全文获取类型
收费全文 | 26737篇 |
免费 | 3107篇 |
国内免费 | 3513篇 |
专业分类
化学 | 9258篇 |
晶体学 | 105篇 |
力学 | 2371篇 |
综合类 | 217篇 |
数学 | 10114篇 |
物理学 | 11292篇 |
出版年
2024年 | 73篇 |
2023年 | 283篇 |
2022年 | 427篇 |
2021年 | 657篇 |
2020年 | 843篇 |
2019年 | 789篇 |
2018年 | 790篇 |
2017年 | 871篇 |
2016年 | 1018篇 |
2015年 | 910篇 |
2014年 | 1342篇 |
2013年 | 2281篇 |
2012年 | 1442篇 |
2011年 | 1707篇 |
2010年 | 1336篇 |
2009年 | 1770篇 |
2008年 | 1816篇 |
2007年 | 1874篇 |
2006年 | 1565篇 |
2005年 | 1362篇 |
2004年 | 1123篇 |
2003年 | 1148篇 |
2002年 | 1056篇 |
2001年 | 831篇 |
2000年 | 830篇 |
1999年 | 725篇 |
1998年 | 657篇 |
1997年 | 483篇 |
1996年 | 333篇 |
1995年 | 315篇 |
1994年 | 261篇 |
1993年 | 241篇 |
1992年 | 218篇 |
1991年 | 197篇 |
1990年 | 192篇 |
1989年 | 184篇 |
1988年 | 150篇 |
1987年 | 141篇 |
1986年 | 113篇 |
1985年 | 128篇 |
1984年 | 124篇 |
1983年 | 55篇 |
1982年 | 96篇 |
1981年 | 81篇 |
1980年 | 65篇 |
1979年 | 86篇 |
1978年 | 67篇 |
1977年 | 78篇 |
1976年 | 67篇 |
1973年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 609 毫秒
991.
Krati Joshi 《Molecular physics》2015,113(19-20):2980-2991
Finite-temperature behaviour of a hollow golden cage (HGC) plays a crucialrole in its potential applications as a catalyst, drug delivery agent, contrasting agent and so on. This physico-chemical property of HGCs is not well understood so far. In that context, Born–Oppenheimer molecular dynamics (BOMD) simulations are performed on a well-known ‘free-standing’ HGC. The cluster considered in this study is the ground state Au18 cluster (a cage with a diameter of about >5.5 Å). The results thus obtained are compared with the BOMD simulation results reported earlier on Au32 icosahedron cage, a conformation with a diameter of nearly. The sphericity of both the clusters is studied using a shape deformation parameter as a function of time and temperature. These results are supplemented by radial distribution function at various temperatures. The observations and analysis of results indicate that, both the clusters retain an HGC conformation from 300 to 400 K, admitting structural fluxionality by the Au18 cluster. Remarkably, the Au18 cluster is able to maintain its hollowness and sphericity up to a high temperature of 1000 K. Underlying structural and electronic properties influencing the individualistic behaviour of cages are highlighted. Composition of the frontier molecular orbitals and the charge distribution play a crucial role in the finite-temperature behaviour of the Au cages. The conclusions are supplemented by supporting calculations on another degenerate ground state Au18 hollow cage and a well-known pyramidal Au18 cage at 300 and 400 K. 相似文献
992.
Density functional theory (DFT) calculations of nuclear magnetic resonance (NMR) spin–spin coupling constants (SSCCs) provide an important contribution for understanding experimentally observed values. It is known that calculated SSCCs using DFT methods correlate well with those experimentally measured. Unlike most of SSCCs, in fluorine compounds, fluorine–fluorine SSCC JFF shows that the Fermi contact (FC) term is not dominant, particularly for JFF in polyfluorinated organic molecules. In order to devise a DFT approach that would correctly reproduce the variation of SSCCs within a series of fluorine compounds, we test several DFT-based approaches, using different exchange and correlation functionals. Isotropic contributions to NMR fluorine–fluorine coupling constants (FC, spin-dipolar, SD, paramagnetic spin-orbit, PSO, and diamagnetic spin-orbit, DSO) have been calculated. Results show that DFT methods give appropriate values for nJFF (n = 4 to 7), while for geminal and vicinal JFF present large deviations from experimental values. For the latter SSCCs (2JFF and 3JFF), the four contributions (FC, SD, PSO and DSO) are analysed as a function of the local and nonlocal exchange in 1,1- and 1,2-difluoroethylene. Although FC term is not dominant for these SSCCs, the variation of this contribution with exchange is remarkable. On the other hand, SD and PSO contributions can be suitably computed without and with exact exchange, respectively. 相似文献
993.
For the first time, high-level structural and rovibrational data are provided for the hyroxyethynyl anion, CCOH?. CCOH? is a promising molecule for interstellar detection even though no new anions have been observed in the interstellar medium for the past half-decade. The large dipole moment of the corresponding neutral radical may be key for its creation as has been hypothesised and supported for other anions known to exist in various astronomical environments. Highly accurate quartic force fields are employed where previous benchmarks have produced spectroscopic constants and anharmonic vibrational frequencies within 20 MHz and 1 cm?1, respectively, of experiment. This same approach is applied here for CCOH? and its deuterated isotopologue with the goal of assisting laboratory experiments and/or astronomical observers in the potential detection of this anion. 相似文献
994.
研究了一个三阶半线性微分方程的奇摄动非线性混合边值问题.利用边界层函数法构造了该问题的形式渐近解,并采用微分不等式理论证明了解的存在性,给出了渐近解的误差估计,最后得出了边界层函数指数型衰减的结论. 相似文献
995.
Testing the validity of the conditional capital asset pricing model(CAPM) is a puzzle in the finance literatureLewellen and Nagel[14]find that the variation in betas and in the equity premium would have to be implausibly large to explain important asset-pricing anomaliesUnfortunately, they do not provide a rigorous test statisticBased on a simulation study, the method proposed in Lewellen and Nagel[14]tends to reject the null too frequently.We develop a new test procedure and derive its limiting distribution under the null hypothesis.Also, we provide a Bootstrap approach to the testing procedure to gain a good finite sample performanceBoth simulations and empirical studies show that our test is necessary for making correct inferences with the conditional CAPM. 相似文献
996.
Lung-Hui Chen 《复变函数与椭圆型方程》2015,60(5):707-731
We study the inverse spectral problem in an interior transmission eigenvalue problem. The Cartwright’s theory in value distribution theory gives a connection between the distributional structure of the eigenvalues and the asymptotic behaviours of its defining functional determinants. Given a sufficient quantity of transmission eigenvalues, we prove a uniqueness of the refraction index in inhomogeneous medium as an uniqueness problem in entire function theory. The asymptotically periodical structure of the zero set of the solutions helps to locate infinitely many eigenvalues of infinite degree of freedom. 相似文献
997.
Zhiber-Shabat方程,描述许多重要的物理现象,是一类重要的非线性方程,有着许多广泛的应用前景.本文给出Zhiber-Shabat方程的多辛几何结构和多辛Fourier拟谱方法.数值算例结果表明多辛离散格式具有较好的长时间的数值稳定性. 相似文献
998.
Tabbi Hanane Amel Bendjeddou Didier Villemin 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):697-704
AbstractThe synthesis of the title compounds has been carried out by condensation via a Wittig-type reaction of a pyridinium hexafluorophosphate with a phosphonate ester to give the desired (4-nitrophenyl)tetrathiafulvalene the nitro group of which was reduced to an amino group. Reaction of the amine with chlorosulfonyl isocyanate and subsequently with tert-butyl alcohol gave the corresponding open-chain sulfamide. Cyclization under basic conditions and de-protection led to 2-[4-(4′,5′-dipropyltetrathiafulvalen-4-yl)]phenyl-1,2,6-thiadiazinane 1,1-dioxide. Finally, N-alkylated and N-acylated cyclic sulfamides linked to tetrathiafulvalene were obtained. Their electron donor ability was measured by cyclic voltammetry. A detailed DFT study based on B3LYP/6–31G (d,p) of electronic properties is also presented. The calculated molecular electrostatic potential shows that, the negative charge covers the nitro and sulfamide function, while positive charge is located at the hydrogen atoms of the amine and sulfamide rings. The calculated HOMO and LUMO energy reveals that charge transfer occurs within the molecule. The chemical reactivity parameters reveal that tetrathiafulvalene 1 is highly reactive, which facilitates the desired formation of the cyclic sulfamide. The first hyperpolarizability βtot shows that compounds 1 and 5 are good candidates as a NLO material. 相似文献
999.
Brent D. Viers 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(2):131-138
End‐linking poly(dimethylsiloxane) was used to prepare bimodal elastomers networks so as to have inhomogeneous nanostructures, and also to prepare others having very broadly multimodal chain‐length distributions. Macroscopic phase separation, probably high crosslink density clusters, was observed to occur in some of the bimodal networks. The mechanical properties in simple extension and in equilibrium swelling were measured. The bimodal elastomers that were not obviously inhomogeneous showed very good mechanical properties, but the macroscopically phase‐separated networks, and the broadly multimodal network were weak. Analysis of the Mooney‐Rivlin profiles suggests that the reinforcing mechanism could have a structural component in addition to that from the limited extensibilities of the short chains. The mechanical properties and the extents of swelling support the cluster conjecture, in accord with previous morphological studies on spatially‐inhomogeneous polysiloxane elastomers. 相似文献
1000.
C.-W. Cho J. Ranke J. Arning J. Thöming U. Preiss C. Jungnickel 《SAR and QSAR in environmental research》2013,24(10):863-882