STUDIES IN POROSITY AND PERMEABILITY CHARACTERISTICS OF POROUS BODIES1

Definitions are given of porosity, permeability, and weight per unit volume. It is shown that these three terms are not synonymous, and that a change of permeability is not necessarily accompanied by a change in porosity.     

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Rotor resonances of high-speed permanent-magnet brushless machines

For high-speed machines, in particular, it is very important to accurately predict natural frequencies of the rotor at the design stage so as to minimize the likelihood of failure. Finite-element analysis and experimental measurements are used to establish the natural frequencies and modes of the rotor of a high-speed permanent-magnet brushless motor, and to assess the influence of leading design parameters, such as the active length, the shaft diameter and extension, the bearings, and the material properties.     

Effects of age, breast density, ethnicity, and estrogen replacement therapy on screening mammographic sensitivity and cancer stage at diagnosis: review of 183,134 screening mammograms in Albuquerque, New Mexico

PURPOSE: To examine how common patient factors affect screening mammographic sensitivity and cancer stage at diagnosis. MATERIALS AND METHODS: The authors used a population-based database of 183,134 screening mammograms and a statewide tumor registry to identify 807 breast cancers detected at screening mammography. RESULTS: Sensitivity varied significantly with ethnicity, use of estrogen replacement therapy, mammographic breast density, and age. Sensitivity was 54% (13 of 24) in women younger than 40 years, 77% (121 of 157) in women aged 40-49 years, 78% (224 of 286) in women aged 50-64 years, and 81% (277 of 340) in women older than 64 years. Sensitivity was 68% (162 of 237) for dense breasts and 85% (302 of 356) for nondense breasts and 74% (180 of 244) in estrogen replacement therapy users and 81% (417 of 513) in nonusers. Sensitivity was most markedly reduced with the combination of dense breasts and estrogen replacement therapy use; there was little difference when only one factor was present. Median cancer size and the percentage of early cancers showed little change with any factors. CONCLUSION: Age is a minor determinant of mammographic sensitivity in women aged 40 years or older. Sensitivity is substantially decreased with the combination of higher breast density and estrogen replacement therapy use. There was not a notable shift in cancer outcomes in the groups with lower mammographic sensitivity. These data do not support different screening recommendations in women aged 40-49 years or in estrogen replacement therapy users.     

Preparation of nanosize Pt clusters using ion exchange of Pt(NH3) 4 2+ inside mesoporous channel of MCM-41

Mesoporous molecular sieve MCM-41 with a Si/Al ratio of 35 was obtained by hydrothermal synthesis using a gel mixture with a molar composition of 6 SiO₂0.1 Al₂O₃1 hexadecyltrimethylammonium chloride 0.25 dodecyltrimethylammonium bromide 0.25 tetrapropylammonium bromide0.15 (NH₄)₂O1.5 Na₂O300 H₂O. The MCM-41 sample was calcined in O₂ flow at 813 K and subsequently ion exchanged with Ca²⁺. A small Pt cluster has been supported on the MCM-41 sample following a procedure using ion exchange of Pt(NH₃) ₄ ²⁺ . The Pt(NH₃) ₄ ²⁺ ion supported on MCM-41 has been activated in O₂ flow at 593 K and subsequently reduced with Fh flow at 573 K, in the same way used for the preparation of a Pt cluster entrapped inside the supercage of zeolite NaY. The resulting Pt cluster supported on the MCM-41 shows hydrogen chemisorption oftotal two H atoms per Pt at 296 K (based on the total amount of Pt) and high catalytic activity for hydrogenolysis of ethane. The chemical shift in¹²⁹Xe NMR spectroscopy of adsorbed xenon indicates that the Pt cluster is located inside the mesoporous molecular sieve.     

Laser ion source development at Holifield Radioactive Ion Beam Facility

This report describes the efforts made to develop a resonant-ionization laser ion source based on tunable Ti:sapphire lasers for nuclear physics and astrophysics research at Holifield Radioactive Ion Beam Facility. Three Ti:sapphire lasers have been upgraded with individual pump lasers to eliminate laser power losses due to synchronization delays. Ionization schemes for 14 elements have been obtained. Off-line studies show that the overall efficiency of the laser ion source can be as high as 40%. TaC surface coatings have been investigated for minimizing surface and bulk trapping of the atoms of interest.     

Evaluation of superposition technique for calculating cogging torque in permanent-magnet brushless machines

We evaluate the superposition method for calculating the cogging torque in permanent-magnet brushless machines, the resultant torque being synthesized from cogging torque components associated with a pair of magnets. Although finite-element analyses and measurements show that the resultant cogging torque cannot be synthesized directly from the torque components due to a single magnet, we use the concept of a fictitious single magnet to analytically establish the relationship between the cogging torque and key design parameters. The method is particularly useful in assessing the influence of the slot number and pole number combination.