Ultrasound Physics
Welcome
Ultrasound Basics
Vibration and Wave
Ultrasound Parameters
Medium Acoustic Property
Ultrasound Reflection
Ultrasound Refraction
Ultrasound Scattering
Ultrasound Attenuation
Ultrasound Application
Ultrasound Transducer
Piezoelectric Effect
Transducer Cosntruction
Array Transducer
Beamforming
Ultrasound Beamformation
Beam Focus
Beam Steering
Imaging
Pulse-echo Method
Imaging Method
Imaging Resolution
Ultrasound Imaging Artifacts
Signal and Circuit
Unipolar Transmitter
Bipolar Transitter
Transceiverg
Time Gain Control
Conditioning
Preprocessing and Postprocessing
Flow Dection
Doppler Effect
Continue Wave Doppler (CW)
Pulse wave Doppler(PW)
Color Flow Imaging
Safety
Intensity
Mechanical Index
Thermal Index
Cavitation
Regulations
Piezoelectric Effect
Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential in response to applied mechanical stress. The material that shows piezoelectricity is called piezoelectric material. Applied electrical charge on both sides of a piece of piezoelectric material, it will cause stress inside and thus generate deform. If the electrical charge is alternative, the piece of material will oscillate and generate mechanical wave. The piezoelectric material has a special structure that will cause positive and negative charge center mismatch when an external stress is introduced from certain direction. Piezoelectric ceramic have many small regions inside it, called “domain”, and each domain has its own piezoelectric direction. When an external stress is introduced, some domains give positive charge if they are lined up according to the stress direction; some domains may give very minimal charge if its own direction is perpendicular to the stress direction; and some domains will give negative charge if it is against the stress direction. The domains are very small at level of a few microns to hundreds microns, and normally they are randomly distributed, without special processing to line up all the
domains,
the material will not show piezoelectric as a whole piece. The processing is called poling, use a high DC voltage applied on both sides of the piece of material for a short duration of time, such as 1 to 10 seconds. Different material needs different voltage to reverse the domains, and this voltage is called coercive voltage. Pure piezoelectric crystal may be a single domain and doesn’t need poling.
Curie temperature: When temperature is high enough, the piezoelectric domains inside ceramic will have such a high kinetic energy and it will break away from the poling direction and resume to its original random direction. This temperature is called Curie temperature. Piezoelectric ceramic will lose its piezoelectricity when its temperature is above its Curie temperature.
Kt: It is thickness mode mechanical-electrical coupling efficient, the key indicator of piezoelectricity performance of the material in thickness mode. By definition, it is the ratio of energy send out to the energy stored by the material. Without piezoelectricity, a ceramic plate with two sides coated with electrodes will behave as a
capacitor,
the impedance will only have imaginary part, no real part. The current go through it and the voltage applied on it will be always 90 degree to each other and thus no energy is emit out but all stored and released. With piezoelectricity effect, at resonant frequency, the impedance will have real part and imaginary part, the real part will consume electrical energy, convert it into acoustic.
Common piezoelectric materials: commonly used piezoelectric materials are ceramics, crystals and polymers. Crystal usually has lower
Kt,
and it not good for thickness mode, but good in bar mode. Ceramic has a better Kt,
good
in thickness mode. Both of crystal and ceramic have high acoustic impedance, usually above 30Mryls. Matching layers are required to transmit acoustic