全文获取类型
收费全文 | 23518篇 |
免费 | 5008篇 |
国内免费 | 8071篇 |
专业分类
化学 | 17882篇 |
晶体学 | 1084篇 |
力学 | 1677篇 |
综合类 | 908篇 |
数学 | 3717篇 |
物理学 | 11329篇 |
出版年
2024年 | 82篇 |
2023年 | 267篇 |
2022年 | 833篇 |
2021年 | 832篇 |
2020年 | 868篇 |
2019年 | 839篇 |
2018年 | 771篇 |
2017年 | 996篇 |
2016年 | 901篇 |
2015年 | 1239篇 |
2014年 | 1386篇 |
2013年 | 1915篇 |
2012年 | 1898篇 |
2011年 | 2043篇 |
2010年 | 1898篇 |
2009年 | 2016篇 |
2008年 | 2329篇 |
2007年 | 2002篇 |
2006年 | 1974篇 |
2005年 | 1660篇 |
2004年 | 1408篇 |
2003年 | 1056篇 |
2002年 | 1030篇 |
2001年 | 1054篇 |
2000年 | 1074篇 |
1999年 | 693篇 |
1998年 | 380篇 |
1997年 | 313篇 |
1996年 | 321篇 |
1995年 | 289篇 |
1994年 | 279篇 |
1993年 | 253篇 |
1992年 | 206篇 |
1991年 | 181篇 |
1990年 | 179篇 |
1989年 | 181篇 |
1988年 | 159篇 |
1987年 | 117篇 |
1986年 | 96篇 |
1985年 | 70篇 |
1984年 | 86篇 |
1983年 | 86篇 |
1982年 | 74篇 |
1981年 | 55篇 |
1980年 | 41篇 |
1979年 | 34篇 |
1978年 | 21篇 |
1977年 | 11篇 |
1976年 | 13篇 |
1975年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
192.
XinZHANG DeHuaHE QiJiangZHANG BoQingXU QiMingZHU 《中国化学快报》2003,14(10):1066-1069
Comparatively high CH3OH selectivity (60.0%) and yield (6.7%) were obtained on MoOx/(LaCoO3 Co3O4) catalysts in selective oxidation of methane to methanol using molecular oxygen as oxidant. The interaction between MoOx and La-Co-oxide modified the molecular structure of molybdenum oxide and the ratio of O^-/O^2- on the catalyst surface, which controlled the catalytic performance of MoOx/(LaCoO3 Co3O4) catalysts. 相似文献
193.
Chemical modification on the stereo‐regular poly(styrene‐co‐4‐methylstyrene) (sPS‐PMS) was attempted in this study. Metallocene copolymerization of styrene (St) and 4‐methylstyrene (MSt) was performed by using η5‐pentamethylcyclopentadienyl‐titanium(IV)tributoxide (Cp*Ti(OBu)3)/methylaluminoxane (MAO)/tri‐iso‐butylaluminum (TIBA) catalyst in the bulk state. Cobalt(II) catalyst was then applied to oxidize the benzylic methyl group on the MSt units of the resulting sPS‐PMS copolymer. Both aldehyde and carboxylic acid in the oxidized products were resolved by the FTIR and 1H NMR. The oxidized sPS‐PMSs exhibit a low and a high‐temperature Tg and Tm corresponding to the transitions in the amorphous and the crystalline regions. Hydrogen‐bond and polar interactions between the aldehyde and carboxylic acids tend to interrupt the regular chain packing of the oxidized sPS‐PMS, resulting in the lowering of Tm with oxidation level. The oxidized sPS‐PMS showed better adhesion to glass fiber than pure sPS‐PMS copolymer as evaluated from the respective SEM fractured micrographs. 相似文献
194.
Regueiro-Ren A Borzilleri RM Zheng X Kim SH Johnson JA Fairchild CR Lee FY Long BH Vite GD 《Organic letters》2001,3(17):2693-2696
[reaction: see text]. A series of 12alpha,13alpha-aziridinyl epothilone derivatives were synthesized in an efficient manner from epothilone A. The final semisynthetic route involves a formal double-inversion of stereochemistry at both the C12 and C13 positions. All aziridine analogues were tested for effects on tubulin binding polymerization and cytotoxicity. The results indicate that the aziridine moiety is a viable isosteric replacement for the epoxide in the case of epothilones. 相似文献
195.
硝酸根离子介质中偏钛酸钡的水热合成 总被引:1,自引:0,他引:1
以硝酸和钛酸四丁酯为原料,在240℃的碱液中成功地制备了四方相BaTiO3运用XRD、DSC和IR等技术碱度、硝酸根离子、钛源活性、填充度等因素对BaTiO3形成、粒度和物相的作用,KOH最适宜浓度为1.0mol/L,碱液中BaTiO3是BaO-TiO2体系中最稳定的物相,硝酸根离子参与了BaTiO3晶体的成核和生长,钛源的高活性和高填充度有利于四方BaTiO3的形成。 相似文献
196.
series of chitosan samlpes with 91% deacetylation but different molecular weight were obtained by varying the time of chitosan solution in acetic acid aqueous solution at 70℃.The viscosities and flow behavior of the concentrated solutions of chitosan were investigated in 10% (vol%) formic acid aqueous solution after the solutions were prepared for two days by using RPX-705 Rheopexy Analyzer.For each chitosan sample,at the concentration of 3.5 g/100ml,the log (ηr/η0)versus lgγη0,M curves superimpose each other within the same rheogramme. Then the relationship between log nr and lgM was studied under different shear rates.For the highest M sample, the experimental data deviate from the straight lines greatly. This was because the chitosan chain degradated in acid solution during dissolving. The molecular weights were measured in acid solution after two days of dissolving, they were right on their own straight lines. However,no obvious degradation was observed after two days in the same solution for the other lower M chitosan sam-ples. In addition, the plot of nr against M was not a straight line,probaly due to particular interaction of chitosan. 相似文献
197.
Time-resolved surface-enhanced Raman scattering (SERS) was applied to study the response of Raman bands from 4-cyanopyridine (PyCN) adsorbed on a Ag electrode to variation of the potential; the temporal resolution was 0.1 s. The response of the SERS signals of PyCN was instantaneous to the oxidation potential of Ag electrode. However, delay of the SERS signals was observed while AgCl was reducing. The decay and growth of the SERS bands look place within 1 s in the cases of desorption and adsorption of PyCN on the electrode. It took much longer for PyCN to alter from one adsorption geometry to another on the electrode. 相似文献
198.
Concise total syntheses of the bioactive mesotricyclic diterpenoids jatrophatrione and citlalitrione
The highly functionalized [5.9.5] tricyclic framework resident in jatrophatrione (1) and citlalitrione (2) has been synthesized. The route begins with the tandem anionic oxy-Cope rearrangement/methylation/transannular ene cyclization of 21 and subsequent introduction of a conjugated enone double bond. Hydroxyl-directed 1,4-reduction of this functionality in 25 with LiAlH(4)/CuI/hexamethylphosphoramide/tetrahydrofuran sets the stage for the implementation of a Grob fragmentation and expedited generation of 27. Stereocontrolled intramolecular hydrosilylation allows for the subsequent introduction of a cyclic carbonate as in 53. This intermediate undergoes remarkably efficient, fully regiocontrolled Treibs reaction to generate 54, with this maneuver serving as a pivotal step for making 1 available five steps later. Treatment of 1 with m-chloroperbenzoic acid leads to 2, with attack occurring preferentially on a alpha-face of the double bond more remote to the carbonyl. 相似文献
199.
A series of novel two-dimensional (2D) and three-dimensional (3D) praseodymium coordination polymers, namely, {[Pr3(PDA)4(HPDA)(H2O)8] x 8H2O}n (2), {[Pr2(PDA)3(H2O)3] x H2O}n (3), {[Pr(PDA)(H2O)4] x ClO4}n (4), and { [Pr2(PDA)2(H2O)5SO4] x 2H2O}n (5) (PDA = pyridine-2,6-dicarboxylic anion), was designed and synthesized under hydrothermal conditions. Complexes 1-3 (chainlike polymer, {[Pr(PDA)(HPDA)(H2O)2] x 4H2O}n (1) was also obtained independently by us, although it has been reported recently by Ghosh et al.) were fabricated successfully by simply tuning the Pr/PDA ratio and exhibited various and intriguing topological structures from a 1D chain to a 3D network. While the synthetic strategy of 5 was triggered and further performed only after 1 was structurally characterized. The complexes were characterized by X-ray single-crystal determination, spectroscopic, and variable-temperature magnetic susceptibility analyses. In complex 2 an unusual nanosized square motif as a building block constructed by eight Pr ions was further assembled into a highly ordered 2D grid compound. In complex 3 the decanuclear Pr metal-based structure as a repeat unit interpenetrated to form a novel 3D polymer. Complex 4 was a 3D network polymer fabricated through a hexanuclear Pr ring as a building block, and ClO4- anions as guests were trapped in the cavity. In complex 5 six Pr atoms, two SO4(2-) anions, and carboxylic oxygen bridges constructed an intriguing rectangle structure as a repeat unit in the grid to form a 2D coordination polymer in which the unique bi-bidentate coordination mode of SO4(2-) anion was observed. 相似文献
200.
ZENG Xiang-Chao② XU Shi-Hai LIU Po-Runa DENG Qin-Ying 《结构化学》2005,24(3):299-302
1 INTRODUCTION Indole and its derivatives have attracted much at-tention due to their chemical properties as well asbiological activities[1, . They have been widely used 2]as the materials for producing pigment, perfume,plant growth regulators, etc. Recently, it has alsobeen found that some indole derivatives present anti-tumor and antiviral activities[3~5]. During our sear-ches for bioactive compounds, a series of indole deri-vatives were synthesized, among … 相似文献