全文获取类型
收费全文 | 1364篇 |
免费 | 47篇 |
国内免费 | 4篇 |
专业分类
化学 | 943篇 |
晶体学 | 10篇 |
力学 | 45篇 |
数学 | 147篇 |
物理学 | 270篇 |
出版年
2023年 | 7篇 |
2022年 | 11篇 |
2021年 | 14篇 |
2020年 | 17篇 |
2019年 | 21篇 |
2018年 | 13篇 |
2017年 | 10篇 |
2016年 | 27篇 |
2015年 | 32篇 |
2014年 | 61篇 |
2013年 | 71篇 |
2012年 | 73篇 |
2011年 | 101篇 |
2010年 | 42篇 |
2009年 | 47篇 |
2008年 | 94篇 |
2007年 | 84篇 |
2006年 | 77篇 |
2005年 | 65篇 |
2004年 | 56篇 |
2003年 | 35篇 |
2002年 | 41篇 |
2001年 | 15篇 |
2000年 | 26篇 |
1999年 | 18篇 |
1998年 | 13篇 |
1997年 | 9篇 |
1996年 | 16篇 |
1995年 | 11篇 |
1994年 | 11篇 |
1993年 | 15篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 18篇 |
1979年 | 16篇 |
1978年 | 11篇 |
1977年 | 15篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 19篇 |
1973年 | 12篇 |
排序方式: 共有1415条查询结果,搜索用时 15 毫秒
121.
A Poly(cobaloxime)/Carbon Nanotube Electrode: Freestanding Buckypaper with Polymer‐Enhanced H2‐Evolution Performance 下载免费PDF全文
Dr. Bertrand Reuillard Dr. Julien Warnan Jane J. Leung David W. Wakerley Dr. Erwin Reisner 《Angewandte Chemie (International ed. in English)》2016,55(12):3952-3957
A freestanding H2‐evolution electrode consisting of a copolymer‐embedded cobaloxime integrated into a multiwall carbon nanotube matrix by π–π interactions is reported. This electrode is straightforward to assemble and displays high activity towards hydrogen evolution in near‐neutral pH solution under inert and aerobic conditions, with a cobalt‐based turnover number (TONCo) of up to 420. An analogous electrode with a monomeric cobaloxime showed less activity with a TONCo of only 80. These results suggest that, in addition to the high surface area of the porous network of the buckypaper, the polymeric scaffold provides a stabilizing environment to the catalyst, leading to further enhancement in catalytic performance. We have therefore established that the use of a multifunctional copolymeric architecture is a viable strategy to enhance the performance of molecular electrocatalysts. 相似文献
122.
123.
Nasuh C. Buyukkaramikli J. Will M. Bertrand Henny P. G. van Ooijen 《Annals of Operations Research》2013,209(1):159-178
In this paper, we study a make-to-order production system with parallel, identical processing units. Each order needs to be satisfied on a single processing unit that is run by a crew. The inter-arrival time and the service time for each order are random variables. The system operates under a lead time performance constraint, which demands the completion of each order within a pre-determined lead time with a certain probability. The minimum number of processing units needed to satisfy this constraint is determined at the tactical level. Our research focuses on the cost savings that can be realized with the use of flexible crews via contractual hiring agreements with an External Labor Supply Agency (ELSA). The ELSA can periodically provide an agreed number of crews. The cost incurred for a flexible crew is higher than that for a permanent crew, and is decreasing in the period length. We model and analyze this system using the transient behavior analysis of multi-server queues and propose several empirically testable functions for the cost of flexible crews. In our computational study, we demonstrate possible cost savings of 2-level, threshold type hiring policies, relative to the fixed capacity system, under 9 scenarios with three demand-to-processing rate ratios and three lead time performance constraints, each of which reflects a different level of ambition. We observe that the maximum savings occur when the cost of a flexible crew is same as that of a permanent crew, and range from 29.38% to 50.56%. However, as the flexible crews become more expensive, the system may choose to employ permanent crews only. We observe that cost savings consist of two parts: savings due to the cancellation of the sclerosis of capacity discreteness, and savings due to the use of workload information in hiring actions. The latter part is higher for more ambitious lead time performance constraints, and for higher mean processing times. Finally, when there is an additional cost for transacting an agreement with the ELSA, we observe that the capacity flexibility option loses its charm, especially if the transaction cost is higher than the cost of a permanent crew. 相似文献
124.
The present work discusses the grafting by electron beam irradiation of poly(ethylene oxide) (PEO) star-shaped polymers onto porous expanded polytetrafluoroethylene (EXPTFE) surfaces. The resulting materials are intended to combine the good biocompatible properties of PEO with the outstanding mechanical properties of PTFE. The star-shaped PEOs were synthesized via anionic polymerization. 3 Mev electron beam irradiation was applied to graft these PEO stars onto porous EXPTFE surfaces. The hydrophobic EXPTFE surface had to be pre-modified with N-vinylpyrrolidone. ESCA was used to quantify the amount of grafted star-shaped PEO. Unmodified EXPTFE surfaces are well known, when implanted in a body, to be rapidly covered by a layer of cells and fibrin. The EXPTFE coated with PEO were implanted in the peritoneal cavity of rats (or under the back skin). This implantation did not induce any inflammation reactions and SEM analysis had attested the absence of adsorbed cells and fibrin. The glucose diffusion properties of these membranes were studied by a lag time analysis method and compared to those of pure PEO hydrogels. As expected, glucose diffuses through the hydrogel coated membrane and diffusion is not affected by the presence of the EXPTFE membrane. 相似文献
125.
The trans-fusarinine backbone is a common feature encountered in many fungal siderophores. This monomer is notably the structural base of Nα-methyl coprogen B and dimerumic acid. Both siderophores are known to be secreted by Scedosporium apiospermum, an emerging pathogenic fungus studied for its high involvement in invasive infections of immunocompromised patients. The strategy developed here for the synthesis of the trans-fusarinine scaffold relies on the preparation of both N-hydroxyornithine and 3-anhydroxymevalonic acid subunits starting from l-ornithine and 3-butyn-1-ol, respectively. The coupling of these two building blocks led to the expected protected backbone. 相似文献
126.
The structure of P,P-bis(diisopropylamino)-4-phenyl-1,3,2lambda(5)-diazaphosphete, 1a, has been determined by a single-crystal X-ray diffraction study (C(19)H(33)N(4)P, monoclinic system, space group P2(1), a = 9.482(1) ?, b = 11.374(3) ?, c = 9.668(2) ?, beta = 97.16(1) degrees, Z = 2). According to quantum chemical calculations at an RHF level of optimization utilizing the 6-31g(d,p) basis set, 1a has a zwitterionic structure with the negative charge delocalized on the NCN allylic fragment and the positive charge localized at the phosphorus. Heterocycle 1a reacts with water and benzaldehyde affording N-phosphoranylbenzamidine 3 (95% yield) and the expected aza-Wittig adduct 4 (85% yield), respectively. Addition of 1 equiv of methyl trifluoromethanesulfonate and of 2 equiv of BH(3).THF to 1a affords cyclic phosphonium salt 5 (94% yield) and the bis(borane) adduct 6a (90% yield), respectively. Dimethyl acetylenedicarboxylate slowly reacts with 1a giving rise to 1,3,4lambda(5)-diazaphosphinine, 9, in 70% yield. The X-ray crystal structures of products 2,3, and 6a are reported (2: C(26)H(38)N(5)P, monoclinic system, space group C2/c, a = 16.337(8) ?, b = 19.810(2) ?, c = 8.800(2) ?, beta = 117.68(2) degrees, Z = 4. 3: C(19)H(35)N(4)OP, orthorhombic system, space group P2(1)2(1)2(1), a = 9.090(1) ?, b = 12.955(2) ?, c = 17.860(3) ?, Z = 4. 6a: C(19)H(39)B(2)N(4)P, orthorhombic system, space group P2(1)2(1)2(1), a = 10.340(1) ?, b = 13.247(1) ?, c = 16.996(1) ?, Z = 4). 相似文献
127.
The chemical behaviour of sila-2 butadienes, formed as transient intermediates either by thermolysis or by photolysis of various 1-vinylsilacyclobutanes, was studied with respect to hydroxylated compounds of different pKa values Two mechanisms can explain the nature of the products obtained on the co-thermolysis of the cyclic compounds with phenol, one with 1-silacyclobut-1-ene intermediate and the other involving an allylic silicenium cation. In both hypothetical mechanisms, the 2-silabutadienes behave as a conjugated system since they lead either to (2+2) cycloaddition or to (1,2)- and (1,4)-electrophilic addition. This evidence for a conjugation phenomenon through a silicon atom is supported by the calculation of the delocalisation energies of butadiene and 2-silabutadiene. 相似文献
128.
A phenanthridinium salt was prepared in four steps, including an intramolecular CH-arylation and a hydride abstraction reaction. Treatment with sterically demanding bases does not lead to the corresponding carbene, but rather to addition products: the planar-constrained geometry significantly enhances the electrophilicity over the acidity of C-aryl iminium salts. 相似文献
129.
Dr. Damien Lhenry Dr. Manuel Larrouy Dr. Claire Bernhard Dr. Victor Goncalves Dr. Olivier Raguin Dr. Peggy Provent Dr. Mathieu Moreau Dr. Bertrand Collin Dr. Alexandra Oudot Dr. Jean‐Marc Vrigneaud Dr. François Brunotte Dr. Christine Goze Prof. Franck Denat 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(37):13091-13099
In molecular imaging, multimodal imaging agents can provide complementary information, for improving the accuracy of disease diagnosis or enhancing patient management. In particular, optical/nuclear imaging may find important preclinical and clinical applications. To simplify the preparation of dual‐labeled imaging agents, we prepared versatile monomolecular multimodal imaging probe (MOMIP) platforms containing both a fluorescent dye (BODIPY) and a metal chelator (polyazamacrocycle). One of the MOMIP was conjugated to a cyclopeptide (i.e., octreotide) and radiolabeled with 111In. In vitro and in vivo studies of the resulting bioconjugate were conducted, highlighting the potential of these BODIPY‐based bimodal probes. This work also confirmed that the biovector and/or the bimodal probes must be chosen carefully, due to the impact of the MOMIP on the overall properties of the resulting imaging agent. 相似文献
130.