Internal segregation of nanoparticles irradiated by laser radiation

The effect of the removal of zinc from brass nanoparticles has been experimentally discovered upon irradiating their suspension in ethanol by laser radiation. The analysis of the absorption spectra of nanoparticles shows that brass nanoparticles are transformed to copper nanoparticles during irradiation. The results are interpreted in terms of the high-pressure-induced modification of the phase diagram of nanoparticles. This pressure is caused by, first, the small radius of nanoparticles and, second, the pressure of the surrounding-liquid vapors upon the laser heating of nanoparticles.     

An approximation property of harmonic functions in Lipschitz domains and some of its consequences

An extension of an inequality of J. B. Garnett (1979), with improvements by B. E. J. Dahlberg (1980), on an approximation property of harmonic functions is proved. The weighted inequality proved here was suggested by the work of J. Pipher (1993) and it implies an extension of a result of S. Y. A. Chang, J. Wilson and T. Wolff (1985) and C. Sweezy (1991) on exponential square integrability of the boundary values of solutions to second-order linear differential equations in divergence form. This implies a solution of a problem left open by R. Bañuelos and C. N. Moore (1989) on sharp estimates for the area integral of harmonic functions in Lipschitz domains.

     

Growth of Sb-Bi gradient single crystals

The growth conditions and structural quality of Sb-Bi gradient single crystals with Bi content from 2 to 18 at %, grown by the Czochralski method with solid phase feed, are investigated. Bi distribution in the crystals along their pulling direction are studied by electron probe microanalysis and the change in the interplanar spacing is analyzed by double-crystal X-ray diffraction. It is established that the pulling rate and feed mass affect the Bi distribution in Sb-Bi single crystals.     

Thermal-performance evaluation of coolant passages with staggered arrays of pin fins

Three-dimensional laminar forced convection including steady-periodic transition is investigated up to periodic-chaotic transition in the fully developed region of coolant passages with staggered arrays of pin fins. Comparative examples concern overall pressure losses and heat transfer characteristics of circular, square and elliptical pins made of nickel and copper. In the numerical model, transient conjugate heat transfer is assumed and space periodicities in pressure, velocity components and temperatures are taken into account. In the range of operative conditions investigated, overall friction factors increase almost linearly with the Reynolds number, while the increase of overall Nusselt numbers with the Reynolds number is characterized by two slope changes connected with the onset of streamwise vortices, and the shedding of transverse vortices, respectively. The use of copper, instead of nickel, increases the overall Nusselt number with all shapes, but is particularly beneficial to the elliptical section. Square pins are characterized by the highest values of friction factors, but are also the best performers as far as convection enhancing is concerned. The reverse is true for the elliptical pins which are characterized by the lowest values of friction factors, but are the worst performers as far as convection enhancing is concerned. On the basis of overall performances, the elliptical pins made of copper are the best choice, at least in the upper range of Reynolds numbers investigated.     

On automation of the procedure for crystal structure model refinement

The methods of automation of the procedure for crystal structure model refinement from experimental diffraction data, implemented in the ASTRA program package, are described. Such tools as statistical tests, parameter scanning, and data scanning reduce the time necessary for structural investigation. At strong correlations between parameters, especially when the data set is limited, parameter scanning has an advantage over the full-matrix refinement.     

Fatigue Loading and Life Prediction in Three Fretting Fatigue Fixtures

Three fixtures for conducting laboratory fretting fatigue tests are described and their respective testing methods and the results of the analysis are compared. Each of these fixtures has been used to investigate the effects of various parameters of interest in fretting fatigue. These fixtures include a unique apparatus in which all load applied to the specimen is transferred to the fretting pads, an apparatus similar to many found in the literature where partial load transfer occurs across the pads, and a simplified dovetail fixture in which the clamping load, P, and the shear load, Q, are varied in phase. Select test conditions from prior experiments performed on identical material and resulting in similar lives ranging from one to ten million cycles from these fixtures are identified. The various testing conditions were used to compute the unique stress field for each case. The resulting contact stresses were used to calculate crack initiation based criteria, and to calculate stress intensity factors. The three fixtures were shown to be able to accommodate a range of loads, fretting pad contours, and specimen geometries that produced a variety of stress fields. A crack-initiation-based criterion was shown to predict the failure lives of thinner specimens accurately. The stress intensity factor calculations showed the possibility of a crack arresting for a stress field that decays rapidly and the possibility of a local minimum for K as a function of depth. The fixtures are shown to be complementary in generating data for development of robust fretting fatigue models that use these criteria.     

Modeling and simulation of first and second sound in solids

We investigate the propagation of mechanical and thermal waves in solids at cryogenic temperatures. The latter are known as the second sound phenomenon. It occurs, e.g., in dielectric solids and differs greatly from the classical case in which the heat transport proceeds by diffusion. Since Fourier’s law of heat conduction fails for modeling second sound, we apply a non-classical one. During the last two decades, the non-classical thermoelastodynamic theory of Green and Naghdi enjoys steadily growing research activities.     

Whirling frequencies and amplitudes due to deviations of generator shape

Earlier measurements in large synchronous generators indicate the existence of backward whirling motion, and also relatively large deviations of shape in both the rotor and the stator. These non-symmetric geometries produce an attraction force between the rotor and the stator, called unbalanced magnetic pull (UMP). The target of this paper is to analyse the whirling frequencies and amplitude of the response for large synchronous generators with a high number of poles, due to deviations of shape in the rotor and stator. A mathematical model is developed to describe the shapes of the rotor and stator, and the corresponding UMP is obtained by using the law of energy conservation. The UMP is analysed due to different deviations of shape. The result gives the average angular frequency and the magnitude of the UMP for certain deviations of shape. From this result, the whirling frequency and the amplitude of the corresponding response can be approximated. Simulations of the response of a Jeffcott rotor model show good agreement with the theoretical results of the UMP for some generator geometries. The conclusion is that different whirling frequencies, both backward and forward whirling, can occur in these machines due to deviations in shape of the generator. Therefore, the shape of the generator can excite resonance vibrations on several other frequencies than the rotation frequency. During maintenance of hydropower generators the shapes of the rotor and stator are frequently measured. The results from this paper can be used to evaluate such measurements and to explain the existence of complicated whirling motion.