T.D. Shen 的文献

GRAPHITE PRODUCEDBY BALL MILLING
395
by a double monochromator (Spex-1403). The graphite powder scan was repeated four times. The
Raman scattering instrument was equipped with a Spex 103 Datamata computer control system. APhilips EM420 TEM equipped with EDAX detector was used for the TEM observations. A small amount of powder for the TEM observation was ground in anhydrous alcohol, a few drops of which were placed onto a carbon-supported copper grid.
INTRODUCTION Recently, there has been an increasing interest in materials with nanocrystalline grain size, which is typically in the range of a few to a few tens of nanometers (l-7). Among the methods to prepare nanostructured materials, mechanical milling/alloying offers the possibility of producing nanocrystalline structures in different alloy systems, and of producing relatively larger powders (>lO ltrn) with a homogeneous nanocrystalline structure, often difficult to obtain with alternative techniques, such as inert gas condensation. The mechanical milling/alloying technique has been used to prepare various hinds of nanostructured materials, such as pure metallic elements (8- ll), solid solutions (12-22), intermetallic compounds (8,23-27), and dispersion-strengthened alloys (29-32). The starting materials employed for the mechanical milling process are single phase, whereas those for mechanical alloying are typically two, or more than two, phases. As for the semiconductor elements, such as Si and Ge, it has been found recently that mechanical milling can be used to obtain crystalline-to-amorphous and/or crystal-to-nanocrystalline transformations (3336). In this report, based on structural investigations by x-ray diffraction, Raman scattering and transmission electron microscopy, we sought to study the structural disorder and phase transition in graphite produced by ball milling. The study of structure, phase transformation and properties
(Accepted January 1996) Abstract-Nanocrystalline graphite with a crystallite size of about 2 nm isformed after 8 h of ball milling. Further milling produces a mixture of nanocrystalline and amorphous phases. The nanocrystalline graphite is relatively stable against heating when compared with some nanocrystalline metals.
Pergamon
NmaSbudured Materials, Vol. I. No. 4. pp. 393-399.1996 Elsevier Science Ltd Copyright @ 1996 Acta Metallurgica Inc. Printed in the USA. AU rights reserved 0965-9113196 $15.00 + .OO
RESULTS AND DISCUSSION Figure 1 shows the XRD profiles of ball-milled graphite for various milling times. Also shown in this figure is the XRD pattern of heat-treated graphite powder ball-milled for 16 h. The heat treatment is carried out by differential thermal analysis (DTA) in vacuum (better than 1.3 x 10m4 whereby the ball-milled graphite powder is heated to 1300°C at 20 K/min. As shown in Pa) Figure 1, the ball milling results in the shift of the position and the broadening of the Bragg peak. After 4 h and 8 h of milling, the (002) reflection peak is asymmetric and may be divided into two components, one of which might correspond to an interlayer spacing, dm2, of 0.340 nm of relatively ordered graphite crystals, and another which relates to a larger interlayer spacing of lattice-damaged crystals (37). After 16 and 40 h of milling, do02 of about 0.363 nm and (3.368 nm are estimated for milled graphite from Figure 1. After 16 h of milling, the (002) reflectlon peak is so broadened that the linewidth at one-half the maximum intensity is larger than seven degrees. The “crystallite size Lc,” which refers to the mean crystallite height (37) calculated from the Scherrer formula, is 1.2 and 1.1 nm, respectively, after 16 h and 40 h of milling. The implication is that the average crystallite is just three unit cells thick! The real crystallite size may be somewhat larger, since Kti and instrumental broadening corrections were not made. The (002) reflection peak for the graphite which is milled for 16 h and heated to 13OO”C, shown in Figure l(f), is still as very broad, indicating that the grain growth is very limited. Two small XRD peaks at about 30.2 and 31.6 degrees in Figure l(f) may result from possible carbon oxide (38) and/ or carbon oxynitride structures. Lattice defects in graphite break down the hexagonal symmetry of the graphite lattice and modify the optical selection rules for the lattice vibrational modes which can be observed in Raman scattering (39). Asingle Raman line is theoretically expected for the hexagonal lattice of graphite and has been observed around 1580cm-’ in natural graphite and stress-annealed pyrolytic graphite, while an additional Raman line is observed around 1360 cm-’ in glassy carbon, polycrystalline graphite and the lattice-damaged graphite induced by ion-irradiation (39-4 1). The Raman line at 1360 cm-l has been shown to arise from graphite containing disorganized regions near crystal edges and lattice defects such as edge dislocations, lattice vacancies (39) and interstitial carbon atoms between the basal planes of carbon atoms, sp2 bonded as in the graphite-crystal structure (4 1, 42). The Raman spectra of ball-milled and heat-treated graphite powder are shown in Figure 2. In the unmilled graphite, there is a weak Raman line at 1360 cm-l besides a sharp Raman line at 1580 cm-l. This reveals that the initial graphite crystal, although it is rather ordered, is not completely ordered (39). Both the 1360 cm-’ and 1580 cm-’ lines broaden continuously with milling. The above mentioned XRD profiles and Raman spectra show that the lattice-disorder of graphite induced by ball milling may be similar to that of other imperfect carbonaceous materials.