Research Interests - Gas Sensors



My Ph.D. focused on Atmospheric Pressure Chemical Vapour Deposition (APCVD) of main group metal phosphides and oxides. The metal oxides were deposited onto gas sensor chips and compared with their respective screen printed compounds.

I am interested in modifying metal oxide semi conductor gas sensors using overlayers and mixtures. I am also interested in novel ways to influence sensor material microstructure as this has a profound experience on the gas sesning properties of the devices.

The information on this page is a very quick summary of some of my work for further information please contact me and/or look at my publications.

For a more detailed explanation of the figures, click on the figure or the link in the figure caption.

 

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Metal Oxide Semi Conductor Gas Sensors

Metal oxide semi conductor gas sensors are an increasingly important technology. Most of the applications use tin dioxide or to a lesser extent tungsten oxide devices. I am interested in using new materials such as chromium titanium oxide (CTO) or gallium oxide for gas sensing applications. Sensors are made by making inks of the sensor material and screen printing these onto sensor chips. The sensor chips are small alumina tiles, typically 3x3 mm square (figure 2). The coated chips then get baked in a furnace and bound up into devices (Figure 1).

Figure 1. A CTO gas sensor device - explanation
Figure 2. Schematic of gas sensor chip - explanation

The chips are then placed in a test rig and exposed to small amounts of test gas, typically 1 to 100 ppm in air. The test gas interacts with the surface and causes a change in resistance accross the electrodes which is measured by a computer and can be related to the type of test gas and the test gas concentration.

Gas sensors of this type show great promise for use in electronic noses and can be used for all sorts of monitoring applications from oil rigs, food production and fire safety.

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Zeolite Modified Sensors

One inherent problem with metal oxide semi conductor devices is that they are indiscriminate, that is to say they provide a response to a wide variety of gases. This can be a problem if we only want to measure one specific gas, say carbon monoxide for fire safety applications. In an effort to improve selectivity devices are modified with zeolites (Figure 3 and 4).

Figure 3. Cross sectional schematic of layered and mixed sensors - explanation
Figure 4. Scanning electron microscope images of a layered sensor surface - explanation
The effect of a zeolite transformation layer can be several fold. There are a variety of diffusion effects which can occur, catalytic reactions may take place as well as the zeolite acting as a molecular sieve and preventing large molecules getting to the sensor material surface. Often an enhancement in gas response is seen. Figure 5 shows an example of how a zeolite overlayer can improve the responsivity of a CTO device by as much as two orders of magnitude.
Figure 5. Gas response to ethanol of Zeolite Modified CTO Sensor and Unmodified CTO Sensor at different gas concentrations - explanation

Different zeolite overlayers cause different effects to the measured signal, which is in turn dependent on the gas being evaluated. In this way the sensors become much more specific to particular gases.

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Chemical Vapour Deposition and Gas Sensors

Another important aspect with metal oxide semi conductor gas sensors is the sensor microstructure. All commercially produced metal oxide semi conductor gas sensors are made using thick film technology such as screen printing where microstructure is fairly homogenous and limited in terms of crystallite size. CVD allows for the controlled growth of films that have a wide variety of microstructures which may lead to improved gas sensing properties. Figures 6 and 7 show some examples of films grown by CVD and their screen printed versions.

Figure 6. Scanning electron microscope images of sensor surfaces prepared by chemical vapour deposition.
Figure 7. Scanning electron microscope images of sensor surfaces prepared by screen printing.

explanation

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