全文获取类型
收费全文 | 8110篇 |
免费 | 1319篇 |
国内免费 | 1283篇 |
专业分类
化学 | 6182篇 |
晶体学 | 95篇 |
力学 | 508篇 |
综合类 | 104篇 |
数学 | 998篇 |
物理学 | 2825篇 |
出版年
2024年 | 21篇 |
2023年 | 162篇 |
2022年 | 333篇 |
2021年 | 303篇 |
2020年 | 355篇 |
2019年 | 340篇 |
2018年 | 294篇 |
2017年 | 270篇 |
2016年 | 386篇 |
2015年 | 364篇 |
2014年 | 458篇 |
2013年 | 613篇 |
2012年 | 717篇 |
2011年 | 778篇 |
2010年 | 511篇 |
2009年 | 511篇 |
2008年 | 603篇 |
2007年 | 503篇 |
2006年 | 431篇 |
2005年 | 385篇 |
2004年 | 293篇 |
2003年 | 250篇 |
2002年 | 235篇 |
2001年 | 202篇 |
2000年 | 165篇 |
1999年 | 160篇 |
1998年 | 138篇 |
1997年 | 131篇 |
1996年 | 133篇 |
1995年 | 122篇 |
1994年 | 105篇 |
1993年 | 83篇 |
1992年 | 80篇 |
1991年 | 65篇 |
1990年 | 49篇 |
1989年 | 36篇 |
1988年 | 29篇 |
1987年 | 30篇 |
1986年 | 15篇 |
1985年 | 22篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1959年 | 1篇 |
1957年 | 3篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A sequential one-pot two-step tandem reaction for selective and efficient synthesis of pyrrolo[2,1-a]isoquinoline and its dihydro- and tetrahydro-derivatives has been developed. The tandem reactions of isoquinoline, α-halogenated methylene compounds, aromatic aldehydes, and cyanoacetamide firstly give tetrahydropyrrolo[2,1-a]isoquinolines as main products. The corresponding pyrrolo[2,1-a]isoquinolines and dihydropyrrolo[2,1-a]isoquinolines can be obtained directly by controlling oxidation with DDQ. The mechanism of this tandem reaction involved the 1,3-dipolar cycloaddition of isoquinolinium ylide as the key step. A unique elimination of the amido group preferring to the cyano group has been observed. 相似文献
992.
Yanyan Kong Xingqin Zhou Guoxian Cao Xijie Xu Meifen Zou Xiaofeng Qin Rongjun Zhang 《Journal of Radioanalytical and Nuclear Chemistry》2011,287(1):93-101
Pyrroloquinoline quinone (PQQ), an essential nutrient, antioxidant, redox modulator and nerve growth factor found in a class
of enzymes called quinoproteins, was labeled with 99mTc by using stannous fluoride (SnF2) method. Radiolabeling qualification, quality control and characterization of 99mTc-PQQ and its biodistribution studies in mice were performed and discussed. Effects of pH values, temperature, time and reducing
agents concentration on the radiolabeling yield were investigated. The quality control procedure of 99mTc-PQQ was determined by thin layer chromatography (TLC), radio high-performance liquid chromatography (RHPLC) and paper electrophoresis
methods. The average radiolabeling yield was 94 ± 1% under optimum conditions of 0.99 mg of PQQ, 30 μg of SnF2, 0.5 mg of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and 18.5 MBq of Na99mTcO4 at pH 6 and 25 °C with a response volume of 1 ± 0.1 mL. 99mTc-PQQ was stable and anionic. Lipid–water partition coefficient of 99mTc-PQQ was −1.49 ± 0.16. The pharmacokinetics parameters of 99mTc-PQQ were t
1/2α = 18.16 min, t
1/2β = 100.45 min, K
12 = 0.013 min−1, K
21 = 0.017 min−1, K
e = 0.016 min−1, AUC (area under the curve) = 1040.78 ID% g−1 min and CL (plasma clearance) = 0.096 mL min−1. The dual-exponential equation was Y = 10.88e−0.038t
+ 5.21e−0.0069t
. The biodistribution of 99mTc-PQQ was studied in ICR (Institute for Cancer Research 7701 Burhelme Are., Fox Chase, Philadelphia, PA 1911 USA) mice. In
vitro autoradiographic studies clearly showed that the 99mTc-PQQ radioactivity accumulated predominantly in the hippocampus and cortex, which had a high density of N-methyl-d-aspartate Receptor (NMDAR). The enrichment can be blocked by NMDAR redox modulatory site antagonists-ebselen (EB) and 99mTc-PQQ is therefore a promising candidate for the molecular imaging of NMDAR. To date, however, there have been no studies
characterizing 99mTc-PQQ. 相似文献
993.
994.
995.
Xihan Ma Qin Wei Shanshan ZhangLin Shi Zhong Zhao 《Journal of Analytical and Applied Pyrolysis》2011,91(2):338-343
Based on differences in acidity, organic acids and phenols were enriched by pH gradient extraction method from walnut shell pyroligneous acid. Contents of organic acids and phenols were measured by acid-base titration method and Folin colorimetric assay, respectively to assess the effectiveness of the extraction. In addition, the antimicrobial activity of the organic acids and antioxidant activity of phenols extracted were investigated. Chemical components of the extracts that were from the optimal concentrations of NaHCO3 and NaOH were measured by GC-MS. The results showed that 5% NaHCO3 could enrich the highest amount of organic acids, whereas 4% NaOH could enrich the highest amount of phenols. The enriched organic acids exhibited high antimicrobial activity, and the enriched phenols exhibited antioxidant activity under low concentrations, and demonstrated dosage dependency. 相似文献
996.
A simple, rapid and sensitive CE-ESI-MS method for the simultaneous analysis of seven stimulants and narcotics (amphetamine, ephedrine, methadone, pethidine, tetracaine, codeine and heroin) was developed. The CE-ESI-MS experimental conditions were optimized as follows: 20?mmol/L ammonium acetate with pH 9.0 as running buffer, the separation voltage of 22?kV and the sheath liquid of isopropanol/water (1:1 v/v) containing 7.5?mmol/L acetic acid with 3.0?μL/min flow rate. Under the optimized conditions, the stimulants and narcotics were well separated within 4.6?min using a 70-cm length fused-silica capillary (50?μm id). The detection limits (S/N=3) of the CE-ESI-MS analysis were in the range of 0.40-1.0?ng/mL. Method repeatability of intra-day and inter-day was satisfactory. The recoveries obtained from the analysis of spiked urine samples were between 84.1 and 108%. The developed method was successfully applied for the simultaneous analysis of methadone, pethidine and codeine and their in vitro metabolites. 相似文献
997.
We present a microfluidic droplet method to synthesize a series of tunable poly(N-isopropylacrylamide) (PNIPAM) microparticles by the addition of polyethylene glycols (PEGs). The PEGs are used as porogens and could be removed simply by washing step. By varying molecular weights and concentrations of the PEGs, morphologies and temperature-sensitive properties of the formed PNIPAM microparticles are flexibly tuned. It is found that PEG of lower molecular weight induces smaller micropore sizes, and results in faster response rate. The volume changes prior to and after shrinkage can also be regulated by the addition of PEGs due to tuned homogeneities of micropores. The microparticles tuned by PEG1000 with ratio of added PEGs to NIPAM of 2:1 respond the fastest (120 s), whereas with ratio of added PEGs to NIPAM of 1:1 display largest volume change (1/γ=12.12). This simplicity and controllability of tunable microparticles synthesis are appealing for various applications ranging from chemical delivery, drug release control, to optical applications. 相似文献
998.
999.
Ma M He Z Yang J Chen F Wang K Zhang Q Deng H Fu Q 《Langmuir : the ACS journal of surfaces and colloids》2011,27(21):13072-13081
In this Article, the morphological evolution in the blend thin film of polystyrene (PS)/poly(ε-caprolactone) (PCL) was investigated via mainly AFM. It was found that an enriched two-layer structure with PS at the upper layer and PCL at the bottom layer was formed during spinning coating. By changing the solution concentration, different kinds of crystal morphologies, such as finger-like, dendritic, and spherulitic-like, could be obtained at the bottom PCL layer. These different initial states led to the morphological evolution processes to be quite different from each other, so the phase separation, dewetting, and crystalline morphology of PS/PCL blend films as a function of time were studied. It was interesting to find that the morphological evolution of PS at the upper layer was largely dependent on the film thickness. For the ultrathin (15 nm) blend film, a liquid-solid/liquid-liquid dewetting-wetting process was observed, forming ribbons that rupture into discrete circular PS islands on voronoi finger-like PCL crystal. For the thick (30 nm) blend film, the liquid-liquid dewetting of the upper PS layer from the underlying adsorbed PCL layer was found, forming interconnected rim structures that rupture into discrete circular PS islands embedded in the single lamellar PCL dendritic crystal due to Rayleigh instability. For the thicker (60 nm) blend film, a two-step liquid-liquid dewetting process with regular holes decorated with dendritic PCL crystal at early annealing stage and small holes decorated with spherulite-like PCL crystal among the early dewetting holes at later annealing stage was observed. The mechanism of this unusual morphological evolution process was discussed on the basis of the entropy effect and annealing-induced phase separation. 相似文献
1000.