全文获取类型
收费全文 | 4926篇 |
免费 | 126篇 |
国内免费 | 6篇 |
专业分类
化学 | 3905篇 |
晶体学 | 6篇 |
力学 | 44篇 |
数学 | 642篇 |
物理学 | 461篇 |
出版年
2019年 | 40篇 |
2018年 | 36篇 |
2017年 | 26篇 |
2016年 | 102篇 |
2015年 | 64篇 |
2014年 | 99篇 |
2013年 | 141篇 |
2012年 | 145篇 |
2011年 | 153篇 |
2010年 | 129篇 |
2009年 | 117篇 |
2008年 | 161篇 |
2007年 | 157篇 |
2006年 | 178篇 |
2005年 | 198篇 |
2004年 | 168篇 |
2003年 | 137篇 |
2002年 | 183篇 |
2001年 | 110篇 |
2000年 | 108篇 |
1999年 | 105篇 |
1998年 | 85篇 |
1997年 | 95篇 |
1996年 | 97篇 |
1995年 | 76篇 |
1994年 | 88篇 |
1993年 | 90篇 |
1992年 | 92篇 |
1991年 | 86篇 |
1990年 | 67篇 |
1989年 | 89篇 |
1988年 | 87篇 |
1987年 | 84篇 |
1986年 | 71篇 |
1985年 | 96篇 |
1984年 | 111篇 |
1983年 | 62篇 |
1982年 | 86篇 |
1981年 | 113篇 |
1980年 | 83篇 |
1979年 | 101篇 |
1978年 | 68篇 |
1977年 | 68篇 |
1976年 | 59篇 |
1975年 | 77篇 |
1974年 | 50篇 |
1973年 | 71篇 |
1972年 | 51篇 |
1971年 | 43篇 |
1970年 | 53篇 |
排序方式: 共有5058条查询结果,搜索用时 15 毫秒
911.
Magic-angle sample spinning (MAS) nuclear magnetic resonance (NMR) measurements are treated for heterogeneous systems with nanometer dimensions. An appreciable line narrowing in the MAS NMR spectra of the embedded molecules may be achieved also in the cases when the molecules still possess an appreciable local mobility. It appears that the MAS frequencies are of comparable order of magnitude as the frequencies which characterize the random molecular motional processes and which compete with MAS. It will be shown that this behavior may occur if inhomogeneous local magnetic fields due to susceptibility effects have a dominating influence on the widths and shapes of the resonance NMR lines. Properties of these local fields are described. Spectra simulations are carried for molecules embedded in these heterogeneous systems when the coherent averaging by MAS is superimposed by random local motions. This situation may occur for molecules contained in nanoporous solids and also for heterogeneous systems like membranes and biological tissues with flexible components like water, lipids, and small peptides. Several examples are treated which reveal advantages and limitations of these experiments and their theoretical interpretation. 相似文献
912.
Biopolymers represent an interesting class of polymers whose potential is not yet been fully realised. However, more recently, biopolymers are gaining impetus in the market. Among the biopolymers, polylactic acid (PLA) is being used successfully and is widely accepted because it is produced from natural raw material and is degradable. Furthermore, it has comparable properties to standard thermoplastic polymers and can also be processed using similar process channels as conventional thermoplastic synthetic materials. Biopolymer applications are manifold and range from foils, fruit and vegetable packaging to hygiene and medical products. It can be expected that in future the application spectrum of biopolymers will increase even further. 相似文献
913.
914.
Markus Winkler Katrin Steinke Ramona Oehme Stefan Berger Dieter Sicker Hans‐Ullrich Siehl Klaus‐Peter Zeller 《Chemie in Unserer Zeit》2014,48(3):190-199
Raffinose has been obtained by extraction with 50% ethanol from the seeds of the Sweet Blue Lupine and purified first via a Cation exchange column and finally by analytical HPLC using a RI‐detector. All analytical spectra were recorded and are reproduced either in the main part or in the supporting information. The NMR‐ and mass‐spectra have been interpreted and compared with theoretical calculations of the 13C chemical shifts. The project is a follow up of the recent book “Classics in Spectroscopy” by S. Berger und D. Sicker (Wiley‐VCH 2009). 相似文献
915.
Dieter Whrle 《Chemie in Unserer Zeit》2014,48(5):376-395
Terrorism is directed by terrorist organizations and terrorist individuals with different ways of carrying out acts of violence against the democratically established order with the aim of political or religious changes. The focus is still the increasing number of attacks with explosives, which is discussed in this article more detailed. Also incendiary bombs are a simple possibility of attack. A major potential risk is seen through the spreading of toxic chemicals (chemical warfare agents, products from industrial production). This is also the case of attacks with radioactive materials by help of explosives. The misuse of chemistry is dicussed also under consideration of potential terrorist attacks by biological and nuclear weapons. Then due to the responsibility of our scientific community we can better engage ourselves in the discussion about terrorism, threats through attacks and possible avoidance strategies. 相似文献
916.
Stefan Wyhlidal Dieter Rank Katharina Schott Gerhard Heiss Jason Goetz 《Isotopes in environmental and health studies》2014,50(4):448-460
Results of stable isotope measurements (δ2H, δ18O) of daily grab samples, taken from the Danube River at Tulln (river km 1963) during 2012, show seasonal and short-term variations depending on the climatic/hydrological conditions and changes in the catchment area (temperature changes, heavy rains and snow melt processes). Isotope ratios in river water clearly reflect the isotopic composition of precipitation water in the catchment area since evaporation influences play a minor role. Average δ2H and δ18O values in 2012 are?78‰ and?11.0‰, respectively, deuterium excess averages 10‰. The entire variation amounts to 1.8‰ in δ18O and 15‰ in δ2H. Quick changes of the isotopic composition within a few days emphasise the necessity of daily sampling for the investigation of hydrological events, while monthly grab sampling seems sufficient for the investigation of long-term hydro-climatic trends. 3H results show peaks (half-width 1–2 days, up to about 150 TU) exceeding the regional environmental level of about 9 TU, probably due to releases from nuclear power plants. 相似文献
917.
Efficient generation of polarized single photons or entangled photon pairs is crucial for the implementation of quantum key distribution (QKD) systems. Self organized semiconductor quantum dots (QDs) are capable of emitting on demand one polarized photon or an entangled photon pair upon current injection. Highly efficient single‐photon sources consist of a pin structure inserted into a microcavity where single electrons and holes are funneled into an InAs QD via a submicron AlOx aperture, leading to emission of single polarized photons with record purity of the spectrum and non‐classicality of the photons. A new QD site‐control technique is based on using the surface strain field of an AlOx current aperture below the QD. GaN/AlN QD based devices are promising to operate at room temperature and reveal a fine‐structure splitting (FSS) depending inversely on the QD size. Large GaN/AlN QDs show disappearance of the FSS. Theory also suggests QDs grown on (111)‐oriented GaAs substrates as source of entangled photon pairs. 相似文献
918.
Matthias Mendt Bettina Jee Dieter Himsl Lutz Moschkowitz Tim Ahnfeldt Norbert Stock Martin Hartmann Andreas Pöppl 《Applied magnetic resonance》2014,45(3):269-285
Continuous-wave electron paramagnetic resonance spectroscopy is applied to explore the adsorption of carbon dioxide (CO2) over the metal organic framework (MOF) MIL-53. Therefore, paramagnetic Cr3+ ions, which replace a small amount of the bulk Al3+ ions in MIL-53(Al/Cr), are used as magnetically active probes. CO2 was adsorbed on samples of MIL-53(Al/Cr) at equilibrium pressures between 0 and 2.5 bar. The transformation from the large pore phase to the narrow pore phase of MIL-53 was observed by electron paramagnetic resonance spectroscopy at small CO2 pressures between 0.2 and 0.4 bar, which is in accordance with adsorption results reported in literature. By analyzing the electron paramagnetic resonance signal intensities of the corresponding Cr3+ probes, the ratio between the amount of the narrow pore phase and the large pore phase before and after this phase transformation was quantified. A small fraction of the large pore phase remains even after this phase transition. CO2 adsorption at 77 K indicates the occurrence of the transformation of this MOF from a narrow pore phase to a large pore phase triggered by the adsorbed CO2. Similar observations were already made using powder X-ray diffraction or infrared spectroscopy. But in contrast to these methods electron paramagnetic resonance spectroscopy on Cr3+ seems to be very sensitive not only to large differences between crystallographic conformations like large pores and narrow pores but also to different amounts and configurations of CO2 molecules trapped in the same structural phase of MIL-53, taking advantage of the high sensitivity of the fine structure interaction of Cr3+. 相似文献
919.
Basudev Pradhan Steve Albrecht Burkhard Stiller Dieter Neher 《Applied Physics A: Materials Science & Processing》2014,115(2):365-369
Inverted organic solar cells are fabricated using low-temperature-annealed ZnO film as an electron transport layer. Uniform ZnO films were prepared by spin coating a diethylzinc (DEZ) precursor solution in air, followed by annealing at 100 °C. Organic solar cells prepared on these ZnO films with a 1:1 P3HT:PCBM blend as the active layer show a high power conversion efficiency of 4.03 %, which is more than 10 % higher than the PCE of solar cells comprising ZnO prepared via a high-temperature sol–gel route. 相似文献
920.
While both organocatalysis and gold catalysis have their roots deeply entrenched in the landscape of modern organic chemistry, an exciting trend in the complementary merging of organocatalysis and especially Au(I) catalysis has emerged in the last four years. This niche area has been developing rapidly and this minireview serves to pin-point the fundamental concepts guiding reaction design in these binary catalytic systems. Moreover, the proven synthetic utility of organo/Au(I) multicatalytic systems in accessing molecular frameworks, previously a challenge to single catalytic systems, has resulted in this new concept permeating numerous areas of organocatalysis, such as primary/secondary amine, Br?nsted acid, hydrogen-bonding as well as N-heterocyclic carbene (NHC) catalysis. The first detailed account of these recent developments is systematically presented. 相似文献