1、 附录 1： Semiconductor Gas Sensors Research and development of gas sensors have shown great advances during the past decade. Semiconductor gas sensors mainly using SnO2 elements have been prevailing as detectors or alarms for leakage of LP ( Liquified Propane) gas and town gas, in addition to other ap
2、plications. Gas sensors based on MOSFET, first proposed in 1975,have attracted interests of many researchers, and have been developed to a point of commercialization as a hydrogen detector. Solid electrolytes, represented by stabilized zirconia, have proven to be very promising sensor materials for
3、oxygen, SO2, etc. This paper aims at reviewing briefly recent advances and trends in semiconductor gas sensors which were developed in a recent few years. Semiconductor gas sensors detect gases from a change in electrical resistance of an element made with a semiconductive metal oxide, typically SnO
4、2. Although sensors utilizing -Fe2O3 or -Fe2O3 have been put into practical use, SnO2 sensor still has an overwhelming market share. The production of semiconductor gas sensors has grown into a large industry: more than 5 million pieces were produced in 1983. Most of them have been used in domestic
5、homes as detectors or alarms for gas leakage. Field statistics has shown that gas alarms are quite effective to prevent an outbreak of gas leakage accident； the accident rate sharply decreases with an increase in gas alarm installation rate. It is useful to know actual Ltd. has analyzed the cases wh
6、ere gas alarms were effective for prevention of disastorous gas accidents. The result is shown in Fig.1. The causes are associated dominantly with mishanding or defects of gas appliances (66.8%), and then with stopcocks (11.3%), suicides (7.6%), gas pipes (6.7%), connectors (6.4%), and the others (1
7、.2%). It is rather surprising that suicides are ranked in the third place in this analysis. Anyway semiconductor gas sensors have been so far given much credit as an important device for home security and the installation rate of gas alarms is increasing year after year. Several years ago a problem
8、arose about SnO2 gas sensor when it was adopted at places like kitchens of underground restaurants where various inflammable gases came out into atmosphere from fuels and cookings very frequently during working time. In such a dirty atmosphere, the gas sensor tended to given an alarm at an inflammab
9、le gas concentration which was lowered gradually with an elapse of operation time. It has been pointed out that the problem was contributed by various factors but was associated most profoundly with the lowering of electric resistance of the sensor element due to its sintering at unexpectedly high w
10、orking temperatures. The problem have been almost solved after revisions were made to the sensor element. Semiconductor gas sensors is being applied in various fields, domestic and industrial. Carbon monoxide has often been used as a sample gas for gas sensor researches. However, reliable CO sensors
11、 using SnO2 were Fig.1. Breakdown of causes of gas leakage Fig.2. Heater voltage and output (according to Osaka Gas Company Ltd.) voltage vs.elapsed time for a SnO2 based CO sensor developed only recently. The sensitivity of SnO2 sensor to CO is low at elevated temperatures below 100oC. On the other
12、 hand, operation at such low temperature is easily disturbed by contamination of the sensor surface with interfering gases such as water vapor and others. Thus the recently developed CO sensor adopts intermittent heat cycles during operation； sensor is heated to elevated temperature (300-400oC) to m
13、ake the sensor surface clean while CO detection is made at a low temperature as shown in Fig.2. Combustion monitor sensors have been also developed by using SnO2 and perovskite-type oxide (La1-xSrxCo1-yFeyO3), respectively. These sensors measure the stoichiometric ratio of air to fuel, and applicable to kerosene or gas stoves, hot water supply systems, and so on. The sensor elements so far used practically are all porously sintered blocks of semiconductive oxides. However, thick film elements, which seem to be more compatible with large scale production and sensor
