全文获取类型
收费全文 | 20048篇 |
免费 | 383篇 |
国内免费 | 76篇 |
专业分类
化学 | 6661篇 |
晶体学 | 358篇 |
力学 | 1025篇 |
综合类 | 3篇 |
数学 | 1790篇 |
物理学 | 7847篇 |
无线电 | 2823篇 |
出版年
2023年 | 79篇 |
2022年 | 200篇 |
2021年 | 184篇 |
2020年 | 116篇 |
2019年 | 133篇 |
2018年 | 236篇 |
2017年 | 199篇 |
2016年 | 390篇 |
2015年 | 280篇 |
2014年 | 438篇 |
2013年 | 888篇 |
2012年 | 1012篇 |
2011年 | 1291篇 |
2010年 | 1008篇 |
2009年 | 1042篇 |
2008年 | 1162篇 |
2007年 | 1247篇 |
2006年 | 1254篇 |
2005年 | 993篇 |
2004年 | 851篇 |
2003年 | 673篇 |
2002年 | 634篇 |
2001年 | 1060篇 |
2000年 | 733篇 |
1999年 | 599篇 |
1998年 | 400篇 |
1997年 | 421篇 |
1996年 | 346篇 |
1995年 | 283篇 |
1994年 | 272篇 |
1993年 | 246篇 |
1992年 | 257篇 |
1991年 | 229篇 |
1990年 | 207篇 |
1989年 | 170篇 |
1988年 | 126篇 |
1987年 | 108篇 |
1986年 | 80篇 |
1985年 | 93篇 |
1984年 | 70篇 |
1983年 | 54篇 |
1982年 | 41篇 |
1981年 | 44篇 |
1980年 | 32篇 |
1978年 | 24篇 |
1977年 | 26篇 |
1976年 | 36篇 |
1975年 | 31篇 |
1974年 | 28篇 |
1973年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Fabrication of bamboo-shaped GaN nanorods 总被引:1,自引:0,他引:1
H. Li J.Y. Li M. He X.L. Chen Z. Zhang 《Applied Physics A: Materials Science & Processing》2002,74(4):561-562
Bamboo-shaped GaN nanorods were formed through a simple sublimation method. They were characterized by means of X-ray powder
diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area
electron diffraction (SAED). The TEM image showed that the nanorods were bamboo-like. XRD, HRTEM and SAED patterns indicated
that the nanorods were single-crystal wurtzite GaN.
Received: 8 January 2001 / Accepted: 28 April 2001 / Published online: 20 December 2001 相似文献
102.
103.
A novel mode-size transformer based on interference between guided and leaky modes is proposed and analyzed. Simulation shows significant improvement in spot-size transform efficiency per unit length, in comparison with the conventional tapered waveguide mode size converters based on mode evolution. Owing to its structural simplicity, easy fabrication is another merit of the new spot-size transformer 相似文献
104.
For past I see ibid., vol.4, no.4, p.405 (1995). The use of the image model of Part I is investigated in the context of image compression. The model decomposes the image into a primary component that contains the strong edge information, a smooth component that represents the background slow-intensity variations, and a texture component that contains the textures. The primary component, which is known to be perceptually important, is encoded separately by encoding the intensity and geometric information of the strong edge brim contours. Two alternatives for coding the smooth and texture components are studied: entropy-coded adaptive DCT and entropy-coded subband coding. It is shown via simulations that the proposed schemes, which can be thought of as a hybrid of waveform coding and feature-based coding techniques, result in both subjective and objective performance improvements over several other image coding schemes and, in particular, over the JPEG continuous-tone image compression standard. These improvements are especially noticeable at low bit rates. Furthermore, it is shown that a perceptual tuning based on the contrast-sensitivity of the human visual system can be used in the DCT-based scheme, which in conjunction with the three-component model, leads to additional subjective performance improvements. 相似文献
105.
This paper deals with the optimal control of a one-machine two-product manufacturing system with setup changes, operating in a continuous time dynamic environment. The system is deterministic. When production is switched from one product to the other, a known constant setup time and a setup cost are incurred. Each product has specified constant processing time and constant demand rate, as well as an infinite supply of raw material. The problem is formulated as a feedback control problem. The objective is to minimize the total backlog, inventory and setup costs incurred over a finite horizon. The optimal solution provides the optimal production rate and setup switching epochs as a function of the state of the system (backlog and inventory levels). For the steady state, the optimal cyclic schedule is determined. To solve the transient case, the system's state space is partitioned into mutually exclusive regions such that with each region, the optimal control policy is determined analytically. 相似文献
106.
Experiments for the determination of mono-, di and tri-butyltin (MBT, DBT and TBT) by hydride generation/gas chromatography/atomic absorption spectrometry in various matrices (sediment, suspended matter, mussel, algae and water) have revealed that poor butyltin recoveries are obtained in sediments displaying high sulphur and hydrocarbon contents; very poor recoveries were also observed for TBT in sediments with high chlorophyll pigment contents as well as in algal samples. It was however not clear whether the hydride generatin was inhibited by these infering compounds, as was previously assumed in the case of hydrocarbons, or whether interferences affected the atomization rate. Further studies were performed to solve this problem in order to validate this method in the case of analyses of, for example, oil-contaminated sediment and algae. This paper presents the results obtained. It is concluded here that the poor recoveries were due to an inhibition of hydride generation rather than to interference at the atomization stage. 相似文献
107.
The authors present a method for solving a class of optimization problems with nonsmooth constraints. In particular, they apply the method to the design of narrowband minimum-power antenna array processors which are robust in the presence of errors such as array element placement, look direction misalignment, and frequency offset. They first show that the constrained minimum power problem has a unique global minimum provided that the constraint set is nonempty. Then it is shown how the design problem derived directly from considerations of the sensitivity of the antenna array processor to errors can be transformed into a quadratic programming problem with linear inequality constraints which can be solved efficiently by the standard active set strategy. They also present numerical results for two types of nonsmooth constraints developed to provided robustness. These results confirm the effectiveness of the method 相似文献
108.
L. B. Sjogren H. -X. L. Liu X. -H. Qin C. W. Domier N. C. Luhmann 《International Journal of Infrared and Millimeter Waves》1993,14(6):1201-1216
Device alternatives for millimeter-wave beam control arrays are examined. For the control function, a wide range of semiconductor device as well as bulk material alternatives are considered. In addition, alternatives to the traditional metal strip are considered for the passive electromagnetic array element. This work should provide a broader perspective regarding the choices available for the optimized design of new beam control arrays. 相似文献
109.
The gravitational interaction between grooves machined in a hollow cylindrical mass of uniform density, and an external point mass, is derived in terms of the Associated Legendre functions, and the parametric form of the coupling coefficients is presented. The cross-sections of the grooves, which are regularly spaced in azimuth, are in the form of truncated sectors of the cylinder's end-faces. This theory is applied to the test-masses for the Satellite Test of the Equivalence Principle (STEP) experiment, for which four grooves have been assumed, and an expression for the axialforce is derived which is more than 104 times faster to compute than a Monte-Carlo integration of similar accuracy. Following this analysis it is suggested that the STEP test-masses should carry at least 6 grooves. This theory has wider application to gravitational problems involving general sectored cylindrical bodies. 相似文献
110.
Gabriel C.M.W. Heffels J.J.M. Hou X. Ouibrahim H. 《Electronics & Communication Engineering Journal》1993,5(2):103-112
Introduction of the broadband integrated services digital network (B-ISDN) will enable network users to make use of advanced multimedia, multiparty services. With this aim in view, RACE II project R2044, MAGIC, is investigating long-term signalling requirements, architecture and protocols for B-ISDN. New service concepts are being developed in MAGIC which will provide more flexibility when compared to narrowband ISDN (N-ISDN). The service requirements and other signalling requirements, such as the separation between call and associated connections at the network-node interface, are used to determine the distribution of functions over the network. The signalling protocols which are used to exchange the signalling information are subsequently developed. These signalling protocols are simulated to prove the performance and the feasibility of call handling concepts 相似文献