Irradiation-induced dimensional changes in co-deposited carbon-silicon coatings

Corbnn

1971. Vol. Y. pp. 439-446.

Pegamon

Press.

Printed

in Great

Britain

IRRADIATION-INDUCED DIMENSIONAL CHANGES IN CO-DEPOSITED CARBON-SILICON COATINGS J. C. BORROS, D. W. STEVENS and R. J. ARINS Gulf General Atomic Company, P.O. Box 608, San Diego, California 92112, U.S.A. (Received 14 September 1970) AbstractIrradiations of pure and silicon-alloyed isotropic carbons (deposited below 1500°C) at 910”-1300°C to fast-neutron fluences in the range 1.4-4.8 X lo** n/cm? (E > 0.18 MeV) show that silicon concentrations above about 18 wt-% are effective in reducing the irradiation-induced dimensional changes. For example, at 910°C the lineal shrinkages caused by irradiation are reduced about a factor of 3 by the addition of 34 wt-% silicon. At 1250” to 1300°C the corresponding reduction is about a factor of 2. At YlO”C, irradiation creep strains as high as 5.5%’ per 10” n/cm2 were measured for the unalloyed carbons. At 1250”-13OO”C, total creep strains of 10 per cent were observed for unalloyed carbons, and 8 per cent were observed for the alloyed coatings. Since none of the creep specimens fractured, these are not limiting values. Comparison data for isotropic carbons deposited below 1500°C (LTI carbons) with those for isotropic carbons deposited above 1500°C (HTI carbons) shows that at high fluences the LTI carbons expand at a rate that is substantially less than the corresponding rate for HTI carbons. 1. INTRODUCTION

BLOCHER has reported that carbon coatings co-deposited with silicon on nuclear fuel particles have higher strength (measured in crushing) than similar coatings deposited without silicon[l]. Systematic studies of the mechanical properties of co-deposited carbon -silicon coatings that have been carried out by Kaae[2] are consistent with Blocher’s observations. The superior mechanical properties of carbon-silicon coatings have made them candidate coatings for nuclear fuel particles. Recent results from irradiations of carbonsilicon deposits have shown that in the range 600”-lOOO”C, silicon concentrations up to 15 wt-% do not significantly affect the dimensional behavior [3] of isotropic carbons deposited at temperatures below about 1500°C (LTI carbons). This communication compares the dimensional stability of LTI carbons and silicon-alloyed LTI carbons with

silicon concentrations up to 34 wt-% at irradiation temperatures in the range YOo”- 1300°C. 2. EXPERIMENTAL The LTI and silicon-alloyed LTI carbon specimens were prepared by vapor deposition onto small graphite disks in a fluid bed already described [4,5]. using procedures The deposition conditions and structural parameters that characerize each deposit are listed in Table 1. The irradiation, designated GE-H-13-16, was carried out by A. L. Pitner in the Engineering Test Reactor in a capsule designed at the Battelle Northwest Laboratories. The irradiation temperatures were measured with thermocouples; the fast-neutron fluences (E > 0.18 MeV) were obtained from the analysis of nickel and iron flux monitor wires. The irradiation conditions are listed in Table 2. Two types of specimens were irradiated. One type was an unrestrained disk that was 439

J. C. BOKROS,

440

Table 1. Deposition conditions

D. W. STEVENS

and preirradiation

structural

Specimen No. 4174-109 4 174-95 4174-101 4174-111 4273-75 4322-85 4322-81 4322-89 4322-63 4322-75 4322-83 4174-115 432269 4322-67 4322-79 4322-77 4174-113

Temp. (“C) 1450 1450 1400 1400 1350 1300 1500 1300 1350 1350 1450 1350 1350 1350 1450 1450 1400

BAF

RI*

33 100 100 100 100 100 100 100 100 100 100 100

1.07 1.08 1.08 1.10 I.08 1.14 1.08 1.05 1.27 I.03 1.04 1.04 1.13 1.08 1.05 1.14 1.05

0.651 0.649 0.649 0645 0.649 0.637 0.649 0.656 0,612 0.660 0.658 0.658 0.639 0.649 0.656 0,637 0.656

removed from the graphite substrate disk by grinding. The other was a composite specimen that consisted of an LTI or alloyed LTI coating on the graphite substrate disk (cf. Fig. 1 of Ref. 7). During irradiation, the coating on the composite configuration tends to shrink parallel to the deposition plane but is restrained by the more stable graphite substrate disk. If the restrained coating does not fracture, a comparison of the dimensional changes measured for the unrestrained Table 2. Irradiation

2 10 13

parameters

Density (g/cm3) RI”

L, (A)

Bulk

Carbon

0.674 0.675 0.675 0.677 0.675 0.682 0.675 0.672 0.694 0.670 0.671 0.671 0.681 0.675 0.672 0.682 0.672

27 27 32 32 32 33 33 32 22 32 47 32 34 21 36 27 64

1.51 1.62 1.66 1.79 1.95 2.10 2.11 2.14 2.02 2.11 2.06 2.05 2.15 1.97 2.13 2.26 2.07

1.51 1.62 1.66 1.79 1.95 2.02 1.97 2.00 1.86 1.95 1.89 1.82 1.85 1.59 1.73 1.88 I.54

Si cont. (wt-%) 0 0 0 0 0 7.1 12.1 12.4 12.7 13.4 13.6 18.4 23.4 26.6 28.7 28.8 34.4

I-y(Ref.6).

*RL=&;RII=

Cell No.

for specimens irradiated

Preferred orientation parameters

% He through silane

30 40 30 35 20 40 40 40 20 40 40 40 20 40 20 20 30

parameters

Structural

Deposition conditions

Propane in He (%)

and R. J. AKINS

Fast fluence (E > 0.18 MeV) (IO*’ n/cm’)

conditions Irradiation temperature (“C)

1.4

910

4.8 4.2

1300 1250

disk of the same coating material provides the irradiation creep strain. The creep strain was calculated by subtracting the free shrinkage from the restrained shgnkage, ignoring the elastic strains, which have been shown to be small [El]. 3. RESULTS AND DISCUSSION ( 1) I?-?&&&?2 al 9 10°C

The postirradiation carbon density pf of the unrestrained specimens is plotted in Fig. 1 as a function of the preirradiation carbon density p. for the irradiation at 91oOC.* The data show that although from 7 to 14 wt-% silicon has little effect on the densification, the addition of from 18 to 34 wt-% silicon *Note that the density plotted for the alloyed coating is the density of the carbon matrix-not the bulk density.

IRRADIATION-INDUCED

CHANGES

910°C 2.2

t

1.4 X 102' N/CM2

, Si

1.5l 1.5

CONCENTRATION

1

1

/

1.6

1.7

I.@

ORiGiNAL

CARBON

(WT.%)

23

I

1 I .9

DENSITY

IL 2.1

2.0

I 2.2

(G/CM3)

Fig. 1. Postirradiation carbon density irradiation carbon density for LTI

versus

pre-

and siliconalloyed LTI carbons irradiated at 910°C to 1.4 X lo” n/cm?. Note that the densities plotted for the alloyed coatings are not bulk densities but rather are the densities of the carbon matrix.

reduces the volumetric shrinkage by from one-third to one-half, depending on both the silicon concentration and the original carbon density. Although the curves are linear, they do not extrapolate through the point pf= p0 = 2.22 g/cm” [7]. Hence, the densification is not first order with respect to the density defect. Accordingly, the apparent first-order behavior at 1000°C reported in Ref. 9 for LTI carbons is probably fortuitious. The data in Fig. 1, together with those reported in Ref. 9 and previously unpublished data, are plotted in Fig. 2(a) as a function of Auence. Figure 2(b) (which is identical lvith Fig. 11 of Ref. 3) shows data for HTI carbons deposited from methane at temperatures above 1500°C. Comparison of the LTI and HTI data (Figs. 2(a) and 2(b), respectively) shows several points of difference. The initial densification rate is substantially higher for LTI carbons than for HTI carbons. The

IN CARBON-SILICON

COATINGS

441

volume expansion at higher fluences, is substantially less for the LTI carbons than for HTI carbons. The high initial shrinkage rates are consistent with the fact that small crystallites are more unstable during irradiation than large crystallites, other factors being constant (see, for example, Ref. 10). The high densification rate is accordingly taken to be due to the closure of voids by the r-direction expansion of the crystallites. A1t high fluences, however, the generation of pores in materials bvith small crystallite sizes is less rapid than in materials with larger crystallites. The following factors may be important in interpreting the latter observation. First, the local displacements at crystallite boundaries may be smaller for LTI carbons than for HTI carbons even though the crystallites in LTI carbons are less stable. This factor is likely to be most important at irradiation temperatures above - WO”C, where the rates at \vhich the crvstallites change shape depend less stronaly on crystallite size and. for the relevant ranges of crystallite sizes, approach one another [ lo]. Second, the high strength of LTI carbons suggests that there may be stronger internal binding between crystallites in LTI carbons than in HTI carbons, and this may inhibit pore generation between crystallites at high fluences in the stronger IaT1 carbons. The latter factor has been discussed in Ref. 11. The linear dimensional changes that correspond to the carbon density changes in Fig. 1 are plotted in Fig. 3. The data are consistent with the volumetric changes. The stabilizing effects of silicon additions become important only when the concentration is above about 14 wt-9. Additions of greater than about 18 wt-‘/-;silicon reduce the parallel shrinkage by about 50 per cent or more-. None of the restrained coatings fractured during the irradiation. The total strains accommodated by irradiation creep in LTI and silicon-alloyed LTI carbons at Ylo”C are plotted in Fig. 4. together with data for

J. C. BOKROS,

442

D. W. STEVENS

and R. J. AKINS

2.3 LTI CARBONS 900” loooac 2.2

-

2

0

-;

u

J

6 [ 102’4N/C”2:

FL”ENC:

‘\

1.9

‘4

s z z

HT I 900”

I.8

CARBONS 1000”c

z

t.5

1 0

I

I

I

I

I

I

I

I

2

3

4

5

6

7

FLUENCE

Fig. 2. Density LTI carbons,

vs. fast-neutron and (b) carbons

fluence deposited

(X

IO”

N/Cd)

for irradiation at 900”-1000°C for (a) above about 1500°C (HTI carbons).

8

IRRADIATION-INCITED

CHANCES

IN CARBON-SILICON

443

COATINGS

not seem likely that an important dependence of irradiation creep on the density of unalloyed LTI carbons will be found.

Si

CONCENTRATION

_7-_ I .5

I.6

I.7 ORIGINAL

(WT-%)

/

I

I

I

I

.I

I.8

I.9

2.0

2.1

2.2

CARBON

DENSITY

(G/cM~)

Fig. 3. Length change vs. preirradiation carbon density for LTI and silicon-alloyed LTI carbons irradiated at 910°C to 14 X 10”’ n/cm”. unalloyed LTI carbons from Ref. 9. The data show that at - 900°C the LTI carbons can accommodate an average strain rate of 5.5% per lo”’ n/cm2 by irradiation creep and that at 900” to 1000°C a total strain of at least 8.5 per cent can be accumulated without fracture. The silicon-a.loyed material was so stable that very little creep was accumulated in the restrained samples. This was because the coatings with low silicon concentrations had a high density and did not shrink much, or because the silicon at concentrations above 18 wt-% in the lower-density coatings was a stabilizing influence. It may be noted that the high strains and high strain rates are all accumulated in the low-density LTI carbons. If irradiation creep were found to be a function of density, then the limits quoted above would apply only to the low-density carbons. Creep data reported in Tables 4 and 5 of Ref. 7 show, however, that when high-density HTI carbons were strained by an expanding substrate, the same creep strains were observed in both high- and low?-density HTI carbons. Accordingly, it does

(2) zrru&tions at 1250”-1300”c The postirradiation density of unrestrained specimens are plotted in Fig. 5 as a function of the preirradiation density for the irradiations at 1250” and 1300°C to fluences of 4.2 and 4.8 X lo” n/cm*, respectively. Corresponding data from Ref. 11 for HTI carbons are included for comparison, The data show that when the silicon concentration is less than about 14 wt-lo, he additions to LTI carbons have little influence on the irradiationinduced volumetric changes. However, when the silicon concentration is 18 wt-% or higher, there is a large stabilizing effect. Comparison of the data with corresponding data from Ref. 11 for HTI carbons shows that the unalloyed LTI carbons expand more slowly at high fluences than HTI carbons. The reasons for this that were presented in the previous section apply as well to these results.

YOO”-1000”c 14

E t B f

10

s :: 8

I

0

1

.

LTI

0

Si-LTI

.

LTI REF.

5.5%

PER

102’

N/CM*

CARBONS CARBONS CARBONS 9)

(FROM

*

2 FAST-NEUTRON

3

k FLUENCE

5

b

(N/CM’)

Fig. 4. Total strain accommodated without fracture during irradiation at ~O”-l~OoC.

7

444

J.

C. BOKROS,

D. W. STEVENS

The linear changes that correspond to the volumetric data presented in Fig. 5 are plotted in Fig. 6 as a function of the preirradiation carbon density. As at YlO”C, the data show an important stabilizing effect when the silicon concentration is greater 2.3, 4.2-4.0 X IO" 1250"-1300°C

N/CM2

LTI;

2.2

7-14 WT-%

Si

and R. J. AKINS

more than just a diluent. Its presence in effect retards the densification in a manner analogous to the way in which densification of a carbon coating is restricted when it is irradiated in a restrained condition on an unyielding support (cf. Fig. 6 of Ref. 9). Therefore, the stabilizing effect of silicon may be dependent on the morphology of the silicon in the mixtures. Hence, it may be possible to further enhance the stability of such a mixture by

P EAF C

1.10

I

7-14 WT-% Si; L/

1

I.5

I.6

I.7 ORIGINAL

I .8 CARBON

I .v DENSITY

2.0

2.1

2.2

(G/cM~)

Fig. 5. Postirradiation carbon density vs. preirradiation carbon density for LTI and siliconalloyed LTI carbons irradiated at 1250” and 1300°C to 4.2 and 4.8 X 1021n/cm’, respectively. The corresponding relationship for HTI carbons (Ref. 11) is shown for comparison.

than about 18 wt-%. The data for the carbons with lower concentration merge with those obtained from unalloyed LTI carbon specimens. There are several mechanisms by which silicon additions might stabilize LTI carbons. Since the silicon is present as a dispersion of silicon carbide particles which are relatively stable during irradiation at high temperatures [12, 131, the co-deposited mixtures should be more stable than the pure carbon (other structural parameters being constant). However, since the densities in Figs. 1 and 5 are densities of the carbon matrix, it is clear that the silicon carbide in the structure acts as

Si CONCENTRATION

,&I 1.5

1.6

1.7 ORIGINAL

(WT-%)

TTT

,I82313

T-T-7

1312

1.8

1.9

CARBON

DENSITY

7

2.0

, 2.1

2.2

(G/CM3)

Fig. 6. Length change vs. preirradiation carbon density for LTI and silicon-alloyed LTI carbons irradiated at 1250” and 1300°C to 4.2 and 4.8 X 10” n/cm*, respectively.

IRRADIATION-INDUCED

CHANGES

optimizing the morphology of the silicon carbide through processing. At 1250” and 1300°C none of the restrained coatings fractured. The total strains accommodated are plotted in Fig. 7; these are the first creep data reported for LTI carbons irradiated near 1300°C. The total strains accumulated are similar to those previously reported for HTI carbons [7].

0 Si-LTI 0 LTI

1300°C 125w 1

t b 70

WT-%

Si

07

0 I3 812 @A:

I

Fig. 7. Total strain accommodated in Si-LTI and unalloyed LTI coatings vs. fast-neutron fluence for irradiations near 1300°C. Numbers next to open points refer to wt-% Si in coating.

Substantial creep strains were also accumulated in the silicon-alloyed LTI carbons. This observation means that the addition of silicon (as silicon carbide) can retard densification and provides stronger, more stable coatings but does not significantly reduce the ability of the deposits to creep during irradiation. Accordingly, alloying LTI carbons with silicon may provide a superior family of coatings for use on nuclear fuel particles.

IN CARBON-SILICON

COATINGS

445

4. CONCLUSIONS

The results provide the following significant conclusions: 1. Silicon with isotropic co-deposited carbon at temperatures below about 1500°C enhances the dimensional stability during irradiation at YOO”-1300°C when the silicon concentration is above about 18 vvt-%. For lower silicon concentrations the effects are small. 2. During irradiation in the range WO”13OO”C, LTI carbons expand less rapidly at high fluences than HTI carbons. 3. ,4t YOO”C,creep strains as high as 5.596 per 102’ n/cmY can be sustained in unalloyed LTI carbons without fracture. This value is higher than values reported previously for either LTI or HTI carbons. Since none of the restrained coatings fractured, this is not a limiting value. 4. At irradiation temperatures of 1250”13OO”C, total creep strains of at least 10 per cent can be accumulated in unalloyed LTI carbons; the corresponding value for alloyed carbons is 8 per cent. Since none of the creep specimens fractured, these values are also not limiting. .4cknouledgements-The authors thank W. H. Ellis, R. R. Frazee, C. Whiting, F. J. Gagnon, and L. J. Noble for experimental assistance. The authors are indebted to A. L. Pitner and R. E. Nightingale at the Battelle Northwest Laboratories for including the specimens in the GE-H-13-16 irradiation. We also thank J. P. Howe for stimulating discussions. The work was supported by both Gulf General Atomic and the U.S. Atomic Energy Contract AT(04-3)-167, Project Commission, Agreement 17. REFERENCES Blocher J. M., U.S. Pat. 3,249,509 (1966). Kaae J. L., Gulf General Atomic Incorporated, private communication of work in progress. Bokros J. C. and Stevens D. W., Irradiation Behavior of Isotropic Carbons, Gulf General Atomic Report GA-9900, (to be published in Carbon). Bokros J. C., The Structure of Pyrolytic Carbon Deposited in a Fluidized Bed, Carbon 3, 17-29 (1965).

446

J. C. BOKROS,

D. W. STEVENS

5. Schwartz A. S. and Bokros J. C., Catalytic Graphitization of Carbon by Titanium, Carbon 5,325-30 (1967). 6. Bokros J. C., In Chemistry and Physics qfCarbon, (Edited by Walker P. L.), Vol. 5, p. 1. Marcel Dekker, New York (1969). 7. Bokros J. C., Guthrie G. L., Dunlap R. W. and Schwartz A. S., Radiation-Induced Dimensional Changes and Creep in Carbonaceous Materials,]. Nucl. Muter. 31,25-47 (1969). 8. Price R. J. and Bokros J. C., Mechanical Properties of Neutron-Irradiated Pyrolytic Carbons,J. Nucl. Muter. 41, 158-74 (1967). 9. Kaae J. L. and Bokros J. C., IrradiationInduced Dimensional Changes and Creep of Isotropic Carbon, Gulf General Atomic Report GA-9919, (to be published in Carbon).

and R. J. AKINS

10. Bokros J. C., Guthrie G. L. and Schwartz A. S., The Influence of Crystallite Size on the Dimensional Changes Induced in Carbonaceous Materials by High-Temperature Irradiation, CarbonG, 55-63 (1968). 11. Bokros J. C. and Koyama K., Interpretation of Dimensional Changes Caused in Pyrolytic Carbon by High-Fluence Neutron Irradiation,

J. u/$11.Phys. 41, 2146-55 (1970). 12. Price R. J., Effects of Fast-Neutron Irradiation on Pyrolytic Silicon Carbide, J. Nucl. Muter. 33,17-22 (1969). 13. Thorne R. P., Howard V. C. and Hope B., Radiation-Induced Changes in Porous Cubic Silicon Carbide, Proc. Brit. Ceram. Sot. 7, 449-59 (1967).