全文获取类型
收费全文 | 203853篇 |
免费 | 1962篇 |
国内免费 | 591篇 |
专业分类
化学 | 102305篇 |
晶体学 | 3340篇 |
力学 | 9287篇 |
综合类 | 5篇 |
数学 | 22470篇 |
物理学 | 68999篇 |
出版年
2021年 | 1652篇 |
2020年 | 1797篇 |
2019年 | 1998篇 |
2018年 | 2699篇 |
2017年 | 2601篇 |
2016年 | 3765篇 |
2015年 | 2304篇 |
2014年 | 3613篇 |
2013年 | 9084篇 |
2012年 | 7240篇 |
2011年 | 8734篇 |
2010年 | 6040篇 |
2009年 | 5909篇 |
2008年 | 7911篇 |
2007年 | 8079篇 |
2006年 | 7722篇 |
2005年 | 6890篇 |
2004年 | 6424篇 |
2003年 | 5650篇 |
2002年 | 5491篇 |
2001年 | 6424篇 |
2000年 | 4931篇 |
1999年 | 3794篇 |
1998年 | 3038篇 |
1997年 | 3045篇 |
1996年 | 2791篇 |
1995年 | 2613篇 |
1994年 | 2462篇 |
1993年 | 2337篇 |
1992年 | 2691篇 |
1991年 | 2671篇 |
1990年 | 2600篇 |
1989年 | 2396篇 |
1988年 | 2449篇 |
1987年 | 2448篇 |
1986年 | 2279篇 |
1985年 | 2960篇 |
1984年 | 3215篇 |
1983年 | 2631篇 |
1982年 | 2801篇 |
1981年 | 2771篇 |
1980年 | 2637篇 |
1979年 | 2727篇 |
1978年 | 2821篇 |
1977年 | 2819篇 |
1976年 | 2886篇 |
1975年 | 2670篇 |
1974年 | 2644篇 |
1973年 | 2776篇 |
1972年 | 1763篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
192.
Vijaya Gracias Alan F. Gasiecki Joel D. Moore Irini Akritopoulou-Zanze Stevan W. Djuric 《Tetrahedron letters》2006,47(50):8977-8980
A general method for the preparation of diaza-spirocycles is reported. This method used an olefin metathesis in order to construct the desired spirocyclic framework. Beginning with commercially available protected amino ketones, this strategy ultimately produced pharmacologically relevant diaza-scaffolds in an efficient and high-yielding process. 相似文献
193.
D. C. Oliveira Y. Messaddeq K. Dahmouche S. J. L. Ribeiro R. R. Gonçalves A. Vesperini D. Gindre J.-M. Nunzi 《Journal of Sol-Gel Science and Technology》2006,40(2-3):359-363
Organic-inorganic hybrid materials were prepared from an ureasil precursor (ureapropyltriethoxysilane designated as UPTES)
and acrylic acid modified zirconium (IV) n-propoxide. Thin films containing rhodamine 6G (Rh6G) were prepared by spin-coating on glass substrates with different Zr:Si molar ratios (Zr:Si = 75:25, 50:50 and 25:75). Refractive
index, thickness, number of propagating modes and attenuation coefficient were measured at 543.5, 632.8 and 1550 nm wavelengths
by the prism coupling technique. Distributed feedback (DFB) laser effect was observed and studied as a function of films thickness
and refractive index. 相似文献
194.
High temperature oxidation of metals leads to residual stresses both in the metal and in the growing oxide. In this work, the evolution of this residual stresses is theoretically predicted in the growing oxide layers. The origin of these stresses is based on a microstructural model. Using experimental results providing from the oxidation kinetics, and an analysis proposed to describe the growth strain occurring in the thin layers, a set of equations is established allowing determining the stresses evolution with oxidation time. Then, the model is compared with experimental results obtained on both α-Fe and phosphated α-Fe, oxidised at different temperatures. Numerical data are extracted from experiments either with an asymptotic formulation or with an inverse method. These two methods give good agreement with experiments and allow extracting the model parameters. 相似文献
195.
Shunin D. G. Filippova A. G. Okhotina N. A. Liakumovich A. G. Samuilov Ya. D. 《Russian Journal of Applied Chemistry》2002,75(6):1020-1023
Procedures are discussed for production of rubber-bitumen compounds based on spent rubber items. A possibility of using these binders for paving is studied experimentally. 相似文献
196.
We consider some Sobolev-type spaces and obtain a necessary and sufficient condition for their embedding in a Lebesgue space. 相似文献
197.
The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu. 相似文献
198.
199.
We use the adiabatic hyperspheric expansion and the Faddeev decomposition of the wave function with only s-waves. We derive for a fixed hyperradius an integro-differential equation for the angular eigenvalue and wave function. The
correlations lower the interaction energy for N = 20 by about a factor of 5.
Received October 22, 2001; accepted for publication November 5, 2001 相似文献
200.
LYNDA D. RODWELL EDWARD B. BARBIER CALLUM M. ROBERTS TIM R. McCLANAHAN 《Natural Resource Modeling》2002,15(4):453-486
ABSTRACT. The excessive and unsustainable exploitation of our marine resources has led to the promotion of marine reserves as a fisheries management tool. Marine reserves, areas in which fishing is restricted or prohibited, can offer opportunities for the recovery of exploited stock and fishery enhancement. In this paper we examine the contribution of fully protected tropical marine reserves to fishery enhancement by modeling marine reserve‐fishery linkages. The consequences of reserve establishment on the long‐run equilibrium fish biomass and fishery catch levels are evaluated. In contrast to earlier models this study highlights the roles of both adult (and juvenile) fish migration and larval dispersal between the reserve and fishing grounds by employing a spawner‐recruit model. Uniform larval dispersal, uniform larval retention and complete larval retention combined with zero, moderate and high fish migration scenarios are analyzed in turn. The numerical simulations are based on Mombasa Marine National Park, Kenya, a fully protected coral reef marine reserve comprising approximately 30% of former fishing grounds. Simulation results suggest that the establishment of a fully protected marine reserve will always lead to an increase in total fish biomass. If the fishery is moderately to heavily exploited, total fishery catch will be greater with the reserve in all scenarios of fish and larval movement. If the fishery faces low levels of exploitation, catches can be optimized without a reserve but with controlled fishing effort. With high fish migration from the reserve, catches are optimized with the reserve. The optimal area of the marine reserve depends on the exploitation rate in the neighboring fishing grounds. For example, if exploitation is maintained at 40%, the ‘optimal’ reserve size would be 10%. If the rate increases to 50%, then the reserve needs to be 30% of the management area in order to maximize catches. However, even in lower exploitation fisheries (below 40%), a small reserve (up to 20%) provides significantly higher gains in fish biomass than losses in catch. Marine reserves are a valuable fisheries management tool. To achieve maximum fishery benefits they should be complemented by fishing effort controls. 相似文献