Kinetics of thermal decomposition of a synthetic K–H3O jarosite analog

The thermal decomposition kinetics of a synthetic K–H₃O jarosite analog was determined from thermogravimetric analysis at various heating rates in air. A thermal decomposition mechanism was proposed based on X-ray analysis of partially decomposed material and distinct features observed during thermal decomposition analysis. The decomposition path is complex. The material was treated as a composite of K-jarosite, H₃O-jarosite, and a “vacancy component”. The evolution of (OH)^? and SO₃ from these individual components was modeled. The decomposition is broken into subreactions according to distinct features in the thermoanalytical measurements. The subreactions are arranged sequentially and in parallel according to the evolution of the participating phases. A set of associated apparent activation energies was determined using isoconversion analysis. Kinetic triplets were assigned to each subreaction. A reasonable match with the observed decomposition was achieved by varying pre-exponential factors.     

Evaluation of K–H3O jarosite as thermal witness material

The scope of this work was the assessment of thermo-oxidative deterioration, hydrothermal stability, and crystalline zone deterioration of some bookbinding leathers from some religious books published in XVIII century stored in Romanian libraries. In this purpose, the following thermal analysis methods were employed: thermogravimetry/derivative thermogravimetry (TG/DTG), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The thermo-oxidative damage of investigated leathers was characterized by the rate of the first thermo-oxidation process put in evidence in TG/DTG curves recorded in static air atmosphere. The hydrothermal stability was characterized by shrinkage temperature determined by DSC analysis of leathers in water excess. The damage of the crystalline zone of leathers was determined by DSC in nitrogen flow and DMA analyses. The qualitative damage for each leather and each kind of degradation was evaluated using the criteria resulted by thermal analysis of a large number of collagen-based materials (pure collagens, new and old parchments and leathers). The obtained results could be used for finding the best possible methods for preservation and/or restoration of the investigated bookbinding leathers.     

Model of heating and ignition of conductive polydisperse powder in electrostatic discharge

Heating of a conductive polydisperse powder by electrostatic discharge (ESD) is modelled numerically. Powder packing is described using a discrete element model; powder resistance is defined by geometry of particle contacts and properties of plasma produced by electrical breakdown between neighbour particles. A set of parametric calculations in combination with experimental data is used to determine necessary adjustable model parameters. The model predicts the temperature for each powder particle resulting from its heating by the ESD current. Location and packing of individual particles within the powder affects greatly their achieved temperatures and thus the likelihood of ignition. Consistently with experiments, a trend showing that smaller particles are generally heated to higher temperatures at a given ESD energy is detected for coarser powders; this trend becomes less clear for finer powders with particle sizes less than the breakdown distance given by the Paschen curve in air. Comparison of the experimental data and calculations suggests that the transition from single particle to cloud combustion occurs when the distance between the particles ignited by ESD becomes close to the flame size for the individual burning particle. This distance, inversely proportional to the number of ignited particles, is primarily determined by the ESD energy.     

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at lower temperatures. The model consistently interprets a wide range of the previously published experimental data describing aluminium ignition.     

A multi-step reaction model for ignition of fully-dense Al-CuO nanocomposite powders

A multi-step reaction model is developed to describe heterogeneous processes occurring upon heating of an Al-CuO nanocomposite material prepared by arrested reactive milling. The reaction model couples a previously derived Cabrera-Mott oxidation mechanism describing initial, low temperature processes and an aluminium oxidation model including formation of different alumina polymorphs at increased film thicknesses and higher temperatures. The reaction model is tuned using traces measured by differential scanning calorimetry. Ignition is studied for thin powder layers and individual particles using respectively the heated filament (heating rates of 10³–10⁴ K s^?1) and laser ignition (heating rate ～10⁶ K s^?1) experiments. The developed heterogeneous reaction model predicts a sharp temperature increase, which can be associated with ignition when the laser power approaches the experimental ignition threshold. In experiments, particles ignited by the laser beam are observed to explode, indicating a substantial gas release accompanying ignition. For the heated filament experiments, the model predicts exothermic reactions at the temperatures, at which ignition is observed experimentally; however, strong thermal contact between the metal filament and powder prevents the model from predicting the thermal runaway. It is suggested that oxygen gas release from decomposing CuO, as observed from particles exploding upon ignition in the laser beam, disrupts the thermal contact of the powder and filament; this phenomenon must be included in the filament ignition model to enable prediction of the temperature runaway.     

Electrical conductivity of a metal powder struck by a spark

Equivalent resistance of a polydisperse powder layer struck by an electric spark is evaluated. Particles were computationally created and mixed using discrete element method; the mixing protocol homogenized particle size distribution within the sample. The conductivity was determined from the equivalent resistance network for the simulated powder bed. A plasma streamer attached to a particle on top of the sample. For each particle contact, an electrical breakdown was assumed; each individual contact resistance was calculated considering its geometry and plasma resistivity. Results of calculations compare well with the available experimental data.     

Oxidation and melting of aluminum nanopowders

Recently, nanometer-sized aluminum powders became available commercially, and their use as potential additives to propellants, explosives, and pyrotechnics has attracted significant interest. It has been suggested that very low melting temperatures are expected for nanosized aluminum powders and that such low melting temperatures could accelerate oxidation and trigger ignition much earlier than for regular, micron-sized aluminum powders. The objective of this work was to investigate experimentally the melting and oxidation behavior of nanosized aluminum powders. Powder samples with three different nominal sizes of 44, 80, and 121 nm were provided by Nanotechnologies Inc. The particle size distributions were measured using small-angle X-ray scattering. Melting was studied by differential scanning calorimetry where the powders were heated from room temperature to 750 degrees C in an argon environment. Thermogravimetric analysis was used to measure the mass increase indicative of oxidation while the powders were heated in an oxygen-argon gas mixture. The measured melting curves were compared to those computed using the experimental particle size distributions and thermodynamic models describing the melting temperature and enthalpy as functions of the particle size. The melting behavior predicted by different models correlated with the experimental observations only qualitatively. Characteristic stepwise oxidation was observed for all studied nanopowders. The observed oxidation behavior was well interpreted considering the recently established kinetics of oxidation of micron-sized aluminum powders. No correlation was found between the melting and oxidation of aluminum nanopowders.     

Formation of a high-current pulsed gas discharge as a source ofhighly concentrated energy flux to treat metal surfaces

The properties of a novel, pulsed, high-current gas discharge with minimized energy losses are investigated. The discharge provides a highly concentrated energy flux that can be used to treat metal surfaces and to form thin surface layers with desirable properties. A theoretical treatment of the formation of the discharge is presented, and the limitations on its voltage and interelectrode separation length are considered. Experiments are carried out to test the theoretical predictions of the discharge parameters. The experimental results show that more than 80% of the energy input to the discharge from the power supply is delivered to the metal surface     

Mechanically alloyed Al-I composite materials

Mechanically alloyed aluminum-iodine composites with iodine concentrations from 4 to 17 wt% were prepared from elemental aluminum and iodine. A reference sample was prepared from aluminum and AlI₃. A shaker mill and an attritor mill, operating at both room temperature and liquid nitrogen temperature, were used for preparation. Materials were characterized by electron microscopy and X-ray diffraction. The iodine release upon heating was studied using thermogravimetry. Mechanical alloying was found to be effective for preparation of Al-I composites that do not release iodine until the material is brought to high temperatures. Mechanical alloying in nitrogen gas at liquid nitrogen temperature was more effective in preparing stabilized Al-I composites than milling at room temperature. Iodine was not retained in materials milled directly in liquid nitrogen. In addition to poorly crystalline AlI₃, other iodine compounds were present in the products. Assuming that the products are similar to other mechanically alloyed materials, it is expected that iodine is mixed with aluminum on the atomic scale, forming metastable Al-I compounds where iodine may be bonded to aluminum more strongly than in AlI₃, explaining why their thermal decomposition and respective iodine release occur at higher temperatures compared to decomposition and boiling of AlI₃.