Multi-Layer Insulation (disc)

This puffy, silvery, rustling disc of material bridges an unlikely gap between textiles, mirrors, and mille-feuilles. Multi-layer insulation, or MLI, is a thermal insulator: when wrapped around an object, this composite material helps keep it at a different temperature from its surroundings, just like insulation in your home keeps the inside of the house comfortable regardless of the temperature outside. 

The sample of multi-layer insulation that we have here is a plump cousin of the emergency blanket – that shiny, lightweight foil found in first aid kits. Both our sample and the emergency blanket rely on flexible plastic sheets coated with a layer of aluminium thinner than a micrometre – that’s more than a hundred times thinner than a human hair. This aluminium coating leverages the metal’s ability to reflect away infrared radiation, the invisible form of light that carries heat. A single aluminized sheet provides ample insulation for most temperature differences encountered in daily life, but it still lets a small percentage of heat through. 

For high-tech applications in demanding environments, with much higher temperature differences, this multi-layered aluminium composite goes the extra mile by stacking several of these sheets together. The incoming heat is bounced around so much in the layered structure that almost nothing makes it through to the other side, making MLI an almost-perfect mirror for infrared radiation. Inside our mille-feuille of a material, the metallised layers are separated by a delicate, veil-like non-woven mesh made of an insulating material like polyester or fibreglass. Without these insulating layers, heat could simply crawl through any touching metallic sheets by conduction, defeating the MLI’s purpose. 

This material is specifically engineered for environments where heating happens mainly through infrared radiation, and where other heating modes like conduction or convection are minimal. This is precisely the case in a vacuum, whether it’s found around a satellite in outer space or inside an underground particle accelerator. Interestingly, any holes punched through the MLI’s metallic layers are intentional: they allow trapped air to escape when the material is first exposed to vacuum, during a satellite launch or the commissioning of an accelerator.

This particular sample comes from CERN, the European Laboratory for Particle Physics. It is probably an offcut from the insulation of a superconducting electromagnet. To operate, magnets like this must keep a core temperature just a few degrees above absolute zero. But, as they sit in a tunnel at room temperature, there is a massive temperature difference between the magnets’ cores and the surrounding air. To handle this, the magnets have been designed like large thermos bottles, with thick, hollow walls from which the air has been pumped out. These vacuum barriers are very efficient at preventing heating through conduction or convection, but some heat can still leap across the vacuum in the form of infrared radiation. That’s where our multi-layered material comes in: it’s skilfully lined up inside the vacuum gaps, creating a barrier that reflects any infrared. CERN’s magnets wear these shiny puffer jackets to stay cool – and not just as a fashion statement! Tailoring MLI to snugly fit the complex edges, bumps and joints of a magnet is an art in itself, requiring master-level sewing and judicious use of Velcro.