Magnetostatic Dipolar Energy of Large Periodic Ni fcc Nanowires, Slabs and Spheres

Producción CientíficaThe computational effort to calculate the magnetostatic dipolar energy, MDE, of a periodic cell of N magnetic moments is an O(N2) task. Compared with the calculation of the Exchange and Zeeman energy terms, this is the most computationally expensive part of the atomistic simulations of the magnetic properties of large periodic magnetic systems. Two strategies to reduce the computational effort have been studied: An analysis of the traditional Ewald method to calculate the MDE of periodic systems and parallel calculations. The detailed analysis reveals that, for certain types of periodic systems, there are many matrix elements of the Ewald method identical to another elements, due to some symmetry properties of the periodic systems. Computation timing experiments of the MDE of large periodic Ni fcc nanowires, slabs and spheres, up to 32000 magnetic moments in the periodic cell, have been carried out and they show that the number of matrix elements that should be calculated is approximately equal to N, instead of N2/2, if these symmetries are used, and that the computation time decreases in an important amount. The time complexity of the analysis of the symmetries is O(N3), increasing the time complexity of the traditional Ewald method. MDE is a very small energy and therefore, the usual required precision of the calculation of the MDE is so high, about 10−6 eV/cell, that the calculations of large periodic magnetic systems are very expensive and the use of the symmetries reduces, in practical terms, the computation time of the MDE in a significant amount, in spite of the increase of the time complexity. The second strategy consists on parallel calculations of the MDE without using the symmetries of the periodic systems. The parallel calculations have been compared with serial calculations that use the symmetries.Ministerio de Economía, Industria y Competitividad ( grant MAT2014-54378-R)Junta de Castilla y León (grants VA050U14 and VA124G18

Grand canonical Monte Carlo simulations of the hydrogen and methane storage capacities of novel but MOFs at room temperature

Producción CientíficaHydrogen Fuel Cell Electric Vehicles (HFCEVs) and Natural Gas Vehicles (NGVs) are cleaner alternatives to present oil-based vehicles. The main problem of these technologies is the on-board storage. Metal-organic frameworks (MOFs) is one of the main groups of solid porous materials that can be used to store hydrogen or methane on-board these vehicles at room temperature and low or moderate pressures. The synthesis of these materials is usually expensive. Recently a group of eleven new BUT MOFs (BUT: Beijing University of Technology) has been synthesized using cheap organic precursors. Grand Canonical Monte Carlo simulations (GCMC) of the hydrogen and methane storage capacities and isosteric heats of these BUTs have been carried out and analyzed at 298.15 K and at pressures in the range 0.5–50 MPa. The correlations between the storage capacities and the porosity, the density, the pore size and the isosteric heat of the MOFs are analyzed. According to the simulations, three of the newly developed BUTs demonstrated high storage capacities for both hydrogen and methane. BUT-104 and 105 exhibited useable hydrogen volumetric and gravimetric capacities of approximately 0.023–0.027 kg/L and 4 wt % at 50 MPa. Additionally, they showcased useable methane volumetric and gravimetric capacities of 0.16–0.21 kg/L and 25 wt % at 25–35 MPa. Moreover, BUT-107 achieved the U.S. Department of Energy (DOE) hydrogen target for 2025, with a useable hydrogen gravimetric capacity of 5.5 wt % at 27 MPa. Furthermore, BUT-107 met the corresponding DOE methane targets, with useable methane volumetric and gravimetric capacities of 0.25 kg/L and 33.33 wt % at 50 MPa.Ministerio de Ciencia e Innovación y Ministerio de Universidades (Beca PGC2018-093745-B-I00)Junta de Castilla y León (Beca VA124G18

Simulations of volumetric hydrogen storage capacities of nanoporous carbons: Effect of dispersion interactions as a function of pressure, temperature and pore width

Producción CientíficaSimulations of the hydrogen storage capacities of activated carbons require an accurate treatment of the interaction of a hydrogen molecule physisorbed on the graphitic-like surfaces of nanoporous carbons, which is dominated by the dispersion interactions. These interactions are described accurately by high level quantum chemistry methods such as the Coupled cluster method with single and double excitations and a non-iterative correction for triple excitations (CCSD(T)), but those methods are computationally very expensive for large systems and massive simulations. Density functional theory (DFT) based methods that include dispersion interactions are less accurate, but computationally less expensive. Calculations of the volumetric hydrogen storage capacities of nanoporous carbons, simulated as benzene and graphene slit-shaped pores, have been carried out, using a quantum-thermodynamic model of the physisorption of H2 on surfaces and the interaction potential energy curves of H2 physisorbed on benzene and graphene obtained using the CCSD(T) and second order Møller-Plesset (MP2) methods and the 14 most popular DFT-based methods that include the dispersion interactions at different levels of complexity. The effect of the dispersion interactions on the DFT-based volumetric capacities as a function of the pressure, temperature and pore width is evaluated. The error of the volumetric capacities obtained with the quantum-thermodynamic model and each method is also calculated and analyzed.Ministerio de Economía, Industria y Competitividad ( grant MAT2014-54378-R)Junta de Castilla y León (projects VA050U14 and VA124G18

Analysis of the Symmetry Properties of Large Periodic Magnetic Systems, to Reduce the Computation Time of the Calculation of the Magnetostatic Dipolar Energy

Producción CientíficaThe computational effort to calculate the magnetostatic dipolar energy, MDE, of a periodic cell of N magnetic moments is an O(N 2 ) task. Compared with the calculation of the Exchange and Zeeman energy terms, this is the most computationally expensive part of the atomistic simulations of the magnetic properties of large periodic magnetic systems. To reduce the computational effort, the traditional Ewald method to calculate the MDE of periodic magnetic systems has been analyzed. The detailed analysis reveals that, for certain types of periodic systems, there are many matrix elements of the Ewald method identical to another elements, due to symmetry properties of the periodic systems. Computation timing experiments of the MDE of large systems, such as Ni fcc nanowires up to 31500 magnetic moments in the periodic cell, have been carried out and they show that the number of matrix elements that should be calculated is approximately equal to N, instead of N 2 /2 if these symmetries are used, and that the computation time decreases in an important amount. The time complexity of the analysis of the symmetries is O(N 3 ), which increases the time complexity of the traditional Ewald method and is in contrast with the computation timing experiments. This is explained by the fact that the MDE is a very small energy and therefore, the usual required precision of the calculation of the MDE is so high, about 10 -6 eV/cell, that the calculations of large periodic magnetic systems are very expensive and the use of the symmetries reduces, in practical terms, the computation time of the MDE in a significant amount, in spite of the increase of the time complexity.Ministerio de Economía, Industria y Competitividad (GrantMAT2014-54378-R)Junta de Castilla y León (GrantVA050U14

La felicidad y sus utopías

Comunicación presentada en el Curso "La felicidad humana", dentro de los Cursos de verano UBU 201

Grand Canonical Monte Carlo simulations of the hydrogen storage capacities of slit-shaped pores, nanotubes and torusenes

Producción CientíficaGrand Canonical Monte Carlo, GCMC, simulations are used to study the gravimetric and volumetric hydrogen storage capacities of different carbon nanopores shapes: Slit-shaped, nanotubes and torusenes at room temperature, 298.15 K, and at pressures between 0.1 and 35 MPa, and for pore diameter or width between 4 and 15 Å. The influence of the pore shape or curvature on the storage capacities as a function of pressure, temperature and pore diameter is investigated and analyzed. A large curvature of the pores means, in general, an increase of the storage capacities of the pores. While torusenes and nanotubes have surfaces with more curvature than the slit-shaped planar pores, their capacities are lower than those of the slit-shaped pores, according to the present GCMC simulations. Torusene, a less studied carbon nanostructure, has two radii or curvatures, but their storage capacities are similar or lower than those of nanotubes, which have only one radius or curvature. The goal is to obtain qualitative and quantitative relationships between the structure of porous materials and the hydrogen storage capacities, in particular or especially the relationship between shape and width of the pores and the hydrogen storage capacities of carbon-based porous materials.Ministerio de Ciencia e Innovación (grant PGC2018-093745-B-I00)Junta de Castilla y León (grant VA124G18

Grand Canonical Monte Carlo simulations of hydrogen and methane storage capacities of two novel Al-nia MOFs at room temperature

Producción CientíficaNovel materials capable of storing hydrogen or/and methane at high gravimetric and volumetric densities are required for hydrogen vehicles to be widely employed as a clean alternative to fossil-based vehicles. Metal-Organic Frameworks (MOFs) are considered as promising candidates to achieve the Department Of Energy (DOE) targets for both, hydrogen and methane storage. Using Grand Canonical Monte Carlo (GCMC) simulations, the hydrogen and methane gravimetric and volumetric storage capacities of two recently synthesized Al-nia MOFs have been studied. Their storage capacities have been compared with the storage capacities of other Al-based MOFs and classical and well-known MOFs, such as IRMOF-5. The two novel Al-nia MOFs have shown high hydrogen and methane gravimetric and volumetric storage capacities at room temperature and moderate pressures, 25–35 MPa, comparable or higher than the storage capacities of classical and Al-based MOFs.Ministerio de Ciencia e Innovación (PGC2018-093745-B-I00)Junta de Castilla y León (VA124G18

Magnetostatic dipolar anisotropy energy and anisotropy constants in arrays of ferromagnetic nanowires as a function of their radius and interwall distance

Producción CientíficaMagnetostatic dipolar anisotropy energy and the total dipolar anisotropy constant, ${K}_{{total}}$, in periodic arrays of ferromagnetic nanowires have been calculated as a function of the nanowire radius, the interwall distance of the nanowires in the arrays and the geometry of the array (square or hexagonal), by using a realistic atomistic model and the Ewald method. The simulated nanowires have a radius size up to 175 Å that corresponds to 31 500 atoms, and the simulated nanowire arrays have interwall distances between 35 and 3000 Å. The dependence of total magnetostatic dipolar anisotropy constant on the nanowire radius, their interwall distance and the type of array symmetry has been analyzed. The total dipolar anisotropy constant, which is the sum of the intrananowire dipolar anisotropy constant, ${K}_{{intra}}$, due to the dipolar interactions inside an isolated nanowire and the main responsible of the shape anisotropy, and of the internanowire dipolar anisotropy constant, ${K}_{{inter}}$, due to the magnetostatic dipolar interactions among nanowires in the array, have been calculated and compared with the magnetocrystalline anisotropy constant for three nanowire compositions and their crystalline structures. The simulations of the nanowire arrays with large interwall distances have been used to calculate the intrananowire anisotropy constant, ${K}_{{intra}}$, and to analyze the competition between the intrananowire, internanowire and magnetocrystalline anisotropies. According to some magnetic theories, the ratio $| {K}_{{inter}}/{K}_{{intra}}|$ equals to the areal filling fraction of a nanowire array. Present calculations indicate that the equation for the areal filling fraction matches perfectly for any interwall distance and radius of Ni and Co nanowire arrays. This first equation is used to write a general equation that relates the radius and interwall distance of nanowire arrays with the intrananowire, internanowire and magnetocrystalline anisotropies. This general equation allows to design the geometry of nanowire arrays with the desired orientation of the easy magnetization axis.Ministerio de Economía, Industria y Competitividad (Grants MAT2014–54378-R, MAT2016–76824-C3-3-R and PGC2018–093745-B-I00)Junta de Castilla y León (Ref. project VA124G18