全文获取类型
收费全文 | 2057篇 |
免费 | 339篇 |
国内免费 | 171篇 |
专业分类
化学 | 1414篇 |
晶体学 | 33篇 |
力学 | 147篇 |
综合类 | 15篇 |
数学 | 181篇 |
物理学 | 777篇 |
出版年
2024年 | 6篇 |
2023年 | 35篇 |
2022年 | 79篇 |
2021年 | 88篇 |
2020年 | 75篇 |
2019年 | 60篇 |
2018年 | 79篇 |
2017年 | 72篇 |
2016年 | 99篇 |
2015年 | 119篇 |
2014年 | 127篇 |
2013年 | 167篇 |
2012年 | 198篇 |
2011年 | 205篇 |
2010年 | 118篇 |
2009年 | 127篇 |
2008年 | 131篇 |
2007年 | 128篇 |
2006年 | 105篇 |
2005年 | 84篇 |
2004年 | 45篇 |
2003年 | 33篇 |
2002年 | 36篇 |
2001年 | 33篇 |
2000年 | 40篇 |
1999年 | 34篇 |
1998年 | 24篇 |
1997年 | 26篇 |
1996年 | 32篇 |
1995年 | 19篇 |
1994年 | 19篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1991年 | 15篇 |
1990年 | 25篇 |
1989年 | 16篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 2篇 |
1981年 | 2篇 |
1976年 | 2篇 |
1957年 | 1篇 |
1942年 | 1篇 |
1941年 | 1篇 |
1936年 | 4篇 |
1930年 | 2篇 |
1929年 | 1篇 |
1927年 | 1篇 |
1923年 | 1篇 |
排序方式: 共有2567条查询结果,搜索用时 31 毫秒
981.
An efficient heterogeneous gold-catalyzed oxidative cross-coupling of propargylic acetates with arylboronic acids has been developed that proceeds smoothly in the presence of Selectfluor and provides a general and powerful tool for the preparation of various valuable α-arylenones with moderate to good yields, excellent E-selectivity, and recyclability of the gold catalyst. The reaction is the first example of heterogeneous gold-catalyzed arylative rearrangement of propargylic acetates for construction of complex enones. 相似文献
982.
983.
Guangfu Feng Xingyu Luo Xu Lu Shiyi Xie Lu Deng Wenyuan Kang Fang He Jiaheng Zhang Chunyang Lei Bin Lin Yan Huang Zhou Nie Shouzhuo Yao 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(20):6662-6666
The comprehensive understanding of the mechanisms underlying the interaction of cells with their membrane microenvironment is of great value for fundamental biological research; however, tracking biomolecules on cell surfaces with high temporal and spatial resolution remains a challenge. Herein, a modular strategy is presented for the construction of cell surface DNA‐based sensors by engineering DNA motifs and synthetic cofactors. In this strategy, a stimuli‐reactive organic molecule is employed as the cofactor for the DNA motif, and the self‐assembly of them forms a FRET‐based holo DNA‐based sensor. With the use of the DNA‐based sensors, the versatility of this modular strategy has been demonstrated in the ratiometric imaging of the cellular extrusion process of endogenous signaling molecules, including sulfur dioxide derivatives and nitric oxide. 相似文献
984.
Ming Lei Zijun Nie Mengyang Yang F. Aníbal Fernández Clive Poole 《Liquid crystals》2019,46(6):846-856
This paper proposes a general design method for microstrip devices using liquid crystal (LC) at microwave frequencies. The method employs lumped element modelling to first optimise the filter at the working frequency. Then EM simulation is used for the second optimisation and the full-wave simulation. LC is used as the tunable material since the anisotropy can be changed with low voltages. The approach of modelling LC as a homogeneous anisotropy demonstrated with Computer Simulation Technology (CST) Microwave Studio is an acceptable approximation, which accurately predicts the tuning range. A new tunable inverted microstrip filter using nematic LCs at millimetre-wave frequencies is designed to verify the method. The proposed design utilises interdigital capacitors in parallel with two inductive loops to form a bandpass filter. It is fabricated and measured, and shows that the centre frequency varies from 5.01 to 5.51 GHz (10% tunability) and achieves a 3-dB bandwidth of 450 MHz, which is in good agreement with simulation results. A finite element simulation is used to investigate the behaviour of LC directors in the fully switched state; the effective permittivity is extracted from finite-element simulation and used in CST to achieve an even more accurate agreement with experimental results. 相似文献
985.
涌浪运动是非均匀水流中的一种非线性运动,是常见的海洋运动形式之一.在进行水下量子通信时,会对光量子信号的传输造成极大的影响.然而,有关涌浪运动造成量子通信信道参数变化的研究,迄今尚未展开.为了研究涌浪运动对水下量子通信性能的影响,首先对涌浪运动的传播建立了数学模型并分析了其频谱特性.针对退极化信道,提出了涌浪运动与水下量子通信信道纠缠和信道容量的定量关系,并对量子密钥分发过程中误码率的影响进行了分析.仿真结果表明,当海面风速在0—20.5 m/s变化时,随着传播周期逐渐增大,信道纠缠度由0.0012逐渐增加到0.8426,信道容量由0.8736减小到0.1024,密钥分发过程中,量子误码率由0.1651增加到0.4812.由此可见,涌浪运动对于水下量子通信性能有着明显的影响.因此,在进行水下量子通信时,应根据涌浪运动的不同程度,自适应调整系统参数. 相似文献
986.
Ying Xu Ying Zhou Guo Zheng Nie Daifeng Zou Zhi Min Ao 《Chinese Journal of Physics (Taipei)》2018,56(5):2285-2290
In this letter, the electronic structure properties of Nb, F monodoping and Nb-F codoping are explored by first-principles calculations. Our results show that Nb-F codoping can reduce the band gap notably. The band edge analysis indicates that both conduction band maximum (CBM) and valence band minimum (VBM) move to higher energies, which is desirable for water splitting. The formation energy and pair binding energy calculation shows that this anion-cation codoping is easy to realize in both O-rich and O-poor conditions. The calculated optical absorption spectra indicate that the visible light absorption can be significantly improved by Nb-F codoping in WO3. Therefore, Nb-F co-doped WO3 is predicted to be a promising visible light photocatalyst for water splitting. 相似文献
987.
Three high-performance concrete (HPC) materials with different specimen geometries were characterized using Kolsky compression bar techniques to study the strain rate and specimen size effects on their uniaxial compressive strength. A large-diameter Kolsky bar and recently established annular pulse shaping technique were used to achieve dynamic stress equilibrium and constant strain-rate deformation in the experiments. A complimentary effort was conducted using a 19-mm-diameter Kolsky compression bar to understand the strain rate and specimen size effects on failure strength and dynamic increase factor (DIF) for concrete. It was found that, for all three concrete materials investigated, the failure strength is highly dependent on the specimen geometry, however such a relationship is not apparent for the DIF. The DIF observed in this study shows significantly lower values compared to historical data, which may indicate the importance of well-controlled dynamic testing conditions on the accuracy and validity of experimental results for concrete materials. 相似文献
988.
This paper deals with nonlinear free vibration of reticulated shallow spherical shells taking into account the effect of transverse shear deformation. The shell is formed by beam members placed in two orthogonal directions. The nondimensional fundamental governing equations in terms of the deflection, rotational angle, and force function are presented, and the solution for the nonlinear free frequency is derived by using the asymptotic iteration method. The asymptotic solution can be used readily to perform the parameter analysis of such space structures with numerous geometrical and material parameters. Numerical examples are given to illustrate the characteristic amplitude-frequency relation and softening and hardening nonlinear behaviors as well as the effect of transverse shear on the linear and nonlinear frequencies of reticulated shells and plates. 相似文献
989.
To simulate the multiple scattering effect of target in synthetic aperture radar (SAR) image, the hybrid method GO/PO method, which combines the geometrical optics (GO) and physical optics (PO), is employed to simulate the scattering field of target. For ray tracing is time-consuming, the Open Graphics Library (OpenGL) is usually employed to accelerate the process of ray tracing. Furthermore, the GO/PO method is improved for the simulation in low pixel situation. For the improved GO/PO method, the pixels are arranged corresponding to the rectangular wave beams one by one, and the GO/PO result is the sum of the contribution values of all the rectangular wave beams. To get high-resolution SAR image, the wideband echo signal is simulated which includes information of many electromagnetic (EM) waves with different frequencies. Finally, the improved GO/PO method is used to simulate the SAR image of targets above rough surface. And the effects of reflected rays and the size of pixel matrix on the SAR image are also discussed. 相似文献
990.
JiangHe Yang JunHui Fan Yi Liu YueLian Zhang ManXian Tuo JianJun Nie YuHai Yuan 《中国科学:物理学 力学 天文学(英文版)》2018,61(5):059511
Blazars are a special subclass of active galactic nuclei with extreme observation properties. This subclass can be divided into two further subclasses of flat spectrum radio quasars(FSRQs) and BL Lacertae objects(BL Lacs) according to their emission line features. To compare the spectral properties of FSRQs and BL Lacs, the 1.4 GHz radio, optical R-band, 1 keV X-ray, and 1 GeVy-ray flux densities for 1108 Fermi blazars are calculated to discuss the properties of the six effective spectral indices of radio to optical(α_(RO)), radio to X-ray(α_(RX)), radio to y ray(α_(Ry)), optical to X-ray(α_(OX)), optical to y ray(α_(Oy)), and X-ray to y ray(α_(Xy)).The main results are as follows: For the averaged effective spectral indices, α_(OX_ α_(Oy) α_(Xy) α_(Ry) α_(RX) α_(RO) for samples of whole blazars and BL Lacs; α_(Xy)≈α_(Ry)≈α_(RX) for FSRQs and low-frequency-peaked BL Lacs(LBLs); and α_(OX)≈α_(Oy)≈α_(Xy) for high-synchrotron-frequency-peaked BL Lacs(HBLs). The distributions of the effective spectral indices involving optical emission(α_(RO), α_(OX), and α_(Oy)) for LBLs are different from those for FSRQs, but if the effective spectral index does not involve optical emission(α_(RX), α_(Ry), and α_(Xy)), the distributions for LBLs and FSRQs almost come from the same parent population. X-ray emissions from blazars include both synchrotron and inverse Compton (IC) components; the IC component for FSRQs and LBLs accounts for a larger proportion than that for HBLs; and the radiation mechanism for LBLs is similar to that for FSRQs, but the radiation mechanism for HBLs is different from that for both FSRQs and LBLs in X-ray bands. The tendency of α_(Ry) decreasing from LBLs to HBLs suggests that the synchrotron self-Compton model explains the main process for highly energetic y rays in BL Lacs. 相似文献