全文获取类型
收费全文 | 26588篇 |
免费 | 991篇 |
国内免费 | 745篇 |
专业分类
化学 | 7206篇 |
晶体学 | 326篇 |
力学 | 661篇 |
综合类 | 55篇 |
数学 | 11478篇 |
物理学 | 8598篇 |
出版年
2023年 | 113篇 |
2022年 | 199篇 |
2021年 | 157篇 |
2020年 | 212篇 |
2019年 | 567篇 |
2018年 | 389篇 |
2017年 | 353篇 |
2016年 | 238篇 |
2015年 | 227篇 |
2014年 | 306篇 |
2013年 | 1247篇 |
2012年 | 633篇 |
2011年 | 1248篇 |
2010年 | 772篇 |
2009年 | 1069篇 |
2008年 | 1239篇 |
2007年 | 1253篇 |
2006年 | 1163篇 |
2005年 | 784篇 |
2004年 | 1151篇 |
2003年 | 1285篇 |
2002年 | 1823篇 |
2001年 | 1845篇 |
2000年 | 1690篇 |
1999年 | 1827篇 |
1998年 | 1580篇 |
1997年 | 968篇 |
1996年 | 499篇 |
1995年 | 307篇 |
1994年 | 260篇 |
1993年 | 136篇 |
1992年 | 130篇 |
1991年 | 107篇 |
1990年 | 185篇 |
1989年 | 172篇 |
1988年 | 109篇 |
1987年 | 93篇 |
1986年 | 137篇 |
1985年 | 202篇 |
1984年 | 189篇 |
1983年 | 86篇 |
1982年 | 152篇 |
1981年 | 244篇 |
1980年 | 240篇 |
1979年 | 114篇 |
1978年 | 176篇 |
1977年 | 105篇 |
1976年 | 79篇 |
1975年 | 82篇 |
1974年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
941.
A 3-amino-5-mercapto-1,2,4-triazole (TA) self-assembled monolayer-modified gold electrode (TA SAM/Au) is characterized by
X-ray photoelectron spectroscopy, A.C. impedance, cyclic voltammetry, chronoamperometry and chronocoulometry. The TA SAM/Au
exhibited good promotion of the electrochemical oxidation of dopamine. Some electrochemical parameters of dopamine such as
electron transfer number, exchange current density, standard heterogeneous rate constant, diffusion coefficient, etc., were
measured by different electrochemical methods. The peak currents of dopamine were linearly dependent on its concentration
in the range of 1.5 × 10−6–1.0 × 10−4 mol L−1, with a detection limit of 5.0 × 10−7 mol L−1. The oxidative peak potentials of dopamine and ascorbic acid were well separated at about 190 ± 10 mV in pH 2.0 BR buffers
at TA SAM/Au, the oxidation peak current increases approximately linearly with increasing concentration of both dopamine and
ascorbic acid in the concentration range of 9.98 × 10−6–4.54 × 10−4 mol L−1. It can be used for simultaneous determination of dopamine and ascorbic acid. 相似文献
942.
Ab initio calculations are presented for H2 and D2 relative Raman intensities originating from common rotational levels for both vibrational-rotational and pure rotational transitions. Factors f(J) required to correct measured intensities for molecular non-rigidity (e.g. in temperature measurements) are tabulated. The calculations are compared with literature perturbation-theory equations (significant differences at large J in vibration-rotation) and with experiment. 相似文献
943.
The ionization threshold of fluoranthene and of TMPD in n-pentane was determined by laser two-photon ionization (TPI) and found to be 4.50 ± 0.05 and 3.88 ± 0.05 eV respectively. For both molecules the TPI spectra show distinct structure due to autoionization. For fluoranthene the TPI spectrum suggests that the molecule dissociates via the first excited singlet state. 相似文献
944.
2,3-Dihydroxypyridine loaded (via –N=N–linker) Amberlite XAD-2 (AXAD-2-DHP) was prepared and characterized by elemental analyses, TGA and FT-IR spectra. It (1g packed in a column of 1cm diameter; surface area 135.5m2g–1) was found to be an effective solid phase sorbent for enriching Zn2+, Mn2+, Ni2+, Pb2+, Cd2+, Cu2+, Fe3+ and Co2+ at pH 3.5 to 7.0 using flow rates between 1.0–5.0mLmin–1. For desorption (recovery 97.0–99.8%) of the metal ions, 8 to 10mL of 2.0molL–1 HCl or 1.5molL–1 HNO3 at a flow rate of between 2.0 and 4.0mLmin–1 were found most suitable. The t1/2 (time for 50% sorption) is between 2 and 10min when a 50mL solution (containing a total amount of metal of 2mg) was equilibrated with 0.5g of resin. Sorption of all metal ions except Pb2+ follows the Langmuir model, whereas for Pb the data fits with the Freundlich model. The sorption capacity is between 60.7 (for Cd) and 406.7 (for Cu) µmolg–1. The resin can withstand an acid concentration of 6molL–1 and can be reused for thirty cycles of sorption–desorption. The preconcentration factor varies between 100 and 300. For Cd, Ni and Cu the sorption capacity of 2,3-dihydroxypyridine loaded cellulose is lower than that of the present resin. The tolerance limits of electrolytes, humic acid, complexing agents, Ca2+ and Mg2+ in the enrichment of all metal ions are reported. The limits of detection are 3.88, 5.37, 8.72, 13.88, 4.71, 1.24, 0.59 and 0.30µgL–1 for Zn2+, Mn2+, Ni2+, Pb2+, Cd2+, Cu2+, Fe3+ and Co2+, respectively. The calibration curves for flame AAS determination were linear in the ranges 0.018–1.0, 0.067–5.0, 0.2–5.0, 0.9–20, 0.028–2.0, 0.077–5.0, 0.19–10 and 0.1–3.5µgmL–1, respectively. All the eight metal ions in river and synthetic water samples, Co in vitamin tablets and Zn in milk samples have been quantitatively enriched with Amberlite XAD-2-DHP and determined by flame atomic absorption spectrometry. 相似文献
945.
《Arabian Journal of Chemistry》2022,15(4):103712
Although recent decades have witnessed the synthesis of 1,3,4-thiadiazoles via phosphorus POCl3-promoted cyclization reaction, simultaneous access to 2-amino-1,3,4-thiadiazole and 2-amino-1,3,4-oxadiazole analogs remains unexpected and elusive. Herein, a detailed regiocontrolled synthesis of 2-amino-1,3,4-thiadiazoles in good to high yields with good regioselectivities from readily available thiosemicarbazides using POCl3 was disclosed. Meantime, to establish a comprehensive structure–activity relationship, 2-amino-1,3,4-oxadiazole derivatives as single regioisomers were prepared via EDCI·HCl-triggered cyclization of the thiosemicarbazide intermediates. The in vitro anti-influenza assays proved that the selected compounds with the pyrazine/pyridine ring exhibited certain inhibitory activities against influenza A virus strains A/HK/68 (H3N2) and A/PR/8/34 (H1N1) in MDCK cells. Among them, N-(adamantan-1-yl)-5-(5-(azepan-1-yl)pyrazin-2-yl)-1,3,4-thiadiazol-2-amine (4j) was the most active compound, and exhibited favorable activity with EC50 values of 3.5 μM and 7.5 μM, respectively. In addition, the molecular docking results explained the reason why compound 4j had dual inhibitory activity and revealed the reasonable binding mode of this compound with the M2-S31N and M2-WT ion channels. This compound had the potential to be further developed as an anti-influenza drug. 相似文献
946.
Peter M.W. Gill Darragh P. O'Neill Nicholas A. Besley 《Theoretical chemistry accounts》2003,109(5):241-250
Two-electron distribution functions and intracules are functions of electronic coordinates and occupy an important, and frequently
overlooked, middle ground between the beguiling simplicity of electron densities and the bewildering complexity of wavefunctions.
We survey the functions that have been considered by earlier workers and introduce two new ones, the Wigner intracule and
the action intracule, that have not previously been discussed. To illustrate their usefulness, we consider the intracules
of jellium, a few small atoms and the dissociating hydrogen molecule.
Received: 26 July 2002 / Accepted: 20 October 2002 /
Published online: 30 January 2003
Correspondence to: P.M.W. Gill e-mail: peter.gill@nott.ac.uk 相似文献
947.
A synthetically prepared seleno-peptide (AHPDVLTVXLQMLDDGR) was used as a model system for the acid hydrolysis of selenized yeast proteins. The seleno-peptide is a tryptic
peptide of a heat shock protein 104 from Saccharomyces cerevisiae, was subjected to acid hydrolysis using methanesulfonic acid over a time period of 8 hours. Aliquots of the solution were
sub-sampled at predetermined time intervals and the peptide fragments characterized by reversed phase LC MSn. Similarly, the appearance of amino acid residues in the solution was monitored. It was found that after about 8 hours the
synthetic peptide completely hydrolyzed. The use of a selenopeptide as a model for hydrolysis of selenized yeast hydrolysis
was validated by comparing the decomposition time profile of the synthetic peptide with that of a selenized yeast sample.
The rate of hydrolysis was identical in both systems, suggesting that the employed acid hydrolysis yields to the complete
decomposition of the Se containing proteins in yeast and consequently to the liberation of selenomethionine. 相似文献
948.
A simple, rapid and specific chemiluminescence (CL) method for the determination of glutathione has been developed. The method
is based on the enhanced CL of the reaction between Ru(phen)3
2+ (phen = 1,10-phenanthroline) and KMnO4 by glutathione in HCl medium. Under the optimum conditions, the response is linearly proportional to the concentration of
glutathione between 1.5 × 10−7 and 1.0 × 10−5 mol L−1. The dection limit for glutathione (5.8 × 10−8 mol L−1) is about 10 and 200 times better than those of the spectrophotometric method using Ellman regent and the Lucigenin – CL
method, respectively. The final procedure allows the determination of glutathione in human serum with recoveries of 92%–108%.
A satisfactory agreement was obtained with a mean relative difference of 2.5% compared to the HPLC method. 相似文献
949.
Kagan Kerman Yasutaka Morita Yuzuru Takamura Mehmet Ozsoz Eiichi Tamiya 《Analytica chimica acta》2004,510(2):169-174
The unique binding event between Escherichia coli single-stranded DNA binding protein (SSB) and single-stranded oligonucleotides conjugated to gold (Au) nanoparticles is utilized for the electrochemical detection of DNA hybridization. SSB was attached onto a self-assembled monolayer (SAM) of single-stranded oligonucleotide modified Au nanoparticle, and the resulting Au-tagged SSB was used as the hybridization label. Changes in the Au oxidation signal was monitored upon binding of Au tagged SSB to probe and hybrid on the electrode surface. The amplified oxidation signal of Au nanoparticles provided a detection limit of 2.17 pM target DNA, which can be applied to genetic diagnosis applications. This work presented here has important implications with regard to combining a biological binding event between a protein and DNA with a solid transducer and metal nanoparticles. 相似文献
950.
The decay rate of vinyl unsaturation in high-density polyethylenes irradiated at temperatures from about 310 to 450 K, changes significantly in the melting range up to the crystalline melting point as does free radical mobility and the polymer crystallinity. However, orienting the polymer, or slow cooling or quenching from the melt, prior to irradiation, do not alter the decay process or its rate, although they do alter the rate of increase of insoluble gel and of elastic modulus in the molten state. It is suggested that, below 340 K, the marked deviations from a first-order decay result from the limited mobility of polymeric free radicals in the crystalline phase and from scavenging, by vinyl groups, segregated into the amorphous phase, of radiolytic hydrogen atoms (H). In the melting range, the mobility of polymeric free radicals increases as the crystallinity decreases, reducing the importance of scavenging, so vinyl decay approximates more closely to a first-order relation. In the melt, the vinyl decay relation is not changed qualitatively by H atom scavenging, but the effective vinyl concentration is lower, so the decay rate drops sharply. 相似文献