First-Principles Study of Semiconducting Clathrate Ba<Subscript>8</Subscript>Al<Subscript>16</Subscript>Ge<Subscript>30</Subscript>

We calculated electronic structures of Ba₈Al₁₆Ge₃₀ in some Al-Ge framework configurations without nearest-neighbor Al-Al bonds by using a first-principles method. The calculated band structures are similar in outline but different in detail. We also calculated thermoelectric properties by using the electronic structures to analyze the experimental results on a sintered Ba₈Al₁₆Ge₃₀ sample. The calculated properties nearly agree with the experimental results; however, the calculated temperature dependences of electrical conductivity are slightly different from one another, because of differences in electronic structure. In this paper, we discuss the temperature dependence from the viewpoints of nonparabolic band effects.     

Improving the effectiveness of system verification

This special section presents three extended and revised papers which originally appeared in the proceedings of the 12th edition of the conference “Tools and Algorithms for the Construction and Analysis of Systems, TACAS 2006” held in Braga, Portugal late March 2006 as a constituent event of the “European joint conferences on Theory and Practice of Software”. All three papers deal with very promising extensions and integrations of formal verification techniques.     

Hydroformylierung von Buta-1,3-dien und butadienhaltigen Kohlenwasserstoffraktionen nach dem Zweiphasenverfahren

Two-phase Hydroformylation of Buta-1,3-diene and Hydrocarbon Mixtures Containing Buta-1,3-diene The two-phase hydroformylation of buta-1,3-diene with (HRh(CO)[P(m-C₆H₄SO₃Na)₃]₃ the Kuntz catalyst system with excess P(m-C₆H₄SO₃Na)₃) gives high yields of C₅-monoaldehydes. Main product in this mixture is the reactive trans- and cis-pent-3-enal. In consecutive reactions the pent-3-enal is partially hydrogenated to n-pentanal, but also-favoured by the protolytic milieu of the two-phase reaction- aldol condensated to 2-propenylheptadienal. The hydrogenation product of the propenylheptadienal, 2-propylheptanol-1, is a good plasticizer alcohol with a wanted low vapour pressure. Especially promising is the two-phase hydroformylation of the unrefined C₄-fraction of the naphtha pyrolysis: after a more than 95 per cent conversion of the buta-1,3-diene also more than 80 per cent of the n-but-1-ene in the C₄-fraction is hydroformylated mainly to wanted n-pentanal. Less than 5–10% of the n-but-2-enes and the isobutene in the C₄-fraction react under these conditions to oxo-products (2- and 3-methylbutanal). Acetylenic compounds in the C₄-fraction are converted quantitatively into products.     

Clinical usefulness of helical-scanning CT for the evaluation of arteriosclerotic carotid lesions

We examined arteriosclerotic carotid lesions in 76 patients using helical scanning CT (HES-CT), and evaluated the clinical usefulness of this method. A high speed slip-ring X-ray CT system was used. Scanning of the neck was performed for a 30 second period following intravenous bolus injection of non-ionic contrast medium, while couch top movement was 2.0 mm/sec. Multiplannar reconstruction images (MPR-image) and 3-dimensional surface images (3D-image) were reconstructed from the continuous raw data. MPR-images offered axial, coronal and sagittal images in which the lesion could be seen from any direction, and 3D-images that could be freely rotated were obtained by using a track ball and monitor. Eighteen cases were also evaluated by conventional angiography. Excellent HES-CT images were obtained in 73 cases, showing occlusion in 13, stenosis in 34, plaques without calcification in 15 and plaques with calcification in 74 vessels. A good correlation was obtained between HES-CT and angiogram in most cases, and in 6 cases, HES-CT was superior in the detection of stenosis, because it enabled us to observe the lesion from various directions. These results suggested that HES-CT was a minimally invasive, useful diagnostic method for the evaluation of arteriosclerotic carotid disease.     

Fast-neutron sputtering of iron at various temperatures

The sputtering ratio for fast neutrons (average energy approximately 2 MeV) bombarding iron targets was determined at various temperatures. Over the temperature range of 200°C to 340°C, the ratio varied from 3 × 10^?3 to 8 × 10^?3 with no uniform variation with temperature. These values are one to two orders of magnitude higher than predicted by random sputtering models and yet an order of magnitude higher than predicted by an ordered model.     

Probabilistic reachability for parametric Markov models

Given a parametric Markov model, we consider the problem of computing the rational function expressing the probability of reaching a given set of states. To attack this principal problem, Daws has suggested to first convert the Markov chain into a finite automaton, from which a regular expression is computed. Afterwards, this expression is evaluated to a closed form function representing the reachability probability. This paper investigates how this idea can be turned into an effective procedure. It turns out that the bottleneck lies in the growth of the regular expression relative to the number of states (n ^{Θ(log n)}). We therefore proceed differently, by tightly intertwining the regular expression computation with its evaluation. This allows us to arrive at an effective method that avoids this blow up in most practical cases. We give a detailed account of the approach, also extending to parametric models with rewards and with non-determinism. Experimental evidence is provided, illustrating that our implementation provides meaningful insights on non-trivial models.     

Effect of Heat Treatment in Air on the Surface of a Sintered Ba8Al16Si30-Based Clathrate

A polycrystalline silicon-based clathrate of nominal composition Ba₈Al₁₅Si₃₁ was prepared by a combination of arc melting and spark plasma sintering. Its thermal stability in air and the effect on the surface of heat treatment in air, which are of practical importance for use of the material at elevated temperatures, were examined for different temperatures (873 K, 973 K, and 1073 K) and heating times (0–480 h). Thermogravimetry and differential thermal analysis in air in the range 300–1523 K indicated that Ba₈Al₁₅Si₃₁ in the bulk form had relatively good thermal stability in air at high temperatures. X-ray diffraction measurements, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy revealed that a thin layer of oxide (probably a barium aluminate, BaAl₂O₄) was formed on the surface by heat treatment in air. The thickness of the oxide layer increased with increasing temperature and heating time. Growth of the oxide layer can be explained well in terms of a diffusion mechanism. The activation energy for oxidation was estimated to be approximately 201 kJ/mol, which is comparable with that for thermal oxidation of silicon. The chemical composition of the interior of the Ba₈Al₁₅Si₃₁ was found to be stable to heat treatment in air.     

On the mechanical strength of monocrystalline,multicrystalline and quasi‐monocrystalline silicon wafers: a four‐line bending test study

Quasi‐monocrystalline silicon wafers have appeared as a critical innovation in the PV industry, joining the most favorable characteristics of the conventional substrates: the higher solar cell efficiencies of monocrystalline Czochralski‐Si (Cz‐Si) wafers and the lower cost and the full square‐shape of the multicrystalline ones. However, the quasi‐monocrystalline ingot growth can lead to a different defect structure than the typical Cz‐Si process. Thus, the properties of the brand new quasi‐monocrystalline wafers, based on low and high crystal defect densities, have been for the first time studied from a mechanical point of view, comparing their strength with that of both Cz‐Si monocrystalline and typical multicrystalline materials. The study has been carried out employing the four line bending test and simulating them by means of FE models. For the analysis, failure stresses were fitted to a three‐parameter Weibull distribution. High mechanical strength was found in all the cases. However, the quasi‐monocrystalline wafers characterized by large density of bulk defects, due to the noticeable density of extended defects, showed lower fracture tensions. Copyright © 2013 John Wiley & Sons, Ltd.     

Comparison and evaluation of methods for the determination of flammability limits, applied to methane/hydrogen/air mixtures

Different methods, both experimental and numerical, to determine the flammability limits are compared and evaluated, exemplified by a determination of the flammability limits of methane/hydrogen/air mixtures for hydrogen fuel molar fractions of 0, 0.2, 0.4 and 0.6, at atmospheric pressure and ambient temperature. Two different experimental methods are used. The first method uses a glass tube with visual observation of the flame, whereas the second method uses a closed spherical vessel with a pressure rise criterion to determine whether flame propagation has occurred. In addition to these experiments, the flammability limits are determined numerically. Unsteady planar and spherically expanding flames are calculated with a one-dimensional flame code with the inclusion of radiation heat loss in the optically thin limit. Comparison of the experimental results with the results of the planar flame calculations shows large differences, especially for lean mixtures. These differences increase with increasing hydrogen content in the fuel. Better agreement with the experimental results is found for the spherically expanding flame calculations. A limiting burning velocity of 5 cm/s is found to predict the upper flammability limit determined with the tube method very well, whereas the limiting flame temperature approach was found to give poorer agreement. Further analysis indicates that the neglect of flame front instabilities is the probable cause of the large differences between experimental and numerical results at the lower flammability limit.