15 research outputs found
Dissociative adsorption of methane on surface oxide structures of Pd-Pt alloys
The dissociative adsorption of methane on variously oxidized Pd, Pt and Pd-Pt
surfaces is investigated using density-functional theory, as a step towards
understanding the combustion of methane on these materials. For Pd-Pt alloys,
models of surface oxide structures are built on the basis of known oxides on Pd
and Pt. The methane adsorption energy presents large variations depending on
the oxide structure and composition. Adsorption is endothermic on the bare
Pd(111) metal surface as well as on stable thin layer oxide structures such as
the () surface oxide on Pd(100) and the PtO-like
oxide on Pt(111). Instead, large adsorption energies are obtained for the (100)
surface of bulk PdO, for metastable mixed PdPtO oxide
layers on Pt(100), and for Pd-Pt(111) surfaces covered with one oxygen
monolayer. In the latter case, we find a net thermodynamic preference for a
direct conversion of methane to methanol, which remains adsorbed on the
oxidized metal substrates via weak hydrogen-bond interactions
On the Short-Time Compositional Stability of Periodic Multilayers
The short-time stability of concentration profiles in coherent periodic
multilayers consisting of two components with large miscibility gap is
investigated by analysing stationary solutions of the Cahn-Hilliard diffusion
equation. The limits of the existence and stability of periodic concentration
profiles are discussed as a function of the average composition for given
multilayer period length. The minimal average composition and the corresponding
layer thickness below which artificially prepared layers dissolve at elevated
temperatures are calculated as a function of the multilayer period length for a
special model of the composition dependence of the Gibbs free energy. For
period lengths exceeding a critical value, layered structures can exist as
metastable states in a certain region of the average composition. The phase
composition in very thin individual layers, comparable with the interphase
boundary width, deviates from that of the corresponding bulk phase.Comment: 29 pages including 7 figures, to be published in Thin Solid Film
Podosome-Driven Defect Development in Lamellar Bone under the Conditions of Senile Osteoporosis Observed at the Nanometer Scale
The degradation mechanism of human trabecular bone harvested from the central part of the femoral head of a patient with a fragility fracture of the femoral neck under conditions of senile osteoporosis was investigated by high-resolution electron microscopy. As evidenced by light microscopy, there is a disturbance of bone metabolism leading to severe and irreparable damages to the bone structure. These defects are evoked by osteoclasts and thus podosome activity. Podosomes create typical pit marks and holes of about 300-400 nm in diameter on the bone surface. Detailed analysis of the stress field caused by the podosomes in the extracellular bone matrix was performed. The calculations yielded maximum stress in the range of few megapascals resulting in formation of microcracks around the podosomes. Disintegration of hydroxyapatite and free lying collagen fibrils were observed at the edges of the plywood structure of the bone lamella. At the ultimate state, the disintegration of the mineralized collagen fibrils to a gelatinous matrix comes along with a delamination of the apatite nanoplatelets resulting in a brittle, porous bone structure. The nanoplatelets aggregate to big hydroxyapatite plates with a size of up to 10 x 20 μm2. The enhanced plate growth can be explained by the interaction of two mechanisms in the ruffled border zone: the accumulation of delaminated hydroxyapatite nanoplatelets near clusters of podosomes and the accelerated nucleation and random growth of HAP nanoplatelets due to a nonsufficient concentration of process-directing carboxylated osteocalcin cOC. © 2021 The Authors. Published by American Chemical Society
Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces
The segregation behavior of the bimetallic alloys PtPd and CoCr in the case
of bare surfaces and in the presence of an oxygen ad-layer has been studied by
means of first-principles modeling based on density-functional theory (DFT).
For both systems, change of the d-band filling due to charge transfer between
the alloy components, resulting in a shift of the d-band center of surface
atoms compared to the pure components, drives the surface segregation and
governs the chemical reactivity of the bimetals. In contrast to previous
findings but consistent with analogous PtNi alloy systems, enrichment of Pt
atoms in the surface layer and of Pd atoms in the first subsurface layer has
been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd
compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs
in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr
occupy preferentially surface sites due to their lower electronegativity and
thus stronger oxygen affinity compared to Pt and Co, respectively. In either
cases, the calculated oxygen adsorption energies on the alloy surfaces are
larger than on the pure components when the more noble components are present
in the subsurface layers
From Bottom-up Insights to Feature Ideas: A Case Study into the Office Environments of Older Knowledge Workers
Part 2: EnvironmentInternational audienceGiven recent demographic changes, adapting the office environments of older knowledge workers to their needs has become increasingly important in supporting an extension of working life. In this paper, we present a case study research of older knowledge workers in Romania, with the goal of gaining bottom-up insights that support the ideation, design, and development of features for a smart work environment. Utilizing a multi-method approach, we combine (1) contextual interviews and observations, (2) an analysis of needs and frictions for deriving insights, (3) an ideation workshop for eliciting potential features, (4) an online survey among experts for evaluating the final feature ideas, and (5) early stage prototyping of selected feature ideas. Following this comprehensive yet efficient approach, we were able to gain a rich understanding of the work realities and contexts of older knowledge workers and to transform that understanding into a concrete set of prioritized feature ideas