Triple junction transport and the impact of grain boundary width in nanocrystalline Cu.

Triple junctions (TJ), singular topological defects of the grain boundary (GB) structure, get a dominant role for grain growth and atomic transport in nanocrystalline matter. Here, we present detailed measurements by atom probe tomography, even of the temperature dependence of TJ transport of Ni in nanocrystalline Cu in the chemical regime of interdiffusion. An unexpected variation of the effective width of merging GBs with temperature is detected. It is demonstrated that proper measurement of TJ transport requires taking into account this remarkable effect. TJ diffusion is found to be a factor of about 200 faster than GB diffusion. Its activation energy amounts to only two-thirds of that of the GB.

A quantitative assessment of microelectrodes.

In order to improve the performance and applicability of atom probe tomography, the application of microelectrodes has been suggested and is realized in modern commercial instruments. In contrast to the original proposition by Nishikawa, in practical realization the down scaling of the microelectrode is limited to about 10microm due to the requirements of a stable measurement. In this work, the field enhancement by electrodes of this size was measured in the FIM mode and compared to finite element calculations of the electric field. The experimental data reveal considerable scattering between individual microelectrodes and specimen tips, but on the average the predictions by the finite element calculation are confirmed. Even a microelectrode of 50microm diameter yields a reasonable field enhancement close to a factor of two.

Design of a laser-assisted tomographic atom probe at Münster University.

To benefit from the latest technical improvements in atom probe analysis, a new tomographic atom probe has been built at the University of Münster, Germany. The instrument utilizes a femtosecond laser system with a high repetition rate combined with the ability of using a micrometer-sized extraction electrode and a wide angle configuration. Since field evaporation is triggered by laser pulses instead of high-voltage pulses, the instrument offers the ability to expand the range of analyzed materials to poorly conducting or insulating materials such as oxides, glasses, ceramics, and polymeric materials. The article describes the design of the instrument and presents characterizing measurements on metals, semiconductors, and oxide ceramic.