Thermal Conductivity – definition and related facts
Thermal conductivity definition
The effectiveness of a material as a thermal insulator can be expressed in terms of its thermal conductivity (TC). The energy transfer rate through a body is proportional to the temperature gradient across the body and its cross sectional area.
In the limit of infinitesimal thickness and temperature difference, the fundamental law of heat conduction is:
Q = λAdT/dx
- Q is the heat flow (W)
- A is the cross-sectional area (m2)
- dT/dx is the temperature/thickness gradient (K/m)
- λ is defined as the thermal conductivity value (W/m. K)
Thermal conductivity λ
A substance with a large thermal conductivity value is a good conductor of heat; one with a small thermal conductivity value is a poor heat conductor i.e. a good insulator. Hence, knowledge of the thermal conductivity value (units W/m. K) allows quantitative comparisons to be made between the thermal insulation efficiencies of different materials. The most effective insulation will have very low thermal conductivity values. Our graphs show that Microtherm® has very low thermal conductivity values for a wide range of temperatures - in contrast with all the other material classes presented.
Thermal Resistivity is the reciprocal of Thermal Conductivity
and has units K. m. W-1.
Thermal Resistance (R)
The Thermal Resistance is the ability of a substance to prevent heat transfer. Thermal Resistance is the reciprocal of Thermal Conductance and is used when discussing the effective thermal performance of a system. Because R is dependent on panel thickness, it can be simply increased by increasing the insulation thickness.
Units m2. K. W-1.
For multiple layers of materials in intimate contact Rtot = R1 + R2 + R3 etc.
The Thermal Conductance is the reciprocal of Thermal Resistance and is defined as the quantity of heat that passes in unit time through a section of particular area and thickness when the opposite faces have a temperature difference of 1K.
Thermal Transmittance (U)
The U value of a specified material is a term used in buildings construction whose units are the same as Thermal Conductance, W. K-1. m-2. It is defined as the air to air transmittance of an element of construction.
For multiple layers of materials in intimate contact
Measurement of thermal conductivity
Microtherm obtains thermal conductivity measurements by three different test procedures:
1. Guarded Hot Plate equipment complying with ISO 8302 and ASTM C 177 standards is used for all of our published data.
This produces precise data over a wide temperature range (up to a mean value of 800°C / 1472 °F). Two flat panels are tested and when steady state conditions are reached heat flow is measured between a heated test square and water cooled outer plates. This method is recognized as the most accurate for the measurement of thermal conductivity of high temperature insulation materials. Although we have our own guarded hot plate equipment, all Microtherm® published TC data are produced for us by the National Physical Laboratory in the UK using this measurement technique.
2. Microtherm also uses in-house testing facilities of their own specialized design.
This testing equipment is based on a cylindrical geometry with a central heat source. It is used for routine quality control and product development. The cylindrical test cells also make this equipment particularly suitable for measurements of thermal conductivity at pressures greater than atmospheric, and in gases other than air.
3. Thermal conductivity measurement of vacuum insulation panels.
In the Microtherm materials laboratory, routine thermal conductivity measurement of SlimVac® panels is done on a FOX 600 machine from Lasercomp. This machine measures centre panel thermal conductivity and has a thermal guard to minimise any effects from edge conduction through barrier materials.
Measurements comply with ISO 8301 and ASTM C-518.
Another measurement technique that could be used for thermal conductivity measurement is the "Hot Wire Method" (ASTM C1113). This method is more suitable for measuring powder, fibrous materials, and liquids.
When comparing the thermal conductivities of different insulation materials it is important to compare only data generated by the same measurement technique. This is why all responsible manufacturers of high temperature insulations always quote the guarded hot plate method.