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Critical thicknesses in Nb-H thin films: coherent and incoherent phase transitions, change of precipitation and growth modes and ultrahigh mechanical stress

dc.contributor.advisorPundt, Astrid Prof. Dr.
dc.contributor.authorBurlaka, Vladimir
dc.titleCritical thicknesses in Nb-H thin films: coherent and incoherent phase transitions, change of precipitation and growth modes and ultrahigh mechanical stressde
dc.contributor.refereePundt, Astrid Prof. Dr.
dc.subject.gokPhysik (PPN621336750)de
dc.description.abstractengFor nanoscale systems such as Me-H thin films, changes of physical properties have been suggested to appear below certain critical sizes. The probable existence of these critical sizes and the related changes of thermodynamical and mechanical properties are the central topics of this thesis, using the model system of Nb-H thin epitaxial films adhered on Al2O2 (11-20) sapphire substrate. Since all the effects and, especially, the efficiency of stress release depends on the film thickness (d), d was considered in this study as a tunable size parameter that was varied from 105 nm to 5 nm. Particulary, this work adresses the suggested occurrence of coherent phase transformations and the appearance of hydrogen-induced ultrahigh mechanical stress in the GPa range, as predicted by calculations. These topics were experimentally studied by scanning tunneling microscopy (STM), X-ray diffraction (XRD) and electrical resistance measurements as applied in-situ during hydrogen gas loading experiments. The main goal was to derive the critical film thickness for coherent phase transformations and to investigate the pre-existing theoretical model. Further, this research was accompanied by a systematic study on the stress development arising within the Nb-H films upon hydrogen loading. This was also studied in dependence on the film thickness. The related data were obtained by use of substrate curvature measurements performed in-situ during electrochemical hydrogen loading. Hereby, the regime of linear elastic film expansion and the development of an ultra-high stress state was of interest. The experimental results suggest an existence of three different regimes in dependence on the film thickness. For d ≤ d1 = 37 - 40 nm, coherent phase transformation appeares in Nb-H thin film system. Here, preferential nucleation of hydrides instead of their growth is found. For d > d1, semi-coherent phase transformation and preferential growth of hydride precipitates is detected. For d ≤ d2 ≈ 8 nm – 15 nm no trace of phase separation in the Nb-H thin film system is found at room temperature (Trt). This shows that the critical temperature Tc of the miscibility gap of the Nb-H thin film system drops below Trt, for d ≤ d2. The related destabilization of the hydride phase is attributed to a decrease of the H-H interaction energy EHH and an increasing contribution of mechanical stress in ultrathin films. Besides, the regime of linear elastic film expansion and reproducable ultra-high mechanical stresses is realized for film thicknesses d ≤ d3 = 5 nm. Hydrogen-induced mechanical stress of up to -10 GPa are measured for 5 nm to 10 nm thin Nb films. Because of the fundamental research character, this study gives insights into the physics of decomposition in thin alloy films when crossing the critical dс - values. The general concept can be transferred to other nano-sized systems fixed to stabilizers and offers the possibility to tune the stress state and to affect the stabilities of phases by crossing critical system sizes, particularly in Me-H thin film
dc.contributor.coRefereeHofsäss, Hans Christian Prof. Dr.
dc.contributor.thirdRefereeKrebs, Hans-Ulrich Prof. Dr.
dc.contributor.thirdRefereeSeibt, Michael Prof. Dr.
dc.contributor.thirdRefereeMoshnyaga, Vasily Prof. Dr.
dc.contributor.thirdRefereeKlein, Helmut PD Dr.
dc.subject.engthin filmsde
dc.subject.engcoherent / incoherent phase transitionde
dc.subject.enghydride formationde
dc.subject.engcritical film thicknessde
dc.subject.engmechanical stressde
dc.subject.engcritical temperaturede
dc.affiliation.instituteFakultät für Physikde

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