全文获取类型
收费全文 | 29157篇 |
免费 | 92篇 |
国内免费 | 273篇 |
专业分类
化学 | 10497篇 |
晶体学 | 266篇 |
力学 | 1393篇 |
综合类 | 12篇 |
数学 | 9134篇 |
物理学 | 8220篇 |
出版年
2021年 | 31篇 |
2020年 | 35篇 |
2018年 | 1227篇 |
2017年 | 1467篇 |
2016年 | 724篇 |
2015年 | 570篇 |
2014年 | 492篇 |
2013年 | 692篇 |
2012年 | 3136篇 |
2011年 | 2317篇 |
2010年 | 1818篇 |
2009年 | 1547篇 |
2008年 | 561篇 |
2007年 | 610篇 |
2006年 | 593篇 |
2005年 | 4425篇 |
2004年 | 3907篇 |
2003年 | 2292篇 |
2002年 | 479篇 |
2001年 | 286篇 |
2000年 | 96篇 |
1999年 | 174篇 |
1998年 | 117篇 |
1997年 | 70篇 |
1996年 | 50篇 |
1995年 | 45篇 |
1994年 | 45篇 |
1992年 | 173篇 |
1991年 | 155篇 |
1990年 | 137篇 |
1989年 | 113篇 |
1988年 | 100篇 |
1987年 | 63篇 |
1986年 | 46篇 |
1985年 | 45篇 |
1984年 | 30篇 |
1983年 | 33篇 |
1979年 | 31篇 |
1978年 | 31篇 |
1976年 | 84篇 |
1975年 | 40篇 |
1974年 | 44篇 |
1973年 | 51篇 |
1972年 | 40篇 |
1971年 | 28篇 |
1970年 | 34篇 |
1969年 | 38篇 |
1968年 | 35篇 |
1967年 | 30篇 |
1966年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Titanium tetrachloride promoted reaction of silyl ketene acetals with epoxides, followed by acidic work-up, affords butanolides in moderate/good yields. With epihalohydrins the reaction is regioselective and occurs at the less substituted end of the epoxide; the γ-haloalkyl-γ-butanolides thus obtained can be further transformed into various products. 相似文献
72.
The problem of homoaromaticity in mono-, di- and polyfluorinated- homotropylium cations is addressed by the B3LYP/6-311++G** DFT method. The energetic, structural and magnetic criteria are used for this purpose. They convincingly show that the ground state equilibrium species are aromatic, or in other words that the homoaromaticity is preserved by the (poly)fluorination. In contrast, a considerable decrease in the aromatic stabilization is observed in the transition structures (TS). According to the NICS(0) index, they vary form strongly antiaromatic, via weakly and non-aromatic to slightly aromatic transition states. However, the hierarchy of the aromaticity in fluorinated homotropylium ions predicted by NICS(0) is completely unrelated to that obtained by using the energy criterion assuming a kinetic definition of aromaticity. On the other hand the latter is closely related to geometric parameters of the equilibrium and transition structures. 相似文献
73.
74.
Inés CarrancoJosé Luis D?&#x;az Oscar JiménezRodolfo Lavilla 《Tetrahedron letters》2003,44(46):8449-8452
The Lewis acid-catalyzed interaction of dihydropyridines with ethyl glyoxalate and primary aliphatic amines yields, depending on the reaction conditions, bicyclic aminals or rearranged tetrahydropyridines in one-pot multicomponent reactions. 相似文献
75.
Gravimetric measurements of thermodesorption of n-hexane and n-heptane were performed under quasi-equilibrium conditions. Differential thermodesorption profiles for ZSM-5 and ZSM-11 showed two peaks, but for Y zeolites, only one thermodesorption peak was observed. A model function, derived from the Langmiur adsorption model, was fitted to the experimental data, and the model parameters (the adsorption entropy and enthalpy) were estimated. The two-step desorption profiles observed for ZSM-5 and ZSM-11 were attributed to the commensurate freezing effect, i.e. a transition in the adsorbed phase resulting in ordering of the adsorbed molecules in the zeolite channels. The results observed for ZSM-11 indicate that the zigzag channels typical for ZSM-5 micropore system are not necessary for this transition to occur. 相似文献
76.
R. Ochoa-Land?&#x;n M. Flores-Acosta R. Ram?&#x;rez-Bon H. Arizpe-Chávez F.F. Castillón-Barraza 《Journal of Physics and Chemistry of Solids》2003,64(11):2245-2251
CdS clusters were synthesized in A type zeolite by reaction in alkaline aqueous solution at temperatures from 30 to 70 °C. The optical properties of the samples were studied by diffuse reflectance and photoluminescence spectroscopy. Their crystalline structure and morphology were studied by X-ray diffraction and by transmission and scanning electron microscopy. We found that at lower temperatures the CdS clusters are encapsulated in the zeolite cages. We compared the properties of these clusters with those encapsulated in the cages of zeolites X and Y, prepared by similar methods. CdS clusters smaller than the CdS exciton diameter are also formed outside the cages in the zeolite matrix. The size of these clusters increases with temperature producing a red-shift of the absorption edge in the optical absorption spectra. 相似文献
77.
Mihai Tib?r 《Topology》2003,42(3):629-639
We show that the complex link of a large class of space germs (X,x0) is characterized by its “simplicity”, among the Milnor fibres of functions with isolated singularity on X. This amounts to the minimality of the Milnor number, whenever this number is defined. Such a phenomenon has been first pointed out in case (X,x0) is an isolated hypersurface singularity, by Teissier (Cycles évanescents, sections planes et conditions de Whitney, in: Singularités à Cargèse 1972, Asterisque, Nos. 7 et 8, Soc. Math. France, Paris, 1973, pp. 285-362). 相似文献
78.
Ruili?ZhangEmail author Jianming?Dai Wenhai?Song Yongqing?Ma Jie?Yang Yuping?Sun 《中国科学G辑(英文版)》2004,47(1):113-120
It is helpful to study the photo-induced effect in the perovskite manganites not only for elucidating the mechanism of colossal
magnetoresistance (CMR) effect but also for potential applications in technology. The laser-induced effect in the Co doping
layered perovskite manganites La1.2Sr1.8Mn1.8Co0.2O7, is studied in this paper and the obtained results are also compared with that gained in the Nd-doping manganites with cubic
perovskite structure. 相似文献
79.
The effect of hydrostatic pressure on the ferroelectric phase transition temperature in [NH2(CH3)2]3[Sb2Cl9] (DMACA) has been studied by electric permittivity measurements at pressures up to 400 MPa. The pressure-temperature phase diagram is given. The phase transition temperature (Tc) increases with increasing pressure up to 150 MPa, passes through a maximum and then decreases with a further increase of pressure. The unexpected nonlinear decrease in Tc with pressure increasing above 150 MPa suggests that the mechanism of ferroelectric phase transition in DMACA is different from hitherto assumed. 相似文献
80.
Dragan Vukoti? 《Journal of Mathematical Analysis and Applications》2004,291(1):189-202
For arbitrary composition operators acting on a general Bergman space we improve the known lower bound for the norm and also generalize a related recent theorem of D.G. Pokorny and J.E. Shapiro. Next, we obtain a geometric formula for the norms of composition operators with linear fractional symbols, thus extending a result of C. Cowen and P. Hurst and revealing the meaning of their computation. Finally, we obtain a lower bound for essential norm of an arbitrary composition operator related to the well-known criterion of B. MacCluer and J.H. Shapiro. As a corollary, norms and essential norms are obtained for certain univalently induced noncompact composition operators in terms of the minimum of the angular derivative of the symbol. 相似文献