全文获取类型
收费全文 | 21648篇 |
免费 | 730篇 |
国内免费 | 2301篇 |
专业分类
化学 | 11324篇 |
晶体学 | 201篇 |
力学 | 206篇 |
综合类 | 96篇 |
数学 | 10569篇 |
物理学 | 2283篇 |
出版年
2023年 | 128篇 |
2022年 | 274篇 |
2021年 | 270篇 |
2020年 | 287篇 |
2019年 | 479篇 |
2018年 | 473篇 |
2017年 | 412篇 |
2016年 | 380篇 |
2015年 | 370篇 |
2014年 | 567篇 |
2013年 | 1817篇 |
2012年 | 906篇 |
2011年 | 1503篇 |
2010年 | 1261篇 |
2009年 | 1454篇 |
2008年 | 1652篇 |
2007年 | 1606篇 |
2006年 | 1354篇 |
2005年 | 1019篇 |
2004年 | 936篇 |
2003年 | 750篇 |
2002年 | 655篇 |
2001年 | 550篇 |
2000年 | 563篇 |
1999年 | 515篇 |
1998年 | 415篇 |
1997年 | 433篇 |
1996年 | 382篇 |
1995年 | 474篇 |
1994年 | 420篇 |
1993年 | 339篇 |
1992年 | 308篇 |
1991年 | 215篇 |
1990年 | 185篇 |
1989年 | 157篇 |
1988年 | 111篇 |
1987年 | 87篇 |
1986年 | 109篇 |
1985年 | 115篇 |
1984年 | 105篇 |
1983年 | 52篇 |
1982年 | 88篇 |
1981年 | 95篇 |
1980年 | 83篇 |
1979年 | 67篇 |
1978年 | 83篇 |
1977年 | 47篇 |
1976年 | 49篇 |
1975年 | 21篇 |
1974年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
21.
The graph-theoretic operation of rooted subtree prune and regraft is increasingly being used as a tool for understanding and modelling reticulation events in evolutionary biology. In this paper, we show that computing the rooted subtree prune and regraft distance between two rooted binary phylogenetic trees on the same label set is NP-hard. This resolves a longstanding open problem. Furthermore, we show that this distance is fixed parameter tractable when parameterised by the distance between the two trees.Received March 16, 2004 相似文献
22.
We extend the matrix version of Cochran's statistical theorem to outer inverses of a matrix. As applications, we investigate the Wishartness and independence of matrix quadratic forms for Kronecker product covariance structures. 相似文献
23.
Teresa uczak Radosaw Pankiewicz Bogusawa ska Grzegorz Schroeder Maria Betowska-Brzezinska Bogumil Brzezinski 《Journal of Molecular Structure》2006,800(1-3):140-145
Novel self-assembled monolayers were obtained on silver using 4,7-diazaheptyl-trimethoxy-silane (SiN) and vinyl-trialkoxy-silane (SiVA, where the alkyl group is 3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51,54,57,60-eicozaoxa- hexaheptaconan). It was shown that thus modified metal surface was protected against electrooxidation. A densely packed monolayer remained stable and did not desorb from the Ag electrode on the potential cycling. The structure of SiN and SiVA as well as their complexes with Li+ cations were calculated and visualised by the AM1d and PM5 semi-empirical methods. 相似文献
24.
1 INTRODUCTION The self-assembly of organic-inorganic hybrid ma- terials is an intriguing area, which is yielding new generations of supramolecular architectures[1, 2]. Coor- dination by transition metals with multifunctional ligands is one of the main design principles. As a rigid multi-dentate ligand, 5-aminoisophthalic acid (AIP) has received considerable attention owing to the variety of bridging abilities[3~8]. It can engage in three types of intermolecular interactions: (1) M-L b… 相似文献
25.
Ya. I. Kurys’ N. S. Netyaga V. G. Koshechko V. D. Pokhodenko 《Theoretical and Experimental Chemistry》2007,43(5):334-342
A guest-host nanocomposite based on electroconducting polyaniline doped with 12-phosphotungstic acid and V2O5 as well as its bifunctional analog containing not more than 5 mass% nanosized platinum were obtained. A study was carried
out on the structure of these nanocomposites, their redox characteristics, and electrocatalytic activity in the reduction
of oxygen. These nanocomposites were found to display catalytic properties in the electrochemical reduction of oxygen, while
the presence of even a slight amount of nanosized platinum in the bifunctional composite leads to a significant increase in
its electrocatalytic activity.
__________
Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 5, pp. 307–314, September–October, 2007. 相似文献
26.
Günter F. Steinke 《Results in Mathematics》2007,51(1-2):111-126
Polster and Steinke [Result. Math., 46 (2004), 103–122] determined the possible Kleinewillingh?fer types of flat Laguerre planes. These types reflect transitivity
properties of groups of certain central automorphisms. We exclude three more types from the list given there with respect
to Laguerre homotheties. This yields a complete determination of all possible single types with respect to Laguerre homotheties
that can occur in flat Laguerre planes. Building on results by M?urer and Hartmann to characterize ovoidal or miquelian Laguerre
planes we further characterize certain flat Laguerre planes in terms of their Kleinewillingh?fer types.
Received: January 16, 2007. Revised: July 26, 2007. 相似文献
27.
Extending to r > 1 a formula of the authors, we compute the expected reflection distance of a product of t random reflections in the complex reflection group G(r, 1, n). The result relies on an explicit decomposition of the reflection distance function into irreducible G(r, 1, n)-characters and on the eigenvalues of certain adjacency matrices.Received December 8, 2003 相似文献
28.
29.
报道了新合成的二氰基二硫纶·菲咯啉-5,6-二酮合铜(Ⅱ)配合物CuLL′(L=mnt2-, 1,2-dicyano-1,2-ethylenedithiolate; L′=phen-5,6-dione, 1,10-phenanthroline-5,6-dione)的变温磁化率和电子顺磁共振波谱表征结果. 发现微晶粉体型的标题配合物CuLL′具有一定的顺磁性,形成四配位的近似于方形的结构. 探讨了这种配合物磁学特性与结构的关系. 相似文献
30.
The title compound is a bifunctional receptors including a thiourea group and a crown ether ring. Due to many possible potentials as a new class of reagents for membrane transports,ion-selective electrodes as well as reaction catalysts, the design and synthesis of bifunctional receptors for simultaneous binding of cations and anions is of ongoning interest in srprarnolecular chemistry1-5. In bifunctional receptors, the binding sites for anions and cations are covalently linked so as to exhibit allosteric or cooperative complexation where the binding affinity for anions(cations)is modified as a result of the cation(anion) complexation.Literature[6] reported that the ability of the thiourea group to bind anions is significantly enhanced when Na+ is bound to the crown moiety. To date, however only a few receptors of this class have been reported.6-8In this paper, we report an improved procedure under microwave irradiation that gives higher yields of title compound and needs fewer reaction times than traditional method.The structure of this compound was determined by IR ,element analysis and X-ray analysis.Scheme 1 The reaction equationThe crystal belongs to triclinic crystal system, P-1 space group, a=0.9547(0)nm, b=1.3637(3)nm,c=1.6029(3)nm, α =75.33(3) , β =83.62(3) , γ =70.99(3) ,Z=4,Dc=1.335g/cm3,F(000)=816,R1= 0.0557 ,wR2=0.1281. It is assembled into a three-dimensionalsupramolecule by intermolecular hydrogen bonds. 相似文献