Carbon Fiber
Introduction
Carbon fiber, also known as graphite fiber or carbon graphite, is a material consisting of thin, strong crystalline filaments of carbon. These fibers are used to reinforce materials, particularly in the field of composite materials, due to their high tensile strength, low weight, and high chemical resistance. Carbon fiber is widely utilized in aerospace, civil engineering, military, and motorsports, along with other competition sports.
History
The development of carbon fiber began in the late 19th century when Thomas Edison carbonized bamboo filaments for use in his light bulbs. However, the modern carbon fiber industry began in the late 1950s and early 1960s. The British Royal Aircraft Establishment developed the first high-performance carbon fibers using rayon as a precursor. Later, the Japanese company Toray Industries developed a more efficient process using polyacrylonitrile (PAN) as a precursor, which remains the dominant method today.
Production Process
Precursors
The primary precursors for carbon fiber production are polyacrylonitrile (PAN), rayon, and pitch. PAN-based fibers are the most common due to their superior mechanical properties and ease of processing. Rayon-based fibers are less common and primarily used for specialized applications. Pitch-based fibers, derived from petroleum or coal tar, offer high modulus and thermal conductivity but are more challenging to process.
Stabilization
The stabilization process involves heating the precursor fibers in the air to around 200-300°C. This process causes the fibers to undergo oxidation, transforming the linear polymer chains into a thermally stable ladder structure. This step is crucial for preventing the fibers from melting during the subsequent carbonization process.
Carbonization
During carbonization, the stabilized fibers are heated to temperatures between 1000-3000°C in an inert atmosphere, typically nitrogen. This process removes non-carbon elements, resulting in fibers composed of over 90% carbon. The degree of carbonization affects the final properties of the carbon fiber, with higher temperatures leading to higher modulus fibers.
Surface Treatment
Surface treatment is essential for improving the adhesion between the carbon fibers and the matrix material in composite applications. This process typically involves oxidizing the fiber surface using gases like ozone or liquids like nitric acid. The treated fibers are then coated with a sizing agent to protect them during handling and to enhance compatibility with the matrix material.
Sizing
Sizing agents are applied to the surface-treated fibers to protect them from damage during weaving and handling. These agents also improve the bonding between the fibers and the matrix material. Common sizing agents include epoxy, polyester, and polyurethane.
Properties
Carbon fiber exhibits a unique combination of properties that make it highly desirable for various applications:
Mechanical Properties
Carbon fibers have an exceptionally high tensile strength, typically ranging from 3.5 to 7 GPa. They also possess a high modulus of elasticity, which can exceed 500 GPa for high-modulus fibers. These properties result in materials that are both strong and stiff.
Thermal Properties
Carbon fibers have excellent thermal stability and can withstand temperatures up to 3000°C in non-oxidizing environments. They also have high thermal conductivity, particularly pitch-based fibers, making them suitable for heat dissipation applications.
Electrical Properties
Carbon fibers are electrically conductive, with electrical resistivity values typically ranging from 1.5 to 2.0 µΩ·m. This property is advantageous for applications requiring electromagnetic interference (EMI) shielding or electrostatic discharge (ESD) protection.
Chemical Resistance
Carbon fibers are highly resistant to chemical attack, including resistance to acids, alkalis, and organic solvents. This property makes them suitable for use in harsh chemical environments.
Applications
Aerospace
Carbon fiber composites are extensively used in the aerospace industry due to their high strength-to-weight ratio and excellent fatigue resistance. Applications include aircraft structures, such as fuselages, wings, and tail sections, as well as spacecraft components.
Automotive
In the automotive industry, carbon fiber is used to reduce vehicle weight, improve fuel efficiency, and enhance performance. Applications include body panels, chassis components, and interior parts. High-performance vehicles, such as Formula One cars, extensively use carbon fiber for its superior properties.
Sports Equipment
Carbon fiber is widely used in sports equipment, including tennis rackets, golf clubs, bicycles, and fishing rods. The material's high strength and low weight provide athletes with enhanced performance and durability.
Civil Engineering
In civil engineering, carbon fiber-reinforced polymers (CFRPs) are used for strengthening and retrofitting structures, such as bridges, buildings, and pipelines. CFRPs offer high strength and corrosion resistance, making them ideal for infrastructure applications.
Medical
Carbon fiber is used in medical applications, including prosthetics, orthotics, and surgical instruments. The material's biocompatibility, radiolucency, and high strength make it suitable for these applications.
Future Developments
Research and development in the field of carbon fiber are focused on improving the material's properties, reducing production costs, and expanding its applications. Advances in precursor materials, processing techniques, and surface treatments are expected to enhance the performance and affordability of carbon fiber composites.