Beiträge der Radiotracertechnik zur Rationalisierung einer großtechnischen Anlage zur Caprolactamproduktion

This paper reviews ways in which radioactive tracer techniques can he used to improve the efficiency of a caprolactam production plant, Residence time spectra and material distributions in different parts of the plant were measured by application of sodium 24, bromine 82 and technetium 99m. The investigations led to improved cumeneoxidators and gave useful informations on the catalytic decomposition of the phenolic tar and the hydrogenation process of phenol. It could he shown that the construction of the lactam extractor must he varied and the results of the tracerexperiments made in the distillation unit contributed to a higher quality of the final product.     

Zur Bestimmung von Aktivierungsparametern für die Stoffübergangs- und Stofftransportprozesse an AgCl- und AgBr-Kristallen mittels radioaktiver Nuklide

A report is given on radiochemical measurements carried out by means of the nuclides ^110mAg⁺, ^82mBr⁻, and ³⁶Cl⁻ in order to determine the activation parameters of the mass transfer processes in AgCl and AgBr in the course of the Ostwald Ripening, examining in particular the influence of the thermal pretreatment and the ageing of the AgCl and AgBr upon the activation parameters. Comparative investigations of the systems AgCl_(s)-^110mAg⁺_(aq) and ^110mAgCl_(s)-Ag⁺_(aq) showed that the mass transfer processes on the crystal (incorporation of the ions into the crystal lattice and detachment of the ions from the lattice points, respectively) as well as the mass transport processes in the crystal lattice (diffusion of the Ag⁺ ions through interstitial sites) can be rate determining, the dehydration of the Ag₊ ions, however, should not be dominant, as has also been proved by measurements in mixed solvents.     

Zur quantentheoretischen Berechnung von Aktivierungsenergien für den Stoffübergang an Phasengrenzflächen von Ionenkristallen bei Kristallisationsprozessen (I). Zur Bestimmung von Aktivierungsenergien für den Stoffübergang an Phasengrenzflächen leichtlöslicher Salze mittels radioaktiver Nuklide

A report is given on the determination of the activation energies for the mass transfer processes at phase boundaries of ionic crystals by means of the method of labelled atoms. In order to obtain reproducible activation energies from the temperature dependence of the mass transfer numbers, the influence factors stirring speed, supersaturation, pretreatment and surface condition of the crystals, grain size distribution, and suspension density must be kept constant. For different ranges of temperature, different activation energies were obtained, which are explained by the desolvation of the ions and the incorporation of the ions into the crystal lattice.     

Temperature dependence of the built-in electric field strength of AlGaN/GaN heterostructures on Si(111) substrate

Photoreflectance spectroscopy was used to measure the barrier electric field strength F of as-grown AlGaN/GaN heterostructures on Si(111) substrate with two-dimensional electron gases in the temperature range from 80 up to 295 K. The Al-contents were in the range from 12 to 20%. Despite the difference of Al-contents and the large temperature variation we find only minute changes of F. This behaviour is explained by an almost constant strain state and thus a constant piezoelectric polarisation, which was concluded from the analysis of the GaN free excitonic transitions observed by photoluminescence excitation spectroscopy. Self-consistent conduction band calculations point to a pinning of the potential of the bare surface at 0.6 V, attributed to a large density of surface donor states.     

Carrier relaxation in epitaxial graphene photoexcited near the Dirac point

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.     

Quantum oscillations and the Sermi surface of LuB12

To examine the Fermi surface of LuB₁₂, measurements of the de Haas-van Alphen (dHvA) effect were made at temperatures between 0.35 and 2 K in magnetic fields up to 12 Tesla. Oscillations in the susceptibility occurred above 5 Tesla in any field direction relative to the single crystal sample. From the Fourier transform of the data obtained, we conclude the Fermi surface of both conduction bands to have multiple extremal cross sections. For some of these orbits, the temperature dependence of the dHvA signal was investigated to determine the corresponding cyclotron mass. For a better understanding, a Full Potential Linearized Augmented Plane Wave-(FLAPW-) band structure calculation was carried out and the shapes of the Fermi surfaces were determined. In addition, we investigated the transverse magnetoresistance as a function of the field and the field direction. Its anisotropy, as well as the Shubnikov-de Haas (SdH) oscillations occurring in certain geometries, are in agreement with the results of the dHvA measurements.     

Capacity limits of information transport in fiber-optic networks

The instantaneous optical Kerr effect in optical fibers is a nonlinear phenomenon that can impose limits on the ability of fiber-optic communication systems to transport information. We present here a conservative estimate of the "fiber channel" capacity in an optically routed network. We show that the fiber capacity per unit bandwidth for a given distance significantly exceeds current record experimental demonstrations.     

Functional genetic analysis reveals a 2-Alkyl-4-quinolone signaling system in the human pathogen Burkholderia pseudomallei and related bacteria

Pseudomonas aeruginosa synthesizes diverse 2-alkyl-4(1H)-quinolones (AHQs), including the signaling molecule 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS), via the pqsABCDE locus. By examining the genome databases, homologs of the pqs genes were identified in other bacteria. However, apart from P. aeruginosa, only Burkholderia pseudomallei and B. thailandensis contained a complete pqsA-E operon (termed hhqA-E). By introducing the B. pseudomallei hhqA and hhqE genes into P. aeruginosa pqsA and pqsE mutants, we show that they are functionally conserved and restore virulence factor and PQS production. B. pseudomallei, B. thailandensis, B. cenocepacia, and P. putida each produced 2-heptyl-4(1H)-quinolone (HHQ), but not PQS. Mutation of hhqA in B. pseudomallei resulted in the loss of AHQ production, altered colony morphology, and enhanced elastase production, which was reduced to parental levels by exogenous HHQ. These data reveal a role for AHQs in bacterial cell-to-cell communication beyond that seen in P. aeruginosa.     

De Haas-van Alphen effect in rare-earth hexaborides (RE=Pr,Nd, Gd)

The de Haas-van Alphen (dHvA) effect has been measured for antiferromagnetic PrB₆, NdB₆, and GdB₆. The dHvA data of PrB₆ and NdB₆ show paramagnetic Fermi surfaces which arise from magnetic breakdown through the small antiferromagnetic gaps in high fields very similar to those of LaB₆. In contrast, the antiferromagnetic Fermi surfaces which arise from new magnetic Brillouin zone boundaries introduced by the magnetic order seem to be very different in PrB₆ and NdB₆, respectively. In GdB₆ dHvA oscillations could be observed for the first time. In all three compounds magnetic phase transitions have been observed in the dHvA measurements in the field range of 10–12 T.Dedicated to Professor Hund on the occasion of his 95th birthday