全文获取类型
收费全文 | 225352篇 |
免费 | 32705篇 |
国内免费 | 22758篇 |
专业分类
化学 | 115405篇 |
晶体学 | 1831篇 |
力学 | 11697篇 |
综合类 | 978篇 |
数学 | 21030篇 |
物理学 | 68870篇 |
无线电 | 61004篇 |
出版年
2024年 | 825篇 |
2023年 | 4956篇 |
2022年 | 6459篇 |
2021年 | 8530篇 |
2020年 | 8334篇 |
2019年 | 7644篇 |
2018年 | 6814篇 |
2017年 | 6650篇 |
2016年 | 9590篇 |
2015年 | 10003篇 |
2014年 | 12189篇 |
2013年 | 15732篇 |
2012年 | 18696篇 |
2011年 | 18915篇 |
2010年 | 13440篇 |
2009年 | 13434篇 |
2008年 | 14353篇 |
2007年 | 12944篇 |
2006年 | 12262篇 |
2005年 | 10560篇 |
2004年 | 7423篇 |
2003年 | 6252篇 |
2002年 | 5633篇 |
2001年 | 4798篇 |
2000年 | 4552篇 |
1999年 | 5187篇 |
1998年 | 4552篇 |
1997年 | 4084篇 |
1996年 | 4248篇 |
1995年 | 3534篇 |
1994年 | 3273篇 |
1993年 | 2693篇 |
1992年 | 2476篇 |
1991年 | 2111篇 |
1990年 | 1625篇 |
1989年 | 1184篇 |
1988年 | 950篇 |
1987年 | 741篇 |
1986年 | 726篇 |
1985年 | 616篇 |
1984年 | 427篇 |
1983年 | 308篇 |
1982年 | 263篇 |
1981年 | 175篇 |
1980年 | 128篇 |
1979年 | 72篇 |
1978年 | 40篇 |
1976年 | 42篇 |
1974年 | 50篇 |
1957年 | 48篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Jia-Zhu Wang 《IEEE transactions on bio-medical engineering》1994,41(5):470-479
The minimum-norm least-squares (MNLS) inverse for magnetic field measurements is applied to a representation of a sulcus of the human brain, where one or both walls have regions of neuronal activity. Simulations indicate that the magnetic source image (MSI) is largely confined to the appropriate wall of the sulcus, even for a depth of 4 cm where the distance between walls is only 3 mm. Two nearly oppositely oriented dipoles located 3 mm apart are found to be distinguished. Influences on the quality of the MSI by measurement noise and inaccuracy in determining the image surface are discussed in detail 相似文献
152.
Yanbin Luo Jianhui Zhang Alexandrov P. Fursin L. Zhao J.H. Burke T. 《Electron Device Letters, IEEE》2003,24(11):695-697
This letter reports the design and fabrication of 4H-SiC bipolar junction transistors with both high voltage (>1kV) and high dc current gain (/spl beta/=32) at a collector current level of I/sub c/=3.83A (J/sub c/=319 A/cm/sup 2/). An Al-free base ohmic contact has been used which, when compared with BJTs fabricated with Al-based base contact, shows clearly improved blocking voltage. A specific on-resistance of 17 m/spl Omega//spl middot/cm/sup 2/ has been achieved for collector current densities up to 289 A/cm/sup 2/. 相似文献
153.
Biao Chen Jianping Wang 《Selected Areas in Communications, IEEE Journal on》2003,21(7):1071-1080
One promising switching technology for wavelength-division multiplexing optical networks is optical burst switching (OBS). However, there are major deficiencies of OBS. (1) The delay offset between a control message and its corresponding data burst is based on the diameter of a network. This affects network efficiency, quality-of-service, and network scalability.( 2) OBS adopts one-way resource reservation scheme, which causes frequent burst collision and, thus, burst loss. We address the above two important issues in OBS. In particular, we study how to improve the performance of delay and loss in OBS. To reduce the end-to-end delay, we propose a hybrid switching scheme. The hybrid switching is a combination of lightpath switching and OBS switching. A virtual topology design algorithm based on simulated annealing to minimize the longest shortest path through the virtual topology is presented. To minimize burst collision and loss, we propose a new routing algorithm, namely, p-routing, for OBS network. The p-routing is based on the wavelength available probability. A path that has higher available probability is less likely to drop bursts due to collision. The probability-based p-routing can reduce the volatility, randomness, and uncertainty of one-way resource reservation. Our studies show that hybrid switching and p-routing are complementary and both can dramatically improve the performance of OBS networks. 相似文献
154.
155.
Sensitivity analysis of lossy coupled transmission lines 总被引:3,自引:0,他引:3
An analysis method, based on the numerical inversion of the Laplace transform, is described for the evaluation of the time domain sensitivity of networks that include lossy coupled transmission lines. The sensitivity can be calculated with respect to network components and parameters of the transmission lines. Sensitivity analysis is useful for waveform shaping and optimization. Examples and comparisons with sensitivity determined by perturbation are presented 相似文献
156.
The reduction of chromate by glutathione, its mono- and diethyl esters,
-cysteine, 2-mercapto-propionic acid and mercapto-succinic acid has been studied. The empirical rate law kobs = ab[Thiol]2/(1 + b[Thiol]) is obeyed by all of these thiols, at neutral pH. No evidence could be obtained for a first-order pathway in glutathione under these conditions. These observations are used to comment on the stability of intermediates potentially important in explaining chromate toxicity. 相似文献
157.
Main chain liquid crystal polymers are modelled as either worms or jointed rods. In reality they are composed of mesogenic units (rods) linked by spacers with varying degrees of flexibility. We present a molecular model to describe non-homogeneous nematic polymers. The model takes account of molecular parameters, such as the lengths of the mesogenic group and the spacer units, and the interactions between them. The spacers are found to have an order differing from the mesogenic units. If the spacer is not very long and thus in effect is inflexible, one end of the spacer can retain to some extent the orientation of the other end, allowing orientational correlation between spacers mediated by the intermediate mesogenic unit. This is important in giving the chain a global rod-like behaviour as the nematic field becomes strong or the temperature low. The nematic order of the two components (mesogens and spacers), the nematic-isotropic transition as well as the latent entropy are examined. Furthermore, the anisotropic conformations of the polymers are investigated, which show either rod-like or random walk behaviour. Comparison of our results with experiment is found to be satisfactory. 相似文献
158.
Improvements in the performance of silicon photovoltaic cells for solar applications are adapted for nonsolar photovoltaic applications. Improved monochromatic light efficiencies above 45% are reported including efficiencies close to 40% for relatively long-wavelength (1.064 μm) light as produced by neodymium-doped yttrium-aluminum garnet (Nd:YAG) lasers 相似文献
159.
A novel soft decision-based median subtraction filter is presented for clutter suppression and infrared (IR) point targets enhancement. The decision is made based on a jump Markov model and its state and parameter estimation using a particle filter. The scheme is compared with other conventional clutter background removal techniques and good results are obtained. 相似文献
160.
Ten-user truly asynchronous gigabit OCDMA transmission experiment with a 511-chip SSFBG en/decoder 总被引:8,自引:0,他引:8
Hamanaka T. Xu Wang Naoya Wada Nishiki A. Kitayama K. 《Lightwave Technology, Journal of》2006,24(1):95-102
A ten-user truly asynchronous gigabit coherent-optical-code-division-multiple-access (OCDMA) transmission was experimentally demonstrated without using any timing coordination. The enabling technologies are a record-length 511-chip superstructured-fiber-Bragg-grating (SSFBG) en/decoder and a supercontinuum (SC)-based optical-thresholding technique to significantly suppress the signal interference beat noise as well as the multiple-access-interference (MAI) noise. 相似文献