全文获取类型
收费全文 | 745篇 |
免费 | 49篇 |
国内免费 | 16篇 |
专业分类
化学 | 507篇 |
晶体学 | 7篇 |
力学 | 56篇 |
数学 | 53篇 |
物理学 | 187篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 36篇 |
2021年 | 78篇 |
2020年 | 53篇 |
2019年 | 59篇 |
2018年 | 54篇 |
2017年 | 44篇 |
2016年 | 45篇 |
2015年 | 32篇 |
2014年 | 55篇 |
2013年 | 55篇 |
2012年 | 59篇 |
2011年 | 60篇 |
2010年 | 34篇 |
2009年 | 20篇 |
2008年 | 17篇 |
2007年 | 19篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2004年 | 10篇 |
2003年 | 4篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1996年 | 3篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1970年 | 2篇 |
1968年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有810条查询结果,搜索用时 406 毫秒
71.
Islam Ullah Khan Tehseen Aman Muhammad Asif Iqbal Asrar Ahmad Kazi 《Mikrochimica acta》2000,134(1-2):27-31
Fluoxetine hydrochloride reacts with benzoyl peroxide and potassium iodide, after heating for 1 min at 30 °C, to give a blue
colour having maximum absorbance at 570 nm. The reaction is selective for fluoxetine with 0.01 mg/mL as visual limit of quantitation
and provides a basis for a new spectrophotometric determination. The colour reaction obeys Beer’s law from 0.1 mg/10 mL to
2.0 mg/10 mL of fluoxetine and the relative standard deviation is 0.68%. The qualitative assessment of tolerable amounts of
other drugs is also studied.
Received September 21, 1998. Revision September 10, 1999. 相似文献
72.
We extend a hybrid DSMC/Navier–Stokes (NS) approach to unify the DSMC and the NS simulators in one framework capable of solving the mixed non‐equilibrium and near‐equilibrium flow regions efficiently. Furthermore, we use a one‐way state‐based coupling (Dirichlet–Dirichlet boundary‐condition coupling) to transfer the required information from the continuum region to the rarefied one. The current hybrid DSMC–NS frame is applied to the hypersonic flows over nanoflat plate and microcylinder cases. The achieved solutions are compared with the pure DSMC and NS solutions. The results show that the current hybrid approach predicts the surface heat transfer rate and shear stress magnitudes very accurately. Some important conclusions can be drawn from this study. For example, although the shock wave region would be a non‐equilibrium region, it is not necessary to use a pure DSMC simulator to solve it entirely. This is important when the researchers wish to predict the surface properties such as velocity slip, temperature jump, wall heat flux rate, and friction drag magnitudes accurately. Our investigation showed that our hybrid solution time would be at least 40% (for the flat plate) and 35% (for the cylinder) of the time that must be spent by a pure DSMC solver to attain the same accuracy.Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
73.
Nazakat Ullah 《Pramana》1993,41(2):171-174
A formulation is developed to derive exact analytic expressions for electron-electron correlation and density of electrons
in momentum space using hydrogenic wave functions. It is shown that for large atoms the expression for density of electrons
has a simple form. 相似文献
74.
Imdad Ullah Mohammadzai Tomoko Ashiuchi Satoshi Tsukahara Yasuaki Okamoto Terufumi Fujiwara 《中国化学会会志》2005,52(5):1037-1042
A simple and fast flow method for the trace level determination of p‐toluidine, 2‐methyl‐5‐nitroaniline, and 2,4‐dinitroaniline in aqueous samples is reported. These amino/nitroaromatics are related to trinitrotoluene (TNT) and appear during the degradation process of the explosive. The chemical principles of ion‐pair formation and liquid‐liquid extraction are applied: In aqueous acidic medium, the protonated analyte [HA]+ makes an ion‐pair with the tetrachloroaurate(III) ion, followed by on‐line ion‐pair extraction into the dichloromethane carrier used. After membrane separation, the CH2Cl2 containing the ion‐pair, [HA]+[AuCl4]−, is mixed with the reversed micellar luminescent reagent of luminol (in 0.3 M Na2CO3) prepared from cetyl‐trimethylammonium chloride in CH2Cl2‐cyclohexane and the [AuCl4−‐luminol chemiluminescence (CL) output is recorded. The detection limits (S/N> 3) are: p‐toluidine, 1.0 × 10−4M; 2‐methyl‐5‐nitroaniline, 1.0 × 10−7 M; 2,4‐dinitroaniline, 1.0 × 10−7 M, while the calibration curves are linear between 1.0 × 10−4 — 1.0 × 10−2 M for all the compounds. Although spectral studies indicated the formation and extraction of a very small amount of the ion‐pair species, the reversed micellar‐mediated CL detection system provides an alternative procedure for the determination of degradation products of the explosive TNT in environmental aqueous samples. 相似文献
75.
The Friedel-Crafts acylation of aromatic compounds with carboxylic acids was investigated in the presence of Tf2O. The reaction was carried out efficiently and very rapidly under mild reaction conditions without the need of any catalyst. 相似文献
76.
Nazakat Ullah 《Pramana》1994,42(6):491-494
The distribution of quarks in light nuclei is given using the quark cluster wave function. An analytic expression for the
nucleus4He is obtained. The distribution so obtained is compared with the one obtained using a different theoretical formulation called
mapping. 相似文献
77.
78.
Influence of thermal annealing on the structural and optical properties of maghemite (γ‐Fe2O3) nanoparticle thin films 下载免费PDF全文
Ehsan Parsianpour Mohammad Gholami Nima Shahbazi Feridoun Samavat 《Surface and interface analysis : SIA》2015,47(5):612-617
In this study, maghemite (γ‐Fe2O3) nanoparticles were initially synthesized via chemical co‐precipitation and then deposited by spray pyrolysis as thin films on white glass substrates. The thin films were annealed for 8 h at 400, 450, 500, 550, and 600 °C in an oven. The structural studies of maghemite nanoparticles were carried out using X‐ray diffractometer. Structural properties that we investigated by X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, SEM, and Energy dispersive X‐ray analysis (EDS). Optical properties of the samples were also investigated by ultraviolet‐visible (UV–vis) spectroscopy. The results showed that maghemite nanoparticles have crystalline structure with domain that increases in size with increasing annealing temperature. The optical band gap values were found to reduce from 2.9 to 2.4 eV with increase in annealing temperature. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
79.
Seyedeh Mahsa Kamali Ehsan Arbabi Amir Arbabi Yu Horie Andrei Faraon 《Laser \u0026amp; Photonics Reviews》2016,10(6):1002-1008
Dielectric metasurfaces are two‐dimensional structures composed of nano‐scatterers that manipulate the phase and polarization of optical waves with subwavelength spatial resolution, thus enabling ultra‐thin components for free‐space optics. While high performance devices with various functionalities, including some that are difficult to achieve using conventional optical setups have been shown, most demonstrated components have fixed parameters. Here, we demonstrate highly tunable dielectric metasurface devices based on subwavelength thick silicon nano‐posts encapsulated in a thin transparent elastic polymer. As proof of concept, we demonstrate a metasurface microlens operating at 915 nm, with focal distance tuning from 600 μm to 1400 μm (over 952 diopters change in optical power) through radial strain, while maintaining a diffraction limited focus and a focusing efficiency above 50%. The demonstrated tunable metasurface concept is highly versatile for developing ultra‐slim, multi‐functional and tunable optical devices with widespread applications ranging from consumer electronics to medical devices and optical communications.
80.
Composite membranes based on polyvinyl chloride and acrylonitrile butadiene styrene (ABS) copolymer have been prepared and then filled with 2–8 wt % of silica nanoparticles. Membranes were fabricated by solution casting method using dimethylacetamide. The performance of prepared membranes were studied for methane and ethane at the feed pressure of 1.0, 1.5, 2.0, and 3.0 bar at 35°C. By increasing the percentage of ABS, permeability of methane and ethane increased. In addition, by adding the silica nanoparticles in the membrane, permeability of gas increased in all cases. The highest gas pair selectivity for C2H6/CH4 could be obtained from PVC/BS (20 wt %) which loaded with 8 wt % of silica nanoparticles. The results of this study suggest that high performance gas separation nanocomposite membranes can be attained by adopting a judicious combination of blending technique for polymeric membrane, optimized loading percentage, and feed operating conditions. 相似文献