全文获取类型
收费全文 | 238篇 |
免费 | 13篇 |
国内免费 | 4篇 |
专业分类
化学 | 124篇 |
晶体学 | 1篇 |
力学 | 27篇 |
数学 | 52篇 |
物理学 | 51篇 |
出版年
2023年 | 1篇 |
2022年 | 7篇 |
2021年 | 10篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 6篇 |
2016年 | 10篇 |
2015年 | 6篇 |
2014年 | 14篇 |
2013年 | 18篇 |
2012年 | 14篇 |
2011年 | 33篇 |
2010年 | 9篇 |
2009年 | 24篇 |
2008年 | 16篇 |
2007年 | 8篇 |
2006年 | 10篇 |
2005年 | 7篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 2篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1989年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有255条查询结果,搜索用时 0 毫秒
231.
In this letter,a mild and efficient procedure for synthesis of phenacyl derivatives under homogenous catalysis in the presence of tetrabutylammonium bromide in aqueous media is described.The nucleophilic substitution reactions were performed under ecofriendly conditions and gave the corresponding products in high yields and short reaction times. 相似文献
232.
O. Abdulaziz N.F.M. Noor I. Hashim M.S.M. Noorani 《Chaos, solitons, and fractals》2008,36(5):1405-1411
The Adomian decomposition method (ADM) is treated as an algorithm for approximating the solutions of the Lorenz and Chen systems in a sequence of time intervals, i.e. the classical ADM is converted into a hybrid analytical–numerical method. Comparisons with the seventh- and eighth-order Runge–Kutta method (RK78) reconfirm the very high accuracy of the hybrid analytical–numerical ADM. 相似文献
233.
Water vapor is an important constituent of any gas and in many applications is regarded as a contaminant that needs to be
monitored and controlled. AIN-CuO composites (2 % ≤ CuO ≤ 50 % by weight) have been studied to exploit them as novel humidity
sensors over wide ranges of moisture levels and temperature. Development of benign microstructure with open porosity has been
attempted by varying the composition and firing conditions. The impedance data acquired on the composites over the frequency
range 5 Hz to 13 MHz, revealed a bulk response in terms of a single semicircular relaxation in the complex Z*-plane. A systematic
variation of electrical conductivity with CuO content in the composites has been explained in the light of percolation theory. 相似文献
234.
Nanocrystalline strontium-doped lanthanum manganite (LSM) with a high specific surface area of 70 m2/g was synthesized via spray pyrolysis. The as prepared powder was characterized by ex-situ X-ray diffraction (XRD), in-situ high temperature X-ray diffraction (HTXRD), ex-situ nitrogen adsorption and high resolution scanning electron microscopy (HRSEM). LSM nanopowders with a mean particle size of 40 nm were dispersed in water-based media using ultrasonication. Nanocomposite LSM-GDC (gadolinium doped ceria) thin films were prepared by single step spin coating of co-stabilized LSM and GDC dispersions. The thickness of these thin films (≤ 1 μm) is more than 10 times lower than conventional cathode layers prepared by screen printing. The interfacial polarization resistances were 68, 118 and 220 mΩ cm2 at 850, 800 and 750 °C, respectively. The high performance is attributed to small grain size, high porosity and large specific surface area. This method offers a very cost effective approach for the preparation of electrochemically highly active porous thin films, particularly applicable for micro solid oxide fuel cells. 相似文献
235.
E. Azad 《Heat and Mass Transfer》2012,48(7):1177-1181
A theoretical model has been developed to investigate the thermal performance of a continuous finned circular tubing of an air-to-air thermosyphon-based heat pipe heat exchanger. The model has been used to determine the heat transfer capacity, which expresses the thermal performance of heat pipe heat exchanger. The model predicts the temperature distribution in the flow direction for both evaporator and condenser sections and also the saturation temperature of the heat pipes. The approach used for the present study considers row-by-row heat-transfer in evaporator and condenser sections of the heat pipe heat exchanger. 相似文献
236.
The flow of an Oldroyd 8-constant fluid between coaxial cylinders with variable viscosity is considered. The heat transfer analysis is also taken into account. An analytical solution of the non-linear problem is obtained using homotopy analysis method. The behavior of pertinent parameters is analyzed and depicted through graphs. 相似文献
237.
George K. El Khoury Philipp Schlatter Azad Noorani Paul F. Fischer Geert Brethouwer Arne V. Johansson 《Flow, Turbulence and Combustion》2013,91(3):475-495
Fully resolved direct numerical simulations (DNSs) have been performed with a high-order spectral element method to study the flow of an incompressible viscous fluid in a smooth circular pipe of radius R and axial length 25R in the turbulent flow regime at four different friction Reynolds numbers Re τ ?=?180, 360, 550 and $1\text{,}000$ . The new set of data is put into perspective with other simulation data sets, obtained in pipe, channel and boundary layer geometry. In particular, differences between different pipe DNS are highlighted. It turns out that the pressure is the variable which differs the most between pipes, channels and boundary layers, leading to significantly different mean and pressure fluctuations, potentially linked to a stronger wake region. In the buffer layer, the variation with Reynolds number of the inner peak of axial velocity fluctuation intensity is similar between channel and boundary layer flows, but lower for the pipe, while the inner peak of the pressure fluctuations show negligible differences between pipe and channel flows but is clearly lower than that for the boundary layer, which is the same behaviour as for the fluctuating wall shear stress. Finally, turbulent kinetic energy budgets are almost indistinguishable between the canonical flows close to the wall (up to y ?+??≈?100), while substantial differences are observed in production and dissipation in the outer layer. A clear Reynolds number dependency is documented for the three flow configurations. 相似文献
238.
Chaos Anti-synchronization between Two Novel Different Hyperchaotic Systems 总被引:1,自引:0,他引:1 下载免费PDF全文
We demonstrate that anti-synchronization can coexist in two different hyperchaotic systems of ratchets moving in different asymmetric potentials by active control method. By using rigorous mathematical theory, the sufficient condition is drawn for the stability of the error dynamics, where the controllers are designed by using the sum of the relevant variables in hyperchaotic systems. Numerical results are presented to justify the theoretical analysis strategy. 相似文献
239.
A very sensitive and selective spectrofluorimetric method has been developed for nickel (ΙΙ) determination in environmental samples. The method is based on measuring the decrease in fluorescence intensity of murexide after nickel (ΙΙ) binding. The intensity of the fluorescence emission peak was measured at ex/em 345/431 nm in several solutions with pH interval 3.0–7.0. The fluorescence intensity decrease was found to be linear in the concentration range of 0.007 mg.L?1 to 0.1 mg.L?1 and 0.1 mg.L?1 to 20 mg.L?1 of nickel (ΙΙ) by using 10?4 M murexide at pH 3. The detection limit was found 0.004 mg.L?1. Relatively large excesses of over 20 cations and anions do not interfere. The method was successfully applied to the analysis of nickel (ΙΙ) in sea, rain and ground water. This method is very precise and accurate (R.S.D.?=?0.42 % for the determination of 0.05 mg.L?1 nickel in 10 replicates). 相似文献
240.
Thermal tunability in terahertz metamaterials fabricated on strontium titanate single-crystal substrates 总被引:1,自引:0,他引:1
We report an experimental demonstration of thermal tuning of resonance frequency in a planar terahertz metamaterial consisting of a gold split-ring resonator array fabricated on a bulk single-crystal strontium titanate (SrTiO?) substrate. Cooling the metamaterial starting from 409 K down to 150 K causes about a 43% shift in resonance frequency, and there is very little variation in resonance strength. The resonance shift is due to the temperature-dependent dielectric constant of the strontium titanate. The experiment opens up avenues for designing tunable terahertz devices by exploiting the temperature-sensitive characteristic of high dielectric constant substrates and complex metal oxide materials. 相似文献