全文获取类型
收费全文 | 90767篇 |
免费 | 14146篇 |
国内免费 | 9725篇 |
专业分类
化学 | 47387篇 |
晶体学 | 824篇 |
力学 | 3985篇 |
综合类 | 453篇 |
数学 | 7486篇 |
物理学 | 25690篇 |
无线电 | 28813篇 |
出版年
2024年 | 418篇 |
2023年 | 2327篇 |
2022年 | 3231篇 |
2021年 | 3848篇 |
2020年 | 3754篇 |
2019年 | 3503篇 |
2018年 | 2985篇 |
2017年 | 3098篇 |
2016年 | 4052篇 |
2015年 | 4521篇 |
2014年 | 5280篇 |
2013年 | 6587篇 |
2012年 | 7850篇 |
2011年 | 7683篇 |
2010年 | 5724篇 |
2009年 | 5722篇 |
2008年 | 5978篇 |
2007年 | 5240篇 |
2006年 | 4863篇 |
2005年 | 4187篇 |
2004年 | 3082篇 |
2003年 | 2444篇 |
2002年 | 2255篇 |
2001年 | 1858篇 |
2000年 | 1630篇 |
1999年 | 1852篇 |
1998年 | 1465篇 |
1997年 | 1341篇 |
1996年 | 1295篇 |
1995年 | 1139篇 |
1994年 | 959篇 |
1993年 | 844篇 |
1992年 | 699篇 |
1991年 | 580篇 |
1990年 | 458篇 |
1989年 | 341篇 |
1988年 | 261篇 |
1987年 | 217篇 |
1986年 | 219篇 |
1985年 | 188篇 |
1984年 | 120篇 |
1983年 | 120篇 |
1982年 | 69篇 |
1981年 | 56篇 |
1980年 | 38篇 |
1979年 | 27篇 |
1977年 | 25篇 |
1976年 | 22篇 |
1975年 | 28篇 |
1957年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
针对目标探测类空间红外相机大范围成像、高灵敏度探测、高精度定位等应用需求,文中提出采用像方远心光路和低温光学技术结合的解决方案,设计了物方视场角8°×8°、入瞳口径265 mm、工作温度200 K的像方远心折射式光学系统。镜头最大口径280 mm,采用多级分散的弹性支撑设计,解决大口径低温透镜装框、透镜组件支撑和镜头整体安装各环节的热应力卸载问题。在保证高刚度和低漏热的情况下,使低温下透镜的热应力对镜头能量集中度的影响降低到可接受范围内。镜头完成装调及室温下像质确认后,进行了力学振动试验,并将其制冷到200 K水平测试像质,测试结果表明,镜头能量集中度达到轴上75%,边缘视场72%。 相似文献
83.
84.
Jin Xie Yun-Wei Song Dr. Bo-Quan Li Dr. Hong-Jie Peng Prof. Jia-Qi Huang Prof. Qiang Zhang 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(49):22334-22339
Polysulfide intermediates (PSs), the liquid-phase species of active materials in lithium–sulfur (Li-S) batteries, connect the electrochemical reactions between insulative solid sulfur and lithium sulfide and are key to full exertion of the high-energy-density Li-S system. Herein, the concept of sulfur container additives is proposed for the direct modification on the PSs species. By reversible storage and release of the sulfur species, the container molecule converts small PSs into large organosulfur species. The prototype di(tri)sulfide-polyethylene glycol sulfur container is highly efficient in the reversible PS transformation to multiply affect electrochemical behaviors of sulfur cathodes in terms of liquid-species clustering, reaction kinetics, and solid deposition. The stability and capacity of Li-S cells was thereby enhanced. The sulfur container is a strategy to directly modify PSs, enlightening the precise regulation on Li-S batteries and multi-phase electrochemical systems. 相似文献
85.
Two novel 2′-hydroxychalcone derivatives (i.e., M1 and M2) are explored in this work. We mainly focus on investigating the effects of photoexcitation on hydrogen bonds and on the excited-state intramolecular proton transfer (ESIPT) process. On the basis of calculations of electrostatic potential surface and intramolecular interactions, we verify the formation of hydrogen bond O1 H2···O3 in both S0 and S1 states. Exploring the ultraviolet–visible spectra in the liquid phase, our simulated results reappear in the experimental phenomenon. Analyzing molecular geometry and infrared stretching vibrational spectra, we confirm O1 H2···O3 is strengthened for both M1 and M2 in the S1 state. We further confirm that charge redistribution facilitates ESIPT tendency. Constructing potential energy curves, we find the ultrafast ESIPT behavior for M1, which is because of the deficiency of side hydroxyl moiety comparing with M2. This work makes a reasonable affiliation of the ESIPT mechanism for M1 and M2. We wish this paper could facilitate understanding these two novel systems and promote their applications. 相似文献
86.
In this paper we investigated the BER performance of DS-CDMA using various chip-waveforms, which include three time-limited chip-waveforms and two band-limited chip-waveforms. Closed-form formulae were derived for evaluating the achievable bit-error rate performance with the aid of the standard Gaussian approximation, when communicating over a Nakagami-m channel. The time-limited waveforms impose a low implementational complexity, since they maybe over sampled and read from a look-up table. However, they are outperformed by the frequency-domain raised-cosine waveform as well as the optimum waveform specifically designed by Cho and Lehnert for achieving the lowest possible bit error rate 相似文献
87.
88.
在对铝合金铸件进行X射线实时探伤时,由于采集回来的许多图像的目标物体的轮廓比较模糊,这样在评片时容易造成对铸件的误判和漏判。针对这种现象并结合这类图像噪声源多的特点,在研究常用卷积算法的基础上,采用特殊卷积(特殊的卷积核)操作,同时根据雕刻的艺术底色为灰色,最后给卷积结果加上灰度值128,实验结果表明图像中目标物体的轮廓变得更清楚,同时处理后的图像具有比较好的三维效果,这给工作人员在看图时带来了方便。 相似文献
89.
Yang Aiying Wang Ziyu Zhang Zhaoyi Chen Zhangyuan Wu Deming 《电子科学学刊(英文版)》2003,20(6):461-466
A 10Gbit/s recirculating system is configured with Chirped Fiber Bragg Grating (CFBG) for the dispersion compensation. For the first time, the transmission distance in the loop reaches 1000km with bit error rate of 10-9. The effect of the group delay ripple of the fiber grating is also investigated in the recirculating systems, and it is shown that the transmission distance is limited to 4 cycles (4×167.1km ) in the loop with the power penalty fluctuation below 1.0dB. Thus the group delay ripple should be reduced to allow for the wavelength drift of±5GHz. At the end of this letter, the principles are given for designing long haul recirculating systems with dispersion compensation CFBG. 相似文献
90.
计量用脉冲Nd:YAG倍频激光器及其电源 总被引:2,自引:0,他引:2
介绍了脉冲Nd:YAG倍频激光器双波长(1064/532nm)3种输出的激光光路。1064nm输出的动静比可达0.40;KTP晶体外腔倍频效率可达0.50。介绍了稳定可靠的调Q电路、逻辑控制电路和开关电源的主电路及其参数计算。 相似文献