Thermal conductivity, also called“thermal conductivity”。[1]It is a measure of the thermal conductivity of a substance.The symbol isλOr K.
English: coefficient of thermal conductivity
When the vertical downward gradient of temperature is 1 ℃/mquantity of heat。
It is specifically defined as: take two parallel planes with an area of 1 square meter and a distance of 1 meter perpendicular to the direction of heat conduction inside the object. If the temperature difference between the two planes is 1K, the heat transferred from one plane to the other within 1 second is defined as the thermal conductivity of the material, and its unit is watt · m-1 · k-1 (W · m-1 · K-1).
If there is no heat loss, for a block material with parallel opposite sides
E/t=λA（θ2-θ1）/ι
Where E is the energy transferred in time t, A is the sectional area,ιIs the length,θ2 andθ1 is respectively for two sectionstemperature。In general, there are:
Also called thermal conductivity, it reflects theheat conductionCapability, pressFourier’s law (See heat conduction), which is defined as the transmission of unit temperature gradient (the temperature decreases by 1K within 1m) through unit heat conduction surface in unit timequantity of heat。
Thermal conductivityλLarge objects are excellent thermal conductors;The smaller thermal conductivity isHot bad conductorOr thermal insulator.λThe value is affected by temperature and increases slightly with the increase of temperature.If the temperature difference between the parts of the material is not very large, the whole material can be seen in practiceλIs a constant.When a crystal cools, its thermal conductivity increases very rapidly.
The thermal conductivity of various substances is mainly measured by experiments, and its theoretical estimation is an active subject in modern physics and physical chemistry.Thermal conductivity is generally not related to pressure, but is greatly affected by temperature.The thermal conductivity of pure metals and most liquids decreases with the increase of temperature, except for water;NonmetalThe thermal conductivity of and gas increases with the increase of temperature.The value at the average temperature of the material is usually used for heat transfer calculation.In addition, the thermal conductivity of solid materials is also related to itsMoisture content, structure and porosity.Generally, materials with large moisture content have high thermal conductivity.For example, the thermal conductivity of dry brick is about 0.27W/(m · K) and that of wet brick is 0.87W/(m · K).The density of the material is large, and its thermal conductivity is usually also large.When the metal contains impurities, the thermal conductivity of the alloy is lower than that of the pure metal.The approximate range of thermal conductivity [W/(m · K)] of various substances is: 50~415 for metals and 12~120 for alloys,Thermal insulation material0.03~0.17, 0.17~0.7 for liquid, 0.007~0.17 for gas,Carbon nanotubesUp to more than 1000.The thermal conductivity of diamond is the highest among known minerals.
research method
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In general, the thermal conductivity of materials can be obtained by theoretical and experimental methods.
Theoretically, starting from the microstructure of the material, based on quantum mechanics and statistical mechanics, the thermal conductivity can be obtained through complex mathematical analysis and calculation by studying the thermal conductivity mechanism of the material and establishing a physical model of heat conduction.However, due to the limited applicability of the theory, and with the rapid increase of new materials, people have not yet found a theoretical equation that is accurate enough and applicable to a wide range, so the exploration of experimental test methods and technologies for thermal conductivity is still the main source of material thermal conductivity data.
Data under standard conditions shall be adopted.For gases, the value corresponds to cp.
Prospect and significance
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With the rapid development of science and technology, more and more polymer materials and nano materials are emerging.The experimental measurement of thermal conductivity of various new substances and materials will open a new and unknown field, which will inevitably lead to a new leap in modern physics.It will also be a new typeThermal conductive materialAnd newThermal insulation materialLay a solid theoretical foundation for the development and research of.This will provide strong theoretical and material support for future space exploration activities and ocean exploration activities.People hope to obtain materials with high thermal conductivity and good mechanical properties to solve the important heat dissipation problem of electronic products. Based on the unique properties of carbon nanotubes, researchers from Tsinghua University have prepared high-performanceCarbon nanotube paperIt has a great application prospect as a heat conduction material in the future.[2]
