composites, whereas the TEM assists in evaluating the different types of interfaces and the structure of the matrix. I was shown some pictures taken with the TEM where it was evident that about 50% of the matrix was oriented with its ab planes normal to the fiber's surface. Apparently, there is an ongoing research program in this important arca. To attain such a microstructure, the samples were processed at high pressures and the surfaces of the fibers were preconditioned. Numerous photographs of the microstructure of the composites were viewed. These were taken with the use of the optical microscope to 2000X and the SEM to magnifications from 50X to 200,000X or higher. The energy dispersive analysis by x ray (EDAX) capabilities can detect mass numbers from 5 to 92. In general, these photos showed the impregnation methods to be of a good quality as the porosity was small in size and uniformly distributed. The weaving seemed to be of a good quality. For example, the spacing of 1K yarns for a 2D cloth was 0.028 inch (0.7 mm). For 3D C/Cs, the spacing can be as low as 1/2 mm (0.025 inch). Other architectures that have been studied are 1D, 4D, and 7D.

A portion of their mechanical testing laboratory was seen. The general impression was that these facilities are capable of performing the usual types of tests. Some of these tests can be conducted from liquid nitrogen to >1,500 °C.

The afternoon discussion involved. about 16 persons and was a continuation of the subject matter of the morning and other topics. One of these is their work on oxidation-resistant coatings. Apparently a "shuttle" type processing method is being used for the formation of a SiC coating by the conversion of the SiO, from its reaction with the C/C substrate. Researchers are concerned with the fact that the C/C strengths are reduced with the formation of these coatings, so they are trying to devise other methods of

improving the resistance to oxidation such as coating the fibers after the preforms are woven and ways of filling the cracked coatings that occur due to the differential thermal mismatch between the coating and the substrate. Similar to the U.S. technique, glassbased fillers are being used to fill these differential thermal expansion cracks. In addition, they are trying to fill the spaces between yarns with silicon carbide particles to enhance the depth resistance of the matrix to oxidation. A loss of the composite's mechanical strength is one of the major problems that must be overcome at this time. The oxidation resistance of these coated samples is determined by two methods. The first is a screening test where the sample is heated to 1,300 °C for 1 hour, and it is acceptable if the weight loss is <1%. Then, the sample is subjected to 30 cycles between RT and 1,250°C and still must lose <1% of its initial weight. There are plans being formulated for conducting more research on higher temperature coatings but no further details were given.

In the course of this discussion, a number of items of interest were mentioned without too much discussion and these are as follows:

• The use of a pyrolytic matrix does increase the toughness of C/Cs. At times this PG matrix is used just to rigidize the preforms, after which they are densified with pitch impregnation.

There is interest in determining the different types of weave architectures that are best for particular applications.

Some mechanical property measurements have been made at liquid nitrogen temperatures. But no mechanical property measurements have been made after the samples have been thermal cycled to these low temperatures.

• A large problem is to define a test or a small series of tests that can be performed on C/Cs to insure the reliability of their performance. Furthermore, will the types of tests have to be changed with each different application? At this time there are no such tests nor any standards defined in China that specify the methods that should be used for testing materials. But some efforts have been initiated towards overcoming these deficiencies.

• There is the belief here that China is ahead of the U.S.S.R. in the development of C/Cs.

The counterpart to this organization in the U.S.S.R. is the Composite Material Science and Production Institute.

• An institute like this also trains students who are on its staff. The people who do the training are designated as professors and they also are affiliated with a university. Presumably students who are finished with courses are doing research for their degree on a topic that is important to the institute.

• When I questioned them about other organizations in China that are doing similar work, the answer was, "This is the leading one." But there are other organizations that are designated to be the focal point for complementary activities for different aspects of the total picture, such as fiber development, which has already been mentioned earlier in this report. There are program coordination meetings between these organizations twice a year. Apparently, the general directions and policies are set by the Commission for Science and Technology.

of bonds along the length of the fibers and how observations in one plane can be representative of the third dimension. Another area of concern was whether thermal cycling of C/Cs resulted in fatigue problems, which will ultimately mean a reduction of strength. They were especially interested for cycles that run from RT to 800 or 1,000 °C. The applications were not identified other than such materials are being considered for "shells." Another area being investigated is fracture mechanics and predicting the failure of materials. Apparently they are trying to determine, with limited success, the influence of defects within organic and metal matrix composites on their failure behavior. These defects are identified nondestructively by x-ray and ultrasonic techniques and destructively by metallographic examination where the samples can be polished.

are available for conducting this type of research and certainly there are competent materials researchers here. Their selection of research priorities will probably be an important factor in determining their progress in C/Cs. It is recommended that some form of communication be maintained to keep track of this progress. The topic of cooperation between this university and its counterparts in the United States was discussed. There is interest on their part in having some sort of communication that may develop into more specific activities.

CHUNGNAM NATIONAL UNIVERSITY

Daeduk Science Town
Daejon 305-764, Korea
Tel: (042) 821-5600; 822-7914
Fax: (042) 823-2931
Dates: 1-7 Mar 1991

Prof. Bosung Rhee, who is dean of the Engineering School and professor of chemical engineering, was my host.

As a result of this meeting, Prof. Jiang and I had a further discussion about the size and distribution of pores Background in the microstructure of C/Cs and their influence on the thermal expansion characteristics of C/Cs. He has found that the last two impregnation cycles can have a profound influence on the thermal expansion of certain types of C/Cs. For example, if these two impregnations are only taken to 1,100 °C and the billet has a high density (>1.85 g/cc), the coefficient of thermal expansion value of the C/C can be increased by 75%. Such a result would be very significant for reducing the thermal expansion mismatch between C/C substrates and their ceramic oxidation-resistant layers that are protecting them. Assessment

This department is just beginning to start research in the area of C/Cs, so not much information is to be expected in the near future. It is difficult to estimate when significant research results will be forthcoming because the facilities

The Korean Government has been for the past 10 years in the process of creating a science center in this city by moving various research activities from throughout Korea to the city. In this manner the expertise that exists in these organizations is concentrated into one area. Furthermore, they can take advantage of the capabilities at the university that is located here. The most recent activity to move is the Korea Advanced Institute of Science and Technology (KAIST), similar to the Japanese science center at Tsukuba.

The carbon research activities are headed by Rhee and were started in 1970. Rhee received his training in carbonaceous materials at the University of Karlsruhe under the supervision of Prof. Fitzer. Carbon fibers was the initial arca of research that was started by Rhee at Chungnam University and

it remains the primary thrust of his research even today. The carbon research group consists of three faculty members, including Rhee, and 15 students. The support for the research has varied over the years from the Korean Ministry of Science and Technology, which is interested in starting a national fiber production industry, to the United Nations through their Industrial Development funds, which are being used to develop a national capability for producing pitch-based fibers. Currently, according to Rhee, about 175 tons/yr of PAN and 60 tons/yr of pitch-based fibers are being produced by industrial firms in Korea. Interest in the production of PAN-based fibers started about 1980 and the precursor was obtained from Courtaulds. The initial technology was developed in Rhee's laboratory. In 1985 some steel and chemical company bought the PAN processing technology from Rik Textile Company of the United Kingdom. It is now the process that is being used to produce 115 tons/yr of PAN fibers. The second industrial company, whose name is Taetent Textiles, is the other organization that has been producing PAN fibers since 1987. But these fibers are from a precursor that has been developed in Korea. The tensile strength of these fibers is reported to be between 250 and 700 kpsi with a maximum modulus of 93 Mpsi. The rate of production for all types of fibers is 60 tons/yr. Pitch

based fiber research was also started in Rhee's laboratory in 1985. The R&D on pitch-based fibers is being continued today with some support from the United Nations Industrial Developmental funds since 1987. Currently there is one industrial organization working with him on methods for producing mesophase pitchbased fibers.

According to Rhee, the Korean industry is just beginning be a producer of carbon fibers. Until recently, this country has been a consumer of carbon fibers, which have then been used in composites that have been manufactured

in Korea. Eventually the intent is to produce goods that will be exported abroad.

It is estimated by Rhee that as many as 25 professors from various Korean universities are doing R&D that is concerned with the applications of carbon fibers in composites of various types of matrices, i.e., cement, organic, and metal. In addition, there are 10 to 15 professors who are involved in C/Cs. Korean research work on C/Cs started in Rhee's laboratory in 1975. There are about three textile organizations that are now weaving carbon fiber cloth to be used in fabricating 2D preforms and two companies that are prepregging the cloth, which is then used for forming resin or carbon matrix composites. To date, no industrial firm is weaving 3D preforms that contain

carbon fibers.

C/C composites have also been of interest since 1973 in Korea and again Rhee and his coworkers did the initial R&D investigations. There is research in C/Cs now being conducted in his laboratory as will be explained in the following section.

To promote an international meeting with the Japanese, a joint seminar on carbon fibers and their applications, hosted by Rhee, was held on 24-27 April 1991. In addition to eight professors from Japan, there were guests from France (Erhburger), Germany (Fitzer), and the United States (Edie). This meeting was supported by the Korean Science and Engineering Foundation (KOSEF) and the Japanese Society for the Promotion of Science (JSPS). This is another example of two governments that are supporting international cooperation.

Program Status

Fiber Development. A unique investigation is being conducted to determine the parameters that control the formation of fibers whose cross sections are atypical to those that are

normally produced throughout the world. Consequently, one of the major thrusts of this group is to understand what processing conditions are required for the formation of fibers with a "C" shape or hollow cross sections. Rhee believes that such a cross section provides more surface area for bonding the fiber to its surrounding matrix and the fiber to its surrounding matrix and that larger strengths per unit area of fiber cross section can be obtained because of the fiber's microstructure and enhanced flexibility. Their experimental approach is to determine the proper treatment for preconditioning the precursor coal tar and selecting the optimum spinning conditions. This means there is a minimum of fiber breakage during the spinning operabreakage during the spinning operation. After this step the fibers are stabilized by air oxidation and then carbonized. So far, they have found that the most important parameter is the heat treatment (HT) of the pitch prior to its being spun. Too little HT results in variable viscosity whereas too much. HT means excessive brittleness and fiber breakage. But the quinoline insoluble and benzene soluble contents of these mesophase pitches do not appear to be very important. The resulting fibers are then characterized for their degree of shrinkage during the pyrolysis step, surface area, and mechanical properties. At this time the diameters of these laboratory spun fibers are between 10 and 40 microns. The hollow fibers have tensile moduli of 35 Mpsi and a strength of 220 kpsi whereas the fibers with a solid circular cross section have 93 Mpsi and 700 kpsi values, respectively. The outside and inside diameter dimensions of the hollow fibers are about 40 and 7 microns, respectively. It is expected these dimensions will be reduced with further R&D effort. One investigation was made that compared the mechanical properties of carbonized thick- and thinwalled hollow fibers that were made from isotropic and mesophase types of pitch. The thin wall (~15 microns) stabilized more rapidly and had about 20%

higher mechanical properties than the >30-micron-thick wall fibers. However, the variability of the cross sections of these fibers makes the interpretation of the data very difficult. The C cross section fibers are variations of the hollow fibers. No strength data were available for this type of fiber nor were there any for composites that contained either types of fibers as the samples were in the process of being fabricated. This research effort is part of a collaborative program with Prof. Edie of Clemson University in the United States, who receives funds from the National Science Foundation according to Rhee. Apparently the orientation of the a-b planes is not uniform within the cross sections of these fibers and therefore is another variation that has to be understood in order to have them orient parallel to the surfaces of the fibers. In most cases the pitches that are being used are prepared by heat treating them prior to their being spun. The goal is to obtain a mesophase content that is more than 98%, which is determined optically at a magnification of 300X. Rhee doesn't feel that the quinoline insoluble (QI) content is an important factor to consider in controlling the spinnability of the pitch precursors.

Another aspect of the research program is to evaluate the influence of different surface preparations like electrolytic oxidation on fibers and their influence on the mechanical properties of unidirectional samples that are prepared in this laboratory. Densities of about 1.78 g/cc are achieved through multiple reimpregnation and carbonization steps. At this time none of these samples are being heat treated to graphitization temperatures.

Activated Carbon Fibers. This investigation was initiated in 1988 by Prof. Ryu, who joined Rhee's group about 8 years ago. The purpose of this study is to determine the mechanisms of adsorption and catalysis that occur in the fibers that contain different types of pores.