Feasability of a greenhouse made of sapphire

It's been possible to make artificial sapphires for over a hundred years. While with the right additives it's possible to make them traditional colouration for gemstone quality rubies and sapphires. However without those additives the resulting mineral comes out more or less clear in the visible spectrum (as well as the near infra-red and UV) which makes it a good substitute for glass in certain applications where its strength, hardness or heat resistance make regular silica glass problematic. (It's one possible form of bulletproof glass for example).

At the moment the technique are still rather energy intensive and rather expensive. But if future technology could increase the size of the panes and reduce the cost, would there be any reason (other than cost) why it couldn't be used as the "glass" in making a greenhouse rather than normal glass or a clear acrylic plastic.

Synthetic sapphire is a pretty awesome material all around. It's already used in some heavy-duty industrial applications like die orifices for metal wire manufacturing and spray nozzles for water jet cutting, and like other forms of alumina oxide, it has really, really good wear resistance. Most importantly, it has very low light absorbance and scattering and maintains its optical properties at extreme temperatures, even when immersed in molten steel. This is potentially a double-edged sword, because you don't want your greenhouse material to transmit infrared radiation too well; it needs to reflect/emit some of it back into the interior and keep it warmer than the exterior. It's also much more thermally conductive than regular glass by virtue of its crystalline structure.

You can't change the latter, but you can (kind of) limit the transmission of reflected heat from the interior by taking advantage of its anisotropy. Sapphire is a strongly refractive material, so you can build your panes as composites: one layer with its crystals oriented in one direction, one oriented in another, allowing it to filter out unwanted radiation. Assuming that the interior of the greenhouse has a denser, more humid atmosphere than the exterior, most of the radiation bouncing around inside will probably be out of orientation to make it through the two-layered filter, allowing the greenhouse to keep some of the heat in.

The only other possible drawback is its higher density compared to silica glass, but if your scaffolding is strong enough, then it shouldn't be a problem. And unless your greenhouse is, like, the size of a soccer stadium, the size of the panes shouldn't be a major issue. Like all single crystal/Czochralski process products, it's cheaper to manufacture sapphire in bulk and cut it down to size later, and you can make the panes a lot thinner than their silica glass equivalents.

Edited by eagleoftheninth on Jan 28th 2019 at 3:29:18 AM