全文获取类型
收费全文 | 10336篇 |
免费 | 1615篇 |
国内免费 | 1114篇 |
专业分类
化学 | 6692篇 |
晶体学 | 20篇 |
力学 | 612篇 |
综合类 | 101篇 |
数学 | 2499篇 |
物理学 | 3141篇 |
出版年
2024年 | 9篇 |
2023年 | 86篇 |
2022年 | 163篇 |
2021年 | 198篇 |
2020年 | 403篇 |
2019年 | 263篇 |
2018年 | 331篇 |
2017年 | 331篇 |
2016年 | 485篇 |
2015年 | 516篇 |
2014年 | 678篇 |
2013年 | 820篇 |
2012年 | 653篇 |
2011年 | 712篇 |
2010年 | 601篇 |
2009年 | 767篇 |
2008年 | 882篇 |
2007年 | 773篇 |
2006年 | 703篇 |
2005年 | 602篇 |
2004年 | 563篇 |
2003年 | 544篇 |
2002年 | 294篇 |
2001年 | 238篇 |
2000年 | 242篇 |
1999年 | 193篇 |
1998年 | 156篇 |
1997年 | 194篇 |
1996年 | 149篇 |
1995年 | 123篇 |
1994年 | 72篇 |
1993年 | 47篇 |
1992年 | 49篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 31篇 |
1988年 | 41篇 |
1987年 | 24篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 12篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1957年 | 2篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 626 毫秒
991.
992.
Martin Lamač Jiří Tauchman Sascha Dietrich Ivana Císařová Heinrich Lang Petr Štěpnička 《应用有机金属化学》2010,24(4):326-331
Amide coupling of (Sp)‐2‐(diphenylphosphanyl)ferrocene‐1‐carboxylic acid with appropriate terminal amines mediated by 1‐hydroxybenzotriazole and a carbodiimide affords multi‐donor amides terminally functionalized with planar‐chiral (Sp)‐2‐(diphenylphosphanyl)ferrocen‐1‐yl moieties in good to excellent yields. Palladium catalysts based on these ligands efficiently promote asymmetric allylic alkylation of 1,3‐diphenylallyl acetate with in situ generated dimethyl malonate anion to give the C‐alkylated product with ees up to 93% at room temperature. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
993.
994.
Nanocrystalline cobalt ferrites with nominal composition CoCrxFe2−xO4 ranging from x=0.0 to 0.5 with step increment of 0.25 were prepared by sol–gel auto combustion and chemical co-precipitation techniques. A comparative study of structural, electrical and magnetic properties of these ferrites has been measured using different characterization techniques. Structural and micro-structural studies were measured using X-ray diffraction, Fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy and atomic force microscopy. Crystallite sizes of the series are within the range of 12–29±2 nm. Lattice parameters decrease by increasing Cr3+ concentration. FTIR confirms the presence of two lattice absorption bands. DC electrical resistivity increases to a value of ∼1010 Ω-cm with increase in Cr3+ concentration, but the most significant increase is in samples prepared by sol–gel combustion. Dielectric properties have been measured as a function of frequency at room temperature. Dielectric loss decreases to 0.1037 and 0.0108 at 5 MHz for chemical co-precipitation and sol–gel combustion, respectively. Impedance measurements further helped in analyzing the electrical properties and to separate the grain and grain boundary resistance effects using a complex impedance analysis. Magnetic parameters were studied using a vibrating sample magnetometer in the applied field of 10 kOe. The saturation magnetization decreased from 63 to 10.8 emu/gm with increase in Cr3+ concentration. 相似文献
995.
996.
997.
998.
Secondary ion mass spectroscopy (SIMS) is a powerful method for element distribution examination of conducting and semi-conducting
surfaces at high spatial resolution and with a high sensitivity. Routine surface analysis produces about 8 to 15 images in
a short time, each of which displays the intensity distribution of one mass, thus generating a multispectral SIMS image. Formation
of occlusions, segregations, and the overall location of the elements relative to each other, are difficult to recognise when
looking at n separate 2-D images.
Image fusion is a process whereby images obtained from various sensors, or at different moments of time, or under different
conditions, are combined together to provide a more complete picture of the object under investigation. The process of combining
SIMS images may be viewed as an attempt to compensate for the inherent effect of SIMS to channel the information obtained
from the sample into different images, corresponding to different element phases.
The wavelet transform is a powerful method for fusion of images. This work covers the use of wavelet based fusion algorithms
on multispectral SIMS images, evaluating the performance of different wavelet based fusion rules on different type of image
systems and comparing the results to conventional fusion techniques. An aim of this study is to increase the information,
i.e. the number of masses, which can be merged into one image in order to enhance the perception and interpretation of the
SIMS surface images. 相似文献
999.
Xiuqiao Xiang Jianzhong Zhou Li Mo Zhimeng Luo Chaoshun Li 《Applied mathematics and computation》2009,215(3):881-888
For the purpose of further saving computing time, an improved algorithm about NSFOT is provided in this paper. That is, by introducing the simple operations such as preprocessing or after-processing, Haar and Walsh transforms are performed conveniently on the multiprocessor. As a result, one large size problem is divided into several small size sub-problems, load on every processor not only decreases greatly but also gets so uniform that much time is saved. Both the theoretical analysis and experimental results demonstrate the effectiveness of the proposed approach. 相似文献
1000.
五像限法太阳自动跟踪仪 总被引:2,自引:0,他引:2
介绍由两心圆组成的等面积五像限光测量装置工作原理,即当光线与光电池垂直时其投影光斑经过一透镜后正好安全覆盖五片光电池,当光线与光电池成一夹角时,其投影光斑必然发生偏移,将光主在光电池上的偏移转移成光电流差值的变化,实现装置自动跟踪太阳。 相似文献