全文获取类型
收费全文 | 39149篇 |
免费 | 4751篇 |
国内免费 | 3557篇 |
专业分类
化学 | 20541篇 |
晶体学 | 286篇 |
力学 | 1948篇 |
综合类 | 273篇 |
数学 | 3282篇 |
物理学 | 10647篇 |
无线电 | 10480篇 |
出版年
2024年 | 151篇 |
2023年 | 916篇 |
2022年 | 1014篇 |
2021年 | 1381篇 |
2020年 | 1371篇 |
2019年 | 1280篇 |
2018年 | 1103篇 |
2017年 | 1053篇 |
2016年 | 1512篇 |
2015年 | 1596篇 |
2014年 | 1939篇 |
2013年 | 2558篇 |
2012年 | 3098篇 |
2011年 | 3126篇 |
2010年 | 2117篇 |
2009年 | 2142篇 |
2008年 | 2273篇 |
2007年 | 2147篇 |
2006年 | 1998篇 |
2005年 | 1720篇 |
2004年 | 1246篇 |
2003年 | 1134篇 |
2002年 | 1013篇 |
2001年 | 805篇 |
2000年 | 859篇 |
1999年 | 940篇 |
1998年 | 843篇 |
1997年 | 772篇 |
1996年 | 870篇 |
1995年 | 689篇 |
1994年 | 629篇 |
1993年 | 538篇 |
1992年 | 505篇 |
1991年 | 409篇 |
1990年 | 324篇 |
1989年 | 239篇 |
1988年 | 186篇 |
1987年 | 160篇 |
1986年 | 156篇 |
1985年 | 131篇 |
1984年 | 100篇 |
1983年 | 67篇 |
1982年 | 46篇 |
1981年 | 38篇 |
1980年 | 21篇 |
1978年 | 23篇 |
1977年 | 25篇 |
1976年 | 21篇 |
1975年 | 34篇 |
1974年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Supplementary proof for "Exact and approximate construction of digital phase modulations by superposition of AMP" by P. A. Laurent 总被引:1,自引:0,他引:1
Kaibin Huang 《Communications, IEEE Transactions on》2005,53(2):234-237
In this letter, we supplement the derivation in Laurent's paper by mathematically proving that combining binary continuous phase modulation (CPM) signals in different bit durations results in the decomposed CPM expression consisting of a set of continuous amplitude-modulated pulse functions. 相似文献
12.
A Low-Power CMOS Linear-in-Decibel Variable Gain Amplifier With Programmable Bandwidth and Stable Group Delay 总被引:1,自引:0,他引:1
Tsou S.-C. Li C.-F. Huang P.-C. 《Circuits and Systems II: Express Briefs, IEEE Transactions on》2006,53(12):1436-1440
This brief presents a new circuit architecture for linear-in-decibel, constant-bandwidth variable gain amplifier (VGA). To obtain high linearity under low-voltage operation, this VGA is a closed-loop structure. In loop amplifier design, two techniques are applied: first, the loop amplifier is given finite input impedance. This arrangement keeps the VGA bandwidth constant under different gain setting. Second, a current-buffered compensation is applied for loop stability. Compared to the Miller compensation, this method achieves wider bandwidth. The prototype chip using 0.18-mum CMOS technology demonstrates that -10- to 20-dB gain and 0.5- to 30-MHz bandwidth can be programmed independently. The group delay difference within 30-dB gain control range is smaller than 1%. The total circuit dissipates 1.35 mA from a 1.8-V supply 相似文献
13.
J. Song H. Jiang Z.J. Liu D.Y. Khang Y. Huang J.A. Rogers C. Lu C.G. Koh 《International Journal of Solids and Structures》2008,45(10):3107-3121
A finite-deformation theory is developed to study the mechanics of thin buckled films on compliant substrates. Perturbation analysis is performed for this highly nonlinear system to obtain the analytical solution. The results agree well with experiments and finite element analysis in wavelength and amplitude. In particular, it is found that the wavelength depends on the strain. Based on the accurate wavelength and amplitude, the membrane and peak strains in thin films, and stretchability and compressibility of the system are also obtained analytically. 相似文献
14.
Current methodologies used for the inference of thin film stress through curvature measurement are strictly restricted to stress and curvature states that are assumed to remain uniform over the entire film/substrate system. These methodologies have recently been extended to a single layer of thin film deposited on a substrate subjected to the non-uniform misfit strain in the thin film. Such methodologies are further extended to multi-layer thin films deposited on a substrate in the present study. Each thin film may have its own non-uniform misfit strain. We derive relations between the stresses in each thin film and the change of system curvatures due to the deposition of each thin film. The interface shear stresses between the adjacent films and between the thin film and the substrate are also obtained from the system curvatures. This provides the basis for the experimental determination of thin film stresses in multi-layer thin films on a substrate. 相似文献
15.
16.
17.
关于海面无线传播模型的探讨 总被引:3,自引:0,他引:3
现有文献中一般都认为海面环境的无线传播模型近似于自由空间传播模型,但在实际测试中发现,自由空间模型预测值与实测值差别较大,不能很好地解释此类环境的信号传播。笔者根据多次海面覆盖测试的数据,分析并拟合出了新的海面传播模型。 相似文献
18.
A closed but approximate formula of Green’s function for an arbitrary aggregate of cubic crystallites is given to derive the e?ective elastic sti?ness tensor of the polycrystal. This formula, which includes three elastic constants of single cubic c… 相似文献
19.
Xin‐Gui Li Run‐Feng Chen Mei‐Rong Huang Mei‐Fang Zhu Qun Chen 《Journal of polymer science. Part A, Polymer chemistry》2004,42(9):2073-2092
A new series of copolymers was synthesized through the oxidative polymerization of pyrrole (PY) and o‐phenetidine (PHT) with inorganic oxidants in acidic media. The polymerization parameters including the mixing method of the oxidant with the comonomer, the comonomer ratio, the time, the temperature, the oxidant, the organic medium, and the acid were systematically optimized for the synthesis of copolymers with high yields, intrinsic viscosities, and solubility. The resultant copolymers were characterized by elemental analysis, infrared, ultraviolet–visible, solution high‐resolution 1H NMR and solid‐state high‐resolution 13C NMR, circular dichroism spectroscopy, and cyclic voltammetry. The results showed that the PY observed content in the copolymers was much higher than the PY feed content. The regular variation of the polymerization yield, intrinsic viscosity, solubility, macromolecular structure, and electroactivity of the resulting polymers with the comonomer ratio, together with the complete solubility of a PY/PHT (10/90) polymer in highly polar solvents, indicated the formation of real random copolymers containing both PY and PHT units rather than a mixture of two homopolymers. However, the polymers containing more than 59 mol % PY were not homogeneous copolymers consisting of soluble and insoluble parts. A semiquantitative relationship between the polymerization yield or solubility of the copolymers and the polarity index of the organic solvents was examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2073–2092, 2004 相似文献
20.
Yu‐Hsiang Hu Chuh‐Yung Chen Chen‐Chien Wang Yao‐Hui Huang Shao‐Ping Wang 《Journal of polymer science. Part A, Polymer chemistry》2004,42(19):4976-4993
The bulk polymerization of styrene initiated by ?‐caprolactam (CL) and n‐dodecyl mercaptan (RSH) has been explored. This novel polymerization system shows living characteristics. For example, the molecular weight of the resulting polymers increases with conversion, and the system has the ability to form diblock copolymers and so forth. The polymer chain end contains thiol and lactam structures, which we have investigated with Fourier transform infrared, 1H NMR, and 13C NMR techniques. Electron spin resonance spectra and theoretical calculations by the Hartree–Fock methods have been used to examine the mechanism. The results reveal that the initial polymerization starts from thiol via a chain‐transfer reaction, and the propagation proceeds by the insertion of a monomer between the terminal group and the intermediate structure of lactam. Finally, the polymerization kinetics have been examined. The polymerization rate varies linearly with the concentration of CL and RSH, and this confirms the mechanism. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4976–4993, 2004 相似文献