In this paper, the S-parameters of 2mm silicon wafer, 14.345mm wood I, 14.215mm wood II and 13.80mm wood III were measured based on 220-325 GHz quasi optical Vector Network Analyzer measurement system. Basic theory based on free space electromagnetic component equations, boundary conditions and S parameters. The electromagnetic field of "air-MUT-air" three-layer structure between calibrated reference planes is analyzed in detail, and the formula of complex permittivity is derived. The results of complex permittivity of silicon wafer extracted by this method are basically consistent with those in the literature. The measurement of woods can accurately distinguish heartwood from sapwood. This study is helpful for us to better understand the interaction mechanism between terahertz waves and wood media, and can lay a foundation for the identification of woods.
In this paper, the coating thickness of thermal barrier coating (TBC) on gas turbine blades is measured by terahertz time domain spectroscopy system. At present, the most mature and widely used ceramic layer (TC) material is yttria partially stabilized zirconia (YSZ). In order to measure the coating thickness on TBC surface by terahertz spectrum, it is necessary to determine the optical properties (refractive index and absorption coefficient) of coating material YSZ in terahertz band. The refractive indexes of YSZ samples in terahertz band under different preparation conditions were obtained by transmission THz-TDS. The refractive indexes of these samples were different under the influence of different process parameters and spraying conditions. Then the time domain signal of TBC sample is obtained by reflective THz-TDS, and the coating thickness is obtained according to the time delay of terahertz wave transmission in the sample. When THz wave propagates at different depths of the sample, the dispersion is different, resulting in waveform broadening. We qualitatively analyze the influence of waveform broadening on thickness measurement. The results show that, within the error range, the coating thickness obtained from the time delay between the negative peak of surface reflection signal and the negative peak of interface reflection signal is in good agreement with the thickness measurement results of metallographic method. The optical properties of YSZ material and the thickness of thermal barrier coating are nondestructive tested in THz band.
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