Piezoelectricity Flashcards
How does an ultrasound transducer generate and detect sound waves?
By using an electrical signal and movement (vibration) which is achieved using piezoelectric materials
What do piezoelectric transducers do?
Convert energy from electrical to mechanical or vice versa
What do piezoelectric transducers do in ultrasound?
They transmit and receive acoustic (mechanical) waves
What does a mechanical strain result in (direct piezoelectric effect)?
A net electrical polarisation (a voltage)
What is inverse (converse) piezoelectric effect?
Applying a potential difference results in compression or extension, depending on the polarity
What is generation?
Time varying (AC) voltage results in acoustic wave emission (V -> nm)
What is detected in piezoelectric effect?
Time varying stress due to acoustic wave produces voltage (nm -> mV)
What are examples of piezoelectric materials?
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)
What makes a material piezoelectric?
The polarisation of the material (either strength or direction) changes when a mechanical stress is applied
What happens when the crystal structure of piezoelectric material is deformed?
It produces a dipole moment (separation of the positive and negative electrical charges)
What are ferroelectrics?
Piezoelectric materials that can have the polarity of their domains reorientated through the application of an electric field
What are ferroelectric materials widely used for?
Ultrasound transducers
What does the bulk material, polycrystalline contain?
Domains each with a net dipole moment (and polar axis)
What is the orientation of the domain of ferroelectric materials?
Domains are randomly orientated so weak macroscopic piezoelectric response
What significantly increases the overall piezoelectric response?
Poling
What is the initial state of ferroelectric materials?
Their domains are randomly orientated
Dipole moment cancel
Weak (or no) macroscopic piezoelectric response
What happens during poling?
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
What happens to ferroelectric materials after they have been poled?
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
What is the extra step needed in poling piezoelectric polymers (PVDF)?
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
What are the characteristic properties of piezoelectric materials?
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)
Why is PZT generally used as an ultrasound transmitter but not receiver ?
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)
What makes PVDF a good receive only material?
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)
How is the acoustic impedance matched to water in polymers (e.g. PZT) ?
By embedding piezoelectrical material in polymers so that they are more efficient sources of ultrasound
