Showing papers by "Eindhoven University of Technology published in 1980"

Behavior of metallic iron catalysts during Fischer-Tropsch synthesis studied with Mössbauer spectroscopy, X-ray diffraction, carbon content determination, and reaction kinetic measurements

Abstract: The conversion of unpromoted, unsupported metallic iron catalysts into carbides during Fischer-Tropsch synthesis (CO:H2:He = 1:1:3, 1 atm) was studied with Mossbauer spectroscopy, X-ray diffraction, carbon content analysis, and reaction kinetic measurements. From a comparison between experiments at different temperatures and literature data, it is concluded that both reaction conditions and the nature of the iron catalyst determine the combination of carbides that will be formed. Investigation of single-phase carbides revealed that the X-ray diffraction pattern commonly ascribed to a pseudohexagonal carbide Fe2C actually belongs to the carbide ∈′-Fe2.2C. At synthesis temperatures of 513 K and lower, unknown iron-carbon species were found, referred to as FexC. It is believed that FexC represents poorly defined structures between α-Fe and a crystallographic carbide. The behavior of metallic iron catalysts during Fischer-Tropseh synthesis at 513 K was studied in more detail as a function of time. It was found that the rate of hydrocarbon formation was initially low, passed through a maximum, and decreased thereupon, while the conversion of α-Fe into carbides started at a high rate and decreased rapidly. These results can be understood as the consequence of either a competition between bulk carbidization and hydrocarbon synthesis or a relatively slow activation of α-Fe for the formation of hydrocarbons in which bulk carbidization plays no role. Deactivation is caused by the formation of an excessive amount of inactive carbon at the surface of the catalyst.

On Cutting Planes

Abstract: We give a geometrical description of Chvatal's version of Gomory's cutting plane method. Restricting ourselves to rational spaces, we prove that the derived geometrical objects are polyhedra again, and that the method also works for unbounded polyhedra.

Zeta-potential and radiotracer adsorption measurements on EFG α-Al2O3 single crystals in NaBr solutions

Abstract: Radiotracer adsorption measurements (24Na82Br tracer) and ζ-potential measurements were performed on α-Al2O3 single crystals, grown by the EFG process, in 0.01 and 0.05 mole dm−3 NaBr and with varying pH. Evidence was obtained for the adsorption of sodium as well as of bromide ions in the compact part of the electrical double layer. A gel layer is not formed. Surface charge density curves were derived from the adsorption and ζ-potential data. The point of zero charge is at pH ≈ 4.5, whereas the isoelectric point (iep) occurs at pH 3.1–3.5, depending on the NaBr concentration. This difference between iep and pzc is attributed to a greater complexation strength of Al2O3 for Br− ions than for Na+ ions. This explanation is corroborated by the radiotracer adsorption measurements and the low iep therefore cannot be attributed to silica contamination. An acceptable description of the experimental data can be given in terms of the site-binding model, following the approach of Davis et al. [J. Colloid Interface Sci. 63, 480 (1978)]. The inner layer capacitance, required to describe the pH region of predominating Br− adsorption, is about twice as large as that required to describe the pH region of predominating Na+ adsorption.

Model for 1/f; noise in MOS transistors biased in the linear region

Abstract: 1/ f ; Noise calculations and experiments are presented for conductance fluctuations in inversion layers. The layers are biased in the ohmic region at very low drain-source voltages. The model makes use of an experimental fact that competing scattering mechanisms other than lattice scattering lead to a reduction of 1/ f ; noise, but does not consider trapping of charge carriers in surface states as the source of 1/ f ; noise. The free charge carrier distribution and a mobility profile play an important part in the model. The model describes the measured results well. A reduction of the effective mobility with increasing gate voltage is accompanied by a strong reduction of the noise.

1f Noise in p-n diodes

Abstract: 1 f Noise calculations and experiments are presented for silicon p-n diodes. The experimental results can be explained by assuming that the 1 f noise is due to fluctuations in the mobility of free carriers. The ratio between the 1 f noise density and the shot noise density in p-n diodes is characterized by the corner frequency fc where the 1 f noise equals the shot noise. In long diodes the corner frequency is proportional to Hooge's empirical 1 f noise constant α ≈ 10-3, and inversely proportional to the minority carrier lifetime in the base. In short diodes the frequency fc is related to the constant α, the width of the base, and the contact recombination velocity.

1/f; noise model for MOSTs biased in nonohmic region

Abstract: Relations are derived for the 1/f; noise current and for the equivalent input noise voltage of a MOST biased in the nonlinear region. The experimentally obtained results are in agreement with the calculations. In addition the value of power spectrum of the noise current is related to that in the ohmic region. In the last region the 1/f; noise appears to be caused by mobility fluctuations of the charge carriers. In the nonohmic region, the noise consists of two contributions: (i) mobility fluctuations and (ii) number fluctuations in the charge carriers due to fluctuations of the effective gate voltage induced by mobility fluctuations.

On the uniqueness resp. Nonexistence of certain codes meeting the Griesmer bound

Abstract: It will be shown that the following binary linear codes are unique: (n, k, d) = (2{suk} − 2u, k, 2{suk−1} − 2u−1} 1 ⩽ u ⩽ k − 1, (2{suk} −2{suk−2} − 3, k,2{suk−1} − 2{suk−3} − 2), k ⩾ 6, and (2{suk−1} + k, k, 2{suk−2} + 2), k ⩾ 3, k ≠ 5. Also there are exactly 2 non-isomorphic (21, 5, 10) codes. Using these results the non-existence of binary linear (n, k, d) codes meeting the Griesmer bound is proved for 2{suk−2} + 3 ⩽ d ⩽ 2{suk−1} − 2{suk−3} − 4.

The Genealogical Approach to the Layout Problem

Abstract: The concept of the genealogical approach to the layout problem is presented. The system pursues the idea of flexible modules and is capable of dealing with arbitrarily complex tasks. The genealogical tree of the system provides a mainframe for organizing the information flow. Results of the system routines are described in terms of transitions between flexibility classes of modules.

Synthesis of a nonpolar, chemically bonded stationary phase with low residual hydroxyl group content

Abstract: A microporous silica support, LiChrosorb Si-100, has been silanized with octyldimethylchlorosilane and octylmethyldichlorosilane. The repeatability of the silanization procedure was within about 2%. In general, these nonpolar modified silicas still contain too many residual hydroxyl groups, causing bifunctional behaviour of the adsorbent. A partial condensation of surface silanol groups at a drying temperature >200°C, prior to the chemical modification, decreases the residual hydroxyl group content. With respect to this residual polarity, monochlorosilanes appear to be effective. The concentration of bonded octyl chains remains virtually constant up to a drying temperature of 400°C. Owing to silanization, the specific surface decreases by 15–20%, whereas the pore volume decreases by 25%.

Efficient higher order implicit one-step methods for integration of stiff differential equations

Abstract: A new implicit integration method is presented which can efficiently be applied in the solution of (stiff) differential equations. The given formulas are of a modified implicit Runge-Kutta type and areA-stable. They may containA-stable embedded methods for error estimation and step-size control.

Local motions in vitreous systems

Abstract: The dielectric relaxation phenomena (dielectric losses) which are found in crystals and glasses in the region of 50 K (if measured with medium frequencies (103−106 s−1) must be assigned to local motions of ions. They show activation energies of the order of 0.1 eV. Two types of dielectric losses due to local motions can be distinguished: (1) The inherent dielectric losses due to local motions, since they are inherent to situations existing in the original crystal lattice or glass network; and (2) the induced dielectric losses due to local motions, since they are due to situations induced in the crystal lattice or the glass network, for instance by stress or radiation. There are also induced dielectric losses due to local motions of electrons, for instance in radiation induced colour centers. These losses show activation energies of the order of 0.01 eV. In this paper a number of cases described in the early as well as the recent literature is summed up and their physical background is discussed.

Absorption of hydrogen in reduced nickel oxide

Abstract: By repeated oxidation and reduction of nickel in alkaline solution an oxide film is formed on nickel which cannot be reduced further. In this film hydrogen is absorbed during cathodic polarization. The absorbed hydrogen is manifest in the voltammogram as an anodic peak before the Ni(OH)2 peak. This was proved by H-diffusion experiments through nickel foils. From experiments with Ni electrodes covered withα- orβ-Ni(OH)2 films, it can be concluded that the reduced nickel oxide layer on nickel is most likely aβ-Ni(OH)2 layer.

Calculation of optimum sterilization conditions for packed conduction-type foods

Abstract: The basis of the method is the short-cut calculation procedure developed by Thijssen et al. (1978) for sterilization conditions yielding optimum quality retention for conduction heating of packed foods at constant temperatures of the heating and cooling medium. This method is improved and extended to variable temperatures of the heating and cooling medium as occur in continuous sterilizers. This is done by converting the variable temperature into a block temperature-time relationship. By means of computer simulations of the sterilization process, the variable heating temperature is converted into a constant heating temperature and an increased homogeneous initial product temperature yielding the same lethal effect. The variable cooling temperature is converted into a constant cooling temperature and an additional heating time at constant retort temperature yielding the same lethal effect as is obtained during the actual cooling process. On the block temperature-time diagram thus obtained the short-cut calculation method is again applicable. The error introduced by this method in the calculated logarithm of the reduction of microorganisms is less than 20%.