Structural order and dielectric properties of coal chars

•This work investigated structure changes and dielectric properties of coal chars.•The crystallite array of coal chars was more ordered with increasing temperature.•The complex permittivity of coal chars depended on treatment temperature.•The crystallite structure was responsible for dielectric properties of coal chars.

For studying the influence of the structural evolution on the dielectric properties, X-ray diffraction and Raman spectroscopy were used to examine the phase and the degree of crystallite structure of coal chars prepared in the temperature range of 850–1600 °C. Additionally, structural changes were visualized by high-resolution transmission electron microscopy. The dielectric properties were measured in the 2–18 GHz frequency range using the transmission/reflection method. Generally, the fraction of carbon ordering of coal chars increased, while more parallel-aligned aromatic layers were presented, confirming that the crystallite structure of coal chars became ordered with increasing temperature. The dielectric properties of chars were found strongly dependent on heat-treatment temperature. Increasing temperature could produce higher values of the real part and imaginary part of the complex permittivity. Although mineral matters in coal chars were also observed to transform during heat treatment, the conversion of inorganic matters might have a marginal effect on the complex permittivity. As a result, it is reasonable to conclude that the dielectric properties primarily result from the crystallite structure of coal chars. Furthermore, the dielectric loss may contribute much more than the magnetic loss to the further application of microwave technology.