WO2008074319A2 - Ceramic electric heating element - Google Patents

Abstract

The invention relates to ceramic electric heating elements, such as glow plugs, or even heating pins, which are made of a combination of electrically conductive and electrically insulating ceramic materials. The object of the invention is to provide ceramic electric heating elements, wherein predetermined electric conductivities can be selectively adjusted in a broad range, and which have at least close to no residual stress due to an adjusted sintering and thermal expansion behavior of the ceramic materials used during sintering and the use at high temperatures. In the heating element according to the invention, the parts are made from an electrically conductive and an electrically insulating ceramic composite material. At least the electrically conductive ceramic composite material is formed using at least three ceramic components, wherein one ceramic base component is silicon nitride, SiAION, or aluminum nitride, and the electrical conductivity is influenced by at least two ceramic components, wherein one or more components (n) comprise an electrically conductive silicon of a metal selected from Mo, W, Ta, Nb, Ti, Zr, Hf, V, and Cr, and a further ceramic component having a lower electric conductivity as opposed to this/these component(s), and wherein the respective portion of further ceramic components determines the electric conductivity of the ceramic composite material(s).

Description

Ceramic electric heating element

The invention relates to ceramic electrical heating elements, such as glow plugs or heating pins, which are formed with a combination of electrically conductive and electrically insulating ceramic materials. This material combination can preferably be used at high temperatures.

For glow plugs or heating pins, ceramic materials which are formed from silicon nitride and molybdenum silicide or another silicide are used in conventional form. The electrical conductivity is influenced only by the proportion and the grain size of the molybdenum silicide. The electrically insulating ceramic material is likewise formed from silicon nitride and molybdenum silicide. The volume fraction of the electrically conductive component (molybdenum silicide) is then, however, in relation to the electrically solubilizing component (silicon nitride) smaller. However, the different composition of the two materials used has a disadvantageous effect on the production and the service life of such heating elements, since the two main components

(Silicon nitride and molybdenum silicide) are very different in their thermo-physical properties. In particular, sintering results in differences in the compaction behavior, which in turn can lead to geometry instabilities or crack formation.

It is also problematic that the thermal expansion coefficients of these two materials differ significantly, so that even when using residual stresses can not be avoided. As already mentioned, the electrical conductivity can be influenced only by the proportions and the grain size of the molybdenum silicide. However, since this is only possible within limits, only a limited range of electrical conductivity can be covered with these two materials and these can be precisely adjusted. Such heating elements can therefore not be produced in any geometric shapes and used universally.

From DE 101 51 617 B4 a heatable ceramic element is known, which is formed with an electrically insulating and at least one electrically conductive zone of ceramic material.

The technical solution described in DE 198 60 919 C1 relates to a ceramic heating element and suitable manufacturing method. It should be an inner insulation layer and an outer layer

Have conductive layer. Both layers should be next to other components include silicon nitride and molybdenum silicide.

A glow plug for a diesel engine is described in DE 36 21 216 C2.

From DE 697 00 797 T2 a ceramic heating element is also known.

It is therefore the object of the invention to provide ceramic electrical heating elements in which predetermined electrical conductivities can be set in a targeted manner over a wide range and, as a result of an adapted sintering and thermal expansion behavior of the ceramic materials used, during sintering and use high temperatures have at least almost no residual stresses.

According to the invention, this object is achieved with heating elements having the features of claim 1.

Advantageous embodiments and further developments of the invention can be achieved with features described in the subordinate claims.

In the heating element according to the invention, the electrically conductive are formed with an electrically conductive and the electrically insulating part with an electrically insulating ceramic composite material, wherein at least the electrically conductive part is formed with three ceramic components. In this case, in both ceramic composite materials as the electrically conductive main component one or more electrically conductive silicide (s) is present, which is bonded to a metal selected from Mo, W, Ta, Nb, Ti, Zr, Hf, V and Cr - det is / are, with molybdenum silicide being preferred. The ceramic base material is silicon nitriride, sialon or aluminum nitride.

The electrical properties of the two ceramic composite materials are adjusted by the proportion of an additionally electrically less conductive ceramic component, such as SiC or B ₄ C. These have similar thermo-physical properties as the silicon nitride.

By virtue of almost equal proportions of the electrically conductive main component, both ceramic composite materials (electrically conductive and insulating) also have very similar thermo-physical properties. For this reason, occur only during sintering and during use only negligible residual stresses in a heating element according to the invention, which has a simplified production and a significantly improved leh lifetime behavior with improved reliability and increased life result.

The heating element according to the invention is particularly preferably formed of two ceramic composite materials with silicon nitride, silicon carbide and molybdenum silicide, wherein the proportion of molybdenum silicide in both ceramic composite materials (electrically conductive and insulating) should be almost the same.

Thus, the thermal expansion coefficient of SiC with 4 * 10 ^{~ 6} K ^{"1 is} approximately midway between that of silicon nitride (2.5 * 10 ^{~ 6} K ^-1 ) and that of MoSi ₂ (8.25 * 10 ^{~ 6} K ¹ ), which has an advantageous effect on the thermal behavior, in particular under thermal cycling. The resistance of MoSi ₂ is 2 * 10 ⁵ Ωcm, that of SiC in the range 0.1 to 1000 Ωcm (depending on the C content) and that of silicon nitride at l * 10 ¹² Ωcm.

It has surprisingly been found that the required electrical conductivity can already be achieved by a combination with a ceramic material having a significantly higher specific electrical resistance, even at a proportion of molybdenum silicide which is below the usually required threshold for percussion.

In addition, with certain proportions of silicide and carbide selected, the resistivity can be set in a range of 0.001 to 10 Ω · cm for an electrically conductive ceramic composite without reaching the percula- tion threshold.

The three components are directly milled and homogenized during the treatment of the starting powders and not formed by reaction sintering. Thus, there is no reactive formation of one of the three components. When producing a ceramic composite material according to the invention, the three components can be used in powder form. A silicon nitride powder should have an average particle size d50 in the range 0.2 to 2.5 μm, molybdenum silicide in the range 0.5 to 2.4 μm and silicon carbide in the range 0.5 to 2 μm. In particular, molybdenum silicide and silicon carbide powder should be as fine as possible.

Characteristic is the formation of an electric conductive network structure formed with MoSi ₂ and SiC in the volume of the base ceramic SIaN ₄ .

An electrically conductive ceramic composite can be used in proportions of 40% by mass of silicon nitride, 30

Mass% silicon carbide and 30 mass% molybdenum silicide be formed. It has a specific electrical resistance of 0.2 Ω cm, which can be adjusted so targeted.

An electrically insulating (dielectric) ceramic composite may be formed in proportions of 65% by mass of silicon nitride, 5% by mass of silicon carbide, and 30% by mass of molybdenum silicide. It has a specific electrical resistance of at least 100 Ω cm.

Conventional sintering additives can be used for both ceramic composite materials.

With the silicon carbide, a finer structure can be formed, which in addition to the favorable thermal expansion behavior also causes improved mechanical properties. The oxidation resistance is also improved.

The difference in the proportions of molybdenum silicide in the electrically conductive and in the insulating part of a heating element should not be greater than 10% by mass.

Thus, a conventional heating elements inserted ceramic material, the hergestelltist with 34 mass% SiSn ₄ and 66% by mass of MoSi2, see a thermal expansion coefficient of 4.28 * 10 ^-6 K ^"1 and a specific electrical resistance of from 0.005 Ωcm on.

In the invention applicable electrically conductive Keranαikkompositwerkstoffe with a share of 48 mass% Si ₃ N ₄ , 5 mass% SiC and 47 mass% MoSi ₂ have a thermal expansion coefficient of 3.21 * 10 ^{~ 6} K ^"1 and a specific electrical resistance of 0.162 Ωcm, a ceramic composite having a content of 36 mass% Si ₃ N ₄ , 28 mass% SiC and 36 mass% MoSi ₂ has a thermal expansion coefficient of 3.32 * 10 ^{~ 6} K ^"1 and a specific electrical resistance of 0.03 Ωcm, a proportion of 61 mass% Si ₃ N ₄ , 3 mass% SiC and 36 mass% MoSi ₂ , with a content of 35 mass% Si ₃ N ₄ , 18 mass -% SiC and 47% by mass MoSi ₂ have a thermal expansion coefficient of 3.69 * 10 ^{~ δ} K ^"1 and a specific electrical resistance of 0.01 ohm-cm and an electrically insulating Keramikkompositwerkstoff with 36% by mass of MoSi ₂ has a coefficient of thermal expansion of 2.68 * 10 ^{~ 6} K ^-1 and a specific elect resistance of 142030 Ωcm.

At a constant proportion of 47% by mass of MoSi ₂ may be taken by changing proportions of SiC influence on the electrical resistivity. Thus, it can be assumed that without additional SiC the specific electrical resistance goes to infinity. With a SiC content of 5 mass% at 0.2 Ωcm, with a proportion of 9 mass% at 0.06 Ωcm, with a proportion of 14 mass% at 0.002 Ωcm, with a proportion of 18 Mass% at 0.002 Ωcm and at a proportion of 23% by mass at 0.002 Ωcm.

With a constant proportion of 36% by mass of MoSi _{2, it is} possible to influence the specific electrical resistance by changing the proportion of SiC. Thus, it can be assumed that, without additional and up to a proportion of 3 mass% SiC, the specific electrical resistance also approaches infinity, which is desirable in the case of an electrically insulating ceramic composite material. The specific electrical resistance is at a SiC content of 6% by mass at 2282 Ωcm, with a share of 9% by mass at 10 Ωcm, with a share of 11% by mass at 0.03 Ωcm and a share of 17 Mass% at 0.03 Ωcm.

At a constant proportion of 6% by mass of SiC, changes in the specific electrical resistance occur as a result of altered proportions of MoSi ₂ . Thus, it can be assumed that with a share of up to about 30 mass% MoSi _2, the specific electrical resistance goes to infinity. With a share of 32% by mass at 1573 Ωcm, with a share of 36% by mass at 18 Ωcm, with a share of 38% by mass at 18 Ωcm, with a share of 42% by mass at 1 Ωcm a proportion of 47% by mass at 0.05 Ωcm and is at a proportion of 51% by mass at 0.005 Ωcm.

The invention will be explained in more detail with reference to examples.

Example 1: In a first example of a heating element according to the invention a Kermikkompositwerkstoff with electrically conductive main component of 47 mass% molybdenum disilicide, with 18 mass% SiC and 35 mass% silicon nitride is formed for the electrically conductive part. It has a specific electrical resistance of 0.01 Ωcm and a thermal expansion coefficient of 3.94 * 10 ^"6 K ^{" 1} .

For the non-electrically conductive part, a composition having 47% by mass of molybdenum disilicide, 0% by mass of SiC and 52% by mass of silicon nitride was selected. The specific electrical resistance is infinite and the thermal expansion coefficient is 3.20 * 10 ^"δ K ^{" 1} .

Sintering additives containing 10% by mass of SC ₂ O ₃ based on the proportion of silicon nitride and silicon carbide as the main component are used.

In the preparation, a mixture of the pulverulent starting components is prepared in an isopropanol suspension, which is then dried in a rotary evaporator. Kneading with added polymers and surfactants achieves an injection-moldable consistency. A heating element in the form of a glow plug with a diameter of 3 mm and a length of 45 mm can be produced in a two-component injection molding process. Then organic components are expelled during a heat treatment. The sintering takes place in a nitrogen atmosphere at temperatures up to max. 1800 ⁰ C.

A heating element produced in this way can be used in continuous operation at an application temperature of 1500 ^° C. without causing damage or destruction.

Example 2:

In a second example of a heating element according to the invention a Kermikkompositwerkstoff with electrically conductive main component of 20 mass% molybdenum disilicide with 30 mass% SiC and 45 mass% silicon nitride is formed for the electrically conductive part. It has a specific electrical resistance of 0.2 Ωcm and a thermal expansion coefficient of 2.9 * 10 ^{~ 6} K ^-1 .

For the non-electrically conductive part was a

Composition with 20% by mass of molybdenum disilicide, 10% by mass of SiC and 69% by mass of silicon nitride. The specific electrical resistance goes to infinity and the thermal expansion coefficient is 2.81 × 10 ^"6 K ^'1.

Sintering additives with Y ₂ O ₃ / Al ₂ O ₃ (with 7% by mass of Y ₂ O ₃ and 5% by mass of Al ₂ O ₃ based on silicon nitride and silicon carbide) are used as the main component.

In the preparation, a mixture of the pulverulent starting components is prepared in a water suspension, which is then dried by spray granulation. A heating element can be provided in the form of a plate with a jet-shaped heating zone by isostatic pressing of both ceramic composites in a defined structure and subsequent green processing. In this case, an electrically insulating part is surrounded on the outside by an electrically conductive part in a U-shaped manner. Then, organic components such as pressing aids are expelled in a heat treatment. The sintering takes place in a nitrogen atmosphere at temperatures up to max. 1750 ⁰ C.

A heating element produced in this way can be used in continuous operation at an application temperature of 1350 ^° C. without damaging or destroying it.

Example 3:

In a third example of a heating element according to the invention a Kermikkompositwerkstoff with electrically conductive main component of 30 mass% molybdenum disilicide, with 35 mass% SiC and 35 mass% silicon nitride is formed for the electrically conductive part. It has an electrical resistivity of 0.05 ohm-cm and a thermal expansion coefficient of 3.21 * 10 ^"6 K ^'1.

For the non-electrically conductive part, a composition of 24 mass% molybdenum disilicide, 10 mass% SiC and 66 mass% silicon nitride was chosen. The electrical resistivity approaches infinity and the thermal expansion coefficient is 2.95 * 10 ^-6 K ^-1 .

Sintering additives with Y ₂ O ₃ (15% by weight based on the proportions of silicon nitride and silicon carbide) are used as the main component.

During production, a mixture of the pulverulent starting components is prepared in a water suspension, the film-pourable suspension is prepared by film-casting the two ceramic composites in the Finished structure, with subsequent lamination and green processing to a heating element processed (a variety of ^' geometries are possible). Then organic components, such as dispersants, are expelled during a heat treatment. The sintering takes place in a nitrogen atmosphere at temperatures up to max. 1800 ⁰ C.

A heating element manufactured in this way can be used in continuous operation at an application temperature of 1400 ^° C. without damaging or destroying it.

At sintering temperatures from 1,750 ° C, it is favorable to increase the nitrogen pressure with argon.

Claims

claims 1. Ceramic electric heating element, wherein the parts are each formed with an electrically conductive and an electrically insulating ceramic composite, while at least the electrically conductive Keramikkompositwerkstoff is formed with at least three ceramic components, this is a ceramic base component silicon nitride, SiAlON or Alumniumnitrid and the electrical conductivity influenced by at least two ceramic components; wherein one or more component (s) is an electrically conductive silicide of a metal selected from Mo, W, Ta, Nb, Ti, Zr, Hf, V, and Cr; the proportion of silicide (s) is below the threshold of percussion; In addition, a further ceramic component with respect to this / these component (s) of smaller electrical conductivity are included and the respective proportion of further ceramic component determines the electrical conductivity of the / the Keramikkompositwerkstoffe (s). 2. Heating element according to claim 1, characterized in that the further ceramic component is silicon carbide or B ₄ C. 3. Heating element according to claim 1 or 2, characterized in that at least one sintering additive is included. 4. Heating element according to one of the preceding claims, characterized in that the proportions of silicide differ in the electrically conductive and in the lektrisch insulating ceramic composite material by a maximum of 10% by mass. 5. Heating element according to one of the preceding claims, characterized in that it is formed with silicon nitride, molybdenum silicide, silicon carbide and a sintering additive. 6. Heating element according to one of the preceding claims, characterized in that the electrically conductive ceramic components form an e-lektrisch conductive network structure within the volume of the ceramic base component. 7. Heating element according to one of the preceding claims, characterized in that the electrical conductivity is determined by the respective proportion of the further ceramic component. 8. Heating element according to one of the preceding claims, characterized in that none of the components contained in the ceramic composite components has been reactively formed during sintering.