Browsing by Author "Candan, Abdullah"
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Article Citation Count: 51CaXH3 (X = Mn, Fe, Co) perovskite-type hydrides for hydrogen storage applications(Wiley, 2020) Işık, Mehmet; Gencer, Aysenur; Güllü, Hasan Hüseyin; Gullu, Hasan H.; Isik, Mehmet; Department of Electrical & Electronics EngineeringHydrogen storage is one of the attractive research interests in recent years due to the advantages of hydrogen to be used as energy source. The studies on hydrogen storage applications focus mainly on investigation of hydrogen storage capabilities of newly introduced compounds. The present paper aims at characterization of CaXH3 (X: Mn, Fe, or Co) perovskite-type hydrides for the first time to understand their potential contribution to the hydrogen storage applications. CaXH3 compounds have been investigated by density functional theory studies to reveal their various characteristics and hydrogen storage properties. CaXH3 compounds have been optimized in cubic crystal structure and the lattice constants of studied compounds have been obtained as 3.60, 3.50, and 3.48 angstrom for X: Mn, Fe, and Co compounds, respectively. The optimized structures have negative formation enthalpies pointing out that studied compounds are thermodynamically stable and could be synthesized experimentally. The gravimetric hydrogen storage densities of X: Mn, Fe, and Co compounds were found in as 3.09, 3.06, and 2.97 wt%, respectively. The revealed values for hydrogen storage densities indicate that CaXH3 compounds may be potential candidates for hydrogen storage applications. Moreover, various mechanical parameters of interest compounds like elastic constants, bulk modulus, and Poisson's ratio have been reported throughout the study. These compounds were found mechanically stable with satisfying Born stability criteria. Further analyses based on Cauchy pressure and Pugh criterion, showed that they have brittleness nature and relatively hard materials. In addition, the electronic characteristics, band structures, and associated partial density of states of CaXH3 hydrides have been revealed. The dynamic stability behavior of them was verified based on the phonon dispersion curves.Article Citation Count: 17First principles study on the structural, electronic, mechanical and lattice dynamical properties of XRhSb (X = Ti and Zr) paramagnet half-Heusler antimonides(Iop Publishing Ltd, 2019) Güllü, Hasan Hüseyin; Candan, Abdullah; Erkisi, Aytac; Gencer, Aysenur; Gullu, Hasan Huseyin; Department of Electrical & Electronics EngineeringThe half-Heusler TiRhSb and ZrRhSb alloys in the formation of face-centered cubic MgAgAs-type structure, which conforms to the F (4) over bar 3m space group with 216 as the space number, have been investigated using Generalized Gradient Approximation (GGA) implemented in Density Functional Theory (DFT). The calculated formation enthalpies and the plotted energy-volume curves of different types of structural phases (alpha, beta, and gamma) in these alloys indicate that the gamma-phase structure is the best energetically suitable structure. In addition, TiRhSb and ZrRhSb alloys have been found as paramagnetic (PM) with the investigation of antiferromagnetic (AFM), ferromagnetic (FM), and paramagnetic (PM) orders in the most stable gamma-phase structure. Therefore, their electronic, mechanical, and dynamical properties have been examined in the gamma structural phase and paramagnetic order. These alloys have semiconducting nature due to the observed same band gaps in both the majority and minority spin channels of the calculated spin-polarised electronic band structure. These calculated band gaps are 0.75 eV for gamma-TiRhSb and 1.18 eV for gamma-ZrRhSb. The predicted elastic constants indicate that the alloys in this study are mechanically stable and show nearly isotropic behavior in the gamma structural phase. Also, the minimum and the diffuson thermal conductivites have been determined for these alloys. Finally, the calculated phonon dispersion spectras for the gamma-TiRhSb and gamma-ZrRhSb half-Heusler antimonide alloys show the dynamic stability of these systems.Article Citation Count: 49First-principle investigation for the hydrogen storage properties of NaXH3 (X= Mn, Fe, Co) perovskite type hydrides(Pergamon-elsevier Science Ltd, 2019) Işık, Mehmet; Candan, Abdullah; Gencer, Aysenur; Isik, Mehmet; Department of Electrical & Electronics EngineeringIn the present study, NaXH3 (X = Mn, Fe, Co) perovskite type hydrides have been investigated by performing first-principles calculation. The results of the structural optimizations show that all these compounds have negative formation energy implying the thermodynamic stability and synthesisability. The mechanical stability of these compounds has been studied with the elastic constants. Moreover, the polycrystalline properties like bulk modulus, Poisson's ratio, etc. have been obtained using calculated elastic constants of interest compounds. The electronic properties have been studied and band structures have been drawn with the corresponding partial density of states. These plots indicated that NaXH3 hydrides show metallic characteristics. The charge transfer characteristics in these compounds have been studied with the Bader partial charge analysis. The phonon dispersion curves and corresponding density of states indicated that NaXH3 compounds are dynamically stable compounds. The investigation on hydrogen storage characteristics of NaXH3 compounds resulted in hydrogen storage capacities of 3.74, 3.70 and 3.57 wt% for X = Mn, Fe and Co, respectively. The present study is the first investigation of NaXH3 perovskite type hydrides as known up to date and may provide remarkable contribution to the future researches in hydrogen storage applications. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.Article Citation Count: 6First-principles studies of Tin+1SiNn (n=1, 2, 3) MAX phase(Taylor & Francis Ltd, 2020) Güllü, Hasan Hüseyin; Gullu, Hasan Huseyin; Candan, Abdullah; Yildiz, Bugra; Erkisi, Aytac; Department of Electrical & Electronics EngineeringIn this study, the structural, electronic, mechanical, lattice dynamical and thermodynamic characteristics of ( 1, 2 and 3) phase compounds were investigated using the first principle calculations. These ternary nitride compounds were found to be stable and synthesisable, and the results on the stability nature of them were also evaluated for the possible and phases. -was found to be the most stable one among these new class of layered phases for which limited works are available in the literature. The band structures, that are essential for the electronic properties, were determined along with the partial density of states (PDOS) indicating the metallic behaviour of these compounds. The polycrystalline elastic moduli were calculated based on the single-crystal elastic constants and the mechanical stabilities were verified. Some basic physical parameters, such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, Debye temperature, and sound velocities, were also predicted. Furthermore, the anisotropic elastic properties were visualised in three dimensions (3D) for Young's modulus, linear compressibility, shear modulus and Poisson's ratio as well as with the calculation of the anisotropic factors. - phase showed the most isotropic characteristics with minimum deviations. These theoretical values were also used to identify the stiffness and ionic characteristics. The phonon dispersion curves and corresponding PDOS indicated that compounds were dynamically stable. Moreover, thermodynamic properties obtained from phonon dispersion curves were investigated in detail.Article Citation Count: 37Investigation of structural, electronic, magnetic and lattice dynamical properties for XCoBi (X: Ti, Zr, Hf) Half-Heusler compounds(Elsevier, 2020) Güllü, Hasan Hüseyin; Isik, Mehmet; Işık, Mehmet; Wang, Xiaotian; Gullu, Hasan Huseyin; Department of Electrical & Electronics EngineeringStructural, electronic, magnetic, mechanical and lattice dynamical properties of XCoBi (X: Ti, Zr, Hf) Half-Heusler compounds have been investigated according to density functional theory and generalized gradient approximation. Among alpha, beta and gamma structural phases, gamma-phase structure has been found as the most stability characteristics depending on the calculated formation enthalpies, energy-volume dependencies and Cauchy pressures. Energy-volume plots of possible magnetic orders of gamma-phase XCoBi compounds have been analyzed and the most stable order has been found as paramagnetic nature. The theoretical studies on gamma-phase structures resulted in band gap energies of 0.96, 0.99 and 0.98 eV for TiCoBi, ZrCoBi and HfCoBi semiconducting compounds, respectively. Born-Huang criteria applied on elastic constants of interest compounds has indicated that gamma-phase is also mechanically stable for all studied compounds. In addition, various mechanical, lattice dynamical and thermodynamical parameters of XCoBi compounds have been calculated in the present study.