Volume 83, Issue 9, 01 May 1998
Index of content:
- CLASSICAL PHENOMENOLOGY: ELECTRICITY, MAGNETISM, OPTICS, ACOUSTICS, HEAT, MECHANICS (PACS 41-52)
83(1998); http://dx.doi.org/10.1063/1.367241View Description Hide Description
We have determined the nonlinear ultrasonic parameter and the ultrasonic longitudinal phase velocity for a series of martensitic steel specimens which varied in carbon content. The specimens were measured in the as-quenched state to ensure that the carbon was present primarily as an interstitial in the martensite. increased monotonically with carbon content and hardness over the range 0.10–0.40 mass % C (39.0–57.5 Rockwell C hardness). However, remained virtually the same for all specimens. Therefore we conclude that is sensitive to microstructural variations between the specimens, but is not. X-ray diffraction experiments indicate that the dislocation density in the specimens is large and increases with increasing carbon content. These results support the hypothesis that the observed increase in can be attributed to dislocation-related effects in the specimens.
83(1998); http://dx.doi.org/10.1063/1.367242View Description Hide Description
A flexural plate wave resonator was constructed by patterning current lines on a silicon nitride membrane suspended on a rectangular silicon frame. Eigenmodes of the rectangular membrane were excited using Lorentz forces generated between alternating surface currents and a static in-plane magnetic field. Preferential coupling to a particular membrane mode was achieved by positioning current lines along longitudinal mode antinodes. An equivalent-circuit model was derived which characterizes the input impedance of a one-port device and the transmission response of a two-port device over a range of frequencies near a single membrane resonance. Experiments were performed to characterize the device’s response to changes in dcmagnetic field strength, ambient gas composition, gas pressure, and input power.
83(1998); http://dx.doi.org/10.1063/1.367243View Description Hide Description
A spatially resolved experimental investigation of the electron energy distribution function(EEDF) and wave propagationcharacteristics in a high frequency surface wave (SW) discharge in nitrogen has been performed. The measurements reveal the specific changes of the EEDF and its integrals which occur as a result of a different coupling between the electrons and the inhomogeneous SW electric field as a function of gas pressure. At a pressure of 0.5 Torr it was found that the EEDF and corresponding integrals depend on the spatial position, which means that local plasma response occurs. As a result, a strong radial inhomogeneity of the discharge is observed which relates to the radial variation of the wave field intensity. On the contrary, at 0.05 Torr nonlocal plasma response is observed. In a molecular plasma, electron energy relaxation occurs faster than in inert gases, this being why the transition from the local to the nonlocal regime takes place here at a lower gas pressure. A turning back of the axial wave number due to the simultaneous effects of collisions and the radial inhomogeneity is also observed. A notable feature of the experiments is that the minimum electron density for the wave to propagate may be smaller than the minimum density for propagation in the collisionless approximation. A strong correlation between the behavior of the plasma parameters and the wave electrodynamics is pointed out.