Important disclaimer: Several of the reagents used here are very harmful and produce toxic gases. If you try to repeat it, at least please wear eye protection — no project is worth permanent blindness.
Making things that glow in the dark is fun, but I wanted to do something more than just buy glow powder from china.
The undisputed king of glow in the dark materials is Strontium Aluminate (SrAl2or4) that glows brightly for hours after exposure to light. Even better, all the ingredients are easy to get: Oxygen is everywhere, Aluminum is quite common, and while Strontium sounds exotic, it is easy to buy for making ceramics.
Since it is almost impossible to mix metal oxides at the atomic level, strontium aluminate is usually produced by the decomposition of soluble nitrate salts:
2 Sr. (NO3)2 + heat (~600 C) ➙ 2 SrO + 4 NO2 (gas) + Or2 (gas)
4 Al (NO3)3 + heat (~200 C) ➙ 2 Al2or3 + 12 NO2 (gas) +3O2 (gas)
SrO+Al2or3 + heat (~1200 C) ➙ SrAl2or4
To make aluminum nitrate, it was not possible to simply dissolve aluminum metal in nitric acid, so I first made aluminum hydroxide from potassium alum (a food preservative):
2 KAl (SO4)2 + 5 NaOH ➙ 2 Al(OH)3 (it’s raining) + 3 Already there2SO4 + K2SO4
To begin, I dissolved some alum in water, and then added dilute sodium hydroxide to precipitate the aluminum hydroxide. This is what the product looks like after it has been filtered and dried at 120 C to remove any traces of water:
Although chemically simple, this is the most annoying step in the entire process. For one, if too much base is added, the aluminum will dissolve again, so I have to keep checking the pH. Second, the reaction produced a very irritating clay like product that took days to filter and wash.
After that mess, I weighed out 1.74g (10 mmol) of the aluminum hydroxide I had just made, 0.73g (5 mmol) of strontium carbonate, 0.01g (0.005 mmol) of Europium (III) oxide, and 0.02 g (0.01 mmol). ) of Dysprosium (III) oxide. After combining everything, I added water and nitric acid until everything was dissolved.
Lanthanides are doppants: small amounts are needed to make the material glow, and different colors are produced. I chose Eu+Dy because it produces the brightest glow, and that classic glow-in-the-dark green color. (which happens to have a wavelength around the peak sensitivity of the human eye)
I then evaporated the nitrate solution until only a salt mixture remained. It is very important to get all the acid out at this point. It should be heated to above 120 C, and kept there until no fumes are produced, even if the pieces are crushed.
Then I added 8.41 grams (140 mmol) of urea and enough water to dissolve everything:
This urea both interferes with crystallization, which can separate different salts, and can act as a fuel in a combustion reaction. (They can also form explosive salts with residual acid, so it’s important to remove them all.)
After heating to 300 C, the mixture turned into a white foam. When lit, it burns slowly and almost without flame. Because it contains a significant excess of fuel, the reaction also reduces Europium to the +2 state, which is necessary for it to glow.
In theory, this combustion should also produce enough heat to form strontium aluminate, I can’t do it. Instead, I simply heat the ashes of the combustion with an oxy-propane torch:
When the partially melted blobs were cooled, they were able to glow brightly, but only for a few minutes as opposed to the hours that commercial items are able to do. I blame it on reagent purity: While I couldn’t find any data on Strontium Aluminate specifically, many phosphors are very sensitive to stray metal ions, such as iron.
Many papers also calcine the final product at 1300 C, but without a furnace that goes up to the top, I have to be content with making things grain by grain with a torch.
On a related note, here’s a cool phosphorescent rock from my brother’s collection:
The photo was taken one second after being hit by the xenon flash lamp. I think it is agate with traces of zinc and manganese ions (Willemite). Trace uranium ions can also cause green fluorescence, but without brightness.
2025-01-19 16:30:00
