Structural and textural changes from polyimide Kapton to graphite: Part II. Magnetoresistance and x-ray diffraction

Structural and textural changes in carbon films prepared from the polyimide Kapton with heat-treatment temperatures up to 3000 °C were followed by measuring the magnetoresistance at 77 K, the ratio of electrical resistivity at room temperature to that at 77 K, and x-ray powder patterns. The maximum magnetoresistance (Δρ/ρ)_max as a measure of graphitization changed its sign from negative to positive around 2500 °C and then increased its value abruptly. The resistivity ratio ρ_R.T./ρ_{77 K} increased above 2500 °C treatment. The values of (Δρ/ρ)_max and ρ_R.T./ρ_{77 K} after 3000 °C treatment reached 275% and 0.93, respectively. The average interlayer spacing d₀₀₂ after the 3000 °C treatment was 0.3354 nm, the same as natural graphite. The changes in structure and texture in the film were consistent with the model proposed in the previous paper from observation by transmission electron microscopy.