全文获取类型
收费全文 | 770篇 |
免费 | 38篇 |
国内免费 | 5篇 |
专业分类
化学 | 452篇 |
晶体学 | 2篇 |
力学 | 33篇 |
数学 | 173篇 |
物理学 | 153篇 |
出版年
2022年 | 14篇 |
2021年 | 14篇 |
2020年 | 28篇 |
2019年 | 15篇 |
2018年 | 21篇 |
2017年 | 8篇 |
2016年 | 21篇 |
2015年 | 14篇 |
2014年 | 27篇 |
2013年 | 54篇 |
2012年 | 57篇 |
2011年 | 51篇 |
2010年 | 30篇 |
2009年 | 23篇 |
2008年 | 45篇 |
2007年 | 30篇 |
2006年 | 34篇 |
2005年 | 35篇 |
2004年 | 28篇 |
2003年 | 26篇 |
2002年 | 23篇 |
2001年 | 12篇 |
2000年 | 7篇 |
1999年 | 10篇 |
1998年 | 5篇 |
1996年 | 5篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1931年 | 4篇 |
1914年 | 4篇 |
1906年 | 4篇 |
1905年 | 3篇 |
1904年 | 11篇 |
1901年 | 5篇 |
1880年 | 3篇 |
1873年 | 3篇 |
排序方式: 共有813条查询结果,搜索用时 15 毫秒
781.
Niels H. Andersen David A. McCrae Douglas B. Grotjahn Satish Y. Gabhe Louis J. Theodore Robert M. Ippolito Tarun K. Sarkar 《Tetrahedron》1981,37(23):4069-4079
Two aspects of organosilicon and -tin chemistry will be dealt with in this paper: the reactivity of allylmetalloids and the potential of the C-MR3 function as a protected carbanion which can be unmasked to provide synthetically useful carbon nucleophiles with the primary focus on the latter. 相似文献
782.
Acoustoelectric current fluctuations in n-type CdS are measured under pulsed bias conditions. The experimental data are interpreted in terms of a model where electrons are trapped in potential troughs associated with spontaneously generated and acoustoelectrically amplified waves. 相似文献
783.
784.
785.
R W Burnett A K Covington N Fogh-Andersen W R Külpman A H Maas O Müller-Plathe O Siggaard-Andersen A L Van Kessel P D Wimberley W G Zijlstra 《Journal of the International Federation of Clinical Chemistry》1994,6(4):115-120
Pre-analytical variables, e.g., specimen collection, transport, and storage, can contribute significantly to inaccurate pH, blood gas, and electrolyte values. The International Federation of Clinical Chemistry (IFCC), through its Committee on pH, Blood Gases and Electrolytes, has developed specific recommendations to minimize the undesirable effects of pre-analytical variables. The Committee has drawn upon the experiences of its own members as well as published data by others. Specifically, the Committee has included pertinent guidelines and suggestions by the IFCC Working Group on Selective Electrodes (WGSE), the National Committee on Clinical Laboratory Standards (NCCLS), and the Electrolyte/Blood Gas Division of the American Association for Clinical Chemistry (AACC). This paper will familiarize the reader with the effect of different types of specimen containers and anticoagulants. It discusses important aspects of specimen collection procedures including patient status and special precautions during specimen collection from indwelling catheters or cannulae. The paper also identifies different requirements in storage and transport of specimens for blood gas and electrolyte analysis. 相似文献
786.
Time-integral measurements have been made of the vacuum deorientation of the 2+ and 4+ states in 150Sm and the 6+ and 8+ states in 156Gd Coulomb-excited by 133 MeV 35Cl ions. The 150Sm results were deduced from γ-ray angular distributions measured in coincidence with backscattered 35Cl ions. In the 156Gd measurements, a fixed counter array and a sandwich target were used to measure directly the differences between the γ-ray angular distributions from nuclei recoiling in vacuum and the unperturbed γ-ray angular distributions from nuclei stopped in a thick 156Gd target. The measured deorientation time constants in ps are τ2(2+) = 27±4; τ2(4+) = 14±4; τ2(6+) = 24±3; τ2(8+) = 23+10?6; τ4(2+) = 12±2; τ4(4+) = 5±3; τ4(6+) = 7.6±2.5. The 8+ data were analyzed assuming τ2/τ4 = as predicted by the Abragam-Pound theory for essentially independently of the degree of quadrupole admixture. The other results are consistent with this except for the 2+ level which shows some quadrupole effect. The present results, which show strong deorientation of high spin levels, are in contrast to earlier work on neutron deficient Er isotopes. In light of our findings we suggest that the failure to observe deorientation in the high spin states in Er is primarily due to anomalously low g-factors associated with the backbending observed in these nuclei. 相似文献
787.
A software package for quantifying elements in solids from energy-dispersive x-ray fluorescence data is described. The algorithm is based on the fundamental parameter approach and facilitates the simultaneous determination of several elements at widely varying concentrations. The software comprises three programs. One program, for spectrometer control and data acquisition, resides in a dedicated microcomputer. The two other programs, for spectra deconvulution and data evaluation, reside in a larger central computer. On-line communication between the computers is feasible, and the system facilitates the implementation of new improved programs. The performance of the system was tested on standard reference materials, and satisfactory accuracy was obtained for up to 21 elements. 相似文献
788.
Hans Vigeland Lerum Niels Højmark Andersen Dag Øistein Eriksen Eddy Walther Hansen Dirk Petersen Grethe Wibetoe Jon Petter Omtvedt 《Journal of solution chemistry》2018,47(8):1395-1417
A large number of model solutions with high ionic strength were synthesised to mimic industrial conditions and were used as a first approach to study Cd extraction in the presence of chloride at high salinity, as experienced in real industrial solutions. The extractant used throughout in this work was Aliquat 336, a quaternary ammonium salt well known to the hydrometallurgical industry. The effects of some selected anions in addition to chloride (i.e., perchlorate, nitrate, and sulfate) were studied. The distribution of cadmium was measured using 109Cd as a tracer. Liquid-scintillation spectroscopy quantified the concentration of 109Cd in both phases. Raman and NMR spectroscopy were employed to gain further insight into the extraction chemistry. A careful analysis of all Cd extraction data showed that within specific windows of the reactant concentrations the chemical reactions could be represented by simplified equations, as discussed thoroughly in the text. Equilibrium constants for the extraction of \({\text{CdCl}}_{3}^{ - }\) from chloride and chloride/sulfate media were determined to be log10Kext?=?4.9?±?0.8 and log10Kext?=?5.7?±?0.5, respectively. For the nitrate environment, an exchange reaction involving a LiNO3 ion pair is proposed and agrees with the experimental data, but was not proven. 14N-NMR and Raman spectroscopy confirmed that the relative affinity of Aliquat 336 for the relevant anions followed the order: perchlorate?>?nitrate?>?chloride?>?sulfate. Finally, 14N-NMR enabled the equilibrium constant of the exchange reaction between nitrate and chloride for Aliquat 336 to be determined. 相似文献
789.
Joachim Thrane Sebastian Kuld Niels D. Nielsen Anker D. Jensen Jens Sehested Jakob M. Christensen 《Angewandte Chemie (International ed. in English)》2020,59(41):18189-18193
Catalytic methanol synthesis is one of the major processes in the chemical industry and may grow in importance, as methanol produced from CO2 and sustainably derived H2 are envisioned to play an important role as energy carriers in a future low‐CO2‐emission society. However, despite the widespread use, the reaction mechanism and the nature of the active sites are not fully understood. Here we report that methanol synthesis at commercially applied conditions using the industrial Cu/ZnO/Al2O3 catalyst is dominated by a methanol‐assisted autocatalytic reaction mechanism. We propose that the presence of methanol enables the hydrogenation of surface formate via methyl formate. Autocatalytic acceleration of the reaction is also observed for Cu supported on SiO2 although with low absolute activity, but not for Cu/Al2O3 catalysts. The results illustrate an important example of autocatalysis in heterogeneous catalysis and pave the way for further understanding, improvements, and process optimization of industrial methanol synthesis. 相似文献
790.
New methods have been developed and applied successfully for the determination of dissolved inorganic, organic and total carbon in water samples. The new methods utilize two instrumental setups, Reagent-Free™ Ion Chromatography (RF™-IC) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Dissolved inorganic carbon (DIC) was measured in untreated samples along with Cl−, F− and SO42− using RF™-IC and by in-line mixing with 0.1 M HNO3 to enhance CO2 removal in the nebulizer, followed by ICP-AES analysis. Total dissolved carbon (TDC) was measured by in-line mixing with 0.1 M NaOH following ICP-AES analysis. Dissolved organic carbon (DOC) was obtained as the difference between DIC and TDC. Only non-volatile organic carbon could be detected by the present method. The workable limits of detection obtained in the present study were 0.5 mM (RF™-IC) and 0.1 mM (ICP-AES) for dissolved inorganic and organic carbon, respectively. The power of the new methods lies in routine analysis of DIC and DOC in samples of natural waters of variable composition and salinity using analytical techniques and facilities available in most laboratories doing water sample analysis. The techniques are sensitive and precise, can be automated using gas-tight sample vials and auto-samplers, and are independent of most elemental interferences with the exception of chloride overload by saline samples when using RF™-IC. The new methods were successfully applied for analysis of DIC and DOC in selected samples of natural and synthetic waters. 相似文献