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Influence of sulphate impurities on the IR transmission of InF3-based glasses
/OURNA L OF
Journal of Non-Crystalline Solids 140 (1992) 77-81 North-Holland
NON-CRYSTALLINE SOLIDS
Influence of sulphate impurities on the IR transmission of InF 3based glasses Y. M e s s a d d e q and M. Poulain Centre d'Etude des Matdriaux Avancis, Laboratoire de Chimie Minfrale, University de Rennes-Beaulieu, Avenue du G~n~ral Leclerc, 35042 Rennes C~dex, France
Fluoroindate glasses are more transparent in the IR spectrum than standard fluorozirconate glasses. The shift of the multiphonon absorption edge makes it necessary to reduce the residual content of sulphate impurities which induce a strong optical absorption between 8 and 9 ~m. T h e chemical elimination of sulphate has been implemented by a zirconium-assisted reduction or by the incorporation of fluoropolymers before melting.
1.
Introduction
Fluoroindate glasses attract an increasing interest because they could extend the possibilities of the standard fluorozirconate glasses already in use. Their extended infrared transmission range will allow the manufacturing of optical fibres operating up to 5.5 ~m, making possible the delivery of CO laser power [1] and increasing the sensitivity of fibre thermometry systems. As low multiphonon absorption also implies low multiphonon emission, these glasses have a potential for laser fibres and optical amplification, especially for wavelengths longer than 2 ~m. Ultimately, they shohld offer a theoretical minimum value of optical transmission losses lower than that of standard fluorozirconate glasses by one order of magnitude. However, the physical and chemical problems limiting the performances of actual glasses remain basically the same as for fluorozirconates: inhomogeneities, impurities, nucleation and crystallization. Among chemical impurities, complex anions make a special group since their main IR absorption bands are located in the multiphonon absorption edge. While less stable nitrate and carbonate anions are likely to be thermally de-
composed in fluoride melts above 700 ° C, phosphates and sulphates are more difficult to remove. Because hydrofluorhydric acid often contains traces of sulfuric acid and earth alkali sulfates precipitate with fluoride in synthesis processes, sulphate impurities are more common. This paper describes various attempts at reducing the sulphate concentration in bulk fluoroindate glasses using oxido-reduction processes.
2. Experimental In this study, the basic glass composition is 40ZnF2-20BaF2-20SrF2-20InF3. As reported in a previous paper [1], samples of several mm in thickness may be prepared from this composition using ammonium bifluoride processing [2] with a slow and progressive fluorination step in order to avoid the formation of oxyfluorides. Synthesis and annealing are carried out at room atmosphere. Starting materials are ZnF 2 and NH4HF 2 from Riedel de HaEn, BaF 2 Rectapur from Merck, SrF 2 from BDH, In20 3 from Preussag and ZrO 2 from MEL. Infrared absorption measurements are carried out using a Perkin-Elmer 1330 spectrophotometer.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
78
Y. Messaddeq, M. Poulain / IR transmission of InF3-based glasses
ing glass prepared in the same conditions does not exhibit any extrinsic absorption band, extending by almost 1 txm the cut-off wavelength corresponding to 50% of maximum transmission. The role of zirconium in the removal of sulphate impurities may be described on the basis of the oxido-reduction processes occurring for glass synthesis and fining. Early studies emphasize the grey or black inclusions contained in fluorozirconate glass samples prepared in neutral atmosphere [3]. One of the positive effects of reactive atmosphere processing is precisely to prevent the formation of these inclusions which have been found to contain low valence zirconium, according to the general equations
3. Results
The general method for removing sulphate anions from fluoride melts is based upon their chemical reduction: as sulphites are less stable thermally than sulphates, reduction will enhance the elimination of SO 4 groups as volatile SO 2. This may be achieved in various ways.
3.1. Reduction assisted by zirconium cations A first method was suggested by an accidental observation: while attempting to stabilize fluoroindate or fluorozincate glasses by incorporation of zirconium fluoride, it appears that the intrinsic absorption band around 9 ~zm is reduced or even removed. Consequently, several glass samples have been synthesized with ZrF4 as a dopant up to 2 mol%. Figure 1 shows, as a result, the comparison between the IR transmission curve of the basic glass and that of a glass doped with 0.5% ZrF4. The OH band at 2.9 txm is related to the incomplete control of water vapor during synthesis. The strong absorption band observed for the parent glass between 8 and 9 ixm is attributed to SO 4 anions because it is broader than that of phosphates. The zirconium-contain-
m r 4 + + e----> mr 3+,
(1)
m r 4 + + 2e---* Zr2+;
(2)
while Zr 3+ cations are identified by ESR in irradiated fluorozirconate glasses [4,5], microprobe analysis showed the the F / Z r ratio could be lower in reduced zones by a factor of two [3]. These observations indicate that zirconium IV may be reduced in fluoride glasses. This reduction may occur at low temperature during fluoride glass synthesis and the melt appears dark
~, (micrometer) 2,5 100
3
4
I
I
I
6
7
8
I
I
I
10
|
I
12 I
I
16 20 I
I
I
30 50
80 I
z
0
1L
60
I
e=3mm i
40
z<
I
\
F-
2O \ 4000
3000
200
1600
1200
800
400
V (cm'l) Fig. 1. IR transmission spectra for the basic glass (continuous line) and the glass doped with 0.5% ZrF4.
Y. Messaddeq, M. Poulain / IR transmission of InF3-based glasses
low valency zirconium fluorides, even beyond 1000 ° C, i.e., when ZrF4 is in the molten state. In order to confirm the role of reduced zirconium in the removal of sulphates, we have implemented the same series of preparations using hafnium in place of zirconium. While these two elements are almost identical chemically, hafnium is much less reductible than zirconium. Indeed glass samples doped with 0.5, 1 and 1.5% H f F 4 do not show any change in the magnitude of the sulphate absorption band.
when batch melting is just completed. The melt becomes clear only after the fining stage in which oxidation occurs. Thus, low valence zirconium is formed in a first step, probably around 500 ° C, then it reacts with sulphate anions at higher temperature, during the fining stage. A possible reaction scheme is 2Zr3++ SO42- ~ 2Zr4++ SO 2 + 202-.
(3)
Previous work by Ohsawa and Shibata [6] reports that sulphate anions are efficient for decreasing the optical scattering originated by reduced zirconium. The mechanism of formation of reduced zirconium in glass melt is still a matter of discussion. While some researchers suggest that it simply results from the chemical equilibrium 1 Zr4++ F - ~ Zr3++ gF2,
79
4. Discussion
4.1. Fluorocarbide processing The use of zirconium cations for the decomposition of sulphate anions has the disadvantage of limiting IR transmission. Figure 2 shows, as an example, the shift of the multiphonon absorption edge induced by the incorporation of zirconium fluoride in a fluoroindate glass. Among various possibilities, we have tested fluoropolymers. Early attempts by Kumar sug-
(4)
the present authors believe that it is greatly enhanced by reducing impurities in the raw starting materials. Several observations support this hypothesis: first, dark species are much less numerous when very pure reagents are used, and second it appears extremely difficult to synthesize
(micrometer) 5
6
7
80
g
8
9
.
10
.
12
.
. .......
16
20
30
. I%ZrF 4
[
60
{ 40 20
2000
160{
1200
V (cm
800
50
400
)
Fig. 2. Evolution of the IR transmission spectra as a function of the ZrF4 content.
[
80
Y. Messaddeq, M. Poulain / IR transmission of InF3-based glasses
( micrometer ) 2.5
3
4
I
100
I
5
~
7 I
8
9 10
I
I
I
12
|
16 20 30 I I I
50
"% 80
z 0
60
iI
e=4mm
\
4o rr
\
2O
4000
3000
2000
1600
1200
800
400
( c m "* ) Fig, 3. IR transmission spectrum of a sulphate containing glass (continuous line) after treatment by powdered PTFE (dashed line).
gested that the addition of teflon to glass melt could have a chemical action similar to that of reactive atmospheres [7]. A set of experiments has been carried out using P T F E powder as an additive in the batch. Starting with a sulphate containing sample, 5 wt% P T F E in weight is incorporated. Black fumes are observed at melting. Molten glass is exposed to room atmosphere until fumes disappear and the melt becomes clear. Then the glass preparation is completed in the normal way. As shown in fig. 3, no sulphate band is observed in the IR transmission of the treated glass. Chemical reactions occurring with P T F E may be written as 1/n(CF2),, + SO42--+ l e E 4 q- I c o 2
-}- S O 2 -~ O 2 - ,
(5)
3/rt(eP2) n + SO 2-
CF 4 + SO 2 + C + 2 F - + CO 2.
(6)
The excess of fluoropolymer undergoes a cracking reaction: ( m + 1 ) / n ( C F 2 ) --+ CmF2~+2 + C.
(7)
The carbon which is produced in reactions (6) and (7) will react with atmospheric oxygen and is eliminated as gaseous CO:. Alternatively, it may be oxidized by a reactive atmosphere. Reaction (6) would be ideal as anionic oxygen is not produced. Reactions (3) and (5) emphasized the formation of anionic oxygen which has been found to originate or to increase optical losses in fluoride glasses. Consequently, reaction (6) would be much more favorable, and it would be interesting to assess which reaction - (5) or (6) - preponderant.
5. Conclusion
Sulphate anions significantly limit the IR transmission of fluoroindate glasses. For windows of several mm in thickness, the IR cut-off may be decreased by almost 1 Ixm, while absorption losses of several d B / m are induced in the 4-5 txm transmission range of optical fibres. Residual sulphate impurities are eliminated by a chemical reduction in the glass melt, either via a small addition of zirconium fluoride in the
l< Messaddeq, M. Poulain / IR transmission of InF3-based glasses starting b a t c h or using P T F E powder. This latter m e t h o d offers two advantages: it does n o t i n d u c e any significant c h a n g e in the m u l t i p h o n o n abs o r p t i o n edge a n d the excess of r e a g e n t is easily e l i m i n a t e d by cracking a n d b u r n i n g in dry air atmosphere.
References [1] Y. Messaddeq and M. Poulain, Mater. Sci. Forum 67-68 (1991) 161.
81
[2] M. Poulain, J. Non-Cryst. Solids 56 (1983) 1. [3] M. Robinson, R.C. Pastor, R.R. Turk, D.P. Devor, M. Braunstein and R. Braunstein, Mater. Res. Bull. 15 (1980) 735. [4] R. Cases, D.L. Griscom and D.C. Tran, J. Non-Cryst. Solids 72 (1985) 51. [5] A. Abgrall, M. Poulain, G. Boisd~, V. Cardin and G. Maze, Proc. SPIE 618 (1986) 63. [6] K. Ohsawa and T. Shibata, J. Lightwave Technol. LT2 (1984) 602. [7] B. Kumar, in: Proc. 1st Int. Symp. on Halide Glasses, Cambridge, March 1982, paper 2C.
Journal of Non-Crystalline Solids 140 (1992) 77-81 North-Holland
NON-CRYSTALLINE SOLIDS
Influence of sulphate impurities on the IR transmission of InF 3based glasses Y. M e s s a d d e q and M. Poulain Centre d'Etude des Matdriaux Avancis, Laboratoire de Chimie Minfrale, University de Rennes-Beaulieu, Avenue du G~n~ral Leclerc, 35042 Rennes C~dex, France
Fluoroindate glasses are more transparent in the IR spectrum than standard fluorozirconate glasses. The shift of the multiphonon absorption edge makes it necessary to reduce the residual content of sulphate impurities which induce a strong optical absorption between 8 and 9 ~m. T h e chemical elimination of sulphate has been implemented by a zirconium-assisted reduction or by the incorporation of fluoropolymers before melting.
1.
Introduction
Fluoroindate glasses attract an increasing interest because they could extend the possibilities of the standard fluorozirconate glasses already in use. Their extended infrared transmission range will allow the manufacturing of optical fibres operating up to 5.5 ~m, making possible the delivery of CO laser power [1] and increasing the sensitivity of fibre thermometry systems. As low multiphonon absorption also implies low multiphonon emission, these glasses have a potential for laser fibres and optical amplification, especially for wavelengths longer than 2 ~m. Ultimately, they shohld offer a theoretical minimum value of optical transmission losses lower than that of standard fluorozirconate glasses by one order of magnitude. However, the physical and chemical problems limiting the performances of actual glasses remain basically the same as for fluorozirconates: inhomogeneities, impurities, nucleation and crystallization. Among chemical impurities, complex anions make a special group since their main IR absorption bands are located in the multiphonon absorption edge. While less stable nitrate and carbonate anions are likely to be thermally de-
composed in fluoride melts above 700 ° C, phosphates and sulphates are more difficult to remove. Because hydrofluorhydric acid often contains traces of sulfuric acid and earth alkali sulfates precipitate with fluoride in synthesis processes, sulphate impurities are more common. This paper describes various attempts at reducing the sulphate concentration in bulk fluoroindate glasses using oxido-reduction processes.
2. Experimental In this study, the basic glass composition is 40ZnF2-20BaF2-20SrF2-20InF3. As reported in a previous paper [1], samples of several mm in thickness may be prepared from this composition using ammonium bifluoride processing [2] with a slow and progressive fluorination step in order to avoid the formation of oxyfluorides. Synthesis and annealing are carried out at room atmosphere. Starting materials are ZnF 2 and NH4HF 2 from Riedel de HaEn, BaF 2 Rectapur from Merck, SrF 2 from BDH, In20 3 from Preussag and ZrO 2 from MEL. Infrared absorption measurements are carried out using a Perkin-Elmer 1330 spectrophotometer.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
78
Y. Messaddeq, M. Poulain / IR transmission of InF3-based glasses
ing glass prepared in the same conditions does not exhibit any extrinsic absorption band, extending by almost 1 txm the cut-off wavelength corresponding to 50% of maximum transmission. The role of zirconium in the removal of sulphate impurities may be described on the basis of the oxido-reduction processes occurring for glass synthesis and fining. Early studies emphasize the grey or black inclusions contained in fluorozirconate glass samples prepared in neutral atmosphere [3]. One of the positive effects of reactive atmosphere processing is precisely to prevent the formation of these inclusions which have been found to contain low valence zirconium, according to the general equations
3. Results
The general method for removing sulphate anions from fluoride melts is based upon their chemical reduction: as sulphites are less stable thermally than sulphates, reduction will enhance the elimination of SO 4 groups as volatile SO 2. This may be achieved in various ways.
3.1. Reduction assisted by zirconium cations A first method was suggested by an accidental observation: while attempting to stabilize fluoroindate or fluorozincate glasses by incorporation of zirconium fluoride, it appears that the intrinsic absorption band around 9 ~zm is reduced or even removed. Consequently, several glass samples have been synthesized with ZrF4 as a dopant up to 2 mol%. Figure 1 shows, as a result, the comparison between the IR transmission curve of the basic glass and that of a glass doped with 0.5% ZrF4. The OH band at 2.9 txm is related to the incomplete control of water vapor during synthesis. The strong absorption band observed for the parent glass between 8 and 9 ixm is attributed to SO 4 anions because it is broader than that of phosphates. The zirconium-contain-
m r 4 + + e----> mr 3+,
(1)
m r 4 + + 2e---* Zr2+;
(2)
while Zr 3+ cations are identified by ESR in irradiated fluorozirconate glasses [4,5], microprobe analysis showed the the F / Z r ratio could be lower in reduced zones by a factor of two [3]. These observations indicate that zirconium IV may be reduced in fluoride glasses. This reduction may occur at low temperature during fluoride glass synthesis and the melt appears dark
~, (micrometer) 2,5 100
3
4
I
I
I
6
7
8
I
I
I
10
|
I
12 I
I
16 20 I
I
I
30 50
80 I
z
0
1L
60
I
e=3mm i
40
z<
I
\
F-
2O \ 4000
3000
200
1600
1200
800
400
V (cm'l) Fig. 1. IR transmission spectra for the basic glass (continuous line) and the glass doped with 0.5% ZrF4.
Y. Messaddeq, M. Poulain / IR transmission of InF3-based glasses
low valency zirconium fluorides, even beyond 1000 ° C, i.e., when ZrF4 is in the molten state. In order to confirm the role of reduced zirconium in the removal of sulphates, we have implemented the same series of preparations using hafnium in place of zirconium. While these two elements are almost identical chemically, hafnium is much less reductible than zirconium. Indeed glass samples doped with 0.5, 1 and 1.5% H f F 4 do not show any change in the magnitude of the sulphate absorption band.
when batch melting is just completed. The melt becomes clear only after the fining stage in which oxidation occurs. Thus, low valence zirconium is formed in a first step, probably around 500 ° C, then it reacts with sulphate anions at higher temperature, during the fining stage. A possible reaction scheme is 2Zr3++ SO42- ~ 2Zr4++ SO 2 + 202-.
(3)
Previous work by Ohsawa and Shibata [6] reports that sulphate anions are efficient for decreasing the optical scattering originated by reduced zirconium. The mechanism of formation of reduced zirconium in glass melt is still a matter of discussion. While some researchers suggest that it simply results from the chemical equilibrium 1 Zr4++ F - ~ Zr3++ gF2,
79
4. Discussion
4.1. Fluorocarbide processing The use of zirconium cations for the decomposition of sulphate anions has the disadvantage of limiting IR transmission. Figure 2 shows, as an example, the shift of the multiphonon absorption edge induced by the incorporation of zirconium fluoride in a fluoroindate glass. Among various possibilities, we have tested fluoropolymers. Early attempts by Kumar sug-
(4)
the present authors believe that it is greatly enhanced by reducing impurities in the raw starting materials. Several observations support this hypothesis: first, dark species are much less numerous when very pure reagents are used, and second it appears extremely difficult to synthesize
(micrometer) 5
6
7
80
g
8
9
.
10
.
12
.
. .......
16
20
30
. I%ZrF 4
[
60
{ 40 20
2000
160{
1200
V (cm
800
50
400
)
Fig. 2. Evolution of the IR transmission spectra as a function of the ZrF4 content.
[
80
Y. Messaddeq, M. Poulain / IR transmission of InF3-based glasses
( micrometer ) 2.5
3
4
I
100
I
5
~
7 I
8
9 10
I
I
I
12
|
16 20 30 I I I
50
"% 80
z 0
60
iI
e=4mm
\
4o rr
\
2O
4000
3000
2000
1600
1200
800
400
( c m "* ) Fig, 3. IR transmission spectrum of a sulphate containing glass (continuous line) after treatment by powdered PTFE (dashed line).
gested that the addition of teflon to glass melt could have a chemical action similar to that of reactive atmospheres [7]. A set of experiments has been carried out using P T F E powder as an additive in the batch. Starting with a sulphate containing sample, 5 wt% P T F E in weight is incorporated. Black fumes are observed at melting. Molten glass is exposed to room atmosphere until fumes disappear and the melt becomes clear. Then the glass preparation is completed in the normal way. As shown in fig. 3, no sulphate band is observed in the IR transmission of the treated glass. Chemical reactions occurring with P T F E may be written as 1/n(CF2),, + SO42--+ l e E 4 q- I c o 2
-}- S O 2 -~ O 2 - ,
(5)
3/rt(eP2) n + SO 2-
CF 4 + SO 2 + C + 2 F - + CO 2.
(6)
The excess of fluoropolymer undergoes a cracking reaction: ( m + 1 ) / n ( C F 2 ) --+ CmF2~+2 + C.
(7)
The carbon which is produced in reactions (6) and (7) will react with atmospheric oxygen and is eliminated as gaseous CO:. Alternatively, it may be oxidized by a reactive atmosphere. Reaction (6) would be ideal as anionic oxygen is not produced. Reactions (3) and (5) emphasized the formation of anionic oxygen which has been found to originate or to increase optical losses in fluoride glasses. Consequently, reaction (6) would be much more favorable, and it would be interesting to assess which reaction - (5) or (6) - preponderant.
5. Conclusion
Sulphate anions significantly limit the IR transmission of fluoroindate glasses. For windows of several mm in thickness, the IR cut-off may be decreased by almost 1 Ixm, while absorption losses of several d B / m are induced in the 4-5 txm transmission range of optical fibres. Residual sulphate impurities are eliminated by a chemical reduction in the glass melt, either via a small addition of zirconium fluoride in the
l< Messaddeq, M. Poulain / IR transmission of InF3-based glasses starting b a t c h or using P T F E powder. This latter m e t h o d offers two advantages: it does n o t i n d u c e any significant c h a n g e in the m u l t i p h o n o n abs o r p t i o n edge a n d the excess of r e a g e n t is easily e l i m i n a t e d by cracking a n d b u r n i n g in dry air atmosphere.
References [1] Y. Messaddeq and M. Poulain, Mater. Sci. Forum 67-68 (1991) 161.
81
[2] M. Poulain, J. Non-Cryst. Solids 56 (1983) 1. [3] M. Robinson, R.C. Pastor, R.R. Turk, D.P. Devor, M. Braunstein and R. Braunstein, Mater. Res. Bull. 15 (1980) 735. [4] R. Cases, D.L. Griscom and D.C. Tran, J. Non-Cryst. Solids 72 (1985) 51. [5] A. Abgrall, M. Poulain, G. Boisd~, V. Cardin and G. Maze, Proc. SPIE 618 (1986) 63. [6] K. Ohsawa and T. Shibata, J. Lightwave Technol. LT2 (1984) 602. [7] B. Kumar, in: Proc. 1st Int. Symp. on Halide Glasses, Cambridge, March 1982, paper 2C.