U.S. patent application number 10/568288 was filed with the patent office on 2007-09-27 for electrophoretic method for the production of ceramic structures.
This patent application is currently assigned to Forschungzentrum Karlsruhe GmbH. Invention is credited to Melanie Dauscher, Jurgen Hausselt, Holger Von Both.
Application Number | 20070221500 10/568288 |
Document ID | / |
Family ID | 34201583 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070221500 |
Kind Code |
A1 |
Hausselt; Jurgen ; et
al. |
September 27, 2007 |
Electrophoretic Method for the Production of Ceramic Structures
Abstract
The invention relates to a method for producing ceramic
structures and gradient structures whose particle size distribution
has at least partly smaller values than the particle size
distribution of the suspension from which the same are produced.
According to the inventive method, a suspension containing ceramic
particles is provided between a couple of electrodes, whereupon an
electric field is applied to the couple of electrodes while the
electrode surfaces of the couple of electrodes are placed
perpendicular to a component of a gravitational field such that a
fraction of the particles is deposited on the electrode of the
couple of electrodes, which is located upstream of the component of
the gravitational field, in the form of a ceramic structure.
Inventors: |
Hausselt; Jurgen;
(Germersheim, DE) ; Dauscher; Melanie; (Bamberg,
DE) ; Von Both; Holger; (Pforzheim, DE) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20043-9998
US
|
Assignee: |
Forschungzentrum Karlsruhe
GmbH
Karlsruhe
DE
D-76021
|
Family ID: |
34201583 |
Appl. No.: |
10/568288 |
Filed: |
August 5, 2004 |
PCT Filed: |
August 5, 2004 |
PCT NO: |
PCT/EP04/08768 |
371 Date: |
February 6, 2007 |
Current U.S.
Class: |
204/490 |
Current CPC
Class: |
C25D 13/02 20130101 |
Class at
Publication: |
204/490 |
International
Class: |
C25D 13/02 20060101
C25D013/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 16, 2003 |
DE |
103 37 688.7 |
Claims
1. A method for producing ceramic structures by applying an
electric field between the electrodes of respectively one electrode
pair, submerged in a suspension that is positioned in a
gravitational field and contains ceramic particles in a
distribution of particle sizes, such that on one of the electrodes
of the respective electrode pair only the particle size fraction of
the ceramic particles is deposited, which is smaller than a
critical particle size that results from the balance between the
forces of the electrical field and the forces of the gravitational
field.
2. The method for producing ceramic structures as defined in claim
1, characterized in that the electrodes of the respective electrode
pair are arranged parallel to each other and horizontal in the
gravitational field of the earth and that the ceramic structure is
deposited on the upper electrode.
3. The method for producing ceramic structures as defined in claim
1, characterized in that the gravitational field is generated with
the aid of a rotating centrifuge.
4. The method for producing ceramic structures as defined in claim
3, characterized in that the electrodes of the respective electrode
pair are arranged parallel to each other and perpendicular to the
direction of the gravitational field, generated by a rotating
centrifuge, and that the ceramic structure is deposited on the
inner electrode.
5. The method for producing ceramic structures as defined in one of
the claims 1, characterized in that the electrical field and the
gravitational field can be varied independent of each other with
respect to intensity and time.
6. The method for producing ceramic structures as defined in one of
the claims 1, characterized smaller values for the distribution of
particle sizes in the ceramic structure than the distribution of
particle sizes in the suspension.
7. The method for producing ceramic structures as defined in claims
1, characterized in that the ceramic particles comprise: structural
ceramics such as Al.sub.2O.sub.3, ZrO.sub.2, mullite, SiC,
Si.sub.3N.sub.4 and/or functional ceramics such as BaTiO.sub.3 or
lead-zirconate-titanate (PZT) and/or bio-ceramics such as hydroxyl
apatite (Ca(OH) (PO.sub.4).sub.3) and/or mineral glass.
8. The method for producing ceramic structures as defined in claims
1, characterized in that the suspension contains at least two
different types of ceramic particles.
9. The method for producing ceramic structures as defined in claims
1, characterized in that the ceramic particle sizes range from 5 nm
to 500 .mu.m, preferably from 10 nm to 100 .mu.m, and especially
preferred from 10 nm to 10 .mu.m.
10. A ceramic structure produced in accordance with claims 1.
11. A ceramic gradient structure produced in accordance with claims
5.
Description
[0001] The invention relates to a method for producing ceramic
structures (for example layers, filters, or micro-structures), as
well as to structures and gradient structures produced with this
method.
[0002] Ceramic (micro-) structures, ceramic layers, and
two-dimensional structures such as tiles, substrates, or filters
are gaining in importance in many fields of technology. In
particular, this is true for the so-called structural ceramics such
as Al.sub.2O.sub.3, ZrO.sub.2,mullite, SiC, Si.sub.3N.sub.4, the
functional ceramics such as BaTiO.sub.3 or PZT
(lead-zirconate-titanate), and the so-called bio-ceramics, such as
hydroxyl apatite Ca(OH) (PO.sub.4).sub.3, but also for mineral
glass materials. Depending on the form, size and area of use for
the parts or layers to be produced, the following production
methods are used: dry pressing, powder-technological injection
molding, hot molding, slip casting, foil casting, electrophoretic
deposition from powder suspensions, and other methods which are
followed by a sintering step.
[0003] All known methods have in common that so-called feedstocks
are used for the molding, wherein these feedstocks consist of
ceramic powders and binders, dispersing agents, as well as slip
additives for improving the workability. With the pressing methods,
only small percentages by volume of these additives are added to
the powders. With the injection molding, hot molding, slip molding,
and foil casting methods, on the other hand, much higher
percentages by volume of binders, dispersing agents, slip agents,
polymers, waxes and suspension liquids such as water and alcohol
are added. With these methods, the powder share ranges from 30 to
70 percentages by volume. For the electrophoretic deposition from
watery or alcoholic suspensions, the percentage by volume of
ceramic powder can range from approximately 5% to 50%.
[0004] All methods furthermore have in common that the distribution
of particles in the powder for the so-called green product is
approximately the same as in the starting powder, the slip, the
feedstock, or the suspension. In general, powders are used which
have a relatively broad distribution in the form of so-called
mono-modal powders, wherein these frequently follow normal
distributions, logarithmic normal distributions, or so-called
Rosin-Rammler distributions. In part, powders are also used which
are present in the form of complex multi-modal distributions.
[0005] The roughness of the produced parts and layers, as well as
their pore sizes and to some degree also their structure following
the sintering, are influenced by the distribution of particle
sizes. For example, the rough components of the powder that is used
determine the surface roughness. The distribution of pore sizes in
the filter membranes is correlated to the particle size: the larger
the powder particles, the larger the pores that develop. For that
reason, only particles below a specific size, e.g. 500 nm, can
therefore be used with traditional production methods to achieve,
for example, especially smooth layers or micro-structures or to
obtain extremely small pore sizes. For this, the powders must first
be fractionated and graded in a complicated manner before the
starting feedstock is produced, for example by means of screening
or wind-sifting, and only the desired powder fraction should be
introduced into the feedstock.
[0006] For cost reasons, these additional and very involved
processing steps are out of the question for most applications.
Particularly smooth layers with peak-to-valley heights below 1
.mu.m and micro-structures with surface details in the .mu.m range
can therefore not be produced by using traditional, commercially
available powders with powder sizes that are generally in the range
above 1 .mu.m.
[0007] Starting from this point, it is the object of the present
invention to provide methods for producing ceramic structures which
are not subject to the aforementioned disadvantages and
restrictions.
[0008] This object is solved with the method as defined in claim 1.
The dependent claims describe advantageous embodiments of the
invention.
[0009] The method according to the invention is based on combining
the electrophoretic deposition and the sedimentation due to
gravitational forces and/or centrifugal forces. The electrophoretic
deposition of ceramic particles from particle suspensions as a
method for producing ceramic layers is known (Heavens, S.N.:
Electrophoretic Deposition as a Processing Route for Ceramics; in
Binner, J. (Ed.), Advanced Ceramic Processing and Technology, Vol.
1, Noyes Publ., Park Ridge, N.J. U.S.). Attempts have recently been
made to use this technique also for realizing ceramic
micro-structures (Both, H. von; Hausselt, J.: 1.sup.st Intern.
Conf. on Electrophoretic Deposition, Banff, Canada, 2002). For
this, an electric field is applied between two electrodes submerged
in the powder suspension, causing a flow of charged particles to
move toward one of the two electrodes and causing the particles to
be deposited thereon.
[0010] The principles of electrophoresis have long been known.
Corresponding theoretical descriptions disclose that in the size
range for ceramic powders used for technical applications, meaning
in the range between 10 nm and 100 .mu.m, the electrophoretic
mobility of the powder particles for the most part does not depend
on their size (Nitzsche, R.; Simon, F.: "TECHNISCHES MESSEN"
[Technical Measuring] Volume 64, pages 106-113, 1997). All particle
sizes occurring in the suspension should therefore be deposited
with substantially the same speed on the electrically conductive
substrate. The deposited layers consequently should have the same
distribution of particle sizes as the suspension, albeit packed
with substantially higher density.
[0011] Our own measurements confirm that the migration rate V.sub.E
in the electric field E depends not only on the suspension medium
(e.g. watery or alcoholic), the chemical composition of the powder
(e.g. Al.sub.2O.sub.3, ZrO.sub.2, SiO.sub.2), and the added
dispersing and binding agents, but can also depend to a small
degree on the existing particle size. Depending on the system, it
is possible to observe different, but always small, dependencies of
the migration rate on the particle size.
[0012] Experiments with alcoholic Al.sub.2O.sub.3 suspensions have
shown that smaller particles are deposited slightly faster than
larger ones, but that this effect is not sufficient for a technical
use, for example for the electrophoretic in-situ-fractionizing. For
the electrophoretic deposition, the mobility .mu. and the migration
rate v.sub.E consequently depend on the particle size (radius r) in
the electric field E, wherein: dv.sub.E/dr.ltoreq.0 (1a) and, owing
to v.sub.E =.mu.E d.mu./dr.ltoreq.0 (1b) applies.
[0013] In watery suspensions with spherical SiO.sub.2 particles
having diameters between 200 nm and 1200 nm, on the other hand,
larger particles will be deposited slightly faster than finer
particles. Thus, for the electrophoretic deposition, the mobility
.mu. and the migrating rate v.sub.E depend on the particle size
(radius r) in the electrical field E, wherein: dv.sub.E/dr.gtoreq.0
(1c) and, owing to of v.sub.E =.mu.E d.mu./dr.gtoreq.0 (1d)
applies.
[0014] For a number of applications, it is desirable to have a
deposition of only specific fractions, such as the share of fine
particles in a predetermined distribution of particle sizes. For
the above-described reasons this cannot be achieved with the
electrophoretic deposition according to the prior art. The known
technique of electrophoretic deposition from powder suspension
furthermore cannot be used for applications using a single
deposition and without changing the powder suspension, for which
initially only rough particles are deposited and, with progressing
time and layer thickness, smaller powder particles are deposited
gradually or continuously.
[0015] According to the invention, a field which causes a particle
speed that depends on the particle size is superimposed on the
electrical field which causes a particle speed that is mostly
independent of the particle size in the direction of the electrical
field. For this type of application, the deposit by sedimentation
which depends on the particle size is suitable, either in a
constant, locality-independent gravitation field (gravity
sedimentation) or in a variable and locality-dependent gravitation
field (centrifuging).
[0016] This is in contrast to the standard electrophoresis where
the effective gravitational force resulting from the earth's
gravitational field, which is undesirable for specific
applications, is eliminated with the aid of suitable measures such
as a special arrangement of the electrodes and especially by
stirring the suspension.
[0017] For individual spherical particles with a radius r and a
density p, which are dispersed (suspended) in a liquid with the
density p.sub.F and the viscosity .eta., this results in a
sedimentation rate v based on the Stokes law, under the effect of
an acceleration b: 6.pi..eta.rv=(4.pi./3)r.sup.3(p-p.sub.F)b
(2)
[0018] The sedimentation rate v is therefore proportional to
r.sup.2 if the viscosity .eta. is constant. Even in cases of
deviations from the equation 2 because of particle shapes which
generally differ from the spherical form and at higher
concentrations for the suspensions, the qualitative relation
remains in place, which indicates that the sedimentation rate
increases with increasing particle size and increasing difference
in the density values. In any case, the following applies: dv/dr
>0. (3)
[0019] If the direction of the migration rate in the electric field
is counter to the sedimentation rate in the gravitational field, a
critical particle size r.sub.c is obtained for each electrical
field intensity E and for each acceleration b in the gravitational
field, wherein the effects of both fields cancel each other out and
the particle is suspended. All particles with r>r.sub.c move in
the direction of the gravitational field while all particles with r
<r.sub.c move in the direction of the electrical field.
Depending on the selection of the electrical field intensity E and
the acceleration b (e.g. by varying the speed in a centrifuge),
freely selectable fractions of the original distribution of
particle sizes can for the most part be deposited on an
electrically conductive substrate.
[0020] In general, the angle between the directions of the
electrical field and the gravitational field is selected such that
a speed component which depends on the particle size can be added
or subtracted for a mostly particle-size independent speed
distribution in the electric field. The effect of the constant and
locality- independent acceleration due to gravity by itself makes
it possible to ensure that the share of fine particles in a
distribution is advantageously deposited on an electrode by varying
the electrical field intensity and the angle between both field
directions. The electrical and the gravitational fields are
preferably arranged parallel to each other, meaning the electrodes
are positioned substantially perpendicular to the direction of the
gravitational field (e.g. horizontally in the gravity field).
[0021] In contrast to the traditional electrophoresis, our
invention provides that one fraction of the suspended particles is
deposited under the effect of gravity on the upper electrode. The
fraction deposited in the form of a ceramic structure generally is
distinguished in that its distribution of particle sizes differs
from the distribution of particle sizes in the suspension, which is
not the case with the standard electrophoresis. Since the finer
particles are preferably deposited, the distribution of particle
sizes in the ceramic structure has lower values than the
distribution of particle sizes in the suspension.
[0022] According to one preferred embodiment of the invention, the
distribution of the particle sizes to be deposited can be
influenced by freely selecting not only the absolute value, but
also the point in time for superimposing the sedimentation by
gravity on the electrical field. The respectively desired limit for
the deposited size fraction can be adjusted by varying the
electrical field intensity.
[0023] A particularly preferred embodiment of the invention is
obtained by superimposing a gravitational field with variable
absolute value on an electrical field with variable absolute value,
as shown in particular with the centrifuging operation where
centrifugal forces occur. While activated, the generated
gravitational field is directed toward the outside, relative to the
axis of rotation of the centrifuge. As a result of this
arrangement, the share of micro particles in the suspension is
deposited according to the invention in the form of a ceramic layer
on the inner electrode.
[0024] The distribution of particle sizes to be deposited can
furthermore be influenced by not only selecting the absolute values
of the electrical field and the gravitational field of the
centrifugal acceleration over broad ranges, but also by freely
selecting the points in time when both fields are activated and/or
shut down.
[0025] The present invention can furthermore be used for
suspensions (dispersions) containing a mixture of different
particles. If such particle mixtures differ in their specific
electrical charge, their electrophoretic mobility and their
electrophoretic deposition rates also differ. Thus, if such
particle mixtures differ in their density, their sedimentation
rates differ in the gravitational and/or the centrifugal field
since in both cases the sedimentation rate according to the
equation 2 is proportional to the difference between the density of
the particles and the density of the suspension liquid.
[0026] Superimposing a gravitational field on an electrical field
therefore permits a far-reaching influencing of the conditions for
depositing particle mixtures which differ not only in size, but
also in their surface charge and/or density. Furthermore, the
method according to the invention can also be used to separate
particle mixtures which do not differ in particle size, but in
their surface charge and/or density.
[0027] The method according to the invention can be used for
creating ceramic structures by means of a multiple and/or
continuous variation of the electrical field and/or (in the case of
centrifuging) the gravitational field during a deposition, without
having to change the powder suspension, wherein the structures have
a gradient with respect to their composition and/or pore depth.
Ceramic gradient structures of this type are suitable, for example,
for use as filter membranes.
[0028] In addition, the method according to the invention is
suitable not only for producing layers for which the distribution
of particle sizes or, if several different powders are used,
composition can be varied widely, but also for separating
suspensions with an expanded variation range as compared to the
pure deposit by sedimentation or the centrifuging methods.
[0029] The invention is explained further in the following, with
the aid of five exemplary embodiments.
EXEMPLARY EMBODIMENT 1
[0030] An Al.sub.2O.sub.3 layer was produced by superimposing
gravitational sedimentation on electrophoretic deposition. Counter
electrode and substrate were arranged horizontally, meaning both
surfaces of the electrode pair were oriented perpendicular to the
direction of the gravitational field. In order to show that
increasingly finer particles are deposited by means of superimposed
sedimentation, the peak-to-valley height was measured optically
with the aid of a surface measuring device (FRT MicroGlider). For a
comparison, a layer was deposited for which the deposit by
sedimentation was suppressed through stirring, according to prior
art, and the electrodes were arranged vertically.
[0031] An ethanol Al.sub.2O.sub.3 suspension (d.sub.50=1300 nm)
starting batch was prepared, with 30 percent by volume solid
material and a dispersing agent content of 2 percent by mass,
relative to the powder mass. The layers were deposited from this
suspension under the following conditions (in the case of a
horizontal arrangement of the upper electrode): TABLE-US-00001 with
stirring without stirring (200 rpm) electrode arrangement
horizontal vertical current 1000 .mu.A 100 .mu.A deposition time 7
hours 51 minutes 30 minutes average peak-to-valley 80 nm 341 nm
height
[0032] The spacing between electrodes was 13 mm. Respectively four
profiles were analyzed in one ceramic layer and the average value
listed. The result showed a clear reduction in the peak-to-valley
height with superimposed sedimentation, respectively determined on
the basis of DIN 4678 and/or ISO 4287.
EXEMPLARY EMBODIMENT 2
[0033] Using different field intensities, Al.sub.2O.sub.3 layers
were produced by superimposing gravitational sedimentation on
electrophoretic deposition. Counter electrode and substrate were
arranged horizontal, relative to each other. To show that the
particles are separated based on their diameter by varying the
field intensity during the elecrophoretic deposition, the
peak-to-valley height was measured optically with the aid of a
surface measuring device (FRT MicroGlider).
[0034] A batch of an ethanol Al.sub.2O.sub.3 suspension
(d.sub.50=1300 nm) was prepared for this, with 5 percent by volume
solid material and a dispersing agent content of 2 percent by mass,
relative to the powder mass. The layers were deposited from this
suspension under the following conditions: TABLE-US-00002 field
intensity 2500 V/m 250 V/m deposition time 2 minutes 30 minutes
average peak-to-valley 106 nm 74 nm height
[0035] Respectively four profiles were analyzed in one layer and
the average value listed. Based on DIN 4678 and/or ISO 4287, a
higher peak-to-valley height was measured for the layers deposited
at higher field intensities. This result confirms the separation of
particles based on their diameter.
EXEMPLARY EMBODIMENT 3
[0036] A SiO.sub.2 layer was produced by superimposing the
gravitational sedimentation on the electrophoretic deposition.
Counter electrode and substrate were arranged horizontally. The
peak-to-valley height was measured optically with a surface
measuring device (FRT MicroGlider) to show that increasingly finer
particles are deposited with the aid of sedimentation. For a
comparison, an additional layer was deposited for which the
sedimentation was suppressed by stirring the suspension,
corresponding to the prior art, and for which the electrodes were
arranged vertically. A starting batch of ethanol SiO.sub.2
suspension (d.sub.50=15 .mu.m) was prepared, with 5 percentages by
volume solid material and a dispersing agent content of 2
percentages by mass, relative to the powder mass. The layers were
deposited from this suspension under the following conditions:
TABLE-US-00003 with stirring without stirring without stirring (200
rpm) electrode horizontal horizontal vertical arrangement voltage
50 V 10 V 50 V average peak- 1.20 .mu.m 0.12 .mu.m 1.77 .mu.m
to-valley height
[0037] The spacing between electrodes was 13 mm. Respectively four
profiles were analyzed in one layer and the average value listed. A
clear reduction in the peak-to-valley height resulted for the
superimposed sedimentation, detected according to DIN 4678 and/or
ISO 4287.
EXEMPLARY EMBODIMENT 4
[0038] An Al.sub.2O.sub.3 layer was produced according to the
invention by superimposing the gravitational sedimentation on the
electrophoretic deposition. The electrodes were arranged
horizontal. The electrophoretic deposition occurred on the upper
electrode. An optical laser granulometer was used to determine the
distribution of particle sizes in the suspension and in the layer
to show that increasingly finer particles are deposited when
superimposing the sedimentation rather than suppressing the
sedimentation by stirring the suspension.
[0039] For this, an ethanol Al.sub.2O.sub.3 suspension batch was
prepared, with 30 percent by volume solid material and a dispersing
agent content of 2 percent by mass, relative to the powder mass.
The layers were deposited from this suspension under the following
conditions: TABLE-US-00004 with stirring without stirring current
1000 .mu.A 1000 .mu.A total deposition time 11 hours 45 min 11
hours 45 min d.sub.50 in the suspension 1.45 .mu.m 1.45 .mu.m prior
to the deposit d.sub.50 in the suspension 1.50 .mu.m 1.25 .mu.m
after 7 hours d.sub.50 in the suspension 1.61 .mu.m 0.51 .mu.m
after 11 h and 45 min d.sub.50 in the re-dispersed 1.40 .mu.m 0.8
.mu.m layer after 11 hours and 45 min
[0040] In each case, the distribution of particle sizes in the
suspension was analyzed prior to, during, and after the deposition,
as well as in the deposited layer following the re-dispersion in
pure ethanol. The differential and cumulative distributions of
particle sizes and the d.sub.50 values for all measurements show
that starting with a nearly symmetrical distribution of particle
sizes with d.sub.50=1.45 .mu.m and in clear contrast to the
standard electrophoresis, the layer deposited with the method
according to the invention has a noticeably reduced average value
for the distribution of particle sizes as well as a clearly visible
powder share with particle sizes below 500 nm.
EXEMPLARY EMBODIMENT 5
[0041] A PZT layer (lead-zirconate-titanate layer) was produced
according to the invention by superimposing gravitational
sedimentation on electrophoretic deposition. Counter electrode and
substrate were arranged horizontally. To show that increasingly
finer particles are deposited by means of the sedimentation, the
peak-to-valley height was measured optically with the aid of a
surface measuring device (FRT MicroGlider) and based on DIN 4678
and/or ISO 4287. An additional layer was produced for comparison,
for which the deposit by sedimentation was suppressed according to
the prior art by stirring the suspension and for which the
electrodes were arranged vertically.
[0042] A starting batch of an ethanol PZT suspension (d.sub.50=2.5
.mu.m) was prepared, with 5 percent by volume solid material and a
dispersing agent content of 2 percent by mass, relative to the
powder mass. The layers were deposited from this suspension under
the following conditions: TABLE-US-00005 with stirring without
stirring (200 rpm) electrode arrangement horizontal vertical
voltage 5 V 50 V average peak-to-valley 79 nm 142 nm height
[0043] The spacing between electrodes was 13 mm. Respectively four
profiles were analyzed in one layer and the average value listed.
The result showed a clear reduction in the peak-to-valley depth of
the layer with superimposed deposit by sedimentation.
* * * * *