Tin dioxide gas sensors: use of the seebeck effect

Sensors and Actuators,

TIN DIOXIDE

251 - 257

GAS SENSORS:

J F McALEER*, STEPHAN??

P

Materrals Development (Received 1985)

8 (1985)

T

USE OF THE SEEBECK

MOSELEY,

DWLVO~, Harwell,

July 30, 1985,

251

P

BOURKE?,

Dldcot,

Oxon

m revised form November

J

0

W

EFFECT NORRIS

and R

OX1 1 ORA (U K) 1, 1985, accepted

November

26,

Abstract This paper describes a novel type of gas sensor, which relies on the thermovoltage generated when one region of a porous semiconductor IS heated by the reactlon of a combustible gas urlth oxygen One of the mam attractions of this type of sensor 1s that the power requirements are mmlmal Use is made of a voltage measurement, which dlstmgulshes the device from other semiconductor gas detectors that depend upon the measurement of resistance Key words combustion

tm oxide,

semiconductor

gas sensing, Seebeck voltage, catalytic

Introduc tlon Increasmg demands for high standards of environmental monitoring and a growing enthusiasm for electromc control of mdustrlal processes have @ven rise to a need for a new generatlon of sensmg devices One of the pnmary areas of need 1s for sensors capable of momtormg the concentration of hazardous gases (both explosive and toxic) Both types of hazard can be monitored with currently available equipment, but at rather high cost, and much current research and development effort has as its aim the production of low cost devices that can be widely deployed Gas sensors based on the sensltlve resistance changes induced by gas chemlsorptlon or reactions with adsorbed molecules on high surface area semiconductor artifacts are thought to offer promise for the detection of toxic gases [l] Flammable gases may be detected m a similar way, or alternatively by using the measured resistance change m a wire heated by the *Present address Genetics International, 11 Nuffleld Way, Abmgdon, U K ‘Present address Physics Department, Impertal College of Science and Technology, London SW7, U K t-t Present address Laboratowe de Chlmle de Sohde, 14032 Caen Ckdex, France 0250-6874/85/$3

30

@ Elsevler SequolajPrmted

m The Netherlands

252

catalytic combustion of the gas of Interest (the ‘pelhstor prmclple) [Z] In both types of device, the sensmg elements are heated to temperatures of several hundreds of degrees (C) and consequently have a substantial power requirement A device capable of operating effectively at room temperature would offer slgmflcant advantages for portable apphcatlons where power supplies result m a weight penalty, and m apphcatlons where power sources are limited because of the risk of lgmtlon of flammable gas Salient aspects of the function of tm dloxlde m resistance modulating gas detectors [3] and the role of applied precious metal catalysts m modlfymg their function [ 41 have recently been described The present paper describes a novel way m which tm dloxlde can be used to afford an electrical response to the presence of reducing gases without recourse to an external source of heat

The Seebeck effect The fluxes of electric charge and of heat through a semiconductor are closely interrelated phenomena and represent two hmltmg cases of a more general formulation [ 51 Heat fluxJQ=

(K1~~Ko)

J, + (K12io:KZ)

g

and electric flux J, = q [K,, ( qE -+(K,--E~K~) where K, K,

$

g]

are the so-called transport integrals [5]

-1 = 4X3 sss

afo -z

em v2d3 12

f. 1s the eqmhbrmm Fermi dlstrlbutlon function, e and v are the energy and velocity of conduction electrons respectively, 12IS the electromc vector, Ef IS the Fermi energy, q 1s the charge on an electron and E 1s the electric field From these two equations, the expressions for isothermal electromc conduction (J, when dT/& = 0), electronic thermal conduction (JQ when J, = 0), the PeItler effect (Ja when dT/dz = 0) and the Seebeck effect (E when J, = 0) can be derived We are here concerned only mth the last case, for which we may write E=

dV dz

=Ly-

dT dz

where (Y 1s the Seebeck coefficient, (K1 - efKo)/qKoT Thus, if the experimental arrangement prevents current flow, the establishment of a temperature differential between different regions of a semiconductor will result m a potential difference due to the Seebeck effect

253

In the experiments described here the source of local heating ISthe heat of combustion generated when an am/reducing gas mixture 1s passed over a semiconductor havmg one region of its surface treated unth a precious metal catalyst The heat of combustion causes the treated region to be heated, while untreated parts of the surface do not catalyse the reaction and thus remam cold A voltage 1sthen measurable between the hot and cold regions

Expenmental

Porous pellets of tin dioxide were prepared as described previously [3] Sensor pellets were then constructed by sputtering gold onto one flat surface of a tm dioxide pellet and platmum onto the other Pressure contacts were made to the two flat surfaces of the pellet and voltages were recorded either with an electrometer or with a recording voltmeter For measurements at elevated temperature, the sensor was contmuously exposed to an infrared lamp

Results and dlscusslon

The voltage response of a pellet prepared as above when exposed to a~ contammg 1% hydrogen 1s shown m Fig 1 The voltage 1s posltlve at the platinum side of the pellet and the response 1s reversible In order to establish that the ongm of the voltage measured is heat generated by catalytic combustion, a pair of SnO, pellets was placed m the field of view of a thermal imaging camera One pellet was untreated, the second presented a surface treated unth both platinum and palladium to the camera and a thermal lme scan was run across the pair As shown m Fig 2, 1% lip 0l-l

Off

1 1

----LOG Increase

05

> E

00 1%

0

10

Mina

20 c

Fig 1 The voltage response pulse of 1% hydrogen m air

of a pellet

Fig 2 Temperature profile across pellets stream of air contammg 1% hydrogen

of SnOz, treated

on one side with platinum,

of SnOz and SnO#t,

to a

Pd) set side by side m a

254

when air contammg 1% hydrogen was passed over the pellets there was no temperature shift across the untreated pellet, but the surface bearing precious metals offered a substantial temperature rise Attempts to measure the temperature increase with a thermistor were unsuccessful This result emphasizes that the amount of heat involved 1s quite small and indicates that temperature measurement usmg the Seebeck voltage 1s very sensitive The variation m voltage response mth the concentration of hydrogen m the air stream IS shown m Fig 3 The dependence 1s not linear We note that at higher concentrations the voltage would be expected to deviate from linearity due to mass transfer effects as the rate of reaction becomes hmlted by diffusion PH *(K)

IO ‘I

1021

1031

1041

1061

IOf.1

1071

IOQ

I

1’01 Pt

negatlvl -mV

t

I

10

rnlll8 I

t 10 Fig 3 Voltage response of SnOa/SnOz(Pt, centratlon m air

Pd) sensor as a function

of the hydrogen

con-

The variation m voltage response with temperature 1s shown m Fig 4 For reasons that are probably connected mth a lack of control of the microstructural Qstrlbutlon of precious metals (discussed below), the responses m Fig 4 are substantially larger than those m Fig 3 However, it 1s clear that the size of the hydrogen response decreases as the temperature increases A X

\

X

x\

\

I21 20

,

, LO

30

X

, 50

T [“Cl Fig 4 The temperature hydrogen in air between

dependence of the response 25 and 50 “C

of SnOz/SnO&‘t,

Pd) sensor to 1%

255

number of factors may contribute to this Both the Seebeck coefflclent and the adsorption isotherm for hydrogen on the surface m question are temperature dependent and, m addition, the size of the potential measured across a pellet of a semiconductor ~111 be a function of the resistance of the material and will fall as the temperature rises For these several reasons it IS dlfflcult to interpret the temperature dependence of response m terms of hmitmg gas reactions A correlation between the sample temperature and the thermovoltage was made by comparing the data from Fig 3 with the temperature reached by the treated surface of the pellet under the various hydrogen partial pressures In Fig 5 it 1s seen that the voltage measurements and the temperatures follow very similar trends including the departure from linearity at the upper end The value of the Seebeck coefficient derived from these data (180 (IV K-l) 1s rather lower than values reported elsewhere m the hterature [6], probably because the microstructure used here 1s far from optimized

Fig 5 A comparison SnO?(Ft, Pd) sensor

of the

voltage

and temperature

responses

of a compostte

SnOz/

The responses of devices of this type depend rather sensltlvely on mlcrostructure In the preparation it 1s necessary to sputter an optimum quantity of precious metal onto the porous tm dloxlde (this can only be estimated by sputtermg time, but was not more than 300 ii) Too little precious metal leaves too little catalytic activity, while too much results m a layer with a reduced catalyst/Sn& interface length Surface area 1s also very important both m respect of presenting a large contact with the gas and also for reducing thermal leakage A range of response behavlour was observed even from pellets prepared according to nominally identical procedures, as evidenced by the difference m the magmtude of the response to 1% hydrogen m air between Fig 3 and Fig 4 It 1s clear that low (ambient) temperature operation of the device limits its use to those gases that are oxidized on metal oxlde/catalytlcally active metal surfaces at such temperatures It 1s unhkely, for example, that methane will be oxidized at this temperature

256

Conslderatlon of the data presented here suggests that the sensltlvrty to hydrogen may be hmlted to concentrations above around 100 ppm Other semlconductmg oxides may also be used as supports Indeed, tm dioxide IS only modestly placed m the ranking of matenals according to Seebeck coefficient (for example, see Table 1) TABLE

1

Seebeck

coefflclents

of some semlconductmg

oxldes

Material

cu(l_~VK-’ )

Temperature measurement

MnO SnOz

1700 930 700

120 20 45

wo3

of (“C)

Reference

7 6 8

Conclusion An entirely new concept in gas sensing practice has been demonstrated m which the thermal voltage measured as a result of catalytic combustion provides a measure of the concentration of a flammable gas The advantages of the new design are slmphclty of operation and mammal power requtrements The ‘Seebeck Sensor’ 1s dlstmgulshable from other semlconductmg gas sensors in which the analyte gas perturbs some property such as resistivlty or permlttzvlty In the present device, the semiconductor remains largely unchanged and the size of the response 1s determmed by the rate of combustion, the heat of combustion and the Seebeck coefflclent of the semiconductor

Acknowledgements Financial support from the Electromcs and Avlonlcs Reqmrements Board of the Department of Trade and Industry IS gratefully acknowledged The authors are also grateful to Dr B C Tofleld for helpful dlscusslons and to Mr M Hale for assistance mth the thermal lmaglng experiments

References 1 P T Moseley and B C Tofleld,Muter Scz Technol, 1 (1985) 505 - 509 2 J G Forth, A Jones and T A Jones, Combust Flame, 21 (1973) 303 - 11 3 S R Morrison, Sensors and Actuators, 2 (1982) 329 - 341 and G Helland, Sensors and Actuators, 2 (1982) 343 - 361 4 (1983) 283 Y Kurokawa and T Selyama, Sensors and Actuators, 4 N Yamazoe, 289

257 5 T C Harman and J M Homg, Thermoelectrw and Thermomagnetw Applrcatzons, Lincoln Laboratory/McGraw-Hill, New York 1967 6 Y Kutoml and T Nobusawa, Tech Rep Kansal Unrv, 18 (1977) 19 7 Ya M Ksendzov and V V Makarov,Sovlet Phys Solid State, 12 (11) 2562 2 J M J3erak and M J Slenko,

J Solrd State Chem , 2 (1970)

Effects

(1971)

and

2559

-

109

Biographies Pat MoseZey was born m Buckmghamshn-e, England m 1943 He studied chemlstrl at the Umverslty of Durham, recelvmg the BSc degree m 1965 and PhD m 1968 Since 1968 he has worked at Harwell, where his prmclpal research interests have been m crystal structure and solid-state chemistry In recent years these interests have been mcreasmgly applied to the development of energy storage devices and to sensors Jerry McAZeer studied chemistry at the University of Southampton where he received the degrees of BSc m 1976 and MSc m 1977 Followmg a study of the anodlc oxldatlon of tltanmm, he was awarded the degree of PhD, again from the University of Southampton, in 1979 He spent two years at the Wolfson Centre for electrochemical science, where his studies included corrosion and the development of non-aqueous batteries He spent the period 1981 - 1983 at the Unlverslty of Alberta teaching undergraduate chemistry and studying the electrochemistry of tetrapyrroles At Harwell from 1983 until 1985, he carried out an mvestlgatlon into the mechanisms of response of semiconductor gas sensors He 1s now with Genetics International developing sensors for medical diagnostics

Penny Bourke was born m Oxford, England m 1966 She worked at Harwell during 1984 - 1985 on the development of gas sensors and 1s now studying for the degree of BSc at Imperial College, London studied chemistry at the Urnverslty of Oxford and was awarded the BA degree m 1977 and the D Phil m 1980 His degree research was a study of the spectroscopic properties of the neptunyl ion From 1980 until 1983 he was a post-doctoral research fellow at Oxford and studled energy tranfer and lummescence properties of morgamc systems In 1983 Dr Norris Joined the catalyst unit at Harwell, and since then has added the development of conductance-modulating sensors to his portfolio of research interests

John

Noms

Ronan Stephan was born m Brest, France m 1960 and studied mathematics and materials science at the Unlverslty of Caen He was awarded the DlpliZime National d’Ing&neur and Dlpl6me d’Etudes Approfondles in 1984, during which year he also spent some time at the Harwell Laboratory He 1s presently preparmg his doctoral thesis m solid-state physics at the Instltut des Sciences de la Mat&e et du Rayonnement, Caen