233

OVERVIEW OF GAS SENSORS FOR ENVIRONMENTAL USE

b b Tetsuro Seiyamaa, Takeshi Nakahara and Takashi Takeuchi

aProfessor Emeritus, Kyushu University (Mailing Adress: Tokuyama Soda Co., Ltd., Fukuoka Branch, Sanwa-bldg., 1-10-24 Tenjin, Chuoku, Fukuoka-city 810, Japan)

bFujisawa Research Laboratory, Tokuyama Soda Co., Ltd., 2051 Endo, Fujisawa-city 252, Japan

1. IMPORTANCE OF ENVIRONMENTAL PROBLEMS AND GROWING NEEDS FOR GAS SENSING

Recently, air pollution became a serious problem again as global environmental problems such as greenhouse effect, acid rain and ozone layer destruction. Of these, the greenhouse effect is the most important and difficult problem. Many scientists warned global warming due to greenhouse effect originated from the increase of CO, concentration in the air. The increase of the atmospheric temperature for past twenty years was 0.3"C whereas it is said to be another 0.3"C for next ten years and 3°C at the end of the twenty-first century, which will bring the sea level rising of 65cm. The second global environmental problem is the acid rain. Some components, especially NO, and SO,, of the exhaust gas from combustion engines, boilers and furnaces make rain acidity (acid rain) to destroy trees and forests. The damage by acid rain has become serious especially in Europe and the east coast of US. The third problem is ozone layer destruction. Chloro-fluorocarbon is decomposed into C1 atom by ultraviolet light and induces chain reactions to destroy ozone molecules in the stratosphere, which results the increase, of ultraviolet light fallen to the ground and causes the increase of skin cancer.

Under the background of growing interest to the global environmental problems, countermeasures and regulations for not only problems directly related to the global environment but also more local environmental problems are now being actively reinforced. In Japan, NO, pollution especially due to Diesel engine vehicles is considered to be one of the most important and urgent problems. The rapid reduction of NO, emission from Diesel engines, however, is considered to be difficult so that besides the control of emission from individual Diesel engines, the control of vehicle type to promote replacing high emission Diesel engines by lower emission engines is being planned to be introduced in the large cities in Japan. In US also, the clean air act was revised in 1990 and the regulations and standards for NO,, CO, HC, SO, emissions were widely altered to diminish urban air pollution, acid rain and ozone layer destruction.

Typical air pollution problems are summarized in Table 1, in which the related gases (both harmful and beneficial gases) and the harmful emission sources are listed. To solve these pollution problems, various countermeasures, which are also given in the table, are trying to be taken. The monitoring and measurement of gases related to the air pollution are the most fundamental to performing many of these countermeasures successfully, so that handy and

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Table 1 Air pollution problems and related gases

Air pollution Related gas Main cause or source Countermeasure problem

Global problem Greenhouse effect C02, CFC, CH4, N20, Combustion of fuel

Decrease of forest Eruption of volcano Use and loss of CFC Substitution and collection of CFC

Acid rain SO2, NO,, NH3, HCI Combustion of fuel

0 3

Ozone layer CFC, BCFC, BFC, Decomposition of O3 destruction CH3CC13 CCI4, O3 with C1 in W-l ight

Local problem Smog sO2, HCI, CO, Industrial waste gas

Combustion of coal Photochemical NO,,SO,, Combustion of fuel

Sulfuric acid mist

smog Non-methane HC

Saving energy Improving efficiency Changing fuel & energy Forest conservation

Desulfurization & denitration from fuel & exhaust gas

Improvement of combustion

Substitution & collection of CFC

Smoke treatment

Same as acid rain

CFC: Chlorofluorocarbon BCFC: Bromochlorofluorocarbon BFC: Bromofluorocarbon

reliable gas sensors are strongly desired to be developed. The needs of gas sensing for controlling air pollution can be classified in three cases. The first one is for grasping the actual circumstances of the air pollution, for example, understanding the details of the generation and consumption of CO, and CH, on global scale. The second is the routine watching of pollutant gases in the atmosphere and in combustion exhaust gases. For this purpose, conventional analytical instruments are used at present but more simple and easy methods are desired t o increase the number of monitoring stations and emission sources. The third need of gas sensing is for closed-loop control of combustion engines, boilers and furnaces. The application of oxygen sensors to the feedback control of the air-fuel ratio of automobile engines is the only one example in which gas sensors are widely used at present for the use of closed-loop control.

Table 2 shows the concentration range and Japanese standard for several pollution gases existing in the air and exhaust gases, which are attracting special interest recently. Considering from the standpoint of sensor development, pollution gas sensors can be divided into two groups with different required specification. The one is for sensing in the atmosphere. For

Table 2 Concentration range and Japanese standard for gases related to environmental problems

co 0 3 CFC(*l)

Concentration Air pollution range to be (0.01-0.5ppm) detected Boiler,combuster

(10-2OOOppm) Automotive exhaust (10-5OOOppm)

water heater (105OOppm)

Heater,

Air pollution Air pollution Air pollution Air pollution Air pollution (0.001-2ppm) (0.01-1Oppm) on the ground (200-4OOppm) (lppt-lppb)

Coal or Gas poisoning (0.01-lppm) Boiler, burner Leak from oil burner (5-1OOOppm) Stratosphere (1-10%) refrigerator (1-1OOOppm) Tunnel of mine (0.1-lOppm Automotive blppm)

Incinerator (1-1OOppm) or 109-10* exhaust (10-500ppm) Boiler, burner molecules/cm3) (5-20%)

(10-1OOOppm) Automotive exhaust (0.1-10%)

Japanese Standard Air pollution NO2

<O.OPO.OGpprn (dayly average)

Exhaust gas Fixed facility 60-8OOp~m(*~) Automobile 0.48gflrm

(passenger car)

(heavy died) 6.8-7.8&Wh

General 5ppm (NO21 allowable limit 25ppm (NO)

<0.04ppm <lOppm oxidant (dayly average) (dayly average) <0.06ppm <O.lppm 4OpPm (hr. average) (hr. average) (8hrs. average)

0. 1-5m3/hr(*3) 2 . 7 g h (passenger car) 9.2gkWh (heavy diesel)

2PPm 5Oppm O.lppm 5000Ppm

(*1) CFC: Chlorofluorocarbon (*2) depend on kind and scale of facility (*3) depend on height of chimney and location of facility N

W wl

236

this purpose, the required concentration range is very low, in general, under 1 ppm so that extremely high sensitivity is required for sensors. The response time and the resistance against the environment are not so severe. The other group of sensors is for sensing in exhaust gases from emission sources. In this case, fast response and strong resistance against high temperature exhaust gases are required for the sensors. The concentration range is generally above 1 ppm, which is rather high in comparison with the sensing in the air. The accuracy and selectivity are equally required for both sensing in the air and in exhaust gases.

2. METHODS OF GAS SENSING RELATED WITH ENVIRONMENTAL PROBLEMS

2.1. Conventional Analytical Methods Conventional analytical methods for environmental use have been

established for the routine watching of pollutant gases in the atmosphere and in combustion exhaust gases. Equipments based on various analytical methods have been put into practice and are widely used in the field. Table 3 presents conventional analytical methods for measuring gases related to environmental pollution. The objective gases can be classified into two groups, i.e., gases at relatively high concentration in the emission and those a t extremely low concentration diffused in the atmosphere. In the table, therefore, measurable gases of conventional analytical methods are categorized by concentration ranges. Classifying various kinds of these methods, there are three main types, that is, optical type which utilize the change of absorption spectrum or luminescence spectrum of gas, chemical analysis based on the chemical reaction of an absorbed gas with a specific reagent, and gas chromatography which use a variety of detectors. Of these, chemical analysis, which is a wet method, has a few disadvantages such as low reaction efficiency between the absorbed gas and the specific reagent, and difficulty for the continuous measuring of the gas. For these reasons, chemical analysis is being displaced by dry methods i.e. optical type or gas chromatography. Among the dry methods, infrared spectroscopy, an optical type, has some features such as variety of measuring gases and simplicity of the measuring principle. Therefore, the applied fields of this method must be further widespread if high sensitive infrared detector and infrared laser which enable both to enhance the sensitivity and to miniaturize is developed.

Analytical conventional methods presented in the table are basically used to measure gases in one spot. It is, therefore, difficult to monitor the degree of the environmental pollution in terrestrial scale. Recently interesting new approaches have been tried in order to extend the monitoring area. For example, perpendicular distribution evaluation of ozone concentration in the stratosphere by a laser radar, total ozone monitoring in extended area by a total ozone mapping spectrometer equipped in an artificial satellite, multi-gas measurement such as CO,, CH, and N,O by an atmospheric periphery infrared spectrometer, high sensitive and accurate analysis of various pollutant gases by a semiconductor laser infrared spectrometer have been carried out lately. Gas monitoring in terrestrial scale must become more important in the future but further details of the new methods is omitted in this paper.

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Table 3 Conventional analytical methods for measuring gases related to environmental Dollution

Analyticalmethod NOx SOX C02 0 3 CFC' HCb CO

Infrared 8 Q Q. Q Q c m spectroscopy

Ultraviolet 8 8 8. spectroscopy

(asoxl a ant) Absorptiometry 43.

0 . Chemiluminescence 0 analysis

(as oxidant) Coulometry Q . Conductometric @a Q

Thermal conductivity 8 analysis

detector Flame ionization detector Q Q c m

rn Flame photometric

a:Chlorofluorocarbon b:Hydrocarbon 8 :Available for monitoring gases in the emission. 0 :Available for monitoring diluted gases in the atmosphere.

detector

2.2. various of Gas sensors Equipments which utilize conventional analytical methods exhibit excellent

accuracy and reproducibility but generally are large, heavy and expensive. Moreover, they are complicated in operation and require much labor in maintenance. Gas sensors which are remarkably smaller, cheeper, simpler and easier than conventional equipments are, therefore, urgently desired to get more data a t more points. Various types of gas sensors which applied a variety of principles and materials have been proposed. Table 4 shows types of gas sensors which is specified by detecting principles and their detectable environmental pollutant gases. Most gas sensors listed in the table have not been put to practical use yet but are on the way to development, so that the sensors in the table were chosen by the feasibility judgment of their detectable gases and concentration ranges. All kinds of environmental pollutant gases except for SO,, chlorofluorocarbon and hydrocarbon in the atmosphere can be detected by the gas sensors as shown in the table. It is, therefore, expected that they will greatly contribute to solve the environmental problems in near future. Principles and features of gas sensors are briefly given as under.

2.2.1. Semiconductor Qpe Semiconductor gas sensor is based on electrical conduction changes, caused

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Table 4 Gas sensors for measuring gases related to environmental pollution

Type of gas sensor NOx SOX C02 0 3 CFCa HCb GO

Semiconductor @a O Q Q Q

Electrochemical Q 0 (liquid electrolyte) O Q

Electrochemical 8 8 -

Catalytic combustion Q Q (solid electrolyte)

Optical Q Q 8 Q

Oscillator G3.Q Q

a:Chlorofluorocarbon b:Hydrocarbon :Available for monitoring gases in the emission.

0 :Available for monitoring diluted gases in the atmosphere.

by contact with gases, of metal oxide semiconductors such as SnO, or ZnO and organic semiconductors such as phthalocyanine or polypyrrole. A number of studies have been done on semiconductor gas sensors since they were developed independently by Seiyama et al.[ll and Taguchi[21. Gas sensors consisting of SnO,, in particular, have been first put to use to detect leakage of combustibles by Figaro Engineering Inc., Japan, in 1968. Since then the applications of SnO, gas sensors have been widespread in various fields for detecting leakage of toxic gases, checking for alcohol, monitoring for cigarette smoke and sensing for odor substances.

The relationship between sensor resistance of semiconductor type and gas concentration can be expressed by the exponential function

In R = k + aoln C

where R is the resistance of the sensor, k and a are the constants and C is the concentration of gas. This equation means that the semiconductor gas sensor is able t o detect wide concentrations of gases, with high accuracy at low concentrations. Furthermore it can detect a variety of gases, electron donative and attractive gases, which undergo the electronic reaction with semiconductor surfaces. Fig. 1 shows the sensitivity characteristics for several gases of the commercialized SnO, gas sensor(TGS822, Figaro Engineering Inc.). The resistance of the sensor decreases as a logarithmic function of the concentration of gases. The sensor can detect various electron donative gases, e.g. ethanol, H,, CO and so on. On the other hand, the lack of selectivity become a problem in case of detecting a specific gas in mixed gases. In order to enhance the selectivity to a specific gas, optimization of the operating temperature, addition of sensitizers or dopants or control of the morphology are

239

Figure 1. Resistivity variations of comercialized Sn02 semiconductor sensor for various gases.

Gas Conc. / ppm

widely investigated. Semiconductor gas sensors are especially expected to be applied for detecting environmental pollutant gases such as NO,, CO, ozone, chlorofluorocarbon and hydrocarbon.

2.2.2. Solid Electrolyte Qp Electrochemical gas sensors detect gases based on the electromotive

force(EMF) o r the current of an electrochemical cell due to the electrochemical reaction of a particular gas. Solid electrolyte which a specific ion can selectively permeate is used as a diaphragm. Potentiometric type gas sensors have been most widely adopted. Among them potentiometric oxygen sensors composed of partial stabilized zirconia have already had practical application and been extensively used for the feedback control of the air-fuel ratio of automobile engines. The oxygen sensor elements are composed of the following electrochemical cell.

EMF is represented as

E = (RT/4F)ln(Po2 JJ/PO, I )

where Po, I and Po, II are the partial pressure of oxygen in reference gas and the one in sample gas respectively, and RT/F has usual meaning. As far as

is kept constant, PO2n can be obtained by monitoring the value of the

Numbers of detectable gases have been increased since Gauthier et a1.[31 first proposed the feasibility of solid electrolyte gas sensors for SO,, NO,, CO, and so on. Moreover the couple of a metal oxide solid electrolyte and an auxiliary electrode, investigated recently, makes it possible for stable operation and disuse of a reference gas. Electrochemical gas sensors has a strong point of detecting gases selectively because only one kind of ion can permeate through

%id.

240

the electrolyte. In the present stage, however, there are unsettled problems such as the discrepancy of EMF from the Nernst's equation at a low temperature and the insufficiency of sensitivity to gases at low concentrations. To overcome these problems, further researches on materials of solid electrolytes and electrodes should be needed. Solid electrolyte gas sensors are anticipated to be developed for detecting SO,, NO, and CO,.

2A3. Liquid Electrolyte Type Liquid electrolyte gas sensors, which employ electrolytic liquid solution, detect

gases by taking out the current as sensor signals, which generates when these gases are electrochemically oxidized or reduced a t an electrolytic liquid/electrode interface, This type has earliest been put into practice among gas sensors. There are two main types, constant potential electrolysis type and galvanic cell type. The former is more excellent because the sensor of this type is available for detecting various gases related to environmental pollution such as NO,, SO,, CO and ozone. Furthermore, because of diversity of the electrolytic potential for each gases, constant potential electrolysis type is capable of detecting a gas selectively by applying proper voltage. Short life caused by the evaporation and leakage of electrolytic liquid have been pointed out, these disadvantages was fairly solved by modifying electrolytic liquid, electrodes and gas permeable membranes.

22.4. Catalytic Combustion Type Catalytic combustion gas sensor is based on the resistance change of an

element composed of a platinum coil covered with a highly oxidative catalyst. When inflammable gases contact with the element, the resistance change is caused from the temperature rise due to a combustion reaction and is converted into the electrical signal. From the practical point of view, a bridge circuit consisting of a sensing element and a compensating element is adopted in order to get rid of the influence of the ambient temperature change. The sensitivity of catalytic combustion gas sensors t o a inflammable gas is determined by the molecular heat of combustion of the gas. Therefore, it is easy for this type to measure single gas but difficult to detect a specific gas among mixed gases. In recent years, many investigations on stabilizing the catalyst have enabled the long term stability of the sensor. Catalytic combustion gas sensors are applicable for monitoring relatively high concentration of CO, chlorofluorocarbon and hydrocarbon in exhaust gases.

22.6. Optical 'Qpe Optical gas sensors utilize the optical change due to the interaction between a

gas and a sensing material. The development of this type of sensor is being forwarded through the improvement of fiber optic communication and its circumferential techniques. Optical fiber, therefore, is often used for this type of sensor. Fiber optic sensors can be classified into two types. That is, unfunctional type in which an optical fiber only works as an optical propagation path, and functional type in which the whole or partial path of the fiber plays a role for gas sensing. In the former, the material in charge of the interaction with a gas is usually applied on the edge surface of an optical fiber, and the change of the intensity or phase of light is propagated through the fiber. In the latter, the method which utilizes evanescent waves is most widely used.

24 1

Gas sensing materials are doped in the cladding of an optical fiber. Refractive indices of core and cladding are adjusted as to satisfy the condition for perfect reflection. In this system, the objective gases are detected from the change of evanescent waves caused by the interaction between a gas and a sensing material. The sensor can be sensitized by repeating perfect reflections many times.

Optical gas sensors generally have some characteristics such as a high insulating capacity, independence of electrical noises and operation in safety. Moreover it is possible for them to detect a variety of gases by choosing a gas sensing material. For environmental use of optical sensors, applications for NO,, SO,, CO, and CO are expected.

2.2.6. Oscillator Oscillation type gas sensors consist of a material capable of selectively

adsorbing and desorbing objective gases and the slight change in the weight of the material due to adsorption or desorption is converted into electrical signals by piezo-electric actuators. These sensors can be classified into two types, i.e., the one utilizes bulk acoustic waves of piezo-electric oscillators and the other makes use of SAW(surface acoustic wave) devices. The interaction between a gas and an adsorbent occurs at the surface of a sensor, so that the SAW type sensor responds t o the surface state more sensitively than the bulk acoustic wave type sensor. SAW sensors generally consist of a dual delay line of which two interdigital electrodes were fabricated onto a piezo-electric actuator as shown in Fig. 2[4]. The sensor layer, an adsorbent for a objective gas, was formed on one delay line. The other line is used to compensate the temperature change. Oscillator type gas sensors are being investigated for detecting NO,, SO,, hydrocarbon, NH, and H,S.

2.2.7. Other Types Besides the sensors mentioned before, FET(fie1d effect transistor) type and

RF amplifier Sensor layer

RF amplifier

Low pass u+ Mixier . Acoustic absorber

filter

Figure 2. Surface acoustic wave gas sensor consisting of dual delay line oscillator[4].

242

capacitor type are interesting regarding the detection of gases related to an environmental pollution. The former, first proposed by Lundstrom[51, utilizes the gate potential change of a metal-oxide-semiconductor field effect transistor due to contact with a gas . NO,, NH, and H,S sensors were fabricated by selecting the proper gate material. The latter detects gases by the capacitance change of a metal oxide capacitor or a MOS(meta1-oxide-semiconductor) capacitor and so far CO, and NH, sensors have been investigated.

3. RECENT RESEARCHES AND DEVELOPMENTS OF GAS SENSORS FOR ENVIRONMENTAL USE

3.1. General lkends Recently gas sensors for environmental use have been focused on the

atmospheric monitoring and the emission control as increasing demands in the world. In order to digest trends of researches and developments on gas sensors for environmental use, the sensors are picked out among reports which were presented in latest five years at two main international conferences on sensors, "International Meeting on Chemical Sensors" and "International Conference on Solid-state Sensors and Actuators". The total 132 gas sensors concerning environmental problems, reported a t the conferences, were classified by their types and detectable gases and the results are shown in Fig. 3. As for the type of sensor, semiconductor type, solid electrolyte type and liquid electrolyte type are predominant and occupy above 70% of total numbers of the reports. Optical type and SAW type based on new detection principles also hold 20% and the number of the reports is increasing year by year. On the other hand, speaking of gases to be detected, five kinds of gas i.e., 0,, NO,, CO, hydrocarbon and CO, occupy above 80% in the whole reports. Among those gases, 0, as for the emission control of automobiles, CO and hydrocarbon as for the detection of the leakage of combustible or toxic gases have intensively been studied in the past. A number of reports on these gases indicate the movement of research from security use to environmental use. Furthermore, the fact that the number of reports on NO, and CO, have rapidly increased in several years reflects the concern about environmental problems. On the other hand, there are only a few reports concerning SO,, ozone and chlorofluorocarbon. This is due to the lack of enough sensitivity of the present gas sensors to detect these gases diffused in the atmosphere at extremely low concentrations.

Now, gas sensors are required to have several characteristics as listed below in order to detect gases related to environmental pollution. (1)sensitivity: excellent sensitivity is necessary to detect gases especially those of very low concentrations in the atmosphere. (2)selectivity: gas sensors are required high selectivity so as to detect a specific gas in the exhaust where various kinds of inteference gases coexist and their concentrations widely change. Of cource, selectivity is also necessary in case of monitoring a gas in the atmosphere because the concentration of coexisting gases are generally much higher than that of the gas to be detected. (3)response time: fast response is needed especially for the monitoring in the exhaust. In order to control the emission of gases, shorter response time than the time constant of actuator systems is demanded. (4)reliability: because of the lack of long-term stability or rather large

243

Fig.3 Numbers of gas sensors for environmental use presented a the International Meeting on Chemical Sensors (1986,1990) and the International Conference on Solid-state Sensors and Actuators

(1987,1989,1991).

dependency on temperature and humidity, practical applications are limited only in a few fields though many types of gas sensors have been proposed. Reliability of the sensor must be decisive for the practical application in environmental use.

In order to satisfy these characteristics, many attempts are being made for various types of sensors. As for semiconductor type, the improvement in sensitivity is proceeded by thinning a sensor layer using new manufacturing techniques such as sputtering, evaporating and langmuir-blodgett method. Regarding the enhancement of selectivity, not only new sensing materials such as metal phthalocyanines, TiO, and In,O,, but also processing of sensor signals obtained from a multi-sensor array enable to detect a specific gas selectively. In solid electrolyte type, the measuring range of oxygen has been enlarged by limiting current type oxygen sensors and those sensors are being applied for controlling the emission from lean burn engines. NO,, SO, and CO, sensors, using the combination of chemically and thermally stable metal oxide electrolytes and auxiliary electrodes composed of metal salts which enable the stable operation, are also been widely studied. Also in liquid electrolyte type, interesting researches such as chlorofluorocarbon sensor with the combination of a pyrolyzer and a sensor, and NO, and SO, sensors using a new electrolyte have been reported. Furthermore a variety of sensors based on new principles e.g. optical type, oscillator type, pattern recognition method and so on have been investigated. Further studies on sensor materials, sensing principles and signal processing are desired to put gas sensors in practical use in this field.

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3.2. Individual Environmental Gas Sensors 3.2.1. N0,SenSor

Acid rain is mainly caused by NO, and SO, emitted from combustion facilities such as automobiles and boilers. Of these gases, NO is generated in the combustion flame by the reaction of N, and 0, and gradually converts to oxides of higher order, which are mainly NO,, in the atmosphere as the time elapsed. There are, therefore, two applications of NO, sensor, the one for detecting NO which is a predominant component of NO, in the combustion emission and the other for monitoring NO, in the atmosphere. NO, is, in general, chemically active and gives rise to electronic, optical and thermal reactions with a variety of materials due to the adsorption on their surface. By utilizing those characteristics, various types of NO, sensors have been proposed. Present status of NO, sensing summarized by Takeuchi[6] is shown in Table 5, where

Table 5 Present status of NOx sensing [6]

Concentration range Accuracy of NOx (%I

1PPb IPPm 1% 0.1 1 10 0 1 . . . . . I . I I I

Requirment for:

Analytical mefhod:

Handy sensors:

MePc a - H PolYPyrrole

SnO2 TiOz H mp

Wo3 H Inz03-Sn02 H hO3-v206 H CnO3-Nb206 H

phosphoric acid - rm Na-P/P”-AhOdNaN03 H Ra NASICON/NaN03-Ba(NOs)z H rn RbAg416 H Nafion H

metal porphine H MePca H

MePc a - ma

Sn02(FETC ) H m MePca (FE‘f ,seebeck effect) - czz?a

air pollution monitor - em combustion gas H ma

chemiluminescence - infrared absorption H

Semiconductor type

l5a mp

5 5

H eaa

rma rrm mp rn

Electrochemical type

rza prm

Optical type

SAW type

Others

Em ma

a:Metal phthalocyanine b:Surfase acoustic wave c:Field effect transistor

245

Figure 4. Molecular structure of metal phthalocyanine.

the concentration range and accuracy of various kinds of NO, sensors developed are listed together with conventional analytical methods and with requirements for monitoring of air pollution and of combustion gas. As for air pollution monitoring, semiconductor sensors [7-151 and SAW sensors[4,16-181 using metal phthalocyanines and solid electrolyte sensor utilizing RbAgI5[ 191 are expected because of their high sensitivity. Especially, phthalocyanines are chemically stable and are, therefore, most feasible materials for NO, monitoring in air. For detecting NO, in combustion exhaust, metal oxide semiconductor sensors[20-291, electrochemical sensors[30-331, optical sensors[34-361 and FET sensors[37-39] have been developed. Among them, sensors using ceramics as NO, sensitive materials have the ability of in-situ monitoring due to their thermal, chemical and mechanical stability. Judging from the performance of NO, sensors, several significant sensors are described in some detail in the following.

Metal phthalocyanines, macrocyclic compounds in which various metals cdinate at the center as shown in Fig. 4, have been extensively studied on NO, sensors since Sadaoka et al. reported the p-type semiconductance of metal phthalocyanines being changed by the adsorption of NO, on their surfaces[40,411. Detailed studies on coordinated metals done by Jones et a1.[42,431 revealed that some metal phthalocyanines, like Pb phthalocyanine, prepared by vacuum sublimation could detect NO, in the concentration range of 1 to lOOppb and were promising materials for monitoring NO, in the atmosphere. There were, however, some disadvantages such as large influence of humidity, slow response especially in recovery and the lack of reproducibility. Many attempts was carried out to overcome the disadvantages, and the following several noticeable results were obtained.

Sadaoka et a1.[10,11] of Ehime University, Japan, found that the response to NO, is much improved by annealing in a proper condition after preparing metal phthalocyanine films by vacuum sublimation. For example, Pb phthalocyanine annealed in air or N2 at 603K showed the excellent response characteristic i.e., 20 seconds in response and a few minutes in recovery even in operating at room temperature. The fast response, which allow the operation in room temperature, owes to the morphology of Pb phthalocyanine films consisting of fine triclinic crystals with mean diameters of the order of 0.2pm. The influence of preparation conditions of Pb phthalocyanine on NO, sensing characteristics was also studied with spectroscopic and electrical techniques by Mockert et all71 of University of Tubingen, F.R.G. Fig. 5 shows 0 1s XPS spectra of Pb phthalocyanine films after different treatments, that is, (a) as preparation under UHV(u1tra-high-vacuum) conditions, (b) after exposure of UHV-prepared films to air and (c) after heating the air-exposed films in

lo' gatzys isorbeci 102- chemisorbed

IOH-

I

1 0'- lead oxide

I I I I

f ' '

a I 1 1 1 I 1 I

Figure 6. Configuration of the coplanar sensor and the current Daths through

540 536 532 528 524 Binding Energy I eV

Figure 5 . 0 Is XPS spectra of Pb phthalocyanine thin films after different treatments. a:as preparation under UHV(ultra-high-vacuum) conditions, b:after exposure of UHV-prepared films to air, c:after the air-exposed filmes in 300mbar 0 2 a t 423KL71.

300mbar 0, at 423K. Literature data of the binding-energy values of O,(gas), O,(physisorbed), Oi(chemisorbed), OH- and 02-(as in PbO) are also indicated. During first exposure of the UHV-prepared thin films to air, small amounts of surface OH groups are formed, which are a prerequisite for subsequent reversible conductivity changes in sensor application. By this preparation procedure, the formation of lead oxide at the surface, which is the cause of the deterioration in sensor performance, can be avoided. Pb phthalocyanine sensor in this study can detect NO, at ppb orders of concentrations reproducibly with fast response.

To improve sensor performances, a new sensor structure was proposed by Ruihua et a1.[131 of Semiconductor Institute of The Chinese Academy of Science, China, and by &in et all141 of University of Tongji, China, taking into account of the fact that the electronic interaction between metal phthalocyanines and NO, occurs at the surface of metal phthalocyanines. The configuration of the coplanar sensor and the current paths through the sensor layer are illustrated in Fig. 6. A pair of interdigital electrodes are formed on the one face of the layer whereas a control electrode is deposited on the other face, and they were then connected as shown in the figure. When the same voltage is applied across the interdigital electrodes and across the phthalocyanine layer, most of the electric current through the bulk phthalocyanine flows into the control electrode and the rest of the electric current through the surface region flows into the interdigital electrode. Therefore, the current flowing between two interdigital electrodes is sensitive to the change in the surface state. This is the

Pb(ok Cu) phthalotyanine 1 I Pb(or Cu) phthalocyaniie film[13,1&

247

reason why the sensor with the coplanar structure shows high sensitivity and fast response.

To enhance the heat-resistance of metal phthalocyanines, the polymerization of silicon and germanium phthalocyanines was investigated by Jeffery et a1.181 of Admiralty Research Establishment, U.K. Metal phthalocyanine chlorides were autoclaved to form hydrates, followed by condensated to obtain thin polymer films.

130"Chbar 400°C PcMCl&M:Si,Ge)+PcM(OH), d ( P c M O ) , ( n 2 5 0 )

2hr lhr

Both polymers respond to ppb levels of NO, at a sensitivity comparable to that of Pb phthalocyanine. The polymers exhibit good heat-resisting characteristics and are stable even in operating in the 200-250°C temperature region. Moreover the silicon polymer exhibits a negligible response to water vapor.

Another important NO, sensors using metal phthalocyanines are SAW gas sensors. SAW devices are attractive for gas sensor applications because of their high sensitivity and reliability, so that NO, sensors using SAW devices with metal phthalocyanines have been extensively studied by many researchers. Nieuwenhuizen et al. of Prins Maurits Laboratory TNO , Netherlands, have actively studied on the SAW gas sensors and as an example of the researches they reported the response of SAW gas sensors for NO,, in which different metal phthalocyanines were deposited on one delay-line of a dual delay-line oscillator[l61. Fig. 7 indicates the response of the sensors as a function of NO, concentration at 150°C. The response is defined as frequency differences divided by the thickness of a phthalocyanine layer. As shown in the Figure. Co phthalocyanine is most sensitive to NO,, followed by Cu phthalocyanine and H,

2 m 1 - 1 ' 1 . 1 - . H2Pc -k MgPc

COPC

x CUPC A FePc,

r(

. Figure 7. Response variations of metal phthlocyanine SAW sensors with NO2 concentration[l61.

248

phthalocyanine. Co phthalocyanine is also preferred for selectivity and Cu phthalocyanine is preferred for response time. It is concluded that the transduction mechanisms of SAW sensors are a combination of changes in mass and in conductivity caused by several chemical and physical processes. The Langmuir-Blodgett film deposition technique, a new preparation technique for metal phthalocyanine films, was applied on the fabrication of SAW sensors. Tetra-4-tert butyl Si phthalocyanine chloride films was formed on a SAW oscillator by using LB technique and the NO, sensing behavior was studied by Holcroft et a1[41 of University of Oxford, U.K. The room temperature response of the sensor coated with monolayer showed response and recovery times comparable with those reported for other phthalocyanine-based sensors operated at much higher temperature. The detection limit of the LB film sensors was found to be 40 ppb of NO,.

Various kinds of metal oxide semiconductors have been investigated since SnO, was found to be able t o detect NO, sensitively by Chang[44,451. At the beginning of the researches and developments, SnO, was focused as an active material, but recently other metal oxides have been studied intensively rather than SnO, because of the problems of SnO, such as a lack of sensitivity and a drift of sensor signals.

For the use of combustion monitoring, a TiO, semiconductor NO, sensor has recently been developed by Satake et al.[251 of Tokuyama Soda, Japan, and is

0.4 I I I 1

-0.8 1 Ta

0.0 0.4 0.8 1.2

Figure 8. Sensitivity of Ti02 semiconductor sensor doped with various elements[251. sensitivity : Sg = log (Rb/Ra)

S NO a:Oppm b:200ppm(5%,02) SC3H6 a:Oppm b:500ppm(500ppm,NO + 5%,02)

249

0.0

Figure 9. Sensitivity variations of In-doped T i 0 2 semiconductor sensor with NOx concentration[251.

1 10 loo loo0 loo00

NOx Conc. / ppm

going to be used practically. The TiO, sensor has great advantages of the ability of in-situ monitoring, being stable, easy handling and maintenance free. Thesensing characteristics of the TiO, sensor was found to be further improved by doping with three valent elements such as Al, Sc, Ga and In. Fig. 8 shows the relationship between the sensitivity to C,H, under co-existing NO and that to NO. In the figure, the sensitivity S is defined as logarithm of the ratio of the resistance Rb in the atmosphere composed of the gas species g of the concentration b to the resistance Ra in the atmosphere consisting of the same gas species of the concentration a.

Sg = log(Rb/Ra)

The sensitivities to NO of TiO, are enhanced by adding the elements both in case of three valent elements and five valent elements. The sensitivities to C,H,, in turn, become higher by doping with five valent elements, lower with three valent elements. NO and NO, concentration dependencies of the sensitivity of TiO, doped with In are shown in Fig. 9. The sensitivity to NO of the sensor is much higher than that to NO,, and increases linearly in a wide concentration range of NO. The sensor also exhibited the fast response to NO with 90% response time of shorter than 30 seconds. These results indicate that TiO, doped with three valent elements are the promising material for in-situ detection of NO, in combustion gases. For other metal oxide sensors, WO, thick filmsr241, IT0 thin films(90%In2O,+10%Sn0,) prepared by magnetron sputtering technique[26,27], mixed oxides such as Al,0,-V,06[281 and Cr,0,-Nb,O6[291 with proper compositions and are also investigated to utilize as NO, sensors.

As for electrochemical NO, sensor, liquid electrolyte type has been developed for a long time, but these days solid type, which solid electrolyte is adopted instead of electrolytic liquid, is widely studied. Ba(NO,), was used as a solid electrolyte in the beginning of investigations[3], but there were some disadvantages like EMF drifts and necessity of reference gas. A new sensor composed of P/P"-Al,O, as a solid electrolyte and NaNO, as an auxiliary

250

Septum

Argon gas

Na-reference electrode

Glass tube Nd-solid Na-P/P'-A1203 as solid electrolyte[311.

Figure 10. NOx sensor using

electrolyte Platinum NaX mesh (+electro. cond.)

electrode was proposed by Hotzel et a1.[31]. of Max-Planck-Institute, F.R.G. The structure of the galvanic cell is illustrated in Fig. 10 and the cell arrangement is shown below.

Pt/Na/JVa-P/P" - Al,O,/NaNO,/Pt ,NO, ,O,

And the overall cell reaction is expressed in the following.

Na + NO, +1/20, = NaNO,

Also in the sensor, reference gas is unneeded by using Na as the solid reference electrode. The EMF of the cell to the NO, partial pressure is in excellent agreement with the values calculated from Nernst's equation. At 430K NO, can be measured down to less than 0.1 ppm and the response time varies from a few seconds up to some minutes for high and low concentrations of NO,, respectively. There are, however, some disadvantages in the sensor operation. That is, the limitation in operating temperature due to the melting point of NaNO3(307"C) auxiliary electrode and large dependence on water resistance. Shimizu et a1.1321 of Kyushu University, Japan, investigated materials for an auxiliary electrode and indicated that NO, sensing performance is further improved by using Ba(NO,),-NaNO, or Ba(NO,), as an auxiliary electrode. These nitrates enable the operation a t higher temperature, the decrease of humidity dependence and fast response because of their high melting points and the water resistances. The use of the binary nitrate electrode of Ba(NO,),- NaNO,, for example, made it possible to operate at temperature up to 500°C without deteriorating the sensor performance. The sensor is expected for in- situ monitoring of NO, in exhaust gases from the viewpoints of stability and selectivity. The overall development of NO, sensors described above is in advance of other

global environmental gas sensors. Present major problems for developing simple and handy sensors are in their durability and reliability, so that detailed studies to improve their durability and reliability should be urgently carried out.

25 1

3.2.2. SO, Sensor SO,, one of main causes of acid rain, is exhausted by the combustion of sulfur

compounds in fuels. Therefore, SO, sensors are required for monitoring both in the atmosphere and in the emission from combustion facilities. Table 6 shows the present status of the research and development of SO, sensor in comparison with conventional analytical equipments and the requirement of environmental SO, monitoring[61, in the same manner as Table 6 for NO, sensors. As is seen in the table, highly sensitive SO, sensors suitable for air pollution monitoring are hardly obtained yet even in the form of prototype. Simple and handy sensors, in the present stage, are for monitoring SO, more than 1 ppm corresponding to the concentration in exhaust gases. Among them, solid electrolyte gas sensors are actively proposed and examined. In the beginning of the researches and developments, alkali-metal sulfates such as &SO,, Na,SO, and Li,SO, have been evaluated as solid electrolyte materials. Their cell potentials, however, unstable because microcracks were generated in the electrolyte due to the phase change during thermal cycling. Difficulties with gas reference electrodes such as maintaining a constant reference gas composition were also pointed out. To overcome these disadvantages, multi- phase sulfate-electrolytes[46-501 with a solid reference electrode and metal

Table 6 Present status of SOX sensing 161

Concentration range Accuracy of sox (%)

1PPb 1PPm 1% 0.1 1 10 e m . e . ( . I I

Requirment for:

Analytical method:

air pollution monitor 1-1 ma combustion gas H FZza

solution conductivity - flame photometry - infrared absorption H ultraviolet absorption H

Em Ez4 md Ea

Handy sesors: Electrochemical type

phosphoric acid - Fa K2So4 - ap Li~S04-AgzSO4 - P-Ak?03/NazS04 - - - - N ~ Z S O ~ - L ~ Z S O ~ - Y ~ ( S O ~ ) C ~ - - Cu(PBza )thiophenolate H pyrenelperylene H

Dzi mzr eua ma

Dzi em

NASICON/NazS04 CaF2-CaS04 NazS04-Y2(S04)3

SiOfliS04-NiO Optical type

nm rn

____ ____ ~ _ _ _ _

a:Tribenzylphosphine

252

Silver embeded Two phase e'ectrolytF / in two phase electrolyte

Platinum mesh -[g Silica tubes I Themocouple

Gas in

Gold leads Catalyst

Figure 11. 'ho-phase sulfates electrolyte SOX sensorC481.

oxides solid electrolytes[51,52] or a metal fluoride solid electrolyte[531 together with sulfates like Na,SO, as an auxiliary electrode have been recently proposed.

Fig, 11 illustrates the schematic diagram of a successful example of the two- phase sulfate-electrolyte sensors developed by Worrell et al.[48] of University of Pennsylvania, USA. The cell composition can be expressed as,

Ag/Li,S0,-23mol%AgzS04/S0,,0,,S0,

As shown at the top of Fig. 11, silver metal is embedded in the right-hand side of the two-phase electrolyte to form the solid reference electrode which enable the disuse of the reference gas. The left-hand side of the two-phase electrolyte is exposed to the gas mixture containing SO,. The Ag,SO, chemical potential is the same in both sides of the two phase electrolyte, resulting no chemical interaction between the reference electrode and the two-phase electrolyte. The cell potentials for the two-phase sulfate-electrolyte sensor are in excellent agreement with the values calculated from Nernst's equation for the range of 3 to 10000 ppm of SO, concentration. The lower detection limit for SO, is determined by the thermodynamic decomposition of Ag,SO,, which is around

ppm SO, in air at 803K. Pt and V,O,, oxidation catalysts, are used t o minimize the equilibration time and improve the response time of the sensor because the response time of the sensor depends upon the equilibrium reaction of S0,-0,-SO, gas mixture. Results for long-term tests showed excellent stability that the measured cell potentials(EMF) were maintained within f 3 mV of the calculated values for 100 days.

Solid reference electrode, another key component of the solid electrolyte SO, sensors, was also widely investigated to disuse the reference gas and to simplify the sensor structure. S o far metal-sulfate(e.g., Ag-Ag,SO,) and oxide- sulfate(e.g., MgO-MgSO,) were proposed and among them most successful solid reference electrodes are Ag-Ag,SO, mixture and NiO-NiSO, mixture. For example, the EMF of the cell using NiO-NiSO, mixture as the solid reference electrode shows good agreement with the calculated values according t o Nernst's equation for the range of 30 ppm to 1 % of SO, at 923m49,50].

253

There are some optical SO, sensors which utilize a change of photo transmittance caused by the interaction of SO, with photoactive material[54] and a quenching of fluorescence from fluorescent materials like pyrene- perylene charge transfer couple due to SO, adsorption[551. These sensors, however, have rather low sensitivity to SO, and are considered not to be suitable for environmental monitoring.

3.2.3.C0, Sensor Carbon dioxide is chemically inactive so that some kind of sensors such as the

semiconductor sensor is not adequate to detect CO, because the semiconductor sensor operates on the principle based on chemical reaction of gases a t the surface. Most of simple and handy sensors are based on the effect of adsorption, desorption and electrochemical reaction. sensors are summarized together with the requirement of environmental monitoring and conventional analytical equipments[6]. Recently, electrochemical sensors, especially solid electrolyte CO, sensors have been actively studied and developed[56-631.

A metal oxide solid electrolyte with an auxiliary electrode, investigated recently, enable the stable operation and the disuse of a reference gas.

In Table 7 , current status of CO,

Table 7 Present status of CO2 sensing [6]

Concentration range Accuracy of c02 (%I

1PPb 1PPm 1% 100%0.1 1 10 * . . . . ] . I I I

Requirment for:

Analytical mefhod:

Handy sensors:

air pollution monitor n Fza combustion gas H ma

infrared absorption - thermal conductivity H

ma zzan

Electrochemical type P-f%03/Na2C03 - Qza NASICON/NazC03 - nm YSZ a INASICONMazCO3 - mp NASICONDJazCOs-Bacos - Em Li+ conductor/LizCOs H am

K2COdPEG H

fluoresceine/F’EG H Em LiTaOs H rm

polyethyleneimine H P a

mixed oxides (capacitor) H Rza zeolite (thermal conductivity) H rn

eza nm

h ydroxya patitdCaClz H

Optical type

SAW type

Others

a:Y203-Zr02 b:polyethyleneglycol

254

Maruyama et a1.[57] of Tokyo Institute of Technology, Japan, reported CO, sensing characteristics of galvanic cells composed of Na+ conductor like NASICON or P-A1,0,, and Na,CO, as an auxiliary electrode. The cell composition is expressed as follows.

Au,C0,,0,/Na2C03/NASICON or P-Al,O,(Na,O)/O,,Au

The EMF of the cell depends on the partial pressure of CO, and is good agreement with the values calculated from Nernst's equation because the activity of Na,O is constant in NASICON or P-Al,O,. Furthermore, the simplification and the miniaturization were accomplished by using a solid reference electrode.

One of the problem of the Na,CO, auxiliary electrode is the degradation of EMF response caused by a water vapor interference[59,60]. The interference occurs because Na,CO, tends to change chemically or electrochemically t o other compounds such as NaOH, NaHCO, and Na,CO,.xH,O. New auxiliary electrodes employing binary carbonates composed of Na,CO, and alkaline earth metal carbonates was found to remarkably diminish the water vapor interference by Miura et a1.[59,601 of Kyushu University, Japan. Fig. 12 illustrates the structure of the sensor element using Na&O,-BaCO, as an auxiliary electrode. Binary carbonate was fixed to NASICON disk tightly by melting and crystallizing method. The EMF is perfectly linear to the logarithm of CO, concentration in the whole range tested(4-400000 ppm) at 823K as shown in Fig. 13. It is again noted that the presence of water vapor(2.7kPa) scarcely

Inorganic adhesive

Na'conductor

Figure 12. C02 sensor using BaC03-Na2C03 as auxiliary electrode and NASICON as solid electrolyte[59,60].

255

-150 I I I I I

A wet C 0 2 (2.7 kPa-H2O) 0 dry C02 //! -250 -

-350 - \

-450 -

-550 - / w

-650 I I I 1 I

Figure 13. EMF variations of the COZ sensor using BaCOs-Na2C03 as auxiliary electrode and NASICON as solid electrolyte for dry and wet C02[59,601.

affect the EMF values as seen in the figure. The solubility of BaCO, in water is extremely smaller than that of Na,CO, so that BaCO, enhances to the water vapor resistance of the binary electrode, and in the result the sensor exhibits the excellent stability against water vapor. The rate of response and the long- term stability are also improved by using the binary electrodes. Judging the CO, sensing characteristics of solid electrolyte sensors, the sensor using the binary electrode is considered to be closest to practical applications.

A new type of CO, sensor, a capacitive type sensor consisting of BaTiO, mixed with a metal oxide, was recently proposed by Ishihara et a1.[64,65] of Oita University, Japan. The CO, sensing characteristics of the mixed capacitor were examined and the results is summarized in Table 8 . Both sensitivity and optimum operating temperature strongly depend on the kind of oxides mixed with BaTiO,. In particular, CuO mixed with BaTiO, exhibits the high sensitivity a t the CO, concentration up to 2% so that i t is suitable for air pollution monitoring. On the other hand, NiO-BaTi03 is able to detect CO, at the higher concentration up to 20%, showing to be applicable for combustion monitoring. The optimum operating temperature for CO, sensing gives a good correlation with the decomposition temperature of carbonate corresponding to the metal oxide mixed with BaTiO,. Capacitance change is, therefore, considered to be caused by carbonation of the metal oxide so that a high selectivity is expected. Other examples of CO, gas sensors are, polyethylene grycol-inorganic salt sensor based on the conductivity changer661, hydroxyapatite-CaC1, sensor based on the ion conductivity change[67], Ca2+- exchanged zeolite sensor utilizing the thermal conductivity changeI681, fluoresceine dispersed fiber optic sensor[671 and polyethyleneimine coated SAW sensor[701. They have their own features but there are many problems to be solved for accomplishing the sensors.

All of the sensors described above are chemical sensors utilizing the chemical interaction between CO, and a sensing material. In point of conventional analytical equipments, a extremely miniaturized IR sensor for CO, was recently developed by Shibata et.a1[71,721 of Sanyo Electric, Japan. Miniaturization was achieved by using a new type of pyroelectric IR detector composed of LiTaO,, a solid-state chopper and anoptical filter in a same

256

Table 8 CO2 sensing characteristics of mixed oxide capacitor [641

Oxide mixeda Dielectricb ~~~~~~e Sensitivityd Upper limit o@ with BaTiOs (CcwCair) detection (%I constant (K)

CaO 19.64 >1173g 0.891 8 22.21 ll40 0.329 10 MgO

La203 11.28 1039 0.451 8

Y203 10.21 1032 0.794 10

CeO2 22.09 934 0.410 8

NiO 47.81 828 0.441 20

zro2 17.45 915 0.740 10

Fez03 27.17 614 0.678 2

Biz03 27.83 718 0.824 2

v 2 0 6 19.86 1,000 0 Si02 13.42 1.000 0

Nd203 11.27 823 0.641 6

PbO 66.94 774 0.711 6

CUO 109.61 7% 2.892 6

c0304f 171.93 801 0.362 6

a:BaTiO3:oxide=l:l(molar ratio). b:Dielectric constant of element a t 573K in air. c:Optimum temperature for C02 detection. d:Sensitivity to 2% C02 e:A linear relationship between sensitivity and COZ concentration exists below

this concentration. f:BaTiOa:C~04=3:l(molar ratio). g:Higher than 1173K

package. The volume of the gas sensor is 1/4 smaller than that of the conventional IR equipment. The accuracy of measurement is within + 5 % for the concentration range of 0 to 20% CO,. It is expected the application for environmental monitoring because of high sensitivity and accuracy.

36.4. Ozone Sensor Table 9 shows the summary of the present status of ozone sensing[61. From

the view point of global environment, a major need for ozone sensing lies in the measurement of ozone concentration in high altitude of atmosphere. Especially, small and light sensors which can be mounted on a sonde or a rocket are desired. Liquid electrolyte sensors are being used now for such

251

Table 9 Present status of 0 3 sensing [61

Concentration range Accuracy of 0 3 (S)

Requirment for:

Analytical mefhod:

Handy sesors:

air pollution monitor

chemiluminescence ultraviolet absorption

Semiconductor type CeOz.-Inz03/SiOz Pd-SnOda-Ab03 c0304

H H H

Electrochemical type galvanic cell H Nafionherchlobc acid H

purpose but are restricted to use up t o 35 km height because they utilize electrolytic liquid. Solid-state semiconductor sensors, such as Iq0,[731, SnO,r74] and Co,O,[75] sensors are expected to overcome the restriction. Among the sensors, an In,O, thin film sensor developed by Takada[73] of New Cosmos Electric, Japan, has highest sensitivity and superior response speed. The structure of the sensor is illustrated in Fig. 14. In,O, thin film is formed by physical vapor deposition. In order to enhance the sensitivity and selectivity, CeO, is impregnated as a form of (NH,),Ce(NO,),, followed by hexamethyldisiloxane(HMDS), which would convert to SO, , is deposited by chemical vapor deposition. It is reported that the sensor can detect ozone below 1 ppb and is good in humidity dependence of the sensitivity and in long-term stability.

HMDS-CVD 1-03 thin film impregnated with (m4)2Ce(N03)6

Pt film electrodes

A1203 substrate

P t film heater

Figure 14. Schematic diagram of 111203 thin film ozone sensor[73].

258

Table 10 Present status of CFC sensing [61

Concentration range Accuracy of CFC (a)

1 . . 1 . . 1 . . . 1 I I I lppt 1PPb 1PPm 1% 1 10 100

Requirment for:

Analytical mefhod:

Handy sesors:

air pollution monitor 1-1 m leak detection H - gas chromatograph 1-1 mp infrared absorption H m

Halogen detector H eza Semiconductor type S-Sn02 I--------I Qza V-Mo-AhOdZnO H ma LazOa-LaFs H em pyrolyzer+SnOz H E?a

pyrolyzer+galvanic cell H lGz4 Electrochemical type

3.2.6. Chlomfluorocarbon Sensor There are two major needs for chlorofluorocarbon sensing. The one is the

detection of leaked chlorofluorocarbon from refrigerators, air conditioners, etc. The other is the monitoring chlorofluorocarbon concentration diffused in the atmosphere. Chlorofluorocarbon sensors being developed now are available only for the detection of the leakage because the level of the atmospheric chlorofluorocarbon concentration is too low to be detected by the present sensors, as is seen in Table 10, which shows the present status of chlorofluorocarbon sensing[6]. Halogen leak detectors have been used widely to check the leakage of chlorofluorocarbon from refrigerators. Recently, semiconductor type chlorofluorocarbon sensors have been developed.

Semiconductor materials being used are Sn0,[761,Zn0[771 and LqO,-LaF,[781. As chlorofluorocarbon is rather stable, the former two sensors are assisted by a catalyst or catalytic dopant to enhance the selectivity to chlorofluorocarbon. A variety of dopants were added to SnO, and sulfur was found to be effective for sensitivity by Nomura et a1.[76] of Figaro Engineering, Japan. Fig. 15 shows the sensitivity to various gases of the sensor doped with 1 mol% of sulfur together with that of the sensor without sulfur. As is seen in the figure, sulfur enhances the sensitivity to chlorofluorocarbons whereas it decreases the sensitivity to alcohols which are interference gases. It is considered that the sensitizing effect of sulfur is due t o the acceleration of SO:- species on the SnO, surface on the cleavage of C-C1 bond in chlorofluorocarbon molecule. The sensor has already been put into practice and is utilized in the detection of leaked chlorofluorocarbon and in the chlorofluorocarbon recovery system which will be described in section 4.2.

Chlorofluorocarbon can be measured with usual electrochemical halogen sensors after decomposing chlorofluorocarbon with a platinum heater. Fig. 16

259

10 t J - 9

0 Without sulfur With sulfur

j 8

8 6

‘ 7

h 5 ‘5 4

TI 3

c,

.H c,

m 8 2

1 R-11 R-12 R-113 R-22 Ethanol n-Octane i-Propanol

Figure 15. Sensitivity of SnOz sensor doped with and without sulfur for chlorofluorocarbons and interference gasesl761.

illustrates the sensor system reported by Komiya et a1.[791 of Bionics Instrument, Japan. Sample gas containing chlorofluorocarbon is introduced into a pyrolyzer and decomposed by a heated Pt filament. The decomposed gases are detected by the electrochemical sensor. All kinds of chlorofluorobabon could be detected by the combination of the pyrolyzer and the electrochemical HF sensor.

3.2.6.0ther Sensors

measurements have been made mainly on non-methane hydrocarbon because From the view point of former environmental hydrocarbon sensing,

sensor

Flow meter

Power source Pump

Figure 16. Chlorofluorocarbon sensor system composed of a pylolyzer and an electrochemical sensor[791.

260

unsaturated hydrocarbon is one of the source material of photochemical smog whereas methane is hardly related to phtochemical reaction. In the field of security, rather high concentration (several thousands of ppm) of leaked methane and propane has been needed to be detected. On the other hand, relatively low concentration (several ppm) of methane in the atmosphere becomes a problem in relation to global environment because methane makes the second largest contribution to the greenhouse effect. Unfortunately, we do not have any sensors which are simple, handy and are able to detect ppm level of hydrocarbon including methane selectively, even in the research and development stage. Semiconductor sensors have the highest potential to detect such a low concentration of hydrocarbon among various hydrocarbon sensors being investigated at present. To improve the sensitivity and selectivity to hydrocarbon of semiconductor sensors, some proposals have been made by

several researchers, for example, SnO, sensors assisted by various sensitizers[80,81] or a catalyst[82], multi-layered SnO, sensorC831 and 'y-Fe,O, thick film sensor[84].

Also for CO sensing, the present sensors are available only for the field of security not for environmental use because of the insufficient sensitivity and selectivity to monitor CO in the atmosphere. Examples of CO sensor which have been improved their sensitivity and selectivity are, for example, SnO, semiconductor sensors operated under periodic temperature cycle[85-871, a electrochemical sensor using nafion membranel881, a catalytic combustion sensor composed of catalysts and hydrophobic polymer[891, a SnO, diode sensor doped with Pd[901 and an optical fiber catalytic sensor with Au/Co,O, as combustion catalys t[9 11.

Oxygen sensors have played an important roll to control the emission of NO,, CO, and hydrocarbons from combustion facilities. Especially, electrochemical sensors using partial stabilized zirconia and TiO, semiconductor sensors have been extensively studied and had practical applications as h-sensors for automobiles. A new type of oxygen sensors, limiting current type oxygen sensors which utilize the oxygen concentration dependence of the limiting current characteristics of zirconia electrolyte cells, with wider measuring concentration range of oxygen have been developed[92-971. Practical limiting current type oxygen sensors are divided into two groups according t o gas diffusion methods, that is, pinhole type sensors[92-941 and porous layer type sensors[95-971. In the former sensors, a cover with a pinhole attached to the outside of the cathode of a zirconia cell acts as an oxygen diffusion rate- determining device. The limiting current of the sensor depends only on the dimension of the pinhole and the ambient oxygen concentration. In the latter sensors, the porous layer attached to the outside of the cathode of the cell play the same role as the cover with the pinhole of the pinhole type sensor. The primary advantage of the porous layer type sensor is quick response because the inner space on the cathode side is very small in comparison with the pinhole type sensor. Because of many features of the limiting current type oxygen sensor such as small size, simple structure, low cost besides wide measuring concentration range, the sensor is promised t o be applied for environmental monitoring.

H,S and NH, are not only poisonous but also offensive-odored so that the sensors for environmental use are required enough sensitivity comparable to the human olfaction. Many types of sensor have been proposed by many

26 1

researchers. SnO, semiconductor sensor doped with sensitizers[98] is preferred for H,S sensing because the sensor detect H,S selectively down to 10 ppb. Electrochemical sensor[99] and FET sensor[100] are preferred for NH, sensing because of their high sensitivity.

4. APPLICATION TO POLLUTION CONTROL

4.1. Automobilea The first practical application of oxygen sensors to automobiles was made in

1978 for three-way catalytic converter systems by several automobile makers in Europe, Japan and US to meet Japanese or California emission standards. In these systems, the aidfuel (A/F) ratio of engine exhaust gas is detected by an oxygen sensor and controlled at an optimum value near stoichiometric A/F ratio, where NO,, CO and HC are simultaneously reduced by a three-way catalyst[lOl]. The realization of the three-way catalytic converter system gave not only a real solution for Japanese and US regulations but also the following three impacts to automobiles. First, it showed that low emission can be compatible with low fuel consumption and good drivability, whereas the other low emission systems were difficult to. Second, it was the first introduction of microprocessors to automobiles and later developed into various computer- controlled systems in automobiles. Third, understanding of ceramics was greatly improved because key components of the system, the catalyst and the oxygen sensor were composed of ceramics and found to be durable even in the severe condition of the automobile exhaust gas. In Japan, the three-way catalyst system is now used in most passenger cars because of such excellent performance.

Most of oxygen sensors being used in this system are zirconia concentration- cell type oxygen sensors, whereas a part of oxygen sensors are titania semiconductor type oxygen sensors. The basic researches and developments of these oxygen sensors were almost completed and further researches and developments are being tried in relation to the improvements on detailed characteristics and durability.

Since the second oil crisis in 1978, saving energy became the most important subject in the world and low fuel consumption vehicles were actively developed. As one of these trials, a closed loop control lean burn engine system shown in Fig.17 was developed and put into practical use by Toyota in 1984(Kimbara et a1.[102] Matsushita et a1.[103]). The system was the first real lean burn system which satisfies low fuel consumption without deteriorating drivability. In the case of Toyota 1600 cc vehicle, fuel economy was improved about 18% from 14.4kmA to 17kmA in Japanese ten mode test by introducing the lean A/F control[l02]. In this system, a zirconia limiting current type oxygen sensor developed also by Toyota (Saji et a1.[95], Kamo et a1.[1041) was installed. The sensor can detect A/F continuously t o control the A/F of exhaust gas to predetermined values in lean A/F. The limiting current type oxygen sensor has many varieties and was studied by many workers since then. They are summarized in the reference [105].

After the conquest of the second oil crisis, the trend of the automotive market turned and headed from low fuel consumption vehicles to high-grade and high- performance vehicles. As a result, the lean burn system with the limiting

262

Swirl Control Valve Sequential Injection

/ Distributor

Lean A/F Ratio Sensor

F P I I Pressuren I Ignition, Coil I@

Figure 17. Automotive lean combustion system with lean A/F(oxygen) sensor[l02].

current type oxygen sensor used in only part of vehicles. Recently, as the interest in global environmental problems increased, lean combustion engine systems with lean A/F sensors began to be actively developed again by several Japanese makers to reduce CO, emission. One of the most important problems for the lean combustion systems to be used widely is that the systems are applicable only for small vehicles at present because the heavier the vehicles, the more deficient in power and also the larger in the amount of harmful emission. Therefore, for the lean burn system to spread more widely, a more efficient lean burn engine has to be developed. One of approaches solving the problem is t o control the engine to its optimum A/F ranging all part of A/F according t o the driving conditions. For this purpose, wide range A/F sensors are being developed also.

Applications of oxygen sensors to systems other than AD' control are also being tried. An example is a closed loop control of the EGR rate of an engine to reduce NO, generation, where EGR (Exhaust Gas Recirculation) rate is the ratio of the mass flow rate of exhaust gas recirculated into the intake manifold to the air mass flow rate in the intake manifold. To investigate the performance of the closed-loop EGR control system, Nishida et a1 al. of Mitsubishi group used a wide range limiting current type oxygen sensor, whose output varies nearly linearly with oxygen concentration and does not depend appreciably on pressure. They showed the effectiveness of the precise control of EGR ratio to reduce NO, emission[l061.

42. Industry A major need for industrial applications of simple and handy gas sensors

exists in detecting gases in the emission from industrial facilities. Gas sensors

263

Housing Sensing Inlet Lead element I I Connector

Figure 18. T i 0 2 semiconductor NOx sensor probe designed for in-situ monitoring of exhaust gases[l071.

for this use are classified into two groups. The one is for sensing NO,, SO,, CO, CO,, hydrocarbons and oxygen emitted from combustion facilities like boilers o r turbines for monitoring combustion conditions and for controlling the combustion or exhaust treatments. The other is for detecting chlorofluorocarbon, hydrocarbon, ozone, H,S and NH, generated from oil refining plants, semiconductor factories and food manufacturing factories to detect the leakage of gases and to control exhaust treatment. In both cases, fast response and selectivity to a specific gas are required for the sensors. Moreover in the former case, strong resistance against high temperature exhaust gases is peculiarly required for the sensors. Because the sensors in researches and developments hardly satisfy these requirements, irrespective of a lot of needs described above, only a few sensors have been put in a practical application, of which some examples will be given in the following from the practical standpoint.

Handy NO, analyzer for in-situ exhaust monitoring has been first developed byTokuyama soda applying TiO, NO, sensor described in section 3.2.1.[251. The structure of a sensor probe designed for in-situ monitoring is shown in Fig. 18[107]. The sensor element composed of TiO, with a dopant is incorporated with a heater and a thermistor and is mounted in stainless housing so as to protect the element from particulates in exhaust gases. The thermistor is used to maintain the operating temperature of the sensor element. That is, applied voltage on the heater is controlled so as to maintain the thermistor output constantly when the temperature or the flow rate of the exhaust gas changes. Fig. 19 shows the relationship between the sensitivity of TiO, sensor and NO, concentration simultaneously measured by a chemiluminescence type NO, analyzer for the exhaust gases of various combustion facilities. In the figure, results obtained by model gas experiments(N0+5%,0 +lO%,H,O+N,) are also drawn as the solid line. As seen in the figure, the sensigvity to exhaust gases of NO, sensor is in good agreement with that to model gas, indicating NO, can accurately detected by the sensor irrespective of the kind of combustion apparatuses and fuels. Furthermore, results of long-term operating test in the exhaust gas of a kerosene heater showed that the sensor was stable over 2000 hours. Commercialized handy NO, analyzer is shown in Fig. 20. The analyzer consists of a sensor probe and a controller. By directly inserting the sensor

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1.0 I

'

, A WaterheateraPG) 0 Boiler (Fuel oil A)

Modelgas 0.8 - 0 Stove(Kerosene)

0.6 - (/3 Figure 19. Correlation between

0.4 - sensitivities of Ti& semiconductor NOx sensor to NOx in exhaust gases of various combustion facilities and the sensitivities to NOx in model gas[lO71.

. -

0.0 I L

10 100 loo0 NOx Conc. / ppm

Figure 20. Handy NOx analyzer using Ti02 semiconductor NOx sensor.

probe into a exhaust gas, NO, concentration for the range from 10 to 3000 ppm in the exhaust gas is displayed at the controller. The analyzer is much smaller and lighter than conventional analytical equipments and needs no maintenance.

A new type of liquid electrolyte gas sensors capable of simultaneously monitoring SO, and NO in flue gases at elevated temperature was constructed by Bergman et al.[108] of Health and Safety Executive, U.K. A mixture of pyro- and meta-phosphoric acids as liquid electrolyte and a metallized-membrane electrode were employed to cope with the corrosive substances in flue gases. The upper operating temperature, 280°C, of the sensor is determined by the heat-resist ing temperature of the gas-permeable PTFE(po1y- tetrafluoroethylene) membrane. Two gold electrodes, separated by a narrow strip, were sputtered on to the PTFE membrane. A pair of potentials of 0.70 and 0.78 V were selected to maximize the differential response to SO, and NO, while to avoid any significant interference from coexisting CO. Fig. 21 shows the response curves of the sensor to stepwise changes in the concentration of SO, and NO. The system shows temporary cross-talk peaks, but no long-term interferences remain. A metal-sinter filter and a thermistor were also adopted

265

1200

loo0

r

400ppm 130ppm 850ppm 3%pm - so2 N O SO2 N O .!l P .

Figure 21. Response variations of the liquid electrolyte sensor with step change in concentration of SO2 and NO[ 1081.

so as to protect the device from abrasive flue dusts and compensate the temperature dependence of cell responses, respectively. The accuracy for SO, and NO is zk 10 ppm or 5 % of the reading between 60 and 280"C, which cover the range of 0 to 50000 ppm with 90 % response time shorter than 1 minute.

A chlorofluorocarbon recovery system incorporated with SnO, sensor was proposed by Setoguchi et a1.[109] of Figaro Engineering, Japan. Fig. 22 illustrates the configuration of recovery system for R- 113(CC1F,-CC1F2), which is one of main chlorofluorocarbons used as a cleaning solution and a solvent. The recovery of R-113 is achieved by adsorbing R-113 on activated charcoal and separating it from air. In this method, however, the recovery efficiency of R-113 decreases as the capacity of activated charcoal deteriorates. Gas sensors in the system are utilized not only for checking the capacity of activated charcoal by monitoring R-113 at the outlet of exhaust gases, but also for selecting the flow path of gases by measuring R-113 both at the inlet of gases to be treated and the outlet of separated R-113. The sensor, which was enhanced the sensitivity and selectivity to R-113 by doping with sulfur, is further improved in the long-term stability and in corrosion resistance considering the condition which the sensor contacts constantly with concentrated R-113.

43. Other Applications In consumer's use, applications of gas sensors related to environmental

pollutant gases are carried out mainly for security and amenity. Examples of the applications are, for instance, in the protection from a incomplete burning of combustion apparatus for home use and in the ventilation or condition of indoor atmosphere. A combustion monitoring sensor using SnO, was developed by Tanaka et a1.[1101 of Figaro Engineering, Japan. The gas water-heater

266

Air, R-113

R-113

Figure 22. Chlorofluorocarbon recovery system using SnO2 semiconductor sensors[l09].

configuration fitted with a SnO, sensor is shown in Fig. 23. The sensor is placed in exhaust gases and is operated by an intermittent heating cycle between 400°C for 20 seconds and 150°C for 40 seconds in order to sensitize to reducing gases in the emission. The resistance of the sensor is measured 39 seconds after heating to 150°C. At 150"C, the sensor resistance changed considerably with an increasing amount of reducing molecules generated by the incomplete burning. Actual tests for trial sensors in the exhaust gases of the water-heater confirmed that the sensing characteristics are quite stable for a long period of time.

Applications of simple and handy sensors under research and development to atmospheric air monitoring are considered to be restricted because of the lack of sufficient sensitivity. Only an electrochemical sensor used for the monitoring of ozone in high altitude atmosphere and an 1%03 semiconductor sensor for monitoring ozone in open air have had practical application. Although conventional analytical equipments are being utilized in this field, requirements for the application of simple and handy sensors are getting increased. The sensors feasible for atmospheric air monitoring, therefore, are chosen among a variety of sensors in the research and development stage. As for NO, sensing in the atmosphere, semiconductor sensors and SAW sensors using metal phthalocyanines are most feasible owing to their excellent sensitivities to NO,. Solid electrolyte sensors and capacitor type sensors are most preferable for detecting CO, in air because of their sufficient sensitivity. The most important key point to put them to practical use is considered to be in reliability and durability.

Water +-

267

Exhaust a T + gas

1: Sensor 2: Exhaust gas assembler

FT 1 5: Heat absorber

Air Gas

Figure 23. Water-heater equipped with SnO semiconductor sensor[ll0].

5. FUTURE SCOPE

In the above sections, the state of the developments of gas sensors related to environmental problems have been discussed. The final subject which should be discussed will be that how the gas sensors will play a role in satisfying the growing needs of gas sensing related to environmental problems in future. As for the first need of gas sensing, that is, grasping the actual circumstances of the air pollution, highly precise and selective measurements such as those of the decomposed or reacted products of chlorofluorocarbon in the stratosphere will progress by means of the improvement of analytical instruments rather than sensors as before. Here, we distinguish 'sensor' from 'analytical instrument' by that the 'sensor' means handy and small scale sensing device although it is difficult to draw a definite boundary line between them. On the other hand, sensors will play an important role on multiple and wide area measurements such as NO, and CO, distribution in a specified region.

The second need is the routine watching of pollutant gases in the atmosphere or in combustion exhaust gases. In the present, analytical instruments are mainly used for this purpose. Handy and low-cost sensors, on the other hand, are being developed actively and will probably be used widely in place of conventional analytical instruments in future. It is likely that the handy sensors will expand their use and extend into a place where the gas detection has never been made before.

For the third need of gas sensing, that is, the closed-loop control of exhaust gas, realization of handy and tough gas sensors are especially expected. Real closed-loop control systems installed on boilers or combustion engines will never be realized without sensors. Generally speaking, however, it will take several years until the realization of the practical application of gas sensors for this use with the exception of a few kind of sensors because the durability of the sensor in exhaust gas is difficult to attain.

268

The next subject is what types of sensors will be promising in future to satisfy the various needs mentioned above. Unfortunately we do not know the definitive type of sensors yet, which will surely satisfy the above needs and will be superior to any other type of sensors in future. From the standpoint of 'practical application within two to five years', semiconductor type and electrochemical type (both solid and liquid) will still maintain the leading position among various types of gas sensors. As for the research and development of gas sensors, new types of sensors such as FET, junction, optical, SAW and capacitive type gas sensors will be studied more and more actively. Of these new types of gas sensors, FET, junction, SAW and capacitive type sensors operate based on a mechanism common with the semiconductor type gas sensor. That is; the adsorption and desorption of gas molecules occur on the surface of sensing materials at the first step of sensing processes and finally the electrical or electro-acoustical properties of the sensing materials changes with gas concentration. Therefore, these types of sensors have common problems such as rather poor selectivity and necessity of heating, which essentially come from adsorption and desorption process. These new types of sensors, however, have their own advantages. For example, both the junction and FET type sensors are very sensitive and the SAW type sensor has frequency output, which is adequate for digital signal processing. Future success of these new types of sensors will depend on how their advantages can surpass their weak points.

The optical type gas sensor is in different situation from other new type of sensors described above. There are two types in the optical type gas sensors. One is the sensor based on an interaction between optical material and objective gases, where adsorption and desorption process play an important role too. The key point of the development of this type of sensor is in finding and utilization of new materials. The other type of optical gas sensor is based on the principle which measures the characteristics of objective gases directly with optical means. This type of optical sensor has the following special features which many of other type gas sensors do not have. The first one is that it is possible to operate without contacting with objective gases directly, which means i t does not necessarily have the problems originated from adsorption and desorption. The second is its high selectivity, which comes from the ability of utilizing spectrum information. The third is that an optical sensor which operates without any aid of electricity is possible to be fabricated if necessary. Thus the latter type optical sensors seem to be very attractive and in fact optical methods are used successfully in conventional analytical instruments although they are too large in size and complicated t o apply to sensors a t present. Trials to miniaturize the infrared spectrometer have been made actively as described in previous sections. These efforts are bringing the image of analytical instruments closer to that of sensors. Further drastic simplification, in future, will realize handy optical sensors, which are completely different from the concept of the analytical instrument.

As for the utilizing technique of sensors, multi-sensor systems composed of several gas sensors and multi-signal processing circuits can greatly enhance the selectivity even if the individual sensor has low selectivity. Furthermore, the instability of sensors is also possible to be compensated or reduced by the systems. Many efforts have been made for investigating multi-sensor systems. At the present level of the research and development, the identification of a gas species in simple gases is possible but that in mixed gases is difficult in

269

laboratory experiments. The practical application of these trials will be realized at first in a use where the objective gases exist to be easily distinguishable.

The combination of the technique of the multi-sensor system with thin or thick film- and micromachining- technology makes it possible to produce array sensors which enable us to get a sensing system being drastically miniaturized and having the same functions as the multi-sensor systems. The array sensors are ideal sensors in a sense. There are, however, the following several problems which have to be solved to realize the practical application. (1) In principle, the array sensors are promising to attain the high selectivity and stability, but it is not sure what extent it can. (2) Generally speaking, less durability is a weak point of the sensors fabricated with thin-film and micromachining technique. (3) The cost of the array sensors are expected to be very low when they are mass-produced. It, however, is really difficult to produce low cost sensors at present because of being in the dilemma of low yield, huge investment and not large quantity. To solve the problem, great progress and maturity will be necessary not only in the research and development of the array sensor itself but also in the manufacturing technique of IC and the related fields. The goal seems to lie far away, but the image of the sensor will completely be changed if the above problems are solved and the practical array sensors are realized.

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