Earth Sciences & Env.
Engineering & Tech.
Information & Comm. Tech.
Life Sciences & Biotech
Use this search facility to find out more about the profile of our HPC-Europa2 visitors, the type of work they have been doing, and their project achievements.
Ion-ion interaction will be probed using large-scale density-functional calculations (DFT) for graphite intercalation compounds. Such compounds are known to have a lattice constant that is dependent on the charge state . Hence, intercalated Lithium strains the graphite lattice which will be used to probe long ranged Lithium-Lithium interactions. A look at the so-far most extensive studies, Refs.  and , shows that the activation energy for hopping decreases dramatically with increasing dilution. Values of 1.105 eV, 0.883 eV and 0.832 eV have been obtained for LiC6, LiC20 and LiC32, respectively. However, these barriers have been obtained from for collective processes only. In this study, the system sizes will be increased beyond LiC32. Also, non-collective motion of Li will be probed.
 G. Sun, M. Kertesz, J. Kürti, R.H. Baughman, Phys. Rev. B 68, 125411 (2003) R.C. Boehm, A. Banerjee, J. Chem. Phys. 96, 1150 (1992) K. Toyoura, Y. Koyama, A. Kuwabara, F. Oba, I. Tanaka, Phys. Rev. B 78, 214303 (2008)
For different charge states of the Lithium intercalation compound the energy barrier for Lithium migration was determined at a fixed graphite layer distance. We found a decrease in barrier with decreasing dilution in accordance with earlier studies [1,2]. However, this barrier is extremely sensible to the distance of the graphitic layers. From c=3.4A to c=4.0A the barrier drops from abover 1.0 eV to around 0.2 eV. This shows that the actual Lithium migration in graphite might be sensitive to the pressure applied to the basal planes. In particular, the material's response to a pressure in the basal plane is very soft, leading to large variation in the respective lattice constant upon basal load. Since electrodes are typically manufactured from a powder technological route, this pressure is a quantity that is difficult to control technologically.
Additionally, we computed the chemical potential of Lithium adsorpted on graphene edges  and defects. All carbonaceous electrodes are nanostructured as to increase their surace area. The area of non-basal planes exposed to the electrodes increases with the surface area, hence for highly porous graphitic material Lithium adsoprtion on edge structures might become important. It is found, that the chemical potential of Lithium (measured vs. bulk Li) can be as low as -2 eV depending on the nature of the respective edge or defect. A Bader analysis reveals that the oxidation state of the Lithium is in this case similar to Lithium in graphite intercalation compounds (LiC6). Even on a hydrogen terminated edge, an applied voltage is hence able to drive a substitution reaction leading to a Lithium decorated edge or defect. This process might be a contribution to the initial formation of the battery. Furthermore, the charged edge leads to long-range interactions on the non-basal graphite surface. This might lead to a considerable upfolding of such surfaces.
 R.C. Boehm, A. Banerjee, J. Chem. Phys. 96, 1150 (1992) K. Toyoura, Y. Koyama, A. Kuwabara, F. Oba, I. Tanaka, Phys. Rev. B 78, 214303 (2008) P. Koskinen, S. Malola, H. Häkkinen, Phys. Rev. Lett. 101, 115502 (2008)
The possibility to enhance the thermopower factor of merit Z is increased with the introduction of interfaces in nanostructured materials. Up to now it has been shown that superlattice consisting of several alternating nano-sized layers, each less than five nanometers thick, block the travel of atomic vibrations that produce heat flow but still let the electrons to stream as current. Adding interfacial phonon scattering sites, the thermal conductivity can be reduced. High performance thermoelectric materials with high electrical conductivity and low thermal conductivity can be achieved with 1D and 2D Silicon and Germanium alloys (SiGe) and heterostructures (Si/Ge): SiGe compounds are indeed completely CMOS-compatible and they can sustain very high temperatures up to 1000 K. In particular 2D SiGe heterostructure have produced interest for possible direct integrated on-chip energy harvesting. Si/Ge superlattices are easy to growth; their electronic properties moreover can be controlled by optimally tuning the thickness of the different layers. This project aims therefore to investigate by ab initio calculations how the Si/Ge interface can affect the transport properties and consequently the thermoelectric efficiency of 2D systems. This project aims to investigate:i) how much the Si/Ge interface affect the electronic transport in an heterostructure;ii) how thick should be the layers of Si and Ge in order to preserve high thermopower factor and low thermal conductivity at the same timeiii) how the concentration of defects in the Si/Ge heterostructure affects the electrical conductivity
The heterostructures have been grown like in the experiments along the  direction taking silicon as substrate. To investigate the role of interface we have taken into account several heterostructures with a Ge part embedded between two Si layers. The thickness of the Ge part have been increase from one single layer up to ten. We have minimized the lattice constant along the growth direction z according to the Martin-Van de Walle scheme; the atomic coordinates have been fully relaxed for all the structures with the code Siesta. For the transport approach we have used the transport code Transiesta. For each configuration we have calculated ground state electronic properties and ballistic transport conductance. The conductance varies according to the number of Ge layers in the heterostructure: it increases w.r.t. the pure silicon system up to saturate when the number of Ge layers is around 5. This could be interpret as a “critical” thickness beyond which the electric conductance cannot been improved. At the moment we are evaluating for the heterostructure with lower number of atoms (due to computational cost issues) how the electronic properties change when defects such as vacancies are introduced in the system.
The objectives of this research program are the following:• Production by the application of integrated computational models with multiscale approach based on data from the literature, thus providing a basis for future research. The verification of the developed models will be achieved through the comparison with literature data or with a synergistic collaboration with the experimental sector. The models will also be applied to the selected pure materials • Using simulations on various scales for the detailed analysis of the distribution of voids and the morphology of high free volume polymers (HYFLON, TEFLON, PIM-PIs). • Identify at least two different nanofillers by analyzing the structure at the molecular level and morphology of the materials under study: the chosen particles are indicated to be suitable for the preparation of mixed matrix membranes (MMM) for specific pairs of gas separations.• Calculation of theoretical values of solubility, permeability and diffusion of gaseous species to be separated, together with detailed analysis of the interactions on the nano- and micro-scales of permeant with nanoparticles and polymer matrix.
takin into account that the period of my visit has been reduced to only one month the achievements are reduced compared to the expected results. Meanwhile I was able to achieve the following objectives:
1) Investigation of the morphology of complex copolymer systems (Hyflon ion for fuell cell application) with a combined Molecular Dynamics/ Mesoscale modelling approach by using the new software Culgi 7.0. developed in the host research group and interfcaed with the Huygens supercomputer of SARA.
2) Validation of the DDFT simulated models by comparison with experimental data.