全文获取类型
收费全文 | 2540篇 |
免费 | 99篇 |
国内免费 | 8篇 |
专业分类
化学 | 2003篇 |
晶体学 | 34篇 |
力学 | 29篇 |
数学 | 142篇 |
物理学 | 439篇 |
出版年
2023年 | 16篇 |
2022年 | 19篇 |
2021年 | 25篇 |
2020年 | 18篇 |
2019年 | 45篇 |
2018年 | 36篇 |
2017年 | 23篇 |
2016年 | 67篇 |
2015年 | 62篇 |
2014年 | 64篇 |
2013年 | 119篇 |
2012年 | 155篇 |
2011年 | 173篇 |
2010年 | 94篇 |
2009年 | 90篇 |
2008年 | 160篇 |
2007年 | 134篇 |
2006年 | 162篇 |
2005年 | 174篇 |
2004年 | 135篇 |
2003年 | 130篇 |
2002年 | 96篇 |
2001年 | 45篇 |
2000年 | 42篇 |
1999年 | 33篇 |
1998年 | 10篇 |
1997年 | 31篇 |
1996年 | 32篇 |
1995年 | 13篇 |
1994年 | 17篇 |
1993年 | 18篇 |
1992年 | 33篇 |
1991年 | 25篇 |
1990年 | 22篇 |
1989年 | 20篇 |
1988年 | 8篇 |
1987年 | 18篇 |
1986年 | 13篇 |
1985年 | 31篇 |
1984年 | 30篇 |
1983年 | 14篇 |
1982年 | 18篇 |
1981年 | 24篇 |
1980年 | 24篇 |
1979年 | 15篇 |
1978年 | 20篇 |
1977年 | 23篇 |
1976年 | 21篇 |
1975年 | 20篇 |
1974年 | 8篇 |
排序方式: 共有2647条查询结果,搜索用时 19 毫秒
31.
A new optical CO2 sensor based on the luminescence intensity change of the europium(III) complex tris(thenoyltrifluoroacetonato) europium(III) dihydrate ([Eu(tta)3]) caused by the absorption change of various pH indicators—thymol blue, phenol red, or cresol red—with CO2 was developed and its CO2 sensing properties were investigated. For all the CO2 sensors using pH indicators the observed luminescence intensity from [Eu(tta)3] at 613 nm increased with increasing CO2 concentration. The linear calibration method based on the plot of (I100–I0)/(I–I0) versus the inverse of CO2 concentration was suggested, where I0 and I100 were luminescence intensities at 613 nm of the CO2 sensor film in 100% nitrogen and 100% gaseous CO2. In all cases the plots showed good linearity and the correlation factors of the plots, r2, were 0.991 for thymol blue, 0.990 for phenol red, and 0.998 for cresol red. The slopes of the plots (A/B) for thymol blue, phenol red, and cresol red were 2.2, 5.2, and 9.0%, respectively. The response times of the CO2 sensor film were 4.0 s for thymol blue, 4.4 s for phenol red, and 8.8 s for cresol red for switching from nitrogen to CO2, and the recovery times of films were 36 s for thymol blue, 39.2 s for phenol red, and 56.6 s for cresol red for switching from CO2 to nitrogen. The signal changes were fully reversible and hysteresis was not observed during the measurements. The highly sensitive CO2 sensor was developed using thymol blue as an indicator for the CO2-sensing probe. 相似文献
32.
Naoki KakusawaYoshinori Tobiyasu Shuji YasuikeKentaro Yamaguchi Hiroko SekiJyoji Kurita 《Tetrahedron letters》2003,44(47):8589-8592
The reaction of 12-arylethynyl-6-methyl-5,6,7,12-tetrahydrodibenzo[c,f][1,5]-azastibocines with organic halides such as acyl halides and aryl halides in the presence of PdCl2(PPh3)2 as a catalyst led to the formation of cross-coupling products, alkynyl ketones and diaryl acetylenes, in good yields. The reactivity of the ethynyl group on the 1,5-azastibocines was far superior to that on diphenyl(phenylethynyl)stibane, which brought about marked improvement in the reaction conditions (lower temperature and shorter reaction time) and in the yields of the cross-coupling products. Single-crystal X-ray analysis of the ethynyl-1,5-azastibocine showed the presence of intramolecular Sb?N interaction which should be responsible for the remarkable reactivity enhancement of the 1,5-azastibocines for this type of reaction. 相似文献
33.
Osamu Sangen Singo Masuda Hidehiko Nakano Kazusige Yamana Kenichi Sasaki Naoki Ikeda 《Journal of polymer science. Part A, Polymer chemistry》1992,30(6):1171-1175
A new synthetic pathway of A–B–A tri-block copolyether which is composed of a hydrophilic poly(oxyethylene) unit as an A part and a hydrophobic poly(oxy-2-methyl-trimethylene) unit as a B part is proposed. Telechelic α-tosyl-ω-tosyloxypoly(oxy-2-methyl-trimethylene) derived from tosylation of poly(oxy-2-methyl-trimethylene glycol) (PMTG) was allowed to react with poly(ethylene glycol) (PEG) in the presence of sodium hydroxide. Tg of the resulting A–B–A tri-block copolyether (PEMG) (M?n = 1600) was ?72°C and its specific gravity [D415] was 1.055. 相似文献
34.
Toru Takagishi Naoki Matsui 《Journal of polymer science. Part A, Polymer chemistry》1987,25(9):2511-2520
The extent of binding of chrome violet, which is a monoazo dye and involves two hydroxyl groups in o and o′ positions to azo group, by polyvinylpyrrolidone is markedly enhanced in the presence of Co2+ ion. The amount of binding in the presence of 1 × 10?4 mol/L of Co2+ ion increases by a factor of about 10 compared to that in the absence of the metal ion. Ni2+ and Zn2+ ions do not perceptively influence the binding affinity of the dye. Cu2+ ion, in contrast, suppresses the binding. To investigate further the action of added metal ions, a cobalt–complex dye was prepared and its binding property for the polymer was compared to that of chrome violet in the presence of metal ions. Some possible mechanisms for the enhancement of chrome violet binding by the addition of Co2+ ion are described. 相似文献
35.
A high-performance liquid chromatographic method is described for the selective determination of taurine in biological fluids by post-column fluorescence reaction. Taurine was separated on an adsorption-distribution type Shodex Ionpac KC-811 column. Then it was converted with hypochlorite into the corresponding N-chloramine, which was allowed to react with thiamine to give fluorescent thiochrome. As little as 6 ng per injection of taurine could be determined. The average recoveries of spiked taurine in serum and urine were 99.5 +/- 2.7 and 101.8 +/- 2.9%, respectively. The method could be applied to the assay of taurine in human serum and urine with simple pretreatment. 相似文献
36.
Several 2,4-diaminothiazoles and 2,4-diaminothiophenes could be easily obtained by metallation using LDA from aminomethylthiomethylenecyanamides and aminomethylthiomethylenemalononitrile, respectively. 相似文献
37.
Sano S Takehisa T Ogawa S Yokoyama K Nagao Y 《Chemical & pharmaceutical bulletin》2002,50(9):1300-1302
Tetrasubstituted (Z)-alkenes were readily prepared through the Horner-Wadsworth-Emmons reactions of methyl 2-[bis(2,2,2-trifluoroethyl)phosphono]propionate with aryl alkyl ketones by employing Sn(OSO(2)CF(3))(2) and N-ethylpiperidine. 相似文献
38.
Matsumura Y Nishimura M Hiu H Watanabe M Kise N 《The Journal of organic chemistry》1996,61(8):2809-2812
Oxidative coupling of phenylacetic acid esters was easily achieved by treating the esters with TiCl(4) and then adding Et(3)N to the resulting solution. The products consisted of dl- and meso-2,3-diphenylsuccinic acid esters with the Claisen condensation product, and the ratio of these products depended on the reaction conditions. Reaction conditions suitable for high dl selectivity were determined, and a dimer of titanium enolate was postulated as an intermediate responsible for the high dl selectivity. The selectivities were compared with those in known oxidative couplings in which titanium enolate intermediates are prepared through lithium enolates and silyl enol ethers. The results suggest that the reactivities of titanium enolates intermediates depend on how they are generated. 相似文献
39.
Polymer-protected platinum/ruthenium colloidal dispersions were prepared by refluxing mixed solutions of hexachloroplatinic(IV) acid and ruthenium(III) chloride in a mixture of ethanol/water (1/1 v/v) in the presence of poly(N-vinyl-2-pyrrolidone). The electronic spectra and transmission electron micrographs suggested that the colloidal dispersions are almost composed of the mixture of the small monometallic Pt and Ru clusters over all the ratio of Pt/Ru compositions. Extended X-ray absorption fine structure analyses and high resolution electron microprobe analyses indicated that no Pt/Ru alloy clusters exist in the dispersions, and the aggregation occurs between small monometallic Pt clusters (diameter ca. 15 A) and partially oxidized Ru microclusters (diameter less than 10 A). Electron diffraction measurements also suggested that the diffraction pattern of aggregated Pt/Ru cluster particles prepared by the simultaneous reduction of Pt and Ru ions is the same as that of the physical mixture of the small monometallic Pt and Ru clusters separately prepared. Therefore, it can be concluded that the aggregated Pt/Ru cluster particles, with 10 to 60 A in diameter, are built up by small monometallic Pt clusters and partially oxidized Ru microclusters, and that Pt/Ru alloy clusters are not formed. 相似文献
40.
Ichikawa K Shimomura N Yamada M Ohkubo N 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(14):3235-3241
In vitro biomimetic mineralization by means of nanotechnology allows the formation of calcium carbonate polymorphs at low temperatures (<25 degrees C) under a CO(2) atmosphere of 500-1500 ppm. A two-dimensional zinc-ion ordered array (zinc array), which acts as an active-site mimic of carbonic anhydrase, has been prepared by immersing the self-organized monolayer of 3-(2-imidazolin-1-y)propyltriethosilane on mica (ImSi substrate) into aqueous zinc solution. The zinc array mounted on the ImSi substrate catalyzed the conversion from CO(2) to HCO(3) (-), and accelerated the formation of calcium carbonate. In situ X-ray diffraction data of the formed calcium carbonate on the poly(L-aspartate)-coated chitin substrate (pAsp substrate), with calcium ion-recognition sites, demonstrated that the interaction between the zinc array and pAsp substrates formed both vaterite and calcite at low temperature (15 degrees C) and mainly vaterite at 25 degrees C; this interaction also controlled the morphology of calcium carbonate formed on pAsp substrate. 相似文献