Density functional theory of the effect of different directional strains on the physical properties of coal.

• The effects of different axial strains on the physical properties of coal were studied. • The study was carried out at the atomic level using first principles. • Coal is most significantly affected by stress in the Y direction. • Strain changes the electrical, optical and other physical properties of coal. The effects of different directional strains on the physical properties of coal were studied using the first-principles density functional theory (DFT). The results show that the effect of strain on the physical properties of coal is mainly in the range of 0–8 % strain and greater than 8 % strain. When the strain is less than 8 %, in the elastic range, strain leads to an increase in the strain energy and stress of the coal, a decrease in deformation resistance, and better thermal stability. The bandgap of coal is 1.42 eV; the bandgap width decreases and the electrical conductivity increases with an increase in strain. Charge transfer in coal mainly occurs around O and N atoms due to their stronger electronegativity; strain has a great effect on the relative positions of the atoms. In this range, the effect of strain on the optical properties of coal is regular. With strain exceeding 8 %, the coal structure loses stability and is in the plastic range. This leads to significant changes in the mechanical properties, stability, electronic structure, and optical properties of coal with poor regularity. The results provide a theoretical basis for understanding the relationship between the molecular structure of coal and its macroscopic properties. [ABSTRACT FROM AUTHOR]