🚀 go-pugleaf

thermal conductivity can vary quite a bit with temperature (things
generaly become much more conductive at lower temperatures).

I'm not sure there has been much research done on the thermal conductivity
of materials at very low temp.

                          Jim Bowers

In article <7926@dirac.physics.purdue.edu> parks@gibbs.physics.purdue.edu (Chris Parks) writes:
>
>What is a good material (good=price + availability) which is
>an electrical insulator and a good conductor of heat?
>
>It will be used at temperatures as low as 4.2 Kelvin and it needs
>to hold up to about 200 volts / mm, heat conductivity needs to
>be comparable to copper.  Does it exist?
>
>Note: Diamond may be ideal but not practical!
>
>Chris Parks
>parks@physics.purdue.edu


Sapphire has been mentioned to me as such a material.  You do not mention in
what form: shape, size this material must be or how much of it you need.  
I believe that you can get man-made sapphire substrates.  They are probably
not cheap, but surely cheaper than diamond.

For cryogenic temperatures, you could try intrinsic silicon (at low
temperatures, it is a very poor electrical conductor (not enough
energy to put any electrons up into the conduction band).  I believe
that most of the heat conduction in silicon is carried by phonons
(lattice vibrations), however, so it should still be a fairly good
conductor of heat (I once saw figures comparing thermal conductivities
of silicon doped at different concentrations, and the thermal conductivity
did not vary much).  Silicon might be one to think of also, since it has
the diamond structure, and is just below diamond in the periodic table.

Hope this helps

John
snyder@henry.ece.cmu.edu

In article <92209.222732FQV@psuvm.psu.edu> FQV@psuvm.psu.edu (jim bowers) writes:
>thermal conductivity can vary quite a bit with temperature (things
>generaly become much more conductive at lower temperatures).
>
>I'm not sure there has been much research done on the thermal conductivity
>of materials at very low temp.
>
>                          Jim Bowers


Actually, you would be surprised:  quite a bit of research has been done on
thermal conductivity of materials at very low temperatures.  Unfortunately,
what people building equipment for cryogenic research are looking for
is usually materials with *LOW* thermal conductivity, not high conductivity.
However, tables exist of the thermal conductivities of many common materials
at cryogenic temperatures.  I had a paper here somewhere a few years ago,
I'll see if I can find the reference.

John
snyder@henry.ece.cmu.edu

Crystalline quartz is a good electrical insulator and a good thermal 
conductor at cryogenic temperatures.  Its thermal expansion coef. matches
that of copper so it will not crack upon cooling.  It is relatively
inexpensive and is available from a number of companies.  We use it
as a thermal strap for the center conductor of microwave coaxial cable.

Hope this helps.

In article <7926@dirac.physics.purdue.edu> parks@gibbs.physics.purdue.edu (Chris Parks) writes:
>
>What is a good material (good=price + availability) which is
>an electrical insulator and a good conductor of heat?
>
>It will be used at temperatures as low as 4.2 Kelvin and it needs
>to hold up to about 200 volts / mm, heat conductivity needs to
>be comparable to copper.  Does it exist?

If it weren't for the low temperature requirement, I would recommend some
of the silicone compounds, which are very good heat conductors.  Mica is
right out.  Are there any silicones that are useful at temperatures that
low?
--scott

In article <7926@dirac.physics.purdue.edu>, parks@gibbs.physics.purdue.edu (Chris Parks) writes:
|>
|>What is a good material (good=price + availability) which is
|>an electrical insulator and a good conductor of heat?
|>
|>It will be used at temperatures as low as 4.2 Kelvin and it needs
|>to hold up to about 200 volts / mm, heat conductivity needs to
|>be comparable to copper.  Does it exist?
|>
|>Note: Diamond may be ideal but not practical!
|>
|>Chris Parks
|>parks@physics.purdue.edu

You may try Tin oxide or Ruthenium oxide based glaze materials. We have
made thick film resistor with these materials. Their heat conductivity
is very good. These materials may be obtained as 'glaze' or as 'ink.
Thanks,
	Anis Rahman
	EECE (Materials Sc.)
	Marquette University
	5916rahmank@vms.csd.mu.edu

In sci.materials, parks@gibbs.physics.purdue.edu (Chris Parks) writes:


>What is a good material (good=price + availability) which is
>an electrical insulator and a good conductor of heat?

>It will be used at temperatures as low as 4.2 Kelvin and it needs
>to hold up to about 200 volts / mm, heat conductivity needs to
>be comparable to copper.  Does it exist?

>Note: Diamond may be ideal but not practical!

>Chris Parks
>parks@physics.purdue.edu

Don't know about conditions at cryogenic temperatures, but assuming that
the heat conductivity is not that far from what it is at room temp
then ceramics such as aluminum nitride and beryllium oxide have very
respectable heat transfer. Certainly almost any insulator will be much 
better than 200 V/mm, so that part is easy.

If you can deal with much less thermal conductivity then you could consider
common alumina (~1/5 thermal cond of copper) or sapphire.

Too bad you can't use diamond.

Stay away from plastics and glasses and rubbers and the like- all are
very poor conductors.

I would suggest a very thin layer of good thermally conductive insulator
then a metal for the bulk of the application. This is standard in
the electronics industry, with one common and cheap solution being
anodized aluminum. This would have one major advantage in that aluminum
IS very good structurally at cryogenic temps.

Stan Bischof
SRSD

In article <7926@dirac.physics.purdue.edu> parks@gibbs.physics.purdue.edu (Chris Parks) writes:

>What is a good material (good=price + availability) which is
>an electrical insulator and a good conductor of heat?

	If your electricity is in superconductors, 'insulator' is a fair
description of copper...

>It will be used at temperatures as low as 4.2 Kelvin and it needs
>to hold up to about 200 volts / mm, heat conductivity needs to
>be comparable to copper.  Does it exist?

	Your best bet is diamond, but you've already ruled that 
out (why, I can't imagine...).  Do you honestly want comparable
heat conductivity at 4.2K to the conductivity of copper at 4.2K?
Thermal conductivity of Cu is about 4W/cm/K at room temperature,
but 113W/cm/K at helium temperature.  Except for diamond, your
best bet is BeO (highly toxic, by the way!) but the only data I have
is for higher temperatures (125 BTU-in/hr/sq. ft/F).

	John Whitmore

parks@gibbs.physics.purdue.edu (Chris Parks) writes:
> 
> What is a good material (good=price + availability) which is
> an electrical insulator and a good conductor of heat?
> 
> It will be used at temperatures as low as 4.2 Kelvin and it needs
> to hold up to about 200 volts / mm, heat conductivity needs to
> be comparable to copper.  Does it exist?
> 
> Note: Diamond may be ideal but not practical!
> 
> Chris Parks
> parks@physics.purdue.edu

i don't know why the diamond is excluded. the synthetic diamond
film (polycrystalline film grown by chemical vapor deposition
or other means) may be a good candidate, i think.

benjamin s. m. lin

---------------------------------
smlin@uz.nthu.edu.tw
dept. of materials sci. & eng.
national tsing hua university
hsinchu 30043, taiwain
republic of china
---------------------------------

In sci.materials, whit@milton.u.washington.edu (John Whitmore) writes:

Sorry, but I have to disagree with two of your points:

>	If your electricity is in superconductors, 'insulator' is a fair
>description of copper...

Copper is an excellent conductor at cryogenic temps, by any definition.
Please no pedantic drivel like ' compared to ZERO resistivity, anything
is infinitely less conductive and hence an insulator'.

>but 113W/cm/K at helium temperature.  Except for diamond, your
>best bet is BeO (highly toxic, by the way!) but the only data I have
                  ^^^^^^^^^^^^^^^^^^^^
Nonsense. This is pure urban legend. You could eat BeO on your breakfast
cereal and have no problems.

The only problems with BeO come in when you powder it very finely and 
inhale it into your lungs. The small particles cause some nasty reactions
similar to what happens with inhaled asbestos. Basically a mechanical
irritation. 

Stan Bischof
SRSD

*************************************************
Merely my own opinions, in no way related to the
opinions of my employer, Hewlett-Packard
*************************************************

In article <17760004@hpnmdla.sr.hp.com> stanb@hpnmdla.sr.hp.com (Stan Bischof) writes:
>In sci.materials, whit@milton.u.washington.edu (John Whitmore) writes:

>>best bet is BeO (highly toxic, by the way!)

>Nonsense. This is pure urban legend. You could eat BeO on your breakfast
>cereal and have no problems.

>The only problems with BeO come in when you powder it very finely and 
>inhale it into your lungs. The small particles cause some nasty reactions
>similar to what happens with inhaled asbestos. Basically a mechanical
>irritation. 

	Are you certain of that?  My trusty TLV-TWA table shows acceptable 
dust levels of arsenic at 0.5 mg/m^3, and of beryllium at 0.002 mg/m^3;
beryllium is a very toxic light-metal poison, and I always thought
the oxide was just as toxic.

	John Whitmore

kludge@grissom.larc.nasa.gov ( Scott Dorsey) writes:

>In article <7926@dirac.physics.purdue.edu> parks@gibbs.physics.purdue.edu (Chris Parks) writes:
>>
>>What is a good material (good=price + availability) which is
>>an electrical insulator and a good conductor of heat?
>>
>>It will be used at temperatures as low as 4.2 Kelvin and it needs
>>to hold up to about 200 volts / mm, heat conductivity needs to
>>be comparable to copper.  Does it exist?

>If it weren't for the low temperature requirement, I would recommend some
>of the silicone compounds, which are very good heat conductors.  Mica is
>right out.  Are there any silicones that are useful at temperatures that
>low?
>--scott
 You don't specify how big, but diamond might be a candidate if the insulator 
is only a millimetre or so across and less than that thick. For instance,
from the Chemical Rubber Company Handbook:

		Material                   K (W/cm.k) - at 273 K

		  Cu			      4.03

		Type IIa diamond	      26.2

	  	Type IIb diamond	      15.2

		Type I   diamond	      9.94

I have in the past seen adverts for diamond heat sinks at remarkably low 
prices (tens of dollars if I remember correctly), so they might not be right
out for your application.
      /~~~\   !                           
      \___    !---   ___ .    .  ___       ###########################
          \   !     /___> \  /  /___>      # smadden@titan.trl.OZ.AU #
      \___/   \__/  \___   \/   \___       ###########################

In sci.materials, whit@milton.u.washington.edu (John Whitmore) writes:


>>The only problems with BeO come in when you powder it very finely and 
>>inhale it into your lungs. The small particles cause some nasty reactions
>>similar to what happens with inhaled asbestos. Basically a mechanical
>>irritation. 

>	Are you certain of that?  My trusty TLV-TWA table shows acceptable 
>dust levels of arsenic at 0.5 mg/m^3, and of beryllium at 0.002 mg/m^3;
>beryllium is a very toxic light-metal poison, and I always thought
>the oxide was just as toxic.

>	John Whitmore

Yep. Only problem with BeO is in very fine powders. Which means you 
definitely DON'T want to inhale the stuff- can cause death in extreme
cases, ,cancer in some, etc.

Perfectly safe handled properly though. WON'T turn you into the
'toxic avenger' if you touch it. Just be careful about any usage
which would abrade it- sawing, laser trim, etc. need to be done
under well controlled conditions.

I'm sure that any vendor who would sell you the stuff would have
handling advisories.

In any case, in answer to the original question- anodized aluminum
would be my best recommendation.

Stan Bischof
HP-SRSD

*************************************************
Merely my own opinions and in no way related to
those of my employer- Hewlett Packard.
*************************************************

<Lots of arguments about BeO deleted>

As a low temp kind o' guy, why not use copper with a covering of
cigarette papers.  Every cryogenic tool kit has a set of rolling papers.
You can hold them in place with the standard GE varnish (7031).  This
has a breakdown voltage of several (~200-300) V/mil (that's 0.001 inch).

The easiest way to get thermal conductivity is to make the insulator
much thinner.  The problem you might have at high voltage is the ease
with which He gas breaks down.  There is a minimum in the breakdown
voltage around 10 torr and so make sure your leads are well insulated
above the liquid near the top of the cryostat.

Scott Hannahs, F. Bitter National Magnet Lab, sth@slipknot.mit.edu

In article <1992Jul28.075319.9973@fs7.ece.cmu.edu> snyder@henry.ece.cmu.edu (John Snyder) writes:
>In article <7926@dirac.physics.purdue.edu> parks@gibbs.physics.purdue.edu (Chris Parks) writes:
>>
>>What is a good material (good=price + availability) which is
>>an electrical insulator and a good conductor of heat?
>>
>>It will be used at temperatures as low as 4.2 Kelvin and it needs
>>to hold up to about 200 volts / mm, heat conductivity needs to
>>be comparable to copper.  Does it exist?
>>
>>Note: Diamond may be ideal but not practical!


>Sapphire has been mentioned to me as such a material.  You do not mention in
>what form: shape, size this material must be or how much of it you need.  
>I believe that you can get man-made sapphire substrates.  They are probably
>not cheap, but surely cheaper than diamond.
>

Actually, Sapphire is quite cheap.  A polished two-inch wafer costs
about 125 bucks.  It is used in UHV viewports up to at least 4 inch 
diameters.  Berylium oxide is another option.  Not transparent, but
a great heat conductor.  Deadly toxic though.

Peter Rosenthal


-- 
Peter Rosenthal       Email rosenthal@cmg.eeel.nist.gov

In article <4857@dove.nist.gov> rosentha@bldrdoc.gov (Peter Rosenthal 303-497-5844) writes:
>In article <1992Jul28.075319.9973@fs7.ece.cmu.edu> snyder@henry.ece.cmu.edu (John Snyder) writes:
>>In article <7926@dirac.physics.purdue.edu> parks@gibbs.physics.purdue.edu (Chris Parks) writes:
>>>
>>>What is a good material (good=price + availability) which is
>>>an electrical insulator and a good conductor of heat?
>>>
>>>It will be used at temperatures as low as 4.2 Kelvin and it needs
>>>to hold up to about 200 volts / mm, heat conductivity needs to
>>>be comparable to copper.  Does it exist?
>>>
>>>Note: Diamond may be ideal but not practical!
>
>
>>Sapphire has been mentioned to me as such a material.  You do not mention in
>>what form: shape, size this material must be or how much of it you need.  
>>I believe that you can get man-made sapphire substrates.  They are probably
>>not cheap, but surely cheaper than diamond.
>>
>
>Actually, Sapphire is quite cheap.  A polished two-inch wafer costs
>about 125 bucks.  It is used in UHV viewports up to at least 4 inch 
       ^^^^^^^^^

I guess I should also have said, "It depends on your definition of cheap."
I generally work with 1 inch diameter wafers, either Corning 0211, 7059,
Silicon wafers, or polished fused silica.  The latter is most expensive,
and costs, (as of about 2 years ago, about $16.00 each).  So compared
to them, sapphire is not cheap.


>diameters.  Berylium oxide is another option.  Not transparent, but
>a great heat conductor.  Deadly toxic though.
>
>Peter Rosenthal
>
>
>-- 
>Peter Rosenthal       Email rosenthal@cmg.eeel.nist.gov
>