全文获取类型
收费全文 | 4415篇 |
免费 | 199篇 |
国内免费 | 43篇 |
专业分类
化学 | 2915篇 |
晶体学 | 41篇 |
力学 | 148篇 |
数学 | 450篇 |
物理学 | 1103篇 |
出版年
2024年 | 19篇 |
2023年 | 47篇 |
2022年 | 244篇 |
2021年 | 231篇 |
2020年 | 172篇 |
2019年 | 170篇 |
2018年 | 175篇 |
2017年 | 137篇 |
2016年 | 208篇 |
2015年 | 147篇 |
2014年 | 218篇 |
2013年 | 383篇 |
2012年 | 309篇 |
2011年 | 320篇 |
2010年 | 246篇 |
2009年 | 200篇 |
2008年 | 214篇 |
2007年 | 184篇 |
2006年 | 144篇 |
2005年 | 84篇 |
2004年 | 101篇 |
2003年 | 60篇 |
2002年 | 60篇 |
2001年 | 35篇 |
2000年 | 29篇 |
1999年 | 31篇 |
1998年 | 20篇 |
1997年 | 29篇 |
1996年 | 26篇 |
1995年 | 12篇 |
1994年 | 26篇 |
1993年 | 21篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 18篇 |
1985年 | 24篇 |
1984年 | 24篇 |
1983年 | 15篇 |
1982年 | 23篇 |
1981年 | 13篇 |
1980年 | 38篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 15篇 |
1976年 | 11篇 |
1973年 | 11篇 |
1969年 | 9篇 |
排序方式: 共有4657条查询结果,搜索用时 0 毫秒
991.
A fast and accurate analytical method, which uses commercially available adsorbents (Tenax TA, Carbotrap B and C, and Carbosieve S-III), was developed for the sampling and determination of aromatic hydrocarbons, chloroaromatic compounds, and 2,3,7,8-tetrachlorodibenzo-p-dioxin. The breakthrough volume data show that Carbotrap C has a good capacity for compounds of high molecular weight, whereas Carbosieve S-III and Tenax TA are efficient for volatile compounds. The organic components are thermally desorbed and transferred to a gas chromatograph/mass spectrometer. Importantly, thermal desorption avoids conventional solvent extraction procedures and also allows reuse of adsorbent tubes. Preliminary results for recovery of analytes from tubes packed with single adsorbent prove that a single-adsorbent bed is not capable of sampling a wide range of compounds. The best method to obtain the desired collection and desorption properties is to use adsorbent tubes containing several different materials. The results of optimization studies are summarized. 相似文献
992.
993.
Spectrophotometric Determination of Prochlorperazine Maleate in Pure and Pharmaceutical Preparations
Tehseen Aman Zeeshan Majeed Asrar Ahmad Kazi Islam Ullah Khan 《Mikrochimica acta》2002,138(1-2):13-17
Prochlorperazine maleate reacts with 1-naphthylamine and sodium nitrite, after heating for 110 s at 80 °C to give an orange
red colour having maximum absorbance at 460 nm. The reaction is selective for prochlorperazine maleate with 0.01 mg/mL as
visual limit of quantitation and provides a basis for a new spectrophotometric determination. The colour reaction obeys Beer’s
law from 0.01 mg/10 mL to 0.33 mg/10 mL of prochlorperazine maleate and the relative standard deviation is 0.68%. The quantitative
assessment of tolerable amounts of other drugs is also studied.
Received September 22, 2000. Revision June 19, 2001 相似文献
994.
The electrooxidation of hyoscine N‐butylbromide (HBB) was investigated by rotating disk electrode voltammetry, cyclic voltammetry and controlled potential coulometry in 0.1 M HNO3 and in 0.1 M tetrabutylammonium perchlorate (TBAP) solutions of acetonitrile at a platinum (Pt) electrode. Based on the results obtained, it is suggested that a bromide ion of HBB was oxidized in one reversible step in aqueous solutions and in two reversible steps in acetonitrile. A differential pulse voltammetric (DPV) method at a Pt electrode was developed for the determination of HBB in the concentration range of 1.0 × 10?6‐1.0 × 10?3 M. The procedure was applied to the determination of HBB in its formulations as well as its recovery from blood serum and urine samples. 相似文献
995.
Le Guen K Ahmad M Céolin D Lablanquie P Miron C Penent F Morin P Simon M 《The Journal of chemical physics》2005,123(8):084302
To get further insight into the CH2BrCl site-selective fragmentation previously observed upon inner-shell ionization, we have performed high-resolution Br 3d and Cl 2p Auger and spin-orbit resolved Br 3d Auger spectra, and studied the dissociation properties of the CH2BrCl2+ dication formed at threshold by means of threshold electron pair-ion coincidence measurements. The key point is that the origin of site-specific bond breaking is found in the Auger decay itself, as it preferentially populates selected dication states. Whereas the predominance of the C-Br bond breaking is observed in both threshold and inner-shell studies, no signature of selective C-Cl rupture is reported for the dication formed at threshold. 相似文献
996.
S. Waheed J. H. Zaidi S. Ahmad M. Saleem 《Journal of Radioanalytical and Nuclear Chemistry》2002,254(3):597-605
Twenty-three individual food items commonly consumed in a high altitude region (Kashmir) have been investigated using instrumental neutron activation analysis. The concentration of Ca, Cl, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Se, Sn, and Zn is reported for these food articles. For quality assurance of our data, IAEA and NIST standard reference materials have also been studied. The dietary intake of these elements was calculated in accordance with the weekly food selection habits of the residents of this area. The dietary contribution of most of these elements was found to be in agreement to the RDA/ESadi levels. The purpose was to design a database of essential elements for this area and to study the adequacy levels of these elements in different dietary items. Fairly adequate intake of Ca, Co, Cr, Cu, Fe, Mg, Mn, Mo, Se, Sn, and Zn through these foodstuffs is reported, whereas, lower intakes through these items were found for Cl, K, and Na. 相似文献
997.
Ahmad Shaabani Abbas Rahmati Masoumeh Sharifi Jafar Mogimi Rad Behnaz Aghaaliakbari Elham Farhangi Donald G. Lee 《Monatshefte für Chemie / Chemical Monthly》2007,138(7):649-651
Summary. The oxidation of arenes and sulfides by potassium permanganate was accomplished in good yields under solvent free and heterogeneous
conditions when manganese(II) sulfate is used as a solid support. After extraction of the organic products, the inorganic
products can be reoxidized to permanganate. This result is important because it provides an approach to oxidation reactions
that is, in theory, infinitely sustainable. 相似文献
998.
S. Ahmad A. Thorén G. Skarnemark 《Journal of Radioanalytical and Nuclear Chemistry》1984,86(4):247-253
A solvent extraction procedure for rapid separation of uranium from complex nuclear reaction product mixtures is suggested. The procedure has been tested in batch experiments with tracer amounts of representative elements. It has also been tested with fission products and uranium tracer using the continuous chemical separation system SISAK at the Mainz TRIGA reactor. 相似文献
999.
Temperature-sensitive composite polymer particles were prepared by seeded emulsion copolymerization of dimethylaminoethyl methacrylate and ethylene glycol dimethacrylate with 0.17 m-sized monodispersed polystyrene seed particles. The adsorption and desorption behaviors of low molecular weight cationic emulsifier as well as albumin were examined to determine the variation of surface properties as a function of temperature below and above 35°C. 相似文献
1000.
A rapid, sensitive and highly selective spectrophotometric method for the determination of manganese oxide content of Portland cement and cement raw meal is developed. The method is based on the reaction of manganese(II) with 1,2,4 trihydroxyanthraquinone (purpurin, PURP) in 50% v/v ethanol-water solution at pH 8.5. The solution equilibria of manganese chelates are demonstrated and characterized for delineating optimal conditions of the complexation reaction and analytical aspect of the Mn-PURP system. The analysis of cement materials of variable manganese content is feasible over the concentration range 1.67-8.13 microg ml(-1) Mn, the limit of detection (at the 95% confidence level) of the method is 68 ng ml(-1) for manganese. Under optimum conditions, the use of first derivative spectrophotometry has the advantage of high sensitivity than normal spectrophotometric method and allows the determination of 0.5 microg ml(-1) of manganese. 相似文献