Multilayer insulation
Multilayer Insulation ( MLI ), also super insulation called one is thermal insulation material (Engl. Insulation ), which was originally developed for space travel, but now also for the insulation of the cryostat is used superconducting magnets and cables for cryogenic liquids or gases. It consists of a few to several dozen layers ( multilayer ) of metal-coated plastic films, which are kept at a distance by a suitable perforation or gauze (spacer). MLI foils hinder the transport of heat through thermal radiation. While in space due to the vacuum no heat transfer by convection can take place and gas heat conduction, in the earthly MLI only in conjunction with an application isolation vacuum applied
principle
A warm body gives off heat in the form of thermal radiation to a colder environment. An additional impermeable layer as an obstacle reduces the heat transport through radiation according to the Stefan-Boltzmann law . It warms up until half of the radiation is reflected back to the body in thermal equilibrium. He gives the other half to the environment. With n layers, the radiation loss is ideally reduced (flat geometry and the same emissivity) by a factor of 1 ⁄ (n + 1) .
With reflective layers, there is an additional reduction due to the low emissivity. The factor is only added once, not increased by the number of layers.
execution
In the simplest case, an MLI consists of metal-coated plastic films. Usually a polyester film is used , for higher temperatures a polyimide film is used. The individual layers resemble a rescue blanket . The metallization consists of aluminum or silver, which takes on a golden color with a thickness of a few atom layers. For critical applications, gold is used, which has a reflectance of more than 99% in the infrared.
The metal layer acts as a mirror, the plastic layer as a mechanical carrier. The thermal insulation between the mirror layers improves significantly if the foils are kept at a distance. A coarse-meshed net made of silk or plastic between the foils reduces the thermal bridges to the few connecting bars. With a double layer, values of 0.1 to 0.05 are achieved for ε * . Ten layers bring it down to 0.01. Even with more than 30 layers, the lower limit is in practice around 0.003.
operation area
Without special measures, spacecraft are exposed to large temperature gradients. There is no atmosphere that could bring about temperature equalization through convection. The heat conduction is limited to bodies that are firmly connected to one another. In the vacuum of space, heat is transmitted almost exclusively via radiation.
Since electronic devices require an operating temperature of around 20 ° C, spacecraft are often wrapped in MLI film. It protects the spacecraft from cooling down through reflection and supports the even distribution of heat. It also protects against heating up by radiation from the outside.
Another area of application is cryotechnology , for example the isolation of cryostats .
Individual evidence
- ↑ WG Fastowski, JW Petrowskit, AE Rowinski: cryogenics , Akademie-Verlag, Berlin 1970