Silver chloride use in technique for fusion bonding of graphite to silver, glass, ceramics, and certain other metal

Faulkner, R. D.

English

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NOTICE 
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The invention is owned by NASA and is therefore available for 
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BONDING GRAPHITE. 
eth 
T h i s  i n v e n t i o n  relates t o  a method for bonding graphite t o  
o t h e r  materials such  as s i l v e r ,  glass ,  ceramics, and metals below 
silver i n  the slectro-motive force sriaa as w 11 as bonding one 
g r a p h i t e  a r t ic le  t o  a n o t h e r  and t o  the composi te  ar t ic le  produced 
by such  bonding. The i n v e n t i o n  g e n e r a l l y  compr ises  the a p p l i c a t i o n  
of a s i l v e r  chloride l a y e r  between the graphite and the material t o  
be bonded thereto, m e l t i n g  the s i l v e r  chloride and thereafter c o o l i n g ,  
The bonding method encompassed by t h i s  i n v e n t i o n  i s  practiced 
by a number of a l t e r n a t i v e  t e c h n i q u e s  p a r t i c u l a r l y - a s  regards the 
placement  of the s i l v e r  chloride on the base g r a p h i t e  and the sub- 
sequen t  f u s i o n  bond ing .cond i t ions .  It  is g e n e r a l l y  p r e f e r r e d  t o  
u t i l i z e  a wafer of si lver chloride of approximately 1 mm. t h i c k n e s s  
d i r e c t l y  la id  on the g r a p h i t e  b u t  other t echn iques  such as powder, 
s p r a y  or pre-heated base g r a p h i t e  t echn iques  may be u t i l i z e d .  A f t e r  
the material t o  be bonded is p l a c e d  ove r  the s i l v e r  chloride, the 
' e n t i r e  composi te  s t r u c t u r e  is heated t o  above 45SoC., the f u s i o n  
t empera tu re  of s i l v e r  chloride, under a tmospher ic ,  i n e r t ,  or vacuum 
c o n d i t i o n s .  The p a r t i c u l a r  mode of h e a t i n g  may be by s t a n d a r d  fu rnace  
{ techniques  or by i n d u c t i o n  methods, I n  the bonding of g l a s s  or ceramics 
it is  g e n e r a l l y  desirable t o  u t i l i z e  an i n n e r  l a y e r  of moble metal be- I 
tween  the s i l v e r  chloride and the ceramic or g l a s s .  This i n n e r  l a y e r  
t echn ique  c a n  a lso be u t i l i z e d  w i t h  other metals of above s i l v e r  i n  
the e l ec t ro -mot ive  force series 
The n o v e l t y  of t h i s  i n v e n t i o n  r e s i d e s ' i n  t h e  concep t  of u t i l i z i n g  
s i l v e r  chloride t o  bond g r a p h i t e  t o  i tself ,  s i l v e r ,  metals below s i l v e r  
i n  the e l ec t ro -mot ive  series, g l a s s ,  and ceramics, By t h i s  i n v e n t i o n  
the a p p l i c a n t  has so lved  a problem long  p l agu ing  the i n d u s t r y  which 
before t h i s  i n v e n t i o n  had t o  r e l y  upon v a r i o u s  mechanical  t echn iques ,  
a d h e s i v e s  or direct m e t a l  m e l t i n g  t echn iques  on the g r a p h i t e ,  These . 
a l t e r n a t i v e  t e c h n i q u e s  either r e q u i r e d  e x t e n s i v e  e x t e r n a l  s t r u c t u r e ,  
w e r e  non-conductive and weak and/or w e r e  very  expens ive .  By the i n -  
s t a n t  i n v e n t i o n  g r a p h i t e  may be bonded w i t h o u t  the above n o t e d  draw- 
backs the reby  r e p r e s e n t i n g  a n  improvement of c o n s i d e r a b l e  s i g n i f i c a n c e  
i n  such f ie lds  as p roduc t ion  of e l e c t r o n i c  components p a r t i c u l a r l y  of 
the m i n i a t u r i z e d  v a r i e t y  as w e l l  as other environments  where spaceb 
c o n d u c t i v i t y ,  c o r r o s i o n  r e s i s t a n c e ,  and cost are impor t an t ,  
d *  
I n v e n t o r  : Richard)lFaulkner 
Employer : Radio Corpora t ion  of A m e r i c a  
I n  
3,453,172 United States Patent Office patented July 1, 1g6g 
tics and Space Administration with respect to an inven- 
tion of Richard D. Faulkner, Laneaster, Pa. 
No Drawing. Filed Oct. 8, 1965, Ser. No. 494,282 
Int. c1. B32b 9/04 
US. CIS 161-182 12 Claims 
ABSTRACT OF THE DISCLOSURE 
It is a further object of the present invention to pro- 
vide a bopding technique for graphite which obviates the 
need for bulky external mechanical structure or weak- 
ening apertures or cut outs in the graphite structure. 
It is another object of this invention to provide a meth- 
od of bonding graphite in which the bonding material is 
thermally and electrically conductive with good stability 
characteristics under corrosive conditions. 
It is still another object of the present invention to 
provide a bonding technique which may be effected under 
atmospheric or controlled conditions to produce a water 
and vacuum tight seal between graphite articles or be- 
I tween graphite articles and noble metals, ceramics, or glass. ~" ~ "I - 
5 tt is still a further object of this invention to provide a 
method for bonding graphite which may be effected with- 
out requiring pre-coatings to the graphite. 
It is still another object of this invention to provide a 
graphite structure which may be easily bonded to a num- 
ORIGIN OF THE INVENTION 20 ber of materials and a composite structure so produced. 
The invention described herein was made in the per- These and other objects will become apparent from a 
formance of work under a NASA contract and is subject consideration of the following description and claims. 
to the provisions of Section 305 of the National Aero- Briefly, the method of the instant invention comprises 
nautics and Space Act of 1958, Public Law 85-568 the placement Of silver chloride (A&]) between the 
(72 Stat. 435,42 U.S.C. 2457). 25 graphite and ;the article to which it is to be bonded, heat- 
 hi^ invention relates to a method for bonding graphite ing the silver chloride to a temperature equal to or above 
tion of silver &loride in such bonding. ~ ~ ~ ~ i f i ~ ~ l l ~ ,  this thereafter cooling to atmospheric temperature. This tech- 
invention provides a bonding technique whereby firm nique is applica~ble for bonding one graphite surface to 
bonds of silver Chloride are provided between graphite 30 another or to bond a graphite surface to silver or metal 
elemnts and between and between graphite and silver, below silver in the electro-motive series. It is also a p  
glass, ceramics and metals below silver in the electro- plicable to bonding glass or ceramic materials to graphite 
motive force series. either directly or preferably through use of an inner noble 
The problem of adheping graphite to itself or other 
materials has resulted in a myriad of proposals and tech- 35 7'0 accomplish the above defined method it is generally 
niques to effectuate such bondng. ~ ~ ~ ~ ~ d l ~ ,  a mechani- preferred to utilize a wafer of silver chloride between 
cal soltion has proved most satisfactory, such techniques the to be bonded- Such wafer should have a 
as clamping, bolting and riveting, tapping and threading, surface area apPrOXimatelY equal to the area of contact 
force and shrink fitting, and powder tamping being most between the mating Parts. However, in some instances 
common expedients. However, such techniques as clamp- 40 Slight overlaps Of the wafer area are desirable while 
ing or tamping require external structure resulting in adequate bonds may be provided on a point rather than 
wasted space while bolting, riveting, tapping or thread- a full Surface contact basis. The wafer should be approxi- 
ing results in a weakened product and increased manufac- mately 1 mm. in thickness for optimum bond strength 
turing expense. Force and shrink fitting are also limited with most graphite although thicknesses as small as .25 
in application not only by external structure but also by 45 mm. Provide good bonding action. Thicknesses .greater 
requisite low use temperatre. to maintain the fit. Besides than 3 mm. can be used but without an increase in bond- 
fie above, any such mechanical may result in ing strength. The greater tbicknesses are used with lower 
a product which is neither vacuum nor water tight. density graphite articles because of the higher perme- 
with little favor because the bonding material lacks Besides the wafer application technique it is also pos- 
thermal and electrical conductivity and lacks the ability sible to apply the silver chloride by both powder and 
to stand UP under high temperatures and corrosive con- spray methods. The powder application merely involves 
ditions. placing the graphite so that the surface to be bonded is 
to the above proposa~s it has been 55 horizontal, pouring the silver chloride in particulate form 
suggested that graphite be bonded by soldering or b r a -  thereon, Placing the material to be bonded to the graphite 
ing techniques. However, most molten metals do not wet above* At the fusion 
graphite SO it has been necessary to first coat the graphite temperature, the Silver chloride forms a constant depth 
with metal by electroplating or by metal spraying prm- film between the bonded elements so that upon cooling 
ems and thereafter solder or braze to the coated graphite. 60 the thickness is constant. 
Even with such an elaborate system the bond is not strong The Spray technique may comprise either melt or solid 
so ,the connection cannot be utilized in environments application. Zn the melt technique, the silver chloride is 
where high strength is a requisite. In more recent years it $Pray& in molten form on the graphite substrate with the 
has been suggested that under controlld atmos- material to be bonded placed thereover before solidifica- 
pheres may be utilized for melt bonding graphite articles 65 tion. Alternatively, the mdted material-graphite sub- 
but expense and construction render such a techique un- Strate combination may be cooled and thereafter utilized 
suitable for normal graphite applications. in a subsequent fusion bonding step. A reverse procedure 
Therefore, it is a principal object of the present inven- may be utilized wherein the silver chloride is sprayed on 
tion to provide a method for bonding graphite to itself the material to be bonded to the graphite and thereafter 
or other materials which maintains good strength even 70 the graphite applied either before solidification or in a 
under relatively high temperature operating condition subsequent fusion bonding step. Solid application by the 
while requiring minimal expenditure for bonding material. spray technique involves forming either a slurry or solu- 
to other materials and more particularly to the utili=- its fusion temperature (approximately 455" c.1 and 
Adhesive bonding has also been attempted but has met 50 
an 
and heating as 
3,453,172 
" 3  
tion of 'the silver chloride, spraying the graphite base 
and thereafter evaporating the carrier solution. 
Another alternative method includes preheating the 
graphite base above the silver chloride fusion temperature 
and applying solid silver chloride thereto which immedi- 
ately melts to form the desired wafer' structure. 
The heating of the silver chloride layer may be effected 
by placing the entire structure in a furnace or by induc- 
tion or conduction electrical heating methods. In order 
to protect some of the bonded materials from oxidation 
it is often desirable to utilize an inert atmospheric during 
heating. Atmospheres found suitable include nitrogen, 
argon and helium. Also the heating may be carried out 
under vacuum to alleviate the above problem. 
It is not generally necesary to provide any clamping 
of the structure during bonding since the weight of the 
material to be bonded to the graphite will be sufficient 
to provide contact between the silver chloride, the base 
graphite and the material to be bonded. However, in 
some instances it may be desirable to temporarily clamp 
the composite article during heating particularly when 
the material to be bonded is very thin or the heating is 
to be effected while the composite structure is vertically 
stacked. 
As pointed out above, when it is desired to bond 
graphite to glass or ceramic materials it is preferable to 
metallize these latter materials with one of the noble 
metals such as gold, silver, platinum, palladium, iridium, 
rhodium, ruthenium or osmium. These metals may be 
applied by standard spray or electrochemical techniques 
which are well known and form no part .of the instant 
invention. 
Example I 
A graphite block was placed on a horizontal surface 
and a wafer of high purity silver chloride i mm. thick 
placed thereover; said wafer being cut to exactly cover 
the surface of the block. A thin sheet of silver (.3 mm. 
thick) of similar area was placed over the wafer and the 
thus formed sandwich (was heated to 460" C. in a nitrogen 
atmosphere. Upon cooling the resultant composite ex- 
hibited high strength and good thermal and electrical 
conductivity. Temperature up to 400" C.  did not appreci- 
ably diminish the strength. 
Example 11 
A similar block as in Example I was placed on a hori- 
zontal plane and powdered silver chloride poured and 
spread thereon to a thickness of about 3 mm. A sheet 
of glass which had been silver plated was placed there- 
over and the composite heated under vacuum to 457" C. 
This product exhibited high tensile strength and corro- 
sion resistance, the tensile strength not varying at tem- 
peratures up to 300' C. 
Example I11 
A similar block as in Example I was placed in a hori- 
zontal plane and a wafer of high purity silver chloride 
2 mm. thick placed thereover. This composite was placed 
in a nitrogen atmosphere furnace, heated to 456" C. to 
melt the silver chloride and cooled. This method provided 
a completely adherent coating of the silver chloride of 
the graphite. A sheet d silver was then 
silver chloride layer side of the graphite 
efitire structure was reheated in nitrogen about 455" C. 
at which temperature the silver chloride melted. Subse- 
quent cooling provided a. structure having similar prop- 
erties to that of Example I. 
The technique of the instant invention finds particular 
utility in the production of electronic components partic- 
ularly in miniaturized parts where space is a critical 
factor. Other 'applications include strength and corrosion 
resistance, the tensile strength not varying at tempera- 
tures up to 400" C. This invention is applicable to all 
4 
forms of graphite including the low permeability, high 
density types as used in nuclear reactors, as well as the 
Iower density, higher permeability types normally found 
in dry cells. It is also applicable to bonding of graphite 
to many other metals, plastics, etc. provided these other 
materials are first bonded to the described noble metals. 
As many widely different embodiments of this inven- 
tion may be made without departing from the spirit and 
.scope thereof, it is understood that the invention is not 
lo limited to the specific embodiments thereof. 
What is claimed is: 
1. A composite article consisting of a base of graph- 
ite, an inner layer of silver chloride fused directly to said 
graphite, and an outer layer of a member selected from 
15 the group consisting of graphite, silver, metals below 
20 
Iected from the group consisting of graphite, silver, metals 
below silver in the electromotive series, glass and ceramics 
25 comprising placing a silver chloride layer between the 
graphite and the material, heating the thus formed sand- 
wich to a temperature of at least about 455" C. and cool- 
ing. 
4. A method according to claim 3 wherein the mate- 
5. A method according to claim 3 wherein the silver 
chloride layer is a solid wafer. 
6. A 'method according to claim 3 wherein the silver 
chloride layer is formed of particulated material. 
7. A method according to claim 3 wherein the silver 
chioride layer is pre-bonded to the graphite layer. 
8. A method according to claim 3 wherein the glass 
and ceramics are coated with a noble metal before bond- 
9. A method of bonding graphite to a material se- 
lected from the group consisting of graphite, silver, metals 
below silver in the electromotive series, glass and ceramics 
comprising placing a wafer of silver chloride about .25 
to 3 mm. thick between the graphite and the material, 
45 heating the thus formed sandwich in an inert atmos- 
phere to a temperature of at least about 455" C. where- 
by the silver chloride forms a melt and thereafter cool- 
ing. 
10. A method according to claim 9 wherein the ma- 
11. A method according to claim 9 wherein the glass 
and ceramics are coated with a noble metal before bond- 
12. A method according to claim 9 wherein the inert 
30 rial is silver. 
35 
ing. 
40 
50 teiial is silver. 
ing. . 
55 atmosphere is nitrogen. 
References Cited 
UNITED STATES PATENTS 
G~ 1,660,650 2/1928 Mailey _ _ _ _ _ _ _ _ _ _  287-20.2 X 
2,573,380 10/1951 Ambrose et al. _ _ _ _  161-225 X 
3,089,799 5/1963 Domicon ________-__ 161-192
3,096,218 7/1963 Lieb et al. _ _ _ _ _ _ _ _ _ _  136-121 
OTHER REFERENCES 
"Pile-Type Silver Chloride Cells," an abstract of Japan 
4919 (1957) in chemical abstracts 52: p. 16092 g (1958). 
65 
, EARL M. BERGERT, Primary Examiner. 
70 R. A. KILLWORTH, Assistant Examiner. 
U.S. C1. X.R. 
. 16.1-225; 136-121 


Bonding graphite with fused silver chloride

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