U.S. patent application number 17/228313 was filed with the patent office on 2021-07-29 for manufacturing process for striae-free multicomponent chalcogenide glasses via convection mixing.
The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Shyam S. Bayya, Gryphon A. Drake, Daniel J. Gibson, Mikhail Kotov, Vinh Q. Nguyen, Jasbinder S. Sanghera.
Application Number | 20210230040 17/228313 |
Document ID | / |
Family ID | 1000005510356 |
Filed Date | 2021-07-29 |
United States Patent
Application |
20210230040 |
Kind Code |
A1 |
Nguyen; Vinh Q. ; et
al. |
July 29, 2021 |
MANUFACTURING PROCESS FOR STRIAE-FREE MULTICOMPONENT CHALCOGENIDE
GLASSES VIA CONVECTION MIXING
Abstract
A high optical quality multicomponent chalcogenide glasses
without refractive index perturbations due to striae, phase
separation or crystal formation made using a sealed ampoule with
chemical components enclosed inside, a two-zone furnace, a
convection heating/mixing step, and multiple fining steps.
Initially, the sealed ampoule is oriented vertically within the
two-zone furnace and heated to melt the chemical components
contained within, and a temperature gradient is created between the
top zone and the bottom zone such that the bottom zone has a higher
temperature. This temperature gradient causes convection currents
within the viscous liquid until it is sufficiently mixed due to the
convective flow. Then the temperature gradient is reversed such
that the top zone now has a higher temperature and the convective
flow ceases. The furnace temperatures are then reduced over a
period of time, with holds at multiple temperatures for fining and
cooling to form a solid glass.
Inventors: |
Nguyen; Vinh Q.; (Fairfax,
VA) ; Sanghera; Jasbinder S.; (Ashburn, VA) ;
Gibson; Daniel J.; (Falls Church, VA) ; Kotov;
Mikhail; (Silver Spring, MD) ; Drake; Gryphon A.;
(Potomac, MD) ; Bayya; Shyam S.; (Ashburn,
VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Family ID: |
1000005510356 |
Appl. No.: |
17/228313 |
Filed: |
April 12, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16226871 |
Dec 20, 2018 |
10974984 |
|
|
17228313 |
|
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62609559 |
Dec 22, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C03C 1/00 20130101; C03B
5/235 20130101; C03B 2201/86 20130101; C03B 5/06 20130101; B32B
17/00 20130101 |
International
Class: |
C03B 5/235 20060101
C03B005/235; B32B 17/00 20060101 B32B017/00; C03B 5/06 20060101
C03B005/06; C03C 1/00 20060101 C03C001/00 |
Claims
1. A striae-free chalcogenide glass made by the method comprising:
loading chalcogenide glass precursors into an ampoule, sealing the
ampoule, and placing the ampoule in a furnace, wherein said furnace
has an upper furnace element corresponding to a top zone and a
lower furnace element corresponding to a bottom zone, and wherein
the furnace is fixed without undergoing any rocking or external
mechanical agitation; heating the ampoule to form a glass melt from
the chalcogenide glass precursors, wherein said heating creates a
negative temperature gradient between the top zone and the bottom
zone, wherein the bottom zone has a higher temperature than the top
zone, and wherein said temperature gradient creates convection
currents in said glass melt; reversing said negative temperature
gradient and creating a positive temperature gradient between said
top zone and said bottom zone by adjusting said upper furnace
element to heat said top zone and adjusting said lower furnace
element to cool said bottom zone, wherein said bottom heat zone has
a lower temperature than said top heat zone; cooling said glass
melt while maintaining said positive temperature gradient, wherein
during said cooling there are holds at multiple temperatures; and
removing said ampoule from said furnace and water quenching said
glass melt resulting in a chalcogenide glass with uniform optical
quality and without refractive index perturbations due to striae,
phase separation or crystal formation.
2. The striae-free chalcogenide glass of claim 1, wherein the
chalcogenide glass precursors comprise arsenic, sulfur, and
selenium, tellurium, germanium, or any combination thereof.
3. A high-performance IR fiber made from the striae-free
chalcogenide glass of claim 1.
4. A refractive optical element made from the striae-free
chalcogenide glass of claim 1.
Description
PRIORITY CLAIM
[0001] The present application is a divisional application of U.S.
application Ser. No. 16/226,871, filed on Dec. 20, 2018 by Vinh Q.
Nguyen et al., entitled "MANUFACTURING PROCESS FOR STRIA-FREE
MULTICOMPONENT CHALCOGENIDE GLASSES VIA CONVECTION MIXING," which
claimed the benefit of U.S. Provisional Application No. 62/609,559,
filed on Dec. 22, 2017 by Vinh Q. Nguyen et al., entitled
"STRIA-FREE MULTICOMPONENT CHALCOGENIDE GLASSES VIA CONVECTION
MIXING." The entire contents of these applications and all
references cited within the present application are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to making striae-free
multicomponent chalcogenide glasses with uniform refractive index
using convection mixing.
Description of the Prior Art
[0003] Chalcogenide glasses comprise at least one chalcogen element
(S, Se or Te) and other elements including, but not limited to, Ge,
As, Ga, Sn, Sb and transmit infrared light (IR) from between about
1 .mu.m, or lower, to about 12 .mu.m or greater, depending on
composition. The infrared transmitting chalcogenide glasses and
optical fibers encompass the IR region of interest with numerous
applications including thermal imaging, temperature monitoring, and
medical applications. Also, chalcogenide glass fibers may be used
for IR missile warning systems and laser threat warning systems to
provide superior aircraft survivability, and high energy IR power
delivery using for example, but not limited to, CO (5.4 .mu.m) and
CO.sub.2 (10.6 .mu.m) lasers (Sanghera et al., "IR fiber optics
development at the naval Research Laboratory," SPIE, 3950, 180-185
(2000) and Sanghera et al., "Applications of Chalcogenide Glass
Optical Fibers at NRL," J. Optoelectronics and Advanced Materials,
3, No. 3, 627-640 (2001)). In addition, these fibers may be used
for remote fiber optic chemical sensor systems for military
facility clean-up and other industrial applications. High quality
infrared transmitting optical fibers enable applications in remote
chemical sensors to detect contaminants in groundwater,
environmental pollution monitoring, Raman amplifiers, optical
ultra-fast switches for telecommunications, fiber sources in the
infrared for sensors, biomedical surgery and tissue diagnostics,
and other civil/industrial process monitoring applications.
Chalcogenide glasses may also be used as bulk optical elements,
including windows, lenses, prisms, beam splitters and the like, and
must have high compositional uniformity and homogeneity in order to
maintain accurate control of light rays passing through the glass
and to achieve satisfactory optical results.
[0004] The properties of the chalcogenide-based glasses, including
optical, physical and thermal properties, such as refractive index,
dispersion, thermo-optic coefficient, glass transition temperature,
viscosity profile, hardness, fracture toughness, thermal expansion,
density, nonlinear index, fluorescence, wavelengths of transmission
and others, can be tailored through composition. However, some
chalcogenide glass compositions with technologically useful
properties may be thermodynamically unstable whereby crystallites
or other inhomogeneities, including phase-separated glassy regions
or devitrified regions, form within the glass during synthesis,
melting or processing. These inhomogeneities can manifest as
striae, cords, bands, veins, inclusions, bubbles, areas of stress
birefeingence and others ("TIE-25: Striae in optical glass,"
Technical Information Sheet, Optics for Devices, Schott AG, (2006)
http://www.us.schott.com/advanced_optics/english/knowledge-center/technic-
al-articles-and-tools/tie.html). When synthesized using the methods
of prior art, this thermodynamic instability limits the physical
size of the glass that may be fabricated (such as Ge.sub.30
As.sub.22 Se.sub.23 Te.sub.25), and in some cases optical quality
glass may not be made in any size due to crystal formation (such as
Ge.sub.13 As.sub.32 Se.sub.25 Te.sub.30) (Kokorina, Glasses for
Infrared Optics, CRC Press, Inc. (1996)). It is well-known in the
art of glass making that thermodynamically unstable glasses can be
synthesized by rapidly cooling the melt, but the glasses are not
optical quality due to striations that form upon rapid cooling.
[0005] It is well-known in the art of glass making that chemical
species may evaporate or sublime from the surface of a glass melt
at temperature resulting in a compositional inhomogeneity or
depletion layer near the surface of the glass melt. For this
reason, it is common to remove and discard the glass near the top
surface of a boule when optical homogeneity is desired. This
problem is especially prevalent in chalcogenide glasses where the
vapor pressures of the constituent elements can be quite high at
the glass melting temperature, as shown graphically in FIG. 1 for
sulfur 80, selenium 81, arsenic 82, tellurium 83, and germanium 84,
and the depletion layer may be very thick if pressure is not
controlled. For this reason, it is common practice to synthesize
chalcogenide glasses inside of sealed containers, such as a quartz
or silica glass ampoule, that has been evacuated or purged with an
inert gas, such as nitrogen, argon, or similar. Additionally, an
inert crucible can be placed inside the silica ampoule to prevent
contact of the melt with the ampoule if there is a detrimental
interaction or chemical reaction between them. Examples of inert
materials include vitreous carbon, graphite, and other suitable
materials. The sealed vessel contains the gaseous chemical elements
under positive pressure and prevents excessive evaporation. A
common practice in the synthesis and melting of chalcogenide
glasses is to rock, invert, shake gently, or otherwise agitate the
ampoule and the liquefied glass melt contained within in order to
thoroughly mix or homogenize the glass melt. As the homogenized
glass melt is subsequently cooled, chemicals in the vapor phase
above the melt must condense as the equilibrium partial pressure
decreases, and it is common that the resulting condensate deposits
on the surface of the glass melt. This condensate can be of varying
chemical composition, dictated by the specific vapor
pressure-temperature curves of the elements, resulting in a
compositional inhomogeneity near the surface of the glass melt. It
is common practice to remove and discard the top surface of the
glass boule to remove condensation and the depletion layer. For
some glass compositions, however, the glass melt may be very fluid
at the melt temperature, and either the condensate, the depletion
layer, or both may percolate deeper within the glass boule as it
solidifies carrying with it inhomogeneity in the form of striae or
cord due to variations in chemical composition. This is especially
true for glasses with multiple chemical components or with multiple
stable phases. An annealing step, heating the glass for an extended
period at a temperature near the glass transition temperature, is
typically performed to alleviate stress in the glass, which can be
another source of strain-induced optical inhomogeneity or stress
birefringence. It is well understood however that the viscosity of
the glass is sufficiently high during annealing that compositional
inhomogeneity cannot be removed by annealing. Such glasses are
difficult or impossible to fabricate with high homogeneity using
the prior art methods.
BRIEF SUMMARY OF THE INVENTION
[0006] The aforementioned problems are overcome in the present
invention which provides a method for synthesizing high optical
quality multicomponent chalcogenide glasses without refractive
index perturbations due to striae, phase separation or crystal
formation using a sealed ampoule with chemical components enclosed
inside, a two-zone furnace, a convection heating/mixing step, and
multiple fining steps. Initially, the sealed ampoule is oriented
vertically within the two-zone furnace and heated to melt the
chemical components contained within, and a temperature gradient
(in the vertical direction) is created between the top zone and the
bottom zone such that the bottom zone has a higher temperature.
This temperature gradient causes convection currents within the
viscous liquid until it is sufficiently mixed due to the convective
flow. Then the temperature gradient is reversed such that the top
zone now has a higher temperature and the convective flow ceases.
The furnace temperatures are then reduced over a period of time,
with holds at multiple temperatures for fining (removal of bubbles)
and cooling to form a solid glass.
[0007] The present invention provides a method for the synthesis of
high-purity chalcogenide glasses with excellent clarity and no
apparent optical inhomogeneity or striae without the need for
mechanical agitation, stirring or rocking. The glasses of this
invention may be used for infrared optical elements, lenses,
windows and infrared optical fibers.
[0008] The present invention enables synthesis of homogeneous,
optical quality glasses for some glass compositions that are not
possible using methods of the prior art. The chalcogenide glasses
and fibers described herein, and more specifically glasses and
fibers containing primarily arsenic, sulfur, selenium, tellurium,
and germanium with dopants including antimony, gallium aluminum,
indium, bismuth, tin, iodine, bromine, chlorine, fluorine,
lanthanum, and other elements up to about 10% atomic each, may be
synthesized according to the method of the present invention in
forms suitable for optical quality fibers and geometric optics
including windows, lenses and other devices.
[0009] The process of the present invention has the following
advantages over the processes of the prior art: [0010] Glass melt
homogenization is not derived from external mechanical agitation or
rocking as is done in the prior art, but from convective flow
within the glass melt and a evaporation-reflux cycle near the
surface of the glass melt, established by a negative temperature
gradient (bottom hotter than the top) within the two-zone furnace.
[0011] No mechanism of rocking is required in the furnace resulting
in more economical and more durable processing facilities for lower
cost glass. [0012] Glass is uniform optical quality and free of
striae or inclusions. This enables reliable manufacture of
high-performance IR fibers and refractive optical elements. [0013]
Glass synthesis cycle times can be greatly reduced by the present
invention.
[0014] These and other features and advantages of the invention, as
well as the invention itself, will become better understood by
reference to the following detailed description, appended claims,
and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows vapor pressure (in Pascal, Pa) curves of five
elements commonly used in the production of chalcogenide glasses
for infrared applications: sulfur, selenium, arsenic, tellurium,
and germanium as a function of temperature in degrees centigrade
(.degree. C.).
[0016] FIG. 2 is a schematic overview of the glass melting and
homogenization step of the process to synthesize infrared glasses
by melt processing using the present invention, specifically
without mechanical agitation. A sealed ampoule containing a glass
melt is inside a vertical furnace having two independently
controllable heaters with the top zone set to a temperature about
350.degree. C. lower than that of the bottom zone to provide a
negative temperature gradient. Convection currents and
evaporation-condensation provide a means of mixing the melt.
[0017] FIG. 3 is a schematic overview of the fining and cooling
steps of the present invention. A sealed ampoule containing a
well-mixed, homogenized glass melt is inside a two-zone furnace
oriented so as to have a common vertical central axis. The
temperature profile is measured using thermocouples along the
length of the ampoule. The upper heater was set to a temperature
about 100.degree. C. higher than that of the lower heater to ensure
a positive temperature gradient.
[0018] FIG. 4 shows temperature profiles of the furnace top zone,
furnace bottom zone, and the temperature gradient during the
synthesis of As.sub.39Se.sub.61 glass described in Example 1, Steps
1-7.
[0019] FIG. 5A shows a photograph of a sealed ampoule containing a
solid As.sub.39Se.sub.61 glass ingot with no soot or glass adhered
to the ampoule above the ingot. The top and bottom of the ingot
were removed at the dashed lines. FIG. 5B shows the resulting
cylinder from the removal of the ingot top and bottom. The end
faces of the resulting cylinder were ground and polished flat and
parallel.
[0020] FIG. 6A shows an infrared image of a 80.degree. C.
perforated steel plate viewed through a 55 mm diameter, 110 mm
thick face-polished cylinder of As.sub.39Se.sub.61 showing no
striae in the glass. FIG. 6B shows an infrared image of a human
hand and fingers viewed through a 55 mm diameter, 110 mm thick
face-polished cylinder of As.sub.39Se.sub.61 showing no striae in
the glass.
[0021] FIG. 7 is a schematic representation of an As--Se glass
cylinder showing the locations where the glass transition
temperature was measured.
[0022] FIG. 8 shows temperature profiles of the furnace top zone,
furnace bottom zone, and the temperature gradient during the
synthesis of As.sub.39Se.sub.61 glass described in Example 2, Steps
1-4.
[0023] FIG. 9 is a schematic overview of the glass melting and
homogenization steps used in the prior art (Examples 3, 4, and 5).
A sealed ampoule containing a glass melt is inside a rocking
furnace with a .+-.45.degree. angle of inclination.
[0024] FIG. 10 is a schematic diagram of the glass fining and
cooling steps of the prior art described in Example 3. A sealed
ampoule containing a glass melt is inside a rocking furnace in a
vertical position. Thermal gradients within the furnace cause
thermal convection currents in the glass melt, and evaporation and
condensation that drip back into the melt cause striae.
[0025] FIG. 11 is a photograph of a sealed ampoule containing a
solid ingot of As.sub.39S.sub.61 glass of the prior art described
in Example 3 with solid glass droplets adhered to the ampoule.
[0026] FIG. 12 is an infrared photograph of a human hand and
fingers viewed through an As.sub.39S.sub.61 glass of the prior art
described in Example 3 showing striae within the glass.
[0027] FIG. 13 is a schematic overview of the glass fining and
cooling steps of the prior art described in Examples 4 and 5. A
sealed ampoule and the glass melt contained therein is inside a
rocking furnace positioned vertically. The temperature profile is
measured by thermocouples positioned along the length of the
ampoule.
[0028] FIGS. 14A and 14B are infrared photographs comparing the
clarity of As.sub.39Se.sub.61 glass of the present invention with
that of the prior art. FIG. 14A shows a human hand viewed through
As.sub.39Se.sub.61 glass of the present invention. FIG. 14B shows a
human hand viewed through As.sub.39Se.sub.61 glass of the prior
art.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention provides a method for synthesizing
high optical quality multicomponent chalcogenide glasses without
refractive index perturbations due to striae, phase separation or
crystal formation. The method uses a sealed ampoule (typically a
quartz or silica ampoule) with chemical components enclosed inside,
a two-zone furnace, a convection heating/mixing step, and multiple
fining steps. Initially, the sealed ampoule is oriented vertically
within the two-zone furnace and heated to melt the chemical
components contained within, and a temperature gradient (in the
vertical direction) is created between the top zone and the bottom
zone such that the bottom zone has a higher temperature. This
temperature gradient causes convection currents within the viscous
liquid until it is sufficiently mixed due to the convective flow.
Then the temperature gradient is reversed such that the top zone
now has a higher temperature and the convective flow ceases. The
furnace temperatures are then reduced over a period of time, with
holds at multiple temperatures for fining (removal of bubbles) and
cooling to form a solid glass.
[0030] FIG. 1 shows vapor pressure (in Pascal, Pa) curves of five
elements commonly used in the production of chalcogenide glasses
for infrared applications: sulfur 80, selenium 81, arsenic 82,
tellurium 83, and germanium 84 as a function of temperature in
degrees centigrade (.degree. C.). The curves are plotted in
logarithmic scale on both axes.
[0031] The glass melting and homogenization step of the process to
synthesize infrared glasses by melt processing using the present
invention without mechanical agitation is shown schematically in
FIG. 2. As shown in FIG. 2, a sealed ampoule 10 containing a glass
melt 11 is placed inside a furnace 12 having two separate heating
elements, specifically a top, or upper, heating element 13 and a
bottom, or lower, heating element 14. These heating elements are
controlled separately such that they provide two distinct
temperature zones within the furnace, a top, or upper, zone 15 and
a bottom, or lower, zone 16. The furnace and ampoule are oriented
such that their common central axis 17 is approximately vertical
and stationary. During the homogenization or mixing step of the
present invention, the bottom zone is heated to be significantly
hotter than the top zone, in the example presented here, the
temperature difference between the two zones is about 350.degree.
C. The temperature gradient imparts a convective flow 18 within the
glass melt 11 that acts to mix the glass melt 11. Chemical elements
leave the surface of the glass melt 19, and enter the vapor phase
20 above the glass melt 11. Liquid droplets 21 containing the glass
constituents condense on the upper portion of the ampoule 10 and
may fall as condensation droplets 22 back into the glass melt 11
where they are reincorporated therein. Bubbles 23 may form within
the glass melt 11 or at the ampoule-glass melt interface and
migrate through the glass melt 11 to the surface 19. The migration
of bubbles 23 through the glass melt 11 can be vigorous and provide
significant mixing and agitation to the glass melt 11 without
external mechanical agitation, for example rocking, inversion or
shaking of the furnace 12, the ampoule 10, or both.
[0032] This invention has been demonstrated using
As.sub.39Se.sub.61 glasses in the examples but can also be applied
to other two-component and multi-component chalcogenide glasses
such as but not limited to arsenic, sulfur, selenium and tellurium
based glasses and other multi-component chalcogenide and
chalcohalide glasses containing germanium, antimony, gallium
aluminum, indium, bismuth, tin, iodine, bromine, chlorine,
fluorine, lanthanum, and other elements. 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.
Example 1: Process of the Present Invention to Make Striae-Free and
Crystallite-Free As.sub.xSe.sub.y Glass without Mechanical Rocking
or Stirring of the Glass Melt
[0033] The process of the present invention to make striae-free and
crystallite-free As.sub.xSe.sub.y glass (where x and y are atomic
percentages and x+y=100) without mechanical rocking or stirring of
the glass melt is shown in FIG. 2, and the steps are detailed in
Table 1 and demonstrated here by example as the fabrication of a
chalcogenide glass comprising 39% atomic As and 61% atomic Se and
having no observable optical inhomogeneity or striae. A total of
800 grams of elemental chemicals, arsenic and selenium, were loaded
inside a quartz-silica ampoule 10 under an inert nitrogen gas
atmosphere. The ampoule was connected to a vacuum pump, evacuated,
sealed, and placed inside a fixed vertical cylindrical bore, or
tube, furnace 12 having an upper furnace element 13 and a lower
furnace element 14 respectively corresponding to a top heat zone 15
and a bottom heat zone 16, and where each furnace zone was heated
according to a glass melting schedule, an example of which is shown
for As.sub.39Se.sub.61 glass in Table 1. The furnace in this
example is fixed in a vertical position throughout the process,
with no rocking or external mechanical agitation.
TABLE-US-00001 TABLE 1 Glass melting schedule for an As39Se61 glass
composition in a two-zone furnace using the present invention with
a second furnace for annealing. Heating Temperature Temperature
Rate (.degree. C.) (.degree. C.) Dwell Step (.degree. C./min) Top
Zone Bottom Zone (Hours) 1 +2 450 800 24 2 +0.6(T), 750 650 5
-0.6(B) 3 -0.6 650 550 5 4 -0.6 550 450 5 5 -0.6 450 350 5 6 -0.6
350 250 5 7 -0.6 250 150 5 8 Remove ampoule from furnace and quench
in 25.degree. C. water 9 165 165 10
[0034] In Step 1, shown schematically in FIG. 2, the bottom zone 16
and top zone 15 of the furnace 12 were heated at a rate of
+2.degree. C./min from 20.degree. C. (room temperature) to
800.degree. C. and 450.degree. C., respectively, to establish a
negative temperature gradient (wherein the bottom of the furnace is
hotter than the top), and these temperatures were held for 24
hours. During this step, the chemical constituents, As and Se, were
heated to form a glass melt 11, and the large temperature gradient
(350.degree. C.) caused significant convective currents 18 within
the glass melt 11. There was also a high rate of vaporization of
chemicals from the surface of the glass melt 19 into the vapor
phase 20 in the space above the melt, and bubbles 23 formed deep
within the glass melt and traveled to the surface 19 adding to the
gas phase material above the melt. This gas-phase material
contacted the upper surface of the ampoule where it cooled and
condensed forming droplets 21 on the ampoule 10 that subsequently
dropped or refluxed 22 falling back into the glass melt 11. These
ongoing processes, especially the boil-reflux cycle, were quite
vigorous and provided mixing of the glass melt without external
mechanical agitation as is required in the prior art. As shown in
FIG. 1, the equilibrium vapor pressures of selenium 81 and arsenic
82 are significantly higher at 800.degree. C., the temperature of
the glass melt, than they are at 450.degree. C., the temperature of
the top of the ampoule, supporting the continuous
evaporation-reflux cycle during this step.
[0035] Steps 2-7 are shown schematically in FIG. 3, which is a
schematic overview of the glass fining and cooling steps of the
process to synthesize infrared glasses by melt processing using the
present invention. A sealed ampoule 10 containing a well-mixed,
homogenized glass melt 24 is vertically oriented inside a furnace
12 having two separate heating elements, specifically a top, or
upper, heating element 13 and a bottom, or lower, heating element
14. These heating elements are controlled separately such that they
provide two distinct temperature zones within the furnace, a top,
or upper, zone 15 and a bottom, or lower, zone 16. The furnace 12
and ampoule 10 are oriented such that their common central axis 17
is approximately vertical and stationary. Thermocouples 101, 102,
103, 104, 105 are affixed to the ampoule 10 at five different
positions to monitor the temperature of the ampoule 10 during the
process. After the mixing step, the temperature gradient is
reversed such that the top zone is hotter than the bottom zone to
eliminate the convective currents and stop mixing of the glass.
[0036] Step 2 serves to stop the homogenization process and start
the fining, or bubble removal process in preparation for cooling
and solidification. In Step 2, the top zone 15 of the furnace was
heated at a rate of +0.6.degree. C./min to 750.degree. C. The
bottom zone 16 was cooled at a rate of -0.6.degree. C./min to
650.degree. C. to establish the positive temperature gradient of
100.degree. C. between the top and bottom zones. Because the
temperature gradient is now inverted from the prior step, the
convective current slows and eventually stops when thermal
equilibrium is reached. Also, the equilibrium vapor pressures of
arsenic 82 and selenium 81 (see FIG. 1) are now lower in the glass
melt, 650.degree. C., than near the top of the ampoule, 750.degree.
C.; and condensation at the top of the ampoule stops, ending the
vigorous evaporation/condensation cycle. This effectively stops the
mixing/homogenization process of Step 1 within the now well-mixed
homogeneous glass melt 24. Both zones were held for 5 hours to
enable fining or settling and removal of bubbles within the
well-mixed homogeneous melt 24.
[0037] Steps 3-7 serve to cool the glass melt slowly, while
maintaining a positive temperature gradient (where the top is
hotter than the bottom) to prevent unwanted convective currents.
Slow cooling minimizes the rate of condensation at the top of the
ampoule.
[0038] In Step 3, the top zone and bottom zone were cooled at a
rate of -0.6.degree. C./min to 650.degree. C. and 550.degree. C.,
respectively, and held for 5 hours while maintaining a temperature
gradient of 100.degree. C. between the top and bottom zones.
[0039] In Step 4, the top zone and bottom zone were cooled at a
rate of -0.6.degree. C./min to 550.degree. C. and 450.degree. C.,
respectively, and held for 5 hours while maintaining a temperature
gradient of 100.degree. C. between the top and bottom zones.
[0040] In Step 5, the top zone and bottom zone were cooled at a
rate of -0.6.degree. C./min to 450.degree. C. and 350.degree. C.,
respectively, and held for 5 hours while maintaining a temperature
gradient of 100.degree. C. between the top and bottom zones.
[0041] In Step 6, the top zone and bottom zone were cooled at a
rate of -0.6.degree. C./min to 350.degree. C. and 250.degree. C.,
respectively, and held for 5 hours while maintaining a temperature
gradient of 100.degree. C. between the top and bottom zones.
[0042] In Step 7, the top zone and bottom zone were cooled at a
rate of -0.6.degree. C./min to 250.degree. C. and 150.degree. C.,
respectively, and held for 0.5 hours while maintaining a
temperature gradient of 100.degree. C. between the top and bottom
zones. The temperature profile of the ampoule during the dwell
portion of Step 7 was measured with thermocouples 101, 102, 103,
104, 105 (FIG. 3) and is reported in Table 2.
TABLE-US-00002 TABLE 2 Ampoule temperature profile during Step 7
Measurement point (See FIG. 3) Temperature 101 251.degree. C. 102
250.degree. C. 103 152.degree. C. 104 151.degree. C. 105
150.degree. C.
[0043] FIG. 4 shows the temperature profiles of the furnace top
zone 74, the furnace bottom zone 75, and the temperature difference
between the furnace top and bottom zones, or the temperature
gradient 76, during the synthesis of the As.sub.39Se.sub.61 glass
of Example 1, Steps 1 through 7. A negative gradient or inverse
gradient is used for the first .about.35 hours of the example and
indicates that the temperature of the bottom zone is greater than
that of the top zone. A positive gradient indicates that the
temperature of the top zone is greater than that of the bottom zone
and reduces the drive for convective currents in the glass melt. A
positive gradient is used in this example from about hour 35 until
the ampoule is removed for quenching in Step 8.
[0044] In Step 8, the hot ampoule is removed from the furnace,
submerged in a room temperature (-25.degree. C.) water bath for
between 1-2 seconds to quench the glass melt into a solid
glass.
[0045] In Step 9, the ampoule and glass contained within are placed
in a second vertically oriented cylindrical bore, tube, furnace at
165.degree. C. for 10 hours to anneal or remove stress introduced
by quenching the solid glass. After 10 hours, the furnace, ampoule,
and glass are then cooled to room temperature and removed from the
annealing furnace. FIG. 5A shows a photograph of the solid glass
ingot 25 within the ampoule 10. Inside the sealed ampoule 10 is a
homogeneous, striae-free, infrared glass ingot 25 with a
composition of 39% atomic As and 61% atomic Se. The surface of the
ampoule above the glass ingot is clean and free of condensed beads
of glass. The glass ingot 25 was removed from the ampoule 10, and
cut at the marked positions 26, 27 in FIG. 5A. As shown in FIG. 5B,
the top and bottom faces of the resulting As--Se glass cylinder 28
were ground flat and parallel and subsequently polished.
[0046] A 110-mm tall 55-mm diameter As.sub.39Se.sub.61 glass
cylinder of the present invention was inspected using an infrared
camera. FIG. 6A shows an infrared photograph of a heated perforated
steel plate 29 viewed through the As--Se glass cylinder with
polished end faces 28. FIG. 6B shows an infrared photograph of a
human hand 31 viewed through the As--Se glass cylinder with
polished end faces 28, where the camera is focused on the distal
surface of the glass cylinder. There are no detectable striae,
inhomogeneity, or refractive index perturbations in the bulk
glass.
[0047] The glass transition temperature of a 2-component glass,
such as the As.sub.39Se.sub.61 glass of this example, is very
sensitive to the chemical makeup of the glass such that a 1% atomic
difference in composition can result in a 10.degree. C. difference
in glass transition temperature. Measurement of glass transition
temperature is therefore often used as a surrogate for
compositional variation in glass making. The As.sub.39Se.sub.61
glass from this example, shown schematically in FIG. 7, was cut
into pieces and the glass transition temperature was measured at
the 10 locations indicated in FIG. 7. Five samples were taken from
the top face of the cylinder 28 including the top left edge 33, top
left center 34, top dead center 35, top right center 36, and top
right edge 37, and five samples were taken from the bottom face
including the bottom left edge 38, bottom left center 39, bottom
dead center 40, bottom right center 41 and right edge bottom 42.
The glass transition temperatures measured at each location ranged
from 181.0.degree. C. to 182.1.degree. C. The accuracy of the glass
transition temperature measurement is .+-.0.5.degree. C. Table 3
lists the glass transition temperatures measured in this example.
The glass transition temperatures are within .+-.1.degree. C.,
indicating a uniform and homogeneous glass without compositional
variations in the bulk glass.
TABLE-US-00003 TABLE 3 Glass transition temperature measured along
the cross section of the As.sub.39Se.sub.61 bulk glass Measurement
location Glass Transition (see FIG. 7) Temperature, .degree. C. Top
33 181.7 Face 34 181.5 35 181.4 36 181.2 37 181.0 Bottom 38 182.0
Face 39 181.5 40 181.6 41 182.1 42 181.0
Example 2: Process of the Present Invention to Make Striae-Free and
Crystallite-Free As.sub.39S.sub.56Se.sub.5 Glass without Mechanical
Rocking or Stirring of the Glass Melt
[0048] Elemental chemicals arsenic, sulfur, and selenium sufficient
to make 80 g of As.sub.39S.sub.56Se.sub.5 glass (percentages atomic
basis) were placed inside an ampoule under a dry nitrogen
atmosphere. The ampoule was heat sealed under vacuum using an
oxygen-methane flame and placed inside a two-zone vertical tube
furnace having an upper heating element and a lower heating element
as shown in FIG. 2. The chemicals were melted, forming a glass
melt, and cooled to form a solid glass according to the schedule in
Table 4. During Step 1, when the top zone temperature was about
550.degree. C. and the bottom zone temperature was about
800.degree. C., a vigorous continual evaporation-reflux cycle could
be observed visually and audibly. FIG. 8 shows graphically the
temperature profiles of the furnace top zone 71, the furnace bottom
zone 72, and the temperature difference between the furnace top and
bottom zones, or the temperature gradient 73, during Steps 1
through 4 of the process in this Example. As in the previous
example of the present invention, the equilibrium vapor pressures
of the constituent elements, arsenic 82, sulfur 80, and selenium 81
(see FIG. 1) are lower at the top of the ampoule than in the glass
melt throughout Steps 1 and 2 driving the evaporation/condensation
reflux cycle that encourages rapid mixing of the melt. A negative
gradient or inverse gradient is used for the first .about.4.5 hours
of the example and indicates that the temperature of the bottom
zone is greater than that of the top zone. Vigorous
evaporation/reflux cycling is observed visually and audibly in this
Example when the magnitude of the inverse temperature gradient
exceeds about 250.degree. C., in other words from about hour 2
though about hour 3.5. A positive gradient indicates that the
temperature of the top zone is greater than that of the bottom zone
and reduces the drive for convective currents in the glass melt. A
positive gradient is used in this example from about hour 4.5 until
the ampoule is removed for quenching in Step 5.
TABLE-US-00004 TABLE 4 Glass melting schedule for
As.sub.39S.sub.56Se.sub.5 glass without mechanical agitation or
stirring Step Top Zone Bottom Zone duration Rate Temp. Rate Temp.
Step (hr) (.degree. C./min) (.degree. C.) (.degree. C./min)
(.degree. C.) Note 1 3.3 +2 420 +4 800 Melt precursors and
establish inverse temperature gradient for convection/reflux
mixing. 2 1 +5 700 0 800 Heat top zone to slow reflux 3 1 0 700 -5
600 Cool bottom zone to establish positive temperature gradient and
stop convection and evaporation/reflux cycle 4 5.6 -1 360 -1 260
Slow cooling for fining 5 Remove ampoule from furnace and quench in
25.degree. C. water 6 10 165 165 Anneal
[0049] The resulting glass was free of striae and transmitted
infrared light.
[0050] The prior art methods to synthesize a chalcogenide glass
from a melt are demonstrated here by Examples 3, 4, and 5.
Example 3: Prior Art Process to Make Arsenic Sulfide-Based
(As.sub.xS.sub.y) Glasses with Homogenization by Mechanical
Agitation
[0051] The common prior art practice for synthesis of bulk
chalcogenide glasses commonly employed in the industry is
demonstrated here by example and comprises five steps: 1)
melting/homogenization, 2) fining, 3) cooling, 4) quenching and 5)
annealing (Sanghera et al., "IR fiber optics development at the
Naval Research Laboratory," SPIE, 3950, 180-185 (2000); Sanghera et
al., "Applications of Chalcogenide Glass Optical Fibers at NRL," J.
Optoelectronics and Advanced Materials, 3 (No. 3), 627-640 (2001);
Kokorina, V. F., Glasses for Infrared Optics, CRC Press, Inc.
(1996); and Sanghera et al., "Development of Low-Loss IR
Transmitting Chalcogenide Glass Fibers," SPIE vol. 2396, 71-77
(1995)). FIG. 9 is a schematic overview of the glass melting and
homogenization step used in the processes to synthesize infrared
glasses by melt processing in the prior art (Examples 3, 4, and 5).
The rocking furnace 48 comprises an upper heating element 49 and a
lower heating element 50. In the prior art of Example 3, the
furnace elements are not necessarily independently controlled. In
the prior art of Examples 4 and 5, the furnace elements may be
independently controlled, but are set to the same temperature
during the homogenization step. A sealed quartz ampoule 51 with a
glass melt 52 therein is positioned within the furnace 48 such that
the furnace 48 and ampoule 51 share a common central axis 53. The
furnace 48, the ampoule 51, and the glass melt 52 are rocked
through a range of motion 54 of about .+-.45.degree. with a center
of rotation 55 located within the center of the furnace 48.
[0052] The melting/homogenization step of this Example 3 is shown
schematically in FIG. 9. Arsenic and sulfur precursors required to
make a glass with the composition of 39% atomic As and 61% atomic S
were loaded in a silica ampoule 51 under an inert nitrogen gas
atmosphere. The ampoule 51 was connected to a vacuum pump,
evacuated, sealed, and placed inside a rocking furnace 48 with a
.+-.45.degree. angle of inclination 54 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 5 (Sanghera et al.,
"Development of Low-Loss IR Transmitting Chalcogenide Glass
Fibers," SPIE vol. 2396, 71-77 (1995)). The rocking furnace 48 may
have one or more furnace elements that may or may not be controlled
independently of one another. In this example, the furnace 48 is
shown to have two heating elements: and upper or top element 49 and
a lower or bottom element 50 corresponding to a top and bottom
heating zone respectively. The sealed silica ampoule 51 and the
chemicals within were inserted into the rocking furnace 48 such
that they share a common central axis 53, and the lengthwise center
of the ampoule 55 corresponds to the center of rotation of the
rocking furnace.
TABLE-US-00005 TABLE 5 Example of a prior art glass melting
schedule for As.sub.39Se.sub.61 glass composition in a two-zone
furnace Tem- Temperature Heating perature (.degree. C.) Rate
(.degree. C.) Bottom Dwell Furnace Step (.degree. C./min) Top Zone
Zone (Hours) Position 1 3 750 750 10 Rocking at .+-. 45.degree.
inclination 2 -- 750 750 1 Vertical 90.degree. fixed 3 -5 440 440 2
Vertical 90.degree. fixed 4 Remove ampoule from furnace and quench
in 25.degree. C. water 5 180 180 10 Annealing furnace
[0053] In Step 1, as shown schematically in FIG. 9 and detailed in
Table 5, the top zone and bottom zone of the furnace were
controlled together, as is commonly done in the prior art, and
heated at a rate of 3.degree. C./min from 20.degree. C. (room
temperature) to 750.degree. C. at which point the rocking action of
the furnace was started. The furnace then remained at 750.degree.
C. for 10 hours and was actively rocked at an inclination angle of
.+-.45.degree. to facilitate mixing and homogenization of the glass
melt 52.
[0054] In Step 2, the fining step, the furnace motion was stopped
and the furnace was set to a vertical position (90.degree. fixed
angle), as shown schematically in FIG. 10, and held at temperature
(750.degree. C.) for 1 hour to facilitate fining and settling of
the glass melt. It is in this step that any bubbles introduced by
rocking migrate to the surface.
[0055] FIG. 10 is a schematic overview of the glass fining and
cooling steps of the process to synthesize infrared glasses by melt
processing in the prior art. The sealed quartz ampoule 51
containing a well-mixed, homogenized glass melt 56 is inside a
rocking furnace 48 that is in a vertical position. The rocking
furnace 48 comprises one or more heating elements 49, 50 that, in
this case, are controlled only as a group and are set to the same
temperature or are set to a temperature difference of less than
about 50.degree. C. Convective heat loss within the furnace 57
ensures that the upper portion of the furnace 58 is cooler than the
lower portion 59 and imparts a temperature gradient. Additionally,
during cooling, the glass melt at the bottom of the ampoule retains
heat and cools more slowly than the portion of the ampoule above
the melt, resulting in a cooling lag that contributes to the
temperature gradient. The temperature gradient was measured using
thermocouples affixed to the ampoule 51 at five positions 201, 202,
203, 204, 205 during the process. During the fining step, this
temperature gradient imparts convective currents 60 within the
glass melt 56 which has a low viscosity. Chemical elements leave
the upper surface 61 of the glass melt, and enter the vapor phase
62 above the glass melt. During the cooling step of the process,
the top portion of the silica ampoule, with a low thermal mass,
cools faster than the bottom portion of the silica ampoule that is
in contact with the glass melt 56, which has a high thermal mass,
and as such, liquid droplets 63 containing the glass constituents
condense on the upper portion of the silica ampoule 51. These
droplets have a different composition than the overall glass melt
and may fall 64 back into the glass melt. This condensation occurs
through the duration of the cooling step while the viscosity of the
glass melt gradually increases. The droplets that fall later in the
process are less likely to be completely reincorporated back into
the glass melt resulting in striae and other inhomogeneity in the
final glass.
[0056] In Step 3, the cooling step, the temperatures of both the
top and bottom zones were reduced at a rate of 5.degree. C./min to
440.degree. C. and the temperature was held at 440.degree. C. for 2
hours while monitoring the temperature profile of the ampoule at
five locations along its length 201, 202, 203, 204, 205) as
indicated in FIG. 10. The temperature profile measurement, shown in
Table 6, indicates an inverse temperature gradient, where the
bottom is hotter than the top, of -12.degree. C., in spite of the
two furnace zones being set to the same temperature.
TABLE-US-00006 TABLE 6 Ampoule temperature profile during the
cooling step, Step 3 Measurement point (See FIG. 10) Temperature
201 430.degree. C. 202 435.degree. C. 203 438.degree. C. 204
441.degree. C. 205 442.degree. C.
[0057] In Step 4, the quench step, the hot ampoule and the glass
melt contained therein were removed from the furnace and submerged
in a room temperature (about 25.degree. C.) water bath for 30
seconds to rapidly cool the melt forming a solid glass.
[0058] In Step 5, the annealing step, the ampoule and the solid
glass ingot contained therein were then placed in another furnace
at 180.degree. C. for 10 hours to remove residual stress introduced
by the rapid cooling in Step 4 from the solid glass.
[0059] FIG. 11 shows a photograph of the solid glass ingot 66
within the sealed silica quartz ampoule 65. The surface of the
ampoule above the glass ingot has condensed beads of glass 67 and
soot adhered to it. The glass ingot was removed from the ampoule,
and the top and bottom portions of the ingot were removed using a
diamond saw and the end faces of the resulting As.sub.39S.sub.61
glass cylinder were ground flat and polished. The glass was
inspected using an infrared camera. FIG. 12 shows an infrared image
of a human hand and fingers 68 viewed through the 65 mm thick glass
cylinder 69 and reveals significant striae and optical
inhomogeneity 70.
[0060] The origin of the striae in the glass of this prior art
example can be attributed to the inverse temperature gradient
apparent in the fining and cooling steps, steps 2 and 3. In Step 3
of this example, although the top and bottom zones of the furnace
are both set at the same temperature, 440.degree. C., the actual
measured temperature along the length of the ampoule containing the
glass melt varies due to 1) convective heat loss within the furnace
and 2) the cooling lag resulting from the larger thermal mass of
the glass melt in the bottom of the ampoule compared to the rapidly
cooling portion of the ampoule above the glass melt. A temperature
gradient (.DELTA.T) of 12.degree. C. has been measured in the
example as detailed in Table 6. This gradient causes thermal
convection currents 60 within the glass melt 56 when the
temperature is high and the glass melt viscosity is low (FIG.
10).
[0061] At elevated temperatures, the vapor pressures of arsenic and
sulfur are high and both elements are present as gasses 62 in the
atmosphere above the glass melt within the sealed ampoule (FIG.
10). During cooling, the equilibrium vapor pressures for both
elements decrease and droplets of liquid As--S 63 condense on the
cooler upper portion of the ampoule. However the glass melt, as it
cools, is still hotter than the top of the ampoule, material may
continue to evaporate from its surface 61 continuing the
evaporation/condensation cycle. These glass droplets may then drip
back into the melt 56. The condensation droplets 64 may have a
different composition than the rest of the glass melt, and this
continual mass fluxing cycle can cause a compositional
non-uniformity throughout the entire melt as thermal convection
currents 60 distribute them deeper into the glass melt. As the
glass cools further during Step 3, the composition of the glass
near the surface is changing as condensation of gaseous components
(e.g. sulfur) from the closed system settle on the surface of the
glass melt forming a depletion layer. Thermal convection currents
within the glass allow this depletion layer, with a slightly
different composition, to become somewhat reincorporated into the
bulk glass. The convection currents are not sufficient to
thoroughly distribute or homogenize the glass, resulting in
compositional gradients within the glass. During water quenching of
Step 4, it took between 26-30 seconds to quench the glass. The
viscosity of the glass increased rapidly as the glass melt cooled
and the compositional gradients became frozen resulting in striae
in the bulk glass. The viscosity of the glass during the annealing
step, Step 5, is too high to allow any removal of compositional
non-uniformity. Consequently, there are refractive index
perturbations in the striae-containing glass that degrade the
quality of the glass and resulting optics and optical fiber made
from the glass of this prior art example.
Example 4: Prior Art Process to Make Striae-Free Arsenic Sulfide
(As.sub.xS.sub.y) Glass Compositions in a Two-Zone Furnace with
Homogenization by Mechanical Agitation
[0062] Nguyen et al. in a prior art invention teach a method to
synthesize striae-free arsenic sulfide-based chalcogenide glass
(As.sub.39S.sub.61) and other chalcogenide glasses (Nguyen et al.,
"Striae-Free Chalcogenide Glasses," U.S. Pat. No. 9,708,210 (Jul.
18, 2017)). Their invention comprises six steps as detailed in
Table 7, and uses a two-zone rocking furnace 48 as shown
schematically in FIGS. 9 and 13, with an upper zone 58 and a lower
zone 59. The key feature of this prior art method is the
establishment of a controlled temperature gradient within the
furnace, in order to suppress the convection currents within the
glass melt and the evaporation-condensation cycle that introduces
striae. Specifically, in this example for synthesis of
As.sub.39S.sub.61 glass the temperature of the upper zone is hotter
than the lower zone by 100.degree. C. during steps 1-4 of the
melting schedule shown here in Table 7 (Nguyen et al., "Striae-Free
Chalcogenide Glasses," U.S. Pat. No. 9,708,210 (Jul. 18, 2017)).
The temperature gradient along the length of the ampoule 51 was
measured at five locations 201, 202, 203, 204, 206 and is shown in
Table 8. This positive temperature gradient eliminates the main
causes of striae and therefore reduces compositional variations in
the molten glass but requires mechanical agitation of the glass
melt, by means of an oscillatory rocking furnace in this
example.
TABLE-US-00007 TABLE 7 Example of a prior art glass melting
schedule for striae-free As.sub.39Se.sub.61 glass in a two-zone
furnace Tem- Temperature Heating perature (.degree. C.) Rate
(.degree. C.) Bottom Dwell Furnace Step (.degree. C./min) Top Zone
Zone (Hours) Position 1 3 850 750 1 Horizontal 0.degree. fixed 2 --
850 750 10 Rocking at .+-. 45.degree. inclination 3 -1 800 700 24
Vertical 90.degree. fixed 4 -0.6 360 260 12 Vertical 90.degree.
fixed 5 Remove ampoule from furnace and quench in 25.degree. C.
water 6 180 180 10 Annealing furnace
TABLE-US-00008 TABLE 8 Ampoule temperature profile during the
cooling step, Step 4 Measurement point (See FIG. 13) Temperature
201 361.degree. C. 202 360.degree. C. 203 262.degree. C. 204
261.degree. C. 206 260.degree. C.
Example 5: Prior Art Process of Glass Melting to Make
Ge.sub.xAs.sub.ySe.sub.zTe.sub.(100-x-y-z) Glass in a Two-Zone
Furnace with Homogenization by Mechanical Agitation
[0063] Nguyen et al. in another prior art invention teach a method
to synthesize striae-free and crystallite-free
Ge.sub.xAs.sub.yS.sub.(100-x-y-z)Se.sub.z glasses (Nguyen et al.,
"Manufacturing Process for Striae-Free Multicomponent Chalcogenide
Glasses via Multiple Fining Steps," U.S. patent Ser. No. 10/131,568
(Nov. 20, 2018)). Their invention builds upon the prior art in
Example 4, comprises nine steps as detailed in Table 9, and uses, a
two-zone rocking furnace 48 as shown schematically in FIGS. 9 and
13, with an upper zone 58 and a lower zone 59.
[0064] FIG. 13 is a schematic overview of the glass fining and
cooling steps of the process to synthesize infrared glasses by melt
processing. The sealed quartz ampoule 51 containing a well-mixed,
homogenized glass melt 56 is inside a rocking furnace 48 that is in
a vertical position. The rocking furnace 48 comprises an upper or
top heating element 49 and a lower or bottom heating element 50
that are independently controlled, and in these prior art examples
are set to different temperatures during the glass fining and
cooling steps such that the temperature of the upper or top zone 58
is about 100.degree. hotter than the lower or bottom zone 59. The
temperature gradient was measured using thermocouples affixed to
the ampoule 51 at five positions 201, 202, 203, 204, 206 during the
process. During the fining step, this temperature gradient prevents
convective currents within the glass melt 56.
[0065] During fining and cooling of the glass melt, steps 3-7, the
temperature of the upper zone is hotter than the lower zone by
100.degree. C. This method also provides multiple cooling steps,
steps 5-7, with rapid cooling rates between them. Rapid cooling
between these steps prevents formation of crystallites by cooling
quickly through the temperatures where crystals nucleate. The
temperature profile of the cooling is shown in Table 10. These
features provide for high optical quality glasses by reducing
striae and preventing formation of crystallites during cooling.
This prior art however requires mechanical agitation of the glass
melt, by means of an oscillatory rocking furnace in this
example.
TABLE-US-00009 TABLE 9 Example of a prior art glass melting
schedule for Ge.sub.xAs.sub.yS.sub.(100-x-y-z)Se.sub.z glass
compositions in a two-zone furnace Tem- Tem- perature Heating
perature (.degree. C.) Rate (.degree. C.) Bottom Dwell Step
(.degree. C./min) Top Zone Zone (Hours) Furnace Position 1 3 680
680 1 Horizontal 0.degree. fixed 2 -- 680 680 15 Rocking at .+-.
45.degree. inclination 3 +0.6(T), 750 650 1 Rocking at .+-.
45.degree. -0.6(B) inclination 4 -- 750 650 3 Vertical 90.degree.
fixed. Fining. 5 -10.0 650 550 3 Vertical 90.degree. fixed.
Cooling. 6 -10.0 550 450 3 Vertical 90.degree. fixed. Cooling. 7
-10.0 450 350 0.5 Vertical 90.degree. fixed. Cooling 8 Remove
ampoule from furnace and quench in 25.degree. C. water 9 195 195 10
Annealing furnace
TABLE-US-00010 TABLE 10 Ampoule temperature profile at the end of
the cooling step, Step 7 Measurement point (See FIG. 13)
Temperature 201 451.degree. C. 202 450.degree. C. 203 352.degree.
C. 204 351.degree. C. 205 350.degree. C.
[0066] FIGS. 14A and 14B are infrared photographs comparing the
clarity of As.sub.39Se.sub.61 glass of the present invention with
that of the prior art. FIG. 14A shows a human hand 31 viewed
through a 55 mm thick As--Se glass cylinder with polished end faces
28 of the present invention where the camera is focused on the
distal surface of the glass cylinder showing no striae or optical
inhomogeneity within the glass. FIG. 14B shows a human hand 31
viewed through a 55 mm thick As--S glass cylinder with polished end
faces 46 of the prior art as detailed in Example 3 where the camera
is focused on the distal surface of the glass cylinder revealing
significant striae or optical inhomogeneity 47.
[0067] The above descriptions are those of the preferred
embodiments of the invention. Various modifications and variations
are possible in light of the above teachings without departing from
the spirit and broader aspects of the invention. It is therefore to
be understood that the claimed invention may be practiced otherwise
than as specifically described. Any references to claim elements in
the singular, for example, using the articles "a," "an," "the," or
"said," is not to be construed as limiting the element to the
singular.
* * * * *
References