- Email: [email protected]
IR transmission and corrosion of lead-bismuth gallate glasses
] O U R N A L OF
ELSEVIER
Journal of Non-Crystalline Solids 171 (1994) 299-303
IR transmission and corrosion of lead-bismuth gallate glasses H o n g y L i n *, L a r r y W . D e c h e n t ,
Delbert
E. Day, James O. Stoffer
Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401, USA
Received 26 February 1993; revised manuscript received 21 February 1994
Abstract Fibers with an average diameter of 27 Ixm were successfully drawn from a heavy metal oxide (HMO) melt and investigated for use in the mid-IR. The 69PbO-13BieO3-18Ga203 (wt%) glass fibers were relatively easy to prepare and had an average tensile strength of 472 MPa and a Weibull modulus of 7.6. The cut-off in the visible and infrared was 0.48 and 7 ~zm, respectively. The surface morphology of the corroded glass fiber varied with the pH of the corrosion solution. The dissolution rate of this glass decreased from 4.9 x 10 -6 to 9.6 x 10 -7 to 1.3 x 10 -7 g cm -2 min 1 for basic (pH = 10), acidic (pH = 4) and neutral (pH = 7) solutions, respectively. The techniques for preparing the fiber of this glass are also reported.
1. Introduction The discovery of heavy metal oxide ( H M O ) glasses (CdO, PbO and Bi203) has attracted interest because of their high non-resonant optical non-linearity and their good infrared transmittance up to about 7 txm [1-3]. The glass-forming tendency of H M O glasses is greatly improved with the addition of a stabilizer, such as G a 2 0 3 , which does not significantly reduce the optical properties [2]. Unlike silicate glasses, infrared (IR) transmitting glasses such as fluoride, chalcogenide and H M O glasses are difficult to pull into fibers because of their high crystallization tendency, fluidity and highly t e m p e r a t u r e - d e p e n d e n t
* Corresponding author. Present address: Mo-Sci Corporation, 4000 Enterprise Drive, PO Box 2, Rolla, MO 65401. Tel: + 1-314 364 2338. Telefax: + 1-314 364 4589.
viscosity which leads to a small working temperature range [1,4-6]. Heavy metal oxide glasses have I R transmission comparable to heavy metal fluoride glasses, but the H M O glasses are easier to melt and pull into fibers, and usually have a better chemical durability. In the present study, fibers of an H M O glass were prepared by direct pulling from the melt. The tensile strength of the fiber, and the chemical durability and optical properties of the glass in bulk form are reported.
2. Experimental Reagent grade powders of lead oxide (99.0%), bismuth oxide ( > 99.8%) and gallium oxide (99.99%) (Alrich Chemical Corporation) were mixed to yield a homogeneous mixture with a composition of 69.0PbO-13.0BizO3-18.0Ga203
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00239-J
300
H. Lin et al. /Journal of Non-Crystalline Solids 171 (1994) 299-303
(wt%). About 10 g of the batch were melted in a high purity alumina crucible at 950°C for 0.5 h. After swirling the melt several times, the melt was cast into a mold and annealed at 350°C for 1 h. A piece (5 mm x 5 mm × 20 mm) was cut from the cast bar and then remelted at 600°C in a P t / R h bushing. The melt was cooled rapidly to 540°C where fibers were pulled at a speed of 3 m / s . Fibers with a mean diameter of 27.2 _+ 5.8 Ixm were collected on a rotating drum, removed and stored in a vacuum desiccator prior to testing. The transmittance of the bulk (2 mm thick) glass in the visible (VIS) region was measured with a spectrophotometer (model 26, Beckman Instrument Inc.) at a scan rate of 100 n m / m i n . The IR spectra was measured by F T I R (PerkinElmer; model 1760-X). The chemical durability of the glass was determined by measuring the weight loss of the glass after immersion for different periods of time in buffer solutions (Fisher Scientific Company, SOB-101, SOB-107 and SOB-116) with pH values of 4, 7 or 10 (at 25°C). The chemical composition of the fibers was determined by energy dispersive x-ray spectroscopy. The surface morphology of the fibers was examined by a scanning electron microscope (SEM) (JEOL; model 35CF) after being immersed in the above-mentioned buffer solutions for various time periods. The tensile strength of individual glass fibers was measured using a gauge length of 5 cm and a cross-head speed of 0.5 m m / m i n (Instron; model TMSM, 500 g load cell).
100
,
,
,
,
,
,
,
,
BO z
70
oo.'°°°°
. . . . . . .
9 6o (/1 -- 5 o ,5
~ +o
~ 3o
,o 10
.
0 I "1 I 0.4 0.8
"'"
i .,oF c,...
I 0,8
I 1.0
I 2.0
/ I 3.0
WAVELENGTH
I 4-.0
I 5.0
I 8.0
7.0
8.0
(urn)
Fig. 1. Visible and infrared spectra of H M O glass (69PbO13BizO3-18Ga203) ( ) line) and (b) H M O F glass ( 6 1 P b O - 1 3 B i 2 0 3 - 1 8 G a 2 0 3 - 8 P b F 2 ) ( . . . . . . ). Sample thickness is 2.0 mm.
the cut-off in the VIS to shorter wavelengths with increasing PbF 2. For a glass containing 7.9 wt% PbF2, the VIS cut-off was 0.45 p.m. The IR spectra of these two glasses is almost identical as shown in Fig. 1. The failure probability versus fracture (tensile) stress for the H M O fibers indicates a Weibull modulus of 7.6 as shown by Fig. 2. The average tensile strength of these fibers is 472 + 60 MPa. The mean fiber diameter was 27.2 + 5.8 ixm. The tensile strength of the H M O fiber is comparable
/ r./"
9(: v
3. Results
,
90
=.
5C
o
Fig. 1 shows a typical V I S - I R transmittance curve for the H M O glass (2.0 mm thick) which is similar to previously reported measurements [1]. The polished bulk glass had a IR transmittance > 50% up to 6.5 txm and a cut-off at 7 Ixm. The large absorption at 3 ~m is due to the O H ions dissolved in the glass. The small absorption at 4.3 Ixm is unidentified. The cut-off in the VIS is 0.48 Ixm. Replacing the oxygen with fluorine (3.0 wt% PbO replaced by PbF 2) changed the color of the glass from reddish brown to yellow and shifted
o.
== =,
10
m
5
1 500
,,500 FRACTURE
STRESS
700
800
(MPo)
Fig. 2. Weibull plot for tensile strength of H M O glass fiber (69PbO-13Bi2 O3-18Ga203).
H. Lin et al. / Journal of Non-Crystalline Solids 171 (1994) 299-303 1.00
,
,
E o 0.75 v
,
tion for 1.5 h at 25°C. Surface cracks appeared after longer immersion times ( ~ 5 h) and some flakes (shell) with a thickness of 1 ~Lm and rich in bismuth separated from the fiber as shown in Fig. 4(a). A fine fibrous structure appeared on the surface of the fibers after they were immersed in the pH = 7 solution for 24 h as shown in Figs. 4(b) and (c). For the fibers immersed in the pH = 4 solution for 24 h, the surface was relatively smooth (Fig. 4(d).
,
V pH=lO.O [] p H = 7 . 0 0 pH-4.0
E
o, 0 . 5 0
m la.i
0.25
0 . 0 0 "---~ 0
~' 5
.. ~_ 10
I 15
301
.r'l
I 20
25
4. Discussion
IMMERSION TIME (h) aT 25aC
Fig. 3. Weight loss as a function of time at 25°C for H M O glass immersed in pH = 4, 7 and 10 buffer solutions.
to that of fluoride glass which is about 500 MPa
[8]. The H M O glass is quite stable in the ambient atmosphere. The H M O glass fibers showed no visible change nor any detectable change in their surface, when examined by SEM, after being exposed to the laboratory atmosphere for 6 months. The weight loss of the H M O glass after immersion in the three buffer solutions (Fig. 3) shows that corrosion was most rapid in the pH = 10 solution where the dissolution rate at 25°C was 4.9 x 10 -6 g cm -2 min -1. When immersed in the acidic (pH = 4) or the neutral solution (pH = 7), the dissolution rate was 9.6 and 1.3 × 10 -7 g c m - 2 m i n - 1, respectively. The surface morphology of corroded fibers varied with the pH of the solution as shown in Fig. 4. Flake-like particles approximately 5-10 I~m in size were present on the surface of the H M O fiber after immersion in the pH = 10 solu-
Table 1 Chemical composition (wt%) at different positions on H M O fiber after 5 h in p H = 10 solution and for as-pulled fiber Location
PbO
Bi20 3
Ga20 3
Surface layer Core As-pulled fiber Batch composition
12.2 65.3 68.7 69.0
85.2 16.4 13.2 13.0
2.6 18.3 18.1 18.0
In lead silicate glasses, the water content can be decreased by reacting with fluorine and water through the reaction: XF 2 + H 2 0 ~ XO + 2HF at high temperature, where X is a cation [7]. However, the reduction in the intensity of 3 ~m absorption band was not significant when 7.9 wt.% of PbF 2 was introduced, which suggests that a reduction of water content in the H M O glass cannot be achieved by a single process. The tensile strength of the H M O glass fiber is lower than that of silica or silicate fibers (tensile strength usually > 1000 MPa), most likely because of surface defects such as small crystals and the low strength of the cation-oxygen ( P b - O ) bond. It is possible that the strength of the H M O fibers can be increased by eliminating surface defects by improved pulling conditions. No comparison with other H M O glass fibers is currently possible due to the lack of previously reported data. However, it is noteworthy that the strength depends strongly on measuring conditions such as the strain rate, the gauge length, fiber diameter and testing environment. The above strength data for different types of fiber is intended only as a general comparison. The pH-dependent surface morphology of corroded H M O fibers, as shown by the SEM photos in Fig. 4, indicates different corrosion mechanisms. The chemical composition of the flakes (shell) and fiber core after immersion in the pH = 10 solution (see Table 1) indicated that the surface layer (shell) contained 85.2 wt% Bi203, by contrast with the fiber core which contained 16.4 wt% BizO3, the same as the as-pulled fiber.
302
H. Lin et al. /Journal of Non-Crystalline Solids 171 (1994) 299-303
T h e results o f the c h e m i c a l d u r a b i l i t y test show that the largest weight loss occurs in t h e p H = 10 solution w h e n P b O is p r e f e r e n t i a l l y l e a c h e d f r o m t h e surface, leaving b e h i n d a c o r r o d e d layer rich in Bi203. T h e relative e a s e o f l e a c h i n g Pb from the glass is b e c a u s e t h e Pb is t h e least tightly b o n d e d c o m p o n e n t since its b o n d s t r e n g t h is the lowest of t h e t h r e e c a t i o n s in the glass. T h e p r e f e r e n t i a l l e a c h i n g of Pb is also r e s p o n s i b l e for the f o r m a t i o n o f shell-core s t r u c t u r e o f the corr o d e d fiber. P o r e s f o r m e d on the surface layer of the fiber a f t e r Pb was p r e f e r e n t i a l l y l e a c h e d . T h e
liquid a b s o r b e d in this p o r o u s layer p r o d u c e d high c a p i l l a r y forces a n d cracks f o r m e d d u r i n g drying w h e n fibers w e r e t a k e n out o f the i m m e r sion solution. T h e fine fibrous d e p o s i t on t h e fiber surface suggests t h a t dissolution o f t h e glass followed by a p r e c i p i t a t i o n p r o c e s s occurs for H M O fibers i m m e r s e d in the p H = 7 solution. A u n i f o r m etching o f the fiber o c c u r r e d for the fibers i m m e r s e d in t h e p H = 4 solution, as indic a t e d by Fig. 4(d). T h e fiber surface is relatively s m o o t h w h e n c o r r o d e d in acidic solution a n d this is b e c a u s e t h e solubility o f t h e m e t a l h y d r o x i d e is
Fig. 4. (a) Appearance of the shell-core structure of HMO glass fibers after immersion in pH = 10 buffer solution for 5 h at 25°C. The outer shell is rich in bismuth while the composition of core was the same as the as-pulled fiber. (b), (c) Typical appearance of fine fibrous material on the surface of HMO glass fiber after immersion in pH = 7 buffer solution for 24 h. (d) Relatively smooth surface appearance of HMO glass fiber after immersion in pH = 4 buffer solution for 24 h. The white bar in lower right corner is 10 I~m in (a), (b) and (d) and 1 I~m in (c).
H. Lin et aL / Journal of Non-Crystalline Solids 171 (1994) 299-303
high in acidic solution. N o p r e c i p i t a t i o n o f l e a d or b i s m u t h h y d r o x i d e is e x p e c t e d in p H = 4 solution. T h e s m a l l e r d i a m e t e r o f t h e s e fibers also i n d i c a t e s t h a t a c o n s i d e r a b l e a m o u n t o f dissolution has o c c u r r e d in t h e p H = 4 solution.
5. Summary H e a v y m e t a l oxide glass fibers with an a v e r a g e tensile s t r e n g t h o f 472 M P a w e r e p u l l e d directly f r o m t h e m e l t at 540°C. This glass t r a n s m i t s to ~ 7 Izm in the I R region. T h e dissolution r a t e of t h e glass d e c r e a s e d from 4.9 x 10 - 7 to 9.6 a n d 1.3 x 10 - 6 g cm - 2 min -~ for basic ( p H = 10), acidic ( p H = 4) a n d n e u t r a l ( p H = 7) solutions, respectively. P r e f e r e n t i a l l e a c h i n g of Pb in the basic s o l u t i o n c a u s e d a shell-core s t r u c t u r e while fiber-like m a t e r i a l s r e p r e c i p i t a t e d on the fiber surface in t h e n e u t r a l solution. In acidic solution, dissolution o c c u r r e d uniformly.
This w o r k was s u p p o r t e d by D A R P A contract DSAG60-91-C-0150.
303 under
References [1] W.H. Dumbaugh, Phys. Chem. Glasses 19 (1978) 121. [2] J.C. Lapp, W.H. Dumbaugh and M.L. Powley, Riv. Staz. Sper. Vetro. 1 (1989) 91. [3] J.E. Shelby, J. Am. Ceram. Soc. 71 (1988) C254. [4] J.A. Wysocki and M.J. Liepmann, SPIE 1327 (1990) 238. [5] J.M. Parker and P.W. France, in: Fluoride Glass Optical Fibers, ed. P.W. France (CRC, Boca Raton, FL, 1990) ch. 2. [6] G.W. Morey, The Properties of Glass, 2nd Ed. (Reinhold, New York, 1954). [7] R.V. Adams, Phys. Chem. Glasses 2 (2) (1961) 50. [8] S.F. Carter, in: Fluoride Glass Optical Fibers, ed. P.W. France (CRC, Boca Raton, FL, 1990) ch. 9, p. 219.
ELSEVIER
Journal of Non-Crystalline Solids 171 (1994) 299-303
IR transmission and corrosion of lead-bismuth gallate glasses H o n g y L i n *, L a r r y W . D e c h e n t ,
Delbert
E. Day, James O. Stoffer
Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401, USA
Received 26 February 1993; revised manuscript received 21 February 1994
Abstract Fibers with an average diameter of 27 Ixm were successfully drawn from a heavy metal oxide (HMO) melt and investigated for use in the mid-IR. The 69PbO-13BieO3-18Ga203 (wt%) glass fibers were relatively easy to prepare and had an average tensile strength of 472 MPa and a Weibull modulus of 7.6. The cut-off in the visible and infrared was 0.48 and 7 ~zm, respectively. The surface morphology of the corroded glass fiber varied with the pH of the corrosion solution. The dissolution rate of this glass decreased from 4.9 x 10 -6 to 9.6 x 10 -7 to 1.3 x 10 -7 g cm -2 min 1 for basic (pH = 10), acidic (pH = 4) and neutral (pH = 7) solutions, respectively. The techniques for preparing the fiber of this glass are also reported.
1. Introduction The discovery of heavy metal oxide ( H M O ) glasses (CdO, PbO and Bi203) has attracted interest because of their high non-resonant optical non-linearity and their good infrared transmittance up to about 7 txm [1-3]. The glass-forming tendency of H M O glasses is greatly improved with the addition of a stabilizer, such as G a 2 0 3 , which does not significantly reduce the optical properties [2]. Unlike silicate glasses, infrared (IR) transmitting glasses such as fluoride, chalcogenide and H M O glasses are difficult to pull into fibers because of their high crystallization tendency, fluidity and highly t e m p e r a t u r e - d e p e n d e n t
* Corresponding author. Present address: Mo-Sci Corporation, 4000 Enterprise Drive, PO Box 2, Rolla, MO 65401. Tel: + 1-314 364 2338. Telefax: + 1-314 364 4589.
viscosity which leads to a small working temperature range [1,4-6]. Heavy metal oxide glasses have I R transmission comparable to heavy metal fluoride glasses, but the H M O glasses are easier to melt and pull into fibers, and usually have a better chemical durability. In the present study, fibers of an H M O glass were prepared by direct pulling from the melt. The tensile strength of the fiber, and the chemical durability and optical properties of the glass in bulk form are reported.
2. Experimental Reagent grade powders of lead oxide (99.0%), bismuth oxide ( > 99.8%) and gallium oxide (99.99%) (Alrich Chemical Corporation) were mixed to yield a homogeneous mixture with a composition of 69.0PbO-13.0BizO3-18.0Ga203
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00239-J
300
H. Lin et al. /Journal of Non-Crystalline Solids 171 (1994) 299-303
(wt%). About 10 g of the batch were melted in a high purity alumina crucible at 950°C for 0.5 h. After swirling the melt several times, the melt was cast into a mold and annealed at 350°C for 1 h. A piece (5 mm x 5 mm × 20 mm) was cut from the cast bar and then remelted at 600°C in a P t / R h bushing. The melt was cooled rapidly to 540°C where fibers were pulled at a speed of 3 m / s . Fibers with a mean diameter of 27.2 _+ 5.8 Ixm were collected on a rotating drum, removed and stored in a vacuum desiccator prior to testing. The transmittance of the bulk (2 mm thick) glass in the visible (VIS) region was measured with a spectrophotometer (model 26, Beckman Instrument Inc.) at a scan rate of 100 n m / m i n . The IR spectra was measured by F T I R (PerkinElmer; model 1760-X). The chemical durability of the glass was determined by measuring the weight loss of the glass after immersion for different periods of time in buffer solutions (Fisher Scientific Company, SOB-101, SOB-107 and SOB-116) with pH values of 4, 7 or 10 (at 25°C). The chemical composition of the fibers was determined by energy dispersive x-ray spectroscopy. The surface morphology of the fibers was examined by a scanning electron microscope (SEM) (JEOL; model 35CF) after being immersed in the above-mentioned buffer solutions for various time periods. The tensile strength of individual glass fibers was measured using a gauge length of 5 cm and a cross-head speed of 0.5 m m / m i n (Instron; model TMSM, 500 g load cell).
100
,
,
,
,
,
,
,
,
BO z
70
oo.'°°°°
. . . . . . .
9 6o (/1 -- 5 o ,5
~ +o
~ 3o
,o 10
.
0 I "1 I 0.4 0.8
"'"
i .,oF c,...
I 0,8
I 1.0
I 2.0
/ I 3.0
WAVELENGTH
I 4-.0
I 5.0
I 8.0
7.0
8.0
(urn)
Fig. 1. Visible and infrared spectra of H M O glass (69PbO13BizO3-18Ga203) ( ) line) and (b) H M O F glass ( 6 1 P b O - 1 3 B i 2 0 3 - 1 8 G a 2 0 3 - 8 P b F 2 ) ( . . . . . . ). Sample thickness is 2.0 mm.
the cut-off in the VIS to shorter wavelengths with increasing PbF 2. For a glass containing 7.9 wt% PbF2, the VIS cut-off was 0.45 p.m. The IR spectra of these two glasses is almost identical as shown in Fig. 1. The failure probability versus fracture (tensile) stress for the H M O fibers indicates a Weibull modulus of 7.6 as shown by Fig. 2. The average tensile strength of these fibers is 472 + 60 MPa. The mean fiber diameter was 27.2 + 5.8 ixm. The tensile strength of the H M O fiber is comparable
/ r./"
9(: v
3. Results
,
90
=.
5C
o
Fig. 1 shows a typical V I S - I R transmittance curve for the H M O glass (2.0 mm thick) which is similar to previously reported measurements [1]. The polished bulk glass had a IR transmittance > 50% up to 6.5 txm and a cut-off at 7 Ixm. The large absorption at 3 ~m is due to the O H ions dissolved in the glass. The small absorption at 4.3 Ixm is unidentified. The cut-off in the VIS is 0.48 Ixm. Replacing the oxygen with fluorine (3.0 wt% PbO replaced by PbF 2) changed the color of the glass from reddish brown to yellow and shifted
o.
== =,
10
m
5
1 500
,,500 FRACTURE
STRESS
700
800
(MPo)
Fig. 2. Weibull plot for tensile strength of H M O glass fiber (69PbO-13Bi2 O3-18Ga203).
H. Lin et al. / Journal of Non-Crystalline Solids 171 (1994) 299-303 1.00
,
,
E o 0.75 v
,
tion for 1.5 h at 25°C. Surface cracks appeared after longer immersion times ( ~ 5 h) and some flakes (shell) with a thickness of 1 ~Lm and rich in bismuth separated from the fiber as shown in Fig. 4(a). A fine fibrous structure appeared on the surface of the fibers after they were immersed in the pH = 7 solution for 24 h as shown in Figs. 4(b) and (c). For the fibers immersed in the pH = 4 solution for 24 h, the surface was relatively smooth (Fig. 4(d).
,
V pH=lO.O [] p H = 7 . 0 0 pH-4.0
E
o, 0 . 5 0
m la.i
0.25
0 . 0 0 "---~ 0
~' 5
.. ~_ 10
I 15
301
.r'l
I 20
25
4. Discussion
IMMERSION TIME (h) aT 25aC
Fig. 3. Weight loss as a function of time at 25°C for H M O glass immersed in pH = 4, 7 and 10 buffer solutions.
to that of fluoride glass which is about 500 MPa
[8]. The H M O glass is quite stable in the ambient atmosphere. The H M O glass fibers showed no visible change nor any detectable change in their surface, when examined by SEM, after being exposed to the laboratory atmosphere for 6 months. The weight loss of the H M O glass after immersion in the three buffer solutions (Fig. 3) shows that corrosion was most rapid in the pH = 10 solution where the dissolution rate at 25°C was 4.9 x 10 -6 g cm -2 min -1. When immersed in the acidic (pH = 4) or the neutral solution (pH = 7), the dissolution rate was 9.6 and 1.3 × 10 -7 g c m - 2 m i n - 1, respectively. The surface morphology of corroded fibers varied with the pH of the solution as shown in Fig. 4. Flake-like particles approximately 5-10 I~m in size were present on the surface of the H M O fiber after immersion in the pH = 10 solu-
Table 1 Chemical composition (wt%) at different positions on H M O fiber after 5 h in p H = 10 solution and for as-pulled fiber Location
PbO
Bi20 3
Ga20 3
Surface layer Core As-pulled fiber Batch composition
12.2 65.3 68.7 69.0
85.2 16.4 13.2 13.0
2.6 18.3 18.1 18.0
In lead silicate glasses, the water content can be decreased by reacting with fluorine and water through the reaction: XF 2 + H 2 0 ~ XO + 2HF at high temperature, where X is a cation [7]. However, the reduction in the intensity of 3 ~m absorption band was not significant when 7.9 wt.% of PbF 2 was introduced, which suggests that a reduction of water content in the H M O glass cannot be achieved by a single process. The tensile strength of the H M O glass fiber is lower than that of silica or silicate fibers (tensile strength usually > 1000 MPa), most likely because of surface defects such as small crystals and the low strength of the cation-oxygen ( P b - O ) bond. It is possible that the strength of the H M O fibers can be increased by eliminating surface defects by improved pulling conditions. No comparison with other H M O glass fibers is currently possible due to the lack of previously reported data. However, it is noteworthy that the strength depends strongly on measuring conditions such as the strain rate, the gauge length, fiber diameter and testing environment. The above strength data for different types of fiber is intended only as a general comparison. The pH-dependent surface morphology of corroded H M O fibers, as shown by the SEM photos in Fig. 4, indicates different corrosion mechanisms. The chemical composition of the flakes (shell) and fiber core after immersion in the pH = 10 solution (see Table 1) indicated that the surface layer (shell) contained 85.2 wt% Bi203, by contrast with the fiber core which contained 16.4 wt% BizO3, the same as the as-pulled fiber.
302
H. Lin et al. /Journal of Non-Crystalline Solids 171 (1994) 299-303
T h e results o f the c h e m i c a l d u r a b i l i t y test show that the largest weight loss occurs in t h e p H = 10 solution w h e n P b O is p r e f e r e n t i a l l y l e a c h e d f r o m t h e surface, leaving b e h i n d a c o r r o d e d layer rich in Bi203. T h e relative e a s e o f l e a c h i n g Pb from the glass is b e c a u s e t h e Pb is t h e least tightly b o n d e d c o m p o n e n t since its b o n d s t r e n g t h is the lowest of t h e t h r e e c a t i o n s in the glass. T h e p r e f e r e n t i a l l e a c h i n g of Pb is also r e s p o n s i b l e for the f o r m a t i o n o f shell-core s t r u c t u r e o f the corr o d e d fiber. P o r e s f o r m e d on the surface layer of the fiber a f t e r Pb was p r e f e r e n t i a l l y l e a c h e d . T h e
liquid a b s o r b e d in this p o r o u s layer p r o d u c e d high c a p i l l a r y forces a n d cracks f o r m e d d u r i n g drying w h e n fibers w e r e t a k e n out o f the i m m e r sion solution. T h e fine fibrous d e p o s i t on t h e fiber surface suggests t h a t dissolution o f t h e glass followed by a p r e c i p i t a t i o n p r o c e s s occurs for H M O fibers i m m e r s e d in the p H = 7 solution. A u n i f o r m etching o f the fiber o c c u r r e d for the fibers i m m e r s e d in t h e p H = 4 solution, as indic a t e d by Fig. 4(d). T h e fiber surface is relatively s m o o t h w h e n c o r r o d e d in acidic solution a n d this is b e c a u s e t h e solubility o f t h e m e t a l h y d r o x i d e is
Fig. 4. (a) Appearance of the shell-core structure of HMO glass fibers after immersion in pH = 10 buffer solution for 5 h at 25°C. The outer shell is rich in bismuth while the composition of core was the same as the as-pulled fiber. (b), (c) Typical appearance of fine fibrous material on the surface of HMO glass fiber after immersion in pH = 7 buffer solution for 24 h. (d) Relatively smooth surface appearance of HMO glass fiber after immersion in pH = 4 buffer solution for 24 h. The white bar in lower right corner is 10 I~m in (a), (b) and (d) and 1 I~m in (c).
H. Lin et aL / Journal of Non-Crystalline Solids 171 (1994) 299-303
high in acidic solution. N o p r e c i p i t a t i o n o f l e a d or b i s m u t h h y d r o x i d e is e x p e c t e d in p H = 4 solution. T h e s m a l l e r d i a m e t e r o f t h e s e fibers also i n d i c a t e s t h a t a c o n s i d e r a b l e a m o u n t o f dissolution has o c c u r r e d in t h e p H = 4 solution.
5. Summary H e a v y m e t a l oxide glass fibers with an a v e r a g e tensile s t r e n g t h o f 472 M P a w e r e p u l l e d directly f r o m t h e m e l t at 540°C. This glass t r a n s m i t s to ~ 7 Izm in the I R region. T h e dissolution r a t e of t h e glass d e c r e a s e d from 4.9 x 10 - 7 to 9.6 a n d 1.3 x 10 - 6 g cm - 2 min -~ for basic ( p H = 10), acidic ( p H = 4) a n d n e u t r a l ( p H = 7) solutions, respectively. P r e f e r e n t i a l l e a c h i n g of Pb in the basic s o l u t i o n c a u s e d a shell-core s t r u c t u r e while fiber-like m a t e r i a l s r e p r e c i p i t a t e d on the fiber surface in t h e n e u t r a l solution. In acidic solution, dissolution o c c u r r e d uniformly.
This w o r k was s u p p o r t e d by D A R P A contract DSAG60-91-C-0150.
303 under
References [1] W.H. Dumbaugh, Phys. Chem. Glasses 19 (1978) 121. [2] J.C. Lapp, W.H. Dumbaugh and M.L. Powley, Riv. Staz. Sper. Vetro. 1 (1989) 91. [3] J.E. Shelby, J. Am. Ceram. Soc. 71 (1988) C254. [4] J.A. Wysocki and M.J. Liepmann, SPIE 1327 (1990) 238. [5] J.M. Parker and P.W. France, in: Fluoride Glass Optical Fibers, ed. P.W. France (CRC, Boca Raton, FL, 1990) ch. 2. [6] G.W. Morey, The Properties of Glass, 2nd Ed. (Reinhold, New York, 1954). [7] R.V. Adams, Phys. Chem. Glasses 2 (2) (1961) 50. [8] S.F. Carter, in: Fluoride Glass Optical Fibers, ed. P.W. France (CRC, Boca Raton, FL, 1990) ch. 9, p. 219.