Introduction
CFRP (carbon fiber-reinforced plastic) and GFRP (glass fiber-reinforced plastic) are indispensable in numerous high-tech applications due to their unique material properties. Their key characteristics are high strength combined with low weight. This, along with their low Thermal ConductivityThermal 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, makes them ideal for high-tech applications in aerospace, automotive and electronics. Their directional (anisotropic) thermal properties play a special role in their application, as the Thermal ConductivityThermal 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 along the fibers is higher than across them. The layered structure allows the fibers to be oriented to either dissipate heat in a targeted manner or to effectively insulate areas. This flexibility enables tailor-made solutions such as the minimization of temperature variations in satellites or the regulation of heat in batteries.
Measurement Conditions and Results
For the determination of thermal properties, Laser/ Light Flash Analysis is particularly well suited. Initially, the Thermal DiffusivityThermal 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 – which is a function of direction – is determined using an instrument such as the LFA 717 HyperFlash®. Subsequently, the data on DensityThe mass density is defined as the ratio between mass and volume. density and Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity can be applied in order to calculate the Thermal ConductivityThermal 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, which is also a function of direction. The measurement conditions are detailed in table 1.
Table 1: Measurement parameters
| Analysis instrument | LFA 717 HyperFlash® |
|---|---|
| Sample size | 10 mm x 10 mm x 2.5 mm – through-plane Several strips of 10 mm x 2.5 mm – in-plane |
| Sample holders | 10 mm square – through-plane 10 mm laminate sample holder – in-plane |
Temperature points | 20 to 150°C in steps of 10 K |
| Atmosphere | 100 ml/min, N2 |
Figure 1 shows the Thermal DiffusivityThermal 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 of GFRP in the through-plane direction (perpendicular to the fiber) and in the in-plane direction (parallel to the fiber). The Thermal DiffusivityThermal 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 decreases slightly with increasing temperature. Between 110°C and 130°C, a small change in gradient can be seen, indicating the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition of the polymer matrix. The in-plane Thermal DiffusivityThermal 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 is about 35 to 40% higher than in the through-plane direction.
A CFRP material is similarly shown in Figure 2. Again, the in-plane Thermal DiffusivityThermal 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 is higher than the through-plane Thermal DiffusivityThermal 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.
For the CFRP material, the difference between the directions is considerably greater than for the GFRP material. It’s not 35 to 40% as for the GFRP sample, but 500 to 600%. This striking difference is due to the carbon fibers, which possess a much higher Thermal DiffusivityThermal 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 than the glass fibers. This is particularly clear in figure 3, which summarizes all of the measurements.
Conclusion
The LFA method can also determine Thermal DiffusivityThermal 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 and Thermal ConductivityThermal 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 as a function of direction, providing important data for the design and construction of high-tech applications.