全文获取类型
收费全文 | 2055篇 |
免费 | 247篇 |
国内免费 | 162篇 |
专业分类
化学 | 1457篇 |
晶体学 | 22篇 |
力学 | 85篇 |
综合类 | 14篇 |
数学 | 167篇 |
物理学 | 719篇 |
出版年
2024年 | 3篇 |
2023年 | 32篇 |
2022年 | 48篇 |
2021年 | 57篇 |
2020年 | 65篇 |
2019年 | 59篇 |
2018年 | 82篇 |
2017年 | 62篇 |
2016年 | 115篇 |
2015年 | 104篇 |
2014年 | 121篇 |
2013年 | 185篇 |
2012年 | 152篇 |
2011年 | 146篇 |
2010年 | 150篇 |
2009年 | 137篇 |
2008年 | 133篇 |
2007年 | 124篇 |
2006年 | 108篇 |
2005年 | 83篇 |
2004年 | 54篇 |
2003年 | 71篇 |
2002年 | 60篇 |
2001年 | 39篇 |
2000年 | 39篇 |
1999年 | 28篇 |
1998年 | 26篇 |
1997年 | 20篇 |
1996年 | 22篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1993年 | 11篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 8篇 |
1989年 | 11篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1972年 | 4篇 |
1971年 | 3篇 |
排序方式: 共有2464条查询结果,搜索用时 15 毫秒
991.
基于电介质光栅的光谱合成是实现高功率高光束质量激光的重要途径. 在电介质光栅的光谱合成系统中, 光栅色散效应是影响合成激光光束质量的重要因素. 本文推导了单光栅和双光栅光谱合成系统中由于光栅色散引起M2因子的变化公式; 详细讨论了这两种合成系统中单路激光线宽、单路激光光斑半径、相邻两路激光波长差、相邻两路激光间距以及光栅周期对光束质量的影响. 研究表明对于单光栅合成系统, 在合成过程中若保持光束质量M2因子的大小不变, 则单路激光带宽随光斑半径的增加而减小; 在双光栅光谱合成系统中, 在保持光束质量的前提下, 单路激光带宽可随光斑半径的增大而相应增加. 数值计算表明, 若要满足合成光束的光束质量M2 ≤1.2的要求, 在单光栅系统中激光线宽需窄于亚纳米量级, 在双光栅系统中激光带宽可为亚纳米. 本文为高功率、高光束质量的光纤激光光谱合成系统的搭建提供了理论指导. 相似文献
992.
以葡聚糖为模板,钛酸四正丁酯、硝酸铁和硝酸镧为前驱体采用模板法制备了一系列铁、镧单掺杂及共掺杂纳米TiO2光催化剂. 利用SEM、XRD、BET比表面积测定和UV-Vis等技术对其形貌、晶体结构及表面结构、光吸收特性等进行了表征. 以甲基橙溶液的光催化降解为模型反应,考察了不同掺杂的样品在紫外和可见光下的光催化性能. TiO2材料具有较大的比表面积(约150 m2/g),铁和镧共掺杂纳米TiO2在可见光区域有较强的吸收,在紫外和可见光条件下较纯TiO2和单掺杂TiO2对甲基橙溶液具有更好的光催化降解效果,且铁和镧的掺杂量显著影响该材料的催化性能. 当铁掺杂量为0.5mol%、镧掺杂量为0.3mol%,在500 ℃焙烧2 h所得光催化材料的催化性能最佳,焙烧4 h即可使甲基橙的降解率达98.8%,且该复合材料有较高的循环回收利用率,重复使用4次仍可使甲基橙的降解率保持在88%以上. 相似文献
993.
Evgenii Glushkov Anastasiia Glushkova Vladimir I. Man’ko 《Journal of Russian Laser Research》2016,37(3):236-243
We discuss the known construction of two interacting superconducting circuits based on Josephson junctions, which can be precisely engineered and easily controlled. In particular, we use the parametric excitation of two circuits realized by an instant change of the qubit coupling to study entropic and information properties of the density matrix of a composite system. We obtain the density matrix from the initial thermal state and perform its analysis in the approximation of small perturbation parameter and sufficiently low temperature. We also check the subadditivity condition for this system both for the von Neumann entropy and deformed entropies and check the dependence of mutual information on the system temperature. Finally, we discuss the applicability of this approach to describe the two coupled superconducting qubits as harmonic oscillators with limited Hilbert space. 相似文献
994.
Julio A. López-Saldívar Armando Figueroa Octavio Castaños Ramón López–Peña Margarita A. Man’ko Vladimir I. Man’ko 《Journal of Russian Laser Research》2016,37(4):313-327
We propose the discretization of the density matrix as a nonlinear positive map for systems with continuous variables. We use this procedure for calculating the entanglement between two modes through different criteria, such as Tsallis entropy, von Neumann entropy and linear entropy, and the logarithmic negativity. As an example, we study the dynamics of entanglement for the two-mode squeezed-vacuum state in the parametric amplifier and show good agreement with the analytic results. Also we address the loss of information on the system state due to the discretization of the density matrix. 相似文献
995.
On Multi‐component Ermakov Systems in a Two‐Layer Fluid: Integrable Hamiltonian Structures and Exact Vortex Solutions 下载免费PDF全文
By introducing an elliptic vortex ansatz, the 2+1‐dimensional two‐layer fluid system is reduced to a finite‐dimensional nonlinear dynamical system. Time‐modulated variables are then introduced and multicomponent Ermakov systems are isolated. The latter is shown to be also Hamiltonian, thereby admitting general solutions in terms of an elliptic integral representation. In particular, a subclass of vortex solutions is obtained and their behaviors are simulated. Such solutions have recently found applications in oceanic and atmospheric dynamics. Moreover, it is proved that the Hamiltonian system is equivalent to the stationary nonlinear cubic Schrödinger equations coupled with a Steen‐Ermakov‐Pinney equation. 相似文献
996.
14‐Vertex Heteroboranes with 14 Skeletal Electron Pairs: An Experimental and Computational Study 下载免费PDF全文
Dr. Alasdair P. M. Robertson Nicholas A. Beattie Dr. Greig Scott Dr. Wing Y. Man John J. Jones Prof. Stuart A. Macgregor Dr. Georgina M. Rosair Prof. Alan J. Welch 《Angewandte Chemie (International ed. in English)》2016,55(30):8706-8710
Three isomers of [(Cp*Ru)2C2B10H12], the first examples of 14‐vertex heteroboranes containing 14‐skeletal electron pairs, have been synthesized by the direct electrophilic insertion of a {Cp*Ru+} fragment into the anion [4‐Cp*‐4,1,6‐RuC2B10H12]?. All three compounds have the same unique polyhedral structure having an approximate Cs symmetry and featuring a four‐atom trapezoidal face. X‐ray diffraction studies could confidently identify only one of the two cage C atoms in each structure. The other C atom position has been established by a combination of i) best fitting of computed and experimental 11B and 1H NMR chemical shifts, and ii) consideration of the lowest computed energy for series of isomers studied by DFT calculations. In all three isomers, one cage C atom occupies a degree‐4 vertex on the short parallel edge of the trapezium. 相似文献
997.
Hong-ni?Teng Teng?Shen Yi-xiao?Hou Yong?ChenEmail author Xiao-yan?Kou Xiu-yun?Wang Man?Hai 《Colloid Journal》2016,78(5):690-697
Mesoporous silica materials were synthesized using tetraеthoxysilane as precursor and liquid crystals formed in aqueous mixtures of cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) as templates, without and with the addition of NaBr or Na2SO4. For this purpose, the formation of liquid crystals as a function of the ratio of CTAB and SDS under different conditions was studied. It was found that liquid crystals formed in the mixed system of CTAB and SDS at certain mixing ratios are well-structured templates for the synthesis of mesoporous silicas. The synthesized silica materials were characterized by transmission electron microscope and nitrogen adsorption/desorption analysis. The pore size of mesoporous silicas could be controlled between 3 to 6 nm by simply changing the concentration of NaBr in solution. The mesoporous silicas exhibited lamellar structure and the order of structural arrangement was promoted with addition of NaBr. However, addition of Na2SO4 led to ink-bottle type pores of mesoporous silica with a narrow pore size distribution of around 2 nm and a higher specific surface area of 610 m2 g–1. 相似文献
998.
Asymmetric Construction of Pyrrolidines Bearing a Trifluoromethylated Quaternary Stereogenic Center via CuI‐Catalyzed 1,3‐Dipolar Cycloaddition of Azomethine Ylides with β‐CF3‐β,β‐Disubstituted Nitroalkenes 下载免费PDF全文
Dr. Li‐Wei Tang Bao‐Jing Zhao Li Dai Man Zhang Prof. Dr. Zhi‐Ming Zhou 《化学:亚洲杂志》2016,11(17):2470-2477
A direct and convenient method has been developed for the synthesis of optically active pyrrolidines bearing a quaternary stereogenic center containing a CF3 group at the C‐3 position of the pyrrolidine ring. The synthesis system, CuI/Si‐FOXAP‐catalyzed exo‐selective 1,3‐dipolar cycloaddition of azomethine ylides with β‐CF3‐β,β‐disubstituted nitroalkenes, provides pyrrolidines with high diastereoselectivities (up to >98:2 d.r.) and excellent enantioselectivities (up to >99.9 ee) and performs well for a broad scope of substrates under mild conditions. 相似文献
999.
Dr. Man‐Rong Li Maria Retuerto Prof. Peter W. Stephens Prof. Mark Croft Dr. Denis Sheptyakov Vladimir Pomjakushin Zheng Deng Dr. Hirofumi Akamatsu Venkatraman Gopalan Dr. Javier Sánchez‐Benítez Felix O. Saouma Prof. Joon I. Jang Prof. David Walker Prof. Martha Greenblatt 《Angewandte Chemie (International ed. in English)》2016,55(34):9862-9867
Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic‐scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2FeMoO6 at unparalleled low temperatures of 150–300 oC. The irreversible ionic motion at ambient pressure, as evidenced by real‐time powder synchrotron X‐ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3TeO6–type to an ordered‐ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures. 相似文献
1000.