Measurement of Porosity in a Composite High Explosive as a Function of Pressing Conditions by Ultra‐Small‐Angle Neutron Scattering with Contrast Variation

We have used ultra‐small‐angle neutron scattering (USANS) with contrast variation to measure the porosity over length scales 0.1–20 μm in a composite high explosive, PBX 9501, formulated with a deuterated binder. Here, we explore the effect of varying the pressing intensity on the PBX 9501 microstructure. Samples of PBX 9501 were die‐pressed with applied pressures ranging between 69 and 200 MPa at 90 °C. Five samples were prepared at each pressure that differed in the fraction of binder that was deuterated, resulting in a change in the neutron scattering length density contrast (Δρ) of the binder relative to that of the high explosive crystallites and voids. By using this approach to discriminate scattering from voids from that due to the binder, we determined microstructure and composition that otherwise would not have been apparent. The sample composition was determined by calculating the Porod Invariant as a function of Δρ and comparing it with compositional estimates obtained from the bulk sample density. Structural modeling of the USANS data, assuming both spherical and cylindrical morphologies, allowed the mean size and size distribution of voids and binder‐filled regions to be determined.     

Design of an extreme-ultraviolet monochromator free from temporal stretching

High-order harmonic generation in gases by use of femtosecond lasers is a source of ultrashort pulses in the extreme-ultraviolet (XUV). For many applications it is necessary to select radiation of only one specific harmonic order without affecting the duration of the ultrashort pulse. A three-grating monochromator that meets this demand has been designed and modeled by ray tracing as well as by wave-optical simulations. The only remaining temporal stretching of an XUV pulse is due to distortion of the pulse front on the gratings and is predicted to be approximately 1 fs. The design has been successfully tested in the near infrared. Finally, the monochromator is also capable of eliminating any existing linear chirp in the harmonic pulses, thus compressing them to shorter durations.     

Analytical parameterization of canopy directional emissivity and directional radiance in the thermal infrared. Application on the retrieval of soil and foliage temperatures using two directional measurements

This work is aimed at deriving canopy component (soil and foliage) temperatures from remote sensing measurements. A simulation study above sparse, partial and dense vegetation canopies has been performed to improve the knowledge of the behaviour of the composite radiative temperature and emissivity. Canopy structural parameters have been introduced in the analytical parameterization of the directional canopy emissivity and directional canopy radiance:namely, the leaf area index (LAI), directional gap fraction and angular cavity effect coefficient. The parameterization has been physically defined allowing its extension to a wide range of Leaf Inclination Distribution Functions (LIDF). When single values are used as leaves and soil temperatures, they prove to be retrieved with insignificant errors from two directional measurements of the canopy radiance (namely at 0 and 55 from nadir), provided that the canopy structure parameters are known. A sensitivity study to the different parameters shows the great importance of the accuracy on LAI estimation (an accuracy of 10 per cent is required to retrieve the leaves temperature with an accuracy better than 0.5 degK, the same requirement being 5 per cent for the retrieval of soil temperature). The radiometric noise is important too, but its effects may be limited by using very different angles for the measurements: for 0 and 55, the effect of a Gaussian noise (NEDeltaT 0.05deg K) is lower than 0.5degK on the retrieved soil and foliage temperatures). Uncertainties on the leaf and soil emissivities (Delta epsilon 0.01) cause little errors in the retrieval (lower than 0.5degK). If the inclination dependence of the leaves temperature is considered, a 1 degK error is observed in the retrieved soil and foliage temperatures. This error is due to the fact that the effective foliage temperature varies with the view angle (a few 10 -1 deg K at 55 ), which implies errors in the inversion scheme. This effect may be corrected for by using an angular corrective term delta depending only on the off-nadir angle used.     

Image-Based Modeling of the Human Eye

Rendering realistic organic materials is a challenging issue. The human eye is an important part of nonverbal communication which, consequently, requires specific modeling and rendering techniques to enhance the realism of virtual characters. We propose an image-based method for estimating both iris morphology and scattering features in order to generate convincing images of virtual eyes. In this regard, we develop a technique to unrefract iris photographs. We model the morphology of the human iris as an irregular multilayered tissue. We then approximate the scattering features of the captured iris. Finally, we propose a real-time rendering technique based on the subsurface texture mapping representation and introduce a precomputed refraction function as well as a caustic function, which accounts for the light interactions at the corneal interface.     

机载雷达的空时发射与编码

论述远程机载雷达,阐明了宽带波形和多通道的同时需求开通了走向新的空时自适应信号与处理技术的道路,从而改进对多个地面和空中目标的探测与分类。     

A new hybrid method based on differential evolution to determine the temperature-dependent parameters of single-diode photovoltaic cells

With renewable energy currently making the headlines, photovoltaic technology has shown significant potential as one of the best energy sources. It thus becomes necessary to predict the performance of photovoltaic systems by modeling it accurately and optimally. We propose a new hybrid algorithm for extracting PV cell parameters to improve their performance and efficiency when subjected to temperature variations. A metaheuristic approach is combined with an analytical approach to improve the accuracy and robustness. We call this approach improved differential evolution (IDE). The performance of the proposed method is evaluated for selected cell data. For validation, several analyses and comparisons are made with other methods, and the results illustrate the accuracy and precision of IDE. The proposed technique can estimate the parameters in an optimal way at any temperature, together with high convergence speed and a short simulation time.

     

On the extension of the grain loop concept from 2D to 3D granular assemblies

Granular Matter - We study the local structural changes along the jamming transitions in asymmetric bidisperse granular packings. The local structure of the packing is assessed by the contact...     

Coupling Ferroelectric to colloidal Nanocrystals as a Generic Strategy to Engineer the Carrier Density Landscape

The design of infrared nanocrystals-based (NCs) photodiodes faces a major challenge related to the identification of barriers with a well-suited band alignment or strategy to finely control the carrier density. Here, this study explores a general complementary approach where the carrier density control is achieved by coupling an NC layer to a ferroelectric material. The up-and-down change in ferroelectric polarization directly impacts the NC electronic structure, resulting in the formation of a lateral pn junction. This effect is uncovered directly using nano X-ray photoemission spectroscopy, which shows a relative energy shift of 115 meV of the NC photoemission signal over the two different up- and down-polarized ferroelectric regions, a shift as large as the open circuit value obtained in the diode stack. The performance of this pn junction reveals enhanced responsivity and reduced noise that lead to a factor 40 increase in the detectivity value.