Electron microscopic method of glass investigation: Techniques and results

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E L E C T R O N M I C R O S C O P I C M E T H O D OF GLASS I N V E S T I G A T I O N : T E C H N I Q U E S AND R E S U L T S

A REVIEW N. M. VAISFELD State Institute of Glass, B. Semenovskaya 10, Moscow E-23, U.S.S.R.

Received 12 July 1971 In recent years electron microscopic methods of glass investigation have been used to a great advantage by glass research laboratories and institutes. It is at present impossible to imagine a scientific research of glass structure without an electron microscope. In 1965 methods of electron microscopic study of glasses and possibilities of its application were discussed in detail1). After 1965, in the literature in this field we may distinguish two main trends: in a number of works electron microscopy is used as a complementary method, the authors do not attempt to improve or to renew the techniques, they only extend the range of glasses studied by the known replica method. The second trend deals with papers which purpose to enlarge the possibilities of electron microscopic investigation of glass structure using new procedures, improved apparatus, or apparatus which is new in principle.

1. Replica method in the electron microscopic investigation of phase separation and crystallization of glasses Some years ago the procedure itself of obtaining high-quality electron micrographs of glass with the replica method could be a subject of an independent investigation. Therefore, the technique of replica preparation and stripping has been described in detail almost in every paper. At present the detailed description is not obligatory, as in most cases a well-known method of self-shadowed C + Pt, C + Pt + Pd, or C + Pt + Ir replica techniques is used. In this connection, procedure problems are not discussed in the first section of the present review. It is necessary, however, to note a wellknown feature of the replica method, i.e. every new object to be studied belonging to the unexplored system requires certain procedure variations. Even the use of a tested method, sometimes leads to problems dealing with the choice of sample etching and replica stripping conditions to reveal the structure, at the same time excluding details which may be erroneously interpreted.

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Some works will be discussed below, where the replica method was successfully used to investigate phase separation, spinodal decomposition and crystallization processes in glass. In 1965 Tran2), reviewing a number of papers on the structure of binary glasses in the Na20-SiO2 system, pointed out that the problem is still far from being resolved; he carried out an electron microscopic investigation of glass structure as a function of composition and heat treatment. The electron micrographs showed that the phase separation process in these glasses with the same temperature and duration of heat treatment, resembles the related process in sodium borosilicate glasses. The relative volume of the separated phases, being independent of microheterogeneous region sizes, is associated only with glass composition rather than with heat treatment. The matrix phase after the heat treatment does not contain heterogeneous regions of the order of 200/~ that are observed in the original glass. It means that small heterogeneous regions coalesce forming larger ones. Simultaneously performed X-ray phase analysis of glasses containing 9-19.5 mol~ Na20 reveals peaks close to that ofcristobalite as well as non-identified lines which disappear with increasing Na20 content. In glasses with more than 28 mol~ Na20 the lines of sodium silicate and sodium trisilicate appear. The presence of crystallites is evidenced by means of X-ray analysis only. Porai-Koshits et al. 3-5) discuss in detail the problems of primary and secondary phase separation of alkali silicate and alkali borosilicate glasses. Electron microscopic investigation of sodium silicate glasses 4) allowed to establish the presence of secondary separation in the glasses and to support the general nature of such kind of separation. It is suggested that certain sodium borosilicate glasses which in some authors' opinion have three-phase structure, are in fact two-phase systems, and small droplets interpreted as a third phase are heterogeneous regions of the secondary nature as in binary sodium silicate glasses. Averjanov 6) supported the concept about chemically heterogeneous structure of sodium silicate and lithium silicate glasses and obtained certain quantitative data about precipitated phases: their shape, size, relative volume and approximate chemical composition. During heat treatment, continuous growth of region sizes occurs, and with changing glass composition, the volume ratio of regions enriched in alkali to regions enriched in silica varies. At the same time phase conversion takes place: a droplet phase becomes a matrix one and vice versa. All the properties of heterogeneous regions are accounted for by their separation nature, in contrast to Vogel 7), who considered lithium silicate glass separation as a preliminary stage of crystallization process. Investigating the structure of binary alkali sicate glasses, Charles s) relates

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the shape and particle sizes of the precipitated phase with glass properties; in particular, with electr0c0nductivity attempting to find out, on this basis, the nature of the mixed alkali effect. The examination of the separation process in low alkali borosilicate glasses containing alkali-earth oxides and AI2039), proved the existence of two types of phase separation patterns in glasses with RO oxides and various A1203 contents. When A1203 is absent, or when its content amounts up to 2~, the heterogeneities have a labyrinthine shape. When the AI203 content varies from 3 to 5-7~o, the heterogeneous regions have the shape of close droplets. The phenomena observed were accounted for by the interaction of Na20 and A1203, which led to the mutual suppression of the homogenizing effect of these oxides. Later, the structural peculiarities of these glasses have been related to their properties 10). Certain quantitative data were obtained in connection with electron microscopic investigations of the glassy phase, enriched in SIO2, in separated silicate glasses 11). The nucleus formation time curves as a function of temperature are in agreement with theoretical conclusions. The study of the influence of that treatment on glass structure in the Na20-CaO-SiO 2 system 12) evidenced a droplet size growth, the droplets having a typical shape observed earlier in many borosilicate compositions. As a rule, the fracture surface propagates through the droplets, and they lose their sphericity as their size grows. In a number of glasses in the PbO-B203 system 13) the sizes of the spherical particles were measured and their number was counted. The obtained values of the average radii of the particles and of the distribution curves were in good agreement with small angle X-ray scattering data. In glasses of the Li20-SiO 2 system, or in ternary glasses in the Li20Na20-SiO 2 and Li20-K20-SiO 2 systems, the substitution of Li20 by Na20 or K20 increases their tendency to separation 14). The authors believe that in the glass of 10Li20.10Na20- 80SiO 2 composition a spinodal decomposition occurs. With increasing duration of heat treatment, growth of the heterogeneous region sizes occurs, the volume of separated phases remaining the same. The sizes of the observed particles are 200 A and more. Using light scattering data, the sizes of the separation re'~ions in borosilicate glasses were estimated 15). These values are in good agreement with electron microscopic data for the region sizes ranging from 500 to 2000 A. A sufficiently good agreement between results obtained by two different methods allows us to extrapolate the results to smaller separation regions (less than 200/~), which cannot be studied by the replica method. In papers dealing with metastable immiscibility of glasses in the B203-SIO 2 and Na20-B203-SiO 2 systems 16,17), large size and unusual droplet shape

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of the precipitated phase after heat treatment at 600 °C during tens of hours was stated. The droplets are 1-21am in size and have a flat rather than a spherical shape. The phenomenon is not yet explained in the articles concerned. The authors emphasize that the glass structure can be investigated by the replica method when the sizes of the structural units are not less than 0.02 I~m. The unusual flat shape of the droplets, observed by the authors, can be explained by the nature of the fracture (a cleavage surface runs within the droplets). Heterogeneity sizes in glasses of the SiO2-A1203-TiO2-PbO system is) have been studied by two independent methods, i.e. by electron microscopy and by small angle X-ray scattering. The obtained results are in good quantitative agreement. Electron microscopic investigations made a great contribution to the theory of spinodal decomposition of glasses. It was Charles 19) who first supposed a connection between the continuous labyrinthine structure of the borosilicate glasses, revealed by means of electron microscope, and the spinodal phase decomposition. In this case, an initial phase separation may be due to the high concentration of the heterogeneities which grow uniformly, and which simultaneously link with one another to form a continuous structure. In this case there is no sharp boundary between heterogeneous regons of defined composition and matrices. The existence of a continuous network structure is also supported by electroconductivity changes. The mechanism of spinodal decomposition was also used to explain the structure observed in glasses of the AI203-SiO 2 system2°). Initially, the decomposition is described by infinitesimal wavy composition fluctuations which grow in amplitude until the formation of the phase boundary is complete. According to Charles 19), characteristic of such a spinodal decomposition mechanism is that the "ripening" stage, when phase boundaries are being established through an increase in composition gradient, must, at least at an early stage, involve a continuity of the two forming phases. Investigations, however, indicate that a continuous interconnected structure can convert into a structure with isolated spheres 21), thus connectivity itself is not a spinodal decomposition criterion. Such a connectivity usually occurs at equal volumes of co-existing phases, as was stated in the above mentioned paper6). In ref. 22, however, where the electron microscopic method has been used to great advantage to obtain immiscibility cupola in the SiO2-A1203-MgO system, the interconnectivity is observed in glasses with 17.5~o MgO where the conditions of equal phase volumes are not satisfied. For compositions with 25~o and 32.5~ MgO at 1000°C lying near the volume phase ratio 50:50, the structural connectivity is not detected. The authors

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believe that a more important criterion for the interconnectivity of structure is a temperature viscosity curve and related diffusion coefficient, rather than equal volumes of coexistent phases. The recent report by Ohlberg and Golob 23) somewhat differs from the above-mentioned works dealing with the nature of the heterogeneity in glasses studied by the electron microscopic method. The authors state the existing discrepancies in defining the heterogeneous region sizes, as well as in interpreting their nature, and that is why they so thoroughly describe the experimental procedure which allowed them to reveal microheterogeneities in a variety of complex silicate glasses, such as commercial plate, sheet and ophthalmic glasses. All the glasses examined are characterized by an undulating structure, the period of "waviness" being of the order of 300-500 A. A low contrast between peak and valley on the micrographs represents low compositional fluctuations of corresponding heterogeneous regions. Reliable experimental data prove that the observed wavy changes of the image density are not artifacts, but reflect the heterogeneous nature of glass. The authors believe that the same chemical forces which lead to phase separation in a simple soda-lime-silica glass are responsible for cluster formation in the more complex silicate glasses. It is not necessary to discuss in detail papers dealing with the investigation of glass crystallization processes. On the whole they are similar, both from the point of view of the problem and the procedure, but even their brief enumeration allows to indicate an extensive range of the systems studied and of the modifiers used. For the BaO-SiO2 glasses 24) in the barium disilicate region, it is shown that metastable phase separation in the glassy phase is an important stage in the nucleation process. The addition of surface-active agents to glasses in the CaO-MgO-A120 3SiO2-TiO2 and MgO-A1203-SiO2-TiO2 systems 25) made it possible to identify and to examine more thoroughly separate stages prior to crystallization. The authors succeeded in indicating that the final structure of glass-ceramics* is determined by process rates at the stages prior to crystallization and does not depend on the nature of the addition. The glass necessarily passes all the stages of the crystallization process without missing any of them and the role of the addition is to shift the temperature of onset and termination of every stage. A number of papers deals with glass crystallization in the lithium alumosilicate system with and without TiO 226-2s), as well as with the role of ZnO in phase separation and crystallization of glasses in the Li20-ZnO-SiO 229) and ZnO-AI203-SiO 230, 31) systems. Synthesis of glass-crystalline materials * I n t h e U.S.S.R. glass-ceramics is usually called "sitall".

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with homogeneous and fine-grained structure and high mechanical strength in the CaO-MgO-A1203-SiO 2 system with TiO2 a2) and F23a) was also studied by means of electron microscopy. Discussing papers on electron microscopy of glasses published during the last five years, it is necessary to emphasize a well-defined tendency for more profound research of the phase separation and crystallization reasons. Only a small part of papers is devoted to the manufacture problems of glasscrystalline materials or to glass surface treatment34-36). Lately there is a growing tendency to study early stages of phase separation which has been impossible some years ago. Such works are characterized by complex investigation, i.e. application of other physical structural methods - X-ray analysis, optical methods, including infra-red spectroscopy, DTA - both in combination with electron microscopy, including elements of quantitative electron microscopy, and in conjunction with the investigation of glass properties. Moreover, the development of modern electron microscopic instruments made the direct electron microscopic observation of appropriately prepared thin glass samples (direct methods) more approachable. The direct methods are often used simultaneously with the replica method, and in a number of papers the former are the main methods of investigation. They will be described in the next section of the present review.

2. Direct methods in the electron microscopic investigation of glass In addition to the afore-mentioned investigations of structure in various glass systems using the replica technique, there appears a number of papers, attempting to resolve by electron microscopic method the general problems af glass structure and of the nature of the vitreous state. This becomes possible, due to improved electron microscopes with resolving power better than 5 A. For the realization of such a resolving power, however, elaboration of new methods of specimen preparation, suitable for direct examination in the electron beam, is required. Methods of glass specimen thinning by blowing, polishing, etching etc. were partly refined lately, but they did not undergo radical changes. Therefore the absence of qualitatively new results in this field is quite natural. Works may be mentioned in which direct methods are used in combination with the replica method 7,14). As in earlier worksZ7), the advantages of transmission methods are pointed out, but the authors do not suggest new information in comparison with data obtained by the replica method. As a rule there is a rather detailed description of preparation technique in these papers. A further thinning of glass blown films of 100-200 ~tm thickness is achieved

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by etching in a solution of 5~ HF-27oHC1-93~ H203s), the specimen being rinsed by the solution using a special pipette, and the thickness being controlled by interference colours. Mechanical thinning of a sample by polishing until the first holes occur, made it possible to study a variety of glasses such as fused silica, sodium borosilicate glasses and certain binary silicate glasses 39,40). The authors identify heterogeneous regions in borosilicate glasses with cesium, rubidium and potassium, whereas noncrystalline heterogeneous regions with sizes of 50-150/~ are revealed in glasses containing sodium and lithium. Phase separation with shape transition from interconnected channels to spherical droplets, and subsequent phase conversion, are revealed in binary bariumsilicate glasses with various BaO contents. In the latter two works, it is pointed out that etching was not used to avoid distortions of the original simple structure, meanwhile the polishing to thicknesses of hundreds of angstrSms involves, of coarse, an extended water attack, which for the alkali phase in alkali borosilicate glasses acts as an etchant with particular effectiveness for such small thicknesses. Therefore a number of conclusions in these papers may be questioned. No wonder that Carrier opposes these conclusions concerning interpretation of micrographs 41) and suggests to use a dry method of observing of edge parts of fine powder particles. However, this method can lead to controversial interpretation of the results too, as contrast on micrographs may be based on both the two-phase structure of the sample, and its thickness variations. While a number of authors caution the investigators against controversial interpretation of experimental results in connection with etching, others 14) advise to etch a sample up to its thinning, considering the direct method being more reliable though more laborious than the replica method. A dry preparation technique, involving the crushing of small pieces of a sample between two microscope slides, allowed to research the structure of quartz and zircon crystals4Z). The method of observing thin edges of glass wedges was used by a number of investigators41,as); in particular its advantages were repeatedly pointed out by Vogel. Mechanical and chemical thinning of glass plates in the Li20-SiO 2 system, previously heat-treated to reveal crystallization centres, in combination with new high-voltage electron microscopes with accelerating voltages of 200 and 600 kV enabled to perform a correct quantitative investigation of particle sizes and their concentration in the glass at early stages of crystallization, as well as to plot particle size distribution curves, and to obtain volume percentages of the precipitated phase44). Excellent stereo pairs of electron micrographs enabled the authors with great certainty to estimate the particle arrangement within the sample and the nature of their distribution as well as the average thickness of the sample.

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In the Soviet Union direct methods of glass structure investigation are being successfully used by Alejnikov et al., who lately improved the technique of dry ultra-thin section preparation 45-48). Reviewing different techniques of preparation of thin glass samples, we must refer to a recent report 49) in which Seward and Uhlmann put under a doubt some conclusions about glass structure based on direct electron microscopic observations made by a number of authors. Studying the reasons of the "wavy" structure of fused silica in the range 50-100 A revealed by direct electron microscopic investigation, it is concluded that the observed heterogeneities are associated with surface diffraction effects and do not reflect the true structure of the bulk glass. It is indicated that the heterogeneities are only sometimes seen and are found to depend on the technique of sample preparation, and are not changed by extended heat treatment of the bulk glass in its glass transition region. It is shown that the variations in density and internal potential, needed to explain the heterogeneities, far exceed (by two or three orders) those expected from fluctuation theory or observed experimentally in small angle X-ray scattering studies. Thus the "waviness" observed cannot be taken as a structural characteristic of the bulk glass, but must be considered as a typical artifact. As usual, new ideas in glass investigation by direct electron microscopic methods favoured the development of preparation techniques and as a consequence a large number of papers in this field has been published. But the results did not justify hope, as clear, unquestionable and, chiefly, new information about glass structure has not yet been obtained. Direct transmission micrographs of glass of any composition represent closed or interconnected microheterogeneities, the combination of shadowed and enlighted parts of image determining by phase transparency degree in an electron beam. Phase identification and thus micrograph interpretation cannot be considered as unquestionable because of inevitable and unforeseen changes which the glass sample undergoes during preparation. In the recent two to three years scanning electron microscopes are used for glass investigation. Their main advantage is that there is nearly no need for special sample preparation (for glass in addition to usual preliminary etching it is necessary to deposit a gold or a carbon film to remove static charge). Distortions arising due to electron beam effects on the sample surface are considerably weaker than in transmission electron microscopic study of the sample 50). Scanning microscopes are characterized by a great depth of focus and therefore they can be used to study such porous samples as certain ceramics 51), glass-gypsum composites 52), BeO whiskers appearing on Be droplets 53), etc. After the first publications concerning application of a scanning

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electron microscope for studying glass and glass-ceramic surfaces 54), there were published more extensive works dealing with phase separation in borosilicate glasses 5s, 56), where the porous leached samples were examined. It follows from the micrographs that a porous silica-rich skeleton is a continuous three-dimensional network, the pore sizes depending on the glass composition and heat treatment. These results are in good agreement with known data obtained by the classical replica method and earlier by small angle X-ray scattering and absorption methods 57, 58). Unfortunately the resolution of a scanning electron microscopic method is as low as 250-300 A, thus its application to glass structure research is expedient only in combination with the usual electron microscopic methods with a resolution of the order of 25 A. Electron microscopy has limited possibilities in identification of observable particles; the identification of their chemical and phase composition is possible using certain indirect data (particle morphology, their behaviour under etching etc). The application of electron probe X-ray microanalysers would to some extent solve this problem; however, these instruments are at present suitable only to examine rather large defects in glasses, impurities, and crystals in glass near refractory59-62). Because of low resolving power, qualitative analysis is possible for inclusions of more than 0.05 mm in size with higher atomic number than silicon has. That is why the most attractive idea of identification of crystalline phases in glass-ceramics, and determination of separated phase compositions in glasses by means of X-ray microanalysers, is at present impracticable. 3. Conclusion

In conclusion it is necessary to distinguish problems which can be solved by the replica method and direct electron microscopic research of glass structure. The replica method is a powerful tool which facilitates glasstechnologists in their every-day work to create new glasses and materials with specified properties. Meanwhile direct methods are advantageous in realizing general problems of vitreous state, revealing the reasons of phase separation and crystallization processes, and for such an important purpose, difficulties arising during sample thinning are not considered as great disadvantage. If the further application of the replica method serves to enlarge the number of glasses studied by electron microscopy, the new methods such as direct examination of glass in transmission electron microscope, scanning electron microscopy and X-ray microanalysing contribute to the extension of our knowledge of the vitreous state. However, the modern state of sample

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p r e p a r a t i o n for direct research is so imperfect t h a t vast possibilities o f new i n s t r u m e n t s with high resolving p o w e r c a n n o t be used to the full extent a n d the m a i n a d v a n t a g e o f the electron m i c r o s c o p i c m e t h o d , i.e. its obviousness, c a n n o t be realized. T h u s the m a i n p r o b l e m which the scientists face when d e a l i n g with direct m e t h o d s o f the study o f glass structure, consists in develo p m e n t o f reliable m e t h o d s o f s a m p l e t h i n n i n g to thicknesses c o m p a r a b l e with the d i m e n s i o n s o f the glass structural units. Besides, the electron microscopic m e t h o d like o t h e r m e t h o d s o f structure e x a m i n a t i o n has certain l i m i t a t i o n s a n d to o b t a i n t h o r o u g h a n d reliable i n f o r m a t i o n , c o m p l e x invest i g a t i o n s are required.

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