What is a transducer?
device converting non-electrical into electrical signals
i.e. electrical transmitted pulses into US pulses & US echo to electrical signals
in the piezoelectric effect; materials expand or …. when a postive (lead, ….. & titanium) or a …. (O2 ions) voltage is applied
1) contract
2) Zirconium
3) negative
When is a drop produced in relation to the probe and material?
When material is stretched or compressed by external force
Describe Dipolar molecules
+ve charge one end, -ve at the other
What does +ve poles move towards, and what does this do to alignment?
moves towards -ve electrodes. If voltages change across element so does alignment = effecting thickness
What is the most common material used for manufacturing medical imaging PZT probes?
synthetic polycrystalline (crystalline grains)/ ceramic material (lead, zirconate, titanate)
Why is this PZT plate/probe used (crystalline grains)?
- high sensitivity
- cope w/ large acoustic powers
What is an advantage of crystalline grains PZT?
- can be formed to a curve or flat shape
- elements fired to make rigid (original powder form)
After firing, +ve charged lead, zirconium and titanium ions and -ve charged O2 ions are …… so each …. contains +ve net and -ve net charge. They are fixed…. but equal distance apart.
1) arranged
2) net
3) opposite
What are PZT divided into and polarised (orientated in the same direction)?
regional numbers (AKA domain)
How is PZT polarised to make a uniform direction? (image 3.3a - no net polarisation)
- materials heated >200 degrees C, applying strong electrical field across it = poling
- Makes the dipoles (in the domain) switch to direction closest to direction of poling electric field
- when cooled ‘frozen’ to form near orientation –> parallel to poling field.
what are the advantages of modified PZT?
- increased BW
2. Increased sensitivity
With modified PZT, there are narrow spaces/channels filled with polymer (image 3.34). What does this help with?
the columns can vibrate more efficiently, reduced density of polymers resulting in reduced overall density and reduced Z
Why is the thickness of the plate chosen to be 1/2 of λ at the centre f of transmit pulse?
makes it expand and contract strongly when alternating voltages at that f are applied = 1/2 Resonance
Why does 1/2 resonance occur (producing a large amplitude)?
reverberating wave traveling back and forth across the plate will travel exactly 1 λ when arriving back at starting position.
What are PDVF (plastic polyvinyldifluroide) used in and its properties?
used in: membrane hydrophones
- wide BW
- short impulse response
- greater focusing
- increased sensitivity
What elements are single crystal transducers doped with?
either 1) lead, magnesium and niobium (PZT-PT)
2) lead, zinc and niobium (PZN-PT)
What does single crystal transducer do that ceramic material PZT transducer can’t?
during polarisation of ceramic PZT, structure limits alignment of domains
Single crystal - has no grain boundaries can be poled to give perfect alignment of dipoles
What advantages does single crystal transducer have?
- wider BW
- improved axial resolution
- -> used for multi-f and harmonic imaging
mirocrmachined US transducers (MUT’s) have better …., wider …. providing shorter US pulses, this improving ….. …..
1) Z
2) BW
3) axial res
What does this image show and what is required?
capacitive micro-machined transducer (CMUT)
steady bias voltage between electrodes
What can happen if increasing the bias voltage in CMUT?
increased beyond a limit (collapse voltage limit) causes membrane to contact with the base = electrical breakdown or damage
When is the > efficiency obtained?
when membrane is close to base (collapse voltage). Solved with placing small structure beneath membrane
What is this image?
peizoelectric micro-machined US transducer (PMUT)
