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   284529篇
  免费   9875篇
  国内免费   9446篇
测绘学   8117篇
大气科学   24791篇
地球物理   60961篇
地质学   102908篇
海洋学   24361篇
天文学   57645篇
综合类   3980篇
自然地理   21087篇
  2022年   2282篇
  2021年   3388篇
  2020年   3556篇
  2019年   3950篇
  2018年   5222篇
  2017年   4798篇
  2016年   7310篇
  2015年   5371篇
  2014年   8360篇
  2013年   15561篇
  2012年   7874篇
  2011年   9410篇
  2010年   8560篇
  2009年   11092篇
  2008年   9821篇
  2007年   9347篇
  2006年   10612篇
  2005年   8557篇
  2004年   8357篇
  2003年   7884篇
  2002年   7400篇
  2001年   6683篇
  2000年   6824篇
  1999年   6543篇
  1998年   6302篇
  1997年   6118篇
  1996年   5644篇
  1995年   5284篇
  1994年   4894篇
  1993年   4506篇
  1992年   4145篇
  1991年   4007篇
  1990年   4060篇
  1989年   3792篇
  1988年   3566篇
  1987年   4001篇
  1986年   3555篇
  1985年   4330篇
  1984年   4807篇
  1983年   4472篇
  1982年   4393篇
  1981年   3987篇
  1980年   3681篇
  1979年   3543篇
  1978年   3505篇
  1977年   3292篇
  1976年   3056篇
  1975年   2964篇
  1974年   2919篇
  1973年   3075篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
271.
In the upper Chesapeake Bay (Maryland, U.S.A.) field surveys were conducted at 18 multiple longshore sand bar sites. The multiple bar systems were found in water depths less than approximately 2 m (mean sea level), and exhibited mild bottom slopes of 0·0052 or less. The number of bars composing each system ranged from four to 17 and the spacing between the crests typically increased in the offshore direction, ranging from 12 to 70 m. Bar height also typically increased with distance offshore and ranged from 0·03 to 0·61 m. A grain size analysis of crest and trough sediment did not reveal any significant differences and the sediment was categorized as ‘fine sand’. A review of the literature data indicated that the Chesapeake Bay multiple bars possessed similar characteristics to those found in Gelding Bay (Baltic Sea); similarities in fetch, wave height and tidal range between the two bays may account for this finding. The surf-scaling parameter indicated that the multiple bar systems were extremely dissipative with regard to wave energy, and wave height appeared to be an important factor in controlling bar spacing and bar height. A multiple wave break point hypothesis was discussed as a possible mechanism for the formation of Chesapeake Bay multiple longshore bars, and limited observational evidence appeared to support such a mechanism.  相似文献   
272.
The metasediments in the Chamba region experienced three phases of deformation: DF1, DF2 and DF3.Folded quartz veins are co-folded with the F2 crenulation folds. Their geometric and tectonic significance is studied by microstructures and shortening adjacent to the discrete crenulation cleavage, S2. Microstructures of folded vein segments, their geometric changes and truncation to cleavage (S2) are mainly due to pressure-solution phenomena and the estimated volume loss from reconstructed vein segments range from 16 to 25.5%,which is closely related to volume decrease (26%) calculated from the polydeformed slates of North Wales areas.  相似文献   
273.
Abstract— A large impact event 500 Ma ago shocked and melted portions of the L‐chondrite parent body. Chico is an impact melt breccia produced by this event. Sawn surfaces of this 105 kg meteorite reveal a dike of fine‐grained, clast‐poor impact melt cutting shocked host chondrite. Coarse (1–2 cm diameter) globules of FeNi metal + sulfide are concentrated along the axis of the dike from metal‐poor regions toward the margins. Refractory lithophile element abundance patterns in the melt rock are parallel to average L chondrites, demonstrating near‐total fusion of the L‐chondrite target by the impact and negligible crystal‐liquid fractionation during emplacement and cooling of the dike. Significant geochemical effects of the impact melting event include fractionation of siderophile and chalcophile elements with increasing metal‐silicate heterogeneity, and mobilization of moderately to highly volatile elements. Siderophile and chalcophile elements ratios such as Ni/Co, Cu/Ga, and Ir/Au vary systematically with decreasing metal content of the melt. Surprisingly small (?102) effective metal/silicate‐melt distribution coefficients for highly siderophile elements probably reflect inefficient segregation of metal despite the large degrees of melting. Moderately volatile lithophile elements such K and Rb were mobilized and heterogeneously distributed in the L‐chondrite impact breccias whereas highly volatile elements such as Cs and Pb were profoundly depleted in the region of the parent body sampled by Chico. Volatile element variations in Chico and other L chondrites are more consistent with a mechanism related to impact heating rather than condensation from a solar nebula. Impact processing can significantly alter the primary distributions of siderophile and volatile elements in chondritic planetesimals.  相似文献   
274.
张威  朱进 《天文学进展》2002,20(4):379-383
对1995年至2001年《Natue》和《Science》上发表的天文学论文的统计表明,行星天文学领域的论文数量明显超过天文学的其它分支学科,占天文学总论文数的1/3左右。从这个角度来看,行星天文学是天文学最活跃和重要的分支学科之一。对这些论文具体内容的分析可以给出当前行星天文学领域的若干热点问题。相比之下,我国对这一重要领域的关注和投入还远远不够。  相似文献   
275.
地震前后垂直形变场动态演化的量化指标   总被引:3,自引:2,他引:1  
提出一种表达垂直形变场动态演化过程的量化指标——区域应变率、应变集中度。在此基础上,对南北地震带各水准监测区近30年的垂直形变资料进行了实际计算.并结合具体震例进行了对比研究。结果表明:量化指标在一定程度上能够反映地震前后形变场的动态演化过程和地壳运动状态,对地震的中长期预报有一定的积极意义。  相似文献   
276.
In this paper we analyse error-removing techniques used without sufficient theoretical support in a previous paper, where Chandrasekhar's higher-order perturbation theories were developed for either uniformly or differentially-rotating polytropic stars.  相似文献   
277.
An analysis of the UV spectra of 28 asteroids obtained with the Internal Ultraviolet Explorer (IUE) satellite is presented. The spectra lie within the range 2100–3200 Å. Our results are examined in terms of both asteroid classification and of current ideas concerning the surface mineralogy of asteroids. For all the asteroids examined, UV reflectivity declines approximately linearly toward shorter wavelengths. In general, the same taxonomic groups are seen in the UV as in the visible and IR, although there is some evidence for asteroids with anomalous UV properties and for UV subclasses within the S class. No mineral absorption features are reported of strength similar to the strongest features in the visible and IR regions, but a number of shallow absorptions do occur and may provide valuable information on the surface composition of many asteroids.  相似文献   
278.
An empirical kinematic method developed by Hakamada and Akasofu (1982) is calibrated on the basis of a one-dimensional MHD solution. The calibrated results are used to simulate the stream-stream interaction and the background corotating structure in a simple situation and also during 22 November–6 December 1977. The solar wind disturbances caused by solar activities during this period are then introduced into the above background stream in simulating the heliospheric disturbance event which was observed by an aligned set of spacecraft at distances between 0.6 and 1.6 a.u. The observations and the simulated results are satisfactory, and a little more refinement in the simulation could reconstruct reasonably well the data by filling the data gaps in the solar wind speed, the density and the IMF magnitude.  相似文献   
279.
Abstract— Fifty-odd years of tektite research are reviewed, proceeding from the discovery of the first North American tektites in 1936. This included the early recognition that tektites were terrestrial objects rather than meteorites and that the glassy particles in tektites were fused quartz (lechatelierite). Later, during National Science Foundation-supported research, it was found that some tektites appeared to have formed as puddles of melt, that the content and character of bubbles in lechatelierite can be used as a relative temperature scale, that rayed bubbles in tektites formed from hydrous minerals, that bubbles in tektites formed chiefly from water which was absorbed into the walls of the bubbles leaving vacuums, and that “fingers” in the surficial part of some tektites may have formed by differential volatilization. Some unpublished observations and adventures are briefly reported.  相似文献   
280.
Abstract— Characterization of the microstructural features of the metal of the Santa Catharina meteorite was performed using a variety of electron optical techniques. Sample USNM#6293 is chemically homogeneous on the micron scale and has a Ni content of 28.2 wt.%. Its microstructure is similar to that of the Twin City ataxite and contains clear taenite II, i.e., fcc taenite with domains of tetrataenite, < 10 nm in size. Sample USNM#3043 is a more typical Santa Catharina specimen with dark and light regions as observed with the light optical microscope. The dark regions are inhomogeneous and contain 45–50 wt.% Ni and 7–12 wt.% O. The light regions are homogeneous and contain 35 wt.% Ni and no detectable oxygen. The microstructure is that of cloudy zone, i.e., islands of tetrataenite, ~20 nm in size, in a honeycomb matrix. The honeycomb phase contains Ni rich oxide in the dark regions and contains metal, fcc taenite, in the light regions. The original metal structure of USNM#3043 is cloudy zone which formed during cooling into the low temperature miscibility gap of the Fe-Ni phase diagram. The dark regions were developed from the metal by selective corrosion of the honeycomb structure, transforming it into Ni containing oxides, possibly non-stoichiometric Fe2NiO4 while retaining the tetrataenite islands. Using the results of this study, many of the existing discrepancies concerning the microstructure of Santa Catharina can be explained.  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