全文获取类型
收费全文 | 2217篇 |
免费 | 52篇 |
国内免费 | 8篇 |
专业分类
化学 | 1690篇 |
晶体学 | 21篇 |
力学 | 34篇 |
数学 | 263篇 |
物理学 | 269篇 |
出版年
2023年 | 21篇 |
2022年 | 66篇 |
2021年 | 73篇 |
2020年 | 50篇 |
2019年 | 30篇 |
2018年 | 48篇 |
2017年 | 42篇 |
2016年 | 49篇 |
2015年 | 78篇 |
2014年 | 62篇 |
2013年 | 124篇 |
2012年 | 130篇 |
2011年 | 143篇 |
2010年 | 96篇 |
2009年 | 68篇 |
2008年 | 141篇 |
2007年 | 130篇 |
2006年 | 123篇 |
2005年 | 110篇 |
2004年 | 89篇 |
2003年 | 101篇 |
2002年 | 68篇 |
2001年 | 33篇 |
2000年 | 27篇 |
1999年 | 19篇 |
1998年 | 17篇 |
1997年 | 17篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 15篇 |
1993年 | 11篇 |
1992年 | 20篇 |
1991年 | 25篇 |
1990年 | 15篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 14篇 |
1985年 | 21篇 |
1984年 | 16篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 13篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1972年 | 7篇 |
1968年 | 5篇 |
排序方式: 共有2277条查询结果,搜索用时 15 毫秒
41.
The consensus string problem for a metric is NP-complete 总被引:1,自引:0,他引:1
Given a set S of strings, a consensus string of S based on consensus error is a string w that minimizes the sum of the distances between w and all the strings in S. In this paper, we show that the problem of finding a consensus string based on consensus error is NP-complete when the penalty matrix is a metric. 相似文献
42.
This paper investigates the role of CDS volatility in providing information concerning the credit quality of a company. In Castellano and D’Ecclesia (J. Financ. Decis. Mak. 2:27, 2011) a first analysis of how CDS quotes respond to rating announcements is provided and it showed that market participants do not rely much on Rating Agencies, especially during periods characterized by very high volatility, i.e. during a financial crisis. Here, a more accurate analysis of the CDS’s ability to provide timely information on the creditworthiness of reference entities is performed, estimating the volatility of CDS quotes by using Exponential GARCH(1,1) models. The event study methodology is applied to a sample of CDS quotes for US and European markets, over the period 2004–2009. Results provide an accurate understanding of market behavior in the presence of news released by Rating Agencies. Overall, market participants seem to provide timely reactions around the event date and we show that the key element of signaling is represented by the changing volatility in CDS quotes, before and after the rating event. 相似文献
43.
We here introduce a laser-driven process to pattern transparent thin films on transparent substrates. This method utilizes a pre-patterned metal film as the dynamic release layer and the transparent thin film is selectively removed by a thermo-elastic force laser-induced in the underlying metal layer. High-fidelity indium tin oxide (ITO) thin film patterns were fabricated on plastic and glass substrates using a pulsed Nd:YAG laser. Tens of square centimeters could be patterned with several pulse shots. We fabricated a pentacene thin film transistor with ITO source and drain electrodes and observed a very low off-current level. This tells that the channel area between ITO electrodes was completely etched out by this laser-driven process. Combined with the absence of photoresist and chemical etching steps, this method provides a simple high-resolution route to pattern transparent thin films over large areas at low temperatures. 相似文献
44.
Novice students have difficulty with the topic of limits in calculus. We believe this is in part because of the multiple perspectives and shifting metaphors available to solve items correctly. We investigated college calculus instructors' personal concepts of limits. Based upon previous research investigating introductory calculus student metaphorical reasoning, we examined 11 college instructors' metaphorical reasoning on limit concepts. This paper focused on previous research of metaphor clusters observed among students to answer the following: (a) Do college instructors use metaphorical reasoning to conceptualize the meaning of a limit? (b) Can we characterize instructor metaphorical reasoning similar to those observed among students? (c) Will an instructor's self‐identification of metaphor clusters be consistent with our metaphor coding? We found that college instructors' perspectives vary, either graphical or algebraic, in their explanations of limit items. All the instructors used metaphors, and instructor metaphorical reasoning aligned with student metaphor clusters. Instructors tended to change their metaphors with respect to the limit item. Instructors were not aware of their use of metaphors, nor were they aware of their inconsistency in their choice of metaphor. We believe that instructor awareness of their own distinct perspectives and metaphors would assist students' understanding of limit concepts. 相似文献
45.
Ariel Carreño-Montero Luis A. Maldonado María Isabel Chávez Simón Hernández-Ortega Guillermo Delgado 《Tetrahedron letters》2019,60(48):151282
The synthesis of ethyl (2′-hydroxy-4′,5′-methylendioxophenyl)acetate, a fragment of the antihyperglycemic natural coumarin subcoriacin, is reported. We found an expeditious route to the title compound in five steps. Final metal catalyzed acid ethanolysis of the vinylic 1,1-methylthio methylsulfoxide derivative afforded the required aryl acetic ester, but in the absence of metal catalyst, an unexpected Pummerer rearrangement produced the 2,3-dimethylthiofuran derivative as the major product. This last result provides an alternative entry to 2,3-dimethlythiobenzofurans. 相似文献
46.
[Structure: see text] Unique core-modified porphyrinoids, such as oxabenziporphyrins, oxapyriporphyrins, and thiapyriporphyrins, bearing exocyclic C-C double bonds at meso-positions, have been synthesized and characterized. The synthesis was accomplished by utilizing typical "3+1"-type condensation. Two different stable tautomeric forms were isolated, and the two tautomeric forms can be interconvertible upon treatment with base. In contrast, only the structure bearing an exocyclic double bond was isolated in the case of oxapyriporphyrin and oxabenziporphyrin. 相似文献
47.
A robust tandem HPLC method coupling size-exclusion (Shodex Asahipak GS-320HQ) and reversed phase (Vydac 218TP54) columns with ultraviolet detection was developed for quantitative determination of interferon-gamma (IFN-gamma) in Chinese hamster ovary cell culture supernatant. The 2D-HPLC system was linked up by a 6-port 2-position low hold-up volume switch valve. Compared to a commercial ELISA kit for IFN-gamma, the coupled column LC approach was able to detect and quantify soluble IFN-gamma, regardless of the glycoprotein's molecular/conformational variability and sample background. Each LC-LC analysis took 90 minutes inclusive of column regeneration. The relative standard deviation of measurements (n = 5) was less than 3%. The limit of detection (LOD) was determined to be 0.35 microg IFN-gamma. 相似文献
48.
de Castro RA Canotilho J Barbosa RM Redinha JS 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2007,67(5):1194-1200
The molecular structure of conformational isomorphs given by X-ray diffraction for racemic and enantiomeric atenolol were optimized at the HF/6-31G* level of theory and the infrared spectra of the structure were calculated. These spectra are used to characterize the differences between the various atenolol conformers. The spectra of the (R,S)- and S-atenolol solid forms were recorded and the bands corresponding to the functional groups identified with the aid of the calculated spectra, fitting analysis, temperature effect and H/D isotopic exchange. Particular attention was paid to the stretch vibration modes of the functional groups present in the atenolol. 相似文献
49.
Melanie Miller William E. Robinson Ana Rita Oliveira Nina Heidary Nikolay Kornienko Julien Warnan Inês A. C. Pereira Erwin Reisner 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(14):4649-4653
The integration of enzymes with synthetic materials allows efficient electrocatalysis and production of solar fuels. Here, we couple formate dehydrogenase ( FDH ) from Desulfovibrio vulgaris Hildenborough (DvH) to metal oxides for catalytic CO2 reduction and report an in‐depth study of the resulting enzyme–material interface. Protein film voltammetry (PFV) demonstrates the stable binding of FDH on metal‐oxide electrodes and reveals the reversible and selective reduction of CO2 to formate. Quartz crystal microbalance (QCM) and attenuated total reflection infrared (ATR‐IR) spectroscopy confirm a high binding affinity for FDH to the TiO2 surface. Adsorption of FDH on dye‐sensitized TiO2 allows for visible‐light‐driven CO2 reduction to formate in the absence of a soluble redox mediator with a turnover frequency (TOF) of 11±1 s?1. The strong coupling of the enzyme to the semiconductor gives rise to a new benchmark in the selective photoreduction of aqueous CO2 to formate. 相似文献
50.
The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S1 and T1 excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 相似文献