Steady-state and diffuse reflectance laser flash photolyses have been camed out to elucidate the mechanism of photodegradation of 1,3-diphenylisobenzofuran (DPBF) on solid surfaces of AlzO3, TiOz, and ZnO. In the absence of oxygen, the semiconductor supports TiOz and ZnO catalyze the photodegradation by accepting electrons from excited DPBF. The fluorescence of degassed DPBF on Ti02 and ZnO is quenched relative to that on alumina, thereby offering independent codmation of charge transfer. In oxygenated samples, the primary mechanism of photodegradation involves reaction with singlet oxygen. This is confirmed by the studies on the insulator surface A1203, where significant degradation is observed only in the case of air-equilibrated samples. The dependence of the rate of DPBF photodegradation on the surface coverage indicates that only the molecules that are in direct contact with the surface undergo photodegradation. The results that highlight the role of the support material in guiding the course of a photochemical reaction are described here.
High‐temperature aero‐thermal heating in a 30 kW inductively coupled plasma torch was used to replicate the effects of harsh oxidizing environments during hypersonic atmospheric entry on fracture behavior and microstructure of two‐dimensional woven SiC fibers. Hi‐Nicalon SiC woven cloths were exposed to surface temperatures over 1400°C with different high‐enthalpy dissociated oxygen and nitrogen plasma flows, and were subsequently deformed in pure tension at room temperature. Changes in fiber microstructure and surface chemistry after thermal exposure were examined by scanning electron microscopy. Pure nitrogen plasmas resulted in a 50% decrease of strength in woven SiC fibers with minimal effects on the fiber structure, except for highly localized surface pitting caused by partial decomposition of silicon oxycarbonitride phase at high temperature. In contrast, exposure to dissociated oxygen and air plasmas led to severe strength reduction and embrittlement over significantly short time scales, corresponding to degradation rates up to 200 times higher than those reported with static heating at equivalent temperatures. The origin of accelerated embrittlement at microscopic scale was found related to complex gas‐surface interactions and high‐temperature oxidizing processes involving the formation of SiO2 bubbles and microcracks on the surface. These findings are important for the development of outer fabric materials for new flexible thermal protection systems in space applications.
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