Heat sink question

We've tested both, and find either an acceptable solution. The factor that is most important is that the zinc oxide paste is 'universal' and can be used on any device typically, while a pad needs to be cut to fit.

If it needs isolation sil pads are pretty nice. I try not to use grease because eventually it gets anywhere, even places where it really shouldn't be.

People are very different. My wife can grab the aluminum foil when the barbecued pizza is done without any problems. I'd be yelping.

Yup. It's good for lots of stuff.

Examples at:

;-)

Zinc oxide is also useful for diaper rash... thus most likely found in both Greasy's and Slowman's medicine cabinets ;-) ...Jim Thompson

Are you sure that Sloman doesn't keep it in his purse?

I didn't want to get too "pursonal" :-) ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

I can see November from my house :-)

I don't blame you. No telling what nasty diseases he's picked up on the streets. :(

Its called a European hand bag..;)

That's odd - not knowing a thing about what he is talking about hasn't stopped him venting his imagination in the past.

-- Bill Sloman, Nijmegen

=A0 =A0 ...Jim Thompson

But - as an Australian - I don't use one. Not that that's going to discourage Mike and Jim from elaborating their little fantasies. they seem to get quite excited about it.

-- Bill Sloman, Nijmegen

I use Dow Corning 340, exclusively. I don't slather it, though, any that isn't between the device and the heatsink is wasted. As long as you can see the edge of it all round, that's enough. Any more is just a goddamn nuisance.

The way to tell if it's zinc oxide is to heat a small sample, say in a gas flame. If it turns yellow when hot, then back to white when it cools, it's ZnO2. If it doesn't, it isn't.

Fred Abse wrote

I did your test and now I am homeless. Could it have been RDX ?

No. Not if pure. RDX is only tetchy when crystalline and cooled. It will burn very quickly rather than detonate when ignited.

See for example:

Regards, Martin Brown

Gas mantles of both of the noted varieties turn yellow when hot (short of glowing as they are intended to), back to white when cool. The variety with thorium and cerium even turns a "toasty golden-brown" when heated in direct sunlight and just short of glowing on its own to extent of making its glow seen in direct sunlight.

For that matter, at least one variety of leaded or "crystal" glass turns yellow when the glass gets to some temperature of "pliably-soft hot".

Zinc oxide is (and I guess as you say, a lot of other materials are) thermochromic. A number of ordinary salts exhibit this behavior near room temperature, most of them ridiculously toxic, like:

I suppose thermochromicity should be a property of any nonmetallic material, defining "nonmetallic" to mean, the conduction band lies above the Fermi energy. Thus, conduction electrons available for absorption and reflection (metallic appearance) are thermally sensitive. The material might be nearly metallic at room temperature (as many high-Z chalcogenide semiconductors are), or an excellent insulator (diamond's bandgap is something stupid like 6eV, and MgO has 8.7eV, which should be similar to ThO2 and CeO2). The important quality being, how much semi-insulating-conducting-ness it has is a property of temperature, which is a ratio of thermal energy to bandgap (from the Fermi distribution).

It occurs to me that red, yellow and green LED dies are colored at room temperature. I suppose if you heated them up, the color would drop -- probably going from yellow, to "toasty golden-brown" as your thoria-ceria example, to something darker (eventually black, like GaAs).

It's interesting that the properties don't necessarily change much in the molten state. Sodium chloride is very conductive in the liquid state, probably for the same reason water is conductive at all (the ionic dissociation constant being different for the two liquids, obviously NaCl's being larger). But both are transparent and colorless, so there's little change in the "bandgap" for photons to interact with. I suppose molten GaAs and Si look about the same as they do at room temperature, maybe GaAs gets a little shinier.

The other side of it is, bandgap of course doesn't stand still, nor do the band distributions. And we're talking large temperature ranges (>3000K for diamond), lots of room for crazy things to happen.

Sometimes, I hate statistical mechanics...

Tim

What about the "binder" - you know, the goo that makes it a paste? ZnO2 is solid, isn't it? Or is it itself the paste?

Thanks, Rich