全文获取类型
收费全文 | 2820篇 |
免费 | 467篇 |
国内免费 | 328篇 |
专业分类
化学 | 2043篇 |
晶体学 | 36篇 |
力学 | 225篇 |
综合类 | 6篇 |
数学 | 335篇 |
物理学 | 970篇 |
出版年
2024年 | 10篇 |
2023年 | 64篇 |
2022年 | 90篇 |
2021年 | 107篇 |
2020年 | 147篇 |
2019年 | 112篇 |
2018年 | 129篇 |
2017年 | 101篇 |
2016年 | 140篇 |
2015年 | 174篇 |
2014年 | 182篇 |
2013年 | 216篇 |
2012年 | 237篇 |
2011年 | 244篇 |
2010年 | 165篇 |
2009年 | 141篇 |
2008年 | 164篇 |
2007年 | 155篇 |
2006年 | 150篇 |
2005年 | 111篇 |
2004年 | 86篇 |
2003年 | 83篇 |
2002年 | 89篇 |
2001年 | 84篇 |
2000年 | 57篇 |
1999年 | 54篇 |
1998年 | 54篇 |
1997年 | 38篇 |
1996年 | 46篇 |
1995年 | 33篇 |
1994年 | 24篇 |
1993年 | 24篇 |
1992年 | 18篇 |
1991年 | 19篇 |
1990年 | 16篇 |
1989年 | 6篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1964年 | 1篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有3615条查询结果,搜索用时 31 毫秒
61.
铜(Ⅱ)和锌(Ⅱ)分别在0.1mol/LKH_2PO_4-Na_2HPO_4缓冲溶液(pH6.5)和0.25mol/LNH_4Cl溶液中,与氟哌酸形成良好的络合吸附波,峰电位分别为-0.26V和-1.28V(vs;SCE);络合比分别为1:3和1:2;峰电流与铜(Ⅱ)和锌(Ⅱ)的浓度均在4.0×10 ̄(-7)~5.0×10 ̄(-6)mol/L范围内呈线性关系,检测限分别为7.0×10 ̄(-8)和5.0×10 ̄(-8)mol/L。用线性扫描和循环伏安法等手段研究体系的行为,表明均具吸附性,而铜(Ⅱ)-氟哌酸体系属络合物中铜(Ⅱ)的两电子还原的可逆过程。 相似文献
62.
The synthesis and anion binding properties of new ruthenium(II) and cobalt(II) phenanthroline complexes, containing two amide
subunits are described. Evidence for anion binding in dimethyl sulfoxide (DMSO) solution was obtained from u.v.–vis titration
experiments. Results indicated that these receptors showed strong affinity for F− and AcO−, and showed weak affinity for OH− and H2PO
4
−
, and showed no affinity for Cl−, Br−, I−. These receptors interacted with various anions examined through hydrogen-bond formation. 相似文献
63.
The hydrogen peroxide-oxidation of o-phenylenediamine (OPD) catalyzed by horseradish peroxidase (HRP) at 37 °C in 50 mM phosphate buffer (pH 7.0) was studied by calorimetry. The apparent molar reaction enthalpy with respect to OPD and hydrogen peroxide were −447 ± 8 kJ mol−1 and −298 ± 9 kJ mol−1, respectively. Oxidation of OPD by H2O2 catalyzed by HRP (1.25 nM) at pH 7.0 and 37 °C follows a ping-pong mechanism. The maximum rate Vmax (0.91 ± 0.05 μM s−1), Michaelis constant for OPD Km,S (51 ± 3 μM), Michaelis constant for hydrogen peroxide Km,H2O2 (136 ± 8 μM), the catalytic constant kcat (364 ± 18 s−1) and the second-order rate constants k+1 = (2.7 ± 0.3) × 106 M−1 s−1 and k+5 = (7.1 ± 0.8) × 106 M−1 s−1 were obtained by the initial rate method. 相似文献
64.
Xiaofeng Wu Jong Min An Jizhen Shang Eugene Huh Sujie Qi Eunhye Lee Haidong Li Gyoungmi Kim Huimin Ma Myung Sook Oh Dokyoung Kim Juyoung Yoon 《Chemical science》2020,11(41):11285
Acetylcholinesterase (AChE) is an extremely critical hydrolase tightly associated with neurological diseases. Currently, developing specific substrates for imaging AChE activity still remains a great challenge due to the interference from butyrylcholinesterase (BChE) and carboxylesterase (CE). Herein, we propose an approach to designing specific substrates for AChE detection by combining dimethylcarbamate choline with a self-immolative scaffold. The representative P10 can effectively eliminate the interference from CE and BChE. The high specificity of P10 has been proved via imaging AChE activity in cells. Moreover, P10 can also be used to successfully map AChE activity in different regions of a normal mouse brain, which may provide important data for AChE evaluation in clinical studies. Such a rational and effective approach can also provide a solid basis for designing probes with different properties to study AChE in biosystems and another way to design specific substrates for other enzymes.In this work, a new approach was developed for designing the representative P10 with high selectivity and sensitivity for imaging AChE activity in the cells and normal mouse brain. 相似文献
65.
66.
67.
Yang M Wang GJ Wang SJ Li XT Xu YP Wang SP Xiang JD Pan SR Cao GX Ye WC 《Rapid communications in mass spectrometry : RCM》2005,19(12):1619-1623
23-Hydroxybetulinic acid is a newly isolated derivative of betulinic acid. The agent exhibits potential anti-tumor activity and functions in this regard via apoptosis. In support of pharmacokinetic and toxicological evaluations, a new assay based on liquid chromatography/mass spectrometry (LC/MS) was developed for the quantitative analysis of 23-hydroxybetulinic acid. Sample preparation consisted of extraction of the plasma by the addition of methylene chloride followed by centrifugation. Aliquots of the supernatant were analyzed using an isocratic reversed-phase high-performance liquid chromatography (HPLC) system coupled to a negative ion electrospray mass spectrometer. Molecules of 23-hydroxybetulinic acid and the internal standard limonin were detected using selected ion monitoring at m/z 471 and 469, respectively. The limit of detection of 23-hydroxybetulinic acid was 0.05 pg (0.11 fmol) injected on-column (10 pg/mL, 5 microL injection volume), and the limit of quantitation was 10 pg (21.19 fmol, 2 ng/mL, 5 muL injection volume). 23-Hydroxybetulinic acid was stable in plasma samples at -20 degrees C for at least 3 weeks. The intra-day and inter-day coefficients of variation of the assay were 3.0 and 4.8%, respectively. The utility of the assay was demonstrated by measuring 23-hydroxybetulinicacid in mouse plasma following intragastric administration (IG) in vivo. Pharmacokinetic parameters were calculated using the 3P97 pharmacokinetic software package. A two-compartment, first-order model was selected for pharmacokinetic modeling. The result showed that after IG of 200 mg/kg 23-hydroxybetulinic acid, the plasma concentrations reached peaks at 2 h with C(max) of 3.1 microg/mL. The 200 mg/kg 23-hydroxybetulinic acid suspension IG doses were found to have long elimination half-lives of 25.6 h and low bioavailability of 2.3%. No interference was noted due to endogenous substances. These analytical methods should be of value in future studies related to the development and characterization of 23-hydroxybetulinic acid. 相似文献
68.
Understanding effect of wall structure on the hydrothermal stability of mesostructured silica SBA-15
Zhang F Yan Y Yang H Meng Y Yu C Tu B Zhao D 《The journal of physical chemistry. B》2005,109(18):8723-8732
Mesostructured silica SBA-15 materials with different structural parameters, such as pore size, pore volume, and wall thickness, etc., were prepared by varying the postsynthesis hydrothermal treatment temperature and adding inorganic salts. The hydrothermal stabilities of these materials in steam (100% water vapor) were systematically investigated using a variety of techniques including powder X-ray diffraction, transmission electron microscopy, nitrogen sorption, and (29)Si solid-state NMR. The effect of the pore size, microporosity or mesoporosity, and wall thickness on the stability was discussed. The results show that all of the SBA-15 materials have a good hydrothermal stability under steam of 600 degrees C for at least 24 h. N(2) sorption measurements show that the Brumauer-Emmett-Teller surface area of SBA-15 materials is decreased by about 62% after treatment under steam at 600 degrees C for 24 h. The materials with thicker walls and more micropores show relatively better hydrothermal stability in steam of 600 degrees C. Interestingly, we found that the microporosity of the mesostructured silica SBA-15 is a very important factor for the hydrothermal stability. To the materials with more micropores, the recombination of Si-O-Si bonds during the high-temperature steam treatment may not cause direct destruction to the wall structure. As a result, SBA-15 materials with more micropores show better stability in pure steam of 600 degrees C. Nevertheless, these materials are easily destroyed in steam of 800 degrees C for 6 h. Two methods to effectively improve the hydrothermal stability are introduced here: one is a high-temperature treatment, and another is a carbon-propping thermal treatment. Thermal treatment at 900 degrees C can enhance the polymerization degree of Si-O-Si bonds and effectively improve the hydrothermal stability of these SBA-15 materials in 800 degrees C steam for 12 h. But, this approach will cause very serious shrinkage of the mesopores, resulting in smaller pore diameter and low surface area. A carbon-propping thermal treating method was employed to enhance the polymerization of Si-O-Si bonds and minimize the serious shrinkage of mesopores at the same time. It was demonstrated to be an effective method that can greatly improve the hydrothermal stability of SBA-15 materials in 800 degrees C steam for 12 h. Furthermore, the SBA-15 materials obtained by using the carbon-propping method possess larger pores and higher surface area after the steam treatment at 800 degrees C compared to the materials from the direct thermal treatment method after the steam treatment. 相似文献
69.
Three photocatalytic oxidation (PCO) systems: C7H16-O2, SO2-O2 and C7H16-SO2-O2 were carried out with the aid of UV-illuminated TiO2 nanoparticles at room temperature in a batch reactor. In C7H16-O2-TiO2 system, no catalyst deactivation was observed, while for SO2-O2-TiO2 and C7H16-SO2-O2-TiO2 systems, the photocatalytic activity of used TiO2 powder showed decreasing and eventually no activity after used consecutively. The reaction products such as sulfur trioxide or sulfuric acid adsorbed onto the surface of TiO2 catalyst were poisoning species. Photocatalytic activity of the deactivated TiO2 powder could be regenerated by sonicating treatment with water and methanol for the two systems, respectively. 相似文献
70.
Lifang Yang Haifeng He Hongqing Cao Xinhua Wan Qi-feng Zhou Department of Polymer Science & Engineering College of Chemistry & Molecular Engineering Peking University Beijing China 《高分子科学》2002,(5):401-407
Four new rod-coil diblock molecules, which had well-defined para-phenylene oligomers (PPP, from biphenyl toquinquephenyl) as rigid segments and poly(ethylene oxide) (PEO, M_w ca. 750) as flexible segments, were synhesized. Thechemical structure and the number of repeating unit in PEO coil and PPP rod were characterized by ~1H-NMR and MALDI-TOF mass spectromeny. The diblock oligomers showed good solubility in common organic solvents and aggregated in wateras characterized by a blue shift in UV-vis absorption spectra, a red shift in emission spectra, quenching of the fluorescenceand formation of the microfibrilla and fractal-like structures. The formation of different supramolecular structures meant thatthis effort might lead to commercial important highly organized functional materials. 相似文献