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Study on the optical and electrical parameters of porcelain material based on Terahertz wave

Terahertz wave; nondestructive testing; transmission mode; reflection mode; optical and electrical parameters.

Abstract- Terahertz wave, which is considered as a potential technique to be applied in nondestructive testing of electrical insulation equipment, is a sort of electromagnetic wave whose frequency is located between microwave and infrared wave with considerable good physical characteristics such as: 1)typical pulse width in picoseconds; 2)good temporal and special coherence; 3)no photon energy, no harmful ionization of biological macromolecules; 4)good penetration; 5)many large molecules have rotational or vibrational levels in the terahertz band, which means certain material structure corresponds to certain response under the function of Terahertz wave. In order to pave the way for Terahertz to the application of defect testing in the area of electrical insulations, research on the Terahertz spectrum characteristics of porcelain material based on the system of Terahertz time domain spectroscopy is presented in this paper. Firstly, based on the equipment fabricated by PICOMETRIX corporation of the U.S, the system is well set up and the test platform is appropriately made up to be capable of generating, capturing and analyzing Terahertz wave. Secondly, flat samples of porcelains are customized with certain sizes. Thirdly, with preparations done, test of samples under Terahertz transmission mode is implemented and typical analyzing method of terahertz wave’s propagation in medium and transform between time domain and frequency domain is utilized to extract fundamental parameters of ceramics. To be concrete, the reference signal and sample signal are respectively received as time domain waves and transformed into frequency domain signal by Fourier transform. Then the information on frequency domain is collected and compared to the theoretical formula based on the fundamental electromagnetic principles until the parameters finally be extracted. Through the test, three major parameters are extracted respectively as refractive index spectrum, absorption coefficient spectrum and relative permittivity spectrum. However, due to the limits of the normal typical method, the deviation of parameters exists. For better use of parameters, an innovative method of analyzing is proposed with simplified operation process and better precision of parameters. With the novel method, reference signal is unnecessary and the focuses are the main pulse and echo pulse of sample signal. To make further study, the inner relationship between certain parameters and material structure is simply analyzed. Absorption coefficient is relevant to the loss during dielectric polarization, while refractive index and permittivity are corresponding to the strength of the molecular polarity. Fourthly, the test under Terahertz reflection mode is conducted to seek for distinct of response from transmission mode and explore more diversity and feasibility of Terahertz wave’s application. Finally, according to the result of reflection mode, the possibility of Terahertz reflection mode applied in testing defects happening in interface of multiple, layered materials is discussed and derived in this paper. To conclude, masses of information is contained in both the time domain and frequency domain of Terahertz wave, the main work remains to be done is to integrate all the information and figure out systematic methods of handling and analyzing them so that the tight connections can be made between them and material structure. Only in this way can Terahertz wave make it to be finally applied in actual operation and

maintenance of power system.