Which Method is the Best Suited to my Particular Sample?
The presented methods for Conductivité ThermiqueThermal 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.thermal conductivity and Diffusivité ThermiqueThermal 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.thermal diffusivity differ from one another primarily with regard to the recommended sample geometry and the achievable thermal diffusivity and thermal conductivity ranges.
An overview of suitable sample sizes is shown in table 1.
| LFA | GHP | HFM* | ||
|---|---|---|---|---|
| Sample shape | Round or rectangular | Square | Round or rectangular | |
| Number of pieces per sample | 1 | 2 | 1 | |
| 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 | |
| Max. thickness | 6 mm | 100 mm | 100 mm (or. 200 mm) | |
| Min. thickness | 0.01 mm, dependent upon sample properties | Approx. 1 mm, dependent upon sample | Approx. 5 mm |
* Three models of HFM are available for different sample sizes
Table:1 Established sample geometries
Due to their relatively large sample capacities, HFMs (Heat Flow Meters) and GHPs (Guarded Hot Plates) – the methods for direct determination of thermal conductivity – are those primarily used for inhomogeneous sample materials (insulation materials).
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.
