engine Olfaction gismo (Mod) Sensors (Part Three)

Quartz Crystal Microbalance

The Quartz Crystal Microbalance (Qcm/Qmb) is an very sensitive mass sensor, capable of measuring mass changes in the nanogram range [1].

Piezoelectric Pressure Sensor

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 potential to excite the crystal into oscillation by applying an Ac voltage over 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 outside in order to add element of selectivity to the sensor [2]. A number of dissimilar types of sensor control under similar basic principles, such as "Bulk Acoustic Wave (Baw)" and "Surface Acoustic Wave (Saw) sensors". Both sensors need an A.C. Voltage for configurations/operation. Baw sensors use the galvanic field in order to excite the quartz crystal to oscillate, and Saw sensors use wave propagation on the outside sensor [1].

a. Manufacturing Process

After being cut along distinct 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]:

1. Spray coating.
2. Growth of Langmuir-Blodgett films.
3. Self-Assembled Monolayers (Sams).

The coating will contribute 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 distinct frequency.

On the outside 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 growth in mass, which causes a slowing in the frequency of oscillation.

Qcm are very sensitive to any itsybitsy changes in their mass, and for this presume the Qcm can portion 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 gismo utilises outside 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 carefully by the spacing of the Idt fingers.

One of the Idt surfaces will develop and covenant when an alternating current applied to it. The movement of the outside generates a wave (some scientists/researchers call it a "Rayleigh Wave"), which will pass straight through the substrate. A frequency counter located in the Idt receiver will then description 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 outside can affect the wavelength/frequency of the outside 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 outside wave is not just affected by the convert 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 dependable 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 private sensor, instead of using a central system controller. In addition to this, ordinary sensors production raw data; but only beneficial data is produced by a smart sensor. Many of the smart sensors can be really programmed and/or reprogrammed, thus rescue time and expense.

The feasibility of using such kind of sensors in any Mod depends on how small the gismo will be and on the final application(s), as well as the final cost of the gismo itself.

Najib Altawell

References

[1] Lee-Davey, J., (2004) "Application Of machine Olfaction system 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 synthetic 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.

© Altawell 2008

engine Olfaction gismo (Mod) Sensors (Part Three)

Absolute Encoders Optical Encoder

machine Olfaction expedient (Mod) Sensors (Part One)

There are a whole of dissimilar types of sensors which can be used as vital components in dissimilar designs for machine olfaction systems.

1. Electrochemical sensors.

Piezoelectric Pressure Sensor

2. Metal oxide semiconductors.

3. Schottky diode-based sensors.

4. Calorimetric sensors.

5. Quartz crystal microbalances.

6. Optic sensors.

Electronic Nose (or eNose) sensors fall into five categories [1]: conductivity sensors, piezoelectric sensors, Metal Oxide Field follow Transistors (Mosfets), Optic sensors, and these employing spectrometry-based sensing methods.

Conductivity sensors may be composed of metal oxide and polymer elements, both of which exhibit a change in resistance when exposed to volatile Organic Compounds (Vocs) [1].

In this article only Metal Oxide Semi-conductor (Mos), Conducting Polymer (Cp) and Quartz Crystal Microbalance (Qcm) will be examined, as they are well researched, documented and established as leading element for varied types of machine olfaction devices. The application, where the proposed gadget will be trained on to analyse, will greatly work on the selection of sensor.

The response of the sensor is a two part process [3]:

1. The vapour pressure of the analyte usually dictates how many molecules are present in the gas phase and consequently how many of them will be at the sensor(s).
2. When the gas-phase molecules are at the sensor(s), these molecules need to be able to react with the sensor(s) in order to yield a response.

Sensors types used in any machine olfaction gadget can be mass transducers e.g. Qmb "Quartz microbalance" or chemoresistors i.e. Based on metal- oxide or conducting polymers. In some cases, arrays may consist of both of the above two types of sensors [4].

Metal-Oxide Semiconductors

These sensors were originally produced in Japan in the 1960s and used in "gas alarm" devices.

Metal oxide semiconductors (Mos) have been used more extensively in electronic nose instruments and are widely available commercially [1].

Mos are made of a ceramic element heated by a heating wire and coated by a semiconducting film. They can sense gases by monitoring changes in the conductance while the interaction of a chemically sensitive material with molecules that need to be detected in the gas phase. Out of many Mos, the material which has been experimented with the most is tin dioxide (SnO2) - this is because of its stability and sensitivity at lower temperatures. dissimilar types of Mos may consist of oxides of tin, zinc, titanium, tungsten, and iridium, doped with a noble metal catalyst such as platinum or palladium.

Mos are subdivided into two types [4]: Thick Film and Thin Film

Limitation of Thick Film Mos: Less sensitive (poor selectivity), it require a longer time to stabilize, higher power consumption. This type of Mos is easier to yield and therefore, cost less to purchase.

Limitation of Thin Film Mos: unstable, difficult to yield and therefore, more high-priced to purchase. On the other hand, it has much higher sensitivity, and much lower power consumption than the thick film Mos gadget [5].

a. Manufacturing process [5]

Polycrystalline is the most tasteless porous material used for thick film sensors. It is usually prepared in a "sol-gel" process [5]:

Tin tetrachloride (SnCl4) is prepared in an aqueous solution, to which is added ammonia (Nh3). This precipitates tin tetra hydroxide which is dried and calcined at 500 - 1000°C to yield tin dioxide (SnO2). This is later ground and mixed with dopands (usually metal chlorides) and then heated to recover the pure metal as a powder.

For the purpose of screen printing, a paste is made up from the powder.

Finally, in a layer of few hundred microns, the paste will be left to cool (e.g. On a alumina tube or plain substrate).

b. Sensing Mechanism

Change of "conductance" in the Mos is the basic principle of the doing in the sensor itself. A change in conductance takes place when an interaction with a gas happens, the conductance varying depending on the attentiveness of the gas itself.

Metal oxide sensors fall into two types [2]:

1. n-type (zinc oxide (ZnO), tin dioxide (SnO2), titanium dioxide (TiO2) iron (Iii) oxide (Fe2O3).
2. p-type (nickel oxide (Ni2O3), cobalt oxide (CoO).

The n type usually responds to "reducing" gases, while the p-type responds to "oxidizing" vapours.

Operation (n-type) [2]:

As the current applied in the middle of the two electrodes, via "the metal oxide", oxygen in the air start to react with the surface and accumulate on the surface of the sensor, consequently "trapping free electrons on the surface from the conduction band" [2]. In this way, the electrical conductance decreases as resistance in these areas increase due to lack of carriers (i.e. increase resistance to current), as there will be a "potential barriers" in the middle of the grains (particles) themselves.

When the sensor exposed to reducing gases (e.g. Co) then the resistance drop, as the gas usually react with the oxygen and therefore, an electron will be released. Consequently, the publish of the electron increase the conductivity as it will cut "the possible barriers" and let the electrons to start to flow [2].

Operation (p-type):

Oxidising gases (e.g. O2, No2) usually remove electrons from the surface of the sensor, and consequently, as a follow of this fee carriers will be produced.

c. Limitation of Mos sensors [4]

1. Poor Selectivity - In particular when a thick film Mos gadget is used. The poor selectivity can be reduced by the deposition of a suitable catalyst layer of noble metals like Pd, Pt, Au and Ag.

2. Mos need high temperatures (around 300°c) to control efficiently; this follow higher power consumption.

3. Sensitive to humidity and to compounds such as ethanol and Co2.

d. Advantages [4]

1. Widely available in a variety of types and sensitivities.

2. Very sensitive to a whole of organic vapours (e.g. Oil).

3. Fast response, usually less than 10 seconds.

Altawell

© Altawell 2008

References

[1] Nagle, H. T., Schiffman, S. S., Gutierrez-Osuna, R.(1998) "The How and Why of

Electronic Noses" Ieee Spectrum September 1998, Volume 35, whole 9, pp. 22-34.

[2] Arshak K., Moore E., Lyons G.M., Harris J., Clifford S "A retell of gas

sensors employed in electronicnose applications". (2004).

[3] Hurst, W. J., (1999) "Electronic Noses & Sensory Array Based Systems".

Technomic Publishing Company, Isbn No. 1-56676-780-6.

[4] Sberveglieri D., (1999) "Metal-Oxide Semicondictors" Asteq Technologies for sensors 1999

[5] Nose Office (2003) "Nose Ii - The Second Network on synthetic Olfactory Sensing".

machine Olfaction expedient (Mod) Sensors (Part One)

Magnetic Encoder Fundamentals Optical Encoder Differential Pressure Sensors