DE10313847B4 - Process for the production of ceramic moldings with a sudden structure gradient and ceramic moldings with a sudden structural gradient - Google Patents

Abstract

Process for producing ceramic shaped bodies from at least two layers, which
Are arranged completely or partially above one another,
- Have the same or different thickness
- and each differ in at least one structural feature by a sudden change in the structural feature of at least 10%,
in which
A suspension of ceramic powder materials is produced,
- in an electrophoresis device
a. contacting an electrode with an electrically conductive fiber structure in the form of a knitted, knitted, laid, felt or nonwoven fabric or
b. an electrode is produced from an electrically conductive fiber structure in the form of a knitted, knitted, laid, felt or nonwoven fabric and subsequently a further fiber structure with a different fiber structure and / or fiber arrangement is applied to the conductive fiber structure,
- Then by electrophoresis, the suspended ceramic powders are deposited on and in the fiber or the fiber structures with complete filling of all cavities in the fiber or the structures
- and to ...

Description

The This invention relates to the field of ceramics and relates to Process for producing ceramic shaped bodies with a sudden structural gradient and ceramic molding with an abrupt structural gradient, as for example as a filter, Burner elements, catalyst carrier or thermal insulation panels can be used.

The Electrophoresis is one of the four basic electrokinetic phenomena next to 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.

The Electrophoretic shaping is a low cost process which used for many years to 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 u. a., Key Engineering Mat. 132-136, 2013 (1997); N. Koura u. a., J. Ceram. Soc. Jpn, Int. Edition 104, 810 (1996)).

Especially This shaping method is suitable for producing uniform particle layers with high green densities (50-60% of 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 A 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 the WO 9523246 A1 describes the electrophoretic deposition of defect-free metal oxide layers from colloidal sols. The electrolytic decomposition of the water contained in the sol above a voltage of 3 V is also presented as problematic due to the formation of gas bubbles at the electrodes during the deposition and the associated formation of voids, voids and uneven layers and thereby avoided that a stream of Gas bubbles are passed over 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 solids contents 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 B1 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 A described. The bodies produced in this way can be used as filters, burner elements, catalyst carriers and others. 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 the WO 01/86030 A1 For example, electroconductive fibers or bundles of fibers are used to electrophoretically deposit ceramic particles thereon from non-aqueous suspensions to produce porous hollow ceramic membranes. The hollow core of the membranes is formed by the burnout of the fiber or fiber bundle carried out following the electrophoretic deposition. The hollow membranes may consist of alumina or zirconia. The porosity of the membranes can be adjusted by adding carbon black, carbon, graphite, different polymer powders or cellulose-based powders added to the starting ceramic powders.

all cited Publications is common that either dense moldings 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.

On the production of laminated green bodies via electrophoresis is in the WO 9848084 A1 received. 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, u. a., Acta mater. 49, 1189-1197 (2001) is the production of carbon fiber reinforced Aluminum oxide matrix composites based on nickel-coated Carbon fibers that over infiltrated an electrophoretic deposition process 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.

Of the Invention is based on the object, a process for the preparation ceramic molding to indicate with a sudden structural gradient, the most diverse Combine material properties in a molding, as well as such a shaped body.

The The object is achieved by the invention specified in the claims. Trainings are Subject of the dependent claims.

• – ganz oder teilweise übereinander angeordneten sind,
• – gleiche oder unterschiedlicher Dicke aufweisen
• – und sich jeweils in mindestens einem Strukturmerkmal sprunghaft durch eine Änderung des Strukturmerkmals von mindestens 10% unterscheidenden, wird
• – eine Suspension aus keramischen Pulvermaterialien hergestellt,
• – in einer Elektrophoresevorrichtung
• a. eine Elektrode mit einem elektrisch leitfähigen Fasergebilde in Form eines Gewirkes, Gestrickes, Geleges, Filzes oder Vlieses kontaktiert oder
• b. eine Elektrode aus einem elektrisch leitfähigen Fasergebilde in Form eines Gewirkes, Gestrickes, Geleges, Filzes oder Vlieses hergestellt und anschließend auf das leitfähige Fasergebilde ein weiteres Fasergebilde mit anderer Faserstruktur und/oder Faseranordnung aufgebracht,
• – dann werden auf elektrophoretischem Wege die suspendierten keramischen Pulver auf und in dem oder den Fasergebilden unter vollständiger Auffüllung aller Hohlräume in dem oder den Fasergebilden abgeschieden
• – und es wird zur Realisierung einer zweiten oder dritten abgeschiedenen Schicht die Abscheidung auf der ganzen oder teilweisen Oberfläche der ersten oder zweiten Schicht mit den gleichen oder anderen keramischen Materialien fortgesetzt, wobei andere keramische Materialien durch Wechseln der Suspension eingebracht werden
• – und anschließend wird nach der Trocknung des Grünkörpers die Sinterung zu einem Formkörpern durchgeführt.

In the inventive method for producing a ceramic molding of at least two layers, the

• Are arranged completely or partially above one another,
• - have the same or different thickness
• - And in each case in at least one structural feature suddenly by a change in the structural feature of at least 10% different, is
• A suspension made of ceramic powder materials,
• - in an electrophoresis device
• a. contacting an electrode with an electrically conductive fiber structure in the form of a knitted, knitted, laid, felt or nonwoven fabric or
• b. an electrode made of an electrically conductive fiber structure in the form of a knitted, knitted, laid, felt or fleece and then applied to the conductive fiber structure another fiber structure with different fiber structure and / or fiber arrangement,
• - Then, the suspended ceramic powders are deposited on and in the one or more fiber structures with complete filling of all cavities in the fiber or the structures by electrophoresis
• - And it is the realization of a second or third deposited layer deposition on the whole or partial surface of the continued first or second layer with the same or other ceramic materials, wherein other ceramic materials are introduced by changing the suspension
• - And then the sintering is carried out after the drying of the green body to a shaped bodies.

advantageously, become ceramic powdery Materials with particle sizes between 10 nm and 3 μm used, even more advantageously ceramic powdery materials with particle sizes between 10 nm and 250 nm are used.

Farther Advantageously, a suspension with a solids loading from 0.1 to 65% by weight.

And also advantageously, the suspension of the ceramic Materials known dispersing aids, polyelectrolytes, ions and / or other known materials added.

Advantageous It is also used when using an electrically conductive fiber structure is made of carbon or carbon containing materials or made of electrically conductive Plastics or consists of a fiber material, which by a surface treatment with electrically conductive material has been provided.

Also it is advantageous if the electrophoretic deposition under Applying a DC voltage is performed.

Farther It is advantageous if the deposition under constant tension in the range of 5 to 500V is performed.

And It is also advantageous if the deposition under constant current in the range of 5 to 100 mA performed becomes.

From It is also an advantage if for complete or partial conservation of the fiber structure in the ceramic molding sintering in inert or reducing atmosphere or performed under vacuum becomes.

Farther It is advantageous if the fiber structure in the sintering or in a sintering preceded thermal treatment step is removed oxidatively.

Of the ceramic according to the invention Moldings, produced by the process according to the invention has layers with different porosity and / or Density and / or pore structure.

advantageously, The at least two layers consist of the same ceramic Material or different ceramic materials.

Also is advantageously the covering of the at least first layer of the at least second Layer between 20 and 100%.

Farther advantageously, the layer thicknesses are at least 1 μm.

And also advantageously, 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, and Mixtures of these materials.

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 be. 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 ..

there can also be a comparatively small step change of a structural feature of 10% to comparatively large effects lead a property. For example, the change in porosity lead by 10% that the layer with the 10% increased porosity now not more translucent but opaque. Or an electrically insulating one Layer has a porosity of 0 and a density equal to the theoretical layer material density corresponds to, 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.

Due to the abrupt change in the structure of the shaped bodies, the density, the porosity and / or the pore structure at the spatial transition is changed abruptly from one layer to the other. The jump height can be controlled by choice of fiber formation and deposition conditions, as well as which structural features or properties resulting from structural features should be changed by leaps and bounds.

at 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 also be changed with another fiber structure Fiber structure and fiber assembly laminated or assembled be.

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

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 Completely fill.

In the case where an aqueous suspension is used, it is also advantageous to use one according to EP 0 107 352 A1 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.

there In this case, the fiber structure in front of the membrane or on the Membrane each arranged in front of the deposition electrode. In the event of of watery 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 insertion the electrodes in the suspension to perform a vacuum treatment so the possible complete degassing of the fiber material and the suspension is achieved.

To the application of the electrical DC voltage to the electrodes the deposition of the ceramic materials and the simultaneous Infiltration of the fiber structure. This is either a constant Voltage or a constant current 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.

To the possible complete Infiltration of the fiber structure is either 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.

at sintering under oxidizing conditions or sintering preceded thermal treatment under oxidizing conditions becomes a complete one Burning out the fibers reached. As a result, are on Place the burnt fibers each cavities, making 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 can be carried out under an inert or reducing atmosphere or under vacuum and thus a complete Burning out of the fiber material can be completely or partially prevented, whereby the production of a molded article with a sudden transition in density and / or porosity is possible.

By the use of, for example, laminated composite different Fiber structure with respect to different fiber type, fiber diameter, fiber shape, fiber spacing are also other 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% of Structural feature or property resulting from a structural feature to understand.

Of the erratic transition takes place 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 including individual layers the total porosity be.

The Pore structure of a layer is determined by 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 metrologically detectable and evaluable.

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 are set by the sintering. The Sintering conditions are for the desired To set properties.

From It is particularly advantageous in the method according to the invention that a molding having a or a plurality of discontinuous structural gradients in a deposition process can be realized.

in the Furthermore, the invention will be explained in more detail in several embodiments.

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 Piano) 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 pro Be 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 voltage of U = 30 V over a period of 20 min carried out, wherein the electrodes were again poled so that the deposition took place on the electrode with the nonwoven fabric. The resulting green body was removed after deposition from the electrode, dried and at 1450 ° C sintered in air. The fibers were made of the ceramic material burned out. The sintered body pointed a ca. 2 mm thick porous Layer with a porosity of 22%, the perfect of a 1 mm thick dense layer was covered. Both layers immediately adjoined each other without cracking were firmly connected.

Claims (16)

Process for the preparation of ceramic shaped bodies at least two layers that - completely or partially on top of each other are arranged - one have the same or different thickness - and yourself each in at least one structural feature by a sudden change of the structural feature differ by at least 10%, at the - one Suspension is made of ceramic powder materials, - in a electrophoresis a. an electrode with an electric conductive Fiber structure in the form of a knitted fabric, knitted fabric, fabric, felt or fleece contacted or b. an electrode from an electrical conductive Fiber structure in the form of a knitted fabric, knitted fabric, fabric, felt or nonwoven fabric and then onto the conductive fiber structure another fiber structure with a different fiber structure and / or fiber arrangement is applied, - then electrophoretically suspended ceramic powders on and in the fibrous structure (s) completely filling all the cavities in the body or the fibrous structures are deposited - and to realize a second or third deposited layer on the deposition the whole or partial surface of the first or second Layer continued with the same or other ceramic materials is, other ceramic materials by changing the suspension be introduced - and subsequently after drying the green body the Sintering into moldings is carried out. The method of claim 1, wherein the ceramic powdery materials with particle sizes between 10 nm and 3 μm be used. The method of claim 2, wherein the ceramic powdery materials with particle sizes between 10 nm and 250 nm are used. The method of claim 1, wherein a suspension is used with a solids loading of 0.1 to 65 wt .-%. The method of claim 1, wherein the suspension dispersing agents known from the ceramic materials, Polyelectrolytes, ions and / or other known materials added become. The method of claim 1, wherein an electrically conductive Fiber structure is used, which consists of carbon or carbon containing materials or electrically conductive plastics or consists of a fiber material, which by a surface treatment with electrically conductive Material has been provided. The method of claim 1, wherein the electrophoretic Deposition is carried out under application of a DC voltage. The method of claim 1, wherein the deposition under constant voltage in the range of 5 to 500V is performed. The method of claim 1, wherein the deposition under constant current in the range of 5 to 100 mA is performed. A method according to claim 2, 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 2, wherein the fibrous structure in the sintering or in a sintering preceded thermal Treatment step is removed by oxidation. Ceramic molded body, made after one the claims 1 to 11 with layers of different porosity and / or density and / or Pore structure. Ceramic molding according to claim 12, in which the at least two layers consist of the same ceramic material or of different ceramic materials. A ceramic molding according to claim 12, wherein the covering of a first layer of a second layer between 20 and 100%. A ceramic molding according to claim 12, wherein the layer thicknesses are at least 1 micron. The ceramic molding according to claim 12, wherein the ceramic materials are 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.