Defect and Diffusion Forum Vol. 436

Abstract: Earth-Air Heat Exchanger (EAHE) is a device that cools or heats air as it flows through underground ducts, helping to lower energy usage while ensuring comfortable thermal conditions. This investigation is designed to assess the thermoenergetic performance of an EAHE in Social Housing (SH) when submitted to different operating and constructive parameters and suggest strategies to improve energy efficiency in buildings. The study involves thermal and energy evaluations of a device integrated into SH, exploring varying air velocity, duct length, and duct diameter. Dynamic simulations using EnergyPlus software were employed for these evaluations. The findings show that incorporating the EAHE to the SH enhances the thermoenergetic efficiency of built environments. The system's best performance was achieved by a duct measuring 34 m long and 110 mm in diameter with a 3.3 m/s airflow, striking a balance between thermal and energy considerations. This contributes to providing validated references and parameters for future projects in the pursuit of enhanced efficiency.

Abstract: The Cahn-Hilliard equation, known for describing the evolution of interfaces in multicomponent systems, can also be employed to noise reduction in mathematical functions and concentration-dependent heat transfer simulations. This work presents a finite difference method discretization of the Cahn-Hilliard equation and explores its applications. For noise reduction, three different noisy functions are simulated, demonstrating effective recovery of original functions despite significant noise levels. In heat transfer simulations, three initial temperature distributions are explored with concentration-dependent thermal diffusivity. Results show that concentration significantly affects thermal diffusivity and heat propagation, leading to non-uniform temperature distributions. Comparative simulations without concentration influence highlight the distinct impact of concentration on thermal behavior. The study underscores a reliable approach to noise reduction and insight into concentration-dependent heat transfer dynamics.

Abstract: This paper presents an efficient and accurate method for solving the time-dependent Hartree-Fock equations for the helium atom in the ground and single excited states. The radial coordinate is discretized by the discrete variable representation (DVR), which is constructed from Coulomb wave functions. To solve the equation, the spectral method with Coloumb wave functions is used as a basis. We illustrate that the calculated electronic energies for the helium atom are in good agreement with the best available experimental values. The CWDVR method proves to be more economical and efficient as it uses the optimal few numbers of grid points compared to other numerical calculations.

Abstract: This study is based on the density functional theory and employs the projected augmented wave method within the VASP program package. It investigates the variation of lattice constants in Fe₂P-type compound FeMnP_0.67Si_0.33 in the ferromagnetic (FM) and antiferromagnetic (AFM) states, with the presence of Mn and Fe vacancy defects and Mn and Fe anti-site defects. the defect formation enthalpy of compounds containing vacancy and substitution defects were calculated using the Wagner-Schottky point defect thermodynamic model. It also investigates the relationship between the equilibrium concentration of point defects and the Mn content in the compound, as well as the variation of defect equilibrium concentration with temperature T. The calculation results show that the presence of point defects in the compound affects the lattice constants. In the FM and AFM states, the formation enthalpies of Fe anti-site and Mn anti-site defects is lower than that of Fe vacancy and Mn vacancy defects. The concentration of point defects increases with increasing temperature. The calculated results provide valuable theoretical references for the experimental preparation, defect analysis, and mechanical properties improvement of the Fe₂P-type iron-manganese-based FeMnP_0.67Si_0.33 compound.

Abstract: Within the conceptual framework of the De Launay model, this study delineates the formulations for elements constituting the dynamical matrix of hexagonal close-packed (HCP) crystals, incorporating the effect of interatomic forces exerted by the first eight nearest neighboring atoms. To validate these derived formulations, the phonon dispersion of magnesium was computed and juxtaposed against the empirical phonon dispersion results reported by other researchers. Subsequently, leveraging the fit of experimental phonon dispersion data of yttrium, as reported by secondary sources, this research delineates the radial and tangential force constants pertaining to the first three neighboring atoms. These constants were then employed to approximate the elastic constants of yttrium. The calculated phonon dispersion branches along the Γ-M and Γ-A directions, as well as the derived values of yttrium's elastic constants, exhibit congruence with the corresponding empirical observations reported in literature.

Abstract: The existing discrepancy between theoretical models and experimental results in describing the elastic properties of ultra-thin nanofilms (less than 10 nm) is primarily attributed to the oversight of the surface layer thickness impact. To address this, a new model incorporating a surface layer with thickness is proposed in this article. Utilizing a layered model, the Young’s modulus of nanofilms approaches that of bulk materials as the film thickness becomes infinitely large, equating to the Young’s modulus of the bulk material in both layered and unlayered models. The dimensional unit of the surface elastic coefficient in the layered model differs from that of the unlayered model, approximately by the thickness of the film. Numerically, the former is more than double the latter. Predictions using the layered model for ultra-thin films comprising only two surface layers reveal a hardening effect in materials such as Si, Ge, InAs, and GaAs. The increase in Young’s modulus for these materials is 20.81%, 95.28%, 79.03%, and 84.04%, respectively, compared to their bulk counterparts. Moreover, a continuous increase in the Young’s modulus is observed as the thickness further decreases.

Abstract: The interaction of short laser pulses with hydrogen atoms will be discussed in this paper. We chose laser pulses with photon energies lower than the ionization threshold energy of hydrogen atoms. Here, the close coupled channel method is used to solve the time-dependent Schrödinger equation. Our goal is to demonstrate how this approach can be used to calculate the phenomenon of multiphoton absorption-induced excitation to the 2S and 2P states. It was also shown how the probability of excitation depends on the duration and strength of the laser pulse. This approach exhibits good agreement when compared to the findings of other studies.