This Heat conductivity article is a work in progress. Information available is confusing and sometimes contradictory. I try to use primary research for my information but it's heavy reading and I simply don't always get it. New information is appearing continually as are new products. Enjoy!

Thermal Conductivity of Carbon Fiber, and other Carbon Based Materials

Carbon Fiber and other carbon based materials are being developed to replace other more traditional heat conducting materials.

What is meant by THERMAL CONDUCTION?

Thermal Conductivity is the ability of a material to conduct heat. It is quantified by Fourier Law of Heat Conduction:

When there exists a temperature gradient within a body, heat energy will flow from the region of high temperature to the region of low temperature. This phenomenon is known as conduction heat transfer, and is described by Fourier's Law.

Heat transfer across materials of high thermal conductivity occurs at a higher rate than across materials of low thermal conductivity. In other words materials with high conductivity conduct heat better than materials with high thermal resistivity which are used as thermal insulators.

Units of Thermal Conductivity

In Imperial units, thermal conductivity is measured in BTU/(hr·ft·F).

In SI units (International System of Units, Metric), it is measured in watts per meter kelvin (W·m-1·K-1).

Several industries are concerned with the heat conductivity and resistance of materials and different scales have been devised to suit their needs. Thus, the construction folks use R-value to rate insulating materials, while the clothing industry uses togs and clo to define the insulating values of textiles.

How is Heat Conducted?

Thermal conductivity is different for non metals and metals.

In metals conductivity is mostly due to free electrons. This is the reason metals with high electrical conductivity also have high thermal conductivity. Thermal conductivity is subject to variations as the temperature changes, often decreasing as the temperature increases.

Non metals heat conductivity is primarily due to lattice vibrations (phonons). Except for high quality crystals at low temperatures, there is not much difference and thermal conductivity remains approximately constant.

Comparing the Thermal Conductivity of Various Materials.

The Units in this table are W/m*K for the conductivity, and g/cm(3) for the density.

Note: The following table is for comparison only. Heat conductivity will vary with chemical composition, types of wood, crystalline structure, methods of measurement, alignment of fibres, temperature gradient, precursor materials. It is presented to show the relative conductivity of materials. Carbon fiber in its various forms is so variable that it is really not possible to just list it without explanations, that's why the heat conducting property of carbon fiber is rarely seen in a table.

Here are some more extensive tables for Heat Conductivity: Wikipedia table of thermal conductivity, Engineering Toolbox's table are alternatives

I got the figures for THERMAL PROPERTY MEASUREMENT OF CARBON-FIBER/EPOXY COMPOSITE MATERIALS from a dissertation for PHD from University of Nebraska. Skip to page 128 for the results, but have a look at the process. It is a good illustration of the kind of work that has to be done to measure thermal conductivity in non metal composites.

What is graphene?

Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, (think of miniature chicken wire structure, ) and is a basic building block for graphitic materials. It can be wrapped up into fullerenes (another name for carbon nanotubes), or stacked into 3D graphite.

Graphene Article from Graphene Industries. Short readable page.

Graphene sheets stack to form graphite. Graphene sheets have recently been produced and are the subject of intense study. They are not widely used yet but will be coming to an industry near you soon!

NOTE, There is a huge number of articles and research papers on the heat conductivity of carbon fiber, carbon nanotubes, graphene. What comes out of this is:

• There is a significant range of measured values. It can be quite low in the case of carbon fiber composite measured across the sheet, or really high for materials such as pyrolytic graphite and Diamond.
• Carbon Fiber, Graphite, and other carbon derivatives vary greatly depending on the plane measured. Along the fiber, the conductivity is high, against the plane the conductivity is highly reduced.
  The higher the carbon content of the carbon fibers, level of carbonization, the higher the thermal conductivity.
• Carbon composites are difficult to measure because a great deal depends on the manufacturing method, exact composition of the matrix, air, alignment of fibers, method of measurement, preparation of samples.
• There have been several experiments where the thermal conductivity of carbon composites have been increased by doping with carbon nanotubes and other carbon based materials.
• Because technology is not developed some of the figures reported are theoretical and have not been achieved yet.

Why bother with carbon based thermal materials?

What are the advantages of using Carbon fiber, graphite etc.

Dimensionally Stable

Carbon fiber has the advantage over copper and other metals because it has a very low Coefficient of Thermal expansion. When a material is heated it expands then shrinks again as it cools down. This can be a significant issue when tolerances are very critical. Optical systems, and micro electronics are examples.

Copper has a coefficient of 16.6 (10-6 m/m K) while carbon fiber can be as low as 0. For this reason copper has been combined with carbon fiber/graphitic materials to create a material with significantly smaller Coefficient of linear thermal expansion.

Aluminium and carbon have been tried but the mixture forms a galvanic couple causing corrosion. Copper is a better choice. Corrosion is not a problem and the actual thermal conductivity can be higher than copper alone if the carbon fiber is highly graphitic.

Significant larger thermal conductivity

Some graphites and diamond are much higher than copper and silver. as much as 5 times more conductive. Usually the cost is prohibitive. High conductivity graphite is quite fragile and this is a disadvantage.

Weight and strength

Carbon materials are significantly lighter than metals. Carbon fiber also has a higher strength to weight ratio.

Any carbon fiber in regular epoxy can only be subjected to temperatures that will not damage the epoxy matrix. For this reason regular composites have a limited use. High temperature epoxy has been developed but it is really not very high. There exists methods of manufacturing panels of carbon fiber without embedding them in epoxy and this extends the range of temperature considerably. See the side panel for a link to the Characterization of high termal conductivity carbon fibers.

Graphite can be used in conditions that would damage conventional materials. It is highly resistant to corrosion and has good non-contaminating properties.SGL Group offers heat exchangers made from graphite

So...Is Carbon Fiber a good heat conductor?

As usual the answer is "it depends." The short answer is NO not when regular carbon fiber is made up in regular epoxy and expected to conduct heat across the thickness. IF a highly carbonized pan fiber with graphite or diamond added, is measured for heat transmission in the length of the fiber it is very good and can rival and exceed copper. Graphite is a common material for heat exchangers.

Other carbon materials such as diamond or some graphites, such as pyrolytic graphite, are stellar and can be 5 times better than copper.

There has been research to improve the thermal conduction of glass fibre composite by adding graphene. A 50% improvement was noted.The Abstract is available here You can also pay to read the complete article.

Graphene sheets and Carbon Nanotubes have very amazing potential but are not yet in common use.To be continued...

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