Showing papers by "Chalmers University of Technology published in 1974"

Contribution to the cohesive energy of simple metals: Spin-dependent effect

Abstract: We find that within local-density schemes for calculating the cohesive energy of simple metals greater sophistication in treating the atom is required. The outermost electron in, e.g., the sodium atom has an unpaired spin. For this and the many similar cases a generalization of the scheme to a spin-density-functional formalism is needed. Application of the local-spin-density approximation gives, e.g., the energy of the hydrogen atom within 1.6% of the exact value, while the local-density approximation is 10% off. The improvement is due to our use of a better model system, i.e., the spin-polarized electron liquid, in the local approximation. We elaborate on the factors leading to the smallness of the error, and we find that there is a systematic partial cancellation between too attractive and too repulsive contributions to the binding for valence electrons in hydrogen and similar atoms. When we extend Tong's calculation for sodium metal along these lines, we find the cohesive energy to lie within 4% of the experimental value. A similar improvement is found for lithium. The spin-density scheme should be a very useful practical method for a large range of applications, including the calculation of chemisorption and charge transfers.

On the Calibration Process of Automatic Network Analyzer Systems (Short Papers)

Abstract: Formulas are presented for the direct calculation of the scattering parameters of a linear two-port, when it is measured by an imperfect network analyzer. Depending on the hardware configuration of the test set, the measurement system is represented by one of two flowgraph models. In both models presented, leakage paths are included. The error parameters, i.e., the scattering parameters of the measuring system, are six respective ten complex numbers for each frequency of interest. A calibration procedure, where measurements are made on standards, will determine the error parameters. One of many possible calibration procedures is briefly described. By using explicit formulas instead of iterative methods, the computing time for the correction of the scattering parameters of the unknown two-port is significantly reduced. The addition of leakage paths will only have a minor effect on computational complexity while measurement accuracy will increase.

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. II. Structural changes occuring in low-density polyethylene at an oxygen content less than 0.0005%

Abstract: Samples of low-density polyethylene, free from additives, were heated at temperatures between 284° and 355°C under high-purity nitrogen. Changes in molecular weight distribution (MWD), molecular weight averages, and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). At 284° and 315°C, the MWD's were shifted toward higher molecular weights and the Mw values increased. At 333° and 355°C, the MWD's shift toward lower molecular weight, but the high molecular weight, tail is largely retained. Mw decreases slowly at 333°C. At 355°C, Mw undergoes a rapid initial drop which levels off. Mw/Mn and the degree of LCB increase with heating time and temperature. Olefinic unsaturation increases. The vinyl groups show a larger relative increase than do the trans-vinylene and vinylidene groups. At 355°C, the peak of the unimodal DSC thermogram is shifted to ∼3°C higher temperature. A lower melting peak then develops, and after 72 and 90 min the two peaks are about equal in size. The density increases from 0.922 g/cm3 to 0.930 g/cm3 for samples heated at 355°C, and the weight loss was 1.5% after 90 min. A reaction scheme for the thermal degradation of polyethylene is discussed. Initiation is suggested to be accomplished by scission of allylic CC bonds. Propagation proceeds by both intra- and intermolecular hydrogen abstraction, followed by β-scission. Termination can occur by both combination and disproportionation. Combination reactions are suggested to account for the observed formation of LCB and high molecular weight material. Due to changes in the degree of LCB during the degradation, viscometry alone will not give a proper measure of the changes in molecular weight.

Saturation capacitance of thin oxide MOS structures and the effective surface density of states of silicon

Abstract: The capacitance vs voltage curve of thin oxide (30–40 A) MOS structures in strong accumulation was studied. The results were interpreted in terms of equivalent surface density of state masses, which was found to be 0·2 m 0 for the silicon valence band and 0·06 m 0 for the conduction band, for both 111 and 100 surfaces. The experimental density of state masses were shown to be much lower than the bulk values. Equivalent density of states masses were calculated from a surface quantization model and in this case agreement with the experiments was obtained for the valence band only.

Photon Emission during Chemisorption of Oxygen on Al and Mg Surfaces

Abstract: Chemisorption of oxygen on Al and Mg was found to cause photon emission with a probability of about ${10}^{\ensuremath{-}7}$ per molecule. The photon intensity was continuously recorded for oxygen exposures ${10}^{\ensuremath{-}8}$-${10}^{\ensuremath{-}1}$ Torr sec. The intensity is closely related to the chemisorption rate and a rate ${l10}^{\ensuremath{-}3}$ monolayer/sec was easily detected. Spectral analysis revealed broad emission bands with maxima at 5100 and 5900 \AA{} for Al + ${\mathrm{O}}_{2}$ and at 4700 and 5600 \AA{} for Mg + ${\mathrm{O}}_{2}$.

Analysis of air-bubble plumes

Abstract: Models are developed to describe the gross behavior of air-bubble plumes generated by point and line sources of air-bubbles released in stagnant water bodies of uniform density. The models predict plume width, velocities, and induced flow rates as a function of elevation above the source. The analysis is confined to the plume mechanics and does not include the horizontal flow created at the surface by the plume. An integral similarity approach, similar to that used for single-phase buoyant plumes, is employed. Governing equations are found by applying conservation of mass, momentum, and buoyancy. The compressibility of the air and the differential velocity between the rising air bubbles and water are introduced in the buoyancy flux equation. Generalized solutions to the normalized governing equations are presented for both point and line sources of air-bubbles. The results of the analyses are compared with existing large-scale experimental data. The comparisons indicate that the models predict the gross behavior of plumes well and yield estimates of the entrainment coefficients and lateral spreading ratios.

Cerebral reactions during intrauterine asphyxia in the sheep. I. Circulation and oxygen consumption in the fetal brain.

Abstract: Extract: Cerebral blood flow (CBF) with the 133Xe clearance technique and cerebral oxygen consumption were measured in the fetal lamb using chioralose-anesthetized ewes and acutely exteriorized fetuses with intact umbilical circulation. To induce fetal hypoxia at different pH levels, three different procedures were used: (1) the ewe was ventilated with a hypoxic gas mixture, containing 8–15% O2 in N2; (2) 5–10% CO2 was added to the hypoxia gas mixture; (3) during hypoxia a continuous rapid infusion of NaHCO3 solution was given intravenously to the fetus. Blood gas tensions, pH, and oxygen saturation were measured repeatedly. Multiple regression analysis was carried out on the data with the variables SaO2, PaO, PaO2, and pH considered as independent variables and flow as the dependent variable. All complete sets of observations were used amounting to 74 observations in 11 animals. No significant correlation was found between CBF and pH. SaO2 showed the highest correlation to CBF. No further improvement of this correlation was obtained when PaCO2 was added as independent variable. PaO2 and PaCO2 described jointly the variations of CBF as well as SaO2 alone. The oxygen consumption of the brain decreased during hypoxia when SaO2 was reduced below 40%. The decrease of the metabolic rate for oxygen was a function both of the degree of hypoxia and of pH. When an acidosis was added to the hypoxia a significant reduction of the oxygen consumption of the brain resulted compared with the same degree of hypoxia at normal or only moderately reduced pH levels. Speculation: The cerebral blood flow of the fetal lamb is regulated mainly via the amount of oxygen available to the brain, and changes of CO2 tension affect CBF via a displacement of the hemoglobin dissociation curve. Cerebral oxygen consumption is reduced to dangerously low levels when a severe acidosis is combined with hypoxia because of a failure to extract the oxygen available in the arterial blood.

Polymer-glass separation in the spiral mold test

Abstract: The spiral mold method (moldability test) was used in a study of phase separation occurring in glass-containing polyethylene and polyamide melts. Pronounced separation effects were recorded for low density polyethylene containing 25 percent glass spheres with diameters ranging approximately from 50 to 100 μm, the glass concentration at the tip of the spiral exceeding 40 percent by weight. For melts containing smaller spheres or fibers the effect was of the order of a few percent. There was a decrease in glass concentration near the center of the spiral.

Optical Properties of Metals and Alloys

Abstract: Publisher Summary This chapter discusses most of the experimental data on optical absorption in metals and metallic alloys up to approximately 1973. The optical wavelength region includes strictly only the visible region—that is, wavelengths from 4000 to 7000 A corresponding to photon energies from 2 to 3 eV. The optical properties, i.e. phenomena such as reflection and absorption of electromagnetic radiation, are not however specific to the above-mentioned energy range. The “optical” properties of materials are therefore in practice investigated over a much broader energy range, namely where the phenomena still can be described within essentially the same theoretical framework. Thus a low wavelength limit of the order of the lattice constant is obtained that takes it into the hard X-ray region (10 A, 1 keV). The upper limit has been taken where the wavelength is of macroscopic size, in the millimeter region (107 A, 10 meV). This chapter concentrates on the central part of this energy range, from the near infrared (0.1 eV) to the uv region (10 eV). Much of the pure uv data have thus been left out mainly because these have more in common with X-ray and atomic physics than with solid state physics which is of primary interest here. The purpose of this chapter is to present a collection of most of the data on interband transitions.

Torsion-bending-shear interaction for concrete beams

Abstract: A rational, simple method is presented for analysis of the torsion-bending-shear interaction at failure for reinforced concrete beams. Equilibrium equations are studied for observed failure mechanisms. Three mechanisms can occur depending on where the concrete compression zone is formed: in the top (mode t ), in the bottom (mode b ), or in one vertical side (mode s ). In modes t and b , the torsion-bending and the shear-bending interaction are governed by second-degree parabolas, whereas the torsion-shear interaction is governed by an ellipse. In mode s , the interaction surface is a cylinder with an elliptic base in the torsion-shear plane. The interaction in this mode is not influenced by the value of the bending moment, M . Approximations regarding the lengths of the moment lever arms and regarding the shear-carrying capacity are examined. Some test results are presented to demonstrate the accuracy of the method. The main feature of the method is that the vertical shear forces are considered in a simple but rational way.

Amorphous gallium-a free electron metal

Abstract: The optical properties of gallium quench condensed onto substrates at 10K, 30K and 80K have been studied. It is found that films condensed at 10K had a spectrum which could be described extremely well by the Drude equations. At higher temperatures two different spectra were observed, which the authors believe belong to the orthorhombic alpha dn and the monoclinic beta phases of gallium.

Wave packets in strongly dispersive media

Abstract: An analytical study of the evolution of slowly varying wave pulses in strongly dispersive media which takes into account dispersive correction terms involving higher derivatives of the group velocity is given. A higher order differential equation for the envelope function is derived and solved recursively and by means of a procedure based on an analogy with the Schrodinger equation. The equation for the envelope function is used to obtain generalizations of the velocity of the pulse defined as the velocity of the center of inertia, and expressions are derived which determine the spreading of the pulse. Finally, we discuss how the presence of other wave modes affects the primary mode in the multimode case.

Some Convergence Results for Galerkin Methods for Parabolic Boundary Value Problems

Abstract: Publisher Summary This chapter discusses the parabolic initial-boundary value problem. The solution to this problem can be represented in the form of eigenvalues and eigenfunctions of the corresponding two-point boundary value problem. The initial-value problem shares some simple properties with the original parabolic problem. The chapter also discusses the error in the Galerkin method by comparing the eigenfunction expansion with a corresponding discrete eigenfunction expansion.

The rectangular plane stress element by Turner, Pian and Wilson

Abstract: One of the first finite elements presented was that by Turner et al.1 Pian2 presented a hybrid plane stress element and Wilson et al.3 presented a non-conforming displacement element. It is shown that the three resulting stiffness matrices are identical.

Properties of Semiconductors

Abstract: The basic material of a field-effect transistor is a chemically pure and perfect single crystal of silicon. In such a crystal the atoms are ordered in a periodic pattern such that they are all lined up perfectly throughout the entire lattice. Silicon is a tetravalent element so that Si atoms sit at the apices and at the centre of tetrahedra in the lattice. Therefore a single crystal of silicon when seen from a particular direction appears as shown in figure 1.1. The electrical characteristics of such crystals are best described with the help of energy band models, tools derived from band theory.

Mechanical and ventilatory parameters in healthy and diseased newborn infants

Abstract: Neonatal pulmonary disease is a major clinical problem. Different pathogenetic patterns present themselves in a rather uniform clinical picture with increased respiratory frequency, cyanosis, retractions and often “grunting” breathing, often referred to as “respiratory distress”. Increasing knowledge of radiology, pathology, physiology and the clinical course in this syndrome has led to a differentiation in diagnoses. Idiopathic respiratory distress syndrome (IRDS) or hyaline membrane disease (HMD) (39) and aspiration syndrome (34, 40) or type I1 respiratory distress syndrome (37, 43) are dominant but other lung disease, congenital lung malformations and heart diseases often appear witk similar symtoms. Clinical respiratory disturbances may also originate from delayed functional adaptation immediately after birth and from disturbances of the central nervous system. A great deal of controversy exists about clinical diagnostic criteria in neonatal pulmonary disease. The main diagnostic methods used are lung X-ray, estimation of the degree of right-to-left shunt, determination of acid-base status and physical status. Pulmonary function testing has had no place in routine diagnostic work in neonates with pulmonary disease because of the lack of suitable methods. However, as described in the preceding paper, it is possible to analyse the pulmonary mechanics in a neonate without requiring any cooperation of the patient. Analysis of pulmonary mechanics in the neonates is of special interest as one factor of considerable importance in the pathogenesis of IRDS is considered to be an interference with the formation of surfactant (1 7) with consequences for the mechanical properties of the lung. It has also been shown that IRDS Aim of the study