Fig. 4. Comparison of the XRD peak intensities (between 2θ = 29.0 and 33.0°) for (a) the CuO + Fe₂O₃ + kaolinite system and (b) the CuO + Fe₂O₃ + Al₂O₃ system sintered at 650–1250 °C for 3 h, and (c) the variation of the peak positions with respect to the sintering temperature for both systems.