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Electron-microscopical studies of glass
Journal of Non-Crystalline Solids 49 (1982) 221-240 North-Holland Publishing Company
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Part H L Characterization of microstructure ELECTRON-MICROSCOPICAL STUDIES OF GLASS W. V O G E L , L. H O R N , H. REISS a n d G. V O L K S C H
Otto-Schott-lnstitute, Department of Chemistry, Friedrich-Schiller-University of Jena, GDR
During the past 25 years, the use of the electron microscope in glass research has substantially expanded our knowledge of the microstructure of glasses, for instance of the phase separation processes in glasses. The use of electron microscope in glass research calls for special knowledge the lack of which would certainly lead to misinterpretations and wrong conclusions. Direct electron transmission of the specimen: low voltage beams create images of high contrast. On the other hand, there is always the risk of altering the specimen. If very high beam voltages are employed this is at the expense of image contrast. The optimum beam voltage range in the dirext electron transmission of glass is 50 to 120 kV, depending very much on the kind of glass to be examined. The replica technique: this technique in electron microscopical investigations of glasses has more advantages than disadvantages but a highly developed technique is necessary. The most important factors will be described. Surface treatment of glass samples prior to making the replica: suitable etching processes of a freshly fractured glass surface sometimes help to enhance the visibility of structural peculiarities but faulty preparations have to be avoided. Use of the scanning electron microscope and the electron-beam microprobe: enormous advantages and problems will be described. Finally selected examples of electron-optical examinations of glass will be given.
1. Contributions of electron microscopy to glass structure research D u r i n g the past 25 years, the use of the electron microscope in glass research has s u b s t a n t i a l l y e x p a n d e d our knowledge of the microstructure of glasses [1,2]. T o d a y we k n o w that most glass melts u n d e r g o more or less a d v a n c i n g phase separation processes d u r i n g cooling, which lead to a microheterogeneous structure of the vitreous solid. There exist all degrees of transitions to glasses whose fine structure c a n best be described either by the network or the crystallite theory. These glasses should, however, be regarded as limiting cases as far as their fine structure is concerned. Obviously, the m a i n reason for microphase separation d u r i n g the cooling of glass melts is the f o r m a t i o n of molecular structural groups that occupy widely differing volume fractions. I n a way we can prove analogies b e t w e e n crystallization b e h a v i o u r a n d the segregation b e h a v i o u r of melts. I n one case, mixed crystals are formed with the guest c o m p o n e n t distributed statistically in the crystal lattice of the host c o m p o n e n t . This would correspond to a h o m o g e n e o u s glass structure that can best be described by the Zacharias e n - W a r r e n theory. I n a n o t h e r case we have the freezing of a eutectic melt; i.e. 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
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two crystal phases of different lattice structures are formed. This would correspond to a glass of microheterogeneous structure, i.e. with phase separation. Electron-microscopical examination further shows that base glasses having rather awkward structural elements, such as phosphate glasses with chain structures, are capable of accomodating into their large cavities foreign structural elements of smaller dimensions without undergoing phase separation. Typical examples are the basic phosphate glasses, which display little tendency to segregate. The situation is altogether different where the guest (or minor) component itself is of a very unwidely structure, such as SiO2 glass. That is why glasses of the binary P205-SIO2 system exhibit a pronounced segregation tendency. An analogous phenomenon Js observed if foreign structural elements are introduced into highly acidic P205 glasses of network structure, i.e. of a higher packing density than chain-structured glasses containing P205. In such a case, microphase separation will readily occur, the same as in silicate or borate glasses. If base glasses of microheterogenous structure are doped with highly charged ions such as Fe, Ni, Ti, Ag, Cu, Au, Nd, Zr or similar ions in small doping concentrations, these ions are enriched almost completely in the microphase offering the best coordination possibilities. In the case of higher doping concentrations the distribution approaches equilibrium, which is, however, still shifted towards the microphase offering better coordination possibilities. Electron microscopy has developed into an enormously broad spectrum of techniques during the past 25 years. Its use in glass research calls for special knowledge, the lack of which would certainly lead to serious misinterpretations and wrong conclusions. The present paper deals with a number of phenomena which are considered to be imperative in electron-microscopical glass examinations.
2. Techniques of electron-microscopical glass examination 2.1. Direct electron transmission of the specimen
The direct transmission of the specimen by electrons has the advantage that the resolving power of the electron microscope can be fully utilized. Ultrami] ~
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Fig. 1. Direct electron transmission through a wedge-shaped glass splinter (schematic). (a) The droplet-shaped segregation regions have a higher density than the embedding glass matrix. On the electron micrograph, the droplets appear as dark areas. (b) The density of the matrix is higher than that of the droplets, which appear as bright areas.
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Fig. 2. Direct electron transmission through a lithium-silicate glass splinter. The SiO2-rich droplet phase has a higher packing density than the surrounding Li20-rich matrix phase and appears darker due to higher electron scattering. c r o t o m e specimens c a n n o t b e m a d e from glass as a rule, so that the p r i m a r y technique a p p l i c a b l e is t r a n s m i s s i o n t h r o u g h the w e d g e - s h a p e d edges of glass splinters. D i r e c t t r a n s m i s s i o n t h r o u g h t h i n - b l o w n glass foils is of no signifi-
Fig. 3. Direct electron transmission through a lithium-silicate glass splinter. Here, the SiO2-rich phase is the matrix, so that the Li20-rich droplets appear brighter.
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cance, because the structure of a rapidly cooled thin glass foil will never be like the structure of a bulk glass sample. Fig. I schematically represents the state of affairs in the direct electron transmission of a glass splinter, in the case when the droplet phase has a higher density than the matrix (fig. l a), and when the density of the matrix phase is higher than that of the droplets (fig. 1b). In the electron micrograph, the phase of higher density will always appear darker. Figs. 2 and 3 are direct experimental evidence of this. In the direct transmission of glass specimens, the electron beam voltage is of substantial importance. Low voltage beams will transmit very thin specimens only, but render images of high contrast. On the other hand, there is always the risk of changing the specimen. The high absorption of electrons in case of low beam voltages frequently heats the specimen up to remelting. Easily reducible ions in the glass, e.g. Pb ions, may be reduced to the pure metal. Consequently, effects may appear that have nothing to do with the true glass structure. If very high beam voltages are employed in direct transmission, the glass specimen used may be thicker, but this is at the expense of image contrast. The optimum beam voltage range in the direct electron transmission of glass is 50 to 120 kV, depending very much on the kind of glass to be examined. Since we can never fully exclude interactions between electron beam and specimen, several electron-optical techniques should be employed in cases of doubt. Sufficient mass differences provided, the direct transmission method can detect structural objects smaller than 50 ~,. 2.2. The replica technique
The replica technique is an indirect method of detecting structural peculiarities in glass. It requires the provision of a freshly fractured surface. The fracture process is to reveal structural inhomogeneities that can be made visible by as deep a relief as possible. A very rapid crack may smoothly shear off structural inhomogeneities so that the resulting fracture surface is rather flat, which leads to a misinterpretation of the observed image because it may wrongly suggest a homogeneous structure of the specimen. Therefore, the fracture process employed to produce a fresh replica surface is of some importance. The sudden cracks caused by impact or heat have proven unsuitable for the detection of fine structural inhomogeneities by the replica technique. The very slow flexural cracking process is far more suitable. Fig. 4 shows the equipment for producing a fresh flexural crack surface in air or in a high vacuum of 10 -6 T o r r [1]. A high vacuum is an absolute necessity in some cases, because in certain glasses the contact of the fresh fracture surface with air will cause undefined changes. After the fresh fracture surface has been provided, it is vapour-coated with a mixed P t - I R - C layer in a single operation. The evaporation electrode arrangement shown in fig. 5 has definitely advanced the method. A carbon electrode bears a welded-on P t / I R bead,
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which acts as a point source of metal vapour and ensures excellently defined shadows. Depending on the surface profile, the most favourable evaporation angle has been found to be between 30 ° and 60 °. In the past, a replica film had to be evaporated in two operations, i.e. perpendicular carbon evaporation followed by oblique shadowing with metals or WO a. The arrangement described is a considerable inprovement that permits far better image qualities to be attained. This preparation technique is based on the fact that in oblique shadowing at an angle of 30 ° to 60 ° using the described P t - I r - C electrode, the carbon particles deposited from the vapour phase on to the specimen surface are still mobile enough to form a continuous carbon film even in the shadow areas, whereas the Pt and Ir particles (mixed with carbon ones) are deposited only on specimen regions facing the vapour source. The mixed P t - I r - C layer is of extremely fine grain; even in electron transmission, i.e. under possible thermal stress, it shows little tendency towards recrystallization and grain enlargement. Fig. 6 schematically represents the process of generating the replica film. Separation of the film from the glass surface, as a rule, is effected by immersing the specimen in water, acids or alkaline solutions, which diffuse below the film, slightly attacking the glass and thus separating the replica from the glass surface. Where the structural inhomogeneities to be micrographed approach the order of magnitude of the vapour-deposited grains, the acquisition of reliable information from a replica micrograph becomes problematic. Here, the application of individual, razor-blade shaped MoO 3 crystals on the freshly cracked specimen surface prior to evaporation has greatly advanced the technique [1,3]. MoO a crystals have an ideally smooth surface. After evaporation they appear on the micrograph as test surfaces; a comparison of the grain on the specimen
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and on the test suface permits a clear decision to be made about the small grains found on the specimen indicate structural inhomogeneities or are just deposited grains. In the latter case, specimen and test surfaces show the same grain. The application of MoO 3 test surfaces in the preparation of replicas makes an additional operation necessary after the replica film has been stripped. The stripped film has to be washed in diluted sodium hydroxide solution in order to dissolve the MoO 3 crystals, which usually stick to the stripped replica film. In the dissolution, sodium molybdate is formed. The electron-optical resolving power for replicas without a test surface is
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Fig. 7. Spark flashover m applying MoO 3 crystals on to the surface of a tellurite glass cuts a "drainage pattern" into the specimen by localized evaporation.
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227
Fig. 8. Crystal phase on the surface of a tellurite glass surface, formed by condensed vapour. These crystals bear no relationship to the microstructure of the glass.
about 120-150 A. With a test surface, structural inhomogeneities of about 50 ,~ can be reliably identified. The application of MoO 3 crystals on the freshly fractured surface is effected by briefly exposing the surface to MoO 3 fumes if the specimen is prepared in air. In the case of vacuum preparation, the nose of the cracking lever has to be
Fig. 9. As for fig. 8.
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studded with MoO 3 crystals before the vacuum is generated (see fig. 4, top right). With the lever nose and the specimen surface facing each other and a high tension applied, a spark discharge from the lever nose to the specimen clamp jaws will hurl a sufficient number of MoO 3 crystals on to the glass fracture surface, which can then be vapour-coated as described. In preparing specimens of glasses having some electric conductivity (e.g. tellurite glasses or chalcogenide glasses) it has been found that the spark employed to apply MoO 3 crystals under vacuum will frequently flash over to
Fig. I1. Replica electron micrograph of a tellurite glass with a MoO 3 crystal test surface. The comparison between the grain of the test surface and that of the glass fracture surface enables reliable information to be obtained concerning the microheterogeneous structure of the tellurite glass sample.
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the glass surface rather than to the clamp jaws, giving rise to gross misinterpre. tations of the electron micrograph. Fig. 7 shows traces of such a spark flashover to a freshly fracture surface of tellurite glass. As a result of partial glass evaporation and condensation of the evaporation products, the glass surface is evenly covered with microcrystals which have nothing to do with the structure of the glass sample (see figs. 8 and 9). A wholly reliable preparation technique for electrically conductive glasses has become possible with a modified cracking device (see fig. 10). Here the M o O 3 crystals are applied to the glass surface by means of a additional platium electrode studded with MoO 3 crystals [4]; the spark flashes over from the Pt electrode to the cracking lever. Fig. 11 is a typical example of such a faultless preparation. It is made from the same glass sample as that of figs. 9 and 10. By comparison with the MoO 3 crystal test surface the minute grain on the specimen surface can be unmistakeably identified as structural inhomogeneity. Many optical and engineering glasses have an electrical resistance of 10 t4 tO 1017 ~ cm. They can readily be prepared by the older technique. Glasses having resistance between 10 s and 1012 f~ cm or even below that range should be prepared with the modified cracking device only. 2.3. Surface treatment of glass samples prior to making the replica [5-7] Suitable chemical treatments (etching processes) of a freshly fractured glass surface prior to vapour deposition permit the differential removal of inhomo-
Fig. 12. Replica electron micrograph of a lithium silicate glass (Pt-Ir-C replica technique). The fracture surface produced in a high vacuum reveals microphase separation to the experienced observer only.
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Fig. 13. Replica electron micrograph as in fig. 12. The glass surface was treated with water for 30 s prior to P t - I r - C evaporation. After chemical surface treatment, the droplet-shaped segregation regions are much more conspicuous.
geneities or the surrounding glass matrix because of different chemical solubilities. This will sometimes help to enhance the visibility of structural peculiarities. Sometimes even qualitative or semi-quantitative information can be derived on the composition of microphases. Especially in case of densely
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Fig. 14. SiO2-rich droplets in a barium borosilicate glass, isolated by chemical treatment. Direct electron transmission.
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packed droplet regions, e.g., this method penetrates into ranges that are inaccessible to the microprobe. Fig. 12 shows a replica electron micrograph of an unetched Li20-SiO2 glass of microheterogeneous structure. A micrograph of the same glass previously etched with water for one minute shows the microdroplet regions much more distinctly (fig. 13). In some cases, microphases in glass can be entirely isolated by etching and thus examined separately (fig. 14). Eligible etchants are water, CH3OH (for etching glasses of high B203 content), other alcohols, diluted HF, HNO 3, H2SO 4, NH4OH or mixtures thereof. The type and concentration of the etchant and the duration and temperature of etching primarily depend on the glass type to be examined. The most favourable conditions have to be ascertained specifically for each glass family. Glass etching prior to a replica preparation offers great advantages but may equally involve serious risks of misinterpretation of the resulting electron micrographs. In particular, we have to consider the possibility of the formation of low-solubility compounds between etchant and glass constituents. Fig. 15 is an electron micrograph of a PbO-B203-SiO 2 glass etched in 1 n HNO 3 for 1 min. In addition to droplet-shaped segregation regions, the picture shows cubic crystals. This is a typical example of a faulty preparation. The cubic crystals are Pb(NO 3)2 formed by etching with HNO 3 and crystallized on the specimen surface during drying. The black spheres are SiO 2 droplets fully
Fig. 15. Replica electron micrograph of a lead borosilicate glass after I min etching of the fracture surface with 1 n HNO 3. Apart from segregation droplets (bright droplets inside the glass; dark ones have been isolated before and stick to the replica film), the micrograph shows cubic crystals. They result from faulty preparation and bear no relationship to the glass structure.
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Fig. 16. Replica electron micrograph of the same glass as in fig. 15. After chemical etching the specimen has been cleaned with water/alcohol/acetone and cellite foil. It shows true structural inhomogeneities only.
Fig. 17. Etching liquid sucked off the etched glass surface shown in fig, 15 and left to dry on a neutral substrate. Cubic Pb(NO3) 2 crystals as in fig. 15 have formed.
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isolated by etching but adhering to the replica and transmitted by electrons together with it. If the etched glass surface is cleaned, by a sequence of solvents comprising water, alcohol and acetone in addition by applying and stripping a film of cellite, the prepared replica will result in a micrograph as shown in fig. 16, where both the cubic Pb(NO3) 2 crystals and the isolated black droplets have disappeared. That the cubic crystals are, in fact, a secondary effect, is shown in fig. 17. Here, a drop of the etching liquid was sucked off the surface of the PbO-B203-SiO 2 glass and left to dry on a specimen slide. The result show the same cubic Pb(NO3) 2 crystals as in fig. 15. We should also bear in mind the well-defined shadows behind the cubic crystals, which testify to the efficiency of the evaporation and replica techniques described. As a supplementary method to the previously described chemical etching technique we should consider ion etching [8]. Under certain conditions it is a valuable alternative. We should not overlook, however, that fact that the ion beam strongly interacts with the glass surface, so that the hazard of changing the specimen by the formation of artefact structures is particularly great. 2.4. Use of the scanning electron miroscope and the electron-beam microprobe
In about 1970, scanning electron microscopy experienced an extraordinary spreading of its application due to enormous advances in instrumentation. Classical electron-optical examination had primarily relied on replica preparation. The function of the scanning electron microscope and the microprobe are
Fig. 18. Scanning radiograph (BaL,, radiation) of a multiple-segregationbarium borosilicate glass, showing that Ba ions are enriched in the primary, large droplet phase.
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Fig. 19. Scanning electron micrograph of a calcium borate gla.,.s, with an inscribed X-ray intensity curve of the CaK,~ radiation. The electron microprobe beam has been run across the specimen along the white horizontal line. The intensity curve clearly sh.~ws that the droplet phase is rich in Ca.
Fig. 20. Scanning electron micrograph of the "burning track" (cf. horizontal line in fig. 19) of an electron beam in electron beam microanalysis.
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based on the utilization of three other types of signals, i.e. back-scattered electrons, secondary electrons, and characteristic X-rays. These are signals formed by the interaction between the specimen material and electrons irradiated into it, i.e. signals that originate at a certain depth inside the specimen. A replica of a good surface relief (the quality of which greatly depends on the cracking speed) only yields indirect information only about structural inhomogeneities, whereas the scanning electron microscope reveals the structure of the specimen at a certain depth, which is a great advantage and a great step ahead. In addition, the SEM image usually provides a much better impression of depth. Contrast in the SEM is produced either by differences of electron back-scattering properties (material contrast) or by the geometrical texture of the surface (topographic contrast). Scanning electron microscopy has some disadvantages also, which should not be overloooked: (i) one drawback is that the specimen will interact with the radiation by absorbing electrons and is thus subjected to a thermal stress that is incomparably higher than that in the classical replica technique. In certain glasses, this always involves the risk of changing the specimen. In many cases the glass surface has been found fused. (ii) Another drawback is that the SEM has a lower resolving power. Objects have to be greater than 50 ,~ in order to be reliably resolved. (iii) Finally, electrically non-conductive specimens have to be vapour-coated
Fig. 21. Sectional enlargement of fig. 20. Fracture surface across the "burning track". Structural changes in the immediate vicinity caused by thermal stress (electron absorption) are clearly visible.
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with carbon or gold in order to avoid electric charges which would greatly disturb examination. A carbon coat will provide a good material contrast, whereas a gold coat will primarily produce a good topographic contrast. The analysis of the third kind of signals mentioned, the occurrence of X-rays characteristic of a particular element, yields qualitative and quantitative information on the distribution of an element in glass. This is an advancement of extraordinary value. Both the scanning radiograph and the X-ray intensity curve of the natural radiation of a certain element of interest unambiguously indicate the distribution of that element over the microphases of the glass. Figs. 18 and 19 represent typical examples. Problems and uncertainties in using the microprobe arise if the object to be analyzed, e.g. a droplet microphase, is smaller than 3 to 5/~m, constituting only part of the excitation volume, or if the microphases are densely packed. Particular caution is recommened in microprobe studies of relatively unstable glasses. It is necessary to point out that in microprobe studies of glasses the electron beam usually is guided across the specimen at a scanning speed of 2 to 50 ~m per min. Often this causes the glass to melt along the beam track, which leads to substantial changes in the structure and concentration of the specimen. This has been observed especially in glasses containing Na ions. A typical case is shown in figs. 20 and 21. The electron beam guided across the glass specimen in fig. 20 has left a deep trace in which certain parts of the glass are evaporated. Fig. 21 is a scanning micrograph of a glass section across the trace, which clearly shows structural changes in the glass. The glass in the immediate vicinity of the trace is relatively homogeneous. It adjoins a ring of more intensive phase separation. Only the glass below this, i.e. at a greater depth, shows no sign of being influenced by the electron beam. These results have to be specifically considered in all microprobe examinations of glasses. Conditions may vary widely from glass to glass. 2.5. Selected examples of electron-optical glass examination
Fig. 22 is a classical replica micrograph of a segregated glass, showing a good surface relief after a slow flexural cracking process. In addition to the crack-tailed droplets, even the sheared-off droplets are clearly visible. Fig. 23 is a replica micrograph of a barium borosilicate glass which was slightly etched with HNO 3. After multiple segregation, the glass contains six microphases of different composition. Fig. 24 shows a barium borosilicate glass after more intensive etching with 1 n of HNO 3 for 2 s. This replica micrograph reveals two shells around the droplet. The outer one, which is rich in B203, has largely been dissolved by the HNO 3, whereas the inner, SiO2-rich shell has not been affected as such, but shows wrinkles caused secondarily by the etching process. Fig. 25 is a scanning micrograph by secondary electrons, showing a multisegregated turbid glass of very high reflectivity ( > 98%) after HNO 3 etching.
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Fig. 22. Classical replica electron micrograph of an unetched two-phase glass after slow flexural rupture in a high vacuum. Some of the droplets have remained in the glass (convex)~others have been dislodged, leaving a cavity; still others have been torn apart. Fig. 26 is a s c a n n i n g electron micrograph of an u n e t c h e d specimen of a m a c h i n e a b l e phlogopite vitroceram. Fig. 27 shows the same type of vitroceram after weak etching with hydrofluoric acid. The s c a n n i n g micrograph reveals a new c o n f i g u r a t i o n of the lamellar
Fig. 23. Replica electron micrograph of a barium borosilicate glass. After segregation in several steps, 6 glass microphases of different compositions have formed.
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Fig. 24. Replica electron micrograph of a barium borosilicate glass after 2 s etching of the fracture surface with 1 n H N O 3. The droplet is surrounded by an outer B203 shell and an inner SiO 2 shell. The B203 shell has been largely dissolved by the H N O 3, whereas the SiO 2 shell has remaied unaffected.
Fig. 25. Scanning electron micrograph (back-scattered electrons) of a barium borosilicate glass etched with l n H N O 3 for 2 s. The glass shows multiple segregation.
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Fig. 26. Scaning electron micrograph of a machineable phlogopite vitroceram. The unetched specimen shows the lamellar mica crystals.
Fig. 27. Scanning electron micrograph of an excellently machineable phlogopite vitroceram after weak HF etching. Here, the phlogopite lamellae are arranged in a "head-of-cabbage" structure. The micrograph shows a cross section through such a globular aggregate.
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p h l o g o p i t e crystals, which is the reason for the c o n s i d e r a b l y i m p r o v e d m a c h i n a b i l i t y of this vitroceram.
3. Concluding remarks In the p a s t 25 years, the e l e c t r o n - m i c r o s c o p i c a l techniques of glass e x a m i n a tion and the related p r e p a r a t i o n m e t h o d s have d e v e l o p e d into an e n o r m o u s l y b r o a d spectrum. It is imperative, however, to clearly realize the c a p a b i l i t i e s a n d limitations o f these techniques; otherwise, faulty results a n d m i s i n t e r p r e t a t i o n s are unavoidable.
References [1] W. Vogel, Struktur und Kristallisation der Gl~iser(VEB Deutscher Verlag fiir Grundstoffindustrie, Leipzig, 1965) 1st ed. [2] W. Vogel, Glaschemie (VEB Deutscher Verlag fiir Grundstoffindustrie Leipzig, 1979) 1st ed. [3] W. Skatulla, H. Wessel and W. Vogel, Silikattechnik 9 (1958) 51. [4] L. Horn and H, Reiss, Tagung Elektronenmikroskopie, Dresden (1978) p. 175. [5] W. Vogel, W. Schmidt and L. Horn, Z. Chem. 9 0969) 401. [6] H. Reiss and L. Horn, Tagung Elektronenmikroskopie, Dresden (1978) p. 171. [7] H. Reiss and L. Horn, Vergleichende Untersuchungen zur Struktur eines mikroheterogenen Glases mittels Vakuumbruch, chemisch- und ionengeiitzen Bruchfliichen, 10. Tagung Elektronenmikroskopie, Leipzig ( 198! ). [8] G. Schimmel and W. Vogel, Methodensammlung der Elecktronenmikroscopie (Wiss. Verlagsgesellschaft, Stuttgart) Chs. 2.4.2.1 by H. Bach: Die Anwendbarkeit des Ionenstrahl~itzens bei der Pr~iparation ftir die Elektronenmikroskopie. [9] D. Briimmer, Mikroanalyse mit Elektronen- und Ionensonden (VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1978). [10] G. VOlksch, H. Reiss and L. Horn, Silikanechnik 32 (1981) 52.
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Part H L Characterization of microstructure ELECTRON-MICROSCOPICAL STUDIES OF GLASS W. V O G E L , L. H O R N , H. REISS a n d G. V O L K S C H
Otto-Schott-lnstitute, Department of Chemistry, Friedrich-Schiller-University of Jena, GDR
During the past 25 years, the use of the electron microscope in glass research has substantially expanded our knowledge of the microstructure of glasses, for instance of the phase separation processes in glasses. The use of electron microscope in glass research calls for special knowledge the lack of which would certainly lead to misinterpretations and wrong conclusions. Direct electron transmission of the specimen: low voltage beams create images of high contrast. On the other hand, there is always the risk of altering the specimen. If very high beam voltages are employed this is at the expense of image contrast. The optimum beam voltage range in the dirext electron transmission of glass is 50 to 120 kV, depending very much on the kind of glass to be examined. The replica technique: this technique in electron microscopical investigations of glasses has more advantages than disadvantages but a highly developed technique is necessary. The most important factors will be described. Surface treatment of glass samples prior to making the replica: suitable etching processes of a freshly fractured glass surface sometimes help to enhance the visibility of structural peculiarities but faulty preparations have to be avoided. Use of the scanning electron microscope and the electron-beam microprobe: enormous advantages and problems will be described. Finally selected examples of electron-optical examinations of glass will be given.
1. Contributions of electron microscopy to glass structure research D u r i n g the past 25 years, the use of the electron microscope in glass research has s u b s t a n t i a l l y e x p a n d e d our knowledge of the microstructure of glasses [1,2]. T o d a y we k n o w that most glass melts u n d e r g o more or less a d v a n c i n g phase separation processes d u r i n g cooling, which lead to a microheterogeneous structure of the vitreous solid. There exist all degrees of transitions to glasses whose fine structure c a n best be described either by the network or the crystallite theory. These glasses should, however, be regarded as limiting cases as far as their fine structure is concerned. Obviously, the m a i n reason for microphase separation d u r i n g the cooling of glass melts is the f o r m a t i o n of molecular structural groups that occupy widely differing volume fractions. I n a way we can prove analogies b e t w e e n crystallization b e h a v i o u r a n d the segregation b e h a v i o u r of melts. I n one case, mixed crystals are formed with the guest c o m p o n e n t distributed statistically in the crystal lattice of the host c o m p o n e n t . This would correspond to a h o m o g e n e o u s glass structure that can best be described by the Zacharias e n - W a r r e n theory. I n a n o t h e r case we have the freezing of a eutectic melt; i.e. 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
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two crystal phases of different lattice structures are formed. This would correspond to a glass of microheterogeneous structure, i.e. with phase separation. Electron-microscopical examination further shows that base glasses having rather awkward structural elements, such as phosphate glasses with chain structures, are capable of accomodating into their large cavities foreign structural elements of smaller dimensions without undergoing phase separation. Typical examples are the basic phosphate glasses, which display little tendency to segregate. The situation is altogether different where the guest (or minor) component itself is of a very unwidely structure, such as SiO2 glass. That is why glasses of the binary P205-SIO2 system exhibit a pronounced segregation tendency. An analogous phenomenon Js observed if foreign structural elements are introduced into highly acidic P205 glasses of network structure, i.e. of a higher packing density than chain-structured glasses containing P205. In such a case, microphase separation will readily occur, the same as in silicate or borate glasses. If base glasses of microheterogenous structure are doped with highly charged ions such as Fe, Ni, Ti, Ag, Cu, Au, Nd, Zr or similar ions in small doping concentrations, these ions are enriched almost completely in the microphase offering the best coordination possibilities. In the case of higher doping concentrations the distribution approaches equilibrium, which is, however, still shifted towards the microphase offering better coordination possibilities. Electron microscopy has developed into an enormously broad spectrum of techniques during the past 25 years. Its use in glass research calls for special knowledge, the lack of which would certainly lead to serious misinterpretations and wrong conclusions. The present paper deals with a number of phenomena which are considered to be imperative in electron-microscopical glass examinations.
2. Techniques of electron-microscopical glass examination 2.1. Direct electron transmission of the specimen
The direct transmission of the specimen by electrons has the advantage that the resolving power of the electron microscope can be fully utilized. Ultrami] ~
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Fig. 1. Direct electron transmission through a wedge-shaped glass splinter (schematic). (a) The droplet-shaped segregation regions have a higher density than the embedding glass matrix. On the electron micrograph, the droplets appear as dark areas. (b) The density of the matrix is higher than that of the droplets, which appear as bright areas.
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Fig. 2. Direct electron transmission through a lithium-silicate glass splinter. The SiO2-rich droplet phase has a higher packing density than the surrounding Li20-rich matrix phase and appears darker due to higher electron scattering. c r o t o m e specimens c a n n o t b e m a d e from glass as a rule, so that the p r i m a r y technique a p p l i c a b l e is t r a n s m i s s i o n t h r o u g h the w e d g e - s h a p e d edges of glass splinters. D i r e c t t r a n s m i s s i o n t h r o u g h t h i n - b l o w n glass foils is of no signifi-
Fig. 3. Direct electron transmission through a lithium-silicate glass splinter. Here, the SiO2-rich phase is the matrix, so that the Li20-rich droplets appear brighter.
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cance, because the structure of a rapidly cooled thin glass foil will never be like the structure of a bulk glass sample. Fig. I schematically represents the state of affairs in the direct electron transmission of a glass splinter, in the case when the droplet phase has a higher density than the matrix (fig. l a), and when the density of the matrix phase is higher than that of the droplets (fig. 1b). In the electron micrograph, the phase of higher density will always appear darker. Figs. 2 and 3 are direct experimental evidence of this. In the direct transmission of glass specimens, the electron beam voltage is of substantial importance. Low voltage beams will transmit very thin specimens only, but render images of high contrast. On the other hand, there is always the risk of changing the specimen. The high absorption of electrons in case of low beam voltages frequently heats the specimen up to remelting. Easily reducible ions in the glass, e.g. Pb ions, may be reduced to the pure metal. Consequently, effects may appear that have nothing to do with the true glass structure. If very high beam voltages are employed in direct transmission, the glass specimen used may be thicker, but this is at the expense of image contrast. The optimum beam voltage range in the direct electron transmission of glass is 50 to 120 kV, depending very much on the kind of glass to be examined. Since we can never fully exclude interactions between electron beam and specimen, several electron-optical techniques should be employed in cases of doubt. Sufficient mass differences provided, the direct transmission method can detect structural objects smaller than 50 ~,. 2.2. The replica technique
The replica technique is an indirect method of detecting structural peculiarities in glass. It requires the provision of a freshly fractured surface. The fracture process is to reveal structural inhomogeneities that can be made visible by as deep a relief as possible. A very rapid crack may smoothly shear off structural inhomogeneities so that the resulting fracture surface is rather flat, which leads to a misinterpretation of the observed image because it may wrongly suggest a homogeneous structure of the specimen. Therefore, the fracture process employed to produce a fresh replica surface is of some importance. The sudden cracks caused by impact or heat have proven unsuitable for the detection of fine structural inhomogeneities by the replica technique. The very slow flexural cracking process is far more suitable. Fig. 4 shows the equipment for producing a fresh flexural crack surface in air or in a high vacuum of 10 -6 T o r r [1]. A high vacuum is an absolute necessity in some cases, because in certain glasses the contact of the fresh fracture surface with air will cause undefined changes. After the fresh fracture surface has been provided, it is vapour-coated with a mixed P t - I R - C layer in a single operation. The evaporation electrode arrangement shown in fig. 5 has definitely advanced the method. A carbon electrode bears a welded-on P t / I R bead,
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225
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which acts as a point source of metal vapour and ensures excellently defined shadows. Depending on the surface profile, the most favourable evaporation angle has been found to be between 30 ° and 60 °. In the past, a replica film had to be evaporated in two operations, i.e. perpendicular carbon evaporation followed by oblique shadowing with metals or WO a. The arrangement described is a considerable inprovement that permits far better image qualities to be attained. This preparation technique is based on the fact that in oblique shadowing at an angle of 30 ° to 60 ° using the described P t - I r - C electrode, the carbon particles deposited from the vapour phase on to the specimen surface are still mobile enough to form a continuous carbon film even in the shadow areas, whereas the Pt and Ir particles (mixed with carbon ones) are deposited only on specimen regions facing the vapour source. The mixed P t - I r - C layer is of extremely fine grain; even in electron transmission, i.e. under possible thermal stress, it shows little tendency towards recrystallization and grain enlargement. Fig. 6 schematically represents the process of generating the replica film. Separation of the film from the glass surface, as a rule, is effected by immersing the specimen in water, acids or alkaline solutions, which diffuse below the film, slightly attacking the glass and thus separating the replica from the glass surface. Where the structural inhomogeneities to be micrographed approach the order of magnitude of the vapour-deposited grains, the acquisition of reliable information from a replica micrograph becomes problematic. Here, the application of individual, razor-blade shaped MoO 3 crystals on the freshly cracked specimen surface prior to evaporation has greatly advanced the technique [1,3]. MoO a crystals have an ideally smooth surface. After evaporation they appear on the micrograph as test surfaces; a comparison of the grain on the specimen
226
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and on the test suface permits a clear decision to be made about the small grains found on the specimen indicate structural inhomogeneities or are just deposited grains. In the latter case, specimen and test surfaces show the same grain. The application of MoO 3 test surfaces in the preparation of replicas makes an additional operation necessary after the replica film has been stripped. The stripped film has to be washed in diluted sodium hydroxide solution in order to dissolve the MoO 3 crystals, which usually stick to the stripped replica film. In the dissolution, sodium molybdate is formed. The electron-optical resolving power for replicas without a test surface is
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Fig. 7. Spark flashover m applying MoO 3 crystals on to the surface of a tellurite glass cuts a "drainage pattern" into the specimen by localized evaporation.
W. Vogel et al. / Electron-microscopical studies of glass
227
Fig. 8. Crystal phase on the surface of a tellurite glass surface, formed by condensed vapour. These crystals bear no relationship to the microstructure of the glass.
about 120-150 A. With a test surface, structural inhomogeneities of about 50 ,~ can be reliably identified. The application of MoO 3 crystals on the freshly fractured surface is effected by briefly exposing the surface to MoO 3 fumes if the specimen is prepared in air. In the case of vacuum preparation, the nose of the cracking lever has to be
Fig. 9. As for fig. 8.
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studded with MoO 3 crystals before the vacuum is generated (see fig. 4, top right). With the lever nose and the specimen surface facing each other and a high tension applied, a spark discharge from the lever nose to the specimen clamp jaws will hurl a sufficient number of MoO 3 crystals on to the glass fracture surface, which can then be vapour-coated as described. In preparing specimens of glasses having some electric conductivity (e.g. tellurite glasses or chalcogenide glasses) it has been found that the spark employed to apply MoO 3 crystals under vacuum will frequently flash over to
Fig. I1. Replica electron micrograph of a tellurite glass with a MoO 3 crystal test surface. The comparison between the grain of the test surface and that of the glass fracture surface enables reliable information to be obtained concerning the microheterogeneous structure of the tellurite glass sample.
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the glass surface rather than to the clamp jaws, giving rise to gross misinterpre. tations of the electron micrograph. Fig. 7 shows traces of such a spark flashover to a freshly fracture surface of tellurite glass. As a result of partial glass evaporation and condensation of the evaporation products, the glass surface is evenly covered with microcrystals which have nothing to do with the structure of the glass sample (see figs. 8 and 9). A wholly reliable preparation technique for electrically conductive glasses has become possible with a modified cracking device (see fig. 10). Here the M o O 3 crystals are applied to the glass surface by means of a additional platium electrode studded with MoO 3 crystals [4]; the spark flashes over from the Pt electrode to the cracking lever. Fig. 11 is a typical example of such a faultless preparation. It is made from the same glass sample as that of figs. 9 and 10. By comparison with the MoO 3 crystal test surface the minute grain on the specimen surface can be unmistakeably identified as structural inhomogeneity. Many optical and engineering glasses have an electrical resistance of 10 t4 tO 1017 ~ cm. They can readily be prepared by the older technique. Glasses having resistance between 10 s and 1012 f~ cm or even below that range should be prepared with the modified cracking device only. 2.3. Surface treatment of glass samples prior to making the replica [5-7] Suitable chemical treatments (etching processes) of a freshly fractured glass surface prior to vapour deposition permit the differential removal of inhomo-
Fig. 12. Replica electron micrograph of a lithium silicate glass (Pt-Ir-C replica technique). The fracture surface produced in a high vacuum reveals microphase separation to the experienced observer only.
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Fig. 13. Replica electron micrograph as in fig. 12. The glass surface was treated with water for 30 s prior to P t - I r - C evaporation. After chemical surface treatment, the droplet-shaped segregation regions are much more conspicuous.
geneities or the surrounding glass matrix because of different chemical solubilities. This will sometimes help to enhance the visibility of structural peculiarities. Sometimes even qualitative or semi-quantitative information can be derived on the composition of microphases. Especially in case of densely
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Fig. 14. SiO2-rich droplets in a barium borosilicate glass, isolated by chemical treatment. Direct electron transmission.
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231
packed droplet regions, e.g., this method penetrates into ranges that are inaccessible to the microprobe. Fig. 12 shows a replica electron micrograph of an unetched Li20-SiO2 glass of microheterogeneous structure. A micrograph of the same glass previously etched with water for one minute shows the microdroplet regions much more distinctly (fig. 13). In some cases, microphases in glass can be entirely isolated by etching and thus examined separately (fig. 14). Eligible etchants are water, CH3OH (for etching glasses of high B203 content), other alcohols, diluted HF, HNO 3, H2SO 4, NH4OH or mixtures thereof. The type and concentration of the etchant and the duration and temperature of etching primarily depend on the glass type to be examined. The most favourable conditions have to be ascertained specifically for each glass family. Glass etching prior to a replica preparation offers great advantages but may equally involve serious risks of misinterpretation of the resulting electron micrographs. In particular, we have to consider the possibility of the formation of low-solubility compounds between etchant and glass constituents. Fig. 15 is an electron micrograph of a PbO-B203-SiO 2 glass etched in 1 n HNO 3 for 1 min. In addition to droplet-shaped segregation regions, the picture shows cubic crystals. This is a typical example of a faulty preparation. The cubic crystals are Pb(NO 3)2 formed by etching with HNO 3 and crystallized on the specimen surface during drying. The black spheres are SiO 2 droplets fully
Fig. 15. Replica electron micrograph of a lead borosilicate glass after I min etching of the fracture surface with 1 n HNO 3. Apart from segregation droplets (bright droplets inside the glass; dark ones have been isolated before and stick to the replica film), the micrograph shows cubic crystals. They result from faulty preparation and bear no relationship to the glass structure.
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Fig. 16. Replica electron micrograph of the same glass as in fig. 15. After chemical etching the specimen has been cleaned with water/alcohol/acetone and cellite foil. It shows true structural inhomogeneities only.
Fig. 17. Etching liquid sucked off the etched glass surface shown in fig, 15 and left to dry on a neutral substrate. Cubic Pb(NO3) 2 crystals as in fig. 15 have formed.
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isolated by etching but adhering to the replica and transmitted by electrons together with it. If the etched glass surface is cleaned, by a sequence of solvents comprising water, alcohol and acetone in addition by applying and stripping a film of cellite, the prepared replica will result in a micrograph as shown in fig. 16, where both the cubic Pb(NO3) 2 crystals and the isolated black droplets have disappeared. That the cubic crystals are, in fact, a secondary effect, is shown in fig. 17. Here, a drop of the etching liquid was sucked off the surface of the PbO-B203-SiO 2 glass and left to dry on a specimen slide. The result show the same cubic Pb(NO3) 2 crystals as in fig. 15. We should also bear in mind the well-defined shadows behind the cubic crystals, which testify to the efficiency of the evaporation and replica techniques described. As a supplementary method to the previously described chemical etching technique we should consider ion etching [8]. Under certain conditions it is a valuable alternative. We should not overlook, however, that fact that the ion beam strongly interacts with the glass surface, so that the hazard of changing the specimen by the formation of artefact structures is particularly great. 2.4. Use of the scanning electron miroscope and the electron-beam microprobe
In about 1970, scanning electron microscopy experienced an extraordinary spreading of its application due to enormous advances in instrumentation. Classical electron-optical examination had primarily relied on replica preparation. The function of the scanning electron microscope and the microprobe are
Fig. 18. Scanning radiograph (BaL,, radiation) of a multiple-segregationbarium borosilicate glass, showing that Ba ions are enriched in the primary, large droplet phase.
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Fig. 19. Scanning electron micrograph of a calcium borate gla.,.s, with an inscribed X-ray intensity curve of the CaK,~ radiation. The electron microprobe beam has been run across the specimen along the white horizontal line. The intensity curve clearly sh.~ws that the droplet phase is rich in Ca.
Fig. 20. Scanning electron micrograph of the "burning track" (cf. horizontal line in fig. 19) of an electron beam in electron beam microanalysis.
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based on the utilization of three other types of signals, i.e. back-scattered electrons, secondary electrons, and characteristic X-rays. These are signals formed by the interaction between the specimen material and electrons irradiated into it, i.e. signals that originate at a certain depth inside the specimen. A replica of a good surface relief (the quality of which greatly depends on the cracking speed) only yields indirect information only about structural inhomogeneities, whereas the scanning electron microscope reveals the structure of the specimen at a certain depth, which is a great advantage and a great step ahead. In addition, the SEM image usually provides a much better impression of depth. Contrast in the SEM is produced either by differences of electron back-scattering properties (material contrast) or by the geometrical texture of the surface (topographic contrast). Scanning electron microscopy has some disadvantages also, which should not be overloooked: (i) one drawback is that the specimen will interact with the radiation by absorbing electrons and is thus subjected to a thermal stress that is incomparably higher than that in the classical replica technique. In certain glasses, this always involves the risk of changing the specimen. In many cases the glass surface has been found fused. (ii) Another drawback is that the SEM has a lower resolving power. Objects have to be greater than 50 ,~ in order to be reliably resolved. (iii) Finally, electrically non-conductive specimens have to be vapour-coated
Fig. 21. Sectional enlargement of fig. 20. Fracture surface across the "burning track". Structural changes in the immediate vicinity caused by thermal stress (electron absorption) are clearly visible.
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with carbon or gold in order to avoid electric charges which would greatly disturb examination. A carbon coat will provide a good material contrast, whereas a gold coat will primarily produce a good topographic contrast. The analysis of the third kind of signals mentioned, the occurrence of X-rays characteristic of a particular element, yields qualitative and quantitative information on the distribution of an element in glass. This is an advancement of extraordinary value. Both the scanning radiograph and the X-ray intensity curve of the natural radiation of a certain element of interest unambiguously indicate the distribution of that element over the microphases of the glass. Figs. 18 and 19 represent typical examples. Problems and uncertainties in using the microprobe arise if the object to be analyzed, e.g. a droplet microphase, is smaller than 3 to 5/~m, constituting only part of the excitation volume, or if the microphases are densely packed. Particular caution is recommened in microprobe studies of relatively unstable glasses. It is necessary to point out that in microprobe studies of glasses the electron beam usually is guided across the specimen at a scanning speed of 2 to 50 ~m per min. Often this causes the glass to melt along the beam track, which leads to substantial changes in the structure and concentration of the specimen. This has been observed especially in glasses containing Na ions. A typical case is shown in figs. 20 and 21. The electron beam guided across the glass specimen in fig. 20 has left a deep trace in which certain parts of the glass are evaporated. Fig. 21 is a scanning micrograph of a glass section across the trace, which clearly shows structural changes in the glass. The glass in the immediate vicinity of the trace is relatively homogeneous. It adjoins a ring of more intensive phase separation. Only the glass below this, i.e. at a greater depth, shows no sign of being influenced by the electron beam. These results have to be specifically considered in all microprobe examinations of glasses. Conditions may vary widely from glass to glass. 2.5. Selected examples of electron-optical glass examination
Fig. 22 is a classical replica micrograph of a segregated glass, showing a good surface relief after a slow flexural cracking process. In addition to the crack-tailed droplets, even the sheared-off droplets are clearly visible. Fig. 23 is a replica micrograph of a barium borosilicate glass which was slightly etched with HNO 3. After multiple segregation, the glass contains six microphases of different composition. Fig. 24 shows a barium borosilicate glass after more intensive etching with 1 n of HNO 3 for 2 s. This replica micrograph reveals two shells around the droplet. The outer one, which is rich in B203, has largely been dissolved by the HNO 3, whereas the inner, SiO2-rich shell has not been affected as such, but shows wrinkles caused secondarily by the etching process. Fig. 25 is a scanning micrograph by secondary electrons, showing a multisegregated turbid glass of very high reflectivity ( > 98%) after HNO 3 etching.
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Fig. 22. Classical replica electron micrograph of an unetched two-phase glass after slow flexural rupture in a high vacuum. Some of the droplets have remained in the glass (convex)~others have been dislodged, leaving a cavity; still others have been torn apart. Fig. 26 is a s c a n n i n g electron micrograph of an u n e t c h e d specimen of a m a c h i n e a b l e phlogopite vitroceram. Fig. 27 shows the same type of vitroceram after weak etching with hydrofluoric acid. The s c a n n i n g micrograph reveals a new c o n f i g u r a t i o n of the lamellar
Fig. 23. Replica electron micrograph of a barium borosilicate glass. After segregation in several steps, 6 glass microphases of different compositions have formed.
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Fig. 24. Replica electron micrograph of a barium borosilicate glass after 2 s etching of the fracture surface with 1 n H N O 3. The droplet is surrounded by an outer B203 shell and an inner SiO 2 shell. The B203 shell has been largely dissolved by the H N O 3, whereas the SiO 2 shell has remaied unaffected.
Fig. 25. Scanning electron micrograph (back-scattered electrons) of a barium borosilicate glass etched with l n H N O 3 for 2 s. The glass shows multiple segregation.
VIL Vogel et a L / Electron-microscopical studies of glass
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Fig. 26. Scaning electron micrograph of a machineable phlogopite vitroceram. The unetched specimen shows the lamellar mica crystals.
Fig. 27. Scanning electron micrograph of an excellently machineable phlogopite vitroceram after weak HF etching. Here, the phlogopite lamellae are arranged in a "head-of-cabbage" structure. The micrograph shows a cross section through such a globular aggregate.
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p h l o g o p i t e crystals, which is the reason for the c o n s i d e r a b l y i m p r o v e d m a c h i n a b i l i t y of this vitroceram.
3. Concluding remarks In the p a s t 25 years, the e l e c t r o n - m i c r o s c o p i c a l techniques of glass e x a m i n a tion and the related p r e p a r a t i o n m e t h o d s have d e v e l o p e d into an e n o r m o u s l y b r o a d spectrum. It is imperative, however, to clearly realize the c a p a b i l i t i e s a n d limitations o f these techniques; otherwise, faulty results a n d m i s i n t e r p r e t a t i o n s are unavoidable.
References [1] W. Vogel, Struktur und Kristallisation der Gl~iser(VEB Deutscher Verlag fiir Grundstoffindustrie, Leipzig, 1965) 1st ed. [2] W. Vogel, Glaschemie (VEB Deutscher Verlag fiir Grundstoffindustrie Leipzig, 1979) 1st ed. [3] W. Skatulla, H. Wessel and W. Vogel, Silikattechnik 9 (1958) 51. [4] L. Horn and H, Reiss, Tagung Elektronenmikroskopie, Dresden (1978) p. 175. [5] W. Vogel, W. Schmidt and L. Horn, Z. Chem. 9 0969) 401. [6] H. Reiss and L. Horn, Tagung Elektronenmikroskopie, Dresden (1978) p. 171. [7] H. Reiss and L. Horn, Vergleichende Untersuchungen zur Struktur eines mikroheterogenen Glases mittels Vakuumbruch, chemisch- und ionengeiitzen Bruchfliichen, 10. Tagung Elektronenmikroskopie, Leipzig ( 198! ). [8] G. Schimmel and W. Vogel, Methodensammlung der Elecktronenmikroscopie (Wiss. Verlagsgesellschaft, Stuttgart) Chs. 2.4.2.1 by H. Bach: Die Anwendbarkeit des Ionenstrahl~itzens bei der Pr~iparation ftir die Elektronenmikroskopie. [9] D. Briimmer, Mikroanalyse mit Elektronen- und Ionensonden (VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1978). [10] G. VOlksch, H. Reiss and L. Horn, Silikanechnik 32 (1981) 52.