- Email: [email protected]
ZnO-modified high modulus glass fibers
~L[t~
] O U R N A L OF
Journal of Non-Crystalline Solids 152 (1993) 279-283 North-Holland
Letter to the Editor
ZnO-modified high modulus glass fibers Frederick T. Wallenberger Department of Materials Science and Engineering, University of Illinois, 1340 West Green Street, Urbana, ILL 61801, USA
Sherman D. Brown Department of Materials Science and Engineering, University of Illinois, 205 South Goodwin Avenue, Urbana, ILL 61801, USA
George Y. O n o d a Jr. IBM Corporation, Thomas Watson Research Center, Box 218, Yorktown Heights, NY 10598, USA Received 9 September 1992 Revised manuscript received 16 November 1992
It was found that zinc oxide is a modifier that effectively increases the modulus of aluminate and silicate glass fibers. Glass fibers having a higher fiber modulus than that of S-Glass were prepared by modifying aluminate and silicate composition with 5-25 mol% ZnO under conventional glass-forming conditions by melting the mixed oxide powders above the liquidus. The highest fiber modulus obtained in the ZnO-modified aluminate system (44CaO-30A12Oa-10ZnO-5MgO5Li20-4SiO 2) was 1.44 x that of S-Glass, but a new drawing process would be required. The highest modulus obtained in the silicate system (46.8SiO2-16.9ZnO-12.1CaO-11.8MgO-5.3TiO2-5.3Li20-0.9ZrO2-0.9CeO-0.01Fe203) was 1.20x that of S-Glass and no new process is required.
I. Introduction
An intensive, government-sponsored effort (~ 100 man years) during the 1960s was aimed at identifying a silicate a n d / o r aluminate glass fiber having a higher modulus than S-Glass, a fiber which had, and still has, the highest modulus of any commercial glass fiber [1]. A higher fiber modulus affords equal stiffness at a lower part weight, assuming equal fiber density and part construction. In addition to low part weight, low fiber cost would be an additional driver for new glass fiber reinforced composites technology in Correspondence to: Dr F.T. Wallenberger, Department of Materials Science and Engineering, University of Illinois, 1340 West Green Street, Urbana, ILL 61801, USA. Tel:+ 1-217 244 3208. Telefax: + 1-217 333 2763.
weight-sensitive transportation (aerospace, aircraft and automobile) applications. The highest modulus in the silica system (1.30 × that of S-Glass) was obtained with HM-glass, a toxic, BeO modified glass fiber [2,3] but this fiber was too toxic to warrant commercialization. About the same modulus was reported for an aluminate fiber [4] whose fabrication would have required the expense of developing a new process. Fibers could not be spun through an orifice (like silicate fibers) because their melt viscosity above and at the liquidus was too low [5]. They were drawn from supercooled melts [6]. In the 1970s, the development of high modulus glass fibers neither containing beryllia nor requiring a new process was halted, but technology is now becoming available that may finally afford a new generation of non-toxic high modulus glass
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
F. T. WaUenberger et al. / ZnO-modified high modulus glass fibers
280
fibers. In 1987, silicon nitride modified (oxynitride) glass fibers were found to a higher modulus than S-Glass [7], and in this letter we report an alternate route to high modulus glass fibers. It consists of modifying typical aluminate and silicate compositions with 5-25 mol% ZnO.
on a cardboard tab with epoxy. Weights were hung on the paper tab to apply the load to the fiber. Strain was measured with a cathetometer having a resolution of 0.001 cm focused on small markers near the top and bottom of the specimen. Weights were added stepwise until the total elongation was 0.1 cm. From these measurements, load-elongation plots were made. The most critical aspect relating to accurate modulus measurement is in determining the fiber geometry. Seldom does a fiber have a uniform cross-section throughout the 25 cm length. Variations in diameter (usually a slight taper) of up to 10% are not uncommon and must be taken into consideration if reliable modulus values are to be obtained. In theory, the cross-sectional area must be measured along the entire fiber. It was determined at four points along the fiber, at both ends and at the 1 / 3 length positions. Each cross-section was photographed under 300 x magnification and the area was computed from the photograph. An error analysis revealed that the modulus determined by this procedure is within a 2% error band if the area variance along the length of the fiber does not exceed 20%.
2. Experimental procedures Reagent-grade oxides and carbonates served as raw materials. About 30 g of a particular composition was melted in a platinum crucible in an oxygen-acetylene pot furnace for several hours above the liquidus at temperatures in the range 1400-1550°C. From those compositions which formed glasses, fibers approximately 6 m long were hand-drawn from the top of the cooling melt. The cooling rates were estimated to be in the order of 103°C/s. After the fibers were obtained, the glass in the crucible was remelted, then poured into a copper mold to make button-shaped specimens to measure density. Fibers obtained by this method were subjected to modulus measurement. The elastic fiber modulus was determined by direct load-elongation measurements. The experimental apparatus consisted of a stand for hanging fibers, 25 cm in length, vertically. The upper end of each fiber was mounted on an aluminum tab with epoxy and the tab was secured to the stand. The lower end of the fiber was mounted
3. Aluminate glass fibers It was the purpose of the following experiments to determine the magnitude of the ZnO
Table 1 ZnO-modified aluminate glass fibers Four- and six-component glasses (tool%) A1203
SiO 2
CaO
ZnO
55 50 45 40 35
5 10 15 20 25
54 49 44 44 44
10 15 20 10 10
MgO
BaO
LizO
Fiber properties tensile modulus (GPa)
lO%~lica composition 30 30 30 30 30
10 10 10 10 10
91.0 106.9 106.2 106.9 111.7
4%silica composition 32 32 32 32 32
4 4 4 4 4
106.9 106.9 115.8 5 5
5
108.2 122.7
F.T. Wallenberger et al. / ZnO-modified high modulus glass fibers Table 2 ZnO-modified silicate glass fibers
Fiber composition
YM-31A (mol%)
This study (tool%)
SiO2 BeO ZnO CaO MgO TiO2 Li20 Zr02
46.8 16.9 12.1 11.8 5.3 5.3 0.9
46.8 16.9 12.1 11.8 5.3 5.3 0.9
CeO 2 Fe203
0.9 0.1
0.9 0.1
Modulus (GPa): Density (g/cm3):
110.0 2.89
102.3 3.26
281
Replacing CaO by Z n O increases the liquidus of the glass. The glass containing 20% Z n O in the low silica composition in table 1 has a liquidus of ~ 1490°C, compared with ~ 1390°C for the glass without ZnO. The highest liquidus observed so far observed for any aluminate melt ( ~ 1550°C) was obtained with both six-component compositions (table 1). This result is noteworthy in the light of Weyl's [8] observation that glass formation in the t e m p e r a t u r e region above 1550°C is rare.
4. Silicate glass fibers
effect on selected aluminate compositions. To this end, C a O was incrementally replaced by Z n O in calcium aluminate compositions. The results of incrementally substituting Z n O for C a O in a high silica calcium aluminate composition, 30A1203-10SiOa-60CaO , is shown in table 1. Up to 30% Z n O could be substituted for C a O while retaining the glassy state. Up to 30% Z n O could also be substituted for C a O in a low silica composition, 32A12Oa-4SiO2-64CaO. A marked increase in modulus was found with increasing amounts of Z n O (table 1). The highest modulus observed for a high silica aluminate glass fiber (111.7 GPa) was obtained with a composition containing 25% ZnO. T h e highest modulus observed for a low silica aluminate glass fiber (115.8 GPa) was obtained with a composition containing 20% ZnO. Finally, the ultimate modulus observed so far for any aluminate glass fiber (122.7 GPa) was obtained with a six-component aluminate composition.
Having observed the powerful effect of Z n O as a replacement for C a O in the aluminate system, we realized [4] that it should be an even more effective replacement for BeO in the aluminate system [6]. BeO is known to increase the modulus of silicate glass [2,3]. Its effectiveness is believed to be the result of the high field strength of the Be + 2 ion and its ability to coordinate four oxygen ions tightly to it. Specifically, the structure of BeO is of the wurtzite type, where the Be ÷ 2 ion is tetrahedrally coordinated by four oxygen ions and each oxygen ion is shared between four BeO 4 tetrahedra. Crystal chemistry data of cation-oxygen compounds reveals that the only other cation forming the wurtzite structure with oxygen is Zn ÷2. Also, Zn2SiO 4 is isomorphous with Be2SiO 4. Prompted by the similarities between Zn ÷ 2 and Be ÷ 2, Zn ÷ 2 was incorporated both into aluminate and silicate glass fibers. Prior to 1970, attention was focused on silicate
Table 3 New high modulus glass fibers Fiber designation
E-Glass S-Glass New HM-Glass New Goal
Major components (mol%) A1203
SiO 2
CaO
14.0 25.0
53.0 65.1 46.8 46.8 4.0
20.0
32.0
12.1 12.1 44.0
BeO
ZnO
16.9 16.9 10.0
Modulus GPa
10 3 km
Process comments
72.4 85.0 102.3 110.0 122.7 137.8
2.8 3.4 3.2 3.8 4.2 4.2
commercial commercial conventional toxic/abandoned new process conventional
LETTER TO THE EDITOR
282
F.T. Wallenberger et aL / ZnO-modified high modulus glass fibers
glass fibers containing 6-17% beryllia. These glass fibers had modulus values between 89.6 and 110.3 GPa. Examples are HM (high modulus) glass fibers including the Owens Corning Glass X-2285 [3] having a modulus of 102.0 GPa, the Aerojet General Glass 970-S [2] having a modulus of 103.4 GPa, and the Owens Coming Glass YM31A [3] having a modulus of 110.3 GPa. The latter contains 16.9% BeO. The effect of ZnO on calcium aluminate glass fibers, and its apparent similarity to BeO, prompted an examination of the effect of replacing BeO by ZnO in the Owens Corning Glass YM-31A on a mole-for-mole basis (table 2). A very workable glass resulted affording a glass fiber with a specific gravity of 3.26 g / c m 3 and a modulus of 102.3 GPa, the highest modulus reported in the published literature for any nonBeO silicate glass fiber.
5. Tensile properties Tensile strength is essentially a criterion for fiber uniformity. Strength increases as surface cracks a n d / o r internal flaws decrease. Consequently, hand drawn fibers and fibers obtained from an early stage of a process development tend to be weak but, as the process continuity (and therefore fiber uniformity) improves in a fully developed process, ultimate strength levels are obtained. For a fiber to have commercial value, a tensile strength of 1.5-6 GPa is required. All fibers discussed in this letter were hand-drawn. They met the minimum criterion (1.5 GPa), but only extensive process development can afford ultimate goal strength levels (6.0 GPa). At this stage of the development, no scientific insights can be gained from an analysis of tensile strength data obtained thus far. Tensile modulus is essentially a materials property. It reflects order in a given system and is therefore affected by crystallinity and morphology. Unlike strength, it does not change much with form, shape and uniformity of a material. The absolute modulus is experimentally measured; it characterizes the stiffness of a fiber. The
specific modulus is a derived function (modulus divided by density); it characterizes the relative value of a fiber in weight-sensitive transportation (automobile, aircraft and aerospace) applications. The absolute modulus of ZnO-modified aluminate glass fibers was up to 1.44X of that of S-Glass, and the absolute modulus of ZnO-modified silicate glass fibers was up to 1.20 × that of S-Glass. An error analysis revealed that the modulus determined by the procedure described in this paper is within a 2% error band. The specific modulus of the ZnO-modified aluminate fiber shown in table 3 is 1.24 × that of S-Glass, and considerable weight and energy savings can be anticipated for fiber reinforced composites of equal weight. The specific modulus of the ZnO-modified silicate fiber shown in table 3 is still lower than that of S-Glass, and more work is needed to identify ZnO silicate compositions yielding fibers with a higher modulus. All fibers described in this letter were X-ray amorphous, and the observed differences in modulus (stiffness) among the various compositions discussed below reflect various degrees of order (on a submicrometer scale) within a generally disordered (X-ray amorphous) system [6]. Another key property is the break elongation of a fiber. In general, X-ray amorphous (glass) fibers have relatively high break elongations (> 4.5%), while the break elongations of aramid and carbon fibers are lower (< 3.0%). High break elongations improve the toughness of the fiber and a resulting composite. Toughnessby this definition is a materials property. It represents the work required to break a fiber in the tensile mode and it is obtained by integrating the area under a given experimental stress strain curve. All ZnO modified aluminate and silicate fibers described in this letter had break elongations > 4.0% and, by inference, materials toughness higher than than that of many other high performance composite reinforcing fibers.
6. Summary and conclusions We found that zinc oxide is a modifier that effectively increases the modulus of aluminate
F. T. Wallenberger et al. / ZnO-modiped high modulus glass fibers
a n d silicate glass fibers. T h e a b s o l u t e m o d u l u s o f t h e s e fibers was u p to 1.44 × t h a t o f S-Glass, a c o m m e r c i a l p r o d u c t having t h e highest m o d u l u s o f any glass f i b e r c u r r e n t l y available. T h e specific m o d u l u s of t h e s e fibers was up to 1.20 x h i g h e r t h a n t h a t o f S-Glass, suggesting t h a t Z n O - m o d ified glass fibers have significant p o t e n t i a l for i n c r e a s i n g t h e stiffness o f f i b e r - r e i n f o r c e d p a r t s at e q u a l w e i g h t or r e d u c i n g the p a r t w e i g h t in sensitive t r a n s p o r t a t i o n a p p l i c a t i o n s at e q u a l p a r t stiffness.
References [1] P.K. Gupta, in: Fibre Reinforcements for Composite Materials, ed. A.R. Bunsell, Composite Materials Series 2 (Elsevier, Amsterdam, 1988) p. 19.
283
[2] A. Lewis, C.F. Neumeyer, L.W. Ragoriewicz and P. Duwez, Aerojet General, Report IR-479-7 (III), USAF Contract No. F33615-67-C-1656, Jan. 1968. [3] W.C. Brady, R.L. Tiede and F.M. Veazy, Owens Coming Fiberglass, Report IR-9-465(II), USAF Contract No. F33615-67-C-1590, Sept. 1967. [4] G.Y. Onoda Jr. and S.D. Brown, J. Am. Ceram. Soc. 53 (1970) 311. [5] F.T. Wallenberger, N.E. Weston and S.A. Dunn, J. NonCryst. Solids 124 (1990) 116. [6] F.T. Wallenberger and S.D. Brown, in: Proc. Int. Conf. on Advances in Inorganic Fiber Technology, Melbourne, Australia, Aug. 14, 1992; Compos. Sci. Technol. (1993) in press. [7] D.R. Messier, Int. J. High Technol. Ceram. 3 (1987) 33. [8] W.A. Weyl and E.C. Marboe, The Constitution of Glasses, Vol. 1 (Interscience-Wiley, New York, 1962) p. 241.
] O U R N A L OF
Journal of Non-Crystalline Solids 152 (1993) 279-283 North-Holland
Letter to the Editor
ZnO-modified high modulus glass fibers Frederick T. Wallenberger Department of Materials Science and Engineering, University of Illinois, 1340 West Green Street, Urbana, ILL 61801, USA
Sherman D. Brown Department of Materials Science and Engineering, University of Illinois, 205 South Goodwin Avenue, Urbana, ILL 61801, USA
George Y. O n o d a Jr. IBM Corporation, Thomas Watson Research Center, Box 218, Yorktown Heights, NY 10598, USA Received 9 September 1992 Revised manuscript received 16 November 1992
It was found that zinc oxide is a modifier that effectively increases the modulus of aluminate and silicate glass fibers. Glass fibers having a higher fiber modulus than that of S-Glass were prepared by modifying aluminate and silicate composition with 5-25 mol% ZnO under conventional glass-forming conditions by melting the mixed oxide powders above the liquidus. The highest fiber modulus obtained in the ZnO-modified aluminate system (44CaO-30A12Oa-10ZnO-5MgO5Li20-4SiO 2) was 1.44 x that of S-Glass, but a new drawing process would be required. The highest modulus obtained in the silicate system (46.8SiO2-16.9ZnO-12.1CaO-11.8MgO-5.3TiO2-5.3Li20-0.9ZrO2-0.9CeO-0.01Fe203) was 1.20x that of S-Glass and no new process is required.
I. Introduction
An intensive, government-sponsored effort (~ 100 man years) during the 1960s was aimed at identifying a silicate a n d / o r aluminate glass fiber having a higher modulus than S-Glass, a fiber which had, and still has, the highest modulus of any commercial glass fiber [1]. A higher fiber modulus affords equal stiffness at a lower part weight, assuming equal fiber density and part construction. In addition to low part weight, low fiber cost would be an additional driver for new glass fiber reinforced composites technology in Correspondence to: Dr F.T. Wallenberger, Department of Materials Science and Engineering, University of Illinois, 1340 West Green Street, Urbana, ILL 61801, USA. Tel:+ 1-217 244 3208. Telefax: + 1-217 333 2763.
weight-sensitive transportation (aerospace, aircraft and automobile) applications. The highest modulus in the silica system (1.30 × that of S-Glass) was obtained with HM-glass, a toxic, BeO modified glass fiber [2,3] but this fiber was too toxic to warrant commercialization. About the same modulus was reported for an aluminate fiber [4] whose fabrication would have required the expense of developing a new process. Fibers could not be spun through an orifice (like silicate fibers) because their melt viscosity above and at the liquidus was too low [5]. They were drawn from supercooled melts [6]. In the 1970s, the development of high modulus glass fibers neither containing beryllia nor requiring a new process was halted, but technology is now becoming available that may finally afford a new generation of non-toxic high modulus glass
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
F. T. WaUenberger et al. / ZnO-modified high modulus glass fibers
280
fibers. In 1987, silicon nitride modified (oxynitride) glass fibers were found to a higher modulus than S-Glass [7], and in this letter we report an alternate route to high modulus glass fibers. It consists of modifying typical aluminate and silicate compositions with 5-25 mol% ZnO.
on a cardboard tab with epoxy. Weights were hung on the paper tab to apply the load to the fiber. Strain was measured with a cathetometer having a resolution of 0.001 cm focused on small markers near the top and bottom of the specimen. Weights were added stepwise until the total elongation was 0.1 cm. From these measurements, load-elongation plots were made. The most critical aspect relating to accurate modulus measurement is in determining the fiber geometry. Seldom does a fiber have a uniform cross-section throughout the 25 cm length. Variations in diameter (usually a slight taper) of up to 10% are not uncommon and must be taken into consideration if reliable modulus values are to be obtained. In theory, the cross-sectional area must be measured along the entire fiber. It was determined at four points along the fiber, at both ends and at the 1 / 3 length positions. Each cross-section was photographed under 300 x magnification and the area was computed from the photograph. An error analysis revealed that the modulus determined by this procedure is within a 2% error band if the area variance along the length of the fiber does not exceed 20%.
2. Experimental procedures Reagent-grade oxides and carbonates served as raw materials. About 30 g of a particular composition was melted in a platinum crucible in an oxygen-acetylene pot furnace for several hours above the liquidus at temperatures in the range 1400-1550°C. From those compositions which formed glasses, fibers approximately 6 m long were hand-drawn from the top of the cooling melt. The cooling rates were estimated to be in the order of 103°C/s. After the fibers were obtained, the glass in the crucible was remelted, then poured into a copper mold to make button-shaped specimens to measure density. Fibers obtained by this method were subjected to modulus measurement. The elastic fiber modulus was determined by direct load-elongation measurements. The experimental apparatus consisted of a stand for hanging fibers, 25 cm in length, vertically. The upper end of each fiber was mounted on an aluminum tab with epoxy and the tab was secured to the stand. The lower end of the fiber was mounted
3. Aluminate glass fibers It was the purpose of the following experiments to determine the magnitude of the ZnO
Table 1 ZnO-modified aluminate glass fibers Four- and six-component glasses (tool%) A1203
SiO 2
CaO
ZnO
55 50 45 40 35
5 10 15 20 25
54 49 44 44 44
10 15 20 10 10
MgO
BaO
LizO
Fiber properties tensile modulus (GPa)
lO%~lica composition 30 30 30 30 30
10 10 10 10 10
91.0 106.9 106.2 106.9 111.7
4%silica composition 32 32 32 32 32
4 4 4 4 4
106.9 106.9 115.8 5 5
5
108.2 122.7
F.T. Wallenberger et al. / ZnO-modified high modulus glass fibers Table 2 ZnO-modified silicate glass fibers
Fiber composition
YM-31A (mol%)
This study (tool%)
SiO2 BeO ZnO CaO MgO TiO2 Li20 Zr02
46.8 16.9 12.1 11.8 5.3 5.3 0.9
46.8 16.9 12.1 11.8 5.3 5.3 0.9
CeO 2 Fe203
0.9 0.1
0.9 0.1
Modulus (GPa): Density (g/cm3):
110.0 2.89
102.3 3.26
281
Replacing CaO by Z n O increases the liquidus of the glass. The glass containing 20% Z n O in the low silica composition in table 1 has a liquidus of ~ 1490°C, compared with ~ 1390°C for the glass without ZnO. The highest liquidus observed so far observed for any aluminate melt ( ~ 1550°C) was obtained with both six-component compositions (table 1). This result is noteworthy in the light of Weyl's [8] observation that glass formation in the t e m p e r a t u r e region above 1550°C is rare.
4. Silicate glass fibers
effect on selected aluminate compositions. To this end, C a O was incrementally replaced by Z n O in calcium aluminate compositions. The results of incrementally substituting Z n O for C a O in a high silica calcium aluminate composition, 30A1203-10SiOa-60CaO , is shown in table 1. Up to 30% Z n O could be substituted for C a O while retaining the glassy state. Up to 30% Z n O could also be substituted for C a O in a low silica composition, 32A12Oa-4SiO2-64CaO. A marked increase in modulus was found with increasing amounts of Z n O (table 1). The highest modulus observed for a high silica aluminate glass fiber (111.7 GPa) was obtained with a composition containing 25% ZnO. T h e highest modulus observed for a low silica aluminate glass fiber (115.8 GPa) was obtained with a composition containing 20% ZnO. Finally, the ultimate modulus observed so far for any aluminate glass fiber (122.7 GPa) was obtained with a six-component aluminate composition.
Having observed the powerful effect of Z n O as a replacement for C a O in the aluminate system, we realized [4] that it should be an even more effective replacement for BeO in the aluminate system [6]. BeO is known to increase the modulus of silicate glass [2,3]. Its effectiveness is believed to be the result of the high field strength of the Be + 2 ion and its ability to coordinate four oxygen ions tightly to it. Specifically, the structure of BeO is of the wurtzite type, where the Be ÷ 2 ion is tetrahedrally coordinated by four oxygen ions and each oxygen ion is shared between four BeO 4 tetrahedra. Crystal chemistry data of cation-oxygen compounds reveals that the only other cation forming the wurtzite structure with oxygen is Zn ÷2. Also, Zn2SiO 4 is isomorphous with Be2SiO 4. Prompted by the similarities between Zn ÷ 2 and Be ÷ 2, Zn ÷ 2 was incorporated both into aluminate and silicate glass fibers. Prior to 1970, attention was focused on silicate
Table 3 New high modulus glass fibers Fiber designation
E-Glass S-Glass New HM-Glass New Goal
Major components (mol%) A1203
SiO 2
CaO
14.0 25.0
53.0 65.1 46.8 46.8 4.0
20.0
32.0
12.1 12.1 44.0
BeO
ZnO
16.9 16.9 10.0
Modulus GPa
10 3 km
Process comments
72.4 85.0 102.3 110.0 122.7 137.8
2.8 3.4 3.2 3.8 4.2 4.2
commercial commercial conventional toxic/abandoned new process conventional
LETTER TO THE EDITOR
282
F.T. Wallenberger et aL / ZnO-modified high modulus glass fibers
glass fibers containing 6-17% beryllia. These glass fibers had modulus values between 89.6 and 110.3 GPa. Examples are HM (high modulus) glass fibers including the Owens Corning Glass X-2285 [3] having a modulus of 102.0 GPa, the Aerojet General Glass 970-S [2] having a modulus of 103.4 GPa, and the Owens Coming Glass YM31A [3] having a modulus of 110.3 GPa. The latter contains 16.9% BeO. The effect of ZnO on calcium aluminate glass fibers, and its apparent similarity to BeO, prompted an examination of the effect of replacing BeO by ZnO in the Owens Corning Glass YM-31A on a mole-for-mole basis (table 2). A very workable glass resulted affording a glass fiber with a specific gravity of 3.26 g / c m 3 and a modulus of 102.3 GPa, the highest modulus reported in the published literature for any nonBeO silicate glass fiber.
5. Tensile properties Tensile strength is essentially a criterion for fiber uniformity. Strength increases as surface cracks a n d / o r internal flaws decrease. Consequently, hand drawn fibers and fibers obtained from an early stage of a process development tend to be weak but, as the process continuity (and therefore fiber uniformity) improves in a fully developed process, ultimate strength levels are obtained. For a fiber to have commercial value, a tensile strength of 1.5-6 GPa is required. All fibers discussed in this letter were hand-drawn. They met the minimum criterion (1.5 GPa), but only extensive process development can afford ultimate goal strength levels (6.0 GPa). At this stage of the development, no scientific insights can be gained from an analysis of tensile strength data obtained thus far. Tensile modulus is essentially a materials property. It reflects order in a given system and is therefore affected by crystallinity and morphology. Unlike strength, it does not change much with form, shape and uniformity of a material. The absolute modulus is experimentally measured; it characterizes the stiffness of a fiber. The
specific modulus is a derived function (modulus divided by density); it characterizes the relative value of a fiber in weight-sensitive transportation (automobile, aircraft and aerospace) applications. The absolute modulus of ZnO-modified aluminate glass fibers was up to 1.44X of that of S-Glass, and the absolute modulus of ZnO-modified silicate glass fibers was up to 1.20 × that of S-Glass. An error analysis revealed that the modulus determined by the procedure described in this paper is within a 2% error band. The specific modulus of the ZnO-modified aluminate fiber shown in table 3 is 1.24 × that of S-Glass, and considerable weight and energy savings can be anticipated for fiber reinforced composites of equal weight. The specific modulus of the ZnO-modified silicate fiber shown in table 3 is still lower than that of S-Glass, and more work is needed to identify ZnO silicate compositions yielding fibers with a higher modulus. All fibers described in this letter were X-ray amorphous, and the observed differences in modulus (stiffness) among the various compositions discussed below reflect various degrees of order (on a submicrometer scale) within a generally disordered (X-ray amorphous) system [6]. Another key property is the break elongation of a fiber. In general, X-ray amorphous (glass) fibers have relatively high break elongations (> 4.5%), while the break elongations of aramid and carbon fibers are lower (< 3.0%). High break elongations improve the toughness of the fiber and a resulting composite. Toughnessby this definition is a materials property. It represents the work required to break a fiber in the tensile mode and it is obtained by integrating the area under a given experimental stress strain curve. All ZnO modified aluminate and silicate fibers described in this letter had break elongations > 4.0% and, by inference, materials toughness higher than than that of many other high performance composite reinforcing fibers.
6. Summary and conclusions We found that zinc oxide is a modifier that effectively increases the modulus of aluminate
F. T. Wallenberger et al. / ZnO-modiped high modulus glass fibers
a n d silicate glass fibers. T h e a b s o l u t e m o d u l u s o f t h e s e fibers was u p to 1.44 × t h a t o f S-Glass, a c o m m e r c i a l p r o d u c t having t h e highest m o d u l u s o f any glass f i b e r c u r r e n t l y available. T h e specific m o d u l u s of t h e s e fibers was up to 1.20 x h i g h e r t h a n t h a t o f S-Glass, suggesting t h a t Z n O - m o d ified glass fibers have significant p o t e n t i a l for i n c r e a s i n g t h e stiffness o f f i b e r - r e i n f o r c e d p a r t s at e q u a l w e i g h t or r e d u c i n g the p a r t w e i g h t in sensitive t r a n s p o r t a t i o n a p p l i c a t i o n s at e q u a l p a r t stiffness.
References [1] P.K. Gupta, in: Fibre Reinforcements for Composite Materials, ed. A.R. Bunsell, Composite Materials Series 2 (Elsevier, Amsterdam, 1988) p. 19.
283
[2] A. Lewis, C.F. Neumeyer, L.W. Ragoriewicz and P. Duwez, Aerojet General, Report IR-479-7 (III), USAF Contract No. F33615-67-C-1656, Jan. 1968. [3] W.C. Brady, R.L. Tiede and F.M. Veazy, Owens Coming Fiberglass, Report IR-9-465(II), USAF Contract No. F33615-67-C-1590, Sept. 1967. [4] G.Y. Onoda Jr. and S.D. Brown, J. Am. Ceram. Soc. 53 (1970) 311. [5] F.T. Wallenberger, N.E. Weston and S.A. Dunn, J. NonCryst. Solids 124 (1990) 116. [6] F.T. Wallenberger and S.D. Brown, in: Proc. Int. Conf. on Advances in Inorganic Fiber Technology, Melbourne, Australia, Aug. 14, 1992; Compos. Sci. Technol. (1993) in press. [7] D.R. Messier, Int. J. High Technol. Ceram. 3 (1987) 33. [8] W.A. Weyl and E.C. Marboe, The Constitution of Glasses, Vol. 1 (Interscience-Wiley, New York, 1962) p. 241.