and Glass-
-Ceramics SPECIAL COMPOSITIONS
Glass to approximately the properties of transparent fused silica has been produced. Borosilicate glass, fairly rich in alkalimetal compounds, is leached with aaid, which removes almost every component except’ the remaining hydrated silica. On reheating, the opalescent mass re-fuses, retaining its original shape and becoming clear, but having shrunk about 15%. Fused alumina and very-high-alumina glasses are used as jewel bearings. Aluminum phosphate glass resists attack by hydrofluoric acid. Photosensitive glass is a fascinating new development. Gold ruby glass has been known for many years, and it was known that colorless gold, silver, copper, and selenium glasses would develop color on reheating. More recently it has been found that the colorlessglassescanbecovered~vith a mask or negative andprinted by exposure for 10 minutes or more under arc light, with subsequent development by heating below the softening point of the glass €or an hour or more. The photographic effects within the gl:ass, which depend on the size of colloidal particles, are astounding, and a variety of colors results, depending on the intensity of reflected or transmitted light in which the photograph or pattern is viewed. More recently still, a clear colorless glass has been produced which becomes milk-white when treated as indicated above. The parts are selectively soluble in chemicals, and with proper exposure, patterns may be eaten entirely through the glass in places. The pattern partly etched may serve for embossing or engraving plates. The possibilities are far beyond conjecture for both art and technology. CONSTITUTION
X-ray studies and the use of absorption spectra have disclosed much regarding the structure and composition of glass. Glass-
forming and glass-modifying elements have been classified. .4 knomledge of atomic and ionic radii, of chemical bonds, and of magnetic and electronic relationships enables the mathematical physicist so to predict, that considerable experimental work is eliminated. The chemist, the physicist, and the engineer have brought us into a new glass era, one that might well be christened “The Glass Age” after the material which is so indispensable to modern man. ART
I n the Catholic Shrine in the nation’s c,ipital reposes a beautiful glass mosaic replica of Murillo’s “Immaculate Conception” from the Vatican in Rome. The Vatican studios have over 50,000 color tones available Though not comparable in art or beauty, other glass mosaics find wide application in architecture Examples are found in the murals of the City Hall in Stockholm, Sweden, in the Union Station in Cincinnati, and in the murals and columns of the new structure which serves as AMERICAN C~IEVICAL SOCIETY headquarters a t this moment in Los Angeles. America and Europe both produce fine crystal, some of which is engraved, cut, and decorated by the foremost artists of our times. Use of the diamond point for stippling and line drawing has been revived in Sweden. The introduction of color in art ware is gaining popularity Attractive art glass is on display periodically at glass shows in our larger cities. Stained glass of .4merican manufacture is found in beautiful windows designed by American artists and installed by rlnierit an artisan7 in our churches. Much is new in the age of glass. RECEIVED for review Spril 11, 1953.
ACCEPTED October 30, 1953.
Microscope ALBERT F. PREBUS, The Ohio State University, Columbus, Ohio JOHN W. MICHENER, Owens-Corning Fiberglas Corp., 1V~i4~ark. Ohio U
Evidence of structural inhomogeneities in glass up to several hundred Angstroms in maximum dimension has been obtained, using the electron microscope. Direct transmittance electron micrographs show the structure with a high degree of contrast. There is a wide variation in structure among the various glasses investigated. The structural evidence is interpreted as indicating the possibility of a relatively high degree of order over distances ranging from 20 A. up to at least 200 A.
A
CHASCE observation of a tapered fiber of borosilicate glass revealed a granular or spotty appearance in that part of the fiber which was small cnough to allow transmission of the electron beam, thus iendering a transmission photograph of the fiber-as opposed to a shadowgraph-which is obtained for larger fiber diameters. This first fiber was one of a much larger group which was being shadowgraphed as a means of determining fiber diameter. This particular fiber tapered from a diameter of well over 1000 A4.to less than 100 -1.and was increasingly transparent with decreasing diameters brloiv 1000 A. This first direct indication of observable structure led to the beginning of a program directed toward an understanding of this structure. The progress January 1954
of this program to the present time is reported here. Severd glasses in different forms which have different thermal histories have been investigated. Previous investigators studying various physical properties of glass have arrived a t widely varying ideas of the structure of glass. Many have estimated inhomogeneities in structure ranging from nearest neighbor separations up to several hundred Angstrom units. Warren’s (a)x-ray diffraction work appeared to give the most direct indication of this inhomogeneity of glass structure and indicated that there x-as no crystalline order in one dimension beyond a distance of approximately 7 A. This conclusion has been broadly interpreted by others as indicating
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Fibers with diameters less than 1000 A. are found in satisfactory quantities in Fiberglaa AAA superfine fiber. The mounting technique for these fine fibers is relatively simple. A very small amount of the AAA fiber material is pressed between two specimen mounting screens, having a high proportion of open area. In this manner, the fibers are anchored sufficiently over most of the screen. This mounting technique is especially satisfactory, as there is practically no chance for contamination or damage to the sample. A careful search of the entire screen area will usually reveal at least one fiber fine enough for study. Samples of massive glass and larger fibers must be ground or crushed in order to produce small flakes thin enough for transmission. Usually, the sample is crushed, and the part that will pass through a 400-mesh screen is then carefully dusted onto a specimen mounting screen which has first been dipped into a dilute solution of the adhesive from cellophane tape in benzene, and then dried. After the powder has been dusted onto the screen, all excess powder is removed by brushing. Much of the powder is still too coarse for the purpose of this work; however, several wedge-shaped edges of chips will usually be visible on each screen. The occasional very small angle wedges are, of course, the best for study. The cellophane tape adhesive provides good adherence of the powder and does not flow over it. Good adherence is necessary, as glass flakes are nonconducting and, hence, take on a charge in the electron beam; consequently, they are subject to large forces tending to cause motion. Other adhesives such as gelatin have been tried, but they have all had a tendency to flow over the samples, thus obscuring structural details. It is not practical to Figure
1.
(Left) Quartz
Crystalline
A thick surface film is visible but does not obscure underlying detail
Figure 3. Figure 2. (Right) Fused Silica The structure elements are here as large as any observed in pure fused silioa
a random structure] in agreement with the earliest concepts of glass as being a perfect supercooled liquid. Milligan (1) and Weber (3), using the techniques of neutron diffraction, have been able to carry out their Fourier analysis of the data to greater distances, and have found indications of a fairly high degree of order a t somewhat greater distances, with some estimates as high as 20 A. In the present work, structural units up to a t least 200 A. have been observed. .
(Left) Fragment of E-Glass
The structure elements are small in thie sample
Figure 4. (Below) Leached EGlass Fragment
EXPERIMENTAL
Because the observed indications of structure 'were obviously in the interior of the fiber and were a t the limit of resolution of most replication techniques, and because samples of glass thin enough for direct transmittance micrographs are readily available] most of the work to date has been a direct study of the glass, itself, without use of surfaoe replicas. A little work has been done with replicas and the resulting micrographs have shown nothing at variance with the direct transmittance pictures. All work has been done on an RCA Type EMU microscope.
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-Ceramics and Glass-
Figure 5 . (Left) Fiber of E-Glass The severe surface flaw a t one end indicates that the fiher probably collided with another during the forming process
Figure 6. (Above) Fibers of E-Glass after Leaching i n Hydrochloric Acid There is some evidence of a chain-type structure
Figure 7.
( R i g h t ) Leached E-Glass Fibers
I n the smaller fiber chains can be traced for a length equivalent to several fiher diameters
Figure 8. (Left) Lead Glass The structure is shown with greater contrast than in the other glasses
Figure 9. (Above) F r a g m e n t of CGlass The uniform small structure oharaeteristic of C-glass is apparent
Figure 10. (Right) F r a g m e n t of C-Glass Similar to Figure 9 but a t lower magnification. A etriated structure is evident
January 1954
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Figure 11. 16% Sodium Oxide-84% Silica
Figure 12. 20770 Sodium Oxide-80770 Silica
Figure 14. 30% Sodium Oxide70% Silica
Figure 15. 35% Sodium Oxide65% Silica
Figure 13. 2570 Sodium Oxide75qo Silica
Figure 16. 35% Sodium Oxide-65% Silica Here the structure is large and clear
Figure 17. 3577' Sodium Oxide -65770 Silica In this sample the structure is very clear hut small as compared t o that of Figure 16
150
Figure 18. 45% Sodium Oxide-55q0 Siliea
Figure 19. 50% Sodium Oxide-50% Silica
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-Ceramics
Figure 20. 16q0 Sodium Oxide-84q' Silica After being quenched from 1500° C.
Figure 21.
and GIass-
16% Sodium Oxide84Yo Silica
Figure 22. 16% Sodium Oxide-84% Silica
From glassy region of a liquidus boat
From crystallized region of a liquidus boat
Figure
23. (Left) 35% Sodium Oxide-65% Silica
After being quenched from 1500' C.
Figure 24. (Aboae) 35% Sodium Oxide-65% Silica From glassy region near the liquidus line in a liquidus boat
Figure 25. (Right) 35% Sodium Oxide-65o/c Silica From glassy region near the liquidus line in a liquidus boat
ieposit the powder on any film, as the film w d provide a back. ground with structure approximating that of the glass. Another mounting procedure for powdered samples is similar to that used for fine fibers. A specimen screen is dusted with the powder, and after the excess powder has been removed, mother screen is firmly pressed against it in a hydraulic press. The x-ray diffraction results referred to below were obtained on an early model of Philips 90" Geiger counter spectrometer. January 1954
RESULTS
Most of the work has been done on a nominal alkali-free commercial glass (Owens-Corning &glass), a series of two-componentsoda-silicaglasses (Naz0-Si02),an acid-resistant glass (OwensCorning C-glass), and a lead glass. The Owens-Corning E-glass has been investigated in both the fibrous and massive forms before and after various thermal treatments as well as various leaching procedures. The soda-silica series has. been observed
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in the beam, diffraction bands are clearly observable. In contrast to the absence of structure in crystalline quartz, Figure 2 shows the granular structure observed in several samples of fused silica. The size of the granular or micellar structure in fused silica ranges from below the limit of resolution of the microscope (15 to 25 A.) up to a t least 100 A. A micrograph of a fragment of Eglass, showing a very fine structure close to the limit of resolution, is shown in Figure 3. The size of the micelles in E-glass is very variable. The effect of leaching a powder fragment of E-glass is shown in Figure 4. This sample was leached in 4N hydrochloric acid a t 90' C. 4 hours, and then dried a t 40" C. in vacuum for ti days. This leaching procedure leaves a silica skeleton consisting of more than 95% silica, and showing the residual structure of the silica very clearly. Figure 26. ( L e f t ) 35% Sodium There have been indications that the Oxide-65% Silica leaching and drying procedure aff ecta From glassy regionof a liquidus boat, 150° C. above liquidus temperature the structure; consequently, the structure observed in leached material cannot be definitely considered as being Figure 27. (Right) 35% Sodium Oxide-65% Silica identical to that of the material before leaching. Figure 5 shows an E-glass From glassy region of a liquidus boat, 150' C. above liquidus temperature fiber. Structure is evident here on a small scale. Apparently, this fiber collided with another during the forming process, as indicated by the severe surface flaw a t one end. Figures 6 and 7 are micrographs of similar E-glass fibers which have been leached in concentrated hydrochloric in the range from 16 to 50% soda, both as produced and after acid a t room temperature 24 hours, and then dried in air a t controlled thermal treatments. The compositions of E- and C65' C. Here, the structure is definitely chainlike. I n Figure 7 some of the chains can be traced to a length equivalent to glass are given in Table I. The results are best presented by referring to the actual micrographs, which are presented here. several fiber diameters. The chain structures seem to be more characteristic in fibers than in powdered bulk glass, indicating All photographs except Figure 10 are a t a magnification of 91,OOOX ; thus 0.9 cm. corresponds to 1000 A. In order to indicate the effects of photographic grain size and thickness variations in the sample, an electron micrograph of crystalline quartz is shown in Figure 1. In this picture a thin film on the sample can be clearly seen. This film is deposited in the microscope and is characteristic of many commercial microscopes. In most of the other photographs shown here, the film is absent or so thin as to be unnoticeable. This film, as it becomes thicker, tends to obscure the detailed structure. However, a thin film tends to facilitate observation, for it serves to conduct away charge from the sample and t o stabilize it in the electron beam. In other micrographs of crystalline quartz, properly oriented
TABLE I. GLASSCOYPOSITIONS Si02
A1203 FeaOa CaO
3: NazO Kz0
-~
152
C-Glass, % 63.6 3.8 0.2) 14.0 2.6 6.7
E-Glass, SZ 54.6 14.8
17.4 4.5 8.0 0.6
Figure 28. 45% Sodium Oxide-55% Silica
Figure 29. 45% Sodium Oxide559%Silica
After being quenched from 150O0 C.
From crystallized region of a liquidus boat
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-Ceramics that the micelles evident in the bulk structure may be drawn out as threads or chains in the process of fiber formation. Figure 8 shows a sample of lead glass that has a structure not significantly different from the other glasses, but shows it with greater contrast. In a f e r areas the micelles seem to be aligned in a regular fashion. Figures 9 and 10 are copies of the same micrograph of a sample of C-glass. Figure 9 shows the rather small scale but uniform structure characteristic of C-glass. Figure 10 a t lower magnification shows a series of striations at an angle of about 45" to the long edge. These striations may be either an indication of a larger scale structure or a very fine type of hackle mark. Such striations have been observed in only a few samples and are not a characteristic of C-glass. Figures 2 t o 10 indicate that there is a definite observable structure in the readily available commercial glasses investigated. However, as the observed structures are all close to the limit of resolution of the microscope, it is difficult to note any characteristic differences. For this reason it seemed desirable to investigate a simple two-component glass system with varying amounts of the two components. The soda-silica system was chosen, and a number of samples ranging from 16% to 50% sodium oxide were produced. The results of this investigation are shown in Figures 11 to 19, which are representative of a large number of micrographs of each sample in the series. Themost significant observation is that the amount of the material in the micelles increases as the percentage of soda increases. The structure observed in the 35% sodium oxide sample is the most clearly developed and defined. However, the micelles are not necessarily the largest a t this composition, as shown in Figure 17. This study is being continued on samples having lower percentages of soda. ,4n x-ray diffraction pattern was obtained for each of these glasses in order to determine the effect of composition. The glass containing 84% silica showed the normal wide band which is characteristic of silica glasses. The intensity of the band decreased with decreasing silica content. The glass containing 50% silica showed no evidence of this characteristic broad band. In order to determine the effect of thermal treatment on the structure, three of the soda-silica glasses were given special heat treatments. One sample of each was held a t a temperature of 1500' C. for 0.5 hour, and then quenched by dropping the shallow crucible containing it into sand. Another sample of each was held in a liquidus boat a t the appropriate temperature for each composition for 48 hours, and then allowed to cool in air. Samples for observation from the liquidus boat were removed by drilling from the desired region. Figures 20 to 22 show the effect of heat treatment on the sample containing 16% sodium oxide. The quenched sample shown in Figure 20 appears to have little and ill-defined structure. Figure 21 shows a sample in the glassy region of the liquidus boat. Here, the structure is about the same as that of the normal sample shown in Figure 11. A sample from the crystallized region of the boat is shown in Figure 22. There is some evidence of crystalline material but practically none of the type of structure seen in Figures 11 and 21. A nample of glass containing 35% sodium oxide which was quenched is shown in Figure 23 and has structure similar to that of the normal sample. Two samples from the glassy region of the liquidus boat, but very close to the liquidus line, are shown in Figures 24 and 25. Here, the structure is less well defined but is still present. Two samples which were a t a temperature 150" C. above the liquidus temperature are shown in Figures
January 1954
and Glass-
26 and 27. Again, the structure is not as well defined, although it may be slightly more pronounced than in the samples closer to the liquidus line. A quenched sample of the glass contsining 45% sodium oxide pictured in Figure 28 has a structure which again is similar to that of the normal glass. A sample from the glassy region of the liquidus boat has a smaller scale structure, as shown in Figure 29. Although the thermal treatments do not show any large effect on structure, there is a consistent correlation between the heat treatment and the observed structure of all three compositions. CAUSE OF PHOTOGRAPHIC DENSITY DIFFERENCES
Thus far, the micellar structures in glasa have been discussed without stating what actual differences in the glass from one region to another can cause the photographic density differences. Following are three explanations:
1. The darker portions may represent regions of greater thicknesses. However, this is doubtful, as in the extremely thin sections the observed contrast would require that there be very great differencesin thickness occurring over a very short distance. Thus, there would be spikes projecting from the surface, but none have been observed. Furthermore, the number of dark areas increases as the sample thickness increases, and this should not be the case, if they merely represent differences in thickness. 2. The darker regions may consist of material of a higher density. This may be partially true, as the photographic density contrast is greater in the case of the lead glass. However, it cannot be the complete explanation, as very great density differences would be required in order to produce the contrast shown in the micrographs of fused silica. 3. The darker areas may be caused by a greater degree of COherent scattering by the regions having a higher degree of order than that of the surrounding material. If this is the case, it is to be expected that most of the partially ordered regions would show up as dark areas in the photograph, since there is a greater probability that they will be oriented in such a manner as to produce coherent scattering away from the electron beam axis. Those few regions which are oriented in such a way that no scattering occurs should appear as light areas in the photographs. That the effects observed in the micrographs cannot be due t o diffraction effects has been proved by the fact that changing the focus of the microscope does not affect the observations other than to reduce definition. Diffraction effects would be greatly affected as the focus is changed. Any of the explanations listed above require that there be an inhomogeneous structure in the glass, However, a combination of the second and third explanations seems most likely. Thus, glass contains micellar regions having a degree of order greater than that of the completely disordered random glassy structure, but less than that of the perfectly ordered crystal. Since the order is not perfect and the amount of partially ordered material is a relatively small percentage of the total material, it will not give rise to any well defined x-ray diffraction pattern. Neutrons which are absorbed to a lesser degree in the disordered glassy material will show the effect of this greater degree of order, because the boundary between partially ordered and completely disordered material is not likely to be sharp. Small-angle scattering diffraction patterns cannot be expected to give a definite indication of structure. LITERATURE CITED
(1) Milligan, W. O., Levy, H. A,, end Peterson, S. W., P h w . Rev., 83,
226 (1951).
(2) Warren, B. E., J. Am. Ceram. Soc., 24, 262 (1941). (3) Weber, A. H., Nucleonics, 7, 31 (1950). RECEIVED for review April 14, 1953.
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