Plastic and nonmetallic article shaping or treating: processes – Carbonizing to form article – Controlling varying temperature or plural heating steps
Reexamination Certificate
1998-08-24
2001-05-08
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Carbonizing to form article
Controlling varying temperature or plural heating steps
C264S638000, C264S640000, C264S641000, C264S645000, C264S650000, C264S651000, C264S658000, C264S682000
Reexamination Certificate
active
06228293
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims the priority of German Application No. 197 36 560.4, filed Aug. 22, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for producing a body having a porous matrix from at least one recrystallized material, by shaping a raw material batch which contains a raw material powder and then sintering. The invention also relates to a body which can be produced by the process. The invention further relates to a process for producing a fiber-reinforced body having a porous matrix and to a body which can be produced according to the process.
Sintered silicon carbide (SSiC) is obtained from compact grained SiC particles with sintering additives for activation and carbon additives for dense sintering. Additives may be, for example, additions of boron and carbon or aluminum and carbon. A batch of raw material is shaped producing a green body, and the green body is sintered without pressure at approximately 2,000° C. A dense product with more than 96% theoretical density is obtained. However, densification during sintering results in shrinkage.
Recrystallized materials, such as recrystallized silicon carbide (RSiC) and recrystallized boron carbide (RB
4
C) are generally known. Recrystallized aluminum nitride (RAlN), recrystallized titanium carbide (RTiC) and recrystallized boron nitride (RBN) are also known. They are generally obtained by sintering batches of raw material which contain raw material particles and optionally additional substances.
Recrystallized silicon carbide is the best known recrystallized material. In comparison to conventional SSiC, it advantageously does not shrink during sintering. Recrystallized silicon carbide is obtained by sintering silicon carbide particles, preferably with bimodal grain distributions. Processes of this type are known from U.S. Pat. No. 2,964,823, German Patent Document DE-OS 2 837 900 and German Patent Document DE 31 49 796. The average particle size of the coarse grain is on the order of about 100 &mgr;m. The size of the fine grain is approximately 1.5 to 8 &mgr;m. These particles are sintered to a porous body at temperatures of above 2,000° C. No sintering additives are required. The sintering mechanism is known. The presence of free carbon is harmful to the consolidation of the structure because it hinders or even prevents the sintering mechanism. No densification takes place during sintering and no shrinkage. A pure undensified porous silicon carbide is obtained.
Silicon carbide is a constituent of fiber-reinforced ceramics, which are preferably produced by the polymer route, by chemical vapor infiltration (CVI) or by liquid silicating. During liquid silicating of fiber-reinforced ceramics, a carbon matrix in contact with liquid silicon is converted to silicon carbide. The carbon matrix to be converted is normally produced by the pyrolysis of resins with a high carbon yield.
The production of fiber-reinforced ceramics by silicating C-matrices—mainly carbon-fiber-reinforced matrices—is known per se. Fibers are mixed with the resin, and optionally with additional substances, shaped and hardened. A resulting green compact is pyrolized. A porous fiber-reinforced silicated carbon body results.
Three problems are usually encountered in the process. First, an irregular pore distribution is usually observed, and very large pores or gaps are formed. During liquid silicating, silicon will fill the pores and remain. Remaining silicon can not be converted to silicon carbide.
Second, the resin which supplies carbon creates relatively massive irregular carbon matrix regions. During the reaction with liquid silicon, a silicon carbide layer forms on the surface around larger C-regions. The resulting approximately 5-10 &mgr;m thick boundary layer prevents further conversion of the carbon enclosed by it. Conversion is only possible by way of diffusion processes over the boundary layer. If it takes place at all, it takes place very slowly. The carbon is therefore not completely converted.
Third, the resulting pore structure is not continuously open, or pore ducts may be closed by the converting reaction. Regions therefore exist into which liquid silicon cannot penetrate. In this case, no conversion of the carbon matrix can take place.
In order to provide a remedy, it has been attempted to create a crack structure into which the liquid silicon can penetrate, by way of, for example, pyrolysis of the resin. Pyrolysis creates an artificial duct porosity or open-pore condition. However, conversion to silicon carbide takes place only in the ducts.
Other attempts have been made to influence pore structure by resin additions. Complete carbon matrix conversion has not been achieved in these cases either.
In the case of the known process, fiber-reinforced silicon carbide ceramics are therefore obtained in which there remains residual carbon content and residual silicon content. Such ceramics, however, are not suitable for high-temperature applications in an oxidizing atmosphere because the residual carbon decomposes in air starting at approximately 400° C. For applications above 1,400°, there must also not be any free silicon because silicon starts to melt at these temperatures.
It is therefore an object of the invention to provide a process and products of the above-mentioned type by means of which, in a simple and reasonably priced manner, bodies can be obtained which have a regular pore structure, particularly fiber-reinforced silicon carbon ceramics, in which the silicon as well as the carbon are completely converted to silicon carbide.
This and other objects and advantages are achieved by the process wherein a raw material powder is used during shaping and sintering which contains a fine grain fraction of an average grain size of at most approximately 2 &mgr;m, and a coarse grain fraction of an average size of approximately 1.5 &mgr;m to approximately 30 &mgr;m, and wherein the sintering process is carried out at a temperature of at most about 1800° C. The process permits the production of recrystallized ceramic materials having a defined, particularly fine and regular pore structure. As a result of a lowered sintering temperature, the process and product are more reasonably priced and suitable for industrial applications.
This object is also achieved by producing a raw material batch which contains a raw material powder having a grain size distribution with a fine grain fraction of an average grain size of at most about 2 &mgr;m and a coarse grain fraction of an average grain size of about 1.5 &mgr;m to approximately 30 &mgr;m, mixing reinforcing fibers with the raw material batch, shaping the raw material producing a green body, sintering the green body at a temperature of at most about 1,800° C. so that a fiber-reinforced body is formed having a porous matrix made of a recrystallized ceramic material, impregnating the fiber-reinforced body with a carbon-supplying substance and converting the carbon-containing substance to carbon so that a fiber-reinforced body is formed which has a carbon-containing porous matrix. This process permits adjusting of the structure and of the porosity of porous matrices such that a liquid infiltration medium, such as a silicon or carbon precursor, can homogeneously penetrate the whole body.
The object is also achieved with the products resulting from either of the above two processes.
The processes according to the invention are based on the recognition that problems, as they occurred in the previously mentioned processes, are the result of matrix shrinkage during pyrolyzing.
Although it is known that recrystallized materials, such as recrystallized silicon carbide, exhibit no shrinkage during sintering at lower temperatures, sintering temperatures had heretofore been much too high for using this material for production of fiber-reinforced ceramics. Conventional ceramic reinforcing fibers may be subjected to maximum temperatures of 1,600° C. Fibers still under development, such as SiBNC fibers, may possibly be
Kriegsmann Jochen
Meistring Rolf
Neumann Nicole
Nixdorf Reinhard
Daimler-Chrysler AG
Derrington James
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
LandOfFree
Process for producing a body having a porous matrix from at... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for producing a body having a porous matrix from at..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing a body having a porous matrix from at... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2481492