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From the Electronic Structure of Point Defects to Functional Properties of Metals and Ceramics
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Point defects are an inherent part of crystalline materials and they influence important physical and chemical properties, such as diffusion, hardness, catalytic activity and phase stability. Increased understanding of point defects enables us to tailor the defect-related properties to the application at hand. Modeling and simulation have a prominent role in acquiring this knowledge. In this thesis thermodynamic and kinetic properties of point defects in metals and ceramics are studied using first-principles calculations based on density functional theory. Phenomenological models are used to translate the atomic level properties, obtained from the first-principles calculations, into functional materials properties. The next paragraph presents the particular problems under study.

The formation and migration of vacancies and simple vacancy clusters in copper are investigated by calculating the energies associated with these processes. The structure, stability and electronic properties of the low-oxygen oxides of titanium, TiOx with 1/3 < x < 3/2, are studied and the importance of structural vacancies is demonstrated. We develop an integrated first-principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on vacancy-induced ordering of the substoichiometric

carbonitride phase, TiCxNy (x+y < 1). The possibility of forming higher oxides of plutonium than plutonium dioxide is explored by calculating the enthalpies for nonstoichiometric defect-containing compounds and the analysis shows that such oxidation is only produced by strong oxidants. For ceria (CeO2) doped with trivalent ions from the lanthanide series we probe the connection between the choice of a dopant and the improvement of ionic conductivity by studying the oxygen-vacancy formation and migration properties. The significance of minimizing the dopant-vacancy interactions is highlighted. We investigate the redox thermodynamics of CeO2-MO2 solid solutions with M being Ti, Zr, Hf, Th, Si, Ge, Sn or Pb and show that reduction is facilitated by small solutes.

The results in this thesis are relevant for the performance of solid electrolytes, which are an integral part of solid oxide fuel cells, oxygen storage materials in automotive three-way catalysts, nuclear waste materials and cutting tool materials.

Place, publisher, year, edition, pages
Stockholm: Materialvetenskap , 2007.
Keyword [en]
first principles, ab initio, density functional theory, Calphad, point defects, diffusion, solid electrolytes, oxygen storage materials
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:kth:diva-4309ISBN: 978-91-7178-590-9 (print)OAI: oai:DiVA.org:kth-4309DiVA, id: diva2:11732
Public defence
2007-03-30, F2, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2012-03-22Bibliographically approved
List of papers
1. Monovacancy and divacancy formation and migration in copper: a first-principles theory
Open this publication in new window or tab >>Monovacancy and divacancy formation and migration in copper: a first-principles theory
2004 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 11, p. 115108-Article in journal (Refereed) Published
Abstract [en]

The formation and migration of monovacancies and divacancies in copper have been studied from first-principles in order to resolve the discrepancies between previously published experimental and theoretical data. The monovacancy and divacancy formation, migration and binding enthalpies as well as the formation volumes have been calculated in the framework of a plane-wave pseudopotential implementation of the density functional theory, with full structural relaxations included. The monovacancy and divacancy formation entropies have been estimated from experimental data by performing a least-squares analysis. We show that the complete set of first-principles data, taking into account the presence of both vacancies and divacancies as well as the temperature dependence of the formation enthalpies and entropies allow one to reproduce the Arrhenius plot of the total vacancy concentration and the diffusion coefficient, both in good agreement with the most accurate experiments.

National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-6884 (URN)10.1103/PhysRevB.70.115108 (DOI)000224209500028 ()2-s2.0-19744381786 (Scopus ID)
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved
2. Thermodynamics of structural vacancies in titanium monoxide from first principles calculations
Open this publication in new window or tab >>Thermodynamics of structural vacancies in titanium monoxide from first principles calculations
2005 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 14, p. 144101-Article in journal (Refereed) Published
Abstract [en]

The structure, stability and electronic properties of the low oxygen oxides of titanium, TiOx with 1/3 <= x <= 3/2, have been studied by means of accurate first-principles calculations. In both stoichiometric and nonstoichiometric TiO there are large fractions of vacant lattice sites. These so-called structural vacancies are essential for understanding the properties and phase stability of titanium oxides. Structures with an ordered arrangement of vacancies were treated with a plane wave pseudo-potential method, while calculations for structures with disordered vacancies were performed within the framework of the Korringa-Kohn-Rostoker Green's function technique. The relaxed structural parameters in general compare well with experimental data, though some discrepancies exist for stoichiometric TiO in the ideal B1 structure, i.e., without any vacancies. The equation of state as well as the elastic properties are also derived. A monoclinic, vacancy-containing, structure of stoichiometric TiO is confirmed to be stable at low temperature and pressure. Experimentally a transition from a stoichiometric cubic structure with disordered vacancies to the ideal B1 structure without any vacancies has been observed at high pressure. It is discussed how this experimental observation relates to the present theoretical results for defect-containing and defect-free TiO.

Keyword
Total-Energy Calculations, Augmented-Wave Method, Electronic-Structure, Lattice-Dynamics, Basis-Set, Tio, Metals, Oxides, Films, Vanadium
National Category
Ceramics Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-6885 (URN)10.1103/PhysRevB.71.144101 (DOI)000228758700017 ()2-s2.0-28744436258 (Scopus ID)
Note

QC 20100622

Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved
3. Oxidation of plutonium dioxide
Open this publication in new window or tab >>Oxidation of plutonium dioxide
2004 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 3, no 4, p. 225-228Article in journal (Refereed) Published
Abstract [en]

The physics and chemistry of the actinide elements form the scientific basis for rational handling of nuclear materials(1-3). In recent experiments(4), most unexpectedly, plutonium dioxide has been found to react with water to form higher oxides up to PuO2.27, whereas PuO2 had always been thought to be the highest stable oxide of plutonium(2,3). We perform a theoretical analysis of this complicated situation on the basis of total energies calculated within density functional theory(5,6) combined with well-established thermodynamic data. The reactions of PuO2 with either O-2 or H2O to form PuO2+delta are calculated to be endothermic: that is, in order to occur they require a supply of energy. However, our calculations show that PuO2+delta can be formed, as an intermediate product, by reactions with the products of radiolysis of water, such as H2O2.

National Category
Other Engineering and Technologies not elsewhere specified Condensed Matter Physics Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-6886 (URN)10.1038/nmat1095 (DOI)000220747700015 ()2-s2.0-1842630582 (Scopus ID)
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved
4. Optimization of ionic conductivity in doped ceria
Open this publication in new window or tab >>Optimization of ionic conductivity in doped ceria
Show others...
2006 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 103, no 10, p. 3518-3521Article in journal (Refereed) Published
Abstract [en]

Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present a quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy-dopant interactions, represented by association (binding) energies of vacancy-dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately.

Keyword
density functional theory; diffusion; point defects; solid oxide fuel cells; CeO2
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-6887 (URN)10.1073/pnas.0509537103 (DOI)000236225300006 ()2-s2.0-33644866625 (Scopus ID)
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved
5. Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism
Open this publication in new window or tab >>Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism
Show others...
2007 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, no 3, p. 035109-Article in journal (Refereed) Published
Abstract [sv]

The electronic structure and thermodynamic properties of CeO2 and Ce2O3 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method, as implemented in the ab initio total-energy and molecular-dynamics program VASP (Vienna ab initio simulation package). The local density approximation (LDA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized Ce 4f electrons. We discuss how the properties of CeO2 and Ce2O3 are affected by the choice of U as well as the choice of exchange-correlation potential, i.e., the local density approximation or the generalized gradient approximation. Further, reduction of CeO2, leading to formation of Ce2O3 and CeO2-x, and its dependence on U and exchange-correlation potential have been studied in detail. Our results show that by choosing an appropriate U it is possible to consistently describe structural, thermodynamic, and electronic properties of CeO2, Ce2O3, and CeO2-x, which enables modeling of redox processes involving ceria-based materials.

National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-6888 (URN)10.1103/PhysRevB.75.035109 (DOI)000243895400042 ()2-s2.0-33846442083 (Scopus ID)
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved
6. Redox properties of CeO2-MO2 MO2 (M=Ti, Zr, Hf or Th) solid solutions from first principles calculations
Open this publication in new window or tab >>Redox properties of CeO2-MO2 MO2 (M=Ti, Zr, Hf or Th) solid solutions from first principles calculations
Show others...
2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, no 3, p. 031909-Article in journal (Refereed) Published
Abstract [en]

The authors have used density functional theory calculations to investigate how the redox thermodynamics and kinetics of CeO2 are influenced by forming solid solutions with TiO2, ZrO2, HfO2, and ThO2. Reduction is facilitated by dissolving TiO2 (largest improvement), HfO2, or ZrO2 (least improvement), while ThO2 makes reduction slightly more difficult. The migration barrier is much lower in the neighborhood of a Ti (largest decrease), Hf, or Zr (least decrease), while the binding energy of solute ions and vacancies increases in the same sequence. They rationalize the properties of ceria solid solutions in terms of defect cluster relaxations.

National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-6889 (URN)10.1063/1.2431775 (DOI)000243582400031 ()2-s2.0-33846459357 (Scopus ID)
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved
7. Theoretical study of CeO(2) doped with tetravalent ions
Open this publication in new window or tab >>Theoretical study of CeO(2) doped with tetravalent ions
Show others...
2007 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 76, no 7, p. 174119-Article in journal (Refereed) Published
Abstract [en]

We have used density functional theory calculations within the LDA+U formulation to investigate how small amounts of dissolved SiO2, GeO2, SnO2, or PbO2 affect the redox thermodynamics of ceria (CeO2). Compared to pure ceria, reduction is facilitated and the reducibility increases in the sequence of CeO2-SnO2, CeO2-GeO2, and CeO2-SiO2, which correlates with the decrease of the ionic radii of the solutes. For low solute concentrations, there is an inverse relation between high reducibility and the solution energy of tetravalent solutes. CeO2-PbO2 is unique in the sense that the initial reduction occurs by Pb(IV)double right arrow Pb(II) instead of the usual Ce(IV)double right arrow Ce(III) reaction. Among the investigated ceria compounds, CeO2-PbO2 has the lowest reduction energy and rather low solution energy. We have studied how the solution and reduction energies depend on the concentration of Si, Ge, Sn, Pb, Ti, Zr, Hf, and Th solute ions. While the solution energy increases monotonously with concentration, the reduction energy first decreases, as compared to pure ceria (except for Th, which exhibits a small increase), and with further increase of solute concentration, it either remains almost constant (Zr, Hf, and Th) or slightly increases (Ti, Si, Ge, and Sn).

National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-6890 (URN)10.1103/PhysRevB.76.174119 (DOI)000251326600043 ()2-s2.0-36648998537 (Scopus ID)
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved
8. First-principles based calculation of binary and multicomponent phase diagrams for titanium carbonitride
Open this publication in new window or tab >>First-principles based calculation of binary and multicomponent phase diagrams for titanium carbonitride
2008 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 32, no 3, p. 543-565Article in journal (Refereed) Published
Abstract [en]

In this paper we have used a combined first principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on the vacancy-induced ordering of the substoichiometric carbonitride phase, TiCxNy (x + y <= 1). Results from earlier Monte Carlo simulations of the low-temperature binary phase diagrams are used in order to formulate sublattice models for TiCxNy within the compound energy formalism (CEF) that are capable of describing both the low temperature ordered and the high-temperature disordered state. We parameterize these models using first-principles calculations and then we demonstrate how they can be merged with thermodynamic descriptions of the remaining Ti-C-N phases that are derived within the Calphad method by fitting model parameters to experimental data. We also discuss structural and electronic properties of the ordered end-member compounds, as well as short range order effects in the TiCxNy phase.

Keyword
First-principles calculations; CALPHAD; Titanium carbonitride; Order-disorder reactions; Phase diagrams
National Category
Condensed Matter Physics Physical Chemistry Ceramics
Identifiers
urn:nbn:se:kth:diva-6891 (URN)10.1016/j.calphad.2008.04.007 (DOI)000260324400014 ()2-s2.0-51649091367 (Scopus ID)
Note
QC 20100622Available from: 2007-03-14 Created: 2007-03-14 Last updated: 2017-12-14Bibliographically approved

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