全文获取类型
收费全文 | 448篇 |
免费 | 26篇 |
国内免费 | 67篇 |
专业分类
化学 | 372篇 |
晶体学 | 7篇 |
力学 | 15篇 |
综合类 | 10篇 |
数学 | 58篇 |
物理学 | 79篇 |
出版年
2024年 | 1篇 |
2023年 | 7篇 |
2022年 | 23篇 |
2021年 | 21篇 |
2020年 | 19篇 |
2019年 | 21篇 |
2018年 | 11篇 |
2017年 | 15篇 |
2016年 | 20篇 |
2015年 | 22篇 |
2014年 | 29篇 |
2013年 | 28篇 |
2012年 | 45篇 |
2011年 | 22篇 |
2010年 | 27篇 |
2009年 | 26篇 |
2008年 | 28篇 |
2007年 | 26篇 |
2006年 | 24篇 |
2005年 | 19篇 |
2004年 | 16篇 |
2003年 | 15篇 |
2002年 | 11篇 |
2001年 | 7篇 |
2000年 | 16篇 |
1999年 | 6篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有541条查询结果,搜索用时 93 毫秒
511.
The spatiotemporal structure of brain oscillations are important in understanding neural function. We analyze oscillatory episodes from isotropic preparations from the middle layers of a mammalian cortex which display irregular and chaotic spatiotemporal wave activity, within which spontaneously emerge spiral and plane waves. The dimensionality of these dynamics shows a consistent decrease during the middle of these episodes, regardless of the presence of simple spiral or plane waves. It is important to define the relevant biological order parameters which govern these dynamical bifurcations. 相似文献
512.
An exact approach is used to investigate Love waves in functionally graded piezoelectric material (FGPM) layer bonded to a semi-infinite homogeneous solid. The piezoelectric material is polarized in z-axis direction and the material properties change gradually with the thickness of the layer. We here assume that all material properties of the piezoelectric layer have the same exponential function distribution along the x-axis direction. The analytical solutions of dispersion relations are obtained for electrically open or short circuit conditions. The effects of the gradient variation of material constants on the phase velocity, the group velocity, and the coupled electromechanical factor are discussed in detail. The displacement, electric potential, and stress distributions along thickness of the graded layer are calculated and plotted. Numerical examples indicate that appropriate gradient distributing of the material properties make Love waves to propagate along the surface of the piezoelectric layer, or a bigger electromechanical coupling factor can be obtained, which is in favor of acquiring a better performance in surface acoustic wave (SAW) devices. 相似文献
513.
A novel water-soluble beta-CD-derivatized erythromycin (EM) was synthesized and used as an effective chiral selector for the resolution of chiral compounds in CZE. The purpose of substitution at the primary hydroxyl site of beta-CD with 1-oxygen-2,3-epoxypropane is to produce a compound having functions of both beta-CD and EM. beta-CD-derivatized EM exhibited excellent enantioselectivities compared with single beta-CD and EM for chiral separation in CE. We also investigated the influence of pH and concentration of BGE, concentration of chiral selector, applied potential, and organic modifier on chiral compounds' separation. 相似文献
514.
A new approach is presented for preparing biomorphological mesoporous TiO2 templated by mimicking bamboo inner shell membrane via supercritical CO2 (SCCO2) transportation through titanium tetrabutyloxide (TTBO). The analysis of wide-angle X-ray powder diffraction (XRD) showed the prepared TiO2 in phase of anatase, and the small-angle XRD revealed the presence of mesopores without periodicity. The product exhibited the shape of crinkled films and extended in two dimensions up to centimeters. The electron microscopic observation showed that the TiO2 films were around 200 nm in thickness, and across the films there were numerous round or ellipse-shaped mesopores, being 10-50 nm in diameter, which were formed by the close packing of TiO2 particles. High-resolution transmission electron microscope (HRTEM) displayed that the single TiO2 particle size was about 12.5 nm. The UV-vis absorption spectrum was transparent in the wavelength of 320-350 nm for suspensions of the prepared mesoporous TiO2 in ethanol at the concentration of 5.0 mg/l. The mesoporous TiO2 prepared with the aid of SCCO2 exhibited an obvious blue shift compared with the TiO2 prepared by sol-gel infiltration. The possible mechanism for the formation of the mesoporous TiO2 is summarized into a biomimetic mineralization pathway. First, TTBO was transported to the membrane surface via SCCO2, and then condensed. Hydrolysis reactions between the functional groups of organic membrane and TTBO took place to form the nuclear TiO2, and the TiO2 seeds grew around the organic membrane into TiO2 mesoporous materials. The approach provides a low-cost and efficient route for the production of ceramics nanomaterials with unique structural features, which may have potential application in designing UV-selective shielding devices [S. Zhao, X.H. Wang, S.B. Xin, Q. Jiang, X.P. Liang, Rare Metal Mater. Eng. 35 (2006) 508-510]. 相似文献
515.
A highly sensitive, specific and simple LC‐MS/MS method was developed to investigate in vivo bio‐transformation of oleuropein in rat. Rat urine samples collected after the intravenous administrations were determined using liquid chromatography coupled to tandem mass spectrometry with electrospray ionization in the negative‐ion mode. The assay procedure involves a simple liquid–liquid extraction of parent oleuropein and the metabolite from rat urine with ethyl acetate. Chromatographic separation was operated with 0.1% formic acid aqueous and methanol in gradient program at a flow rate of 0.80 mL/min on an RP‐C18 column with a total run time of 30 min. This method has been successfully applied to simultaneous determination of oleuropein and its metabolite in rat urine. Oxygenation was found to be the major metabolic pathway of the oleuropein in rat after intravenous administration. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
516.
The manganese phosphate with high P-O-Mn connectivity shows a two-step relaxation process in two distinct spin glass states. 相似文献
517.
Influence of Inoculum Source and Pre-incubation on Bio-Methane Potential of Chicken Manure and Corn Stover 总被引:1,自引:0,他引:1
Yeqing Li Lu Feng Ruihong Zhang Yanfeng He Xiaoying Liu Xiao Xiao Xinxin Ma Chang Chen Guangqing Liu 《Applied biochemistry and biotechnology》2013,171(1):117-127
In order to investigate the effects of inoculum source and pre-incubation on methane production performance of chicken manure (CM) and corn stover (CS), two sets of bio-methane potential tests using non- and pre-incubated inocula (digested sludge from a municipal wastewater treatment plant (DSMW) and digested sludge from a chicken manure treatment plant (DSCM)) were conducted at 37 °C. Modified Gompertz and first-order models were used to evaluate the kinetic parameters. The results revealed that DSMW was better than DSCM in digesting organic substrates (CM and CS), since the average ultimate methane yields were 351 mL g?1 volatile solid (VS)added for CM and 300 mL g?1 VSadded for CS when DSMW was used as inoculum, and 298 mL g?1 VSadded for CM and 218 mL g?1 VSadded for CS when DSCM was used as inoculum, respectively. Nevertheless, there was no significant difference (p?>?0.05) in the ultimate methane yields between non- and pre-incubated inoculum for digesting CM and CS, regardless of the inoculum source. However, when evaluating the kinetic parameters of anaerobic digestion, the correlation coefficient, maximal methane production rate, and hydrolysis rate constant were slightly higher using pre-incubated inoculum as compared to non-incubated inoculum. 相似文献
518.
Yu Sheng Min Lin Xiangwei Li Hongxia Hao Xiaoying Lin Hongchen Sun Hao Zhang 《Particle & Particle Systems Characterization》2014,31(7):788-793
Gold (Au) nanomaterials are promising photothermal agents for the selective treatment of tumor cells owing to the strong capability to convert near‐infrared (NIR) irradiation into heat energy. One basic issue for practical photothermal therapy is the enhancement of photothermal effect in NIR region. Here, various low‐molecular‐weight thiols are applied to induce one‐dimensional (1D) self‐assembly of Au nanorods (NRs), which leads to the redshift of absorption peak towards NIR region. As a result, the 1D assembled Au NRs exhibit improved photothermal effect at 808 nm in comparison to unassembled Au NRs. 相似文献
519.
520.
Graphene oxide coated capillary for the analysis of endocrine‐disrupting chemicals by open‐tubular capillary electrochromatography with amperometric detection 下载免费PDF全文
Xiaoying Jiang Yuehuang Jiang Guoyue Shi Tianshu Zhou 《Journal of separation science》2014,37(13):1671-1678
A graphene oxide‐coated capillary was fabricated by using 3‐aminopropyltriethoxysilane as the cross‐linking agent. It was used for the separation and detection of three endocrine‐disrupting chemicals, including bisphenol A, 4‐nonylphenol, and 4‐octylphenol by capillary electrochromatography. Due to the hydrophobicity, hydrogen bonding, and π–π interaction between graphene oxide and the analytes, the three analytes could be well separated in pH = 11.0, 20 mmol/L Na2B4O7‐NaOH/methanol mobile phase (50:50, v/v) within 950 s. After preconcentration, the detection limits were 6.7 × 10?10, 3.3 × 10?9, and 6.7 × 10?10 mol/L (S/N = 3) for bisphenol A, nonylphenol, and octylphenol, respectively. The developed method was successfully applied to the determination of the above analytes in water samples. The satisfactory result demonstrated that the graphene oxide coated capillary used in capillary electrochromatography with amperometric detection was convenient to prepare, highly stable, and had good reproducibility. 相似文献