Piezoelectricity

Question 1

How does an ultrasound transducer generate and detect sound waves?

Answer 1

By using an electrical signal and movement (vibration) which is achieved using piezoelectric materials

Question 2

What do piezoelectric transducers do?

Answer 2

Convert energy from electrical to mechanical or vice versa

Question 3

What do piezoelectric transducers do in ultrasound?

Answer 3

They transmit and receive acoustic (mechanical) waves

Question 4

What does a mechanical strain result in (direct piezoelectric effect)?

Answer 4

A net electrical polarisation (a voltage)

Question 5

What is inverse (converse) piezoelectric effect?

Answer 5

Applying a potential difference results in compression or extension, depending on the polarity

Question 6

What is generation?

Answer 6

Time varying (AC) voltage results in acoustic wave emission (V -> nm)

Question 7

What is detection?

Answer 7

Time varying stress due to acoustic wave produces voltage (nm -> mV)

Question 8

What are examples of piezoelectric materials?

Answer 8

Quartz (weakly piezoelectric, no intrinsic dipole moment)

Piezoelectric ceramics (PZT, strongly piezoelectric and requires poling to become piezoelectric)

Piezoelectric polymers (PVDF, thin film that has close acoustic impedance match to tissue)

Piezocomposites (pieces of PZT embedded in polymer material)

Question 9

What makes a material piezoelectric?

Answer 9

The polarisation of the material (either strength or direction) changes when a mechanical stress is applied

Question 10

What happens when the crystal structure of piezoelectric material is deformed?

Answer 10

It produces a dipole moment (separation of the positive and negative electrical charges)

Question 11

What are ferroelectrics?

Answer 11

Piezoelectric materials that can have the polarity of their domains reorientated through the application of an electric field

Question 12

What are ferroelectric materials widely used for?

Answer 12

Ultrasound transducers

Question 13

What does the bulk material, polycrystalline contain?

Answer 13

Domains each with a net dipole moment (and polar axis)

Question 14

What is the orientation of the domain of ferroelectric materials?

Answer 14

Domains are randomly orientated so weak macroscopic piezoelectric response

Question 15

What significantly increases the overall piezoelectric response?

Answer 15

Poling

Question 16

What is the initial state of ferroelectric materials?

Answer 16

Their domains are randomly orientated

Dipole moment cancel

Weak (or no) macroscopic piezoelectric response

Question 17

What happens during poling?

Answer 17

The temperature is elevated ( > Curie temperature , ~ 200 degrees)

Using application of large DC voltage (2000 V/mm) which causes domains to align with electric field produced by voltage

Question 18

What happens to ferroelectric materials after they have been poled?

Answer 18

Domain remains frozen in near-alignment

Material is now strongly piezoelectric as dipole moments are better aligned

They can now be polarised in any direction

Question 19

What is the extra step needed in poling piezoelectric polymers (PVDF)?

Answer 19

The material needs to be mechanically stretched (as non polar) in one or both lateral directions which changes molecular conformation

Then material can be poled to align polarisation

Question 20

What are the characteristic properties of piezoelectric materials?

Answer 20

d: transmission constant (strain constant, m V^-1)

g: receiving constant (voltage constant, VmN^-1)

k: electromechanical coupling factor (effectiveness of conversion of electrical energy to mechanical energy)

Question 21

Why is PZT generally used as an ultrasound transmitter but not receiver ?

Answer 21

High d value and k value

Low g value makes it less suitable for receive only application

It has large impedance mismatch to water leading to narrow bandwidth (high-Q)

Question 22

What makes PVDF a good receive only material?

Answer 22

High g value

but has low d and k value so not used as transmitter

Impedance is close to water which leads to broad bandwidth (low-Q)

Question 23

How is the acoustic impedance matched to water in polymers (e.g. PZT) ?

Answer 23

By embedding piezoelectrical material in polymers so that they are more efficient sources of ultrasound