Positron annihilation study of nanocrystalline iron

NanoSmchued

Pergamon

Materials, Vol. 12, pp. 1059-1062, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965-9773/99/.$-m front matter

PI1 SO9659773(99)00299-S

POSITRON

ANNIHILATION

STUDY OF NANOCRYSTALLINE IRON

D. Segers’, S. Van Petegem’, J. F. Liiffler’, H. Van Swygenhoved, W. Wagner*, C. Dauwe’ ‘RUG, Department

of Subatomic

and Radiation Physics, NUMAT, Proeftuinstraat Gent, Belgium ‘Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

86, B-9000

Abstract - Positron lifetime measurements on d@erent nanocrystalline iron samples show that the microstructure, with respect to the inherent free volumes in the samples, can be determined. Three distinct defect types can be detected: i) a vacancy-like defect in the inter$ace, ii) a microvoid at the intersection of interfaces and iii) larger free volumes (missing grains) where the annihilation of ortho-positronium is observed. The eflect of the crystallite size and preparation conditions of the samples are discussed. 01999 Acta Metallurgica Inc.

INTRODUCTION Positron annihilation is an ideal tool to investigate the microstructure of materials. In iron the positron lifetime varies from the bulk value rb =106 ps in defect-free regions to rlv = 175 ps (1) for the mono-vacancy lifetime up to -500 ps in multi-vacancy clusters (2). Positrons trapped in even larger cavities can form positronium atoms (Ps), in both the para- (pPs) and the ortho-state (o-Ps). The o-Ps gives rise to a long lifetime of the order of nanosecondis. Schaefer and Wtirschum (3,4) performed lifetime measurements on nanocrystalline iron and proposed a scheme for analysis based on four possible states of the positron: i) the free positron state (rr) ii) a trapped state into a vacancy-like defect in the interface (z,) iii) a trapped state into a vacancy cluster at the intersection of interfaces (Q) and iv) a state in large free volumes (q) such as missing grains. Similar results were also reported for nanocrystalline Cu and Pd (5)

EXPERIMENTAL Positron lifetime measurements were carried out with a standard fast-fast lifetime spectrometer with a resolution (FWHM) of 225 ps. The measured lifetime spectra contained at least 5~10~ counts. They were analyzed with the multi-component pogram LT by Kansy (6). As aipositron source, “NaCl of about 10 pCi was sealed between two kapton foils with a thickness of 7.5 urn, which has to be surrounded by two identical samples in the so called 1059

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FOURTHINTERNATIONAL GINFERENCE ON NAKSTRUCTURED MATERIALS

sandwich configuration. The thickness of the samples varies between 0.15 and 0.43 mm, thick enough to stop all the positrons. The samples were produced by inert-gas condensation and consolidation in-situ under high vacuum. The characteristics of the samples are summarized in table 1. Unfortunately, the sample processing for the eight samples was different. However we choose sample pairs (table 1: Al+A2, Bl+B2, Cl+C2, Dl+D2) with almost identical grain sizes as determined by the SANS and XRD mean coherence diameter (table 1: DsANsand DXRD).

DISCUSSION The results of the multi-component analysis, after correction for annihilation in the kapton foils, are summarized in table 2. For the samples A, B and C all the obtained lifetime components are longer than the bulk positron lifetime rb in iron (106 ps). For sample D the shortest lifetime zf is smaller than rb. ln samples A, B and C the crystallite size is much smaller than the positron diffusion length L+ = 100 nm measured in crystalline bulk metals (7). This means that in those samples all the positrons thermalized in the crystallites will end up at the traps outside the crystallites and saturation trapping occurs. Comparing the samples A, B and C, sample A is the only set prepared at room temperature. In sample A the measured lifetime rl is longer than the mono-vacancy lifetime zlv in iron, whereas for the samples B and C the lifetime r, is shorter than z,v. We can conclude that in sample A the vacancy-like defects are larger than a mono-vacancy, while in the samples B and C the defects are smaller than a mono-vacancy. This behaviour of z1 is inconsistent with the statements of reference (3) and (4) which claims that open spaces smaller than a mono-vacancy are non-effective positron traps. Our measurements clearly show that those open volumes effectively trap positrons. The measured lifetimes rz in the three samples are comparable. The value ‘c2 = 430 ps corresponds to a microvoid containing about 26

a.p. = as prepared, w.c.= warm compacted, n.d.=not determined

FOURTH INTERNATIONAL CONFERENCEON NAN~STRUCTUREDMATERIALS

I

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TABLE 2 Summary Sample Name

Of The Multi-component r, (ps)

r2 (ps)

‘53(ps)

Al+A2

193 k3

417 +9

994+ 40

Bl+B2

161 *1

447 f2

1154 + 14

c1+c2

157 fl

424 f3

867 f 13

164 f3

369 fl

1108 +7

Dl+D2

rf (p)

Analysis Results Of The Positron Lifetime Spectra Shown In Figure 1

.54 :t1

r3’ (ps)

I, (%)

I* (%)

$)

5491 f 60

24 f 1

13

I, (%)

W) 48 + 1

45 + 1

7fl

49.5 + 0.1

43.6 fO.l

4.9f 0.1

70.8 * 0.2

26.1 fO.l

3.1 f 0.2

57.8 + 0.1

16.8 fO.l

1.4* 0.1

2.0+ 0.1

vacancies (2). The lifetime spectrum of sample B shows an extra very long component. It could only be analysed with four components, where both r3 and rtj’ are to be ascribed to the annihilation of o-Ps from larger free volumes. Normally, positronium cannot exist in metals, because the electron density is much too high. However when the positron is trapped into a very large cavity, the electron density seen by the positron is low enough so that an isolated positronelectron atomic system, called positronium (Ps) can be formed. The natural lifetimes of the two possible states, p-Ps and o-Ps, are respectively 124 ps and 142 ns. In the cavity the o-Ps lifetime is shortened to a few nanoseconds due to frequent collisions of the o-Ps with the inner walls of the free volume. Although samples Bl and B2 have the same crystallite size, two different o-Ps lifetimes (rc, and z,‘) appear. In analogy with the results of the sets A, C and D where only one long lifetime rj is measured, we assume that each sample Bl and B2 gives rise to only one long lifetime (r3 or rg’) and that samples Bl and B2 having the same crystallite sizes (as determined b:y DXRo and DSANS)have different concentrations and sizes of Ps-trapping holes. So in disagreement with the results of (3) and (4) we do not interpret the results as being a consequence of the presence of a broad lifetime distribution. This is also a strong indication that the sample preparation (consolidation and processing) has a different influence on grain size and free .volume. In sample D, where the crystallite size is of the same order of the positron diffusion length, a very short lifetime rr = 54 ps < rb is observed. This is consistent with the trapping model (8, 9). It is known that, for unsaturated trapping of positrons, a number of lifetime components

appear, where the shortest is given by rf = (~~+~~iCi~,wi*xiandCi

respectively the trapping rates and concentration for all possible trapping centres. Whether trapping is s,aturated or not, the longer lived components (ri > rb) are the defect trapped lifetimes. r, lis the lifetime of the positrons trapped at vacancy-like defects in the interfaces (smaller than a mono-vacancy), r2 is the lifetime in the microvoids, containing about 15 vacancies (2) and ‘s3is again ascribed to the annihilation of o-Ps in larger free volumes. Here

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the microvoid lifetime r2 is smaller than in the samples A, B and C, indicating that due to crystallite growth (annealing at 400°C and 550°C see table 1) shrinkage of the microvoids is observed. In order to elucidate more clearly the effects of the different thermal treatments, each sample is being measured individually ( 10).

CONCLUSIONS

In this work positron lifetime results are presented on nanocrystalline iron samples, not specially prepared for positron annihilation studies. From the measurements, it can be concluded that, in contradiction with the results of reference (3) and (4) : i) open volumes smaller than a mono-vacancy can indeed act as an effective positron trap in n-Fe, and ii) the ops lifetime is not distributed according to a very broad lifetime distribution. It has also been demonstrated that : i) the sample preparation has a different influence on grain size and free volume distribution and ii) that in samples where the grain size is larger than 85 nm effects of delocalized positron states (no saturation trapping) are seen. The model as proposed in reference (3) and (4), where three different positron trapping states are considered, adequately explains the experimental results for positron localisation in n-Fe.

ACKNOWLEDGEMENTS

This research is supported by the Belgian Federal Government through the office of the prime minister program Interuniversity Attraction Pole IUAP P4/10 and by the Flemish Government program FWO G0006-96.

REFERENCES

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