Theory of He trapping, diffusion, and clustering in UO2

We have performed ab initio total energy calculations to investigate the behavior of helium and its diffusion properties in uranium dioxide (UO₂). Our investigations are based on the density functional theory within the generalized gradient approximation (GGA). The trapping behavior of He in UO₂ has been modeled with a supercell containing 96-atoms as well as uranium and oxygen vacancy trapping sites. The calculated incorporation energies show that for He a uranium vacancy is more stable than an oxygen vacancy or an octahedral interstitial site (OIS). Interstitial site hopping is found to be the rate-determining mechanism of the He diffusion process and the corresponding migration energy is computed as 2.79 eV at 0 K (with the spin-orbit coupling (SOC) included), and as 2.09 eV by using the thermally expanded lattice parameter of UO₂ at 1200 K, which is relatively close to the experimental value of 2.0 eV. The lattice expansion coefficient of He-induced swelling of UO₂ is calculated as 9 × 10⁻². For two He atoms, we have found that they form a dumbbell configuration if they are close enough to each other, and that the lattice expansion induced by a dumbbell is larger than by two distant interstitial He atoms. The clustering tendency of He has been studied for small clusters of up to six He atoms. We find that He strongly tends to cluster in the vicinity of an OIS, and that the collective action of the He atoms is sufficient to spontaneously create additional point defects around the He cluster in the UO₂ lattice.     

3He Anomalously High Spin Diffusion in Solid Para-Hydrogen

NMR measurements of ³He spin diffusion coefficient in solid para-H₂ are carried out at the temperatures 0.45–1.5 K. The crystals have been grown under constant pressure 20–130 bar. The ³He concentrations in the initial para-H₂–³He gas mixtures were 0.1% and 0.3%. It is found out that the decay of echo amplitude vs both magnetic field gradient G and time interval τ between RF pulses is of non-exponential character, typical of one-dimensional diffusion in restricted geometry. The values of true spin-diffusion coefficient D _S measured are found to be ∼10⁻⁴ cm²/s at 20 bar. At 108 bar D _S value is one order of magnitude less. D _S does not depend on temperature. Such spin diffusion coefficient values seem to be anomalously high in comparison with well-known values of D _S =10⁻⁵ cm²/s for bulk liquid ³He at 27 bar and D _S =10⁻⁸ cm²/s for bulk solid ³He at 108 bar. The special experiments with the crystal annealing make it clear that the high spin diffusion here is connected with fast diffusion along dislocation lines.     

Ni induced lateral crystallization of high density Ge-dots/Si heterostructures

In this work, we propose a novel method for obtaining high-density Ge-dots/Si multilayered heterostructures. The high-density self-assembled Ge dots are firstly grown on a-Si layer using low-pressure chemical vapor deposition (LPCVD), and then low-temperature recrystallized by Ni based metal induced lateral crystallization (MILC). According to optical micrograph, microprobe Raman spectroscopy and transmission electron microscopy (TEM) observations, it has been found that the Ni induced lateral crystallized Si film has large leaf-like grains elongated along the MILC direction with (110) preference. The strain shift of Ge dots reveals the formation of high quality interface between the crystallized Si and Ge dot.     

High-pressure high-temperature (HPHT) synthesis and magnetism of MSr2Y1.5Ce0.5Cu2O10 compounds with M = Ru, Mn, and Cr

We report synthesis and magnetization of MSr₂Y_1.5Ce_0.5Cu₂O₁₀ (M/Y-1222) compounds with M = Ru, Mn and Cr by HPHT. They are synthesized at high-pressure of 6 GPa in optimized temperatures of up to 1450 °C, for Ru, Mn and 1550 °C in the case of Cr based Y/1222, for a time duration of three hours for all the samples. All the samples crystallized with tetragonal structure in I4/mmm space group. Few minor impurities are also seen in the case of Mn,Cr/Y-1222 samples. DC magnetic susceptibility measurements exhibited ferromagnetic like transition for all the three compounds at low temperatures. At 5 K, the magnetization M(H) experiments showed clear ferromagnetic like hysteresis loops for Ru and Mn samples with sizeable returning moment (M_r) and coercive field (H_c). In the case of Cr/Y-1222 system though, ferromagnetism is visible below around 45 K in susceptibility measurements, the characteristic M_r and H_c are not seen. To the best of our knowledge the Mn,Cr/Y-1222 phases are synthesized for the first time and the quality of our earlier reported Ru/Y-1222 is improved.     

A model of preferential (100) crystal orientation of Si grains grown by aluminium-induced layer-exchange process

The aluminium-induced layer-exchange process allows to grow thin large-grained polycrystalline Si films on foreign substrates. A characteristic feature of these films is the preferential (100) orientation of Si grains, favourable for subsequent epitaxial thickening at low temperatures. In this work, a model based on the preferential nucleation is proposed, which elucidates a possible origin of the preferential (100) orientation and its sensitivity to the preparation and process conditions. The probability of Si nuclei to have respective orientation is attributed to the nucleation barrier, i.e. the critical value of the change of the Gibbs energy during nucleation. The preferential orientation is formed statistically by the nuclei having the lowest nucleation barriers.     

Influence of grain size on nitrogen diffusivity in austenitic stainless steel

Ion nitriding of austenitic stainless steel with the aim to improve the tribological properties while retaining the excellent corrosion resistance is a well-established method. At the same time, strongly varying microstructures can be obtained depending on the pretreatment. In this work, the influence of prior heat treatment in the temperature range between 950 and 1200 °C on the microstructure, especially the grain size, and the corresponding observed nitrogen diffusivity in austenitic stainless steel DIN 1.4301 (AISI 304, X6CrNi18.10) after nitrogen plasma immersion ion implantation (PIII) is studied. Cross-section and plan view samples were prepared and investigated. With increased annealing temperature, both larger grains and slower diffusion was observed, despite diffusion ranges much smaller than the average grain size. Another, still hidden effect of dislocation densities or other defects on both secondary parameters is suggested.     

Characterization of electroplated copper self-annealing with investigations focused on incorporated impurities

At room temperature electroplated copper exhibits changes in microstructure widely known as self-annealing. To investigate this phenomenon we simultaneously determined resistivity, residual stress, microstructure evolution, and behavior of organic impurities in three Cu layers of 600, 1000, and 2000 nm thickness. The examination of Cu layer impurities presupposed an extensive work of identification and elimination of contamination sources. After developing and applying several cleaning procedures it was possible to qualify and quantify incorporated C as indicator for organic impurities. The investigation of Cu self-annealing led to the conclusion that the microstructure evolution has to be divided into two periods. The first period of inhibited grain growth shows an impurity diffusion out of the metallization layer combined with a significant stress relaxation. In the following second period a forced grain growth evolution starts forming up a coarse grain microstructure.     

Quantitative electron energy loss spectroscopy of Si nanoclusters embedded in SiOx

We have used a methodology, based on electron energy loss spectroscopy combined with energy filtered images, which allows us to quantify the clustered silicon concentration in annealed sub-stoichiometric silicon oxide layers (SiO_x). This information was coupled to the chemical silicon and oxygen concentrations determined from Rutherford backscattering analyses. The silicon agglomeration kinetics was investigated as a function of the gas flows used during the plasma enhanced chemical vapor deposition process. Although the clustered Si concentration and the average cluster radius do not follow a well defined dependence on Si, O, or N concentration, separately, the ratio between the former parameter and the silicon excess concentration decreases monotonically as a function of the nitrogen concentration approaching a saturation value equal to 0.2 for nitrogen concentrations above 10 at.%.     

Fabrication of ZnO films consisting of densely accumulated mesoporous nanosheets and their dye-sensitized solar cell performance

Nanocrystalline and mesoporous ZnO films approximately 4 μm in thickness were fabricated through a simple chemical deposition of Zn₄CO₃(OH)₆·H₂O in an aqueous solution of zinc nitrate and urea and subsequent pyrolysis at a low temperature of 300 °C. Microscopic observation of the films revealed that they were composed of mesoporous nanosheets accumulating with submicrometer-order spacing between them on conducting glass substrates. Adsorption of N-719 dye onto the ZnO surface, for application to dye-sensitized solar cells, led to dye-loading of 1.1 × 10^− 7 mol/cm² in spite of the relatively small thickness. The resultant ZnO/N-719 photoanode exhibited a short-circuit photocurrent density of 13.8 mA/cm², which is of the highest level ever reported for ZnO. An overall light-to-electricity conversion efficiency of 3.3% was achieved under 1 sun AM1.5 illumination without any optimization in view of the other cell components.     

Effects of 0.28-2.80 MeV proton irradiation on GaInP/GaAs/Ge triple-junction solar cells for space use

GaInP/GaAs/Ge triple-junction solar cells were irradiated with 0.28, 0.62 and 2.80 MeV protons with fluences ranging from 1 × 10¹⁰ cm⁻² to 1 × 10¹³ cm⁻². Their performance degradation is analyzed using current-voltage characteristics and spectral response measurements. The degradation rates of the short circuit current, open circuit voltage, and maximum power output increase with fluence, but decrease with increasing proton energy. It was also observed that the spectral response of the GaAs middle cell degrades more significantly than that of the GaInP top cell.