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Sept., l!)til
FORMATIOS OF COLOR CENTERS I N SAPPHIRE BY SOLAR RADIATIOS BY A4.F. GABRYSH, H. EYRING ASD TAIKYNE A. REE Instztutefor the Study of Rate Processes, University of Utah, Salt Lake Czty, Utah Received March 14, 1961
An appreciable increase in light absorption along the “C” axis in Corundum (a-AlzO,), after its exposure t o an oxygen atmosphere followed by exposure to solar energy, is found over the range from 230 to 290 mp. Color centers are induced by heating xn oxygen and an abaorption peak a t 260 mp is further enhanced after exposure to solar energy. Sssuming an oscillator strength of unity for the absorbing center, the optical density is estimated t o be increased by about 4.3 X loi6 centers per cc. after exposure t o solar energy. The absorption curves show that a color center concentration a t the 236 rng energy level is shifted to a higher energy level, 233 mp, by the solarization.
A . number of authors have reported data for the transmission of sapphire in the ultraviolet, visible arid infrared regi0ns.l-j Measurements have also been made in the Shuman region of the ultraviolet.6 Rieke and Daniels have studied7 light-induced thermoluminescence of various crystal phases of alunlinum oxide, Wooda and Rindoneg have reported on the “antagonistic behavior” of short and long wave lengths, and solar energy, in forming color centers in glass. Yo data concerning the optical properties of synthetic sapphire, after its oxygenation” and “solarization,” have beeii found in the literature. The terms “oxygenation” and “hydrogenation” are used in this paper to describe the effect of heating crystals at high temperatures in these atmospheres. The term “solar ization”’ relates to the net effect of the ultraviolet, visible and infrared components of solar radiation in producing changes in the light transmission of oxygenated sapphire. Measurements of the transmission of synthetic single-crystal sapphire spheres’O have been made after exposures to (a) oxygen atmosphere, a t high temperatures (1525”) and then (b) to solar radiation. These absorption data are compared with non-oxygenated, non-solarized disc used as a control. While the conclusions drawn from these data are regarded as tentative, they are considered of sufficient interest, from the stand point of sapphire (1) G. Calingaert, 9. D. Heron and R. Stair, SOC.Automotive Eng. J., 39, 448 (1936). (2) R. A. H u n t and R. H. Schuler, Phys ReE., 8 9 , 664 (1953). (3) P. W. Levy a n d G. J. Dienes, “Report of the Conference of Defects on Crystalline Solids,” H. H. Wills Physical Laboratory, University of Bristol, July 1954. -41~0,Phya. Rev., 94, 1409(.4) (1954). (4) National Bureau Standards, NBS Test. 461d-14/47. (5) R. W. Kebler, “Optical Properties of Synthetic Sapphire, ”F8727, Linde Air Products Company, Indianapolis, Ind. ( 6 ) R. Bauple, A. Gillrs, J. Ramand a n d B. Vodar, J . Opt. SOC.,4 0 , 788 (1950). (7) J. K. Rieke and F. Daniels, J . Phus. Chem., 61, 629 (1967). ( 8 ) A. R. Wood and &I. N. Leathwood, Nature, 124, 441 (1929). (9) G. E. R.indone, Report; Reprinted from the Travaux d u I V Congress International du Verre, Paris, 1956. (10) The optically polished spherical specimens were obtained from the Linde Company, Crystal Products Division. T h e samples were fabricated f r o m aluiniria powder which showed the following speotroscopic analysiii:
CaO Cr2Op CUO Gaz03 Fe20s PbO MgO
0.0030~o .0008 ,0003 ,0020 ,0025 ,0040 .QOO1
MnlOa NO SiO, Na10 &4g20 SnOn
0.0001 ,0020 ,0010 .OQlO .0001 .0002
as a precision lens element, to warrant reporting at the present time. The C and A axes in the single-crystal spheres were determined by the back-reflection method. l1 The Greninger chart and Wulff net mere used to orient the axes of the spheres to within 3” of coincidence with the X-ray beam line and the axis of a specially designed holder, The spheres were ground so that two parallel, optically polished, facets were formed perpendicular (within 3’) to the axes. Facets were also ground a t 4.5’ to these axes and at right angles to both C and A axes. The crystal thickness along the “C” axis was 8.8 mm. Reflection loss for normal incidence at the surface was approximately 0.076. The absorption spectra were measured rrith the aid of a Beckman DU Quartz spectrophotometer located in a temperature controlled (25’) room, over the wave length range of 220 mp (5.78 e.v.) to 1,200 mp (1.035 e.v.). Beginning a t 1,200 mp the data were taken a t intervals of 50 down to 700 mp; from 700 to 500 a t intervals of 20 m p ; from 500 to 300 a t 10 mp intervals and from 300 t o 220 mp at intervals of 5 my. The sensitivity of the phototube did not permit reliable measurements above 1,200 my. I n Figs. 1 and 2 are shown the optical density with energy for light passing along the C axis in the spheres. Measurements were also made along the norms to all other pairs of parallel facets and are recorded elsewhere.12 ii’one of these showed the “solarization” effect observed along the C axis.13 Figure l a shows the method of heating the specimens in continuously flowing gas. The system was sealed by a sintered alumina plug containing a gas inlet and outlet. Gas entering the system was passed through a drying column containing calcium chloride, Ascarite, magnesium perchlorate and activated alumina. The escaping hot gas passed through a water-bath. The tubings and sample-holding “boat” were obtained from Morganite and were specified as fabricated from material showing a chemical analysis of 99.7% h120,. Specimens mere rested, out of contact with the alumina boats, on sections of singlecrystal polished rod. The temperature was measured with an optical pyrometer. The maximum (11) B. P. Cullity, “Elements of X-Ray Diffraction,” AddisonWesley Publishing Co., Boston, Mass., 1936, Chapter 8. (12) .?. F. Gabrvsh (unpublished); Institute for the Study of R a t e Processes, University of Utah. (13) It ha5 been found in other samples t h a t the absorption was decreased somewhat if the oxygenated AhOa sample was heated in a hydrogen atmosphere (hydrogenated) a t aiound 1500*,
A. F. GABRYSH, H. EYILIYG ASD T. A. REG
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-
As Received
--- Heated in 02 Atmos.
0 6
“C o N T R G LS” (C) - - L .5. L I ’ I---! L _ iI 4 3 -i--2
Energy (ev).
Fig. 1.-(a) Method of exiosing samples t o oxygen atmosphere; (b) heating history; (e) control curves represent three readings on sphere 2 and two readings on a disc (‘‘as received,” solid curve; after heating in 0 2 atmosphere, broken curve). The readings were taken in parallel with readings on sphere 1 each time the oxygenated sample was exposed to solar energy.
No. I
, Solarizsd
0.02
t i
0.8 0.6 0.4: 0.2 5.0 0.8 0.6 0.4 0.2 4.0 Energy (e.v.). Fig. 2.-(a) Curve 1 Ehows increase in light absorption in the 230-290 m p range after the oxygenated sample (curve 2 ) was solarized. (b) Amplified plot of optical density ( -log 1/10)versus energy: The light curve results when the “Background” optical density is subtracted from the optical density after oxygenation (curve 2 minus curve 3 in Fig. 2a). The heavy curve is the result of subtracting the background optical density from the optical density found in the crystal after its exposure t o oxygen atmosphere followed by the exposure to solar energy( curve 1minus curve 3 in Fig. 2a).
temperature was 1525” for a period of 1 hour, Fig. lb. The solid line curves, Fig. IC, show the optical density versus energy for the specimens “as received” from the manufacturer (and after the crystals were X-rayed, facets ground and polished). All readings were referenced to 100% transmission, for each wave length, in an open (free air) path. The broken lines shorn the increased absorption after the samples were exposed to a dry oxygen atmosphere in the manner given in l a and lb.
T-ol. 63
Curve 1 in Fig. 2a shows the final optical density curve of sphere #1 after its direct exposure to the rays of the sun for a total of 24 hours in three periods of 8 hours each. The average temperature of the crystals was about 30”. The temperature was recorded on a mercury thermometer placed near the crystals. After the first exposure of sphere #I to the solar energy, readings showed some divergence in the 290-230 mp range but remained the same (within 0.2%) for the other wave lengths as those observed after oxygenation. Readings from the control (non-solarized) sphere were taken in parallel with sphere #l and were within 0.2% of previous (oxygenated) readings. Because of the nonavailability of another sphere, a disc, non-oxygenated and non-solarized, was added as a further control. Parallel readings were taken on all three specimens after the second and third exposures of sphere $1 to the sun’s rays. The data for the controls, sphere #2 and the disc remained xithin 0.2% of previous readings. Curve 3 of Fig. 2a is the “background,” 2 is the curve resulting after the sphere had been exposed to the oxygen atmosphere, and 1 shows the data after oxygenated sphere g l was exposed for the third time to solar energy. From this controlled experiment one can conclude that the increase in absorption is not due to a time effect, but is influenced by solar energy. Figure 2b is an amplified plot of optical density vs. energy for the 290-280 nip region. The compound curves show that both processes, oxygenation and solarization, influence a common absorption peak a t 260 my (4.78 e.v.); however, a 236 my peak is merely shifted, after solarization, to a higher energy absorption, 233 mp. At 260 mp the absorption coefficient, after oxygenation, is approximately 0.301 cm.-I while after solarization it is approximately 0.327 cm. -1. The absorption coefficients for the 236 my peak (before solarization) and the 233 mp peak (after solarization) are approximately 0.352 cm. -I. Using the formulas14 of Smakula, as corrected by Hilsch and Pohl, and assuming an oscillator strength of unity for the absorbing center, the color center concentration a t 260 nip in the oxygenated sphere, before solarization, is 2.98 X lo1’ per cc., and 3.41 X 1017per cc. after solarization. The color center concentration of 1.24 X lo1’ per cc., appearing before solarization at 236 mp, remains the same as the absorbing center shifts to 233 mp after solarization. This study reveals that (1) both oxygenation and solarization influence a common absorption peak (260 mp), (2) the absorption curve in reality is compound in nature and (3) the color-center with lesser density is not influenced in magnitude by solar radiation but is shifted t o a higher energy level. It is thought that the absorption a t 260 mp 1.31 X IO” n’/(n’Z -I- 2)s a,W, where No is the density (14) No! of absorbing centers, f is the oscillator strength of the transition, n’ is the index of refraction, am is the absorption coefficient in cm. - 1 a t t h e maximum a n d W is t h e width of t h e absorption curve in electron volts. a t half maxima. The cylindrical volume in the crystal traversed b y the light was about 4.4 X 10-8 om.’. (For a complete discussion Bee, F. Seitz, “The Modern Theory of Solids,” 1st ed., p. 664. MoGraw Hill Book, Co., New York, N. Y., 1940.)
Sept., 1961
ELECTROLYSIS OF SODIUM AMALGAMS
results (as suggested in other studies)15 from a photosensitive couple of oxygen and the A1208 crystal lattice inhomogeneities such as aluminum vacancies and dislocations. The shift of the lesser peak to a higher energy could, perhaps, be traced to the direct involvement of trace impurities, after the crystal is exposed to solar energy, and latticr?strains. All these samples used above were subsequently treated differently. For a more detailed analysis, data will be obtained from a solarized, non-oxygenated sample; also, data with respect to the decay with time of the color-centers induced by solar energy will be of great importance to the analysis. A microscope examination of the sphere specimens which were used in the above experiment showed
1549
a small amount of cavitation appearing in the crystals exposed to oxygenation and solarization. Cavitation also was produced in straight-rod samples which were merely flame polished. A better controlled study should show to what extent each treatment is responsible for the cavltation and it should help in the understanding of the role that cavitation has in influencing the formation of color centers. Acknowledgment.-We thank the National Science Foundation for supporting this work, Mrs. B. Staker for assistance in calculations and Miss Nola McKee for assistance with the calculations, preparation of drawings and the manuscript. (15) (a) F. P. Claike, P h d . Mag., 2, 607 (1957): (b) R. Chang, Report NAA-SR-3339, Atomic8 International, a Division of North American Aviation, Inc., Nov. 13, 1958.
TIIE ELECTROLYSIS OF SODIUX AJfL4LGA?dS’ BY JOHNC. ANGUS*AND EDWARD E. HUCKE Departtr~entof Chemical and Matallurgwal Engineermg, Unaversily of XLchrgan, Ann Arbor, MLchigun Receaued March 16, 1061
The electrolysis of 0.097 and 0.485 weight yosodium amalgams was studied a t temperatuies up to 344” Below approumately 290’ the sodium is transported to the anode; above 290” thc direction of transport reverbes and the sodium niigratrs to the cathode. This effect, which has heretofore never been observed, is postulated as caused by the tiiermal deconiposition of “compounds” or associations which persist in the liquid amalgam.
Introduction Kremann3 showed that when Na amalgams were electrolyzed, the direction of transport of the Ka and Hg depended on the concentration of the amalgam. That is, a t 240” for Na concentrations greater than about 2.0 weight 70E a , the Na migrates to the cathode; when the Na concentration is below 2.0 weight the Ea migrates to the anode. This curious effect has been observed in the K-Hg,4 Ba-Hg4 and Ea-ICi6 systems. I n each case the concentration of the reversal occurs at, or very close to, a composition where compounds are present in the solid phase. Moreover, in each case the component in stoichiometric excess of the compound composition migrates t’o the cathode. It has been pointed out6 that in 38 of 40 binary and ternary alloy systems, the component with the smallest atomic mass moves to the cathode upon electrolysis. On the basis of this empirical correlat’ioii the authors postulated that the reversal was caused by “compounds:” or associations, existing in the liquid metal. I n the Na-Hg (1) Taken f i o m a portion of t h e dissertation submit,ted by John C. Angus t o t h e Rackham Graduate School, University of hlichigan. Ann Arbor, Michigan. in partial fulfillment of t h e requirements of t h r Ph.D. degree. (2) Minnesota Mining a n d Manufacturing Co., St. Paul, blinnesota. (3) R. Kremann, A. Vosrin a n d H. Scheibel, Monatsh. Chem.. 57, 323 (1931). (4) For a review of these experiments see K. Schwara, “Elektrolytische Wanderung in flussigen und festen Metallen,” J. A. Barth. Leipaig, 1940. (5) S. I. Drakin a n d A. K. Maltsev. Z h w . Fie. Khim., S1, 2038 (1957). (0) J. C. Anfius, J. D. Verhoeven a n d E. E. Hucke, Paper presented t o t h e International Symposium on t,he Physical Chemistry of Process Metallurgy, A1:ME. Pittsburgh, 1959.
system, for example, if an undissociated species Ka,Hg, exists 111 the liquid state, one mould expect to find predominantly ?ia,Hgm and Na on thc high S a side of the “compound” composition and NanRg,, and Hg 011 the high Hg side. The correlation then suggests that in these cases a reversal should occur in the direction that is, in fact, observed. If this interpretation is correct, one might expect the reversal to vanish at sufficiently high temperatures if the associations are broken down by thermal agitation. In the present work t n o amalgams (0.483 a n d S a ) that show transport of the ?;a 0.097 w i g h t to the anode at 240” were electio!yzed at temperatures approaching the normal boiling point of the amalgams. Experimental Materials.-Mallinckrodt analytical reagent grade KC'^ with a reported purity of 99.9% and Rlerck and Mallinckrodt A.C.S. Reagent grade niercury was used without further purification. Preparation of Amalgams.--Amalgams of known comgosition were prepared in a glove box under dry nitrogen. odium metal was added to previously weighed polyethylene bottles which were then tightly stoppered, taken out of the dry box, and reweighed. The Na and enough FIg to make the desired amalgam composition were then brought back into the dry box where the amalgamation was performed in a Pyrex beaker. At, room temperature the amaigams could be handled in air for short periods of time with negligible scum formation. Equipment and Procedure.-The electrolyses were carried out in thin walled Pyrex tubes approximately 15 cm. long and with an inside diameter of 0.075 cm. The tubes were closed a t one end and open a t the other. A 0.008 inch diameter tungsten electrode wire Tas Realed into the closed end. The electrolysis tube was situated within a cell consisting of a vertical, 60 mm. diameter Pvrex tubr which was scaled at the bottom. The necessary vacuum and elec-
