coatings. Samples of tin-plated iron were analyzed to determine the thickness of the tin cover. I n Figure 7 the spectrum from a typical sample is compared with that of polished iron. The shift in the position of the iron edge is clearly indicated, but in the esample shown the tin coating was so thick, about 0.55 mg./cm.*, that the alpha particles scattered from the iron showed an appreciable energy spread due to straggling. The inflexion point corresponding to the iron in the energy spectrum was then used as the reference point. Because the sharpness of the plateau step in the energy spectrum deteriorates with increasing coating thicknesses, the error inherent in defining the position of the inflexion point increases, but the relative error in ascertaining the coating thickness remains about *20j,. Measurement of Geological Samples. Homogeneous samples infinitely thick with respect to the alpha particle range can be analyzed for their major components by alpha particle scattering. Each component is identi-
fied by a step in the energy spectrum, as shown in the examples in Figure 8. The position of the step determines the element and its concentration is obtained from the height. A variety of geological samples were analyzed and some spectra are shown in Figure 8. The results are comparable with those obtained when large sources of alpha-emitting radioisotopes were used (5). The sample of galena was submitted as a pure lead ore. but the analysis showed it to contain copper and/or zinc (these elements are irresolvable by the method used) in relatively high concentrations, a common occurrence in this mineral. ACKNOWLEDGMENl
The authors acknowledge the willing assistance of Ren6 Pretorius and Albert Bottega. Hermann Rohm is thanked for his analysis of the anodized aluminium targets. Geological samples were kindly loaned by the Geology Department, University of Stellenbosch.
LITERATURE CITED
(1) Blignaut, E., Kritzinger, J. J., Nucl. fnst. Methods 36, 176 (1965). (2),Buechner, W. W., Kobertshaw, J. E., ?runs. Am. Nucl. SOC.5 , 197 (1962). (3) Dearnaley, G., Whitehead, A. B., At. Energ. Res. Estab. (Gt. Brit.) Rept. R 3437 (1960;. (4)Green, F. I,., Cooper, hl. D., Robertshaw, J. E., Trans. Am. Nuel. SOC.5 , 197 (1962). (5) Pattersoii, J. H., Turkevich, A . L., Franzgrote, E., J . Geophys. Res. 7 0 , 1311 (1965). (6) Peisach, AI., Poole, L). O., J . S . AjTzcan Chem. Inst. 18,61 (1965). ( 7 ) Peisach, hZ., Poole, D. O., Proc. 1965 Intern. Conf. Modern Trends in .4ctivation hnalysis, College Station, Texas, ICAA-I1/37, in press. (8) Rubin, S., Passell, T. O., Bailey, L. E., ANAL. CHEM.29, 736 (1957). (9) Sippel, R. F., Phys. RW. 115, 1441 (1959).
RECEIVEDMarch 21, 1966. Accepted May 20, 1966. One of us, (D.O.P.) thanks the South African Atomic Energy Board for permission to include his work in this publication.
Characterization by Optical and Electron Microscopy Techniques of Oxidation Products Formed on Dual Porosity Sintered Nickel Electrodes I
WILLIAM R. LASKO and WARREN
K. TlCE
United Aircraft Research laboratories, East Hartford, Conn.
LEE SCHULMEISTER Pratt and Whitney Aircraft Division, United Aircraff Corp., East Hartford, Conn.
b
Optical and electron microscope cross-sectional techniques were developed to characterize the oxidation products formed on dual porosity sintered nickel electrode structures after operation in a hydrogen-oxygen fuel cell. The cross-sectional optical micrographs showed an electrode structure consisting of both fine and coarse pore layers of irregular-shaped nickel particles containing a coating which gradually diminished in thickness when traversing from the electrolyte (potassium hydroxide) to the gas (oxygen) sides of the electrode. Electron diffraction analysis of the coating gave the nickel oxide structure. Crosssectional replication examinations, particularly at the electrolyte side, revealed bridging of the oxide layer between various nickel particles. Replication analysis also showed that the oxide coating consisted of discrete particles which varied in size depending on the section of the electrode.
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ANALYTICAL CHEMISTRY
D
nickel fuel cell electrodes are currently being employed in the fuel cell designed for the Apollo lunar spacecraft, Basically, the cell is a modified version of the Bacon cell (1) which employs H2as the fuel and O2as the oxidant, with KOH as the electrolyte. The gas-liquid interface is established by a balance between the gas pressure (3) in the coarse pore section and the surface tension forces of the electrolyte in the fine pore region of the dual porous electrode (2). This paper is concerned primarily with the light and electron microscope techniques developed to obtain a better understanding of the true morphological makeup of fine and coarse pore regions of the electrode as well as the nature of the adsorbed species formed at the Ni particle interface before and after operation in the HrOn fuel cell. Light and electron microscope techniques are particularly useful in studies of this type since in the former a complete macrostructural UAL POROUS
history of the electrode can be obtained. In the latter technique it is possible to observe fine microstructural changes at the Xi particle interface which could aid in deducing a growth mechanism of the coating. EXPERIMENTAL
The biporous electrodes analyzed in this study were prepared by pressureless sintering of two layers of different size Ni powders and rolling to form a sheet of the desired thickness. The pore size of electrode facing the electrolyte side was smaller than the coarse pore layer facing the gas phase. The coating formed on the used electrode typifies the type of deposit obtained a t the surface of Ni particles after operation as an O2electrode in a KOH electrolyte maintained a t 450" F. Prior to microscopic analysis the used electrode wa.s thoroughly washed to remove adsorbed KOH electrolyte. In order to determine the morphclogical makeup of an untreated elec-
-
-
,lvvle 1. Cross-sectional lioht microaraph of a n un" oxidized oxygen electrode Figure 2. Cross-sectional replication electron micrograph of a coarse pore region in a n unoxidired oxygen electrode
MogniRcaHm 50X
trode as well as information relative to the nature of the adsorbed layer formed on a cleaned used electrode after operation in the fuel cell, it was necee sary to prepare cross sections without dam*ng the brittle porous StNCtUre. The biporous electrodes were cut with a silicon carbide cutoff wheel cooled continuously with water. The cut specimens were then impregnated in Bakelite resin B R d o l 4 as follows: The specimens were soaked briefly in acetone and then transferred to a closed jar containing the resin diluted with acetone (2 part4 resin to 1 part acetone) to increase the fluidity for better penetration. The specimens were submergd overnight in this solution. The specimens were then removed from the solution and set up quickly with the flat faces of the electrodes on top in a horizontal position in small metal covered cans. The surface was immediately coated with undiluted resin and baked for 2 hours at 85' C. under vacuum. Resin was pericdically added to the specimen as it soaked into the porous surface until the surface remained completely coated with resin. The oven temperature was increased to 9 S 1 0 0 O C. and held under vacuum overnight. After this the resin was allowed to cool slowly to m m temperature. The samples were then rough ground on 240, 320, 400, and 600 wet S i c papers and polished using 6micron diamond followed with h a 1 polishing with Linde B (0.1micron) powder. Replicas were made of cross sections of the hiporous electrode structures employing a negative twwtage collodion, platinum-carbon replica tecbnique. The samples were lightly etched with a solution prepared of acetic, hydrochloric, sulfuric, chromic acids, c o p per chloride, and water to produce some contrast between the Ni particles and the resin. The technique consists in flooding the sample surface with a 4% solution of collodion in amyl ace= tate. A 200mCqh specimen support screen was then placed in the solution and brought into contact with the sample. The collodion was permitted to dry on the specimen without draining. When dry, moisture was condensed from the breath onto the collodion film with embedded screens by breathing
M q n H k a H m 22WX
directly onto the surface of the specimen. Scotch Brand cellophane tape WBS attached to the end of the film and the film was removed by dry stripping. The detached replica was then placed in a vacuum evaporator where a P t C mixture was evaporated at a suitable angle onto t,he replica surface. Immediately after the Pt-C deposition, a reinforcing layer of c a r h n was evaporated onto the specimen at normal incidence. The collodion primary replica was dissolved in amyl acetate leaving the secondary negative C-Pt-C replica in place on the specimen support mids. RESULTS AND DISCUSSONS
A complete cross section of a bipomus 0 s electrode as Seen by the light microscope at 5OX is shown in Figure 1 The light, irregularly-shaped ureas represent the distribution of the Ni particles. The tiner particle size material (fine pore) which is more densely packed (labeled A) represents the liquid (KOH) side of the eleetrode, while (B) typifies the cnaree pore area, and (0represents the gas (0%) side of the eleetrode. The l w e white hemispheric e o n on the photomirrograph ripresents aportion of the rrinfomng wire3 which are r~nployed
ELECTROLYTE SIDE
Figure 3.
to aid in supporting the sintered structure. The dark regions typify the pore areas which have been tilled with resin. An electron micrograph of a section of the coarse pore layer (B) nf an unoxidised sintered Ni electrode is shown in Figure 2. The irregularly-shaped smooth regions (labeled 2 on the m i c w graph) typify the pores which have been filled with resin, while the Ni particles (labeled 1) appear somewhat mottled in appearance. This same number designation will be used in all the electron micrographs prepared from replicas of c r o s sections. The Ni resin interface is relatively smooth (shown hy untailed arrows) and free of any coating. An optical cross-sectional analysis of an 0 2 electrode after operation in a H, OZfuel cell is shown in Figure 3. In this c a ~ ethe magnification was increased to 5OOX to obtain a better insight into the distribution of the coating around the Ni particles at the liquid (KOH), middle, and gas (0,)side of the electrode. From these optical micrographs it is apparent that the coating thickness varies from approximately 1 to 2 microns on the electrolvte side (KOH) to 0.5 M 1.0 micron o n i h r g%?&le (0;)of the r l e r t d c .
MIDDLE
GAS SIDE
Cross-sectional light micrographs of sections of a used oxygen electrode MeonkaHrrr 500X
VOL 38. NO. 10, SEPTEMBER 1966
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Figure 4. Cross-sectional replication electron micrograph of an area near the electrolyte side of a used oxygen electrode
Figure 5. Cross-sectional replication electron micrograph of an area near the electrolyte side of a used oxygen electrode MagnHicaUon; 2200X
MqnHlcaHax 2700X
The advantaxe of employing electron microscope techniques in place of optical methods with respect to resolving more structural detail of the adsorbed layer is illustrated by the crosiwxtional replication micrographs taken at the electrolyte side (4)as shown in Figures 4 and 5. Electron diffraction analysis of the coating gave primarily N O as shown by the data in Table I. The adsorbed layer represents both the small amount of Xi0 film farmed during the electrde fabrication process, as well as the more mg-
nificsnt oxide build-np introduced during electrochemical oxidation in the fuel cell. This is dramatically seen in Figures 4 and 5 where the oxide film (shown by untailed arrows) has bridged across several Ni particles. Close examination of the oxide layer also shows discrete particle boundaries with an a p proximate particle size range of 0.5 to 1.5 microns. The tailed arrows on the micrograph in Figure 5 illustrate regions in which the NiO coating has broken down to afiner product, perhaps through
Table I. Electron Diffraction Analysis of Used Biporour Oxygen Electrode
3.12 2.97 2.40 . ..
2.w 1.48 1.26 1.21 1.06
3.13 2.93
...
... ... ... ... ...
... ...
2.415 tronr 2.09 6trona 1.48 strong 1.'26 1.21 1.04
1'.
!
-Figure 6. Cross-sectional replication electron micrograph of an area near the middle of a used oxygen electrode Magnmcau-
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27oox
Figure 7.
Cross-sectional replication electron micrograph
of an area near the gas side of a used oxygen electrode MagniRcmkm 2200X
solubilization in the molten electrolyte. Further replication analysis of a section near the middle of the electrode ( B ) illustrated in Figure 6, reveals a decrease in the oxide film thickness with little or no evidence of oxide bridging between the Ni particles. At the gas side (C) of the electrode shown in Figure 7 , the morphological nature of the adsorbed film appears to be different from that shown in Figures 4, 5, and 6. No simple explanation can be given a t this time for the change in particle morphology at the gas side of the electrode.
From these studies it has been shown that both optical and cross-sectional replication electron microscopy can be employed to define the nature of oxidation products formed on biparous sintered electrode structures after operation in the fuel cell, particularly such phenomena as bridging and changes in the morphology of the oxide coating with location in the electrode. Electron microscopy can also be effectively employed to determine oxidation mechanisms and the results of changing cell operating conditions.
ACKNOWLEDGMENT
w.
The authors thank J. &rmann for PreParingthelightmicrograPhs and G. p. McCarthy for assisting in the preparation of some of the electron micrographs. LITERATURE CITED
(1) Bacon, F. T.2 Am. Chem. SoC. s m posium on Fuel Cells Vol. I, “FuelCe$s,; G, J. Young, ed,, pp, 51-77, ACS, Chicago, 111, (1959). (2) Oswin, H. G., Chodosh, S. M., Advan. Chem. Ser., 47, 61-71 (1965). ( 3 ) Shaw, M., sot. Of Automotive Eng., 5325, pp. 1-5 (1962). RECEIVEDfor review May 31, 1966. Accepted July 13, 1966.
Limitations of Tritium Measurements by Liquid Scintillation Counting of Emulsions ROYAL H. BENSON Process Technology and Engineering Department, Monsanto Co., Texas City, Texas
b Considerable error can result from the use of the emulsion counting technique when applied to undefined systems. Important variables are shown to be phosphor concentration, sample concentration, temperature, agitation, sample distribution, cooling time, chemical nature of the sample, and methods used for quench correction. Two methods of sample preparation are given and the errors associated with each are defined. The toluene-Triton X-1 00 phosphor gives a solubilized system with a discontinuous transition to a soluble plus emulsion system as water sample size is increased.
M
of low energy beta emitters in aqueous solutions by liquid scintillation counting of emulsions was reported by Patterson and Green (a). The figures of merit obtained by these workers pointed out the potential value of the method and the excellent sensitivity which this technique offers. Further, if emulsions are considered as two-phase systems, emulsion counting should offer reduced chemical quenching for compounds remaining in the aqueous emulsion droplets and, consequently, a route to higher counting efficiency for these cases. The apparent simplicity of the emulsion-counting procedure is completely misleading. The system is extremely complex, and serious errors can result from its application to radioassay of sample sizes which have not been carefully defined. These errors can be very large in the case of tritium measurements. The limitations of the method described in this paper are, EASUREMENT
however, less serious for the more energetic beta emitters. Nevertheless, they will contribute to substantial errors in measurement, depending upon the energy of the particles, the degree of quenching, and the chemical nature of the radioactive sample. The emulsion-counting technique can be used with precision only when the many variables of the system are understood. These include phosphor concentration, sample concentration, temperature, agitation, sample distribution in the vial, time of cooling, chemical nature of the sample, and the type of quenching correction used by the investigator. All of these factors are important in obtaining accurate and precise radioassay data. EXPERIMENTAL
Apparatus and Reagents. Samples were assayed in a Model 314 EX Packard Tri-Carb Liquid Scintillation Spectrometer (Packard Instrument Co., Downers Grove, Ill.). The instrument was modified to permit use of external standardization. Scintillators and vials were obtained from the same company. Triton X-100 (Rohm & Haas, Philadelphia, Pa.) was purified by passage through a column of 20- to 40-mesh activated carbon (2 x 20 cm.) followed by a column of 30- to 60mesh activated silicic acid (2 X 20 cm.). The product thus obtained was waterwhite and showed reduced phosphe rescence compared to material purified by silica gel alone (a). PHOSPHOR. The results reported here are based on a phosphor consisting of two parts of toluene and one part of Triton X-100 containing 5.5 grams of 2,Miphenyloxazole and 100 mg. of
7759 1 POPOP per liter. Unless otherwise stated, the sample sizes used routinely were 4.0 ml. of tritiated water in 15 ml. of phosphor. Figure 1 shows the effect of diphenyloxazole concentration on counting efficiency using the above volumes. SAMPLE PREPARATION.In these studies, both phosphor and tritiated water were accurately measured into vials for radioassay. Preparatory to counting, the samples were treated by one of the two methods outlined. Selection of the particular technique to be used is left to the individual. However, the errors associated with each are defined. Method 1. The phosphor and sample are measured into the counting vial at room temperature, shaken, and placed in the freezer. The samples are shaken several times during the cooling period of at least 1 hour. Finally, the contents of each vial are tapped down to distribute the contents uniformly in the bottom of the vial and remove excess sample from the inside of the cap and walls. The samples are then counted. Sample cooling, with shaking, to the freezer temperature is important in order to obtain reproducibility. Figure 2 shows the variation in count rate with temperature of a room-temperature sample placed in the freezer and counted consecutively with intermittent shaking. The curve shows that the freezer should be operated a t about -2’ C. At least 1 hour of cooling is necessary to bring the sample to thermal equilibrium. The effect of the amount of shaking on the counting characteristics was found to be important but not critical. A 4.0-ml. sample of tritiated water was added to 15 ml. of phosphor, cooled for 1 hour without mixing, and counted. VOL 38, NO. 10, SEPTEMBER 1966
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