Terahertz Time-Domain Spectroscopic Study of Aircraft Composite and Matrix Resins

Nowadays, many kinds of advanced composite have been applied in manufacture of aircraft. And composites with different matrix resin have different performance superiorities, its physical properties influence the design, employment and detection of composites. The purpose of this study was to investigate the spectral characteristics and dielectric properties of aircraft glass fiber composites and matrix resins in the terahertz (THz) frequency range. Terahertz time-domain spectroscopy was employed to examine the spectral absorption and dielectric dispersion characteristics of three major types, namely, epoxy resins 3238A/glass fiber EW180A, cyanate ester (CE) resin 9915/glass fiber QW120A, and bismaleimide (BMI) resins QY8911/glass fiber ZW100A, in the 0.2 similar to 1.0 THz frequency range. The refractive index n, the absorption coefficient a and the real part epsilon' and imaginary part epsilon '' of the dielectric constant of each composite and resin were calculated. Additionally, the influence of added modifying groups in resins was analyzed and the property of composites with glass fiber was compared. On this basis, the relaxation process of the dipoles in each resin system was theoretically calculated and fitting analyzed using the Debye model. The results showed the following points. The sequence of epsilon '' and alpha is QW120A/9915<ZW100A/QY8911<EW180A/3238A and 9915<QY8911<3238A. The reason is that, for a resin system, the more polar functional groups and molecules it has, the higher its polarity is, the more intense the relaxation motions caused by the orientation polarization of the dipoles, the greater the energy loss is. With the increasing frequency of the alternating electric field, the change of dipoles lags behind the change in the electric field, the relaxation also becomes more prominent, and more energy is required to overcome the viscous resistance inside the material. Consequently, the dielectric loss increased, resulting in an increase in epsilon '' and alpha. In addition, no significant absorption peaks appeared for any of the composites and resins. Because of the difference in molecular chain structure and content of polar groups, the CE resin had the lowest epsilon' and n, and exhibited the highest and most stable dielectric performance, followed by the BMI resins, whereas the epoxy resins exhibited fluctuating and slightly less stable dielectric performance. And the sequence of epsilon' and n is QW120A/9915>ZW100A/QY8911>EW180A/3238A and 9915>QY8911>3238A. In polymers, due to their relaxation motions, the polarization behavior of the dipoles was affected by damping, resulting in a relative decrease in the extent of orientation polarization. epsilon' and n each exhibited an abnormal dispersion, i.e., epsilon' and n decreased with increasing frequency. However, the influence of polarization behavior is weakened by the glass fiber of composites, which don't exhibit an abnormal dispersion. Due to the difference in the mechanism of polarization response between groups differing in polarity in the mixed resin systems, the fitting of the data of epsilon' and n using the Debye equation had good results, whereas there was an error between the fitted and measured values of epsilon '' and alpha. This study reports, for the first time, the difference in dielectric properties between various types of composites and matrix resins, and their spectral characteristics and patterns in the THz frequency range. Additionally, it gives the basic parameters of five aircraft glass fiber composites with epoxy, CE and BMI resins in the THz frequency range, adds new content to the research on solid dielectric materials in the THz frequency range, and provides an important reference for THz nondestructive testing of aeronautical composites and improving the performance of aeronautical composites and matrix resins as well as technologies currently in rapid development, such as THz and wave-absorbing materials, opto-semiconductors and high-performance radomes.