U.S. patent application number 10/311850 was filed with the patent office on 2004-12-09 for glass ceramic composites.
Invention is credited to Conzone, Samuel David, Hayden, Joseph S, Marker, Alexander J III, Simpson, Robert D.
Application Number | 20040247826 10/311850 |
Document ID | / |
Family ID | 24390347 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040247826 |
Kind Code |
A1 |
Conzone, Samuel David ; et
al. |
December 9, 2004 |
Glass ceramic composites
Abstract
A method for joining glass ceramic surfaces to each other and/or
other types of surfaces using a silicate liquid is disclosed. The
products are suitable for use as, e.g. mirror blanks or
microlithography stages, at low temperatures. Component pieces are
polished then joined at low temperature using a silicate-containing
joining liquid. Assembly is then performed in such a way that the
joining liquid forms an interface between each component. After a
period of low or slightly elevated temperature curing, rigid joints
are formed throughout and the composite is dimensionally,
vibrationally, and temperature stable and can withstand tensile
stresses >4000 psi. The room-temperature cured composite can be
heat treated using a slow, systematic temperature increase to
dehydrate the joints. A sealing coating may optionally be provided
to prevent excess dried joining liquid from flaking off the formed
joint.
Inventors: |
Conzone, Samuel David;
(Clarks Green, PA) ; Marker, Alexander J III;
(Springbrook Township, PA) ; Hayden, Joseph S;
(Clark Summit, PA) ; Simpson, Robert D; (Dunmore,
PA) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Family ID: |
24390347 |
Appl. No.: |
10/311850 |
Filed: |
June 23, 2003 |
PCT Filed: |
June 20, 2001 |
PCT NO: |
PCT/US01/41042 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10311850 |
Jun 23, 2003 |
|
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|
09597157 |
Jun 20, 2000 |
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Current U.S.
Class: |
428/131 ;
156/89.11; 428/426; 65/33.5; 65/33.6 |
Current CPC
Class: |
Y10T 428/24322 20150115;
Y10T 428/24273 20150115; Y10T 428/21 20150115; C03C 27/06 20130101;
C03C 27/044 20130101 |
Class at
Publication: |
428/131 ;
428/426; 156/089.11; 065/033.5; 065/033.6 |
International
Class: |
B32B 017/06 |
Claims
We claim:
1. A low-temperature process for the fabrication of lightweight
composite glass ceramic structures, comprising: providing a
plurality of glass ceramic components, said glass ceramic
components each being polished to form at least one joining surface
on each component, providing a silicate-containing joining liquid
to at least one of the joining surfaces, bringing the joining
surfaces of the plurality of glass components together to form a
joint, allowing the joint to cure for a period of time of at least
20 hours at a temperature of at least 20 degrees C., whereby a
cured joint is formed between said plurality of glass components,
and dehydrating the cured joint gradually by raising the
temperature slowly and maintaining an elevated temperature below
the glass transition temperature of the glass ceramic for at least
20 hours.
2. A method as claimed in claim 1, wherein the step of dehydrating
comprises gradually raising the temperature at a rate which allows
dehydration of the joint without imparting damage to the joint from
accelerated H.sub.2O evaporation.
3. A process as claimed in claim 1, wherein the glass ceramic
components are Zerodur.RTM. components.
4. A process as claimed in claim 1, wherein the glass ceramic
components include a low expansion material prepared by partially
ceramizing a lithium alumino silicate glass.
5. A process as claimed in claim 1, wherein the glass ceramic
components include a material selected from the group consisting of
Nexcera, Clearceram, VO2, Astrositall and Sitall.
6. A process as claimed in claim 1, wherein the silicate containing
liquid is a lithium silicate containing liquid or a sodium silicate
containing liquid.
7. A process as claimed in claim 1, wherein the silicate containing
liquid is an alkali silicate, alkaline earth silicate, mixed alkali
silicate, mixed alkaline earth silicate, or mixed alkali/alkaline
earth silicate liquid.
8. A process as claimed in claim 2, wherein the step of gradually
raising the temperature comprises increasing the temperature at a
rate less than or equal to 3 degrees K/hour.
9. A process as claimed in claim 8, wherein the temperature is
periodically stabilized and held.
10. A process as claimed in claim 9, wherein the temperature is
stabilized at intervals of from 5 to 20 degrees C. and held for at
least 10 hours.
11. A process as claimed in claim 10, wherein the temperature is
stabilized at intervals of about 10 degrees C. and held for about
20 hours.
12. A process as claimed in claim 1, wherein the components include
at least one component with a hollowed out light-weighted
portion.
13. A process as claimed in claim 1, wherein the components
comprise segments of a light-weighted core.
14. A process as claimed in claim 1, wherein the components include
a mirror blank.
15. A process as claimed in claim 1, wherein the period of time is
at least one hour.
16. A process as claimed in claim 1, wherein the period of time is
at least one day.
17. A process as claimed in claim 1, wherein the period of time is
at least one week.
18. A process as claimed in claim 1, wherein the step of providing
the silicate containing liquid comprises applying the silicate
containing liquid in an amount of at least 0.01
microliter/cm.sup.2.
19. A process as claimed in claim 2, wherein the temperature is
from 60 to 600 degrees C.
20. A process as claimed in claim 19, wherein the temperature is
below 120 degrees C.
21. A glass ceramic composite, comprising: a plurality of glass
ceramic components, said components being rigidly connected by a
silicate-containing layer formed by curing a silicate-containing
liquid between the glass ceramic components at a temperature of at
least 20 degrees C. for at least 20 hours.
22. A glass ceramic composite as claimed in claim 21, wherein the
cured silicate-containing layer is dehydrated at a temperature at
or below 600 degrees C.
23. A glass ceramic composite as claimed in claim 22, wherein the
cured silicate-containing layer is dehydrated at a temperature at
or below 120 degrees C.
24. A glass ceramic composite as claimed in claim 21, wherein the
plurality of glass ceramic components comprise Zerodur
components.
25. A glass ceramic composite as claimed in claim 21 wherein the
plurality of glass ceramic components include a glass ceramic
material prepared by partially ceramizing a lithium alumino
silicate glass.
26. A glass ceramic composite as claimed in claim 20, wherein the
plurality of glass ceramic components include a material selected
from the group consisting of Nexcera, Clearceram, VO2, Astrositall
and Sitall.
27. A glass ceramic composite as claimed in claim 21, wherein at
least one of the glass ceramic components contains a hollowed out
space.
28. A glass ceramic composite as claimed in claim 21, wherein at
least one of the components comprises a mirror blank.
29. A glass ceramic composite as claimed in claim 21, wherein the
light weighted section is prepared by assembling separate segments
of glass ceramic.
30. A glass ceramic composite as claimed in claim 16, wherein the
rigid connection can withstand tensile stresses during usage of at
least 750 psi.
31. A mirror blank suitable for use in space or flight applications
comprising a plurality of glass ceramic components joined by a
silicate-containing liquid and cured at a temperature of at least
20 degrees C.
32. A mirror blank as claimed in claim 31, wherein the mirror blank
is formed from a face plate, a back plate, and at least one
supporting element.
33. A mirror blank as claimed in claim 32, wherein there are a
plurality of supporting elements.
34. A mirror blank as claimed in claim 33, wherein the plurality of
supporting elements comprise crossed vertical walls, said vertical
walls being provided with slots to form a lattice.
35. A mirror blank as claimed in claim 34, wherein each lattice
opening forms a cell, and the cell is provided with a ventilation
hole or is open to the surrounding environment.
36. A composite glass ceramic structure suitable for use as a
microlithography stage comprising a plurality of glass ceramic
components joined by a silicate-containing liquid and cured at a
temperature of at least 20 degrees C.
37. A method of joining lightweight composite glass ceramic
surfaces comprising applying to at least one of said surfaces a
silicate-containing solution and curing for an extended period at a
slightly elevated temperature above room temperature.
38. A method as claimed in claim 37, wherein the temperature is
from 30 to 50 degrees C., and the extended period is at least 20
hours.
39. A glass ceramic composite as claimed in claim 21, wherein said
silicate-containing layer comprises, in weight %, H.sub.2O 50-99.9
SiO.sub.2 0.01-40 Al.sub.2O.sub.3 0-10 Li.sub.2O 0-20 Na.sub.2O
0-20 K.sub.2O 0-25 MgO 0-10 CaO 0-10 BaO 0-10 SrO 0-10.
40. A light weight glass-ceramic mirror blank, comprising: a face
plate which is circular in shape, a back plate which corresponds in
shape and size to the face plate, a plurality of support elements
located between said face and back plates, said plurality of
supporting elements forming a latticework said latticework and face
plate and back plate defining a plurality of cells, said cells each
being provided with a ventilation aperture or being open to the
environment, said face plate, plurality of supporting elements, and
back plate being joined by a silicate-containing liquid in a
plurality of joining steps to form a plurality of joints, and at
each step, the joint cured at a temperature of at least 20 degrees
C. for a period of at least 20 hours.
41. A mirror blank as claimed in claim 1, wherein the joint is
further dehydrated at a temperature of from 30 to 50 degrees C. for
at least 20 hours.
42. An optical or electrooptical device comprising a composite of
claim 21.
43. A device of claim 42 which is an etalon.
44. A vacuum chamber comprising a composite of claim 21.
45. A composite of claim 21 which has a low coefficient of
expansion.
46. A low-temperature process for the fabrication of a lightweight
composite glass ceramic structure, comprising: providing at least
two surfaces at least one of which is a glass ceramic, and the
other of which is a glass-ceramic or another material having a
surface group which reacts with a silicate joining liquid to form a
stable bond, said surfaces each being polished to form joining
surfaces on each, providing a silicate-containing joining liquid to
at least one of the joining surfaces, bringing the joining surfaces
together to form a joint, allowing the joint to cure for a period
of time of at least 20 hours at a temperature of at least 20
degrees C., whereby a cured joint is formed between said surfaces,
and dehydrating the cured joint gradually by raising the
temperature slowly and maintaining for at least 20 hours an
elevated temperature below the lowest temperature at which one of
the joined components is adversely affected.
47. A process of claim 46 wherein at least one of the surfaces is
SiO.sub.2, quartz, Al.sub.2O.sub.3, multi component silicate glass,
BK-7, ULE, antimony silicate glass, a low expansion metallic alloy,
optionally coated with a thin layer of SiO.sub.2, germanium glass,
tellurium glass or borosilicate glass.
48. A process of claim 46 where at least one of the surfaces is a
glass and said elevated temperature is below the lowest glass
transition temperature of said joined surfaces.
49. A process as claimed in claim 46, wherein the glass ceramic
component is a Zerodur.RTM. component.
50. A glass ceramic composite, comprising: at least two surfaces at
least one of which is a glass ceramic and the other of which is a
glass-ceramic or another material having a surface group which
reacts with a silicate joining liquid to form a stable bond, said
surfaces being rigidly connected by a silicate-containing layer
formed by curing a silicate-containing liquid between the glass
ceramic surface and said other surface at a temperature of at least
20 degrees C. for at least 20 hours.
51. A glass ceramic composite as claimed in claim 21, wherein the
glass ceramic surface comprises Zerodur.RTM..
52. A glass ceramic composite of claim 21 wherein said other
surface is SiO.sub.2, quartz, Al.sub.2O.sub.3, multi component
silicate glass, BK-7, ULE, antimony silicate glass, a low expansion
metallic alloy, optionally coated with a thin layer of SiO.sub.2,
germanium glass, tellurium glass or borosilicate glass.
53. A process of claim 46 wherein said joining liquid comprises
ground or fritted silica-containing glass-ceramic or glass.
Description
BACKGROUND
[0001] I. Field of the Invention
[0002] The invention relates to the field of glass ceramic
composite structures and methods for forming composite glass
ceramic structures using a low temperature joining process.
[0003] II. Description of the Prior Art
[0004] Glass ceramics, which generally consist of an inorganic,
non-porous material having a crystalline phase and a glassy phase,
are known for specialized applications. Such glass ceramics are
manufactured by selecting suitable raw materials, melting,
refining, homogenizing, and then hot forming the material into a
glassy blank. After the glassy blank is cooled and annealed, a
temperature treatment follows whereby the glassy blank is
transformed into a glass ceramic by controlled volume
crystallization (ceramization). Ceramization is a two-step process;
nuclei are formed within the glass at one temperature, and then
grown at a higher temperature. The dual structure of the glass
ceramic material can impart very special properties, including a
very low coefficient of thermal expansion (CTE).
[0005] One preferred material, Zerodur.RTM. (available from Schott
Glass Technologies, Duryea, Pa.) contains about 65-80 weight
percent crystalline phase with a high quartz structure, which
imparts a negative linear thermal expansion. The remaining glassy
phase (which surrounds the crystals) has a positive thermal
expansion. The resulting behavior from the negative-CTE crystalline
phase and the positive-CTE glassy phase is a material with an
extremely low CTE.
[0006] Glass ceramics are useful in a wide variety of applications,
such as mirror substrates for astronomical telescopes; mirror
substrates for X-ray telescopes in satellites, optical elements for
comet probes, weather satellites, and microlithography; frames and
mirrors for ring-laser gyroscopes; distance gauges in laser
resonators; measurement rods as standards for precision measurement
technology, and other uses where very low CTE is important.
[0007] Large segments of monolithic glass ceramic are often used
for many of the applications listed above. However, these large
segments of monolithic glass ceramic are often very massive. For
instance, larger astronomical telescopes can contain mirrors that
exceed 3.6 meters, and the appropriate glass ceramic for use in
such telescopes can exceed several tons. Thus, there is a need to
develop light-weighted glass ceramic materials to overcome the
problems associated with the massive nature of large monolithic
segments of glass ceramic.
[0008] Various joining methods for optical materials are known;
e.g. heat fusion, or frit bonding, however, none provides a
low-temperature solution such as is provided for in the present
invention.
[0009] The known prior art for fabricating light-weighted blanks
(i.e. heat fusion, frit bonding) suffers from several drawbacks.
For example, pressure and temperature are typically required to
form strong joints at high temperature. Developing
loading/unloading fixtures for operation at T>600.degree. C. is
complex and expensive. Further, heat fusion and frit bonding
processes are conducted at temperatures near or above the glass
transition temperature (T.sub.g) of the starting material (i.e.,
glassy Zerodur.RTM., or ULE). The viscosity of glass is
sufficiently low at these temperatures, such that limited flow
(deformation) can occur. This deformation can cause gross
dimensional changes, which can yield a defective mirror blank.
[0010] Additionally, large, high temperature furnaces (up to 2.0 m
in diameter) with stringent thermal tolerances are required for
heat fusion and frit bonding. Such furnaces must be custom-made and
are often very expensive. Furthermore, glassy Zerodur.RTM. shrinks
by approximately 3% during ceramization. This shrinkage can cause
joint stresses that result in deformation and/or catastrophic
failure of the mirror blank during joining.
[0011] Yet another difficulty encountered in the prior art is that
light-weighted mirror blanks can fail during high temperature
joining when thermally induced stresses form at the joint
interfaces (especially during cooling to room temperature). Such
joint failure results in a 100% loss after a tremendous amount of
value has been added to the mirror blank (i.e., machining,
polishing, water-jet cutting, assembly, high temperature fixturing,
etc.).
[0012] A solution to this problem is needed, to allow for the
low-temperature joining of fabricated mirror blanks in a step-wise
manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A is a perspective view of a preferred embodiment of a
lightweight mirror blank according to the present invention.
[0014] FIG. 1B is a top plan view of a preferred embodiment of a
lightweight mirror blank according to the present invention.
[0015] FIG. 1C is a side view of a preferred embodiment of a
lightweight mirror blank according to the present invention.
[0016] FIG. 1D is a perspective view of a support element of a
preferred embodiment of a lightweight mirror blank according to the
present invention.
[0017] FIG. 2 is a diagrammatic representation of the molecular
structure of a surface of a glass ceramic material suitable for
joining according to the present invention.
[0018] FIG. 3 is a diagrammatic representation of the molecular
structure of a joined glass ceramic material according to the
present invention.
[0019] FIG. 4 is a graphical representation of the average flexural
strength of monolithic Zerodur.RTM. compared to various joined
Zerodur.RTM. components.
[0020] FIG. 5 is a graphical representation of the average CTE of
joined Zerodur.RTM. rods.
[0021] FIG. 6 is a diagrammatic representation of the vapor
pressure build up between bulk Zerodur.RTM. blocks.
[0022] FIG. 7 is a perspective view of a joined flexural
sample.
[0023] FIG. 8 is a perspective view of a joined CTE rod.
SUMMARY OF INVENTION
[0024] The present invention relates to a low-temperature process
for the fabrication of lightweight composite glass ceramic
structures, comprised, e.g., of a plurality (e.g., two or more) of
glass ceramic components, said glass ceramic components each being
polished sufficiently to form at least one joining surface on each
component, providing a silicate-containing joining liquid to at
least one of the joining surfaces, bringing the joining surfaces of
the plurality of glass components together to form a joint, and
allowing the joint to cure for a period of time at ambient
temperature, whereby a cured joint is formed between said plurality
of glass components. It also relates to a glass ceramic composite,
comprising a plurality of glass ceramic components, said components
being rigidly connected by a silicate-containing layer formed by
curing a silicate-containing liquid between the glass ceramic
components. The composites are preferably dehydrated by a carefully
controlled dehydration process.
[0025] For example, a low-temperature process for the fabrication
of lightweight composite glass ceramic structures, comprises:
[0026] providing a plurality of glass ceramic components, said
glass ceramic components each being polished to form at least one
joining surface on each component,
[0027] providing a silicate-containing joining liquid to at least
one of the joining surfaces,
[0028] bringing the joining surfaces of the plurality of glass
components together to form a joint,
[0029] allowing the joint to cure for a period of time of at least
20 hours at a temperature of at least 20 degrees C., whereby a
cured joint is formed between said plurality of glass components,
and
[0030] dehydrating the cured joint gradually by raising the
temperature slowly and maintaining an elevated temperature below
the glass transition temperature of the glass ceramic for at least
20 hours.
[0031] This invention also relates to:
[0032] A glass ceramic composite, comprising:
[0033] a plurality of glass ceramic components, said components
being rigidly connected by a silicate-containing layer formed by
curing a silicate-containing liquid between the glass ceramic
components at a temperature of at least 20 degrees C. for at least
20 hours;
[0034] A mirror blank suitable for use in space or flight
applications comprising a plurality of glass ceramic components
joined by a silicate-containing liquid and cured at a temperature
of at least 20 degrees C.;
[0035] A composite glass ceramic structure suitable for use as a
microlithography stage comprising a plurality of glass ceramic
components joined by a silicate-containing liquid and cured at a
temperature of at least 20 degrees C.;
[0036] A method of joining lightweight composite glass ceramic
surfaces comprising applying to at least one of said surfaces a
silicate-containing solution and curing for an extended period at a
slightly elevated temperature above room temperature; and
[0037] A light weight glass-ceramic mirror blank, comprising:
[0038] a face plate which is circular in shape,
[0039] a back plate which corresponds in shape and size to the face
plate,
[0040] a plurality of support elements located between said face
and back plates, said plurality of supporting elements forming a
latticework
[0041] said latticework and face plate and back plate defining a
plurality of cells,
[0042] said cells each being provided with a ventilation aperture
or being open to the environment,
[0043] said face plate, plurality of supporting elements, and back
plate being joined by a silicate-containing liquid in a plurality
of joining steps to form a plurality of joints, and at each step,
the joint cured at a temperature of at least 20 degrees C. for a
period of at least 20 hours.
[0044] A light-weighted mirror blank according to the present
invention can be comprised of thin (preferably <5 cm) face and
back-plates that are fused, joined or bonded to a light-weighted
core. FIGS. 1A, 1B, 1C, and 1D illustrate some of the separate
components that may comprise a light-weighted Zerodur.RTM. mirror
blank. While much of the description herein is in terms of the
preferred Zerodur.RTM. material and mirror blanks, any glass
ceramic material in general for any application is included in the
invention.
[0045] With reference to FIG. 1A, the face 10 and back plates 14
may be flat or curved sheets of Zerodur.RTM. that are polished on
both sides. The light-weighted core may be a polished
honeycomb-shaped component, which may be prepared by water-jet
cutting segments from a solid slab of Zerodur.RTM.. It may also
comprise supporting elements 12 arranged to support and space the
face and back plates.
[0046] After assembly and fusion, a light-weighted mirror blank
often has an overall mass/volume ratio that can be 30% of that for
a standard (solid) Zerodur.RTM. blank, or even as low as 5 or 10%
or even lower.
[0047] FIG. 1B is a top view illustrating a preferred arrangement
including the supporting elements 16 and ventilation holes 18.
[0048] FIG. 1D illustrates a preferred way of joining two
supporting elements 20, 22 using corresponding slots 24, 26.
Optionally, the slots may be prepared with polished mating surfaces
and joined according to the present invention.
[0049] Applicants have developed a novel method for fabricating
composites, especially light-weighted glass ceramic mirror blanks
and microlithography stages, at low temperatures. By low
temperatures, as used herein, is meant temperatures generally under
the glass transition temperature of the glass ceramic. Temperatures
under 500.degree. C. may be suitable for practice of the invention,
more preferably below 140.degree. C., even more preferably below
130.degree. C., most preferably, the temperature is
.ltoreq.120.degree. C. The light-weighted glass ceramic mirror
blanks are intended for applications where dimensional stability,
adequate strength and low mass/volume are critical. Such
applications include, but are not limited to, optical systems
intended for space flight and airborne based applications such as
satellites, navigation systems, and remote sensing, land-based
systems, and precision structures such as microlithography
stages.
[0050] The light-weighted structures according to the present
invention should be rigid when compared to a monolithic piece of
glass ceramic that the structures are replacing. By "rigid" is
meant having essentially the same vibrational characteristics as
the monolithic piece, being vibrationally, thermally, and otherwise
dimensionally stable so as to have essentially indistinguishable
opto-mechanical and thermo-optical characteristics from those of
the monolithic piece of glass ceramic.
[0051] In a preferred embodiment, a low temperature joining
technique is provided which allows light-weighted mirror blanks to
be fabricated in a step-wise manner.
[0052] In a preferred embodiment, a polished face-plate, back-plate
and light-weighted core are first prepared from the commercially
available glass ceramic known as Zerodur.RTM..
[0053] The polished components are cleaned in various aqueous and
non-aqueous solutions before being dried and placed in a clean room
environment.
[0054] A small volume of an aqueous based, silica-containing,
joining liquid is applied to the polished joining surfaces of each
component Assembly is then performed in such a way that the joining
liquid forms an interface between each component (i.e., between the
faceplate and the light-weighted core). After a period of room
temperature curing, rigid joints are formed throughout the
light-weighted mirror blank, which can withstand tensile stresses
>4000 psi. Furthermore, the dimensional stability of the joined
mirror blank is similar to that of pure, monolithic Zerodur.RTM.
(i.e., the coefficient of thermal expansion of the blank is <75
ppb/K, is unaffected by the joining, and is the same as the
monolithic glass ceramic that is being replaced). Such
room-temperature-cured mirror blanks are then preferably heated to
temperatures ranging from 60 to 120.degree. C. to promote joint
dehydration and accelerate the curing process.
[0055] In a particularly preferred embodiment, the joints are
subjected to an extended duration of slight temperature increase to
dehydrate the joints. The removal of water from the joints over an
extended period of time dramatically increases the strength,
reliability, and rigidity of the structures according to the
present invention over the strength, reliability, and rigidity of
joints that are quickly dehydrated (e.g. by microwave). Preferable
temperatures for the extended duration dehydration include from
20-50 degrees C. for at least 20 hours, more preferably from 30-40
degrees C. for at least 20 hours up to three weeks.
[0056] Several advantages arise out of the novel low-temperature
joining process. First, a light-weighted mirror may now be
constructed piece by piece, thus eliminating some of the risk
associated with the high temperature processes. During high
temperature processing the entire mirror blank could deform during
heating or fracture during cooling, with no indication until
furnace unloading. Second, a drying furnace capable of temperatures
up to .apprxeq.120.degree. C. may be required for low-temperature
joining. Such low temperature drying furnaces are commercially
available and much less expensive than high temperature furnaces.
Third, no material deformation occurs, since low temperature
joining is conducted at temperatures well below the T.sub.g, where
glass ceramics are elastic solids. Fourth, fully ceramized glass
ceramics are utilized for low temperature joining. This eliminates
the problems associated with the shrinkage that occurs when the
glass is converted to a glass ceramic during heat fusion. Fifth,
pressure is not typically required for low temperature joining.
Thus, no complicated loading/unloading fixtures are necessary.
[0057] These and other advantages will become apparent to one of
skill in the art having regard for this disclosure.
[0058] A lightweight mirror blank represents a substantial
investment of time is and materials, which makes the mirror blank
very expensive. If a defect arises or a catastrophic failure occurs
within the unit during high temperature bonding (e.g. heat fusion
or frit bonding), the entire value-added process is a loss and the
process must start anew. The applicants have developed a novel
process which provides for the component by component assembly of
large, light-weighted structures, and may be accomplished at low
temperatures (i.e. well below the Tg of typical alkali-alumino
silicate glasses, <500.degree. C.).
[0059] Glass ceramics that are joined according to the instant
invention are first prepared by selecting the monolithic pieces to
be fabricated into a finished composite structure. Various
elements, e.g. mirror components, face plates, back plates, and
structural connections are fabricated and sized to yield a
composite, light-weighted structure which has a low thermal
expansion.
[0060] A light-weighted mirror blank, for example, as used herein,
is a physical structure onto which a reflective coating may by
applied to form a functional mirror. Mirrors are typically used
during flight or space applications where remote sensing is being
conducted. Satellites which image portions of the earth's surface,
or other space-based observation platforms use such mirrors. A high
level of stability, structural and thermal, is required to obtain
an acceptable image. Space-based applications may be exposed to
extremes in temperature, which require an extraordinarily reliable
and sound structure.
[0061] Glass ceramics are suitable materials for such applications,
since they exhibit excellent stability during temperature
fluctuations. One such glass ceramic is Zerodur.RTM., which is an
example of an especially preferred mirror blank material because it
has excellent dimensional stability, owing to a CTE of less than 75
ppb/K from 0-50.degree. C. Other glass ceramics, based on
SiO.sub.2--containing glasses, including Nexcera, a glass ceramic
made by Nippon Steel, Tokyo, Japan, VO2 or V2O, a lithium alumino
silicate glass ceramic produced in VR China Xinhu Factory,
Shanghai, China, Clearceram, Astro-Sitall and Sitall, which are
lithium alumino silicate glass ceramics produced by Lytkarino,
Moscow, Russia are suitable for the practice of this invention.
[0062] Normally, monolithic (i.e. solid) mirrors are utilized to
fabricate precision optics due to their ease of manufacture,
dimensional stability, and excellent mechanical strength. A
monolithic mirror blank is fabricated from a single piece, having
no joints, and such mirror blanks are usually massive structures.
For example, a standard, cylindrical Zerodur.RTM. mirror blank with
a diameter of 1.5 m and a thickness of 0.25 m will have a mass of
greater than 1,100 kilograms (i.e. greater than 2,400 pounds).
Lifting and/or automating such a massive weight often prevents the
use of glass ceramic mirror blanks in flight or space-based
applications.
[0063] The present invention enables a series of components to be
assembled, and can overcome the drawbacks associated with the large
mass of monolithic mirror blanks. Lightweight mirror blanks, which
preferably consist of thin (preferably less than 5 cm) face and
back plates, that are attached to a light-weighted core, as shown
in FIG. 1. These three components, i.e. face plate, back plate, and
light-weighted core, are joined according to the present invention
to form a stable, rigid, light-weighted mirror blank at low
temperature.
[0064] With reference to FIG. 1, a novel light-weighted mirror
blank according to the present invention is seen. The outer
surfaces of the mirror blank 10 may be flat, curved, concave,
convex, or any geometrically desired shape. The face and back plate
are preferably between 0.5 cm and 5 cm in thickness, most
preferably about 1-3 cm thick.
[0065] Beneath the face plate (from the perspective of FIG. 1) is a
lattice of supporting elements 12 forming a light weighted core.
Each of these elements is also preferably a glass-ceramic to yield
a uniform coefficient of thermal expansion for the unit. The
supporting elements are illustrated as aligned in a crosshatched
manner, but any suitable arrangement, which provides dimensional
stability, is acceptable. The supporting elements are preferably
designed for sufficient stability without excessive weight. The
elements may have voids drilled within (e.g. by water cutting or
mechanical drilling) to further reduce the mass of material. Other
shapes and styles of supporting elements may be designed by one of
skill in the art having regard for this disclosure.
[0066] In an alternative embodiment, the light-weight core is
formed from monolithic Zerodur.RTM. by water-jet cutting,
ultrasonic drilling, laser cutting, diamond saw drilling, etc.
[0067] When such a light weight mirror blank is finished, the
overall mass may be reduced by as much as 70% or more when compared
to a monolithic structure, yet retaining sufficient CTE and
strength characteristics to be perfectly suitable for airborne or
space borne applications where high tolerances are expected.
[0068] Appropriate components (including a face plate, back plate,
and light weighted core) should be selected to obtain the maximum
benefits of the instant invention. For example, a plurality of
components to be fabricated into a larger structure would be
selected as follows.
[0069] A plurality of components of glass ceramic with nearly
identical CTE's are selected. By way of example, three slabs of
Zerodur.TM. with nearly identical CTE (i.e., each within .+-.25
ppb/K) will be fabricated. In this example, a 30 cm mirror blank is
to be fabricated.
[0070] Two slabs will have nominal dimensions of 30 cm in diameter
and 3 cm thick. These two slabs will form the face and back-plates
of a light-weighted mirror blank according to the instant
invention. The third slab will have a diameter and thickness of
approximately, 30 cm and 4 cm, respectively. This third slab will
be used to fabricate the light-weighted core.
[0071] The two slabs intended for the face and back-plates are cut
and/or ground to an appropriate thickness, and then polished. The
surfaces intended for joining are prepared with a surface figure of
1/3 .lambda.(.apprxeq.200 nm peak to valley). Although the face and
back-plates may be flat, other geometric shapes are possible (i.e.,
they may be concave, convex, or of other suitable shape).
[0072] A plurality of holes are drilled through the back-plate to
reduce the overall weight of the mirror blank, and to provide an
opening for each cell within the light-weighted core.
[0073] The slab intended for the light-weighted core is cut and
ground to an appropriate thickness and then polished. Both surfaces
of the core have a surface figure of 1/3 .lambda.(.apprxeq.200 nm
peak to valley). Again, the core may be flat or of other geometric
shape (i.e., either side could be concave, convex, or other
suitable shape as appropriate to provide surfaces that will "mate"
with the face and back plates during assembly).
[0074] The core is then weight-reduced by removing excess material
without compromising the structural integrity of the core. For
example, it may be converted into a honeycomb-like shape by
water-jet cutting, or any other method in the cutting art that can
be used to transform a solid piece of glass into a honeycomb-like
structure (i.e., laser cutting, ultrasonic drilling, drilling and
cutting with conventional equipment, etc.). The machined core will
be referred to as the light-weighted core hereafter. If necessary,
the surfaces of the light-weighted core are re-finished to remove
any surface damage that results from the weight reducing step.
[0075] After the component pieces are selected and prepared, the
next step of the instant novel process preferably includes
cleaning. Each component (face-plate, back-plate, light-weighted
core) is, if necessary, cleaned simultaneously or independently,
using the following procedure:
[0076] First, the components will be immersed in a cleaning
solution comprised of 10 vol % Micro Solution and 90 vol %
deionized H.sub.2O for approximately 30 minutes. This solution is
preferably agitated by stirring, rocking, vibration or forced
circulation. The cleaning solution is then removed and the
component immersed in deionized H.sub.2O for approximately 5
minutes.
[0077] The component is then removed from the H.sub.2O and immersed
in a KOH.sub.(aq) solution (0.01 to 9.0 M) for approximately 5
minutes. Subsequently, the components are removed from the KOH(aq)
solution and immersed in deionized H.sub.2O for approximately 5
minutes.
[0078] Finally, each component is rinsed with high purity (low
residue) ethanol, then rinsed with high purity (low residue)
methanol, and transported into a clean room environment. After
transporting the components into the clean room, deionized N.sub.2
gas will preferably be used to remove any residual methanol from
the joining surfaces. Optionally, a CO.sub.2 sno-gun is used to
remove any dust or particulate debris from the joining
surfaces.
[0079] At this point, all joining surfaces are chemically clean and
free from particulate debris. The clean, dry components are then
stored, such that their joining surfaces are not in contact with
any source of particulate or chemical contamination.
[0080] Once the component pieces are polished and cleaned to the
requisite degree, the surfaces to be joined should be free of
debris and contain a large concentration of silanol (SI--OH)
groups, as may be seen in FIG. 2.
[0081] An exemplary silicate "joining liquid" with the general
oxide composition shown in Table I is prepared and is used for low
temperature joining according to the present invention. The joining
liquid typically contains silanol (Si(OH).sub.4 (aq)), alkali
(Na.sup.+.sub.(aq)) and other aqueous species (i.e.,
H.sub.2O.sub.(aq), H.sub.3O.sup.+.sub.(aq), OH.sup.31 .sub.(aq)).
Although not wishing to be bound by the theory of joint formation,
the notations under "comments" give a theoretical explanation for
the functionality of species in the joining liquid.
1TABLE I Compositional Space for the Joining Liquid Oxide Component
Wt % Comments H.sub.2O 50-99.9 H.sub.2O is the solvent, and
H.sub.2O molecules may contribute to joint strength by forming
hydrogen bonds across the joint interface SiO.sub.2 0.01-40 The
SiO.sub.2 component of the liquid is primarily responsible for
joint rigidity (i.e., -Si--O--Si cross links) Al.sub.2O.sub.3 0-10
Al.sub.2O.sub.3 could be added to the liquid to increase the
chemical durability of the resulting joint, and to make the
interfacial region more chemically similar to Zerodur .TM. (a
lithium alumino-silicate glass ceramic) Li.sub.2O 0-20 Li.sup.+
ions could contribute to joint strength by forming Li.sup.+/NBO
(non-bridging oxygen, SiO-) bonds. Na.sub.2O 0-20 Na.sup.+ ions
could contribute to joint strength by forming Na.sup.+/NBO bonds.
K.sub.2O 0-25 K.sup.+ ions could contribute to joint strength by
forming K.sup.+/NBO bonds. MgO 0-10 Mg.sup.2+ ions could contribute
to joint strength by forming Mg.sup.2+/NBO bonds. CaO 0-10
Ca.sup.2+ ions could contribute to joint strength by forming
Ca.sup.2+/NBO bonds. BaO 0-10 Ba.sup.2+ ions could contribute to
joint strength by forming Ba.sup.2+/NBO bonds. SrO 0-10 Sr.sup.2+
ions could contribute to joint strength by forming Sr.sup.2+/NBO
bonds.
[0082] Other suitable joining liquids include lithium silicate
liquid, potassium silicate liquid, magnesium silicate liquid,
calcium silicate liquid, barium silicate liquid, strontium silicate
liquid, mixed alkali silicate liquid, mixed alkali/alkaline earth
silicate liquid, mixed alkaline earth silicate liquid, and silicate
liquid, where "liquid" refers to an aqueous solution.
[0083] Joining is initiated by sandwiching the appropriate liquid
between two clean surfaces, as shown in FIG. 3. Only a small volume
(preferably .apprxeq.0.5 .mu.l/cm.sup.2) of the joining liquid is
required to form a strong joint. Larger or smaller volumes may be
used by one of skill in the art having regard for this disclosure.
It is preferred that at a minimum there be at least 0.01
.mu.l/cm.sup.2.
[0084] In another embodiment of the present invention, ground or
fritted glass-ceramic or glass, e.g., Zerodur.RTM. or
silica-containing glass or glass-ceramic, could be added to the
joining liquid to form a stronger joint, fill in voids at the joint
interface, and promote joint dehydration.
[0085] A joining liquid with the general composition shown in Table
I is used to join the face and back plates to the light-weighted
core. First, particulate debris is preferably removed from the
joining liquid using filtration and/or centrifugation. The two
joints required to fabricate a light-weighted mirror blank
(face-plate to light-weighted core, and back-plate to
light-weighted core) could be fabricated simultaneously, but
preferably they are fabricated separately.
[0086] Joining liquid is applied to one or both surfaces intended
for joining (i.e., to the back of the faceplate and the mating side
of the light-weighted core) by any of a number of coating
techniques. For example, spin coating may be used, where an excess
of joining liquid is to be applied to the surface, and then the
majority removed by high-RPM spinning.
[0087] Another application technique is dip coating, where the
surface is dipped into a vat of joining liquid of appropriate
viscosity and then removed at an appropriate speed to ensure that
an adequate volume (at least 0.01 .mu.l/cm.sup.2) of joining liquid
remained.
[0088] Another suitable method is pipetting or auto-dispensing
drops of joining liquid at various points on the joining surface.
Upon joining, these drops would spread and provide an adequate
volume of joining liquid. Yet another method contemplated by the
instant invention includes joining the components while submerged
in a vat of joining liquid, i.e. assembling the mirror blank while
submerged. Another method includes bringing the two components into
contact and allowing the joining liquid to be introduced from the
side and be drawn between the two surfaces to be joined by
capillary action. Capillary forces will "pull" the liquid into the
interfacial region and a joint will be formed.
[0089] One skilled in the art having regard for this disclosure may
envision the use of any automated technique commonly used to
dispense liquid in, e.g., the adhesives industry. In a preferred
embodiment, reproducible volumes of joining liquid are applied to
the joining surfaces.
[0090] In an especially preferred embodiment, premature curing
(hardening) of the liquid prior to joining is avoided by performing
the joining in a humid environment. The silicate liquid quickly
dries under a dry atmosphere (<30% relative humidity) to form a
residue, which results in an imperfect joint
[0091] For example, when making a joint at a low relative humidity
<30% RH, the following problems can occur:
[0092] 1) Premature drying of the joining liquid resulting in a
thick joint interface >>1 micron.
[0093] 2) Premature drying of the joining liquid resulting in joint
turbidity.
[0094] 3) Premature drying around the edges of a drop of joining
liquid that is applied to the joining surface resulting in a
ring-shaped defect at the joint interface.
[0095] 4) Debonding after curing at room temperature.
[0096] 5) Debonding after heat treatment at temperatures ranging
from 30.degree. C. to 600.degree. C.
[0097] 6) Premature drying of the joining liquid resulting in the
inability to align the pieces once in contact.
[0098] 7) Near impossibility of evenly coating a complicated light
weighted structure prior to joining without encountering problems
mentioned above.
[0099] The problem is intensified as the volume of joining liquid
is reduced (for instance from 0.5 .mu.L/cm.sup.2 to 0.05
.mu.l/cm.sup.2).
[0100] In order to minimize these problems and to achieve defect
reduced, strong joints, bonding should be conducted in a controlled
humidity environment ranging from about 30% R.H. to about 100%
R.H., preferably from about 60% R.H. to about 100% R.H., more
preferably from about 75% R.H. to about 95% R.H. A particularly
preferred condition is at about 80% relative humidity. A suitable
time period range between solution application and bonding contact
of the surface, i.e., bonding, is from immediately after
application to about 10 minutes after application, preferably from
immediately after application to about 5 minutes after application,
more preferably from immediately after application to about 1
minute after application. It is particularly preferred to contact
the surfaces as soon as possible after solution application.
[0101] In a preferred embodiment, the faceplate is brought into
contact with the light-weighted core using an alignment apparatus.
The joining liquid forms an interface between the two clean,
polished (e.g., course or finely ground) surfaces. The joined
component should be allowed to cure for at least 1 hour before
attempting to fabricate the second joint.
[0102] After allowing at least 1 hour for the first joint (face
plate to light-weighted core) to cure, joining liquid will then be
applied to the back plate. The back plate is then brought into
contact with opposite side of the light-weighted core using an
alignment apparatus. The joining liquid then spreads and forms an
interface between the two components. In an especially preferred
embodiment, each hole in the back-plate is aligned with each cell
(void) in the light-weighted core.
[0103] Aligning the holes in the back-plate with the cells in the
light-weighted core ensures that no air is trapped within the
light-weighted mirror. Trapped air can be problematic, especially
when the mirror blank is used for an application where appreciable
temperature fluctuation can occur. Temperature fluctuations would
change the air pressure within an airtight compartment and could
cause local variations in the surface figure of the front
faceplate. Such local variations in surface figure could compromise
the performance of the mirror.
[0104] Although not wishing to be bound by the following
theoretical explanation, it is believed that the following
mechanism explains the development of such mechanically strong, low
CTE joints by the low-temperature joining process.
[0105] Once the liquid is sandwiched between the two surfaces,
condensation reactions (Equation (1)) begin as silanol groups
combine to form --Si--O--Si-- linkages, while releasing
(condensing) H.sub.2O.
--Si--OH+OH--Si.fwdarw.--Si--O--Si--+H.sub.2O (1)
[0106] A rigid joint is formed when the --Si--O--Si-- cross-linking
spreads throughout the interfacial region and eventually binds the
two joining surfaces together, as shown in FIG. 3. The resulting
joint is composed of a hydrated, silicate solid. It is a sodium
silicate solid if a sodium silicate liquid is used for the joining
process.
[0107] It is believed that the Si--O--Si-- linkages are primarily
responsible for joint strength. While the H.sub.2O present in the
interfacial region (i.e., that formed by condensation reactions and
that originally present in the joining liquid) can evaporate from
the joint interface, diffuse into the joining material, or form
hydrogen bonds across the interface, it is believed that H.sub.2O
can also contribute to the overall joint strength. Finally, the
Na.sup.+ ions are expected to form chemical bonds between
non-bridging oxygen (NBO) atoms within the joint interface. The
result of --Si--O--Si-- cross-linking, hydrogen bonding and
Na.sup.+/NBO bonding is a rigid joint that typically has a strength
.gtoreq.4000 psi.
[0108] The residual H.sub.2O that remains in the interfacial region
after room temperature curing may contribute to the overall joint
strength by providing hydrogen bonding. However, in an alternative
embodiment of the present invention, the joint is dehydrated. If,
for example, the end application for the joint requires stability
at temperatures >100.degree. C., removal of H.sub.2O will reduce
the risk of failure.
[0109] The fully joined, light-weighted mirror blank would be
allowed to cure at room temperature for at least 1 day, and
preferably for 7 days before a dehydration should be attempted.
[0110] Not all forms of dehydration will work in preparing joints
according to the instant invention. For example, when
Zerodur.RTM.--Zerodur.RTM. joints are prepared at room temperature
and then subjected to microwave treatment, catastrophic failure
generally occurs. Dehydration without catastrophic failure can best
be achieved by using a slow, controlled heat-treatment schedule
according to the present invention.
[0111] When small samples of glass-ceramic Zerodur.RTM.
(30.times.20.times.10 mm, joining surface 10.times.10 mm) are
fabricated according to the present invention, after 1-week room
temperature cure, these joints are strong enough that they cannot
be fractured by hand.
[0112] When excessive force is applied, the resulting fracture does
not necessarily occur at the joint interface. When small
Zerodur.RTM.--Zerodur.RTM. samples are fractured by impacting them
on a hard surface, the fracture often results in pullout of
material on either side of the joint interface.
Zerodur.RTM.--Zerodur.RTM. joints formed at room temperature
according to the instant invention show excellent strength
characteristics as evident from the flexural strength testing
results shown in FIG. 4. A room temperature cured
Zerodur.RTM.--Zerodur.R- TM. joint has a flexural strength, which
can exceed 9,000 psi.
[0113] The large flexural strengths (5,000 to 10,000 psi) achieved
by joining and curing Zerodur.RTM. at room temperature are
sufficient for space-based applications (where the allowable stress
for Zerodur.RTM. is 750 psi). However, the strength and stability
of such room-temperature-cured joints is deleteriously affected by
temperature ramps (>10 K/h).
[0114] Quickly heating room temperature cured
Zerodur.RTM.--Zerodur.RTM. joints to temperatures >50.degree. C.
(i.e., by subjecting the joint to excessive microwave radiation)
will often cause catastrophic failure. Although not wishing to be
bound by this theory, it is believed that residual H.sub.2O at the
joint interface is likely responsible for the strength degradation
and proneness for failure when Zerodur.RTM.--Zerodur.RTM. joints
are quickly heated.
[0115] When a room temperature cured, Zerodur.RTM.--Zerodur.RTM.
joint is heated at >10 K/h to temperatures >50.degree. C.,
the vapor pressure of H.sub.2O within the hydrated joint interface
is increased. This increase in vapor pressure cannot necessarily be
accommodated by H.sub.2O diffusion into the bulk Zerodur.RTM.. As
noted previously, Zerodur.RTM. is a glass ceramic, which contains
.apprxeq.70 wt % of .apprxeq.50 nm lithium alumino silicate
crystals in a matrix of silica-rich glass.
[0116] Most diffusion models for glass-ceramics assume that the
crystalline phase is impermeable to diffusing species, such as
OH.sup.-, H.sub.3O.sup.+ or H.sub.2O. Thus, the crystalline nature
of Zerodur.RTM. likely blocks H.sub.2O diffusion into the bulk, and
it is believed that the only way for H.sub.2O to escape the
hydrated joint interface is by surface diffusion to the edge of the
sample, see FIG. 6. Once the H.sub.2O reaches the edge of the
joint, it can easily be removed by evaporation.
[0117] When a Zerodur.RTM.--Zerodur.RTM. joint is rapidly heated,
and the vapor pressure within the joint interface is not offset by
H.sub.2O surface diffusion, interfacial damage or joint failure can
occur. Interfacial damage is generally apparent in the form of
small (1 mm) bubbles that form at the joint interface. Such damage
usually occurs when a room-temperature-cured,
Zerodur.RTM.--Zerodur.RTM. joint is heated at .gtoreq.5 K/h in a
furnace or oven. Sufficiently rapid heating (i.e., subjecting a
room temperature cured Zerodur.RTM.--Zerodur.RTM. joint to
microwave radiation, in a commercial microwave oven) often causes
catastrophic failure, as the joint simply "pops" apart.
[0118] A novel slow and systematic heat-treating technique has been
developed to dehydrate room-temperature-cured glass-ceramic and
Zerodur.RTM.--Zerodur.RTM. joints. The heat-treatment schedule is
shown in Table II. This heat-treatment schedule is suitable for
joint dehydration where the mean free path for H.sub.2O surface
diffusion is preferably no greater than 12.5 mm. This exemplary
treatment schedule is a preferred schedule; slower treatments are
possible and may be preferable for different types of joints. Such
treatment regimens may be easily developed by one skilled in the
art having regard for this disclosure.
2TABLE III Preferred Heat Treatment Schedule for Dehydrating 25
.times. 25 mm Joints Step Ramp Rate (K/h) Temperature (.degree. C.)
Hold (h) 1 0.001-2 40 10-40 2 0.001-3 50 15-60 3 0.001-3 60 20-80 4
0.001-3 70 20 5 0.001-3 80 20 6 0.001-3 90 20 7 0.001-3 100 20 8
0.001-3 120 20
[0119] The strength of heat-treated Zerodur.RTM.--Zerodur.RTM. and
other glass-ceramics joints may be slightly less than those cured
at room temperature (FIG. 4), and this is likely due to the
depletion of hydrogen bonding at the joint interface (as H.sub.2O
is removed by evaporation). However, the strength and dimensional
stability of such heat-treated joints are more than sufficient for
use in lighted-weight mirror blank applications and others
requiring at least 750 psi strength. FIG. 4 shows that the flexural
strength of Zerodur.RTM.--Zerodur.RTM. joints is always at least
five times greater than the allowable stress for Zerodur.RTM.,
regardless of heat-treatment temperature from 60 to 120.degree. C.
Furthermore, the dimensional stability does not seem to be affected
by heat-treatments up to 120.degree. C., since the CTE of
Zerodur.RTM.--Zerodur.RTM. joints were not statistically different
from the average CTE of a monolithic Zerodur.RTM. slab (i.e.,
-26.+-.10 pb/K) as shown in FIG. 5.
[0120] Heat-treatment at temperatures above 120.degree. C. may
further increase the strength of a Zerodur.RTM.--Zerodur.RTM.
joint. The interfacial region that joins two Zerodur.RTM. sections
together is essentially a sol-gel derived material (i.e., the
sodium silicate liquid (sol) was transformed to a rigid interfacial
region (gel) by curing). Sol gel materials derived from sodium
silicate liquid can be heat-treated at temperatures exceeding
600.degree. C., to increase the density and strength. However, the
primary goal of a low-temperature joining process is to avoid as
much excessive heat treatment as is possible.
[0121] In yet another preferred embodiment, such joined components
may be overcoated with a polymeric coating to prevent the loss of
dried joining liquid after the joint has been cured. In many
applications, the avoidance of particulate is debris, such as
flaked off dust, is highly desirable and the addition of a
polymeric or other sealant coating is preferred.
[0122] Although the foregoing discussion has been framed in terms
of joining two or more glass ceramic structures, it is also
possible to join a glass ceramic surface to other types of surfaces
using the same procedures and considerations discussed above.
Suitable other surfaces which can be joined to glass ceramic
surfaces using the same silicate joining liquid and considerations
include surfaces of a glass, metallic, metallic alloy, crystalline,
polymeric, etc. nature. Typically, a surface can be joined to a
glass ceramic surface in accordance with this invention as long as
it has surface reactive groups which react with components of the
joining liquid to form bonds to both surfaces, e.g., which permit
the formation of Si-bonds with the silicon atoms in the joining
liquid, e.g., via --O--, --N--, --C-containing, --P-- etc. bonds,
e.g., surface OH, NH.sub.2, PO.sub.4, etc. groups, or carbonaceous
groups containing, e.g., --O-- or --P-- atoms, etc. Suitable such
surfaces include, for example,
[0123] SiO.sub.2 (glass, non crystalline)
[0124] quartz
[0125] Al.sub.2O.sub.3 (aluminum oxide, crystalline)
[0126] multi component silicate glass
[0127] BK-7
[0128] ULE, a titanium silicate glass produced by Corning, Inc,
Corning N.Y.
[0129] antimony silicate glasses
[0130] low expansion metallic alloys (e.g., Invar) optionally
coated with a thin layer of SiO.sub.2
[0131] germanium glasses
[0132] tellurium glasses
[0133] borosilicate glasses
[0134] Thus, where a glass substrate is used, it is preferably a
multi-component oxide glass, a non-oxide glass, or a mixed oxide
glass. Preferred multi-component oxide glasses include silicate,
borate, germanate, telluride, phosphate or aluminate glasses.
Preferred non-oxide glasses are chalcogenide, fluoride, heavy metal
fluoride (e.g., ZBLAN.RTM.) or sulfide (e.g., As.sub.2 S.sub.3)
glass. A preferred mixed oxide/fluoride glass is a fluorophosphate
glass. Where a crystalline material is used it is preferably a
single crystalline material, e.g., a non-semiconducting material
such as LiNbO.sub.3, a fluoride such as CaF.sub.2 or LiF, a
chloride such as NaCl or AgCl, an iodide such as KI, a bromide such
as AgBr, or an oxide such as Sapphire (Al.sub.2O.sub.3).
Alternatively, the single crystalline material may be a
semiconducting material such as GaAs, InP, ZnS, ZnSe, ZnTe, Si or
Ge.
[0135] For joints to surfaces other than glass ceramics, the rate
of heat treatment discussed above can be modified according to the
nature of the non-glass ceramic surface. For example, for glasses
such as silica and other materials which have open networks through
which water can escape from the joint interface or which are
hygroscopic (e.g., phosphate later glasses) rates higher than the
approximate value of 5.degree. K/hr mentioned above may be
applicable, e.g., 6, 7, 8, 10.degree. K/hr etc., depending on the
substances involved. Routine experiments will readily determine
suitable values.
[0136] Moreover, the nature of the non-glass-ceramic material will
also straightforwardly affect the temperature of heating. When
glasses are joined, the maximum temperature should not exceed the
lowest glass transaction temperature of the jointed materials.
Analogously, the maximum temperature should not exceed any value
which would adversely affect the properties of the joined
materials. For example, when one material has a CTE lower than the
others, routine care will be taken to ensure that temperatures are
avoided which would cause one or more bonded surfaces to rupture or
be stressed unacceptably.
[0137] Although the foregoing discusses primarily mirror surfaces,
virtually any kind of device can benefit from this invention's
surface joints, e.g., also etalons, vacuum chambers, low expansion
structures in general, etc.
[0138] Without further elaboration, it is believed that one skilled
in the art can, using the preceding description, utilize the
present invention to its fullest extent. The following preferred
specific embodiments are, therefore, to be construed as merely
illustrative, and not limitative of the remainder of the disclosure
in any way whatsoever.
[0139] In the foregoing and in the following examples, all
temperatures are set forth uncorrected in degrees Celsius; and,
unless otherwise indicated, all parts and percentages are by
weight.
[0140] The entire disclosures of all applications, patents and
publications, cited above, and U.S. Ser. No. 09/597,157 filed Jun.
20, 2000, are hereby incorporated by reference.
EXAMPLES
Example 1
[0141] Zerodur.RTM. samples with approximate dimensions of
60.times.25.times.25 mm were used for all joining experiments. One
of the 25.times.25 mm faces on each sample was polished, with a
surface figure of at least 1/3 .lambda. (200 nm peak to valley,
this surface will be referred to as the joining surface), while the
other 25.times.25 mm face was simply polished to a transparent
finish.
[0142] The joining surfaces were cleaned using various solutions.
First, the samples were ultrasonically cleaned in a 10:90
volumetric ratio of Micro Cleaning Solution and deionized H.sub.2O.
The samples were then rinsed with deionized H.sub.2O before being
ultrasonically cleaned with 3.0 M KOH.sub.(aq) solution. After
cleaning with KOH.sub.(aq) solution, the samples were rinsed with
deionized H.sub.2O, ethanol and methanol before being dried with
deionized N.sub.2 gas. Finally, a CO.sub.2 sno-gun was used to
remove particulate debris from the joining surfaces. The clean
samples were placed within a clean, dust-free enclosure until
joining was conducted.
[0143] The Zerodur.RTM. samples were joined using a low temperature
joining process. When making a joint by the low temperature joining
process, sodium silicate liquid is sandwiched between two clean,
polished, joining surfaces. A mechanically strong joint is formed
when the liquid cures and forms a rigid interface between the two
joining surfaces.
[0144] A sodium silicate liquid containing .apprxeq.15 wt %
SiO.sub.2 and .apprxeq.6 wt % Na.sub.2O was used for all
Zerodur.RTM.--Zerodur.RTM. joining experiments conducted for
strength evaluation. The joining liquid was prepared by diluting
commercially available sodium silicate liquid with deionized water.
After dilution, the resulting liquid was filtered with a 0.2 .mu.m
filter, before centrifugation was used to separate and remove any
remaining particulate debris. A 3 .mu.l drop of the centrifuged
sodium silicate joining liquid was applied to the joining surface
of a Zerodur.RTM. sample using a pipette. A second joining surface
was then brought down upon the 3 .mu.l drop to spread and sandwich
the joining liquid between the two Zerodur.TM. surfaces. All of the
joining experiments were conducted in a clean, dust free
enclosure.
[0145] The joint assembly (two 60.times.25.times.25 mm samples
joined with the sodium silicate liquid) was allowed to cure for at
least 7 days at room temperature (15 to 30.degree. C.). Some joint
assemblies were also heat-treated at temperatures ranging from 60
to 120.degree. C. Flexural samples were machined from the room
temperature cured and heat-treated joint assemblies, as shown in
FIG. 7.
[0146] Care was taken during machining to ensure that the samples
were not subjected to elevated temperatures (>50.degree. C.) or
corrosive liquids (i.e., acid-based pitch remover). The goal was to
machine samples from the joint assemblies without deleteriously
affecting the joint strength. The resulting flexural samples were
nominally 3.times.4.times.45 mm, in accordance with the ASTM
standard C1161-94"Standard Test Method for Flexural Strength of
Advanced Ceramics at Ambient Temperature." The 4.times.45 mm faces
of the flexural specimens were ground with 20 .mu.m Al.sub.2O.sub.3
grinding media before flexural strength tests were conducted.
[0147] Flexural strength testing was conducted in accordance with
ASTM standard C1161-94. Flexural strength was measured for
Zerodur.RTM.--Zerodur.RTM. joints that had been cured at room
temperature, and those heat-treated at 60, 90, or 120.degree. C.
for 20 h. The flexural strength of monolithic Zerodur.RTM. was also
measured for comparative purposes (note that the monolithic
Zerodur.RTM. flexural surfaces were also prepared with a 20 .mu.m
Al.sub.2O.sub.3 grinding media).
[0148] The results from the flexural strength testing are shown in
FIG. 4. The average flexural strength of the
Zerodur.RTM.--Zerodur.RTM. joints was greater than 5,000 psi,
regardless of the heat-treatment temperature. However, the
Zerodur.RTM.--Zerodur.RTM. joints always yielded a lower flexural
strength than the monolithic Zerodur.RTM. samples, which had an
average flexural strength of >13,000 psi. A flexural strength of
5000 psi is more than six times greater than the allowable working
stress (750 psi), and more than three times greater than the
ultimate handling stress (1,500 psi) for Zerodur.RTM.. Thus,
Zerodur.RTM.--Zerodur.RTM. joints prepared by the low temperature
joining process and heat treated at temperatures ranging from 60 to
120.degree. C. are expected to be suitably strong for use
light-weighted Zerodur.RTM. mirror blanks.
Example 2
[0149] The CTE of components joined according to the present
invention is most preferably the same as that of monolithic glass
ceramic materials. The minimization or elimination of joint-induced
CTE variation ensures dimensional stability when the mirror-blank
is used for applications, such as remote sensing.
[0150] Testing was conducted to determine whether a composite glass
ceramic joint has the same CTE as monolithic glass ceramics. All
samples intended for CTE evaluation were obtained from the same
slab of Zerodur.RTM. (Melt Number F-9831). The intrinsic variation
of CTE in the slab was determined by measuring the CTE at three
positions. The three values of CTE were -33, -25 and -20 ppb/K
(each CTE reported herein is an average value, measured from 0 to
50.degree. C.). Note that the typical measurement accuracy for the
CTE in the temperature range of 0 to 50.degree. C. is .+-.10 ppb/K
and the precision under repeatability conditions is .+-.5 ppb/K.
Therefore, the average CTE of the Zerodur.RTM. slab from 0 to
50.degree. C. (Melt Number F-9831) was -26.+-.10 ppb/K.
[0151] Samples with the nominal dimensions of 60.times.25.times.25
mm were used to prepare Zerodur.RTM.--Zerodur.RTM. joint assemblies
for CTE evaluation. These samples were cleaned and joined using the
same procedures described in the Mechanical Strength section
(above). Two Zerodur.RTM.--Zerodur.RTM. joint assemblies were
prepared for CTE evaluation and allowed to cure for at least 7 days
at room temperature. A CTE rod (6 mm diameter) was core drilled
from each of the two joint assemblies, as shown in FIG. 8.
[0152] The CTE of the first rod was measured before and after 20
hour heat-treatments at 60, 100 and 120.degree. C., while the CTE
of the second rod was measured before and after 20 hour
heat-treatments at 60, 90 and 120.degree. C. FIG. 4 compares the
CTE data collected from each of the joined CTE rods with the
average CTE of the Zerodur.TM. slab (melt number F-9831). Note that
all CTE data are within 10 ppb/K of the average CTE measured for
the Zerodur.RTM. slab. Thus, the low temperature joining process
does not affect CTE, and when the process according to the present
invention is used to fabricate light-weighted Zerodur.RTM. mirror
blanks, remarkably superior and stable composite structures are
formed.
Example 3
[0153] One general procedure that can be used to prevent the
silicate-containing joining liquid from prematurely drying prior to
bonding is as follows.
[0154] Strong bonds between Zerodur.RTM. pieces were made in a
class 100 clean box with humidity control (.+-.2% Relative
Humidity) at ambient temperature. In one experiment, a polished
face plate and polished back plate were jointed to a latticed
2-part polished rib structure assembled with slots as shown in FIG.
1D. In a second experiment, a water-jet cut light-weighted core was
joined to a polished face plate and polished back plate. The
bonding surface of the core was coarse ground. The face plate had
four ventilation holes as illustrated in FIG. 1B to allow
dehydration of the joints. In a third experiment, 3 mm.times.4
mm.times.45 mm parallelpipeds were joined on a 4 mm.times.45 mm
fine ground side to a polished plate. The relative humidity was
varied from 25-90% R.H. at ambient temperature. The time period
between solution application and bonding was varied as well from
immediately after application to 10 min. after application.
[0155] At RH values of 30% and higher, the joints were excellent,
e.g., at RH less than 30%, the joints had imperfections.
[0156] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0157] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention
and, without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *