On the accuracy of oxygen pressure measurements

Zirconia-based oxygen sensors are used in many applications and in research. However, the measurement of oxygen partial pressures with these devices is not necessarily correct. Reasons for deviation between sensor reading and nominal composition of the gas mixture are reviewed in this work. The sensor electrochemical permeability is introduced as a starting point. The impact of this oxygen leak on the composition of the surrounding atmosphere is evaluated from mass balances to all species in the system. This procedure shows that even the use of mixtures like CO+CO₂ to establish a given oxygen partial pressure has a limited range of applicability. This is due to the small buffer behavior of some of these mixtures. The gas phase transport of oxygen away from the sensor electrode is also studied. High concentration overpotentials are expected to develop under the circumstances where the buffer behavior of the gas mixture is small. In fact, high concentrations of the CO/CO₂ couple are required both to optimize the gas-mixture buffer effect and the oxygen transport in the gas phase. The latter instead of consisting exclusively of simple diffusion of free oxygen molecules might also involve combined diffusion and counter diffusion of both carbon oxides as oxygen carriers.     

On the accuracy of EPI-based phase contrast velocimetry

For over a decade, echo-planar imaging (EPI) has been used in both the medical and applied sciences to capture velocity fields of fluid flows. However, previous studies have not rigorously confirmed the accuracy of the measurements or sought to understand the limitations of the technique. In this study, a bipolar gradient was added to a flow-compensated EPI pulse sequence to obtain rapid phase contrast images of steady and unsteady flows through two step stenoses. For steady Re = 100 and 258 flows, accuracy was measured through systematic comparisons with CFD simulations, mass flow rate measurements, and spin echo phase contrast images. On average, the EPI image data exhibited velocity errors of 5 to 10 percent, while mass was conserved to within 5.6 percent at each axial position. Compared to spin-echo phase contrast images, the EPI images have 50 percent lower signal-to-noise ratio, larger local velocity errors, and similar mass conservation characteristics. An unsteady flow was then examined by starting a pump and allowing it to reach a steady Re = 100 flow. Accuracy in this case was measured by the consistency between mass flow rate measurements at different axial positions. Images taken at 0.3 s intervals captured the velocity field evolution and showed that 50 to 100 percent errors occur when the flow changes on a time scale faster than the image acquisition time.     

On the accuracy of Hartree-Fock calculations in light nuclei

Kato's theory provides rigorous mathematical estimates for the accuracy of approximate solutions of the eigenvalue equation of an arbitrary Hermitean operator H, the mean-square deviation of H representing a measure for the quality of an approximate solution. Using a quadrupole-quadrupole interaction, we calculate the mean-square deviations of HF, HFP, and PHF solutions for some nuclei in the sd shell, and check the accuracy of the Hartree-Fock solutions by applying Kato's estimates.     

On the performance and accuracy of 2D navigator pulses.

The purpose of this study was to investigate and to optimize the performance of two-dimensional spatially selective excitation pulses used for navigator applications on a clinical scanner. The influence of gradient imperfections, off-resonance effects, and incomplete k-space covering on the pencil beam-shaped spatial excitation profile of the 2D RF pulse was studied. The studies involved experiments performed on phantoms and in vivo. In addition, simulations were carried out by numerical integration of the Bloch equations. The accuracy of positioning of the pencil beam was increased by a factor of three by employing a simple correction scheme for the compensation of gradient distortions. The spatial selectivity of the 2D RF pulse was improved by taking sampling density corrections into account. The 2D RF pulse performance was found to be sufficient to monitor the diaphragm motion even at moderate gradient strength. For applications, where a high spatial resolution is required or a less characteristic contrast is present a strong gradient system is recommended.     

On the accuracy of the wavefunctions calculated by LAPW method

The accuracy and the convergence properties of LAPW wavefunctions are studied using Be metal as an example. We show that radial part of thes-component of wavefunction inside the muffin-tin sphere differs substantially from the exact solution to the radial Schrodinger equation. We find that LAPW method underestimates the energy expectation value in the interstitial region. The inaccuracy of the well-converged wavefunctions occasionally produces significant errors in momentum matrix elements.     

圆介质参考波导特征方程的近似式及其应用

本文将作为参考波导的圆介质波导模式的特征方程进行了有效的近似简化,使之成为独立的四个简单特征方程组。利用这些方程组作为参考波导的特征方程来计算各种复杂波导的传输参数时,能够大大简化计算工作。     

Revealing how network structure affects accuracy of link prediction

Link prediction plays an important role in network reconstruction and network evolution. The network structure affects the accuracy of link prediction, which is an interesting problem. In this paper we use common neighbors and the Gini coefficient to reveal the relation between them, which can provide a good reference for the choice of a suitable link prediction algorithm according to the network structure. Moreover, the statistical analysis reveals correlation between the common neighbors index, Gini coefficient index and other indices to describe the network structure, such as Laplacian eigenvalues, clustering coefficient, degree heterogeneity, and assortativity of network. Furthermore, a new method to predict missing links is proposed. The experimental results show that the proposed algorithm yields better prediction accuracy and robustness to the network structure than existing currently used methods for a variety of real-world networks.     

一类新型激波捕捉格式的耗散性与稳定性分析

高超声速流动是高复杂性的可压缩黏性流动, 其中存在激波、剪切层、激波/激波干扰、激波/边界层干扰、旋涡与分离流动等复杂流场结构. 对其进行准确模拟需要使用低耗散、强鲁棒性的激波捕捉方法. 本文基于一类新型的通量项分裂方法, 提出了一种耗散低且鲁棒性好的激波捕捉格式K-CUSP-X. 对该格式的耗散性和激波稳定性进行了详细的理论分析, 得到了格式激波稳定的数值条件. 推论认为, 迎风格式激波稳定的充分条件为速度扰动量具有衰减性, 数值实验验证了该推论. 研究表明, 该格式与Toro提出的通量分裂格式K-CUSP-T相比, 在保证精确捕捉接触间断的同时, 又具有更好的稳定性, 在激波处不会产生“红玉”现象.     

On the accuracy of soft-photon approximations for resonance free-free transitions

The connection between resonances occuring in the scattering of electrons on atoms (molecules) with or without the presence of a classical laser field is demonstrated for aN-state model-atom in 1st order perturbation theory with respect to the laser field. It is possible to predict the position, the shape and the background of resonances in the laser field from the sole knowledge of the corresponding scattering amplitudes without the laser field. At first, a soft-photon approximation for individual partial wave amplitudes is derived. This approximations is then tested numerically for a 2-state square well potential model. Details about the accuracy of the soft-photon approximation are obtained by comparing with the numerically exact results. Finally our findings are applied to give an interpretation of recent experimental data.     

On the accuracy of the WKB approximation in optical dielectric waveguides

Eigenvalues and field distributions for optical waveguides obtained from the WKB approximation have been compared with those found from geometrical optics and from more rigorous weakly-guiding LP mode theories in the cases of the step-index and parabolic-index profiles. In all cases it is found that the zero-order WKB approximation yields very accurate eigenvalues provided care is taken in the choice of phase factors in the eigenvalue equation. Expressions are deduced for the required phase factors for guides of arbitrary index profile in both two- and three-dimensions, and physical interpretations are given in terms of ray optics. The first-order WKB field distributions are found to give good agreement with the mode fields everywhere except in the vicinity of the caustics.     

On the conformation and the structure of polymacromonomers

Using poly( -norbornenyl polystyrene) as a model we studied the conformation and the structure of branched polymer chains each of whose backbone constitutive units bear a branch. The parameters taken into account are: the polymerisation degree of the backbone, the molar mass of the branches and the solution concentration. The effect of the global conformation of these polymacromonomers, either spherically or cylindrically (vermiform) symmetric, is considered. The compactness of the polymer constitutive unit distribution, the branches and the backbone conformations are considered as functions of the first two parameters above. Significant results are obtained on all of these three points.Received: 24 May 2004, Published online: 21 September 2004PACS: 61.12.Ex Neutron scattering - 61.41. + e Polymers, elastomers, and plastics - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

On the accuracy of wavefunctions obtained by the Fourier grid Hamiltonian method

The quality of wavefunctions obtained by the Fourier grid Hamiltonian (FGH) method is analyzed. The criteria used for judging the quality are the extent to which virial, hypervirial and Hellmann-Feynman theorems are satisfied by the numerically computed FGH-wavefunction. The quality of the FGH-wavefunction is also examined from the point of view of local error in the wavefunction. It is shown that high quality wavefunctions can be obtained from the FGH recipe if the grid length (L) and grid spacings are chosen after properly examining the range of the potential and its nature.     

On the accuracy of double scattering approximation for atmospheric polarization computations

Interpretation of multi-angle spectro-polarimetric data in remote sensing of atmospheric aerosols requires fast and accurate methods of solving the vector radiative transfer equation (VRTE). The single and double scattering approximations could provide an analytical framework for the inversion algorithms and are relatively fast; however accuracy assessments of these approximations for the aerosol atmospheres in the atmospheric window channels have been missing. This paper provides such analysis for a vertically homogeneous aerosol atmosphere with weak and strong asymmetry of scattering. In both cases, the double scattering approximation gives a high accuracy result (relative error ～0.2%) only for the low optical path ～10^?2. As the error rapidly grows with optical thickness, a full VRTE solution is required for the practical remote sensing analysis. It is shown that the scattering anisotropy is not important at low optical thicknesses neither for reflected nor for transmitted polarization components of radiation.     

On the formation of structure and electronic transport

By systematic studies of amorphous systems (and most of the results are transferable to liquid systems) we are able to show that structure formation at early stages is to a high degree the effect of self-organized and hence optimized resonances between macroscopic subsystems. There is, for example, a spherical-periodic resonance, based on momentum exchange between the valence electrons in total as one subsystem, and the forming static structure as the other one. It causes spherical structural periodicity of nearest-neighbour shells at medium distances and is a global effect, giving rise to similar effects as described by Bloch's theorem in crystals. Resonances based on an exchange of angular momentum became apparent too. Accordingly, together with the local quantum chemical effects, global resonances are important as well and both will cooperate to get the most optimal energetic situation for the total system. Occasionally, the global effects even dominate structure formation. We report on different scenarios where the total system is able to optimize the resonances. The resonance model explains major structural features of many liquid and amorphous systems of different types as there are pure elements, binary as well as ternary alloys, metallic glasses, glassy semiconductors, glassy Zintl systems, and light-weight Al–TM alloys (TM: from Ti to Cu). Spherical-periodic order causes pseudogaps or even gaps at the Fermi energy and hence has dramatic influences on any electronic transport. Accordingly, understanding structure formation on the basis of resonances also triggers a deeper understanding of electronic transport properties.     

On the electronic structure of graphite

Comprehending investigations of the electronic structure of polycrystalline as well as monocrystalline graphite have been performed by means of X-ray emission, self absorption and X-ray induced photoemission techniques. On the basis of these combined investigations a model for the determination of the relative cross sections for the photoemission process has been established and applied to graphite, where it yields _s / _p =32. The anisotropy and polarization of the CK-radiation of monocrystalline graphite is discussed in terms of the binding properties of the graphite lattice. The predictions are verified by measurements of the CK-emission employing a crystal monochromator which acts simultaneously as a nearly perfect analyzer for the polarization of the monochromatized radiation. By means of the self absorption technique the unoccupied part of the conduction band has been investigated.     

On the band structure of CdSb

The paper consists of two parts. Throughout the first part only the symmetry properties and the existence of the gap are employed for a study of electron bands in CdSb by means of group theory. A general structure of valence bands is obtained, the possible locations of extrema limiting the gap are discussed, and a criterion for the conservation of the gap after switching off the spin-orbit interaction is deduced.In the second part a simple version of thel-dependent pseudopotential suitable for crystals with many atoms in the elementary cell is proposed and the corresponding structure of energy levels in the point is calculated. The results indicate the starting approximation of free electrons to be reasonable and support the hypothesis that the gap arises even before the spin-orbit effects are accounted for.     

On the structure of analytic renormalization

A direct proof is given that analytic renormalization has an additive structure and hence may be implemented by counterterms in the Lagrangian.