From: tiq@fennel.cc.uwa.oz.au (Todd Green)
Newsgroups: sci.physics.fusion
Subject: Loading Measurements
Date: 13 Nov 92 12:34:14 +0800
Organization: University of Western Australia
NNTP-Posting-Host: fennel.cc.uwa.edu.au

Originally-From: ames!FNALD.FNAL.GOV!DROEGE
 
>Dick Blue says that he does not see a good explanation as to how high gas
>loadings are measured. I don't know how to do it either.
 
>Tod Green gives a good discussion and picks the four point resistance method.
>I too used this for a while.  I now measure the excess oxygen.  As far as I
>can see neither method is very good.  But they are both better than anything
>else that I can figure out.
 
>I watch the skyscraper and count the number of people coming in and out the
>front door to estimate how many people are inside.  Tod monitors the elevator
>traffic.  My method is bad because there is a subway under the building, and
>also a back door.  I think Tod's method fails because we really don't know
>the relation between elevator traffic and people in the building when the
>building is occupied by more people than the fire department allows.

Tom is right, neither method is totally satisfactory, but we have no reasonable
alternatives. At least they are both useful in a relative sense, i.e. enable us
to see under what conditions the cathode is gaining or losing deuterium, even
if the absolute amount is a bit suspect. McKubre has allegedly checked the
resistance calibration against a volumetric method, so I really think it is
fairly well established. Volumetric measurements are direct and should be more
reliable, but I'm not so sure. There is the possibility of gas losses via leaks
and also the occurrence of unanticipated and weird chemical processes, and Tom
appears to has some evidence for the latter. One thing that worries me is that
nearly all the "papers" (usually conference reports) that report loadings > 1.0
were done volumetrically, and I just don't see that such high values are likely
(see later).

In another post Tom writes:
 
>I have measured 1190 relative volumes.  Because oxygen is slowly lost due to
>the formation of the Platinum compound (yellow crud) the excess oxygen method
>tends to undermeasure.  There is a simultaneous energy measurement that tends
>to confirm that something is going on, but if anything it indicates that the
>loadins is also under measured.  Moore (Trans Electrochemical Soc., 75 [1939])
>measured 2800 relative volumes.  I cannot find anything wrong with his
>method.  But Moore thought that more than half of the gas existed not in the
>crystal lattice but in "rifts in the slip planes".  Moore used very fine
>wires.  He would then pass a current through the wire and heat it up to drive
>off the hydrogen.  Hard to go wrong with such a simple method.
 
Tom, I don't really think that loadings greater than 1.0 are possible via 
electrolysis and I am a tiny bit suspicious of Moore's 2800 relative volumes. 
It would basically mean that you are populating tetrahedral sites and this is
energetically very unfavorable. The alternative is that the D is going into
rifts or voids (as D2) but Moore's "rift" hypothesis is generally considered
wrong these days. There is a lengthy discussion about it in T. Flannagan's book
"The palladium hydrogen system". Basically, TF notes that there is very
little  difference in the solubility of hydrogen in palladium prepared in
various ways (cold worked, annealed, cast etc.) and this seems to exclude that
idea that LARGE amounts of deuterium can be contained in voids or other sites
such as defects, vacancies and dislocations.

From Dieter Britz: 
 
>Tom Droege and Todd Green have discussed the problem of measuring the loading.
>I still like the excess oxygen method Tom used, even with the interference by
>the yellow crud. You could largely account for the effect of that. For one
>thing, you can weigh the Pt anode before and after, which gives you the Pt
>content of the crud. At the end, carefully collect as much crud as you can,
>and have it analysed quantitatively for Pt, O and D (or H; Bockris tells us a
>lot of H gets in there! {:] ) and you can then estimate the total amount formed
>from the actual Pt lost. Then calculate how much oxygen was swallowed by that
>and correct the excess.
 
>There is a problem here. If the crud were to eat oxygen, Tom actually
>underestimated the excess oxygen - i.e. there would have been more, if not for
>the crud. The way I see this happening (and I do wonder at so much Pt
>dissolving) is the corrosion reaction, in a strongly alkaline solution:
 
>Pt + 4OD-  ---> Pt(OD)4 + 4e-                  (1)
 
>(Never mind the extra D2O's attached to the stuff). This reaction competes
>with the predominant one taking place at the Pt,
 
>4OD- ---> O2  + 2D2O + 4e-                     (2)
 
>so while D2 is being produced at the Pd cathode, reaction (1) robs us of a
>bit of O2 supposed to be made at the Pt anode. I hate to say it - because
>frankly I don't believe in loadings far above 0.8 - but it looks to me as if
>you might have underestimated the loadings. Please correct me, somebody, if
>I have this wrong (Todd?).

I'd agree that such a mechanism might be operating and that it would produce
an oxygen deficit, but it seems odd that you could produce so much of the Pt 
crud. There does seem to be some corrosion of the anode but the amount is rather
small. I had some electrolyte solutions analysed for various elements (looking
for transmutation products mainly :->) and platinum was  detected at only a 
few ppm. This was in a cell that had been operating  for about 40 days at high
current densities. Tom uses (I think) saturated LiOD and maybe the corrosion
is accelerated under more basic conditions. 

Dieter also wonders what loadings are possible. Under most conditions I get
between 0.8 and 0.85, but using the McKubre pretreatment (only approximately as
I don't have the full details), it ranges from 0.88 to 0.92. Anything below
1.0 is, I think, possible. Tom's polishing trick is certainly worth a try -
0.94 is way above the alleged threshold for initiating XS heat.

----
Todd
 
Newsgroups: sci.physics.fusion