Large-angle Tilt Boundaries Research Articles

Nanoscopic analysis of oxygen segregation at tilt boundaries in silicon ingots using atom probe tomography combined with TEM and ab initio calculations.

We have developed an analytical method to determine the segregation levels on the same tilt boundaries (TBs) at the same nanoscopic location by a joint use of atom probe tomography and scanning transmission electron microscopy, and discussed the mechanism of oxygen segregation at TBs in silicon ingots in terms of bond distortions around the TBs. The three-dimensional distribution of oxygen atoms was determined at the typical small- and large-angle TBs by atom probe tomography with a low impurity detection limit (0.01 at.% on a TB plane) simultaneously with high spatial resolution (about 0.4 nm). The three-dimensional distribution was correlated with the atomic stress around the TBs; the stress at large-angle TBs was estimated by ab initio calculations based on atomic resolution scanning transmission electron microscopy data and that at small-angle TBs were calculated with the elastic theory based on dark-field transmission electron microscopy data. Oxygen atoms would segregate at bond-centred sites under tensile stress above about 2 GPa, so as to attain a more stable bonding network by reducing the local stress. The number of oxygen atoms segregating in a unit TB area NGB (in atoms nm-2 ) was determined to be proportional to both the number of the atomic sites under tensile stress in a unit TB area nbc and the average concentration of oxygen atoms around the TB [Oi ] (in at.%) with NGB ∼ 50 nbc [Oi ].

A theoretical evaluation of the temperature and strain-rate dependent fracture strength of tilt grain boundaries in graphene

Based on molecular dynamic simulations, we investigate the effects of temperature and strain rate on the strength of single layer graphene with tilt grain boundaries under tension. The simulation results show that temperature plays an important role in the strength of graphene with grain boundaries. The strength of graphene with grain boundaries decreases significantly as temperature increases. In particular, we confirm a previous report that graphene with large angle tilt boundaries (which has a high density of defects) may be much stronger than that with low angle boundaries. This finding holds true for temperatures from 10 to 1800K and strain rates from 0.0001 to 0.01ps−1.

Structural Characteristics of Oriented Mesostructured Silica Thin Films

Mesoporous thin films synthesized via an electrochemical strategy (ref 1) generally show granular domains, each of which is composed of hexagonally packed one-dimensional channels oriented uniquely perpendicular to the film surface. Grain boundaries either parallel or normal to the channel direction might affect the properties and subsequent application of the film. In this study, the structural details of oriented mesostructured silica thin films have been examined by transmission electron microscope. The pore structures are characterized using the traditional crystallographic concepts but show different structural properties from that of polycrystalline materials. The boundary structures vary much depending on the residual internal stress and the orientation relationship between the bounded grains. A variety of structural features, typically near the large-angle tilt boundaries, have been observed including coincidence site lattices, lattice distortion, lattice displacement, and dislocations. According to the present structural analysis, microstructure evolution and potential applications have been discussed with respect to the oriented mesoporous films.

The effect of colonies of aligned grains on critical current in high-temperature superconductors

X-ray microdiffraction has been used to map the orientational distribution of grains in a 4 × 5mm2 polycrystalline film of Tl1−yBa2Cu2Cu3Ox (Tl-1223) grown on yttira-stabilized zirconia (YSZ). The film consists of “colonies”, each containing ∼ 104 grains with their a-axes aligned; the c-axes of all grains are aligned normal to the film, but there is no overall a-axis alignment. While the grain boundaries within a colony are small angle, colonies are separated by large-angle tilt boundaries. The typical colony size is 0.4 mm. Measured critical currents are compared with simulations based on observed distributions of grain alignment; we find that variations in grain alignment account for the observed variations in the critical current density. Simulations over longer distances indicate that a colony structure can substantially increase the critical current, especially in magnetic fields.

Josephson-like properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin film weak links

Thin-film YBa/sub 2/Cu/sub 3/O/sub 7/ microbridges have been fabricated by photo- and e-beam lithography in polycrystalline thin films grown on MgO substrates and containing a controlled density of large-angle tilt boundaries, thereby isolating individual tilt-boundary weak links. Depending on the growth parameters of the film, such weak links have critical current densities J/sub c/ ranging from 10/sup 6/ A mod cm/sup 2/. The higher J/sub c/ microbridges exhibit behavior indicative of one-dimensional flux flow and flux creep in the weak links, while the lower J/sub c/ microbridges have RSJ-like properties. Detailed comparisons have been made between the predictions of the resistively shunted junction (RSJ) model including the calculated effects of thermal noise rounding and the measured DC and microwave response of these weak links. Measurements have also been made of magnetic field effects on the critical current density. An unexpected scaling behavior has been observed in the I-V characteristics of certain types of these microbridges containing 45 degrees tilt boundaries that indicates that in those cases I/sub C/R/sub N//sup 2/ is a constant for the tilt-boundary weak link. The implications of these results for developing a successful description of the superconductive properties of YBa/sub 2/Cu/sub 3/O/sub 7/ grain boundary weak links, which electron microscopy reveals to be clean, stoichiometric, and abrupt, are discussed.

Growth of YBa2Cu3O7 thin films on MgO: The effect of substrate preparation

We discuss the results of a study of the effect of substrate preparation on the microstructure and superconductive properties of YBa2Cu3O7 thin films formed by laser ablation on (001) MgO substrates. Thermal annealing of the substrates is found to be highly effective in producing at fairly low growth temperatures (670 °C), epitaxial, c-axis normal films with good superconductive properties. Alternative surface treatments result in the formation of large angle tilt boundaries and inferior superconductive properties.

Scaling behavior of YBa2Cu3O7−δ thin-film weak links

The superconductive weak link properties of microbridges formed in c-axis normal YBa2Cu3O7−δ polycrystalline thin films containing a variable amount of large angle tilt boundaries have been studied. In the low critical current density limit these weak links have current-voltage (I-V) characteristics that are accurately modeled by the resistively shunted junction model. The I-V’s are found to accurately follow a simple scaling law with the product of the critical current and weak link resistance Rn varying linearly with the weak link conductance.