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Results from Pt model catalyst surfaces have demonstrated that surface defects, in particular surface concavities, can improve the oxygen reduction reaction (ORR) kinetics. It is, however, a challe...

Friction stir welding is an innovative welding technology that is often employed to weld aluminum alloys of the 2xxx-series. These alloys are primarily used in the aerospace industry to join structural parts or tanks. For these applications, high quality requirements have to be met. In a previous study, it was shown that controlling the welding temperature is one key to ensuring the quality of the welds. It was observed that the tensile strength varies depending on the welding temperature and the welding speed. In this paper, Positron Annihilation Spectroscopy (PAS) with a scanning positron beam was applied to detect crystallographic defects and changes of the microstructure of friction stir welds. The results were compared to optical microscopy and to hardness maps. Hereby, vacancy formation in the cross section of friction stir welds could be observed. Furthermore, it could be demonstrated that the state of the precipitates is the primary determinant for the mechanical properties of the welds. Finally, the observations made through PAS were employed to explain the trends in the mechanical properties, which were observed in the previous study. In particular, the varying ultimate tensile strength (UTS) could be clearly attributed to dissolution and re-precipitation phenomena, which occur during and after welding.

We measured the temperature dependent equilibrium vacancy concentration using in-situ positron annihilation spectroscopy in order to determine the enthalpy $H_\text{f}$ and entropy $S_\text{f}$ of vacancy formation in elementary fcc-La. The Arrhenius law applied for the data analysis, however, is shown to fail in explaining the unexpected high values for both $S_\text{f}$ and $H_\text{f}$: in particular $S_\text{f}=17(2)~k_\text{B}$ is one order of magnitude larger compared to other elemental metals, and the experimental value of $H_\text{f}$ is found to be more than three standard deviations off the theoretical one $H_\text{f}=1.46~\text{eV}$ (our \acs{dft} calculation for La at $T=0~\text{K}$). A consistent explanation is given beyond the classical Arrhenius approach in terms of a temperature dependence of the vacancy formation entropy with $S_\text{f}^\prime=-0.0120(14)~k_\text{B}/\text{K}$ accounting for the anharmonic potential introduced by vacancies., Comment: 5 pages, 3 figures

The fraction of positronium (Ps) emitted from a surface of a germanium single crystal at high temperature is usually assumed to approach unity at zero positron implantation energy. In the experiment, however, the determination of the absolute Ps fraction is not straightforward since recording a reference spectrum with $100%$ Ps formation remains demanding. We use geant4-simulated detector responses to $2\ensuremath{\gamma}$ and $3\ensuremath{\gamma}$ radiation sources mimicking positron and Ps annihilation inside the (coincidence) Doppler-broadening spectrometer at NEPOMUC, FRM II, in order to derive a reliable value for the Ps fraction reemitted from a Ge(100) target heated close to its melting point. Analysis of the measured spectra by fitting the simulated spectra shows an absolute value of $72\ifmmode\pm\else\textpm\fi{}4%$ maximum Ps formation, contradicting the $100%$ assumption.

Microstructural changes of a commercial Al–Mg–Si alloy were studied during artificial aging by in situ Doppler broadening spectroscopy using a high-intensity positron beam. The in situ positron annihilation characteristics at high temperatures differ considerably from the conventionally applied ex situ measurements at low temperatures. Therefore, a more comprehensive view of precipitation processes in Al–Mg–Si alloys is obtained. Further, in situ positron–electron annihilation techniques allow for an investigation of aging processes with increased sensitivity. For the artificial aging temperatures of 180 °C and 210 °C pronounced variations of the Doppler broadening S-parameter reveal (i) the evolution of clusters into larger precipitates and (ii) the time of the formation of ” precipitates and the role of vacancies in connection to this. Towards higher aging times, the transformation from coherent ” to semi-coherent ’ precipitates could be verified. Additional insights are gained by in situ measurements of the S-parameter during the solution heat treatment of the previously overaged sample. Here, the S-parameter reveals both the dissolution of precipitates starting from temperatures of 364 °C and the thermal generation of vacancies.

The CDB-spectrometer at NEPOMUC enables spatially resolved Doppler Broadening Spectroscopy of the electron-positron annihilation line with a lateral resolution of up to 200~$\mu$m (FWHM). We have applied this technique for studying single particles of silica gel dependent on loading with an altering amount of the ionic liquid [C2MIM][NTf2]. Our results reveal a strong correlation between the observables of DBS and the loading factor which presumably arises from a smaller inner surface of voids in loaded silica gel.

In this study the structure and evolution of vacancy type defects in lithium ion batteries are investigated in respect of crystallographic properties. The relation between positron annihilation and electronic structure is discussed in terms of structural dynamics during the lithiation process. Samples of Li1−xNi1/3Mn1/3Co1/3O2 (NMC-111) electrodes with decreasing lithium content (x = 0–0.7) covering the whole range of state of charge were electrochemically prepared for the non-destructive analysis using positron coincidence Doppler broadening spectroscopy (CDBS). The positron measurements allowed us to observe the evolution of the defect structure caused by the delithiation process in the NMC-111 electrodes. The combination of CDBS with X-ray diffraction for the characterization of the lattice structures enabled the analysis of the well-known kinetic-hindrance-effect in the first charge-discharge cycle and possible implications of vacancy ordering. In particular, CDBS revealed the highest degree of relithiation after discharge to 3.0 V at 55 °C. For the first time, we report on the successful application of CDBS on NMC-111 electrodes yielding new insights in the important role of defects caused by the delithiation process and the kinetic hindrance effect.

A brief report is given on recent studies of the atomistic processes during charging of battery cathode material LixCoO2 by means of magnetometry and positron annihilation. A set-up for operando magnetometry is implemented which, for the first time, allowed to continuously monitor the distinct variation of the magnetic susceptibility χ of LixCoO2 which occurs during consecutive charging and discharging cycles. The variation of χ with Li+ content in the concentration range 1>x≥0.77 arises from a variation of the electronic density of states and from electronic correlation effects. The χ (x)-behaviour for xx)-variations in this concentration regime. First measurements on LixCoO2 thin-films performed at the positron beam line NEPOMUC of FRM II at the Heinz Maier-Leibnitz neutron source will be presented.

Different ODS RAF steels with nominal composition Fe–14Cr–2W–0.3-Ti–0.3Y2O3 (wt%) and a model Fe–14Cr (wt%) alloy have been subjected to single Fe+ or He+ ion irradiations to simulate the effect of a fusion environment at different doses and temperatures. The irradiation induced open volume defects have been characterised by positron annihilation spectroscopy and transmission electron microscopy. For low temperature Fe+ irradiations, positron annihilation spectroscopy results show a high concentration of irradiation induced vacancy clusters, much more evident in the model alloy as compared to the ODS steels. The extent of damage due to He+ irradiations at RT and high temperatures is considerably lower than for the Fe+ irradiations. Regardless of the temperature He+ irradiations introduced changes in the chemical environment of open-volume positron traps possibly due to the creation of He-vacancy complexes. Positron annihilation spectroscopy results also suggest that irradiation induced vacancy-type defects in ODS steels are not associated with nanoparticles but are mainly in the ferritic matrix. The presence of irradiation induced bubbles was studied after He+ irradiation at high temperatures. Small bubbles were observed, being smaller in the ODS steels as compared to the model alloy.

We developed a new scanning positron microbeam for spatially resolved (coincident) Doppler broadening spectroscopy ((C)DBS) at the neutron induced positron source Munich (NEPOMUC). For this purpose, positrons are moderated three times: First, positrons generated by pair production are self-moderated in annealed polycrystalline Pt foils inside the beam tube housing the positron source NEPOMUC. Then, the brightness of the primary 1 keV positron beam with 109 moderated positrons per second is enhanced by a W(100) single crystal remoderator operated in back reflection geometry. Finally, in the upgraded CDB spectrometer we apply a 100 nm thin Ni(100) foil acting as transmission remoderator to generate a positron microbeam with a minimum beam diameter of 33 µm. We demonstrate the potential of laterally resolved DBS by imaging the defect distribution around a laser beam weld of the high-strength age-hardenable Al alloy AlCu6Mn. CDBS was applied with unprecedented spatial resolution to reveal the dissolution of Cu rich precipitates and the formation of Al-vacancy-Cu-complexes.We developed a new scanning positron microbeam for spatially resolved (coincident) Doppler broadening spectroscopy ((C)DBS) at the neutron induced positron source Munich (NEPOMUC). For this purpose, positrons are moderated three times: First, positrons generated by pair production are self-moderated in annealed polycrystalline Pt foils inside the beam tube housing the positron source NEPOMUC. Then, the brightness of the primary 1 keV positron beam with 109 moderated positrons per second is enhanced by a W(100) single crystal remoderator operated in back reflection geometry. Finally, in the upgraded CDB spectrometer we apply a 100 nm thin Ni(100) foil acting as transmission remoderator to generate a positron microbeam with a minimum beam diameter of 33 µm. We demonstrate the potential of laterally resolved DBS by imaging the defect distribution around a laser beam weld of the high-strength age-hardenable Al alloy AlCu6Mn. CDBS was applied with unprecedented spatial resolution to reveal the dissolution of C...

The oxygen deficiency $\ensuremath{\delta}$ in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) plays a crucial role for affecting high-temperature superconductivity. We apply (coincident) Doppler broadening spectroscopy of the electron-positron annihilation line to study in situ the temperature dependence of the oxygen concentration and its depth profile in single crystalline YBCO film grown on ${\mathrm{SrTiO}}_{3}$ (STO) substrates. The oxygen diffusion during tempering is found to lead to a distinct depth dependence of $\ensuremath{\delta}$, which is not accessible using x-ray diffraction. A steady state reached within a few minutes is defined by both, the oxygen exchange at the surface and at the interface to the STO substrate. Moreover, we reveal the depth-dependent critical temperature ${T}_{\mathrm{c}}$ in the as prepared and tempered YBCO film.

We present studies on the free volume of silica gel dependent on water absorption and on loading with a different amount of the ionic liquid (IL) [C2 MIM][NTf2] using positron lifetime spectroscopy (PLS). The found two long lifetime components (2 and 10–20 ns) are attributed to the annihilation of ortho-positronium (o-Ps) in micropores of different size. The application of a semi-empirical quantum mechanical model allowed us to correlate the measured o-Ps lifetimes to the absolute mean size of the micropores by approximation with a spherical shape. The mean diameter of the pores in the pure silica gel could, hence, be determined to 1.72(6) nm. IL loading with up to 70 vol% lead to the formation of smaller voids with a mean diameter of 1.28(4) nm according to a reduction of the mean pore volume by about 60%. In addition, the accumulation of H2 O molecules from the surrounding air inside the pores could be observed in-situ by time dependent PLS.

Heusler compounds exhibit a wide range of different electronic ground states and are hence expected to be applicable as functional materials in novel electronic and spintronic devices. Since the growth of large and defect-free Heusler crystals is still challenging, single crystals of Fe2TiSn and Cu2MnAl were grown by the optical floating zone technique. Two positron annihilation techniques -Angular Correlation of Annihilation Radiation (ACAR) and Doppler Broadening Spectroscopy (DBS)- were applied in order to study both, the electronic structure and lattice defects. Recently, we succeeded to observe clearly the anisotropy of the Fermi surface of Cu2MnAl, whereas the spectra of Fe2TiSn were disturbed by foreign phases. In order to estimate the defect concentration in different samples of Heusler compounds the positron diffusion length was determined by DBS using a monoenergetic positron beam.

Outstanding crystalline perfection is a key requirement for the formation of new forms of electronic order in a vast number of widely different materials. Whereas excellent sample quality represents a standard claim in the literature, there are, quite generally, no reliable microscopic probes to establish the nature and concentration of lattice defects such as voids, dislocations and different species of point defects on the level relevant to the length and energy scales inherent to these new forms of order. Here we report an experimental study of the archetypical skyrmion-lattice compound MnSi, where we relate the characteristic types of point defects and their concentration to the magnetic properties by combining different types of positron spectroscopy with ab-initio calculations and bulk measurements. We find that Mn antisite disorder broadens the magnetic phase transitions and lowers their critical temperatures, whereas the skyrmion lattice phase forms for all samples studied underlining the robustness of this topologically non-trivial state. Taken together, this demonstrates the unprecedented sensitivity of positron spectroscopy in studies of new forms of electronic order., 8 pages, 4 figures

The damage profile of oxide dispersion strengthened steels after single-, or simultaneous triple-ion irradiation at different conditions has been characterized using a low energy positron beam in order to provide information on microstructural changes induced by irradiation. Doppler broadening and coincident Doppler broadening measurements of the positron annihilation line have been performed on different Fe-Cr-(W,Ti) alloys reinforced with Y2O3, to identify the nature and stability of irradiation-induced open-volume defects and their possible association with the oxide nanoparticles. It was found that irradiation induced vacancy clusters are associated with Cr atoms. The results are of highest interest for modeling the damage induced by 14 MeV neutrons in reduced activation Fe-Cr alloys relevant for fusion devices. This investigation was supported by the Ministerio de Ciencia e Innovación (Contract ENE2010-17462), and the European Commission through the European Fusion Development Agreement (Contract No. 09-240) and 7th FP through the “Research infrastructures” action of the “Capacities” Program, NMI3-II (Grant 283883). The authors acknowledge the JANNUS-Saclay team for their scientific and technological advice.

In this paper, a dual-polarized antenna set for ultrawideband (UWB) handheld applications is presented. The proposed structure is intended for communication with circularly-polarized antennas located in base stations. It consists of two orthogonal lightly skewed dipoles with elliptic branches. In the operating frequency range, the antennas show good impedance matching and negligible group delay and gain variations. Therefore, the structure is suitable for transmitting and receiving UWB signals in the frequency range between 3.5 GHz and 4.5 GHz.

In summer 2012, the new NEutron induced POsitron Source MUniCh (NEPOMUC) was installed and put into operation at the research reactor FRM II. At NEPOMUC upgrade 80% 113Cd enriched Cd is used as neutron-gamma converter in order to ensure an operation time of 25 years. A structure of Pt foils inside the beam tube generates positrons by pair production. Moderated positrons leaving the Pt front foil are electrically extracted and magnetically guided to the outside of the reactor pool. The whole design, including Pt-foils, the electric lenses and the magnetic fields, has been improved in order to enhance both the intensity and the brightness of the positron beam. After adjusting the potentials and the magnetic guide and compensation fields an intensity of about 3·109 moderated positrons per second is expected. During the first start-up, the measured temperatures of about 90°C ensure a reliable operation of the positron source. Within this contribution the features and the status of NEPOMUC upgrade are elucidated. In addition, an overview of recent positron beam experiments and current developments at the spectrometers is given.

We present an extension of an existing outdoor UWB scatterer detection algorithm [1], [2], making it usable for indoor scenarios. The algorithm is extended with the capability to find virtual sources, which can explain reflections from extended, plane surfaces. The extension thus removes the restriction to just single-bounce signal paths. Single-bounce scatterers, as well as virtual sources extracted from the channel impulse response, are assigned to positions in 2-dimensional space and can be matched to given floor plan information. The functionality is demonstrated with indoor measurement data obtained in a large office environment that represents a challenging dense multipath scenario. Results show that estimated locations of virtual sources and scatterers fit very well with the expected locations.

We present a detailed analysis of an indoor UWB channel measurement campaign. The focus is on the modeling of the deterministic part of the multipath channel using a-priori known relevant reflections and scatterers, found from an available floor plan. Our approach uses virtual signal sources, whose locations and visibilities can be calculated using simple ray-launching techniques. The channel analysis steps exploit these results, using an effective multipath cancellation method that introduces virtually no artifacts. We show that the corresponding multipath-components can explain up to 90% of the UWB channel impulse responses in terms of energy capture. This is important for multipath-aided indoor localization, which provides robust position fixes using a single base station only.

The IEEE 802.15.4a standard for wireless sensor networks is designed for highly accurate ranging with impulse radio Ultra Wideband (UWB) signals. The standard allows a large number of different system parameters for the preamble, which are analyzed in detail for energy detectors. Energy detectors have low complexity and promise low cost and low energy consumption. The focus of this work is on statistical modeling of inter-pulse interference (IPI), noise terms and correlation properties of two despreading methods. One method shows perfect crosscorrelation properties and the other one shows a zero-mean noise floor. An IPI intensive LOS industrial environment was chosen to verify the theoretical results and to evaluate both despreading methods for different system settings. It is shown that the first method performs better if IPI is the limiting factor and the other one is suitable if noise is dominant.

In this contribution we present a modular IEEE 802.15.4a UWB demonstrator platform. The demonstrator supports a one-way communication link and allows for ranging and communication experiments.

A chip manufacturing process requires extensive support of CAD-tools in order to predict the behavior of the embedded circuitry and to ensure the intended system functionality. Past experience shows that the overall performance of UHF RFID systems is mainly limited by multipath propagation and detuning. In this context, system-level simulations are vital to assess the overall performance and improve the embedded circuit. We present a simulator framework capable of handling chip-level tag models, fading MIMO radio channels, and interrogator building blocks on signal level. It is based on highly flexible behavioral tag-models instead of highly accurate but static ASIC models. In contrast to other UHF RFID simulators, it is explicitly designed to handle wideband signals, fading channels, nonlinearities, and detuning effects. The simulator is currently used to develop and evaluate the performance of ranging and realtime channel estimation systems. The presented results emphasize the feasibility of our framework in the evaluation of a range estimation approach between a standard UHF RFID transponder and an interrogator.

Single crystalline YBa$_{\text{2}}$Cu$_{\text{3}}$O$_{\text{7}-\delta}$ (YBCO) thin films were grown by pulsed laser deposition (PLD) in order to probe the oxygen deficiency $\delta$ using a mono-energetic positron beam. The sample set covered a large range of $\delta$ (0.191

In 2012, the new neutron induced positron source NEPOMUC upgrade was put into operation at FRMII. Major changes have been made to the source which consists of a neutron-?-converter out of Cd and a Pt foil structure for electron positron pair production and positron moderation. The new design leads to an improvement of both intensity and brightness of the mono-energetic positron beam. In addition, the application of highly enriched 113Cd as neutron-?-converter extends the lifetime of the positron source to 25 years. A new switching and remoderation device has been installed in order to allow toggling from the high-intensity primary beam to a brightness enhanced remoderated positron beam. At present, an intensity of more than 109 moderated positrons per second is achieved at NEPOMUC upgrade. The main characteristics are presented which comprise positron yield and beam profile of both the primary and the remoderated positron beam.

Experimental results of Coincident Doppler Broadening Spectroscopy (CDBS) in Si, Cr, Ni, Cu, Pt and Au references were compared with ab-initio calculations. These calculations were performed by use of the MIKA Doppler software package, which is provided for the community by Aalto University. This software calculates electron wavefunctions by an atomic superposition method. It is shown that despite this simple approach, the element specific information obtained by CDBS can be calculated with high reliability for the examined elements.

At NEPOMUC, four different experiments are installed permanently and one beam port is open for positron experiments provided by external users. To match the different demands of these experiments on the phase space density of the positron beam, the established NEPOMUC remoderator was relocated and two novel beam switches were installed to provide the primary high-intensity beam or the remoderated high-brightness beam within short switching times. At the present state, the remoderated beam has a mean diameter of 1.85(35) mm and an intensity of 3.0(3) 107 e+/s. For the transformation of the longitudinal phase space density a new buncher setup was developed which converts the continuous positron beam into a pulsed beam.

A new high resolution coincidence Doppler spectrometer is under development at the positron beam facility NEPOMUC at the research neutron source FRM II. A Ni(100) re-moderation foil will be installed at the new spectrometer in transmission geometry for producing a positron beam with high brightness. Therefore, a magnetic lens for focusing the positrons onto the re-moderator and an electrostatic lens system for focusing the beam onto the sample have to be designed and simulated. The aim is to reach a beam diameter below 10 μm at the sample position for high resolution experiments. The current status of the project will be described and presented.