Fakultäten der Universität Bonn

Exploring Phase-Change Materials for Heat-Storage from First Principles

2026-06-26T12:30:41Z

Exploring Phase-Change Materials for Heat-Storage from First Principles Jütten, Stefan The transition towards a carbon-neutral energy landscape necessitates the development of efficient thermal energy storage systems to bridge the temporal gap between renewable energy supply and thermal demand. The polymorphic ceramic trititanium pentoxide (Ti₃O₅) has emerged as a promising candidate for latent heat storage, capable of storing thermal energy in a metastable high-temperature phase indefinitely. Here, a comprehensive first-principles investigation of the Ti₃O₅ heat-storage system is presented, ranging from the electronic structure of the bulk material to the complex thermodynamic and kinetic behavior of doped systems, interfaces, surfaces and nanoparticles.
The initial part of this work establishes a robust theoretical framework for describing the open-shell transition metal oxide. It is demonstrated that the meta-GGA functional r²SCAN, augmented with the D3 dispersion correction, provides an accurate description of the structure of Ti₃O₅ polymorphs, superior to standard hybrid functionals. In the bulk, the thermodynamic ground state of the β-phase is identified as an antiferromagnetic semiconductor, while the metastable λ-phase is shown to be a ferromagnetic semiconductor. The transition state is characterized by a rotation of a central Ti-dimer, and predicted r²SCAN-D3 phase transition enthalpy and phase transition temperature are in good agreement with experiment.
Building on this foundation, the modulation of heat-storage properties via aliovalent cation doping (Sc, Al, Mg) is explored. The results reveal that doping lowers the phase transition temperature and enthalpy, primarily through local lattice distortions rather than direct electronic effects. Substitution turns the semiconducting bulk materials into metals, with significant electron density accumulation at the defect site. At low dopant concentration the β-Ti₃O₅ to λ-Ti₃O₅ barrier remains unchanged while the substitution stabilizes the λ-phases relative to β-Ti₃O₅. This provides a theoretical basis for tuning the operational temperature window of the material for specific waste-heat recovery applications.
A central challenge in the theoretical description of Ti₃O₅ has been the discrepancy between calculated and experimental pressures required to induce the phase transition. This thesis resolves this issue by moving beyond bulk models. First, the close degree of lattice matching between β- and λ-phases results in stable optimized phase interfaces along the (100), (010) and (001) grain boundaries between the phases. A more refined picture of the phase transition pathway is obtained by varying the β/λ ratio in large supercell models, revealing a distinct anisotropy in the phase transition pathway involving the (001) interface, which proceeds with a significantly lower barrier as compared to the other considered interfaces, confirming experimental trends. Hydrostatic pressure simulation on these mixed-phase models reveals pressure to destabilize high λ-phase fraction systems, however, these pressures of several GPa still overestimate experimental values by orders of magnitude. Second, the influence of particle size and morphology is quantified. By calculating surface free energies and applying the Wulff construction, it is shown that surface effects stabilize the λ-phase in particles smaller than 43 nm in diameter, providing a thermodynamic explanation for the experimentally observed thermal hysteresis and the persistence of the metastable phase at room temperature. The phase transition temperature is also shown to be influenced by particle size, with nanoparticles exhibiting diameters in the experimentally synthesized regime displaying phase transition temperatures in excellent agreement with experiment.
Finally, the atomistic mechanism of the pressure-induced phase transition is elucidated using machine-learned potentials driven on-the-fly probability enhanced sampling simulations. Simulated annealing simulations reveal a favorable surface reconstruction of the (001) λ-phase surface, which is then subject to repulsive harmonic potentials to model pressure effects. A comprehensive, universal and transferable framework for the translation of the slab compression to a pressure value is introduced. By modeling the uniaxial compression of nanoparticle surfaces rather than hydrostatic bulk compression, the predicted transition threshold of ≈700 bar is brought into better agreement with experimental values (≈600 bar). The mechanism is shown to involve a sequential, system size independent nucleation and a resulting layer-by-layer transformation, which is resolved in detail from direct molecular dynamics simulations under pressure at ambient temperatures.
Collectively, this work bridges the gap between quantum-chemical predictions and experimental observations in Ti₃O₅. It establishes a validated computational workflow for the discovery and optimization of phase-change materials, highlighting the critical importance of the correct choice of computational method, uncovering the fundamental effects of cation substitution in the modulation of material properties and the use of realistic models accounting for finite-size effects for accurate predictions of thermodynamic material properties.

Psychologische Sicherheit von Assistenzärzten im Deutschen Gesundheitssystem

2026-06-26T11:17:06Z

Psychologische Sicherheit von Assistenzärzten im Deutschen Gesundheitssystem Etti, Nicola Constanze Hintergrund
Psychologische Sicherheit hat einen entscheidenden Einfluss auf die Performanz von Teams, erhöht die Arbeitsplatzzufriedenheit und reduziert Kündigungsabsichten. Eine erhöhte Adhärenz kann Patientensicherheit durch Erhalt von Expertise erhöhen und Kosten innerhalb des Gesundheitssystems senken. Diese Studie evaluiert das Ausmaß Psychologischer Sicherheit deutscher Assistenzärzte und dessen Einflüsse auf Arbeitsplatzzufriedenheit und Kündigungsabsichten.
Methoden
In einer Querschnittsstudie wurden deutsche Assistenzärzte Organisations- und Fach-übergreifend über die Online Studienplattform MediLearn zu einer Online-Umfrage eingeladen. Psychologische Sicherheit wurde auf Ebene der Vorgesetzten, Peers und des Teams als Ganzes erfasst. Mittels etablierter Fragebögen wurde die Arbeitsplatzzufriedenheit gemessen sowie die Intention den Arbeitgeber aufgrund dessen Arbeitsplatzkultur zu verlassen erfragt. Neben deskriptiver Statistik wurden Korrelations- und Regressionsanalysen durchgeführt, um uni- und multivariate Assoziationen der drei Dimensionen Psychologischer Sicherheit mit Arbeitsplatzzufriedenheit und Kündigungsabsicht zu analysieren.
Ergebnisse
Die Stichprobe bestand aus 432 Assistenzärzten in frühen Phasen ihrer Weiterbildung (77,5% in den ersten 3 Weiterbildungsjahren) mit einem Frauenanteil von 78,2% und überwiegender Vollzeit Beschäftigung (85,6%). Der Mittelwert Psychologischer Sicherheit unterschied sich zwischen den Dimensionen der Führungsebene (6,01, 95% Konfidenzintervall 5,81-6,21), Peers (7,95, 95% Konfidenzintervall 7,78-8,12) und dem Team als Ganzes (7,30, 95% Konfidenzintervall 7,11-7,49). Die Korrelationsanalysen ergaben signifikante Assoziationen aller Dimensionen Psychologischer Sicherheit mit Arbeitsplatzzufriedenheit und der Intention den Arbeitgeber zu verlassen. In den multiplen Regressionsanalysen zeigten weibliches Geschlecht und höheres Alter negative Assoziationen mit Arbeitsplatzzufriedenheit. Psychologische Sicherheit auf Führungs- und Teamebene, nicht aber auf Ebene der Peers war sowohl positiv mit Arbeitsplatzzufriedenheit als auch negativ mit Kündigungsabsichten assoziiert.
Zusammenfassung
Die Studie trägt Organisations- und Fach-übergreifend zum Verständnis Psychologischer Sicherheit deutscher Assistenzärzte bei. Die differenzierte Betrachtung auf Führungs-, Peer- und Teamebene ermöglicht neue Einblicke mit Führungskräften als bedeutende Einflussgröße auf Arbeitsplatzzufriedenheit und Kündigungsabsichten, wobei die Psychologische Sicherheit auf dieser Ebene unter den Befragten am niedrigsten ausgeprägt war. Hier ansetzend können Folgestudien einen wertvollen Beitrag zum Erhalt personeller Ressourcen im deutschen Gesundheitssystem leisten.

Dynamics of microcyst-like epithelial changes associated with Belantamab mafodotin therapy in a patient with multiple myeloma-a case report

2026-06-26T10:31:39Z

Dynamics of microcyst-like epithelial changes associated with Belantamab mafodotin therapy in a patient with multiple myeloma-a case report Schlößer, Lukas; Löffler, Karin U.; Heine, Annkristin; Holz, Frank G.; Herwig-Carl, Martina C.

Grossing of non-neoplastic globes, including fetal eyes

2026-06-26T10:16:48Z

Grossing of non-neoplastic globes, including fetal eyes Herwig-Carl, Martina C.; Ussem, Leticia; Holz, Frank G.; Müller, Annette M.; Löffler, Karin U. Grossing and sectioning of fetal and adult globes are critical steps in the histopathologic evaluation of ocular diseases. Proper handling of the globe is essential for accurate diagnosis of conditions such as tumors and non-neoplastic changes, including trauma, infections, and previous surgical procedures. Orientation of the globe is followed by external examination, documenting characteristics including size, shape, and any visible lesion or abnormality. Specific attention must be paid to the presence of tumors, infectious or degenerative changes, as well as signs of trauma or prior surgery. Measurements of the globe and associated structures, including the cornea and optic nerve, and – in fetal eyes – anatomical and timely development are recorded. Transillumination is essential to detect a shadow, which may be caused by a mass or hemorrhage as well as increased light transmission in areas of atrophy, such as coloboma.
Fixation of the globe is routinely performed in 4% paraformaldehyde to preserve the tissue. After fixation for 24 h, the globe is sectioned systematically to examine intraocular structures, such as the anterior chamber, lens status, uvea, retina, vitreous, and optic nerve. The adult globe is usually trisected along the horizontal or – depending on the clinical indications – vertical/oblique plane to create a pupil-optic nerve (PO) section. Additional sections of the calottes may be taken to evaluate areas of interest, such as suspicious masses or areas of atrophy. For research purposes, various fixation and sectioning protocols may be considered.
Each gross section is carefully inspected, documented, and processed for microscopic examination to ensure that any pathologic finding is adequately sampled. Accurate grossing and sectioning are critical for correlating clinical and histologic findings, facilitating diagnosis, guiding treatment decisions, and ultimately improving patient outcomes. Standardization of grossing and sectioning ensures a comprehensive evaluation of the eye globe and contributes to the advancement of ophthalmology.