Tag: Materials Science

39th ICACC: Day 2

Today I had to get to work myself, unlike the first day of the ICACC where I was only a spectator. At 8h30 I kicked of the GYIF as the first speaker in a session called “Theoretical Modeling and Applications” with my presentation: Computational Materials Science: Where Theory meets Experiment. Intended for a general audience of (expectedly mainly) experimental materials scientist this presentation was aimed to show the audience that computational materials science has in the last decades developed to the level where results relevant for real-life applications in materials science can be obtained. For this I used three examples of my own work: (1) Pt nanowires on Ge(001), where I used simulated STM to build an accurate atomistic model of these wires, (2) doped cerium oxides, where the influence of doping on lattice parameter and thermal expansion coefficients was studied for the purpose of matching them to those of other materials, and (3) Metal-Organic Frameworks, where I showed that the spin-configuration of the MIL-47(V) MOF is linked to the transition pressure inducing breathing.

Later on during the morning, I had the pleasure of chairing the session “Additive Manufacturing” together with Valerie Wiesner from NASA. During this session Mahref Vali and Lisa Rueschoff presented their most recent work on 3D printing of ceramic materials, a technique which will allow the printing of ceramic components in the future. The third speaker of this session, Rumi Kitazawa, delighted us with an inspiring talk on the “Engineering applications of Menger sponges”. A Menger sponge is a fractal related to the Cantor set, as such a fully developed Menger sponge can not be build with any real material, however, using 3D printing it is possible to build a structure with Menger sponge like features (i.e. with holes down to a certain size). By comparing experimental stress tests on such 3D-printed systems with calculations of the strain energy in such a structure, Rumi was able to show these Menger materials have a peculiar, albeit very organised, strain pattern along the main diagonal of the material. The combination of large and small pores present in these Menger sponge materials may make this behavior relevant for MOF (and other porous) materials, where the large pores reflect inter-grain pores, while the small pores are the pores of the MOF. So this is definitely a topic to remember.

Next to presentations, conferences also contain social events. Today, there were two social events; at noon there was a luncheon by the GYIF where the young investigators could mix with people from industry and senior group leaders. In the evening there was the first poster session and booth-stand where companies try to sell their services and lab equipment. Somehow, as a theoretician, I am always a bit at a loss at such events. To draw in more people, there was also a shot-glass contest. No, it was not the goal to drink as much as possible, but to build a protective structure around a shot-glass using only 15 drinking straws (no tape, wire, paper, staples,… allowed). To find the most protective structure, the shot-glass and straw constructions were dropped from various heights. Twenty four teams started at a drop height of 3 feet (~1 m), where already the first shot-glass didn’t survive the drop. Every round, the drop height was increased by 3 feet. For a drop of 20 feet (~6 m) there were only two teams remaining, including our team. Altough our glass survived its first bounce, the second bounce unfortunately broke our glass (darn). Then it was the turn of our remaining competitor, who’s shot-glass exploded into shards on first impact. Officially the result was a draw, although it is clear our construct had clearly the upper hand 😎 .

Our mixed Theoretical-Experimental "international-multi-university" research  team. Left to right: me (UGhent, Belgium), Bert Conings (UHasselt, Belgium), and Chenxin Jin  (Dalhousie University, Canada)  On the right hand side, you can see our construct of straws  around the shot glass.

Our mixed Theoretical-Experimental “international-multi-university” research team. Left to right: me (UGhent, Belgium), Bert Conings (UHasselt, Belgium), and Chenxin Jin (Dalhousie University, Canada)
On the right hand side, you can see our construct of straws around the shot glass.

39th ICACC: Day 1

Today the 39th International Conference  and exposition on Advanced Ceramics and Composites (ICACC) started in Daytona Beach, Florida. Here, scientist from all over the world will be discussing their latest work and findings in the field of ceramic materials during the coming week. Although my project on ceramic materials has been completed for over 2 years now, culminating in a second PhD, I was invited to present my work here. As with many of this kind of conferences, I am afraid that, as a computational materials scientist, I belong to a minority, strongly outnumbered by the experimental (materials) scientists present. This is an aspect that I will need to consider preparing my presentations.

The first morning session consisted of four plenary lectures (general overview presentations in which celebrated group leaders present the overall picture of the work done in their group and their hopes/views on the future). We started of an interesting lecture on “Thermal Barrier Coatings for Gas Turbines” by Prof. David Clarke, where we learned that, since a major part of the world wide energy production is gas based, improving gas-turbine efficiency by only 1% would produce more energy than all renewable production currently in play. This efficiency improvement can be obtained by operating the gas-turbines at higher temperatures. Unfortunately, the metal fans of such a turbine start to degrade if temperatures are too high. By coating them with materials that have a low thermal conductivity, it is possible to operate at the required high temperatures, while the metal fans experience an acceptable operating temperature which is a few hundred degrees lower. Next Prof. Sanjay Mathur from the university of Cologne presented his  groups work on the development of precursor libraries (these ideas are similar as those behind computational high-throughput projects). In precursor chemistry, where changing functional groups or doping leads to changes in the surface morphology, such libraries would then present an interesting tool for designing new materials for energy and health applications. As this apparently is a very hot topic, the fire-alarm of the conference center went into overdrive and everyone needed to be evacuated. When the conference resumed half an hour later Prof. Mathur reassured us he did not intend to fire things up like this. His presentation was followed by that of Prof. Cato Laurencin, who showed us how ceramic materials could be used in a new field he wishes to launch: “Regenerative engineering”. Here, combinations of micro- and nanostructured ceramics are used as matrices to grow and differentiate stem cells intended to heal fractured bones and cartilage. The final presentation by Prof. Kazushige Ohno discussed next generation filters for diesel particulates, which should provide us with a cleaner future.

In the afternoon the parallel symposia started, where I followed the 4th Global Young Investigator Forum (GYIF). Here, Prof. Ricardo Castro presented an interesting method for experimentally obtaining the surface energy of nanoparticles. His quest originated from the simple observation that existing phase diagrams for bulk materials no longer hold when one is working with nano-particles. In such systems, the energy contributions due to the surface of the particle become comparable to those of the inner bulk. Interestingly, one of the example systems Prof. Castro looked into was Mn3+ doped CeO2. In his work he found that the Mn was mainly located at the surface of the CeO2 particles, something I also expected from my own work on aliovalent doped CeO2, based on the defect formation energies of Cu and Co doping. Further presentations discussed Organometal trihalide perovskite solar cells. Although these solar cells still are rather unstable, they do show promise with regard to their efficiency.(The origin of this efficiency is unfortunately not really understood. Maybe other perovskite MOFs are more stable?)

Comment on ‘Europium doping induced symmetry deviation and its impact on the second harmonic generation of doped ZnO nanowires’

Authors: Danny E. P. Vanpoucke
Journal: Nanotechnology 25(45), 458001 (2014)
doi: 10.1088/0957-4484/25/45/458001
IF(2014): 3.821
export: bibtex
pdf: <Nanotechnology>

Abstract

In Dhara et al. 2014 Nanotechnology 25 225202, the authors reported on the synthesis of Eu-doped ZnO nanowires (NWs) and investigated the influence of Eu doping on the second harmonic generation (SHG). Maximum SHG was found to correlate strongly with the structural deformation attributed to Eu3+ doping. In this comment, we show the deformation of interest is due to the presence of Eu2+ dopants, based on both the experimental data presented by Dhara et al. and ab-initio density functional theory calculations.

Quasi-1D physics in metal-organic frameworks: MIL-47(V) from first principles

Authors: Danny E. P. Vanpoucke, Jan W. Jaeken, Stijn De Baerdemacker, Kurt Lejaeghere
and Veronique Van Speybroeck
Journal: Beilstein J. Nanotechnol. 5, 1738-1748 (2014)
doi: 10.3762/bjnano.5.184
IF(2014): 2.670
export: bibtex
pdf: <Beilstein> (open access)
Graphical Abstract: (left) Spin density of anti-ferromagnetic MIL-47(V) with ferromagnetic chains. (right) Electronic band structure and density of states.
Graphical Abstract: The MIL-47(V) MOF has one unpaired electron per V site. As a result, different spin configurations are possible, several of which lead to an anti-ferromagnetic state. The spin density of an antiferromagnetic state, containing only ferromagnetic chains is shown on the left. On the right, the electronic band structure of the same system is presented.

Abstract

The geometric and electronic structure of the MIL-47(V) metal-organic framework (MOF) is investigated by using ab initio density functional theory (DFT) calculations. Special focus is placed on the relation between the spin configuration and the properties of the MOF. The ground state is found to be antiferromagnetic, with an equilibrium volume of 1554.70 Å3. The transition pressure of the pressure-induced large-pore-to-narrow-pore phase transition is calculated to be 82 MPa and 124 MPa for systems with ferromagnetic and antiferromagnetic chains, respectively. For a mixed system, the transition pressure is found to be a weighted average of the ferromagnetic and antiferromagnetic transition pressures. Mapping DFT energies onto a simple-spin Hamiltonian shows both the intra- and inter-chain coupling to be antiferromagnetic, with the latter coupling constant being two orders of magnitude smaller than the former, suggesting the MIL-47(V) to present quasi-1D behavior. The electronic structure of the different spin configurations is investigated and it shows that the band gap position varies strongly with the spin configuration. The valence and conduction bands show a clear V d-character. In addition, these bands are flat in directions orthogonal to VO6 chains, while showing dispersion along the the direction of the VO6 chains, similar as for other quasi-1D materials.

Aliovalent Doping of CeO2: DFT study of oxidation state and vacancy effects

Authors: Danny E. P. Vanpoucke, Patrick Bultinck, Stefaan Cottenier, Veronique Van Speybroeck, and Isabel Van Driessche,
Journal: J. Mater. Chem. A 2(33), 13723-13737 (2014)
doi: 10.1039/C4TA02449D
IF(2014): 7.443
export: bibtex
pdf: <JMaterChemA> <arXiv>

Abstract

The modification of CeO2 properties by means of aliovalent doping is investigated within the ab initio density functional theory framework. Lattice parameters, dopant atomic radii, bulk moduli and thermal expansion coefficients of fluorite type Ce1-xMxO2-y (with M = Mg, V, Co, Cu, Zn, Nb, Ba, La, Sm, Gd, Yb, and Bi) are presented for 0.00 ≤ x ≤ 0.25. The relative stability of the doped systems is discussed, and the influence of oxygen vacancies is investigated. It is shown that oxygen vacancies tend to increase the lattice parameter, and strongly decrease the bulk modulus. Defect formation energies are correlated with calculated crystal radii and covalent radii of the dopants, and are shown to present no simple trend. The previously observed inverse relationship between the thermal expansion coefficient and the bulk modulus in group IV doped CeO2 [J. Am. Ceram. Soc. 97(1), 258 (2014)] is shown to persist independent of the inclusion of charge compensating vacancies.

Tetravalent Doping of CeO2: The impact of valence electron character on group IV dopant influence

Authors: Danny E. P. Vanpoucke, Stefaan Cottenier, Veronique Van Speybroeck, Isabel Van Driessche, and Patrick Bultinck
Journal: J. Am. Ceram. Soc. 97(1), 258-266 (2014)
doi: 10.1111/jace.12650
IF(2014): 2.610
export: bibtex
pdf: <J.Am.Ceram.Soc.> <arXiv>

Abstract

Fluorite CeO2 doped with group IV elements is studied within the density functional theory (DFT) and DFT + U framework. Concentration-dependent formation energies are calculated for Ce1−xZxO2 (Z = C, Si, Ge, Sn, Pb, Ti, Zr, Hf) with 0 ≤ x ≤ 0.25 and a roughly decreasing trend with ionic radius is observed. The influence of the valence and near valence electronic configuration is discussed, indicating the importance of filled d and f shells near the Fermi level for all properties investigated. A clearly different behavior of group IVa and IVb dopants is observed: the former are more suitable for surface modifications and the latter are more suitable for bulk modifications. For the entire set of group IV dopants, there exists an inverse relation between the change, due to doping, of the bulk modulus, and the thermal expansion coefficients. Hirshfeld-I atomic charges show that charge-transfer effects due to doping are limited to the nearest-neighbor oxygen atoms.

Modeling 1D structures on semiconductor surfaces: Synergy of theory and experiment

Authors: Danny E. P. Vanpoucke
Journal: J. Phys.: Condens. Matter 26(13), 133001 (2014)
doi: 10.1088/0953-8984/26/13/133001
IF(2014): 2.346
export: bibtex
pdf: <J.Phys.Condens.Matter> <arXiv>

Abstract

Atomic scale nanowires attract enormous interest in a wide range of fields. On the one hand, due to their quasi-one-dimensional nature, they can act as an experimental testbed for exotic physics: Peierls instability, charge density waves, and Luttinger liquid behavior. On the other hand, due to their small size, they are of interest not only for future device applications in the micro-electronics industry, but also for applications regarding molecular electronics. This versatile nature makes them interesting systems to produce and study, but their size and growth conditions push both experimental production and theoretical modeling to their limits. In this review, modeling of atomic scale nanowires on semiconductor surfaces is discussed, focusing on the interplay between theory and experiment. The current state of modeling efforts on Pt- and Au-induced nanowires on Ge(001) is presented, indicating their similarities and differences. Recently discovered nanowire systems (Ir, Co, Sr) on the Ge(001) surface are also touched upon. The importance of scanning tunneling microscopy as a tool for direct comparison of theoretical and experimental data is shown, as is the power of density functional theory as an atomistic simulation approach. It becomes clear that complementary strengths of theoretical and experimental investigations are required for successful modeling of the atomistic nanowires, due to their complexity.

Cover Image of Journal of Computational Chemistry : Extending Hirshfeld-I

Authors: Danny E. P. Vanpoucke
Journal: J. Comput. Chem. 34(5), i-ii (2013)
doi: 10.1002/jcc.23239
IF(2013): 3.601
export: bibtex
pdf: <J.Comput.Chem.>

Abstract

The image shows an isosurface of Hirshfeld-I “atoms in molecules” for Ti-doped CeO2, taken at an electron density of 0.03e/Å3, as presented by Danny E. P. Vanpoucke, Patrick Bultinck, and Isabel Van Driessche on page 405. The cubic Ce0.75Ti0.25O2 unit cell is shown along the 111 direction. The different atoms are still clearly distinguishable at this iso-surface level, and show the Ti atom in the corners to be much smaller than the Ce atoms on the sides. In this issue, this implementation of the Hirshfeld- I method for solids is published back to back with a Comment from Thomas A. Manz and the authors’ Reply.


Cover of Journal of Computational Chemistry: Volume 34, Issue 5, feb. 15, 2013

Reply to ‘Comment on “Extending Hirshfeld-I to bulk and periodic materials” ‘

Authors: Danny E. P. Vanpoucke, Isabel Van Driessche, and Patrick Bultinck
Journal: J. Comput. Chem. 34(5), 422-427 (2013)
doi: 10.1002/jcc.23193
IF(2013): 3.601
export: bibtex
pdf: <J.Comput.Chem.> <arXiv>
Graphical Abstract: Hirshfeld-I atoms-in-molecules atoms in Ti doped CeO2. Graphical Abstract:The issues raised in the preceding comment are addressed. It is shown why Hirshfeld-I is, from a theoretical point of view, a good method for defining AIM and obtaining charges. Charges for a set of ionic systems are calculated using our presented method and shown to be chemically feasable. Comparison of pseudo-density to all-electron based results shows the pseudo-densities to be sufficient to obtain all-electron quality results. Timing results for systems containing hundreds of atoms.

Abstract

The issues raised in the comment by Manz are addressed through the presentation of calculated atomic charges for NaF, NaCl, MgO, SrTiO3, and La2Ce2O7, using our previously presented method for calculating Hirshfeld-I charges in solids (Vanpoucke et al., J. Comput. Chem. doi: 10.1002/jcc.23088). It is shown that the use of pseudovalence charges is sufficient to retrieve the full all-electron Hirshfeld-I charges to good accuracy. Furthermore, we present timing results of different systems, containing up to over 200 atoms, underlining the relatively low cost for large systems. A number of theoretical issues are formulated, pointing out mainly that care must be taken when deriving new atoms in molecules methods based on “expectations” for atomic charges.

Extending Hirshfeld-I to bulk and periodic materials

Authors: Danny E. P. Vanpoucke, Patrick Bultinck, and Isabel Van Driessche,
Journal: J. Comput. Chem. 34(5), 405-417 (2013)
doi: 10.1002/jcc.23088
IF(2013): 3.601
export: bibtex
pdf: <J.Comput.Chem.> <arXiv>
Graphical Abstract: Hirshfeld-I atoms-in-molecules carbon atoms in a graphene sheet. Graphical Abstract: The Hirshfeld-I method is extended to solids, allowing for the partitioning of a solid density into constituent atoms. The use of precalculated density grids makes the implementation code independent, and the use of pseudo-potential based electron density distributions is shown to give qualitatively the same results as all electron densities. Results for some simple solids/periodic systems like cerium oxide and graphene are presented.

Abstract

In this work, a method is described to extend the iterative Hirshfeld-I method, generally used for molecules, to periodic systems. The implementation makes use of precalculated pseudopotential-based electron density distributions, and it is shown that high-quality results are obtained for both molecules and solids, such as ceria, diamond, and graphite. The use of grids containing (precalculated) electron densities makes the implementation independent of the solid state or quantum chemical code used for studying the system. The extension described here allows for easy calculation of atomic charges and charge transfer in periodic and bulk systems. The conceptual issue of obtaining reference densities for anions is discussed, and the delocalization problem for anionic reference densities originating from the use of a plane wave basis set is identified and handled.