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Thermal properties of metals

1. Melting point

The melting point is the temperature at which a crystalline substance melts, that is, the temperature at which the solid and liquid states of the substance can coexist in equilibrium. The melting points of various metal crystals vary greatly because there is more than one factor that affects the melting point. In addition to the basic similarity of the structures of various metal crystals, there are other factors, such as the mass of atoms, the size of nuclear charge, the arrangement of atoms, etc. For the same metal crystal, the melting point is related to the pressure it is subjected to. Under a certain pressure, the melting point of a metal crystal is the same as its solidification point. Melting point is one of the main indicators to be considered when using metal materials in the production of electric light sources.

2. Thermal conductivity

Thermal conductivity represents the ability of metal materials to transfer heat, and its unit is W/(℃.cm) [watt/(degree·cm)]. In the production of electric light sources, proper heat conduction can ensure the stability of the operating temperature of certain parts (such as the electrodes of discharge lamps). Thermal conductivity determines the speed of heat conduction on metal parts.

Generally speaking, metals are good conductors of heat, but their abilities vary. The relative order of thermal conductivity of some metals is as follows:

Ag>0u>Al>Zn>r'e>Pb>Hg. Silver and copper have the best thermal conductivity, so copper is often used as the outer conductor of electric light sources.

The thermal conductivity of metals is the result of the exchange of electron energy between the high-temperature part and the low-temperature part of the metal. When a part of the metal is heated, the energy of the free electrons there increases, and the movement speed also increases. Therefore, through collision, the free electrons transfer energy to other metal ions. The metal transfers energy from the high-temperature part to the low-temperature part through the movement of free electrons, so that the whole piece of metal reaches the same temperature.

3. Specific heat

Specific heat is the amount of heat that a unit amount of substance needs to absorb when the temperature rises by 1°C. Its unit is J/(kg.℃) [joule/(kilogram·degree)].

The specific heat of various substances is different. For the same substance, the size of the specific heat is related to the conditions during heating, such as the temperature, pressure and volume changes. Even the same substance has different specific heats in different physical states. As the heat radiator (filament) of an incandescent lamp, it is better to have a lower specific heat. As the anode of a discharge lamp, it is better to have a higher specific heat.

4. Thermal evaporation rate

The vaporization of liquid substances or the sublimation of solid substances into gaseous substances is called evaporation. The amount of substances evaporated from a unit area in a unit time is called the evaporation rate, and its unit is g/(cm².s) [gram/(centimeter·second)].

In electric light sources, metal parts often work in a vacuum or high temperature state, which is conducive to the evaporation of materials. If the evaporation rate is too large, the service life of the electric light source will be shortened or the stability of the working parameters will be affected. The evaporation material condenses on the colder parts, which will destroy the insulation and cause the glass shell to lose transparency. The evaporation rate of the metal is related to its structure, pressure and temperature. As a filament and electrode, it is hoped that the evaporation rate is as small as possible. In fact, the evaporation temperature is generally used to represent the evaporation characteristics of the substance. At this temperature, the evaporation rate of the substance can no longer be ignored. It is roughly equivalent to the temperature when the saturated vapor pressure of the metal is about 133.3x10- Pa (1x10- Torr).

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