By using electrochemical and EHD impedances, CaCO3 accelerated deposition from carbonically pure ... more By using electrochemical and EHD impedances, CaCO3 accelerated deposition from carbonically pure waters on a rotating platinum disc electrode can be characterized through the kinetics of oxygen reduction on a partially blocked surface. An approximate model of ...
The efficiency of chemical antiscale treatments was assessed by various techniques: chronoamperom... more The efficiency of chemical antiscale treatments was assessed by various techniques: chronoamperometry, electrogravimetry and impedance measurement. Electrogravimetry gave data directly readable by the user. The particular chemistry of the additives may give results more difficult to interpret through chronoamperometry, where the residual current often increases when the quantity of antiscale chemical species increases.
The efficiency of physical antiscale treatments of water was evaluated by various techniques, suc... more The efficiency of physical antiscale treatments of water was evaluated by various techniques, such as chronoamperometry, electrochemical impedance and chronoelectrogravimetry. It was found that the antiscale electrolytic treatment is more efficient than magnetic treatment, at least for the commercial devices used here, and that it also has a longer lasting effect. The electrolytic process generates CaCO3 nuclei in suspension in the bulk water, which provokes homogeneous scaling rather than heterogeneous scaling on the walls which is to be avoided. If the small crystals generated in the bulk are eliminated by filtration, surface scaling is again observed.
Calcareous deposits were investigated by electrochemical impedance spectroscopy for electrochemic... more Calcareous deposits were investigated by electrochemical impedance spectroscopy for electrochemically accelerated scaling. Firstly, the oxygen reduction was studied in a solution of low ionic strength similar to the usual scaling water solutions. Its reaction mechanism can be described by an electron transfer limited by the mass transport in the solution bulk. Secondly, by measuring the impedance on a totally covered
IEEE Transactions on Microwave Theory and Techniques, 2000
A Method of Moments (MoM) integral equation formulation of a Generalized Scattering Matrix (GSM) ... more A Method of Moments (MoM) integral equation formulation of a Generalized Scattering Matrix (GSM) is presented for the full-wave analysis of interactive planar electric and magnetic discontinuities in waveguide. This was developed to efficiently handle a variety of waveguide-based strip-to-slot transitions especially on thin substrates. This single matrix formulation replaces the problematic procedure of cascading individual GSMs of an electric (strip) layer, a thin substrate, and a magnetic (slot) layer.
IEEE Transactions on Microwave Theory and Techniques, 1999
Microwave and millimeter-wave systems have generally been developed from a circuit perspective wi... more Microwave and millimeter-wave systems have generally been developed from a circuit perspective with the effect of the electromagnetic (EM) environment modeled using lumped elements or N-port scattering parameters. The recent development of the local reference node concept coupled with steady-state and transient analyses using state variables allows the incorporation of unrestrained EM modeling of microwave structures in a circuit simulator. A strategy implementing global modeling of electrically large microwave systems using the circuit abstraction is presented. This is applied to the modeling of a quasi-optical power-combining amplifier
IEEE Transactions on Microwave Theory and Techniques, 1999
An electric-field integral-equation formulation discretized via the method of moments (MoM) is pr... more An electric-field integral-equation formulation discretized via the method of moments (MoM) is proposed for the analysis of arbitrarily shaped planar conductive layers in a shielded guided-wave structure. The method results in a generalized scattering matrix (GSM) for the planar structure and can be used with other GSM's, derived using this or other techniques, to model cascaded structures in waveguide. The Kummer transformation is applied to accelerate slowly converging double series expansions of impedance matrix elements obtained in the MoM solution. In this transformation, the quasi-static part associated with a singularity of the electric-type Green's function in the region of a conductive layer is extracted and evaluated in terms of modified Bessel functions, resulting in a dramatic reduction of terms in a double series summation. The proposed technique permits the modeling of a variety of conductive frequencyselective surfaces, including quasi-optical grids and patch arrays for application to spatial power combining.
An electric eld integral equation formulation discretized via the method of moments (MoM) is prop... more An electric eld integral equation formulation discretized via the method of moments (MoM) is proposed for the analysis of arbitrarily shaped planar conductive lay- ers in a shielded guided-wave structure. The method re- sults in a Generalized Scattering Matrix (GSM) for the pla- nar structure and can be used with other GSMs, derived using this or other techniques, to model
IEEE Transactions on Microwave Theory and Techniques, 1999
The method of moments (MoM) in conjunction with the generalized scattering matrix (GSM) approach ... more The method of moments (MoM) in conjunction with the generalized scattering matrix (GSM) approach is proposed to analyze transverse multilayered structures in a metal waveguide. The formulation incorporates ports as an integral part of the GSM formulation, thus, the resulting model can be integrated with circuit analysis. The proposed technique permits the modeling of interactive discontinuities due to the consideration of a large number of modes in the cascade. The GSM-MoM method can be successfully applied to the investigation of a variety of shielded multilayered structures, iris coupled filters, determining the input impedance of probe excited waveguides, and of waveguide-based spatial power combiners.
An integrated electromagnetic and circuit simulation environment is developed for the simulation ... more An integrated electromagnetic and circuit simulation environment is developed for the simulation of spatial power combining systems. The analysis incorporates surface modes, nonuniform excitation and full nonlinear effects. The simulation tools are used to predict the performance of a 2 by 2 quasi-optical grid amplifier
IEEE Transactions on Microwave Theory and Techniques, 1998
A spatially distributed radio-frequency (RF) circuit, microwave, or millimeter-wave circuit does ... more A spatially distributed radio-frequency (RF) circuit, microwave, or millimeter-wave circuit does not have a global reference node as required in conventional nodal analysis. Instead, local reference nodes associated with ports are required. This paper adapts modified nodal analysis to accommodate spatially distributed circuits, allowing conventional harmonic balance and transient simulators to be used.
Medium (200 to 400°C) to high (600 to 800°C) temperature corrosion of technical carbons (Acheson ... more Medium (200 to 400°C) to high (600 to 800°C) temperature corrosion of technical carbons (Acheson graphites) have been investigated in alkali chloride melts at chlorine evolving anodes. At low temperature in chloride melts containing free Lewis acid (AlCl3) no chlorine is evolved — even at high current densities — because chlorine, together with aluminium chloride, instantaneously form intercalation compounds with graphite and, as a consequence, the carbon desintegrates very rapidly. At 200°C carbon is consumed anodically in a C/Cl of molar ratio ≈ 70/1. With increasing temperature Scheson graphites become more stable so that at 700°C short term destruction cannot be observed in melts which contain free Lewis acid. Chlorine corrosion of carbon electrodes in purified basic alkali chloride melts, which are free of oxygen carriers and, in particular, free of water at temperatures between 600 and 800°C in basic chloride melts, is an electrochemical reaction proceeding at low current densities slower than anticipated from thermodynamic data for carbon chlorination equilibria. The anodic carbon corrosion reaction has an activation energy of only 50 kJ mol−1 and its rate increases with increasing anode potential, or anodic current densities (rate: α exp (i)). At a technical current density of 0.4 A cm−2 at 700°C the corrosion rate is estimated to be of the order of centimeters per year, rendering carbon anodes dimensionally unstable. Most important is to note that apart from CCl4, chlorinated carbon compounds (olefins and arenes) are generated as side-products which are noxious and ecologically dangerous and must not be released from processes which use carbon anodes for chlorine evolution from salt melts.
By using electrochemical and EHD impedances, CaCO3 accelerated deposition from carbonically pure ... more By using electrochemical and EHD impedances, CaCO3 accelerated deposition from carbonically pure waters on a rotating platinum disc electrode can be characterized through the kinetics of oxygen reduction on a partially blocked surface. An approximate model of ...
The efficiency of chemical antiscale treatments was assessed by various techniques: chronoamperom... more The efficiency of chemical antiscale treatments was assessed by various techniques: chronoamperometry, electrogravimetry and impedance measurement. Electrogravimetry gave data directly readable by the user. The particular chemistry of the additives may give results more difficult to interpret through chronoamperometry, where the residual current often increases when the quantity of antiscale chemical species increases.
The efficiency of physical antiscale treatments of water was evaluated by various techniques, suc... more The efficiency of physical antiscale treatments of water was evaluated by various techniques, such as chronoamperometry, electrochemical impedance and chronoelectrogravimetry. It was found that the antiscale electrolytic treatment is more efficient than magnetic treatment, at least for the commercial devices used here, and that it also has a longer lasting effect. The electrolytic process generates CaCO3 nuclei in suspension in the bulk water, which provokes homogeneous scaling rather than heterogeneous scaling on the walls which is to be avoided. If the small crystals generated in the bulk are eliminated by filtration, surface scaling is again observed.
Calcareous deposits were investigated by electrochemical impedance spectroscopy for electrochemic... more Calcareous deposits were investigated by electrochemical impedance spectroscopy for electrochemically accelerated scaling. Firstly, the oxygen reduction was studied in a solution of low ionic strength similar to the usual scaling water solutions. Its reaction mechanism can be described by an electron transfer limited by the mass transport in the solution bulk. Secondly, by measuring the impedance on a totally covered
IEEE Transactions on Microwave Theory and Techniques, 2000
A Method of Moments (MoM) integral equation formulation of a Generalized Scattering Matrix (GSM) ... more A Method of Moments (MoM) integral equation formulation of a Generalized Scattering Matrix (GSM) is presented for the full-wave analysis of interactive planar electric and magnetic discontinuities in waveguide. This was developed to efficiently handle a variety of waveguide-based strip-to-slot transitions especially on thin substrates. This single matrix formulation replaces the problematic procedure of cascading individual GSMs of an electric (strip) layer, a thin substrate, and a magnetic (slot) layer.
IEEE Transactions on Microwave Theory and Techniques, 1999
Microwave and millimeter-wave systems have generally been developed from a circuit perspective wi... more Microwave and millimeter-wave systems have generally been developed from a circuit perspective with the effect of the electromagnetic (EM) environment modeled using lumped elements or N-port scattering parameters. The recent development of the local reference node concept coupled with steady-state and transient analyses using state variables allows the incorporation of unrestrained EM modeling of microwave structures in a circuit simulator. A strategy implementing global modeling of electrically large microwave systems using the circuit abstraction is presented. This is applied to the modeling of a quasi-optical power-combining amplifier
IEEE Transactions on Microwave Theory and Techniques, 1999
An electric-field integral-equation formulation discretized via the method of moments (MoM) is pr... more An electric-field integral-equation formulation discretized via the method of moments (MoM) is proposed for the analysis of arbitrarily shaped planar conductive layers in a shielded guided-wave structure. The method results in a generalized scattering matrix (GSM) for the planar structure and can be used with other GSM's, derived using this or other techniques, to model cascaded structures in waveguide. The Kummer transformation is applied to accelerate slowly converging double series expansions of impedance matrix elements obtained in the MoM solution. In this transformation, the quasi-static part associated with a singularity of the electric-type Green's function in the region of a conductive layer is extracted and evaluated in terms of modified Bessel functions, resulting in a dramatic reduction of terms in a double series summation. The proposed technique permits the modeling of a variety of conductive frequencyselective surfaces, including quasi-optical grids and patch arrays for application to spatial power combining.
An electric eld integral equation formulation discretized via the method of moments (MoM) is prop... more An electric eld integral equation formulation discretized via the method of moments (MoM) is proposed for the analysis of arbitrarily shaped planar conductive lay- ers in a shielded guided-wave structure. The method re- sults in a Generalized Scattering Matrix (GSM) for the pla- nar structure and can be used with other GSMs, derived using this or other techniques, to model
IEEE Transactions on Microwave Theory and Techniques, 1999
The method of moments (MoM) in conjunction with the generalized scattering matrix (GSM) approach ... more The method of moments (MoM) in conjunction with the generalized scattering matrix (GSM) approach is proposed to analyze transverse multilayered structures in a metal waveguide. The formulation incorporates ports as an integral part of the GSM formulation, thus, the resulting model can be integrated with circuit analysis. The proposed technique permits the modeling of interactive discontinuities due to the consideration of a large number of modes in the cascade. The GSM-MoM method can be successfully applied to the investigation of a variety of shielded multilayered structures, iris coupled filters, determining the input impedance of probe excited waveguides, and of waveguide-based spatial power combiners.
An integrated electromagnetic and circuit simulation environment is developed for the simulation ... more An integrated electromagnetic and circuit simulation environment is developed for the simulation of spatial power combining systems. The analysis incorporates surface modes, nonuniform excitation and full nonlinear effects. The simulation tools are used to predict the performance of a 2 by 2 quasi-optical grid amplifier
IEEE Transactions on Microwave Theory and Techniques, 1998
A spatially distributed radio-frequency (RF) circuit, microwave, or millimeter-wave circuit does ... more A spatially distributed radio-frequency (RF) circuit, microwave, or millimeter-wave circuit does not have a global reference node as required in conventional nodal analysis. Instead, local reference nodes associated with ports are required. This paper adapts modified nodal analysis to accommodate spatially distributed circuits, allowing conventional harmonic balance and transient simulators to be used.
Medium (200 to 400°C) to high (600 to 800°C) temperature corrosion of technical carbons (Acheson ... more Medium (200 to 400°C) to high (600 to 800°C) temperature corrosion of technical carbons (Acheson graphites) have been investigated in alkali chloride melts at chlorine evolving anodes. At low temperature in chloride melts containing free Lewis acid (AlCl3) no chlorine is evolved — even at high current densities — because chlorine, together with aluminium chloride, instantaneously form intercalation compounds with graphite and, as a consequence, the carbon desintegrates very rapidly. At 200°C carbon is consumed anodically in a C/Cl of molar ratio ≈ 70/1. With increasing temperature Scheson graphites become more stable so that at 700°C short term destruction cannot be observed in melts which contain free Lewis acid. Chlorine corrosion of carbon electrodes in purified basic alkali chloride melts, which are free of oxygen carriers and, in particular, free of water at temperatures between 600 and 800°C in basic chloride melts, is an electrochemical reaction proceeding at low current densities slower than anticipated from thermodynamic data for carbon chlorination equilibria. The anodic carbon corrosion reaction has an activation energy of only 50 kJ mol−1 and its rate increases with increasing anode potential, or anodic current densities (rate: α exp (i)). At a technical current density of 0.4 A cm−2 at 700°C the corrosion rate is estimated to be of the order of centimeters per year, rendering carbon anodes dimensionally unstable. Most important is to note that apart from CCl4, chlorinated carbon compounds (olefins and arenes) are generated as side-products which are noxious and ecologically dangerous and must not be released from processes which use carbon anodes for chlorine evolution from salt melts.
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Papers by Ahmed Khalil