The development of advanced materials and thermal analysis methods increasingly requires data on thermal conductivity and thermal diffusivity to achieve improved thermal design of components. Energy saving, heat protection, thermal insulation, increased lifetime of high temperature components - e.g. in turbine engines - are a few examples of the manifold applications. Below is an example of the application of CMCs, valued for their low mass combined with high thermal performance.
Heat protection and component improvement at high temperatures
Ceramic matrix composites (CMCs) with low porosity are obtained in one cycle via the liquid silicon infiltration process, which is characterised by short processing times and low manufacturing costs. Besides aerospace applications, such as hot structures for re-entry vehicles, more and more applications beyond this classic field of CMCs are of increasing interest - e.g. weight-saving in high power brake discs.
It is essential to know the thermal conductivity so as to calculate the maximum temperature achieved in operation, which has a crucial impact on component lifetime. At high or very high temperatures the thermal diffusivity can be measured and, if density and specific heat capacity are known, converted to thermal conductivity.
CMCs are ideal materials for thermal protection applications in aerospace/space due to their low weight, high strength and thermal resistance.
CMC materials can be used to make high power disc brakes.
The scientific work related to the above study was carried out by IKE, University of Stuttgart and DLR, Stuttgart (DLR, Institut fŘr Bauweisen und Konstruktionsforschung). See the two references below for further details.
References R Brandt, M Frie▀, G Neuer, 2003, High Temp - High Press, 35, 169-177 M Frie▀, W Krenkel, R Kochend÷rfer, R Brandt, G Neuer, H-P Maier, in: Basic Research and Technologies for Two-Stage-to Orbit Vehicles. D Jacob, G Sachs, S Wagner (Ed), 2005, Wiley-VCH Verlag, Weinheim, Germany, ISBN 3-527-27735-8, pp 499-525
Super-insulation for highest thermal performance
In porous insulation materials, heat is transferred via conduction through the solid material structure, the intervening gas and by thermal radiation. But in super-insulation materials heat normally conducted via the gas inside the pores is prevented by means of evacuation and by microporous materials. By evacuating insulation systems with porous fillings, super-evacuated insulation materials can be produced which have a thermal conductivity of less than 0.005 W/(mĚK) at 20░C. Compare this with air which has a thermal conductivity of 0.026 W/(mĚK). The insulation system's vacuum-tight casing comprises, for example, multi-layered compound film or thin stainless-steel sheeting.
Evacuated foil insulation has outstanding insulation properties at low temperatures, making it most suitable for cryo-applications (T< -180░C). Effective thermal conductivity values of far less than 0.001 W/(mĚK) can be achieved, using layered, highly-reflective metallic foils separated with spacers.
Silica aerogel, a super-insulating material with outstanding thermal insulation properties.
Infrared image of a renovated fašade with integrated vacuum insulation panels.
For high temperature applications, heat transport via the gas molecules is effectively reduced by employing microporous silicates which lead to restricted movement within the microporous structure. Thermal conductivity values of less than 0.06 W/(mĚK) at 1000░C can be realised here. Thermal radiative heat transfer in insulation systems is reduced by using highly-reflective foils that act as radiation shields. There are also so-called infrared opacifiers comprising, for instance, infrared active oxides or carbides, which efficiently diffuse or absorb the thermal radiation in the insulation system.Super-insulation materials are employed in high-tech fields such as cryotechnology, space technology and fusion research, but also in domestic refrigerators and innovative buildings. In applications such as these, it is even more important that a maximum insulating effect is achieved for a given thickness.