Pyrolitic Carbon in PC Cooling

[ChalkChalkson]

TLDR; Say one happened to have a pad of pyrolitic carbon with the graphene sheets being coplanar to the pad. Then maybe it would be cool to do something with it where the conductivity in the direction the pad is actually conductive in mattered and not in the direction the conductivity sucks. For example making a heat conducting ribbon cable to cool a chip remotely.

 

I don't want to spoil anything, so let's just say @AlexTheGreatish was at it again with cool cooling stuff.

 

Why Graphite Conductivity is Anisotropic

Super short background on what graphene, graphite and pyrolitic carbon are (sheets and how they stack):

Spoiler

When you have a whole bunch of carbon and nothing else, the structure it tends to form is hexagons. Hexagons are pretty cool because they happen to tesselate euclidean planes - so the carbon forms large sheets made up of a bunch of hexagons. Individual sheets are called "Graphene". In nature graphene doesnt come up much, usually you have a bunch of sheets - take a handful of small sheets at a time and stack them on top of each other, then take those small "blocks" and throw them together in random orientations and you get graphite. The stuff your pencil uses to write. When you have a fancy machine that can make macroscopic neatly stacked sheets, those are called pyrolitic carbon.

So, what is all the fuzz about? Well the graphene sheets are really cool. Because of they are conjugated systems (read: because their chemistry is cool) electrons can move freely through the entire sheet - almost exactly like a metal! You might have heard that moving charges is what conducts electricity, so you would assume that graphene sheets are really good at conducting heat along their surface. And you'd be correct! Now, much more interesting for this subforum, the "charge" for heat conduction is just energy, and basically anything can transmit energy, including electrons. That's why graphene (and metals) are so good at conducting both.

That's all nice, but graphene is one of those sci-fi materials that, may not be the stuff of dreams anymore, but are very much still the stuff of research papers, not commodity items.

 

Graphite however is. The issue is that electrons have a much harder time to jump between layers than they have moving along them. This means, if you have a stack of graphene sheets oriented in the X-Y plane, you will measure excellent conductivity in those directions and bad conductivity in the Z direction.

In ordinary graphite, by the time the heat or current has traveled through the material, it will have passed many zones, in some it will have traveled along the sheets in some it had to travel across them, So in effect you will measure a conductivity that is something inbetween.

With pyrolitic carbon however all of them are aligned. So the pad itself will have an X and Y direction where the heat travels really well and a direction where it travels really badly.

 

So let's wildly imagine having such a pad is large in the X and Y directions and thin in the Z direction. You would expect it to be so good at conducting in the X-Y plane that touching it at two far away points in that plane would be enough to melt your way through an ice cube. You would also expect it to be really electrically conductive along that surface. But, it would really suck as a thermal conductor in the Z direction. (just making things up tough)

 

When something varies with the orientation it's called anisotropic (an means not, iso equal, tropos means place or path/way or something like that). The anisotropy of graphite (meaning just one sheet stack here, because we physicists like to pretend cows are spheres....) is very well known. Here is a paper in nature from the 30s (lower left hand side). In fact it is pretty much *the* example of macroscopic anisotropy resulting from crystal structures for intro to condensed matter physics courses

 

If It's a Shit Thermal Pad, What Can You Do With It?

 

Let's say for some reason you bought some pyrolitic carbon on @LinusTech's credit card and wanted to use it for somehting that works to make him happy.... A thing you could try is to cut a strip as wide as a chip and a couple times as long. Glue it onto the chip and stick a cooler on the other end of the strip. If the thermal conductivity is really above 500W/mK then cooling a chip through a 10cm x 5cm x 0.1mm ribbon cable should "only" add like 1K per 5W. So hypothetically, cooling something like a Pentium G5XXX with a decent cooler and the ribbon cable could work. 500W/mK is around 30% of the theoretical performance of pyrolitic carbon, so there is a chance.... (Plus it'd look dope through a thermal camera). If the thing performs at closer to 60% of theoretical performance, maybe you can even make the strip long enough to put the cooler on the back....

 

Even if the obect is never to be seen again. I'd love to hear if the electric conductivity through the pad is noticably worse, since that would give me more reason to believe that this anisotropy was what caused the poor result.

 

If It Ain't Graphene Then What Is the Sci-Fi Material For Thermal Pads?

The ideal themal pad in not electrically conductive along the X-Y plane and has awesome thermal conductivity in the Z direction. It also needs to be flexible. From the discussion above it should be cleat that it would be great to have a 1D equivalent for graphene. This material exists - and much to Riley's enjoyment - it's carbon nano tubes. So my guess for the best thermal pad would be carbon nanotubes all aligned in the Z direction and stuck together with silicone rubber or something like that. Thinking about it, those glue coated graphene sheets are basically the exact opposite of the ideal thermal pad... funny.

 

You folks might actually be able to simulate this with the materials you have. Cut the sheets into thin 5cm long strips and glue them together into a 5x5cm block, then slice a thing layer off with a very sharp razor. That way the good conduction direction is actually aligned with the direction you need it to conduct in. Not sure if the fabrication works, I'm a theoretical physicist, someone who knows how to actually manufacture stuff like @AlexTheGreatish...

Though with the 16µm sheets you have that'd be like 3000 layers. But, if you for example alternated these sheets with 0.1mm double sided tape, that'd go down to a couple hundred layers. And if they do actually conduct at 1000W/mK along the sheet, the resulting composite would have a conductivity of around 80-100W/mK in the relevant direction without being electrically conductive in that relevant direction... Maybe worth a try on a small scale.

[For Science!]

Realistically speaking though, the plane of interest should also be the cleavage plane of pyrolytic carbon, and so with any kind of meaningful pressure would the pyrolytic carbon not be a crumbled mess?

 

And if you don't apply meaningful pressure, how do you achieve adequate contact between the fairly rough ends of the carbon to the heat source, ultimately resulting in the same situation of needing something that can actually bridge the air gap.

 

Since the conductivity of metal-to-metal contact is what we want to promote, I still maintain that any pad solution that presents a physical wall between two interfaces to be less superior than a paste that moves out of the way if the metal-to-metal contact can be achieved.

[ChalkChalkson]

5 minutes ago, For Science! said:

Realistically speaking though, the plane of interest should also be the cleavage plane of pyrolytic carbon, and so with any kind of meaningful pressure would the pyrolytic carbon not be a crumbled mess?

In the ribbon cable example you'd apply the mounting pressure in the Z direction, so there it should be fine. For a theoretical compound thermal pad where the X direction goes from the IHS to the cooler, you'd hope that the matrix gives the needed mechanical properties. IIRC that's what they do with graphene fragments in epoxy. But this is super outside my wheel house. You'd need to ask a composite materials person.

5 minutes ago, For Science! said:

Since the conductivity of metal-to-metal contact is what we want to promote, I still maintain that any pad solution that presents a physical wall between two interfaces to be less superior than a paste that moves out of the way if the metal-to-metal contact can be achieved.

Well, these high tech carbon based materials have stupidly high conductivities, so the increased resistence where the contact might otherwise be metal to metal could be negligable. And where the pad actually fills gaps it is a lot better than even diamond particles in suspension (IC diamond). But let's be honest here, pyrolitic carbon composites are kinda overengineering a very simple problem. That's why I kinda like the idea with the ribbon cable, at leat that is something where the high conductivity actually gives a meaningful advantage over metal. In fact, the theoretical performance is so high, it should even beat heat pipes...

[For Science!]

5 minutes ago, ChalkChalkson said:

In the ribbon cable example you'd apply the mounting pressure in the Z direction, so there it should be fine. For a theoretical compound thermal pad where the X direction goes from the IHS to the cooler, you'd hope that the matrix gives the needed mechanical properties. IIRC that's what they do with graphene fragments in epoxy. But this is super outside my wheel house. You'd need to ask a composite materials person.

Perhaps, although I guess with most non-flat mounting I would expect for any mica-like substance to fray as the force is applied onto the middle (for a convex item pressing against it). Did you see the micrographs of IC diamond graphite pad vs TG? there graphite tubes are supposedly suspended in slightly different orientations.

 

 

[ChalkChalkson]

15 minutes ago, For Science! said:

Perhaps, although I guess with most non-flat mounting I would expect for any mica-like substance to fray as the force is applied onto the middle (for a convex item pressing against it). Did you see the micrographs of IC diamond graphite pad vs TG? there graphite tubes are supposedly suspended in slightly different orientations.

 

 

Wouldn't you expect a suspension to have an even distribution of orientations? I'd love to see some scattering graphs of these TIMs.

EDIT: just looked at the SEM pictures in that review. Pretty interesting! I have to say when I said IC Diamond I was thinking about the thermal paste with diamond particles. Would also like to know what the structure of the carbon fibers in this material is, expecting somehting polycrystaline. Again, I what I'd love most would be to see someone take those pads and pastes and do some scattering experiments. If only I had gone down the route of applied physics, could have been a fun BSC thesis