封标Thermal conductivity
In physics, thermal conductivity, , is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per kelvin-meter (W·K−1·m−1, i.e. W/(K·m) or in IP units (Btu·hr−1·ft−1·F−1, i.e. Btu/(hr·ft⋅F). Multiplied by a temperature difference (in kelvins, K) and an area (in square meters, m2), and divided by a thickness (in meters, m), the thermal conductivity predicts the rate of energy loss (in watts, W) through a piece of material. In the window building industry "thermal conductivity" is expresd as the U-Factor [1], which measures the rate of heat transfer and tells you how well the window insulates. U-factor values are generally recorded in IP units (Btu/(hr·ft⋅F)) and usually range from 0.15 to 1.25. The lower the U-factor, the better the window insulates.
The reciprocal of thermal conductivity is thermal resistivity.
Measurement
There are a number of ways to measure thermal conductivity. Each of the is suitable for a limited range of materials, depending on the thermal properties and the medium temperature. There is a distinction between steady-state and transient techniques.
In general, steady-state techniques are uful when the temperature of the material does not change with time. This makes the signal analysis straightforward (steady state implies constant signals). The disadvantage is that a well-engineered experimental tup is usually needed. The Divided Bar (various types) is the most common device ud for consolidated rock samples.
The transient techniques perform a measurement during the process of heating up. Their advantage is quicker measurements. Transient methods are usually carried out by needle probes.
Standards
•IEEE Standard 442-1981, "IEEE guide for soil thermal resistivity measurements", ISBN 0-7381-0794-8. See also soil thermal properties. [2] [3]
•IEEE Standard 98-2002, "Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials", ISBN 0-7381-3277-2 [4] [5]
•ASTM Standard D5334-08, "Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure" [6]
•ASTM Standard D5470-06, "Standard Test Method for Thermal Transmission Properties of Thermall
y Conductive Electrical Insulation Materials" [7]
•ASTM Standard E1225-04, "Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique" [8]
•ASTM Standard D5930-01, "Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique" [9]
•ASTM Standard D2717-95, "Standard Test Method for Thermal Conductivity of Liquids" [10]
•ISO 22007-2:2008 "Plastics -- Determination of thermal conductivity and thermal diffusivity -- Part 2: Transient plane heat source (hot disc) method" [11]
•Note: What is called the k-value of construction materials (e.g. window glass) in the U.S., is called λ-value in Europe. What is called U-value (= the inver of R-value) in the U.S., ud to be called k-value in Europe, but is now also called U-value in Europe.
Definitions
The reciprocal of thermal conductivity is thermal resistivity, usually measured in kelvin-meters per wa
tt (K·m·W−1). When dealing with a known amount of material, its thermal conductance and the reciprocal property, thermal resistance, can be described. Unfortunately, there are differing definitions for the terms.
Conductance
For general scientific u, thermal conductance is the quantity of heat that pass in unit time through a plate of particular area and thickness when its opposite faces differ in temperature by one kelvin. For a plate of thermal conductivity k, area A and thickness L this is kA/L, measured in W·K−1 (equivalent to: W/°C). Thermal conductivity and conductance are analogous to electrical conductivity (A·m−1·V−1) and electrical conductance (A·V−1).
There is also a measure known as heat transfer coefficient: the quantity of heat that pass in unit time through unit area of a plate of particular thickness when its opposite faces differ in temperature by one kelvin. The reciprocal is thermal insulance. In summary:
•thermal conductance = kA/L, measured in W·K−1
•thermal resistance = L/(kA), measured in K·W−1 (equivalent to: °C/W)
•heat transfer coefficient = k/L, measured in W·K−1·m−2
•thermal insulance = L/k, measured in K·m²·W−1.
The heat transfer coefficient is also known as thermal admittance
Resistance
When thermal resistances occur in ries, they are additive. So when heat flows through two components each with a resistance of 1 °C/W, the total resistance is 2 °C/W.
A common engineering design problem involves the lection of an appropriate sized heat sink for a given heat source. Working in units of thermal resistance greatly simplifies the design calculation. The following formula can be ud to estimate the performance:
where:
•R
is the maximum thermal resistance of the heat sink to ambient, in °C/W
hs
•is the temperature difference (temperature drop), in °C
is the thermal power (heat flow), in watts
•P
th
is the thermal resistance of the heat source, in °C/W
•R
s
For example, if a component produces 100 W of heat, and has a thermal resistance of 0.5 °C/W, wh
at is the maximum thermal resistance of the heat sink? Suppo the maximum temperature is 125 °C, and the ambient temperature is 25 °C; then the is 100 °C. The heat sink's thermal resistance to ambient must then be 0.5 °C/W or less.
Transmittance
A third term, thermal transmittance , incorporates the thermal conductance of a structure along with heat transfer due to convection and radiation. It is measured in the same units as thermal conductance and is sometimes known as the composite thermal conductance . The term U-value is another synonym.
雷雨是谁写的Summary
In summary, for a plate of thermal conductivity k (the k value [12] ), area A and thickness t :
•
thermal conductance = k /t , measured in W·K −1·m −2;•
thermal resistance (R-value ) = t /k , measured in K·m²·W −1;•
thermal transmittance (U-value ) = 1/(Σ(t /k )) + convection + radiation, measured in W·K −1·m −2.•K-value refers in Europe to the total insulation value of a building. K-value is obtained by multiplying the form factor of the building (= the total inward surface of the outward walls of the building divided by the total volume of the building) with the average U-value of the outward walls of the building. K value is therefore expresd as (m 2·m −3)·(W·K −1·m −2) = W·K −1·m −3. A hou with a volume of 400 m³ and a K-value of 0.45 (the new European norm. It is commonly referred to as K45) will therefore theoretically require 180 W to maintain its interior
temperature 1 K above exterior temperature. So, to maintain the hou at 20 °C when it is freezing outside (0 °C),3600 W of continuous heating is required.
Examples
In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law,as freely moving valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the incread importance of phonon carriers for heat in non-metals. As shown in the table below, highly electrically conductive silver is less thermally conductive than diamond, which is an electrical insulator.
Thermal conductivity depends on many properties of a material, notably its structure and temperature. For instance,pure crystalline substances exhibit very different thermal conductivities along different crystal axes, due to differences in phonon coupling along a given crystal axis. Sapphire is a notable example of variable thermal conductivity bad on orientation and temperature, with 35 W/(m·K) along the c-axis and 32 W/(m·K) along the a-axis.[13]
Air and other gas are generally good insulators, in the abnce of convection. Therefore, many insulating materials function simply by having a large number of gas-filled pockets which prevent large-scale convection. Examples of the include expanded and extruded polystyrene (popularly referred to as "styrofoam") and silica aerogel. Natural,biological insulators such as fur and feathers achieve similar effects by dramatically inhibiting convection of air or water near an animal's skin.
Ceramic is ud for its low thermal conductivity on exhaust systems to prevent heat from reaching
nsitive components
Light gas, such as hydrogen and helium typically have high thermal
conductivity. Den gas such as xenon and dichlorodifluoromethane
have low thermal conductivity. An exception, sulfur hexafluoride, a
den gas, has a relatively high thermal conductivity due to its high
heat capacity. Argon, a gas denr than air, is often ud in insulated止痛风湿丸
glazing (double paned windows) to improve their insulation
characteristics.
Thermal conductivity is important in building insulation and related
fields. However, materials ud in such trades are rarely subjected to
chemical purity standards. Several construction materials' k values are listed below. The should be considered approximate due to the uncertainties related to material definitions.
The following table is meant as a small sample of data to illustrate the thermal conductivity of various types of substances. For more complete listings of measured k -values, e the references.Experimental values
Experimental values of thermal conductivity.燕麦米怎么煮粥
This is a list of approximate values of
thermal conductivity, k , for some
common materials. Plea consult the
list of thermal conductivities for more
accurate values, references and
detailed information.Material Thermal conductivity
W/(m·K)
Silica Aerogel 0.004 - 0.04
Air 0.025
Wood
0.04 - 0.4Hollow Fill Fibre Insulation 0.042函数的最值
赞美朋友的句子Alcohols and oils
0.1 - 0.21Polypropylene
0.25 [14]Mineral oil
0.138Rubber
0.16LPG
0.23 - 0.26Cement, Portland
0.29Epoxy (silica-filled)
0.30Epoxy (unfilled)
0.59Water (liquid)
0.6Thermal grea
0.7 - 3Thermal epoxy
1 - 7Glass
1.1Soil
1.5Concrete, stone
1.7Ice
每一个明天2Sandstone
2.4Stainless steel 12.11 ~ 45.0
Lead35.3
Aluminium237 (pure)
120—180 (alloys)
Gold318
Copper401成熟男人网名
Silver429
Diamond900 - 2320
Graphene(4840±440) - (5300±480)
Physical origins
Heat flux is exceedingly difficult to control and isolate in a laboratory tting. Thus at the atomic level, there are no simple, correct expressions for thermal conductivity. Atomically, the thermal conductivity of a system is determined by how atoms composing the system interact. There are two different approaches for calculating the thermal conductivity of a system.
•The first approach employs the Green-Kubo relations. Although this employs analytic expressions which in principle can be solved, in order to calculate the thermal conductivity of a den fluid or solid using this relation requires the u of molecular dynamics computer simulation [15].
•The cond approach is bad upon the relaxation time approach. Due to the anharmonicity within t
he crystal potential, the phonons in the system are known to scatter. There are three main mechanisms for scattering:•Boundary scattering, a phonon hitting the boundary of a system;
•Mass defect scattering, a phonon hitting an impurity within the system and scattering;
•Phonon-phonon scattering, a phonon breaking into two lower energy phonons or a phonon colliding with another phonon and merging into one higher energy phonon.
Lattice waves
Heat transport in both glassy and crystalline dielectric solids occurs through elastic vibrations of the lattice (phonons). This transport is limited by elastic scattering of acoustic phonons by lattice defects. The predictions were confirmed by the experiments of Chang and Jones on commercial glass and glass ceramics, where mean free paths were limited by "internal boundary scattering" to length scales of 10−2 cm to 10−3 cm. [16][17]
The phonon mean free path has been associated directly with the effective relaxation length for process without directional correlation. Thus, if V
g
is the group velocity of a phonon wave packet, then the relaxation length is defined as:
where t is the characteristic relaxation time. Since longitudinal waves have a much greater pha velocity than
transver waves, V
long is much greater than V
trans
, and the relaxation length or mean free path of longitudinal
phonons will be much greater. Thus, thermal conductivity will be largely determined by the speed of longitudinal phonons. [16][18]
Regarding the dependence of wave velocity on wavelength or frequency (dispersion), low-frequency phonons of long wavelength will be limited in relaxation length by elastic Rayleigh scattering. This type of light scattering form small particles is proportional to the fourth power of the frequency. For higher frequencies, the power of the frequency will decrea until at highest frequencies scattering is almost frequency independent. Similar arguments were subquently generalized to many glass forming substances using Brillouin scattering. [19][20][21][22]
