全文获取类型
收费全文 | 2734篇 |
免费 | 89篇 |
国内免费 | 9篇 |
专业分类
化学 | 1918篇 |
晶体学 | 7篇 |
力学 | 28篇 |
数学 | 413篇 |
物理学 | 339篇 |
无线电 | 127篇 |
出版年
2021年 | 29篇 |
2020年 | 48篇 |
2019年 | 49篇 |
2018年 | 23篇 |
2017年 | 19篇 |
2016年 | 72篇 |
2015年 | 79篇 |
2014年 | 82篇 |
2013年 | 101篇 |
2012年 | 126篇 |
2011年 | 144篇 |
2010年 | 95篇 |
2009年 | 71篇 |
2008年 | 104篇 |
2007年 | 131篇 |
2006年 | 134篇 |
2005年 | 122篇 |
2004年 | 83篇 |
2003年 | 77篇 |
2002年 | 90篇 |
2001年 | 58篇 |
2000年 | 37篇 |
1999年 | 40篇 |
1998年 | 40篇 |
1997年 | 29篇 |
1996年 | 24篇 |
1995年 | 32篇 |
1994年 | 34篇 |
1993年 | 29篇 |
1992年 | 45篇 |
1991年 | 26篇 |
1990年 | 27篇 |
1989年 | 29篇 |
1988年 | 35篇 |
1987年 | 41篇 |
1986年 | 31篇 |
1985年 | 44篇 |
1984年 | 31篇 |
1983年 | 22篇 |
1982年 | 22篇 |
1981年 | 30篇 |
1980年 | 33篇 |
1979年 | 22篇 |
1978年 | 30篇 |
1977年 | 23篇 |
1976年 | 28篇 |
1975年 | 32篇 |
1974年 | 34篇 |
1973年 | 32篇 |
1971年 | 20篇 |
排序方式: 共有2832条查询结果,搜索用时 9 毫秒
141.
Dr. Mario Leypold Lukas Schuh Prof. Roland Fischer Dr. Ana Torvisco Prof. Michaela Flock Prof. Harald Stueger Dr. Michael Haas 《Angewandte Chemie (International ed. in English)》2017,56(28):8089-8093
Herein, we report on the first sila-aldol reaction, which emphasizes the tight connection between silicon and carbon chemistry. This novel synthetic method provides straightforward access to 2-oxahexasilabicyclo[3.2.1]octan-8-ide, a structurally complex silicon framework, in quantitative yield. Its structure was confirmed by NMR spectroscopy and X-ray crystallography, and it displays a distinctive charge-transfer transition. The complete mechanism of this highly selective rearrangement cascade is outlined and supported by density functional theory (DFT) calculations, which highlight the thermodynamic driving force and the low activation barriers of this powerful silicon–carbon bond-forming strategy. 相似文献
142.
143.
The 1D- and 2D-6Li, 6Li-INADEQUATE experiments are described as new tools for the detection of scalar coupled nonequivalent 6Li nuclei in organolithium clusters. Practical applications of these sequences are demonstrated for the 6Li-NMR spectra of (E)-1-lithio-2-(2-lithiophenyl)-1-phenylhex-1-ene ( 1 ) and (E)-2-lithio-1-(2-lithiophenyl)-1-phenylpent-1-ene ( 2 ), where signals due to dimers and monomers can be distinguished. The performance of the 2D-6Li, 6Li-INADEQUATE and the 6Li, 6Li-COSY-45-LR experiment are compared. The 6Li chemical shifts of 1 and 2 are discussed. 相似文献
144.
Harald Rennhofer Janea Khnke Jozef Keckes Johannes Tintner Christoph Unterweger Thomas Zinn Karl Deix Helga Lichtenegger Wolfgang Gindl-Altmutter 《Molecules (Basel, Switzerland)》2021,26(7)
Application of low-cost carbon black from lignin highly depends on the materials properties, which might by determined by raw material and processing conditions. Four different technical lignins were subjected to thermostabilization followed by stepwise heat treatment up to a temperature of 2000 °C in order to obtain micro-sized carbon particles. The development of the pore structure, graphitization and inner surfaces were investigated by X-ray scattering complemented by scanning electron microscopy and FTIR spectroscopy. Lignosulfonate-based carbons exhibit a complex pore structure with nanopores and mesopores that evolve by heat treatment. Organosolv, kraft and soda lignin-based samples exhibit distinct pores growing steadily with heat treatment temperature. All carbons exhibit increasing pore size of about 0.5–2 nm and increasing inner surface, with a strong increase between 1200 °C and 1600 °C. The chemistry and bonding nature shifts from basic organic material towards pure graphite. The crystallite size was found to increase with the increasing degree of graphitization. Heat treatment of just 1600 °C might be sufficient for many applications, allowing to reduce production energy while maintaining materials properties. 相似文献
145.
146.
Harald Schfer 《无机化学与普通化学杂志》1982,489(1):154-160
Structure and Properties of Cesium Hydroxide Monohydrate, a Compound Characterized by Layered [H3O2?] Polyanions in its High Temperature Form Cesium hydroxide monohydrate, which was formed at the synthesis of cesium hydride as a by-product, was obtained in form of single crystals by recrystallization from ammonia in high pressure autoclaves. Temperature dependent X-ray structure investigations and measurements of the specific heat show the occurance of several modifications. At 293 K X-ray data prove that it is possible to distinguish between OH? ions and H2O molecules. This can also be confirmed by IR spectroscopy. At 355 K and 400 K the investigations on single crystals show layered [H3O2?] polyanions, which are separated by layers of cesium ions in a hexagonal unit cell. 相似文献
147.
Raphael Rataj Matthias Werneburg Dr. Harald Below Prof. Dr. Juergen F. Kolb 《Chemphyschem》2023,24(19):e202300143
The production of hydrogen peroxide (H2O2) is a key parameter for the performance of pulsed discharges submerged in water utilized as advanced oxidation process. So far, any related assessment of the underlying mechanism was conducted for the application of several hundred discharges, which did not allow for a correlation with physical processes. Moreover, the production was rarely investigated depending on water conductivity as one of the most important parameters for the development of submerged discharges. Accordingly, hydrogen peroxide generation was investigated here for individual single discharge events instigated with 100 ns high-voltage pulses in water with three different conductivities and was associated with the discharge development, i. e. spatial expansion and dissipated electrical energy. The approach necessitated the improvement of an electrochemical flow injection analysis based on the reaction of Prussian blue with H2O2. Hydrogen peroxide concentrations were quadratically increasing with propagation time and stable for different water conductivities. H2O2 production per unit volume of a discharge was constant over time with an estimated rate constant of 3.2 mol ⋅ m−1 s−1, averaged over the crosssectional area of all discharge filaments. However, the individually dissipated energy increased with conductivity, hence, the production efficiency decreased from 6.1 g ⋅ kWh−1 to 1.4 g ⋅ kWh−1, which was explained by increased resistive losses within the bulk liquid. 相似文献
148.
149.
Material– and flow properties of magnetic fluids can be influenced by applying an external magnetic field. In this work we will particularly consider the onset of convection in magnetic fluids which is influenced by a magnetic force. In a horizontal magnetic fluid layer the force arises if a temperature gradient and an external magnetic field is applied. The behaviour of the onset of convection is investigated for a static and a time–modulated magnetic field. For the case of a static magnetic field the onset of convection depends on the strength of the field and for a time–modulated magnetic field an additional dependence on the frequency of magnetic field variation is found. The experimental results presented here confirm in principle the theoretical predictions about the influence of static and time–modulated magnetic forces on the onset of convection. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
150.
This paper analyzes the level and cyclicality of regulatory bank capital for asset portfolio securitizations in relation to the cyclicality of capital requirements for the underlying loan portfolio as under Basel II/III. We find that the cyclicality of capital requirements is higher for (i) asset portfolio securitizations relative to primary loan portfolios, (ii) Ratings Based Approach (RBA) relative to the Supervisory Formula Approach, (iii) given the RBA for a point-in-time rating methodology relative to a rate-and-forget rating methodology, and (iv) under the passive reinvestment rule relative to alternative rules. Capital requirements of the individual tranches reveal that the volatility of aggregated capital charges for the securitized portfolio is triggered by the most senior tranches. This is due to the fact that senior tranches are more sensitive to the macroeconomy. An empirical analysis provides evidence that current credit ratings are time-constant and that economic losses for securitizations have exceeded the required capital in the recent financial crisis. 相似文献