全文获取类型
收费全文 | 101782篇 |
免费 | 15729篇 |
国内免费 | 11107篇 |
专业分类
化学 | 55331篇 |
晶体学 | 725篇 |
力学 | 4991篇 |
综合类 | 428篇 |
数学 | 8899篇 |
物理学 | 30555篇 |
无线电 | 27689篇 |
出版年
2024年 | 439篇 |
2023年 | 2515篇 |
2022年 | 3162篇 |
2021年 | 3936篇 |
2020年 | 3990篇 |
2019年 | 3735篇 |
2018年 | 3209篇 |
2017年 | 3084篇 |
2016年 | 4397篇 |
2015年 | 4658篇 |
2014年 | 5541篇 |
2013年 | 7020篇 |
2012年 | 8662篇 |
2011年 | 8535篇 |
2010年 | 6244篇 |
2009年 | 6084篇 |
2008年 | 6486篇 |
2007年 | 5802篇 |
2006年 | 5442篇 |
2005年 | 4618篇 |
2004年 | 3551篇 |
2003年 | 3133篇 |
2002年 | 3050篇 |
2001年 | 2593篇 |
2000年 | 2306篇 |
1999年 | 2342篇 |
1998年 | 1961篇 |
1997年 | 1751篇 |
1996年 | 1739篇 |
1995年 | 1454篇 |
1994年 | 1263篇 |
1993年 | 1023篇 |
1992年 | 928篇 |
1991年 | 797篇 |
1990年 | 637篇 |
1989年 | 447篇 |
1988年 | 397篇 |
1987年 | 304篇 |
1986年 | 279篇 |
1985年 | 239篇 |
1984年 | 179篇 |
1983年 | 129篇 |
1982年 | 96篇 |
1981年 | 76篇 |
1980年 | 77篇 |
1979年 | 39篇 |
1978年 | 35篇 |
1977年 | 46篇 |
1976年 | 39篇 |
1973年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
981.
Kerr介质中级联三能级原子与压缩相干态相互作用的光子统计演化 总被引:6,自引:0,他引:6
本文研究了存在Kerr介质的高Q腔中压缩相干态与级联三能级原子相互作用的光子统计演化规律,讨论了光场参数、Kerr介质及相干振幅对平均光子数的影响. 相似文献
982.
L. B. Sjogren H-X. L. Liu X-H. Qin C. W. Domier N. C. Luhmann Jr. 《Journal of Infrared, Millimeter and Terahertz Waves》1993,14(8):1509-1529
New array design concepts are described for the phase and amplitude control of millimeter and submillimeter-wave beams. Phase shifter array designs providing increased phase range and wider bandwidth are described. Techniques involving the integration of gain-producing elements as well as tuning elements on a single array are proposed for application to high-performance beam control and beam shaping. These concepts should facilitate the further development of quasi-optical solid state device-based arrays for application to millimeter-wave electronic systems. 相似文献
983.
Esarey E. Ting A. Sprangle P. Umstadter D. Liu X. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1993,21(1):95-104
A linearly polarized, ultraintense laser field induces transverse plasma currents which are highly relativistic and nonlinear, resulting in the generation of coherent harmonic radiation in the forward direction (i.e., copropagating with the incident laser field). A nonlinear cold fluid model, valid for ultrahigh intensities, is formulated and used to analyze relativistic harmonic generation. The plasma density response is included self-consistently and is shown to significantly reduce the current driving the harmonic radiation. Phase detuning severely limits the growth of the harmonic radiation. The effects of diffraction are considered in the mildly relativistic limit. No third-harmonic signal emerges from a uniform plasma of near-infinite extent. A finite third-harmonic signal requires the use of a semi-infinite or finite slab plasma. For an initially uniform plasma, no second-harmonic radiation is generated. Generation of even harmonics requires transverse gradients in the initial plasma density profile 相似文献
984.
Shealy J.B. Hashemi M.M. Kiziloglu K. DenBaars S.P. Mishra U.K. Liu T.K. Brown J.J. Lui M. 《Electron Device Letters, IEEE》1993,14(12):545-547
A technology for increasing both the two-terminal gate-drain breakdown and subsequently the three-terminal-off-state breakdown of AlInAs/GaInAs high-electron-mobility transistors (HEMTs) to record values without substantial impact on other parameters is presented. The breakdown in these structures is dependent on the multiplication of electrons injected from the source (channel current) and the gate (gate leakage) into the channel. In addition, holes are generated by high fields at the drain and are injected back into the gate and source electrodes. These phenomena can be suppressed by increasing the gate barrier height and alleviating the fields at the drain. Both have been achieved by incorporating a p+-2DEG junction as the gate that modulates the 2DEG gas and by utilizing selective regrowth of the source and drain regions by MOCVD. The 1-μm-gate-length devices fabricated have two-terminal gate-drain and three-terminal-off-state breakdown voltages of 31 V and 28 V, respectively 相似文献
985.
Liu Xianlong 《数理统计与管理》1998,(6)
本文从理论上讨论了费歇判别与回归模型的等价,指出了利用费歇判别值对企业的综合经济效益进行排序是不合理的,并提出了改进的综合评价方法 相似文献
986.
Insensitive current/voltage-mode filters using FTFNs 总被引:1,自引:0,他引:1
Shen-Tuan Liu Jiin-Long Lee 《Electronics letters》1996,32(12):1079-1080
A new configuration for realising insensitive current/voltage-mode filters using two four-terminal floating nullors (FTFNs) is presented. It can provide the curren/voltage-mode filters with reduced sensitivities. Moreover, the current-mode filters can have high output impedance and only grounded capacitors. The proposed universal voltage-mode filter with dual outputs can realise lowpass, bandpass, highpass, notch and allpass filtering functions. Simulation results are given to verify the theoretical analysis 相似文献
987.
本文介绍了一种新的自动调焦方法准直小孔法.对自动调焦的原理和后继电路处理方法作了详细的分析.它具有响应时间短、灵敏度高,能够很好的在激光直接写入系统中克服物镜工作台运动中导轨的不平和轴向窜动所造成的离焦。 相似文献
988.
Wanhui Liu Jianghong Tang Ying Wang Guoli Shen R. Yu 《Analytical and bioanalytical chemistry》1998,362(4):387-390
An optical fiber sensor for the continuous determination of hydrochloric acid is presented. It is based on the fluorescence quenching of a flavone containing copolymer membrane by hydrochloric acid. The quenching efficiency is greatly enhanced in the presence of Fe(III). This enhancement is attributed to the primary inner filter effect, as well as the formation of a complex between the 4′-N,N-dimethylaminoflavone group in the copolymer and the Fe(III) species extracted from hydrochloric acid solution. The optical response is linear and reversible for 0.10–6.00 mol L–1 HCl with a response time of the order of a second. The standard deviations for repeated alternative measurements of 0.20 and 2.00 mol L–1 hydrochloric acid are 0.32% and 0.46% (n = 10), respectively, indicating a good reproducibility. Because of the covalently bonding of the dye to polymer, the sensor exhibits also a good stability. Selectivity has also been evaluated for some potential interferents. The sensor in conjunction with a flow-injection system can be used for on-line determination of hydrochloric acid. 相似文献
989.
加热到1173K的Cu-25at;%Al合金块状试样经723K60S等温淬火后其组织由片状的“羽毛状贝氏体(9R结构)和β_1'马氏体(18R结构)组成。当试样被离子束轰去进一步薄化后,在足够薄的区域内生成了许多2H结构的薄片马氏体。初步分析认为该薄片马氏体形成的原因是由于贝氏体生成时在试样内留下了高的应力场,当试样薄化到一临界厚度以下时,试样区的三维约束条件松弛使试样内的应力场触发了薄片马氏体的生成。这些薄片马氏体是通过位于贝氏体界面上位错的重新启动而长大的。 相似文献
990.
Pu-Kun Liu Zhong-Hai Yang Shang-Jie Qian Sheng-Gang Liu 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1995,23(2):156-162
Using the Laplace transformation, the kinetic theory of the high-relativistic electron cyclotron maser having a novel open-resonator-the axisymmetrical quasioptical cavity of oblique rotation at arbitrary angle, is carried out within the framework of the linearized Vlasov-Maxwell equations. The beam-wave interaction power, starting current and frequency shift have been derived and numerically calculated. The influence of rotation angle on the beam-wave interaction is also studied. A series of important conclusions obtained reveal the characteristics of this novel quasioptical electron cyclotron maser 相似文献