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   4630篇
  免费   889篇
  国内免费   1232篇
测绘学   524篇
大气科学   575篇
地球物理   1070篇
地质学   2590篇
海洋学   857篇
天文学   150篇
综合类   365篇
自然地理   620篇
  2024年   30篇
  2023年   101篇
  2022年   261篇
  2021年   311篇
  2020年   249篇
  2019年   292篇
  2018年   270篇
  2017年   260篇
  2016年   251篇
  2015年   284篇
  2014年   293篇
  2013年   328篇
  2012年   355篇
  2011年   374篇
  2010年   363篇
  2009年   341篇
  2008年   277篇
  2007年   298篇
  2006年   242篇
  2005年   223篇
  2004年   207篇
  2003年   147篇
  2002年   182篇
  2001年   160篇
  2000年   119篇
  1999年   69篇
  1998年   68篇
  1997年   65篇
  1996年   39篇
  1995年   43篇
  1994年   29篇
  1993年   30篇
  1992年   35篇
  1991年   18篇
  1990年   11篇
  1989年   19篇
  1988年   9篇
  1987年   10篇
  1986年   4篇
  1985年   7篇
  1984年   6篇
  1983年   6篇
  1982年   5篇
  1981年   8篇
  1980年   10篇
  1979年   9篇
  1978年   10篇
  1976年   4篇
  1975年   5篇
  1973年   3篇
排序方式: 共有6751条查询结果,搜索用时 15 毫秒
1.
The economic development in China and the rising of the living standard need to speed up the urbanisa-tion.The development of small towns is an important way to Chinese urbanization.Land use plays a very important role in the development of small towns.However there are many problems in the development of small towns,esp.in land use.The paper first discusses the land problems in the development of small towns.Such as much cultivated land lies idle,under-utilization and waste of land,increasing illegal use of land,unstable contractural relationship for land use.The relationship between the development of small towns and land use is also studied.Then the guidelines for the land system innovations of small towns are put forward.Namely the sustainability of social and economic development,the bal-ance between land reservation and land utilization,the provision of service to village ,agriculture and farmer,the manage-ment of land resource and land assets,the parsimonious andlegal use of land.The basic framework of land system innova-tions of small towns is put forward finally.It include the land replacement policy for small towns and the permanent tenan-cy of farmland,the overall plan for land uses and other plans in harmony,the establishment of a flexible system of land supplies,using land with payments ,the transfer of agricultural land and refining the land law related to the construction of small towns.  相似文献   
2.
研究了当X为S-闭空间时,C(X)的子集F为紧的充要条件,从而扩充了著名的Ascoli定理。  相似文献   
3.
上扬子地台震旦系铅锌矿床类型及找矿方向   总被引:13,自引:0,他引:13  
上扬子地台含铅锌矿地层主要为震旦系。震旦系在上扬子地台主要发育上统灯影组,次有下统陡山沱组。将上扬子地台震旦系铅锌矿床成因划分为一级为叠生矿床,二级为层控矿床,三级为热水沉积改造矿床和热水沉积再造矿床四级类型,该类型按铅锌比例进一步划分为富铅型、富锌型、铅锌共生型。研究区内最重要的铅锌矿床成因类型为南郑马元、会东大梁子铅锌矿为代表的富锌型。陕西南郑、紫阳—镇坪地区,湖北武当、神农架、黄陵地区,以及地台西缘的安宁河、甘洛—小江、峨边—寻甸等地区的震旦系是寻找铅锌矿的最有利地段。  相似文献   
4.
In view of the situation of excavation of open coal mine for the underground water disaster,we should carry out simulation studies for the numerical value of the water lowering project and improve the accuracy and the level of the water lowering project.On the basis of the hydrological geological conditions of certain open mine digging,a more reasonable seepage numerical model was built according to MODFLOW.It was simulated in advance that the process of the confined water level descending with the time,and combining with the actual observations to test the correctness of the model.The calculation showed that the results coincided well with the results of actual measurement.Based on this,different water lowering numerical simulations were built for the open coal mine digging.It could be simulated and forecast that the changes of the groundwater level in drainage process within and outside the mine pit,and it was quantitatively assessed that the possible water lowering result of the opencast water drainage process,which provide an important basis for the actual water lowering project and the possible project disposal.  相似文献   
5.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。  相似文献   
6.
We derive the classical Delaunay variables by finding a suitable symmetry action of the three torus T3 on the phase space of the Kepler problem, computing its associated momentum map and using the geometry associated with this structure. A central feature in this derivation is the identification of the mean anomaly as the angle variable for a symplectic S 1 action on the union of the non-degenerate elliptic Kepler orbits. This approach is geometrically more natural than traditional ones such as directly solving Hamilton–Jacobi equations, or employing the Lagrange bracket. As an application of the new derivation, we give a singularity free treatment of the averaged J 2-dynamics (the effect of the bulge of the Earth) in the Cartesian coordinates by making use of the fact that the averaged J 2-Hamiltonian is a collective Hamiltonian of the T3 momentum map. We also use this geometric structure to identify the drifts in satellite orbits due to the J 2 effect as geometric phases.  相似文献   
7.
CORROSION AND ADHERENCE AT SEA OIL-PLATFORM   总被引:1,自引:0,他引:1  
Measurements of the adhesive corrosion and protective potentials of 3 Bohai Sea oilplatforms under different cathodic protection showed corrosion still occurred on these platforms undertheir designed protective potentials. Preliminary conclusions from indoor experiments on the relation betweenadhesive corrosion and cathodic protection showed adhering organisms caused the corrosion onthese platforms.  相似文献   
8.
饶长辉  姜文汉  凌宁 《天文学报》2001,42(2):134-139
分析以观测系统焦面上和探测器测到的太阳表面米粒结构的对比度与观测系统口径,大气湍流相干长度以及系统探测灵敏度的关系,给出了不同口径,不同大气湍流相干长度以及不同系统探测灵敏度时的米粒结构对比值计算结果,此外还给出了实验结果。  相似文献   
9.
10.
Energy is stored when the force-free magnetic field in an active region departs from a potential field, the departure showing up as a shear in the field. As soon as the field untwists, energy will be released to produce flares. Based on this idea, we derived an analytical solution of the equation of force-free field under the assumption of a constant force-free factor, and found expressions for seven important quantities for quadrupolar sunspots: the magnetic energy of the twisted field, that of potential field, the extractable free energy ΔM, the magnetic flux, the total current, the force-free factor and the field decay factor, in terms of three observables: the field intensity, the twist angle and the distance between two spots of the same polarity. The expression for ΔM can be useful in solar prediction work. For the active region of August, 1972, we found ΔM up to 6 × 1032 erg, sufficient to supply the energy of the observed flare activity. Observations of this active region are in good general agreement with our theoretical expectations: in the entire twisting of the quadrupolar sunspot group, each spot assumes the form of a complete spiral in the clockwise direction for each of the four spots.  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