Structural transformations in poly(di-n-alkylsiloxane)s with alkyl side groups containing 7 to 10 carbon atoms

Temperature dependent X-ray diffraction studies of poly(di-n-alkylsiloxane)s substituted with side chains containing 7 to 10 carbon atoms have been carried out in order to elucidate the crystalline structure of these materials. In contrast to their lower substituted homologues, no evidence for the presence of a hexagonal columnar mesophase could be found for the investigated materials. At low temperatures, far below the melting temperature, the polysiloxane backbones are assumed to pack in an orthorhombic lattice, with the n-alkyl side chains in a planar all-trans conformation and oriented perpendicularly to the backbone. The paraffinic n-alkyl side chains were found to crystallize in an orthorhombic lattice. The thermal behaviour of the investigated materials has been observed to show an odd-even effect. Whereas poly(di-n-heptylsiloxane) and poly(di-n-nonylsiloxane) were found to melt in a single step, an intermediate state of order was observed for poly(di-n-octylsiloxane) and poly(di-n-decylsiloxane) upon heating from the crystalline to the isotropic state. For this intermediate state, a pseudohexagonal packing of the n-alkyl side chains is proposed

Firm valuation and the uncertainty of future tax avoidance

The paper studies the effect of uncertainty in tax avoidance on firm value. We first show in a clean surplus valuation model that expected tax rates interact with expectations about future profitability. This paper builds and tests a valuation framework that incorporates two outcome dimensions of corporate tax avoidance strategies: the stability and the level of expected tax rates. We develop a tax planning score that captures these two dimensions. The measure improves the prediction of future tax avoidance. We finally show that the tax planning score strengthens the effect of pre-tax earnings on firm value. Firms with effective and persistent tax planning have a stronger effect of pre-tax earnings on firm value while firms with poor tax planning or volatile effective tax rates receive a discount on their earnings

Hydrosilylation of 1-alkenes with dichlorosilane

Symmetrically and unsymmetrically substituted diorganodichlorosilanes have been prepared by hydrosilylation with dichlorosilane using two different platinum catalysts, i.e., hexachloroplatinic acid (Speier's catalyst) and a platinum cyclovinylmethylsiloxane complex. Hydrosilylation of unsubstituted 1-alkenes proved to be very efficient, yielding anti-Markonikov substituted di-n-alkyldichlorosilanes. However, no reaction was observed when electron-deficient 1-alkenes were used. Octacarbonyldicobalt enabled formation of the monoadduct of 1H,1H,2H-perfluoro-1-hexene with dichlorosilane, which was employed in a second hydrosilylation of the olefin. Thus, the anti-Markovnikov diadduct was obtained in 40% overall yield. The two-step synthesis has also been applied successfully to obtain unsymmetrically substituted diorganodichlorosilanes containing nitrile and ether groups

Interactive Augmentation of Voice Quality and Reduction of Breath Airflow in the Soprano Voice

SummaryIn 1985, at a conference sponsored by the National Institutes of Health, Martin Rothenberg first described a form of nonlinear source-tract acoustic interaction mechanism by which some sopranos, singing in their high range, can use to reduce the total airflow, to allow holding the note longer, and simultaneously enrich the quality of the voice, without straining the voice. (M. Rothenberg, “Source-Tract Acoustic Interaction in the Soprano Voice and Implications for Vocal Efficiency,” Fourth International Conference on Vocal Fold Physiology, New Haven, Connecticut, June 3–6, 1985.) In this paper, we describe additional evidence for this type of nonlinear source-tract interaction in some soprano singing and describe an analogous interaction phenomenon in communication engineering. We also present some implications for voice research and pedagogy

Integrating bottom-up and top-down reasoning in COLAB

The knowledge compilation laboratory COLAB integrates declarative knowledge representation formalisms, providing source-to-source and source-to-code compilers of various knowledge types. Its architecture separates taxonomical and assertional knowledge. The assertional component consists of a constraint system and a rule system, which supports bottom-up and top-down reasoning of Horn clauses. Two approaches for forward reasoning have been implemented. The first set-oriented approach uses a fixpoint computation. It allows top-down verification of selected premises. Goal-directed bottom-up reasoning is achieved by a magic-set transformation of the rules with respect to a goal. The second tuple-oriented approach reasons forward to derive the consequences of an explicitly given set of facts. This is achieved by a transformation of the rules to top-down executable Horn clauses. The paper gives an overview of the various forward reasoning approaches, their compilation into an abstract machine and their integration into the COLAB shell

Tree biomass equations from terrestrial LiDAR : a case study in Guyana

Large uncertainties in tree and forest carbon estimates weaken national efforts to accurately estimate aboveground biomass (AGB) for their national monitoring, measurement, reporting and verification system. Allometric equations to estimate biomass have improved, but remain limited. They rely on destructive sampling; large trees are under-represented in the data used to create them; and they cannot always be applied to different regions. These factors lead to uncertainties and systematic errors in biomass estimations. We developed allometric models to estimate tree AGB in Guyana. These models were based on tree attributes (diameter, height, crown diameter) obtained from terrestrial laser scanning (TLS) point clouds from 72 tropical trees and wood density. We validated our methods and models with data from 26 additional destructively harvested trees. We found that our best TLS-derived allometric models included crown diameter, provided more accurate AGB estimates (R-2 = 0.92-0.93) than traditional pantropical models (R-2 = 0.85-0.89), and were especially accurate for large trees (diameter > 70 cm). The assessed pantropical models underestimated AGB by 4 to 13%. Nevertheless, one pantropical model (Chave et al. 2005 without height) consistently performed best among the pantropical models tested (R-2 = 0.89) and predicted AGB accurately across all size classes-which but for this could not be known without destructive or TLS-derived validation data. Our methods also demonstrate that tree height is difficult to measure in situ, and the inclusion of height in allometric models consistently worsened AGB estimates. We determined that TLS-derived AGB estimates were unbiased. Our approach advances methods to be able to develop, test, and choose allometric models without the need to harvest trees

Charakterisierung zeitvarianter HF-Impedanzen im Hinblick auf die EMV-Modellierung leistungselektronischer Systeme

Zukünftige leistungselektronische Systeme werden von dem Einsatz von Wide-Bandgap- Halbleitern wie Siliciumcarbid (SiC) oder Galliumnitrid (GaN) profitieren. Gegenüber Silicium-Halbleitern sind deutlich gesteigerte Taktfrequenzen möglich, was sich positiv auf die Effizienz und den Bauraum der Systeme auswirkt. Aus Sicht der EMV stellen die sehr steilen Schaltflanken, die bei GaN-Leistungshalbleitern mehr als 100 kV/μs und 50 kA/μs betragen können, jedoch eine besondere Herausforderung dar. Häufig ist eine Modellierung des leistungselektronischen Systems u.a. hinsichtlich seiner HF-Impedanz sinnvoll, um die Leistungselektronik und gegebenenfalls erforderliche EMV-Maßnahmen bereits vor der Realisierung des Gesamtsystems optimal auslegen zu können. Hierdurch können der Designprozess verkürzt und Entwicklungskosten eingespart werden. In [1] werden beispielsweise die Gleich- und die Gegentaktimpedanz eines im Betrieb befindlichen Umrichter-Motor-Systems durch Einkopplung eines HF-Signals und Messung der resultierenden HF-Spannungen und HF-Ströme charakterisiert. Diese Messdaten werden danach zur Modellierung einer Ersatzimpedanz genutzt. Falls die gemessene HF-Impedanz in dem untersuchten Szenario kaum vom Betriebszustand des Systems abhängt, ist die Nachbildung leicht möglich. Der zeitliche Verlauf der HF-Impedanzen leistungselektronischer Systeme ergibt sich aber durch das periodische Umschalten der Halbleiter und ist daher im Allgemeinen direkt durch den jeweiligen Betriebszustand der Leistungselektronik festgelegt. Es ist zu untersuchen, wie eine Modellierung solcher Systeme auch dann sinnvoll erfolgen kann, wenn sich die EMV-relevanten HF-Eigenschaften bei verschiedenen Betriebszuständen deutlich ändern. Im Rahmen dieser Veröffentlichung erfolgt eine grundlegende Betrachtung zur Charakterisierung zeitvarianter HF-Impedanzen. Anstelle eines realen leistungselektronischen, zu untersuchenden Systems (system under test, SUT) wird hier ein generischer Laboraufbau genutzt, mit dem unterschiedliche Impedanzverläufe mit Taktfrequenzen bis 100 kHz reproduzierbar nachgebildet werden können. Der Laboraufbau wird in Abschnitt 2 dieses Beitrags vorgestellt. Abschnitt 3 veranschaulicht, welchen Einfluss die Zeitvarianz von HF-Impedanzen auf deren Messung mit den in der EMV etablierten Messmethoden besitzt. Ausgehend von diesen Erkenntnissen wird in Abschnitt 4 ein schalttaktsynchronisiertes Messkonzept vorgestellt, mit dessen Hilfe der tatsächliche zeitliche Verlauf der HF-Impedanz von leistungselektronischen Systemen analysiert werden kann. Mit diesem Verfahren aufgenommene Messkurven werden mit Referenzmessungen verglichen, um die Messgenauigkeit zu bewerten. Abschnitt 5 gibt abschließend einen kurzen Ausblick auf zukünftige, weiterführende Untersuchungen zu zeitvarianten HF-Impedanzen und mögliche Erweiterungen des hier vorgestellten Messkonzepts

Phasenrichtige HF-Störstrommessung und HF-Stromeinspeisung mit kalibrierten Impedanzzangen

Der Einsatz schnell schaltender Wide-Bandgap-Halbleiter mit dem Ziel der Effizienzsteigerung leistungselektronischer Systeme erfordert präzise EMV-Messtechnik. Nicht nur um sicherzustellen, dass die Grenzwerte eingehalten werden, sondern auch zur Unterstützung des Designprozesses im Sinne einer ganzheitlichen Effizienzoptimierung, die auch EMV-Maßnahmen wie Filter und Schirmungen mit einbezieht. Eine solche Optimierung erfordert eine umfangreiche Modellierung des Gesamtsystems und dazu die möglichst genaue Kenntnis der Hochfrequenzeigenschaften der beteiligten Komponenten unter realen Randbedingungen. Am Institut für EMV wird dazu an einem kontaktlosen Messverfahren geforscht, das auf der Verwendung so genannter Impedanzzangen basiert, die zusammen mit dem dazugehörigen Kalibrierverfahren in [1] präsentiert wurden. Die Impedanzzange ist eine spezielle Form der Stromzange, die nur eine einzige oder sehr wenige Sekundärwindungen besitzt. Idealerweise wird so die Impedanz des Systems unter Test (SUT) direkt auf den Anschluss der Impedanzzange abgebildet. Dies ermöglicht die rückführbare 3-Term-Kalibrierung der Zange unter Verwendung dreier Kalibrierstandards mit Hilfe von Reflexionsmessungen, wodurch das komplexe Übertragungsverhalten der Impedanzzange ermittelt werden kann [1]. Dieses Verfahren ermöglicht breitbandige kontaktlose Impedanzmessungen [1] und kontaktlose vektorielle Netzwerkanalyse [2, 5]. In diesem Beitrag geht es darum, inwiefern dieses Verfahren auch zur Hochfrequenzstörstrommessung und Hochfrequenzstromeinspeisung verwendet werden kann, ob sich dadurch Vorteile gegenüber den klassischen Methoden ergeben und welche Probleme dabei auftreten

The Nature of Study Programmes in Vocational Education: Evaluation of the Model for Comprehensive Competence-Based Vocational Education in the Netherlands

In a previous series of studies, a model of comprehensive competence-based vocational education (CCBE model) was developed, consisting of eight principles of competence-based vocational education (CBE) that were elaborated for four implementation levels (Wesselink et al. European journal of vocational training 40:38–51 2007a). The model thus consisted of 32 cells, all defined by text. It was developed to provide study programme teams working in vocational education with an instrument to assess the actual and desired “competentiveness” of their study programmes. “Competentiveness” refers to the extent to which study programmes are based on the principles of CBE that we formulated. The model is an instrument for analysing the alignment of study programmes with the defining principles of CBE and clarifying programme teams’ intentions, i.e. the extent to which they wish to achieve higher levels of implementation of the different principles. This article presents the results of two studies, the aim of which was to identify adjustments the teachers felt were necessary to make the CCBE model a valid instrument for assessing the actual and desired “competentiveness” of their study programmes. In study A, 57 teachers evaluated the model during focus group discussions, resulting in a revised CCBE model consisting of ten principles for five levels of implementation. In study B, 151 teachers completed a questionnaire to evaluate the comprehensibility of the revised model. The study showed that teachers understood and interpreted the revised model as intended, were able to position their study programmes by using the revised model and that the content validity of the revised model was good

Abstractions of linear dynamic networks for input selection in local module identification

In abstractions of linear dynamic networks, selected node signals are removed from the network, while keeping the remaining node signals invariant. The topology and link dynamics, or modules, of an abstracted network will generally be changed compared to the original network. Abstractions of dynamic networks can be used to select an appropriate set of node signals that are to be measured, on the basis of which a particular local module can be estimated. A method is introduced for network abstraction that generalizes previously introduced algorithms, as e.g. immersion and the method of indirect inputs. For this abstraction method it is shown under which conditions on the selected signals a particular module will remain invariant. This leads to sets of conditions on selected measured node variables that allow identification of the target module.Comment: 17 pages, 7 figures. Paper to appear in Automatica, Vol. 117, July 202