Influence of microstructure on electrical resistivity of hydrothermal carbonization-derived carbon spheres

The numerous applications of carbon spheres (CS) in the field of new technologies, prompt a deep analysis of such objects. In the present work, CS were synthesized by hydrothermal carbonization (HTC) based on glucose with the surfactant. In the first part of the work, the effect of the concentration of the surfactant used in the synthesis on the efficiency of the process, the size of the obtained spheres and the relationship between their structural properties and electrical resistivity were studied. In the second part, based on a fixed surfactant concentration, it was shown how to obtain carbon spheres with high monodispersity in an easy, efficient and reproducible way, controlling parameters such as temperature and synthesis time. A detailed analysis of selected three sizes of carbon spheres annealed over a temperature range of 600 to 1000 °C was then carried out. The CS results showed that their surface morphology, microstructure, porosity and electrical resistivity strongly depend on the processing temperature. These conclusions are based on scanning electron microscopy, Raman spectroscopy, X-ray diffraction, thermogravimetric analysis, nitrogen adsorption analysis and resistivity measurements. The specific surface area (SSA) and pore volume varied with annealing temperature and sphere size, significantly influencing the development of the porous structure. In the 200–400 nm size range, sphere size had no clear effect on sample resistivity. The dominant factors were graphitization processes and changes in SSA and pore volume. Across all annealing temperatures, samples with the highest resistivity consistently exhibited the largest SSA.