Help needed with Diamond synthesis (really)

[bandu]

I'm working with a television production company on a new project - we're looking at using "backyard methods" to produce diamonds (nothing special, we'll be happy with industrial grade for the show), namely using an unconventional technique reportedly used by the soviets during the cold war.

Our aim is to build a firing tube into the ground and pressurise carbon (probably graphite or charcoal) to the point at which it produces diamonds with a dead weight - propelled by explosives or a rocket.
I am aware of the HPHT process and we did look into building a press until it was decided against for visual reasons (a big rocket is a lot more interesting than a press).
Our preliminary design is to fire a 1kg weight with a frontal area of around 10cm^2 at up to 1000mph at a couple of kilos of carbon, heated to approximately 1200°c.
I am under the impression that we will need to subject the carbon to a pressure of around 100 tons (metric) per cm^2 to create the diamond, but again this is all internet-based suggestion...

I admit that the numbers are pulled out of a hat, but we need to figure out how much pressure would be exerted on the carbon - the team is mechanically minded, but none of us are scientists, so any help with the equations would be greatly appreciated.

 

[MagnetDave]

I think you're going to have a kinetics issue - the impact load will be over and done with before the recrystallization process can happen. It might work better, perhaps, if you set a very high-thrust rocket motor directly on the parts, and let the thrust of the rocket press the parts for the duration of the burn. It won't produce nearly the force of the impact.

It's an interesting idea, but the concept is pretty out there, and I'm have trouble wrapping my head around it.

For a back of the envelope calculation - if you know the velocity of the the weight (1000mph or 1467ft/s) and the length of the space you have to decelerate (call it 1 ft) you can assume, for a first order, a linear deceleration (not true at all). You will have an average speed of 1467ft/s divided by 2, or 733 ft/s. This will cover your deceleration distance in 1ft divided by 733 ft/s, or 1.36x10^-3 s. This is an acceleration of 1.08x10^6 ft/s/s, or 3.3x10^5 m/s/s. By Newton's second law, F=ma. Your 1kg weight will require (and extert) a 330 kN force. On a 10 cm^2 frontal area, that is 3.3 kN/cm^2 of force, or about 3 metric tons per cm^2.

Granted - this is a first-order approximation. the real system will be nonlinear, more force will be exerted as you approach the compression limit, but more force will ALSO be dispersed by whatever this is sitting on on the ground. You might do better with a smaller, heavier slug fired out of a cannon.

 

[mikegem]

I agree you'll have a kinetics problem. To transform graphite to diamond, you need to heat the graphite (gives the energy needed to overcome a potential energy barrier in rearranging sp2 (graphitic) carbon bonds to sp3 (diamond) carbon bonds. You also need simultaneously to increase the pressure to the region where diamond, not graphite, is the thermodynamically stable form of carbon. Your 100MT/cm^2 is in the right pressure range, and maybe the projectile impact will give you the temperature, I don't know.

But then, having transformed some graphite to diamond, you need to cool the diamond while the pressure remains high. If the pressure drops while the diamonds are still hot, they will transform to graphite because graphite is the stable form of carbon at ordinary pressure. I don't think the impactor scheme will do this for you.

Your experiment is in the direction of a shock method of diamond synthesis that does work, and which can be done in a backyard (if your yard is really, really large - this will annoy your neighbors!).

In this method, a block of explosive (C4, TNT, etc.) is stacked on an aluminum plate that sits in turn on top of a plate of graphite. The whole assembly is placed over a big pool of water. The explosive detonates, sends a shock wave into the aluminum, melting it and pushing it at very high speed into the graphite. As it impacts the graphite, it heats and pressurizes it so that a pretty big fraction of it turns into tiny diamond particles. The moving mass of mixed aluminum and carbon slams into the water and gets cooled really fast, before the pressure drops below the range where diamond is the stable carbon species. This prevents the diamond from reverting to graphite. The aluminum/carbon fragments are recovered, the aluminum is dissolved, and the diamonds are recovered.

This method was developed by deCarli and Jamieson, and is still used today to make fine diamond grit. It's limited to very small (think submicron) diamond particles because the process is over quickly and there's not time for diamond crystals to grow much bigger than a micron. It's a spectacular process! I've seen it done with a 55 gallon barrel of water for the quench liquid, and it worked fine.

For a more successful diamond synthesis you could actually do at home, you might look at chemical vapor deposition. Google the work of Russ Hemley and his group at Carnegie Institute. Basically, with a microwave oven (microwave plasma-assisted CVD), you could grow gem quality diamonds in your kitchen. Not as dynamic as explosives or projectile synthesis, but it really works, and it uses materials and equipment that are readily available. Good luck, sounds like a fun project.

 

[mikegem]

One more thought - The impactor method does actually work, it's just that the impactor has to be really big, moving very fast. Diamond particles are routinely found at meteorite impact sites, where they're created in the extreme pressures and temperatures produced when something traveling a few miles/sec hits the earth. Not exactly a backyard experiment.

The only impactor apparatus I can think of that might make diamonds would be a gas gun or a railgun. These can give projectile speeds in the low miles/sec range, and maybe if you shot a piece of graphite sitting on water to give a rapid quench, you might make some diamond dust. Of course, it's a rare backyard that will have a gas gun or a rail gun, so this wouldn't be helpful in your project.

 

[Shockman]

You need a hidraulic pressuresystem and more than 3000K. To keep the permanent pressure on this temperature is not a backyardproject. sorry

 

[Phrak]

You need a hidraulic pressuresystem and more than 3000K. To keep the permanent pressure on this temperature is not a backyardproject. sorry

How long do I have to maintain my pressure system to get some nice gems, say 3mm on a side? I mean, in the past, I've given it a few minutes and nothing happens. Does it take weeks, months or years??

 

[Shockman]

I think the exact pressure/temperature/cristallisationvelocity are industrial seecret. You need to experiment with it if you hawe the equipment. I hawe seen once a Pressure/Temperature diagram from Enydine. It is a factory for industrial diamonds. This diagram was very complex. If you want to grov monocristaline diamands, you need extremely high p and t parameters and time mesured in days. Try to google "HPHT technology". There is a a wiki site to.

http://en.wikipedia.org/wiki/Synthetic_diamond