Indian Journal of Pure & Appl ied Physics VoL 37, April 1999, pp. 252-254

Investigation of Ag2S04:MS04 (M = Zn, Ni, Co and Cu) binary systems from S02 gas sensor view point

S S Bhoga, PO Borkar# & K Singh*

Department of Physics, Nagpur University. Nagpur 440 0 I 0

* Department of Phys ics. Amravati Uni versity. Amravati 444 602

#Department of Physics, Sindhu Mahav idya laya. Nagpur

Received 3 February 1999

Electrical conducti vity of new Ag2S0~:MS04 (M = Zn. Ni. Co and eu) binary systems preparcd by slow cooling techniquc is evaluated using complex impedance spectroscopy. A conductivity max imum at about 20 mo le % or MSO~ is due to the dispersion of second insolub le phase in Ag2S04 matr ix. Optimi sed apt co mposi tions (with Ag + Ag2S04 as so lid rcft:rcnce) arc used to engineer a few galvanic sensors. The sensor characterised at 500°C ex hibits good reversibil ity with res OIN': time or about 30 scconds. al so shows im proved long term stab il ity. The cross sensiti vity to CO} andmuisllI re is fOlindlO be negligible .

1 Introduction Pollution of the air by S02, is widespread since it

ex ists wherever fossi I ftl e ls are burn t. The concentration of th is gas in the atmos phere is norm ally far below the leve l which cannot be detected by odour. Bein g present to some extent at a ll t imes, it does produce detectab le effects: ( i) it is harmfu l to stone'W ork and metalwork . ( ii ) ca uses major di srupti on in agricultural prod ucti vity (damages plant ti ss ues), ( iii ) aggravates the illness like bronchit is. Since the presence of above po llutants in atm osphere is harm ful both from the hea lth and the economy point of view, a knowledge of the leve l at whi ch these gases are present in the air is important. The increasing demand of environmenta l pollution and pub­lic hea lth considerati ons due to tremendous growth of industria lization, there is a strong incenti ve to deve lop a technique for the detect ion of this po llutant gas. In addi ti on to this, in recent past sensors have attrac ted a great dea l of attention of sc ient ists and engineers and for that matter in near fu ture it is expected to ga in much attraction in view of the construct:on of more or less " intell igent" ensembles which integrate actuating, sens­ing and computing subsystems.

The rapid advancement in the ga lvanic sensors is ch iefl y I inked with the di scovery of advanced materi als and processin g techniques. Cu rrently, multi -component systems have gained much of an impol1ance because of thei r chem ica l stab i I ity and inherent high ion ic conduc­ti vity as compared to mono-component systems'- In

addition to thi s, ever since the concept of usi ng met­al/metal sul phate reference electrode in so li d electro­chem ical gas sensors, si lver ion cond ucto rs have evoked a remark ab le attrac tion}. Sil ver sulphate based electro­lyte pal1icul arly exhi bits many fo ld adva ntages over other sul phates (e.g. , Na2S04, K2SO.I, Rb"SO.d in engi­neerin g SO, gas sensors li ke: (i) coexistence of Ag-O-S phase in Ag/Ag2S04, (ii) eq uili bra ti on of antago nist SO.I 2- (so lid ) with S02/S0, (gas), (i ii ) hi gh ion ic con­ducti vity in variant over SO, environment etc"' The eh ief object of the present in ves ti ga ti on is to prov ide a !llulti ­phase so lid e lect ro lyte useful for so li d state sensors. Prac ti ca lly, Ag2S04-MSb.1 (M = Zn, N i, Co and ClI) t\\ O phase system is focused.

2 l<=xperimentaJ Details The ini tia l ingred ients Ag2S04 and MSO-1 (where.

M = Zn, Ni, Co and Cu) with pur ity greater than 99 % were procured from Aldri ch Chemica ls (USA) Appro­priate mole fractions of the above chem ica ls in (I -x)

Ag2S04:(X) MS04 ( where x = 0.1 to 1.0 ) mo le ratio were prepared by s iow coo ling the me lt at a predeter­mined coo ling rate of 20°C/min . Thus prepared samples were charac terised by X- ray povvder diffraction as d is­cussed eb ewhere' .

For electr: cal cond ucti vity measure ments. the Sill ll­

pies were obta ined in the form of circu lar discs of aboui 9 111m diameter and 1.5 mm thi ck ness by press in g the powder in a sta inless stee l d ie-punch . Thus obtained pell ets were s intered at 500°C for J hI'. A good ohmic

-r

.., contact was ensured by using a quality silver paint (Elteck, India) on both the flat surfaces fol lowed by baking at 200°C for 2 hr. The above process was care­fully carried out in the dark room so as to avoid photo­decomposition of A�S04'

Prior to impedance measurements, the sample was heated to 440°C for two hours in order to homogenize the charge carriers. Real and imaginary parts of the impedance were measured as a parametric function of frequency and temperature in the range from 5 Hz to 13MHz and from 450 to 250°C respectively using a computer controlled HP 4 1 92A If impedance ana­lyser.

A few galvanic cells were constructed using opti­mised solid electrolyte and Ag+Ag2S04 reference elec­trode as discussed earlier

4. The sensor was characterised

at 500°C. The test gas of pre-decided partial pressures (S02 in fixed 2 1 % 02 and remaining N 2 gas) was ob­tained using electronic mass flow meters-cum-control­lers (Teledyne Hestings, USA). The response time and reversibility of sensor is tested by toggling the concen­tration ofS02 in 02 and N2 between 1 % to 500 ppm, and simultaneously recording the cell emf as a parametric function of time. Additionally, the long term stability of the sensor at fixed S02 partial pressure was investi­gated.

3 Results and Discussion

The tiulk conductivity of each sample is obtained by fol lowing a complex impedance analysiss. The plots of log (an versus 103 IT, for A�S04:ZnS04 binary system (Fig. 1), obey the Arrhenius law:

(an = (ano exp(-E)kn . . . ( 1 ) where, Ea = Ef + Em, Ef and Em are the energies of defect formation and migration respectively. Thus, the conduc­tivity is governed by thermally activated defects, n, given by:

n = exp(-Er/kn . . . (2) Similarly, the conductivity plots for the ( I-x)

A�S04:(x)MS04(M = Ni, Co and Cu) systems obeyed Arrhenius law. Fig. 2 displays the concentration depend­ent conductivity for all series under study. It can be seen from Fig. 2 that as the concentration ofMS04 increases the conductivity also increases and exhibits maximum at about 20 mole %. Beyond this concentration it de­creases.

The above conductivity behaviour could be explained as fol lows:

The room temperature X-ray powder diffraction pat­terns showed characteristic lines corresponding only to

2·---------------------,==9

t log[oT ·1

(SKI em)] ·2

.3 1---�-------------.. l.4 1.S \.6 1.7' 1.11 1.9

253

Fig. 1- Arrhenius plots for Ag2S04:ZnS04 binary system

-4

t 5 log[o :Slcm)} 6

, ,

Mn '" _ 0

Zn Mg I�i .7�--��--------------�

o 20 40 eo 80 100

Concentration, x (mol%) �

Fig. 2 - Variation of log a with MS04 concentration

Ag2S04 and MS04 in respective systems. There were no lines due to new intermediate phases which rule out the formation and dispersion of other high conducting phase. The partial solid solubility of divalent sulphate into Ag2S04 lattice, however, cannot be ruled oue. Thus the present system can be visualized as the dispersion of second insoluble MS04 phase in Ag2S04 solid solution (SS) matrix.

The total conductivity of such hi-phase systems is due to the contributions from: (i) Ag2S04 solid solution, (ii) MS04, (iii) homo-junction (interface between Ag2S041 Ag2S04), and (iv) hetero-junction (interface be­tween Ag2S04/MS04). Since maximum conductivity of optimised samples is higher than those of the individual phases, hetero-junction chiefly contributes towards an enhancement in conductivity. The surface interaction at the Ag2S04/MS04 interface gives rise to highly disor­dered space charge layer in its vicinity. The formation of homo-junction and hetero-junction for the systems under consideration is analogous to that discussed pre-

254 INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1999

1811

t emf 17Q

(mV)

1l1li

1~ ~------------------------~ o 101 202 303 .04 ~06 606 707

Time (Sl-4

Fig. 3 - Variation of sensor emf with time after toggling the S02 partial pressure. at 500a e

viousl/ 6. The highly disordered space charge layer

facilitates ion migration along and across the interface leading to increase in overall conductivity of spec imen .

As the concentration of second phase increases, the insol uble phase precipitates out and subseq uently dis­perses into Ag2S04 SS matrix . Such di spersion increases hi ghly conducting hetero-junctions, forming ion pe~co­lating paths thereby enhancement in the conductivity. The maximum conductivity at x = 30 is due to the percolation threshold concentration. The decrease in conducti vity beyond percolation threshold concentra­tion is attributed to the di sruption of perco lati on paths due to agg lomeration of grains7.

Fig.3 depicts the variation ofsensor emf as a function of tim e after toggling the SOl partial pressure from 1% to 500 ppm . Ev idently, the emf of sensor initi ally in­creases rapidly and atta ins a saturation value . The time required to attain 90% of saturation va lue is defined as th e response tim e. The response tim e for the present se nsor is found to be 45 secs . A close look of Fig. 3 also revea ls that the sensor attains the same saturation va lue after change in gas concentration show ing perfect re­\ tTsibi lity of the se nsor. Eve n after a few thermal cy­cl ing th e (heating and coo ling) se nsor shows a s imilar trend . The reproducibility of the above results is con­finned after fabrication and evaluati on of a new identi ­ca I sensor.

The va ri ation of sensor emf with tim e at a fi xed SOc partial pressure is shown in Fig. 4. As seen there is no chan ge in the emf' with time indicati ng a good long term stability. Increased stab ility is due to an improved sin-

140

.~ ------ ..... t 110

emf (mV) 80

50

Time (S)-4

Fig. 4 - Variation of sensor emf with tIlne for fix ed S02 part ial pressure. at 500°C

terability of the bi-phase so lid electrolyte. The cross sensitivity to CO2, CO, H2 and moisture was determined by introducing these gases indi vi dually along with the known partial pressure of S02 and recordin g the emf with time . The cross sensiti vity of the sensor to other gases e.g. , CO2, Hl and moisture is fo und to be neg li­gible.

4 Conclusion Conductivity of binary systems is higher than the host

systems. Enhancement in conduct ivi ty in the tvvo phase system is clue to the hetero-j unction fo rm ing ion perco­lating path s. The optimi sed two phase compos itions are suitable for S02 gas sensor app licat ion .

Acknowledgement Authors are thankful to the Departm ent of Sc ience

and Tec hnology, New Delhi (No. III (4-42)93-ET) for prov iding fin ancia l ass istance to carry ou t thi s work .

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