The structure and phase composition of stabilized zirconia-based nanosystems exposed to shock-wave treatment

The special features of the structure and phase composition of nanocrystalline zirconia-based powders subjected to shock-wave treatment are studied. The investigations show that zirconia with small amounts of yttria and/or alumina is in nanocrystalline and quasi-amorphous states representing a nonequilibrium solid solution of ZrO2 (Y, Al) and that an increase in the monoclinic phase abundance is associated with a reduction in the critical size of tetragonal crystallites due to an accumulation of lattice microdistortions. The monoclinic phase in powders with yttria and alumina additions is not formed even with shock compression at pressures up to 20 GPa. This is attributed to the fact that the resultant lattice microdistortion level is inadequate to destabilize the nanocrystalline tetragonal phase. Relaxation of microdistortions on annealing causes the critical size of tetragonal crystallites to increase. As this takes place, the monoclinic phase is converted into the tetragonal one.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 61–70, September, 2004.     

Controlled excitation of resonance self-oscillations in one-dimensional distributed systems

On the basis of the method of equivalent linearization combined with the method of moments, laws of self-oscillation excitation are obtained that provide the modes with maximum intensity of resonance (or quasi-resonance) oscillations in one-dimensional systems with distributed parameters. A restriction of a general type is imposed on the law of excitation. In the particular case of an integral quadratic restriction, the law of excitation leads to the generation of purely harmonic self-oscillations. The use of an extended (multiplicatively stabilizing) control provides the uniqueness and stability of the quasi-optimal mode of self-oscillation.     

Reduction of the error of hydrophone calibration with respect to the field in a hydroacoustic tank by the reciprocity method

A hydrophone calibration procedure that considerably reduces the error caused by the acoustic field distortions in a hydroacoustic tank is proposed. The procedure is based on the definition of the reduced electric transfer impedances of transducers (i.e., the electric transfer impedances reduced to the spherical wave propagation law) and consists in measuring the electric transfer impedances for different distances between hydroacoustic transducers. The sensitivity of the hydrophone under calibration is calculated from the far-field values of the reduced electric transfer impedances. The latter are determined using a mathematical model of the hydrophone in the form of a system that contains a point sensing element and a finite number of point sources of acoustic signals (point reflectors). A method of determining the number and coordinates of the point reflectors from the analysis of the acoustic “images” of the hydrophone’s reflecting surface is proposed. The measuring technique, the algorithms of mathematical processing, and the results of experimental studies are considered. A comparative analysis of the results of the hydrophone calibration with respect to the field by the reciprocity method is performed for the cases of using the conventional technique and the proposed method.