- Email: [email protected]
Glass experiments in space
Journal of Non-Crystalline Solids 80 (1986) 587-593 North-Holland, Amsterdam
587
GLASS EXPERIMENTS IN SPACE
Norbert J. K R E I D L Universities of Missouri-Rolla and University of New Mexico, College of Santa Fe, USA
Current and potential glass research under microgravity includes studies of minimum cooling rate, homogeneous nucleation, diffusion, surface tension and enrichment, fining without buoyancy, avoidance of liquid phase separation, gel synthesis, composites and uniform microballoons. The experiments do not just anticipate space fabrication, but will also provide information and innovation encouraging advanced processing on earth.
1. Microgravity science
The opportunities provided by the space environment for materials science and engineering are now generally recognized: virtual weightlessness (microgravity), unlimited vacuum, unique radiation fields, and the essentially infinite dimensions of space. Microgravity is the most important of these features, offering the absence of density-driven convection, freedom from gravity-driven deformation, and the containerless management of large volumes of liquids. The original motivation for a materials program in space, namely the future processing of novel materials unattainable on earth, has become overshadowed by the recognition of microgravity as a potential for the acquisition of unique knowledge applicable to materials processing either on earth or in space. This trend has been clearly symbolized by the recent change in the name of NASA's materials processing in space (MPS) program to Microgravity Science and Applications (MSA) [1]. Accordingly, the majority of ongoing projects have become peer-reviewed studies of high scientific quality. The applications issue is addressed by attractive formats available for industry-NASA cooperation. An example is the long-range comprehensive joint research program 3M-NASA at first emphasizing crystal growth and thin films. Two projects have progressed beyond basic research (monodisperse latex spheres, electrophoresis). This paper represents an attempt to briefly survey microgravity studies in the field of glass science and technology. At first glance, glass appears a less likely candidate for microgravity experiments since the processing of high viscosity liquids should be less significantly affected by the relatively weak 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
588
N.J. Kreidl / Glass experiments in space
gravity forces. Yet, glass science increasingly expands to include low viscosity melts (e.g. fluoride systems for communication fibers, or metal glass systems). And containerless melting is attractive for the undisturbed study of e.g. nucleation, surface or diffusion phenomena, as well as for work at extremely high temperatures. For this reason a significant spectrum of microgravity glass investigations can be reported at this time.
2. Glass experiments 2.1. General
Since 1969 the USA materials processing program in space has been, in part, guided by advice from the University Space Research Association (USRA) - composed of more than 50 universities - through topical committees, with whose early recommendations [2a] most current USA experiments seem to correspond. The present trend follows the general trend discussed in the introduction - peer-reviewed quality research with an eye on industrial cooperation and utilization. Attempts to increase international interaction are made. A brief review of significant glass studies follows. 2.2. Microballoons
Manipulation without physical contact to form special shapes has been given some attention [1]. For example, the formation of microballoons serving as nuclear fusion target containers [3] was examined in containerless and microgravity environments [5-9]. The investigation of the basic properties governing the process unaffected by gravity has been carried out on metal glasses [7] as well as on more conventional glasses [10]. Metal glass (Au55Pbz2.sSb22.5) spheres of 2000/.Lm were obtained with superbly smooth (+250.4,) surfaces [5]. A typical wall thickness is 20/zm. Gel synthesis of such balloons was explored by [11]. 2.3. Critical cooling rate
One of the most important objectives of glass processing in space is to establish, and if established, to utilize any difference in critical cooling rate between 1 G and microgravity environments. A smaller critical cooling rate in space would encourage the development of novel glass systems, e.g. extreme optical glasses. So far no such difference has been convincingly demonstrated, but expectations are based on the observation that it is usually heterogeneous nucleation, caused, for instance, by contact with the container, which limits glass formation in exotic systems. In the case of metal glasses, this could be a critical factor in the attempt to obtain promising compositions in bulk form [34].
N.J. Kreidl / Glass experiments in space
589
In the field of oxide glasses, attempts to verify a difference in critical cooling rates between 1 G and near zero G now concentrate on systems whose 1 G critical cooling rates are well known (or determined by a suitable simplified technique [12]) and which can be processed in temperature ranges and at cooling rates now available in existing space facilities. I am particularly revolved in the program of Day [2b], Ray and Day [13] who chose the systems Na20-SiO2, PbO-SiO2 and BaO-TiO2-SiO2. The latter system, for example, forms glasses with critical cooling rates of from 20 to 200 K/s. If, and when, a significant difference will have been ascertained, interesting borderline systems (such as the systems CaF2-AIF3-X) which promise control of the secondary spectrum in lenses in the past provided by crystalline CaF2, have been scheduled for investigation. New laser heating techniques have been devised to evaluate critical cooling rates by Ethridge [ 14] and Ethridge and Curreri [ 15]. Similarly, boundaries of glass formation in fluoride glass candidates for communication fibers (e.g. ZrF4-BaF2-X) are being scrutinized by Doremus [16] and Bansal et al. [17]. Fluoride glasses permit communication at longer wave lengths at which theoretical scattering losses are much lower, and so require less frequent repeats. Organic systems (e.g. O-terphenyl) are reported on by Trinh [18]. 2.4. Phase separation
In systems showing a large above-liquidus immiscibility dome, two liquids are obtained, the heavier one on the bottom, the lighter one on top. In glass-forming immiscible systems such as CaO-SiO2 and BaO-B203, two glass layers will form and can be demonstrated by doping with CoO which enters the Ca, or Ba-rich phase nearly 100%. In the absence of gravity, new and perhaps microdisperse glasses may be obtained, although it has not been well established to date what the relative roles of thermodynamic driving force, surface tension, interface tension with the container, and gravity might be (Weinberg, oral communication). Indeed, while metallic alloys with new microstructures were obtained, some of the specimens showed surprising configurations [ 19]. Day's [2b/program contains experiments with BaO-B203 glasses. Petrovskii et al. [20] report the useful increase of Nd-doping in a laser glass in which phase separation was suppressed and/or homogeneity improved in microgravity. 2.5. Floating zone
The floating-zone process for producing ceramics, including glasses, from corrosive melts of low surface tension suffers from many difficulties. Naumann [1] expects relief by microgravity processing.
590
N.J. Kreidl / Glass experiments in space
2.6. Gel glass
The raw material for any glass processing in space has to be delivered in some form. Day [2b] has devised low-temperature-sintered bodies as precursors. For the near future, low-temperature-synthesized gels look promising [1,21-23]. Gel synthesis of glasses has become a favorite subject. This affects space experimenters, who are not discouraged by problems of pre-precipitation, segregation on drying [1], or crystallization in the presence of H20 [23]. Gel processing is a prospect beyond providing clean precursors. Gel glass processing in space promises ultrapure specimens [23]. Also, gel processing in space is not confronted by the fining problem: the absence of convection is not a significant factor in contrast to the case of melt forming. 2.7. Transport
To avoid interference by gravity-driven convection [24,25], diffusion profiles on two sandwiched samples of sodium and sodium-rubidium trisilicate glasses were evaluated (tracer technique) in the microgravity environment of the T E X U S rocket (1982). In contrast to the contact zone distorted by convection on earth, the contact zones in the microgravity experiment were smooth. The 1 G self-diffusion coefficient (D'a) was found to be too high because of convection. The microgravity experiment provided what may be considered an intrinsic D*a. 2.8. Glass ceramics and composites
Specimens of special glass ceramics whose precursor glass or remaining vitreous matrix would sag at the required reheating temperature could be studied in microgravity. Similarly, reinforcements in composites could be arranged undisturbed by gravity in low-viscosity matrices. Hardly any experiments of this kind are underway at this time. t'etrovskii et al. [20] have reported even distribution of the active iron oxide component in the matrix of a magneto-optic borate glass composite. A proposal [26] involving the placement of metal spheres in a glass matrix aims at the achievement of certain electrical properties. 2.9. Surface
Last, but not least, microgravity is an environment favoring unencumbered surface studies. One aspect is the quantitative evaluation of surface enrichment in glass components lowering surface tension. Knowledge of such surface concentrations or gradients are also of practical interest (substrates, amplifiers, controlled crystallization). The phenomenon of
N.J. Kreidl / Glass experiments in space
591
enrichment has been described extensively by Weyl and Marboe [3,5] at a time when surface analysis was not developed to the point of going beyond qualitative statements. Mo, V and Wo are outstanding. Pb and F are technologically important examples. Microgravity experiments are expected to avoid interference by segregation and convection. Uhlmann and Tuller [27] have started to determine some of the gradients as a function of temperature and time. Another, larger area of research concerns problems of fining glass (the glass technologist's term for removing residual bubbles). Under 1 G the fining process involves two simultaneous mechanisms (1) solution and exsolution of gases, including the function of so-called fining agents and (2) buoyancy. Microgravity, on the one hand, permits an undisturbed study of the solution mechanism. Weinberg [28], on the other hand, calls for a new fining mechanism in the absence of buoyancy. An elegant alternative fining procedure was introduced and studied by Subramanian and coworkers [29-32]. It is based on a phenomenon termed thermocapillary convection or the Marangani effect, i.e. flow and bubble motion induced by gradients in interracial tension. Theory (Shankhar et al. [33]) was confirmed by experiments showin~ bubble migration towards a hot spot [31] (see also ref. [20]). This alternative fining method, now also called "thermal fining", is based on the theory of Young et al. [52] V =
OX
a/2~,
(1)
where V = bubble velocity, OT/OX in K cm ~ is the temperature gradient, Oo'/c?T is the temperature gradient of the surface tension in dyn cm -~ K i a = the bubble diameter in cm, "0 = the viscosity in P. The analytical solution by Subramanian contains eq. (1) as a first term. For experimental verification, a composition with large Oo'/OT such as a borate had to be chosen.
3. Conclusions
Within the scope of materials processing in space, glass projects have so far represented a minor effort in an initial stage. Nevertheless, important and interesting programs are now underway or planned. The verification of possible differences in critical cooling rates is of fundamental and practical value. The exploration of surface and diffusion phenomena without gravity interference is another example of significant experimentation in progress. The emphasis at this point is in two only seemingly opposite directions: (1) gaining scientific and engineering knowledge by space experiments; (2) mobilizing the interest of industry to participate in space research closer related to potential applications. Attractive formats of cooperation have become available to industry. Immediate critical requirements from the
592
N.J. Kreidl / Glass experiments in space
viewpoint of the glass community are facilities permitting higher temperature ranges, faster cooling rates, stabilized levitation, availability of astronaut intervention, and, eventually, a manned space laboratory.
Reterences [1] R. Naumann, Microgravity Science and Applications, Program Description Document, NASA (G. Marshall Space Flight Center, Alabama, February 1984). [2] (a) N. Kreidl, G. Rindone, E. Snitzer, D. Uhlmann and R. Nichols, Preparation of Glasses and Ceramic Materials, Summary Report (Nat. Acad. Engng., 1974); (b) D. E. Day, Mat. Processing in Space Ecosystems International (January 1980) Flight STS7. [3] C. Hendricks, ICF Targets, Technical Digest. Conf. on Inertial Confinement Fusion (1980) p. 80. [4] N. Kreidl, Symp. on Industrialization of Materials Processing in Space (1984) to be published. [5] M. Lee, J. Kendall, D. Elleman, W.-K. Rhim, R. Helizon, CH. Youngberg, I.-A. Feng and T. Wang, Materials Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 95. [6] T. Wang, in: Materials Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 105. [7] T. Wang, NASA, Materials Processing in Space (G. Marshall Space Flight Center, April 1983) (additional references in memorandum) p. 129. [8] S. Bjorksten, Mat. Proc. in Space, Ecosystems (January 1984) #24. [9] J. Rush, W. Stephens and E. Ethridge, Mat. Processing in the Reduced Gravity Environment of Space (North-Holland, New York, 1982) p. 131. [10] S. Dunn, R. Nagler, E. Paquette, A. Pomplun and E. Crosby, Mat. Processing in Space, Ecosystems International (January, 1984) # 111. [11] R. Nolan, R. Downs and A. Ebner, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 3.43. [12] C. Ray and D. Day, J. Am. Ceram. Soc. 67 (1984) 806. [13] C. Ray and D. Day, Nat. Sampe Tech. Conf. Ser. 25 (1983) 135. [14] E. Ethridge and P. Curreri, Electromagnetic Moldless Casting, ed., J. Wallace (NorthHolland, New York, 1983). [15] E. Ethridge and P. Curreri, Mat. Processing in Space, Ecosystems International (January 1984) #31. [16] R. Doremus, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement, p. 31. [17] N. Bansal, R. Doremus, A. Bruce and C. Moynihan, J. Am. Ceram. Soc. 66 (1983) 233. [18] E. Trinh, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 35. [19] S. Gelles, A. Markworth and G. Mobley, Proc. 4th Eur. Symp. Mat. Sci. Under Microgravity ESA SP 191 Paris-Codex 15 (June 1983) p. 307. [20] G. Petrovskii, V. Ryumin and T. Semeshkin, Steklo i Keramika 1 (1983) 5. [21] M. Weinberg, Mat. Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 38. [22] R. Downs and W. Miller, Materials Processing in Space, Ecosystems International (January 1984) #108. [23] S. Mukherjee, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 321. [24] M. Braedt and G. Frischat, J. Am. Ceram. Soc. (4) (1984) C-54.
N.J. Kreidl / Glass experiments in space
593
[25] G. Frischat, M. Braedt and W. Beier, Proc. 4th Europ. Symp. on Materials Science Under Microgravitv, ESA-SP 191 Publ. Madrid (1983). [26] J. Zarzycki, oral communication (1983). [27] D. Uhlmann and H. Tuller, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 37. [28] M. Weinberg, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 301. [29] R.S. Subramanian and R. Cole, Mat. Proc. in Space, Ecosystems Intern. (1984) #86. [30] T.J. McNeil, R. Cole and R.S. Subramanian, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 289. [31] H.D. Smith, D.M. Mattox, W.R. Wilcox, R.S. Subramian and M. Meyappan, Mat. Proc. in the Reduced Gravity Requirement of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 279. [32] P. Annamalai, R. Subramanian and R. Cole, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 187. [33] N. Shankar, R. Cole and R- Subramanian, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 249. [34] F. Spaepen, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 34. [35] W. Weyl and E. Marboe, The Constitution of Glasses (Interscience, New York, 1967). [36] J. Cahn, Mat. Trans. 10A (1979) 119. [37] J. Carruthers, Materials Processing in the Reduced Gravity Environment of Space (North-Holland, Amsterdam, 1982) p. 3. [38] D. Day, Mat. Processing in Space Ecosystems International (January 1980) Flight STS 7. [39] N. Kreidl, G. Rindone, E. Snitzer, D. Uhlmann and R. Nichols, Preparation of Glasses and Ceramic Materials, Summary Report for National Ac. of Engineering (1974) summer study. [40] N. Drehman and D. Turnbull, Mat. Processing in Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 81. [41] E. Ethridge and P. Curreri, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 121. [42] R. Lolen, Mat. Proc. in Space, Ecosystems Int. (January 1984) #66. [43] S.H. Mukherjee, Materials Processing in Space Ecosystems International (January 1984) #5. [44] C.A. Rey, R.R. Whymark, T.J. Danley and D.R. Merkley, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982). [45] H.D. Smith, D.M. Mattox and D.P. Partlow, Space Processing Applications, Rocket Project SPAR VIII Final Report (June 1984). [46] H.D. Smith, D.M. Mattox, W.R. Wilcox and R.S. Subramian, Final Rep. Contr. NAS832351 (June 30, 1977). [47] R.P. Chassay, ed., Space Processing Applications Rocket Project Spar VVI Final Report NASA Sci. and Tech. Inf. Branch (June 1984). [48] R.S. Subramanian, AIChE J. 27 (14) (1982) 646 [49] THREE-M (Release) American Glass Review (May 1984) p. 7. [50] D. Turnbull, Mat. Proc. in Space, Ecosyst. Intern. (January 1984) #5; Mat. Processing m Space (G. Marshall Flight Center, Alabama, April 1983) p. 127. [51] D. Uhlmann, Mat. Processing in the Reduced Gravity Environment of Space, ed., O. Rindone (North-Holland, New York, 1982) p. 269. [52] N. Young, J. Goldstein and M. Block, J. Fluid Mech. 6 (1959) 350.
587
GLASS EXPERIMENTS IN SPACE
Norbert J. K R E I D L Universities of Missouri-Rolla and University of New Mexico, College of Santa Fe, USA
Current and potential glass research under microgravity includes studies of minimum cooling rate, homogeneous nucleation, diffusion, surface tension and enrichment, fining without buoyancy, avoidance of liquid phase separation, gel synthesis, composites and uniform microballoons. The experiments do not just anticipate space fabrication, but will also provide information and innovation encouraging advanced processing on earth.
1. Microgravity science
The opportunities provided by the space environment for materials science and engineering are now generally recognized: virtual weightlessness (microgravity), unlimited vacuum, unique radiation fields, and the essentially infinite dimensions of space. Microgravity is the most important of these features, offering the absence of density-driven convection, freedom from gravity-driven deformation, and the containerless management of large volumes of liquids. The original motivation for a materials program in space, namely the future processing of novel materials unattainable on earth, has become overshadowed by the recognition of microgravity as a potential for the acquisition of unique knowledge applicable to materials processing either on earth or in space. This trend has been clearly symbolized by the recent change in the name of NASA's materials processing in space (MPS) program to Microgravity Science and Applications (MSA) [1]. Accordingly, the majority of ongoing projects have become peer-reviewed studies of high scientific quality. The applications issue is addressed by attractive formats available for industry-NASA cooperation. An example is the long-range comprehensive joint research program 3M-NASA at first emphasizing crystal growth and thin films. Two projects have progressed beyond basic research (monodisperse latex spheres, electrophoresis). This paper represents an attempt to briefly survey microgravity studies in the field of glass science and technology. At first glance, glass appears a less likely candidate for microgravity experiments since the processing of high viscosity liquids should be less significantly affected by the relatively weak 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
588
N.J. Kreidl / Glass experiments in space
gravity forces. Yet, glass science increasingly expands to include low viscosity melts (e.g. fluoride systems for communication fibers, or metal glass systems). And containerless melting is attractive for the undisturbed study of e.g. nucleation, surface or diffusion phenomena, as well as for work at extremely high temperatures. For this reason a significant spectrum of microgravity glass investigations can be reported at this time.
2. Glass experiments 2.1. General
Since 1969 the USA materials processing program in space has been, in part, guided by advice from the University Space Research Association (USRA) - composed of more than 50 universities - through topical committees, with whose early recommendations [2a] most current USA experiments seem to correspond. The present trend follows the general trend discussed in the introduction - peer-reviewed quality research with an eye on industrial cooperation and utilization. Attempts to increase international interaction are made. A brief review of significant glass studies follows. 2.2. Microballoons
Manipulation without physical contact to form special shapes has been given some attention [1]. For example, the formation of microballoons serving as nuclear fusion target containers [3] was examined in containerless and microgravity environments [5-9]. The investigation of the basic properties governing the process unaffected by gravity has been carried out on metal glasses [7] as well as on more conventional glasses [10]. Metal glass (Au55Pbz2.sSb22.5) spheres of 2000/.Lm were obtained with superbly smooth (+250.4,) surfaces [5]. A typical wall thickness is 20/zm. Gel synthesis of such balloons was explored by [11]. 2.3. Critical cooling rate
One of the most important objectives of glass processing in space is to establish, and if established, to utilize any difference in critical cooling rate between 1 G and microgravity environments. A smaller critical cooling rate in space would encourage the development of novel glass systems, e.g. extreme optical glasses. So far no such difference has been convincingly demonstrated, but expectations are based on the observation that it is usually heterogeneous nucleation, caused, for instance, by contact with the container, which limits glass formation in exotic systems. In the case of metal glasses, this could be a critical factor in the attempt to obtain promising compositions in bulk form [34].
N.J. Kreidl / Glass experiments in space
589
In the field of oxide glasses, attempts to verify a difference in critical cooling rates between 1 G and near zero G now concentrate on systems whose 1 G critical cooling rates are well known (or determined by a suitable simplified technique [12]) and which can be processed in temperature ranges and at cooling rates now available in existing space facilities. I am particularly revolved in the program of Day [2b], Ray and Day [13] who chose the systems Na20-SiO2, PbO-SiO2 and BaO-TiO2-SiO2. The latter system, for example, forms glasses with critical cooling rates of from 20 to 200 K/s. If, and when, a significant difference will have been ascertained, interesting borderline systems (such as the systems CaF2-AIF3-X) which promise control of the secondary spectrum in lenses in the past provided by crystalline CaF2, have been scheduled for investigation. New laser heating techniques have been devised to evaluate critical cooling rates by Ethridge [ 14] and Ethridge and Curreri [ 15]. Similarly, boundaries of glass formation in fluoride glass candidates for communication fibers (e.g. ZrF4-BaF2-X) are being scrutinized by Doremus [16] and Bansal et al. [17]. Fluoride glasses permit communication at longer wave lengths at which theoretical scattering losses are much lower, and so require less frequent repeats. Organic systems (e.g. O-terphenyl) are reported on by Trinh [18]. 2.4. Phase separation
In systems showing a large above-liquidus immiscibility dome, two liquids are obtained, the heavier one on the bottom, the lighter one on top. In glass-forming immiscible systems such as CaO-SiO2 and BaO-B203, two glass layers will form and can be demonstrated by doping with CoO which enters the Ca, or Ba-rich phase nearly 100%. In the absence of gravity, new and perhaps microdisperse glasses may be obtained, although it has not been well established to date what the relative roles of thermodynamic driving force, surface tension, interface tension with the container, and gravity might be (Weinberg, oral communication). Indeed, while metallic alloys with new microstructures were obtained, some of the specimens showed surprising configurations [ 19]. Day's [2b/program contains experiments with BaO-B203 glasses. Petrovskii et al. [20] report the useful increase of Nd-doping in a laser glass in which phase separation was suppressed and/or homogeneity improved in microgravity. 2.5. Floating zone
The floating-zone process for producing ceramics, including glasses, from corrosive melts of low surface tension suffers from many difficulties. Naumann [1] expects relief by microgravity processing.
590
N.J. Kreidl / Glass experiments in space
2.6. Gel glass
The raw material for any glass processing in space has to be delivered in some form. Day [2b] has devised low-temperature-sintered bodies as precursors. For the near future, low-temperature-synthesized gels look promising [1,21-23]. Gel synthesis of glasses has become a favorite subject. This affects space experimenters, who are not discouraged by problems of pre-precipitation, segregation on drying [1], or crystallization in the presence of H20 [23]. Gel processing is a prospect beyond providing clean precursors. Gel glass processing in space promises ultrapure specimens [23]. Also, gel processing in space is not confronted by the fining problem: the absence of convection is not a significant factor in contrast to the case of melt forming. 2.7. Transport
To avoid interference by gravity-driven convection [24,25], diffusion profiles on two sandwiched samples of sodium and sodium-rubidium trisilicate glasses were evaluated (tracer technique) in the microgravity environment of the T E X U S rocket (1982). In contrast to the contact zone distorted by convection on earth, the contact zones in the microgravity experiment were smooth. The 1 G self-diffusion coefficient (D'a) was found to be too high because of convection. The microgravity experiment provided what may be considered an intrinsic D*a. 2.8. Glass ceramics and composites
Specimens of special glass ceramics whose precursor glass or remaining vitreous matrix would sag at the required reheating temperature could be studied in microgravity. Similarly, reinforcements in composites could be arranged undisturbed by gravity in low-viscosity matrices. Hardly any experiments of this kind are underway at this time. t'etrovskii et al. [20] have reported even distribution of the active iron oxide component in the matrix of a magneto-optic borate glass composite. A proposal [26] involving the placement of metal spheres in a glass matrix aims at the achievement of certain electrical properties. 2.9. Surface
Last, but not least, microgravity is an environment favoring unencumbered surface studies. One aspect is the quantitative evaluation of surface enrichment in glass components lowering surface tension. Knowledge of such surface concentrations or gradients are also of practical interest (substrates, amplifiers, controlled crystallization). The phenomenon of
N.J. Kreidl / Glass experiments in space
591
enrichment has been described extensively by Weyl and Marboe [3,5] at a time when surface analysis was not developed to the point of going beyond qualitative statements. Mo, V and Wo are outstanding. Pb and F are technologically important examples. Microgravity experiments are expected to avoid interference by segregation and convection. Uhlmann and Tuller [27] have started to determine some of the gradients as a function of temperature and time. Another, larger area of research concerns problems of fining glass (the glass technologist's term for removing residual bubbles). Under 1 G the fining process involves two simultaneous mechanisms (1) solution and exsolution of gases, including the function of so-called fining agents and (2) buoyancy. Microgravity, on the one hand, permits an undisturbed study of the solution mechanism. Weinberg [28], on the other hand, calls for a new fining mechanism in the absence of buoyancy. An elegant alternative fining procedure was introduced and studied by Subramanian and coworkers [29-32]. It is based on a phenomenon termed thermocapillary convection or the Marangani effect, i.e. flow and bubble motion induced by gradients in interracial tension. Theory (Shankhar et al. [33]) was confirmed by experiments showin~ bubble migration towards a hot spot [31] (see also ref. [20]). This alternative fining method, now also called "thermal fining", is based on the theory of Young et al. [52] V =
OX
a/2~,
(1)
where V = bubble velocity, OT/OX in K cm ~ is the temperature gradient, Oo'/c?T is the temperature gradient of the surface tension in dyn cm -~ K i a = the bubble diameter in cm, "0 = the viscosity in P. The analytical solution by Subramanian contains eq. (1) as a first term. For experimental verification, a composition with large Oo'/OT such as a borate had to be chosen.
3. Conclusions
Within the scope of materials processing in space, glass projects have so far represented a minor effort in an initial stage. Nevertheless, important and interesting programs are now underway or planned. The verification of possible differences in critical cooling rates is of fundamental and practical value. The exploration of surface and diffusion phenomena without gravity interference is another example of significant experimentation in progress. The emphasis at this point is in two only seemingly opposite directions: (1) gaining scientific and engineering knowledge by space experiments; (2) mobilizing the interest of industry to participate in space research closer related to potential applications. Attractive formats of cooperation have become available to industry. Immediate critical requirements from the
592
N.J. Kreidl / Glass experiments in space
viewpoint of the glass community are facilities permitting higher temperature ranges, faster cooling rates, stabilized levitation, availability of astronaut intervention, and, eventually, a manned space laboratory.
Reterences [1] R. Naumann, Microgravity Science and Applications, Program Description Document, NASA (G. Marshall Space Flight Center, Alabama, February 1984). [2] (a) N. Kreidl, G. Rindone, E. Snitzer, D. Uhlmann and R. Nichols, Preparation of Glasses and Ceramic Materials, Summary Report (Nat. Acad. Engng., 1974); (b) D. E. Day, Mat. Processing in Space Ecosystems International (January 1980) Flight STS7. [3] C. Hendricks, ICF Targets, Technical Digest. Conf. on Inertial Confinement Fusion (1980) p. 80. [4] N. Kreidl, Symp. on Industrialization of Materials Processing in Space (1984) to be published. [5] M. Lee, J. Kendall, D. Elleman, W.-K. Rhim, R. Helizon, CH. Youngberg, I.-A. Feng and T. Wang, Materials Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 95. [6] T. Wang, in: Materials Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 105. [7] T. Wang, NASA, Materials Processing in Space (G. Marshall Space Flight Center, April 1983) (additional references in memorandum) p. 129. [8] S. Bjorksten, Mat. Proc. in Space, Ecosystems (January 1984) #24. [9] J. Rush, W. Stephens and E. Ethridge, Mat. Processing in the Reduced Gravity Environment of Space (North-Holland, New York, 1982) p. 131. [10] S. Dunn, R. Nagler, E. Paquette, A. Pomplun and E. Crosby, Mat. Processing in Space, Ecosystems International (January, 1984) # 111. [11] R. Nolan, R. Downs and A. Ebner, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 3.43. [12] C. Ray and D. Day, J. Am. Ceram. Soc. 67 (1984) 806. [13] C. Ray and D. Day, Nat. Sampe Tech. Conf. Ser. 25 (1983) 135. [14] E. Ethridge and P. Curreri, Electromagnetic Moldless Casting, ed., J. Wallace (NorthHolland, New York, 1983). [15] E. Ethridge and P. Curreri, Mat. Processing in Space, Ecosystems International (January 1984) #31. [16] R. Doremus, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement, p. 31. [17] N. Bansal, R. Doremus, A. Bruce and C. Moynihan, J. Am. Ceram. Soc. 66 (1983) 233. [18] E. Trinh, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 35. [19] S. Gelles, A. Markworth and G. Mobley, Proc. 4th Eur. Symp. Mat. Sci. Under Microgravity ESA SP 191 Paris-Codex 15 (June 1983) p. 307. [20] G. Petrovskii, V. Ryumin and T. Semeshkin, Steklo i Keramika 1 (1983) 5. [21] M. Weinberg, Mat. Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 38. [22] R. Downs and W. Miller, Materials Processing in Space, Ecosystems International (January 1984) #108. [23] S. Mukherjee, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 321. [24] M. Braedt and G. Frischat, J. Am. Ceram. Soc. (4) (1984) C-54.
N.J. Kreidl / Glass experiments in space
593
[25] G. Frischat, M. Braedt and W. Beier, Proc. 4th Europ. Symp. on Materials Science Under Microgravitv, ESA-SP 191 Publ. Madrid (1983). [26] J. Zarzycki, oral communication (1983). [27] D. Uhlmann and H. Tuller, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 37. [28] M. Weinberg, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 301. [29] R.S. Subramanian and R. Cole, Mat. Proc. in Space, Ecosystems Intern. (1984) #86. [30] T.J. McNeil, R. Cole and R.S. Subramanian, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 289. [31] H.D. Smith, D.M. Mattox, W.R. Wilcox, R.S. Subramian and M. Meyappan, Mat. Proc. in the Reduced Gravity Requirement of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 279. [32] P. Annamalai, R. Subramanian and R. Cole, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 187. [33] N. Shankar, R. Cole and R- Subramanian, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 249. [34] F. Spaepen, Materials Processing in Space (G. Marshall Flight Center, Alabama, October 1983) Supplement p. 34. [35] W. Weyl and E. Marboe, The Constitution of Glasses (Interscience, New York, 1967). [36] J. Cahn, Mat. Trans. 10A (1979) 119. [37] J. Carruthers, Materials Processing in the Reduced Gravity Environment of Space (North-Holland, Amsterdam, 1982) p. 3. [38] D. Day, Mat. Processing in Space Ecosystems International (January 1980) Flight STS 7. [39] N. Kreidl, G. Rindone, E. Snitzer, D. Uhlmann and R. Nichols, Preparation of Glasses and Ceramic Materials, Summary Report for National Ac. of Engineering (1974) summer study. [40] N. Drehman and D. Turnbull, Mat. Processing in Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 81. [41] E. Ethridge and P. Curreri, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982) p. 121. [42] R. Lolen, Mat. Proc. in Space, Ecosystems Int. (January 1984) #66. [43] S.H. Mukherjee, Materials Processing in Space Ecosystems International (January 1984) #5. [44] C.A. Rey, R.R. Whymark, T.J. Danley and D.R. Merkley, Mat. Processing in the Reduced Gravity Environment of Space, ed., G. Rindone (North-Holland, New York, 1982). [45] H.D. Smith, D.M. Mattox and D.P. Partlow, Space Processing Applications, Rocket Project SPAR VIII Final Report (June 1984). [46] H.D. Smith, D.M. Mattox, W.R. Wilcox and R.S. Subramian, Final Rep. Contr. NAS832351 (June 30, 1977). [47] R.P. Chassay, ed., Space Processing Applications Rocket Project Spar VVI Final Report NASA Sci. and Tech. Inf. Branch (June 1984). [48] R.S. Subramanian, AIChE J. 27 (14) (1982) 646 [49] THREE-M (Release) American Glass Review (May 1984) p. 7. [50] D. Turnbull, Mat. Proc. in Space, Ecosyst. Intern. (January 1984) #5; Mat. Processing m Space (G. Marshall Flight Center, Alabama, April 1983) p. 127. [51] D. Uhlmann, Mat. Processing in the Reduced Gravity Environment of Space, ed., O. Rindone (North-Holland, New York, 1982) p. 269. [52] N. Young, J. Goldstein and M. Block, J. Fluid Mech. 6 (1959) 350.