Graphite - Structure, Properties and Manufacture
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Transcript of Graphite - Structure, Properties and Manufacture
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.