Quartz Crystal Microbalance
The Quartz Crystal Microbalance (QCM/QMB) is an extremely sensitive mass sensor, capable of measuring mass changes in the nanogram range [1].
QCMs are piezoelectric devices fabricated from a thin plate of quartz with electrodes affixed to each side of the plate.
A QCM-D (Quartz Crystal Microbalance with Dissipation monitoring) consists of a thin quartz disc sandwiched between a pair of electrodes.
Due to the piezoelectric properties of quartz, it is possible to excite the crystal into oscillation by applying an AC voltage across the electrodes. Changes to this oscillation are directly proportional to mass changes on the crystal [1].
Various sorbent coatings can be used on the crystal surface in order to add element of selectivity to the sensor [2]. A number of different types of sensor operate under similar basic principles, such as "Bulk Acoustic Wave (BAW)" and "Surface Acoustic Wave (SAW) sensors". Both sensors require an A.C. voltage for configurations/operation. BAW sensors use the electric field in order to excite the quartz crystal to oscillate, and SAW sensors use wave propagation on the surface sensor [1].
a. Manufacturing Process
After being cut along certain crystallographic axis, the thin plates of the single piezoelectric crystal quartz are covered with thin gold electrodes on both sides [4].
The two sides of the crystal are then coated with polymer films. The coating technique could be any of the following [4]:
- Spray coating.
- Growth of Langmuir-Blodgett films.
- Self-Assembled Monolayers (SAMs).
The coating will provide the conductivity and changing of mass.
b. Sensing Mechanism
The QCM is basically a thin quartz wafer with electrode pads on each side [5].
The QCM oscillates mechanically, when connected to an amplifier.
At the same time the amplifier oscillates electronically, with a certain frequency.
On the surface of the QCM there is a coating of a sensitive chemical. Exposure of which to analyte vapour, cause the molecules of the analyte inter into the coating. The result will be an increase in mass, which causes a slowing in the frequency of oscillation.
QCM are very sensitive to any minute changes in their mass, and for this reason the QCM can measure changes in its frequency to 1 part in 108 [5]. Normal operating frequencies are in the range from 10 to 30 MHz. [4].
Surface Acoustic Wave Sensors (SAW)
As in the QMB (i.e. QMC) this sensor is based on the same principle i.e. when mass changes, frequency changes. The device utilises surface acoustic waves, with a frequency of about 600 MHz [4].
a. Manufacturing Process
Two inter-digital transducers (IDT) are usually made up from thin metal electrodes and fitted on "a polished piezoelectric substrate", located in the centre and enclosed by resonators [4].
The wavelength is determined by the spacing of the IDT fingers.
One of the IDT surfaces will expand and contract when an alternating current applied to it. The movement of the surface generates a wave (some scientists/researchers call it a "Rayleigh Wave"), which will pass through the substrate. A frequency counter located in the IDT receiver will then record the frequency of the wave.
To minimise noise and temperature, as well as lower the frequency to be measured, a dual SAW set up may be constructed, and therefore, the reference signal from the SAW (uncoated) will be mixed with the sensor signal.
b. Sensing Mechanism
The physical properties of the surface can affect the wavelength/frequency of the surface wave itself. A thin layer of polymer coats the substrate, which is located between the two IDTs. The absorption of gas changes the mass of the polymer, and consequently the properties of the sensitive layer. The surface wave is not just affected by the change of mass; it is affected by other factors, such as temperature, pressure, dielectric constant and viscosity.
Smart Sensors
Smart sensors are simply sensors with microprocessors attached to them. When it comes to a system design, a smart sensor can be:
Easier.
Cheaper.
More reliable and more scaleable.
Higher performance.
More rapid to design.
Obviously, these benefits are all obtained when microprocessors or computing resources are embedded on the sensor. Therefore, the processing of data is performed on the spot i.e. within each individual sensor, instead of using a central system controller. In addition to this, ordinary sensors output raw data; but only useful data is produced by a smart sensor. Many of the smart sensors can be easily programmed and/or reprogrammed, thus saving time and expense.
The feasibility of using such kind of sensors in any MOD depends on how small the device will be and on the final application(s), as well as the final cost of the device itself.
References
[1] Lee-Davey, J., (2004) "Application Of Machine Olfaction Principles For The Detection Of High
Voltage Transformer Oil Degradation"Cranfield University.
[2] Perera, A., Sundic T., Pardo A., Gutierrez-Osuna R., Marco S., (2002)"A Portable Electronic Nose Based on Embedded PC Technology and GNU/Linux: Hardware, Software and Applications"
IEEE SENSORS JOURNAL, VOL. 2, NO. 3, JUNE 2002 235
[3] K. Persaud, G. Dodd, Nature 1982, 299, 352-355.
[4] Nose Office (2003) "NOSE II - The Second Network on Artificial Olfactory Sensing" University of Tuebingen Dec 2003 - Germany
[5] Finklea, H. O., lecture notes ( ) "Gas Phase Sensors"
Department of Chemistry West Virginia University
Morgantown, WV 26506-6045.
Najib Altawell
© Altawell 2008
