Jan A. Puszynski

Magnetic field enhanced thermal conductivity in heat transfer nanofluids containing Ni coated single wall carbon nanotubes

In this paper, we report that the thermal conductivity (TC) of heat transfer nanofluids containing Ni coated single wall carbon nanotube can be enhanced by applied magnetic field. A reasonable explanation for these interesting results is that Ni coated nanotubes form aligned chains under applied magnetic field, which improves thermal conductivity via increased contacts. On longer holding in magnetic field, the nanotubes gradually move and form large clumps of nanotubes, which eventually decreases the TC. When we reduce the magnetic field strength and maintain a smaller field right after TC reaches the maximum, the TC value can be kept longer compared to without magnetic field. We attribute gradual magnetic clumping to the gradual cause of the TC decrease in the magnetic field. We also found that the time to reach the maximum peak value of TC is increased as the applied magnetic field is reduced. Scanning electron microscopy images show that the Ni coated nantubes are aligned well under the influence of a ...

Processing and Ignition Characteristics of Aluminum-Bismuth Trioxide Nanothermite System

During the past few years, significant progress has been made in the development of new nanoenergetic materials consisting of mixtures of metal and oxidizer nanopowders. It has been found that such reacting mixtures release energy by 2 to 3 orders of magnitude faster than similar systems consisting of micron-size reactants. In some cases, combustion-front velocities reach hundreds of meters per second. These new reacting systems find applications in both civilian and military sectors, including fast vaporization of active chemical components, fast heating of thermal batteries and main ingredients of new environmentally benign percussion primers or electric matches. This paper presents experimental results on ignition and combustion front propagation characteristics in the Al-Bi 2 O 3 nanothermite system. The effect of different coatings and inhibitors of the reaction of aluminum with water in the presence and without the presence of bismuth trioxide is discussed. In addition, thermodynamic analysis of the Al-Bi 2 O 3 reacting system and reaction kinetics measurements using differential scanning calorimetry are presented. Electrostatic discharge sensitivities of Al-Bi 2 O 3 , Al-MoO 3 , and Al-Fe 2 O 3 nanothermite systems were determined. It was found that all investigated nanothermite systems showed very high levels of electrostatic discharge sensitivity in the form of dry and loose powder.

Experimental Study on the Synthesis of Titanium and Tantalum Nitrides in the Self-Propagating Regime

Abstract A detailed experimental study of the synthesis of transition metal nitrides via combustion reactions between gaseous nitrogen and the respective metal powders at elevated pressures is presented. Two representative combustion reactions were studied: between tantalum and nitrogen, which proceeds without the formation of any liquid phase, and between titanium and nitrogen, which is accompanied by melling of the reactant titanium powder. The effects of nitrogen pressure, metal particle size, porosity of the metal sample and dilution of the solid phase on the combustion characteristics were studied and optimum conditions for the synthesis of the respective nitrides were determined. Values of the apparent activation energies for the two reactions were extracted, based on combustion characteristics measurements.

On the Dynamics of Equations Describing Gasless Combustion in Condensed Systems

Abstract Abstract— The stability of the propagating front in condensed phase gasless combustion was investigated numerically. The combustion process may be stable or unstable depending on the values of the activation energy and the heat of reaction. In the case of pulsating combustion, regimes of single as well multiple oscillations were observed. A regime characterized by very complex oscillatory patterns and lack of periodicity was identified as chaotic.

Experimental study of ignition and extinction waves and oscillatory behavior of a tubular nonadiabatic fixed bed reactor for the oxidation of carbon monoxide

Abstract An experimental study of axial temperature profiles in a nonadiabatic tubular fixed bed reactor has been made under the transient operation. The catalytic carbon monoxide oxidation occurring on a Pt/alumina catalyst has been used. Ignition and extinction waves have been studied both in a short and a long catalyst bed. It is shown that in a short bed ignition and extinction occur at the reactor outlet while for a long bed the ignition and extinction process takes place inside the reactor and is not influenced by the outlet part of the reactor. A systematic measurement indicates that in a narrow range of inlet parameters the integral nonadiabatic nonisothermal reactor may operate in an oscillatory state. These oscillations can be periodic or aperiodic.

H2 generation from two-step thermochemical water-splitting reaction using sol-gel derived SnxFeyOz

H2 has a great potential to replace fossil fuels and contribute to clean energy by reducing the environmental carbon foot-print. This study reports H2 generation from a thermochemical water-splitting reaction using sol-gel derived SnxFeyOz powders. The sol-gel synthesis involved the addition of SnCl2 · 2H2O and FeCl2 · 4H2O in ethanol followed by gelation using propylene oxide. As-synthesized gels were aged, dried, and heated rapidly upto different temperatures and quenched in air or N2 environment. The calcined powders were characterized using powder x-ray diffraction, BET surface area analyzer, and scanning electron microscopy (SEM). Calcination temperature and environment were found to have a significant effect on phase composition and specific surface area (SSA). The calcined SnxFeyOz powders were placed in a tubular Inconel reactor and four consecutive thermochemical cycles were performed. Water-splitting and regeneration steps were carried out at 900 °C and 1100 °C, respectively. The powder calcined...

H2 generation from thermochemical water-splitting using yttria stabilized NiFe2O4 core-shell nanoparticles

This investigation reports synthesis of core-shell NiFe2O4/Y2O3 nanoparticles by sol-gel technique and their H2 volume generation ability via thermochemical water-splitting reaction at 900 °C–1100 °C. Thermochemical water-splitting process involves a cyclic operation of a low-temperature water-splitting step and relatively high temperature regeneration step using redox materials. Because of the cyclic nature of the process, the redox materials undergo thermal fatigue leading to grain growth or sintering, consequently, steady H2 production is not realized in multiple thermochemical cycles. In this study, attempts were made to achieve steady H2 volume generation in multiple thermochemical cycles using core-shell nanoparticles, which were synthesized using precursors such as NiCl2, FeCl2, and YCl3, and pluronic P123 surfactant template. H2 volume generated by NiFe2O4/Y2O3 core-shell nanoparticles was found to be relatively stable over multiple thermochemical cycles. Contrasting to this, the H2 volume generat...

Thermochemistry and Kinetics

Self-propagating high-temperature synthesis (SHS), also called combustion synthesis, is a novel and simple method for making certain advanced ceramic and intermetallic materials (Merzhanov, 1993; Pampuch, Stobierski and Lis, 1994). This method has received considerable attention as an alternative to conventional furnace technology. The principal concept of this technique is that, once initiated, a highly exothermic reaction can become self-sustaining and will propagate through the reactant mixture in the form of a combustion wave. For this to happen, a chemical reaction must have a relatively high activation energy and must also generate a sufficient amount of heat. Synthesis of refractory high-temperature materials (e.g. borides, carbides, nitrides and silicides) from elemental constituents exhibits both of the characteristics mentioned above.

Reaction front propagation characteristics in non-catalytic exothermic gas-solid systems

Abstract Synthesis of transition metal nitrides and hydrides can be accomplished in the self propagating high temperature regime. A one dimensional model is proposed to study the effects of various operating parameters on the stability of the combustion front, to which gaseous oxidizer is fed by a process of filtration through the permeable solid. Different propagation phenomena can exist depending on the pressure of the gaseous reactant. The stability criteria derived for solid-solid systems can be extended for gas-solid systems at high pressures. At very low pressures filtration combustion is always stable. Systems which show unstable front propagation under adiabatic conditions have been observed to become more unstable due to heat losses.

Use of 2-D-adaptive mesh in simulation of combustion front phenomena

Abstract Solutions to models with different length scales may contain regions such as shocks, steep fronts and other near discontinuities. Adaptive meshing strategies, in which a spatial mesh network is adjusted dynamically so as to capture the local behavior accurately, will be described. The algorithm will be tested on an example of solid-solid combustion.