The field of chemical sensors has grown rapidly in the past decade. Various types of chemical sensors have been devised for the detection and monitoring of chemical substances contained in gases, solutions, and organisms. Besides the basic researches on sensing mechanism, selectivites, sensitivities and so on, efforts have also been directed to searches for new sensor materials as well as to finding new application fields. The chemical sensor is a combination of a receptor portion for molecule recognition and a transducer portion for electrical signal output. In order to enhance the selectivity, molecular shape-selective function is required for the receptor. As for the transducer, there are two major methods to detect electrical signal, that is, amperometry and potentiometry. In both cases, it is very important to perceive the relation of output signal to the concentration of molecule to be recognized. This paper reviews recent advances in chemical sensors, including their principle and characteristics, and refers to some future scopes.
Chemical sensors currently used and under development are reviewed. It is pointed out that more detailed considerations are needed for the performances in specialty respect to the enviroments in which the sensors are practical by used. From this standpoint, discussions are made on the chemical sensors used in gas leak detectors, humidity monitors, combustion controllers, and biosensors. The detection principle is based on direct or indirect conversion of chemical quantity into electric signal. The direct conversion utilizes electrochemical processes, whereas the indirect conversion uses some energy modulating process coupled with chemical reactions. Detection mechanisms are reviewed following this classification and examples are described. Prospects of chemical sensors are discussed and it is remarked that multi-functional and “chemically smart” sensors should be developed.
Physical sensors are reviewed. What is the physical sensor? How does the physical sensor differ from the chemical sensor? What kinds of physical effects can be applied to the physical sensors? These questions are answered in this paper. The present status and the future of physical sensors, optical, magnetic, temperature, and, pressure sensors, are described.
Electrical conduction in anodized aluminum layers has been studied under various environmental conditions. The anodization was carried out in oxalic acid and sulfuric acid baths. All the oxide layers were found to be amorphous using X-ray diffraction. A thin aluminum film was evaporated onto the oxide layer and thus a sample gas sensor with Al/Al2O3/Al structure was fabricated. Temperature dependences of the conductance of the sample sensors were measured in vacuum in the temperature range from 20°C to 100°C and activation energies for conduction were calculated. The sample sensors showed resistance-decrease associated with vapor adsorption of alcohols such as methylalcohol, ethylalcohol, etc. The sensitivity is higher as a whole with the layers anodized in the oxalic acid solution than with those anodized in the sulfuric acid solution. The conductivitychange associated with thermally stimulated desorption of alcohols was measured, and the desorption behavior of the adsorbate was interpreted in terms of the conductivity-change.
The humidity-dependent impedance of sintered Sb2ZrOx was studied. Most of the open pore are throughpores. The volume of voids, average pore radius, and average particle size were estimated to be 0.21 cm3·cm-3, 2×10-6, and 1×10-5cm, respectively. No distinct peaks could be detected in x-ray diffraction patterns. In the high humidity region (P/P0>0.2), it was found that the impedance is expressed as Z=Z0exp(-Eapp/kT*)exp(Eapp/kT). The decrease of Eapp with an increase in the amount of adsorbed water was interpretable in terms of the decrease in Coulomb interaction for carrier (proton) formation. In addition, the value of Eapp in high coverage region (θ>2.5) was in fair agreement with that in liquid water trapped from the ambient air.
Influences of preparation procedure of tin dioxide catalyst on H2 oxidation selectivity are investigated. The tin dioxide which was prepared by calcination of precipitate obtained by neutralization of SnCl2 solution with K2CO3 solution, was found to be a high selective H2 oxidation catalyst. This catalyst is employed as a cover for the sample gas electrode of CO sensor element made of stabilized ZrO2. The output of this sensor element for 200 ppm of CO is higher than that for 2000 ppm of H2.
The electrical conduction mechanisms of three types, of films have been investigated in relation to humidity-electrical resistance characteristics and their variations with time. Type I film consisted of heat-treated methylphenyl silicone and SiO2. Type II film consisted of a heat-treated mixture of silicone, SiO2, and Na2CO3. Type III film was a heat-treated mixture of silicone, SiO2, and graphite. The dominant charge carriers for the surface electrical conduction were thought to be protons in type I, hydrated sodium ions in type II, and protons and electrons in type III, respectively. The electrical resistance of type I increases and that of type II decreased with time of exposure to the laboratory air. The time dependence of the humidity-resistance characteristic of type III was less than that of other two types. However, the humidity-sensitivity of type III was not very good.
Works of conventional tin oxide gas sensors are often disturbed by a contact with ethanol vapor. In order to remove such a disturbance, it was tried to insensitize the sensor to ethanol by use of oxidation catalysts. Of the twelve different catalysts tested, alumina supported Pt catalyst (Pt(1%)/Al2O3) was found to be the most effective for total oxidation of ethanol vapor; ethanol was completely oxidized above ca. 200°C under the experimental conditions. The effects of Pt(1%)/Al2O3 catalyst in the actual sensing performance were examined by using three types of sensor elements. As a result, the best data were obtained with the coated element which was prepared by coating the conventional sensor element (base element) with Pt(1%)/Al2O3 catalyst. The sensitivity of the base element to ethanol vapor was higher than those to CH4 and C3H8 below 400°C and was still significant at 500°C, The coated element, however, was almost insensitive to ethanol vapor in the temperature range 300-500°C, while it was as sensitive to CH4 and C3H8 as the base element. It was also confirmed that the rate of sensor response was little affected by the catalyst coating.
The offset voltage drift of silicon pressure sensors is analyzed, assuming that in semiconductor-diffused resistance the variation with temperature is approximated by the 2nd power of temperature. The mechanism of resistance variation from the initial value is investigated and its procedure is displayed in an analytical form. It is found that the internal stress or strain introduced in the various processes from wafer fabrication to bonding and packaging of chips is mainly responsible for the offsed drift. The discussion is also referred to the experiment of bonding, in which the temperature drift is greatly influenced by the thermal stress from the backplate.
The recent progress in cost and reliability of microprocessors and the interface circuits have brought more needs for every kind of reliable sensing devices. The requirements for controlling humidity as well as temperature in an ambient atmosphere have been extended to various fields such as industry, agriculture, housing and so on. Definition and its standard are first given. Next, the present status of the art and the future trend in humidity sensors and also the related instruments are reviewd. Included are smart sensors or intelligent sensors composed of silicon micro-sensors and processing circuits, and electrolyte or ceramic sensors. Finally, the relationships between solid surface and its moisture sensitive characteristics, that is, chemisorption and physisorption of water vapour on the solid surface are discussed.
Alkali metal sulfates are basically applicable to SOx sensors as solid electrolytes. The electro-motive force (emf) of the solid electrolyte cells is expressed by the Nernst equation. The lowering of the emf is noticed at a large SOx pressures difference between the anode and the cathode. This is caused by the physical permeation of gases in the electrolyte through cracks and pores which arise from volume change during the III-I phase transition and from low sinterability, The use of NASICON (Na3Zr2Si2PO12) which is easily sintered to high densities provides an SOx sensor with no permeability. The generation of emf is essentially due to the formation of Na2SO4 both on the anode and on the cathode. The solid reference electrodes for SOx sensors are reviewed briefly. A small insert-type CO2 sensor has been constructed using Na2CO3 with a tip of NASICON or β-alumina, and is proved to function satisfactorily. The activity of Na2O in NASICON or β-alumina is taken as a reference state in this sensor.
A small biosensor is required in medical field and bioindustries. It is possible to prepare a small biosensor using semiconductor as a transducer. A micro enzyme sensor has been developed for clinical analysis. The micro enzyme sensor consists of immobilized enzyme and an ion sensitive field effect transistor (ISFET) which has been developed for a pH sensor. A micro enzyme sensor for neutral lipids has been developed using immobilized lipoprotein lipase (LPL) and ISFET, LPL was immobilized on an ISFET. The chip was modified with γ-aminopropyltriethoxysilane and thin organic membrane consisting of triacetyl cellulose, 1, 8-diamino-4-aminomethyloctane and glutaraldehyde. pH in the solution was changed following the lipoprotein lipase reaction. This pH change depends on the concentration of neutral lipids. The calibration curve was linear in the range 0.5 mM-2.0 mM. Furthermore, a micro enzyme sensor for urea was also developed using immobilized urease and FET. A micro oxygen sensor (Clark type) was prepared using semiconductor technologies. The micro oxygen sensor can be used as a transducer of the biosensor.
An FET sensor is an integrated device of the insulated gate field effect transistor (IGFET) and the chemical sensor and therefore is named as chemically sensitive field effect transistor (CHEMFET). In the CHEMIFET, the gate metal is replaced a more complex structure having chemically sensitive layer. CHEMFETs are new type of chemical sensors and have potential advantages over conventional chemical sensors in miniatuarization, robust solid state nature, mass productivity etc. According to the nature of the interaction between the species to be detected and the chemically sensitive layer, CHEMFETs can be divided into two groups : one that will measure gas concentrations, for example Pd gate FET (H2 gas sensor) and the other that will measure ion concentrations in the solution. The latter called an ion sensitive FET (ISFET). This paper describes the present status of ISFET, that is, its histoical survey, principle, fabrication method and ion selectivities.
As a result of the expanding applications of electronic devices for consumers appliances, thermistor sensors have rapidly begun to be used in many homes. It is supposed that the total quantity of the thermistor sensors produced in Japan in a year amounts to 30 million pcs. Since the mass-production of accurate thermistors for instruments has been achieved to meet the needs timely. The applications of thermistors are expanding, applications for microwave oven ranges, kerosene for heaters, plain paper copy machines, and electronic clinical thermometers are explained and also the thermistor humidity sensors with which absolute humidity can be measured are introduced. By detecting sudden increase in water vapor pressure with the thermistor humidity sensors, progress of the cooking can be detected, and very good controlled results are obtained.
Recently, in this country, town gas and LPG have come into wide use as fuel, giving us clean energy in burning. These gases have, however, potential hazards that might cause explosion when they leak out accidentally or mistakenly. To cope with such the risk, gas sensors have been put into practical application and thusspread quickly. These are two kinds of practical gas sensors, semiconductor gas sensor and catalytic combustion type gas sensor. Although they are both working well at present, further improvement would be necessary because their gas sensing capabilities are required to be steady and stable in a long run even under severe environmental stresses, and they should not be supposed to make any wrong alarms. This paper is intended to report on analysis of the chemical mechanism of the interaction between the sensors and fuel gases, and also some physical property changes observed at the sensor surfaces, as acted upon by both the fuel gases and environmental stresses, such as temperature, humidity, miscellaneous gasses, and so on.