I just saw a show with a brief description of graphene sheets. Apparently it conducts heat really, really quickly/well. Of course its also very expensive.
What do you think about mounting it between power output devices and the usual heatsink? It would seem to have 2 benefits 1) keeps all the output devices at the same temp and 2) moves more heat to a larger area of the back of the heatsink - don't have to worry about the "slow" heat transfer on aluminum from the devices to the fringes of the heatsink.
What do you think about mounting it between power output devices and the usual heatsink? It would seem to have 2 benefits 1) keeps all the output devices at the same temp and 2) moves more heat to a larger area of the back of the heatsink - don't have to worry about the "slow" heat transfer on aluminum from the devices to the fringes of the heatsink.
The heat conductivity of graphene sheet is along the plane, which is what a heat spreader does. Bonding a device to a heatsink requires heat conductivity in the Z axis - through the plane. Isothermy in the plane is less important than conduction from the device to the heatsink. So, while it's a neat material, it's unique feature, anisotropy, is a problem here. You're better off with any of a number of non-directional materials.
Expensive? My Head Prestige Power tennis racquet made from Graphene only cost me $250. it weighs 270 grams. I reckon I should be able to get thousands of insulators (TO247 size) out of 1 racquet. Anybody know what's the melting point for Graphene? Should I pour the melted graphene into 1 sheet and cut it up or make tiny molds?
I don't think it melts. It's a hexagonal planar crystal of carbon, sort of like graphite, but the crystals are a lot longer. I think it goes straight to plasma at some handful of thousands of degrees. This bond structure is why it's such a directional thermal conductor - tons of strong double bonds along the plane, only weak Van Der Waals bonds between the planes.
Found some stuff at digikey. A 60mm x 90mm piece is only a few $. Larger pieces cost much much more.
Claims to be several times more thermally conductive than copper. Probably a very good electrical conductor too unfortunately, but i think some come with an insulating backing.
One of the brochures mentions using it as a replacement for thermal grease.
For a veteran (someone who could do the setups and measurements easily) amp builder it might be worth some experimenting.
Thermal - Pads, Sheets | Fans, Thermal Management | DigiKey
Claims to be several times more thermally conductive than copper. Probably a very good electrical conductor too unfortunately, but i think some come with an insulating backing.
One of the brochures mentions using it as a replacement for thermal grease.
For a veteran (someone who could do the setups and measurements easily) amp builder it might be worth some experimenting.
Thermal - Pads, Sheets | Fans, Thermal Management | DigiKey
Ok, found that they do make product(s) with electric insulation properties they call SSM elastomer (EYEG product numbering). Some of their other products also appear to have elecltrical insulation.
http://media.digikey.com/pdf/Data Sheets/Panasonic Electronic Components/PGS_Br.pdf
https://industrial.panasonic.com/cdbs/www-data/pdf/AYA0000/AYA0000C16.pdf
http://media.digikey.com/pdf/Data Sheets/Panasonic Electronic Components/PGS_Br.pdf
https://industrial.panasonic.com/cdbs/www-data/pdf/AYA0000/AYA0000C16.pdf
The most whiz-bang of heat sink goop (TIM in other parlance) is diamond-loaded paste. Graphene is sure great, but diamond dust works very well thermally. Either case, you still need an insulator to not tie your heatsink to VEE or VCC (depending on the arrangement).
And, as great as all this is, it might save you a degree or two of die temperature versus more mundane mounting methods?
And, as great as all this is, it might save you a degree or two of die temperature versus more mundane mounting methods?
The key to high thermal conductivity is a bond structure that has a lot of very strong double bonds in a compact arrangement. The idea is that when one atom is mechanically 'bumped' by its hotter neighbor through the bonds that attach the atoms, the stronger the bond, the better the 'bump' is translated. That 'bump' is what happens when a hot atom conducts its heat (mechanical, kinetic energy) to its neighbor - the stronger the bond, the better the heat conductivity.
You can rightly infer from this that a stronger bond will transfer more energy; this results in better thermal conduction. The nicest geometry is thus a tetrahedral structure, like diamond. A great advantage is that a tetrahedron is rigid in all directions, and the bond lengths are low, so its heat conductivity is high regardless of the orientation of the crystal to the heat. A cubic structure is also good, but less ideal, and a hexagonal flat plate structure like graphite or graphene is great along the plane of the bonds, but relatively poor when conducting energy perpendicular to the bond plane where only weak bonds attach the hexagonal crystals to each other. (This is what makes graphite slippery - the weak Van Der Waals bonds between the planes are easily sheared, but the hexagonal lattice is not. Thus, the graphite retains is solidity while being slippery in one dimension.)
One of the oddities is that electrical conductivity is not exclusively related to the bond structure - a hexagonal boron nitride lattice is a great insulator, whereas graphite, in the same crystal structure, is a great electrical conductor, and both are excellent thermal conductors.
So, long and short, graphene is a nice thermal conductor, but it's also an electrical conductor, so it's an annoyance to use as a heatsink or spreader. Some other compounds such as boron nitride or aluminum nitride are also very good thermal conductors, but are electrical insulators. They share the same bond structures, but do not have free electrons for conductivity, so they are insulators, with very low loss because of their compact, tightly bonded structure.
So, graphene is useful, but if you have to put a layer of Kapton over it, then you're throwing away a lot of its thermal conductivity. And, given that BN or AlN are also really good thermal conductors, but nearly ideal insulators, there's no point in trying to insulate graphene and expecting to retain its exceptional thermal properties.
Sorry for the long winded explanation, but a little solid state chemistry can explain a lot sometimes.
You can rightly infer from this that a stronger bond will transfer more energy; this results in better thermal conduction. The nicest geometry is thus a tetrahedral structure, like diamond. A great advantage is that a tetrahedron is rigid in all directions, and the bond lengths are low, so its heat conductivity is high regardless of the orientation of the crystal to the heat. A cubic structure is also good, but less ideal, and a hexagonal flat plate structure like graphite or graphene is great along the plane of the bonds, but relatively poor when conducting energy perpendicular to the bond plane where only weak bonds attach the hexagonal crystals to each other. (This is what makes graphite slippery - the weak Van Der Waals bonds between the planes are easily sheared, but the hexagonal lattice is not. Thus, the graphite retains is solidity while being slippery in one dimension.)
One of the oddities is that electrical conductivity is not exclusively related to the bond structure - a hexagonal boron nitride lattice is a great insulator, whereas graphite, in the same crystal structure, is a great electrical conductor, and both are excellent thermal conductors.
So, long and short, graphene is a nice thermal conductor, but it's also an electrical conductor, so it's an annoyance to use as a heatsink or spreader. Some other compounds such as boron nitride or aluminum nitride are also very good thermal conductors, but are electrical insulators. They share the same bond structures, but do not have free electrons for conductivity, so they are insulators, with very low loss because of their compact, tightly bonded structure.
So, graphene is useful, but if you have to put a layer of Kapton over it, then you're throwing away a lot of its thermal conductivity. And, given that BN or AlN are also really good thermal conductors, but nearly ideal insulators, there's no point in trying to insulate graphene and expecting to retain its exceptional thermal properties.
Sorry for the long winded explanation, but a little solid state chemistry can explain a lot sometimes.
Please don't apologise! We love that kind of fascinating insight - thank you Monte.
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