Influence of silica fillers during the electron irradiation of DGEBA/TETA epoxy resins, part II: Study of the thermomechanical properties

Experimental and Theoretical Study of Gamma Radiolysis and Dose Rate Effect of o-Cresol Formaldehyde Epoxy Composites

Gamma radiolysis behaviors and mechanisms of silica-filled o-cresol formaldehyde epoxy are studied at 2.20 × 10^-5 to 1.95 × 10^-1 Gy/s. The radiolysis-induced changes in chemical structures do not severely affect its thermostability. The slightly deteriorated mechanical strength at temperature exceeding 100 °C is accompanied by the declining glass transition temperature (from 185.9 to 172.2 °C) and enhanced damping ability. The gas yields of hydrogen, methane, and carbon dioxide manifest a remarkable dose rate effect. Based on the Schwarzschild law, their yields at an extremely low dose rate are accurately predicted by the established master curves. Besides, the latent radiolysis of gas products and postradiation effect are found with caution. The radiation-caused residual spin species are proved to be composed of silica defects and a phenoxy-type free radical with a tert-butyl group, according to the experimental results, theoretical calculations, and spectra simulations. The lower vertical ionization potential (7.6 eV) and adiabatic ionization potential (7.1 eV) are primarily due to the ionization of the benzene ring moiety with the tert-butyl group, which is likely to suffer from radiolysis. The calculated bond dissociation energy (260.8-563.5 kJ/mol) of the typical chemical bonds of epoxy is consistent with its radiolytic vulnerability and degradation mechanisms.

Characterization of an epoxy filler for piezocomposites compatible with microfabrication processes

Miniature ultrasound transducer arrays that can operate at frequencies above 30 MHz are needed for high-resolution medical imaging. One way to achieve this is with a kerfless structure based on 1-3 connectivity piezocomposite with the electrodes defined by photolithography. To achieve this, not only does the composite need planar, parallel, and smooth surfaces, but it must also be made with an epoxy filler compatible with the chemicals, heat, and vacuum required for photolithography. This paper reports full characterization of an epoxy suitable for fine-scale kerfless array fabrication, including photolithographic processing. Material properties have been investigated as a function of cure temperature and for compatibility with solvents. By increasing the cure temperature, the crosslinking between the epoxy and the hardener in- creases, resulting in a higher glass transition temperature. The cured epoxy consequently has better resistance to both heat and solvents. An elevated cure temperature, near 100°C, is required to optimize material properties for photolithography on 1-3 piezocomposites. The acoustic properties of the epoxy have also been studied. These are similar to other epoxies used in piezocomposite fabrication and no significant changes have been observed for the different cure temperatures.