時間：2011-03-11 23:11來源：藍天飛行翻譯 作者：admin 點擊：次
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dichromate solution. A solution of potassium or sodium dichromate used to form a hard, airtight film on the surface of magnesium parts to prevent corrosion. dichromate treatment (corrosion protection). A corrosion-preventive treatment for magnesium alloys. A tight oxide film is formed on the surface of a magnesium alloy when it is treated with a solution of potassium or sodium dichromate. die (metal-cutting tool). A cutting tool used to cut threads on the outside of a piece of round stock. die (permanent mold). A permanent mold used to cast parts from metal or plastic resin. Material in its plastic state is forced into the die under pressure. die casting. A method of forming metal parts by forcing metal in its plastic state into a high-strength die, using a large amount of force. Die-cast parts have more strength than parts cast by pouring the molten metal into a mold. dielectric. An insulating material that can store electrical energy in an electrostatic field. The valence electrons in a dielectric material can be pulled away from their atom only with a strong electrical force, but the orbits of these electrons can be distorted by an electric field. Electrical energy is stored in this distorted field. Glass and ceramics are examples of good dielectrics. dielectric constant. A number used to indicate the ease with which a particular dielectric material concentrates lines of electrostatic force. A vacuum is used as the reference and is assigned a dielectric constant of one. The dielectric constant of air is very near that of a vacuum; it is 1.0006. Mica allows the lines of electrostatic force to pass much more easily, and it has a dielectric constant of 7. The dielectric constant of the insulating material in a capacitor determines its capacity. dielectric heating. A process used to heat certain types of nonconductive plastic materials. The material to be heated forms the dielectric, or insulator, of a capacitor, and two metal electrodes act as the plates. Alternating current with a frequency of between 20 and 80 megahertz is placed across the plates. The material between the plates is heated by the dielectric losses caused by the friction of the molecules moving under the influence of the rapidly changing electrostatic field. Dielectric heating is used to seal bags made of vinyl film and to cure certain types of plastic resins. dielectric loss. Electrical energy converted into heat in a dielectric material. Dielectric loss occurs when the material is subjected to a rapidly changing electrostatic field. Power is lost, and heat is produced. dielectric strength. The maximum amount of voltage that can be placed across a dielectric material without it breaking down and conducting. Dielectric strength is normally measured in volts per millimeter of thickness. diesel engine. A form of compression-ignition reciprocating heat engine. In a diesel engine, the fuel-air mixture inside the cylinder is ignited by the heat of compression rather than by an electric spark. |
