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The transformation behavior of free-volume defect in (80GeS2-20Ga2S3)100-x (CsI)x (x = 0, 5, 10, 15 mol%) chalcogenide glasses was studied by employing positron annihilation spectroscopic technique, which could reveal valuable information for in-depth understanding of nano-structural defects in glassy matrix. The results indicate that the structural changes caused by CsI additives can be adequately described by positron trapping modes determined with two-state model. The initial addition of CsI (x = 5 mol%) led to a void contraction, whereas, the void agglomeration occurred with the increase of CsI and the free-volume defects of the glasses were obviously reduced. The atomic density ρ is inversely proportional to the number of these defects. Meanwhile, the UV cut-off edge shifts toward short-wavelength with increasing of CsI. This study provides the valuable information of defects evolution in GeS2-Ga2S3-CsI glasses.

Evolution of free-volume positron trapping defects caused by crystallization process in (80GeS 2 –20Ga 2 S 3 ) 100−х (СsCl) x , 0 ≤ x ≤ 15 chalcogenide-chalcohalide glasses was studied by positron annihilation lifetime technique. It is established that CsCl additives in Ge–Ga–S glassy matrix transform defect-related component spectra, indicating that the agglomeration of free-volume voids occurs in initial and crystallized (80GeS 2 –20Ga 2 S 3 ) 100−х (СsCl) x , 0 ≤ x ≤ 10 glasses. Void fragmentation in (80GeS 2 –20Ga 2 S 3 ) 85 (СsCl) 15 glass can be associated with loosing of their inner structure. Full crystallization in each of these glasses corresponds to the formation of defect-related voids. These trends are confirmed by positron-positronium decomposition algorithm. It is shown, that CsCl additives result in white shift in the visible regions in transmission spectra. The γ-irradiation of 80GeS 2 –20Ga 2 S 3 base glass leads to slight long-wavelength shift of the fundamental optical absorption edge and decreasing of transmission speaks in favor of possible formation of additional defects in glasses and their darkening.

Background Hydrogel and silicone-hydrogel polymeric materials are widely used in ophthalmology for the manufacture of contact lenses. An important aspect is the investigation of the structure of these materials. Objectives This study has been conducted in order to compare the degree of disorder and presence of free volumes in the internal structure of the polymeric soft contact lenses Omafilcon A (hydrogel) and Comfilcon A (silicone-hydrogel). Differences in the occurrence of trapping centers for positrons and free volumes between the types of investigated contact lenses have been demonstrated. Material and methods Two types of polymeric contact lenses were used as materials: Omafilcon A (hydrogel) and Comfilcon A (silicone-hydrogel). The study was performed using positron annihilation lifetime spectroscopy (PALS). Results When the results of the measurements has been obtained, a graphical curve has created to describe the relationship of the number of annihilation acts in time. Significant changes were observed between the contact lenses investigated in positron trapping in macropores (based on a two-state model) and the presence of free volumes (based on the Tao-Eldrup model). Conclusions The use of the positron annihilation two-state model made it possible to demonstrate that a higher positron trapping rate in macropores occurs in the silicone-hydrogel contact lens. Additionally, calculations using the Tao-Eldrup model show the existence of free volumes in both types of materials. The size and fraction of free volumes is much larger in the silicone-hydrogel contact lens.

Positron annihilation lifetime spectroscopy combined with Doppler broadening of annihilation radiation was applied to study free-volume entities in GeS2-Ga2S3glasses affected by Ga additions. It is shown that Ga-related void sub-system plays a decisive role in positron trapping process, while the overall density variation is defined mainly by Ge-related sub-system. These results serve as basis for new characterization route for inner free-volume structure of these glasses.

Free volume redistribution, caused by structural relaxation during long-term natural physical ageing, is revealed by positron annihilation lifetime technique for AsxSe100-x and GeySe100-y glasses. It is shown, that two-state positron trapping model is appropriate for an adequate description of changes caused by long-term physical ageing in the atomic-deficient subsystem of these materials. The observed behaviour of defect-related component in the fit of the experimentally measured PAL spectra for aged and rejuvenated glasses testifies in favour of fragmentation of larger free volume entities.

The positron irradiation effect in high density polyethylene at ambient temperature is studied by positron annihilation lifetime method. The positron spectra are analyzed with a model which takes into account positron trapping and so called delayed formation of positronium (DFP). A good relationship is noticed between the rate of positron trapping and the rate of DFP.

Structural transformations caused by coarse-grained powdering and fine-grained mechanochemical milling in a dry mode were probed in high-temperature modification of tetra-arsenic tetra-sulfide known as β-As4S4. In respect to X-ray diffraction analysis, the characteristic sizes of β-As4S4 crystallites in these coarse- and fine-grained powdered pellets were 90 and 40 nm, respectively. Positron annihilation lifetime spectroscopy was employed to characterize transformations occurred in free-volume structure of these nanoarsenicals. Experimentally measured positron lifetime spectra were parameterized in respect to three- or two-term fitting procedures and respectively compared with those accumulated for single crystalline realgar α-As4S4 polymorph. The effect of coarse-grained powdering was found to result in generation of large amount of positron and positronium Ps trapping sites inside arsenicals in addition to existing ones. In fine-grained powdered β-As4S4 pellets, the positron trapping sites with characteristic free volumes close to bi- and tri-atomic vacancies were evidently dominated. These defects were supposed to originate from grain boundary regions and interfacial free volumes near aggregated β-As4S4 crystallites. Thus, the cumulative production of different positron traps with lifetimes close to defect-related lifetimes in realgar α-As4S4 polymorph was detected in fine-grained milled samples.

The annealing of submicrocrystalline copper produced by the equal channel angular pressing followed by rolling was studied using positron annihilation and XRD analysis. In as-prepared samples, positrons are trapped at vacancies, concentration of which is very high (∼1.6 × 10−4) and dislocation type defects; however, a few percent of positrons annihilate from a free state. Increasing annealing temperature leads to the formation of vacancy complexes. The main positron trap centers in the temperature range ΔT = 20–300°C are vacancies and their small complexes of two or three vacancies. The dominant centers of positron trapping in the temperature range ΔT = 300–670°C are dislocation-type defects.

The report about subsurface zone (SZ) created during dry sliding in pure bismuth is presented. The positron lifetime measurements revealed the well defined subsurface zone where the concentration of created during sliding vacancy clusters decreases exponentially with the depth from the worn surface increases. The experimental results are quite well described by the two state trapping model which allows to determine the positron trapping rate and hence deduce about the depth dependency of vacancy voids concentration. Despite of the low recrystallization temperature ca. 115 °C in this semimetal, determined also by the positron lifetime measurements, no significant effect of temperature rise due to sliding on the SZ constitution is observed.

Positron annihilation depth profiling is applied as a sensitive probe to investigate the defect evolution of hydrogenated amorphous silicon (a-Si:H) absorber layers fabricated by the PE-CVD method with typical thicknesses of 300-500 nm. The Doppler broadening lineshape parameter S of the layers was found to depend significantly on the applied hydrogen-to-silane flow ratio during their deposition, related to hydrogen-induced changes in the microstructure and local composition at the positron trapping site. Light-soaking degradation induces a time-dependent variation in the S parameter which seems to correlate with the evolution of defect densities extracted from FT-Photocurrent Spectroscopy measurements. (© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Extended defects in insulating nanoporous MgAl2O4 spinel-type ceramics which trap positrons are characterized by positron annihilation lifetime spectroscopy before and after water-immersion treatment. It is shown that in both cases the same type of positron traps prevails, but positron trapping in defects near grain boundaries occurs more efficiently in water-moistened ceramics. It is shown the more perfect structure of ceramics, the more considerable changes occur in the positron trapping parameters of water-moistened ceramics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It is shown that extended free-volume defects in sintered functional spinels can be well characterized with positron annihilation lifetime spectroscopy, provided obtained data are treated in terms of unified multi-channel positron annihilation model involving both positron trapping and ortho-positronium decay modes. Crystallographical specificity of spinel structure reflects in the shortest lifetime component, while the middle component corresponds to vacancy-like defects near grain boundaries. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass.

Positron lifetime and Doppler broadening measurements were performed to study the precipitation process of MgCa alloys. Isochronal studies revealed changes in the precipitate nucleation and growth rates. The evolution of the ageing treatment was studied by observing the positron trapping at the matrix–Mg 2 Ca precipitate interfaces.

The defect properties in as-grown and deformedp-type GaAs with different concentration of dopants and different growth method have investigated by positron lifetime measurement. The result indicates that no positron trapping was observed in LEC-grown Zn-dopedp-type GaAs. However, in HB- and FZ-grown Zn-doped GaAs, positron trapping into vacancy type defects was observed. In deformed samples, clusters were formed during deformation. Positron detected shallow positron traps and the dominant shallow positron traps were attributed to Zn acceptors in Zn-doped GaAs.

We have used a low-energy positron beam to investigate the defects in N-doped and undoped ZnSe layers grown on GaAs:Si or ZnSe substrates. In N-doped ZnSe we observe positron trapping at vacancies, identified as VSeNSe complexes, and at negative ions. Based on positron data, we give estimates for the concentrations of vacancies and negative ions. The results suggest that in addition to forming isolated NSe acceptors or VSeNSe pairs, incorporated nitrogen also forms donor-type defects leading to compensation of holes. In undoped ZnSe layers, the results show presence of Zn vacancies in concentrations 10–10 cm. The Zn vacancy concentration correlates with the dislocation density at the ZnSe/GaAs interface.

Fachbereich Physik der Martin-Luther-Universita¨t Halle-Wittenberg, Experimentelle Physik III, Friedemann-Bach-Platz 6,D-06108 Halle (Saale), Germany~Received 11 August 1998!Isochronal annealing investigations in electron-irradiated 6H-SiC were correlated with temperature-dependent measurements of positron lifetime. It turned out that the positron trapping at temperatures up to 300K was dominated by the trapping in shallow positron traps. These defects were already present in the unirra-diated material and could be attributed to the antisite defects. The annealing of the irradiation-induced mono-vacancies and divacancies took a continuous course up to 1740 K. In the wide-temperature range between 300and 1200 K, different monovacancylike defects annealed and above 1200 K the divacancy disappeared. Theweak increase of the average positron lifetime between 750 and 1200 K detected at measurement temperaturesbelow 300 K must be caused by partial annealing of shallow traps, probably irradiation-induced shallow traps.After the final annealing at 1740 K all of the irradiation-induced defects vanished.@S0163-1829~99!00516-0#I. INTRODUCTION

The YBaCuO and LaSrCuO systems are well known as examples of ceramic superconductors with high transition temperatures. The purpose of the present study is to investigate explosively packed YBa2Cu3O7−x and statically packed systems LaNdBaCuO and LaNdPbCuO. The investigation was carried out at room and liquid nitrogen temperatures using positron annihilation spectroscopy. The results are interpreted in the light of positron trapping at defects. Changes in the lifetime values correspond to changes in defect type, whilst changes in the intensity reflect changes in the defect concentration. The data show that the sample preparation remains a crucial factor. The present results indicate that if the understanding of positron properties in high Tc superconductors can be developed to the same extent as has been done for conventional metals, positron annihilation may become a useful material-characterisation tool for these materials just as it certainly has become for conventional metals.

Positron annihilation spectroscopy on nanocrystalline Pd85Zr15 shows the segregation of Zr at the interfaces and a decrease of the fraction of nanovoids the size of 10 to 15 missing atoms upon isochronal annealing up to Ta=1100 K by which the grain size increases up to d=185 nm. In this state, the positron trapping at interfacial free volumes reversibly changes contingent upon the temperature whereas the interfacial composition is unchanged when the measuring temperature varies.

Positron annihilation has been applied to study thermally oxidized 4H- and 6H-SiC. The SiC/SiO2 interface is found to contain a high density of open-volume defects. The positron trapping at the interface defects correlates with the charge of the interface determined by capacitance–voltage experiments. For oxides grown on n-SiC substrates, the positron annihilation characteristics at these defects are nearly indistinguishable from those of a silicon/oxide interface, with no discernable contribution from C-related bonds or carbon clusters. These results indicate that those defects at the SiC/oxide interface, which are visible to positrons, are similar to those at the Si/oxide interface. The positron annihilation characteristics suggest that these defects are vacancies surrounded by oxygen atoms.

Basing on the approach to the calculations of the positron trapping rate presented by Shirai and Takamura [1], an attempt is made to explain the prevacancy effects observed in positron annihilation measurements in real metals. The calculations are done for the point defect and for the dislocation, taking into account the thermal distribution of the positron energy and the thermal expansion of the crystal lattice. In spite of the thermal distribution of energy of thermalized positrons the resonance trapping of positrons can be responsible for prevacancy effects in metals and can help solve some problems in the study of positron annihilation. Auf der Basis einer Naherung der von Shirai und Takamura [1] vorgestellten Berechnungen zur Positronen-Einfangrate wird der Versuch unternommen, die Vorleerstelleneffekte zu erklaren, die bei Positronenannihilationsmessungen in realen Metallen beobachtet wurden. Die Berechnungen werden untcr Berucksichtigung der thermischen Verteilung der Positronen-Energie und der thermischen Gitterausdehnung fur Punktdefekte und Versetzungen durchgefuhrt. Der resonante Positroneneinfang konntc trotz der thermischen Verteilung der thermalisierten Positronen verantwortlich fur die Vorleerstelleneffekte sein. Er konnte bei der Losung einiger Probleme im Rahmen der Untersuchung der Positronenannihilation nutzlich sein.

Positron lifetime measurements have been performed to study vacancy defects in vapour-liquid-solid (VLS) grown PbSe and Pb 1- x Sn x Se ( x = 0.07, 0.1, 0.3). Post-growth annealing under various vapour pressure conditions has been used to change the number of Pb vacancies, resulting in the determination of the specific positron trapping rate μ Pb v = (1.0±0.1)×10 14 s -1 . The sensitivity range of the positron annihilation method to the Pb vacancies was found to be 10 17 Pb ] 20 cm -3 . Positron lifetimes in perfect PbTe, PbSe and PbS crystals have been calculated. Moreover, we have predicted lifetimes of positrons trapped by vacancies. The calculated lifetimes in bulk and defects correspond well to the experimentally determined values.

One-dimensional angular correlation of annihilation radiation was measured in a series of amorphous (Fe100-xCrx)85B15 alloys (x = 0-32) exposed to ,-irradiation from intensive "Co source. The relevant shape parameters were compared with those in non-irradiated alloys. Irradiation-induced changes of those parameters were found to depend on Cr contents x. The opposite signs of these changes were observed in x 4 ranges, while near x = 4 the parameters stayed unchanged. We try to interpret the results assuming that γ-quanta are able to knock out boron atoms from their positions into large cavities, thus preventing positron trapping. The crystalline embryos mode1 of amorphous alloys suggested by Hamada and Fujita is also used for explanation of the observed effects. PACS numbers: 61.43.Dq, 61.80.Ed, 78.70.Bj

A brief introduction to the subject of positron annihilation techniques (PAT) will be presented. The application of PAT to the characterization of radiation induced defects in metals and to investigate defect kinetics during post irradiation isochronal annealing is discussed. Typical examples are given showing the processes of positron trapping, detrapping, helium-vacancy complex formation, vacancy-impurity interaction, bubble characterisation and positronium formation in radiation induced, defect rich metals. Some recent results of PAT indicating the presence of vacancy-like defects in quasicrystals of Al-Mn, Al-Mn-Si and Al-Cu-Fe will be presented.

The defect structure in fusion neutron irradiated Ni and Ni+25 at% Fe has been studied by positron lifetime spectroscopy. The positron trapping is used as a defect indicator. Two lifetime components τ 1 and τ 2 are resolved. It is found that the number of vacancies in vacancy clusters is larger than 11. The recovery stage in Ni+25 at% Fe is shifted from 750 to 850 K.

The sintering process of CuO under isochronal annealing has been investigated by positron lifetime spectroscopy. A long-lived component of 370±20 psec, observed in as-pressed samples and samples annealed at T≤900 K, has been attributed to continuous channels along three-grain junctions. A short-lived component of about 224 psec is attributed to positron traps at two-grain interfaces. The annihilation parameters exhibit recovery stages which may be correlated with the stages of sintering. A drastic recovery, starting at 975 K, is correlated with the final stage of sintering. The results suggest a lifetime of approximately 147 psec for positrons in the bulk of CuO. © 1991 Chapman and Hall Ltd., The present work was partially supported by C.I.C.Y.T. (Spain) under grant no. MAT 30/89.

In this work, a three state trapping model has been applied to several metal elements. A good linear trend was found in both E 1v F vs. Tc, E 1v F vs. Tm and Tc vs. Tm curves. Some indication of positron trapping in divacancies is also reported in some elements.

Upon hydrogenation of KC8 to KC8H2/3 at 295 K, the free-annihilation lifetime of positrons is shortened. The lifetime of positrons trapped in defects changes from 455 to (380 to 405) ps and their intensity increases from 30 to 50% of the total intensity. With an increase in temperature from 295 to 320 K, the intensity due to trapped positrons in KC8H2/3 increases to 60%. Domain edges formed through the structural change of KC8 and thermally generated vacancy-type defects work as trapping centers for positrons. Nach Hydrierung von KC8 zu KC8H2/3 bei 295 K wird die Annihilationslebensdauer der Positronen verringert. Die Lebensdauer von in Defekten eingefangenen Positronen andert sich von 455 zu 380 bis 405 ps und ihre Intensitat nimmt von 30 auf 50% der Gesamtintensitat zu. Mit der Zunahme der Temperatur von 295 auf 320 K nimmt die Intensitat der eingefangenen Positronen in KC8H2/3 auf 60% zu. Die durch die strukturelle Veranderung von KC8 gebildeten Domanenecken und die thermisch erzeugten Defekte vom Leerstellentyp wirken als Einfangszentren der Positronen.

Results of a Doppler broadening positron annihilation study on crystalline, quasicrystalline and amorphous Al-Cu-T (T=FeV) alloys are presented. For the first time it is shown that a quasicrystal (Al65Cu20Fe15) can be prepared with a positron trapping site concentration below the sensitivity of positron annihilation spectroscopy. The results and those of similar studies previously reported for quasicrystalline Al-Mn-Si and Al-Cu-Li are discussed in conjunction with possible stabilisation mechanisms.