全文获取类型
收费全文 | 855篇 |
免费 | 38篇 |
国内免费 | 4篇 |
专业分类
化学 | 587篇 |
晶体学 | 4篇 |
力学 | 17篇 |
数学 | 160篇 |
物理学 | 129篇 |
出版年
2023年 | 10篇 |
2022年 | 6篇 |
2021年 | 8篇 |
2020年 | 18篇 |
2019年 | 24篇 |
2018年 | 14篇 |
2017年 | 12篇 |
2016年 | 24篇 |
2015年 | 26篇 |
2014年 | 34篇 |
2013年 | 34篇 |
2012年 | 51篇 |
2011年 | 62篇 |
2010年 | 28篇 |
2009年 | 28篇 |
2008年 | 52篇 |
2007年 | 52篇 |
2006年 | 54篇 |
2005年 | 37篇 |
2004年 | 44篇 |
2003年 | 44篇 |
2002年 | 33篇 |
2001年 | 19篇 |
2000年 | 20篇 |
1999年 | 12篇 |
1998年 | 15篇 |
1996年 | 12篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 8篇 |
1988年 | 3篇 |
1986年 | 4篇 |
1983年 | 4篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1970年 | 2篇 |
1968年 | 3篇 |
1967年 | 4篇 |
排序方式: 共有897条查询结果,搜索用时 15 毫秒
1.
Jalal M. Nawash Brendan Twamley Kelvin G. Lynn 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(8):i66-i68
Our investigations into the ZnO–TeO2 system have produced a new phase, zinc(II) hexatellurium(IV) tridecaoxide, ZnTe6O13, with trigonal (R) symmetry, synthesized by repeated heating and cooling to a maximum temperature of 1053 K. The asymmetric unit consists of a Zn atom coordinated in a distorted octahedral fashion by two unique tellurium(IV) oxide units that form trigonal–bipyramidal TeO4 and TeO3+1 corner‐ and edge‐shared polyhedra. Except for the Zn and an O atom, which occupy 6c positions, all atoms occupy 18f general positions. 相似文献
2.
3.
Charles W. Wolgemuth Raymond E. Goldstein Thomas R. Powers 《Physica D: Nonlinear Phenomena》2004,190(3-4):266-289
Certain bacteria form filamentous colonies when the cells fail to separate after dividing. In Bacillus subtilis, Bacillus thermus, and Cyanobacteria, the filaments can wrap into complex supercoiled structures as the cells grow. The structures may be solenoids or plectonemes, with or without branches in the latter case. Any microscopic theory of these morphological instabilities must address the nature of pattern selection in the presence of growth, for growth renders the problem nonautonomous and the bifurcations dynamic. To gain insight into these phenomena, we formulate a general theory for growing elastic filaments with bending and twisting resistance in a viscous medium, and study an illustrative model problem: a growing filament with preferred twist, closed into a loop. Growth depletes the twist, inducing a twist strain. The closure of the loop prevents the filament from unwinding back to the preferred twist; instead, twist relaxation is accomplished by the formation of supercoils. Growth also produces viscous stresses on the filament which even in the absence of twist produce buckling instabilities. Our linear stability analysis and numerical studies reveal two dynamic regimes. For small intrinsic twist the instability is akin to Euler buckling, leading to solenoidal structures, while for large twist it is like the classic writhing of a twisted filament, producing plectonemic windings. This model may apply to situations in which supercoils form only, or more readily, when axial rotation of filaments is blocked. Applications to specific biological systems are proposed. 相似文献
4.
Symmetry is one of the most important aesthetic criteria in graph drawing because it reveals structure in the graph. To draw
graphs symmetrically, we employ two steps. The first step is to find appropriate automorphisms. The second step is to draw
the graph to display the automorphisms. Our aim in this paper is to construct maximally symmetric straight line drawings of
triconnected planar graphs in linear time. Previously known algorithms run in quadratic time. We show that an algorithm of
Fontet can be used to find an embedding in the plane with the maximum number of symmetries, and present a new algorithm for
finding a straight line drawing that achieves that maximum. Both algorithms run in linear time. 相似文献
5.
Brendan D. McKay 《Combinatorica》1990,10(4):367-377
LetRT(n), ED(n) andEOG(n) be the number of labelled regular tournaments, labelled loop-free simple Eulerian digraphs, and labelled Eulerian oriented simple graphs, respectively, onn vertices. Then, asn,, for any>0. The last two families of graphs are also enumerated by their numbers of edges. The proofs use the saddle point method applied to appropriaten-dimensional integrals. 相似文献
6.
Evaluation of gas chromatography-tandem quadrupole mass spectrometry for the determination of organochlorine pesticides in fats and oils 总被引:1,自引:0,他引:1
Patel K Fussell RJ Hetmanski M Goodall DM Keely BJ 《Journal of chromatography. A》2005,1068(2):289-296
A gas chromatography-tandem quadrupole mass spectrometry multi-residue method for the analysis of 19 organochlorine pesticides in fats and oils has been developed. Gel permeation chromatography was employed to remove lipid material prior to GC-MS/MS analysis. Average recoveries of the pesticides spiked at 10 and 50 microg kg(-1) into fish oil, pork fat, olive oil and hydrogenated vegetable oil were typically in the range 70-110% with relative standard deviations generally less than 10%. Calculated limits of detection are between 0.1 and 2.0 microg kg(-1) and results obtained for the analysis of proficiency test materials are in good agreement with assigned values. The higher selectivity of the GC-MS/MS compared to electron capture detection and GC-MS in selective ion monitoring mode allowed unambiguous identification and confirmation of all the target pesticides at low microg kg(-1) levels in fats and oils in a single analysis. 相似文献
7.
Huynh MH Witham LM Lasker JM Wetzler M Mort B Jameson DL White PS Takeuchi KJ 《Journal of the American Chemical Society》2003,125(2):308-309
fac-[RuII(Cl)(dpp)(L3)]+ (L3 = tris(pyrid-2-yl)methoxymethane (tpmm) = [1A]+ and tris(pyrid-2-yl)pentoxymethane (tppm) = [1B]+ and dpp = di(pyrazol-1-yl)propane) rapidly undergo ligand substitution with water to form fac-[RuII(H2O)(dpp)(L3)]2+ (L3 = tpmm = [2A]2+ and tppm = [2B]2+). In the structure of [2A]2+, the distorted octahedral arrangement of ligands around Ru is evident by a long Ru(1)-O(40) of 2.172(3) A and a large angle O(40)-Ru(1)-N(51) of 96.95(14) degrees . The remarkably short distance between O(40) of H2O and H(45a) of dpp confirms the heteroscorpionate ligand effect of dpp on H2O. [2B]2+ aerobically catalyzes methyl p-tolyl sulfide to methyl p-tolyl sulfoxide in 1,2-dichlorobenzene at 25.0 +/- 0.1 degrees C under 11.4 psi of O2. Experimental facts in support of this aerobic sulfide oxidation are the absence of H2O2 and the oxidative reactivity of the putative Ru(IV)-oxo intermediate toward methyl p-tolyl sulfide, 2-propanol, and allyl alcohol. This study provides the first documented example of aerobic-sulfide oxidation catalyzed by the remarkably labile heteroscorpionate Ru(II)-aqua complex without the formation of a highly reactive peroxide as an intermediate. 相似文献
8.
Reactions of two preformed trinuclear W/Cu/S clusters, [A](2)[WS(4)(CuCN)(2)] (1: A = Et(4)N; 2: A = PPh(4)), with different concentrations of acetic acid in MeCN generate two interesting 2D polymeric clusters [Et(4)N](3)[(WS(4)Cu(2))(2)(mu-CN)(3)].2MeCN (3), and [PPh(4)][WS(4)Cu(3)(mu-CN)(2)].MeCN (4), respectively. Compound 4 can also be readily obtained in a high yield from the reaction of 2 with equimolar [Cu(MeCN)(4)]PF(6) in MeCN. These compounds have been characterized by elemental analysis, IR spectra, thermal analysis, and single-crystal X-ray diffraction. An X-ray analysis reveals that compound 3 retains the WS(4)Cu(2) cluster core, which serves as a 3-connecting node to link equivalent nodes via single cyanide bridges, forming an anionic 2D (6,3) net. Compound 4 consists of a T-shaped WS(4)Cu(3) core, which also acts as a 3-connecting node, with links to 3 equivalent clusters either through single or double cyanide bridges, affording a different anionic 2D (6,3) network. The acetic acid induced aggregation of 3 and 4 from the two cluster precursors 1 and 2 suggests that this simple synthetic strategy is likely to be applicable to many related systems. 相似文献
9.
Assembly of a supramolecular cube, [(Cp*WS3Cu3)8 Cl8(CN)12Li4] from a preformed incomplete cubane-like compound [PPh4][Cp*WS3(CuCN)3 总被引:2,自引:0,他引:2
Lang JP Xu QF Chen ZN Abrahams BF 《Journal of the American Chemical Society》2003,125(42):12682-12683
A supramolecular cube has been formed by linking WS3Cu3 clusters with bidentate ligands. Eight WS3Cu3 clusters, which have an incomplete cubane-like structure, serve as the three connecting nodes of the cube, while 12 bridging cyanide anions coordinated to copper centers lie along the edges of the cube. Eight chloride anions and four lithium cations are located inside the cube. 相似文献