Nonlinear Thermopower Behaviour of N-Type Carbon Nanofibres and Their Melt Mixed Polypropylene Composites

The temperature dependent electrical conductivity σ (T) and thermopower (Seebeck coefficient) S (T) from 303.15 K (30 °C) to 373.15 K (100 °C) of an as-received commercial n-type vapour grown carbon nanofibre (CNF) powder and its melt-mixed polypropylene (PP) composite with 5 wt.% of CNFs have been analysed. At 30 °C, the σ and S of the CNF powder are ~136 S m−1 and −5.1 μV K−1, respectively, whereas its PP/CNF composite showed lower conductivities and less negative S-values of ~15 S m−1 and −3.4 μV K−1, respectively. The σ (T) of both samples presents a dσ/dT < 0 character described by the 3D variable range hopping (VRH) model. In contrast, their S (T) shows a dS/dT > 0 character, also observed in some doped multiwall carbon nanotube (MWCNT) mats with nonlinear thermopower behaviour, and explained here from the contribution of impurities in the CNF structure such as oxygen and sulphur, which cause sharply varying and localized states at approximately 0.09 eV above their Fermi energy level (EF).

First-Principles Study of Vacancy and Impurities Defects in Graphene

In this work, we have studied the electronic and magnetic properties of 1C atom vacancy defects in graphene (1Cv-d-G), 1N atom impurity defects in graphene (1NI-d-G) and 1O atom impurity defects in graphene (1OI-d-G) materials through first principles calculations based on spin-polarized density functional theory (DFT) method, using computational tool Quantum ESPRESSO (QE) code. From band structure and density of states (DOS) calculations, we found that supercell structure of monolayer graphene is a zero bandgap material. But, electronic bands of 1Cv-d-G, 1NI-d-G and 1OI-d-G materials split around the Fermi energy level and DOS of up & down spins states appear in the Fermi energy level. Thus, 1Cv-d-G, 1NI-d-G and 1OI-d-G materials have metallic properties. We have studied the magnetic properties of pure and defected materials by analyzing density of states (DOS) and partial density of states (PDOS) calculations. We found that graphene and 1OI-d-G materials have non-magnetic properties. On the other hand, 1C vacancy atom and 1N impurity atom induced magnetization in 1Cv-d-G & 1NI-d-G materials by the rebonding of dangling bonds and acquiring significant magnetic moments of values -0.75μB/cell & 0.05μB/cell respectively through remaining unsaturated dangling bond. Therefore, non-magnetic graphene changes to magnetic 1Cv-d-G and 1NI-d-G materials due to 1C atom vacancy defects and 1N atom impurity defects. The 2p orbital of carbon atoms has main contribution of magnetic moment in these defected structures.

First principles band structure investigation of cubic spinel CuMn2Te4Compound for thermoelectric applications

Spinel CuMn2Te4 material is investigated for its nonmagnetic and ferromagnetic phases by first principles calculation using the FP-LAPW method. Exchange and correlation are treated with GGA. T otal energy calculations reveal that CuMn2T e4 compound tend to stabilise at ferromagnetic phase. Non vanishing bands at the Fermi Energy level illustrates the metallic nature of CuMn2Te4at both of its nonmagnetic and ferromagnetic phases. Transport properties are studied using the BoltzTraP code at 325K. Thermo power of -26.22μv/k is estimated for nonmagnetic CuMn2Te4. At ferromagnetic phase, these values are predicted as 84.85μv/kand 53.39μv/k respectively for spin-up and spin-down states. Computed power factor values are 2.29 x1014Wm-1s-1K-2for NM phase and 2.33×1017Wm−1s−1K−2/1.001×1010Wm−1s-−1K-−2respectivelyfor FM up-spin/down-spin phases. A total magnetic moment of 7.11003 μB/F.U. is obtained for the energetically favourable ferromagnetic phase of CuMn2Te4 compound. High pressure investigations reveal the possibility of electronic topological transition that may lead to changes in the Fermi Surface topology and hence changes in the physical properties of nonmagnetic CuMn2Te4.

Numerical Investigation of Graphene and STO Based Tunable Terahertz Absorber with Switchable Bifunctionality of Broadband and Narrowband Absorption

A graphene metamaterial and strontium titanate (STO)-based terahertz absorber with tunable and switchable bifunctionality has been numerically investigated in this work. Through electrically tuning the Fermi energy level of the cross-shaped graphene, the bandwidth of the proposed absorber varies continuously from 0.12 THz to 0.38 THz with the absorptance exceeding 90%, which indicates the functionality of broadband absorption. When the Fermi energy level of the cross-shaped graphene is 0 eV, the proposed absorber exhibits the other functionality of narrowband absorption owing to the thermal control of the relative permittivity of STO, and the rate of change of the center frequency is 50% ranging from 0.56 THz to 0.84 THz. The peak intensity of the narrowband absorption approximates to nearly 100% through adjusting the Fermi energy level of the graphene strips. The calculated results indicate that it is not sensitive to the polarization for wide incidence angles. The proposed absorber can realize tunable bifunctionality of broadband absorption with a tunable bandwidth and narrowband absorption with a tunable center frequency, which provides an alternative design opinion of the tunable terahertz devices with high performance for high-density integrated systems.

Электронные и теплофизические свойства газовых гидратов: результаты моделирования из первых принципов

The results of an ab-initio molecular dynamics study of the electronic and thermophysical properties of methane hydrate with a cubic sI structure are presented. Good agreement of the simulation results for heat capacity at constant volume and density with experimental data is found. Based on the analysis of the density of electronic states, the temperature dependences of the electronic properties of methane hydrate, including the Fermi energy level, width and boundaries of the band gap are determined. For the empty framework of the hydrate (water clathrate framework), the electron energy spectra E(k) were calculated along the directions M-X, X-G, G-M, and G-R. It was found that the presence of CH4 molecules in an aqueous clathrate leads to an increase in the Fermi energy of the hydrate from 2.4 to 3.0 eV.

The effect of elastic strains on the adsorption energy of H, O, and OH in transition metals

The adsorption energies of H, O, and OH on 11 transition metals were calculated as a function of strain. They were related to the Fermi energy level which only depended on the change of the area of the hole due to strain following a general law.

Abnormal Fano Profile in Graphene-Wrapped Dielectric Particle Dimer

We give a theoretical study on the near field enhancement and far field spectrum of an adjacent graphene-wrapped sphere dimer with different radii. The Fano profile is found in the near field enhancement spectrum of such a symmetry-broken dimer system, which is, however, hidden in the far field spectrum. We demonstrate that this kind of Fano profile is rising from the coupling of dimer’s plasmon hybridization modes by analyzing the dipole moments of each sphere. Moreover, different orientation of incident wave polarization will lead to the different plasmon hybridization coupling, thus giving rise to a different Fano profile. By changing the Fermi energy level, we could achieve tunable Fano profile in near field enhancement.

Ti3O5 and Al2TiO5 Crystals Flotation Characteristics from Ti-bearing Blast Furnace Slag: A Density Functional Theory and Experimental Study

Anosovite crystalline is an ideal mineral for flotation from the Ti-bearing blast furnace (TBBF) slag. Ti3O5 crystal and Al2TiO5 crystal are two kinds of anosovites, and the Al element significantly affects the electronic structure and flotation performance of anosovite. The floatability of Ti3O5 and Al2TiO5 crystals were studied by Mulliken populations, energy bands, and density of states (DOS). In addition, the flotation experiment of the two kinds of anosovite crystals (Ti3O5 and Al2TiO5) was conducted and proved that the density functional theory (DFT) calculation results were accurate. Compared with Ti3O5 crystal, the Fermi energy level of Al2TiO5 crystal shifts around 2 eV in a negative direction by DOS analysis, which is beneficial to flotation. And Al2TiO5 crystal possesses a larger value of bond population, which is 0.41, for Ti-O bonds than Ti3O5 crystal and the bond length of Ti-O in Al2TiO5 crystal is shorter, therefore Al2TiO5 crystal shows a stronger covalency. The changes of the Fermi energy level and the covalency bonds in Al2TiO5 crystal both demonstrated that doping the Al component into the Ti3O5 crystal was beneficial to improve the flotation effect. Moreover, the Al2TiO5 crystal had a higher flotation efficiency compared to the Ti3O5 crystal when the dosages of salicylhydroxamic acid (SHA) and sodium oleate were the same. Therefore, both DFT calculation and experiment show that the flotation effect of the Al2TiO5 crystal is better than that of the Ti3O5 crystal.

Configurational Effects on Strain and Doping at Graphene-Silver Nanowire Interfaces

Graphene shows substrate-dependent physical and electronic properties. Here, we presented the interaction between single-layer graphene and silver nanowire (AgNW) in terms of physical straining and doping. We observed a snap-through event for single-layer graphene/AgNW at a separation of AgNWs of 55 nm, beyond the graphene suspended over the nanowires. The adhesion force between the Atomic Force Microscopy (AFM) tip apex and the suspended graphene was measured as higher than the conformed one by 1.8 nN. The presence of AgNW modulates the Fermi energy level of graphene and reduces the work function by 0.25 eV, which results in n-type doping. Consequently, a lateral p-n-p junction is formed with single AgNW. The correlation Raman plot between G-2D modes reveals the increment of strain in graphene of 0.05% due to the curvature around AgNW, and 0.01% when AgNW lies on the top of graphene. These results provide essential information in inspecting the physical and electronic influences from AgNW.

A Broadband Tunable Terahertz Metamaterial Absorber Based on Single-Layer Complementary Gammadion-Shaped Graphene

We present a simple design of a broadband tunable metamaterial absorber (MMA) in the terahertz (THz) region, which consists of a single layer complementary gammadion-shaped (CGS) graphene sheet and a polydimethylsiloxane (PDMS) dielectric substrate placed on a continuous metal film. The Fermi energy level (Ef) of the graphene can be modulated dynamically by the applied DC bias voltage, which enables us to electrically control the absorption performance of the proposed MMA flexibly. When Ef = 0.8 eV, the relative bandwidth of the proposed MMA, which represents the frequency region of absorption beyond 90%, can reaches its maximal value of 72.1%. Simulated electric field distributions reveal that the broadband absorption mainly originates from the excitation of surface plasmon polaritons (SPPs) on the CGS graphene sheet. Furthermore, the proposed MMA is polarization-insensitive and has wide angles for both transverse-electric (TE) and transverse-magnetic (TM) waves in the broadband frequency range. The broadband absorption capacity of the designed MMA can be effectively adjusted by varying the Fermi energy level of graphene. Lastly, the absorbance of the MMA can be adjusted from 42% to 99.1% by changing the Ef from 0 eV to 0.8 eV, which is in agreement with the theoretical calculation by using the interference 41theory. Due to its simple structure and flexible tunability, the proposed MMA has potential application prospects in tunable filtering, modulators, sensing, and other multispectral devices.