A Dual Threat: Redox-Activity and Electronic Structures of Well-Defined Donor–Acceptor Fulleretic Covalent-Organic Materials

The effect of donor (D)–acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two- and three-dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight-fold conductivity enhancement. The first evaluation of redox behavior of buckyball- or tetracyanoquinodimethane-integrated crystalline was conducted. In parallel with tailoring the D-A alignment responsible for “static” changes in materials properties, an external stimulus was applied for “dynamic” control of the electronic profiles. Overall, the presented D–A strategic design, with stimuli-controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.     

Electron and positron pair emission by low energy positron impact on surfaces

The emission of electron pairs from surfaces has the power to reveal details about the electron–electron interaction in condensed matter. This process, stimulated by a primary electron or photon beam, has been studied both in experiment and theory over the last two decades. An additional pathway, namely positron–electron pair emission, holds the promise to provide additional information. It is based on the notion that the Pauli exclusion principle does not need to be considered for this process.We have commissioned a laboratory based positron source and performed a systematic study on a variety of solid surfaces. In a symmetric emission geometry we can explore the fact that positron and electron are distinguishable particles. Following fundamental symmetry arguments we have to expect that the available energy is shared unequally among positron and electron. Experimentally we observe such a behavior for all materials studied. We find an universal feature for all materials in the sense that on average the positron carries a larger fraction of the available energy. This is qualitatively accounted for by a simplified scattering model. Numerical results, which we obtained by a microscopic theory of positron–electron emission from surfaces, reveal however that there are also cases in which the electron carries more energy. Whether the positron or the electron is more energetic depends on details of the bound electron state and of the emission geometry. The coincidence intensity is strongly material dependent and there exists an almost monotonic relation between the singles and coincidence intensity. These results resemble the findings obtained in electron and photon stimulated electron pair emission. An additional reaction channel is the emission of an electron pair upon positron impact. We will discuss the energy distributions and the material dependence of the coincidence signal which shows similar features as those for positron–electron pairs.     

Laser ablation of paper: Raman identification of products

Old paper samples are bleached using pulsed laser radiation with a wavelength of 532 nm. The ablation products of five paper samples that differ by composition and production dates are studied using Raman microspectroscopy. Cellulose, protein, calcite, titanium dioxide (anatase, rutile, and brookite), quartz, lazurite, bonattite, and dolomite are identified as ablation products.     

Hard Thermal Loops in the n-Dimensional ? 3 Theory

We derive a closed-form result for the leading thermal contributions which appear in the n-dimensional ? ³ theory at high temperature. These contributions become local only in the long wavelength and in the static limits, being given by different expressions in these two limits.     

The synthetic preparation of naturally-occurring aromatase inhibitors,morachalcone A,isogemichalcone B,and isogemichalcone C

A convergent synthesis applicable to the preparation of oxidized prenylchalcones is reported that relies on key Claisen–Schmidt, Mitsunobu, and vinyl/benzyl Stille coupling operations. The synthetic strategy was applied towards the preparation of the natural products morachalcone A and isogemichalcones B & C, allowing their preparation in less than 10 steps and 6–8% overall yield.     

Elements of Design‐Based Science Activities That Affect Students' Motivation

The primary purpose of this study was to examine the ways in which a 12‐week afterschool science and engineering program affected middle school students' motivation to engage in science and engineering activities. We used current motivation research and theory as a conceptual framework to assess 14 students' motivation through questionnaires, structured interviews, and observations. Students reported that during the activities they perceived that they were empowered to make choices in how to complete things, the activities were useful to them, they could succeed in the activities, they enjoyed and were interested in the hands‐on activities and some presentations, they felt cared for by the facilitators and received help when they were stuck or confused, and they put forth effort. Based on our examination of data across our three data sources, we identified motivating opportunities that were provided to students during the activities. These motivating opportunities can serve as examples to help both formal and informal science educators better connect motivation theory to practice so that they can create motivating opportunities for students. Furthermore, this study provides a methodological example of how students' motivation can be examined during the context of authentic science and engineering instruction.     

Einflu\ der TemperaturabhÄngigkeit der Stoffwerte auf den WÄrmeübergang an der laminar überströmten ebenen Platte

Zusammenfassung Der Einflu\ der TemperaturabhÄngigkeit der Stoffwerte auf den WÄrmeübergang ist zur Zeit noch nicht befriedigend geklÄrt. Da Messungen des WÄrmeübergangs stets mit mehr oder weniger gro\en Fehlern behaftet sind, und da der Einflu\ der TemperaturabhÄngigkeit für die einzelnen Stoffwerte nicht getrennt untersucht werden kann, ist es experimentell praktisch nicht möglich, Korrekturfaktoren zu bestimmen. Es bleibt daher nur der Versuch, rechnerisch dieses Problem anzugehen, wobei es wichtig ist, Verfahren zu benutzen, die nicht zu lange Rechenzeiten erfordern, um eine gro\e Anzahl von Parameterstudien durchzuführen.über eine solche Rechnung für eine lÄngsangeströmte, ebene Platte bei laminarer Strömung wird berichtet. Dabei konnte auch der Dissipationsterm berücksichtigt werden.Bereits aus der Form der zu lösenden Differentialgleichungen ergibt sich, da\ die VerhÄltnisse der Stoffwerte einzeln berücksichtigt werden müssen, da sich die Stoffwerte in den Differentialgleichungen nicht zu einer Prandtl-Zahl zusammenfassen lassen. Die Ergebnisse der Parameterstudien werden diskutiert und mit den bisher verwendeten Korrekturfaktoren verglichen.

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Influence of temperature dependent physical properties on the heat transfer in the laminar boundary layer of a parallel affluxed flat plate

The influence of temperature dependent properties on the heat transfer is not jet satisfying claryfied. Measurements of heat transfer contain more ore less great faults, and it is practically not possible to investigate the influence of the temperature dependence for particular physical properties separately. Therefore, the only possibility is to examine this problem numerically. Hereby it is important to use methods which don't require too much computing time in order to study a great number of parameters.In the following article such a numerical method for the calculation of a laminar boundary layer will be introduced. By this method it is also possible to consider the dissipation.Already the form of the differential equations, which are to be solved, shows, that the influence of a variable property fluid has to be considered separately, because it is not possible to combine all physical properties with the Pr-number. The results of the parameter studies will be discussed and will be compared with the well known correction terms.

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Bezeichnungen C dimensionslose Stoffwertkorrektur - c _p spezifische WÄrmekapazitÄt - Eckertsche Kenngrö\e - dimensionslose Stromfunktion - erste Ableitung der dimensionslosen Stromfunktion - n Exponent des ZÄhigkeitstermes (Gl. (43)) - Nusseltsche Kennzahl - Pr=c_p/ Prandtlsche Kennzahl - WÄrmestromdichte - Reynoldsche Kennzahl - r Recovery-Faktor (Gl. (50)) - T Temperatur - t _E Eigentemperatur - u,u Strömungsgeschwindigkeit inx-Richtung - Strömungsgeschwindigkey-Richtung - WÄrmeübergangskoeffizient - dimensionslosey-Koordinat - dimensionslose Temperatur - Dichte - dynamische ViskositÄt - v kinematische ViskositÄt - WÄrmeleitfÄhigkeit Indizes au\erhalb der Grenzschicht - w an der Wand Vorgetragen auf der Sitzung des GVC-Fachausschusses WÄrme- und Stoffübertragung am 30. 3. 1981 in Freiburg