全文获取类型
收费全文 | 5433篇 |
免费 | 434篇 |
国内免费 | 422篇 |
专业分类
化学 | 1831篇 |
晶体学 | 32篇 |
力学 | 1814篇 |
综合类 | 56篇 |
数学 | 1247篇 |
物理学 | 1309篇 |
出版年
2024年 | 8篇 |
2023年 | 56篇 |
2022年 | 89篇 |
2021年 | 109篇 |
2020年 | 129篇 |
2019年 | 107篇 |
2018年 | 135篇 |
2017年 | 185篇 |
2016年 | 206篇 |
2015年 | 137篇 |
2014年 | 200篇 |
2013年 | 496篇 |
2012年 | 281篇 |
2011年 | 251篇 |
2010年 | 195篇 |
2009年 | 253篇 |
2008年 | 265篇 |
2007年 | 281篇 |
2006年 | 299篇 |
2005年 | 263篇 |
2004年 | 274篇 |
2003年 | 231篇 |
2002年 | 199篇 |
2001年 | 180篇 |
2000年 | 174篇 |
1999年 | 137篇 |
1998年 | 135篇 |
1997年 | 146篇 |
1996年 | 127篇 |
1995年 | 108篇 |
1994年 | 94篇 |
1993年 | 86篇 |
1992年 | 57篇 |
1991年 | 56篇 |
1990年 | 42篇 |
1989年 | 30篇 |
1988年 | 19篇 |
1987年 | 21篇 |
1986年 | 45篇 |
1985年 | 35篇 |
1984年 | 29篇 |
1983年 | 10篇 |
1982年 | 51篇 |
1981年 | 17篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有6289条查询结果,搜索用时 15 毫秒
211.
212.
Abraham Sanenga Galefang Allycan Mapunda Tshepiso Merapelo Ludo Jacob Leatile Marata Bokamoso Basutli Joseph Monamati Chuma 《Entropy (Basel, Switzerland)》2020,22(11)
The open nature of radio propagation enables ubiquitous wireless communication. This allows for seamless data transmission. However, unauthorized users may pose a threat to the security of the data being transmitted to authorized users. This gives rise to network vulnerabilities such as hacking, eavesdropping, and jamming of the transmitted information. Physical layer security (PLS) has been identified as one of the promising security approaches to safeguard the transmission from eavesdroppers in a wireless network. It is an alternative to the computationally demanding and complex cryptographic algorithms and techniques. PLS has continually received exponential research interest owing to the possibility of exploiting the characteristics of the wireless channel. One of the main characteristics includes the random nature of the transmission channel. The aforesaid nature makes it possible for confidential and authentic signal transmission between the sender and the receiver in the physical layer. We start by introducing the basic theories of PLS, including the wiretap channel, information-theoretic security, and a brief discussion of the cryptography security technique. Furthermore, an overview of multiple-input multiple-output (MIMO) communication is provided. The main focus of our review is based on the existing key-less PLS optimization techniques, their limitations, and challenges. The paper also looks into the promising key research areas in addressing these shortfalls. Lastly, a comprehensive overview of some of the recent PLS research in 5G and 6G technologies of wireless communication networks is provided. 相似文献
213.
蔡体录 《浙江大学学报(理学版)》1987,14(4):493-501
本文根据北仑港区的水文资料,运用三种不同方法进行初步的冲淤计算。计算成果与北仑港区地形测量资料比较一致。分析了水动力条件,床面切应力以及地形等资料,认为目前本区是处于稳定阶段,水流足能把来沙带走。 相似文献
214.
In our previous paper, we derived a new single bubble model including the effect of bulk viscosity. To confront it to experiments, single bubble dynamics was measured here in 30% (v/v) glycerol-water mixture under different acoustic amplitudes and compared to models including or not the effect of bulk viscosity. The results showed that calculated bubble dynamics were not significantly affected by the bulk viscosity within the experimental conditions used in this study. However, there was a noticeable delay for the first rebound when bulk viscosity was considered. The corresponding sonoluminescence intensities were collected and compared with theoretical predictions. The results did not allow to discriminate between the two models (one includes the effect of bulk viscosity, the other does not), confirming the negligible effect of bulk viscosity in this condition (30% (v/v) glycerol-water mixture). Due to the instability of a single bubble in higher viscosity solutions, we could not implement experiments that can discriminate between the two models. 相似文献
215.
Dong-Yang Jing 《中国物理 B》2023,32(1):17401-017401
One-dimensional s-wave superconductor with spin-orbit coupling is a platform for the realization of Majorana zero modes. The spin-exchange with the magnetic skyrmion lattice can induce spin-orbit coupling in a s-wave superconductor system and the effects are different from the constant spin-orbit coupling. The strength of the effective spin-orbit coupling as well as the rich topoloigcal phase diagram are directly connected to the radius of the skyrmion lattice R. We obtain the rich topological phase diagram of this system with different skyrmion lattice radii by numerically evaluating the spectrum of the system under the periodic boundary condition, and we also find the Majorana zero modes under the open boundary condition to verify the bulk-edge correspondence. 相似文献
216.
The acceleration effect of poly(ethylene oxide) on nucleophilic reactions was investigated. The enhancement of the reaction rate was interpreted by the cooperative solvation of alkali metal ions with ethereal oxygens of PEO resulting in active nucleophilic anions. In relation to the complex formation of alkali metal ions with PEO, the oligo(ethylene oxide) derivatives were prepared as the synthetic ionophores, which were able to transport alkali metal ions selectively through a liquid membrane against the alkali metal ion concentration. 相似文献
217.
Mandeep Singh Bakshi 《Journal of Dispersion Science and Technology》2013,34(5):615-631
The conductances of sodium perfluorooctanoate (SPFO), sodium dodecylsulphate (SDS), dodecyltrimethylammonium bromide (DTAB), and tetradecyltrimethylammonium bromide (TTAB) in 18-crown-6 ether + water (CR+W), p-cyclodextrin + water (CY+W), and 1,10-phenanthroIine + water (Phen+W) mixtures with fixed 4 mM of each additive were determined over the temperature range of 5-55 °C. The conductivity plots for all the surfactants showed single break from which the critical micellization concentration (cmc) and degree of micelle ionization (x) were computed. From the pre and the post micellar slopes of the conductivity curves, the equivalent conductivities of the monomeric (Aass) and the micellar states (Amjc), respectively, were calculated and discussed with respect to the surfactant-additive complexation. It was observed that the micelle formation of all the ionic surfactants irrespective of the nature of their head groups were delayed in CYC+W in comparison to that in CR+W and Phen+W systems over the temperature range studied. The micelle formation of SPFO and SDS in CR+W and Phen+W systems showed stabilization of the respective micelles due to the adsorption of Na+-CR and Na+-Phen complexes at the micelle solution interface in comparison to that of DTAB and TTAB. 相似文献
218.
A novel system of symmetric and non-symmetric dimers containing azobenzene groups has been synthesized and studied in an attempt to understand further the molecular origins of the intercalated smectic phases. For the non-symmetric dimers, the lack of symmetry was derived solely from the differences in length of the two terminal alkyl chains. Both the spacer and terminal chain lengths were varied. The spacer length was found to exert a profound influence on the clearing temperatures of these materials and a large odd-even effect was observed for the series. The smectic A phase stability was observed to increase with the terminal chain length, yet decrease with increasing spacer length. X-ray diffraction has revealed the structure of the smectic A phase of both the symmetric and non-symmetric azobenzene dimers to be of the monolayer type and not intercalated. The existence of the intercalated phase has previously been explained in terms of either a charge-transfer interaction, or by an electrostatic quadrupolar interaction. However, it has been thought that it may also be the result of an excluded volume or space filling constraint. For the non-symmetric liquid crystal dimers described here, a charge-transfer interaction should be minimal, as should the stabilization from the quadrupolar interaction between the two mesogens. However, it appears that some sort of specific interaction is required to stabilize the intercalated structure. 相似文献
219.
A numerical scheme is developed to obtain the flow field around one, two and five ellipsoidal objects inside a cylindrical tube. The scheme uses the Galerkin finite element technique and the primitive variable(u–v–p) formulation. The two-dimensional incompressible Navier–Stokes equations are solved numerically by using the direct mixed interpolation method. A Picard iteration scheme is used for the solution of the resulting system of non-linear algebraic equations. The computer code is verified by checking with known analytical solutions for the flow past a sphere. Results for the shear stress distributions along the ellipsoids, forces and drag coefficients are obtained for different geometric ratios and Reynolds numbers. Some of the intermediate computational results on the velocity fields developed are also reported. 相似文献
220.
ABSTRACT The structure of liquid bismuth was studied by X-ray diffraction and computer simulation methods. The contraction of the atomic structure within the first coordination sphere in the temperature interval of 575-1225?K is reported. The temperature dependencies of the coordination numbers and of the free volume are analysed. On the basis of the temperature dependencies of the free volume, the temperature dependencies of viscosity and the self-diffusion coefficient were calculated to be in the ranges from 1.17 to 0.86?mPa?s and from 2.18 × 10?9 to 5.44 × 10?9?m2/s, respectively. The free volume – extracted results are in fair agreement with the experimental data and with the results obtained in the molecular dynamics simulations. 相似文献