CHAPTER 2 2. COMPOSITION, STRUCTURE AND PROPERTIES OF INORGANIC AND ORGANIC The scientific investigation of composition and properties of inorganic glass started in the last century. By comparison, the development of organic glass is just in its early stages. The question of the structure of glass led to a discussion of whether it exists in a microcrystalline or in an amorphous state From the thermodynamic point of view, all condensed substances at zero temperature in equilibrium conditions should be crystal- line. There are, however, also noncrystalline solids in a metastable state. Relaxation times for crystallisation of these solids are extremely long and they therefore remain amorphous in practice. Crystals are rare and their structures are limited in number and can be reduced to only 14 Bravais lattices. The number of possible non-crystalline structural arrangements, however, is infinite. This diversity of the positions of atoms and molecules does not affect their thermodynamic and transport properties. On the whole, disordered structures are macroscopically presented as homogeneous and iso- tropic media. It is generally assumed today that glass belongs to the predominant non- crystalline solids 2.1 GLASS-FORMING INORGANIC MATERIALS The glassy state is known in some elements, notably selenium and tellurium. Sele nium also forms glassy mixtures with phosphorus. There are also some semiconductive glasses of compounds like AS2, Se3 A number of salts may exist as glass. The best known is BeF 2. Complex types of glass have been prepared containing BeF2 together with NaF, KF, LiF, CaF2, Mg F2 and AlF3. Some nitrates(Na, K), sulfates and chlo- rides have been obtained as small glassy droplets by spraying the molten material onto cold plates. More details on such glasses can be found in ref [1]. Most of these types of glass, however, have little technical significance. This is not true of the glassy metals Amorphous alloys or metallic glass containing two or three components such as PdSi FeB, TiNi, NiPB etc., are new materials that have interesting mechanical and magnetic properties that are highly desirable in modern technical applications. They are produced in the form of strips by quenching the melt extremely rapidly [2]. The most important material termed as glass is formed, however, by oxides [1, 3, 4] The central difference between metallic, semiconductive and oxide glasses lies in the relative strengths of their chemical bonds as measured by the energy gap between occupied and unoccupied electronic states. In oxide glasses this gap is more than 5 electron volts, that is, it lies in the vacuum ultraviolet so that such glasses are transpar-
CHAPTER 2 2. COMPOSITION, STRUCTURE AND PROPERTIES OF INORGANIC AND ORGANIC GLASSES The scientific investigation of composition and properties of inorganic glass started in the last century. By comparison, the development of organic glass is just in its early stages. The question of the structure of glass led to a discussion of whether it exists in a microcrystalline or in an amorphous state. From the thermodynamic point of view, all condensed substances at zero temperature in equilibrium conditions should be crystalline. There are, however, also noncrystalline solids in a metastable state. Relaxation times for crystallisation of these solids are extremely long and they therefore remain amorphous in practice. Crystals are rare and their structures are limited in number and can be reduced to only 14 Bravais lattices. The number of possible non-crystalline structural arrangements, however, is infinite. This diversity of the positions of atoms and molecules does not affect their thermodynamic and transport properties. On the whole, disordered structures are macroscopically presented as homogeneous and isotropic media. It is generally assumed today that glass belongs to the predominant noncrystalline solids. 2.1 GLASS-FORMING INORGANIC MATERIALS The glassy state is known in some elements, notably selenium and tellurium. Selenium also forms glassy mixtures with phosphorus. There are also some semiconductive glasses of compounds like As2. Se3 A number of salts may exist as glass. The best known is BeF2. Complex types of glass have been prepared containing BeF2 together with NaF, KF, LiF, CaF2, MgF2 and A1F3. Some nitrates (Na, K), sulfates and chlorides have been obtained as small glassy droplets by spraying the molten material onto cold plates. More details on such glasses can be found in ref. [ 1 ]. Most of these types of glass, however, have little technical significance. This is not true of the glassy metals. Amorphous alloys or metallic glass containing two or three components such as PdSi, FeB, TiNi, NiPB etc., are new materials that have interesting mechanical and magnetic properties that are highly desirable in modern technical applications. They are produced in the form of strips by quenching the melt extremely rapidly [2]. The most important material termed as glass is formed, however, by oxides [1,3,4]. The central difference between metallic, semiconductive and oxide glasses lies in the relative strengths of their chemical bonds as measured by the energy gap between occupied and unoccupied electronic states. In oxide glasses this gap is more than 5 electron volts, that is, it lies in the vacuum ultraviolet so that such glasses are transpar-
ent and colourless, apart from impurities. The semiconductive glasses have energy gaps near 1, 5 ev and are coloured yellow or red, while in metallic glasses the energy gap is Typical and possible glass-forming oxides are listed in table 1 TABLE 1 GLASS-FORMING OXIDES Typical glass formers 02 Bi20 ZrO V Silica is the constituent material in technical glass. Commercial glass is almost exclusively silicate glass Oxides that apparently do not form glass but may be included in glass to obtain special properties such as chemical durability, low electrical conduc tivity, high refractive index and dispersion, increase in hardness and melting point, etc are listed in Table 2. To obtain glass that transmits infrared, some special components such as AS2S3 and TeO2, are used [51 TABLE 2 GLASS-PROPERTY-MODIF YING OXIDES ALO Pb,o Sro Cdo In term of bond strength between the cation and the oxygen all glass formers have values greater than 5 eV and the typical modifiers have lower values in the range of about 2.8eV[6] 2.1.1 CRYSTALLITE THEORY When coolit from the melting point, many materials pass through a ter perature range in which the liquid becomes unstable with respect to one or more crys- talline compounds. An increase in viscosity, however, may partially or completely prevent the discontinuous change into the crystalline phase. Studies of refractive index changes by heat treatment and investigations of other physical properties of glass led Lebedev [1, 4, 7] to the conclusion that glass contains ordered zones of small crystallites
ent and colourless, apart from impurities. The semiconductive glasses have energy gaps near 1,5 eV and are coloured yellow or red, while in metallic glasses the energy gap is zero. Typical and possible glass-forming oxides are listed in Table 1. TABLE 1 GLASS-FORMING OXIDES Typical glass formers: B203 SiO2 P205 As203 GeO2 As203 Sb203 Possible glass formers: Bi203 ZrO2 V205 Silica is the constituent material in technical glass. Commercial glass is almost exclusively silicate glass. Oxides that apparently do not form glass but may be included in glass to obtain special properties such as chemical durability, low electrical conductivity, high refractive index and dispersion, increase in hardness and melting point, etc. are listed in Table 2. To obtain glass that transmits infrared, some special components such as As2S3 and TeO2, are used [5]. TABLE 2 GLASS-PROPERTY-MODIFYING OXIDES Na20 ZnO A1203 SnO2 K20 BeO La203 TiO2 Pb20 PbO Y203 ThO2 Cs20 MgO In203 CaO CrO SrO BaO CdO In term of bond strength between the cation and the oxygen all glass formers have values greater than 5 eV and the typical modifiers have lower values in the range of about 2.8 eV [6]. 2.1.1 CRYSTALLITE THEORY When cooling down from the melting point, many materials pass through a temperature range in which the liquid becomes unstable with respect to one or more crystalline compounds. An increase in viscosity, however, may partially or completely prevent the discontinuous change into the crystalline phase. Studies of refractive index changes by heat treatment and investigations of other physical properties of glass led Lebedev [ 1,4,7] to the conclusion that glass contains ordered zones of small crystallites
In the crystallite hypothesis, it is assumed that glass may contain both amorphous and crystalline zones which are linked by an intermediate formation. These remarkably small crystallites of about 10 A in size consisting of 3-6 atoms are assumed to be of irregular form with distortions in their lattice. Unfortunately, no stringent experimental evidence can be found to support this hypothesis because even X-ray and electron diffraction structural analysis is unable to detect the possible existence of crystals in the range of about 10A. 2.1.2 RANDOM NETWORK THEORY Extensive X-ray structural analyses of glass as well as studies of the melting pro allowed Zachariasen [8]to explain glass as an extended molecular network without ymmetry and periodicity. The glass-forming cations such as Si and B are sur- rounded by oxygen ions arranged in the shape of tetrahedra or triangles. Regarding the oxygen ions, a distinction must be made between bridging and non-bridging ions. In the first case, two polyhedra are linked together over an oxygen ion, and in the second case the oxygen ion belongs only to one polyhedron and has one remaining negative charge In this way, a polymer structure consisting of long chains crosslinked at intervals is produced. The unbalanced negative charge is compensated by low charge and large size cations, e.g. Na, K, Ca, Ba located in the holes between the oxygen polyhedra Substitution of silicon ions in the network by other large charge and small size cations is possible. The network theory was supported by further X-ray investigations by War ren in 1933 and 1937[9a], and in investigations by other scientists. In Fig. 1, a two- dimensional drawing shows the crystalline state of SiO2(a), the glass network of SiO2 (b)and the glass network of a sodium silicate glass(c) Structure of crystallised silica(A), of fused silica(B)and of sodium silicate glass(C) More recent investigations of chalcogenide glasses such as As2Se3 [9b] and also of oxide glass [9b] in transmission electron microscopy suggest that there exist structural domains, large macromolecules or clusters which are, in the case of silica and other oxide glasses, between 60 and 100 A in diameter [9c]. The domain structure in oxide
In the crystallite hypothesis, it is assumed that glass may contain both amorphous and crystalline zones which are linked by an intermediate formation. These remarkably small crystallites of about 10 A in size consisting of 3-6 atoms are assumed to be of irregular form with distortions in their lattice. Unfortunately, no stringent experimental evidence can be found to support this hypothesis because even X-ray and electrondiffraction structural analysis is unable to detect the possible existence of crystals in the range of about 10/~. 2.1.2 RANDOM NETWORK THEORY Extensive X-ray structural analyses of glass as well as studies of the melting process allowed Zachariasen [8] to explain glass as an extended molecular network without Si 4+ B 3+ symmetry and periodicity. The glass-forming cations such as and are surrounded by oxygen ions arranged in the shape of tetrahedra or triangles. Regarding the oxygen ions, a distinction must be made between bridging and non-bridging ions. In the first case, two polyhedra are linked together over an oxygen ion, and in the second case the oxygen ion belongs only to one polyhedron and has one remaining negative charge. In this way, a polymer structure consisting of long chains crosslinked at intervals is produced. The unbalanced negative charge is compensated by low charge and large size cations, e. g. Na +, K +, Ca ++, Ba ++ located in the holes between the oxygen polyhedra. Substitution of silicon ions in the network by other large charge and small size cations is possible. The network theory was supported by further X-ray investigations by Warren in 1933 and 1937 [9a], and in investigations by other scientists. In Fig. 1, a twodimensional drawing shows the crystalline state of SiO2 (a), the glass network of SiO2 (b) and the glass network of a sodium silicate glass (c). 9 Si4 § 9 0 2. ~ Na § A B C Fig. 1 Structure of crystallised silica (A), of fused silica (B) and of sodium silicate glass (C). More recent investigations of chalcogenide glasses such as As2Se3 [9b] and also of oxide glass [9b] in transmission electron microscopy suggest that there exist structural domains, large macromolecules or clusters which are, in the case of silica and other oxide glasses, between 60 and 100/~ in diameter [9c]. The domain structure in oxide
glass is difficult to observe because of the possible polymerisation of the domain inter faces by ambient moisture In this sense, glass can be viewed as an assembly of subunits [10] which is not in pposition with the random network theory Zachariasen [8] has carefully distinguished between random orientation on a local level and cluster formation on a lager scale. The idea of continuous large scale random networks is thus merely a considerable oversimplification of his ideas. Today the network theory is generally accepted for ordinary glass. It appears, however, that some complex multicomponent types of glass may also consist to some extent of very small ordered zones in an amorphous network matrix. This is especially true after heat treatment, which can induce phase separation and crystallisation 2.1.3 PHASE SEPARATION DEVITRIFICATION Glassy materials can be considered as frozen-in liquids, which consist, in the case of xidi materials, of polymer chains with branches and cross linkages. With the excep- tion of quartz glass, all types of industrial glass are multicomponent systems. The fact that glass is a multicomponent material leads, however, to the formation of very com plicated structures. These are characterised by the presence of glass-former skeletons of Intensity curves of X-ray scattering of sodium silicate glass in various states according to Valenkov and Porai-Koshits [Il] a)Original glass, b) glass annealed for 2 hours at 420.C various shapes and also by a varied form of microheterogeneity. a variable short-range order in the distribution of ions and atoms exists, however, inside the microregions of the chemical and structural heterogeneities. The microheterogeneous structure of glass was discovered and studied first for two-component glass by Valenkov and Porai- Koshits [11]. They found that the X-ray diffraction pattern of sodium silicate glass depended on the thermal treatment of the sample, as seen in Fig. 2
10 glass is difficult to observe because of the possible polymerisation of the domain interfaces by ambient moisture. In this sense, glass can be viewed as an assembly of subunits [10] which is not in opposition with the random network theory. Zachariasen [8] has carefully distinguished between random orientation on a local level and cluster formation on a lager scale. The idea of continuous large scale random networks is thus merely a considerable oversimplification of his ideas. Today the network theory is generally accepted for ordinary glass. It appears, however, that some complex multicomponent types of glass may also consist to some extent of very small ordered zones in an amorphous network matrix. This is especially true after heat treatment, which can induce phase separation and crystallisation. 2.1.3 PHASE SEPARATION, DEVITRIFICATION Glassy materials can be considered as frozen-in liquids, which consist, in the case of oxidic materials, of polymer chains with branches and cross linkages. With the exception of quartz glass, all types of industrial glass are multicomponent systems. The fact that glass is a multicomponent material leads, however, to the formation of very complicated structures. These are characterised by the presence of glass-former skeletons of a) 200 100, o b) 2OO .8- 100' _.= O. c) lOOO 600 2o0 o ~ .... Fig. 2 Intensity curves of X-ray scattering of sodium silicate glass in various states according to Valenkov and Porai-Koshits [11] a) Original glass, b) glass annealed for 2 hours at 420~ c) glass after devitrification 0,05 0,15 0,25 0,35 sin 19 various shapes and also by a varied form of" microheterogeneity. A variable short-range order in the distribution of ions and atoms exists, however, inside the microregions of" the chemical and structural heterogeneities. The microheterogeneous structure of glass was discovered and studied first for two-component glass by Valenkov and PoraiKoshits [11]. They found that the X-ray diffraction pattern of sodium silicate glass depended on the thermal treatment of" the sample, as seen in Fig. 2
he Interpretation of the diffraction patterns showed a clear deviation from the Zacharlasen-Warren concept, according to which the Na ions have a random distribu- tion in the holes between the en ions of the disorderly continuous silica network The pattern indicated a micro-heterogeneous structure [12], that consisted of microre- gions with a sodium metasilicate composition embedded in the glassy silica structure Similar results were also obtained with binary borosilicate glass, three-component sodium borosilicate glass [13] and other types of glass Phase separation in certain optically clear types of glass was also indicated in elec- tron-optical investigations. The phase separation that occurs in some glass, however, does not provide evidence for the crystallite theory. In ma nuclei and crystallites can be found which appear as the result of imperfections in the production technology or of the subsequent devitrification process. If, during the working or annealing processes, the glass is held too long in the temperature region in which crystallisation takes place most readily, it will devitrify and be destroyed. De vitrification is the main factor which limits the composition range of practical types of glass. It is an ever-present danger in all glass manufacture and working. The devitrifi- cation in ordinary glass takes place chiefly on the glass surface [14-17]and manifests itself in different ways: from almost indiscernible microcrystals to a fully developed crystallisation. It appears, however, that devitrification does not always start on a sur- face; it seems to be much more dependent on surface pre-treatment 2.1.4 GLASS-FORMING ORGANIC MATERIALS Organic glass or transparent plastics are synthetic solid materials consisting of polymer compounds that are formed mainly by the elements C, H, O and N. The poly- mer macromolecules are obtained by polymerisation, polycondensation or polyaddition reactions between monomers [18] We distinguish here between thermosets, which undergo a destructive chemical change upon application of heat, and thermoplastics, which can be resoftened repeat edly without any change in chemical composition Thermoplastics are generally pre- ferred for optical applications [19, 20]. Like inorganic glass, they have no fixed melting point but rather a softening region. The plastics can be made fluid and shaped by the application of heat and pr Plastic is increasingly used as a substitute for inorganic glass and other materials. It is, however, often necessary to retain the appearance of the substituted material by special surface treatments. High-impact strength organic polymers have a rapidly ex panding market in applications as diverse as ophthalmic lenses, architectural glass, electronic equipment packaging, various shaped form parts in automotive industry and in the form of foils as substrates for various types of thin films
11 The Interpretation of the diffraction patterns showed a clear deviation from the Zacharlasen-Warren concept, according to which the Na + ions have a random distribution in the holes between the oxygen ions of the disorderly continuous silica network. The pattern indicated a micro-heterogeneous structure [12], that consisted of microregions with a sodium metasilicate composition embedded in the glassy silica structure. Similar results were also obtained with binary borosilicate glass, three-component sodium borosilicate glass [ 13] and other types of glass. Phase separation in certain optically clear types of glass was also indicated in electron-optical investigations. The phase separation that occurs in some glass, however, does not provide evidence for the crystallite theory. In many types of glass, crystalline nuclei and crystallites can be found which appear as the result of imperfections in the production technology or of the subsequent devitrification process. If, during the working or annealing processes, the glass is held too long in the temperature region in which crystallisation takes place most readily, it will devitrify and be destroyed. Devitrification is the main factor which limits the composition range of practical types of glass. It is an ever-present danger in all glass manufacture and working. The devitrification in ordinary glass takes place chiefly on the glass surface [ 14-17] and manifests itself in different ways: from almost indiscernible microcrystals to a fully developed crystallisation. It appears, however, that devitrification does not always start on a surface; it seems to be much more dependent on surface pre-treatment. 2.1.4 GLASS-FORMING ORGANIC MATERIALS Organic glass or transparent plastics are synthetic solid materials consisting of polymer compounds that are formed mainly by the elements C, H, O and N. The polymer macromolecules are obtained by polymerisation, polycondensation or polyaddition reactions between monomers [ 18]. We distinguish here between thermosets, which undergo a destructive chemical change upon application of heat, and thermoplastics, which can be resoftened repeatedly without any change in chemical composition Thermoplastics are generally preferred for optical applications [ 19,20].Like inorganic glass, they have no fixed melting point but rather a softening region. The plastics can be made fluid and shaped by the application of heat and pressure. Plastic is increasingly used as a substitute for inorganic glass and other materials. It is, however, often necessary to retain the appearance of the substituted material by special surface treatments. High-impact strength organic polymers have a rapidly expanding market in applications as diverse as ophthalmic lenses, architectural glass, electronic equipment packaging, various shaped form parts in automotive industry and in the form of foils as substrates for various types of thin films