全文获取类型
收费全文 | 1603篇 |
免费 | 37篇 |
国内免费 | 4篇 |
专业分类
化学 | 1087篇 |
晶体学 | 20篇 |
力学 | 32篇 |
数学 | 44篇 |
物理学 | 461篇 |
出版年
2023年 | 9篇 |
2021年 | 14篇 |
2020年 | 15篇 |
2019年 | 13篇 |
2018年 | 10篇 |
2017年 | 19篇 |
2016年 | 19篇 |
2015年 | 28篇 |
2014年 | 21篇 |
2013年 | 52篇 |
2012年 | 62篇 |
2011年 | 84篇 |
2010年 | 49篇 |
2009年 | 53篇 |
2008年 | 85篇 |
2007年 | 75篇 |
2006年 | 67篇 |
2005年 | 70篇 |
2004年 | 75篇 |
2003年 | 68篇 |
2002年 | 63篇 |
2001年 | 52篇 |
2000年 | 50篇 |
1999年 | 17篇 |
1998年 | 24篇 |
1997年 | 18篇 |
1996年 | 20篇 |
1995年 | 20篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 28篇 |
1991年 | 25篇 |
1990年 | 16篇 |
1989年 | 21篇 |
1988年 | 26篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 27篇 |
1984年 | 29篇 |
1983年 | 18篇 |
1982年 | 22篇 |
1981年 | 24篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 21篇 |
1975年 | 20篇 |
1974年 | 11篇 |
1968年 | 7篇 |
排序方式: 共有1644条查询结果,搜索用时 31 毫秒
101.
Yoshihiro Shigemasa Takaaki Ueda Hiroyuki Saimoto 《Journal of carbohydrate chemistry》2013,32(4):669-673
Abstract The formose reactions2 in N,N-dimethylformamide (DMF) catalyzed by 2-(dimethylamino)ethanol and thiamine hydrochloride, have been found to give rise to dihydroxyacetone and DL-glycero-tetrulose selectively at 1.1 M and 3.0 M of formaldehyde concentration, respectively. In our consecutive study on the formose reaction in DMF, it has been fortunately found that the distribution of products is able to be controlled by the amount of water added to the reaction mixture. We describe herein the first example of the favored formation of DL-2-C-hydroxymethyl-3-pentu-lose (GP-191) in the formose reaction using DMF-H2O solvent, and it's isolation and structure elucidation. 相似文献
102.
Various types of crystalline cellulose consisting of group I (cell I, IIII, IVI) and group II (cell II, IIIII, IVII) prepared from cotton linter were adjusted for their degree of polymerization (DP) as starting materials. These celluloses were then treated by semi-flow hot-compressed water (HCW) at 230–270 °C/10 MPa/2–15 min to study their decomposition behaviors. The treatments performed resulted in residues of celluloses and water-soluble (WS) portions. Consequently, the crystallinity of the residues was found to remain the same, but the DP was reduced as the temperature increased. Additionally, X-ray diffractometry and Fourier transform-infrared analyses demonstrated that crystallographic changes occurred for residues of cell IIII, IVI and IIIII. Despite these changes, the overall results of the residues showed that group I has higher resistance to decomposing than group II. As for the WS portions, the yields of the hydrolyzed and degraded products were higher in group II than group I, indicating that group II is less resistant to decomposition by HCW treatment. Results for both the residues and WS portions are in agreement with each other, showing that the degree of difficulty of decomposition was higher in group I than group II. Therefore, the decomposition behaviors of the celluloses are due to differences in the crystalline forms. 相似文献
103.
An Okinawan soft coral Cespitularia sp. has proven to be a good source of cytotoxic metabolites having a carbon skeleton of the seco-type variety of xenicin. This soft coral yielded alcyonolide, the major constituent, and other related compounds, all of which have proven to be cytotoxic. Reinvestigation of the cytotoxic ethyl acetate extracts of the coral yielded two new alcyonolide congeners, trisnorditerpenoid 1 and diterpenoid 2, possessing the same carbon skeleton. Their structures were elucidated by a detailed analysis of spectroscopic data (1D, 2D NMR, and MS). Metabolites 1 and 2 showed cytotoxicity against HCT116 cells (IC50 6.04 and 47.0 μM, respectively) and a dose dependent, anti-inflammatory effect in LPS/IFN-γ-stimulated inflammatory RAW 264.7 macrophage cells. 相似文献
104.
Synthesis,Crystal Packing,and Ambipolar Carrier Transport Property of Twisted Dibenzo[g,p]chrysenes 下载免费PDF全文
Dr. Yasuyuki Ueda Dr. Hayato Tsuji Dr. Hideyuki Tanaka Prof. Dr. Eiichi Nakamura 《化学:亚洲杂志》2014,9(6):1623-1628
A versatile method for the synthesis of dibenzo[g,p]chrysene (DBC) derivatives based on regio‐ and stereoselective stannyllithiation to diarylacetylenes is described. This method affords a variety of DBCs possessing both electron‐donating and electron‐withdrawing functional groups. These twisted molecules take brickwork packing structures in single crystals. Thus, ambipolar carrier transport properties with mobility values of up to 10?3 cm2 V?1 s?1 in the amorphous state were achieved. Functional groups on DBC frameworks are considered to increase carrier mobility through the enhancement of intermolecular interactions in the brickwork packing structures. 相似文献
105.
Jahnke T Czasch A Schöffler M Schössler S Käsz M Titze J Kreidi K Grisenti RE Staudte A Jagutzki O Schmidt LP Weber T Schmidt-Böcking H Ueda K Dörner R 《Physical review letters》2007,99(15):153401
We investigate the interatomic Coulombic decay (ICD) of neon dimers following photoionization with simultaneous excitation of the ionized atom (shakeup) in a multiparticle coincidence experiment. We find that, depending on the parity of the excited state, which determines whether ICD takes place via virtual dipole photon emission or overlap of the wave functions, the decay happens at different internuclear distances, illustrating that nuclear dynamics heavily influence the electronic decay in the neon dimer. 相似文献
106.
Laser-diode pumped heavy-doped Yb:YAG ceramic lasers 总被引:1,自引:0,他引:1
Laser performance of heavy-doped Yb:YAG ceramics was investigated using a two-pass pumping miniature laser configuration. Slope efficiency of 52% and optical-to-optical efficiency of 48% have been achieved for 1-mm-thick YAG ceramic doped with 20 at.% ytterbium ions. Laser spectra of Yb:YAG ceramic and single-crystal lasers were addressed under different intracavity laser intensities. Heavy-doped Yb:YAG ceramic is more suitable for a thin disk laser than a single-crystal with the same Yb(3+)-ion lasants. 相似文献
107.
Converging-axicon-based radially polarized ytterbium fiber laser and evidence on the mode profile inside the gain fiber 总被引:3,自引:0,他引:3
The radially polarized mode is achieved from an active Yb fiber by utilizing of an intracavity converging axicon, where the axicon acts as a TM(01) mode selector based on not only its Brewster convex surface but also the distance between its vertex and plane output coupler. The polarization state of the annular laser beam is checked by using a home-made eight-hole aperture. Furthermore, an uncoated plane glass plate is inserted into the cavity, and the reflected beam points to the existence of an annular lasing mode inside the gain fiber. The issues for developing high-power radially polarized fiber lasers also are considered. 相似文献
108.
109.
Raku Watanabe Dr. Nobuki Kato Kengo Hayashi Sho Tozawa Dr. Yusuke Ogura Prof. Shigefumi Kuwahara Prof. Minoru Ueda 《ChemistryOpen》2020,9(10):1008-1017
An efficient and stereoselective syntheses of all the possible stereoisomers of coronafacic acid (CFA) has been developed. The stereochemistries of C3a and C7a were controlled in a diastereoselective Diels-Alder type cycloaddition using a chiral auxiliary. CFA and 6-epi-CFA were synthesized by hydrogenation of a common intermediate. During the synthesis of 6-epi-CFA, we established that its cis-fused configuration is important for the introduction of C4-C5 double bond by dehydration. This report is the first practical synthesis of both 6-epi-CFA, and its enantiomer. 相似文献
110.