PROCESOS LABORALES PERIÓDO

The tables summarise the experiments described in the literature. The survey is biased towards tin dioxide sensors as these are of particular interest to this project. A number of generalisations can be made from this survey.

From the tables it can be seen that the most common catalyst additives for SnC^ sensors are Pt and Pd. For these sensors, there was a wide range of preparation methods. These could be generally divided into two types: thin films o f tin oxide with a layer of metal (0.1 to 300 ML) deposited on the surface; and thick films or pellets impregnated with a Pt or Pd solution (concentration 0.2 to 5 wt%), fired between 300 and 1000 °C. Both catalysts were used primarily for detection of CO, and there was a general agreement that the catalysts improved gas response. There were some claims of quicker response times and lower operating

temperatures. The most common operating temperature for Pt-Sn02 was 200-300 °C, compared with 300-400 °C for Pd-Sn02.

Much o f the literature described above is aimed at improving the sensitivity of SnC^ to CO. However, only three groups have been found in the literature to have developed a semiconducting metal oxide sensor for CO operating at room

temperature, and all utilize Pt-Sn02- The first reference to such a system was by Yamazoe and co-workers [48] (Japan, 1983). This was followed by the work carried out by W illiams and co-workers [4] (Harwell, UK, 1988), and finally Ambrazeviciené et al [6] (Lithuania, 1993). A very similar MIS capacitor has also been described by Kang and Kim [54-56, 59] (U.S.A, 1993). The work by all these groups is described briefly below.

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preparation method used was as follows. Tin(IV) oxide was calcined in air at 600 °C for 10 h. It was then impregnated with the metal chloride or nitrate and calcined again at 600 °C for 10 h. A pellet was then pressed and calcined a third time at 600 °C for 5 h. It was found that 0.5 wt% Pt addition gave the best response to 0.02% CO of all the transition metals tested, and further that the best response was obtained for operation at room temperature. The response to 0.8%

H2 was very similar.

A large amount of literature has been produced by Williams and co-workers, on many types of gas sensor. Their room temperature CO sensor was prepared by calcining tin dioxide at 1000 °C, pressing, and sintering at 1000 °C for 16 h in air. The resulting pellet was then impregnated with an aqueous solution of the metal salt followed by immersion in a non-aqueous solvent to precipitate the metal compound. The solvent was removed by vacuum drying, and the deposited metal compound was thermally decomposed in situ, at the lowest temperature necessary to yield a very finely divided catalyst. The concentration of the impregnation solution was chosen so that 10% of the available tin oxide surface area was covered with catalyst. It was found that both Pt- and Pd-Sn02 yielded good réponse at room temperature to up to 1 % CO. Similar results were obtained on exposure to hydrogen.

Ambrazeviciené et al investigated 1 fxm thick films prepared by coating a

substrate of Si + Si02 with an aqueous solution of SnCl2.H20 and HCl. The

catalysts were added to the solutions in the form of H2PtClg.H2 0 and SbiOs, and

dried. The samples were then annealed in air at a temperature in the range 300 to 900 °C. It was found that Sb-Sn02 alone shows a low gas response at 100 °C. The magnitude of response and response time for Pt-Sn02 were both found to depend on annealing temperature. The Pt-Sn02 annealed below 400 °C was sensitive but slow in its response, with the response time being 50 to 500 s.

When fired at 700 °C or more, there was a reduction in sensitivity of Pt-Sn02-

However, Pt-Sb-SnOz was very sensitive to CO and H2. It exhibited a rapid

response and recovery at operating temperatures of 100 °C or less, with response times at 2 s or less. These sensors were also found to detect ethanol,

acetaldehyde and acetic acid, but not O2, CO2, SO2, or NO2.

A slightly different system is a MIS capacitor which detects CO, O2 and H2,

which has been proposed by Kang and Kim. The system investigated was: catalyst(Pd, Pt or Ag)-adsorptive oxide(Sn02 or ZnO)-SigN^-Si02-Si-A1. The sensor was built on p-type silicon, and the polished surface coated with 50 Â of Si02, followed by 400 Â of SigN^. The 800 Â adsorptive oxide layer was grown by deposition of Sn or Zn, followed by thermal oxidation at 400 °C for 1 h. A 400 Â layer of catalytic metal gate was evaporated onto the SnO^ or ZnO. The

response to O2, H2 and CO was measured in the range 27 to 100 °C. At 50 °C,

for detection of CO, only the Pt-Sn02 device gave a good response in a

background of oxygen. The Pd-Sn02 was much less sensitive, and Ag-Sn02 and Pd-ZnO were insensitive to CO at this temperature. Pt-Sn02 also gave a good

response to H2 in oxygen. In the absense of oxygen, the response decreased

drastically. The device was also insensitive to CO in the absence of oxygen. The response time (here, time taken to reach 90% of final saturated value of voltage shift) of this sensor to CO was 2 min, and the recovery time (time needed to return to 90% of the base line in air) was 3.2 min. However, the response and

recovery time of the same sensor to H2 was of the order of a few seconds. It was

found that the response to CO decreased with decreasing operating temperature, so that the best response was obtained at 100 °C.

The closest preparation method to the one utilised in this work is that developed by Williams and co-workers, and their work forms a basis for the studies

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additive tested to reduce the operating temperature of SnOi, they did not further investigate the effects of changes in the preparation method, although some predictions were made. This work, therefore, takes their initial studies a stage further by investigating in detail and explaining the effect of gas response on changing the following: the surface coverage of platinum; the platinum particle size (related to firing temperature), and the dopant density in the tin oxide. Ambrazeviciené et al have mentioned two of these topics: they noted a decrease in gas response with increasing firing temperature, and an improvement in gas response on doping with antimony.

Returning to the tables, it can be seen that there is some disagreement over which catalyst has what effect. There is a wide variety of sensor preparations, so it is difficult to know which effects are due to the geometry of the sensor or its preparation method, or which effects are due to the catalyst. This confusion is possibly linked to the fact that approximately half of the papers describe

characterisation of sensors only by gas response, and do not mention any other characterisation methods, such as XPS or ac impedance. If there was more attention paid to characterisation of the sensors, some of these effects may be better understood.

MATERIAL