首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8206篇
  免费   1273篇
  国内免费   750篇
化学   5683篇
晶体学   132篇
力学   495篇
综合类   72篇
数学   787篇
物理学   3060篇
  2024年   44篇
  2023年   189篇
  2022年   318篇
  2021年   317篇
  2020年   379篇
  2019年   350篇
  2018年   304篇
  2017年   254篇
  2016年   412篇
  2015年   401篇
  2014年   533篇
  2013年   606篇
  2012年   672篇
  2011年   693篇
  2010年   457篇
  2009年   374篇
  2008年   498篇
  2007年   425篇
  2006年   379篇
  2005年   321篇
  2004年   214篇
  2003年   218篇
  2002年   166篇
  2001年   124篇
  2000年   154篇
  1999年   151篇
  1998年   129篇
  1997年   131篇
  1996年   151篇
  1995年   110篇
  1994年   108篇
  1993年   89篇
  1992年   78篇
  1991年   69篇
  1990年   72篇
  1989年   47篇
  1988年   49篇
  1987年   37篇
  1986年   37篇
  1985年   26篇
  1984年   22篇
  1983年   15篇
  1982年   19篇
  1981年   15篇
  1980年   11篇
  1979年   8篇
  1975年   5篇
  1974年   5篇
  1970年   12篇
  1937年   5篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
In a multi-wire proportional chamber detector(MWPC), the anode and signal wires must maintain suitable tension, which is very important for the detector's stable and accurate performance. As a result, wire tension control and measurement is essential in MWPC construction. A high pressure3 He MWPC detector is to be used as the thermal neutron detector of the multi-functional reflectometer at China Spallation Neutron Source, and in the construction of the detector, we have developed a wire tension measurement system. This system is accurate,portable and time-saving. With it, the wire tension on an anode wire plane has been tested. The measurement results show that the wire tension control techniques used in detector manufacture are reliable.  相似文献   
42.
We consider the arrival time distribution defined through the quantum probability current for a Gaussian wave packet representing free particles in quantum mechanics in order to explore the issue of the classical limit of arrival time. We formulate the classical analogue of the arrival time distribution for an ensemble of free particles represented by a phase space distribution function evolving under the classical Liouville's equation. The classical probability current so constructed matches with the quantum probability current in the limit of minimum uncertainty. Further, it is possible to show in general that smooth transitions from the quantum mechanical probability current and the mean arrival time to their respective classical values are obtained in the limit of large mass of the particles.  相似文献   
43.
The observations of three-photon-induced frequency-upconversion fluorescence and the highly directional stimulated visible emission in two dyes, 4-[p-(dicyanoethylamino) styryl]-N-methylpyridinium iodide (abbreviated as CEASP) and the complex of CEASP and Ce(NO3) (abbreviated as CEASP-Ce), are reported. The photographs of the forward amplified spontaneous emissions spots, pumped by an optical parametric oscillator idler with a pulse width of 8 ns and a wavelength of 1.3 μ m, are shown. The upconversion fluorescence produced both in dimethyl formamide solution and 2-hydroxyethyl methacrylate (HEMA) polymer spans from green to red, with a cubic dependence on the pump light intensity. The experimental results imply that the existence of the lanthanide ion Ce3 + sensitizes the nonlinear absorption and emission.  相似文献   
44.
First-principle calculations based on density function theory (DFT) are used to clarify the roles of γ-Fe2O3 in fly ash for removing mercury from coal-fired flue gases. In this study, the structure of key surface of γ-Fe2O3 is modeled and spin-polarized periodic boundary conditions with the partial relaxation of atom positions are employed. Binding energies of Hg on γ-Fe2O3 (0 0 1) perfect and defective surfaces are calculated for different adsorption sites and the potential adsorption sites are predicted. Additionally, electronic structure is examined to better understand the binding mechanism. It is found that mercury is preferably adsorbed on the bridge site of γ-Fe2O3 (0 0 1) perfect surface, with binding energy of −54.3 kJ/mol. The much stronger binding occurs at oxygen vacancy surface with binding energy of −134.6 kJ/mol. The calculations also show that the formation of hybridized orbital between Hg and Fe atom of γ-Fe2O3 (0 0 1) is responsible for the relatively strong interaction of mercury with the solid surface, which suggests that the presently described processes are all noncatalytic in nature. However, this is a reflection more of mercury's amalgamation ability.  相似文献   
45.
王永龙  卢伟涛  蒋华  许长谭  潘洪哲 《中国物理 B》2012,21(7):70501-070501
By using the Faddeev-Senjanovic path integral quantization method, we quantize the composite fermions in quantum electrodynamics (QED). In the sense of Dirac’s conjecture, we deduce all the constraints and give Dirac’s gauge transformations (DGT). According to that the effective action is invariant under the DGT, we obtain the Noether theorem at the quantum level, which shows the fractional charges for the composite fermions in QED. This result is better than the one deduced from the equations of motion for the statistical potentials, because this result contains both odd and even fractional numbers. Furthermore, we deduce the Noether theorem from the invariance of the effective action under the rotational transformations in 2-dimensional (x, y) plane. The result shows that the composite fermions have fractional spins and fractional statistics. These anomalous properties are given by the constraints for the statistical gauge potential.  相似文献   
46.
Liu XH  Pan L  Weng JQ  Tan CX  Li YH  Wang BL  Li ZM 《Molecular diversity》2012,16(2):251-260
To develop novel inhibitors of ketol-acid reductoisomerase, a series of (oxdi/tri)azoles derivatives was synthesized and characterized by (1)H NMR, MS, elemental analyses, and crystallography. According to the biological activities of these compounds obtained both in vivo and in vitro, compound 4-cyclopropyl-3-((4-fluorobenzyl)thio)-5-methyl-4H-1,2,4-triazole showed excellent KARI inhibitory activity (100% at 100?μg?mL?(-1) in vitro). In addition, most of the title compounds exhibited good herbicidal activity against Brassica campestris in vivo.  相似文献   
47.
孔文婕  吕力  张殿琳  潘正伟 《中国物理》2005,14(10):2090-2092
The $1/f$ noise in multiwalled carbon nanotubes bundles has been investigated between the frequency range of 0.1 to 30 Hz. At room temperature the noise spectrum is standard 1/f, and its level is proportional to the square of the bias voltage. With decreasing temperature the noise level also decreases. At 4.2 K the noise level follows a non-monotonic dependence against the bias voltage, showing a peak at a certain bias voltage, meanwhile its frequency dependence also deviates from the 1/f trend. This anomalous behaviour is discussed within the picture of environmental quantum fluctuation of charge transport in the samples.  相似文献   
48.
系统研究了表面润湿法合成β沸石的体系中,干晶种、润湿态晶种、异晶晶种(Y沸石、ZSM 5沸石)以及碱处理β沸石晶种在此晶化体系中的作用.采用β沸石产品及异晶作晶种时,可使产品的相对结晶度稍有提高,但对沸石微晶核的结构导向作用不明显.经浓碱液处理20min后的β沸石产品(碱处理晶种)作晶种引入晶化体系后,可以显著提高产品的相对结晶度,降低模板剂的用量,缩短晶化诱导期,成为表面润湿法合成β沸石体系的有效晶种形式.  相似文献   
49.
Electronic structure and magnetic properties of metastable SmCo_7 compound   总被引:1,自引:0,他引:1  
1 Introduction The rare-earth transition-metal intermetallic compounds have been widely investi-gated for many years, among them the Sm-Co series compounds with 1:5 and 2:17 crys-tal structures. These compounds have been used as sintered and bonded permanent magnets since the 1960s[1,2]. Interest recently has been focused on the TbCu7-type struc-ture Sm-Co intermetallic compounds with a strong uniaxial magnetocrytalline anisot-ropy and a low temperature coefficient (β = ?0.11%)[3―6] due t…  相似文献   
50.
王艳红  郭磐  倪国强 《光学技术》2012,38(3):357-361
由于空间环境中宇宙辐射无处不在,工作在空间环境的光纤系统必须考虑辐致损伤特性。建立了10GHz的射频光纤链路传输系统,分别采用单模光纤和保偏光纤作为传输媒质。利用60 Co`γ射线源,在剂量率1rad/s总剂量50krad下,对射频光纤链路进行辐照实验,对两种系统做对比分析。实验结果表明,单模光纤链路的辐致损耗绝对值较大,保偏光纤链路的辐致损耗绝对值较小;单模光纤的幂律模型指数较保偏光纤大,即同样的辐照剂量,单模光纤受影响程度大。在辐照起主要影响因素的阶段,单模光纤延时差受辐照影响程度较保偏光纤稍小。辐致损耗和辐致延时差对光纤长度都有线性累加作用。从整体水平看,50krad的辐射总剂量对两种光纤造成的损伤并不大,传输的信号仍在可用的范围之内。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号