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   2524篇
  免费   404篇
  国内免费   593篇
测绘学   128篇
大气科学   447篇
地球物理   711篇
地质学   1375篇
海洋学   247篇
天文学   92篇
综合类   246篇
自然地理   275篇
  2024年   17篇
  2023年   43篇
  2022年   126篇
  2021年   122篇
  2020年   118篇
  2019年   125篇
  2018年   167篇
  2017年   155篇
  2016年   135篇
  2015年   120篇
  2014年   159篇
  2013年   172篇
  2012年   145篇
  2011年   151篇
  2010年   159篇
  2009年   158篇
  2008年   119篇
  2007年   132篇
  2006年   83篇
  2005年   72篇
  2004年   85篇
  2003年   70篇
  2002年   64篇
  2001年   70篇
  2000年   97篇
  1999年   88篇
  1998年   69篇
  1997年   74篇
  1996年   90篇
  1995年   61篇
  1994年   62篇
  1993年   47篇
  1992年   31篇
  1991年   21篇
  1990年   24篇
  1989年   20篇
  1988年   17篇
  1987年   13篇
  1986年   6篇
  1985年   8篇
  1984年   3篇
  1982年   5篇
  1981年   5篇
  1980年   3篇
  1978年   1篇
  1976年   2篇
  1975年   2篇
  1973年   1篇
  1936年   1篇
  1933年   1篇
排序方式: 共有3521条查询结果,搜索用时 15 毫秒
51.
This paper proposes a scheme of axial triple-well optical dipole trap by employing a simple optical system composed of a circular cosine grating and a lens. Three optical wells separated averagely by ~37 μm were created when illuminating by a YAG laser with power 1 mW. These wells with average trapping depth ~0.5 μK and volume ~74 μm3 are suitable to trap and manipulate an atomic Bose--Einstein condensation (BEC). Due to a controllable grating implemented by a spatial light modulator, an evolution between a triple-well trap and a single-well one is achievable by adjusting the height of potential barrier between adjacent wells. Based on this novel triple-well potentials, the loading and splitting of BEC, as well as the interference between three freely expanding BECs, are also numerically stimulated within the framework of mean-field treatment. By fitting three cosine functions with three Gaussian envelopes to interference fringe, the information of relative phases among three condensates is extracted.  相似文献   
52.
Saishitang Cu-polymetallic deposit is located in the southeast section of Late Paleozoic arcfold in the southeastern margin of Qaidam platform. Accoring to the geological process of the deposit,four mineralization episodes were identified: melt/fluid coexisting period(O),skarn period(A),first sulfide period(B) and second sulfide period(C),and 10 stages were finally subdivided. Three types of inclusions were classified in seven stages,namely crystal bearing inclusions(type I),aqueous inclusions(type II) and pure liquid inclusions(type III). Type I and II inclusions were observed in stage O1,having homogenization temperature from 252 to 431°C,and salinities ranging from 24.3% to 48.0%. Type I inclusion was present in stage A1,having homogenization temperature from 506 to 548°C,and salinities ranging from 39.4% to 44.6%. In stage B1,type II and III inclusions were observed,with homogenization temperature concentrating between 300–400°C,and salinities from 0.4% to 4.3%. Type II inclusions were present in stage B2,with homogenization temperature varying from 403 to 550°C. In stage C1,type I and II inclusion commonly coexisted,and constituted a boiling inclusion group,having homogenization temperatures at 187–463°C,and salinities in a range of 29.4%–46.8% and 2.2%–11.0%. Type II and III inclusions were developed in stage C2,having homogenization temperature at 124–350°C,and salinities ranging between 1.6% and 15.4%. In stage C3,type II and III inclusions were presented,with a homogenization temperature range of 164–360°C,and salinities varying from 4.0% to 11.0%. The results of micro-thermal analysis show that fluids are characterized by high temperature and high salinity in stage O1 and A1,and experienced slight decrease in temperature and dramatic decrease in salinity in stage B1 and B2. In stage C1,the salinity of fluid increased greatly and a further decrease of temperature and salinity occurred in stage C2 and C3. Fluids boiled in stage C1. With calculated pressure of 22 MPa from the trapping temperature of 284–289°C,a mineralization depth of 2.2 km was inferred. Results of Laser Raman Spectroscopy show high density of H2 O,CH4 and CO2 were found as gas composition. H-O isotope study indicates the oreforming fluids were the mixture of magmatic water and meteoric water. Physicochemical parameters of fluids show oxygen and sulfur fugacity experienced a decrease,and redox state is weakly reducing. Along with fluid evolution,oxidation has increased slightly. Comprehensive analysis shows that melt exsolution occurred during the formation of quartz diorite and that metal elements existed and migrated in the form of chlorine complex. Immiscible fluid separation and boiling widely occurred after addition of new fluids,bringing about dissociation of chlorine-complex,resulting in a great deal of copper precipitation. In conclusion,Saishitang deposit,controlled by regional tectonics,is formed by metasomatism between highly fractionated mineralization rock body and wall rock,and belongs to banded skarn Cu-polymetallic deposit.  相似文献   
53.
大水沟独立碲矿床的直接赋矿围岩为低绿片岩相变质岩。前人认为其原岩为陆源碎屑岩。通过对该赋矿岩系的地质特征(包括岩相学特征与副矿物组合标型特征)、岩石化学特征、硅同位素地球化学特征和稀土元素地球化学特征(包括元素钪的地球化学特征)的研究,证明其原岩为分异程度较低的幔源拉斑玄武岩,并具有典型岩浆玄武岩的演化趋势。  相似文献   
54.
本文对频率测深电偶源发射时的波区视电阻率响应进行了较系统的讨论,指出了国内现有理论中的一些错误。研究发现,在进行波区视电阻率响应的理论推导时,当基底电阻率有限时进行零级近似是正确的,但当基底电阻率为无限大时,进行零级近似是不完善的,必须进行修改。利用正确的公式进行了视电阻率响应的正演计算,结果表明,利用零级近似得出的公式与利用一级近似得出的公式计算的视电阻率响应曲线首支部分符合较好,但对于有高阻基底存在的地电断面,响应曲线尾支差异很大。  相似文献   
55.
近太湖新银鱼不仅在形态上与太湖新银鱼很相似,而且它们的个体大小、生长速度、食性等方面也十分相近。近几年在湖北地区的湖泊、水库相继发现了近太湖新银鱼,并在湖北省广水市徐家河水库成为优势种类,形成了较大生产规模。从目前研究的结果来看,近太湖新银鱼也是适宜我国湖泊、水库移植的银鱼种类。  相似文献   
56.
中国板块构造与盐类矿产   总被引:5,自引:1,他引:5  
中国大陆的华北、塔里木、准噶尔-兴安、华南及甘青藏等五大板块发展过程中于地块边缘活动带、陆间裂谷、陆内裂谷等场所生成的含盐盆地,沉积了各时期(O、C、T、J-E和Q)盐类矿床。通过分析对比认为,甘青藏板块兰坪-思茅拗陷带(J2-E1、Q),塔里木板块(C、K-E、Q),华北板块(O2),华南板块(T1-2)为成钾有利地带,中国东部众多的K-E红层盆地中成钾条件较差  相似文献   
57.
Groundwater plays a key role in arid regions as the majority of water is supplied by it. Groundwater pollution is a major issue, because it is susceptible to contamination from land use and other anthropogenic impacts. A study was carried out to build a vulnerability map for the Ordos Plateau using the DRASTIC model in a GIS environment. The map was designed to show the areas of the highest potential for groundwater pollution based on hydrogeological conditions. Seven environmental parameters, such as depth to water table, net recharge, aquifer media, soil media, topography, impact of the vadose zone media, and hydraulic conductivity of the aquifer, were incorporated into the DRASTIC model and GIS was used to create a groundwater vulnerability map by overlaying the available data. The results of this study show that 24.8 % of the study area has high pollution potential, 24.2 % has moderate pollution potential, 19.7 % has low pollution potential, and the remaining 31.3 % of the area has no risk of groundwater pollution. The regional distribution of nitrate is well correlated with the DRASTIC vulnerability index. In contrast to this, although the DRASTIC model indicated that the western part had no risk, nitrate concentrations were higher in some of these areas. In particular, higher nitrate concentrations were recorded along river valleys and around lakes, such as the Mulin River valley. This is mainly caused by the intensive agricultural development and favorable conditions for recharge along river valleys.  相似文献   
58.
一株产铁载体细菌的筛选及其与云母的相互作用   总被引:2,自引:0,他引:2  
        产铁载体细菌与云母类矿物相互作用的研究有助于了解矿物生物风化和土壤形成的演化规律和机理。采用纯培养法 自南京龙山废钾矿区酸模根际土壤分离筛选到一株高产荧光铁载体的细菌Z6,通过16S rDNA 序列分析和生理生化反应将其 鉴定为假单胞菌属(Pseudomonas sp.);通过室温静置培养试验研究Z6 菌株与云母的相互作用结果表明,细菌生长代谢前期 (8~15 d)快速产生大量铁载体,菌株与矿物共培养75 d 后,产铁载体细菌对云母类矿物确实产生了风化作用,黑云母组发 酵液中Fe 含量增加了211 倍,Si 含量增加了约27.8 倍,SEM 显示云母表面出现溶蚀坑,细菌可以在矿物表面形成生物膜或 矿物- 细菌复合物。不同矿物对共培养体系中细菌数量、荧光铁载体含量、溶液pH 的影响不同。  相似文献   
59.
Reservoirs are the most important constructions for water resources management and flood control. Great concern has been paid to the effects of reservoir on downstream area and the differences between inflows and dam site floods due to the changes of upstream flow generation and concentration conditions after reservoir’s impoundment. These differences result in inconsistency between inflow quantiles and the reservoir design criteria derived by dam site flood series, which can be a potential risk and must be quantificationally evaluated. In this study, flood frequency analysis (FFA) and flood control risk analysis (FCRA) methods are used with the long reservoir inflow series derived from a multiple inputs and single output model and a copula-based inflow estimation model. The results of FFA and FCRA are compared and the influences on reservoir flood management are also discussed. The Three Gorges Reservoir (TGR) in China is selected as a case study. Results show that the differences between the TGR inflow and dam site floods are significant which result in changes on its flood control risk rates. The mean values of TGR’s annual maximum inflow peak discharge and 3 days flood volume have increased 5.58 and 3.85% than the dam site ones, while declined by 1.82 and 1.72% for the annual maximum 7 and 15 days flood volumes. The flood control risk rates of middle and small flood events are increased while extreme flood events are declined. It is shown that the TGR can satisfy the flood control task under current hydrologic regime and the results can offer references for better management of the TGR.  相似文献   
60.
黄土堆积作为气下沉积,任何一个深度都曾经暴露于地表,因而必然受到相关地表过程的作用.由于这些过程均发生于特定的环境条件下,形成的特征多数具有明确的环境意义;而黄土在沉积后也可能受到各种地质过程的改造,从而对研究中常用的气候代用指标有一定影响.文章基于野外、微形态等分析,结合前人成果,对我国北方新近纪风尘堆积中常见的同沉积和沉积后改造特征、形成过程及环境意义进行研究.由于一些特征在黄土堆积中具有普遍性,可作为识别风成堆积的标志和环境事件研究的指标,并有助于全面理解常用的替代指标的环境意义.  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