全文获取类型
收费全文 | 560篇 |
免费 | 36篇 |
国内免费 | 2篇 |
专业分类
化学 | 448篇 |
晶体学 | 1篇 |
力学 | 14篇 |
数学 | 51篇 |
物理学 | 84篇 |
出版年
2024年 | 1篇 |
2023年 | 7篇 |
2022年 | 24篇 |
2021年 | 31篇 |
2020年 | 27篇 |
2019年 | 18篇 |
2018年 | 12篇 |
2017年 | 6篇 |
2016年 | 30篇 |
2015年 | 29篇 |
2014年 | 23篇 |
2013年 | 42篇 |
2012年 | 45篇 |
2011年 | 50篇 |
2010年 | 17篇 |
2009年 | 17篇 |
2008年 | 37篇 |
2007年 | 32篇 |
2006年 | 30篇 |
2005年 | 34篇 |
2004年 | 16篇 |
2003年 | 11篇 |
2002年 | 21篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有598条查询结果,搜索用时 312 毫秒
121.
Bagno A Claeson S Maggini M Martini ML Prato M Scorrano G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(5):1015-1023
The substituent effect of the dihydro[60]fullerenyl group and its hydrophobic parameters have been evaluated quantitatively. The substituent constant has been determined from the pK value of a fullerene-based, para-substituted benzoic acid 1 in 80% dioxane/water (v/v) by NMR spectroscopy. The resulting Hammett sigma value of 0.06, consistent with a small electron-withdrawing effect of C(60), is a consequence of the fact that only inductive effects can be transmitted through the two tetracoordinate carbon atoms between the fullerene pi system and the para-position of the benzoic acid moiety in 1. The parameter pi, which describes the hydrophobic character of the substituent C(60), has been evaluated as the difference between that of 1 and model compound 2. The pi value, which is larger than 3, indicates that the fullerene cage imparts high hydrophobicity to the molecule to which it is attached. Finally, we have evaluated how the fullerene spheroid influences the acid-base properties and nucleophilicity of the pyrrolidine nitrogen in a suitably functionalized fulleropyrrolidine. The fulleropyrrolidine 4 (pK(BD)(+)=5.6) is six orders of magnitude less basic and 1000 times less reactive than its model 3 (pK(BD)(+)=11.6). This may be related to through-space interactions of the nitrogen lone pair and the fullerene pi system. 相似文献
122.
123.
Naeem A. Tahir Alexander Shutov Antonio R. Piriz Paul Neumayer Igor V. Lomonosov Vincent Bagnoud Sofia A. Piriz 《等离子体物理论文集》2019,59(4-5)
This paper presents detailed 2D hydrodynamic simulations of implosion of a multi‐layered cylindrical target that is driven by an intense uranium beam. The target is comprised of a thick, high‐Z, high‐ρ cylindrical shell that encloses a sample material (Fe in the present case). Two options have been used for the focal spot geometry: an annular form and a circular form. The purpose of this work is to show that an intense heavy‐ion beam can induce the extreme physical conditions in the sample material similar to those that exist in the planetary cores. In this study, we use parameters of the beam that will be generated at the Facility for Antiprotons and Ion Research (FAIR), Darmstadt, in a few years' time. Production of these high‐energy‐density (HED) samples will allow us to study planetary physics in the laboratory. It is to be noted that planetary physics research is an important part of the FAIR HED physics program. A dedicated experiment named LAboratory PLAnetary Sciences (LAPLAS) has been proposed for this purpose. These simulations show that in such experiments an Fe sample can be imploded to the Earth's core conditions and to those in more massive rocky planets called Super‐Earths. Similarly, implosion of hydrogen and water samples will generate the core conditions of solar and extrasolar hydrogen‐rich gas giants and water‐rich icy planets, respectively. The LAPLAS experiments will thus provide very valuable information on the equation of state and transport properties of matter under extreme physical conditions, which will help scientists understand the structure and evolution of the planets in our solar system as well as of the extrasolar planets. 相似文献
124.
Sofia Trakhtenberg Ramya Kumar Jason Bianchini Savin Thor Debora M. Martino 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(12):1311-1315
Photocrosslinking of thymine‐based water‐soluble polymer films was investigated at varying preparation conditions. Adding salt or decreasing the pH of the solution from which the films were cast resulted in the decreased efficiency of photoimmobilization, while increasing the pH was found to increase the photoimmobilization efficiency. A mechanistic rationale for the observed effects is proposed. 相似文献
125.
Omid Yousefzade Sofia Valenti Jordi Puiggalí Hamid Garmabi Roberto Macovez 《Journal of Polymer Science.Polymer Physics》2019,57(4):222-233
Temperature‐modulated differential scanning calorimetry (TMDSC) and broadband dielectric spectroscopy (BDS) were employed to study the glass transition, size of the cooperative rearranging regions (CRRs), crystallization kinetics, and dielectric relaxation response of nanocomposites constituted by chain‐extended poly(L‐lactide) (PLLA) and carboxylated carbon nanotubes (f‐CNTs). The CRR size and the number of relaxing structural units decreased in the presence of crystals during isothermal crystallization. All samples displayed both a primary (α) and secondary (β) relaxation in BDS spectra. The relaxation dynamics of PLLA chains was barely affected by the presence of the f‐CNT. Constrained polymer chains and thickness of interphase (t i) were measured using dielectric spectra in tan δ representation. t i values were found to be 46 and 24 nm for sample containing 0.2 and 0.5% weight fraction of f‐CNT, respectively. All samples underwent partial crystallization (with roughly 30% of final crystalline fraction) some 15 or 20° above their glass‐transition temperature (T g). Crystallization leads to a fragile‐to‐strong transition in the temperature dependence of the cooperative α relaxation and to the increased visibility of a Maxwell–Wagner–Sillars (MWS) interfacial relaxation, which appears to be present in all samples. The heterogeneity of the polymeric samples was quantified in terms of a new parameter, the heterogeneity index (H). © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 222–233 相似文献
126.
Sofia Målberg Dildare Basalp Anna Finne‐Wistrand Ann‐Christine Albertsson 《Journal of polymer science. Part A, Polymer chemistry》2010,48(5):1214-1219
The catalytic activities of Bi(III) acetate (Bi(OAc)3) and of creatinine towards the ring‐opening polymerization of L ‐lactide have been compared with those of a stannous (II) ethylhexanoate ((SnOct)2)‐based system and with those of a system catalyzed by enzymes. All four were suitable catalysts for the synthesis of high and moderate molecular weight poly(L ‐lactide)s and the differences in reactivity and efficiency have been studied. Linear and branched poly(L ‐lactide)s were synthesized using these bio‐safe initiators together with ethylene glycol, pentaerythritol, and myoinositol as coinitiators. The polymerizations were performed in bulk at 120 and 140 °C and different reactivities and molecular weights were achieved by adding different amounts of coinitiators. A molecular weight of 105,900 g/mol was achieved with 99% conversion in 5 h at 120 °C with a Bi(OAc)3‐based system. This system was comparable to Sn(Oct)2 at 140 °C. The reactivity of creatinine is lower than that of Bi(OAc)3 but higher compared with enzymes lipase PS (Pseudomonas fluorescens). A ratio of Sn(Oct)2Mo/Io 10,000:1 was needed to achieve a polymer with a reasonable low amount of tin residue in the precipitated polymer, and a system catalyzed by creatinine at 140 °C has a higher conversion rate than such a system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1214–1219, 2010 相似文献
127.
128.
Sofia Lindblad Flóra Boróka Németh Dr. Tamás Földes Dr. Daniel von der Heiden Herh G. Vang Zakarias L. Driscoll Emily R. Gonnering Prof. Imre Pápai Prof. Nathan Bowling Prof. Mate Erdelyi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(55):13748-13756
[Bis(pyridine)iodine(I)]+ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N−I−N]+ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their 15N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]+ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N−I−N]+ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations. 相似文献
129.
Martin Brodrecht Bharti Kumari Dr. A. S. Sofia Lilly Thankamony Dr. Hergen Breitzke Dr. Torsten Gutmann Prof. Dr. Gerd Buntkowsky 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(20):5214-5221
The structure and surface functionalization of biologically relevant silica-based hybrid materials was investigated by 2D solid-state NMR techniques combined with dynamic nuclear polarization (DNP). This approach was applied to a model system of mesoporous silica, which was modified through in-pore grafting of small peptides by solid-phase peptide synthesis (SPPS). To prove the covalent binding of the peptides on the surface, DNP-enhanced solid-state NMR was used for the detection of 15N NMR signals in natural abundance. DNP-enhanced heterocorrelation experiments with frequency switched Lee–Goldburg homonuclear proton decoupling (1H–13C and 1H–15N CP MAS FSLG HETCOR) were performed to verify the primary structure and configuration of the synthesized peptides. 1H FSLG spectra and 1H-29Si FSLG HETCOR correlation spectra were recorded to investigate the orientation of the amino acid residues with respect to the silica surface. The combination of these NMR techniques provides detailed insights into the structure of amino acid functionalized hybrid compounds and allows for the understanding for each synthesis step during the in-pore SPPS. 相似文献
130.
Sofia Curland Leah Javitt Isabelle Weissbuch David Ehre Meir Lahav Igor Lubomirsky 《Angewandte Chemie (International ed. in English)》2020,59(36):15570-15574
By performing icing experiments on hydrophilic and hydrophobic surfaces of pyroelectric amino acids and on the x‐cut faces of LiTaO3, we discovered that the effect of electrofreezing of super cooled water is triggered by ions of carbonic acid. During the cooling of the hydrophilic pyroelectric crystals, a continuous water layer is created between the charged hemihedral faces, as confirmed by impedance measurements. As a result, a current of carbonic acid ions, produced by dissolved environmental CO2, flows through the wetted layer towards the hemihedral faces and elevates the icing temperature. This proposed mechanism is based on the following: (i) on hydrophilic surfaces, water with dissolved CO2 (pH 4) freezes at higher temperatures than pure water of pH 7. (ii) In the absence of the ionic current, achieved by linking the two hemihedral faces of hydrophilic crystals by a conductive paint, water of the two pH levels freeze at the same temperature. (iii) On hydrophobic crystals with similar pyroelectric coefficients, where there is no continuous wetted layer, no electrofreezing effect is observed. 相似文献