Piezoelectricity Flashcards
What is the range of charge coefficients on ultrasonic receivers?
2-400 pC/N
What is the range of electromechanical coupling factors for ultrasonic transmitters?
0.1-0.8
What is the range of electrical permittivity for ultrasonic transmitters?
5-2,000
Explain how the piezoelectric effect occurs in terms of dipole moments.
- At rest dipoles are evenly distributed, no net charge on the surface.
- When stress is applied, the moments reorientate themselves causing a change in the surface charge density.
- When a voltage is applied, the dipole shapes change causing a change in the crystal dimensions.
What is the chemical formula for PZT?
PbZr_xTi_x-1O_3
x is usually 0.52 for ultrasound elements as theis represent the morphotropic phase boundary.
What is the density and speed of sound of PZT? From that calculate it acoustic impedance.
p=7,500kg/m^3, c=4,500m/s
Z=pc= 34MRayl
Describe the construction of a single element transducer.
- A thin circular piezoelectric plate (PZT) is coated on both sides with metallic paint to act as electrodes.
- A quarter-wavelength matching layer is attached to the front (waves sum on the transmit side, and cancel on the PZT side).
- A backing layer is stuck behind to dampen ringing of the plate.
- A tuning coil is attached between the two electrodes so that the applied alternating voltage can be adjusted to match the resonance frequency of the plate.
- The whole device is encased in a plastic shield to reduce leakage current, and then encased in a metal shield to protect it from damage.
Explain transducer resonance.
A Transducer is most efficient at halfwave resonance. At this point, the ultrasound wave travelling through the PZT will constructively interfere with the contractions and expansions of the crystal driven by the applied voltage. Creating a higher amplitude wave for less energy input.
What is the equation for ultrasonic focusing using a lens?
F=R/{(c2/c1)-1}
- F is the focal length,
- R is the radius of curvature.
- c1 and c2 are the speeds of sound for the lens and the focusing medium respectively.
Apart from using a lens, how else can a single element transducer be focused?
Using a convex element.
What is the ideal acoustic impedance of the quarter wave matching layer?
It should be the geometric mean of the two adjacent media (PZT and Tissue or Lens).
Z_ml=(Z_pzt*Z_lens)^0.5
At what point is the transducer bandwidth measured?
-3dB points either side of the peak.
How do the backing and matching layers affect the pulse bandwidth, amplitude, and duration?
Broaden the bandwidth, increase the amplitude, shorten the duration.
What are the geometric requirements for a rectangular thickness expanding element?
w»d and L»d
What are the geometric requirements for a circular thickness expanding element?
2r»d
