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   7459篇
  免费   1600篇
  国内免费   2263篇
测绘学   866篇
大气科学   1282篇
地球物理   1807篇
地质学   4175篇
海洋学   1192篇
天文学   207篇
综合类   760篇
自然地理   1033篇
  2024年   35篇
  2023年   135篇
  2022年   387篇
  2021年   446篇
  2020年   343篇
  2019年   435篇
  2018年   474篇
  2017年   465篇
  2016年   500篇
  2015年   459篇
  2014年   528篇
  2013年   548篇
  2012年   585篇
  2011年   599篇
  2010年   645篇
  2009年   538篇
  2008年   523篇
  2007年   467篇
  2006年   400篇
  2005年   325篇
  2004年   261篇
  2003年   232篇
  2002年   218篇
  2001年   240篇
  2000年   190篇
  1999年   229篇
  1998年   156篇
  1997年   158篇
  1996年   112篇
  1995年   121篇
  1994年   114篇
  1993年   110篇
  1992年   75篇
  1991年   49篇
  1990年   37篇
  1989年   38篇
  1988年   39篇
  1987年   21篇
  1986年   16篇
  1985年   10篇
  1984年   8篇
  1983年   6篇
  1982年   11篇
  1981年   7篇
  1980年   12篇
  1979年   4篇
  1976年   2篇
  1958年   4篇
  1954年   5篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
信息孤岛是信息化建设的必经过程,信息共享是智慧城市建设所要解决的重点问题。作为智慧城市的重要组成部分,智慧国土的建设必然需要对国土资源数据进行整合。本文基于现有研究成果,结合增城市国土资源数据的实际情况,提出基于数据仓库的国土资源数据整合方案,并对增城市1∶2 000的空间基础数据进行了有效整合。  相似文献   
942.
The improvement on the calculation of anti-Stokes energy transfer rate is studied in the present work. The additional proportion coefficient between Stokes and anti-Stokes light intensities of quantum Raman scattering theory as compared with the classical Raman theory is introduced to successfully describe the anti-Stokes energy transfer. The theoretical formula for the improvement on the calculation of anti-Stokes energy transfer rate is derived for the first time in this study. The correctness of introducing coefficient exp{ΔE / kT} from well-known Raman scatter theory is demonstrated also. Moreover, the experimental lifetime measurement in Er0.01YbxY1 - 0.01 - xVO4 crystal is performed to justify the validity of our important improvement in the original phonon-assisted energy transfer theory for the first time.  相似文献   
943.
从储层岩性、成岩作用、隔夹层研究等方面入手,利用渗透率变异系数、突进系数、级差、隔夹层参数以及孔隙度、渗透率的变化对桩74块低渗油藏进行了储层非均质性研究。Es_3下Ⅱ油组储层为中孔、特低渗的湖底扇碎屑岩,渗透率与主流孔喉半径具有良好的相关性,成岩作用的差异是造成储层非均质性差异的一个因素。自下而上层内非均质性逐渐加强。夹层不甚发育,对储层非均质性影响不大。层间非均质较强,隔层发育,对储层非均质性影响较大。  相似文献   
944.
JCZ-1     
分析讨论了  相似文献   
945.
A meso‐scale particle model is presented to simulate the expansion of concrete subjected to alkali‐aggregate reaction (AAR) and to analyze the AAR‐induced degradation of the mechanical properties. It is the first attempt to evaluate the deterioration mechanism due to AAR using the discrete‐element method. A three‐phase meso‐scale model for concrete composed of aggregates, mortar and the interface is established with the combination of a pre‐processing approach and the particle flow code, PFC2D. A homogeneous aggregate expansion approach is applied to model the AAR expansion. Uniaxial compression tests are conducted for the AAR‐affected concrete to examine the effects on the mechanical properties. Two specimens with different aggregate sizes are analyzed to consider the effects of aggregate size on AAR. The results show that the meso‐scale particle model is valid to predict the expansion and the internal micro‐cracking patterns caused by AAR. The two different specimens exhibit similar behavior. The Young's modulus and compressive strength are significantly reduced with the increase of AAR expansion. The shape of the stress–strain curves obtained from the compression tests clearly reflects the influence of internal micro‐cracks: an increased nonlinearity before the peak loading and a more gradual softening for more severely affected specimens. Similar macroscopic failure patterns of the specimens under compression are observed in terms of diagonal macroscopic cracks splitting the specimen into several triangular pieces, whereas localized micro‐cracks forming in slightly affected specimens are different from branching and diffusing cracks in severely affected ones, demonstrating different failure mechanisms. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
946.
采用室内水泥池及活饵培育配套技术进行尖吻鲈苗种阶段工厂化育苗。投放刚孵化出膜的仔鱼160万尾,经室内池8d的培育,成活率为90%;经13d的培育其规格超过1cm,成活率为56.9%;经24d的培育平均规格达2cm左右,平均成活率为50%,共育出80万尾。苗种生长快,体质健壮,成活率高。关键技术是:基础设施配套;采用不同活饵,做到适口、及时转化、足量;定期过筛分级培育;水质控制良好;注意防病。2cm以上鱼种阶段采用多种方式培育,经15~20d的饲养,规格达3~4cm,平均成活率为65%~75%。  相似文献   
947.
Mineral inventories of enstatite chondrites; (EH and EL) are strictly dictated by combined parameters mainly very low dual oxygen (fO2) and sulfur (fS2) fugacities. They are best preserved in the Almahata Sitta MS‐17, MS‐177 fragments, and the ALHA 77295 and MAC 88136 Antarctic meteorites. These conditions induce a stark change of the geochemical behavior of nominally lithophile elements to chalcophile or even siderophile and changes in the elemental partitioning thus leading to formation of unusual mineral assemblages with high abundance of exotic sulfide species and enrichment in the metallic alloys, for example, silicides and phosphides. Origin and mode of formation of these exotic chondrites, and their parental source regions could be best scrutinized by multitask research experiments of the most primitive members covering mineralogical, petrological, cosmochemical, and indispensably short‐lived isotopic chronology. The magnitude of temperature and pressure prevailed during their formation in their source regions could eventually be reasonably estimated: pre‐ and postaccretionary could eventually be deduced. The dual low fugacities are regulated by the carbon to oxygen ratios estimated to be >0.83 and <1.03. These parameters not only induce unusual geochemical behavior of the elements inverting many nominally lithophile elements to chalcophile or even siderophile or anthracophile. Structure and mineral inventories in EL3 and EH3 chondrites are fundamentally different. Yet EH3 and EL3 members store crucial information relevant to eventual source regions and importantly possible variation in C/O ratio in the course of their evolution. EL3 and EH3 chondrites contain trichotomous lithologies (1) chondrules and their fragments, (2) polygonal enstatite‐dominated objects, and (3) multiphase metal‐rich nodules. Mineralogical and cosmochemical inventories of lithologies in the same EL3 indicate not only similarities (REE inventory and anomalies in oldhamite) but also distinct differences (sinoite‐enstatite‐graphite relationship). Oldhamite in chondrules and polygonal fragments in EL3 depict negative Eu anomaly attesting a common cosmochemical source. Metal‐dominated nodules in both EL3 and EH3 are conglomerates of metal clasts and sulfide fragments in EH3 and concentrically zoned C‐bearing metal micropebbles (≥25 μm ≤50 μm) in EL3 thus manifesting a frozen in unique primordial accretionary metal texture and composition. Sinoite‐enstatite‐diopside‐graphite textures reveal a nucleation and growth strongly suggestive of fluctuating C/O ratio during their nucleation and growth in the source regions. Mineral inventories, sulfide phase relations, sinoite‐enstatite‐graphite intergrowth, carbon and nitrogen isotopic compositions of graphite, spatial nitrogen abundance in graphite in metal nodules, and last but not least 129I/129Xe and 53Mn/53Cr systematics negate any previously suggested melting episode, pre‐accretionary or dynamic, in parental asteroids.  相似文献   
948.
Lou  Yidong  Gong  Xiaopeng  Gu  Shengfeng  Zheng  Fu  Feng  Yanming 《GPS Solutions》2017,21(1):177-186
GPS Solutions - Carrier phase ambiguity resolution over long baselines is challenging in BDS data processing. This is partially due to the variations of the hardware biases in BDS code signals and...  相似文献   
949.
汤池沟热田大地构造位置处于辽阳-本溪凹陷与凤城凸起的交接部位,属浅埋藏水热对流型热田。热储由中生代燕山期花岗斑岩、凝灰岩组成。热源为岩浆活动的余热。热流体主要靠侧向基岩裂隙水及断裂充水带的水补给,自北东向南西方向以脉状裂隙水运动为主,热流体直接受北东、北北东及北西向断裂控制。该热田储存有较丰富的地热资源,热能总量计算为 58.05 TJ,目前的热水开采量每年为 414 000 t,相当于消耗燃烧值为 29.3 MJ的标准煤每年 1 833.43 t,如果按此潜力开发,其开采年限预计可持续 80年以上。  相似文献   
950.
水流挟沙力分析与探讨   总被引:4,自引:0,他引:4  
水流挟沙力是悬沙输运计算中非常重要的问题之一,其计算形式常采用基于重力理论的经验和半经验公式.本文针对目前广泛采用的2种挟沙力关系式,选用4组实验水槽数据和2组天然河道实测资料分别讨论其关联性.结果表明,不论指数m在其取值范围内取何值,2个表达式皆存在一定的函数关联性.且当指数m取1和2/3时,直接表现出很强的线性关联.在挟沙力计算中采用流速的平方关系式,物理意义清晰,形式简单.在实际应用时,适当选取系数,挟沙力采用形式简单的平方关系式是完全可行的.  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