全文获取类型
收费全文 | 6534篇 |
免费 | 943篇 |
国内免费 | 467篇 |
专业分类
化学 | 1058篇 |
晶体学 | 20篇 |
力学 | 1160篇 |
综合类 | 52篇 |
数学 | 313篇 |
物理学 | 5341篇 |
出版年
2024年 | 5篇 |
2023年 | 40篇 |
2022年 | 98篇 |
2021年 | 101篇 |
2020年 | 122篇 |
2019年 | 94篇 |
2018年 | 95篇 |
2017年 | 253篇 |
2016年 | 281篇 |
2015年 | 247篇 |
2014年 | 407篇 |
2013年 | 408篇 |
2012年 | 376篇 |
2011年 | 455篇 |
2010年 | 321篇 |
2009年 | 409篇 |
2008年 | 495篇 |
2007年 | 469篇 |
2006年 | 407篇 |
2005年 | 333篇 |
2004年 | 323篇 |
2003年 | 300篇 |
2002年 | 256篇 |
2001年 | 247篇 |
2000年 | 212篇 |
1999年 | 175篇 |
1998年 | 163篇 |
1997年 | 157篇 |
1996年 | 115篇 |
1995年 | 100篇 |
1994年 | 65篇 |
1993年 | 73篇 |
1992年 | 52篇 |
1991年 | 46篇 |
1990年 | 36篇 |
1989年 | 39篇 |
1988年 | 31篇 |
1987年 | 28篇 |
1986年 | 14篇 |
1985年 | 27篇 |
1984年 | 19篇 |
1983年 | 14篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有7944条查询结果,搜索用时 46 毫秒
71.
We present a numerical method for reconstructing the coefficient in a wave equation from a single measurement of partial Dirichlet boundary data. The original inverse problem is converted to a nonlinear integral differential equation, which is solved by an iterative method. At each iteration, one linear second‐order elliptic problem is solved to update the reconstruction of the coefficient, then the reconstructed coefficient is used to solve the forward problem to obtain the new data for the next iteration. The initial guess of the iterative method is provided by an approximate model. This model extends the approximate globally convergent method proposed by Beilina and Klibanov, which has been well developed for the determination of the coefficient in a special wave equation. Numerical experiments are presented to demonstrate the stability and robustness of the proposed method with noisy data.© 2014 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 31: 289–307, 2015 相似文献
72.
Sergio GUTIERREZ 《数学年刊B辑(英文版)》2015,36(5):843-854
An optimal design method for two materials based on small amplitude
homogenization is presented. The method allows to use quite general
objective functions at the price that the two materials should have
small contrasts in their relevant physical parameters. The following
two applications are shown: Stress constrained compliance
minimization and defect location in elastic bodies. 相似文献
73.
Long distance atomic teleportation (LDAT) is of prime importance in long distance quantum communication. Scheme proposed by Bose et al. (1999) in principle enables us to have LDAT using cavity decay. However it gives message state dependent fidelity and success rate. Here, using interaction of entangled coherent states with atom–cavity systems and a two-step measurement, we show how, LDAT can be achieved with unit fidelity and as good success as desired under ideal conditions. The scheme is unique in that, the first measurement predicts success or failure. If success is predicted then second measurement gives perfect teleportation. If failure is predicted the message-qubit remains conserved therefore a second attempt may be started. We found that even in presence of decoherence due to dissipation of energy our scheme gives message state independent success rate and almost perfect teleportation in single attempt with mean fidelity of teleportation equal to 0.9 at long distances. However if first attempt fails, unlike ideal case where message-qubit remains conserved with unit fidelity, in presence of decoherence the message-qubit remains conserved to some degree, therefore mean fidelity of teleportation can be increased beyond 0.9 by repeating the process. 相似文献
74.
The Through-SiliconVias (TSV) is a key component of three dimensional electronic packaging. Obtaining its stresses is very important for evaluating its reliability. A micro-infrared photoelasticity system with a thermal loading function was built and applied to characterize the stresses of the TSV structure. Through testing it was found that the stress of each TSV is different even if their fabrication technology is exactly the same, that different TSVs obtain their stress free states at different elevated temperatures, and that their stress free states are maintained even when the temperature is further elevated. A finite element model was used to quantitatively determine the stresses of a TSV under different stress-free temperatures. Different virtual photoelasticity fringe patterns were then created based on the principle of photoelasticity and the simulated stresses. Comparing the virtual fringe patterns with the experimental pattern, an appropriate virtual photoelasticity fringe pattern and the corresponding stresses of TSV were determined 相似文献
75.
A novel optical extensometer is developed to estimate the local uniform strain on planar surface accurately. The proposed system consists of a shared large format lens and two image sensors, which acquire pairs of images of two isolated small regions on the object surface simultaneously. Digital image correlation (DIC) algorithm is applied to determine the relative displacement between gauge points designated on recorded pairs of images. Then local strain can be extracted after dividing the relative displacement by the scale distance. Moreover, a special experimental setup called “correction sheet” is used to eliminate the virtual strain induced by out-of-plane motions. Uni-axial tensile experiments are performed to validate the reliability and resolution of the optical extensometer, and the measurement results demonstrate that the resolution of the optical extensometer achieves 2–3 με. 相似文献
76.
报道了激光诱导热光栅光谱测温技术的研究. 通过两束相干交叉的脉冲抽运光, 在NO2/N2混合气中诱导出热光栅, 一束满足布拉格散射条件的连续探测光在交叉区域激励出相干的热光栅信号, 经过空间和光谱滤波的信号光由光电倍增管探测, 并由数字示波器显示和存储. 该信号携带了丰富的流场信息, 通过频域分析, 对气体的温度进行了测量, 热光栅光谱技术测量的温度与热电偶温度符合得很好. 同时还利用热光栅光谱技术进行了气体声速的直接测量, 在一定的温度范围内, 测量结果与理论曲线基本一致, 显示了该技术具有较高的测量精度与多参数同时测量的能力. 对影响信号波形的因素进行了分析, 结果表明, 热光栅光谱测温技术在高压强环境下应用具有独特的优势, 是一种应用前景广阔的激光燃烧诊断技术. 相似文献
77.
78.
79.
Polystyrene latex (PSL) nanoparticle (NP) sample is one of the most widely used standard materials. It is used for calibration of particle counters and particle size measurement tools. It has been reported that the measured NP sizes by various methods, such as Differential Mobility Analysis, dynamic light scattering (DLS), optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM), differ from each other. Deformation of PSL NPs on mica substrate has been reported in AFM measurements: the lateral width of PSL NPs is smaller than their vertical height. To provide a reliable calibration standard, the deformation must be measured by a method that can reliably visualize the entire three dimensional (3D) shape of the PSL NPs. Here we present a method for detailed measurement of PSL NP 3D shape by means of electron tomography in a transmission electron microscope. The observed shape of the PSL NPs with 100 nm and 50 nm diameter were not spherical, but squished in direction perpendicular to the support substrate by about 7.4% and 12.1%, respectively. The high difference in surface energy of the PSL NPs and that of substrate together with their low Young modulus appear to explain the squishing of the NPs without presence of water film. 相似文献
80.
Describes the functions, background applications and features of the resolver, in order to calculate the angle position of the resolver signals, this paper choose AD7656 precision chip, proposed the resolver algorithm for calculating resolver angle by measuring the output voltage and excitation voltage of the resolver. Mainly introduce AD7656 and its hardware circuit design, and flowchart of the algorithm for arctangent. This algorithm is simple and reliable. Experimental results show that the system design is reasonable, measurement algorithms of resolver angle meets the requirements. Analyze the error for this result and prepare for the study. 相似文献