Why this might work when other attempts have failed

Several readers pointed out that others have tried to fixate Nitrogen using ultrasonics, and none succeeded.

Supeno's paper "Sonochemical Fixation of Nitrogen" reviews the previous attempts and has a good explanation of the difficulties involved. If you're interested, it's an easy read and provides good background information for this project.

Of course you don't do what others have done and expect different results. For different results, you have to do something no one has tried. Preferably with a rationale for why the differences are important, and how they might lead to success.

So here at last is the secret sauce: what I intend to do that is different from previous attempts!

Difference 1: Focused ultrasonics

Previous attempts used ultrasonics at low power density. More specifically, previous attempts made no attempt to focus the ultrasonic energy to high density.

Supeno experimented with a tuned chamber that, as far as I can tell, generates standing [planar] waves. The energy within the chamber is the transducer power divided by the base area times the Q of the tuned system.

He used roughly(*) 50 watts from a transducer 2.125" in diameter, which is 2.2 watts/cm^2, which is not a high energy density. His paper doesn't list the tuned chamber specs, but if the Q is 20 then the energy density would still be only 44 watts/cm^2.

(*) Various depending on the experiment, but in this range.

For my experiments, I've milled a rectangular end horn and a parabolic reflector anvil.

In theory, the rectangular horn should generate plane waves which will be concentrated by the parabolic reflector. This should produce enormous energy densities and pressures, which are different experimental conditions from the other attempts.

Difference 2: Directly sonicating the gas

Previous attempts used Nitrogen and Hydrogen dissolved in water. Specifically, in previous attempts Nitrogen and Hydrogen were bubbled through water using an air stone, and sonicated.

Hydrogen and Nitrogen are largely insoluble in water, so one would expect such a system to have a poor yield.

...but note that previous experiments did produce tiny amounts of ammonia. That's important!

It takes 3 moles of Hydrogen to produce 1 mole of ammonia, so dividing the Hydrogen solubility by 3 tells us that the most ammonia we can get from 100cc of water using this method is about 50 micromoles(*).

Further, the molar proportion of dissolved Nitrogen to Hydrogen is 0.8:1, which is nowhere near the ammonia ratio of 1:3. This reduces the maximum yield: with less Hydrogen available than needed, fewer activated Nitrogens will encounter Hydrogens to form ammonia.

Supeno experimented on 100cc batches and produced around 5 nanomoles of ammonia(**). Perhaps ultrasonic production of ammonia might work with a different setup.

(*) Gloss gloss, such as continually replenished gases and not accounting for sonification time.

(**) Various, depending on experiment, see paper.

My system uses a capillary tube to hold a small bubble of gas (mixed to molar proportions) at the focal point of the parabolic reflector.

The hope is that focused ultrasound will compress the bubble to high pressures, generated ammonia will dissolve into the surrounding water, the bubble will shrink, and gas will be replaced as needed by a mass flow controller.

For now I'll manually control the mass flow via serial port, but if ammonia is generated it seems reasonable to automate this using a webcam and OpenCV.

Modification of parameters

Experimentation is a form of search, and to increase the chances of finding something you need a wide search area.

With this setup I can vary the ultrasonic power delivered to the bubble, the size of the bubble (a little), the temperature of the water, and the gas mixture (proportions, or adding a buffer gas such as Argon). I can also dissolve things in the water which might catalyze the reaction, and I can try to make other compounds such as oxides of Nitrogen.

My plan is to take measurements under various conditions, look for trends in ammonia formation (if any), and zero in on the most effective experimental regime. At that point I can estimate the efficiency and compare to the Haber process.

Video of the system in action

Here's a video of the reaction chamber showing horn and anvil. It doesn't have a capillary tubing mount (under construction), but I have an air bell for collecting gas, Hydrogen and Nitrogen generators, and a mass flow controller.

This the same effect as the 2KW cavitation mentioned in the a previously mentioned video, at a smaller scale.

The cloudy wisps extending from the bottom of the horn are flows of cavitation bubbles. I think. Power output around 50 watts, through a cylindrical horn sculpted to a 44x12 mm rectangular(ish) end.