A fused-quartz mirror is attached to a conductive metal substrate by the following steps: tinning one surface of a fused-quartz mirror with a solder of substantially pure indium; tinning a metallic substrate with an indium eutectic alloy consisting essentially of indium bismuth, lead and tin having a melting point substantially below that of indium; heating the eutectic alloy to a temperature substantially above its melting point, but below that of the solder; floating the mirror into place, and subsequently cooling the alloy to a temperature substantially below its melting point

Mckeown, D.

Sonnenschein, G.

English

NASA Technical Reports Server

United States Patent 
TINNING SURFACE 
OF FUSED-QUARTZ MIRROR 
WITH PURE INDIUM SOLDER 
i 
Fletcher et al. 
[ I l l  4,033,503 
[GI July 5, 1977 
[54] METHOD FOR ATTACHING A 
FUSED-QUARTZ MIRROR TO A 
CONDUCTIVE METAL SUBSTRATE 
Inventors: James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of; George Sonnensehein, 
San Diego; Daniel McKeown, La 
Jolla, both of Calif. 
[ 761 
[22] Filed: Aug. 27, 1976 
1211 Appl. No.: 718,267 
[52] U.S. C1. ................................ 2281124; 2281263 
[ 5 11 Int. Cl.2 ........................................... B23K 1/02 
[SS] Field of Search .......... 228/121, 122, 123, 124, 
2281263 
1561 References Cited 
UNITED STATES PATENTS 
2,746,140 511956 Belser ............................ 228/263 X 
3,100,339 8/1963 Bondley ......................... 2281263 X 
3,1 11,741 11/1963 Allen et al. .................... 228/122 X 
Primary Examiner-Al Lawrence Smith 
Assistant Examiner-K. J. Ramsey 
Attorney, Agent, ar Firm-L. D. Wofford, Jr.; J. H. 
Beumer; John R. Manning 
r571 ABSTRACT 
A method for attaching a fused-quartz mirror to a con- 
ductive metal substrate which comprises the steps of 
tinning one surface of a fused-quartz mirror with a 
solder of substantially pure indium, tinning a metallic 
substrate with an indium eutectic alloy consisting es- 
sentially of indium, bismuth, lead and tin having a melt- 
ing point substantially below that of indium, heating the 
eutectic alloy to a temperature substantially above its 
melting point, but below that of the solder, floating the 
mirror into place, and subsequently cooling the alloy to 
a temperature substantially below its melting point. 
4 Clahns, 2 Drawing Figures 
OVEN - H EAT1 NG TINNED SUBSTRATE ABOVE 
MELTING POINT OF EUTECTIC ALLOY 
I I 
I 
t 
FLOATING THE TINNED SURFACE OF 
FUSED-QUARTZ MIRROR INTO 
SUPERIMPOSED RELATION WITH HEATED SUBSTRATE 
I 
I 1 
COOLING TO EFFECT BOND 
OF MIRROR AND SUBSTRATE 
https://ntrs.nasa.gov/search.jsp?R=19770022316 2020-03-20T07:47:40+00:00Z
U.S. Patent July 5,  1977 
/6 
////// 
Fig. I 
. 
Fig 2 
TINNING SURFACE 
OF FUSED-QUARTZ MIRROR 
TINNING A METAL SUBSTRATE 
WITH EUTECTIC ALLOY 
OVEN-HEATING TINNED SUBSTRATE ABOVE 
MELTING POINT OF EUTECTIC ALLOY 
4,033,503 
FLOATING THE TINNED SURFACE OF 
FUSED-QUARTZ MIRROR INTO 
SUPERIMPOSED RELATION WITH HEATED SUBSTRATE 
COOLING TO EFFECT BOND 
OF MIRROR AND SUBSTRATE 
1 
4,033,503 
3 
METHOD FOR ATTACHING A FUSED-QUARTZ 
ROR TO A CONDUCTIVE METAL SUBSTRATE 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 
85-568 (72 Stat. 435; 42 U.S.C. 2457). 
BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The invention generally relates to a method for at- 
taching a fused-quartz mirror to a metal substrate, and 
more particularly to a method in which one surface of 
a fused-quartz mirror is coated with a film of solder, of 
a class including pure indium solder, and a metal sub- 
strate is tinned with a film of indium eutectic alloy, 
consisting essentially of indium, bismuth, lead and tin 
and subsequently united with the mirror at a solder/al- 
loy interface. 
2. Description of the Prior Art: 
A major contaminant found aboard spacecraft is 
water vapor. Water vapor, of course, is present as a 
consequence of it having been absorbed in or deposited 
on substances and structures found aboard spacecraft, 
or as a result of operations of fuel cells frequently in- 
cluded in spacecraft systems. 
As can be appreciated by those familiar with the 
spacecraft industry, precise determination of spatial 
distribution and density of water vapor in spacecraft 
systems and experiments is important to successful 
operation. 
Water vapor often is monitored by cooling a quartz 
crystal microbalance (QCM) to temperatures below 
140" K and observing its mass loading rate. In a fully 
equipped laboratory, cooling of a QCM readily can be 
accomplished cryogenically using liquid nitrogen or 
helium. However, it is generally accepted that radiative 
cooling is potentially the most practical technique for 
cooling a QCM in a spacecraft environment, because of 
the general unavailability of cryogenic fluids aboard 
such craft. 
Radiant coolers usually include a second-surface, 
fused-quartz mirror bonded to a metal substrate. Un- 
fortunately, the coefficient of thermal expansion of a 
metal substrate, such as one formed of aluminum, cop- 
per and the like, is from fifteen to twenty-five times that 
of fused quartz. As a consequence, separation of a 
mirror from its substrate may be expected to occur. 
In an effort to overcome the propensity of mirrors to 
separate from their substrate, radiant coolers have 
been fabricated utilizing spectrally selected paints and 
adhesive coatings of metallized plastics in forming the 
reflective surfaces. Moreover, attempts have been 
made to bond the mirrors to the substrates utilizing 
silicone rubber adhesives. Unfortunately, those materi- 
als heretofore employed in attempting to avoid the 
consequences of the differentials of coefficients of 
expansion which in operation occurs between the sub- 
strate and the mirror, have poor outgassing characteris- 
tics and thus tend to introduce inaccuracies in the mon- 
itoring of water vapor. 
Use of thin layers of gold applied to each non-wetta- 
ble surface of a pair of surfaces to be joined has been 
proposed for overcoming the aforementioned difficul- 
ties. When employing layers of gold for this purpose the 
L, 
layers are positioned in face-to-face contact, and a 
length of indium wire is placed along one edge of the 
interface between the layers and heated in a vacuum. 
The indium is thus caused to flow along the interface 
5 between the layers. For example, see the method dis- 
closed in United States Letters Pat. No. 3,857,161. The 
disadvantages of such a technique is also discussed in 
the aforementioned patent. For example, it is pointed 
out that gold applied to the non-wettable surfaces, to 
10 serve as a wetting agent for the indium, is highlv soluble 
in indium. Thus, unless a sufficiently thick film of gold 
is applied to the surface, it may completely dissolve in 
the molting indium, resulting in a de-wetting of the 
surfaces and consequent failure of the seal. Addition- 
15 ally, the strength of the bond thus produced tends to be 
less than often is desired. Furthermore, it should be 
apparent that not only is the material utilized economi- 
cally expensive, the technique is in itself rather com- 
plex. 
Therefore, as can be appreciated, there currently 
exists a need for a practical method for bonding se- 
cond-surface mirrors to radiant coolers which over- 
comes the aforementioned difficulties and disadvan- 
tages. 
It is, therefore, the general purpose of the instant 
invention to provide a method for attaching a fused- 
quartz mirror to a conductive metal substrate and 
which has particular utility in the fabrication of radia- 
tively-cooled QCM's for space application. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the instant invention to provide a 
method for attaching a fused-quartz mirror to a metal- 
It is another object to provide an improved method 
for attaching a fused-quartz mirror to a metallic sub- 
strate which overcomes the difficulties and disadvan- 
tages experienced as a consequence of the existing 
4o differentials in thermal expansion for a mirror and its 
substrate. 
It is another object to provide a method for attaching 
a fused-quartz mirror to a conductive metal substrate 
which avoids the difffculties and disadvantages previ- 
It is another object to provide a method in which a 
fused-quartz mirror is attached to a metallic substrate, 
although not necessarily limited in use thereto since the 
method can be employed equally as well when bonding 
50 ceramic meterials, such as silicone, to metallic sub- 
strates. 
These and other objects and advantages are achieved 
through a method in which one surface of a fused- 
quartz mirror is tinned with a solder of pure indium, a 
5 5  planar surface of a metallic substrate is then tinned 
with an eutectic alloy having a melting point substan- 
tially below that of solder and consisting essentially of 
indium, bismuth, lead and tin; the alloy is thus heated 
to its melting temperature and the tinned surface of the 
60 fused-quartz mirror is floated into place and, thereaf- 
ter, the resulting structure is permitted to cool. 
2o 
25 
30 
35 lic substrate. 
45 ously experienced. 
BRIEF DESCRIPTION OF THE DRAWING 
FIG. P is a perspective view illustrating a fused-quartz 
65 mirror mounted on a metal substrate. 
FIG. 2 is a flow diagram illustrating in sequence the 
steps of the method which embodies the principles of 
the instant invention. 
4,03 3,503 
2 
d 
DESCRIPTION OF PREFERRED EMBODIMENT 
As shown in the drawings, the substrate 10 has depos- 
ited thereon a thin film 12 of eutectic alloy. The sub- 
strate 10 is formed of a suitable conductive material, 
such as aluminum, copper and the like, while the film 
12 comprises an eutectic quarternary alloy consisting 
essentially of indium, 20%; bismuth, 50%; lead, 18%, 
and tin, 12%, by weight, having a melting point of 58" 
C. 
As shown in the drawings, a film 14 of pure indium 
solder having a melting point of 157" C is interposed 
between a surface of fused-quartz mirror, designated 
16, and the film 12. Through the films 12 and 14, the 
mirror 16 is bonded to its substrate 10. 
In order to deposit the films 12 and 14 between the 
mirror 16 and its substrate 10, one planar surface of the 
fused-quartz mirror 16 is tinned by depositing thereon 
the film 14 of pure indium solder of the film 14, utiliz- 
ing a suitable tinning technique well understood by 
those familiar with the art of joining adjacent surfaces 
of ceramics and metal. Briefly, however, it is sufficient 
to understand that to achieve tinning, the plan surface 
of the mirror 16 is wetted by applying thereto the pure 
indium in its molten state. 
Subsequent to the tinning of the mirror 16, the metal 
substrate 10 is tinned with the eutectic alloy of the film 
12, and then permitted to cool. The tinning of the sub- 
strate 10, also, is effected in a manner well known, 
similar to that aforementioned. 
The next step requires that the temperature of the 
alloy of the film 12 be raised sufficiently for causing it 
to again attain a molten condition. This, where desired, 
is achieved simply by placing the tinned substrate in an 
oven and raising the temperature thereof to 75" C. 
Where desired, a lip, not shown, is extended along the 
peripheral portions of the substrate 10 for purposes of 
assuring that in cooperation with the effects of surface 
tension of the alloy in its molten state the alloy remains 
in place on the surface of the substrate. The tinned 
surface the mirror 16 is now deposited on the upper 
surface of the molten alloy and floated into place. 
Where desired, slight vibratory motion may be im- 
parted to the mirror, through a gentle tapping thereof, 
for purposes of assuring that all irregularities of the 
surfaces of the mirror 16 and its substrate 10 are com- 
pletely filled with the eutectic alloy. Subsequently, the 
temperature of the alloy is slowly reduced to ambient 
temperature. Of course, the film of indium 14 and the 
film of solder 12 achieve a bonded condition with the 
mirror 16 and substrate 10, respectively, as well as with 
each other, for thus effecting a bonding of the mirror to 
its substrate. 
It has been found through testing that excellent adhe- 
sion between the mirror and substrate thus is achieved. 
For example, in testing the adhesion of a mirror 16 to 
its substrate 10, the mirror and indium solder separated 
at the quartzjsolder interface before fracturing of the 
solder occurred. Moreover, temperature cycling of the 
soldered mirror, between 300" K and 100" K, resulted 
in no apparent deterioration of the bond existing be- 
tween the fused-quartz mirror 16 and its substrate 10. 
4 
In operation, even though the thermal expansion 
coefficient of the substrate is from fifteen to twenty- 
five times greater than that of the fused-quartz mirror, 
fracture which normally would be experienced, be- 
5 cause of the differential in expansion coefficients, is 
avoided simply because of the ductility of the film of 
indium through which temperature-induced thermal 
stress is absorbed. Thus the large sheer stresses which 
occur in a space environment due to the severity of the 
In view of the foregoing, it should be apparent that 
the low melting temperatures of solders of the tin- 
indium family permit a successful bonding of second- 
surface fused-quartz mirrors to metal substrates. More- 
15 over, self-contamination due to outgassing by organic 
cements and the like is thus eliminated, and since ther- 
mal conductivity of the indium solders is several orders 
of magnitude greater than organic cements lower radi- 
ant cooling operating temperatures can be provided 
20 for. Thus the method of the instant invention has par- 
ticular utility in fabricating radiant coolers for use 
aboard operational spacecraft. 
Finally, it is contemplated that while the method of 
the instant invention is particularly useful in the fabri- 
25 cation of radiant coolers, utility also can be found in 
bonding solar cells to panel substrates because of the 
elimination of inherent problems resulting from outgas- 
sing. Furthermore, increased conversion of efficiency 
resulting from a minimizing of the thermal resistance 
30 between the cell and the substrate can be expected to 
occur. 
I O  tempertures operationally encountered are absorbed. 
We claim: 
1. A method for attaching a fused-quartz mirror to a 
A. tinning one substantially planar surface of a fused- 
quartz mirror with a solder of a class including pure 
indium solder; 
B. tinning a metal substrate with an indium eutectic 
alloy consisting essentially of indium, bismuth, lead 
and tin, having a melting point substantially below 
that of the solder; 
C. heating the alloy to a temperature substantially 
above its melting point, but below that of the sol- 
der; 
D. establishing a contiguous relationship between the 
tinned surface of the mirror and a surface of the 
alloy while maintaining the temperature of the 
alloy substantially above the melting point of the 
alloy; and 
E. cooling the alloy to a temperature substantially 
below its melting point while maintaining the estab- 
lished contiguous relationship between the surfaces 
of the mirror and the alloy. 
2. The method of claim 1 wherein said eutectic alloy 
5 5  consists essentially of twenty parts by weight indium, 
fifty parts by weight bismuth, eighteen parts by weight 
lead, and twelve parts by weight tin. 
3. The method of claim 1 further comprising the step 
of vibrating the mirror prior to cooling the alloy while 
60 the contiguous relation between the alloy and the 
tinned surface is maintained. 
4. The method of claim 1 wherein said metallic sub- 
strate includes an endless peripheral lip. 
conductive metal substrate comprising the steps of: 
35 
40 
45 
50 
* * * * *  
65 


Method for attaching a fused-quartz mirror to a conductive metal substrate

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