全文获取类型
收费全文 | 24233篇 |
免费 | 2846篇 |
国内免费 | 3268篇 |
专业分类
化学 | 9687篇 |
晶体学 | 104篇 |
力学 | 2026篇 |
综合类 | 177篇 |
数学 | 7751篇 |
物理学 | 10602篇 |
出版年
2024年 | 76篇 |
2023年 | 253篇 |
2022年 | 488篇 |
2021年 | 593篇 |
2020年 | 755篇 |
2019年 | 701篇 |
2018年 | 701篇 |
2017年 | 789篇 |
2016年 | 941篇 |
2015年 | 836篇 |
2014年 | 1211篇 |
2013年 | 2025篇 |
2012年 | 1305篇 |
2011年 | 1549篇 |
2010年 | 1216篇 |
2009年 | 1579篇 |
2008年 | 1624篇 |
2007年 | 1674篇 |
2006年 | 1397篇 |
2005年 | 1185篇 |
2004年 | 961篇 |
2003年 | 979篇 |
2002年 | 911篇 |
2001年 | 721篇 |
2000年 | 736篇 |
1999年 | 619篇 |
1998年 | 563篇 |
1997年 | 429篇 |
1996年 | 294篇 |
1995年 | 285篇 |
1994年 | 237篇 |
1993年 | 238篇 |
1992年 | 223篇 |
1991年 | 182篇 |
1990年 | 200篇 |
1989年 | 189篇 |
1988年 | 167篇 |
1987年 | 166篇 |
1986年 | 138篇 |
1985年 | 140篇 |
1984年 | 140篇 |
1983年 | 80篇 |
1982年 | 112篇 |
1981年 | 106篇 |
1980年 | 94篇 |
1979年 | 99篇 |
1978年 | 77篇 |
1977年 | 76篇 |
1976年 | 70篇 |
1973年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 122 毫秒
961.
The Fe3Al alloy with D03 structure exhibits large recoverable strains due to reversible slips. Tension and compression experiments were conducted on single crystals of Fe3Al, and the onset of slip in forward and reverse directions were obtained utilizing high-resolution digital image correlation technique. The back stress provides the driving force for reversal of deformation upon unloading, resulting in a superelastic phenomenon as in shape memory alloys. Using density functional theory simulations, we obtain the energy barriers (GSFE – generalized stacking fault energy) for {1?1?0}〈1?1?1〉 and {1?1?2}〈1?1?1〉 slips in D03 Fe3Al and the elastic moduli tensor, and undertake anisotropic continuum calculations to obtain the back stress and the frictional stress responsible for reversible slip. We compare the theoretically obtained slip stress magnitudes (friction and back stress) with the experimental measurements disclosing excellent agreement. 相似文献
962.
For the specific slip geometry of two sets of coplanar systems (a total of four systems) in fcc metals, the range of dislocation networks in boundaries aligned with one of the two active slip planes is predicted from the Frank equation for boundaries free of long-range elastic stresses. Detailed comparison with experimental data for eight dislocation boundaries in cold-rolled aluminium grains of the 45° ND rotated Cube orientation is conducted. It is concluded that the boundaries are Low-Energy Dislocation Structures, which are in good agreement with the Frank equation while also lowering the energy by dislocation reactions. Cross slip plays a role in the boundary formation process. 相似文献
963.
The behaviour of a dislocation pileup with a finite-strength source is investigated in the presence of various stress gradients within a continuum model where a free-dislocation region exists around the source. Expressions for dislocation density and stress field within the pileup are derived for the situation where there are first and second spatial gradients in applied stress. For a pileup configuration under an applied stress, yielding occurs when the force acting on the leading dislocations at the pileup tips reaches the obstacle strength, and at the same time, it is required that the source be at the threshold stress for dislocation production. A numerical methodology is presented to solve the underlying equations that represent the yielding conditions. The yield stress calculated for a pileup configuration is found to depend on stress gradients, obstacle spacing and source/obstacle strengths. It increases with increasing the first stress gradient, yet dependent on the second stress gradient. Furthermore, while the dependency of yield stress on the obstacle spacing intensifies with increasing the first stress gradient, it diminishes with an increase of second stress gradient. Therefore, the second stress gradient, as a newly introduced parameter, can provide a new physical insight into the size-dependent plasticity phenomena at small length scales. 相似文献
964.
The phase transitions, lattice dynamical and thermodynamic properties of BeS, BsSe and BeTe at high pressure have been investigated with the density functional theory. The calculated equilibrium structural parameters agree well with the available experimental and theoretical values. The phase transition pressures from the zinc-blende (ZB) to the nickel arsenide (NiAs) phase of these compounds are determined. The calculated phonon dispersion curves of these compounds in ZB phase at zero pressure do not show any anomaly or instability. Dynamically, the ZB phase of BeS, BeSe and BeTe is found to be stable near transition pressures PT. Within the quasiharmonic approximation, the thermodynamic properties including the thermal expansion coefficient, heat capacity at constant volume, heat capacity at constant pressure and entropy are predicted. 相似文献
965.
Responsive mechanism of 2-(2'-hydroxyphenyl)benzoxazole-based two-photon fluorescent probes for zinc and hydroxide ions
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Response theory is used to investigate one- and two-photon absorption(TPA) as well as the emission properties of a series of potential zinc ion and pH sensitive materials containing 2-(2’-hydroxyphenyl)benzoxazole(HPBO) end groups.Special emphasis is placed on the evolution of their optical properties upon combining with zinc ions or deprotonation.Our calculated results indicate that upon combining with zinc ions or deprotonation,these HPBO derivatives show drastic changes in their one-photon absorption(OPA),emission,and TPA properties.Moreover,the mechanisms of the probes are analyzed and found to be an intramolecular charge transfer.These compounds are thus proved to be excellent candidates for two-photon fluorescent zinc and pH probes. 相似文献
966.
《Current Applied Physics》2015,15(10):1256-1261
P-type conductivity in MOCVD grown ZnO was obtained by directional thermal diffusion of arsenic from semi-insulating GaAs substrate. The films were single crystalline in nature and oriented along (002) direction. Ab initio calculations in the framework of density functional theory have been carried out with different chemical states of arsenic in ZnO. Present calculations suggested AsZn–2VZn defect is a shallow acceptor and results in ferromagnetism in ZnO. The magnetic measurements of the samples indeed showed ferromagnetic ordering at room temperature. X-ray photoelectron spectra confirmed the presence of AsZn and VZn. The core level chemical shift in binding energy of AsZn indicated the formation of AsZn–2VZn. Diffused arsenic substitutes zinc atom and creates additional zinc vacancies. The zinc vacancies, surrounding the oxygen atoms, result in unpaired O 2p electrons which in turn induce ferromagnetism in the samples. 相似文献
967.
968.
采用密度泛函理论计算方法模拟了简单钴卟啉过氧中间体PCo-O2与环己烷C6H12的作用,分析了反应路径中各驻点能量和反应过渡态分子构型。研究结果表明,PCo-O2向底物环己烷夺氢的反应可以延正方向进行,二线态PCo-O2更具反应活性,反应过程中Co-O键得到加强,O-O键被削弱。依据理论计算结果,探讨了四苯基钴卟啉催化环己烷氧化生成环己醇和环己酮的反应机理,指出反应延Lyons高价金属氧代物机理生成环己醇,而反应循环中产生的烷基自由基可以延烷基过氧化过渡金属配合物反应机理进行生成环己酮。 相似文献
969.
The structural, elastic, mechanical, electronic, optical properties and effective masses of CuMIIIBO2 (MIIIB = Sc, Y, La) compounds have been investigated by the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory under local density approximation. The equilibrium structural parameters are in good agreement with previous experimental and theoretical data. To our knowledge, there are no available data of elastic constants for comparison. The bulk, shear and Young's modulus, ratio of B/G, Poisson's ratio and Lamé's constants of CuMIIIBO2 have been studied. The electronic structures of CuMIIIBO2 are consistent with other calculations. The population analysis, charge densities and effective masses have been shown and analyzed. The imaginary and real parts of the dielectric function, refractive index and extinction coefficient of CuMIIIBO2 are calculated. The interband transitions to absorption of CuMIIIBO2 have been analyzed. 相似文献
970.
Two States Are Not Enough: Quantitative Evaluation of the Valence‐Bond Intramolecular Charge‐Transfer Model and Its Use in Predicting Bond Length Alternation Effects
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Peter D. Jarowski Prof. Yirong Mo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(51):17214-17221
The structural weights of the canonical resonance contributors used in the Two‐state valence‐bond charge‐transfer model, neutral (N, R1) and ionic (VB‐CT, R2), to the ground states and excited states of a series of linear dipolar intramolecular charge‐transfer chromophores containing a buta‐1,3‐dien‐1,4‐diyl bridge have been computed by using the block‐localized wavefunction (BLW) method at the B3LYP/6‐311+G(d) level to provide the first quantitative assessment of this simple model. Ground‐ and excited‐state analysis reveals surprisingly low ground‐state structural weights for the VB‐CT resonance form using either this Two‐state model or an expanded Ten‐state model. The VB‐CT state is found to be more prominent in the excited state. Individual resonance forms were structurally optimized to understand the origins of the bond length alternation (BLA) of the bridging unit. Using a Wheland energy‐based weighting scheme, the weighted average of the optimized bond lengths with the Two‐state model was unable to reproduce the BLA features with values 0.04 to 0.02 Å too large compared to the fully delocalized (FD) structure (BLW: ca. ?0.13 to ?0.07 Å, FD: ca. ?0.09 to ?0.05 Å). Instead, an expanded Ten‐state model fit the BLA values of the FD structure to within only 0.001 Å of FD. 相似文献