Unique feature of vacuum arc is the concentration of all rather high (~100A and more) discharge current in microscopic (~10 mkm) cathode spots (CS) on the cathode. Current and power density in these cathode spots as well as concentration and pressure of formed plasma may reach superior high values (100 MA/cm2 and 109 W/cm2, 1020 cm-3 and 1010 Pa accordingly) at CS life time ~10-7 s.
Cathode spots shape in the result of explosive electron emission from micrononuniformity of cathode surface from exposure to high (~10MV/m) local electric field. After microexplosion from one point of cathode surface (from one CS) discharge voltage restores and initiates microexplosion from another point of cathode surface (from another CS). So CS is a powerful microscopic reactor with very short life time that provides powerful generation of high ionized cathode material vapor as well as generation of accompanying microdrops from region of superior high pressure and temperature to vacuum. Such conditions are very favourable for nanopowder production. Unlike other methods of nanopowder production (where ensuring of acceptable conditions is one of the main and hard problem for realization) these conditions are given by nature itself in the case of vacuum arc.
Two mechanisms of nanopowder production in vacuum arc are possible:
In accordance with first mechanism, nanoparticles form from liquid microdrops (with dimensions of 0,1 – 1,0 mkm) that are generated in vacuum arc. When reaching some critical electrical charge microdrop starts to be divided in the result of Relay instability. Conditions of microdrop critical charge accumulation may be controlled by means of arc plasma parameters. Cooling rate of nanodrops (that were formed in the result of microdrop division) is no less than 107 K/s at vacuum arc conditions. This is sufficient for amorphous transformation at nanodrops solidifying and prevent from following particles amalgamation at the expense of their coalescence. Taking into consideration that microdrops quantity (generated by a cathode spot) may reach 105 and frequency of cathode spots formation may reach 107 s-1, rate of microdrops generation may reach 1011.s-1. This value exceeds output of laser dispersion (at pulse frequency ~10 Hz) by a factor of 104 at least.
In accordance with second mechanism, nanoparticles form from cathode material vapor generated by cathode spot. Nanoparticles synthesis is harnesses in the process of vapor natural expansion from cathode spots to vacuum, vapor cooling and nucleation. It is important that stay duration of nanoparticles in zone of nucleation and growth is very limited as a cathode spot size is very small (~10 mkm). In combination with active and uniform cooling of vapor (that is the result of small cathode spot size too) this allow to reach superior small sizes of synthesized nanoparticles as well as superior small of their sizes variance. No interacting (in the process of vapor natural expansion) ionized vapor fraction may be transported by means of electromagnetic fields to a zone of secondary interaction where secondary nanoparticles synthesis may be realized. In this case range of materials of synthesized nanoparticles may be increased to a considerable degree with the introduction of reactive gases (oxygen, nitrogen, acetylene, etc.) in a zone of secondary interaction (with nanoparticles synthesis of oxides, nitrides, carbides of any metals in the result of interaction these reactive gases with transported ionized vapor flow accordingly).
Proposed method of nanopowder production has high manufacturability (it doesn’t need in precursor and cooling gases in comparison with other plasma and laser methods) and capacity (that may be increased by arc current increase without any limit). Moreover nanopowder manufacturing takes place in vacuum condition that is guarantee of high quality of product.
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