全文获取类型
收费全文 | 37971篇 |
免费 | 5484篇 |
国内免费 | 4712篇 |
专业分类
化学 | 21038篇 |
晶体学 | 349篇 |
力学 | 1718篇 |
综合类 | 309篇 |
数学 | 3350篇 |
物理学 | 10745篇 |
无线电 | 10658篇 |
出版年
2024年 | 125篇 |
2023年 | 888篇 |
2022年 | 1089篇 |
2021年 | 1332篇 |
2020年 | 1427篇 |
2019年 | 1323篇 |
2018年 | 1045篇 |
2017年 | 1132篇 |
2016年 | 1548篇 |
2015年 | 1628篇 |
2014年 | 1995篇 |
2013年 | 2585篇 |
2012年 | 3089篇 |
2011年 | 2975篇 |
2010年 | 2250篇 |
2009年 | 2272篇 |
2008年 | 2530篇 |
2007年 | 2398篇 |
2006年 | 2229篇 |
2005年 | 1894篇 |
2004年 | 1482篇 |
2003年 | 1378篇 |
2002年 | 1225篇 |
2001年 | 954篇 |
2000年 | 921篇 |
1999年 | 945篇 |
1998年 | 796篇 |
1997年 | 664篇 |
1996年 | 692篇 |
1995年 | 557篇 |
1994年 | 515篇 |
1993年 | 439篇 |
1992年 | 374篇 |
1991年 | 326篇 |
1990年 | 305篇 |
1989年 | 193篇 |
1988年 | 148篇 |
1987年 | 114篇 |
1986年 | 120篇 |
1985年 | 85篇 |
1984年 | 41篇 |
1983年 | 47篇 |
1982年 | 43篇 |
1981年 | 24篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1973年 | 1篇 |
1968年 | 1篇 |
1959年 | 1篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
1-三甲硅氧基膦酸酯是一类具有显著生物活性的有机硅磷化合物。具有药效长、毒性低等特点,采用含P—O—Si键的亚磷酸酯与羰基化合物反应的方法进行合成,反应条件温和,产率比较高,但需要先制备化学性质活泼的含硅基亚磷酸酯,而且反应要在惰性气体下进行,实施有所不便。本文直接采用三烷基亚磷酸酯(1)、α、β-不饱和醛(2)与三甲基氯硅烷进 相似文献
952.
Jiaming L Wenqi L Xuelin L Hailing C Qiaomei L Longdi L Zhimin L Guohui Z Cuilian C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(13-14):3077-3081
Luminescent particles of lead carboxymethyl cellulose (Pb(CMC)2) containing salicyl fluorones (THBF), Pb(CMC)2-THBF were synthesized by the sol-gel method, using sodium carboxymethyl cellulose (NaCMC) as precursor and Pb2+ as precipitant. Pb(CMC)2-THBF can emit the intense and stable solid substrate room temperature phosphorescence (SS-RTP) on filter paper. And EDTA can chelate Pb2+ in Pb(CMC)2-THBF, causing it to decompose into aqueous soluble components PbY2-, CMC- and THBF, which can react with Ag+ to form Ag(CMC)2-THBF, causing the decrease of phosphorescence intensity. Based on the facts above, a new method for the determination of trace silver by SS-RTP quenching method was established. The linear range of this method is 8.0-40.0 fg spot(-1) (20.0-100.0 pg ml(-1)), with a detection limit (LD) of 2.2 fg spot(-1) (corresponding to a concentration range of 5.5 x 10(-13) g ml(-1)), and the regression equation of working curve is DeltaI(p) = 12.56 + 0.5527C(Ag+) (fg spot(-1), 0.4 microl spot(-1)), n = 8, r = 0.9992. This method has been applied to the determination of trace silver in human hair and tea sample with satisfactory results. The mechanism of SS-RTP emission is also discussed. 相似文献
953.
Determination of DNA using sodium 9,10-anthraquinone-2-sulfonate as an in situ photochemical fluorescence probe 总被引:2,自引:0,他引:2
Wen-You Li Xiang-Qun Guo Jin-Gou Xu Qing-Zhi Zhu Yi-Bing Zhao 《Analytica chimica acta》1997,340(1-3):291-296
An in situ photochemical fluorescence probe method for the determination of DNA with sodium 9,10-anthraquinone-2-sulfonate (AQS) as a photochemical fluorescence probe was developed. It was based on the conversion of AQS into an intensively fluorescent product by irradiating with UV radiation. The photochemical reaction is retarded by DNA. The determination can be carried out by measuring the fluorescence intensity at a fixed time. The calibration graph was linear in the range 0–80 ng ml−1 calf thymus (CT) DNA (r = 0.9991), the limit of detection was 3.2 ng ml−1 CT DNA (n = 9). The kinetic behaviour of the photochemical reaction and the effects of experimental conditions were investigated and discussed in detail. The results of absorption spectra and competitive binding experiments suggested the interaction between AQS and DNA to be intercalative. 相似文献
954.
955.
In the system of SDS/n-C5H11OH/n-C7H16/H2O with the weight ratio of SDS/n-C5H11OH/H2O system at5.0/47.5/47.5, the upper phase of the system was W/O microemulsion, and the lower phase was the bicontinuous microemulsion. When the n-heptane content was less than 1%, with the increase of the n-heptane content, the capacitance (Co, Cod) in the upper phase (W/O) dropped, the capacitance (CB1, CBld) in the lower phase (BI) raised. At the same time, the W/O-BI inteffacial potential (ΔE), capacitance (Ci), and charge-transfer current (ict) decreased.After the n-heptane content reached 1%, with the increase of the n-heptane content, ΔE, Ci and ict demonstrated no significant change. 相似文献
956.
We describe the use of the polymeric surfactant poly(sodium undecylenic sulfate) (poly-SUS) as a stationary phase coating in open-tubular capillary electrochromatography (OT-CEC) coupled with electrospray ionization-mass spectrometry (ESI-MS) for the analysis of beta-blocker and benzodiazepine analytes. The production of a polymeric surfactant coating on the capillary inner wall involves (i) adsorption of the cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) to the inner surface of capillary, and (ii) adsorption of the negatively charged poly-SUS onto the cationic polymer layer via strong physical interaction of the two polymer layers. As compared with micellar electrokinetic chromatography (MEKC) coupled with ESI-MS, the main advantage of this proposed method is minimization of introduction of the monomeric or polymeric surfactant into the mass spectrometer, thus avoiding the interference of the nonvolatile micelle in ESI-MS. The effects of buffer pH and applied voltage on the separation of the analytes are also discussed. Under optimum conditions, four of the five beta-blockers and four benzodiazepines are separated. 相似文献
957.
Lu W Mi BX Chan MC Hui Z Zhu N Lee ST Che CM 《Chemical communications (Cambridge, England)》2002,(3):206-207
Tridentate cyclometalated platinum(II) complexes bearing sigma-alkynyl ligands exhibit tunable photoluminescence and enhanced stability during vacuum deposition; OLEDs based on these materials display orange to red electrophosphorescence with low turn-on voltages (approximately 4 V), maximum luminance approaching 10,000 cd m-2 and efficiency up to 4.2 cd A-1. 相似文献
958.
Xiu-Kui Qu Lan-Ying Zhu Ling Li Xi-Lian Wei Feng Liu De-Zhi Sun 《Journal of solution chemistry》2007,36(5):643-650
As a continuation of our previous investigation, interactions between cyclodextrin (β-CD), γ-cyclodextrin (γ-CD) and alkyl trimethylammonium bromides in aqueous solutions have been studied with titration calorimetry and 1H NMR at 298.15 K. The results are discussed in terms of the amphiphilic interaction of CD with surfactants and the iceberg
structure formed by water molecules existing around the hydrophobic tail of surfactant molecules. The stoichiometry of the β-CD–surfactant system is 1:1 whereas that of the γ-CD–surfactant system is 1:2. The corresponding formation enthalpy (negative) of the complexes of the two systems decreases
with an increase in the number of carbon atoms (n) in hydrophobic chain of surfactant molecule, C
n
H2n+1, whereas the entropy increases with the enlargement of n. 相似文献
959.
An electron-donating heteroatom substituent at position-2 of a furan promotes regiospecific opening of the 7-oxa bridge of the Diels-Alder cycloadduct with hexafluoro-2-butyne, producing a 4-heterosubstituted 2,3-di(trifluoromethyl)phenol building block in a single step. The phenol and heteroatom substituent are easily transformed to the corresponding iodide or triflate that readily undergoes Heck, Suzuki, and Stille reactions to install a variety of substituents in high yields. This methodology provides a facile and general synthesis of 1,4-disubsituted 2, 3-di(trifluoromethyl)benzenes. 相似文献
960.
Wenhua Chen Xue Yao Zhenghui Huang Fei Mao Longfei Guan Yun Tang Hualiang Jiang Jian Li Jin Huang Lubin Jiang Jin Zhu 《中国化学快报》2019,30(1):250-254
A series of novel 2,4-diaminopyrimidine-modified compounds was designed and synthesized. Compound 14 showed micromolar dual inhibitory effect on both FP-2 and PfDHFR, and potential inhibition to the proliferation of P. falciparum 3D7 strain and chloroquine-resistant P. falciparum Dd2 strain. 相似文献