Browsing by Subject "Thermodynamic properties"

Now showing 1 - 6 of 6

Open Access
Compressibility of a two-dimensional electron gas in a parallel magnetic field
(Elsevier B.V., 2007) Subaşi, A. L.; Tanatar, Bilal
The thermodynamic compressibility of a two-dimensional electron system in the presence of an in-plane magnetic field is calculated. We use accurate correlation energy results from quantum Monte Carlo simulations to construct the ground state energy and obtain the critical magnetic field Bc required to fully spin polarize the system. Inverse compressibility as a function of density shows a kink-like behavior in the presence of an applied magnetic field, which can be identified as Bc. Our calculations suggest an alternative approach to transport measurements of determining full spin polarization.
Open Access
Correlation effects in a one-dimensional electron gas with short-range interaction
(Pergamon Press, 1999) Demirel, E.; Tanatar, Bilal
We study the correlation effects in a one-dimensional electron gas with repulsive delta-function interaction. The correlation effects are described by a local-field correction which takes into account the short-range correlations. We find that the ground state energy is in good agreement with the exact result up to intermediate coupling strengths, showing an improvement over the STLS approximation. The compressibility, the static structure factor and the pair-correlation function are also calculated within the present approximation.
Open Access
Effects of field plate on the maximum temperature and temperature distribution for gan HEMT devices
(American Society of Mechanical Engineers, 2016) Kara D.; Donmezer N.; Canan, Talha Furkan; Şen, Özlem; Özbay, Ekmel
Field plated GaN high electron mobility transistors (HEMTs) are widely preferred amongst other GaN HEMT devices because of their ability to regulate electric field at high power densities. When operated at high power densities, GaN HEMTs suffer significantly from the concentrated heating effects in a small region called hotspot located closer to the drain edge of the gate. Although; the stabilizing effect of field plate on the electrical field distribution in HEMTs is known by researchers, its effect on temperature distribution and the hotspot temperature is still not studied to a greater extend. For this purpose, finite element thermal modelling of devices with different sizes of field plates are performed using the joule heating distribution data obtained from 2D electrical simulations. Results obtained from such combined model show that the existence of a field plate changes the electrical field, therefore the heat generation distribution within device. Moreover; increasing the size of the field plate has an effect on the maximum temperature at the hotspot region. The results are used to analyze these effects and improve usage of field plates for high electron mobility transistors to obtain better temperature profiles. Copyright © 2016 by ASME.
Open Access
Lyotropic liquid-crystalline mesophases of [Zn(H2O)6](NO3)2-C12EO10-CTAB-H2O and [Zn(H2O)6](NO3)2-C12EO10-SDS-H2O systems
(2008) Albayrak, C.; Soylu, A. M.; Dag, Ö.
The mixture of two surfactants (C12EO10-CTAB and C 12EO10-SDS) forms lyotropic liquid-crystalline (LLC) mesophases with [Zn(H2O)6](NO3)2 in the presence of a minimum concentration of 1.75 H2O per C 12EO10. The metal ion/C12EO10 mole ratio can be increased up to 8.0, which is a record high metal ion density in an LLC mesophase. The metal ion concentration can be increased in the medium by increasing the CTAB/C12EO10 or SDS/C12EO 10 mole ratio at the expense of the stability of the LLC mesophase. The structure and some thermal properties of the new mesophase have been investigated using XRD, POM, FTIR, and Raman techniques. © 2008 American Chemical Society.
Open Access
Structural, microstructural and thermal properties of lead-free bismuth-sodium-barium-titanate piezoceramics synthesized by mechanical alloying
(2013) Amini, R.; Ghazanfari, M.R.; Alizadeh, M.; Ardakani H.A.; Ghaffari, M.
Bismuth-sodium-barium-titanate piezoceramics with a composition of (Bi 0.5Na0.5)0.94Ba0.06TiO3 (BNBT) were prepared by mechanical alloying (MA). Structural analysis and phase identification were performed by X-ray diffraction (XRD). Microstructural studies and chemical composition homogeneity were performed by scanning electron microscope (SEM) coupled with energy dispersive X-ray analysis (EDX). Furthermore, thermal properties of the as-milled powders were evaluated by thermogravimetry/differential thermal analysis (TG/DTA). During the initial milling, the constituents were transformed to the perovskite, pyrochlore, and BNT phases; in addition, partial amorphization of the structure appeared during the milling cycle. As MA progressed, transformation of pyrochlore-to-perovskite and crystallization of the amorphous phase occurred and also, the BNBT phase was significantly developed. It was found that the MA process has the ability to synthesize the BNBT powders with a submicron particle size, regular morphology, and uniform elemental distribution. © 2012 Elsevier Ltd.
Open Access
Vibrational and thermodynamic properties of α-, β-, γ-, and 6, 6, 12-graphyne structures
(Institute of Physics Publishing, 2014) Perkgöz, N. K.; Sevik, C.
Electronic, vibrational, and thermodynamic properties of different graphyne structures, namely α-, β-, γ-, and -graphyne, are investigated through first principles-based quasi-harmonic approximation by using phonon dispersions predicted from density-functional perturbation theory. Similar to graphene, graphyne was shown to exhibit a structure with extraordinary electronic features, mechanical hardness, thermal resistance, and very high conductivity from different calculation methods. Hence, characterizing its phonon dispersions and vibrational and thermodynamic properties in a systematic way is of great importance for both understanding its fundamental molecular properties and also figuring out its phase stability issues at different temperatures. Thus, in this research work, thermodynamic stability of different graphyne allotropes is assessed by investigating vibrational properties, lattice thermal expansion coefficients, and Gibbs free energy. According to our results, although the imaginary vibrational frequencies exist for β-graphyne, there is no such a negative behavior for α-, γ-, and -graphyne structures. In general, the Grüneisen parameters and linear thermal expansion coefficients of these structures are calculated to be rather more negative when compared to those of the graphene structure. In addition, the predicted difference between the binding energies per atom for the structures of graphene and graphyne points out that graphyne networks have relatively lower phase stability in comparison with the graphene structures. © 2014 IOP Publishing Ltd.