Crystallinity is an essential indicator for evaluating the quality of fiber materials. Terahertz spectroscopy technology has excellent penetrability, no harmful substances, and commendable detection capability of absorption characteristics. The terahertz spectroscopy technology has great application potential in the field of fiber material research, especially for the characterization of the crystallinity of cellulose. In this work, the absorption peak of wood cellulose, microcrystalline cellulose, wood nano cellulose, and cotton nano cellulose were probed in the terahertz band to calculate the crystallinity, and the result compared with XRD and FT-IR analysis. The vibration model of cellulose molecular motion was obtained by density functional theory. The results showed that the average length of wood cellulose (WC) single fiber was 300 μm. The microcrystalline cellulose (MCC) was bar-like, and the average length was 20 μm. The cotton cellulose nanofiber (C-CNF) was a single fibrous substance with a length of 50 μm, while the wood cellulose nanofiber (W-CNF) was with a length of 250 μm. The crystallinity of cellulose samples in THz was calculated as follows: 73% for WC, 78% for MCC, 85% for W-CNF, and 90% for C-CNF. The crystallinity values were obtained by the three methods which were different to some extent. The absorption peak of the terahertz spectra was most obvious when the samples thickness was 1 mm and mixed mass ratio of the polyethylene and cellulose was 1:1. The degree of crystallinity was proportional to the terahertz absorption coefficients of cellulose, the five-movement models of cellulose molecules corresponded to the five absorption peak positions of cellulose.

Iron oxides have been investigated by employing mostly Fourier-transform infrared spectroscopy (FTIR). However, absorption spectra sometimes are difficult to obtain in situ, where accessibility to the location is limited. In this paper, we propose using emission spectra to investigate the purity of hematite (α-Fe₂O₃) and magnetite (Fe₃O₄). We used an infrared beam (wavelength of 1064 nm) from a pulsed laser to obtain emission spectra of the α-Fe₂O₃ and Fe₃O₄ for two different purities (99.99% and 96%) ranging from visible to near-infrared regions. The average values of full width at half maximum (FWHM) for the 99.99% purity were found to be 0.15 nm and 0.17 nm for the hematite and the magnetite, respectively; the average values of FWHM for the 96% purity were found to be 0.19 nm and 0.23 nm for the hematite and the magnetite, respectively. In addition, it is found that lower purity iron oxides exhibited a higher amplitude in the broader emission spectrum when compared to the iron oxides with high purity under the same experimental conditions. Therefore, by observing the amplitude of the broader emission spectra, it is possible to differentiate qualitatively between hematite and magnetite purities; moreover, from the average value of the FWHM of the spectral lines, it is possible to evaluate percentage purity content of hematite and magnetite, which are useful for different mechanical and biomedical applications. FTIR measurements also confirmed that the purity of the oxides can be found by measuring the peaks’ shifts. However, the proposed technique can be used in place of FTIR, where accessibility to the measurement’s location becomes problematic.