全文获取类型
收费全文 | 228篇 |
免费 | 23篇 |
国内免费 | 1篇 |
专业分类
化学 | 131篇 |
力学 | 19篇 |
数学 | 53篇 |
物理学 | 49篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 8篇 |
2020年 | 14篇 |
2019年 | 5篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 15篇 |
2015年 | 14篇 |
2014年 | 22篇 |
2013年 | 36篇 |
2012年 | 20篇 |
2011年 | 14篇 |
2010年 | 15篇 |
2009年 | 12篇 |
2008年 | 3篇 |
2007年 | 3篇 |
2006年 | 6篇 |
2005年 | 4篇 |
2004年 | 4篇 |
2003年 | 3篇 |
2002年 | 5篇 |
2001年 | 3篇 |
1999年 | 1篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有252条查询结果,搜索用时 15 毫秒
71.
Many researchers have shown that plant-derived polyphenolic compounds are helpful nutraceuticals in restraining various disorders such as neoplastic diseases. In this study two linear furanocoumarins, aviprin and aviprin-3″-O-D-glucopyranoside (A3G), were isolated from methanol extract of Prangos uloptera roots. The evaluation of free radical scavenging capacity of the compounds showed that aviprin is a more effective antioxidant than A3G with RC50 of 0.54?mg?mL?1. The biological and antiproliferative activities of the furanocoumarins were examined using human cervical carcinoma HeLa cell line and LNCaP prostatic cell line. Cell membrane integrity and cell viability were evaluated by measuring trypan blue exclusion assay and reduction of the tetrazolium blue compound, respectively. Treating the LNCaP cell line with various concentrations of the furanocoumarins showed that IC?? of aviprin and A3G were 0.4 and 6.6?mg?mL?1, whereas their CC50 values were 0.7 and 11?mg?mL?1, respectively. These results indicated that 42.7% of LNCaP cells were not dead by necrosis. Treating the HeLa cells by the furanocoumarins showed the greater sensitivity of the HeLa cell line than the LNCaP cell line. A morphological analysis and the study of DNA fragmentation provided further some evidence for the inhibition of the LNCaP cell line via apoptosis induction. 相似文献
72.
Saber M. Elsayed Ruhul A. Sarker Daryl L. Essam 《Computational Optimization and Applications》2013,54(3):771-790
A considerable number of differential evolution variants have been proposed in the last few decades. However, no variant was able to consistently perform over a wide range of test problems. In this paper, propose two novel differential evolution based algorithms are proposed for solving constrained optimization problems. Both algorithms utilize the strengths of multiple mutation and crossover operators. The appropriate mix of the mutation and crossover operators, for any given problem, is determined through an adaptive learning process. In addition, to further accelerate the convergence of the algorithm, a local search technique is applied to a few selected individuals in each generation. The resulting algorithms are named as Self-Adaptive Differential Evolution Incorporating a Heuristic Mixing of Operators. The algorithms have been tested by solving 60 constrained optimization test instances. The results showed that the proposed algorithms have a competitive, if not better, performance in comparison to the-state-of-the-art algorithms. 相似文献
73.
74.
The effective ultrasonic energy radiation into the air of piezoelectric transducers requires using multilayer matching systems with accurately selected acoustic impedances and the thickness of particular layers. One major problem of ultrasonic transducers, radiating acoustic energy into air, is to find the proper acoustic impedances of one or more matching layers. This work aims at developing an original solution to the acoustic impedance mismatch between transducer and air. If the acoustic impedance defences between transducer and air be more, then finding best matching layer(s) is harder. Therefore we consider PZT (lead zirconate titanate piezo electric) transducer and air that has huge acoustic impedance deference. The vibration source energy (PZT), which is used to generate the incident wave, consumes a part of the mechanical energy and converts it to an electrical one in theoretical calculation. After calculating matching layers, we consider the energy source as layer to design a transducer. However, this part of the mechanical energy will be neglected during the mathematical work. This approximation is correct only if the transducer is open-circuit. Since the possibilities of choosing material with required acoustic impedance are limited (the counted values cannot always be realized and applied in practice) it is necessary to correct the differences between theoretical values and the possibilities of practical application of given acoustic impedances. Such a correction can be done by manipulating other parameters of matching layers (e.g. by changing their thickness). The efficiency of the energy transmission from the piezoceramic transducer through different layers with different thickness and different attenuation enabling a compensation of non-ideal real values by changing their thickness was computer analyzed (base on genetic algorithm). Firstly, three theoretical solutions were investigated. Namely, Chebyshev, Desilets and Souquet theories. However, the obtained acoustic impedances do not necessarily correspond to a nowadays available material. Consequently, the values of the acoustic impedances are switched to the nearest values in a large material database. The switched values of the acoustic impedances do not generally give efficient transmission coefficients. Therefore, we proposed, in a second step, the use of a genetic algorithm (GA) to select the best acoustic impedances for matching layers from the material database for a narrow band ultrasonic transducer that work at frequency below the 2.5 MHz by considering attenuation. However this bank is rich, the results get better. So the accuracy of the propose method increase by using a lot of materials with exact data for acoustic impedance and their attenuation, especially in high frequency. This yields highly more efficient transmission coefficient. In fact by using increasing number of layer we can increase our chance to find the best sets of materials with valuable both in acoustic impedance and low attenuation. Precisely, the transmission coefficient is almost equal to unity for the all studied cases. Finally the effect of thickness on transmission coefficient is investigated for different layers. The results showed that the transmission coefficient for air media is a function of thickness and sensitive to it even for small variation in thickness. In fact, the sensitivity increases when the differences of acoustic impedances to be high (difference between PZT and air). 相似文献
75.
Saber Azizi Mohammad-Reza Ghazavi Siamak Esmaeilzadeh Khadem Ghader Rezazadeh Cetin Cetinkaya 《Nonlinear dynamics》2013,73(1-2):853-867
The impetus of this study is to investigate the nonlinear chaotic dynamics of a clamped–clamped micro-beam exposed to simultaneous electrostatic and piezoelectric actuation. The micro-beam is sandwiched with piezoelectric layers throughout its length. The combined DC and AC electrostatic actuation is imposed on the micro-beam through two upper and lower electrodes. The piezoelectric layers are actuated via a DC electric voltage applied in the direction of the height of the piezoelectric layers, which produces an axial force proportional to the applied DC voltage. The governing differential equation of the motion is derived using Hamiltonian principle and discretized to a nonlinear Duffing type ODE using Galerkin method. The governing ODE is numerically integrated to get the response of the system in terms of the governing parameters. The results show that the response of the system is greatly affected by the amounts of DC and AC electrostatic voltages applied to the upper and lower electrodes. The results show that the response of the system can be highly nonlinear and in some regions chaotic. Evaluating the K–S entropy of the system, based on several initial conditions given to the system, the chaotic response is distinguished from the periodic or quasiperiodic ones. The main objective is to passively control the chaotic response by applying an appropriate DC voltage to the piezoelectric layers. 相似文献
76.
M. Tahani M. S. Karimi A. Mahmoudi Motlagh S. Mirmahdian 《Heat and Mass Transfer》2013,49(10):1369-1384
In this investigation, the effects of spike as retractable drag and aerodynamic heating reduction into the reentry Earth’s atmosphere for hemispherical body flying at hypersonic flow have been numerically studied. This numerical solution has been carried out for different length, shapes and nose configuration of spike. Additional modifications to the tip of the spike are investigated in order to obtain different bow shocks, including no spike, conical, flat and hemispherical aerodisk mounted. Unsteady compressible 3-D Navier–Stokes equations are solved with k ? ω (SST) turbulence model for a flow over a forward facing spike attached to a heat shield for a free stream Mach number of 6. The obtained numerical results are compared with the experimental ones, and the results shows acceptable verification. This analysis shows that the aerodisk is more effective than aerospike. The designs produced 60 and 15 % reduction in drag and wall temperature responses, respectively. 相似文献
77.
We survey some of the fundamental results on the stability and asymptoticity of linear Volterra difference equations. The method of Z-transform is heavily utilized in equations of convolution type. An example is given to show that uniform asymptotic stability does not necessarily imply exponential stabilty. It is shown that the two notions are equivalent if the kernel decays exponentially. For equations of nonconvolution type, Liapunov functions are used to find explicit criteria for stability. Moreover, the resolvent matrix is defined to produce a variation of constants formula. The study of asymptotic equivalence for difference equations with infinite delay is carried out in Section 6. Finally, we state some problems. 相似文献
78.
Laura K. Braydich-Stolle Nicole M. Schaeublin Richard C. Murdock Jingkun Jiang Pratim Biswas John J. Schlager Saber M. Hussain 《Journal of nanoparticle research》2009,11(6):1361-1374
Certain properties that nanoparticles possess differentiate them from their bulk counterparts, and these characteristics must
be evaluated prior to nanoparticle studies and include: size, shape, dispersion, physical and chemical properties, surface
area, and surface chemistry. Early nanotoxicity studies evaluating TiO2 have yielded conflicting data which identify either size or crystal structure as the mediating property for nano-TiO2 toxicity. However, it is important to note that none of these studies examined size with the crystal structure composition
controlled for or examined crystal structure while controlling the nanoparticle size. The goal of this study was to evaluate
the role of size and crystal structure in TiO2 nanotoxicity while controlling for as many other nanoproperties as possible using the HEL-30 mouse keratinocyte cell line
as a model for dermal exposure. In the size-dependent studies, all the nanoparticles are 100% anatase, and aggregate sizes
were determined in order to take into account the effect of agglomeration on size-dependent toxicity. In addition, varying
crystal structures were assessed while the size of the nanoparticles was controlled. We were able to identify that both size
and crystal structure contribute to cytotoxicity and that the mechanism of cell death varies based on crystal structure. The
100% anatase TiO2 nanoparticles, regardless of size, induced cell necrosis, while the rutile TiO2 nanoparticles initiated apoptosis through formation of reactive oxygen species (ROS). 相似文献
79.
Akpan N. Ikot H. Hassanabadi E. Maghsoodi Saber Zarrinkamar N. Salehi 《Physics of Particles and Nuclei Letters》2014,11(4):443-457
In this paper, we calculate the energy spectra and the corresponding wavefunction for the symmetric and asymmetric trigonometric Rosen-Morse potential of the Dirac equation within the framework of spin and pseudospin symmetry limits including the tensor interaction using the supersymmetric quantum mechanics (SUSYQM) formalism. We have also reported some numerical results and figures to show the effect of tensor interaction. 相似文献
80.
Motlagh Mahboube Mohaghegh Hassanzadeh-Tabrizi S. A. Saffar-Teluri Ali 《Journal of Sol-Gel Science and Technology》2015,73(1):9-13
Journal of Sol-Gel Science and Technology - Nanocomposite of Mn2O3/Al2O3/SiO2 was prepared through an in situ sol–gel process, in which Mn2O3 nanocrystals were dispersed in the silica-alumina... 相似文献