Conducts carbon fiber heat

Adjustable thermal conductivity in composite materials - from heat insulators to heat conductors

The physical properties of GRP / CFRP and their special thermal design options: When it comes to thermal conductivity, the following applies: CFRP and GFRP can be combined very flexibly in terms of their properties. The conductivity of the fibers, like that of the plastic, offers a wide range of possible configurations. This is how excellent heat insulators / insulating materials can be created. But also components that even outperform stainless steel in terms of their thermal conductivity.

What is the difference between the thermal conductivity of CFRP and GFRP?

CFRP thermal conductivity high or low - both are possible
Carbon fibers have one very good thermal conductivity. Depending on whether the thermal conductivity of a composite made of fiber composite material should be high or low, the plastic used can support the desired property. So it is possible through highly thermally conductive plastics to exploit the carbon thermal conductivity.
There is also the option of using components with a integrated heating function manufacture: This is used for De-icing of wing leading edges, of rotor blades in wind turbines or the front hood of rail vehicles.
GRP and thermal conductivity: thermal insulator and material for thermal insulation
Glass fiber reinforced plastics have one very low coefficient of thermal conductivity. So GRP are good Heat insulators. This is because both the thermal conductivity of the glass fiber and the thermal conductivity of plastic in the area of ​​the thermal insulation lie. Combined with other special properties (electrical insulator, light and strong, weather-resistant and easy-care surface ...), GRP can be used for many applications. Another possibility is to use GRP with insulation: GRP can act as a chemically resistant outer shell that protects the insulation material inside against weather, mechanical stress, etc. When dealing with food, GRP is a valued material because it can insulate temperatures and is easy to clean in terms of hygiene.

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Where does the thermal conductivity of fiber composites play an important role?

Use of GRP in construction due to its low thermal conductivity
Due to their low thermal coefficient, GRP are suitable for use in construction. Classic metal facades are largely determined in their structure by the thermal separation of the interior and the environment. The construction of such facades is generally very complex, and alternatives made of wood or thermoplastics are only suitable to a limited extent.
GRP, on the other hand, is a very resistant building material with low thermal conductivity that can withstand high loads over the long term. This allows the construction of load-bearing structures that can also penetrate the thermal envelope of a building, which makes planning and construction easier and more flexible. Certain types of GRP also have such a high glass fiber content that the temperature expansion of such components is very similar to that of glass.
In contrast to conventional steel structures, this enables the building glazing to be fastened directly to the surrounding posts. This combination of glass and GRP was first used in 2002 at the “glasstec” trade fair on a glass pavilion.

How do you measure thermal conductivity? What is a heat flow?

What does thermal conductivity / specific thermal conductivity mean?
The thermal conductivity or the thermal conductivity coefficient indicates the current that passes through a layer of 1m at 1 Kelvin3 flows (so-called heat flow). The term thermal conductivity generally describes how well thermal energy is transported through a body. Thermal conductivity is therefore a property of a substance - one speaks of thermal properties. The transport happens because there is a temperature gradient. According to the second law of thermodynamics, heat always flows in the direction of the lower temperature in the gradient.
In general, a lower value of the thermal conductivity means a higher insulation capacity of the material. For most materials, the thermal conductivity increases slightly as the temperature increases. The specific thermal conductivity of a material is a constant that does not depend on the temperature.
What does the thermal conductivity depend on?
The following factors are important for determining the thermal conductivity:
  • The thermal conductivity of the material itself
  • The pores (type, arrangement, size)
  • Gas filling in the pores (pressure)
  • Structure of the solid components
  • temperature
  • Bulk density
  • Moisture content
Good conductors of heat and bad conductors of heat
Metals such as copper and aluminum are considered to be good heat conductors. The thermal conductivity of steel is rather low for a metal. Concrete, glass, styrofoam and air count as poor heat conductors. Since air is a poor conductor of heat, materials with a high degree of air inclusion are often used as insulating materials, such as glass wool and polystyrene.

Comparison: How is the thermal conductivity of different substances pronounced?

Thermal conductivity of plastic
Plastics can be seen as important heat insulators. The thermal conductivity of plastics is between 0.1 and 0.8 W / m * K. One reason for this is that the heat transport in them takes place through vibrations of polymer chains. With epoxy resin, the thermal conductivity is between 0.5 and 1.6 W / (m * K), depending on the condition / pretreatment. The same basically applies to the thermal conductivity of polymers / bulk plastics.

Thermal conductivity glass

Glass also acts as a heat insulator / insulating material. Ordinary window glass has a lambda value of 0.75. An important figure for windows is the U-value for the heat transfer coefficient. The lower it is, the more heating energy the windows can save.

Thermal conductivity glass fiber

The conductivities differ slightly between the individual types of glass fibers (E-glass, AR-glass, ECR-glass, R-glass, C-glass, S-glass, M-glass, D-glass). In principle, however, they all move in the range of normal glass.

Thermal conductivity carbon

The value of the carbon fiber itself is 17 W / (m * K) here. Compared to fiberglass, a certain amount of heat can be passed on here. However, significantly higher values ​​are possible for CFRP with hardened carbon laminate. Carbon fiber reinforced plastics can be better heat conductors than stainless steels - with higher strength and much lower weight.

Thermal conductivity metals

Aluminum with 230 W / (m * K) and copper have the best thermal conductivity. The thermal conductivity of steel is significantly lower and is heavily dependent on the specific composition. Standard values ​​are around 50 W / (m * K). The thermal conductivity of stainless steel is often significantly lower.

Types of heat transfer

There are three different forms:
  • Heat conduction: Heat is transferred within a substance.
  • Heat transfer: The heat is transferred from one substance to the other, e.g. when water is heated in a saucepan or when the burner in a heater heats the water in the heating circuit.
  • Heat transfer: It takes place when heat is transferred from a warm material through another material to a third material, e.g. when heat radiates outside through the window.

Measurement of heat conduction

In order to determine the thermal conductivity of insulation, heat flow meters and heat flow calorimeters are used. With heat flow sensors it is possible to measure heat flows non-invasively. In this way, together with the heat conduction, the heat radiation and heat convection can also be determined.

What is thermal insulation?

If it is undesirable for heat to transfer between bodies of higher and lower temperature, thermal insulation measures are required. This can also be seen as “reducing heat transfer”. Thermal insulation can help save fuel or it can protect heat-sensitive bodies from heat.

Similar to the three options for heat transfer, options for its insulation can also apply:
  • A prevention of Conduction can be done by thermal insulation materials. Examples are mats made of glass wool, thermal insulation panels made of Styrofoam or windows with a double layer of glass.
  • The Heat flow between bodies can be reduced by preventing air flows between them, e.g. by using seals or walls.
  • Thermal radiation can be prevented by using bright, smooth, reflective surfaces. In this way, the rays are reflected, e.g. on the inner surface of a thermos flask.

How is the thermal conductivity / coefficient of thermal conductivity calculated?

If you want to calculate the coefficient of thermal conductivity, a formula that is well explained on Wikipedia helps.

Calculation of thermal conductivity (heat flow formula)
The unit of thermal conductivity is given in watt by meter times Kelvin W / (m * K). If you calculate the reciprocal of this, you get the specific thermal resistance. There is a temperature dependency in thermal conductivity: For most materials it increases slightly when the temperature of the material increases.
The hot to cold directed heat flow of a body is created when a certain temperature gradient (temperature difference hot to cold divided by the length of the body) flows through a body that has a certain length and average area.

Further material advantages in addition to the adjustable thermal conductivity of CFRP and GFRP materials