THE DEPENDENCE OF THERMAL CONDUCTIVITY OF BINDER RESIDUES ON HEAT

TREATMENT TEMPERATURE?

P.M:L WAGNER

Loa Alamos Scientific Laboratory. University of California, Los Alamos, NM 87545, U.S.A.

(Receiced 29 September 1975)

Abstract-The dependence of the thermal conductivity of carbon residues of one pitch and one polyfurfuryl alcohol binder on the heat treatment temperature has been investigated. Specially prepared graphites made with these two binders were used for the experiments. The thermal conductivity data were analyzed in terms of a two-component system and the binder residue thermal conductivity calculated. Both binder residues show an increase in thermal conductivity with increasing heat treatment temperature

1. INTRODUCTION

Analysis of heat transfer in the high temperature gas cooled reactor (HTGR) fuel rods has indicated that an increase in the thermal conductivity (A) of fuel rods

would result in a significant improvement in the thermal performance of the reactor core[l]. Whereas it has long been recognized that A of artificial graphite can be affected by the thermal treatment, quantitative informa- tion on the influence of heat treatment temperature on A for graphites of interest to us was not to be found in the literature. In particular. since fuel rods in the HTGR are made using pitch or a similar binder, it is the thermal

conductivity of the binder residue that is of primary interest. The experiments performed to determine the A of the binder residues as a function of heat treatment temperature are described in this paper.

?. EXPERIMENTAL

Two molded graphites. S-7 and S-8 were used for the experiments.f Although both graphites were made using the same filler flout. S-7 was made with 35 parts per hundred (pph) pitch, S-8 with 28 pph partially polymerized polyfurfuryl alcohol (PFA)[2] catalyzed with 4% maleic

As so often is the case with graphites, it is difficult to

establish a unique cause-and-effect correlation. The work described here is no exception to this rule. Direct measurement of A for a specimen made using a petroleum pitch or a thermosetting resin binder (the two binders of interest) is an impossible task since the pyrolysis and the subsequent onset of graphitization of the material with increasing heat treatment tend to result in degradation of the structural integrity of the sample. Therefore, we decided to use compacts made from a well graphitized graphite filler and a binder of interest. We reasoned that since the filler had been graphitized at temperatures in excess of those to be used in the A experiments, any increase in A should be due to changes in the A of the binder residue component only.

tWorkcompletedundertheauspicesof theU.SEnergyResearch and Development Administration.

fThese graphites were prepared by R. J. Imprescia.

anhydride. The specimens were hot-molded at 13.8 MPa (2000 psi) to 1173°K in graphite dies, then baked to 1’773°K in He. Some properties of these graphites are tabulated below:

Bulk density (g/cm’) Binder residue

Percent of original binder by weight 51.3 35.3 Percent of total weight of baked

specimen IS5 9.(1 Preferred orientation+

Bacon anisotropy factor I.188 1.147-1.IsI Electrical resistivity (JL ohm-cm)

WG$ 1762 I?!‘8 AG 207 1713

Thermal conductivity (Wm ’ K ‘) WC 59 ;‘o AG 45 (IO

tThese measurements were made by J. A. O’Rourke. $ WC = with grain; AG = across grain.

After each heat-treatment, the samples were removed from the furnace and their thermal diffusivities measured at room temperature using a thermal transient method 141. The thermal diffusivity (a) is related to the specimen dimensions by

Four specimens were used for the study. These were oriented with and across the grain for the two graphites.

The specimens were heat-treated together. the heat- treatments consisted of holding for 3.6 ks (one hour) in

vacuum at each of the temperatures: 2046, 2270. 2491. 2674 and 2869°K. The geometry of the heater assembly was such that the temperature uniformity throughout the samples and from sample to sample should be within the 0-5°K tange[3]. The accuracy of the temperature readings are probably within + 5°K at the lower temperatures and + 15°K at the highest.

Q = constant x (thickness)’ x (characteristic time) ‘.

The thermal conductivity is obtained from A =z opC,,,

71

72 P. WAGNER

where p = density, C, = specific heat. During the course of the heat-treatments the densities and the thicknesses of specimens changed; this was probably true also of the specific heat. Since however the change is expected to be small, a constant value (0.721 J g-’ K-‘) was used for C,. Linear interpolation based on initial and final values was used to obtain p and the thickness for the individual temperatures.

3. RESULTS

The obtained thermal conductivities are plotted against heat-treatment temperature in Fig. 1. As expected, a monotonic increase of A with heat-treatment temperature is found. The rate of increase of A with heat-treatment temperature is greater for the pitch bonded graphite than for that made with PFA. This is more important than the fact that A for the pitch-bonded material is initially lower than for the PFA graphite. This lower initial value results from the use of non-optimized binder-flour ratios for the experimental graphites. The reasons for the deviations of the experimental points from the smoothed curves are not clear.

To obtain the change in thermal conductivity for the binder alone, we first corrected the measured A’s for porosity[5] by using the relation:

2+P A (for 100% density) = A (measured) -

2(1-P)

where P = 1 - p/p0 = fractional porosity; p = density of the artifact; p0 = theoretical density. The so corrected A’s

100

90

80

70

Y

5 60

z

T 100

Y T

gw

i

80

70

60

50

t

t

c L

I I I I I I

Flour 100 ports

Pitch 35 pOrtS

I I I I I I 1773 2073 2213 2473 2673 2873 K

Heat Treatment Temperature

Fig. 1. Dependence of the thermal conductivity (300°K) of two artificial graphites on heat treatment temperature.

were then used to calculate the binder residue thermal conductivity by assuming that the materials are two-phase systems with the graphite particles as the dispersed “phase” and the binder residue as the continuous matrix. The thermal conductivity of the system is related to those of the components by[6]

A =A,,

where Ab = thermal conductivity of the binder (the continuous phase); A, = thermal conductivity of the graphite (the dispersed phase); and V, = the volume fraction of the graphite. Since the graphite flour had been fully graphitized A, was assumed to be 150 W/mK. The binder residue weight fractions after pyrolysis were 0.155 for the pitch and 0.09 for the PFA. Assuming these values represent also the volume fractions of the nonporous material V, was taken as 0.845 for the pitch bonded graphite and as 0.91 for that made with the PFA. Using these values, the above equation was solved for Ah.

The thermal conductivities for the two binder residues are shown as functions of heat-treatment temperature in Fig. 2. Published data on pitch residues[7] show A at 300°K to be about 10 W mm’ Km’ for materials which have been heat treated at 2073°K. This is the value used for A in the HTGR fuel rods after 2073°K thermal treatment (the fuel rods are largely pitch). Our values for pitch residue

I I I I I I I I 100

t Polyfurfurol Alcohol

50 -

Y 0

4

z i

; Y 100

t Pitch

T

50

0 1773 2073 2273 2473 2673 2873 K

Heat Treatment Temperature

Fig. 2. Dependence of the thermal conductivity (300°K) of two binder residues on heat treatment temperature.

‘The dependence of thermal conductivity on heat treatment temperature 73

heated to 2073°K are I I and 13 W mu’ K ‘. On the average. our A values for the PFA binder residue are

lower than corresponding values of A for the pitch binder. This is to be expected since the natural product (pitch) graphitizes more readily than the synthetic polymer at these temperatures. As can be seen from Fig. 2, the intluence of heat treatment on the thermal conductivity of the binders is very marked. Both binders are affected about equally by the thermal treatment (i.e. increase in A i\ of 7-10X).

REFERENCES

I. Balcomb .I. D. and Wagner P., Proc. of the BNES international Conference, London (Nov. 26, 1973).

2. Wagner P. and Dauelsberg L. B., Curbon 7, 273 (1969). 3. Wagner P., Reu. Sci. Inst. 8, 1054 (1966). 4. Wagner P. and Dauelsberg I.. B.. Carbon 5. 271 (1967). 5. Wagner P.. O’Rourke J. A. and Armstrong P. E.. 1. 4nr. Cer.

sot. 55. 214 (1972). 6. Cheng S. C. and Vachon R. I.. Inl. .1. HM Mrtss ~~NIISJ~~ 12.

249 (1969). 7. Thermophysical Properties of Mufter (Edited by Y. S.

Touloukian ef al.), Vol. 2. Plenum. Nelc York 11970).