全文获取类型
收费全文 | 156563篇 |
免费 | 12883篇 |
国内免费 | 6898篇 |
专业分类
电工技术 | 8746篇 |
技术理论 | 3篇 |
综合类 | 8356篇 |
化学工业 | 24825篇 |
金属工艺 | 8545篇 |
机械仪表 | 10173篇 |
建筑科学 | 9423篇 |
矿业工程 | 3281篇 |
能源动力 | 5294篇 |
轻工业 | 11008篇 |
水利工程 | 2802篇 |
石油天然气 | 4886篇 |
武器工业 | 1286篇 |
无线电 | 22128篇 |
一般工业技术 | 21646篇 |
冶金工业 | 9791篇 |
原子能技术 | 2178篇 |
自动化技术 | 21973篇 |
出版年
2024年 | 375篇 |
2023年 | 1730篇 |
2022年 | 3194篇 |
2021年 | 5208篇 |
2020年 | 3802篇 |
2019年 | 3394篇 |
2018年 | 3902篇 |
2017年 | 4304篇 |
2016年 | 4496篇 |
2015年 | 5496篇 |
2014年 | 7425篇 |
2013年 | 9780篇 |
2012年 | 10671篇 |
2011年 | 12223篇 |
2010年 | 10754篇 |
2009年 | 10623篇 |
2008年 | 10469篇 |
2007年 | 9740篇 |
2006年 | 9031篇 |
2005年 | 7350篇 |
2004年 | 5864篇 |
2003年 | 4988篇 |
2002年 | 4828篇 |
2001年 | 3983篇 |
2000年 | 3334篇 |
1999年 | 2725篇 |
1998年 | 3174篇 |
1997年 | 2260篇 |
1996年 | 1870篇 |
1995年 | 1506篇 |
1994年 | 1125篇 |
1993年 | 1014篇 |
1992年 | 708篇 |
1991年 | 670篇 |
1990年 | 554篇 |
1989年 | 527篇 |
1988年 | 397篇 |
1987年 | 337篇 |
1986年 | 308篇 |
1985年 | 256篇 |
1984年 | 220篇 |
1983年 | 181篇 |
1982年 | 171篇 |
1981年 | 154篇 |
1980年 | 146篇 |
1979年 | 112篇 |
1978年 | 101篇 |
1977年 | 136篇 |
1976年 | 170篇 |
1975年 | 88篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ascorbic acid and glutathione (GSH) are antioxidants and free radical scavengers that provide the first line of defense against oxidative damage in the CNS. Using HPLC with electrochemical detection, we determined tissue contents of these antioxidants in brain and spinal cord in species with varying abilities to tolerate anoxia, including anoxia-tolerant pond and box turtles, moderately tolerant garter snakes, anoxia-intolerant clawed frogs (Xenopus laevis), and intolerant Long-Evans hooded rats. These data were compared with ascorbate and GSH levels in selected regions of guinea pig CNS, human cortex, and values from the literature. Ascorbate levels in turtles were typically 100% higher than those in rat. Cortex, olfactory bulb, and dorsal ventricular ridge had the highest content in turtle, 5-6 mumol g-1 of tissue wet weight, which was twice that in rat cortex (2.82 +/- 0.05 mumol g-1) and threefold greater than in guinea pig cortex (1.71 +/- 0.03 mumol g-1). Regionally distinct levels (2-4 mumol g-1) were found in turtle cerebellum, optic lobe, brainstem, and spinal cord, with a decreasing anterior-to-posterior gradient. Ascorbate was lowest in white matter (optic nerve) in each species. Snake cortex and brainstem had significantly higher ascorbate levels than in rat or guinea pig, although other regions had comparable or lower levels. Frog ascorbate was generally in an intermediate range between that in rat and guinea pig. In contrast to ascorbate, GSH levels in anoxia-tolerant turtles, 2-3 mumol g-1 of tissue wet weight, were similar to those in mammalian or amphibian brain, with no consistent pattern associated with anoxia tolerance. GSH levels in pond turtle CNS were significantly higher (by 10-20%) than in rat for several regions but were generally lower than in guinea pig or frog. GSH in box turtle and snake CNS were the same or lower than in rat or guinea pig. The distribution GSH in the CNS also had a decreasing anterior-to-posterior gradient but with less variability than ascorbate: levels were similar in optic nerve, brainstem, and spinal cord. The paradoxically high levels of ascorbate in turtle brain, which has a lower rate of oxidative metabolism than mammalian, suggest that ascorbate is an essential cerebral antioxidant. High levels may have evolved to protect cells from oxidative damage when aerobic metabolism resumes after a hypoxic dive. 相似文献
992.
Young -Wook Kim Jin -Soo Song Sang -Whan Park June -Gunn Lee 《Journal of Materials Science》1993,28(14):3866-3868
A new, faster process was developed for the fabrication of Nicalon-fibre-reinforced SiC composites by combining polymer solution infiltration (PSI) and chemical vapour infiltration (CVI). The process led to the near-net-shape fabrication of fibre-reinforced ceramic-matrix composites and reduced infiltration time. Typical flexural strength and fracture toughness of these composites were 296 MPa and 10.9 MPa m1/2 at room temperature (RT) and 252 MPa and 9.6 MPa m1/2 at 1000 °C, respectively. The composites exhibited load-carrying capability after crack initiation. 相似文献
993.
B~(10)(n,α)Li~7裂变反应的径迹显微照相技术被用来研究含硼Fe-30%Ni合金高温热变形再结晶过程中硼在晶界上的偏聚行为,实验表明,含硼Fe-30%Ni合金1000℃热变形20%再结晶过程中,运动晶界前方发生硼偏聚而后方发生贫硼现象,而在静止晶界上看不到硼偏聚,随着再结晶新晶粒的长大,运动晶界上的硼偏聚变弱。本文提出,可以用晶界对溶质原子热运动散射的量子理论来解释溶质原子在运动晶界上的偏聚现象。 相似文献
994.
掺富勒烯SiO2气凝胶的制备与发光性质研究 总被引:3,自引:0,他引:3
本文利用溶胶-凝胶工艺成功地将富勒烯掺入了SiO2气凝胶的纳米孔洞中,在Ar^+离子激光激发下,观察到了很强的可见光发射,对该现象作了初步的解释。 相似文献
995.
996.
Motion based object tracking with mobile camera 总被引:4,自引:0,他引:4
Kyu Won Lee Seong Won Ryu Soo Jong Lee Kyu Tae Park 《Electronics letters》1998,34(3):256-258
An efficient algorithm which detects and tracks a moving object is proposed. The global motion energy caused by camera movement is eliminated by finding the maximal matching position with the cross-correlation value between two consecutive frames using the spiral scanning technique. The region of a moving object is segmented exactly by combining the results of the temporal derivatives and edge map from one image, and the actual centroid of the moving object is determined as the mid-point of the cumulative distribution of the projection profile of the region 相似文献
997.
998.
A wide variety of speech recognition distortion measures have been proposed and tested, including some especially effective ones. It is shown that there is a general framework, based on the concepts of information theory, linking most of these measures. The distortion measure between any two speech spectra can be defined in terms of the distortions between the associated probability distributions. This general framework defines three broad families of distortion measures for speech recognition and provides a consistent way of combining the energy and the spectral information of a phonetic event. In addition, the cepstral-domain representation for several distortion measures is derived, allowing comparison of these measures in a domain that also yields convenient equations for their practical implementation 相似文献
999.
1000.
Numerical prediction of the wind flow and pollutant dispersion over two-dimensional hilly terrain is presented. The wind tunnel
experiments are conducted to validate the numerical results of the flow field. Measured mean velocity profiles, turbulence
characteristics, and surface pressure distributions show good agreement with the numerical predictions. The hypothesis of
Reynolds number independency for an atmospheric boundary layer flow over aerodynamically rough terrain is numerically confirmed.
The linear theory provides generally good prediction of speed-up characteristics for gently sloped low hills. The effect of
two-dimensional double hills on the dispersion of pollutants from continuously or temporally released line source of different
emission heights and locations is also investigated. The ground-level concentrations are considerably reduced as emission
heights are increased. The variances of ground-level concentration with respect to time from a temporally released source
are strongly influenced by the flow separation. 相似文献