全文获取类型
收费全文 | 22760篇 |
免费 | 3483篇 |
国内免费 | 2442篇 |
专业分类
化学 | 16072篇 |
晶体学 | 276篇 |
力学 | 1353篇 |
综合类 | 187篇 |
数学 | 2319篇 |
物理学 | 8478篇 |
出版年
2024年 | 68篇 |
2023年 | 423篇 |
2022年 | 739篇 |
2021年 | 810篇 |
2020年 | 850篇 |
2019年 | 836篇 |
2018年 | 713篇 |
2017年 | 738篇 |
2016年 | 1015篇 |
2015年 | 1036篇 |
2014年 | 1175篇 |
2013年 | 1610篇 |
2012年 | 1869篇 |
2011年 | 1991篇 |
2010年 | 1298篇 |
2009年 | 1267篇 |
2008年 | 1463篇 |
2007年 | 1349篇 |
2006年 | 1217篇 |
2005年 | 970篇 |
2004年 | 774篇 |
2003年 | 658篇 |
2002年 | 633篇 |
2001年 | 461篇 |
2000年 | 456篇 |
1999年 | 481篇 |
1998年 | 422篇 |
1997年 | 423篇 |
1996年 | 463篇 |
1995年 | 365篇 |
1994年 | 381篇 |
1993年 | 274篇 |
1992年 | 254篇 |
1991年 | 233篇 |
1990年 | 165篇 |
1989年 | 129篇 |
1988年 | 117篇 |
1987年 | 97篇 |
1986年 | 95篇 |
1985年 | 85篇 |
1984年 | 58篇 |
1983年 | 49篇 |
1982年 | 38篇 |
1981年 | 28篇 |
1980年 | 31篇 |
1979年 | 18篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1975年 | 7篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
981.
格雷码辅助相移技术可以实现具有较强鲁棒性与抗噪能力的三维(3D)形貌测量。为解决由待测物体不均匀的表面反射率、噪声和物体运动等因素造成的级次边沿误码问题,提出了一种基于错位格雷码的动态3D形貌测量方法。将传统格雷码图案在投影前预先移动半个条纹周期得到错位格雷码图案,再采用传统格雷码解码方法对二值化后的错位格雷码图案解码,可得到与截断相位完全错开的解码结果。对该解码结果进行修正后即可利用得到的正确的相位级次辅助截断相位成功展开。同时,为了提高测量精度,引入了一个虚拟相位平面以进一步拓展投影条纹周期数。实验结果表明,所提方法在使用N帧格雷码图案的情况下,可以编码周期数为2N+1的投影条纹进行3D测量,其无需任何附加图案即可避免级次边沿误码问题,并有效提升了测量精度。复杂动态场景的3D重建结果证明,所提方法能够以2381 frame/s的速率实现高精度、高效率和高速的3D形貌测量。 相似文献
982.
Margarida S. Miranda Luís Pinto da Silva Joaquim C. G. Esteves da Silva 《Journal of Physical Organic Chemistry》2014,27(1):47-56
2‐Ethylhexyl 4‐methoxycinnamate (EHMC) is a very commonly used UVB filter that is known to isomerize from the (E) to the (Z) isomer in the presence of light. In this study, we have performed high level quantum chemical calculations using density functional theory (DFT) with the B3LYP density functional and extended basis sets to study the gas‐phase molecular structure of EHMC and its energetic stability. Calculations were also performed for related smaller molecules cinnamic acid and 4‐methoxycinnamic acid. Charge delocalization has been analyzed using natural charges and Wiberg bond indexes within the natural bond orbital analysis and using nucleus independent chemical shifts. Density functional theory calculations reveal that the (E) isomer of EHMC is more stable than the (Z) by about 20 kJ mol?1 in both the gas and aqueous phases. The enthalpy of formation in the gas phase of (E)‐EHMC was derived from an isodesmic bond separation reaction. Long‐range corrected DFT calculations in implicit water were made in order to understand the excited state properties of the (E) and (Z) isomers of EHMC. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
983.
984.
In this work, poly(fluorocyclopentenyl)siloxane (FPCS) was obtained via a single electron transfer addition reaction of poly(cyclopentadienyl)siloxane (PCS) and perfluoroalkyl iodides, and reduction reaction of the intermediates. PCS was prepared by substitution and hydrolysis reactions using methyltrichlorosilane and sodium cyclopentadienide (NaCp) as raw materials. Fourier transform infrared (FTIR), 1H NMR, and 19F NMR indicated the structures of the target polymers. The XPS results showed that the thin films prepared by dip-coating were fluorine enriched at the surface. Atomic force microscopy (AFM) image showed that on the rough surface of films, there were many pinnacles which were generated through the migration of side chain fluoroalkyl groups. The relative static contact angles of water and n-hexadecane on FPCS and PCS indicated that sodium dithionite initiated the reaction of perfluoroalkyl iodides and PCS so that FPCS was successfully synthesized. The measured surface energy of PCS was 2.57 × 10?2 N/m; while FPCS was 2.14 × 10?2 N/m, which represented better liquid repellent property compared to PCS. 相似文献
985.
Wenhe Du Jingxuan Yang Zhongmin Yao Junguo Lu Daosen Liu Quanling Cui 《Journal of Russian Laser Research》2014,35(4):416-423
Both increasing experimental evidence and some results of theoretical investigation have shown that there exist two kinds of turbulence in the aerosphere, Kolmogorov and non-Kolmogorov turbulence. Thus, it is necessary to improve the theory of optical wave propagation through atmospheric turbulence, namely, study the laser-beam propagation in non-Kolmogorov turbulence, before analyzing the joint influence of the Kolmogorov turbulence and non-Kolmogorov one on satellite laser communication. The beam wander will lead to the performance degradation of satellite laser communication systems and exert an influence on the achievement and stability of its links. In this paper, we consider a theoretical power spectrum of refractive-index fluctuations with a generalized power law in order to derive the variance of Gaussian-beam wave wander in weak turbulence for a horizonal path and analyze the influence of spectral power-law variations on the beam wander. We show that the expression for the beam-wander variance is of concise closed form and independent of the optical wavelength. 相似文献
986.
Temperature memory effect and its stability revealed via differential scanning calorimetry in ethylene‐vinyl acetate within glass transition range
下载免费PDF全文
![点击此处可从《Journal of Polymer Science.Polymer Physics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Tao Xi Wang Wei Min Huang Hongmei Chen Rui Xiao Hai Bo Lu Shu Feng Kang 《Journal of Polymer Science.Polymer Physics》2016,54(17):1731-1737
In this article, we reveal the temperature memory effect (TME) in a commercial thermoplastic polymer, namely ethylene‐vinyl acetate (EVA), within its glass transition range via a series of differential scanning calorimeter (DSC) tests. In addition, we investigate the influence of heating holding time and also compare the observed TME in current study with that of shape memory alloys (SMAs). It is concluded that the TME via DSC (without any macroscopic shape change) is achievable within the glass transition range of a polymer. Conversely, although the observed TME shares the many similar features as those in SMAs, due to the nature of micro‐Brownian motion in the glass transition of polymers, the resulted TME is strongly affected by the heating holding time. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1731–1737 相似文献
987.
988.
Ingrid Daubechies Jianfeng Lu Hau-Tieng Wu 《Applied and Computational Harmonic Analysis》2011,30(2):243-261
The EMD algorithm is a technique that aims to decompose into their building blocks functions that are the superposition of a (reasonably) small number of components, well separated in the time–frequency plane, each of which can be viewed as approximately harmonic locally, with slowly varying amplitudes and frequencies. The EMD has already shown its usefulness in a wide range of applications including meteorology, structural stability analysis, medical studies. On the other hand, the EMD algorithm contains heuristic and ad hoc elements that make it hard to analyze mathematically.In this paper we describe a method that captures the flavor and philosophy of the EMD approach, albeit using a different approach in constructing the components. The proposed method is a combination of wavelet analysis and reallocation method. We introduce a precise mathematical definition for a class of functions that can be viewed as a superposition of a reasonably small number of approximately harmonic components, and we prove that our method does indeed succeed in decomposing arbitrary functions in this class. We provide several examples, for simulated as well as real data. 相似文献
989.
Isovalent cation substitution into rocksalt oxides, MO, has been investigated using atomistic simulation. A strain related parameter, ε, is established that relates the size of a substitutional cation to the host lattice ion for which it has been substituted. This has allowed us to identify relationships between solution energy, defect volume and a strain parameter, which are general for any rocksalt oxide host lattice and as such are predictive for any combination of a divalent cation and rocksalt host lattice. 相似文献
990.