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Epitome
Journal of Non-Crystalline Solids 71 (1985) 1-4 North-Holland, Amsterdam
1
EPITOME Norbert K R E I D L Santa Fe, New Mexico, USA
1. Introduction This Conference may well serve as a unique, if not the first, comprehensive assessment of the effects of modes of formation on the structure and properties of glasses with an emphasis on what happens above the glass transition range. Obviously, the latter effects cannot be separated from, indeed they interact with, events below the upper limit of the glass transition, the elucidation of which has been the subject of numerous and intensive investigations for several decades. Some consideration, therefore, had to be given to the glass transition at the start of the Conference. Today, most people are aware of the need to use standards or report details. It is almost half a century since Roetger appealed to the glass communi D, to do that. Pioneers like Lillie, Tool and others provided the proper scientific basis. At the present meeting, Cooper * offered a nice tribute to Liilie in calling the Lillie number the one-number parameter relating the state of a sample to the inverse cooling rate and relaxation time. H e did that in an interesting essay in which he supplemented the old story and introduced the topic by analyzing what he calls the freezing-in transition from the stable liquid via metastable and unstable states to the frozen state. Yet Kinser tells us papers are still arriving for review which lack this characterization and must be rejected. T o this in a way introductory subtopic, G u p t a *, Haus and Kerner *, made valuable contributions, including models of structure and relaxation.
2. Background It was found necessary to introduce a spectrum of information to this Conference before assessing the effects of modes of synthesis - the ConfereneCs main subject. This task may be exemplified here by a few notable presentations of tutorial value. Defects and associated electronic states were introduced to the Conference by Davis * and Shimizu *. From their detailed analysis the # a s s chemist could
* Papers by these authors are listed in the Table of Contents. 0022-3093/85/$03.30 © Elsevier Science Pubhshers B.V. (North-Holland Physics Publishing Division)
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N. Kreidl / Epitome
clearly deduce the difference in behavior of amorphous solids containing Si or Ge. In chalcogenides, species normally 2 or 3 coordinated, e~g. Se or As, can at low cost of energy form 1 and 3, or 2 and 4 coordinated defects of decisive influence on the properties. But for normally 4-coordinated Si or Ge the electronic structure forbids 5 coordination, allowing only 3 or 2 coordinated defects. Films deposited from all such substances will differ in many properties from those of the bulk obtained by melting, certainly in some response to defect type and concentration differences. More familiar structural probes like X-ray, Raman, or magnetic resonance spectroscopy are increasingly being supplemented by newer approaches requiring some explanation to some of us. EXAFS was introduced in a thorough and competent manner by Greaves *, using the system Si-O as a model. Nuclear magnetic resonance is now making available much more powerful instrumentation. The prospects of pinpointing O, Si, Na, etc. sites were revealed by Bray * D u t t a - D a y * discussed the thermally stimulated polarization current as a probe. Other methods were introduced in closer connection with specific aspects of modes of preparation which were discussed in this context. In close connection with the general theme, specific studies of an entire spectrum of diverse materials served as examples of future investigation of the influence of type of synthesis. Among such studies were those of optical waveguide materials (Yasui and Inoue *, Friebele et al. *), of phosphorus and nitrogen centers (Hosono et al. *, Watanabe et al. *), of peroxy centers (Freund *), and within the context of several other presentations, of differing sol-gel preparation techniques (Rabinovich *), and of nuclear waste disposal (Antonini et al. *, Sales et al. *). A large effort is now being made to exploit indications first given by Jack in England more than a decade ago towards incorporating the Si-N or P - N bond to strengthen oxide glasses (e.g. Mittl et al. *). Tiny iron particles were precipitated in H 2 treated SiO 2 glass containing iron oxide around 100 ppm (Weeks and Chatelain *).
3. Redox
Among the first problems that come to mind when we consider the effects of modes of preparation are those connected with redox equilibria. This is true whether we produce filters and sun glasses or equations for ligand roles. An overview was presented by Paul * by means of an illustration of redox series for simple well-defined systems, and supplemented by Schreiber and Balazs * (for borosilicate glasses). Bri~ckner * prefers to speak of the redox ratio rather than equilibrium since a true equilibrium is practically unattainable. The influence of the melting process including atmospheric and dissolved gases
N. Kreidl / Epitome
3
present not only a technological difficulty, but on the contrary, also a way of control. The redox ratio was shown to be a crucial factor in electrical transport. It may well, according to Bri~ckner *, determine possible or impossible hopping processes which thus can be controlled by partial substitution of one redox pair by another. Among effects on other properties not at all connected with color is that of Cu ÷ on low expansion. The effect on diffusion and electrical transport of changes in Fe3+/Fe 2÷ is large (Frischat and Baier *. The redox ratio determination can be used as a structural probe (Yoko et al. *). Balta * used Pb as a probe for basicity. In borate glasses containing halides and transition elements (Cu) a relation to the appearance and disappearance of nonbridging oxygen and BO4 groups respectively is demonstrated (Araujo *). Wherever point defects critically affect exacting requirements, knowledge of the dependence of their nature and concentration on preparation technique is needed. These effects were extensively discussed by Kawazoe * in reference to melted and ion-exchange (included reannealed) oxide glasses containing various group III, IV or V cations. Weeks' * creates new SiO2defect centers by laser illumination. Defect centers in GeS and AsS glasses depend on preparation (Kordas et al. *), (Wright et al. *). However, Wright et al. * propose to drop the prevalent interpretation that the difference between film and bulk ms2S 3 glass is based on a film structure consisting of realgar (As4Sn)-like molecular buildup in contrast to the orpiment (crystalline AszS3)-like layer structure of bulk ms2S 3 glass. The bulk glass may well have a 3-dimensional structure devoid of the layers typical of orpiment. As2S 3 does, indeed, form very stable glasses. Sputtered SiO 2 films are found to differ in defect type depending on formation details containing e.g. N centers or NO 2 centers, respectively (Wolf et al. *). SiO 2 layers obtained by O diffusion into crystalline Si substrates are largely dependent on substrate structure ("quasi epitaxy") (Revesz *). "Buried" layers are obtained in Si by implementation with a microstructure sensitive to substrate temperature (Holland et al. *). For all such films, structural particularities must be anticipated and will form a continuous theme of the discussions raised at this conference. Galeener * described the difference of "Thermal Oxide" SiO 2 films from "fused silica". The difference is dominated by the influence of preparation on OH content. No important use has been found for high pressure (higher than or equal to 20 kb) synthesis, but structural and property differences are well established (Kushiro et al. *). Future discussions should most likely include more evidence on more common melted glasses whose sensitivity to process variables had been pointed out years ago by Rindone. At this conference, Magruder et al. * reported significant differences in electrical resistivity with fusion temperature. Clearly such apparent contradictions towards the concept of constant fictive temperature above the glass transition are predominantly due to specific defect concentrations. A fascinating theme addressed through Ramar~ techniques is
4
N. Kreidl / Epitome
the existence and variability of rings in S i O 2 s t r u c t u r e s (Galeener, Sharma et al. *). Last, but not least, the conference had to deal, as future conferences will have to do, with assessing differences between high temperature melted and lower temperature sol-gel glasses. The investigation, preparation and application of gel glasses has become a major preoccupation of glass laboratories. The topic was extensively introduced by Brinker *. Important questions include the following: If structures and properties differ, will they persist? Are phase separation phenomena common? Do edge-shared tetrahedra result from drying? As long as the gel structure remains more open, the low density state could be nicknamed a "pseudofictive" one. Certainly, in many cases, a lower Tg will be observed experimentally. The distribution of relaxation times may be found broader. Yet, above Tg, glasses made in both ways were found to be remarkably similar. Brinker * concludes that gel and melt glasses can, but need not be, identical. Among many other aspects of sol-gel glasses, the following were treated. Transition elements in gel glasses may resemble those in melt glasses (Cr, Mn, Ni) suggesting the establishment of the SiO2 structure around 500°C. The difference in Fe, Co and Cu is attributed to the effect of temperature range on their particular redox behavior a n d / o r coordination changes (Sakka et al. *). For magnetic signals of Ti 3+ the difference is large (Dave et al. *). Different gel techniques (e.g. water or alkyl etc.) also yield different properties (Mukherjee et al.), (Bonner et al. *). Defects in gel glasses are different (Kordas et al. *, Wolf et al. *). An entirely different but important mode of glassmaking is provided by nature. Geological origins and variations, especially of the curious Libyan desert glass, were discussed by Seebaugh et al. * and by Galeener *. A paper by Pye et al. offering a different interpretation was withdrawn. It may be concluded that the Conference was a stimulating sample of the future changes arising from the recognition of the critical nature of preparation details. A greater participation of people involved in conventional glass melting would be desirable. A compromise between need, enthusiasm, economy and charity might suggest another blue grass convention around 1987.
1
EPITOME Norbert K R E I D L Santa Fe, New Mexico, USA
1. Introduction This Conference may well serve as a unique, if not the first, comprehensive assessment of the effects of modes of formation on the structure and properties of glasses with an emphasis on what happens above the glass transition range. Obviously, the latter effects cannot be separated from, indeed they interact with, events below the upper limit of the glass transition, the elucidation of which has been the subject of numerous and intensive investigations for several decades. Some consideration, therefore, had to be given to the glass transition at the start of the Conference. Today, most people are aware of the need to use standards or report details. It is almost half a century since Roetger appealed to the glass communi D, to do that. Pioneers like Lillie, Tool and others provided the proper scientific basis. At the present meeting, Cooper * offered a nice tribute to Liilie in calling the Lillie number the one-number parameter relating the state of a sample to the inverse cooling rate and relaxation time. H e did that in an interesting essay in which he supplemented the old story and introduced the topic by analyzing what he calls the freezing-in transition from the stable liquid via metastable and unstable states to the frozen state. Yet Kinser tells us papers are still arriving for review which lack this characterization and must be rejected. T o this in a way introductory subtopic, G u p t a *, Haus and Kerner *, made valuable contributions, including models of structure and relaxation.
2. Background It was found necessary to introduce a spectrum of information to this Conference before assessing the effects of modes of synthesis - the ConfereneCs main subject. This task may be exemplified here by a few notable presentations of tutorial value. Defects and associated electronic states were introduced to the Conference by Davis * and Shimizu *. From their detailed analysis the # a s s chemist could
* Papers by these authors are listed in the Table of Contents. 0022-3093/85/$03.30 © Elsevier Science Pubhshers B.V. (North-Holland Physics Publishing Division)
2
N. Kreidl / Epitome
clearly deduce the difference in behavior of amorphous solids containing Si or Ge. In chalcogenides, species normally 2 or 3 coordinated, e~g. Se or As, can at low cost of energy form 1 and 3, or 2 and 4 coordinated defects of decisive influence on the properties. But for normally 4-coordinated Si or Ge the electronic structure forbids 5 coordination, allowing only 3 or 2 coordinated defects. Films deposited from all such substances will differ in many properties from those of the bulk obtained by melting, certainly in some response to defect type and concentration differences. More familiar structural probes like X-ray, Raman, or magnetic resonance spectroscopy are increasingly being supplemented by newer approaches requiring some explanation to some of us. EXAFS was introduced in a thorough and competent manner by Greaves *, using the system Si-O as a model. Nuclear magnetic resonance is now making available much more powerful instrumentation. The prospects of pinpointing O, Si, Na, etc. sites were revealed by Bray * D u t t a - D a y * discussed the thermally stimulated polarization current as a probe. Other methods were introduced in closer connection with specific aspects of modes of preparation which were discussed in this context. In close connection with the general theme, specific studies of an entire spectrum of diverse materials served as examples of future investigation of the influence of type of synthesis. Among such studies were those of optical waveguide materials (Yasui and Inoue *, Friebele et al. *), of phosphorus and nitrogen centers (Hosono et al. *, Watanabe et al. *), of peroxy centers (Freund *), and within the context of several other presentations, of differing sol-gel preparation techniques (Rabinovich *), and of nuclear waste disposal (Antonini et al. *, Sales et al. *). A large effort is now being made to exploit indications first given by Jack in England more than a decade ago towards incorporating the Si-N or P - N bond to strengthen oxide glasses (e.g. Mittl et al. *). Tiny iron particles were precipitated in H 2 treated SiO 2 glass containing iron oxide around 100 ppm (Weeks and Chatelain *).
3. Redox
Among the first problems that come to mind when we consider the effects of modes of preparation are those connected with redox equilibria. This is true whether we produce filters and sun glasses or equations for ligand roles. An overview was presented by Paul * by means of an illustration of redox series for simple well-defined systems, and supplemented by Schreiber and Balazs * (for borosilicate glasses). Bri~ckner * prefers to speak of the redox ratio rather than equilibrium since a true equilibrium is practically unattainable. The influence of the melting process including atmospheric and dissolved gases
N. Kreidl / Epitome
3
present not only a technological difficulty, but on the contrary, also a way of control. The redox ratio was shown to be a crucial factor in electrical transport. It may well, according to Bri~ckner *, determine possible or impossible hopping processes which thus can be controlled by partial substitution of one redox pair by another. Among effects on other properties not at all connected with color is that of Cu ÷ on low expansion. The effect on diffusion and electrical transport of changes in Fe3+/Fe 2÷ is large (Frischat and Baier *. The redox ratio determination can be used as a structural probe (Yoko et al. *). Balta * used Pb as a probe for basicity. In borate glasses containing halides and transition elements (Cu) a relation to the appearance and disappearance of nonbridging oxygen and BO4 groups respectively is demonstrated (Araujo *). Wherever point defects critically affect exacting requirements, knowledge of the dependence of their nature and concentration on preparation technique is needed. These effects were extensively discussed by Kawazoe * in reference to melted and ion-exchange (included reannealed) oxide glasses containing various group III, IV or V cations. Weeks' * creates new SiO2defect centers by laser illumination. Defect centers in GeS and AsS glasses depend on preparation (Kordas et al. *), (Wright et al. *). However, Wright et al. * propose to drop the prevalent interpretation that the difference between film and bulk ms2S 3 glass is based on a film structure consisting of realgar (As4Sn)-like molecular buildup in contrast to the orpiment (crystalline AszS3)-like layer structure of bulk ms2S 3 glass. The bulk glass may well have a 3-dimensional structure devoid of the layers typical of orpiment. As2S 3 does, indeed, form very stable glasses. Sputtered SiO 2 films are found to differ in defect type depending on formation details containing e.g. N centers or NO 2 centers, respectively (Wolf et al. *). SiO 2 layers obtained by O diffusion into crystalline Si substrates are largely dependent on substrate structure ("quasi epitaxy") (Revesz *). "Buried" layers are obtained in Si by implementation with a microstructure sensitive to substrate temperature (Holland et al. *). For all such films, structural particularities must be anticipated and will form a continuous theme of the discussions raised at this conference. Galeener * described the difference of "Thermal Oxide" SiO 2 films from "fused silica". The difference is dominated by the influence of preparation on OH content. No important use has been found for high pressure (higher than or equal to 20 kb) synthesis, but structural and property differences are well established (Kushiro et al. *). Future discussions should most likely include more evidence on more common melted glasses whose sensitivity to process variables had been pointed out years ago by Rindone. At this conference, Magruder et al. * reported significant differences in electrical resistivity with fusion temperature. Clearly such apparent contradictions towards the concept of constant fictive temperature above the glass transition are predominantly due to specific defect concentrations. A fascinating theme addressed through Ramar~ techniques is
4
N. Kreidl / Epitome
the existence and variability of rings in S i O 2 s t r u c t u r e s (Galeener, Sharma et al. *). Last, but not least, the conference had to deal, as future conferences will have to do, with assessing differences between high temperature melted and lower temperature sol-gel glasses. The investigation, preparation and application of gel glasses has become a major preoccupation of glass laboratories. The topic was extensively introduced by Brinker *. Important questions include the following: If structures and properties differ, will they persist? Are phase separation phenomena common? Do edge-shared tetrahedra result from drying? As long as the gel structure remains more open, the low density state could be nicknamed a "pseudofictive" one. Certainly, in many cases, a lower Tg will be observed experimentally. The distribution of relaxation times may be found broader. Yet, above Tg, glasses made in both ways were found to be remarkably similar. Brinker * concludes that gel and melt glasses can, but need not be, identical. Among many other aspects of sol-gel glasses, the following were treated. Transition elements in gel glasses may resemble those in melt glasses (Cr, Mn, Ni) suggesting the establishment of the SiO2 structure around 500°C. The difference in Fe, Co and Cu is attributed to the effect of temperature range on their particular redox behavior a n d / o r coordination changes (Sakka et al. *). For magnetic signals of Ti 3+ the difference is large (Dave et al. *). Different gel techniques (e.g. water or alkyl etc.) also yield different properties (Mukherjee et al.), (Bonner et al. *). Defects in gel glasses are different (Kordas et al. *, Wolf et al. *). An entirely different but important mode of glassmaking is provided by nature. Geological origins and variations, especially of the curious Libyan desert glass, were discussed by Seebaugh et al. * and by Galeener *. A paper by Pye et al. offering a different interpretation was withdrawn. It may be concluded that the Conference was a stimulating sample of the future changes arising from the recognition of the critical nature of preparation details. A greater participation of people involved in conventional glass melting would be desirable. A compromise between need, enthusiasm, economy and charity might suggest another blue grass convention around 1987.