Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Viscosity
Patent
1997-09-25
1999-12-28
Brock, Michael
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Viscosity
73 32A, 310316, G01N 1110
Patent
active
060065893
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION OF THE INVENTION
Background of the Invention
The invention relates to a device for measurement of the resonant frequency and the dissipation factor of a piezoelectric resonator, preferentially of the quartz crystal microbalance (QCM) type (where quartz crystal constitutes the sensing piezoelectric material). This type of device (the QCM) uses a single crystal quartz disk coated with electrodes of appropriate shape. The electrodes are connected to electrical circuitry that induces oscillations in the quartz.
The quartz crystal microbalance, QCM, is known to be an extremely sensitive weighing device. It is often used to weigh small amounts of matter which has been either removed from the quartz electrode(s) or may have been deposited onto one (or both) of the electrodes, usually as a thin film. (For details see, e.g., Applications of piezoelectric quartz crystal microbalances, C. Lu and A. W. Czanderna, Editors. 1984, Elsevier: Amsterdam; J. F. Alder and J. J. McCallum, The Analyst, 1983. vol. 108 p. 1291; G. G. Guilbaut Ion-Selective Electrode Review, 1980. vol. 2: p. 3; and D. A. Buttry in Electroanalytical Chemistry, A. J. Bard, Editor. 1991, Marcel Dekker, Inc.: New York. p. 1.)
Below we exemplify the function of the device using as an example a so-called AT-cut quartz sensor, FIG. 1b (for an illustration of the crystallographic axes of quartz see FIG. 1a). This is not to be contrued as a limitation on the invention. Other sensor materials, and/or quartz sensors cut to other crystallographic direction(s)--e.g. BT-cut, SC-cut, etc.--will function in a similar manner. The AT-cut sensor vibrates in a shear mode with an amplitude of about 1 nm at a fundamental resonant frequency, .function., given by: .function. is approximately 10 MHz. The quartz sensor starts to oscillate if an AC electric field with a frequency centered close to the fundamental resonant frequency of the sensor is applied perpendicularly to its surfaces. Usually, electrodes on each side of the sensor plate are deposited by evaporation and are subsequently contacted to an external AC field generator (for example to a signal generator, or to an oscillator drive circuit, or the like). Under favorable condition this arrangement is capable of sensing mass changes smaller than 1 ng/cm.sup.-2.
Ideally the mass changes, .DELTA.m, at the sensor electrode(s), induce a shift in the resonance frequency of the sensor, .DELTA..function., proportional to the mass changes: quartz plate.
Equation (2) is valid provided that the mass is attached rigidly to the electrode and follows the oscillatory motion of the sensor without dissipative losses. Equation (2) may fail when the added mass is viscous or is not rigidly attached to the electrode(s) and can thus suffer elastic or plastic deformation(s) during oscillations. The relation between added mass and the shift of the resonant frequency then becomes more complex. The latter situation arises when for example a water droplet is deposited onto an electrode of the quartz sensor.
When the validity of Eq. (2) can be questioned, it becomes particularly important to obtain additional information about the properties of the added matter such as viscosity and/or the strength with which the added matter {film, particle(s) droplet(s)} is attached to the sensor electrode. The dissipation factor, D, of the QCM contains at least part of this information (the dissipation factor is defined as an inverse of the quality factor, Q,--a quantity commonly used in the oscillating devices). Thus, it would be of importance for the QCM technique if both .DELTA..function. and .DELTA.D (the changes in the resonant frequency and the dissipation factor, respectively) could be measured simultaneously, especially in complex situations when Eq. (2) is no longer valid.
It is possible that additional information on the properties of a deposit and on its interaction with the sensor electrode can be obtained by exciting the quartz plate to oscillate in its overtone modes (3rd, 5th, etc.) and simultaneously measure .DELTA..functi
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Hook Fredrik
Kasemo Bengt
Krozer Anatol
Rodahl Michael
Brock Michael
Kwok Helen C.
O-Sense AB
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