Aluminum foil ⁑ DernocuaAluminum foil<br>Kragen Javier Sitaker, 02021-05-24 (updated 02021-09-11)<br>(14 minutes)<br>Kitchen aluminum foil is a remarkable material.
It’s typically 10 μm thick and 400 mm wide, giving it an aspect ratio<br>of 40000 in that dimension, and rolls are commonly some ten meters in<br>length, for an aspect ratio of 1 000 000; heavy-duty versions can<br>reach 30 μm or more. Despite their thinness, foils of 25 μm or<br>more are impermeable to oxygen, water, and light, though Wikipedia<br>claims thinner foils typically are plagued with pinholes. It comes in<br>a fully annealed state, so it rapidly work-hardens when bent, and<br>because of its thinness can be bent at deep submillimeter scales to<br>form metamaterials. It’s highly reflective (88% on the bright side<br>across the visible spectrum and even higher in the infrared) and<br>conductive, rivaling copper. It resists corrosion for years in<br>weather, it’s nontoxic, it’s light (2.71 g/cc), and it’s damn cheap,<br>under 50¢/m².
Robert Lang recommends laminating tissue paper on one or both<br>sides of kitchen aluminum foil to make “tissue foil”, which for years<br>he considered the ideal origami material. Notably, he uses a weak<br>sacrificial adhesive layer to hold the foil in place for the<br>lamination process.
Typical alloys include especially 1100 and 1200, but also 8111,<br>8015, and 8006, with 0.06%–0.6% silicon and 0.4%–1.6% iron, and in<br>some cases also some copper or manganese, under 0.5%. (1100 is<br>sometimes described as an “unalloyed aluminum grade” but it’s<br>specified to contain 0.05%–0.20% of copper, and it unavoidably has<br>other impurities.) Room-temperature yield strengths of these alloys<br>range from 30–170 MPa, with ultimate tensile strengths of 70–200 MPa,<br>and of course they all have a Young’s modulus around 70 GPa. Because<br>its crystal structure is fcc, it remains ductile down to absolute<br>zero, making it suitable for cryogenic applications; indeed, aluminum<br>becomes stronger at cryogenic temperatures. And, although it<br>weakens dramatically at higher temperatures, it doesn’t melt until<br>almost 650°, enabling it to be used at higher temperatures than<br>organic materials.
If oxidized (for example, with a soda solution, an arc, or<br>anodization) it yields amorphous sapphire, which if crystallized is an<br>excellent insulator, refractory, and abrasive. The oxidation process<br>produces a great deal of heat, making aluminum a<br>very-high-energy-density fuel, and, thanks to aluminum’s sternly<br>trivalent nature, electrical current; aluminum-foil fuel cells are<br>routinely produced by amateurs, though these typically oxidize the<br>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<br>noticeably cheaper than photovoltaic cells, currently around 18¢/Wp,<br>360 times more expensive. (However, the foil number there is sunlight<br>watts; if you’re making a PV solar concentrator you have to divide by<br>the efficiency of the solar cells, say 21%, which gives you 0.24¢/Wp<br>electric.) A large aluminum-foil assembly would be<br>vulnerable to significant deflections, but many small assemblies could<br>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<br>making the equivalent of corrugated cardboard out of it, maybe using<br>aqueous boric acid (US$1.70/kg according to Potential local sources and prices of refractory materials) or<br>borax as the glue. The surface tension of water is ample to hold<br>aluminum foil in place until the water dries.
The feature that currently attracts my attention is the possibility of<br>work-hardening, which suggests the tempting possibility of making<br>tooling from aluminum foil that can itself work aluminum foil at room<br>temperature, a possibility reinforced by the immense aspect ratios<br>routinely available. As a simple example, you can in theory roll some<br>foil into a cone, and the point of this cone can dent, form a rib in,<br>or even pierce more of the same foil; but this is much easier in<br>practice if you first fold the foil 16 layers thick, form ribs<br>converging to a point on the last-formed fold, then roll the cone<br>around that point. If the last-formed fold is reversed, the aluminum<br>along the outer edge of the fold is the aluminum that was most<br>strained previously, having been bent double with as small a radius as<br>possible, and so will be the most work-hardened.
I was able to use such a cone to pierce not just aluminum foil but the<br>skin of an apple. I folded it from some foil which, folded 256 layers<br>thick, measured 2.57 mm in my shitty digital calipers; the resulting<br>square measured 27–29 mm on each side and weighed 1.8 g, giving a<br>density of only 0.8–1.0 g/cc, so it’s probably more than half air,<br>though it rapidly sinks in water, so probably the density is a<br>little higher than that.
Using such a cone point to form ribs without piercing foil is tricky,<br>because it tends to have significant asperities around the tip, which<br>tend to tear the foil if it...