全文获取类型
收费全文 | 401686篇 |
免费 | 4055篇 |
国内免费 | 1434篇 |
专业分类
化学 | 219964篇 |
晶体学 | 6408篇 |
力学 | 16609篇 |
综合类 | 6篇 |
数学 | 43369篇 |
物理学 | 120819篇 |
出版年
2018年 | 3519篇 |
2017年 | 3406篇 |
2016年 | 5670篇 |
2015年 | 3990篇 |
2014年 | 5871篇 |
2013年 | 17621篇 |
2012年 | 12322篇 |
2011年 | 15341篇 |
2010年 | 10165篇 |
2009年 | 10075篇 |
2008年 | 13807篇 |
2007年 | 14000篇 |
2006年 | 13042篇 |
2005年 | 12096篇 |
2004年 | 10950篇 |
2003年 | 9773篇 |
2002年 | 9575篇 |
2001年 | 11714篇 |
2000年 | 8841篇 |
1999年 | 7006篇 |
1998年 | 5695篇 |
1997年 | 5738篇 |
1996年 | 5483篇 |
1995年 | 5116篇 |
1994年 | 4896篇 |
1993年 | 4855篇 |
1992年 | 5566篇 |
1991年 | 5415篇 |
1990年 | 5199篇 |
1989年 | 5224篇 |
1988年 | 5227篇 |
1987年 | 5140篇 |
1986年 | 4832篇 |
1985年 | 6603篇 |
1984年 | 6798篇 |
1983年 | 5574篇 |
1982年 | 6056篇 |
1981年 | 5905篇 |
1980年 | 5692篇 |
1979年 | 5881篇 |
1978年 | 6281篇 |
1977年 | 6023篇 |
1976年 | 6137篇 |
1975年 | 5624篇 |
1974年 | 5651篇 |
1973年 | 5956篇 |
1972年 | 3790篇 |
1971年 | 3030篇 |
1968年 | 3266篇 |
1967年 | 3223篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
881.
The highest resolution in a lithographic process is often determined by the properties of the resist material. With the currently used polymeric resists, a resolution of better than 100 nm has been achieved under manufacturing conditions, but the future nanoscale devices will require a 10 times superior resolution. In this paper we present an overview of the resist materials, especially with regard to limiting resolution. In principle, inorganic resists should have higher limiting resolution than polymer resists due to smaller fundamental structural units and stronger bonds in the former. However, compositional and/or structural inhomogeneities may limit their ultimate resolution. New results are presented that indicate chalcogenide glasses as promising photo and electron beam resists, which also have the advantages of greater hardness, resistance to acids, easy fabrication in thin film form, and the unique phenomena like radiation enhanced diffusion. 相似文献
882.
A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl2F2 as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl2F2/SiCl4 ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si–O–Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum ultra-violet (VUV) regions. The maximum refractive index depression of ?0.5 × 10?3 is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl2F2 as a dopant precursor with suitable flow of SiCl4 vapor along with O2 through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70–1.79 mol% fluorine incorporated by the MCVD process based on the analyses of 19F MAS spectra done by high-resolution 19F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO1.5F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO2F polyhedral which become increased with increasing the fluorine content. 相似文献
883.
884.
Bikshandarkoil R. Srinivasan 《Journal of Chemical Sciences》2004,116(5):251-259
The complexes [MoX4]2− (M = Mo; X = O or S) exist as the monomeric tetrahedral species in aqueous alkaline solutions. Acidification of tetraoxomolybdate
results in the condensation of the tetrahedral units via a series of polyoxomolybdates leading to the ultimate formation of
the trioxide MoO3. Heptamolybdate [Mo7O24]6− is the first major polyanion of the acidification reaction. In contrast, acidification of tetrathiomolybdates leads to the
formation of amorphous molybdenum trisulphide via a dinuclear Mo(V) complex. The formation of the dinuclear Mo(V) complex
precludes the formation of any higher nuclearity Mo(VI)-S complexes in aqueous solution. Thus it is shown that the all-sulphur
analogue of heptamolybdate [M07S24]6− does not exist in alkaline medium and also cannot be isolated from aqueous acidic medium 相似文献
885.
A new co‐ordinate invariant streamwise upwind formulation for convection dominated flows is developed. The eddy diffusivity/viscosity is added directly to the equations in order to remove the oscillations in the solution. The equations then can be solved by any high‐order scheme and the solution retains the accuracy of the high‐order scheme. The accuracy and reduced lateral thickness growth rate are demonstrated with several numerical examples, including pure convective flows and lid‐driven cavity flow. The lateral spreading due to the numerical diffusion is controlled by the anisotropic tensor. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
886.
In order to study cross flow induced vibration of heat exchanger tube bundles, a new fluid–structure interaction model based on surface vorticity method is proposed. With this model, the vibration of a flexible cylinder is simulated at Re=2.67 × 104, the computational results of the cylinder response, the fluid force, the vibration frequency, and the vorticity map are presented. The numerical results reproduce the amplitude‐limiting and non‐linear (lock‐in) characteristics of flow‐induced vibration. The maximum vibration amplitude as well as its corresponding lock‐in frequency is in good agreement with experimental results. The amplitude of vibration can be as high as 0.88D for the case investigated. As vibration amplitude increases, the amplitude of the lift force also increases. With enhancement of vibration amplitude, the vortex pattern in the near wake changes significantly. This fluid–structure interaction model is further applied to simulate flow‐induced vibration of two tandem cylinders and two side‐by‐side cylinders at similar Reynolds number. Promising and reasonable results and predictions are obtained. It is hopeful that with this relatively simple and computer time saving method, flow induced vibration of a large number of flexible tube bundles can be successfully simulated. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
887.
This paper attempts to reproduce numerically previous experimental findings with opposed flows and extends their range to quantify the effects of upstream pipes and nozzles with inviscid, laminar and turbulent flows. The choice of conservation equations, boundary conditions, algorithms for their solution, the degree of grid dependence, numerical diffusion and the validity of numerical approximations are justified with supporting calculations where necessary. The results of all calculations on the stagnation plane show maximum strain rates close to the annular exit from the nozzles and pipes for lower separations and it can be expected that corresponding reacting flows will tend to extinguish in this region with the extinction moving towards the axis. With laminar flows, the maximum strain rate increased with Reynolds number and the maximum values were generally greater than with inviscid flows and smaller than with turbulent flows. With large separations, the strain rates varied less and this explains some results with reacting flows where the extinction appeared to begin on the axis. The turbulent‐flow calculations allowed comparison of three common variants of a two‐equation first‐moment closure. They provided reasonable and useful indications of strain rates but none correctly represented the rms of velocity fluctuations on the axis and close to the stagnation plane. As expected, those designed to deal with this problem produced results in better agreement with experiment but were still imperfect. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
888.
Model based online diagnosis of unbalance and transverse fatigue crack in rotor systems 总被引:7,自引:0,他引:7
A model based technique for online identification of malfunctions in rotor systems is discussed. Presence of fault changes the dynamic behavior of the system. This change is taken into account by equivalent loads acting on the undamaged system model. Equivalent loads are fictitious forces and moments acting on the undamaged system model, which generate a dynamic behavior identical to that of the real damaged system. The mathematical representation of equivalent loads is referred to as Fault Model. The work focuses on developing a fault model for a transverse fatigue crack in shaft and testing it through simulated studies. The basic principle of the technique is validated for unbalance identification, through numerical simulations as well as by experiments on a real rotor system. 相似文献
889.
S. Han H. Kühn F. W. Hsing K. Münnemann B. Blümich S. Appelt 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,167(2):187
We have visualized the melting and dissolution processes of xenon (Xe) ice into different solvents using the methods of nuclear magnetic resonance (NMR) spectroscopy, imaging, and time resolved spectroscopic imaging by means of hyperpolarized 129Xe. Starting from the initial condition of a hyperpolarized solid Xe layer frozen on top of an ethanol (ethanol/water) ice block we measured the Xe phase transitions as a function of time and temperature. In the pure ethanol sample, pieces of Xe ice first fall through the viscous ethanol to the bottom of the sample tube and then form a thin layer of liquid Xe/ethanol. The xenon atoms are trapped in this liquid layer up to room temperature and keep their magnetization over a time period of 11 min. In the ethanol/water mixture (80 vol%/20%), most of the polarized Xe liquid first stays on top of the ethanol/water ice block and then starts to penetrate into the pores and cracks of the ethanol/water ice block. In the final stage, nearly all the Xe polarization is in the gas phase above the liquid and trapped inside the pores. NMR spectra of homogeneous samples of pure ethanol containing thermally polarized Xe and the spectroscopic images of the melting process show that very high concentrations of hyperpolarized Xe (about half of the density of liquid Xe) can be stored or delivered in pure ethanol. 相似文献
890.
Winoto S. H. 《显形杂志》2004,7(3):257-262
The 7th Asian Symposium on Visualization (7ASV) was successfully held in Singapore from 3rd to 7th November 2003. This event was originally scheduled from 26th to 30th May 2003, but had to be postponed because of the outbreak of SARS (Severe Acute Respiratory Syndrome) in some parts of the
world. A total number of 122 participants attended the Symposium of which, 107 were foreign participants from 16 countries
(excluding Singapore) worldwide. There were 104 papers covering broad range of topics presented at the Symposium which were
delivered in 2×7 long sessions, and 9 keynote papers. 相似文献