全文获取类型
收费全文 | 8074篇 |
免费 | 369篇 |
国内免费 | 1837篇 |
专业分类
化学 | 4906篇 |
晶体学 | 5篇 |
力学 | 576篇 |
综合类 | 50篇 |
数学 | 1301篇 |
物理学 | 3442篇 |
出版年
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 6篇 |
2020年 | 13篇 |
2019年 | 8篇 |
2018年 | 10篇 |
2016年 | 19篇 |
2015年 | 49篇 |
2014年 | 20篇 |
2013年 | 91篇 |
2012年 | 103篇 |
2011年 | 1322篇 |
2010年 | 674篇 |
2009年 | 675篇 |
2008年 | 109篇 |
2007年 | 118篇 |
2006年 | 185篇 |
2005年 | 421篇 |
2004年 | 1267篇 |
2003年 | 751篇 |
2002年 | 640篇 |
2001年 | 556篇 |
2000年 | 212篇 |
1999年 | 223篇 |
1998年 | 258篇 |
1997年 | 188篇 |
1996年 | 116篇 |
1995年 | 121篇 |
1994年 | 94篇 |
1993年 | 429篇 |
1992年 | 482篇 |
1991年 | 297篇 |
1990年 | 308篇 |
1989年 | 303篇 |
1988年 | 68篇 |
1987年 | 10篇 |
1986年 | 25篇 |
1985年 | 21篇 |
1984年 | 17篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1979年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1962年 | 2篇 |
1938年 | 3篇 |
1937年 | 2篇 |
1931年 | 2篇 |
1893年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 703 毫秒
991.
992.
I show the formulation of de Sitter Special Relativity (dS-SR) based on Dirac-Lu-Zou-Guo's discussions. dS-SR quantum mechanics is formulated, and the dS-SR Dirac equation for hydrogen is suggested. The equation in the earth-QSO framework reference is solved by means of the adiabatic approach. It's found that the fine-structure "constant" α in dS-SR varies with time. By means of the t-z relation of the ΛCDM model, α's time-dependency becomes redshift z-dependent. The dS-SR's predictions of Δα/α agree with data of spectra of 143 quasar absorption systems, the dS-space-time symmetry is SO(3,2) (i.e., anti-dS group) and the universal parameter R (de Sitter ratio) in dS-SR is estimated to be R≈2.73×1012 ly. The effects of dS-SR become visible at the cosmic space-time scale (i.e., the distance ≥ 109 ly). At that scale, dS-SR is more reliable than Einstein SR. The α-variation with time is evidence of SR with de Sitter symmetry. 相似文献
993.
The characteristics of ion beam extraction and focused to a volume as small as possible were investigated with the aid of computer code SIMION 3D version 7. This has been used to evaluate the extraction characteristics (accel-decel system) to generate an ion beam with low beam emittance and high brightness. The simulation process can provide a good study for optimizing the extraction and focusing system of the ion beam without any losses and transported to the required target. Also, a study of a simulation model for the extraction system of the ion source was used to describe the possible plasma boundary curvatures during the ion extraction that may be affected by the change in an extraction potential with a constant plasma density meniscus. 相似文献
994.
995.
Micromegas prototypes with thermo-bond film separators 总被引:1,自引:0,他引:1
In this paper we report the results of Micromegas prototypes constructed by attaching micromesh to an anode using thermo-bond films. The excellent metal attaching ability and good dielectical property of this kind of film make it a promising material to be used as avalanche gap spacers. Several prototypes are successfully made. The electron transmission properties are first studied and then the gas gain is measured in argon-isobutane mixtures. The maximum gain of more than 104 is easily obtained. The energy resolutions for 55Fe 5.9 keV Kα ray can be better than 20% over one magnitude in gain for different operational gas mixtures and the best energy resolution of 13.7% (FWHM) can be achieved with the gas mixture of 94% argon concentration. The preliminary test results of the prototypes with sensitive area of 45 mm×45 mm without internal support show good uniformity across the sensitive area. 相似文献
996.
Based on the multi-coincidence measurement, the time resolution of three liquid scintillation detectors (BC501A) were determined strictly by solving the coincidence equations, where the influence from electronics estimated by self coincidence measurement as well as the background had been considered. The result of this work agreed well with the result that was deduced from the traditional method, and it will be helpful to analyze the energy resolution of neutron time of flight spectra measured by using such detectors at CIAE (China Institute of Atomic Energy). 相似文献
997.
A novel simulation method to evaluate the collection performance of a monolithic active pixel sensor
A novel simulation method is presented in this paper to evaluate the collection performance of monolithic active pixel sensor (MAPS) devices for minimum ionizing particle tracking. A simplified 3D matrix pixel structure is built using the computer aided design software Sentaurus. The virtual device is then divided into hundreds of parts and an independent customized X photon model is involved in each part to simulate the conditions under 55Fe radiation. After data processing and analysis, charge collection efficiency, collection time and diffusion conditions can be estimated in detail. In order to verify the reliability of the method, comparisons are made between the simulations and experiments. Although there are some defects, it can be concluded that the proposed idea is a feasible method for the evaluation of the MAPS collection performance. 相似文献
998.
The Gamow-Teller transitions for pf shell nuclei with proton number less than 40 and neutron number larger than 40 were believed to be blocked, due to the full filling of the neutron orbit. However, recent experimental research shows that the Gamow-Teller transitions for these kinds of nuclei are not blocked. In this paper, we systematically calculate the GT transition of pf shell nuclei 76Se in different truncations, and the results are compared with experimental results. It is shown that, due to correlations, the believed blocked GT transition occurs, and the shell model calculations reproduce the experimental GT strength. In addition, the electron capture rates in a stellar environment are calculated and discussed. 相似文献
999.
Properties of prompt fission neutrons from 238U(n, f) are calculated for incident neutron energies below 6 MeV using the multi-modal model, including the prompt fission neutron spectrum, the average prompt fission neutron multiplicity, and the prompt fission neutron multiplicity as a function of the fission fragment mass υ(A) (usually named “sawtooth” data) The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental fission fragment data. The predicted results are in good agreement with the experimental data. 相似文献
1000.
Fission cross sections strongly depend on the ratio of the level density parameter in fission to neutron emission, af/an. In this work, a cascade-exciton model implemented in the code CEM95 has been used to observe this effect for proton induced fission cross sections of tungsten, lead and bismuth. The method was employed using different level density parameter ratios for each fission cross section calculation. The calculated fission cross sections are compared with the available experimental data in the literature. It has been observed that a change of the ratio of the level density parameter, af/an, is necessary with the incident energy of the proton, to best estimate the fission cross sections in CEM95. 相似文献