Substituent Effect on the Photoreduction Kinetics of Benzophenone

The kinetics of the photoreduction of four benzophenone derivatives by isopropyl alcohol was examined in acetonitrile, namely tetra-meta-trifluoromethyl-, di-para-trifluoromethyl-, di-para-methoxy benzophenone and for comparison the unsubstituted molecule itself. The basic spectroscopic (absorption and phosphorescence spectra) and photophysical (quantum yields, excited state energies) properties were established, and the key kinetic parameters were determined by the laser flash photolysis transient absorption technique. The rate coefficients of both the primary and secondary photoreduction reaction show remarkable dependence on ring substitution. This substantial effect is caused by the considerable change in the activation energy of the corresponding process. The experimental results as well as DFT quantum chemical calculations clearly indicate that these benzophenone derivatives all react as n-p* excited ketones, and the rate as well as the activation energy of the reduction steps change parallel with the reaction enthalpies, the determining factor being the stability of the forming aromatic ketyl radicals. The secondary photoreduction of benzophenones by the aliphatic ketyl radical formed in the primary step occurs via a hydrogen bonded complex. The binding energy of the hydrogen bonded complex between the aliphatic ketyl radical reactant and a solvent molecule is a critical parameter influencing the observable rate of the secondary photoreduction

Particle Impact Behavior of Silicon-Nitride Ceramic at Elevated Temperature

By using a novel particle launch system, the impact tests of silicon nitride ceramic have been carried out. Particle of 1 mm dia. and 4 mg weight was impacted on the ceramic specimen at velocities up to 1 000 m s-1 under the temperatures up to 1 350°C and tensile loads up to 100 MPa. The results show a significant influence of the temperature on the impact fracture behavior. At the room temperature, a map of fracture modes in terms of material bending strength and thickness is shown. In the last, non-dimensional parameter required to estimate critical velocity for strength degradation is obtained.Des essais d'impact de céramique en nitrure de silicium ont été réalisés à l'aide d'un nouveau système de lancement. Les échantillons de céramique ont été impactés à l'aide de particules de 1 mm de diamètre et 4mg de masse jusqu'à des vitesses de 1000 ms-1, des températures de 1350°C et des chargements en tension de 100 MPa. Les résultats montrent une influence significative de la température sur le comportement de la rupture sous impact. Pour la température ambiante, une cartographie des modes de rupture en fonction de la résistance du matériau en flexion et de l'épaisseur est indiquée ainsi qu'un paramètre sans dimension estimant la vitesse critique de l'endommagement