U.S. patent application number 12/709045 was filed with the patent office on 2010-09-30 for solar reflecting mirror having a protective coating and method of making same.
This patent application is currently assigned to PPG INDUSTRIES OHIO, INC.. Invention is credited to Abhinav Bhandari, Harry Buhay, William R. Siskos, James P. Thiel.
Application Number | 20100242953 12/709045 |
Document ID | / |
Family ID | 42782612 |
Filed Date | 2010-09-30 |
United States Patent
Application |
20100242953 |
Kind Code |
A1 |
Bhandari; Abhinav ; et
al. |
September 30, 2010 |
SOLAR REFLECTING MIRROR HAVING A PROTECTIVE COATING AND METHOD OF
MAKING SAME
Abstract
A solar reflecting mirror includes a shaped glass substrate
having a focal area, a reflective coating over its convex surface
and a sodium ion barrier layer over its concave surface. The shaped
substrate has a strain pattern having a radial tension strain at
the bottom area, and circumferential compression strain at the
periphery of the substrate. As the distance from the periphery of
the shaped substrate increases, the circumferential compression
strain decreases to a "transition line" where circumferential
tension strain begins. As the distance from the transition line in
a direction toward the bottom area of the glass substrate
increases, the circumferential tension increases. To compensate for
the strain pattern in the shaped glass substrate to avoid buckling
of, and surface cracks of, the barrier layer, the barrier layer
including an oxide of silicon and aluminum thickness, among other
things is varied on. A method of making the solar mirror from
shaped sections is also discussed.
Inventors: |
Bhandari; Abhinav;
(Cranberry, PA) ; Buhay; Harry; (Allison Park,
PA) ; Siskos; William R.; (Delmont, PA) ;
Thiel; James P.; (Pittsburgh, PA) |
Correspondence
Address: |
PPG INDUSTRIES INC;INTELLECTUAL PROPERTY DEPT
ONE PPG PLACE
PITTSBURGH
PA
15272
US
|
Assignee: |
PPG INDUSTRIES OHIO, INC.
Cleveland
OH
|
Family ID: |
42782612 |
Appl. No.: |
12/709045 |
Filed: |
February 19, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61164047 |
Mar 27, 2009 |
|
|
|
Current U.S.
Class: |
126/684 ;
29/592 |
Current CPC
Class: |
C03C 2218/365 20130101;
Y02E 10/40 20130101; G02B 1/105 20130101; C03C 17/3644 20130101;
G02B 19/008 20130101; Y10T 29/49 20150115; G02B 5/0808 20130101;
F24S 23/79 20180501; F24S 23/82 20180501; C03B 23/0357 20130101;
F24S 23/70 20180501; G02B 19/0023 20130101; C03C 17/3663 20130101;
G02B 1/14 20150115; F24S 23/71 20180501; G02B 19/0042 20130101;
C03C 17/36 20130101 |
Class at
Publication: |
126/684 ;
29/592 |
International
Class: |
F24J 2/10 20060101
F24J002/10 |
Claims
1. A solar reflecting mirror having a curved reflective surface,
comprising: a transparent substrate having a convex surface and an
opposite concave surface, and a reflective coating over the convex
surface and an alkali barrier layer over the concave surface
wherein the reflective coating reflects selected wavelengths of the
electromagnetic spectrum.
2. The solar mirror according to claim 1 wherein the alkali barrier
layer has mechanical and chemical protective properties.
3. The solar mirror according to claim 1 wherein the barrier layer
is on the concave surface of the substrate and comprises an oxide
of silicon and aluminum.
4. The solar mirror according to claim 3 wherein the barrier layer
has a weight percent of silicon greater than a weight percent of
aluminum.
5. The solar mirror according to claim 4 wherein the barrier layer
comprises 15 atomic percent aluminum and 85 atomic percent silicon,
and the film is deposited by magnetron sputtering vacuum
deposition.
6. The solar mirror according to claim 5 wherein the barrier layer
has a thickness in the range of 700-950 nanometers.
7. The solar mirror according to claim 1 wherein the transparent
substrate is a soda-lime-silica shaped glass substrate having a
focal area and the barrier layer is a sodium ion barrier layer.
8. The solar mirror according to claim 7 wherein the barrier layer
has a first surface and an opposite second surface, and the first
surface of the barrier layer is in surface contact with the concave
surface of the shaped glass substrate and the second surface of the
barrier layer is facing away from the concave surface of the shaped
glass substrate.
9. The solar mirror according to claim 8 wherein the barrier layer
comprises an oxide of silicon and aluminum, and the first surface
of the barrier layer has a first weight percent of silicon and the
second surface of the barrier layer has a second weight percent of
silicon, wherein the first weight percent of silicon is different
than the second weight percent of silicon.
10. The solar mirror according to claim 7 wherein the shaped glass
substrate comprises at least two shaped glass segments maintained
together to provide the shaped glass substrate.
11. The solar mirror according to claim 10 wherein each segment
comprises (1/(total segments of the parabolic shaped glass
substrate)) part of the parabolic shaped glass substrate.
12. The solar mirror according to claim 7 wherein perimeter of the
shaped glass substrate comprises four corners and four sides.
13. The solar mirror according to claim 7 wherein the shaped glass
substrate has a strain pattern comprising a radial tension strain
at a bottom area of the shaped glass substrate, and circumferential
compression strain at a periphery of the shaped glass substrate;
wherein as the distance from the periphery of the shaped glass
substrate increases in a direction toward the bottom area of the
shaped glass substrate, the circumferential compression strain
decreases to an area designated as a "transition line" where
circumferential tension strain and the radial tension strain are
present in the glass, and as the distance from the transition line
in a direction toward the bottom area of the shaped glass substrate
increases, the circumferential tension strain increases.
14. The solar mirror according to claim 13 wherein the barrier
coating covers the concave surface of the glass shaped substrate
and has a constant thickness.
15. The solar mirror according to claim 14 wherein the barrier
layer has a thickness in the range of 60 to 100 nanometers, and a
composition comprising an oxide of silicon and aluminum, and the
reflective coating is a silver coating.
16. The solar mirror according to claim 13 wherein the barrier
coating increases in thickness as the distance from the periphery
of the shaped glass substrate toward the bottom area of the shaped
glass substrate increases
17. The solar mirror according to claim 16 wherein the barrier
coating is in the thickness range of 40 to 100 nanometers.
18. The solar mirror according to claim 13 wherein the barrier
coating has a first constant thickness from the perimeter of the
shaped glass substrate to the transition line of the shaped glass
substrate, and a second constant thickness from the transition line
of the shaped glass substrate to the bottom area of the shaped
glass substrate, wherein the first constant thickness is different
from the second constant thickness.
19. The solar mirror according to claim 18 wherein the first
constant thickness of the barrier coating is in the range of 40 to
60 nanometers, and the second constant thickness is in the range of
greater than 60 to100 nanometers.
20. A method of making a solar reflecting mirror having a curved
reflective surface, comprising: providing a flat transparent sheet;
shaping the sheet to provide a shaped transparent substrate having
a convex surface and an opposite concave surface and a focal area;
applying a reflective coating over the convex surface of the
substrate, and providing an alkali barrier layer over the concave
surface of the substrate.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefits of U.S. Provisional
Patent Application Ser. No. 61/164,047 filed Mar. 27, 2009 and
titled "ALKALI BARRIER LAYER." Application Ser. No. 61/164,047 in
its entirety is incorporated herein by reference.
[0002] This application is related to U.S. patent application Ser.
No. ______ filed even date in the name of James P. Thiel and titled
A SOLAR REFLECTING MIRROR AND METHOD OF MAKING SAME. Application
Ser. No. ______ in its entirety is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates to a solar reflecting mirror, e.g. a
parabolic shaped solar reflecting glass mirror having a protective
coating, e.g. an alkali barrier layer and method of making same,
and more particularly, to an alkali barrier layer on the concave
surface of the mirror to prevent alkali ions, e.g. sodium ions from
precipitating on the concave surface of the mirror. The preferred
alkali barrier layer of the invention has scratch resistant and
chemical resistant properties to prevent abrasive damage to the
concave surface of the mirror.
[0005] 2. Description of the Available Technology
[0006] At the present time, there is interest to increase the
efficiency of solar collectors, e.g. and not limiting to the
discussion, improve the efficiency of solar mirrors, e.g. parabolic
shaped mirrors, used to reflect the sun's rays to a device located
at the focal point of the parabolic mirror. The device is usually
of the type known in the art to convert the sun's energy to another
form of useable energy, e.g. electric energy. In another embodiment
of the prior art, the parabolic mirror is a primary mirror
reflecting the sun's rays to a secondary mirror positioned relative
to the focal point of the primary mirror to reflect the sun's rays
to the converting device.
[0007] In general, the parabolic shaped mirror includes a parabolic
shaped substrate having a reflective surface, e.g. a silver coating
on the convex surface of the shaped substrate. The preferred
material of the shaped substrate is soda-lime-silica glass because
of the high yield in shaping a flat glass sheet to a parabolic
sheet or substrate; the low cost of making flat glass sheets, and
the high yield and low cost of applying a solar reflective coating
on a surface of the shaped glass substrate.
[0008] Although soda-lime-silica glass is an acceptable material
for the substrate for solar reflecting mirrors, there are
limitations to the use of glass. More particularly, in the shaping
process, a flat glass sheet is heated to temperatures above
1200.degree. Fahrenheit (hereinafter also referred to as "F") and
shaped into the parabolic shape. During the heating and shaping of
the glass sheet, the alkali ions, e.g. the sodium ions in the glass
sheet diffuse, or leech, out of the glass sheet. Further, during
exposure of the parabolic shaped glass substrate to solar energy,
e.g. long-term environmental exposure, additional sodium ions leech
out of the glass substrate. As is appreciated by those skilled in
the art, the leeching or diffusion of the sodium ions from the
glass is an expected occurrence, and at low temperatures is a slow
process. However, heating the glass and/or the long term
environmental exposure of the glass to solar energy accelerates the
leeching or diffusion of sodium ions out of the glass, and
increases the amount of sodium ions that leech out of the glass.
The sodium ions leeching out of the glass react with moisture in
the atmosphere, and convert from sodium ions to sodium compounds,
e.g. sodium hydroxide and sodium carbonate. The sodium compounds
can etch the surface of the glass and can deposit as a precipitate
on the surface of the glass. The sodium compound precipitates
decrease the transmission of visible light through the glass, e.g.
in the case of the parabolic shaped glass substrate, decrease
transmission of solar energy to the reflective coating on the
convex surface of the shaped glass substrate, and decrease the
transmission of the solar energy reflected from the reflective
coating through the shaped glass substrate to the concave surface
of the shaped glass substrate.
[0009] Further as is appreciated by those skilled in the art, the
surface of the shaped glass substrates is a specular surface, and
the solar energy is incident on the concave surface of the glass
substrate as parallel light rays. The parallel light rays are
reflected from the concave surface, and reflected from the
reflective coating, as convergent light rays. The sodium compound
precipitate on the concave glass surfaces converts the specular
surface to a non-specular or diffusing surface directing the light
rays reflected from, and passing through, the precipitate away from
the focal point of the primary mirror. The term "specular surface"
as used herein means a light reflective surface where a light ray
incident on the reflective surface has an angle of incidence equal
to the angle of reflection. The term "non-specular or diffusing
surface" as used herein means a reflective surface where a light
ray incident on the reflective surface has an angle of incidence
different from the angle of reflection.
[0010] Another limitation of glass is that care has to be exercised
to avoid scratching the glass surfaces. Scratches on the glass
surfaces can also change a specular surface to a non-specular or
diffusing surface. As is appreciated by those skilled in the art,
as the reflective concave surface changes from a specular surface
to a non-specular or diffusing surface, the percent of reflective
solar light rays incident on the focal point of the parabolic
shaped mirror are reduced, lowering the efficiency of the solar
reflective mirror.
[0011] Present techniques to remove and/or to eliminate the sodium
compound precipitate from the concave surface of a parabolic mirror
include cleaning the surfaces and/or enclosing the concave surface
of the mirror in a sealed chamber having an inert gas to prevent
the sodium ions from forming the precipitate. Present techniques
for removing scratches include buffing the surfaces of the glass
sheet having the scratches. All of these techniques to ensure the
surfaces of the solar mirror remain a specular surface are
expensive.
[0012] Barrier layers are known in the art, e.g. disclosed in U.S.
Pat. Nos. 4,238,276; 5,270,615; 5,830,252 and 6,027,766, and U.S.
patent application Ser. No. 08/597543, and U.S. Publication
200710275253A1. One of the limitations of the presently available
alkali barrier layers and/or scratch resistant layers is that they
are efficient for use on flat or shaped surfaces of glass
substrates, but are not efficient for use on a flat surface that is
subsequently shaped to a curved surface, e.g. a concave surface of
a parabolic mirror. There is little, if any, recognition or
discussion in the prior art of the problems that have to be solved
when a substrate coated with a barrier layer and/or a scratch
resistant layer is shaped from a flat-coated substrate to a
parabolic shaped coated substrate. More particularly, there is
little, if any, discussion in the prior art of eliminating the
cracks in, and/or the buckling of, the coating as the contour of
the coated glass is changed from a glass piece having flat surface
to a shaped glass substrate having a concave surface. As is
recognized by the instant application, when the barrier coating is
stressed, the coating cracks and the sodium ions are exposed to the
atmosphere and form the sodium compound precipitate on the surfaces
of the glass substrate, and/or when the barrier coating and/or the
scratch resistant coating buckles the surface changes from a
specular surface to a non-specular or diffusing surface.
[0013] As can now be appreciated by those skilled in the art, it
would be advantages to provide an alkali barrier coating or layer,
e.g. a sodium ion barrier coating that has scratch resistant
properties to prevent the concave surfaces of the primary and
secondary mirrors from changing from a specular surface to a
non-specular or diffusing surface.
SUMMARY OF THE INVENTION
[0014] This invention relates to a solar reflecting mirror having a
curved reflective surface, including, among other things, a
transparent substrate having a convex surface and an opposite
concave surface, and a reflective coating over the convex surface
and an alkali barrier layer or coating over the concave surface.
The reflective coating reflects selected wavelengths of the
electromagnetic spectrum.
[0015] Further, the invention relates to a method of making the
solar reflecting mirror having a curved reflective surface by,
among other things, providing a flat transparent sheet; shaping the
sheet to provide a shaped transparent substrate having a convex
surface and an opposite concave surface and a focal area; applying
a reflective coating over the convex surface of the substrate, and
providing an alkali barrier layer over the concave surface of the
substrate.
[0016] Still further, the invention relates to an alkali barrier
coating including, among other things, an oxide of silicon and
aluminum.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is an elevated plan view of a prior art array of
solar collectors.
[0018] FIG. 2 is an isometric view of a prior art solar collector,
and FIG. 2A is an enlarged view of a sun's ray incident on the
concave surface of the solar collector.
[0019] FIG. 3 is a view similar to the view of FIG. 2 showing a
solar mirror of the invention.
[0020] FIG. 4 is an isometric view of a piece of glass having a
coating of the invention, the coating in FIG. 4 having portions
removed for purposes of clarity.
[0021] FIG. 5A is a side elevated view of a vacuum mold having the
piece of glass of FIG. 4 mounted on the open end of the vacuum
mold, and FIG. 5B is a cross sectional view of the vacuum mold
having the shaped glass substrate of the invention in the interior
of the vacuum mold.
[0022] FIG. 6 is an elevated top view of the shaped glass substrate
of the invention showing the pattern of circumferential compressive
strains at the periphery of the shaped glass substrate.
[0023] FIG. 7 is a view taken along line 7-7 of FIG. 6 showing,
among other things, the transition strain line of the shaped glass
substrate.
[0024] FIG. 8 is a view taken along line 8-8 of FIG. 7 showing the
circumferential tensile strain and the radial tensile strain of the
shaped glass substrate.
[0025] FIG. 9A is an isometric view of a segment of the glass piece
shown in FIG. 4; FIG. 9B is an isometric view of the segment shown
in FIG. 9A after the glass piece is shaped into the shaped glass
substrate, the coating having peaks and valleys, and FIG. 9C is a
view similar to the view of FIG. 9B showing a segment of the shaped
glass substrate made according to the teachings of the invention,
the coating having reduced number of peaks and valleys, reduced
heights of peaks and reduced depths of valleys.
[0026] FIG. 10 is a view similar to the view of FIG. 4 showing
another embodiment of the invention to make the shaped solar mirror
of the invention that includes cutting a coated glass into
segments.
[0027] FIG. 11 is an isometric top view of a glass sheet pressing
arrangement that can be used in the practice of the invention to
shape the segments cut from the coated glass of FIG. 10.
[0028] FIG. 12 is a top view of a shaped solar mirror of the
invention made by joining shaped glass segments.
[0029] FIG. 13 is a view similar to the view of FIG. 3 showing the
shaped solar mirror of the invention made with the shaped glass
segments
[0030] FIG. 14 is view similar to view of FIG. 4 showing a coating
shield over a circular glass piece.
[0031] FIG. 15 is an elevated cross sectional top view of the
shaped glass substrate at a position between the transition strain
line and the bottom of the shaped glass substrate, the view showing
fissures in the circumferential tension and radial tension areas of
the shaped glass substrate, the cross hatching of the coating is
not shown for purposes of clarity.
[0032] FIGS. 16-19 are cross sectional side views of sections of
flat glass pieces of the type shown in FIG. 4 having a barrier
coating and/or a scratch resistant coating of the invention on one
or both surfaces of the glass pieces, and optionally a reflective
surface over one surface of the glass pieces.
[0033] FIG. 20 is a side view of a section of a photovoltaic cell
having the barrier layer of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0034] In the following discussion, spatial or directional terms,
such as "inner", "outer", "left", "right", "up", "down",
"horizontal", "vertical", and the like, relate to the invention as
it is shown in the drawing figures. However, it is to be understood
that the invention can assume various alternative orientations and,
accordingly, such terms are not to be considered as limiting.
Further, all numbers expressing dimensions, physical
characteristics, and so forth, used in the specification and claims
are to be understood as being modified in all instances by the term
"about." Accordingly, unless indicated to the contrary, the
numerical values set forth in the following specification and
claims can vary depending upon the desired properties sought to be
obtained by the present invention. At the very least, and not as an
attempt to limit the application of the doctrine of equivalents to
the scope of the claims, each numerical parameter should at least
be construed in light of the number of reported significant digits
and by applying ordinary rounding techniques. Moreover, all ranges
disclosed herein are to be understood to encompass any and all
sub-ranges subsumed therein. For example, a stated range of "1 to
10" should be considered to include any and all sub-ranges between
(and inclusive of) the minimum value of 1 and the maximum value of
10; that is, all sub-ranges beginning with a minimum value of 1 or
more and ending with a maximum value of 10 or less, e.g., 1 to 6.7,
or 3.2 to 8.1, or 5.5 to 10. Also, as used herein, the terms
"applied over", or "provided over" mean applied, or provided on but
not necessarily in surface contact. For example, a material
"applied over" a substrate or a substrate surface does not preclude
the presence of one or more other materials of the same or
different composition located between the deposited material and
the substrate or substrate surface.
[0035] Before discussing several non-limiting embodiments of the
invention, it is understood that the invention is not limited in
its application to the details of the particular non-limiting
embodiments shown and discussed herein since the invention is
capable of other embodiments. Further, the terminology used herein
to discuss the invention is for the purpose of description and is
not of limitation. Still further, unless indicated otherwise, in
the following discussion like numbers refer to like elements.
[0036] The barrier coating or layer of the invention is a
silicon-aluminum oxide coating discussed in detail below. The
silicon-aluminum oxide coating of the invention also provides
protection against mechanical damage, e.g. scratches, and chemical
damage, e.g. chemical etching from materials having a pH in the
range of 7-14, and in particular in the range of 9-14. The
following discussions relating to the barrier properties of the
coating of the invention are applicable to the scratch resistant
properties of the coating of the invention unless indicated
otherwise. In this regard, at coating thickness below 50 nanometers
(hereinafter also referred to as "nm"), the silicon-aluminum oxide
coating of the invention loses resistance to mechanical damage and
chemical damage.
[0037] For clarity of discussion, the terms "alkali barrier layer
or coating" and "sodium ion barrier layer or coating" means a layer
or coating that acts as a barrier to prevent, or limit the
formation of alkali or sodium precipitates on, and optionally has a
resistance to prevent or limit mechanical and/or chemical damage
to, the surface over which, or on which, the layer or coating is
applied. "Protective layer or coating" means a layer or coating
that has a resistance to prevent or limit mechanical and/or
chemical damage to, and/or can limit the formation of alkali or
sodium precipitates on, the surface over which, or on which, the
layer or coating is applied.
[0038] Non-limiting embodiments of the invention will be discussed
using magnetron sputtering vacuum deposition (hereinafter also
referred to as "MSVD") coating process to apply a coating or layer
or film over, or on, a substrate surface that is a barrier to
alkali ions, e.g. prevents the sodium ions from reacting with
moisture in the atmosphere and converting the sodium ions to sodium
compounds, e.g. sodium hydroxide and sodium carbonate, which
compounds precipitate on the surface of the glass as discussed
above. As is appreciated, the invention is not limited to the
coating process, and the coating process can be any coating process
that applies or coats an alkali ion, e.g. a sodium ion, barrier
film or layer on, or over, a glass surface.
[0039] The following discussion is directed to non-limiting
embodiments of applying an alkali ion barrier coating or layer.
Unless indicated otherwise, the discussion is applicable to scratch
resistant coatings or layers.
[0040] As is appreciated, the glass substrate or piece is not
limiting to the invention, and the glass can be a glass of any
composition; the glass can be clear or colored glass, and/or the
glass can be annealed, heat strengthened or tempered glass. The
glass piece or substrate can have any shape, thickness and size.
The non-limiting embodiments of the invention are presented as the
embodiments relate to solar reflecting mirrors. The invention,
however, is not limited thereto, and the invention can be practiced
in the manufacture of commercial and residential windows; glass
shower doors; transparencies for air, space, land and water
vehicles; coated bottles; coated glass for thin film photovoltaic
applications; electrically heated glass for anti-fog commercial
refrigerators, and glass for furniture use.
[0041] In the following discussion, the shaped solar reflecting
mirror is referred to as a parabolic shaped reflecting mirror,
however, the invention is not limited thereto, and the invention,
unless indicated other wise can be practiced with any mirror having
a curved reflective surface and a focal point or focal area, e.g.
but not limited to the invention, a parabolic shaped mirror, and
spherical shaped mirror. A "focal point" and "focal area" is
defined as a position where more than 80% of the solar rays
reflected from the mirror converge. The size and the location of
the "focal area" is not limiting to the invention, and in one
non-limiting embodiment of the invention, the focal area is less
than one fifth (1/5) of the reflecting area of the mirror.
[0042] Shown in FIG. 1 is an array 18 of shaped solar collectors 20
(see FIG. 2) of the prior art to convert solar energy to electric
energy. The invention is not limited to the manner of joining the
solar collectors 20 in the array 18 and any techniques known in the
art can be used to join the solar collectors 20 in the array 18.
Further, the invention is not limited to the number of solar
collectors 20 in the array 18, e.g. the invention can be practiced
on one solar collector 20 and an of array of 2, 3, 4, 5, 10, 20,
greater than 50 and any number combination of solar collectors.
Still further, the invention contemplates an array 18 of solar
collectors 20 mounted in any convenient manner in a stationary
position, or an array 18 of solar collectors 20 mounted in any
convenient manner to follow the path of the sun to maximize
exposure of the solar collectors to solar energy. Each of the solar
collectors 20 can have the same or have different designs to direct
the solar energy to a particular area where the solar energy is
converted to an alternative energy source, e.g. electric energy or
heat.
[0043] With reference to FIG. 2, each of the solar collectors 20
includes a shaped reflective mirror, e.g. a parabolic shaped mirror
22 (also referred to herein as a "primary mirror") to focus the
solar energy on device 26 to convert the solar energy to electric
energy. The parabolic shaped mirror 22 includes a parabolic shaped
glass substrate 28. The glass substrate 28 preferably has a total
iron content of less than 0.020 weight percent, a 90% transmission
in the visible range, e.g. 350 to 770 nanometers ("nm"), of the
electromagnetic spectrum, and in the infrared ("IR") range, e.g.
greater than 770 nm to 2150 nm of the electromagnetic spectrum, and
a low absorption, e.g. below 2% in the visible range and the IR
range. Glasses having the preceding optical properties are
disclosed in U.S. patent application Ser. No. 12/275,264 filed Nov.
21, 2008 and U.S. Pat. No. 5,030,594, which documents in their
entirety are incorporated herein by reference. PPG Industries, Inc.
sells glasses having the above properties under the trademarks
STARPHIRE and SOLARPHIRE PV. The shaped glass substrate 28 has a
concave surface 30 and an opposite convex surface 32. The periphery
of shaped glass substrate 28 is shaped to provide sides 33. As
shown in FIG. 1, the sides 33 of adjacent solar collectors 20
contact one another to maximize coverage of a given area with
reflective surfaces. A reflective coating, layer or film 34
(clearly shown in FIG. 2) is over and preferably on the convex
surface 32 of the shaped glass substrate 28. The reflective film 34
can be metal, e.g. but not limited to silver, aluminum, nickel,
stainless steel or gold. Usually the reflective film 34 is
silver.
[0044] With continued reference to FIG. 2, the parallel solar
energy rays represented by rays 36 are incident on the concave
surface 30. A portion 37 of the rays 36 is reflected from the
concave surface 30 to the converting device 26, and a portion 38
passes through the concave surface 30, and through the shaped glass
substrate 28, and is reflected from surface 42 of the reflective
film 34 back through the shaped glass substrate 28 as reflected ray
43 (see FIG. 2A) to the converting device 26. The solar energy rays
are shown in FIG. 2 as two rays 36 for purpose of clarity and
simplicity instead of the infinite number of parallel solar energy
rays incident on the concave surface 30. Further, as is appreciated
by those skilled in the art, there is reflection of the solar rays
between the concave surface 30 and the convex surface 32 of the
shaped glass substrate 28; however, a detailed discussion of the
transmission, absorption and reflection of the solar energy rays
incident on, and passing through a transparent substrate is well
known in the art and no further discussion is deemed necessary.
[0045] In the embodiment shown in FIGS. 1 and 2, the converting
device 26 includes a shaped secondary mirror 44 positioned relative
to the focal point of the parabolic shaped mirror or primary mirror
22, and an optical rod or light bar 46 (clearly shown in FIG. 2) at
the focal area of the primary mirror 44. Multi-junction solar cells
48 are positioned at end 50 of the light bar 46. With this
arrangement the reflected rays 37 and 43 (see FIG. 2A) are incident
on the secondary mirror 44; the secondary mirror reflects the rays
37 and 43 to end 52 of the light bar 46 (clearly shown in FIG. 2).
The rays 37 and 43 pass through the light bar 46 and out of the end
50 of the light bar 46, and are incident on the solar cells 48 to
convert the solar energy to electric energy. As is appreciated by
those skilled in the art, the solar cells 48 can be positioned at
the focal point of the primary mirror 22 to eliminate the secondary
mirror 44.
[0046] The invention is not limiting to the shape of the secondary
mirror 44. More particularly, the secondary mirror in the practice
of the invention preferably has a flat reflective surface. In the
practice of the invention, the secondary mirror was a circular
piece of flat glass having a solar reflecting coated surface, e.g.
a silver coated surface. The invention, however, can be practiced
using a shaped secondary mirror having concave and convex surfaces
and a reflective coating on at least one of the surfaces, e.g. the
convex surface.
[0047] With reference to FIG. 1, a cover 60 (partially shown in
upper left hand corner of FIG. 1) is supported over the array of
solar collectors to prevent dust and water from depositing on the
concave surface 30 of the parabolic shaped mirror 22 of the solar
collectors 20. As is known in the art, the cover 60 is transparent
to the visible and IR wavelength ranges of the electromagnetic
scale. Optionally the shaped glass substrate 28 of the primary
mirror 22 has a cut out 64 (clearly shown in FIG. 2) at the bottom
of the glass shaped substrate 28 to provide access to the light bar
46 and the solar cells 48.
[0048] As discussed above in the section titled "Description of The
Available Technology," a limitation of the presently available
solar collectors is the use of soda-lime-silica glass substrates
for the primary mirror 22 and for the secondary mirror 44. The
glass substrates are usually cut glass pieces cut from a continuous
glass ribbon made by the float glass process, e.g. the glass making
process disclosed in U.S. Pat. Nos. 3,333,936 and 4,402,722, which
patents in their entirety are hereby incorporated by reference. As
is well known in the art, the soda-lime silica glass contains
sodium ions. The long term environment exposure, e.g. to the solar
rays 36 impinging on the primary mirror 22 heats the shaped glass
substrate 28, and the heating of the glass to form the parabolic
shaped substrate 28, provides energy for sodium ions to diffuse or
leech out of the shaped glass substrate 28. The sodium ions
leeching out of the shaped glass substrate 28 at the surfaces 30
and 32 react with the moisture in the atmosphere, and convert the
sodium ions to sodium compounds, e.g. sodium hydroxide and sodium
carbonate. The sodium compounds deposit as a precipitate on the
surfaces of the shaped glass substrate 28. The sodium compound
precipitate on the concave surface 30 of the shaped glass substrate
28 decreases the visible light transmission of the shaped glass
substrate 28 and makes portions of the concave surface 30 having
the sodium compound precipitate a non-specular or diffusing surface
directing the reflected rays 37 and 43 away from the focal point of
the primary mirror 22, or away from the secondary mirror 44. There
is minimal, if any, sodium compound precipitate on the convex
surface 32 of the primary mirror 22 because the convex surface has
the reflective coating 34 and a protective plastic coating or film
53 (shown only in FIG. 2) over the reflective coating. As is known
in the art, the protective coating 53 protects the reflective
coating 34 from the environment, and in the practice of the
invention, the protective coating 53 limits sodium ions at the
convex surface 32 of the glass substrate 28 from reacting with the
environment to form the sodium precipitates. Although the
protective coating 53 for the reflective coating 34 prevents the
formation of sodium compound precipitates, the invention
contemplates the practice of the invention on the convex surface 32
of the glass substrate 28. As can now be appreciated, the secondary
mirror 44, which is made of soda-lime silica glass, can have the
same drawbacks as the primary mirror 22 except that the sodium
compound precipitate on the secondary mirror directs the reflected
rays from the primary mirror 22 away from the light rod 46.
[0049] With reference to FIG. 3, in one non-limited embodiment of
the invention, the concave surface 30 of the shaped glass substrate
28 of the primary mirror 22 has a sodium barrier coating or layer
or film 66.
[0050] With reference to FIG. 4, the sodium barrier coating 66 is
applied over and preferably on surface 68 of a circular shaped flat
glass piece 70. The surface 68 of the glass piece 70 is designated
to be the concave surface 30 of the shaped glass substrate 28. In
the practice of the invention the barrier layer 66 preferably
transmits greater than 90%, more preferably greater than 95% and
most preferably 100% of the visible and IR spectrum of the
electromagnetic wavelength. The barrier layer 66 preferably can
withstand temperatures greater than the shaping or bending
temperature of the glass, e.g. temperatures greater than
1220.degree. Fahrenheit ("F") for soda-lime silica glass. Further,
the barrier layer 66 preferably does not crack and/or buckle during
shaping of the glass piece 70 to the extent that alkali ions, e.g.
sodium ions, can not move through the cracks in the barrier coating
66, and the buckling does not significantly deflect the rays 37 and
43 away from the focal point of the parabolic shaped mirror 22. A
discussion of cracks in the barrier coating 66 and buckling of the
barrier coating 66 is presented in more detail below.
[0051] In one non-limiting embodiment of the invention, the
circular flat glass piece 70 had a diameter of 18 inches (45.72
centimeters ("cm")) and a thickness of 0.083 inch (2.1 millimeters
("mm")). An 800 angstrom thick barrier coating 66 of an oxide of 85
atomic percent silicon and 15 atomic percent aluminum was deposited
on the surface 68 of the glass piece 70 (designated to be the
concave surface 30 of the shaped glass substrate 28) by the MSVD
coating process. The surface 72 of the coating glass piece
designated to be the convex surface 32 of the shaped glass
substrate 28 was placed on open end 74 of a vacuum-shaping mold 76
(see FIG. 5A). The glass piece 70 and the mold 76 were heated in a
furnace (not shown) to heat the glass piece to a temperature of
1220.degree. F. (660.degree. centigrade ("C")). The coated glass
piece 70 and the vacuum-mold 76 were uniformly heated in any usual
manner. After the coated glass piece 70 and the vacuum-mold 76 were
heated to 1220.degree. F. (660.degree. C.), air was evacuated from
the interior 78 of the mold 76 by way of spaced holes 77 to force
the heated glass piece 70 into the interior 78 of the vacuum mold
76 to provide the shaped glass substrate 28 having the coating 66
(see FIG. 5B). The heated shaped glass substrate was controllable
cooled to anneal the shaped glass substrate. As can be appreciated,
the invention contemplates heating the glass piece 70 and the
vacuum mold 76 separately, and thereafter placing the glass piece
70 on the open end 74 of the vacuum mold 76, and shaping the glass
piece 70 as described above. Processes and equipment for heating
glass, shaping glass in vacuum molds, for annealing glass and
coated glass are well known in the art and no detailed discussion
is deemed necessary.
[0052] During the shaping process, as the flat glass piece 70 (see
FIG. 4) is biased or pulled into the interior 78 of the vacuum mold
76, center portion 79 of the flat glass piece 70 is stretched. As a
result of the stretching, the thickness at bottom area 80 of the
shaped glass substrate 28 (see FIG. 5B) (corresponding to the
center portion 79 of the glass piece 70 in FIG. 4 and the hole 64
in FIG. 3) is 80% of the thickness of the center portion 79 of the
flat glass piece 70 (see FIG. 4), and the thickness of the marginal
edge 81 of the shaped glass substrate 28 (see FIG. 5B) is 105% of
the thickness of marginal edge 82 of the flat glass piece 70 (see
FIG. 4). As can be appreciated, the marginal edge 81 of the shaped
glass substrate 28 is highly strained and has optical distortion.
In the practice of the invention, but not limited thereto, a
segment 83 of the shaped glass substrate 28 (see FIG. 5B) was cut
off to remove portions of the highly strained and optically
distorted glass and to position the sides 33 of adjacent ones of
the shaped solar mirror 20 against one another as shown in the
array 18 (see FIG. 1). In the practice of the invention, but not
limiting to the invention, a section of about 2 inches measured
from peripheral edge 84 toward the bottom 80 (see FIG. 5B) of the
shaped glass substrate 28 was cut off. Additional portions of the
peripheral edge of the shaped glass substrate were removed to
provide the sides 33 (see FIG. 3) of the shaped glass substrate 28.
The cut out or hole 64 (see FIG. 3) was cut in the bottom area 80
(see FIG. 5B) of the shaped glass substrate 28. Thereafter, the
reflective coating, e.g. a silver layer 34 was applied over the
convex surface 32 of the shaped glass substrate 28 (see FIG. 3),
and the protective film 53 (see FIG. 2) was applied on the
reflective coating 34.
[0053] As is appreciated, the invention is not limited to the
process of cutting the hole 64 in the bottom area 80 (see FIG. 5B)
of the shaped glass substrate 28, cutting the peripheral edge 24 of
the shaped glass substrate, or to the coating process to apply the
reflective coating 34 and the protective coating 53 over the convex
surface 32 of the shaped glass substrate 28, and any cutting and/or
coating techniques known in the art can be used in the practice of
the invention.
[0054] At a temperature in the range of 1200.degree.-1300.degree.
F. (649.degree.-704.degree. C.), the glass piece 70 is heat
softened or viscous; on the other hand, the barrier coating 66 of
the invention, e.g. the oxide of aluminum and silicon is a
refractory material and remains dimensionally stable at a
temperature in the range of 1200.degree.-1300.degree. F.
(649.degree.-704.degree. C.). As used herein, the term
"dimensionally stable" means that the physical dimensions of the
coating during and/or after heating of the glass piece does not
change more than .+-.5% and preferably not more than .+-.2%. During
the shaping of the flat glass piece 70 to the shaped glass
substrate 28, the strain pattern shown in FIGS. 6-8 develops in the
shaped glass substrate 28. With reference to FIGS. 6-8, as needed,
radial tension strain shown by number 90 are present at the bottom
portion of the shaped glass substrate (see FIG. 8), and
circumferential compression strain shown by the number 92 are
present at the periphery 84 of the shaped glass substrate 28. The
barrier coating 66 experiences the stresses due to being adhered to
the concave surface of the glass substrate. As the distance from
the periphery 84 of the shaped glass substrate 28 increases in a
direction toward the bottom area 80 of the shaped glass substrate
28 (see FIG. 7), the radial tension strain 90 generally remains the
same, and the circumferential compression strain 92 decreases to a
location designated as the "transition line" and identified by the
number 94 in FIG. 7 where circumferential tension strain designated
by the number 102 (see FIG. 8) begins in the glass and the radial
tension strain 90 (see FIG. 8) is present in the glass. For the
shaped glass substrate 28 under discussion, e.g. the shaped glass
substrate 28 made from the flat glass piece 70 having a diameter of
18 inches (45.72 cm) and a thickness of 0.083 inch (2.1 mm), the
transition line 94 is at a position on the shaped glass substrate
28 that corresponds to a position on the flat glass piece 70 about
3 inches (7.62 cm) from the center, i.e. from the center of the
center portion 79, of the flat glass piece 70. As the distance from
the transition line 94 in a direction toward the bottom area 80 of
the shaped glass substrate 28 increases, the shaped glass substrate
has increasing circumferential tension strain designated by the
number 102 and has the radial tension strain 90 (see FIG. 8).
[0055] As is appreciated by those skilled in the art, the strains
in the shaped glass substrate 28 can be measured in any convenient
manner. In the practice of the invention, the strains of the shaped
glass piece 28 under discussion were calculated using the ANSYS
finite element computer program.
[0056] The sodium barrier coating 66 in the circumferential
compression area 103 of the shaped glass substrate 28, i.e. the
area between the periphery 84 and the transition line 94 of the
shaped glass substrate 28 (see FIG. 7) was observed to have
buckling in the radial direction perpendicular to the compressive
strain in the glass. In the location of the transition line 94, the
barrier coating 66 was observed to have an area of radial cracks.
In the circumferential tension area 104 of the shaped glass
substrate 28, i.e. the area between the transition line 94 and the
bottom area 80 of the shaped glass substrate 28 (see FIG. 7), the
barrier coating 66 was observed to have small random fissures or
cracks.
[0057] As discussed above, the maximum compressive stresses are at
the marginal edge portions 81 of the shaped glass substrate 28 (see
FIGS. 5B and 7), and it is expected that maximum buckling of the
barrier coating 66 will be present at the marginal edge portions
81. It has also been observed that very few of the suns rays
impinging on the marginal edge portions 81 of the initially shaped
glass substrate 28 are directed to the focal point or focal area of
the shaped glass substrate 28. In view of the foregoing, the
marginal edge portion 81 of the initially shaped glass substrate 28
extending a distance from the peripheral edge 84 of the shaped
glass substrate 28 equal to 10-15% of the distance measured from
the peripheral edge 84 to the center of the bottom area 80 of the
initially shaped glass substrate was removed. In one non-limiting
embodiment of the invention, for a shaped glass substrate 28 shaped
from a flat glass piece 70 having a diameter of 18 inches (45.72
cm), a section of about 2 inches (5.08 cm) measured from peripheral
edge 84 toward the bottom 80 (see FIG. 5B) of the shaped glass
substrate was cut off to remove portions of the highly strained and
optically distorted glass. Additional portions of the peripheral
edge of the shaped glass substrate were removed to provide the
sides 33 (see FIG. 3) of the shaped glass substrate 28,
[0058] The discussion is now directed to the observed and/or
expected defects caused by the fissures and/or cracks in the
barrier coating 66, and the observed and/or expected defects caused
by buckling of the barrier coating. It is expected that cracks or
fissures that extend through the thickness of the barrier coating
66 will provide passageways for moisture in the atmosphere and the
sodium ions leeching out of the glass to interact with one another
to form sodium compound precipitates which can deposit on surface
108 of the barrier coating 66 (see FIG. 7) and/or between the
barrier coating 66 and the concave surface 30 of the shaped glass
substrate 28. The sodium compounds on the surface 108 of the
barrier coating 66 can change the specular surface of the barrier
coating 66 to a non-specular or diffusing surface, and the sodium
compound precipitates between the barrier coating 66 and the convex
surface 30 can cause separation of the barrier coating 66.
[0059] The defect of buckling can change the surface 108 of the
barrier coating 66 from a specular surface to a non-specular or
diffusing surface, and severe cases of buckling can, in addition,
cause cracks in the barrier coating. The following discussion is
directed to the barrier coating 66, and the discussion, unless
indicated otherwise, is applicable to the scratch resistant
properties (discussed above) of the barrier coating.
[0060] With reference to FIGS. 9A-9C as needed, the barrier coating
66 on a segment 110 of the glass piece 70 (FIG. 9A) expected to be
in the area of circumferential compression 103 (see FIG. 7) has a
length measured between sides 112 and 113, and a width measured
between sides 116 and 117. After the glass piece 70 is shaped into
the shaped glass substrate 28, the segment 110 of the flat glass
piece 70 corresponds to segment 118 of the shaped glass substrate
28. The convex surface 32 of the segment 118 of the shaped glass
substrate 28 has a length as measured between the sides 112 and 113
of the segment 118 that is slightly greater than the length
measured between sides 112 and 113 of the segment 110 of the flat
glass piece 70, and the convex surface 32 of the segment 118 of the
shaped glass substrate 28 has a width as measured between sides 116
and 117 of the segment 118 that is less than the width of the
segment 110 of the flat glass piece 70 as measured between sides
116 and 117 of the segment 118. The concave surface 30 of the
segment 110 of the shaped glass substrate 28 has a length as
measured between sides 112 and 113 of the segment 118 that is
slightly greater than the length measured between sides 112 and 113
of the segment 110 of the flat glass piece 70, and the concave
surface 30 of the segment 118 of the shaped glass substrate 28 has
a width as measured between sides 116 and 117 of the segment 118
that is less than the width of the flat glass piece 70 as measured
between sides 116 and 117 of the segment 118.
[0061] The difference in the increase between the length of the
convex surface 32 and the length of the concave surface 30 as
measured between the sides 112 and 113 of the segment 118 is small.
The difference in the decrease between the width of the concave
surface 30 as measured between the sides 116 and 117 of the segment
118 is greater than the difference between the length of the
concave side and convex side of the segment 118. By way of
illustration and not limiting to the invention, a measured
expansion between the sides 112 and 113 of the segment 110 and the
sides 112 and 113 of the segment 118 was 2-6% for both the concave
side and the convex side. The contraction between the sides 116 and
118 of the segment 110 and the sides 116 and 118 of the segment 118
measured at the perimeter of the shaped glass substrate 28 was 14%
with the concave side 30 having a contraction of 14% and the convex
side 32 having a contraction of 13%. At the bottom 80 of the shaped
glass substrate 28, the elongation for the convex and concave sides
was 5% and 4%, respectively.
[0062] The length and width of the barrier coating 66, on the other
hand, remains the same and buckles because of the reduction of the
width of the concave and convex surfaces of the shaped glass
substrate 28 compared to the corresponding width of the flat glass
piece 70 commonly referred to as strain. More particularly, the
glass is viscous during the shaping process, and the buckling of
the barrier coating 66 changes the contour of the concave surface
30 of the shaped glass substrate 28 to a surface having folds 120,
e.g. a corrugated surface (see FIG. 9B) to accommodate the decrease
in the width of the surface 72 of the flat glass piece 70. The
folds 120 change the surface 108 of the barrier coating 66 and the
concave surface 30 of the shaped glass substrate 28 from a specular
surface in FIG. 9A to a non-specular or diffusing surface in FIG.
9B. In the first instance (FIG. 9B), as the thickness of the
barrier coating 66 increases, e.g. the barrier coating increases to
a thickness of 160 nanometers ("nm"), while the amount of shrinkage
of the width of the flat glass piece remains the same, the number
of folds 120 and the height of the folds 120 increases, increasing
the percentage of diffused reflected sun rays 37 and 43 (see FIGS.
2 and 2A). In the second instance (FIG. 9C) as the thickness of the
barrier coating 66 decreases, e.g. the barrier coating 66 decreases
to a thickness of 60 nm, while the amount of shrinkage of the flat
glass piece 70 remains the same, the number of folds 120 and the
height of the folds in the second instance (FIG. 9C) is less than
the number of folds 120 and the height of the folds 120 in the
first instance (see FIG. 9B), decreasing the percentage of diffused
reflected sun rays 37 and 43 (see FIGS. 2 and 2A). As mentioned
above, the area 103 of the circumferential compression (see FIG. 7)
decreases as the distance from the periphery 84 of the shaped glass
substrate 28 increases (see FIGS. 6-8); therefore the percent
shrinkage of the circumferential width of the concave surface 30 of
the shaped glass substrate 28 decreases as the distance from the
periphery 84 of the shaped glass substrate 28 increases, and the
thickness of the barrier coating 66 can be increased without
increasing the number of folds 120 and the amplitudes of the folds
120 (see FIGS. 9B and 9C).
[0063] In one non-limiting embodiment of the invention, the
thickness of the barrier coating 66 is selected to have sodium
barrier properties and to minimize buckling. More particularly, the
minimum thickness of the barrier coating 66 is selected to prevent
the sodium ions from reacting with moisture in the atmosphere to
convert the sodium ions to sodium compound precipitates and to
minimize buckling. As is appreciated by those skilled in the art,
the mechanism of sodium ions moving out of the glass is a diffusion
process and for purposes of this invention the parameter of
interest is the amount of sodium ions present in the glass. The
diffusion rate, size of the alkali ion, e.g. the sodium ion, and
the energy to drive the sodium ion to the surface of the shaped
glass substrate 28 is not considered relevant to the present
discussion because the use of the solar mirror is a long term use,
e.g. 30 years.
[0064] Based on the forgoing, the amount of alkali ions or sodium
ions in glass is a function of the glass composition and the
thickness of the glass piece, e.g. as the thickness of the glass
piece 70 or of the shaped glass substrate 28 increases, the number
of sodium ions in the glass piece increases, and the thickness
and/or density of the barrier coating is preferably increased. For
a soda-lime-silica glass the sodium concentration is generally 14
weight percent. In one non-limiting embodiment of the invention the
parabolic shaped mirror 22 is made of a glass substrate having a
thickness of 0.083 inch (2.1 millimeter). In this non-limiting
embodiment of the invention, the barrier coating is an MSVD coating
of an oxide of 85 atomic percent silicon and 15 atomic percent
aluminum. The minimum coating thickness to prevent sodium ions from
reacting with moisture in the environment to convert the sodium ion
to sodium compound precipitates is 40 nm. As is appreciated, any
thickness above the minimum thickness prevents sodium ions from
reacting with moisture in the environment; however, as the
thickness of the barrier coating 66 increases, the severity of the
buckling increases. In the practice of the invention, the barrier
coating 66 in the circumferential tension area 104 (see FIG. 7) is
preferably in the range of 40-100 nm, more preferable in the range
of 60-100 nm, and most preferably in the range of 60-80 nm. The
same coating composition having coating thicknesses in the range of
40-100 nm provides a protective coating against mechanical and
chemical attack and/or damage.
[0065] As discussed above, the flat glass piece 70 is shaped using
the vacuum mold 76 (see FIGS. 5A and 5B). After the flat glass
piece 70 is shaped, the shaped glass substrate is removed from the
mold 76 when the glass is dimensionally stable and is annealed. For
purposes of the invention, the glass is considered to be
dimensionally stable when the shaped glass can support its own
weight without changing its shape. For the glass disclosed in U.S.
patent application Ser. No. 12/275,264 filed Nov. 21, 2008 and U.S.
Pat. No. 5,030,594, the glass is dimensionally stable at a
temperature of 1050.degree. F. The annealing process reduces the
intrinsic stresses in the barrier coating 66 and in the shaped
glass substrate 28 to minimize residual stresses so that the
barrier coating and the shaped glass substrate 28 can be cut
without shattering the substrate 28 or fracturing the barrier
coating. The annealing equipment and rate at which the flat glass
substrate 28 is annealed is not limiting to the invention, and any
equipment for, and method of, and rate of, annealing known in the
art can be used in the practice of the invention. Annealing coated
and uncoated glass articles is well known in the art and no further
discussion is deemed necessary.
[0066] The invention is not limited to the thickness of the glass
piece 70, and the glass piece can be any thickness. In the
preferred practice of the invention, the glass piece 70 is
preferably thin to provide a light-weight shaped glass substrates
28. Although thin glass is preferred, the glass thickness should be
sufficient thick to have structural stability. As used herein the
term "structural stability" means the glass has to be processed
from the flat glass piece 70 (see FIG. 4) to the parabolic shaped
mirror 22 (see 3) using a vacuum mold or a pressing mold with
minimal glass breakage. In the practice of the invention, the glass
thickness is preferably in the range of 0.075-0.126 inch (1.9-3.2
mm), more preferably in the range of 0.078-0.110 inch (2.0-2.8 mm),
and most preferably in the range of 0.083-0.091 inch (2.1-2.3
mm).
[0067] In the preferred practice of the invention, the barrier
coating 66 is an oxide of 15 atomic percent aluminum and 85 atomic
percent silicon. Increasing the atomic percent of aluminum makes
the coating stiffer. Although a stiffer coating reduces buckling,
it is prone to cracking. The cracks in the coating can result in
moisture in the atmosphere reacting with the sodium ions converting
the sodium ions to sodium compounds. For barrier coatings of an
oxide of aluminum and silicon, the coatings preferably include
30-100 atomic percent silicon and 0-70 atomic percent aluminum,
more preferably 50-95 atomic percent silicon and 5-50 atomic
percent aluminum, e.g. 30 to less than 100 atomic percent silicon
and greater than 0 to 70 atomic percent aluminum, and most
preferably includes 60-90 atomic percent silicon and 10-40 atomic
percent aluminum. As can be appreciated, the invention is not
limited to a barrier coating or film of an oxide of aluminum and
silicon, and any sodium barrier film of the type known in the art
can be used in the practice of the invention. Types of barrier
coatings that can be used in the practice of the invention include,
but are not limited to, the coatings or films disclosed in United
States Publication 2007/0275253A1, which document in its entirety
are hereby incorporated by reference.
[0068] As is appreciated by those skilled in the art of MSVD
coating, the deposition parameters can be altered to reduce
intrinsic stresses in the coated barrier film; however, as
discussed above, the barrier film and the shaped glass substrate
are annealed at the same time to minimize residual stresses so that
the shaped glass substrate 28 can be cut without shattering the
substrate 28. Therefore reducing the intrinsic stress in the
barrier coating during the deposition of the coating is optional
and not limiting to the invention.
[0069] The invention contemplates reducing the strain in the shaped
glass substrate 28 by reducing the time to shape the glass piece 70
(see FIG. 4) into the shaped glass substrate 28 (see FIG. 5B). As
can be appreciated as the temperature of the glass piece 70
increases, the viscosity of the glass decreases, and the amplitude
of the buckling of the barrier coating 66 increases because the
coating has time to buckle to its full extent, and the glass has
time to flow in the plane of the coating, e.g. the glass has time
to flow into the folds of the barrier coating 60 or 120 (see FIG.
9C). Further, increasing the shaping time, i.e. the time it takes
to pull the glass piece 70 into the cavity of the shaping mold 76,
increases the amplitude of the buckling of the barrier coating 66
because the coating 66 has time to buckle to its full extent, and
the glass has time to flow into the folds of the barrier coating 66
(see FIG. 4) or 120 (see FIG. 9C).
[0070] In the practice of the invention, the glass piece 70 at the
time of forming preferably has a viscosity in the range of
1.00.times.10.sup.7.8 poise to 5.36.times.10.sup.9 poise, when the
glass piece is pulled into the vacuum mold 76. At this viscosity
range, minimum buckling of the barrier coating 66 was found to
occur when the shaping time is three seconds, and maximum buckling
of the barrier coating 66 was found to occur when the shaping time
is 25 seconds. Based on the forgoing, it is expected that minimum
buckling of the barrier coating 66 is greater than zero to five
seconds and preferably three seconds, and maximum buckling of the
barrier coating 66 is 25 or more seconds for glass in the viscosity
range of 1.00.times.10.sup.7.6 poise to 5.36.times.10.sup.9
poise.
[0071] As is appreciated by those skilled in the art the
temperature versus viscosity curve for glass depends on the glass
composition. It has been determined that soda-lime-silica glass of
the type sold by PPG Industries, Inc. under the registered
trademark STARPHIRE has a viscosity in the range of
1.00.times.10.sup.7.8 poise to 5.36.times.10.sup.9 poise at
temperatures in the range of 1200.degree. to 1300.degree. F. In the
practice of the invention, the piece 70 of STARPHIRE glass was
heated in a furnace set at 1300.degree. F. to heat the glass piece
70 to an expected temperature of 1220.degree. F. The glass had a
viscosity of 2.60.times.10.sup.9 poise, and minimum buckling of the
barrier coating 66 was found to occur when the shaping time is
three seconds, and maximum buckling of the barrier coating 66 was
found to occur when the shaping time is 25 seconds.
[0072] As can now be appreciated by those skilled in the art, the
strain patterns for the convex side of the shaped glass piece 28
are similar to the strain patterns for the concave side of the
shaped glass piece 28.
[0073] With reference to FIGS. 10-13, as needed, the invention also
contemplates reducing the strain in the shaped glass substrate 28
by cutting segments from a flat glass sheet, shaping the segments
and joining the shaped segments together to provide a shaped glass
substrate similar in shape to the shaped glass substrate 28 (see
FIG. 3). In one non-limiting embodiment of the invention, surface
124 of a flat glass sheet 126 is coated with the barrier coating 66
(see FIG. 10). The surface 124 of the glass sheet 126 is expected
to be the concave surface 128 of the shaped glass substrate 130
(see FIGS. 12 and 13). Four flat segments 132-135 are cut from the
glass sheet 126. Each of the flat segments 132-135 includes a
radiused corner 136 joining sides 138 and 140; a flat end 142
joining sides 144 and 146; side 138 is joined to side 144 at corner
148, and side 140 is joined to side 146 at corner 149.
[0074] Each of the segments 132-135 are sized such that shaping the
segments 132-135 as discussed below provides 1/4 of the shaped
glass substrate 130 (see FIGS. 12 and 13) such that joining the
shaped segments 132-135 together in a manner discussed below forms
the shaped glass substrate 130, which is similar to the shaped
glass substrate 28 (see FIG. 3).
[0075] The invention is not limited to the manner in which the
segments 132-135 are cut from the glass sheet 126, and any of the
cutting or scoring techniques known in the art can be used in the
practice of the invention. The edges of the segments 132-135 can be
seamed as is known in the art for purposes of safety. Each of the
flat segments 132-135 are shaped in any convenient manner using any
of the pressing methods and equipment known in the art, e.g. but
not limited to press bending using a solid upper mold having a
shaping surface and a lower mold having a flexible supporting
surface; a solid upper mold having a shaping surface and a lower
ring mold, and a vacuum upper mold having a shaping surface, e.g.
as disclosed in U.S. Pat. Nos. 7,240,519 and 7,437,892 which
patents in their entirety are hereby incorporated by reference.
[0076] In the preferred practice of the invention, the segments
132-135 are shaped using an upper vacuum mold having a shaping
surface. With reference to FIG. 11, one of the segments 132-135,
e.g. the segment 132 is heated to a viscosity in the range of
1.00.times.10.sup.7.8 poise to 5.36.times.10.sup.9 poise and
provided on curved surface 156 of lower support member 157. Upper
vacuum shaping mold 158 having a shaped surface and the support
member 157 are moved relative to one another, e.g. the upper mold
158 moved toward the lower support member 157 to bring the segment
132 into contact with the shaping surface 159. Vacuum is pulled
through the shaping surfaces 159 of the upper mold 158 to shape the
segment 132. The process is repeated to shape the remaining three
segments 133-135 to provide four shape segments 160-163.
Optionally, the four segments can be shaped simultaneously by
providing a shaping mold with four shaping areas.
[0077] The reflective coating 34 and the protective coating 53 (see
FIG. 2) is applied to the convex surface of the shaped segments
160-163.
[0078] In the preferred practice of the invention, the barrier
coating 66 is applied to the surface 124 of the flat glass sheet
126 before the segments 132-135 are cut from the glass sheet 126.
The invention, however, contemplates applying the barrier coating
66 to the flat segments 132-135 or the shaped segments 160-163. In
the practice of the invention, the reflective coating 34 and the
protective coating 54 are applied to the convex surface of the
shaped segments 160-163; the invention, however, contemplates
applying the reflective coating 34 and the protective coating 53 to
the surface of the glass sheet 126 opposite to the surface 124 of
the glass sheet. As can be appreciated, if the reflective coating
34 and the protective coating 54 are applied before the segments
132-135 are shaped, the reflective coating 34 and the protective
coating 54 have to withstand the temperatures at which the glass
segments 132-135 are shaped. Optionally the protective coating 54
can be applied after the segments are shaped.
[0079] The invention is not limited to the number of segments
132-135 joined to make the shaped glass substrate 130, and the
shaped glass substrate 130 can be formed by joining 2, 3, 4, 5 or
more segments. As can now be appreciated, the greater the number of
shaped segments joined to form the shaped glass substrate 130, the
greater will be the reduction in the strain in the shaped glass
substrate 28 or 130.
[0080] With reference to FIGS. 12 and 13, the shaped glass segments
160-163 are joined together in any convenient manner. In one
non-limited embodiment of the invention, the segments 160-163 are
positioned together to form the shaped glass substrate 130, and a
pair of rings 166 and 168 (shown only in FIG. 12) are secured to
the reflective coating 34 by an adhesive. In another non-limiting
embodiment of the invention, the rings 166 and 168 are joined to
the convex surface 32 of the shaped glass substrate. Thereafter,
the convex surface of the joined shaped segments 160-163 and the
rings 166 and 168 are coated in any convenient manner with the
reflective coating 34 and the protective coating 53. In still
another non-limiting embodiment of the invention, the sides of the
shaped segments are joined together by an adhesive, e.g. an
adhesive joins the sides 140 of adjacent ones of the shaped
segments together, and the sides 138 of adjacent ones of the shaped
segments together as shown in FIG. 12. As viewed in FIGS. 10 and
13, the radiused corners 136 form the cut out 64 of the shaped
substrate 130.
[0081] The invention is not limited to manner in which the
dimensions of the flat segments 132-135 are derived. For example
and not limiting to the invention, the dimensions of the flat
segments can be derived from a computer program, and from
constructing the shaped parabolic substrate, cutting the shaped
substrate into the desired number of segments, and measuring the
sides of the segments.
[0082] As can now be appreciated, employing the above techniques
will reduce the strain in the glass and will reduce the buckling
and fracturing of the barrier coating 66; however, as long as
strains remain in the glass the barrier coating 66 will have
degrees of buckling and cracking. In view of the forgoing, the
invention contemplates further reducing the fracturing and buckling
of the barrier coating 66 by providing barrier coatings 66 of
different thickness over selectively surface portions of the flat
glass piece 70 designated to be the concave surface 30 of the
shaped glass substrates 28 (see FIG. 3) and the shaped glass
substrate 126 (see FIG. 13), In the following discussion, the
embodiments of the invention are practiced on the flat glass piece
70 to provide the shaped glass substrate 28 shaped from the flat
glass piece 70. The discussion, however unless indicated otherwise
is applicable to applying the barrier coating 66 to the glass
segments 132-135, or the shaped glass segments 160-163.
[0083] In a first non-limiting embodiment of the invention, the
barrier coating 66 has a constant thickness over the surface 68 of
the flat glass piece 70 (see FIG. 4) designated to be the concave
surface 30 of the shaped glass substrate 28 (hereinafter referred
to as "Coating Technique No. 1"). In a second non-limiting
embodiment of the invention, the changing of the circumferential
strains in the concave surface 30 of the shaped glass substrate 28
is compensated for by applying or depositing a barrier coating or
layer 66 that has varying thickness, e.g. a thickness that
increases as the distance from perimeter 150 of the circular flat
glass piece 70 (see FIG. 4) increases in a direction toward the
center portion 79 of the flat glass piece 70 (hereinafter referred
to as "Coating Technique No. 2"). In a third non-limiting
embodiment, the changing of the circumferential strains in the
concave surface 30 of the shaped glass substrate 28 is compensated
for by applying or depositing the barrier layer 66 to have a first
constant thickness from the perimeter 170 of the flat glass piece
70 to the expected position of the transition line 94 (see FIG. 7),
and a second constant thickness from the transition line 94 to the
center portion 79 of the flat glass piece 70, with the second
thickness of the barrier coating thicker than the first thickness
of the barrier coating (hereinafter referred to as "Coating
technique No. 3").
[0084] The variation of coating thickness for making the shaped
glass substrate 28 (see FIGS. 3 and 5B) can be accomplished by
masking areas of the flat piece 70 to have a thin coating, e.g.
using a shield 170 to cover the surface of the glass piece 70 (see
FIG. 14) expected to be in the circumferential compression area 103
(see FIG. 7) as the center portion 79 of the flat glass piece 70 is
coated.
[0085] Coating Technique No. 1 is practiced to provide the segments
160-163 by coating the surface 124 of the flat glass sheet 126
before, or after, cutting the outline of the segments 132-136 in
the sheet. Coating technique No. 2 is practiced to provide the
segments 160-163 by coating the segments after the segments 132-136
are outlined in the flat glass sheet 126 by cut lines, or after the
segments 132-136 are removed from the glass sheet. The thickness of
the coating 66 for Coating Technique No. 2 increases as the
distance from the flat end 142 (see FIG. 10) increases in a
direction toward the radiused corner 136. Coating technique No. 3
is practiced to provide the segments 160-163 by coating the
segments after the segments 132-136 are outlined in the flat glass
sheet 126 by cut lines, or after the segments 132-136 are removed
from the glass sheet. The coating 66 for Coating Technique No. 3 is
applied to the segments 132-135 to have a first constant thickness
from the sides 144 and 146 of the flat segments 132-136 to the
expected position of the transition line 94 (see FIG. 7), and a
second constant thickness from the transition line 94 to the
radiused end 136 of the segments 132-136.
[0086] The barrier coating 66 for Coating Technique No. 1 has a
constant thickness in the range of 40-100 nm, or in the range of
80-100 nm. In one non-limiting embodiment of the invention, the
barrier coating 66 included an oxide of 85 atomic percent silicon
and 15 atomic percent aluminum. The barrier coating 66 having a
thickness of 80 nm was deposited by the MSVD on the surface 72 of
the flat piece glass 70. The glass was of the type disclosed in
U.S. patent application Ser. No. 12/275,264 filed Nov. 21, 2008 or
U.S. Pat. No. 5,030,594. The flat glass piece 70 was a circular
piece of glass having a diameter of 17.75 inches; a total iron
content of less than 0.020 weight percent, a 90% transmission in
the visible range, and the IR range, of the electromagnetic
spectrum, and below 2% absorption in the visible range and the IR
range. The flat glass piece 70 was shaped in a vacuum mold to
provide the shaped glass substrate 28, e.g. a bending time of less
than 25 seconds. After the shaped glass substrate cooled, the
periphery of the shaped glass substrate 28 was shaped as discussed
above to provide the shaped glass substrate 28 with the sides 33
and the center hole 28 (see FIG. 3). A reflective silver coating 34
was applied over the convex surface 32 of the shaped glass
substrate 28 to provide the parabolic shaped mirror 22.
[0087] The Coating Technique No. 2 provides a barrier coating 66
that increases in thickness as the distance from the periphery of
the flat glass piece 70 toward the center portion 79 increases,
e.g. the barrier coating 66 increases, preferably, but not limiting
to the invention, from a thickness of 40 nm at the periphery 172 of
the flat glass piece 70 to a thickness of 80 nm at the center
portion 79 of the flat glass piece 70. In this manner the thickness
of the barrier coating 66 increases as the circumferential strains
in the glass decrease and the % width shrinkage of the concave
surface 30 of the shaped glass substrate 28 decreases to reduce the
buckling. Passing the transition line 94 toward the center portion
80 of the shaped glass substrate 28, the thickness of the barrier
coating 66 increases as the circumferential tension increases. With
reference to FIG. 15, there is shown a cross section of the shaped
glass substrate 28 in the circumferential tension area 104, which
is between the transition line 94 and the center area 80 (see FIGS.
7 and 15). The barrier coating 66 has fissures 174, however the
barrier coating 66 is thick enough, e.g. 80 nm such that the
fissures 154 do not extend to the surface 108 of the barrier
coating 66.
[0088] The barrier coating 66 for Coating Technique No. 3 has a
first constant thickness from the periphery 172 of the flat glass
piece 70 to the expected position of the transition line 94 of the
shaped glass substrate 28, and a second constant thickness from the
transition line 94 to the center portion 79 of the flat glass piece
70, with the first thickness of the barrier coating 66 thinner than
the second thickness of the barrier coating 66. In one non-limiting
embodiment of the invention, the first constant thickness of the
barrier coating 66 is in the range of 40-60 nm, more preferably 40
to 50 nm, and the second constant thickness is in the range of
greater than 60 to 100 nm, more preferably in the range of greater
than 60 to 80 nm. With this arrangement, the buckling of the
barrier coating 66 is minimized in the circumferential compression
area 103, and the thickness of the barrier coating 66 is thick
enough in the circumferential tension area 104 such that the
fissures 174 do not extend to the surface 108 of the barrier
coating 66. Further, with this arrangement, the thickness of the
barrier coating 66 is thinner between the peripheral edge 84 and
the transition line 94, i.e. in the area of increased glass
thickness to reduce buckling of the barrier coating 66, and the
thickness of the barrier coat 66 is thicker between the transition
line 94 and the bottom area 80 of the shaped glass substrate 28,
i.e. in the area of the thinner glass where buckling is not as
severe as in the circumferential compression area 103 and the
fissures 174 are a concern. As can be appreciated, the invention is
not limited to the coating thickness change in the area of the
transition line 94, and the coating thickness change can be a
gradual change, or a step change.
[0089] As can now be appreciated, in the instance when the
secondary mirror 44 includes a shaped substrate, the technique of
preventing buckling of the barrier coating 66 can be practiced to
make a shaped secondary mirror.
[0090] Additional embodiments of the invention include, but are not
limited to: [0091] 1. applying the barrier layer 66 and/or the
scratch resistant coating over the surface 68 of the flat glass
piece 70 designated to be the concave surface 30 of the shaped
glass substrate 28 and the barrier layer 66 over the surface 72 of
the flat glass piece 70 (see FIG. 16) designated to be the convex
surface, and shaping the flat glass sheet 70 to the shaped glass
substrate 28. Thereafter the reflective layer 34 and optionally the
protective coating 53 are applied over the barrier layer 66 on the
convex surface 32 of the shaped glass substrate 28; [0092] 2.
applying the barrier layer 66 and/or the scratch resistant coating
over the surface 68 of the flat glass piece 70 designated to be the
concave surface of the shaped glass substrate 28, and the barrier
layer 66 over the surface 72 of the flat glass piece 70 designated
to be the convex surface of the flat glass piece 70, and applying
the reflective coating layer 34 over the barrier layer 66 on the
surface 72 (see FIG. 17), and thereafter shaping the flat glass
sheet 70 to the shaped glass substrate 28; [0093] 3. shaping the
flat glass piece 70 to a parabolic shaped glass substrate 28, and
applying the barrier layer 66 and/or the scratch resistant coating
over the concave surface 30, and the reflective coating 34 over the
convex surface 32 of the parabolic shaped glass substrate 28 (see
FIG. 18), and [0094] 4. shaping the flat glass piece 70 to the
shaped glass substrate 28, and applying the barrier layer 66 over
the convex surface 32, and the barrier layer and/or the scratch
resistant coating over the concave surface 30 of the shaped glass
substrate 28, and applying the reflective coating 34 over or on the
barrier layer 66 over or on the convex surface 32 (see FIG.
19).
[0095] As can be appreciated, when the reflective layer 34 and/or
the barrier layer 66 and/or the scratch resistant coating are
applied to the flat glass piece 70, and the coated flat glass is
heated and shaped in the practice of a non-limiting embodiment of
the invention, e.g. as discussed above, the reflective layer 34 and
the barrier layer 66 and/or the scratch resistant coating have to
be able to withstand the elevated temperatures of shaping, e.g.
above 1200.degree. F. Reflective coatings that can withstand
elevated temperatures are known in the art, e.g. see U.S. Pat. No.
7,329,433 which patent in its entirety is hereby incorporated by
reference. The patent discloses primer films that are deposited on
a reflective layer to protect the reflective layer during high
temperature processing.
[0096] In the preferred practice of the invention, the barrier
layer 66 is applied using MSVD equipment. As is appreciated by
those skilled in the art, the cathodes for MSVD coating have to be
electrically conductive. To provide a silicon cathode that is
electrically conductive, aluminum is added to the silicon, e.g.
greater than 5 weight percent. The invention, however, is not
limited to MSVD application of the barrier layer, and any known
coating process for applying the barrier layer can be used in the
practice of the invention. Further, the invention is not limited to
having a homogenous barrier layer, and the invention contemplates a
barrier layer having varying compositions of oxides of silicon and
aluminum. For example in one non-limiting embodiment of the
invention, a first barrier layer of an oxide of 60 atomic weight
percent of aluminum and 40 atomic weight percent of silicon is
applied to the surface of the glass and a second barrier layer of
an oxide of 85 atomic weight percent of aluminum and 15 atomic
weight percent of silicon is applied on the first barrier
layer.
[0097] As can now be appreciated the barrier layer 66 of the
invention can be used to prevent sodium ions from damaging
conductive layers of photovoltaic devices. More particularly, and
with reference to FIG. 20, there is shown a photovoltaic device 184
having a conductive coating 186 over the barrier layer 66 of the
invention. The barrier layer 66 is applied to surface 188 of glass
sheet 190. The barrier layer 66 prevents the sodium ions forming
sodium compound precipitates that attack and damage the conductive
coating 186 of the photovoltaic cell 184.
[0098] As discussed in detail above, the barrier layer of an oxide
of silicon and aluminum in addition to providing a barrier to
prevent sodium ions from moving out of the glass, also provides a
protective layer for the glass to prevent mechanical and chemical
damage to the glass surface.
[0099] It will be readily appreciated by those skilled in the art
that modifications can be made to the non-limiting embodiments of
the invention without departing from the concepts disclosed in the
foregoing description. Accordingly, the particular non-limiting
embodiments of the invention described in detail herein are
illustrative only and are not limiting to the scope of the
invention, which is to be given the full breadth of the appended
claims and any and all equivalents thereof.
* * * * *