Piezoelectric EffectCrystals which acquire a charge when compressed, twisted or distorted are said to be piezoelectric. This provides a convenient transducer effect between electrical and mechanical oscillations. Quartz demonstrates this property and is extremely stable. Quartz crystals are used for watch crystals and for precise frequency reference crystals for radio transmitters. Rochelle salt produces a comparatively large voltage upon compression and was used in early crystal microphones. Barium titanate, lead zirconate, and lead titanate are ceramic materials which exhibit piezoelectricity and are used in ultrasonic transducers as well as microphones. If an electrical oscillation is applied to such ceramic wafers, they will respond with mechanical vibrations which provide the ultrasonic sound source. The standard piezoelectric material for medical imaging processes has been lead zirconate titanate (PZT). Piezoelectric ceramic materials have found use in producing motions on the order of nanometers in the control of scanning tunneling microscopes. The word piezo is Greek for "push". The effect known as piezoelectricity was discovered by brothers Pierre and Jacques Curie when they were 21 and 24 years old in 1880. There is a magnetic analog where ferromagnetic materials respond mechanically to magnetic fields. This effect, called magnetostriction, is responsible for the familiar hum of transformers and other AC devices containing iron cores. |
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Scanning Tunneling MicroscopeThe subject of a 1986 Nobel Prize, the scanning tunneling microscope is currently the most powerful and versatile tool for imaging individual atoms. If a pointed metal probe is placed sufficiently close to a solid sample and a voltage of say 10 millivolts is applied between the probe and the surface, then electron tunneling can occur. The separation between the mini-tip of the probe and the sample must be on the order of nanometers. The exponential variation of the tunneling current with separation can show surface variations in the range 0.01 nanometers. The sharpening of the mini-tip of the probe is accomplished with electrochemical etching. This etching sharpens it to only about 1000 nm, but since the sharpening is not perfectly smooth, it leaves a surface with many mini-tips which are on the scale of nanometers. Positioning a probe over the nanometer range can be done with piezoelectric ceramic wafers. Gert Binnig and Heinrich Rohrer were awarded the Nobel Prize in 1986 along with Ernst Ruska for his work in developing electron microscopy. |
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