Striae-Free Chalcogenide Glasses

Abstract

A striae-free chalcogenide glass with uniform refractive index.

Claims

1. A product of the process comprising the steps of: melting chalcogenide glass inside a sealed silica ampoule; providing a 2-zone furnace comprising an upper zone and a lower zone wherein the upper zone is at a higher temperature than the lower zone and wherein the zones are independently controllable temperature zones; mixing by rocking the sealed silica ampoule inside the 2-zone furnace; placing the sealed silica ampoule in a vertical position; forming a glass melt as the upper zone is at a higher temperature than the lower zone; positioning the glass melt such that the glass melt is within the lower zone; and cooling slowly and quenching the temperature; reducing the temperature of the upper zone at a rate of 0.6 C./min to 370 C.; reducing the temperatures of the lower zone at a rate of 0.6 C./min to 260 C.; holding these temperatures for 12 hours; and thereby forming the striae-free chalcogenide glass.

2. The product of the process of claim 1 further comprising the step of preventing convection currents within the glass melt as the glass melt solidifies forming the chalcogenide glass.

3. The product of the process of claim 2 further comprising the step of preventing condensation of glass on the ampoule as the glass melt cools.

4. The product of the process of claim 1 further comprising the steps of: allowing for an initial melting of the chalcogenide glass prior to the step of mixing by rocking; and allowing for homogenization of the chalcogenide glass.

5. The product of the process of claim 4 further comprising the step of avoiding abrasion of the ampoule during the rocking step.

6. The product of the process of claim 1 further comprising the steps of: maintaining the temperature of the lower zone at a temperature of about 700 C. for about 24 hours; and maintaining the temperature of the upper zone at a temperature of about 100 C. greater than the temperature of the lower zone for about 24 hours.

7. A product of the process comprising the steps of: loading arsenic and sulfur precursors sufficient to constitute a 120 gram batch of glass with the composition of 39% at. arsenic (As) and 61% at. sulphur (S) or about 71.88 grams As and 48.12 grams S into a silica ampoule under an inert gas atmosphere; connecting the ampoule to a vacuum pump; evacuating the ampoule for 4 hours at 110.sup.5 Torr; sealing the ampoule; placing the ampoule inside a rocking furnace with a 45 angle of inclination wherein the furnace has a top zone and a bottom zone and wherein the zones are independently controllable temperature zones; heating and rocking the ampoule; heating the top zone and the bottom zone of the furnace at a rate of 3 C./min from room temperature; heating the top zone to 850 C.; heating the bottom zone to 750 C.; holding constant the temperature of the top zone (850 C.) and bottom zone (750 C.) for 10 hours; rocking the furnace at an inclination angle of 45 to facilitate mixing and homogenization of the elemental components; stopping the furnace motion; setting the furnace to a vertical position or 90 fixed angle; decreasing the temperature of the top zone at a rate of 1 C./min to 800 C.; decreasing the temperature of the bottom zone at a rate of 1 C./min to 700 C.; holding the furnace position and temperature profile for 24 hours to facilitate fining and settling of the glass melt; reducing the temperature of the top zone at a rate of 0.6 C./min to 370 C.; reducing the temperatures of the bottom zone at a rate of 0.6 C./min to 260 C.; holding these temperatures for 12 hours; forming a chalcogenide glass; removing the ampoule from the furnace; submerging the ampoule in a room temperature water bath for 10 seconds to quench the chalcogenide glass; annealing the chalcogenide glass by placing the ampoule in another furnace at 180 C. for 10 hours; and forming a striae-free chalcogenide glass.

8. A chalcogenide glass comprising a striae-free and high optical quality chalcogenide glass that is uniform and homogeneous.

9. The chalcogenide glass of claim 8 wherein the striae-free chalcogenide glass has no refractive index perturbations.

10. The product of the method to synthesize striae-free chalcogenide glass using melt processing comprising melting chalcogenide glass inside a sealed silica ampoule, providing a 2-zone furnace comprising an upper zone and a lower zone wherein the upper zone is at a higher temperature than the lower zone, mixing by rocking the sealed silica ampoule inside the 2-zone furnace, placing the sealed silica ampoule in a vertical position, forming a boule as the upper zone is at a higher temperature than the lower zone, positioning the boule such that the chalcogenide glass is within the lower zone, and cooling slowly and quenching the temperature and thereby forming the striae-free chalcogenide glass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 illustrates a schematic overview of the prior art process to synthesize chalcogenide glasses by melt processing. A sealed quartz ampoule (101) containing arsenic sulfide (201) glass melt precursors inside a rocking furnace (301) with a 45 inclination angle (401).

[0023] FIG. 2 illustrates a schematic overview of rocking furnace (302) in vertical (90) fixed position during Step 3 of the prior art process and measured temperatures at various points (501, 502, 503, 504, 505) along the length of the ampoule (102) containing As.sub.39S.sub.61 glass melt (302) in the prior art example immediately prior to glass quenching Step 4. In this example the top zone (312) and bottom zone (322) are set to the same temperature (440 C.) and beads of condensed glass (602) are seen to form at the top of the ampoule.

[0024] FIG. 3A illustrates a schematic diagram of thermal convection current (703) in the bulk As.sub.39S.sub.61 glass (203) and glass condensation drops (603) on top of the cooler ampoule (103) inside the furnace (303) of the prior art process.

[0025] FIG. 3B illustrates a photo of said ampoule with glass condensation above the glass melt.

[0026] FIG. 4 illustrates an IR-image of a human hand and fingers viewed through a 1 inch diameter, 2.5 inches thick disk (both faces polished) of As.sub.39S.sub.61 glass showing striae in the prior art bulk glass.

[0027] FIG. 5 illustrates a schematic overview of the furnace used in the current invention to synthesize chalcogenide glasses by melt processing. A sealed quartz ampoule (105) containing arsenic sulfide (205) glass melt precursors inside a rocking furnace (305) having two independently controllable temperature zones (315, 325) with a 45 inclination angle (405).

[0028] FIG. 6 illustrates a schematic overview of a rocking furnace (306) in vertical (90) fixed position during Step 5 of the process of the present invention and measured temperatures at various points (506, 507, 508, 509, 510) along the length of the quartz ampoule (106) containing arsenic sulfide glass melt (206). The top zone (316) was set to 360 C. and the bottom zone (326) was set to 260 C. in this example.

[0029] FIG. 7A illustrates a photo of ampoule with no glass condensation above the glass melt.

[0030] FIG. 7B illustrates an IR-image of a human hand and fingers viewed through a 1 inch diameter, 2.5 inches thick disk (both faces polished) of an As.sub.39S.sub.61 glass of the present invention showing no striae in the uniform bulk glass.

[0031] FIG. 8 illustrates an IR image of human hand and fingers viewed through arsenic sulfide glass disks produced from (left) new process with striae-free glass and (right) old conventional process with striae swirl in the glass.

DETAILED DESCRIPTION

[0032] Described herein is a new method to synthesize striae-free chalcogenide glass using melt processing.

[0033] One embodiment is described in the example using As.sub.39S.sub.61 glass.

Example 1

Process of the Present Invention to Make Striae-Free Arsenic Sulfide Glasses

[0034] Arsenic and sulfur precursors sufficient to constitute a 120 gram batch of glass with the composition of 39% at. As and 61% at. S (71.88 grams and 48.12 grams respectively) were loaded in a silica ampoule under an inert gas atmosphere.

[0035] The ampoule was connected to a vacuum pump and evacuated for 4 hours at 110.sup.5 Torr.

[0036] The ampoule was sealed using a methane/oxygen torch and placed inside a rocking furnace with a 45 angle of inclination and two independently controllable temperature zones (shown in FIG. 5) where it was heated and rocked according to a glass melting schedule, an example of which is shown for As.sub.39S.sub.61 glass in Table 2.

[0037] In Step 1, the top and bottom zones of the furnace were heated at a rate of 3 C./min from 20 C. (room temperature) to 850 C. (top) and 750 C. (bottom).

[0038] In Step 2, the temperature of the top zone (850 C.) and bottom zone (750 C.) were held constant for 10 hours while the furnace was rocked at an inclination angle of 45 to facilitate mixing and homogenization of the elemental components.

[0039] In Step 3, the furnace motion was stopped and the furnace was set to a vertical position (90 fixed angle). At the same time, the temperatures of the top zone and bottom zone were decreased at a rate of 1 C./min to 800 C. (top) and 700 C. (bottom). This furnace position and temperature profile were held for 24 hours to facilitate fining and settling of the glass melt.

[0040] In Step 4, the temperatures of the top zone and the bottom zone were reduced at a rate of 0.6 C./min to 370 C. (top) and 260 C. (bottom). These temperatures were held for 12 hours.

[0041] In Step 5, the hot ampoule was removed from the furnace, submerged in a room temperature water bath for 10 seconds to quench the glass, and was placed in another furnace at 180 C. for 10 hours to anneal the solid glass.

TABLE-US-00002 TABLE 2 Glass melting schedule for As.sub.39S.sub.61 glass composition in a two-zone furnace using the present invention. Temper- Temperature Heating ature ( C.) Rate ( C.) Bottom Dwell Step ( C./min) Top Zone Zone (Hours) Furnace Position 1 3 850 750 1 Horizontal 0 fixed 2 850 750 10 Rocking at 45 inclination 3 1 800 700 24 Vertical 90 fixed 4 0.6 360 260 12 Vertical 90 fixed 5 Water quench

[0042] Step 1 of the present invention allows for an initial melting of precursor materials prior to rocking for homogenization and reduces the potential of abrasion of the ampoule by solid precursors during the next step, which is not a part of the prior art process.

[0043] Step 2 here allows for a temperature gradient in the ampoule to encourage mixing and homogenization during rocking.

[0044] In Step 3 of the process of the present invention, the ampoule containing the glass melt is positioned such that the glass melt is largely confined within the bottom zone of the furnace and it is being fined at high temperature (700 C.) for a longer time than in the prior art method (24 hours in this example compared to 1 hour in the prior art method) which encourages homogenization.

[0045] The temperature of the top zone in this step is set to a higher temperature (800 C.) than the bottom zone, which has two benefits: 1) convection currents within the glass melt are reduced and 2) condensation and mass fluxing within the glass melt are prevented.

[0046] This temperature gradient eliminates the main causes of striae and therefore reduces compositional variations in the molten glass compared with the prior art.

[0047] In Step 4, the temperatures of the top zone and bottom zones are decreased slowly (0.6 C./min compared to 5 C./min in Step 3 of the prior art method) while keeping the top zone (360 C.) 100 C. hotter than the bottom zone (260 C.). Note that this differs from Step 3 of the prior art method, which allows for a natural temperature gradient within the furnace permitting the bottom of the glass to be hotter than the top as shown in FIG. 2.

[0048] This slow ramp rate and a consistent 100 C. higher temperature in the top zone prevent thermal convection within the glass in this stage which allows the uniform conditions in the molten glass to remain as the glass cools and prevents the reincorporation of surface glass into the bulk glass during this step.

[0049] FIG. 6 shows the actual measured temperatures of the top and bottom zones of the furnace during the dwell portion of Step 4 in this example.

[0050] During water quenching of Step 5, the viscosity of the glass increases rapidly as the glass melt cools but thermal stresses are less compared to the method of the prior art due to the slow cool rate and long dwell in Step 4 and the shorter quench time in the method of the present invention.

[0051] FIG. 7A shows a photo of an As.sub.39S.sub.61 glass of the present invention inside an ampoule with no glass condensation above the glass melt, and FIG. 7B shows an IR-image of a human hand and fingers viewed through a 1 inch diameter, 2.5 inches thick disk (both faces polished) of an As.sub.39S.sub.61 glass of the present invention with no detectable striae or refractive index perturbations in the bulk glass.

[0052] The process of the present invention produces striae-free and high optical quality chalcogenide glasses. The uniform and homogeneous glasses are free from refractive index perturbations.

[0053] The process of the present invention has several advantages over the conventional process of the prior art. For example, thermal convection heat loss, convection current and mass flux are eliminated within the bulk molten glass by setting the temperature of the top zone at least 100 C. (or thereabouts) higher than the bottom zone through all steps of the process.

[0054] Another advantage is the controlled slow cooling enables thermal equilibrium and steady state to occur in the molten glass melt throughout the process. This contributes to a striae-free, lower energy, and stable state of the glass melt just before quenching.

[0055] Still another advantage is striae-free and uniform compositions in the bulk glass eliminate refractive index perturbations enabling glass with higher optical quality for high-performance IR fibers and refractive optical elements. FIG. 8 shows a side-by-side comparison of striae free glasses prepared using the method of the present invention (left) and the method of the prior art (right).

[0056] This invention has been demonstrated using As.sub.39S.sub.61 glass in the above example but can also be applied to other chalcogenide glasses such as, but not limited to, AsS-based glasses with different compositions, AsSe, GeAsSe and GeAsSeTe-based glasses and other multi-component chalcogenide and chalcohalide glasses. The present invention could also be applied to the fabrication of other glasses, for example silicates, borates, fluorides, phosphates and others, or processing of viscous liquids, for example polymer melts, metals, salts and other liquids, where homogeneity is desired.

[0057] Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles a, an, the, or said is not construed as limiting the element to the singular.