.

Fig. 4: Thermal conduction in conventional thermal insulation materials: Thermal conduction via the solid pore walls (yellow arrow), thermal conduction via the filling gases (blue arrows) and thermal radiation between the pore walls (red arrows). The contribution made by convection within the pores (green arrow) is negligible.

Fig. 5: An enclosed volume of approximately ten litres: Conventionally insulated (white) and insulated with VIP of the same insulation quality (silver).
© ZAE Bayern

Fig. 5a: Thermal insulation materials in comparison
© BINE Informationsdienst
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Heat transfer in thermal insulation materials

The aim of thermal insulation materials is to reduce the transfer of heat caused by temperature differences. The thermal conductivity is the significantmaterial parameter: the lower it is, the better the insulating effect.

Convection, a form of heat transfer that is linked to the transport of a gas or a liquid, is a very effective mechanism. It is used, for example, in the circulation of water in heating systems. Suppressing this is the primary task of thermal insulation, which is also optimally fulfilled by conventional thermal insulation materials thanks to their finecelled structure.

The second thermal transportmechanism is thermal conduction, in which individual atoms or molecules transfer heat to neighbouring atoms or molecules. This mechanism can be found not just in solids but also in quiescent liquids and gases. Since the thermal conductivity of gases is generally considerably lower than for solids, thermal insulation materials are highly porous. Plastics are particularly good at reducing the amount of heat conducted via the solid structure. With such highly porous thermal insulation materials, it is nevertheless the heat conducted by the gas in the cavities that dominates the overall thermal transport (proportion greater than 60%). The type of gas and – with nano-structured materials – the gas pressure in the pores considerably influence not just the heat conducted by the gas but, as a result, the overall thermal transport.
A third, frequently underestimated contribution to the thermal transport is provided by infrared radiation. This transport mechanism does not depend on the presence of matter at all. All surfaces emit thermal radiation in accordance with their own temperature, whereby they also absorb and scatter incident radiation. The transport of infrared radiation in a porous system is influenced by the density (and structure) of the material: the higher the density, the more it is attenuated – the requirements for diminishing it are therefore exactly contrary to the requirements for reducing the solid conductivity. Additives – so-called infrared opacifiers – help diminish the infrared radiation transport even more for a given material density.

Conventional thermal insulation materials achieve thermal conductivities between 0.030 W/(m K) and 0.040 W/(m K). In order to improve the thermal insulation properties, a particular focus is on reducing gaseous thermal conductivity. One possibility is to use heavier gases with a lower thermal conductivity than air. Polyurethane foams filled with heavy gas achieve thermal conductivities less than 0.022 W/(m K). However, permeating air causes the thermal conductivity to increase over time. Another approach is to make the structure so fine that the gas particles under atmospheric pressure collide not so much with one another but with a diverse number of walls. To achieve this, the pores must be smaller than a few tenths of a micrometre in size. Nano-structured fumed silica or aerogels have measured values as small as 0.015 W/(m K).
Vacuum insulation is taking a different approach: lowering the gas pressure largely eliminatesthermal conduction via the gas. The thermal conductivity of evacuated thermal insulation materials loaded with atmospheric pressure therefore ranges between 0.002 W/(m K) and 0.008 W/(m K).

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