Alexander P. Ilyin

The mechanism of combustion of superfine aluminum powders

An experimental study of the combustion of superfine aluminum powders (average particle diameter as 0.1 m) in air is reported. The formation of aluminum nitride during the combustion of aluminum in air and the influence of combustion conditions on the structures and compositions of the final products are addressed. The experiments were conducted in static air at 1 atm. Superfine aluminum powders were produced by exploding an electrically heated wire. Such a superfine aluminum powder is stable in air, but once ignited can burn in a self-sustaining way due to its low bulk density (0.1 g/cm 3 ) and low thermal conductivity. During combustion, the temperature and radiation were measured; also the burning was recorded by a video camera. Scanning electron microscopy, X-ray diffraction and chemical analysis were performed on both the initial powders and final products. It was found that the powders ignited by local heating and burned in a two-stage self-propagating regime. The products of the first stage consisted of unreacted aluminum (70 mass %) and amorphous oxides with traces of AlN. After the second stage, the AlN content exceeded 50% and the residual Al content decreased to 10%. A qualitative discussion is given of the kinetic limitations for the oxidation of AlN due to rapid condensation and encapsulation of gaseous AlN.

Reactivity of superfine aluminum powders stabilized by aluminum diboride

It is known that superfine aluminum powders (SFAP) can successfully replace the micronsized aluminum powders (10–100 m) in propellants. Such substitution leads to an increase in the combustion efficiency of aluminum and also leads to decreased agglomeration of the combustion products and reduction of twophase losses [1, 2]. The decrease in the size of aluminum particles and increase in the reaction’s surface area, considerably increase the combustion rate of the propellant composition [1]. SFAP obtained by the electrical explosion of wires (EEW) has been studied in detail [3–9], and interest in such SFAP continues to rise [10]. The formation of particles under conditions of electrical explosion (power density 10 W/cm, time of process 1–10 s) can lead to the stabilization of metastable energy-saturated structures, which relax at relatively low temperatures and increase the reactivity of SFAP [11]. An increase in the dispersiveness of SFAP leads to an increase in their reactivity. But the basic problem of using SFAP is the relatively low content of metallic aluminum (93–97 mass %) [5, 6] simultaneously with the high reactivity of SFAP. Another problem of using superfine powders as additives to propellants is the original agglomeration of SFAP when produced by EEW. The original agglomeration of SFAP is connected with the reactivity of the particles’ surfaces, and the necessity for particles to concentrate and collect in the gas-phase. The agglomerates of SFAP in the porous structures consist of superfine particles in the state of the primary stage of sintering. Between the particles (d 0.05 m) are contact zones which slightly change the shape of the particles. The presence of agglomerates of particles leads to heterogeneity of the mixtures and to coalescence of agglomerates in the heat penetration zone during combustion. In this case large drops are formed, so the advantages of using SFAP are lost. It has been experimentally established that additives to the Ar gas used in EEW, such as chemically active gases (O2 and N2), lead to the products from EEW being more dispersed [4]. Reduction of particle size in this case is because of a decrease in agglomeration and sintering during EEW. The presence of high-melting point non-metallic compounds (AlN, Al2O3) on Al particles also reduces agglomeration during the heating of SFAP during combustion, analogous to when aluminum particles are encapsulated by high-melting point metals (Cu, Ni, Fe) [12]. If the stabilization of SFAP is because of the formation of an oxide film, it leads to the loss of 3 to 5 mass % of aluminum and to a decrease in the combustion enthalpy of the powder. In other words, to stabilize SFAP it is necessary to cover them by a film obstructing future oxidation. If such a film is Al2O3 the content of metallic aluminum is just 93 to 97 mass %. Additionally, the Al2O3 is the substance containing aluminum in its highest degree of oxidation (Al ), which is inert during combustion. By adding nitrogen to argon during EEW, the dispersity of the powders obtained increases, and a coating of AlN can be produced in the electrical explosion [4]. During passiva*Corresponding author. E-mail: rrc@uou.ulsan.ac.kr

Estimation of the reactivity of aluminum superfine powders for energetic applications

A new quantitative method of estimating the reactivity of aluminum superfine powders for energetic applications has been suggested. The method is based on differential thermal analysis-thermogravimetry (DTA-TG) data analysis in air as oxidizers for measurement first of four reactivity parameters (temperature of intensive oxidation onset, maximum rate of oxidation, degree of conversion of metallic aluminum, and specific heat release). After the comprehensive testing of the powders, the most reactive powder was identified. The special features of the oxidation process for the most reactive samples of aluminum superfine powder, obtained with a wires electrical explosion method, are explained and a theoretical mechanism for a new phenomenon, “pulsed oxidation” of superfine aluminum powder in air, is proposed.

Electroexplosive Technology of Nanopowders Production: Current Status and Future Prospects

The current situation of the nanopowders production technology based on the process of electrical explosion of wires is described. The advantages and disadvantages of the electroexplosive technology are indicated. The results of studies characterizing the effect of the electrical explosion conditions on the nanopowders properties are presented, including latest results: conditions of nanopowders passivation, conditions of nanopowders production having narrow size distribution, the methods of nanopowders diagnostic and standartization. In addition, the application and area of future research on this technology are proposed.

Thermal Stability of Iron Micro- and Nanopowders after Electron Beam Irradiation

Chemical activity of micro-and nanosized powders during open air heating after electron beam (up to 360 keV of electron kinetic energy) irradiation has been studied. A differential thermal analysis disclosed that an initial oxidation temperature for iron powders has been decreased to ~ 30°C after electron beam irradiation. Thus, the thermal oxidative stability of iron powders in air was improved without any detected changes in other activity parameters.

The Energy Stored in the Aluminum Nanopowder Irradiated by Electron Beam

The influence of the electron beam irradiation on the parameters of aluminum nanopowder oxidation by heating in air was studied. It was found that the oxidation starts at the temperature in the range from 410° C to 460° C and independent on the radiation dose. The degree of oxidation varied from 44.4 % to 58.3 % and its dependence on the radiation dose was not established. The heat energy release occurred in two stages: at the first stage (up to ~ 660° C) in general the increase of the thermal effect was observed. At the second oxidation stage of irradiated aluminum nanopowder the growth of the thermal effect also observed. The peak of heat effect achieved by irradiation (45.0 kGy absorbed dose) was 2576 J/g higher than the thermal effect for non-irradiated aluminum nanopowder. The energy stored is an additional motivating factor in the synthesis of composite materials, intermetallic compounds, hydrogen producing reactions and synthesis of various kinds.

Influence of Ultra-Violet Radiation on Sublimation Energy of Silver Chloride (AgCl)

Influence of silver chloride synthesis conditions on its heat of sublimation after ultra-violet irradiation was explored. It was determined that silver chloride synthesized in excess of silver ions after irradiation was characterized by heat of sublimation about 6813 J/g, which meant that the original value increased in ~ 1.8 times. At the same time silver chloride synthesized in excess of chloride ions was characterized by decrease in value of heat of sublimation by ~ 2 times. Growth of heat of sublimation of silver chloride with excess of silver ions is connected with energy expenditure on sublimation due to decrease in proportion of surface which is not covered with silver. Reduction of heat of sublimation of silver chloride with excess of chloride ions is explained by absence of silver solid surface film and catalytic effect of silver klasters formed during the process of irradiation.

The Electric Field and Ultrasonic Treatment Casing of Titanium Dioxide

This article deals with the study of direct current (DC), ultrasonic, and of electrolyte of the influence on the change of titanium dioxide sorption properties. The TiO2 prepared by the TiCl4 hydrolysis method. Attention is drawn to charging exchange processes the surface of titanium dioxide particles after they pass through a layer of counter ions in an aqueous medium under the influence of a DC. The stages of the end product’s formation using methods of the X-ray diffraction, the differential thermal and the infrared spectrometric and the gas adsorption analysis were proposed. Dependence of redistribution sorption active centers at the surface of TiO2 was proved.

Effect of Electrical Parameters and Surrounding Gas on the Electroexplosive Tungsten Nanopowders Characteristics

Tungsten nanopowders were produced by the method of wires electrical explosion in the different gases. The study of phase and dispersed composition of the powders was carried out. The influence of electrical parameters such as the value of energy input in wire and the arc stage of the explosion was discussed. The factors that make for decreasing the particles size are the lower pressure of surrounding gas and the use of addition of chemically reactive gas.

Characteristics of nanopowders produced by electrical explosion of copper wires in argon with air additives

In this paper the dispersed, phase and chemical compositions of nanopowders produced by electrical explosion of wires in various gaseous medium were investigated. It is found that electrical explosion of copper wires in argon with little additives of air (1...2 vol.%) results in producing the nanopowders having high specific surface area (10.5...11 m2/g) and containing ~80 % of metal copper. The size of copper nanopowders decreases with increasing the content of reactive gas in argon (up to 30 vol. %) and their phase composition changes: the output of CuO is observed.