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Dang-Hui W.,Xi'an Shiyou University | Yue H.,Xidian University | Sheng-Rui X.,Xidian University | Tian-Han X.,Xi'an Shiyou University | And 3 more authors.
Journal of Alloys and Compounds | Year: 2013

In this study, we have reported on growth of thick AlGaN layer on the c-plane sapphire substrate with low-temperature AlN (LT-AlN) nucleation layer by low-pressure metal-organic chemical vapor deposition (LPMOCVD). High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements have been employed to study the crystal quality, threading dislocation density, surface morphology, and optical properties of thick AlGaN layer. Results indicate that the insertion of LT-AlN nucleation layer between sapphire substrate and high-temperature AlN nucleation layer effectively improves the thick AlGaN crystal quality, reduces the surface roughness and eliminates the threading dislocation density. © 2012 Elsevier B.V. All rights reserved.

Liu W.,Northwest University, China | Chang Q.-R.,Northwest University, China | Guo M.,Northwest University, China | Xing D.-X.,Northwest University, China | And 2 more authors.
Guang Pu Xue Yu Guang Pu Fen Xi/Spectroscopy and Spectral Analysis | Year: 2011

The hyperspectral reflectance of soil was measured by a ASD FieldSpec within 400~1000 nm. Next, its first derivative of spectra were acquired and de-noised by the threshold de-noising method based on wavelet transform. From the de-noised derivative spectra, absorption areas used as indicatoresas for soil organic matter content were acquired by numerical integration. Results show that: (1) Because of much noise, it is difficult to identify spectra contour and features in the first derivative of soil spectra resulting from different organic content levels. (2) When the scale of wavelet decomposition was 3, the threshold de-noising method based on wavelet transform can keep the balance between smoothing curve and holding spectra features. (3) Absorption area S(538, 586) is extracted from de-noised first derivative of soil spectra, and the coefficient of correlation between it and organic matter content is 0.8963.

Shi H.-S.,Northwest University of Politics and Law | Han L.-N.,XianYang Normal College
Applied Mechanics and Materials | Year: 2012

In Linux embedded system, multimedia application is becoming more and more popular based on the ARM process, therefore, it is very urgent that the development of audio and video in one multi-functional player. Using audio and video media library function, IIS interface, DMA control technology and LCD display technology, to development multimedia player function, using Qt complete embedded graphical user interface development so that the system has a good interface. The audio and video data Input/output by the study, using the software decoding, reduces the system development cost, solve the media play the normal player function. The design principle is clear, easy to realize, has a certain practical value. © (2012) Trans Tech Publications, Switzerland.

Wang L.-G.,Xidian University | Wu Z.-S.,Xidian University | Wang M.-J.,Xianyang Normal College
Wuli Xuebao/Acta Physica Sinica | Year: 2013

Rytov theory combined with the ABCD ray matrix is used to analyzes the statistical property of a reflected wave from a retro-reflector caused by atmospheric turbulence. The covariance function and the scintillation index of the receiving field caused by the combined effect of the turbulence and the random shaking of the reflector array are deduced on the basis of the mutual statistical independence of the two kinds of effects. The aperture factor and the scintillation of a receiving aperture with a finite size are calculated, and those of the retro-reflector and the array are compared to prove that the array does not change distribution of the intensity fluctuation but amplifies the fluctuation by a factor of 2. © 2013 Chinese Physical Society.

Li Y.-Q.,Xidian University | Wu Z.-S.,Xidian University | Zhang Y.-Y.,Xidian University | Wang M.-J.,Xianyang Normal College
Chinese Physics B | Year: 2014

Based on the modified Rytov theory and the international telecommunication union-radio (ITU-R) slant atmospheric structure constant model, the uniform scintillation index of partially coherent Gaussian - Schell model (GSM) beam propagation in the slant path is derived from weak- to strong-turbulence regions considering inner- and outer-scale effects. The effects of wavelength of beams and inner- and outer-scale of turbulence on scintillation are analyzed numerically. Comparison between the scintillation of GSM beams under the von Karman spectrum and that of beams under the modified Hill spectrum is made. The results obtained show that the scintillation index obtained under the von Karman spectrum is smaller than that under the modified Hill spectrum. This study can find theory bases for the experiments of the partially coherent GSM beam propagation through atmospheric turbulence. © 2014 Chinese Physical Society and IOP Publishing Ltd.

Xiang N.,Xidian University | Wu Z.,Xidian University | Wang M.,Xianyang Normal College
Hongwai yu Jiguang Gongcheng/Infrared and Laser Engineering | Year: 2013

Based on the extended Huygens-Fresnel integral, the cross-spectral density and Rytov's phase structure function, average intensity was derived by a formula for the Fourier transform of Gaussian function. Then mean squared root beam width, beam wander of a partially coherent Gaussian-Schell-model (GSM) beam in turbulent atmosphere were found out. It shows that beam spread and beam wander have relation with initial beam radius, initial coherence width, wavelength and transmitter hight in the atmospheric turbulence. At last, some measures were brought to decrease the effect of atmospheric turbulence.

Wang L.,Xidian University | Wu Z.,Xidian University | Wang M.,Xianyang Normal College
Guangxue Xuebao/Acta Optica Sinica | Year: 2013

Based on complex Gaussian-function expansion method, the transmitting wave is decomposed into a series of Gaussian beam components. ABCD ray-matrix theory combined with Rytov approximation is used to derive the expressions of the mean intensity and the backscattering enhancement coefficient of the reflected wave from a finite circular mirror in the atmospheric turbulence, which reveals the existence of the oscillation zone. The degree of coherence function and coherence length are calculated and the results prove that the turbulence in the oscillation zone has a larger effect on the wave front.

Dong Q.,Xianyang Normal College | Li Y.,Xianyang Normal College | Xu J.,Northwestern Polytechnical University | Wang M.,Xianyang Normal College
2012 10th International Symposium on Antennas, Propagation and EM Theory, ISAPE 2012 | Year: 2012

The generalized Lorentz-Mie formulas are used to study the scattering characteristics when a femtosecond pulse illuminates a spherical sand and dust particle. Efficiencies for extinction and scattering of pulse wave by sand and dust particles are calculated for different radii and pulse duration. The calculations show that even for pulses with a constant carrier wavelength and pulse-filling coefficient, the efficiencies for extinction and scattering differ very much between the carrier wave and the different pulses. © 2012 IEEE.

Li Z.,Northwestern Polytechnical University | Li H.,Northwestern Polytechnical University | Li W.,Northwestern Polytechnical University | Wang J.,Northwestern Polytechnical University | And 2 more authors.
Applied Surface Science | Year: 2011

A novel ZrC-SiC coating was prepared on carbon/carbon (C/C) composites surface by solid phase infiltration and the ablation properties of the ZrC-SiC coated C/C composites under oxyacetylene flame were studied. The results show that the coating prepared on the condition of optimum process parameters exhibits dense surface and outstanding anti-ablation ability. After ablation for 20 s, the mass ablation rates of the coated C/C composites can be lowered to 2.36 × 10-3 g/s, 37.1% reduction compared with uncoated C/C composites. The oxide layer composed of ZrO2 and SiO2 acts as oxygen diffusion barrier and the evaporation of ZrO2 and SiO 2 absorbs a great amount of heat from the flame and reduces the erosive attack on the coating. © 2011 Elsevier B.V. All rights reserved.

Wang R.,Xidian University | Wu Z.,Xidian University | Zhang Y.,Xidian University | Wang M.,Xianyang Normal College | Gong L.,Xi'an Technological University
Qiangjiguang Yu Lizishu/High Power Laser and Particle Beams | Year: 2014

As the extension of the microwave and millimeter-wave, the terahertz wave provides much greater communication bandwidth and capacity than millimeter wave. The THz pulses appeare the temporal and spatial deformation because of the waves scattered by the incident particles, and the pulses signal attenuation occurs. Based on the Mie theory, using the discrete distribution of particles with the random wave transmission and scattering theory, the extinction coefficient of fog droplet in the THz wave is calculated. Considering the fog particle spectral distribution, we obtain the average scattering characteristics with different incident wavelengths. The multiple scattering property of the THz signal from advection fog, e.g. the transmissivity and reflectivity, is investigated with Monte Carlo method. The forward and backward scattering intensities with the scattering angle by different visibility are analyzed. The result shows that in the atmospheric environment with a lower visibility, the absorption and attenuation caused by fog in terahertz wavelength can not be ignored. The research in the paper is of great significance to the space transmission and communication application of the terahertz technology. ©, 2014, Editorial Office of High Power Laser and Particle Beams. All right reserved.

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