Comprehensive nuclear materials 2 10 graphite properties and characteristics

Comprehensive nuclear materials 2 10 graphite properties and characteristics Comprehensive nuclear materials 2 10 graphite properties and characteristics Comprehensive nuclear materials 2 10 graphite properties and characteristics Comprehensive nuclear materials 2 10 graphite properties and characteristics Comprehensive nuclear materials 2 10 graphite properties and characteristics

AGR Advanced gas-cooled reactor

CTE Coefficient of thermal expansion

FoM Figure of merit

RMS Root mean square

K Ic Critical stress-intensity factor

K T Thermal conductivity at temperature T

l a Mean graphite crystal dimensions in the

a-direction

l c Mean graphite crystal dimensions in the c-direction

m Charge carrier effective mass

N Charge carrier density

a? Synthetic graphite coefficient of thermal expansion perpendicular to the molding or extrusion direction

D th Thermal shock figure of merit

g Cosine of the angle of orientation with respect

to the c-axis of the crystal

u D Debye temperature

l Charge carrier mean-free path

285

m Charge carrier mobility

Graphite occurs naturally as a black lustrous mineral

and is mined in many places worldwide This natural

form is most commonly found as natural flake graphite

and significant deposits have been found and mined in

Sri Lanka, Germany, Ukraine, Russia, China, Africa,

the United States of America, Central America, South

America, and Canada However, artificial or synthetic

graphite is the subject of this chapter

in a planar network in which the carbon atom is

apart in a given plane to form the hexagonal graphene

p-type are present, causing a shorter bond length

than that in the case of the tetrahedral bonding

orbital hybridization only) observed in

diamond Thus, in its perfect form, the crystal

(covalent) sheets of carbon atoms in a hexagonal

van der Waals type bonds in an ABAB stacking

sequence with a separation of 0.335 nm

Acheson in 1895 facilitated the development of the

process for the manufacture of artificial (synthetic)

polygranular graphite Excellent accounts of the

properties and application of graphite may be found

2.10.2 Manufacture

Detailed accounts of the manufacture of polygranular

Figure 2summarizes the major processing steps in

the manufacture of synthetic graphite Syntheticgraphite consists of two phases: a filler material and

a binder phase The predominant filler materials arepetroleum cokes made by the delayed coking process

or coal–tar pitch-derived cokes The structure, shape,and size distribution of the filler particles are majorvariables in the manufacturing process Thus, theproperties are greatly influenced by coke morphol-ogy For example, the needle coke used in arc furnaceelectrode graphite imparts low electrical resistivityand low coefficient of thermal expansion (CTE),resulting in anisotropic graphite with high thermalshock resistance and high electrical conductivity,which is ideally suited for the application Suchneedle-coke materials would, however, be whollyunsuited for nuclear graphite applications, where apremium is placed upon isotropic behavior (see

coke is usually calcined (thermally processed)

The calcined filler, once it has been crushed,milled, and sized, is mixed with the binder (typically

a coal–tar pitch) in heated mixers, along with certainadditives to improve processing (e.g., extrusion oils).The formulations (i.e., the amounts of specific ingre-dients to make a specified grade) are carefully fol-lowed to ensure that the desired properties areattained in the final products The warm mix is trans-ferred to the mix cylinder of an extrusion press, and

c 0.670 nm

0.246 nm a

Reproduced from Burchell, T D In Carbon Materials for Advanced Technologies; Burchell, T D., Ed.; Elsevier Science: Oxford, 1999.

the mix is extruded to the desired diameter andlength Alternately, the green mix may be moldedinto the desired form using large steel molds on avertical press Vibrational molding and isostaticpressing may also be used to form the green body.The green body is air- or water-cooled and thenbaked to completely pyrolyze the binder.

Baking is considered the most important step inthe manufacture of carbon and graphite The pitchbinder softens upon heating and goes through a liquidphase before irreversibly converting into a solid car-bon Consequently, the green articles can distort

or slump in baking if they are not properly packed

in the furnace If the furnace-heating rate is toorapid, the volatile gases evolved during pyrolysiscannot easily diffuse out of the green body, and itmay crack If a sufficiently high temperature is notachieved, the baked carbon will not attain the desireddensity and physical properties Finally, if the bakedartifact is cooled too rapidly after baking, thermalgradients may cause the carbon blocks to crack Forall of these reasons, utmost care is taken over thebaking process

Bake furnaces are usually directly heated (electricelements or gas burning) and are of the pit design.The furnaces may be in the form of a ring so that thewaste heat from one furnace may be used to preheatthe adjacent furnace The basic operational stepsinclude (1) loading, (2) preheating (on waste gas),(3) gas heating (on fire), (4) cooling (on air), and(5) unloading Typical cycle times are of the order of

stacked into the furnace and the interstices filled withpack materials (coke and/or sand) Thermocouplesare placed at set locations within the furnace to allow

Load furnace Power on

Cool

Unpack

Reload

Cool Unpack Repair Reload

Figure 3 Typical time versus temperature cycles for baking and graphitizing steps in the manufacture of graphite.

direct monitoring and control of the furnace

temper-ature More modern furnaces may be of the

car-bottom type, in which the green bodies are packed

into saggers (steel containers) with ‘pack’ filling the

space between the green body and the saggers

The saggers are loaded onto an insulated rail car

and rolled into a furnace The rail car is essentially

the bottom of the furnace Thermocouples are placed

within the furnace to allow direct monitoring and

control of the baking temperature

The furnaces are unpacked when the product has

cooled to a sufficiently low temperature to prevent

damage Following unloading, the baked carbons are

cleaned, inspected, and certain physical properties

determined The carbon products are inspected,

usually on a sampling basis, and their dimensions,

bulk density, and specific resistivity are determined

Measurement of the specific electrical resistivity is of

special significance since the electrical resistivity

correlates with the maximum temperature attained

during baking Minimum values of bulk density

and maximum values of electrical resistivity are

specified for each grade of carbon/graphite that is

manufactured

Certain baked carbon products (those to be

fur-ther processed to produce synthetic graphite) will be

densified by impregnation with a petroleum pitch,

followed by rebaking to pyrolyze the impregnant

pitch Depending upon the desired final density,

pro-ducts may be reimpregnated several times Useful

increases in density and strength are obtained with

up to six impregnations, but two or three are more

common The final step in the production of graphite

is a thermal treatment that involves heating the

Graphitization is achieved in an Acheson furnace in

which heating occurs by passing an electric current

throughout the baked products and the coke pack

that surrounds them The entire furnace is covered

with sand to exclude air during operation

Longitu-dinal graphitization is increasingly used in the

indus-try today In this process, the baked forms are laid

end to end and covered with sand to exclude air

The current is carried in the product itself rather

than through the furnace coke pack During the

terms, carbon atoms in the baked material migrate

to form the thermodynamically more stable graphite

lattice

Certain graphite require high chemical purity

This is achieved by selecting very pure cokes,

by including a halogen purification stage in themanufacture of the cokes or graphite, either duringgraphitization or as a postprocessing step Graphitemanufacture is a lengthy process, typically 6–9 months

in duration

Graphite structure is largely dependent uponthe manufacturing process Graphites are classified

(containing grains in the starting mix that are

forming process will tend to align the grains to impart

‘texture’ to the green body The extrusion processwill align the grains with their long axis parallel

to the forming axis, whereas molding and vibrationalmolding will tend to align the long axis of theparticles in the plane perpendicular to the formingaxis Thus, molded graphite has two perpendicularwith-grain (WG) orientations and one against-grain(AG) orientation, whereas extruded graphite hasone WG orientation (parallel to the billets longaxis) and two AG orientations Isostatically pressedgraphite does not exhibit a preferred orientation.Examples of various graphite microstructures are

in nuclear applications or been candidates for

used as the moderator in the earliest nuclear reactors

in the United States Pile grade A (PGA) graphite(Figure 5) was used as the moderator in the early air-cooled reactors and Magnox reactors in the United

next generation of high-temperature reactors GradeIG-110 (Figures 7 and 8) is the moderator material

500mm

AGOT graphite

Figure 4 Grade AGOT graphite microstructure (viewed under polarized light).

500 mm

PGA graphite WG

Figure 5 Grade PGA graphite (with-grain) microstructure

(viewed under polarized light).

500mm

NBG-18 graphite WG

Figure 6 Grade NBG-18 graphite (with-grain)

microstructure (viewed under polarized light).

500mm

IG-110 graphite

Figure 7 Grade IG-110 graphite microstructure (viewed

under polarized light).

in the high-temperature test reactor in Japan, and

grade 2020 graphite (Figures 9 and 10) was a candidate

for the core support structure of the modular

high-temperature gas-cooled reactor in the United States

shows the structure of AGOT graphite, an extruded

medium-grained, needle-coke graphite (maximum

struc-ture of IG-110 graphite, an isostatically pressed,

Similarly, grade 2020 (Figures 9 and 10) is also

a fine-grained, isostatically pressed graphite The

UK graphite PGA is extruded needle-coke graphite

individual needle-coke filler particles (named needle

coke because of their acicular structure) can clearly

be distinguished in this graphite Another dominant

feature of graphite texture can easily be distinguished

inFigure 5, namely porosity Graphite single

the porosity that is distributed throughout the

to the surface, while the remainder is closed In the

case of PGX graphite, large pores in the structure

result in relatively low strength The formation of

pores and cracks in the graphite during manufacture

adds to the texture arising from grain orientation and

causes anisotropy in the graphite physical properties

Three classes of porosity may be identified in

syn-thetic graphite:

 Those formed by incomplete filling of voids in the

green body by the impregnant pitch; the voids

originally form during mixing and forming

 Gas entrapment pores formed from binder

phase pyrolysis gases during the baking stage of

manufacture

 Thermal cracks formed by the anisotropic

shrink-age of the crystals in the filler coke and binder

Isotropic behavior is a very desirable property in

Effects in Graphite) and is achieved in modern

isotropic structure in the initial formulation

Coke isotropy results in large measure from the

optical domain structure of the calcined coke The

optical domain size is a measure of the

extended-preferred orientation of the crystallographic basal

planes Essentially, the optical domain size and

struc-ture (domains are the isochromatic regions in the

coke and binder revealed when the structure isviewed at high magnification on an optical micro-scope under polarized light) controls the isotropy

of the filler coke Anisotropic ‘needle’ cokes haverelatively large extended optical domains, whereas

‘isotropic’ cokes exhibit smaller, randomly orientateddomains The domain structure of a coke is devel-oped during delayed coking through pitch pyrolysischemistry (mesophase formation) and coking trans-port phenomena At the atomic scale, orientation

of the crystallographic structure is characterizedusing X-ray diffraction analysis The crystal spacingwithin the graphitized artifact may be determined

extent to which the basal planes are parallel to one

mean height over which the layers are stacked in a

define the perfection of the crystal (contained withinthe graphitized coke and binder) and the degree ofgraphitization

An important feature of artificial graphite ture, which has a controlling influence upon thematerial properties, is that the structural featuredimensions span several orders of magnitude Thecrystal lattice parameters are fractions of a nanometer

thermal microcracks between planes are typicallythe size of crystallites Within the graphite, the opti-cal domain (extended orientation of crystallites) may

the isotropy of synthetic graphite As discussed lier, graphite grain size (usually refers to largest fillerparticles) is a manufacturing variable and is typically

depending upon the category and location (porescould be within filler or binder phases) is commensu-rate with grain size The largest pores (excluding ther-mal cracks between the crystal layers) are typically

2.10.3 Physical Properties

The thermal behavior of a solid material is controlled

by the interatomic forces through the vibrationalspectrum of the crystal lattice The properties are

generally insensitive to the spectrum details because

they derive from a wide range of wavelengths in the

spectrum However, the graphite crystal is highly

anisotropic because of the in-plane, strong covalent

bonds and out-of-plane, weak van der Waals bonds,

so the above generalizations are not necessarily

applicable Moreover, the electronic contributions to

the thermal behavior must be considered at low

temperatures

A complete and comprehensive review of the

thermal properties of graphite has been written by

the form of lattice vibrations These vibrations are

considered to be standing waves and thus can only

have certain permitted frequencies (density of states

of waves) These waves produce atomic

displace-ments, which can be resolved so as to be parallel to

the wave vector (longitudinal waves) and in two

directions perpendicular to it (transverse waves)

The Debye equation thus gives the specific

allow-ing the upper limit in the integral to go to infinity

/15), and ondifferentiating we get

Thus, at low temperatures, the specific heat is

(eqn [2] At high temperatures,z is

hence on integrating we get the Dulong–Petit value

con-cerned only with the specific heat at temperatures

specific heat should rise exponentially with

temperature The specific heat of graphite is shown

in Figure 11 over the temperature range 300–

3000 K Experimental data have been shown to be

all graphite

11:07T1:644þ 0:0003688T0 :02191 Jkg1K ½3The hexagonal graphite lattice has two principal

expansion coefficient parallel to the hexagonal

the crystal parallel to the basal plane (a-axis) Thethermal expansion coefficient in any direction at an

constant at temperatures up to 2500 K The thermalexpansion coefficients of synthetic graphite are a func-tion of the (1) crystal anisotropy, (2) the orientation

of the crystallites (i.e., textural effects arising duringmanufacture), and (3) the presence of suitably orientedporosity A billet of molded or extruded graphitewould exhibit the same symmetry as the graphitecrystal due to alignment of the crystallites duringthe forming process, with the thermal expansion

(perpendic-ular to the molding or extrusion direction), tively However, the thermal expansion coefficients ofpolycrystalline graphites are typically significantly

was the first to associate this phenomenon with thepresence of pores and cracks in the polycrystallinegraphite that were preferentially aligned with thegraphitic basal planes, thereby preventing the high

400 800 1200 1600 2000 2400

Figure 11 The temperature dependence of the specific heat of graphite, a comparison of calculated values and literature data for POCO AXM-5Q graphite Sources: ASTM C781, 13 data from Hust, J G NBS Special Publication 260-89; U.S Department of Commerce, National Bureau of Standards, 1984; p 59.

c-axis crystal expansion from contributing fully to the

observed bulk expansion The thermal closure of

aligned internal porosity results in an increasing

instantaneous and mean CTE with temperature and,

significantly, an increasing strength with temperature

iso-tropic graphite, the CTE more closely approaches the

graphite crystallite value

Figure 12illustrates the above thermal expansion

the average CTE All three graphite grades (Poco,

PCEA, and IG-110) show increasing expansivity with

increasing temperature due to the thermal closure of

internal porosity PCEA, the extruded grade, displays

greater thermal expansion and a greater average

CTE in the AG direction than in the WG direction,

reflecting the preferred orientation of the filler-coke

particles due to the forming process Poco graphite is

particularly high-density isostatically pressed

graph-ite (hence the relatively large expansion and CTE)

PCEA is a medium-grain extruded grade and clearly

displays different thermal expansion behavior ing upon the orientation (WG or AG) Grade IG-110

depend-is a fine-grain, depend-isostatically pressed grade but ddepend-isplays alower density (and CTE) than Poco graphite.Graphite is a phonon conductor of heat Conse-quently, the thermal conductivity of a graphite singlecrystal is highly anisotropic, reflecting the differentbond types within and between the carbon basal

the basal plane), the atom bonding is of the primary,covalent type, whereas between the basal planes

much weaker secondary or van der Waals type nons (elastic waves) may thus travel considerably

within a graphite single crystal

conductivity of natural and pyrolytic graphite (singlecrystal similes) The room-temperature thermal con-ductivity parallel to the basal planes is typically

planes, the room-temperature thermal conductivity

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0 (a)

Figure 12 Thermal expansion behavior of various graphite grades (a) thermal expansion versus measurement

temperature and (b) average coefficient of thermal expansion verses temperature.

is typically<10 W m1K The thermal conductivity

of graphite shows a maximum with temperature

At higher temperatures, above the maxima, the

thermal conductivity decreases with increasing

tem-perature due to phonon scattering Measurements on

the maxima in thermal conductivity parallel to

max-imum thermal conductivity (perpendicular to the

basal planes) for a pyrolytic graphite to be located

con-ductivity is dominated by an electronic contribution

that is proportional to temperature

The temperature dependence of the in-plane

various forms of pyrolytic graphites Substantial

improvements in thermal conductivity caused by

thermal annealing and/or compression annealing

are attributed to increased crystal perfection and

increases in the size of the regions of coherent

ordering (crystallites), which minimizes the extent

of phonon-defect scattering and results in a larger

phonon mean-free path With increasing

tempera-ture, the dominant phonon interaction becomes

phonon–phonon scattering (Umklapp processes)

Therefore, the observed reduction in thermal ductivity with increasing temperature and the con-

to the dominant effect of Umklapp scattering inreducing phonon mean-free path

A popular method for determining the thermalconductivity of carbon and graphite is the ther-mal ‘flash’ technique in which a small specimen isexposed to a thermal pulse, usually from a xenonflash lamp or a laser, and the back face of the specimenobserved with an infrared detector The specimen’sthermal conductivity is then determined from theback face temperature-rise transient The thermal

Figure 14shows data for the temperature dence of thermal conductivity of typical near-isotropic synthetic graphite The data were obtainedusing the laser-flash method over the temperature

illustrate the reduction of thermal conductivity withincreasing temperature and textural effects in anextruded graphite because of filler-coke orientation

The electrical conductivity, s, for a given group of

Figure 13 The temperature dependence of thermal

conductivity for pyrolytic graphite in the as-deposited,

annealed, and compression-annealed condition.

Data from Roth, E P.; Watson, R D.; Moss, M.; Drotning,

W D Sandia National Laboratory Report No

SAND-88-2057, UC-423; 1989.

30 50 70 90 110 130 150 170

Figure 14 The temperature dependence of a typical extruded synthetic graphite in the with-grain (parallel to extrusion) and against-grain (perpendicular to extrusion) directions.

whereN is the charge carrier density, q the electric

charge, m the carrier mobility, t the relaxation time,

The relaxation time, (t), is related to the carrier

mean-free path, l, and is defined as the time elapsed

between two collisions, such that

the charge carrier velocity at the Fermi surface The

carrier mean-free path is the distance between two

scattering centers

In synthetic graphite, the dominant charge

car-riers are electrons, and the dominant scattering

effects are intrinsic (phonon–electron scattering) at

(lattice defects, crystallite edges, irradiation induced

In tion to electrons being scattered by defects, vacancies

addi-may act as electron traps

The influence of intrinsic and defect scattering

is approximately independent of temperature An

important parameter for characterizing scattering

well-ordered graphite is effectively controlled by

For industrial applications of graphite, such as arc

furnace electrodes and aluminum smelting cell

cath-ode blocks, the electrical resistivity is an important

parameter Excess power consumption due to the

resistance of the graphite will impact the economics

effect of temperature on the electrical resistivity

For well-graphitized materials, the resistivity is seen

to initially fall with increasing temperature,

reach-ing a minimum at 800–1000 K, the resistivity then

increases in an almost linear fashion to temperatures

electrical resistivity also initially falls as

tempera-ture increases However, the scattering of charge

temperature-dependent Initially, scattering is dominated by

extrinsic defects, but as the mean-free path for sic scattering becomes comparable with that fromdefect scattering, an increase in resistivity will be

carrier mobility outweighs the increase in charge rier density, and resistivity is seen to increase inFigure 15

car-Other factors may influence the electrical ity of synthetic graphite, such as anisotropy in thefiller particle and texture (orientation and distribu-tion of filler particle and porosity) Contributions tothe overall resistivity will come from the binderphase and the interface resistance between filler-coke and binder phase Increases in crystallinity ofthe graphite, through judicious selection of raw mate-rials, or higher graphitization temperatures, willdecrease the room-temperature electrical resistivity.Generally, the greater the room-temperature resis-tivity, the greater the temperature at which the resis-tivity minima occurs; the steeper the initial decrease

resistiv-in resistivity, the larger the magnitude of the resistiv-initialdrop in resistivity Typical physical properties forseveral graphite grades with a range of filler-particle

2.10.4 Mechanical Properties

The bulk density of synthetic graphite varies ing to the manufacturing process Increases in den-sity are achieved by utilizing fine filler particles(although this limits the forming size) Also, theforming method and number of impregnations can

accord-0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0