全文获取类型
收费全文 | 644篇 |
免费 | 35篇 |
国内免费 | 15篇 |
专业分类
化学 | 404篇 |
力学 | 55篇 |
数学 | 100篇 |
物理学 | 135篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 11篇 |
2021年 | 33篇 |
2020年 | 30篇 |
2019年 | 37篇 |
2018年 | 32篇 |
2017年 | 39篇 |
2016年 | 42篇 |
2015年 | 25篇 |
2014年 | 49篇 |
2013年 | 87篇 |
2012年 | 65篇 |
2011年 | 61篇 |
2010年 | 47篇 |
2009年 | 30篇 |
2008年 | 19篇 |
2007年 | 21篇 |
2006年 | 12篇 |
2005年 | 6篇 |
2004年 | 2篇 |
2003年 | 3篇 |
2002年 | 1篇 |
2001年 | 4篇 |
2000年 | 1篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1993年 | 2篇 |
1991年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有694条查询结果,搜索用时 343 毫秒
91.
A Wiener-Hermite functional series expansion method for response analysis of nonlinear systems under random excitations is presented. The formal procedure for the derivation of the deterministic equations governing the Wiener-Hermite kernel functions is described. A singleterm expansion is used to analyze the non-stationary responses of a damped Duffing oscillator subjected to several modulated white noises. An iterative procedure for evaluating the kernel functions and the mean-square responses of the Duffing oscillator is developed. For several values of non-linearity strength and different damping coefficients the non-stationary variances and autocorrelations of the response are obtained. The results are compared with those found by the digital simulations and the existing methods. 相似文献
92.
Composite of conducting polyaniline-isobutylated urea formaldehyde: Preparation and characterization
Bakhshali Massoumi Raheleh Magssoudi Elmira Kashani Ali Akbar Entezami 《Polymer Science Series A》2011,53(8):727-732
Composite of conductive polyaniline-isobutylated urea formaldehyde have been prepared by chemical oxidative emulsion polymerization
of aniline in the presence of isobutylated urea formaldehyde resin (BUFR) in toluene-water solvents at room temperature. The
mass loading of polyaniline was controlled by varying the BUFR/aniline charging ratio as well as oxidant (ammonium persulfate)/aniline
molar ratio. Some factors capable of affecting the yield and conductivity of composite, such as amount of the oxidant, type
of the dispersants (span-80 and span-20), and amount of resin and organic acid (para-toluene sulfonic acid) were investigated.
The prepared composites were characterized by FTIR spectroscopy and scanning electron microscopy (SEM). 相似文献
93.
The present study reports the development, validation and application of a new green liquid chromatographic method for the determination of glutathione (GSH) in vegetable samples. In this work we introduce—for the first time—ethyl propiolate (EP) as an advantageous post-column derivatization reagent for thiolic compounds. GSH (tR = 6.60 min) and N-acetylcysteine (NAC, internal standard) (tR = 11.80 min) were separated efficiently from matrix endogenous compounds by using a 100% aqueous mobile phase (0.1%, v/v CH3COOH in 1 mmol L−1 EDTA, QV = 0.5 mL min−1) and a Prevail® reversed phase column that offers the advantage of stable packing material in aqueous mobile phases. The parameters of the post-column reaction (pH, amount concentration of the reagent, flow rates, length of the reaction coil and temperature) were studied. The linear determination range for GSH was 1–200 μmol L−1 and the LOD was 0.1 μmol L−1 (S/N = 3). Total endogenous GSH was determined in broccoli, potato, asparagus and Brussels sprouts using the standards addition approach. The accuracy was evaluated by both recovery experiments (R = 91–110%) and comparison to an o-phthalaldehyde/glycine corroborative post-column derivatization fluorimetric method. 相似文献
94.
Considering the thermodynamic aspects and reaction pathways of chemical adsorption of NH3 molecule at the open ends of boron nitride nanotubes (BNNTs), theoretically, it was found that the open-ended BNNTs are able
to cleave the N–H bond of NH3 via a one- or two-stepwise mechanism. The N-enriched and B-enriched open-ended BNNTs show a nucleophilic and electrophilic
behavior toward the NH3, respectively. Besides, some effects of this chemical adsorption on the electronic properties of BNNTs were explored. 相似文献
95.
Zarrin Es’haghi Zohreh Rezaeifar Gholam-Hossein Rounaghi Zahra Alian Nezhadi Mazyar Ahmadi Golsefidi 《Analytica chimica acta》2011,(1):8313
A novel solid-phase microextraction technique using a hollow fiber-supported sol–gel combined with multi-walled carbon nanotubes was employed in the determination of phenobarbital in wastewater. In this new technique, a silica-based, organic–inorganic polymer containing functionalized multi-walled carbon nanotubes (MWCNTs) was prepared with sol–gel technology via the reaction of tetraethylorthosilicate (TEOS) with an acidic catalyst (HCl). This sol was injected into a polypropylene hollow fiber segment for in situ gelation. This device operated in direct immersion sampling mode. The experimental setup is simple and affordable, and the device is disposable, so there is no risk of cross-contamination or carry-over. Parameters affecting extraction such as pH of the aqueous solution, ageing and extraction times, aqueous sample volume, agitation speed and carbon nanotube amount were optimized. Linearity was observed over a range of 0.50–5000 ng mL−1, with an estimation coefficient (r2) higher than 0.982. The limit of detection (LOD) was 0.32 ng mL−1 (n = 5), and repeatability (RSD% = 2.9) was from the average of three levels of analyte concentrations (1, 1000 and 4500 ng mL−1) with three replicates for each within a single day. Finally, a pre-concentration factor of 2100 was obtained for phenobarbital. 相似文献
96.
We describe the catalytic activity of the first chimeric ligase containing a foldameric sequence of β- and γ-amino acids. The chimeric backbone provides for the spatial arrangement of all functional groups involved in the formation of the catalytic site to allow efficient catalysis to take place. Our finding indicates significant progress in the field of functionally active artificial motifs. 相似文献
97.
Ahmadi F Alizadeh AA Shahabadi N Rahimi-Nasrabadi M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,79(5):1466-1474
In this work a complex of Al3+ with curcumin ([Al(curcumin) (EtOH)2](NO3)2) was synthesized and characterized by UV-vis, FT-IR, elemental analysis and spectrophotometric titration techniques. The mole ratio plot revealed a 1:1 complex between Al3+ and curcumin in solution. For binding studies of this complex to calf thymus-DNA various methods such as: UV-vis, fluorescence, circular dichroism (CD), FT-IR spectroscopy and cyclic voltammetry were used. The intrinsic binding constant of ACC with DNA at 25°C was calculated by UV-vis and cyclic voltammetry as 2.1×10(4) and 2.6×10(4), respectively. The thermodynamic studies showed that the reaction is enthalpy and entropy favored. The CD results showed that only the Δ-ACC interacts with DNA and the Δ-ACC form has not any tendency to interact with DNA, also the pure curcumin has not any stereoselective interaction with CT-DNA. Fluorimetric studies showed that fluorescence enhancement was initiated by a static process in the ground state. The cyclic voltammetry showed that ACC interact with DNA with a binding site size of 2. From the FT-IR we concluded that the Δ-ACC interacts with DNA via partial electrostatic and minor groove binding. In comparison with previous works it was concluded that curcumin significantly reduced the affinity of Al3+ to the DNA. 相似文献
98.
99.
Mohammadpour R Iraji zad A Hagfeldt A Boschloo G 《Physical chemistry chemical physics : PCCP》2011,13(48):21487-21491
In this work, we report on fabrication and characterization of dye-sensitized solar cells based on TiO(2) nanotube/nanoparticle (NT/NP) composite electrodes. TiO(2) nanotubes were prepared by anodization of Ti foil in an organic electrolyte. The nanotubes were chemically separated from the foil, ground and added to a TiO(2) nanoparticle paste, from which composite NT/NP electrodes were fabricated. In the composite TiO(2) films the nanotubes existed in bundles with a length of a few micrometres. By optimizing the amount of NT in the paste, dye-sensitized solar cells with an efficiency of 5.6% were obtained, a 10% improvement in comparison to solar cells with pure NP electrodes. By increasing the fraction of NT in the electrode the current density increased by 20% (from 11.1 to 13.3 mA cm(-2)), but the open circuit voltage decreased from 0.78 to 0.73 V. Electron transport, lifetime and extraction studies were performed to investigate this behavior. A higher fraction of NT in the paste led to more and deeper traps in the resulting composite electrodes. Nevertheless, faster electron transport under short-circuit conditions was found with increased NT content, but the electron lifetime was not improved. The electron diffusion length calculated for short-circuit conditions was increased 3-fold in composite electrodes with an optimized NT fraction. The charge collection efficiency was more than 90% over a wide range of light intensities, leading to improved solar cell performance. 相似文献
100.
Dynamics of tractor lateral overturn on slopes under the influence of position disturbances (model development) 总被引:1,自引:0,他引:1
Iman Ahmadi 《Journal of Terramechanics》2011,48(5):339-346
In order to investigate the effects of forward speed, ground slope and wheel–ground friction coefficient on lateral stability of tractor at the presence of position disturbances, a tractor dynamic model was developed. In this model two types of instability were considered: instability due to overturn and skid and for each case the stability index was determined. Different geometries and mass specifications of tractor MITSUBISHI-2501D were used to examine the model. According to the results of this model forward speed and ground slope had a reverse effect on all stability indexes. Moreover stability of this tractor was more affected by tractor skidding than overturning. Therefore to improve the overall stability of this tractor, preference should be on increasing the tractor stability index derived from skid dynamics of tractor. 相似文献