G. L. Zhao

Electromagnetic wave absorption properties of carbon nanotubes-epoxy composites at microwave frequencies

This paper focuses on the absorption properties of electromagnetic waves, both electric and magnetic, from multiwalled carbon nanotubes (MWCNTs)-epoxy composites. A microwave resonant cavity was utilized as a probe. Six groups of MWCNT-epoxy composite samples with different MWCNT outside diameters (ODs) were fabricated and tested. The weight percentages of MWCNTs in each sample group were in the range of 1%–10%. The line widths at half-power-maxima, the shifts in resonant frequency under load, and the amplitudes of the absorption profiles for the six groups of composite samples were compared. In general, the MWCNTs with smaller ODs had higher microwave absorption efficiency at 9.968 GHz than the large diameter ones. However, the sample group M5 (OD<8u2002nm) showed unusual results, a lower microwave absorption than other sample groups, even though the OD of the MWCNTs in the sample group M5 was smaller than those of other groups. The scanning electron microscope analysis and microwave absorption measurements ...

Ab-initio local density approximation description of the electronic properties of zinc blende cadmium sulfide (zb-CdS)

Abstract Ab-initio, self-consistent electronic energy bands of zinc blende CdS are reported within the local density functional approximation (LDA). Our first principle, non-relativistic and ground state calculations employed a local density potential and the linear combination of atomic orbitals (LCAO). Within the framework of the Bagayoko, Zhao, and Williams (BZW) method, we solved self-consistently both the Kohn–Sham equation and the equation giving the ground state density in terms of the wavefunctions of the occupied states. Our calculated, direct band gap of 2.39xa0eV, at the Γ point, is in accord with the experiment. Our calculation reproduced the peaks in the conduction and valence bands density of states, within experimental uncertainties. The calculated electron effective mass agrees with experimental findings.

Comment on “Band gap bowing and electron localization of GaXIn1−XN” [J. Appl. Phys. 100, 093717 (2006)]

Some previous density functional theory (DFT) calculations of the band gap of wurtzite and cubic InN, before the work of Lee and Wang [J. Appl. Phys. 100, 093717 (2006)], are in agreement with the screened-exchange findings of these authors and with experiment. These previous findings point to an intrinsic capability of DFT, in the local density approximation, to correctly describe the band gap of semiconductors. These comments also discuss some recent results [Phys. Rev. B 76, 037101 (2007)] on an extensive hybridization of the In 4d and N 2s bands that is lost when the d electrons are included in the core. Our discussions in these comments indicate that when the two inherently coupled equations of DFT are both solved self-consistently, the resulting bands, including low-lying conduction ones, appear to have much more physics content than previously believed.

Measurements of electromagnetic wave absorption properties of carbon nanotube–epoxy composites at microwave frequencies around 8.43 GHz

Microwave (8.43 GHz) absorption properties have been measured for multiwalled carbon nanotube (MWCNT)–epoxy composites. A microwave resonant cavity technique was utilized as a probe. Six groups of MWCNT–epoxy composite samples with different MWCNT outside diameters were fabricated and tested. The weight percentages of MWCNTs in each sample group were in the range of 1%–10%. The amplitudes of the absorption profiles, the linewidths at half-power maxima, and the shifts in resonant frequencies under load for the six groups of MWCNT–epoxy composite samples were compared. Similar to our previous results measured at 9.968 GHz [Z. Ye et al., J. Appl. Phys. 108, 054315 (2010)], the microwave absorption properties of our samples strongly rely on two parameters: the number of nanotubes and the density of nanotube bundles. It was found that the measured microwave absorption properties of the samples around 8.43 GHz showed a more sensitive response to the magnetic component of the microwave field rather than the elec...

First principle local density approximation description of the electronic properties of ferroelectric sodium nitrite

Abstract The electronic structure of the ferroelectric crystal, NaNO 2 , is studied by means of first-principles, local density calculations. Our ab-initio, non-relativistic calculations employed a local density functional approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). Following the Bagayoko, Zhao, Williams, method, as enhanced by Ekuma and Franklin (BZW-EF), we solved self-consistently both the Kohn-Sham equation and the equation giving the ground state charge density in terms of the wave functions of the occupied states. We found an indirect band gap of 2.83xa0eV, from W to R. Our calculated direct gaps are 2.90, 2.98, 3.02, 3.22, and 3.51xa0eV at R , W , X , Γ , and T , respectively. The band structure and density of states show high localization, typical of a molecular solid. The partial density of states shows that the valence bands are formed only by complex anionic states. These results are in excellent agreement with experiment. So are the calculated densities of states. Our calculated electron effective masses of 1.18, 0.63, and 0.73 m o in the Γ–X, Γ–R, and Γ–W directions, respectively, show the highly anisotropic nature of this material.

AB-INITIO CALCULATIONS OF SUPERCONDUCTING PROPERTIES OF YBa2Cu3O7

We present ab-initio calculations for the electronic structure and superconducting properties of YBa2Cu3O7 (YBCO). The electronic structure was calculated using a self-consistent ab-initio LCAO method. We solved the anisotropic Eliashberg gap equation numerically. The strong coupling of the high energy optical phonons around 60-73 meV, with the electrons at the Fermi surface, leads to a high Tc in YBCO. The calculated Tc is about 89 K for μ*=0.1. The good agreement of the calculated results with experimental measurements and the ab-initio nature of the calculations support the scenario of an anisotropic s-wave superconductor for YBCO.

The Gap Function in YBa2Cu3O7

We have solved the four-dimensional anisotropic Eliashberg gap equation for YBa2Cu3O7 (YBCO) using the calculated electronic structure and the electron–phonon interaction matrix elements. The calculated Tc for YBCO is about 89 K or μ*= 0.1. At or slightly above the transition temperature Tc, the real part of the gap function Δ(k, 0), for all the k-points on the Fermi surface, becomes zero and the material is not superconducting. However, the energy gap function Δ(k,ω) is still nonzero for ω > 0 for some electronic states, leading to a pseudo-gap behavior in YBCO.

AB-INITIO CALCULATIONS OF ELECTRONIC PROPERTIES OF InP AND GaP

We present results from ab-initio, self-consistent local density approximation (LDA) calculations of electronic and related properties of zinc blende indium phosphide (InP) and gallium phosphide (GaP). We employed a LDA potential and implemented the linear combination of atomic orbitals (LCAO) formalism. This implementation followed the Bagayoko, Zhao and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW–EF). This method searches for the optimal basis set that yields the minima of the occupied energies. This search entails increases of the size of the basis set and the related modifications of angular symmetry and of radial orbitals. Our calculated, direct band gap of 1.398 eV (1.40 eV), at the Γ point, is in excellent agreement with experimental values, for InP, and our preliminary result for the indirect gap of GaP is 2.135 eV, from the Γ to X high symmetry points. We have also calculated electron and hole effective masses for both InP and GaP. These calculated properties also agree with exp...

New carbon nanotube-epoxy composite for dampening microwave cavity resonance

We studied the dampening effect of multiwalled carbon nanotubes (MWNTs)-epoxy composite to microwave cavity resonance. Six groups of MWNT-epoxy composite samples with different MWNT outside-diameters (OD) were fabricated and tested. The weight percentages of MWNTs in each sample group were in the range of 1-10%. In general, the MWNTs with smaller outside-diameters had higher microwave dampening efficiency at 9.968 GHz than the larger ones. The SEM analysis and microwave measurements showed that the microwave dampening efficiency of the CNTs was also affected by the morphologies of MWNTs in individual bundles. A higher density of MWNTs in CNT bundles results in relatively lower efficiency in dampening the microwave fields at the central frequency of 9.968GHz.

Predictive ab-initio computations of properties of ferroelectric materials

We present a newly developed, ab-initio, self-consistent procedure for predictive calculations of electronic and related properties of ferroelectric materials. Known as the Bagayoko, Zhao, amd Williams (BZM) procedure, this approach resolves the long-standing disagreement between experimental and theoretical conduction bands, in general, and band gaps, in particular, for ferroelectrical materials and other semiconductors. We discuss applications to tetragonal BaTiO3 and implications for molecules and band-gap engineering as well as the nuclear shell model.