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Carbon fibers andnanofibers

D.D.L. ChungUniversity at Buffalo

State University of New York

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Crystal forms of carbon

Graphite

Diamond

Fullerene

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Types of

carbon nanofiberNanofiber with fish-bone morphology

Multi-walled nanotube (concentriccylinders in shell)

Single-walled nanotube (chirality)

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Nomenclature

Fiber (diameter 1 micron or above,

typically around 10 microns)Nanofiber (also called filament,

diameter below 1 micron, typically

0.1 micron or less)

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Skin structures

Smooth laminar structure

Rough laminar structure

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Bonding in graphite

In-plane:

covalent and metallic bonding

Out-of-plane:

van der Waals bonding

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Properties of graphite

Anisotropic

Easy shear between carbon layerslimiting the strength

High electrical and thermal

conductivity and high modulus in theplane of the carbon layers

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Reactivity of graphite

Reacts with oxygen to form gases

Functional groups on the edgesurface

Reacts with intercalates to form

intercalation compounds

Inert compared to many solids

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Carbon

Non-crystalline, turbostratic

Metastable form

graphitizes upon heating above

2000 degrees C.

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Properties of carbon

compared to graphite

Less conductive

Lower in modulus

Higher in strength

Lower in oxidation resistance

Cannot be intercalated

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Carbon fiber precursors

Pitch

Polyacrylonitrile (PAN)

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Carbon fiber

fabricationStabilization

Carbonization (pyrolysis)

Graphitization

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Grades of pitch

Isotropic

Mesophase pitch

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Grades of carbon fiber

High-strength carbon fiber

High-modulus carbon fiber

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Nanofiber group

morphology

IntertwinedParallel

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Fabrication of

carbon nanofibersCatalytic growth from

carbonaceous gas

Arc discharge

Laser evaporation

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Catalytic method

Carbonaceous gases: acetylene,

ethylene, methane, natural gas,benzene, etc.

Catalyst: iron, nickel, etc.

(particles typically 10 nm, fromsalts or organometallics)

Reducing gases: CO, hydrogen

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Advantage of catalytic

methodLow cost

Suitability for massproduction

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Disadvantage of

catalytic method

Catalyst particleremaining at the tip

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Structural

characterization

Diffraction (crystal structure, crystallite size,

texture)

Microscopy (microstructure)

Raman spectroscopy (disordered structure)

X-ray photoelectron spectroscopy (functional

groups on surface) Surface area and pore size analysis

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Raman scattering

Crystalline graphite

Disordered graphite

Crystalline diamond

Disordered diamond

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Microscopy

Transmission/scanning

electron microscopyAtomic force microscopy

(or scanning tunnelingmicroscopy)

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Electrical characterization

Single fiber volume electrical resistivity

Fiber compact volume electrical resistivity

Fiber composite volume electrical

resistivity

Fiber-matrix contact electrical resistivity Interlaminar contact electrical resistivity

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I1 V1

V2 I2

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I3

V3

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I1 V

1 V

2 V

3 V

4 V

5 V

6 V

7 V

8 V

9 V

10 I

2

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Mechanical characterization

Single fiber tensile testing

Single fiber pull-out testing

(shear bond strength measurement)

Single fiber fragmentation testing

Composite tensile testing

Composite flexural testing

Single fiber tensile

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Single fiber tensile

strengthCarbon fiber 3.5 GPa

Kevlar fiber 3.6 GPa

E-glass fiber 3.4 GPa

Steel 1.3 GPa

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Single fiber tensile

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Single fiber tensile

modulusCarbon fiber 230 GPa

Kevlar fiber 60 GPa

E-glass fiber 22 GPa

Steel 210 GPa

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Thermal characterization

Thermogravimetric analysis (weight

loss measurement upon heating)

Thermal expansion measurement

Specific heat measurement

Thermal conductivity measurement

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Difficulty of nanofiber

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Difficulty of nanofiber

characterizationDifficulty of testing an individual

nanofiber

Variation among nanofibers in thesame sample (particularly for single-walled carbon nanotubes)

High cost of nanofiber (particularlyfor single-walled nanotubes)

R d i

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Recommendation

Start with nanofibers that are

not single-walled carbonnanotubes

Compare with carbon fibersand other competing materials