1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 DOI: 10.1002/prep.201800178 Parameters of Aluminum Nanopowders Activity after Long-Term Storage in an Airtight Container Alexander Ilyin, [a] Dmitriy Tikhonov, [a] and Andrei Mostovshchikov* [a] Abstract: The work has established the change in the activ- ity parameters of aluminum nanopowders produced by electric explosion of a wire during their storage in an air- tight container. The change in the characteristics of the nanopowders using four activity parameters was measured by benchmarking their activity after the production, passi- vation and keeping for 10–20 days and after their storage for 16 years. It was also established that after long-term storage, the temperature of oxidation initiation for the nanopowders in air has increased; the increment of mass during oxidation at up to 800 8C for all specimens has in- creased, while the content of metallic aluminum has de- creased. The electrical double layer on the surface of alumi- num nanoparticles during storing even in an airtight container changes with time and does not provide the pro- tective function. In this connection, the future application of aluminum nanopowders in high-energy materials is not feasible without solving the problem of their stability. Ac- cording to the results, electric-explosion aluminum nano- powder is an unstable aluminum-air system even in an air- tight container. Keywords: electric explosion of wire · aluminum nanopowder · chemical activity parameters · storage · airtight container 1 Introduction One of the primary problems of practical application of alu- minum nanopowders is conservation of metallic aluminum and its reactivity [1–4]. Therewith, the production of alumi- num nanopowder that is stable in air requires partial oxida- tion combined with a reduction of aluminum content at least down to 92–93 mas. %. The replacement of conven- tional aluminum powders in pyrotechnical compositions and rocket fuels by aluminum nanopowders with low con- tent of metallic aluminum for the sake of improved per- formance is not evident [5–7]. Any nanopowders are unstable or meta-stable solid body-gas systems. As compared to compounds with low dispersity, metallic nanopowders have prominent reactivity [8–10] and interact with their environment [11,12]. The main oxidizer of metallic nanopowders are protons that form after dissociation of water molecules on the surface of nanoparticles [13, 14]. The structure of metallic nano- particles [15] formed after passivation in air is peculiar for an electrical double layer (EDL): adsorbed water molecules dissociate into protons that diffuse into a nanoparticle and hydroxide ions that stabilize on the surface. The latter chemically bond with the nanoparticle surface, while pro- tons oxidize the metal yielding atoms and molecules of hy- drogen [13]. For a number of metals (aluminum, titanium, zirconium and hafnium), protons form respective hydrides [13]. With the decrease in the diameter of metal particles, the state of atoms – initially on the surface, and from the diameter of 30 Æ 10 nm, inside the particles as well – be- comes unstable [14]. The formation and nature of pro- tective films on the surface of aluminum particles depends on their diameter (surface curvature). When the diameter di- minishes down to 1 mm, the diffusion mechanism of pro- tective film formation persists, which thickness varies from 20 to 40 mm. For the aluminum particles with the diameter less than 100 nm, due to the change in atom state on the surface, the nature of the protective film also changes. The process of aluminum oxidation in nanostate decelerates af- ter the formation of electrical double layer having pseudo- capacitance with internal positively charged layer [13, 15]. The stability of the double layer determines the stability of aluminum nanopowders during storage. Indirect evidence of the electrical double layer is the much lesser thickness of the protective layer of 2–10 nm [13, 15]: with a decrease in the diameter of aluminum nanoparticles, the thickness of the electrical double layer drops. The heating (destruction of electrical double layer) of aluminum nanopowder in air and its oxidation is accompanied by a threshold effect [13, 15]. Nazarenko et al [16] demonstrated that the electric explosion nanopowders for 10–27 years storage generally conserve their thermo chemical parameters. However, they are not given in detail depending on the conditions of the energy parameters of the electric explosion. The goal of the present work is to determine the regu- larities of change in chemical activity parameters of a series [a] Prof. A. Ilyin, Prof. D. Tikhonov, A. Mostovshchikov National Research Tomsk Polytechnic University, 30, Lenin Ave., 634050, Tomsk, Russia *e-mail: pasembellum@mail.ru Short Communication These are not the final page numbers! ÞÞ Propellants Explos. Pyrotech. 2018, 43, 1–6 © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1