Microtexture of highly crystallized graphite as studied by galvanomagnetic properties and electron channeling contrast e
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Microtexture of highly crystallized graphite as studied by galvanomagnetic properties and electron channeling contrast effect Yutaka Kaburagi, Akira Yoshida, and Yoshihiro Hishiyama Musashi Institute of Technology, 1-28-1, Tamazutsumi, Setagaya-ku, Tokyo 158, Japan (Received 26 May 1994; accepted 23 October 1995)
The relationship between microtexture and crystallinity of highly crystallized graphites with the residual resistivity ratio r300Kyr4.2K of 3.45–5.50 was investigated. The graphite crystals studied were kish graphite (KG), highly oriented pyrolytic graphite (HOPG), and highly crystallized graphite films prepared from carbonized aromatic polyimide films. The study was made by the observations of an electron channeling pattern and electron channeling contrast image (ECI) under scanning electron microscope and the measurements of x-ray diffraction, magnetoresistance, and Hall coefficient. The values of the mean free path of the carriers l, which approximates the mean crystal grain size, were estimated to be 2.6 –6.1 mm from the magnetoresistance at 4.2 K for the highly crystallized graphites. The values of the average crystal grain diameter D in the basal plane evaluated from ECI were several hundred microns or more for KG, 60 mm for HOPG, and 6 and 12 mm for the graphite films. The difference between the values of l and D for each crystallized graphite was discussed in relation to other results obtained.
I. INTRODUCTION
Nearly perfect crystals of graphite have been obtained artificially. They are kish graphite (KG), highly oriented pyrolytic graphite (HOPG), and highly crystallized graphite (HCG) film. KG is excess carbon precipitated as a flake from carbon-rich iron melt, and is purified to remove iron impurity.1 HOPG is prepared from pyrolytic carbon by heat treatment at very high temperature under mechanical stress.2 The HCG film is prepared from aromatic polyimide films, such as Kapton and Novax, by carbonizing them and then graphitizing at high temperatures.3–6 On transport properties, large KG flakes purified reveal fairly well crystal perfection,1,7–11 and HOPG is also a highly crystallized and well-oriented graphite material, but the degree of crystal perfection of HOPG is lower than that of KG.2,7–15 The HCG film exhibits relatively high crystallinity, but the crystallinity is lower than that of HOPG.3–6 The residual resistivity ratio of the in-plane electrical resistivity RRR, which is the ratio of the resistivity at room temperature to that at liquid helium temperature srRTyr4.2K d, has been used as a measure of the crystallinity of these highly crystallized graphites. The representative values of RRR are 10 –106 for KG,7–11,13 4–18 for HOPG,2,7–9,12–15 and 2–3 for the HCG film.3–6 The crystallinity depends on the microtexture of graphite crystal. Electron channeling contrast effect under scanning electron microscope (SEM) has been developed to observe the crystal perfection, and to evaluate the a-axis distribution of crystal grains and their mean J.
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