review toward innovations of gas sensor technology · 2005. 7. 27. · sensors, such as electrolyte...

Sensors and Actuators B 108 (2005) 2–14

Review

Toward innovations of gas sensor technology

Noboru Yamazoe∗

Professor Emeritus of Kyushu University, Shimanoe Laboratory, Faculty of Engineering Sciences,Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan

Received 14 July 2004; received in revised form 7 December 2004; accepted 7 December 2004Available online 15 February 2005

Abstract

Although gas sensors have been almost matured in some application fields, there are a variety of newly emerging markets and poten-tial markets which will be substantiated when gas sensors are innovated sufficiently. The importance of materials design in innovatinggas sensors are demonstrated by taking semiconductor gas sensors and solid electrolyte gas sensors as examples. In addition, attemptsto make the sensor devices more intelligent and more quantitative are also important for further advancements of gas sensor technol-ogy.
© 2005 Elsevier B.V. All rights reserved.
Keywords:Gas sensor technology; Solid electrolyte; Semiconductor; Potential markets

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32. Overview of the technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1. Mature markets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. Emerging markets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3. Challenging markets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. Materials design for gas sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64. Semiconductor gas sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65. Solid electrolyte gas sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86. For innovations of gas sensor technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.1. Innovative sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.2. Intelligent sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.3. Hybrid systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7. Concluding remarks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Further reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Biography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

∗ Tel.: +81 925837537; fax: +81 925837538.E-mail address:[email protected].

0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.snb.2004.12.075

N. Yamazoe / Sensors and Actuators B 108 (2005) 2–14 3

1. Introduction

There are a variety of gas sensors so far developed. Somesensors, such as electrolyte solution based electrochemicalsensors and catalytic combustion type ones, were developeda long time ago for professional uses. A real sensor era hasstarted in 1970s during which semiconductor combustiblegas sensors, solid electrolyte oxygen sensors and humiditysensors were commercialized for non-professional uses. Onthe occasion of the first IMCS held at Fukuoka, Japan in1983, major topics of gas sensors were comprised of thesegas sensors. In the two decades since, extensive efforts havebeen compiled not only for advancing these sensors but alsofor developing various new gas sensors, which have been ingreat demand to make sure safety, health, amenity, environ-mental reservation, energy saving and so on.Fig. 1shows thestatistics of the whole sales of the sensors produced in Japanfor the year of 1998. Large shares are occupied by groupsof physical sensors, the group of gas and humidity sensorscounting only 1.6% of the whole sales. Unfortunately, thesestatistics failed to include oxygen sensors for car emissioncontrol, which are produced in a massive scale exceeding 10million sets yearly. The real share of this group should thenbe considerably larger than that. Even so it is true that the gassensors markets are still minor. Nevertheless, the importanceof gas sensors for our modern society is never so minor, ase s fora to bei ad-v er toi hisa crib-i donea

2. Overview of the technology

The atmospheric air we live in contains numerous kindsof chemical species, natural and artificial, some of whichare vital to our life while many others are harmful more orless.Fig. 2 illustrates the concentration levels of typical gascomponents concerned. The vital gases like O2 and humid-ity should be kept at adequate levels in living atmospheres,while hazardous gases should be controlled to be under thedesignated levels. As for lower hydrocarbons and H2, whichare used as fuels, their explosion after leakage into air is amajor concern for gas sensors, and 1/10 of lower explosionlimit (LEL) for each gas is taken as an alarming level forgas sensors. For toxic gases, offensive odors, volatile organiccompounds (VOCs) and other air pollutants, their standardshave been legislated by various laws based on the strengthof toxicity or offensiveness of each gas in Japan, as indi-cated by star marks in the figure. The full line shown foreach gas indicates the range of concentration safely coveredby a commercial gas sensor, while the broken line indicatesthat reportedly covered in a laboratory test. In the recent twodecades, these dilute components in air have emerged as tar-gets for sensory detection, but many of them are left yet tobe challenged. Some standards of VOCs such as benzene areseen to be less than 0.1 ppm, far out of reach by the presentgas sensors. Current trends in gas sensor technology are de-s

2

gass ch asc stione ing

nsors

asily exemplified by the importance of oxygen sensorutomobiles. Gas sensor technology has already grown

ndispensable for various aspects in our life. Yet furtherancements of the technology are strongly needed in ordmprove sustainability of our society and quality of life. Trticle aims at contributing to those advancements by des

ng the author’s personal opinions about what has beennd what is yet to be done in this field of technology.

Fig. 1. Sales of se

cribed in more detail below.

.1. Mature markets

With a history of more than 3 decades, markets ofensors have become almost mature in some fields suombustible gas monitoring, oxygen sensing for combuxhaust control and humidity sensing for amenity of liv

for 1998 in Japan[1].

4 N. Yamazoe / Sensors and Actuators B 108 (2005) 2–14

Fig. 2. Concentration levels of typical gas components concerned. Star marks indicate the standards of the gases legislated in Japan by (1) EnvironmentalStandard, (2) Ordinance on Health Standards in the Office, (3) Offensive Odor Control Law, (4) Working Environment Measurement Law, and (5) Ordinanceby Ministry of Health, Labour and Welfare.

spaces. As a matter of course, such mature markets can bea great base for developing new sensors to replace conven-tional ones. A good example can be picked up in the field ofcombustible gas monitoring. This field was first opened withsemiconductor gas sensors. However, catalytic combustiontype gas sensors, which have been popular traditionally inEurope, have been improved to satisfy the requirements ofgas alarms for combustible gases including CO in air. Com-petitions are serious in the field of CO gas monitoring where alimiting current type sensor using a proton conducting mem-brane shown inFig. 3 has emerged as a second competitor.The last one is an extended version of the mixed potentialtype sensors for H2 and CO we reported about 15 years ago[2]. This sensor operates at room temperature so that it isfavored by some users as a battery-driven, cordless sensor.

2.2. Emerging markets

There are several emerging markets of gas sensors as well.Fig. 4illustrates how a domestic house will be equipped withgas sensors in Japan. Various kinds of sensors to monitorCO2, air quality, odors and humidity are in increasing de-mand for various purposes. For example, a large number ofair cleaners equipped with an air-quality sensor are producedyearly to be installed not only in houses but also in car cab-

ins. It is noted that most of these applications are concernedwith combustible gases at fairly low levels and therefore bestmet by semiconductor gas sensors. An auto-damper system(ADS) for car ventilation is obtained by coupling two semi-conductor gas sensors which respond to hydrocarbons andNOx (NO and NO2), respectively. It opens or closes air inletdepending on whether the outside air is clean or polluted. As

Fig. 3. Structure and working mechanism of mixed potential type CO sensoroperative at room temperature.


Fig. 4. Various kinds of gas sensors equipped at various sites in a house[3].

shown inFig. 5, production of ADS is increasing year-by-year, exceeding 3 millions in quantity in 2001. Yet anotherexample is given by combination type fire alarms. Tradition-ally, fire alarms have used a smoke detector or a heat detector.In the event of fire, however, various combustible gases arealso produced and those gases, particularly hydrogen, diffusemore rapidly than smoke or heat does. In many cases sensingthose gases is useful for earlier detection of fire. Because ofthis advantage, a semiconductor gas sensor has been incorpo-

F paredw

rated additionally into the conventional fire alarms. Numberof combined type fire alarms is increasing steeply in recentyears, as shown inFig. 6. These examples indicate the highpotentiality of gas sensors in our society.

2.3. Challenging markets

Once a gas sensor (seed) is developed to meet a strongdemand from our society (need), a prosperous new marketwould be created. Numerous examples can be cited for suchdemands. Car emission control is going to be more and morestringent. Onboard sensors to monitor NOx, hydrocarbonsand CO are in strong demands to meet the stringent emis-sion control. The sensors are to be used not only to check the

F larmsi

ig. 5. Annual production of auto-damper systems for cars as comith that of cars in the world[4].

ig. 6. Annual sales of conventional type and combination type fire an Japan[5].


emission levels but also to diagnose the activity of three-waycatalysts. Onboard sensing of NH3 is also demanded for real-izing urea-based selective reduction of NOx. Car coolers aregoing to change to the ones mediated by CO2 in near futureso that CO2 leakage monitoring will be demanded for safetypurpose. Apart from car-related demands, monitoring of var-ious toxic or offensive compounds indoor and outdoor wouldbecome very important for securing health and amenity, as al-ready mentioned. Particularly, sensing of oxygen containinggases such as CO2, NOx and SO2 is badly needed for variousapplications. In addition, special attention should be directedto the various hazardous gases present in working places.For example, N2O and ethylene oxide gas frequently usedin hospitals are beyond sensory detection at present. Variouschemical reagents such as NH3, HNO3, HF and PH3 are usedin silicon machining factories but the vapors of these reagentsor their reaction products in the closed spaces are mostly leftas targets of future sensory detection. From a broader view-point, we are also requested to think of ubiquitous sensors toimplement the development of ubiquitous information tech-nology. All these examples indicate that our goal of the sensordevelopment is still far away.

3. Materials design for gas sensors

fulfillm tionsa ents,p tedo andr ingg terialsu ateri-a andd mate-r thus

for innovations of gas sensor technology. Here it is tried todemonstrate this for two types of gas sensors.

4. Semiconductor gas sensors

The gas sensors using n-type oxide semiconductors likeSnO2 detect gases from a change in the electrical resistanceof a porous sensing body. These sensors are best suited for de-tecting combustible gases at low concentration levels in viewof sensitivity, stability, robustness and so on. Despite the sim-ple working principle, however, the gas sensing mechanisminvolved is fairly complex. Sensing performances, especiallysensitivity, are controlled by three independent factors of re-ceptor function, transducer function and utility, as illustratedin Fig. 7. Receptor function concerns the ability of the oxidesurface to interact with the target gas. Chemical propertiesof the surface oxygen of the oxide itself is responsible forthis function in a neat oxide device, but this function can belargely modified to induce a large change in sensitivity whenan additive (noble metals, acidic or basic oxides) is loaded onthe oxide surface[6–10]. Transducer function concerns theability to convert the signal caused by chemical interactionof the oxide surface (work function change) into electricalsignal. This function is played by each boundary betweengrains, to which a double-Schottky barrier model can be ap-p theno s es-s iam-e e(sf g[ no yd de isl

F l as the s.

In order to be used in practice, a gas sensor shouldany requirements which depend on the purposes, locand conditions of sensor operation. Among the requiremrimarily important would be sensing performance-relanes (e.g., sensitivity, selectivity and rate of response)eliability-related ones (e.g., drift, stability and interferases). These are all connected with the sensing mased so that selection and processing of the sensing mls (materials design) have key importance in researchevelopment of gas sensors. The author believes thatials design is a base for new innovative sensors and

ig. 7. Receptor function, transducer function and utility factor as wel

lied. The resistance depends on the barrier height andn the concentration of the target gas. This situation ientially unchanged with a change in the grain size (dter,D) of the oxide unlessD is kept above a critical valuDc) which is just equal to twice the thickness (Ls) of surfacepace charge layer of the oxide. ForD smaller thanDc (6 nmor SnO2), sensitivity increases sharply with decreasinD11–13]. Since usuallyLs is a function of the concentratiof electron donors in the bulk oxide,Dc can be changed boping the base oxide with a foreign oxide. When the oxi

oaded with a foreign additive, the additive can modifyLs as

physicochemical or materials properties involved for semiconductor gas sensor


Fig. 8. Depth profiles of gas concentration inside a porous film at variousvalues ofm.

well if it interacts electronically with the oxide. In fact sucha change inLs or barrier height explains marked sensitizingeffects of certain noble metals like Pd for the sensors of thistype [14–17]. In the case of Pd-loaded SnO2, for example,under exposure to air Pd is oxidized into PdO, which acts astrong acceptor of electrons from SnO2. In this state, eachgrain of SnO2 is covered with a strongly electron-deficientspace charge layer, giving rise to a high resistance. Uponcontact to a combustible gas in air, PdO is reduced to Pdwhich is no longer an electron acceptor, resulting in a sharpdrop in the electrical resistance. It is noted that the sensitiz-ing effects come out through coupling a redox change of theadditive with a change in its electronic interaction with theoxide grains.

The last factor, utility, concerns the accessibility of inneroxide grains to the target gas. The importance of this factor ismade obvious when one considers that the target gas (reduc-ing gas) reacts with the oxide surface on the way of diffusinginto the balk of device. If the rate of reaction is too large com-pared with that of diffusion, the gas molecules cannot accessthe grains located at inner sites, leaving them un-utilized forgas sensing and thus resulting in a loss in sensor response.The existence of this factor was suspected a fairly long timeago from familiar volcano-shaped correlations between sen-sor response and operating temperature, but quantitative un-derstanding of it was made possible only recently for thin filmdp ncen-t filmcs adys intdniK

Fig. 9. Utility factor as correlated withm.

depth profiles of concentration depend markedly on the mag-nitude ofm. Form< 1, significant part of target gas can reachthe bottom of the film. Form> 3, however, most part of the gasis consumed before arriving at the bottom and at extremelylargemonly the surface region is accessible to the gas. Whenthe increase of sheet conductance at givenx is assumed tobe proportional to the target gas concentration at that pointas a first approximation, sensor response of the film can bederived easily by integrating the sheet conductance over thefilm. It is noted that the proportionality constant, called sen-sor response coefficient, used in the assumption is related tothe surface reaction and thus steeply increases with increas-ing temperature. The utility factor, defined as the ratio of thesensor response to that expected under the optimal conditionof m= 0, can be formulated to be equal to (1/m)tan hm. Asshown inFig. 9, the utility factor decreases with increasingmso thatmshould be kept small, or, smallL and smallk/Dkratio should be combined to keep the utility factor close tounity. SinceDk = (4r/3)(2RT/πM)1/2, wherer is pore radius,M the molecular mass of target gas and RT has its usual mean-ing, the film has to haver as large as possible andL as smallas possible to obtain higher utility factor. This is in agree-ment with the experimental result that the responses of SnO2thin films to H2 decreased with increasingL [18]. Further,ris known empirically to be roughly comparable to the size(D) of grains involved so that the utility factor can also bec en-s ervedf e inm

pedc s. Thes -c oef-fi oesd ats lyt d

evices derived from SnO2 sols[18,19]. When a thin film withores of a uniform radius is exposed to a target gas at a co

ration of Cs, the relative concentration (C/Cs) inside thean be formulated to be,C/Cs = cos h[1− (x/L)]m/cos hm, byolving a simple diffusion-reaction equation under the stetate conditions. Here,C is the concentration of target gashe film, Cs the concentration of target gas atx= 0, x theistance from the surface,L the film thickness andm is aon-dimensional quantity defined bym=L(k/Dk)1/2 wherek

s rate constant of a first order surface reaction andDk thenudsen diffusion coefficient. As illustrated inFig. 8, the

ontrolled throughD. Strikingly sharp dependence of sor response on the grain (crystallite) size has been obsor H2S, which is seemingly more susceptible to a changicrostructure than H2, as shown inFig. 10 [20].It would be of interest to consider how the volcano-sha

orrelation between resistance and temperature appearensor response under optimal condition (m= 0) tends to inrease with increasingT because the sensor response ccient increases. The utility factor, on the other hand, gown with increasingT, eventually approaching to zeroufficiently highT, because the termk/Dk increases sharpo take a very large value at highT. The volcano-shape


Fig. 10. Sensor responses to 5 ppm H2S for the thin film devices derivedfrom SnO2 sols different in crystallite size as correlated with operating tem-perature (constant film thicknesses of about 200 nm).

correlation results when these phenomena are coupled to-gether, as simulated for different film thicknesses inFig. 11[18]. Apparently the temperature at the response maximumcan be taken as a measure to estimate the openness of mi-crostructure. These results indicate how important the mi-crostructure control is for promoting the sensor response ofa thin film device. For a thick film device or a bulk type de-vice, microstructure is more complicated with the presence ofsecondary particles of grains and macro-pores. Nevertheless,almost the same conclusions can be drawn for these devicesif L is redefined as the radius of secondary particles involved.More complicated situation prevails for the devices loadedwith foreign additives where all of the three independent fac-tors mentioned before should be optimized. Even in such acase, however, the importance of microstructure control re-mains intact. For instance, the sensor response to CO for athick film device using Co3O4 (0.5 mass%) loaded SnO2 goesthrough a fairly sharp maximum on increasing mixing time ina satellite ball mill for the composite prior to screen-printing,as shown inFig. 12 [21]. The sensor response after mixing

F eratingt

Fig. 12. Sensor responses to 100 ppm CO for Co3O4 (0.5 wt.%)-SnO2 com-posite thick film devices as correlated with the time of ball-milling.

the optimum time is about 1000 to 100 ppm CO, which is al-most two orders of magnitude higher than the highest valuesso far achieved with various devices. As evaluated form thepore size distribution analysis for the powder samples, thevolume of pores in the radius range of 10–35 nm increases inthe initial mixing time up to 6 h. This suggests that the pro-motion of sensor response is brought about by the increase ofporosity, though the deterioration of sensor response with thelater mixing is yet to be investigated. Optimization of higherorder structure for a composite system would be one of themost important subjects in the research and development ofgas sensors for very dilute components like benzene.

5. Solid electrolyte gas sensors

In recent years, one of major topics in solid electrolyte gassensors has been research and development of type III sensorsfor which an electrochemical cell made of a typical solidelectrolyte like NASICON (Na3Zr2Si2PO12, Na+ conductor)which is attached with an oxyacid salt like Na2CO3 as anauxiliary phase (foreign receptor). The devices of this typewere first proposed for detection of SO2 and CO2 by the groupof Saito and co-workers[22,23]. For example, the CO2 sensorwas constructed as follows.

••

•

•-

i ord-
ig. 11. Volcano-shaped correlations between sensor response and op
emperature simulated for thin film devices different in film thickness.

Ambient air, Au| NASICON| Na2CO3 | Au, CO2 in air;Sensing electrode reaction:

2Na+ + CO2 + (1/2)O2 + 2e− = Na2CO3

Reference electrode reaction:

2Na+ + (1/2)O2 + 2e− = Na2O(inNASICON)

Overall reaction; Na2O (in NASICON) + CO2 = Na2CO3

Upon exposure to CO2 of partial pressurePCO2, theoretcally this cell generates electromotive force (EMF) acc


ing to the Nernst’s equation, EMF= E0 + (RT/2F )ln PCO2,when the oxygen partial pressures over the sensing andreference electrodes are the same. HereF is Faraday constantandRThas the usual meaning.E0 is a constant which is de-termined by chemical activities of Na2O (in NASICON) andNa2CO3. Obviously the sensor forms a galvanic cell (trans-ducer) by combining a CO2-sensitive half-cell (right) withan O2-sensitive one (left). Similarly a sensor for NO2 andSO3 (or SO2) can be formulated easily by replacing NA2CO3by NaNO3 or Na2SO4, respectively. Although these deviceswork reasonably well under ideal conditions at elevated oper-ating temperatures, gas-sensing properties become unstableunder humid conditions. Because of such weaknesses, noneof these sensors have been put in practice, in spite of the inten-sive research efforts devoted. From a viewpoint of practicaldevices, both of the half-cells should be improved substan-tially, as described mainly for the case of CO2 sensors below.

As for the auxiliary phase used for the gas-sensitive halfcell in CO2 sensing devices, we found that binary systemsbetween Na2CO3 and alkaline earth metal carbonate suchas Na2CO3–BaCO3 (1:2 in molar ratio) were superior to theneat phase of Na2CO3 [24,25]. When the auxiliary phase wasattached to the surface of NASICON disk by a melting-and-quenching method, Na2CO3 was found to corrode the NASI-CON surface very seriously, while the binary systems werefar less corrosive. In addition, the binary systems made it pos-s hichw es toC -m adyon im-pw hu-m hu-m nsingc thatLa( sa mida -s SO[ meo owni aryp ryp hea zir-c ea ionso s-i toe ow[ F ina the

Fig. 13. Sensing capabilities of type III solid electrolyte sensors to CO2,NO2 or SO2 at respective operating temperatures indicated.

concerned target gas dissociated thermally from the auxiliaryphase used. Optimization of the auxiliary phase to lower thebase EMF is thus a way to improve LDL. It is concluded thatthe auxiliary phase determines all the important properties torespond the target gas (receptor function).

Improvement of the reference half-cell was found to bealso indispensable for the devices using NASICON solidelectrolyte. It has been revealed that NASICON is vulnerableto attack by humid air at ambient temperature, eluding out theNa3PO4 impurity present at grain boundaries to be depositedon the surface as shown inFig. 14. In the presence of CO2,this impurity can act as a source to derive Na2CO3, whichprovides the reference electrode with CO2 sensing proper-ties and thus deprives the devices of gas sensing capabilities.In addition, it is also possible in the presence of humid airand CO2 that the Na2O component in NASICON reacts withCO2 to result in a decrease in its activity, which is also unfa-vorable for the reference half-cell. These phenomena are notvisible during steady sensor operation at elevate temperature

F ICONd ngt

ible to obtain a fine, porous layer of auxiliary phase, was beneficial indeed for quick responses of the devicO2. The devices exhibited excellent CO2 sensing perforances without being disturbed by humidity under steperation at elevated temperatures (400–500◦C). Unfortu-ately, however, these devices turned out to be totallyractical because the auxiliary phases containing Na2CO3ere too deliquescent at room temperature to survive inid atmospheres; once kept switched off overnight inid atmospheres, for example, the devices lost the se

apability almost completely. Fortunately it was foundi2CO3, non-deliquescent carbonate, could replace Na2CO3nd that the binary systems including it like Li2CO3–BaCO31:2 in molar ratio) gave excellent CO2 sensing propertiet elevated temperature as well as good stability in hutmospheres at room temperature[26,27]. Such material deign was also essential for developing type III sensors for228–30]and NO2 [31–34]. Gas sensing capabilities of sof type III sensors developed in our laboratory are sh

n Fig. 13, together with the solid electrolytes and auxilihases used. For the SO2 sensor, a ternary system auxiliahase including SiO2 was useful to improve adhesion of tuxiliary phase to the substrate of magnesia-stabilizedonia (MSZ). For the NO2 sensor, NaNO2 was found to bfar better auxiliary phase for detecting low concentratf NO2 than NaNO3 [32,34]. Especially a binary compo

te NaNO2–Li2CO3 (9:1 in molar ratio) made it possiblextend lower detection limit (LDL) down to 5 ppb or bel34]. It is remarked that each type III sensor exhibits EMir (base EMF), which is most probably the response to

ig. 14. Deposits spontaneously formed on the surface after a NASisk was kept in humid air containing CO2 at ambient temperature for a lo

ime (13 days).


Fig. 15. Stability of glass-coated NaCoO2 electrode and conventional Au electrode during a series of heat-cycle tests as observed with a three-electrode deviceindicated.

but become conspicuous as a drift in base EMF or a loss ofCO2 sensing properties when the devices are kept at roomtemperature in humid atmospheres. In order to overcome thisproblem, the reference half-cell should be improved to beresistant to the change of NASICON surface. In principle,this is achieved by introducing either an oxide ion conductorsuch as BICUBOX[35] or a solid reference material such asNa0.6CoO2 (cobalt bronze)[36] between the reference elec-trode and NASICON to construct a new reference half-cellas follows:

• Ambient air, Au| BICUVOX or Na0.6CoO2 | NASICON

The former material would allow the half cell to befixed at the potential determined by the partial pressure ofoxygen (air reference), while the latter would to that de-termined by the electrode reaction of the material itself,xNa0.6CoO2 =xNa0.6−1/xCoO2 + Na+ + e−, (solid reference).Actually the latter material turned out to be unstable to theattack by CO2 as it was. Fortunately however, it could be sta-bilized effectively when coated with a layer of an inorganicglass. Both of the above materials were effective in stabiliz-ing the EMF response to CO2 from drifting even after thedevices were exposed to humid atmospheres at room tem-perature.Fig. 15compares the behavior of the glass-coatedNa0.6CoO2 with that of a bare Au reference electrode duringa heat cycle test; Temperature was switched between operat-i also n ofC de-

pendent intervals as shown. For this test, a three-electrodedevice was fabricated as also shown and the potential of eachelectrode was measured relative to the third electrode (Au)which was always exposed to clean synthetic air. It is seen thatthe potential of the bare Au electrode continued to float upand down during the heat-cycle test, reflecting the instabilityof the NASICON surface. On the other hand, the potential ofthe other electrode tended to converge to a steady value afteran ageing period in the beginning, confirming its reliabilityas a reference electrode. From the same figure, the potentialof this electrode is seen to reach the steady value quickly atthe heating up stage. Such fast warm-up characteristics arerequested especially for applications to non-stationary facil-ities like cars. It is remarked that the introduction of a solidreference material should be also important for the NO2 sen-sor using NASICON as well, though this is yet to be verified.

Conventionally the sensing electrode for type III CO2 sen-sors is provided with metals such as Au. When the metalelectrode is replaced by an oxide electrode such as indium-tin-oxide (ITO), utterly different CO2 sensing properties areobtained. That is, the device fabricated as shown inFig. 16exhibits excellent CO2 sensing performances at room temper-ature (30◦C) in the presence of humidity at 30% and abovein relative humidity, as shown in the same figure[37]. In thisdevice, the same auxiliary phase as used previously is locatedbetween NASICON and a porous layer of ITO, while the Aur anicat on the

ng temperature (450◦C) and room temperature at intervf one to several days, while, under a fixed concentratioO2, humidity of the gas flow was altered stepwise at in

eference electrode is covered with a layer of an inorgdhesive. The sensing electrode reaction involving CO2 in

his case appears to be mediated by adsorbed water


Fig. 16. Structure and sensing performance of type III CO2 sensor usingindium-tin-oxide (ITO) sensing electrode at room temperature (30◦C) undervarious humidity conditions.

surface or in the pores of ITO grains. This result suggests thefeasibility of room temperature sensors for CO2.

Finally it is pointed out that the reference half cell men-tioned above can be replaced by a MISFET chip as a newtransducer, as illustrated for the case of NO2 sensing inFig. 17[38]. The NO2-sensitve half-cell consisting of a NaNO2 basedcomposite and Au electrode is now deposited over the gatearea of the device. This device responds to NO2 in air by achange in the threshold voltage of FET and the mechanismand performance of NO2 sensing have been shown to be ex-actly the same as those of the type III NO2 sensor mentionedabove[32].

6. For innovations of gas sensor technology

Gas sensors are an indispensable interface through whichwe acquire chemical information out of our surroundings inreal time. The information provides a base on which we con-struct feedback control systems for chemical processes, au-tomatic operation systems for gas-related facilities, alarmingsystems for gas appliances, security systems to protect work-ers from chemical hazards, etc. When gas sensors are usedcollectively as a network, on the other hand, chemical infor-mation can be acquired as a function of location and time.The distribution of particular chemicals over an urbane areaa ppedi theq un-t there

Fig. 17. Structure and sensing capabilities of an NO2 sensor for which aNO2-sensitive half-cell is combined with a field effect transistor (FET) chip.

has been a plan to diagnose the flow or congestion of vehi-cles in urbane area by placing along the traffic roads a seriesof gas sensors which respond to the chemicals emitted fromthe vehicles. Various gas sensors have been developed andput into practical use in many fields of applications so far.These achievements are however, rather limited, consideringthe intrinsic potentiality of gas sensor technology. There area variety of new or emerging fields of sensor applications, asmentioned previously. Meeting these needs with advanced ornew sensors one by one would lead to new frontiers of gassensor technology. The followings are personal opinions ofthe author on what is important for innovations of the tech-nology.

6.1. Innovative sensors

Innovations of sensor devices are no doubt the key to opennew frontiers. Here innovations can be conceived in variousaspects, i.e., development of a new sensor device capable ofdetecting a target gas which has so far been out of sensory de-tection, upgrading the sensitivity of a known device by morethan one order of magnitude, extending the lower detection

s well as its change with time, for example, can be man this way. The maps would be useful for diagnosinguality of atmospheric air in that area or setting forth a co

ermeasure against air pollutions. As a similar example,


limit to a target gas by more than one order of magnitude, real-ization of room temperature gas sensors, development of gassensors compatible with micro-machining, etc. Most of cur-rent sensors have been designed based on empirical knowl-edge rather than materials science. Basic studies to elucidategas sensing mechanism and key designing factors are neces-sary to establish a secure base for these innovations. This isespecially so for gas sensors using ceramic materials whichchange their physicochemical properties drastically depend-ing on the way and condition of processing used. It seemsworth trying to develop ultimate sensors which are extremein certain features such as sensitivity, selectivity and size,because possibilities and limitations would be made clear inthose processes.

6.2. Intelligent sensors

In most cases sensor output contains a constant term (airbase) which is irrelevant to target gas. When the air baseis not stable but tends to shift or drift with time, reliabil-ity of gas sensing is undermined. This is serious especiallywhen absolute values of target gas concentration are required.If it is made possible to diagnose or calibrate the shift ordrift, gas sensors would find a lot of new users in variousapplication fields. In semiconductor gas sensors, the shifto ft isb ma-t asesi is inm e thes n (onet ima-t beenp tible-g ch ise ) ex-c e thed are

Fig. 18. Simultaneous analysis of quality and strength of odors by a coupleof semiconductor gas sensors[39]. Sensor A is more sensitive to light gasesthan to heavy gases while sensor B behaves just oppositely.

requested to elaborate new ways of self-diagnosis and self-calibration.

Semiconductor gas sensors respond to combustible gasesindirectly through their oxidation reactions over the sensingmaterials used. Generally speaking, the sensor responseis a function of two variables, i.e., kind of gas and itsconcentration so that one cannot determine both variablessimultaneously from the output of a single sensor. Whenplural sensors significantly different in gas sensing propertyare combined, one can obtain information about chemicalspecies. This is well exemplified by an odor analyzer recentlycommercialized[39]. It adapts two gas sensors (A and B) ofwhich A is more sensitive to light odorants (NH3, H2S, etc.)than to heavy odorants (aromatic compounds, large unsatu-rated hydrocarbons, etc.) and B shows just opposite sensingproperties. The quality and strength of a given odor are evalu-

n adso sensor.

f air base is caused mainly by moisture, while the driy a change in physicochemical properties of sensing

erials. For gas alarms which detect rather quick incren target gas concentration, the air base (resistance)

any cases approximated to be the highest resistancensor has experienced during a designated time spao several hours) precedent. Unfortunately this approxion cannot always be verified in general cases. It hasroposed to compensate the moisture effect in combusas sensing by referring to a compensation device whixactly the same as the sensing device (thick film typeept for a sheet of glass placed on top in order to makevice sensitive to moisture only. More investigations

Fig. 19. Response transients to 5 ppm NH3 in air for a system in which a
rbent oscillating in temperature is coupled with a semiconductor gas


ated simultaneously by correlating the output of sensor A withthat of B, as shown inFig. 18. It is noteworthy that this kind ofinformation can be acquired by using just a couple of sensors.

6.3. Hybrid systems

Standard instrumental analysis is carried out based on var-ious analytical techniques such as densification, extractionand separation of chemical species involved. Some of thesetechniques can also be combined effectively with gas sensorsto construct a hybrid system. For example, a target gas can becaptured easily by adsorption if adequate adsorbent is avail-able, while the captured gas can be freed back on heating theadsorbent. In other words, the target gas, even if too dilute,can be concentrated up to a level sufficient for sensory detec-tion by choosing the conditions of adsorption and desorption.Thus a hybrid system using such temperature-modulated ad-sorption would be useful for detecting very dilute gases, al-though the sensory data are obtained only intermittently, asillustrated inFig. 19 [40]. It is also possible to combine a gaschromatography technique with a gas sensor. The resultingsystem would be suited for detecting individual componentsin gaseous mixtures. These new functions are, however, ac-quired at the cost of the important feature of gas sensors, i.e.,providing real time and continuous information.

7

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[6] S. Matushima, T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Roleof additives on alcohol sensing by semiconductor gas sensor, Chem.Lett. 5 (1989) 845.

[7] J. Tamaki, T. Maekawa, S. Matsushima, N. Miura, N. Yamazoe,Ethanol gas sensing properties of Pd-La2O3-In2O3 thick film ele-ment, Chem. Lett. 3 (1990) 477.

[8] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Sensing behaviorof CuO-loaded SnO2 element for H2S detection, Chem. Lett. (1991)575.

[9] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Gold-loaded tung-sten oxide sensor for detection of ammonia in air, Chem. Lett. (1992)639.

[10] T. Maekawa, Y. Anno, J. Tamaki, N. Miura, N. Yamazoe, Y. Asano,K. Hayashi, Development of semiconductor gas sensor to discernflavors of consomme soup, Sens. Actuators B 13/14 (1993) 713.

[11] C.N. Xu, J. Tamaki, N. Miura, N. Yamazoe, Correlation between gassensitivity and crystallite size in porous SnO2-based sensors, Chem.Lett. 3 (1990) 441.

[12] C.N. Xu, J. Tamaki, N. Miura, N. Yamazoe, Effect of crystallite sizeupon gas sensitivity of porous SnO2-based sensors, in: TechnicalDigest of Ninth Sensor Symposium, Tokyo, 1990, p. 95.

[13] C.N. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects ongas sensitivity of porous SnO2-based elements, Sens. Actuators B 3(1991) 147.

[14] S. Matsushima, Y. Teraoka, N. Miura, N. Yamazoe, Electric inter-action between metal additives and tin dioxide in tin dioxide-basedgas sensors, Jpn. J. Appl. Phys. 27 (10) (1988) 1798.

[15] S. Matsushima, J. Tamaki, N. Miura, N. Yamazoe, TEM observationof the dispersion state of Pd on SnO2, Chem. Lett. 9 (1989) 1651.

[16] S. Matsushima, Y. Teraoka, N. Miura, N. Yamazoe, Metal-supportinteractions in semiconductor gas sensors, in: Proceedings of the

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. Concluding remarks

Gas sensor technology is an indispensable tool tote new technologies and new life-styles which are comle with sustainable society. Further advancement of itoubt desired strongly worldwide. To enhance it, researcould be requested to pay attention to a few more sug

ions. Gas sensor technology is interdisciplinary indeehat collaborations among people working in broadly difnt disciplines, ranging from materials scientists to maevelopers, would be necessary to open new frontiersearchers should be well acquainted with the needs hmerged or newly emerging in industry and society. Forurpose, one should listen to opinions of users and mevelopers carefully. Importance of carrying out fieldannot be overstated when a new sensor device haseveloped. It is important not only because it confirms

easibility of the device in practice but also because itirect way to demonstrate the potentiality of the new fron

he device aims at opening. Finally, challenging spiritackbone of every successful researcher. The authorincerely that gas sensor technology will be innovated toribute more and more to the society in the future.

eferences

[2] N. Miura, T. Harada, N. Yamazoe, Sensing characteristics womechanism of four-probe type solid-state hydrogen sensor usinton conductor, J. Electrochem. Soc. 136 (7) (1989) 1216.

MRS International Meeting on Advanced Materials, vol. 2, Tok1989, p. 349.

17] N. Yamazoe, S. Matsushima, T. Maekawa, J. Tamaki, N. MControl of Pd dispersion in SnO2-based sensors, Catal. Sci. Tech1 (1991) 201.

18] G. Sakai, N. Matsunaga, K. Shimanoe, N. Yamazoe, Theory ofdiffusion controlled sensitivity for thin film semiconductor gas ssor, Sens. Actuators B 80 (2001) 125.

19] N. Matsunaga, G. Sakai, K. Shimanoe, N. Yamazoe, Diffuequation-based study of thin film semiconductor gas sensor-restransient, Sens. Actuators B 83 (2002) 216.

20] D.D. Vuong, G. Sakai, K. Shimanoe, N. Yamazoe, Hydrogenfide gas sensing properties of thin films derived from SnO2 solsdifferent in grain size, Sens. Actuators B 105 (2) (2005) 4442.

21] S. Abe, U.-S. Choi, K. Shimanoe, N. Yamazoe, Influences ofmilling time on gas-sensing properties of Co3O4-SnO2 compositesSens. Actuators B 107 (2) (2005) 516–522.

22] Y. Saito, T. Maruyama, Y. Matsumoto, K. Kobayashi, Y. Yano,plicability of sodium sulfate as a solid electrolyte for a sulfur oxisensor, Solid State Ionics 14 (1984) 273.

23] T. Maruyama, S. Sasaki, Y. Saito, Potentiometric gas sensocarbon dioxide using solid electrolyte, Solid State Ionics 23 (1107.

24] S. Yao, Y. Shimizu, N. Miura, N. Yamazoe, Solid electrolCO2 sensor using binary carbonate electrode, Chem. Lett. (12033.

25] N. Miura, S. Yao, Y. Shimizu, N. Yamazoe, Carbon dioxide seusing sodium ion conductor and binary carbonate auxiliary phaElectrochem. Soc. 139 (1992) 1384.

26] S. Yao, S. Hosohara, Y. Shimizu, N. Miura, H. Futata, N. YamaSolid electrolyte CO2 sensor using NASICON and Li-based bincarbonate electrode, Chem. Lett. (1991) 2069.

27] S. Yao, Y. Shimizu, N. Miura, N. Yamazoe, Solid electrolytebon dioxide sensor using sodium-ion conductor and Li2CO3-BaCO3

electrode, Jpn. J. Appl. Phys. 31 (2B) (1992) L197.


[28] Y. Yan, Y. Shimizu, N. Miura, N. Yamazoe, Solid-state sensor forsulfur oxides based on stabilized zirconia and metal sulphate, Chem.Lett. (1992) 635.

[29] Y. Yan, Y. Shimizu, N. Miura, N. Yamazoe, Characteristics and sens-ing mechanism of SOx sensor using stabilized zirconia and metalsulphate, Sens. Actuators B 12 (1993) 77.

[30] Y. Yan, Y. Shimizu, N. Miura, N. Yamazoe, High-performancesolid-electrolyte SOx sensor using mgo-stabilized zirconia tube andLi2SO4-CaSO4-SiO2 auxiliary phase, Sens. Actuators B 20 (1994)81.

[31] Y. Shimizu, Y. Okamoto, S. Yao, N. Miura, N. Yamazoe, Solidelectrolyte NO2 sensors fitted with sodium nitrate and/or bariumnitrate electrodes, DENKI KAGAKU 59 (6) (1991) 465.

[32] S. Yao, Y. Shimizu, N. Miura, N. Yamazoe, Use of sodium nitriteauxiliary electrode for solid electrolyte sensor to detect nitrogen ox-ides, Chem. Lett. (1992) 587.

[33] N. Miura, S. Yao, Y. Shimizu, N. Yamazoe, Development of high-performance solid-electrolyte sensors for NO and NO2, Sens. Actu-ators B 13/14 (1993) 387.

[34] N. Miura, S. Yao, Y. Shimizu, N. Yamazoe, New auxiliary sensingmaterials for solid electrolyte NO2 sensors, Solid State Ionics 70/71(1993) 572.

[35] Y. Miyachi, G. Sakai, K. Shimanoe, N. Yamazoe, Stabilization ofCounter Electrode for NASICON Based Potentiometric CO2 Sensor,Ceram. Eng. Sci. Proc. 25 (3) (2004) 471–476.

[36] Y. Miyachi, G. Sakai, K. Shimanoe, N. Yamazoe, Improvement ofwarming-up characteristics of potentiometric CO2 sensor by usingsolid reference counter electrode, Sens. Actuators B 108 (2005)365.

[37] K. Obata, K. Shimanoe, N. Miura, N. Yamazoe, NASICON devicesattached with LiCO -BaCO auxiliary phase for COsensing under

[38] S. Nakata, K. Shimanoe, N. Miura, N. Yamazoe, NO2 sensing prop-erties of FET devices attached with NaNO2-based binary auxiliaryphase, in: Proceedings of the International Symposium on Chem-ical and Biological Sensors and Analytical Methods II, 2001, pp.414–422.

[39] T. Okano, From Leaflet of Futaba Electronics Co. Ltd., 2004.[40] G. Sakai, Y. Ogata, N. Miura, N. Yamazoe, Design of gas-sensing

system using gas-enriching scheme, Electrochem. Soc. Proc. 99-23(1999) 113.

Further reading

[1] From data of Japan electronics and information technology industriesassociation (1998).

[3] T. Nakahara, Development of gas sensors and cultivation of new mar-kets for air quality, in: Proceedings of the 38th Chemical SensorSymposium, 2004, p. 73.

[4] From data of World Motor Vehicle Statistics (in 2000), Research re-port of Japan Automobile technology (No. 9 in 2000) by DevelopmentBank of Japan and Marketing Research by Figaro Engineering Inc.

[5] From data of Japan Fire Equipment Inspection Institute (2003).

Biography

Noboru Yamazoehad been a professor at Kyushu University since 1981until he retired in 2004. He received his BE degree in Applied Chemistryin 1963 and PhD in Engineering in 1969 from Kyushu University. Hisresearch interests were directed mostly to development and applicationo

2 3 3 2

ambient conditions, J. Mater. Sci. 38 (2003) 4283.
f functional inorganic materials.

review toward innovations of gas sensor technology · 2005. 7. 27. · sensors, such as electrolyte...

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