piezoelectric and pyroelectric transducers · piezoelectric and pyroelectric transducers ... • in...

Piezoelectric and Pyroelectric

transducers

UNIVERSITÀ DEGLI STUDI DI CATANIA DIPARTIMENTO DI INGEGNERIA ELETTRICA, ELETTRONICA E INFORMATICA

(DIEEI)

•Piezoelectric transducers are based on the property of accumulating charges if stressed (direct effect) and to strain in case of an electric signal is applied across their electrodes (inverse effect).

Piezoelectric transducers

Both effects were discovered by Jacques and Pierre Curie in 1880-1881.

Historical summary

Pyroelectric effect (Brewster, 1824 )

Piezoelectric materials (J. and P. Curie, 1880) Ferroelectric materials (Valasek, 1920, Rochelle salt)

Slater’s theory (polarization due to atoms of hydrogen, 40’s) until BaTiO3 has been discovered

Perovskite (KNbO3, KTaO3, LiNbO3 ,LiTaO3 e PbTiO3 , 50’s); from microscopic approach to macroscopic one (as the termodynamic one, Ginziburg and Devonshire)

PZT ceramics (1956)

60’s – 70’s return to crystal structure phase transition concept and first applications

Crystallographic classes

anisotropic

Piezoelectricity is related to the crystalline (ionic) structure.

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Polarization in the same direction as the stress

Perpendicular polarization

Null polarization

Piezoelectricity is due to asymmetries in the crystallographic structure.

Piezoelectric materials

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•An example: the barium titanate (BaTiO3) ceramic


• The natural piezoelectric materials most frequently used are quartz and tourmaline. • The synthetic materials more extensively used are not crystalline but ceramics.

Piezoelectric materials •An example: the barium titanate (BaTiO3) ceramic

The Curie point is about 130°C. Above 130°C, a nonpiezoelectric cubic phase is stable, where the center of positive charge (Ba2+ and Ti4+) coincides with the center of the negative charge (O2–) (Figure a). When cooled below the Curie point, a tetragonal structure (shown in Figure b) develops where the center of positive charge is displaced relative to the O2–ions, leading to the formation of electric dipoles.

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Piezoelectric Cristals

• Piezoelectric behavior in lithium niobate (LiNbO3) and lithium tantalate

(LiTaO3) was first studied in the mid-1960s. Both have ε values of approximately 40. If cut correctly, they have coupling coefficient (k) values of 0.65 and 0.4, respectively. In addition, the Curie points for both are extremely high (T0 ~ 1210°C for LiNbO3, and 620°C for LiTaO3).


Piezoelectric Ceramics Perovskites • Perovskite is the name given to a group of materials with general formula ABO3

having the same structure as the mineral calcium titanate (CaTiO3), barium titanate (BaTiO3), lead titanate (PbTiO3), lead zirconate titanate (PbZrxTi1-xO3, or PZT), lead lanthanum zirconate titanate [Pb1-xLax(ZryT1-y)1-x/4O3, or PLZT], and lead magnesium niobate [PbMg1/3Nb2/3O3, or PMN].

• These single-crystal relaxor materials are now being intensively investigated and show great promise for future generations of piezoelectric transducers and sensors.

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Piezoelectric Ceramics

• Originally piezoelectric ceramics shows a random orientation of dipoles leading to a null polarization

• In order to obtain a preferential axis (polar axis) a polling process is required..

•1) heating the material close to the Curie temperature (slightly above)

•2) applying an electric field (10kV/cm) parallel to the polar axis (the field depends on the

material thickness)

•3) cooling the material exposed to the electric field


Some polymers lacking central symmetry also display piezoelectric properties with a value high enough to consider them for those applications where because of the size and shape required it would be impossible to use other solid materials. The most common is polyvinylidene fluoride (PVF2 or PVDF), whose piezoelectric voltage coefficient is about four times that of quartz, and its copolymers. Electrodes are screen printed or vacuum deposited.

Polymers

• In order to improve the mechanical properties for piezoelectric sensors, piezoelectric ”composite” materials are used. They are heterogeneous systems consisting of two or more different phases , one of which at least shows piezoelectric properties.

Modeling…..


Piezoelectric equations describe the relationship between electric and mechanical quantities

In case of a dielectric nonpiezoelectric material:

strain F

compliance, (1/s is Young’s modulus)

stress T=F/A

A potential difference applied between plates creates an electric field E

displacement vector or electric flux density

dielectric constant

polarization vector

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constant ricpiezoelect(C/N)d

field-E constantatcompliances

stressconstant at constantdielectric

dETsS

EdTD

E

T

E

T

Modeling…..

Electric field Stress Polarization

Strain


In case of a piezoelectric the mutual effects come to play:

In real devices 6 possible axis must be considered: 3 for stress due to compression/expansion 3 for torsional stress

Modeling…..


the first index defines the polarization axis while the second index states for the stress direction

Modeling…..


g = ̂d/ɛT is the piezoelectric voltage coefficient.

electromechanical coupling coefficient

Ex. In case of a torsional stress of 1N/m2 applied to axis 2 (direction 5) will produce

a charge density of 515 pC/m2 along direction 1.

The d15 and d33 coefficients of BaTiO3 are 270 and 191 10–12 C N–1, respectively. The k for BaTiO3 is approximately 0.5. Calcium-doped PbTiO3 has a relative dielectric constant e33 of 200, a d33 of 65 10–12 C/N, and a k of approximately 0.5. The addition of calcium results in a lowering of the Curie point to 225°C.

PbTiO3 shows to posses excellent piezoelectric properties when oriented along the [001] direction. The piezoelectric charge coefficient d33 of 25 × 10–10 C N–1 , coupling coefficient k of more than 0.9, and ultrahigh strain of 1.7% were achieved in Pb(Zn1/3Nb2/3)O3-PbTiO3 solid solution.


Examples

Lead Titanate

Barium Titanate

Modeling…..


PbTiO3

Modeling…..


h

w

Fz

l

Ez

Dh

Amplifier

cable

E1 Eo

Piezoelectric transducers Modeling…..

•In the high frequency domain the relationship between V and F is frequency

independent

•Forcing the device with a known force and measuring V it is possible to

estimate d33

•In case of a unknown C a high value capacitor can be added in parallel

Piezoelectric transducers Modeling…..

The high output impedance of the piezoelectric can cause coupling problem with the

amplifier

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Rosettes of piezoelectrics exist to detect deformation in two or three direction


Piezoelectric transducers Advantages •High stiffness, to measure force. •High resonant frequency (up to 500 kHz) • Stability, reproducibility and linearity •Large operating temperature range •Low sensitivity to external magnetic field Drawbacks •Curie Temperature, Tc •Resonant behavior •High output impedance but never infinite: a constant stress initially generates (or, better, displaces) a charge that will slowly drain off as time passes. There is no dc response. Hence: •Cannot be used to detect static quantities

Pyroelectric sensors • The pyroelectric effect is analogous to the piezoelectric effect, but now it refers to

change in temperature causing change in spontaneous polarization and resulting change in electric charge.

If ΔT is uniform throughout the material, the pyroelectric effect can be described by the equation:

spontaneous polarization pyroelectric coefficient

When the detector absorbs radiation, its temperature and hence its polarization changes, thus resulting in a surface charge on the capacitor plates charge induced:

thermal radiation

Cd Ad

b

resulting voltage:

Pyroelectric materials

• pyroelectricity is based on crystal anisotropy

linear

The polarization of linear materials cannot be changed by inverting the E-field

tourmaline

lithium sulfate

cadmium and selenium sulfides

lithium tantalate

strontium and barium niobate

lead zirconate-titanate (PZT)

triglicine sulfate

• Pyroelectric properties disappear at the Curie temperature

Pyroelectric materials

Common applications

detection of thermal radiation at ambient temperature: • pyrometers (noncontact temperature meters in furnaces, melted glass or metal,

films, and heat loss assessment in buildings) • radiometers (measurement of power generated by a radiation source)

IR analyzers Intruder and position detection Fire detection High-power laser pulse detection High-resolution thermometry

piezoelectric and pyroelectric transducers · piezoelectric and pyroelectric transducers ... • in...

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