Exploding wire method
Exploding Wire Method (also known as EWM) is a high energy density process 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, high intensity light source, and production method for metal nanoparticles.
HistoryExploding Wire Method has a surprisingly long history for a process only recently appropriated. Progress on the comprehension of the mechanism was intermittent, and even at present day there are many aspects that remain not fully understood. One of the first documented cases of using electricity to melt a metal occurred in the late 1700s and is credited to Martin van Marum who melted 70 feet of metal wire with 64 Leyden Jars as a capacitor. Van Marum's generator was built in 1784, and is now located in the Teylers Museum in the Netherlands. Years later, Benjamin Franklin vaporized thin gold leaf to burn images onto paper. 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, Michael 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 1800s. Spectrography investigation of the process, led by J.A. Anderson, became widespread in the 1900s. 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 H. Johnston of the Manhattan Project found use for EWM in the development of nuclear detonators. Current day research focuses on utilizing EWM to produce nanoparticles as well as better understanding specifics of the mechanism such as the effects of the system environment on the process.
MechanismThe 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 25 kWh/kg and discharges a pulse of charge density 104 - 106 A/mm2, leading to temperatures up to 100,000 K. The phenomenon occurs over a time period of only 10−5-10−8 seconds. The process is as follows:
- A rising current, supplied by the capacitor, is carried across the wire.
- The current heats up the wire through ohmic heating until the metal begins to melt. The metal melts to form a broken series of imperfect spheres called unduloids. The current rises so fast that the liquid metal has no time to move out of the way.
- The unduloids vaporize. The metal vapor creates a lower resistance path, allowing an even faster current increase.
- An electric arc is formed, which turns the vapor into plasma. A bright flash of light is also produced.
- The plasma is allowed to expand freely, creating a shock wave.
- Electromagnetic radiation is released in tandem with the shock wave.
- The shock wave pushes liquid, gaseous and plasmatic metal outwards, breaking the circuit and ending the process.