全文获取类型
收费全文 | 3676篇 |
免费 | 584篇 |
国内免费 | 478篇 |
专业分类
化学 | 2740篇 |
晶体学 | 20篇 |
力学 | 178篇 |
综合类 | 34篇 |
数学 | 311篇 |
物理学 | 1455篇 |
出版年
2024年 | 13篇 |
2023年 | 88篇 |
2022年 | 119篇 |
2021年 | 98篇 |
2020年 | 141篇 |
2019年 | 139篇 |
2018年 | 110篇 |
2017年 | 106篇 |
2016年 | 157篇 |
2015年 | 152篇 |
2014年 | 202篇 |
2013年 | 282篇 |
2012年 | 357篇 |
2011年 | 373篇 |
2010年 | 251篇 |
2009年 | 208篇 |
2008年 | 281篇 |
2007年 | 231篇 |
2006年 | 211篇 |
2005年 | 169篇 |
2004年 | 131篇 |
2003年 | 103篇 |
2002年 | 99篇 |
2001年 | 88篇 |
2000年 | 63篇 |
1999年 | 64篇 |
1998年 | 60篇 |
1997年 | 59篇 |
1996年 | 68篇 |
1995年 | 77篇 |
1994年 | 39篇 |
1993年 | 26篇 |
1992年 | 35篇 |
1991年 | 30篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 16篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1974年 | 1篇 |
1940年 | 1篇 |
1902年 | 1篇 |
1898年 | 1篇 |
排序方式: 共有4738条查询结果,搜索用时 17 毫秒
961.
Following an initial cleaning-up step on the D101 macroporous resin, a preparative high-speed counter-current chromatography (HSCCC) with a two-phase solvent system composed of n-hexane-ethyl acetate-methanol-water (1:0.9:0.9:1, v/v) was used to isolate and separate schizandrin and gomisin A from Schisandra chinensis. A total of 107 mg schizandrin and 36 mg gomisin A with purities of 99.5% and 99.1% were obtained from 400 mg crude extract in one-step elution and less than 3 h, and the structure identification was performed by UV, IR, MS, 1H NMR and 13C NMR. 相似文献
962.
AbstractSchisandra propinqua subsp. sinensis is a traditional medicinal plant used in Chinese folk medicine. Melanoma is the most dangerous form of skin cancer. To discover bioactive phytochemicals for preventing human melanoma, we have investigated the inhibitory effects of schisantherin F in Schisandra propinqua subsp. sinensis on human melanoma A375 cells and relevant mechanisms. The results showed that schisantherin F can inhibit A375 cells through inducing apoptosis. Further investigations have demonstrated schisantherin F attenuated the overproduction of ROS, depolarization of MMP, and mPTP opening. Meanwhile, schisantherin F inhibited the activity of Caspase-3 and up-stream Caspase-9, down-regulated Bcl-2 and up-regulated Bax. These findings propose the inhibitory mechanisms of schisantherin F in A375 cells include induction of mitochondrial dysfunction and mitochondria-mediated apoptosis. 相似文献
963.
A new route to podolactones functionalized in the A ring has been achieved. Two key steps were employed in this synthesis, the construction of the bicyclic skeleton via a Mn(III)-mediated radical cyclization and the transformation of this bicyclic intermediate into the tetracyclic podolactone skeleton through a Pd (II)-mediated bislactonization of the corresponding conjugate diene. The reported synthesis of 3beta-hydroxy-13,14,15,16-tetranorlabda-7,9(11)-dien-(19,6beta),(12,17)-diolide (1) let us reassign the structure of wentilactone B, for which structure 1 was wrongly reported. [reaction: see text] 相似文献
964.
聚吡咯纳米阵列电极的光电化学 总被引:1,自引:0,他引:1
以多孔的铝阳极氧化膜(AAO)为模板制备了直径约为80 nm聚吡咯(PPy)纳米线的阵列电极, 并研究了它的光电化学响应. 结果表明, 在电极电位低于-0.1 V(vs Ag/AgCl)时出现的阴极光电流是由聚吡咯纳米线的p型半导体性质引起的, 其平带电位为-0.217 V. 聚吡咯纳米线的长度对光电流的影响较大, 最佳长度为42 nm. 这是因为在很短的聚吡咯纳米线阵列中PPy太少, 产生的光电流弱, 而在过长的聚吡咯纳米线阵列中光生电子在到达电极基底前易于与光生空穴复合而消失. 聚吡咯纳米线有可能作为纳米光电器件用于未来微器件系统. 相似文献
965.
Yuhui Qu Xiufang Bian Hongwei Tang Pengchao Si 《International journal of quantum chemistry》2005,101(4):381-388
In this work, we present the results for the first time of our study on hydrogen‐bonded H2CO3–HO2 complexes (structures 1, 2) by means of ab initio molecular orbital theory. These complexes are important intermediates in the reaction of the hydroperoxy radical and cabonic acid. We calculated that these structures are a six‐membered ring. We found that the binding energy of two complexes are 5.8 and 9.3 kcal/mol using the CCSD(T) method. We also calculated the vibrational and rotational frequencies for these complexes. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
966.
用自行安装的精密静弹燃烧量热计测定了四苯基卟啉(TPP)、四甲苯基卟啉(T_((PCH)_3)PP)、四对甲氧苯基卟啉(T_((P-OCH)_3)PP)和四邻氨苯基卟啉(T_((O-NH)_2)PP)的标准燃烧能,并计算出标准生成焓。样品 1.将苯甲醛加入沸腾的丙酸中迥流30分钟后冷至50~60℃,加入乙醇,冷却,过滤,紫色产物用甲醇洗,干燥后得TPP粗品。将粗品溶于迥流的二氯甲烷,加入溶于苯的DDQ迥流30分钟,蒸除溶剂,残渣溶于氯仿,氧化铝-氯仿柱层析,氯仿-甲醇重结晶制得纯TPP. 相似文献
967.
A novel high-yielding Zn/CuI-mediated coupling method of alkyl halides with vinyl sulfones, vinyl sulfonates, and vinyl sulfonamides is described. This protocol is applicable for primary, secondary, and tertiary alkyl iodides and bromides. Alkyl chlorides and aryl and vinyl halides were unreactive under the reaction conditions. Formamide was found to be a superior solvent for obtaining high yields. 相似文献
968.
969.
The nucleus-independent chemical shifts (NICS), anisotropy of the induced current density (AICD), and molecular orbital (MO) analyses demonstrated the non-aromaticity of the β-diketiminate-coordinated Ae compounds. The DFT calculations revealed relatively high isomerization energies (ISE) for β-diketiminatecoordinated Ae compounds, which contribute to their stability. 相似文献
970.
熔融碳酸盐燃料电池研究 总被引:5,自引:0,他引:5
以烧结Ni作电极,以LiAlO2无机膜作电池隔膜,组装成了电极而积28cm^2的小型熔融碳酸盐燃料电池,试验了各种工作条件对电池性能的影响,电池经多次启动性能无衰减,放电电流密度100mA/cm^2,电池电压0.95V,放电电流密度125mA/cm^2,电池输出功率114mW/cm^2,燃料气利用率80%,电池能量效率31%。 相似文献