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All,<br>
Very interesting.<br>
I appreciate Lief's recall of Bob Hamilton's (UC Berkeley)<br>
experience with polymerized diborane and Steve Paolimi's (Harvard)<br>
experience with polymerized diborane (decaborane) formation possibly<br>
being caused by Joule-Thomson cooling.<br>
<br>
New to me is Steve's information that diborane mixtures with
hydrogen are <br>
not subject to this phenomena. At question then is the following:<br>
1. Does dilution in H2 avoid the problem because hydrogen is one of
the<br>
few gases that does not experience Joule-Thomsen cooling; or,<br>
2. Does dilution in H2 avoid the problem because hydrogen prevents<br>
polymerization by constantly saturating any potential cross link
sites;<br>
i.e., it always drives the equilibrium toward monomer not polymer.<br>
<br>
If number1 is correct, then dilution in helium should also solve the
problem<br>
- and I suspect it does not.<br>
I expect number 2 is more important and Joule-Thomsen cooling is
only<br>
partially (if at all) responsible for polymerization. I expect
polymerization<br>
of stored diborane, enhanced by the cylinder experiencing
temperature<br>
extremes (> 40 C) in an outside gas bunker is the issue. (Note,
residues<br>
are common on the regulator high pressure side also - not just the
low pressure side.)<br>
I expect cylinders are cooled to slow the polymerization reaction,
not to<br>
decrease Joule-Thomsen cooling. Cooling cylinders buys time, but
that's all.<br>
Some information on diborane decomposition is attached.<br>
This gas phase analysis article does not even address the non
volatile decomposition <br>
products (polymers like decaborane) but the temp/decomposition
relationship is clear.<br>
<br>
Suggestion:<br>
What if you use 3% B2H6 / 4% H2 / balance N2<br>
The hydrogen pushes the equilibrium to diborane but<br>
the concentration does not raise any H2 handling concerns.<br>
<br>
Bill Flounders<br>
UC Berkeley<br>
<br>
<br>
<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">Paolini, Steven wrote:<br>
</div>
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<div class="WordSection1">
<p class="MsoNormal"><span style="color:#1F497D">I have had
extensive grief with this same scenario when I was in
industry. To make a long story short, Diborane in the
absence of Hydrogen will form solid particles because of the
cooling effect (Joule-Thomson) of the regulator reducing
the pressure. The solids are what is known as Decaborane
(B10H6). Unfortunately, they form mostly on the seat of the
bonnet and allow the regulator to “creep” resulting in the
high pressure leaking past the regulator and into the low
pressure side. Some engineers have tried to chill the bottle
to reduce the Joule Thomson effect but that just resulted in
the material forming elsewhere in the circuit.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Your regulator
is certainly suited for handling Diborane but you must get
used to the fact that they will degrade over time as this
material is formed. My suggestions are as follows:<o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l1 level1 lfo5"><!--[if !supportLists]--><span
style="color:#1F497D"><span style="mso-list:Ignore">1)<span
style="font:7.0pt "Times New Roman""> </span></span></span><!--[endif]--><span
style="color:#1F497D">Replace the regulator with a two stage
unit, this will extend the failure time.<o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l1 level1 lfo5"><!--[if !supportLists]--><span
style="color:#1F497D"><span style="mso-list:Ignore">2)<span
style="font:7.0pt "Times New Roman""> </span></span></span><!--[endif]--><span
style="color:#1F497D">Specify a large CV on all new
regulators, this will extend the life because the effect
will be on a larger surface area.<o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l1 level1 lfo5"><!--[if !supportLists]--><span
style="color:#1F497D"><span style="mso-list:Ignore">3)<span
style="font:7.0pt "Times New Roman""> </span></span></span><!--[endif]--><span
style="color:#1F497D">“Stack” regulators in series, The less
that the regulator has to reduce pressure, the less Joule
Thomson effect will happen. This is expensive and clumsy but
a colleague of mine tells me that he has run this set up for
three years without incident.<o:p></o:p></span></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l1 level1 lfo5"><!--[if !supportLists]--><span
style="color:#1F497D"><span style="mso-list:Ignore">4)<span
style="font:7.0pt "Times New Roman""> </span></span></span><!--[endif]--><span
style="color:#1F497D">Best method for an absolute cure is to
(if you can) switch your mixture to H2 instead of N2, you
may not be able to do this depending on your process but if
you can, it will eliminate the problem.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.25in"><span
style="color:#1F497D">I used to run 5000 PPM B2H6 in N2 as a
dopant to a furnace, H2 was out of the question because the
process was close to the auto ignition point of H2. I used
Tescom 1/2” bonnet regulators and swapped them out every
three months. If we didn’t change them regularly, they would
begin to creep and ultimately fail. It is a phenomenon that
you have to deal with in an expensive manner.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.25in"><span
style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:.25in"><span
style="color:#1F497D">Steve Paolini<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:.25in"><span
style="color:#1F497D">Harvard University Center for
Nanoscale Systems<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #B5C4DF
1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">
<a class="moz-txt-link-abbreviated" href="mailto:labnetwork-bounces@mtl.mit.edu">labnetwork-bounces@mtl.mit.edu</a>
[<a class="moz-txt-link-freetext" href="mailto:labnetwork-bounces@mtl.mit.edu">mailto:labnetwork-bounces@mtl.mit.edu</a>] <b>On Behalf Of
</b>Leif Johansen<br>
<b>Sent:</b> Friday, November 16, 2012 7:25 AM<br>
<b>To:</b> Lab Network (<a class="moz-txt-link-abbreviated" href="mailto:labnetwork@mtl.mit.edu">labnetwork@mtl.mit.edu</a>)<br>
<b>Subject:</b> [labnetwork] Issue with diborane in
nitrogen mixture<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Dear all,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Here at DTU Danchip we are facing some
difficulties with diborane in nitrogen mixtures. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I the recent months we have made the
following observations with our 3% diborane in nitrogen
mixture:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph"
style="margin-bottom:12.0pt;text-indent:-.25in;mso-list:l2
level1 lfo2"><!--[if !supportLists]--><span
style="font-family:Symbol"><span style="mso-list:Ignore">·<span
style="font:7.0pt "Times New Roman"">
</span></span></span><!--[endif]-->Approximately half year
ago, the safety valve opened up due to a too large
overpressure on the secondary side of the regulator. Usually,
the secondary pressure is set to 1-2 Bars overpressure. The
safety valve will open up at pressures exceeding ca. 7 Bars. <o:p></o:p></p>
<p class="MsoListParagraph"
style="margin-bottom:12.0pt;text-indent:-.25in;mso-list:l2
level1 lfo2"><!--[if !supportLists]--><span
style="font-family:Symbol"><span style="mso-list:Ignore">·<span
style="font:7.0pt "Times New Roman"">
</span></span></span><!--[endif]-->At that time we
believed that the incident was caused by a faulty gas panel
regulator. So we changed the regulator and continued to use
the gas. <o:p></o:p></p>
<p class="MsoListParagraph"
style="margin-bottom:12.0pt;text-indent:-.25in;mso-list:l2
level1 lfo2"><!--[if !supportLists]--><span
style="font-family:Symbol"><span style="mso-list:Ignore">·<span
style="font:7.0pt "Times New Roman"">
</span></span></span><!--[endif]-->However, in the past
month or so we have seen that the pressure rises from the
setpoint of 1.5 Bars to around 3 Bars. If we then consume gas
from the line, the pressure has been observed to drop in
discrete steps rather than in a smooth way. <o:p></o:p></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l2 level1 lfo2"><!--[if !supportLists]--><span
style="font-family:Symbol"><span style="mso-list:Ignore">·<span
style="font:7.0pt "Times New Roman"">
</span></span></span><!--[endif]-->This week the pressure
suddenly increased dramatically again, leading to a new safety
valve release. We have now shut down the gas line. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">My questions are : <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph"
style="margin-bottom:12.0pt;text-indent:-.25in;mso-list:l0
level1 lfo4"><!--[if !supportLists]--><span
style="mso-list:Ignore">1.<span style="font:7.0pt
"Times New Roman""> </span></span><!--[endif]-->The
bottle is from March 2010. I have once read that over time
diborane decomposes into higher order borohydrides, which,
unlike diborane, are solid and can clog orifices and nozzles.
This phenomenon has already been addressed by Bob Hamilton in
this forum. Is our diborane bottle simply too old and emits
“ear wax” into the gas line?<o:p></o:p></p>
<p class="MsoListParagraph"
style="margin-bottom:12.0pt;text-indent:-.25in;mso-list:l0
level1 lfo4"><!--[if !supportLists]--><span
style="mso-list:Ignore">2.<span style="font:7.0pt
"Times New Roman""> </span></span><!--[endif]-->The
bottle is located in a gas bunker outside, where the
temperature can vary quite a bit over the four seasons. The
specified temperature range is 0°C to 40°C (32°F to 104°F ).
What happens if the temperature exceeds this range?<o:p></o:p></p>
<p class="MsoListParagraph"
style="text-indent:-.25in;mso-list:l0 level1 lfo4"><!--[if !supportLists]--><span
style="mso-list:Ignore">3.<span style="font:7.0pt
"Times New Roman""> </span></span><!--[endif]-->The
gas panel regulator is a “Rotatrex SI 240 V” gas regulator
(240 Bar primary side, 7 Bar secondary side). I have read that
certain materials should be avoided inside valves and
regulators used for diborane gas. As far as I can read on the
data sheet, the body is made of 316 L steel, the diaphragm is
made of Hasetlloy and the valve seat is made of Vespel or
PVDF. Can anyone comment on whether our present regulator is
suited at all to handle diborane? If not, can anybody
recommend a better alternative?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">These issues are causing considerable
down-times of our diborane consuming equipment, so I would
very much appreciate any information you could share which
could help us solve the problem. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<table class="MsoNormalTable"
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<p class="MsoNormal"><b><span
style="font-size:8.0pt;font-family:"Arial","sans-serif";color:black">Leif
S. Johansen <o:p></o:p></span></b></p>
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href="http://www.danchip.dtu.dk/">www.danchip.dtu.dk/</a>
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<pre wrap="">_______________________________________________
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