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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 in
some material properties (ZPTA ceramics). These and other
measures taken to optimise properties permit the achievement of
outstanding figures for



bending strength
modulus of elasticity and
thermal behaviour
that 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 permittivities, therefore
finding application as capacitor dielectrics. They are also used as
piezoelectric
ceramic
materials.
Barium carbonate, titanium dioxide and other raw materials for
doping purposes are sintered at between 1,200°C and 1,400°C 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 as
a 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 mains
voltage, 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 lead
zirconate 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 charge
depends 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 of
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 in
length, 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 methods
are 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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Figure Diagram of the domains in lead zirconate titanate before,
during
and
after
polarisation
S = 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.
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