Stirlingengineforum.com

Has anyone tried using boron nitride powder (amphorous, not compressed)?
PHYSICAL FORM: Solid metal COLOR: White
ODOR: None
MELT POINT: 2700
SOLUBILITY IN WATER: Insoluble
SPECIFIC GRAVITY: N/A
VOLATILE BY WEIGHT: Essentially zero
VAPOR PRESSURE: N/A
DENSITY: 2.25 g/cm


Its nickname is "white graphite" even though it is clearly not graphite, but acts like it. I doesn't burn well at all, either, and doesn't decompose.
This makes it an excellent choice as a high temperature lubricant and casting release agent for metal casting.
sold in aerosol cans or direct powder.
http://i.ebayimg.com/00/s/MTUwMFgxMDAw/ ... W/$_57.JPG

Thanks for posting the info. I am not a material scientist; but I am wondering if this material can be form into any shape through sintering. It would make a perfect piston.

Well, if you compress and sinter it, it becomes something much harder.....just like graphite. Except in this case, it is about 0.25 softer than diamond. The compressed, hardened form will cut even silicon carbide grinding wheels, and is used to dress them to refresh the surface. The powdered form is used to lubricate gun barrels, too. Bullet velocity even increases considerably. If fact, if I remember right, it is slicker than graphite by a small amount. Fun fact: used in cosemetics to replace talc powder.

I looked into mixing it with molten aluminum, but it is non-wetting to the aluminum/magnesium family of alloys. They do sell brush-on or spray-on coatings, though, and coating a cast iron piston looks promising even if it flakes off and travels inside the motor. It would increase the thermal conductivity of the working gas as well as being inert to oxidation at operating temperatures.
The coatings are pricey, though.
Oh, the graphite-like stuff is called Hexagonal Boron Nitride. The super hard is Cubic Boron Nitride. So if it says "HBN" on the label, that is what you want.

even if it flakes off and travels inside the motor

That is not good. It will jam up the regenerator and/or heat exchanger. In my design, the air passage is about 100 (and at most 200) microns wide.

This, obviously, is a good material for stirling engines and we will have a handful of work if we want to learn what can be done with it. Seems like you have worked with it. Is it brittle in alloy form? Also, the melting point you posted. Is it in centigrade? Use this material as a matrix with tungsten fibers would be an excellent composite for heat exchangers and seal rings. If it works, we can kiss those polyurethane seal roller rings good by.

Edit: More info

Ceramic coatings offer an alternative
to bone ash. These foundry coatings,
which are applied as liquids and dry
to form a solid film, should have high
thermal and chemical stability and a
similar thermal expansion to the coated
surface in order to prevent the coating
flaking off. A material that ideally
meets all these requirements is boron
nitride (BN).

Download http://www.esk.com/uploads/tx_userjspre ... ting_e.pdf and search for "Boron nitride plus binder"

I think the coarsest size is about 14 micron, so if used, I don't anticipate a big problem. I really haven't worked with the stuff, but I do my homework. From what I have seen, it is alot like talcum powder--greasy, forms ultrafine dust, and technically can be hot formed into spongy, sintered-like shapes like you want. (I was wrong, earlier) I wish I had the spare cash and time to play around with it. It is rather expensive. My first practical application will be the hunting rifle I've got. (have to justify spending the money)
I believe it was degrees 2973C, at which it sublimes, and skips the liquid phase. http://en.wikipedia.org/wiki/Boron_nitride
I just need to snag some of the stuff and tinker. It looks very promising.

OK. Just figured out. To reduce cost, ring seals can be made out of boron nitride the same way the carbon seals used turbine engines. The ring consist of 3 pieces and they are spring loaded from the piston side against the cylinder wall.

I believe it was degrees 2973C

That's perfect.

Do these ring seals have overlapping ends like a transmission seal? It would only make sense if they did. I imagine a thin supporting (backing) ring would have to be used, if the compressed stuff is as soft as I imagine. I was thinking a trapped ring held in place with a snap ring in a groove on the cylinder wall or two-piece piston might do well. Trapping it would give it plenty of strength no matter what the toughness of the material.
Seems I sparked a thought process that has a life of it's own. We seem to have different plans for the stuff, heh.
I was hoping to leave powdered hBN in the stirling engine, swishing about all over, increasing both the heat capacity of the gas as well as thermal conductivity. hBN can be bought in 100um grain sizes or even finer, if you like. Superfine dust running around the engine, lubricating and conducting heat, sounds really good to me. Apparently, hBN has excellent thermal conductivity, from what I've read, slightly better than cast iron, but less than nickel.

Found some more detailed info:
" Boron nitride is often referred to as “white graphite” because it is a lubricious material with the same platy hexagonal structure as carbon graphite. Unlike graphite, BN is a very good electrical insulator. It offers very high thermal conductivity and good thermal shock resistance. BN is stable in inert and reducing atmospheres up to 2000°C, and in oxidizing atmospheres to 850°C. "

Yes. They do.

http://youtu.be/u49cTC-lwAE

I like this design. Hydraulic/pneumatic cast iron seal. Would be nice if coated with hBN.

I am afraid the bonding between the two being not strong and due to different heat expansion and the bending of ring, the coating might fall off. A groove might be able to hold the hBN in the ring. Actuators are different from engines; but I think we can make an exception here since stirling engine cylinders' temperatures do not fluctuate much.

FYI. Amazon is selling it for $100 per lb. http://www.amazon.com/Hexagonal-Boron-N ... B00BM8KM20

Edit: hBN is being use as coating on piston seals for break in, so they are not expensive. The coating eventually burnt off. It should be fine for stirling engines as the temperature will never be as high as in the combustion chamber of an IC engine.

Interesting material ! If someone is interested and want to test It, Just wanted to note that there are much smaller quantities being sold on amazon. In a smaller quantity it isn't really that expensive.

I found companies in China that can make custom parts. Inexpensive as well.
This is a refractory in China that will make custom hBN parts
http://stauniverse.en.alibaba.com/

Edit: I send the above company about custom hBN parts and I got an email back from them the same day.

Very glad to receive your inquiry for our boron nitride parts.

Pls send us one drawing, or provide the size you need, we can customize

for you as per the specification you provide.

Looking forwrad to your favorable reply.

Best wishes,
chris zhang

I should definitely order some from amazon for sure. alibaba web links tend to be geared towards mass production, it seems. For now, I am all about the prototype.

Apparently, the hBN is also hygroscopic (water absorbing), and may make a nice dessicant of sorts for a stirling engine, too. Getting moisture out of a working gas like air should really improve efficiency.

One of my posts years ago involved using borax as a coating to protect pistons from high temperature air. I mentioned that borax is a super high temperature flux for welding iron that was used by blackmiths. I took an acetalyne torch to the stuff, and tried coating some metal with it, but I had too much moisture in it and it crackled and sputtered very unevenly. I need to revisit the test, and try to brush it on while it is in a liquid state. One thing I could never do: destroy it. I could only melt it, nothing more. As a flux, it will dissolve all metal oxides into it and preserve the metal, even in the rain. I believe boron compounds will be key in perfecting the practical stirling engine.

Borax is actually the flux used for brazing, I use it instead of the stuff that you buy in the welding supply shop because it's so cheap. When the Blacksmith does forge welding the flux is sand which is also the basis of glass, as in windows, bottles ect.
The coating I am, interested in is Xlan, which is a form of Teflon that is baked on, it can go on an aluminium piston.
Ian S C

I got an other email from the company in China for small quantities and they said they can make as few as 10pcs. Just need the engineering file or a file from Blender will do.

Edit: Sintered hBN has about the same strength as cast iron (4% carbon). Probably good coating for piston rings.