Number density

Abstract: Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length ( approximately 2-4 nm for YBCO at temperatures < or =77 K) and when the areal number density of defects is of the order of (H/2) x 10(11) cm(-2), where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of approximately 8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77 K) by a factor of two to three for high magnetic fields.

Abstract: Pool boiling of saturated water at 1 atm pressure has been investigated. In the experiments, copper surfaces prepared by following a well-defined procedure were used. The cumulative number density of the cavities and their shapes were determined with an optical microscope. The surface had a mirror finish and had a surface Ra (centerline average) value of less than 0.02 μm. The wettability of the surface was changed by controlling the degree of oxidation of the surface. In the experiments with the primary surface, the wall heat flux and superheat were determined with the help of thermocouples embedded in the test block. The density, spatial distribution, local distribution, and nearest-neighbor distance distribution of active nucleation sites in partial and fully developed nucleate boiling were determined from still pictures.

Abstract: A method for estimating structural properties of random media is described. The size, number density, and scattering strength of particles are estimated from an analysis of the radio frequency (rf) echo signal power spectrum. Simple correlation functions and the accurate scattering theory of Faran [J.J. Faran, J. Acoust. Soc. Am. 23, 405-418 (1951)], which includes the effects of shear waves, were used separately to model backscatter from spherical particles and thereby describe the structures of the medium. These methods were tested using both glass sphere-in-agar and polystyrene sphere-in-agar scattering media. With the appropriate correlation function, it was possible to measure glass sphere diameters with an accuracy of 20%. It was not possible to accurately estimate the size of polystyrene spheres with the simple spherical and Gaussian correlation models examined because of a significant shear wave contribution. Using the Faran scattering theory for spheres, however, the accuracy for estimating diameters was improved to 10% for both glass and polystyrene scattering media. It was possible to estimate the product of the average scattering particle number density and the average scattering strength per particle, but with lower accuracy than the size estimates. The dependence of the measurement accuracy on the inclusion of shear waves, the wavelength of sound, and medium attenuation are considered, and the implications for describing the structure of biological soft tissues are discussed.