全文获取类型
收费全文 | 21052篇 |
免费 | 3647篇 |
国内免费 | 2553篇 |
专业分类
化学 | 15137篇 |
晶体学 | 265篇 |
力学 | 1255篇 |
综合类 | 187篇 |
数学 | 2075篇 |
物理学 | 8333篇 |
出版年
2024年 | 79篇 |
2023年 | 480篇 |
2022年 | 683篇 |
2021年 | 777篇 |
2020年 | 833篇 |
2019年 | 808篇 |
2018年 | 684篇 |
2017年 | 644篇 |
2016年 | 999篇 |
2015年 | 1008篇 |
2014年 | 1152篇 |
2013年 | 1517篇 |
2012年 | 1916篇 |
2011年 | 1987篇 |
2010年 | 1332篇 |
2009年 | 1164篇 |
2008年 | 1352篇 |
2007年 | 1350篇 |
2006年 | 1177篇 |
2005年 | 1034篇 |
2004年 | 745篇 |
2003年 | 640篇 |
2002年 | 584篇 |
2001年 | 439篇 |
2000年 | 417篇 |
1999年 | 472篇 |
1998年 | 355篇 |
1997年 | 369篇 |
1996年 | 309篇 |
1995年 | 296篇 |
1994年 | 304篇 |
1993年 | 247篇 |
1992年 | 197篇 |
1991年 | 200篇 |
1990年 | 179篇 |
1989年 | 112篇 |
1988年 | 86篇 |
1987年 | 89篇 |
1986年 | 60篇 |
1985年 | 44篇 |
1984年 | 43篇 |
1983年 | 24篇 |
1982年 | 25篇 |
1981年 | 18篇 |
1980年 | 14篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1957年 | 4篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
针对正则化方法在解决实际反演问题时既能克服问题的不适定性又可以很大程度上抑制噪声和误差的传播, 本文提出了利用遗传算法结合正则化方法的新算法, 在遗传算法适应度函数中引入正则化项来反演波导参数; 然后对算法进行仿真试验, 结果表明新算法与传统遗传算法相比具有较高的反演精度, 并指出当噪声误差小于10%时, 算法具有较强的“去噪”性能; 最后利用机载雷达在Wallops岛探测的海表面处局部回波资料进行反演试验, 将反演结果与实测大气折射率廓线进行比较, 说明该算法的有效性. 新方法为海洋大气波导反演研究提供了一种新思路. 相似文献
202.
数字域时间延迟积分时间CMOS相机高分“凝视”成像设计分析 总被引:2,自引:1,他引:2
为实现凝视卫星高分跟踪成像,设计了数字域时间延迟积分(TDI)互补金属氧化物半导体(CMOS)传感器在凝视姿态下的成像匹配模型,推导了卫星凝视跟踪成像时相对轨道坐标系的姿态变化,采用坐标变换方法实时计算其在凝视过程中随姿态变化的行转移时间,利用蒙特卡罗方法统计计算了凝视模式下姿态指向精度和稳定度对成像的影响。利用数字域TDI CMOS原理样机和小卫星姿态控制系统全物理仿真平台对成像进行了仿真分析。结果表明,卫星的姿态控制精度在成像过程中会引起纵向的像移速度失配和横向匹配的残余像移,成像积分级数越高,图像信噪比越大。积分级数的增加对卫星姿态提出较高要求,仿真平台姿态角和姿态角速度控制精度分别优于0.05°,0.005°/s时,采用积分级数为48级能较好地满足成像质量要求。 相似文献
203.
Xuewen Wu Dalian Ding Haiyan Jiang Xiaowei Xing Suping Huang Hong Liu Zhedong Chen Hong Sun 《Journal of nanoparticle research》2012,14(1):708-13
Hydroxyapatite nanoparticles (nHAT) are known to have excellent biocompatibility, and have attracted increasing attention
as new candidates of non-viral vectors for gene therapy. In our previous studies, nHAT carrying a therapeutic gene and a reporter
gene were successfully transfected into the spiral ganglion neurons in the inner ear of guinea pigs in vivo as well as in
the cultured cell lines, although the transfection efficiencies were never higher than 30%. In this study, the surface modification
of nHAT with polyethylenimine (PEI) was made (PEI–nHAT, diameter = 73.09 ± 27.32 nm) and a recombinant plasmid carrying enhanced
green fluorescent protein (EGFP) gene and neurotrophin-3 (NT-3) gene was constructed as pEGFPC2–NT3. The PEI modified nHAT
and the recombinant plasmid was then connected to form the nHAT-based vector–gene complex (PEI–nHAT–pEGFPC2–NT3). This complex
was then placed onto the intact round window membranes of the chinchillas for inner ear transfection. Auditory brainstem response
(ABR) was tested to evaluate auditory function. Green fluorescence of EGFP was observed using confocal microscopy 48 h after
administering vector–gene complexes. There was no significant threshold shift in tone burst-evoked ABR at any tested frequency.
Abundant, condensed green fluorescence was found in dark cells on both sides of the crista and around the macula of the utricle.
Scattered EGFP signals were also detected in vestibular hair cells, some Schwann cells in the cochlear spiral ganglion region,
some outer pillar cells in the organ of Corti, and a few cells in the stria vascularis. The density of green fluorescence-marked
cells was obviously higher in the vestibular dark cell area than in other areas of the inner ear, suggesting that vestibular
dark cells may have the ability to actively engulf the nHAT-based vector–gene complexes. Considering the high transfection
efficiency in the vestibular system, PEI–nHAT may be a potential vector for gene therapy of inner ear diseases, especially
vestibular disorders, and deserves further study. 相似文献
204.
Kai Jiang Jinzhong Zhang Wenlei Yu Zhigao Hu Junhao Chu 《Applied Physics A: Materials Science & Processing》2012,106(4):877-884
The Ba0.4Sr0.6−x
Mn
x
TiO3 (BSMT) ceramics with different Mn composition (from 1% to 10%) have been prepared via the conventional solid-state reaction
sintering. The X-ray diffraction analysis shows that the ceramics are polycrystalline with the single perovskite phase. The
lattice vibrations and optical properties have been investigated using Raman scattering, spectroscopic ellipsometry (SE),
and infrared reflectance spectra. It was found that the optical bandgap for the BSMT ceramics is varied between 3.40 and 3.65
eV. The three first-order Raman-active phonon modes can be observed, and the frequency of the A
1(LO3)/E(LO) mode shows a blue shift of 8 cm−1 with the Mn composition, which can be attributed to the distortion of the TiO6 octahedron. With increasing Mn composition, the frequency of the infrared-active TO4 mode decreases from 532 to 520 cm−1, owing to the local variation of the lattice constant induced by the Mn incorporation. Moreover, the optical functions of
the ceramics from the far-infrared to ultraviolet region are obtained based on the SE and reflectance spectra, which is useful
for the potential applications in ferroelectric-based optoelectronic devices. 相似文献
205.
从实际应用的角度出发,概括性地描述了高重复频率脉冲功率的主要技术特点,如工作次数多、脉冲间隔时间短和平均输出功率高等。通过介绍高平均功率准分子激光、常压脉冲气体放电、感应式回旋加速器等有代表性应用实例,论述了高重复频率脉冲功率技术的学术意义。高重复频率脉冲功率技术将进一步完善脉冲压缩理论和电路设计方法,从而在短脉冲和波形控制等方面取得进展,同时高重复频率脉冲功率发生器将进一步趋向多样化和极限化。高重复频率脉冲功率技术的发展与脉冲功率器件性能的持续提高之间相互促进,会不断将高重复频率脉冲功率技术的成果带向工业生产和日常生活。 相似文献
206.
研究了一种基于螺旋波纹波导特殊色散特性对输入微波脉冲进行压缩的新方法。利用3维全电磁粒子模拟软件对X波段螺旋波纹波导进行了建模,模拟分析了该波导的特殊色散特性及提高功率增益的方法,并将已有模型脉冲压缩增益提高了6.65。模拟结果表明:通过优化输入微波的频率调制特性,可以获得更加符合被动式脉冲压缩所需要的调频形式;采用改进后的椭圆波导,可以得到更有利于被动式脉冲压缩的本征模式;通过延长渐变段长度,可以减少规则段和渐变段之间不均匀性造成的微波反射,从而实现更高的脉冲压缩增益。 相似文献
207.
为了减小常规L波段高功率微波测量接收天线的结构尺寸及增益,设计了一种基于轴向缝隙馈电的小型化同轴扩张型天线.通过理论分析和数值模拟,选择了较优的结构尺寸,得到天线的增益及方向图特性:在1.3~1.6 GHz范围内,增益从-2.0 dBi变化至0.8 dBi;天线最大辐射方向在物理结构轴向.基于矢量网络分析仪E8362B的天线特性测量结果与数值计算结果基本一致:工作频率从1 3~1 6GHz变化时,增益从-2.3 dBi变化至1.2 dBi;E面方向图主瓣宽度大于 60°,轴向轴比大于35dB,结果表明设计的天线能够满足L波段高功率微波渊量天线低增益小型化要求. 相似文献
208.
209.
210.
S-transform proposed in 1996 by Stockwell R.G is a simple and popular technique for the time–frequency analysis. It has been introduced in optical three-dimensional shape measurement, recently. In this paper, a study about applications of S-transform in the demodulation of deformed fringe patterns is performed. We focus on discussing not only the S-transform spectrum filtering technique, the S-transform ridge technique and the phase gradient calculation method based on S-transform used in fringe pattern demodulation, but also the phase unwrapping technique. In addition, a generalized S-transform was introduced to analyze fringe patterns, which is helpful to improve the measurement accuracy and flexibility of the method based on S-transform. The reconstruction results based on S-transform were compared with that on wavelet transform and windowed Fourier transform in fringe analysis. 相似文献