Some recent advances in glasses and glass-ceramics

S o m e R e c e n t A d v a n c e s in G l a s s e s and Glass-Ceramics (3 P Smith, Coming Glass Works, Coming, New York 14831, USA

Abstract Glass is a very old material with many uses, although it's commonly prized for what it may contain, or what may be seen through it. With the establishment within the last century of laboratories concerned with glass research and developmen~ many n e w kinds of glass, with n e w uses, or n e w methods for production fo glass articles, have appeared Nine which are familiar to many people are listed in this survey. Three of these relate to inventions of n e w compositions, and six to n e w processes for forming glass into a usable product. This article discusses some newer developments in glasses, as an indication of directions of some current or recent research. They include durable low temperature glasses, durable far infra.red transmitting glasses, full-colour photosensitive glasses, glass microlens arrays, precision direct molding, specialized glass-ceramics for dental restorations, for film or paper, or for tough materials. Finally, I m a k e some brave predictions about future directions of research and development in that fascinating materials, glass.

Introduction Glass has been manufactured for more than four thousand years, and pieces of naturally-occurring glass were used as tools even earlier. So, what's new in glass? The first indus~ial laboratory in the world devoted to glass was established only about 80 years ago, and out of it and similar industrial laboratories have flowed many discoveries in materials, process, or product, which have touched and presumably in s o m e specific way improved the lives of millions of people, or of at least most of us who will read this article. Let me suggest s o m e of these familar inventions, with the dates associated with them going from about 1914 to 1970. • borosilicate glasses - glasses resistant to thermal shock, with application as diverse as signal lenses, laboratory apparatus, telescope mirrors, and baking ware. • the "ribbon machine", making envelopes for incandescent lamps and other kinds of bulbs from a molten ribbon of glass at rates up to 2000 per minute; a couple of orders of magnitude faster than previously possible.

54

• fiber glass, for insulation or for reinforcement of plastics or concrete. • continuous melting of ultra-high quality optical and ophthalmic glasses. • centrifugal casting, a high-speed, precise process for producing the tapered funnels of cathode-ray (television) tubes. • "float glass", a continuous process for high-speed, high quality sheet glass by floating and cooling the molten glass on a bed of molten metal. • glass-ceramics, crystallized from glasses. They may be opaque or transparent or coloured. • photochromic glasses, to protect eyes against bright sunlight. • optical waveguides; specialized multicomponent fibers for optical communication or power transmission. And there are many more, s o m e not so familiar, which have unique applications in technological or general consumer uses. But, you ask, what have you done for m e lately? I've selected a few examples of current or recent work at

the same laboratory (Coming) responsible entirely or to a major degree for all but one (float glass developed by Pilkington in the UK) of the highlights listed above, which may help to answer that question. Perhaps they will suggest to you s o m e new applications of these new variations on a very old theme. Durable Glasses with Low Melting Temperatures One of the major problems in many glass-forming operations is that hot, molten glass is a very corrosive liquid, which places severe restrictions on the choice of mold materials. A glass which can be formed at lower temperatures would have real advantages. Unfortunately, in glasses there exists a general relationship between viscosity and durability. The glass systems which are the least refractory, ie borates, phosphates, halides, etc are most readily attacked by water. The degradation mechanism in these glasses usually involves dissolution of the entire glass network. A new family of glasses in the Pb-Sn-P-O-F system, in which there is a significant departure from the normal relationship between viscosity and durability, has been

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

discoveredL Melting is at relatively low temperatures, and forming of these glasses is possible within the temperature ranges normally associated with plastics. The composition space in which clear, durable glasses are obtained is approximately defined by certain component ratios: 8
Table I Durability and figure of merit (FOM) Glass

Tg (c'C)

Lead ultraphosphate Lead borate solder glass S-95 Optical crown 59Sn-6.4Pb-34.0P-6O.4F-121.50 56.7Sn-6.2Pb-37.0 P-49.5 F-130.70 56.5Sn-47.0Pb-38.7P-52.8F-131.60

205 394 574 600 95 125 120

Solution Rate (mm/day, 20~'C) 1.9 2.5 1.9 7.0 2.4 4.6 2.4

x x x x x x x

10 .4 10 .5 10 .5 10 .6 10 .5 10 .5 10 .5

FOM 26 101 92 238 439 175 347

(Compositions are shown as atomic percent for the cations and as atoms :)er 100 cations for the anions)

Fig 1

Low temperature glass; wooden mould

Bi203

50 ~////

\\

50

Cation %

'\

\'~\, 2 5 '\

'\ \

PbO

Fig 2

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

25

5O

75

Ga203

Glass formation in the Ga203 -PbO-Bi203 system

55

Full-colour Photosensitive Glasses The colouration of glasses by colloidal suspensions of metals, particularly of gold, copper or silver has been practiced for a long time, as in the spectacularly beautiful gold ruby glasses 5. Photosensitive glasses, with photoenhanced colouration, requiring

of the physical and optical properties for some of these glasses are listed, along with those for a soda-lime-silica glass. The high electro-optical or Kerr effect, shown by the high Verdet constant, in addition suggests these glasses as candidates for non-linear optical devices 4,

Bi203

50/~/

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75

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Fig 3

\~"/(iG/aeS~ts,)\~50

C a t i o n O/o

f

I

i

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25

50

75

Ga203

Glass formation in the Gae03 -CdO-Bi203 system

Table II Infra-red transmitting glasses Cation percent

Glass

Bi203 Ga203 PbO CdO Expansion Coefficienl (25-200~C),/~C x 107 Annealing point '% Density, gm/cm 3 Verdet constant, min/oe.cm

EO

ES

IV

IY

35 25 40

5 40 55

70 15

60 25

111 319 8.2 021

84 383 7.6 0.11

15

15

112 349 8.3 1.18

101 386 8.0 0.20

Loo|

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56

91 510 2.5 0.01

-

sol

Fig 4

Sodalime glass

4

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5 6 WAVELENGTH (/am)

Infra-red transmittance of oxide glasses

\ 7

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an exposure to light followed by a heat treatment, were the subject of patents by Stookey, and photochromic glasses, with collidal silver halide particles the active component, by Armistead and Stookey 6.7.8.The fullcolour glass described here results from a controlled secondary ultraviolet exposure and heat treatment, of glasses similar in composition to the earlier photosensitive glasses 9. These so-called polychromatic glasses are sodium silicates with added zinc and aluminium oxides and significant amounts of F- and Br- and the sensitising ions Ag +, Ce3+, Sn 2+, and S b 3+. Addition of sufficient fluorine is necessary so that on cooling they become supersaturated solutions of NaF; a secondary halide must be present in addition to fluoride. A typical glass composition is shown in table 111. The sequence of steps in the photochemical process shown in table IV produced the developed microcrystals shown in the electron micrographs, figures 5 and 6. They are observed to be highly elongated pyramids in a range of crystal sizes, small (less than 0.25 microns) for the non-scattering transparent glasses and larger (0.3-1.0 microns) for opaque glasses. Silver particles are observed at the elongated tips of all the pyramidal crystals (figure 6). The anisotropy of the pyramid is related to the resultant colour of the glass; the lengthwidth ratio is greatest, as high as 10/1, for low first exposures, tending toward unity at the highest exposures. The colours progress correspondingly from green (high aspect ratio, low exposure) through blue, violet, red and orange to yellow (spherical silver particles at tip), as seen in figure 7. Figure 8 shows pieces of the glass laid on a CIE colour plot. A typical polychromatic glass before processing is colourless and transparent, having physical, chemical, and optical properties similar to those of window glass. Its unique characteristic is its potential, when activated by ultraviolet light and heat, to produce within the same glass any desired transparent co]our or combination of colours, any desired pattern of white or coloured opacity, to employ either two or three-dimensional geometry in the patterns, and to produce permanent full-colour photographic images. After processing, physical properties

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

Table III Typical polychromatic glass composition Glass C o m p o n e n t

Weight Percent

SiO 2 Na20 ZnO AI203

69.0 15.8 4.8 6.8

FBr

2.3 1.0

Ag +

0.01

CeO 2

0.05

Sb203 SnO

0.20 0.05

and chemical durability remain essentially unchanged, and the new optical properties (colours, opacity, etc.) b e c o m e f'~xed. Because absorption and scattering characteristics are due to silver particles and inorganic crystals in a stable glass matrix, they are as permanent as the glass. This is believed to be the first colour photographic medium having true colour permanence. Only by heating the material above 400 ° or 450°C, where atomic diffusion can occur, will the colours deteriorate.

Function Glass matrix

Crystal constituents (with Na and Ag)

Sensitiser and colourant Optical sensitiser Thermal sensitisers,

redox, refining agents

o7~

36~95~

/ - 477k / / Fig7

Transmission curves for fully developed polychromatic glass for various first exposure times

U~l

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.~ o~-/ Microcrystals in a polychromatic glass

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Microcrystal in a polychromatic glass

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

Fig 8

Polychromatic glasses on CIE colour plot

57

Table IV The photochemical process

Fig 9

Event

Effect

First ultraviolel (300 nm) exposure (5 sec. to 5 min.)

Lalent silver image in glass

First heat, 450-500:C

a) subcolloidal photosilver nuclei b) nucleated pyramidal NaF

Second ultraviolet (300 nm) exposure ( 1 0 m i n t o 2 h)

Latent silver image in crystallite apices

Second heat, 300-460 C

Colour image, anisotropic silver aarticles

Crystals in photosensitive glass after treatment

:

Applications for these glasses may include photomurals, decorative windows (cathedrals, anybody?), portrait and scenic photographs, art reproductions and art ware, decorative table-ware and permanent information storage for archives. G l a s s M i c r o l e n s Arrays As noted above, the colour in photosensitive glasses is due to the absorption of small noble metal colloids which are produced by exposure to ultraviolet light followed by a heat treatment. In one compositional class of such photosensitive glasses the noble metal specks, when attaining a certain critical size, can serve as nuclei for the growth of a crystalline microphase, in this case lithium metasilicate, from the initially homogeneous glass. Figure 9 shows an electron micrograph of the photoinduced crystalline phase after exposure and thermal development. The photoinduced phase has a density greater than that of the unexposed homogeneous glass. Therefore, if the photosensitive glass is exposed through a photomask as shown in the upper right corner inset in figure 10, the exposed region will densify during the thermal cycle, which reaches a temperature above the softening temperature of the original glass, and squeeze the unexposed cylindrical region of soft glass, pushing it out beyond the surface. Surface tension then causes the new surface shape to be spherical ~0.~L.Figure 10 shows a schematic representation of the stress

GSOO @©©

F - r -

f

-

#

Fig I0 Photomask and dimensional changes in glass

58

Fig 11 Spherical lenses. Lens 400 micron diameter, ~20 micron height

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

resulting in the squeezing effect on the glass. Figure 11 shows a perspective, obtained with use of an electron microscope, of the spherical surface. Equally important, another photochemical process can be made to occur simultaneously with the lenslet formation, with further silver reduction in the exposed crystallized region, producing an optically absorbing surround to each lens. The net result is an array of spherical lenslets surrounded by an optically dense absorbing region. Figure 12 is an optical micrograph showing the resulting structure after exposure and thermal development. One possible application could be as a linear lens array in a camera autofocus module. For this application the critical parameters are the precise location of the cent~e-to-centTe distance of the lenses within the array so as to permit accurate registration to the CCD detector chip, and the uniformity of the radius of curvature of the lenses across the array. The process has been able to control the former to within a standard deviation of 0.2/.u-n and the latter to _0.1 pro. Another possible application is an array for one-to-one erect imaging. Typical dimensions would be an array of 0.5 m m diameter lenses, hexagonally close-packed on 0.55 m m centres. Figure 13 is of a xerographic copy of a photograph, made in a standard Canon copier, with such a micro-lens array.

Fig 12 Lenses surrounded by an optically opaque lregion

Precision Optical Elements by Direct Moulding For more than 2000 years, glass articles have been made by pressing or blowing into moulds of various materials, but optical quality of the Fig 13 Copiedphotograph by xerography using lens array surfaces has always been far below that required for most optical u s e s . And over the past few centuries, the technology of grinding and polishing the surfaces of pieces of glass to table V Properties of glasses and moulded lenses produce lenses of the desired shape, with high precision and high surface Molded glasses 1.5/60,1.6/50,1.7/30 Index/dispersion quality has been continually refined. <5x10 ~ Lens index inhomogenei|y It's relatively easy to produce a < :1:5 x 10 -4 Batch-batch index inhomogeneity < ~/100 spherical surface, but difficult to Birefringence 100-200 x 10 -7/~' C Thermal expansion produce the newly developed and highly desirable aspheric shapes, Lenses 5-60 mm especially if they are of large and Diameter <_+ 0.1 mm Cen|erthickness accuracy rapidly changing curvature. Spheric and aspheric lenses have been directly Figure Accuracy < 4 fringes moulded from glass, with the necesPower < 1 fringe Irregularity sary precision and surface quality n. There are only two requirements:

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

59

suitable glass and suitable moulds. These two requirements interact; both present formidable problems. At Coming, three glasses were developed to cover most of the range of optical properties needed. They're listed in table V. Refractive indices in available and suitable glasses can range from about ] .45 to 1.80. Figure 14 gives an idea of the range of sizes of lenses already achieved with this technology. Lenses in cameras of all types and in video disk players represent two of many possible applications. It's expected that because correction of spherical aberration with conventional spherical optics requires use of multiple elements, the use of aspheric elements will lead to simplified mounting, lower weight, lower light losses and more compact design in many other optical systems. Chain Silicate Glass-Ceramics Three general classes of glass-ceramics with chain silicates: polymeric crystals in which single or higher order multiple chains of silica tetrahedra form the mineral backbone, have now been developed. ~3.The result of the particular crystalline microstmcture is that these glass-ceramics are both strong and tough. Enstatite glass-ceramics Refractory, tough, and fine-grained glass-ceramics based on enstatite (MgSiO3) have been produced in the SiO2-MgO-ZrO2 and SiO2-MgOAl203-Li2o-z~ systems. These materials

Fig 14 Direct - moulded lenses. Coin diameter 19mm

contain from 50-85 weight percent Fracture toughness values as high enstatite with auxiliary phases zircon, as 5 MPa m ~n were achieved in these B-spodumene, minor tetragonal zir- highly crystalline glass-ceramics. These conia, and small amounts of glass. have use temperatures approaching 1525°C, the minimum ternary eutectic The actual toughening mechanism appears to involve crack deflection in the SiO2-MgO-ZrO2 phase diagram. from fine polysynthetic twinning in- Potassium fluor-richterite cluding possible penny-shaped cracks glass- ceramics along twin boundaries caused by the Glass-ceramics with the amphibole shrinkage. Splintering due to the potassium fluor-richtedte (KNaCaMg5 intersection of cleavage and twin Si8022F2) as the principal crystalline planes is also believed a factor. See phase have the strength and toughfigure 15. ness of a random microstructure of

Fig 15 Enstatite-zircon glass-ceramic. Photograph ~4x6 microns

60

Fig16 Potassium fluoro.richterite glass- ceramic

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

glass-ceramics. From two areas in this broad range of compositions, materials have been produced which can be made into teeth or into paper. Simple quartemary glasses containing K20, MgF2, MgO, and SiO2 were found which, when properly heat treated for internal nucleation and crystallization, will produce tetrasilicic

mica as the precipitated phase ~4 Typical compositions are given in table VI. The microplatelets into which the mica forms con~bute to the relatively high strength of the material compared to that of tooth enamel and dental porcelain; the average compressive strengths, moduli of rupture, and microhardness are in table VII.

Table VI Compositions of tetrasilicic mica glass-ceramics

Fig 17 Canasite glass-ceramic

tightly interlocked fine-grained acicular amphibole crystals with aspect ratios of 10:1 in a matrix of cristobalite, mica, and residual glass (figure 16). Fractures follow a tortuous path around rod-like crystals accounting for the high strength and toughness of these K-F-richterite glass-ceramics. The abraded moduls of rupture is ] 50 _+ 15 MPa; fracture toughness is 3.2 _ 2 MPa m ~n. The toughening mechanism is due to rod reinforcement by the K-F-richterite crystals.

Canasite glass.ceramics Fluorocanasite, CasNa3~K2.3Si~2030F4, has been synthesized from glasses close to its stoichiometry. A highly crystalline microstructure of interpenetrating blades produces strong and tough materials with high fracture strain. Such a microstructure is depicted in figure 17. Cleavage splintering causes energy absorption through crack branching and deflection. The thermal expansion of canasite is highly anisotropic. Considerable stress is thereby developed along grain boundaries on cooling. This suggests another toughening mechanism, namely, anisotropic thermal expansion stress microcracking. Canasite glass-ceramics with 300 MPa flexural strength have a fairly low Young's modulus; namely, 82 GPa. Therefore the strain at rupture is approximately 0.37%; high for a silicate material. The measured fracture toughness is 5.0 MPa m ~n. Dental Restorations Another family of glass-ceramics is based on mica as the crystalline phase, and from these have been produced the well-known machinable

Constituent

I

Amount II

K20 MgF 2 MgO SiO 2 ZrO 2

11.0 10.6 16.4 62.0

13.8 10.6 13.8 61.8

(wt%) III

IV

15.8 10.4 13.5 60.3

15.4 10.3 12.5 59.9 1.9

Table VII Mechanical properties of dental materials

Material

Microhardness

Modulus of rupture

Compressive strength

Glass-ceramic Tooth enamel Dentine Porcelain Gold alloy Amalgam

360 (K100) 340 70 460 200 110

150 MPa 10 50 25 400 70

830 MPa 400 300 170 420 380

Fig 18 Glass-ceramic tooth

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

61

The concept for restorative dentist]~y is to produce the desired shape by the lost wax process now used for gold restorations. The usual finishing to mesh with opposing teeth is done with the usual dental instruments. The material is non-porous, so a high gloss can be obtained by polishing. The thermal conductivity is about the same as that of enamel or porcelain, unlike that of silver amalgam which is more than 20 times, and that of gold which is more than 300 times higher; we can drink our uncivilizedly cold beer in the United States without any sensation - at least to the tooth. However, teeth have a surprisingly large range of colour, depending on several factors. Colourants may be added to the surface so that the resultant glass-ceramic restoration closely matches adjacent teeth; the glass-ceramic is also translucent, as is tooth enamel, and not opaque as porcelain, and is, in contra-distinction to metals, transparent to x-rays. The middle tooth in figure 18 is made of the glass-ceramic; the adjacent tooth with white spots has a typical porcelain inlay.

Inorganic Film or Paper If the parent glass is a fluorosilicate containing lithium instead of potassium, the proper heat treatment will permit the crystallization of relatively large amounts of a lithia-fluodde-hectorite type of crystal, Li20.MgF2.MgO.4SiO~2 ~'~. These crystals disintegrate rapidly in water or other polar liquids to produce a non-settling dispersion of platelets. To increase the speed of the reaction, the glass or glass-ceramic is crushed to a powder before exposure to water.

decade or two, in glass and glassceramic research and development? I am emboldened to append s o m e suggestions; a prophet is not without honour, save in his own country _~7 One thing we may look for is a significant advance in our knowledge of the structure of glasses. There is much current work to determine glass structure; especially its defect structure - even the simplest of binary glasses, such as SiOz and B~O3, are not so simple. The many newer techniques of analysis, both theoretical and instrumental, will give new insight into the structure of glasses. That knowledge will be used to devise new glasses or to develop desired new properties in known ones. New methods of glass manufacture, such as the sol-gel method, will permit formation of glasses not possible by conventional melting

The delamination of the mica crystals is very effective; they are usually very thin (at most a few unit cells) and of high aspect ratio (>100:1). Mixing the lithium hectorite gel with a solu~on of KC1 results in a stable potassium mica, a potassium-lithium-magnesium silicate, by rapid exchange of K+ for some of the Li+ ions. Flocculation follows and a coherent and continuous film, which can be less than 25 microns thick, of the flocculated material can be made by forcing a thin fiat stream of the gel into a salt bath. If the floc in the form of fibrils is deposited on the moving screen of a conventional papermaking machine, a thin paper can be the result (figure 19). This mica paper, with flakes which in general lie in the plane of the paper as a result of the method of formation is, after washing and drying, flexible and flexible enough to be creasable. The process, a schematic of which is in figure 20, allows incorporation of other materials, such as fibres, into the paper. Board, foam, or beads can be made by process modifications. Tensile strengths range up to 50 MPa and temperatures of over 500°C can be tolerated without embrittlement. Dielectric strengths are in excess of 20 KV/mm, and good chemical durability exists over a wide range of pH. Envisioned applications include fire-proof paper and board, capacitor dielectric, circuit boards, substrates and flexible electrical & thermal insulation.

18,19

By far the most common glasses produced today, mainly in sheet, container or fibre form, are alkalilime-silicates. Specialized applications require other compositional areas: borosilicate, aluminosilicate, pure silica or high silica. Newer glass compositions are evolving: nitrogen glasses, halide glasses, hydrosilicate glasses, semiconducting glasses, ionconducting glasses, chalcogenide glasses which contain no oxides, biocompatible glasses, and I should include here metallic glasses or more properly glassy metals - metals rapidly cooled so that they retain an amorphous structure. All these will

Future developments What can we expect in the next

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L~~.tNAT.EI Fig 19 Glass-ceramic paper

62

ICOMPOSITE]

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Fig 20 Glass-ceramic film or paper process

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

continue to evolve, and find wider use in applications, eg, long wavelength transparency, for which they are peculiarly suitable. There will be new classes of glassceramics: harder, stronger, tougher, more refractory. One long-recognized need is for an all-ceramic, or nearlyall-ceramic engine. Both Otto-cycle and Diesel engines with ceramic components will be running, at high temperatures, without cooling and without lubrication. They'll be much simpler, and much more efficient, than the highly refined but by comparison cumbersome and complicated engines they will replace. Stirling and rotary or Wankel engines may well have an important place in this scenario. One of the most significant words in the paragraph above is "tougher". Many metals, and many plastics, yield before rupture, and accommodate large local stresses by deformation. Glasses, in general, are perfectly elastic, and show only catastrophic failure, in contrast to what is sometimes termed the graceful failure of metals. Composites, therefore, will also be one of the answers. The incorporation of an inorganic, high elastic modulus, strong material in particle, fibre or whisker form, or a combination of them, in an inorganic matrix is now receiving much research and development attention, which will almost certainly produce new classes of high-temperature engineering materials. Glass-ceramics can themselves be considered to be a composite, with one or more crystalline materials in a residual glass matrix. Producing those crystals in a fibre or plate geometry has already been demonstrated to produce glassceramics with significantly increased toughness. Transformation-toughening, presently demonstrated in zirconia, will be another method of toughening ceramics. As new plastics, especially those which will survive at higher temperatures, are developed, there .will be new organic-inorganic composites finding specialized applications with

requirements which the ubiquitous and highly successful glass fibrereinforced plastics now cannot meet. Ogden Nash repeats the quote "Ifyou can't lick 'em, jine 'em" and we will certainly see more incorporation of dissimilar materials in useful composites, taking advantage of desirable properties of each of the components. This will extend past the conventional concept of composites to include organic materials dissolved in soft glasses or materials in which organics are a part of the silicate structure, the so-called Ormosil materials 2°. Glass is brittle, and intrinsically strong. It is strong in compression 2'. Hollow, buoyant glass structures have been taken to more than four miles below the surface of the sea without imploding 2z~. Is there a glass submarine 24 in your future?

Acknowledgements Many people at Corning, in addition to those specifically referenced, have contributed to the success of the projects reviewed here. I also wish to acknowledge the help of, and note my gratitude to, the many members of the Coming staff who suggested, supplied material for, and reviewed and edited specific sections of this summary in the areas of their own special technical competence. References

1 P A Tick. Water Durable Glasses with Ultralow Melting Temperatures. Physics and Chemistry of Glasses 25, 6, pp 149-154, (1984). 2 W H Dumbaugh. Infrared Transmitting Glasses. Optical Engineering 24 (2), pp 257-262, (1985). 3 W H Dumbaugh. Infrared Optical Materials and Fibres IV. Proceedings of the SPIE 618, pp 160-164, (1986). 4 N F Borrelli, W H Dumbaugh. Flectro-and Magneto-optic F_ffectsin HeavyMetal Oxide Glasses. Infrared Optical Materials and Fibres V. Proceedings of the SPIE 843, pp. 6-9 (1987). 5 S D Stookey, Colouration of Glass by Gold, Silver and Copper. Jour. Amer. Cer. Soc. 32 (8), pp. 246-249 (1949). 6 S D Stookey. (_IS Patent 2,515,937, July

1950. 7

S D Stookey. US Patent 2,651,145, September 1953.

MATERIALS & DESIGN Vol. 10 No. 2 MARCH/APRIL 1989

8 W H Armistead, S D Stookey. US Patent 3,208,860, September 1965. 9 S D Stookey, G H Beall, J E Pierson. Fullcolour Photosensitive Glass. Jour AppI Phys 49 (10), pp 5114-5123 (1978). 10 r'l F Borrelli, D L Morse, R H Bellman, W L Morgan. Photolytic Technique for Producing Microlenses in PhotosensitiveGlass.Applied Optics, 24, 16, pp 2520-2525 (1985) 11 N F Borrelli, D L Morse. Microlens Arrays Produced by a PhotolyticTechnique. Applied Optics, 27, 8, pp 476-479 (1988). 12 R O Maschmeyer, C A Andrysick, T W Geyer, H A Meissner, C J Parker, L M Sanford. Precision Moulded-glass Optics. Applied Optics 22, 16, Aug 15 (1983). 13 G H Beall. Tough Glass-ceramics Based on Chain Silicates. International Symposium on New Glasses, December 1-2, 1987, Tokyo, Japan. 14 D G Grossman. Processing a Dental Ceramic by Casting methods. Ceramic Engineering and Science Proceedings, 6 (1-2), pp 19-40 (1985). 15 S N Hoda, G H Beall. Alkaline Earth Mica Glass-ceramics, Advances in Ceramics (The American Ceramic Sodety) 4, (1982). 16 S N Hoda, G H Beall, D G Grossman, R J Schlaufman. Poster paper at XIII International Congress on Glass, Hamburg, Germany, July 1983. ] 7 An outstanding con~butor to glass science for more than half' a century is Dr Norbert J Kreidl. In honour of his eightieth birthday in July, 1984, a Symposium on Glass Science and Technology: Problems and Prospects for 2004 was held in his birthplace, Vienna. The tithE-eightpapers contributed by his friends are collected in Volume 13 (1985) of' The Journal of Non-crystalline Solids. 18 R Roy. Aids in Hydrothermal Experimentation: II, Methods of Maldng Mixtures for Both "Dry" and "Wet" Phase Equilibrium Studies". Jour Am Cer Soc 39, 145

(] 956). 19 G Carturan, V Gottardi, M Graziani. Physical and Chemical Evaluations Occurring in Glass Formation from Alkoxides of Silicon, Aluminium and Sodium. Jour of Non-crys Solids 29, 41 (1978). See also the entire Vol 48 (1982) of the Journal of Non-crystalline Solids for proceedings of a 1981 workshop on glasses and glass ceramics from gels. 21 P W Bridgman, I Simon. Effects of Very High Pressureson Glass.Jour of Appl Phys. 24, 4 (1953). 20 H Schmidt, H Scholze, A Kaiser. Jour of Non-crys Solids 48, 65 (1982). 22 J D Stachiw. A Practical Glass Deep Sea Vehicle. Undersea Technology 5 7 (1964). 23 H A Perry. The Argument for Glass Submersibles. Undersea Technology 5, 9

(1964). 24 G P Smith. Deep Submergence Vehicles. Geo-marine Technology 5, pp 179-184 (1965).

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