Influence o f time on emission o f counts in a fresh and an electro'con dition ed tube under d- c. excitation *

I n d i a n J . P k y $ . 40, 390-<400 (1975)

Influence o f time on emission o f counts in a fresh and an electro'con dition ed tube under d- c. excitation *

S . G. Pim p ALE

Dtpatinient oj Physics^ Barsi College, Barsi-4iVS4;{)l, Vist, Sholapur, Maharashtra

{Received 25 September

Chem isorption o f ad m ixtu re o f gases (air) on glass surface as a d ete r­

m in an t o f th e current pulses (counts) p assin g th rou gh an ozonizer ex c ited by a definite high valu e o f direct current im p ulse p o ten tia l, viz. 1.5 kV , has been in v e stig a ted p rev io u sly cold w orked co a x ia l cylin drical (fresh) and an electro-con d ition ed discharge tu b e. I n a fresh tu b e, th e co n d u c tiv ity o f discharge cou n ts is fou n d to be e x c e e d ­ in g ly large; it a tta in s satu ration w ith in an ap p reciab ly lon g in terv a l o f tim e and rem ains u naltered w ith in th e exp erim en tal accuracy.

Com pared w ith th e d ata observed in fresh tu b e, th e cou n t rate rem ains sa tu ra ted w ith in an ap preciab ly sh ort tim e-in terv a l. T hus, for aged ail’, th e discharge cou n ts reached to th e m in im um sta tio n a ry v a lu e w ith in

6

T he v a ria tio n o f discharge cou n ts in dark w ith con tin u ou s agin g roferj’ed to as th e pulsed em ission p henom en on, follow s th e p o stu la ­ tio n o f an ad sorp tion -typ e boundary layer on th e annulai' w alls and its ch aracteristic work fu n ction . On th is con sideration , th e e q u a ­ tio n s y ield .

(0 I I

.

.

cidx

® ~~

d —fi.

(xdx

S

k{t)^

(Hi) (Slg) =

w ith

7,3; Isi OL

x

;

t

1. In t r o d u c t io n

The effec t o f sorbed gases on p h oto- and thorm o-clcctric currents h as been w ell stu d ied . Cantor (1893) and K n ob lau ch (1899) observed th a t su b stan ces w h en exp osed t o u ltra v io let ligh t, lo st som e n eg a tiv e charges, Chi islcr (1908) fou n d th a t th is plienam onon, v iz., photo-elootric effect, could be elim in a ted co m p letely , in certain cases, by th e rem oval o f occlu d ed gases b y scrapping it in vacu u m . N o p h oto-electj’ic effect in m etals w as e v id en t (K u stn er 1914). Sim ilarly, w hen all

work i’oj)ortc(l hero was carried out in the Chemistry Department, U niversity of Poona.

390

E m ission of counts under dx , excitation 391

occluded gases from p otassiu m are rem oved , th e photo-olectrip effect disappeared en tirely (W iedem ann & H allaw ach s 1914). E ssen tia lly sim ilar results were ob tained b y num ber o f o th er workers (H en n ings 1914, S tn m f 1914). W hile th e data o f th e a b ove in v estig a to rs em p h asised th e favourable influence o f adsorbed gases on p h o to-electric effect, con trary findings in d icatin g th eir inh ib itory action on electron lib eration from m etal b y irrad iation , wore also recorded. T hus, S toletow (1889) fou n d th a t th e p h oto-electric effect from th e platinum incrased when th e adsorbed gases were rem oved b y h ea tin g it to 200°C, a t 7()0°C th e effect w as tw ic e th a n th a t a t lab oratory tem p eratu re (Z oleny 1901). I t w as shown th a t th e p h oto-electric sen sitiv en ess after em ission o f occluded gases was definitely larger th a n th e effec t n oted before th e ex p u lsio n o f th e gas (Pearsol 1916). The above con trad ictory resu lts received ex p la n a tio n from th e findings du(^ to Peach (1914) th a t th e p h o to -electric effec t o f th e m etal w as d ep en d en t upon th e nature o f th e gas occlu d ed or adsorbed su ch as hydrogen, carbon m on oxide, oxygen , etc. Tt h as l>een em p h asised b y M allaw achs (1932) th a t electron egative gases cause a decrease and elec tro p o sitiv e gases, an increase. F urther, a d istin ction was m ade b etw een th e gas adsorbed on th e surface, and th o se absorbed w ith in th e body, o f th e m etal. A dsorbed gases retard th e escape o f electrons, w hile th e absorbed gases aid th e lib eration o f electrons, ap p aren tly b y causing som e internal a gitation s (H allaw ach s 1932). T hese view s are su b sta n tia te d b y th e d ata on ih evm o-electric effect. T his phenom en on w as w ell in v e stig a ted b y Langm uir (1913, 1914). I t w as n oted th a t therm o-electric effect o f tu n gsten w as m arkedly inhibited by sorption o f O

2

2000^0

W and o x y g en . M uch a d van cem en t in th e stu d ies o f sorption w as ach ieved b y tile fu n d am en tal in v e stig a tio n s o f L angm uir. A ccording to Langm uir, there ex ists a fixed num ber o f atom ic groups or sites per u n it area o f th e solid surface, on w hich alone ad sorp tion o f a gas atom or a m olecule can ta k e p lace. T hese adsorbed atom s, dep en din g u pon th eir electron affinity, retard, as m en tion ed b y H allaw ch s (1932), th e escape o f electron s.

W hile p h o to -electric and therm o-electric currents and th e influence thereon of adsorbed film s received m uch a tten tio n , th e variation o f th e discharge current pulses w ith th e surface nature o f th e electrod es esp ecially w ith solid dielectric m aterial, w as n o t fu lly in v e stig a ted . Tt has, how ever, been m en tion ed (Loeb 1939) th a t th e discharge current and th e m echanism s responsible therefore are m arkedly d ep en d en t upon th e work fu n ction o f th e cathod e (chiefly th e gas adsorbed on it). T his factor has n ow been in v estig a ted in som e d etails.

2 . Th e o r y a n d Me t h o d of In v e s t ig a t io n

In general, th ere are th ree p rocesses w hich occur w hen a gas is brought in contact w ith a solid, ab sorp tion, d iffu sion and adsorption (M oBain 1938, Laidler

392 S. G. PimpaJe

1949). The phenom enon o f absorption refers to th e in tak e o f g a s by th e en tire bulk, w hile diffusion deals w ith th e passage o f gas atom s or m oled u les through th e intorm olecular capillaries, o f th e solid substan ce (Lennard 1924). In th e la st process, adsorption, a know n q u a n tity o f th e gas is tak en up b y th e surface and surface alone o f th e solid. T hese three phenom en a are sum m arised b y a single nam e sorption (M cBain 1938). A m ong th ese, adsorption is o f m arked im portance. I t is th e on ly process w hich is found to occur a t ordinary tem p era­

tures, w ith su bstan ces like Cu, Al, Au, Ag, P t, glass, etc. which are in com m on use as electrode m aterials and has therefore been stu d ied on ly in th is paper.

A dsorption is principally o f tw o tjrpes, th e in stan tan eou s and slow process.

W hen a gas is allow ed to com e in con tact with a clear solid surface, a know n q u a n tity o f it, depending upon th e tem perature and pressure thereof, is tak en up in sta n ta n eo u sly (Laidler 1949); in th is case, th e gas e x ists in th e form o f m u lti­

layers (Brunauer

et al

to as V an dor W aal’s m ultim olocular or p h ysical adsorption. T his is follow ed b y \ the slow process; th e m axim um tim e n ecessary for th e a tta in m e n t o f satu ration is o f th e order o f a few hours to years (Swan & U rquhart 1927) T he large tim e recorded w as n o t due to slow diffusion, since th e corresponding con stan ts varied ch aracteristically w ith pressure. Tt w as further em phasised (Bangham 1928) th a t th e lon g in tervals n oted a t th e saturation tim e in th e slow processes w ere n ot u nten ab le from th e follow in g th eoretical considerations. I t is sum m arised g en e­

rally (B am gham 1928, T om pkins & Crawford 1948), follow ing Langm jiir, th a t th ere are a d efin ite num ber o f atom ic groups or sites (sm all elem en try areas) per sq. cm . o f solid surface on w hich sorption o f a gas particle can ta k e place.

I t is further assum ed th a t th ese atom ic groups are unable in th eir norm al con d i­

tio n to adsorb and require first t o be a ctiv a ted before th e y a ctu a lly com bine w ith or adsorb a m olecule or an atom , th e n ecessary en ergy being ob tained from th e ex c ited m olecules or atom s arriving from th e gas phase to th e solid surface duo to th e tlierm al m om en ts. I t h as been m en tion ed ab ove th a t no sooner th a t gas is introduced in to th e sy stem con tain ing nude surface, p h ysical adsorption in w hich th e gas e x ists in th e from o f m ulti m olecular layers ak in to th ose o f liquid phase, tak es place in stan stan eou sly. A m olecule or a gas a to m w ith enhanced e x c ita tio n energy should therefore d rive in to th ese layers, roach th e Langm uir site transfer its energy to th is last. T he rate (w henever th e jr a te o f sorp tion is m ention ed , it refers to th e slow process o n ly and n o t to th e oth er k in d o f sorption v iz,, p hysical or V an der W a a l’s adsorption) o f sorption d ep en d s upon th e p ro­

b ab ility o f th e ex cited atom s or m olecules reaching th e solid surface con tain ing free Langm uir species, sm all elem en tary areas), sorption a tta in s satu ration w hen th ese are com p letely occupied. B ecau se o f th is, th is process is also referred to as chem isorption, activated adsorptio^

ox

E m ission of counts under d.c. excitation 393

As cited a^bove, th e rate o f a c tiv a te d ad sorp tion depends th e p rob ab ility factor regarding th e a cc essib ility o f the. e x c ited gas particles to th e solid surface (R am aiah 1952). U nder con traction (discharge), th e p opulation o f ex c ited atom s or m olecules p resu m ab ly th eir reach in g th e surface, are exceed in gly m arked;

on accou n t o f th is sorption under (W illow & G eorge 1916, Johnson 1923, Taylor 1928 & R am aiah 1962) co n traction is ex p e c te d to be very rapid as a ctu a lly o b ­ served. F rom th is it follow s th a t w h atever p rocesses th a t occur under ordinary con d ition s and during con traction , refer to on e and th e sam e phenom enon viz,, a ctivated adsorption or Langm uir adsorpton.

(

T he influence o f ch em isorption on th e electrical co n d u c tiv ity o f a gas was now in v e stig a ted . For th is purpose th e discharge tu b e w as h eated for rem oval adsorbed gases and a know n q u a n tity o f a gas w a s introd uced therein and ex cited b y a d efin ite p o ten tia l. T he v ariation o f its cou n t rate w as stu d ied a t regular intervals o f tim e during th e progress o f sorp tion under discharge.

3. E

Description of the discharge vessels

Heat treatment:

m itten t, p resu m ab ly due to local changes on surface o f th e glass w alls. I t is, therefore, n ecessary to rem ove th e traces o f oxid e on th e electrode-surface n ecessary to rem ove th e traces o f o x id e on th e electrode-surface to ob tain accurate values o f cou n ts. T he surfaces o f th e discharge tu b e were freed from adsorbed gases b y th e follow in g process. T he discharge tu b e w as h ea te d in a h eater b ox fitted w ith h eater coils and th en cooled slow ly to th e room tem p eratu re. T his h eatin g a t 200'^C w ip es ou t its p reviou s h istory. Thus h ea tin g seem s to h elp in bringing th e con d ition o f th e v essels nearer to th a t o f th e fresh one an d referred to fresh tu b e. T he tu b e w as th e n su b jected t

6

1.6

con d itionin g or agin g b y earlier workers. T he electrod e con d ition o f th e tu b e

394 S. G. Pim pale

w as cheeked b y observing th e cotint rate a t a definite valu e o f d iscrim inator bias (50V) an d a t a fixed d,c. ex c itin g p oten tial.

WorJeing 'potential.

.5

van om eter a ctu a ted b y a crystal det^jctor. The d etails o f those were given in a num ber o f earlier com m unications (Pim pale 1972, 1973. 1974), and are n o t reiterated here. W hile using th e ozonizer, th e follow in g m a y be ta k en into consideration. The ozonizer was Avorked a t p o ten tia ls ab ove th e th reshold poten- t.ial

Vga

(V —Vga)

where

V

(V —Vgo),

Vga

present; su td ies w as m ain tained very high to th e threshold p o ten tia l, such th a t th e ab ove change Avould be m axim um .

Furtherm ore, th is w ould fa cilita te th e stu d ies o f discharge m o stly concerned w ith th e surface o f th e electrodes, since it was show n (H ennings 1914) th a t a t X)()tentialH near

Vga.

{pOg)

V

Vga

(fi)

et al

Qas investigated

1

o f m ercury enclosed in an annular space o f th e ozonizer w as stu d ied . T he exp eri­

m ental set u p for th e in v estig a tio n o f th e variation o f cou n t rate Avith tim e in h eat-treated and u n treated discharge tu b e is show n in figure

1

T w o series o f exp erim en ts were carried ou t using fresh and electrocond ition ed ozonizer. The form er had been in use for w ork on tim e-variation s o f discharge counts in air over a period o f 15 hours, th e latter AA^as electrocond ition ed .

In th e first series o f exp erim en ts, th e count rate a t a g iv en cou n tin g tim e, A^iz., 3 m in, av^s m easured w ith an electron ic scaler. A t a con stan t ap plied c o n ti­

n uous potential o f 1.5. kV , th e cou n ts were observed a t in tervals o f 30 m in w hen th e ozonizer w as in th e dark; th ose d a ta are p resented grap h ically b y a solid curve in figure 2. The course o f reaction w as ex p ec ted to be eith er a progressive d im i­

n u tion o f cou n ts or its con stan cy, d ep en din g upon w hether th e tem p eratu re o f th e sy stem w as low enough. A ctu ally, it w as found th a t under th e con tin uou s d is­

charge (c/. a solid curve in figure

2

E m ission o f eounte under d.c. excitation 395

a^ain to rise and fall th ereafter th e c o n d u c tiv ity reached th e m inim um sta tio n a ry value. T he con d ition s for th e op tim u m d evelop m en t o f th is periodic effect arc such th a t th e corresponding progress o f th e change is rather slow; th u s for instance, in one case as m a n y as

3

sals and o f th e oth er electrical q u a n tities m en tion ed already w ore ob tained n eces­

sita tin g an exp osu re o f 900 m in u tes to th e discharge. A fter th is tim e, there w as slow ing d ow n o f th e change, in d ica tiv e o f a near com pletion o f th e reaction.

H.T.

O il NqCI

GAS

L .T .

' zo

■H

<

. o<

- fC

a:

1

A fter one set o f exp erim en ts, th e v essels was u n excited for 24 hours, th e dis- t^harge w as th en restarted and th e variation o f th e discharge cou n ts a t intervals o f 30 m in w as n oted . A d o tte d curve o f figure 2 g iv es th e d a ta ob tain ed w ith in tervals o f 30 m in. I t w as in terestin g to n o te th a t th e co n d u c tiv ity o f th e d ischarge cou n ts was in itia lly ex c eed in g ly largo a t th e first in stan ce o f recording and d ecreased rapidly w itli tim e . T hus in air, cou n t rate w as 164 a t —

0

Com pared w ith th e d a ta ob served in fresh tu b e, th e cou n t rate a tta in e d sa tu ­ ration w ith in and ap p reciab ly sh ort tim e in terval. T hus, for aged air, th e dis- (jharge coiu its reached th e m inim um sta tio n a ry valu e w ith in

6

2

2

396 S. 0 . Pim pale

P re v io u sly b y cold w orked ozonizer, th e c o n d u c tiv ity o f th e discharge cou n ts w as fou n d to be ex c eed in g ly large, th e c o n d u c tiv ity a tta in e d satu ration w ith in an ap preciab ly lon g tim e in terv a l and th e c o n d u c tiv ity o f th e discharge cou n ts rem ained u n altered w ith in th e exp erim en tal acou ran cy (c/. a solid curve in figure

2

TIUB IH HIN

Fig. 2. Current-time chai'aoteristicB showing pulsed emission decay

3. D

T he follow in g are th e im p ortan t con clu sions from th e foregoing resu lts :

1

I

2

3)

ported in th e earlier com m u n ication s (L oeb

1939,

1948,

cl al 1953),

a —/?.exp (a—y^)® (

1

H ere,

I

n^.e

H

tio n o f electron avalan ch es. T he T ow n sen d ’s first coefficient a d ep en d s u pon th e

Em ission of counts under (2.c. excitation 397

p osition X in th e n on tin ifon n field. Fu rth er, w hen a is m uch greater th an th e second coefficient eq. (

1

r __ r

cxdx

" ® a —/^.exp /

adx

From th is, it is clear th a t th e cou n t rate and th e creation o f electron a v a ­ lanches are fu n d am en tally governed b y t

I

,

a

fl

The th eoretical con sideration s on con traction in gases su ggest th a t a decrease in gas pressure increases th e current as a c tu a lly observed (R am aiah 1961). T he result (

1

buted t o th e decrease o f p rim ary electron s on a ccou n t o f progressive sorption o f gases under con traction (discharge). T his is in accord w ith th e ob servation th a t p h oto- an d th erm o-electric currents decreased w ith th e adsorption o f electron egative m olecules (or atom s). T hese la st on accoun t o f their electron affinit, capture electrons and th u s retard th eir escape from th e surface (HaUawach 1932).

T he ab ove con sideration su ggests th a t i f

S

m entary area (site) on clean glass surface, sorption o f air atom s thereon and su b se­

quent capture o f electron m a y be represented as 8

+ A * -^

S

... (3)

T his form ation w ou ld em ph asise th a t atom s adsorbed on glass retain th eir electron egative character. T his, how ever, in d icates th e current v iew th a t atom s are held b y a ctiv a ted adsortpion on glass b y sharing an electron from alkali atom in glass.

T he rate o f d ecay in cou n ts g iv es a strik ing support for th e a b ove v ie w th a t th e adsorbed gas is responsible for th e finding (

1

troscopic m eth od s T aylor (192$) ob served th a t th e disappearance o f in hydrogen 9

398 S. G. Pimpale

n eon tu b es w as aooom panied w ith in

5

As

Further, B angh am (1928) h as show n th a t the rate o f sorption o f a gas on glass is w ell represented b y th e follow ing relationship,

... (4)

whore 8 is th e am ou n t adsorbed u p to tim e £,

m

k

From th e ab ove argum ent, if we m ake th e assum ption th a t th e decrease

(SI

)

t

(SI

)

k't^.

I t follow s, therefore, th a t

log

[SI

)

t\

(5)

(

6

I t is in stru ctive to n ote th a t as required b y tlxis relation (

6

(Slg)

versus log

t

I t m a y be m ention ed th a t p reviou sly b y cold w orked tu b e altered m arkedly th e ab ove d ata. Sim ilar results were obtained b y R am aiah (1954) in th e stu d y o f Josh i-effeot (Ai). W hile in olectroconditionod ozonizer, th e effect could be observed im m ed iately, in h eat-treated vessles, a certain am ou n t o f aging w as fou n d n ecessary to in itia te th e rapid d ecay in cou n ts. The d isp arity w as a ttr i­

b uted to th e presence o f alkali on th e surface o f former ty p e o f ozonizer, w hich enhanced sorption to aid th e occurrence o f d ecay in acou n ts. E v en in th e m easure­

m ents o f sorption o f gases, th e presence o f alkali on glass surface p layed an im por­

ta n t role (MoBain 1938). F arad ay (1830) observed largo adsorption o f w ater vapour on glass and attrib u ted it to th e ex isten ce o f alkali. W hen th is w as rem oved b y w ashing glass surface w ith acids an appreciable sorption w as n oticed . T his ca ta ­ ly tic co n d u ctiv ity is how ever, a general feature o f such polar-substances. K n o w ­ ledge o f num ber o f chem ical reaction s w hich are controlled b y th e presence o f traces o f som e polar su b stan ce, is availab le (D ixon 1884, 1898; Miller & R u ssel 1902). N orrish (1923) has em ph asised th a t th e increased a c tiv ity o f gas reaction s in th e presence o f sm all q u a n tities o f N aO H , K C l, etc;, is due to d istortion on accoun t o f th e strong local disturbin g forces available a t th e surface o f th e polar

E m ission o f counts under d.c. excitation 399

substance, in the stable configuration of the molecule such that these last become more vulnerable to be attacked. Lennard-Jones & Dent (1924) envisaged the various forces existing outside the surface of the polar substance; (a) forces of attraction because of induced dipole in the centre of an atom or ion and the elec­

trostatic field; (6 ) forces due to polarization in the substance by the free charges outside (c) direct electrostatic force between ions in the solid and the free state outside it and (d) Van der Waal’s forces. Of course, Van der Waal’s forces play a primary role, they act as the first agents in the process of adsorption of atom or ion and the electrostatic forces serve to clinch the capture of ultimate adsorption.

Ao k n o w l b d g m n t

The author wishes to express his grateful thanks to Prof. S, S, Joshi and Prof.

H. J. Arnikar for giving him encouragement and facilities for carrying out the present study. He wishes also to express his gratitude to Mr. S. S, Sardesai for the assistance in the experimental work.

Bb f e b b n o b s

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Dixon 1884 Phil Trans.; 1894 Bake Jour. Chem. Soc. 65, 611.

Faraday 1830 Ptdl. Trana. Roy. Soc. 1, 49.

Francis & Burt F . P. 1927 Ibid 116, 686.

Grog & Jacobs 1948 Ibud, 45, 616.

Hallawachs 1932 Photoelectric Phenomena, Mc-Graw H ill Book Co., N .Y ., pp. 80.

Hennings 1914 Phys. Rev. 8, 228.

Hill 1047 J . Ghem Phys. 15, 767, Johnson 1923 Proo. Roy, Soc., 123, 633.

Joshi S. S. 1928 Trans Farad. Soc. 25, 127; 140.

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Joshi S. S. 1945 Curr. Sci. 14, 67.

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Kuetner 1914 Zeit. f. Phys. 2, 68.

Laidlor 1949 J . Phys. Sc Colloid. Ohem. 62, 713.

Langmuir 1913 Phys. Rev.

2

Lennard>Jones 1924 Trans. Farad. Soc., Appendix, 24, 92.

Lennard Jones & D ent 1924 Trans. Farads Soc., 24, 02.

Loeb L. 1939 Fundamental Processes in Electrical Discharges in Oases. John Willey.

Mo Bain 1938 Sorption of Oases by Solids, Boutledge.

Miller A. B . & Bussol 1902 J . Ohem Soc. 81, 1272.

Korrishl923 J . Ohem. Soc. 3006.

400 S. G. Pimpale

Peach 1914 A nn. der. Phya*

36.

Pearsol 1016 Phya. Rev* 8, 238.

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B am aiah N. A. 1961 J . Sci. Rea, 1, 91.

Bam aiah N. A. 1952 «7. Chem, Phya (Paris)

328.

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B am anm urty M. V. 1948 Ibid 25, 226.

atoletow 1889 Oompt. Rend. 108, 1241.

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Taylor H. 8. 1928 Nature 121, 70.3; 122, 347.

Tompkins & Crawford 1948 Trans. Farad. Soe.

698.

Townsend 1948 Electron in Oases^ Hudrinaon. Sci Sti Technical Puhlicationa., W iedemann & Hallawach 1914 Verhandle, dent. d. Phya. Qea, 5, 291.

Willows & George 1916 Proc. Phya, Soc. 28, 124.

Zeleny 1901 Phya. Rev. 12. 321.