全文获取类型
收费全文 | 3733篇 |
免费 | 762篇 |
国内免费 | 746篇 |
专业分类
化学 | 1705篇 |
晶体学 | 49篇 |
力学 | 170篇 |
综合类 | 113篇 |
数学 | 424篇 |
物理学 | 995篇 |
无线电 | 1785篇 |
出版年
2024年 | 11篇 |
2023年 | 51篇 |
2022年 | 79篇 |
2021年 | 98篇 |
2020年 | 86篇 |
2019年 | 105篇 |
2018年 | 100篇 |
2017年 | 127篇 |
2016年 | 135篇 |
2015年 | 176篇 |
2014年 | 157篇 |
2013年 | 208篇 |
2012年 | 234篇 |
2011年 | 260篇 |
2010年 | 237篇 |
2009年 | 258篇 |
2008年 | 264篇 |
2007年 | 282篇 |
2006年 | 248篇 |
2005年 | 202篇 |
2004年 | 201篇 |
2003年 | 309篇 |
2002年 | 377篇 |
2001年 | 367篇 |
2000年 | 227篇 |
1999年 | 113篇 |
1998年 | 58篇 |
1997年 | 35篇 |
1996年 | 35篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 20篇 |
1992年 | 21篇 |
1991年 | 12篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1965年 | 1篇 |
1963年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有5241条查询结果,搜索用时 710 毫秒
71.
72.
Implicit polynomials (IPs) are considered as a powerful tool for object curve fitting tasks due to their simplicity and fewer parameters. The traditional linear methods, such as 3L, MinVar, and MinMax, often achieve good performances in fitting simple objects, but usually work poorly or even fail to obtain closed curves of complex object contours. To handle the complex fitting issues, taking the advantages of deep neural networks, we designed a neural network model continuity-sparsity constrained network (CSC-Net) with encoder and decoder structure to learn the coefficients of IPs. Further, the continuity constraint is added to ensure the obtained curves are closed, and the sparseness constraint is added to reduce the spurious zero sets of the fitted curves. The experimental results show that better performances have been obtained on both simple and complex object fitting tasks. 相似文献
73.
针对变压器故障诊断准确率低和稳定性差的问题,文中提出了一种改进麻雀搜索算法优化贝叶斯网络的变压器故障诊断方法。首先,通过计算互信息建立最大支撑树并进行定向处理得到贝叶斯网络初始结构即初始种群。然后,在算法中引入一种新的合作机制和正弦余弦算法,提高算法收敛速度和全局搜索能力,并利用油中溶解气体分析,创建基于改进麻雀搜索算法优化贝叶斯网络的变压器故障诊断模型。最后,为了证明所提方法的优越性,将所提的方法与现有变压器故障诊断方法进行对比。结果表明,文中所提出的方法故障诊断率最高,可以更精准地对变压器进行故障诊断。 相似文献
74.
75.
Shou‐Ri Sheng Xiao‐Ling Liu Yong‐Fen Tong Lie Chen Hong‐Li Wen Cai‐Sheng Song 《中国化学会会志》2005,52(5):943-946
Several new methyl substituted poly(aryl ether ketone)s containing sulfone linkage with inherent viscosities of 0.62–0.84 dL/g have been prepared from 4,4′‐bis(2‐methylphenoxy)diphenylsulfone and 4,4′‐bis(3‐methylphenoxy)diphenylsulfone with terephthaloyl chloride and isophthaloyl chloride by electrophilic Friedel‐Crafts acylation in the presence of DMF with anhydrous AlCl3 as a catalyst in 1,2‐dichloroethane, respectively. These polymers having weight‐average molecular weight in the range of 71,000–49,000 are all amorphous and show high glass transition temperatures ranging from 167 °C to 191 °C, excellent thermal stability at temperatures over 400 °C in air or nitrogen, high char yields of 51–58% in nitrogen and good solubility in CHCl3 and polar solvents such as DMF, DMSO and NMP at room temperature. 相似文献
76.
Three categories of tea, black, green and oolong tea, with those varieties in each category were analyzed for their contents of cellulose, hemicellulose, lignin, polyphenols, caffeine and amino acids. The data were subjected to multivarige analysis. Principal component analysis and principal component classification provide discrimination between the different categories and varieties. The quality index, assigned to these tea standards by experts, could be predicted form the principal component scores. 相似文献
77.
A composite film of ethylenediamine tetraacetic acid (EDTA)‐ZrO2 organic‐inorganic hybrid was prepared based on the chelation between Zr(IV) and EDTA. The direct electrochemical behavior of cytochrome c (cyt. c) at the hybrid film modified glassy carbon electrodes was investigated. The immobilized EDTA can promote the redox of heme in horse heart cyt. c which gives rise to a pair of reversible redox peaks with a formal potential of 40 mV (vs. SCE). The peak current increased linearly with the increase of cyt. c concentration in the range of 1.6 × 10?6—8.0 × 10?5 mol·L?1 with the correlation coefficient of 0.996. Further investigation shows that metal ions can impede the electron transfer of cyt. c. The impediment capability of metal ions depends on their coordination capability with EDTA and their valence number. 相似文献
78.
TONG Yi-Ping 《结构化学》2007,26(2):143-148
79.
The spectral evolution of the surface-enhanced Raman scattering (SERS) of 4-tert-butylbenzylmer-captan(4-tBBM)on gold nanopanlcles assembly under laser irradiation is reported.The reIative intensities of typical peaks in the spectrum of 4-tBBM gradually change with irradiation time.Comparison of the rate of spectral changes under several experimental conditions indicates that the surface plasmon resonance(SPR)induced heat in the gold nanoparticles assembly is the origin of the spectraI evolution.During the process of self-assembly,4-tBBM molecules do not form a compact ordered monolayer because of the spatial hindrance of the 4-tert-butyl end group.The heat induced by laser irradiation drives the 4-tBBM molecules to rearrange to a more stable orientation. 相似文献
80.
Shan Shan Tang Chang Lu Shao Shou Zhu Li 《中国化学快报》2007,18(4):465-468
Nanofibers of poly(vinyl pyrrolidone) (PVP)/Eu^3+ with diameters of 300-900 nm were prepared by using sol-gel processing and electrospinning technique. The products were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence (PL). The results indicated that, Eu^3+ was successfully embedded in the onedimensional hybrid nanofibers, and the PVP/Eu^3+ hybrid nanofibers had favorable photoluminescence properties. 相似文献