Limited stability of the pH reference material sodium tetraborate decahydrate (“borax”)

Thermogravimetric and X-ray powder diffraction analyses revealed that the pH reference material sodium tetraborate decahydrate (“borax”) is not a stable substance but, loosing some of its water of crystallization, transforms slowly into the pentahydrate. The connected pH changes of the solutions are opposite to and larger than, those expected according to the dilution value of the decahydrate as shown by differential potentiometric measurements. The transformation rate depends on the storing time in the closed original container, the time of and after the first exposure of the material to the atmosphere, the frequency of and the relative humidity during subsequent exposures, and the temperature. The transformation reaction is not understood in detail. However, the experiments showed that sodium tetraborate decahydrate is a reliable pH reference material, whose pH is constant within ΔpH=±0.005 within at least two years after preparation, even if occasionally exposed to the atmosphere. Further work will show whether this time limit can be somewhat extended.     

Oxidation of polyethylene surfaces by remote plasma discharge: A comparison study with alternative oxidation methods

The reaction rates and products of remote oxygen plasma treatment, corona discharge, and ozone treatment of high and low density polyethylenes have been examined using x-ray photoelectron spectroscopy. The oxygen uptake by remote plasma treatment was faster than that of other surface treatments using excited oxygen species. A steady state concentration of 18 ± 1% oxygen can be attained within 1 s of exposure in the remote plasma.     

Konvexität in halbgeordneten Ebenen

Let (E, (ie63-01),) be a halfordered plane and call convex if M contains for any two points a,bM all pointsx between a and b. It is shown that segments [a,b] are convex if and only if (E, (ie63-02)) is the affine plane of order 3, (E, (ie63-03),) is an ordered plane or is near-trivial.

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Herrn Professor Johannes Böhm zum 70. Geburtstag gewidmet     

A public-key cryptosystem utilizing cyclotomic fields

While it is well-known that the RSA public-key cryptosystem can be broken if its modulusN can be factored, it is not known whether there are other ways of breaking RSA. This paper presents a public-key scheme which necessarily requires knowledge of the factorization of its modulus in order to be broken. Rabin introduced the first system whose security is equivalent to the difficulty of factoring the modulus. His scheme is based on squaring (cubing) for encryption and extracting square (cube) roots for decryption. This introduces a 14 (19) ambiguity in the decryption. Various schemes which overcome this problem have been introduced for both the quadratic and cubic case. We generalize the ideas of Williams' cubic system to larger prime exponents. The cases of higher prime order introduce a number of problems not encountered in the quadratic and cubic cases, namely the existence of fundamental units in the underlying cyclotomic field, the evaluation of higher power residue symbols, and the increased difficulty of Euclidean division in the field.     

Electromagnetic fission of238U at 600 and 1000 MeV per nucleon

Electromagnetic fission of²³⁸U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsäcker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.Communicated by V. Metag     

Zum mechanismus des strahlenchemischen abbaus von polytetrafluorethylen

The present work studies the changes in polymer structure and the mechanism of the decomposition of polytetrafluoroethylene resin (PTFE) exposed to high energy radiation (electron beam). Spectroscopic and kinetic observations are used to interpret the degradation process. For the first time the decomposition of PTFE has been carried out on a preparative scale and new results obtained by analysing the degradation products. The radiation-induced degradation of PTFE is accompanied by thermal degradation under certain irradiation conditions. This is due to an increase in temperature of the polymer caused by retardation of highly accelerated electrons (heat accumulation effect).The kinetics are discussed in terms of the reactions and recombination of radicals produced by high-energy radiation both in the polymer melt and the polymer surface. These are related to the overall rate of decomposition.The primary radicals formed by decomposition of PTFE in an inert atmosphere (N₂, Ar) react to produce perfluorinated alkanes and alkenes. In the presence of reactive gases the decomposition fragments originated will react rapidly; e.g. if oxygen is present in the reactive area the radicals form perflourinated peroxyl and oxyl radicals which finally stabilize themselves by CC-scission to perfluorocarbon acid fluorides and carbonyldifluorides.     

Modelling the electric double layer at electrode/electrolyte interfaces

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Singulett-Sauerstoff-induzierte spaltung von spiro[1,8a]dihydroindolizinen

The ¹O₂-induced degradation of spiro[1,8a]dihydroindolizines $\text{1}$ - a new photochromic system - may involve dioxetanes as intermediates giving fulvenylesters $\text{3}$.     

General alloy analogs of itinerant magnetic systems

Magnetically disordered, strong correlated itinerant electron systems are studied within the Hubbard model. Following an alloy analogy, the approximated system hamiltonian contains local vectoral magnetic fields at the atomic sites, which can be oriented into all space directions. The statistical system of the localized magnetic moments (due to the fields) is treated by the aid of a generalized CPA-technique. Supposing a macroscopic magnetic structure of sublattice antiferromagnetism, the formation of incomplete antiferromagnetic ordering of the localized moments is examined. The results show, that the electron density plays an essential roll for the question, whether or not it is possible to go beyond the pure (itinerant) antiferromagnetic state. Including the results of earlier papers for homogeneous macroscopic magnetic structure, on the whole scale of electron densities it is summed up (at zero temperature), where the different states (without-, with incomplete- or with complete magnetic order) are able to exist. It is shown, for fixed density respectively, which different phases are passed through on changing the coupling constant.Work supported by the Deutsche Forschungsgemeinschaft     

Berechnung der Bandstruktur vom Vanadiumcarbid VC

Zusammenfassung Die Bandstruktur von stöchiometrischem Vanadiumcarbid VC wird mit Hilfe der quasi-selbstkonsistenten APW-Methode ermittelt. Als Austauschpotential wird ein parametrisiertes Slatersches Austauschpotential verwendet. Die Energie-Eigenwerte werden für 256k-Punkte in der 1. Brillouin-Zone berechnet. Zur Berechnung der Zustandsdichte werden weitere Energiewerte (für insgesamt 16384k-Punkte) nach der LCAO-Interpolationsmethode von Slater und Koster bestimmt und mit diesen die Zustandsdichte ermittelt. Die partiellen LCAO-Zustandsdichten zeigen weitgehende Übereinstimmung mit den APW-Charakterdichten und geben Aufschluß über denl-Charakter der Energiebänder. Die Ladungsverteilung im Kristall weist auf eine Ladungsüberführung vom Vanadium zum Kohlenstoff und damit auf eine gewisse Ionizität der Verbindung VC hin.

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Calculation of the band structure of vanadium carbide VC

The energy band structure of stoichiometric VC has been calculated by the quasi-self-consistent APW method. For the exchange potential a parametrized Slater-free-electron approximation has been used. The energy eigenvalues have been calculated for 256k-points in the first Brillouin zone. Further energy values (for a total number of 16384k-points) have been obtained by the LCAO interpolation method of Slater and Koster. From these energy values the density-of-states histogram has been calculated. The partial LCAO-densities-of-states are largely in agreement with the APW-character-densities. Both quantities give information about thel-character of the energy bands. The charges in the APW-spheres indicate a charge transfer from vanadium to carbon.

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Résumé La structure des bandes énergétiques du VC de composition stoichiométrique a été calculée par la méthode APW quasiautoconsistante. On s'est servi d'un potentiel d'échange paramétrisé selon Slater. Les eigen-valeurs d'énergie sont déterminées pour 256 pointsk dans la premiere zone de Brillouin. Pour dériver la densité d'états énergétiques ainsi que les densités d'états partielles (densités d'états du caractère 2s, 2p et3d) des valeurs d'énergie pour d'autres pointsk (en somme 16384) sont trouvées avec l'aide de la méthode d'interpolation LCAO de Slater et Koster. Il n'y a pas des différences significantes entre ces quantités et les densités des caractèresl calculées directement par la méthode APW. Des densités d'états partielles on peut dériver le caractèrel des bandes énergétiques.De la distribution des charges dans le cristal résulte un transfer de charge du vanadium au carbon ce qui signifie que le VC possède une certaine ionicité.

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Dem Gedenken an Herrn Prof. Dr. K. H. Hansen gewidmet.

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Dem Jubiläumsfonds der österreichischen Nationalbank sind wir für die gewährte finanzielle Unterstützung zu großem Dank verpflichtet. Herrn Prof. Dr. H. Nowotny danken wir für die Förderung, die er dieser Arbeitsrichtung angedeihen läßt. Die große Hilfe von Herrn Dr. K. Schwarz, University of Florida, Gainesville, wird dankbar anerkannt.