Microstructural characterisation and thermal properties of model high

temperature reactor graphite composites

Department of Chemistry, IAM, University of Pretoria

Energy Postgraduate Conference 2013, 11-14 August 2013iThemba Labs, Cape Town

Mr. Philemon Magampa (Ph.D student)Supervisor: Prof. N. Manyala

Co-supervisor: Prof. W. Focke

INTRODUCTION

Graphitic composites made from isostatically moulded compound containing natural flake graphite (64 wt.%), synthetic graphite (16 wt.%) and a phenolic novolac binder (20 wt.%) are used together with several layers of various forms of pyrolytic carbon and SiC as encasement of the fuel particles (see Figure 1). They retain the fission products released at elevated reactor temperatures and provide structural integrity.1-4

The composite must possess suitable properties to perform these functions, i.e. density > 1.7 g.cm-3

; high mechanical strength, high thermal conductivity, high oxidation resistance and low thermal expansion coefficient.5

Figure 1: A fuel sphere encasing TRISO coated fuel kernel.

The main aim of this project is to study the variation in microstructure and thermal

properties with composition of the uniaxiallly cold compression moulded graphite

composites(at fixed resin content, 20 wt.%).

1. Nicholls D.R., Trans. Royal Soc. S. Afr., 2001, 56, 125-130.2. Nicholls D.R., S. Afr. J. Sc., 2002, 98, 31-35.3. Tang et al., Nucl. Eng. Des., 2002, 218, 91-102.4. Zhao et al., Nucl. Eng. Des., 2006, 236, 643-647.5. Cohn et al., Graphite Artificial, in : Kirk-Othmer (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, 2005, 713-771.

EXPERIMENTAL: FABRICATION

Z-blade Mixing(1 h)

Phenolic resin solution in methanol

Graphite powder mix

Resinated graphite paste

Dry at 70 °C(6 h)

Mill

Graphite-carbon composite powder

Natural graphite

Synthetic graphite

Compression mould

(13 MPa, 1 h)

Annealing (1800 °C for 2 h in He)

Sieve: < 212 µm

Carbonisation(900 °C in N2)

Figure 2: Fabrication steps for the graphitic composites.

Figure 3: An unheated test sample obtained after uniaxial compression moulding.

RESULTS: CHARACTERISATIONX-ray diffraction (XRD): The diffraction patterns were recorded using a BRUKER’s D8 ADVANCE diffractometer. A Cu Kα radiation source (λ = 0.15418 nm) was used.

Patterns for the annealed composites and SG powder were consistent with hexagonal crystalline phase of graphite6.

A pattern for the NG was consistent with the graphite-3R containing both hexagonal and rhombohedral phases7. (43.4°, 46.2° and 56.6°)

Figure 4: XRD patterns of the raw graphite powders and the annealed composites heated to 1800°C.

6.Trucano P. and Chen R., Nature, 1975, 258, 136.7. Lipson H. and Stokes A.R., Proc. Phys. Soc. London., 1942, A181, 101.

RESULTS: CHARACTERISATIONRaman Spectroscopy: (Degree of ordering)The Raman spectra were recorded with T64000 series II triple spectrometer system from HORIBA Scientific. A 514.3 nm wavelength laser line of a coherent Innova® 70 Ar+ was used.

Figure 5: The Raman spectra of neat graphite powders and annealed graphitic composites.

Raman spectra exhibited two peaks; i.e. Disorder induced D-peak ~1350 cm-

1 and structural order induced G-peak ~1580 cm-1. (raw graphites and composites were polycrystalline)

The intensity ratio ID:IG is a measure of structural ordering in the carbon material8-10

Composition

ID/IG

0:100:0 0.12

0:80:20 0.59

80:0:20 0.37

100:0:20 0.18

8. Tuinstra F. and Koenig J. L., J. Chem. Phys., 1970, 53, 1126-1130.9. Ko et al., J. Appl. Polym. Sci., 2001, 81, 1084-1089.10. Nakamizo et al., Carbon, 1974, 12, 259-267.

RESULTS: CHARACTERISATIONa b

c

50 m

d

C

PF

R

Figure 6: Polarised optical micrographs of annealed graphite composites: (a) 0:80:20 (b) 40:40:20 (c) 64:16:20 and (d) 80:0:20 (NG:SG:novolac wt.%).

The composites comprised compacted

particles due to pressing and binding

effect of the resin.

The particles were partially aligned in

the direction perpendicular to

pressing.

A composite containing no natural

graphite appeared more porous than other composites

RESULTS: PROPERTIES Thermogravimetric analysis (TGA): (air oxidation)Air oxidative stability of the raw materials and annealed composites was determined using a Mettler Toledo A851 TGA/SDTA machine. 20 mg samples were heated from 18-1000 °C at a heating rate of 5 °C/min in instrument grade air flowing at 50 mL/min.

Figure 7: The TGA (a) and DTG (b) profiles of the annealed graphite composites.• The composites were stable in air below 650 °C• Composites containing mainly NG (64 & 80 wt.% NG) were less stable in air• The mass loss curves (DTG) were bimodal; the resin derived carbon is lost first ca. 700 °C followed by the loss of graphitic carbon ca. 850 °C

RESULTS: PROPERTIES Thermomechanical analysis (TMA): (linear CTE)The linear CTE of the annealed composites were determined using a TA instruments TMA Q400 V22.1 analyser. The runs were performed from 20 to 600 °C at a heating rate of 5 °C/min in N2 flowing at 50 mL/min and applying a force of 0.02 N. The instantaneous CTE values were calculated using equation (1) as follows11: CTE, α = 1/L0 × dL/dT…………………………………………………………………………(1)

Composition(NG:SG:Novolac)

wt.%

CTE, αp

(×10-6 K-1)

CTE, αN

(×10-6 K-1)

Anisotropic ratioαp: αN

0:80:20 6.25±0.89 2.06±0.50 3.0

16:64:20 5.70±0.80 1.74±0.31 3.3

40:40:20 5.82±0.75 2.08±0.44 2.8

64:16:20 7.88±1.40 2.00±0.17 3.9

80:0:20 6.58±0.71 1.91±0.45 3.4

Table 1: Average linear CTE values of the annealed graphite composites obtained in the temperature range 20 to 600 °C.

Composites containing mainly SG as a major component expanded less in the pressing direction than those containing mainly NG as a major component.

A composite containing 64 wt.% NG showed slightly higher average αP value than the other composites.

All the composites showed similar and constant average αN values at ca. 2 × 10-6 K-1. Anistropic ratio in expansion, αp: αN > 2

The CTE values reported in this work were similar to those reported for nuclear graphites11-12.11. Hacker et al., J. Phys. D: Appl. Phys., 2000, 33, 991-

99812. Cohn et al., Graphite Artificial, in : Kirk-Othmer (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, John

Wiley & Sons, 2005, 713-771.

RESULTS: PROPERTIES Xenon Flash photolysis: (thermal conductivity)Thermal diffusivity and conductivity of the composites samples were measured with an Anter Flashline 3000 fitted with a Xenon Flash under nitrogen flow. Measurements were performed at 100, 400, 700 and 1000 °C. The thermal conductivity was calculated from the measured diffusivity, density and specific heat of the samples at each temperature using equation (2) below13:

κ = α.ρ.Cp………………………………………………………………………………….(2)

13. Clark L.M. and Taylor R.E., J. Appl. Phys., 1975, 46, 714-719.

Figure 8: Temperature dependence of the thermal conductivity of the annealed graphite composites in the moulding direction

Generally κ values decreased with increase in measurement temperature; in graphite heat conduction occurs through lattice vibrations and as such as the temperature rises phonon scattering become pronounced

A composite containing only SG and binder (0:80:20) showed lower κ values at each temperature because it had high open porosity (41 %) and more structural defects than other composites.

The compression moulded graphite composites exhibited different microstructures. Composites containing mainly NG showed flake-like particles whilst those containing mainly SG showed needle-like and fine-grained particles. The needle-like particles were indistinguishable from the flake-like particles on the micrographs of the annealed composites. These particles aligned themselves in the direction normal to pressing direction.

The annealed graphite composites were stable in air up to 650 °C. The raw graphites showed complementary properties. The SG powder was purer and thermally stable, a composite of this graphite (0:80:20) showed lower CTE values than other composites containing mainly NG. Also it had lower thermal conductivity values than other composites.

On the other hand, the NG graphite powder was less pure and thermally less stable. A composite of this graphite (i.e. 80:0:20 and 64:16:20) showed higher CTE values than composites containing mainly SG. Also they had higher thermal conductivity values than composites containing mainly SG.

CONCLUSIONS

ACKNOWLEDGEMENTS

My supervisors: Prof. Ncholu Manyala and Prof. Walter Focke

PBMR and NECSA for provision of samples and financing my studies

My colleagues in SARChI Chair in Carbon Materials and Technology and other departments at UP

Colleagues from other institutions such as the CSIR, NECSA, UJ, etc

Family and Friends

This work has been published in the following Journal: P. P. Magampa et al., Journal of Nuclear Materials, 2013, 436, 76-83