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991.
992.
A flow past a circular-section cylinder with a perforated conic shroud, in which the perforation is located at the peak of the conic disturbance as the shroud installed on the cylinder and uniformly distributed with several circular holes, is numerically simulated at a Reynolds number of 100. Two factors in the perforation are taken into account, i.e. the attack angle relative to the direction of incoming flow and diameter of holes. The effect of such perforation on the drag, lift and vortex-shedding frequency is mainly investigated. Results have shown that variation of the attack angle has a little effect, especially on the drag and vortex-shedding frequency, except in certain cases due to the varied vortex-shedding patterns in the near wake. The increasing hole diameter still exhibits a little effect on the drag and frequency of vortex shedding, but really reduces the lift, in particular at larger wavelength, such as the lift reduction reaching almost 66%–68% after introducing the perforation. 相似文献
993.
Cherts in the Zhongba mélange of the western Yarlung Zangbo Suture Zone (YZSZ) contain well preserved radiolarian assemblages. These radiolarian assemblages indicate that the Zhongba mélange has middle Jurassic–early Cretaceous remnant, are coeval with those from the central and eastern parts of the YZSZ. Cherts from the Najiu area yield Aalenian to Aptian radiolarians, while cherts interbedded with siliceous mudstones from the Bielongjiala area yield Aptian radiolarians, indicating that terrigenous-derived sediments were deposited during early Aptian. The above observations indicate that the entire YZSZ have a similar geochronological framework and thus they underwent similar geological evolution: (1) during the Jurassic, the Neo-Tethys was a wide ocean with pelagic sediments distal from continents; (2) during the Cretaceous (around 130–120 Ma), the Neo-Tethys started to subduct along the southern margin of the Lhasa block, and terrigenous-derived siliceous mudstone began deposition. 相似文献
994.
基于少量控制点的Radarsat-2影像快速几何纠正技术研究 总被引:1,自引:0,他引:1
Radarsat-2卫星依据其搭载的GPS接收机可实现3倍中误差小于60m的精确实时定轨。由此本文提出依据其影像元数据信息实现快速几何纠正的方法,该方法利用少量的几个控制点来消除Radarsat-2影像与待纠正参考系间的系统误差,从而实现Radarsat-2影像的快速几何纠正。本文并依据SAR斜距成像原理的纠正公式和实地采集的GPS数据,验证了元数据中所提供RPF模型的内部精度和外部符合精度。通过实验验证了本快速纠正技术可以获得中误差小于2个像素的平面几何纠正精度。 相似文献
995.
中国数字地震台网(CDSN)经过第3期升级改造后,硬件系统性能有了质的飞跃,设备的可维护性及数据稳定性全面提升,系统的复杂程度也逐渐攀升。本文详细介绍CDSN技术系统的构成及升级过程,阐述技术规范和实施细节及对CDSN技术改造发挥的作用。 相似文献
996.
乌鲁木齐冬季大气细颗粒物水溶性离子特征及来源 总被引:2,自引:0,他引:2
为了探讨乌鲁木齐冬季大气细颗粒物的污染水平及其水溶性离子的特征,于2013年1-3月采集大气PM2.5样品,并利用离子色谱仪分析其中的水溶性离子,采用硫转化率、离子相关性分析及后向轨迹模型对其可能来源进行了讨论。结果表明:观测期间采样点PM2.5平均质量浓度为170.13±51.39 μg·m-3,水溶性离子总浓度平均值为53.47±23.76 μg·m-3,其中3种二次离子(SO42-、NO3-和NH4+)是水溶性离子的主要组分;不同天气类型下PM2.5和离子浓度差异较大,雾、霾天气二次离子浓度占总浓度的81.99%和86.24%,硫转化率均大于0.1;春节期间由于燃放大量的烟花爆竹,使得PM2.5可溶性离子K+和Cl-浓度急剧上升;NH4+与SO42-、NO3-相关系数分别为0.975和0.748,(NH4)2SO4、NH4HSO4和NH4NO3是细颗粒物水溶性组分的可能结合方式,Cl-和K+的相关性显著,说明两者具有同源性;固定排放源仍然是乌鲁木齐大气污染物的主要来源,局地大气输送会使大气污染加重。 相似文献
997.
998.
CHEN Zhong HUANG ChiYue WU BiHao ZHAO MeiXun & YAN Wen Key Laboratory of Marginal Sea Geology South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou China 《中国科学:地球科学(英文版)》2010,(3)
Aluminum is an active amphoteric metal element, but native aluminum (Al°) can be formed and preserved in special condi- tions so that its occurrence has unique geological significance. In this paper, the coarse fractions ( 63 μm) of 269 samples of surface sediments and of 165 sediment samples in four cores from the South China Sea (SCS) were picked out and analyzed. Al° particles were found in only five surface samples and three samples in two sedimentary cores. The particles are grayish or silver white in color with a strongly metallic luster and high plasticity, and they occur as irregular plats, elongated shapes and spherules with botryoidal structures. The identified Al° particles belong to cubical system, Fm3m space group with a varying from 4.028×10?1 nm to 4.059×10?1 nm. Their chemical compositions consist mainly of Al (95.07%–99.84%) and minor amounts of Si, Fe, Ti, S, Zn, Mg, Ca and trace amounts of K, Na, Cu, Co, P and Mn. Our results show that the identified Al° is authigenic, formed in situ in the prospective areas of gas hydrates, and that it is an unreported type of Al° occurrence. A possi- ble mechanism of Al° formation is proposed: under the strongly reducing micro environments at cold seeps, dissolved Al3+ in sediments is reduced to its metallic state by the strongly reducing agent of H2 through microbial processes and enzyme action. As a result, the Al° particles from the prospective gas hydrate areas in the SCS are the special products, representing the first report on Al° from the sediments at cold seeps. The proposed mechanism provides a theoretical basis for further studies on the special biogeochemical processes at cold seeps. 相似文献
999.
1000.