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Some recent new applications of glass technology
Journal of Non-Crystalline Solids 80 (1986) 69-82 North-Holland, Amsterdam
SOME RECENT NEW APPLICATIONS
69
OF G L A S S T E C H N O L O G Y
H.M. G A R F I N K E L * Research and Development Division, Coming Glass Works, Coming, New York 14831, USA
Currently, at Corning Glass Works we are involved in several new, interesting applications of glass technology, which have the potential of impacting a wide diversity of applications. Three such areas presently under investigation will be reviewed, viz., molded optics, dental restorative materials, and inorganic paper. The common denominator relating these three examples is the fact that each relies on a unique glass composition as the starting material. The first development is a special area of glass composition capable of being molded directly into optical elements without grinding and polishing. The second involves a new castable micaceous ceramic dental material derived from special glass compositions for application in several phases of restorative dentistry. Finally, the third development relates to a ceramic paper based upon a mica-like crystal, which is precipitated from glass. The current status of the technology will be discussed for each in the context of the potential applications.
1. Introduction
A great deal of optimism was expressed recently about the state of glass science and technology at a symposium at the University of Vienna on Glass Science and Technology entitled "Problems and Prospects for 2004" honoring the 80th birthday of Dr Norbert G. Kreidl. New and exciting advances are being made in a number of fundamental areas relating to glass structure, electrical, optical, and mechanical properties, nucleation and crystallization, and new glassforming compositions and methods, just to mention a few. The general consensus is that a healthy technology picture should continue out to the year 2004 [1]. To continue this general theme, this paper will describe three glass-related programs currently under development at Corning Glass Works. Each program started with research on new glass compositions, although at the present state of development process understanding and control appear to be keys to success. While none of the programs to be described is yet a commercial reality, each is viewed by us as having significant potential.
*Current address: Engelhard Corp., Menlo Park, CN 40, Edison, NJ 08818, U.S.A. 0022-3093/86/$03.50 O Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
H.M. Garfinkel / New applications of glass technology
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2. Molded-glass optics 2.1. Background The first development is a special area of glass composition capable of being molded directly into optical elements without grinding and polishing. Only plastics have so far realized much of the potential advantages of molding in the fabrication of precision optics. Glass, however, does have a clear advantage over plastics as an optical material in the index-dispersion range available. A number of reports have appeared already on precision molded glass lenses describing the formation of surface figures of various levels of accuracy [2-4]. However, none of the reports describes the formation of complete lenses.
2.2. Glass compositions A long-term commitment has been made at our laboratory to the formulation and study of low-temperature glasses. Essentially, these are glasses exhibiting low Tg'S with good durability under ambient conditions [5-9]. The glass used most extensively is a noncommercial specialty composition derived from the studies cited above. It has a refractive index of 1.604 and an Abbe number of about 50. Other moldable compositions exhibiting good chemical stability are available with indices ranging from 1.45 to 1.80 and dispersions comparable to commercial optical glasses. Work still continues on these very interesting low-temperature glasses for a number of applications other than molded optics.
2.3. Lens performance Because of the ease of measurement on spherical surfaces, some effort was directed at a 0.4-N.A. 8 mm diameter lens with spherical surfaces as shown in fig. 1 [10], although our primary interest is in aspherics. Typical design tolerances achieved are shown in table 1 as a measure of the molding process. Fig. 2 shows interferograms of the [/0.8 surfaces of a mold and a typical lens
SPHERICAL
fOB J
~""~'f
2 0 SPHERICAL
Fig. 1. Schematic of molded spherical lens shown in cross section.
H.M. Garfinkel / New applications o[ glass technology
71
Table 1 Selected measured molding tolerances Parameter
Measurement
Tolerance
Lens thickness Lens diameter Lens wedge Birefringence Index homogeneity Relative surface figure
Traveling stage microscope Traveling stage microscope Newton interferometry Friedel polarimeter Immersion refractrometer Fizeau interferometry
± 10 txm ± 10 ~m < 10-~ rad < 0.01 wave/cm < 5 × 10 ~' < 0.06 ~m p-p
Fig. 2. Fizeau interferograms of the ]70.8 surface of a mold and typical lens from that mold.
f r o m that mold. T h e lens replicates the mold to within 1/10 w a v e or 0.06 ~ m p e a k - t o - p e a k . Replication of the f/2.0 surface is c o m p a r a b l e . A l t h o u g h surface finish was not e v a l u a t e d quantitatively, it a p p e a r e d a d e q u a t e for typical optical applications. M o r e interestingly, diffraction-limited m o l d e d glass biaspheric lenses were o b t a i n e d by the same m o l d i n g process. Results have been r e p o r t e d for a biaspheric singlet a p p r o x i m a t e l y 6 m m in diameter, tested at a numerical a p e r t u r e of 0.45 [11]. A typical m o l d e d lens is shown in fig. 3 and described schematically in fig. 4. T h e lens surfaces are distinctly aspheric. For example, o v e r the clear aperture, b o t h surfaces deviate f r o m the best-fit spheres by 2 0 / ~ m . O v e r the entire m o l d e d surface, deviation f r o m best-fit spheres a p p r o a c h e s 3 5 / x m . With the same fabrication tolerances as described previously for the spherical example, the e x p e c t e d on-axis transmitted
72
H.M. Garlinkel New applications of glass technology
Fig. 3. Molded glass biaspheric lens.
SCHEMATIC OF MOLDED BIASPHERIC LENS
i/
f 0 . 6 6 SPHERICAL
f
\ ~ f 5 SPHERICAL
Fig. 4. Schematic drawing of the biaspheric lens. The total lens diameter exceeds optical diameter. The f/nos, indicate surface optical speeds as if the surface was spherical.
wave-front performance was calculated to be 0.06 wave rms tested at 0.45 NA and 0.6328/xm. Typical performance was found to be 0.05-0.08 wave rms optical path difference. (For diffraction-limited performance the accepted Marechal criterion is 0.074 wave rms.) Not surprisingly, experimental results involving mold rotation indicated that lens performance is primarily limited by the surface figure accuracy of the mold. 2.4. Outlook Work is continuing at our laboratory in this very promising new area with results indicating the availability of a wide range in lens size. Success with this development should offer the optical designer a much wider degree of freedom in design. Properly designed aspheric surfaces can minimize spheri-
H.M. Garfinkel / New applications of glass technology
73
cal aberration and off-axis coma whose correction with conventional spherical optics requires use of additional elements. It is not unusual for an aspheric element to be capable of performance equivalent to three or more spherical elements in the appropriate design. Reducing the number of elements can lead to simplified mounting, lower weight, lower light sources, and more compact designs.
3. Dental restoratives
3.1. Background The second development, a program in the field of bioceramics, relies on a new castable glass-ceramic dental material, which has the potential for use in many phases of restorative dentistry. MacCulloch recognized the potential for glass-ceramics in dentistry by producing denture teeth in the Li20ZnO-A1203 system [12]. Hench et al. investigated the use of Li20-SiO2 glassceramic for preparing dental restorations [13], but found these materials too weak and too poor in chemical durability for practical application in the oral environment. Kasloff [14] reported on transparent castings, which were heat treated at 200 to 250°C to yield an opaque ceramic; composition of these glass-ceramics was not disclosed.
3.2. Glass compositions Much work has been carried out at Corning on glass-ceramics based on the crystallization of fluorine-containing mica crystals. The interlocking, "house-of-cards" microstructure of these sheet-silicate minerals results in materials which can be machined to tight tolerances with conventional metal-working equipment. The present castable ceramic for dental applications is a special member of the family of machineable glass-ceramics consisting essentially of tetrasilicic fluormica crystals (K2MgsSi802oF4) dispersed in a minor glassy phase [15-17]. Although the crystal phase is mica, it contains no boron or aluminum as is found in most micas. The refractive index of the crystals is not too different from that of the surrounding residual glass, thereby enhancing translucency. Durability, hardness, and other properties of this material make it quite suitable for use in restoring lost tooth structure.
3.3. Biomaterial performance The process used to fabricate a cast ceramic restoration relies on a mold prepared by the conventional "lost-wax" technique used to make gold crowns. The glass is melted at about 1370°C and cast into the mold, which is spun so that centrifugal force will pull the glass down tightly into the cavity.
74
H.M. Garjinkel / New applications of glass technology
Upon cooling, the investment material is broken away leaving a glass crown, which is then converted into a glass-ceramic by heat treatment. The final microstructure can be seen in the SEM in fig. 5 after heat treatment for six hours at 1075°C. The residual glass phase, which occupies approximately 45 vol.% of the glass-ceramic, has been etched away. Strength and reinforcement of the cast ceramic result from the interlocking crystalline microstructure; these crystals help deflect and divert fractures on a microscopic level. Fig. 6 shows the casting before and after ceramming. Comparison is made in table 2 of physical properties of the cast ceramic with other important dental materials. The glass-ceramic material has twice the modulus of rupture and over four times the compressive strength of conventional dental porcelain. The modulus of rupture and the compressive strength of the glass-ceramic are about three times those of the natural tooth. The low conductivity of the glass-ceramic impedes the transfer of heat and cold to the underlying tooth structure. The similarity of density and hardness between the glass-ceramic and enamel helps to reduce wear. Similarity in radiographic density of the ceramic and natural enamel facilitates radiographic detection of dental caries. The cast ceramic compared favorably to dental porcelains in both strong acids and water. Finally, the ceramic material has shown complete biocompatibility following the recommended procedures outlined in ANSI/ADA document No. 41 in tests performed by an independent laboratory.
Fig. 5. Cast ceramic microstructure.
H.M. Garfinkel / New applications of glass technology
75
Fig. 6. The casting before and after heat treatment.
Table 2 Comparison of selected physical properties of dental materials Properties
Cast Enamel b) Dentine h) Porcelain ") ceramic .~
Density (g/cm 3) 2.7 Refractive index 1.52 Translucency 0.56 Thermal conductivity (cal/sec/cm2/°C/cm) 0.0040 Coefficient of expansion × 10-~'/°C 7.2 MOR (MPa) 152 Compressive strength 828 (MPa) Modulus of elasticity GPa 70.3 Microhardness KHN 362
3.0 1.65 0.48
2.2 -
0.0022 (11.4) 10.3 400
84.1 343
2.4 0.27
0.0015 51 297
18.3 68
0.0030 8.0 75.9 172
82.8 460
Gold alloy b) 14.0 0 0 0.7 14.4 448 -
Amalgam ")
11.0 t) 0 0.055 22-28 69 379
90 62 90-220 110
~ Measurements made at Coming Glass Works. b~ Restorative dental materials, ed. R.G. Craig, C.V. Mosby Co. (1980).
Clinical tests have been carried out with both anterior and posterior crowns. The final results have been very encouraging with the restorations p l a c e d h a v i n g a v e r y n a t u r a l a p p e a r a n c e as s h o w n in fig. 7. B o t h t h e fit a n d t h e t i s s u e r e s p o n s e h a v e b e e n n o t e d as e x c e l l e n t b y t h e c l i n i c i a n s .
76
H.M. Garfinkel
New applications of glass technology
Fig. 7. Examples of anterior and posterior cast ceramic crowns.
3.4. Outlook Continued d e v e l o p m e n t of the glass-ceramic has resulted in a method of fabricating and shading restorations with r e m a r k a b l e accuracy of form and superior aesthetics. T h e s e restorations have displayed excellent fit, ease of
H.M. Garfinkel / New applications of glass technology
77
adjustment, and superior tissue response. This material offers a viable alternative to metal-ceramics used in fixed restorative dentistry. Currently, Dentsply International and Coming Glass Works are jointly engaged in an effort to perfect marketable systems. This effort includes extensive in vitro and clinical testing programs.
4. Inorganic paper 4.1. Background
The last program, which is the least developed of the three, involves a ceramic paper based upon a mica-like crystal precipitated from glass. Paper-like products have been manufactured from various inorganic materials for a number of years. Most commonly, the starting materials for such products have been either asbestos or fibrous forms of alumina, basalt, glass, wollastonite, zirconia, etc., bonded together with an inorganic or organic binder. However, none of these papers has exhibited the smoothness, flexibility, and the mechanical strength of a conventional wood pulp paper. Because of the other very desirable properties demonstrated by inorganic paper, especially the electrical and thermal insulating characteristics and the resistance to weathering and chemical attack shown by micaceous papers, research has continued to demonstrate even better inorganic papers. The primary goal of this work has been to develop inorganic papers, which retain the desirable chemical and physical properties of commercially available inorganic papers and yet display the smoothness, flexibility, and mechanical strength of conventional wood pulp paper. The crystal-containing gels that are used to make paper can also be used to prepare films, fibers, boards, beads, and coatings. 4.2. Glass composition
As described in the previous section on dental restoratives, a long-term effort has been ongoing related to mica containing glass-ceramics. This work has focused on glass compositions, which can be crystallized to yield lithium and/or sodium water-swelling micaceous materials. Although the primary crystalline phase employed has been lithium fluorhectorite (LiMg2LiSi4OmF2), other water-swelling micas and solid solutions of these micas with other structurally compatible species can be used as starting materials for papers, films, etc. [8]. 4.3. Process and properties
Paper, film, fiber, and board can be prepared from crystal-containing gels derived from water-swellable mica-containing glass-ceramics. The basic
78
H.M. Garfinkel / New applications of glass technology
process employed to make the micaceous inorganic paper and some of the available intermediate forms are shown in fig. 8. In this process, a fluormica or hydroxylmica glass-ceramic is crystallized by heat treating the appropriate glass composition. The glass-ceramic is reacted with water (or other polar liquid), yielding very small platelets of exceptionally high aspect ratio (500:1). Flocculation of the gel is accomplished by exchange of the interlayer Li + and/or Na + ions with larger inorganic cations (e.g., K +, Rb +, Cs +, Ca +2, Sr +2, Ba +2, NH], etc) or certain organic polycations (e.g., primary, secondary, or tertiary amines solubilized with acid; quaternary ammonium or phosphonium salts; or tertiary sulfonium salts [19]; and aminosilanes or organic chrome complexes [20]). Flocculation occurs because the negatively charged mica particles, which have been separated due to hydration and partial loss of interlayer cations into the surrounding medium, are electrically neutralized. At this point the organic polycation exchanged complex can exhibit marked hydrophobic behavior [ 19,20]. This
MELTING ]
I CERAMMING I
WATER SWELLING ]
SOL (Gel)
RAPID ION EXCHANGE i
BINDERS ) I COATINGS
POWDER ELECTRODEPOS ITED FILMS
~
I FILMS FOAMS BEADS -- COMPOSITES
FLOC SLURRY
I FORMING PROCESSES
> I BINDERS COATINGS >
" ~
POWDER ELECTRODEPOSITED FILMS
MOLDED SHAPES EXTRUSIONS
Fig. 8. Schematic representation of the basic process to make inorganic paper.
H.M. Garfinkel / New applications of glass technology
Fig. 9. Thirty cm-wide ceramic paper by the Fourdrinier process.
Fig. 10. Ceramic film, paper, and board.
79
80
H.M. Garfinkel / New applications of glass technology
floc can then be processed into paper with conventional Fourdrinier papermaking equipment as shown in fig. 9. Film can be made by extruding the gel into an aqueous KC1 solution or by electrophoretic deposition [21] from a mica sol. Board can be prepared by a variety of process options including: heating organic exchanged floc under pressure; compression molding, injection molding, or extruding spray dried floc; laminating paper made on conventional papermaking equipment with organic resin; or direct deposition of the slurry on wire-forming equipment followed by firing to, say, 900°C and impregnating with resin to yield a rigid, nonporous board. Foams can be prepared by frothing an ion exchanged gel with added surfactant by a shearing action and setting the gel. Figure 10 shows film, paper, and board and fig. 11 shows ceramic foam and board.
Fig. 11. Ceramic foam and board.
H.M. Garfinkel / New applications of glass technology
81
Table 3 Selected typical properties of mica paper and film
Thickness (/xm) Bulk density (g/cm 3) Porosity (%) Pore size (mm) Tensile strength (MPa), machine direction Tensile strength (MPa), cross direction Dielectric strength, kV/cm, 25°C Loss tangent 100 Hz, 25°C a~ 300°C 10 Hz, 25°C ~) 300°C Dielectric constant 100 Hz, 25°C ") 300°C 1(1Hz, 25°C a) 300°C DC volume resistivity, ohm-cm, 25°C
Paper with 20% fiberglass
Extruded film
130 0.80 55 >20
20 1.10 55 >5
8.9
9.0
6.9
3.1
120 0.3 0.8 0.2 0.4 8 20 5 5 > 10 ">
200 0.3 0.8 0.06 0.2 9 7
> 1 0 l"
,l) Data taken on samples in vacuum.
Table 4 Typical foam properties Strength: M O R CRUSH Porosity (open cells) Cell size Density Thermal conductivity at 38°C Thermal expansion (0-300°C) Use temperature
7-35 MPa 2-14 MPa 95% 100-200/zm 0.64-0.96 g/cm 3 0.05-0.12 W m LK t 50 x 10 7 cm/cm/OC 850°C
Chemical and physical properties of these synthetic mica materials can be modified readily by addition of particulate and/or fibrous fillers and reinforcements. These can be organic and/or inorganic additives. Paper and film are hygroscopic showing about a 5% weight gain at 75% relative humidity. They are stable in most acids to pH = 3 and most bases to pH = 13; and are unaffected up to 400°C, gradually embrittling in the range
H.M. Gar]inkel / New applications of glass technology
82
500-700°C, and melting above 1000°C. Some typical properties of paper and film are shown in table 3, and typical foam properties are listed in table 4. The unique characteristics of these materials are: high use temperature; good dielectric properties; excellent durability in strong acid and alkali; can form composites with organic and/or other inorganic materials; and can be prepared in a variety of particulate and monolithic forms. 4.4. Outlook
Presently several potential applications, which rely on the most unique characteristics of these materials, are under investigation in joint research and development programs between Corning Glass and other industrial firms.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
J.R. Hutchins, J. Mat. Sci., to be published. G.E. Blair, US Patent 4 168 961. C.K. Wu, US Patent 4 073 654 (1978). R.R. Turk, Proc. Soc. Photo-Opt. Instr. Eng. 297 (1981) 204. L.M. Sanford and P.A. Tick, US Patent 4 285 730 (1981). P.A. Tick and L.M. Sanford, US Patent 4 323 654 (1982). A.R. Olszewski, P.A. Tick and L.M. Sanford, US Patent 4 362 819 (1982). P.A. Tick, US Patent 4 405 724 (1983). D.L.J. Leroy and J.P. Mazeau, US Patent 4 447 550 (1984). R.O. Maschmeyer, C.A. Andrysick, T.W. Geyer, H.E. Meissner, C.J. Parker and L.M. Sanford, Appl. Optics 22 (1983) 2410. R.O. Maschmeyer, R.M. Hujar, L.L. Carpenter, B.W. Nicholson and E.F. Vozenilek, Appl. Optics 22 (1983) 2413. W.T. MacCulloch, Brit. Dent. J. 124 (1968) 361. L.W. Hench et al., Presented at Meeting of IADR, Chicago, I11. (March 19, 1971). Z. Kasloff, in: Dental Porcelain: The State of the Art, ed. H.N. Yamada (1977) p. 241. D.G. Grossman, US Patent 3 732 087 (1973). P.J. Adair, US Patent 4 431 420 (1984). D.G. Grossman, presented at W.K. Kellogg Foundation Institute, Graduate and Postgraduate Dentistry, University of Michigan (1983). G.H. Beall, D.G. Grossman, S.N. Hoda and K.R. Kubinski, US Patent 4 239 519 (1980); US Patent 4 297 139 (1981); US Patent 4 339 530 (1982). S.H. Wu, US Patent 4 455 382 (1984). S.N. Hoda and R. Olszewski, US Patent 4 454 237 (1984). F.P. Fehlner and W.J. Wein, US Patent 4 432 852 (1984).
SOME RECENT NEW APPLICATIONS
69
OF G L A S S T E C H N O L O G Y
H.M. G A R F I N K E L * Research and Development Division, Coming Glass Works, Coming, New York 14831, USA
Currently, at Corning Glass Works we are involved in several new, interesting applications of glass technology, which have the potential of impacting a wide diversity of applications. Three such areas presently under investigation will be reviewed, viz., molded optics, dental restorative materials, and inorganic paper. The common denominator relating these three examples is the fact that each relies on a unique glass composition as the starting material. The first development is a special area of glass composition capable of being molded directly into optical elements without grinding and polishing. The second involves a new castable micaceous ceramic dental material derived from special glass compositions for application in several phases of restorative dentistry. Finally, the third development relates to a ceramic paper based upon a mica-like crystal, which is precipitated from glass. The current status of the technology will be discussed for each in the context of the potential applications.
1. Introduction
A great deal of optimism was expressed recently about the state of glass science and technology at a symposium at the University of Vienna on Glass Science and Technology entitled "Problems and Prospects for 2004" honoring the 80th birthday of Dr Norbert G. Kreidl. New and exciting advances are being made in a number of fundamental areas relating to glass structure, electrical, optical, and mechanical properties, nucleation and crystallization, and new glassforming compositions and methods, just to mention a few. The general consensus is that a healthy technology picture should continue out to the year 2004 [1]. To continue this general theme, this paper will describe three glass-related programs currently under development at Corning Glass Works. Each program started with research on new glass compositions, although at the present state of development process understanding and control appear to be keys to success. While none of the programs to be described is yet a commercial reality, each is viewed by us as having significant potential.
*Current address: Engelhard Corp., Menlo Park, CN 40, Edison, NJ 08818, U.S.A. 0022-3093/86/$03.50 O Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
H.M. Garfinkel / New applications of glass technology
70
2. Molded-glass optics 2.1. Background The first development is a special area of glass composition capable of being molded directly into optical elements without grinding and polishing. Only plastics have so far realized much of the potential advantages of molding in the fabrication of precision optics. Glass, however, does have a clear advantage over plastics as an optical material in the index-dispersion range available. A number of reports have appeared already on precision molded glass lenses describing the formation of surface figures of various levels of accuracy [2-4]. However, none of the reports describes the formation of complete lenses.
2.2. Glass compositions A long-term commitment has been made at our laboratory to the formulation and study of low-temperature glasses. Essentially, these are glasses exhibiting low Tg'S with good durability under ambient conditions [5-9]. The glass used most extensively is a noncommercial specialty composition derived from the studies cited above. It has a refractive index of 1.604 and an Abbe number of about 50. Other moldable compositions exhibiting good chemical stability are available with indices ranging from 1.45 to 1.80 and dispersions comparable to commercial optical glasses. Work still continues on these very interesting low-temperature glasses for a number of applications other than molded optics.
2.3. Lens performance Because of the ease of measurement on spherical surfaces, some effort was directed at a 0.4-N.A. 8 mm diameter lens with spherical surfaces as shown in fig. 1 [10], although our primary interest is in aspherics. Typical design tolerances achieved are shown in table 1 as a measure of the molding process. Fig. 2 shows interferograms of the [/0.8 surfaces of a mold and a typical lens
SPHERICAL
fOB J
~""~'f
2 0 SPHERICAL
Fig. 1. Schematic of molded spherical lens shown in cross section.
H.M. Garfinkel / New applications o[ glass technology
71
Table 1 Selected measured molding tolerances Parameter
Measurement
Tolerance
Lens thickness Lens diameter Lens wedge Birefringence Index homogeneity Relative surface figure
Traveling stage microscope Traveling stage microscope Newton interferometry Friedel polarimeter Immersion refractrometer Fizeau interferometry
± 10 txm ± 10 ~m < 10-~ rad < 0.01 wave/cm < 5 × 10 ~' < 0.06 ~m p-p
Fig. 2. Fizeau interferograms of the ]70.8 surface of a mold and typical lens from that mold.
f r o m that mold. T h e lens replicates the mold to within 1/10 w a v e or 0.06 ~ m p e a k - t o - p e a k . Replication of the f/2.0 surface is c o m p a r a b l e . A l t h o u g h surface finish was not e v a l u a t e d quantitatively, it a p p e a r e d a d e q u a t e for typical optical applications. M o r e interestingly, diffraction-limited m o l d e d glass biaspheric lenses were o b t a i n e d by the same m o l d i n g process. Results have been r e p o r t e d for a biaspheric singlet a p p r o x i m a t e l y 6 m m in diameter, tested at a numerical a p e r t u r e of 0.45 [11]. A typical m o l d e d lens is shown in fig. 3 and described schematically in fig. 4. T h e lens surfaces are distinctly aspheric. For example, o v e r the clear aperture, b o t h surfaces deviate f r o m the best-fit spheres by 2 0 / ~ m . O v e r the entire m o l d e d surface, deviation f r o m best-fit spheres a p p r o a c h e s 3 5 / x m . With the same fabrication tolerances as described previously for the spherical example, the e x p e c t e d on-axis transmitted
72
H.M. Garlinkel New applications of glass technology
Fig. 3. Molded glass biaspheric lens.
SCHEMATIC OF MOLDED BIASPHERIC LENS
i/
f 0 . 6 6 SPHERICAL
f
\ ~ f 5 SPHERICAL
Fig. 4. Schematic drawing of the biaspheric lens. The total lens diameter exceeds optical diameter. The f/nos, indicate surface optical speeds as if the surface was spherical.
wave-front performance was calculated to be 0.06 wave rms tested at 0.45 NA and 0.6328/xm. Typical performance was found to be 0.05-0.08 wave rms optical path difference. (For diffraction-limited performance the accepted Marechal criterion is 0.074 wave rms.) Not surprisingly, experimental results involving mold rotation indicated that lens performance is primarily limited by the surface figure accuracy of the mold. 2.4. Outlook Work is continuing at our laboratory in this very promising new area with results indicating the availability of a wide range in lens size. Success with this development should offer the optical designer a much wider degree of freedom in design. Properly designed aspheric surfaces can minimize spheri-
H.M. Garfinkel / New applications of glass technology
73
cal aberration and off-axis coma whose correction with conventional spherical optics requires use of additional elements. It is not unusual for an aspheric element to be capable of performance equivalent to three or more spherical elements in the appropriate design. Reducing the number of elements can lead to simplified mounting, lower weight, lower light sources, and more compact designs.
3. Dental restoratives
3.1. Background The second development, a program in the field of bioceramics, relies on a new castable glass-ceramic dental material, which has the potential for use in many phases of restorative dentistry. MacCulloch recognized the potential for glass-ceramics in dentistry by producing denture teeth in the Li20ZnO-A1203 system [12]. Hench et al. investigated the use of Li20-SiO2 glassceramic for preparing dental restorations [13], but found these materials too weak and too poor in chemical durability for practical application in the oral environment. Kasloff [14] reported on transparent castings, which were heat treated at 200 to 250°C to yield an opaque ceramic; composition of these glass-ceramics was not disclosed.
3.2. Glass compositions Much work has been carried out at Corning on glass-ceramics based on the crystallization of fluorine-containing mica crystals. The interlocking, "house-of-cards" microstructure of these sheet-silicate minerals results in materials which can be machined to tight tolerances with conventional metal-working equipment. The present castable ceramic for dental applications is a special member of the family of machineable glass-ceramics consisting essentially of tetrasilicic fluormica crystals (K2MgsSi802oF4) dispersed in a minor glassy phase [15-17]. Although the crystal phase is mica, it contains no boron or aluminum as is found in most micas. The refractive index of the crystals is not too different from that of the surrounding residual glass, thereby enhancing translucency. Durability, hardness, and other properties of this material make it quite suitable for use in restoring lost tooth structure.
3.3. Biomaterial performance The process used to fabricate a cast ceramic restoration relies on a mold prepared by the conventional "lost-wax" technique used to make gold crowns. The glass is melted at about 1370°C and cast into the mold, which is spun so that centrifugal force will pull the glass down tightly into the cavity.
74
H.M. Garjinkel / New applications of glass technology
Upon cooling, the investment material is broken away leaving a glass crown, which is then converted into a glass-ceramic by heat treatment. The final microstructure can be seen in the SEM in fig. 5 after heat treatment for six hours at 1075°C. The residual glass phase, which occupies approximately 45 vol.% of the glass-ceramic, has been etched away. Strength and reinforcement of the cast ceramic result from the interlocking crystalline microstructure; these crystals help deflect and divert fractures on a microscopic level. Fig. 6 shows the casting before and after ceramming. Comparison is made in table 2 of physical properties of the cast ceramic with other important dental materials. The glass-ceramic material has twice the modulus of rupture and over four times the compressive strength of conventional dental porcelain. The modulus of rupture and the compressive strength of the glass-ceramic are about three times those of the natural tooth. The low conductivity of the glass-ceramic impedes the transfer of heat and cold to the underlying tooth structure. The similarity of density and hardness between the glass-ceramic and enamel helps to reduce wear. Similarity in radiographic density of the ceramic and natural enamel facilitates radiographic detection of dental caries. The cast ceramic compared favorably to dental porcelains in both strong acids and water. Finally, the ceramic material has shown complete biocompatibility following the recommended procedures outlined in ANSI/ADA document No. 41 in tests performed by an independent laboratory.
Fig. 5. Cast ceramic microstructure.
H.M. Garfinkel / New applications of glass technology
75
Fig. 6. The casting before and after heat treatment.
Table 2 Comparison of selected physical properties of dental materials Properties
Cast Enamel b) Dentine h) Porcelain ") ceramic .~
Density (g/cm 3) 2.7 Refractive index 1.52 Translucency 0.56 Thermal conductivity (cal/sec/cm2/°C/cm) 0.0040 Coefficient of expansion × 10-~'/°C 7.2 MOR (MPa) 152 Compressive strength 828 (MPa) Modulus of elasticity GPa 70.3 Microhardness KHN 362
3.0 1.65 0.48
2.2 -
0.0022 (11.4) 10.3 400
84.1 343
2.4 0.27
0.0015 51 297
18.3 68
0.0030 8.0 75.9 172
82.8 460
Gold alloy b) 14.0 0 0 0.7 14.4 448 -
Amalgam ")
11.0 t) 0 0.055 22-28 69 379
90 62 90-220 110
~ Measurements made at Coming Glass Works. b~ Restorative dental materials, ed. R.G. Craig, C.V. Mosby Co. (1980).
Clinical tests have been carried out with both anterior and posterior crowns. The final results have been very encouraging with the restorations p l a c e d h a v i n g a v e r y n a t u r a l a p p e a r a n c e as s h o w n in fig. 7. B o t h t h e fit a n d t h e t i s s u e r e s p o n s e h a v e b e e n n o t e d as e x c e l l e n t b y t h e c l i n i c i a n s .
76
H.M. Garfinkel
New applications of glass technology
Fig. 7. Examples of anterior and posterior cast ceramic crowns.
3.4. Outlook Continued d e v e l o p m e n t of the glass-ceramic has resulted in a method of fabricating and shading restorations with r e m a r k a b l e accuracy of form and superior aesthetics. T h e s e restorations have displayed excellent fit, ease of
H.M. Garfinkel / New applications of glass technology
77
adjustment, and superior tissue response. This material offers a viable alternative to metal-ceramics used in fixed restorative dentistry. Currently, Dentsply International and Coming Glass Works are jointly engaged in an effort to perfect marketable systems. This effort includes extensive in vitro and clinical testing programs.
4. Inorganic paper 4.1. Background
The last program, which is the least developed of the three, involves a ceramic paper based upon a mica-like crystal precipitated from glass. Paper-like products have been manufactured from various inorganic materials for a number of years. Most commonly, the starting materials for such products have been either asbestos or fibrous forms of alumina, basalt, glass, wollastonite, zirconia, etc., bonded together with an inorganic or organic binder. However, none of these papers has exhibited the smoothness, flexibility, and the mechanical strength of a conventional wood pulp paper. Because of the other very desirable properties demonstrated by inorganic paper, especially the electrical and thermal insulating characteristics and the resistance to weathering and chemical attack shown by micaceous papers, research has continued to demonstrate even better inorganic papers. The primary goal of this work has been to develop inorganic papers, which retain the desirable chemical and physical properties of commercially available inorganic papers and yet display the smoothness, flexibility, and mechanical strength of conventional wood pulp paper. The crystal-containing gels that are used to make paper can also be used to prepare films, fibers, boards, beads, and coatings. 4.2. Glass composition
As described in the previous section on dental restoratives, a long-term effort has been ongoing related to mica containing glass-ceramics. This work has focused on glass compositions, which can be crystallized to yield lithium and/or sodium water-swelling micaceous materials. Although the primary crystalline phase employed has been lithium fluorhectorite (LiMg2LiSi4OmF2), other water-swelling micas and solid solutions of these micas with other structurally compatible species can be used as starting materials for papers, films, etc. [8]. 4.3. Process and properties
Paper, film, fiber, and board can be prepared from crystal-containing gels derived from water-swellable mica-containing glass-ceramics. The basic
78
H.M. Garfinkel / New applications of glass technology
process employed to make the micaceous inorganic paper and some of the available intermediate forms are shown in fig. 8. In this process, a fluormica or hydroxylmica glass-ceramic is crystallized by heat treating the appropriate glass composition. The glass-ceramic is reacted with water (or other polar liquid), yielding very small platelets of exceptionally high aspect ratio (500:1). Flocculation of the gel is accomplished by exchange of the interlayer Li + and/or Na + ions with larger inorganic cations (e.g., K +, Rb +, Cs +, Ca +2, Sr +2, Ba +2, NH], etc) or certain organic polycations (e.g., primary, secondary, or tertiary amines solubilized with acid; quaternary ammonium or phosphonium salts; or tertiary sulfonium salts [19]; and aminosilanes or organic chrome complexes [20]). Flocculation occurs because the negatively charged mica particles, which have been separated due to hydration and partial loss of interlayer cations into the surrounding medium, are electrically neutralized. At this point the organic polycation exchanged complex can exhibit marked hydrophobic behavior [ 19,20]. This
MELTING ]
I CERAMMING I
WATER SWELLING ]
SOL (Gel)
RAPID ION EXCHANGE i
BINDERS ) I COATINGS
POWDER ELECTRODEPOS ITED FILMS
~
I FILMS FOAMS BEADS -- COMPOSITES
FLOC SLURRY
I FORMING PROCESSES
> I BINDERS COATINGS >
" ~
POWDER ELECTRODEPOSITED FILMS
MOLDED SHAPES EXTRUSIONS
Fig. 8. Schematic representation of the basic process to make inorganic paper.
H.M. Garfinkel / New applications of glass technology
Fig. 9. Thirty cm-wide ceramic paper by the Fourdrinier process.
Fig. 10. Ceramic film, paper, and board.
79
80
H.M. Garfinkel / New applications of glass technology
floc can then be processed into paper with conventional Fourdrinier papermaking equipment as shown in fig. 9. Film can be made by extruding the gel into an aqueous KC1 solution or by electrophoretic deposition [21] from a mica sol. Board can be prepared by a variety of process options including: heating organic exchanged floc under pressure; compression molding, injection molding, or extruding spray dried floc; laminating paper made on conventional papermaking equipment with organic resin; or direct deposition of the slurry on wire-forming equipment followed by firing to, say, 900°C and impregnating with resin to yield a rigid, nonporous board. Foams can be prepared by frothing an ion exchanged gel with added surfactant by a shearing action and setting the gel. Figure 10 shows film, paper, and board and fig. 11 shows ceramic foam and board.
Fig. 11. Ceramic foam and board.
H.M. Garfinkel / New applications of glass technology
81
Table 3 Selected typical properties of mica paper and film
Thickness (/xm) Bulk density (g/cm 3) Porosity (%) Pore size (mm) Tensile strength (MPa), machine direction Tensile strength (MPa), cross direction Dielectric strength, kV/cm, 25°C Loss tangent 100 Hz, 25°C a~ 300°C 10 Hz, 25°C ~) 300°C Dielectric constant 100 Hz, 25°C ") 300°C 1(1Hz, 25°C a) 300°C DC volume resistivity, ohm-cm, 25°C
Paper with 20% fiberglass
Extruded film
130 0.80 55 >20
20 1.10 55 >5
8.9
9.0
6.9
3.1
120 0.3 0.8 0.2 0.4 8 20 5 5 > 10 ">
200 0.3 0.8 0.06 0.2 9 7
> 1 0 l"
,l) Data taken on samples in vacuum.
Table 4 Typical foam properties Strength: M O R CRUSH Porosity (open cells) Cell size Density Thermal conductivity at 38°C Thermal expansion (0-300°C) Use temperature
7-35 MPa 2-14 MPa 95% 100-200/zm 0.64-0.96 g/cm 3 0.05-0.12 W m LK t 50 x 10 7 cm/cm/OC 850°C
Chemical and physical properties of these synthetic mica materials can be modified readily by addition of particulate and/or fibrous fillers and reinforcements. These can be organic and/or inorganic additives. Paper and film are hygroscopic showing about a 5% weight gain at 75% relative humidity. They are stable in most acids to pH = 3 and most bases to pH = 13; and are unaffected up to 400°C, gradually embrittling in the range
H.M. Gar]inkel / New applications of glass technology
82
500-700°C, and melting above 1000°C. Some typical properties of paper and film are shown in table 3, and typical foam properties are listed in table 4. The unique characteristics of these materials are: high use temperature; good dielectric properties; excellent durability in strong acid and alkali; can form composites with organic and/or other inorganic materials; and can be prepared in a variety of particulate and monolithic forms. 4.4. Outlook
Presently several potential applications, which rely on the most unique characteristics of these materials, are under investigation in joint research and development programs between Corning Glass and other industrial firms.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
J.R. Hutchins, J. Mat. Sci., to be published. G.E. Blair, US Patent 4 168 961. C.K. Wu, US Patent 4 073 654 (1978). R.R. Turk, Proc. Soc. Photo-Opt. Instr. Eng. 297 (1981) 204. L.M. Sanford and P.A. Tick, US Patent 4 285 730 (1981). P.A. Tick and L.M. Sanford, US Patent 4 323 654 (1982). A.R. Olszewski, P.A. Tick and L.M. Sanford, US Patent 4 362 819 (1982). P.A. Tick, US Patent 4 405 724 (1983). D.L.J. Leroy and J.P. Mazeau, US Patent 4 447 550 (1984). R.O. Maschmeyer, C.A. Andrysick, T.W. Geyer, H.E. Meissner, C.J. Parker and L.M. Sanford, Appl. Optics 22 (1983) 2410. R.O. Maschmeyer, R.M. Hujar, L.L. Carpenter, B.W. Nicholson and E.F. Vozenilek, Appl. Optics 22 (1983) 2413. W.T. MacCulloch, Brit. Dent. J. 124 (1968) 361. L.W. Hench et al., Presented at Meeting of IADR, Chicago, I11. (March 19, 1971). Z. Kasloff, in: Dental Porcelain: The State of the Art, ed. H.N. Yamada (1977) p. 241. D.G. Grossman, US Patent 3 732 087 (1973). P.J. Adair, US Patent 4 431 420 (1984). D.G. Grossman, presented at W.K. Kellogg Foundation Institute, Graduate and Postgraduate Dentistry, University of Michigan (1983). G.H. Beall, D.G. Grossman, S.N. Hoda and K.R. Kubinski, US Patent 4 239 519 (1980); US Patent 4 297 139 (1981); US Patent 4 339 530 (1982). S.H. Wu, US Patent 4 455 382 (1984). S.N. Hoda and R. Olszewski, US Patent 4 454 237 (1984). F.P. Fehlner and W.J. Wein, US Patent 4 432 852 (1984).