Fang Xiao-Yong

Modification of Band Gap of β-SiC by N-Doping

The geometrical and electronic structures of nitrogen-doped β-SiC are investigated by employing the first principles of plane wave ultra-soft pseudo-potential technology based on density functional theory. The structures of SiC1−xNx (x = 0, 1/32, 1/16, 1/8, 1/4) with different doping concentrations are optimized. The results reveal that the band gap of β-SiC transforms from an indirect band gap to a direct band gap with band gap shrinkage after carbon atoms are replaced by nitrogen atoms. The Fermi level shifts from valence band top to conduction band by doping nitrogen in pure β-SiC, and the doped β-SiC becomes metallic. The degree of Fermi levels entering into the conduction band increases with the increment of doping concentration; however, the band gap becomes narrower. This is attributed to defects with negative electricity occurring in surrounding silicon atoms. With the increase of doping concentration, more residual electrons, more easily captured by the 3p orbit in the silicon atom, will be provided by nitrogen atoms to form more defects with negative electricity.

A simulation of the quasi-standing wave and generalized half-wave loss of electromagnetic wave in non-ideal media

In this paper, the phenomena of reflection towards normal incident electromagnetic wave on double loss-medium are analyzed, and the superposition wave mechanism that is similar to standing wave in transparent medium is exposed. In this case, it is more suitable that wave impedance of media is used to determine the thin or thick medium and half-wave loss. Based on the thought mentioned above, the new concept of quasi-standing wave and generalized half-wave loss are put forward for the first time. Moreover, the basic criteria of generalized half-wave loss are given. Especially, the basic characters and generalized law of quasi-standing wave is simulated by means of computations, and their physical mechanisms are also described.

Theoretical analysis of 2D laser angle sensor and several design parameters

Having adopted several sets of new technology and techniques, such as cube-corner prism of higher refractive index, orthogonal holographic grating, non-linear compensation, and fringe fractionization, etc., we have developed a new type of laser angle sensor that can be used to measure two-dimensional angle of moving target. The sensor has high sensitivity, high precision and longer measuring range compared with the original sensor, and its measuring range will achieve ±35°, while its minimum resolution angle is 0.004°. The experimental results show that the measuring error is not greater than ±0.01°. In this paper, several design parameters and measuring results for 2D laser angle sensor are given.

Microwave Absorbing Materials: Solutions for Real Functions under Ideal Conditions of Microwave Absorption

We present general equations for metal-substrate single-layer microwave absorbing materials. The optimum electromagnetic parameters and matching thicknesses for electric-loss, magnetic-loss, and electromagnetic-loss materials are given, and possible solutions for the equations and the corresponding theoretic curves are discussed as well. The results may be helpful for designing high-performance microwave absorbing materials.

Polarization Mechanism of Oxygen Vacancy and Its Influence on Dielectric Properties in ZnO

We report on the mechanism of the dielectric properties of oxygen vacancy in ZnO, using ab initio numerical simulations of oxygen vacancy on the band structure and dielectric properties, to develop photoelectric material applications. It is revealed that the appearance of oxygen vacancies leads to wider energy band gap, obvious blue shift and increase in the peak of dielectric function as compared to the intrinsic ZnO simulation. We explain these unusual phenomena and analyze the dielectric changes with the mechanism of polarization in the semiconductors. It is shown that the main mechanism of influencing dielectric properties is the electron displacement polarization. The result may be helpful for development of photoelectric materials.

High-Temperature Permittivity and Data-Mining of Silicon Dioxide at GHz Band

The high-temperature permittivity of quartz fibre-reinforced silicon dioxide (SiO2/SiO2) nano-composites is studied on the basis of the multi-scale theoretical model. We obtain the permittivity of the SiO2/SiO2 at high temperature, which is dependent on the temperature by data-mining. The result shows that the permittivity and loss tangent obtained by data-mining are well consistent with the measured ones. The high-temperature permittivity can be well predicted for SiO2/SiO2 by the as-proposed model and the data-mining method.

Dielectric Response and Broadband Microwave Absorption Properties of Three-Layer Graded ZnO Nanowhisker/Polyester Composites

We design and prepare three-layer graded ZnO nanowhisker/polyester composites. The dispersion configuration of ZnO nanowhiskers in the polyester is investigated, and their microwave reflectivity curves are also measured. Experimental results have shown that the graded dispersion with ZnO nanowhiskers contributes to broadband microwave absorption. In other words, the absorption band depends on the graded dispersion configuration of ZnO nanowhiskers in polyester matrix.

Different Roles of a Boron Substitute for Carbon and Silicon in β-SiC

The first-principles numerical simulation is employed to calculate the effect of replacement of carbon and silicon with boron on the electronic structure and optical properties of β-SiC. Mulliken analysis shows that the B impurity bond lengths shrink in the case of BSi, while they expand with reference to BC. In addition, BSi contains C—C, Si—Si and B—Si bonds. The calculated results show that the two systems of BC and BSi apply different dispersion. BC is in accordance with the Lorentz dispersion theory while BSi follows the Drude dispersion theory. Theoretic analysis and quantitative calculation are used for conductivity spectra in the infrared region.

Influence of Non-uniform Temperature Field on Spectra of Fibre Bragg Grating

We simulate the spectrum characteristics of fibre Bragg grating (FBG) with non-uniform temperature using the transmission matrix method, and the results are analysed. It is found that firstly the modulated coefficient of average refractive index is a very important parameter that influences the spectrum characteristic of the fibre Bragg grating, and secondly the spectrum curves are different in different temperature fields at the same parameter. Hence, we can determine the metrical temperature by analysing the spectrum of fibre Bragg grating.

Fine structure in Fresnel diffraction patterns and its application in optical measurement

The function of light intensity distribution of Fresnel diffraction from an opaque fibre is derived, and the fine structure in the diffraction pattern is theoretically described. In this paper, the relationship between fibre diameter and fine structure in the diffraction pattern is given. The fine structure can be used to measure the diameter of fibres, and measuring subscript limits may be greatly squeezed. Several fibres of different diameters are measured by means of the fine interference fringe in the diffraction pattern, and experimental results show the improved diffraction technique is highly reliable with higher precision compared with the original technique.