Scanning tunneling microscope study of polyacrylonitrile-based carbon fibers
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Scanning tunneling microscope study of polyacrylonitrile-based carbon fibers Dongxia Shi, Ning Liu, Haiqiang Yang, Juning Gao, Yueshan Jiang, and Shijin Pang Beijing Laboratory of Vacuum Physics, Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, China
Xubiao Wu and Zhen Ji University of Science and Technology Beijing, Beijing 100083, China (Received 12 February 1996; accepted 30 April 1997)
Scanning tunneling microscopy (STM) was used to characterize the surface topography of polyacrylonitrile (PAN)-based carbon fibers before and after electrochemical treatment, stretch resistance test, and high-temperature treatment. A new kind of spiral structures was found, which was not only on the surface but also in the inner layer. The spiral structure of the fibers was caused by the spinning process. The fiber structure contained the shape of the precursor. There were some large cracks in the carbon fibers after the stretching resistance test. The large cracks can result in carbon fiber breaking under certain stress conditions. The difference in the structures of the carbon fibers before and after the high-temperature treatment was determined.
I. INTRODUCTION
Carbon fiber composites have been extensively used in industry such as aerospace, astronautical, and sporting goods because of low density, high strength and modulus, and superior strength-to-weight properties.1 PAN-based carbon fibers are produced from polyacrylonitrile (PAN) which are more resistant to the compressive forces applied in flexure than liquid crystal-based materials such as carbon fibers from pitches or lyotropic polymers. In the past years the PAN-based carbon fibers have received much attention and have maintained their predominance as a reinforcing material. Many analytical methods have been employed to investigate the structure of the carbon fibers, such as x-ray diffraction (XRD) techniques,2,3 scanning electron microscopy (SEM), transmission electron microscopy (TEM),4 high-resolution electron-microscopy (HREM),5,6 and polarized-light microscopy.3,7 Except HREM, other analytical methods can investigate the structure of carbon fibers only on a micron scale. Although HREM can investigate the structure on a submicron scale, the sample preparation is generally sophisticated and the obtained images are not in real space. However, scanning tunneling microscopy (STM) can image the surface structure of carbon fibers down to the nanometer scale and in real space.8–11 Sample preparation is easy and the sample can be observed nondestructively in air. In this paper, STM was used to analyze the fiber surface structures and morphology before and after electroJ. Mater. Res., Vol. 12, No. 10, Oct 1997
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chemical treatment, strength resistance test, and hightemperature treatment. II. EXPERIMENTAL
To prepare the samples, one bundle of 1K fibers was cut into four parts. The first part without treatment was directly ob
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