为实现准等熵压缩的波阻抗梯度飞片的实验研究

采用粉末冶金方法,制备出波阻抗沿厚度方向呈特定分布的WMoTi体系梯度飞片,在二级轻气炮上进行了梯度飞片击靶实验的初步研究.结果表明,利用梯度飞片可以实现对靶样品的准等熵压缩,击靶后产生的波阵面是平缓、连续上升的,明显不同于冲击压缩的陡峭波阵面,靶面压力峰值最高达到了167GPa.波阻抗按二次函数分布的梯度飞片能够获得最佳的击靶波形 关键词： 波阻抗梯度飞片 准等熵压缩     

固体炸药的磁驱动准等熵压缩实验研究

较宽压力范围内未反应炸药的本构关系和状态方程对于深入和精确认识压缩波作用下炸药组分间相互作用的力学过程和起爆热点的形成机制具有重要意义。较之冲击压缩,磁驱动准等熵压缩加载(无冲击压缩)是获取较宽压力范围内未反应炸药的动态压缩力学特性更有效的手段。基于大电流产生的电磁力作用原理,在国内率先实现了炸药的磁驱动无冲击压缩实验技术。通过对负载电极、炸药样品参数的优化设计和安装工艺的控制,实现了5 GPa载荷内JO-9159炸药的磁驱动准等熵压缩加载。基于激光位移干涉测量技术和Lagrange数据处理方法,获得了JO-9159炸药的速度响应历史和准等熵压缩线。所得结果与文献数据进行了比较,结果表明,在实验压力范围内,JO-9159炸药的等熵压缩线与PBX9501炸药的等熵压缩线一致。     

A GENERALIZED MODELING FOR CEMENT-BASED MATERIALS UNDER LARGE RANGE OF COMPRESSION STATE

The aim of this study is to describe the main behavior of cement-based materials under large compression state based on the recent experimental research. In this paper, the strainstress relations are firstly analyzed and confining pressure state is regarded as low/medium/high state. A generalized cup modeling is introduced by a coupled deviatoric shearing, pore collapse and damage mechanism within thermodynamic framework. A series of numerical simulations are performed for the considered cement paste and concrete. Comparisons between numerical predictions and experimental results show that the proposed model is able to describe the main features of mechanical behavior under large range of compression state.     

Mechanical Behavior of Polymer Gels for RDCs and RCSAs Collection: NMR Imaging Study of Buckling Phenomena

Anisotropic NMR parameters, such as residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs or Δν_Q), appear in solution‐state NMR when the molecules under study are subjected to a degree of order. The tunable alignment by reversible compression/relaxation of gels (PMMA and p‐HEMA) is an easy, user‐friendly, and very affordable method to measure them. When using this method, a fraction of isotropic NMR signals is observed in the NMR spectra, even at a maximum degree of compression. To explain the origin of these isotropic signals we decided to investigate their physical location inside the NMR tube using deuterium 1D imaging and MRI micro‐imaging experiments. It was observed that after a certain degree of compression the gels start to buckle and they generate pockets of isotropic solvent, which are never eliminated. The amount of buckling depends on the amount of cross‐linker and the length of the gel.     

Computational studies of the cone and 1,2,3 alternate calix[6]arene bis‐crown‐4 isomers: structures,NMR shifts,atomic charges,and steric compression

The cone and 1,2,3 alternate isomers of calix[6]arene bis‐crown‐4 were investigated computationally. Structural optimizations, energies, bond distances, and Mulliken charges were calculated by the application of the B3LYP/6‐31g(d) method/basis, followed by NMR calculations via both B3LYP/6‐31g(d) and HF/6‐31g(d). Calculations were completed at three different levels of imposed symmetry, and two calculations investigated the chloroform solvent effects. Better NMR results were obtained from HF/6‐31g(d) calculations that did not impose molecular symmetry constraints. Consideration of solvent effects improved ground state energies, but other improvements were minimal and not significant enough to justify the added computational expense of solvent calculations. Overall results are consistent with known experimental assignments and were valuable for assigning previously unknown NMR peaks. Net charges, electrostatic forces, and local dipoles – but not bond lengths – are strongly correlated to spectroscopic manifestations of steric compression. Copyright © 2009 John Wiley & Sons, Ltd.     

An improved partial SPIHT with classified weighted rate-distortion optimization for interferential multispectral image compression

Based on the property analysis of interferential multispectral images, a novel compression algorithm of partial set partitioning in hierarchical trees (SPIHT) with classified weighted rate-distortion optimization is presented.After wavelet decomposition, partial SPIHT is applied to each zero tree independently by adaptively selecting one of three coding modes according to the probability of the significant coefficients in each bitplane.Meanwhile the interferential multispectral image is partitioned into two kinds of regions in terms of luminous intensity, and the rate-distortion slopes of zero trees are then lifted with classified weights according to their distortion contribution to the constructed spectrum.Finally a global ratedistortion optimization truncation is performed.Compared with the conventional methods, the proposed algorithm not only improves the performance in spatial domain but also reduces the distortion in spectral domain.     

Experimental study on generation of high energy few cycle pulses with hollow fiber filled with neon

25 fs pulses with energy up to 0.8 mJ from a multi-pass amplifier system have been spectrally broadened from 460 nm to 950 nm due to strong self-phase modulation (SPM) effect in a gas filled hollow fiber. Using a set of chirped mirrors, the ultra-broadband dispersion compensation was achieved, and the compressed pulses reached their transform limit. Under optimized conditions we achieved pulses with duration of 5.1 fs and with energy of 400 μJ, corresponding to the peak power up to 80 GW. Supported by the National Natural Science Foundation of China (Grant Nos. 60608003, 60490280, 60225005 and 60621063)     

Spectral compression of femtosecond pulses via frequency intermodulation

A new technique of spectral compression of femtosecond pulses, based on their frequency intermodulation in the frequency mixing process, is demonstrated experimentally. In the exploited phenomenon of frequency intermodulation the phase of the reference pulse is superimposed on the signal instead of phase self- or cross-modulation used in traditional spectral compressors. In the process of frequency mixing a twelve-fold aberration-free spectral compression of 100 fs pulses has been achieved experimentally.     

Near Best Tree Approximation

Tree approximation is a form of nonlinear wavelet approximation that appears naturally in applications such as image compression and entropy encoding. The distinction between tree approximation and the more familiar n-term wavelet approximation is that the wavelets appearing in the approximant are required to align themselves in a certain connected tree structure. This makes their positions easy to encode. Previous work [4,6] has established upper bounds for the error of tree approximation for certain (Besov) classes of functions. This paper, in contrast, studies tree approximation of individual functions with the aim of characterizing those functions with a prescribed approximation error. We accomplish this in the case that the approximation error is measured in L ₂, or in the case p2, in the Besov spaces B _p ⁰(L _p ), which are close to (but not the same as) L _p . Our characterization of functions with a prescribed approximation order in these cases is given in terms of a certain maximal function applied to the wavelet coefficients.     

基于子波变换的光谱信息数据压缩方法

本文介绍一种基于子波变换的光谱信息数据压缩方法，利用子波变换的多尺度分析原理，将原始光谱数据分解成集中源信号绝大部分能量的模糊信号和反映源信号变化特性的锐化信号。由于锐化信号只有源信号变化梯度大的区域系数值才较大，其他区域都接近零，只需保存少量的系数，就可以实现数据压缩，用本文方法，对２１种典型地物光谱数据进行了数据压缩实验，在１．０～１．７均方根误差情况下，若压缩结果不编码，压缩比一般为４：１～