GoKawiilAluminum foil
Kragen Javier Sitaker, 02021-05-24 (updated 02021-09-11) (14 minutes)
Kitchen aluminum foil is a remarkable material.
It’s typically 10 μm thick and 400 mm wide, giving it an aspect ratio of 40000 in that dimension, and rolls are commonly some ten meters in length, for an aspect ratio of 1 000 000; heavy-duty versions can reach 30 μm or more. Despite their thinness, foils of 25 μm or more are impermeable to oxygen, water, and light, though Wikipedia claims thinner foils typically are plagued with pinholes. It comes in a fully annealed state, so it rapidly work-hardens when bent, and because of its thinness can be bent at deep submillimeter scales to form metamaterials. It’s highly reflective (88% on the bright side across the visible spectrum and even higher in the infrared) and conductive, rivaling copper. It resists corrosion for years in weather, it’s nontoxic, it’s light (2.71 g/cc), and it’s damn cheap, under 50¢/m².
Robert Lang recommends laminating tissue paper on one or both sides of kitchen aluminum foil to make “tissue foil”, which for years he considered the ideal origami material. Notably, he uses a weak sacrificial adhesive layer to hold the foil in place for the lamination process.
Typical alloys include especially 1100 and 1200, but also 8111, 8015, and 8006, with 0.06%–0.6% silicon and 0.4%–1.6% iron, and in some cases also some copper or manganese, under 0.5%. (1100 is sometimes described as an “unalloyed aluminum grade” but it’s specified to contain 0.05%–0.20% of copper, and it unavoidably has other impurities.) Room-temperature yield strengths of these alloys range from 30–170 MPa, with ultimate tensile strengths of 70–200 MPa, and of course they all have a Young’s modulus around 70 GPa. Because its crystal structure is fcc, it remains ductile down to absolute zero, making it suitable for cryogenic applications; indeed, aluminum becomes stronger at cryogenic temperatures. And, although it weakens dramatically at higher temperatures, it doesn’t melt until almost 650°, enabling it to be used at higher temperatures than organic materials.
If oxidized (for example, with a soda solution, an arc, or anodization) it yields amorphous sapphire, which if crystallized is an excellent insulator, refractory, and abrasive. The oxidation process produces a great deal of heat, making aluminum a very-high-energy-density fuel, and, thanks to aluminum’s sternly trivalent nature, electrical current; aluminum-foil fuel cells are routinely produced by amateurs, though these typically oxidize the aluminum to the chloride rather than the hydroxide or the oxide.
50¢/m² is 50¢/kWp in a solar concentrator, or 0.05¢/Wp, which is noticeably cheaper than photovoltaic cells, currently around 18¢/Wp, 360 times more expensive. (However, the foil number there is sunlight watts; if you’re making a PV solar concentrator you have to divide by the efficiency of the solar cells, say 21%, which gives you 0.24¢/Wp electric.) A large aluminum-foil assembly would be vulnerable to significant deflections, but many small assemblies could be placed on a hard, stable surface such as a rock or an adobe wall.
Alternatively, though, it might be possible to stiffen the foil by making the equivalent of corrugated cardboard out of it, maybe using aqueous boric acid (US$1.70/kg according to Potential local sources and prices of refractory materials) or borax as the glue. The surface tension of water is ample to hold aluminum foil in place until the water dries.
The feature that currently attracts my attention is the possibility of work-hardening, which suggests the tempting possibility of making tooling from aluminum foil that can itself work aluminum foil at room temperature, a possibility reinforced by the immense aspect ratios routinely available. As a simple example, you can in theory roll some foil into a cone, and the point of this cone can dent, form a rib in, or even pierce more of the same foil; but this is much easier in practice if you first fold the foil 16 layers thick, form ribs converging to a point on the last-formed fold, then roll the cone around that point. If the last-formed fold is reversed, the aluminum along the outer edge of the fold is the aluminum that was most strained previously, having been bent double with as small a radius as possible, and so will be the most work-hardened.
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