Valley current characterization of high current density resonant tunnelling diodes for terahertz-wave applications

We report valley current characterisation of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTDs) grown by metal-organic vapour phase epitaxy (MOVPE) for THz emission, with a view to investigate the origin of the valley current and optimize device performance. By applying a dual-pass fabrication technique, we are able to measure the RTD I-V characteristic for different perimeter/area ratios, which uniquely allows us to investigate the contribution of leakage current to the valley current and its effect on the PVCR from a single device. Temperature dependent (20 – 300 K) characteristics for a device are critically analysed and the effect of temperature on the maximum extractable power (PMAX) and the negative differential conductance (NDC) of the device is investigated. By performing theoretical modelling, we are able to explore the effect of typical variations in structural composition during the growth process on the tunnelling properties of the device, and hence the device performance

Theory of Broensted acidity in zeolites

The nature of the chem. bond of protons in a zeolite is analyzed from theor. and spectroscopic results. Of interest is the dependence on zeolite structure as well as compn. The zeolitic OH bond is mainly covalent. Proton attachment to the zeolite lattice causes a weakening of neighboring Si-O and Al-O bonds. The effective increase in vol. of the bridging O atom causes a local deformation, that changes the strength of the lattice-chem. bonds over a few bond distances. Proton concn. effects as well as lattice-compn. effects can be understood from the lattice-relaxation model. The energetics of proton transfer is controlled by the need to stabilize the resulting Zwitter-ion. The pos. charge on the cation becomes stabilized by contact with basic lattice-O atom

Powdered Crystalline Silicotitanate (CST) Isotherms for SRS Wastes

One of the primary inputs for modeling an ion exchange column is the equilibrium driving force for mass transfer between the solution and the solid phase. The equilibrium relationship is typically known as an isotherm. This document contains the predicted isotherms for the various Savannah River Site (SRS) waste types in equilibrium with powdered (ungranulated) crystalline silicotitanate (CST)

Spectroscopy, energetics and siting of NH+4 in zeolites; theory and experiment

The adsorption of an NH3 mol. on an acidic zeolite and the proton transfer from the zeolite to the NH3 are studied by quantum chem. methods. The NH+4 is adsorbed with 2 or 3 H bonds to the zeolite. The calcd. vibrational frequencies explain the exptl. IR spectra. [on SciFinder (R)

Improving Prolog Programs: Refactoring for Prolog

Refactoring is an established technique from the OO-community to restructure code: it aims at improving software readability, maintainability and extensibility. Although refactoring is not tied to the OO-paradigm in particular, its ideas have not been applied to Logic Programming until now. This paper applies the ideas of refactoring to Prolog programs. A catalogue is presented listing refactorings classified according to scope. Some of the refactorings have been adapted from the OO-paradigm, while others have been specifically designed for Prolog. Also the discrepancy between intended and operational semantics in Prolog is addressed by some of the refactorings. In addition, ViPReSS, a semi-automatic refactoring browser, is discussed and the experience with applying \vipress to a large Prolog legacy system is reported. Our main conclusion is that refactoring is not only a viable technique in Prolog but also a rather desirable one.Comment: To appear in ICLP 200

Fourier-transform infrared and inelastic neutron-scattering study of hy zeolites

A combination of FTi.r. and INS spectroscopy is used in a vibrational study of the bending and stretching vibrations of the acidic hydroxyl groups of Y zeolites. The influence of the number of acidic Bronsted sites and the Si/Al ratio is discussed. Out-of-plane hydroxyl bending modes are assigned to vibrations centered around 419 cm-1 and in-plane hydroxyl bending modes are assigned to vibrations centered around 1089 cm-1. Upon dealumination, these bands are shifted by approximately 30 cm-1 to lower values. The less intense bands at 319,470,565,765, and 1130 cm-1 are assigned to proton- coupled framework vibrations. Upon dealumination, the mode at 319 cm-1 is shifted to lower frequencies and the modes at 565 and 1130 cm-1 are shifted to higher frequencie

Modeling of structure and vibrational spectra of aluminophosphate AlPO4 5 and its silica analog SSZ 24

In this study, the structural and vibrational properties of the AlPO4 structure AlPO4-5 and the silica structure SSZ-24 are compared. Lattice energy calcns. are done using existing potential parameter sets suitable for silicas and AlPO4's. For the computation of vibrational spectra of silica systems, force consts. derived by Etchepare et al. are used. For AlPO4 spectra simulations, a new force field is presented that is based on a fit on vibrational frequencies of a-berlinite, the AlPO4 analog of a-quartz. Lattice energy calcns. result in a symmetry of AlPO4-5 and SSZ-24 that is lower than derived exptl. A shift of layers is obsd. for both structures when a potential with partial charges is used. These results are indicative for an underestn. of the charges used in the partial charge model. The influence of structure on spectra is shown to be rather weak. The main differences between the spectra of AlPO4-5 and SSZ-24 are due to the interat. force const

Doelmatigheidswinst van minder hypotheekrenteaftrek

We bespreken enkele welvaartsdividenden van het versoberen van de subsidie op de eigen woning. Deze dividenden doen zich in de eerste plaats voor op de woningmarkt. Daarnaast kan zich een welvaartsdividend voordoen op de arbeidsmarkt. De omvang van dit tweede dividend is evenwel sterk afhankelijk van de elasticiteit van het aanbod van woningen