[Diy_efi] Possible method of partial repair of damaged O2 sensor (was designed for O2)

[Mike]

At 12:14 AM 4/06/05, you wrote:
>WARNING: boring non-efi technical stuff follows -  you have been warned <G>

<chuckle> In other words, multivaried abstract challenged peasants ..go away..

>Mike,
>You raise some interesting questions (at least for me, a ceramist who has worked with ZrO2 and PbZrO3) The fact that the sensor is affected by the presence of Pb would seem to indicate that there is some kind of reaction. The details of any reaction, however, are in question. I would guess that the kinetics of the reaction are rapid enough at 600 C that a few hours would be adequate for reaction to occur. When the volume of the reaction product is similar to that of the reactants, you can get a protective coating similar to the surface oxidation of aluminum. Here the molar volume of PbZrO3 is much larger than that of ZrO2 (ignoring the PbO vapor) so it may not be a stable coating. Also, any difference in coefficient of thermal expansion between ZrO2 and PbZrO3 would tend to spal off a coating when thermally cycled (don't have any data here for PbZrO3) Thermal cycling would happen during every startup - shutdown so I don't see the torch as offering much new.

Yes and this is assuming we just have PbZr03 and in an ideal world possibly. I think it a
fair guess that the compound which upsets the sensor surface has tendrils of "crap" and
other organic partially burned complexes in some way bonded with PbZr03 hence a liberal use
of heating in a very oxygen rich environment and perhaps a purge with pure O2 when really hot
may go a long way to combust the complexes in such a way most of the PbZr03 is 'cast out'
and henceforth not to sully our sensors with excrement...

>The point that I was trying to make was that PbZrO3 is stable in a strongly oxidizing environment at high temperatures - it doesn't volatilize or 'burn off' as it is already a stable oxide.

mmm Maybe but,
Its the other multi-farious compounds that could well cause local complexes to combust and form
intermediaries or any other temporary probabilistic compounds that could ameliorate the surface
in such a way as to allow heating and a dip in pure O2 worthwhile, i'm just guessing but from
the perspective that nothing in (chemical) processes have a std dev of zero and there are heaps
of other reactions going on an incremental asymptotic approach might be viable.

>>Is the PbZr03 acting as a short or just occluding the sensor area, so a higher
>>impedance output results. ie. If a lower impedance output is expected
>>and the pre-amp designed accordingly and contamination occurs the output
>>will fall away. If a much higher impedance pre-amp were used instead would
>>the contamination be less likely to affect the output in that some area occlusion
>>could be tolerable ?
>
>
>If there is PbZrO3 on the surface it would act as an oxygen barrier - it is not an ionic oxygen conductor like ZrO2. Since the ZrO2 sensor is acting like a battery (Nernst cell) conducting oxygen ions from one side to the other driven by the difference in oxygen content (partial pressure) between the inside and outside, any ZrO2 surface that is free of PbZrO3 should be adequate to produce the cell voltage. Current would scale with surface area but, since  high impedance amps are the rule rather than the exception, this probably would not be a problem until the bitter end.

Interesting, thanks for the reminder - its been 20yrs since I read the basic blurb from
Bosch on the sensor operation, that makes a lot more sense to me now.

>All of this discussion assumes that there is a surface reaction rather than diffusion down grain boundaries. If the grain boundaries are coated with PbZrO3 stopping oxygen conduction, the time to sensor death would probably be much shorter.

mmm True, I wonder if anyone has had the chance to place a contaminated or even 'clean ULP'
used O2 sensor under an electron microscope or even an atomic array microscope - I guess
in the dark depths of Bosch they might have, I think they have at least one e-microscope in Germany...

>Are you still awake? (and I don't mean the time zone difference)

Sure, well it is almost 1am but, I do enjoy technical dialectic inspired by more
than a smattering of diversity free from personal tone or tangential reference to
ones abilities - all the way from Australia - ripper ! u done great mate ;-)

And food for technical diversion from my usual humdrum design planning :)

As I've only had a Carlton Cold (beer), I'm ok - if I had a couple of glasses of red whine
I'd be digressing into the woolies of mystical and esoteric meanderings, oooops
gotta be careful, slap my left wrist <chuckle>

>I'll try to do a literature search and see if I find the real answer.

Yeah interesting, for your area of interest I'd do that also, not being that
much of a ceramist I'm looking at this from the perspective of the consumer
or  rather the better informed consumer being empowered to act a bit
more independently and not necessarily be dependent upon wild generalisations
from multinationals inspired by commercial advantage as part of the paradigm
of the throw away society... (end of tangential rant).

Though I think I'm losing the battle, the cost of mass manufacture has dropped
so much in the last 30 years its amazing what high technology might be capable
of in another 30 years !

Rgds

Mike








>Jay
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Regards from


Mike Massen
Perth, Western Australia
VL Commodore Fuse Rail that wont warp or melt !
http://niche.iinet.net.au