The piezoelectric effect focusing on PZT

1. AGH
University of Science and Technology
Materials News
Nicola Ergo
THE PIEZOELECTRIC
EFFECT
Author: Nicola Ergo
FOCUS ON PZT

2. SUMMARY
 HISTORY
 INTRODUCTION
 PZT
 POLARIZATION OF A CERAMIC
 PIEZOELECTRIC EFFECT
 HYSTERESIS CURVE FOR POLARIZATION
 BUTTERFLY LOOP
 BASIC BEHAVIOUR OF A PIEZOELECTRIC
CERAMIC BODY
 DEPOLARIZATION
 PIEZOELECTRIC CONSTANTS
 PERMITTIVITY Ɛ
 CHARGE CONSTANT d
 VOLTAGE CONSTANT g
 COUPLING FACTOR k
 COMPARING PIEZOELECTRIC MATERIALS
 MANUFACTURING OF PZT
 POPULARITY OF PZT AND APPLICATIONS
 APPLICATIONS OF PIEZOELECTRIC
MATERIALS
 REFERENCES
 QUESTIONNAIRE

3. HISTORY
The piezoelectric effect was discovered by
Jacques and Pierre Curie in 1880. They found
that if certain crystals were subjected to
mechanical strain, they became electrically
polarized and the degree of polarization was
proportional to the applied strain.
The Curies also discovered that these same
materials deformed when they were exposed to
an electric field. This has become known as the
inverse piezoelectric effect.

4. INTRODUCTION
Piezoelectricity behaviour
 Mechanical stimulus induces current (direct effect)
 Electrical stimulus induces deformation (inverse effect)
Piezoelectric materials in nature
 Naturally-occurring crystals
(quartz, tourmaline and sodium potassium tartrate)
 Member of ferroelectrics
 Ionic or partly ionic bonds
 No polarities of charges in the neutral conditions
 Usually perovskite structure (1)
 Asymmetry of the structure (2) when mechanical or
electrical stimuli are applied
the molecular structure is oriented such
that the material exhibits a local charge
separation, known as electric dipole
(1)
(2)
a tetragonal/rhombahedral structure
very close to cubic

5. INTRODUCTION
No piezoelectric effect
 Cubic, with a center of symmetry
 Covalent bonds
 The electric dipoles always add up to
zero
Piezoelectric effect
 Lack of a center of symmetry
 Ionic or partly ionic bonds
 Strain shifts the relative positions of
the positive and negative charges,
giving rise to a net electric dipole

6. INTRODUCTION
NATURAL SYNTHETIC
Quartz Load zirconate titanate (PZT)
Rochelle Salt Zinc oxide (ZnO)
Topaz Barium titanate (BaTiO3)
Tendon Gallium orthophosphate (GaPO4)
Sucrose Potassium niobate (KNbO3)
Silk Lead titanate (PbTiO3)
Enamel Lithium tantalate (LiTaO3)
Dentin Langasite (La3Ga5SiO14)
DNA Polyparaxylene
Tourmaline Polyvinylidene (PVDF)
Man made piezoelectric ceramics
Man made piezoelectric polymers

7. INTRODUCTION
A stress (tensile or compressive) applied to such a
crystal will alter the separation between the
positive and negative charge sites in each
elementary cell leading to a net polarization at the
crystal surface.
The effect is:
 Practically linear
 Direction-dependent
 Reciprocal
so that compressive and tensile stresses
will generate electric fields and hence
voltages of opposite polarity
so that if the crystal is exposed to an
electric field, it will experience an elastic
strain causing its length to increase or
decrease according to the field polarity
that is the polarization varies directly with
the applied stress

8. PZT
An important group of piezoelectric materials are the piezoelectric ceramics, of which
PZT is an example. These are polycrystalline ferroelectric materials with the perovskite
crystal structure.
PZT have the general formula:
A2+B1+02-
3,
 A denotes a large divalent metal
ion such as barium or lead
 B denotes a tetravalent metal ion
such as titanium or zirconium

9. PZT
Below the Curie point
 Tetragonal symmetry
 Positive and negative charge sites no longer
coincide, so each elementary cell then has a
built-in electric dipole which may be reversed,
and also switched to certain allowed
directions by the application of an electric
field
Above the Curie point
 Simple cubic symmetry
 Positive and negative charge sites coinciding,
so there are no dipoles present in the material
Curie point can be thought of as the “melting” temperature of the piezoelectric
properties, because the material cannot sustain polarization at a
temperature above the Curie point.

10. POLARIZATION OF A CERAMIC
 Before the polarization electric dipoles in the artificial piezoelectric materials
composition are randomly oriented, so the material does not exhibit the
piezoelectric effect
 When a strong electrical field is applied (i.e. poling treatment), the electric
dipoles reorient themselves and the material will also lengthen in the direction of
the field
 Once the electric field is extinguished, the dipoles maintain their orientation and
the material then exhibit the piezoelectric effect so that an electrical
voltage can be recovered along any surface of the material when the material is
subjected to a mechanical stress. However, the alignment of the dipole moments
may not be perfectly straight because each domain may have several allowed
directions.

11. PIEZOELECTRIC EFFECT
The electric field E and the polarization P are connected in a dielectric medium by the
relation:
D = ε() E + P,
 ε() Permittivity of free space
 D Electric displacement
 E Electric field
 P Polarization
For a ferroelectric material like PZT, however, P is itself a function of E.

12. HYSTERESIS CURVE FOR POLARIZATION
Dielectric hysteresis of a “soft” PZT.
The electric displacement D(E) is obtained by addition
of ε() E to the polarization P(E) in accordance with Eq.
D = ε()E + P.
 If an initially unpolarized sample of PZT is
subjected to an increasing electric field at a
temperature slightly below its Curie point, the
dipoles become increasing aligned with the field
and the polarization will follow the “initial curve”.
 When the field has increased beyond a certain
value, no further increase in polarization will be
observed because the dipoles are then all aligned
with the field. The material is then said to have
reached its saturation polarization Ps.
 If the field is now reduced to zero, the dipoles
become less strongly aligned, however, they don’t
return to their original alignment since there are
several preferred directions within the crystallites
and they remain in the ones most closely aligned
with the original field. Since there is still, therefore,
a very high degree of alignment, the polarization
does not fall back to zero but to a value somewhat
lower than the saturation polarization known as
the remanent polarization Pr.
PolarizationP[C/m2]
Electric field E [kV/mm]
0
0 0,5 1,51
0,1
-0,5-1,5 -1
0,2
0,3
-0,1
-0,2
-0,3
Ps
-Ps
-Pr≡-Dr
Pr≡Dr

13. BUTTERFLY LOOP
It can be seen that this also exhibits a hysteresis effect corresponding precisely with the effect
observed for polarization.
Since the volume of the sample remains roughly constant, a relative increase (or decrease) in
S3 will be accompanied by a relative decrease (or increase) in the sample's dimension
perpendicular to the field (S1 and S2) equal to about half the change in S3.
Electric field E [kV/mm]
0 0,5 1,51-0,5-1,5 -1
5
10
15 ·10-4
S3
10
20
30 ·10-4
-S1
P≡ Pr
Mechanical deformation S3 in the direction
of polarization and field, as well as S1 and
S2 normal to this direction as a
function of field strength for a “soft” PZT.
The S1 curve is based on measurement,
S3 is given by S3 -2S1 : -2S2.

14. BASIC BEHAVIOUR OF A PIEZOELECTRIC CERAMIC BODY
The cylinder under no-load conditions.
If an external force produces compressive or tensile strain in the
material, the resulting change in dipole moment causes a voltage to
appear between the electrodes.
If it is stretched, the voltage across the electrodes will have opposite
polarity to the poling voltage.
If the cylinder is compressed so that it resumes its original form, i.e.
before poling, the voltage will have the same polarity as the poling
voltage.
Generator action conversion of mechanical energy into electrical energy

15. BASIC BEHAVIOUR OF A PIEZOELECTRIC CERAMIC BODY
If a voltage of opposite polarity to the poling voltage in applied to the
electrodes, the cylinder will shorten.
If an alternating voltage is applied to the electrodes, the cylinder will
grow and shrink at the same frequency as that of the applied voltage.
If the applied voltage has the same polarity as the poling voltage, the
cylinder will lengthen.
Motor action conversion of electrical energy into mechanical energy

16. DEPOLARIZATION
After its poling treatment a PZT ceramic will be permanently polarized, and care must
therefore be taken in all subsequent handling to ensure that the ceramic is not depolarized,
since this will result in partial or even total loss of its piezoelectric properties.
The ceramic may be depolarized:
 Electrically
 Mechanically
 Thermally
Exposure to a strong electric field of opposite polarity to
the poling field will depolarize a piezoelectric element
Mechanical depolarization occurs when the mechanical
stress on a piezoelectric element becomes high enough
to disturb the orientation of the domains and hence
destroy the alignment of the dipoles
If a piezoelectric element is heated to its Curie point, the
domains become disordered and the element becomes
completely depolarized

17. PIEZOELECTRIC CONSTANTS
Since piezoelectric ceramics are anisotropic, their physical constants (elasticity, permittivity
etc.) are tensor quantities and relate to both the direction of the applied stress, electric field
etc., and to the directions perpendicular to these.
For this reason the constants are generally given two subscript indices which refer to the
direction of the two related quantities (e.g. stress and strain for elasticity, displacement and
electric field for permittivity). Furthermore a superscript index is used to indicate a quantity
that's kept constant.
Rectangular coordinate system:
 the direction of positive polarization is usually
chosen to coincide with the Z-axis
 the directions of X, Y and Z are represented by 1, 2
and 3 respectively
 the shear about these axes by 4, 5 and 6
respectively

18. PIEZOELECTRIC CONSTANTS – PERMITTIVITY Ɛ
The permittivity, or dielectric constant, ε, for a piezoelectric ceramic material is the dielectric
displacement per unit electric field.
 εT permittivity at constant stress
 εS permittivity at constant strain
• The first subscript to ε indicates the direction of the dielectric displacement
• The second is the direction of the electric field
e.g.
permittivity for dielectric displacement and electric field in
direction 1 (perpendicular to direction in which ceramic element is
polarized), under conditions of constant stress
ƐT
11

19. PIEZOELECTRIC CONSTANTS – CHARGE CONSTANT d
The piezoelectric charge constant, d, is the polarization generated per unit of mechanical
stress (T) applied to a piezoelectric material or, alternatively, is the mechanical strain (S)
experienced by a piezoelectric material per unit of electric field applied.
• The first subscript to d indicates the direction of polarization generated in the material
when the electric field, E, is zero or, alternatively, is the direction of the applied field
strength.
• The second subscript is the direction of the applied stress or the induced strain,
respectively. Because the strain induced in a piezoelectric material by an applied electric
field is the product of the value for the electric field and the value for d, d is an important
indicator of a material's suitability for strain-dependent (actuator) applications.
e.g.
induced polarization in direction 3 (parallel to direction in which
ceramic element is polarized) per unit stress applied in direction 3
or
induced strain in direction 3 per unit electric field applied in
direction 3
d33

20. PIEZOELECTRIC CONSTANTS – VOLTAGE CONSTANT g
The piezoelectric voltage constant, g, is the electric field generated by a piezoelectric material
per unit of mechanical stress applied or, alternatively, is the mechanical strain experienced by
a piezoelectric material per unit of electric displacement applied.
• The first subscript to g indicates the direction of the electric field generated in the material,
or the direction of the applied electric displacement.
• The second subscript is the direction of the applied stress or the induced strain,
respectively.
e.g.
induced electric field in direction 3 (parallel to direction in which
ceramic element is polarized) per unit stress applied in direction 1
or
induced strain in direction 1 per unit electric displacement applied
in direction 3
g31

21. PIEZOELECTRIC CONSTANTS – COUPLING FACTOR k
The electromechanical coupling factor, k, is an indicator of the effectiveness with which a
piezoelectric material converts electrical energy into mechanical energy, or converts
mechanical energy into electrical energy.
• The first subscript to k denotes the direction along which the electrodes are applied.
• The second denotes the direction along which the mechanical energy is applied, or
developed.
k2
eff =
𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑
𝑖𝑛𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦
e.g.
factor for electric field in direction 3 (parallel to direction in which
ceramic element is polarized) and longitudinal vibrations in
direction 3
k33
k does not account for dielectric
losses or mechanical losses, nor for
recovery of unconverted energy
theoretical efficiency

22. COMPARING PIEZOELECTRIC MATERIALS
SOME PIEZOELECTRIC MATERIALS
Symbol Unit BaTiO3 PZT PVDF
Density 103 kg/m3 5,7 7,5 1,78
Relative permittivity Ɛ/Ɛ0 1700 1200 12
Piezoelectric constant d31
10-12 C/N
78 110 23
Voltage constant g31
10-3 Vm/N
5 10 216
Electromechanical constant k31 % at 1 kHz 21 30 12
The piezoelectric constant is lower for polymers as compared to ceramic based piezoelectric
materials.
• When the same amount of voltage applied to polymer and ceramic piezoelectric materials,
the shape change of ceramic based materials are larger than polymers.
• The piezoelectric voltage coefficient of PVDF is about 21 times higher than that of PZT and
40 times higher than that of BaTiO3, therefore PVDF is better for sensor applications.
• The electromechanical coupling constants k31 of PZT is approximately 2.5 times larger than
the electromechanical constant of PVDF which means it is able to convert 2.5 times more
mechanical stress into electrical energy than that PVDF.
Man made piezoelectric ceramics
Man made piezoelectric polymers

23. MANUFACTURING OF PZT
Batch Weighing
High-purity raw materials are evaluated, selected and sourced throughout the world.
Selection criteria, in addition to purity, include material activity and limits on specific
deleterious impurities. Once each material is selected and approved for use, it is precisely
weighed, according to the formulation being manufactured.
Wet Milling
These ingredients are wet-milled together in their proper proportions to achieve a uniform
particle size distribution. Precise control over particle size distribution is necessary to ensure
appropriate material activity during the calcination.
Drying
Following the wet milling process, the product is dried and prepared for calcining.
Calcining
The product must be calcined in high-purity crucibles to guarantee no chemical contaminants
are present in the final product. The calcining operation is carried out in air at about 1000°C,
where the desired PZT phase is formed.

24. MANUFACTURING OF PZT
Wet Milling and Binder Addiction
PZT powder is returned to the mill to ensure homogeneity and to prepare the material for the
addition of an organic binding agent.
Spray Drying
The binder-containing slurry is then fed to a spray dryer, where water is evaporated.
The purpose of spray drying the PZT powder material is to provide a free-flowing product in
the form of binder-containing hollow spheres with a narrow particle size distribution. The
morphology of the PZT material is crucial to consistently fill die cavities in the dry pressing
process when manufacturing piezoelectric ceramics.
Pressing to form “green” piezoelectric ceramic elements
The uniform PZT spheres of appropriate particle size distribution allow for air escapement
throughout the compaction process, yielding lamination-free green ceramic shapes.

25. POPULARITY OF PZT AND APPLICATIONS
PZT, lead zirconate titanate, is the most commonly used piezo ceramic today. In general, piezo
ceramics are the preferred choice because they are:
 physically strong
 chemically inert
 relatively inexpensive to manufacture
 greater sensitivity
 high operating temperature (high Curie point)
 high dielectric constant
 high coupling factor
 high charge sensitivity
 high density with a fine grain structure
 a clean, noise-free frequency response
 flow or level sensors
 ultrasonic nondestructive testing/evaluation (NDT/NDE)
applications
 accurate inspections of automotive, structural or aerospace
products
 ultrasonic cleaners
 sonar devices

26. APPLICATIONS OF PIEZOELECTRIC MATERIALS
Piezoelectric Sensors
A piezoelectric sensor converts a physical parameter,
such as acceleration or pressure, into an electrical
signal. In some sensors the physical parameter acts
directly on the piezoelectric element; in other devices
an acoustical signal establishes vibrations in the
element and the vibrations are, in turn, converted into
an electrical signal. Often, the system provides a visual,
audible, or physical response to the input from the
piezoelectric sensor (e.g. automobile seatbelts lock in
response to a rapid deceleration, piezoelectric pickups
for electrically amplified guitars).
Piezoelectric Generators
Piezoelectric ceramics can generate voltages sufficient to
spark across an electrode gap, and thus can be used as
ignitors in fuel lighters, gas stoves, welding equipment, and
other such apparatus. Piezoelectric ignition systems are small
and simple.

27. APPLICATIONS
Piezo Actuators
A piezo actuator converts an electrical signal into a
precisely controlled physical displacement, to finely
adjust precision machining tools, lenses, or mirrors.
Actuators also are used to control hydraulic valves, act as
small-volume pumps or special-purpose motors, and in
other applications.
Piezoelectric Transducer
Piezoelectric transducers convert electrical energy into
vibrational mechanical energy (often sound or
ultrasound) and vice versa. Because the piezoelectric
effect is reversible, a transducer can both generate an
ultrasound signal from electrical energy and convert
incoming sound into an electrical signal.
Piezoelectric transducers are used to generate ultrasonic
vibrations for cleaning, atomizing liquids, drilling or
milling ceramics or other difficult materials, welding
plastics, medical diagnostics, integrated into park
distance control and other use.

28. APPLICATIONS
Power generating sidewalk
• Charging pads under the cross walk
collect energy from the vibrations.
• Piezoelectric charging panels channel
energy to lithium ion batteries (which
can be used further).
Floor mats and people powered dance clubs
• Series of crystals can be laid below the floor
mats, tiles and carpets.
• One footstep can only provide enough electrical
current to light two 60watt bulbs for one
second.
• When mob uses the dance floor, an enormous
voltage is generated.
• This energy is used to power the equipment of
nightclubs.

29. REFERENCES
• Alternative Resources for Renewable Energy: Piezoelectric and Photovoltaic Smart Structures - D.
Vatansever, E. Siores and T. Shah
• Materials Science and Engineering An Introduction - William D. Callister, Jr.
• Piezoelectricity: Basics and applications - Petar Jurcevic
• http://didel.script.univ-paris-
diderot.fr/claroline/backends/download.php?url=L0FyY2hpdi90dXRvcmlhbF9waWV6b18yLnBkZg%3D
%3D&cidReset=true&cidReq=36UAHB543
• https://www.americanpiezo.com/
• http://www.piceramic.com/piezo-technology/fundamentals.html
• http://knowledge.ulprospector.com/2689/pe-piezoelectric-materials/
• http://piezotechnologies.com/knowledge-desk

30. QUESTIONNAIRE
1. What does it mean piezoelectric effect?
2. Which are the conditions for a material to exhibit the piezoelectric effect?
3. Why is necessary to polarize a material to make it piezoelectric?
4. How is it possible to depolarize a piezoelectric material?
5. Why piezo ceramics are usually most used?
6. Please write some common applications of piezoelectric materials.

31. THANKS FOR THE ATTENTION