全文获取类型
收费全文 | 772篇 |
免费 | 25篇 |
国内免费 | 1篇 |
专业分类
化学 | 526篇 |
晶体学 | 17篇 |
力学 | 16篇 |
数学 | 44篇 |
物理学 | 195篇 |
出版年
2023年 | 3篇 |
2021年 | 7篇 |
2019年 | 12篇 |
2018年 | 12篇 |
2017年 | 7篇 |
2016年 | 11篇 |
2015年 | 16篇 |
2014年 | 15篇 |
2013年 | 28篇 |
2012年 | 36篇 |
2011年 | 42篇 |
2010年 | 19篇 |
2009年 | 18篇 |
2008年 | 51篇 |
2007年 | 39篇 |
2006年 | 23篇 |
2005年 | 30篇 |
2004年 | 34篇 |
2003年 | 27篇 |
2002年 | 31篇 |
2001年 | 19篇 |
2000年 | 21篇 |
1999年 | 15篇 |
1998年 | 14篇 |
1997年 | 7篇 |
1996年 | 11篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1992年 | 18篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 15篇 |
1988年 | 13篇 |
1987年 | 12篇 |
1986年 | 14篇 |
1985年 | 10篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1967年 | 4篇 |
排序方式: 共有798条查询结果,搜索用时 15 毫秒
91.
This paper considers a Lotka–Volterra predator–prey model with predators receiving an environmental time-variation. For such a system, a unique interior equilibrium is shown to be globally asymptotically stable if the time-variation is bounded and weakly integrally positive. Our result tells that the equilibrium can be stabilized even by nonnegative functions that make the limiting system structurally unstable. Numerical simulations are also shown to illustrate the result and to suggest that cases with time-variation acting on predators have larger-scale convergence to the equilibrium than population dynamics with time-variation acting on prey. 相似文献
92.
Sano S Okubo Y Handa A Nakao M Kitaike S Nagao Y Kakegawa H 《Chemical & pharmaceutical bulletin》2011,59(7):885-888
Isoliquiritigenin [ILG, (E)-1] was readily prepared via the Horner-Wadsworth-Emmons reactions using β-ketophosphonates 5a, b. An improved protocol for the synthesis of (E)-1 via the Claisen-Schmidt condensation was also presented. 相似文献
93.
94.
(7)Li NMR spectra of DEME-TFSA [DEME=N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium; TFSA=bis(trifluoromethanesulfonyl)amide], EMI-TFSA (EMI=1-ethyl-3-methylimidazolium), MPP-TFSA (MPP = N-methyl-N-propylpyridinium), DEME-PFSA [PFSA=bis(pentafluoroethanesulfonyl)amide], and DEME-HFSA [HFSA=bis(heptafluoropropanesulfonyl)amide] ionic liquid (IL) solutions containing LiX (X=TFSA, PFSA, or HFSA) and C211 (4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane) were measured at various temperatures. As a result, it was found that the uncomplexed Li(I) species existing as [Li(X)(2)](-) in the present ILs exchange with the complexed Li(I) ([Li·C211](+)) and that the exchange reactions proceed through the bimolecular mechanism, [Li·C211](+) + [*Li(X)(2)](-)=[*Li·C211](+) + [Li(X)(2)](-). Kinetic parameters [k(s)/(kg m(-1) s(-1)) at 25 °C, ΔH(++)/(kJ mol(-1)), ΔS(++)/(J K(-1) mol(-1))] are as follows: 5.57×10(-2), 69.8 ± 0.4, and -34.9 ± 1.0 for the DEME-TFSA system; 5.77×10(-2), 70.6 ± 0.2, and -31.9 ± 0.6 for the EMI-TFSA system, 6.13×10(-2), 69.0 ± 0.3, and -36.7 ± 0.7 for the MPP-TFSA system; 1.35 × 10(-1), 65.2 ± 0.5, and -43.1 ± 1.4 for the DEME-PFSA system; 1.14×10(-1), 64.4 ± 0.3, and -47.1 ± 0.6 for the DEME-HFSA system. To compare these kinetic data with those in conventional nonaqueous solvents, the exchange reactions of Li(I) between [Li·C211](+) and solvated Li(I) in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were also examined. These Li(I) exchange reactions were found to be independent of the concentrations of the solvated Li(I) and hence proposed to proceed through the dissociative mechanism. Kinetic parameters [k(s)/s(-1) at 25 °C, ΔH(++)/(kJ mol(-1)), ΔS(++)/(J K(-1) mol(-1))] are as follows: 1.10 × 10(-2), 68.9 ± 0.2, and -51.3 ± 0.4 for the DMF system; 1.13×10(-2), 76.3 ± 0.3, and -26.3 ± 0.8 for the DMSO system. The differences in reactivities between ILs and nonaqueous solvents were proposed to be attributed to those in the chemical forms of the uncomplexed Li(I) species, i.e., the negatively charged species ([Li(X)(2)](-)) in ILs, and the positively charged ones ([Li(solvent)(n)](+)) in nonaqueous solvents. 相似文献
95.
Yamaoka Y Taniguchi Y Yasuzuka S Yamamura Y Saito K 《The Journal of chemical physics》2011,135(4):044705
Quasi-binary (QB) picture of thermotropics, which regards a neat thermotropic liquid crystal as a binary system consisting of (semi)rigid core and molten alkyl chain, was assessed experimentally for the most famous (and representative) thermotropic mesogenic series nCB. By adding n-heptane as solvent, the smectic A phase was induced in 7CB-n-heptane system. Small angle x-ray diffraction showed that the QB picture holds not only in the phase behavior but also in the structure. It is suggested that the melting of 8CB and 9CB to isotropic liquid via smectic and nematic liquid crystals can be understood as a thermotropic-lyotropic crossover. 相似文献
96.
The diiron carbonyl cluster is held by a native CXXC motif, which includes Cys14 and Cys17, in the cytochrome c sequence. It is found that the diiron carbonyl complex works well as a catalyst for H(2) evolution. It has a TON of ~80 over 2 h at pH 4.7 in the presence of a Ru-photosensitizer and ascorbate as a sacrificial reagent in aqueous media. 相似文献
97.
Ferrocenylruthenocene was oxidized in sulfuric acid containing NH4BF4, giving a diamagnetic monocationic salt determined as ferrocenylruthenocenium+BF
4
–
.sd[1]Ferroceno[1]ruthenocenophane was oxidized with BF3-Et2O containing benzoquinone, giving a paramagnetic monocationic salt determined as [1]ferroceno[1]ruthenocenophanium+BF
4
–
. Based on the results of57Fe-Mössbauer spectroscopic studies of the salts, it was concluded that an antiferromagnetic Ru(III)-Ru(III) bond is formed in the solid state. 相似文献
98.
99.
Yuji Sano Kotaro Shirai Naoto Takahata Takafumi Hirata Neil C Sturchio 《Analytical sciences》2005,21(9):1091-1097
Concentrations of minor (Mg and Sr) and trace (Ba and U) elements in four natural calcium carbonate samples were first analyzed by inductively coupled plasma mass spectrometry (ICP-MS) after chemical dissolution and calibrated against a standard dolomite. Their homogeneities were checked by in situ laser ablation (LA) ICP-MS with 10-20 spots. The carbonate samples were measured by using a high lateral resolution secondary ion mass spectrometer (Nano-SIMS NS50). A approximately 4 nA O- primary beam was used to sputter a 5-6-microm diameter crater on the sample surface, and secondary positive ions were extracted for mass analysis using an accelerating voltage of 8 kV and a Mattauch-Herzog geometry. A multi-collector system was adjusted to detect 26Mg+, 43Ca+, 88Sr+, 138Ba+, 238U16O+ and 238U16O2+ ions at the same time. A resolving power of 2500-5000 at 10% peak height was attained by an entrance slit set at 40 microm, and each exit slit at 50 microm with adequate flat-topped peaks. The observed 26Mg/43Ca, 88Sr/43Ca, 138Ba/43Ca and 238U16O2/43Ca ratios agreed well with those measured by LA-ICP-MS. Foraminifera shells were analyzed at 5-6 microm scale by Nano-SIMS. There was a large variation of the Mg/Ca ratios, up to +/- 38%, even in a single fragment of the shell, suggesting that although the ratios provide a useful paleoceanographic proxy at bulk scale, they may reflect a more complex pattern at < 10 microm scale. 相似文献
100.
In 1′, 1‴-dibutylbiferrocenium triiodide crystals A show temperature-independent trapped-valence state, while crystals B show
a valence-delocalization of the mixed-valence state. The mixed-valence states of both crystals are discussed in relation to
their crystal structures.
Address after October 1991: Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima
724, Japan 相似文献