全文获取类型
收费全文 | 15627篇 |
免费 | 1567篇 |
国内免费 | 563篇 |
专业分类
化学 | 4726篇 |
晶体学 | 15篇 |
力学 | 870篇 |
综合类 | 81篇 |
数学 | 7172篇 |
物理学 | 4893篇 |
出版年
2024年 | 20篇 |
2023年 | 148篇 |
2022年 | 212篇 |
2021年 | 356篇 |
2020年 | 403篇 |
2019年 | 411篇 |
2018年 | 281篇 |
2017年 | 333篇 |
2016年 | 568篇 |
2015年 | 466篇 |
2014年 | 710篇 |
2013年 | 1199篇 |
2012年 | 684篇 |
2011年 | 740篇 |
2010年 | 671篇 |
2009年 | 920篇 |
2008年 | 1107篇 |
2007年 | 1099篇 |
2006年 | 913篇 |
2005年 | 667篇 |
2004年 | 570篇 |
2003年 | 630篇 |
2002年 | 617篇 |
2001年 | 483篇 |
2000年 | 487篇 |
1999年 | 416篇 |
1998年 | 407篇 |
1997年 | 272篇 |
1996年 | 237篇 |
1995年 | 184篇 |
1994年 | 185篇 |
1993年 | 157篇 |
1992年 | 145篇 |
1991年 | 96篇 |
1990年 | 92篇 |
1989年 | 92篇 |
1988年 | 103篇 |
1987年 | 72篇 |
1986年 | 68篇 |
1985年 | 104篇 |
1984年 | 81篇 |
1983年 | 28篇 |
1982年 | 51篇 |
1981年 | 47篇 |
1980年 | 50篇 |
1979年 | 39篇 |
1978年 | 34篇 |
1977年 | 37篇 |
1976年 | 30篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Bauer T Schulz S Nieger M Krossing I 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(7):1729-1737
Reactions of tBuP(NH(2))(2) with Group 13 trialkyls MR(3) (M=Al, Ga, In; R=Me, tBu) were investigated in detail. According to variable-temperature (VT) NMR investigations, the reaction proceeds stepwise with the initial formation of aminophosphane adducts, which subsequently react to give iminophosphorane adducts and finally the heterocyclic metallonitridophosphinates. BP86/TZVPP (DFT) calculations were performed to verify this reaction pathway, to elucidate the influence of the central Group 13 element on the stability of the reaction intermediates and the heterocycles, as well as to assess the thermodynamics of their formation. The relative stability of free and complexed aminophosphane RP(NH(2))(2) and iminophosphorane R(H(2)N)(H)P=NH (adducts) with P(III) and P(V) centers was studied in more detail with DFT and MP2 methods. In addition, the influence of the substituent R was investigated by variation of R from H to Me, tBu, F, and NH(2). In general, the aminophosphane form was found to be favored for the free ligand, however, upon complexation with MR(3) (M=Al, Ga; R=alkyl) both forms are almost equal in energy. 相似文献
952.
Devaux A Minkowski C Calzaferri G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(10):2391-2408
Fluorenone (C13H8O) was inserted into the channels of zeolite L by using gas-phase adsorption. The size, structure, and stability of fluorenone are well suited for studying host-guest interactions. The Fourier transform IR, Raman, luminescence, and excitation spectra, in addition to thermal analysis data, of fluorenone in solution and fluorenone/zeolite L are reported. Normal coordinate analysis of fluorenone was performed, based on which IR and Raman bands were assigned, and an experimental force field was determined. The vibrational spectra can be used for nondestructive quantitative analysis by comparing a characteristic dye band with a zeolite band that has been chosen as the internal standard. Molecular orbital calculations were performed to gain a better understanding of the electronic structure of the system and to support the interpretation of the electronic absorption and luminescence spectra. Fluorenone shows unusual luminescence behavior in that it emits from two states. The relative intensity of these two bands depends strongly on the environment and changes unexpectedly in response to temperature. In fluorenone/zeolite L, the intensity of the 300 nm band (lifetime 9 micros) increases with decreasing temperature, while the opposite is true for the 400 nm band (lifetime 115 micros). A model of the host-guest interaction is derived from the experimental results and calculations: the dye molecule sits close to the channel walls with the carbonyl group pointing to an Al3+ site of the zeolite framework. A secondary interaction was observed between the fluorenone's aromatic ring and the zeolite's charge-compensating cations. 相似文献
953.
Corbett PT Otto S Sanders JK 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(13):3139-3143
We present a versatile computer model of diverse dynamic combinatorial libraries, and examine how molecular recognition between library members and a template can be used to amplify the best binders. The correlation between host-guest binding and amplification was examined for a set of 50 libraries with >300 components each over a wide range of template and building block concentrations. Depending on these concentrations correlations vary from poor (when using a large excess of template) to good (for very dilute libraries and/or substoichiometric template concentrations), highlighting the need to choose the experimental conditions for dynamic combinatorial libraries thoughtfully. 相似文献
954.
Consider the delay difference system
955.
This paper considers the problem of the robust H
filtering for a class of nonlinear discrete-time Markovian jump systems with real time-varying norm-bounded parameter uncertainty. For each mode, the nonlinearity is assumed to satisfy the global Lipschitz conditions and appears in both the state and measured output equations. The problem that we address is the design of a nonlinear filter which ensures robust stochastic stability and a prescribed H
performance level of the filtering error system for all admissible uncertainties. A sufficient condition for the solvability of this problem is obtained in terms of a set of linear matrix inequalities; an explicit expression of a desired nonlinear H
filter is also given. Finally, an example is provided to demonstrate the effectiveness of the proposed approach. 相似文献
956.
We study the degenerate ecological models where are positive numbers. The structure of positive solutions of the models is discussed via bifurcation theory and monotone techniques. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
957.
Let X be a Polish space and P a Markov operator acting on the space of Borel measures on X. We will prove the existence of an invariant measure with respect to P, provided that P satisfies some condition of a Prokhorov type and that the family of functions is equi-continuous with respect to the Prokhorov distance at some point of the space X. Moreover, we will construct a counterexample which show that the above equi-continuity condition cannot be dropped. 相似文献
958.
Zeng-Qi Ou 《Journal of Mathematical Analysis and Applications》2004,291(1):203-213
An existence theorem of homoclinic solution is obtained for a class of the nonautonomous second order Hamiltonian systems , ∀t∈R, by the minimax methods in the critical point theory, specially, the generalized mountain pass theorem, where L(t) is unnecessary uniformly positively definite for all t∈R, and W(t,x) satisfies the superquadratic condition W(t,x)/|x|2→+∞ as |x|→∞ uniformly in t, and need not satisfy the global Ambrosetti-Rabinowitz condition. 相似文献
959.
Xiong Li 《Proceedings of the American Mathematical Society》2004,132(7):2057-2066
In this paper we will study the boundedness of all solutions for second-order differential equations
where and satisfies the sublinear growth condition. Since the system in general is non-Hamiltonian, we have to introduce reversibility assumptions to apply the twist theorem for reversible mappings. Under some suitable conditions we then obtain the existence of invariant tori and thus the boundedness of all solutions.
where and satisfies the sublinear growth condition. Since the system in general is non-Hamiltonian, we have to introduce reversibility assumptions to apply the twist theorem for reversible mappings. Under some suitable conditions we then obtain the existence of invariant tori and thus the boundedness of all solutions.
960.
We consider smooth multimodal maps which have finitely many non-flat critical points. We prove the existence of real bounds. From this we obtain a new proof for the non-existence of wandering intervals, derive extremely useful improved Koebe principles, show that high iterates have `negative Schwarzian derivative' and give results on ergodic properties of the map. One of the main complications in the proofs is that we allow to have inflection points.