全文获取类型
收费全文 | 1380篇 |
免费 | 65篇 |
国内免费 | 7篇 |
专业分类
化学 | 1147篇 |
晶体学 | 7篇 |
力学 | 27篇 |
数学 | 130篇 |
物理学 | 141篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 15篇 |
2021年 | 36篇 |
2020年 | 36篇 |
2019年 | 30篇 |
2018年 | 31篇 |
2017年 | 15篇 |
2016年 | 40篇 |
2015年 | 53篇 |
2014年 | 51篇 |
2013年 | 83篇 |
2012年 | 101篇 |
2011年 | 131篇 |
2010年 | 62篇 |
2009年 | 57篇 |
2008年 | 104篇 |
2007年 | 96篇 |
2006年 | 84篇 |
2005年 | 70篇 |
2004年 | 75篇 |
2003年 | 47篇 |
2002年 | 50篇 |
2001年 | 22篇 |
2000年 | 21篇 |
1999年 | 8篇 |
1998年 | 13篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 15篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 1篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 8篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 8篇 |
1980年 | 1篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1975年 | 1篇 |
1962年 | 2篇 |
1956年 | 1篇 |
1937年 | 1篇 |
排序方式: 共有1452条查询结果,搜索用时 359 毫秒
71.
High-power all-normal-dispersion femtosecond pulse generation from a Yb-doped large-mode-area microstructure fiber laser 总被引:3,自引:0,他引:3
We report on an all-normal-dispersion mode-locked fiber laser based on a large-mode-area Yb-doped microstructure fiber and using a high nonlinear modulation depth semiconductor saturable absorber mirror. The laser delivers 3.3 W of average output power with positively chirped 5.5 ps pulses at a center wavelength of 1033 nm. The pulse repetition rate is 46.4 MHz, which results in an energy per pulse of 71 nJ. These pulses are extracavity dechirped down to 516 fs by using bulk gratings. The average power of the dechirped pulses is 2.3 W, which corresponds to a peak power of more than 96 kW. 相似文献
72.
73.
Dr. Alexander Sachse Caroline Wuttke Dr. Elízio Lissner Dr. Michèle Oberson de Souza 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(46):14996-14999
Hierarchically porous ZSM‐5 was achieved by using a simple bottom‐up strategy combining zeolite seeds with imidazolium‐based ionic liquids. The bimodal ZSM‐5 with hexagonal arranged mesopores (3 nm) shows important activity in the acid catalysis of bulky compounds relative to conventional ZSM‐5. 相似文献
74.
Inherently Chiral Macrocyclic Oligothiophenes: Easily Accessible Electrosensitive Cavities with Outstanding Enantioselection Performances 下载免费PDF全文
Prof. Francesco Sannicolò Prof. Patrizia R. Mussini Prof. Tiziana Benincori Dr. Roberto Cirilli Prof. Sergio Abbate Serena Arnaboldi Simone Casolo Dr. Ettore Castiglioni Prof. Giovanna Longhi Dr. Rocco Martinazzo Dr. Monica Panigati Dr. Marco Pappini Dr. Elsa Quartapelle Procopio Dr. Simona Rizzo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(47):15298-15302
Linear conjugated oligothiophenes of variable length and different substitution pattern are ubiquitous in technologically advanced optoelectronic devices, though limitations in application derive from insolubility, scarce processability and chain‐end effects. This study describes an easy access to chiral cyclic oligothiophenes constituted by 12 and 18 fully conjugated thiophene units. Chemical oxidation of an “inherently chiral” sexithiophene monomer, synthesized in two steps from commercially available materials, induces the formation of an elliptical dimer and a triangular trimer endowed with electrosensitive cavities of different tunable sizes. Combination of chirality with electroactivity makes these molecules unique in the current oligothiophenes literature. These macrocycles, which are stable and soluble in most organic solvents, show outstanding chiroptical properties, high circularly polarized luminescence effects and an exceptional enantiorecognition ability. 相似文献
75.
On the Synthesis,Characterization and Reactivity of N‐Heteroaryl–Boryl Radicals,a New Radical Class Based on Five‐Membered Ring Ligands 下载免费PDF全文
Dr. Mohamad‐Ali Tehfe Dr. Stéphane Schweizer Dr. Anne‐Caroline Chany Dr. Cédric Ysacco Dr. Jean‐Louis Clément Dr. Didier Gigmes Dr. Fabrice Morlet‐Savary Prof. Jean‐Pierre Fouassier Dr. Markus Neuburger Dr. Théophile Tschamber Dr. Nicolas Blanchard Prof. Jacques Lalevée 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(17):5054-5063
The synthesis and physical characterization of a new class of N‐heterocycle–boryl radicals is presented, based on five membered ring ligands with a N(sp2) complexation site. These pyrazole–boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; B?H) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(B?H) by the fine tuning of the N‐heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O2, oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N‐heterocycle–boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions. 相似文献
76.
Juliano Fiorelli Catarina Abdala Gomide Francisco Antonio Rocco Lahr Maria Fátima do Nascimento Diogo de Lucca Sartori Julian Eduardo Mejia Ballesteros Stephen Bonila Bueno Ugo Leandro Belini 《Cellulose (London, England)》2014,21(5):3269-3277
Anatomical and physico-chemical properties of residual natural fibers (sugarcane bagasse, coconut fibers and peanut hulls) were characterized in order to evaluate their potential for use in the production of particleboard. The bulk density was determined by helium pycnometer and the chemical characteristics by using an electronic pH meter (for pH determination) on fibers dissolved in acidic and neutral detergents (to determine the levels of cellulose, hemicellulose and lignin). The anatomical characteristics were established using scanning electron microscopy coupled with an X-ray detector system, as well as energy dispersive X-ray spectroscopy. Results indicated similarities and differences between physico-chemical and anatomical characteristics of the residual lignocellulosic fibers when compared with the Pinus sp. wood commercially employed in particleboard production. Bulk density and pH for residual lignocellulosic fibers and Pinus sp. wood presented analogous values. Similar amounts of cellulose and lignin were identified between waste fibers and Pinus sp. wood. The presence of silica was identified in coconut fiber, peanut hull and sugarcane bagasse waste fibers, and may affect the mechanical characteristics of panels. Coconut and sugarcane bagasse fibers show surface pores with diameters ranging from 1.2 to 2.1 μm, below the 5 μm identified for Pinus sp. wood. Both fibers present pores distributed over their entire surface, whereas peanut hull fibers have no pores on their surface. This characteristic contributes to resin dispersion among particles, reflecting positively on the physical–mechanical properties of the panels. Particleboards produced with residual lignocellulosic fibers present similar physical–mechanical properties to those of Pinus sp. wood panels. 相似文献
77.
Caroline Ferrari Eric Olmos Frédérique Balandras Nguyen Tran Isabelle Chevalot Emmanuel Guedon Annie Marc 《Applied biochemistry and biotechnology》2014,172(2):1004-1017
The extensive use of mesenchymal stem cells (MCS) in tissue engineering and cell therapy increases the necessity to improve their expansion. Among these, porcine MCS are valuable models for tissue engineering and are classically expanded in static T-flasks. In this work, different processes of stirred cultures were evaluated and compared. First, the effect of glucose, glutamine, antioxidant, and growth factors concentrations on porcine MSC expansion were analyzed in a suitable medium by performing kinetic studies. Results showed that a lower glucose concentration (5.5 mM) enabled to increase maximal cell concentration by 40 % compared with a higher one (25 mM), while addition of 2 to 6 mM of glutamine increased maximal cell concentration by more than 25 % compared with no glutamine supplementation. Moreover, supplementation with 1 μM thioctic acid increased maximal cell concentration by 40 % compared with no supplementation. Using this adapted medium, microcarriers cultures were performed and compared with T-flasks expansion. Porcine MSC were shown to be able to proliferate on the five types of microcarriers tested. Moreover, cultures on Cytodex 1, Cytopore 2, and Cultispher G exhibited a MSC growth rate more than 40 % higher compared with expansion in T-flasks, while MSC metabolism was similar. 相似文献
78.
Dr. Caroline M. Storey Dr. Matthew R. Gyton Dr. Rhiann E. Andrew Dr. Adrian B. Chaplin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(64):14715-14723
Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands—which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C2H4)][BArF4] (n=12, 14, 16; ArF=3,5-(CF3)2C6H3)—have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr’ (Ar’=3,5-tBu2C6H3) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C−C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr’, and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2)Ar’ is encumbered relative to head-to-head coupling of HC≡CAr’ and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C2H4)][BArF4] and solvent effects. 相似文献
79.
Mary‐Ann J. Siegert Caroline H. Knittel Roderich D. Süssmuth 《Angewandte Chemie (International ed. in English)》2020,59(14):5500-5504
The toxic bicyclic octapeptide α‐amanitin is mostly found in different species of the mushroom genus Amanita, with the death cap (Amanita phalloides) as one of the most prominent members. Due to its high selective inhibition of RNA polymerase II, which is directly linked to its high toxicity, particularly to hepatocytes, α‐amanitin received an increased attention as a toxin‐component of antibody‐drug conjugates (ADC) in cancer research. Furthermore, the isolation of α‐amanitin from mushrooms as the sole source severely restricts compound supply as well as further investigations, as structure–activity relationship (SAR) studies. Based on a straightforward access to the non‐proteinogenic amino acid dihydroxyisoleucine, we herein present a robust total synthesis of α‐amanitin providing options for production at larger scale as well as future structural diversifications. 相似文献
80.
L-Lactide was polymerized in toluene with various alkoxide initiators. These initiators were prepared in situ from potassium t.-butoxide and primary or secondary alcohols such as tetradecanol, diethylene glycol butyl ether, menthol or testosteron. All these alcohols were incorporated as ester endgroups into the polylactide chain. However 1H-NMR spectroscopy proves the existence of more OH-endgroup than ester endgroups. This finding and 10–20% racemization observed for all anionic polymerizations suggest that chain-transfer reactions with the monomer via deprotonation take place. When poly(ethylene glycol) monomethyl ether in combination with KOtBu was used as initiator, twoblock copolymers were obtained. With poly(ethylene glycol) A-B-A-triblock copolymers could be synthesized. The quantitative reaction of the poly(ethylene glycol)s with L-lactide could be proven by both 1H NMR spectroscopy and gelpermeation chromatography. DSC-measurements show that depending on their lengths either the polylactide or the poly(ethylene oxide)blocks can crystallize. Due to partial racemization the melting temperatures (Tm) of the poly(L-lactide) blocks did not exceed 155°C. 相似文献