No abstract

No abstract

Quartz crystals in sandstones at depths of 1200 m–1400 m below the surface appear to reach a solubility equilibrium with the 4He‐concentration in the surrounding pore‐ or groundwater after some time. A rather high 4He‐concentration of 4.5 · 10−3 cc STP 4He/cm3 of water measured in a groundwater sample would for instance maintain a He pressure of 0.47 atm in a related volume. This value is equal within analytical error to the pressure deduced from the measured helium content of the quartz and its internal helium‐accessible volume. To determine this volume, quartz crystals of 0.1 to 1 mm were separated from sandstones and exposed to a helium gas pressure of 32 atm at a temperature of 290°C for up to 2 months. By crushing, melting or isothermal heating the helium was then extracted from the helium saturated samples. A volume on the order of 0.1% of the crystal volume is only accessible to helium atoms but not to argon atoms or water molecules. By monitoring the diffusive loss of He from the crystals at 350°C an effective diffusion constant on the order of 10−9 cm2/s is estimated. Extrapolation to the temperature of 70°C in the sediments at a depth of 1400 m gives a typical time of about 100 000 years to reach equilibrium between helium in porewaters and the internal He‐accessible volume of quartz crystals. In a geologic situation with stagnant pore‐ or groundwaters in sediments it therefore appears to be possible with this new method to deduce a 4He depth profile for porewaters in impermeable rocks based on their mineral record.

The virtual crystal approximation has been used to determine the temperature-composition dependence of the GaN-AlN, GaN-InN, and InN-AlN band gap energies. Also, the thermodynamic instability states in the mixed crystals were studied. The expression for the band gap of mixed A-B crystals has been derived: EgAB = (1−x)EgA + xEgB − bSxx, where EgA and EgB are the direct gaps for compounds A and B, respectively, and x is the alloy concentration. The term Sxx ~ T0/(∂2G/∂x2) where G is the thermodynamic potential of the mixed crystal, b is a bowing parameter, and T0 has the meaning of a growth temperature.

Аннотация. Экспериментально изучается энергетический порог пробоя сквозных отверстий в металлических фольгах разной толщины мощным лазерным излучением. Выявляются свойства вещества фольг, характерные для фазовых переходов второго рода «жидкий металл-газ». Контролирующим параметром порога пробоя является выходное (пороговое) отверстие с теневой стороны мишени, возникающее при минимальной энергии импульса излучения, необходимой лишь только для возникновения выходного отверстия для данной толщины фольги и энергия импульса лазерного излучения. Ключевые слова: мощное лазерное излучение, лазерный пробой, пороговый пробой, фазовый переход второго рода «жидкий металл-газ», теплопроводность, металлические фольги.