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Versions of the material containing higher concentrations of

zirconium oxide, if appropriately processed, develop platelets in

the microstructure, which contributes to an improvement insome material properties (ZPTA ceramics). These and other

measures taken to optimise properties permit the achievement of

outstanding figures for

y bending strengthy modulus of elasticity andy

thermal behaviourthat are superior to those of Y-ZTP (tetragonal zirconium oxide

stabilised with yttrium oxide).

Figure 17: ZTA with a high proportion of zirconium oxide,shown as a breakage

microstructure in order to reveal the platelets

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Barium Titanate

Barium titanates are amongst the materials used as functional

ceramics. They possess extremely high permittivitys, therefore

finding application as capacitor dielectrics. They are also used as

piezoelectric ceramic

Barium carbonate, titanium dioxide and other raw materials for

doping purposes are sintered at between 1,200C and 1,400C to

form polycrystalline barium titanate. This exhibits

semiconducting properties together with a positive temperature

coefficient of the ohmic resistance, for which reason it is used asa positive temperature coefficient resistor (PTC). This effect is

characterised by a very sharp rise of electrical resistance

several powers of ten starting at a reference temperature (Tb).

Figure 19: Resistance curves of PTC ceramic

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PTC ceramics are used as temperature sensors in

instrumentation and control technology, and as limit sensors for

motor and machine protection. The material is also applied for

self-regulating heating elements operating from low and mainsvoltage, as switching delay elements (for electric motor starting

and de-magnetisation), and for overload protection.

Figure: Components manufactured from PTC ceramics

Lead zirconate titanate

Currently the most important piezoelectric ceramic materials

are based on mixed oxide crystal system consisting of leadzirconate and lead titanate known as lead zirconate titanate

(PZT).

The specific properties of these ceramics, such as the high

dielectric constant, are dependent on the molar ratio of lead

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zirconate to lead titanate as well as on substitution and doping

with additional elements. A wide range of modifications can be

implemented in this way, creating materials with highly varied

specifications.The piezoelectric effect

The piezoelectric effect links both electrical and mechanical

properties.

Piezoelectricity refers to a linear electro-mechanical interplay

between the mechanical and electrical states of a crystal.

The direct piezoelectric effect refers to an electrical charge,detectable as a voltage, being created in proportion to

mechanical deformation of the crystal.

Figure Piezoelectric effect resulting

from an external force. The

polarity of the electric chargedepends on the direction of the

applied force.

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The reciprocal or inverse piezoelectric effect refers to a

deformation that arises in proportion to an external electrical

field created by the application of an electrical voltage.

Figure The inverse piezoelectric

effect under the influence o

external electric fields. The

dimensions of the body vary

with the change in voltage.

Principles

The piezoelectricity of ferroelectric materials is a consequence

of the existence of polar areas (domains) whose orientation

changes as result of the polarisation, i.e. of the application of an

electrical voltage. The polarisation is associated with a change inlength, S.

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Figure 23: Electric dipoles in a piezoelectric material before

and after polarisation.

Lead zirconium titanate Pb(Zrx Ti(1-x)) O3 is processed in

polycrystalline form. The two most common shaping methodsare pressing and the tape casting process. The green body

acquires its ceramic properties through firing.

The piezoceramic, however, only acquires its technically

interesting piezoelectric properties through a polarisation

process.

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FigureDiagram of the domains in lead zirconate titanate before,

during and after polarisationS = change in length during polarisation

Sr = residual changing length after the polarisation

process

Figure 25: Fundamental vibration modes in piezoceramic

components

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Figure : Isostatic pressing with regions of different compression

(grey levels)

Figure : Extrusion

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Figure: Substrates of aluminium oxide

Figure: Various piezo-ceramic parts.