全文获取类型
收费全文 | 781篇 |
免费 | 25篇 |
国内免费 | 1篇 |
专业分类
化学 | 644篇 |
晶体学 | 5篇 |
力学 | 7篇 |
数学 | 80篇 |
物理学 | 71篇 |
出版年
2023年 | 3篇 |
2022年 | 52篇 |
2021年 | 47篇 |
2020年 | 25篇 |
2019年 | 23篇 |
2018年 | 24篇 |
2017年 | 18篇 |
2016年 | 37篇 |
2015年 | 19篇 |
2014年 | 31篇 |
2013年 | 37篇 |
2012年 | 57篇 |
2011年 | 57篇 |
2010年 | 34篇 |
2009年 | 35篇 |
2008年 | 32篇 |
2007年 | 43篇 |
2006年 | 29篇 |
2005年 | 34篇 |
2004年 | 46篇 |
2003年 | 32篇 |
2002年 | 21篇 |
2001年 | 7篇 |
2000年 | 3篇 |
1999年 | 7篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有807条查询结果,搜索用时 31 毫秒
771.
Renata de Jesus Coelho Castro Fausto S. Sobrinho Marco A. Sundfeld da Gama Patrícia M. de Castro Barra Rosemar Antoniassi Marcone A. L. de Oliveira 《Central European Journal of Chemistry》2013,11(8):1286-1296
An alternative method for extraction optimization of C18:2 n-6 and C18:3 n-3, the main precursors for the synthesis of conjugated linoleic acid (CLA), in Brachiaria ruzizienses forages was proposed. Three methods of lipid extraction were tested: 1. Hara & Radin, 2. Micro Folch and 3. Bligh & Dyer. The preliminary test showed the Hara & Radin method as the most promising procedure. Then, a 33 Box Behnken design with triplicate in the central point was applied in Hara & Radin method in order to optimize the extraction procedure. The optimization extraction was monitored by quantification of C18:2 n-6 and C18:3 n-3 through capillary zone electrophoresis (CZE). The results obtained by CZE were compared to gas chromatography (AOCS official method) in real samples using the paired t-test. No significant difference between methods was found within a 95% confidence interval (p-value= 0.937). The alternative CZE method for Brachiaria ruzizienses forages analysis has some advantages in comparison with official GC method such as, short analysis time (10 min), no derivatization step for sample preparation, absence of specific separation columns, lower analytical cost and high throughput. 相似文献
772.
Renata Mansini Wlodzimierz Ogryczak M. Grazia Speranza 《European Journal of Operational Research》2014
Markowitz formulated the portfolio optimization problem through two criteria: the expected return and the risk, as a measure of the variability of the return. The classical Markowitz model uses the variance as the risk measure and is a quadratic programming problem. Many attempts have been made to linearize the portfolio optimization problem. Several different risk measures have been proposed which are computationally attractive as (for discrete random variables) they give rise to linear programming (LP) problems. About twenty years ago, the mean absolute deviation (MAD) model drew a lot of attention resulting in much research and speeding up development of other LP models. Further, the LP models based on the conditional value at risk (CVaR) have a great impact on new developments in portfolio optimization during the first decade of the 21st century. The LP solvability may become relevant for real-life decisions when portfolios have to meet side constraints and take into account transaction costs or when large size instances have to be solved. In this paper we review the variety of LP solvable portfolio optimization models presented in the literature, the real features that have been modeled and the solution approaches to the resulting models, in most of the cases mixed integer linear programming (MILP) models. We also discuss the impact of the inclusion of the real features. 相似文献
773.
The paper concerns the density points with respect to the sequences of intervals tending to zero in the family of Lebesgue measurable sets. It shows that for some sequences analogue of the Lebesgue density theorem holds. Simultaneously, this paper presents proof of theorem that for any sequence of intervals tending to zero a relevant operator ? J generates a topology. It is almost but not exactly the same result as in the category aspect presented in [WIERTELAK, R.: A generalization of density topology with respect to category, Real Anal. Exchange 32 (2006/2007), 273–286]. Therefore this paper is a continuation of the previous research concerning similarities and differences between measure and category. 相似文献
774.
Renata Studzińska Marcin WróblewskiAleksandra Karczmarska-Wódzka Renata Kołodziejska 《Tetrahedron letters》2014
2-Bromomethyl- and 2-iodomethyl-2,3-dihydrothiazolo[3,2-a]pyrimidin-5-ones are prepared via the reaction of 3-allyl-2-thiouracil derivatives with bromine or iodine chloride, respectively. The 6-bromo and 6-nitro derivatives are synthesized by an electrophilic substitution at C-6 of the thiazolopyrimidine system. As a result, novel 2,3-dihydrothiazolo[3,2-a]pyrimidin-5-one derivatives are obtained. Hydrogen halide elimination from the 2-halomethyl-2,3-dihydrothiazolo[3,2-a]pyrimidin-5-ones is also reported. 相似文献
775.
The total content of polyphenols and flavonoids determined in the same plant and their corresponding antioxidant activities may vary widely, depending on the extraction conditions applied. This study was conducted to optimise the extraction conditions of phenolics and flavonoids from the mistletoe plant. Various extraction methods, i.e. ultrasound-assisted extraction technology, maceration, maceration with stirring, accelerated solvent extraction (ASE), and extraction under reflux were evaluated for their percentage extraction of polyphenols (TPC) and flavonoids (TFC) from Viscum album subsp. abietis. In addition, the anti-radical activity of extracts was analysed using the 2,2-diphenyl-1-picrylhydrazyl method. The effects of temperature, solvent type, and concentration on the phenolic extraction efficiency and antioxidant activity were studied using chemometric and statistical methods. The results showed that the extracts of V. album subsp. abietis contained large amounts of polyphenols and flavonoids (up to 57.673 mg g?1 and 9.955 mg g?1 of dry extract, respectively) and exhibited potent antioxidant activity, hence representing promising sources of powerful antioxidants. Due to its high extraction efficiency and considerable saving of time and solvent, ASE was more effective than the other extraction techniques. Extracts prepared with water-polar solvent mixtures displayed the highest TPC, TFC, and antioxidant activity, while organic polar solvents were the least efficient extractants. 相似文献
776.
Przemysław Rybiński Grażyna Janowska Renata Dobrzyńska Agnieszka Kucharska 《Journal of Thermal Analysis and Calorimetry》2014,115(1):771-782
This paper presents the results of investigating the thermal stability, flammability, and fire hazard of cross-linked EVM/NBR blends unfilled and filled with halogenless flame-retardant compounds such as melamine cyanurate or magnesium hydroxide. The thermal analysis of the blends was carried out in the atmosphere of air. The activation energy of the composite destruction was determined by two non-isothermal methods: Flynn–Wall–Ozawa’s and Kissinger’s methods. The flammability of the composites obtained was determined by the method of oxygen index and on the basis of their combustion in air. The fire hazard of the vulcanizates investigated was determined with the use of a cone calorimeter and on the basis of toxicometric parameters W LC50SM. The test results have shown that the flame retardants used increase the thermal stability of the cross-linked blends and decrease their flammability, and thereby allow one to obtain self-extinguishing or non-flammable polymeric materials. The cross-linked EVM/NBR blends filled with these flame-retardant compounds are characterized by good mechanical properties and reduced fire hazard. 相似文献
777.
Renata M. Braga Tiago R. Costa Julio C. O. Freitas Joana M. F. Barros Dulce M. A. Melo Marcus A. F. Melo 《Journal of Thermal Analysis and Calorimetry》2014,117(3):1341-1348
The Elephant Grass (Pennisetum purpureum Schum) was pretreated by two independent processes, through washing with hot water (W-EG) and acid solution (AW-EG) to improve its energy properties to apply it in a thermochemical process conversion into fuel. The biomasses were analyzed by proximate and ultimate analysis; and the pyrolysis kinetics, before and after pretreatments, were evaluated by the apparent activation energy (E a) for decomposition in the temperature range of greater volatile matter through the Model-free kinetics using thermogravimetric analysis data. The kinetics of the microcrystalline cellulose Avicel PH-101 was performed to evaluate the E a result of pure cellulose. The pretreatments were efficient in increasing the volatile matter and heating value, decreasing moisture and ash content, and improving its energetic power to the application in fast pyrolysis process for bio-oil production. The TG results have shown that the reduction in ash content facilitates the pyrolysis process, increasing the volatile matter and decreasing the apparent activation energy required to biomasses degradation, due to less diffusional resistances to heat and mass transfer of W-EG and AW-EG. The Avicel PH-101 showed the highest value of apparent activated energy (E a = 276.2 kJ mol?1) which could be explained by its crystallinity, suggesting that crystalline cellulose regions are less accessible to heat diffusion than amorphous regions, requiring more energy to its degradation. 相似文献
778.
Improved Cyclopropanation Activity of Histidine‐Ligated Cytochrome P450 Enables the Enantioselective Formal Synthesis of Levomilnacipran
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Z. Jane Wang Dr. Hans Renata Nicole E. Peck Christopher C. Farwell Dr. Pedro S. Coelho Prof. Dr. Frances H. Arnold 《Angewandte Chemie (International ed. in English)》2014,53(26):6810-6813
Engineering enzymes capable of modes of activation unprecedented in nature will increase the range of industrially important molecules that can be synthesized through biocatalysis. However, low activity for a new function is often a limitation in adopting enzymes for preparative‐scale synthesis, reaction with demanding substrates, or when a natural substrate is also present. By mutating the proximal ligand and other key active‐site residues of the cytochrome P450 enzyme from Bacillus megaterium (P450‐BM3), a highly active His‐ligated variant of P450‐BM3 that can be employed for the enantioselective synthesis of the levomilnacipran core was engineered. This enzyme, BM3‐Hstar, catalyzes the cyclopropanation of N,N‐diethyl‐2‐phenylacrylamide with an estimated initial rate of over 1000 turnovers per minute and can be used under aerobic conditions. Cyclopropanation activity is highly dependent on the electronic properties of the P450 proximal ligand, which can be used to tune this non‐natural enzyme activity. 相似文献
779.
Andrej Oriňák Ivan Talian Evtim V. Efremov Freek Ariese Renata Oriáaková 《Chromatographia》2008,67(3-4):315-319
Hyphenation of thin layer chromatography (TLC) with surface-based spectral methods requires a homogeneous surface for direct
and quantitative analysis on the chromatographic plate after separation. Since most chromatographic materials do not produce
strong background signals in Raman spectroscopy (RS) or surface-enhanced RS (SERS), we tested the suitability of two different
chromatographic substrates and one interface for coupling SERS with TLC. This was carried out by using a chromatographic thin
layer, specially produced for RS measurements, and a monolithic silica thin layer. A typical TLC plate with a modified aluminium
backplate foil on one side was used as an interface. Three biologically active diterpenes, namely gibberellic acid (GA), abietic
acid (AA) and kaurenoic acid (KA), were used as test analytes. Stock solutions were applied directly onto the surface, followed
by the addition of silver colloid and measurements were taken by SERS. The strongest signal (excitation at 514.5 nm) was obtained
for GA using a Raman treated thin layer where the enhancement factor value was determined to be 102. Several fundamental Raman bands for GA were found at 1622, 1593, 1570, 1542, 1366 and 1236 cm−1. When the monolithic silica layer was used, no useful SERS signals were observed. The SERS spectra on modified aluminium
backplate for AA and GA were quite similar and no SERS spectrum was obtained for KA. Future research will be concerned towards
the use of nanostructured surfaces for SERS analysis.
An erratum to this article can be found at 相似文献
780.
The aim of the present study was to attempt to describe the procedure of isolation, purification, enrichment and determination of 4-n-nonylphenol (4-n-NP) and 4-tert-octylphenol (4-t-OP) in water and biological samples (fish tissue). There were five procedures of solid phase extraction (SPE) tested using different sorbents for the isolation of analytes from water samples. Moreover, we isolated these chemicals from biological matrices with the aid of various extraction methods. The purpose of it was to perform an optimisation of ultrasonic bath, accelerated solvent extraction (ASE) and solid phase extraction process of alkylphenols from biological samples, through the choice of selective sorbents (octadecyl, octadecyl end-capped and amine) and search solvents (methylene chloride, methanol, hexane). Reversed-phase HPLC with diode array detection was used for the determination of 4-n-NP and 4-t-OP in water and fish tissue samples. Sensitivity was evaluated by determining the limit of detection (LOD=0.06 and 0.04ng microL(-1)) and limit of quantification (LOQ=0.18 and 0.16ng microL(-1)) of 4-NP and 4-t-OP, respectively. A series of standard solutions for 4-n-NP and 4-t-OP provided the basis for plotting an analytical curve and obtaining a linear dependence in the range of approximately 1-25ng microL(-1). The best efficiencies obtained for 4-n-NP and 4-t-OP in water samples were 76.65% (+/-1.49) and 83.08% (+/-3.73), respectively. In the case of fish tissue, different situation was observed because the obtained values were considerably lower, being 68.32% for 4-t-OP using hexane (program 1) as solvent and 72.35% (program 2) for 4-n-NP using acetonitrile. 相似文献