ISOTROPIC GRAPHITE FROM NEEDLE COKE

Doug Miller, Irwin Lewis*, Mark SantanaGrafTech International Ltd., 12900 Snow Road, Parma, OH 44130, USA

*Consultant Key Words: needle coke, isotropic coke, optical domain, coherent domain, nuclear

Corresponding author e-mail address: Doug.Miller@graftech.com

Introduction

Isotropic high coefficient of thermal expansion (CTE) graphites, such as those used for nuclear reactors, are typically produced using an isotropic coke filler. Such cokes are generally less graphitizable with lower aspect ratios than needle cokes used for producing graphite electrodes. Since high isotropy is a required property for nuclear graphites, they are produced through conventional processing using isotropic coke and pitch binder. However, it has been suggested that a high degree of graphite crystallinitywould be advantageous for isotropic nuclear graphite [1, 2, 3]. Such a result can be achieved by using a needle coke filler with modified processing. GrafTech has used this approach to produce nuclear graphite by a process known internally as the “BAN” process, which stands for British Acheson Nuclear [4].

The objective of this study was to investigate the structural effects at different stages of the BAN process and to explore the interaction between crystalline structure, anisotropy and product CTE. For this purpose, a series of graphite artifacts was produced using a single source of electrode grade needle coke at the green and calcined state.Processing was varied to produce low CTE anisotropic and high CTE isotropic samples. Filler particles and finished graphite samples were evaluated by optical microscopy and x-ray diffraction (XRD).

Experimental

I. Graphite Artifact Production - The following 5 steps define the BAN processing procedures used in this study.

1. Mill raw needle coke through 325 mesh to produce filler. (A sample of the same coke was “batch” calcined in the laboratory and processed for comparison).

2. Mix the fillers with coal tar pitch binder and extrude green rods. (All extrusions were 19 mm diameter).

3. Bake the rods from step 2 to ~ 900 C.4. Crush and mill the baked rods to produce particles and flour.

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5. Use the flours from step 4 as filler to produce graphite. (The rods were extruded, baked, and graphitized to 3000 C.Because of the small scale of the experiments, “all flour” mixes were used. The flour was sized to at least 50% passing 200 mesh).

II. Structural Studies

Bright field and polarized light microscopy were used to examine the flour particles at each step of the process. The aspect ratio of the step 1 flour particles was determined by scanning electron microscopy with automated image analysis. Samples of flour from steps 1 and 4 and rods from step 3 were graphitized to 3000 degrees so that latticeparameters could be measured by XRD. Graphitized step 3 rods and the BAN process graphite samples from step 5 were examined by optical microscopy and XRD and also measured for coefficient of thermal expansion (CTE) (25-100 C). With-grain CTEs weredetermined using GrafTech’s capacitance bridge method [5]. Against-grain CTEs were estimated using thermomechanical analysis.

All samples for XRD were prepared as powders. Step 1 powders were cast in epoxy to minimize orientation of the anisotropic particles. Graphitized rods from steps 3 & 5 were also measured as machined specimens with 3 selected orientations. The reported results are average measurements for all sample configurations.

Results and Discussion

Polarized light optical microscopy shows that particles fractured along domain lines duringcalcining. Figures 1 and 2 below show calcined and uncalcined particles.

Figure 1. Calcined particle. Figure 2. Raw particle.

When the calcined particles were milled, the cracks tended to propagate. This resulted in particles with a higher aspect ratio than those produced by milling raw coke, since the raw coke particles fractured across optical domains. Figure 3 shows the -325 mesh flour derived from the calcined coke. Figure 4 shows the -325 mesh flour from the raw coke.

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Figure 3. -325 mesh calcined coke (aspect ratio = 3.7).

500x Bright Field 500x Polarized Light

Figure 4. -325 mesh raw coke (aspect ratio = 2.3).

500x Bright Field 500x Polarized Light

X-ray diffraction measurements on graphitized samples of -325 mesh flours (step 1) and of graphitized step 3 artifacts are given in the Table I below. The differences in La and Lc between the raw and calcined flour are within the scatter of the data. The differences in aspect ratio are significant.

Table I Properties of Graphitized Step 1 Flours and Graphitized Step 3 Rods

Sample Description d (002) Lc

(002)

AveLa (110)La (100)

ParticleAspectRatio CTE wg*

Raw -325 Flour (graphitized) 3.363 967 412 2.3

Calcined -325 flour (graphitized) 3.363 733 660 3.7

Raw -325 Rod (graphitized) 3.368 464 568 2.08

Calcined -325 Rod (graphitized) 3.366 516 474 0.43*(in/in x 10-6/ C, 25-100 C)

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When step 3 baked rods were milled to >50% passing 200 mesh, the raw coke and es 4

igure 4. Step 4 flour particle from calcined Figure 5. Step 4 flour particle from raw

calcined coke derived materials were distinguishable by microscopy as seen in Figurand 5 below.

Fcoke (aspect ratio = 1.8). coke (aspect ratio = 1.7).

500x Polarized Light 500x Polarized Light

he flours from step 4 were mixed with binder pitch, extruded, baked and graphitized to

e

Table II Comparison of Step 5 Prod Raw and Calcined Cokes

T3000 C (step 5). The lattice parameters and CTE were measured on these products. The results presented in Table II show that the lattice parameters are comparable for thtwo materials despite the large difference in CTE.

ucts from

Sample Description d (002) Lc (002)

AveLa (110)La (100) CTE wg* CTE ag*

Raw Coke BAN Rod 3.369 410 452 3.59 <4.0

Calcined Coke BAN Rod 3.37 435 440 1.79 <4.0*(in/in x 10-6/ C, 25-100 C)

icroscopy of the BAN rods reveals more orientation in the calcined coke derived cles.

Mmaterial. This is consistent with the examination of the step 1 and step 4 flour partiFigures 6 and 7 below show the step 5 products from calcined and raw cokes respectively.

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Figure 6. Calcined coke - BAN process rod. Figure 7. Raw coke - BAN process rod.

500x Polarized Light 500x Polarized Light

attice parameters for the BAN process rods indicate somewhat lower crystallinity than

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Table III BAN Process Needle Coke vs. C nally Processed Isotropic Coke

Lfor the starting flours that were graphitized. This may be due to the binder not being as well graphitized as the needle coke filler, the repeated milling operations, or strain on particles due to shrinkages during baking and graphitizing. The raw coke derived BANprocess material has a CTE of 3.6. GrafTech nuclear graphite grade PCEA, which is produced using isotropic coke and conventional processing, has a CTE of 3.5. Table Ibelow compares the BAN processed graphite with grade PCEA.

onventio

Sample Description d (002) Lc (002) CTE wg CTE ag

Raw Coke BAN Rod 3.369 410 3.59 <4.0

PCEA 3.376 343 3.5 3.7*(in/in x 10-6/ C, 25-100 C)

he data indicates that BAN processed needle coke has resulted in a graphite with more

nother indication of higher crystallinity for the BAN graphite when compared to other of

Tcrystalline perfection than isotropic nuclear graphite with comparable CTE.

Acommercial graphites is shown in Figure 8 where Lc is plotted vs. CTE for a wide rangegraphites. The BAN graphite appears to deviate from the correlation by showing a higherCTE for its Lc value.

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Figure 8. Comparison of Lc and CTE for a range of commercial graphites.

250

300

350

400

450

500

550

600

650

700

-1.000 0.000 1.000 2.000 3.000 4.000 5.000

CTE

Lc

(002

)

Lc (002)Raw BANLinear (Lc (002))

Figure 9 compares the optical textures of the BAN and PCEA graphites. It is evident that the optical domain size of the BAN processed graphite is much larger than that of the PCEA graphite produced with isotropic coke.

Figure 9. Comparison of optical domains for BAN processed raw needle coke and grade PCEA graphite derived from isotropic coke. The graphites have equivalent CTEs.

PCEA 400x PL(largest particles are 800 microns, largest domains are 10 microns)

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BAN 400x PL (largest particles are 60 microns, largest domains are the size of particles)

Conclusions

The BAN process produced high CTE graphite from needle coke. Milling the coke in the raw state was a key factor. The aspect ratio of milled raw particles was less than milled calcined particles. Lattice parameters were the same for raw and calcined derived samples. There appears to be more crystalline perfection in BAN processed material than for graphite produced from semi-isotropic coke having similar CTE. The optical domain size of BAN processed material was much larger than nuclear graphite made from isotropic coke with similar CTE.

References

[1] Kelly B.T. and Brocklehurst J. R., “ High Dose Fast Neutron Irradiation of Highly Oriented Pyrolytic Graphite”, Carbon 9, 783, (1971).

[2] Engle G. B., “Irradiation Behavior of Nuclear Graphite at Elevated Temperatures”,Carbon 9, 539, (1970).

[3] Thrower P., “The Structure of Reactor Graphites and its Relation to High Temperature Irradiation Dimensional Stability”, Carbon 9, 265, (1970). [4] Bradwell K., Harvey J.,”Improvements in the Production of Graphite”, British Patent

1,098,882, January 1968. [5] Wagoner G., Sprogis G., and Proctor D. G., “Capacitance Bridge Measurements of

Thermal Expansion”, Carbon ’86. Baden-Baden, June 30-July 4, 1986, P. 234-236.

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