Functional Ceramics

7/28/2019 Functional Ceramics

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Functional Ceramics

Chapter 4:


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Outline of Chapter 4

4-1Introduction to Functional Ceramics

4-2 Fabrication Processes for Functional Ceramics

4-3Application of Ferroelectric Ceramics


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Questions for Chapter 41 What are Ceramics and Functional Ceramics?

2 What are the application fields of Functional Ceramics?

3 The types of Functional Ceramics and their applications.

4 How to fabricate functional ceramics?

5 The crystal structure of ferroelectric ceramics (BaTiO3 ceramics).

6 Why are ferroelectric ceramics so important?

7 The applications of ferroelectric ceramics.


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What are Ceramics?

comes from the Greece word keramicos, which means burnt stuff

broadly classed as inorganic, non-metallic materials

usually a compound, or a combination of compounds, between

metallic and nonmetallic elements (mainly O, N, C, B)

always composed of more than one element (Al2O3, SiO2, SiC, etc.)

bonds are either totally ionic, or combination of ionic and covalent.

A ceramic is an inorganic, nonmetallic solid prepared by the action of

heat and subsequent cooling. Ceramic materials may have a

crystalline or partly crystalline structure, or may be amorphous (e.g., a

glass) .

4.1 Introduction to Functional Ceramics


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4.1 Introduction to Functional Ceramics

Ceramic artware

Piezoelectric ceramics

Ceramic components

Ceramic knife

Bioceramics Ceramic components


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Classification of Ceramics

Traditional ceramics

Clay based products

Structural ceramics

Used for their mechanical properties

Functional ceramics

Used for other properties than mechanical strength, i.e. electrical,

optical, magnetic properties

Ceramic bearing and bolts

Ceramic dishware

Piezoelectric ceramic components


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Classification of Functional Ceramics

Based on Chemical composition:

Silicate Ceramices: presence of glassy phase in a porous structureClay ceramics (with mullite - 3Al2O3.2SiO2)

Silica ceramics (with cordierite 2MgO.2Al2O3.2SiO2)

Oxide Ceramics: dominant crystalline phase, with small glassy phaseSingle oxide (Al2O3), doped oxide, mixed oxide (BaTiO3)

Non-oxide Ceramics:

Carbon, SiC, BN, Ti3N4, sialon(Si3N4-Al2O3 )


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Properties and Applications of Functional Ceramics

Properties

Optical

Electronic and ion conducting Catalytic

Magnetic

et al.

Applications of functional ceramics

Information and communication technology

Energy technology

Process technology - catalysis

Environmental technology

Medical technology

Sensor technology


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Types of Functional Ceramics

Ceramic insulators

Ferroelectric Ceramics

Piezoelectric ceramics

Thermosensitive ceramics

Pressure-sensitive ceramics

Gas-sensitive ceramics

et al.


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Ceramic insulators

electrical resistivity (> 109 cm)

Functions:

The primary function is physical separation of conductors and

regulation or prevention of current flow between them.

Other functions are to provide mechanical support, heat dissipation,

and environmental protection for conductors.

Classification:

Oxide-based Ceramics: SiO2, Al2O3, MgO-Al2O3-SiO2, BaO-Al2O3-SiO2

Nitride-based Ceramics: BN, AlN, Si3N4, Ti3N4


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Ceramic insulators

Advantages:

The advantage of ceramics as insulators is their capability forhigh-

temperature operation.

High voltage insulators of ceramic materials are mainly used in

outdoor switching stations and outdoor lines.

Disadvantages:

brittle and easily chipped or broken

Ceramic insulator supporting a power lineDifferent shaped ceramic insulators


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Ferroelectric ceramics is a special group ofminerals that have

ferroelectric properties: the strong dependence of the dielectric constant

oftemperature, electrical field, the presence ofhysteresis and others.

The ferroelectric effect: the polarity can be reversed under the

influence of an electric field of the appropriate orientation.

Crystal Structure:

The ABO3 perovskite type materials are by far the most important

category for ferroelectric ceramics.

For example, BaTiO3, CaTiO3, PbTiO3, KNbO3


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Applications:

Ferroelectric random access memory (FRAM): this offers very fast

storage and retrieval of data, with the advantage that the stored data is

preserved when there is no power supply.

Ferroelectric ceramics are suitable for use in capacitors, for example

in ultrasound imaging and high sensitivity infrared cameras.

Thin-film ferroelectric ceramics, which can be used in optical

waveguides and optical memory displays.


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Piezoelectric Ceramics

Piezoelectricity is the charge which accumulates in certain solid

materials in response to applied mechanical stress.

A ceramic, such as lead zirconate titanate (PZT), that converts an

electrical field to a mechanical strain or a mechanical strain to an

electrical charge.

Applications:

Actuators

Sensors

Generators

Transducers


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Other Functional Ceramics

Thermosensitive ceramics: PTC ((Ba2Pb)TiO3, (Sr,Ba)TiO3)

NTC (MnO-CuO-O2, Mn-Nio-O2)

Pressure-sensitive ceramics: (ZnO ceramics)

Humidity sensitive ceramics: (Si-Na2O-V2O5, ZnO-Li2O-V2O5 ceramics)

Gas-sensitive ceramics: (SnO2, ZnO, Fe2O3 ceramics)


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4.2 Fabrication Processes for Functional Ceramics

General Fabrication Processing for Functional Ceramics

Synthesis of powder(critical step: monophasic powders, with fine

and homogeneous particle size distribution)

Milling, usually with additive mixing (lubricant, plasticizers, binders)

Drying

Forming

Sintering (the reduction of the porosity)

Finishing (including slicing, lapping, polishing, electroding,encapsulation and poling)

Evaluation


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Outline of fabrication process for multi-layer ceramic capacitor

Capacitors could provide functions

such as interdicting DC current, storingcharges, filtering waves, differentiating

frequencies and resonating circuit for

electronic circuits. Therefore capacitor

ceramics are the most often used

materials among all functional ceramics,in which Multilayer Ceramic Capacitors

(MLCCs) are the most often used devices

among all capacitors.

BaTiO3


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Outline of fabrication process for PZT pieoelectric ceramics

lead zirconate titanate

(PbZrTiO3, PZT),


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Raw Materials and Powder preparation

Raw materials should be properly selected to meet demands from

functional ceramic performance, production processing and facilities,

as well as economical concern. (raw materials are the foundation for

ceramics of excellent performance.)

purity

reaction activity

Powder preparation methods

oxide synthesis (solid state reaction)

co-precipitation

sol-gel method

hydrothermal methods

spray pyrolysis


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Ball mill mixing and grinding

a conventional and often used technique

agent balls (higher density induce more remarkable effect of impacting

and grinding)

types of millsNormal ball mills, Stirring ball mills, Planetary ball mills, Jet mills.


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Powder preparation methods

Powder preparation by oxide synthesis

Synthesis is generally conducted in solid states, and thermal analysis and

X ray diffraction analysis could be used to analyze composition of synthesized

powders and phase evolution during synthesis.

impurity, particle size > 0.5-2.0 um

low reactive activities


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Powder preparation by co-precipitation

Solutions of metal ion salt are mixed according to mole ratio, and precipitators

are then introduced in the solution to precipitate metal ions. Powders of specific

composition could be obtained by drying and firing the sediments.

Fabrication of BaTiO3 by co-precipitation method

Co-precipitation of BaTiO3 powder:

1. Inorganic salts of Ba and Ti as precursors:


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Powder preparation by co-precipitation

3. Organic metal salts as precursors:

2. Co-precipitation in oxalate is the major technique for mass production of

BaTiO3 powder.

Advantages and disadvantages of co-precipitation method:

Satisfy the features including high purity, high fineness, high homogeneity,

and low sintering temperature.

Problem of possible residue of Cl-

and particle agglomeration.


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Powder preparation by hydrothermal method

Example of BaTiO3 powder:

Advantages:

fine grain size (


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Comparison among the above preparation methods

Precipitation of oxalate and hydrothermal methods are very important

wet chemical techniques for powder preparation. Hydrothermal method

is promising in the mass production of ferrite powders.


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Powder preparation by sol-gel method

Powder obtained with sol-gel technique is more homogeneous and itsadditive dispersion is also more homogeneous, thus composition could be

better controlled and thin films could be easily obtained.

Example of BaTiO3 powder:

Synthesis temperature (550~600 oC)

Oxide processing (900~1000 oC)


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Powder preparation by spray pyrolysis

newly developed technique for fabricating multi-element powders.

provide higher homogeneity than other wet chemical methods.

ZrO2, PZT, superconductor, mullite.


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Questions:1 What is sintering and what is the role of sintering in fabrication

process?

2 Comparison of the sintering mechanisms?

3 Hysteresis loop of ferroelectrics.

4 Which materials can be used for ferroelectric ceramics?

5 Why are ferroelectric ceramics so important?

6 The applications of ferroelectric ceramics.


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Shaping and Forming of Functional Ceramics

Conventional film processes

Forming is a significant fabricating process for ceramic materials.

Current film processes

Sputtering

CVD


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Conventional forming processes for powder compression


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Conventional forming processes for powder compression

Spray granulation

Mechanisms of two types of spray drying

(a) Pressure spray nozzle dryer;

(b) High speed rotary spray dryer

Spherical particles from spray drying

a b


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Sintering

The densification process of ceramic powders at a high temperature is

called sintering.

Sintering could eliminate most pores in porous green body and compress

residual pores, and bring grain growth and improve binding among grains.

Energy is required during sintering to advance mass transfer. Heat energy

is the major source of energy, while energy gradation from particle contact

and surface tension could also provide energy.

Sintering mechanisms

Gas phase sintering

Solid phase sintering

Liquid phase sintering

Activated liquid phase sintering


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Gas phase sintering

The driving force: the differentiation in vapor pressure.

the curvature is larger for smaller particles, and the driving force for vapor

transfer is correspondingly larger.

could enhance material strength and reduce open pores, but it does

not lead to contraction.

densification can not be achieved with only gas phase transfer.


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Solid phase sintering

the driving force for solid phase sintering lies at the differentiation of free

energy or chemical potential between free surface of particles and its

interface with neighboring particle.

The diffusion could be gas phase diffusion, surface diffusion, grain

boundary diffusion or internal diffusion.

the factors for enhancing sintering

small particle size and narrow distribution a properly wide distribution of granularity to allow dense packing and to

reduce interstitial volume

green body should have a uniform distribution of density

an optimal pre-calcining temperature


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Liquid phase sintering

If liquid phase is generated to wet powder particles at sintering temperature,

liquid phase sintering will occur.

Liquid phase surrounding particles will bring capillary pressure, and it could

promote densification.

Particles could be better realigned to achieve denser packing

Contact pressure between particles will be improved to promote mass

transfer through dissolution and precipitation.

Liquid phase sintering strongly depends on temperature since slight

increase of temperature might produce a large amount of liquid phase.

liquid phase sometimes will promote densification, but sometimes will lead

to abnormal grain growth or deformation that impedes densification.


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Activated liquid phase sintering

is also called as transient liquid phase sintering.

liquid phase promotes densification during sintering, but its composition

would be altered or the liquid phase could totally disappear at the end of

sintering.

Particles could be better realigned to achieve denser packing

Three methods could be used to obtain transient liquid phase sintering.

proper powders or additives could be added to generate one or moreliquid intermediate products after various chemical reactions, but the final

products are solid state

certain powders could be added to generate liquid phase and turn into

solid solution finally.

glass phase could be crystallized with heat treatment to eliminate liquidphase products.


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Sintering Mechanism Summary


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Sintering Process

Sintering is a key process during fabrication of functional ceramics.

During sintering, the shrinkage

and porosity decreased, and the

density increased.

Sintering is actually a process of

densification through mass transfer

mechanism.


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1. Linear expansion stage (25~400 oC)

2. Solid reaction stage (400~1000 oC)

3. Shrinkage stage (700~above 1000 oC)

4. Grain growth stage

Sintering process could be divided into four stages:

Sintering Process


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Effects of Pressure and Atmosphere on Sintering

Pressure

Almost all ceramics have pores that mainly contain air (O2 and N2),

thus sintering under reduced pressure or vacuum could promote the

elimination of pores. Sintering in vacuum: PLZT, ferrite, Al2O3.

Atmosphere

Perovskite materials (ABO3): sintering in oxygen could promote oxygendiffusion via oxygen vacancy and increase densification.

Sintering in oxygen: PZT, PLZT and some niobate ceramics.


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Pressure Sintering

Hot pressing (HP): 10000~30000 kPa

Hot iso-static pressing (HIP): 150000 kPa

Ordinary sintering: 100~700 kPa

large pressure could facilitate elimination

of pores and vacancies through diffusion

along grain boundaries.


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Microwave sintering

Microwave sintering could bring uniform densification and promote

ionic diffusion without causing abnormal grain growth.

Advantages of microwave sintering

Semiconductor doping range is broader, which could facilitate the

fabrication of ceramics with a low resistivity.

Sintering temperature could be decreased to avoid abnormal grain

growth, which could improve dielectric strength

Sintering time could be shortened from 8~10 h to 0.5~1.5 h.

Lead volatilization could be reduced.


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4.3 Application of Ferroelectric Ceramics

Ferroelectric ceramics were born in the early 1940s with the discovery ofthe phenomenon of ferroelectricity.

Ferroelectricity is a property of certain materials which possess a

spontaneous electric polarization that can be reversed by the application of

an external electric field.

Variety of ferroelectric ceramics used in piezoelectric and electrostrictive

applications, such as sonar, accelerometers, actuators, and sensors.


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Abbreviations Used for Ferroelectric Ceramics

PZT: Lead zirconate titanate

PLZT: Lead lanthanum zirconate titanate

PMN: Lead magnesium niobate

PT: Lead titanate

PZN: Lead zinc niobate

PSZT: Lead stannate zirconate titanate

PZ: Lead zirconate

BST: Barium strontium titanate

SBT: Strontium bismuth titanate


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Ferroelectrics: Ferroelectric domains

Ferroelectric domains are generated by coupling between dipolemoments of atoms.

When subjected to electric field, the domains pointing towards its

direction start to grow over its neighbouring domains.


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Ferroelectrics: hysteresis loop

Saturation and remanent

polarization

Coercive field

Possibility to reverse the

polarization

Smart material: it keeps

information (remanent

poalrization)


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Ferroelectrics: phase transition

Ferroelectricity is a phase transition (Curie point)

Ferroelectric phase has always lower symmetry

Example: BaTiO3, PbTiO3 (cubic changes into tetragonal)

Pi l i i i F l i C i


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Piezoelectricity in Ferroelectric Ceramics

two effects are operative in piezoelectric crystals, in general, and inferroelectric ceramics, in particular.

Piezoelectric effects in ferroelectric ceramics

Basis for Ferroelectricity in Ceramics


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Basis for Ferroelectricity in Ceramics

TheABO3 perovskite type materials are by far the most importantcategory for ferroelectric ceramics.

For example, BaTiO3, PZT, PLZT, PT (lead titanate), PMN, (Na,K)NbO3

Perovskite ABO3 unit cell for

PZT or PLZT, illustrating

180polarization reversal for

two of the six possible

polarization states produced by

displacement of the central

cation in the tetragonal plane

f


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Properties of Ferroelectric ceramics

Mechanical: poor toughness (under study)

Electrical: semiconductors, superconductors, piezoelectrics,

pyroelectrics, ferroelectrics (BaTiO3, PZT)

High resistance to abrasion

Excellent hot strength

Chemical inertness

We can tailor properties for specific applications


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Why are ferroelectric ceramics so important?

Ferroelectrics

High permittivities

Spontaneus polarization Electric conducticity can be

controlled

Piezoelectric and pyroelectric

effect Optical anisotropy, electrooptic

an photorefractive deffect

Ceramics

Broad range of chemical

composition Control of grain size, porosity

Possibility of varying its shape

and size.

High resistance to abrasion Excellent hot strength

Chemical inertness

All this properties lead to a lot of potential

aplications!

Applications of Ferroelectric Ceramics


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Applications of bulk and film ceramic electronic materials

Ferroelectric ceramics are used in a very broad range of functional ceramics

and form the materials base for the majority of electronic applications. These

electronic applicators account for more than 60% of the total high technology

ceramics market worldwide.

A li ti f F l t i C i


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Why ferroelectric ceramics have been so successful over the years in

finding an increasing number of applications?

Their simplicity, compact size, low cost, and high reliability are very

attractive features tothe design engineer.

Why the trend in the industry is toward film devices?

(1) lower operating voltages

(2) size and weight compatibility with integration trends

(3) better processing compatibility with silicon technology

(4) ease of fabrication

(5) lower costs through integration


Capacitors


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Capacitors

Basic principle

'C' is the capacitance, is the permittivity of free space, is the

relative dielectric permittivity, 't' is the distance between the electrodes,

'A' is the area of the electrodes.

0( )rC

t

=

0 r

To get a high volumetric efficiency (capacitance per unit volume), the dielectric

material between the electrodes should have a large dielectric constant, a large

area and a small thickness.

BaTiO3 ceramic based disk capacitors have captured more than 50% of the

ceramic capacitor market.


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Multilayer Ceramics (MLC)

the volumetric efficiency can be further enhanced .

consists of alternate layers of dielectric and electrode material.

0( )rn A

Ct

=

MLC capacitors are made by the tape casting process.

Ferroelectric Memories


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Ferroelectric Memories

FRAM (Ferroelectric Random Access Memory) is a non-volatile

memory combining both ROM and RAM advantages in addition to non-

volatility features. It has higher speed in write mode, lower power

consumption and higher endurance.

PZT thin film

Pyroelectric Detectors


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Pyroelectric Detectors

Pyroelectric detectors are current sources with an output

proportional to the rate of change of its temperature.

PbTiO3, (Pb,La)TiO3 and PZT

Surface Acoustic Wave Substrates


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Surface Acoustic Wave Substrates

An elastic wave generated at the input interdigital transducer (IDT)travels along the surface of the piezoelectric substrate and it is

detected by the output interdigital transducer. These devices are

mainly used for delay lines and filters in television and microwave

communication applications

LiNbO3, or LiTaO3 were used as SAW substrates

Gas Ignitors


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Gas Ignitors

It consists of two oppositely poled ceramic cylinders attached end

to end in order to double the charge available for the spark.

Usually PZT ceramic disks are used for this application

A piezoelectric spark generator

Accelerometers


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Accelerometers

An accelerometer is a device which gives an electrical outputproportional to the acceleration.

The transducer is a piezoelectric cylinder which is poled along its

axis but has its poling electrodes removed and the sensing

electrodes applied to its inner and outer surfaces.

PZT ceramics

A shear mode accelerometer

Piezoelectric Transformers


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Piezoelectric Transformers

Low voltage to high voltage transformation can be done by using

a piezoelectric plate.

A length mode resonance is excited by applying a low AC voltage

source between the larger face electrodes.

A piezoelectric transformer with the arrows indicating the poling directions

Impact Printer Head


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the printing pin element consists of a piezoactuator, a stroke

amplifier operated on the lever principle and a printing wire

Schematic of a printing pin element

Summary of Functional Ceramics


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Summary of Functional Ceramics

Definition of functional ceramics.

Application fields of functional ceramics.

General fabrication processing for functional ceramics.

The synthesis methods of raw powders.

Sintering mechanism and process.

Ferroelectricity of ferroelectric ceramics.

Applications of ferroelectric ceramics.

Functional Ceramics

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