.

8.

9.

A. A. Zenin, A. G. Merzhanov, and G. A. Nersesyan, Fiz. Goreniya Vzryva, 17, No. i, 79 (1981). V. S. Vol'kenshtein, High-Speed Method for Determination o~ Thermophysical Characteris- tics of Materials [in Russian], Energiya, Leningrad (1971). A. F. Chudnovskii, Thermophysical Characteristics of Dispersed Materials [in Russian], Fizmatgiz, Moscow (1962).

EFFECTS OF HEATING CONDITIONS ON THE AGGLOMERATION

OF ALUMINUM POWDER IN AIR

A. K. Lokenbakh, N. A. Zaporina, A. Z. Knipele, V. V. Strod, and L. K. Lepin'

Aluminum particle agglomerationis accompanied by preflame processes and ignition in condensed systems. Recently, there have been many papers on the mechanisms and the quanti- tative description of metal particle agglomeration and sintering [1-4]. The reason for the agglomeration is that the oxide shells on the particles crack because of the difference in expansion coefficient between the metal and the oxide [5, 7]. The data of [8-11] indicate that thecracking starts over a wide temperature range (600-2090°K), which is determined by the particle size; the less the size, the lower the cracking temperature.

Equal importance attaches to the thermal stresses, which are dependent on the tempera- ture rise rate. However, the effects of this factor have been examined in less detail, as have those of the state of the initial oxide layer. Kinetic studies show that the structure of the surface oxide has a considerable effect on the oxidation kinetics and is one of the major factors determining aluminum particle agglomeration [12]. On the other hand, not much is known [9] about the phenomenology of oxide-layer cracking.

This series of studies was designed to examine the effects of heating rate on oxide layer failure for aluminum particles in relation to layer structure and gas composition. Here we give results from electron microscopy applie~ to the oxide shells of aluminum particles during heating in air.

We used polydisperse aluminum powder with a mean particle size of i0 ~m. In the initial state, the particles had surface relief (Fig. la). The oxide films (Fig. ib) consisted of an amorphous matrix containing numerous inclusions of finely divided y-AI203 crystallites. Also, the films were inhomogeneous in structure because of numerous folds due to the shrinkage of the metal core during passage to the solid state [6].

The aluminum powder was placed as a thin layer (l =~)in a corundum or Nichrome holder, which was placed in a flow of air with a given temperature (I173-1473°K), The temperatures of the gas and specimen were measured with Pt--Pt/Rh thermocouples, whose emf pass to a KSP-4 potentiometer. The heating rate was determined from the slope of the initial linear part on the temperature curve. The values were 2"103-1.5"10 ~ °K/min under various conditions. When the specimen had reached the gas temperature, it was rapidly cooled and was examined by transmission and scanning electron microscopy with BS-540 and JEM-IOOC microscopes with the ASID-4D scanning attachment.

Here we describe the electron micrographs for particles after interaction with air at various temperatures Tgand heating rates q.

q ~ 2000°K/min, Tg = I173°K. The small particles agglomerate without substantial chang~ in shape and with the retention of the metal in the oxide shells. The large particles lose their metal partially or completely, and the oxide shells show considerable deformation (de- pressions and the production of agglomerates of indefinite shape, Fig. 2a). Some parts of the surface show a network of fine cracks overgrown by oxide (Fig. 2b), and whisker crystals grow (Fig. 2c).

Riga. Translated from Fizika Goreniya i Vzryva, Vol. 21, No. i, pp. 73-82, January- February, 1985. Original article submitted November 9, 1983.

0010-5082/85/2101-0069509.50 © 1985 Plenum Publishing Corporation 69

70

r=l

~-I

71

72

r~

73

74

TABLE i

973 1073 1173

1273 1373 1473

0,54 0,51 0,45

0,37 0,28 0,24

K.10', SeC --it

0,05 0,08 0,75

5,42 4,28 4,61

E, k I / mole

99,8

84,7

q ~ 4000°K/min, Tg = 1273°K. The degree of agglomeration increases, while the initial shapes of the small particles are mainly retained. The damage to the oxide shells on the large particles becomesmore notable, and !argecracks~nay appear (Fig. 3a). The thickness and crystallinity of the oxide films arehigher than at Tg = I173°K, and one can see places where the damage has been healed by thinner oxide layers (Fig. 3b).

q ~ 5000°K/min, Tg ffi 12230K (Fig. 4a) or 1373°K (Fig. 4b). No matter what the gas temperature, there is considerable damage to the oxide shells in the main on the large particles at this heating rate. The small particles still retain their shape, no matter whether they agglomerate with one another or with the large particles. Often, the cavities in the disrupted oxide shells show the oxide surface of the remaining metal. The oxide layer becomes thick and struc£ured. Whisker crystals are formed before the oxide layer is dis- rupted.

q ~ 6000°K/min, Tg = 1273°K. The disruption of the oxide shells leads to the liquid metal from the particles coalescing into droplets of irregular shape. The oxide coating is of highly developed structure, and whisker crystals are seen at the surface.

q ~ 8500°K/min, Tg = 1323°K. There is pronounced particle agglomeration without visible damage to the oxide shells. Necks between particles are formed (Fig. 5a). Figure 5b shows the structure of the contact after separating the agglomerated particles. The whisker cry- stals grow vigorously and become appreciably larger.

q ~ 15,500°K/min, Tg ffi 1473°K. The agglomeration involves broad necks and occurs without visible damage to the oxide layers. Figure 5c shows thick and highly structured oxide layers, on which are many crystalline formations.

These results show that cracking in the oxide layer and consequent agglomeration occur throughout the range of nonstationary heating. However, the picture is substantially depend- ent on the conditions. In [12] it was shown that the oxidation kinetics for aluminum powder may be described by

dm K dr = ~ (too - - mr),

where m0 is the initial mass of the metal phase in the powder, mT is the mass of metal oxidized up to time T, K is the generalized oxidation rate constant, and o is the oxide layer thick- Hess.

With da/dT = K'/o x, while the number of agglomerated particles is w, the degree of oxi- dation is

a ~- t - - exp ( - k ~ ' ) ,

where n = x/(x--l) (~ = const) or n = I(I + ~) (x = const).

The kinetic characteristics given in Table i may be compared with our electron-micro- scope data, which shows that there are substantial changes in the system at Tg > I173°K. There is considerable disruption of the oxide shells, which, on the one hand, produces pro- nounced particle agglomeration, while, on the other, there is contact between the clean surface of the molten metal and the oxidizing agent. This makes itself felt as a reduction in the characteristic constant n, which implies a marked increase in the effects of agglo- meration on the oxidation kinetics. Above I173°K, there is also a reduction in the effective oxidation activation energy, which indicates facilitated contact between the metal and the

75

l i t

soo ~ Z

o ~o 2"0 Fig. 6

iO00

f

30 ~ sec

oxygen in the air.* The heating curves after the specimen has attained a temperature of I150-1200°K show an increase in the rate of temperature rise, which indicates self-heating due to the increasing contribution from the heat of oxidation (Fig. 6, where curve i is for Tg = 1373°K and q = 5025=K/min, and curve 2 is for Tg = 1473°K, q = 15,500°K/min).

This threshold temperature (~I173°K) is to be considered as due to phenomena in the large number of metal particles, where there is local temperature rise and uneven heating due to the holder. In spite of this, the results agree well with [4], where contact links be- tween particles were also found at I173°K. According to [16], the temperature at which the oxide layer is disrupted in a mixed condensed system and at which the particles begin to heat spontaneously is somewhat higher at about 1300°K.

The oxidizing medium is of relatively low temperature, and the experiment is of sub- stantial duration (T = 15-60 see), which enable us to determine the form of the damage to the shells and the degree of particle agglomeration as influenced by various factors (T, q, and T).

All the observed phenomena can be interpreted as due to two competing processes: thermo- mechanical interaction between the metal core and the oxide shell, and physicochemical pro- cesses in the oxidation. Thermal expansion and melting increase the metal volume by about 10% in the temperature range used. Under these conditions, the linear-expansion coefficient for the oxide layer is much less than that for the metal [6, 7]. The oxide layer has initial structural inhomogeneity, and it is damaged the more readily the higher the heating rate, which determines the effects of the thermal shock on the oxide layer. At comparatively low heating rates, small cracks are produced, which may be healed by the newly formed oxide (Fig. 2). At higher heating rates, the rapidly expanding metal core damages or completely dis- rupts theoxide layer (Fig. 4). The heating rate plays the main part in a certain temperature range.

Aluminum has a high oxygen affinity (AG 0 = --1583 kJ/mole [7]), which predetermines the rapid oxidation under any conditions. A feature of the present conditions is that the direct metal--oxygen contact is determined by the damage to the oxide shell and the oxidation rate. As the temperature rises, the liquid metal becomes less viscous and more fluid (by a factor 1.5-2) [6], while the vapor pressure increases (from 6.25 × 10 -2 to 2.93 Pa in the range 1273- 1473°K [5]), and aluminum suboxides are produced [17].

Therefore, the oxidation of aluminum powder is quite complicated, and the initial stage can be divided into several substages, as is evident from the heating curve (Fig. 6): crystal- lization and growth in the stressed oxide shell (oxidation by diffusion, part I), ongoing oxide layer growth by diffusion in conjunction with direct oxidation of the liquid aluminum or the vapor in the microcracks (part II), and the occurrence of large defects in the oxide layer, which lead to rapid oxidation of the liquid metal (self-heating in part III). As the heating rate increases, the disruption of the oxide layer in parts II and III increases (Figs. 3 and 4), which leads to the metal escaping. The conditions observed with Tg = 1223-1273°K

*The effectiveactivation energy for the oxidation of finely divided aluminum powder is much less than that for the molten metal in the compact state (330-500 kJ/mole [14, 16]). This also indicates that the oxidationof finely divided aluminum powder does not involve the classical diffusion mechanism.

76

and q ~ 6000°K/min are evidently the most favorable for metal ignition. At T ~ 1373°K, the thermal stresses in the oxide shells increase considerably (q = 5000-15,500°K/min), but the oxidation rate is sufficiently large to heal even larger defects. Therefore, one observes only the formation of large agglomerates with readily visible necks between particles (Fig. 5).

Whisker crystals are formed throughout the temperature range, and microdiffraction indi- cates that they consist of y-Alz03. One assumesthat the whisker crystals produced before the formation of largecracks may facilitate the agglomeration to a certain extent, as they guide the metal emerging from the cracks. A separate discussion is required for the factors responsible for the occurrence and growth of the whisker crystals, particularly at low temperatures. Also, the initial state of the oxide film is very important in the agglomeration of aluminum powder particles.

We are indebted to Yu. V. Frolov for interest and valuable discussions.

LITERATURE CITED

i. V. D. Gladun, Yu. V. Frolov, et al. Fiz. Goreniya Vzryva, 12, No. 2, 191 (1976). 2. V. D. Gladun, Yu. V. Frolov, and L. Ya. Kashporov, Fiz. Goreniya Vzryva, 13, No. 5, 705

(1977). 3. V. G. Grigor'ev, K. P. Kuznetsov, and V. E. Zarko, Fiz. Goreniya Vzryva, 17, No. 4, 9

(1981). 4. A. A. Razdobreev, A. I. Skorik, et al., Fiz. Gorenlya Vzryva, 17, No. 6, 63 (1981). 5. Metallurgists' Handbook on Nonferrous Metals [in Russian], Metallurgiya, Moscow (1971). 6. Metallurgists' Handbook on Nonferrous Metals [in Russian], Metallurgiya, Moscow (1955). 7. G. V. Samsonov (ed.), The Physicochemical Properties of Oxides (Handbook) [in Russian],

Metallurgiya, Moscow (1978). 8. Yu. I. Petrov, Fiz. Tverd. Tela, 5, No. 9, 2461 (1963). 9. J. E. Crump, J. L. Prentice, and K. J. Kraeutle, Combust. Sci. Technol., i, No. 3, 205

(1969). i0. V. A. Ermakov, A. A. Razdobreev, et al., Fiz. Goreniya Vzryva, 18, No. 2, 141 (1982). ii. M. Ya. Gen, Yu. V. Frolov, et al., in: Burning Processes in Chemical Technology and

Metallurgy [in Russian], Chernogolovka (1973). 12. A. K. Lokenbakh, V. V. Strod, et al., Izv. Akad. Nauk Latv. SSR, Ser. Khim., No. i, 50

(198l). 13. W. C. Sleppy, J. Electrochem. Soc., I!I, No. 8, 903 (1964). 14. B. S. Mitin and V. V. Samoteikin, Zh. Fiz. Khim., 45, No. 3, 730 (1971). 15. E. Sturm and H. Winterhagen, Aluminum, 54, No. 6, 380 (1978). 16. P. F. Pokhil, A. F. Belyaev, et al., The Combustion of Metal Powders in Active Media

[in Russian], Nauka, Moscow (1972). 17. L. Brewer and A. W. Searcy, J. Am. Chem. Soc., 73, No. ii, 5308 (1951).

77