Decaborane Handling Issues:  Design Modifications for pB11 Fusion Research

LPPX is starting to plan for the modification of the chamber to use the decaborane gas.  For this purpose, the chamber has to be heated and has to be maintained at 100-120oC to keep the decaborane gas from condensing.  A separate chamber is needed to store the solid decaborane.  Several other design modifications are required such as protection of the pumping system against fouling by boron particles.

Below is an email string that addresses issues related to decaborane handling.

Email Exchange

Hi Alex,

Nice to hear from you.  I am Dr. S. Krupakar Murali (call me Murali) and am designing the chamber and heating system to handle decaborane.  Since this material, as you know, is a solid at room temperature we intend to heat it up to fill the chamber with the gas.  However, I am not sure how to handle this material.  We need some special hoods, do you have any recommendations as to what we need to take into consideration before we order a hood for this application?  I have shortlisted the following ductless hood, but am not sure if it would work http://www.air-science.com/portable.html  what is your opinion?

Thanks,

Murali

PS:  Do you have any references that speak specifically about decaborane and other gasesous forms of boron compounds

Hi Murali,

Greetings from Massachusetts.  Wow!  I’ve never seen one of these portable fume hoods.  I’m impressed how compact it is.  I think it’s fine for decaborane.  As you may know decaborane needs to be in a fume hood both because of its toxicity and because it is hygroscopic.  After opening the bottle we used to put tape around the bottle seal after closing it.  Over time it starts to get clumpy and when heating a sample for the first time, it outgasses a fair amount of water.  If you have an RGA on your system that would be very helpful.  Before we had a fume hood we just minimized exposure time and avoided breathing directly next to it.  As long as you’re careful there shouldn’t be much of a threat to anyone’s health.  I don’t want to minimize its toxicity but any sort of air flow from a fume hood will be enough to protect people from breathing exposure when working with powder from a bottle.  Some obvious points: wear gloves, wash hands after use, maybe even wear a simple face mask. Cleaning dirtied parts requires more protection.

If my memory is correct, decaborane at 100degC has a chamber pressure of about 10mtorr.  One of the more aggravating aspects of working with the material was that any gas lines between your vaporizer and chamber must be at a temperature greater than the vaporizer temperature or else it will condense and clog up.  Also very important: above ~300-350degC decaborane decomposes.  Also important:  at what ever temperature you operate, it’s messy stuff.  It will coat everything up and maybe affect your plasma operating conditions.  Anyone working with the material will get more decaborane exposure cleaning the chamber than handling material from the bottle.  I believe we used hydrogen peroxide to clean it off chamber walls.  That requires a mask.  If you can put any parts that need cleaning under the fume hood your better off. 

Hope this is helpful.  Let me know if you have further questions.

Regards,
Alex

On different boron containing compounds:

Other boron compounds in solid form at room temp are octadecaborane and carborane.  Octadecaborane has 18 boron atoms/molecule.  It has a similar toxicity level and vapor pressure as compared with decaborane.  Carborane is C2B10H12.  If you can tolerate a little carbon, it’s easier to handle, less toxic, as compared with deca.  It decomposes at a much higher temp, near ~700degC.  In the semiconductor industry working with a gas is more reliable than a solid so we typically use boron trifluoride or diborane.  Both are very toxic and require gas sniffing alarm systems.

I have a lot of references on decaborane but most are geared to using it for ion implantation.  There are a some papers from M. Sosnowski of NJIT from the 80s and 90s and I. Yamada but I’m not sure they’re what you’re looking for.

Regards,
Alex