WO2005056494A1

WO2005056494A1 - Method for producing a precursor ceramic

Info

Publication number: WO2005056494A1
Application number: PCT/DE2004/002234
Authority: WO
Inventors: Alexander Klonczynski; Ralf Riedel; Martin Koehne; Herwig Schiefer; Rahul Harshe
Original assignee: Robert Bosch Gmbh
Priority date: 2003-12-09
Filing date: 2004-10-08
Publication date: 2005-06-23
Also published as: DE10357354A1; EP1704129A1

Abstract

The invention relates to a method for producing a precursor ceramic by pyrolyzing oxygen-containing element-organic precursor polymers. The precursor polymers contain aluminum as an additive.

Description

Process for producing a precursor ceramic

The invention relates to a method for producing a precursor ceramic according to the preamble of the independent claim.

State of the art

In the manufacture of ceramic glow plugs from ceramic composites, amorphous SiOC ceramics are obtained through the pyrolysis of organic precursors. Advantages of the precursor thermolysis process compared to conventional manufacturing processes for ceramics (sintering) are the significantly lower process temperatures and the easy processability and formability of polysiloxane resins.

The production of moldings is only possible with the use of additional fillers, otherwise shrinkage cracks and pores will occur during pyrolysis. In this way, the properties (coefficient of thermal expansion, thermal conductivity, specific electrical resistance) of the composite can be precisely set using suitable fillers. It is possible, as disclosed in EP-B-0 412 428, to use reactive fillers in order to achieve better binding of the fillers to the matrix or to use inert fillers.

Through the choice of fillers, the electrical and physical property profile of the ceramic composite material of the glow plug resulting after pyrolysis is tailored exactly to the requirement profile. The use of an oxygen-containing polysiloxane precursor as the starting material enables easy processing in air and thus the production of inexpensive products. The pyrolysis product of the filled polysiloxane has good strength, high chemical stability (oxidation, Corrosion) and is harmless to health. In general, one of the great advantages of the precursor thermolysis process over the conventional manufacturing processes (sintering) for ceramic composite materials is the possibility that a larger spectrum of fillers is available. Firstly, because pyrolysis generally takes place at much lower temperatures than the sintering process, which means that at sintering temperatures liquid or volatile fillers are still used in the precursor pyrolysis process and, in addition, phase reactions occurring at higher temperatures can be avoided; on the other hand, because the polysiloxane resins, as meltable thermosetting polymers and soluble in organic solvents, enable simple and extremely homogeneous incorporation of fillers into the precursor (kneading, dissolving). This is so interesting because the properties of the precursor composite material can be adjusted over a wide range with a large selection of fillers.

However, in order to ensure that the properties are set via the fillers, the influence of the matrix on the respective property must be as low as possible. Since the matrix forms a coherent network in all of the composites of the ceramic glow plug, a problem arises for the production of the insulating intermediate layer of the glow plug if the matrix has a too low specific electrical resistance after the production process. Another problem arises in the event that the matrix or the composite loses high-temperature strength and thermal shock resistance due to phase changes, crystallization and oxidation. The problem of the low specific electrical resistance and the undesired crystallization of the matrix material could be significantly reduced by using boron-containing fillers. However, the presence of an amorphous glass can lead to insufficient creep resistance of the matrix material at high temperatures. This can have an effect in particular on the local deformation of the material in the hot areas of the glow pencil.

The aim of the present invention is therefore to increase and stabilize the specific electrical resistances of the material used and to achieve an increase in the high-temperature creep resistance. Advantages of the invention

The process according to the invention for the production of precursor ceramics by pyrolysis of oxygen-containing elemental organic precursor polymers has the advantage over the prior art that the resulting material has an increased specific resistance.

It is also advantageous that it shows a significantly improved high-temperature creep resistance.

Another advantage is that the resulting material does not undergo phase changes in the material that lead to its mechanical destruction (durability).

In addition, an advantage of the method according to the invention is that there is no aging of the specific electrical resistance and therefore no aging of the functional properties of the resulting material.

Advantageous developments of the invention result from the measures mentioned in the subclaims.

embodiments

The essence of the invention is the use of aluminum as an additive by modifying the polymer and / or by adding as an additive in the form of aluminum-containing fillers. The modification is a synthesis (for example sol-gel synthesis) of an aluminum-containing polymer. When used as an additive, due to the low melting point of aluminum, use as the finest nanopowder is a basic requirement to achieve the desired effects. The reaction of the aluminum with the oxygen from the SiOC matrix can be regarded as decisive here. This reaction leads to the formation of a mullite which is significantly more resistant to high temperatures and especially more resistant to high temperatures than amorphous SiOC glass. This improves the durability of the resulting material and the aging of the electrical resistance is reduced.

There are aluminum-containing SiO (Al) C samples, which were produced either by adding aluminum nanopowder to the polysiloxane or by modifying the polymeric precursors, in an atmosphere intended for the application (i.e., under

Argon, H ₂ , N ₂ , CH ₄ , etc.) pyrolyzed in the temperature range between 600 ° C - 1400 ° C. The procedure is as follows: In insulation materials, ie materials that are electrically insulating after heat treatment (ceramic materials with a specific electrical resistance R> 10 ³ Ωcm) or conductive materials, ie materials that are electrically conductive after heat treatment (ceramic materials with a specific electrical resistance R <10 ° Ωcm), the glow plug is incorporated with aluminum-containing additives during processing.

The amount of aluminum incorporated is in the range from 0.1 to 60% by mass, preferably between 0.1 and 5% by mass.

The pyrolysis of the insulation materials were all carried out under standard conditions (heating from 20 ° C. to 1300 ° C. at 30 K / h, holding for 1 h at 1300 ° C., and cooling at 300 K / h) in order to compare them with the property profile more conventionally To ensure standard dimensions.

Example 1 :

Production of two masses with the same volume of fillers:

Composition 1: 65% by volume of polymer (MK-polymer polysilsesquioxane) / 30% by volume of SiO _2/5 vol% aluminum nanopowder mass 2: 65% by volume of polymer (MK-polymer polysilsesquioxane) / 25% by volume of SiO ₂ / 10 vol% aluminum nanopowder The masses were prepared by grinding the powders in the planetary ball mill and then sieving them with a mesh size between approximately 100 μm and approximately 500 μm. The samples were then shaped and cross-linked using a hot pressing process. The pyrolysis of the samples was carried out at heating rates in the range of 25 K / h in order to ensure compact samples.

In order to observe the phase development with increasing pyrolysis temperature, the pyrolysis was carried out at 1100 ° C, 1200 ° C, 1300 ° C and 1400 ° C.

The X-ray diffraction studies on the materials show the reaction of the aluminum with the oxygen of the SiOC matrix to form mullite and the simultaneous formation of SiC. This indicates that the use of aluminum nanopowder makes it possible to bind the excess carbon out of the matrix. As a result, carbon apparently makes no contribution to conductivity.

Since the separation of the carbon can be regarded as the main reason for the reduction in the specific electrical resistance, a measurement correlating with the specific electrical resistance of the samples showed a correlation with the excretion or development of the free carbon in the SiOC ceramic. It can be seen that the specific electrical resistance of the samples does not decrease even at pyrolysis temperatures above 1300 ° C. It can also be seen that the level of the specific resistance at 10 ⁶ Ωcm is 3-4 orders of magnitude higher than for aluminum-free SiOC / SiO ₂ composites.

Example 2:

Aluminum-containing conductive masses and insulation masses for a ceramic glow plug with a diameter of approximately 3 mm were produced. The production was carried out by grinding the fillers in the planetary ball mill and then sieving with a mesh size of 150 μm. The samples were then shaped and crosslinked using a hot press process. The compositions of the ceramic starting materials were in the following range:

50-80% by volume of polysiloxane with an amount of 5% by volume of aluminum nanopowder already contained in the polymer and 1% by weight of zirconium acetylacetonate (based on the amount of polymer) 0-10% by volume SiC 0-20% by volume Al ₂ O ₃ 0-30% by volume MoSi ₂ 0-5% by mass boron

The pyrolysis was carried out with a heating rate of 25K / h to 1300 ° G, an hour of holding time at the final temperature and an argon flow of 2 l / h. The degree of filling in a graphite furnace from FCT was 12%. The samples were then exposed to air in a Nabertherm oven for 8h / 1350 ° C.

It could be shown that the specific electrical resistance of the

Insulation material can be stabilized to a value above 10 ohm cm even when aluminum is added. Even after aging at 1350 ° C, no resistance aging can be seen.

The very low post-shrinkage of the material after aging at 1350 ° C indicates a significantly higher creep resistance of the matrix material.

A large part of the aging of the electrical properties of the conductive material results from the post-shrinkage of the matrix material and the associated convergence of the MoSi ₂ particles (shift in the percolation curve). Therefore, a lower post-shrinkage also results in a slight aging of the specific electrical resistance of the guide masses.

This is the decisive advantage of using aluminum as an additive to increase the high temperature stability. The crystallization behavior with regard to the formation of cristobalite was examined on the basis of dilatometric measurements. This shows that even at temperatures of 1350 ° C in air, no cristobalite has formed in the bulk of the different materials. Cristobalite has a much higher one

Coefficient of thermal expansion than the other materials in the corresponding masses. Compared to the systems without aluminum, these results indicate a significantly improved thermal cycling resistance.

Example 3:

An aluminum modified resin was made using a sol-gel process. For this purpose, MK polymer with 1% by mass of zirconium acetylacetonate as catalyst was dissolved in isopropanol and a proportion of 9.1% by mass of alumatran (AKA005 from ABCR) (based on the polymer) was added. After gelling, the gel was dried at 120 ° C for 5 hours.

The dried gel was then ground and then hot pressed at 180 ° C. The

Pyrolysis temperature was 1100 ° C in an argon atmosphere.

The samples were kept for about 10 to about 50 hours after pyrolysis

Treated temperatures of 1200 ° C - 1700 ° C in an argon atmosphere.

It can be seen in the X-ray diffraction diagrams for different aging temperatures that a mullite / SiC structure is formed in an amorphous matrix. At the same time, it should be noted that a large proportion of the material is amorphous even at temperatures of 1700 ° C.

The samples were free of cracks after the thermal treatment. They showed no signs of carbothermal reduction, indicating a significant improvement in the

High temperature resistance in comparison to aluminum-free SiOC materials indicates.

With aluminum-modified polymers and with the use of aluminum nanopowder as a filler, significantly improved high-temperature properties can be achieved at temperatures> 1600 ° C. A significant increase in the specific electrical resistance of the SiOC matrix is possible by using aluminum nanopowder. The reaction of the aluminum with the oxygen from the SiOC matrix leads to the formation of a stable mullite / SiC compound.

According to the literature, the glass transition temperature of aluminosilicate glasses is T> 1500 ° C and thus at least 150 ° C higher than that of SiO ₂ or borosilicate glasses. At the same time, the achievable strength level of a mullite / SiC composite is considerably higher (approximately 400 MPa) than that of pure SiOC ceramics (approximately 150 MPa).

When using aluminum additives in the composite material of glow plugs, for example, no other oxidation products (e.g. MoO ₃ , Mo ₅ Si ₃ , SiO ₂ ) could be detected in the bulk of the material even after longer aging times (1400 ° C / 50h).

The improvements that can be achieved with the method according to the invention by incorporating aluminum into the polymer matrix are:

Formation of aluminum oxide with the oxygen from the SiOC matrix and the elemental aluminum as filler. From the additional free bonds on silicon that were previously occupied by oxygen, silicon carbide forms with the excess carbon from the SiOC matrix. This leads to an increase in the specific electrical resistance of the material regardless of the fillers used. Substantially improved high-temperature creep resistance due to the formation of a mullite / SiC composite with significantly increased glass transition temperature [values?] No formation of cristobalite and thus an improved durability, in particular due to the higher Due to the low tendency to re-shrink, the glass transition temperature results in no aging of the specific electrical resistance of the conductive material and therefore no aging in the functional properties of the resulting material with regard to the heating time and annealing temperature

Claims

Expectations

1. A method for producing a precursor ceramic by pyrolysis of oxygen-containing elemental organic precursor polymers, characterized in that the precursor polymers contain aluminum as an additive.

2. The method according to claim 1, characterized in that the aluminum is added to the precursor polymers in the form of aluminum nanopowder as a filler.

3. The method according to claim 1, characterized in that aluminum-modified element-organic precursor polymers are used as starting materials.

4. The method according to any one of claims 1 to 3, characterized in that the aluminum reacts with the oxygen of the elemental organic precursor polymer to form mullite.

5. The method according to any one of claims 1 to 4, characterized in that the excess carbon of the elementary organic precursor polymer reacts to form SiC.

6. The method according to claim 4 and 5, characterized in that a stable mullite / SiC composite is formed.

7. The method according to any one of the preceding claims, characterized in that the specific resistance of the precursor ceramic is> 10 ⁶ ohm cm.

8. Use of aluminum-containing precursor ceramics for the production of glow plugs.