Development of tellurium oxide and lead-bismuth oxide ... · below in figure 3. Table 2 follows with a summary of the DSC curves for this group of candidate TeO2-BaO-ZnO glass compositions

Development of tellurium oxide and lead-bismuth oxide glasses for mid-wave infra-red transmission optics

Beiming Zhou1, Charles F. Rapp1, John K. Driver1, Michael J. Myers1, John D. Myers1, Jonathan Goldstein2, Rich Utano3, Shantanu Gupta3

1 Kigre, Inc., 100 Marshland Road, Hilton Head Island, SC 29926 Ph# 843-681-5800, E-mail: [email protected]

2AFRL/RXPSO, 3005 Hobson Way, WPAFB, OH 45433 Ph# 937-255-9785, E-mail: [email protected]

3Fibertek, Inc., 13605 Dulles Technology Drive, Herndon, VA 20171 Ph# 703-471-7671, E-mail: [email protected]

ABSTRACT

Heavy metal oxide glasses exhibiting high transmission in the Mid-Wave Infra-Red (MWIR) spectrum are often

difficult to manufacture in large sizes with optimized physical and optical properties. In this work, we researched and developed improved tellurium-zinc-barium and lead-bismuth-gallium heavy metal oxide glasses for use in the

manufacture of fiber optics, optical components and laser gain materials. Two glass families were investigated, one based upon tellurium and another based on lead-bismuth. Glass compositions were optimized for stability and high

transmission in the MWIR. Targeted glass specifications included low hydroxyl concentration, extended MWIR transmission window, and high resistance against devitrification upon heating. Work included the processing of high purity raw materials, melting under controlled dry Redox balanced atmosphere, finning, casting and annealing. Batch melts as large as 4 kilograms were sprue cast into aluminum and stainless steel molds or temperature controlled bronze tube with mechanical bait. Small (100g) test melts were typically processed in-situ in a 5%Auo/95%Pto crucible. Our group manufactured and evaluated over 100 different experimental heavy metal glass compositions during a two year period. A wide range of glass melting, fining, casting techniques and experimental protocols were employed. MWIR glass applications include remote sensing, directional infrared counter measures, detection of explosives and chemical

warfare agents, laser detection tracking and ranging, range gated imaging and spectroscopy. Enhanced long range mid-infrared sensor performance is optimized when operating in the atmospheric windows from ~ 2.0 to 2.4µm, ~ 3.5

to 4.3µm and ~ 4.5 to 5.0µm. Keywords: Tellurium oxide glasses, Lead bismuth oxide glasses, Mid-wave infra-red transmission glasses, Heavy metal glasses, Glass multiphonon relaxation rate and energy gap, MWIR glass melting, MWIR glass processing

1. INTRODUCTION High refractive index heavy metal glasses may provide other unique properties besides high MWIR spectral transmission. For example, tellurite glass fibers provide nonlinear properties capable of stimulated Raman amplification [1]. Lead-bismuth glasses exhibit high non-resonant optical nonlinearity capable of Faraday effect optical switching [2,3]. We began this work with a tellurium-zinc-barium (65TeO2-13ZnO-10BaO-7Li2O) TBZ glass composition. It was chosen for its excellent stability with respect to crystallization. Work on lead-bismuth glasses began with a Dumbaugh (57.2PbO-25Bi2O3-17.8Ga2O3) PBG glass composition [4,5,6]. The heavier the metal and the weaker the M-O bond, the further the glass phonon edge is moved out. The infrared cutoff (near 0% transmission for 2 mm thickness) for these and other glasses is shown in table 1 below [7].

Glass Fundamental Absorption Cutoff Former of Network Former M-O Bond Wavelength

Silicate 8.4 - 11.5 micron 5 microns Gemanate 10.4 – 23.2 6 CaO –Al2O3 11.8 6 Tellurite 13.0 6.5 PbO-Bi2O3 > 30 8.75

Table 1 - Glass fundamental absorption and infrared cutoff wavelength

The long wavelength transmission limit of a glass is usually determined by the multiphonon absorption edge. This multiphonon edge is determined by the highest energy vibration of the glass host and may be increased through the use of compositions that include heavy metal oxides as the glass former. For the common glass forming oxides, the multiphonon edge and infrared transmission increase in the order B2O3 < P2O5 < SiO2 < GeO2 = Al2O3 < TeO2 < Bi2O3. Figure 1 shows the relationship between multiphonon relaxation rate and energy gap for various crystals and glasses [8]. A significant absorption generally begins about 1 to 2 microns before the “cutoff” wavelength. The shape of this absorption edge is greatly affected by other components in the glass. Small amounts of an ion with a high charge and/or light weight can affect the transmission at shorter wavelengths. Dry tellurite oxide glass samples based on TeO2-BaO-ZnO manufactured during this study provided high transmission out to 4.5 microns. This compares with a theoretical cutoff wavelength of 6.5 microns. Partially dry PbO-Bi2O3-Ga2O3 glasses showed (after removing the water bands from the spectra) an infrared wavelength transmission cutoff of 7.0 microns.

Fig. 1- Multiphonon relaxation rate vs. energy gap

2. TeO2 GLASS COMPOSITION STABILITY

Our first goal was to develop highly stable TeO2-BaO-ZnO (TBZ) glass compositions exhibiting low liquidus temperatures and high spectral transmission in the MWIR. Well stabilized glasses show a high resistance to crystallization during the glass casting, quenching and annealing processes. Candidate glass compositions did not include light weight or highly charged ions such as Nb5+ or W6+ that may contribute to a shorter IR wavelength absorption edge. Work began with melting small 100 and 200g TBZ glass compositions in uncontrolled air atmosphere. One test melt composition (PbBiGa1) did not include tellurium. Compositions were initially batched, melted and fined in-situ in Pto crucibles and later in Auo/Pto crucibles. We utilized a TA Instruments Q10 Differential Scanning Calorimeter (DSC) to measure transition (Tg) crystallization (Tx) and fluidity (Tf) temperatures. The DSC curves provided us with quantitative evidence for glass stability. Glasses were prepared and analyzed by DSC for their tendency to crystallize.

Heating and cooling rates of 20, 5 and 1 oC / min were used to evaluate different glass compositions. A highly stable glass DSC curve for TeO8-23 is shown in figure 2.

Fig. 2 - DSC curve for stable TeO8-23 composition, 20o/min to 300oC followed by 1o/min

A DSC curve taken on a less stable TeO8-22 composition exhibiting a crystallization temperature of 527 oC is shown below in figure 3. Table 2 follows with a summary of the DSC curves for this group of candidate TeO2-BaO-ZnO glass compositions. Figure 4 is a TBZ composition phase diagram plot establishing area of highest stability glasses. A glass stability summary for the initial 28 tellurite glass melts is shown in Table 3.

Fig. 3 - DSC curve for less stable TeO8-22 composition

20o/min to 300oC followed by 1o/min

Table 2 - TBZ glass composition data table with DSC performance results 20o/min to 300oC followed by 1o/min

Te08-23 glass composition containing 60% TeO2, 20% BaO, and 20% ZnO (mole%) was found to be the most stable. Figure 4 plot illustrates this showing this glasses location near the center of the most stable glasses in the ternary system. The glass composition was chosen for further characterization in terms of its liquidus temperature, TL. We measured the liquidus temperature of the glass Te08-23 via a quench technique. A sample of glass was melted in a Pto crucible and held at various temperatures for about 16 hours. The results of the test were as follows:

485 oC No Crystals 426 oC Heavy Crystals

The approximate liquidus temperature for this glass is about 460 oC. This is expected to be sufficiently low enough for homogenization and casting of good quality glass.

Fig. 4 - TBZ composition plot establishing area of highest stability glasses

Melt# Mole% Comments Tg Tx

Te08-1 70TeO2-13ZnO-10BaO-7Li2O Excellent 306 None

Te08-2 65TeO2-10ZnO-25BaO Good 100g 358 487


Te08-4 70TeO2-18PbO-12BaO Crystallized

Te08-5 70TeO2-30PbO Crystallized

Te08-6 73TeO2-17PbO-10BaO Crystals cooling

Te08-7 75TeO2-16PbO-9BaO Crystals cooling

Te08-8 80TeO2-14PbO-6BaO Good 200g 307 449

Te08-9 70TeO2-13ZnO-10BaO-7K2O Excellent 305 None

Te08-10 65TeO2-13ZnO-15BaO-7K2O Good 100g 306 (?)

PbBiGa1 57.2PbO-25Bi2O3-17.8Ga2O3 Good 100g

Te08-12 40PbO-17.5Bi2O3-12.5Ga2O3-30TeO2 Crystallized

Te08-13 70TeO2-20ZnO-10BaO Good 100g 333 None


Te08-15 60TeO2-15ZnO-25BaO Good 100g 364 (?)

Te08-16 60TeO2-10Ga2O3-13ZnO-10BaO-7Li2O Crystallized

Te08-17 60TeO2-10Bi2O3-20ZnO-10BaO Crystallized







Te08-26 55TeO2-22.5ZnO-22.5BaO Good 100g 359 None

Te08-27 50TeO2-25ZnO-25BaO Crystallized Phase Separate

Te08-28 85TeO2-7.5ZnO-7.5BaO Good 100g

Table 3 - TBZ glass stability summary of 28 tellurite glass melts

3. GLASS DRYING

Extra care was taken in glass batching, handling and processing to insure isolation of the glass batch and final glass from atmospheric moisture during handling. For example, all batch components were dried separately before use. The glass batch was then weighed, mixed and ground in a glove box with a dry air atmosphere to avoid water absorption. The grinding step is used to prevent "melt segregation" and to give a more homogeneous melt. Halide additions are often included to help remove water. Glass melting was performed under high purity ultra-dry air. The air is REDOX (REDuction OXidation) balanced specifically for each glass composition, crucible material and melt processing parameters. Micro Torr air filters from SAES were used to lower hydroxyl levels to low ppb levels over the glass melt atmosphere.

Multiphonon

Bulk Glass Absorption due to OH Absorption Measured

Peak Absorption at about 3 m Estimated at 4.5 m Fiber Loss

ln(1/T) log10(1/T) [OH] log10(1/T) at 1.0 m

(cm‐1) (cm

‐1) dB/m (ppm) (cm

‐1) dB/m

Massera, et al Glasses

Te‐Bi‐ZnO 3.77 1.64 1,640 213 0.118 25

Te‐Bi‐ZnF2 2.05 0.89 890 115

Te‐Bi‐Zn(O2 atm.) 1.39 0.60 600 78 12

Te08‐232 (4 % BaF2)* 0.016 0.007 7 1 *** 0.091 2 **

Te08‐23hm23 (4% BaF2)* 0.018 0.008 8 1 0.089

Te08‐57 (4% BaF2)* 0.014 0.006 6 1 0.090

Te08‐52 (20 % ZnCl2)* 0.021 0.009 9 1 0.061

Te08‐42hm4 (50 % ZnCl2)* 0.039 0.017 17 2 0.047

Te08‐63 (50 % ZnCl2) 0.021

Te08‐70 (40 % ZnCl2) 0.022

* Samples sent to Fibertek for Raman testing.

** Fibertek Measurement

*** 2 ppm estimated previously (see Kigre report dated May 24, 2009)

A dry/inert gas atmosphere is also employed during fiber processing. Glass absorption coefficients can be less than 0.02 / cm at 4.5 microns and possibly less than 0.005 / cm at 4.5 microns with the use of such dry atmosphere processing. We expect that with the employ of a double crucible fiber manufacturing process, we may readily manufacture low loss high quality single mode TBZ glass fiber exhibiting ~5dB/Km [9]. These absorption coefficients figures are defined as I/I0 = 10- α t, where t is in cm and α = 1/t x log10(I/I0). Therefore an absorption coefficient of 0.001/cm = 1 dB/m. Recently published work by Massera, et al. on development of low OH content tellurite glasses report OH levels in the 78 to 213 ppm region [10]. This may be compared to a number of “dry” TBZ glass compositions manufactured in this study. For example, oxyhalide TBZ glasses such as Te08-23α2 are ~100 times drier than Massera, et al. and any other TeO2 glasses reported to date. Table 4 provides an OH concentration comparison for dry TBZ glasses.

Table 4 - Dry TBZ glass OH concentration comparison

An example drying process used during this investigation utilized 4 mole % BaF2 added to a 4 kg melt of the Te08-23 composition. This produced melts with very low water concentration showing about 2 %/cm absorption at 3.45 µm (the peak of the OH band). It was estimated that this corresponded to about 2 ppm OH. The base glass absorption (absorption with no water) for Te08-23 and similar compositions continued to show a region of relatively high loss between 4.5 and 5.0 microns. These losses are primarily due to an absorption band that occurs between about 4.5 and 5.5 microns. The absorption did not appear to be associated with OH concentration. A spectral curve showing the 4.5µm shoulder is shown in figure 5. Figure 6 shows the FTIR absorption spectra for Te08-23.

Fig. 5 - Absorption shoulder on TBZ glass IR cutoff

Fig. 6 – FTIR absorption spectra for Te08-23 TBZ glass IR cutoff

Mole %

TeO2 ZnO BaO Na2O BaCl2 ZnCl2 Comments Tg Tx

Te08‐23 60 20 20 357 None

Te08‐26 55 22.5 22.5 359 None

Te08‐28 85 7.5 7.5

Te08‐29 65 12.9 17.1 5 310 429 (small)

Te08‐30 60 17.5 17.5 5 322 None

Te08‐31 60 21.9 13.1 5 327 None

Te08‐32 60 20 15 5 346 None

Te08‐33 60 20 10 10 349 None

Te08‐34 60 20 20 328 None

Te08‐35 80 20 347 491 (very large)

Te08‐36 60 20 20 See Below 371 454 (very large)

Te08‐37 65 13 15 7 309 525 (very small)

Te08‐40 40 30 30 Completely Crystallized, no glass.

Te08‐41 40 30 30 Completely Crystallized, no glass.

Te08‐42 50 25 25 See Below 324 473 (small)

Te08‐43 60 20 20 Same as ‐36 328 none

Te08‐44 50 25 10 15 349 480 (small)

Notes: Te08‐36 had a thick crystal layer at the Pt interface. Some glass present away from Pt.

The first Te08‐42 had a slight amount of crystals at one point on the surface.

The second Te08‐42 melt used dried BaF2 and therefore had slightly more BaF2. This melt

apparently separated into two layers, the top half of the melt was completely crystallzed,

while the bottom half was completely glass.

4. TBZ OXY-HALIDE GLASS DEVELOPMENT

We set out to determine if this band was due to an intrinsic TeO2 multiphonon absorption, or if some impurity exists in the TeO2 batch material such as CO2, SiO2, or P2O5. We developed new compositions to reduce the intensity of this band by a factor of ~ 2.5X. Our approach to reduce the base glass absorption was to reduce the concentration of TeO2 in the glass. Stable compositions with lower levels of TeO2 were expected to reduce the number of closely associated Te-O bonds and thereby reduce the intensity of the multiphonon absorption edge. Evidence of this is referenced in oxy-halides such as TeO2-Li2O-LiCl and TeO2-PbO-PbCl2 [11,12]. These heavy metal glass systems were shown to have very large glass forming regions extending below 30 mole % TeO2 [1]. We prepared and evaluated a series of TeO2-BaO-ZnO-BaF2-ZnF2 glasses for stability and IR transmission. The compositions evaluated are given in table 5.

Table 5 - TBZ Glass stability and composition data for TeO8-23 through TeO8-44

Glasses Te08-35, -36, and -40 correspond to glasses #9, #4, and #7 respectively in the paper by Plotnichenko, et al [1]. These glass compositions were prepared for comparison since they were reported to have excellent IR transmission and properties. TBZ oxy-halide glasses containing only TeO2-BaO-BaCl2 were found to be far less stable than those with similar amounts of Cl but also containing ZnO (or ZnCl2). This is expected since ZnCl2 is itself a glass former. Good comparison was made between the glasses Te08-34 (with Zn) and Te08-36 (with no ZnO). The -34 glass was very stable with no evidence of crystallization in the melt, or in the DSC run. On the other hand, glass -36 had a very heavy layer of crystallization at the Pt interface. Nearly half the melt was crystallized. A photograph of this melt is shown in figure 7. DSC runs of Te08-34 and -36 are shown in figures 8 and 9.

Fig. 7 - Photograph of the melt Te08-36 (Glass containing no ZnO)

Fig. 8 - DSC run Te08-36 (20% BaO) shows large crystallization peak

Fig. 9 - DSC run Te08-34 (20% ZnO) no crystallization peak

Melt Te08-42 was apparently on the very edge of an immiscibility region. The first melts of this composition used an anhydrous BaCl2 “as received”. This material actually contained 6.7 % water so that the first two melts contained a slightly lower BaCl2 than expected. The first melts were also quite stable with only a very slight region of crystallization apparent on the top of the cooled melt. Another melt was made with dried BaCl2 so the batched composition was very near the target. This melt contained several percent higher BaCl2 and acted entirely different than the first melt. After cooling and annealing, the melt with slightly higher BaCl2 was crystallized on the top half and glassy on the bottom half. A photograph of this melt is shown in figure 10.

Fig. 10 -Te08-42 with higher BaCl2 and evidence of crystallization

0.00

0.10

0.20

0.30

0.40

0.50

0.60

3.5 4.0 4.5 5.0 5.5 6.0 6.5

O.D. (cm

‐1)

Wavelength (m)

Comparison of Te08‐23 and Tr08‐42 Absorption Spectra (‐log10)(Corrected for Reflection and Water Absorption).

0

20

40

60

80

100

3.5 4.0 4.5 5.0 5.5 6.0 6.5

% T

Wavelength (m)

Comparison of % Transmission for Te08‐23 and Te08‐42(Corrected for Reflection and Water Absorption)

t = 1 cm

The IR transmission of the above glasses was seen to increase significantly as the TeO2 was decreased and the Cl increased. This is illustrated in figures 11 and 12.

Fig. 11 - Absorption spectra, red line = Te08-42 Te08-23 (60% TeO2 & 0% Cl) and Te08-42 (50% TeO2 & 25% BaCl)

Fig. 12 – Transmission spectra, red line = Te08-42

Te08-23 (60% TeO2 & 0% Cl) and Te08-42 (50% TeO2 & 25% BaCl) This same trend was seen for most of the other glasses as well. The optical density values were determined for different wavelengths for the glasses shown in the table 6.

Optical Density at:

4.545 m 4.762 m 5.00 m 5.263 m 5.556 m [Te] [O] [O] x [Te]

0 110.0 0.0

Te08‐32hm2 0.1064 0.1636 0.2054 0.2517 0.3476 60.2 155.5 9362.3

Te08‐32hm3 0.1053 0.1586 0.2033 0.2519 0.3367 60.2 155.5 9362.3

Te08‐33 0.0843 0.1278 0.17 0.2251 0.3031 60.4 151.0 9120.4

Te08‐34 0.0585 0.102 0.1535 0.2064 0.282 60.8 141.9 8628.3

Te08‐42hm2 0.0494 0.0822 0.1248 0.1698 0.2001 50.9 127.1 6463.0

Te08‐44 0.0611 0.1121 0.1448 0.1813 0.2251 50.0 135.0 6750.0

Te08‐26 0.1065 0.1667 0.1915 0.2176 0.2831 55.0 155.0 8525.0

Te08‐23hm4 0.1093 0.1628 0.1997 0.229 0.3016 60.0 160.0 9600.0

Te08‐23a1,2,3 0.1052 0.157 0.1984 0.242 0.3179 60.0 160.0 9600.0

Te08‐23hm11 0.1139 0.1709 0.2011 0.2429 0.3152 60.0 160.0 9600.0

Te08‐23hm5 0.1058 0.1557 0.1988 0.2402 0.3169 60.0 160.0 9600.0

Te08‐28 0.106 0.1726 0.2615 0.3645 0.483 85.0 185.0 15725.0

Te08‐35 0.0608 0.1085 0.1695 0.2377 0.3529 81.1 162.2 13151.2

Table 6 – TBZ glass optical density values for different wavelengths

A comparison was made of high Cl melts fond in literature with TBZ, oxy-halide melt Te08-32α2. The transmission spectra shown in the published paper by Plotnichenko, et al [1] suggest the BaCl2 compositions have a greatly improved IR transmission than the non-chloride compositions. In order to confirm this, a 1 mm thick sample of Te08-32α2 was prepared and the transmission spectrum measured for comparison. The measured spectrum of Te08-32 α2, with no correction for reflection, thickness, etc. is shown in figure13. The Te08-32 transmission is above all but the Plotnichenko Glass #7. The reason for such a great difference between the Te08-32 transmission and the published non chloride glasses is not known. One possibility is that the published spectra were measured on highly contaminated glasses. The spectra start at 5 µm so it is not possible to evaluate the water in the glass. The Plotnichenko paper notes that the glasses were melted in either Pt or ceramic crucibles. Ceramic crucible contamination may also contribute to the intense absorptions in the BaO-TeO2 and ZnO-TeO2 glasses. The Te08-23 glass has a lower TeO2 than the published BaO-TeO2 and ZnO-TeO2 glasses. Therefore, the transmission expected for a 1 mm thick piece of Te08-28 (85 % TeO2) was calculated from a 1 cm spectrum. No other correction for water, etc. was made. A 20 % reflection loss was assumed for the corrected spectrum. As can be seen in the figure 10 specta, the transmission is below that of Te08-32 but well above the published spectra of the Cl free glasses. Spectra (figure 10) raise concerns about very minor contamination with oxides such as SiO2. The specifications for SiO2 contamination in the TeO2 used in our melts was measured at < 1 ppm. However, chemical analyses showed approximately 16 ppm and 25 ppm Si in two of our alpha melts. Therefore, a melt was made in which 100 ppm SiO2 was added to the batch (10 mg SiO2 in a 100 g melt). The spectrum for these glasses is shown in figures 14 and 15. It is possible that a small absorption can be seen in the 3.5 to 4.0 µm region indicating SiO2 contamination. However, no significant difference is seen in the 4.5 to 5.5 µm region of concern.

‐0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

O.D. (cm

‐1)

Wavelength (m)

Absorption Spectrum (‐log10) of Te08‐23hm19 (100 ppm SiO2)(Corrected for Thickness and Water Absorption)

Fig. 13 - Comparison of IR transmission of TeO2-BaO-ZnO

glasses to Plotnixhenko, et al.

Fig. 14 - IR transmission of 100ppm SiO2 contaminated glass

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50

O.D

. (-l

og

10T

) (cm

-1)

Wavelength (microns)

Plot of Absorption Spectrum (-log10T) for Ave. of Te08-231,2&3(Corrected for reflection, thickness, and OH)

Fig. 15 - IR Transmission average of Te08-23α1, 2, & 3 glasses

5. TBZ GLASS MELTING & CASTING A 4.2 kg melt of glass composition Te08-23 was processed in a custom designed furnace. The batch included 4 mole% BaF2 added to help the removal of hydroxide. The batch was pre-melted at 750 oC and transferred to a grain stabilized platinum lined furnace for mixing, fining and casting operations. High purity dry air atmosphere (20% O2 & 80% N2) was introduced as both a cover gas and melt “bubbler.” The melt temperature was adjusted to 600 oC and a mechanical glass stirring mechanism was employed as a glass homogenizer. This was followed by 3 hours of fining at 650 oC and sprue casting at 550 oC. The casting mold we utilized is a custom built temperature controlled device comprised of a bronze tube and mechanical bait. Glass billets were then subjected to a rough fine anneal resulting in ~ 15 to 20nm/cm strain birefringence. Common multicomponent glasses such as silicates, phosphates and fluorophosphates exhibit densities in the ~ 2.5 to 3.5 g/cc range. A 4.0 kg melt of one of these glasses will result in an “as cast” glass billet ~ 2” dia. x 12 to 15” long as shown in figure 16. The tellurite and lead bismuth heavy metal oxide glasses have densities in the ~ 6 to 8 g/cc range. A 4 kg melt of these glasses produce 2” dia. x 6 to 8” long billets as shown in figure 17. Typically we find that glass processed in this manner yields a high percentage of “laser grade” optical quality material with good index homogeneity and few seeds, crystals and other defects.

Fig. 16 - Phosphate glass “as cast” billet ~ 2” dia. x 12 to 15” long

Fig. 17 - Tellurite glass “as cast” billet ~ 2” dia. x 8” long

6. TBZ GLASS PREFORM FABRICATION

Preform fabrication was done at Kigre, Inc. and fiber pulling at Clemson University. Two TBZ (Te08-23) glass preform rods (1” dia. x 6” long) were fabricated from as cast billets. A “dummy” preform glass rod was used for calibration of the fiber tower glass processing parameters as shown in figure 18 as seen before the completion of fabrication. A second TBZ glass preform used to produce coated fiber for testing and evaluation is shown in figure 19. A standard 0.5” diameter x 0.5” high notch was machined on the end of each bulk glass fiber preform rod to facilitate hanging in the Clemson’s Heathway 6.5 meter optical fiber draw tower. The fiber pulling tower is shown in figure 20.

Fig. 18 - TBZ dummy bulk glass preform

Fig. 19 – OD polished & notched TBZ bulk glass fiber preform

Fig. 20 - Clemson’s Heathway 6.5 meter optical fiber draw tower

7. TBZ GLASS FIBER PULLING

TBZ glass fiber preform Te08-23 alpha2 (25mm dia. x 70 mm long) was chosen for practice to obtain precision draw parameters due to its shorter length. TBZ glass preform Te08-23 alpha1 (25mm dia. x 110mm long) was designated for optimized fiber pulling operations. Clemson engineers pulled 500 meters of high quality TBZ glass fiber with various diameters. 100 meters of fiber with targeted fiber diameters were drawn continuously with no breaks. The fiber strength appeared to be good. Table 7 displays pulled fiber diameter statistics for each draw. Table 8 is an example of the fiber tower pulling and coating specifications. Figure 12 is a micrograph of the cleaved TBZ fiber end face.

Table 7 – Pulled TBZ glass fiber glass and coating diameter specifications

Table 8 – Draw#2 example fiber pulling and coating specifications

Fig. 21 – Cleaved TBZ glass 125µm dia. fiber end face

8. PbO-Bi2O3 GLASS COMPOSITION DEVELOPMENT

W. H. Dumbaugh has done a great deal of work in the development of infrared transmitting glasses based on the PbO-Bi2O3 system [2, 4, 7, 13, 15, 16, 17, 19, 20]. Of the common glass forming oxides, the multiphonon edge and infrared transmission of lead bismuth glasses exhibit the longest wavelength transmission out to 8.75um into the infrared [7]. This is due to bismuth’s metal oxide network former bonds. Simple PbO-Bi2O3 glasses are unstable with respect to crystallization. In order to form a stable glass additional components are required. Relatively stable glasses were found in the Bi2O3-PbO-ZnO-BaO and B2O3-PbO-Tl2O-CdO systems. Since the additional oxides all have relatively low fundamental vibrations in the glass, these compositions gave excellent infrared transmission. However, in order to form very stable glasses it is necessary to add a trivalent oxide such as Al2O3, Fe2O3 or Ga2O3. Because of their higher charge, these ions have a measureable effect on the infrared transmission of the glass. Al2O3 significantly degrades the infrared transmission. Ga2O3 has a smaller but significant effect on the IR transmission. Ga2O3 also has the greatest effect on the glass stability [20]. PbO-Ga2O3-Bi2O3 glasses were made in sizes up to 9 inch diameter x 1

inch thick with no crystals. This is apparently due to the fact that Ga2O3 also acts as a glass former. Binary PbO-Ga2O3 glasses can be formed that are quite stable [15, 18]. Therefore, the very stable PbO-Ga2O3-Bi2O3 glasses were chosen for further development in this study. Our glass composition development work began with Dumbaugh “EO” composition with (mole%) 57.2 PbO, 25.0 Ga2O3 and 17.8 Bi2O3. This composition had been cast into large samples with no crystallization. It was also used to measure the glass viscosity from about 10 to over 106 poise [15]. During this test, the glass was cooled from a high temperature to about 400 oC at a rate of 2 oC / minute. No crystals were found in the glass after the test. This would suggest that the glass is very stable. Sun, et al [28] recently reported stable glasses in the Li2O-BaO-PbO-Bi2O3 system. These glasses are interesting since they contain no highly charged ions (such as Ga3+) and should have improved IR transmission. Melting procedures were developed that produced PBG glasses with less scatter using Pb2O4 rather than PbO. In addition, stirring the melt in a Pt-Au crucible helped to eliminate scattering sites and improve the glass optical quality. Melts prepared in Al2O3 crucible showed very high continuous scatter due to an induced two liquid phase separation in the glass. Ga2O3 free glass compositions were manufactured to improve the IR transmission but unfortunately showed a high tendency towards crystallization. Most published work by Dumbaugh describes PBG glasses melted in Pt crucibles [2, 4, 7, 15, 20]. This is also the approach taken at Kigre as it typically provides for production of the highest optical quality glasses. Dissolution of platinum oxide in the glass shifts the short wavelength cutoff to longer wavelengths resulting in a change in color from yellow to red [20, 21]. This is illustrated in the glass samples of figure 22.

Fig 22 - PBG glass samples melted in platinum crucibles

Most other crucible materials have drawbacks related to the dissolution of the crucible. These include Au [20, 21, 27], Al2O3 [27, 28, 29], ZrO2 [27], and SnO2 [22]. Scattering sites have also been attributed to Pt [16], and elemental Bi [30]. An initial 34 small PBG glass test melts were manufactured, evaluated and compared in terms of melt temperature, color, optical quality, scatter sites, inclusions, bubbles and tendency for crystallization. Evaluation and analysis of spectra and DSC were used to further optimize the glass melt processing, stability and optical properties. Data on modified PBG glass compositions is shown in table 9. Glass 1hm10 looked good. Glasses 14hm, 15hm and 16hm exhibited heavy crystallization. Glasses hm17 and 18 looked cloudy. 19hm would not make a glass.

Table 9 - Summary of modified PBG glass test melt data

A summary of the PBG1 glass properties is shown in table 10.

Table 10 - Summary of PBG1 glass test melt data

Figure 23 shows the glass formation region in the PbO-Ga2O3-Bi2O3 as reported by Dumbaugh [33]. A series of glasses were melted in this system to determine their relative stability. Kigre measured values for Tg and the coefficient of thermal expansion are given for several of the glasses in the figure. The units for Tg and the CTE are oC and 10-7/oC respectively.

Fig. 23 - Glass formation region for PbO-Ga2O3-Bi2O3 [33]

An attempt was made to modify the glass composition with oxides such as Li2O, ZnO, BaO, Cs2O, and GeO2 to determine their effects on glass stability and other properties. A substitution of Cs2O for PbO was the one compositional modification that was found that accomplished all of the desired property changes. These included increasing the glass stability with respect to crystallization and lowering the refractive index, as well as increasing the thermal expansion and decreasing the softening point to compensate for the changes produced by an increase in Ga2O3. We found that a 10 mole % substitution of BaO, ZnO, or Li2O for PbO resulted in very unstable glasses that were completely crystallized on cooling. A 10 mole % substitution of GeO2 proportionally for all glass components resulted in a very good glass with excellent stability as judged by the DSC crystallization peaks. Almost no crystallization peak could be seen. A substitution of Cs2O for PbO resulted in an increase in the glass stability with an optimum stability at about 6 mole % Cs2O. Above 6 mole % the glass stability began to decrease again.The Cs2O for PbO also increased the coefficient of thermal expansion and decreased the Tg. This was a desirable effect in that it compensated for changes produce by an increase in Ga2O3 in the glass. The DSC traces for the glasses containing 0, 2, 4, and 6 mole % Cs2O showed the best stability for 2 mole% Cs2O. In summary, we started with Dumbaugh PbO‐Ga2O3‐Bi2O3

glass with the following Kigre composition designation PbBiGa1. Numerous hand melts of PbBiGa glass were manufactured, compared and evaluated for stability. PbBiGa26 composition with 2% Cs2O was chosen as exhibiting the best stability as indicated from DSC and scatter measurements.

9. PBG GLASS MELTING, CASTING & PULLING PbBiGa26 composition was used in the subsequent manufacture of bulk PBG glass. We fabricated three (3) bulk PBG glass fiber preforms. One was designated for use in fiber tower calibration and the other two for fiber pulling. The cast PBG 26 bulk glass “bars” are shown in figure 24. Fabricated PBG glass preforms are shown in figure 25. Two of the “notches” or “nubs” machined into the end of the glass fiber preforms (for hanging in the tower) were shattered during shipment to Clemson. The preforms were returned to Kigre and repaired. It was not until the pulling of the glass fiber did we realize just how brittle or inelastic this glass composition was. This particular composition must have produced a volume reduction in glass interatomic distance that dramatically increased the strain or brittleness of the glass [34]. PbBiGa26 glass composition and processing data are summarized in table 11.

Mol% Wt% Raw Material 100g Glass

PbO 55.14 45.8 Pb3O4 46.89

Bi2O3 21.43 37.16 Bi2O3 37.16

Ga2O3 21.43 14.95 Ga2O3 14.95

Cs2O 2.00 2.10 Cs2CO3 2.42 Density: d = 7.9110g/cm3 Pt crucible Premelt in Pt at 1000oC minimum time period Transfer to alpha Stir 4hrs at 1000oC Stir at 900oC and cast Cast Temperature: 825-850 oC Anneal Temperature: Tg = 342 oC Tf = 368 oC Crystallization Temperature: 804 oC

Table 11 - PbBiGa26 glass composition and processing data

PbBiGa26 (Mole %) PbO 55.14 Bi2O3 21.43 Ga2O3 21.43 Cs2O 2.00

PbBiGa1 (Mole %) PbO 57.2 Bi2O3 25.0 Ga2O3 17.8

Fig. 24 - Alpha Melt High Quality PBG Glass “Bars” 1/5 x 1.5 x ~ 8”

Fig. 25 - 1” Diameter Fibertek PBG Glass Preforms w/Broken Nubs

10. PBG GLASS FIBER PULLING

Three initial draw attempts were made using the 25mm dia. X 130mm long PbBiGa26 glass composition with little success as the required draw start temperatures of ~ 550 oC was found to be above the glass crystallization temperature. Various atmosphere cover gases were employed to reduce preform/fiber surface crystallization including dry air, nitrogen and argon. The preforms were reworked from 25mm to 10mm diameter and the tower was changed to accommodate Clemson’s small dual zone furnace. Furnace atmosphere was controlled with dry air filtered through desiccant chamber. This smaller furnace provided for more precise control of atmosphere, as well as a more narrow heat zone. The small dual zone furnace is limited to smaller preforms ~ 10mm OD X 100mm length. Three 10mm dia. preforms were fabricated from the remaining 25mm OD PBG preform. Temperature trials were completed by Clemson to verify actual heat

zone temperature versus set point. This helped to account for offset and insure a more precise draw temperature reducing the potential for crystallization. The offset of the actual furnace temp in the center of the heat zone was found to be approximately 40 oC cooler than that measured by the furnace controls when the preheat zone was set to 300oC Draw# 4 and 5 were also found to be problematic. Draw #6 produced coated fiber but was too weak to wind on the spool. Several meter long lengths of draw 6, 250µm glass/400µm coated fiber was set aside and saved. Draw #7 was successful in that 125 meters of coated PBG glass fiber was collected on a spool. The glass exhibited extreme brittleness and was very difficult to handle without breakage. Details from the draw report are as follows: No ramp was used for the preheat and main zone furnace settings. Preheat was set to 300oC and the main zone set to 490oC. Draw 7 fiber was readily coated. Feed rate was set at 3.0mm/min, with a line speed of ~ 18 meters/minute we finally reached the target 125 micron fiber. After the fiber was initially coated, the fiber diameter was increased to approximately 250 microns for the first 50 meters on the spool. The line speed was then increased in order to decrease the fiber diameter to approximately 125 microns. This enabled the fiber to be fed into the furnace at a higher rate, reduce the amount of crystallization and insure the strength of the fiber was sufficient to be coated. Approximately 75 meters of 125 micron fiber was then collected. The fiber strength was very low. A total of approximately 125 meters of coated PBG glass fiber was spooled.

CONCLUSION Tellurium-barium-zinc (TBZ) and lead-bismuth-gallium (PBG) heavy metal oxide glass compositions were developed for optimum stability and high transmission into the MWIR spectrum. A wide range of glass melting, fining, casting techniques and experimental protocols were employed. Glass melt process controls including dry air reduction-oxidation balanced atmosphere were employed to suppress crystallization, insure low hydroxyl concentration and extended the MWIR transmission window. Over 100 experimental glass compositions were manufactured and evaluated during a two and a half year period. High purity raw materials and platinum lined furnaces were utilized to minimize contamination and insure maximum transmission window IR cutoff. Our optimum TBZ glass composition provided properties suitable for the fabrication of various high quality optical components such as widows, lenses and fiber optics. A most promising PBG glass composition exhibited high transmission and good stability against crystallization. Unfortunately, this amorphous material’s extreme brittleness made it ill-suited for the manufacture of fiber optics.

ACKNOWLEDGEMENTS We wish to thank John Ballato, Paul Foy and Wade Hopkins of Clemson University, Center for Optical Materials Science and Engineering Technologies (COMSET). Thank you for your technical support, instrumental analysis and hard work. COMSET is one of the very few organizations in the world with the intelligence, technical prowess and advanced capabilities to pull uncooperative exotic materials into 100’s of meters of high quality fibers. We also wish to thank the United States Air Force Research Laboratory for provided financial support for this work under contract # FA8650-09-C-5406

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Development of tellurium oxide and lead-bismuth oxide ... · below in figure 3. Table 2 follows with a summary of the DSC curves for this group of candidate TeO2-BaO-ZnO glass compositions