全文获取类型
收费全文 | 378771篇 |
免费 | 3889篇 |
国内免费 | 1145篇 |
专业分类
化学 | 165192篇 |
晶体学 | 5382篇 |
力学 | 15540篇 |
综合类 | 7篇 |
数学 | 38561篇 |
物理学 | 109831篇 |
无线电 | 49292篇 |
出版年
2021年 | 3524篇 |
2020年 | 3800篇 |
2019年 | 4261篇 |
2018年 | 5739篇 |
2017年 | 5889篇 |
2016年 | 8091篇 |
2015年 | 4393篇 |
2014年 | 7686篇 |
2013年 | 17116篇 |
2012年 | 13133篇 |
2011年 | 15710篇 |
2010年 | 11980篇 |
2009年 | 12463篇 |
2008年 | 15142篇 |
2007年 | 15656篇 |
2006年 | 14540篇 |
2005年 | 12783篇 |
2004年 | 11923篇 |
2003年 | 10705篇 |
2002年 | 10613篇 |
2001年 | 12302篇 |
2000年 | 9556篇 |
1999年 | 7638篇 |
1998年 | 6598篇 |
1997年 | 6407篇 |
1996年 | 5932篇 |
1995年 | 5226篇 |
1994年 | 5142篇 |
1993年 | 4984篇 |
1992年 | 5414篇 |
1991年 | 5519篇 |
1990年 | 5234篇 |
1989年 | 4963篇 |
1988年 | 4642篇 |
1987年 | 4429篇 |
1986年 | 4114篇 |
1985年 | 5145篇 |
1984年 | 5188篇 |
1983年 | 4314篇 |
1982年 | 4320篇 |
1981年 | 4024篇 |
1980年 | 3908篇 |
1979年 | 4156篇 |
1978年 | 4125篇 |
1977年 | 4117篇 |
1976年 | 4067篇 |
1975年 | 3880篇 |
1974年 | 3778篇 |
1973年 | 3819篇 |
1972年 | 2734篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The saturation of the 2.71 μm laser output has been measured in erbium-doped ZBLAN singlemode fibres with Er3+ concentrations of 1000, 5000 and 10000 ppm mol. Limits of the single-mode-laser output are discussed 相似文献
992.
Mastrapasqua M. Luryi S. Belenky G.L. Garbinski P.A. Cho A.Y. Sivco D.L. 《Electron Devices, IEEE Transactions on》1993,40(8):1371-1377
A monolithic multiterminal logic device that functions both optically and electrically as an ORNAND gate, is demonstrated for the first time. The device, based on the real-space transfer of hot electrons into a complementary collector layer, has been implemented in an InGaAs/InAlAs/InGaAs heterostructure grown by molecular beam epitaxy. Excellent performance is obtained at room temperature. The collector current and the optical output power both exhibit the OR and the NAND functions of any two of the three input terminals, these functions being interchangeable by the voltage on the third terminal 相似文献
993.
Characteristics of thermally expanded core fiber 总被引:1,自引:0,他引:1
Thermally expanded core (TEC) fiber is expected to reduce fiber-to-fiber and fiber-to-laser diode connection loss. This paper describes the characteristics of TEC fiber theoretically and experimentally. We reveal theoretically that when fabricating TEC fiber the mode field diameter (MFD) is enlarged more effectively by increasing the heating temperature rather than the heating time. In the 1300-1600°C temperature range with heating times between 0 and 60 min, it is necessary to control the temperature accurately so that no deviation from the target temperature is more than ±30°C. This is in order to ensure that any connection loss caused by MFD mismatch is less than 0.1 dB. We show experimentally that the propagation loss of TEC fiber is dependent on the heating region and wavelength by using a micro burner with a propane/oxygen flame. Based on the relationship between the loss characteristics and the expanded MFD, we suggest a method for nondestructively measuring the MFD in TEC fibers 相似文献
994.
Ku T.K. Chen S.H. Yang C.D. She N.J. Wang C.C. Chen C.F. Hsieh I.J. Cheng H.C. 《Electron Device Letters, IEEE》1996,17(5):208-210
Undoped and phosphorus (P)-doped diamond-clad Si field emitter arrays have been successfully fabricated using microwave plasma chemical vapor deposition (MPCVD) technology. The electron emission from the blunt diamond-clad microtips are much higher than those for the pure Si tips with sharp curvature due to a lower work function. Furthermore, the characteristics of emission current against applied voltage for the P-doped diamond-clad tips show superior emission at lower field to the undoped ones. After the examination of Auger electron spectroscopy (AES) and electrical characteristics of as-grown diamond, such a significant enhancement of the electron emission from the P-doped diamond-clad tips is attributed to a higher electron conductivity and defect densities 相似文献
995.
WKB近似下的Fourier衍射成象方法 总被引:1,自引:0,他引:1
对于介质目标微波衍射成象,本文引入了WKB近似来模拟目标内部总场。基于这种近似,我们导出了Fourier衍射公式,并采用了广义滤波逆传播方法由目标空间谱实现目标特性的重建。计算机模拟结果表明采用WKB近似重建目标特性较Born近似有明显改善。 相似文献
996.
M. Zandian J. G. Pasko J. M. Arias R. E. De Wames S. H. Shin 《Journal of Electronic Materials》1995,24(5):681-684
Measurements of 77K RoA and 300K reverse bias dynamic impedance (RdA) products at one volt reverse bias has been carried out to assess the degree of correlation of this figure of merit. Planar
P-on-n heterostructures were grown on near lattice-matched CdZnTe substrates with Hg1-xCdxTe (0.20< x <0.30) by molecular beam epitaxy. These devices were passivated with CdTe and doped with indium and arsenic as
n- and p-type dopants, respectively. Current-voltage characteristic of these devices exhibit thermally generated dark currents
at small and modest reverse bias. We have observed that RoA values of these long wavelength infrared P-on-n heterostructure photodiodes at 77K correlate with room temperature RdA values. Diode arrays with high room temperature RdA values at one volt reverse bias also have high RoA values at 77K. Similarly, low RdA values at room temperature indicate poor performance at 77K where deviation from diffusion current occurs at reverse bias
of 0.2 to 1 volt at room temperature. The results presented here, for a small samples of devices, demonstrate that room temperature
measurements of current-voltage characteristics to evaluate Hg1-xCdxTe (0.22< x <0.28) diode performance and array uniformity at lower temperatures can be used. This provides an acceptable criteria
for further study at lower temperatures. 相似文献
997.
Bosacchi A. Franchi S. Gombia E. Mosca R. Fantini F. Menozzi R. 《Electronics letters》1993,29(8):651-653
Outstanding stability has been observed in Al/Al/sub x/Ga/sub 1-x/As and Al/GaAs/Al/sub x/Ga/sub 1-x/As (x=0.25) Schottky barriers prepared by depositing Al in situ by MBE on annealing up to 400 degrees C. Conventionally evaporated barriers have been fabricated and compared with epitaxial ones. The changes in barrier height and ideality factor induced by annealing are reported.<> 相似文献
998.
A precoding scheme for noise whitening on intersymbol interference (ISI) channels is presented. This scheme is compatible with trellis-coded modulation and, unlike Tomlinson precoding, allows constellation shaping. It can be used with almost any shaping scheme, including the optimal SVQ shaping, as opposed to trellis precoding, which can only be used with trellis shaping. The implementation complexity of this scheme is minimal-only three times that of the noise prediction filter, hence effective noise whitening can be achieved by using a high-order predictor 相似文献
999.
A wideband low-noise pseudomorphic HEMT MMIC variable-gain amplifier has been designed and fabricated. The amplifier has a nominal gain of 13 dB across the band 2-20 GHz, with gain flatness better than ±0.4 dB. The noise figure is less than 3 dB across the band 6-16 GHz. An on-chip temperature-sensing diode is used to provide a linear temperature correction which has been used to reduce the gain variation of the amplifier by a factor of 2 across the temperature range -50°C to +95°C 相似文献
1000.
Yayla G. Krishnamoorthy A.V. Marsden G.C. Esener S.C. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1994,82(11):1749-1762
We report the implementation of a prototype three-dimensional (3D) optoelectronic neural network that combines free-space optical interconnects with silicon-VLSI-based optoelectronic circuits. The prototype system consists of a 16-node input, 4-neuron hidden, and a single-neuron output layer, where the denser input-to-hidden-layer connections are optical. The input layer uses PLZT light modulators to generate optical outputs which are distributed over an optoelectronic neural network chip through space-invariant holographic optical interconnects. Optical interconnections provide negligible fan-out delay and allow compact, purely on-chip electronic H-tree type fan-in structure. The small prototype system achieves a measured 8-bit electronic fan-in precision and a calculated maximum speed of 640 million interconnections per second. The system was tested using synaptic weights learned off system and was shown to distinguish any vertical line from any horizontal one in an image of 4×4 pixels. New, more efficient light detector and small-area analog synapse circuits and denser optoelectronic neuron layouts are proposed to scale up the system. A high-speed, feed-forward optoelectronic synapse implementation density of up to 104/cm2 seems feasible using new synapse design. A scaling analysis of the system shows that the optically interconnected neural network implementation can provide higher fan-in speed and lower power consumption characteristics than a purely electronic, crossbar-based neural network implementation 相似文献