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A bright future for glass-ceramics

Cost-efficient nanofabrication of high-temperature superconducting ceramic wire • Materials mingle for mini windmill • Rustum Roy remembered • MS&T’10 Final Program • ICACC’11 (Daytona Beach) & Electronic Materials and Applications 2011 meetings previews • 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 - lization of certain – 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 (114 articles), Schott Glaswerke (69), expansion coefficient with transparency route also is attractive to produce glass- IBM (65), Co. in the visible 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, , 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 -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 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 sinter-crystallization process is that such a narrow field within all the numer- disilicate glass with precipitated 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 . These glass- toughness of that material. Stookey ceramics can be pat- had accidentally created the first glass- terned by Fig. 2 Glass-ceramic teeth. 17 ceramic, denominated Fotoceram. 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 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, , 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 group.shef.ac.uk/tc7.html) about three whether Fuji Photo, Japan; Pittsburg between 60 and 75 percent crystallin- decades ago. Figure 1 shows some past Plate Glass, U.S.; NGK Insulators, ity, the overall negative expansion of and present TC-7 members and guests. Japan; Sklo Union, Czech Republic; the crystal phase(s) and the positive CBP Engineering, U.K.; International expansion of the residual glass phase Commercial glass-ceramic Ceramics, U.K.; and Konstantinovskii, cancel each other. Thus, the glass- products Russia; remain active in the glass- ceramic as a whole has a TCE that is The first commercially viable glass- ceramic business.11 very close to zero. But such a balance ceramic was developed in the aerospace A range of commercially successful is not straightforward, because the rela- industry in the late 1950s as radomes glass-ceramics for consumer applica- tive stiffness of the glass and crystal to protect radar equipment in the tions include famous brands of low- phases also is important. nosecones of aircraft and rockets. Glass- expansion products that are resistant Glass-ceramics also can be adjusted ceramics used in these applications to thermal shock: CorningWare; and to match the CTE of the material to must exhibit a challenging combina- Vision – a transparent glass-ceramic. which they will be bonded. LAS glass- tion of properties to withstand critical Cooktop plates, such as Schott’s Ceran, ceramics were originally developed conditions resulting from rain erosion Eurokera’s Kerablack and Nippon for use in mirrors and mirror mounts and atmospheric reentry: homogeneity; Electric Glass’ Neoceram also are avail- of astronomical telescopes. They now low dielectric constant; low coefficient able. These products rely on their rela- have become known and have entered of ; low dielectric tively high toughness (compared with the domestic market through their use loss; high mechanical strength; and glasses), appealing aesthetics and very in cooktops, as high abrasion resistance. Glass-ceramics low thermal expansion coefficient. well as high-performance reflectors for now are used in nosecones of high- The most important system commer- digital projectors. Other well-known performance aircraft and missiles. No cially is the Li2O–Al2O3–SiO2 (LAS) brands of these low-expansion glass- glass, metal or single crystal can simul- system with additional components, ceramics are Ceran, Kerablack and taneously meet all of these relevant such as CaO, MgO, ZnO, BaO, P2O5, Neoceram (cooktops); and Robax, 10 specifications. Na2O and K2O. Fining agents include Keralite and Neoceram (stoves and fire-

Another class of traditional, but still As2O5 and SnO2. ZrO2 in combinations places). Nippon Electric Glass’s related

American Ceramic Society Bulletin, Vol. 89, No. 8 21 A bright future for glass-ceramics

voltages, various frequencies and high temperatures. When properly baked out, machinable glass-ceramics will not out- gas under vacuum environments. They can be machined to complicated shapes and precision parts with ordinary metal- working tools, quickly and inexpensively. Machinable glass-ceramics require no postfiring after machining. This means that specifications can be met without having to resort to costly machining with diamond tools. Typical applica- tions include insulators and supports for vacuum environment feed-troughs; spac- ers, headers and windows for microwave tube devices; sample holders for micro- scopes; aerospace components; welding nozzles; fixtures; and medical equipment. Some dental and some bioactive glass- ceramics also are machinable using mod- Fig. 3. Micrographs of open and partially blocked dentin tubules by Biosilicate glass- ern CAD–CAM techniques.10–12 cermic powder. RHS – results of a clinical study of dentin sensitivity level of 160 teeth: Initial and after 1 to 6 applications of Biosilicate. (Reprinted from a research report for Vitrovita – Glass-Ceramic Innovation Institute.) Construction materials Several authors, especially in the products in this area include Firelite applications include precision optics, United Kingdom, Eastern Europe, fire-rated glass. The same class of mate- mirror substrates for large astronomi- Cuba, Italy and Brazil have developed rial was also used until the late 1990s cal telescopes, mirror substrates for many glass-ceramics made from a wide as CorningWare dishes, which could X-ray telescopes, optical elements for variety of waste materials, such incin- be taken from the freezer directly to comet probes, ring laser erator ashes, blast furnaces slags, steel the oven with no risk of thermal shock and standards for precision measure- slags and sugar-cane ashes. Their com- damage. These low-CTE glass-ceramics ment technology. Other great low-CTE position and predominant crystal phases are the most successful commercial glass-ceramics success stories are Ceran vary widely. These low-cost, dark- glass-ceramics thus far developed.10–12 panels for cooktops and Robax for fire- colored (because of the high level of places and stoves.10–12,14 transition elements in wastes) materials Thermal uses of glass-ceramics Another relevant thermal property are generally strong, hard and chemi- Another particularly important mate- of glass-ceramics is their limiting use cally resistant. Their intended use is for rial is , a semitransparent, non- temperature. Because of their residual abrasion and chemically resistant parts -ceramic made by Schott. glass phase, most glass-ceramics flow and or floor and wall tile used in chemi- Zerodur has an extremely low CTE (0.00 deform at relatively low temperatures, cal, mechanical and other heavy-duty ± 0.02 × 10–6/K between 0°C and 50°C), typically below about 700°C. However, industries or construction. which can even become zero or slightly some notable exceptions exist. An A high-end-use construction and negative in some temperature ranges. example is a celsian glass-ceramic in the architecture glass-ceramic is Neopariés, which was pioneered by Nippon Another unique characteristic of this SrO–BaO–Al2O3–SiO2 system, which glass-ceramic is its exceptionally good has use temperatures as high as 1,450°C Electric Glass about 20 years ago and homogeneity. Even in large material and CTEs that match silicon, SiC and continues to be used. This glass-ceramic is one of the few commercial products blocks, it is almost impossible to mea- Si3N4. This material is meltable at com- sure the fluctuations in mechanical and mercial temperatures (1,650°C).18 made by sintering, and its main crystal thermal properties. Its good transparency phase is wollastonite (calcium metasili- in the range 400 to 2,300 nanometers Machinable glass-ceramics cate). Neopariés is a pore-free, partially allows a verification of internal quality. Macor, Dicor, Vitronit, Photoveel crystallized material with a soft rich Therefore, it can be ensured that neither and other brands of machinable glass- appearance similar to marble and gran- bubbles nor inclusions go undetected. ceramics rely on mica in their ite. However, it has none of the main- Because of its unique properties, microstructure. Their high CTE readily tenance problems of natural stone and Zerodur is a preferred material for matches most metals and sealing glasses. is an attractive material for exterior and lightweight honeycomb mirror mounts They exhibit zero porosity and, in gen- interior building walls and table tops. made for satellite mirrors. Other typical eral, are excellent insulators at high Because of the growing concern

22 American Ceramic Society Bulletin, Vol. 89, No. 8 about sustainability and exhausting conductivity makes Table I. Relevant properties of bioactive glass-ceramics reserves of natural stones, the use of them comfort- Glass-ceramic glass-ceramics as a construction materi- able in the mouth. —————————————————————— al deserves much attention. References Moreover, the mate- Highly bioactive Property Cerabone Bioverit I glass ceramic 11 and 19–21 discuss these uses further. rial is extremely † † ‡ durable, and it is Bioactivity class B B A High-strength glass-ceramics relatively easy to Machinability Low Good Fair Density (g/cm3) 3.1 2.8 2.6 The average fracture strength (Sf ≈ manufacture to cus- Three-point flexural strength (MPa) 215 140–180 210 100–250 MPa) and toughness (KIc ≈ tomized units. All- 1/2 1–2.5 MPa∙m ) of most glass-ceramics ceramic restorations Young’s modulus (GPa) 120 70–90 70 are generally higher than those of com- can be used to cover Vickers hardness (HV) 680 500 600 mercial glasses (S 50–70 MPa; K 1/2 f ≈ Ic even dark tooth Toughness (MPa∙m ) 2.0 1.2–2.1 0.95 0.7 MPa∙m1/2). Glass-ceramics with ≈ cores (e.g., if the Slow crack growth index 33 especially high strength and tough- tooth is severely dis- † class B bonds only to . ‡Biomaterial class A bonds to hard (bone) and soft (cartilage) ness have been reported by George colored or a titanium tissues. Beall and colleagues10 for canasite abutment is used). glass-ceramic (Sf ≈ 300 MPa; KIc ≈ 5 MPa.m1/2). Somewhat smaller but Current lithium disilicate glass-ceram- in Table I. still impressive values of strength and ics – e.g., Ivoclar’s IPS e.max – are ideal Cerabone – developed by Tadashi for fabricating single-tooth restorations Kokubo and produced by Nippon toughness (Sf ≈ 350–400 MPa; KIc ≈ 2.3–2.9 MPa∙m1/2) have been reported (Fig. 2). This innovative glass-ceramic Electric Glass Co. Ltd. – is prob- for the lithium disilicate IPS e.max produces highly esthetic results. Its hard- ably the most widely used bioactive Press developed by Wolfram Hoeland ness is similar to that of natural teeth, glass-ceramic for bone replacement. and other Ivoclar researchers.10 The and it is two to three times stronger than Numerous clinical trials have shown common feature of these glass-ceramics other dental glass-ceramics. The mate- intergrowth between this glass-ceramic is their lath-shaped crystals that lead to rial can be either pressed or machined and human bone. Tadashi informed us crack deflection and toughening. to the desired shape in the dental labo- in 2009 that about 50,000 successful Other successful strategies to implants already have been made using ratory. Because of its high strength (Sf increase strength and toughness include Cerabone. Bioverits are machineable ≈ 360–400 MPa) and toughness (KIc ≈ fiber reinforcement, chemical strength- 2.3–2.9 MPa∙m1/2(single-edge V-notched glass-ceramics that are very useful, ening by ion-exchange methods and beam)), restorations fabricated with this because they can be easily modified development of a thin surface layer material can be cemented by various during clinical procedures. Bioverit II is with a lower thermal expansion than techniques. These glass-ceramics possess especially good in this respect.10,11,25–27 the interior to induce a compressive true-to-nature shade behavior, natural- A different type of highly bioactive surface layer. All these concepts have looking esthetics, natural-looking light glass-ceramic was developed by Peitl et been demonstrated for a few glass- transmission, versatile applications and a al.28 in 1995. This is a low-density glass- ceramic compositions. However, they comprehensive spectrum of indications. ceramic in the Na-Ca-Si-P-O system are far from being fully explored, and, that has a Young’s modulus closer to thus, there is much scope for further Bioactive glass-ceramics that of cortical bone and much higher research. Another important aspect bioactivity than previous bioactive glass- that needs further study is effect of Bioactive glass-ceramics form in-situ the type (compressive versus tensile) a biologically active layer of hydroxy- ceramics. This particular combination of and magnitude of the internal residual carbonate (the mineral phase of properties is desired for several applica- stresses that are always present and are bone and teeth) that bonds to bone and tions. This glass-ceramic is about 30 to maximum at the crystal–glass inter- teeth and sometimes even to soft tissue. 50 percent crystalline, and its main phase faces. Values of 0.1 to 1.0 gigapascals Moreover, load-bearing applications is Na2O∙2CaO∙3SiO2. The first clinical have been reported in some glass- require excellent mechanical properties. trials for middle-ear bone replacements ceramics. These stresses certainly affect Many products have reached commer- in 30 patients yielded very positive the overall mechanical performance of cial success: Cerabone A-W (apatite– results. Table I summaries the main prop- the material. A few papers have dealt wollastonite), Ceravital (apatite–devit- erties of some bioactive glass-ceramics. with residual stresses in glass-ceramics, rite), Bioverit I (mica–apatite), Bioverit A new glass-ceramic based on the including References 16 and 22–24. II (mica) and Ilmaplant L1 and AP40. same Na-Ca-Si-P-O system (Biosilicate) They have been used as granular fillers, but with some compositional modifica- Dental glass-ceramics artificial vertebrae, scaffolds, iliac spac- tions and greater than 99.5 percent All-ceramic dental restorations are ers, spinous spacers, intervertebral spac- crystallinity recently was developed by attractive to dentists and patients, ers, middle-ear implants and as other Zanotto and colleagues.29–32 This glass- because they are biocompatible, have types of small-bone replacements. Some ceramic is as bioactive as the “gold stan- superior aesthetics and their low thermal of their interesting properties are listed dard” invented by Larry

American Ceramic Society Bulletin, Vol. 89, No. 8 23 A bright future for glass-ceramics

sated by additional Li+ ions, leading to

the Li1+xMxTi2–x(PO4)3 system. Some authors claim that the main advantage of obtaining such materials by the glass-ceramic route would be a decreased porosity if compared with ceramic materials obtained by the clas- sical sintering route. However, one feature that has been scarcely explored, is that, if the parent glass presents internal nucleation, one may easily and effectively control, for instance by dou- ble-heat treatments, the microstructure of the glass-ceramic to further increase its conductivity.36 Solid oxide fuel cells are ceramic solid-state energy conversion devices Fig. 4. From left to right: parent glass, glass-ceramic with 97 percent crystallinity that produce electricity by electrochemi- and glass-ceramic with 50 percent crystallinity. Grain size is about 20 micrometers. cally combining fuel (e.g., hydrogen gas or natural gas) and oxidant (e.g., air) Hench. Clinical tests of treatment with route for tumor treatment has been fol- gases across an ionic conducting oxide at Biosilicate powder for dentin hypersensi- lowed by several authors. Several other operating temperatures of about 800°C. tivity in 160 sensitive teeth conducted by compositions have been and are pres- The planar SOFC configuration provides dentist Jessica Cavalle are shown in Fig. ently being tested in various laboratories. a simple manufacturing process and high 3. After the first treatment, one-third current densities, but it requires hermetic of the teeth lost their sensitivity. After Electrically conducting and sealing to prevent fuel–oxidant mixing six applications of Biosilicate powder, insulating glass-ceramics and to electrically insulate the stack. 94 percent of the teeth were cured. This Electrically insulating materials, such A suitable sealing material must powdered glass-ceramic also can be use- as spinel–enstatite, canasite and lithium meet several criteria: chemical stability ful for making small sintered and disilicate glass-ceramics (made by at 800°C under oxidizing and reducing bioactive scaffolds, such as those shown Corning) as well as TS-10 glass-ceramic wet atmospheres (air, hydrogen gas); in the studies of Enrica Verné33 and Aldo substrates (made by Ohara) are used electrically insulating; chemical com- Boccaccini34 and their colleagues. in magnetic media disks for hard disk patibility (i.e., must not poison other Another interesting class of bioactive drives. These materials offer the key cell components); ability to form a seal glass-ceramics is heat-generating bioac- properties necessary for today’s higher at about 900°C that results in a her- tive or biocompatible glass-ceramics areal density, smaller, thinner drive metic bond with high strength; CTE of intended for use for hyperthermic treat- designs. These glass-ceramics have 10–12 ppm/K; and long-term reliability ment of tumors. For instance, in one high toughness, provide low surface during high-temperature operation and study by Koichiro et al.,35 glass plates of roughness and good flatness, ultralow during thermal cycles to room tempera- ture. In the scientific literature phos- the chemical composition CaO–SiO2– glide heights and excellent shock resis- 10 phosilicate, boron-free alkaline-earth Fe2O3–B2O3–P2O5 were ceramized. The tance. resulting glass-ceramic containing mag- On the other hand, lithium-ion- and -ceramics, netite and wollastonite crystals showed conducting glass-ceramics are promising for instance SrO–La2O3–Al2O3–B2O3– high-saturation magnetization. This solid electrolytes for lithium batteries. SiO2, have been suggested for SOFC glass-ceramic formed a calcium- and Sufficiently high conductivity at ambi- sealing applications.37 Several research phosphorous-rich layer on its surface and ent temperature (10–3 (Ω∙cm)–1) has groups in the world are attempting to tightly bonded with bone within about been demonstrated when precursor develop such materials. eight weeks of implantation. The par- glasses crystallize in the highly conduc- Some groups have experimentally ent glass did not form the calcium- and tive nanoscale application specific inte- demonstrated the possibility of produc- phosphorous-rich layer and did bond grated circuit structure. Systems derived ing glass fibers of the famous BISCO with bone at 25 weeks. Under an exter- (Bi Sr CaCu O ) system – which are not from the LiTi2(PO4)3 composition have 2 2 2 8 nal magnetic field, granules of this glass- been extensively studied. In fact, partial superconducting – and then crystallizing ceramic filled in rabbit tibias heated substitution of Ti4+ by a trivalent cation, them to produce glass-ceramic supercon- surrounding bone to more than 42°C M3+, such as Al3+, Ga3+, In3+, Sc3+, Y3+, ductors.38 and maintained this temperature for 30 La3+, Cr3+ or Fe3+, generates a deficiency Last, but not least, several glass- minutes.35 Since then, this promising in positive charges, which is compen- ceramics-containing piezoelectric and

24 American Ceramic Society Bulletin, Vol. 89, No. 8 ferroelectric phases have been studied. als that absorb UV, reflect IR and are less than about 200 nanometers and has These areas for glass-ceramics applica- transparent to visible light; materials a small or moderate crystallized frac- tions have not yet been fully explored. that absorb UV and fluoresce in red/IR; tion of 1 to 70 percent. However, an substrates for arrayed grat- interesting new discovery was recently Transparent glass-ceramics ing; solid-state lighting – white light; reported by Berthier da Cunha and Some successful and many trial opti- and laser pumps. Ohara’s WMS-15 colleagues.42,43 They developed a large- cal applications have been proposed for glass-ceramic substrates, a commer- grain (about 10–50 micrometer), highly transparent glass-ceramics: cookware cially available product, have improved crystalline (97 percent) and transparent (Vision) that allows continuous visual- transmittance and exceptionally low glass-ceramic, as shown in Fig. 4. ization and monitoring of the cooking surface roughness values. They enable Mark Davis16 mentions in his 2008 process); fireplace protection; transpar- manufacturers to produce leading-edge review paper that “many of the exotic ent armors for visors or vehicle windows; dense wavelength division multiplexer optical features now demonstrated in substrates for LCD devices; ring laser and gain-flattening filters. The WMS glass-ceramics (e.g., SHG, lasing, elec- gyroscopes; missile noses; fiber grating substrates facilitate the production of trooptical effect) have not yet been athermalization; precision photolithog- special filters.10,11 made into a commercial product.” raphy; printed optical circuits; and small One interesting case of optical appli- or very large telescope mirrors (Zerodur). cation is the photothermal refractive Glass-ceramic armor In this last example, the telescope’s glass-ceramic produced by photother- Some patents have been filed and optical components are required to over- mo-induced crystallization. PTR glass, others have been granted for inven- come distortions caused by temperature invented by Stookey, is an bro- tions related to armor materials for fluctuations. Therefore, glass-ceramics mine sodium zinc glass the protection of people or equipment with zero expansion are highly suitable. doped with silver, , tin or anti- against high-speed projectiles or frag- The keen interest in glass-ceramics mony that can be locally crystallized ments. Ceramic materials are used for optical applications is caused by by UV exposure in selected regions fol- particularly in armors for which low their advantages over glasses, single- lowed by heat-treatment above its glass weight is important: bullet-proof vests; crystals and sintered transparent transition temperature. and armor for automobiles, aircraft and ceramics. Unlike glasses, glass-ceramics However, PTR glass-ceramic has helicopters, especially in cockpits or demonstrate properties similar to those reached the consumer market during seats and for protection of functionally of single crystals. In contrast with the past 15 years thanks to the develop- important parts. The first and still-used single crystals or sintered ceramics, ment of Bragg gratings and other types ceramic armor materials consist of high- glass-ceramics can be made in intricate of volume holograms for laser devices modulus and hard Al2O3, although its shapes and sizes by fast and cost-effi- (narrow-band spectral and angular fil- density is quite high, about 4 grams per cient glass-manufacturing processes. ters, laser beam deflectors, splitters and cubic centimeter. Other very hard, but Transparent glass-ceramics based on attenuators). Leon Glebov and his team less dense materials, such as SiC and fluoride, chalcogenide and oxyfluoride at the College of Optics and Photonics B4C, can be produced only at very high doped with rare-earth ions have been (CREOL), University of Central Florida, temperatures by costly manufacturing successfully used for wavelength up- have been responsible for this develop- processes and are, hence, expensive. conversion devices for europium-doped ment. At least two companies produce Most glass-ceramics have lower hard- waveguide amplifiers. Transparent such Bragg gratings. This glass-ceramic ness and Young’s modulus than the mullite-, spinel-, willemite-, ghanite- is interesting because of the low amount above-described ceramics, but have and gelenite-based glass-ceramics of its crystal phase, less than 1 percent the great advantage of low density doped with transition-metal ions have of nanosized NaF, and the intricate pho- and much lower cost. Moreover, glass- been developed for use in tunable and tothermal crystallization mechanism of ceramics can be transparent to visible infrared lasers, solar collectors and PTR glass is yet scarcely known!40–41 light. Alstom’s Transarm, a transparent high-temperature lamp applications. To be transparent in the visible glass-ceramic armor is based on lithium Glass-ceramics that exhibit second har- range, a glass-ceramic must have one or disilicate. It originally was developed for monic generation and materials with a combination of the following charac- protective visors for bomb disposal work. high Kerr constant for electrooptical teristics: the crystal size must be much Another example is Schott’s Resistan, devices have been developed as well. less than the wavelength of visible a range of low-expansion glass-ceramics The combination of several properties light (i.e., less than 200 nanometers); that can be opaque or transparent and is the hallmark for their success.39 and the birefringence must be very low are intended for substrates for vehicular Other optically active applications or there must be negligible difference and personal armor systems. include luminescent glass-ceramics for between the refractive indexes of the Little has been published and patent- solar concentrators, up-conversion and residual glass matrix and the crystals. ed on this particular use of glass-ceram- amplification devices; illumination The vast majority of existing transpar- ics, compared with other applications, devices using IR; heat-resistant materi- ent glass-ceramics relies on crystal size because of the sensitive nature of this

American Ceramic Society Bulletin, Vol. 89, No. 8 25 A bright future for glass-ceramics military-related research. For more infor- • Optical properties: Articles are listed above (and not listed) and their mation the reader is referred to patents translucent or opaque, opalescent, fluo- exciting potential applications, glass- granted to Michael Budd and colleagues. rescent, and colored and photo-induc- ceramics have indeed a bright future! n tion nucleations are possible. Concluding remarks Mark Davis mentions in his review Editor’s note An impressive variety of glass- article, that “… as George Beall pointed All of the glass-ceramic products ceramics has been developed during the out to me some years ago, although the cited in this article have registered past six decades. Yet, many others with number of glass-ceramics in total that trademarks held by the companies that unusual and unforeseen properties and [make it] into commercialization is quite produced them. applications are likely to be discovered small, once they do make it, they exist in the future. as a viable product typically for decades.” About the author Glass-ceramics possess many favor- This is certainly true: From several Edgar Dutra Zanotto is chair of the able features. thousand patents, only a few dozen glass- Nucleation, Crystallization and Glass- • Composition: 1052 compositions ceramics products have reached the mar- Ceramics Committee (TC 7) of the can, in principle, be vitrified by com- ket. However, most of them – or their International Commission on Glass bining and varying by 1 mole percent updated versions – remain there, and and head of the Vitreous Materials of all the 80 “friendly” elements of the some have sold millions! Laboratory, Department of Materials periodical table, which could then be Much is already known about glass- Engineering, Universidade Federal crystallized to form a glass-ceramic.44 ceramic technology, but many challeng- de São Carlos, São Carlos, SP, Brazil es in glass-ceramic research and devel- • Forming: Articles of any shape (www.lamav.ufscar.br). opment are ahead. They include the can, in principle, be made by rolling, search for new compositions (and there casting, pressing, blowing, drawing or Acknowledgments are many alternatives to explore), other by any other glass-processing method I am indebted to Donald Stookey and more potent nucleating agents, and that already exists or may be invented. for discovering glass-ceramics! I also new or improved crystallization pro- • Thermal treatment: Crystallization deeply thank my former and present cesses. Challenges include microwave TC-7 colleagues and friends – from is induced on the cooling path, in one heating, biomimetic microstructures, step or multiple steps. whom I learned much about the intri- textured crystallization demonstrated by cacies of glass crystallization, proper- • Microstructure: Articles can Christian Russel of the OSI in and ties and applications of glass-ceramics be engineered from nanograins, laser crystallization demonstrated by during the past three decades: Peter micrograins or macrograins; low or high Taka Komatsu of Tohoku University in James, Mike Weinberg, Stvan Szabo crystallinity; zero, low or high porosity; Japan. A deeper understanding and con- (all deceased), Tadaski Kobubo, one or multiple crystal phases; random trol of photothermal-induced nucleation Klaus Heide, Wolfgang Pannhorst, or aligned crystals; and surface-induced associated or not with chemical etch- Linda Pinckney, Ian Donald, Guenter or internal crystallization. ing; the development of harder, stiffer, Volsksh, Taka Komatsu, Akihiko • Thermal properties: Thermal stronger and tougher glass-ceramics; and expansion can be controlled – nega- glass-ceramics with increased transpar- Sakamoto, Michael Budd, Maria tive, zero or highly positive; stability can ency or conductivity are also timely. Pascual, Miguel Prado, Vlad Fokin, Jeri range from about 400°C to 1,450°C; and A wide range of potential properties Sestak, Mark Davis, Gilles Querel, Jojo low thermal conductivity is common. of glass-ceramics is possible because Deubener, Ralf Mueller, Falk Gabel and • Mechanical properties: Articles of the ability to design their composi- Robert Hill. have much higher strength and tough- tion, thermal treatment and resulting Thanks also to Christian Ruessel for ness than glasses, but the limits are far microstructure. This, combined with hosting several dozen internship students from being reached, possibility to be the flexibility of high-speed hot-glass from LaMaV at the famous Otto-Schott further strengthened by fiber addition, forming will ensure continued growth Institute in Jena. To Leon Glebov, chemical and thermal methods. They of glass-ceramic technology. As in their Larissa Glebova, Julien Lumeau, Vlad are hard, some are machinable. serendipitous discovery, “luck” based on Fokin, Gui Parent and all the members • Chemical properties: Articles are systematic exploratory research, solid of the photothermo-induced nucle- resorbable or highly durable. understanding of glass structure, relax- ation research team. To Larry Hench, • Biological properties: Articles are ation, crystallization and properties, as Oscar Peitl, Murilo Crovacce, Renato biocompatible (inert) or bioactive. well as knowledge of the vast literature Siqueira, Alex Fraga, Marina Ismael • Electrical and magnetic properties: and deductive reasoning, may allow and Bruno Poletto – LaMaV’s bio glass- Articles have low or high dielectric other great inventions to bear fruit. ceramics team. To Miguel Prado, Ralf constant and loss, high breakdown volt- From their glorious past, starting with Muller, Catia Fredericci, Edu Ferreira, age, ionic conducting or insulating, their accidental discovery, to their very Anne Barbosa, Vivi Oliveira and Rapha superconducting, piezoelectric and fer- successful commercial products as well (Bode) Reis – the sinter-crystallization romagnetic properties. as their impressive range of properties team of LaMaV. To Ana Rodrigues, JL

26 American Ceramic Society Bulletin, Vol. 89, No. 8 and their students – the ionic conduct- 12P.W. Mc Millan, Ed., Glass Ceramics, 2nd ed. Biomed. Mater. Res., 82A, 545–57 (2007). ing glass-ceramics team of LaMaV. To Academic Press, New York, 1979. 32C. Tirapelli, H. Panzeri, E.H.G. Lara, R.G.Soares, Vlad Fokin, Juern Schmelzer, Dick 13P.F. James, “Glass-Ceramics: New Compositions O. Peitl and E.D. Zanotto, “The Effect of a Novel and Uses,” J. Non-Cryst. , 181, 1–15 (1995). Crystallised Bioactive Glass-Ceramic Powder on Brow, Joe Zwanziger, Aldo Boccacini, 14W. Pannhorst, “Glass-Ceramics: State-of-the- Dentine Hypersensitivity: A Long-Term Clinical Alicia Duran, M.J. Pascual, Marcio Art,” J. Non-Cryst. Solids, 219, 198–204 (1997). Study,” J. Oral Rehab., (2010), accepted for publica- Nascimento, Mariana Villas Boas, tion. 15G.H. Beall and L.R. Pinckney, “Nanophase Glass- 33 Daniel Cassar, Leo Gallo, Tiago Mosca, Ceramics,” J. Am. Ceram. Soc., 82, 5–16 (1999). C. Vitale-Brovarone, F. Baino and E. Verné, “High-Strength Bioactive Glass-Ceramic Scaffolds Diogo Alves and all past collaborators 16 M.J. Davis, “Practical Aspects and Implications for Bone Regeneration,” J. Mater. Sci.-Mater. Med., on fundamental studies on nucleation of Interfaces in Glass-Ceramics: A Review,” Int. J. 20, 643–53 (2009). Mater. Res., 99, 120–28 (2008). and crystallization. To Gui Parente, 34O. Bretcanu, C. Samaille and A.R. Boccaccini, Michael Budd, Mark Davis and George 17S.D. Stookey, Ed., Explorations in Glass. American “Simple Methods to Fabricate Bioglass®-Derived Beall for their valuable critical review Ceramic Society, Westerville, Ohio, 2000. Glass-Ceramic Scaffolds Exhibiting Porosity 18 of this manuscript. Finally, I thank the G.H. Beall, “Refractory Glass-Ceramics Based on Gradient,” J. Mater. Sci., 43, 4127–34 (2008). Alkaline-Earth ,” J. Eur. Ceram. 35 Brazilian funding agencies Capes, CNPq O. Koichiro, I. Minoru, N. Takashi, Y. Takao, Soc., 29, 1211–19 (2009). E. 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