The piezoelectric effect

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 (BaTiO 3 ) Tendon Gallium orthophosphate (GaPO 4 ) Sucrose Potassium niobate (KNbO 3 ) Silk Lead titanate (PbTiO 3 ) Enamel Lithium tantalate (LiTaO 3 ) Dentin Langasite (La 3 Ga 5 SiO 14 ) 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: A 2 +B 1 +0 2- 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 T etragonal 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 D ielectric 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 P s . 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 P r . Polarization P [C/m 2 ] Electric field E [kV/mm] 0,5 1,5 1 0,1 -0,5 -1,5 -1 0,2 0,3 -0,1 -0,2 -0,3 P s -P s -P r ≡-D r P r ≡D r

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 S 3 will be accompanied by a relative decrease (or increase) in the sample's dimension perpendicular to the field (S 1 and S 2 ) equal to about half the change in S 3 . Electric field E [kV/mm] 0,5 1,5 1 -0,5 -1,5 -1 5 10 15 ·10 -4 S 3 10 20 30 ·10 -4 -S 1 P≡ P r Mechanical deformation S 3 in the d irection of polarization and field, as well as S 1 and S 2 normal to this direction as a function of field strength for a “soft” PZT. The S 1 curve is based on measurement, S 3 is given by S 3 -2S 1 : - 2S 2 .

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 : t he direction of positive polarization is usually chosen to coincide with the Z-axis t he 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. ε T permittivity at constant stress ε S permittivity at constant strain The first subscript to ε indicates the direction of the dielectric displacement T he 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 Ɛ T 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 d 33

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 g 31

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. T he second denotes the direction along which the mechanical energy is applied, or developed. k 2 eff = e.g.   factor for electric field in direction 3 (parallel to direction in which ceramic element is polarized) and longitudinal vibrations in direction 3 k 33 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 BaTiO 3 PZT PVDF Density 10 3 kg/m 3 5,7 7,5 1,78 Relative p ermittivity Ɛ/Ɛ 1700 1200 12 Piezoelectric constant d 31 10 -12 C/N 78 110 23 Voltage constant g 31 10 -3 Vm /N 5 10 216 Electromechanical constant k 31 % 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 BaTiO 3 , therefore PVDF is better for sensor applications. The electromechanical coupling constants k 31 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 d eleterious 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 What does it mean piezoelectric effect? Which are the conditions for a material to exhibit the piezoelectric effect? Why is necessary to polarize a material to make it piezoelectric? How is it possible to depolarize a piezoelectric material? Why piezo ceramics are usually most used? Please write some common applications of piezoelectric materials.

31. THANKS FOR THE ATTENTION