Effects of scattering on optical parameters of terahertz time-domain spectroscopy of coal based on effective medium model

Abstract

The optical parameters measured by terahertz time-domain spectroscopy (THz-TDS) are directly analyzed without considering the scattering effect, which is a common issue in the quantitative analysis of coal using THz-TDS. In this study, the optical parameters, microstructure, and true density of the samples were measured using THz-TDS, Scanning Electron Microscopy (SEM), and true density meter. The effective permittivity of a pure high-density polyethylene (HDPE) tablet sample was determined using the Maxwell-Garnett (MG), Bruggeman (BR), Landau-Lifshitz-Looyenga (LLL), and linear absorption (LA) models. The discrepancies between the effective permittivity and the true permittivity of HDPE were compared, leading to the determination that the LA model is the optimal approach. The irregular morphology of particles and the significant difference in permittivity between HDPE and coal are responsible for larger discrepancies observed in other effective medium models. The amplitude of the sample is inversely proportional to the size of coal particles in the time-dependent THz signal spectra. The effective absorbance and the extinction coefficient of coal are increased by scattering as the size of particles increases. Larger coal particles tend to agglomerate, resulting in a greater error in the effective refractive index. A quadratic function can be utilized to model the correlation between coal's absorbance caused by scattering and relative size. The results have demonstrated the validity of the LA model in calculating effective permittivity in tablet samples and provided guidance for minimizing scattering errors in THz-TDS.