- Email: [email protected]
Emerging applications of glass
CHAPTER 23
Emerging applications of glass 23.1 Introduction From glass windows and containers to lenses and optical fiber, glass has proven to be one of the most important materials for enabling the development of contemporary human civilization. Owing to this indelible impact of glass on modern society, it has been proposed that we are now living in The Glass Age [1]. The positive influence of glass on our world continues to grow as new glass products and processes are developed to address global challenges in energy, the environment, health care, information/communication technology, and more [2]. In this closing chapter, we review some new emerging applications of glass that promise to address many of these grand challenges. We hope that these new and anticipated applications of glass will inspire students to pursue dedicated research in these areas to invent the new materials and processes required for solving these grand challenges and improving the quality of life for all of humanity. To understand which emerging applications of glass could have the greatest impact on our society, it is helpful to put previous glass innovations into some historical context. To this end, the authors commissioned a survey of professionals in the glass field. A total of 91 respondents ranked the top 10 glass innovations that have had the greatest impact on humankind. The score for each innovation is calculated by assigning 10 points for each first place ranking, 9 points for each second place ranking, etc., and 1 point for each 10th place ranking. Total scores for all 91 respondents are plotted in Fig. 23.1. According to the survey results, the top 10 glass innovations having the greatest impact on humankind are: (1) glass lenses, (2) optical fiber, (3) glass windows, (4) the Pilkington float process, (5) glass lightbulb envelopes, (6) glass containers, (7) laboratory glass, (8) television glass (for cathode ray tubes), (9) glass-ceramics, and (10) bioactive glasses.1 Product innovations dominate the list, and the Pilkington float process is justifiably ranked as the most impactful industrial glass process. Of course, many of these 1
Varshneya and Mauro’s ranking: (1) windows, (2) lenses, (3) light bulb, and (4) optical fiber.
Fundamentals of Inorganic Glasses https://doi.org/10.1016/B978-0-12-816225-5.00023-7
© 2019 Elsevier Inc. All rights reserved.
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Glass lenses (ophthalmic, microscope, telescope) Optical fiber Glass windows (for architecture and transportation) Pilkington float process Glass lightbulb envelopes Glass containers (beverages, packaging, pharmaceutical) Laboratory glasses Television glass (cathode ray tubes) Glass-ceramics Bioactive glasses Chemical strengthening process Thermal tempering process Liquid crystal display substrates High purity fused silica Glass mirrors Strengthened cover glass for mobile devices Fusion draw process Glass insulation wool Laser glass Siemens regenerative continuous glass-melting tank... Glass optical amplifiers Ultra-low expansion glass Stained glass Glass fiber-reinforced composites Glass baking dishes Glass tubing Ribbon machine Chemical vapor deposition process Space shuttle glass Sealing glasses Anti-reflective coatings Chalcogenide glass based memory and threshold switches Lead glass (”crystal”) Infrared transmitting glasses Laminated glasses Window self-cleaning process Bulk metallic glasses Sol-gel process Foam glass Vello/danner processes for tube making Other
Fig. 23.1 Compiled survey responses from 91 experts in glass science and technology, answering the question, “What are the top 10 glass products/processes that have had the greatest impact on humankind?” Product innovations are colored blue, and process innovations are colored red.
important products rely on the optical transparency of silicate glasses. Other important properties for many of the products include mechanical strength, chemical durability, and dimensional stability. Finally, it is interesting to note that nearly all of these top innovations involve silicate glass compositions.
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The remainder of this chapter is organized into sections dealing with various grand challenges for glass science and technology in the fields of energy, the environment, information technology, architecture, transportation, pharmaceutical packaging, and bioactive glasses for healthcare applications.
23.2 Glass for energy applications As the world demand for energy increases, new technologies are required to enable renewable energy generation and more efficient energy storage. One of the most promising sources of renewable energy is, of course, the sun. Energy from the sun can be harvested via a variety of technologies, including photovoltaics, solar thermal energy generation, and photobioreactors. Glassy materials play a key role in each of these technologies for enhancing the solar energy conversion efficiency and enabling the overall functionality of the device. An excellent review of glasses for solar energy conversion is provided by Deubener et al. [3] Photovoltaic technologies are based on the photoelectric effect, where photons are absorbed by a material to excite electrons from the valence band to the conduction band. A variety of photoelectric materials can be used in photovoltaic solar cells, including crystalline and amorphous silicon, as well as thin-film semiconductors such as CdTe and copper indium gallium sulfide (CIGS). Fig. 23.2 shows a compilation of the maximum reported solar energy conversion efficiencies for photovoltaic solar cells using different material families. Glass design plays an important role in improving the solar energy conversion efficiency by increasing the fraction of solar energy that can be imparted on the semiconductor, for example, through increased transparency, light trapping, or antireflective coatings. Glasses also provide mechanical and chemical protection for ensuring the long-term functionality of the photovoltaic cells. Solar thermal devices offer an alternative method for harvesting energy from the sun. With solar thermal energy, parabolic glass mirrors reflect the sun’s rays, directing the light onto a glass tube along the focal path of the mirrors. The solar energy heats a fluid inside of the glass tube, which then powers a generator for the production of electricity. Glasses for solar thermal collectors must have high mechanical strength, chemical durability, and dimensional stability with respect to large changes in temperature. Photobioreactors are another approach for harvesting energy from the sun. With this approach, phototrophic microorganisms such as green algae are grown inside of glass tubes. When the microorganisms are exposed to
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Fig. 23.2 Plot of the highest confirmed conversion efficiencies for research solar cells, from 1976 to the present, for various photovoltaic technologies, as compiled by the National Renewable Energy Laboratory (NREL), US Department of Energy.
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solar radiation, they convert the solar energy into chemical energy through natural photosynthesis. The conditions of the photobioreactor, including the optical transmission and temperature, are optimized to maximize the rate of solar energy conversion by the microorganisms. Beyond solar energy, glass also plays a critical role in enabling efficient conversion of wind energy to electricity. The efficiency of windmills increases with longer turbine blades, which are made from fiberglassreinforced composites. Longer blades require the development of fiberglass compositions with higher Young’s modulus to increase the stiffness of the blades. The strength of the fiberglass-reinforced composite is also a critical property to enable larger, more efficient, and more reliable windmills. New glassy materials are also being developed for energy storage, including glass-based solid-state batteries. Here the goals are to improve energy storage density, reduce charging time, and increase the number of possible recharging cycles (i.e., improved cycling performance). The long-term safety of solid-state batteries is also of paramount concern. The performance of solid-state batteries requires having a charged species with sufficiently high mobility. Typically, Li+ or Na+ is used as the ion conducting species. Both glass-based and glass-ceramic-based materials have been proposed as appropriate solid-state electrolyte materials [4–7].
23.3 Glass and the environment Nuclear power generation results in both high- and low-level radioactive waste requiring geological time scales to decay fully. Hence, the safe (very) long-term storage of nuclear waste is of critical concern for the protection of our environment and to ensure the safety of humans and wildlife living near the waste disposal sites. One effective technique for storing liquid nuclear waste is to immobilize it through vitrification in a glassy matrix [8]. A schematic diagram of the steps involved with nuclear waste vitrification is shown in Fig. 23.3. Of course, the glass must be stable and chemically durable over thousands of years to ensure the long-term safety of the waste storage. A significant amount of research is currently being performed to develop glass chemistries that can enable higher solubility of the radioactive waste, while simultaneously ensuring a sufficiently high durability. Glassceramic materials have also been proposed for nuclear waste immobilization and may enable an even higher density of waste storage compared to glasses [9].
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Fig. 23.3 Schematic diagram of nuclear waste vitrification process. (Courtesy T. Kokot, Saxon Glass Technologies.)
While the past century has seen an unprecedented rise in living conditions of billions of people, many hundreds of millions of people still live in abject poverty with no or little access to clean water. The development of porous filter materials offers opportunities to sanitize water for safe consumption.
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Porous glasses for such applications may include foam glasses or glasses that have been phase separated by spinodal decomposition. In a phase-separated glass, typically one of the phases has a lower chemical durability than the other. This lower durability phase can be leached out using an appropriate acid bath treatment. This leaves an interconnected network of porosity, where the width of the pores reflects the width of the spinodal phases. Such filters may also be utilized for air purification, which is especially urgent issue in many developing countries. Glass is an environmentally friendly material in many ways. Most glasses are made from safe, readily available raw materials such as sand, soda ash, and limestone. Also, glasses can be reused and recycled any number of times. However, the large energy consumption required for glass melting has a negative impact on our environment. It is thus crucial for the glass industry to develop more energy-efficient melting technologies. One approach to reduce the environmental impact of glass manufacturing is to develop alternative glass compositions with lower melting temperatures compared to traditional soda lime silicate glasses. This is a challenging problem, since a lower melting point glass would typically have a lower concentration of silica, which may adversely affect properties such as chemical durability and optical transparency. Significant work is required in both glass composition development and melt engineering to improve the sustainability of industrial glass production, hopefully ultimately achieving the goal of carbon-neutral glass manufacturing.
23.4 Glass for information technology The invention of low-loss glass optical fibers was essential in the development of the Internet, enabling exponentially growing levels of communication across the globe [10]. As the demand for bandwidth continues to grow, new fiber optic technologies should be developed to enable more data to be transmitted over longer distances, while minimizing the need for signal amplification. This will involve further reductions in fiber attenuation, which may be achieved through improved material design or process optimization. Photonic crystal fibers may also enable lower-loss optical transmission, although significant process challenges must be overcome before this technology becomes viable for long-haul transmission [11]. Finally, new fibers must be developed to enable quantum communication, which can lead to secure communication via quantum entangled photons [12].
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Glass has also played a revolutionary role in information display, from early televisions based on cathode ray tubes to more recent flat panel displays [13]. As the resolution of these displays improves by the use of smaller pixel sizes, the requirements on the high-tech glass substrates become more and more stringent [14]. New glasses must be developed to improve dimensional stability during the display fabrication process, that is, to minimize the magnitude of volume relaxation as the thin film electronics that constitute the display are deposited onto the glass substrates. New ultrathin glasses are also being developed to enable bendable or even foldable displays [15]. Advanced glasses are also being developed for visualizing information through augmented and virtual reality devices, which perhaps represent the next revolution in information display technology [16]. In addition to their critical role in the transmission and display of information, glasses have also revolutionized data storage. Chalcogenide-based phase change memories have enabled rewritable data storage by toggling localized regions between glassy and crystalline states, which represent either 1 or 0 bits [17]. High-density magnetic memory disks are built on highstrength, high-stiffness glass substrates to enable faster rotational speeds and higher-density memory storage [18]. Finally, glasses are promising materials to enable next-generation holographic memories, which could enable exceptionally high data storage densities [19].
23.5 Glass for architecture While often taken for granted, glass windows enable visible light into homes and offices while protecting the occupants from harsh weather conditions [20]. Today, glass is used in architecture both for its practical functionality and for its appealing aesthetics. Some newly developed glasses for architectural applications include photochromic [21] and electrochromic [22] materials, which enable windows to dynamically adapt to sunlight conditions to improve energy efficiency. Vacuum insulated glazing is another technology to enable windows with improved energy efficiency [23]. Traditional double-pane windows are filled with a noble gas such as argon between the two glass panes, which reduces the transfer of heat across the window. With vacuum insulated glazing, a vacuum is used in place of the noble gas to further reduce heat transfer. The strength of the glass window panes becomes even more important with vacuum insulated
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glazing, since small spacers are required to maintain the plane parallel geometry of the two glass panes, and localized stresses are generated near the contact points of the glass with these spacers. Also, the window panes must be perfectly sealed to prevent the diffusion of air into the vacuum. Also in the architectural space, new laminated glasses are being developed to increase acoustic damping, that is, to reduce “noise pollution” in homes and offices. For example, a recent patent includes a viscoelastic acoustic damping layer between two panes of glass to increase the sound absorption [24]. Finally, glass substrates and fibers will play a critical role for enabling the “Internet of Things,” that is, the interconnection of everyday appliances and objects, such as stovetops, refrigerators, etc., within buildings. This connectivity will enable these objects to send and receive data, and perhaps even include built-in smart displays.
23.6 Glass for transportation Glass technologies have been essential in the development of modern transportation, including cars, trains, and airplanes. For example, chemically strengthened glasses are essential for the safety of airplane cockpit windshields. The fuel efficiency of automobiles can be improved through the use of thinner, lightweight strengthened glasses, while simultaneously enhancing the safety of the vehicles [25]. Glassy materials will also play a more prominent role in automotive interiors, especially with the development of selfdriving vehicles. With humans playing a less active role in the driving of the vehicle, glass displays and touch screens will provide new options for entertainment and connectivity on the road.
23.6.1 Antiballistic glass Resistance to high-velocity projectile impacts is a newer area of interest in stronger glass products. The use of glass for transparent armor as windows in military vehicles is a prime example. A ballistic event requires consideration of the event time of the order of a few microseconds. Sharp projectile traveling at speeds exceeding 600 m/s deliver energy to the target at rates greater than those absorbed by a plastic deformation due to local atomic motion around the indent (intrinsic), by flexing, or by displacement of the housing (extrinsic). Upon impact along the x-axis, two systems of waves are created: Rayleigh waves on the surface and P (primary or longitudinal)- and
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S (secondary or transvers)-body waves. The P-waves (compressional or dilatational) are polarized in the x-direction and the S-waves are polarized either parallel to the horizontal x-z plane (“S-H waves”) or parallel to the vertical x-y plane (“S-V waves”). The Rayleigh waves travel at 3390 m/s in silica glass, and 3300 m/s in soda lime glass and carry 67% of the total energy; longitudinal waves travel at 5860 m/s carrying 7% energy; and transverse waves travel at 3570 m/s and carry 26% energy. A complex system of cracks nucleates at the impact site. The longitudinal compressive wave is reflected back as a longitudinal tensile wave by the nonimpact (back) side of the glass pane and causes fracture at the back surface a few microseconds after the impact. Almost always, the ballistic armor is a laminate composed of some glass layers, transparent ceramic layers, polymer interlayers, and a polycarbonate backing. The purpose of the polymer interlayers and polycarbonate is to absorb a portion of the impacting energy and to keep the shards from landing into the vehicle interior. Measurements of ballistic impact and damage from cracking in glass are conducted using a split-Hopkinson bar-type setup [26] or an edge-onimpact described by Strassburger et al. [27, 28], both fitted with high-speed videography. There are at least five primary issues in the ballistic application of glass: (i) There is no such thing as an unbreakable glass. (ii) If the glass survives the first bullet, how would it behave against the second bullet? (iii) Will there be any vision left after glass cracks such that an occupant can see the enemy fire? (iv) The gross weight of the laminate. One does not want the vehicle axle to break or the brake lining to wear out rapidly. (v) The cost of the laminate. Currently used ballistic glass laminates are traditionally composed of several well-annealed thick glass layers bonded with polymers such as polyvinyl butyrate. Military vehicles often have five to seven glass layers each 1/2 in. thick. The weight of such assembly can be quite high. Strassburger et al. [28] show damage wave propagation in a four-layer glass laminate with polycarbonate backing captured by a high-speed camera at 90°. In the first three layers of glass, a damage wave traveling 1650 m/s is generated by the transverse mode, the delay in the interlayers, and then a damage wave in the fourth glass layer due to a reflected wave. Given that a high-velocity sharp projectile generates complex cracking due to both the hydrostatic and the deviatoric components of the stress field, the options to improve performance of the glass layer appear limited:
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(1) Reduce projectile energy by having a hard, high fracture toughness transparent ceramic as strike-face. From his experiments using 76 51 mm AP projectile impacting at 850 m/s velocity, Strassburger [27] showed that a 4-mm thick transparent Mg-spinel could effectively abrade the projectile to block its penetration; with a 2.2-mm thickness, the residual velocity was 120 m/s. (2) Use extremely high surface compression-containing glass, such as the Ion-Armor (marketed by Saxon Glass Technologies, Inc.) with nearly 1 GPa surface compression and 1 mm DOL as intermediate plies. The impact facing surface of Ion-Armor has a high dynamic hardness to help blunt the impacting projectile [29] and a back surface which avoids cracking due to the reflected tensile wave [30, 31]. The rather high internal tension in the Ion-Armor is currently an issue for vision after glass fracture and needs to be addressed.
23.7 Glass for pharmaceutical packaging Chemical inertness of vials, cartridges, and ampoules is of paramount importance for pharmaceutical applications to prevent the medicine from interacting with the glass during long-term storage [32]. The relatively high permeability to air and the sintering aids used in most molded plastics greatly reduce the shelf life for medicines. Likewise, metals which suffer from oxidation and reactivity with medicine are not usable for packaging. Glass containers for storing expensive cosmetics also follow pharmaceutical guidelines. Pharmaceutical glasses are classified into four types: • Type I pharmaceutical packaging is made from borosilicate glass having a low-thermal expansion coefficient and high chemical durability. Such borosilicate glass can safely store medicines that are either strong acids or strong bases. • Type II packaging is made from a surface-treated soda lime silicate glass. The surface treatment removes leachable ions from the glass surface to improve chemical durability compared to untreated soda lime glass. However, the chemical durability is still not as high as Type I borosilicate glass. • Type III packaging is made from standard soda lime silicate glass. Such packaging is suitable for storage of less corrosive medicines. • Finally, Type IV pharmaceutical packaging is a low-quality soda lime silicate having the poorest chemical durability. Although borosilicate glass compositions are the packaging of choice for most injectables, glass can be attacked by several potent medicines. The problem is particularly acute when glass vial forming temperatures fall in
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the phase separation region (Chapter 4). It is possible that the sodium borate phase is dissolved out and the fluid reaches subsurface to release silica-rich flakes [33]. Suspended flakes in medicines have resulted in expensive recalls in some cases. The problem is termed delamination [34]. An additional problem that often plagues glass packaging is chipping during medicine filling operation and glass cracking due to transportation damage as well as due to handling or during medicine delivery. The answer in some cases is to use chemically strengthened glass containers. An example of one such product is the EpiPen auto-injector cartridge to treat emergencies from severe allergic reactions such as beestings. The glass used for the EpiPen cartridge must be both chemically durable and mechanically strong to withstand the high pressures associated with the injection process. A proprietary chemical strengthening process developed and commercialized by Saxon Glass Technologies (founded by the senior author of this textbook) has proved extraordinarily successful for reducing failure rates to the sub-ppm level. As of the printing of this book, several million devices have been strengthened through this process, with no known failures.
23.8 Biocompatible and bioactive glasses Also within the realm of health care, biocompatible and bioactive glasses and glass-ceramics have been developed to address a wide range of medical problems, including bone repair, cancer therapy, soft tissue repair, and dental restorations [35]. Such materials are already improving the lives of millions of patients across the world. Glass-ceramic materials have revolutionized the field of restorative dentistry, replacing traditionally used metal-based solutions for more than 200 million people worldwide [36]. Example applications of glass-ceramics include dental inlays, onlays, veneers, and bridges. Glass-ceramic materials for such applications must have high mechanical strength and toughness, as well as high chemical durability. The optical properties of the glassceramics, such as coloration and translucency, should also match those of natural teeth as closely as possible. Bioactive glass powders have recently been incorporated into toothpaste to stimulate the remineralization of dental enamel. Such remineralization both protects the teeth and reduces sensitivity to hot and cold liquids. Biocompatible and bioactive glass scaffolds are also having a transformative effect on hard tissue repair applications such as bone tissue engineering.
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The regeneration of bone defects requires materials that are mechanically strong and tough, without being rejected by the body’s immune system. More recent bioactive glasses can even help stimulate the body’s natural healing process to regrow natural bone tissue [37]. Newly developed bioactive glasses are also being used for soft tissue repair applications. Deep, persistent wounds are a serious health concern, especially in diabetic patients. Newly developed borate glass fibers have demonstrated the remarkable ability to heal soft tissue wounds where no previous therapy had succeeded [38]. The borate glass fibers are packed into the wound, degrading over a period of several days. As the glass degrades, it releases chemicals that the body needs to enable soft tissue repair. Moreover, the fibrous morphology of the glasses provides pathways along which the body can regrow tissue. The degradation rate of the borate glasses is a key parameter to ensure that the chemicals are delivered to the body at an appropriate rate for the tissue growth processes [39]. Further research is still required to optimize glass compositions for soft tissue repair and to understand the fundamental mechanisms governing the healing process. The mention of “research” reminds the authors of the large backlog that has piled up during the writing of this third edition. Time for us to get back to work!
23.9 Glass greats: Elias Snitzer Elias Snitzer (1925–2012, see Fig. 23.4) was born in Lynn, Massachusetts, and graduated from Tufts University with a BS in electrical engineering (1945) and from the University of Chicago with MS (1950) and PhD (1953) degrees in physics. Snitzer began his work on Nd-doped glass lasers while working at the American Optical Company. In 1961 he published a theoretical description of optical modes in glass fibers, and his report on the first successful glass laser soon followed. Snitzer served as director of corporate research at American Optical until 1977, when he joined United Technologies Research Center and later Polaroid (1984). While at Polaroid, Snitzer invented the double-clad fiber laser, which is widely used in many optical communications systems. Snitzer joined the faculty at Rutgers University in 1989, where he was a devoted teacher and continued his research in fiber lasers, amplifiers, and glass Bragg gratings. Snitzer was the winner of numerous international awards for his research, including the Otto Schott Research Award (1999).
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Fig. 23.4 Elias Snitzer (1925–2012), inventor of the fiber laser.
References [1] D.L. Morse, J.W. Evenson, Int. J. Appl. Glas. Sci. 7 (2016) 409. [2] J.C. Mauro, Curr. Opin. Solid State Mater. Sci. 22 (2018) 58. [3] J. Deubener, G. Helsch, A. Moiseev, H. Bornh€ oft, J. Euro. Ceram. Soc. 29 (2009) 1203. [4] A. Hayashi, T. Ohtomo, F. Mizuno, K. Tadanaga, M. Tatsumisago, Electrochem. Commun. 5 (2003) 701. [5] K. Takada, N. Aotani, K. Iwamoto, S. Kondo, Solid State Ionics 86–88 (1996) 877. [6] S.W. Martin, J. Am. Ceram. Soc. 74 (1991) 1767. [7] M.H. Braga, A.J. Murchison, J.A. Ferreira, P. Singh, J.B. Goodenough, Energy Environ. Sci. 9 (2016) 948. [8] J.D. Vienna, Int. J. Appl. Glas. Sci. 1 (2010) 309. [9] J.S. McCloy, A. Goel, MRS Bull. 42 (2017) 233. [10] R.D. Maurer, Proc. IEEE 61 (1973) 452. [11] P. Russell, Science 299 (2003) 358. [12] A. Vaziri, G. Weihs, A. Zeilinger, Phys. Rev. Lett. 89 (2002). 240401. [13] A. Ellison, I.A. Cornejo, Int. J. Appl. Glas. Sci. 1 (2010) 87. [14] Q. Zheng, J.C. Mauro, J. Chem. Phys. 146 (2017). 074504. [15] S.M. Garner (Ed.), Flexible Glass: Enabling Thin, Lightweight, and Flexible Electronics, John Wiley & Sons, 2017. [16] C.W. Chen, T.J. Ho, I.C. Khoo, Laser Science, JTu2A-9, 2017. [17] M. Wuttig, H. Bhaskaran, T. Taubner, Nat. Photonics 11 (2017) 465. [18] T. Takahashi and M. Ueda, U.S. Patent 9,583,128 (2017). [19] P. Cheben, M.L. Calvo, Appl. Phys. Lett. 78 (2001) 1490.
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[20] M. Arbab, J.J. Finley, Int. J. Appl. Glas. Sci. 1 (2010) 118. [21] N. F. Borrelli, N. T. L€ onnroth, M. Prassas, P. M. Then, and L. A. Zenteno, U.S. Patent Application 20170075049 A1 (2017). [22] C.G. Granqvist, Thin Solid Films 564 (2014) 1. [23] E. Cuce, P.M. Cuce, Renew. Sust. Energ. Rev. 54 (2016) 1345. [24] C. Payen and D. Fournier, U.S. Patent 9,994,001 (2018). [25] T. Leonhard, T. Cleary, M. Moore, S. Seyler, W.K. Fisher, SAE Int. J. Passenger CarsMechan. Sys. 8 (2015) 95. [26] B. Hopkinson, Philos, Trans. R. Soc. (London) A 213 (437) (1914). [27] E. Strassburger, J. Euro. Ceram. Soc. 29 (2009) 267. [28] E. Strassburger, S. Bauer, G. Popko, Defense Tech. 10 (2014) 226. [29] P. Jannotti, G. Subhash, P. Ifju, P.K. Kreski, A.K. Varshneya, J. Euro. Ceram. Soc. 32 (2012) 1551. [30] P. Jannotti, G. Subhash, A.K. Varshneya, Int. J. Impact Eng. 75 (2015) 53. [31] A. K. Varshneya and P. K. Kreski, U.S. Patent 9,162,426 B2 (2015). [32] V. Dimbleby, J. Pharm. Pharmacol. 5 (1953) 969. [33] P.K. Kreski, A.K. Varshneya, Ceram. Trans. 231 (2012) 85. [34] R.G. Iacocca, M. Allgeier, J. Mater. Sci. 42 (2007) 801–811. [35] G. Kaur, Bioactive Glasses: Potential Biomaterials for Future Therapy, Springer, 2017. [36] W. H€ oland, G.H. Beall, Glass-Ceramic Technology, second ed., John Wiley & Sons, 2012. [37] Q. Fu, E. Saiz, M.N. Rahaman, A.P. Tomsia, Mater. Sci. Eng. C 31 (2011) 1245. [38] P. Wray, Am. Ceram. Soc. Bull. 90 (2011) 24. [39] M.N. Rahaman, D.E. Day, B.S. Bal, Q. Fu, S.B. Jung, L.F. Bonewald, A.P. Tomsia, Acta Biomater. 7 (2011) 2355.
Emerging applications of glass 23.1 Introduction From glass windows and containers to lenses and optical fiber, glass has proven to be one of the most important materials for enabling the development of contemporary human civilization. Owing to this indelible impact of glass on modern society, it has been proposed that we are now living in The Glass Age [1]. The positive influence of glass on our world continues to grow as new glass products and processes are developed to address global challenges in energy, the environment, health care, information/communication technology, and more [2]. In this closing chapter, we review some new emerging applications of glass that promise to address many of these grand challenges. We hope that these new and anticipated applications of glass will inspire students to pursue dedicated research in these areas to invent the new materials and processes required for solving these grand challenges and improving the quality of life for all of humanity. To understand which emerging applications of glass could have the greatest impact on our society, it is helpful to put previous glass innovations into some historical context. To this end, the authors commissioned a survey of professionals in the glass field. A total of 91 respondents ranked the top 10 glass innovations that have had the greatest impact on humankind. The score for each innovation is calculated by assigning 10 points for each first place ranking, 9 points for each second place ranking, etc., and 1 point for each 10th place ranking. Total scores for all 91 respondents are plotted in Fig. 23.1. According to the survey results, the top 10 glass innovations having the greatest impact on humankind are: (1) glass lenses, (2) optical fiber, (3) glass windows, (4) the Pilkington float process, (5) glass lightbulb envelopes, (6) glass containers, (7) laboratory glass, (8) television glass (for cathode ray tubes), (9) glass-ceramics, and (10) bioactive glasses.1 Product innovations dominate the list, and the Pilkington float process is justifiably ranked as the most impactful industrial glass process. Of course, many of these 1
Varshneya and Mauro’s ranking: (1) windows, (2) lenses, (3) light bulb, and (4) optical fiber.
Fundamentals of Inorganic Glasses https://doi.org/10.1016/B978-0-12-816225-5.00023-7
© 2019 Elsevier Inc. All rights reserved.
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What are the top ten glass products/processes that have had the greatest impact on humankind? 0
100
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300
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Glass lenses (ophthalmic, microscope, telescope) Optical fiber Glass windows (for architecture and transportation) Pilkington float process Glass lightbulb envelopes Glass containers (beverages, packaging, pharmaceutical) Laboratory glasses Television glass (cathode ray tubes) Glass-ceramics Bioactive glasses Chemical strengthening process Thermal tempering process Liquid crystal display substrates High purity fused silica Glass mirrors Strengthened cover glass for mobile devices Fusion draw process Glass insulation wool Laser glass Siemens regenerative continuous glass-melting tank... Glass optical amplifiers Ultra-low expansion glass Stained glass Glass fiber-reinforced composites Glass baking dishes Glass tubing Ribbon machine Chemical vapor deposition process Space shuttle glass Sealing glasses Anti-reflective coatings Chalcogenide glass based memory and threshold switches Lead glass (”crystal”) Infrared transmitting glasses Laminated glasses Window self-cleaning process Bulk metallic glasses Sol-gel process Foam glass Vello/danner processes for tube making Other
Fig. 23.1 Compiled survey responses from 91 experts in glass science and technology, answering the question, “What are the top 10 glass products/processes that have had the greatest impact on humankind?” Product innovations are colored blue, and process innovations are colored red.
important products rely on the optical transparency of silicate glasses. Other important properties for many of the products include mechanical strength, chemical durability, and dimensional stability. Finally, it is interesting to note that nearly all of these top innovations involve silicate glass compositions.
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The remainder of this chapter is organized into sections dealing with various grand challenges for glass science and technology in the fields of energy, the environment, information technology, architecture, transportation, pharmaceutical packaging, and bioactive glasses for healthcare applications.
23.2 Glass for energy applications As the world demand for energy increases, new technologies are required to enable renewable energy generation and more efficient energy storage. One of the most promising sources of renewable energy is, of course, the sun. Energy from the sun can be harvested via a variety of technologies, including photovoltaics, solar thermal energy generation, and photobioreactors. Glassy materials play a key role in each of these technologies for enhancing the solar energy conversion efficiency and enabling the overall functionality of the device. An excellent review of glasses for solar energy conversion is provided by Deubener et al. [3] Photovoltaic technologies are based on the photoelectric effect, where photons are absorbed by a material to excite electrons from the valence band to the conduction band. A variety of photoelectric materials can be used in photovoltaic solar cells, including crystalline and amorphous silicon, as well as thin-film semiconductors such as CdTe and copper indium gallium sulfide (CIGS). Fig. 23.2 shows a compilation of the maximum reported solar energy conversion efficiencies for photovoltaic solar cells using different material families. Glass design plays an important role in improving the solar energy conversion efficiency by increasing the fraction of solar energy that can be imparted on the semiconductor, for example, through increased transparency, light trapping, or antireflective coatings. Glasses also provide mechanical and chemical protection for ensuring the long-term functionality of the photovoltaic cells. Solar thermal devices offer an alternative method for harvesting energy from the sun. With solar thermal energy, parabolic glass mirrors reflect the sun’s rays, directing the light onto a glass tube along the focal path of the mirrors. The solar energy heats a fluid inside of the glass tube, which then powers a generator for the production of electricity. Glasses for solar thermal collectors must have high mechanical strength, chemical durability, and dimensional stability with respect to large changes in temperature. Photobioreactors are another approach for harvesting energy from the sun. With this approach, phototrophic microorganisms such as green algae are grown inside of glass tubes. When the microorganisms are exposed to
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Fig. 23.2 Plot of the highest confirmed conversion efficiencies for research solar cells, from 1976 to the present, for various photovoltaic technologies, as compiled by the National Renewable Energy Laboratory (NREL), US Department of Energy.
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solar radiation, they convert the solar energy into chemical energy through natural photosynthesis. The conditions of the photobioreactor, including the optical transmission and temperature, are optimized to maximize the rate of solar energy conversion by the microorganisms. Beyond solar energy, glass also plays a critical role in enabling efficient conversion of wind energy to electricity. The efficiency of windmills increases with longer turbine blades, which are made from fiberglassreinforced composites. Longer blades require the development of fiberglass compositions with higher Young’s modulus to increase the stiffness of the blades. The strength of the fiberglass-reinforced composite is also a critical property to enable larger, more efficient, and more reliable windmills. New glassy materials are also being developed for energy storage, including glass-based solid-state batteries. Here the goals are to improve energy storage density, reduce charging time, and increase the number of possible recharging cycles (i.e., improved cycling performance). The long-term safety of solid-state batteries is also of paramount concern. The performance of solid-state batteries requires having a charged species with sufficiently high mobility. Typically, Li+ or Na+ is used as the ion conducting species. Both glass-based and glass-ceramic-based materials have been proposed as appropriate solid-state electrolyte materials [4–7].
23.3 Glass and the environment Nuclear power generation results in both high- and low-level radioactive waste requiring geological time scales to decay fully. Hence, the safe (very) long-term storage of nuclear waste is of critical concern for the protection of our environment and to ensure the safety of humans and wildlife living near the waste disposal sites. One effective technique for storing liquid nuclear waste is to immobilize it through vitrification in a glassy matrix [8]. A schematic diagram of the steps involved with nuclear waste vitrification is shown in Fig. 23.3. Of course, the glass must be stable and chemically durable over thousands of years to ensure the long-term safety of the waste storage. A significant amount of research is currently being performed to develop glass chemistries that can enable higher solubility of the radioactive waste, while simultaneously ensuring a sufficiently high durability. Glassceramic materials have also been proposed for nuclear waste immobilization and may enable an even higher density of waste storage compared to glasses [9].
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Fig. 23.3 Schematic diagram of nuclear waste vitrification process. (Courtesy T. Kokot, Saxon Glass Technologies.)
While the past century has seen an unprecedented rise in living conditions of billions of people, many hundreds of millions of people still live in abject poverty with no or little access to clean water. The development of porous filter materials offers opportunities to sanitize water for safe consumption.
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Porous glasses for such applications may include foam glasses or glasses that have been phase separated by spinodal decomposition. In a phase-separated glass, typically one of the phases has a lower chemical durability than the other. This lower durability phase can be leached out using an appropriate acid bath treatment. This leaves an interconnected network of porosity, where the width of the pores reflects the width of the spinodal phases. Such filters may also be utilized for air purification, which is especially urgent issue in many developing countries. Glass is an environmentally friendly material in many ways. Most glasses are made from safe, readily available raw materials such as sand, soda ash, and limestone. Also, glasses can be reused and recycled any number of times. However, the large energy consumption required for glass melting has a negative impact on our environment. It is thus crucial for the glass industry to develop more energy-efficient melting technologies. One approach to reduce the environmental impact of glass manufacturing is to develop alternative glass compositions with lower melting temperatures compared to traditional soda lime silicate glasses. This is a challenging problem, since a lower melting point glass would typically have a lower concentration of silica, which may adversely affect properties such as chemical durability and optical transparency. Significant work is required in both glass composition development and melt engineering to improve the sustainability of industrial glass production, hopefully ultimately achieving the goal of carbon-neutral glass manufacturing.
23.4 Glass for information technology The invention of low-loss glass optical fibers was essential in the development of the Internet, enabling exponentially growing levels of communication across the globe [10]. As the demand for bandwidth continues to grow, new fiber optic technologies should be developed to enable more data to be transmitted over longer distances, while minimizing the need for signal amplification. This will involve further reductions in fiber attenuation, which may be achieved through improved material design or process optimization. Photonic crystal fibers may also enable lower-loss optical transmission, although significant process challenges must be overcome before this technology becomes viable for long-haul transmission [11]. Finally, new fibers must be developed to enable quantum communication, which can lead to secure communication via quantum entangled photons [12].
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Glass has also played a revolutionary role in information display, from early televisions based on cathode ray tubes to more recent flat panel displays [13]. As the resolution of these displays improves by the use of smaller pixel sizes, the requirements on the high-tech glass substrates become more and more stringent [14]. New glasses must be developed to improve dimensional stability during the display fabrication process, that is, to minimize the magnitude of volume relaxation as the thin film electronics that constitute the display are deposited onto the glass substrates. New ultrathin glasses are also being developed to enable bendable or even foldable displays [15]. Advanced glasses are also being developed for visualizing information through augmented and virtual reality devices, which perhaps represent the next revolution in information display technology [16]. In addition to their critical role in the transmission and display of information, glasses have also revolutionized data storage. Chalcogenide-based phase change memories have enabled rewritable data storage by toggling localized regions between glassy and crystalline states, which represent either 1 or 0 bits [17]. High-density magnetic memory disks are built on highstrength, high-stiffness glass substrates to enable faster rotational speeds and higher-density memory storage [18]. Finally, glasses are promising materials to enable next-generation holographic memories, which could enable exceptionally high data storage densities [19].
23.5 Glass for architecture While often taken for granted, glass windows enable visible light into homes and offices while protecting the occupants from harsh weather conditions [20]. Today, glass is used in architecture both for its practical functionality and for its appealing aesthetics. Some newly developed glasses for architectural applications include photochromic [21] and electrochromic [22] materials, which enable windows to dynamically adapt to sunlight conditions to improve energy efficiency. Vacuum insulated glazing is another technology to enable windows with improved energy efficiency [23]. Traditional double-pane windows are filled with a noble gas such as argon between the two glass panes, which reduces the transfer of heat across the window. With vacuum insulated glazing, a vacuum is used in place of the noble gas to further reduce heat transfer. The strength of the glass window panes becomes even more important with vacuum insulated
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glazing, since small spacers are required to maintain the plane parallel geometry of the two glass panes, and localized stresses are generated near the contact points of the glass with these spacers. Also, the window panes must be perfectly sealed to prevent the diffusion of air into the vacuum. Also in the architectural space, new laminated glasses are being developed to increase acoustic damping, that is, to reduce “noise pollution” in homes and offices. For example, a recent patent includes a viscoelastic acoustic damping layer between two panes of glass to increase the sound absorption [24]. Finally, glass substrates and fibers will play a critical role for enabling the “Internet of Things,” that is, the interconnection of everyday appliances and objects, such as stovetops, refrigerators, etc., within buildings. This connectivity will enable these objects to send and receive data, and perhaps even include built-in smart displays.
23.6 Glass for transportation Glass technologies have been essential in the development of modern transportation, including cars, trains, and airplanes. For example, chemically strengthened glasses are essential for the safety of airplane cockpit windshields. The fuel efficiency of automobiles can be improved through the use of thinner, lightweight strengthened glasses, while simultaneously enhancing the safety of the vehicles [25]. Glassy materials will also play a more prominent role in automotive interiors, especially with the development of selfdriving vehicles. With humans playing a less active role in the driving of the vehicle, glass displays and touch screens will provide new options for entertainment and connectivity on the road.
23.6.1 Antiballistic glass Resistance to high-velocity projectile impacts is a newer area of interest in stronger glass products. The use of glass for transparent armor as windows in military vehicles is a prime example. A ballistic event requires consideration of the event time of the order of a few microseconds. Sharp projectile traveling at speeds exceeding 600 m/s deliver energy to the target at rates greater than those absorbed by a plastic deformation due to local atomic motion around the indent (intrinsic), by flexing, or by displacement of the housing (extrinsic). Upon impact along the x-axis, two systems of waves are created: Rayleigh waves on the surface and P (primary or longitudinal)- and
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S (secondary or transvers)-body waves. The P-waves (compressional or dilatational) are polarized in the x-direction and the S-waves are polarized either parallel to the horizontal x-z plane (“S-H waves”) or parallel to the vertical x-y plane (“S-V waves”). The Rayleigh waves travel at 3390 m/s in silica glass, and 3300 m/s in soda lime glass and carry 67% of the total energy; longitudinal waves travel at 5860 m/s carrying 7% energy; and transverse waves travel at 3570 m/s and carry 26% energy. A complex system of cracks nucleates at the impact site. The longitudinal compressive wave is reflected back as a longitudinal tensile wave by the nonimpact (back) side of the glass pane and causes fracture at the back surface a few microseconds after the impact. Almost always, the ballistic armor is a laminate composed of some glass layers, transparent ceramic layers, polymer interlayers, and a polycarbonate backing. The purpose of the polymer interlayers and polycarbonate is to absorb a portion of the impacting energy and to keep the shards from landing into the vehicle interior. Measurements of ballistic impact and damage from cracking in glass are conducted using a split-Hopkinson bar-type setup [26] or an edge-onimpact described by Strassburger et al. [27, 28], both fitted with high-speed videography. There are at least five primary issues in the ballistic application of glass: (i) There is no such thing as an unbreakable glass. (ii) If the glass survives the first bullet, how would it behave against the second bullet? (iii) Will there be any vision left after glass cracks such that an occupant can see the enemy fire? (iv) The gross weight of the laminate. One does not want the vehicle axle to break or the brake lining to wear out rapidly. (v) The cost of the laminate. Currently used ballistic glass laminates are traditionally composed of several well-annealed thick glass layers bonded with polymers such as polyvinyl butyrate. Military vehicles often have five to seven glass layers each 1/2 in. thick. The weight of such assembly can be quite high. Strassburger et al. [28] show damage wave propagation in a four-layer glass laminate with polycarbonate backing captured by a high-speed camera at 90°. In the first three layers of glass, a damage wave traveling 1650 m/s is generated by the transverse mode, the delay in the interlayers, and then a damage wave in the fourth glass layer due to a reflected wave. Given that a high-velocity sharp projectile generates complex cracking due to both the hydrostatic and the deviatoric components of the stress field, the options to improve performance of the glass layer appear limited:
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(1) Reduce projectile energy by having a hard, high fracture toughness transparent ceramic as strike-face. From his experiments using 76 51 mm AP projectile impacting at 850 m/s velocity, Strassburger [27] showed that a 4-mm thick transparent Mg-spinel could effectively abrade the projectile to block its penetration; with a 2.2-mm thickness, the residual velocity was 120 m/s. (2) Use extremely high surface compression-containing glass, such as the Ion-Armor (marketed by Saxon Glass Technologies, Inc.) with nearly 1 GPa surface compression and 1 mm DOL as intermediate plies. The impact facing surface of Ion-Armor has a high dynamic hardness to help blunt the impacting projectile [29] and a back surface which avoids cracking due to the reflected tensile wave [30, 31]. The rather high internal tension in the Ion-Armor is currently an issue for vision after glass fracture and needs to be addressed.
23.7 Glass for pharmaceutical packaging Chemical inertness of vials, cartridges, and ampoules is of paramount importance for pharmaceutical applications to prevent the medicine from interacting with the glass during long-term storage [32]. The relatively high permeability to air and the sintering aids used in most molded plastics greatly reduce the shelf life for medicines. Likewise, metals which suffer from oxidation and reactivity with medicine are not usable for packaging. Glass containers for storing expensive cosmetics also follow pharmaceutical guidelines. Pharmaceutical glasses are classified into four types: • Type I pharmaceutical packaging is made from borosilicate glass having a low-thermal expansion coefficient and high chemical durability. Such borosilicate glass can safely store medicines that are either strong acids or strong bases. • Type II packaging is made from a surface-treated soda lime silicate glass. The surface treatment removes leachable ions from the glass surface to improve chemical durability compared to untreated soda lime glass. However, the chemical durability is still not as high as Type I borosilicate glass. • Type III packaging is made from standard soda lime silicate glass. Such packaging is suitable for storage of less corrosive medicines. • Finally, Type IV pharmaceutical packaging is a low-quality soda lime silicate having the poorest chemical durability. Although borosilicate glass compositions are the packaging of choice for most injectables, glass can be attacked by several potent medicines. The problem is particularly acute when glass vial forming temperatures fall in
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the phase separation region (Chapter 4). It is possible that the sodium borate phase is dissolved out and the fluid reaches subsurface to release silica-rich flakes [33]. Suspended flakes in medicines have resulted in expensive recalls in some cases. The problem is termed delamination [34]. An additional problem that often plagues glass packaging is chipping during medicine filling operation and glass cracking due to transportation damage as well as due to handling or during medicine delivery. The answer in some cases is to use chemically strengthened glass containers. An example of one such product is the EpiPen auto-injector cartridge to treat emergencies from severe allergic reactions such as beestings. The glass used for the EpiPen cartridge must be both chemically durable and mechanically strong to withstand the high pressures associated with the injection process. A proprietary chemical strengthening process developed and commercialized by Saxon Glass Technologies (founded by the senior author of this textbook) has proved extraordinarily successful for reducing failure rates to the sub-ppm level. As of the printing of this book, several million devices have been strengthened through this process, with no known failures.
23.8 Biocompatible and bioactive glasses Also within the realm of health care, biocompatible and bioactive glasses and glass-ceramics have been developed to address a wide range of medical problems, including bone repair, cancer therapy, soft tissue repair, and dental restorations [35]. Such materials are already improving the lives of millions of patients across the world. Glass-ceramic materials have revolutionized the field of restorative dentistry, replacing traditionally used metal-based solutions for more than 200 million people worldwide [36]. Example applications of glass-ceramics include dental inlays, onlays, veneers, and bridges. Glass-ceramic materials for such applications must have high mechanical strength and toughness, as well as high chemical durability. The optical properties of the glassceramics, such as coloration and translucency, should also match those of natural teeth as closely as possible. Bioactive glass powders have recently been incorporated into toothpaste to stimulate the remineralization of dental enamel. Such remineralization both protects the teeth and reduces sensitivity to hot and cold liquids. Biocompatible and bioactive glass scaffolds are also having a transformative effect on hard tissue repair applications such as bone tissue engineering.
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The regeneration of bone defects requires materials that are mechanically strong and tough, without being rejected by the body’s immune system. More recent bioactive glasses can even help stimulate the body’s natural healing process to regrow natural bone tissue [37]. Newly developed bioactive glasses are also being used for soft tissue repair applications. Deep, persistent wounds are a serious health concern, especially in diabetic patients. Newly developed borate glass fibers have demonstrated the remarkable ability to heal soft tissue wounds where no previous therapy had succeeded [38]. The borate glass fibers are packed into the wound, degrading over a period of several days. As the glass degrades, it releases chemicals that the body needs to enable soft tissue repair. Moreover, the fibrous morphology of the glasses provides pathways along which the body can regrow tissue. The degradation rate of the borate glasses is a key parameter to ensure that the chemicals are delivered to the body at an appropriate rate for the tissue growth processes [39]. Further research is still required to optimize glass compositions for soft tissue repair and to understand the fundamental mechanisms governing the healing process. The mention of “research” reminds the authors of the large backlog that has piled up during the writing of this third edition. Time for us to get back to work!
23.9 Glass greats: Elias Snitzer Elias Snitzer (1925–2012, see Fig. 23.4) was born in Lynn, Massachusetts, and graduated from Tufts University with a BS in electrical engineering (1945) and from the University of Chicago with MS (1950) and PhD (1953) degrees in physics. Snitzer began his work on Nd-doped glass lasers while working at the American Optical Company. In 1961 he published a theoretical description of optical modes in glass fibers, and his report on the first successful glass laser soon followed. Snitzer served as director of corporate research at American Optical until 1977, when he joined United Technologies Research Center and later Polaroid (1984). While at Polaroid, Snitzer invented the double-clad fiber laser, which is widely used in many optical communications systems. Snitzer joined the faculty at Rutgers University in 1989, where he was a devoted teacher and continued his research in fiber lasers, amplifiers, and glass Bragg gratings. Snitzer was the winner of numerous international awards for his research, including the Otto Schott Research Award (1999).
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Fig. 23.4 Elias Snitzer (1925–2012), inventor of the fiber laser.
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