Thermal conductivity (ceramics)

[George Herold]

There's been a design for a little transistor/diode temp. sensor
floating around in my head.

In one version, there'll be a direct ground connection.

I wanted to have an electrically floating version too.
I want to try a circuit on flex (kapton).
Using the kapton as the insulator looks like a good idea.
But the thermal conductivity 'stinks'. (~0.15 W/m-K)
So driving home tonight I was thinking about using
an Al2O3 washer instead.
"What's the thermal cond. I said to myself, 100, 50 W/m-K?"
Googleing Al2O3, thermal conductivity Coorstek, I found this,

http://www.coorstek.com/resource-library/library/8510-1042_ceramic_material_properties.pdf
(I might have to print it out and pin it up.)

So for this application I need a 'decent' electrical insulator.
Which looks to rule out all the carbide's.
But what about the direct sintered SiC?

And then what about Aluminum and Silicon Nitrides?
Has any one used those?

I'm looking for a little thin insulating washer, that will go from
77K to 400K. (Maybe 500?).
A hole for a 2-56 screw.

George H.

 

[George Herold]

AlN is great stuff, and not usually very expensive. Thermal
conductivity is close to that of common aluminum alloys.

I've seen references to it decomposing on contact with water, into Al
and ammonia or some such, so I left a slab in water for a week.
Nothing happened. I think it's OK as regards outgassing in most apps.

I've seen stock TO220 insulators in AlN.

Neat, I'm already using TO220 transistors as temp sensors.
(Prepacked hole who could resist?)

Hard anodized aluminum is a good insulator and a good thermal
conductor, especially as a 1 mil thick layer is usually enough.

Interesting. Does anyone sell anodized Al washers?

Another trick is to use a filler (microsphere, glass bead, fine sand,
even monofilament) in epoxy as a spacer between an alum or copper heat
spreader and a baseplate. Epoxy is OK as long as you keep it thin.

No, I need a sensor that students can move around.
I'm thinking I need some sort of spring (Belville washer?) in the
stack-up.

George H.

 

[Jon Kirwan]

There's been a design for a little transistor/diode temp. sensor
floating around in my head.

In one version, there'll be a direct ground connection.

I wanted to have an electrically floating version too.
I want to try a circuit on flex (kapton).
Using the kapton as the insulator looks like a good idea.
But the thermal conductivity 'stinks'. (~0.15 W/m-K)

If it is very thin (and I think you can get .0005" thickness
tapes -- certainly .001" because I use that, myself) then it
may not be so bad. What would be the area? (I can't work out
the W/K without that.)

So driving home tonight I was thinking about using
an Al2O3 washer instead.
"What's the thermal cond. I said to myself, 100, 50 W/m-K?"
Googleing Al2O3, thermal conductivity Coorstek, I found this,

http://www.coorstek.com/resource-library/library/8510-1042_ceramic_material_properties.pdf
(I might have to print it out and pin it up.)

So for this application I need a 'decent' electrical insulator.
Which looks to rule out all the carbide's.
But what about the direct sintered SiC?

And then what about Aluminum and Silicon Nitrides?
Has any one used those?

I'm looking for a little thin insulating washer, that will go from
77K to 400K. (Maybe 500?).
A hole for a 2-56 screw.

George H.

Have you considered the usual suspects from Cotronics? Their
810 potting compound is about 0.67 W/m-K, I think. It's
volume resistivity is on the order of 10^11 (ohm-cm.)

Jon

 

[George Herold]

If it is very thin (and I think you can get .0005" thickness
tapes -- certainly .001" because I use that, myself) then it
may not be so bad. What would be the area? (I can't work out
the W/K without that.)

Yeah, I ran the numbers with thin tape. I worry about it breaking
with repeated use. Alumina has 100 times the conductivity, so the
same area, but ten times thicker, (A 10 mil washer) still has 10 times
the conductivity.

Have you considered the usual suspects from Cotronics? Their
810 potting compound is about 0.67 W/m-K, I think. It's
volume resistivity is on the order of 10^11 (ohm-cm.)

I'd like something moveable.

(Ge varnish is a classic low temp, thermal 'tie down'.)

George H.

 

[Spehro Pefhany]

If it is very thin (and I think you can get .0005" thickness
tapes -- certainly .001" because I use that, myself) then it
may not be so bad. What would be the area? (I can't work out
the W/K without that.)

Film? Adhesive on polyimide tapes is normally 1.5+ mils thick (per
coated side, of course).

AlO2 99.8% washers 0.25 O.D. 0.08" I.D. 0.03" thick $3 apiece
Polyimide " 0.25 O.D. 0.09" I.D. 0.005" thick 70 cents ea.

The AlO2 would have to be about 6x better conductivity to make up
the increased thickness. Plus, it doesn't conform, so it won't
make as good contact. (Un?) fortunately, it's about 60x better,
so, ignoring contact issues, it will be 10x better overall, at
4.3x the cost. If the surface is not very flat, and the applied
pressure fairly even, the ceramic will probably break.



Best regards,
Spehro Pefhany

 

[George Herold]

Film? Adhesive on polyimide tapes is normally 1.5+ mils thick (per
coated side, of course).

AlO2 99.8% washers 0.25 O.D. 0.08" I.D. 0.03" thick  $3 apiece
Polyimide       "  0.25 O.D. 0.09" I.D. 0.005" thick  70 cents ea.

The AlO2 would have to be about 6x better conductivity to make up
the increased thickness. Plus, it doesn't conform, so it won't
make as good contact. (Un?) fortunately, it's about 60x better,
so, ignoring contact issues, it will be 10x better overall, at
4.3x the cost. If the surface is not very flat, and the applied
pressure fairly even, the ceramic will probably break.

Best regards,
Spehro Pefhany

Cracking the ceramic is an issue.
For the tape, as far as I can tell the glue on the tape is worse than
the tape itself.

Any 'pinions on anodized Al?

George H.

 

[Jon Kirwan]

Film? Adhesive on polyimide tapes is normally 1.5+ mils thick (per
coated side, of course).

http://www.kaptontape.com/1_Mil_Kapton_tape.php

I only barely recall seeing 1/2 mil somewhere (didn't want it
at the time.) I'm using the 1 mil variety. You are right, as
the silicone adhesive is way too thick and can be a problem.
I just imagine it possible to get the film without the
adhesive. Someone must have it.

AlO2 99.8% washers 0.25 O.D. 0.08" I.D. 0.03" thick $3 apiece
Polyimide " 0.25 O.D. 0.09" I.D. 0.005" thick 70 cents ea.

The AlO2 would have to be about 6x better conductivity to make up
the increased thickness. Plus, it doesn't conform, so it won't
make as good contact. (Un?) fortunately, it's about 60x better,
so, ignoring contact issues, it will be 10x better overall, at
4.3x the cost. If the surface is not very flat, and the applied
pressure fairly even, the ceramic will probably break.

Pretty much agree. But without knowing exactly what George is
about (I don't have a clear picture to be honest), I can't
say if Cotronics stuff _could_ be useful or not. I've used
it, though, and like it. And it is AlO2.

Jon

 

[Jon Kirwan]

Cracking the ceramic is an issue.
For the tape, as far as I can tell the glue on the tape is worse than
the tape itself.

Yes, the silicon glue is as thick or thicker than the film
itself. As Spehro immediately pointed out. As I also said, I
suspect that the film by itself should be available from
someone. Just not "as available" as the silicone glued tapes
are. Could mechanically thin the adhesive, I suppose. I might
try that here.

Any 'pinions on anodized Al?

George H.

None from me except that I'd be worried about bumps and air
acting as a thermal insulator.

Jon

 

[miso]

Glass bead is what they use in chips for electrical isolation with heat
transfer. Though I've used it in production, I never opened a package to
see what the bead material looks like.

 

[Spehro Pefhany]

Any 'pinions on anodized Al?

George H.

I like it, but don't know where to get hard anodized alumin(i)um
washers in small quantities. I'm pretty sure my favorite OEM washer
supplier could make them in moderate quantity, but I don't think that
helps you.

DIY is possible (these guys just the other side of Rochester have all
the stuff) http://www.caswellplating.com but I bet your H&S folks
would make things complicated.


Best regards,
Spehro Pefhany

 

[SoothSayer]

---
A beryllium oxide washer is probably what you're looking for, and
here's one source among many:

http://www.aavid.com/product-group/interface/insulators/beryllium

Google Beryllium Oxide washer for lots more

Just make sure to keep them from making contact with a cut, or any
other internal contact.

 

[Syd Rumpo]

On 03/01/2013 03:14, Jon Kirwan wrote:

I only barely recall seeing 1/2 mil somewhere (didn't want it
at the time.) I'm using the 1 mil variety. You are right, as
the silicone adhesive is way too thick and can be a problem.
I just imagine it possible to get the film without the
adhesive. Someone must have it.

Jon

RS Components, for one...

http://uk.rs-online.com/web/p/thermal-insulating-films/5363946/

....that's one thou/mil and is sold as thermal insulating film!

Cheers

 

[SoothSayer]

On 03/01/2013 03:14, Jon Kirwan wrote:




RS Components, for one...

http://uk.rs-online.com/web/p/thermal-insulating-films/5363946/

...that's one thou/mil and is sold as thermal insulating film!

Cheers

There are tapes made for this. They are thermal paste coated.

One applies the tape (a pre-cut piece), and then removes the film
separator, and applies the device.

The thermal coat melds into all the surface deformities. One ends up
with a serviceable, replaceable 'gap tight' thermal mating.

I will look at the branding on the roll at work and post it.

 

[George Herold]

I like it, but don't know where to get hard anodized alumin(i)um
washers in small quantities. I'm pretty sure my favorite OEM washer
supplier could make them in moderate quantity, but I don't think that
helps you.

Well I guess that depends on what's a 'moderate' quantity.
Hundreds certainly and maybe 1,000's. But that's 'small potatoes' for
washers.

But we have a metal 'stamping' supplier that could do the aluminum and
then have them anodized by someone else...

DIY is possible (these guys just the other side of Rochester have all
the stuff)http://www.caswellplating.combut I bet your H&S folks
would make things complicated.

Yuck... No, (but thanks) let someone else do the messy bits.

George H.

 

[George Herold]

http://www.kaptontape.com/1_Mil_Kapton_tape.php

I only barely recall seeing 1/2 mil somewhere (didn't want it
at the time.) I'm using the 1 mil variety. You are right, as
the silicone adhesive is way too thick and can be a problem.
I just imagine it possible to get the film without the
adhesive. Someone must have it.



Pretty much agree. But without knowing exactly what George is
about (I don't have a clear picture to be honest), I can't
say if Cotronics stuff _could_ be useful or not. I've used
it, though, and like it. And it is AlO2.

Jon

The Dupont site has all sorts of 'stuff'.
http://www2.dupont.com/Kapton/en_US/products/index.html

(I bet you didn't know there were some many types of Kapton.)

George H.

 

[SoothSayer]

There are tapes made for this. They are thermal paste coated.

One applies the tape (a pre-cut piece), and then removes the film
separator, and applies the device.

The thermal coat melds into all the surface deformities. One ends up
with a serviceable, replaceable 'gap tight' thermal mating.

I will look at the branding on the roll at work and post it.

Forgot to mention... The tape to which I refer is Aluminum.

That should change your image of what I offered.

 

[George Herold]

---
A beryllium oxide washer is probably what you're looking for, and
here's one source among many:

http://www.aavid.com/product-group/interface/insulators/beryllium

Google Beryllium Oxide washer for lots more

Thanks John, I don't think I need the best, nor even the penultimate
thermal conductor. (The MSDS is a bit off putting for student use.)

George H.

 

[George Herold]

I like it, but don't know where to get hard anodized alumin(i)um
washers in small quantities. I'm pretty sure my favorite OEM washer
supplier could make them in moderate quantity, but I don't think that
helps you.

Hey, Seastrom will make Al washers with an anodized finish.
http://www.seastrom-mfg.com/

George H.

 

[Spehro Pefhany]

Well I guess that depends on what's a 'moderate' quantity.
Hundreds certainly and maybe 1,000's. But that's 'small potatoes' for
washers.

You might want to try asking Bokers then- they can do that sort of
small order, maybe they can do or arrange for the hard (Type III)
anodize?

http://www.bokers.com

 

[Spehro Pefhany]

Hey, Seastrom will make Al washers with an anodized finish.
http://www.seastrom-mfg.com/

George H.

Thanks for the link.

They don't list Type III (hard) anodize, but maybe they can do it.