Note: Descriptions are shown in the official language in which they were submitted.
1085146
This invention relates to a novel method for sealing
the ends of porous ceramic elements and to the bonding of
metallic end-pieces to said extremities.
A barrier for the separation of the gasecus elements
of a mixture is usually composed o~ a porous cylindrical
support having high mechanical strength, the internal wall
of which is coated with a thin layer of co~pacted ~icroporous -
powder which ensures the separation of these gas isotopes.
After construction of a barrier of the ceramic type,
it must be fitted at both ends with two metallic end-pieces
which will ser~e to mount the barrier between two diffuser
plates or to secure it to other barriers.
The material employed for bonding the end-pieces and
the method of application adopted assume special importance
since they must confer high mechanical ~trength on the barrier
which may be subjected to high tensile, torsional and vibra-
-tional stresses during its operation.
The abo~e-mentioned bonding operation must not give
rise in particular to embrittlement of the ceramic support,
for example to e~en very partial crack formation at the grain
boundaries which constitute the porous material nor must it
cause dislocation or weakening o~ the di~using layer.
The bond thus formed must also achieve perfect leak-
tightness of the extremity o the barrier in order to ensure
that the entire gas stream to be separated essentially passes
thrcugh the diffusing layer without of course by-passing said
layer so as to escape through the support or within the thick-
ness of this latter.
This requirement entails the need in particular to
ensure that the adhesion of the sealing and bonding material
is as gool within the interior of the barrier or in other
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35146
words on the microporous diE~using layer as on the end face
or the ex-terior o~ the support which has a high degree of
surface roughness.
It is also necessc~ry to achieve per~ect adhesion
with respect to the metallic end-piece. For reasons of
efficiency of isotope sepæ ation, the lengths of overlay of
the material e~ployed ~or sealing the barrier and bonding the
end-pieces must be approxLmately equal both on the inside (on ..
the side corresponding to the dif~using layer) and on the
outside (on the side corresponding to the support) of the
barrier.
It can thereEore be sta-ted that this operation has a
double objective : on the one hand that o~ ens~ing perfect
leak-t:Lghtn~ss o~ -the e~xtremi-ty of the barr:ler and on the other
hand that o.E securln~ the~ metalllc end-p:Lece ko said extremity
in such a manner as to ensure high mechanical strength.
A large number of materials and methols have already
been employecl ~or achie~ing the objectives de~ined in the
~oregoing.
It has been endea~ored to employ certain metals or
metal alloys modi~ied by additions ~or the purpose o~ "wetting"
the ceramic portlons. These metallic substances whlch are
melted on the extremity oE the barrier and of the end-piece
usually dislocate the di~using layer and result in considerable
embrittlement o~ the ceramic support which is liable to break
spontaneously as a result o~ a binding or hooping action. These
alloys o~ten have a complex composition and o~er poor
resistance to the ~luorinated agents to which they are sub-
jected.
.30.~- It has also been proposed to carry out this operation
by deposition o~ a ~luorinated resin such as tri~luorochloro-
1~5146
ethylene which is mol~en and polymerized in the hot state, the
barrier and the end-piece being equipped with an external
mol~ with an internal punch ; these la-tter can readily be.
opened and e~tracted after heat treatment. However, this ~ !
resin deposit does not have good adhes.Lon to the end-piece -
and this disadvantage results in relatively low values of
mechanical strength o~ the en-tire assembly. In addition,
adhesion to the microporous layer is often de-fective and :~
leakages are frequently liable to be detected between the
resin deposit and this layer. Furthermore, the viscosity of
these deposits decreases as a function of the temperature, the
deposit undergoes a certain creep de~ormation and the
mechanical properties and lealc~ticJhtness are considexably
impa:Lred during use of the bc~rrler :ln the hot state and under
hlgh mechanical stresses.
Slnce the operation which consists in sealing the
extremity o~ the barrier and bonding the end-piece at the same
time cannot readily be perormed in practice, it has been
proposed to dissociate these two operations.
Sealing o~ the extremities o~ the barrler is carried
out by dipping these latter in a suspension d powdered glass
.in a suitable llqu.td, in drying and subjecting to heat treat-
ment at a temperature which results in ~usion oE the glass
layer. This enamelling o~ the extremities is carried out with
a glass which is chosen for its high resistance in a ~luorinated
corrosive atmosphere, ~or example a glass having a base o~
phosphates and alu~ina.
The glass layer which is deposited on the exter:ior
o~ ~he support is then subjected to sand-blasting with
. corundum in such manner as to endow.this latter with a certain
degree o~ sur~ace rcughness. This roughened surEace is then
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~351~6
coated with a metallic laye~ such as a layer of copper, for
example, which is deposi-ted by projection in the hot sta-te by
means of a flame-spray torch.
Ihe metallic end-piece ls then brough-t in-to contact
with the extremity of the barrier which has thus been coated
and is bonded by means of a fusible braziny alloy such as, for
exampletin-lead-silver or tin-lead-cadmium which "wets" both
the end-piece and the metallic layer which is deposited on the
enamel at the extremities of the barrier.
This method entails a læge number of operations
which makes it ~airly costly and is subject to major dis-
ad~antages whlch make it inapplLcable at relatLvely hiyh
o~eratincJ -tQmperatures o~ the barriers. Glass a~fords only
very :~nperf~ct res:Lstance to the severe conditions oE corrosion
by t'ne fluorinated gases and the same applies to the metal
deposited on the glass and the brazing alloy which serves to
bond the end-piece. The mechanical properties of an assembly
of this type become impaired after a relatively short period
of operation.
I~ another method which has been adopted, a firs-t
step conslsts ln ensuring leak-tlyhtness o the barrier by
means of a deposit o a re~ractory mineral substance such as
aluminum oxidet ~or example,or the solid solutions or
compounds of said oxide with other oxides, thls ma-terial being
deposited by projection in the hot state by means of a
chemical ~lame-spray torch or a plasma-arc torch.
The angle o pLojection is chosen so as to ensure
that the internal portion o~ -the barrier (on the side corre-
sponding to the di~fusing layer) t the end ~ace and the
external portion (on -the side correspondiny to the support)
are coated with a layer of the- material.
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1~85~6 `
In a ~econd step following this sealing operation,
the end-piece is placed in contact with the extremity of the
barrier and the complete assembly is joined together by means
of a deposit of metal which is also projected in the hot state
onto the previously sanded end-piece and onto the extremity
of -the barrier.
This operation is attended by a certain number of
drawbacks which set a considerable lirnitation on its practical . '
value.
During the operation of oxide deposition which is
intended to ensure leak-tightness of the barrier, it is
necessary to place withln the interior of this :Latter a
rubber p.lu~ wh:lch l.imits the length o.E deposit since rebound
ing o~ the mo:Lten re:~racto:r,y particles results in a la,yer of
substantial leng-th as well as irregular and discontinuous
thickness which impairs the separation efficiency of the
barrier. The presence of said plug which limi~s the length
o~ the internal deposit results in the presence of a bead or
overthickness, thus producing a hlgher pressure arop in the
longitudinal stream within the barrier.
The adhesion of said oxide deposit on the dif:Euslng
la,yer, especially when thls latter has a very fine structure
(short pore radii) is very poor and a leak can frequently be
detected between the diffusing layer and the oxide deposit.
~he oxide deposit on the end face of the barrier is
very delicate and sensitive to mechanical shocks. When the
metallic end-piece is brought into contact with the barrier
by mechanical means r precautions must accordingly be taken
to prevent either chipping or rupture of the deposit.
30, FinallyL the metallic layer which is intended to
ensure attachment.of the end-piece to ~he barrier i,s not
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~L~851~6
alw~ys strictly leak-tiyht and if the end-piece is not
perfectly applied against the extremity o~ the barrier,
leakage is liable -to take place between them through the
bonding me-tal.
It should be pointed out that all the methods of the
prior art as described in the foregoing are applied to
completed barriers, that is, to ceramic supports on which the
microporous separating layer has been ~eposited.
If the sealing and bonding operation is defective,
the completely manufacturea barrier must therefore be elimina-ted
and these rejects are particularly costly.
The method in accordance with the invention consists
in carrying out the operation for ensuring leak-tlghtness of
the extr~m:ltles Oe the ceramlc support on wh:Lch ls subse~uently
cL~po.s:lt~d ~he thln mlcroporous separatlng :Layer whlch ls sub-
jected to the operations of compactingr heat treatment and
any necessary thickness adjustments and consists in fixing the
metallic end-pieces on this completed barrier.
Thls method is inexpensive, readily permits of manu-
facture on an industrial scale and the number oE operationsis very lim~ted. It ensures total con-tainment of the gas
stream wlthin sald barr:ler and ma~lmum separatlon efEiclency.
The method pr~vldes exceptional propertles of
resistance to mechanical stresses~ particularly good behavior
in a corrosive and especlally a fluorinated atmosphere and the
barrier which is thus ecluipped can be subjected to high
operating temperatures and aerodynamic stresses without under-
going any detectable degradation in time.
Finally, this method of sealing and bonding does not
introduce any additional pressure drop in the lorlyitudinal
s-trea~ which circulates within the barrier.
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10~5~L6
The invention has for its object a method of fabrica-
tion of porous elements constituted by ceramic oxide materials
and characterized by the s-teps of metallization of the
extremities of the porous tube which cons-tit:utes the support
of said element by spraying in the hot state, deposi-tion on
the internal wall of the support of a thin ]ayer of micro-
porous powder which is intended to ensure the separation of
the gaseous elements, compacting of the deposited layer and
bonding of metallic end-pieces to the extremities of said
element by spraying a metal in the hot state on the surfaces
of the element and the end-piece adjacent the abutting
surfaces.
The porous tube which constitutes -the support of the
separation element i8 subjected to a first operation which
consists in depositing a metallic layer on its extremities by
spraying in khe hot state by means of a suitable spray-torch.
This layer covers the external wall, the end face and the
internal wall of the support over a limited distance.
The internal wall of the support which has thus been
treated is then coated by any known means with a thin layer of
microporous material.
The layer thus deposited is compacted by isostatic
compression.
The element as thus prepared may be subjected to a
heat treatment and if necessary to adjustment of its ~ -
permeability, a cylindrical end-piece is then placed in
intimate contact with the extremity of the element which has
jus-t been produced and is attached to said element by means of
a metallic deposit which is sprayed at the level of the
junction between element and end-piece by means o-f a suitable
spray-torch.
The invention is illustrated by means of the following
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10~51~6
embcdiments ~Ihich are given by way o:E example and not in any
limiting sense, reference being made to the accompanying
clrawings, wherein : -
- Fig. 1 illustrates the modLe of deposition of a
metallic layer a-t the extremity of th.e ceramic support ;
- Fig. 2 is an enlarged view of the extremity of the
support with its metallic sealing layer and the subsequently
deposited microporous layer ;
- Fig. 3 shows the extremity of the completed separa-
tion element after attachment of the metallic end-piece.
In Fig. 1, the support 1 which is usually provided
in the form oE a cylindrical tube having h:lgh porosity, sub-
stAntlal permeability and constltuted by gra:lns o~ one or a
n~ber of mekal ox~des consolidated by s:lnterlng at high
temperature ls subjected to a movement of rotation on a
suitable mechanical devlce.
A spray-torch 2 deposits a metallic layer 3 on the
extremity of the support 1.
The spray-torch is usually a gun which serves to melt
a wire by means oE propane~ butane-oxygen or acetylene-oxygen
flames.
The rnetal-sprayincJ means could also consist of a gun
for maintaining an electric ar between two wires of the metal
to be deposited which is sprayed onto the part to be coated
with a jet of gas or alternatively a plasma-arc torch fed
with po~der of the metal to be sprayed.
The choice of the deposited metal assumes special
significance since it must aff.ord perfect resistance to the
chemical action of the corrosive fluorinatecd gases to which
30- the elemen-t will be subjected during use. Furthermore~ the
. metal in the state in which it is deposited must:retain a
.
_9_
10~5~L46
certain degree of ductility ~7hich will enable it to increase
in density during subsequent operations and to form a
practically leak-tigh-t coating between the microporous layer
and the support.
A metal of this type can be nickel or one of its
alloys but aluminum is particularly suitable and offers the
requisite advantages.
The spraying distance between the torch and the
support is chosen so as to ensure that the metal par-ticles
reach the support in liquid form without resulting in
excessive heating of this latter. With a view to preventing
any temperature rise, a compressed-a:ir jet can be d:lrected
on~o the extremity of the support in thQ direct:Lorl opposite
to that o~ tLIe torch~
The spraying angle is detenmined so as to limit the
length of the deposit within the tube to the desirea length.
However, a certain number of particles rebound upon the
internal wall of the tube and result in a degraded and
partially discontinuous deposit which i~ liable to impair the
enrichmen-t efficacy of the barrier and to produce local
defect~. In order to prevent deposition of these incldent
particlest there can be admitted at the opposite end of the
barrier a stream of gas and especially of compressed air, the
flow rate of which is calculated so as to thrus-t back said
incident particles but without cooling the jet of particles
which are discharged directly from the torch.
In order to limit the length o~ the metal deposit,
it is prefercible to place the spray-torch as close as possible
to the support tube. Howeverr since it is neessary to
prevent rebounding of the particles and especially excessive
heating of the t~e, the flow rates oE fuel gas and oxidizerr
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L4~
the flow rate of air as well as the r~tes of feed of the wire
within the torch are set a-t low values. The grains of metal
and more particularly of aluminum which are sprayed wi-th
-these particular settings are in any case of larger size and
oxidi~ed to a lesser extent than those obtained with the
settings usually recommended by flame-spray torch manufacturers
and are conducive to subsequent densification of the deposit.
These settings can in any case be maintained in the end-piece
bonding operation which will be described below.
In order to limit the length of the deposit on the
external wall oE the support tube, a screen 4 is placed
between the support tube and the torch Ln order to collect
p~rt oE the sprayed partLcl~. Sa:L~ screen is constltuted by
a strlp of meta:L or plastic res:Ln or by a traveling strip oE
paper or else by a plate o~ vitreous carbon, in accordance
with a preferr~d variant of the invention.
Af-ter this operation, the support is co~ered at both
ends with a continuous metal layer which ~orms a coating on
the external wall, the end face and the internal wall, the
lengths oE deposit on the external and internal walls being
approximately equal. It is worthy o~ note that these metal
layers provlde a mechanical rein~orcement for the extremities
of the support which can then be subjected to the subsequent
operations invol~ed in the fabrication o~ the separation
element with greatly reduced sensitivity to shocks or other
stresses.
The ~ollowing operation consists in depositing the
thin layer o~ a ~icroporous powder which will ensure separa-
tion of the gaseous compounds.
This powder is deposited by filtration1through the
support\of a suspension oE said powder in a sui-table solvent.
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1(~85~6
It .should be observed that the me-tal layer deposited
during the previous operation has a decreasing thicknes~ from
the extremity to the interior of the support and can form a
gradual transition zone from the leak-tight metallic portion
to the ceramic porous portion and that the di~fusing layer
can be deposited on the porous metal.lic portion, thus ensuring
freedom from any defect in the layer at the metal-ceramic
~oundary.
After deposition o~ the separating layer and thermal
elimination of any organic binders containing the suspension
which has ser~ed to deposit said layer, the complete assembly
is then suhjected to isostatic compression ~or the purpose of
comp~cting the layer on th~ support and endowing this latter
w:L-th the desired properties oE textur~ (pore .radlus, porosity,
permeablllty~.
Fig. 2 shows the end portion o~ the element obtained
in which the reference numeral 1 designates the porous ceramic
support, the reference numeral 3 designa-tes the metal deposit
formed by spraying and the reference numeral S designates the
separating layer which is overlaid on the metal deposit over
a short distance.
~ter this campression operation, the assemhly ls
subjected i~ necessary to a thermal treatment for removing
certain ~oreign substances from the layer by desorption or for
developing the texture and the characteristics o~ said layer
by thermal evolution o~ its elementary crystalli-tes. This
operation can also ensure annealing of the deposited metal
which thus relieves all mechanical stresses to which the
support may otherwi.se be subjected by said deposit.
The above-mentioned operation which consi.sts in
adjustment of permeability can also be carriea out prior to
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~35~6
heat treatment.
~ s indica-ted in Fiy. 3, the porous barrier constitute
by the support 1 together with its metallic end deposit 3 and
i-ts internal separating layer 5 is fitted with a metallic end-
piece 6 having an ex-ternal diameter ~Ihich is slightly smaller
than the external diame-ter of the elemen-t.
The cylindrical end-piece aforesaid is placed in the
axis of the element and in close contact with this latter.
5aid metallic end-piece is bonded by means of a metal'
layer 7 which is deposited by means of a spray~torch on the
two elements to be joined.together.
On the 'side corresponding to the suppor-t, it is
:lmportant to ensure that the lencJth o:E metal deposited does not
exceed that o~ the layer ak the -time o:E init:ial Spray-COatin~J
of -the support. To this end, positioning of the torch bo-th in
distance and jet discharge angle will be adjusted and it will
also be possible to make use of shields for limiting said
length.
As in the case of the operation involving metal
deposition on the ends of the support, the metal must be chos.en
so as to afford perfect resistance -to severe conditions of
chemical corrosion at the time oE operation oE the element.
Said metal can be either nickel or a nickel alloy '
but, in a preferred alternative embodiment of the invention,
use.will be made of aluminum since this latter fully satisfies
the requirements of chemical and mechanical resistance which
are laid do~m.
The end-piece'is also constituted by a metal w~ich ''
oFfers. a high degree of resistance to fluorinated gases and
can consist of a copper alloy, ~or example. In this case~ the
'surface of said end-.piec.e which will be coated with the
3L~8~46
sprayed bonding metal must previously be treated by sand-
blas-ting with corundum or shot-blasting in order to be endowed
wi-th a certain degree of surface roughness which will permit
mechanical adhesion of -the sprayed metal.
In accordance with a preferred alternative embodiment
of the invention, the sprayed metal is aluminum or one of its
alloys and the end-piece is formed of nickel or nickel alloy
and in the case last mentioned, the end-piece will not be
sand-blas-ted but simply cleaned with a wire brush, for example.
In fact, the molten aluminum grains discharged from the torch
react exothermically with the nickel so as to produce inter-
metallic compounds oE the nickel aluminide type which endow
the junction between end-ptece and depos:lted met:al w:lth out-
~tand:Lng mechan:Lcal strength clue to the metallurqlcal dlffuslon
of the alumlnum within the nickel.
As in the case of the operation which consists in
depositing the metal layer on the extremity of the support,
the metal which ser~es to bond the end-piece onto the separa-
tion element can be deposited without thereby departlng from
the scope o~ the invention by any known spray-coating means
and Eor example by means of a chemlcal Elame-spray torch, an
electric-arc torch or a plasma-arc torch.
The element for the separation of gas compounds as
fabricated in accordance with the method of the in~ention
ensures perfect compliance with the conditions of separation
efficiency, mechanical strength and chemical inertia which a
product of this type is intended to provide.
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