AT528252A1 - CERAMIC INSULATOR-CONDUCTOR COMPOSITE MATERIAL - Google Patents

Abstract

Electrically conductive ceramic insulator conductor composite material (1) with a silicate ceramic conductive cuboid honeycomb body (2), wherein the honeycomb body (2) comprises at least 60 vol% of an insulating silicate, 9 to 38 vol% of a carbide and at least 2% of a sinter additive, wherein electrodes (3) in the form of metallic layers are applied to two opposite lateral surfaces of the cuboid honeycomb body.

Description

CERAMIC INSULATOR-CONDUCTOR COMPOSITE MATERIAL

The present invention relates to a method for producing an electrically conductive ceramic insulator-conductor composite material comprising a silicate ceramic conductive, cuboid honeycomb structure. The invention further relates to an electrically conductive ceramic insulator-conductor composite material comprising a silicate ceramic conductive, cuboid honeycomb structure. Finally, the invention relates to uses of this composite material.

BACKGROUND OF THE INVENTION

The structure and properties of honeycomb cores, particularly their large surface-to-volume ratio, which promotes efficient mass and heat transfer, allow for a wide range of applications. Honeycomb cores are therefore used, for example, in exhaust gas purification and chemical reactors. In the exhaust gas purification of vehicles with combustion engines, ceramic honeycomb cores have been used for decades due to the limited space available. These cores typically have a circular cylindrical shape. The round cross-section enables even load distribution, resulting in improved mechanical stability, especially under pressure. Enclosing honeycomb cores with a round cross-section in a metallic housing and connecting them to inlet and outlet pipes is also simpler compared to other shapes. Cordierite, a crystalline material from the group of silicate ceramics, is characterized by its low thermal expansion, high temperature resistance, and low density, making it ideally suited for honeycomb core applications. For the efficient reduction of pollutants, as with any other chemical process, a specific reaction temperature is required, which is usually significantly higher than the ambient temperature. Cordierite, or silicate ceramics in general, exhibit insulating properties and are therefore unsuitable for resistance heating. In the prior art, these honeycomb structures are thus heated indirectly, for example, via external gas burners, by flowing through them with elevated-temperature gas, via embedded heating elements, or by infrared radiators. Direct heating of the honeycomb structure, especially in the form of resistance heating, allows for optimization of energy efficiency. The material required for this must have good electrical conductivity as well as high thermal and chemical resistance.

exhibit durability.

distributed around the circumference and provided over the entire length of the substrate.

Furthermore, US 2012/0076699 A1 describes a circular cylindrical honeycomb structure with a pair of electrodes formed on an outer circumferential surface such that the electrodes are opposite each other in the direction of the diameter of the honeycomb body and have an electrode connection formed in a central part of each electrode.

where the thickness of each electrode decreases from the center outwards.

WO 2020/102536 A1 discloses conductive ceramic bodies with a carbide phase and a silicide phase, each phase containing a metal. The carbide phase consists of SiC in a proportion of 45 to 90 wt.% and the silicide phase consists of MoSiz and MosSiz in a proportion of 10 to 55 wt.%.

AT 525.295 Al discloses conductive silicate ceramic bodies comprising at least 50 wt.% of a silicate and at least 5 wt.% of a boride, and optionally carbides.

and silicides.

EP 2 140 927 A1 describes a cuboid honeycomb body made of insulating ceramic, in particular silicate ceramic, porcelain or aluminium-magnesium silicate, for thermal and catalytic exhaust gas purification, with several channels through which a fluid can flow.

Fluid channels with a coating of catalytically active nanoparticles.

The conductive ceramic honeycomb structures described in the prior art are largely based on a composite of exclusively conductive ceramics, which, unlike silicate ceramics, have disadvantages in their material properties: for example, they are brittle or exhibit low thermal shock stability. Furthermore, due to their cylindrical honeycomb geometry, homogeneous current flow is limited, and therefore...

Temperature distribution becomes more difficult.

The object of the present invention is therefore a method for producing a conductive ceramic composite body which, despite effective electrical conductivity, retains the excellent material properties of silicate ceramics and exhibits a homogeneous

To ensure temperature distribution.

The problem is solved by a method for producing an electrically conductive ceramic insulator-conductor composite material with a silicate ceramic conductive cuboid honeycomb body, wherein the honeycomb body comprises at least 60 vol% of an insulating silicate, 9 to 38 vol% of a carbide, and at least 2% of a sintering additive, wherein electrodes in the form of metallic layers are applied to two opposing lateral surfaces of the cuboid honeycomb body, the method comprising the steps of: a) providing a mixture of the silicate, the carbide, and the sintering additive with a binder; b) extruding the mixture from step a) into a cuboid honeycomb body; c) drying the formed mixture from step b); d) heating the formed mixture from step c) to 200°C–400°C; and e) sintering in a controlled atmosphere at 1150°C to 1300°C,

f) Mechanical and/or chemical cleaning of the surfaces intended for the electrodes, g) Activation of the surfaces intended for the electrodes,

h) Applying the electrodes by metallizing the cleaned surfaces.

The silicate, carbide, and sinter additive are preferably in powder form. The particle sizes of the powder are preferably between 1 µm and 40 µm. Binders and additives used to achieve the desired rheological properties for extrusion include, for example, water, methylcellulose, PEG, PVA, bentonite, surfactants, and release agents.

Application. Cordierite, steatite and/or mullite are preferably used as silicates.

upset.

The extrusion of the mixture in step b) into a cuboid honeycomb body can be achieved by extruding the mixture from a) through a rectangular or square die into a square or rectangular strand. The strand is then cut at the ends from the

The tool is brought to the desired length using a cutting agent.

The drying step of the shaped mixture from step b) in step c) takes place under controlled conditions (humidity, temperature), preferably in a climate chamber, to slowly reduce the moisture content of the ceramic body and

To avoid cracking.

Preferably, step d) is carried out at 250–300 °C with a low heating rate of a maximum of 1.5 K/min during the heating from room to process temperature. This step serves to remove organic additives from the binder.

The sintering in step e) takes place in a controlled, oxygen-free atmosphere.

Preferably, step f) is carried out by sandblasting followed by chemical degreasing, e.g. with a solution of disodium metasilicate, sodium hydroxide, sodium carbonate and, if necessary, a surfactant at 50°C — 80°C for 5 — 15 min.

Steps d) and e) are preferably carried out in one furnace run.

Step g) preferably takes place by contact with citric acid dissolved in water for

approximately 60 s at room temperature. The metallization of the surfaces in step h) is preferably carried out by electroplating.

chemical coating, by applying a liquid or powdered coating

coating material, soldering or printing processes (e.g. screen printing).

Electrodes are applied in the form of metallic layers.

The electrodes, in the form of metallic layers, are preferably arranged over the entire length or sectorally on the opposite surfaces of the cuboid honeycomb body.

upset.

Preferably, the sum of carbide and sinter additive is at least 18 vol.% and the proportion of carbide in the carbide-sinter additive mixture is at least 50 vol.%.

The silicate is preferably selected from the group consisting of cordierite, steatite, mullite, or mixtures thereof. Preferably, the silicate comprises cordierite. The carbide is preferably selected from the group consisting of SiC and/or TiC. The sintering additive is preferably

Carbon or boride, especially preferably TiB2, ZrB2 or mixtures thereof.

The honeycomb structure preferably has a rectangular or square cross-section.

Cross-section. For electrically conductive bodies, the requirement is that the metal layer of the electrode has a lower electrical resistance than the ceramic. The thickness of the

The metal layer is a maximum of 500 µm, preferably between 20 and 200 µm.

The electrodes, in the form of metallic layers, are partially or completely covered.

upset.

which is produced by a material-joining process, e.g. soldering.

The ceramic, conductive honeycomb body with metallization exhibits permanent thermal stability up to 1000 °C, and up to 1100 °C for short periods. The ceramic material according to the invention has a specific resistance at 30 °C between 1:10⁻⁵ and 1:10⁻⁵ m. Furthermore, the conductive body exhibits excellent thermal shock resistance, i.e.,

The body remains stable after a sudden cooling from 800 to 0°C.

Finally, the invention relates to the use of the conductive body of the aforementioned type as a carrier for exhaust gas purification.

The invention further relates to the use of the conductive body of the aforementioned type.

as a chemical reactor.

The invention also relates to the use of the conductive body of the aforementioned type.

as a carrier for temperature change adsorption or absorption.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be explained in more detail below using examples and figures.

It shows in the figures:

Fig. 1 shows a cross-section of an electrically conductive, ceramic insulator-conductor composite material according to the state of the art.

Fig. 2 shows an electrically conductive ceramic insulator-conductor composite material according to the invention.

Figures 3 to 6 show embodiments of electrically conductive ceramic insulator-conductor composite materials according to the invention in oblique projection.

Fig. 7 shows a cross-section through an electrically conductive ceramic insulator-conductor composite material according to the invention.

Fig. 8 shows a cross-section through another electrical device according to the invention.

conductive, ceramic insulator-conductor composite material.

Oblique crack.

The development of an electrically conductive, ceramic insulator-conductor composite with a silicate ceramic base in the form of a cuboid honeycomb structure offers the possibility of creating a material that combines the excellent properties of silicate ceramics and their comparatively low sintering temperature with additional material properties such as good electrical and thermal conductivity. The combination of different materials into an electrically conductive, ceramic insulator-conductor composite requires similar surface energies for sufficient wetting and similar coefficients of thermal expansion for joint processing.

as well as sufficient compaction of the ceramic composite during sintering.

High porosity of the composite material would prevent a continuous conductive network within the honeycomb structure and thus effective conductivity. The transition from insulating to conductive for an insulator-conductor composite occurs abruptly (percolation threshold) at a critical volume concentration of the conductive phase in the insulating silicate ceramic. At low concentrations, the distance between the electrically conductive particles distributed across the insulating matrix is large, and there is little or no connection. The possibility of forming a connected network increases with

increasing volume fraction of conductive particles.

Due to the largest volume fraction of silicate ceramic, the sintering temperature of the composite material relating to the present invention is in the range of silicate ceramics (-1200°C), which is significantly lower than the sintering temperature for carbides (>1900°C). To nevertheless achieve sufficient densification of the composite material, a sintering additive of at least 2 vol.% is added, for example, carbon, boride, or aluminum.

Composite materials have a positive influence. Compared to metallic honeycomb structures, ceramic honeycomb structures have the advantage that

on ceramic honeycomb bodies due to a comparatively increased surface roughness

simply a functional coating that increases the inner surface area was applied.

up to 1100 °C.

Most ceramic and electrically conductive materials, however, exhibit a negative temperature coefficient; their electrical resistance decreases with increasing temperature. In contrast to metallic honeycomb structures, these materials lack self-regulating properties. As the temperature rises, the resistance decreases further, which can lead to increased current flow, further heating, and ultimately overheating. For this reason, the contacting and homogeneous current introduction (avoiding point contacts and hot spots) described in this invention into a ceramic,

Conductive composite material is crucial for its functionality.

Due to the comparatively high surface roughness of ceramic surfaces and partial surface oxidation, a high contact resistance results. This can lead to very high resistance when current flows through the conductive ceramic at a single point, resulting in high power at the contact point and consequently local heating of the contact, potentially leading to failure. Therefore, for low-loss and uniform current application into the ceramic, conductive honeycomb structure, a surface metallization (electrode) of the ceramic at the point of current application is necessary. This electrode must, on the one hand, guarantee a metallurgical bond with the ceramic to prevent oxidation between the ceramic and the metal. On the other hand, for uniform current application, the electrode must have a lower resistance than the ceramic. Due to the different coefficients of thermal expansion of the metallic electrode and the ceramic, it is also necessary that the metal layer (electrode) does not exceed a certain thickness (approx. 500 µm) in order to prevent breakage of the ceramic honeycomb structure due to shear or tensile stresses during rapid heating and cooling processes. For the quality of the

A material-bonded connection requires prior mechanical and chemical cleaning of the

by means of solder or printing of metallic pastes (e.g. screen printing process).

Another challenge in this context is the typically round cross-section of the honeycomb structure 2 in composite bodies 1. The round cross-section allows for a more uniform load distribution, leading to improved mechanical stability, especially under compressive loads. Enclosing honeycomb structures 2 with a round cross-section, as per the prior art (Fig. 1), in a metallic housing and connecting them to inlet and outlet pipes is also simpler compared to other shapes. However, if the current is introduced via electrodes 3 on the cylindrical surface of a round cross-section, the current seeks the shortest path. In combination with the negative temperature coefficient of ceramic conductors, this results in only a few short paths being heated significantly, while longer paths along the cross-section remain currentless. It has been shown that a round cross-section provides an inhomogeneous current and power density because the current paths (indicated by the differently shaded arrows) are of varying lengths. In a cuboid honeycomb structure 2, the rectangular cross-section results in a homogeneous current and power density, since the current always has to travel the same distance (Fig. 2). The electrodes 3 are metallic.

Layers applied to two opposite surfaces of the hull.

By applying the electrodes 3 over the entire surface or partially, the silicate ceramic honeycomb body 2 can be heated completely (Fig. 4), almost completely (Figs. 3 and 5) or only partially (sectoral, Fig. 6).

Contacting the end faces is not possible, as this would create direct contact between electrode 3 and the flowing fluid, posing a significant safety risk and potentially leading to corrosion of electrode 3. To ensure a uniform current and temperature distribution along the cross-section while still avoiding contact between electrode 3 and the flowing fluid, the design presented here, with a rectangular, preferably square, cross-section of the honeycomb body and contacting (Fig. 2), either across the entire surface or partially along its length (Figs. 3 to 5), was implemented on two

opposite lateral surfaces designed.

Furthermore, contacting the lateral surfaces, as opposed to the end face, enables the most efficient heating of the honeycomb structure. This allows for sectoral heating along the length of the honeycomb, enabling either the heating of individual segments (Fig. 6) or the adjustment of the heating power in individual segments along the length of the honeycomb. With the aid of intelligent control systems, the power can thus be adjusted to the demand at any given time (e.g., reaction enthalpy depending on the rate), resulting in a drastic energy reduction compared to conventional systems with heating of the entire honeycomb.

Honeycomb structure allowed.

The current supply to electrode 3 can be effected, for example, via a mechanically pressed metallic mesh or knitted fabric 4 in order to compensate for different thermal expansions between the ceramic honeycomb body, electrode 3, and electrical connection 5 (Fig. 7). Another possibility for the current supply is a

point connection, which is produced by a material-bonded joining process 6

(Fig. 8).

In addition to homogeneous current flow, the cuboid honeycomb structure allows for the comparatively simple arrangement of multiple elements compared to other shapes.

Honeycomb bodies 1 side by side or one behind the other (Fig. 9). Tables 1-2 below describe various ceramic, conductive composite materials that were investigated within the scope of the invention.

Examples not in accordance with the invention are also provided for comparison purposes.

Table 1: Composition of samples according to the invention

type TiC Vol.% SIC Vol.% CVol.% TiBz Vol.% ZıB2 Vol.% MI1 72.0 - 25.5 - 2.5 7 M2 72.0 - 25.5 2.5 - 7 M3 70.0 14.0 14.0 2.0 - 7 M4 72.0 - 14.0 - 14.0 M5 72.0 12.5 12.5 - 3.0 7 M6 72.0 26.0 - - 2.0 7

M7 70.0 - 16.0 - 14.0 M8 75.0 - 15.0 - - 10.0 M9 72.0 - 14.0 - 10.0 4.0

M10 75.0 - 15.0 - 8.0 2.0

MI11 80.0 - 10.0 - 10.0 -

MI12 65.0 - 33.0 2.0 - -

M13 76.0 - 12.0 - 12.0 -

Table 2: Composition of comparative examples

Silicate carbide silicide graphite

Vol.% Vol.% Vol.% Vol.% VI 57.0 33.0 10.0 V2 60.0 24.0 16.0 v3 64.0 13.0 23.0 V4 70.0 - 30.0 V5 50.0 50.0 - V6 77.0 - - 23.0

The specific electrical resistance of the samples according to the invention, as well as the comparative examples, is between 1:10 and 1:10* m² at 30 °C. In contrast to the comparative examples, however, the samples according to the invention exhibit continuous thermal stability up to 1000 °C, and up to 1100 °C for short periods. The comparative examples V1-V4, on the other hand, exhibit thermal stability up to a maximum of 300 °C under oxidizing conditions.

°C and V5 and V6 have a resistance only up to 400 °C.

REQUIREMENTS

1. Method for producing an electrically conductive ceramic insulator-conductor composite material (1) comprising a silicate ceramic conductive cuboid honeycomb body (2), wherein the honeycomb body (2) comprises ° at least 60 vol% of an insulating silicate, ° 9 to 38 vol% of a carbide and ° at least 2% of a sintering additive,

wherein electrodes (3) in the form of metallic layers are applied to two opposite lateral surfaces of the cuboid honeycomb body, the process comprising the steps a) providing a mixture of the silicate, the carbide and the sinter additive with a binder, b) extruding the mixture from step a) into a cuboid honeycomb body (2), c) drying the formed mixture from step b), d) heating the formed mixture from step c) to 200°C - 400°C, e) sintering in a controlled atmosphere at 1150°C to 1300°C, f) mechanically and/or chemically cleaning the surfaces intended for the electrodes, g) activating the surfaces intended for the electrodes (3), h) applying the electrodes (3) by metallizing the cleaned surfaces.

2. The method of claim 1, characterized in that steps d) and e) are performed in a

Oven operation will be carried out.

3. Method according to claim 1 or claim 2, characterized in that step f)

This includes sandblasting and subsequent chemical degreasing.

4. Method according to one of claims 1 to 3, characterized in that step g)

the treatment with citric acid for approximately 60 seconds at room temperature. 5. Method according to any one of claims 1 to 4, characterized in that step h)

by electroplating, chemical coating, by applying a liquid or

powdered coating material or a soldering or printing process is used.

6. Electrically conductive ceramic insulator-conductor composite material (1) with a silicate ceramic conductive cuboid honeycomb body (2), wherein the honeycomb body (2) comprises at least 60 vol.% of an insulating silicate, 9 to 38 vol.% of a carbide and at least 2% of a sinter additive, wherein on two opposite lateral surfaces of the cuboid honeycomb body

Electrodes (3) are applied in the form of metallic layers.

7. Electrically conductive ceramic insulator-conductor composite material according to claim 6, characterized in that the silicate is selected from the group consisting of

Cordierite, steatite, mullite or mixtures thereof.

8. Electrically conductive ceramic insulator-conductor composite material according to claim 6 or claim 7, characterized in that the carbide is selected from the group

consisting of SiC and/or TiC.

9. Electrically conductive ceramic insulator-conductor composite material according to one of claims 6 to 8, characterized in that the sintering additive consists of carbon or a

Boride from the group consisting of TiB2, ZrB2 or mixtures thereof.

10. Electrically conductive ceramic insulator-conductor composite material according to claim 9, characterized in that the sum of carbide and sinter additive is at least 18 vol.% and that the proportion of carbide in the carbide-sinter additive mixture is at least 50 vol.%.

11. Electrically conductive ceramic insulator-conductor composite material according to one of claims 6 to 10, characterized in that the honeycomb body has a rectangular shape.

12. Electrically conductive ceramic insulator-conductor composite material according to one of the

Claims 6 to 10, characterized in that the electrodes (3) are in the form of metallic

Layers are applied partially or completely.

13. Electrically conductive ceramic insulator-conductor composite material according to one of claims 6 to 12, characterized in that the electrode (3) has a connection,

which comprises a metallic mesh or knitted fabric (4).

14. Electrically conductive, ceramic insulator-conductor composite material according to one of claims 6 to 12, characterized in that the electrode (3) has a connection,

which is attached by a material-bonding joining process (6).

15. Arrangement comprising several electrically conductive, ceramic insulator-conductor composite materials according to any one of claims 6 to 14, wherein the composite materials are arranged serially

or are connected in parallel.

16. Arrangement according to claim 15, characterized in that at least two composite materials are arranged side by side and at least two composite materials are arranged one behind the other.

are arranged.

17. Use of an electrically conductive, ceramic insulator-conductor

Composite material according to one of claims 6 to 14 as a support for exhaust gas purification. 18. Use of an electrically conductive, ceramic insulator-conductor composite material according to one of claims 6 to 14 as a support for temperature-changing ad-

or absorptions.

19. Use of an electrically conductive, ceramic insulator-conductor

composite material according to one of claims 6 to 14 as a chemical reactor.

Claims (19)

REQUIREMENTS 1. Method for producing an electrically conductive ceramic insulator-conductor composite material (1) comprising a silicate ceramic conductive cuboid honeycomb body (2), wherein the honeycomb body (2) comprises ° at least 60 vol% of an insulating silicate, ° 9 to 38 vol% of a carbide and ° at least 2% of a sintering additive, wherein electrodes (3) in the form of metallic layers are applied to two opposite lateral surfaces of the cuboid honeycomb body, the process comprising the steps a) providing a mixture of the silicate, the carbide and the sinter additive with a binder, b) extruding the mixture from step a) into a cuboid honeycomb body (2), c) drying the formed mixture from step b), d) heating the formed mixture from step c) to 200°C - 400°C, e) sintering in a controlled atmosphere at 1150°C to 1300°C, f) Mechanical and/or chemical cleaning of the surfaces intended for the electrodes, g) Activation of the surfaces intended for the electrodes (3) using an acid, h) Applying the electrodes (3) by metallizing the cleaned surfaces. 2. The method of claim 1, characterized in that steps d) and e) are performed in a Oven operation will be carried out. 3. Method according to claim 1 or claim 2, characterized in that step f) This includes sandblasting and subsequent chemical degreasing. 4. Method according to one of claims 1 to 3, characterized in that step g) The treatment includes applying citric acid for approximately 60 seconds at room temperature. 5. Method according to any one of claims 1 to 4, characterized in that step h) is carried out by electroplating, chemical coating, by applying a liquid or powdered coating material or a soldering or printing process is used. 22/24 LAST CLAIMS SUBMITTED 6. Electrically conductive ceramic insulator-conductor composite material (1) with a silicate ceramic conductive cuboid honeycomb body (2), wherein the honeycomb body (2) comprises at least 60 vol.% of an insulating silicate, 9 to 38 vol.% of a carbide and at least 2% of a sinter additive, wherein on two opposite lateral surfaces of the cuboid honeycomb body Electrodes (3) are applied in the form of metallic layers. 7. Electrically conductive ceramic insulator-conductor composite material according to claim 6, characterized in that the silicate is selected from the group consisting of Cordierite, steatite, mullite or mixtures thereof. 8. Electrically conductive ceramic insulator-conductor composite material according to claim 6 or claim 7, characterized in that the carbide is selected from the group consisting of SiC and/or TiC. 9. Electrically conductive ceramic insulator-conductor composite material according to one of claims 6 to 8, characterized in that the sintering additive consists of carbon or a Boride from the group consisting of TiB2, ZrB2 or mixtures thereof. 10. Electrically conductive ceramic insulator-conductor composite material according to claim 9, characterized in that the sum of carbide and sinter additive is at least 18 vol.% and that the proportion of carbide in the carbide-sinter additive mixture is at least 50 vol.%. 11. Electrically conductive ceramic insulator-conductor composite material according to one of claims 6 to 10, characterized in that the honeycomb body has a rectangular shape. cross-section or has a square cross-section. 12. Electrically conductive ceramic insulator-conductor composite material according to one of claims 6 to 10, characterized in that the electrodes (3) are in the form of metallic Layers are applied partially or completely. 23/24 LAST CLAIMS SUBMITTED 13. Electrically conductive ceramic insulator-conductor composite material according to one of claims 6 to 12, characterized in that the electrode (3) has a connection, which comprises a metallic mesh or knitted fabric (4). 14. Electrically conductive, ceramic insulator-conductor composite material according to one of claims 6 to 12, characterized in that the electrode (3) has a connection, which is attached by a material-bonding joining process (6). 15. Arrangement comprising several electrically conductive, ceramic insulator-conductor composite materials according to any one of claims 6 to 14, wherein the composite materials are arranged serially or are connected in parallel. 16. Arrangement according to claim 15, characterized in that at least two composite materials are arranged side by side and at least two composite materials are arranged one behind the other. are arranged. 17. Use of an electrically conductive, ceramic insulator-conductor Composite material according to one of claims 6 to 14 as a carrier for exhaust gas purification. 18. Use of an electrically conductive, ceramic insulator-conductor composite material according to one of claims 6 to 14 as a support for temperature-changing ad- or absorptions. 19. Use of an electrically conductive, ceramic insulator-conductor composite material according to one of claims 6 to 14 as a chemical reactor. 24/24 LAST CLAIM SUBMITTED