Coulomb explosion

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The Coulomb explosion is a process in which, e.g. B. by a focused laser beam , a solid material changes into the plasma state.

Mode of action

The physical mechanism is that the electrons are excited by the strong electromagnetic field of the beam and their excitation energy is coupled to the atomic motion. Atomic motion can break the bonds that hold solid bodies together. The outer valence electrons , which are responsible for the chemical bond, can easily be separated from the atom and leave it positively charged. Atoms whose bonds are broken in this way repel each other, and the material explodes into a small cloud of high-energy ions that travel faster than if they were vaporized with heat.

For the explosive reactions of most alkali metals with water, a kind of naturally occurring Coulomb explosion is postulated as the trigger on the basis of observations made with high-speed cameras. The rapid transition of the valence electrons into the water creates strong repulsive forces between the metal cations within the alkali metal, which results in a strong increase in the reaction surface.

Cnidarians may use the principle of the Coulomb explosion when shooting down the contents of their nettle cells .

Technical design

The Coulomb explosion is one of the mechanisms by which laser metalworking ( laser cutting , laser drilling , laser ablation , sometimes laser marking , etc.) becomes possible.

Coulomb explosions for industrial material processing are triggered by short laser pulses (in the picosecond or high femtosecond range). The required extremely high beam intensities (10 to 400 TW / cm 2 ) can only be handled in practice for a very short period of time. The impulses are short enough to localize the thermal removal: the energy supply (the impulse) ends before the heat is passed on.

Comparison with melting processes

The Coulomb explosion is a “cold” alternative to the prevailing process of thermal ablation , which is based on the local heating, melting and evaporation of the molecules and atoms by means of less intense radiation (see e.g. laser marking). With such lower-energy lasers, thermally removed materials can close pores, which are important for catalysts or battery operation, and recrystallize or even burn the substrate, thus changing the physical and chemical properties of the machined area. In contrast, even light foams remain unsealed when processed by Coulomb explosions.

application

Coulomb explosions can be used on any material to drill holes, remove surface layers and give surfaces a texture or microstructure; z. B. to control the amount of ink adhering to the roller in printing machines .

Individual evidence

  1. M. Hashida, H. Mishima, S. Tokita, S. Sakabe: Non-thermal ablation of expanded polytetrafluoroethylene with an intense femtosecond-pulse laser . In: Optics Express . tape 17 , no. 15 , 2009, p. 13116-13121 , doi : 10.1364 / OE.17.013116 .
  2. PE Mason, F. Uhlig, V. Vaněk, T. Buttersack, S. Bauerecker, P. Jungwirth: Coulomb explosion during the early stages of the reaction of alkali metals with water. In: Nature chemistry. Volume 7, No. 3, 2015, pp. 250-254, doi: 10.1038 / nchem.2161 , PMID 25698335 .
  3. ^ Stefan Berking, Klaus Herrmann: Formation and discharge of nematocysts is controlled by a proton gradient across the cyst membrane. In: Helgoland Marine Research. Volume 60, No. 3, 2006, pp. 180-188, doi: 10.1007 / s10152-005-0019-y
  4. Dirk Müller: Picosecond Lasers for High-Quality Industrial Micromachining. In: Photonics Spectra. Volume 43, No. 11, 2009, pp. 46-47 ( HTML version )