Thermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.The relationship between transported heat per unit of time (dQ/dt or heat flow Q) and the temperature gradient (ΔT/Δx) through Area A (the area through which the heat is flowing perpendicularly at a steady rate) is described by the thermal conductivity equation.
Thermal diffusivity (a with the unit mm2/s) is a material-specific property for characterizing unsteady heat conduction. This value describes how quickly a material reacts to a change in temperature.
In order to predict cooling processes or to simulate temperature fields, the thermal diffusivity must be known; it is a requisite for solving the Fourier Differential Equation for unsteady heat conduction.
An overview of the thermal conductivity and thermal diffusivity values of selected materials is shown in table 1.
Table 1: Thermal conductivity and thermal diffusivity values for various materials
|Material||Thermal Conductivity / W/(m•K)||Thermal Diffusivity / mm2/s|
Source: www.chemie.de/lexikonInformation about the thermal conductivity of further materials can be found in the “Thermal Properties of Elements", "Thermal Properties of Ceramics" and "Thermal Properties of Polymers” posters.
Table 1: Established sample geometries
|Sample shape||Round or rectangular||Square||Round or rectangular||Rectangular (cuboid)|
|Number of pieces per sample||1||2||1||2|
|Diameter and/or edge lengths||6 mm to 25.4 mm||300 mm x 300 mm||150 mm x 150 mm to 300 mm x 300 mm (or 305 mm x 305 mm to 610 mm x 610 mm||No less than 200 mm x 100 mm (Standard: 230 mm x 114 mm)|
|Max. thickness||6 mm||100 mm||100 mm (or 200 mm)||76 mm|
|Min. thickness||0.01 mm, dependent upon sample properties||Approx. 1 mm, dependent upon sample||Approx. 5 mm||50 mm|
* Two models of HFM are available for different sample sizes
Due to their relatively large sample capacities, HFMs (Heat Flow Meters), GHPs (Guarded Hot Plates) and TCTs (Thermal Conductivity Tester, hot wire technique) – the methods for direct determination of thermal conductivity – are those primarily used for inhomogeneous sample materials (insulation materials). The TCT 426 was specially developed for tests on refractory materials and ceramics.
The Laser or Light Flash Apparatuses (LFAs) are configured to handle only much smaller sample sizes. Standard samples have a size of 12.7 mm and a thickness of 2 to 3 mm.
An overview of the various thermal conductivities depending on the used method can be seen in figure 1 and for temperature ranges in figure 2.
The laser or light flash method dates back to studies by Parker et al. in 1961.
In carrying out a measurement, the lower surface of a plane parallel sample (see fig. 1) is first heated by a short energy pulse. The resulting temperature change on the upper surface of the sample is then measured with an infrared detector. The typical course of the signals is presented in figure 2 (red curve). The higher the sample’s thermal diffusivity, the steeper the signal increase.
Using the half time (t1/2, time value at half signal height) and sample thickness (d), the thermal diffusivity (a) and finally the thermal conductivity (λ) can be calculated by means of the formula in figure 2. Furthermore, the specific heat (cp) of solids can be determined using the signal height (ΔTmax) compared to the signal height of a reference material.
LFA investigations generally take much less time than thermal conductivity measurements by means of GHP (Guarded Hot Plate), HFM (Heat Flow Meter) or TCT (Thermal Conductivity Tester).
For the precise measurement of thermal diffusivity a, the flash method has established itself as a rapid, versatile and precise direct measuring method. NETZSCH offers a total of three models (LFA 467 HyperFlash, LFA 457 MicroFlash® and LFA 427), which cover the total a broad spectrum of materials and temperature ranges.
The thermal conductivity (λ) of insulation materials can be determined directly with plate instrumentation (HFM = Heat Flow Meter or GHP = Guarded Hot Plate): Included here are the HFM 436 Lambda with its new, expanded measurement capabilities and the GHP 456 Titan® guarded hot plate, which is an absolute method and thus requires no calibration.
For determining the thermal conductivity of refractory materials, the hot wire method is best suited (TCT = Thermal Conductivity Tester).
The instruments listed above operate in accordance with relevant instrument and usage norms.
Specifically, these include:
For LFA: ASTM E1461, DIN EN 821-2, DIN 30905, ISO 22007-4, ISO 18755
For HFM: ASTM C518, ISO 8301, DIN EN 12667, JIS A 1412
For TCT: ISO 8894, ASTM C1113, EN 993-14/15, DIN 51046
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