Savings in the air

In a bid to help the car industry keep production costs down scientists at

are developing nano-structured micro-power gas sensors.

These are often used in 'car air cabin quality' tests to detect pollutant gases, such as carbon monoxide and nitrous oxides.

While conventional sensors are handmade and can cost up to £20 each, the researchers hope to produce one that would use very low power and be electronically mass-produced to ultimately bring down car manufacturing costs.

'Typically, you are paying £17 to £20 a sensor, but we would get way below £6 and our target is around 70p each,' said Prof Julian Gardner of Warwick's electronic engineering department. 'By using microelectronic technology, similar to the production of silicon wafers, you can mass produce them at a low cost.'

Furthermore, Gardner said the sensors will be fully compatible with complementary metal-oxide semiconductor (CMOS) technology.

'CMOS allows you to integrate all the electronics on to the chip, so it is much cheaper,' he said.

Traditional sensors have a thick film of paste, made up of ground tin oxide and palladium oxide materials, which can detect high levels of gas concentrations. The Warwick team, however, is developing a material that is even more sensitive to gases and are exploring the use of carbon nanotubes, zinc oxide nanowires and mesoporous (a material with pores with diameters between two and 50nm ) tungsten oxide for this purpose.

'While conventional materials will be able to detect one part of a million of gas, the new material would offer a superior sensitivity to gases by one order of magnitude. This means it will be able to detect 100 parts of a billion in comparison. Depending on the gas, compared with existing sensor materials, this one will also be able to sense at lower concentrations,' said Gardner.

'We're looking at new sensing materials, such as nano needles, carbon nanotubes and nano rods, for their greater sensitivity to gas. When a gas reacts with oxygen in the air, it forms a depletion layer and the width of this layer is about the same as that of the rod.

'When you react the gas, they start to conduct electricity and it is the changes in electric resistance that indicates the level of gas.'

Another advantage of using carbon is its ability to function and detect gases such as carbon monoxide, at low temperatures. The new sensor, however, would also be able to run at very high temperatures.

'Conventional sensors run at around 400ºC, but to detect some gases, for example methane or hydrogen, they need to run at 550º C,' said Gardner.

'We're showing for the first time that this technology is capable of going to even higher temperatures — up to 750º C.'

To make the materials more selective, so that initially, they respond to only one gas, the researchers would add catalysts to modify the surface of the carbon nanotube or nanowires.

At only 2mm sq, the sensors will have hotplates that are just tens of microns in size. Although standard sensors also have hotplates, according to Gardner, one of the innovative features of his sensor is the silicon on insulator (SOI) technology and its integration on to the chip.

'The sensors use ultra-low power, which is possible because of SOI technology, whereby little electrical heaters heat up the microhotplates on the sensor to power them.'

Although the microhotplates will be made smaller, Gardner claims the new sensor would be just as efficient as traditional ones.

'The smaller hotplate will be as efficient as you are just measuring the change in electrical resistance — it does not matter what size it is. But for other types, for example microcalorimeters, which measures changes in heat to indicate the gas concentration level, you cannot go too small as they would lose their sensitivity,' he said.

Now at the electronics integration stage of the project, Gardner hopes the sensor will be commercially available in three years.

'We don't think there is anything out there that really compares, particularly cost-wise, and that's why companies, such as

, are interested in it,' he said.