全文获取类型
收费全文 | 1329篇 |
免费 | 44篇 |
国内免费 | 33篇 |
专业分类
化学 | 146篇 |
晶体学 | 5篇 |
力学 | 123篇 |
综合类 | 12篇 |
数学 | 648篇 |
物理学 | 472篇 |
出版年
2023年 | 39篇 |
2022年 | 18篇 |
2021年 | 13篇 |
2020年 | 16篇 |
2019年 | 12篇 |
2018年 | 15篇 |
2017年 | 42篇 |
2016年 | 49篇 |
2015年 | 47篇 |
2014年 | 73篇 |
2013年 | 97篇 |
2012年 | 88篇 |
2011年 | 63篇 |
2010年 | 53篇 |
2009年 | 106篇 |
2008年 | 69篇 |
2007年 | 81篇 |
2006年 | 76篇 |
2005年 | 52篇 |
2004年 | 50篇 |
2003年 | 34篇 |
2002年 | 37篇 |
2001年 | 41篇 |
2000年 | 29篇 |
1999年 | 23篇 |
1998年 | 16篇 |
1997年 | 25篇 |
1996年 | 26篇 |
1995年 | 14篇 |
1994年 | 8篇 |
1993年 | 14篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1978年 | 4篇 |
1977年 | 1篇 |
1976年 | 6篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有1406条查询结果,搜索用时 468 毫秒
51.
Extracellular biosynthesis of monodispersed gold nanoparticles by a SAM capping route 总被引:1,自引:0,他引:1
Li Wen Zhonghua Lin Pingying Gu Jianzhang Zhou Bingxing Yao Guoliang Chen Jinkun Fu 《Journal of nanoparticle research》2009,11(2):279-288
Monodispersed gold nanoparticles capped with a self-assembled monolayer of dodecanethiol were biosynthesized extracellularly
by an efficient, simple, and environmental friendly procedure, which involved the use of Bacillus megatherium D01 as the reducing agent and the use of dodecanethiol as the capping ligand at 26 °C. The kinetics of gold nanoparticle
formation was followed by transmission electron microscope (TEM) and UV-vis spectroscopy. It was shown that reaction time
was an important parameter in controlling the morphology of gold nanoparticles. The effect of thiol on the shape, size, and
dispersity of gold nanoparticles was also studied. The results showed that the presence of thiol during the biosynthesis could
induce the formation of small size gold nanoparticles (<2.5 nm), hold the shape of spherical nanoparticles, and promote the
monodispersity of nanoparticles. Through the modulation of reaction time and the use of thiol, monodispersed spherical gold
nanoparticles capped with thiol of 1.9 ± 0.8 nm size were formed by using Bacillus megatherium D01. 相似文献
52.
为了抑制分布式光纤拉曼温度系统的温漂和瑞利散射光窜扰反斯托克斯散射光,在传感光纤前端盲区后放置光纤取样环,用瑞利散射光解调反斯托克斯散射光及用光纤取样环的温度计算光纤线上其它点的温度,提高了系统的测温精度和稳定性。采用功率100 mW,波长1.55 μm,脉宽10 ns脉冲激光器和15 dB前置光纤放大器,100M14bitA/D转换卡及DSP作数字平均构建光纤拉曼温度系统的实验,实现了测温误差在±0.03 ℃内。 相似文献
53.
Under small-signal assumption, equivalent noise figure normalized by fiber nonlinearities (NENF) of bi-directionally pumped distributed fiber Raman amplifiers (BiDFRA) is derived. Amplified spontaneous Raman scattering noise and double Rayleigh scattering noise are both included. The relation between NENF and Raman gain, forward Raman gain percentage is investigated under different polarization factor and Rayleigh scattering coefficient. It agrees well with numerical simulation results. Based on the analytical expression, the optical signal-to-nose ratio (OSNR) improvement of hybrid pre-BiDFRA/erbium-doped fiber amplifier (EDFA) compared with sole EDFA is studied. It is shown that there is an optimum Raman gain and forward Raman gain percentage to maximize the OSNR improvement. Finally, some guidelines on BiDFRA design are proposed. 相似文献
54.
In this paper the calculation of resonance frequencies and threshold gains of distributed feedback (DFB) lasers with strong modulations is discussed. The theory is based on the Floquet solutions of a complex Hill differential equation. The dispersion relation of the above solutions is shown to be of importance for the behaviour of the resonances of the DFB laser. 相似文献
55.
本文给出了掺CdSxSe1-x玻璃平面波导分布反馈(DFB)光学双稳器件的设计和制备方法,实现了低功耗、快速的本征光学双稳。 相似文献
56.
We study a family of control problems that arise in the design of dynamically loaded bearings which have a minimum power loss; in particular, we study the design of piston rings with minimum parasitic power loss.This research was partially supported by NSF Grant No. MCS-77-05624. 相似文献
57.
Modern lubricants often exhibit shear-thinning due to the presence of high molecular weight polymers as additives. Therefore the influence of such non-Newtonian effects on the performances of lubricating systems must be predicted. The corresponding fluid film flow is governed by a non-linear partial differential equation, which generalizes the classical Reynolds equation. Having prescribed adequate boundary conditions, this equation is solved by a finite element method with optimal control. The problem of the square slider bearing lubricated by the Rabinowitsch fluid is solved in order to test the accuracy of the numerical scheme. The pressure and velocity fields are given and compared with the corresponding ones obtained for the Newtonian fluid. 相似文献
58.
High-performance InGaAs/InGaAlAs multiple-quantum-well vertical-cavity surface-emitting lasers (VCSELs) with InGaAlAs/InP distributed Bragg reflectors are proposed for operation at the wavelength of . The lasers have good heat diffusion characteristic, large index contrast in DBRs, and weak temperature sensitivity. They could be fabricated either by metal-organic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE) growth. The laser light-current characteristics indicate that a suitable reflectivity of the DBR on the light output side in a laser makes its output power increase greatly and its lasing threshold current reduce significantly, and that a small VCSEL could output the power around its maximum for the output mirror at the reflectivity varying in a broader range than a large VCSEL does. 相似文献
59.
Long-duration experiments with clouds of microparticles are planned for the ICAPS facility on board the International Space Station ISS. The scientific objectives of such experiments are widespread and are ranging from the simulation of aerosol behaviour in Earths atmosphere to the formation of planets in the early solar system. It is, however, even under microgravity conditions, impossible to sustain a cloud of free-floating, microscopic particles for an extended period of time, due to thermal diffusion and due to unavoidable external accelerations. Therefore, a trap for dust clouds is required which prevents the diffusion of the particles, which provides a source of relative velocities between the dust grains and which can also concentrate the dust to higher number densities that are otherwise not achievable. We are planning to use the photophoretic effect for such a particle trap. First short-duration microgravity experiments on the photophoretic motion of microscopic particles show that such an optical particle-cloud trap is feasible. First tests of a two-dimensional trap were performed in the Bremen drop tower. 相似文献
60.
Counterexamples are constructed for some plausible conjectures.
Typical of these: as the Maximum Principle ensures that positive boundary data
give a positive state at time T from 0 initial data, one might
(plausibly, but falsely) conjecture that all positive terminal states
should be approximately reachable in this way, i.e., subject to the
requirement that the boundary data stays nonnegative. 相似文献