J.P. de Villiers

GAUSSIAN PROCESS MODELING OF CPW-FED SLOT ANTENNAS

Gaussian process (GP) regression is proposed as a structured supervised learning alternative to neural networks for the modeling of CPW-fed slot antenna input characteristics. A Gaussian process is a stochastic process and entails the generalization of the Gaussian probability distribution to functions. Standard GP regression is applied to modeling S11 against frequency of a CPW-fed second- resonant slot dipole, while an approximate method for large datasets is applied to an ultrawideband (UWB) slot with U-shaped tuning stub | A challenging problem given the highly non-linear underlying function that maps tunable geometry variables and frequency to S11=input impedance. Predictions using large test data sets yielded results of an accuracy comparable to the target moment-method-based full-wave simulations, with normalized root mean squared errors of 0.50% for the slot dipole, and below 1.8% for the UWB antenna. The GP methodology has various inherent beneflts, including the need to learn only a handful of (hyper) parameters, and training errors that are efiectively zero for noise-free observations. GP regression would be eminently suitable for integration in antenna design algorithms as a fast substitute for computationally intensive full-wave analysis.

Rietveld and pair distribution function study of Hagg carbide using synchrotron X-ray diffraction

Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe(5)C(2) Hägg carbide (χ-HC) during FT synthesis. Arguably, χ-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of χ-HC to iron oxide, a step-wise decarburization to cementite (θ-Fe(3)C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of χ-HC for the understanding of the synthesis process and for comparison with the first-principles ab initio modelling. Here the results of high-resolution synchrotron X-ray powder diffraction data are reported. The atomic arrangement of χ-HC was confirmed by Rietveld refinement and subsequent real-space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of χ-HC correspond well with that of a pseudo-monoclinic phase of space group Pī [a = 11.5661 (6) Å, b = 4.5709 (1) Å, c = 5.0611 (2) Å, α = 89.990 (5)°, β = 97.753 (4)°, γ = 90.195 (4)°], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure.

On the convexity of the LCCM cost function for DS-CDMA blind multiuser detection

The linearly constrained constant modulus algorithm (LCCMA) applied to blind DS-CDMA multiuser detection was widely considered in the last few years. In some papers, global convergence of the linearly constrained constant modulus (LCCM) algorithm was incorrectly claimed, after which corrections were published by other authors (see the Introduction section for references). In this letter, analysis of the LCCM criterion is extended to prove convexity of the LCCM cost function between any point and the global minimum, subject to certain conditions, by using the definition of a continuous convex function.

Disintegration in high grade titania slags: low temperature oxidation reactions and associated fracture mechanics of pseudobrookite

Abstract Cooling of solidified titania slag in air results in the oxidation of a pseudobrookite phase (M3O5 where M = Fe, Ti) to form phases that are associated with the disintegration of the solid slag. Extensive disintegration of the slag at temperatures below 773 K is accompanied by the formation of a disordered MOx phase, which is a unit cell intergrowth of anatase (TiO2), and M3O5 structural elements. In situ X-ray diffraction measurement and Rietveld refinement of the pseudobrookite unit cell parameters yielded values for spontaneous strain associated with the oxidation of the slag. In the most reactive samples oxidised isothermally at 473 K there is 0·6% contraction along the a-axis, a 0·3% expansion along the b-axis and the c-axis expands by 1%, while still retaining the pseudobrookite structure. Values for the elastic constants and Youngs moduli for M3O5 were calculated using the GULP crystal optimisation code. These values, together with experimental values for the spontaneous strain tensor and the fracture toughness of M3O5, were used to calculate the critical crack size for crack propagation. Crack sizes of 0·9–9 μm are sufficiently large to explain the cracking that preceded the extensive formation of MOx and the subsequent segmentation of the solid slag into very small particle sizes. Lattice energy calculations of the compounds anatase (TiO2), FeTi2O5, and FeTi5O11 with GULP showed that the latter is unstable by 173 kJ mole−1 as compared to a mixture of anatase (TiO2) and FeTi2O5 and will eventually revert to anatase (TiO2) and M3O5.

Disintegration in high-grade titania slags: low temperature oxidation reactions of ferro-pseudobrookite

Abstract The predominant phase in commercial high-grade solidified titania slags is a compound with the pseudobrookite or M3 O5 structure in which a substantial proportion of the titanium is in the trivalent oxidation state. Cooling of the slag in air results in the oxidation of the pseudobrookite to form phases that are associated with the disintegration of the solid slag. The presence of excessive amounts of fine-grained oxidised material renders it unsuitable for subsequent fluidised bed chlorination for the production of TiO2 pigment. Depending on the degree of reduction, the pseudobrookite phase has a composition with variable iron content. In high-grade slag, the stoichiometry of this phase is approximating that of (Fe0.27 Mg0.07 Al0.04 Ti3+ 1.35Ti4+1.35)O5. Single crystal structure analysis revealed no significant change in the room temperature structure at temperatures of 250°C and 350°C in air. At temperatures above 700°C, the oxidation of the slag is associated with the formation of mainly anatase, rutile, and oxidised M3O 5. No discernible disintegration takes place during this reaction. Between ~550°C and 700°C, the reaction is a slow one involving the formation of anatase and oxidised M3O5. Very little disintegration is associated with this reaction. Below ~550°C, the dominant reaction changes to a fast reaction involving the formation of a single disordered phase (MOx), related to both M3 O5 and anatase, and this results in extensive disintegration. This oxidation takes place in two steps. Initially, large cracks form parallel to the a-crystallographic axis direction in the precursor M3 O5 crystals. Triangular domains of the oxidised phase having a composition MOx form on the crack surfaces. Secondary cracks form along the c-axis directions, dissecting the domains. The cracks originating from adjacent domains join up to segment the crystal into small rectangular fragments as small as 10μm in size.

The effect of coating parameters and annealing on the crushing strength of TRISO coated particles

Abstract The crushing strength (load at fracture) of coated particles was measured by compression between soft metal anvils. The method requires no sample preparation and can be used as a quality control method as well as a valuable tool for comparing different coating conditions during the manufacture of TRISO particles. Batches of coated particles manufactured with different coating parameters were prepared and tested. Batches prepared under different conditions exhibited significant differences in crushing strength. Higher argon concentrations in the coater gas mixture resulted in higher crushing strength. Anomalies in the crushing strength of particles are related to defects and possibly residual stresses produced during coating. The influence of annealing at 1950°C on crushing strength was also investigated. The average crushing strength of batches of particles decreased with annealing. Different preparation methods showed a marked difference in the level of deterioration of the particles with annealing. Batches produced with 80% argon gas mixture at 1300°C showed the greatest reduction in crushing strength. An inverse correlation was found between the crushing strength and the uranium that is leached from batches. The particles with the lowest crushing strength would also be more susceptible to mechanical damage during handling and consequently also to leaching of the uranium during leach tests.

An assessment of the production of fine material in iron ore sinter

Abstract Iron ore sinter is produced from fine grained ore in order to provide a direct charge to the blast furnace. During the sinter production process, fine sinter is produced that is not acceptable as feedstock for the blast furnace. The aim of this study is to compare the composition and phase chemistry of the fine and normal sinter to determine if chemical or operational changes can be made to reduce the amount of fine sinter formed. This will be achieved through the analysis of real industrial sinter samples and sinter test pot samples. It has been concluded that the production of fine sinter is a direct function of the amount of haematite present in the sinter. Suggestions to decrease the amount of fine sinter formed include: uniform heat distribution during ignition, pO2 alteration by reductant addition, lower ignition temperature, regulating the cooling regime and decreasing the grain size of lime.AbstractIron ore sinter is produced from fine grained ore in order to provide a direct charge to the blast furnace. During the sinter production process, fine sinter is produced that is not acceptable as feedstock for the blast furnace. The aim of this study is to compare the composition and phase chemistry of the fine and normal sinter to determine if chemical or operational changes can be made to reduce the amount of fine sinter formed. This will be achieved through the analysis of real industrial sinter samples and sinter test pot samples. It has been concluded that the production of fine sinter is a direct function of the amount of haematite present in the sinter. Suggestions to decrease the amount of fine sinter formed include: uniform heat distribution during ignition, pO2 alteration by reductant addition, lower ignition temperature, regulating the cooling regime and decreasing the grain size of lime.

XRD analysis and evaluation of iron ores and sinters

Powder X-ray diffraction is increasingly being used for the characterization of iron ores, sinters, and pellets. This is the result of continuing developments in instrumentation and software that enabled scientists and engineers to quickly analyze and quantify minerals and phases in often nonhomogeneous materials. The method is briefly introduced and the various applications of the method are described together with its advantages and limitations. Typical applications in iron ore processing and in sinter production are discussed with practical examples.

Analysis of the LCDCM criterion for DS-CDMA blind multiuser detection

In this paper we will see that the linearly constrained differential constant modulus (LCDCM) cost criterion is well suited for blind multiuser detection. Unlike the linearly constrained constant modulus (LCCM) cost function, it is not dependent on desired user amplitude to ensure a single optimum point. The global minimum of the LCDCM cost function will be established by proving the Hessian matrix positive definite at the single solution where the gradient function is zero.

Theoretical evaluation and simulation of a novel phase locked loop with low phase noise and fast settling times

This paper contains the theoretical description and simulation results of a novel phase detector. This method, originally developed by Zuta (1998) for frequency synthesizers, can be extended to carrier synchronisation, greatly increasing the pull-in range and reducing phase noise and settling time compared to conventional PLL techniques. This makes it ideally suited for frequency hopping and burst mode mobile digital communication applications.