Magnesium electrical resistivity

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

A reduction in the ohmic drop across the electrolyte can be achieved either by a decrease of the electrical resistivity of the electrolyte or by a decrease of the interelectrode distance or lower current density. Table 12 shows that sodium and lithium salts increase the electrical conductivity of the electrolyte. The electrolysis products, magnesium and chlorine, get more easily in contact with each other and recombine by a decrease of the interelectrode distance. This so-called recombination reaction,... 

The preparation and electrical properties of magnesium mercury, MgjHgj, have been reported. The intermetallic compound was prepared by heating the elements together above 562 °C, at which temperature the compound melts, in an evacuated silica tube. Electrical resistivity and Hall-effect measurements were made from 2.4 to 297 K. No phase transition was observed over this range. The Hall constant was low and the ideal resistivity (the difference between total and residual resistivity) had the form p a T , where a = 1.9 and 1.0 at 11.50K and... 

Lustrous, silver-white surface (when freshly cut) face-centered cubic structure below 300 C. Ignites in air when finely divided, then burns with crimson flame. Much harder than sodium, but softer than aluminum or magnesium. Acquires bluish-gray tarnish on exposure to moist air. dj° 1.54. mp 85C. bp 1440. Electrical resistivity at 20 3.5 ohm cm. Brinell hardness 17. Heat of combustion 15l.9cal/g. sp ht (0-100 ) 0,149 cal/g. Considerably less reactive than sodium. E (aq) CaJ + /Ca —2.87 V. Reacts with water, alcohols, and dd acids with evolution of hydrogen. Reacts with halogens. Dissolves in liquid ammonia to form a blue soln. Contact with alkali hydroxides or carbonates may cause detonation. Bums in air. Calcium salts impart brick red color to a flame. Insol in and inert towards benzene, kerosene. 

Silvery-white metal hexagonal dose-packed structure-Slowty oxidizes in moist air. Available as bars, ribbons, wire and powder, mp 651°. bp 1100°. dM 1,738. Sp heat (20°) 0.245 cal/g. Heat of fusion 88 cal/g. Electrical resistivity 4.46 fiohm-cm, E° (aq) MgJ + /Mg —2.37 V. Reads very slowly with water at ordinary temp, less slowly at 100°. Reacts readily with dil adds with liberation of hydrogen reacts with aq solns of ammonium salts, forming a double salt. Reduces carbon monoxide, carbon dioxide, sulfur dioxide, nitric oxide, and nitrous oxide at a red heat. Burns in air continues to bum in a current of steam. Combines directly with nitrogen, sulfur, the halogens, phosphorus, and arsenic, Reacts with methyl alcohol at 200° giving magnesium methylate. 

The electrical resistivity of magnesium, at 273 K, is 4.05 x 10 m. Magnesium adopts the A3 stmcture with hexagonal lattice parameters of oq = 0.3209 nm, and cq = 0.5211 nm. The velocity of electrons at the Fermi surface is 1.58 x 10 ms. Each magnesium atom contributes two electrons to the stmcture. Calculate the relaxation time, T, and the mean free path. A, of the electrons. Compare A with the interatomic spacing of magnesium atoms in the crystal. 

Reasons for use abrasion resistance, cost reduction, electric conductivity (metal fibers, carbon fibers, carbon black), EMI shielding (metal and carbon fibers), electric resistivity (mica), flame retarding properties (aluminum hydroxide, antimony trioxide, magnesium hydroxide), impact resistance improvement (small particle size calcium carbonate), improvement of radiation stability (zeolite), increase of density, increase of flexural modulus, impact strength, and stiffness (talc), nucleating agent for bubble formation, permeability (mica), smoke suppression (magnesium hydroxide), thermal stabilization (calcium carbonate), wear resistance (aluminum oxide, silica carbide, wollastonite)... 

Specific fillers may be added to add or enhance specific properties in magnetic materials. Alumina, antimony trioxide or magnesium carbonate may be added to improve the flame retardant properties of the composite. The addition of lead oxide or carbide lead is suggested for improve resistance to nuclear radiation. To improve thermal conductivity, alumina, aluminium powder or silicates can be added. Silica, talc, mica or kaolin are indicated to increase the electrical resistance. The inclusion of metal powders, metal oxides or silicates are suggested to thermal absorption improvement. To increase the tensile strength in NR-based materials, carbon black in general, fibres or polymeric materials may be added. 

Kaschnitz, E., Reiter, P., and Potdacher, G. (2005) Spedfic heat, electrical resistivity, and linear thermal expansion coefficient of the magnesium alloy AE42 measured hy suhsecond pulse heating. Int.J. Thermophys., 26,1229-1237. 

The design and installation method for a buried structure using magnesium or any other suitable anode material as the source of current requires that the wiring system be well insulated with a good electrical resistance material otherwise, the insulation may be destroyed by the anodic reaction products once the anode is energized by a power supply. Also, moisture must exists at the backfiU-soil interface to provide a path for uniform current flow toward the active anode bed. 

In resistance thermometers (or RTDs) one uses the dependency of electrical resistance on the temperature [2). The resistance of metals almost always rises with temperature. In practice one uses platinum thermometers. A light platinum wire on a ceramic support is covered with a protective layer of glass, enamel, or magnesium oxide. This measurement resistance is the end of one someasurement element, which ends in connecting terminals. The whole assembly is in a sleeve, which is solidly connected to the measuring point in the apparatus with flanges or screws (Figure 12.2). 

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