Thermal conductivity of metals and its application

Metals are substances that havecrystal structure. When heated, they are able to melt, that is, go into a fluid state. Some of them have a low melting point: they can be melted by placing them in a regular spoon and holding candles above the flame. It's lead and tin. Others can be melted only in special ovens. Copper and iron have a high melting point. To reduce it, additives are added to the metal. The resulting alloys (steel, bronze, cast iron, brass) have a melting point lower than the parent metal.

On what does the melting point of metals depend? All of them have certain characteristics - heat capacity and thermal conductivity of metals. Heat capacity refers to the ability to absorb heat when heated. Its numerical index is the specific heat. It means the amount of energy that can absorb a unit of mass of metal, heated by 1 ° C. From this indicator depends fuel consumption for heating the metal preform to the desired temperature. The heat capacity of most metals is in the range 300-400 J / (kg * K), metal alloys - 100-2000 J / (kg * K).

Thermal conductivity of metals is the transfer of heatfrom the hotter particles to the colder ones according to the Fourier law for their macroscopic immobility. It depends on the structure of the material, its chemical composition and the type of interatomic bond. In metals, heat is transferred by electrons, in other solid materials by phonons. The thermal conductivity of metals is the higher, the more perfect the crystal structure they have. The more metal has impurities, the more distorted the crystal lattice, and the lower the thermal conductivity. Doping introduces such distortions into the structure of metals and lowers the thermal conductivity relative to the base metal.

All metals have good thermal conductivity, butsome higher than others. An example of such metals is gold, copper, silver. The lower thermal conductivity is found in tin, aluminum and iron. Increased thermal conductivity of metals is a virtue or a disadvantage, depending on the scope of their use. For example, it is necessary for metal dishes for rapid heating of food. At the same time, the use of metals with high thermal conductivity to make the handles of dishes makes it difficult to use - the knobs heat up too quickly and can not be touched. Therefore, heat insulating materials are used here.

Another characteristic of the metal that affects itsproperties - thermal expansion. It looks like an increase in the volume of the metal when it is heated and decreases with cooling. This phenomenon must be taken into account when manufacturing metal products. For example, the lids of the pots are made overhead, the kettles also have a gap between the lid and the casing so that the lid does not get stuck when heated.

For each metal, the coefficientthermal expansion. It is determined by heating to 1 ° C of a prototype having a length of 1 m. Lead, zinc, and tin have the largest coefficient. It is smaller with copper and silver. Even lower - iron and gold.

By chemical properties, metals are divided intoseveral groups. There are active metals (for example, potassium or sodium) capable of instantly reacting with air or water. The six most active metals that make up the first group of the periodic table are called alkaline. They have a small melting point and are so soft that they can be cut with a knife. Connecting with water, they form alkaline solutions, hence their name.

The second group consists of alkaline earth metals - calcium, magnesium, etc. They are part of many minerals, more solid and refractory. Examples of metals of the next, third and fourth groups can be lead and aluminum. These are fairly soft metals and they are often used in alloys. Transition metals (iron, chromium, nickel, copper, gold, silver) are less active, more forging and are often used in industry in the form of alloys.

The position of each metal in the activity seriescharacterizes his ability to react. The more active the metal, the easier it takes the oxygen. They are very difficult to isolate from compounds, while low-active types of metals can be found in pure form. The most active of them - potassium and sodium - are stored in kerosene, outside it they are immediately oxidized. Of the metals used in industry, copper is the least active. It makes tanks and pipes for hot water, as well as electrical wires.