WO1994002430A1

WO1994002430A1 - METHOD FOR STABILIZING POLYCARBOSILANE DERIVED Si-C-O CONTAINING FIBERS

Info

Publication number: WO1994002430A1
Application number: PCT/US1993/006855
Authority: WO
Inventors: Craig P. Jacobson; Lutgard C. Dejonghe
Original assignee: The Regents Of The University Of California
Priority date: 1992-07-24
Filing date: 1993-07-22
Publication date: 1994-02-03

Abstract

Disclosed is a method for converting polycarbosilane derived Si-C-O containing fibers such as Nicalon or Tyranno, to polycrystralline silicon carbide ceramic fibers by means of heating the fibers at a heating rate of at least about 5 °C per minute in a non-oxidizing atmosphere in the presence of boron vapor to temperatures in excess of about 1500 °C. Also diclosed are the fibers made by the method.

Description

Method For Stabilizing Polycarbosilane Derived Si-C-0 Containing Fibers

Background of the Invention

This invention was made with Government support under Contract No. DE-AC03-76SF00098 between the U.S. Department of Energy and the University of California for the operation of Lawrence Berkeley Laboratory. The Government has certain rights in this invention.

1. Field of Invention

The present invention is concerned with a method for thermally stabilizing polycarbosilane derived Si-C-0 containing fibers, and to the fibers produced by the method. In particular, the invention concerns a method for converting polycarbosilane derived Si-C-0 containing fibers to polycrystalline fibers in a nonoxidizing atmosphere in the presence of boron vapor.

Continuous ceramic fibers are used in a variety of applications such as reinforcement for plastic, ceramic or metal matrices or as high temperature insulation material . Silicon carbide ceramic fibers, in particular polycarbosilane derived fibers such as NICALON (Nippon Carbon, Tokyo, Japan) or TYRANNO (UBE Industries, Yamaguchi, Japan) , are known for their high strength and high temperature resistance relative to other high performance fibers. Unfortunately, during the curing process oxygen is incorporated into the fiber and this leads to thermal degradation at temperatures above about 1200°C due to weight loss and surface pitting (see Man et al . , J. Mat. Sci . 19, 1191-1201 (1984)) . This is particularly undesirable since silicon carbide fibers are compatible with a variety of matrices and are the fibers of choice for high-performance composites. However, processing temperatures for these advanced composite systems are typically above lOOOoc and often in excess of 1700oc. Thus, a more stable (oxidation and corrosion resistant) , higher temperature (>1200°C) fiber is needed if continuous fiber composites are to attain their full potential of high-temperature performance.

2. Related Art

Dow Corning Corporation (European Patent Application #91100412.5, 1991; authors D. Deleeuw, J. Lipowitz, and P. Lu.) has developed a process for the preparation of thermally stable, substantially polycrystalline silicon carbide fibers formed from a preceramic polymer comprising a polycarbosilane resin having at least about 0.2% by weight boron. The fibers are pyrolyzed at a temperature greater than about 1600°C in a non-oxidizing environment. The application also covers forming fibers from a polycarbosilane resin, infusibilizing the fibers, and pyrolyzing them at a temperature of greater than about 1600°C in a nonoxidizing environment, wherein at least about 0.2% by weight boron is incorporated during the infusibilization or pyrolysis. The thermal stability of^"- these fibers is achieved by the incorporation of boron prior to or during ceramification.

The method described herein converts ceramified fibers, such as described in the Dow Corning application, to a more stable fiber and uses the shrinkage of the fibers as an advantage in composite processing. Summary of Invention

It is an object of this invention to provide a method of stabilizing polycarbosilane derived Si-C-0 containing fibers .

It is another object of the invention to provide a method of increasing the thermal stability of polycarbosilane derived Si-C-0 containing fibers.

It is still another object of the invention to provide polycarbosilane derived Si-C-0 containing fibers having improved thermal stability.

The present invention relates to a method for converting polycarbosilane derived Si-C-0 containing ceramic fibers to polycrystalline SiC fibers. In the method polymer precursor derived silicon carbide fibers are heated at a heating rate above about 5oC/min to a temperature above about 1500oC in a nonoxidizing atmosphere such as argon, nitrogen, or helium in the presence of boron vapor for a period of time sufficient to yield stabilized polycrystalline silicon carbide fibers . The treated fibers have superior thermal stability over the untreated fibers as will be seen in the attached figures.

Brief Description of the Drawings

Fig. 1 is a micrograph of a Nicalon fiber heat treated in accordance with the method of the invention in a boron vapor.

Fig. 2 is a micrograph of a Nicalon fiber heat treated in an identical manner as the fiber in Fig. 1, except no boron vapor was used. Fig. 3 is a micrograph of a composite of Nicalon fibers in a matrix heat treated in accordance with the method of the invention.

Description of the Preferred Embodiments

The present invention is based on the discovery that heating polycarbosilane derived silicon carbide fibers in the presence of boron vapor in an inert atmosphere converts these fibers to smooth, polycrystalline silicon carbide fibers. During this conversion the fibers undergo a reduction in size and it is believed that this is due to a decrease in the porosity and oxygen content of the fiber. The reduction or elimination of oxygen from the fiber prevents the carbothermic reaction that produces carbon monoxide and silicon monoxide gas that leads to weight loss and surface pitting and hence degradation of the fibers properties. The shrinkage of these fibers has significant advantages for processing of composite systems.

The polycarbosilane fibers which are stabilized in accordance with the process of the invention, are described in an article entitled "Polymer-Derived Ceramic Fibers", by Jonathan Lipowitz in Ceramic Bulletin, Vol. 70, No. 12, 1991, which article is incorporated by reference herein.

In that article is described a process for converting a cured polycarbosilane fiber to a ceramic fiber by pyrolysis. The method of this invention differs from the process described in the Libowitz's article in the sense that the method of the invention relates to the stabilization of ceramic fibers which have already under gone pyrolysis, as described in the Lipowitz article.

Silicon carbide fibers are also described in the Handbook of Composites, Vol. 1-Stong Fibers, Chapter 6 edited by . Watt and D.V. Perov [1985, Elscbier Sciences Publishers B.V.] . Also, European patent application No. 91100412.5 filed 15.01.91, incorporated by reference herein also describes the preparation of substantially crystalline silicon carbide fibers from polycarbosilane. The fibers produced in accordance with the European patent application are the starting fibers from which the process of this invention is derived.

In carrying out the method of this invention, the silicon carbide fibers are heated to a temperature within the range of about 1500°C to about 2300oC, preferably from about 1700°C to about 2100oc, and most preferably from about 1750OC to about 2050°C.

The fibers are heated for a period of time ranging from a few seconds to about 240 minutes, preferably from about 1 minute to about 90 minutes, and most preferably from about 5 minutes to about 60 minutes.

The fibers are desirably heated at a rate ranging from about 5 to about 100°C per minute, preferably from about 10 to about 75°C per minute, and most preferably from about 20 to about 60oC per minute.

The fibers are normally heated at atmospheric pressure, for ease of processing. However, they can be heated in an atmosphere ranging from about zero atmosphere to infinity in terms of the upper limit, with the preferred range being from about 0.3 to 10 atmospheres. Most preferred, as indicated previously, is one atmosphere for ease of ^{~ ~} processing.

The atmosphere must be a non-oxidizing one, preferably being nitrogen or argon gas. Other non-oxidizing gases could be used however.

Boron is the preferred source of boron vapor. Other boron containing compounds, such as boron carbide, titanium diboride, or zirconium diboride, can be used however. The boron source may be separate from the fibers or it may be contained in a powder around the fibers.

The boron vapor pressure is established by heating the boron source. The boron vapor pressure depends upon the temperature of the boron source, with respect to the processing temperature of the fibers . The operable temperature range can be from about 1400 to about 2400°C, the preferred range from about 1650 to about 2200oc, and the most preferred range from about 1700 to about 2100oC.

The fibers treated in accordance with the method of the invention retain their flexibility as well as their smooth surface as seen in a scanning electron microscope [SEM] . The effectiveness of the method can be measured by the grain size of the silicon carbide polycrystals . Fibers with grains larger than about 0.5 micrometers in diameter can be considered to be unstable. In addition the effectiveness of the method may be measured by the linear shrinkage of the fiber, with shrinkage greater than about 7% indicating dense, stable fibers.

In carrying out the method of the invention, the most critical processing parameters are the heating rate and the temperature. A minimum heating rate of about 5oC per minute and a minimum temperature of about 1500oc are needed for this process or method to be effective in stabilizing the fibers.

This invention will be more fully understood by reference to the following examples. All materials used were obtained from the following commercial sources : Ceramic grade Nicalon fiber from Nippon Carbon Co., Ltd.; Tyranno fibers from UBE Industries; elemental boron from Gallery Chemical Co.; titanium diboride from Union Carbide; and boron carbide from Alfa Products. Scanning electron microscopy was performed on an 151 DS 130 at 10 kev accelerating voltage. X-ray ' diffractometry and subsequent crystallite size analysis was carried out on a Siemens D500 Diffractometer at 40 keV and 30 mA.

Example 1

NICALON fibers and boron were obtained from commercial sources and used as received. A sample of the fibers was placed in a graphite crucible along with the boron. The crucible was loaded in an Astro graphite element resistance furnace and the chamber was evacuated to less than 100 millitorr. Argon gas was then used to backfill the chamber and a continuous flow of gas was established. A constant heating rate of 30°C/min was used until a temperature of about 1000°C was obtained. At this time an optical pyrometer was sighted on the sample and the heating run continued at a rate of 60oc/min. When the sample reached 2100°C the temperature was held for 30 in. The furnace power was then turned off and the chamber allowed to cool with a continuous flow of gas maintained. The fibers were removed and examined under SEN. Samples of Nicalon fibers, identically heat treated, in the presence of boron vapor (Figure 1) and without boron vapor (Figure 2) are shown.

The stabilized fibers had a linear shrinkage of about 12%, a mass loss of about 27%, and a crystallite size less than 0.5 μm.

Example 2

TYRANNO fibers and elemental boron were obtained from commercial sources and used as received. A sample of the fiber was placed in a graphite crucible along with the boron and this was placed in an Astro graphite furnace. The chamber was evacuated to less than 100 millitorr and Argon gas was then used to backfill the chamber and a continuous flow of gas established. The furnace was heated to about 1000°C at which time an optical pyrometer was sighted on the sample and the heating run continued at a rate of 60oc/min. When the sample reached 1650°C the temperature was held for 60 min. The furnace power was turned off and the furnace allowed to cool. The fibers were removed and examined. The fibers were flexible, smooth and shiny. They had a linear shrinkage of about 15%, a mass loss of 35%, and a crystallite size of less than 50 nanometers.

Example 3

NICALON fibers and boron carbide were obtained from commercial sources and used as received. A sample of the fiber was placed in a graphite crucible along with the boron carbide and this was placed in an Astro graphite furnace. The chamber was evacuated to less than 100 millitorr and Nitrogen gas was then used to backfill the chamber and a continuous flow of gas established. The furnace was heated to lOOOoc at which time an optical pyrometer was sighted on the sample and the heating run continued at a rate of 60oC/min. When the sample reached 2000°C the temperature was held for 10 min. The furnace power was turned off and the furnace allowed to cool . The fibers were removed and examined. The fibers were flexible, smooth and shiny. They had a linear shrinkage of about 10% and a mass loss of about 25%.

Example 4

A sample of NICALON fiber was placed in a graphite crucible along with boron and this was placed in an Astro graphite furnace. The chamber was evacuated to less than 100 millitorr and Nitrogen gas was then used to backfill the chamber and a continuous flow of gas established. The furnace was heated to 1000°C at which time an optical pyrometer was sighted on the sample and the heating run continued at a rate of 10°C/min. When the sample reached 1750°c the temperature was held for 10 min. The furnace power was turned off and the furnace allowed to cool. The fibers were removed and examined. The fibers were flexible, smooth and shiny. They had a linear shrinkage of about 5%, a mass loss of 21%, and a crystallite size of less than 20 nanometers.

Example 5

A sample of NICALON fiber was placed in a graphite crucible along with titanium diboride and this was placed in an Astro graphite furnace. The chamber was evacuated to less than 100 millitorr and Argon gas was then used to backfill the chamber and a continuous flow of gas established. The furnace was heated to 1000°C at which time an optical pyrometer was sighted on the sample and the heating run continued at a rate of 30oc/min. When the sample reached 1500°C the temperature was held for less than 1 min. The furnace power was turned off and the furnace allowed to cool. The fibers were removed and examined. The fibers were flexible, smooth and shiny. They had a linear shrinkage of less than 5% (the short hold time inhibited further densification) , a mass loss of about 17%, and a crystallite size of less than 20 nanometers.

Example 6

A sample of NICALON fiber was chopped into 3mm lengths and dispersed in toluene. Into this solution submicron silicon carbide, boron, and carbon were added so that the slurry had the following solid content: 34 wt% chopped NICALON fiber, 59 wt% SiC, 4 wt% C, 2 wt% B, and 1 wt% oleic acid. The slurry was stir dried and the resulting powder was ground with a mortar and pestle. Samples were made by uniaxially pressing the ground powder to about 10,000 psi . The resulting pellets were placed in a graphite crucible and this was placed in an Astro graphite furnace. The chamber was evacuated to less than 100 millitorr and Argon gas was then used to backfill the chamber and a continuous flow of gas established. The furnace was heated to 1000°C at which time an optical pyrometer was sighted on the sample and the^ heating run continued at a rate of 60°C/min. When the sample reached 2050°C the temperature was held for 30 min. The furnace power was turned off and the furnace allowed to cool. The resulting composite was removed then fractured. The exposed surface was examined under the SEM. As can be seen by Figure 3, the fibers are smooth and dense. This example demonstrates the advantages of using these fibers for processing composite systems. The simultaneous densification of the fibers and matrix allows pressureless sintering of composite systems .

The treated fibers may be substituted where polycarbosilane derived silicon carbide fibers are currently being used. In addition these fibers can be used where higher processing temperatures currently limit the untreated fibers . The most important potential use for this process is in the production of ceramic fiber ceramic matrix composites. Here, the shrinkage of the fibers during processing can be used to a great advantage, where other experimental polycrystalline SiC fibers would limit processing. Using these fibers in ceramic matrix composites could have significant cost savings over current techniques such as chemical vapor infiltration or liquid polymer impregnation where repeated steps or pressure is needed.

Thermally stable ceramic fibers have value as the reinforcing phase in high performance composite materials for applications in energy and related industrial technologies such as gas turbine power generators, heat recovery equipment, waste incineration systems, burners and combustors. Smooth, thermally stable fibers have been obtained at temperatures in excess of 2000°C, allowing the processing of many advanced ceramic and metal matrix composites.

Claims

WHAT IS CLAIMED IS:

1. A method for increasing the heat stability of polycarbosilane derived Si-C-0 containing fibers which compromise heating the fibers in an non-oxidizing atmosphere

5 containing a boron vapor at a temperature of at least about 1500°C and maintaining the fibers.at that temperature for a period of time sufficient to stabilize them.

2. The method of claim 1, wherein said temperature ranges from about 1500°C to about 2300°C.

10 3. The method of claim 1, wherein said non-oxidizing atmosphere is a gas selected from the group consisting of argon, nitrogen, and helium.

4. The method of claim 1, wherein said temperature ranges from about 1750oc to about 2050°C.

15 5. The method of claim 1, wherein said temperature is

6. The method of claim 1, wherein said non-oxidizing atmosphere is argon.

7. The method of claim 1, wherein said non-oxidizing 20 atmosphere is nitrogen.

8. The method of claim 1, wherein said non-oxidizing atmosphere is helium.

9. The method of claim 1, wherein said heating rate for said fibers is at least about 5 degrees centigrade per

25 minute.

<

10. The method of claim 1, wherein said heating rate ranges from about 10 degrees centigrade per minute to about 75 degrees centigrade per minute.

11. A ceramic or metal matrix composite comprising stabilized polycarbosilane derived ceramic fibers.

12. The composite of claim 11, wherein said stabilized fibers comprise Tyranno.

13. The composite of claim 11, wherein said ceramic fibers comprise Nicalon.

14. Stabilized polycarbosilane derived silicon carbide fibers which have been stabilized by heating said fibers to a temperature of at least about 1500°C, and maintaining said fibers at that temperature in the presence of boron vapor and a non-oxidizing atmosphere for a period of time sufficient to stabilize said fibers.

15. The fibers of claim 14, wherein said oxidizing atmosphere is argon gas.

16. The method of claim 1, wherein said fibers are heated at a heating rate ranging from about 20 to about 60 degrees centigrade per minute to a temperature of from about 1750°C to about 2050°C for a period of time to form stabilized fibers having a grain size less than about 0.5 in diameter.

17. The method of claim 1, wherein the source of the boron vapor is titanium diboride.

18. The method of claim 1, wherein the source of the boron is boron carbide.