WMO Education and Training Programme

What Is Carbon Fiber?

Carbon fiber is a composite material known for its remarkable strength-to-weight ratio. Composed of minuscule strands of carbon atoms bonded together in a crystalline arrangement, carbon fibers are approximately 5–10 micrometers in diameter. The material is a mainstay in sectors like aerospace, automotive, and sports equipment, mainly due to its high tensile strength, low weight, corrosion resistance, and dimensional stability. Furthermore, carbon fiber exhibits electrical and thermal conductivity, albeit to varying degrees.

Conductive Principle of Carbon Fiber

The conductive properties of carbon fiber arise from its unique atomic structure and bonding. Carbon atoms form covalent bonds, creating extended chain molecules. This structure contains delocalized electrons, similar to graphite, albeit with fewer free electrons. These electrons facilitate limited charge transmission, categorizing carbon fiber as a semiconductor. While not as conductive as metals like copper or aluminum, carbon fiber’s conductivity is sufficient for specific applications, offering a lightweight alternative with additional functional benefits.

Factors Affecting the Conductive Properties of Carbon Fiber

The electrical conductivity of carbon fiber is influenced by several factors:

Raw Materials and Preparation Process: Different raw materials produce fibers with varying resistivity levels. Mesophase pitch-based fibers generally exhibit lower resistivity than PAN-based fibers.

Heat Treatment Temperature: Higher graphitization levels enhance electron transmission due to expanded graphite layers and better alignment along the fiber axis, improving conductivity.

Microstructural Defects: The presence of crystal defects hampers electron mobility, increasing resistivity. Purity is crucial as impurities obstruct electron flow.

Composite Materials: Carbon fibers in composites enhance conductivity based on factors like fiber orientation and volume fraction, with the polymer matrix often acting as an insulator.

Applications of Conductive Carbon Fiber

Conductive carbon fiber is indispensable across multiple industries:

Electromagnetic Shielding: Carbon fiber is used in EMI shielding, offering reflection akin to metals when fibers align parallel to the electric field, and absorption at other angles. This capability is invaluable in aerospace and defense.

Electronic Devices: Its conductivity finds applications in batteries and capacitors, enhancing thermal management and electrical performance.

Aerospace and Aviation: Carbon fiber’s lightweight yet conductive nature is ideal for aircraft wings and reinforcements, providing lightning protection and structural integration.

Types of Carbon Fibers

l Activated Carbon Fibers

l PAN-Based Carbon Fibers

l Pitch-Based Carbon Fibers

Myths About Carbon Fiber Conductivity

Several myths surround carbon fiber conductivity:

Uniform Conductivity: Conductivity varies among carbon fibers depending on raw materials and processing, with high modulus fibers showing better conductivity post-graphitization.

Mechanical Properties Correlation: The assumption that conductivity and mechanical strength are interlinked is false; they rely on different structural aspects.

Composite Conductivity: Unlike pure carbon fibers, composites rely on fiber networks for conductivity. Low fiber content drastically reduces this property.

Environmental Impact: Conductivity increases with temperature but decreases with humidity. Chemical corrosion can also degrade fibers.

Poor Conductivity Equals Insulation: Despite higher resistivity than metals, carbon fiber is not an insulator and suits applications like antistatic materials.

Replacement of Metal Conductors: Although carbon fiber offers lightweight conductivity, it is unsuitable for carrying large currents compared to metals.

EMI Shielding in All Composites: Effective electromagnetic shielding requires a continuous fiber network, often needing over 30% fiber content for efficiency.

In conclusion, carbon fiber’s conductivity, while not matching metals, offers unique advantages for various applications, combining lightweight properties with sufficient electrical transmission capabilities. These characteristics drive its widespread use across advancing technological fields, dispelling myths about its limitations.