全文获取类型
收费全文 | 1094篇 |
免费 | 22篇 |
国内免费 | 5篇 |
专业分类
化学 | 742篇 |
晶体学 | 6篇 |
力学 | 25篇 |
数学 | 167篇 |
物理学 | 181篇 |
出版年
2022年 | 27篇 |
2021年 | 27篇 |
2020年 | 27篇 |
2019年 | 19篇 |
2018年 | 24篇 |
2017年 | 15篇 |
2016年 | 32篇 |
2015年 | 23篇 |
2014年 | 49篇 |
2013年 | 87篇 |
2012年 | 69篇 |
2011年 | 82篇 |
2010年 | 43篇 |
2009年 | 58篇 |
2008年 | 68篇 |
2007年 | 50篇 |
2006年 | 57篇 |
2005年 | 58篇 |
2004年 | 41篇 |
2003年 | 35篇 |
2002年 | 30篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 17篇 |
1998年 | 10篇 |
1997年 | 9篇 |
1996年 | 16篇 |
1995年 | 7篇 |
1994年 | 15篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1968年 | 2篇 |
1967年 | 2篇 |
排序方式: 共有1121条查询结果,搜索用时 15 毫秒
51.
Bogdan Boduszek Tomasz Olszewski Waldemar Goldeman Magdalena Konieczna 《Phosphorus, sulfur, and silicon and the related elements》2013,188(4):787-795
The synthesis of a series of new pyridine aminomethylphosphinic acids is described. These compounds were obtained in the reaction of the corresponding pyridine aldehydes with primary amines and with ethyl phenylphosphinate, or methylphosphinate, in the presence of bromotrimethylsilane. In aqueous, strong acid solutions, pyridine aminophosphinic acids were split, forming the phenyl-, or methylphosphonic, acid and the corresponding secondary pyridyl-alkylamines. The kinetics of some observed cleavages were measured, and a mechanism of the cleavage has been proposed. 相似文献
52.
Dumitru Tita Tunde Jurca Adriana Fulias Eleonora Marian Bogdan Tita 《Journal of Thermal Analysis and Calorimetry》2013,112(1):407-419
This study is part of a research project aimed to find and optimize methods by which drug-excipient compatibility can be reliably and quickly assessed. The objective of the present study was to evaluate the compatibility of the acetylsalicylic acid (ASA), an non-steroidal anti-inflammatory drug, with pharmaceutical excipients of common use including diluents, binders, disintegrants, lubricants and solubilising agents. In order to investigate the possible interactions between ASA and eleven excipients differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry analysis completed by Fourier transform infrared spectroscopy (FT-IR) and X-ray powder diffraction were used for compatibility study. The DSC has proven to be, among the selected analytical techniques, the most sensitive and specific in assessing the compatibility. The samples, as physical mixtures, were prepared by mixing the analyte and excipients in a proportion of 1:1 (w:w). On the basis of thermal results (especially DSC), confirmed by FT-IR and X-ray analysis, a possible chemical interaction was found between the ASA with polyvinylpyrrolidone K30 (PVP) and magnesium stearate, respectively a possible physical interaction with colloidal silicon dioxide and stearic acid (Ac. St.). 相似文献
53.
Bogdan Tutunaru Adriana Samide Catalin Negrila 《Journal of Thermal Analysis and Calorimetry》2013,111(2):1149-1154
The influence of different ions NO3 ? and SO4 2? on the carbon steel corrosion in ammonium chloride was investigated using mass loss measurements and potentiodynamic polarization. Corrosion products were analyzed using X-ray photoelectron spectroscopy (XPS) and simultaneous thermal and differential scanning calorimetry (TG/DSC). XPS analysis shows that the main product of corrosion is a non-stoichiometric Fe3+ oxyhydroxide, consisting of a mixture of FeO(OH) and FeO(OH) containing inclusions of these anions, species such as Fe3+O(OH,Cl?); Fe3+O(OH,SO4 2?); and Fe3+O(OH,NO3 ?). TG/DSC confirms the decomposition of the rusty products formed by chemical corrosion, compounds like Fe3+ oxyhydroxides, with β-FeOOH as the major phase, crystal structure of which may contain Cl?, NO3 ?, and SO4 2?—e.g., akaganeite [Fe3+O(OH,A)]. 相似文献
54.
Bogdan Tita Tunde Jurca Dumitru Tita 《Journal of Thermal Analysis and Calorimetry》2013,113(1):291-299
Thermal analysis is a routine method in the solution of pharmaceuticals problems such as the control of raw materials, to the determination of purity, to the qualitative and quantitative analysis of drug formulation, tests of thermal stability and compatibility, the determination of kinetic parameters, etc. The evaluation of thermal stability in the solid state is mostly made by analyzing their decomposition under isothermal and non-isothermal conditions. The present work reports the study on the thermal behavior of pentoxifylline—active substance and tablets, respectively, the determination of the kinetic parameters for the decomposition process under non-isothermal conditions and in a nitrogen atmosphere at five heating rates: 2.5, 5, 7.5, 10 and 15 °C min?1. For the determination of kinetic parameters from the TG/DTG curves, the following differential methods were utilized: Friedman isoconversional and Chang, respectively, integral methods: Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, Li–Tang, and Starink. Thermoanalytical curves showed that the active substance is thermally more stable than the tablets. The decrease in stability was attributed to the presence of excipients. 相似文献
55.
Acid-catalyzed cyclization of D-mannitol followed by isopropylidenation furnished 2,5-anhydro-1,3-O-isopropylidene-D-glucitol, conveniently isolated by dry column vacuum chromatography. 相似文献
56.
57.
Adriana?SamideEmail author Bogdan?Tutunaru 《Central European Journal of Chemistry》2014,12(9):901-908
Quinine sulfate dihydrate (QNS), IUPAC name: (8S,9R)-6-methoxy-4-quinolenyl-5-vinyl-2-quinuclidinyl methanol sulfate dihydrate, was tested as corrosion inhibitor for carbon steel in 1.5 mol L?1 HCl solution using the potentiodynamic polarization and the electrochemical impedance spectroscopy (EIS) associated with UV-Vis spectrophotometry. The electrochemical results showed that, the inhibition efficiency (IE) increased with the increase in QNS concentration, reaching a maximum value of 93.35±0.25%. The polarization resistance (R p) followed the same trend, obtaining the highest value of 659.7 Ω cm2, while the corrosion current density (i corr) reached the lowest level of 195 µA cm?2. The action mechanism of QNS was proposed considering the ability of quinine (QN) to be adsorbed on the metal surface via the lone pairs of electrons from hydroxyl oxygen atom, and/or from quinoline and quinuclidinic nitrogens. The occurrence of the complexes between inhibitor and iron ions was considered an additional process, which may contribute to protective layer formation. The Temkin adsorption isotherm was found as the best fitting for the degree of surface coverage (θ) values. In order to elucidate the mechanism of protective layer formation, the free energy of adsorption (ΔG o ads) value was calculated. This indicates that the inhibitor acts by chemical adsorption on the steel surface. 相似文献
58.
Mihaela Tertis Oana Hosu Bogdan Feier Andreea Cernat Anca Florea Cecilia Cristea 《Molecules (Basel, Switzerland)》2021,26(11)
Food safety and quality control pose serious issues to food industry and public health domains, in general, with direct effects on consumers. Any physical, chemical, or biological unexpected or unidentified food constituent may exhibit harmful effects on people and animals from mild to severe reactions. According to the World Health Organization (WHO), unsafe foodstuffs are especially dangerous for infants, young children, elderly, and chronic patients. It is imperative to continuously develop new technologies to detect foodborne pathogens and contaminants in order to aid the strengthening of healthcare and economic systems. In recent years, peptide-based sensors gained much attention in the field of food research as an alternative to immuno-, apta-, or DNA-based sensors. This review presents an overview of the electrochemical biosensors using peptides as molecular bio-recognition elements published mainly in the last decade, highlighting their possible application for rapid, non-destructive, and in situ analysis of food samples. Comparison with peptide-based optical and piezoelectrical sensors in terms of analytical performance is presented. Methods of foodstuffs pretreatment are also discussed. 相似文献
59.
We present a comparative study of the application of a recently introduced heuristic algorithm to the optimization of transport on three major types of complex networks. The algorithm balances network traffic iteratively by minimizing the maximum node betweenness with as little path lengthening as possible. We show that by using this optimal routing, a network can sustain significantly higher traffic without jamming than in the case of shortest path routing. A formula is proved and tested with numerical simulation that allows quick computation of the average number of hops along the path and of the average travel times once the betweennesses of the nodes are computed. Using this formula, we show that routing optimization preserves the small-world character exhibited by networks under shortest path routing, and that it significantly reduces the average travel time on congested networks with only a negligible increase in the average travel time at low loads. Finally, we study the correlation between the weights of the links in the case of optimal routing and the betweennesses of the nodes connected by them. 相似文献
60.
Kyle M. McCall Bogdan M. Benin Michael Wörle Thomas Vonderach Detlef Günther Maksym V. Kovalenko 《Helvetica chimica acta》2021,104(1):e2000206
Inorganic, lead-free metal halides are widely sought after following the rise of the halide perovskites as outstanding optoelectronic materials, due to their enhanced stability and reduced toxicity. Herein, we report on the solvothermal synthesis of Rb7Sb3Br16, which exhibits a 0D structure comprised of [SbBr6]3− octahedra and edge-sharing bioctahedra [Sb2Br10]4− dimers that order into layers along the c-axis. This all-inorganic material is air-stable and exhibits weak orange photoluminescence (PL) at room temperature. Low-temperature PL and PL excitation (PLE) measurements reveal the presence of two distinct emission bands that originate from these structural units, with the high-energy emission quenching as temperature rises beyond 150 K. We are also able to obtain Rb7Bi3Br16 and Rb7Bi3I16 which both crystallize in orthorhombic symmetry, with Rb7Bi3Br16 presenting weak low-temperature luminescence while Rb7Bi3I16 is non-luminescent. This work expands the library of emissive inorganic metal halides and provides further evidence for the efficacy of low-dimensional Sb−X luminescent centers based on octahedral and edge-sharing [Sb2X10]4− dimers. 相似文献