全文获取类型
收费全文 | 507篇 |
免费 | 26篇 |
国内免费 | 1篇 |
专业分类
化学 | 326篇 |
晶体学 | 2篇 |
力学 | 16篇 |
数学 | 94篇 |
物理学 | 96篇 |
出版年
2022年 | 4篇 |
2021年 | 13篇 |
2020年 | 7篇 |
2019年 | 15篇 |
2018年 | 6篇 |
2017年 | 6篇 |
2016年 | 13篇 |
2015年 | 17篇 |
2014年 | 17篇 |
2013年 | 28篇 |
2012年 | 39篇 |
2011年 | 43篇 |
2010年 | 26篇 |
2009年 | 19篇 |
2008年 | 27篇 |
2007年 | 36篇 |
2006年 | 24篇 |
2005年 | 31篇 |
2004年 | 23篇 |
2003年 | 13篇 |
2002年 | 18篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 3篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1970年 | 3篇 |
1966年 | 2篇 |
1923年 | 2篇 |
1913年 | 2篇 |
排序方式: 共有534条查询结果,搜索用时 468 毫秒
471.
A new aldol ester named 17-O-triacontanoylheptadecanal (1) was isolated from the aerial part of Mimosa invisa (Mimosaceae) together with eight known compounds identified as β-sitosterol (2), α-amyrine (3), lupeol (4), 4'-O-methylepinumisoflavone (5), alpinumisoflavone (6), betulinic acid (7), 3-O-β-D-glucopyranoside of sitosterol (8) and epirobinetinidol (9). The structures of compounds were determined on the basis of NMR and mass spectrometry data as well as by comparing the data reported in the literatures. The antimicrobial activities of the crude extract and compounds 1 and 9 were investigated against seven microbial species. The natural products showed moderate activities compared to that of the crude extract. 相似文献
472.
Virus-cell membrane fusion requires a critical transition from positive to negative membrane curvature. St.?Vincent et?al. (2010), in PNAS, designed a class of antivirals that targets this transition. These rigid amphipathic fusion inhibitors are active against an array of enveloped viruses. 相似文献
473.
Emília Illeková Helene Lefaix Frederic Prima Dušan Janičkovič Peter Švec 《Journal of Thermal Analysis and Calorimetry》2010,99(3):957-963
The influence of the solidification rate, the thermal stability, and devitrification process of the rapidly solidified Ti45Zr38Ni17 alloy have been examined on ribbons prepared by the planar flow casting method. Differential scanning calorimetry in the
continuous heating mode, X-ray diffraction, and electron microscopy techniques were applied. Comparison of the so-obtained
microstructures revealed the competition between icosahedral quasi-crystalline and β-Ti(Zr) phases, both dispersed in an amorphous
matrix. It has been found that the decomposition process of rapidly quenched ribbons consists of the sequence of several independent
exothermic and endothermic reactions involving the additional precipitation of quasi-crystalline nanoparticles and both irreversible
and reversible changes between the unstable high-temperature β and stable low-temperature α phases. The formation of the intermetallics
as well as the transformation of quasi-crystals into Laves phase has been observed at higher temperatures in all ribbons. 相似文献
474.
We model a market in which suppliers bid step-function offer curves using agent-based modeling. Our model is an abstraction of electricity markets where step-function offer curves are given to an independent system operator that manages the auctions in electricity markets. Positing an elementary and computationally accessible learning model, Probe and Adjust, we present analytic results that characterize both the behavior of the learning model and the properties of step-function equilibria. Thus, we have developed a framework for validating agent-based models prior to using them in situations that are too complicated to be analyzed using traditional economic theory. In addition, we demonstrate computationally that, by using alternative policies, even simple agents can achieve monopoly rewards for themselves by pursuing more industry-oriented strategies. This raises the issue of how participants in oligopolistic markets actually behave. 相似文献
475.
We report the development of two methods for determining the effective electroacoustic tensor of 1-3 piezocomposite material, one experimental and one theoretical based on homogenization techniques. The main aim was to compare and validate the results provided by these approaches. The slowness surfaces of bulk wave were computed in the large wavelength domain and were fitted to obtain the effective properties of the composite. Model predictions are discussed and compared with the Smith's model. The experimental method is an inversion technique comparing measurement of transmission coefficient through the piezocomposite plate with the simulated coefficient. The accuracy and stability of the minimization procedure is discussed. Experimental results obtained from two piezocomposite test plates are presented and compared with theory. 相似文献
476.
Godfrey JD Fox RT Buono FG Gougoutas JZ Malley MF 《The Journal of organic chemistry》2006,71(22):8647-8650
Conversion of an alpha,alpha-dichloroester to the corresponding alpha-keto acid was unexpectedly complicated by a novel 1,4-homofragmentation. Investigation of the kinetics of this reaction revealed a mechanism involving an alpha-lactone intermediate, which can lead to both the desired alpha-keto acid and the 1,4-homofragmentation, with the product distribution being dependent upon reaction conditions. This information allowed development of a process that affords the alpha-keto acid exclusively and should be generally applicable to the preparation of alpha-keto acids from alpha,alpha-dichloroesters or acids. 相似文献
477.
Nonlinear effects are known to occur in Compton scattering light sources, when the laser normalized potential A approaches unity. In this Letter, it is shown that nonlinear spectral features can appear at arbitrarily low values of A, if the fractional bandwidth of the laser pulse Δ??1 is sufficiently small to satisfy A2Δ??1. A three-dimensional analysis, based on a local plane wave, slow-varying envelope approximation, enables the study of these effects for realistic interactions between an electron beam and a laser pulse, and their influence on high-precision Compton scattering light sources. 相似文献
478.
In this experimental investigation, we explore the feasibility of using wide-field illumination for time-resolved fluorescence molecular tomography. The performance of wide-field patterns with a time-resolved imaging platform is investigated in vitro and in a small animal model. A Monte Carlo-based forward model is employed to reconstruct fluorescence yield based on time-gated datasets. An improvement in resolution and quantification when using the time-gate data type compared to the commonly used cw data type is demonstrated in vitro. Furthermore, the feasibility of wide-field strategies for fluorescence preclinical applications is established by an accurate localization of a fluorescent inclusion implanted in the chest cavity of a murine model. 相似文献
479.
Nazarov AA Risse J Ang WH Schmitt F Zava O Ruggi A Groessl M Scopelitti R Juillerat-Jeanneret L Hartinger CG Dyson PJ 《Inorganic chemistry》2012,51(6):3633-3639
Anthracene derivatives of ruthenium(II) arene compounds with 1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane (pta) or a sugar phosphite ligand, viz., 3,5,6-bicyclophosphite-1,2-O-isopropylidene-α-d-glucofuranoside, were prepared in order to evaluate their anticancer properties compared to the parent compounds and to use them as models for intracellular visualization by fluorescence microscopy. Similar IC(50) values were obtained in cell proliferation assays, and similar levels of uptake and accumulation were also established. The X-ray structure of [{Ru(η(6)-C(6)H(5)CH(2)NHCO-anthracene)Cl(2)(pta)] is also reported. 相似文献
480.
F Poineau EV Johnstone PM Forster L Ma AP Sattelberger KR Czerwinski 《Inorganic chemistry》2012,51(17):9563-9570
The cesium salts of [Tc(2)X(8)](3-) (X = Cl, Br), the reduction product of (n-Bu(4)N)[TcOCl(4)] with (n-Bu(4)N)BH(4) in THF, and the product obtained from reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C have been characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. For the [Tc(2)X(8)](3-) anions, the Tc-Tc separations found by EXAFS spectroscopy (2.12(2) ? for both X = Cl and Br) are in excellent agreement with those found by single-crystal X-ray diffraction (SCXRD) measurements (2.117[4] ? for X = Cl and 2.1265(1) ? for X = Br). The Tc-Tc separation found by EXAFS in these anions is slightly shorter than those found in the [Tc(2)X(8)](2-) anions (2.16(2) ? for X = Cl and Br). Spectroscopic and SCXRD characterization of the reduction product of (n-Bu(4)N)[TcOCl(4)] with (n-Bu(4)N)BH(4) are consistent with the presence of dinuclear species that are related to the [Tc(2)Cl(8)](n-) (n = 2, 3) anions. From these results, a new preparation of (n-Bu(4)N)(2)[Tc(2)Cl(8)] was developed. Finally, EXAFS characterization of the product obtained from reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C indicates the presence of amorphous α-TcCl(3). The Tc-Tc separation (i.e., 2.46(2) ?) measured in this compound is consistent with the presence of Tc═Tc double bonds in the [Tc(3)](9+) core. 相似文献