Nanophase Ni particles produced by a blown arc method
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Nanophase Ni particles produced by a blown arc method M.H. Teng, J.J. Host, J.-H. Hwang, B.R. Elliott, J.R. Weertman, T.O. Mason, V. P. Dravid, and D. L. Johnson Department of Materials Science and Engineering, Northwestern University, 2225 North Campus Drive, Evanston, Illinois 60208-3108 (Received 6 June 1994; accepted 22 October 1994)
Nanophase Ni particles (50 nm) mixed with the smaller near-average can be seen. Figure 2 is a TEM micrograph of the 56 m/s specimen with the same magnification as Fig. 1. It shows the same chain structure but with a much smaller average particle size. Note that all the particles have about the same size, and no oversize particles are seen. The sizes of the Ni particles from the five experiments were measured manually from TEM micrographs, and the results are shown in Fig. 3. Most particles were close to spherical shape; the diameters of the nonspherical particles were determined by averaging the lengths of the longest and shortest axes. The accuracy of the measurements was limited by the resolution of the micrographs, which resulted in an uncertainty of about 2 nm at lower magnification down to about 0.5 nm at the magnification used for measuring the 56 m/s specimen. The size distributions of all five samples are close to log-normal. The curves shown superimposed on the histograms in Fig. 3 represent the log-normal distributions; they match reasonably well with the histograms. The average particle diameters of these five specimens are listed in Table I. Figure 4 shows the average particle size versus velocity, illustrating clearly the size reduction that occurs with a blown arc. The average sizes of the 0, 1, and 9 m/s specimens show no significant differences, while at 20 ms the entire distribution begins to shift to
lower sizes. At 56 m/s the average particle size is much lower and the standard deviation is narrower. The fitted log-normal curves are plotted against particle size in Fig. 5. This clearly shows the effectiveness of a blown arc in reducing the particle size and also demonstrates the existence of a threshold gas velocity below which the particle size is not measurably affected by the gas velocity. The threshold lies between 9 and 20 m/s. The five geometric standard deviations a (defined as the size at 84.13% divided by the size at 50% in a log-probability plot) derived from the lognormal distribution curves lie within the empirical range proposed by Granqvist and Buhrman.6 Their empirical rule for crystalline inert-gas evaporated particles is 1.36 =£ a «£ 1.60; our range is from 1.36 to 1.53. A common problem with the arc method is the occurrence of large particles, usually 100 nm and sometimes larger, caused by instability in the arc. Because large particles were found in the production chamber, we compared the samples collected from the cold trap where a few larger particles were present in the lower velocity cases. However, few large particles were found in the 56 m/s experiment (Fig. 2), so a high gas velocity may be the solution to this problem.
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