Durability of polyurethanes elastomers

The effect of thermal aging on three different elastomeric polyurethane adhesives was studied. Consequently, an attempt was made to predict the polyurethanes' service lifetimes from the changes in tensile properties, hardness of bulk specimens and adhesion strength properties (lap shear and butt joints) of polycarbonate bonded joints. Aging temperatures ranged from ambient to 70 °C (at 50% relative humidity) for durations from 20 days to 18 months. Experimental results were analyzed according to a kinetic rate theory and using limit values for the various properties. Consequently, service lifetimes were determined and compared to results from actual service life. Predictions of long-term performance from accelerated tests proved to be reasonable for the properties and materials studied.     

Superconductivity of niobium nitride single crystals after implantation of carbon and nitrogen

Single crystals of δ-NbN_0.85 with a superconducting transition temperature T_c of 14.3 K were implanted with nitrogen and carbon ions at room temperature and subsequently annealed at high temperatures. Implantation was also performed at high substrate temperatures. After implantation at about 920°C maximum T_c-values of 16.5 and 17.8 K were obtained with N- and C-ions respectively. Disorder observed after room temperature implantation consisted of displaced Nb-atoms which could not be completely annealed in an isochromous annealing process up to 1000°C. For annealing temperatures above 1100°C nitrogen diffusion out of the implanted layers resulted in a reduction of T_c.     

Some mappings of pfriodic groups

For the rational integera and the rational primeb, letP(a,b) be the set of primesp such that . A natural integerq satisfies (i) iffq is a power product fromP(a,b). In the (additively written) Abelian groupG, g→ag permutes the elements ofG ^# in cycles whose lengths are multiples ofb, but not ofb ², iffG is aπ-group withπ⊂P (a,b). The casea=−2,b=2 has combinational applications.     

Research problems

This section appears in nearly every issue. Contributions are invited, and should be submitted to R. C. Thompson, Mathematics Department, University of California, Santa Barbara, CA. 93106, USA.     

Remarks on a principle of localization

We prove that the Brauer class of a crossed product is a sum of symbols iff its “local” components are. Analogously we show that a solution of the “Goldie rank conjecture” would follow from the “local” statements; an extension of a result of Cliff-Sehgal is an easy corollary.     

A study of the second excited state of8Be by the10B(d,α2)8Be-reaction

The second excited state of⁸Be has been studied by analysis of the coincidence spectra of the reaction¹⁰B(d, α)2α forE _d =1.0-2.0 MeV. The excitation energy was found to be 12.0±0.4 MeV, and the reduced width 0.85±0.12 MeV. Differential crosssections of the (d, α ₂)-reaction were deduced.     

Dipole moment of interacting rare-gas atoms

The translational absorption spectra of HeAr and NeAr have been analyzed with a hard-sphere potential to obtain the explicit dependence of the interaction dipoles on interatomic distance. The dipoles were found to consist of a short-range part varying almost exponentially with the distance and of a long-range part which varies more slowly. ^μHe Ar^(r) exhibited an unexpected maximum at small distances.     

Proximal-isometric (P J) flows

We develop some new tools for the study of minimal transformation groups (flows) and apply these tools to extend the classification theory of minimal flows to a larger class than has previously been classified. The tools we develop arise from a simplification and unification of the techniques used by Furstenberg, Ellis, and Veech in their proofs of structure theorems about distal and point distal flows. This simplification is made possible largely through the use of recent results of Glasner on the structure of 2^X (whose points consist of closed subsets of the phase space X of a given flow).