thermal conductivity

Low coefficient is called insulation material

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This entry is made by China Science and Technology Information Magazine Participate in editing and review Science Popularization China · Science Encyclopedia authentication.

The thermal conductivity refers to the thermal conductivity of the material with a thickness of 1 meter and a surface temperature difference of 1 degree (K, ℃) on both sides under the condition of stable heat transfer, which is transmitted through an area of 1 square meter within 1 second quantity of heat , the unit is watt/meter · degree (W/(m · K), where K can be replaced by ℃).^[1]

The thermal conductivity is only applicable to the heat transfer form with heat conduction. When other forms of heat transfer exist, such as radiation , convection, mass transfer and other heat transfer forms. The composite heat transfer relationship is usually referred to as the apparent thermal conductivity, dominant thermal conductivity or effective thermal conductivity of material. In addition, the thermal conductivity is determined for Homogeneous material In fact, there are porous, multi-layer, multi structure and anisotropic materials. The thermal conductivity obtained by such materials is actually a performance of comprehensive thermal conductivity, also known as average thermal conductivity coefficient 。

Chinese name: thermal conductivity
Foreign name: thermal conductivity
Discipline: physics

Correlation law: Fourier’s law
application area: physics
Discipline scope: Condensed matter physics

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▪ Gas
five Thermal insulation material

Fourier’s law

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according to Fourier’s law , the definition formula of thermal conductivity is

Among them, x Is the direction of heat flow.

Is in this direction Heat flux ，W/m two 。

Is the temperature in this direction gradient , the unit is K/m.

about Isotropy The thermal conductivity in all directions is the same.^[3]

influence factor

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The thermal conductivity of different materials is different; The thermal conductivity of the same material is related to its structure, density, humidity, temperature, pressure and other factors. When the water content of the same material is low and the temperature is low, the thermal conductivity is small. Generally speaking, the thermal conductivity of solid is greater than that of liquid, while that of liquid is greater than that of gas. This difference is largely due to the different molecular spacing between the two states. The coefficient values used in engineering calculation are determined by special tests.

With the increase of temperature or moisture content, the thermal conductivity of the five typical building materials measured showed an increasing trend. This is analyzed from the microscopic mechanism below. yes porous material When it is wet, the liquid water will replace the original air in the micropore; At normal temperature and pressure, the thermal conductivity of liquid water (about 0.59W/(m · K)) is much greater than that of air (about 0.026W/(m · K)). Therefore, the thermal conductivity of wet materials will be greater than that of dry materials, and the higher the moisture content, the greater the thermal conductivity. If water condenses into ice at low temperature, the overall thermal conductivity of the material will also increase because the thermal conductivity of ice is up to 2.2W/(m · K).

Different from the effect caused by dampness, temperature rise will cause Molecular thermal motion Accelerate the heat conduction of solid skeleton and convection heat transfer of fluid in pores. In addition, the radiation heat transfer between the hole walls will also be strengthened due to the temperature rise. If the material contains moisture, the temperature gradient may also have an important impact: the temperature gradient will form a vapor pressure gradient, which will make the water vapor migrate from the high temperature side to the low temperature side; Under certain conditions, water vapor may condense at the low temperature side, and the formed liquid water will migrate from the low temperature side to the high temperature side under the driving of capillary pressure. Such a cycle is similar to the enhanced heat transfer effect of heat pipe, which significantly increases the thermal conductivity of the material.^[2]

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, through studying the heat conduction mechanism of the material, the physical model of heat conduction is established, and the thermal conductivity can be obtained through complex mathematical analysis and calculation. However, due to the limited applicability of the theory, and with the rapid increase of new materials, people have not yet found theoretical equations that are accurate enough and applicable to a wide range, so the exploration of experimental testing methods and technologies for thermal conductivity is still the main source of material thermal conductivity data.^[3]

The measurement of thermal conductivity is divided into dynamic method and steady method, and the steady method is divided into heat flow meter method and protective hot plate method. Considering the accuracy of the instrument and the temperature control range, refer to the GB/T10294-2008 standard, and use the protective hot plate method for testing.

The experimental instrument is shown in Figure 1, including the main body, cold and heat source control system and intelligent measuring instrument.

Figure 1

The main body is composed of hot plate, cold plate and specimen clamping system. The hot plate consists of three main parts: the main heating plate, the protective heating plate and the back protective heating plate. The temperature of the main heating plate and the protective heating plate is controlled by the resistance heater and the intelligent measuring instrument, and the temperature of the back protective heating plate is controlled by the precision constant temperature water tank to keep the temperature of the three heating plates consistent. The cold plate is composed of aluminum plate, semiconductor refrigerant and cooling water jacket, which can accurately control the temperature of the cold plate at the set value. The intelligent measuring instrument is used for the temperature measurement and control of the entire testing system to achieve full automatic testing.

Three to six specimens with a size of 30cm × 30cm × 3-5 cm were prepared for each material, and the thermal conductivity was tested 12 to 35 times at different temperatures and moisture contents. Before the test, the test piece shall be cultured to different moisture content, and then all sides of the test piece shall be wrapped with 4 layers of plastic film. The water vapor permeability of the film Sd>1.5m can be regarded as impermeable. Its thickness and thermal resistance are 0.0225mm and 0.000537m respectively two K/W， Can be ignored.^[2]

Thermal conductivity of material

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solid

Solids are composed of free electron and atom The atoms are constrained in a regularly arranged lattice. Accordingly, the transmission of heat energy is realized by two functions: the migration of free electrons and the vibration wave of the lattice. When regarded as a quasi particle phenomenon, the lattice vibrator is called phonon 。 In pure metals, electrons contribute the most to heat conduction, while Nonconductive The contribution of phonons plays a major role in.^[3]

See Table 1 for commonly used solid thermal conductivity. The thermal conductivity of pure metals generally decreases with the increase of temperature. However, the purity of metal has a great impact on the thermal conductivity, such as the ordinary carbon steel The thermal conductivity of stainless steel is only 16 W/m · K.

Table 1 Thermal conductivity of common solid materials
solid	Temperature, ℃	Thermal conductivity λ, W/m · K
aluminum	three hundred	two hundred and thirty
cadmium	eighteen	ninety-four
copper	one hundred	three hundred and seventy-seven
Wrought iron	eighteen	sixty-one
cast iron	fifty-three	forty-eight
lead	one hundred	thirty-three
nickel	one hundred	fifty-seven
silver	one hundred	four hundred and twelve
Steel (1% C)	eighteen	forty-five
Marine metal	thirty	one hundred and thirteen
bronze		one hundred and eighty-nine
stainless steel	twenty	sixteen
graphite	zero	one hundred and fifty-one
Asbestos board	fifty	zero point one seven
asbestos	0~100	zero point one five
concrete	0~100	one point two eight
refractory bricks		one point zero four
Insulating brick	0~100	0.12~0.21
Building brick	twenty	zero point six nine
Fluff blanket	0~100	zero point zero four seven
Cotton wool	thirty	zero point zero five zero
Glass	thirty	one point zero nine
mica	fifty	zero point four three
Hard rubber	zero	zero point one five
Sawdust	twenty	zero point zero five two
cork	thirty	zero point zero four three
Glass wool	--	zero point zero four one
85% magnesium oxide	--	zero point zero seven zero
TDD (rock wool) insulation integrated board	seventy	zero point zero four zero
TDD (XPS board) insulation integrated board	twenty-five	zero point zero two eight
TDD (vacuum insulation) insulation integrated board	twenty-five	zero point zero zero six
TDD vacuum insulation board	twenty-five	zero point zero zero six
ABS	--	zero point two five

liquid

Liquids are divided into metallic liquids and non-metallic liquids. The former has a higher thermal conductivity and the latter has a lower thermal conductivity. In non-metallic liquids, the thermal conductivity of water is the largest. Except for water and glycerin, the thermal conductivity of most liquids decreases slightly with the increase of temperature. Generally speaking, the thermal conductivity of solution is lower than that of pure liquid. Table 2 lists the thermal conductivity values of several liquids.

Table 2 Thermal conductivity of liquid
liquid		Temperature, ℃	Thermal conductivity λ, W/m · K
acetic acid	50%	twenty	zero point three five
acetone		thirty	zero point one seven
aniline		0~20	zero point one seven
benzene		thirty	zero point one six
Calcium chloride brine	30%	thirty	zero point five five
ethanol	80%	twenty	zero point two four
glycerol	60%	twenty	zero point three eight
glycerol	40%	twenty	zero point four five
Heptane		thirty	zero point one four
Mercury		twenty-eight	eight point three six
sulphuric acid	90%	thirty	zero point three six
sulphuric acid	60%	thirty	zero point four three
water		thirty	zero point six two

Gas

The thermal conductivity of gas increases with temperature. In the normal pressure range, its thermal conductivity changes little with the pressure, only when the pressure is greater than 196200kN/m two , or the pressure is less than 2.67kN/m two (20mmHg), the thermal conductivity increases with the increase of pressure. Therefore, the influence of pressure on gas thermal conductivity can often be ignored in engineering calculation.

The thermal conductivity of gas is very small, so it is not good for heat conduction, but it is good for heat preservation. See Table 3 for the thermal conductivity of several common gases.

Table 3 Thermal conductivity of gas
Gas	Temperature, ℃	Thermal conductivity λ, W/m · K
hydrogen	zero	zero point one seven
carbon dioxide	zero	zero point zero one five
atmosphere	zero	zero point zero two four
atmosphere	one hundred	zero point zero three one
methane	zero	zero point zero two nine
Water vapor	one hundred	zero point zero two five
nitrogen	zero	zero point zero two four
ethylene	zero	zero point zero one seven
oxygen	zero	zero point zero two four
ethane	zero	zero point zero one eight

Thermal insulation material

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Generally, materials with low thermal conductivity are called Thermal insulation material (According to the national standard of our country, the materials whose thermal conductivity is not greater than 0.12W/(m · K) when the average temperature is not higher than 350 ℃ are called insulation materials), while the materials whose thermal conductivity is less than 0.05 W/(m · K) are called efficient insulation materials.

Materials with high thermal conductivity have excellent thermal conductivity. stay Heat flux With the same thickness, the temperature difference between the high temperature side wall and the low temperature side wall decreases with the increase of thermal conductivity. For example, the boiler tube is still open Incrustation Due to the high thermal conductivity of the steel, the temperature difference between the inner and outer walls of the steel pipe is small. The temperature of the inner wall of the steel pipe is close to the water temperature in the pipe, so the temperature difference of the pipe wall (the average temperature of the inner and outer walls) will not be very high. However, when scaling occurs on the inner wall of the furnace tube, due to the small thermal conductivity of the scale, the temperature difference between the inside and outside of the scale increases with the increase of the scale thickness, thus rapidly raising the metal temperature of the tube wall. When the scale thickness reaches a considerable value (generally 1~3mm), the furnace tube wall temperature will exceed the allowable value, resulting in overheating and damage of the furnace tube. yes Boiler wall As for thermal insulation materials of pipes, the lower the thermal conductivity is, the better.

Generally, materials with thermal conductivity less than 0.2W/(m · K) are called thermal insulation materials. For example, asbestos, perlite, etc.^[3]

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