Note: Descriptions are shown in the official language in which they were submitted.
Ketcham-sanderson~st. Julien~Wexell g-l-2-2A
~ ~ c3 ~ ,,3
THIN FLEXIB1E SXNTERED STRUC~URES
This application is a Continuation-In-Part application
of Serial No. 07/393,53?, filed August 11, 1989.
Background of the Invention
The present invention is directed toward flexible
sintered structures. More specifically, the invention
relates to flexible high strength inorganic structures such
as inorganic sheets or tapes, made by combining powdered
metallic, metalloid or, most preferably, oxide powders with
appropriate liquid v~hicle components and casting or
otherwise shaping and sintering the resultant powder
batches. Sheets, foils, ribbons, or other high aspect
ratio products made in accordance with the invention can
exhibit high hardness, flexibility, and toughness with
excellent thermal stability over a wide range of tempera-
tures.
Thin flexible sintered structures are useful for a
multitude of productive applications. They may be employed
for electronic and/or electrooptic uses, such as waveguides,
or as substrates for electronic coatings, superconductors,
or high temperature superconductors.
With improved mechanical properties, flexible ceramics
could be useful as a protective layer for glass or other
substrate materials where a layer of protection is needed
to resist scratches. With sufficient structural flexibility
in the flexible ceramic, the object to be protected could
simply be wrapped for protection.
Flexible inorganics, especially flexible ceramics,
would offer unigue advantages as chemically stable substrate
-2- ~ t~
materials. Porous ceramic materials are known to provide
high suf~ace areas. High surface area substrates provide
desirable receiving surfaces for a variety of coatings.
Alumina, for example, provides in its many crystalline
S forms an excellent surface for the application of catalysts.
Porous or dense alumina which could be provided as a
flexible ceramic foil and subsequently coated with a base
or noble metal and/or oxide catalyst, or treated with
zeolites, would have uni~ue advantages for a varie~y of
chemical applications.
Sintered porous metallic foils, e.g., porous stainless
steel foils, can be made and optionally oxidized or other-
wise treated to provide high surface area metal-based
substrates. Coated substrates of metallic or oxid~ type,
formed into any desired honeycomb or other circular,
laminar, and/or trapezoidal structures, would offer stable
support in harsh environments where flexibility in combina
tion with a specific substrate geometry would be particu-
larly advantageous.
Since the discovery of high temperature oxide super~
conductors, there has ~een widespread interest in combining
these relatively brittle materials with strong flexible
substrate materials to provide supexconducting wires.
Those skilled in the superconductor art have struggled to
identify useful substrates for these superconductors.
One suggestion has been to use metallic components to
provide supporting substrates or jacketing for the super-
conductors. A particular disadvantage of metals, however,
is the diffusivity of the metals at th~ sintering tempera-
tures required for ceramic superconductor application,which could undesirably modify the compositions of the
applied superconductor materials.
Unlike metals, ceramic substrates are conventionally
sintered at a higher temperatures than any of the yttrium
barium copper oxide (YBCO), bismuth strontium copper oxide
(BSCO) and/or thallium copper oxide families o high
tem~erature superconductors, thus minimizing thP diffusivity
-3~ 3~ ~
problem. Additionally, ceramics are more compatible with
oxide s~erconductor coatings, due perhaps to improved
wetting of the substrates by the coatings during coating
application. Thus decreased interfacial discontinuities
and increased substrate/layer stability are attainable. As
those skilled in this art can appreciate, other metal
and/or oxide and/or ceramic coatings would also benefit
from this improved coating compatibility.
of course the production of thin and flexible ceramic
ibers such as silicon carbide fibers and aluminosilicate
fibers ls well known. Ceramic fibers of these types are
generally produced by spinning techniques or variations
thereof f For example, NicalonR (silicon oxycarbide)
fibers, NextelR (Al2O3-SiO2-B2O3) fibers, and even r-alumina
fibers are typically produced by spinning a fiber of a
pyrolyzable precursor material and then pyrolyzing the spun
fiber. Alternatively, fibers of alumina and zirconia can
be produced by spinning a precursor material comprising
fine oxide powder, followed by sintering to an integral
oxide fiber product.
Still other methods o fiber manufacture include the
vapor deposition of precursors onto a starting or substrate
filament and/ox the spinning and optional heat treatment of
glass fibers ~rom molten glass. Although none of the
fibers produced from precursors as above described are
perfectly cylindrical, almost all are of very low aspect
ratio, i.e., below 2:1. For a further discussion of the
major fibers and their use in composites, reference may be
made to Frank K. Ko, "Preform Fiber Architecture for
Ceramic-Matrix Composites," Am. Ceram. Soc. Bull., 68 ~2]
401-414 (1989)o
Unfortunately, while formed of inherently strong
materials, long fibers of these cer~mic materials are very
weak. The weakness of fibers is ~imply due to the flaw
populations in the fibers and the statistical laws which
insure that most long fibers will include at least one
--4-- ~ f ~ 3
defect of sufficient magnitude to cause failure at stress
levels ~ll below the inherent strength of the material.
While the strength levels attainable depend of course
on the number and size of the defects introduced into the
fibers from batch or manufacturing process sources, the
defect population needed to sustain successful production
of strong long fibers is very small. Thus, for example, it
can be calculated that, for fibers of 10 microns diameter
comprising defect particles or voids of similar size,
defect levels below 1 defect per each one hundred million
parts of volume are needed to yield reasonable selections
of strong kilometer-long lengths of fiber.
Prior work in the field of thin film ceramics includes
U. S. Patent No. 4,710,227 disclosing the preparation of
thin flexible 'igreen" (unfired) ceramic tapes from
solutions, the tapes being coated and cut, stacked and
fired to form thin-dielectric capacitors. This process is
further described in published European applications EP
0302972 and EP 0317676. Capacitors with ceramic layers of
1-50 microns can be made; however the capacitor fabrication
process which is disclosed does not utilize the production
or handling of sintered or fired (binder~free) flexible
tapes in unstacked or unsupported Eorm. In addition, the
range of useful materials is limited by the ceramic process
employed.
Summary of the Invention
The present invention solves many of the problems
associated with prior applications and methodologies of
flexible sintered structures. The present invention
provides a thin f lexible sintered structure for a wide
field of uses, herebefore deprived of a suitable sintered
structural flexible material.
The product of this invention is useful in any envi-
ronment where a hard tough thin refractory flexible
substrate and/or layer is needed. The flexibility will
-5~
depend on layer thickness to a large measure and therefore
can be ~ilored as such, for a specific use. Generally,
the thicker the substrate the less flexible it becomes.
Thinner substrates can be flexible to the point where
S toughened and hardened sintered materials may waff in a
slight breeze, yet remain hard and tough to mechanical
and/or thermal abuses. A use of this kind or strong
flexibili~y could be as a diaphragm in a pump or valve.
High surface areas can be created by manipulating
porosity. Porosities are increased by manipulating
sintering temperatures and/or including higher loadings of
materials within the batch that burn out at firing tempera-
tures. Porosities of the present invention can be as low
as zero or as high as about 60%. Both porous and dense
foils will maintain flexibility, due to the slight thick-
ness of the product. Differential porosities in these
materials can be useful for flltration and/or membrane
operations.
The chemical inertness as well as the surface morphol-
ogy, thermal expansion, and flexibility of the presentflexible inorganic substrates make them promising substrates
for superconductor materials.
This invention thus provides a means for applying
ceramic process technology to a wide variety of materials
for the production of flexible inorganic, preferably
ceramic, products. Thin materials can be formed in the
green state in a molded configuration and subsequently
sintered to a dense or porous structure with a large
measure of flexibilityO
In the method of the present invention a thin preform,
for example a thin sheet or layer comprising the green
material, is first produced. The material is then sintered
to provide a thin sintered structure with a flexibility
sufficient to permit a high degree of bending without
breakage under an applied force. Flexibility in the
sintered material is sufficient to permit bending to an
effective radius of curvature of less than 20 centimeters
-6- ~t) ~
ox some Pquivalent measure, preferably less than 5 centi-
meters ~ some equivalent measure, more preferably less
than 1 centimeter or some equivalent measure, and most
preferably less than 0.5 centimeter or some equivalent
measure.
By an "effective" radius of curvature is meant that
radius of curvature which may be locally generated by
bending in a sintered body in addition to any natural or
inherent curvature provided in the sintered configuration
of the material. Thus the curved sintered ceramic products
of the invention are characterized in that they can be
furthex bent, straightened, or hent to reverse curvature
without breakage.
The cross-sectional thickness of the sintered structure
on axes parallel to axes of applied force easily relieved
by bending of the structure preferably will not exceed
about 45 microns, and most preferably will not exceed about
30 microns. The lower limit of thickness is simply the
minimum thickness required to render the struc~ure amenable
to handling without breakage. Sintered thicknesses of 4
microns can readily be achieved, and thicknesses on the
order of 1 micron appear quite feasible.
For thin sheet or tape structures, depending on the
composition of the material, a single layer or a plurality
of layers up to 500 ~m in thickness can in some cases be
made or assembled while still retaining some flexibility.
However, for the desired low bending radius, sintered sheet
or segment thicknesses will most preferably not exceed 30
~m, or even 10 ~m, with thicker members being provided by
layering the thin sintered sheets or other segments~
Description of the Drawinq
The drawing is a plot of electrical resistivity versus
temperature for a superconducting oxide coating disposed on
a flexible ceramic substrate in accordance with the
invention.
Detailed _escr~ption
Generally, the green material used .in the invention is
comprised of zirconias, aluminas, titanias, silicas
(including æirconates, aluminates, titanates and silicates),
rare earth metals and/or their oxides, alkalis and alkaline
earth metals, and/or their oxides, steels, stainless
steels, aluminides, intermetallics, aluminum and its
alloys, the first, second, and third transition series of
metals, their oxides, borides, nitrides, carbides,
silicides, and/or combinations thereof and therPbetween.
Optional additions of sintering aids, dispersants, binders,
plasticizers, toughening and hardening agents, and solvents
can be advantageously present~ The materials of interest
especially include brittle materials. It is a particular
advantage of the invention that structural flexibility can
be achieved in sintered structures composed of materials
which are normally considered to be brittle and inflexible.
Utilizing extrusion, tape casting or other known
ceramic batch shaping technology, a selected combination of
the above components is mixed into a plastic batch, formed
into an elongated green body of any desired cross-sectional
shape, and sintered. While the preferred cross-sectional
shape of the sintered structure is linear (as for thin
sheet or tape), other shapes including rectangular, cylin
drical ~tubular), trapezoidal, I-shapes, H-shapes, or
dumbbell shapes may be provided. In each case, however,
the cross-sectional shape is characterized by at least one
high-aspect-ratio segment, such as a straight or curved web
or connecting segment or an extending fin or other protru-
ding segment, which is suf f iciently thin to be flexible in
sintered form. By a high-aspect-ratio segment is meant a
segment having an aspect ratio ~segment length to thickness)
of at least 2:1, more preEerable at least 3:1.
For the manufacture of the preferred green sheet or
tape, a slurry or slip is preferably made from the green
batch by the addition of sufficient solvent to obtain a
fluid viscosity. The slurry or slip is ~hen formed into a
uniform~hin sheet by a thin sheet forming means, for
example doctor blading, rolling, mashing, extrusion or any
means those skilled in the art use to make thin sheets or
foils. The thin sheet is then heated to sintering tempera-
tures. The r~sultant structure is a sintered strong
material with multi-directional flexibility.
Preferred ceramic compositions suitable for flexible
substrate production in accordance with the invention
include zirconia-based compositions. As is known, zirconia-
based ceramic materials may optionally include ~he oxides
of the transition series metals and the rare earth metal
oxides. Stabilized zirconias, such as those stabllized by
additions of alkaline earth metal oxides including for
example magnesia and/or calcia, titanium and tin oxides,
are preferred embodiments. Those compositions stabilized
with yttria, are more preferred embodiments.
Some other useful examples of stabilizers are those
selected from indium oxide and the oxides of the rare earth
metals such as lanthanum, cerium, scandium, praseodynium,
neodynium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, and
lutetium. The crystalline geometries of zirconia such as
tetragonal, monoclinic, and/or cubic and their combinations
are all important physical parameters of this structural
material.
Ceramic sheets or so-called ceramic foils can be made
tougher by selecting certain toughening agents known to
those skilled in this art. Particularly useful and
preferred toughening agents are the oxides of tantalum and
niobium which can be advantageously added to the above
stabilizers. Reference to these toughening materials is
made in published European patent application EP 0199459,
published October 2S, 198~, the subs~ance of which is
herein incorporated by re~erence in its entirety.
That patent also discloses the properties of useful
bulk materials such as a-alumina, ~-alumina, ~"-al~ina,
g ~ ~ 2 ~
A12O3-Cr2O3 solid solution, mullite, and spinel. These
same ma~erials can be usefully employed as ceramic body
components and/or as companions to zirconia and the before-
stated toughening agents.
Combinations of titania and zirconia consisting
essentially of 45 to 94.75 mole percent zirconia, 5 to 45
mole percent titania, and 0.25 to 10 mole percent rare
earth metal oxides are found to be advantageous compositions
for forming flexible substrates in accordance with the
invention. Toughness and hardness properties are disclosed
in U. S. Patent 4,753,902, the disclosure of which is
incorporated herein by reference in its entirety.
Combinations of molybdenum and tungsten oxides with
magnesia, calcia, zirconia and rare earth rnetal oxides have
also been found to provide useful ceramic materials. For
instance, zirconia/hafnia~based compositions consisting
essentially of about 79-99.5 mole percent of oxide
components selected from the group consisting of ZrO2,
HfO2, partially stabilized ZrO2, partially stabilized HO2,
ZrO2-HfO2 solid solution, and partially stabilized ZrO2-HfO2
solid solution, together with 0.25 to 15 mole percent of
the before stated rare earth metal sxides and 0.25 to 6
mole percent of the oxides of molybdenum and/or tungsten,
are found to be useful compositions. Optional supplemental
additions of 0.5-10 mole percent of rare earth vanadates
are useful in these formulations.
The ahove zirconia/hafnia based compositions are
disclosed in commonly assigned U. S. patent application
Serial NoO 07/245,523, filed Septembar 19, 1988, the
disclosure of which, as filed, is herein incorporated by
reference as filed. Further materials therein descrlbed
include compositions consisting essentially of about 40 to
94.75 mole percent of the above zirconia/hafnia oxide
components, 5 to 45 mole percent of SnO2, and 0.25 to 15
mole percent of rare earth metal oxides, these providing
particularly hard and tough ceramic matexials.
Also useful to provide flexible ceramics are composi-
tions c~sisting essentially of about of 82 to 99 mole
percent of one or more of oxides selected from the group
consisting of ZrO2, HfO2, and ZrO2-HfO2 solid solutions,
0.5 to 10 mole percent of a stabilizer selected from the
group yttria, scandia, rare earth metal oxides, ceria,
titania, tin oxide, calcia, and magnesia, and 0.5-8 mole
percent of toughening agents selected from the group
consisting of yttrium and rare Parth metal niobates,
tantalates, and vanadates, and magnesium and calcium
tungstates and molybdenates. These ceramics, characteriz-
able as hard and tough ceramics exhibiting psuedo-plasticity,
are disclosed in commonly assigned U. S. patent application
Serial No. 3~8,532 filed March 24, 1989.
The invention also comprises thin flexible sintered
ceramic structures as above described composed of certain
recently developed hard refractory ceramic alloysO The
ceramic alloys consist essentially of a novel zirconia
alloy alone or in combination with a conventional refractory
ceramic, the zirconia alloy constituting a least 5% and up
to 100% by volume of the ceramic alloy.
The conventional refractory ceramics for these alloys
are selected from known materials. Typically, one or more
ceramics selected from the group consisting of a-alumina,
~-alumina, ~ " -alumina, alumina-chromia solid solutions,
chromia, mullite, aluminum mulllte-chromium mullite solid
solutions, chromium mullite, sialon, nasicon, silicon
carbide, silicon nitride, spinels, titanium carbide,
titanium nitride, titanium diboride, zircon and/or zirconium
carbide are used.
The novel zirconia alloy, present alone or in a
proportion of at least 5 volume percent as a toughening
addition to the conventional refractory ceramic r will
consist essentially of about: 35-99.75 mole % of oxides
selected from the group consisting of zirconia, hafnia, and
zirconia-hafnia solid solution and 0.25-45 mole % of one or
more oxide additives selected in the indicated proportions
--1 1--
from the following groups of additives. The first yroup
consist~ of 5-45 mole % of titania and/or tin oxide. The
second consists of 0-20 mole % total of metal oxides
selected in the indicated proportions from the groups
5 consisting of ~i) 0-4 mole % of MoO3 and/or WO3, (ii) 0-10
mole % ~o~al of oxide compounds of the formula MM'O4+/ ~
whexein M' is V, Nb, Ta, or combinations thereof, M is Mg
Ca, Ti, Sn, Sc, Y, La, Ce, the rare earth metals, or
combinations thereof, and ~ is in the range of 0-1, and
(iii) 0-6 mole % total of oxide compounds of the formula
M''M'''O4~/ ~ wherein M''' is W and/or Mo, M'' is Mg, Ca,
Ti, Sn, Sc, Y, La, Ce, the rare earth metals, or combina-
tions thexeof, and ~ is in the range 0-1.
In addition to one of the essential additives set
forth above, the zirconia alloy may comprise, as optional
additives, 0-20 mole % of cerium oxide, and 0-10 mole %
total of oxides of one or more metals selected from the
group consisting of Mg, Ca, Sc, Y, La, and the rare earth
metals.
In a more specific embodiment the zirconia alloy
consists essentially of 35-94.75 mole % of oxides selected
from the group consisting of zi~conia, ha~nia, and zirconia~
hafnia solid solution, 5-45 mole % of titania and/or tin
oxide, and 0.25-20 mole % total of oxides selected in the
indicated proportions from the group consisting of 0-20
mole % cerium oxide and 0-10 mole % total of oxides of
metals selected from the group of Mg, Ca, Sc, Y, La, and
the rare earth metals.
~n yet another specific embodiment the zirconia alloy
consists essentially of 70-99.5 mole % of oxides selected
from the group consisting of zirconia, hafnia, and zirconia-
hafnia solid solution, 0.5-10 mole % total of oxide
compounds of the formula MM'O4+/ ~ wherein M' is selected
from the group of V, Nb, Ta, and combinations thereof, M is
selected from the group of Mg, Ca, Ti, Sn, Sc, Y, La, the
rare earth metals, and combinations thereof, and ~ is in
the range of 0-1. Optional additions to these alloys
-12- ~ ~ r~
include 0-20 mole % cerium oxide and 0-10 mole ~ of oxides
of meta~s selected from the group of Mg, Ca, Sc, Y, La, the
rar~ earth metals, and combinations thereof.
In yet another specific embodiment the zirconia alloy
consists essentially of 79-99.75 mole ~ of oxides selected
from the group consisting of zirconia, hafnia, and zirconia-
hafnia solid solutions, 0-15 mole % total of compounds
selected in the indicated proportions from the group
consisting of 0-7 mole % of oxides of Mg, Ca, Sc, Y, La,
and the rare earth metals and 0-15 mole % of CeOz, TiO2,
and/or SnO2. The alloys further comprise one or more
toughening agents selected from the group consisting of
0.25-6 mole % total of compounds of the formula
M''M'''O4+/ ~, wherein M''' is W and/or Mo, M'' is selected
from the group consisting of Mg, Ca, Ti, Sn, Sc, Y, La, the
rare earth metals and combinations thereof, and ~ is in the
range of 0-1, and 0.25-4 mole % of MoO3 and/or WO3.
As noted, in addition to providing alloying additives
to harden conventional ceramics as set forth above, the
zlrconia alloys alone may constitute the hard ceramic alloy
material used to make flexible ceramic substrates in
accordance with the invention.
In the preferred method the invention takes compounds
such as above described and produces thin flexible sintered
sheets, foils, or ribbons thererom. Of course, flexible
whiskers and/or fibers may also be made from these
materials, with good strength, but the very high strengths
needed to provide strong, flexible ceramics providing
dependable support properties in long lengths are not
readily attainable in fiber and/or whisker configurations.
In order to manipulate these compositions into flexible
structures, novel processing methods are required.
Heretofore, similar compositions were used for cutting tool
inserts as disclosed in U. S. Patent 4,770,673, the
disclosure of which is incorporated by rererence. Due to
their hardness and toughness ater sintering, these composi-
tions provided unlikely candidates for flexible ceramics.
- 1 3 ~ l 3
Nevertheless, it is now found that the following methodolo-
gies can~successfully be used to embrace these materials
and bodies within the family of thin sintered flexible
materials.
To provide high quality thin sheet materials, fine
powders of the component composition are needed. Pre~erred
particle sizes are less than 5 ~m in diameter, most prefer-
ably less than 1.5 ~m in diameter. The powder can be
milled and separated to obtain the preferred powder size.
To provide ceramic batches amenable to the appropriate
forming techniques, the powdered batch materials are
generally mixed with fugitive organic or inorganic vehicle
formulations, most frequently formulations comprising one
or more organic solvents. Examples of preferred organic
solvents include mixtures of methanol and 2-methoxy ethanol.
Other organic solvents that may be useful for this purpose
are alcohols, ethers, aromatics, ketones, aldehydes,
esters, alkanes, alkenes, alkynes, and or combinations
thereof and therebetween. Inorganic solvents, particularly
water, may additionally or alternatively be used as
solvents.
Also useful in the preparation of ceramic batches in
accordance with the invention are powder dispersants. A
large number of dispersants can be utilized for this
purpose, including, for example, phosphate esters, polyether
alcohols, polymeric fatty esters, polyelectrolytes,
sulfonated polyesters, fatty acids and their alkali and
ammonium salts, and combinations thereof and therebetween.
An example of a specific and preferred dispersant is Emphos
PS-21A dispersant~ a phosphate ester dispersant commercially
available from ~he Witco Chemical Co., New York, NY.
Various plasticizers and binders known for use in the
preparation of ceramic powder batches may also be included
in the batch formulations of the invention. An example of
a specific plasticizer which has been used is dibutyl
phthalate, while a preferred binder is Butvar B-98 binder,
a polyvinyl butyral binder commercially available from the
Monsanto Company of St. Louis, Mo. Other binders that may
be usef~l for this purpose include polyalkyl carbonates,
acrylic polymers, alkyds, polyesters, cellulosic ethers,
cellulosic esters, nitrocellulose, polyvinyl ethers,
polyethylene glycol, polyvinyl butyral, polyvinyl alcohol,
polyvinyl acetate, and silicones as well as copolymers,
blends, or other combinations of the foregoing binder
materials.
When mixing with metals care must be taken to avoid
pyrophoricity. Additionally, when sintering the metal
compositions, an inert and/or reducing atmosphere, or a
vacuum is necessary to enable the metals to sinter without
oxidation. Advantageously, after sintering, the metals can
then be oxidized as disclosed in U. S. patent application
Serial No. 07/219,985 filed July 15, 1988 the disclosure of
which is herein incorporated by reference as filed.
Once compounded and uniformly mlxed, the batch is next
formed into thin sheets or other preforms having thin
flexible segments. This for~ing can be done by any means
whereby a thin layer, sheet or web can be configured.
Means such as doctor blading, pressing, rolling, extruding,
printing, molding, casting, spraying, drawing, blowing, and
combinations thereof and therebetween can provide green
bodies incorporating thin segments or thin sheet configura-
tions. Narrow ribbons or sheets many meters wide can beprovided.
Two methods have been found which improve the strength,
formability, and handleability of the green structures~ In
the first, the extrusion and/or drawing of low vlscosity
slips is combined with immediate and direct contact between
the thin extrudate and a gelling and/or drying liquid.
This technique has been found to be advantageous for
imparting green strength to the extruded or otherwise
configured green bodies. Binder, solvent, and gelling
liquid combinations may be chosen 50 that one or more of
the solvents in the ceramic slip is highly miscible ~ith
the gelling and/or drying medium. Preferably, the binder
~15~ s~
employed for batches to be ~hus treated is not be miscible
with th~gelling and/or drying medium, to avoid binder loss
during drying or gelling.
Flocculation, gelation and/or drying are particularly
s useful for the extrusion of low-viscosity batch formula-
tions. Slips with low initial viscosity can be extruded
through fine orifices of complex shape into a gelling or
drying liquid at relatively low extrusion pressures. With
prompt gelling after extrusion, the extrudate gains strength
and resists slumping and loss of shape definition. Thus
shapes of complex configuration not otherwise extrudable,
such as I-beam cross-sections or the like, can through
rapid gelation be extruded with excellent shape retention
in the green product.
Gelation can be facilitated by pRa or pKb (pKs)
adjustments of the slip or through the use of combinations
of extrudate treating media and slip vehicle combinations
which promote rapid gelation of the extrudate. Examples of
suitable media/vehicle combinations include the following:
Batch Vehicle Extrudate reatment
polyvinyl butyral/alcohol water
polybutyl methacrylate/
/isopropanol methanol
polymethyl methacrylate/
/tetrahydrofuran hexane
~5 polymethyl methacrylate/
/toluene hexane
PKs adjustments can be effected by use of strong acids
or bases and weak acids or bases, for example diethylamine.
Weak acids such as propionic or acetic acid are preferred.
The acid or base can be either organic or inorganic. A
buffered system incorporated to adjust the pKs and/or
maintain it within a certain range will also be effective.
It has also found particularly useful to form the
green material on or in contact with one or more fugitive
polymer layers or sheets. The processability and handle-
ability of the green body are greatly enhanced through the
support provided by such a polymer shee~. The material for
the she~ or layer can if desired be selected ~uch that it
provides initial support for the green body during subse-
~uent sintering to a product, yet vaporizes without damage
S to the product in the same manner as the organic binders,
dispersants and other organic constituents of the batch are
vapori~ed.
Vaporization of the fugitive polymer sheet or layer
can occur before, during, or after other organic components
of the green material are vaporized. Fugitive polymers
which may be useful to provide such layers or sheets
include acrylic polymers and co-polymers and polyalkyl
carbonate polymers; optional sheet or layer components
include plasticizers and waxes. These are generally though
not necessarily free of inorganic powder additives.
Green structures produced as described, whether
provided in long continuous lengths or relatively short
sheets, are typically sintered by treatment in a high
temperature furnace. Long dwell times in the furnace are
seldom required due to the low mass of green material
present at any one time.
For lony continuous lengths of tape or ribbon configur-
ation, the strength of the sintered material is frequently
suf f icient that the material itself can provide the drawing
force needed to continuously draw unsintered green material
through the furnace hot zone. As an aid to this process,
it is useful to provide supporting setters within the
sintering furnace which are angled downwardly in the
direction of drawing. This provides a gravitational assist
for the transport of the material through the furnace and
reduces the draw tension required.
The sintered structure of the invention can be used as
a substrate for catalysis. Catalysts of interest for this
purpose are the base me~al and/or oxide catalysts, such as
titanium, vanadium, chromium, cobalt, copper, iron, manga-
nese, molybdenum, nickel, niobium, tantalum, tungsten,
zinc, rare earth metals, alloys thereof and therebetween.
-17~ 3
Additionally, the noble metal catalys~s, such as platinum,
palladi~, silver, rhodium, gold can be combined with the
substrate. In combining the catalyst with the substrate,
the combination can be by chemical vapor deposition, by
coating with a high surface area base coating with a
subsequent catalyst overcoat, by impregnating the substrate
with the catalyst, or simply mixing the catalyst with the
batch prior to sintering.
The present invention can be incorporated as a struc-
tural material within other compositions as in a composite.For example, by drawing in narrow elongated form, the
sintered material can be made part sf another structural
material, adding new strength and/or toughness to the
material. Both metals and ceramic materials can be used in
this manner.
The following examples are illustrative of the various
means to practice the invention herein disclosed, and are
not intended to limit the scope of the invention.
Example 1
To prepare a green ceramic material, 100 grams of
yttria-stabilized zirconia powder (commercially available
as TZ-2Y powder from the Tosoh Chemical Company of Tokyo,
Japan and coMprising 2 mole percent Zro2 as a stabilizer)
was milled with a mixture of 30 grams of methanol and 24
grams of 2-methoxy ethanol containing 0.25 grams o~ a
phosphate ester dispersant. The dispersant is commercially
available as Emphos PS-21A dispersant from the Witco
Chemical Co. of New York, NY. This batch was designated as
Batch A. Batch B was comprised of 100 grams of the zirconia
powder, 16 grams of 2-methoxy ethanol, 20 grams of methanol,
and 0.25 grams of Emphos PS~21A dispersant. Th~ batches
were milled with 1/2-inch zirconia balls.
~5 The milled batches were placed in 250 ml wide mouth
NalgeneTM polyethylene bottles and then subsequently placed
on a vibratory mill for 76 hours. The particle size
1 8 ~ ~ t..~ ~ -J ~ 3
distribution in the final batches averaged from about
O.88~ , as measured on a Leeds and Northrup Microtrac
particle size analyzer. Similar particle size data obtained
using a ~oriba capa-500 analyzer from Horiba LTD~ of Kyoto,
Japan suggest that particle sizes produced by the described
milling procedure could be lower than the above repor~ed
values by a factor of from 3 to 5, but in any case particle
sizes of the order of 0.1-1.2 ~m predominate in these
batches~
The viscosity of the slip after milling ranged from
about 4.2 cps to 11.5 cps at 39.2 sec 1, with slips made
from oven dried powders showing ~he lowest values. A
viscosity within the range of 3 to 15 is preferred.
Coarse particles were removed from the slip by settling
for 3 days, then removing the fluid portion from the
settled sludge. An alternative separation procedure is to
centrifuge the slip at about 2000 rpm for 10 minutes.
Next added to the slip of Batch A was 2.5 grams of
glacial acetic acid, with 2.25 grams of glacial acetic acid
being ~dded to Batch B. The addition of the acid h~lped to
develop a flocculated state which is evinced by thixotropy.
The degree of thixotropy was dependent upon the amount of
acid added.
The slip of Batch A was then mixed with 6 grams of
polyvinyl butyral binder, commercially available as Butvar
B-98 binder from the Monsanto Company, and 3 gra~s o~
dibutyl phthalate. ~atch B was mixed with 10 grams of
methanol~ 8 grams of 2-methoxy ethanol, 6 grams of the
binder and 2.9 grams of dibutyl phthalate. The acid was
added before the binder, making the bind~r easier to
dissolve. The slips were shaken vigorously for 5 minutes,
placed on a roller, and turned slowly for several hours to
de-air. Some of the samples of the slip batch were further
de-aired in vacuo. The final viscosity of the slips as
measured on a Brookfield viscometer was within the range of
1500 to 5000 cps at 8.7 sec 1. Batch A, specifically, had
a viscosity of 3470 cps in this test.
19 2 ~
Tapes were cast from the batches produced a~ described
using a ~tandard 2, 4, or 6 mil doctor blade to cast onto a
2 mil Mylar~ polyester carrier film. In general, the
smoothness of the substrate can determine the smoothness of
the casting. Thus plastic-coated paper can alternatively
be used, but typically provides a surface roughness similar
to paper fibers. Smoother substrates including polyester,
fluorocarbon, polyethylene and/or polypropylen~ films are
therefore used when a smooth product surface is desired.
The cast tapes thus provided were then allowed to air
dry from 5 minutes to several hours, then placed in a
drying oven at about 70~C and/or 90C for 5 minutes to an
hour. The tape was less brittle and the adhesion to the
carrier film lessened after oven drying.
The dried green tape was next released from the
carrier film by pulling the film over a sharp edge.
Removal of the tape by this or equivalent means prior to
cutting of the tape is preferred. The tape was then cut
into strips 0.5 to 100 mm in width. The cut tape was then
placed on a flat setter plate for sintering, oriented so
that the portion of the tape which had contacted the film
was facing toward the setter. Alumina and zirconia setters
were used.
The tapes were then fired according to the following
schedule:
Room temperature to 200C in 1 hour
200C to 500C in 1 hour
500C to 1450C in 3 hours
1450C hold for 2 hours
1450C to room temperature for 5 hours
The heating rates used were not critical; both faster
heat-up and faster cool down rates were successfully tried.
However, uniform heating of the tape is preferred to avoid
warping during the binder burnout or sintering.
Properties of the tape products thus provided are
reported below in Table 1. The table includes a number of
-20~ Ç~
samples and their geometrical dimensions produced from
Batch A~ Once fired the tapes were strong. This was
demonstrated by the bend radius achievable for the sintered
ribbons. The actual strengths may be calculated from the
bending radius attainable without breakage using the bend
radius equation, known to those skilled in this art. The
accepted elastic modulus of 200 GPa and and Poisson's ratio
of 0.25 for this zirconia material were used in the
equation. The porosity of the sintered tape samples was
less than 5 volume percent.
Table_1
Width Thickness Bend Radius Strength
15Sample (10 3 m) (10-6 )~10 3 m~ (GPa)
1 1.80 20 1.70 1.25
2 1.80 18 1.78 1.07
3 1.80 18 1.54 1.24
4 1.32 18 1.70 1.12
1.32 18 1O71 1.11
6 1.02 23 2.10 1.1~
7 1.02 ~3 2.36 1.03
8 0.99 20 1.83 1.16
The as-fired surfaces of the tapes which had been in
contact with the carrier film were very flat and smooth,
providing an excellent surface for coating. The averaye
surface roughness of these as-fired tape surfaces was 8.99
nrn for the tape cast on MylarR polyester film, as measured
by WYKO surface analysisO
-21- "" ,'
ExamPle 2
In Example 2, ribbon samples of green material from
Batch A were sintered in accordance with a process of
continuously firing the green ribbon. Green ceramic
ribbons with lengths up to 30 centimeters were fired by
drawing the ribbons through a platinum wound furnace heated
to 1350C. The furnace had a small hot zone. The support-
ing surface for the ribbon within the furnace was set at an
incline of between 12 and 20 degrees downwardly from the
entrance toward the exit end of the furnace, to provide a
gravitational assist for the drawing process.
As the green tape was drawn through the hot zone, the
tape sintered to a dense structure that could be easily
manipulated. The time in the hot zone was less than 5
minutes, with a rate of sintering of about 2 cm of ribbon
length per minute. ~igher sintering rates can be achieved
by increasing the sintering temperature, e.g., to about
1500C.
Example 3
The use of a fugitive polymer base layer in the tape
casting procedures of Examples 1 and 2 is advantageous
because it makes the thin green material easier to handle.
To provide such a layer, a fugitive polymer solution was
prepared in a polyethylene bottle by dissolving 40 parts by
weight polymethyl methacrylate ~fugitive polymer) in 60
parts of ethyl acetate. The solution was placed on a
roller mill ~o mix.
The acrylic pol~mer solution thus provided was then
cast onto a polyester substrate ~ilm using a doctor blade
to form thin acrylic sheet. The polymer-coated substrate
was then placed in a 60-70C drying oven for 30 to 60
minutes.
A slip containing yttria-stabilized tetragonal zirconia
was then prepared utilizing the materials and procedures
-22- f~
used to make zirconia Batch A of Example 1. The ceramic
slip wa~'then cast over the acrylic layer using a doctor
blade. The carrier film with the acrylic and ceramic
layers of coating was transferred to a drying oven for 30
to 60 minutes.
The thickness of the fired films was varied as a
function of the height of the doctor blades. Thinner or
thicker ~heets were made by the proper choice of doctor
blades and slip viscosity. The lower viscosity slips and
smaller blade heights yielded thinner tapes.
The thinnest sheets were produced by thinniny a slip
such as Batch A of Example 1 with solvents. To 10 grams of
the slip were added 1.11 grams of methanol and 0.88 grams
of 2-methoxyethanol. Slips with a viscosity of 1500 cps or
less may be advantageously made by this technique.
The thinned slip thus provided was cast with a 2 or 3
mil blade on top of a fugitive acrylic layer cast as above
described with a 4 mil blade. This produced a composite
tape with about a 6 ~m green ceramic layer which sintered
to around 5 ~m. The firing schedule was as reported in
Table 2 below:
Table 2
25 Start Temperature End Temperature Time
Room Temperature 200 120 min
200 500 360 min
500 1420 375 min
1420 1420 120 min
301420 Room Temperature 120 min
Data which were obtained for 1 to 2 mm wide ribbons of
sintered ceramic made in accordance with the Example are
given below in Table 3:
-23~
Table 3
e~ ~
ThicknessBend Radius Strength
Sam~le (10-6 ) (10-6 ) (GPa?
9 6.0 362 1.77
1~ 5.2344 1.61
11 11.5710 1.72
12 11.5725 1.69
13 11.591~ 1.34
10 1~ 11.5 850 1.4
15 16.01400 1.22
16 16.51520 1.15
Samples 9 and 10 above were made from a thinned slip,
while the remaining samples were made with the standard
Batch A slip. The scatter in the measurements increased
for wider samples, such as the 3.5 mm wide sample as shown
in Table 4.
Table 4
ThicknessBend Radius Strength
Sam~ (1o-6 ) _ tlO 6 m) (GPa~
17 11.5 1020 1.20
18 11.5 737 1.66
19 11.5 1180 1.04
The thicknesses of the 5 to 6 ~m ribbons were deter-
mined from optical micrographs. The other thicknesses were
measured with a micrometer.
Sintered eight-micron-thick sheets as large as 9.5 cm
by 9.5 cm and a four-micron-thick sheet 6 cm by 7 cm have
been made with the fugitive polymer. Such sheets were
transparent enough to read through. Even thinner sheets
can be made by setting the doctor blade for the ceramic
slip casting step at zero clearance. Under this condition
only the bulk of the ceramic slip causes greater than zero
-24~
clearance on the blade; thus a residual, very thin, slip
layer i~ provided.
Example 4
Small 1.8 mm x 4.8 mm x 8 ~m rectangular pieces of
ceramic tape were formed by screen printing an ink consist-
ing of ceramic slip onto a fugitive polymer which had been
cast on a MylarR casting film. The ink used was prepared
by mixing a slip conforming to Batch A of Example 1 with
excess binder and enough 2-methoxyethanol to yield a thin
consistency. After the printed images were dry, the
printed images with fugitive polymer were rele~sed from th~
MylarR casting film. The images were then placed polymer
side down onto a zirconia setter and sintered to 1420C for
2 hours. The sintering schedule shown below in Table 5 was
used:
Ta~le 5
Start Temp. Stop Temp. Time
(C) (C) (minutes)
. .
Room Temp. 150 60
150 500 420
500 1420 375
1~20 1420 120
1420 Room Temp. 120
The ceramic tape samples produced by this process were
strong and flexible.
25~ 2,~
~xample 5
Ceramic sheet samples approximately 1 cm wide were
formed by printing with a rubber stamp onto a fugitive
polymer as in Example 4, using the Batch A slip with enough
additional t-butyl alcohol solvent to form a printable ink.
After sintering using the schedule of Table 5, the resulting
ceramic pieces accurately reflected the original image.
These pieces retained image details with widths of 140 ~m.
Example 6
Narrow tapes of yttria-stabilized Zro2 were formed
from a slip of the composition of Batch A of Example 1
utilizing a combination bead extrusion/doctor blading
method. This method was as follows:
a) A doctor blade was brought into contact with a
polymer sheet carrier;
b) A narrow continuous bead of slip was placed
before the advancing blade ~nd the blade spread the extruded
bead to form a narrow tape from the slip;
c) The narrvw tape was dried and removed from the
carrier film; and,
d) The tape was sintered in the manner described in
Example 1 above.
The polymer sheet carrier in this Example was a 2-mil
MylarR carrier film. The bead was extruded using a 10 cc
syringe fitted wi~h a 21 gauge needle. Either ~ mil or 6
mil blades were used to spread the beads into narrow thin
tapes. The tapes had a high degree of transparency
indicating little porosity.
Results from the bend-testing of the ceramic tapes
produced in accordance with the Example are set forth in
Table 6 below-
~26~ '3
Table 6
c
WidthThickness Bend Radius Strength
Sample(10-3 (1o-6 (10-3 (GPa)
18 1 25 3.0 0.89
19 3 33 3.5 1.00
Thus high strength in combination with good flexibility in
the tape samples were achieved.
To provide cross-sectional configurations other than
thin sheet or tape, extrusion processes can be used. Using
extrusion, it is possible to provide low aspect ratio
ceramic products, even including ceramic fibers. As
previously noted, fibers do not exhibit the strength and
flexibility of products with high aspect ratio cross-
sections or segments. Nevertheless the following example
illustrates that fibers can be successfully formed by this
technique.
Exam~le 7
.
Ceramic fibers were prepared by extruding a ceramic
slip into a gelling liquid. The slip had the composition
of Batch A of Example 1, and was used to fill a syringe
fitted with a stainless steel 21, 25, and/or 26 gauge
needle. The needle ~nd of the syringe was subm~rged in a
gelling liquid, in this case cold water being the preferred
agent, and was extruded out through the needle.
The slip gelled upon contact with the water and formed
a gelled fiber which reflected the shape of the needle
orifice. Details of orifice configuration as fine as 5
microns have been produced. The gelled fiber was draw
through 5 to 30 cm of cold water and was then pulled from
the water bath and dried by exposure to air.
Green fiber was made by this process at a rate of 2 to
20 cm per second and could be made in very long lengths.
Smaller diameter green fiber was produced by using a
-27-
smaller diameter orifice in combination with lower viscosity
slips. The process can be operated in a continuous fashion
by wrapping the extruded material on a rotating spool
placed approximately one meter above the gelling liquid.
In the process as described, the diameter of the fiber
is determined in part by the relative rates at which the
material is pushed through the orifice and/or the rate at
which it is drawn from the orifice. If material is drawn
slowly, a larger diameter is achieved, while if the material
is drawn more quickly a thinner diameter is achieved.
The dried fiber samples thus provided were finally
fired for 2 houxs at 1430C to yield sinter d fiber from ~5
to 150 ~m in diameter.
Example 8
Tape 50 ~m thick by 250 ~m wide was produced by
extrusion through an approximately rectangular orifice into
a gelling liquid as in Example 7. The orifice was prepared
by compressing the end of a syringe needle perpendicular to
the needle's long axis and then b~ grinding the needle tip
flat. The slip and gelling liquid of Example 7 may be
used. Tape as thin as 5 ~m could be made by this method
using slip thinned to an appropriate consistency.
Ceramic formulations similar in composition to Batch A
of Example 1 but comprising other zirconia powders are also
preferred materials for making strong flexible tape, as
illustrated in the following Examples.
-28~
Example 9
e;
A slip was prepared from a ceramic powder using
zirconia comprising 4 mole percent Y2O3 as a stabilizer.
The powder was first dried in a vacuurn furnace for 90
minutes at 200C, and then combined into a formulation
containing the following ingredients:
Ceramic powder 40 g
Ethanol 9.2 g
2-Methoxy ethanol 6.0 g
Methyl isobutyl ketone4.0 g
Di-butyl phthalate 3.9 ~
Emphos PS 21A dispersant 3.0 g
~illing media 39 g
The above mixture was milled in a SPEX 8000
Miller/Mixer for 45 minutes. To the resulting mixture was
then added 3.0 g of Butvar B-98 binder, with continued
milling for an additional 45 minutes.
Tape was cast from the resulting slip onto a plastic
coated paper film carrier using a 6-mil doctor blade
clearance. The tape was next dried and ribbon was cut from
the dried tape using a razor blade. The tape was sintered
between zirconia or alumina setter sheets to 1450C.
A tape 7.3 mrn wide, 43 ~m thick and 8 cm long produced
as described could be bent to a curvature radius of 8~5 rnm,
for a calculated strength of 538 MPa. Other structures
made from slips of this zirconia powder included sintered
ribbon 35 ~m thick by 1 cm wide by 10 cm long, and a 3 cm x
3 cm by 75 ~m square zirconia sheet.
-29~ 3
Exam~le 10
A slip was made containing ZrO2 comprising 6 mole
percent Y2O3. The slip was prepared following the procedure
used to make Batch ~ of Example 1. The mean particle size
of the zirconia after milling was 1.1 ~m with 50% of the
particles finer than 0.91 ~m.
Tapes were then prepared from the slip following the
procedures of Example 1. The properties of the tapes thus
provided are reported below in Table 7.
Table 7
WidthThickness Bend Radius Strength
Sample(10 3 m)(10-6 ) (10 3 m) ~MPa)
22 1.1 33 8.0 439
23 2.5 33 8.5 409
The average strength for these samples was 424 MPa,
and the tapes exhibited transparency indicative of low
porosity.
Example 11
A slip was made comprising ceramic powder containing
Zr2 with 2 mole percent Y2U3 and 2 mole percent Y~bO4.
The composition of the slip was as follows:
Ceramic powder 61 g
Methanol 40 g
2-methoxy ethanol 32 g
Emphos PS21A dispersant 1.0 g
Acetic acid 1O6 g
Milling media 450 g
The mix was vibramilled overnight, and an additional 2
g of methanol was added. To 139.8 g of the resulting slip
the following binder components were added:
-30- ~ t~
Poly vinyl butyral 4.86 g
Di-butyl phthalate 2.43 g
Tapes were cast from the resulting slip with a 4 mil
doctor blade clearance, cut into strips, and the cut
samples fired for 2 hours to 1390C, 1420C or 1500C.
Additional tape was made by casting with a 2 mil blade over
an acrylic layer which had been cast with a 4 mil blade.
The ~ollowing properties were obtained:
Table 8
Firing Bend
Temperature Thickness Radius Strength
15 Sample (C) (1o-6 ) (10 m) (GPa)
24 1390 10 0O26 *
1420 30 2.0 1.60
26 1420 36 3.1 1.23
27 1420 6 1.7 0.375**
28 1500 33 3.5 1.00
* This sample demonstrated porosity and the elastic
modulus used in calculating the strength (200 GPa)
would tend to overestimate the actual strength.
** These data are for a 2 cm x 2 cm sheet. The remaining
data are for 1 to 2 mm wide ribbons.
To account for the bend radius observed in sample 24
above and still have an inherent material strength of order
1.6 GPa, an effective elastic modulus of 80 GPa could be~ used in the bend radius equation.
In addition to flexibility, the cut sample of the
above tape fired at 1500C exhibited a high degree of
transformation (psuedo-) plasticity, as evidenced by
transformation bands both alon~ the fracture surface and a~
probable areas of stress concentration away from ~he
fracture surface. These transformation bands have been
associated with transforma~ion plasticity in materials of
-31~ 2~
this and similar compositions. The present invention thus
combines both flexibility and transformation plasticity in
one ceramic body.
Example 12
A slip was made containing alumina powder with the
following ingredients:
Alumina 67 y
Methanol 30 g
2-methoxy ethanol 24 g
Acetic acid 0.3 g
Milling media 450 g
(zirconia balls)
The alumina powder used was Alcoa A-1000 SG, lot 4BD
6742. The above mixture was vibramilled for 3 days, with
the mean particle size after milling being 1.05 ~m as
measured on a Microtrac analyzer. After milling, the
following constituents were added to the slip:
Acetic acid 1.08 g
Poly vinyl butyral 1.30 g
Di-butyl phthalate 0.65 g
Tape was then cast from this slip, dried, and sintered
at 1600C for two hours. The sintered tape was sufficiently
transparent to serve as a clear overlay through which
printed material could easily be read. The following data
were obtained from bend tests of the tape, using an elastic
modulus of 380 GPa for the sintered alumina:
Table 9
Width Thickness Bend Radius Strength
Sam~le(10-3 ) (lO _ m) (10~3 ) (MPa)
35 29 1.0 38 25 307
1.3 36 13 535
-32- ~3r~
~= ~
A slip comprisiny a mixture of alumina and yttria-
stabilized zirconia was made from the following ingredients:
Batch A zirconia slip16.66 g
Alumina slip ~Example 12) 4~55 g
Acetic acid 0.19 g
Butvar B-98 0.22 g
Di-butyl phthalate 0.11 g
This mixture was milled on a SPEX mill for 10 minutes,
and tapes were then cast from the mixture onto MylarR
polymer sheets using 4-mil and 6-mil doctor blade clearances.
Strips were cut from the cast tapes and the cut samples
fired to about 1430C for 2 hours.
The following strengths were calculated from tape bend
tests using an elastic modulus value of 230 GPa.
Table 10
WidthThickness Bend Radius Strength
Sample (10-3 )(10 6 m) ~10-3 ) (MPa)
31 1.2 36 4.24 1030
32 1.3 20 2.95 829
33 1.3 20 2.89 8
Thus high levels of strength and flexibility in the
ceramic tape were achieved.
_33_ ~ 7~ ~ 3
~ ExamPle 14
A slip containing mullite powder was prepared from the
following ingredients:
Mullite powder 55 g
Methanol 30 g
2-methoxy ethanol 24 g
Milling media 450 g
The mullite powder used was Baikowski 1981 Ref 193
Mullite Powder Deagglomerated l~pe CR. The mixture was
vibramilled for 3 days, after which the mean particle size
of the mullite powder was determined to be 1.36 ~m as
measured on a Microtrac analyzer.
The milled mixture was allowed to settle overnight and
the supernatant slip, retaining about 49 weight percent of
dispersed mullite, was recovered by decantation. To
25.01 g of this slip were added:
Acetic acid 0.57 g
Poly vinyl butyral 1.37 g
Di-butyl phthalate 0.69 g
The resulting slip was thoroughly mixed, and tapes
were cast and subsequently fired to about 1600C for 2
hours. Mullite ribhon 38 microns in thickness prepared
from this slip could be bent to a radius of curvature below
3.0 cm without breakage.
-34
Ex~le 15
A slip was made containing magnesium aluminate spinel
powder with the following ingredients:
Spinel 61 g
Methanol 30 g
2-methoxy ethanol24 g
Acetic acid 0.4 g
Milling media 450 g
~0
The spinel powder used was Baikowski 8293264 log 822
8/82 powder with an alumina content of 72.75%. The mixture
of ingredients was vibramilled for 3 days, following which
an additional 7.5 g of methanol and 6 g of 2-methoxyethanol
were added. The mean particle size after milling was 1.78
~m as measured on a Microtrac analyzer.
To a 20 g portion of the spinel slip thus prepared the
following were added:
Methanol 3.00 g
2-methoxy ethanol2.40 g
Acetic acid 0.43 g
Poly vinyl butyral 1.04 g
Di-butyl phthalate0.52 g
Tapes were cast from this slip using a 6-mil blade
clearance, and the cast tape was then cut into ribbon and
fired to 1600C. The fired ribbon had a thickness of 43 ~m
and could be bent to a radius of curvature of 2.5 cm
without breakage.
Example 16
A laminar structure was formed by casting a tape of
yttria-stabilized ZrO2 comprising 2 mole percent Y2O3 over
a tape of stabilized ZrO2 comprising 6 mole percent Y2O3.
The 6 mole percent Y2O3 slip was cast on a MylarR carrier
film using a 2 mil doctor blade~ The tape was allowed to
dry for about 5 minutes. Next a slip containing zirconia
with 2 mole percent Y2O3 was cast on top of th~ first tape
using a 6 mil doctor blade. The laminar tape was allowed
to dry in an oven at 90C, released from the carrier film,
cu~ into ribbons and fired at about 1430C for 2 hours. It
was apparent from a cross-sectional photomicrograph that
the sintered laminar ribbons had two distinct layers which
exhibited different fracture behavior and scattered light
differently. The 6 mole percent Y2O3 layer of one sintered
ribbon was 12 ~m thick while the 2 mole percent Y203 layer
was 25 ~m thick.
Results for bend tests conducted on the laminated
samples are reported in Table 11 below. Included in Table
11 for each of the samples tested are the dimensions and
properties of the tapes, as well as an indication, by
composition, of which surface or side of each tape sample
was the side in tension in the bend test.
Table 11
Bend
Side in Width Thickness Radius Strength
3~ Sample Tension10 3 m) (10_ m) (10_ m) (MPa)
34 6 mol/o0.8g 42 9 495
2 molto0.89 42 6.5 687
6 mol/o0.86 41 7.25 602
2 mol/o0.86 41 6.5 671
36 6 mol/o1.8 43 12.5 366
-36- 2~ 3
As the data indicate, the laminar structures of this
Example exhibited a strength anisotropy. Hence, strength
values obtalned with the 6 mole percent Y2O3 layer in
tension (averaging 466 MPa) were generally lower than those
obtained with the 2 mole percent Y2O3 layer in tension
(averaging 679 MPa). These values can be compared with the
average values obtained for 6 mole percent Y2O3 ribbons
(424 MPa) and for similar 2 mole percent Y2O3 ribbons (1.11
GPa).
Example 17
Flexible ceramic tapes for composite superconducting
wires have been produced. In one procedure, a zirconia
lS ribbon substrate was coated with a slip containiny
Y1Ba2Cu3O7 O (where ô equals 0 to 1.0, the compound being
referred to as a 123 superconductor)~ The sintered tape
had a thickness of approximately 75 ~m and was coated with
approximately 30 ~m of superconductor-containing slip, and
then fired. Firing was for a period of 4 hours at 940C to
sinter the superconductor and give good adhesion between
the coating and the substrate.
X-ray diffraction patterns indicated a high degree of
orientation for the coating, with the crystallographic b
axis being perpendicular to the plane of the ribbon. The
degree of orientation was indicated by the enhanced in-
tensity of the lin~s with Miller Indices 010, as shown in
Table 12 below which reports X-ray line intensities for the
123 superconductor sintered on the ZrO2 tape substrate, and
3~ for the bulk 123 powder. The x-ray pattern was indicative
of a reasonably good superconducting material, orthorhombic
Y1Ba2Cu3O7 ~. Flexibility was also evident after sintering
of the coating onto the substrate.
~s~ t''3
-37-
Table 12
Relative Intensities
Miller 123 on sulk Lines with
Index* Zirconia 123 Enhanced IntensitY
010 5 -
020 14 - ~
001, 030 49 9 +
100 1 2
021, 120 5
031 _# 28
130, 110 100 100
041, 140, 050 79 14
131 12 25
002~ 060 92 30 +
200 14 20
151 2 6
161, 132 28 48
231 5 _ 21 _ -
0 * The indexin~ scheme used designates the lon~ axis as
the b axis.
_ Indicates line not discerned by computer program used.
# This line appears as a shoulder and was not given an
intensity value by the computer program used.
Thick superconductor films of 123 composition tend to
react with the zirconia substrate to form a thin layer of
barium zirconate at the interface. A sheet of ZrO2 was
coated with a slurry containing 123 powder. When this
coated zirconia was heated to 950C for 20 minutes, the
thick film of 123 sintered to itself but did not adhere
well to the zirconia. Reaction ~or sintering) times of 2
hours at 927C were acceptable, improving adherence.
-38~
Example 18
Fluoride-enhanced thick film superconductor composi-
tions also form superconduc~ing coatings on the flexible
ceramic substrates of the invention. Compositions success-
fully applied include the fluorine-containing materials
disclosed in U. S. patent application Serial No. 07/207,170
filed June 15, 1988 the disclosure of which, as filed, is
incorporated herein by reference.
As an illustrative procedure, a slurry containing
ethyl acetate and a powdered ceramic superconductor
consisting essentially of Y1Ba2Cu3O7 ~F was prepared by
mixing 3 g of powder with 3 g of ethyl acetate. The slurry
was pipetted onto a flexible zirconia sheet and the excess
slurry allowed to run off. The substrate used was zirconia
comprising 4 mole percent of a yttria stabilizer and having
a sheet thickness of approximately 70 ~m. After firing
onto the substrate, the coating demonstrated superconductive
behavior; the resistivity of the coating at 7iK decxeased
at least 3 orders of magnitude from the room temperature
resistivity. A 50% reduction in resistivity is more
typical of con~entional conductive materials.
Voltage/current data for the superconducting coating
of the Example is set forth in Table 13 below. The Table
records the voltages needed to induce cuxrent flow over a
range of current magnitudes (0.1-10000 mAl for the sample
at room temperature (25 C) and at 77 K.
Table 13
_ _ .
Current(mA)
0.1 1 10 100 1000 2000 5000 10000
Voltages (mV)at 25 C
0.000 0.006 0.047 0.~62 4.625 - - -
35 at 77 K
0.000 0.000 0.000 0.000 __0.000 0.011 0.194 0.943
- indicates not measured
-39- 2 ~
Example 19
High temperature superconductor coating were applied
to flexible zirconia substrates using a laser ablation
technique, known to those skilled in this art. Laser
ablation targets of two distinct compositions were used,
denominated Compositions X and U, the compositions being of
the form YlBa2-xAgxCU37-~ wherein the mole proportions of
the components were as follows:
lO Composition X U
(coefficient in formula)
Y 1.0 1.0
Ba 2.0 1.85
Ag(x) 0.0 0.15
Cu 3.0 3.0
These compositions are disclosed in U. S. Patent
Application Serial No. 07/315,326, filed February 24, 1989,
the disclosure of which, as filed, is herein incorporated
by reference. These materials were deposited on both
flexible ceramic tape and single crystal cubic zirconia
using the laser ablation technique. The samples were then
annealed according to the following schedule:
Table 14
Starting Set Next Set Time
Temperature C Temperature C Minutes
Room Temperature 600 60
600 600 60
600 700 15
700 700 60
700 850 15
850 850 360
850 Room Temperature 180
4 o
Following annealing, silver metal was evaporated onto the
~c
coated substrates in order to form electrical contacts for
measurements and the coated substrates were heated again in
oxygen to 300C.
Data indicating the electrical resistivities for
Compositions X and U on tape and on single crystal cubic
zirconia are shown in the Drawing. The Drawing plots the
normalized resistance o~ the samples as a ~unction of
temperature over the temperature range from near 0~ K to
100 K, setting unit resistance at the higher temperature.
Curve A plots data for Composition U on a flexible ZrO2
substrate~ Curve B for Composition U on single crystal
ZrO2, Curve C for Composition X on flexible ZrO2, and Curve
D for Composition X on single crystal ZrO2. As is clearly
demonstrated by the data presented, continuous high tempera-
ture superconductor coatings have been provided on these
flexible substrates.
Example 20
Four narrow (1 mm to 2 mm wide) zirconia ribbons of 20
~m thickness were coated with Composition U of the previous
example using laser ablation. The samples were annealed in
flowing oxygen using the schedule of the previous example,
except that temperatures measured were 661C for the 600C
set, 761C for the 700C set, and 865 C for the highest
set temperature (for which the set point was 810C).
Silver electrodes were then laid down on the coatings by
vacuum vapor deposition and the samples heated in oxygen to
300C.
one coated and annealed ribbon from this example was
bent to test its strength after heat treatment. The
thickness of the substrate was 20 ~m~ the sample was bent
with the coating in compression to a radius of 1.9 mm
without breakingO From this data the calculated strengthwas at least 1.11 GPa. No visible degradation occurred to
the coating or the substrate.
-41~ L ~.
Exam~le 21
A 0.5 ~m coating Nb3Sn coating was applied to zirconia
ribbon substrates by co-sputtering Nb metal and Sn metal
using a CVC rf sputtering apparatus. Niobium metal and tin
metal were used for targets. The substrates were 25 ~m x
1.65 mm x 2.5 cm zirconia ribbons which contained 2 mole %
yttria.
The coated ribbons were annealed at 960C in vacuum
for 1 hour in order to homogenize the alloy. The annealed
alloy adhered to the substrates and could be bent to a
radius of less ~han S mm in either direc~ion (with the
coating in either compression or tension) without either
the coating or the substrate suffering visible damage. The
x-ray diffraction pattern for a film produced concurrently
on an alumina substrate showed a cubic material with
lattice parameter of 5.34 angstroms, close to the literature
value of 5.291 for Nb3Sn. The film on the flexible zirconia
substrate was found to superconduct with a Tc of 18K.
ExamPle 22
Flexible zirconia tapes were coated with silver metal
by evaporative vapor deposition, followed by heatin~ of the
composite to 300C. The product was a non-ductile, flexible
conductiny composite. Thus flexible conductors which may
be fatigue resistant composites were provided.
A 1.7 mm wide sintered ribbon of ZrO2 with 2 mole
percent Y~O3 was adhered to a 3 mm thick 4.4 cm OD sintered
zirconia ring using a zirconia slip having the composition
of Batch A of Example 1. The body so formed was sintered
to 1430C for 2 hours. After sintering, the ribbon was
affixed to the tube.
42 ~ . t . . A .
ExamPle 24
Thin green metal tape was prepared by a tape casting
process. 167 grams of stainless steel powder was mixed
with 42.4 grams of a binder, 10.0 grams of a plasticizer,
and 54.5 grams of 1~1,1 trichloroethane. The binder was
commercially available 5200 MLC binder made by the E. I.
duPont Company and the plasticizer was Santicizer 150
commercially available from the Monsanto Company. These
materials were mixed and cast onto MylarR polymer film to
form a green stainless steel tape layer about 28 mils
thick.
To form a covering ce.ramic layer, 100 grams of ZrO2
powder comprising 2 mole percent of a Y2O3 stabilizer, 6.48
grams of Butvar B-98 binder, 19.52 grams of ethanol, 31.24
grams of xylene, and 8.24 grams of dibutyl phthalate were
mixed into a slurry and applied over the surface of the
green stainless steel to provide a green laminar tape
configuration. The laminar green tape was then sintered
for 2 hours at 1300C in a vacuum furnace to provide an
strong, flexible integral metal/ceramic composite tape.
Composite sintered tapes of less than 30 ~m thickness may
be made by this process.
-43-
Example 25
c
A slip was made with zirconia comprising 2 mole percent
Y203 as a stahilizer. The initial batch was pr~pared in a
250 ml polyethylene bottle and contained 100 g of ceramic
powder which had be~n dried in an oven at 400C, 24 g of
2-methoxy ~thanol, 28 g of methanol, 1.0 g of Emphos
PS-21A, and 400 g of ZrO2 milling media. The batch was
milled for 70 hours, poured into a 125 ml polyethylene
bottle and left to settle for 168 hours. The batch was
pipetted off the sediment into another 125 ml bottle and
left to settle for an additional 24 hours. The batch was
again pipetted off the sediment and into a 125 ml bottle.
The twice-settled batch contained approximately 74.9 g of
ceramic powder. The mean particle size was 0.38 ~m as
measured on a Horiba Capa-550. To this slip were added
1.69 g of acetic acid, 2.27 g of dibutyl phthalate, and
4.50 g of polyvinyl butyral. The slip was rolled on a ball
mill to dissolve the binder and homogeni~e the slip. The
final vis~osity was 598 cps at 8.7 sec-1.
Two fugitive polymer solutions were prepared in 60 ml
polyethylene bottles. One solution was prepared by first
adding 0.05 g of water to 16 g of medium molecular weight
polymethyl methacrylate from Aldrich Chemical Co., Inc. of
Milwaukee, Wisconsin, rolling overnight, and warming to 60
C in an oven. To this was added 32 g of ethyl acetate and
2 g of dibutyl phthalate. Likewise a second fugitive
polymer solution was prepared but with 0.05 g of water,
15.6 q of polymer, 29.4 g of ethyl acetate, and 5 g of
dibutyl phthalate.
The first fugitive polymer solution was cast on a 2 mil
Mylar~ polymer carrier using a doctor blade with a 1 mil
clearance. This was dried in an 60 C oven or several
minutes. The slip containing ceramic powder was then cast
over the first fugitive polymer layer using a blade with a
1 mil clearance. This was also dried for several minutes
in a 60C oven. The second fugitive polymer solution wa~
-44~ ~ ~J~ L~
then cast over both of the previous layers, again with a
.~
blade with a 1 mil clearance, thus forming a 3-layer
sandwich structure.
The cast structure was released from the MylarR carrier
film, cut. to size with a rotary blade and fired to 1450 C
for 2 hours. In this way strong, flexible, refractory 10
cm by 12 cm sheets were prepared which were 8 - 10 ~m thick
when measured with a micrometer.
While the invention has been particularly described
above with respect to specific materials and specific
procedures, it will be recognized that those materials and
procedures are presented for purposes of illustration only
and are not intended to be limiting. Thus numerous modifi-
cations and variations upon the compositions and processes
specifically described herein may be resorted to by those
skilled in the art within the scope of the appended claims.
