Papers by Sergey Rashkeev
Idaho National Laboratory (INL) has an ongoing project that supports the development of high-temp... more Idaho National Laboratory (INL) has an ongoing project that supports the development of high-temperature electrolysis for efficient hydrogen production from steam using solid oxide electrolysis cells (SOECs). To accomplish this objective, technical and degradation issues associated with the SOECs must be addressed. This report covers various approaches being pursued to model degradation issues in SOECs.
Metastable (transition) aluminas, in particular gamma -Al_2O3 and eta -Al_2O_3, are widely used a... more Metastable (transition) aluminas, in particular gamma -Al_2O3 and eta -Al_2O_3, are widely used as catalysts as well as supports for different transition metals. When doped with Cr, the two modifications exhibit remarkably different aging behavior: gamma -Al_2O_3-supported catalysts degrade two orders of magnitude faster than those based on eta -Al_2O_3. Here we use a combination of Z-contrast aberration-corrected scanning transmission
nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalys... more nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers 1 . For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous γ-Al 2 O 3 , a widely used catalyst and catalytic support 2-6 , transforms to non-porous α-Al 2 O 3 at ~1,100°C (refs 7-10). Doping with La raises the transformation temperature 8-11 to ~1,250°C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds 10-15 . Here, we use direct imaging by aberrationcorrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and firstprinciples calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to α-Al 2 O 3 .
Bulletin of the American Physical Society, 2005
Nature Materials, 2004
nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalys... more nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers 1 . For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous γ-Al 2 O 3 , a widely used catalyst and catalytic support 2-6 , transforms to non-porous α-Al 2 O 3 at ~1,100°C (refs 7-10). Doping with La raises the transformation temperature 8-11 to ~1,250°C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds 10-15 . Here, we use direct imaging by aberrationcorrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and firstprinciples calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to α-Al 2 O 3 .
Le Journal de Physique IV, 1993
The change in slope of the Arrheliius curve for the diffusion coefficient a.s a. function of temp... more The change in slope of the Arrheliius curve for the diffusion coefficient a.s a. function of temperature is discussed and its occurance is explained by vacancies concentra.tion clependence of the forma.tion energy. It is shown that a sign of the ,mixing energy parameter of the r,eytilar solid solu.tion of vacancies in pure llletals is closely coliiiected with the sign of tlie curva,ture of the diffusion plot. For the analysis of this plot as well as for the estimatioil of other parameters of self diffusion of bcc Cs we used the total energy calculations based on the Full Potential LMTO method.
Computational Materials Science, 2001
Phase modulations of periodic dislocation patterns observed in transmission electron microscopy (... more Phase modulations of periodic dislocation patterns observed in transmission electron microscopy (TEM) studies of some single crystalline metals, are analyzed using the methods of nonlinear dynamics. The Ginzburg±Landau (GL) equation for the soft mode instability in the weakly nonlinear regime is derived for the Walgraef±Aifantis (WA) model for a coupled system of two populations of dislocations. The bulk of results is obtained using the GL equation, and, therefore, the results are more general than the WA-model itself. We demonstrate that phase modulations of dislocation patterns can be described using the concept of the Eckhaus instability which describes one of the most fundamental``generic'' mechanisms of wavelength-changing. The timescale of wavelength-changing processes in dislocation systems can be very large when the system is close to the Eckhaus stability limit. This means that metastable phase modulations of dislocation patterns can survive nearly unchanged for a long time. The results of numerical simulations for realistic values of the parameters show that the Eckhaus instability could be the underlying physical reason of modulated ladder structures of persistent slip bands (PSBs) in cyclically deformed metallic alloys. Ó
Aps Meeting Abstracts, Mar 1, 2004
The design of catalysts that operate at high temperatures and are stable against structural trans... more The design of catalysts that operate at high temperatures and are stable against structural transformations is a major challenge for scientists and engineers. Porous γ -Al_2O3 is widely used as catalyst and catalyst support because of its large surface-to-volume ratio. However, it transforms to non-porous α -Al_2O3 (corundum) at ˜ 1100r C. Loss of surface area via sintering at operating temperatures can often be inhibited by the incorporation of certain impurities. For example, doping with La raises the transformation temperature to ˜ 1250r C, but it has not even been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds. We use a combination of first-principles calculations, direct imaging of individual La atoms by Z-contrast scanning transmission electron microscopy, and extended X-ray absorption fine structure to understand the atomic-scale mechanism of stabilization of γ -alumina by La. We show that La atoms always remain atomically dispersed on γ -alumina surfaces. Sintering and transformation to α -alumina is inhibited to avoid La incorporation in bulk γ -alumina or chemisorption on α -alumina. This work was supported in part by NSF Grant DMR-0111841 and DOE Grants DE-FC02-01CH11085 and DE-AC05-00OR22725.
Nat Mater, 2004
nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalys... more nature materials | VOL 3 | MARCH 2004 | www.nature.com/naturematerials 143 T he design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers 1 . For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization. For example, porous γ-Al 2 O 3 , a widely used catalyst and catalytic support 2-6 , transforms to non-porous α-Al 2 O 3 at ~1,100°C (refs 7-10). Doping with La raises the transformation temperature 8-11 to ~1,250°C, but it has not been possible to establish if La atoms enter the bulk, adsorb on surfaces as single atoms or clusters, or form surface compounds 10-15 . Here, we use direct imaging by aberrationcorrected Z-contrast scanning transmission electron microscopy coupled with extended X-ray absorption fine structure and firstprinciples calculations to demonstrate that, contrary to expectations, stabilization is achieved by isolated La atoms adsorbed on the surface. Strong binding and mutual repulsion of La atoms effectively pin the surface and inhibit both sintering and the transformation to α-Al 2 O 3 .
MRS Proceedings, 2002
We employ a combination of Z-contrast scanning transmission electron microscopy (Z-STEM) and firs... more We employ a combination of Z-contrast scanning transmission electron microscopy (Z-STEM) and first-principles density-functional calculations to investigate the interaction between metal atoms and cubic alumina catalytic supports. We show that there are two observed La sites at the porous γalumina surface, and that single La atoms do not exhibit a tendency to cluster. Cr behaves very differently from La -it has a tendency to cover the alumina surface in ordered, periodic raft-like patches. The degradation of the chromia/alumina catalyst is related to the possibility for the chromium to move away from the surface into the bulk, and the activation barrier for such a process is higher in ηalumina than in γalumina, i.e., ηalumina is a more durable support for chromium catalyst.
Physical Review B, 2003
Transition metals ͑Pt, Pd, Rh, Cr, etc.͒ are widely used as catalysts on ␥and -alumina, two phase... more Transition metals ͑Pt, Pd, Rh, Cr, etc.͒ are widely used as catalysts on ␥and -alumina, two phases of Al 2 O 3 , but the catalytic activity of Cr on ␥-alumina degrades rapidly. We report density-functional calculations that trace the origin of the effect to differences in surface reconstruction caused by different distributions of bulk cation vacancies. On -alumina, Cr atoms remain on the surface, threefold-coordinated and reactive; on ␥-alumina, they get trapped in subsurface octahedral sites and become inactive. Some metal atoms ͑e.g., Mn͒ also get trapped, but the other catalytic elements ͑Pt, Pd, Rh͒ do not. Steric constraints, size effects, or other qualitative considerations are not adequate to account for the results.
physica status solidi (b), 1997
Phys Rev B, 1999
A first-principles study of the birefringence and the frequency-dependent second-harmonic generat... more A first-principles study of the birefringence and the frequency-dependent second-harmonic generation (SHG) coefficients of the ternary pnictide semiconductors with formula ABC2 (A=Zn, Cd; B=Si, Ge; C=As, P) with the chalcopyrite structures was carried out. The zero-frequency limits of χ(2)123 were found to be in reasonable agreement with available experimental data for all the considered materials. We found that substitution of P by As, Si by Ge, and Zn by Cd is favorable to get a higher value of χ(2)(0). An analysis of the different contributions shows that the anomalously high value of the zero-frequency SHG in CdGeAs2 appears as a result of a very small interband term in the zero-frequency limit which, contrary to most of the other materials of this class, does not compensate the large intraband contribution. Simple inverse power scaling laws between gaps and χ(2) values are not supported by our results. We find that the (001) oriented 1+1 superlattice structure has significantly lower gaps than the chalcopyrite and correspondingly higher χ(2). However, this smaller gap structure is characterized by a large alternatingly compressive and tensile lateral strain in the layers, which makes it unfavorable. The calculated values of the birefringence for ZnGeP2 and CdGeAs2 are in fair agreement (discrepancies being rather constant and of the order of 10%) with experiment in the frequency range corresponding to the middle of the gap.
Phys Rev B, 2001
We calculated the nuclear spin-lattice relaxation rate in the (Mg,Al)B2 system and found that the... more We calculated the nuclear spin-lattice relaxation rate in the (Mg,Al)B2 system and found that the orbital relaxation mechanism dominates over the dipolar and Fermi-contact mechanisms in MgB2, whereas in AlB2 due to a smaller density of states and strong anisotropy of boron p orbitals the relaxation is completely determined by Fermi-contact interaction. The results for MgB2 are compared with existing experimental data, whereas our results for the doped alloy and AlB2 can be used in future experiments to verify the theoretical predictions about the electronic structure of this system.
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Papers by Sergey Rashkeev