DE10313847A1 - Shaped ceramic body with irregular structure gradient, e.g. useful for filter, comprises at least two layers partially or completely one on another, with changing structural characteristic at transition between layers - Google Patents

Abstract

A shaped ceramic body with an irregular structure gradient comprises at least two layers partially or completely one on the other of the same or different thickness. The transition between the layers comprises at least one irregularly changing structural characteristic. An independent claim is included for a process for preparation of the shaped body which involves preparation of a suspension of the ceramic material, separation of the suspended ceramic powder, production of two or three layers, drying and sintering to give the required shaped body.

Description

The invention relates to the Field of ceramics and relates to shaped ceramic bodies with a sudden structural gradient, as for example as a filter, burner elements, catalyst support or thermal insulation panels can be used and a process for their preparation.

Electrophoresis is one of the four basic electrokinetic phenomena besides the flow potential, electroosmosis and sedimentation potential. Under electrophoresis is the migration of electrically charged particles in an electrical Field understood. In the fields of classical ceramics and the High-performance ceramics, the electrophoretic deposition is superficially charged Powder particles from aqueous or non-aqueous Suspensions in the DC field to an electrode manifold as low-defect forming method applied.

Electrophoretic shaping is a cheap Process that has been used for many years to produce ceramic Green body and Layers of different shapes for applications as ceramic / ceramic and metal / ceramic composites or for thin or thick layers for electronic applications Sarkar et al., J. Am. Ceram. Soc., 79, 1987 (1996); L. Vandeperre et al., Key Engineering Mat. 132-136, 2013 (1997); N. Koura et al., J. Ceram. Soc. Jpn, Int. Edition 104, 810 (1996)).

This shaping method is particularly suitable for the production of uniform particle layers with high green densities (50-60 % of the theoretical material density) and adjustable thickness. basic requirements for the Electrophoretic deposition are always stable suspensions from powder particles which have to carry a sufficient surface charge to be transported in the electric field to the deposition electrode to be able to. The particles can be positively or negatively charged and migrate depending on the charge to the cathode or anode.

In the EP 0 713 931 A2 describes a process for producing thin, dense ZrO ₂ films by electrophoretic deposition. Partially stabilized ZrO ₂ powders are dispersed in organic solvents and a surface charge is set by adding iodine. By immersing electrodes and applying a direct electrical voltage, the charged powder ponds move in the direction of one of the electrodes and deposit there as a thin layer. After sintering, dense ceramic layers are achieved. Depending on the deposition conditions, the layer thicknesses were between 1 and 100 μm.

The production of thin, dense ceramic layers on precious metals via electrophoresis is described in EP 0 381 179 A2 used to reduce excessive evaporation of platinum or platinum alloys in the glass melting process. SiO ₂ , CeO ₂ , TiO ₂ , Al ₂ O ₃ and mixtures of these oxides were used as ceramic materials. The deposition was carried out from non-aqueous suspensions and led to the formation of pore-free, uniform protective layers after sintering.

The deposition of dense ceramic parts from aqueous suspensions is in the US 5,194,129 described. Thus, ceramic optical ferrules can be generated by electrophoresis. The powder particles positively charged in the suspension are deposited on a cathode consisting of a material which absorbs the hydrogen produced by the electrolytic decomposition of the water at the cathode. Preferred H ₂ getter materials are called palladium or palladium alloys. The electrophoretic deposition of the ferrules takes place on a cathode connected palladium wire with a diameter of 125-150 microns. The deposition voltage is in a range of 25 to 200 V and allows the generation of a layer thickness of 2.5 mm on the palladium wire in about 60 seconds.

In WO 9523246, the electrophoretic Deposition of defect-free metal oxide layers from colloidal sols described. The electrolytic decomposition of the sol contained in the sol Water above a voltage of 3 V is due to the formation of gas bubbles at the electrodes during the deposition and the related Formation of gaps, Defects and uneven layers also presented as problematic and avoided by passing a stream of gas bubbles across the substrate, to remove the resulting hydrogen from the substrate.

Another possibility of producing dense alumina shaped bodies from aqueous dispersions is disclosed in US Pat EP 0 107 352 A1 specified. In this case, aqueous suspensions with Feststoftgehalten between 0.1 and 40.0 wt .-% are applied in an electrophoretic cell between two electrodes with a DC voltage. In addition, between the two electrodes is a microporous membrane or an ion exchange membrane on which the deposition of the powder takes place. The formation of hydrogen gas bubbles thus takes place behind the membrane at the cathode, whereby the production of defect-free shaped body succeeds on the membrane.

In the US 6,270,642 the production of gas-tight electrolyte layers on porous electrode materials is described, which are suitable for use as electrodes in fuel cells.

The production of ceramic bodies with conical pores via electrophoretic shaping is described in US 5,472,583 described. The on this Way generated body can be used as a filter, burner elements, catalyst support, inter alia. The conical pore shape is characterized in that the narrower openings have a diameter of about 0.5-5 microns, the other openings have a diameter of about 20-200 microns. The suspension medium used is water in which the powder particles are dispersed together with a dispersing aid, a defoamer and various ions. The ion concentration is so high that the conductivity of the suspension is sufficient to support the electrolysis of the water and thus produce conical pores in the deposited solid.

In WO 0186030 are electrically conductive Fibers or bundles used by fibers to electrophoretically non-aqueous Suspensions ceramic particles deposit on it and on this Way porous produce ceramic hollow membranes. The hollow core of the membranes arises from the subsequent electrophoretic Deposition carried out Burning out the fiber or fiber bundle. The hollow membranes can be made Alumina or zirconia exist. The porosity of the membranes can by adding carbon black, Carbon, graphite, different polymer powder or cellulose-based Powders that are added to the ceramic starting powders set become.

All cited publications are common, that either dense molded body or layers or exclusively porous Moldings or Layers are prepared by electrophoretic deposition. In case of porous Moldings or Layers can be used to adjust the porosity either by the at Electrolysis of the water resulting gases or by burning out particulate or fibrous Carbon or carbonaceous materials.

The production of laminated green bodies by electrophoresis is discussed in WO 9848084. Ceramic layers of different composition ratios of Al ₂ O ₃ / ZrO ₂ are deposited in successive steps in layer thicknesses of 20 to 40 μm and 30 to 50 μm on an electrode made of an organic suspension. The densities of the deposited layers amount to up to 70% of the theoretical density. In order to separate the respective subsequent layer, the suspension must be changed.

In C. Kaya, et al., Acta mater. 49, 1189-1197 (2001) becomes the production of carbon fiber reinforced alumina matrix composites based on nickel-coated carbon fibers, which have a electrophoretic deposition process infiltrated under vacuum were reported. This technique allows the ceramic particles very effective the gaps between the individual fibers. The ceramic particles need present either as sol particles or nanoscale powder particles, to allow penetration into the fiber structures. The nickel coating gives the carbon fibers excellent electrical conductivity and good wettability.

The invention is based on the object, ceramic moldings indicate with a sudden structural gradient, the most diverse material properties in a shaped body can unite and a process for their preparation.

The task is through in the claims specified invention solved. Further developments are the subject of the dependent claims.

The ceramic molding according to the invention with leaky structure gradient consists of at least two whole or partially over each other arranged layers of equal or different thickness, wherein the transition between the layers, each by at least one step change Characteristic feature is characterized.

Advantageously, the at least two exist Layers of the same ceramic material or different ceramic materials.

Further advantageously, the coverage is the at least first layer of the at least second layer between 20 and 100%.

It is also beneficial if the Layer thicknesses at least 1 um.

It is also advantageous if the erratic transition the structure of the layers by a change of at least 10% a structural feature or a structural feature resulting property is characterized.

It is also advantageous if the structural changes in terms of porosity and / or the density and / or the pore structure.

In the method according to the invention for producing a ceramic shaped body, a suspension of ceramic powdery materials is produced. The ceramic powdery materials are advantageously ZrO ₂ , Al ₂ O ₃ , TiO ₂ , Y ₂ O ₃ , SiC, SiO ₂ , Si ₃ N ₄ , AlN, mullite, cordierite, PZT, BaTiO ₃ , SnO ₂ , YBaCu and mixtures thereof Materials.

In an electrophoresis device, an electrode is then contacted with an electrically conductive fiber structure or an electrode made of an electrically conductive fiber structure. Thereafter, either another fiber structure with a different fiber structure and / or fiber arrangement is applied to the conductive fiber structure or not. Subsequently, the deposition of the suspended ceramic powders is carried out by electrophoresis on and in the fiber structure (s), with substantially complete filling of all voids in the fiber structure (s). In order to realize the at least second or third layer completely or partially on the surface of the first or second layer, the deposition is then continued with the same or other ceramic materials, wherein other ceramic materials are introduced by changing the suspension. Subsequently, after the drying of the green body, the sintering is carried out to form a shaped body.

It is also advantageous if the ceramic powdery Materials particle sizes between 10 nm and 3 μm, particularly advantageous between 10 and 250 nm.

It is also advantageous if a Suspension based on polar organic solvents or water is used, the suspension advantageously has a solids loading of 0.1 to 65 wt .-%.

Also advantageously be the suspension of the ceramic materials known dispersing aids, Added polyelectrolytes, ions and / or other known materials, being also over a pH adjustment influences the stability of the suspension can be.

It is also beneficial if an electrically conductive Fiber structure is used, which consists of carbon or carbon-containing Materials or made of electrically conductive plastics or a fibrous material which passes through a surface treatment with electrically conductive Material has been provided.

Furthermore, it is advantageous if a fiber structure is used, which is a knitted fabric, a knitted fabric, a scrim, a felt or a fleece is.

And it is also beneficial if the electrophoretic deposition under application of a DC voltage carried out is, wherein the deposition under constant stress in the range of 5 to 500 V or under constant current in the range of 5 to 100 mA performed can be.

Furthermore, it is advantageous if to the full or partial preservation of the fiber structure in the ceramic molding body Sintering in an inert or reducing atmosphere or under vacuum.

And it's also an advantage when the fiber structure is sintered or sintered preceded thermal treatment step removed by oxidation becomes.

By the solution according to the invention it is possible ceramic shaped bodies due to the transient transitions in their structure also sudden transitions in their material properties have and thus in a molding, for example, also opposite properties can unite. For example, a layer of the molding may be gas-permeable and the second or another layer has been formed gastight his. Or one layer is translucent and the other layer opaque. Or the one layer is in complete or partial preservation fiber structure well electrically conductive and the other layer is electrically insulating. Or the one layer is well thermally conductive and the other layer is thermally insulating etc ..

It can also be a comparatively small leap change of a structural feature of 10% to comparatively large effects lead a property. For example, the change in porosity of 10% cause that the layer with increased by 10% Porosity now no longer translucent but opaque. Or an electric one insulating layer has a porosity of 0 and a density that corresponds to the theoretical layer material density, on and the subsequent Layer by still minor Presence of carbon or carbon containing fibers has a 10% lower density and electrical conductivity on. Such drastic property changes can be made for the use of such materials very big Have meaning.

By the sudden change the structure of the moldings is primarily the density, the porosity and / or the pore structure at the spatial transition Changed from one layer to another. The jump height can be controlled by choice of fiber structure and deposition conditions, as well, which structural features or structural features resulting properties are to be changed abruptly.

In the method according to the invention In an electrophoresis device two electrodes, for example made of precious metal or graphite, introducing the shape of the electrodes arbitrarily and in dependence from the desired Moldings are chosen can. Deposition electrode is an electrode that has a sign opposite charge compared to surface charge has the particles in the suspension. The electrode, The deposition takes place from a conductive fiber structure or is completely or partially with such a conductive fiber structure partially covered. The conductive Fiber structure may be made of carbon or carbon containing materials or electrically conductive Plastics or consist of a fiber material, which by a surface treatment with electrically conductive Material has been provided.

The conductive fiber structure can thereby also with a further fiber structure of modified fiber structure and fiber arrangement laminated or assembled.

The suspension of the ceramic powdery materials should, as well known in the prior art, have the highest possible stability, which can be achieved, for example, by adding known dispersing aids. In addition to high stability, the suspension should also be known to be relatively low Have viscosity.

Regarding the particle size of the ceramic powdery Materials must be taken into account that these depend on are to be selected from the size of the fiber spaces. In any case, even finer particles should be included in order to be as possible all cavities preferably Completed fill.

In the case where an aqueous suspension is used, it is also advantageous to use one according to EP 0 107 352 Positioning described ion exchange membrane between the electrodes to prevent the formation of gas bubbles due to the electrolytic decomposition of the water at the deposition electrode.

In this case, the fiber structure in this Fall in front of the membrane or on the membrane before the deposition electrode arranged. In the case of aqueous Suspensions, the deposition takes place at the deposition electrode opposite side of the membrane. In this case, a simultaneous deposition of particles on the membrane and the electrode is prevented the space between the membrane and the electrode at which the deposition would be done filled with a balance electrolyte without solids content.

It is beneficial after inserting the electrodes in the suspension to carry out a vacuum treatment, so that the possible complete degassing of the fiber material and the suspension is achieved.

After applying the electrical DC voltage is applied to the electrodes, the deposition of the ceramic Materials and the simultaneous infiltration of the fiber structure. In this case, either a constant voltage or a constant current is applied. In any case, is for the desired Thickness of the deposition and the materials used in each case Hold time of voltage or current to select which either known is or determined by a small number of experiments each can be.

After the most complete infiltration of the Fibrous material is either made with the same ceramic material further deposited, leaving on the infiltrated fiber structure a dense top layer of the same material is formed. But it can also the suspension can be changed and after only partially Infiltration of the fiber structure with the first ceramic material the complete Infiltration with a second ceramic material and then, for example, by changing the suspension a cover layer be applied from a third material.

Surprisingly can by further deposition after complete infiltration of the Fasergebildes a homogeneous cover layer are generated, the one green density from 50 to 70% of the theoretical density, and by a subsequent sintering be converted into a dense layer can.

During sintering under oxidizing Conditions or one of the sintering preceded thermal Treatment under oxidizing conditions will be a complete burnout reaches the fibers. As a result, instead of the burned out fibers each have cavities, causing this part of the molding a significantly different porosity and / or density and / or pore structure, than the other Form part of the body. The thicknesses of the respective layers depend on the type and the geometric dimensions of the fiber material, according to the suspension properties, the Properties of the ceramic materials and according to the deposition parameters.

Likewise, the sintering process under inert or reducing atmosphere or under vacuum and thereby a complete burnout the fiber material are wholly or partially prevented, whereby the production of a shaped body with a jumpy transition in density and / or porosity becomes possible.

By using, for example laminated composite fiber structure with respect to different Fiber type, fiber diameter, fiber shape, fiber spacing are also More options given, shaped body with sudden changes indicate the pore structure and / or the porosity.

Regarding the erratic change is to say that according to the invention, a change of at least 10% a structural feature or a structural feature to understand resulting property.

The erratic transition is carried out according to the invention either at the interface two different fiber structures in the green body or at the interface between the fiber structure in the green body and the overlying fiber-free layer, which in the sintered state a dense layer will form.

With regard to the structural features porosity, density and pore structure, the following is to be understood according to the invention:
According to the invention, porosity is to be understood as meaning the total porosity of the individual layers, which are composed of the open and closed porosity.

The density should be the outer material density of the including individual layers the total porosity his.

The pore structure of a layer becomes from the roundness of the pores and / or the aspect ratios the pores and / or the orientation of the pores and / or the pore diameter certainly.

The direction, shape and size of the pores in the molding is determined by the arrangement of the fibers in the green body.

All these data of the structural features porosity, density and pore structure are metrological can be detected and evaluated.

The drying to be carried out after the deposition can advantageously still of a further thermal oxidizing Treatment step followed, in which the organic components the suspension medium, the dispersing aid and the fiber structure be removed.

The material properties become set by the sintering. The sintering conditions are included for the desired To set properties.

Of particular advantage is it at the method according to the invention, that a shaped body with one or more abrupt structural gradients in one Deposition process can be realized.

In the following, the invention is attached several embodiments explained in more detail.

example 1

For electrophoretic infiltration, a carbon soft felt (SIGRATHERM, Fa. SGL Carbon) was cut with a thickness of 10 mm and a basis weight of 1050 g / m ² to the size of 25 × 25 × 10 mm ³ and using Leit carbon (Fa Plano) glued on a flat platinum electrode. The thus-prepared electrode was placed in parallel to a second platinum electrode in an electrophoresis cell, the distance between the two electrodes being 25 mm. Thereafter, the electrophoresis cell was filled with a ceramic suspension prepared by dispersing 150 g of ZrO ₂ TZ-3Y (TOSOH Corp.) in 150 g of ethanol (98%, undegraded) with the addition of 2 g of benzoic acid followed by stirring for 30 minutes , To vent the felt and the suspension, the entire electrophoresis cell was evacuated in a vacuum <5000 Pa.

The electrophoretic deposition of the ceramic material was at a constant current of I = 10 mA over a period of 20 minutes. These were the electrodes poled so that the deposition on the electrode with the carbon felt took place. After deposition, the resulting green body of The electrode is removed, dried and air in the following regime sintered: 1 hour at 500 ° C, then 1 hour at 900 ° C, then 1 hour at 1500 ° C. During sintering, carbon felt became ceramic Burned out material. The sintered body had an approx. 7 mm thick porous Layer with a porosity of 45%, the perfect of a ca. 2 mm thick dense layer covered with a closed porosity <1%. Both layers immediately adjoined each other without cracking and were firmly connected.

Example 2

From a flax fiber fleece (FLAXOPROPP GmbH) with a thickness of 4 mm and a basis weight of 665 g / m ² , a sample of size 30 × 30 × 4 mm ^{3 was} cut out. The sample was pyrolyzed under nitrogen atmosphere at 1600 ° C. After pyrolysis, a carbon fiber nonwoven fabric of dimensions 23 × 23 × 3 mm ³ and a mass of 0.12 g was obtained. This was adhered to a platinum electrode according to Example 1 for electrophoretic infiltration and introduced into an electrophoresis cell. For the ceramic suspension, 100 g of ZTA powder TZ-3Y20A (TOSOH Corp.) was dispersed in 150 g of ethanol (98%, undegraded) with the addition of 1.5 g of benzoic acid and stirred for 30 minutes.

The electrophoretic deposition of the ceramic material was at a constant stress of U = 30V over a period of 20 min, with the electrodes in turn were poled so that the deposition on the electrode with the Nonwoven fabric was made. The resulting green body was after deposition from the electrode, dried and stored at 1450 ° C in air sintered. The fibers were made of the ceramic material burned out. The sintered body had a ca. 2 mm thick porous Layer with a porosity of 22%, the perfect of an approx. 1 mm thick dense Layer was covered. Both layers immediately bordered crack-free together and were firmly connected.

Claims (21)

Ceramic moldings with a sudden structural gradient, consisting of at least two completely or partially on top of each other arranged layers of equal or different thickness, wherein the transition between the layers, each by at least one step change Characteristic feature is characterized. Ceramic moldings according to claim 1, wherein the at least two layers of the same ceramic material or different ceramic Materials exist. Ceramic moldings according to claim 1, wherein the covering of the at least first layer of the at least second layer is between 20 and 100%. Ceramic moldings according to claim 1, wherein the layer thicknesses are at least 1 micron. Ceramic shaped bodies according to claim 1, in which the abrupt transition of the structure of Layers by a change of at least 10% of a structural feature or a property resulting from a structural feature. Ceramic moldings according to claim 1, wherein structural changes in terms of Porosity and / or the density and / or the pore structure are present. Ceramic shaped bodies according to claim 1, in which the ceramic materials ZrO ₂ , Al ₂ O ₃ , TiO ₂ , Y ₂ O ₃ , SiC, SiO ₂ , Si ₃ N ₄ , AlN, mullite, cordierite, PZT, BaTiO ₃ , SnO ₂ , YBaCu, as well as mixtures of these materials. Process for the preparation of ceramic moldings according to one of the claims 1 to 7, in which a suspension made of ceramic materials in an electrophoresis device, an electrode with a electrically conductive Fibrous contacted or an electrode from an electrical conductive Fiber structure is produced, then either on the conductive fiber structure another fiber structure with a different fiber structure and / or fiber arrangement is applied or not and then the deposition of the suspended ceramic powder by electrophoresis on and in or the fibrous structures with substantially complete filling of all cavities in the or the fiber structure and the deposition for the realization of the at least second or third layer in whole or in part on the surface of the first or second layer with the same or different ceramic materials is continued, with other ceramic materials by changing the suspension are introduced and then after drying the Green body sintering to a molded article carried out becomes. A method according to claim 8, wherein the ceramic powdery Materials particle sizes between 10 nm and 3 μm. The method of claim 9, wherein the ceramic powdery Materials particle sizes between 10 nm and 250 nm. The method of claim 8, wherein the suspension made of ceramic powdery Materials based on polar organic solvents or water is produced. The method of claim 8, wherein a suspension is used with a solids loading of 0.1 to 65 wt .-%. The method of claim 8, wherein the suspension dispersing agents known from the ceramic materials, polyelectrolytes, Ions and / or other known materials are added. The method of claim 8, wherein an electrically conductive Fiber structure is used, which consists of carbon or carbon-containing Materials or made of electrically conductive plastics or a fibrous material which passes through a surface treatment with electrically conductive Material has been provided. A method according to claim 8, wherein a fiber structure is used, which is a knitted fabric, a knitted fabric, a clutch, a Felt or a fleece is. The method of claim 8, wherein the electrophoretic Deposition is carried out under application of a DC voltage. The method of claim 8, wherein the deposition under constant voltage in the range of 5 to 500V is performed. The method of claim 8, wherein the deposition under constant current in the range of 5 to 100 mA is performed. The method of claim 8, wherein a suspension used on the basis of polar organic solvents or water becomes. A method according to claim 9, wherein the complete or Partial preservation of the fiber structure in the ceramic molded body Sintering in an inert or reducing atmosphere or under vacuum. The method of claim 9, wherein the fiber structure in the sintering or in a sintering preceded thermal Treatment step is removed by oxidation.