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Environmental effects on the strength of fluorozirconate glass fibers
Journal of Non-Crystalline Solids 74 (1985) 229-236 North-Holland, Amsterdam
ENVIRONMENTAL
229
EFFECTS ON THE STRENGTH OF
FLUOROZIRCONATE GLASS FIBERS A . M . N A K A T A , J. L A U * a n d J.D. M A C K E N Z I E Department of Materials Science and Engineering, University of California, Los Angeles, California 90024, USA Received 26 October 1984
The tensile strength of Teflon FEP jacketed fluorozirconate glass fibers was found to be severely affected by the environment. The strenght of fibers placed in a humid atmosphere dropped to about half of their original strength after 4 days while the strength remained unchanged in a dry environment. Microscopy revealed substantial damage to the fiber surface due to the presence of moisture. It was concluded that Teflon FEP does not provide adequate protection as a moisture barrier.
1. Introduction H e a v y m e t a l h a l i d e glasses have excellent i n f r a r e d t r a n s m i s s i o n p r o p e r t i e s a n d have b e e n p r o p o s e d as p o s s i b l e c a n d i d a t e s for o p t i c a l waveguides. Of p a r t i c u l a r interest is the f l u o r o z i r c o n a t e system which has b e e n p r e d i c t e d to have theoretical m i n i m u m losses - 10 - 2 d b k m -1 [1]. In o r d e r to be conside r e d for such an a p p l i c a t i o n , one of the r e q u i r e m e n t s is that the m a t e r i a l m u s t either be sufficiently d u r a b l e o r a d e q u a t e l y p r o t e c t e d f r o m the e n v i r o n m e n t in o r d e r to preserve its m e c h a n i c a l strength a n d t r a n s m i s s i o n properties. C u r r e n t r e p o r t s on the chemical d u r a b i l i t y of f l u o r o z i r c o n a t e glasses consist largely of s o l u b i l i t y studies a n d exist for b u l k o r p o w d e r s a m p l e s only. T h e y show the s o l u b i l i t y o f f l u o r o z i r c o n a t e glasses in water to be - 1 0 - 4 g c m - 2 h - 1 [2] versus - 1 0 - 9 to 10 - t ° g c m - 2 h -1 for vitreous silica [3]. W a t e r has a l r e a d y b e e n shown to cause an increase in t r a n s m i s s i o n losses [4] a n d a decrease in the tensile strength o f silica o p t i c a l fibers [5]. Since the surface area to v o l u m e ratio of a glass fiber is r o u g h l y 500 times that of a b u l k piece of glass, one w o u l d expect to see m o r e p r o n o u n c e d t r a n s m i s s i o n losses a n d d r o p in m e c h a n i c a l strength in f l u o r o z i r c o n a t e glass fiber strength d u e to the p o o r e r c h e m i c a l d u r a b i l i t y as c o m p a r e d with vitreous silica. Teflon F E P has b e e n c o n s i d e r e d as a possible p r o t e c t i v e c o a t i n g for f l u o r o z i r c o n a t e glass fibers [6]. This p a p e r is c o n c e r n e d with the effect of a * Present address: W.R. Grace & Co., Washington Research Center, 7379 Route 32, Columbia, Maryland 21044, USA. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
230
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
Table 1 Glass composition (mol.%) 51% ZrF4 20% LiF 15% BaF2 5% PbF2 5% LaF3 3% A1F3
Distilled from Cerac 99.5% pure Alfa ultrapure grade Alfa ultrapure grade Alfa ultrapure grade Alfa 99.9% pure Cerac 99.5% pure
humid atmosphere on the strength-one type of fluorozirconate glass fiber coated with Teflon FEP.
2. Experimental procedure The composition of the glass used in this study is given in table 1. The procedures for glass preparation and fiber fabrication were described in a previous paper [7]. The glass was chosen for its favorable viscosity behavior around the fiber drawing temperature region. Both heating zones in the two-coil furnace [7] were held at 290°C during fiber drawing. The glass fibers obtained were between 100-170 /~m in diameter with a typical Teflon FEP coating thickness of 25-35/~m. Samples were cut into half meter lengths, and the ends were sealed with Teflon FEP. The fibers were simultaneously placed into either a chamber with a - 100% relative humidity atmosphere or into a dry box with a 99.998% dry nitrogen atmosphere. Both were kept at room temperature (23°C). Fiber samples were periodically removed for tensile testing on an Instron using a 25 m m testing length. Approximately 30 specimens were tested for each condition. Infrared absorption and scanning electron microscopy were also used in this study.
3. Results and discussion The average strength values for the " w e t " and " d r y " populations as a function of time are shown in fig. 1, the error bars indicate 95% confidence limits. The initial strength of the fibers in the 1 0 0 - 1 7 0 / t m diameter range was 150 MPa. The strength of the fibers stored in the humid atmosphere dropped to half of their original value in 4 days and then levelled off, while fibers stored in the dry atmosphere retained their strength. The reduction in strength is clearly related to the presence of moisture. According to manufacturer's literature, Teflon FEP is thermally stable at processing temperatures up to 370°C [8]. Thus, the thermal cycle through the fiber drawing temperature of 290°C should not affect the physical properties of Teflon FEP to any drastic degree. The permeation by vapors and gases in Teflon FEP is much lower than most organic polymers. Molecular diffusion
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
231
200 r--
150 "I" F-
c~
Teflon FEP jacketed fluoride fiber in dry environment
z
LU
n"
I00
LU
c~ nUJ
Teflon FEP jacketed 50
fluoride fiber in humid environment
0
5 TIME
10 (DAYS)
15
Fig. 1. Graph of average tensile strength versus time for fibers held in either a humid or dry atmosphere.
does however occur, and the water vapor transmission rate for Teflon FEP is 1.4 g / m 2 / 2 4 h for a 25.4 ~t thick film [9]. By assuming the average diameter of the fibers studied to be 135 /~m and that the fibers were coated with a 25.4 # m thick jacket, the time required to form a monolayer of water (approximate area of water molecule to be 0.44 A2) on the surface of the fiber was estimated to be less than 6 min. Therefore, in principle, an ample supply of water is available to attack the fiber within the time period of testing ( - 10 d). Permeation of water through the Teflon FEP jacket was also studied by infrared absorption. Three different spectra are shown in fig. 2. As can be seen, after exposure to the humid environment, new absorption bands appeared at 2.9/~m and 6/~m. The 2.9 # m band corresponds to the -OH stretching and the 6 /~m band is attributed to lattice water or water of hydration [10]. The intensity of the 2.9 ~tm decreased by a few per cent after the fibers were removed from the humid atmosphere and placed in ambient for a few hours. On the other hand, the intensity of the 6/~m band remained unchanged even after the fibers were placed in a vacuum for 18 h. The initial decrease in the 2.9/~m band has been attributed to the evaporation of surface moisture. The evidence seems to indicate, however, that a substantial amount of water remains trapped inside the Teflon FEP cladding, probably due to irreversible chemical reaction with the fluoride glass fiber. The infrared study also showed that the amount of water absorbed increased with the number of days in the wet atmosphere.
232
2.5
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers 4.0
3.0
5,0
MICRONS 8.0 7.0
8.0
10
20
30 50
A
4000
35'00 30'00 2 ~
20'00 18'(~0 lebO 14bO 12'00 10~
e~O
6~0
460
200
WAVENUMBER (cr'n"1) Fig. 2. Infrared spectra of A) Teflon FEP film; B) Teflon FEP jacketed fluoride glass fibers before exposure to a humid environment; and C) Teflon FEP jacketed fluoride glass fibers after five days in a humid environment.
SEM work was carried out to examine the effect of water on the fluoride fibers. The fiber surface was exposed by stripping the Teflon cladding away with a razor. A micrograph of a freshly drawn (pristine) stripped fiber is shown in fig. 3. A classic fracture pattern with a mirror region is evident and the surface of the fiber is smooth. Fibers that were stored in a dry atmosphere for 2 d revealed
Fig. 3. SEM micrograph of a stripped pristine fluoride glass fiber.
.4.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
233
Fig. 4. SEM micrograph of a bare fluoride glass fiber that has been exposed to a humid atmosphere for two days at room temperature (23°C). fracture p a t t e r n s identical to those of the pristine samples. Figure 4 is a n example of a bare fluoride glass fiber that was exposed to a h u m i d environm e n t for 2 days. C o n s i d e r a b l e damage a n d corrosion has taken place. The
Fig. 5. SEM micrograph of a Teflon FEP jacketed fluoride glass fiber that has been exposed to a humid environment for two days at room temperature (23°C).
234
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
Fig. 6. SEM micrograph of a Teflon FEP jacketed fluoride glass fiber that has been stored in a 99.998% dry nitrogen atmosphere for 24 days at room temperature (23°C).
mud-crack patterns observed are similar to those seen in other studies [2]. A Teflon FEP-clad fiber that was also exposed to the wet atmosphere for 2 days is shown in fig. 5. Although extensive corrosion has not occurred as in the case of the bare fiber, hackles on the surface of the fiber caused by the razor upon stripping away the Teflon coating can be seen. These hackles are not found on any of the pristine or dry samples and they indicate that a soft layer has formed on the surface. A direct comparison of Teflon FEP-clad fibers that were both stored for 24 days in either a wet or dry atmosphere is shown in figs. 6 and 7. The fiber that was kept dry still exhibits clear fracture patterns and smooth surfaces which are not found in any of the wet fiber samples. The wet fiber, on the other hand, showed a flat fracture face and substantial surface damage. In conclusion, it is believed that the water that permeated through the Teflon FEP jacket reacted with the glass fiber surface to form hydrated fluorides or hydroxides, thereby creating surface damage. This is reflected by the decrease in strength of the fibers. Probable paths for the hydration reaction are listed below: 1. MF. + x H 2 0 ---* MF n • xH20, or 2. MF. + x H 2 0 ~ M(OH). + nHF, or 3. MF. + x H 2 0 ~ M ( O H ) . - y ( H 2 0 ) + n H F + (x - n - y ) H 2 0 . Although Teflon FEP is less permeable to water than most organic polymers, it is still inadequate in as far as providing a protective coating for fluoride glass fibers. It could be argued that thicker coatings can provide better protection. However, if the useful life of a fiber is targeted at ] 0 9 S ( - 20 y),
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
235
Fig. 7. SEM micrograph of A) Teflon FEP jacketed fluoride glass fiber that has been stored in a humid atmosphere for 24 days at room temperature (23°C); and B) an enlargement of A).
this w o u l d i m p l y c o a t i n g thicknesses - 10-~ m, at the very least, are required. C l e a r l y m u c h m o r e w o r k is r e q u i r e d to develop m a t e r i a l s which will offer b e t t e r p r o t e c t i o n to the fibers as well as being c o m p a t i b l e with fiber f a b r i c a t i o n methods. T h e a u t h o r s wish to a c k n o w l e d g e s u p p o r t of this w o r k b y the A i r F o r c e
236
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
Office of Scientific Research, Directorate of Chemical and Atmospheric Sciences u n d e r G r a n t N o . A F O S R - 8 3 - 0 0 2 4 0 .
References [1] M.G. Drexhage, B. Bendow and C.T. Moynihan, Laser Focus 16 (1980) 62. [2] (a) C.T. Simmons, H. Sutter, J.H. Simmons and D.C. Tran, Mat. Res. Bull. 17 (1982) 1203; (b) M. Kiepmann, Durability of Fluorozirconate Glass in Water, Masters Thesis, UCLA, 1981. [3] W. Stober, Adv. Chem. Ser. 67 (1967) 161. [4] N. Uesugi et al., Electron. Lett. 19 (1983) 762. [5] S. Sakaguchi, T. Kiumura and S. Takahashi, Phys. Fiber Opt. Adv. Ceram. 2 (1982) 140. [6] S. Mitachi, S. Shibata and T. Manabe, Electron. Lett. 17 (1980) 128. [7] J. Lau, A.M. Nakata and J.D. Mackenzie, J. Non-Crystalline Solids 70 (1985) 233. [8] S.V. Gangal, Enc. Chem. Technol. 11 (1980) 24. [9] J. Teflon 11 (1970) 8. [10] R.A. Nyquist and R.D. Kael, Infrared Spectra of Inorganic Compounds (Academic Press, London and New York, 1971).
ENVIRONMENTAL
229
EFFECTS ON THE STRENGTH OF
FLUOROZIRCONATE GLASS FIBERS A . M . N A K A T A , J. L A U * a n d J.D. M A C K E N Z I E Department of Materials Science and Engineering, University of California, Los Angeles, California 90024, USA Received 26 October 1984
The tensile strength of Teflon FEP jacketed fluorozirconate glass fibers was found to be severely affected by the environment. The strenght of fibers placed in a humid atmosphere dropped to about half of their original strength after 4 days while the strength remained unchanged in a dry environment. Microscopy revealed substantial damage to the fiber surface due to the presence of moisture. It was concluded that Teflon FEP does not provide adequate protection as a moisture barrier.
1. Introduction H e a v y m e t a l h a l i d e glasses have excellent i n f r a r e d t r a n s m i s s i o n p r o p e r t i e s a n d have b e e n p r o p o s e d as p o s s i b l e c a n d i d a t e s for o p t i c a l waveguides. Of p a r t i c u l a r interest is the f l u o r o z i r c o n a t e system which has b e e n p r e d i c t e d to have theoretical m i n i m u m losses - 10 - 2 d b k m -1 [1]. In o r d e r to be conside r e d for such an a p p l i c a t i o n , one of the r e q u i r e m e n t s is that the m a t e r i a l m u s t either be sufficiently d u r a b l e o r a d e q u a t e l y p r o t e c t e d f r o m the e n v i r o n m e n t in o r d e r to preserve its m e c h a n i c a l strength a n d t r a n s m i s s i o n properties. C u r r e n t r e p o r t s on the chemical d u r a b i l i t y of f l u o r o z i r c o n a t e glasses consist largely of s o l u b i l i t y studies a n d exist for b u l k o r p o w d e r s a m p l e s only. T h e y show the s o l u b i l i t y o f f l u o r o z i r c o n a t e glasses in water to be - 1 0 - 4 g c m - 2 h - 1 [2] versus - 1 0 - 9 to 10 - t ° g c m - 2 h -1 for vitreous silica [3]. W a t e r has a l r e a d y b e e n shown to cause an increase in t r a n s m i s s i o n losses [4] a n d a decrease in the tensile strength o f silica o p t i c a l fibers [5]. Since the surface area to v o l u m e ratio of a glass fiber is r o u g h l y 500 times that of a b u l k piece of glass, one w o u l d expect to see m o r e p r o n o u n c e d t r a n s m i s s i o n losses a n d d r o p in m e c h a n i c a l strength in f l u o r o z i r c o n a t e glass fiber strength d u e to the p o o r e r c h e m i c a l d u r a b i l i t y as c o m p a r e d with vitreous silica. Teflon F E P has b e e n c o n s i d e r e d as a possible p r o t e c t i v e c o a t i n g for f l u o r o z i r c o n a t e glass fibers [6]. This p a p e r is c o n c e r n e d with the effect of a * Present address: W.R. Grace & Co., Washington Research Center, 7379 Route 32, Columbia, Maryland 21044, USA. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
230
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
Table 1 Glass composition (mol.%) 51% ZrF4 20% LiF 15% BaF2 5% PbF2 5% LaF3 3% A1F3
Distilled from Cerac 99.5% pure Alfa ultrapure grade Alfa ultrapure grade Alfa ultrapure grade Alfa 99.9% pure Cerac 99.5% pure
humid atmosphere on the strength-one type of fluorozirconate glass fiber coated with Teflon FEP.
2. Experimental procedure The composition of the glass used in this study is given in table 1. The procedures for glass preparation and fiber fabrication were described in a previous paper [7]. The glass was chosen for its favorable viscosity behavior around the fiber drawing temperature region. Both heating zones in the two-coil furnace [7] were held at 290°C during fiber drawing. The glass fibers obtained were between 100-170 /~m in diameter with a typical Teflon FEP coating thickness of 25-35/~m. Samples were cut into half meter lengths, and the ends were sealed with Teflon FEP. The fibers were simultaneously placed into either a chamber with a - 100% relative humidity atmosphere or into a dry box with a 99.998% dry nitrogen atmosphere. Both were kept at room temperature (23°C). Fiber samples were periodically removed for tensile testing on an Instron using a 25 m m testing length. Approximately 30 specimens were tested for each condition. Infrared absorption and scanning electron microscopy were also used in this study.
3. Results and discussion The average strength values for the " w e t " and " d r y " populations as a function of time are shown in fig. 1, the error bars indicate 95% confidence limits. The initial strength of the fibers in the 1 0 0 - 1 7 0 / t m diameter range was 150 MPa. The strength of the fibers stored in the humid atmosphere dropped to half of their original value in 4 days and then levelled off, while fibers stored in the dry atmosphere retained their strength. The reduction in strength is clearly related to the presence of moisture. According to manufacturer's literature, Teflon FEP is thermally stable at processing temperatures up to 370°C [8]. Thus, the thermal cycle through the fiber drawing temperature of 290°C should not affect the physical properties of Teflon FEP to any drastic degree. The permeation by vapors and gases in Teflon FEP is much lower than most organic polymers. Molecular diffusion
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
231
200 r--
150 "I" F-
c~
Teflon FEP jacketed fluoride fiber in dry environment
z
LU
n"
I00
LU
c~ nUJ
Teflon FEP jacketed 50
fluoride fiber in humid environment
0
5 TIME
10 (DAYS)
15
Fig. 1. Graph of average tensile strength versus time for fibers held in either a humid or dry atmosphere.
does however occur, and the water vapor transmission rate for Teflon FEP is 1.4 g / m 2 / 2 4 h for a 25.4 ~t thick film [9]. By assuming the average diameter of the fibers studied to be 135 /~m and that the fibers were coated with a 25.4 # m thick jacket, the time required to form a monolayer of water (approximate area of water molecule to be 0.44 A2) on the surface of the fiber was estimated to be less than 6 min. Therefore, in principle, an ample supply of water is available to attack the fiber within the time period of testing ( - 10 d). Permeation of water through the Teflon FEP jacket was also studied by infrared absorption. Three different spectra are shown in fig. 2. As can be seen, after exposure to the humid environment, new absorption bands appeared at 2.9/~m and 6/~m. The 2.9 # m band corresponds to the -OH stretching and the 6 /~m band is attributed to lattice water or water of hydration [10]. The intensity of the 2.9 ~tm decreased by a few per cent after the fibers were removed from the humid atmosphere and placed in ambient for a few hours. On the other hand, the intensity of the 6/~m band remained unchanged even after the fibers were placed in a vacuum for 18 h. The initial decrease in the 2.9/~m band has been attributed to the evaporation of surface moisture. The evidence seems to indicate, however, that a substantial amount of water remains trapped inside the Teflon FEP cladding, probably due to irreversible chemical reaction with the fluoride glass fiber. The infrared study also showed that the amount of water absorbed increased with the number of days in the wet atmosphere.
232
2.5
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers 4.0
3.0
5,0
MICRONS 8.0 7.0
8.0
10
20
30 50
A
4000
35'00 30'00 2 ~
20'00 18'(~0 lebO 14bO 12'00 10~
e~O
6~0
460
200
WAVENUMBER (cr'n"1) Fig. 2. Infrared spectra of A) Teflon FEP film; B) Teflon FEP jacketed fluoride glass fibers before exposure to a humid environment; and C) Teflon FEP jacketed fluoride glass fibers after five days in a humid environment.
SEM work was carried out to examine the effect of water on the fluoride fibers. The fiber surface was exposed by stripping the Teflon cladding away with a razor. A micrograph of a freshly drawn (pristine) stripped fiber is shown in fig. 3. A classic fracture pattern with a mirror region is evident and the surface of the fiber is smooth. Fibers that were stored in a dry atmosphere for 2 d revealed
Fig. 3. SEM micrograph of a stripped pristine fluoride glass fiber.
.4.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
233
Fig. 4. SEM micrograph of a bare fluoride glass fiber that has been exposed to a humid atmosphere for two days at room temperature (23°C). fracture p a t t e r n s identical to those of the pristine samples. Figure 4 is a n example of a bare fluoride glass fiber that was exposed to a h u m i d environm e n t for 2 days. C o n s i d e r a b l e damage a n d corrosion has taken place. The
Fig. 5. SEM micrograph of a Teflon FEP jacketed fluoride glass fiber that has been exposed to a humid environment for two days at room temperature (23°C).
234
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
Fig. 6. SEM micrograph of a Teflon FEP jacketed fluoride glass fiber that has been stored in a 99.998% dry nitrogen atmosphere for 24 days at room temperature (23°C).
mud-crack patterns observed are similar to those seen in other studies [2]. A Teflon FEP-clad fiber that was also exposed to the wet atmosphere for 2 days is shown in fig. 5. Although extensive corrosion has not occurred as in the case of the bare fiber, hackles on the surface of the fiber caused by the razor upon stripping away the Teflon coating can be seen. These hackles are not found on any of the pristine or dry samples and they indicate that a soft layer has formed on the surface. A direct comparison of Teflon FEP-clad fibers that were both stored for 24 days in either a wet or dry atmosphere is shown in figs. 6 and 7. The fiber that was kept dry still exhibits clear fracture patterns and smooth surfaces which are not found in any of the wet fiber samples. The wet fiber, on the other hand, showed a flat fracture face and substantial surface damage. In conclusion, it is believed that the water that permeated through the Teflon FEP jacket reacted with the glass fiber surface to form hydrated fluorides or hydroxides, thereby creating surface damage. This is reflected by the decrease in strength of the fibers. Probable paths for the hydration reaction are listed below: 1. MF. + x H 2 0 ---* MF n • xH20, or 2. MF. + x H 2 0 ~ M(OH). + nHF, or 3. MF. + x H 2 0 ~ M ( O H ) . - y ( H 2 0 ) + n H F + (x - n - y ) H 2 0 . Although Teflon FEP is less permeable to water than most organic polymers, it is still inadequate in as far as providing a protective coating for fluoride glass fibers. It could be argued that thicker coatings can provide better protection. However, if the useful life of a fiber is targeted at ] 0 9 S ( - 20 y),
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
235
Fig. 7. SEM micrograph of A) Teflon FEP jacketed fluoride glass fiber that has been stored in a humid atmosphere for 24 days at room temperature (23°C); and B) an enlargement of A).
this w o u l d i m p l y c o a t i n g thicknesses - 10-~ m, at the very least, are required. C l e a r l y m u c h m o r e w o r k is r e q u i r e d to develop m a t e r i a l s which will offer b e t t e r p r o t e c t i o n to the fibers as well as being c o m p a t i b l e with fiber f a b r i c a t i o n methods. T h e a u t h o r s wish to a c k n o w l e d g e s u p p o r t of this w o r k b y the A i r F o r c e
236
A.M. Nakata et al. / Environmental effects on strength of fluorozirconate glass fibers
Office of Scientific Research, Directorate of Chemical and Atmospheric Sciences u n d e r G r a n t N o . A F O S R - 8 3 - 0 0 2 4 0 .
References [1] M.G. Drexhage, B. Bendow and C.T. Moynihan, Laser Focus 16 (1980) 62. [2] (a) C.T. Simmons, H. Sutter, J.H. Simmons and D.C. Tran, Mat. Res. Bull. 17 (1982) 1203; (b) M. Kiepmann, Durability of Fluorozirconate Glass in Water, Masters Thesis, UCLA, 1981. [3] W. Stober, Adv. Chem. Ser. 67 (1967) 161. [4] N. Uesugi et al., Electron. Lett. 19 (1983) 762. [5] S. Sakaguchi, T. Kiumura and S. Takahashi, Phys. Fiber Opt. Adv. Ceram. 2 (1982) 140. [6] S. Mitachi, S. Shibata and T. Manabe, Electron. Lett. 17 (1980) 128. [7] J. Lau, A.M. Nakata and J.D. Mackenzie, J. Non-Crystalline Solids 70 (1985) 233. [8] S.V. Gangal, Enc. Chem. Technol. 11 (1980) 24. [9] J. Teflon 11 (1970) 8. [10] R.A. Nyquist and R.D. Kael, Infrared Spectra of Inorganic Compounds (Academic Press, London and New York, 1971).