Recent applications of glass science

Journal of Non-CrystallineSolids 123 (1990) 377-384 North-Holland

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R E C E N T A P P L I C A T I O N S O F GLASS SCIENCE Norbert J. K R E I D L Professor Emeritus, University of Missouri, Rolla, MO, USA

Recent, often exciting, applications of glass science include: solid state batteries, electronic switches and memories, electrophotography, solar cells, microspheres for optical strengthening and medical uses, novel glass ceramics such as machinable and bioactive materials, solder glasses, composites, sol-gel glasses, gradient index optics, communication fibers, sensors, non-linear and active optics, and, perhaps, digital optics. The large field of special coatings is not covered in this review.

1. Introduction

Some historians claim the renaissance of the past 150 years for the birth and systematic application of science. But no, science and scientific innovation have existed almost from the beginning of the human race. Just take agriculture, husbandry, ceramics and metallurgy. While even now chance often leads to an applicable discovery, scientific groundwork at least fortifies and expands useful applications. What is relatively new is publication and patent protection instead of family tradition and shop secrecy, as well as the wide spread and speed of information. Most recently, however, one can observe a counterforce, the administrative retardation of technology transfer. This study deals with the most fascinating applications of glass science to present and future technological advances, generally excluding the vast field of coatings. It is designed as a concise review with pertinent references * to specific fields.

2. Fast ion conduction

Traditionally, glasses have been known and used as electrical insulators, at least at ambient * The wide scope of this review enforces an arbitrary narrow selection.

temperatures. More recently, it has become possible to increase dramatically the mobility of cations in oxide and other glasses by manipulating the anionic network to facilitating their transport. An increase in ionic conductivity by up to ten orders of magnitude has been achieved in oxide glasses replacing some of the oxygen by halogens (e.g. I) creating zones through which, for example, Li ÷ or Ag ÷ can diffuse easily [1-5]. The existence of such zones is evidenced, for example, in borate glasses by nmr signals revealing the coordination of boron. It is found that halogen does not replace oxygen in its characteristics position next to boron. Halogen rather predominately associates with the cation so that e.g. Ag ÷ moves through A g - I channels in the intact borate structure. Fast-ion conducting glasses (conductivity 10 -3, sbda lime - 10 -13 f~ cm -1) now include borate, phosphate, germanate, arsenate, vanadate, molybdate, niobate, selenite, tellurite, selenide, telluride glasses doped with C1, Br or I, with mostly Li ÷ or Ag + the mobile species. A chief aim of applied research in this area is the solid state battery for cars. The main problems are corrosion and softness [6]. The main prospect seems to be the development of compromise systems such as aluminoborate bases. Other possible applications might be sensors, electrochromic displays, and energy converters.

0022-3093/90/$03.50 © 1990 - Elsevier SciencePublishers B.V. (North-Holland)

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3. Amorphous semiconductors Electronic vitreous conductors, no longer considered recent developments, are only listed for completeness. Their discovery and potential application can be traced to Kolomiets [7]; the basic underlying physics was developed by Mott, who eventually received the Nobel prize for this work, and others [8,9]. Many applications, especially to switching and memory devices or electrostatic copying, were considered. A host of conferences and publications have treated progress in the field, most recently 'Non-Crystalline Semiconductors 86' (1987) and the 1988 conference in Oxnard (to be published). An extensive review of applications is by Hudgens [10]. One of the earliest applications of chalcogenide glasses (Se : Te) was electrophotography which still seemingly remains the most economically important [10]. The original expectation of a large area of applications in switches and memories which had spawned and supported this extensive scientific interest was, however, never fulfilled. Competition by conventional crystalline technology proved superior (e.g. MiOS gate memory). Optical memories based on laser-light-induced phase changes, however, are expected to become commercially useful. The most recent extension of this field is the discovery and utilization of amorphous silicon, (section 4).

4. Amorphous silicon alloys The full exploitation of the sun as a source of energy depends on the efficiency and economy of solar cell production. Especially in regions of continuous sunshine, such as in New Mexico, solar energy will become the major source. Until a short time ago, expensive crystalline silicon was the main photovoltaic material. While it was soon obvious that amorphous (glassy) silicon would be much more economic, it was found to be strained, defective and resistant to controlled doping. The solution was the accidental discovery that hydrogen contamination from the starting material Sill# improved defect structure and doping ability [9]. While as late as 1975 Physics Reviews Letters

rejected Spear and Lecomber's proof of Si: H doping [9], soon flexible amorphous solar cells based on H- and F-doped silicon were being produced [11-13]. Japan is pioneering the field [15], but a wide spread is expected for example in the USA, if and when a cost of not much over $2/kW and an efficiency of 25% (1988 commercial 8%, laboratory 13%) will have been achieved in flexible multiple films, and aging problems will have been controlled [13]. Further improvements expected comprise sophisticated (luminescent) collectors of diffused light [14] which make tracking unnecessary. Other potential applications of amorphous silicon alloys include [11,15] watches, printers, color sensors, light sensors, strain gauges, image sensors, antireflection thin films, field effect transistors (FETS), displays (TV), diodes, and X-ray mirrors.

5. Microspheres While high-refractive-index solid glass spheres used in highway signs have been well-known for a long time, the very large amount (several tons per day) of small hollow glass spheres as stiffening lightweight fillers in plastic insulators is a relatively new development. The glass melt is doped with a well-controlled amount of sulfate which blows the droplets while they are dropping from a nozzle. The delicate process was designed and further refined for the application to nuclear fusion [16]. To contain the gases used in high power laser nuclear fusion experiments, ultraprecise hollow microspheres (50-500)xm) are now bein~ produced with surfaces smooth to less than 100 A deviation and wall uniformity of 1% in a range of 0.5 to 30 )~m [17]. For this application an aqueous glass solution is the starting material for drops from which water first evaporates in a crust on the inside of which residual material then deposits, forming a hollow balloon. Solid soluble glass beads containing short-lived radioactive elements are injected into malignant tumors to be destroyed by radiation. After fulfilling its function the glass is dissolved in the body [18]. The process has been used clinically in

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Canada for about two years and is now being considered in the USA.

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Sil2030F4 have been ceramized to precipitate crystalline canasite and exhibit a room temperature toughness of close to 5 Mpa m -~/2, about 5 times that of glass ceramics known before [27].

6. Advanced glass ceramics 6. 4. Solder glass ceramics

The use of glass ceramics in cooking utensils and stove tops has been long established. Also, the glass community is well aware of the advantages of shaping an object like a glass and converting it to a strong, non-porous ceramic of specific properties. The underlying science and technology has been reviewed competently by, for example, McMillan [19] Beall and Duke [20a]. More recent advanced technologies and applications include the following. 6.1. Mica glass ceramics [21]

Machinable glass ceramics are based on compositions for which ceramization results in the formation of mica-like phlogopite crystals whose sheet structure facilitates drilling and machining. It is fascinating to find that compositional refinements can cause these crystals to change from a planar to a spherical ('cabbage-like') arrangement improving machinability [22]. From a gel of mica-type glass, ceramic paper is being produced [20b,23]. 6.2. Astronomical mirrors

The combination of crystal phases obtained on heat treating Li20-MgO-ZnO-P2Os-A1203-SiO2 glass using nucleation agents such as TiO 2, ZrO 2 leads to a glass ceramic (e.g. Zerodur brand) of practically zero expansion, suitable for large precision mirrors used in astronomical observatories [24,25]. For exacting specifications some small aging effects were eliminated by the omission of Mg 2÷ whose mobility might have been involved in the problem [26]. 6.3. Canasite glass ceramics

Up to very recently, glass ceramics have been known to be strong but not very tough. Glasses close to the approximate composition CasNa4K-

In the field of solder glasses, increasing use is made of soft glasses converting on sealing to glass ceramics of the proper coefficient of expansion (CTE) and increased strength [28]. Range and control of CTE are comparable to that for solder glasses. Types and techniques are applicable to coatings. 6.5. Bioactive glass ceramics

Glass ceramics serving as clinical implants are based on parent glasses yielding apatite on proper nucleation and growth treatment. Apatite is the chief constituent of human bones and teeth. The resulting glass ceramic is bioactive, i.e. it reacts with body fluids to deposit bone substance [29]. These glass ceramics have been in clinical use in Europe for about three years [30]. A variety contains, in addition to apatite, phlogopite crystals making the material machineable thus enabling the surgeon to make fine-adjustments while operating. More recently, a third phase which is piezoelectric (a modification of aluminum phosphate) multiplies the rate of bioactivity. A panel discussion of this extraordinary field of glass science and technology was scheduled for this 15th Glass Congress under the guidance of Vogel.

7. Composites Among the many combinations leading to new composite materials with desirable properties are those with glass or glass ceramic matrices [3133a,b]. The classical composites with glass fibers reinforcing plastics are not covered in this review. In this area one new idea deals with obtaining high module non-crystalline fibers (e.g. CaOA1203) by direct spinning from the melt ('inviscid

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melt-spun fibers') [34]. Reinforcing a glass or glass ceramic by materials with higher modulus, such as graphite or silicon carbide, leads to superior strength at higher service temperatures. Processes often still requiring better understanding include impregnating the reinforcing material as a fabric with a glass slurry prior to ceramization [33c] and sol-gel techniques [35]. There remains room for research on filler materials other than graphite or silicon carbide. Examples are silicon nitride [36], mullite, zirconia [37]. Remaining problems include high temperature degradation or limited strain-to-failure. However, in the immediate future most research will have to be concerned with detail on interphase and process problems [32].

8. Sol-gel glasses Wet-chemical synthesis instead of high temperature fusion promises vitreous materials of superior purity and homogeneity as well as novel structures, even compositions. Accordingly, research and development is moving from a laboratory curiosity to a major field of inquiry foreshadowing a multiplicity of applications [38]. First attention was focused on fabricating high purity SiO2 glass, the unusual optical properties of which are still a subject of some controversy. At present an unlimited array of multicomponent systems containing e.g. A1, Zr, Ti, Mg, Fe, V, Nb is being investigated. Hundreds of publications testify to the intense activity in sol-gel glass and glass ceramic product and process research and development (see, for example, the Proceedings of the 4th International Workshop on Glasses and Glass Ceramics from Gels [39a]. The 1989 Workshop took place in Brazil [39]. A review of this vast effort is impossible within the frame of this presentation. Among excellent recent reviews are: Sakka [39b], Zarzycki [40], Brinker [41a,b] and Scherer [41b], Johnson [42], and Brinker [41c]. The principal process steps are the preparation of a suitable precursor-hydrolysis and polymerization-gelation-drying-densification to a glass (glass-ceramic, ceramic) in bulk, powder, fiber or film configuration.

For obtaining bulk glass in desirable larger crack-free sizes the drying process is most critical [43]. Among methods to alleviate the problem are (a) hypercritical drying, (b) DCCA (drying control by chemical additives) [44] and, most recently, (c) osmotic extraction (sol in a membrane is immersed in, for example, alcohol) [45]. It is hoped as improved drying processes emerge, to obtain monoliths of increasing size, e.g. of SiO2 [46]. Considering this situation, it is not surprising that the emphasis so far is on sol-gel coatings which can be used to modify optical (antireflection), electrical, mechanical and chemical responses. Even fine patterning might be achieved by the sol-gel method [47]. Another promising aspect is the preparation of porous materials omitting final densification (ultrafilters, gas separators, fiber precursors, insulators, catalyst substrates). Glass ceramics and composites of novel properties can be produced by sol-gel techniques. Entirely new materials obtained by sol-gel techniques incorporate organic groups into inorganic networks [48,49]. A first application was to contaCt lenses.

9. Gradient index optics (grin rods) A lens system may, for some applications, conveniently be replaced by a piano-piece of glass in which variations in refraction are induced by the diffusion of suitable ions [50,51]. For instance, a glass cylinder may be immersed at elevated temperatures in a salt bath to promote the exchange of, for example, sodium for caesium. The resulting gradient index rod possesses lens action and is called a grin rod. Of course, many patterns of index variation can be achieved. The technique also allows the fabrication of channel waveguides [52]. More recently both Moore's and Japanese laboratories have produced gradient index optics by sol-gel techniques [53]. See also the proceedings of the 1987 Workshop [39a]. From a gel a more soluble species may be gradually leached to obtain a gradient index preserved in the densified end-product.

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10. Communication fibers

One of the most exciting recent applications of glass science is the transfer of communication from electricity to light. The concept of fast transmission of verbal information was patented by Bell as early as 1880 [54]. The light intensity of a laser source required for a practical system was, of course, not available before the 1970s. Also a glass wire sufficiently long to transmit the light message without an excessive number of repeats had to be free of absorbing impurities to a degree surpassing the best optical glass. The most successful vapor deposition processes achieving this goal have been described by Klein [55] and in minute detail by Li [56]. The confinement of the light message within the fiber had to be achieved by higher index clad which required rigorously controlled compositional changes during deposition of the preform from which the fiber is drawn. More complicated profiles than the first simple step or gradient coreclad design have become necessary to control modes and dispersion of light propagation. Communication by light based on glass waveguides will soon expand from dosed systems, of which there are now many in the USA, to the major national and international telephone systems [57-61]. Obviously, a minimum of repeaters was found to be desirable from the start. However, even the cleanest silica or silicate glass retains an inevitable loss of about 0.5 db km-1 due to Raleigh scattering. Since, on principle, this loss decreases with the 4th power of wavelength, hopes were raised to arrive below 0.01 db km -1 and eventually to achieve a no-repeat transocean cab!e. An intensive search for glasses transmitting further out in the IR than silica was soon undertaken. The chance discovery of stable IR transmitting fluoride glasses [62] generated a flood of research, publications and international conferences. Yet, loss mechanisms other than Raleigh scattering, and difficulties arising from the working and service properties of these glasses, have so far not reduced the loss to much below 1 db km -1, let alone below 0.1 db km -1. This did not represent

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enough of an advantage to accept the difficulties of production. However, new fields of short fiber applications (see following sections) in the higher wavelength range have opened up, and research on the fluoride glass system remains active. The basis of first-found and of most of currently studied glasses is the ability of ZrF4 (also HfF4, ThF4) to form glasses with many additives, particularly BaF2, LaF3, A1F3, NaF [63]. Short fibers will also be increasingly used in flexible bundle image transfer, e.g. endoscopy and traffic control.

11. Optical sensors Very small mechanical, chemical, electrical or magnetic disturbances will have a very strong effect on transmission, frequency or polarization of light in a glass fiber. On this basis, many ultrasensitive instruments (sensors) can be and have been developed [60,64,65]. A recent (1985) estimate for the next five years was a market of $200 million, and high technology estimates have often been surpassed by reality! A few examples will illustrate this fast-expanding field. Temperature can be sensed to at least 0.001 K [6]. A slight change in the level of a liquid or melt can be measured by the effect of a slight change in the angle of reflection on a fiber [67]. A minute strain will have a large effect on the polarization of light in a glass fiber which thus serves as a sensitive gauge. In aviation, a fiber system implanted into aircraft components such as wings to warn of any structural changes is now being considered. A fiber may be coated with a substance (e.g. Pd) reacting with the chemical environment and thus transmitting a change in ihis environment to the fiber (chemical sensor) [68]. A vast field opens to non-oxide glasses whenever specific laser light calls for transmission in their IR transmission range. For the short fibers used in these applications, a loss < 1 db kin-1 is not required. Therefore the exploration of nonsilicate fluoride and chalcogenide continues [65].

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Medical applications (e.g. in endoscopy, ophthalmology) are expected to expand rapidly [69]. Estimates for five years from now are not much below 10 l° dollars of systems requiring not much below 10 s dollars worth of fibers.

12. Glass in active devices Glass lasers continue to be developed and used [14b] ever since their discovery by Snitzer and their dramatic use in the exploration of nuclear fusion [70]. Their function is based on doping with rare earth luminescent ions, particularly neodymium. More recently rare earth and semiconductordoped glasses are also considered for the conversion of wavelength (e.g. 1.06 to 0.53 ~tm) and amplification as proposed earlier by Ross and Snitzer [71]. The introduction of rare earths into the now available low-loss oxide fibers is difficult. New processes and/or non-oxide glasses are required [72], also because the range of the lasing wavelengths in demand is being extended. Another new development is the sensitization of one lasing ion by a second doping, e.g. erbium by ytterbium [73]. Glasses doped with highVerdet-constant elements (polarization response to magnetic field) can be used as sensors for magnetic fields [74].

13. Non-linear refraction A fast-expanding novel aspect of sensing in general is the potential use of non-linearity of refraction under the high intensity of radiation now commonly available from laser sources. For such high intensities the second term in the equation n -~ n I + H 2 1 ,

(n refractive index, n 1 constant index at low intensity, I intensity, n 2 the 'non-linear index') cannot be neglected. Thus a high n 2 can, indeed, be utilized for switches, memories, modulators (signal processing elements).

While it is true that semiconducting crystals and organic compounds have generally much higher n2 values than inorganic glasses, the fast response time and their attractive material properties and process options keeps research on high n 2 ( > 10 -11 esu) glasses alive [75]. Generally n 2 increases with nl, which in turn calls for high atomic weight and/or high polarizability of constituents. In practice n 1 should be > 2. In practice, n 2 at least two orders of magnitude higher than that of silica (0.02 × 10 -la) can be obtained in e.g. lead thallium silicate, lead thallion tellurite and in titanium niobium glasses. There are at least three other ways to combine property and process assets of glasses with the advantage of polymers and semiconductor crystals: (1) inorganic-organic hybrid glasses; (2) porous glass filed with organics and (3) semiconductordoped glasses known in principle for a long time as CdS-CdSe doped photographic filter glasses. In the last case, fascinating to the traditional glass scientist, it has been found that non-linearity is tied to a surface to bulk ratio (i.e. smaller size) of the CdS-(Se) crystals larger than that of the classical filter glasses. This criterion is associated with quantum confinement effects [76].

14. Integrated optics and digital optics Integrated optics [61,77] will be based on glass waveguides in slab, strip, or film rather than fiber form, but systems may also include fibers. While many switching candidates are crystalline, glasses offer important advantages: (1) very small index differences obtainable by ion exchange [53]. (2) transparency, (3) low refraction, (4) chemical and mechanical integrity. As the conversion of predominantly electronic to increasingly integrated and miniaturized optical systems progresses, the entirely optical computer is in sight [78,79]. Its advent was first visualized by Miller and Chynoweth [57]. Huang [81] expects a convincing prototype by 1990 [80], and a large activity can be foreseen by 1995. The driving force for this development is the expectation to process

N.J. Kreidl / Recent applications of glass science

two o r d e r s o f m a g n i t u d e m o r e bits two o r d e r s of m a g n i t u d e faster t h a n n o w possible. This e x p e c t a tion is fortified b y events like the recent establishm e n t of a C e n t e r for O p t o e l e c t r o n i c C o m p u t i n g Systems w i t h a b u d g e t of $5 000 0 0 0 / y e a r in Denver, C o l o r a d o .

15. Epilogue W e are in a m o s t exciting p e r i o d of i n n o v a t i o n in glass science, engineering, p r o d u c t i o n , a n d utilization. O n the o t h e r h a n d , the e n o r m o u s size o f the effort, especially in large countries, has ind u c e d a d e c e l e r a t i o n of t e c h n o l o g y t r a n s f e r a n d s o m e waste of funding. It also challenges the e d u c a t i o n a l system, especially the c h a n n e l i n g to r e q u i r e d skills. T h e news o f a d v a n c e s in such f a s c i n a t i n g fields as s o p h i s t i c a t e d windows, m e d i cal glass devices, u n e x p e c t e d electrical uses, superi o r t e l e c o m m u n i c a t i o n ( v i d e o p h o n e ) , ultrasensitive i n s t r u m e n t s , and, eventually, o p t i c a l c o m p u t e r s , calls for b e t t e r d i s t r i b u t i o n of scientific i n f o r m a tion to the p u b l i c a n d to youth. Small enterprises s h o u l d b e e n c o u r a g e d to challenge the deficiencies of giant o r g a n i z a t i o n s . ( T h e glass c o m m u n i t y fort u n a t e l y enjoys a t r a d i t i o n of c o r d i a l i n t e r c h a n g e as witnessed b y this 15th I n t e r n a t i o n a l C o n g r e s s in the city of L e n i n g r a d . )

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