Complex Time Dependence Research Articles

Strain-aging behavior of irradiated and denitrided mild steel

Influences of neutron irradiation and dry hydrogen treatment on yield point phenomena in mild steel were examined. Static strain-aging studies were performed on vacuum-annealed, dry-hydrogen-treated and neutron-irradiated samples. Hardening due to neutron radiation damage exhibited a cube root dependence of the fluence. In addition to an increase in the lower yield stress, the Lüder strain increased with dose up to 10 18 neutrons cm −2, and at higher doses an apparently rounded yield was observed because of fracture during the Lüder straining. The increase in the Lüder strain followed a (Φt) 1 3 law, similar to that for radiation hardening. Static strain-aging studies revealed a complex time dependence of the strain-aging index f and seem to illustrate a two-stage process: initially, atmosphere locking and, for long aging times, locking due to precipitation. The effect of neutron irradiation was very similar to the effect of dry hydrogen treatment; the rate of aging decreased and, at high fluences (or longer exposures to dry hydrogen), essentially non-aging steel was obtained. However, the kinetics and/or mechanism of strain aging are not affected by neutron radiation (or dry hydrogen treatment).

Voltage and time dependence of restitution in heart.

Between beats, cardiac muscle gradually recovers the ability to contract in response to a stimulus. We examined the time and voltage dependence of this "restitution" process using sheep cardiac Purkinje fibers that were voltage clamped by the two-micro-electrode technique. The rate and time course of restitution depended on the voltage and duration of preceding conditioning depolarizations. As the voltage of a conditioning depolarization was made less negative, the early rate of restitution increased, the process reached a greater peak value, and oscillations in the time course of restitution became more likely. When conditioning depolarizations were to voltages near or above the normal action potential plateau, increasing their duration had much the same effect as increasing voltage. However, with conditioning depolarizations to -40 or -50 mV, prolongation had little or no effect on the subsequent rate of restitution. The voltage during restitution affected both the time course and final extent of restitution. Steady-state restitution was an approximately sigmoid function of voltage. The early rate and peak value of restitution increased as the voltage during restitution was made more negative. Oscillations of restitution were dramatic at voltages between -70 and -50 mV. The behavior of restitution confirms that most of the calcium that activates contraction comes from intracellular stores. The complex time and voltage dependence of restitution suggests that the process reflects the time course of calcium reaccumulation in an internal store.

Time-resolved emission spectroscopy of the dansyl fluorescence probe.

Using a cavity-dumped laser system as the light source, we have recorded time-resolved fluorescence spectra for the dansyl group in a variety of systems. The fluorescence spectrum of dansylphosphatidylethanolamine incorporated into a variety of lipid bilayer systems at ambient temperatures shows very marked time dependence, with red shifts of up to 40 nm over a 40-ns period. Similarly, large red shifts are observed for dansylamide in 1-butanol at low temperatures, which can be tentatively attributed to the initial rapid formation of an intramolecular charge transfer state, followed by reorientation of the polar solvent molecules. The time-resolved behavior in the lipid bilayer system can, by comparison, also be at least partly explained by solvent reorientation, but the possibility of heterogeneity of the binding site must also be considered. The complex time dependence of the fluorescence of the dansyl group in these lipid bilayer systems considerably complicates the interpretation of fluorescence quenching and fluorescence energy transfer experiments.

Relaxation oscillations in stimulated Raman scattering

Light pulses created by stimulated Raman scattering have been found to exhibit a complex time dependence which resembles relaxation oscillations. A focused laser pulse generated both forward and backward Raman emissions which appeared as a series of pulses with durations much shorter than the incident laser pulse. Time dependence of the Raman emission was observed directly by use of a streak camera. The number of observed pulses increased with the intensity of the incident pulse, while separation of the pulses in time depended on the length of the focal region. Beam focusing was incorporated in the coupled wave equations for stimulated Raman scattering. These rate equations were then solved numerically, and the results are in good qualitative agreement with the experimental observations. The short Raman pulses are created by a process associated with depletion of the incident laser pulse. This process occurs under a broad range of conditions.

Mass Spectrometry of Background Gases in Glow-Discharge Sputtering of Tantalum Thin Films

The composition of the ambient atmosphere during the sputtering of tantalum films in argon glow discharges has been studied by means of a mass spectrometer adapted both for residual analysis of vacua between 10−9−10−5 Torr, and process monitoring at pressures of (10−50)×10−5 Torr. The partial pressures of a number of residuals were found to increase immediately upon initiation of the discharge in both an oil diffusion and a turbomolecular pump system. The total background gas levels during the discharge were in the 1×10−5−5×10−4 Torr range, with the major component being hydrogen, and smaller contributions from water vapor and methane. The pressure increase of these species resulted from ion bombardment decomposition of adsorbed surface layers, and exhibited two maxima. The first is attributed to desorption from the electrodes, and the second to desorption from the bell jar wall. The spectrometer was used for detailed studies of the gettering of deliberate additions of N2, CH4 and H2 during sputtering in argon. Considerable differences were found in the effect of the glow discharge on those gases, with the gettering of CH4 and H2 exhibiting a complex time dependence. Tantalum films sputtered without deliberate gas additions, were found to be of the beta-tantalum form even when the discharge-generated background was reduced to ∼1×10−6 Torr by long presputtering.