of molccular hydrogen, hydrogen peroxide, or hydrogcn atoms. Thc dccompositioii of isopropyl alcohol rcsiilts in thc formatmi of hydrogen. At low concentrations of isopropyl alcohol the dehydrogenation seems to bc an intramolecular process in which any two hydrogcn atoms have ari equal probability to bc climinatcd Thc intramolecwlar production of a hydrogen molcculc which iiivolvrs the a-hydrogen of isopropyl alcohol yields mainly acetolie. The dccomposition of isopropyl
alcohol a t high concentrations involves an additioiial bimolecular reaction with a prcft:rcntial abstraction of t h e U-hydrogeii. These findings may he interpreted by the thernial decomposition of the alcohol inside the cavities i i i contrast to chemical changcs of nonvolatile solutes which take placc by "indirect action.” The sonolytic behavior of volatile solutes canriot therefore be correlated with the sonochcmistry of water arid is a furictioii of their volatility and thermal st,ahility.
The Effect of‘ Oxygen on the Electrical Resistance of Evaporated Silver Films
by A. W. Smith
‘I‘he electrical resistance of thin films of silver was determilied in the prcsencc of, various gases betwecii 200 a i d 300’. Osygcn incrcased t h e resistnncc! as a qiiadrat,ic funct~ion with time. Iteducing gases such as hydrogen, ethylene, or carbon monoxide returned the resistaiice of the film to its normal value. The el‘fect is interprctcd ilot as duc to clcctroiiic interactions but as due to rccrystallizatio~iof the film uiidcr the influcnce of t h e various gases.
Introduction Thcrc have hccn two investigations reported in the litcraturr on thc effect of oxygen and other gases on the elcctrical rcsistancc of silver films 1 , 2 Botli of t l m c authors uscd chcmically deposited silver on glass fibcrs. Thc measiircmcnt of resistance was made ill both iiivcstigations by prcssing a inat of these fibers bet wc(111 silvrr plates. In both stiidics the work was related to thc oxidation of ethylene on silvclr. 1Iore rcctiitly, Dixon and l,ongfield3 suggested that more work bc donr with this system. 13rcausc of expcricnce here in the dctcrrninatioii of c.Ioctrica1 rcsistance and the effect of adsorption on evaporatcd films on copper oxide, it was dccidrd to study t hr cffcct of adsorption on the electrical rcsistaiicc of evaporated silvcr films. ‘The advantagt. of using evaporated filrns is that the uncertain contacts b c t w c n fibcrs which were prcsciit in the previous studics would not bc prcsciit
Experimental The cxpcrimriital apparatus was that rcpovtcd previously for staudies011 copper oxide films4 with t’hr rxccption of t h e ovaporatioii tube aiid the equipment for mcasuring lesistaiice. Thc evaporation tube n-as construct,rd as follows. A four-wire t~iingst,cii-throuRtiglass “press” was cut pcrpeiidicular to the tiiiigst,eii lcads through the glass aiid the leads, and thcn this e i i d was polished. T w o coats of “bright plat,inuni” aiid thcn two coats of chcmically deposited silver covcred this end of the press iiicluding the ends of the four tungsten leads. This mctallic coating was removed
A. W. SMITH
1466
from the space between the middle leads. This press was then installed in a Pyrex tube, and silver was evaporated from a filament onto the end of the press. The filament was prepared by winding three strands of 0.38-mm. molybdenum wire and one strand of 0.18-mm. silver wire. The resistance was measured by the four probe method. A constant current of 1 ma. was passed through the outer two leads and the voltage was measured across the center two leads with a Leeds and Northrup microvolter.
Results and Discussion The increase in resistance upon admission of oxygeii to silver-plated fiben was held by Lyuharskii* to be due to removal of electrons from the silver by the oxygen. Twigg,' on the other hand, held that oxygen interpolated itself between the contacts of one fiber to the next. Dixon and LongfieIda assume that this is an electronic effect in suggesting that this measurement be used in studying the oxidation of ethylene on silver. The use of evaporated films can help to decide between these different mechanisms for this effect because the evaporated films form a continuous path and a four probe method may be used to eliminate contact effects. Evaporated silver films of appropriate thickness showed large increases in resistance when exposed to oxygen at temperatures above about 200". Films which were too thick showed only a small effect while films which were too thin lost metallic conductivity
1.8(
I
TIME^
(MINUTES$)
Increase in resiSt:uire uf 35l)-.-\. evaporated silvez film when exposed to 100 torr preasure of oxygen at 254'.
Figure 1.
after one or more oxygen and hydrogen trratments. By comparing the temprrature coefficient of resistance of the films with the data of Raker and Caldwell,j a value of 3001100 A. was obtained for the thickness of appropriate films. There is no correlation between the rate of oxygen adsorption and the resistance changes. The adsorption of oxygen on silver above 200' is virtually complete in a matter of minutes.6 The electrical resistance of these films, however, increased steadily, proportional to the square root of time, for long periods. In one attempt to observe saturation the resistance was followed for a week. At the end of this period the resistance was still increasing according to this function. Figure 1 shows a typical result for these films. During this run the resistance approximately doubled. Treatment with reducing gases returns the resistance to its original value in a matter of minutes with a nearly exponential function. In one series of experiments at 3W0, after a 15-min. exposure to hydrogen, 99% of the resistance change was restored, whereas after a 4.5-min. exposure to carbon monoxide only 70% of the change was restored. The effect of ethylehe was intcrmcdiate between that of carbon monoxide and hydrogen. The oxygen-hydrogen cycle has been repeated many times with nearly identical resistance chauges on each cycle. ( 5 ) B. B. Baker and W. C. Cnldwell. Ameq Lahoratory. 15C-215, 1. S. Atomic Energy Commis-ion. March 20. 1952.
(GI W. W. Smeltser. E.L.Tollefson. and A. Carnbron. Con. .I. 34. 1046 (1956).
Chrm..
EFFECTOB OXYGENON ELECTRICAL RESISTANCE OF EVAPORATED SILVERFILMS
16
18
20
TEMPERATURE-’
(OK-’
22 x
IO4)
Figure 3. Arrhenius plot of rateoof resistance ncrease V B . temperatme for 350-A. evaporated silver film exposed to 100 torr pressure of oxygen.
It is known that the alternate exposure to oxygen and hydrogen causes rapid roughening and facetiiig of many metal surfaces. Wilson, et uL,7 have shown extensive recrystallization of silver when exposed to a mixture of ethylene and oxygen. Observation of the films used here with the electron microscope shows that, after much treatment, the originally smooth film formed agglomerated to separate particles. This is shown in Fig. 2. Under moderate treatments these particles
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are probably still connected by necks as shown for copper fiIms.4 From the magnitude of the resistance change and from the lack of correspondence with the oxygen uptake, it seems obvious that this effect is not electronic. The observation that treatment with oxygen causes crystal growth indicates that the correct reason for this resistance change is crystallite growth with the removal of silver from the necks between crystallites. The role of oxygen is to provide a change in surface energy for the different crystal faces making such growth possible. A small electronic effect may also be present, but it would be hidden by the large nonelectronic effect. The rate of grain boundary diffusion for silver has been determined t o be 20 kcal./mole.* The rate coilstant K for the resistance change due to oxygen obtained from the expression, dR/dt = K / R , was determined a t a number of temperatures, AH Arrheiiius plot of this data is shown in Fig. 3, This leads to an activation energy of 24 kcal./mole. This value indicates that the activation energy of the process responsible for the resistance change is of a reasoilable magnitude for a diffusion-controlled process. Apparently the diffusion process is slower on an oxygencovered surface than on a reduced surface, The fact that “resintering” goes at a different rate depending on the reducing gas used shows that the actual rate of oxygen removal may be important. Chalmers, King, and Shuttle\.rrorthg show that, at higher temperatures, oxygen causes striations produced by the exposure of 111 planes and that these disappear in a nitrogen atmosphere. This is strong evidence that the mechanisin of recrystallization is responsible for the increase in resistance of evaporated silver films and presumably for plated fibers as well. ~~
(7) J. N. Wilson, H H Voge, D. 1’. Stevenson, A. E, Smith, and L. T. Atkins, J . P i i y s . Chem., 63, 463 (1959) (8) D. Turnbull i n ”Atom Movements.” American Society of Metals, Cleveland, Ohio, 1951, p. 129. (9) B. Chalmers, Ii. King, and R. Shuttleworth, P,,oc. Roy. Soc. (London), A193, 465 (1948).
Volume 66, Sumher 6
June, 1964
