We consider wave generation by turbulent convection in a plane parallel, stratified atmosphere that sits in a gravitational field, g. The atmosphere consists of two semi-infinite layers, the lower adiabatic and polytropic and the upper isothermal. The adiabatic layer supports a convective energy flux given by mixing length theory; Fe ""'pv~, where pis mass density and v 8 is the velocity of the energy bearing turbulent eddies. Acoustic waves with ro > roac and gravity waves with ro < 2khHirob propagate in the isothermal layer whose acoustic cutoff frequency, roaco and Brunt-Vliisiilii frequency, rob, satisfy ro:C = ygf4H; and ro: = (y-1)gfyH;, where y and H; denote the adiabatic index and scale height. The atmosphere traps acoustic waves in upper part of the adiabatic layer (p-modes) and gravity waves on the interface between the adiabatic and isothermal layers (f-modes). These modes obey the dispersion relation ro 2 ~ ; gk{ n + ~) , for ro < roac. Here, m is the polytropic index, kh is the magnitude of the horizontal wave vector, and n is the number of nodes in the radial displacement eigenfunction; n = 0 for /-modes. Wave generation is concentrated at the top of the convection zone since the turbulent Mach number, M = v 8 jc, peaks there; we assume M, ~ 1. The dimensionless efficiency, '1• for the conversion of the energy carried by convection into wave energy is calculated to be '1 ""'Mf 512 for p-modes,J-modes, and propagating acoustic waves, and '1 ""' M, for propagating gravity waves. Most of the energy going into p-modes, /-modes, and propagating acoustic waves is emitted by inertial range eddies of size h""' M: 12 H, at ro""' roac and kh""' 1/H,. The energy emission into propagating gravity waves is dominated by energy bearing eddies of size ""'H, and is concentrated at ro""' v,/H,""' M,roac and kh""' 1/H,. We find the power input to individual p-modes, E,, to vary as ro< 2 m 2 + 7 m-3 Ji

Negative bias temperature instability is studied in thick and thin gate oxide p-MOSFETs. The relative contributions of interface-and bulk-trap generation to this device degradation mode are analyzed for a wide range of stress bias and stress temperature. The effects of gate voltage and oxide field, as well as those of inversion layer holes, impact ionized hot holes, and hot electrons on interface-and bulk-trap generation, are identified. The bulk-trap generation process is interpreted within the modified anode-hole injection model and the mechanism of interface-trap generation is modeled within the framework of the classical reaction-diffusion theory. The diffusion species for interface-trap generation is unambiguously identified. Moreover, a high-temperature, diffusion-triggered, enhanced interface-trap generation mechanism is discussed for thin gate oxide p-MOSFETs. Finally, a novel scaling methodology is proposed for interface-trap generation that helps in obtaining a simple, analytical model useful for reliability projection.

We relate entropy and magnetic field perturbations to variations of solar p-mode eigenfrequencies. The frequency variations result from changes in path length and propagation speed. These produce shifts of opposite sign. Path length changes dominate for entropy perturbations, and propagation speed changes dominate for most types of magnetic field perturbations. The p-mode frequencies increased along with solar activity between 1986 and 1989. The frequency shifts exhibit a rapid rise with increasing frequency followed by a precipitous drop. The positive component signals a strengthening of the photospheric magnetic field to an rms value of order 200 G. The sudden drop at high frequency is due to a combination of a resonance and an increase in temperature in the chromospheric cavity. The magnetic stress perturbation decays above the top of the convection zone on a length scale comparable to the pressure scale height and grows gradually with depth below. The former characteristic implies that the stress is mainly due to small magnetic elements of the enhanced network, a conclusion supported by our analysis of Kitt Peak magnetograms. The latter property suggests that the flux tubes which pierce the photosphere strengthen with depth, at least to a pressure level of 10 8 dynes em-2. The presence of a resonance in the chromospheric cavity means that the transition layer maintains enough coherence to partially reflect acoustic waves even near cycle maximum. The fractional chromospheric temperature rise implies a much larger fractional increase in the rate of mechanical heating, as indicated by the variation of the Ca 11 H and K lines.

No abstract

A number of orthopedic disorders and bone defect issues are solved by scaffold-based therapy in tissue engineering. The biocompatibility of chitosan (polysaccharide) and its similarity with glycosaminoglycan makes it a bone-grafting material. The current work focus on the synthesis of chitosan and chitosan-gelatin scaffold for hard tissue engineering. The chitosan and chitosan-gelatin scaffold have shown improved specific surface area, density, porosity, mechanical properties, biodegradability and absorption. These scaffolds can lead to the development or artificial fabrication of hard tissue alternates. The porous scaffold samples were prepared by freeze-drying method. The microstructure, mechanical and degradable properties of chitosan and chitosan-gelatin scaffolds were analyzed and results revealed that the scaffolds prepared from chitosan-gelatin can be utilized as a useful matrix for tissue engineering.