A bright future for glass-ceramics From their glorious past, starting with their accidental discovery, to successful commercial products, the impressive range of properties and exciting potential applications of glass-ceramics indeed ensure a bright future! by Edgar Dutra Zanotto lass-ceramics were discovered – somewhat accidently G – in 1953. Since then, many exciting papers have been published and patents granted related to glass-ceram- ics by research institutes, universities and companies worldwide. Glass-ceramics (also known as vitro- cerams, pyrocerams, vitrocerâmicos, vitroceramiques and sittals) are produced by controlled crystal- lization of certain glasses – generally induced by nucleating additives. This is in contrast with sponta- neous sur- face crys- tallization, which is normally not wanted in glass manufacturing. They always con- tain a residual glassy phase and one or more embedded crystalline phases. The crystallinity varies between 0.5 and 99.5 percent, most frequently between 30 and 70 percent. Controlled ceramization yields an (Credit: Schott North America.) array of materials with interesting, sometimes unusual, combinations of properties. American Ceramic Society Bulletin, Vol. 89, No. 8 19 A bright future for glass-ceramics Unlike sintered ceramics, glass- ceramics are inherently free from poros- ity. However, in some cases, bubbles or pores develop during the latter stages of crystallization. Glass-ceramics have, in principle, several advantages. • They can be mass produced by any glass-forming technique. • It is possible to design their nano- structure or microstructure for a given application. Fig. 1. Standing, from left to right, TC-7 members Ralf Muller, Guenter VoelKsch, Linda • They have zero or very low porosity. Pinckney, Edgar Zanotto, Wolfgang Pannhorst, Takayuki Komatsu, Miguel Prado, • It is possible for them to combine Michael Budd, Joachim Deubener, Wolfram Hoeland and Ian Donald. Sitting are distin- a variety of desired properties. guished guests George Beall and Donald Stookey. Picture taken in Jackson Hole, Wyo., One example of the fourth advan- September 2006. tage is combining very low thermal hot-pressing techniques. The sintering (114 articles), Schott Glaswerke (69), expansion coefficient with transparency route also is attractive to produce glass- IBM (65), Nippon Electric Glass Co. in the visible wavelength range for ceramics from reluctant glass-forming (30), Ivoclar Vivadent AG (29), NEC cooking ware. Another is combining compositions, which could be made as Corp. (24), Aerospace Corp. (20) and very high strength and toughness with a “frit,” molded and sinter-crystallized. Toyota TI (18). translucency, biocompatibility, chemi- Commercial applications of sintered There are far too many papers cal durability and relatively low hard- glass-ceramics include devitrifying frit and patents to be cited in this short ness for dental applications. solder glasses for sealing TV tubes, “insight” article. Thus, we will direct Glass-ceramics are normally pro- cofired multilayer substrates for elec- the interested reader to a limited num- duced in two steps. First, a glass is tronic packaging, marblelike floor and ber of key books and papers, including formed by a standard glass-manufactur- wall tile (Neopariés and similar brands) some of our own. The fundamentals ing process. Second, the glass article and some bioactive glass-ceramics. behind the understanding and control is shaped, cooled and reheated above References 1–6 provide a list of recent of glass crystallization concern the its glass transition temperature. The articles and reviews on the fundamen- mechanisms, thermodynamics and second step is sometimes repeated as a tals of the sinter-crystallization process. kinetics of crystal nucleation, growth third step. In these heat treatments, the and overall crystallization. Several article partly crystallizes in the interior. Patents and papers groups have focused on such studies In most cases, nucleating agents (e.g., An idea of the scientific and com- during the past century. Interested noble metals, fluorides, ZrO2, TiO2, mercial importance of glass-ceramics readers are invited to check References P2O5, Cr2O3 or Fe2O3) are added to the comes from a search on Free-patents 7–9 for reviews on the basics of inter- base glass composition to boost the Online, which comprises granted pat- nal and surface nucleation in glasses. nucleation process. ents or applications in the United Readers are referred to classical text- A less frequently used method is to States, Europe and Japan. About 2,400 books in References 10–12 and review induce and control internal crystalliza- granted or filed U.S. patents appear articles in References 13–16 for more tion during the cooling path of a molten with the keywords “glass ceramic” in detailed information on glass-ceramics. viscous liquid. This process is used some- the abstract. There also are about 1,500 times to form relatively coarse-grained European and 2,700 Japanese patents. Discovery of glass-ceramics glass-ceramics from waste materials to be There is some overlap in these num- Natural glass-ceramics, such as used in the construction industry. bers, because the same patent is often some types of obsidian, “always” have Glass-ceramics also can be produced deposited in different countries. existed. Synthetic glass-ceramics were by concurrent sinter-crystallization of A similar search for published papers serendipitously discovered in 1953. glass-particle compacts. In this case, in the Scopus database with the same Stanley Donald Stookey, then a young crystallization starts at glass–particle keywords yields about 10,000 articles. researcher at Corning Glass Works, interfaces. A main advantage of the These are very impressive numbers for meant to anneal a piece of a lithium sinter-crystallization process is that such a narrow field within all the numer- disilicate glass with precipitated silver nucleating agents are not necessary, ous materials classes and types. This sug- particles (meant to form a permanent because the particle surfaces provide gests that plenty is already known about photographic image) in a furnace at nucleation sites. A disadvantage of this glass-ceramics technology. A similar 600°C. He accidentally overheated the method is 0.5 to 3.0 percent residual search in Scopus indicates that, since glass to about 900°C. “Damm it, I’ve porosity. However, this can be some- 1960, the most prolific companies in ruined a furnace!” Stookey thought. times minimized or even eliminated by glass-ceramics research are Corning Inc. Instead of a melted pool of glass, the 20 American Ceramic Society Bulletin, Vol. 89, No. 8 astonished Stookey observed a white modern and very inter- material that had not changed shape. esting glass-ceramics, He then accidentally dropped the is represented by piece on the floor, but it did not shat- Corning’s Fotoceram ter, contrary to what might normally (also invented by have been expected from a piece of Stookey) and Schott’s glass! He was surprised by the unusual Foturan. These glass- toughness of that material. Stookey ceramics can be pat- had accidentally created the first glass- terned by ultraviolet Fig. 2 Glass-ceramic teeth. 17 ceramic, denominated Fotoceram. light and selectively crystallized by ther- with TiO2 are the most commonly used In their book, Volfram Hoeland and mal treatments. The crystallized regions nucleation agents. The main crystalline George Beall mention that “knowledge then are completely dissolved by acid phase is a β-quartz solid solution, which of the literature, good observation skills etching. The patterned glass can be used is highly anisotropic and has an overall and deductive reasoning were clearly as-is or can be heated once more to form negative TCE. LAS glass-ceramics can evident in allowing the chance events polycrystalline glass-ceramic plates that sustain repeated and quick tempera- to bear fruit.” This glass-ceramic was have high-precision holes, channels or ture changes of 800°C to 1000°C. The later known also as Pyroceram. This first any desired intricate pattern. The prod- dominant crystalline phase of these synthetic glass-ceramic eventually led ucts are used in electronics, chemistry, glass-ceramics, β-quartz solid solution, to the development of CorningWare in acoustics, optics, mechanics and biology has a strong negative CTE. 1957.17 It also influenced the develop- in applications that include microchan- Keatite solid solution (β-spodumene) ment of Vision, a transparent cookware. nels in optical fibers, ink-jet printer also has a negative CTE, higher than CorningWare entered the consumer heads, substrates for pressure sensors and β-quartz solid solution. The negative marketplace in 1958 and became a mul- acoustic systems in head-phones.10–12 CTE of the crystal phase contrasts timillion dollar product. with the positive CTE of the residual The scientific and commercial Consumer products glass. Adjusting the proportion of these importance of glass-ceramics was recog- Corning, Schott, St. Gobain, phases offers a wide range of possible nized by the International Commission Nippon, Ohara, Ivoclar and few others CTEs in the finished composite. For on Glass, which established TC-7, presently produce commercial glass- most current applications, a low or zero the “Nucleation, Crystallization and ceramics for consumer and special- CTE is desired. A negative CTE also is Glass-Ceramic Committee” (www.icg. ized markets. We could not confirm possible. At a certain point, generally