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First-principles electronic structure and positron-state calculations for perfect and defected 3C-and 6H-SiC polytypes of SiC have been performed. Monovacancies and divacancies have been treated; the influence of lattice position and nitrogen impurities have been considered in the former case. Positron affinities and binding energies have been calculated; trends are discussed, and the results compared with recent atomic superposition method calculations. Experimental determination of the electron and positron work functions of the same 6H-SiC allows an assessment of the accuracy of the present first-principles calculations, and to suggest further improvements. ͓S0163-1829͑96͒01828-0͔

The mean positron lifetime r. positron diffusion length L, and the positron and electron work functions (d+ and d -) for polycryst,alline Tic have been experimentally determined. The results were i = 160(2) ps. L+ = 138(27) nm and $-= 3.96(0.08) eV; 4+ w a shown to be almost certainly positive. These results svongly support the suggestion from recenl first-principles e l e m n i c StNCNre and positron state calcul~tions that positions are trapped by and annihilate in metal vacancies in this material. XPS measurements indicate that the trapping sites may be predominantly in thin carbon-rich layen between grains, a picture which may also explain the long near-surface diffusion length. may coexist and it is very hard to resolve their sometimes competing roles in the physical

Pt films on alumina substrates, both in their as-received and annealed states, have been extensively characterized by slow positron implantation spectroscopy ͑SPIS͒. Bulk Pt samples have been investigated by conventional positron annihilation spectroscopy ͑PAS͒ as well as by SPIS. A variety of state-of-the-art theoretical calculations have been performed to aid the interpretation of experimental findings. The research shows that a re-interpretation of earlier defect studies of bulk Pt by PAS is required in order to achieve a satisfactory agreement with the present experimental findings and theory.

T he reconstruction of the precipitati on structure after solution treatment has been in vesti gated by positron annihi lation spectroscopy . I n this study the behaviour of samples ta ken from an aircraf t \A irbus 300", being in op eration for 18 years, is compared to a ref erence material supplied by A lcoa. T he results of position annihil ati on spectroscopy show that there are no signi Ùcant di˜erences in b oth materials. T his Ùnding is in agreement w ith the results obtained from other metho ds w ithin the scop e of the general research program w hich points to a materials behaviour as predicted for a safe op eration of this typ e of aircraf t. PACS numb ers: 78. 70. Bj 1. I n t r o d u ct io n T h ese i n vesti gati ons are part of a l arg er pro g ram establ i shed by Ai rb us Industri es to chara cteri ze the a ging b ehavi our of the al l oy Al -2024 used i n the fabri cati on of the aircra ft \ Ai rbus" . The chem ical com p ositi on of thi s al l oy i s gi ven i n T abl e.Po sitro n anni hi l ati on spectro scopy (P A S) is a metho d whi ch i s sensiti ve to defects i n a m i croscopi c and ato mi c scal e. Po sitro ns may b e tra pp ed by vacanci es, vo i ds, di slocati ons, semi coherent and i ncoherent pa rti cl es, and coherent preci pita tes such as Gui ni er{ Presto n zones (G PZ). Theref ore duri ng the l ast years the p ositro n m etho d ha s turned out to b e a p otenti a l to ol for studyi ng decomp ositi o n pheno m ena i n al loys, especiall y i n Al [1{ 3].The f orm ati on of the G PZ al rea dy occurs at (o r near to ) ro om tem p erature. D uri ng ag ing m etasta bl e pha ses are form ed and then di sapp ear or are tra nsform ed i nto more stabl e preci pi ta tes whi ch are ful l y i ncoherent to the m atri x. The sequence (4 41)

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