Showing papers by "Zhen Li published in 2002"

Investigation of the microwave absorbing properties of carbon aerogels

Abstract: Carbon aerogel is a new kind of functional material. In this paper, carbon aerogel is prepared by solvent exchange instead of supercritical-drying. The catalyst concentration influences the microstructure of carbon aerogel greatly. By changing the resorcinol per catalyst (R/C) molar ratio, polymeric and colloidal carbon aerogel are formed. The absorbing properties of carbon aerogel are influenced by its microstructure and density. The maximum reflection coefficient is about −22 and −32 dB for the polymeric and colloidal carbon aerogels, respectively. The bandwidth corresponding to the reflection coefficient below −10 dB is more than 5 GHz for both structures. With increasing thickness, the peak value of the reflection coefficient moves to lower frequency.

Arylaldehydes-pentafluorophenyl Hydrazones as Second-order Nonlinear Optical Chromophores: A Novel Approach for Remarkably Defeating the Nonlinearity-transparency Trade-off

Abstract: A series of new arylhydrazone chromophores containing pentafluorophenyl as electronic acceptor are synthesized and found to exhibit significantly blue-shifted absorption in comparison with the corresponding 4-nitrophenyl analogues while keeping β value in the same order, and this may provide a new opportunity for defeating nonlinearity-transparency trade-off in designing second-order nonlinear optical chromophores.

Measuring soil moisture change with vegetation cover using passive and active microwave data

Abstract: Two basic microwave approaches are used to measure soil moisture, one is passive which is based on radiometry and the other is active and uses radar. Both approaches utilize the large contrast between the dielectric constant of dry soil and water. A total backscattering amount for a vegetated surface include volume, surface, and surface-volume interaction scattering terms. The backscattering model here is based on without surface-volume interaction scattering terms. In attempt to use active microwave remote sensors in estimation of soil moisture, two major problems, effects of surface roughness and vegetation cover, are faced. For a given sensor, we assume the roughness under the condition of no change during data acquisitions. The main problem for retrieval of surface dielectric properties is separate the volume scattering item from total backscattering. With the time-serial soil moisture map from L band passive microwave radiometry, the Electronically Scanned Thinned Array Radiometer (ESTAR) at Southern Great Plains 1997 (SGP97), we calculated the surface reflectivity with 800 m resolution. The volume scattering items at 800 m resolution can be derived using multi-temporal resample calibration Radarsat SAR and surface reflectivity data. Weighting the ratio of NDVI at different resolution from NOAA/AVHRR and TM, the surface reflectivity change with 50 m resolution can be estimated according to the total backscattering and volume scattering, then soil moisture change be mapped at 50 m resolution.

Performance evaluation and analysis of protocols for IP mobility support: a quantitative study

Abstract: With the increasing number of mobile users, the network traffic for tracking mobile hosts is expected to increase dramatically, thus an efficient protocol for IP mobility support is of great value. We study the performance of such protocols. Starting from identifying the shortcomings in current evaluation methods and presenting the unfitness to our topic in current network simulators, we make three major contributions in this paper: (1) we present an integrated development environment we have developed and utilized for protocol evaluation; (2) we show the simulation results of two benchmark protocols with our simulation environment, and we give our analysis to the results; (3) based on the analysis, we propose a new protocol and demonstrate its efficiency through the comparison with the two benchmarks.

Arylaldehydes-pentafluorophenyl Hydrazones as Second-Order Nonlinear Optical Chromophores: A Novel Approach for Remarkably Defeating the Nonlinearity-Transparency Trade-off.

Abstract: A series of new arylhydrazone chromophores containing pentafluorophenyl as electronic acceptor are synthesized and found to exhibit significantly blue-shifted absorption in comparison with the corresponding 4-nitrophenyl analogues while keeping β value in the same order, and this may provide a new opportunity for defeating nonlinearity-transparency trade-off in designing second-order nonlinear optical chromophores.

The monitoring of land-cover change in western China from remote sensing data

Abstract: There are serious environmental problems, such as deforestation, soil erosion, salinization, and desert encroachment in the western China, its natural conditions is very delicate. But little is known about change in this region. The remote sensing data and digital image processing techniques provide consistent, reliable and quantifiable regional scale land-cover for environmental change research. The objective of this study is analyzing the environment change in a year in western China by using remote sensing data. In this paper, three typical experiment sites, which are a desert and oasis area, a loess plateau area and the source area of the Yellow River, are selected for detail change analyses. The change of NDVI, land-cover classifications between 1980s and present are detected from the satellite data. The datasets of remote sensing include Landsat-TM, ETM, SPOT and ASTER, all of these image are normalized to 30 m resolution and same coordination system for change analyses. The results show that industry development, cultivated land increment, soil erosion, forest and grassland decrement are the main reason of environmental deterioration.