全文获取类型
收费全文 | 1969篇 |
免费 | 93篇 |
国内免费 | 13篇 |
专业分类
化学 | 1369篇 |
晶体学 | 13篇 |
力学 | 44篇 |
数学 | 167篇 |
物理学 | 482篇 |
出版年
2023年 | 11篇 |
2022年 | 16篇 |
2021年 | 34篇 |
2020年 | 28篇 |
2019年 | 34篇 |
2018年 | 20篇 |
2017年 | 23篇 |
2016年 | 57篇 |
2015年 | 57篇 |
2014年 | 63篇 |
2013年 | 114篇 |
2012年 | 102篇 |
2011年 | 133篇 |
2010年 | 77篇 |
2009年 | 74篇 |
2008年 | 110篇 |
2007年 | 106篇 |
2006年 | 82篇 |
2005年 | 73篇 |
2004年 | 72篇 |
2003年 | 52篇 |
2002年 | 67篇 |
2001年 | 42篇 |
2000年 | 64篇 |
1999年 | 30篇 |
1998年 | 35篇 |
1997年 | 24篇 |
1996年 | 24篇 |
1995年 | 34篇 |
1994年 | 32篇 |
1993年 | 32篇 |
1992年 | 29篇 |
1991年 | 37篇 |
1990年 | 32篇 |
1989年 | 21篇 |
1988年 | 24篇 |
1987年 | 19篇 |
1986年 | 20篇 |
1985年 | 26篇 |
1984年 | 25篇 |
1983年 | 17篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 9篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 11篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 6篇 |
排序方式: 共有2075条查询结果,搜索用时 15 毫秒
81.
Neutral oxorhenium(V) complexes with thiosemicarbazones derived from 2‐pyridine formamide, HL1, are formed when [ReOCl3(PPh3)2] reacts with equimolar amounts of the ligands. Reduction of the metal and the formation of rhenium(III) complexes of the composition [Re(L1)2]+ occurs when an excess of thiosemicarbazones is used and the reaction is performed in boiling toluene for a prolonged period of time. The thiosemicarbazones deprotonate and act as tridentate ligands as has been confirmed by an X‐ray structure of [ReOCl2(L1b)], where HL1b is 2‐pyridineformamide‐N(4)‐ethylthiosemicarbazone and the ligand occupies the equatorial coordination sphere of the complex together with one of the chloro ligands. 相似文献
82.
83.
84.
Catechins (catechin and other derivatives) are naturally occurring flavonoids present in a number of plants and foods. They are also part of numerous nutraceutical formulations because they are believed to have antioxidant, cancer chemo-preventative, anti-inflammatory and antimicrobial properties. The determination of catechins has traditionally been performed by HPLC. However, this methodology is both time and sample intensive and generates large amounts of organic solvent waste. In the current report, an application of MEKC using a PDMS microchip is presented for the analysis of catechins. The system uses pulsed amperometric detection for direct analysis of important naturally occurring catechins. The effect of pH, surfactant concentration, detection potential and signal stability were analyzed. Linear relationships were found between the concentration and peak current, with good stability and limits of detection of 8 [micro sign]M for catechin, epigallocatechin gallate and epicatechin, and 14 [micro sign]M for epicatechin gallate. Optimum conditions were applied to the detection of selected catechins in a commercially available green tea extract nutraceutical and the results were compared to HPLC analysis. The analysis using microchip micellar electrokinetic chromatography and pulsed amperometric detection was completed in 4.5 min, 10 times faster than the HPLC analysis. 相似文献
85.
Reaction of a (237)Np(V) stock solution in the presence of oxalic acid, calcium chloride, and sodium hydroxide under hydrothermal conditions produces single crystals of a neptunium(V) oxalate, Na(2)NpO(2)(C(2)O(4))OH.H(2)O. The structure consists of one-dimensional chains running down the a axis and is the first example of a neptunium(V) oxalate compound containing hydroxide anions. 相似文献
86.
Summary Mercury(II) in the presence of a large excess of thiocyanate forms a violet colour with Rhodamine B. The complex formed can be stabilized by addition of poly(vinyl alcohol), and forms the basis for a spectrophotometric method for determination of trace amounts of mercury. The calibration graph for measurement at 610 nm is linear in the range 1–15g of mercury per 25 ml, with a molar absorptivity of 1.1×105l· mole–1·cm·. The effect of foreign ions has been studied and the method can be applied to the determination of mercury in air with reliable analytical results.
Spektrophotometrische Bestimmung von Quecksilber mit Rhodanid und Rhodamin B
Zusammenfassung In Gegenwart eines großen Überschusses von Rhodanid bildet Hg(II) mit Rhodamin B eine violette Färbung. Durch Zusatz von Polyvinylalkohol kann dieser Komplex stabilisiert werden und bietet somit die Grundlage für die spektrophotometrische Bestimmung von Hg-Spuren. Die Eichkurve für die Messung bei 610 nm verläuft für 1–15g Hg/25 ml linear. Die molare Absorptivität beträgt 1,1×105 l·mol–1·cm–1. Die Fremdionenwirkung wurde untersucht. Das Verfahren kann zur Hg-Bestimmung in Luft verwendet werden.相似文献
87.
M. M. Minor W. K. Hensley M. M. Denton S. R. Garcia 《Journal of Radioanalytical and Nuclear Chemistry》1982,70(1-2):459-471
An automated delayed neutron counting and instrumental neutron activation analysis system has been developed at Los Alamos
National Laboratory's Omega West Reactor (OWR) to analyze samples for uranium and 31 additional elements with a maximum throughput
of 400 samples per day. The system and its mode of operation for a large reconnaissance survey will be described. 相似文献
88.
Summary The distribution equilibria of 3-phenyl-3-methyl-2-mercaptopropenoic acid (PhMMP), pyridine and the titanium (IV)-PhMMP complex in the presence of pyridine have been studied in the chloroform-water system. An extraction-photometric method for the determination of micro amounts of titanium has been developed. The synergistic effect of pyridine, and the influence of pH, the reagent concentration and the presence of electrolytes and masking agents on the equilibrium have been determined. From slope analysis of the distribution curves, the composition of the extracted species has been found to be Ti(OH)2(HR)2·py. The complex is extracted quantitatively. Beer's law is obeyed over the range 0.09–4.30g/ml titanium (IV) and the molar absorptivity is 1.30×1041·mole–1·cm–1 at 447 nm. The method has been applied to the determination of titanium in ceramics.
Über die Extraktion und spektrophotometrische Bestimmung von Titan(IV) mit 3-Phenyl-3-methyl-2-mercaptopropensäure mit Hilfe der synergistischen Wirkung des Pyridins
Zusammmenfassung Das Verteilungsgleichgewicht von 3-Phenyl-3-methyl-2-mercaptopropensäure (PhMMP), Pyridin und Titan-PhMMP-Komplex in Gegenwart von Pyridin wurde in einem Wasser-Chloroform-System untersucht. Eine extraktionsphotometrische Methode für die Bestimmung von Mikromengen Titan(IV) wurde ausgearbeitet. Die synergistische Wirkung des Pyridins sowie der Einfluß des pH, des Reagens und der Gegenwart von Elektrolyten und maskierenden Reagenzien in der wäßrigen Phase auf das Gleichgewicht wurden bestimmt. Aus der Richtung der Verteilungskurven wurde für den extrahierten Komplex die Zusammensetzung Ti(OH)2(HR)2·py ermittelt. Der Komplex wird quantitativ extrahiert. Das Beersche Gesetz ist von 0,09–4,30g·ml–1 Ti(IV) erfüllt, der molare Absorptionskoeffizient ist 1,13·1041·mol–1·cm–1; die Sandell-Empfindlichkeit beträgt 3,7ng·cm–2 bei 447 nm. Diese Methode wurde für die Bestimmung von Titan in keramischem Material verwendet.相似文献
89.
New synthetic “tricks”. Triphenylphosphine-mediated amide formation from carboxylic acids and azides
Equimolar amounts of carboxylic acids, aryl or alkyl azides, and Ph3P in refluxing benzene (hexane, toluene) afford amides in good yields. Insolubility of zwitterions Ph3P+-NH(CH2)nCOO-, arising from μ-azido acids and Ph3P, limits the utilization of the method for lactame formation. 相似文献
90.
Garcia CB Zhang Y Mahajan S DiSalvo F Wiesner U 《Journal of the American Chemical Society》2003,125(44):13310-13311
In the present study poly(isoprene-block-ethylene oxide), PI-b-PEO, block copolymers are used to structure iron oxide and silica precursors into reverse mesophases, which upon dissolution of the organic matrix lead to well-defined nanoparticles of spheres, cylinders, and plates based on the original structure of the mesophase prepared. The hybrid mesophases with sphere, cylinder, and lamellar morphologies containing the inorganic components in the minority phases are characterized through a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). After heat treatments the respective nanoparticles on mica surfaces are characterized by scanning force microscopy (SFM). X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) magnetometer measurements are performed to demonstrate that the heat treatment leads to the formation of a magnetic gamma-Fe2O3 crystalline phase within the amorphous aluminosilicate. The results pave the way to functional, i.e., magnetic nanoparticles where the size, shape, and iron oxide concentration can be controlled opening a range of possible applications. 相似文献