Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SINGLE-CRYSTAL-LIKE MATERIALS
BACKGROUND OF THE INVENTION
The present invention relates to polycrystalline materials having Single-
Crystal-Like
properties, in which a plurality of Single-Crystal Particles are assembled
together with their
crystallographic axes aligned in at least one, but also possibly in two or
three dimensions. The
present invention also relates to methods for forming the polycrystalline
materials of the present
Single crystal materials have applications in mechanical, electronic,
electromechanical,
optical and magnetic devices. However, the growth and processing of large
single crystals is
difficult, time-consuming and expensive. The growth of ceramic single crystals
from high
temperature melt or liquid solution often require expensive and energy
consuming furnaces. The
required melt or liquid solution is contained by crucibles often consisting of
expensive precious
metals such as Pt or Pd. Single crystals must be cooled from their growth
temperatures, and can
be damaged upon cooling by stresses induced by a variety of factors such as
polymorphic phase
transformation or anisotropic contraction of the lattice. Stresses can induce
cracks or significant
changes in crystal properties. These induced stresses can make it difficult,
if not impossible, to
manufacture useful crystals in large sizes. Other problems associated with
high temperature
crystal growth arise from phenomena that alter the composition of the crystal
such as volatility
of one or more of the components and incongruent melting behavior. In
addition, molten
solvents can introduce impurities into the crystal that cannot be eliminated
by conventional
purification processes.
Single crystals are typically grown as large boules. These boules are
processed by cutting,
dicing and polishing prior to incorporation into a device. These steps are
time-consuming and
may introduce defects. The size of the finished crystal is limited by the
processing operation. A
lower limit in size of hundreds of microns is typical. The upper size limit is
governed by the size
and quality of the crystal boule. The size varies greatly with composition.
For example, Si can
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be grown up to a diameter of about 10 inches, while the diameter of YIG may be
only on the
order of 0.5 inch .
Furthermore, single crystals have lower fracture toughness than their
polycrystalline
counterparts. Thus, single crystals can be extremely brittle, and their
strength can be greatly
diminished with surface damage (e.g., scratches) and this diminishes their
reliability for many
types of applications.
There exists a need for low cost materials with performance properties
comparable to
single crystals yet which overcome the limitations of single crystals
described above and low cost
methods for their preparation.
SUMMARY OF THE INVENTION
This need is met by the present invention. A cost-effective method of
fabricating
polycrystalline single phase and composite materials has been developed that
addresses the
deficiencies of single crystals while capturing some, if not all of the
performance advantages.
The present invention takes Single-Crystal Particles and packs and aligns them
with respect to
one, two or three dimensions to form a polycrystalline microstructure that for
all practical
purposes captures the performance of at least one important property of a
large single crystal.
Therefore, according to one aspect of the present invention, a polycrystalline
material is
provided in which a plurality of Single-Crystal Particles having self-
orientation are bonded
together to fix their orientation along at least one crystallographic
direction. The particles
interact with one another or with a substrate surface to align their
crystallographic axes in one,
two or three dimensions. The preferred degree of alignment will depend on the
device
application for the material. For purposes of alignment, it is essential that
the particles have
uniform shapes with dominant planar surfaces in a suitable orientation,
preferably perpendicular
to or parallel to, with respect to the desired direction of alignment.
According to one embodiment of this aspect of the invention, the
polycrystalline materials
comprise a plurality of Single-Crystal seed Particles aligned in at least one
direction, in which
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the voids therebetween have been filled with a nutrient matrix of
nanoparticles of the same or
different material and heated at a temperature sufficient to induce solid
state diffusion until the
nanoparticles bond together adjacent Single-Crystal Particles. Preferably,
this embodiment is
heated at a temperature sufficient to induce grain boundary mobility, so that
the Single-Crystal
Particles grow by consumption of the nanoparticles until impingement of
adjacent crystal grain
boundaries prevent further growth. The net result is a polycrystalline
microstructure, the grains
of which for all practical purposes are aligned in one, two or three
dimensions so that it performs
like a single crystal with respect to some desired property.
According to one preferred embodiment of this aspect of the invention, cube-
shaped
Single-Crystal Particles are packed and aligned with respect to one, two or
three dimensions to
form a polycrystalline microstructure that for all practical purposes captures
the performance of
at least one important property of a large single crystal. According to this
preferred embodiment
of this aspect of the invention, the cube-shaped Single-Crystal Particles are
bonded together by
filling the voids with nanoparticles, after which the filled array is heated
to at least a temperature
sufficient to induce solid state diffusion between the nanoparticles and the
crystal particles until
the nanoparticles bond together adjacent Single-Crystal Particles. According
to an even more
preferred embodiment, the temperature of the heating step is sufficient to
induce grain boundary
mobility, so that the cube-shaped Single-Crystal Particles grow by consumption
of the
nanoparticles until impingement of the grain boundaries of adjacent single
crystal regions occurs.
According to another preferred embodiment of the present invention, the
polycrystalline
materials comprise a plurality of Single-Crystal Particles that are aligned in
at least one
crystallographic direction and bonded together by a polymer phase. This
embodiment of the
present invention incorporates the discovery that aligned Single-Crystal
Particles produce a net
Single-Crystal-Like behavior, even when the particles are bonded together by a
non-ceramic
material without impingement of the Single-Crystal Particle surfaces.
Accordingly, essentially
any thermoplastic or thermosetting polymer is suitable for use with this
embodiment of the
present invention.
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For example, applying an oscillating electric field across a polymer-ceramic
composite
consisting of an aligned array of piezoelectric Single-Crystal Particles
causes each individual
crystalline particle to generate an acoustic wave, which combines with waves
from other particles
to form a net acoustic wave characteristic of a single crystal having the same
size as the array.
It is the particle alignment that produces the net Single-Crystal-Like
behavior. This embodiment
of the invention thus provides a polymer-bonded Single-Crystal-Particle
composite material with
Single-Crystal-Like properties, which does not require the high temperature
steps necessary for
making a ceramic single crystal..
The present invention also includes methods by which the polycrystalline
materials of the
present invention are made. In particular, a film-forming process has been
developed for making
large-area single-crystal-like films from Single-Crystal Particles.
In this process a microarray of Single-Crystal Particles is self-assembled at
the interface
of two irmniscible fluids. The first fluid is a supporting fluid, such as
water, upon which the
microarray assembles. The second fluid is a particle dispersion containing
three components that
provide the means for self-assembly. The three components are (1) the Single-
Crystal Particles,
(2) a dispersant liquid that aids in the dispersion of the particles in the
second fluid and also
provides a spreading tension that causes a film of the second fluid to spread
on the surface of the
first fluid; and (3) a liquid that provides a capillary force that acts on the
powder particles to form
an aligned-particle microarrray.
The inventive method, however, is not limited to the formation of oriented
Single-Crystal
Particle microarrays, but finds utility in the preparation of powder particle
monolayer films in
general. Therefore, according to another aspect of the present invention, a
method of forming
a monlayer of particles is provided including the steps of: (A) contacting a
particle dispersion
with the surface of an immiscible supporting fluid, wherein the particle
dispersion has a lower
density (specific gravity) than the supporting fluid and is a dispersion of
particles capable of
floating without wetting on the supporting fluid in a mixture of (1) a
dispersant liquid that aids
in the dispersion of the particles in the dispersion and provides a spreading
tension that causes
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a film of the fluid dispersion to spread on the surface of the supporting
fluid, and (2) a capillary
fluid that acts on the particles to form a monolayer particle film; and (B)
removing the dispersant
liquid so that a particle monolayer assembles at the interface of the
supporting liquid and the
particle dispersion. The particles can have a higher density than the
supporting fluid or the
5 dispersant liquids.
Preferred methods according to the present invention employ Single-Crystal
Particles,
which assemble a microarrays oriented in at least the <001> direction.
Particles less than about
1000 microns in size are preferred, with particles less than about 100 microns
in size being more
preferred, and nanosized particles less than one micron in size being most
preferred.
The contacting is preferably performed by incremental addition of the particle
dispersion
to the supporting fluid. The dispersant liquid is preferably removed by
evaporation, and is also
preferably a volatile liquid. At the end of the addition step, a particle
monolayer film remains
suspended at the interface of the supporting fluid and the particle
dispersion. The film is flexible
and conformable and can be removed from the interface and deposited intact
onto any solid
substrate surface. The capillary forces that form the monolayer also hold the
monolayer together
as it is transferred from the surface of the supporting fluid to the substrate
surface. Therefore,
methods according to one embodiment of this aspect of the present invention
will further include
the step of transferring the particle monolayer to the surface of a solid
substrate. Preferred
methods bond the particle film to the substrate surface.
The present invention also includes particle monolayers prepared by the method
of the
present invention. The particle film may be formed into the single phase and
composite materials
of the present invention. According to one embodiment of this aspect of the
invention, the
particles are Single-Crystal seed Particles and the voids therebetween are
filled with a nutrient
matrix of nanoparticles of the same or different material and heated at a
temperature sufficient
to induce solid state diffusion until the nanoparticles bond together adjacent
Single-Crystal
Particles. Preferably, this embodiment is heated at a temperature sufficient
to induce grain
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boundary mobility, so that the Single-Crystal Particles grow by consumption of
the nanoparticles
until impingement of adj acent crystal grain boundaries prevent further
growth.
According to one preferred embodiment of this aspect of the invention, cube-
shaped
Single-Crystal Particles are used. The cube-shaped Single-Crystal Particles
are bonded together
by filling the voids with a nutrient matrix of nanoparticles, after which the
filled array is heated
to at least a temperature sufficient to induce solid state diffusion between
the nanoparticles and
the crystal particles until the nanoparticles bond together adjacent Single-
Crystal Particles.
According to an even more preferred embodiment, the temperature of the heating
step is
sufficient to induce grain boundary mobility, so that the cube-shaped Single-
Crystal Particles
grow by consumption of the nanoparticles until impingement of the grain
boundaries of adjacent
single crystal regions occurs.
According to another preferred embodiment of the present invention,
polycrystalline
materials are prepared in which the particles are Single-Crystal Particles
forming a microarray
film in which the Particles are aligned in at least the <001> direction and
bonded together by a
polymer phase. This embodiment of the invention thus provides a method by
which polymer-
bonded Single-Crystal-Particle composite materials with Single-Crystal-Like
properties, can be
made, which does not require the high temperature steps necessary for making a
ceramic single
crystal.
The polycrystalline Single-Crystal-Like composite materials of the present
invention, are
suitable for many applications where a single crystal would be useful.
Accordingly, the present
invention also includes mechanical, electronic, electromechanical, optical and
magnetic devices
incorporating the Single-Crystal-Like polycrystalline materials of the present
invention. The
polycrystalline materials of the present invention are particularly useful in
the preparation of
piezoelectric devices, which are included among the electromechanical devices
of the present
invention.
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It is conceivable that the present invention could be utilized for the
purposes of making
a structural material, where the mechanical properties alone sufficiently
justify its utility. Thus,
in one preferred embodiment of this invention, a highly filled polymer-ceramic
composite
consists of aligned ceramic cubes in a low volume fraction matrix of organic
polymer. In another
preferred embodiment, a high volume fraction of ceramic single crystal cube-
shaped particles are
dispersed in a low volume fraction of non-aligned fine grains of the same
ceramic composition.
This invention takes advantage of the higher fracture toughness that
polycrystalline textured
materials have in comparison to their single crystal counterparts.
Accordingly, the
polycrystalline Single-Crystal-Like materials of the present invention are
much more
mechanically durable than comparable single crystals of the prior art. Thus,
the present invention
also provides materials having the functional electrical and optical
properties of single crystals
but which can also endure mechanical shock, vibration, and the like. Further
features and
advantages of the invention will become more readily apparent from the
following detailed
description when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts the preparation of a polycrystalline composite material
according to one
embodiment of the present invention in which the individual cube-shaped Single-
Crystal Particles are
aligned in one crystallographic direction;
FIG. 2 depicts the preparation of a polycrystalline composite material
according to another
embodiment of the present invention, wherein the individual cube-shaped Single-
Crystal Particles are
aligned in three crystallographic directions; and
FIG. 3 depicts a PZT cube microarray with lateral particle faces in contact
assembled according
to the method of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purposes of the present invention, "Single-Crystal Particles" are
defined as single
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g
crystals with controlled size on a size scale of fractions of a millimeter or
less and controlled
morphology where all the particles have essentially the same shape or habit.
Furthermore, for
purposes of the present invention, the term "Single-Crystal Particle" also
includes polycrystalline
agglomerates having low-angle grain boundaries between the individual single-
crystal grains of
the agglomerate, such that the polycrystalline agglomerate exhibits
essentially single crystal
behavior. Perfect Single-Crystal Particles are nevertheless preferred. By
controlled morphology,
we mean that the Single-Crystal Particles have a single large (dominant)
crystal face in a suitable
orientation, preferably perpendicular to or parallel to, with respect to the
desired direction of
alignment. More than one large crystal face is acceptable, provided that these
faces are
symmetrically equivalent. In certain circumstances, more than one
crystallographic direction
would be acceptable, thus, in this case, a mixture of crystals having the
desired dominant crystal
faces would be acceptable. In the case, where there is no dominant crystal
face, such as in a
cylindrical single crystal fiber, then the required dominant direction is
along the length of the
fiber and this must also be the desired direction of alignment.
Single crystals are defined as macroscopic crystals that are inches,
centimeters, or many
millimeters in size. "Single-Crystal-Like" materials are aligned arrays of
Single-Crystal Particles
whose positions and orientation are fixed by bonding to a substrate and/or
embedding in a
polymer, metallic or ceramic matrix. A reactive embedding matrix can consist
of nutrient that
allows the Single-Crystal Particles to be grown to a larger size.
Alternatively, the embedding
matrix can be inert and only bond the Single-Crystal Particles together. All
of the above
mentioned Single-Crystal-Like materials can also be referred to as textured
materials. The
embedding matrix can play a functional (e.g., optical, electrical, magnetic)
and/or structural role
in influencing the materials properties.
The Single-Crystal Particles of the present invention have at least "Self-
Aligning"
morphologies, which may also be highly "Space Filling." For purposes of the
present invention,
"Self-Aligning" morphologies are defined as including any particles that are
capable of self-
organizing to form a polycrystalline structure wherein the Single-Crystal
Particles are aligned
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along at least one crystallographic direction so that the particles perform
collectively like a
monolithic single crystal. If the particle morphology permits the packing of
particles at higher
volume fractions than normally encountered for randomly oriented and packed
equiaxed
particles, then these morphologies are also "Space Filling". Volume. fractions
typically
encountered for equiaxed randomly oriented and packed particles correspond to
a value of about
0.65 or less. The present invention in a particular embodiment, where cube-
shaped particles are
aligned and packed can produce packing fractions that approach 1.0 when 3-
dimensional
alignment is achieved. There are numerous systems with Single-Crystal Particle
morphologies
that offer both Self-Aligning and Space-Filling morphologies. Examples of
Single-Crystal
Particle morphologies with Self Aligning Space-Filling morphologies include
cubic particles,
hexagonal platelets, hexagonal fibers, rectangular platelets, rectangular
particles, octahedral
particles, and the like.
Certain Self-Aligning Space-Filling morphologies provide reliable self
alignment in one
direction, so that when the particles sit on a surface, at a minimum they
align with at least one
common crystallographic direction. For example, cubes can align along the
<001>-direction yet
be randomized with respect to the <010>- and <100>-directions. Platelets and
fibers will align
similarly. Cubes can also align perfectly with respect to all three
crystallographic axes. Fibers
cannot align in all three directions if they have a round cross-section, but
can if their cross-
section occupies a two dimensional Space-Filling geometry such as a square,
rectangle or
hexagon. Platelets can align in three dimensions if their morphology is
uniform with respect to
3 dimensions. For instance, a hexagonal platelet can align in 3 dimensions,
provided it has
sufficient thickness to prevent platelets from overlapping. However,
regardless of the thickness,
a round platelet cannot align in 3 dimensions.
There are many applications where precise self alignment is not important. For
example,
Self-Aligning morphologies may establish a preferred orientation that could be
10 degrees from
the desired alignment direction, yet still sufficiently capture the desired
properties of a single
crystal. Thus, particles having such morphologies include particles that
essentially have the
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desired morphology. For instance, for particles that are cubes, the particles
need not be perfect
cubes. The axes need not be at perfect 90 degree angles, nor exactly equal in
length. Corners
may also be cut off of the particles. Furthermore, "cube" or "cubic" is
intended to refer to
morphology, and is not intended to limit the particles to cubic crystal
systems. Instead, Single-
5 Crystal Particles that have orthorhombic, tetragonal or rhombohedral crystal
structure may also
be employed as cubes if they possess the defined cubic morphology. In other
words, any
essentially orthogonal Single-Crystal Particles in which the faces are
essentially square,
essentially rectangular, or both, that possess an essentially cubic morphology
are considered
cubes for purposes of the present invention.
10 The use of Self-Aligning Space-Filling single-crystal particles for
monolithic structures
can consist of a single layer of crystals. In addition, Single-Crystal-Like
materials can also consist
of multiple layers of crystals as well. The layer or layers can be conformal
to curved surfaces,
layers can be deposited on surfaces of complex geometry, and layers can be
wound into complex
geometries such as spirals, circles, and ellipes, so that the substrate
surface need not be flat. It
can also be curved, stepped or patterned. If the film is deposited on a solid
but flexible surface
such as a polymer film, it can be further shaped by folding or stretching the
polymer film. In this
way the particle film layer can be shaped and prepared for further processing,
such as heat
treating or sintering. Stacked layers of particles can be prepared by folding
layers of polymer
films.
Single-Crystal Particles suitable for use with the present invention are
materials whose
properties are strongly dependent on crystallographic direction, which are
also known as Vector
properties or more generally Tensor properties. Examples of Tensor properties
include mechani-
cal, electronic, electromechanical, optical and magnetic properties. For
electromechanical single-
crystal end-uses, including piezoelectric uses, exemplary compounds include
lead zirconate
titanate (PZT) compounds having the formula Pb(ZrxTil_X) 03 with a perovskite
structure wherein
0.20<x<0. ~0, with x preferably being greater than about 0.52, and other
materials with perovskite
structure and properties that depend on crystallographic direction, such as
lead zinc niobate
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doped with lead titanate, lead magnesium niobate doped with lead titanate,
sodium potassium
bismuth titanate doped with barium and zirconium, bismuth ferrite doped with
lead titanate, and
the like. PZT piezoelectric compounds are particularly preferred.
Optical Single-Crystal Particle compounds include doped alumina (sapphire),
yttrium
aluminum garnets (YAG), yttrium iron garnets (YIG), lead lanthanum zirconate
titanate (PLZT),
zinc oxide, rhodium doped barium titanate, gallium nitride, cadmium sulphide,
titania, calcium
fluoride, rare earth doped lanthanum chlorides, rare earth doped lanthanum
fluorides, yttrium
orthophosphate, terbium phosphate, and the like. Magnetic single crystal
compounds include
manganese zinc ferrite, strontium ferrite, barium ferrite, yttrium iron
garnet, samarium cobalt
alloys, neodymium-iron-boron alloys and the like.
In one embodiment of the present invention, the Single-Crystal Particles are
grown to a
larger size so that the entire microstructure consists of aligned Single-
Crystal Particles. The
Single-Crystal Particles are considered "seeds" if they are grown to a larger
size via solid state
or liquid phase sintering. . In order to grow the seeds larger, there must
always be a substantial
size difference between the nutrient particles and the Single-Crystal
Particles. The Single-
Crystal Particles are much larger than the nanoparticles of the nutrient
matrix, which are less than
one micron in size and preferably less than 0.1 micron in size. This size
difference enables the
Single-Crystal Particles to grow at the expense of the nutrient particles. If
the nutrient matrix
nanoparticles and the Single-Crystal Particles were comparable in size, then
growth of the Single-
Crystal Particles would not consume the nutrient matrix and the fraction of
aligned material
would not be increased.
The Single-Crystal-Like polycrystalline materials of the present invention are
prepared
by packing and aligning a plurality of Single-Crystal Particles into an
aligned array, and then
bonding the particles together. Packing and aligning can be done in many ways.
There are
physical methods such as the use of screens and micromolds, which provide a
mechanism to
achieve one, two and three-dimensional alignment. These methods use a physical
template,
wherein Single-Crystal Particles can be passed through an opening of
comparable dimension and
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having a controlled shape to precisely position and align the particles. The
particles can be
settled onto an adhesive-coated surface that secures the particles thereto
upon contact of the
dominant face of the Single-Crystal Particle with the adhesive.
A simpler approach settles the particles on a flat surface, thereby aligning
the particles
one-dimensionally perpendicular to the surface. In this case, the
crystallographic alignment of
the particles is randomized in the plane of the substrate and the alignment is
one dimensional.
For some systems such as piezocrystals, this is all that is necessary in order
to get the required
Single-Crystal-Like performance from the polycrystalline materials of the
present invention.
Three-dimensional alignment, however, maximizes the capture of Single-Crystal-
Like properties.
Other packing and aligning methods include self-assembly methods.
Electrostatic or
magnetic forces may be used to align the Single-Crystal Particles. Another
approach is to coat
the seed crystals with wax in a manner such that each crystal is
unagglomerated. The wax-coated
single-crystal-particles can be mixed with water to make a suspension that can
be poured onto
a heated surface. When the water/wax-coated-Single-Crystal Particle suspension
becomes hot,
the wax layer become molten and serves to bond the dominant crystal faces of
the particles
together as they approach one another. Essentially any means by which
alignment of the particles
can be obtained is suitable for use with the present invention.
Microarrays may also be packed and aligned from suspensions of Single-Crystal
Particles
processed at volume fractions of about 0.01 to about 80% in a liquid. High
volume fractions are
preferred, i.e., about 50% or greater. The suspension is lightly agitated to
increase packing
density and to order the Single-Crystal Particles. The liquid is then decanted
and the structure
is dried. As drying proceeds, surface tensional forces, which arise during
drying, further
consolidate the Single-Crystal Particles using the dominant crystal faces to
align their
orientations with one another.
2~ A particularly preferred approach uses two immiscible liquids and can be
used to prepare
monolayers of essentially any small particle. A particle dispersion is
prepared at particle volume
fractions of about 0.01 to about 80%. Light agitation is used with Single-
Crystal Particles to
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increase packing density and to order the Particles. The dispersion is
contacted with the surface
of an immiscible supporting fluid, preferably by incremental addition.
The particle dispersion is prepared by dispersing the particles in a mixture
of (1) a
dispersant liquid that aids in the dispersion of the particles and as is it is
removed provides a
spreading tension that causes a film of the dispersion to spread on the
surface of the supporting
fluid and (2) a capillary liquid that acts on the particles to form the
particle monolayer, drawing
the particles together using the immiscible lower fluid as a lubricating
surface. The lubricating
action of the surface contributes to the alignment of the Single-Crystal
Particles, permitting the
particles to freely rearrange by rotation and translation.
Particle dispersions will contain between about a 1:99 and about a 99:1 volume
ratio of
dispersant liquid to capillary liquid. About a 50:50 ratio is typically
preferred. In general, the
amount of dispersant liquid must be sufficient to supply an adequate particle-
spreading force
before significant removal occurs, especially when the dispersant liquid is
volatile and is
removed by evaporation.
The supporting fluid is typically water. However, other high surface tension
fluids on
which particles can float without wetting, such as non-wetting metals with low
melting points
(e.g., gallium, mercury, and the like) can be used to support the particles,
so that the particles can
rearrange on the liquid metal surface. The surface tension must be sufficient
to support the
particles.
The solvents of the particle dispersion are selected so that a dispersion is
provided with
a density lower than that of the supporting fluid, which wets the particles,
is immiscible with and
does not react with the underlying supporting fluid layer, and draws the
particles together. When
the particles are Single-Crystal Particles, the dispersion interacts with the
dominant crystal faces
of the Single-Crystal Particles to form a consolidated, dense, aligned
structure.
Thus, when the supporting fluid is water, the capillary liquid is selected
from non-polar
immiscible hydrocarbon solvents selected for being lighter than and immiscible
with water to
float the particles thereon. The hydrocarbon solvents may be selected from
single alkanes and
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mixtures thereof capable of drawing the particles together by capillary action
as the dispersant
liquid is removed, examples of which include mineral oil, decalin, decane,
hexane, and the like.
According to one embodiment of the inventive method, the capillary liquid may
be
functionalized, for example, with moieties that polymerize upon exposure to UV
light, or with
other functional groups that interact with the particles. The capillary liquid
may also contain
nano-sized particles that a hierarchic structure with larger particles in the
dispersion as the
dispersant liquid evaporates.
The particle dispersion components must be selected so that the particles are
incapable
of being wetted by the supporting fluid. This may be an inherent property of
the particle, or the
particle may be coated to prevent wetting, such as with a hydrophobic
surfactant when the
supporting fluid is water. In the alternative, either the dispersant liquid,
the capillary liquid, or
both, may interact with the particle surface to prevent particle wetting by
the supporting fluid.
When the supporting fluid is water, the dispersant liquid is a polar organic
solvent such
as ethanol, isopropanol, acetone, and the like. Without being bound by any
particular theory, it
is believed that molecules of the dispersant liquid attach themselves to the
surface of the particles
via their polar end, leaving the hydrocarbon non-polar end of the dispersant
liquid molecules free
to intermix with the non-polar capillary liquid, thereby preventing particle
wetting and aiding in
the dispersion of the particles in the particle dispersion. This is in
addition to the dispersant
liquid molecules providing the spreading tension that spreads a film of the
dispersion across the
surface of the supporting fluid as the dispersant liquid is removed..
For example, when the dispersant liquid is isopropanol, the aprticles are PZT
and the
capillary liquid is mineral oil, the alcohol group bonds to the PZT surface,
thereby presenting the
organic part of the isopropanol structure to the particle exterior and coating
the particle with the
non-polar portion of the molecule, thereby changing the nature of these
surfaces from hydrophilic
to hydrophobic. When a small amount of mineral oil is added to a mixture of
PZT particles in
isopropanol, the adsorbed isopropanol molecules preferentially interact with
mineral oil
molecules to trap a layer of mineral oil on the cube surfaces (i.e. the
mineral oil wets the coated
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PZT particles), thereby increasing their hydrophobic character.
The mineral oil (being both lighter than water and immisible in water) helps
to float the
PZT particles on the supporting fluid. As the particle dispersion touches the
waterlair interface,
the spreading tension of the isopropanol causes it to spread across the water
surface. The
5 spreading tension provides a force that compresses the PZT particles against
the container walls
and against each other. As the particles come into close proximity with each
other, the mineral
oil (with its relatively high surface tension and good wetting of the alcohol-
coated particles)
provides a strong capillary force that stabilizes the microarray. The system
energy is minimized
when the PZT particles are oriented face-to-face and in contact with one
another in the plane of
10 the film. Thus, a microarray of densely packed particles forms, and floats
like a raft on the water's
surface. Domains within a PZT cube microarray are readily observed in Figure
3, where PZT
cubes are packed by the inventive process with their lateral faces in contact.
The above example shows how the inventive process can be performed with
conventional
fluids that can be readily found in any household. It is also possible to use
alternative fluids, such
15 as molten metals, glasses, polymers, monomers, and inorganic melts that
crystallize or solidify
upon cooling. Thus, it should be possible to draw thin layers of a variety of
different materials.
For instance, it is possible to fabricate particulate reinforced glass sheets
where silicon carbide
could be dispersed in a borosilicate glass to make a high strength tough
glass. A metal matrix
composite can be made with silicon-coated alumina dispersed in a matrix of
aluminum to make
a high strength metal matrix composite. A PZT powder can be dispersed in a
matrix of
polyurethane) to make a transducer material. Copper powder could be dispersed
in
polyethylene) to make a conductive tape. It should be mentioned that the
particulate phase in the
material could be randomly oriented or oriented with a high degree of texture
depending on the
morphology of the particulate phase and whether or not each particle is a
single crystal. Thus the
method of the present invention is not necessarily limited to methods in which
oriented or Single-
Crystal Particle films are prepared. The particles need not be Single-
Crystals, and even if Single
-Crystals, the Particles need not be oriented. Instead, the inventive method
can be used to make
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essentially any monolayer particle film dispersed in a matrix formed by
removal of the dispersant
liquid of the particle dispersion.
This method can also be used to make hierarchic structures. For example,
spherical,
micron-sized particles can be packed as a monolayer on film, and the capillary
liquid can contain
nanoparticles that are orders of magnitude smaller, that will pack in the
intersticies of the larger
particles as the dispersant liquid is removed. The net result is a film with
dense hierarchic
particle packing. The method of the present invention can be used to pack
cubes very densely to
achieve exceptional particle packing fractions of 80 volume percent and
greater.
There are also many methods suitable for bonding together an array of Single-
Crystal
Particles. For example, the voids of a particle array can be filled with a
nanoparticle suspension
by casting the suspension on top of the array. Single-Crystal Particles can
also be co-mixed with
nanoparticles using a high volume fraction of Single-Crystal Particles and a
smaller fraction of
nanoparticles that act as a lubricating and binding phase that helps the
Single-Crystal Particle
ensemble organize while maintaining adhesion between the Single-Crystal
Particles.
Single-Crystal Particles of the foregoing compounds are prepared by
essentially
conventional means, such as by precipitation from molten salt solvent or
hydrothermal solutions,
microwave-hydrothermal synthesis, vapor phase synthesis, aqueous
precipitation, precipitation
from homogeneous solution, sonochemistry, spray pyrolysis, biomemitic
processing, emulsion
processing, microemulsion processing, plasma synthesis, and the like. The
particles are then
aligned as described above, and bonded together, either by filling the voids
therebetween with
nanoparticles of the same or different material or polymer, and then heating
the composite
structure. The nanoparticles are prepared by physical grinding, by
conventional sol-gel
techniques or other techniques well known to those skilled in nanoscience.
Monolithic polycrystalline composite materials according to the present
invention are
prepared by heating the packed, aligned and filled Single-Crystal Particle
array to a temperature
at which solid state diffusion between the nanoparticles and the Single-
Crystal Particles occurs,
until the Single-Crystal Particles of the, array are bonded together. To
obtain a level of solid state
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17
diffusion sufficient to bind together adjacent particles, a temperature above
about half of the
melting temperature is required, which is most generally in the range 200
°C to 2000 °C. The
temperature range selected will depend upon the material being bonded, but can
be readily
determined by those of ordinary skill in the art without undue experimentation
within the defined
range. For example, a temperature up to 2000 °C is suitable for ceramic
oxides. Temperatures
as low as 200 °C can be used to sinter and grow as grains materials
such as fluorides.
Temperatures even as low as 150 °C may be used, but with high pressure
(e.g., on the order of
gigapascals) to sinter materials that densify at high temperatures.
To induce grain boundary mobility, a temperature higher than that employed for
sintering
is required. For example, the conditions used for processing polycrystalline
ceramics may
require a temperature between 1000 °C and 1500 °C. Conventional
crystal growth methods
typically will require much higher temperatures to melt oxides to single phase
liquids. The
present invention does not melt the material, but can, in some cases, form a
liquid phase that
coexists with the solid phase and thus, makes the material partially molten
(liquid phase
sintering). For example, lead oxide can be used to dissolve PZT and
recrystallize it onto the seed
Single-Crystal Particles. Other liquid phases can be envisioned, as long as
their melting
temperature is below that of the seed crystals and nutrient, and they can
dissolve and recrystallize
the nutrient and redeposit it onto the seed Single-Crystal Particles. In some
cases, it is desirable
to densify the entire structure so that there are no pores therein, and then
let the Single-Crystal
Particles grow through consumption of the nutrient by solid state processes
(i.e., no liquid phase,
but rather solid phase, sintering), or even by using a liquid phase. In other
circumstances, the
material may densify while the grains are growing, that is, the porosity
disappears as the Single-
Crystal Particles grow. It should be noted that the nutrient nanoparticles can
differ
compositionally from the seeds, e.g., the seeds can be strontium titanate
cubes while the nutrient
nanoparticles are PZT. Or, the seeds can be PZT and the nutrient nanoparticles
can be lead
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1g
magnesium niobate doped with lead titanate. These are just two examples of how
the seeds and
the nutrient nanoparticles can differ compositionally.
No matter how the polycrystalline material is formed, the preparation steps
may be
repeated to form a polycrystalline material having multiple layers of aligned
cube-shaped
particles. The resulting material is essentially a three-dimensional object
with one , two, or three
dimensional alignment of Single-Crystal Particles within.
The preparation of a polycrystalline array with one-dimensional alignment is
shown in
FIG. 1. Single-Crystal Particles 12 are aligned in one crystallographic
direction, i.e., normal to
the plane of surface 14. The crystallographic orientation of the particles in
the other two-
dimensions is completely randomized. For purposes of illustration, particles
12 are PZT Single-
Crystal Particles, deposited on a flat substrate, which can be any type of
polymer material such
as MylarTM or any type of metal such as platinum or any type of ceramic such
as alumina. An
adhesive (any kind) can be used to anchor the particles down, or even the
surface tension of
residual processing liquid, such as water, can accomplish this task. Voids 16
between the
particles 12 are then filled with PZT nanoparticles 18. The nanoparticles can
be prepared by the
method disclosed by Das et al., "Low Temperature Preparation of Nano-
crystalline Lead
Zirconate Titanate Using Triethanolamine," (J. Am. Ceram. Soc.), 81(12), 3357-
60 (1998). The
assembly 20 is then heated t a temperature of 1200 °C for 2 hours,
resulting in growth of the
Single-Crystal Particles 12 by consumption of the nanoparticles 18 until
impingement of adj acent
grain boundaries 22 occurs. This produces a polycrystalline material 24
consisting of
piezoelectric Single-Crystal Particles aligned in one crystallographic
direction but producing a
net Single-Crystal-Like piezoelectric effect.
The preparation of a polycrystalline material 124 consisting of cubic Single-
Crystal
Particles 112 aligned in all three crystallographic directions is shown in
FIG. 2. As in FIG.1, the
particles 112 are aligned in one crystallographic direction normal to the
plane of surface 114.
However, the particles, again PZT cubes, have been aligned three-dimensionally
by being passed
through a photomask such as those used in semiconductor manufacturing (not
shown). The voids
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19
116 between the Single-Crystal Particles 112 are again filled with PZT
nanoparticles 118 and the
assembly 120 is again heated at 1200 °C for 2 hours to obtain growth of
the Single-Crystal
Particles by consumption of the nanoparticles until impingement of adjacent
grain boundaries
122 occurs. However, because of the three-dimensional alignment, exceptionally
low grain
boundary angles are obtained between individual Single-Crystal Particles,
significantly enhancing
a. the net Single-Crystal-Like behavior in the polycrystalline material 124.
A polymer binder can be substituted for the nanoparticles 18 or 118 to obtain
a polymer-
ceramic composite that exhibits a net Single-Crystal-Like effect due to the
oriented Single
Crystal Particles. Powder particles of polymer may be employed, which are then
heated to melt
or sinter the polymer powder to form a binder bonding together the Single-
Crystal Particles. Or
a solvent solution of polymer may be used to fill the void between the Single-
Crystal Particles,
which is then heated to evaporate the solvent and form a polymer binder
bonding together the
individual Single-Crystal Particles. For example, a composite structure may be
prepared by
infiltrating microarrays with a 3% acetone solution of a polymer such as
poly(methylmethacrylate), epoxy resin (Envirotex Lite from Environmental
Technology, Inc.),
polyurethane (9130A prepolymer/9130B curative from Epoxical, Inc.), and the
like. The Single-
Crystal Particles may also be packed and aligned onto the surface of an
adhesive-coated polymer
film or embedded into the surface of molten melted polymer film.
When a polymer binder is employed, the net Single-Crystal-Like effect is
obtained
independent of the polymer properties. Essentially any thermoplastic or
thermosetting polymer
may be used as the polymer binder, as well as any polymer that is capable of
being sintered to
form a binder bonding the Single-Crystal Particles together. Because the net
Single-Crystal-Like
effect is independent of the polymer properties, the polymer is instead chosen
for the end use
property desired for the polycrystalline composite. Thus, the polymer can be
chosen as a passive
component (e.g., a structural polymer can provide strength to a composite
comprised of electro-
optical Single Crystal Particles for use as an optical switch) and as an
active component (e.g.,
an electrostrictive polymer can be used for a a composite comprised of
piezoelectric Single
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Crystal Particles for an actuator). For instance, a polymer capable of forming
a flexible film, such
as poly(urethane), poly(vinylidene fluoride), and the like, may be chosen as a
matrix in which
piezoelectric Single= Crystal Particles can be packed and aligned to form a
transducer useful to
make a speaker having a micron-dimensioned thickness capable of being rolled
up and applied
5 as a film. Polymers may also be chosen for end-use properties such as
rigidity, impact-resistance,
heat resistance, cold resistance, optical transparency, electrical
resistivity, and the like.
It will thus be appreciated that the present invention can be extended to
essentially any
present and future end-use for a single crystal. Essentially any Single-
Crystal Particle having
utility in a mechanical, electronic, electromechanical, optical or magnetic
end-use application
10 can be employed in the present invention to produce a polycrystalline
composite material of
macroscopic dimensions having the same utility as a single crystal. In
addition to the
piezoelectric applications for speakers and microphones discussed above,
piezoelectric materials
are also widely used in transducers, in general, as well as in dynamic random
access memories
(DRAMs), decoupling capacitors, acoustic sensors, optical filters, actuators
and modulators.
15 Polycrystalline composite replacements for the single crystals used in
laser and photon detectors
can also be prepared. Optical shutters can be prepared using polycrystalline
composites of PLZT,
which turns opaque black upon the application of a voltage.
Doped semiconductors for microelectronics can be prepared using Single-Crystal
Particles of silicon can be doped to be p- or n-type semiconductors.
Photorefractive materials
20 for optical switching and memory storage can be prepared using oriented
Single-Crystal Particles
of Rh-doped barium titanate. Optical lasers can be prepared using oriented
Single-Crystal
Particles of doped sapphire.
The following non-limiting example set forth hereinbelow illustrates certain
aspects of the
invention. All parts and percentages are by weight unless otherwise noted and
all temperatures are in
degrees Celsius. The stoichiometric values for the Single-Crystal Particle
materials are approximate.
EXAMPLE
PREPARATION OF PZT SINGLE-CRYSTAL PARTICLE ARRAY
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Materials. All water used was de-ionized water purified using a MILLIPORE
system. Nitric acid
and sodium hydroxide were of analytical grade and directly used as received.
Sodium polyacrylate have
a weight-average molecular weight of 5100 daltons was purchased from Fluka Co.
and used as received.
Cube-shaped PZT particles were synthesized using a PARR pressure reactor
according to the procedure
disclosed by Cho, et al. "Hydrothermal Synthesis of Acircular Lead Zirconate
Titanate (PZT)," J. Cryst.
Growth, 226 2-3 , 313-326 (2001).
The PZT particle size distribution was measured using a MICROTRAC 9200 Full
Range
Analyzer (Leeds & Northrup). Particle morphology was determined using Field
Emission
Scanning Electron Microscopy (FESEM, LEO Electron Microscopy, Inc.). The
particle size
analysis indicated that the PZT particles had a mean volume diameter of 4.45
microns, a mean
number diameter of 3.32 microns and a standard deviation of 1.37. The
calculated specific
surface area of the PZT particles was 1.51 m2/g. The PZT particles were
dispersed into water and
ultrasonicated for two minutes before being loaded into the particle size
analyzer.
An electroacoustic analyzer (MATEC 8000, Matec Applied Sciences) was used to
measure the
Zeta potential of the PZT particles suspension, the pH of which was adjusted
using 0.01 M HNO3 and
0.01 M NaOH as titrants. After each addition of base or acid, fifteen minutes
was allowed for the entire
solution to equilibrate to a stable pH value. Several aqueous
sodiumpolyacrylate solutions with different
concentrations were prepared under vigorous stirnng under room temperature.
The polymer solutions,
used soon after the polymer completely dissolved, were added to a suspension
of 22 g PZT particles into
220 mL water for obtaining the function of the Zeta potential against polymer
concentration. A
suspension containing 22 g PZT powder and 27 g sodiumpolyacrylate powder
dispersed in 220 mL water
was vigorously stirred for four hours and then used for the Zeta potential
measurement for obtaining the
function of the Zeta potential again pH.
4 g of PZT powder and 4.92 g of sodium polyacrylate powder were added to 40 mL
water
and vigorously stirred for four hours. The suspension was divided into several
portions and the
pH of each portion was titrated using fresh ammonium hydroxide solution (NH3,
28.5 wlw°7o,
Fisher Scientific) and 0.01 M HN03. Suspensions of pH 3.6, 6.0, 8.4, 9.7 and
11.0, measured
with a small pH-meter (Fisher Scientific, Model 505 MP), were transferred onto
glass slides
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22
separately. The surfaces of each glass slide were first washed with alcohol
and acetone and finally
washed with water. The cleaned glass slides were carefully dried and checked
with an optical
microscope to ensure that there was not any dust contaminants on the surfaces.
Samples were air dried
under a hood and covered during the drying process to prevent dust
contamination. Samples made from
suspensions of polymer were also prepared for comparison to samples containing
polymer.
Polymer suspension samples having a pH of nine and above formed one-
dimensional particle
arrays. The particles were well-arranged, forming a planar array of very
smooth surfaces, with virtually
no defects observed. The particle array was of homogenous thickness, about 6.5
microns thick. The high
degree of alignment improves electromechanical performance, including
piezoelectricity, among other
properties.
It will thus be appreciated that the inventive method is useful for processing
nanopowders and
micron-size powders into ultra-thin monolayers, down to micro- and nanometer
sizes. The only
restriction on powder particle size arises from gravity. If the particles are
too heavy they will not remain
at the fluid interface but fall to the bottom of the container.
The ability to process small particle sizes is useful, for example, in the
manufacture of multilayer
capacitors where the process enables dielectric layers of unprecedented
thinness. However, the
applications can be numerous for many different materials where thin layers of
particulate materials are
required, where the particulates can be metallic, ceramic, or polymeric. Thus,
in addition to a wide range
of electronic applications, there are optical, magnetic, catalytic,
structural, and many other application
areas for materials made from the present invention.
The present application claims priority benefit of U.S. Provisional
Application No. 60/380,353
and U.S. Patent Application No. 10/145,372. Both Applications were filed on
May 13, 2002, and the
disclosures of both Applications are incorporated herein by reference.
The foregoing example and description of the preferred embodiment should be
taken as
illustrating, rather than as limiting the present invention as defined by the
claims. As will be readily
appreciated, numerous variations and combinations of the features set forth
above can be utilized
without departing from the present invention as set forth in the claims. All
such variations are intended
to be included within the scope of the following claims.
