全文获取类型
收费全文 | 2436篇 |
免费 | 130篇 |
国内免费 | 13篇 |
专业分类
化学 | 1883篇 |
晶体学 | 23篇 |
力学 | 56篇 |
数学 | 295篇 |
物理学 | 322篇 |
出版年
2023年 | 22篇 |
2022年 | 59篇 |
2021年 | 63篇 |
2020年 | 50篇 |
2019年 | 62篇 |
2018年 | 55篇 |
2017年 | 26篇 |
2016年 | 97篇 |
2015年 | 79篇 |
2014年 | 83篇 |
2013年 | 167篇 |
2012年 | 197篇 |
2011年 | 181篇 |
2010年 | 92篇 |
2009年 | 89篇 |
2008年 | 144篇 |
2007年 | 157篇 |
2006年 | 111篇 |
2005年 | 132篇 |
2004年 | 119篇 |
2003年 | 81篇 |
2002年 | 127篇 |
2001年 | 28篇 |
2000年 | 23篇 |
1999年 | 24篇 |
1998年 | 19篇 |
1997年 | 33篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 15篇 |
1993年 | 18篇 |
1992年 | 8篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 6篇 |
1987年 | 9篇 |
1986年 | 12篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 4篇 |
1974年 | 2篇 |
1973年 | 4篇 |
排序方式: 共有2579条查询结果,搜索用时 328 毫秒
141.
Chesnick IE Todorov TI Centeno JA Newbury DE Small JA Potter K 《Magnetic resonance imaging》2007,25(7):1095-1104
Paramagnetic manganese (II) can be employed as a calcium surrogate to sensitize magnetic resonance microscopy (MRM) to the processing of calcium during bone formation. At high doses, osteoblasts can take up sufficient quantities of manganese, resulting in marked changes in water proton T(1), T(2) and magnetization transfer ratio values compared to those for untreated cells. Accordingly, inductively coupled plasma mass spectrometry (ICP-MS) results confirm that the manganese content of treated cell pellets was 10-fold higher than that for untreated cell pellets. To establish that manganese is processed like calcium and deposited as bone, calvaria from the skull of embryonic chicks were grown in culture medium supplemented with 1 mM MnCl(2) and 3 mM CaCl(2). A banding pattern of high and low T(2) values, consistent with mineral deposits with high and low levels of manganese, was observed radiating from the calvarial ridge. The results of ICP-MS studies confirm that manganese-treated calvaria take up increasing amounts of manganese with time in culture. Finally, elemental mapping studies with electron probe microanalysis confirmed local variations in the manganese content of bone newly deposited on the calvarial surface. This is the first reported use of manganese-enhanced MRM to study the process whereby calcium is taken up by osteoblasts cells and deposited as bone. 相似文献
142.
Dr. Víctor Laina-Martín Dr. Jose A. Fernández-Salas Dr. José Alemán 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(49):12509-12520
Cycloaddition reactions, in particular Diels-Alder reactions, have attracted a lot of attention from organic chemists since they represent one of the most powerful methodologies for the construction of carbon-carbon bonds. In particular, inverse-electron-demand hetero-Diels-Alder reactions have been an important breakthrough for the synthesis of heterocyclic compounds. Among all their variants, the organocatalytic enantioselective version has been widely explored since the asymmetric construction of diversely functionalized scaffolds under reaction conditions encompassed within the green chemistry field is of great interest. In this review, a profound revision on the latest advances on the organocatalytic asymmetric inverse-electron demand hetero-Diels-Alder reaction is shown. 相似文献
143.
Inés Sánchez-Movellán Jorge Moreno-Ceballos Dr. Pablo García-Fernández Prof. Jose Antonio Aramburu Prof. Miguel Moreno 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(54):13582-13590
In the search for new high-temperature superconductors, it has been proposed that there are strong similarities between the fluoroargentate AgF2 and the cuprate La2CuO4. We explored the origin of the possible layered structure of AgF2 by studying its parent high-symmetry phase and comparing these results with those of a seemingly analogous cuprate, CuF2. Our findings first stress the large differences between CuF2 and AgF2. Indeed, the parent structure of AgF2 is found to be cubic, naturally devoid of any layering, even though Ag2+ ions occupy trigonal sites that, nevertheless, allow the existence of a Jahn-Teller effect. The observed Pbca orthorhombic phase is found when the system is cooperatively distorted by a local E⊗e trigonal Jahn-Teller effect around the silver sites that creates both geometrical and magnetic layering. While the distortion implies that two Ag2+−F− bonds increase their distance by 15 % and become softer, our simulations indicate that covalent bonding and interlayer electron hopping is strong, unlike the situation in cuprate superconductors, and that, in fact, exfoliation of individual planes might be a harder task than previously suggested. As a salient feature, these results prove that the actual magnetic structure in AgF2 is a direct consequence of vibronic contributions involved in the Jahn-Teller effect. Finally, our findings show that, due to the multiple minima intrinsic to the Jahn-Teller energy surface, the system is ferroelastic, a property that is strongly coupled to magnetism in this argentate. 相似文献
144.
Rosa L. Camacho-Mendoza Eliazar Aquino-Torres Julian Cruz-Borbolla Jose G. Alvarado-Rodríguez Oscar Olvera-Neria Jayanthi Narayanan Thangarasu Pandiyan 《Structural chemistry》2014,25(1):115-126
DFT studies of both the Fe4 cluster and the Fe(110) surface interaction with pyrrole, furan, thiophene, and selenophene showed that selenophene forms a stabler adsorbate iron complex than the other heterocyclic molecules; this is consistent with the binding energy data that were calculated between the Fe cluster and the Fe(110) surface with the heterocycles. Furthermore, when the adsorption of the compounds with the iron cluster was analyzed by molecular orbital studies, the orbitals of selenophene overlapped more strongly with the Fe atom than that of the other molecules. In TD-DFT, the π → π* peak observed for the molecules disappeared when they formed complexes, and there appeared a charge transfer band (ligand to metal), thus confirming the coordination of these molecules with the cluster. The data suggest that the chemisorption is an exothermic process. 相似文献
145.
Drop calorimetric measurements on a versatile monazite phase loaded with simulated radioactive waste
R. Asuvathraman Kitheri Joseph Jose Joseph M. V. Krishnaiah K. V. Govindan Kutty 《Journal of Thermal Analysis and Calorimetry》2014,117(3):1151-1156
Monazite is one of the candidate ceramic matrices for the immobilization of high level radioactive waste (HLW) from the reprocessing of spent nuclear fuel. The monazite phase, Ce0.8Ca0.2PO4, can accommodate cations of different valences due to the mixed valence state (+3 and +4) of Ce in this compound, by facilitating the oxidation and reduction of the Ce3+ and Ce4+ as required by the in-coming cation. This will assist in accommodating HLW of different compositions in the monazite crystal structure even if the average valence of the HLW elements is other than 3. Therefore, the monazite phase, Ce0.8Ca0.2PO4, can be a versatile host for the immobilization of HLW. The enthalpy increment and heat capacity of this versatile monazite phase and a simulated waste form based on it with 20 mass% HLW oxides were measured by drop calorimetry in the temperature range from 373 to 873 K, and the results are compared with those measured for CePO4. 相似文献
146.
Confinement of CuII–Phthalocyanine in a Bioinspired Hybrid Nanoparticle‐Assembled Structure Yields Selective and Stable Epoxidation Catalysts
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Joydeb Manna Arlin Jose Amali Dr. Rohit Kumar Rana 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(27):8453-8457
Herein, we demonstrate that a bioinspired assembly of silica nanoparticles with polyamines as structure‐directing agents similar to that known for the biosilicification of diatoms can pave the way for the efficient encapsulation of sulfonated copper–phthalocyanine in a hybrid microcapsule structure, in which the organic component provides a capable environment for its catalytic activity in epoxidation reactions and the nanoassembled structure imparts stability. 相似文献
147.
Dr. Arlin Jose Amali Dr. Hideto Hoshino Dr. Chun Wu Dr. Masanori Ando Prof. Qiang Xu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(27):8279-8282
Highly photoluminescent carbon nanodots (CNDs) were synthesized for the first time from metal–organic framework (MOF, ZIF‐8) nanoparticles. Coupled with fluorescence and non‐toxic characteristics, these carbon nanodots could potentially be used in biosafe color patterning. 相似文献
148.
Development of a Halide‐Free Aluminium‐Based Catalyst for the Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Jose A. Castro‐Osma Prof. Michael North Dr. Xiao Wu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(46):15005-15008
Kinetic studies of the synthesis of glycerol carbonate from glycidol and carbon dioxide have been carried out. These showed that under suitable reaction conditions, bimetallic aluminium(salen) complex 4 is able to catalyse the conversion of epoxides into the corresponding cyclic carbonates without the need for a co‐catalyst. 相似文献
149.
Gonzalo Abellán Jose A. Carrasco Eugenio Coronado Helena Prima-García 《Journal of Sol-Gel Science and Technology》2014,70(2):292-299
Pure and highly crystalline FeNi3 alloy nanoparticles (NPs) were synthesized via sol–gel route with benzyl alcohol, using hydrazine as a reduction reagent without the usage of additional surfactant molecules nor further annealing processes. The structural studies revealed that the particle size is of ca. 200 nm, whose structure consisted on aggregation of small crystallites of about 13 nm. The magnetic properties of the as-synthesized NPs were similar to the bulk with a saturation magnetization of 95 emu g?1. Moreover, the coercive field was ca. 50 G, exhibiting a M r /M s ratio of 0.03, indicative of soft ferromagnetism. The electrical transport in the temperature range 2–300 K exhibits a typical ferromagnetic metallic behaviour. Finally, similar FeNi3 NPs were synthesized in EtOH/H2O mixtures in the presence of sodium dodecyl sulphate molecules as surfactant for comparative purposes, exhibiting a typical half hard magnetic behaviour, highlighting the interest of the reported benzylic route. 相似文献
150.
Dr. Jamie Hicks Dr. Petra Vasko Andreas Heilmann Prof. Jose M. Goicoechea Prof. Simon Aldridge 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(46):20556-20560
The reactivity of the electron-rich anionic AlI aluminyl compound K2[(NON)Al]2 (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) towards mono- and disubstituted arenes is reported. C−H activation chemistry with n-butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer-type transition state. Selectivity is therefore based on similar electronic factors to classical SNAr chemistry, which implies the destabilisation of transition states featuring electron-donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C−H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C−H activation at a site which is neither ortho nor para to a methyl substituent. 相似文献