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Method and equipment for magnetic pulse pressing of powders

For the first time in the world practice, the laboratory of applied electrodynamics developed the use of magnetic pulse pressing (MPP) to produce bulk nanostructured materials. The principle of this method consists in the electromechanical conversion of the energy of the primary capacitive storage into the kinetic energy of the press tool, performing work on the compression of the powder. The method is characterized by soft pulsed compression waves in powders with amplitude of up to 2 GPa (with repeated use of the press tool) and duration in the range of 10-500 µs. The method allows generating higher pulse pressures, about 10 GPa, but with a single use of the press tool.

Soft pulsed compression waves prove to be a very effective tool for compacting powders with particle sizes < 100 nm. When pulse compression of nanopowders successfully combined simultaneous actions of the following significant factors.

  • High pulse pressure contributes to the force compaction of nanoparticles.
  • The influence of a large mechanical pulse of particles is expressed in a significant decrease in the role of potential interparticle interaction, which prevents the mutual movement of nanoparticles, which can be considered as an increase in the mobility of particles, and at the macro level - as a decrease in internal friction. This allows you to obtain the pressing of nanopowders with higher density. Moreover, the role of this effect is enhanced by reducing the average particle size in the powder.
  • Due to the adiabatic nature of the process, soft pulsed pressing is characterized by a significant pulsed heating of the compressible powder, which improves its compressibility and is able to stimulate structural and phase transformations.
  • Due to the transience of pulse pressing in some cases it is possible to maintain metastable structural-phase States of the powders preferred for the bulk nanomaterial being formed.

The equipment for magnetic pulse pressing of nanopowders of various materials has been developed and created. The fundamental difference between our designs pulse of presses is the possibility of compaction of nanosized powders.

Experimental facilities are economical; allow automation of the process and a significant increase in productivity. Pulse pressing of nanopowders can be carried out at temperatures up to 500°C in a vacuum after thermal degassing, which ensures the removal of adsorbed substances from the surface of the particles.

In the existing MPP installations of two types, pressing is implemented using flat and radially converging compression waves.

The principle of operation of a flat (uniaxial) MP press is explained by the scheme. The uniaxial press contains a flat spiral inductor and a nearby mechanical concentrator (massive conductive plate), separated from the inductor by a thin insulating gap. The inductor and the concentrator together represent an axisymmetric inductor system. When the discharge current i of the storage device passes through the inductor in the gap between it and the concentrator, a pulsed magnetic field B is created, inducing a current of density j in the conductive surface of the concentrator. The resulting pulse force fM, pushing the concentrator out of the magnetic field, is the result of the interaction of the induced current j and the magnetic field. Hub, accumulating the mechanical momentum, leads to the movement of the punch compresses the powder in the matrix.

Characteristics of the uniaxial magnetic-pulsed press:

  • amplitude of impulse forces is up to 900 kN;
  • the duration of the pulse power is 200 – 500 µs;
  • the energy intensity of the primary drive is 30 kJ;
  • current in the discharge circuit is up to 50 kA;
  • the amplitude of the magnetic field of the inductor is up to 15 T;
  • diameter of pressing of nanopowders is up to 40 mm;
  • pressing height is less than diameter;
  • degassing before pressing
    - at temperatures up to 600°C,
    - when the degree of vacuum to 1 Pa.

The main nodes of the setup: a generator of pulse currents; the press frame with the inductor system; a degassing unit; press tool; a sensor to measure the current in the inductor, the pressing pressure and displacement of press tool.

The appearance of the press and the block diagram are shown in the following figures.


The characteristic time dependences of the pressing pressure, P(t), and the current in the inductor, i(t), on the example of pressing a disk of nanopowder Al2O3 with a diameter of 30 mm, are shown in the graph.

The technique of magnetic pulse compression of thin-walled copper tubular shells is worked out for pressing long blanks of nanopowders in the form of pipes and rods. This shell performs the function of a radially convergent cylindrical punch. The principle of radial compression of the shell under the pressure of the magnetic field generated by a large pulse current flowing along the pipe (z-pinch circuit) is used. The scheme of pressing of powder tubular billets is shown in the figure. Before pressing, the powder 3 is placed in a tubular cavity between the copper pipe 1 and the rod 2 of a solid material that performs the function of forming a channel. After degassing the powder inside a special box is carried out in-situ gas-tight encapsulation of the mold with gaskets 4 and 5. This allows you to manipulate the mold in the air, without fear of secondary penetration into the pressed powder adsorbed substances. The prepared mold is connected to a pulse current generator with a capacitive energy storage device C. During pressing, a large pulse current flows along the copper shell and returns to the generator through an axisymmetric massive external metal pipe.

The characteristics of the radial magnetic-impulse press:

  • the amplitude of the pulse pressure is up to 2 GPa;
  • duration of the pulse pressure is 10-50 µs;
  • power consumption of primary storage is 135 kJ;
  • current in the discharge circuit is up to 2 MA;
  • the amplitude of the working magnetic field is up to 50 T;
  • diameter of pressing of nanopowders is up to 20 mm;
  • pressing length is up to 250 mm;
  • degassing before pressing
    - at temperatures up to 600°C,
    - when the degree of vacuum to 1 Pa.
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