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Allgemeine Untersuchungsverfahren und Laboratoriumstechnik

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Novel Nanorod Precipitate Formation in Neodymium and Titanium Codoped Bismuth Ferrite

The discovery of unusual nanorod precipitates in bismuth ferrite doped with Nd and Ti is reported. The atomic structure and chemistry of the nanorods are determined using a combination of high angle annular dark field imaging, electron energy loss spectroscopy, and density functional calculations. It is found that the structure of the BiFeO₃ matrix is strongly modified adjacent to the precipitates; the readiness of BiFeO₃ to adopt different structural allotropes in turn explains why such a large axial ratio, uncommon in precipitates, is stabilized. In addition, a correlation is found between the alignment of the rods and the orientation of ferroelastic domains in the matrix, which is consistent with the system's attempt to minimize its internal strain. Density functional calculations indicate a finite density of electronic states at the Fermi energy within the rods, suggesting enhanced electrical conductivity along the rod axes, and motivating future investigations of nanorod functionalities.     

Relationship Between Microstructure,Strength, and Fracture in an Al-Zn-Mg Electron Beam Weld: Part I: Microstructure Characterization

This work presents a detailed, multiscale, spatially resolved study of the microstructure of an electron beam butt weld of the EN-AW 7020 (Al-Zn-Mg) alloy. Using a combination of optical, scanning and transmission electron microscopy, differential scanning calorimetry, and small-angle X-ray scattering, the distribution of phases in the different areas of the heat-affected zone and of the fusion zone is quantitatively characterized, for two different aging states: naturally aged after welding and artificially aged at 423 K (150 °C). The heat-affected zone consists of regions experiencing different levels of precipitate dissolution and coarsening during welding as well as new precipitation during post-welding heat treatment (PWHT). The microstructure of the fusion zone is typical from a fast solidification process, with a strong solute segregation in the interdendritic zones. The precipitate distribution after PWHT follows this solute distribution, and the resulting hardness is much lower than the relatively homogeneous value in the base metal and the heat-affected zone.     

Room-temperature operation of InAs quantum-dash lasers on InP (001)

The first self-assembled InAs quantum dash lasers grown by molecular beam epitaxy on InP (001) substrates are reported. Pulsed room-temperature operation demonstrates wavelengths from 1.60 to 1.66 μm for one-, three-, and five-stack designs, a threshold current density as low as 410 A/cm² for single-stack uncoated lasers, and a distinctly quantum-wire-like dependence of the threshold current on the laser cavity orientation. The maximal modal gains for lasing in the ground-state with the cavity perpendicular to the dash direction are determined to be 15 cm^-1 for single-stack and 22 cm^-1 for five-stack lasers     

High-power AlGaInAs strained multiquantum well lasers operating at 1.52 /spl mu/m

1.75 W CW power in AlGaInAs-InP strained QW lasers is demonstrated. Room temperature threshold current densities are 410 A/cm/sup 2/, and the characteristic temperature is 69 K. The variation in the external differential efficiency with cavity length and temperature reveal the optimum length and show how nonradiative recombination mechanisms limit the performance.     

Very low threshold current density room temperature continuous-wavelasing from a single-layer InAs quantum-dot laser

Continuous-wave (CW) lasing operation with a very low threshold current density (J_th=32.5 A/cm²) has been achieved at room temperature by a ridge waveguide quantum-dot (QD) laser containing a single InAs QD layer embedded within a strained InGaAs quantum well (dot-in-well, or DWELL structure). Lasing proceeds via the QD ground state with an emission wavelength of 1.25 μm when the cavity length is longer than 4.2 mm. For a 5-mm long QD laser, CW lasing has been achieved at temperatures as high as 40°C, with a characteristic temperature T₀ of 41 K near room temperature. Lasers with a 20 μm stripe width have a differential slope efficiency of 32% and peak output power of >10 mW per facet (uncoated)     

Exploiting design patterns to automate validation of class invariants

In this paper, techniques are presented that exploit two design patterns, the Visitor pattern and the Decorator pattern, to validate invariants about the data attributes in a C++ class automatically. To investigate the pragmatics involved in using the two patterns, a study of an existing, well‐tested application, keystone, a parser and front‐end for the C++ language, is presented. Results from the study indicate that these two patterns provide flexibility in terms of the frequency and level of granularity of validation of the class invariants, which are expressed in the Object Constraint Language (OCL). The quantitative results measure the impact of these approaches and the additional faults uncovered through validation of the case study. Copyright © 2005 John Wiley & Sons, Ltd.