Formation of mismatched layers in pentagonal nanorods

A new mechanism is presented to model the relaxation phenomena in pentagonal nanorods (PNRs) – elongated multiple twinned crystals. It is demonstrated that a shell possessing crystal mismatch with respect to the PNR core region will reduce the internal energy of the PNR associated with wedge disclinations of strength 7°20′ lying along the PNR axis. We predict the existence of an optimal magnitude for core/shell crystal lattice mismatch and an optimal shell thickness providing maximum energy release for this mechanism of mechanical stress relaxation. The considered relaxation mechanism can be realized by the diffusion of impurities in the shell region without change of the PNR radius or by growth of a thin mismatched shell layer with the corresponding thickening of PNR. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Background noise in piezoresistive,electret condenser,and ceramic microphones

Background noise studies have been extended from air condenser microphones to piezoresistive, electret condenser, and ceramic microphones. Theoretical models of the respective noise sources within each microphone are developed and are used to derive analytical expressions for the noise power spectral density for each type. Several additional noise sources for the piezoresistive and electret microphones, beyond what had previously been considered, were applied to the models and were found to contribute significantly to the total noise power spectral density. Experimental background noise measurements were taken using an upgraded acoustic isolation vessel and data acquisition system, and the results were compared to the theoretically obtained expressions. The models were found to yield power spectral densities consistent with the experimental results. The measurements reveal that the 1/f noise coefficient is strongly correlated with the diaphragm damping resistance, irrespective of the detection technology, i.e., air condenser, piezoresistive, etc. This conclusion has profound implications upon the expected 1/f noise component of micromachined (MEMS) microphones.     

Intervalley conversion at a boundary. Microscopic model

A simple model is proposed which makes it possible to obtain correctly the boundary conditions on the envelope wave functions in the case of contact of materials with substantially different electron spectra (presence or absence of a side valley or substantial difference in its position). The passage of an electron through such a boundary is considered, and analytical expressions for the transmission coefficient and intervalley transition coefficient are found. The dependence of these coefficients on the parameters of the bounding materials and on the nature of the boundary turn out to be decisive for vertical transport phenomena. Possible consequences of efficient conversion at the boundaries in multilayer structures are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 839–843 (May 1997)     

Low-temperature plasticity in nanocrystalline titanium and copper

The stress-strain compressive curves, temperature dependences of the yield stress, and small-inelastic-strain rate spectra of coarse-grained and ultrafine-grained (produced by equal-channel angular pressing) titanium and copper are compared in the temperature range 4.2–300 K. As the temperature decreases, copper undergoes mainly strain hardening and titanium undergoes thermal hardening. The temperature dependences of the yield stress of titanium and copper have specific features which correlate with the behavior of their small-inelastic-strain rate spectra. Under the same loading conditions, the rate of microplastic deformation of ultrafine-grained titanium is lower than that of coarse-grained titanium and the rate peaks shift toward high temperatures. The deformation activation volumes of titanium samples differing in terms of their grain size are (10–35)b ³, where b is the Burgers vector magnitude. The dependences of the yield stress on the grain size at various temperatures are satisfactorily described by the Hall-Petch relation.     

Diffractive beam splitter characterization via a power-recycled interferometer

We used the high-precision laser interferometer technique of power recycling to characterize the optical loss of an all-reflective grating beam splitter. This beam splitter was used to set up a Michelson interferometer with a power-recycling resonator with a finesse of 883. Analyzing the results obtained, we determined the beam splitter's total optical loss to be (0.193+/-0.019)%. Low loss all-reflective beam splitters might find application in future high-power laser interferometers for the detection of gravitational waves.     

On binding energy of trions in bulk materials

We study the negatively $T^{?}$ and positively $T^{+}$ charged trions in bulk materials in the effective mass approximation within the framework of a potential model. The binding energies of trions in various semiconductors are calculated by employing Faddeev equation in configuration space. Results of calculations of the binding energies for $T^{?}$ are consistent with previous computational studies and are in reasonable agreement with experimental measurements, while the $T^{+}$ is unbound for all considered cases. The mechanism of formation of the binding energy of trions is analyzed by comparing contributions of a mass-polarization term related to kinetic energy operators and a term related to the Coulomb repulsion of identical particles.     

Synthesis and structure study of ordered arrays of ZnSe nanodots

A novel approach is presented for synthesis of ZnSe nanodot arrays by physical vapor deposition on porous aluminum oxide templates with ordered channels. The structure of nanodots was studied by scanning electron microscopy and EXAFS spectroscopy. Data were obtained for the sizes of nanodots in the array and local atomic structure parameters, i.e., the interatomic distances and coordination numbers, in comparison with the data for the ZnSe film synthesized on a smooth surface of nonporous Al₂O₃.     

Influence of turbulence on train noise

The subject of this paper is the long distance propagation of train noise. The sound exposure level of train noise L_AE was measured. To describe the results of measurements, a semi-analytical model was used. It takes into account the wave-front divergence, air absorption, ground effect, and the turbulence destroying the coherent nature of the ground effect. The model contains three adjustable parameters that must be estimated at the site. To verify the model, we performed measurements of L_AE at the distance D = 450 m from the train track center. The difference between the calculated and measured mean values of L_AE equals 1.3 dB.