Chapter 8: Piezoelectricity and transducers

Question 1

What is the direct piezoelectric effect?

Answer 1

A voltage can be generated by applying either a compressive or tensile force to a piezoelectric material where the voltages are opposite in polarity

Question 2

the separation of two charges is often called a ____ _____(such a dipole will try to align itself with an external electric field.)

Answer 2

A dipole moment

Question 3

in some crystal structures the symmetry of the molecules means that every dipole is ___ by an __and ___dipole so these materials arent piezoelectric

Answer 3

cancelled,
equal
opposite

Question 4

What is pyroelectric?

Answer 4

they have a temperature dependent net polarisation even in the absence of an applied stress or electric field.

Question 5

what is the electric equivalent of a bar magnet

Answer 5

an electret

Question 6

what happens when a pyroelectric material is heated?

Answer 6

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.

Question 7

How quickly does the heating of the pyroelectric material need to be carried out so that it can be detected?

Answer 7

faster than the leakage current

Question 8

Ferroelectric materials - what are they?

Answer 8

their polarisation can be changed by applying an external electric field

Question 9

What is poling?

Answer 9

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?)

Question 10

How do you go about the process of poling?

Answer 10

1. Heat the material beyond Curie temperature.
2. Apply a strong electric field to orient the domains in the same direction.
3. Cool the material.
4. Remove the electric field, leaving the polarisations of the domains more or less aligned.

Question 11

Which particular piezoceramic is used mostly in ultrasound?

Answer 11

PZT. ( Lead zirconate titanate)

Question 12

What are the three most important physical constraints for piezoelectric materials to consider?

Answer 12

1. The Transmission Coefficient (Strain constant) - strain produced per unit of applied electric field
2. Recieving constant (Voltage constant) - electric field generated per unit of applied stress.
3. 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.

Question 13

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?

Answer 13

high
high
low

It is a great transmitter however a terrible receiver.

Question 14

why does the PZT have such a small bandwidth?

Answer 14

because of its bad impedance mismatch to water, there are many reflections

Question 15

PVDF - as an ultrasound transducer? it is a good idea or not?

Answer 15

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.

Question 16

How can piezoelectric ceramics improve its mismatch in acoustic impedance with water?

Answer 16

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.

Question 17

the two ways in which the frequency response of a transducer is altered are?

Answer 17

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.

Question 18

The sensitivity is determined by what?

Answer 18

by the choice of piezoelectric element as well as the properties of the connected electronics.

Question 19

What is a matched load transducer?

Answer 19

A thought experiment in that the surrounding medium has the same acoustic properties as the piezoelectric element.

Question 20

What is the FWHM and what is it used for?

Answer 20

The full width of the lobe at half the maximum amplitude.

Question 21

what waves are involved in the Air-backed transducer?

Answer 21

3 waves, the outward wave, the inward waves from the rear face and the front face of the transducer.

Question 22

what is a limitation of the quarter wave matching layer?

Answer 22

will only transmit perfectly at the one frequency that satisfies lambda/4