全文获取类型
收费全文 | 158613篇 |
免费 | 9372篇 |
国内免费 | 6998篇 |
专业分类
化学 | 61040篇 |
晶体学 | 1369篇 |
力学 | 9541篇 |
综合类 | 370篇 |
数学 | 37800篇 |
物理学 | 33804篇 |
无线电 | 31059篇 |
出版年
2024年 | 261篇 |
2023年 | 1629篇 |
2022年 | 2086篇 |
2021年 | 2704篇 |
2020年 | 2593篇 |
2019年 | 2389篇 |
2018年 | 13406篇 |
2017年 | 13073篇 |
2016年 | 9423篇 |
2015年 | 3917篇 |
2014年 | 4043篇 |
2013年 | 5095篇 |
2012年 | 9789篇 |
2011年 | 17022篇 |
2010年 | 10744篇 |
2009年 | 10741篇 |
2008年 | 11474篇 |
2007年 | 13495篇 |
2006年 | 4083篇 |
2005年 | 4863篇 |
2004年 | 4436篇 |
2003年 | 4335篇 |
2002年 | 3122篇 |
2001年 | 2001篇 |
2000年 | 1879篇 |
1999年 | 1724篇 |
1998年 | 1535篇 |
1997年 | 1349篇 |
1996年 | 1482篇 |
1995年 | 1193篇 |
1994年 | 1105篇 |
1993年 | 993篇 |
1992年 | 874篇 |
1991年 | 728篇 |
1990年 | 645篇 |
1989年 | 492篇 |
1988年 | 458篇 |
1987年 | 374篇 |
1986年 | 303篇 |
1985年 | 347篇 |
1984年 | 285篇 |
1983年 | 236篇 |
1982年 | 201篇 |
1981年 | 161篇 |
1980年 | 168篇 |
1979年 | 125篇 |
1978年 | 105篇 |
1976年 | 92篇 |
1974年 | 92篇 |
1973年 | 98篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Crosslinking behaviors of acrylonitrile butadiene rubber (NBR)/poly (vinyl chloride) (PVC) alloy, filled with anhydrous copper sulfate (CuSO4) particles, were investigated for the first time by dynamic mechanical analysis (DMA) under hetero and isothermal modes, respectively. In the heterothermal testing, (NBR/PVC)/CuSO4 compound showed two marked increases in the storage modulus (E′), corresponding to coordination crosslinking of NBR-CuSO4 and self-crosslinking of NBR and PVC respectively. During the isothermal testing, a dramatic increase in E′ was found at the initial stage while that of original NBR/PVC alloy was not detected. The marked increase in E′ of (NBR/PVC)/CuSO4 compound was mainly due to the crosslinking induced by coordination between CN and Cu2+. The increasing extent of E′ increased with the rise of CuSO4 content, suggesting the formation of a greater number of crosslinks. Moreover, the activation energy (Ea) of crosslinking process was about 139 kJ/mol. In this work, fourier transform infrared spectrum (FT-IR) and equilibrium swelling method were also performed for the characterization of the compound. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 41–51, 2007 相似文献
72.
In this paper, we discuss the local and global existence ofweak solutions for some hyperbolicparabolic systems modellingchemotaxis. 相似文献
73.
The electromagnetic properties of electrically thick rectangular microstrip antennas were investigated experimentally. Antennas were fabricated with different patch sizes and with electrical thicknesses ranging from 0.03 to 0.23 wavelengths in the dielectric substrate. The resonant frequencies were measured and compared to existing formulas. The bandwidth was calculated as a function of electrical thickness and the antenna radiation patterns were measured. 相似文献
74.
This work reports a new synthetic approach for single‐phase TiO2 nanomaterials by solvothermal treatment of titanium tetrachloride in acetone at 80–110 °C. Small, uniform, and yet size‐tunable (5–10 nm) anatase titania nanocrystallites were obtained using a low concentration of TiCl4 in acetone (i.e., at molar ratios of TiCl4/acetone ≤ 1:15) in the temperature range of 80–110 °C, while rutile nanofibers were synthesized using a high concentration of TiCl4 (e.g., TiCl4/acetone = 1:10) at 110 °C. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
75.
76.
Wang K.C. Asbeck P.M. Chang M.F. Sullivan G.J. Miller D.L. Basit H.F. 《Electronics letters》1989,25(17):1111-1112
The authors report a novel noninterfering and simple approach for evaluation of circuits implemented with AlGaAs/GaAs heterojunction bipolar transistors (HBTs). This method makes use of radiative recombination in the base region of current-carrying HBTs. The infrared radiation emitted is 'visible' to the closed-circuit TV (CCTV) cameras. Therefore, one can view the operation of the HBT circuit under test at normal biases with a TV monitor. This method can be used to determine logic states of gates, as well as collector current of individual HBTs within integrated circuits.<> 相似文献
77.
K.W. Chang W.W. Sorin 《Photonics Technology Letters, IEEE》1989,1(3):68-70
A novel type of polarization-independent optical isolator is described which utilizes the polarization walkoff provided by birefringent crystals. Isolation of 44 dB is measured at a wavelength of 1.3 mu m with single-mode fiber insertion loss of 1.5 dB. A temperature-independent scheme is also described that provides at least 39 dB of isolation over the temperature range of 25-75 degrees C.<> 相似文献
78.
A coupled-mode equation for anisotropic waveguide systems of arbitrary cross section and general dielectric distribution is derived. Numerical results comparing the exact calculations to those of the method of Hardy et al. (Opt. Lett., vol.11, 742-4, 1986) show that the same accuracy can be obtained not only for TE, but also for TM mode coupling in the case of anisotropic waveguides, and the improved coupled-mode theory is applicable to the situation when moderately strong coupling occurs under the condition where the edge-to-edge separation of two coupled guides D 2 is about 0.1 μm 相似文献
79.
We report viscometric data collected in a Couette rheometry on dilute, single‐solvent polystyrene (PS)/dioctyl phthalate (DOP) solutions over a variety of polymer molecular weights (5.5 × 105 ≤ Mw ≤ 3.0 × 106 Da) and system temperatures (288 K ≤ T ≤ 318 K). In view of the essential viscometric features, the current data may be classified into three categories: The first concerns all the investigated solutions at low shear rates, where the solution properties are found to agree excellently with the Zimm model predictions. The second includes all sample solutions, except for high‐molecular‐weight PS samples (Mw ≥ 2.0 × 106 Da), where excellent time–temperature superposition is observed for the steady‐state polymer viscosity at constant polymer molecular weights. No similar superposition applies at a constant temperature but varied polymer molecular weights, however. The third appears to be characteristic of dilute high‐molecular‐weight polymer solutions, for which the effects of temperature on the viscosity curve are further complicated at high shear rates. The implications concerning the relative importance of hydrodynamic interactions, segmental interactions, and chain extensibility with increasing polymer molecular weight, system temperature, and shear rate are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 787–794, 2006 相似文献
80.
Anchoring Properties of Nematic Liquid Crystals Studied Using the Attenuated Total Reflection Method
By using the attenuated total reflection method associated with the excitation of surface plasmons, the tilt angle of the liquid crystal director and its gradient at the surface are measured in a planar nematic cell as a function of the applied voltage. The surface anchoring anisotropy δπ of the liquid crystal and the surface elastic constant ks, are found to be δπ = 0.288 erg/cm and ks, = 9·12 × 10-11 erg, respectively, when the boundary condition suggested by Barbero et al is used. The theoretical and experimental values obtained with this boundary condition and that of Mada are discussed. The results show that the boundary condition proposed by Barbero et al is in better agreement with the experiment. 相似文献