Carbon Fiber Chemical Structure
Introduction
Carbon fiber is widely recognized as a high-performance material that combines low density, exceptional tensile strength, and high stiffness. These superior material properties are derived directly from its unique chemical structure of carbon fiber
and the processing steps used to transform raw precursors into continuous carbon filaments.
To fully understand why carbon fibers outperform metals and other materials, it is essential to study their general properties
, molecular bonding, and crystallographic organization. This article explores the chemistry behind carbon fibers, with comparisons to other carbon fiber composites and their diverse applications.
Atomic and Molecular Structure of Carbon Fibers
Carbon Hybridization and Bonding
Carbon atoms in carbon fiber primarily exist in the sp² hybridization state, forming hexagonal graphite-like layers (graphene sheets).
Each carbon atom bonds with three neighbors via σ-bonds, while π-electrons are delocalized, providing high electrical conductivity and stability.
These graphene planes are responsible for high tensile modulus and tensile strength, while interlayer van der Waals forces account for anisotropic mechanical behavior.
Key Data:
C–C bond length: ~1.42 Å
Interlayer spacing (d002) in turbostratic carbon: 0.344–0.350 nm (larger than ideal graphite at 0.335 nm, due to disorder)
For a broader overview of carbon fiber fundamentals, see What is Carbon Fiber？
Crystalline vs. Amorphous Regions
Carbon fibers are semi-crystalline materials with two main structural features:
Ordered Graphitic Domains → aligned graphene planes, responsible for elastic modulus.
Disordered/Turbostratic Regions → misaligned carbon planes, providing toughness and strain tolerance.
These two structural phases explain the balance of strength and ductility seen in modern carbon fiber composites
Precursor Influence on Chemical Structure
PAN-Based Carbon Fibers
Precursor: Polyacrylonitrile (PAN)
Carbon Content: > 92%
Key Chemistry:
Stabilization induces cyclization of nitrile groups (–C≡N).
Carbonization reorganizes into ladder-like aromatic structures.
Structure: Highly oriented, fine crystallites with good balance of strength and modulus.
Applications: Aerospace, automotive, and high-performance carbon fiber applications.
Pitch-Based Carbon Fibers
Precursor: Mesophase pitch
Carbon Content: > 95%
Key Chemistry:
High aromatic content from the start.
Graphitization yields very high modulus (> 900 GPa possible).
Structure: Larger crystallites, highly graphitic, but more brittle.
Applications: Specialized carbon fiber composites for space and civil engineering.
Rayon-Based Carbon Fibers
Precursor: Cellulose-derived rayon
Carbon Content: 80–85%
Structure: Low carbon yield, less ordered structure.
Applications: Historical only, replaced by PAN and pitch.
Processing-Induced Structural Evolution
Stabilization (200–300 °C, Oxidizing Atmosphere)
PAN fibers undergo cyclization, dehydrogenation, and oxidation.
Formation of thermally stable ladder polymer prevents melting.
Carbonization (1000–1500 °C, Inert Atmosphere)
Removal of hydrogen, oxygen, nitrogen.
Aromatic structures condense into turbostratic graphite-like layers.
Carbon content reaches > 90%.
Graphitization (2500–3000 °C)
In PAN and pitch-based fibers, further ordering occurs.
Interlayer spacing decreases, crystallites grow in size.
Increases elastic modulus but can reduce tensile strength due to microcracks.
These transformations directly determine the carbon fiber properties
observed in the final material.
Material Properties and Structure Relationship
| Property | PAN-Based Carbon Fiber | Pitch-Based Carbon Fiber | Notes |
|---|---|---|---|
| Density (g/cm³) | 1.75 – 1.90 | 2.00 – 2.20 | Higher graphitization → higher density |
| Tensile Strength (GPa) | 3.0 – 7.0 | 1.5 – 3.5 | PAN offers higher strength |
| Elastic Modulus (GPa) | 200 – 400 | 400 – 900 | Pitch offers higher modulus |
| Carbon Content (%) | 92 – 95 | 95 – 99 | Pitch closer to graphite |
| d002 spacing (nm) | 0.344 – 0.350 | 0.335 – 0.338 | PAN is more turbostratic |
These values highlight how carbon fiber density, tensile strength, and elastic modulus all depend on chemical structure. A full breakdown of performance data can be found in our guide to carbon fiber properties
Chemical Structure and Failure Mechanisms
Tensile Failure: Crack propagation along turbostratic regions.
Compression Failure: Microbuckling between graphitic layers.
Shear Failure: Weak van der Waals bonding between graphene sheets.
This explains why yield stress of carbon fiber and ultimate tensile strength are highly anisotropic. For real-world relevance, see how this translates into different carbon fiber applications
Applications Linked to Chemical Structure
Aerospace: PAN-based fibers → balance of strength and toughness.
Civil Engineering: Pitch-based fibers → extremely high modulus for prestressed concrete.
Sporting Goods: PAN-based fibers dominate due to lightweight and high impact resistance.
Electronics: Conductive pathways due to sp² bonding used in EMI shielding.
To understand how chemical structure drives practical usage, explore our full section on carbon fiber applications
Applications Linked to Chemical Structure
Aerospace: PAN-based fibers → balance of strength and toughness.
Civil Engineering: Pitch-based fibers → extremely high modulus for prestressed concrete.
Sporting Goods: PAN-based fibers dominate due to lightweight and high impact resistance.
Electronics: Conductive pathways due to sp² bonding used in EMI shielding.
Conclusion
The chemical structure of carbon fiber—a hybrid of ordered graphitic crystallites and disordered turbostratic regions—defines its density, tensile strength, elastic modulus, and thermal stability.
Precursor chemistry (PAN, Pitch, Rayon) and processing conditions (stabilization, carbonization, graphitization) directly control carbon fiber’s material properties. This explains why carbon fibers are the foundation of advanced carbon fiber composites
across aerospace, automotive, construction, and sporting industries.
For a full overview, revisit our main guide: What is Carbon Fiber .