全文获取类型
收费全文 | 534篇 |
免费 | 22篇 |
国内免费 | 6篇 |
专业分类
化学 | 352篇 |
晶体学 | 3篇 |
力学 | 23篇 |
数学 | 59篇 |
物理学 | 125篇 |
出版年
2022年 | 6篇 |
2021年 | 7篇 |
2020年 | 8篇 |
2019年 | 6篇 |
2018年 | 4篇 |
2016年 | 12篇 |
2015年 | 10篇 |
2014年 | 19篇 |
2013年 | 41篇 |
2012年 | 29篇 |
2011年 | 32篇 |
2010年 | 26篇 |
2009年 | 28篇 |
2008年 | 33篇 |
2007年 | 21篇 |
2006年 | 26篇 |
2005年 | 16篇 |
2004年 | 20篇 |
2003年 | 19篇 |
2002年 | 26篇 |
2001年 | 24篇 |
2000年 | 14篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1969年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有562条查询结果,搜索用时 15 毫秒
551.
Oxidation of 5,10,15,20-tetramesitylporphyrinatoiron(III) perchlorate, (TMP)FeIII(ClO4), with ferric perchlorate in acetonitrile gave a metastable species identified as (TMP)FeIV(ClO4)2 that decayed within seconds to the known isomeric species (TMP*+)FeIII(ClO4)2. Irradiation of the metastable species with 355 nm laser light gave a highly reactive transient that reacts with simple organic reductants (alkenes and arylalkanes) 5 orders of magnitude faster than known Compound I analogues, (TMP*+)FeIV(O)(X-). 相似文献
552.
To prevent casein adsorption and improve between-run repeatability, a CE procedure is developed for cation analysis in milk using a modified imidazole/alpha-hydroxyisobutyric acid (HIBA) BGE system to operate at pH 6 to match the pH of milk and elevate imidazole concentration to enhance its buffer action. The procedure is shown to produce a fast, economic and efficient method for cation separation in milk with only simple dilution. Upon direct hydrodynamic injection of diluted milk sample at 8 cm for 30 s in an uncoated column with a BGE consisting of 10 mM imidazole, 10 mM HIBA and 10% methanol at pH 6.0 under +18 kV, baseline separation was achieved for K(+), Na(+), Ca(2+), Mg(2+), Mn(2+), Cd(2+), Co(2+), Ni(2+)and Zn(2+). Agreeable results at 95% confidence level were obtained using CE and inductively coupled plasma-atomic emission spectrometry (ICP-AES) for milk samples after protein removal. Baseline-resolved peaks for essential minerals were obtained for fresh and non-refrigerated reconstituted milks. Long-term stability was demonstrated by repeated determinations without rinsing and improvement in repeatability was shown by rinsing with 60 mM SDS in BGE. Information on metal speciation useful for nutritional assessment was obtained from CE to complement the ICP methods. 相似文献
553.
This article reviews the applications of CE that are relevant to the analysis of small molecules in foods. CE has been applied to a wide range of important areas of food analysis and is rapidly being established as an alternative technique to chromatographic methods including HPLC and GC within analytical food and research laboratories. In recent years the analysis of food by CE has become more frequent and important and as such a variety of compounds have been separated and quantified. Although many other analytes have been detected by CE, this review will highlight areas relating primarily to the rather broad chemical classes of free amino acids, carbohydrates, organic acids, vitamins and a variety of antioxidants. In addition, information relating to the analyte, sample matrix, mode of CE employed, scope of the methodology and the detection and derivatization of the small molecules are considered and discussed. 相似文献
554.
Ching Min HsuRong Fung Huang 《Experimental Thermal and Fluid Science》2011,35(7):1370-1382
The effects of acoustic excitation on the flow behavior, penetration, and spread of the stack-issued and wall-issued transverse jets were studied experimentally. The jet flow was periodically excited by a loudspeaker that was driven with a square wave at resonance Strouhal numbers. The pulsed transverse jet was characterized by jet Reynolds number 2000. Streak pictures of the smoke flow patterns illuminated by the laser-light sheet in the median plane were recorded with a high-speed digital camera to illustrate the evolution process of the characteristic flow behavior within one excitation cycle. The binary edge-detection technique was used to determine penetration height and spread width. The tracer-gas concentration measurement provided jet dispersion information. The evolution processes of both the stack-issued and wall-issued transverse jets were characterized by a leading vortex ring and swing motion of the jet column near the jet exit as the jets were forced at resonance Strouhal numbers. A leading vortex ring appeared near the jet exit during the leading phase of excitation cycle and evolved subsequently to puffs of jet fluids in the upwind shear layer of the deflected jet. The swinging motion of the near-tube tip jet column induced up/down oscillation of the deflected jet. The excited stack-issued transverse jet exhibited significantly larger penetration height and spread width than the excited wall-issued transverse jet. The tracer-gas detection experiment results showed that the excited transverse jet disperses significantly faster and wider than the non-excited transverse jet. Pulsating the transverse jet at low resonance Strouhal numbers produced higher mixing and dispersion effects than pulsating the transverse jet at high resonance Strouhal numbers. 相似文献
555.
Quality assurance and quality control for thermal/optical analysis of aerosol samples for organic and elemental carbon 总被引:1,自引:0,他引:1
Chow JC Watson JG Robles J Wang X Chen LW Trimble DL Kohl SD Tropp RJ Fung KK 《Analytical and bioanalytical chemistry》2011,401(10):3141-3152
Accurate, precise, and valid organic and elemental carbon (OC and EC, respectively) measurements require more effort than
the routine analysis of ambient aerosol and source samples. This paper documents the quality assurance (QA) and quality control
(QC) procedures that should be implemented to ensure consistency of OC and EC measurements. Prior to field sampling, the appropriate
filter substrate must be selected and tested for sampling effectiveness. Unexposed filters are pre-fired to remove contaminants
and acceptance tested. After sampling, filters must be stored in the laboratory in clean, labeled containers under refrigeration
(<4 °C) to minimize loss of semi-volatile OC. QA activities include participation in laboratory accreditation programs, external
system audits, and interlaboratory comparisons. For thermal/optical carbon analyses, periodic QC tests include calibration
of the flame ionization detector with different types of carbon standards, thermogram inspection, replicate analyses, quantification
of trace oxygen concentrations (<100 ppmv) in the helium atmosphere, and calibration of the sample temperature sensor. These
established QA/QC procedures are applicable to aerosol sampling and analysis for carbon and other chemical components. 相似文献
556.
Abstract Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4′-butylaniline (MBBA) and 4-n-pentyl-4′-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature. 相似文献
557.
It was shown previously that long range 2H-13 C dipolar coupling constants in liquid crystals could be obtained from the proton-decoupled 13C NMR spectrum of monodeuterated compounds. This approach has now been applied to the study of 4′-(1-deuteropentyloxy)-4-biphenylcarbonitrile (5OCB-d1). The 113C spectrum of this compound gives nine 2H- 13C dipolar coupling constants for carbon ranging from one to six bonds away from the dueterium nucleus. The 2 H NMR and 5OCB-d1 gives the deuterium quadrupole coupling constant and the 1H- 2H dipolar coupling constant for the α -CHD group. In addition, eleven 2-bond and two 3-bond 1H- 13C dipolar coupling constants have been obtained for 4′-pentyloxy-4-biphenylcarbonitrile (non-deuterated 5OCB) by using the 2D 13C method of proton-encoded local field spectroscopy. 相似文献
558.
Babajan Banaganapalli Chaitanya Mulakayala Gowsia D Naveen Mulakayala Madhusudana Pulaganti Noor Ahmad Shaik Anuradha CM Raja Mohan Rao Jumana Yousuf Al-Aama Suresh Kumar Chitta 《Applied biochemistry and biotechnology》2013,171(7):1639-1657
Resveratrol (RVS) is a naturally occurring antioxidant, able to display an array of biological activities. In the present investigation, a new derivative of RVS, RVS(a), was synthesized, and its biological activity was determined on U937 cells. It was observed that RVS(a) showed pronounced activity on U937 cells than RVS. RVS(a) is able to induce apoptosis in tumor cell lines through subsequent DNA fragmentation. From the EMSA results, it was evident that RVS(a) was able to suppress the activity of NFkB by interfering its DNA binding ability. Furthermore, the molecular interaction analysis (docking and dynamics) stated that RVS(a) has strong association with the IkB-alpha site of NFkB compared with RVS; this binding nature of RVS(a) might be prevent the NFkB binding ability with DNA. The present findings represent the potential activity of propynyl RVS on U937 cells and signifying it as a one of putative chemotherapeutic drugs against cancer. 相似文献
559.
Y. S. Fung Y. Y. Wong 《International journal of environmental analytical chemistry》2013,93(3-4):397-412
Abstract A light and portable organic vapours monitor has been developed using piezoelectric (P/Z) quartz crystal to determine Volatile Organic Compounds (VOCs) commonly found in workplace air. Out of the eight coating material studied, High Vacuum Grease (HVG) was found to be the best for determining commonly encountered organic vapours such as 1,1,1-Trichloroethane, Chloroform, Benzene and Toluene in room air. The optimised coating weight was 23 μg HVG at a flow rate of 200 mL/min. Moisture was found to interfere and the use of a Nafion gas dryer was found to reduce the relative humidity to a constant 37 % for room air with 43–81 % relative humidity. Under optimised conditions, a mean sensitivity of 1.9 × 1?2 Hz/ppm was obtained for total VOCs with 17.2% RSD variation towards different VOCs. Excellent linear working ranges were obtained from 40 to 2400 ppm (v/v), with response time and recovery time within one minute up to 500 ppm (v/v) of analyte. High and relatively constant response factors were obtained for different VOCs. The reliability of the mcthod has been established by parallel method comparison using established GC-FID method. The P/Z detector developed could also be used to monitor individual VOCs. 相似文献
560.
Marijke WA de Backer Maike AD Brans Mieneke CM Luijendijk Keith M Garner Dianne MA van den Heuvel R Jeroen Pasterkamp Roger AH Adan 《BMC neuroscience》2010,11(1):94