全文获取类型
收费全文 | 66790篇 |
免费 | 7382篇 |
国内免费 | 4458篇 |
专业分类
电工技术 | 5788篇 |
技术理论 | 2篇 |
综合类 | 5384篇 |
化学工业 | 9456篇 |
金属工艺 | 3794篇 |
机械仪表 | 4515篇 |
建筑科学 | 5410篇 |
矿业工程 | 2281篇 |
能源动力 | 1979篇 |
轻工业 | 4906篇 |
水利工程 | 1671篇 |
石油天然气 | 3408篇 |
武器工业 | 847篇 |
无线电 | 8488篇 |
一般工业技术 | 6903篇 |
冶金工业 | 2674篇 |
原子能技术 | 940篇 |
自动化技术 | 10184篇 |
出版年
2024年 | 391篇 |
2023年 | 1217篇 |
2022年 | 2352篇 |
2021年 | 3358篇 |
2020年 | 2436篇 |
2019年 | 1906篇 |
2018年 | 2139篇 |
2017年 | 2413篇 |
2016年 | 2236篇 |
2015年 | 3151篇 |
2014年 | 3925篇 |
2013年 | 4584篇 |
2012年 | 5423篇 |
2011年 | 5531篇 |
2010年 | 4969篇 |
2009年 | 4733篇 |
2008年 | 4537篇 |
2007年 | 4286篇 |
2006年 | 3981篇 |
2005年 | 3193篇 |
2004年 | 2239篇 |
2003年 | 1655篇 |
2002年 | 1599篇 |
2001年 | 1235篇 |
2000年 | 1027篇 |
1999年 | 850篇 |
1998年 | 595篇 |
1997年 | 519篇 |
1996年 | 456篇 |
1995年 | 383篇 |
1994年 | 298篇 |
1993年 | 203篇 |
1992年 | 180篇 |
1991年 | 123篇 |
1990年 | 123篇 |
1989年 | 65篇 |
1988年 | 58篇 |
1987年 | 36篇 |
1986年 | 36篇 |
1985年 | 15篇 |
1984年 | 20篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 13篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 18篇 |
1959年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm−1 for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm−1 and 13.62 ± 2.11 μm−1 for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight. 相似文献
42.
43.
Direct sequence code-division multiple access has been shown as the predominant access technology to provide high-speed multimedia services in next-generation wireless cellular networks. The DS-CDMA cellular network significantly differentiates itself from the traditional narrowband TDMA/FDMA system, in which great capacity gain can be explored by taking advantage of its unique characteristics. This article addresses resource management in DS-CDMA cellular networks. We first provide an overview of the DS-CDMA network and discuss the main challenges in the design of resource management schemes. We then examine several critical issues within the scope of resource management, including congestion control, rate and power control, and cell planning, with the emphasis on call admission control. Finally, we offer some insights and discussions on the performance aspects, strengths, and weaknesses of existing proposals. 相似文献
44.
G. Lucovsky J.G. Hong C.C. Fulton N.A. Stoute Y. Zou R.J. Nemanich D.E. Aspnes H. Ade D.G. Schlom 《Microelectronics Reliability》2005,45(5-6):827
This paper uses X-ray absorption spectroscopy to study the electronic structure of the high-k gate dielectrics including TM and RE oxides. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d* states, which define the optical band gap, Eg, and the conduction band offset energy with respect to crystalline Si, EB. Eg and EB scale with the atomic properties of the TM and RE atoms providing important insights for identification high-k dielectrics that meet performance targets for advanced CMOS devices. 相似文献
45.
Polyester‐based polyurethane/nano‐silica composites were obtained via in situ polymerization and investigated by Fourier‐transform infrared spectroscopy (FTIR), or FTIR coupled with attenuated total reflectance (FTIR‐ATR), Transmission electron microscopy (TEM), atomic force microscopy (AFM), an Instron testing machine, dynamic mechanical analysis (DMA) and ultraviolet‐visible spectrophotometry (UV‐vis). FTIR analysis showed that in situ polymerization provoked some chemical reactions between polyester molecules and nano‐silica particles. FTIR‐ATR, TEM and AFM analyses showed that both surface and interface contained nano‐silica particles. Instron testing and DMA data showed that introducing nano‐silica particles into polyurethane enhanced the hardness, glass temperature and adhesion strength of polyurethane to the substrate, but also increased the resin viscosity. UV‐vis spectrophotometry showed that nano‐silica obtained by the fumed method did not shield UV radiation in polyurethane films. Copyright © 2003 Society of Chemical Industry 相似文献
46.
In this paper we report the fabrication of ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films on Si-on-Insulator (SOI) substrates with and without an electrode by pulsed excimer laser deposition combined with rapid thermal annealing. Based on the structural and interfacial characteristics analysis by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), cross-sectional transmission electron microscopy (XTEM) and automatic spreading resistance measurement (ASR), the film structure and orientation were revealed to be dependent on the annealing time and annealing temperature as well as deposition temperature. From RBS spectra and XTEM observation it is shown that the PZT thin films did not interact with the top silicon layers of SOI and that the composition of the film was similar to the target. The ASR measurements showed that the electrical properties of PZT/SOI as well as PZT/Pt/SOI were abrupt, and that the electrical properties of the SOI substrates were still good after the PZT growth. 相似文献
47.
推导出了成叠带状光纤在松套管中余长的两个计算公式,此两公式是带状光纤缆结构设计需要用到的基本公式。此外,还对带状光纤缆结构设计作了初步讨论。 相似文献
48.
本文提出了一种基于拓扑法的多关节机械手无碰路径规划方法,文中重点讨论了三关节机械手路径规划时如何划分状态空间、判别各连通块之间的连通性以及路径搜索等问题,最后我们给出了一个实验结果以示该方法的有效性。 相似文献
49.
本文论述了一种普及型电子式电能表的设计原理和实现方法,具体讨论了时分割乘法器和电流互感器的组成和误差分析,最后给出测试结果。 相似文献
50.