Production of finely ground natural graphite particles with high electrical conductivity by controlling the grinding atmosphere Original Research Article

Natural graphite particles with high crystallinity sieved to obtain a particle size range of under 63 μm were ground with a ball mill, under various well-controlled grinding atmospheres such as N2, O2, He, H2, and vacuum. The ratio, Xdif50/Xst50, i.e. between the 50 wt.% Stokes diameter and the 50 wt.% laser diffraction diameter, of the ground particles, was used as an index of the flakiness of the particles. The specific resistance of films composed of the ground graphite particles was systematically measured. The rate of reduction in the size of the particles by grinding was slow under an O2-rich atmosphere such as 100% O2 and dry air. On the other hand, it was relatively fast in vacuum, or under an N2 or He atmosphere, and a gas mixture of 99% N2 and 1% O2. The rate of size reduction by grinding under a H2 atmosphere was intermediate. In our experimental conditions, the flakiness of the ground particles increased with the decrease in the particles’ sizes. The electrical conductivity of the ground particles, however, tended to decrease with the decrease in their sizes. Under the condition that the Stokes diameter of the ground particles remains constant, the electrical conductivity of films made from the ground particles increases with the increase in the flakiness of the particles. It was finally determined from our systematic grinding experiments that small flaky particles, which had a size, Xst of ∼1 μm, with a high electrical conductivity can be produced by grinding in a gas mixture of 99% N2 and 1% O2. In this case, the flaky shape of the ground particles was visually confirmed by scanning electron microscopy.