SCIENCE/TECHNOLOGY als, coatings, and as precursors that can be pyrolyzed to give ceramics. This last possibility normally might be viewed as the least promising because economic factors have tended to work against the use of organometallics as precursors for structural materials. But Manners has wisely chosen his organometallic starting materials with an eye to economy. His ingredients—ferro
cene and organosilanes—are cheap as organometallics go. And the polymers are easy to make. In fact, he says, "It would be difficult to think of an orgamvmetallic polymer that would be cheaper" to make than his polv(ferrcxxTivlsilane)s. That kind oi thinking, suggest some observers, should be embraced by all organometallic chemists working in materials science.
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New thin-film sensor detects nitrogen dioxide "Dow Corning ? I'd like ar\ antifoam s a m p l e . · · a n d a l a r g e , sharp object."
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OCTOBER 3, 1994 C&EN
Researchers in Wales have developed an inexpensive nitrogen dioxide gas sensor that they hope will eventually be produced commercially for use in a variety of environmental applications. The device is based on tin dioxide and, the researchers claim, "combines the benefits of semiconductor technology and thin-film production routes." The device was described by Gary S. V. Coles, senior lecturer in electrical en gineering at the University of Wales, S w a n s e a , at the 8th I n t e r n a t i o n a l School on Condensed Matter Physics, held last month at Varna, Bulgaria. Over the past 10 years or so, the re searchers have tried "to make semi conductor sensors based on tin diox ide which confer some degree of se lectivity," says Coles. "In the past, we've always looked at thick films which, compared with thin films, are simple to lay down in a binder such as water or an organic medium. For the thin-film device, we use clean-room technology." The Swansea group prepares thin films by a novel, two-step sputteringoxidation route. A film of metallic tin is deposited on a platinum contact arrav on an alumina substrate by radio fre quency sputtering from a tin target heated in an atmosphere of argon. The tin is then oxidized by heating the de vice in an oxygen stream. Coles points out that tin dioxide is a wide band gap semiconducting materi al. Oxygen vacancies present under normal conditions confer η-type prop erties on the material. In ambient air, oxygen is adsorbed onto the oxide sur face in different ionic forms—O", Ο 2 , 0 2 ~, or 0 2 2 ~—depending on tempera ture. The electrons required to form the ions are d r a w n from the conduction band of the tin dioxide. As a result, the material has a lower conductivity than might normally be expected.
At elevated temperatures, the ad sorbed oxygen reacts with reducing gas es such as methane and carbon monox ide with a subsequent release oi elec trons back into the conduction band. "So vou see a drastic rise in the conductivity, and this is the property vou monitor to detect vour gases," explains Coles. At elevated temperatures and in the presence of an oxidizing gas such as N 0 2 , large decreases in conductivity are observed. This is caused by the chemisorption of Ni0 2 with an asso ciated capture oi conduction band electrons: Sn0 2 (e ) + N O , -> S n 0 2 ( N 0 2 ) Because conductivity decreases when N 0 2 is chemisorbed, a material of knver resistance is required to ensure that a
Sensor responds selectively to N0 2
100 Gas concentration, ppm
Tin dioxide-based thin-film sensor responds selectively to N 0 2 at 200 °C with sensitivity measured as the ra tio R„dS/Rair for oxidizing gases such as N 0 2 . R lir is the base resistance of the device in uncontaminated air; R„^ is the resistance of the device in contaminated environments.
According to Coles, his group's sensor is relatively inexpensive to make. "We hope to produce these devices for a few [dollars]," he says. "The standard technology for detecting N0 2 is based on chemiluminescence, but these instruments are expensive—they can cost thousands of [dollars]. Electrochemical cells, which are also used for detecting N0 2 , are cheaper but they have a limited shelf-life." Moseley and Coles both agree that monitoring N0 2 and other gases polluting the atmosphere is becoming increasingly important to society. Coles notes that, "N0 2 is an extremely toxic gas Coles monitors changes in conductivity to detect the presence of oxidizing gases such as N02. which is immediately dangerous to life at concentrations of around 50 ppm. It The Swansea researchers hope their has a threshold limit value of 3 ppm." high concentration of electrons are available for capture by the target N 0 2 device will eventually be produced He adds that "it has received a great molecules. This design ''should result commercially for monitoring N 0 2 deal of media attention in recent years in high sensitivity/7 according to Coles. emissions from large industrial installa- for various problems including acid In addition, a high degree of selectivity tions such as power stations. They also rain and the aggravation of breathing is obtained because resistance chang- hope that it might be used as a catalyt- conditions such as asthma." The gas is es—caused by interference from com- ic converter efficiency monitor on auto- released into the atmosphere by burning mon reducing gases such as CO and mobiles and in the general monitoring fossil fuels. CH4—are masked by the high conduc- of air quality in urban environments. Michael Freemantle tivity of the material. Coles has characterized the sensor with respect to its response to N0 2 , H2, CO, and CH4 mixtures in dry air over a range of operating temperatures. The thin-film sensors show marked sensitivity to N 0 2 with virtually no response to either CO, H2, or CH 4 at 200 °C. At higher temperatures, responses to other gases are observed. Here's why it makes sense to do business Some development work still rewith Reilly Industries, the world's largest mains to be done on the device, howproducer of pyridine, picolines, and ever. 'We haven't yet done a full enviderivatives: ronmental characterization," explains Coles. He points out that there is alThe most reliable supply: | Only Reilly most an infinite number of gases that can serve your needs from two pyridine could be tested. In particular, humidity manufacturing plants on two continents. is a problem. Capteur Sensors & Analysers, based Faster response: When you need fast delivery, rely on Reili in Abingdon, England, markets a thickWe maintain bulk inventories in the U.S. and Europe, both backed film solid-state N 0 2 sensor that, like by direct sales and technical support staff. the thin-film sensor developed at Swansea, is based on a gas-sensitive reKnowledge when you need it: Expect better technical support sistor. However, Capteur does not use from the innovator in pyridine chemistry. No one can share more tin dioxide but another material to pyridine experience and knowledge than Reilly. form the gas-sensitive layer. Patrick T. Moseley, technical director at Capteur, f makes sense to work with the leader. says, "We have taken the view that you can use a wide range of semiconductReilly Industries, Inc., ing oxides and that brings with it the 1500 S.Tibbs Avenue, Indianapolis, IN 46241 benefits of specificity." 317-247-8141, Moseley notes that "humidity is a traFax 317-248-6402. ditional problem of tin dioxide sensors." He says Capteur's sensor has a very CIRCLE 2 3 ON READER SERVICE CARD small moisture response.
WHEN YOU NEED PYRIDINE, CALL THE LEADER.
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OCTOBER 3, 1994 C&EN
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