High-modulus glass fibers for new transportation and infrastructure composites and new infrared uses

Composites Science and Technology 51 (1994) 243-263

H I G H - M O D U L U S GLASS FIBERS FOR N E W T R A N S P O R T A T I O N A N D I N F R A S T R U C T U R E COMPOSITES A N D N E W I N F R A R E D USES F. T. Wallenberger & S. D. Brown University of Illinois, Department of Materials Science and Engineering, 1304 West Green Street, Urbana, Illinois 61801, USA (Received 31 July 1992; accepted 7 June 1993) uses. Hafner et al. t showed that certain calcium aluminate bulk glasses had a sapphire-like transmission window in the infrared (IR) extending to > 5 . 0 # m while those of silica glasses do not go much beyond 4.0/~m. Some of these glasses were commercialized. All the aluminate glasses which were made at that time 1 had a strong hydroxyl band at 2-9/~m, i.e. reduced transparency in the infrared. This band is more prominent than that of silica glasses, and also more difficult to eliminate. Between 1967 and 1978, Davy2 devised a process yielding hydroxyl-free substrates from conventional melts. Some of these IR optical glasses are still on the market.

Abstract

Calcium aluminate glass fibers offer major new opportunities in transportation, infrastructure composites and new infrared sensor applications. They possess a higher modulus than S-glass, an advantage that can yield major weight and energy savings in car, aircraft and aerospace composites, or stiffer composite parts in emerging bridge and construction markets. Their superior alkali resistance is an important advantage in cement composites. They also possess sapphire-like infrared transmission, a property that is obtained at a fraction of the cost of single crystal sapphire fibers. Quaternary non-silica calcium aluminates with 40-50% alumina have high melt viscosities (>>lO Pa s). Experimental fibers were obtained by potentially commercial processes (up-drawing from supercooled melts and down-drawing from preforms). Binary compositions with >50% alumina have low viscosities (<
1.1.2 Glass fibers An equally intensive effort was under way to design new silicate and aluminate glass fibers with a higher modulus than that of S-glass, then and now having the highest known modulus of any commercial glass fiber. 3 In the silicate system, a higher modulus (1.25 times that of S-glass) was obtained in 1955 with HM-glass, a beryllia-containing silicate fiber, 3'4 and an even higher modulus (1.5 times that of S-glass) was obtained with Sialon-glass, a silicon nitride modified silicate fiber. 5 In the aluminate system, the highest modulus (1.45 times that of S-glass) was achieved with a quaternary ZnO-containing calcium aluminate composition. 6-8 Calcium aluminate glass fibers were also made from low viscosity melts. 9 No new high modulus glass fiber was commercialized.

Keywords: high modulus fibers, new glass theory 1.1.3 Purpose In 1987, Wallenberger et al. 1°-32 initiated a major re-investigation of calcium aluminate fibers and their formation from low ~°-2~ and high viscosity melts. 22-32 It is the purpose of this review to: (1) analyze the experimental processes which can be used to prepare calcium aluminate glass fibers from viscous and inviscid melts; (2) describe the structural and optical properties for aluminate fibers which can be obtained by the various experimental processes; (3) develop a new theory with regard to melt viscosity and glassy

1 CALCIUM ALUMINATE GLASS FIBERS 1.1 Introduction

1.1.1 Bulk glasses An intensive effort was under way between 1955 and 1970 to design new bulk glasses for infrared optical

Composites Science and Technology 0266-3538/94/$07.00 © 1994 Elsevier Science Limited. 243

244

F. T. Wallenberger, S. D. Brown

1.2.1 Introduction Fibers can be looked upon as potential products, or as research tools. For the latter function, they have a simple yet demanding shape, and require high materials uniformity before they can be synthesized with reasonable strength and surface uniformity. More basic materials information can be gained from attempts to form fibers from a given melt than from pouring glasses from melts. In this section, the assessment of the behavior of calcium aluminate melts will be based both on a discussion of forming free-standing glass fibers and a discussion of forming bulk glasses from the melt.

(especially those with >55% alumina) have low melt viscosities. Several hypotheses have been advanced since 1968 to explain the structure of these high viscosity compositions. Onoda and Brown 7 attempted to link the melt viscosity to fundamental properties of the ions themselves but adequate investigative tools were not available at that time. Uhlmann 33 advanced the theoretical understanding in this field by providing a kinetic treatment of glass formation for oxide systems free of classical network formers, and Angel134"35 introduced the concept of 'fragile melts' providing further new insights regarding the behavior of aluminate melts. Wallenberger et al. ej found that incorporation of nanosize carbon particles into the surface of a low viscosity calcium aluminate melt increased the surface viscosity by up to a factor of 10. 21 By analogy, the magnitude of the melt viscosity of calcium aluminates in general may therefore also be rheology controlled, 3e i.e. due to differences in the coordination number, size and orientation of nanocrystalline oxide structures and/or nanostructural oxide domains.

1.2.2 High and low viscosities The melts of all silicate glasses and glass fibers have high viscosities. The behavior of aluminate melts is more complex than that of silicate melts. Depending upon the composition, the melts of calcium aluminate glasses and glass fibers can have high 4-8 or low viscosities. 9'1°'2~ For example, Fig. 1 highlights the region in the calcia-alumina phase diagram (45-50% alumina) that yields high viscosity melts especially for ternary and quaternary compositions, while all others

1.2. 3 Glass forming melts Aluminate melts, glasses and glass fibers are being reviewed here with emphasis on systems free of classical glass formers such as silica and in accordance with the kinetic treatment of glass formation advanced in 1972 by D. R. Uhlmann. 33 The glass forming ability of a melt depends on the viscosity at the liquidus and the rate increase in viscosity with falling temperature below the liquidus. Considerable evidence obtained in the high viscosity

modulus that is required to make further progress toward achieving a fundamental and predictive understanding of the generic system; and (4) outline market opportunities for new and affordable high modulus glass fibers to reinforce composites, sapphirelike infrared optical fibers, and alkali resistant cement reinforcing fibers. 1.2 Melt

behavior

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3CoO_AI203\ CaAI204 Ca12AIMO33 Fig. 1. The calcia-alumina-silica phase diagram.

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2020

New opportunities with high-modulus glass fibers calcium aluminate system with over 300 individual compositions indicates that a necessary but not sufficient condition for glass formation is that the ratio of oxygen ions to network forming ions (AI 3+ and Si which is often used as a minor constitutent) is between 2-35 and 2-6, 6-8 a compositional range which in turn affects the specific temperature differential between crystallization temperature. 36 The glass forming ability of low viscosity melts depends more on very high quench rates or chemical stabilization than on chemical compositionfll The relationships between composition and glass forming ability of a given melt in either the high or low viscosity range of the phase diagram are complex, especially since the crystallization and nucleation temperatures wherever measured tend to overlap. 6-8 As a result, no predictive relationship appears to exist between composition and glass forming ability of a melt.

245

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1.2. 4 Fiber forming viscosities An exact knowledge of the relationship between compositions and their melt viscosities above and below the liquidus would be even more important for glass fiber formation since it is more difficult to spin a continuous fiber than to pour a bulk glass. Unfortunately, only few quantitative data of this kind are available, s'37 In Fig. 2, the presently available information for binary systems37 is represented by solid lines, and for ternary and quarternary systems8 by dotted lines. Silicate glasses are known to have a melt viscosity of 10-100Pas (102-103 poise) in the range in which they form fibers 1 and Brown and Onoda 6-8 determined experimentally that a melt viscosity of 10-1130 Pa s was required to support fiber formation from high viscosity calcium aluminate melts. In the absence of quantitative data we assume 32 that the melt viscosity is in the 10-100Pas range if a free-standing glass fiber can be obtained with a given composition at the temperature at which fibers were obtained. A melt viscosity of 10-100 Pa s is therefore the key criterion for a fiber forming melt, but for a given composition it can occur above and/or below the liquidus. Fibers can be obtained from silicate melts above the liquidus by extruding them through an orifice (bushing) or from melts below the liquidus, e.g. by down-drawing them from a preform or up-drawing them from the surface of a melt. In over 1300 recorded attempts, calcium aluminate fibers were found to have fiber forming viscosities only below the liquidus; the melt viscosity was too low above the liquidus to facilitate fiber formation. In summary, the temperature-viscosity relationship for silicate melts below and above the liquidus is relatively flat and Newtonian and, in contrast, calcium aluminate melts have either a very steep temperature-

j// -2

30

I

40

I

50

I

60

I

70

I

80

I

90

100

Alumina Concentration (wt. %)

Fig. 2. Melt viscosity versus melt composition.

viscosity relationship or exhibit a non-Newtonian ('fragile') behavior. 34"35 This relationship will be discussed in greater detail for both viscous and inviscid melts but it suggests that a viscosity of 10-100 Pa s is obtained at that point or in that range of the viscosity-temperature curve where fibers can be obtained by at least one of several methods.

1.2.5 Anhydrous melts Satisfactory high and low viscosity melts for the fabrication of calcium aluminate glasses and glass fiber can be prepared by a number of processes. Some of these yield anhydrous compositions; others yield compositions containing a more or less pronounced hydroxyl band in the infrared spectra. Anhydrous glasses are preferred for optical and for structural uses. (1) The hydroxyl band blocks an important region around 2.9 ltm, is more prominent than that in silica glasses, and more difficult to eliminate especially from larger batchesfl "38 (2) The presence of water may also adversely affect the modulus. 32

246

F. T. Wallenberger, S. D. B r o w n

Two processes are known to yield calcium aluminate melts (and subsequently bulk glasses and glass fibers) having a strong hydroxyl group at 2.9 #m in their IR transmission spectra. One process is the preparation of melts process from mixed carbonate and/or oxide powders in precious metal crucibles. This process has been used as early as in 1958 by Hafner et al.~ and as recently as in 1993 by E. V. Uhlmann et al. 36 The other process that affords hydrated calcium aluminate glasses (and subsequently melts) is the sol-gel process, as reported, for example by Goktas and Weinberg. 39 Three processes are known to yield hydrate-free (anhydrous) calcium aluminate melts, glasses and glass fibers. One method consists of coupling the conventional melt processing step using precious metal crucibles with a rigorous dewatering step using high vacuum techniques. Although only less than 15min of process time is required to produce hydrate-free calcium aluminate glasses, this approach can be used to only with very small (up to 1 g) melt batches and only if the melt does not contain volatile oxide constituents such as Na20. 2 The other method is the Davy process, 2 which was developed between 1967 and 1978 for the commercial preparation of quaternary non-silica containing aluminate bulk glass for missile windows from high viscosity melts in carbon crucibles. 38 It can be used to produce large (up to 10kg) batches of hydrate-free glass in somewhat more than 1 h of processing time. Carbon may serve as a scavenger, but Wallenberger et al. in Ref. 21 pointed out that its role may actually be more complex. The third method yields anhydrous calcium aluminate glasses, actually glass fibers from low viscosity (inviscid) melts as shown by Wallenberger et al. lt~-19 The mechanism of this inviscid melt spinning process as elucidated by Wallenberger et al. in Refs 20 and 21 shows that particulate carbon is incorporated into the surface of the jet and raises its viscosity during inviscid melt spinning, and that an additional hermetic carbon sheath formed on the surface of solidified fiber. This result further documents the powerful effect of carbon in terms of producing hydroxyl-free (anhydrous) calcium substrates. As expected, glass fibers which were down-drawn from anhydrous calcium aluminate glasses which had been made in carbon crucibles were also anhydrous. 3~ Drawing did not introduce hydration. However, hydrate-free glass fibers were obtained by inviscid melt spinning, 2~ even when the mixed oxide melt had originally been prepared in metal crucibles. The effect of the carbon sheath which can be applied to aluminate glass fibers in this process historically anticipates the effect of a hermetic carbon sheath which was recently applied by CVD to optical silicate glass fibers. 4°

In summary, anhydrous calcium aluminate melts, glasses and glass fibers can be obtained by processes which involve exposure of the melt to rigorous techniques of dewatering (high vacuum) or to carbon in one form or another. Conventional fabrication of calcium aluminate melts in precious metal crucibles or by the sol-gel process yield hydrated melts. In practical terms, anhydrous species are not only expected to increase the transmission in the infrared but also the modulus of a given material.

1.3 Glass fibers from high viscosity melts 1.3. 1 Introduction

This section, and the following section on glass fibers from low viscosity melts, looks upon fibers as potential products, not research tools, and attempts to define the processes which are available to form uniform, and free standing fibers from melts having in general high viscosities but viscosities above and below the liquidus which vary from composition to composition. 1.3.2 Formation o f glass fibers

According to Brown and Onoda, 6 it is a necessary but not sufficient condition for the formation of aluminate glasses and glass fibers that the ratio of oxygen ions to network forming ions (AI 3+ and Si, an occasional minor constituent) is between 2.35 and 2.6. 6-8 The melt behavior of such compositions dominates, in turn, the specific temperature differential between crystallization temperature. 36 Wallenberger 32 found that the magnitude of the melt viscosity appears to be rheoiogy controlled, i.e. due to differences in the coordination number, size and orientation of nanocrystalline oxide structures and/or nanostructural oxide domains. This hypothesis is being quantified, az High viscosity calcium aluminate melts have a fiber forming viscosity below their liquidus. Glass fibers are therefore best obtained from melts below their liquidus. 1.3. 3 Fiber f o r m i n g processes

The specific melt behavior of a material has significant consequences with regard to the formation of continuous fibers in a stable process regime. The effect of melt behavior on fiber forming ability can be evidenced from the successive efforts by Machlan, 4 Onoda and Brown, 6-8 Wallenberger et al., 31 and Foy et al. 4~ to fabricate glass fibers from caicia-alumina compositions having high melt viscosities. Figure 3 shows schematic drawings of experimental laboratory processes which have so far been explored. Machlan 4 found that the melt of only one of -1000 similar quaternary, low silica alumina-calcia compositions with 40-45% alumina behaved like a silicate melt. Inasmuch as a glass fiber could be extruded

N e w opportunities with h i g h - m o d u l u s glass fibers

247

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~Ring

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Fig. 3. Potential fiber fabrication processes. (spun) through an orifice. Thus, Machlan's material can be assumed to have had an estimated fiber forming viscosity (10-100 Pa s) above the liquidus. He obtained short and discontinuous fibers, and poor spinning continuity. In this process (Fig. 3 top left) the melt is above the liquidus. The viscosity temperature relationship of the other compositions studied by Machlan 4 differed from that of a typical silicate melt. Today we would call them 'fragile' melts, having high viscosities below the liquidus but low viscosities above the liquidus. Onoda and Brown studied the melt behavior of nearly 300 ternary and quaternary, low silica and non-silica melts which were located in the same high viscosity region in the phase diagram that yielded Machlan's fiber, and were able to up-draw over 100 glass fibers from supercooled melts. 6-a A continuous up-drawing process was designed and operated by Schroeder et al. 5° using a selected aluminate composition. The viscosity of this low silica, calcium aluminate melt was reported a to be 18-2Pas at LT (liquidus temperature) + 200C, 72.0 Pa s at LT - 50°C, and

Fig. 4. Non-silica calcium aluminate fiber from a supercooled melt. about 1000Pas at L T - 2 0 0 ° C (the upper working range limit for fiber drawing). In this up-drawing process (Fig. 3, bottom left), the melt is below the liquidus. While searching for new high modulus infrared optical oxide glass fibers two decades later, Wallenberger et al. 31 fabricated two glass fibers from high viscosity aluminate compositions similar to those made by Machlan 4 and Onoda and Brown. 6-8 One was a low (<4%) silica composition, the other a comparable composition without silica. Both were made by up-drawing from supercooled melts according to the literature, 6-8 but they were, for the first time, also made by down drawing from a preform, thus providing a third process option (Fig. 4). Subsequent work by Foy et al. 4~ confirmed that the down-drawing process previously demonstrated by Wallenberger et al. 31 in the laboratory can be scaled up on process development equipment. In this process (Fig. 3, top right) the melt is below the liquidus. Short (<2 m) fiber lengths of the same composition were occasionally obtained by more than one laboratory method, but a correlation between process concepts and product compositions is not available. 1.3. 4 S u m m a r y

The melt behavior of high viscosity calcium aluminate melts differs from that of high viscosity silicate melts. With one exception, these high viscosity melts have low melt viscosities at and above the liquidus but high melt viscosities below the liquidus, thus requiring a process by which fibers are made from supercooled melts. In contrast to earlier conclusions, 4"6-8 two major insights emerge: (1) down-drawing from a preform was not known in 1970, but today it is the preferred alternative to up-drawing from supercooled melts; (2) the presence of silica is not needed to achieve fiber-forming viscosities, but it may be needed

248

F. T. Wallenberger, S. D. B r o w n

eventually, as a secondary factor, to fine-tune the viscosity/temperature profile so as to enhance the spinning (drawing) continuity. 1.4 Glass fibers from low viscosity melts 1.4.1 Introduction The vast majority of glass forming, binary, ternary and quaternary, low silica and non-silica aluminacalcia compositions with 50-100% alumina have low melt viscosities ( < l P a s ) above and below the liquidus. Cunningham et al. 9 converted low viscosity compositions with up to 80% alumina into glass fibers "1 by a process known as inviscid melt spinning. Compositions with >81% alumina gave crystalline fibers. 15 Wallenberger et al. in Ref. 21 found that this process is rheology controlled, as shown in the following discussion, i.e. better understood from the point of process theory than fiber formation from high viscosity melts. 32 1.4. 2 Fiber f o r m i n g process In the inviscid melt spinning process (Fig. 3, bottom right), the low viscosity melt is ejected through an orifice into a chemically reactive environment such as propane that stabilizes the inviscid jet and consolidates the resulting fiber. Without chemical jet stabilization, Rayleigh waves result which will break up into droplets (shot). Wallenberger et al. 1° began to reevaluate the inviscid melt spinning process in 1989 with the aim of affordable glass fibers for structural and optical uses, and concluded in 19922°'21 that: (1) propane decomposes on the surface of the hot liquid jet; (2) pyrolytic carbon that is generated enters into its surface of the molten jet before it solidifies and raises the surface viscosity to the fiber forming level (i.e. to 36Pas). The stabilization of an inviscid jets was originally believed to be controlled by the surface chemistry6 but was actually found to be controlled by the surface rheologyfl°'21 1.4. 3 Molten jet stabifity The principle governing the formation and the break-up of a jet is well recognized. 9'21 A liquid jet is unstable with respect to surface tension. Jet break-up is due to axisymmetrical surface pressures and proceeds by the growth of Rayleigh waves or periodic variations of increasing amplitude in the jet diameter, ultimately resulting in separate droplets. The droplets become shot. In a velocity range above the critical value but below the onset of turbulence, jet lifetime (t) depends on density (p), diameter (D), viscosity (r/), and surface tension (7). The time (t) for the melt to traverse the continuous length of the jet before the onset of break-up is given by eqn (1). 9"21 The

dominant factors in this equation are viscosity and diameter. Both depend exponentially on temperature. Surface tension and density which are less temperature sensitive are secondary factors: t = 14[(pD~/7)

''2 +

(3r/D/y) ]

(1)

Molten jets of a silicate glass ~ or an organic polymer melt have very high viscosities and relatively long lifetimes. Continuous fibers can be spun or drawn from the melt by conventional methods. Selected jets of calcia-alumina melts with <50% alumina can also have high viscosities and long lifetimes, and amorphous fibers can be drawn from supercooled melts. 8'31 In these cases the jets solidify well before they can form Rayleigh waves and droplets. The vast majority of caicia-alumina melts, however, especially those with >50% alumina, have low melt viscosities and short lifetimes. 9"1°'2~ When they are ejected through an orifice into a neutral, non-reactive environment, they are known to give Rayleigh waves and droplets (shot). i. 4. 4 lnviscid melt spinning Jets from low viscosity alumina-calcia melts with >50% alumina were stabilized when ejected through an orifice into a chemically reactive environment such as propane which decomposes at the hot oxide surface. Particulate carbon is formed, reacts to create a fiber surface morphology that stabilizes the jet and consolidates the fiber before it can deteriorate into Rayleigh waves and droplets and increase in the jet lifetime. Figure 5 shows a stabilized aluminate fiber and a fiber with frozen Raleigh waves. Table 1 lists the liquidus temperatures, melt viscosities, surface tensions and melt densities for a range of alumina-calcia melts and the experimental

J

r

m

Straight Stabilized Fiber

..............

Frozen Rayleigh Wave

Fig. 5. Straight fiber and frozen Rayleigh waves.

249

New opportunities with high-modulus glass fibers Table 1. Properties of alumina jets at the spin temperature z~

Alumina content (wt%)

Fiber structure

51.5 54-6 66.8 80.2 100.0

Liquidus temperature (°C)

Spin temperature (°C)

Jet/fiber diameter (/~m)

Melt density (g/cm ~)

Surface tension (mN/m)

Melt viscosity (Pa s)

Unassisted jet lifetime (s)

1 415 1 390 1 650 1 830 2 050

1 500 1 500 1 700 1 900 2 100

375 190 105 118 350

2.70 2.70 2-68 2.68 2-85

680 680 625 575 570

0.34 0.55 0.14 0-06 0.04

1.4 x 10-2 8-7 x 10-2 2.0 X 10 -3 1.7 x 10-3 7.5 )< 10 -3

Amorphous Amorphous Amorphous Amorphous Polycrystalline

spin temperatures and jet (fiber) diameters. 2~ The effect of diameter can be seen in Fig. 6. Using eqn (1), the unassisted jet lifetimes were then calculated from these literature properties and the experimentally determined jet (fiber) diameters. As expected, the jet lifetime depends primarily on viscosity and diameter. All fibers in Table 1 are X-ray amorphous except the 100% alumina fiber which was crystalline. Among the amorphous fibers in Table 1, the longest unassisted lifetime, 8.7 x 10 -2 s, was obtained for the 54.6% alumina jet which had the highest viscosity and an intermediate diameter. The shortest unassisted lifetime obtained for an amorphous fiber, 1-7 x 10 - 3 s , was obtained with 80.2% alumina jet having a melt viscosity of 0-06 Pa s and a fiber diameter of 118/tin. The lowest fiber diameter, 105/zm, 9A° was obtained with a 66-8% alumina jet having a viscosity of

0-14 Pa s and an unassisted jet lifetime of 2.0 x 10 -3 s. Fibers with > 8 0 % alumina were crystalline as spun. ~5

1.4. 5 Fiber surface morphology Inviscid melt spun fibers have a complex surface morphology. 1°'21 The decomposition of propane forms a carbon-rich skin in the surface of all fibers, and a black carbon sheath 1° or film 9 on the surface most fibers. This sheath was formed under most, but not all process conditions (Fig. 7). The carbon-rich skin in the surface of the fibers which was not detected by scanning electron microscopy (SEM) in earlier work 6 was recently found to be a permanent, i.e. integral, part of the fiber. 21 SEM of polished cross-sections showed that the skin extended 20-50/~m into the fiber from its surface 2~ and that the sheath, whenever it was formed on the surface of the fiber, was up to 600 nm thick. ~°'21 Whenever it was formed, the carbon

lO-5

10 -4

"•

10 -3

z ,,~ lO "2

10 "1

o

o

S-glass i

260

36o

.oo

Fiber Diameter, ~m

Fig. 6. Jet stability calculated for a 54% alumina fiber.

Fig. 7. Fibers with and without carbon sheath.

250

F. T. Wallenberger, S. D. Brown

sheath on the fiber surface could be thermally or mechanically removed. 9""~'2~

100 90 -

1.4. 6 Depth profile analysis Depth profile analyses were obtained by sputtered neutral mass spectrometry (SNMS), a method similar to the more widely known secondary ion mass spectroscopy (SIMS). Successive atomic layers were removed continuously from the surface and the sputtered species were analyzed by mass spectrometry. With SIMS, sputtered secondary ions are measured, but quantitative measurements are extremely difficult because of large matrix effects. Analyses of the neutral species by SNMS are less sensitive, hut matrix effects are substantially eliminated, allowing more meaningful concentration comparisons. Sputtered neutral depth profiles and images were obtained with a mass spectrometer which was built similar to a design of Benninghoven. 42 Two fibers, approximately 225/~m in diameter, were selected for analysis. Quantitative electron probe microanalysis of polished cross-sections revealed uniform compositions for both fibers, i.e. 59-7% A1203, 40.1% CaO and 0-2% MgO, and 53.9% A1203, 39.1% CaO, 3-0% MgO and 4.0% S i O 2 by weight, respectively. The 59.7% alumina fiber had a black sheath as spun, which was earlier shown to be about 600 nm thick by SEM of a polished cross-section. "~ SNMS analysis of the black, 59-7% alumina fiber which had a carbon sheath as spun showed that the thickness of the carbon sheath the 59.7% alumina fiber was about 800 nm and in general agreement with the 600nm thickness determined from SEM images. ~°'2~ SNMS analysis of the translucent 53.9% alumina fiber, which had no carbon sheath as spun, was performed on a 100/~m (transverse) by 250/~m (longitudinal) area. Depth profiles for carbon, oxygen, aluminum and calcium (Fig. 8) are similar to those of the 59.7% alumina fiber without sheath. There was no carbon sheath and the carbon-rich skin was - 5 0 nm thick. 1.4. 7 Chemical jet stabilization A stable jet was obtained by inviscid melt spinning and chemical jet stabilization, and this could only have occurred by a mechanism (or combination of mechanisms) capable of producing a sufficiently large instant bulk or surface viscosity increase that increases the corresponding jet lifetime from 2.0 x 10-3s to 1.2x 10-1s, thus producing an assisted jet lifetime that at least equals the jet cooling time. Inviscid melt spinning is a dynamic process. 21 The temperature of the jet decreases due to radiant and convective cooling. The propane temperature increases near the jet and propane decomposes to give particulate carbon and hydrogen. Carbon enters into

8070_.e 6 0 ¢g

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=-.t

50-

u. o~

40302010-

0

illl 0

it 50

tiii

tl 100

t tit 150

it

I 200

Depth, nrn

l~g. 8. SNMS depth profile: translucent, 53-9% alumina fiber spun without carbon sheath. the surface of the molten jet creating a carbon-rich skin and may in part react with the oxide forming carbides and (oxy)carbides. The jet solidifies, yielding an amorphous or crystalline fiber. Particulate carbon may, but does not always, deposit on the jet surface to form a sheath as the jet (fiber) cools. Whether or not a carbon sheath is formed depends on the process conditions. Consequently, chemical jet stabilization which is capable of converting an otherwise unstable jet into a stable jet might rely on one or more of these mechanisms: (1) formation of a carbon sheath on surface of the jet and fiber; (2) rapid cooling of the jet due to the endothermic decomposition of propane and/or the endothermic surface reaction between oxide and carbon; and/or (3) an increase of viscosity in the surface of the molten jet through incorporation and/or formation of particulates. An early report 9 attributes chemical stabilization of a molten, inviscid ceramic oxide jet to the formation of a 'stabilizing film of pyrolytic carbon', as shown in Fig. 7. However, not all successfully spun, i.e. chemically stabilized, alumina-calcia fibers had a carbon sheath, 9""~ and there are other inconsistencies in the early mechanism proposal. As previously discussed20.2~.28 this mechanism is no longer tenable. Endothermic cooling due to the decomposition of propane (eqn (2)) and/or the reaction of carbon with the hot oxide (eqns (3) and (4)) could contribute to jet stabilization, but was found to be insufficient to stabilize the selected jet at 1700°C. However, the combined temperature drop due to the two endothermic cooling processes (78°C) was found to amount to only 13% of the temperature drop needed (605°C) to

New opportunities with high-modulus glass fibers achieve required jet lifetime of 2.0 x 10 -1

be realistically achievable by carbon insertion in the jet surface.

S: 20'21

C3Hs(g) = 3C(s) + 4HE(g)

(2)

2AIEO3 + 3C = AI404C + 2CO(g)

(3)

CaO + 3C = CaCE + CO(g)

1.4. 9 Assisted lifetime Figure 9 shows how the alumina jet lifetime at 1700°C would increase if the surface viscosity of the melt could be raised by carbon insertion. For comparison, computed jet lifetimes are also shown for the unassisted spinning of E-glass at temperatures between 1100 and 1480°C. Thus, at least a 260-fold increase in surface viscosity from 0.14 to 36.6 Pa s was obtained when the fiber was spun at 1700°C to yield a chemically stabilized jet with an assisted lifetime at least equal to the cooling time of 2.6 x 10 -l s. E-glass is melt spinnable at temperatures ranging from 1100 to 1480°C 1 where a 105/um diameter jet has viscosities ranging from 6.1 to 8 1 6 P a s and a calculated lifetime between 8.8 x 10 -2 and 1.2 x 10 ~s. The assisted lifetime of the equal diameter, 66.8% alumina jet was 2-6 x 10 -~ s, well within the range of the unassisted E-glass jet lifetimes. As a result, the calculated increase in surface viscosity of this alumina jet is judged to be realistic. 2~

(4)

In addition, the average quench rate by heat transfer (3-4 x 103°C/s) was found to be lower than the quench rates (105-107°C/s) achieved by rapid solidification43 and splat cooling. 44 Thus, inviscid melt spinning is at best only a modified rapid solidification process, but the stabilization of the fiber is not entirely due to a traditional rapid solidification phenomenon.

1.4. 8 Jet stabilization mechanism According to eqn (1), viscosity is the major factor in determining jet lifetime, surface tension is a secondary f a c t o L E1 An increase in the surface viscosity of the molten oxide will therefore increase the jet lifetime from that calculated for an unassisted jet (2.0 × 10-3s) to that of calculated for an assisted jet (2.0 x 10 -1 s) which must have been obtained since continuous fibers were obtained. For such mechanism to be viable, 2~ three conditions must be fulfilled. (1) The increase in the jet surface viscosity must afford an assisted jet lifetime that at least matches the jet cooling time. (2) The assisted lifetime resulting from the viscosity increase must be comparable to the actual (unassisted) lifetime of a typical silicate fiber such as E-glass. (3) The surface viscosity increase needed to achieve this lifetime must

100

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d E

1.4.10 Surface viscosity The pyrolytic production of carbon has been said to "create a 'snowstorm' of large, flat molecules containing the hexagonal ring structure of graphite",45 i.e. flakes or fiat aggregates of smaller particles. They can enter into the surface of the molten jet and act as viscosity builders. High melt viscosities have also been

E-glassilielt " i............

U} "0 ¢., 0

~

J

/

/

~

66.8% Alumihe melt

=1,oo.c

~ ~ , l i z e d 1°'1 ..........................

480:e

251

....

1

jet at 1700"C: y,e

s t=z6,.o

. . . . .

° ~

/

.J

./ 4n'2 /tJ

......s '": ...........

.....

I

-

J "~ ~'~ 10 .3

10 "I

4'

Stable Jet -- Unstable Jet -

-

s

Unstabilized jet at 1700"C: ~1= 0.1~ Pe.s yields t = 2.0x10 -3 s (<
101

10 2

10 3

Melt Vlscoslty, Pa.s

Fig. 9. Viscosity of 66.8% alumina jet with 105 #m diameter.

/

F. T. WaUenberger, S. D. Brown

252

observed during the carbothermal reduction of alumina. They were attributed to the crystallization of AI4C3, which has a tendency to form a threedimensional network of needle-shaped crystals. 46 Solid networks of (oxy)carbide particles, or unreacted carbon particles encased in (oxy)carbide are likely responsible for increasing the viscosity in the surface or skin of the jet by this mechanism. Since jet geometry and surface forces would likely constrain particle formation into planar structures parallel to the jet surface, the rheological treatment for flakes is appropriate. The viscosity (r/) of a Newtonian fluid containing solid, suspended particles, relative to that (r/o) of the suspending fluid, is given by the Mooney equation, 47 as shown in eqn (5):

ln(n/~o)

kE~2/[1

=

tj)2/~m]

-

(5)

The Einstein coefficient (kE) for incorporated particles depends on particle shape. It is equal to 5.0, 7.0, 8.75 or 9.5 for flake width-to-thickness ratios of 4, 6, 8 or 9; 48'49 ~2 is the volume fraction of filler, and q~m is the maximum packing fraction for the flakes (a typical value being 0-75). Figure 10 shows the suspension viscosity as a function of the volume Flake width-to-thickness ratio, I_/D :

10 3

66,8% alumina jet at

10 2

,~ / / v , , I I

W 0 0

II .

> 0 (.I

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

6

4

/

1700

• = 0.32-0.45 yields TI = 36.6 Pa.s

a

9 8

.

I , I

I

t ..... J

.

101

.

.

.

.

.

.

.

.

.

.

.

I1// // / l/ I /

=,1 ffl

•

t,

d

ViSCOSity without particles

10 o

,

~/ /. "/

"

-

/ y,',"

J . --

I

III....

I Stable Jet ] -- -- - - Unstable Jet

I

I c 7""

f/ 10 "1

0

0.1

0.2

0.3

1.4.11 Volume fraction The volume fraction of solids can be estimated from SNMS/ESCA data for the translucent fiber without a sheath. The skin was about 50 nm thick. The average mole fraction of CaO within the skin is about 59% of the amount in the alumina-calcia melt. Elemental C and AI4C3 are assumed to be the only carboncontaining species; ESCA data from the etched surface show the mole ratio of C to AI4C3 is 41(3)/51 or 2.41. The combined volume fraction of solids in the oxide skin was calculated by Wailenberger et al. 2°'21 to be 0.508, a value that is well above the amount needed to permit rheological stabilization of the molten jet. Some of the carbon present as carbide may be in solid solution with oxide which would decrease the amount available as particulate materials. In summary, 2°'21 incorporation of particulate reaction products in the surface of the molten jet causes the required surface viscosity increase. 1.4.12 Summary During inviscid melt spinning, the stabilization of alumina jets can be attributed to an increase in surface layer viscosity due to suspension of solid carbon particles in the molten oxide surface and formation of a carbon-rich skin in the resulting fiber. 2°'2~ The increase in surface melt viscosity produces an assisted jet stability which is comparable to that of conventional (unassisted) E-glass jets in their melt spinnable temperature range (Table 2). 1.5 Properties of caldum aimminate glass fibers

/:/l///

~..j......J

fraction of suspended flake-like material with these shape ratios. The viscosity of the suspending fluid (bulk molten) oxide is 0.14Pas and the required 36.6 Pa s viscosity for the surface layer of the jet can be attained with flake shapes having LID between 4 and 9, and volume fractions of solids between 0.32 and 0-45.

0.4

o.s

Volume Fraction of Suspended Particles, ¢2

Fig. 18. Surface viscosity increase due to suspension of flake-like particles.

1.5.1 Introduction For a fiber to be commercially useful, it must have high tensile strength (>2-1 GPa), high tensile modulus (>69GPa), low fiber diameter (<20/~m) and high environmental stability in projected end-uses. In weight-sensitive uses, fibers and/or composites with high specific strength and modulus (i.e. strength and stiffness normalized for weight density) have a high replacement value relative to fibers and/or composites with lower specific strength and modulus. Infrared optical fibers do not require diameters <100 #m, but will require high transparency, a cladded structure, and perhaps an additional, hermetic carbon coating. Calcium aluminate fibers potentially meet these requirements. ~5 A new material, such as a suitable

253

New opportunities with high-modulus glass fibers Table 2. Rheology controlled jet stabilization mechanism (105 pm diameter jet) Composition (wt%) oxide melt 21 Alumina (66-8%)-calcia Silica-alumina (E-glass)

Fiber process

T (°C)

Inviscid Viscous

1 700 1 360

Viscosity (Pa s)

Jet lifetime (s)

Bulk

Surface

Unassisted

Assisted

0-14 18-7

36.6 18-7

2-0 × 1 0 - 3 2.6 x 10-'

2-6 x 10-' --

melts. 6-s T h e m o d u l u s o f t h e s e s u p e r c o o l e d m e l t d r a w n ( S M D ) fibers r a n g e d f r o m 79.3 to 122-7 G P a ( T a b l e 4). A specific, q u a t e r n a r y a l u m i n a - c a l c i a magnesia-silica material (44.3-48.7-3.5-3.5 wt%) was i d e n t i f i e d with a m o d u l u s o f 110 G P a a n d a t e n s i l e s t r e n g t h o f 7.6 G P a . 3-5 It was s e l e c t e d for u p s c a l e d e v a l u a t i o n , a n d a d e v e l o p m e n t p r o c e s s was d e m o n s t r a t e d for u p - d r a w i n g o f fibers f r o m s u p e r c o o l e d melts. 5° T h e h i g h e s t s t r e n g t h level ( 8 . 3 G P a ) w a s o b t a i n e d w h e n t h e low-silica a l u m i n a t e fiber w a s d r a w n in an i n d u c t i o n f u r n a c e . 9 W h e n d r a w n in a n ( o x y ) a c e t y l e n e f u r n a c e 6-8 its m a x i m u m s t r e n g t h was 4.8 G P a . T h e s t r e n g t h d i f f e r e n c e s w e r e a t t r i b u t e d to differences in m e l t h o m o g e n e i t y . 5° I n d u c t i o n h e a t e d m e l t s w e r e c o m p l e t e l y h o m o g e n i z e d . 5°

high viscosity c a l c i u m a l u m i n a t e c o m p o s i t i o n m a y b e c o m e t h e c o r e for existing silicate b a s e d o p t i c a l fibers.

1.5. 2 Melt spun fibers O n l y o n e c o m p o s i t i o n ( 4 6 . 5 % A1203, 9 - 1 % SiO2) t h a t is l o c a t e d in t h e c e n t e r o f t h e high viscosity r e g i o n o n t h e p h a s e d i a g r a m (Fig. 1) c o u l d b e m e l t s p u n t h r o u g h an orifice h a d m o d e r a t e t e n s i l e p r o p e r t i e s . T h e as-spun m o d u l u s o f t h e c o n t i n u o u s l y s p u n fiber was 91.7 G P a , t h e a n n e a l e d fiber m o d u l u s w a s 103.4 G P a , a n d the t e n s i l e s t r e n g t h o f t h e virgin fiber was 2.75 G P a as r e p o r t e d b y M a c h l a n . 4 G l a s s fibers m a d e f r o m inviscid m e l t s a r e s h o w n in T a b l e 3. T h e m o d u l u s o f a l u m i n a t e glass fibers s p u n f r o m inviscid melts r a n g i n g f r o m 69 to 89 G P a w a s l o w e r , p e r h a p s suggesting s t r u c t u r e s with less o r d e r t h a n glass fibers m a d e f r o m high viscosity m e l t s . T e n s i l e s t r e n g t h was also l o w e r ( 0 . 1 6 - 1 . 0 5 G P a ) .

1.5. 4 Down-drawn fibers S t o n g q u a t e r n a r y glass fibers w e r e r e c e n t l y m a d e b y W a l l e n b e r g e r et al. 31 b y d o w n - d r a w i n g t h e m in t h e laboratory from a preform of a commercial non-silica, a l u m i n a - calcia - m a g n e s i a - b a r i a (46-1 - 36.1 - 4.0 1 3 - 8 w t % ) glass, a n d f r o m t h a t o f a c o m m e r c i a l low silica a l u m i n a - c a l c i a - m a g n e s i a - s i l i c a (42.6-47.74.1-5.6wt%) c o n t r o l . T h e c o m m e r c i a l b u l k glass

1.5.3 Up-drawn fibers G l a s s fibers w h i c h w e r e o b t a i n e d f r o m o v e r 100 t e r n a r y a n d q u a t e r n a r y c o m p o s i t i o n s with 2 3 - 5 46.5 w t % a l u m i n a , b y u p - d r a w i n g f r o m s u p e r c o o l e d

Table 3. Low viscosity calcium aluminate fibers '° Fiber

Composition (wt%)

Spin temperature (°c)

no.

Nominal

S-glass 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Measured

AI203

CaO

MgO

SiO2

25 52 54 65 65 54 54 54 54 65 65 65 67 67 79 67 67 81

-41 46 35 35 46 46 46 46 35 35 35 33 33 21 33 33 19

10 7 -----------------

65 ------------------

A1203

C aO

MgO

SiO2

51-5

41-0

7.5

--

54.0 54.1 54.6 54.8 56.5 59.0 59.1 60-8

39-0 38.9 39.0 38.9 43-5 40-8 40.9 39.1

3.0 3.0 2"5 2.5

4.0 4.0 3.9 3-8 -----

66-5 66-8 66-8

33.5 33-2 33.2

---

67-6 80-2

32-4 19.8

---

0-2 0"1

-

-

1 500 1 500 1 700 1 700 1 500 1 500 1 500 1 500 1 700 1 700 1 700 1 700 1 700 1 800 1 70O

1 700 1 800

Fiber diameter (#m)

10 325-430 189-211 170-200 170 190-250 216-295 350-450 190-250 213-222 225-245 141-175 171-315 160-190 102-280 105 167-171 117-150

Maximum fiber length (cm)

30 10 28 20 30 3 20 24 24 30 20 10 20 30 10 12 10

254

F. T. Wallenberger, S. D. B r o w n

Table 4. High viscosity calcium aluminate fibers4"~'a'3~

Tensile modulus GPa

Ref

Composition (wt%)

A1203

SiO2

23.5 23-47 46-5 44-5 46-2 30.0 44.3 42-6 35.2 30-0 32.0 32.0

3.7

CaO

MgO

Process route and comments Other oxide(s)

no.

79-3 80-99 103-4 108-9 109-6 109.6 110-3 110.3 110-3 111.7 115.8 122-7

6 6-8 4 6 31 7 6-8 31 6 7 7 7

9-1 3.5 10.0 3-5 4-1 2-8 10-0 4.0 4.0

56-7 Over 80 compositions 27-5 5.2 Na20, K20 46.6 5.3 36.0 4-0 13.8 BaO 30.0 30-0 ZnO 48.7 3.5 47.7 5.6 36.2 25.7 PbO 35.0 25-0 ZnO 44-0 29.0 ZnO 44.0 29-0 ZnO

samples had been obtained from Sassoon Ltd. -~8 Both fibers (Table 4) had strengths >2.8 GPa, diameters <10/zm, and moduli of 110 GPa. On the basis of these findings,31 Foy et al. 4t demonstrated an upscaled process for the same compositions on standard down-drawing development units otherwise used for fabricating silica optical fibers from blanks. The highest modulus reported for any aluminate composition (123 GPa) was obtained with a ZnO modified composition6-8'3~ demonstrating the powerful effect of ZnO as a modulus modifier. In summary, the highest modulus on record for any glass fiber (125GPa) is that demonstrated5 for a silicon nitride modified silicate fiber, the second highest modulus (123 GPa) is that demonstrated for a zinc oxide modified aluminate glass fiber 6-8'3~ and, excluding the modulus for a toxic beryllia modified silicate fiber ( l l 0 G P a ) , the third highest modulus (102 GPa) is that demonstrated for a ZnO modified silicate fiber. 6-8"22 Contrary to the literature, 6-s Wallenberger et al. 3~ found that the compositional presence a small amount (<10%) of SiO2 was not required as a viscosity builder (network former) for good glass fiber formation. 1.5. 5 lnviscid s p u n fibers

WaUenberger et al ~° showed that inviscid melt spun (IMS) glass fibers are not as strong and rigid as those drawn from viscous melts, despite the fact that they

Melt-drawn, low silica fiber Melt-drawn, low silica fibers Melt spun fiber (through orifice) Melt-drawn low silica fiber Melt-drawn non-silica fiber Melt-drawn high ZnO fiber Melt-drawn low silica fiber Melt-drawn low silica fiber Melt-drawn high PbO fiber Melt-drawn high ZnO fiber Melt-drawn high ZnO fiber Melt-drawn fiber, Li20 added

have a much higher alumina content (51-81%) than those (23-47%) made from high viscosity melts (Table 5) by conventional melt spinning (CMS) or supercooled melt drawing (SMD). IMS glass fibers with 51% and 81% alumina had a modulus of only 70GPa (like E-Glass) and 60GPa, respectively. Inviscid melt spinning (IMS), a rapid solidification process, appears to be capable of affording more uniformly random, i.e. less ordered structures. 1.5. 6 Glass ceramic fibers

Onoda and Brown 7 and Schroeder et al. 5° studied the crystallization behavior of viscous glass fibers with 30-50% alumina between 1968 and 1969 and concluded that uniform nucleation and crystallization, i.e. attainment of high modulus glass ceramic fibers is feasible, but at that time there was no demand for such fibers. Wallenberger32 found that uniform crystallization of inviscid melt spun fibers to obtain glass ceramic fibers is also feasible. Amorphous IMS fibers with an 81% alumina content, which were melt spun under a wide range of heating/annealing conditions, had moduli ranging from 60 to 170 GPa. 51 A fiber with a modulus of 170 GPa was found32 to be X-ray amorphous but it had a pronounced nanocrystaUine phase, suggesting that annealed inviscid melt spun (AIMS) fibers are glass ceramic fibers. Finally, Dunn and Paquette 51 redrew IMS glass

Table 5. High modulus aluminate fibers

Property summary (status) Alumina (wt%) Demonstrated diameters (/~m) Maximum tensile strength (GPa) Maximum tensile modulus (GPa)

Conventional melt spinning (CMS)

Supercooled melt drawing (SMD)

Inviscid melt spinning (IMS)

Redrawing (RIMS) annealing (AIMS)

46-5 >10 2.75 103.4

23-47 >7 8.27 123

51-81 >105 1.04 89

51-81 15-200 TBD 170.0

255

New opportunities with high-modulus glass fibers Table 6. High modulus silicate fibers a''Ja

GPa

Ref. no.

68-9 72.5 83.5 110.3 115.5 125.0 >125-0 170-0

53 3 3 3 22 8 8 53

Process route and comments

Composition (wt%)

Tensile modulus

A1203

SiO2

14.0 25"0

100 53.0 65-1 53.7 46.8

CaO

MgO

20.0

3.0 9-9 9.0 11.8

12.9 12.1

Other oxide(s) 10.0 B203 8 BeO, etc. 16.9 ZnO, etc. 5 SiN >10SIN >15 SiN

Melt spun, fused quartz Melt spun, E-glass fiber Melt spun, S-glass fiber Melt spun, HM-glass fiber Melt spun, Z-glass fiber Melt spun, Sialon fiber Difficult to draw (defects) New high strength process

fibers above the crystallization temperature but below the liquidus and obtained redrawn (RIMS) fibers with a modulus of 70 to 121GPa, thus having a more ordered, but still X-ray amorphous structure. The authors did not look for a nanocrystalline phase but it was most likely present. 32 Moduli of 137-164GPa were also reported 5~'52 but these fibers were essentially11 polycrystalline and brittle due to nonuniform, heterogeneous crystallization.

(Table 6). The recent paper by Suganuma et al. 53 confirms that a glass fiber with a modulus approaching that of pitch carbon fibers and essentially glass fiber economics offers commercial value. Interestingly, the glass fibers with the highest modulus in the silicate and aluminate system share a major structural feature. They are amorphous but highly nanocrystalline.

I. 5. 7 Summary In summary, three routes are available for achieving ultra high modulus aluminate glass and glass ceramic fibers, if a demand for such properties at essentially fiberglass economics emerges in the trade. (1) Modification of high viscosity aluminates with ZnO gave a fiber modulus of 123 GPa and further progress is possible. (2) Nucleation and homogeneous crystallization of high viscosity aluminates with and without ZnO addition. (3) RIMS fibers had a modulus of 137GPa without heterogeneous polycrystallization. (4) Annealing of inviscid melt spun fibers showed that a fiber modulus of 170 GPa, perhaps higher, is also feasible but a structure property study relating compositions to melt behavior and fiber modulus. Aluminate fibers having a modulus of 123 to 170 GPa will compete with silicate fibers having about the same high modulus for the same emerging transportation composites and infrastructure markets

1.6.1 Process selection versus viscosity From a process perspective (Table 7), three discussion points emerge, all worthy of further research inquiry. (1) In the low viscosity range (>55% alumina), only inviscid melt spinning affords adequate process continuity, and therefore continuous fibers. (2) In the high viscosity range (<50% alumina), the methods of choice are either drawing fibers from the supercooled melts below the liquidus or from preforms. (3) In the transition range (50-55% alumina), some compositions may be processable by drawing fibers as discussed above, others may require inviscid melt spinning.

1.6 Structure-property relationships

1.6. 2 Composition versus melt viscosity It is evident that there is no direct correlation between melt viscosity and composition. One potential mechanism to explain the results is related to that which emerged from recent work by Wallenberger et

Table 7. Viscosity-modulus relationshipss'a~

Composition Fibers Weight % Ref. no.

AI203-CaO-MgO-SiO2

A1203-CaO

A1203-CaO

AI203-CaO

From supercooled melt 44.3-48.7-3-5-3.5 8

54.0-46.0 31

By inviscid melt spinning 67.0-33.0 31

80-0-20-0 31

Viscosity (Pa s) LT + 100°C LT LT-200°C

5.8 27-8 1 000.0

0-6 1-5 12.5

0-1 0-2 0-7

0.05 0.07 0.20

Modulus (GPa)

110.0

69-0

79-0

89-0

LT, Liquidus temperature.

256

F. T. Wallenberger, S. D. B r o w n

al. 2~ regarding the stabilization of low viscosity jets

where aggregates of suspended carbon particles enter into an aluminate melt and raise its viscosity. By analogy, one can envision that differences in melt viscosities between viscous and inviscid melts. Thus, viscosity could be rheology controlled. 32 1.6. 3 Composition versus tensile m o d u l u s

Modulus (stiffness) is a materials property. It reflects structural order and is independent of form. Fig. 11 compares the modulus of silica and alumina based fibers with their alumina content. Regarding the crystalline fibers, Sapphicon is a single crystal sapphite (alumina) fiber having the highest modulus (414 GPa). The polycrystalline fibers have a modulus of ranging from 380 to 138GPa. They include in decreasing order of alumina content and modulus the slurry spun fibers Fiber FP, Aimax, Safimax, Altex, and Denka (100-80% alumina), the sol-gel Nextel fibers (80-63% alumina) and a redrawn, inviscid melt spun (RIMS) aluminate fiber (54% alumina). These literature data 53"54 confirm a direct correlation between modulus (order) and composition among crystalline fibers. As evidenced from further inspection of Fig. 11, the modulus of the X-ray amorphous fibers does not correlate with alumina content but varies anywhere

500

Saphicon 400

,,..,,..=..,1.=l

,,..,,.,,..,.,

q ,==

=,ll==

Fiber F P

Almax a.

Safimax

300

._~

•

o 2 0

@ •

Nextel 480 •

•' ~

2

i

200

•

• ,, ..,,.

= ii,

= i,

m,.,,.,,

HM

Denka

312

O R I M S Fiber = lie = w =.,,,.,

• ,t , . ,

• E] C M S

100

SAO •

EL~ 'FOG-M Glass

Nextel

.m I.L

440

Nextel •

o

Altex

• SMD

Fibers

Fiber 0

0

IMS

0

i. 6. 4 Fiber diameter versus tensile strength

Fibers E

Fibers x i

0

20

I

40

between 68-9 and 122.7 GPa. This group of materials includes the silicate fibers which have either no alumina content (Fused Quartz (FQ), HM-glass) or <25% alumina (E-, S-glass), the supercooled melt drawn (SMD) aluminate fibers and the conventionally melt spun (CMS) aluminate fiber having 23-46% alumina; and the inviscid melt spun (IMS) aluminate fibers having 51-81% alumina content. The 1.45 times variation in modulus from low to high suggests a variable order within a disordered X-ray amorphous system but there is no direct correlaion between modulus (order) and composition. 3e Surprisingly, the fiber modulus of X-ray amorphous, inviscid melt spun (IMS) aluminate fibers is lower (Table 7) than that of typical X-ray amorphous SMD aluminate fibers, although they have a much higher alumina content, and thus a higher crystallization potential. It is possible that inviscid melt spinning will freeze a less ordered amorphous structure than possible by drawing fibers from the melt. In contrast, fibers made from high viscosity melts have generally a higher modulus than those spun from inviscid melts. Different compositions occasionally have the same modulus. Table 4 lists five fibers with radically different compositions, all having a modulus of 109.6-110.3 (i.e. 110.2+0-6) GPa. However, the alumina content of these quaternary compositions ranges from 30.0 to 46.2%. In addition, one composition has 13-8% BaO, another 25.7% PbO, the third has 25-0% ZnO, and the remaining two have very high levels (47.7 and 48.7%, respectively) of CaO. There is no direct correlation between the composition and modulus. Since the modulus of X-ray amorphous aluminate fibers does not depend directly on the composition, it must depend on structural variations (aggregates) in an otherwise X-ray amorphous, glassy material (fiber), whether it was derived from a viscous or inviscid melt. The modulus (structural order) may therefore change with a change in process conditions (temperature, time, etc.), and there is indirect evidence that this is the case at least with inviscid melt spun fibers, where cooling rates are more critical. Several potential causes (mechanisms) are possible. Most likely, the nanoscale order that affects the melt viscosity carries over into the glassy fiber structure and affects the modulus. 32

i

i

60

80

100

Alumina (% wt.)

Fig. U. Fiber modulus versus fiber composition.

In the early stages of any fiber development, strength is merely an indicator of product uniformity. Excessive defects cause premature failure. As soon as a stable process is in hand, fiber strength will always be found to be inversely related to fiber diameter. Wallenberger et al. 28 showed that this relationship is also seen with aluminate glass fibers made from

New opportunities with high-modulus glass fibers

1000¸

\

'

257

i

s-gt~s~ \

-6890

i 3445

.,.

2760

,,Glassfibers from h~h -..

viscosity melt~

..

..

-.

..

!

Z070

[

1380

v o

i

100,

I-

o

C~ssf~oersfrom low visc~ity melts

345

276

138

.69

10. 0

1oo

200

3O0

400

FiberDiameter, ~m Fig. 12. Tensile strength versus fiber diameter.

viscous and inviscid melts. However, fibers from viscous melts had generally higher strength than those from inviscid melts (Fig. 12). Unfortunately, a comparison of fibers from both systems at equal diameters is not yet available. However, the tensile strength of inviscid melt spun fibers was generally lower than that of fibers made from high viscosity melts. This difference may be transient, due to a less well developed process, or it may in the end reflect an inherent structural weakness. In that case, the cohesive energy density of a melt and that of the resulting glass fiber might exhibit a quantifiable relationship that the authors will seek to develop.

1.6. 5 Tensile strength versus temperature Inviscid melt spun aluminate glass fibers offer a potential product advantage over conventional silicate fibers; they were found to retain their room temperature tensile strength better at elevated temperatures than typical silicate glass fibers, x~ An amorphous fiber with 80-2% alumina retained 87% of its room temperature strength at 750°C; another fiber with 66.8% alumina retained 86% of its room temperature strength (Fig. 13). In contrast, an S-glass control retained only 25-30% of its room temperature properties at 7 5 0 ° C , 1] suggesting that the strength of calcium aluminate fibers is only limited by their

100

~

ers

=- 80'

~ ~ "':

\

>*~c. 60 n-I"

~

O -,= c "~

Crystallization II e~

"t

S-Glass .ntrol)

I

20

0

o

2go

560

7go

looo

12'5o

Temperature,°C Fig. | 3 . Strength retention at elevated temperatures.

crystallization temperatures ranging from 969 to 1021°C." One may speculate that silicate and aluminate glass fibers fail at high temperatures by a different mechanism. Silicate glass fibers lose their strength gradually as the mobility of their molecular chains increases with increasing test temperature. Aluminate glass fibers lose their strength suddenly at a

258

F. T. Wallenberger, S. D. Brown

temperature where structural changes occur due to crystallization. If confirmed by further work, the results would suggest that aluminate fibers may tolerate 100-200°C higher operating temperatures than silicate fibers in the same fiber reinforced composite, or may offer better optical high temperature sensors. 1.6. 6 Specific strength versus fiber value Functional relationships between materials with different densities are often determined by comparing their specific properties (strength and modulus divided by weight density). The specific properties allow comparison of relative product values, i.e. relative performance at equal weight. These are important parameters for the selection of materials for weight sensitive fiber reinforced composites in the transportation market. A comparison of specific tensile strength is meaningful only if two fibers in question, e.g. an aluminate glass fiber with 44% alumina (SMD-44) and a military fiber optics glass (FOG-M) are aimed at the same applications. Such a comparison has been reported 28 and shows that the specific strength of these fibers is 300 and 260 kin, respectively, suggesting that SMD-44 would have superior value, i.e. being - 1 5 % stronger than FOG-M at equal weight or - 1 5 % lighter at equal weight. If it were fully developed, the aluminate fiber might be suitable for applications similar to those currently being served by FOG-M fibers, except that it would offer a much wider infrared window, i.e. sapphire-like instead of silicalike infrared transmission. 12 1.6. 7 Specific modulus versus composite value Table 8 shows the tensile properties of a composite part (a unibar) reinforced with E-glass, versus those of comparably constructed parts reinforced with (a) SMD aluminate glass fibers having 44.3% alumina, (b) aluminate glass ceramic fibers having 80% alumina, (c) Kevlar T M 49 fibers, and (d) Thornel T M 300 carbon fibers. The composite data were obtained from the literature; 54;55 all except those for the glass

ceramic fiber represent experimental values. The specific properties were calculated and inserted in Table 8 to compare the relative value of these composites at equal stiffness. If designed to possess equal stiffness, the SMD-44 aluminate glass fiber reinforced unibar would have 63% of the weight of the E-glass reinforced unibar, the AIMS aluminate glass ceramic fiber reinforced unibar 44%, the Kevlar 49 fiber reinforced unibar 33%, and the Thornel 300 fiber reinforced unibar 23%. In weight sensitive uses, a smaller payload is preferred. The part value must be judged individually for each application and includes part performance versus materials, fabrication and development cost, and overheads. Glass fiber economics is envisioned for the aluminate glass and glass ceramic fibers. 1.6. 8 Composition versus IR transmission Calcium aluminate glasses are hydrate free when the melts are produced by the Davy process. 2 Like sapphire, a single crystal material, they do not have a band in the infrared spectrum at 2-9#m that characterizes the presence of hydrated species. Most aluminate compositions made by a different process (including sol-gel), and most silicate compositions have a strong hydroxyl band in the IR. These relationships are shown in Fig. 14 showing hydroxylfree sapphire and calcium aluminate glasses and a typical silicate glass that is hydrated. Wallenberger et al. 1~ found the IR transmission for an inviscid melt spun fiber (67% alumina) to be nearly identical with that of the bulk glass (41% alumina). The present authors' samples are hydrate-free since the melts were either prepared by the Davy process, and/or the fibers had a hermetic carbon sheath. 1.6. 9 Composition versus hydrolytic stability Calcium aluminate bulk glasses and glass fibers have excellent alkali resistance, 6-s'31 thus are prime candidates for reinforcing cementitious composites. On the other hand, bare, as-drawn fibers have low hydrolytic stability 31 and require coatings to improve their water and acid resistance. Coatings are

Table 8. High modulus composites

Unidirectional epoxy composite properties Fiber volume fraction (%) Strength (GPa) Modulus (GPa) Density (g/cm 3) Specific modulus (103km) Weight at equal stiffness (%)

Composite reinforcing fiber, 0° Direction 5°'sl E-glass 58% SiO2 60 1.10 39-3 2.05 1-95 100

SMD 44% A1203 61 1.25 68.3 2.26 3.08 63

AIMS calculated

Kevlar 49

Thornel 300

60 TBD 102.0 2.80 3-60 54

60 1.40 82-0 1.41 5.87 33

60 1-38 131.0 1.55 8.62 23

New opportunities with high-modulas glass fibers 11111

1.7

Potential

market

259 opportunities

1.7. I Infrastructures and construction

80t ~

~

For a given part design, a higher fiber modulus can be used to make stiffer parts at equal weight. A higher modulus glass fiber can be used in the design of new infrastructure composites to achieve greater part stiffness and damage resistance in the construction of all composite foot and full service bridges, and in the repair of earthquake damaged bridge columns and buildings. A high specific modulus and weight savings which are required in transportation markets are not required here. A new glass fiber with a higher modulus may have a higher density than the incumbent fiber. Along with a higher modulus, a higher composite weight may actually be preferred. Inspection of Table 9 shows that the aluminate and glass fiber with the highest measured modulus (170 GPa, 2.5 times that of E-glass), approaches the modulus of carbon fibers with projected fiber glass economics. The cost issue is more complex in the field of infrastructure composites where the use of composites is less advanced than that in other composites markets. In spite of the fact that the lifecycle cost is lower, the start-up cost for single structures (e.g. a bridge) is higher than that for conventional materials. That tends to work, at least initially, against the adoption of new materials in this market. Military infrastructure composites technology points into the direction of carbon fiber reinforced composites. But for reasons of initial and lifecycle cost, a high modulus glass fiber may be the preferred glass fiber especially if it approaches the modulus of carbon fibers and has essentially glass fiber economics. For example, Vetrotex (UK) Ltd recently designed an apparently affordable glass fiber reinforced composite bridge 56 for the civilian sector. The fiber glass composites bridge stretches across the River Tay near Aberfeldy, Scotland, and is the

Sapphire

60-,4•

0.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Wave Length (l~rn)

Fig. 14. IR transmission: calcium aluminate, sapphire and quartz.

potentially available from many years of experience with analogous commercial bulk glass applications, 5° but need to be confirmed for fibers. The present authors' found that the hydrolytic stability increases with increasing alumina content. Additional protection can be derived from the application of a hermetic carbon coating that can be applied by a process recently developed for silica optical fibers4° or by the inviscid melt spinning process, m'21 Thus, the present authors found that an aluminate glass fiber with 80% alumina and a hermetic carbon coating31'4° is unaffected by prolonged exposure to steam. In addition, treatment with fluorides was found to successfully prevent water corrosion of calcium aluminate surfaces, 7 and a number of protective and coupling agents were explored for selected fibers, but need to be optimized. 5°

T a b l e 9 . N e w g l a s s a n d g l a s s c e r a m i c fibers

Tensile modulus

Composition: major components

Fiber designation

A1203

SiO2

CaO

ZnO

minor major --

major minor major

minor major minor

10% --

major major

--major

major minor minor

SiN

Glass fibers E-glass (silicate) SMD Z (aluminate) SIALON (silicate)

--

--

->5%

Glass ceramic fibers RIMS (aluminate) AIMS (aluminate) SIALON (silicate)

--

--

->15%

X-ray crystallinity

Absolute (GPa)

Specific (103 km)

69-0 122.7 125.0

2-8 4-2 4-2

amorphous amorphous amorphous

140.0 170.0 170.0

4-6 5-9 5.9

nanocryst. nanocryst. nanocryst.

Ref.

51 32 53

260

F. T. WaUenberger, S. D. Brown

world's largest composite span. Made of interlocking modules of glass reinforced plastic and aramid suspension cables, the bridge was assembled at the remote site without heavy machinery. 56 The Vetrotex venture suggests that composite modules reinforced with presently available glass fibers are the minimum adequate, and that an affordable ultra-high modulus glass fiber would provide higher lifecycle performance and value-in-use at a modest cost premium than either the current low cost (low modulus) silicate glass fibers or the rather costly (higher modulus) carbon reinforcing fibers. As this article goes to press, a new development is being reported. 57 The first full service all-composite bridge is currently being designed by the University of California in San Diego and will be built at an estimated cost of 55 million dollars to connect two parts of the campus over Interstate Highway 1-5 in 1996. The final deck will be made from modular sections of two prototype composite structures. 58 One is a fiberglass reinforced structure containing a minor amount of carbon fiber. It was made by Du Pont-Hardcore. The other consists of a honeycomb core between woven Kevlar composite face sheets. It was made by the Fiberite Corporation. Both prototypes were designed to meet the specification of the University of California in San Diego. They are less than a foot in thickness. This development points the way to the future.

1. 7. 2 Transportation markets For a given part design, a higher fiber modulus can also be used to make parts with equal stiffness at a lower part weight. The use of fiberglass reinforced composites in the transportation market increases, and a new high modulus glass fiber could become a major driver in weight-sensitive automotive uses, provided it has a higher specific (i.e. density considered) modulus. The specific modulus (measured modulus divided by density) is used for design purposes. Thus, if less of a stiffer reinforcing fiber can be used to design a smaller replacement part (or a thinner composite section) of equal weight, the weight and energy savings will be proportional to the ratio of the specific moduli of the composites made from the new and the incumbent reinforcing fiber. As shown in Table 9, the aluminate and glass fibers with the highest measured modulus (170GPa, 2-5 times that of E-glass) also have the highest specific modulus (2-1 times that of E-glass), thus facilitating major weight and energy savings, depending only on the fiber volume fraction (FVF) of the new composite part relative to the incumbent part. At equal stiffness, an array of these high modulus glass fibers weighs 60% less in absolute terms and 50% less in specific terms than a similar array of E-glass fibers. The same

is true for composites at equal stiffness from these two reinforcing fibers (see Tables 8 and 11). Many car manufacturers have recently begun to introduce glass fiber reinforced composites parts into the automotive market using new and cost-effective processes. Suppliers such as Owens Corning have also participated in the development of new composite manufacturing processes. For example, Owens Corning59 began to make a low cost, high strength method of assembly in Europe that produces complete fiberglass reinforced car body and chassis parts in a unit operation. In this process, glass fibers are fed to a robot, combined with a powdered binder, and sprayed onto a mold, where hot air melts the binder. The glass mat is then transferred to a closed mold, where resin is injected to complete the part. In summary, equipment is in place and/or coming on stream which could capitalize on a new and affordable ultra-high modulus glass fiber to affort major weight and energy savings in the automotive composite parts in the future.

1.7. 3 Glass fiber reinforced cement Glass fibers for cement matrix composites must be alkali resistant both during the composite manufacture and after long-term exposure of the composite to atmospheric conditions. The matrix in many instances is ordinary Portland cement. The incumbent glass fibers to reinforce cement are the British alkali resistant (AR) silicate glass fibers CHEMFIL and the Slovak AR silicate glass fiber VVUS. ~ Both have a higher SiO2 content than E-glass (61-62 versus 55%), a much higher Na20 content (14-15 versus ~1%), a much lower A1203 content (<1 versus 15%) and a high ZrO2 content (13-14 versus 0%). Calcium aluminates which are known to have a superior alkali resistance. They may become the material of choice to reinforce cementicious composites. 1.7. 4 Sapphire fiber functionality Selected quaternary calcium aluminate bulk glasses are known to have sapphire-like infrared transmissions spectra, z23'38 More recent work confirmed that calcium aluminate glass fibers having binary to quaternary compositions and containing 46-80% alumina also have sapphire-like infrared transmission spectra. ~2 Fibers from various compositions show only minor differences at the nominal cut-off around 5-5 #m. 32 Two potential applications come to mind which capitalize on this attractive functionality. One potential application is the core of optical silica preforms and therefore optical fibers. 3z This use will not be further discussed here. The other potential application is the manufacture of affordable infrared optical fibers. In summary, selected calcium aluminate glass fibers appear to be capable of replacing commercial single

261

New opportunities with high-modulus glass fibers

crystal sapphire (Sapphicon) fibers, in almost all sensor applications, except those which require the superior high temperature capability of the single crystal fiber (-1400°C) in relation to that of the glass fiber (-950°C). Whenever this substitution is possible it can be made at a fraction of the materials cost. Commercial sapphire fibers are known to have price points exceeding US $4400 kg, while that of calCium aluminate fibers is not expected to exceed US $6.6 kg when fully developed. 28

Table 10. TeHuria-alumlna fiber properties~

50-170 kpsi 10.1-11.5 Mpsi 40-150/~m X-ray amorphous Clear, pale yellow

Tensile strength Tensile modulus Fiber diameter Structure Appearance

100 0.92 mm Glass 80

2 ALUMINUM TELLURITE GLASS FIBERS

0.9

2.1 Background 2. I. 1. Introduction Wailenberger 32 is applying the same approach for making calcium aluminate glass fibers to other non-silica oxide systems having high and low viscosity melts. One example, an aluminum tellurite glass fiber, has already appeared in the literatureY Subsequently, Vogel et al. 6~ reported the preparation of other tellurite fibers.

~

6o

~

4o

2O

0 0.0

2.1.2 The tellurite system Telluria-alumina (tellurite) compositions with 511 wt% alumina are known glass formers. 6°'6] They are valuable optical glasses for designing scientific instruments. 61 The Glass Handbook 37 offers physical properties of 39 binary TeO2 systems with constituents alphabetically ranging from Ag20 to Z n O 27"37 but melt viscosities are listed only for key compositions in the TeO2-Na20 system. 24-27'37 2.2 Fabrications of glass fibers 2. 2.1 Melt viscosities The melt behavior of glass forming alumina-telluria compositions above and below the liquidus was determined to select an appropriate method to fabricate strong fibers. Pure telluria melted at 735°C. Its viscosity was low at and above the liquidus and it crystallized below the liquidus. Telluria-alumina melts containing 5-11 wt% alumina had low melt viscosities above the liquidus. The viscosity increased below the liquidus and transparent glasses resulted. 24-27 Since fibers from these glass forming tellurite melts could be up-drawn from supercooled melts the present authors estimate that their viscosities are 10-100 Pa s below the liquidus. 2. 2. 2 Fiber properties The properties of these experimental fibers which were up to 2 m in length 24-27 are shown in Table 10. The most homogeneous of the hand-drawn tellurite fiber was chosen for spectral transmission measurements from 0.35 to 10.00/zm. Transmission measurements were made by a procedure previously

1.0

2.0

3.b

s:o

do

To

Wave Length (l~m) Fig. 15. Per cent spectral absorption of teilurite bulk glass and glass fiber.

pioneered by Wailenberger et al. 14 for calcium aluminate fibers. The tellurite fiber shown in Fig. 15 has a lower transmission below 2.8/zm than the tellurite bulk glass. The two curves show good agreement in the 2-8-4-0/~m range. Above 4/~m, the fiber transmission is higher, possibly because it contains less alumina. Silica glasses and calcium aluminate glasses often show an absorption band with a transmission minimum at 2-90 #m due to hydroxyl groups. Tellurite bulk glasses and glass fibers have an absorption band with a transmission minimum at about 3-05/~m. It appears to be an inherent materials property not related to hydroxyl groups. The minima are at different wavelengths (2.90 versus 3.05/~m) and the teUurite band is broad while a typical hydroxyl band is narrow. 24-27 3 SUMMARY AND CONCLUSIONS

Calcium aluminate glass fibers offer major new opportunities in transportation, infrastructure and sensor composite markets. They possess a higher modulus than S-glass, an advantage that can yield major weight and energy savings in car, aircraft and aerospace composites, or stiffer composite parts in emerging bridge and construction markets. Their

262

F. T. Wallenberger, S. D. Brown

superior alkali resistance is important advantage in cement composites. They also possess sapphire-like infrared transmission, a property that is obtained at a fraction of the cost of sapphire fibers. Quaternary non-silica calcium aluminates with 4 0 - 5 0 % alumina have high melt viscosities (10-100 Pa s). Fibers were obtained in the laboratory by potentially commercial processes (up-drawing from supercooled melts and down-drawing from preforms). Binary compositions with 50-80% alumina have low viscosities (<1 Pa s). Fibers were obtained by inviscid melt spinning. Fibers drawn from viscous melts were stronger and stiffer than those spun from inviscid melts. Infrared optical aluminum tellurite fibers were, for the first time, obtained from supercooled melts.

REFERENCES 1. Hafner, H. C., Kreidl, N. J. & Weidel, A., Optical and physical properties of some calcium aluminate glasses. J. Am. Ceram. Soc., 41(8) (1958) 315-23. 2. Davy, J. R., Development of calcium aluminate glasses for use in the infrared spectrum. US Patent No. 3 338 694, 1967; Glass Technology, 19(2) (1978) 32-6. 3. Gupta, P. K., Glass fibers for composite materials. In Fibre Reinforcements for Composites Materials, ed. A. R. Bunsell. Composite Materials Series 2, Elsevier, Amsterdam, 1988, Chap. 2, pp. 19-71. 4. Machlan, G. R., The development of fibrous glass having high elastic moduli. Owens Corning Corporation, WADC Report 55-290, 1955 July 3l. 5. Meisel, J. A., Messier, D. R. & Patel, P. J., High modulus glass fibers. J. Non-Cryst. Solids, (1994), in press. 6. Brown, S. D & Onoda, G. Y., Jr, High modulus glasses based on ceramic oxies. Tech. Report R-6692, Contract NOw-65-0426-d. US Department of the Navy, Bureau of Naval Weapons, Washington, DC, Oct. 1966. 7. Onoda, G. Y., Jr & Brown, S. D., High modulus glasses based on ceramic oxides. Technical Report R-7363, Contract N00019-67-C-301. US Department of the Navy, Naval Air Systems Command, Washington, DC, Feb. 1968. 8. Onoda, G. Y., Jr & Brown, S. D., Low silica glasses based on calcium aluminates. J. Amer. Ceramic Soc., 53(6) (1970) 311-16. 9. Cunningham, R. E., Rakestraw, L. F. & Dunn, S. A., Inviscid melt spinning of filaments. In Spinning Wire from Molten Metal, ed., J. Mottern & W. J. Privott. AIChE Symposium Series, 74 (180), 20-32 (1978). 10. Wallenberger, F. T., Weston, N. E. & Dunn, S. A., Inviscid melt spinning: As-spun amorphous alumina fibers. Materials Lett. 2(4) (1990) 121-7. 11. Wallenberger, F. T., Weston, N. E. & Dunn, S. A., Inviscid melt spinning: Crystallization of amorphous alumina fibers, SAMPE Quarterly, 21 30-34 (1990). 12. Wallenberger, F. T., Weston, N. E. & Dunn, S. A., Melt spun calcium aluminate fibers: Infrared transmission. J. Non-cryst. Solids, 12(1) (1990) 116-19. 13. Wailenberger, F. T., Weston, N. E. & Dunn, S. A., Melt spun calcium aluminate fibers: Product value. International Conference on Electronic Materials, Materials Research Society Conference Proceedings, ed. R. P. H. Chang, T. Sugano & V. T. Nguyen. Materials Research Society, Pittsburg, PA, 1990, pp. 295-300.

14, Wallenberger, F. T., Weston, N. E. & Dunn, S. A., Inviscid melt spinning of alumina fibers: Jet stabilization dynamics. SAMPE Quarterly, 22(1) (1990) 15-77. 15, Wallenberger, F. T., Weston, N. E. & Dunn, S. A., Inviscid melt spinning: As-spun crystalline alumina fibers. J. Mater. Res., 5(11) (1990) 2682-6. 16, Wallenberger, F. T., Small diameter alumina fibers made by melt spinning. In Materials and Processing Report. Elsevier Science Publishers, New York, 3-4 Sept. 1990. 17. Wallenberger, F. T., Melt spinning of amorphous alumina fibers. Ceramic Bull., 69(10) (1990) 1646-8. 18. Wallenberger, F. T., Weston, N. E. & Dunn, S. A., Inviscid melt spinning: Strength of amorphous alumina fibers. In Advanced Structural Inorganic Composites, ed. P. Vincencini. Elsevier, Amsterdam, 1991, pp. 47-55. 19. Wallenberger, F. T., Melt spun alumina fibers. Technical Ceramics International, 3(March) (1991) 1. 20. Wailenberger, F. T., Weston, N. E. & Motzfeldt, K., Inviscid melt spinning of alumina fibers: Jet stabilization mechanism. Ceramic Engng. & Sci. Proc., 12(7/8) (1991) 1039-47. 21. Wallenberger, F. T., Weston, N. E., Motzfeldt, K. & Swartzfager, D. G., Inviscid melt spinning of alumina fibers: Chemical jet stabilization. J. Amer. Ceramic Soc., 75(3) (1992) 629-39. 22. Wallenberger, F. T., Brown, S. D. & Onoda, G. Y., ZnO-modified high modulus glass fibers. J. Non-cryst. Solids, 152 (1993) 279-83. 23. Kreidl, N. & Wallenberger, F. T., Development, preparation and application of aluminate glasses--History and prospects. Paper presented at Glass Conf. on the Relaxation of Glass, Aleganti, Spain, 7-13 July 1993. 24. Wallenberger, F. T. & Brown, S. D., Infrared tellurite and aluminate glass fibers. In Ceramics-Adding the Value, Proc. Int. Ceramic Conf. in Australia, Vol. I, ed. M. J. Bannister. Australasian Ceramics Society, Melbourne, 1992, pp. 214-20. 25. Wallenberger, F. T., Weston, N. E. & Brown, S. D., Infrared optical tellurite glass fibers. J. Australasian Ceramic Soc., 28(1) (1992) 9. 26. WaUenberger, F. T. & Brown, S. D., Non-silica oxide glass fibers from high and low viscosity melts. Paper presented at Int. Workshop on Advances in Inorganic Fibre Technology, Melbourne, Australia, 14 Aug. 1992. 27. Wallenberger, F. T., Weston, N. E. & Brown, S. D., Infrared optical tellurite glass fibers. J. Non-cryst. Solids, 144(1) (1992) 107-10. 28. Wallenberger, F. T., Brown, S. D. & Koutsky, J. A., Melt processing of optical and structural alumina fibers: Process review and product outlook. SAMPE Quarterly, 23(2) (1992) 17-28. 29. Wallenberger, F. T., Koutsky, J. A. & Brown, S. D., Melt processing of calcium aluminate glass fibers: Fibers with sapphirelike infrared transmission. In Submolecular Glass Chemistry and Physics, ed. P. Bray & N. J. Kreidl. Society of Photo-Optical Instrumentation Engineers, Bellington, WA, 1991, pp. 7-82. 30. Wallenberger, F. T., Weston, N. E. & Brown, S. D., Melt processed calcium aluminate fibers: Optical and structural properties. In Growth of Materials for Infrared Detectors, ed. R. E. Longshore & J. Baars. Society of Photo-Optical Instrumentation Engineers, Bellington, WA, 1991, pp. 116-24. 31. Wallenberger, F. T., Weston, N. E. & Brown, S. D., Calcium aluminate glass fibers: Drawing from super-

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