Carbon felt is a soft, lightweight sheet material made from carbon fibers that are compressed and bonded into a non-woven fabric. It looks and feels somewhat like thick wool felt, but it’s engineered for extreme environments: vacuum furnaces, energy storage systems, and high-temperature industrial processes. With roughly 98% porosity and thermal conductivity as low as 0.064 W/m·K in a vacuum, it acts as an exceptional insulator at temperatures that would destroy most other materials.
How Carbon Felt Is Made
Carbon felt starts as a synthetic polymer fiber, most commonly polyacrylonitrile (PAN). These fibers are first formed into a non-woven mat, then heated in an oxygen-free environment in a process called carbonization. At around 1,000°C, the polymer fibers convert into carbon fibers with a carbon content of about 90%. The result is standard carbon felt.
To produce graphite felt, the material undergoes an additional step called graphitization. This involves heating it further to 2,000°C or above, again without oxygen present. The higher temperature reorganizes the carbon atoms into a more ordered crystalline structure, pushing carbon purity above 99% and reducing ash content to below 800 parts per million. Both products look similar, but graphite felt has higher purity, better electrical conductivity, and slightly different thermal behavior.
Carbon Felt vs. Graphite Felt
The two are often confused or used interchangeably in casual conversation, but they differ in meaningful ways. Carbon felt, produced at around 1,000°C, has a carbon content of 90% or higher. Graphite felt, produced at 2,000°C or above, reaches 99% or higher. The key property differences for PAN-based versions:
- Carbon content: ≥90% for carbon felt, ≥99% for graphite felt
- Ash content: 1,000–2,000 ppm for carbon felt, ≤800 ppm for graphite felt
- Thermal conductivity: 0.12–0.17 W/m·K for carbon felt, 0.10–0.15 W/m·K for graphite felt
- Bulk density: 0.11–0.16 g/cm³ for carbon felt, 0.10–0.15 g/cm³ for graphite felt
Graphite felt’s higher purity makes it the better choice when contamination is a concern, such as in semiconductor manufacturing or certain chemical processes. Carbon felt is less expensive and sufficient for many insulation and filtration applications.
Thermal and Physical Properties
Carbon felt’s most striking characteristic is its porosity, which can reach about 98%. That means the material is almost entirely open space, with thin carbon fibers forming a loose three-dimensional network. This structure is what makes it such a good thermal insulator: there’s very little solid material to conduct heat through.
In a vacuum, graphite felt has been measured at just 0.064 W/m·K at room temperature. In air, that rises slightly to 0.077 W/m·K. In helium, which conducts heat more readily than air, it jumps to 0.232 W/m·K. The surrounding gas matters because at low temperatures (below about 230°C), heat moves through the felt mainly by solid fiber conduction and gas conduction between fibers. At higher temperatures, radiation between fibers becomes the dominant mechanism, and thermal conductivity can climb significantly, reaching values above 1.0 W/m·K at 1,600°C.
Commercial carbon felt typically comes in thicknesses of 6, 8, or 10 mm, with widths up to 1,200 mm. Bulk densities range from about 50 to 80 kg/m³, which is extremely light. For context, that’s roughly one-fifteenth the density of water.
Where Carbon Felt Is Used
High-Temperature Insulation
The most common application is thermal insulation inside vacuum furnaces and inert-atmosphere furnaces. Because carbon reacts with oxygen at high temperatures, carbon felt can only be used in environments where oxygen is absent or minimal. In a vacuum or under an inert gas like argon, it can withstand temperatures well above 1,000°C. This makes it a standard lining material in furnaces used for heat treating metals, sintering ceramics, and growing crystals.
Energy Storage Electrodes
Carbon felt and graphite felt serve as electrode materials in vanadium redox flow batteries, a type of large-scale energy storage system used for grid applications. The felt’s uniform three-dimensional fiber structure provides a large surface area for the chemical reactions that store and release energy, while its wide operating voltage range makes it compatible with the battery chemistry. Performance depends heavily on the felt’s surface area and the number of chemically active sites on its fibers, which is why researchers often treat the felt with heat or chemicals to roughen the fiber surfaces before use.
The felt also resists corrosion in the acidic electrolyte solutions these batteries use, though long-term contact with the electrolyte does cause gradual degradation. Studies on vanadium battery electrolytes have shown that the carbon surface slowly oxidizes over a period of days, reducing the effective surface area and increasing electrical resistance.
Other Applications
Carbon felt also shows up in fuel cells as gas diffusion layers, in composite manufacturing as a reinforcement preform, and in some filtration systems for molten metals. Its chemical resistance to many acids and alkalis, combined with its temperature tolerance, gives it a role in specialized chemical processing equipment as well.
Handling and Safety
As a solid sheet, carbon felt is not particularly hazardous. The main concern is dust. Cutting, machining, or even rough handling can release fine carbon fiber particles into the air. These particles can irritate your skin, eyes, and respiratory system through mechanical abrasion, similar to fiberglass dust.
Carbon and graphite dust is electrically conductive, which creates a less obvious hazard: dust accumulating on electrical equipment can cause short circuits, potentially leading to electrical shock, fire, or equipment damage. Accumulated dust can also create slippery surfaces and, in sufficient quantities, poses a combustible dust risk.
If you’re cutting or working with carbon felt, use dust collection at the source and wear an N-95 dust mask or equivalent respiratory protection. General ventilation, regular vacuuming or sweeping of work surfaces, and basic hygiene practices (washing hands before eating, keeping food away from the work area) are the standard precautions recommended by manufacturers.