Re: RF Conductivity of thin Mo film

Peter Simon wrote:
> Mark Fergerson <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
>
>
> >   Are you writing a proposal, or doing design work? I ask 
> > because if the former, you have a lot of paper to plow 
> > through to find such arcane data. If the latter, you can do 
> > some preliminary labwork to narrow things down.
>
> Design work.  We are planning on measuring the plane wave return loss
> and insertion loss of some test pieces, but I would like to obtain
> analytic predictions first.

  Right. Is the physical layout absolutely fixed? Can you 
relocate the radome so it's out of the exhaust plume(s)?

> >   Elsewhere I suggested you provide a cold finger to 
> > collect the Mo before it contaminates your radome; why is 
> > that unacceptable?
>
> I apologize.  I have never before heard of a "cold finger" and thought
> that your answer was some kind of jest or sarcastic comment.  Could
> you please enlighten me on this subject?

  Just a piece of refrigerated metal juxtaposed to catch 
the hot stuff. But as Unc points out it likely won't help 
unless the geometry's just right.

  This is why I asked for more info on the actual problem; 
your request was a bit too general.

<snip>

> ...  I fear that duplicating the space
> environment, with its hard vacuum, diurnal temperature variations, and
> very intermittent firing of the plasma thruster for short bursts over
> several years, may be very difficult to replicate in the lab.  But we
> will do the best we can here.

  Well, I meant try to duplicate just the conditions of 
deposition, do a worst-case buildup, and then do your 
transmissivity tests without worrying about other elements' 
possible inclusion in the film (due to outgassing etc). All 
the rest shouldn't have much effect as I can't see 
vacuum-sintering and annealing altering the film's 
properties much. But I could be wrong.

> > > 2. What is the electron mean free path length for Mo?  Does this
> > > depend on temperature?
> >
> > http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975PhLA...54..229A
>
>
> Great reference! Thanks!  How'd you find it?

  I Googled for

 Molybdenum +"electron mean free path"

  I think it was the third or fourth return.

> > > 3. Is it true that the bulk conductivity of metals is inversely
> > > proportional to temperature over my working range (90K to 400K)?
> >
> >   Got a CRC? Plot some graphs.
>
> I only have a very old CRC (1978) here at home, and it doesn't seem to
> have anything relevant, at least that I can find (it lacks a detailed
> table of contents <amazing!>)

  Hm. Organization was never the CRC's strong suit; I 
browse mine regularly just for the hell of it otherwise I'd 
never be able to find anything in it.

  Best I can suggest is to look at the Resistivity Table(s) 
in the Electricity and Magnetism section; mine (the 38th 
edition, '57-'58) lists only drawn Mo, but gives resistivity 
at 20C as 5.7 ohm-cm and at 750C as 21.0 ohm-cm. The 
coefficients at 25C, 100C, and 1000C are in the following 
table, "Temperature Coefficient Of Resistivity". Convert 
units as appropriate. How to extrapolate from bulk drawn Mo 
to films, I have no idea. Unc?

> > Why not prevent 
> > the deposition in the first place?
>
>
> I agree.  My job is to help convince the configuration people that the
> electrical performance of the radome is going to be unacceptably
> degraded and that they need to do something to remove the source of
> the contamination.

  First I'd find out what "unacceptable" means. Is this an 
Earth-communication antenna? What range, ERP, etc?

  Mention to them that _any_ metal on the radome could kill 
the TWT or whatever the RF source is.

  Mark L. Fergerson