Papers by Author: Gerhard Krexner

Abstract: Behavior of sodium metal subject to geometrical constraints has been studied at variable temperatures. A porous medium, Vycor glass of an average pore size of 9.2 nm, was loaded with Na by vacuum vaporization. Synchrotron X-ray powder diffraction of the specimen revealed the existence of close-packed phases such as FCC, HCP, 9R and 4H already at room temperature whereas the bulk BCC phase of Na was completely absent. In contrast, close-packed phases in bulk sodium are only observed at temperatures below 35 K upon cooling. This result can be qualitatively understood in view of earlier zero Kelvin calculations from which it is well known that the differences of structural energies between BCC and close-packed phases are very small. As a consequence, also various close-packed structures can be formed at ambient temperature under certain conditions and environments. The lattice expansion in the restricted geometry has been determined in the range of 160 K to 300 K and compared with the bulk metal. Confined Na exhibits smaller lattice expansion as compared to that of the bulk which likewise can be explained as being due to confinement.

Abstract: Recent investigations on palladium hydride (Pd-H) showed, for the first time, evidence of formation of vacancy-hydrogen (Vac-H) clusters during Severe Plastic Deformation (SPD) effected by High Pressure Torsion (HPT). Vacancy concentrations produced in Pd-H by this method are extraordinarily high. DSC-scans show that the thermal stability range of vacancies is extended by about 150K due to trapping of hydrogen leading to the formation of vacancy-hydrogen clusters. Recent experiments give evidence that the mobility of the H atoms and/or the vacancies is conditional for the formation of Vac-H clusters during HPT. Results furthermore indicate defect stabilization by hydrogen trapping not only for vacancy-type defects but also for dislocations and grain boundaries.