9,538 research outputs found
High Efficiency-High Frequency Transducers for Ceramic Inspection
The compressional-wave ultrasonic transducer is the heart of ultrasonic NDE inspection systems. New piezoelectric ceramics, have been developed with very high electromechanical coupling (kt\u3e70%) and have been used for transmission and reception of ultrasonic waves. This paper discusses ways to improve the signal-to-noise ratio (i.e. efficiency) of \u3e30 MHz transducers (i.e. selection of materials, selection of diameter etc.) Application of the transducers developed to inspect creep-failed siliconized-SiC ceramics are discussed
High Angular Resolution Stellar Imaging with Occultations from the Cassini Spacecraft II: Kronocyclic Tomography
We present an advance in the use of Cassini observations of stellar
occultations by the rings of Saturn for stellar studies. Stewart et al. (2013)
demonstrated the potential use of such observations for measuring stellar
angular diameters. Here, we use these same observations, and tomographic
imaging reconstruction techniques, to produce two dimensional images of complex
stellar systems. We detail the determination of the basic observational
reference frame. A technique for recovering model-independent brightness
profiles for data from each occulting edge is discussed, along with the
tomographic combination of these profiles to build an image of the source star.
Finally we demonstrate the technique with recovered images of the {\alpha}
Centauri binary system and the circumstellar environment of the evolved
late-type giant star, Mira.Comment: 8 pages, 8 figures, Accepted by MNRA
Breaking of ergodicity and long relaxation times in systems with long-range interactions
The thermodynamic and dynamical properties of an Ising model with both short
range and long range, mean field like, interactions are studied within the
microcanonical ensemble. It is found that the relaxation time of
thermodynamically unstable states diverges logarithmically with system size.
This is in contrast with the case of short range interactions where this time
is finite. Moreover, at sufficiently low energies, gaps in the magnetization
interval may develop to which no microscopic configuration corresponds. As a
result, in local microcanonical dynamics the system cannot move across the gap,
leading to breaking of ergodicity even in finite systems. These are general
features of systems with long range interactions and are expected to be valid
even when the interaction is slowly decaying with distance.Comment: 4 pages, 5 figure
Two-stream instability in quasi-one-dimensional Bose-Einstein condensates
We apply a kinetic model to predict the existence of an instability mechanism in elongated Bose-Einstein condensates. Our kinetic description, based on the Wigner formalism, is employed to highlight the existence of unstable Bogoliubov waves that may be excited in the counterpropagation configuration. We identify a dimensionless parameter, the Mach number at T=0, that tunes different regimes of stability. We also estimate the magnitude of the main parameters at which two-stream instability is expected to be observed under typical experimental conditions
Direct path from microscopic mechanics to Debye shielding, Landau damping, and wave-particle interaction
The derivation of Debye shielding and Landau damping from the -body
description of plasmas is performed directly by using Newton's second law for
the -body system. This is done in a few steps with elementary calculations
using standard tools of calculus, and no probabilistic setting. Unexpectedly,
Debye shielding is encountered together with Landau damping. This approach is
shown to be justified in the one-dimensional case when the number of particles
in a Debye sphere becomes large. The theory is extended to accommodate a
correct description of trapping and chaos due to Langmuir waves. Shielding and
collisional transport are found to be two related aspects of the repulsive
deflections of electrons, in such a way that each particle is shielded by all
other ones while keeping in uninterrupted motion.Comment: arXiv admin note: substantial text overlap with arXiv:1310.3096,
arXiv:1210.154
High Frequency-High Temperature Ultrasonic Transducers
Ultrasonic testing is a promising NDE technique for ceramic structural components. The lifetime of such components is controlled by defects and flaws. The critical flaw sizes for high performance ceramics are 25 µm and to detect such small flaws, the frequency of the transducers has to be \u3e30 MHz [1–4]. Such transducers are usually made from ZnO, LiNbO3, LMN composites [5], and PVDF piezoelectric materials However there are no reports of aluminium nitride (AIN) films being used for such applications In this paper the piezoelectric and dielectric properties of A1N films and their application to high frequency devices are discussed and compared with the conventional materials
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