Browsing by Author "Kübel, Christian"

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Open Access
The Fermi energy as common parameter to describe charge compensation mechanisms : a path to Fermi level engineering of oxide electroceramics
(2023) Klein, Andreas; Albe, Karsten; Bein, Nicole; Clemens, Oliver; Creutz, Kim Alexander; Erhart, Paul; Frericks, Markus; Ghorbani, Elaheh; Hofmann, Jan Philipp; Huang, Binxiang; Kaiser, Bernhard; Kolb, Ute; Koruza, Jurij; Kübel, Christian; Lohaus, Katharina N. S.; Rödel, Jürgen; Rohrer, Jochen; Rheinheimer, Wolfgang; De Souza, Roger A.; Streibel, Verena; Weidenkaff, Anke; Widenmeyer, Marc; Xu, Bai-Xiang; Zhang, Hongbin
Chemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering.
Open Access
Olefin ring‐closing metathesis under spatial confinement : morphology-transport relationships
(2020) Tallarek, Ulrich; Hochstrasser, Janika; Ziegler, Felix; Huang, Xiaohui; Kübel, Christian; Buchmeiser, Michael R.
Spatial confinement effects on hindered transport in mesoporous silica particles are quantified using reconstructions of their morphology obtained by electron tomography as geometrical models in direct diffusion simulations for passive, finite‐size tracers. We monitor accessible porosity and effective diffusion coefficients resulting from steric and hydrodynamic interactions between tracers and pore space confinement as a function of λ=dtracer/dmeso, the ratio of tracer to mean mesopore size. For λ=0, pointlike tracers reproduce the true diffusive tortuosities. For λ>0, derived hindrance factors quantify the extent to which diffusion through the materials is hindered compared with free diffusion in the bulk liquid. Morphology‐transport relationships are then discussed with respect to the immobilization, formation, and transport of key molecular species in the ring‐closing metathesis of an α,ω‐diene to macro(mono)cyclization product and oligomer, with a 2nd‐generation Hoveyda‐Grubbs type catalyst immobilized inside the mesopores of the particles.