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   847篇
  免费   118篇
  国内免费   251篇
测绘学   33篇
大气科学   142篇
地球物理   93篇
地质学   342篇
海洋学   261篇
天文学   238篇
综合类   62篇
自然地理   45篇
  2024年   10篇
  2023年   12篇
  2022年   26篇
  2021年   35篇
  2020年   25篇
  2019年   35篇
  2018年   23篇
  2017年   21篇
  2016年   24篇
  2015年   28篇
  2014年   42篇
  2013年   48篇
  2012年   48篇
  2011年   41篇
  2010年   51篇
  2009年   54篇
  2008年   60篇
  2007年   57篇
  2006年   81篇
  2005年   88篇
  2004年   59篇
  2003年   54篇
  2002年   48篇
  2001年   46篇
  2000年   44篇
  1999年   38篇
  1998年   23篇
  1997年   10篇
  1996年   16篇
  1995年   16篇
  1994年   5篇
  1993年   15篇
  1992年   10篇
  1991年   1篇
  1990年   4篇
  1989年   6篇
  1988年   2篇
  1987年   2篇
  1985年   2篇
  1984年   4篇
  1981年   1篇
  1978年   1篇
排序方式: 共有1216条查询结果,搜索用时 15 毫秒
31.
天气学和天气预报的研究进展   总被引:14,自引:4,他引:14       下载免费PDF全文
全面回顾了75年来中国科学院大气物理研究所科研人员在气团、锋面、梅雨、寒潮、阻高、副高、暴雨、高低空急流、亚澳季风区内涡旋和对流等现象的天气学研究进展及其在不同时期所取得的研究成果,总结了在短中期天气预报及短期气候预测领域在模式发展和改进以及在数值模拟等方面所取得的成就和进展.同时指出模式发展和预测在中国数值天气预报及短期气候预测方面所做出的贡献.  相似文献   
32.
任意载荷激励下直角三角形板单元局部动力效应   总被引:1,自引:0,他引:1  
本文推导了在任意载荷激励下直角三角形板单元局部动力效应修正公式。采用单边积分法,对单元边上的单位宽度上的内力进行局部动力修正,代替以往的用加密网格提高精度的方法。计算结果表明,局部动力效应影响不可忽略,本文方法可使计算结果更为精确。这对分析大型复杂结构环境载荷激励下的局部动力效应具有参考价值。  相似文献   
33.
钙膨润土钠化技术   总被引:2,自引:0,他引:2       下载免费PDF全文
利用我国丰富的钙膨润土资源,人工改性和为钠膨润土,钠化原理是利用膨润土中主要矿物成分蒙脱石晶层表面吸附性能,在水存在的条件下进行盐基交换反应。泥浆性能全部达到天然优质钠膨润土的指标,完全符合国际通用API标准。  相似文献   
34.
The investigation of the environmental properties of minerals, i.e., environmental mineralogy, is a branch of science dealing with interactions between natural minerals and spheres of the Earth surface as well as a reflection of global change, prevention of ecological destruction, participation in biomineralogy, and remediation of environmental pollution. Pollutant treatment by natural minerals is based on the natural law and reflects natural self-purification functions in the inorganic world, similar to that of the organic world - a biological treatment. A series of case studies related to natural self-purification, which were mostly completed by our group, are discussed in this paper. In natural cryptomelane there is a larger pseudotetragonal tunnel than that formed by [MnO6] octahedral double chains, with an aperture of 0.462-0.466 nm2, filled with K cations. Cryptomelane might be a real naturally-occurring mineral of the active octahedral molecular sieve (OMS-2). CrⅥ-bearing wastewater can be treate  相似文献   
35.
The annual and semi-annual variations of the ionosphere are investigated in the present paper by using the daytime F2 layer peak electron concentration (NmF2) observed at a global ionosonde network with 104 stations. The main features are outlined as follows. (1) The annual variations are most pronounced at magnetic latitudes of 40–60° in both hemispheres, and usually manifest as winter anomalies; Below magnetic latitude of 40° as well as in the tropical region they are much weaker and winter anomalies that are not obvious. (2) The semi-annual variations, which are usually peak in March or April in most regions, are generally weak in the near-pole regions and strong in the far-pole regions of both hemispheres. (3) Compared with their annual components, the semi-annual variations in the tropical region are more significant.In order to explain the above results, we particularly analyze the global atomic/molecular ratio of [O/N2] at the F2 layer peak height by the MSIS90 model. The results show that the annual variation of [O/N2] is closely related with that of NmF2 prevailing in mid-latitudes and [O/N2] annual variation usually may lead to the winter anomalies of NmF2 occurring in the near-pole region. Moreover, NmF2 semi-annual variations appearing in the tropical region also have a close relationship with the variation of [O/N2]. On the other hand, the semi-annual variations of NmF2 in the far-pole region cannot be simply explained by that of [O/N2], but the variation of the solar zenith angle may also have a significant contribution.  相似文献   
36.
Molecular dynamics (MD) modeling of the 10-Å phase, Mg3Si4O10(OH)2·xH2O, with x=2/3, 1.0 and 2.0 shows complex structural changes with pressure, temperature and water content and provides new insight into the structures and stabilization of these phases under subduction zone conditions. The structure(s) of this phase and its role as a reservoir of water in the mantle have been controversial, and these calculations provide specific predictions that can be tested by in situ diffraction studies. At ambient conditions, the computed structures of talc (x=0) and the 10-Å phases with x=2/3 and 1.0 are stable over the 350-ps period of the MD simulations. Under these conditions, the 10-Å phases show phlogopite-like layer stacking in good agreement with previously published structures based on powder X-ray diffraction data for samples quenched from high-pressure and high-temperature experiments. The calculations show that the 10-Å phase with x=2.0 is unstable at ambient conditions. The computed structures at P=5.5 GPa and T=750 K, well within the known stability field of the 10-Å phase, change significantly with water content, reflecting changing H-bonding configurations. For x=2/3, the layer stacking is talc-like, and for x=1.0, it is phlogopite-like. The calculations show that transformation between these two stackings occurs readily, and that the talc-like stacking for the x=2/3 composition is unlikely to be quenchable to ambient conditions. For x=2.0, the layer stacking at P=5.5 GPa and T=750 K is different than any previously proposed structure for a 10-Å phase. In this structure, the neighboring basal oxygens of adjacent magnesium silicate layers are displaced by b/3 (about 3 Å) resulting in the Si atoms of one siloxane sheet being located above the center of the six-member ring across the interlayer. The water molecules are located 1.2 Å above the center of all six-member rings and accept H-bonds from the OH groups located below the rings. The b/3-displaced structure does not readily transform to either the talc-like or phlogopite-like structure, because neither of these stackings can accommodate two water molecules per formula unit. There is likely to be a compositional discontinuity and phase transition between the b/3-displaced phase and the phase with phlogopite-like stacking. The simulations reported here are the first to use the recently developed CLAYFF force field to calculate mineral structures at elevated pressures and temperatures.  相似文献   
37.
介绍利用屏幕截取技术制作人降作业炮点四周环境坐标示意图的方法。  相似文献   
38.
铝钛柱撑改性膨润土处理两种模拟废水的实验研究   总被引:10,自引:1,他引:10  
以钠基膨润土为原料制备了铝钛无机柱撑、铝钛有机柱撑改性膨润土,确定了改性膨润土的最佳铝钛比和铝土比,并将其应用于模拟废水、化学实验室废水的处理。与原土进行比较,探讨了用量、pH值、搅拌时间等对COD去除率、色度去除率、浊度去除率的影响,通过正交实验对实验条件进行了优化。结果表明:有机柱撑改性膨润土、无机柱撑改性膨润土对废水的处理效果均明显好于原土,对废水色度、浊度的去除率均大于93%;对废水COD的去除效果,有机柱撑改性膨润土比无机柱撑改性膨润土有较大提高,去除率最高达66.31%,结果令人满意。  相似文献   
39.
高速公路沉降计算中e-lg p法修正探讨   总被引:2,自引:1,他引:2  
利用已有的高速公路土工试验资料,通过大量的非线性有限元计算得出一个与路堤填筑高度及路基土体压缩系数相关的沉降综合修正式,使得单向 e-lgp 法的计算结果修正后能够考虑土体侧向变形。实例验证表明:经此式修正后的沉降值误差较小,不但比规范法更合理,而且与实测值以及有限元法结果很接近,具有一定的工程参考价值和借鉴意义。  相似文献   
40.
利用钾长石粉水热合成13X沸石分子筛的实验研究   总被引:11,自引:0,他引:11  
对综合利用钾长石提钾工艺中的重要高附加值副产品13X沸石分子筛的合成进行了实验研究。对福建沙县的钾长石粉加入配料NaCO3进行焙烧实验,确定培烧的最佳工艺参数为:钾长石粉:NaCO3=1:1.30(摩尔比),焙烧温度为845℃,焙烧时间为150min;优化的水热合成条件为M2O/SiO2(mo1)=1.50,H2O/M2O(mo1)=40.0,合成时间为8h,晶种加入量为9.0%。对合成样品化学成分分析、X射线物相分析、红外光谱分析、扫描电镜分析表明,合成13X沸石分子筛结晶完好,性能优良.水热反应经由溶解水合反应和聚合浓缩反应两大阶段。  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