General
Quartz is one member of a family of crystals that experience the piezoelectric effect. The piezoelectric effect has found applications in high power sources, sensors, actuators, frequency standards, motors, etc., and the relationship between applied voltage and mechanical deformation is well known; this allows probing an acoustic resonance by electrical means. Applying alternating current to the quartz crystal will induce oscillations. With an alternating current between the electrodes of a properly cut crystal, a standing shear wave is generated. The Q factor, which is the ratio of frequency and bandwidth, can be as high as 106. Such a narrow resonance leads to highly stable oscillators and a high accuracy in the determination of the resonance frequency. The QCM exploits this ease and precision for sensing. Common equipment allows resolution down to 1 Hz on crystals with a fundamental resonant frequency in the 4 – 6 MHz range. A typical setup for the QCM contains water cooling tubes, the retaining unit, frequency sensing equipment through a microdot feed-through, an oscillation source, and a measurement and recording device.
The frequency of oscillation of the quartz crystal is partially dependent on the thickness of the crystal. During normal operation, all the other influencing variables remain constant; thus a change in thickness correlates directly to a change in frequency. As mass is deposited on the surface of the crystal, the thickness increases; consequently the frequency of oscillation decreases from the initial value. With some simplifying assumptions, this frequency change can be quantified and correlated precisely to the mass change using Sauerbrey's equation. Other techniques for measuring the properties of thin films include Ellipsometry, Surface Plasmon Resonance (SPR) Spectroscopy, and Dual Polarisation Interferometry.
Read more about this topic: Quartz Crystal Microbalance
Famous quotes containing the word general:
“There is absolutely no evidence—developmental or otherwise—to support separating twins in school as a general policy. . . . The best policy seems to be no policy at all, which means that each year, you and your children need to decide what will work best for you.”
—Pamela Patrick Novotny (20th century)
“Amid the pressure of great events, a general principle gives no help.”
—Georg Wilhelm Friedrich Hegel (1770–1831)
“The reputation of generosity is to be purchased pretty cheap; it does not depend so much upon a man’s general expense, as it does upon his giving handsomely where it is proper to give at all. A man, for instance, who should give a servant four shillings, would pass for covetous, while he who gave him a crown, would be reckoned generous; so that the difference of those two opposite characters, turns upon one shilling.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)