全文获取类型
收费全文 | 1772篇 |
免费 | 484篇 |
国内免费 | 229篇 |
专业分类
化学 | 233篇 |
晶体学 | 11篇 |
力学 | 347篇 |
综合类 | 24篇 |
数学 | 607篇 |
物理学 | 1263篇 |
出版年
2024年 | 19篇 |
2023年 | 32篇 |
2022年 | 70篇 |
2021年 | 52篇 |
2020年 | 51篇 |
2019年 | 59篇 |
2018年 | 68篇 |
2017年 | 78篇 |
2016年 | 104篇 |
2015年 | 74篇 |
2014年 | 90篇 |
2013年 | 161篇 |
2012年 | 126篇 |
2011年 | 150篇 |
2010年 | 95篇 |
2009年 | 117篇 |
2008年 | 97篇 |
2007年 | 123篇 |
2006年 | 91篇 |
2005年 | 78篇 |
2004年 | 61篇 |
2003年 | 68篇 |
2002年 | 78篇 |
2001年 | 77篇 |
2000年 | 64篇 |
1999年 | 64篇 |
1998年 | 39篇 |
1997年 | 47篇 |
1996年 | 25篇 |
1995年 | 26篇 |
1994年 | 26篇 |
1993年 | 31篇 |
1992年 | 22篇 |
1991年 | 20篇 |
1990年 | 13篇 |
1989年 | 8篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1973年 | 3篇 |
1957年 | 2篇 |
排序方式: 共有2485条查询结果,搜索用时 15 毫秒
31.
Qian‐Lin Tang 《International journal of quantum chemistry》2013,113(16):1992-2001
Practical copper (Cu)‐based catalysts for the water–gas shift (WGS) reaction was long believed to expose a large proportion of Cu(110) planes. In this work, as an important first step toward addressing sulfur poisoning of these catalysts, the detailed mechanism for the splitting of hydrogen sulfide (H2S) on the open Cu(110) facet has been investigated in the framework of periodic, self‐consistent density functional theory (DFT‐GGA). The microkinetic model based on the first‐principles calculations has also been developed to quantitatively evaluate the two considered decomposition routes for yielding surface atomic sulfur (S*): (1) H2S → H2S* → SH* → S* and (2) 2H2S → 2H2S* → 2SH* → S* + H2S* → S* + H2S. The first pathway proceeding through unimolecular SH* dissociation was identified to be feasible, whereas the second pathway involving bimolecular SH* disproportionation made no contribution to S* formation. The molecular adsorption of H2S is the slowest elementary step of its full decomposition, being related with the large entropy term of the gas‐phase reactant under realistic reaction conditions. A comparison of thermodynamic and kinetic reactivity between the substrate and the close‐packed Cu(111) surface further shows that a loosely packed facet can promote the S* formation from H2S on Cu, thus revealing that the reaction process is structure sensitive. The present DFT and microkinetic modeling results provide a reasonably complete picture for the chemistry of H2S on the Cu(110) surface, which is a necessary basis for the design of new sulfur‐tolerant WGS catalysts. © 2013 Wiley Periodicals, Inc. 相似文献
32.
A compact measurement system based on a novel combination of cantilever enhanced photoacoustic spectroscopy (CEPAS) and optical parametric oscillator (OPO) was applied to the gas phase measurement of benzene, toluene, and o-, m- and p-xylene (BTX) traces. The OPO had a band width (FWHM) of 1.3 nm, was tuned from 3237 to 3296 nm in steps of 0.1 nm and so spectra of BTX at different concentrations were recorded. The power emitted by the OPO increased from 88 mW at 3237 nm to 103 mW at 3296 nm. The univariate detection limits (3σ, 0.951 s) for benzene, toluene, p-, m- and o-xylene at 3288 nm were 12.0, 9.8, 13.2, 10.1 and 16.0 ppb, respectively. Multivariate data analysis using science-based calibration was used to resolve the interference of the analytes. The multivariate detection limits (3σ, 3237–3296 nm, 591 spectral points each 0.951 s) for benzene, toluene, p-, m- and o-xylene in the multi-compound sample, where all other analytes and water interfere were 4.3, 7.4, 11.0, 12.5 and 6.2 ppb, respectively. Without interferents, the multivariate detection limits varied between 0.5 and 0.6 ppb. The sum of the cross-selectivities (3237–3296 nm, 591 spectral points, each 0.951 s) per analyte were below 0.05 ppb/ppb, with an average of 0.038 ppb/ppb. The cross-selectivity of water to the analytes was on average 1.22 × 10−4 ppb/ppb. The OPO is small in size (L × W × H 125 × 70 × 45 mm), commercially available, and easy to operate and integrate to setups. The combination with sensitive CEPAS enables compact measurement systems for industrial as well as environmental trace gas monitoring. 相似文献
33.
Michel E. Marhic Peter A. Andrekson Periklis Petropoulos Stojan Radic Christophe Peucheret Mahmoud Jazayerifar 《Laser \u0026amp; Photonics Reviews》2015,9(1):50-74
The prospects for using fiber optical parametric amplifiers (OPAs) in optical communication systems are reviewed. Phase‐insensitive amplifiers (PIAs) and phase‐sensitive amplifiers (PSAs) are considered. Low‐penalty amplification at/or near 1 Tb/s has been achieved, for both wavelength‐ and time‐division multiplexed formats. High‐quality mid‐span spectral inversion has been demonstrated at 0.64 Tb/s, avoiding electronic dispersion compensation. All‐optical amplitude regeneration of amplitude‐modulated signals has been performed, while PSAs have been used to demonstrate phase regeneration of phase‐modulated signals. A PSA with 1.1‐dB noise figure has been demonstrated, and preliminary wavelength‐division multiplexing experiments have been performed with PSAs. 512 Gb/s have been transmitted over 6,000 km by periodic phase conjugation. Simulations indicate that PIAs could reach data rate x reach products in excess of 14,000 Tb/s × km in realistic wavelength‐division multiplexed long‐haul networks. Technical challenges remaining to be addressed in order for fiber OPAs to become useful for long‐haul communication networks are discussed.
34.
Trends in ultrashort and ultrahigh power laser pulses based on optical parametric chirped pulse amplification 下载免费PDF全文
Since the proof-of-principle demonstration of optical parametric amplification to efficiently amplify chirped laser pulses in 1992,optical parametric chirped pulse amplification(OPCPA)became the most promising method for the amplification of broadband optical pulses.In the meantime,we are witnessing an exciting progress in the development of powerful and ultrashort pulse laser systems that employ chirped pulse parametric amplifiers.The output power and pulse duration of these systems have ranged from a few gigawatts to hundreds of terawatts with a potential of tens of petawatts power level.Meanwhile,the output pulse duration based on optical parametric amplification has entered the range of fewoptical-cycle field.In this paper,we overview the basic principles,trends in development,and current state of the ultrashort and laser systems based on OPCPA,respectively. 相似文献
35.
Pressure-induced insulator-to-metal transition (IMT) has been studied in the van der Waals compound iron thiophosphate (FePS3) using first-principles calculations within the periodic linear combination of atomic orbitals method with hybrid Hartree–Fock-DFT B3LYP functional. Our calculations reproduce correctly the IMT at ∼15 GPa, which is accompanied by a reduction of the unit cell volume and of the vdW gap. We found from the detailed analysis of the projected density of states that the 3p states of phosphorus atoms contribute significantly at the bottom of the conduction band. As a result, the collapse of the band gap occurs due to changes in the electronic structure of FePS3 induced by relative displacements of phosphorus or sulfur atoms along the c-axis direction under pressure. 相似文献
36.
采用基于密度泛函理论的平面波超软赝势方法对本征Zn2GeO4,Mn2+掺杂Zn2GeO4,Mn2+/N2-共掺杂Zn2GeO4超晶胞进行了几何结构优化,计算了掺杂前后体系的晶格常数、能带结构、态密度和光学性质。结果表明,Mn离子掺入后,Mn离子3d轨道与O离子2p轨道之间有强烈的轨道杂化效应,掺杂系统不稳定,而Mn/N离子共掺后,Mn离子和N离子之间的吸引作用克服了Mn离子之间的排斥作用,能够明显地提高掺杂浓度和体系的稳定性。光学性质计算结果表明,Mn离子与N离子共掺杂能改善Zn2GeO4电子在低能区的光学跃迁特性,增强电子在可见光区的光学跃迁;吸收谱计算结果显示,Mn离子与N离子掺入后体系对低频电磁波吸收增加。 相似文献
37.
38.
ABSTRACTThe iridium is an important metal which has excellent resistance to corrosion at high temperature. L12 intermetallic compounds i.e. Ir3Nb and Ir3Zr, with similar lattice parameters are ideal for working at high temperature. They are fully soluble due to their low lattice misfit. A first-principle investigation into the effect of doping Zr with different concentrations on the electronic structure, mechanical and thermodynamic properties of NbIr3 has been studied to prompt the development of novel high-temperature materials. Nine ZrxNb8?xIr24 compounds are carefully considered. The results show that adding Zr into these compounds can strengthen their structural stability and ductility. Nevertheless, it reduces the elastic modulus and elastic stiffness. Simultaneously, with the increase of Zr content, the thermodynamic properties of these compounds decrease. It is also found that the changes of elastic modulus are mainly attributed to the variations of bonds in these compounds. 相似文献
39.
ZENG ZhenHua MA XiuFang DING WuChen & LI WeiXue State Key Laboratory of Catalysis Center for Theoretical Computational Chemistry Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian China 《中国科学:化学》2010,(2)
Combined with third generation synchrotron radiation light sources, X-ray photoelectron spectroscopy (XPS) with higher energy resolution, brilliance, enhanced surface sensitivity and photoemission cross section in real time found extensive applications in solid-gas interface chemistry. This paper reports the calculation of the core-level binding energy shifts (CLS) using the first-principles density functional theory. The interplay between the CLS calculations and XPS measurements to uncover the structures,... 相似文献
40.
Keith W. Buffinton 《Nonlinear dynamics》1990,1(5):359-377
Studied in this work are the formulation of equations of motion and the response to parametric excitation of a uniform cantilever beam moving longitudinally over a single bilateral support. The equations of motion are generated by using assumed modes to discretize the beam, by regarding the support as a kinematic constraint, and by employing an alternate form of Kane's method that is particularly well suited to systems subject to constraints. Instability information is developed using the results of perturbation analysis for harmonic longitudinal motions of small amplitude and with Floquet theory for general periodic motions of any amplitude. Results demonstrate that definitive instability information can be obtained for a beam moving longitudinally over supports based on the frequencies of free transverse vibration of a beam that is longitudinally fixed. 相似文献