Dan Luss

Combustion synthesis and characterization of Sr and Ga doped LaFeO3

Abstract A homogeneous perovskite oxide La 0.5 Sr 0.5 Ga 0.2 Fe 0.8 O 3− δ (LSGFO) has been synthesized by self-propagating high-temperature synthesis (SHS). The homogeneity and the particle size of the combustion product may be increased by decreasing the cooling rate of the sample, either by increasing the sample diameter or by controlling the post-combustion temperature. The particle morphology depends on the gaseous and molten compounds formed as the combustion front passes through the sample. The perovskite oxide maintained its cubic structure at all temperatures in air. However, a decomposition of LSGFO occurred at 860°C under a simulated syngas environment (22% CH 4 +57% H 2 +21% CO 2 , oxygen partial pressure of about 10 −17 atm). The maximum electrical conductivity of a disc prepared from the SHS powder was 142 S/cm at 580°C under oxygen pressure of 1 atm. The LSGFO may be suitable for use as a membrane in syngas production since its thermal expansion in air and reducing environment are rather close at high temperature.

Pattern selection in controlled reaction–diffusion systems

When a chemical reaction is carried out on a catalytic ribbon, the spatial average temperature of which is kept constant by electrical heating, spatiotemporal temperature patterns form when the uniform steady state is unstable at the set temperature. Numerical simulations reveal periodic and aperiodic patterns of moving pulses, ‘‘breathing’’ pulses, or stationary and oscillatory fronts. The transitions between some of these patterns are intricate and proceed via global bifurcations. Bifurcation maps of parameter regions leading to specific patterns are used to gain insight into pattern formation and organization of these parameter regions. The relations among the dynamics of the uncontrolled system, the ribbon length, and the selected pattern are discussed. Similar patterns are expected to evolve in other reaction–diffusion systems subject to control of space‐averaged properties.

Impact of global interaction on pattern formation on a disk

Global interaction introduces a new mode of communication among all surface elements and may lead to the formation of a very rich variety of spatiotemporal patterns on a disk some of which cannot exist in its absence. While some of the motions on a disk are analogous to those observed in one‐dimensional systems, some, such as targets and spirals, exist only in two‐dimensional systems. The global interaction stabilizes motions such as target patterns, which are not stable in its absence. It may, however, destabilize the spiral motion and cause the spiral tip to meander in a rather complex fashion and in some cases to exit the disk. The global interaction also increases the sensitivity of the system to the initial conditions so that qualitatively different patterns may be obtained when different initial conditions are used.

Patterns of temperature pulses on electrically heated catalytic ribbons

Abstract A rich and intricate variety of spatiotemporal temperature patterns is predicted to exist on the surface of a thin catalytic ribbon, the average temperature of which is kept by electrical heating at a preset value, when the catalytic surface undergoes slow activation-deactivation. The control of the average temperature introduces an integral constraint that generates patters that are not observed in its absence. This one-dimensional system exhibits (in addition to a chaotic behavior) four basic motions: unidirectional pulses, back-and-forth pulses, antiphase oscillation, and a source point from which two pulses emanate and move in opposite directions. Other motions we observed are combinations and transitions among these four. Several of the motions have already been observed experimentally. At some transitions, spatial structures appear to change discontinuosly and without hysteresis, suggesting global bifurcations.

Efficient bifurcation analysis of periodically-forced distributed parameter systems

Abstract Changes in the qualitative features of the bifurcation diagrams or the dynamic features of forced periodic systems occur at singular points, which satisfy certain defining conditions. The loci of these singular points may be constructed by a continuation procedure and used to bound parameter regions with qualitatively different features. When the model of a forced periodic system is a set of partial differential equations, construction of these loci may require extensive computational time, making this task often impractical. We present here a novel, very efficient numerical method for construction of these loci. The procedure uses Frechet differentiation to simplify the determination of the defining conditions and the Broyden inverse update method to accelerate the iterative steps involved in the shooting method. The procedure is illustrated first by construction of a map of parameter regions with qualitatively different bifurcation diagrams for an adiabatic reverse-flow reactor (RFR), the direction of feed to which is changed periodically. We then construct a map of parameter regions in which a cooled RFR has qualitatively different dynamic features. Both maps reveal surprising features.

Hot zone formation during carbon monoxide oxidation in a radial flow reactor

Abstract A high-temperature zone formed during the atmospheric oxidation of carbon monoxide on the external surface of a radial-flow reactor, which was a hollow cylinder of reticulated alumina ( 32 mm o.d, 25 mm i.d, and 102 mm high) impregnated with Pd. The reactor was placed horizontally inside a vessel with a sapphire window and a set of gold-plated mirrors enabled continuous monitoring of the entire cylindrical reactor surface temperature by an infrared camera. The high-temperature regions formed close to the extinction of the uniformly ignited state and was separated by a sharp temperature front from the surrounding cold region. The temperature difference between the cold and hot regions was up to 150°C for a feed containing 6 vol% carbon monoxide and 70 vol% O 2 . The size of the hot zone, which was of the order of a few cm, decreased upon cooling of the vessel. The slow motion of the temperature fronts (breathing) caused complex oscillations in the size of the hot zone and the overall reaction rate.

Pattern formation on a nonuniformly active ring

A rich variety of spatiotemporal patterns exists on reactive rings having a nonuniform activity. Simulations of a case with a nonuniformity indicate that when the local phase‐plane features change along the ring, novel motions evolve which cannot form on a uniformly active ring. An example is a rotating pulse which can move only in one direction but not in the other. The nonuniformity increases the number of possible patterns which can exist on a specific ring relative to that existing on a uniformly active one. Thus the nonuniformity increases the sensitivity of the observed patterns to changes in the initial conditions or perturbations.

Hot zones formation in packed bed reactors

Abstract Stationary and complex moving hot zones were observed during atmospheric oxidation of carbon monoxide on the top surface of a shallow packed bed, consisting of several layers of spherical catalytic pellets (Pd/Al2O3). The test reaction was atmospheric oxidation of carbon monoxide. The reactor was run under conditions for which steady-state multiplicity and hot zone existed for some feed temperatures. The size of the hot zones was much larger than that of individual particles. IR imaging revealed that the hot and cold regions (temperature difference of the order of 100°C) were separated by a sharp (about 3 mm wide) temperature front. A very intricate periodic motion in which the hot zone repeatedly split and coalesced was observed in the shallow packed bed reactor. The transition from the branch of uniformly ignited to the states with a hot region was usually supercritical. It is not yet clear which rate processes generate the transversal hot zones in uniform packed bed reactors.

Electric Fields Produced by High-Temperature Metal Oxidation

Transient electric fields of up to 1.5 V were generated during the high temperature oxidation of pure metals (Mg. Al. Ti, Zr, Hf, Cr, Mn, Fe, Co, and Ni) either by gaseous oxygen or by solid peroxides or perchlorates. These time-varying fields are most probably due to ion formation and motion in and behind the moving high temperature reaction zone, which creates an electric double layer within individual particles. The shape and magnitude of the induced temporal electric signal depends upon the reactant properties, mode of the front propagation, and mobility of the ions formed. Unsteady combustion front propagation (occurring during the oxidation of Mg, Al, and Nb) generates an oscillatory electrical signal Metals belonging to the same group in the periodic table (Ti, Zr, and Hf: Ni and Cool and whose products have the same oxidation state generate qualitatively similar electrical signals. The sign and magnitude of the transient electric potential generated during metal oxidation by perchlorates or peroxides are dominated by the chemoioization processes involved in the decomposition of a solid oxidizer.

Chemomagnetic fields produced by solid combustion reactions

We report here the observation of chemomagnetism, generation of a magnetic field by rapid high-temperature solid reactions producing various oxides. The low-level transient magnetic fields were measured with a high-Tc superconducting quantum interference device magnetometer during the synthesis of both nonferromagnetic and ferromagnetic (ferrite) compounds. The magnetic field was most likely produced by chemoionization processes generated by the moving high-temperature reaction zone. The permanent magnetic field formed by the synthesis of ferromagnetic materials depended on the difference between the combustion temperature and the Curie temperature of the product.