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Application of Cerimetric Methods for Determining the Metallic Aluminum Content in Ultrafine Aluminum Powders

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Application of Cerimetric Methods for Determining the Metallic Aluminum Content in Ultrafine Aluminum Powders 160 Propellants, Explosives, Pyrotechnics 32, No. 2 (2007) Full Paper Application of Cerimetric Methods for Determining the Metallic Aluminum Content in Ultrafine Aluminum Powders Tatyana D. Fedotova* Novosibirsk State University, 630090 Novosibirsk (Russia) Oleg G. Glotov, Vladimir E. Zarko Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, 630090 Novosibirsk (Russia) DOI: 10.1002/prep.200700017 * mixtures with gelled water as a promising non-toxic Abstract propellant for space applications [11 – 12]. Ultrafine (nano-sized) aluminum is a promising component for Brief reviews of the basic publications are presented in [1, solid propellants, gel propellants, explosives, etc. The work is 13 – 15]. In all cases, the use of nanoAl requires objective focused upon a problem of determining active (metallic) alumi- num content in nanoAl powders as one of the characteristics of characterization of its properties. One of the most important their reactivity. It is shown that the high reactivity, the presence of characteristics is the content of active (metal) aluminum. It either the gases adsorbed or coating matter on the particle surface is worth noting that the corresponding analysis methods are restricts the traditionally used permanganatometric and volumet- still under development, since conventional methods used ric analytic methods. A new technique determining the active for characterization of micron sized aluminum powders are aluminum content is presented. This is the adaptation of known cerimetric method based on the analytical reaction Ce4þ þ eÀ ¼ either often unsuitable for the analysis of nanoAl or require Ce3þ. The method can be applied for analysis of metallic substantial modification. In the present work, the problem aluminum in the probes of nanoAl (including ones with organic of the determination of metal aluminum in nanoAl particles coating), as well as micron sized aluminum, and their condensed by means of analytical chemistry is considered. combustion products. Keywords: Nanoaluminum, Metal Aluminum Content, Cerimetric Method, Combustion 2 The Difficulties of the Chemical Analysis of NanoAl 1 Introduction A decrease in particle size causes changes in the specific It is known [1] that a decrease in aluminum particle size surface of the powder, its structure, phase composition, and leads to an increase in specific surface area of powder and to relative thickness of the oxide shell. These parameters are the enhancement of its chemical activity. The development also affected by the method of powder production. As a of techniques for producing ultrafine aluminum (below result, the reactivity of nanopowders changes substantially, refer to as nanoAl) with particle size less than 200 nm, such and the oxidation and combustion of nanoAl proceed in a as vaporization/condensation (Gen-Miller method) [2 – 4], manner differing from that for the micrometric aluminum plasma recondensation [5 – 6], and electroexplosion of wire powders [1, 13 – 15]. A very indicative example is the [7 – 10] has initiated a number of works studying the interaction of nanoAl in the stoichiometric mixture with combustion of various propellant compositions containing gelled water in the regime of layer-by-layer self-sustaining nanoAl. combustion [16]. Moreover, nanoAl reacts with water even The possible areas of using nanoAl in chemical propulsion at room temperature [17]. The interaction between fine systems, i. e. in classical and hybrid rocket motors, are: aluminum particles and water is one of the factors adding complexity in the chemical analysis. * solid propellant formulations to change the burning rate Whatever the method of production, the nanoAl powders and vary its dependence on pressure, are subjected to passivation to ensure their stability. The * solid propellant formulations to reduce agglomeration, passivation involves the treatment of surface with either * mixtures with liquid hydrocarbon fuels for jet engines, gaseous or liquid reagents [9 – 10, 18 – 19] that form a protective layer. In the simplest case, this can be an oxide * Corresponding author; e-mail: [email protected] layer. In more complicated cases, a layer of special-purpose 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Application of Cerimetric Method for Determining the Metallic Aluminum Content in Ultrafine Aluminum Powders 161 substance is created on the surface in addition to the oxide The importance of controlling the nanoAl chemical layer. For example, some organic compounds form coatings composition dictates some additional requirements to that prevent particles from conglutination [18]. Others chemical analytical methods: they should be reproducible, render hydrophobic properties and stability against water convenient for multiple analyses, etc. [19]. The latter is important for the nanoAl/gelled H2O propellants [11 – 12]. From the viewpoint of chemical analysis, the presence of coatings other than oxide can be 3 Cerimetry as a Procedure to Solve the Problem a complicating factor to be taken into account. A gaseous “shell” is known to be formed on the surface of The use of redox reactions for analytical purposes implies particles when nanoAl is produced by the electroexplosion a definite difference in the oxidative-reductive potential of wire. The content of adsorbed gases can reach 7% [15]. (ORP) values for the corresponding semi-reactions which Here and below, mass percent values are used. It was shown provide their completeness. However, when calculating the [17] that the presence of adsorbed gases is a factor, which corresponding equilibrium constants for nanoAl powders may lead to overestimated aluminum content determined one cannot use the standard ORP values, because for the by means of gas volumetric analysis in the powder obtained nanopowders produced by electroexplosion method the by electroexplosion method. recorded ORP values are substantially lower than those for In order to eliminate a systematic error of the gas the bulk metal [15]. This is because the fine aluminum volumetric method, some specific preparation procedures behaves as stronger reducing agent than the coarse particles can be used for removing the particles gaseous shell in the or the bulk samples and readily interacts with the protons of tested sample through vacuumization or thermodesorption. acids and water. The finely divided state of the powder However, one cannot be quite sure that this would not lead results in accelerating the process macrokinetics. Thus, the to the partial oxidation of the surface, the changes in the hydrogen evolving at a high rate has not enough time to chemical composition of the sample under analysis and the reduce Fe3þ ions in solution in course of permanganato- distortion of the results of analysis. Therefore, the problem metric analysis. of the chemical analysis of nanoAl requires the application Based on these prerequisites, we decided to use a stronger of independent analysis methods followed by the compar- oxidizing agent than Fe3þ ion (whose standard ORP is ison of the results obtained. 0.77 V) to oxidize aluminum, in order to suppress the The permanganatometric method [21] has been success- participation of protons in this process and to avoid hydro- fully used to analyze both the virgin aluminum powders and gen evolution from the reaction mixture. This idea is the condensed combustion products (in general, consisting realized in cerimetric method, which is widely known [21 – of Al2O3 and unburned Al). Actually, in our practice this 22] but has never been used for systems under study. method, being more convenient and accurate, has com- The cerimetric procedure is based on the analytical pletely replaced the volumetric method. However, when it reaction Ce4þ þ eÀ ¼ Ce3þ, which proceeds in the strong was initially used to analyze a number of nanoAl powders, acid environment. The value of its potential depends on the first of all those with coatings, unsatisfactory results were nature and concentration of an acid. In a 1 M – 4 M solution obtained. The powders exhibited high chemical activity, of sulphuric acid it comprises 1.44 – 1.42 V. The large ORP which resulted in a competitive reaction accompanied with value, the absence of side reactions, the stability of the hydrogen evolution and, as a consequence, caused poor working solution of the titrant and the simplicity of fixing reproducibility and gave a negative systematic error. This the equivalence point are the advantages of this method. finding was confirmed by the results of chelatometric A direct titration by a cerium(IV) solution can be used in determination of the total aluminum content in these principle to determine the content of Fe2þ ions formed in the powders. The oxygen content, calculated as a difference, solution of ferric ammonium sulfate during the oxidation of allowed estimating the content of aluminum oxide as the metal aluminum, instead of potassium permanganate or main compound on the surface of metal particles. With dichromate according to the following reactions: respect to the mass balance, the obtained values indi- cated that the metal alumnium content determined by Al þ 3Fe3þ ¼ Al3þ þ 3Fe2þ permanganatometric procedure was underestimated, even taking into account that the nanoAl powders contain the Ce4þ þ Fe2þ ¼ Ce3þ þ Fe3þ admixtures of other metals and gases adsorbed on the surface. This was tested for the commercial grade Russian Actually, although the passivating coatings protect parti- aluminum powders PA-4 and ASD-6by both methods and ally
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