全文获取类型
收费全文 | 49278篇 |
免费 | 7799篇 |
国内免费 | 5625篇 |
专业分类
化学 | 26233篇 |
晶体学 | 519篇 |
力学 | 2474篇 |
综合类 | 291篇 |
数学 | 4446篇 |
物理学 | 15212篇 |
无线电 | 13527篇 |
出版年
2024年 | 237篇 |
2023年 | 1198篇 |
2022年 | 1487篇 |
2021年 | 1820篇 |
2020年 | 1882篇 |
2019年 | 1728篇 |
2018年 | 1521篇 |
2017年 | 1486篇 |
2016年 | 2081篇 |
2015年 | 2267篇 |
2014年 | 2682篇 |
2013年 | 3301篇 |
2012年 | 4034篇 |
2011年 | 4122篇 |
2010年 | 2919篇 |
2009年 | 3044篇 |
2008年 | 3210篇 |
2007年 | 2911篇 |
2006年 | 2833篇 |
2005年 | 2224篇 |
2004年 | 1696篇 |
2003年 | 1510篇 |
2002年 | 1403篇 |
2001年 | 1180篇 |
2000年 | 1173篇 |
1999年 | 1128篇 |
1998年 | 1021篇 |
1997年 | 908篇 |
1996年 | 932篇 |
1995年 | 814篇 |
1994年 | 700篇 |
1993年 | 545篇 |
1992年 | 565篇 |
1991年 | 420篇 |
1990年 | 350篇 |
1989年 | 279篇 |
1988年 | 238篇 |
1987年 | 188篇 |
1986年 | 176篇 |
1985年 | 162篇 |
1984年 | 112篇 |
1983年 | 74篇 |
1982年 | 68篇 |
1981年 | 39篇 |
1980年 | 17篇 |
1979年 | 10篇 |
1977年 | 2篇 |
1971年 | 1篇 |
1959年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
851.
Wu X Ding S Ding Q Gray NS Schultz PG 《Journal of the American Chemical Society》2002,124(49):14520-14521
Purmorphamine, which is a 2,6,9-trisubstituted purine compound, was discovered through cell-based high-throughput screening from a heterocycle combinatorial library. It differentiates multipotent mesenchymal progenitor cells into an osteoblast lineage. It will serve as a unique chemical tool to study the molecular mechanisms of osteogenesis of stem cells and bone development. 相似文献
852.
Hong Zheng Xiao-Lan ChenMing-Hui Hu Dong-Hui LiJin-Gou Xu 《Analytica chimica acta》2002,461(2):235-242
A new method based on near-infrared (near-IR) fluorescence recovery, employing a two-reagent system which is composed of an anionic heptamethylene cyanine (HMC) and a polycationic phthalocyanine dye, Alcian blue 8GX, is presented for the determination of nucleic acids. With a maximum excitation wavelength at 766 nm and a maximum emission wavelength at 796 nm, the fluorescence recovery is linear with the concentration of nucleic acids added. Factors including the acidity of the medium, the reaction time, the optimal ratio of the two reagents, as well as the influence of foreign substance were all investigated. Meanwhile, the mechanism of fluorescence recovery was also studied. Under the optimal conditions, the linear ranges of the calibration curves were 10-250 ng ml−1 for calf thymus DNA (CT DNA) and 10-200 ng ml−1 for yeast RNA. The detection limits were 6.8 ng ml−1 for CT DNA and 6.3 ng ml−1 for yeast RNA, respectively. The method has been applied to the analysis of practical samples and the recovery results were satisfactory. 相似文献
853.
Chen YW Liu YC Lu SX Xu CS Shao CL Wang C Zhang JY Lu YM Shen DZ Fan XW 《The Journal of chemical physics》2005,123(13):134701
Self-assembled zinc oxide (ZnO) and indium-doping zinc oxide (ZnO:In) nanorod thin films were synthesized on quartz substrates without catalyst in aqueous solution by sol-gel method. The samples were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), Raman-scattering spectroscopy, room-temperature photoluminescence (PL) spectra, and temperature-dependent PL spectra measurements. XRD and Raman spectra illustrated that there were no single In2O3 phase in ZnO lattice after indium doping. The PL spectra of ZnO showed a strong UV emission band located at 394 nm and a very weak visible emission associated with deep-level defects. Indium incorporation induced the shift of optical band gap, quenching of the near-band-edge photoluminescence and enhanced LO mode multiphonon resonant Raman scattering in ZnO crystals at different temperatures. Abnormal temperature dependence of UV emission integrated intensity of ZnO and ZnO:In samples is observed. The local state emission peak of ZnO:In samples at 3.37 eV is observed in low-temperature PL spectra. The near-band-edge emission peak at room temperature was a mixture of excitons and impurity-related transitions for both of two samples. 相似文献
854.
碘酸钾存在下极谱催化波法测定陈皮甙 总被引:2,自引:0,他引:2
基于陈皮甙在KIO3存在下产生的极谱催化波,拟定了测定陈皮甙的新方法。在 0.55 mol/L HAc-0.01mol/L NaAc(pH 4.00±0.05)-1.0×10~(-3)mol/L KIO3缓冲溶液中,陈皮甙催化波的峰电位为-1.35 V(vs,SCE),它的一阶导数峰峰电流与陈皮甙浓度在2.6 × 10~(-7)~2.6 × 10~(-6)mol/L范围内呈线性关系(r=0.9970,n=6)。本方法比现有的电化学方法灵敏度高一个数量级。可用于直接测定中草药贯众水提液中陈皮甙含量。 相似文献
855.
Yang Nan Subhash P. Upadhyaya Wei Xu Kathrine E. Hughes William J. Dunn Ludwig Bauer Hemendra N. Bhargava George A. Doss 《Journal of heterocyclic chemistry》1996,33(2):399-407
A number of (E)-7-arylidenenaltrexones were synthesized by azeotropic distillation of water from a benzene solution of naltrexone and an aromatic aldehyde (benzaldehyde, 4-chloro- and 4-fluorobenzaldehyde, 3-and 4-pyridinecarboxaldehyde and 1-methyl-2-imidazolecarboxaldehyde) using piperidine as a catalyst. In addition, (E)-7-benzylidenenaloxone was prepared by the previously published Claisen-Schmidt condensation using sodium hydroxide in methanol. The stereochemistry of these arylidene derivatives 3–9 was determined to be (E) by means of nuclear Overhauser enhancement experiments. The 13C nmr spectra of (E)- 3–9 are recorded in deuteriochloroform and those of the hydrochlorides in deuteriodimethyl sulfoxide. 相似文献
856.
The basicities of aluminated zeolites βhave been investigated by FTIR spectra of chemisorbed pyrrole. The NH-stretching frequency of pyrrole adsorbed on the samples shifts towards lower wavenumber as the St/Al ratio is decreased and correlates well with the negative charge on the framework oxygen calculated from the Sanderson electronegativity equalization principle, revealing zeolite β to possess a stronger overall basicity after alumination and, therefore, the alumination to be an effective method to improve the basicity. It is also found that the heterogeneity of basic strength occurs and some stronger basic sites appear after alumination. 相似文献
857.
Morikawa T Xu F Ninomiya K Matsuda H Yoshikawa M 《Chemical & pharmaceutical bulletin》2004,52(4):494-497
New dolabellane-type diterpene alkaloids, nigellamines A(3), A(4), A(5), and C, were isolated from the methanolic extract of an Egyptian medicinal food, black cumin (the seeds of Nigella sativa). Their absolute configurations were determined on the basis of chemical and physicochemical evidence. Nigellamines were found to lower triglyceride levels in primary cultured mouse hepatocytes, and in particular, the activity of nigellamine A(5) was equivalent to that of the hypolipidemic agent, clofibrate. 相似文献
858.
Biofouling has posed serious problems in maritime industry including increased fuel consumptions, economic loss from ship-hull maintenances, contamination of drinking water, and serious corrosion for mechanical instruments. Minimizing the attachment of bacteria and formation of biofilm could be advantageous in reducing the early stages of biofouling. Zosteric acid, a natural product present in eelgrass, was found to have ability for preventing the attachment of some bacteria and barnacles. In this study, the antifouling ability of zosteric acid during the early stages of fouling was evaluated using attachment studies of fresh water bacteria. Simultaneously, various methods were sought for incorporating zosteric acid into silicone to prolong the release of the compound. The main results from this study were that zosteric acid exhibited anti-bacterial attachment regardless of whether it dispersed in water or incorporated into a coating. In addition, the release rate of zosteric acid from the incorporated coatings, particularly those where zosteric acid was uniformly dispersed with aggregates size of 4 microm or less, was orders of magnitude slower than those of previous reports. The release results indicate that the service life of our coatings could be far extended even with a small amount of zosteric acid incorporated. 相似文献
859.
860.
Xu J Jiao P Deng D Zhang Q Tsang CW Chan AS 《Rapid communications in mass spectrometry : RCM》2002,16(12):1174-1179
The mass spectrometric fragmentation behaviour of five pairs of (R,R)- and (S,S)-4,5-bis(benzoxazol-2-yl)-2,2-dimethyl-1,3-dioxolane derivatives, one pair of (R,R)- and (S,S)-4,5-bis(benzothiazol-2-yl)-2,2-dimethyl-1,3-dioxolanes, and three pairs of (R,R)- and (S,S)-N,N'-bis(2-hydroxyaryl)-2,2-dimethyl-1,3-dioxolane-4,5-dicarbamides, all important compounds for asymmetric catalysis (P. Jiao et al., Tetrahedron Asymmetry 2001; 12: 3081), has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization conditions. The spectral observations have been rationalized in terms of fragment ion structures and fragmentation mechanisms that will provide an aid to spectral interpretation for new compounds of this type. 相似文献