全文获取类型
收费全文 | 845篇 |
免费 | 85篇 |
国内免费 | 4篇 |
专业分类
化学 | 730篇 |
晶体学 | 1篇 |
力学 | 16篇 |
数学 | 83篇 |
物理学 | 104篇 |
出版年
2024年 | 3篇 |
2023年 | 32篇 |
2022年 | 35篇 |
2021年 | 49篇 |
2020年 | 71篇 |
2019年 | 80篇 |
2018年 | 29篇 |
2017年 | 19篇 |
2016年 | 73篇 |
2015年 | 52篇 |
2014年 | 35篇 |
2013年 | 66篇 |
2012年 | 47篇 |
2011年 | 60篇 |
2010年 | 25篇 |
2009年 | 17篇 |
2008年 | 34篇 |
2007年 | 20篇 |
2006年 | 28篇 |
2005年 | 25篇 |
2004年 | 15篇 |
2003年 | 12篇 |
2002年 | 10篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1997年 | 3篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1988年 | 2篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1982年 | 2篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1973年 | 2篇 |
1934年 | 2篇 |
1932年 | 2篇 |
1931年 | 2篇 |
1930年 | 2篇 |
1928年 | 2篇 |
1919年 | 1篇 |
1913年 | 1篇 |
1894年 | 1篇 |
排序方式: 共有934条查询结果,搜索用时 15 毫秒
151.
Kroeze JE Hirata N Koops S Nazeeruddin MK Schmidt-Mende L Grätzel M Durrant JR 《Journal of the American Chemical Society》2006,128(50):16376-16383
The optimization of interfacial charge transfer is crucial to the design of dye-sensitized solar cells. In this paper we address the dynamics of the charge separation and recombination in liquid-electrolyte and solid-state cells employing a series of amphiphilic ruthenium dyes with varying hydrocarbon chain lengths, acting as an insulating barrier for electron-hole recombination. Dynamics of electron injection, monitored by time-resolved emission spectroscopy, and of charge recombination and regeneration, monitored by transient optical absorption spectroscopy, are correlated with device performance. We find that increasing dye alkyl chain length results in slower charge recombination dynamics to both the dye cation and the redox electrolyte or solid-state hole conductor (spiro-OMeTAD). These slower recombination dynamics are however paralleled by reduced rates for both electron injection into the TiO2 electrode and dye regeneration by the I-/I3- redox couple or spiro-OMeTAD. Kinetic competition between electron recombination with dye cations and dye ground state regeneration by the iodide electrolyte is found to be a key factor for liquid electrolyte cells, with optimum device performance being obtained when the dye regeneration is just fast enough to compete with electron-hole recombination. These results are discussed in terms of the minimization of kinetic redundancy in solid-state and liquid-electrolyte dye-sensitized photovoltaic devices. 相似文献
152.
Lara B. Anderson James Gray Dan Grayson Yang-Hui He André Lukas 《Communications in Mathematical Physics》2010,297(1):95-127
We present a practical, algebraic method for efficiently calculating the Yukawa couplings of a large class of heterotic compactifications
on Calabi-Yau three-folds with non-standard embeddings. Our methodology covers all of, though is not restricted to, the recently
classified positive monads over favourable complete intersection Calabi-Yau three-folds. Since the algorithm is based on manipulating
polynomials it can be easily implemented on a computer. This makes the automated investigation of Yukawa couplings for large
classes of smooth heterotic compactifications a viable possibility. 相似文献
153.
Lukas Pietzonka Thomas Lautenschläger Daniel Spemann Annemarie Finzel Jürgen W. Gerlach Frank Frost Carsten Bundesmann 《The European Physical Journal B - Condensed Matter and Complex Systems》2018,91(10):252
TiO2 thin films were grown by ion beam sputter deposition (IBSD) using oxygen ions, with the ion energy and geometrical parameters (ion incidence angle, polar emission angle, and scattering angle) being varied systematically. Metallic Ti and ceramic TiO2 served as target materials. The thin films were characterized concerning thickness, growth rate, surface topography, structural properties, mass density, and optical properties. It was found that the scattering geometry has the main impact on the film properties. Target material, ion energy, and ion incidence angle have only a marginal influence. Former studies on reactive IBSD of TiO2 using Ar and Xe ions reported equivalent patterns. Nevertheless, the respective ion species distinctively affects the film properties. For instance, mass density and the refractive index of the TiO2 thin films are remarkably lower for sputtering with oxygen ions than for sputtering with Ar or Xe ions. The variations in the thin film properties are tentatively attributed to the angular and the energy distribution of the film-forming particles, especially, to those of the backscattered primary particles. 相似文献
154.
An der Lan L Bartl P Leidlmair C Jochum R Denifl S Echt O Scheier P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(14):4411-4418
Helium atoms bind strongly to alkali cations which, when embedded in liquid helium, form so-called snowballs. Calculations suggest that helium atoms in the first solvation layer of these snowballs form rigid structures and that their number (n) is well defined, especially for the lighter alkalis. However, experiments have so far failed to accurately determine values of n. We present high-resolution mass spectra of Na(+)He(n), K(+)He(n), Na(2)(+)He(n) and K(2)(+)He(n), formed by electron ionization of doped helium droplets; the data allow for a critical comparison with several theoretical studies. For sodium and potassium monomers the spectra indicate that the value of n is slightly smaller than calculated. Na(2)(+)He(n) displays two distinct anomalies at n=2 and n=6, in agreement with theory; dissociation energies derived from experiment closely track theoretical values. K(2)(+)He(n) distributions are fairly featureless, which also agrees with predictions. 相似文献
155.
Danhelova H Hradecky J Prinosilova S Cajka T Riddellova K Vaclavik L Hajslova J 《Analytical and bioanalytical chemistry》2012,403(10):2883-2889
The development and use of a fast method employing a direct analysis in real time (DART) ion source coupled to high-resolution time-of-flight mass spectrometry (TOFMS) for the quantitative analysis of caffeine in various coffee samples has been demonstrated in this study. A simple sample extraction procedure employing hot water was followed by direct, high-throughput (<1 min per run) examination of the extracts spread on a glass rod under optimized conditions of ambient mass spectrometry, without any prior chromatographic separation. For quantification of caffeine using DART-TOFMS, an external calibration was used. Isotopically labeled caffeine was used to compensate for the variations of the ion intensities of caffeine signal. Recoveries of the DART-TOFMS method were 97% for instant coffee at the spiking levels of 20 and 60 mg/g, respectively, while for roasted ground coffee, the obtained values were 106% and 107% at the spiking levels of 10 and 30 mg/g, respectively. The repeatability of the whole analytical procedure (expressed as relative standard deviation, RSD, %) was <5% for all tested spiking levels and matrices. Since the linearity range of the method was relatively narrow (two orders of magnitude), an optimization of sample dilution prior the DART-TOFMS measurement to avoid saturation of the detector was needed. 相似文献
156.
Moravcova E Vaclavik L Lacina O Hrbek V Riddellova K Hajslova J 《Analytical and bioanalytical chemistry》2012,402(9):2871-2883
A sensitive and accurate method utilizing ultra-high performance liquid chromatography (U-HPLC) coupled to high resolution
mass spectrometry based on orbitrap technology (orbitrapMS) for the analysis of nine 3-chloropropane-1,2-diol (3-MCPD) diesters
in vegetable oils was developed. To remove the interfering triacylglycerols that induce strong matrix effects, a clean-up
step on silica gel column was used. The quantitative analysis was performed with the use of deuterium-labeled internal standards.
The lowest calibration levels estimated for the respective analytes ranged from 2 to 5 μg kg−1. Good recovery values (89–120%) and repeatability (RSD 5–9%) was obtained at spiking levels of 2 and 10 mg kg−1. As an alternative, a novel ambient desorption ionization technique, direct analysis in real time (DART), hyphenated with
orbitrapMS, was employed for no separation, high-throughput, semi-quantitative screening of 3-MCPD diesters in samples obtained
by chromatographic fractionation. Additionally, the levels of 3-MCPD diesters measured in real-life vegetable oil samples
(palm oil, sunflower oil, rapeseed oil) using both methods are reported. Relatively good agreement of the data generated by
U-HPLC-orbitrapMS and DART-orbitrapMS were observed. With regard to a low ionization yield achieved for 3-MCPD monoesters,
the methods presented in this paper were not yet applicable for the analysis of these contaminants at the naturally occurring
levels. 相似文献
157.
Decarboxylative Chan-Evans-Lam-type couplings are presented as a new strategy for the regiospecific construction of diaryl and alkyl aryl ethers starting from easily available aromatic carboxylic acids. They allow converting various aromatic carboxylate salts into the corresponding aryl ethers by reaction with alkyl orthosilicates or aryl borates, under aerobic conditions in the presence of silver carbonate as the decarboxylation catalyst and copper acetate as the cross-coupling catalyst. 相似文献
158.
Dipl.‐Chem. Lukas Zöphel Dr. Kunal S. Mali Dr. Puniredd Sreenivasa Reddy Dr. Manfred Wagner Prof. Dr. Steven De Feyter Dr. Wojciech Pisula Prof. Dr. Klaus Müllen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(11):3264-3276
Pyrene‐fused tetraazaporphyrins were synthesized from pyrene‐4,5‐dicarbonitrile precursors using a recently reported procedure as the key step for the asymmetric substitution of pyrene. Metal‐free, zinc‐ and lead‐centered pyrenocyanines were obtained and their optical properties as well as their molecular assembly in the solution and bulk phases and at the liquid/solid interface were studied. The characteristic Q‐band appears broadened, most likely owing to distortion of the molecule introduced by the steric demand of the angularly extended aromatic residue. The angular annulation does not bathochromically shift the Q‐band as far as would have been expected for the linear case. Peripheral substitution with linear and branched alkoxy chains affords solubility of the compounds in organic solvents. The influence of the distinct steric demand of the substituents on aggregation was investigated for metal‐centered pyrenocyanines by using temperature‐dependent 1H NMR and UV/Vis spectroscopy. The self‐assembly at the liquid/solid interface was studied using scanning tunneling microscopy. The alkoxy substituents facilitate the anchoring of these slightly non‐planar molecules on the surface of graphite. Pyrenocyanine molecules form well‐ordered 2D arrays in which the molecules are arranged in rows. The angular annulation of the pyrenocyanine residue leads to characteristic adsorption behavior at the liquid/solid interface, in which the molecules adsorb in two different adsorption geometries. The alkoxy side‐chains give rise to a discotic columnar superstructure and induce distinct thermotropic behavior. Dependent on the steric demand of the branched chains and the central metal atom, the molecules are rotated with respect to each other to form helical organization. 相似文献
159.
Gerber LC Moser N Luechinger NA Stark WJ Grass RN 《Chemical communications (Cambridge, England)》2012,48(32):3869-3871
Lanthanum oxide nanoparticles were utilized to scavenge phosphate from microbial growth media for the use of targeted nutrient starvation as an antimicrobial strategy. Only in phosphate poor environments a toxic effect was observed. The effect was shown on Escherichia coli, Staphylococcus carnosus, Penicillium roqueforti, and Chlorella vulgaris. 相似文献
160.
Nathalie Vermeulen J.E. Sipe Lukas G. Helt Hugo Thienpont 《Laser \u0026amp; Photonics Reviews》2012,6(6):793-801
Recent progress in the fabrication of high‐quality synthetic diamond and of diamond waveguide structures has enabled photonics researchers to start exploiting the unique optical properties of diamond for various applications. In this article the promise of on‐chip diamond ring resonators for wavelength conversion based on Kerr and Raman‐resonant four‐wave mixing is numerically demonstrated. After examining to what extent both dispersion‐engineered phase‐matching and “automatic” quasi‐phase‐matching can be established in diamond ring converters, it is shown that such a “double‐matching” approach can yield high conversion efficiencies for a wide range of wavelengths in the near‐infrared/mid‐infrared domain, as well as in the ultraviolet/visible domain. 相似文献