Note: Descriptions are shown in the official language in which they were submitted.
~'3~
METHOD OF FORMING GLASS OR CERAMIC ARTICLE
Background of the Invention
-
The present invention relates to a method for making pure,-
homogeneous porous glass or ceramic bodies from particulate
oxide materials such as fumed metal oxides. The porous bodies
may be consolidated into dense, void-free glass or ceramic
products.
The concept of making glass products at reduced tempera-
tures by sintering a mass of glass particles is old. A number
of methods for producing the mass have been considered,
including slip casting, compaction, sedimentation and polymeriz-
ation or condensation from solutions.
Perhaps the most common methods are the solution methods
wherein solutions or suspensions of glass-forming constituents
are treated to form a precipitate, polymerization product, or
gel which is then dried and sintered to form glass. U.S~
Patents Nos. 3,535,890, 3,678,144 and 4,112,032 describe one
such approach to glass-forming wherein silicate solutions or
suspensions are gelled, dried and fired to produce glasses.
Disadvantages of such approaches include the need for a very
prolonged and difficult drying process, and only limited
product purity due to the presence of iron and other metallic
impurities in the starting material. Drying treatments
requiring days, weeks or months for completion are not un-
common.
Higher purity can be attained through the use of
starting materials such as alkoxides in the precipitating
solutions, as noted in G.B. Patent No. 2,041,913 disclosing
an adaption of such a process to the manufacture of glass
~t_
9~3~
optical waveguides. Again, however, the drying step is
difficult and prolonged.
Slip~casting methods have long been used to produce
ceramic products from particulate oxide materials, and
much of the published literature deals with the technology
of producing a satisfactory slip. For example, U.S. Patent
No. 2,942,991 teaches the stabilization of an aqueous
casting slip while S. G. Whiteway et al., in "Slip Casting
Magnesia", Ceramic Bulletin, 40 (7) pages 432-433 (1961)
discuss the advantages of non-aqueous slips. In contrast
to the solution methods discussed above, vehicle removal
in slip-casting is relatively quick and easy, due in part
to the relatively large particles sizes of the suspended
oxides. Hence gelation is easily avoided and the problems
of gel fragility and cracking are not encountered,
although product configuration is somewhat limited.
Attempts to adapt slip casting technology to the
handling of high-purity fumed oxides have been made, as
reported in U.S. Patents Nos. 4,0~2,361 and 4,200,445.
The difficulty with fumed oxides arises in part because
they are fluffy, high-surface-area materials (surface
areas in the range of 25-400 m /g with average particle
sizes below l ~m, typically 0.01-0~1 microns) which are
hard to handle and difficult to incorporate into flowable
suspensions. In addition, the cast suspensions typically
crack on drying in the same manner as the solution-made
gels. Thus, the teaching of the aforementioned two patents
is to use aqueous suspensions of fumed oxides to prepare
a partially sintered, comminuted intermediate (particle
30 , sizes of 1-10 um) for use as a starting material in a
conventional slip casting process.
--2--
~q3~
It is propose~ in G.B~ 2,U23,571 to deposit oxide
glasses of contxol~ed composi~ion by drying and sintering
f oxides from fluid suspensions on the inside of a heated
silica tube. While this ~echnique could be used to produce a
composition gradient in ~he deposi~ed glass, the rate of
material deposition is slower than would be desired. Also,
! adherent, crack-free layers are dif~icult to obtain, and the
- method somewha~ limits the configuration of the produc~.
I~ is therefore a principal object of ~he present
invention to provlde an improved method for making glass or
t cer~mic products from fumed oxides by sintering methods.
It is a further object of the invention to provide a
method for making a glass product of optical ~uality by a
_Platively _apid procedure which facilitates the production
of pure, ~oid-~ree pxoducts from a particulate suspension o
fumed oxides.
Other ohjects and advantages or the invention will
become apparent from the following description thereo f .
J 20 The present inven~ion uses stable suspensions of
particulate oxides as starting materials for producing slass
or c~ramic products from gels, th~ suspensions being so
constituted as to substantially simplify subse~uent
processins and to impxove the properties of the resulting
products. Although the gels provided trom these suspensi_-.s
must be dried under controlled conditions, drying is
relatively rapid and relatively large, crack-~ree, po~ous
produc!s exhibiting excellent homogenei.-~ are easi7y
obtained. The porous products can readily be sintered ~o
produce dense, void-free glass or ceramlc articles.
--3--
~ 3~
AdvantaseousLy, no compoqition limitations on the end
product are imposed by the natuxe of the starting
suspensions. Essentially any oxidic material which can be
produced by 1ame oxidation or hydrolysis in an appropriate
particle size range, or otherwise made in an agglomeration
state insurinS sufficien~ly small particle size, can be used.
The method of the inventlon broadly encompasses the
following s~eps. First, a stable, flowable, non-aqueous
suspension o~ a particulate oxidic material is prep~red using
oxides having particle sizes below about 0.5 microns. The
¦ oxides can be prepared, e.g., by flame oxidation and will
- normally have ~ surîace area in the range 9f about 25-400
m2/g. The suspension can be prepared by stirring,
hish-speed mixing, wet-milling or the like, and may be
stabilized using added dispersants if necessarv to prevent
gelling or particle agglomeration and settling.
The use o~ a non-aqueous, water-free liquid vehicle and,
prefera~ly, a water free oxide, are quite important in
attaining the objects of the in~ention. The presence of
water can cause uncontrollable gelation of the suspension.
Also, the exclusion of water rom the vehicle greatly
alleviates pr~form cracking problems during subsequent
processing.
- The second step o~ the process of the invention involves
forming the suspension into a selected confisuration for tke
pro~uct and causing it to gel in that configuration. This is
accomplished by adding a selling agent to the suspension,
; either ~efore or after form~ng, in an amount ef~ective to
cause complete gelation. Through the proper selection of the
gelling agent and its composition, amounts effective to
produce complete gelation can be added without causing
immediate gelling. Tnis means that the suspension can be
--4--
3~
~oth destabili~ed and formed prior to conversion to a gelled
\! intermediate mass~ Casting is normally the preferred
i t~chnique for forming the suspension into the dosired
configuration since the viscosity of the suspension, notwith-
standing a rather high solids content, is normally low.
After the suspension has been formed into a selected
configuration for a product and gelled in that configuration
it is dried in that configuration by expelling the liquid
vehicle from the pore structure thereof. The gelled
configuration is maintained until the gelled intermediate has
i substantially dried, in ordar to minimize the possibility of
gel distortion and crac~ing. ~y virtue of the homogeneity
and relatively large pore size of the gelled material, and
because the vehicle is essentially non-aqueous, drying
stresses are minLmized and dryin~ can be relatively rapid
without -is~ing product cracking, so that an integral product
having the configuration of the gelled suspension, except for
arying shrinkage, can be obtained.
The pr~duct resulting from the above described process
is typically very homogeneous in microstructure. ~t can be
used as dried or after partial sintering for any applic2tion
wherein a pu~e, microporous support s~ructure, filter, or the
like is desired. Alternati~ely, an additional heating ste~
`,. wherein the product is fully sintered to a dense, void-free
glass or ceramic product can ~e employed. The partlcle sizes
in the dried gel are sufficiently small that sintering is
easy, and a t,ansparent glass product is readily obtained.
~elation of the stable non-aqueous suspensions of
! the invention can be very rapid, e.~., occuring almos.
immediately after forming, without detrlmentally affecting
3~
the pore homogeneity or shape of the fired product. This
feature makes the present process particularly useful for
layer casting, wherein a product is built up by a succes-
sion of cast layers, each layer being cast upon a pre-
viously cast underlayer which has gelled sufficiently to
provide a good support for the subsequently applied layers
within a very short time.
Rapid gelation enhances the feasibility of designing
composition gradients into layer-cast products by facili-
tating the forming of many layers of slightly varying com-
position in the product structure. One useful way to
implement such a procedure is to mix the starting suspen-
sion, here termed the first suspension, with at least one
additional suspension containing a particulate oxide of
a different composition than that contained in the first
suspension.
The proportions of each suspension in the mixture
are varied in stepwise or continuous fashion as the layers
are deposited, to obtain a selected oxide composition in
each deposited layer. Mixing can occur before or after
gelling agents have been added to one or both of the sus-
pensions, provided that forming can take place before
gelling occurs. In this way radial or axial composition
gradients can readily be obtained.
Brief Description of the Drawings
The invention may be further understood by reference to
the drawings wherein
FIG 1 is a plot of setting time versus concentration
for a particular gelling agen-t/suspension combination useful
in accordance with the invention; and
--6--
9~
FIG. 2 is a porosimetry eurve for a product provided in
accordance with the.present invention.
~3~d~
: The successful practice of the invention intially
' re~uires the preparation of a stable fluid dispersion of the
oxide particles selec~ed for incorporation into the desired
product. By a sta~le dispersion i5 meant one wherein the
particles will remain in suspension without permanent agglo-
mera~ion or gelling for a time sufficient to permit the
10 suspension to be shaped by castin~ or other means. Of
course, some settling is permitted if the suspension can
readily be es~ablished by simple mixing prior to shaping.
Particulake oxidic materials useful in preparing the
¦ suspensions include glassy or amorphous oxides, e.g., fumed
SiO2, GeO2, P2O5, mixtures of these fumed oxides with each other
or with ~opants such as TiO2, A12O3, SnO2, ZrO2, ZnO, MgO,
Sb205 and any other fumed oxide or oxide combination which can
be prepared by flame sxidation or hydrolysis to form a glassy
soot product of the required particle size. In some cases, the
20 product of the invention will be a crystalllne ceramic body, and
in that case, the suspended par_-culate oxidic material may be
crystalline. Examples of crystalline materials which may be used
J include crystalline forms of the above oxides and any of the other
constituents of conventional or commercial ceramic ?roducts which
can be prepared in the necessary particle size range.
As previously noted, oxide particle size is quite
important in achieving the objects of the invention~ -t is
preferred that particle sizes for the selected oxides ,~all
within the range of about 0.01-0.5 microns, correspcn~ing to
~'Z1~3'~
a surface area for a sample of the material in the range of
about 15-100 m /g. If smaller particle sizes are used, the
drying process is complicated, in some cases requiring
resort to autoclave drying at temperatures above the critical
point of the vehicle to achieve useful drying rates without
cracking.
Although many techniques have been developed for the
production of sub-micron-size oxidic particles, the pre-
ferred method is flame oxidation or hydrolysis because the
fumed products of this method commonly exhibit the necessary
particle size and surface area as made. For example, fNmed
silica particles are typically agglomerates of tiny silica
spheres, having the appearance of grape clusters, but the
agglomerates seldom exceed 0.5 microns in size and readily
form stable colloidal suspensions in appropriate vehicle-
dispersant systems.
The use of fumed oxides may be distinguished from prior
art methods such as the so-called sol-gel method wherein
ceramic products are produced from gels wherein the oxidic
material is polymerized or precipitated from within an
aqueous or water-alcohol solution. With few exceptions,
solution-grown oxidic materials have sufficiently small
particle sizes that gels with very small pores are produced.
Gels of this type dry extremely slowly and are very difficult
to dry and consolidate without cracking, for reasons herein-
after more fully described.
A variety of different ways for providing suspensions
of oxide materials in liquid vehicles are known,both from
the paint industry and from fundamental research in colloid
chemistry. The known mechanisms for stabilizing dispersions
by the prevention of particle agglomeration in the vehicle
--8--
L943'~
include electrical double layer formation, steric dispersion,
and polar screening. The present invention requires the use
of a non-aqueous vehicle, most frequently a hydrophobic
organic liquid, wherein the preferred method for stabilizing
the suspension is that of steric dispersion. In dispersions
of this type, the complete absence of water, including water
absorbed onto the particulate oxide added to the suspension,
is very desirable since even trace amounts of water can
interfere with the stabilization mechanism and promote
flocculation of the particles and gelling of the suspension.
In fact, water can constitute a useful flocculating or
gelling agent in these systems.
To stabilize the dispersion by steric hindrance, a
dispersing agent is added to the suspension, typically
consisting of a chain like molecule comprising both hydro-
philic and hydrophobic groups. The hydrophilic group, e.g.,
an -OH or -COOH group, is attracted to and capable of adsorb-
ing on the hydrophilic oxide particles present in the suspen-
sion. The hydrophobic group can be a carbon chain or the
like which is attracted to the organic vehicle and capable
of preventing agglomeration of the particulate phase by
steric hindrance.
Stabilization by the technique of steric dispersion is
particularly useful because the dispersant layers on the
particles are thin, so that rather highly concentrated but
stable suspensions may be prepared. Of course, other stabi-
lization techniques including electrical double layer and
polar screen methods may be employed, provided stable fluid
dispersions capable of flocculation with a compatible gelling
agent can be prepared therefrom~
Examples of vehicles which can be used with oxidic
particulates such as silica and silicate glasses include
_g_
~3 ~3'~
hydrophobic solvents such as n-hexane, chloroform, methylene
chloride or the like, and also some vehicles which are
miscible with water, e.g., n-propanol. Examples of dispersants
which can be used in solvents such as above described include
stearic acid, stearic alcohol, and even shorter straight-chain
alcohols, liquid at room temperature, such as the normal
alcohols with carbon chains of 3-10 atoms.
Flocculation or gellation of a steric dispersion produced
as above described is thought to involve the displacement of
the dispersant from the surfaces of the oxide particles by
constituents having a stronger affinity therefor than the
dispersant. As already noted, water is one agent which can
accomplish this result, although water deflocculation and
gellation is a rather slow process in cases where the
attachment of a dispersant to the oxide particles is strong.
More rapid flocculation and gellation in these cases can be
achieved by adding small amounts of aqueous acidic or basic
solutions to the suspension, with basic solutions being
preferred. Basic media can cause rather rapid gellation at
low concentrations in the vehicles tried, whereas rather
strong acid solutions are required~
In general, the gelling agent used to destabilize the
suspension can simply be addedto the suspension in liquid
form with mixing to achieve dispersion. Again, however, it
is difficult to obtain a homogenous dispersion of aqueous
solutions where highly hydrophobic vehicles such as chloroform
are employed. One satisfactory technique to aid homogeneous
gelling is to mix the aqueous gelling agent with a quantity
of vehicle-miscible liquid such as methanol, and to then add
the combination to the suspension. This does not diminish
the potency of the aqueous gelling agent in most cases.
10-
9~
A pre.erred d'~stabilizing technique for some applica-
~ions, parti,_ularly in the case of a very hydrophobic ~ehicle
; such as chloroform, is to use a vehicle-miscible or ~ehicle-
soluble gelling agent. A preerred class of gellins agents
of this type includes the alkyl amine gellins aser.ts, for
example heptylamine or 1,3-diamino propane. The quanti~y of
¦ those gelling agents needed for complete gelation is very
small, and the superior misci~ility of the gelling agent ir.
the suspension produces a gel of significantly improved
homogeneitY
It is desirable to cast or otherwise fonm a dest~bilized
suspension before gellation has been substantially initiated,
and for this purpose it is useful to control gelling time by
controlling the amount of gelling agent in~roduced into the
suspension. An example o~ the effect of gelling agent
concentration on setting time is shown in FIG. 1 of the
drawing, which plo~s setting time in seconds 'or a suspension
of SiQ2 in chlorofo~m stabilized with l-decanol as a function
of the concentration of an NH40H gelling agent introduced
~herein. The gelling solution for the case illustrated
consisted of 9'~ ml methanol, 5 ml water, and 0.2-0.6 ml of
30% a~ueous NH40H as the gelling agent, the gellins
solution being combined with the suspension in a volume ratio
~, of 1 to 3. The strong effect of ~H40H concentration on
i setting time, defined as the time interval between the
addition of the gelling agent and the time when a sample o-
the gelling suspension no longer rlowed from an inverted test
tube, is readily apparent.
j An alternative ~orming procedure is to destahili~e and
gel the suspension after it has been formed into the desired
configuration. It is possible to add liquid gelling agents
to the rluid suspension after casting, but a more convenien~
.
~ 1L--
3,~,
proced~_e is to pass a gaseous gellin~ agent, such as NY.3
sas, over, around ox through the shaped suspension to cause
to gel in the cast shape.
Because the gelled intermediate produced after casting
is relatively weak and subject to crac~ing in the course of
the shrinka~e which will accompany the removal of the liquid
phase, it has been fou~d very helpful to cast the suspension
onto a surface which will deform with the intermediate as it
shrinXs during drying. One sui~a~le technique for accomp-
; 10 lishing this is to line the casting mold with a deformable
¦ polymeric layerr such as a thin plastic film, which will
adhere to and deform with the intermediate as it shrlnks
during the early stases of drying. This film can then be
conveniently removed after the intermediate has attained
1 sufficient strensth in the col~rse of the drying process.
t The gelling process, although a~oided in certain prior
solution methods f.or forming glasses, is considered a
~ritical aspect of the present invention because it prevents
d~formation and cracklng of the cast suspension durlng the
20 drying period. As noted in the prior art, even fumed oxide
suspensions, if cast without gellins, tend to d-y at the
outer surfaces thereof, forming a skin which deforms and/or
c_acks during the ensuing slow drying and shrinkase of the
1. interior.
On the other hand, the strongly gelled structure of the
invention does not form a skin but dries uniformly with
transport of the liquid vehicle from the interior to the
outer surface on a continuous basis lS the lntermediato is
! dried. In this way, inhomogeneous dryir.g and nonuniform
30 stress on the drying microporous product is minimized.
1~-
3~
The use of a non-aqueous vehicle and control over the
particle size or surface area of the suspended particulate
oxidic material are particularly important factors during
the drying stage of the present process. After a large part
of the vehicle has been expelled from the gel and most of
the drying shrinkage has occurred, large forces arise in the
gel due to capillary tension which develops as the vehicle
is replaced by air in the pore structure of the gel. This
capillary tension, inversely proportional to the pore size
of the gel, is also directly proportional to the surface
tension of the vehicle, which in the case of selected organic
vehicles can be substantially less than that of water. Thus
it is the use of an organic vehicle in combination with
particle size control over the dispersed oxide which permits
drying of the gelled material without the formation of
cracks in a reasonable period of time.
Sintering of the dried porous glass or ceramic product
body can be conveniently accomplished by heating at tempera-
tures well below~the melting points of the oxides employed.
By virtue of the pore homogeneity of the dried product,
resulting from controlled gellation and the avoidance of
overly large agglomerates during the suspension step of the
process, void-free consolidation is readily accomplished and
transparent amorphous products, and even transparent crystal-
line products, are possible. Thus the production of glass
and ceramic products at temperatures well below the melting
temperatures of their oxide constituents is considerably
facilitated.
The invention may be further understood by reference to
the following illustrative examples demonstrating the pro-
duction of ceramic products in accordance therewith.
-13-
Example_I
A particulate oxidic material consiting of a SiO2 soot
is prepared by flame hydrolysis. An oxygen carrier gas is
bubbled at a rate of 1.55 standard liters per minute (slpm)
through a reservoir of SiC14 maintained at 36C. The resulting
O2-SiC14 vapor stream is fed to a flame oxidation burner of
the type described in U.S. Patent No. 4,165,223, along with
natural gas and oxygen for the burner flame at flow rates
of 2.5 slpm and 2.5 slpm, respectively. An inner shield
oxygen flow of 2.5 slpm and an outer shield oxygen flow of
3.8 slpm arP also provided to the burner as described in the
patent.
The soot resulting from the combustion of the chloride
gases in the burner under the conditions described is directed
down a glass tube where it collects on the wall of the tube
for subsequent removalO As prepared, this soot has several
weight percent of adsorbed H2O and HCl. To remove these
contaminants and obtain a pure dry oxidic product, the soot
is heated to about 800C in flowing dry N2 for a period of
about one hour. The product is a dry SiO2 soot having a
surface area of about 76 m /g, or a particle size averaging
about 0.04 microns.
In order to prepare a stable dispersion, a soot pro-
duced as described is first dried at 200C to remove any
adsorbed water collected in storage. It is then placed in a
dry environment, for example a glove box, to keep it dry
until use.
A chloroform vehicle is purified by distillation and a
l-decanol dispersant is dried by filtering through a mole-
cular sieve. The vehicle for the suspension is then pre-
pared by addlng 14.4 ml of l-decanol to 150 ml of the
chloroform solvent with mixing.
~]4-
~Z19~
The vehicle thus prepared is transferred into the glove
box and 60 g sample of the dry 500t iS mixed with the
vehicle in a Teflon plastic milling jar, to which are also
added a number of Teflon cylinders for mixing. The plastic
jar is then capped, removed from the glove box, and the
mixture ball-milled by rotating the jar for twelve hours to
obtain a good dispersion of the soot in the vehicle. The
product is a stable dispersion of SiO2 in chloroform,
exhibiting good fluidity and excellent resistance to floc-
lC culation and settling.
The gelling agent to be used to gel this suspension is
a water-methanol mixture comprising 95 ml CH30H and 5 ml
H20 by volume to which is added 0.5 ml of 30~ aqueous NH40H
as the principal gellant. To obtain good mixing of the
gelling agent and the suspension, each is loaded into a separate
graduated funnel and exposed to a partial vacuum (20 inches
of mercury) for about 5 minutes. This treatment removes
dissolved gas from the liquids and minimizes bubble evolution
in the course of mixing.
The suspension and gelling agent are separately de-
livered into a mixing chamber containing a magnetic stirring
element, at a ratio of 3 parts of suspension to 1 part of
gelling agent by volume. The suspension/gelling agent
mixture is then dispensed from the mixing chamber into a
plastic-lined mold.
The mold employed is a rectangular container about 6.2
x 7.5 cm in area in which is placed a sheet of 2-mil poly-
ethylene film as a flexible liner. The suspension and
gelling agent fill this mold to form a mass of the same area
about l cm in thickness. Gellation of this casting to a
-15-
:~2~43~
nonflowable mass occurs within an interval of about 2 minutes
after casting.
The gelled intermediate thus provided is dried at a
suitable rate to a porous solid. Drying is carried out in
a circular vented glass evaporation chamber about 30 cm in
diameter which incorporates vent holes having a total surface
area of about 0.8 cm2. This venting controls the evaporation
rate of the liquid vehicle and ensures that cracking will
not occur. Crack-free drying occurs over a period of about
72 hours at room temperature, with shrinkage of about 25%
(linear) in the dimensions of the product.
The dried gel is exposed to vaccum for a few hours and
then heated to 800C in oxygen to remove residual organic
constituents therefrom. A heating rate of about 50C/hr is
used, although much faster heating rates can be employed.
No structural change in the porous intermediate is detec-
table during this drying process.
Mercury penetration porosimetry of the dried gel indi-
cates a porosity of about 65%, with the pore size showing a
bimodal distribution. The structure comprises one group of
pores near 40 nm in diameter and another group in the
200-300 nm diameter range. A typical porosimetry curve for
such a product is shown in FIG 2 of the drawing. Such a
product could be used, for example, as a catalyst or enzyme
support, as a filter, or for any other article wherein
homogeneous porosity is desired.
Sintering of the dried porous product is suitably
accomplished in an inert atmosphere, e.gn, a helium atmos-
phere, and full consolidation can be achieved within an
30 interval of less than 30 minutes at 1350C. Shrinkage
during this step of the process is typically about 40%
-16-
~'~19~3~
(linear?, the product being an essentially clear glass plate
of very pure SiO2 about 2.8 x 3.4 cm in size, containing
only a few small scattered seed defects.
Example II
In an alternative and preferred gelling procedure, the
stabilized silica-chloroform suspension of Example I is
destabilized and gelled using a vehicle-miscible alkyl amine
gelling agent. The selected gelling agent, heptylamine, is
added to a chloroform vehicle for the purpose of dilution,
the final concentration of the amine in the chloroform
gelling solution being 600 parts per million by volume.
This solution is combined with a silica-chloroform suspen-
sion of the type described in Example I in the stirred
mixing chamber of that example in a volume ratio of 3 parts
suspension to 1 part of the gelling solution.
The destabilized suspension thus prepared may be cast
as in Example I, and will gel in the casting mold within
about 4 minutes. The resulting gel may be dried and con-
solidated in accordance with the procedure of Example I to
obtain a clear glass product. However, because of the high
miscibility of the gelling agent, the gel exhibits improved
homogeneity and the consolidated glass product exhibits
improved transparency and complete freedom from seed defects.
While in the foregoing examples the stabilization of
the suspension was accomplished by steric dispersion, the
following example describes a suspension wherein no added
stabilizing agent is present.
_ample III
A 120g sample of a silicasoot produced as described in
Example I, having a surface area of 86 m /g and a particle
-17
9~3'~
size averaging about 0.03 microns, is added to 300 ml of 1-
propanol in the milling jar of Example I and the mixture is
ball-milled in the jar by rotation for 12 hours. The
product is a stable suspension of fumed silica, although in
this case no dispersant has been added.
A solution to be used as the gelling agent for this
suspension is prepared as in Example I, except that 8 ml of
30% aqueous NH40H is added to 95 ml. CH30H and 5 ml of H20
to provide the solutionO This solution is then combined
with the propanol-silica suspension in a ratio of 3 parts of
the suspension to one part of the gelling agent by volume in
the magnetic mixi~g chamber of-Example I, and the mixture is
dispensed into a square mold about 12.5 cm. on a side to a
depth of about 1.2 cm. Gelling of the mixture occurs within
an interval of about 5 minutes.
The gelled mass is dried in the mold in the drying
chamber described in Example I, with substantially complete
evaporation of the vehicle occurring within 144 hours at
room temperature, without any cracking of the cast plate.
The dried mass is about 10 x 10 x 0.9 cm in size.
This dried mass can be sintered to a clear glass
plate, if desired, by heating to a temperature of 1350C
in an inert atmosphere such as helium. Complete consoli-
dation occurs within an interval of about 30 minutes.
Shrinkage during firing is about 40% (linear).
Example IV
The preparation of a product from a doped fused silica
or other pure glass fumed oxide proceeds in a manner sub-
stantially analogous to that of Example I. A doped fused
silica soot consisting essentially of 15% GeO2 and 85%
-~8-
~L9 ~3~
SiO2 by weight is prepared using a flame oxidation procedure
as in Example I. A reactant vapor stream is provided by
bubbling oxygen at a rate of 1.55 slpm through a reservoir
of SiCl4 maintained at 36C, and oxygen at a rate of 0.41
slpm th~ough a reservoir of GeC14 maintained at 60C. The
O2-SiCl4 and O2-GeCl4 vapor streams are mixed and fed to the
oxidation burner along with a flame feed consisting of 1.3
slpm of natural gas and 1.3 slpm of oxygen. The burner is
also provided with outer shield oxygen at a rate of 1.8 slpm
and inner shield oxygen at a rate of 0.8 slpm.
The soot resulting from the combustion of the burner
gases under these conditions is collected in a tube as-in - -
Example I. It is then removed and heated to 800C in air to
remove adsorbed H2O and HCl and stored for subsequent use.
The product is a pure GeO2-SiO2 soot with a surface area of
about 87.5 m2/g.
To prepare a suspension of this fumed oxide, a 60 g
sample is heated to 400C in air to desorb water, and then
quickly mixed with lO0 ml of chloroform containing 17.5 ml
of added 1-decanol as a dispersing agent in a small high-
speed blender.
A 60 ml sample of the suspension thus prepared is
destabilized in an open beaker by adding ~ ml of a gelling
agent solution thereto with magnetic stirring. The gelling
agent solution consists of 1 ml H2O as the gelling agent,
mixed with 79 ml CH30H.
The destabilized suspension thus provided is formed by
casting into a cylindrical mold lined with plastic film
where it gels within an interval of about 5 min. This
casting is then permitted to dry in the open air. Sub-
stantially complete drying of the casting, with some cracking
19-
9~L3;~
due to the speed of the drying process, is accomplished
within an interval of 18 hours. The ~ried product is then
sintered to a clear glass piece at 1400C within 30 minutes.
Example V
The preparation of a stable suspension of a crystalline
fumed oxide material follows substantially the procedure
employed for the amorphous or glassy materials described in
the previous examples. A crystalline fumed titanium dioxide
is prepared using the above-described flame hydrolysis
burner fed with a reactant stream produced by bubbling
nitrogen at a flow rate of 1 slpm through a reservoir of
titanium isopropoxide, Ti (ioC3H7)4, being maintained at
170-180C. and then adding by-pass nitrogen to the bubbler
flow to produce a total reactant stream flow of 1.5 slpm.
Combustion of this stream is achieved by means of a gas-
oxygen mixture consisting of 5.6 slpm of natural gas and 5.6
slpm of oxygen, the burner also beina provided with inner
shield oxygen at a flow rate of 2.8 slpm. and outer shield
oxygen at a flow rate of 2.8 slpm. The product is a pure
TiO2 soot having a surface area of about 14.3 m /g.
To prepare a suspension of this oxide, a 60 g sample of
the material which has first been heated to 400C under
vacuum for 1 hour and thereafter kept in a glove box is
placed in a plastic mixing jar containing several plastic
milling cylinders along with 150 ml. of a chloroform vehicle
and 8 ml of an oleic acid dispersant. This combination is
milled in the jar overnight. The product of this process is
a stable, fluid, colloidal suspension of pure TiO2 in chloroform.
A gelling solution for this suspension is prepared by
combining one part H2O by volume and one part trifluoro-
acetic acid by volume as gelling agents with 10 parts by
-20-
43~
volume of methanol. A 50 ml sample of the TiO2 suspension
is then shaken in a closed container with 5 ml of the gelling
solution and the shaken mixture is cast into a square mold 5
cm on a side to a depth of about 0O4 cm. Gellation of the
cast suspension is rapid and drying of the gelled suspension
is permitted to occur over an interval of about 24 hours.
Some cracking of the gel occurs on drying, presently attributed
to the presence of air bubbles in the casting, but no granu-
lation is observed, the product consisting of three large,
crack-free sections of the original casting.
Example VI
A useful technique for layer casting, e.g., to provide
a succession of gelled layers on a stationary or moving
substrate, is to provide the gelling agent as a gas which
contacts the suspension as the layers are deposited. In a
specific example of this technique, a stable suspension of
SiO2 is prepared by adding a 60 g sample of dry SiO2 soot
produced as described in Example I to a chloroform vehicle
consisting of 110 ml of chloroform and 14.4 ml of 1-decanol,
with jar milling as in Example I.
The SiO2 suspension thus provided is fed into a rotating
mold consisting of a plugged, horizontally positioned glass
cylinder about 7 cm in diameter and 30 cm in length, the
cylinder being rotated about the cylinder axis at about 160
rpm. The suspension feed line is a movable delivery tube
inserted through one end plug which repeatedly traverses the
length of the cylinder along the axis of rotation, depositing
the suspension as a coating on the moving cylinder wall at a
rate of about 48 ml/min.
30The gelling agent for the suspension is gaseous NH3
which is supplied through an opening in the opposite end
-21-
plug at a rate of.about 0~4 slpm. Casting is commenced after
NH3 has displaced most of the air within the cylinder. .Under
2 conditions described, gellins of the flowing suspens~on
is initiated even ~efore the suspension reaches the cylindex
wall, and successive layers of the rapidly gelling suspension
~ can readily be deposited wi~hout disrupting ~he integrity of
¦ underlying layers. The gelling rate may be con~rolled ~y
diluting the ~3 with an inert gas such as ~itrogen, i~
desired.
The aboveode~c_ibed technique would ~ave particulc~
application if it were desired to provide a cylindrical or
other member exhibiting an axial or radial composition
gradient. The composition o~ the suspension could be changed
in continuous or stepwise fashion as the delivery tube
traversed the cylinder, providing succeeding layers o_
preselected composition on the mold wall. Similarly, laye~
casting can be used to produce plates or blanks of graded
composition. Advantageously, there ~s no need to heat the
mold to expel the vehicle; xather the gelled casting can be
dried as a unit after layer deposition has been completed.
Glass products produced by processing as hereina~ove
described have sufficient optical quality that they car. ~e
used as components of low loss glass optical waveguides, 2S
, illustrated by the following examples.
Exam le VII
A vehicle mixture for a fumed oxide suspension is
prepared which consists of about 150 ml. chlorofor~ as the
solvent and 16.8 ml. l-propanol as the dispersant. To this
vehicle is added a f~ned oxide consisting of 72 grams of
SiO2 of Q.06 microns average particle size produced by 1ame
-22-
~ 3L94~3~
oxidation, which had been fired at 1000C for one-half hour
`~ in oxygen. Beforë being used this silica is dried at 200C
~or 1 hour in a vacuum to remove water adsorbed in storage.
The fumed oxide is dispersed in the vehicle mixture by
ball-milling the two together for about 16 hours. The
suspension is then centrifuged at 20C0 rpm for 10 minutes to
J remo~e any large agglomerates.
- The suspension thus pro~ided is pouxed into a separatory
funnel for controlled ~ransfer to a forming mold. A partial
10 vacuum is applied~,to this funnel to de-ai. the suspension.
1 3 mm. Teflon plasti,c tube connects the funnel outlet to a
bottom inlet in the mold, which is for a preform of rod
configuration.
The mold used to form the rod is 2 cylindrical poly-
ethylene bag about 3.2 cm. O.~ r X 35 cm. long. The bottom
,l inlet consists of a 1/8" Teflon plastic tube sealed irto
the ~ottom of the bag. Tne top of the bag is supported and
~eld open by a ground joint at the top of a cylindrical glass
mold holder about 6.4 cmc O.D. X 40 cm. long, into which the
bag is suspended. This holder is equipped with closeable 6
~m. O.D. top vents~
A valve on the separatory funnel is opened and the
suspension is allowed to ~low by means of gravit~ at about 6
ml./min. into the bottom of the bag mold. By delivering f~om
the bottom, trapped air ln the casting is avoidedO Using 180
ml. of the suspension, a casttng approximatelv 3.2 cm. O.D.
22 cm. long is formed.
i Once the bag is filled, the vents are closed and a
gelling agent consisting of gaseous N~3 is flowed into the
mold holder and around and over the open bag at a rate of
about 35 cc./min. for 15 minutes. This treatment is
-23-
1~9 ~3~
suffioien! to completel~ gel the suspensiG-. to a semisolid
gelled castins fo~ a glass -od.
After this cas~ing has gelled, ~he vents in the top of
the mold holder are opened and the casting is pexmitted to
dry. Complete drying to an integral, porous oxlde preform,
with some shrinkage but without crackiny, is accomplished in
¦ approximately 72 hoursO
The dried rod preform thus provided, now about 2.5 cm.
O.D. X 20 cm. long, is placed in a vacuum for 3 hours and
thereafter heated.jto lG00C in oxy~en to remove any res~dual
i organic constituents. I~ is then further hea~ed for
consolidation to a temperature of 1500C~ in an atmosphere of
helium containing 2 vol. % chlorine, to produce a transparent
glass rod approximately 10 cm. long X 15.6 ~mm. O.D.
! The slass rod thus provided, which is a consolidated
i preform for a core element of an optical waveguide fiber, is
drawn into glass ~ane about 4 mm. in diameter. It is then
-oated with a soot layer composed of a particulate boro-
silicate glass by a known flame oxidatio~ pxocess. This soot
~ layer is thereafter consolidated to a transparent cladding
layer a~d the clad preform drawn into an optical waveguide
fiber with a diameter of approximately 125 um.
The fiber thus provided has a core 19 microns in
diameter composed of fus~d silica and cladding of
borosilicate glass having a refractive index lower than that
of ilica, the fiber having a numerical aperture of 0.lO.
The attenuation of the fiber is 14 db/hm at a wavele~gth of
1 850 nm, the majority of this attenuation heing at~xibuted to
J bending losses, due to the relatively low numerical aperture
of the fiber.
-24
.9
~ =
1 A vehicle mix ure for a fumed oxide suspension is
prepared which consi~ts of about 40a ml. chloro~orm as the
solvent and 60.8 ml. 1-propanol as the dispersant. To this
- vehicle is added a particulate fumed oxide consisting o_
196.5 grams of colloidal silica, this silica havins an
¦ average particle size on the order of 0.06 microns and being
commercially availabie as OX~5Q silica from Degussa, Inc.,
Teterbôro, NJ. This silica had been heated to 1000C for 1/2
hour in oxygen to remove bound water, and prior ~o use is
j dried a~ 200C for 1 hour in a vacuum to remove adsorbed
I water.
The silica is disperse~ in the vehicle mixture by
ball-milling the two together for about 16 hours. The
suspension thus provided is then centrifuged at 2000 rpm for
10 minutes. The centrifuged suspension is then pourea into z
separato~ funnel.and de-airea by the application of a
partial vacuum as in Example VII above.
The de-aired suspe~sion is drawn rom the funnel through
a plastic tube to a glass delivery cylinder by means o~ a
motor-driven plastic ~iston in the cylinder. It is 'hen
forced by the piston into a bag mold through a flexible
plastic tube terminating ln a 14-inch lons stainless steel
. delivery tube, the latter tube having an inside diameter of
1/8 inch.
The bag mold is configured to provide a rod-shaped
casting for the core element of an optical wa~egulde fiber,
consisting of a polyethylene bag 1-7/8" O.D. X 14" long. One
I end is heat-sealed shut and the other end is held open by a
ground glass joint at g:he top edge of a 2-1/2" O.D. X 16"
long cylindrical glass mold holder into which the bag 5
suspended. Both the ground glass top for the mold holaer and
~ 3~
the bottom section of the mold holder are equipped with 1/4"
, O.D. vent tubes.
To fill the mold, the stainless deliver~ tube is
inserted thxough the ~op vent of the mold holder and into the
mold, being positioned within 1/2" of the bottom of the bas.
The colloidal suspension is then injected into the bag from
the delivery cylinder at a rate of abou~ 15-30 ml./min. As
the mold is filled the delivery tube is withdrawn so that air
bubbles are not ~rapped in the suspension.
i io After the mold has been filled, the vents are close- and
¦ ammonia is flowed into the mold holder and around the casting
J at a flow ra~e o 3S cc.~min. for 85 minutes. At the end o~
! this interval the suspension has gelled to a semisolid
casting and is ready for drying.
, Drying is carried out by opening the vent holes in the
¦ molc holder, permitting the vehicle to slowly evaporate over
an interval of 16 days. Final drylng is then accomplished by
placing the integral rod preform, now 1-1/2 X a inches in
~ize and essentially free of cracks, in a vacuum for 3 hours,
and then heating it to lOOO~C in oxygen for 90 minutes to
remove residual organics.
Consolidation of the porous preform tc clear glass is
accomplished by further heating the rod to 1500C in a helium
- atmosphere containing 2 vol. ~ C12 gas. The .ransparent
consolidated rod is about 4 inches long and 7/8 inch in
diameter.
The transparent rod is then d~awn into an opticai
I waveguide fiber core element approximately 125 microns in
j diameter. As it is drawn, it is coated with a silicor.e
plastic coating material having a lower refractive index than
that of silica. This coating constitutes a transparent
claading element for the optical waveguide fiber. The
343~
optical attenuati4n of this waveguide is abou-t 3~6 d~/~m at 2
waveleng~h o 850 r~.
¦ By virtue of the flexibiliti of the suspension casting
and gelling techniques of the invention it is possible to
form core elements, cladding elements, csre-cladding com~ina-
tions, and core elements of non-uniform (e.g., graded)
¦ re~ractive index by direct casting as abo~e described. The
latter are obtained using the layer casting technique
discussed above by varying the composi~ion of the suspension
as it is being cast.
¦ Where the axticle to be formed is a glass core element
for an optical wa~eguide, the cl~dding element can be applied
by casting a second suspension around the gelled fi~st
s~spension or, alternatively, b~ othex claddin~ techniques.
For example, a claddins could be applied by flame hydro'~sis
after the castlng for the core element is dried, or after it
is consolidated t~ clear glass. A plastic cladding may be
applied during or after the core element is formed from the
consolidated preform by drawing.
Of course, the foregoing examples are merely representa-
tive of products which could be provided in accordance wi~h
the invention. Numerous variations and modiLications of this
procedure may be resorted to by those skille~ in the art
within the scope of the appended claims.
Thus the invention constitutes a substantial improvement
over the so-called solution methods for formina silicate
slasses wherein fluid compositions are gelled and the g27 lec
inte~meZiate dried over prolonged periods and sintered to
¦ produce mic-oporous or non-porous glasses. Through the use
of non-aqueous suspensions of particulate o~ides of the
above-disclosed particle size, and the process of selling the
suspension after ~orming it into a product of the des_red
j
-2,-
~ 21~3~
con'iguration, cr~ck-free d~ying of the product is consid-
erably facilitate~ and r.elatively large, crack-free products
can more quickly and easily be obtained.
'
-28-
