Alexandre Ermoline

Production of carbon-coated aluminium nanopowders in pulsed microarc discharge

A new technique of metal nanopowder production and in situ coating using a microarc discharge is proposed. The feasibility of this method is demonstrated and preliminary results are presented. The microarc discharge was operated at 1 atm in both pure Ar and natural gas environments. Respectively, aluminium nanoparticles without and with ~1 nm thick carbon surface coating were obtained. The morphology and structures of the nanoparticles were studied using a transmission electron microscope (TEM), and size distribution of the coated particles was determined from the TEM image analyses. It was found that the sizes of the produced carbon-coated aluminium particles were well described by a lognormal distribution with the geometrical mean size of 22.7 nm and standard deviation of 1.35.

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.

Experimental technique for studying high-temperature phases in reactive molten metal based systems

Containerless, microgravity experiments for studying equilibria in molten metal–gas systems have been designed and conducted onboard of a NASA KC-135 aircraft flying parabolic trajectories. An experimental apparatus enabling one to acoustically levitate, laser heat, and splat quench 1–3 mm metal and ceramic samples has been developed and equipped with computer-based controller and optical diagnostics. Normal-gravity testing determined the levitator operation parameters providing stable and adjustable sample positioning. A methodology for optimizing the levitator performance using direct observation of levitated samples was developed and found to be more useful than traditional pressure mapping of the acoustic field. In microgravity experiments, spherical specimens prepared of pressed, premixed powders of ZrO2, ZrN, and Zr, were acoustically levitated inside an argon-filled chamber at one atmosphere and heated by a CO2 laser up to 2800 K. Using a uniaxial acoustic levitator in microgravity, the location of...

Consolidation of Reactive Nanocomposite Powders

There is interest in replacing energetically inert structural components with reactive structures capable of highly exothermic reactions. Consolidated reactive materials are also desired for other applications, including reactive fragments, high density additives to explosives, insensitive pyrotechnic components, etc. Unlike nano-energetic compositions based on mixed nanopowders, reactive nanocomposite powders prepared by Arrested Reactive Milling (ARM) can be readily consolidated to achieve combined characteristics of high reactivity, low porosity, and structural strength. Different consolidation methods can be applied, and as a first step, a simple uniaxial pressing of nanocomposite powders is used in this project. A set of reactive nanocomposite powders with several Al-based thermite compositions prepared by ARM was used to prepare pellet-like consolidated samples. For various mechanical tests, both cylindrical and rectangular pellets were prepared with varied dimensions and varied degrees of compaction. Pellet compaction densities exceeding 90% of theoretical maximum density, were achieved. Despite the presence of Al and oxidizers, including MoO3, CuO, Bi2O3 and others, mixed on the nanoscale in different samples, no reaction was observed to be triggered by the powder compaction at pressures reaching 500 MPa. An experimental technique has been developed to study the thermal ignition initiation of the consolidated samples as a function of their physical and mechanical properties. The experimental technique will be used to develop a theoretical model to describe the ignition behavior of the consolidated materials.

Thermal theory of aluminum particle ignition in continuum, free-molecular, and transition heat transfer regimes

Most studies on nano- and micro- sized aluminum particle ignition have been focused on the processes occuring inside particles. In the current paper, thermal ignition of an aluminum particle in the air is simulated with different heat transfer models: continuum, free-molecular and Fuchs model. A single parabolic oxidation law is assumed in the particle size range from nano- to millimeter diameters. A particle is considered ignited when it reaches the oxide melting point. The criterion defining the limits of validity for each model is the ratio of continuum and free-molecular heat transfer rates. The dependence of ignition temperature

Equations for the Cabrera–Mott kinetics of oxidation for spherical nanoparticles

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Low-temperature exothermic reactions in fully dense Al–CuO nanocomposite powders

on particle size is in a qualitative agreement with the experimental trends:

Reactions leading to ignition in fully dense nanocomposite Al-oxide systems

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Correlation of optical emission and pressure generated upon ignition of fully-dense nanocomposite thermite powders

can have values in the range 700--1500 K for nanoparticles due to the dominating contribution of a free-molecular heat transfer, and sharp growth of

Model of heterogeneous combustion of small particles

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