全文获取类型
收费全文 | 35885篇 |
免费 | 6837篇 |
国内免费 | 9861篇 |
专业分类
化学 | 26380篇 |
晶体学 | 1225篇 |
力学 | 2402篇 |
综合类 | 1143篇 |
数学 | 5533篇 |
物理学 | 15900篇 |
出版年
2024年 | 94篇 |
2023年 | 395篇 |
2022年 | 1098篇 |
2021年 | 1146篇 |
2020年 | 1173篇 |
2019年 | 1215篇 |
2018年 | 1138篇 |
2017年 | 1363篇 |
2016年 | 1432篇 |
2015年 | 1787篇 |
2014年 | 2182篇 |
2013年 | 2832篇 |
2012年 | 2865篇 |
2011年 | 3103篇 |
2010年 | 2756篇 |
2009年 | 2928篇 |
2008年 | 3354篇 |
2007年 | 2936篇 |
2006年 | 2844篇 |
2005年 | 2463篇 |
2004年 | 2001篇 |
2003年 | 1563篇 |
2002年 | 1626篇 |
2001年 | 1483篇 |
2000年 | 1439篇 |
1999年 | 943篇 |
1998年 | 560篇 |
1997年 | 432篇 |
1996年 | 439篇 |
1995年 | 380篇 |
1994年 | 351篇 |
1993年 | 317篇 |
1992年 | 267篇 |
1991年 | 233篇 |
1990年 | 216篇 |
1989年 | 210篇 |
1988年 | 177篇 |
1987年 | 135篇 |
1986年 | 105篇 |
1985年 | 84篇 |
1984年 | 91篇 |
1983年 | 87篇 |
1982年 | 74篇 |
1981年 | 59篇 |
1980年 | 50篇 |
1979年 | 39篇 |
1978年 | 32篇 |
1977年 | 13篇 |
1976年 | 13篇 |
1971年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
A free standing film of polyaniline as large as 18 cm×18cm×0.002 cm can be obtained by evaporation of a solution of the chemically synthesized base in NMP. Its structure was examined by the elemental analysis, IR, U.V.-visible spectra, XPS, DSC, SEM and X-ray scattering and its conducting behavior as well as electrochemical properties were studied. Results show that the composition, structure of main chain, physical properties of the free standing film of polyanilme is similar to that of the powder. However, some differences in its electronic structure, conductivity at room temperature and potential of redox couple between the flee standing film and powder are observed, which may be due to cross-linking of the film of polyaniline. 相似文献
993.
尽管已报道了多种结构类型的手性单或双噁唑啉, 但手性多噁唑啉的合成及其应用的文献报道比较少. 本工作以多元羧酸为原料, 与手性2-氨基-1-丁醇经一步反应合成了8个手性多噁唑啉, 产率为89%~98%, 其结构经1 H NMR谱、IR谱、MS谱和元素分析确证. 相似文献
994.
Two cobalt(II) coordination polymers formed from bte (bte = 1,2-bis(1,2,4-triazol-1-yl)ethane), namely [Co(bte)2(dca)2]n (1) and {[Co(bte)(dca)2] · H2O}n (2), have been synthesized and characterized by elementary analyses, IR, thermogravimetric analyses, X-ray diffraction analyses and magnetic measurements. Compound 1 is a double-chain with Co(II) centers bridged by bte, containing metallocycles of [Co2(bte)2] and trans dca as termination ligands. In 2, each Co(II) center is bonded by two bridging bte ligands and four dca as μ-1,5-dca in different orientations in the 3D network. 相似文献
995.
Gold nanotubule membranes were prepared by using electroless deposition of gold within the pores and surfaces of polycarbonate track-etched membranes.And the gold nanotubule membrane was used as an electrode for determination of uric acid in urine samples for the first time.In Britton-Robinson buffer of pH 4.56,uric acid exhibited well-defined differential pulse voltammograms.And the interference between coexistent ascorbic acid and uric acid was overcome owing to the attractive ability of the gold nanotubule electrode to yield a large anodic peak difference ca.0.404 V(vs.SCE).The proposed method was then applied to the determination of uric acid in urine without any pretreatment. 相似文献
996.
Synthesis of new tetrazene(N-N=N-N)-linked bi(1,2,4-triazole) 总被引:2,自引:0,他引:2
Sheng Hua Li Si Ping Pang Xiao Tong Li Yong Zhong Yu Xin Qi Zhao 《中国化学快报》2007,18(10):1176-1178
The reaction of 4-amino-1,2,4-triazole with sodium dichloroisocyanurate (SDCI) afforded new tetrazene(N-N=N-N)-linked bi(1.2,4-triazole) 2a in excellent yield.Increasing the molar ratio of SDCI to 4-amino-1,2,4-triazole,the chlorinated product 1,5,5′- trichloro-4,4′-azo-1,2,4-triazole (2b) was formed.These new compounds have been characterized by MS,~1H NMR,~(13)C NMR,and elemental analysis. 相似文献
997.
不对称催化是由潜手性反应物合成光活性化合物的有效途径,α,β-不饱和氨基酸的氢化立体选择性已达90%以上,L-Dopa的工业化生产则标志着不对称催化氢化开始走向实际应用。高选择性的催化剂一般是一价铑的手性双膦配体络合物,其中DIOP[2,3-O-异丙叉- 相似文献
998.
凝胶-燃烧法合成YAG∶Eu3+纳米荧光材料的结构和发光性能 总被引:6,自引:0,他引:6
Y3Al5O12∶Eu nanophosphors were synthesized by a gel combustion method. The structure of phosphors was characterized by XRD and FTIR. YAG phase came to occur when YAG∶Eu precursors were sintered at 800 ℃, although the phase was mainly amorphous. The organic groups pyrolyzed completely and pure YAG phase was obtained in the samples sintered at 900 ℃. In the formation of YAG phase, no intermediate phases such as YAP and YAM were detected. Both 5D0 → 7F1 orange and 5D0 → 7F2 red emission could be observed for all the sintered samples. However, the emission of amorphous samples was greatly different from that of crystalline ones. The former was mainly 5D0→ 7F2 red emission, but the latter was 5D0 → 7F1 orange emission. As sintering temperature rises, the ratio of orange to red for phosphors increases. Eu could be doped up to 8% in YAG host lattice, and fluorescence quenching was absent. It indicated that the gel combustion synthesis method can increase emission intensity and quenching concentration due to a good distribution of Eu3+ activators in Y3Al5O12 matrix. 相似文献
999.
Instead of environmentally toxic chromium oxidant, singlet oxygen generated photcchemically was used as environmentally friendly and benign oxidizing agent to accomplish the transformation of pseudodiosgenin diacetate to diosone efficiently in a low toxic and less expensive solvent acetone. Accordingly 16-dehydropregnenolone acetate(16-DPA), an important intermediate for preparation of steroidal drugs, was prepared in good yield (75%) when photoreaction was run in acetone/acetic anhydride/pyridine system. The mild reaction condition as well as simple and environmentally friendly process made the method commercially viable and important for production of 16-DPA in industrial scale. 相似文献
1000.
Oh JY Moon JH Lee YH Hyung SW Lee SW Kim MS 《Rapid communications in mass spectrometry : RCM》2005,19(10):1283-1288
Photodissociation (PD) tandem mass spectra have been obtained at 266 nm for the protonated molecules of a tryptic peptide, ASHLGLAR, and of its phenyl isothiocyanate and 4-sulfophenyl isothiocyanate derivatives, generated by matrix-assisted laser desorption/ionization. Derivatization with the aromatic chromophores greatly reduced the intensity of the laser required for efficient PD. Major fragment ions observed in the three spectra are quite similar. General features of the PD tandem mass spectra and their potential utility for analytical purposes are discussed. 相似文献