferrite unit cell [4]

In the case of magnetic core applications, the Zn2+ ions can be partially substituted into

the Fe3+ octahedral sites increasing the net magnetization by reducing the offset by one

valence. Ni2+ or Mn2+ ions then fill the remaining octahedral sites. The magnetic

moment created by the combinations of Mn-Zn and Ni-Zn with the iron oxide play a

large role in the magnetic properties of the ceramic.

2.2 PROPERTIES

The magnetic, mechanical and thermal properties of the spinel ferrite determine which

industrial applications are available to the ceramic. The strength of the magnetic field,

tensile strength, hardness, core loss and size are some of the determining factors in this

decision.

2.2.1 MAGNETIC PROPERTIES

Ceramic magnets are designed to be cheaper and more efficient than their metallic

alternates. A large requirement of that design is for the ceramics to have magnetic

properties better than or equal to metal magnets. Properties of importance are the

saturation inductance (Bsat), resistivity and relative permeability. Table 1 summarizes

some of the magnetic properties of metal and ceramic magnets used in magnetic cores.

The 99.95% iron material is known as a powdered iron while the Permalloy is a Ni-Fe

combination.

Table 1: Magnetic properties of ceramic and metal magnets used in magnetic cores [5]

Material Saturation Resistivity Relative


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Inductance(T)

(cm) Permeability

99.95% Fe 2.14 10x10-6 5000

Fe-80%Ni

Permalloy0.8 55x10-6 100000

Mn-Zn Ferrite 0.4-0.5 10-103 750-15000

Ni-Zn Ferrite 0.3-0.4 105-1010 10-1000

The saturation inductance of a ceramic is the maximum amount of inductance, or

magnetization, a ceramic can hold. Larger Bsat values mean that a magnet can be made

smaller while still producing the same amount of magnetization as magnets with a lower

Bsat. Metals contain saturation values of approximately 2T (Tesla) while ceramic

transformer cores are between 0.3T and 0.5T. This difference is due to the large volume

of nonmagnetic oxygen ions present in the lattice structure which lowers the overall

magnetic moment as compared to metals. A comparison of different ferrite and metal

magnet saturation inductances are in Table 1.

Core resistivity is another important property of magnets. As with saturation

inductance the resistivity of ferrites is large due to the presence of oxygen ions. This

larger resistance to electric current results in much lower core losses from eddy currents

in ferrites as compared to metal magnets; this allows for high-frequency applications to

be performed with little loss which is not possible with metal magnets. Entries in Table 1

show the resistivity of several metal and ceramic magnets.

The permeability of a magnet is the ability to resist the formation of a magnetic field

within the material. This property is used to relate the applied magnetic field to the

produced inductance in a magnet as seen in the equation below.


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HB =

where: B = Inductance of the material

= Permeability

H = Magnetic Field Strength

Relative permeability is equal to the permeability of a material relative to a vacuum.

Materials with low relative permeabilities will store energy which is undesirable in

transformer applications and is considered to be a parasitic loss. Ferrite magnets have

much smaller permeabilities than either powdered iron (99.95% iron) or Permalloy as

seen in Table 1. The reason for such low permeabilities is the effect of grain boundaries,

grain size and porosity within the crystals microstructure that prevents movement of the

domain walls. Thus a larger magnetic field is required to move the domain wall. The

domain walls are the transition regions where the magnetic moments within a structure

change directions when a magnetic field is applied.

A final parameter of transformer or inductor magnets is core loss. During operation

the wire surrounding a core is exposed to alternating currents which constantly change

the direction of the magnetic field. This changing field results in the formation of eddy

currents which oppose and weaken the inductance within the field according to Lenzs

law. Thus the amount of energy put into making the original inductance is diminished.

Magnets with higher resistivity resist the formation of eddy currents and are therefore

more efficient at converting the applied current into magnetization of the core. As seen


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in the previous paragraphs metals have a very low resistance while ceramics are very

high. Other types of core loss exist in hysteresis. As seen in Figure 1 the energy put into

a material is less than the energy recovered when the magnetization is reversed. This

difference is energy lost to hysteresis. The impact of these core losses will be discussed

further in the Section 2.4 on applications.

2.2.2 MECHANICALAND THERMAL PROPERTIES

Although the magnetic properties of a ceramic are imperative to their eventual use, a

ceramic magnet must be strong enough to handle the everyday wear and tear that is

associated with its application. They also must be able to operate under varying

temperature situations. Tensile strength (T), compressive strength (c), hardness,

compliance (E) and thermal conductivity (k) are some of the more crucial parameters of

the ceramic magnets. The ranges of these properties for the ceramic magnets are

summarized in Table 2.

Table 2: Mechanical and thermal properties of ceramic magnets [5]

Tensile Strength (MPa) 20-60

Compressive Strength

(MPa)200-700

Compliance (MPa) 80-150

Vickers Hardness 600-900

Thermal Conductivity

(W/ms)

0.0035-

0.005

It can be seen from the table that the tensile strengths are quite low while the

compressive strengths are high. The compliance, or Youngs modulus, of the ceramics is

also quite small relative to metals. Based upon these lower values the ceramic magnet


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would not be well suited for transformer applications where vibration or shock of the unit

occurs, such as military applications. The low thermal conductivities are also concerning

as transformers generate heat which needs to be removed for operation.

From Table 2 it can be seen that the overall sturdiness of a ferrite core is low

compared to their metal counterparts. This indicates that application of the core is not

only dependent upon the required magnetic properties but the environment in which the

core is used.

2.2.3 EFFECTOF TEMPERATUREON PROPERTIES

The crystal structure of a ceramic is susceptible to variations in its properties due to

the thermal vibration associated with changes in temperature. Thermal vibration can alter

both the permeability of a ferrite and cause complete disorder above a certain

temperature, known as the Curie temperature.

In Mn-Zn ferrites the permeability can more than double over a 250C span in a non-

linear fashion. Table 3 shows how the permeabilities of several combinations of Mn-Zn

can change over temperature from their initial values. The different initial permeabilities

can be created through varying the amounts of manganese and zinc within the ceramic.

Table 3: Change in the initial permeability of Mn-Zn ferrites over temperature [5]

at -50CInitial at

25C at 50C at 150C at 200C

700 800 850 850 900

800 1200 1150 1500 2000

1500 2000 2100 3200 N/A


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1250 2300 2400 2700 3100

Ferrites also have a unique temperature upon which the magnetic structure will break

down due to thermal vibration. This temperature is called the Curie temperature. Above

this temperature the thermal energy overcomes the magnetic energy resulting in complete

disorder of the internal moments and an overall decrease in magnetization of the ceramic.

Below this temperature the structure remains partially magnetized but the inductance is

reduced the closer the temperature is to the Curie temperature. At 0K the magnetization

would be at a maximum. The below figure expresses the effect of the Curie temperature.

Figure 4: Effect of temperature on the magnetization of a magnet

In powdered iron the Curie temperature is 770C while ferrites based on Fe, Mn, Co,

and Ni ions are 585C, 300C, 747C and 585C respectively. The shows that, in

general, metals will retain their inductance better than ceramics during higher

temperature applications.


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2.3 MANUFACTURING

Properties of ceramics such as the permeability and saturation inductance can be vastly

improved by controlling the composition, grain size, resistivity and internal grain losses

in the manufacturing process. Even minor percentage differences in the composition will

result in severely diminished magnetic and thermal properties as illustrated in Figure 5.

Thus the efficiency of the ceramic can be vastly increased if the manufacturing process is

perfected.

Figure 5: Saturation inductance with minute changes in composition [6]

Ceramic magnets are currently manufactured using the conventional sintering

technique. This begins with mixing of the raw powders for the desired composition (i.e.

MnCO3, ZnO and Fe2O3 powders among other minute molecules). These powders should

be inspected for purity but some sacrifice must be made for cost. Several impurities

could result in accelerated grain growth or other adverse effects in sintering. Mixing of


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sintering is complete the product should be ready for application in transformers and

inductors.

2.4 APPLICATIONS

Ceramic magnets can be used in a variety of situations from magnetic storage to

permanent magnets. Spinel ferrites however, are useful as transformer and inductor

cores. The primary purpose of a magnetic core is to magnetically link two or more

circuits by effectively transmitting magnetic flux through itself to another, adjacent core.

Magnetic fields required to activate this flux are generated by current in a wire coiled

around the magnet. The magnitude of this field is a combination of the current in the

wire and the number of turns of wire around the core. Since alternating current (AC) is

used in the circuit, the inductance generated within the core will be alternating by the

same frequency as the current. Ferrites can then operate for a range of applications with

their effectiveness, compared to metal magnets, largely dependent upon the frequency of

the current. Further development of ferrites is encouraged due to the lower production

costs and lower losses as compared to metal magnets.

In low frequency applications, 1 Hz to 20 kHz, metal magnets such as powdered iron

and Permalloy are preferred to ferrites. This is because metal magnets have similar core

losses to ferrites while maintaining superior saturation inductances (0.8T to 0.4T), which

allows them to be much smaller than ferrites. At low frequencies eddy currents, which

result in core loss, are not as much of a factor as the current alternates at a much slower

rate. Thus the advantage gained by having a higher resistivity is minimal for ferrites in


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low frequency applications. Ferrites also contain larger hysteresis losses than metals

which make them less preferable when eddy currents are not a factor. In addition, ferrites

store a small amount of energy at low frequencies which is an undesirable quality in

transformer applications. This energy storage is due to the low permeabilities associated

with ferrites. In summary, due to similar core losses, metal magnets are superior to

ferrites in low frequency applications due to higher permeabilities and saturation

inductances.

Higher frequency applications ranging from 100 kHz to 100 MHz favour the use of

ferrites to metal magnets. At higher frequencies eddy currents are much more

pronounced and are the primary factor in deciding which material is used for the

application. Core losses in metal magnets are severe and reduce the efficiency of the core

to a fraction of the value seen when operating at low frequencies. Alternatively, ferrites

experience much lower efficiency losses due to their higher resistivities. With a

resistivity of between 10 cm and 1000 cm, Mn-Zn ferrites are used in applications up

to 1 or 2 MHz. Ni-Zn ferrites have a much higher resistivity between 10 kcm and 10

Gcm allowing for eddy current losses to be low enough for applications up to several

hundred MHz.

Intermediate frequency applications in the 20 kHz to 100 kHz range use both metal

and ceramic magnets. Though the core losses in the metals are higher, the better

permeabilities and saturation inductances seen in metals make up for the shortcomings in

some cases. Therefore the choice between metal or ceramic cores in these regions must


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take into consideration the size requirements, environment and strength to a much greater

extent than in low or high frequency situations where losses are prevalent. Figure 6

shows the total core loss as a function of the generated inductance for different

frequencies.

Applications involving heating situations, high shock or vibration are better performed

by metal magnets than ceramics. The low thermal conductivity of a ceramic is unable to

remove heat generated by the transformer in some conditions causing the temperature to

potentially run away. Although heating of the core increases the permeability it will

decrease the magnetization as the Curie temperature is approached. Damage may also

occur to the surrounding wires. Military or large motion applications are also not meant

for ceramic magnets as they have low tensile strengths and compliance which could result

in premature breaking of the component.


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Figure 6: Graph of the core loss (hysteresis + eddy currents) vs B for ferrites [9]

2.5 LIMITATIONS

The limitations of ferrites are a result of several of its properties compared to metals.

Tensile strength, thermal conductivity and lower saturation inductances all reduce the

number of applications available to the material. High shock or vibration situations may

cause the material to break while the low thermal conductivities would prevent the

ceramic from expelling heat, damaging its performance and surroundings. Lower

saturation inductances, Bsat, also limit the minimum size of the created ferrites. Since

metals in general have larger Bsats they can be made smaller while producing the same

power output and are therefore more desirable in compact designs. A lower Bsat and

higher hysteresis losses also prevent ceramics from replacing magnets in the low

frequency applications. As newer ceramics are created these properties can be improved


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but currently they hinder the performance and subsequently the use of ferrites in certain

situations.

3 CONCLUSIONSAND RECOMMENDATIONS

The ceramic magnets used in transformer and inductor cores are various compositions

of (Mn,Zn)Fe3O4 and (Ni,Zn)Fe3O4. Although the majority of the material properties

including saturation inductance, mechanical strength and thermal conductivity are

inferior to those of metals, the ferrites have managed to flourish in several applications.

The specific composition of the ceramic and the manufacturing processes used can

improve these limiting properties of ferrites and enhance their range of applications.

Improvements to the manufacturing process may be possible through cheaper

techniques to attain high purity base powders. As the field naturally advances, powders

will become more pure at a lower cost. This will increase not only the actual properties

of the magnet but reduce costs by increasing the predictability of the mix and requiring

less readjustment of the composition throughout manufacturing. Reducing the impurities

will also prevent the finished bars from being excessively fragile due to cracking.

Applications which require more robust ceramics will benefit from this.

The permeability of ferrites is another property that could be improved. Ceramics

currently have permeabilities far below that of Permalloy or even powdered irons due to

grain boundaries, porosity and grain size within the microstructure impairing movement

of the domain walls during polarization changes. Improving sintering techniques through


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alterations to the sintering time, temperature and oxygen content would optimize grain

growth and assist in reaching 100% theoretical density thus increasing the permeability.

This process is again assisted by the higher predictability associated with improving the

purity of the powders. Increased permeability decreases the magnetic field required to

induce a certain magnetization thus reducing the amount of energy inputted into the

system.

Seemingly opposing the suggestion of removing impurities from the mix powders,

there are many untested possible additions to the Mn-Zn and Ni-Zn ferrites that could

improve the performance of the ceramics. These could range from a very wide variety of

oxides which would then require a re-optimization of the entire manufacturing process.

The precise composition and type of oxide would be hard to determine but could raise the

properties of the ceramic to a point where thermal applications are more possible or the

ceramic is more robust.

Possibilities for improvement to the field of magnetic ceramics are almost limitless.

The relatively low cost of ceramic powders is driving continuous advancement in

industry. This includes investigations into new types of additives, manufacturing

processes and sintering techniques. As technology improves ferrites may eventually be

the material of choice for a majority of magnetic core applications.


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4 REFERENCES

[1] TDK Lambda. Glossary of Power Supply Terms. [Online] http://www.uk.tdk-

lambda.com/public/glossary.aspx?index=H. 2009

[2] Dexter Magnetic Electronics. [Online] http://www.directindustry.com/prod/dexter-magnetic-

technologies/ferrite-core-for-cable-23025-356264.html. 2010

[3] L. Rozenblat. Electrical Power Transformer and Inductor: Design principles, calculation, theory,

tutorials and other information. [Online] http://www.smps.us/magnetics.html. 2009

[4] B.M. Moskowitz. Hitchhikers Guide to Magnetism: Classes of Magnetic Materials. [Online]

http://www.irm.umn.edu/hg2m/hg2m_b/hg2m_b.html. 1991

[5] A. Goldman. Ceramics and Glasses: Vol. 4 Engineered Materials Handbook, ASM. Ferrite Technology

Worldwide Inc. 1991.

[6] G.E. Schaller. Ferrite Processing and Effects on Material Performance . [Online] http://www.cmi-

ferrite.com/News/Papers/ferpro.pdf.

[7] Texas Instruments. Magnetic Core Characteristics . [Online]

http://focus.ti.com/lit/ml/slup124/slup124.pdf.

[8] L. Dixon. Magnetic Core Properties . [Online] http://focus.ti.com/lit/ml/slup128b/slup128b.pdf.

[9] University of Colorado. Fundamental of Power Electronics: Chapter 13:Basic Magnetic Theory .

[Online] http://ecee.colorado.edu/~ecen5797/course_material/Ch13slides.pdf.

Documents

Ceramics Project - Tom