Browsing by Author "Gullu, Hasan H."
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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: 5Influence of layer on the electrical properties of Au/n-4H SiC diodes(indian Acad Sciences, 2018) Güllü, Hasan Hüseyin; Gullu, Hasan H.; Yildiz, Esra D.; Department of Electrical & Electronics EngineeringIn this study, the effect of insulator layer on the electrical characteristics of Au/n-4H SiC diode was investigated. The current-voltage (), capacitance-voltage () and conductance-voltage () measurements were carried out at room temperature condition. Under thermionic emission model, electrical parameters as zero-bias barrier height (), ideality factor (n), interface states (), and series () and shunt () resistances were estimated from forward bias analyses. The values of n and were about 1.305 and 0.796 eV for metal-semiconductor (MS) rectifying diode, and 3.142 and 0.713 eV for metal-insulator-semiconductor (MIS) diode with the insertion of layer, respectively. Since the values of n were greater than the unity, the fabricated diodes showed non-ideal behaviour. The energy distribution profile of of the diodes was calculated by taking into account of the bias dependence of the effective barrier height () and . The obtained values with are almost one order of magnitude lower than those without for two diodes. According to Cheung's model, were calculated and these values were found in increasing behaviour with the contribution of insulator layer. In addition, the plot behaviours with linear dependence between ln() vs. indicated that the dominant conduction mechanism in the reverse bias region was Schottky effect for both MS and MIS diodes. In the room temperature measurements, different from the results of MIS diode, the values of C for MS diode was observed in decreasing behaviour from ideality with crossing the certain forward bias voltage point ( ). The decrease in the negative capacitance corresponds to the increase of G / w.