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The results of investigations of the products of the electrical explosion of aluminum and tungsten wires in liquid media (water and decane) are presented.
Russian PhysicsJournal, Vol. 39, No. 6, 1996

FORMATION OF CHEMICAL IN T H E E L E C T R I C A L

COMPOUNDS

EXPLOSION OF

M E T A L W I R E S IN L I Q U I D S

A. P. Irin, O. B. Nazarenko, and V. Ya. Ushakov

UDC 537.321:541.16

The results of investigations of the products of the electrical explosion of aluminum and tungsten wires in liquid media (water and decane) are presented. It is shown that the phase composition of the powders formed depends on the energy characteristics of the explosion. The mechanism by which the products of the explosion are formed is discussed.

One of the most promising trends in obtaining composition materials and alloys is to use materials in metastable states [1]. When a material is compacted or formed from a metastable state it becomes more stable, and this helps to increase the activity of the reagents during the relaxation period [2]. Ultradispersed powders as materials in metastable states are of considerable interest in the development of powder technology [3]. One of the methods of dispersing metals is the electrical explosion of metal wires, which enables ultradispersed powders to be obtained. Despite the numerous publications on the subject [4, 5], the mechanisms by which metals interact with the surrounding medium have not been investigated to a sufficient extent. The formation of powders occurs under conditions of intense mechanical mixing, heat exchange, diffusion, and the action of electric and magnetic fields, which depend on the parameters of the discharge circuit, the working voltage, the kind of wires used etc. The lack of information on the chemical interactions limits the possibility of controlling the process by which ultradispersed powders of chemical compounds are obtained, and does not enable the composition and characteristics of the f'mal products, the shape of the particles, and the degree of dispersion to be predicted sufficiently fully. In this paper we present the results of investigations of the products, and the mechanism by which they are formed, in the electrical explosion of aluminum and tungsten wires in liquid media. The experiments on the explosion of wires were carried out using the equipment described in [6]. The solid products of the electrical explosion of metal wires were investigated when the explosion parameters were varied, namely, the energy introduced into the wires (e/ec), and the energy of the arc discharge when distilled water and decane were used as the medium. The parameters of the discharge circuit enabled us to vary the level of energy introduced into the wires from 0.4 e/e c to 1.8 e/e c (where e is the energy introduced into the wire and e c is the sublimation energy of the wire material). The solid products obtained as ~ result were separated by precipitation and decantation with subsequent drying at room temperature. The samples of ultradispersed powders were subjected to an x-ray phase analysis using the DRON-3.0 diffractometer employing CuK~ radiation. The degree of dispersion and shape of the particles were determined with a JSM-840 scanning electron microscope. Standard methods were employed to process and interpret the results.

RESULTS An electrical explosion of aluminum wires in water gives rise to the formation of a suspension of products, whose stability increases as the energy introduced into the wire increases up to - 1.2 e/e c, and then decreases when e/e c is increased further. X-ray phase analysis shows (Fig. 1)that the main product of the electrical explosion of aluminum wires is ~-AI203 (the low-temperature modification). The metal phase of aluminum is present in the products, and for e/e c = 0.9 maximum NIIVN, Tomsk Polytechnical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 9-13, June, 1996. Original article submitted May 3, 1995. 510

1064-8887/96/3906-0510515.00 9

Plenum Publishing Corporation

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Fig. 1. X-ray line diagram of the products of the electrical explosion of aluminum wires in distilled water for e/ec = 0.5-0.6 (1), 0.9 (2), 1.2 (3), 1.5 (4), 1.8 (5), and 1.0 (6) at the arc stage, 0.9 (7) - the wire is placed under water, ASTM-AI data (8), and -y-Al203 (9). reflection corresponds to aluminum, and for e/ec = 1.2 it corresponds to 50% aluminum. When e/ec is increased further, the relative content of metallic aluminum decreases. An analysis of the photographs of the powders obtained using an electron microscope showed that the particles formed in the electrical explosion of aluminum wires in water are close to spherical (Fig. 2a). The particle diameter varies from O. 1 #m to 0.5 #m. At high magnifications one can see that the surface of the particles is friable and coated with freer fragments and particles. The electrical explosion of aluminum wires over the surface of water leads to a reduction in the residual aluminum and better preservation of the spherical shape of the particles, which follows from the photographs obtained using an electron microscope (Fig. 2b). Comparison of the particle sizes shows that electrical explosion over the surface (e/ec = 0.9) enables one to obtain powders of -y-AI203, the particles of which have a diameter from 0.16 #m to 0.10 #m and less (within the limits of resolution of the scanning electron microscope). When comparable energy is introduced into the wire and the electrical explosion is carried out in distilled water, powders are obtained whose particles have a diameter from 0.6 ttm to 0.10 ,~m with a considerable fraction of finer particles of irregular form. A measurement of the area of the specific surface (the BET method) shows that the samples of-y-Al203 obtained by the electrical explosion of aluminum wires over the surface of the water and in the volume of the water differ only slightly in value: 34.8 m2/g and 36.3 m2/g, respectively. The results obtained are obviously related to the greater contribution of the finer fractions to the specific surface. When the water is replaced by the hydrocarbon decane, it is found that for the same values of the energy introduced into the aluminum wires the relative yield of the final product (aluminum carbide A14C3) is much lower (Fig. 3). When e/ec is increased from 0.65 to 1.5 there is an increase in the yield of aluminum carbide, but the main final product of the explosion remains aluminum. Additional striking of the electric arc after the electric explosion remains aluminum. Additional striking of the electric arc after the electric explosion of the wire (e/e c = 0.9) does not lead to any appreciable increase in the yield of aluminum carbide. The electrical explosion of aluminum wires in air over the surface of decane leads to oxidation and the formation of ~-A1203, which then does not react with the decane. An analysis of the particle sizes from the photographs (Fig. 2c) shows that the powder consists of particles of a coarse fraction (diameter 1043.1 ttm), the shape of which is close to spherical, while the surface has a friable structure, and a fine fraction which is hard to resolve (particle diameter less than O. l ttm), which forms friable associates. The electrical explosion of tungsten wires in decane for small values of e/e c (e/e c = 0.4), according to the x-ray phase analysis, leads to the formation of two tungsten carbides (IV, WC and II, W2C) with metal tungsten impurity (Fig. 4). When e/er is increased from 0.4 to 1.0 the relative yield of WC increases, while the relative yield of W2C ftrst increases and then 511

Fig. 2. Microphotographs of the powders obtained in the electrical explosion of metal wires (e/e c - 1.0-1.2) in liquids: aluminum wires in distilled water (a), over water (b), in decane (c), and tungsten wires in decane (d).

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Fig. 3. X-ray line diagram of the products of the electrical explosion of aluminum wires in decane for e/e c = 0.65 (1), 0.9 (2), 1.2 (3), 1.5 (4) and 0.9 (5) - from the arc stage, 1.1 (6) - the conductor was placed over the decane, ASTM data - AI (7), A14C3 (8), and ~,a1203 (9). Fig. 4. X-ray line diagram of the products of the electrical explosion of tungsten wires in decane for e/% = 0.4 (1), 0.5 (2), 0.7 (3), 1.0 (4) and 1.16 (5) - from the arc stage, 0.75 (6) - the wire was placed over the deeane, ASTM data - W (7), W2C (8), W C ] - x (9), WO 2 (10). 512

decreases to the impurity level ( < 10 relative percent). The relative content of metal tungsten over the whole range was low, with the exception of sample 2 (Fig. 4), where its presence is obviously due to the fact that with this energy contribution the parts of the wire close to the holders break down with the formation of relatively coarse particles. Dispersion of the tungsten wires from the subsequent arc stage (sample 5) leads to a reduction in the relative content of W2C. The electrical explosion of tungsten wires over the surface of decane helps to increase the content of W2C and leads to the appearance of tungsten oxides. An analysis shows that unlike '),-AI203 and AI4C3, the products of the electrical explosion of tungsten wires are powders, whose particles have a spherical shape and a diameter ranging from 5-3 #m to O. 1 ~tm and less (Fig. 2d). Note that the reduction in the relative content of tungsten in sample 1 (Fig. 4) is a consequence of the fractionation of the products of the electrical explosion when the decane is separated.

CONCLUSIONS The results of an analysis of the composition of the products formed when exploded wires interact with water and decane showed that the shape of the particles is close to spherical with a smooth surface in the case of tungsten and a friable surface in the case of aluminum. The exploding wire initially disperses into spherical particles, which interact with the surrounding medium without changing their shape. It is most probable that the mechanism of this interaction is a reaction with materials in the gaseous state. Unlike gases, liquids are a more dense medium, and this leads to a coarsening of the particles compared with explosion in gases [7], but, at the same time, it also leads to a more thorough conversion of the metals into chemical compounds. The breakdown of the continuity of the expanding wire in the electrical explosion in a liquid obviously occurs at a higher temperature of the metal and the surrounding medium than in the case of an electrical explosion in gases. The preferential formation of tungsten carbide WC in the electrical explosion in decane, unlike the formation of W2C in the electrical explosion in mixtures of acetylene and argon, may also be due to the higher temperature and longer duration of the process in the liquid. The higher temperatures when the products of the electrical explosion of aluminum wires interact with water leads to the formation of a final product of 3,-A1203, which is usually obtained from aluminum hydroxides at a temperature above 400"C. Under these conditions water may participate in the chemical interaction in the vapor state.

REFERENCES .

2. 3. 4. 5. 6. 7.

I. D. Morokhov, L. I. Trusov, and S. P. Chizhik, Ultradispersed Metal Media [in Russian], Atomizdat, Moscow (1977). C. Hauffe, Reactions in Solids and Their Surfaces. Pt. II [Russian translation], IL, Moscow (1963), p. 169. A. P. Win, G. V. Yablunovskii, and N. A. Yavorovskii, Clusters in the Gaseous Phase [in Russian], SO Akad. Nauk SSSR, Novosibirsk (1987), pp. 132-136. E. V. Krivitskii, The Dynamics of Electrical Explosions in a Liquid [in Russian], Naukova Dumka, Kiev (1986), p. 208. V. A. Burtsev, N. V. Kalinin, and A. V. Luchinskii, The Electrical Explosion of Wires and Its Application in Electrophysical Equipment [in Russian], Energoatomizdat, Moscow (1990), p. 288. V. Ya. Ushakov, O. B. Nazarenko, A. P. II'in, et al., The Purification of Water and Drains [in Russian], NIIVN, Tomsk (1994), pp. 52-57. E. Cook and B. Siegel, J. Inorg. Nuci. Chem., 30, 1699-1706 (1968).

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