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Exploding Wire Method (also known as EWM) is a process by which a dense rising current is applied to a thin electrically conductive wire. The heat vaporizes the wire, and an electric arc over that vapor creates a shockwave and explosion. Exploding Wire Method is best known to be used in a detonator in nuclear munitions, as a high intensity light source for high speed photography, and for the production of metal Nanoparticles.

History

The first documented case of using electricity to melt a metal is credited to Martin van Marum who melted 70 feet of metal wire with Leyden Jars as a capacitor. Meanwhile, Benjamin Franklin vaporized thin gold leaf to burn an image onto paper.[1] While neither Marum nor Franklin actually incited the exploding wire phenomenon, they were important steps towards its discovery.

Edward Nairne was the first to note the existence of the exploding wire method in 1774 with silver and copper wire. Subsequently Faraday used EWM to deposit thin gold films through the solidification of vaporized metal on adjacent surfaces. Then, vapor deposits of metal gas as a result of EWM were studied by August Toepler during the 19th century. Spectrography investigation of the process became widespread over the 20th century, which enabled a better understanding and subsequently the first glimpses of practical application. 1946 saw the first production of nanoparticles in aluminum, uranium and plutonium wires. EWM as a light source attracted a lot of research attention during the mid 20th century. Congruently, the Manhattan Project found use for it in the development of nuclear arms detonators. [1] [2]

Current day research focuses on utilizing EWM to produce nanoparticles and further understanding of the phenomena.


Mechanism

The basic components needed for the exploding wire method are a thin conductive wire and a capacitor. The wire is typically gold, aluminum or platinum, and is usually less than 0.05mm in diameter. The capacitor has an energy consumption of about 25kWh/kg and discharges a pulse of charge density 104 - 106 A/mm2[3]. The phenomena occurs over a time period of only a couple of milliseconds.

The process is as follows:

  1. A rising current, supplied by the capacitor, is carried across the wire.
  2. The current heats up the wire until the metal begins to melt. The metal melts to form Unduloids. The current rises so fast that the liquid metal has no time to move out of the way.
  3. The unduloids vaporize. The metal vapor creates a lower resistance path, allowing an even faster current increase.
  4. An electric arc is formed, which turns the vapor into plasma. A bright flash of light is also produced.
  5. The plasma is allowed to expand freely, creating a shock wave.
  6. Electromagnetic radiation is released in tandem with the shock wave.
  7. The shock wave pushes liquid, gaseous and plasmatic metal outwards, breaking the circuit and ending the process.


Practical Application

EWM research has suggested possible applications in the excitation of optical masers, high intensity light sources for communications, spacecraft propulsion, joining difficult materials such as quartz, and generation of high power radio-frequency pulses.[1] The most promising applications of EWM are as a detonator, light source, and for the production of nanoparticles.

Detonator

EWM has found its most use as a detonator, named the Exploding-bridgewire detonator, for munitions from nuclear bombs to amateur plastic rockets. Their precision is advantageous over an alternative chemical fuse as the explosion is consistent and occurs only a few milliseconds after the current is applied.

Light Source

EWM is a great mechanism by which to get a short duration high intensity light source. The peak intensity for copper wire, for example, is 9.6e8 candle power/cm2[4]. The advantage of a flash produced in this is way is that it is easily reproducible with little variation in intensity. The linear nature of the wire allows for specifically shaped and angled light flashes. Different types of wires can be used to produce different colors of light. [5]The light source can be used in interferometry, flash photolysis,quantitative spectroscopy, and high speed photography.

Production of Nanoparticles

Nanoparticles are a relatively new material used in medicine, manufacturing, environmental cleanup and circuitry.

EWM can be used to cheaply and efficiently produce nanoparticles at a rate up to 200g/h[3]. Particles can be as small as .01μm but are most commonly below 100nm in diameter. As the voltage of the capacitor is raised, the particle diameter decreases. The product created is pure, with an impurity of below 1%[3].

When performed in a standard atmosphere containing oxygen metal oxides are formed. Pure metal nanoparticles can also be produced with EWM in an inert gas environment, usually argon. Pure metal nanopowders must be kept in their inert environment because they ignite when exposed to oxygen in air.[3]

Metal oxide nanoparticles are used in Catalysis, sensors, oxygen antioxident, self repairing metal, UV ray protection, odor proofing, improved batteries, printable circuits, optoelectronic materials, and Environmental remediation. [6][7]

References

  1. ^ a b c McGrath, J.R. (May 1966). "Exploding Wire Research 1774 - 1963". NRL Memorandum Report: 17. Retrieved 24 October 2014.
  2. ^ Hansen, Stephen (2011). Exploding Wires Principles, Apparatus and Experiments (PDF). Bell Jar. Retrieved 24 October 2014.
  3. ^ a b c d Kotov, Yu (2003). "Electric explosion of wires as a method for preparation of nanopowders" (PDF). Journal of Nanoparticle Research (5).
  4. ^ Conn, William (October 28, 1949). "The Use of "Exploding Wires" as a Light Source of Very High Intensity and Short Duration". JOURNAL OF THE OPTICAL SOCIETY OF AMERICA. 41 (7). Retrieved 30 October 2014.
  5. ^ Oster, Gisela K.; Marcus, R. A. (1957). "Exploding Wire as a Light Source in Flash Photolysis". The Journal of Chemical Physics. 27 (1): 189. doi:10.1063/1.1743665. Retrieved 2 November 2014.
  6. ^ Boysen, Earl. "Nanoparticles Applications and Uses". understandingnano. Retrieved 2 November 2014.
  7. ^ Oskam, Gerko (24 February 2006). "Metal oxide nanoparticles: synthesis, characterization and application" (PDF). Journal of Sol-Gel Science and Technology. 37 (3): 161–164. doi:10.1007/s10971-005-6621-2.
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