Method of producing a conductive silicon carbide-based...

Details Method of producing a conductive silicon carbide-based... Method of producing a conductive silicon carbide-based...

1998-04-08

2001-02-13

Derrington, James (Department: 1734)

Plastic and nonmetallic article shaping or treating: processes

Outside of mold sintering or vitrifying of shaped inorganic...

Including plural heating steps

C264S674000, C264S676000, C264S682000, C252S504000, C501S090000

Reexamination Certificate

active

06187256

ABSTRACT:

Materials based on silicon carbide have been known for some time and are utilized in a variety of ways for producing components. They have a series of interesting properties, including the low density, the high hardness, the low coefficient of thermal expansion, the good oxidation and corrosion resistance and also a favourable creep behaviour and high thermal conductivity.
Furthermore, pure SiC has semiconducting properties, with the corresponding electrical behaviour. Of particular interest here is pressureless sintered SiC, since it combines most of the abovementioned properties with a very good thermomechanical behaviour (high strength at high temperature).
It is known from the prior art that, only by using sintering aids, SiC can be subjected to pressureless sintering. Possible sintering aids which have been described are a great number of different compounds and material combinations, including, inter alia, metals such as aluminium, iron, lithium or magnesium and also metal oxides such as aluminium oxide, beryllium oxide and rare earth metal oxides. However, only combinations of carbon/boron, carbon/boron carbide and carbon/aluminium have become established as sintering aids in industrial use. It is worth noting that only small amounts of sintering aids are required to achieve virtually complete densification. The values published hitherto for carbon are between 1.5 and 2.6% by weight and for boron or boron carbide between 0.3 and 1% by weight, based on silicon carbide used. During sintering, the carbon acts as a reducing agent and cleans the grain surface of the SiC of SiO
2
. Associated therewith is an increase in the surface energy of the powder and the grain boundary diffusion during sintering. In contrast, boron is incorporated at the grain boundaries and increases the volume diffusion during sintering. At the same time it acts against grain growth. To enable use to be made of these advantageous properties of the sintering aids, they have to be distributed homogeneously in the green ceramic. The necessary homogeneity can be achieved in various ways. Frequently, the powder mixture comprising SiC and sintering aids is subjected to intensive wet milling in the presence of surface-active substances. A particularly high homogeneity is achieved when the individual SiC particles are coated directly with nanosize sintering aids (e.g. nanosize carbon black), as is disclosed in DE-A-42 33 626.
In addition, it is known that production of SiC materials having a good electrical conductivity requires dopants. These dopants include, inter alia, aluminium nitride, molybdenum disilicide, phosphorus, arsenic and antimony. However, these additives have an unfavourable influence on the sintering behaviour of the ceramic, so that sufficient densification can only be achieved by pressure-supported sintering processes (hot pressing, hot isostatic pressing), but SiC ceramics produced by these methods still have a relatively high porosity and have only limited oxidation stability in air at high temperature.
Accordingly, it is an object of the present invention to produce SiC materials having good electrical properties, in particular good electrical conductivity, good oxidation resistance and high strength by pressureless sintering.
It has surprisingly been found that this object can be achieved by means of the system (&agr;-)SiC/B
4
C/carbon if the green bodies are produced by the process described in DE-A-42 33 626 and the green bodies are subjected to a multistage sintering process which is carried out at least in part in the presence of nitrogen.
The present invention accordingly provides a process for producing a conductive sintered body based on silicon carbide, in which
a) (preferably &agr;-)silicon carbide particles, which may have been pretreated with a surface modifier, are dispersed in an aqueous and/or organic medium and positive or negative surface charges are generated on the silicon carbide particles by adjustment of the pH of the dispersion obtained;
b) carbon black and boron carbide are mixed in as sintering aids, where at least the carbon black particles have a surface charge opposite to the surface charge of the silicon carbide particles and the boron carbide can also be added, completely or in part, at a later point in time (stage c′));
c) the slip obtained after stage b) is shaped directly to form a green body or
c′) a sinterable powder is isolated from the slip obtained and is shaped to form a green body, where the above boron carbide (completely or in part) can also be added to this sinterable powder; and
d) the green body obtained is subjected to pressureless sintering to form a sintered body,
where the process is characterized in that said stage d) is carried out in essentially three successive steps, namely (i) preheating to 1200-1900° C., (ii) sintering at 1900-2200° C. and (iii) post-heating at 2150-1850° C. and subsequent cooling to ambient temperature, and said step (i) is carried out in a nitrogen-containing atmosphere, said step (ii) is carried out in a noble gas (preferably argon) atmosphere and said step (iii) is carried out in an atmosphere containing nitrogen and/or carbon monoxide.
In a modification of this process, the slip obtained after stage (b) is applied to a sintering-resistant substrate and dried and the substrate thus coated is sintered as described in stage d).
As already mentioned above, the stages (a) to (c) of the process of the invention are carried out as described in DE-A-42 33 626, which in terms of details is hereby expressly incorporated by reference.
In stage (a), the silicon carbide powder is suspended in water and/or organic media.
Suitable organic dispersion media are especially water-miscible organic solvents such as alcohols, esters, ketones, dimethylfomamide and dimethyl sulphoxide.
The Si—OH groups present on the surface of the SiC particles are converted in the presence of protons or hydroxyl ions into charged groups Si—OH
2
+
or Si—O
−
, which give rise to an electrostatic repulsion of the fine SiC particles and, thus, to a finely dispersed suspension.
Preferably, the formation of negative or positive surface charges is effected or aided by addition of an acid or base. Suitable acids for this purpose are, for is example, inorganic acids such as HCl, HNO
3
, H
3
PO
4
, H
2
SO
4
and also organic carboxylic acids such as acetic acid, propionic acid, citric acid, succinic acid, oxalic acid and benzoic acid. Suitable bases are, for example, NH
3
, NaOH, KOH, Ca(OH)
2
and also primary, secondary and tertiary, aliphatic and aromatic amines and tetraalkylammonium hydroxides. It is likewise possible to use acidic or basic polyelectrolytes such as polyacrylic acid, polymethacrylic acid, polysulphonic acids, polycarboxylic acids and salts (e.g. having Na
+
or NH
4
+
as cations) of these compounds.
The surface charges generated in this way can be measured as the zeta potential. The zeta potential is pH-dependent and is either positive or negative in relation to the isoelectric point of the respective material (e.g. the SiC). As a result of the electrostatic charging with the same polarity, the dispersed individual particles remain stable in suspension.
According to a preferred embodiment of the present invention, the SiC powder is subjected to a surface modification before formation of the surface charges. According to the invention, this surface modification is carried out by coating the SiC with a surface modifier having functional groups which can be converted into negatively or positively charged groups by establishing an appropriate pH.
Suitable surface modifiers are, for example, silanes, acid chlorides, carboxamides, carboxylic anhydrides and carboxylic esters and also organic polyelectrolytes such as polyacrylic acid, polymethacrylic acid, polysulphonic acids, polycarboxylic acids and salts thereof.
Examples of silanes which can be used are mercaptopropyltrimethoxysilane, 3-(trimethoxysilyl)propyl methacrylate, 3-(triethoxysilyl)propylsuccinic anhydride, cyanoethyltrimethoxysilane, 3-thiocyana

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