全文获取类型
收费全文 | 101912篇 |
免费 | 18728篇 |
国内免费 | 9792篇 |
专业分类
化学 | 84429篇 |
晶体学 | 1232篇 |
力学 | 4843篇 |
综合类 | 695篇 |
数学 | 9958篇 |
物理学 | 29275篇 |
出版年
2024年 | 231篇 |
2023年 | 1320篇 |
2022年 | 2187篇 |
2021年 | 2504篇 |
2020年 | 3575篇 |
2019年 | 4739篇 |
2018年 | 3119篇 |
2017年 | 2742篇 |
2016年 | 6191篇 |
2015年 | 6425篇 |
2014年 | 7016篇 |
2013年 | 8642篇 |
2012年 | 8571篇 |
2011年 | 8130篇 |
2010年 | 6944篇 |
2009年 | 6798篇 |
2008年 | 6815篇 |
2007年 | 5861篇 |
2006年 | 5314篇 |
2005年 | 4749篇 |
2004年 | 3867篇 |
2003年 | 3350篇 |
2002年 | 4008篇 |
2001年 | 2915篇 |
2000年 | 2818篇 |
1999年 | 1857篇 |
1998年 | 1273篇 |
1997年 | 1182篇 |
1996年 | 1082篇 |
1995年 | 979篇 |
1994年 | 889篇 |
1993年 | 722篇 |
1992年 | 662篇 |
1991年 | 565篇 |
1990年 | 503篇 |
1989年 | 382篇 |
1988年 | 314篇 |
1987年 | 267篇 |
1986年 | 223篇 |
1985年 | 211篇 |
1984年 | 152篇 |
1983年 | 100篇 |
1982年 | 87篇 |
1981年 | 55篇 |
1980年 | 37篇 |
1979年 | 15篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1957年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Using the method of diagonalization, in which zero-field wavefunctions from a potential model are chosen as a basis, we calculate positions and widths of anticrossings for highly excited Stark states of Rb with principal quantum number n up to 49. We obtain the theoretical positions and widths in good agreement with available experimental values, and we present scaling laws of the field positions and widths of anticrossings that can be used to provide information for future experiments. 相似文献
992.
The polymer bound Schiff‐base ligand (PS‐SalGlu) has been prepared from polystyrene bound salicylaldehyde and glutamic acid, and its complex (PS‐SalGlu‐Co) has also been synthesized. The polymer ligand and its complex were characterized by infrared spectroscopy, small area X‐ray photoelectron spectroscopy, and inductively coupled plasma‐atomic emission spectro (ICP‐AFS). In the presence of the complex, cyclohexene can be effectively oxidized by molecular oxygen without reductant. The long‐chain linear aliphatic olefins, such as 1‐octene, 1‐decene, 1‐dodecene and 1‐tetradecene, can be directly oxidized by molecular oxygen catalyzed by PS‐SalGlu‐Co, which affords the 1,2‐epoxy alkane. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
993.
We report completely optimized ONIOM DFT/AM1 molecular orbital calculations on several collagen-like triple helices based upon the repeating triad, ProProGly. The requirement of Gly as every third amino acid in collagen can be attributed to its enantiomorphic nature, as it behaves as a Damino acid in collagen. We, therefore, explored related collagen-like triple helices with one of the central Gly's mutated to either L or DAla; l-Ala appreciably destabilizes, while d-Ala slightly stabilizes the triple helical structure. Mutation of the same Gly to DSer, which is simply DAla with an OH in place of one of the methyl H's, induces a much greater stabilization due to an additional H-bond formed between this OH and a C=O on an adjacent peptide strand. Energies are presented for the triple helices and their component strands (both optimized and distorted to their triple helical geometries) relative to the component amino acids. The variation of relative energies with the chosen reference is delineated. 相似文献
994.
995.
Employing hot tungsten filament to thermal dissociate molecular hydrogen,we generated gas phase atomic hydrogen under ultra-high vacuum(UHV)conditions and investigated its interaction with Pt(111) surface.Thermal desorption spectroscopy(TDS)results demonstrate that adsorption of molecular hy- drogen on Pt(111)forms surface Had species whereas adsorption of atomic hydrogen forms not only surface Had species but also bulk Had species.Bulk Had species is more thermal-unstable than surface Had species on Pt(111),suggesting that bulk Had species is more energetic.This kind of weakly- adsorbed bulk Had species might be the active hydrogen species in the Pt-catalyzed hydrogenation reactions. 相似文献
996.
Baopei Xu C. Peter Lillya James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1992,30(9):1899-1909
A spiro(benzoxasilole) catalyst, 3,3,3′,3′-tetrakis(trifluoromethyl)-1,1′-(3H,3H′)-spirobis(1,2-benzoxasilole) was used to polymerize 3,3-R,R′-oxetanes: BEMO (R, R′ = ethoxymethyl), AMMO (R = azidomethyl, R′ = methyl), NMMO (R = nitratomethyl, R′ = methyl), BAMO (R, R′ = azidomethyl), and BCMO (R, R′ = chloromethyl) with descending rates in this order. 31P-NMR of polymerization mixtures quenched using Bu3P are consistent with an oxonium ion propagating species. Water is not a cocatalyst because it increases the induction period which is not eliminated by the proton trap 2,6-di-t-bu-tylpyridine. The propagating chains were terminated by transfer with the ether oxygen of the polymer either intermolecularly or intramolecularly. The index of propagation to chain transfer, Kkikp/ktr, varies over more than three orders of magnitude for BEMO > AMMO > NMMO > BAMO. However, kp/ktr for the four monomers differ by less than a factor of five indicating the same factors are affecting propagation and chain transfer. Addition of benzyl alcohol and propandiol produced poly(BEMO) having one and two hydroxyl termini, respectively. These telechelic polymers can be used to synthesize linear triblock or multiblock copolymers of oxetane derivatives. © 1992 John Wiley & Sons, Inc. 相似文献
997.
A silica polymorph, silica-sodalite was synthesized from a nonaqueous system NaOH-SiO2-C2H4(OH)2 and its chemical compositionwas determined to be Si12 O24,2C2H4(OH)2 by the X-fluorescence,thermogravimetric and organic elemental analyses. The single crystal X-ray diffraction structure determination of the silica-sodaliteshowed that the silica-sodalite is cubic, spacing group I23 and unitcell parameter a= 8.8354A. The IF and solid state high resolution Siand 13 C NMR spectra of the silica-sodalite were measured and discussed . 相似文献
998.
999.
1000.
Ab initio valence bond method is employed to quantitatively study the concepts of ionic resonance energy and ionicity of a chemical bond in the cases of hydrides XH(X=Li,Be,B,C,N,O,F) and fluorides XF(X=Li,Be,B),By establishing the relationship between resonance and stability,and comparing the calculated ionicities with Pauling‘s earlier estimations in the above diatomic molecules,the merits of Pauling‘s classical resonance theory were demonstrated at the ab initio level. 相似文献