全文获取类型
收费全文 | 36094篇 |
免费 | 6498篇 |
国内免费 | 9158篇 |
专业分类
化学 | 26809篇 |
晶体学 | 1075篇 |
力学 | 2139篇 |
综合类 | 890篇 |
数学 | 4297篇 |
物理学 | 16540篇 |
出版年
2024年 | 111篇 |
2023年 | 522篇 |
2022年 | 1255篇 |
2021年 | 1264篇 |
2020年 | 1368篇 |
2019年 | 1369篇 |
2018年 | 1228篇 |
2017年 | 1447篇 |
2016年 | 1632篇 |
2015年 | 1947篇 |
2014年 | 2216篇 |
2013年 | 2822篇 |
2012年 | 3024篇 |
2011年 | 3381篇 |
2010年 | 2838篇 |
2009年 | 2907篇 |
2008年 | 3131篇 |
2007年 | 2712篇 |
2006年 | 2702篇 |
2005年 | 2320篇 |
2004年 | 1780篇 |
2003年 | 1383篇 |
2002年 | 1356篇 |
2001年 | 1249篇 |
2000年 | 1257篇 |
1999年 | 796篇 |
1998年 | 499篇 |
1997年 | 455篇 |
1996年 | 411篇 |
1995年 | 343篇 |
1994年 | 317篇 |
1993年 | 299篇 |
1992年 | 275篇 |
1991年 | 170篇 |
1990年 | 168篇 |
1989年 | 139篇 |
1988年 | 122篇 |
1987年 | 96篇 |
1986年 | 85篇 |
1985年 | 71篇 |
1984年 | 56篇 |
1983年 | 62篇 |
1982年 | 32篇 |
1981年 | 32篇 |
1980年 | 14篇 |
1979年 | 33篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1959年 | 6篇 |
1936年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
51.
Analysis of the Coaxial Ridge-Loaded Helical Groove Waveguide 总被引:1,自引:0,他引:1
Yonghai Lan Wenxiang Wang Yubin Gong 《International Journal of Infrared and Millimeter Waves》2002,23(3):425-434
The coaxial ridge-loaded helical groove waveguide is proposed in this paper. As an all-metal slow-wave circuit, it has advantages of good heat dissipation and great size, and thus is suitable for use of millimeter TWT. By means of field theory, the expressions of the dispersion equation and the coupling impedance of the coaxial ridge-loaded helical groove waveguide are obtained. The influence of various circuit dimensions on the dispersion relation and the coupling impedance is investigated by the results of numerical computation. 相似文献
52.
微微秒自动调谐参量激光的研究 总被引:3,自引:3,他引:0
采用一对MgO:LiNbO_3角度调谐覆盖了红近外波段0.7~2.2μm,并配以自动波长扫描。文中分析了影响脉冲宽度与线宽的因素,与实验结果相吻合。脉宽<30ps,线宽~1nm(简并波长附近~10nm),单脉冲峰功率达MW级,参量光总能量转换效率达5.4%。 相似文献
53.
54.
55.
基于BP神经网络的企业未来获利能力智能综合评价 总被引:3,自引:0,他引:3
分析了相关分析——多指标综合评价法在确定企业未来获利能力方面的优点和不足 ;并在其基础上提出了基于 BP神经网络的多指标综合评价法 ;仿真试验证明了基于 BP神经网络的多指标综合评价法的有效性 相似文献
56.
A novel copolymer, poly(N‐hexyl‐3,7‐phenothiazylene‐1,2‐ethenylene‐2,6‐pyridylene‐1,2‐ethenylene) ( P3 ), containing N‐hexyl‐3,7‐phenothiazylene and 2,6‐pyridylene chromophores was synthesized to investigate the effect of protonation, metal complexation, and chemical oxidation on its absorption and photoluminescence (PL). Poly(N‐hexyl‐3,8‐iminodibenzyl‐1,2‐ethenylene‐1,3‐phenylene‐1,2‐ethenylene) and poly(N‐hexyl‐3,7‐phenothiazylene‐1,2‐ethenylene‐1,3‐phenylene‐1,2‐ethenylene) ( P2 ), consisting of 1,3‐divinylbenzene alternated with N‐hexyl‐3,8‐iminodibenzyl and N‐hexyl‐3,7‐phenothiazylene, respectively, were also prepared for comparison. Electrochemical investigations revealed that P3 exhibited lower band gaps (2.34 eV) due to alternating donor and acceptor conjugated units (push–pull structure). The absorption and PL spectral variations of P3 were easily manipulated by protonation, metal chelation, and chemical oxidation. P3 displayed significant bathochromic shifts when protonated with trifluoroacetic acid in chloroform. The complexation of P3 with Fe3+ led to a significant absorption change and fluorescence quenching, and this implied the coordination of ferric ions with the 2,6‐pyridylene groups in the backbone. Moreover, both phenothiazylene‐containing P2 and P3 showed conspicuous PL quenching with a slight redshift when oxidized with NOBF4. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1272–1284, 2004 相似文献
57.
58.
The spin coating method was used to assemble polymer (Poly (2,5-dibutoxy-1,4-phenylenevinylene)) (DBO-PPV) into the pores of porous alumina which was prepared by anodization. Four peaks in the photoluminescence (PL) spectra of the composite, with contributions from the DBO-PPV and porous alumina, were found. It was also found that the light emitting from the porous alumina could excite the photoluminescence of DBO-PPV. The nanometer effect of the porous alumina can lead to a blue shift of 90 nm of the PL peaks of DBO-PPV. 相似文献
59.
The present paper covers a salt of 2:18 heteropoly complex of tungstotitanophosphate prepared with the method of improved sequence preparation. The title compound was characterized by elemental analysis, 31P NMR spectra, polarography, cyclic voltammetry, IR, UV, X-ray powder diffraction , X-ray photoelectron spectra and TG-DTA analysis. The results show that the complex with the formula K8[TiP2W17O62]·12H2O has Dawson structure. Its properties such as spectra, redox behavior and thermal stability are discussed. 相似文献
60.
Meihua Lu H. Gong T. Song Jian-Ping Wang Hong-Wei Zhang T.J. Zhou 《Journal of magnetism and magnetic materials》2006
We present a simple way to synthesize FePt and ZnO (wide-band-gap semiconductor) nanoparticle composites. The FePt nanoparticles were fabricated using the method reported by Sun et al. By controlling the heating rate, 3 nm FePt nanoparticles were synthesized. Well-dispersed FePt and ZnO nanoparticle composites were prepared by further adding zinc acetate and oleyl amine into the 3 nm FePt nanoparticle dispersion. By controlling the molar ratio of the FePt and zinc acetate, FePt and ZnO nanoparticle composites with different FePt particle fractions were obtained. The intensity of photo luminescence spectra of the nanoparticle composites increases very much with decreasing FePt particle fraction, whereas the peak position shifts a little. After annealing at 550 °C for half an hour, the nanoparticle composites become magnetically hard or semi-hard with coercivity much dependent on the FePt particle volume fraction. The coercivity of the composites increases with annealing temperature. The composites hold the promise of applications in new generation recording and/or optical devices. 相似文献