Papers by Keyword: Positron Annihilation Lifetime Spectroscopy (PALS)

Abstract: We present a novel attempt to combine in-situ and ex-situ measurements -- here exemplary for aluminum alloys. For this research we have chosen an Al-1.7Cu-1.3Mg (at.\%) alloy, which has been cast from high purity elements (5N5 Al, 4N Cu and 4N Mg). DSC (differential scanning calorimetry) serves as a basic method, which is employed to determine different precipitation states towards the final S-phase formation: before the formation starts, at the maximum of the exothermal peak, and after the end of the exothermal reaction. By an abrupt truncation of the heating ramp (5 K/min), i.e. cooling quickly to room temperature, the current sample state is frozen-in for a defined temperature. After truncation all samples have been measured without further preparation by X-ray diffraction (XRD), positron annihilation lifetime spectroscopy (PALS). By this treatment we could correlate exactly different sample states, which is impossible by conventional experiments, i.e. heating to a defined temperature and holding for a certain time. This attempt opens new possibilities to investigate defined and comparable sample states by methods, which require extensive sample preparation, like TEM or 3DAP, and in-situ methods like DSC or XRD / EXAFS / SAXS at synchrotron beam lines.

Abstract: To determine the electrically inactive fraction of As or P in heavily doped as-grown Czochralski Si 4-point resistivity and SIMS measurements were carried out. No clear trend for the electrical inactive fraction was found with an increasing dopant concentration, though a mean electrical inactive fraction of 11.5% for As doping could be determined.Experimental results on a dopant-vacancy complex in as-grown Si are scarce, hence temperature-dependent positron annihilation lifetime spectroscopy (PALS) was carried out on several heavily As and P doped as-grown Si samples. The measured average positron annihilation lifetime τ_av is between 218 ps and 220 ps. No temperature dependent effect on τ_av could be observed. Therefore, it can be concluded that in the studied doping range the dopant-vacancy complexes do not exist. The reason for the inactivation of the dopant has to be found elsewhere. A possible explanation can be the formation of dopant precipitates.

Abstract: Kovdor (Karelia) deposite vermiculites modified by acid of different concentrations and chitosan were studied using positron annihilation lifetime spectroscopy, measuring apparent density, dye adsorption, nitrogen adsorption by BET and porometry. It has been shown that after acidizing vermiculite density changes comparing to the samples’ density depending on the acid concentration. The density increases with the increasing acid concentration. Internal volume of the micropores and the value of maximum sorption of brilliant green change directly proportional to the density of the modified vermiculite.

Abstract: This study was focused on commercial oxide-dispersion strengthened (ODS) steels - MA 956 (20%Cr), PM 2000 (19%Cr), ODM 751 (16%Cr) and MA 957 (14%Cr) developed for fuel cladding of GEN IV reactors. The ODS steels are described in order to comparison of their microstructure features. Vacancy defects were observed by Doppler Broadening Spectroscopy (DBS) and Positron Annihilation Lifetime Spectroscopy (PALS). Residual stress proportional to all kinds of defects was investigated by Magnetic Barkhausen Noise (MBN) measurement. The highest presence of open volume defects was found in MA 956 and the lowest defect concentration in MA 957, although this steel contains the largest defects (six-vacancies together with dislocations). Other investigated steels demonstrated probably three- or four-vacancy clusters. Further, results from positron technique indicated proportionality of chromium content to defect concentration. Magnetic Barkhausen noise results also showed that Hpeak value (describing grain size) increased with growth of chromium content. However residual stress was independent on chromium level.

Abstract: Positron annihilation lifetime spectroscopy (PALS) has widely been used for probing open volume defects in various materials. PALS is in principle non-destructive, yet conventional PALS is not strictly non-destructive because cutting out of two specimens from the material is required. Recently we developed a novel method of PALS, which is potentially applicable to non-destructive, onsite material inspection. In order to explore the possibility of onsite monitoring of polymer degradation by this novel method of PALS, we studied variations of positron lifetime and mechanical properties of ultra high molecular weight polyethylene (UHMWPE) subjected to γ-irradiation. Correlations were found between the mechanical properties and ortho-positronium lifetimes, suggesting the feasibility of non-destructive, onsite monitoring of polymer degradation by PALS. The effect of γ-irradiation on positronium formation is discussed.

Abstract: The new organic-inorganic materials, based on polyether chains covalently linked to a silica framework through urea bridges, referred as ureasilicates or ureasils, and semiconducting As₂S₃-ureasil composites are investigated using positron annihilation lifetime spectroscopy (PALS) and scanning electron microscopy (SEM) techniques. The results obtained show that incorporation of the As2S3 clusters into ureasil affects on the ortho-positronium (o-Ps) intensity or positronium formation probability and micro-/nanoscopic structure compared to the pure polymer, the effect is more essential as the loading fraction of As₂S₃ increases.

Abstract: In the last years there have been made several studies, involving many materials, with the aim of trying to explain some physical, chemical and mechanical macroscopic properties of these materials, across the study of the free volumes at the atomic and molecular scale. The positron annihilation lifetime spectroscopy (PALS) is one of the most widely used in this type of studies, and it was here used to characterize the free volumes fraction of a commercial epoxy and trying to understand the mechanism associated with the process of moisture absorption which leads to an increase of mass and volume on this commercial epoxy.

Abstract: Defects are central to any understanding of material characteristics. They determine a host of material properties, including strength, ductility, resistivity and opacity. Positron annihilation spectroscopy (PAS) is a well-established high-sensitivity technique for detecting defects in solids. In this work, we study the influence of plastic deformation on the properties of 5754(AlMg3) aluminum alloy.

Abstract: Positron chemistry refers to chemical processes of high-energy positrons injected into molecular substances, the most interesting of which is the formation of positronium (Ps), the hydrogen-like bound state between a positron and an electron. Ps is formed predominantly by fast intra-track radiation chemical processes. In polymers it tends to be localized in intra/inter-molecular open space in the sparsely packed amorphous structure. Whilst short-lived singlet para-positronium (p-Ps) undergoes self-annihilation, the positron in long-lived triplet ortho-positronium (o-Ps) annihilates with one of the spin opposite electrons bound in the surrounding polymer molecules. This process is called pick-off annihilation. The pick-off annihilation lifetime reflects the polymer chain packing through the size of the volume, where Ps is localized. Positrons are used to probe the amorphous structure of various polymeric systems. In this article, basic concepts and experimental techniques of positron chemistry in polymers as well as applications to the characterization of functional polymeric materials are overviewed.

Abstract: Positron annihilation lifetime spectroscopy (PALS) is an experimental technique whereby the lifetime spectrum of positrons injected into a material is measured. Analysis of this spectrum can be used to characterize defects in the material. While radioisotope positron sources are often used for PALS, there are several advantages of using mono-energetic, slow positron beams. In order to measure lifetime spectra with such beams it is necessary to produce a pulsed beam with an extremely short pulse length (~ 200 ps). In this article we discuss the advantages of using pulsed, slow-positron beams, the various methods of beam production and pulsing methods. We focus in particular on intense beams generated by electron accelerators and describe the current status and future plans for the accelerator based facility at AIST.