全文获取类型
收费全文 | 214篇 |
免费 | 2篇 |
专业分类
化学 | 129篇 |
力学 | 2篇 |
数学 | 23篇 |
物理学 | 62篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 12篇 |
2021年 | 13篇 |
2020年 | 3篇 |
2019年 | 6篇 |
2018年 | 3篇 |
2017年 | 7篇 |
2016年 | 12篇 |
2015年 | 6篇 |
2014年 | 9篇 |
2013年 | 15篇 |
2012年 | 18篇 |
2011年 | 14篇 |
2010年 | 13篇 |
2009年 | 13篇 |
2008年 | 11篇 |
2007年 | 10篇 |
2006年 | 10篇 |
2005年 | 4篇 |
2004年 | 7篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 5篇 |
1997年 | 2篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1982年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有216条查询结果,搜索用时 0 毫秒
51.
Antonia Torres Ávila Pimenta Raimundo Braz‐Filho Piero Giuseppe Delprete Elnatan Bezerra de Souza Edilberto Rocha Silveira Mary Anne Sousa Lima 《Magnetic resonance in chemistry : MRC》2010,48(9):734-737
Two unusual monoterpene indole alkaloids, stachyoside ( 1 ) and nor‐methyl‐23‐oxo‐correantoside ( 2 ), have been isolated from the aerial parts of Psychotria stachyoides. The structural elucidation of both compounds was performed by the aid of HRESIMS, FT‐IR, and 1D‐ and 2D‐NMR techniques including COSY, HSQC, HMBC, and NOESY. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
52.
Asymptotic behavior of solutions to a class of nonlocal non‐autonomous diffusion equations 下载免费PDF全文
F. D. M. Bezerra M. J. D. Nascimento S. H. da Silva 《Mathematical Methods in the Applied Sciences》2015,38(17):4317-4329
In this paper, we consider the nonlocal non‐autonomous evolution problems where Ω is a bounded smooth domain in , N≥1, β is a positive constant, the coefficient a is a continuous bounded function on , and K is an integral operator with symmetric kernel , being J a non‐negative function continuously differentiable on and . We prove the existence of global pullback attractor, and we exhibit a functional to evolution process generated by this problem that decreases along of solutions. Assuming the parameter β is small enough, we show that the origin is locally pullback asymptotically stable. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
53.
Korn Md Andrade JB Jesus DS Lemos VA Bandeira ML Santos WN Bezerra MA Amorim FA Souza AS Ferreira SL 《Talanta》2006,69(1):16-24
Lead is recognized worldwide as a poisonous metal. Thus, the determination of this element is often required in environmental, biological, food and geological samples. However, these analyses are difficult because such samples contain relatively low concentrations of lead, which fall below the detection limit of conventional analytical techniques such as flame atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry. Several preconcentration procedures to determine lead have therefore been devised, involving separation techniques such as liquid-liquid extraction, solid phase extraction, coprecipitation and cloud point extraction. Citing 160 references, this paper offers a critical review of preconcentration procedures for determining lead using spectroanalytical techniques. 相似文献
54.
This article describes the development by response surface methodology (RSM) of a procedure for iron, zinc and manganese determination by flame atomic absorption spectrometry (FAAS) in food samples after digestion employing a focussed microwave system. A Doehlert matrix was used to find optimal conditions for the procedure through response surface study. Three variables (irradiation power and time and composition of oxidant solution—HNO3 + H2O2) were regarded as factors in the optimization study. The working conditions were established as a compromise between optimum values found for each analyte taking into consideration the robustness of the procedure. These values were 12 min, 260 W and 42% (v/v) for irradiation time, irradiation power and percent of H2O2 in solution, respectively. The accuracy of the optimized procedure was evaluated by analysis of certified reference materials and by comparison with a well-established closed vessel microwave dissolution methodology. 相似文献
55.
Liliana Fátima Bezerra de Lira Jose Geraldo Andrade Pacheco Eduardo Homem de Siqueira Cavalcanti Maria Fernanda Pimentel 《Microchemical Journal》2010,96(1):126-34
This paper evaluates analytical methods based on near infrared (NIR) and middle infrared (MIR) spectroscopy and multivariate calibration to monitor the stability of biodiesel. There was a focus on three parameters: oxidative stability index, acid number and water content. Ethylic and methylic biodiesel from different feedstocks were used in experiments of accelerated aging, in order to take into account the wide variety of oilseeds and feedstocks available in Brazil. Partial least squares (PLS) and multiple linear regression (MLR) models were developed. Different pre-processing techniques and spectral variable/regions selection algorithms were evaluated. For MLR models, the successive projection algorithm (SPA) was employed. Interval PLS (iPLS) and selection of variables taking into account the significant regression coefficients were used for PLS models. Results showed that both near and middle infrared regions, and all variable selection methods tested were efficient for predicting these three important quality parameters of B100, the root mean squares error of prediction (RMSEP) values being comparable to the reproducibility of the corresponding standard method for each property investigated. 相似文献
56.
Giovanna Lovato Roberto A. Bezerra José A. D. Rodrigues Suzana M. Ratusznei Marcelo Zaiat 《Applied biochemistry and biotechnology》2012,166(8):2007-2029
The aim of this work was to investigate the effect of different feeding times (2, 4 and 6 h) and applied volumetric organic
loads (4.5, 6.0 and 7.5 gCOD L−1 day−1) on the performance of an anaerobic sequencing batch biofilm reactor (AnSBBR) treating effluent from biodiesel production.
Polyurethane foam cubes were used as inert support in the reactor, and mixing was accomplished by recirculating the liquid
phase. The effect of feeding time on reactor performance showed to be more pronounced at higher values of applied volumetric
organic loads (AVOLs). Highest organic material removal efficiencies achieved at AVOL of 4.5 gCOD L−1 day−1 were 87 % at 4-h feeding against 84 % at 2-h and 6-h feeding. At AVOL of 6.0 gCOD L−1 day−1, highest organic material removal efficiencies achieved with 4-h and 6-h feeding were 84 %, against 71 % at 2-h feeding.
At AVOL of 7.5 gCOD L−1 day−1, organic material removal efficiency achieved with 4-h feeding was 77 %. Hence, longer feeding times favored minimization
of total volatile acids concentration during the cycle as well as in the effluent, guaranteeing process stability and safety. 相似文献
57.
MK Bhide RM Kadam MD Sastry Ajay Singh Shashwati Sen Manmeet Kaur DK Aswal SK Gupta VC Sahni 《Pramana》2002,58(5-6):799-802
Microwave absorption studies have been carried out on MgB2 superconductor using a standard X-band EPR spectrometer. The modulated low-field microwave absorption signals recorded for
polycrystalline (grain size ∼ 10 μm) samples suggested the absence of weak-link character. The field dependent direct microwave
absorption has been found to obey a ✓H dependence with two different slopes, which indicated a transition from strongly pinned lattice to flux flow regime. 相似文献
58.
Stable isotope methods are potentially quite useful for validating natural or enhanced mineral degradation of contaminants. For this reason, a continuous flow gas chromatograph (GC), isotope ratio mass spectrometer (IRMS) has been coupled with a quadrupole mass selective detector (MSD) to allow simultaneous mass spectral and stable carbon isotope ratio data to be obtained from a single chromatographic analysis. This allows the target contaminant and any extra-cellular degradation intermediates to be both qualified and quantified. Previously acceptable limits of precision (0.3 parts per mil) are undesirable given the small fractionation observed during aerobic degradation. To further understand the fate of organic contaminants and to gain information about the metabolic degradative pathway employed by a microorganism, routine isotopic analyses on a range of analytes have been performed. Quantities of sample producing mass-44 ion beam signal (I(44)) of 2 x 10(-10) to 1 x 10(-8) A were analysed. When the IRMS was tuned for high sensitivity, ion source nonlinearities were overcome by peak height correction from an algorithm that was produced using known isotopic standards of varying concentrations. This led to sample accuracy of <0.01 per thousand and sample precision of 0.1 per thousand. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
59.
UP Schlunegger M Stoeckli RM Caprioli 《Rapid communications in mass spectrometry : RCM》1999,13(18):1792-1796
An ion trap has been modified for the analysis of high mass ions generated by matrix-assisted laser desorption/ionization. Samples are deposited on a probe tip and introduced directly onto the hyperbolically shaped surface of one endcap. All three electrodes - both the endcaps and the ring electrode - are insulated so that the radio frequency (Rf) voltage may be applied to the center ring electrode and the inverted Rf voltage to the endcaps. By using low frequencies (below 100 kHz) and low amplitudes (below 200 V), high mass singly charged ions may be trapped and analyzed by a frequency sweep at constant amplitude. In the high mass range (60-160 kDa), this instrument showed good sensitivity, signal-to-noise ratios, and mass resolution. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
60.