Electric arc

An arc is a self-sustaining gas discharge between two electrodes, which must have a sufficiently high electrical potential difference ( = voltage ) to maintain the required high impact ionization current density. The gas discharge is a plasma, in which the particles (atoms or molecules ) are at least partially ionized. The free charge carriers have the effect that the gas is electrically conductive. Most plasmas are almost neutral, i.e., the number of ions and electrons is the same. Since the ions towards the much lighter electrons are much slower, the electrons are to carry electricity often almost entirely relevant.

Occurring in the electric power industry during switching operations arcs are referred to as switching arc. Unwanted arcs, which often result in injuries or damages are called arc.

Characteristics

Characteristic are here

• The relatively small compared to the glow discharge cathode fall (of the order of the excitation or ionization of the atoms involved, about 10 eV),
• A partially falling (not resistive ) current-voltage characteristic ( negative differential resistance),
• In comparison to the high current density in the glow discharge plasma,
• Gas and electron temperature are strongly coupled. It is usually approximately reaches the local thermal equilibrium.
• The gas pressures are relatively high (p > 0.1 bar).
• The gas temperature is 5,000 K to 50,000 K.

The phenomenon of the electric arc was discovered about the same time immediately after the preparation of the first high-performance power sources ( Voltaic pile by Alessandro Volta, 1800) by the British Humphry Davy and the Russians Petrov. Davy held two charcoal pencils, which were each connected to one pole of a voltaic pile, together and she moved to the beginning of the current flow slowly apart. For horizontal arrangement of the pins, which serve as electrodes, the plasma burns through its thermal buoyancy in the form of a characteristic arc which gave its name to this gas discharge. See also Jacob's Ladder.

Arcs through copper pipes need a minimum voltage of about 12 V and a minimum current of about 0.4 A. Next to send high frequency waves also typically intense infrared, visible and ultraviolet radiation.

Depending on the operating parameters of various processes may be largely responsible for the emission of electrons from the cathode material. An important parameter is the work function, which must be made so that electrons can leave the solid. This is in arcs through the existing external field reduced ( Schottky effect or Schottky humiliation ). Other relevant processes in the electron emission can be the following:

• Thermal emission (including thermionic emission, thermionic effect, Edison effect, Richardson effect or Edison - Richardson - called effect ),
• Field emission is that the existing electric field allows the electrons out of quantum mechanical tunneling from the solid.
• Thermionic field emission: Strong electric fields cause other effects that are not covered by the previous questions.
• Secondary electron emission: The cathode drop positive ions to the cathode are accelerated toward. In their impact they cause the release of electrons. Also can be emitted by excited atoms or ions of high energy photons ( in the UV or XUV range) that trigger from the cathode due to the external photoelectric effect secondary electrons.

Current account

In an arc, the plasma is heated by collisions of the accelerated electrons in an electric field with the heavy particles. The heat is transferred to the outside takes place by conduction. In addition, emission and absorption of radiation must be considered in the current account. The current account is:

Considering the pace of a volume element can be written for the enthalpy:

If we now consider a vertically disposed stationary driven cylindrical arc, then the current account can be represented easily. The flow is negligible ( the upward movement of a volume element in this case), and the terms radiation, one obtains a power balance, which describes the rotation-symmetrical heating and heat conduction to the outside:

The temperature profile of the arc depends on the gas used. Molecular gases are dissociated in the arc. In the radial portions, in which the dissociation of the molecules is highly increased and the thermal conductivity of the gas and the temperature gradient is very high and, accordingly, is steeper than in the use of monoatomic inert gases. Furthermore, it may also toward separation ( ambipolar diffusion, cataphoresis ) to come.

Technical Applications

Bulbs

Arcs were first used in lighting technology: arc lamps are the oldest electric light sources. Davy made ​​his first pertinent observations probably already about 1802, but published this until later (1812 ). The arcs were first operated in open air. There graphite electrodes were used, which burned down relatively quickly.

In high pressure mercury lamps, argon is used with a pressure of a few millibar, and mercury. The lamp is ignited by a high voltage pulse forms and a glow discharge from only. With increasing temperature, the mercury evaporates, the pressure increases in accordance with the mercury vapor pressure and the discharge passes into an arc discharge. In the spectrum of the arc to dominate the strong mercury lines.

The xenon short-arc lamp used in movie projectors and powerful spotlights. Xenon is in the visible spectrum, many optical transitions. In conjunction with high discharge pressures, a great line broadening is achieved, so that together with the continuum emission of free electrons overall a fairly continuous, daylight spectrum is emitted. The radiation source has a small spatial extent, and can therefore be well collimated by reflectors and lenses.

In addition, various variations have been established by arcs as radiation standards for certain wavelength ranges.

Electric welding

In TIG welding ( tungsten inert gas ) welding gas-or water -cooled tungsten electrodes are used, and there is a protective argon plasma generated. The welding filler material is melted on the plasma of a long arc which simultaneously melting the edges of the base material.

When electric welding of the burning of the electrode material is, however, desirable. Electrode and the edges of the base material are melted during welding and form the weld.

Steel production

An important application is electric arc furnace for producing steel in electric steel works.

Production of thin metallic layers

Another application is the production of thin metal layers by means of arc evaporation ( Arc- PVD). In this case, the atoms or molecules of a solid material ( target) is released and deposited on a substrate by means of the kinetic energy of the electrons of the arc. This method is used whenever there is wear-reducing titanium nitride films on cutting tools.

Chemical analysis

A classic application learns the arc in the spectral analysis for the determination of major and trace constituents mainly of solids. The material to be analyzed is vaporized in an arc, with the corresponding spectral lines are excited. The determination of the chemical elements on whose emitted lines and the determination of their share in the sample by the intensity of the emission to an optical emission spectrometer ( OES). Mainly direct current arcs with carbon or graphite electrodes are employed.

Drive means

Arc engines use the electrical resistance of an arc to a strong push gas to heat and thereby accelerate from a nozzle at high speed (> 4 km / s). Arc thrusters are used as engine of satellites to track receipt and train change maneuvers to perform. The thrust can be generated is much lower than with chemical combustion engines, the specific impulse, however, significantly better.

Future applications

Waste disposal

The U.S. company Startech operates in Bristol, Connecticut, a pilot plant for plasma gasification of waste by arc. Into the interior of the reaction vessel two protruding electrodes that are under high tension. The high voltage turns the air between them in electrically conductive plasma. Up to 17,000 degrees Centigrade are reached at the walls of the chamber, there are still 1700 degrees. The molecules of the materials introduced disintegrate into atoms: The inorganic components of the waste melt and collect at the bottom of the reactor. The organic substances on the other hand ( eg, plastics ) fizzle gas. In addition to hydrogen contained therein, especially carbon monoxide.

The problem with the method is the exorbitantly high energy consumption. In the near future, it should be economical only when the hazardous waste disposal.

Arc plasma reactor

This is a process for the production of acetylene from coal.

The process was developed in 1980 as a joint project of Huls AG ( Marl ) with DMT Society for the extraction of acetylene. The coal must be before the reaction very finely ground (particle size: 100 microns ) are. At very high temperatures 1000-2000 K in the arc plasma ( cathode made of tungsten doped with ThO2, anode made of copper) is a mixture of hydrogen and carbon particles at short contact times brought (a few ms ) to the reaction, water is formed by quenching with ethyne. The coal throughput of the pilot plant was about 350-500 kg / h at a coal conversion of 50%, an acetylene yield of 20/ 100 kg coal, a current of 1000 A, a voltage of 1250 V. The product mixture is next acetylene ( weight 25, 0 %), still a substantial proportion of carbon monoxide ( 19.9% ​​by weight ) and hydrogen ( 33.6% by weight ).

The method may be interesting in some regions of the world for the production of hydrocarbon compounds in the presence of low-cost coal and cheap electricity.