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Radiation induced surface segregation of impurities in boron carbide

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Page 1: Radiation induced surface segregation of impurities in boron carbide

Nuclear Instruments and Methods in Physics Research B23 (1987) 347-352

North-Holland, Amsterdam

347

RADIATION INDUCED SURFACE SEGREGATION OF IMPURITIES IN BORON CARBIDE

V. SARASWATI

Materials Deveiopment Laborato~, Indira Gandhi Centre for Atomic research, Ka~akka~ 603 IO.?, India

Received 21 July 1986

Surface segregation of impurities and formation of a compound has been observed on heating pellets of boron carbide which were

previously irradiated with 100 keV helium ions. This phenomenon is attributed to oxidation.

1. Introduction 2. Experimental details

Boron carbide is a control rod material for fast reactors. On the absorption of a neutron in “B, helium is emitted. Hence, the presence of helium atoms and their influence on the material properties and the integr- ity of the control rod is of relevance in its usage in reactors. Earlier, we had studied the effect of 100 keV helium ion irradiation on boron carbide pellets [l]. The heat treatment of the irradiated pellets revealed certain

microst~ctur~ and microehemical changes, presumably due to the presence of oxygen; these are discussed in this paper.

Boron carbide pellets were obtained commercially from Elektroschmelzwerk, Kempten, GMBH. The dominant impurities were indicated to be iron and silicon. X-ray diffraction showed characteristic lines of boron carbide with an additional line corresponding to graphite. Energy dispersive spectrum (EDX) attached to SEM, showed peaks corresponding to Fe, Si, Al, K, S, Cl and Ca, at cavities and pore areas, but not within the grains. The amount of iron had been estimated with reference to the susceptibility measurements. The spec- trochemical analysis and EPMA estimation of the quan-

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Fig. 1. Energy deposition distribution as a function of the penetration depth s. for 100 keV helium ions bombarding a boron carbide

pellet. Sa(x) and St,(x) represent the damage profile and Fx( x) depicts the helium profile

0168-583X/87/$03.50 C Elsevier Science Publishers B.V. (North-Holl~d Physics Publis~ng Division)

Page 2: Radiation induced surface segregation of impurities in boron carbide

348 V. Srrmwuir / Radiutron rnduced segregutmn in boron curhrde

tity of iron was 260 ppm. We assume that the amount of other impurities could be of this order. The density of the pellet was 2.32 g/cm3, about 92% of the theoreti- cal, The average grain size in the polished and etched sample was found to be - 5 pm. Small pieces of 0.8 mm thickness were irradiated at room temperature with 100 keV helium ions at normal incidence. The beam current density was about 50 kA/cm’. Irradiation dosages were lo”, lo”, 5 X 10” and lOI ions/cm2 and the area irradiated could be varied from 3 to 5 mm. A computer programme [2] was used for calculating the helium and damage profiles which are shown in fig. 1.

3. Results

On irradiation, no change in the lattice parameters was observed [l]. A slight swelling due to voids could be discerned from the SEM micrograph of fig. 2, for a pellet irradiated to a dosage 10” ions/cm*. First and second generation growth of bubbles and blister were also observed. The growth was preferential around pores and cavities as seen from fig. 3. For pellets irradiated at smaller dosages viz. 10” and 10’s ions/cm’ bubble growth on the surface was low.

On heating these irradiated pellets at 200°C for six

Fig. 2. Scanning electron micrograph of a pellet irradiated at

lOI ions/cm’. The elevation at the interface indicates

swelling.

Fig. 3. Preferential growth of bubbles at cavities.

Fig. 4. Unirradiated pellet - vacuum annealed at 750°C for

three hours.

Fig. 5. Unirradiated pellet - annealed in open air at 750” for three hours.

Page 3: Radiation induced surface segregation of impurities in boron carbide

V. Suruswati / Rod&ion induced segregution in boron curhide 349

hours in open air no change was noticed in the micro- structure.

The heat treatment at 750°C for three hours in

vacuum revealed certain peculiar features. Fig. 4 shows the microstructure of an u&radiated pellet heated at 75O’C in a vacuum of 10m6 Ton. The micrograph of an

Fig. 6. Optical micrographs of specimens post-irradiation annealed at 750°C for three hours. (a) Before heating, (b) fifteen hours

after annealing, (c) at the interface, (d) for a sample with irradiation dosage 10 I7 ions/cm’, (e) irradiation dosage 10” ions/cm2, (f)

at a higher magnification.

Page 4: Radiation induced surface segregation of impurities in boron carbide

350 V. Saraswati / Radiation induced segregation in boron carbide

Fig. 7. Optical micrographs, fourteen days after annealing: (a) interface region, (b) irradiated region.

unirradiated pellet heated at 750°C in open air for three hours is shown in fig. 5. The oxidation of the surface is evident. In fig. 6, a group of micrographs is presented to show the sequence of events that took place on heating at 750°C for three hours under an inadequate vacuum of 10M3 Torr. The poor vacuum happened accidentally when one of the pumps failed. The changes were noticed on the whole surface in both the irradiated and nonirradiated regions, in all the three samples with different incident irradiation dosages viz lo”, lo’* and 1019 ions/cm2. The details on the surface were similar. Fig. 6a shows the microstructure of one of the pellets before heating. Figs. 6b-f show the features, nearly fifteen hours after switching off the heating. The optical micrographs reveal globular swellings of 200 pm size with cracks and blisters. After fourteen days the

globular swellings were observed to have settled (fig. 7), and segregated regions of 100-150 pm could be seen. The interface between the irradiated and nonirradiated region is still clearly visible (fig. 7a). The events were not monitored every day as they were not expected. SEM pictures (fig. 8a) taken after a month showed the surface to be similar to that of fig. 7b. Fig. 8b, shows that the cracks have widened. Fig. 9 is a comparison of

the EDX taken at a cavity in an unirradiated, unheated, pellet and a similar material after irradiation and heat treatment at 750°C as above. The apparant increase of impurities, as in fig. 9b, is considered to be due to surface segregation impurity atoms, present in the bulk, with the migration of helium bubbles.

With subsequent aging there was no change in the microstructure. X-ray diffraction of the pellet showed

Fig. 8. SEM pictures of the specimen showing (a) roughened surface, (b) widened cracks.

Page 5: Radiation induced surface segregation of impurities in boron carbide

V. Saraswati / Radiation induced segregation in boron carbide

Fig. 9. EDX at cavities (a) unirradiated pellet (b) samples as in fig. 6e.

the characteristic boron carbide lines, a line due to graphite as before and four additional lines cor<espond- ing to d spacings, 2.06, 3.235, 3.048 and 6.163 A. These lines have been identified as due to a silicate of the scapolite family - aluminum-calcium-silicate- carbonate. All the atoms in the above molecular unit had been observed as impurities in EDX (fig. 9a).

4. Discussion

The formation of this compound on the surface is interesting for the following reasons. The impurity atoms, which were previously present in insignificant concentrations in the bulk and not perceptible in X-ray diffraction, have become significant after oxidation and formation of the silicate compound. Scapolite has a formula,

Al,Ca,(SiO,),(SO,, COs).

The oxygen content is about 50% of the molecular weight.

Analysis of some of our powder samples showed that Si and B,O, were present in concentrations less than 0.5%. Thermogravimetric analysis done in an argon atmosphere had also shown a slight increase in weight indicating the possibility of some oxygen present in the pellet even before irradiation. A nonirradiated pellet heated in open air at 750” C did not show the formation of the silicate compound because the impurity atoms are not available on the surface in sufficient concentra- tion. It is hence reasonable to assume that the helium atoms in the irradiated material have been responsible for the segregation of the impurity atoms, their oxida-

tion and the formation of silicate compounds. On irradiation with helium ions, vacancies and inter-

stitials are produced in the material. The helium ions are trapped at the vacancies and they grow into bubbles and appear as blisters on the surface when the dosage is high, viz 1019 ions/cm2. At lower dosages, bubbles are smaller and do not show up as blisters on the surface, but nevertheless they are present. These bubbles migrate towards pores and grain boundaries which are the avail- able sinks. There is an 8% porosity in the system and microcracks are widened by thermal stresses. Boron carbide is a ceramic and its thermal conductivity is low. As pores and voids act as scattering centres for phonons thermal conductivity is reduced further in a porous material. This adds to the thermal stress and widens. Further the microcracks which might then act as trans- port paths both for helium as well as for impurities. These could be oxidized at the surface followed by silicate formation if initially the impurities individually were in quantities greater than 1000 ppm. Radiation induced segregation of solute atoms and impurity atoms are known for dilute alloys [3]. The peculiar feature here is that the impurity atoms, probably distributed uni- formly in the material and at a low concentration, have segregated to the surface with an increase in their weight owing to oxidation. The microstructural features are a consequence of this process. The Vickers hardness number was found to have reduced substantially from the initial 3800 + 100 kg/mm2, the value for nonirradi- ated material. The indentation impression was fuzzy at a load of 200 g. It must be mentioned that these trace impurities had no influence on microstructure or harde- ness when heating was performed under good vacuum conditions.

Page 6: Radiation induced surface segregation of impurities in boron carbide

352 V. Saruswur~ / Rud~orron reduced segregurm IPI horon curhrde

5. Conclusion References

Sintered boron carbide pellets, especially hot pressed, were considered stable against oxidation [4] below 1000°C. In the present work we find that it is not true. Further, migration of implanted ions can create compli- cations such as surface segregation of impurity atoms. This results in embrittlement owing to the development of large cracks. Thus, the presence of oxygen even at small pressures is not desirable for retaining the integr- ity of the irradiated pellets.

111

[2l

I31

I thank Dr. Placid Rodriguez for his kind support. I gratefully acknowledge the help given by Dr. Nandcd- kar in the irradiation of the samples. I also thank

Messers. G.V.N. Rao, Varadarajan and Vaidyanathan for their assistance in the X-ray, photography and SPM.

I41

V. Saraswati and G.V.N. Rao Jr., Mat. Sci. Lett. 4 (1985)

260.

I. Manning and G.P. Miiller, Comp. Phys. Commun. 7

(1974) 85.

V.K. Sethi and P.R. Okamoto, in: Phase Stability during

Irradiation, eds., J.R. Holland. L.K. Mansur and D.I. Potter

(Met. Society of AIME, 1981) p. 109;

H. Wiedersich, in: Surface Modification and Alloying by

Laser, Ion and Electron Beams, eds.. I.M. Poate, G. Foti

and D.C. Jacobson (Plenum. New York and London, 1983)

p. 261.

A.LIPP Technische Rundschau, nos. 14, 18, 33 (1965) and

7 (1966) p. 21;

F. Gauzzi, Metall. itaI 12 (1959) 565.