Graphite: Structure, properties and manufacture

Brian Rand

NUCLEAR GRAPHITE MATERIALS TECHNOLOGY –

April 2009

Bonding and the forms of solid carbon

sp -

Carbynes

sp2

-

Graphite

sp3

-

Diamond

π

electrons delocalised over the layer planes

CRYSTALLINE FORMS

GRAPHITE DIAMOND

• Graphite has shorter bond length in the plane• Graphite has stronger ‘in-plane’

bonding

Hexagonal Rhombohedral

CRYSTALLINE GRAPHITE

HIGHEST AND LOWEST BOND ENERGIES-

in 'a' and 'c' directions

GREATEST ANISOTROPY IN PROPERTIESYOUNG MODULUS: -

‘a’

axis = 1050GPa (c11

); ‘c’

axis = 36GPa (c33

)SHEAR MODULUS: ~ 4GPaEXPANSION COEFFICIENT: 'a' ~ -1 ×

10-6; 'c' ~ 30 ×

10-6

THERMAL CONDUCTIVITY: 'a' > 2000 Wm-1K-1

ELECTRICAL RESISTIVITY 0.4 -

0.8 μohm.m

)cos1 (

)1()2(11 224413

433

2211

ϕγ

γγγ

−=

−+++=

where

sss)-γ(sE

GRAPHITE ELASTIC CONSTANTS

• C-axis elastic constant (c33

)• Inter-layer shear (c44

)

DOMINATE AT ALL BUT NARROWEST ANGLES

Therefore generally low moduli except for highly ‘a’

axis

aligned structures, e.g. fibres

‘c’

axis (φ) misalignment

THERMAL EXPANSION• c-AXIS EXPANSION COEFFICIENT, VERY HIGH

•

a-AXIS EXPANSION COEFFICIENT, NEGATIVE TO SLIGHTLY POSITIVE ABOVE ABOUT 300oC

•

SYNTHETIC CARBONS/GRAPHITES HAVE VERY DIFFERENT COEFFICIENTS

SPECIFIC HEAT

From Burchell

Temperature dependence well characterised

THERMAL CONDUCTIVITY

path freemean is locityphonon vel average is

heat specific is C where31

ν

νκ lC=

For given solid specific heats and phonon velocities are same

Therefore conductivity is directly proportional phonon mean free path

For large crystals, where boundary scattering is unimportant, thermal conductivity should follow specific heat –

at low temperatures.

At higher temperatures, phonon-phonon scattering becomes important and conductivity falls

Graphite Oxidation• Basal plane is unreactive

•

Reaction occurs at exposed edge atoms

•

Even within the plane the oxidation rate is anisotropic

•

Etch pits bordered by ‘zig-zag’

or ‘armchair’

planes

•Relative reactivity can change with temperature

Catalyst particles may act on the exposed edges

SUMMARY -

GRAPHITE

• Most anisotropic crystal

•Strongest bonding ‘in-plane’, weakest in c-direction

•High ‘in-plane’

elastic modulus

•High ‘in-plane’

thermal conductivity

•Low ‘in-plane’

expansion coefficient

•Semi-metallic conductor

•

Basal plane is low energy surface –

non-wetting to polar species

•Oxidation is anisotropic –

reaction from edge sites

• Poor shear properties

FABRICATION OF CARBON MATERIALS

CONVENTIONAL METHODS OF FABRICATION SUCH AS SINTERING NOT POSSIBLE because of ultra-strong bond strength, limited atomic mobility at all but highest temperatures

Therefore must use CONTROLLED PYROLYSIS & HEAT TREATMENT OF

APPROPRIATE ORGANIC PRECURSORS

•

Gas Phase•

Liquid Phase

•

Solid Phase DIFFERENT MORPHOLOGYSTRUCTURE& PROPERTIES

CONTROL OF CRYSTAL AND MICRO- STRUCTURE

PYROLYSIS PRODUCTS•

Mostly sp2

hybridisation•

Aromatic layers

BUTgrossly defectivetwisted/distorted

•

Properties different from graphite

MICROSTRUCTURE•

‘Crystallinity’

•

Domains of oriented structure

•

PoresSize distribution

&orientation

(pore shape)

FEATURES

•Diffuse bands in some of the positions of graphite lines

•No. of bands much smaller than no. lines of graphite

•No bands corresponding to hkl diffractions, where both h (or k) as

well as l are non-zero –

i.e. 002, 004 or 100, but no 101

•No stacking sequence, 3D ordering

•Bands are very broad indicating nano-sized diffracting regions -

x-ray

coherence lengths (‘crystallite’ dimensions)

Structure of non- graphitic carbons

GRAPHITE & CARBON STRUCTURES

5 and 7 membered rings bend layers in

different directions

Schematic of 2 piles of layers: L2 is the real layer extent, L, the defect-free part, N the number of layers coherently stacked, la the coherent length in-plane and lc the coherent length along the stacking direction (dashed: coherent domain).

Graphitising Carbons

Oberlin, A. Oberlin, A. Carbon. Carbon. (1984)(1984)

The Marsh/ The Marsh/ Griffiths modelGriffiths model

As the ordering increases, As the ordering increases, properties along the basal axis properties along the basal axis improve;improve;••Thermal / electrical Thermal / electrical

conductivityconductivity••Elastic modulusElastic modulus

LLccLLaa

LMO –

Local molecular ordering of the BSUs

(Basic Structural Units) – VERY IMPORTANT FOR

GRAPHITISATION.

High Resolution Images of a Graphitisation Series

200200ooCC

27302730ooCC 20002000ooCC

600600ooCC 12001200ooCC

All images All images show (002) show (002) fringes.fringes.

Growth in layer stack parameters(x-ray coherence lengths –

“crystallite dimensions)

N.B. Coherence lengths are still in the nano-scale, even after graphitisation!

Carbon/graphite areTHE ULTIMATE

NANOSTRUCTURED MATERIALS!!

bcosθkλ

cL =

NON-GRAPHITISING CARBONS e.g. from Phenolic

Resin

Some carbons do not develop the graphitic structure even when heated to 3000oC.

There is some peak narrowing, but no extensive increase in coherence lengths in either direction.

No stacking sequence

Effect of HTT on Young ModulusPerfection and stiffening of the layer planes

For bulk graphite this is microstructurally

controlled and varies enormously!

pitchPAN

For highly oriented samples, relatively pore-free, the effect of basal plane perfection

(e.g. increase in La

) can be observed and E approaches the theoretical value

Kipling et al

Helium (true solid?) Density

Graphitisable

precursor

Graphitisation ParametersFranklind = 3.440 -0.086(1-p) –

0.064p(1-p)

p = proportion of misaligned layersp < 0.25

Bacond = 3.440 -

0.086 (1-p2)

Turbostratic

Raman Spectroscopy -1

E2g

modes are active in Raman

Raman Spectroscopy -

2

1580 cm-1

peak narrows with graphitisation

1360 cm-1

peak is a disorder peak and gradually disappears

Raman Spectroscopy -

3Relative intensity of two peaks changes on graphitisation.

Can be used empirically to estimate the a direction coherence length, La

Effect of heat treatment temperature on electrical resistivity

Rapid drops in resistivity

1.

On conversion from organic solid to carbon

2.

after Heat Treatment above 2000ºC when graphitisation

begins

Shape of Thermal Conductivity -

Temperature Curves

•

The peak occurs when the mean free path due to Umklapp

scattering becomes larger than the grain

size and grain boundary scattering begins to dominate

•

As the crystallite size increases the thermal conductivity peak occurs both at a lower temperature and has a numerically larger value

(Kelly & Gilchrist 1969)

(Taylor et al 1968)

Temperature

K

~100K

0

100

200

300

400

500

600

700

800

200 400 600 800 1000 1200 1400Temperature (K)

Ther

mal

Con

duct

ivity

(Wm

-1K

-1)

K1100P100P55P25HMAS4

Axial Thermal Conductivity of Fibres

•

Graphite split collar and retaining ring

•

Approx. 20 % volume fraction

Illustrating the effect of increasing the ‘a’

axis coherence length

Increasing HTT

Large polygranular

(polycrystalline) graphite blocks –

manufacture

•

Need graphitisable precursors •

Gas phase deposits limited in dimensions possible –

tend to

have large anisotropy•

Therefore use coke particles bonded with coal tar or petroleum based pitch

•

Coke produced from coal tar pitch or petroleum residues•

Coke directly from coals –

impure and do not graphitise

well

•

Need to be free from inorganic impurities •

Carbonisation of organic precursor has ~ 50% mass loss as volatiles PLUS increase in density from about 1.35 tp

2.0 gcm-3

•

Therefore massive volumetric shrinkage•

Coke is carbonised

(calcined) at ~1200ºC when most shrinkage

has taken place.•

Organic precursor must give appropriate microstructure and morphology as well as being graphitisable

“Green”

Coke Pitch binder

“Calcined”

Coke

Coke filler particle distribution

Blended coke-pitch

mixture

Shaped coke-pitch

blocks

Carbonised

block

Graphitised

block

Re-impregnated block

Heat 1200-1400ºC

Grinding & classification

Mixing at T~ 150ºC

Moulded/extruded

Carbonised

`1000-1400ºC

Graphitised

>2750ºC

PURIFIED GRAPHITE BLOCK

Ready for machiningHigh temperaturehalogenation

Coke filler factors

•

Coke structure and composition

•

Structure means crystallographic and microstructure (texture)

•

Coke ‘grain’

size distribution selected according to block dimensions, space filling ability and rheology appropriate to shaping technique

Pitch Factors

•

Graphitisability

•

Carbon yield

•

Rheology

•

Different for binder and impregnant

•

Optimum binder content

Optimum to binder content

Too low, insufficient bond.

Too high, porosity due to volumetric shrinkage during carbonisation and ‘bubble’

like pores.

Binder content %

Prop

erty

, e.g

. You

ng’s

Mod

ulus

Coke textures

Mesophase deformation during flow gives ‘needle’

coke

characteristics and elongated, anisotropic particles

Coke from coal extract

More reactive mesophases

give more isotropic and finer textures

a c

b d Figure 4.1.1. Optical micrographs of (a) PGA (reflected light) and (b) PGA (polarised light) showing large filler grains. Note the orientation of the individual grains revealed by interference colours and texture within these grains; (c) Gilsocarbon (reflected light) and (d) Gilsocarbon (polarised light) showing large spherical filler grains. Note the resin-filled pores between grains (NC = needle-coke grain; SG=spherical grain; LC=lenticular crack; B=binder; P=pore; GP=globular pore; R=resin)

BULK GRAPHTE MICROSTRUCTURES

HOW DO THESE OPTICAL TEXTURES ARISE?

•

Organic precursors comprise complex mixtures of large polyaromatic

molecules which are approximately planar

•

At temperatures around 400ºC they stack up to form a liquid crystalline phase –

discotic

nemartic

liquid crystal

•

The liquid crystal forms domains in which there is long range preferred orientation of the molecules

•

This domain texture can be viewed by quenching the sample, polishing and viewing under polarised

light in the optical

microscope.•

As temperature increases the molecules condense to form larger units which then become immobile. At this point the texture is fixed and further heat treatment acts to remove the heteroatoms

and

perfect the lamellar array as shown earlier•

The liquid crystalline region is critical to the control of the texture of the coke filleand

the coke that develops from the binder pitch

b

d e

50 µm

a

Mesophase nucleation, growth and coalescence with gradually increasing temperature

After Mochida et al

SCHEMATIC REPRESENTATIONOF MOLECULAR ORGANISATIONWITHIN MESOPHASE SPHERES

SHEAR DEFORMATION can orient mesophase

Deformation is retained if material is cooled more rapidly than relaxation processes (temperature dependent)

Leads to preferred orientation in cokes, fibres matrix in C/C composites

Deformation around bubbles of volatile matter as they rise through the pyrolysis liquid

undeformed

sheared

Coke textures

Mesophase deformation during flow gives ‘needle’

coke

characteristics and elongated, anisotropic particles

Coke from coal extract

More reactive mesophases

give more isotropic and finer textures

Disclinations

in mesophase are retained in the coke/carbon and carried through to the graphite

N.B. Note the dimensions of the oriented domains, tens to hundreds of microns

X-ray coherence lengths are up to 100nm

Coke texture and disclinations

have a strong influence on crack development and propagation

Shrinkage cracks during carbonisation and on cooling may follow lamelliform

texture.

THERMAL EXPANSIONExpansion coefficients very different from crystal values

Partly because of lower macroscopic anisotropy or for nuclear graphite, ideally isotropic behaviour

Also ‘c’

axis expansion is partly accommodated in the lamellar fissures, ‘Mrozowski’

cracks resulting from anisotropic shrinkage during cooling

or carbonisationExamples are extruded graphites

with preferred orientation of ‘needle-like’

coke grains

Range of moduli and strengths of carbon/graphite materials

Wide variation due to varying preferred orientation, graphitic character and porosity

Polygranular

graphites have relatively low

modulus and strength due to porosity, random orientation of lamellar regions (grains) and two phase nature of the composite

The term Grain in this context refers to the coke filler particles not the ‘crystallites’

themselves

Stress-strain behaviour of synthetic graphite –

plastic deformation

Non-linearity and hysteresis

in stress-

strain plot for graphite with evidence of permanent ‘set’. Occurs both in tension and incompression

Due to dislocation movement and basal plane shear

Fracture of graphite (three point loading)

0

20

40

60

80

100

120

0.0 0.1 0.2 0.3 0.4 0.5Displacement, u (mm)

Load, P (N

)

IG110

Gilsocarbon

Ucar

(parallel)

Ucar

(perpendicular)

3-point bending

•Non-catastrophic fracture behaviour

•Residual strength after maximum stress

•Relatively large work of fracture

•Results from crack deflection and weak interfaces

From Fazluddin

PhD thesis

Crack propagation is mostly around grains, where there is relatively weak bonding

Typical tortuous crack pattern in graphite

Propagation around grain and frictional effects during opening

Propagation through ‘needle’

grain

Filler grains protrude from fracture surface

Effect of grain and pore size on strength (after Burchell)

500μm20μm

4μm

As a general rule graphites

with larger maximum grain (filler particle) sizes tend to have lower strengths

Strength and modulus increase with temperature

Anisotropic contraction on cooling opens cracks parallel to layer planes, which affect modulus and strength.

Expansion into cracks on reheating closes them, increasing the modulus and strength.

Conclusions•

Graphite is intrinsically anisotropic in its crystal structure and its properties

•

Synthetic graphite, in bulk form, is a composite produced from coke-pitch mixtures.

•

The two phases both graphitise

but the degree of graphitisation may differ. The dimensions of oriented domains differ

significantly for the two phases.•

The final heat treatment is greatly important to the degre

of

graphitisation.•

The bodies produced are porous and contain lamellar micro-

cracks oriented wrt

the graphene

layers.•

These cracks are of great significance leading to reduced coefficient of expansion and mechanical properties increasing with measurement temperature.

•

As will be seen they are also critically important in the irradiation behaviour.