全文获取类型
收费全文 | 1739篇 |
免费 | 78篇 |
国内免费 | 12篇 |
专业分类
化学 | 1365篇 |
晶体学 | 16篇 |
力学 | 11篇 |
数学 | 102篇 |
物理学 | 335篇 |
出版年
2023年 | 11篇 |
2022年 | 11篇 |
2021年 | 15篇 |
2020年 | 29篇 |
2019年 | 33篇 |
2018年 | 15篇 |
2017年 | 19篇 |
2016年 | 44篇 |
2015年 | 32篇 |
2014年 | 38篇 |
2013年 | 89篇 |
2012年 | 79篇 |
2011年 | 107篇 |
2010年 | 54篇 |
2009年 | 66篇 |
2008年 | 130篇 |
2007年 | 103篇 |
2006年 | 127篇 |
2005年 | 107篇 |
2004年 | 88篇 |
2003年 | 63篇 |
2002年 | 65篇 |
2001年 | 40篇 |
2000年 | 27篇 |
1999年 | 27篇 |
1998年 | 17篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 10篇 |
1993年 | 16篇 |
1992年 | 10篇 |
1991年 | 21篇 |
1990年 | 14篇 |
1989年 | 9篇 |
1988年 | 16篇 |
1987年 | 14篇 |
1986年 | 10篇 |
1985年 | 21篇 |
1984年 | 33篇 |
1983年 | 15篇 |
1982年 | 22篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1978年 | 15篇 |
1977年 | 9篇 |
1976年 | 17篇 |
1975年 | 9篇 |
1974年 | 12篇 |
1973年 | 13篇 |
排序方式: 共有1829条查询结果,搜索用时 15 毫秒
971.
We have developed a useful procedure for borylation using catalytic CuCl/Xantphos and stoichiometric K(O-t-Bu): the highly versatile method is suitable for the synthesis of allylboronates including those with terminal and internal allylic systems, cyclic structures, and functional groups. Optically active allylboronates were synthesized from prochiral substrates with a chiral diphosphine ligand using this procedure. The reaction rate is generally higher than the original Cu(O-t-Bu)/ligand catalyst system, which we previously reported. 相似文献
972.
973.
Dr. Katerina Sordakis Dr. Akihiro Tsurusaki Dr. Masayuki Iguchi Prof. Hajime Kawanami Dr. Yuichiro Himeda Prof. Gábor Laurenczy 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(44):15605-15608
Carbon dioxide may constitute a source of chemicals and fuels if efficient and renewable processes are developed that directly utilize it as feedstock. Two of its reduction products are formic acid and methanol, which have also been proposed as liquid organic chemical carriers in sustainable hydrogen storage. Here we report that both the hydrogenation of carbon dioxide to formic acid and the disproportionation of formic acid into methanol can be realized at ambient temperature and in aqueous, acidic solution, with an iridium catalyst. The formic acid yield is maximized in water without additives, while acidification results in complete (98 %) and selective (96 %) formic acid disproportionation into methanol. These promising features in combination with the low reaction temperatures and the absence of organic solvents and additives are relevant for a sustainable hydrogen/methanol economy. 相似文献
974.
Dr. Alvaro Mayoral Dr. Qing Zhang Yi Zhou Pengyu Chen Dr. Yanhang Ma Dr. Taro Monji Dr. Pit Losch Dr. Wolfgang Schmidt Prof. Ferdi Schüth Dr. Hajime Hirao Prof. Jihong Yu Prof. Osamu Terasaki 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(44):19678-19685
Zeolites are becoming more versatile in their chemical functions through rational design of their frameworks. Therefore, direct imaging of all atoms at the atomic scale, basic units (Si, Al, and O), heteroatoms in the framework, and extra-framework cations, is needed. TEM provides local information at the atomic level, but the serious problem of electron-beam damage needs to be overcome. Herein, all framework atoms, including oxygen and most of the extra-framework Na cations, are successfully observed in one of the most electron-beam-sensitive and lowest framework density zeolites, Na- LTA . Zeolite performance, for instance in catalysis, is highly dependent on the location of incorporated heteroatoms. Fe single atomic sites in the MFI framework have been imaged for the first time. The approach presented here, combining image analysis, electron diffraction, and DFT calculations, can provide essential structural keys for tuning catalytically active sites at the atomic level. 相似文献
975.
976.
M. Niikura E. Ideguchi N. Aoi H. Baba T. Fukuchi Y. Ichikawa H. Iwasaki T. Kubo M. Kurokawa M. Liu S. Michimasa T. Ohnishi T. K. Onishi S. Ota S. Shimoura H. Suzuki D. Suzuki Y. Wakabayashi K. Yoshida Y. Zheng 《The European Physical Journal A - Hadrons and Nuclei》2009,42(3):471-475
In-beam $ \gamma$ -ray spectroscopy of high-spin states in 49-51Ti was performed via the fusion-evaporation reaction using a radioactive beam. By excitation function and $ \gamma$ - $ \gamma$ coincidence analysis, yrast high-spin levels up to I = (21/2-),(11+),(17/2-) in 49-51Ti were determined. The levels were compared with full-pf -shell model calculation. The level structure indicates the persistency of the N = 28 shell gap at yrast states in 49-51Ti . 相似文献
977.
H. J. Ong N. Imai D. Suzuki H. Iwasaki H. Sakurai T. K. Onishi M. K. Suzuki S. Ota S. Takeuchi T. Nakao Y. Togano Y. Kondo N. Aoi H. Baba S. Bishop Y. Ichikawa M. Ishihara T. Kubo K. Kurita T. Motobayashi T. Nakamura T. Okumura Y. Yanagisawa 《The European Physical Journal A - Hadrons and Nuclei》2009,42(3):393-396
We have measured the lifetime of the first excited 2+ state in 18C using an upgraded recoil shadow method to determine the electric quadrupole transition. The measured mean lifetime is 18.9±0.9 (stat)±4.4 (syst) ps, corresponding to B(E2;21 + $ \rightarrow$ 0+ gs) = 4.3±0.2±1.0e 2 fm^4, or about 1.5 Weisskopf units. The mean lifetime of the first 2+ state in 16C was remeasured to be 18.3±1.4±4.8 ps, about four times shorter than the value reported previously. The discrepancy is explained by incorporating the $ \gamma$ -ray angular distribution obtained in this work into the previous measurement. The observed transition strengths in 16, 18C are hindered compared to the empirical values, indicating that the anomalous E2 strength observed in 16C persists in 18C . 相似文献
978.
Hajime Seito Tatsuya Ichikawa Hiroaki Hanaya Yoshishige Sato Hirohisa Kaneko Yasuyuki Haruyama Hiroshi Watanabe Takuji Kojima 《Radiation Physics and Chemistry》2009,78(11):961-965
Characteristics of clear PMMA dosimeter (Radix W) were studied for electron irradiation and compared with the response for gamma irradiation. The dose–response curves were nearly linear up to 30 kGy and become sublinear at higher doses. The radiation-induced absorbance was reduced with 6% within 4 h after irradiation. Dose comparisons were performed for 2, 3, 4 and 5 MeV electron irradiation using cellulose triacetate dosimeter (CTA) (FTR-125) and Radix W dosimeters which were independently calibrated for 2 MeV electrons and 60Co gamma-rays using calorimeter and ionizing chamber, respectively. The doses estimated by CTA and Radix W were different by about 20%. The magnitude of this difference was, however, independent of electron energy. 相似文献
979.
A polymer inclusion membrane (PIM) is reported consisting of 45% (m/m) di(2-ethylhexyl)phosphoric acid (D2EHPA) immobilized in poly(vinyl chloride) (PVC) for use as a solid phase absorbent for selectively extracting Zn(II) from aqueous solutions in the presence of Cd(II), Co(II), Cu(II), Ni(II) and Fe(II). Interference from Fe(III) in the sample is eliminated by precipitation with orthophosphate prior to the extraction of Zn(II). Studies using a dual compartment transport cell have shown that the Zn(II) flux (2.58 × 10−6 mol m−2 s−1) is comparable to that observed for supported liquid membranes. The stoichiometry of the extracted complex is shown to be ZnR2·HR, where R is the D2EHPA anion. 相似文献
980.
A new pyrolysis–GC/MS system incorporating with on-line micro-ultraviolet (UV) irradiation was developed to make rapid evaluation of the synergistic material deterioration during UV irradiation under thermal and oxidative atmospheres. The basic effectiveness of the system was demonstrated by polystyrene, polypropylene and polycarbonate as the test samples. The volatile products evolved during deterioration of the polymers were analyzed on-line by thermal desorption GC/MS, and then the residual degraded polymers were analyzed by evolved gas analysis (EGA) and/or Py–GC/MS to obtain specific thermograms and pyrograms. Based on these results, the deterioration mechanism of the polymeric materials during irradiation under thermal and oxidative atmosphere can be evaluated using a sub-milligram polymer sample within a relatively short period of time. 相似文献