全文获取类型
收费全文 | 43046篇 |
免费 | 6590篇 |
国内免费 | 5096篇 |
专业分类
化学 | 24466篇 |
晶体学 | 512篇 |
力学 | 1710篇 |
综合类 | 338篇 |
数学 | 3360篇 |
物理学 | 11551篇 |
无线电 | 12795篇 |
出版年
2024年 | 178篇 |
2023年 | 973篇 |
2022年 | 1288篇 |
2021年 | 1599篇 |
2020年 | 1579篇 |
2019年 | 1543篇 |
2018年 | 1306篇 |
2017年 | 1368篇 |
2016年 | 1865篇 |
2015年 | 2061篇 |
2014年 | 2455篇 |
2013年 | 3021篇 |
2012年 | 3666篇 |
2011年 | 3741篇 |
2010年 | 2760篇 |
2009年 | 2716篇 |
2008年 | 3056篇 |
2007年 | 2777篇 |
2006年 | 2608篇 |
2005年 | 2172篇 |
2004年 | 1604篇 |
2003年 | 1389篇 |
2002年 | 1324篇 |
2001年 | 1052篇 |
2000年 | 894篇 |
1999年 | 861篇 |
1998年 | 681篇 |
1997年 | 610篇 |
1996年 | 557篇 |
1995年 | 505篇 |
1994年 | 465篇 |
1993年 | 381篇 |
1992年 | 315篇 |
1991年 | 307篇 |
1990年 | 250篇 |
1989年 | 176篇 |
1988年 | 120篇 |
1987年 | 129篇 |
1986年 | 88篇 |
1985年 | 75篇 |
1984年 | 62篇 |
1983年 | 36篇 |
1982年 | 34篇 |
1981年 | 26篇 |
1980年 | 21篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
181.
Two techniques for modelling intermodulation-borne spurious components in fractional-N frequency synthesisers are described. The first techniques establishes the potential frequencies of these components, while the second considers a more comprehensive model to predict their distribution in a realistic system as a result of one particular intermodulation mechanism. Good agreement is found and it is apparent that intermodulation is likely to be a major cause of such spurious components. 相似文献
182.
Measurement of small-signal and large-signal responses of packaged laser modules at high temperature
Ning Hua Zhu Ji Min Wen Hai Peng Song Shang Jian Zhang Liang Xie 《Optical and Quantum Electronics》2006,38(15):1245-1257
In this paper, the pulsed injection method is extended to measure the chip temperature of various packaged laser modules,
such as the DFB laser modules, the FP laser modules, and the EML laser modules. An optimal injection condition is obtained
by investigating the dependence of the lasing wavelength on the width and period of the injection pulse in a relatively wide
temperature range. The small-signal frequency responses and large-signal performances of packaged laser modules at different
chip temperature are measured. The adiabatic small-signal modulation characteristics of packaged LD are first extracted. In
the large-signal measurement, the effects of chip temperature, bias current and driving signal on the performances of the
laser modules are discussed. It has been found that the large-signal performances of the EML modules depend on the different
red-shift speeds of the DFB and EAM sections as chip temperature varying, and the optimal characteristics may be achieved
at higher temperature. 相似文献
183.
Min Yan Min Qiu 《Photonics Technology Letters, IEEE》2007,19(22):1804-1806
The anisotropic finite-element mesh is deployed in a vector finite-element method to numerically derive surface plasmon polaritons (SPPs) more efficiently. The superiority of the anisotropic mesh lies in the fact that SPP modes, predictably, have a sharp field variation in the surface-normal direction, whereas the variation along the surface-tangent direction is comparably gentle. A case study on metallic fiber shows that the employment of anisotropic finite elements reduces the problem size by a factor of seven. 相似文献
184.
Yan Zhao Xingbin Yang Ru Jiang Xiaoli Sun Wenmin Liu Shengyong Zhang 《Analytical sciences》2006,22(5):747-751
The investigation on capillary electrophoretic enantioseparation of six synthetic compounds containing vicinal diol groups has been undertaken to acquire the optimum conditions using native beta-cyclodextrin (beta-CD) as chiral selector and borate as a background electrolyte. The separation was carried out in an uncoated capillary (58.5 cm x 75 microm i.d., effective length 48.5 cm) and the effects of several important factors were investigated in detail. The results showed that beta-CD as a chiral selector exhibited good enantioselectivity and that the enantioseparation was greatly influenced by the structure of the diols, the borate concentration and the buffer pH. The optimum performance was obtained for the chiral vicinal diols under the conditions of 200 mM borate buffer of pH 9.8 containing 1.7% beta-CD at an applied voltage of 15 kV and a capillary temperature of 20 degrees C. Under the conditions, four diols were baseline separated with fast analysis time and the good theoretical plate numbers (above 10 x 10(4)) and favorable migration-time reproducibilities (RSDs below 3.0%) were obtained. The separation results were satisfactory. 相似文献
185.
J.-S. Kang G. Kim S.S. Lee S. Choi S. Cho S.W. Han H.J. Shin B.I. Min 《Journal of magnetism and magnetic materials》2006
The spatial concentration distribution and local electronic structure of ferromagnetic Ge1−xTx (T=Cr, Mn, Fe) DMS single crystals have been investigated by using scanning photoelectron microscopy (SPEM), X-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). It is found that doped T ions in Ge1−xTx crystals are chemically phase-separated, suggesting that the observed ferromagnetism arises from the phase-separated T-rich phases in Ge1−xTx. 相似文献
186.
A simple “chimney” method was used to eliminate the voids in an arrangement of quantum dots sized 2 nm on a solid substrate, which resulted in a large well ordered superlattice of area in the order more than 1 μm2. Based on the principle of speeding up the interparticle interaction of nanoparticles to overcome the particle-substrate one, a lateral centripetal force originated from a glass tube acting as a chimney in a simple evaporation device is imposed. This method allows the packing process to be controlled in a mechanical force field, that is, with the same nanogold dispersion different patterns on a substrate—from separate dots to an ordered compact monolayer or even a multilayer structure—could be easily obtained. 相似文献
187.
188.
环形线电荷的电场分布 总被引:30,自引:10,他引:20
给出了圆环形线电荷电场分布的理论公式,并利用计算机计算出其具体分布规律。 相似文献
189.
双折射双频激光器偏振特性的分析 总被引:6,自引:1,他引:5
用激光原理和晶体光学原理分析了腔内加旋光晶体的激光器输出光的偏振特性,实验和分析表明,激光器输出两束不同频率的垂直线偏振光,激光器两端输出光的偏振面有一夹角a,且此夹角与晶轴和传光方向的夹角θ有关。理论计算结果在双折射双频激光器的实验结果符合很好。 相似文献
190.
Barron C.C. Mahon C.J. Thibeault B.J. Wang G. Jiang W. Coldren L.A. Bowers J.E. 《Quantum Electronics, IEEE Journal of》1995,31(8):1484-1493
We have designed, fabricated, and characterized GaAs-AlGaAs (λ=864 nm) asymmetric Fabry-Perot modulators with ≈37 GHz modulation frequency response, comparable to the fastest waveguide modulators. The modulation response saturates at high optical powers due to saturation of the excitonic absorption and heating effects, but the frequency response is independent of the incident optical intensity, since it depends only on the RC time constant, and not on the carrier transit time. The device design takes advantage of the fact that the quantum-confined Stark effect is more pronounced at some distance from the absorption edge to achieve a modulator with ⩾20 dB contrast and ≈3 dB insertion loss for ±2 V operating voltage, but only 21 fF capacitance. The DC bias used to move the operating point off the absorption edge has the additional benefits of improving the linearity and chirp of the device, as well as the saturation intensity. Here we present measurements of the modulation and photocurrent responses of the modulators, calculate the RC and transit times for the device, analyze the saturation mechanisms, and discuss the linearity and chirp of the device from the perspective of a high-speed digital optical communications system 相似文献