全文获取类型
收费全文 | 260篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
化学 | 183篇 |
晶体学 | 1篇 |
力学 | 3篇 |
数学 | 46篇 |
物理学 | 41篇 |
出版年
2023年 | 1篇 |
2022年 | 5篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 5篇 |
2018年 | 7篇 |
2017年 | 5篇 |
2016年 | 12篇 |
2015年 | 11篇 |
2014年 | 11篇 |
2013年 | 18篇 |
2012年 | 23篇 |
2011年 | 37篇 |
2010年 | 8篇 |
2009年 | 11篇 |
2008年 | 18篇 |
2007年 | 7篇 |
2006年 | 14篇 |
2005年 | 8篇 |
2004年 | 5篇 |
2003年 | 8篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1997年 | 6篇 |
1994年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1968年 | 2篇 |
1963年 | 1篇 |
1958年 | 1篇 |
1956年 | 3篇 |
1954年 | 2篇 |
1948年 | 2篇 |
1938年 | 1篇 |
1937年 | 1篇 |
1936年 | 4篇 |
1934年 | 1篇 |
1933年 | 2篇 |
1932年 | 2篇 |
1931年 | 1篇 |
1929年 | 2篇 |
1928年 | 1篇 |
1923年 | 2篇 |
1893年 | 2篇 |
1892年 | 1篇 |
排序方式: 共有274条查询结果,搜索用时 31 毫秒
91.
92.
Janne Raula Matti Lehtim?ki Maarit Karppinen Maxim Antopolsky Hua Jiang Antti Rahikkala Esko I. Kauppinen 《Journal of nanoparticle research》2012,14(7):1-14
In this article, we describe the synthesis of new and ion-selective nanofiltration (NF) membranes using polyvinylidene fluoride (PVDF) nanofibers and hyperbranched polyethylenimine (PEI) as building blocks. These new nanofibrous composite (NFC) membranes consist of crosslinked hyperbranched PEI networks supported by PVDF nanofibrous scaffolds that are electrospun onto commercial PVDF microfiltration (MF) membranes. A major objective of our study was to fabricate positively charged NF membranes that can be operated at low pressure with high water flux and improved rejection for monovalent cations. To achieve this, we investigated the effects of crosslinker chemistry on membrane properties (morphology, composition, hydrophobicity, and zeta potential) and membrane performance (salt rejection and permeate flux) in aqueous solutions (2,000?mg/L) of four salts (NaCl, MgCl2, Na2SO4, and MgSO4) at pH 4, 6, and 8. We found that an NFC?CPVDF membrane with a network of PEI macromolecules crosslinked with trimesoyl chloride has a high water flux (~30?L?m?2?h?1) and high rejections for MgCl2 (~88 %) and NaCl (~65 %) at pH 6 using a pressure of 7?bar. The overall results of our study suggest that PVDF nanofibers and hyperbranched PEI are promising building blocks for the fabrication of high performance NF membranes for water purification. 相似文献
93.
94.
Wolfgang Haack 《Mathematische Zeitschrift》1932,35(1):66-79
Ohne Zusammenfassung 相似文献
95.
Savolainen J Dijkhuizen N Fanciulli R Liddell PA Gust D Moore TA Moore AL Hauer J Buckup T Motzkus M Herek JL 《The journal of physical chemistry. B》2008,112(9):2678-2685
A caroteno-purpurin dyad molecule was studied by steady-state and pump-probe spectroscopies to resolve the excited-state deactivation dynamics of the different energy levels as well as the connecting energy flow pathways and corresponding rate constants. The data were analyzed with a two-step multi-parameter global fitting procedure that makes use of an evolutionary algorithm. We found that following ultrafast excitation of the donor (carotenoid) chromophore to its S2 state, the energy flows via two channels: energy transfer (70%) and internal conversion (30%) with time constants of 54 and 110 fs, respectively. Additionally, some of the initial excitation is found to populate the hot ground state, revealing another limitation to the functional efficiency. At later times, a back transfer occurs from the purpurin to the carotenoid triplet state in nanosecond timescales. Details of the energy flow within the dyad as well as species associated spectra are disentangled for all excited-state and ground-state species for the first time. We also observe oscillations with the most pronounced peak on the Fourier transform spectrum having a frequency of 530 cm(-1). The dyad mimics the dynamics of the natural light-harvesting complex LH2 from Rhodopseudomonas acidophila and is hence a good model system to be used in studies aimed to further explain previous work in which the branching ratio between the competing pathways of energy loss and energy transfer could be manipulated by adaptive femtosecond pulse shaping. 相似文献
96.
Tiina J. Kauppila Alexander Haack Kai Kroll Hendrik Kersten Thorsten Benter 《Journal of the American Society for Mass Spectrometry》2016,27(3):422-431
In a preceding work with dopant assisted-atmospheric pressure photoionization (DA-APPI), an abundant ion at [M + 77]+ was observed in the spectra of pyridine and quinoline with chlorobenzene dopant. This contribution aims to reveal the identity and route of formation of this species, and to systematically investigate structurally related analytes and dopants. Compounds containing N-, O-, and S-lone pairs were investigated with APPI in the presence of fluoro-, chloro-, bromo-, and iodobenzene dopants. Computational calculations on a density functional theory (DFT) level were carried out to study the reaction mechanism for pyridine and the different halobenzenes. The experimental and computational results indicated that the [M + 77]+ ion was formed by nucleophilic aromatic ipso-substitution between the halobenzene radical cation and nucleophilic analytes. The reaction was most efficient for N-heteroaromatic compounds, and it was weakened by sterical effects and enhanced by resonance stabilization. The reaction was most efficient with chloro-, bromo-, and iodobenzenes, whereas with fluorobenzene the reaction was scarcely observed. The calculated Gibbs free energies for the reaction between pyridine and the halobenzenes were shown to increase in the order I < Br < Cl < F. The reaction was found endergonic for fluorobenzene due to the strong C–F bonding, and exergonic for the other halobenzenes. For fluoro- and chlorobenzenes the reaction was shown to proceed through an intermediate state corresponding to [M + dopant]+, which was highly stable for fluorobenzene. For the bulkier bromine and iodine, this intermediate did not exist, but the halogens were shown to detach already during the approach by the nucleophile. 相似文献
97.
A Comparative Study of Pyrolysis Liquids by Slow Pyrolysis of Industrial Hemp Leaves,Hurds and Roots
Ayobami Salami Jorma Heikkinen Laura Tomppo Marko Hyttinen Timo Keklinen Janne Jnis Jouko Vepslinen Reijo Lappalainen 《Molecules (Basel, Switzerland)》2021,26(11)
This study assessed the pyrolysis liquids obtained by slow pyrolysis of industrial hemp leaves, hurds, and roots. The liquids recovered between a pyrolysis temperature of 275–350 °C, at two condensation temperatures 130 °C and 70 °C, were analyzed. Aqueous and bio-oil pyrolysis liquids were produced and analyzed by proton nuclear magnetic resonance (NMR), gas chromatography–mass spectrometry (GC-MS), and atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry (APPI FT-ICR MS). NMR revealed quantitative concentrations of the most abundant compounds in the aqueous fractions and compound groups in the oily fractions. In the aqueous fractions, the concentration range of acetic acid was 50–241 gL−1, methanol 2–30 gL−1, propanoic acid 5–20 gL−1, and 1-hydroxybutan-2-one 2 gL−1. GC-MS was used to compare the compositions of the volatile compounds and APPI FT-ICR MS was utilized to determine the most abundant higher molecular weight compounds. The different obtained pyrolysis liquids (aqueous and oily) had various volatile and nonvolatile compounds such as acetic acid, 2,6-dimethoxyphenol, 2-methoxyphenol, and cannabidiol. This study provides a detailed understanding of the chemical composition of pyrolysis liquids from different parts of the industrial hemp plant and assesses their possible economic potential. 相似文献
98.
Pyrrolidine-2,4-diones have been identified as a novel starting point for the synthesis of peptide analogues. This paper describes a method for the efficient and diastereoselective incorporation of this moiety into peptide chains to furnish di- and tripeptide analogs. The stability of these pyrrolidinone modified di- and tripeptides was found to be markedly improved when compared to that of a natural peptide. In addition, solid phase peptide synthesis employing a pyrrolidinone containing tripeptide is demonstrated. 相似文献
99.
100.
Emma‐Rose Janeček Dr. Jason R. McKee Cindy S. Y. Tan Dr. Antti Nykänen Dr. Marjo Kettunen Prof. Janne Laine Prof. Olli Ikkala Prof. Oren A. Scherman 《Angewandte Chemie (International ed. in English)》2015,54(18):5383-5388
Hybrid nanocomposites were constructed based on colloidal nanofibrillar hydrogels with interpenetrating supramolecular hydrogels, displaying enhanced rheological yield strain and a synergistic improvement in storage modulus. The supramolecular hydrogel consists of naphthyl‐functionalized hydroxyethyl cellulose and a cationic polystyrene derivative decorated with methylviologen moieties, physically cross‐linked with cucurbit[8]uril macrocyclic hosts. Fast exchange kinetics within the supramolecular system are enabled by reversible cross‐linking through the binding of the naphthyl and viologen guests. The colloidal hydrogel consists of nanofibrillated cellulose that combines a mechanically strong nanofiber skeleton with a lateral fibrillar diameter of a few nanometers. The two networks interact through hydroxyethyl cellulose adsorption to the nanofibrillated cellulose surfaces. This work shows methods to bridge the length scales of molecular and colloidal hybrid hydrogels, resulting in synergy between reinforcement and dynamics. 相似文献