Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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:PATENT :-.'
1799-33-00 ,--
: TITLE: :~LIGHTWEI~GHT~STRUCTURES AND~METHODS FOR THE ..
FABRICATION THEREOF
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- This invent~lon-:was~developed under NASA Contract
No.~ NAS 1-18476.~
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;5 ~BACKGROUND OF THE~INVENTIOW
: l. Field of the Invention
:This invention relates to an improved method of
: fabricating stiff and strong l~ightweight structures, and ;~:. .:
more particuIarly,~to an improved method for the ~;
10 ~ :fabrication~of~silicon~carbide tSiC) and/or silicon (Si)
lightweight structures by the utilization of ~ ~ :
: : conventional vapor deposition~:techniques. Such
lightweight structures;have utility in a variety of
diverse~applications i~ncluding back-up structures for :
: 15 optical components:, as structural components for:.
automobile, aerospace and space appIications, and as
~;; lightweight furniture part~s for space.
;~ : 2. Description of the Prior Art ~ ;
In the field of optics, light detection and ranging
(LIDAR) has come to be recogni2ed as an important
: diagnostic tool~for remote measurement of a diversity of
atmospheric parameters such as minor species of
concentrations pressure, temperature, and water vapor -~
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profiles, aerosol cloud distributions, and wind fields.
LIDAR techniques such as measurement of back scattered
signals, differential absorption, and Doppler shifts
have been used to obtain information about the earth's
atmosphere. ~ ~
The performance of a~LIDAR system depends upon the
opt~ical configurat~ion~of~its receiving telescope.
Often, due to~space limitations such as in a shuttle
b~rne LIDAR;system,~the~length of~the telescope is
10 ~ fixed. There;fore, the~opt}cal designer must select a
particular~shape~and optics~speed of the mirrors to ~ -
; maximize~the;throughput of the telescope. The most
critical element in~the receiving telescope is the
primary mirror~because of its size,~we~ight, fabrication
cost, and thermal exposure to the outside world. Since
the received~signal~ls~directly~proportional to the area
of the primary~mirror, it~is important to use as large a
; primary mirror~as feaslble~to~obtain~reasonable signal
levels for accurate~measurement. This is particularly
true when~a space-borne LIDAR system is used to measure
wind profiles in~the troposphere on a global basis.
; The~conventional~;techniques employed in the prior
art for fabricating~large t>l.0 meter diameter) mirrors
ar~e quite~slow and time consuming. Several months to
years are~reguired-to fabricate~a large mirror from
ultra low expans~ion silica glass or Zerodur, a product
,commerciallyjavailable from Schott Glass Technologies,
Inc., 400 York Avenue, Duryea, PA 18642. Since a
number of space-based LIDAR systems are planned for the
future, considerable attention is currently being given
to the development of techniques for the rapid and
economic production~of large, high performance mirrors.
20~9~97
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Thus, a spin casting technique has been proposed to
fabricate 1.2 meter and 3.5 meter diameter glass mirror ,
blanks containing lightweight honeycomb cells. Although
this technique is relatively faster than the
5~ conventional mirror fabrication methods and produces
lightweight mirr~ors~, the weight of these mirrors is
; still an~order of ma~gnitude more than permissible for
many space~applications. Furtherr~the spin-castlng - --
technique~is~unsultable for fabricating large mirrors of
10~ ~advanced cerami;cs~such~as SiC,~titanium diboride (TiB2),
and boron~carbide (B4C)~that have high melting points. ~ -
These latter materials have properties superior to those
of glass for large lightweight optics.
Other technlques lnvolving~the casting of fiber
reinforced compos~ites containlng epoxy and plastics and
the stretching of membranes over appropriate substrates
are also currently under investigation.
Stlll another techni~que;for ma~lng stiff
; lightweight structures ;is disc}osed in U. S. Patent No. ~`
4,716l064 granted to Robert A. Holzl et al. on
December~29,~ 198~7.~ The Holzl et~al. patent emphasizes a ~-
requirement for two parallel~separated surface defining
mémbers~that~;a~re connected by~stlffeners. Fabrication ~ -
starts with;a~solid graphite disc which defines the
outer envelope~to the part to be produced. Then, by a ;~ -
~: ~ : , ,
series of drillings of bores or holes in the graphite ~
disc~ theluse of plugs and multiple coatings of a i --
~; chemically vapor deposlted material possessing a high
stiffness to weight ratio, the part is constructed. A
disadvantage of this fabrication procedure is that it is
time consuming, complex and costly. Moreover, the many
steps of drilling, plugging, and multiple coating ~ -
involved inherently limit the ability to control figure
stability. This impairs the value of the process where
extreme figure stability retention is of importance, as
in high performance mirrors. Additionally, the Holzl et
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; al. technique is limited to relatively thin structures
because of thé difficulty~of obtaining uniform coatings
~; in the passages~between the spaced~parallel surface
; ; defining members.
~ Thus, there is a need and a demand for improvement
n~the~methods~of~ fabricat~l~on~of~stiff and strong
lightweight structures:to the~end of achieving extreme
f~igure~stab~ ty~retentlon~as~well~as an~amenability to
being~scaled~up~in~-si~ze~wh~ile~at~the same time enabling
~a( ~10~ slmplif~ica;tlon~ n~the~fabrication~procedure and~ ;
reduction~in~the~time;r~equired~for and the cost of such
procedure.
SUMMARY~OF~THE~INVENTION
An~object~of~the;~invention~-is~to~provide an ~;~
15~ improvéd~method~for;~fabricating~sti~ff~and strong
lightweight~structures~that are~characterized by extreme
figure~stability;~retent~ion.~
Another~objeot~ of~;the invention is~to provide an
;imp`roved~method~ënabling~simplification~in,;reduction in
2~0~ time~required~for,~and~cost;~of fabricating stiff and~
strong,lightwei~g~ht~ structures.
A~further~ob~ect~of~the ;lnvention~ lS to provide an
improved~stiff~and~istrong lightweight structure.
An addit~ional~`object~of the invention is to prdvide ; ;i~
~ such~a~structure~having~pa`rticular~utiIity as back-up
structure in the~fabrication o ~lightweight~mirrors.
Another~object~of the~invention is to provide a
stiff ànd~strong Lightweight structure~comprising a - -~
plurality of ribs~each~of which has~ a length and~`a -
height that are greatly in excess of the thickness
` thereof, the ribs being assembled in the form o~a ~-
,
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: structure having a plurality of cells and a stiffening
and strengthening material coated on and enclosing the ;~
ribs, such materiaI comprising a material that is vapor
~ ; deposited on the ribs.
;~ : 5 Still another object of the invention is to provide
:~ a method of fabricating;a lightweight structure from a
plurality of ribs~each~of which have substantially the ~.
~:: :: same height and~thic~kness~with the height and length ;.;
: greatly exceeding the thickness, comprising the steps : -
of~
"
a~ forming~from a first set of said ribs,
~; each of which are of substantially the same length,
a hexagonal cell~havi~ng a depth equal to the height
: of said~ribs~
: ,
~ (bj~:;formlng slots in the center of first, ~ :
second and~third ones of a second set of said :-
ribs~which~ribs~;are all of the same length and : -:~
: substantially~ eq~ual to:the distance between the
- m~st widely spaced corners of said hexagonal cell, ~ :
: 20 : with:a~fi~rst slot from the top of a first one of
said ribs:,~ a:second~slot from the: bottom of said :;;
:second one of said ribs, and:third and fourth
slots from~both the top and~bottom of said third
: one of said ribs,~ with the~ lengths of said first ~ :
:25 ~ and second~slots being greater than half the height
of said:ribs and~the lengths of said third and
fourth slots being less than half the height of
. said rIbs,
(c);lnterlocking said first, second and third
~; 30 ones of said ribs at~the centers thereof by
bringing said first and second slots into
~: cooperative relation with said third and fourth ~- .
~; slots, respectively,
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(d) positioning said interlocked first, second
and third ones of said ribs relatively to said
hexagonal cell to connect the most widely spaced
corners thereof thereby to form a structure core
having six regions each of equilateral triangular
cross section with each such region comprising a
cell, and
(e) exposing the structure core to a vapor
deposition process to deposit and coat thereon a
stiffening and strengthening material thereby to
enclose said structure core in a monolithic
structure of such material.
Still another object of the invention is to provide
an improved method for fabricating such an improved
stiff and strong lightweight structure that is
characterized by the adaptability thereof for
fabrication in various predetermined configurations.
A further object of the invention is to provide an -~
improved method for fabricating stiff and strong
lightweight structures that is characterized by the
adaptability thereof for scaling up in size.
In accomplishing these and other objectives, there
is provided, in accordance with the invention, a four
step process for fabricating lightweight structures out ;-
of SiC and/or silicon ~Si). The lightweight structure
consists of a core to define the shape and size of the
structure, overcoated with an appropriate deposit, such
as SiC or Si, to give the lightweight structure strength - ~
and stiffness and to bond the lightweight structure to ~ ~ -
another surface.
The lightweight structure core is fabricated by
bonding together thin ribs of a suitable material with a ;
compatible bonding agent. The core may consist of many
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honeycomb cells of appropriate shapes. This core
structure may be placed on a suitable substrate the
surface configuration of which may be predetermined.
The substrate may be coated with a release agent. A
desired overcoat material is then deposited on the core
structure by employing conventional or other appropriate
deposition processes. A sufficient thickness of the
overcoat material is deposited to ensure that the core
is totally coated. The lightweight structure so
fabricated is unloaded from the deposition system and
separated from the substrate. I~ necessary or
desirable, the enclosed core material may be removed by
drilling small holes in the walls of the structure,
followed by burning, etching or melting of the core -
material away from the deposited overcoat material.
Fabrication of the lightweight structure in
accordance with the four step process thus is as
follows: (i) fabrication of a lightweight structure
core; (ii) mounting of the lightweight structure core on
a substrate for deposition of the overcoat material;
(iii) deposition of the overcoat material to enclose the
core; and (iv) core removal from the substrate.
The lightweight structure core may be fabricated
using a metal or non-metal as the core material,
; 25 including plastics, ceramics, carbon, glass, polymer,etc. The main requirement for a good candidate core
material is that it should be compatible with the
deposition process and material. Thin ribs of the core
material are obtained and then assembled in the form of
a honeycomb structure. The ribs may be joined together
at the corners and intersections with a suitable bonding
agent, as known to those skilled in the art. Other
joining processes such as welding, brazing, soldering,
may also be used.
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Each cell of the honeycomb structure may be in the
shape of a circle, square, rectangle or a polygon. The
lightweight structure may also be fabricated with a
combination of different cell shapes. The preferred
structure, however, is the one which has the greatest
stiffness for the intended application, such as one
involving hexagonal cells, each of which contain six
triangular cells.
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The invent~ion has particular utility in the
fabrication of~ ghtweight Si/SiC mirrors. Thus, a
complete lightweight mirror substrate may be fabricated ~
directly in a vapor deposition chamber, in a one-step -
process, with~no bonding agent being required to attach ~ ~ -
the SiC back-up structure to the faceplate of the
mirror. ~ -
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The various features of novelty which characterize
the invention~are pointed out with particularity in the
claims annexed to and forming a part-o~ this
specification. For a better understanding of the ~-
invention, its operating advantages, and specific
objects attained~by its use, reference is made to the
accompanying drawings and descriptive matter in which
preferred embodiments of the invention are illustrated. ~-
BRIEF DESCRIPTION OF THE DRAWINGS ~ ~
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2~ With this summary of the invention, a detailed
description~follows with reference being made to-the
accompanying drawings which form part of the ~ -
specification, of which:
Figs. 1 and 2 are plan and front views,
respectively, of a lightweight structure core mounted,
in accordance with a first embodiment of the invention,
for deposition thereon of an overcoat material;
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Fig. 3 is a detailed view of two of the three
intersecting and interlocking ribs of the lightweight
structure core of Figs. 1 and 2;
Fig. 4 is a detailed view of the third one of the
intersecting and interlocking ribs of the lightweight
structure core of Figs. 1 and 2;
Figs. 5 and 6 are plan and front views,
respectively, of a lightweight structure core mounted,
in accordance with a second embodiment of the invention,
for deposition thereon of an overcoat material;
Fig. 7 is a perspective view of a chemically vapor
deposited SiC lightweight structure produced utilizing
the lightweight structure core mounted as shown in Figs.
5 and 6;
Figs. 8 and 9 are plan and front views,
respectively, of a lightweight structure core mounted,
in accordance with a third embodiment of the invention,
for deposition thereon of an overcoat material;
Fig. 10 is a perspective view of a chemically vapor
deposited fabricated SiC lightweight structure bonded to
a SiC faceplate produced utilizing the lightweight
structure core mounted as shown in Figs. 8 and 9;
Fig. 11 is a schematic illustration of a chemically
vapor deposition apparatus that may be employed to
~fabricate SiC and Si lightweight structures, as
illustrated in Figs. 1-10;
Figs. 12 and 13 are plan and side views,
respectively, of a scaled up in size lightweight back-up
structure core assembly having utility in the formation
by chemical vapor deposition of a monolithic lightweight
Si/SiC mirror faceplate and the back-up structure
therefor;
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Figs. 14-30 are side views illustrating the shapes
of the ribs used in the back-up structure core assembly
of Figs. 12 and 13, with the assembly being in
accordance with a first and preferred method;
Figs. 31 and 32 are plan and side views,
respectively, of a scaled up in size lightweight back-up
structure core assembly having utility in the formation -
by chemical vapor deposition of a monolithic lightweight
Si/SiC mirror faceplate and the back-up structure
therefor, with the assembly of the lightweight back-up
structure core being by a second method;
Figs. 33-42 are side views illustrating the shapes ~-
of the ribs used in the back-up structure core assembly
of Figs. 31 and 32 in accordance with a second method of
assembly; and
Fig. 43 is a schematic illustration of a chemical
vapor deposition furnace that may be used to effect SiC
and Si deposits on a mirror faceplate and the back-up
structure therefor as shown in Figs. 12, 13, 33 and 34.
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DESCRIPTION OF THE PREFERRED EM~ODIMENT~
Figs. 1 and 2 of the drawings illustrate a
lightweight structure core 10 that is fabricated from
graphite ribs 12a, 12b, 12c and 14a...14f. The core 10
is fabricated such that the ribs 14a...14f, which are
all of the same length, form a hexagonal cell. The ribs
12a, 12b and 12c intersect in the center and connect the
six corners of the hexagon. Ribs 12a, 12b and 12c also
divide the hexa~on into six triangular parts. Rib~ 12a,
12b and 12c are fabricated with center slots, as
described further hereinafter with reference to Figs. 3
and 4, to interlock them in place.
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In the preferred embodiments of the invention, the
ribs all have a thickness of about 0.5 mm.(0.020 inch).
Further, the ribs are all characterized in having a hiyh
ratio of the length and height thereof to their
thickness. That is to say, the length and the height of
each rib greatly exceeds its thickness.
In the invention embodiment illustrated in Figs. 1
and 2 and those illustrated, also, in Figs. 5 and 6 and
in Figs. 8 and 9, all of the ribs have at least two
adjacent surfaces that form an edge, all portions of
which are located in a single plane, such as that
containing the bottom edges 14g, 14h and 14i shown in
Fig. 2.
To the end that the ribs 12a, 12b and 12c may
interlock with each other at the center thereof, two of
the ribs, 12a and 12b, for example, as shown in Fig. 3,
are provided with a single transverse slot 12d that
extends slightly more than half way through the height
of the rib. The third rib, 12c, as shown in Fig. 4, is
provided at the center thereof with opposed transverse
slots 12e and 12f that extend less than half way through
the height thereof. Assembly of the ribs 12a...12c in
operative relation is effected by placing the slots 12d
of ribs 12a and 12b in interlocking relation with
opposed transverse slots 12e and 12f, each of which
slots extends less than half way through the height of
rib 12c. Ribs 14a...14f are positioned to define the
outer perimeter of the structure 10, that is, to
complete a hexagon, as shown.
The graphite ribs 12a.. 12c and 14a.. 14f may be
joined with a graphite cement. Graphite is a good core
material because it is compatible with most deposition
procedures. Further, several different types of
graphite with different thermal expansion coefficients
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2~9697
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are available. A particular graphite having a thermal
expansion coefficient closely matching that of an
overcoat material to be deposited can be selected. A
disadvantage of graphite is that it is a fragile
material. Thus, difficulties may be encountered in the
fabrication of lightweight structure cores with graphite
rib thicknesses less than 0.5 mm. (0.020 inch). The -~
graphite rib thickness may be reduced to less than 0.5
mm., however, by burning of the rib in air. Other
strong and stiff materials such as Si, SiC, tungsten
(W), molybdenum (Mo), etc. may also be used to
fabricate extremely thin wall lightweight structure
cores.
Mounting of the lightweight structure core 10 in a ~-
deposition system for deposit thereon of a suitable
deposition material depends upon the application for
which the lightweight structure is intended to be used.
If only the lightweight structure core is required
without any plate or substrate at either end, the
lightweight structure core may be mounted on graphite
poles 16a...16f attached to a substrate 18, as shown in
Fig. 2, with the edges of the ribs engaging the tips of
the poles. After the deposition of the overcoat
material is completed, the lightweight structure is
obtained by separating the structure from the poles, as
by cutting.
If a plate of the deposited material is required at
one end of the lightweight structure, the lightweight
structure core 10 either may be loosely bonded to or
placed on a substrate 20 coated with a mold release
substance 22, as shown in Figs. 5 and 6. A suspension
of graphite particles in an organic solvent may be used
as the mold release coating. With such use, deposition
will occur not only on the walls of the lightweight
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structure core 10 but also at the base thereof. On
completion of the deposition process, the lightweight
structure with a base plate 24 of overcoat material
formed thereon is separated from the substrate 20. In
Fig. 7, there is illustrated a perspective view of a SiC
totally enclosed graphite lightweight structure 26
fabricated by this method.
In some applications such as the fabrication of
lightweight Si or SiC mirrors, it may be desirable
simultaneously to fabricate the lightweight structure
and bond it to a faceplate of a desired material. In
such cases, a lightweight structure core 10, as shown in
Figs. 8 and 9, is bonded to a faceplate 28, as by flow
bonding indicated at 30, and the deposition operation is
performed. The material of the faceplate should be
compatible with the deposition process to assure
adherence of the deposited material. Fig. 10
illustrates a SiC enclosed graphite lightweight
structure bonded to a SiC faceplate which has been -
fabricated by the use of this method.
In order to enclose the lightweight structure core,
an appropriate overcoat material may be deposited by any
of the vapor deposition processes that are currently
; available. These processes include physical vapor
deposition, sputtering, chemical vapor deposition and
its different types (plasma assisted vapor deposition,
low pressure vapor deposition, laser assisted vapor
! ' deposition, metal organic vapor deposition, etc.),
evaporation and ion beam implantation. The materials
which can be deposited include metals and nonmetals
(plastics, ceramics, glasses, polymers, etc.).
Fig. 11 schematically illustrates a chemical vapor
deposition apparatus, designated 32, that may be used to
fabricate SiC and Si lightweight structures in
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accordance with the invention~ This apparatus 32
includes a horizontal research furnace 34, specifically
an electrically heated 3-zone Lindberg furnace, a
reactant supply system 36, and an exhaust system 38.
Associated with furnace 34 is an elongated tube 40
of aluminum oxide (A12O3) containing a reaction or
deposition chamber 42 that is substantially coextensive
with zone 2. ~one 2, as shown, is heated by a heating
element 44 while zones 1 and 3 are heated by
individually associated heating elements 46 and 48,
respectively. Blocks of firebrick, designated 50 and
52, are located outside tube 40 in the regions thereof
respectively associated with zones 1 and 3.
The deposition region within chamber 40 is
indicated at 54 and, as shown, has associated therewith
a mandrel 56 consisting of four sides of an open box and
a baffle plate 58. The pressure within chamber 42 is
indicated by a pressure gauge 60.
Mounting, as by bonding, of the lightweight
structure core 10 on the baffle plate 58 for the
deposition thereon of an overcoat material is preferred.
This is for the reason that such mounting provides
minimal deposition nonuniformity from cell to cell in
the lightweight structure core.
The reactant supply system 36 includes a t~nk 60
comprising a source of argon (Ar) under pressure, a
bubbler tank 62 containing methyltrichlorosilane
(CH3SiC13) or trichlorosilane (SiHC13) through which -
argon from source 60 is bubbled under control of valves -
64a and 64b, and a separate source (not shown) of
hydrogen (H2). The SiC and Si material to be deposited
is fabricated by reacting Ch3SiC13 or SiHC13 with H2,
respectively. Other silane and hydrocarbon sources can
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be used to form SiC and Si. Both of these materials
have been fabricated over a wide range of deposition
temperature and reactor pressure, as shown in Table I
below.
TABLE I - NOMINAL CVD PROCESS PARAMETERS
USED TO FABRICATE SiC AND Si
LIGHTWEIGHT STRUCTURES
Si Mater- FLC ~ RATES (Slpm)¦Deposi- Reactor Deposi-
No. ial ~ _ ~3sicl3 Ar tion Pres- tion
Pro- or Temper- sure Rate
duced SiC13 ature
C torr ~m/min.
_ _
1 SiC <10 <2.0 <4.C 13558 - 25-300 <1.25
2 Si <15 <2.0 <5 ~ 182358 - 25-300 <1.75
The reagents may be introduced into the deposition
chamber 42 through a central injector (not shown). The
injector may be cooled with water to ~i) prevent
deposition in the injector and (ii) to keep the
temperature of the reagents low thereby minimizing gas
phase decomposition or nucleation. The deposition
thickness is controlled by varying the chemical vapor
deposition process parameters and the deposition time.
After a sufficient thickness of the material is
deposited, the deposition process is terminated and the
furnace is cooled very slowly to prevent cracking and
distortion of the lightweight structure due to residual
stresses.
The exhaust system 38 shown in Fig. 11 includes a
vacuum pump 64, a scrubber 66, gaseous filters 68 and
an oil filter 70. The exhaust system 38 is provided to
evacuate the gaseous reaction products that are released
in the reaction chamber 42 during the deposition process.
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Removal of the graphite core, as mentioned
previously, is optional. Since the deposited material
completely encloses the core material, it is not
necessary to remove the core material. As those skilled
in the art understand, a core material can be selected
the presence of which will not degrade the performance
of the lightweight structure. Candidate core materials
are graphite, Si, glass, quartz and various metals.
It is noted that when a vapor deposition technique
is used to fabricate a lightweight structure, the
gaseous flow in the lightweight structure, as
illustrated by the arrows in Fig. 2, is a "stagnation"
flow governed by diffusion. This tends to yield
deposition nonuniformity along the cell depth where the
undesired effects of stagnation flow tend to be the
greatest. By the term "stagnation flow" is meant a flow
that is sluggish or lacking in activity, that is, a flow
that has little motion or power of motion.
In accordance with the invention~ such stagnation
flow may be minimized by providing holes 14j, as shown -
in Figs. 6, 7 and 9, in the walls of the lightweight -~
structure core lO, and in particular, the walls of
adjacent cells. This results in a gaseous flow, as
illustrated ~y the arrows in Figs. 6 and 9, and improves
the strength of the lightweight structure that is ; - -
produced. The preferred location for the holes 14j is
on the walls near the base o~ the lightweight structure -~ -
core, that is, adjacent the substrate 20, as seen in Fig.
., ,
6, and adjacent the faceplate 28, as seen in Fig. 9.
EXAMPLE I
:
The SiC enclosed graphite lightweight structure
shown in Fig. 7 was fabricated by the above method -
described in connection with the deposition apparatus
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~ 2019697
-17-
shown in Fig. 11 and involving process parameters as
given in TABLE I. The lightweight structure core was
constructed from graphite ribs about a . 5 mm. thick, 3.25
cm. long and 2.5 cm. high. The deposition thickness was
about O.76 mm. (O.03 inch). The lightweight structure
produced was quite strong and rigid. There were no
apparent stresses or cracks in the structure.
EXAMPLE II
The chemical vapor technology of fabricating a
lightweight back-up structure was demonstrated by
fabricating a one cell SiC structure on the backside of
a faceplate. First, a graphite core consisting of an
outer hexagonal cell with six inner triangular cells, as
illustrated in Figs. 8 and 9, was constructed from
graphite ribs about 0.5 mm. thick. Each side of this
hexagonal cell was 3.25 cm. long and 2.50 cm. high.
This graphite core was placed on the backside of the SiC
faceplate and then coated with SiC. This process
produced a monolithic lightweight SiC structure without
the use of any bonding agent. To avoid residual
stresses in the structure, a grade of graphite was used
which has a thermal expansion coefficient larger than
that of the chemically vapor deposited SiC.
A coating of Si about 0.5 mm. thick on the near-net
shape SiC faceplate was applied to permit fabrication of
the final optical figure. To obtain a more uniform Si
coating, the SiC faceplate was mounted such that the
flow directly impinged on the replicated surface. Since
the Si coating is required only on the front surface
of the mirror, all other areas were masked with grafoil.
The mirror was polished flat to a figure of 1/5th of a
wave at 0.6328 ~m and a finish of <lOA ~MS.
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In accordance with the invention, the
aforementioned procedure may also be extended
to fabricate curved Si/SiC mirrors of scaled up size and
lightweight back-up structures therefor.
When fabricating structure cores for use as back-up
structure for flat mirrors, the assembly of the ribs, as
previously mentioned, is such that all of the ribs have
at least two adjacent surfaces that form an edge, all
portions of which lie in a single plane. Thus,
contiguous edge portions of the plurality of cells
formed by the assembly of the ribs all lie in the same
... .
plane. In the case of the fabrication of structure ;
cores for use as back-up structure for curved mirrors,
contiguous edge portions of the cells of the structure
formed by the ribs, when assembled, lie on a curved
surface.
The fabrication of curved mirrors is more involved,
as is apparent from the description provided~ ~ ~
hereinafter, due to ti) the optical fabrication of a - -
curved surface required, and tii) fabrication and
assembly of a graphite core for the lightweight
structure. In other respects, the fabrication of curved
and flat mirrors is similar. -
,
In order to scale the lightweight SiC back-up
structure, first the graphite core is scaled. Since the
thickness of the graphite ribs is kept the same during
scaling, considerable care is required to assemble a
large size graphite structure core.
Figs. 12 and 13 illustrate plan and side views,
respectively, of a scaled up lightweight structure core
according to the invention. The lightweight structure
core, designated 72, comprising a fourth embodiment of -
the invention, has particular utility as the back-up
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structure for lightweight Si/SiC curved mirrors as
distinguished from flat mirrors, as shown in Figs. 7 and
10. Two methods are disclosed herein for the
fabrication of the lightweight structure core 72.
The lightweight structure core 72, as shown in Fig.
12, is fabricated from six ribs of equal length which
are positioned such that a large hexagonal cell having a
depth equal to that of the ribs covers most of the
backside of a circular faceplate 74. Connecting the six
corners of this hexagon are three large ribs which
intersect at their centers. These ribs also divide the
hexagon into six equal triangular parts. These large
ribs, similarly to ribs 12a, 12b and 12c shown in Figs.
3 and 4, are fabricated with center slots to interlock
them in place.
More specifically, in the fabrication of the
lightweight structure core 72, six outer sides of a
large hexagon comprising ribs of a first set, all of
which have the same length, and three central ribs
comprising ribs of a second set, all of which have the
same length, are bonded together. Next the six
triangular regions that are formed within the hexagon
are filled with ribs of a third set to form smaller
cells of equilateral cross section and bonded together
to complete the inner region. The region outside the
hexagon may then be closed with ribs of a fourth set to
cover as much of the circular area of the faceplate 74,
as possible.
Details of the assembly of the lightweight
structure core 72 of Figs. 12 and 13, according to a
preferred method of assembly, are described herein with
reference to Figs. 14-30. As shown in Fig. 12, ribs 76,
78, 80, 82 and 84 are positioned in parallel in equally
spaced apart relation. The ribs 76...84 all have
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different lengths and are provided with uniformly spaced
slots, designated 86a at the top, as shown in Figs.
14-18, respectively. Each of ribs 76 and 82, as shown
in Figs. 14 and 17, also include two spaced notches,
designated 86b, at the top. There are two pieces for
each of the ribs 78...84, the second piece in each case
being designated by a prime mark (') in Fig. 12. One of
the two pieces in each case is positioned in the upper
half of the large hexagon, as seen in Fig. 12, and the
other piece is positioned in the lower half. Thus, rib
84 is positioned in the top half and rib 84' is
positioned in the bottom half.
Additional parallel positioned and equally spaced
apart ribs, designated 88, 90, 92, 94 and 96, as seen in ~ -
Fig. 12, all have different lengths and are provided
with uniformly spaced slots, designated 98a at the
bottom, as shown in Figs. 19-23, respectively, with ribs
88 and 94 also having two notches, designated 98b, at
the top. Note that the ribs are made up of three parts
when the slots are made into the notches. Thus, rib 88,
as shown in Fig. 19, comprises three parts that are
designated 94, 94' and 94". Similarly rib 94, as shown
in Fig. 22, comprises three parts athat are designated
94, 94' and 94". There are two pieces of each of the
25 ribs 9096, with the second piece being designated
by a prime mark. The two rib pieces, 96 and 96', thus
are positioned at opposite sides of the large hexagon,
as shown.
Further parallel positioned and equally spaced
ribs, designated 100, 102, 104, 106 and 108, as seen in
Fig. 12, all have different lengths and are provided
with uniformly spaced slots, designated 110a, at the
top and uniformly spaced slots, designated 110b, at
the bottom, as shown in Figs. 24-28, respectively,
~i 20196~7
-21-
with two spaced notches, each designated 112, being
provided in the top of ribs 100 and 106. There are
two pieces of each of the ribs 102...108, with the
second piece being designated by a prime mark.
~: :
As shown in Fig. 12, the region outside the large
hexagon may be closed by a total of 12 ribs
designated 114 (or 116) and there are six ribs
designated 118. Ribs 114, 116, as shown in Fig. 29,
and ribs 118, as shown in Fig. 30, are not provided
10~ with any slots. For convenience of illustration, the
closure segments 114, 116 and 118 are not shown in
Fig. 13.
~- Thus, there are a total of 45 pieces that are
required to assemble the lightweight back-up structure
core 72. There are flow holes, designated 120, that are
provided in the ribs. Each cell has such holes.
In accordance with the invention, the scaled up in
size lightweight structure core may be assembled by a
second method. According to this method, which is
; 20 described with reference to Figs. 31 and 32, in the
assembly of a lightweight~structure core 72', three -~
centraI ribs 122,~124 and l26 are first attached at the
centers thereof. ~One of these ribs, 122, has one slot
in the center at the~top, as shown in Fig. 33, another
one, 124, has one slot in the center at the bottom, as
shown in Fig. 34, and the third one, 126, has two slots,
with one being in the center at the bottom and the other
in the center at the top, as shown in Fig. 35. Then six
ribs designated I28, 130, 132, 134, 136 and 138, all of
which are of the same size, as illustrated in Fig. 36,
are bonded to ribs 122, 124 and 126 to complete the
large hexagon.
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Each of the large triangles formed within the
hexagon are then filled with smaller triangular cells.
For example, ribs 140, 142 and 144, as illustrated in
Fig. 37, are bonded. ~ach of ribs 140, 142 and 144 has
a top slit and a bottom slit, which slots are spaced by
a cell length. Then ribs 146, 148 and 150, which are of
the same length, are locked in the center of the
triangle and bonded at the edges. Such locking may be
performed in the same manner as described hereinbefore.
That is to say, one of the ribs 146 may have one slot at
the bottom, another rib 148 may have one slot at the
top, and the third rib 150 may have two slots, one at
the top and one at the bottom, as shown in Fig. 38.
Rib 150 andribs 152 and 154, as shown in Fig. 39, are
then locked and bonded at the edges. Finally, ribs 148
and 154, and a rib 156, also as shown in Fig. 39, are
locked to complete the triangle. Once all six
triangles, and hence, the large hexagon is all filled
up, six outside closer modules are attached utilizing
closure segments 158, 160, 162 as shown in Figs. 40-42,
respectively, and in Fig. 31. For convenience of
illustration, the closure segments have not been shown
in Fig. 32.
As shown in the following TABL~ II, there are a
total of 117 ribs or pieces required in the assembly of
the lightweight structure core 72 utilizing the second
method. The quantity of each piece required is given in
the TABLE.
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TABLE II
Reference No.
of Piece QuantitY Fiq. No.
122 1 33
124 1 34
126 1 35
128... 138 6 36
140... 144 18 37
146... 150 18 38
152... 156 18 39
158 I2 40
160 24 41
1~2 18 42
As contrasted with the rib3 in the lightweight
structure cores 10 shown in Figs. 1-10 in which the
bottom edges of the ribs are all located in the same
plane, the bottom edges of the ribs of the lightweight
structure cores 72 and 72', as best seen in Figs. 13 and
33, respectively, curved, and hence, ail portions
thereo are not located in the same plane. The
structure of Figs. 1-10, as described, is appropriate
for use in the fabrication of back-up structures for
fIat mirrors or other flat members; those of Figs. 12-42
facilitate use in the fabrication of mirrors or other -~
members having curved surfaces. This demonstrates the
adaptability of the lightweight structure core of the
invention for fabrication in various configurations.
Fig. 43 illustrates a chemical vapor deposition
system 164 that may be used to effect SiC and Si
deposits on a mirror faceplate and the back-up structure
therefor. The system 164 includes a furnace 166 -
comprising a vertically positioned graphite tube 168,
electrical heating elements 170 that surround tube 168,
~ ` 2019697
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three mandrels 172, 174 and 176, and three baffle plates
178, 180 and 182.
The mandrels 172, 174 and 176 are arranged in
series and are fabricated from high density graphite
having a thermal expansion coefficient larger than that
of the chemical vapor deposited SiC~ Each graphite
mandrel 172, 174 and 176 is held with four graphite
posts which, in turn, are attached to respectively
associated graphite baffle plates 178, 180 and 182.
Each baffle plate is supported by the circular
graphite tube 168 which encloses the deposition area and
isolates the latter from the graphite heating elements
170.
Reagents, CH3SiC13 and H2, are introduced into
the bottom of the tube 170 from four water-cooled
injectors 184 mounted in the bottom cover 186 of tube -:
168.
In order to increase deposition efficiency and
accommodate three mandrels in the chemical vapor
deposition furnace, the first mandrel 172 is placed
close to the injectors 184. To prevent the injectors
184 from producing "growth marks" on the first mandrel,
a graphite manifold 188 was used which blunted the
injector flow and allowed the reagents to flow uniformly :
through a large central hole. This arrangement provides
a more uniform deposit on all three mandrels 172, 174
and 176.
CH3SiC13 is a liquid at room temperature with a
vapor pressure of about 140 torr at 20C. It is carried
to the deposition region by bubbling argon through two
CH3SiC13 tanks ~not shown). The CH3SiC13 flow Erom
the two tanks is divided into four parts which pass
through the four injectors. The pressure and
~ ,!.. " " . , " ~
~ \
- 20~9697
-25-
temperature of the CH3SiC13 tank and the argon flow
rates are maintained the same for both tanks to obtain a
uniform deposition.
EXAMP~E III
The chemical vapor deposition mirror fabrication
technology was scaled from a small horizontal research
furnace to a pilot-plant size production furnace capable
of fabricating a 40-cm.-diameter mirror. A
40-cm.-diameter mirror was designed. The salient
features of the arrangement are given in TABLE III.
TABLE III - 40-cm.-DIAMETER Si/SiC
MIRROR DESIGN FEATURES
Si Cladded SiC FaceplateInch cm.
Si Cladding Thickness 0.020 0.05
SiC Faceplate Thickness0.0880.22
Faceplate Total Thickness 0.108 0.27
SiC Liqhtweiqht Structure
~all Thickness 0.064 0.163
Cell Height 1.28 3.25 ;
Cell Length 1.97 5.00
Flow Hole Diameter 0.275 0.70
Hole Center Distance From Edge 0.40 1.02 ~ -~
No. of Equilateral Triangular
Cells 96.0 96.0
2$ Cell Aspect Ratio 1.3 1.3
SitSiC Mirror
Mandrel Diameter 16.0 40.48
Radius of Curvature 39.37 100.0
Total Mirror Thickness1.3883.52
Center Depth 0.82 2.09
2~9~97
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The mirror design assumed a polishing load of -1 psi, a
peak-to-valley intercell sag of -0.025 ~m, a
peak-to-valley self-weight gravity distortion between
supports (20 cm. apart) of -0.025 pm, and a minimum
natural frequency of 25 Hz. The weight of the mirror is
2.94 kg which corresponds to a weight specification of
about 19 kg per meter squared.
In order to scaIe the SiC back-up structure, first -~
the graphite core is scaled. The lightweight structure
consisted of 16 hexagonal cells containing a total of 96
triangular cells. The cell aspect ratio, defined as the
cell depth to the diameter of the inscribed circle, is
1.3 for each triangular cell.
The scaling of the chemical vapor deposition
fabrication technology to the required size involves the
following:
(a) Material scaling. The optimum chemical
vapor deposition process conditions which produced
the Si and SiC materials in the research furnace
were scaled to the pilot-plant size furnace. This
scaling was performed keeping the following
parameters unchanged: (1) deposition rate, (2)
deposition setup geomet cally similar to the one
used in the research furnace, (3) deposition
temperature, and (4) furnace pressure. In
addition, nondimensional chemical vapor deposition
process parameters were identified and important
scaling laws were develped. Based on these laws,
reagent 1OW rates, molar ratio, and injector
diameter were fixed. The scaling laws were
validated by fabricating Si and SiC plates o size
32 cm. x 90 cm. and 0.63 cm. in the pilot-plant
size furnace. Important physical, optical,
- -` 20~9697
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mechanical, and thermal properties of this material
were compared with those corresponding to the
research material, and were found to be identical.
(b) Scaling of the chemical vapor deposition
mirror fabrication technology. This involves
scaling of the replicated faceplate.
The scaled graphite core was placed on the backside
of the SiC faceplate and coated with ~iC in the
pilot-plant size furnace. After this was accomplished,
the SiC faceplate was separated from the graphite
mandrel and the front of the faceplate was coated with
chemical vapor deposited Si.
Thus, in accordance with the invention, there has
been provided unique lightweight structures and improved
methods that enable simplification in, reduction of time ~ -
required for, and cost of their fabrication. The
structures provided are comprised of vapor deposited
material such as SiC or Si in a monolithic form. The
structures, while light in weight, are characterized by ;
being very stiff and strong and in having extreme figure
stability retention. The structures are further -
characterized in having an extraordinary adaptability
for fabrication in various predetermined configurations,
for being scaled up in size, and in having utility in a
variety of diverse applications including back-up
structure for mirrors.
With this description of the invention in detail, ~
those skilled in the art will appreciate that modifica- -
tions may be made to the invention without departing
from its spirit. Therefore, it is not intended that the
scope of the invention be limited to the specific
embodiment illustrated and described. Rather, it is
intended that the scope of the invention be determined
by the appended claims and their equivalents.
