全文获取类型
收费全文 | 945篇 |
免费 | 38篇 |
国内免费 | 5篇 |
专业分类
化学 | 688篇 |
力学 | 37篇 |
综合类 | 1篇 |
数学 | 110篇 |
物理学 | 152篇 |
出版年
2023年 | 8篇 |
2022年 | 13篇 |
2021年 | 18篇 |
2020年 | 27篇 |
2019年 | 18篇 |
2018年 | 10篇 |
2017年 | 17篇 |
2016年 | 17篇 |
2015年 | 28篇 |
2014年 | 44篇 |
2013年 | 50篇 |
2012年 | 88篇 |
2011年 | 80篇 |
2010年 | 49篇 |
2009年 | 42篇 |
2008年 | 80篇 |
2007年 | 62篇 |
2006年 | 56篇 |
2005年 | 39篇 |
2004年 | 29篇 |
2003年 | 24篇 |
2002年 | 13篇 |
2001年 | 13篇 |
2000年 | 21篇 |
1999年 | 13篇 |
1998年 | 5篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1991年 | 11篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1958年 | 3篇 |
1956年 | 2篇 |
排序方式: 共有988条查询结果,搜索用时 31 毫秒
931.
932.
A series of imidazole-containing rod-like Schiff's bases and their ionic copper(II) chelates with various lengths of the terminal alkyl chain containing 6, 8, 10, 12, 14 and 16 carbon atoms have been synthesised. The synthesised compounds were characterised by elemental analyses, 1H NMR, IR and UV–vis and mass spectroscopies. Thermotropic smectic C mesophases in the ligands and smectic A mesophases in the copper(II) complexes were identified using POM, DSC and small-angle XRD scattering methods. X-ray diffraction patterns of the prepared imidazole imines indicate to supramolecular self-assembled structures in the liquid crystal state, which are formed by means of intermolecular hydrogen bonds. It was established that both liquid crystal arrangement and supramolecular assemblies in ligands disappeared near 190°C, mainly regardless of the lengths of the terminal alkyl chains. Contrary, assembling of the copper(II) complexes into supramolecular bilayers occurs near 200°C, which causes their transition to a smectic A mesophase. 相似文献
933.
934.
Funding small and medium-sized enterprises (SMEs) to support technological innovation is critical for national competitiveness. Technology credit scoring models are required for the selection of appropriate funding beneficiaries. Typically, a technology credit-scoring model consists of several attributes and new models must be derived every time these attributes are updated. However, it is not feasible to develop new models until sufficient historical evaluation data based on these new attributes will have accumulated. In order to resolve this limitation, we suggest the framework to update the technology credit scoring model. This framework consists of ways to construct new technology credit-scoring model by comparing alternative scenarios for various relationships between existing and new attributes based on explanatory factor analysis, analysis of variance, and logistic regression. Our approach can contribute to find the optimal scenario for updating a scoring model. 相似文献
935.
A double channel structure has been used by depositing a thin amorphous‐AlZnO (a‐AZO) layer grown by atomic layer deposition between a ZnO channel and a gate dielectric to enhance the electrical stability. The effect of the a‐AZO layer on the electrical stability of a‐AZO/ZnO thin‐film transistors (TFTs) has been investigated under positive gate bias and temperature stress test. The use of the a‐AZO layer with 5 nm thickness resulted in enhanced subthreshold swing and decreased Vth shift under positive gate bias/temperature stress. In addition, the falling rate of the oxide TFT using a‐AZO/ ZnO double channel had a larger value (0.35 eV/V) than that of pure ZnO TFT (0.24 eV/V). These results suggest that the interface trap density between dielectric and channel was reduced by inserting a‐AZO layer at the interface between the channel and the gate insulator, compared with pure ZnO channel. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
936.
A Theoretical Investigation into the Luminescent Properties of d8‐Transition‐Metal Complexes with Tetradentate Schiff Base Ligands
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. Glenna So Ming Tong Dr. Pui Keong Chow Dr. Wai‐Pong To Prof. Wai‐Ming Kwok Prof. Chi‐Ming Che 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(21):6433-6443
A theoretical investigation on the luminescence efficiency of a series of d8 transition‐metal Schiff base complexes was undertaken. The aim was to understand the different photophysics of [M‐salen]n complexes (salen=N,N′‐bis(salicylidene)ethylenediamine; M=Pt, Pd (n=0); Au (n=+1)) in acetonitrile solutions at room temperature: [Pt‐salen] is phosphorescent and [Au‐salen]+ is fluorescent, but [Pd‐salen] is nonemissive. Based on the calculation results, it was proposed that incorporation of electron‐withdrawing groups at the 4‐position of the Schiff base ligand should widen the 3MLCT–3MC gap (MLCT=metal‐to‐ligand charge transfer and MC=metal centered, that is, the dd excited state); thus permitting phosphorescence of the corresponding PdII Schiff base complex. Although it is experimentally proven that [Pd‐salph‐4E] (salph=N,N′‐bis(salicylidene)‐1,2‐phenylenediamine; 4E means an electron‐withdrawing substituent at the 4‐position of the salicylidene) displays triplet emission, its quantum yield is low at room temperature. The corresponding PtII Schiff base complex, [Pt‐salph‐4E], is also much less emissive than the unsubstituted analogue, [Pt‐salph]. Thus, a detailed theoretical analysis of how the substituent and central metal affected the photophysics of [M‐salph‐X] (X is a substituent on the salph ligand, M=Pt or Pd) was performed. Temperature effects were also investigated. The simple energy gap law underestimated the nonradiative decay rates and was insufficient to account for the temperature dependence of the nonradiative decay rates of the complexes studied herein. On the other hand, the present analysis demonstrates that inclusions of low‐frequency modes and the associated frequency shifts are decisive in providing better quantitative estimates of the nonradiative decay rates and the experimentally observed temperature effects. Moreover, spin–orbit coupling, which is often considered only in the context of radiative decay rate, has a significant role in determining the nonradiative rate as well. 相似文献
937.
In Sun Hwang So Ra Lee Sooyeul Cho Kab Ryong Chae Hyoung Jun Park Ji Hyun Lee Ok Soon Heo Seung Yi Hong Woo-Seong Kim 《Accreditation and quality assurance》2014,19(1):41-46
South Korea’s Ministry of Food and Drug Safety (MFDS) has been developing programs for the inspection and accreditation of food sanitation inspection institutions. Food sanitation inspection institutions such as MFDS regional offices, the Research Institute of Public Health and Environment and authorized private service providers in South Korea must participate in proficiency testing (PT) programs to comply with the Food Sanitation Act and MFDS Notification No. 2012-112. As the PT provider, the MFDS annually plans various microbiological and chemical PT programs for foods, cosmetics, and pharmaceutical products in accordance with ISO/IEC 17043. The aim of this project was to evaluate the performance of microbiological PT programs to ensure the quality of their routine test results. The test materials used were freeze-dried BioBalls from BTF Pty Ltd. Homogeneity and stability were investigated to assess the adequacy of the selected test materials. This project also contains data from inter-laboratory comparisons organized by MFDS in 2011 and 2012. More than 50 laboratories attended the PT program and submitted their results. Laboratory results were rated with z-scores according to the international standard ISO 13528. The results from 2011 and 2012 revealed that all participating laboratories had similar levels of proficiency. Most of the participants received a rating of “Satisfactory.” Moreover, the percentage of participants who received a rating of “Unsatisfactory” decreased from 3.5 % in 2011 to 2.0 % in 2012. 相似文献
938.
Dr. Siew‐Peng Chia Dr. Emma Carter Dr. Hong‐Wei Xi Dr. Yongxin Li Dr. Cheuk‐Wai So 《Angewandte Chemie (International ed. in English)》2014,53(32):8455-8458
The two‐electron reduction of a Group 14‐element(I) complex [RË?] (E=Ge, R=supporting ligand) to form a novel low‐valent dianion radical with the composition [RË:]. 2? is reported. The reaction of [LGeCl] ( 1 , L=2,6‐(CH?NAr)2C6H3, Ar=2,6‐iPr2C6H3) with excess calcium in THF at room temperature afforded the germylidenediide dianion radical complex [LGe]. 2??Ca(THF)32+ ( 2 ). The reaction proceeds through the formation of the germanium(I) radical [LGe?], which then undergoes a two‐electron reduction with calcium to form 2 . EPR spectroscopy, X‐ray crystallography, and theoretical studies show that the germanium center in 2 has two lone pairs of electrons and the radical is delocalized over the germanium‐containing heterocycle. In contrast, the magnesium derivative of the germylidendiide dianion radical is unstable and undergoes dimerization with concurrent dearomatization to form the germylidenide anion complex [C6H3‐2‐{C(H)?NAr}Ge‐Mg‐6‐{C(H)‐NAr}]2 ( 3 ). 相似文献
939.
Chan PH So PK Ma DL Zhao Y Lai TS Chung WH Chan KC Yiu KF Chan HW Siu FM Tsang CW Leung YC Wong KY 《Journal of the American Chemical Society》2008,130(20):6351-6361
The fluorescein-labeled E166C mutant of the PenPC beta-lactamase (E166Cf) represents a successful model in the construction of "switch-on" fluorescent biosensors from nonallosteric proteins (Chan P.-H. et al.; J. Am Chem. Soc., 2004, 126, 4074). This paper focuses on the study of the biosensing mechanism by which the E166Cf biosensor changes its fluorescence upon beta-lactam binding and hydrolysis. Mass spectrometric and stopped-flow fluorescence studies of E166Cf with cefuroxime, penicillin G, and 6-aminopenicillanic acid reveal that the formation of enzyme-substrate complex enhances the fluorescence of E166Cf, and the subsequent regeneration of the free enzyme restores the weak fluorescence of E166Cf. Molecular modeling studies of E166Cf with penicillin G show that the fluorescein label is likely to share a common space with the beta-lactam and thiazolidine rings of the antibiotic in the active site. This spatial clash appears to cause the fluorescein label to move from the active site to the external aqueous environment upon substrate binding and hence experience higher water exposure. Steady-state fluorescence measurements indicate that the fluorescence of E166Cf can be enhanced by 6-aminopenicillanic acid, which consists of the beta-lactam and thiazolidine rings only. Thermal denaturation experiments of the wild-type enzyme, E166C, and E166Cf reveal that the E166C mutation is likely to increase the flexibility of the Omega-loop. This "modified" structural property might compensate for the possible steric effect of the fluorescein label on substrate binding. 相似文献
940.