Dynamic Low Frequency Research Articles

Torsion studies of relaxation magnetic effects in C60 fullerite in magnetic field

Highly sensitive torsion technique was used in dynamic low-frequency (0.1–1 Hz) oscillatory and static (without oscillations) experiments to study temperature dependences of the magnetic properties of C60 fullerite in the magnetic fields transverse and longitudinal with respect to the oscillation axis. The temperature position of observed oscillation attenuation peaks that are related with phase transitions depends on the direction in which temperature changes (cooling or heating). The most clearly pronounced transitions accompanied by reorientation of magnetic dipole moments have been observed in the region of the structural (Fm3m–Pa3) transition at Т ≈ 260 K and at temperatures Т ≈ 180–200 K. The region of “chaos” has been discovered where magnetic properties and the direction of the “spontaneous” rotation of the fullerite specimen with respect to magnetic field change rapidly. We have shown that under certain conditions the relaxation time of the magnetic moments of molecular rotators, after elapsing of which the fullerite specimen experiences multiple changes in its rotational movement direction, if temperature increases from 77 K to 280 K, coincides with the “magic” time that was observed in static experiments at room temperatures Т = 295 K. An assumption has been made that the phenomena observed are related with relaxation processes in the rotational subsystem of molecular C60 rotators and with the magnetic flux trapped by fullerite.

A Dynamic Low-Frequency Vibration Damper for Bearings of Turbine-Generator Sets

Formulas for calculating the dimensions of a dynamic low-frequency vibration damper are derived. The damper is designed as a circularly curved thin-walled plate. Bench tests demonstrated that the damper effectively reduces the amplitude of vibrations of the bearing of a high-power turbine-generator set in two directions simultaneously.

Dynamic Low-Frequency Electrical Impedance of Biological Materials Subject to Freezing and Its Implementation in Cryosurgical Monitoring

A precise monitoring of the extent of freezing during a cryosurgical process has been an important problem in health-care clinics. Among various existing techniques, the dynamic electrical impedance utilizing the impedance jump to detect the ice moving front, is a suitable way because the impedance of frozen tissue is much higher (3 to 4 orders of magnitude or even larger) than that of unfrozen tissue. Based on two experimental setups, the dynamic low-frequency impedance (DLFI) and impedance changing rate (ICR) for selected biological materials (fresh pork and rabbit tissues) subject to freezing were systematically measured. Their transient behaviors were investigated, and implementations in a practical cryosurgery to detect ice front propagation were analyzed. Furthermore, in vitro experiments were performed on distilled water and phantom gel. The experimental results, obtained with a two-electrode measuring technique, are as follows: (1) The impedance of all samples has a rapid response to the external freezing. (2) The impedance will not jump into an insulation region when the cooling temperature is not low enough. (3) As an alternative to DLFI, ICR imaging can also give important information for the phase-change process, which may lead to an efficient method to detect the ice-ball growth. (4) There is an evident variation in DLFI for different biological tissues when subjected to the same cooling temperatures; this value also differs for the same tissue under different cooling conditions.

RXTEObservations of QPOs in the Black Hole Candidate GRS 1915+105

We report on quasi-periodic oscillations (QPOs) in the black hole candidate GRS 1915+105 seen in 31 observations made by the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. We distinguish three different types: a QPO with constant centroid frequency of 67 Hz, a dynamic low-frequency (10-3-10 Hz) QPO with a large variety of amplitudes and widths, and high-amplitude sputters at frequencies of 10-3-10-1 Hz that are among the most extreme X-ray variations ever seen. We discuss the 67 Hz QPO on the assumption that it arises in the inner accretion disk of a black hole binary. If this QPO represents the rotation frequency of the innermost stable orbit around a nonrotating black hole, then the implied mass is 33 M☉. An alternative interpretation of this QPO as g-mode oscillations in the inner disk (Nowak et al. 1996) implies a black hole mass of 10 M☉. Four selected QPOs at lower frequencies (0.067-1.8 Hz) are tracked continuously in five energy bands. The QPO-folded profiles are mostly sinusoidal. Remarkably, all four investigated QPOs are broadened in frequency by a random walk in oscillation phase. At higher photon energies the QPO profiles show larger amplitude and increasing phase lag. Our QPO profile analysis shows that the phase delays are not caused by scattering effects, and we discuss a more direct relation between these QPOs and the origin of the hard X-ray spectrum. There are at least three general shapes of the broadband power continuum, with a typical persistence time scale of several weeks. The combined characteristics of the power spectra, light curves, and energy spectra during the period between 1996 February 21 and 1996 August 15 are interpreted as a succession of four different emission states. None of these states appears identical to any of the canonical states of black hole binaries.