全文获取类型
收费全文 | 3413篇 |
免费 | 86篇 |
国内免费 | 334篇 |
专业分类
化学 | 3182篇 |
晶体学 | 3篇 |
力学 | 107篇 |
数学 | 405篇 |
物理学 | 136篇 |
出版年
2024年 | 2篇 |
2023年 | 62篇 |
2022年 | 58篇 |
2021年 | 41篇 |
2020年 | 60篇 |
2019年 | 79篇 |
2018年 | 79篇 |
2017年 | 88篇 |
2016年 | 81篇 |
2015年 | 67篇 |
2014年 | 134篇 |
2013年 | 187篇 |
2012年 | 253篇 |
2011年 | 204篇 |
2010年 | 187篇 |
2009年 | 240篇 |
2008年 | 266篇 |
2007年 | 277篇 |
2006年 | 238篇 |
2005年 | 199篇 |
2004年 | 213篇 |
2003年 | 104篇 |
2002年 | 118篇 |
2001年 | 67篇 |
2000年 | 43篇 |
1999年 | 55篇 |
1998年 | 61篇 |
1997年 | 61篇 |
1996年 | 49篇 |
1995年 | 42篇 |
1994年 | 51篇 |
1993年 | 23篇 |
1992年 | 32篇 |
1991年 | 31篇 |
1990年 | 9篇 |
1989年 | 21篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有3833条查询结果,搜索用时 31 毫秒
141.
The voltammetric procedure for determination of traces of Cr(VI) [Anal. Chim. Acta (1992) 262:103] was modified by changing the temperature of the measurements. It was found that at the temperature of 40 °C the time of decrease of the Cr(III) signal was shortened from 30 to 5 min. As a result the total analysis time was drastically shortened. The modified procedure does not show any disadvantage as compared to the original method. The results of Cr(VI) determination by the modified procedure are less affected by Cr(III) as compared to the original method. The detection limit of the method was 2.5 × 10-11 mol L-1 (1.2 ng L-1). The validation of the modified procedure was performed by comparison of the results of analyses of tap and river water samples with those obtained using original procedure. 相似文献
142.
《Electroanalysis》2006,18(12):1193-1201
A chemically modified carbon paste electrode with 2,7‐bis(ferrocenyl ethyl)fluoren‐9‐one (2,7‐BFEFMCPE) was employed to study the electrocatalytic oxidation of ascorbic acid in aqueous solution using cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The diffusion coefficient (D=1.89×10?5 cm2 s?1), and the kinetic parameter such as the electron transfer coefficient, α (=0.42) of ascorbic acid oxidation at the surface of 2,7‐BFEFMCPE was determined using electrochemical approaches. It has been found that under an optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 300 mV less positive than that of an unmodified carbon paste electrode. The catalytic oxidation peak currents show a linear dependence on the ascorbic acid concentration and linear analytical curves were obtained in the ranges of 8.0×10?5 M–2.0×10?3 M and 3.1×10?5 M–3.3×10?3 M of ascorbic acid with correlation coefficients of 0.9980 and 0.9976 in cyclic voltammetry and differential pulse voltammetry, respectively. The detection limits (2δ) were determined to be 2.9×10?5 M and 9.0×10?6 M with cyclic voltammetry and differential pulse voltammetry, respectively. This method was also examined for determination of ascorbic acid in pharmaceutical preparations. 相似文献
143.
A simplified procedure based on ion-exchange separation of selenourea (Se-U) and selenocystamine (Se-CM), which have very close half-wave potential when they are simultaneously analyzed by voltammetric techniques, has been developed and optimized. Thus, selenocystamine remains in the cation exchanger Purolite C 100 H, whereas selenourea is found in the effluent and is determined by square wave cathodic stripping voltammetry using Na2CO3 as electrolyte. Selenocystamine is then eluted from the cation exchanger using 4 M HCl and analyzed by square wave cathodic stripping voltammetry in the HCl solution. For each voltammetric determination the corresponding parameters were investigated and optimized; the obtained detection limits were 0.3 ng Se mL–1 for Se-CM and 2 ng Se mL–1 for Se-U. A flow sheet for the separation of inorganic (Se(IV) and Se(VI)) and organoselenium compounds (Se-U, Se-CM, (CH3)2Se2, and (CH3)2Se) developed for their electrochemical detection is presented and it was successfully applied to a certified reference material, an environmental soil sample, and a urine sample. 相似文献
144.
Flow injection (FI) and sequential injection (SI) systems with anodic stripping voltammetric detection have been exploited for simultaneous determination of some metals. A pre-plated mercury film on a glassy carbon disc electrode was used as a working electrode in both systems. The same film can be repeatedly applied for at least 50 analysis cycles, thus reducing the mercury consumption and waste. A single line FI voltammetric system using an acetate buffer as a carrier and an electrolyte solution was employed. An injected standard/sample zone was mixed with the buffer in a mixing coil before entering a flow cell. Metal ions were deposited on the working electrode by applying a potential of −1.1 V vs Ag/AgCl reference electrode. The stripping was performed by anodically scanning potential of working electrode to +0.25 V, resulting a voltammogram. Effects of acetate buffer concentration, flow rate and sample volume were investigated. Under the selected condition, detection limits of 1 μg l−1 for Cd(II), 18 μg l−1 for Cu(II), 2 μg l−1 for Pb(II) and 17 μg l−1 for Zn(II) with precisions of 2–5% (n=11) were obtained. The SI voltammetric system was similar to the FI system and using an acetate buffer as a carrier solution. The SI system was operated by a PC via in-house written software and employing an autotitrator as a syringe pump. Standard/sample was aspirated and the zone was then sent to a flow cell for measurement. Detection limits for Cd(II), Cu(II), Pb(II) and Zn(II) were 6, 3, 10 and 470 μg l−1, respectively. Applications to water samples were demonstrated. A homemade UV-digester was used for removing organic matters in the wastewater samples prior to analysis by the proposed voltammetric systems. 相似文献
145.
《Electroanalysis》2003,15(10):878-884
Three glassy carbon (GC) samples: undoped and doped with boron or phosphorus, prepared at 1000 °C, were compared in respect to hydrodynamic current‐potential curves in acidic medium, cyclic voltammograms for Fe3+/2+ and Fe(CN)63?/4? and argentometric titrations of halides. Some experiments were also carried out using standard Tokai and Sigri GC and Ag electrode. It appeared that GC doped with boron and phosphorus exhibited significant increase in hydrogen evolution overpotential. As for the electrode kinetics (ΔEp criterion), no significant difference was observed between doped and undoped electrodes. In the potentiometric titrations the phosphorus‐doped electrode was advantageous over the other GC and Ag electrodes as it enabled more precise end‐point detection. 相似文献
146.
Studies of the electrochemical behavior of epinephrine at a homocysteine self-assembled electrode 总被引:6,自引:0,他引:6
The self-assembled electrode with the homocysteine monolayer (Hcy/Au) has been characterized by infrared spectroscopy and ac impedance spectroscopy in electrolyte. The Hcy/Au electrode is demonstrated to promote the electrochemical response of epinephrine (E) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (ks) is 2.1×10−2 cm s−1 at the self-assembled electrode. The reduction peak of E can be used to determine the concentration of E in presence of ascorbic acid (AA) owing to the Hcy/Au also promoting the electrochemical oxidation of AA. 相似文献
147.
H. X. Zong C. J. Cong L. N. Wang G. H. Guo Q. Y. Liu K. L. Zhang 《Journal of Solid State Electrochemistry》2007,11(2):195-200
Yttrium-doped lithium manganese oxide (LiMn0.98Y0.02O2) was prepared by ion exchange of lithium for sodium in NaMn0.98Y0.02O2 precursors obtained by using rheological phase reaction method. This material had small particle size, which was composed of grain size of about 100 nm. Especially, LiMn0.98Y0.02O2 delivered the initial discharge capacity of about 191 mA h g−1 at room temperature when cycled between 2.0 and 4.4 V vs Li/Li+. Moreover, it showed an excellent cycling behavior, its specific capacity remained above 173 mA h g−1 after 20 cycles, and the material did not transform into spinel structure during the electrochemical cycling according to the cyclic voltammograms and X-ray powder diffraction. The electrochemical results revealed that the doping of Y3+ improved the performance of LiMnO2 considerably. 相似文献
148.
149.
The oxidative electrochemistry of 11 chiral bis-phosphinoferrocene ligands, all within the Josiphos class of ligands, was examined in methylene chloride. The oxidation of these ligands displays multiple waves of varying chemical reversibility. Palladium(II) and platinum(II) complexes with the general formula [MCl2(P-P)] (M = Pd or Pt; P-P = Josiphos) were prepared, characterized by NMR and cyclic voltammetry. The electrochemistry simplifies greatly upon coordination of the Josiphos ligands. The X-ray structures of a palladium(II) and platinum(II) complex of the same Josiphos ligand are reported. 相似文献
150.
《Electroanalysis》2003,15(14):1204-1211
Two types of glassy carbon (GC) powder (i.e., Sigradur K and Sigradur G) have been mixed with mineral oil to obtain glassy carbon paste electrodes (GCPE's). The electrochemical behavior of such electrodes at different percentages of glassy carbon has been evaluated with respect to the electrochemistry of ferricyanide as revealed with cyclic voltammetry and the best paste composition was chosen. GC was then modified with Prussian Blue (PB), mixed at different percentages with unmodified GC and with a fixed amount of mineral oil in order to obtain PB modified glassy carbon paste electrodes (PB‐GCPE's). PB‐GCPE's with different percentages of GC modified with PB (PB‐GC) were compared and the dependence on the amount of PB on their performances was evaluated by studying the parameters of cyclic voltammetry (i.e., current peak, ΔEp, anodic and cathodic current ratio, charge density) and the amperometric response to H2O2. Data interpretation based on the GC surface area is presented. GCPE's with a selected amount of PB‐GC were then tested as H2O2 probes and all the analytical parameters together with the dependence on pH were evaluated. Some preliminary experiments with these electrodes assembled as glucose, lysine and lactate biosensors are also reported. 相似文献