全文获取类型
收费全文 | 4119篇 |
免费 | 195篇 |
国内免费 | 51篇 |
专业分类
化学 | 3054篇 |
晶体学 | 17篇 |
力学 | 191篇 |
数学 | 392篇 |
物理学 | 711篇 |
出版年
2023年 | 31篇 |
2022年 | 114篇 |
2021年 | 158篇 |
2020年 | 187篇 |
2019年 | 166篇 |
2018年 | 192篇 |
2017年 | 131篇 |
2016年 | 229篇 |
2015年 | 170篇 |
2014年 | 232篇 |
2013年 | 446篇 |
2012年 | 296篇 |
2011年 | 293篇 |
2010年 | 229篇 |
2009年 | 175篇 |
2008年 | 199篇 |
2007年 | 139篇 |
2006年 | 133篇 |
2005年 | 76篇 |
2004年 | 90篇 |
2003年 | 85篇 |
2002年 | 58篇 |
2001年 | 32篇 |
2000年 | 37篇 |
1999年 | 29篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 24篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 22篇 |
1992年 | 23篇 |
1991年 | 23篇 |
1990年 | 17篇 |
1989年 | 21篇 |
1988年 | 24篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 22篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 18篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有4365条查询结果,搜索用时 15 毫秒
191.
Hassan Keypour Ahmad Ali Dehghani-Firouzabadi Majid Rezaeivala Sadegh Salehzadeh 《合成通讯》2013,43(7):1136-1142
A new Schiff base cryptand (L) has been prepared via [2 + 2] condensation of 1,3,5-tris(2-formylphenoxymethyl)benzene (1) with 1,3,5-tris(aminomethyl)benzene trishydrochloride (2) in the presence of Ba(II) ion. The analytical data show that the Ba(II) cation is not retained by the product. Thus it seems that the role of the Ba(II) ion is to organize the transition state preceding the formation of the cryptand. 相似文献
192.
Hassan Hazarkhani 《合成通讯》2013,43(15):2597-2606
Trichloromelamine was used effectively as a catalyst for thioacetalization of aldehydes and transthioacetalization of acetals and oxathioacetals under mild and almost neutral reaction conditions. By this method, aldehydes, acetals, and oxathioacetals were selectively protected in the presence of ketones as their 1,3-dithiolanes or 1,3-dithianes. 相似文献
193.
Mojtaba Shamsipur Hamid Reza Rajabi Mohammad Hassan Beyzavi Hashem Sharghi 《Mikrochimica acta》2013,180(9-10):791-799
We report on the synthesis of polymeric nanoparticles (PNPs) containing a tetrakis(3-hydroxyphenyl)porphyrin, and their use for the separation of mercury(II) ion. The PNPs were prepared by bulk polymerization from methacrylic acid (the monomer), ethyleneglycol dimethacrylate (the cross-linker), 2,2′-azobisisobutyronitrile (the radical initiator) and the mercury(II) complex of 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin. The Hg(II) ion was then removed by treatment with dilute hydrochloric acid. The PNPs were characterized by colorimetry, FT-IR spectroscopy, and scanning electron microscopy. The material is capable of binding Hg(II) from analyte samples. Bound Hg(II) ions can be eluted with dilute nitric acid and then quantified by cold vapor AAS. The extraction efficiency, the effects of pH, preconcentration and leaching times, sample volume, and of the nature, concentration and volume of eluent were investigated. The maximum adsorption capacity of the PNPs is 249 mg g?1, the relative standard deviation of the AAS assay is 2.2 %, and the limit of detection (3σ) is 8 ng.L?1. The nanoparticles exhibit excellent selectivity for Hg(II) ion over other metal ions and were successfully applied to the selective extraction and determination of Hg(II) ion in spiked water samples. Figure
Schematic presentation of leaching process of mercury(II) ion from the prepared IIP 相似文献
194.
195.
Highly sensitive FRET-based fluorescence immunoassay for aflatoxin B1 using cadmium telluride quantum dots 总被引:1,自引:0,他引:1
Roya Zekavati Shahabeddin Safi Seyed Jamal Hashemi Tavoos Rahmani-Cherati Meisam Tabatabaei Afshin Mohsenifar Mansour Bayat 《Mikrochimica acta》2013,180(13-14):1217-1223
We report on a competitive immunoassay for the determination of aflatoxin B1 using fluorescence resonance energy transfer (FRET) from anti-aflatoxin B1 antibody (immobilized on the shell of CdTe quantum dots) to Rhodamine 123 (Rho 123-labeled aflatoxin B1 bound to albumin). The highly specific immunoreaction between the antibody against aflatoxin B1 on the QDs and the labeled-aflatoxin B1 brings the Rho 123 fluorophore (acting as the acceptor) and the QDs (acting as the donor) in close spatial proximity and causes FRET to occur upon photoexcitation of the QDs. In the absence of unlabeled aflatoxin B1, the antigen-antibody complex is stable, and strong emission resulting from the FRET from QDs to labeled aflatoxin B1 is observed. In the presence of aflatoxin B1, it will compete with the labeled aflatoxin B1-albumin complex for binding to the antibody-QDs conjugate so that FRET will be increasingly suppressed. The reduction in the fluorescence intensity of the acceptor correlates well with the concentration of aflatoxin B1. The feasibility of the method was established by the detection of aflatoxin B1 in spiked human serum. There is a linear relationship between the increased fluorescence intensity of Rho 123 with increasing concentration of aflatoxin B1 in spike human serum, over the range of 0.1–0.6 μmol·mL?1. The limit of detection is 2?×?10?11 M. This homogeneous competitive detection scheme is simple, rapid and efficient, and does not require excessive washing and separation steps. Figure
A nanobiosensor has been fabricated based on a competitive immunoassay for the determination of aflatoxin B1 using fluorescence resonance energy transfer (FRET). In the presence of aflatoxin B1, it will compete with the labeled aflatoxin B1-albumin complex for binding to the antibody-QDs conjugate so that FRET will be increasingly suppressed. 相似文献
196.
Hassan Talaat Ali H. Moussa M. Shalaby El-Wallid S. Sedik M. Tag El-Din Kamal 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2013,87(3):454-460
We apply the time-dependent theory to the collinear exchange reaction F + CH4 → FCH3 + H. We have performed detailed calculations on two-dimensional potential surfaces representing the ground electronic potentials of the collinear F + CH4 → FCH3 + H reaction, at incident energies. Transmission coefficients range from zero to unity, depending upon the incident energy. Normal modes of vibrations are displayed along the reaction path. 相似文献
197.
A new, one-pot multicomponent reaction of two molecules of aromatic aldehydes with 2,7-naphthalenediol and ammonium hydrogen phosphate is described as an efficient and direct procedure for the preparation of novel 1,3-diphenyl-2-azaphenalene derivatives in a mixture of EtOH-H2O (3:1) under reflux conditions. 相似文献
198.
Hassan Zali Boeini 《合成通讯》2013,43(19):2932-2938
1,3-Dibromo-5,5-diphenylimidazolidine-2,4-dione (N,N’-dibromo phenytoin) was efficiently used for the rapid conversion of thiobenzamides to the corresponding 3,5-diaryl-1,2,4-thiadiazoles in water–wet paste conditions. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file.
199.
Nanoparticles of organosilane-based nitrite ionic liquid immobilized on silica, 1-butyl-3-methylimidazolium nitrite, and 1-(3-trimethoxysilylpropyl)-3-methylimidazolium nitrite were used as effective reagents for the preparation of benzotriazole derivatives from 1,2-diaminobenzenes at room temperature under mild solvent-free conditions. These ionic liquids play as nitrosonium sources in this procedure.1,2-Diaminobenzene derivatives have been treated with ionic liquids to give the related diaminobenzenes in very good to excellent yields in short reaction times.
Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the full experimental and spectral details. 相似文献
200.
Ternary Ni–Mo–P thin films have been electrodeposited from citrate‐based electrolyte onto graphite substrates for application as anode catalysts for ethanol electrooxidation. The operating deposition parameters were optimized to produce Ni–Mo–P alloy films of outstanding catalytic activity. The phase structure of the deposits was evaluated employing X‐ray diffraction technique. Morphology and chemical composition of the deposited alloy films were studied using scanning electron microscopy and energy‐dispersive X‐ray analysis, respectively. The results demonstrated that the rate of Ni–Mo–P deposition increases with increasing the ammonium molybdate concentration in the plating electrolyte up to 10 g l?1. Also, the amount of Mo in the deposits increases with increasing the ammonium molybdate concentration up to 7.5 g l?1, and the maximum Mo content in the film was 9.1 at.%. The catalytic activity of Ni–Mo–P/C alloy films has been evaluated towards electrooxidation of ethanol in 1.0 M NaOH solution by using cyclic voltammetry and chronoamperometry. The catalytic performance of the prepared anodes as a function of the amount of Mo was studied. The results showed an increase in the oxidation peak current density of ethanol with increasing the Mo at.% in the deposited alloy films. Additionally, Ni–Mo–P/C electrodes displayed significantly improved catalytic activity and stability towards electrooxidation of ethanol compared with that of Ni–P/C electrode. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献