EP1654404A1 - Electrophoretic method for the production of ceramic structures - Google Patents

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.

Description

 ELECTROPHORETIC METHOD FOR PRODUCING CERAMIC STRUCTURES

The invention relates to a method for producing ceramic structures (such as layers, filters or microstructures) and to ceramic structures and gradient structures produced using this method.

Ceramic (micro) structures, ceramic coatings and two-dimensional structures such as plates, substrates or filters are gaining in importance for many areas of technology. This applies both to so-called structural ceramics such as Al ₂ O ₃ , ZrO, mullite, SiC, Si3 ₄ as well as to functional ceramics such as BaTi0 ₃ or PZT (lead zirconate titanate) and to so-called bioceramics such as hydroxylapatite Ca (OH ) (P0 ₄ ) ₃ , but also for mineral glasses. Depending on the shape, size and area of application of the parts or layers to be manufactured, dry pressing, powder technology injection molding, hot molding, slip molding, film casting, electrophoretic deposition from powder suspensions and other processes with subsequent sintering are used as production processes.

All known methods have in common that so-called feedstocks are used for the shaping, which consist of ceramic powders and binders, dispersants and lubricants to improve the processability. In the pressing process, such additives are only added to the powders in parts by volume of a few percent. In contrast, injection molding, hot molding, slip casting and film casting use far higher volumes of binders, dispersants, lubricants, polymers, waxes and

Suspension liquids such as water and alcohol are added. In these processes, the powder proportions are 30 to 70 percent by volume. In the electrophoretic deposition from aqueous or alcoholic suspensions, the volume fractions of the ceramic powder can be in the range from approx. 5 to 50%.

It is also common to all processes that the powder is roughly the same particle size distribution in which they are in the starting powder, in

Slurry, in the feedstock or in the suspension, also in the so-called green part. In general, powders are used which are present as so-called monomodal powders in a relatively broad distribution, which often follow normal distributions, logarithmic normal distributions or so-called Rosin-Rammler distributions. In some cases, powders are also used, which are in the form of complex multimodal distributions.

The roughness of the resulting parts and layers as well as their pore size and sometimes their structure after sintering are influenced by the particle size distribution. For example, the rough proportions of the powders used determine the surface roughness. The pore size distribution, for example of filter membranes, also correlates with the particle size: the coarser the powder particles, the larger the resulting pores. Therefore, in the conventional manufacturing processes, for example, to achieve particularly smooth layers or microstructures or to achieve a very fine pore size, only particles below a certain size, such as. B. 500 nm can be used. For this purpose, the powders first have to be fractionated and classified in a complicated way, for example by sieving or air screening, and only the desired powder fraction should be introduced into the feedstock before the production of the starting feedstock.

For most applications, these additional, very complex process steps are out of the question for cost reasons. With conventional, commercially available powders, which generally contain powder components in the range above 1 μm, it is therefore not possible to produce particularly smooth layers with roughness depths below 1 μm and microstructures with surface details in the μm range.

Proceeding from this, it is the object of the invention to specify methods for the production of ceramic structures which the named Disadvantages and limitations do not have.

This object is achieved by the method according to claim 1. The subclaims describe advantageous embodiments of the invention.

The method according to the invention is based on the combination of electrophoretic deposition and sedimentation due to gravity or centrifugal forces. The electrophoretic deposition of ceramic particles from particle suspensions is known as a process for the production of ceramic layers (Heavens, SN: Electrophoretic Deposition as a Processing Route for Ceramics; in Binner, J. (Ed.), Advanced Ceramics Processing and Technology, Vol. 1, Noyes Publ., Park Ridge, NJ, USA). More recently, attempts have been made to use this technique to realize ceramic microstructures (Both, H. von; Haußelt, J.: l ^st In tern. Conf. On Elektrophoretic Deposition, Banff, Canada, 2002). For this purpose, by applying an electric field between two electrodes immersed in the powder suspension, a particle stream of charged particles is moved to one of the two electrodes and deposited there.

The principles of electrophoresis have long been known. Corresponding theoretical descriptions state that in the size range of technical ceramic powders, i.e. H. between 10 nm and 100 μm, the electrophoretic mobility of the powder particles is largely independent of their size (Nitzsche, R .; Simon, F .: Technisches Messen, Vol. 64, pp. 106-113, 1997). Therefore, all particle sizes occurring in the suspension should be deposited on the electrically conductive substrate at largely the same speed. The deposited layers should therefore have the same particle size distribution as the suspension, albeit in a much denser packing.

Our own measurements confirm that the rate of migration v _E in the electric field E is not only from the suspension medium (e.g. aqueous or alcoholic), from the chemical composition of the powder (e.g.

B. AI ₂ O ₃ , Zrθ2, S1O ₂ ) and depends on the dispersant and binder additives, but that there may also be a slight dependence on the particle size. Depending on the system, different, but in all cases small, dependencies of the migration speed on the particle size can be observed.

Studies on alcoholic Al ₂ θ ₃ suspensions have shown that smaller particles are separated slightly faster than coarse ones, but that this effect can be used for technical purposes, e.g. B. is not sufficient for electrophoretic in-situ fractionation. In this case, electrophoretic deposition results in a dependence of the mobility μ and the rate of migration v _E on the particle size (radius r) in the electric field E, for which dv _r / dr ≤ 0 da; and because of v _E = μE dμ / dr ≤ O (lb) applies.

In aqueous suspensions with spherical Si0 ₂ particles with diameters between 200 nm and 1200 nm, however, coarser particles separate slightly faster than finer particles, so that during electrophoretic deposition the mobility μ and the rate of migration v _{E depend} on the particle size ( Radius r) results in the electric field E, for which dv _E / dr ≥ 0 (lc) and because of v _E - μE dμ / dr ≥ O (ld).

For a number of applications, it is desirable if only agreed fractions such as the fine fraction of a given

Particle size distribution can be separated. With the

For the reasons described above, this is not possible according to prior art electrophoretic deposition. Also

Applications in which one can be separated in a single process without

Changing the powder suspension, for example, initially only deposits coarse powder particles, which gradually or continuously progressively and with increasing thickness and layer thickness, are not possible with the known technique of electrophoretic powder separation.

According to the invention, a field is superimposed on the electric field, which brings about a particle velocity that is largely independent of particle size in the direction of the electric field, and causes a particle velocity that is dependent on particle size. Particle size-dependent sedimentation is suitable for this either in a constant, location-independent gravitational field (gravity sedimentation) or in a variable and location-dependent gravitational field (centrifugation).

This is in contrast to conventional electrophoresis, in which suitable means such. B. the special arrangement of the electrodes and in particular by stirring the suspension, the undesirable effective gravitational force, which results from the gravitational field of the earth, is switched off for certain applications.

For individual spherical particles with a radius r and a density p, which are dispersed (suspended) in a liquid with a density p _F and a viscosity η, a sinking velocity v results under the effect of an acceleration b, which follows the relationship according to Stokes:

Accordingly, the rate of descent v is proportional to r ² at constant viscosity η. Even if due to particle shapes in usually deviate from the spherical shape, and at high concentrated

Suspensions deviations from equation 2, the qualitative relationship remains, which says that the sedimentation rate increases with increasing particle size and increasing difference in density values. In any case, d v / dr> 0. (3)

If the direction of the speed of migration in the electric field is opposite to the rate of descent in the gravitational field, a critical particle size r _c results for each electric field strength E and for each acceleration b in the gravitational field, at which the effects of both fields cancel and the particle floats. All particles with r> r _c move in the direction of the gravitational field, all particles with r <r _c move in the direction of the electrical field. Depending on the choice of the electric field strength E and the acceleration b (for example by varying the speed in a centrifuge), largely freely selectable fractions of the particle size distribution originally present can thus be deposited on an electrically conductive substrate.

In general, the angle between the directions of the electric field and the gravitational field is chosen such that a velocity component that is dependent on the particle size can be added or subtracted from the velocity distribution in the electric field that is largely independent of particle size. Already under the effect of the constant and location-independent gravitational acceleration, variation of the electric field strength and the angle between the two field directions can ensure that the fine fraction of a particle size distribution is preferably deposited on an electrode. The electrical and the gravitational fields are preferably arranged parallel to one another, ie the electrodes are essentially perpendicular to the direction of the gravitational field (eg horizontally in the gravitational field). - 1 -

In contrast to conventional electrophoresis, a fraction of the suspended particles is deposited in the gravitational field on the upper electrode. The fraction deposited in the form of a ceramic structure is generally distinguished by the fact that its particle size distribution differs from the particle size distribution of the suspension, which is not the case in conventional electrophoresis. Since the finer particles are preferably deposited, the particle size distribution of the ceramic structure has lower values than the particle size distribution of the suspension.

In a preferred embodiment of the invention, the particle size distribution to be separated can be influenced in that not only the absolute amount, but also the point in time at which the electric field from the

Gravity sedimentation is superimposed, is freely chosen. By varying the electric field strength, the desired limit of the separated size fraction can be set.

A particularly preferred embodiment of the invention results from the superimposition of an electric field which is variable in its absolute amount with a gravitational field which is variable in its absolute amount, as is represented in particular by centrifugation, in which centrifugal forces (centrifugal forces) occur. In operation, the resulting gravitational field is directed outwards with respect to the axis of rotation of the centrifuge. As a result, the fine fraction of the suspension in the form of a ceramic layer is deposited according to the invention on the inner electrode in this arrangement.

A further influence on the particle size distribution to be separated can be achieved by not only choosing the absolute amounts of the electrical field and the gravitational field of the centrifugal acceleration over a wide range, but also freely choosing the times at which both fields are switched on and / or off , The present invention is also applicable to suspensions (dispersions) which consist of particles of different compositions. If such particle mixtures differ in their specific electrical charge, their electrophoretic mobilities and their electrophoretic deposition rates are different. If such particle mixtures differ in their density, then their sedimentation speeds are different in the gravitational or centrifugal force field, because in both cases the sedimentation speed according to equation 2 is proportional to the difference between the density of the particles and the density of the liquid of the suspension.

The superimposition of an electric field with a gravitational field consequently permits extensive influence on the deposition conditions in the case of particle mixtures which differ not only in their size, but also in their surface charge and / or in their density. Furthermore, the method according to the invention can also be used to separate those particle mixtures which do not differ in their particle size, but do differ in their surface charge and / or their density.

The method according to the invention can thus be used to produce ceramic structures with a multiple or continuous variation of the electric field and / or (in the case of centrifugation) of the gravitational field in a deposition process without changing the powder suspension, which structures have a gradient in relation to their composition and / or have pore depth. Ceramic gradient structures of this type are suitable, for example, as filter membranes.

In addition, the method according to the invention is suitable not only for the production of layers in which the particle size distribution or, if several different powders are present, the composition can be varied within wide limits, but also for Separation of suspensions with a wider range of variation compared to pure sedimentation or centrifugation processes.

The invention is explained in more detail below on the basis of five exemplary embodiments.

Example 1:

An Al 2 O ₃ layer was produced by superimposing electrophoresis deposition and gravity sedimentation. The counter electrode and substrate were arranged horizontally, ie both surfaces of the pair of electrodes were aligned perpendicular to the direction of the gravitational field. In order to show that increasingly finer particles are separated with overlaid sedimentation, the roughness depth was optically examined with the aid of a surface measuring device (FRT Microglider). For comparison, an additional layer was also produced, in the deposition of which the sedimentation was suppressed in accordance with the prior art by stirring the suspension and the electrodes were arranged vertically. An ethanolic l2θ ₃ solution (dso - 1300 n) with 30 volume percent solids content and a dispersant content of 2 mass percent based on the mass of the powder was prepared. The layers were deposited from this suspension under the following conditions (in the case of a horizontal arrangement on the upper electrode):

The electrode gap was 13 mm. There were four profiles in each examined a ceramic layer. The mean is given. There was a significant reduction in the roughness depth with superimposed sedimentation, which was determined in each case in accordance with DIN 4678 or ISO 4287.

Example 2:

Overlaying electrophoresis deposition and gravity sedimentation were used to produce Al ₂ θ ₃ layers using different field strengths. The counter electrode and substrate were arranged horizontally to one another. In order to show that the particles are separated according to their diameter by varying the field strength during electrophoretic deposition, the roughness depth was optically examined using a surface measuring device (FRT microglider). For this purpose, an ethanolic Al ₂ O ₃ suspension (d ₅₀ = 1300 nm) with 5 volume percent solids content and a dispersant content of 2 mass percent based on the mass of the powder was used. The layers were deposited from this suspension under the following conditions:

Four profiles in each layer were examined. The mean is given. According to DIN 4678 or ISO 4287, the roughness of the layers deposited at higher field strengths was higher. This result confirms the separation of the particles according to their diameter. Embodiment 3

An SiO ₂ layer was produced by superimposing electrophoresis deposition and gravity sedimentation. The counter electrode and substrate were arranged horizontally. In order to show that increasingly finer particles are separated out with sedimentation, the roughness depth was optically examined with the help of a surface measuring device (FRT Microglider). For comparison, an additional layer was also produced, in the deposition of which the sedimentation was suppressed in accordance with the prior art by stirring the suspension and the electrodes were arranged vertically. There was an ethanolisc e Si0 ₂ suspension (dso = 15 microns) with 5 volume percent solids and a dispersant content of 2 mass percent based on the mass of the powder. The layers were deposited from this suspension under the following conditions:

The electrode distance was 13 mm. Four profiles in each layer were examined. The mean is given. There was a significant reduction in the roughness depth with overlaid sedimentation, which was determined in accordance with DIN 4678 and ISO 4287.

Design example:

According to the invention, an Al ₂ O ₃ layer was produced by means of electrophoresis and superimposed gravity sedimentation. The electrodes were arranged horizontally. Electrophoretic deposition was carried out on the upper electrode. In order to show that increasingly finer particles are deposited with overlaid sedimentation than when avoiding sedimentation by stirring, the particle size distribution in the suspension and in the layer was determined optically with a laser granulometer. For this purpose, an ethanolic Al ₂ O ₃ suspension with 30 volume percent solids content and a dispersant content of 2 mass percent based on the mass of the powder was prepared. The layers were deposited from this suspension under the following conditions:

The particle size distribution in the suspension before the deposition, during and after the deposition and in the deposited layer after redispersion in pure ethanol was examined in each case. The differential and cumulative particle size distributions and the dso values of all measurements show that, starting from an almost symmetrical particle size distribution with d ₅₀ = 1.45 μm and in clear contrast to conventional electrophoresis, the layer deposited using the method according to the invention has a significantly reduced average particle size distribution and one just as clearly recognizable Powder content with particle sizes below 500 nm.

Example 5:

According to the invention, a PZT layer (lead zirconate titanate layer) was produced by superimposing electrophoresis deposition and gravity sedimentation. The counter electrode and substrate were arranged horizontally. To show that increasingly finer particles are separated with sedimentation, the roughness depth was optically examined with the help of a surface measuring device (FRT Microglider) in accordance with DIN 4678 or ISO 4287. For comparison, an additional layer was also produced, in the deposition of which the sedimentation was suppressed in accordance with the prior art by stirring the suspension and the electrodes were arranged vertically. An ethanolic PZT suspension (dso ⁼ 2.5 μm) with 5 volume percent solids content and a dispersant content of 2 mass percent based on the mass of the powder was prepared. The layers were deposited from this suspension under the following conditions:

The electrode gap was 13 mm. Four profiles in each layer were examined. The mean is given. There was a significant reduction in the roughness depth with overlaid sedimentation.

Claims

Claims: 1. A method for producing ceramic structures, in which an electric field is applied between each pair of electrodes, which is immersed in a suspension in a gravitational field, which contains ceramic particles with a particle size distribution, in such a way that one of the electrodes of the respective Electrode pairs only separates the size fraction of the ceramic particles, which is smaller than a critical particle size, which results from the equilibrium between the forces resulting from the electric field and from the gravitational field. 2. The method for producing ceramic structures according to claim 1, characterized in that the electrodes of the respective pair of electrodes are arranged parallel to one another and horizontally in the gravitational field of the earth, and the ceramic structure is deposited on the upper electrode. 3. A method for producing ceramic structures according to claim 1, characterized in that the gravitational field is generated by means of a rotating centrifuge. 4. A method for producing ceramic structures according to claim 3, characterized in that the electrodes of the respective pair of electrodes are arranged parallel to one another and perpendicular to the direction of the gravitational field of a rotating centrifuge, and the ceramic structure is deposited on the inner electrode. 5. A method for producing ceramic structures according to one of claims 1 to 4, characterized in that the amounts of the electric field and the gravitational field are varied in time independently of one another. 6. The method for producing ceramic structures according to one of claims 1 to 5, characterized in that the particle size distribution of the ceramic structure has lower values than the particle size distribution of the suspension. 7. The method for producing ceramic structures according to one of claims 1 to 6, characterized in that the ceramic particles - structural ceramics such as Al ₂ O ₃ , Zr0 ₂ , mullite, SiC, Sι ₃ N ₄ and / or - functional ceramics such as BaTi0 _3rd or lead zirconate titanate (PZT) and / or - bioceramics such as hydroxylapatite (Ca (OH) (P0 ₄ ) ₃ ) and / or - mineral glasses. 8. A method for producing ceramic structures according to one of claims 1 to 7, characterized in that the suspension contains at least two different types of ceramic particles. 9. A method for producing ceramic structures according to one of claims 1 to 8, characterized in that the sizes of the ceramic particles are between 5 nm and 500 μm, preferably between 10 nm and 100 μm, particularly preferably between 10 nm and 10 μm. 10. Ceramic structure produced according to one of claims 1 to 9. 11. Ceramic gradient structure, produced according to one of claims 5 to 9.