X-Ray Studies on Lead-Acid Storage Batteries CMAHLBS S. BARRE=, Curnegie Institute of Technology, Pittsburgh, Pa. HE reitctions taking place in lead-acid Ytorage batteries have been the subject of research for more than fifty years, during which period many theories have been proposed and soiiie bitter controversies have arisen. Tliere is little doubt but that much of this confusion would have been avoided if x-ray diffraction had been employed years ago as a means of identifying the compounds present in storage batteries. When the x-ray method vas finally used by Mazza in 1927 ($), it presented clear-cut evidence coiifirming the so-called double sulfate theory of storagc battery operation originally proposed by Gladstone and Tribe in 1882 (1). Several independent lines of investigation lrrtve confirmed this classical theory, and it alone has successfully withstood all criticism, so that a t the present time it is almost universally accepted (4). Mama applied x-ray diffraction as a test of the double sulfate theory by making up a cell and x-raying tlie materials from it after normal charge, prolonged charge, partial discharge, and complete discharge, respectively. X-ray diffraction studies enabled him to identify the compounds present in the active materials and yielded the following informat.ion: The positive plate consisted of lead peroxide in the charged and overcharged condition and, after discharge, consisted of lead peroxide plus lead sulfate; the negative plate consisted of ordinary lead in the charged condition and lead plus lead sulfate in the discharged condition. Thus the x-ray identification of compounds through their crystal structure confirmed the double sulfate theory that lead sulfate forms on both the positive and negative plates upon discharge, and reverts to lead peroxide and lead on the positive and negative plates, respectively, on charge. Mama’s observations also showed all these materials to be in a finely divided state. The investigation reported below is concerned wholly with commercial batteries of common t w e s in h t h normal nnd unhealthy condition. Mama’s experiments were repeated using a somewhat d i f f e r e n t technic, but attention vas directed chiefly toward the problem OF “sulfation,” in an attempt to see what information of value could be obtained by x-rays on this uezata pestio.
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X-RAYTESTS Since some seventy-five samples were x-rayed during this work, it was necessary to use a diffraction camera that would permit rapid exposures and easy preparation of specimens, both in the powdered and plate forms. A simple instrument that proved quite satisfactory for the work was the small hlackreflection camera shown in Figure 1. A pencil of rays passes through a hole in a flat film, sticks the specimen, and is reflected back; thus the reflected rays making angles between 115’ and 170” with the primary beam are registered on the film. The film is mounted in a b r a s s c a s s e t t e u n d e r b l a c k paper;
the specimen is mount.ed on a framework attached to the ette, and a t a fixed distance of 4 em. in front of the To Sacilitate comparison of different spectrograms, tlie Stme size x-ray Ixam was used throughout the series. Tlie beam was defined by a 1-nim. diameter pin hole in the plane of tlie film, and a tube of 2 mm. inside diameter and 2 cm. length cxtending from the film toward the spec’ .mien. In this camera it was easy to irradiate any spot on the surface of a storage battery plate with the plate either dry or wrapped in Cellophane while wet. When samples consisted serapings or filins from tlie battery plates, they were mounted on adliesive tape under Cellophane. The exposures required were about 30 minutes with a Shearer x-ray tubc riirining a t 8 to 10 milliamperes, 50,000 volts peak, with single-wave rectification. A copper tsrget vas used in the tube throughout this series of exposures. The photograms obtained with this instrument consist (if circles concentric about the central hole, if a great number of small cryst,als are bathed in tlie rays. The r i n p are nctually built up of innumerable spots, each spot being a reflection from a single crystallite in the multicrystalline sample. The size of the individual spots increases with increasing size of the reflecting crystals when they exceed 1 0 - 3 em. in diameter (%). On many of the photograms there were distinct spots caused by large particles in addition to comiilete rings from finely divided material. From a measurement of the ring diameters together wit,li tlie knowledge of wave length of the x-rays used, one can learn the spacings between planes of atoms in the reflecting crystals. If the x-ray diffraction data are complete enough, the crystal structure can be carefully deduced, but this is un‘ary in the prcscnt case where merely the identification of compounds is required. It is here suEcient merely t o compare the pattern of rings from the 1111 known substances with calibration pattcrns from known s u b s t a n c e s in order to identify the unknown. The instrument used in this work is satisfactory for this technic even though it yields data much too incomplete for a complete determination of crystal structure. Througliout the work care was taken to manipulate the saniples in such a manner as to avoid oxidation. In some instances the plates werc never brought into contact with air; when this was unavoidablc, the samples were usually thoroughly w a s h e d and immediately dried in vacuum. I)iFX.llACTION
PATTERNS
Typical diffraction patterns obtained with the camera of Figure 1 are reproduced in Yigure 2. Only a few are here
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pat,teros. These are illustratcd by the ulqw four pictures in Figure 2, pattern A being that given by finely divided lead, pattern B finely divided lead peroxide, pattern C finely divided lead sulfate, and patt.crn D coarsely crystalline lead sulfste. The majority of patterns are composed of C and D superimposed upon either A (in the case of the negative plate) or R (in the case of the positive plate). The intensities of the patterns from fine or coarse-grained lead sulfate relative
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was thought some y e a s ago t o Le the case, for its spectrum is identical with that of the ordinary material. A test of the classical double sulfate theory was made by x-raying material from both positive and negative plates taken from cells after complete charge and after discharge. It was observed that the quantity of finely divided lead sulfate was greater io the discharged cell than in the charged cell. This confirms the double sulfate theory and the work of Mazza (S). It was noted, however, that considerable lead sulfate was present even after a 36hour charge a t a low rate on a battery which liad been kept always in a healthy eondition and had previously received about fifteen cycles. By comparing the intensity of the sulfate lines (Figure 2C) with the peroxide lines ( B ) or lead lines ( A ) ,an estimate can be made of the sulSate content of the active material at the place irradiated by the rays. This possibility might be employed to determine the distribution of sulfate tbroughout the active material of a plate and to study the dutnges in this distribution with the changes in battery operation or with changes in plate design or manufacture. Little was done aloug this line in the present investigation, although it was noted that samples scraped from the surface of a negative plate showed a higher sulfate content than samples removed in such a way as to indicate the average condition within 8 plate. It is perhaps not beyond the bounds of practicabi1it.y that the porosity of active materials could be ineasured effectively in this way, or that this method could indicate whether the active material is in suitable contact with the grid structure. N o i r ~ i a ~. ~, N D P E I ~ M C ISULFATX: ~U~
FIOURE2. TYPICAL DIFPRACTION PATTERNS A . Lead, finagmined; i?. Lead peroxide. f i n e - g r i n d :
C. Lesd
sulfate, fine-sdned; D . Lead sulfate. aoarse-srdned; E . 1'518 aubnlarj", DOsitiYe
to the intensities of the lead or lead peroxide patterns depend upon a number of factors which will be discussed later. Spectrogram E is an example showing two superimposed patterns-i. e., finely divided lead peroxide and coarse, crystalline lead sulfate. I t follows at once that the principal ingredients of the several commercial types of batteries investigated are the compounds just enumerated; however, the method used in this work is not sensitive (as is the spectroscopy of x-ray emission lines) to small amounts of other substances which arc present in all commercial batteries. Only in one spectrogram were the other substances present in sufficient quantity to register on a film; the interior of the negative plate of a disoarded automobile ignition batt.ery exhibited nunierous lines from antimony and other unidentified crystals. Only finely divided lead and lead peroxide were observed. I t may nevertheless be true that electrical cycling of a battery increases the size of the individual crystallites of lead and lead peroxide, but the patterns show that such increase is not great. The rings from these active materials werc always uniform in darkening and never of the spotty character obtained with coarse crystalline material. The lead in the negative plate is not an allotropic form of ordinary lead, as
The question of the difference between nornial sulfate and pernicious or unhealthy sulfate is an ideal one for x-ray study. Samples from a badly sulfated automobile battery gave a pattern of type D (Figure 2 ) and obviously was composed of crystallites of considerable size. The spots were not numerous enough to form well-defined rings which could be compared with rings from lead sulfate, so a different type of aiffraction camera was brought into use to identify them. This camera differs from the back reflection camera in that the photographic film is mounted on a revolving disk so constructed that x-rays pass down the hollow axle supporting the disk and stick the specimen placed about 8 cm. in front of the disk (Figure 3). As a result of the rotation of the film, each spot of the pattern is spread into a circle whose diameter can be accurately measured. The instrument also provides for rotation of the specimen for the purpose of increasing the number of reflecting particles. Identical patterns were ohtained from a sulfated negative, a sulfated positive, and commercially pore lead sulfate. The spacings of the atomic planes in the storage battery sulfate differed from the spacings in lead sulfate by less than one part in ten or twenty thousand. X-ray diffraction yields clear proof, therefore, that only one kind of sulfate exists in the lead-acid storage cell, and that the unhealthy sulfate differs from the healthy sulfate only in particle size. A photogram of the material adhering to the separator in a much used battery disclosed only lead sulfate. This was also coarsely crystalline. These conclusions wilt he no surprise to thnse familiar with current opinion on the subject, yet direct evidence seems to have been lacking up to the present time. Since the large grains of sulfate reflect as separate spots, while the normal sulfate gives continuous rings, it should be possible to control conditions so that the number of separate spots would indicate the percentage of material in the form of large crystallites. An attempt was made io this direction, using the camera of Figure 1. Samples were prepared by passing them through a screen of 150 mesh and on to a screen of 250
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nierh. Those particles which p a w d tlie former and failed to pass the latter were mounted on adhesive tape under Cellophane in a layer thick enough to absorb completely the x-ray beam. By using an x-ray beam of the same cross section for each exposure, by placing the samples so that the whole of the beam wa& fully absorbed in tlie specimen, and by giving all of the samples the same screening treatment, it was assured that substantially the same volume of material was irradiated a t each exposure. (This constancy of volume could be obtained only in the case of mixtures of constituents of greatly differing linear absorption coefficients when the linear dimensions of the particles were greater than the effective penetration of the rays.) Under these circumstances the x-ray beam struck approximately the same number of particles each time. Of this number a certain fraction was oriented so as to reflect the rays to the film. The uniform technic of preparing, mounting, and exposing the samples held this fraction to a constant value thronghout the series. The number of individual spots from the large sulfate crystals could accordingly be taken as proportional to the total number of sulfate crystals in the irradiated volume and, in turn, to the percentage of these in the active material. While it would have been possible to evaluate the constant of proportionality from a knowledge of the geometrical conditions in the apparatus, a less hazardous method was thought to be the direct calibration from materials of known eompoaition. A test of the accuracy of the method was undertaken, using active material from the negative plate of a normal cell to which was added various amounts of active material from a completely sulfated negative. The battery choseii as a standard of thorough sulfation was a discarded automolile battery of common make in which tlie positive grids were completcly Plant&formed, and the negative plates had such a high sulfate content that the sulfate spots obscured the lines from lead in the diffraction pattern. Both positive and negative plates fell to pieces with handling. Samples of 0, 5, 10, and 15 per cent concentration of the completely sulfated material, mixed with tlie normal, yielded films readily distinguishable from each other. The numher of individual spots in a given area on a film (the area lying between the inner and outcr diffraction rings from the lead) was counted and plotted against the percentage of sulfated material in the mixture. Considerable deviation from a linear relationship was found, showing that the accuracy of the method was not high; nevertheless it was possible to determine the sulfate content within 2 to 4 per cent. It was observed, incidentally, that the material from the normal negative plate did not show a complete absence of sulfate spots, as was expected, but apparently had a composition between i and 10 per cent of that taken as 100 per cent sulfated. It was subsequently Sound that a laboratory assistant had inadvertently substituted for the cell which was to supply the normal material one which had remained idle for a year in the laboratory before the test and was scarcely in perfect condition. These experiments on the quantitative determination 01 nnhealthy sulfation by no means prove that there is a linear relationship between the sulfation so measured and the loss of electrical capacity of a cell, nor do they suffice to predict the rcmaininF: useful life of the cell. A correlation of the diffraction patterns with electrical or mechanical characteristics would be necessary before conclusions of this type could be drawn. Such a correlation might be d8crent for different makes of batteries and for batteries of the same make but different design. A nnmber of interesting results were obtained from this method of analysis, however, without pushing it to its limit of accuracy and without carrying out a correlation of the sort just mentioned.
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It wa& Sound that spectrograms of the active materials in used cells differed mai,kedly in quantity of coarse crystalline lead sulfato. Even the positive and negative plate8 from the same cell rarely showed the same quantities. A positive plate from a submarine storage battery that had been put in commission i n 1918 and subjected to approximately one thousand cycles gave photogram E uf Figure 2 with a sprinkling of large sulfate spnts. On the other hand, the negative
p1at.e of the same cell yielded a pattern witti a mucli smaller number of sulfate spots, though with a considerable amount of finely divided sulfate. Vinal (4)reports a parallel case of this in a portable battery that had stood 3 years in a discharged condition; microscopic examination showed larger crystals in the positive plate than on the negative. Somewhat similar photograms were obtained from a small battery which had been used to operate a time clock. Its history was as follows: After 4 months of operation the battery suffered a separator failure and was repaired. It subsequently stood idle in a charged condition for 90 days. Diffraction patterns showed a nnmher of sulfate spots for the positive plates (closely resembling photogram E ) , but the negative plates were altogether free of them. I t was diBicult to believe that the electrical history of this batt.ery could explain this situation. I t seemed more probable that it was a result of the method of manufact,ure. It was also difficult to believe that the 1918 snbnvlrine cell was in a serious condition with regard to sulfate, when the pattern from its positive plate so closely resembled the pattern from the slightly used time clock battery just mentioned. This idea was confirmed when the discovery was made that the submarine cell had failed because of puncturing of the separator, and that probably excessive sulfation had not.hing to do with its death. A situation just the reverse of these was observed in a submarine cell of special design, which had seen 2 years of service. Positive plates from this cell yielded a pure lead peroxide pattern with no sulfate spots, yet the photogram of the negative plate disclosed sulfation of an aggrava.ted nature. It was learned that this battery c,ontained grids of abnormally
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high antimony content, and that antimony had deposited on the negative plate during operation to such an extent that concentrations in the negative active material became as high as 1 per cent antimony (shown by chemical analysis). With this evidence available it cannot be doubted but that selfdischarge proceeded rapidly on the negative plate, and that conditions there were favorable to the growth of large sulfate crystals. It might be mentioned that at the time this study was made the battery had deteriorated to such an extent that it was impossible to charge it, and there was a constant vigorous evolution of gas from the self-discharge. The growth of finely divided healthy sulfate into coarsely grained, irreducible sulfate takes place either by spontaneous solution of the smaller particles and deposition on the larger, or by solution and precipitation with periodically changing solubility of lead sulfate in the electrolyte. The solubility of lead sulfate varies both with temperature and with acid concentration. Any factor increasing the rate of self-discharge also increases the likelihood of growth of large sulfate particles. Which of these causes is most important in practice, and what methods can be used to reduce their action are questions worthy of considerable research. The attention that has been given to sulfation in this paper is perhaps out of proportion to its real importance as a cause
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of the failure of storage batteries. It may well be the case, as is sometimes stated, that proper operation of any well-built battery can practically eliminate the sulfation hazard. But the paper will have justified itself if it in any way clarifies current thought on the much misunderstood lead-acid battery, or if it stimulates further research in this field where continued research is urgently needed. ACKKOWLEDGMENT The author is indebted to Commander E. D. Almy, Lieutenant R. W. Abbott, E. G. Lunn, and F. W. Bichowsky, of the Naval Research Laboratory for their active interest and valuable suggestions throughout the course of the work. LITERATURE CITED (1) Gladstone and Tribe, Nature, 25, 221 (1882); Electrician, 9 , 612 (1882). (2) Glocker, “Materialpriifung mit Rontgenstrahlen,” p. 299, Julius Springer, Berlin, 1927; Clark, Metals & Alloys, 1, 153-61 (1929). (3) M a m a , Attiaccad. Lincei, [6] 4,215-18; 5 , 117-19,688-93 (1927). (4) Vinal, “Storage Batteries,” Riley, 1930.
RECEIVED September 1, 1932. This article is published with the permission of the U.9. Navy Department: the research was conducted at Naval Research Laboratory, Washington. D. C.
Distribution of Extractive in Redwood Its Relation t o Durability E. C. SHERRARD AND E. F. KURTH,Forest Products Laboratory, Madison, Wis. AWLEY, F l e c k , a n d The aqueous extractive in young, secondnear the top a point is reached growth redwood trees is comparatively a t which the concentration is Richards’ have demonalmost uniform throughout the that the distributed throughout the heartwood of the cross s e c t i o n of t h e t r u n k . resistancy to decay of various trunk* I n Old, virgin-growfh trees there is a Both t h e c o l d - w a t e r extract woods could belargelyexplained decrease of extractive at the center of the trunk and the hot-water extract show by the toxicity of their hot-water with decrease in height in the tree, and a large the same relative distribution extracts. The material toxic to at the outer edge of the throughout the trunk, although fungi in redwood is readily exincrease of the v a l u e s for the cold-water with hot heartwood with decrease in height in the tree. extract areof smallermagnitude. and dilute alkalies, such as one per cent sodium hydroxide and The durability Of redwood is attributed to the Wherever compression wood is sodiumcarbonate. Lessextract nature of the extractive and caries with the encountered, t h e e x t r a c t i v e content is abnormally low. is removed with cold water and extractice distribution. The average hot-water extracwith ether than with any of t h e c o n t e n t of seven secondthe foregoing s o l v e n t s . An attempt is made in this paper to correlate the amount and growth redwood trees, varying in age from 45 to 64 years, location of the extractives in the tree with the durability of was found for sapwood to be 3.2 per cent a t the 1-foot height, 2.6 per cent a t the %-foot height, and 2.8 per cent at the top the wood from different parts of the trunk. of the trunk; for heartwood the values 12.3, 10.1, and 11.2 OF EXTRACTIVE IN REDWOOD per cent were obtained for the respective heights. DISTRIBUTIOK Studies at the Forest Products Laboratory on virgin redThe distribution of extractive in virgin-growth redwood is wood have demonstrated a clearly defined variation in ex- represented graphically in Figure 1. This graph presents tractive content of the heartwood with height in tree and the amounts of hot-water extractive throughout cross secwith position in cross section of the trunk. Similar studies on tions taken from six heights in a tree. The amount of exyoung, second-growth redwood have revealed that, although tractive in sapwood as shown by the points a t the extreme left a corresponding variation in the extractive is usually per- is much smaller than in the heartwood immediately adjacent. ceptible, the tendency is toward a more uniform distribution. The values for the 88-foot height of this tree are abnormal in I n virgin-growth trees the aqueous extractive is highest in that they are lower than the values for sections of the trunk the heartwood adjoining the sapwood of the butt and de- above this level. creases toward the center of the cross section. I n the outside heartwood there is a gradual decrease of extractive with inTOXICITY OF REDWOOD EXTRACTIVE crease in height of the tree; a t the center it increases until Of different The hot-water extracts from the 1 Hawley, L. F.,Fleck, L. C., and Richards, C. H., IND.ENQ.CHEM., parts of the redwood tree (the extractive distribution of 16. 699 (1924).
