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Exploding Wire Method (also known as EWM) is a process centered around a high energy density phenomena by which a 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 as a detonator in nuclear munitions, as a high intensity light source, and for the production of metal nanoparticles.

History

Exploding Wire Method has a surprisingly long history for a process only recently appropriated. It wasn't until the 1900's that research transitioned from investigation to application. Many famous scientists interested in electricity at one point or another came upon and studied exploding wires. Understanding the phenomena has been a challenge that many have tried their hand at, making incremental steps towards total comprehension. Even at present day, there are many aspects that deserve further study.

The first documented case of using electricity to melt a metal occurred in the 1700's [1] and is credited to Martin van Marum who melted 70 feet of metal wire with 64 Leyden Jars as a capacitor. Shortly after, Benjamin Franklin vaporized thin gold leaf to burn images onto paper.[2][3] While neither Marum nor Franklin actually incited the exploding wire phenomenon, they were both 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 1800's. Spectrography investigation of the process, lead by J.A. Anderson, became widespread in the 1900's. The spectrography experiments enabled a better understanding and subsequently the first glimpses of practical application. The mid 20th century saw experiments with EWM as a light source and for the production of nanoparticles in aluminum, uranium and plutonium wires. Congruently, Luis Álvarez and Lawrence Johnston of the Manhattan Project found use for it in the development of nuclear detonators. [3] [4]

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, iron or platinum, and is usually less than 0.5mm in diameter. The capacitor has an energy consumption of about 25kWh/kg and discharges a pulse of charge density 104 - 106 A/mm2,[5] leading to temperatures up to 100,000K. The phenomena occurs over a time period of only 10-5-10-8 seconds. [6]

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 through ohmic heating 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.[3] 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 an effective 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.[7] J.A. Anderson wrote in his initial spectrography studies that the light was comparable to a black body at 20,000K.[8] The advantage of a flash produced in this 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 and different types of wires can be used to produce different colors of light.[9] The light source can be used in interferometry, flash photolysis, quantitative spectroscopy, and high-speed photography.

Production of Nanoparticles

EWM can be used to cheaply and efficiently produce nanoparticles at a rate of 50-300 grams per hour.[6] The product created is very pure, with an impurity level of below 1%.[5] The particles are created when the ambient gas of the system cools the vaporous metal.[10] The process requires a relatively low energy consumption as little energy is lost in an electric to thermal energy conversion. Environmental effects are minimal due to the process taking place in a closed system. The Particles can be as small as 10nm but are most commonly below 100nm in diameter. Physical attributes of the nanopowder can be altered depending on the parameters of the explosion. For example, as the voltage of the capacitor is raised, the particle diameter decreases. Also, the pressure of the gas environment can change the dispersiveness of the nanoparticles.[6]

When performed in a standard atmosphere containing oxygen, metal oxides are formed. Pure metal nanoparticles can also be produced with EWM in an inert environment, usually argon gas or distilled water.[11] Pure metal nanopowders must be kept in their inert environment because they ignite when exposed to oxygen in air.[5] Often, the metal vapor is contained by operating the mechanism within a steel box or similar container.

Nanoparticles are a relatively new material used in medicine, manufacturing, environmental cleanup and circuitry. Metal oxide and pure metal nanoparticles are used in Catalysis, sensors, oxygen antioxident, self repairing metal, ceramics, UV ray protection, odor proofing, improved batteries, printable circuits, optoelectronic materials, and Environmental remediation. [12][13] The demand for metal nanoparticles, and therefore production methods, has increased as technology continues to progress with an emphasis on nanotechnology. Despite its overwhelming simplicity and efficiency, EWM has not seen widespread utilization in material production industry due to issues in manufacturing quantity.

References

  1. ^ Dibner, [by] Herbert W. Meyer. Foreword by Bern (1972). A history of electricity and magnetism (PDF). Norwalk, Conn.: Burndy Library. p. 32. ISBN 026213070X.
  2. ^ Holcombe, J.A.; Sacks, R.D. (March 16, 1973). "Exploding wire excitation for trace analysis of Hg, Cd, Pb and Ni using electrodeposition for preconcentration" (PDF). Spectrochimica Acta. 22B: 451–467. Retrieved 2 November 2014.
  3. ^ a b c McGrath, J.R. (May 1966). "Exploding Wire Research 1774 - 1963". NRL Memorandum Report: 17. Retrieved 24 October 2014.
  4. ^ Hansen, Stephen (2011). Exploding Wires Principles, Apparatus and Experiments (PDF). Bell Jar. Retrieved 24 October 2014.
  5. ^ a b c Kotov, Yu (2003). "Electric explosion of wires as a method for preparation of nanopowders" (PDF). Journal of Nanoparticle Research (5).
  6. ^ a b c Nazatenko, O (16 September 2007). "Nanopowders produced by electrical explosion of wires" (PDF). Dept. of Exology Tomsk Polytechnic University. Retrieved 6 November 2014.
  7. ^ 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.
  8. ^ Anderson, J.A. (May 22, 1922). "The Spectral Energy Distribution And Opacity Of Wire Explosion Vapors" (PDF). Mount Wilson Observatory, Carnegie Institution of Washington. 8: 1. Retrieved 2 November 2014.
  9. ^ 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.
  10. ^ Mathur, Sanjay; Sing, Mrityunjay (2010). "Nanostructured Materials and Nanotechology III". Ceramic Engineering and Science Proceedings. 30 (7): 92. Retrieved 2 November 2014.
  11. ^ Alqudami, Abdullah. "Fluorescence from metallic silver and iron nanoparticles prepared by exploding wire technique" (PDF). Dpt. of Physics and Astrohpysics New Delhi: 15. doi:110 00. Retrieved 2 November 2014. {{cite journal}}: Check |doi= value (help)
  12. ^ Boysen, Earl. "Nanoparticles Applications and Uses". understandingnano. Retrieved 2 November 2014.
  13. ^ 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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