首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2143篇
  免费   299篇
  国内免费   614篇
测绘学   144篇
大气科学   182篇
地球物理   349篇
地质学   1416篇
海洋学   307篇
天文学   311篇
综合类   137篇
自然地理   210篇
  2024年   3篇
  2023年   28篇
  2022年   39篇
  2021年   55篇
  2020年   66篇
  2019年   81篇
  2018年   63篇
  2017年   73篇
  2016年   71篇
  2015年   101篇
  2014年   102篇
  2013年   122篇
  2012年   125篇
  2011年   143篇
  2010年   152篇
  2009年   168篇
  2008年   147篇
  2007年   157篇
  2006年   173篇
  2005年   125篇
  2004年   130篇
  2003年   127篇
  2002年   84篇
  2001年   115篇
  2000年   87篇
  1999年   84篇
  1998年   61篇
  1997年   64篇
  1996年   49篇
  1995年   44篇
  1994年   31篇
  1993年   42篇
  1992年   35篇
  1991年   35篇
  1990年   17篇
  1989年   9篇
  1988年   14篇
  1987年   8篇
  1986年   4篇
  1985年   6篇
  1984年   2篇
  1983年   3篇
  1982年   5篇
  1980年   2篇
  1979年   1篇
  1978年   2篇
  1954年   1篇
排序方式: 共有3056条查询结果,搜索用时 15 毫秒
991.
流动注射-火焰原子吸收光谱法测定地质样品中的常量金   总被引:4,自引:3,他引:1  
样品经室温烧至650℃高温后灼烧保持1.5 h,用浓王水溶解。用泡塑分两次吸附富集样品中的常量金,将富集同一样品中金的两块泡塑置于盛有10 mL 10 g/L硫脲溶液的玻璃试管中,沸水浴保持20 min,使金释放出来,所得溶液应用原子吸收分光光度计采用流动注射测定。对仪器的测定条件进行了优化比较。方法检出限(3σ)为0.022μg/g,精密度(RSD,n=11)小于8.0%,测定范围为0.05~60μg/g。经国家金矿石分析标准物质验证,其测定值与标准值基本一致。  相似文献   
992.
模拟月壤研究进展及CUG-1A模拟月壤   总被引:4,自引:1,他引:3  
利用我国东北部吉林省辉南县新生代火山岩为初始原料,初步研制了模拟月壤CUG-1A.在研究CUG-1A的化学性质的同时,重点研究了其物理力学性质.数据分析表明,CUG-1A与Apollo14采样点的月壤样品有着相似的化学成分、矿物组成和物理力学性质,是一种理想的低钛型月海月壤的模拟样品.  相似文献   
993.
天然半导体矿物金红石因结构中含有类质同象替代杂质元素V和Fe,具有一定的可见光吸收和光催化活性。为改善金红石的日光光催化性能,在H2还原气氛下,对天然金红石粉末进行500~900℃不同温度的热处理改性研究。紫外-可见漫反射吸收光谱(UV-Vis diffuse reflectance absorption spectra)表明H2还原处理显著改善了金红石在可见光区460~750 nm波段的光吸收,其中900℃处理样品的光吸收提升最为明显。电子顺磁共振(EPR)和X射线光电子能谱(XPS)测试表明,随着还原温度升高,杂质元素V和Fe从高价态(V5+,Fe3+)向较低的价态(V4+,V3+,Fe2+)转化,同时金红石表面的化学吸附水含量也有所增加。本文认为H2还原热处理引起的过渡金属元素价态的改变,尤其是较低氧化态V离子(V4+和V3+)的形成,可能是导致金红石样品可见光吸收显著增加的主要原因。  相似文献   
994.
本文针对火力发电厂供热机组中吸收式热泵的应用,按照供热、节电分别计算的思路,提出一种分析其经济效益的方法。  相似文献   
995.
马营堡井田基本构造形态为一单斜构造,区内不同岩性地层的电阻率、密度、声波时差及伽马值均有明显差异,其中煤层的电阻率、密度及声波时差曲线均表现出出高异常及自然伽马低异常;而砂岩随粒度的变细及泥质含量的增加,自然伽玛逐渐增大。依据该区岩性基本物性特征,确定了井田内新生界与基岩、下石盒子组与山西组分界及主采煤层4、9号等主要标志层的响应特征。这些地层标志在测井曲线上形态,可作为该区地层对比的重要依据。  相似文献   
996.
This paper investigates the effects that geometry and control have on the absorption characteristics of wavemakers. It presents the hydrodynamic coefficients for piston and bottom hinged flap wavemakers and also for two novel wavemaker profiles. Absorption efficiencies are presented for wavemakers using one, two and three control coefficients for reactive control. This is then used to analyse the absorption efficiency of each of the different wavemakers based upon the geometry and the control strategy used. It is shown that the amount of absorption for a given paddle differs greatly depending on the choice of control coefficients used to implement complex conjugate control. Increased absorption can be achieved over a broader bandwith of frequencies when the geometry of the wavemaker is optimised for one specific frequency.  相似文献   
997.
Ichthyoplankton assemblages were compared between regions dominated by the oligotrophic East Australian Current (EAC) and the inner-shelf waters off southeastern Australia, to determine if the early life history of fish was related to the separation of the EAC from the coast, producing different water masses as well as characteristic taxa. Samples were collected at the surface and in sub-surface waters, at 50 and 100 m isobath stations, during two summer research voyages in November 1998 and January 1999. On both voyages the study region was characterized by coastal and EAC waters in the north (∼31°S), and in the south by topographically induced upwelling (∼31°S), associated with narrowing of the continental shelf and separation of the EAC from the coast. Among the 111 families of larval fish, we observed distinctive assemblages of ichthyoplankton associated with the two different water masses. A greater abundance of the Carangidae, Labridae, Lutjanidae, Microcanthidae, Myctophidae and Scombridae was associated with the nutrient poor EAC water mass, while the Callionymidae, Clupeidae, Platycephalidae and Sillaginidae were mostly found in the cooler and/or fresher inner-shelf water mass. We assessed these patterns with opportunistic samples from an unusual, wind-driven upwelling event in the north (∼31°S) earlier in the November voyage. The relative abundance of these 10 characteristic families distinguished this wind-driven upwelling event from the subsequent relaxation and predominance of the EAC assemblage at this location just 6 d later. Distinctive and abundant families such as larval clupeids, relative to larval carangids, could be a useful marker of inner-shelf, EAC and mixed water masses in the absence of robust hydrographic data. This and related studies indicate contrast in early life histories of Sardinops sagax and Trachurus spp., which appear to spawn respectively in the inner-shelf and outer-shelf waters. The post-flexion stages of S. sagax predominate in the outer-shelf and Tasman Front, while post-flexion Trachurus spp. predominate in inner-shelf water masses.  相似文献   
998.
本文对水体吸收光谱的2种测量方法进行了比较,这2种方法分别为基于分光光度计的定量滤膜技术和吸收衰减仪的现场测量方法。为了检验和比较这2种测量方法的不同,同时对两类不同类型水体的吸收系数进行了测量。其中一类水体为实验室培养的单种藻类水体,共有36个样本,包括4大类,分属16个种;另一类水体为黄海水域的水体,包括49个站位数据,基本覆盖了南黄海水域,440 nm的吸收系数在0.05~2.0 m-1之间。结果表明,不同波段实验室培养的单种藻类水体两者测量吸收系数的相关系数平方在0.96~0.98之间,相对均方根误差为14%~16%之间;对于黄海水体的测量,相关系数的平方在0.90~0.94之间,相对均方根误差在14%~27%之间。2种测量方法对这两类水体吸收系数的测量均具有很高的一致性,其中实验室培养的单种藻类水体的测量结果的一致性好于黄海水体的测量结果。  相似文献   
999.
Using infrared sensors to detect ice clouds in different atmospheric layers is still a challenge. The different scattering and absorption properties of longwave and shortwave infrared channels can be utilized to fulfill this purpose.In this study, the release of Suomi-NPP Cross-track Infrared Sounder(Cr IS) full spectrum resolution is used to select and pair channels from longwave(~ 15 μm) and shortwave(~4.3 μm) CO_2 absorption bands under stricter conditions, so as to better detect ice clouds. Besides, the differences of the weighting function peaks and cloud insensitive level altitudes of the paired channels are both within 50 h Pa so that the variances due to atmospheric conditions can be minimized. The training data of clear sky are determined by Visible Infrared Imaging Radiometer Suite(VIIRS) cloud mask product and used to find the linear relationship between the paired longwave and shortwave CO_2 absorption channels. From the linear relationship, the so-called cloud emission and scattering index(CESI) is derived to detect ice clouds. CESI clearly captures the center and the ice cloud features of the Super Typhoon Hato located above 415 h Pa. Moreover, the CESI distributions agree with cloud top pressure from the VIIRS in both daytime and nighttime in different atmospheric layers.  相似文献   
1000.
In this article we discuss the importance of high‐resolution absorption spectroscopy for our understanding of the distribution and physical nature of the gaseous circumgalactic medium (CGM) that surrounds the Milky Way. Observational and theoretical studies indicate a high complexity of the gas kinematics and an extreme multi‐phase nature of the CGM in low‐redshift galaxies. High‐precision absorption‐line measurements of the Milky Way's gas environment thus are essential to explore fundamental parameters of circumgalactic gas in the local Universe, such as mass, chemical composition, and spatial distribution. We shortly review important characteristics of the Milky Way's CGM and discuss recent results from our multi‐wavelength observations of the Magellanic Stream. Finally, we discuss the potential of studying the warm‐hot phase of the Milky Way's CGM by searching for extremely weak [Fe X] λ 6374.5 Å and [Fe XIV] λ 5302.9 Å absorption in optical QSO spectra. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号