Chapter 8: Piezoelectricity and transducers Flashcards
What is the direct piezoelectric effect?
A voltage can be generated by applying either a compressive or tensile force to a piezoelectric material where the voltages are opposite in polarity
the separation of two charges is often called a ____ _____(such a dipole will try to align itself with an external electric field.)
A dipole moment
in some crystal structures the symmetry of the molecules means that every dipole is ___ by an __and ___dipole so these materials arent piezoelectric
cancelled,
equal
opposite
What is pyroelectric?
they have a temperature dependent net polarisation even in the absence of an applied stress or electric field.
what is the electric equivalent of a bar magnet
an electret
what happens when a pyroelectric material is heated?
rearrangement of the charges in the crystal due to thermal expansion and this can be detected as a change in the polarisation of the crystal.
How quickly does the heating of the pyroelectric material need to be carried out so that it can be detected?
faster than the leakage current
Ferroelectric materials - what are they?
their polarisation can be changed by applying an external electric field
What is poling?
The piezoelectricity of macroscopic samples can be permanently increased through applying electric fields which is a process called poling. (aligning all the domains remember the video we saw before?)
How do you go about the process of poling?
- Heat the material beyond Curie temperature.
- Apply a strong electric field to orient the domains in the same direction.
- Cool the material.
- Remove the electric field, leaving the polarisations of the domains more or less aligned.
Which particular piezoceramic is used mostly in ultrasound?
PZT. ( Lead zirconate titanate)
What are the three most important physical constraints for piezoelectric materials to consider?
- The Transmission Coefficient (Strain constant) - strain produced per unit of applied electric field
- Recieving constant (Voltage constant) - electric field generated per unit of applied stress.
- Electromechanical coupling factor k - dimensionless constant characterising the effectiveness of conversion of electrical energy to mechanical energy and vice versa.
Way to remember it - conversion of strain to electricity, vice versa and effectiveness.
PZT - has a (high/low) transmission constant?
- has a (high/low) coupling coefficient?
- has a (high/low) recieving constant?
and what do these mean for its effectiveness as a transducer?
high
high
low
It is a great transmitter however a terrible receiver.
why does the PZT have such a small bandwidth?
because of its bad impedance mismatch to water, there are many reflections
PVDF - as an ultrasound transducer? it is a good idea or not?
PVDF has a very high detection sensitivity and has a closer impedance match to water which make it excellent for recieve only applications.
However low transmission constant and coupling coefficient mean that PVDF is rarely used as an ultrasound transmitter.
How can piezoelectric ceramics improve its mismatch in acoustic impedance with water?
by forming piezoelectric composites!
(Piezoelectric material is diced, into sections, forming either slabs or pillars of piezoelectric material. The spaces are then filled with a polymer material, which is much softer than the piezoelectric material. This gives an effective acoustic impedance much closer to water.
the two ways in which the frequency response of a transducer is altered are?
1) adhering a block of material to the back face of the transducer element and 2) adhering a layer on the front of the transducer element.
The sensitivity is determined by what?
by the choice of piezoelectric element as well as the properties of the connected electronics.
What is a matched load transducer?
A thought experiment in that the surrounding medium has the same acoustic properties as the piezoelectric element.
What is the FWHM and what is it used for?
The full width of the lobe at half the maximum amplitude.
what waves are involved in the Air-backed transducer?
3 waves, the outward wave, the inward waves from the rear face and the front face of the transducer.
what is a limitation of the quarter wave matching layer?
will only transmit perfectly at the one frequency that satisfies lambda/4
