Why carbon is hot news
Tom Shelley reports on advanced heat spreaders and heat insulators – from the very expensive to the more economical – but all carbon-based
Carbon-based materials are up there with the elite when it comes to being employed as heat spreaders. For one thing, they offer several times the thermal conductivity of copper per unit weight. In a slightly different guise, they are equally effective heat insulators and, in nano tube form, serve as a kind of nano ‘Velcro’ for replaceable fastenings.
Some of the materials are certainly expensive, especially those based on diamonds (even if these are of the Chinese synthetic variety). Others, however, are based on good old-fashioned graphite and far more affordable. Many applications are to be found in the aerospace industry, especially in military system electronics, but others are low enough in cost to find their way into consumer durables, especially flat panel displays and laptop electronics where they can eradicate the need for cooling fans.
A recent seminar on a report produced by the DTI Global Watch, ‘Developments and trends in thermal management technologies – a mission to the USA’, highlighted how effective such material can be. One of the top performers for thermal conductivity, for example, is the Thermal Pyrolytic Graphite (TPG) material produced by Momentive Performance Materials in Strongsville, Ohio. It is made by chemical vapour deposition of cracked methane, followed by a combination of pressure and thermal treatment at very high temperatures, to produce a material with an in plane thermal conductivity of more than 1500W/m deg K (Copper is 400 and pure aluminium 235 W/m deg K), but whose through plane thermal conductivity is 10W/m deg K. Piece part price is $15 to $30 per square inch. On its own, it is somewhat fragile and, according to Dr Paul Cooper, manager – new technology at Radstone Technology, “has to be kept in compression all the time”. It is usually supplied embedded in an aluminium plate, in which form it is designated ‘TC1050’ and subject to export controls, since it is used in a US fighter power supply.
Much less expensive are the GrafTech SpreaderShield materials, to be found in the Sony Vaio X505 and Panasonic laptop and notebook computers, as well as various LCD and plasma displays. These are made from Graphite flake converted into sheet by a chemical treatment. In plane thermal conductivity is 140 to 500W/m deg K and through plane 5 to 12 W/m deg K. Although used only 0.2mm thick, it replaced heat pipe and fan units, says Cooper, and reduced the CPU temperatures from more than 77°C above ambient to 60°C – as well as effecting a 50% weight loss in the heat management components and improving battery life.
Dr Chris Stirling, technical support manager for Morgan Carbon, said the materials they had come across fell into four classes: monolithic carbonaceous, carbon-carbon, metal matrix composites and ceramic matrix composites. As far as metal graphite composites were concerned, the team considered the most well established materials as being the Graphmet family of materials from Materials and Electrochemical Research (MER) in Tucson, which are made by squeeze casting in to porous graphite performs. Stirling explained that metals don’t wet carbon - the formation of carbides has to be avoided - so low melting point alloys have to be used, such as aluminium silicon, with the application of coatings to enhance the interface. Graphmet 350 has a thermal conductivity in the range 220 to 360 W/m degK and is machinable, although its bend strength is only 30MPa.
JW Composites in Salt Lake City applies a thin layer of chemical vapour deposited molybdenum to graphite fibre, as this produces a surface that may be wetted by infiltrated copper. Yet molybdenum is effectively insoluble in copper, maintaining its thermal conductivity, and allows the development of a carbide interface that has low interfacial thermal resistance. The resulting composite material has a thermal expansion coefficient that may be controlled within the range 2 to 10 ppm/deg K, with in plane thermal conductivity equivalent to copper at 400 W/m deg K and Z plane conductivity of 200 W/m deg K. The material is of particular interest in power applications for mounting the large IGBT devices favoured for use in motor drives, halving die size. JW Composites has also developed a graphite foam copper composite, with conductivities of 600 to 1200 W/m deg K in ligaments, although developmental materials have thermal conductivities of 342 W/m deg K.
Depending on precursor materials, carbon foams can be made with either extraordinarily high thermal conductivities or extraordinarily low ones, with thermal conductivities of less than 1 W/m deg K, enabling them to be used as thermal insulators. According to Cooper and Stirling, the low thermal conductivity foams are glassy, while the high ones are graphitised. So, if one of the low thermal conductivity thermal foams were to become hot enough, it would graphitise and turn from having a very low to a very high one thermal conductivity.
The lowest thermal conductivity known in a fully dense material, according to Stirling, is not carbon, but thin films of tungsten selenide on silicon, which have a thermal conductivity of only 0.05 W/m deg K. However, the lowest thermal conductivity materials of all are silica-based aerogels, with typical thermal conductivities of only 0.02 W/m deg K. These materials, once used exclusively by NASA, are now affordable enough to be used in glazing panels, in which form they are produced by Cabot in Frankfurt and sold under the brand name Nanogel. Some of the carbon foam developments could follow a similar path, since carbon is not a fundamentally expensive substance.
The same is not so likely to be true of the metal diamond composites being developed both by MER and JW Composites. Chinese synthetic diamonds have driven costs down to only £157 to £560/kg, depending on quality. JW Composites is applying its molybdenum coating technology to subsequent melt processing with copper or silver matrices, building on its Molybonded diamond tooling technology. Diamond loadings for development projects are 50% by volume fraction and it is estimated this could be increased to 75 to 80%, if necessary. Thermal conductivities of 600 to 800 W/m deg K are anticipated.
The highest thermal conductivities of all are along the lengths of multi-walled carbon nanotubes. Stirling says that thermal conductivities of 3000 W/m deg K have been measured and that 6,000 is theoretically possible. Most high tech carbon companies and universities have some kind of carbon nanotube research project going. Dr Norman Stockham, one of the managers at TWI described how carbon nanotube mats on substrates can be made to act as a nano scale ‘Velcro’. Research is being undertaken at the University of California at Berkeley, Purdue University and MER.
Unlike Velcro, the mats of nanotubes bond to a flat surface they have been pressed against almost as well as two mats bond to each other. They bond through Van der Waals interactions with adhesion strengths of up to about 12N/sq cm. Also unlike Velcro, according to Dr Stirling, “If you pull them apart and put them back, the joint loses strength” and, “The smaller the die size, the higher the strength”, which effect is attributed to slight curvature effects at large sizes. The intention of the development is to make good electrical and thermal contact, as much as provide mechanical adhesion.
* It is possible to make carbon derived materials with thermal conductivities of more than 1500W/m deg K (Copper is 400 and pure aluminium 235 W/m deg K)
* It is also possible to make carbon materials with thermal conductivities of less than 1W/m deg K.
* There are materials that were considered exotic and only suitable for aerospace electronics a few years ago that have now become cheap enough to incorporate into building products. The same could happen to some of these materials
* Multiwalled carbon nanotubes have thermal conductivities along their axes of up to 3000 W/m deg C, and theoretically could achieve 6000. They are also showing promise for making dry electrical, thermal contacts and mechanical bonding, acting as a kind of nano scale ‘Velcro’
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