dc electric field
Yu.G. Morozov, O.V. Belousova, M.V. Kuznetsov
In work influence of a dc electric field on formation of nickel oxide nanoparticles with given average sizes, specific surface and oxidation degree is investigated.
Nanopowders produced by M. Ja. Gen's modified levitation-jet method at constant Ni metal wire charge – 2,78∙10-7 kg/s. Identical parameters of oxide nanoparticles managed to be produced under various synthesis conditions when the oxidizer, passing through a zone of condensation of particles, could participate not only in their oxidation, but also influence the sizes of a condensation zone, and accordingly the average sizes of such particles. XRD has shown a presence of crystal lattice reflexes from high – temperature, cubic – NiO for synthesized nanoparticles, and also fcc nickel for nanoparticles with an medium and small degree of oxidation. EDX data, for all nanoparticles, also have shown only the presence of nickel and oxygen, in corresponding rates.
Dependences of specific surface of nanoparticles S and the residual contents of metal nickel v from intensity of an applied electric field, produced in a series of experiments are obtained at the constant helium supply – 1,67∙10-4 m3/s and the dependences received in the combined mode: the charge of helium - the same, the charge of air – 2,06∙10-5 m3/s. For the first series of experiments the appreciable maximum of value of the specific surface of nanoparticles with increase in intensity of the electric field up to 300 kV/m is found out. Thus reduction in the residual contents of metal nickel in the area of the maximal values of the specific surface is observed also. In the second series of experiments reduction of the specific surface of nanoparticles with increase in intensity of the electric field takes place only.
In microphotos of nanoparticles, obtained in above described conditions, strong-oxidated nanoparticles, practically pure NiO, have a strongly pronounced cubic shape and the average size in 11,5 nm. Nanoparticles, obtained under the same conditions, but at the presence of electric field E by intensity in 225 kV/m, are larger (19,2 nm), but their shapes and the nickel oxide contents have remained practically constant.
On dependence of the specific surface of an ensemble of nanoparticles from the electric field, produced in the combined mode (helium supply – 1,67∙10-4 m3/s, oxygen – 1,06∙10-5 m3/s), Is marked the maximum at nonzero intensity of the field though in the greater fields, the specific surface of such nanoparticles nevertheless decreases also their average size, accordingly, increases. The average residual contents of nickel exceeds the value for the case of synthesis with air stream participation, correlation S and v is weaker, however, the peak character of v (E) behavior in the field of drop on S (E) dependence nevertheless takes place. For nanoparticles, produced in the separate mode at similar values of the gases supplies, it is possible to note a presence rarely wide maximum of the specific surface value by increase in the electric field intensity. These results also are in good correlation with behavior of the residual contents of metal nickel in all the area of applied field values.
The analysis of microphotos of two samples of medium-oxidized nanoparticles from a series of experiments presents an influence of the electric field on processes of nanoparticles formation, has allowed to establish, that the average size of separate nanoparticles, obtained by intensity in 250 kV/m, in the presence of the electric field is equal 13,1 nm, without the field – 16 nm. These particles, contain besides NiO also enough amount of metal Ni. The shape of particles quite probably to consider as a result of agglomeration of various cubic and spherical particles.
At comparative study of XRD data for medium-oxidized nanoparticles, produced in the electric field and by it absence, it is possible to allocate the basic peaks for Ni and NiO components and to track after reduction of the metal content for the particles obtained by the field application. On magnetization hysteresis curves for the same nanoparticle ensembles it was observed a reduction of saturation magnetization that also has been interpreted as an increase in the oxidation degree (reduction in the metal core sizes) for ensembles of nanoparticles, produced with application of the electric field