Showing papers by "Chalmers University of Technology published in 1975"

Equations for Microstrip Circuit Design

Abstract: A method for determining the accuracy of computed impedance and wavelength data for microstrip is proposed. It is shown that the standard equations of Wheeler and Schneider have rather large errors. Revised equations for microstrip impedance and wavelength are given both for analysis and synthesis with accuracy better than 1%. A simple, accurate equation for the microstrip open circuit is presented. Experimental data on microstrip T-junctions are compared with existing theories. A new accurate equation is given for the reference plane displacement in the stub arm. Corrections are proposed in the existing equations for the other parameters of the equivalent circuit.

The Catalytic Mechanism of Carbonic Anhydrase

Abstract: A model for the catalytic mechanism of human carbonic anhydrase II involving two ionizing active-site groups is presented. One of these groups is involved in CO2/HCO3 - interconversion. It is thought to be a zinc-bound H2O molecule ionizing to OH-. The other group, presumably His-64, is involved in the transfer of H+ between the active site and the reaction medium. The model can be characterized as a ‘ping-pong’ mechanism with a CO2/HCO3 - interconversion half reaction and a H+ transfer half reaction. Five discrete reaction steps are discussed; (1) the binding (and dissociation) of CO2, (2) the chemical interconversion between CO2 and HCO3 -, (3) the dissociation (and binding) of HCO3 -, (4) a rate-limiting intramolecular H+ transfer between the metal-bound H2O and His-64, and (5) a buffer-mediated H+ transfer between His-64 and the reaction medium. A tentative free- energy profile for this reaction sequence is presented.

Relativistic molecular calculations in the Dirac–Slater model

Abstract: A new method is presented to calculate binding energies and eigenfunctions for molecules, using the Dirac–Slater Hamiltonian. A numerical basis set of four component wavefunctions is obtained from atom‐like Dirac–Slater wavefunctions. A discrete variational method (DVM) has been applied to generate the binding energies and eigenfunctions for the molecule. Results are given for a series of molecules, including dihydrides H2X (X=O, S, Se, Te), diatomic indium halides InX (X=F, Cl, Br, I), and metal chlorides XCl (X=B, Al, Ga, In, Tl). Comparison is made with results from nonrelativistic calculations using the DVM with numerical Hartree–Fock–Slater‐type wavefunctions and with other types of nonrelativistic calculations. In particular, relativistic level shifts and spin–orbit splitting have been analyzed. The theoretical ionization energies are compared with experimental results. Generally a very good agreement is obtained between the experimental and theoretical binding energies for the valence levels, calcu...

On L p −L p′ estimates for the wave-equation

Abstract: The purpose of this note is to present a short proof of some of the results announced by Littman [4], and in particular to obtain a fundamental Lp-Lp , estimate for a class of evolution equations with constant coefficients, in particular containing the stimate for the wave-equation due to Strichartz [-7, 8]. The method used here is particularly apt to deal with approximations to the wave-equation, such as those obtained by finite element or finite difference methods. Details of such applications will appear elsewhere.

DTA and X‐Ray Analysis Study of Nucleation and Crystallization of MgO‐Al2O3‐SiO2 Glasses Containing ZrO2, TiO2, and CeO2

Abstract: Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al2O3-SiO2 system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO2 or TiO2. Up to 10 wt% CeO2 was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce2Ti2O4 (Si2O7). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO2, but no crystalline TiO2 was detected. In the presence of CeO2, TiO2 was a more effective catalyst than ZrO2. Although CeO2 lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.

Anomalous behavior of electrical conductivity and thermal noise in carbon black‐containing polymers at Tg and Tm

Abstract: Pronounced maxima in resistivity–temperature curves have been found at or above Tg for a number of amorphous polymers (PS, PMMA, PVC) containing a small percentage of carbon black. The maxima appeared only after a certain period of storage at T < Tg or at heating rates substantially higher than those used in cooling the samples prior to the measurement. For this reason, no maxima were observed during cooling. In crystalline polymers (PE, PP), maxima at Tm appeared both on heating and cooling. Marked relaxation effects (time-dependent resistivity) were recorded in the Tg and Tm regions. Good agreement was found between resistivity and thermal noise data.

Calculation of the hyperfine interaction using an effective-operator form of many-body theory

Abstract: The effective-operator form of many-body theory is reviewed and applied to the calculation of the hyperfine structure. Numerical results are given for the $2p$, $3p$, and $4p$ excited states of Li and the $3p$ state of Na. This is the first complete calculation of the hyperfine structure using an effective-operator form of perturbation theory. As in the Brueckner-Goldstone form of many-body theory, the various terms in the perturbation expansion are represented by Feynman diagrams which correspond to basic physical processes. The angular part of the perturbation diagrams are evaluated by taking advantage of the formal analogy between the Feynman diagrams and the angular-momentum diagrams, introduced by Jucys et al. The radial part of the diagrams is calculated by solving one- and two-particle equations for the particular linear combination of excited states that contribute to the Feynman diagrams. In this way all second- and third-order effects are accurately evaluated without explicitly constructing the excited orbitals. For the $2p$ state of Li our results are in agreement with the calculations of Nesbet and of Hameed and Foley. However, our quadrupole calculation disagrees with the work of Das and co-workers. The many-body results for Li and Na are compared with semiempirical methods for evaluating the quadrupole moment from the hyperfine interaction and a new quadrupole moment of $^{23}\mathrm{Na}$ is given.

Current Flow in Ion-Acoustic and Langmuir Turbulence Plasma Interaction

Abstract: The process of Langmuir and ion-acoustic turbulence interaction due to turbulent bremsstrahlung collisions is considered. It is shown that the ion-sound turbulence excites the Langmuir turbulence even for isotropic particle distributions but for anisotropic ion-sound turbulent waves. The presence of both ion-sound and Langmuir turbulence may lead to the excitation of currents which cause strong magnetic fields.

Beta-decay energies and masses of short-lived isotopes of rubidium, caesium, francium, and radium

Abstract: Total decay energies have been measured for a number of neutron-deficient Rb and Cs isotopes, as well as for some neutron-rich isotopes of Fr and Ra. Mass separated sources were produced at the ISOLDE on-line separator at CERN. By applying two differentβ-γ coincidence methods,Q values or their lower limits were determined for76–78Rb,80Rb,121–124Cs,222Fr,224–226Fr,229Ra-229Ac. For many of these nuclei, the atomic mass excesses could be derived, allowing the comparison of masses of far unstable nuclei with predictions from mass formulae. The odd-odd nuclei76Rb and78Rb appear to be 1−1 1/2 MeV more strongly bound than expected from the systematics.

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen

Abstract: The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.

Plastic films on the bottom of the Skagerack

Abstract: FISHERMEN from Smogen, Sweden have remarked that when they trawl along the bottom of the Skagerack they almost invariably collect sheets of plastic in their nets.

Density oscillations and density response in systems with nonuniform electron density

Abstract: A formulation for the density response of systems with inhomogeneous electron density is presented. The treatment allows a separation of the response into a classical instantaneous part which is a function of electron density alone, and a retarded dynamic part that depends on the details of the excitation energies and wave functions of the system. This enables one to understand which properties of a system are determined essentially by the classical part and which by the dynamic part. As an application and illustration of studying in a unified manner adverse systems, such as a semi-infinite electron gas, electron distribution in defects and electrons in atoms, the classical part alone has been studied and some simple results related to the density oscillations in the system have been derived.

Paramagnetic and Fluorescent Probes Attached to “Buried” Sulfhydryl Groups in Human Carbonic Anhydrases. Application to Inhibitor Binding, Denaturation and Refolding

Abstract: 1 The single–SH groups in the human carbonic anhydrases B and C have been modified under denaturing conditions. The modified enzymes recover catalytic activity after dilution of the denaturing medium with buffer. By this method a spin label and a fluorescent probe were specifically introduced into the molecules. 2 The modified and reactivated enzymes have similar kinetic properties, inhibitor-binding constants, circular dichroism spectra, and stabilities towards guanidine hydrochloride as the native enzymes. However, the esterase activity of the modified C enzyme is reduced to about 50%. 3 The spectra associated with the probes are insensitive to inhibitor binding in case of the B enzyme, whereas changes of electron paramagnetic resonance spectrum and fluorescence intensity respectively, were observed for the probe-containing C enzymes. The cysteines are located in different parts of the tertiary structures of the homologous B and C enzymes, and these observations suggest that small conformational changes accompanying inhibitor binding are localized to regions of the molecules near the active-site cavity. 4 During denaturation of the spin-labeled B enzyme in 1.7 M guanidine hydrochloride a transient mobilization of the probe occurs, but the mobility is ultimately reduced to a low level. This observation supports previous evidence that denaturation under these conditions, in or near the transition region, mainly yields incorrectly folded molecules rather than stable intermediates between native and fully denatured molecules. 5 During refolding of fully denatured, spin-labeled B and C enzymes the mobility of the probe is drastically reduced within less than 0.1 s after dilution. This would reflect a very short lifetime of the randomly coiled state under these conditions.

Characterization of the reaction between ferrocytochrome c and cytochrome c oxidase.

Abstract: The reaction between cytochrome c oxidase and ferrocytochrome c has been investigated by the stopped-flow method. It has been found that only one electron acceptor, a heme group, in the oxidase is rapidly reduced by cytochrome c. The presence of N3- does not affect the reduction of the acceptor, which supports the hypothesis that this is identical with cytochrome a. The results are consistent with the existence of a simple equilibrium between cytochrome a and cytochrome c: c-2 + a-3+ in equilibrium c-3+ + a-2+ with an equilibrium constant corresponding to an oxidation-reduction potential of cytochrome a 30 mV higher than that for cytochrome c at pH 7.4. The oxidation-reduction potential of the a-3+ /a-2+ couple, 285 mV (based on a potential of 255 mV for cytochrome c), and the optical properties of the reduced form indicate that it is identical with neither of the reduced hemes seen in potentiometric titrations. The oxidase species resulting from the rapid reduction of cytochrome a by cytochrome c is proposed to represent a metastable intermediate state which, under anaerobic conditions, eventually is transformed into a more stable state characterized by a reduced high-potential heme.

Influence of the channel width on the threshold voltage modulation in m.o.s.f.e.t.s

Abstract: The threshold vollage of an m.o.s. field-effect transistor is modulated by the source-to-substrate reverse bias. In the letter, the theory for long- and short-channel transistors is extended to include the influence of the channel width. The result is an analytical expression for the threshold voltage as a function of geometry and bias that agrees well with experimental data.

Vocational electromyography: studies of localized muscle fatigue at the assembly line.

Abstract: Myoelectric signal power spectrum changes have boon estimated during vocational labour to evaluate an objective method for the assessment of the amount of localized muscle fatigue produced by the work. Two assembly line stations, which were considered heavy by workers, hove been studied, viz. assembly work with the arms above the shoulders, and heavy and light wet-rubbing of car bodies. Significant power spectrum changes have been classified as incidents of fatigue and mean values of intensities of fatigue incidents per treated ear body have boon calculated. It is found that heavy work produces statistically significantly higher intensities of fatigue incidents than light work, and that the distribution of intensity values over muscles corresponds to the engagement of the muscles in the work. It is concluded that the method can be used to evaluate the muscle strain of a certain work situation and that the distribution of fatigue incidents over muscles gives hints to how the work situation can be changed t...

Estimating heat transfer rates from mass transfer studies on plate heat exchanger surfaces

Abstract: Tests were conducted on surfaces for plate heat exchangers, of the cross-corrugated type, to determine local and overall mass transfer rates. Analogous heat transfer data was obtained by the method described by Chilton and Colburn using the j-factor method.

Rotation of laser beam polarization in acousto-optic devices

Abstract: When a linear, polarized laser beam is passed through an acousto-optic device, the polarization direction of the outgoing beam is often different from that of the entering beam, and thus a rotation of the polarization angle takes place. This paper shows how the polarization rotation and the intensity can be calculated for different polarization angles. Experimental results obtained with a single crystal germanium frequency shifter in the 10.6 μm region agreed well with theory.