全文获取类型
收费全文 | 1523篇 |
免费 | 158篇 |
国内免费 | 246篇 |
专业分类
化学 | 1794篇 |
晶体学 | 4篇 |
力学 | 3篇 |
综合类 | 8篇 |
数学 | 3篇 |
物理学 | 115篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 47篇 |
2021年 | 67篇 |
2020年 | 79篇 |
2019年 | 51篇 |
2018年 | 52篇 |
2017年 | 50篇 |
2016年 | 83篇 |
2015年 | 67篇 |
2014年 | 62篇 |
2013年 | 88篇 |
2012年 | 131篇 |
2011年 | 79篇 |
2010年 | 66篇 |
2009年 | 83篇 |
2008年 | 66篇 |
2007年 | 85篇 |
2006年 | 60篇 |
2005年 | 76篇 |
2004年 | 62篇 |
2003年 | 53篇 |
2002年 | 97篇 |
2001年 | 46篇 |
2000年 | 37篇 |
1999年 | 43篇 |
1998年 | 47篇 |
1997年 | 25篇 |
1996年 | 27篇 |
1995年 | 22篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 15篇 |
1991年 | 20篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1977年 | 2篇 |
排序方式: 共有1927条查询结果,搜索用时 31 毫秒
51.
The anion-exchange properties of a new organomineral sorbent obtained by modification of clinoptilolite-containing tuffs by polyhexamethyleneguanidine have been studied after different periods of storage (time after synthesis) and numbers of sorption-regeneration cycles. The sorbent can be used as a cation- and an anion-exchanger simultaneously. Selectivity coefficients (exchange constants) for F–, SO4
2–, and HPO4
2– ionsvs. Cl– ions on modified clinoptilolite-containing tuffs have been determined. The modification improves the mechanical properties of clinoptilolite tuffs.For part 1, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1554–1556, September, 1994. 相似文献
52.
Enantioselective, potentiometric membrane electrodes (EPMEs) based on carbon paste impregnated with different maltodextrins {dextrose equivalent (DE) 4.0-7.0 (I), 13.0-17.0 (II) and 16.5-19.5 (III)} as chiral selectors for the assay of S-perindopril is described. The proposed electrodes could be reliably employed in the assay of S-perindopril raw material and from its pharmaceutical formulation, Coversyl® tablets. The electrode based on maltodextrin (I) showed the best enantioselectivity and time-stability. The surfaces of the electrodes are easily renewable by simply polishing on an alumina paper. 相似文献
53.
Summary The dependence of the chloride distribution coefficient on the co-ion of solutions of different alkali fluorides, MF, up to 11M is tested on the strongly basic anion-exchange resin AG1-X10. Under the same experimental conditions the distribution coefficient decreases in the following order for M+: Na+>K+>Rb+>Cs+. This can be explained by the different co-ion-chloride interactions. The consequence of this interaction for a chromatographic separation of chloride is shown with 5M KF and CsF solutions, used as eluants. Depending on the fluoride concentration, the distribution coefficient passes through a minimum value to increase again at higher electrolyte concentration. The non-exchange electrolyte in the resin phase is responsible for this effect. In addition, the bromide and the iodide distribution coefficients up to 10M KF solutions are determined. One results is that the selectivity coefficient between halide ions increases at higher electrolyte concentrations. 相似文献
54.
55.
Ketene silylacetals derived from prenyl and (Z)- and (E)-crotyl 2-methylcyclopentanecarboxylates (9) were subjected to the Ireland-Claisen rearrangement. All three substrates rearranged with complete facial selectivity, but the (Z)- and (E)-crotyl systems gave a mixture comprised of the same diastereomers of 1-(1-methyl-2-propenyl)-2-methylcyclopentanecarboxylic acid (14) in ratios of 2:1 and 1:2, respectively. In contrast, the ketene silylacetals prepared from allyl and prenyl 2-methoxycyclopentanecarboxylates (22) underwent rearrangements with both facial stereochemistries. 相似文献
56.
57.
Ion association in the system Ca(OH)2–H3PO4–KCl–H2O at 37°C has been studied potentiometrically over a range of pH from 3 to 9. The experimental conditions were optimized for the accurate determination of the association constants for the formation of the ion pairs CaH2PO
4
+
, CaHPO4 and CaPO
4
–
which were found to be 27.9±0.1, 591±2, and (1.35±0.02)×106 L-mol–1, respectively. 相似文献
58.
Vinod Kumar Gupta Ashok Kumar Singh Sameena Mehtab Barkha Gupta 《Analytica chimica acta》2006,566(1):5-10
A new PVC membrane electrode for Co2+ based on N,N′-bis(salicylidene)-3,4-diaminotoluene, an excellent neutral carrier, has been fabricated using sodium tetraphenylborate (NaTPB) as an anionic excluder and dioctylphthalte (DOP) as a solvent mediator. The electrode exhibits a linear potential response in the concentration range of 7.9 × 10−8 to 1.0 × 10−1 M with a slope of 30 ± 0.2 mV per decade. The detection limit of the proposed sensor is 5.0 × 10−8 M and it can be used over a period of 5 months. The proposed sensor revealed good selectivity over a wide variety of other cations including alkali, alkaline earth, heavy and transition metals and could be used in the pH range of 2.0-9.0. This electrode was successfully applied for the determination of Co2+in real samples and as an indicator electrode in potentiometric titration of cobalt ions. 相似文献
59.
《Journal of separation science》2002,25(18):1317-1324
Multidimensional gas chromatography (2D GC) is demonstrated as a way to improve limits of detectability of spectrophotometric detectors. UV and IR detectors are generally less sensitive than mass spectrometers or other GC detectors. This has placed some limitations on the useful capabilities provided by spectrophotometric detectors, such as the ability to provide structure‐related information for a particular analyte. In this paper, we report results from interfacing a 2D GC instrument to a UV detector. Symmetry factor and the ratio of retention time divided by peak width did not show deterioration of the quality of chromatography when a megabore column was used with this detector. Furthermore, an increase in the limits of detectability over that attainable in a single‐column system was realized by using the 2D GC system. However, the low flow (1 mL/min) imposed by the use of a microbore column (250 μm ID) caused significant tailing when the UV detector was used. 相似文献
60.
Lin Jia Hui Yang Jun Deng Junmei Chen Yuan Zhou Pan Ding Leigang Li Na Han Yanguang Li 《中国化学》2019,37(5):497-500
Electrocatalytic carbon dioxide reduction holds great promise for reducing the atmospheric CO2 level and alleviating the energy crisis. High‐performance electrocatalysts are often required in order to lower the high overpotential and expedite the sluggish reaction kinetics of CO2 electroreduction. Copper is a promising candidate metal. However, it usually suffers from the issues of poor stability and low product selectivity. In this work, bimetallic Cu‐Bi is obtained by reducing the microspherical copper bismuthate (CuBi2O4) for selectively catalyzing the CO2 reduction to formate (HCOO–). The bimetallic Cu‐Bi electrocatalyst exhibits high activity and selectivity with the Faradic efficiency over 90% in a wide potential window. A maximum Faradaic efficiency of ~95% is obtained at –0.93 V versus reversible hydrogen electrode. Furthermore, the catalyst shows high stability over 6 h with Faradaic efficiency of ~95%. This study provides an important clue in designing new functional materials for CO2 electroreduction with high activity and selectivity. 相似文献