Stability of large vacancy clusters in silicon

Using a density-functional-based tight-binding method we investigate the stability of various vacancy clusters up to a size of 17 vacancies. Additionally, we compute the positron lifetimes for the most stable structures to compare them to experimental data. A simple bond-counting model is extended to take into account the formation of new bonds. This yields a very good agreement with the explicitly calculated formation energies of the relaxed structures for V6 to V14. The structures, where the vacancies form closed rings, such as V6 and V10, are especially stable against dissociation. For these structures, the calculated dissociation energies are in agreement with experimentally determined annealing temperatures and the calculated positron lifetimes are consistent with measurements.Peer reviewe

Co-doping of (Bi0.5Na0.5)TiO3: Secondary phase formation and lattice site preference of Co

Bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT) is considered to be one of the most promising lead-free alternatives to piezoelectric lead zirconate titanate (PZT). However, the effect of dopants on the material has so far received little attention from an atomic point of view. In this study we investigated the effects of cobalt-doping on the formation of additional phases and determined the preferred lattice site of cobalt in BNT. The latter was achieved by comparing the measured x-ray absorption near-edge structure (XANES) spectra to numerically calculated spectra of cobalt on various lattice sites in BNT.(Bi0.5Na0.5)TiO3 + x mol% Co (x = 0.0, 0.5, 1.0, 2.6) was synthesized via solid state reaction. As revealed by SEM backscattering images, a secondary phase formed in all doped specimens. Using both XRD and SEM-EDX, it was identified as Co2TiO4 for dopant levels >0.5 mol%. In addition, a certain amount of cobalt was incorporated into BNT, as shown by electron probe microanalysis. This amount increased with increasing dopant levels, suggesting that an equilibrium forms together with the secondary phase. The XANES experiments revealed that cobalt occupies the octahedral B-site in the BNT perovskite lattice, substituting Ti and promoting the formation of oxygen vacancies in the material

Thermo-optical-measurement technique TOM to characterize sintering behavior of additive manufactured metallic parts under atmospheric control

Following the shrinkage of powder metallurgical samples of unusual shape is possible employing our thermo-optical measurement device with atmospheric control (TOM-AC). Its accuracy of better than 0.5 microns resolution reached during contact-less measurements and its robust contour-detection algorithm allow a data quality making a numerical derivative of the shrinkage possible. Hence, we present here shrinkage and shrinkage rates for spheres of carbonyl iron, metallic foams and a grid produced by screen printing of stainless steel powder. The TOM-AC allows not only measuring samples of arbitrary shapes but also changing the sintering atmospheres (vacuum, inert gas or reducing gases like hydrogen mixtures) during the heat treatment at chosen temperatures. The whole control during the heat treatment is realised via a specific code on a standard PC

Thermo-optical-measurement devices for powder metallurgical applications

Our thermo-optical measurement device with atmospheric control (TOM-AC) allows to follow changes in the shapes of samples (up to 40mm in diameter) with an accuracy of about 0.5 micron by contact-less measurements. Samples of arbitrary shape can be monitored by TOM-AC during the heat treatment at chosen temperatures and atmospheres. Its contour - and thus dimensional changes - are detected by a sophisticated algorithm. Applications range from determining the shrinkage or shape distortion of powder metallurgical products to measuring the wetting angle of melts. Atmospheric control, i.e. controlling and changing the gas atmosphere (e.g. inert gas / reducing gas at different pressures or vacuum) is realized during the heat treatment via a specific code on standard PCs. We will present examples for different application: shrinkage curves for free and load sintering for powder metallurgical products of different shapes and experiments following the de-binding and melting process

On the role of grain boundaries during sintering of carbonyl iron

We present new results in positron annihilation lifetime spectroscopy (PALS), thermo-optical dilatometry and microscopy, which are indicating a strong correlation between grain-boundaries and mass transport during the sintering process of carbonyl iron powder. In this particular system we were able to show that the change in particle shape and size with increasing temperature yields an anisotropy in shrinkage, which manifests itself in a higher shrinkage perpendicular to the compaction axis. In the intermediate stage of sintering, where the major mass transport occurs, the average distance between two grain boundaries could be determined to (3,73 ± 0,18) μm at T = 744°C. This is in good agreement with previous calculations of positron pathways in defect free particles. Furthermore, due to sintering temperatures far above the annealing temperature of dislocations in iron, the existence of dislocations in the bulk of the particles is very unlikely. These claims are reflected by the collected positron data, which exhibit a clear grain boundary signal of ∼ 250ps while no vacancy or dislocation signal (typically ∼ 160 ps) is evident in the intermediate stage of sintering

Nano-structures in Al-based alloys

The usefulness of first-principles calculations for studying solute-atom clustering in metal alloys is discussed. This usefulness stems directly from the properties predicted by the calculations or via a related interpretation of experimental results. In this paper we review the results of our computational studies on small solute clusters in Al-based alloys. The predicted coincidence Doppler broadening spectra of the positron annihilation method are used to analyse experimental results. The calculated binding energies of small solute atom clusters explain why Cu atoms form two-dimensional platelets on the (100) planes in Al whereas Zn forms three-dimensional clusters.Peer reviewe

Impact of higher-order uncertainty

In some games, the impact of higher-order uncertainty is very large, implying that present economic theories may rely critically on the strong common knowledge assumptions they make. Focusing on normal-form games in which the players ’ action spaces are compact metric spaces, we show that our key condition, called “global stability under uncertainty, ” implies that the maximum change in equilibrium actions due to changes in players ’ beliefs at orders higher than k is exponentially decreasing in k. Therefore, given any need for precision, we can approximate equilibrium actions by specifying only finitely many orders of beliefs

Natural ageing of Al Cu Mg revisited from a local perspective

Although Al alloys based on the Al-Cu-(Mg) system have been investigated for decades, information about the evolution of microstructure on the atomistic level during natural ageing is scarce. Therefore, the early stages of natural ageing in laboratory Al-Cu-(Mg) alloys and in AA2024 were investigated using positron annihilation and X-ray absorption spectroscopy. This complementary approach allows for accessing both essential components of Al alloys, namely vacancies and alloying elements. It is found, that during natural ageing the chemical environment of vacancies is formed both of Cu and Mg atoms and that the rearrangement of vacancy surroundings persists the hardness increase of Al-Cu-Mg alloys. During natural ageing two different regimes of vacancy environment are detected and interpreted in terms of cluster growth and vacancy capture. Features of the near edge structure of the X-ray absorption are interpreted using theoretical calculations obtained by the FEFF8.4 c ode. Thus the agglomeration of Cu and Mg, which is accompanied by lattice distortions around Cu atoms, can be probed. A minimal size of Cu-Mg co-clusters is proposed for later stages of natural ageing