Carbon-carbon mirror for space applications

A carbon-carbon mirror for high thermal input space applications is comprised of a plurality of thermally conductive, metallic layers formed as a laminate structure atop a carbon-carbon substrate. Within the laminate structure, a first thin adhesion layer of chromium is formed directly on the surface of a carbon-carbon substrate. Atop the adhesion layer, a plating base layer of gold or copper is deposited, followed by a thick working/smnoothing layer of nickel, which serves as the optically reflective surface of the mirror. The nickel layer is polished to a precision flatness or smoothness. Because the underlying gold and chromium layers possess high thermal conductivity, they enhance the transmission of heat from the cladding layer to the underlying carbon-carbon substrate, thereby preventing the laminate from absorbing heat, which would lead to delamination of the structure. In order to minimize thermally induced distortion of the mirror structure, the metal laminate build-up is preferably formed on opposite sides of the carbon-carbon substrate, thereby affording a thermal balancing of the substrate. However, the nickel layer on the non-reflective side of the mirror is not polished.

FIELD OF THE INVENTION 
The present invention relates in general to mirrors suitable for high 
thermal energy applications, such as solar energy concentrators and laser 
mirrors, and is particularly directed to a metallic laminate mirror 
structure having a carbon-carbon substrate. 
BACKGROUND OF THE INVENTION 
Mirrors designed for use in space and airborne high thermal energy 
applications (e.g. solar concentrators and laser mirrors) must not only be 
lightweight (to satisfy payload launch requirements), but must possess a 
high precision optical smoothness and flatness that is dimensionally 
stable over a wide range of thermal inputs. 
For this purpose, the use of lightweight composites, particularly glass 
laminate structures employing a carbon-carbon substrate, has been 
proposed. One example of such a glass laminate-on-carbon-carbon mirror 
structure is detailed in U.S. Pat. No. 4,451,119 to Meyers et al. As 
described therein, because of its strength, low weight and thermal 
stability properties, a carbon-carbon substrate offers a particularly 
attractive substrate or backing material for supporting a reflective 
surface. However, because of its porosity and generally rough fibrous 
surface, a carbon-carbon substrate offers a surface of poor optical 
quality upon which to form a highly reflective coating. Consequently, the 
patentees propose the use of an intermediate bonding laminate structure of 
silicon carbide and glass that serves to fill in the voids in the 
carbon-carbon substrate and provides an adhesion layer that can be 
polished to optical quality for receiving a reflective coating. 
Unfortunately, such a structure suffers from a number of drawbacks that 
the use of a carbon-carbon substrate seeks to overcome. 
More specifically, the choice of a carbon-carbon substrate is predicated 
upon its thermal stability (low coefficient of thermal expansion) and its 
substantial load-bearing to weight capacity. These performance advantages 
are diminished in the patented mirror structure because of the 
interposition of the glass laminate between the carbon-carbon substrate 
and the optical cladding layer. The glass laminate adds substantial mass, 
on the one hand and, most significantly, constitutes a thermal insulating 
barrier between the optical cladding layer and its underlying 
carbon-carbon support substrate. As a result, the high thermal inputs with 
which the mirror is intended to be used cause a substantial heating of the 
glass laminate structure and eventually lead to a delamination of the 
intermediate adhesion layers, thus effectively warping and degrading the 
reflective surface. 
SUMMARY OF THE INVENTION 
Pursuant to the present invention, the above-mentioned drawbacks of 
conventional carbon-carbon mirror structures, such as the glass-containing 
laminate design described in the Patent to Meyers et al, are obviated by a 
new and improved carbon-carbon mirror configuration which, rather than 
absorb and be distorted by high thermal inputs to the mirror, provides 
instead a heat removal flow path between the optical coating and the 
underlying carbon-carbon substrate, so that the integrity and fidelity of 
the mirror surface is not detrimentally affected. For this purpose, the 
mirror structure according to the present invention is comprised of a 
plurality of thermally conductive, metallic layers formed as a laminate 
structure atop the carbon-carbon substrate. 
In accordance with a preferred embodiment of the invention, within the 
laminate structure, a first thin (on the order of 50 Angstroms thickness) 
adhesion layer of chromium or similar Group VIA, VIIA or VIIIA metal is 
formed directly on the surface of a carbon-carbon substrate and 
effectively replicates the rough fibrous texture of the surface of the 
carbon-carbon substrate. Atop this adhesion layer a plating base layer of 
gold or copper or similar Group VIIIA or IB metal (having a thickness on 
the order of 2000 Angstroms) is deposited, followed by a thick 
working/smoothing layer of nickel or similar Group VIA, VIIA, VIIIA or IB 
(thickness on the order of 0.001-0.003 inch.) which is to serve as the 
optically smooth surface of the mirror on which a reflective metal such as 
aluminum, silver or rhodium may be deposited. The nickel layer is polished 
to a precision smoothness (40 Angstroms, peak-to-peak). Because the 
underlying gold and chromium layers possess high thermal conductivity, 
they augment, rather than effectively impede, (as occurs in the case of 
the thermally insulating glass of a conventional carbon-carbon mirror 
structure described in the above-referenced Meyers et al patent) the 
transmission of heat from the cladding layer to the underlying 
carbon-carbon substrate. 
In order to minimize thermally induced distortion of the mirror structure, 
the metal laminate build-up is preferably formed on opposite surfaces of 
an individual carbon-carbon substrate, thereby affording a thermal 
balancing of the substrate. However, the nickel layer on the 
non-reflective side of the mirror is not polished, to avoid unnecessary 
processing.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is illustrated a cross-sectional 
illustration of a metallic laminate structure for a mirror built up on a 
substrate of carbon-carbon material in accordance with the present 
invention. The carbon-carbon substrate, shown at 11, may comprise a 
commercially available carbon-carbon material, such as that described in 
the above-referenced Meyers et al patent, and may have a thickness on the 
order of 0.045 inches. Using a surface characteristic measurement device, 
such as an Alpha step profilometer measurement device, the roughness of 
the top surface 13 of the substrate 11 was observed to be on the order of 
2.0-2.5 .mu.m with peaks (valleys) 15 of up to 5.4 .mu.m. Using 
conventional polishing equipment, such as a Strasbaugh Model Y-1 polisher, 
the top surface 13 of substrate 11 was ground and polished on a six inch 
steel lap with 5 .mu.m microgrit, thereby reducing the roughness to an 
average on the order of 1.0-1.5 .mu.m with peaks on the order of 2.0-3.5 
.mu.m. 
Following this grinding of surface 13 of the carbon-carbon substrate 11, a 
thin adhesion layer of chromium 21 was deposited on the polished surface 
13. For this purpose, a VEECO VE-7760 high vacuum deposition system was 
employed to form a thin layer of chromium having thickness on the order of 
50 .ANG.-500 .ANG.. As noted above, chromium layer 21 serves to adhere 
overlying metal (gold/nickel) to the underlying carbon-carbon substrate. 
As can be seen in FIG. 1, because of the thinness of the adhesion layer 21, 
its top surface 23 effectively replicates the roughness of the surface 13 
of the underlying carbon-carbon substrate 11. FIG. 1 illustrates a valley 
25 of chromium layer 21 aligned with valley 15 of carbon-carbon substrate 
11. 
Next, a gold layer 31 was vapor deposited on the chromium layer 21 to a 
thickness on the order of 2,000 .ANG.-2,500 .ANG.. As can be seen from 
FIG. 1, the texture of the top surface 33 of the gold layer 31 effectively 
replicates that of the underlying chromium layer 21 and carbon-carbon 
substrate 11, so that a valley 35 of the gold layer 31 is effectively 
aligned with the valley 15 in the underlying carbon-carbon substrate. Gold 
layer 31 provides a plating base for a thick nickel layer 41 which is to 
serve as the optically smooth surface of the mirror. In place of gold 
layer 31, copper or similar Group VIIIA or IB metal may be used as a plate 
base for the nickel layer. In this case vapor deposition is again used to 
deposit 2,000 .ANG.-2,500 .ANG. of metal on the chromium, adhesion layer 
21. 
In order to form the nickel layer 41, a nickel sulfamate solution was 
electrolessly plated on the gold base layer using the following procedure. 
With the gold layer 31 deposited on the chromium adhesion layer 21, the 
metal laminate was electrolytically cleaned in a isopropylene cleaning 
solution having a volume density of 58.6 ounces per gallon in a 
temperature range of 70.degree.-75.degree. C. for a period of 10-30 
seconds and then rinsed in water. Next, the structure was immersed in a 
10-15% solution of sulfuric acid for 20-30 seconds and then again rinsed 
in water. This cleaning procedure may be repeated as desired to assure 
cleanliness of the surface of the gold layer 31. 
Following the cleaning of the surface of the gold layer 31, the laminate 
structure was subjected to a strike in NL electroless nickel 63 at a 
temperature of 75.degree.-80.degree. C., with four parts of NL electroless 
nickel 63 mixed with one part of water, with the electroless strike taking 
place for a period of 10-20 seconds. After an initial deposit, the 
structure remained emerged in the nickel strike for a period of 30 minutes 
to improve the resistance for sulfamate nickel. 
Next, nickel sulfamate was plated upon the strike layer at a temperature of 
45.degree.-50.degree. C., via filter system agitation, at plating rate of 
0.001 mils per hour (25 amps/FT.sup.2) for a period of 3 hours. The 
resulting nickel layer 41 plated up to a thickness on the order of 
0.001-0.003". 
Following this nickel plating step, top surface 43 of the nickel layer 41 
was ground to remove edge build-up and the roughness was measured. Initial 
surface roughness averaged about 0.6 .mu.m with peaks of 1.2-1.8 .mu.m. 
This roughness was reduced by grinding surface 43 using 5 .mu.m grit paper 
to improve the roughness to an average of about 0.4 .mu.m with a peak 
roughness of 0.8-1.0 .mu.m. 
Following this initial roughness reduction step, surface 43 was polished 
with a 3 .mu.m diamond buff, to result in an extremely smooth precision 
surface 43, the surface roughness of which was measured to be about 30-40 
.ANG. with peaks of only 60 .ANG., thereby providing an extremely high 
quality optical surface. Following this, an optically reflective layer 51 
such as aluminum, silver or rhodium may be deposited on the substrate to 
provide maximum reflectivity. 
Because the optical quality layer 41 and underlying laminate structure 
through which the optical quality layer 41 is attached to the 
carbon-carbon substrate 11 is comprised of a series of contiguous highly 
conductive metal layers (chromium-gold-nickel) there is effectively 
provided a high thermal conductivity path between the optical quality 
surface 43 upon which optically reflective layer 51 that receives the high 
thermal energy inputs and the underlying carbon-carbon substrate 11, which 
not only provides the high stiffness to weight ratio for minimizing the 
weight of the mirror structure for required structural rigidity, but 
enjoys high thermal conductivity. The carbon-carbon substrate also 
possesses a low coefficient of thermal expansion, thereby minimizing 
thermal distortion in the surface of the mirror in response to thermal 
fluctuations in the environment to which the mirror is exposed. 
Consequently, the thermal energy of an intense optical input to the 
reflective surface of the mirror is rapidly conducted away through the 
metal laminate structure and into the underlying carbon-carbon substrate. 
This thermal transmission effect of the present invention offers a 
substantial advantage over conventional glass laminate structures, such as 
that described in the above-reference Meyer et al patent, which glass 
structures effectively constitute thermal insulators, that block or impede 
the transmission of thermal energy between the mirror surface and the 
underlying carbon-carbon substrate. As a consequence, in a conventional 
glass-containing laminate structure, the substantial thermal energy inputs 
(which can reach temperatures on the order of 4,000.degree. F.) can cause 
a delamination and cracking of the laminate build-up, thereby warping the 
mirror and destroying its precision geometry. 
Because of the superior performance characteristics of the present 
invention, it has particular utility in space-spaced applications, such as 
solar concentrator mirror configurations. One example of the application 
of the present application to such a solar concentrator structure involves 
its use in a faceted mirror architecture, such as that described in 
copending application Ser. No. 019,699 filed Feb. 27, 1987, entitled 
"Offset Truss Hex Solar Concentrator" by J. White et al, and assigned to 
the assignee of the present application. When employed for the individual 
triangle facets of the mirror structure described in that application, the 
processing of the laminate structure of the mirror structures is carried 
out after the tiles are individually formed and mounted. Then, the 
above-described sequence of steps is carried out, with each tile being 
measured with precise flatness measuring equipment to verify its flatness. 
Following their individual manufacture, the tiles are placed on precision 
ground glass plate having small holes through its thickness to provide a 
vacuum drawing source. The tiles are aligned on a plate in a pattern 
designed for the final facet. A vacuum is then applied to the glass plate, 
so that the tiles are held in place and an underlying graphite-base 
structure is bonded to individual tile standoffs using a graphite-base 
ceramic adhesive. The vacuum provided during the attachment of the tiles 
provides a strain relief during bounding so that the tiles will not move 
out of place during the cure cycle. 
It should be observed that the metallic laminate, carbon-carbon structure 
of the high thermal energy mirror structure of the present invention is 
not limited to use in only the solar energy concentrator structure 
described in the above-referenced copending patent application. That 
application is simply mentioned as an example of the application of the 
invention and a suitable environment in which individual mirror tiles may 
be fabricated and employed. 
As noted previously, in order to minimize thermally induced distortion of 
the mirror structure, the metal laminate build-up is preferably formed on 
each of opposite surfaces of the carbon-carbon substrate, as 
diagrammatically illustrated in FIG. 2, thereby affording a thermal 
balancing of both sides of the substrate. However, the nickel layer on the 
non-reflective side of the mirror is not polished, to avoid unnecessary 
processing. 
While we have shown and described an embodiment in accordance with the 
present invention, it is to be understood that the same is not limited 
thereto but is susceptible to numerous changes and modifications as known 
to a person skilled in the art, and we therefore do not wish to be limited 
to the details shown and described herein but intend to cover all such 
changes and modifications as are obvious to one of ordinary skill in the 
art.