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Showing papers by "Braunschweig University of Technology published in 1990"


Journal ArticleDOI
TL;DR: In this paper, the microkinetics for the liquid phase synthesis of methyl tertiary butyl ether (MTBE) from methanol and isobutene using a macroporous sulfonic acid ion exchange resin as catalyst (Amberlyst 15, CVT resin) were determined experimentally in a continuous stirred tank reactor in the temperature range 323-363 K at 2.1 MPa.

215 citations


Journal ArticleDOI
TL;DR: The temperature dependence of the dipole polarizability α(λ, T) of free atoms and molecules was determined by precise measurements of the refractive index n of gases in the extended temperature range between 300 K and 1100 K for wavelength λ = 632·99 nm, using a specially constructed Michelson twin interferometer as mentioned in this paper.
Abstract: The temperature dependence of the dipole polarizability α(λ, T) of free atoms and molecules is determined by precise measurements of the refractive index n of gases in the extended temperature range between 300 K and 1100 K for wavelength λ = 632·99 nm, using a specially constructed Michelson twin interferometer. α of the noble gases is observed to be independent of T. α of the molecular gases H2, N2, O2, and CH4 increases with increasing temperature by an amount of approximately 1 per cent per 1000 K. These results are in excellent agreement with theoretical predictions. They will be compared to previously measured temperature dependent polarizabilities.

104 citations


Journal ArticleDOI
TL;DR: In this article, the deHoffmann-Teller frame was used to detect the magnetopause normal vector, N, and the average plasma velocity along N is used to deduce a boundary layer thickness of about 1.3 R{sub E}.
Abstract: Convection electric fields, E{sub c} = {minus}V{sub p}{times}B, measured during a magnetopause/boundary layer encounter at low latitude by the AMPTE/IRM spacecraft, are shown to yield an accurate determination of the magnetopause normal vector, N, even though the magnetic shear across the magnetopause was small. The average plasma velocity along N is used to deduce a boundary layer thickness of about 1.3 R{sub E}. Analysis of E{sub c} in the magnetopause itself reveals the existence of a deHoffmann-Teller frame in which the flow is nearly field aligned and approximately Alfvenic, indicating that the magnetopause was a rotational discontinuity; this is interpreted as a signature that magnetic-field reconnection was occurring somewhere on the magnetopause.

81 citations


Journal ArticleDOI
TL;DR: A statistical survey on the balance of total (thermal and magnetic) pressure in the Earth's plasma sheet and tail lobe was conducted in this article, using 8 months of AMPTE/IRM plasma and magnetic field data.
Abstract: Using eight months of AMPTE/IRM plasma and magnetic field data, we have done a statistical survey on the balance of total (thermal and magnetic) pressure in the Earth's plasma sheet and tail lobe. About 300,000 measurements obtained in the plasma sheet and the lobe were compared for different levels of magnetic activity as well as different distances from the Earth. Our data show that lobe and plasma sheet pressure balance very well. Even in the worst case they do not deviate by more than half of the variance in the data itself. Approximately constant total pressure was also seen during a quiet time pass when IRM traversed nearly the whole magnetotail in the vertical direction, from the southern hemisphere lobe through the neutral sheet and into the northern plasma sheet boundary layer.

76 citations


Journal ArticleDOI
TL;DR: Rhodococcus erythropolis is able to synthesize an anionic trehalose-2,2',3,4-tetraester during cultivation on n-alkanes and electron microscopical observations showed that n-C-14,15 (technical grade) grown cells contained numerous lipid inclusions in contrast to n- C-10 ( technical grade) growing cells.

74 citations


Journal ArticleDOI
TL;DR: D Dupin's cyclides are useful in blending conventional solids in solid modeling and can easily be derived from a construction using an ellipse and a string as given 125 years ago by J. Clerk Maxwell.

72 citations


Journal ArticleDOI
TL;DR: In this article, the syenite pegmatites are derived from a depleted mantle source with isotopic signatures (eNd(T)= 5.7 and 5.1) of ocean island basalts (OIB).

70 citations


Journal ArticleDOI
TL;DR: Magnesite is relatively frequent as a mineral, there are, however, only two types of economically important deposits: magnetite and magnetite-rich coal seams as discussed by the authors, both of which are found in the USA.
Abstract: Magnesite is relatively frequent as a mineral, there are, however, only two types of economically important deposits:

69 citations


Book ChapterDOI
28 May 1990
TL;DR: The semantic domain for object-oriented languages and systems outlined in this paper is an amalgamation of two approaches: the objects-as-sheaves approach of the second author published nearly two decades ago, and the more recent ideas of the first and third authors on objects as processes endowed with observation.
Abstract: The semantic domain for object-oriented languages and systems outlined in this paper is an amalgamation of two approaches: the objects-as-sheaves approach of the second author published nearly two decades ago, and the more recent ideas of the first and third authors on objects as processes endowed with observation. The basic insight is that objects in the latter sense correspond to object morphisms in the former sense. After an informal introduction into both approaches, we first elaborate on the sheaf model, using the term "behaviour" for objects in this sense, and avoiding concepts from topology. Behaviours and behaviour morphisms are shown to form a complete category where parallel composition is reflected by limits. Objects are defined to be behaviour morphisms, giving rise to a cocomplete category of objects where colimits reflect object aggregation. Object morphisms reflect different forms of inheritance, and also object reification (implementing objects over objects) is conveniently expressed in this framework.

68 citations


Journal ArticleDOI
TL;DR: This work provides definitive proof that an arctiid moth is able to hydrolyse plant ester alkaloids and re-esterify the resulting necine base with acids which are intermediates of insect metabolism.

63 citations


Journal ArticleDOI
TL;DR: In this article, an experimental set-up is presented for the measurement of steady-state reaction rates in the liquid phase methyl tertiary butyl ether (MTBE) synthesis from isobutene and methanol using ion exchange resin as catalyst.
Abstract: An experimental set-up is presented for the measurement of steady-state reaction rates in the liquid phase methyl tertiary butyl ether (MTBE) synthesis from isobutene and methanol using ion exchange resin as catalyst. This apparatus was used for a separate investigation of the main side reaction, i.e., the formation of isobutene dimers (DIB), in the temperature range between 60 and 90°C. The reaction catalyzed by Amberlyst 15 (A15) showed no steady-state behaviour, but the catalytic activity declined at a rate dependent on the reaction conditions. Time constants for activity loss were determined in the range from 3.5 to 30 h. The deactivated A15 catalyst could be regenerated through MTBE synthesis experiment. Deactivation is assumed to be caused by blocking of the microparticle gel phase by higher isobutene oligomers. The DIB formation, using 1-butene as solvent, was of 2nd order with respect to isobutene and showed an apparent activation energy of about 40 kJ/mol.

Proceedings ArticleDOI
01 Aug 1990
TL;DR: This paper shows that the necessary computational burden to compute range values by means of active triangulation techniques can be reduced drastically by introducing weak geometrical constraints for the projector camera configurations.
Abstract: It has been shown earlier that efficient active triangulation methods for range data acquisition can be devised by employing an appropriate time-space encoding. In this paper we show that the necessary computational burden to compute range values by means of these techniques can be reduced drastically by introducing weak geometrical constraints for the projector camera configurations. Furthermore, we suggest the concept of a virtual measurement coordinate system reducing the triangulation to simple one-dimensional LUT operations. Thus, the range acquisition technique can be implemented very fast in a low cost PC based image processing environment. We show how to calibrate the system and give some experimental results.

Journal ArticleDOI
TL;DR: In this article, the authors extended the microkinetic model for methyl tertiary butyl ether (MTBE) synthesis to the case of macropore diffusion of methanol as the rate-controlling step.

Journal ArticleDOI
TL;DR: Les parametres les plus importants et leurs effets sur le broyage sont demontres, ainsi qu'expliques par un modele et les consequences pour les conditions operatoires des broyeurs a boulets agites sont presentees as mentioned in this paper.
Abstract: Les parametres les plus importants et leurs effets sur le broyage sont demontres, ainsi qu'expliques par un modele et les consequences pour les conditions operatoires des broyeurs a boulets agites sont presentees

Journal ArticleDOI
TL;DR: Three major sesquiterpenes from Encelia actoni and E. asperifolia were studied for toxicity and antifeedant activity against larvae of the polyphagous Spodoptera littoralis and the deleterious effects of encelin were due to toxicity as well as to antifeedsant activities.

Journal ArticleDOI
TL;DR: It was found that compounds with epoxy and peroxo groups in rings A and B had a notable sensitizing potential, suggesting insertion into the lipid bilayer may play an important rôle in contact sensitivity.
Abstract: 9 oxidation products of abietic acid, dehydroabietic acid, and levopimaric acid were prepared synthetically to determine their sensitizing potential in guinea pigs. It was found that compounds with epoxy and peroxo groups in rings A and B had a notable sensitizing potential. The same result was found with 7-oxode-hydroabietic acid identified earlier in rosin (3) and a polar fraction obtained from commercial abietic acid, suggesting the presence of still unidentified oxidation products. Hydroxylation of rings A or B, or conversion to the methyl esters, considerably decreases the sensitizing potential. A model is presented underlining the importance of hydrophobic and polar domains, in addition to chemically reactive groupings, in the allergen. Insertion into the lipid bilayer may play an important role in contact sensitivity.

Journal ArticleDOI
TL;DR: The square lattice and the simple cubic lattice quantum percolation thresholds are calculated by studying the sensitivity of eigenvalues to a change in boundary conditions, and a transition from weakly to strongly localized states is identified.
Abstract: Numerical results are reported for the quantum site-percolation problem. For the square lattice and the simple cubic lattice quantum percolation thresholds ${\mathit{p}}_{\mathit{q}}$ are calculated by studying the sensitivity of eigenvalues to a change in boundary conditions. Observing the energy dependence of the transition from localized to extended states, mobility edge trajectories are calculated. We obtain ${\mathit{p}}_{\mathit{q}}$=0.45 in three dimensions and ${\mathit{p}}_{\mathit{q}}$=0.70 in two dimensions. The latter value of ${\mathit{p}}_{\mathit{q}}$ is identified with a transition from weakly to strongly localized states, according to a similar localization behavior observed for the Anderson problem.

Journal ArticleDOI
TL;DR: In this paper, the photochemistry of α-(2,6-dimethoxyphenoxy)-acetophenone (1 ) was studied using CIDNP spectroscopy; an XeCl excimer laser (308 nm) and a 250 MHz 1 H nuclear magnetic resonance (NMR) spectrometer were employed.
Abstract: The photochemistry of α-(2,6-dimethoxyphenoxy)-acetophenone ( 1 ) was studied using CIDNP spectroscopy; an XeCl excimer laser (308 nm) and a 250 MHz 1 H nuclear magnetic resonance (NMR) spectrometer were employed. The results are compared with the photoreactions of α-hydroxyacetophenone ( 2 ) and α-methoxyacetophenone ( 3 ). The CIDNP spectra of 1 can be interpreted as a result of a β cleavage from the singlet state. With time-resolved CIDNP spectroscopy it is possible to detect the enol of acetophenone in a second reaction path and to observe the tautomerization to acetophenone. Evidence is presented that this reaction path starts with the photoreduction of 1 from the triplet state. Compound 2 shows α cleavage from the triplet state and no β cleavage. As expected no CIDNP spectra are observed from 3 which is known to react via a Norrish type II mechanism.

Journal ArticleDOI
TL;DR: In this article, the influence of solids concentration in the feed on the separation efficiency of hydrocyclones has long been recognized, but a systematic research on the change in pressure drop across the hydrocyclone and the latter's separation characteristics has thus far been lacking.
Abstract: Even though the influence of solids concentration in the feed on the separation efficiency of hydrocyclones has long been recognized, a systematic research on the change in pressure drop across the hydrocyclone and the latter's separation characteristics has thus far been lacking. If all other operating parameters are kept constant, an increase in feed concentration generally leads to a coarser cut size, reduced sharpness of separation and a rise in pressure drop. Apart from the particles hindering one another's radial motion, the limited capacity of the apex valve and the changes in the flow field within the hydrocyclone cause additional particles to be entrained by the overflow. The flow ratio thus becomes an additional parameter in determining the cyclone efficiency. A theoretical model, which takes these effects into account, has been developed and successfully tested against experimental data.

Journal ArticleDOI
TL;DR: In this paper, a reductive amination of reducing mono-and disaccharides with alkylamines was used to synthesize amphiphilic poly(vinylsaccharide)s (poly(7) and poly(8)).
Abstract: New amphiphilic poly(vinylsaccharide)s (poly(7) and poly(8)) were synthesized. Various alkylamino substituted mono- and disaccharides 3 and 5 were obtained by reductive amination of reducing mono- and disaccharides with alkylamines. The reaction of these monofunctionalized sugar derivates with 2-isocyanatoethyl methacrylate (2) leads to well-defined monomers (7 and 8) of the urea type. The monomers were polymerized in aqueous solution to high-molecular-weight polymers, using the redox initiator system (NH4)2S2O8/Na2S2O5. The influence of structural variations by changing the hydrophilic sugar moiety or the lipophilic alkyl part was studied by viscometry, light scattering and measurements of the surface tension.

Journal ArticleDOI
TL;DR: The ability of the microorganism to perform these bioconversions was found to be inducible by D,L-5-IMH as well as to be dependent on the presence of Mn2+.

Journal ArticleDOI
TL;DR: In this article, a four-step preparation of dimethyl 7-oxobicyclo[2.2.1]-heptane-exo-2, exo-3-dicarboxylate (4) in satisfactory yield from the readily available 6,6-dimethylfulvene and maleic anhydride is described.
Abstract: A four-step preparation of dimethyl 7-oxobicyclo[2.2.1]-heptane-exo-2, exo-3-dicarboxylate (4) in satisfactory yield from the readily available 6,6-dimethylfulvene and maleic anhydride is described. Compound 4 represents a well-functionalized intermediate for the synthesis of norbornane derivatives which is used for an improved synthesis of the tricyclic diester 7, precursor of the cage compound 8. An alternative approach to diester 7 from dimethyl 7-isopropylidenebicyclo[2.2.1]heptane-exo-2, exo-3-dicarboxylate (2) is also discussed.

Journal ArticleDOI
TL;DR: The freezing temperature under high external fields is labels as the upper limit of the SG temperature, and indicates a true thermodynamic phase transition; the cusp in the ac susceptibility is not necessarily a confirmation for the establishment of a SG state.
Abstract: Combined ac- and dc-susceptibility, neutron-diffraction, and M\"ossbauer studies of ${\mathrm{UFe}}_{4}$${\mathrm{Al}}_{8}$, ${\mathrm{NpFe}}_{4}$${\mathrm{Al}}_{8}$, and ${\mathrm{ThFe}}_{4}$${\mathrm{Al}}_{8}$ argue for the establishment of a spin-glass (SG) state below ${\mathit{T}}_{\mathrm{SG}}$(B\ensuremath{\rightarrow}0)\ensuremath{\approxeq}130, 120, and 110 K, respectively. The SG temperature ${\mathit{T}}_{\mathrm{SG}}$(B) decreases with application of external magnetic field B. The ac susceptibility (${\mathrm{\ensuremath{\chi}}}_{\mathrm{ac}}$) shows a sharp cusp at ${\mathit{T}}_{\mathrm{SG}}$(B\ensuremath{\approxeq}0). The isothermal and thermoremanent magnetization differ markedly and show temperature-dependent irreversibilities below ${\mathit{T}}_{\mathrm{SG}}$(B). The M\"ossbauer spectra reveal onset of hyperfine splitting, indicating frozen spins below ${\mathit{T}}_{\mathrm{SG}}$(0). The neutron-diffraction studies of ${\mathrm{UFe}}_{4}$${\mathrm{Al}}_{8}$ and ${\mathrm{NpFe}}_{4}$${\mathrm{Al}}_{8}$ show that after switching off the external magnetic field greater than 3 T the U and Np momenta (2a site) are frozen parallel to the direction of the external field. We label the freezing temperature under high external fields ${\mathit{T}}_{\mathrm{OG}}$, and show that it serves as the upper limit of ${\mathit{T}}_{\mathrm{SG}}$(B) with ${\mathit{T}}_{\mathrm{SG}}$(B\ensuremath{\rightarrow}0)\ensuremath{\rightarrow}${\mathit{T}}_{\mathrm{OG}}$. For concentrated SG systems, ${\mathit{T}}_{\mathrm{SG}}$(0) is a reproducible value, independent of the observation time window, which indicates a true thermodynamic phase transition; the cusp in ${\mathrm{\ensuremath{\chi}}}_{\mathrm{ac}}$, however, is not necessarily a confirmation for the establishment of a SG state.

Journal ArticleDOI
TL;DR: In this paper, the general solution for the convolution of an arbitrary rf pulse shape with a time-dependent nuclear spin polarization is derived, and it is shown that correct rate constants are obtained only if the true pulse shape is used in the evaluation.

Journal ArticleDOI
TL;DR: In this paper, a detailed analysis of the critical population for the setting-in of a chain reaction is presented, and it is shown that this critical population is only about 2 to 3 times the current population and could be reached within 20 to 50 years, if spaceflight activities will be continued as in the past.

Journal ArticleDOI
01 Oct 1990
TL;DR: In this paper, a statistical study investigating Pc5 pulsation ionospheric boundary conditions is reported, which utilizes data from the Sweden And Britain Radar-auroral Experiment (SABRE) VHF coherent auroral radar.
Abstract: A statistical study investigating Pc5 pulsation ionospheric boundary conditions is reported. The study utilizes data from the Sweden And Britain Radar-auroral Experiment (SABRE) VHF coherent auroral radar. The ratio of the pulsation radar backscatter intensity modulations to the ionospheric irregularity drift velocity modulations is calculated. This enables the pulsation ionospheric electric field boundary condition to be established, allowing the wave mode in the radar field of view to be determined. Increasingly compressional Pc5 waves are observed as the azimuthal wavenumber increases in the range m = 1–2. One possible interpretation of this result is that the dayside, continuous pulsations observed with SABRE are caused by an impulsive excitation of the magnetosphere. Three compressional pulsations with similar characteristics are examined in more detail and a full case study, which includes data from an extensive ground magnetometer array, as well as the Scandinavian Twin Auroral Radar Experiment (STARE), is conducted on one of these pulsations. The case study event has a constant period and a small phase variation between 51° and 67° N geomagnetic latitude and 78° and 110° E geomagnetic longitude, with a clear amplitude maximum close to 63° N latitude. The ionospheric electric field boundary condition reveals the pulsation to be a fast mode wave in the SABRE and STARE fields of view. These Pc5 pulsation observations are discussed in terms of recent global mode modelling.

Journal ArticleDOI
TL;DR: The major sesquiterpene as well as chlorogenic acid were, in contrast to the chromenes, hardly detectable in cotyledons and in primary leaves but were increasingly accumulated in leaves at higher nodes resulting in a chemical dichotomy during seedling development.

Book ChapterDOI
01 Jan 1990
TL;DR: In this article, the main dislocation mechanisms relevant for plastic instabilities in metals and the patterns involved are reviewed and the possibilities for describing the deformation kinetics on micro- and macroscale by the approaches of viscous glide and of generation rate of dislocations are discussed.
Abstract: The paper reviews the main dislocation mechanisms relevant for plastic instabilities in metals and the patterns involved. The observability of plastic instabilities by various methods and the classification of micromechanisms (M-, h-, S-, T-type instabilities) as well as propagating modes of shear are summarized. Short excursions deal with instabilities by diffusionless structural transformations and with unstable localized deformation in amorphous alloys. Finally, the possibilities for describing the deformation kinetics on micro- and macroscale by the approaches of viscous glide and of generation rate of dislocations are discussed.

Journal ArticleDOI
TL;DR: In this article, non-radiative energy transfer from singlet excited perylene to Co2+ ions has been studied in methanol, ethylene glycol and glycerol by time-correlated single-photon counting and steady state quenching experiments.
Abstract: Non-radiative energy transfer from singlet excited perylene to Co2+ ions has been studied in methanol, ethylene glycol and glycerol by time-correlated single-photon counting and steady state quenching experiments. In highly viscous glycerol the decay curves are represented by the expression given by Forster for long-range energy transfer. The experimental critical distance R0 = 13.2 A is in good agreement with the theoretical value. In methanol and ethylene glycol the decay is modified owing to translational motion and can be described by the function exp(— at — bt 1 2 with reasonable values for the reaction sphere and diffusion coefficients.

Journal ArticleDOI
15 Oct 1990
TL;DR: In this paper, the cyclic photoreaction between 9,10-anthraquinone and N,N-dimethylaniline (DMA) in acetonitrile- d 3 has been studied between 237 and 296 K by time resolved measurements of chemically induced dynamic nuclear polarization (CIDNP).
Abstract: The cyclic photoreaction between 9,10-anthraquinone and N,N-dimethylaniline (DMA) in acetonitrile- d 3 has been studied between 237 and 296 K by time resolved measurements of chemically induced dynamic nuclear polarization (CIDNP). A generally applicable kinetic treatment including the calculation of absolute CIDNP signal intensifies is presented. For the dimethylamino protons in the donor radicals a mean nuclear spin relaxation time of 103 μs (±25%) was found. An activation energy of 11.3 kJ/mol (±10%) was determined for the degenerate electron exchange between DMA and its radical cation.