Peritektische Zersetzung von YBa2Cu3O7−x

During the heating of YBCO a peritectic reaction takes place at 1020C, which can be described by: 2YBa₂Cu₃O_7–xY₂BaCuO₅+L+(1-2x)/2O₂ (1) whereL = 3BaCuO₂ +2CuO is a fluid with limited amount of yttrium.It has been reported, that many parameters can influence the reaction. From one side not only the starting size of grains but also the heating rate have an influence on the resulting Y₂BaCuO₅-phase. From the other side, there is a change of the peritectical temperature caused by changing of the partial pressure of O₂ and the presence of parasitic phase.From general kinetic consideration one can draw the conclusion, that different mechanisms (nucleation, phase-boundary reaction and diffusion) can control the reaction.Using DTA/TG measurements, the peritectic reaction has been examined. Classical kinetic methods (Kissinger and Friedman) has been used. The Friedman method has given the dependence of the activation energy from reaction degree. This suggests many steps reactions. The dependence of the DTA-peaks from the heat rate suggest a parallel steps of reaction. This assumption can be motivated by evaluation of free O₂, one solid and liquid phase formation. Amount of this phases depends on the heating rate. Additionally X-ray and microscopic methods has been used. In this way was shown, that the perovskit structure is stable up to peritectical temperature and than is dramatically destroyed. From microscopic observations has been got information about shape and size of solid phase and it's creation as a function of temperature, time and starting grain size.

Die Arbeit wurde BMBF gefördert und durch Land NRW unterschtützt.     

Zur Theorie des Abbaus von Kettenmolekülen

Zusammenfassung In der Arbeit wird eine Methode zur praktischen Anwendung der mathematischen Ergebnisse der statistischen Theorie des Abbaus vonKettenmolekülen angegeben. Ausgehend von dem Ansatz und der L?sung von R. Simha (16) wird für die Berechnung der in der mathematischen L?sung auftretenden zahlreichen Konstanten ein Schema aufgestellt. Ferner wird der Spezialfall behandelt, da der Abbau nicht bis zum Monomeren verl?uft. Au\erdem wird eine N?herungsl?sung für jene F?lle angegeben, in denen die exakte L?sung infolge des gro\en Rechenaufwandes nicht anwendbar ist. Abschlie\end wird kurz das umgekehrte Problem, die Berechnung der Abbauwahrscheinlichkeiten aus experimentell bestimmten Abbaukurven, behandelt.     

Zwischenmolekulare Kräfte und Molekülbau

Zusammenfassung In einer kürzlich erschienenen Arbeit war unter Benutzung eines einfachen Flüssigkeitsmodelles der Zusammenhang zwischen Oberflächenspannung. Ausdehnungskoeffizient, Schallgeschwindigkeit und einiger weiteren Größen mit den v. d. Waals-Londonschen Kräften (Dispersionskräfte) gezeigt worden. Auf Grund halbempirischer Betrachtungen ist es im Anschluß an die quantenmechanischen Rechnungen von F. London (1.c.) möglich, die Dispersionskräfte aus der Polarisierbarkeit und der Ladung der Moleküle abzuleiten. Unter der Annahme, daß für die Dispersionskräfte in erster Linie die Bindungselektronenpaare und zum Teil die weiteren Elektronenpaare der äußeren Elektronenhülle maßgebend sind, lassen sich die v. d. Waals-Londonschen Kräfte quantitativ berechnen. Die so aus elektrischen und optischen Daten bestimmten Werte stimmen bis auf wenige Prozent mit den aus dem Ausdehnungskoeffizient berechneten Werten überein.     

27Al NMR Study of Al-Speciation in Aqueous Alumina-Sols

A new method of preparing concentrated alumo-sols, the hydrolysis of Al-tri-sec-butylate in acidic aqueous media at 85°C, was studied in detail by varying the H₂O/Al- and NO₃ ⁻/Al-ratios in wide ranges. The components of the sols were characterized by ²⁷Al NMR spectroscopy. The pH-value depends on both the chosen H₂O/Al- and NO₃ ⁻/Al-ratio and on the aging time of the sols and reflects the composition of the sols. Al₁₃ polycations were detected in sols with a pH-value between 3.0 and 3.7. As a new result its presence was shown by NMR below 3.4. The Al₁₃ content of the sols increased with pH and the maximum fraction of Al₁₃ polycations was detected in the sol with the highest pH (3.7). Nearly 65% of the entire aluminium content of this solution is bound in the Al₁₃ polycations. Hence, a new synthetic method for the preparation of Al₁₃ ions containing sols was developed. Aging studies of the sols showed, that the Al₁₃ polycations were more stable in solutions with higher pH-value. Al₁₃ polycations were detected after an aging time of four months only in sols with a pH-value of 3.7. Tempering the aged sols at 40° to 80°C caused formation of Al₁₃ and also of Al₃₀ polycations.     

A laser system for the spectroscopy of highly charged bismuth ions

We present and characterize a laser system for the spectroscopy on highly charged ²⁰⁹Bi⁸²⁺ ions at a wavelength of 243.87?nm. For absolute frequency stabilization, the laser system is locked to a near-infra-red laser stabilized to a rubidium transition line using a transfer cavity based locking scheme. Tuning of the output frequency with high precision is achieved via a tunable rf offset lock. A?sample-and-hold technique gives an extended tuning range of several THz in the UV. This scheme is universally applicable to the stabilization of laser systems at wavelengths not directly accessible to atomic or molecular resonances. We determine the frequency accuracy of the laser system using Doppler-free absorption spectroscopy of Te₂ vapor at 488?nm. Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty of σ _{244 nm}=6.14?MHz (one standard deviation) over six days of operation.     

Graphene on Ru(0001): contact formation and chemical reactivity on the atomic scale

Graphene on Ru(0001) is contacted with Au tips of a cryogenic scanning tunneling microscope. The formation and conductance of single-atom contacts vary within the moiré unit cell. Density functional calculations reveal that elastic distortions of the graphene lattice occur at contact due to a selectively enhanced chemical reactivity of C atoms at hollow sites of Ru(0001). Concomitant quantum transport calculations indicate that the graphene-Ru distance determines the conductance variations.