Jeffery :
Well, there is an additional property involved in dissipating heat,
basically a surface effect between two dissimilar substances, but AFAIK it's
not appreciably different for aluminum to air or copper to air. IMO the
reason you've never seen a copper heat sink is simply because thermal
conductivity isn't all that important a property, the capacity of a given
sink in given circumstances is usually limited by it's surface area. And
because aluminum is easier to extrude, it makes it easier to create heat
sinks with lots of surface area per volume. Plus, heat sinks have really
only become a requirement since the invention of the power transistor in the
50s, when copper was already more expensive than aluminum. However I have
seen copper "heat sinks" on old transmitting tubes.
BTW, "heat sink" is a misnomer, although that's what everyone calls them.
They really work by conducting heat to the air, they could more properly be
called heat conductors or perhaps "thermal impedance matching devices" <g>
I believe that, for pure metals, thermal conductivity is directly
proportional to electrical conductivity. (Durned if I can remember the name
of the law, but there is one.) However that is not necessarily true for
alloys or non-metallic substances. For instance, silicone thermal compound
is an electrical insulator, but also a modest conductor of heat (although
many times poorer than metal).
Gold is actually a poorer conductor than copper, it is valuable mostly for
it's resistance to corrosion and electrolysis. You'll find it mostly in
critical contacts, or where dissimilar metals must be joined. The tarnish
that forms on copper, silver and aluminum is an insulator that louses up the
connection.
AFAIK Silver is the best conductor of heat and electricity, followed by
copper. RF components are sometimes silver plated to improve their high
frequency properties.
Randall
> And for purposes of comparison, is a material's ability to conduct heat
> ALWAYS directly porportional to conducting electricity? Is there any
> material out there that's great at conducting heat, but not electricity or
> vice versa?
>
> Can we assume that if material A conducts electricity better than material
> B, that material A also conducts heat better than material B?
>
> And finally, is the property of a material to conduct heat also make it a
> good property to dissipate heat?
>
> Every heat sink I've ever seen was made of aluminum. And I don't think
> they did it for weight savings. Maybe cost savings?
>
> And for the most part, the VAST majority of wires I've worked with were
> copper, or an alloy that was mostly copper. As I recall, back int the 70s,
> they had a disasterous experiment using aluminum core wiring for homes.
> The result was a lot of melted wires where they met copper termininals.
>
> If it's a super-important installation, we find gold contacts which I
> believe is the BEST conductor of electiricty, but to the best of my
> knowledge, nobody has ever made a radiator of gold.
>
> So I know we have some scientists on this list, what's the scoop? Why does
> aluminum loose to copper for conducting heat ( and electiricy ) but wins (
> I think ) over copper for dissipating heat? Is it simply a cost issue?
> Cost and strength?
/// triumphs@autox.team.net mailing list
/// To unsubscribe send a plain text message to majordomo@autox.team.net
/// with nothing in it but
///
/// unsubscribe triumphs
///
/// or try http://www.team.net/cgi-bin/majorcool
|