Wet chemical nitridation of (100)GaAs surface: Effect on electrical parameters of surface-barrier Au-Ti/GaAs structures

The influence exerted by wet chemical nitridation of the (100)GaAs substrate surface in hydrazinesulfide solutions before fabrication of barrier contacts on the parameters of surface-barrier Au-Ti/GaAs structures has been studied. The structures fabricated on nitrided substrates are characterized by lower reverse currents and higher breakdown voltages. The barrier height in structures of this kind is 0.71±0.02 eV, and the ideality factor, 1.06±0.01. The observed improvement in the electrical parameters of the structures is due to replacement of the natural oxide layer on the substrate surface by a thin coherent GaN film.     

Use of heat emitted by broken optic fibers: A new approach for damage detection in composites

Optic fibers, stripped of their jacket, weakened and embedded in a composite plate structure, cracked due to the occurrence of cracks and delaminations in the composite. At the location of a crack in a fiber, transmitted laser-light energy was converted into thermal energy, causing the temperature in the neighborhood of the crack to rise. The temperature change was detected with the aid of an infrared camera. Dynamic numerical simulations of the heat development in the vicinity of the fiber crack were performed. An analytical solution of the temperature distribution on the surface of the composite plate above a fiber crack was also developed. Both analytical and numerical results showed the feasibility of detecting and monitoring the hot spot on the composite plate caused by optic fiber cracking, by use of infrared emission from the fibers. This was successfully confirmed in experiment. The influence of parameters such as depth of the fiber below the surface of the plate, heat conductivity coefficient and light power, on the temperature distribution was also studied. Ultrasound scan of the damaged plate confirmed the accuracy of the hot-spot method in defining the damaged area.     

Measurements of natural concentrations of I in uranium ores by accelerator mass spectrometry

An accelerator mass spectrometry system is described and utilized for measurements of ¹²⁹I concentrations in natural and environmental samples. We report here on measurements of ¹²⁹I isotopic abundances in iodine reagents and in iodine of mineral origin and of ¹²⁹I concentrations in uranium ores of different origins. The ¹²⁹I isotopic abundances for two measured contemporary iodine reagents and for iodine from a deep underground brine are 1.3 × 10⁻¹³ and about 4 × 10⁻¹⁴, respectively. ¹²⁹I/U ratios in the range 10⁻¹³–10⁻¹² are measured and compared to a simple model of ¹²⁹I production by spontaneous and induced fission of uranium. No clear correlation with the uranium concentrations or residence times is observed.