Ignition and passivation of nanopowders and compact samples made of nanopowders

Abstract

It is experimentally shown that at the temperature lower than 0 °C, Ni nanoparticles do not ignite in dry air; however, passivation occurs and provides the further stability of nanoparticles at room temperature. It is shown that the samples of Ni nanoparticles passivated in dry air contain only nickel. By means of the scanning electronic microscopy and AES method, it is established that passivation at the temperature lower than 0 °C prevents agglomeration and coalescence of Ni nanoparticles; therefore, the size of nanoparticles after passivation at −8 °C is less than for those passivated at 20 °C. This is a further proof that theoretical approaches of the classical macroscopic theory of thermal explosion can be applied for the description of the phenomena of ignition of macroscopic objects of nanoparticles. Nickel nanopowders have been synthesized by the chemicometallurgical method under dynamic conditions, when the composition of the gas medium is changed in the process of synthesis with the retention of the overall duration of synthesis, for the first time. It has been demonstrated that the specific surface area of the nanopowder and the average size of nickel nanoparticles are almost independent of the order of treatment with argon and hydrogen flows. It has been established that the dynamic nickel nanopowder synthesis method provides efficient control over the average size of nanoparticles. A model of the porous sample ignition is proposed, based on an assumption of a limiting role of the oxidizer diffusion in the ignition mechanism. It is shown that the ignition process can have a two-stage character. The duration of the stages is estimated by the methods of combustion theory. The applicability limits of the semi-infinite body model were determined. The role of the finite size of a sample in the ignition process was analyzed. A model of passivation of a layer of a pyrophoric nanopowder was formulated and studied by analytical and numerical methods. Under the assumption that the passivation wave propagation is controlled by the oxidant diffusion, the dependence of the temperature in the heat-release zone on governing parameters was obtained. It was shown that, for the maximum temperature in the nanopowder layer, there is intermediate asymptotic behavior characterized by time-invariant temperature. To decrease the passivation time, while keeping the temperature rise at permissible level, passivation was proposed to be carried out in two stages with increasing oxidant concentration in the gas phase. Analytical expressions were derived for the minimum switch time at given temperature rise. Numerical computations confirmed the correctness of the conclusions drawn from the theoretical analysis and also showed a good quantitative agreement between the results of the approximate and numerical calculations. It was shown that a decrease in initial nanopowder temperature leads to a transition from the layer-by-layer passivation mode to the bulk passivation mode, which enables one to perform passivation even at high oxidizer concentration in the gas. Analytical expressions for determining the boundaries of the layer-by-layer and bulk passivation modes were derived. Numerical calculations confirmed the correctness of the conclusions of the theoretical analysis. Copper nanopowders were synthesized both by a hydrogen reduction method (a method of chemical metallurgy) and thermal decomposition of copper citrate and formate. It is shown that Cu nanopowder synthesized from Cu citrate is not pyrophoric. Its combustion can be initiated with an external source; the velocity of combustion wave makes 1.3 ± 0.3 mm/s. The nanopowder has a specific surface four times greater ~(45 ± 5 m2/g) than the oxide obtained by the reduction method, it does not practically contain oxides, and it is stable in ambient air. Cu nanopowder obtained by the method of chemical metallurgy is pyrophoric; however, its passivation leads to formation of noticeable amounts of Cu oxides. Combustion velocities of the passivated and nonpassivated Cu nanopowder were almost equal (0.3 ± 0.04 mm/s). Tungsten nanopowders were synthesized by a hydrogen reduction method (a method of chemical metallurgy) at 440–640 °C from the WO3 precursors with different specific surface: 2 m2/g (I), 11 m2/g (II) and 0.8 m2/g (III). It has been shown that W nanopowder synthesized at 640 °C from the precursors (I)–(III) is α-W; it is not pyrophoric. Its combustion can be initiated by an external source; the combustion develops in the finger-like mode. The nanopowder synthesized at 480 °C from the precursors (I), (II) is the mixture of α-W, β-W and WO2.9; it is pyrophoric at the expense of β-W. Nanopowder synthesized at 480 °C from the precursor (III) is β-W with the traces of WO3 and WO2.9. The temperature interval of β-W synthesis obtained in the work is very narrow: 470–490 °C. α-W nanopowders have a specific surface 10 ± 2 m2/g; passivated nanopowder of β-W has a specific surface 18 ± 1 m2/g. It was shown that the self-heating of a compacted sample made of nonpassivated iron nanopowder is not uniform, although it begins simultaneously within the entire surface of the sample. It is found that the maximum temperature of self-heating decreases with an increase in relative density of samples, which indicates that the oxidation process is limited by the diffusion supply of oxidant. It is shown that the process of interaction of samples with air occurs in a superficial mode. A qualitative agreement of the results of the theoretical analysis with experimental data is obtained. The dependence of the mode of interaction of samples with the air on the duration of the exposure of weighing bottles to the air is revealed. The possibility of passivation of compacted samples made of iron and nickel nanopowder is experimentally established.