Site: Nagoya University
Department of Crystalline Materials Science
Nagoya 464-01, Japan
Date Visited: 25 July 1997
WTEC: D. Shaw (report author), E. Hu, L. Jelinski, M.C. Roco, C. Uyehara
The X-ray diffraction spectrum of milled graphite was the same as for amorphous material. The density decreased from 2.2 to 1.85 g/cm3 in 36 hours of milling. Neutron diffraction confirmed the disorder. These results were analyzed to calculate the radial distribution factor that gives the coordination number, which reduced from 3.01 to 2.82 in the 36 hours. It is believed that this describes production of dangling bonds in the milled material. Assuming the crystalline structure is unchanged upon milling, only the size of the particles changes and the coordination number decreases. The size was estimated to be about 27 Å.
Transmission electron microscopy (TEM) before milling showed a layered graphite-like structure. After milling the material was amorphous.
Ball milling is done at low temperature. The equipment has a 150 G (x gravity) capability but 10 G is used in work in this department.
Trigonal selenium was also milled. Using the same analytical procedures as above, it was estimated that the particles contained about 22 atoms.
Li and graphite were milled together. The Li incorporates into the graphite and coats the balls to produce a gold-colored LiC6 film. The Li is inserted into the hexagonal C network if the milling intensity is kept low. The potential application is to batteries. One of the technical challenges is to remove the material from the balls.
Prof. Tanaka described nanobeam drilling using e-beam, which showed approximately 1 nm square windows drilled with a cylindrical beam. The lattice was distorted around the periphery of the window.
Tanaka discussed a number of other issues, including granular magnetoresistive structures and mass production of fine particles, but it was not clear whether the work was done at Nagoya University or pulled from the literature.
Tanaka showed a video of two sharp gold tips coming together to create a liquid-like interface. The tips were electrolitically sharpened. As seen in the video, the tip diameter was about 10-15 atoms. As the tips separated, the liquid-like region took on the lattice constant of one of the two tips. Previously, this demonstration had been reported with a tip and a flat surface.
Prof. Inoue has calculated the magnetoresistance ratio vs. surface/volume ratio of FeCr granules in a matrix. The model was independent of matrix material and distance between clusters. Although the assumed spacing was 2-5 Å, interactions were not included. Data from Tohoku University of a tunnel diode with Co-AlO granular barrier had a four order of magnitude drop in resistivity and a constant MR ratio of about 19% at 4.2 K. The barrier thickness was about 1.0 Ám, and the maximum voltage was 1.0 V. Typical barrier thickness reported in the literature is about 10-15 Å, so the Tohoku data applies to fields that are about 1E7 lower than structures designed for memory cells or hard drive heads.
The decrease in resistivity was explained by the charging energy of the single-electron-like granules, although this explanation seems unlikely to this author.