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11.
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Jianwei Wang 《Earth and Planetary Science Letters》2004,222(2):517-527
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
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The shear viscosities and 1 bar heat capacities of glasses and melts along the 67mol% silica isopleth in the system SiO2-Al2O3-Na2O-TiO2 have been determined in the temperature ranges 780-1140 K and 305-1090 K respectively. Anomalous behaviour of both these properties is observed for compositions rich in TiO2 and/or Al2O3, an observation attributed to liquid-liquid phase separation followed by anatase crystallization. For samples which do not show anomalous behaviour, it is found that the partial molar heat capacity of the TiO2 component previously determined in Al-free compositions reproduces our heat capacities to within 1.3%. Viscosity data show that addition of TiO2 tends to increase viscosity and melt fragility at constant temperature. Furthermore, heat capacity and viscosity data may be combined within the framework of the Adam-Gibbs theory to extract values of the configurational entropy of the liquids and qualitative estimates of the variation of the average energy barrier to viscous flow. Configurational entropy at 900K is inferred to decrease upon addition of TiO2, in contrast to previous results from Al-free systems. The compositional limit separating normal from anomalous behaviour, as well as the data for homogenous melts have been used to constrain the structural role of Ti in these samples. Our data are consistent with a majority of Ti in five-fold coordination associated with a titanyl bond, in agreement with previous spectroscopic studies. Furthermore, we find no evidence for a Ti-Al interaction in our samples, and we are led to the conclusion that Al and Ti are incompletely mixed, a hypothesis consistent with the observed reduction of configurational entropy upon addition of TiO2, suggesting an important role of medium range order in controlling the variations in thermodynamic properties. 相似文献
15.
M. Stepanova T. Vucina-Parga E. Antonova I. Ovchinnikov Y. Yermolaev 《Journal of Atmospheric and Solar》2005,67(17-18):1815
Fluctuations of the plasma bulk velocity across the plasma sheet are studied using single-point measurements from the Corall instrument on board the Interball/Tail satellite. Several hour-long intervals of continuous data corresponding to quiet geomagnetic conditions and different phases of isolated substorms are analyzed. The plasma sheet flow appears to be strongly turbulent, i.e. dominated by fluctuations that are unpredictable. Corresponding eddy diffusion coefficients were obtained as a function of the autocorrelation time and rms velocity of the fluctuations. It was found that the amplitude of the turbulence and the values of eddy-diffusion coefficients increase significantly during substorm growth and expansion phases and they decrease to their initial level during the recovery phase. We also studied a relationship between the eddy-diffusion coefficients and the absolute value of the geomagnetic field, also measured by the Interball/Tail satellite. It was found that this relationship varies depending on the phase of substorm, indicating possible change in the turbulence regimen with substorm phase. 相似文献
16.
MT法中利用阻抗相位资料对畸变视电阻率曲线的校正 总被引:11,自引:3,他引:11
从理论上论证了在一定条件下大地电磁响应中的相位资料将不受随机电磁噪声的影响,介绍了由相位资料递推计算视电阻率的方法,通过与远参考资料的比较,证明利用阻抗相位资料可分段恢复校正畸变的实测视电阻率数据。 相似文献
17.
相位激电测深在有色金属矿产勘查中的应用 总被引:2,自引:4,他引:2
激发极化方法一直是有色金属矿产勘查的主要手段,国内大多使用时间域激发极化方法,相位激发极化法应用较少。相位激电法采用选频测量,抗干扰能力相对较强,近些年在有色金属矿产勘查中一直使用相位激电测深方法,取得了较好的实际应用效果。在使用中,根据实际情况选用偶极-偶极或单极-偶极测量装置,其中偶极-偶极测量装置主要用于普查阶段,单极-偶极装置采用常规高密度电法的布极方式,主要用于详查阶段。这里以具有代表性的高阻高极化、低阻高极化矿体上的应用结果为例,说明偶极-偶极和单极-偶极装置相位激发极化测深的应用效果,以及在实际找矿工作中的作用。 相似文献
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对Li_2O-(Mg、Zn、Ni)O-V_2O_5三元体系在500—800℃相图的研究表明,在每个系统内部都出现了一个依端员组分摩尔比为1:2:1的化合物。即橄榄石型的LiMgVO_4,硅铍石型的LiZnVO_4和尖晶石型的LiNiVO_4。不同的是,Li_2O-MgO-V_2O_5中发现了第二个三元相和一个固溶体。LiO_2-NiO-V_2O_6中得出一个固溶体,而LiZnNO_4中未得固溶体。 相似文献
20.
登陆北上台风暴雨突发性增强的一种机制研究 总被引:5,自引:1,他引:5
采用一个三维混合模式对1992年8月30日至9月2日一次登陆北上台风暴雨过程进行了模拟。模式可以较准确地预报出与地面倒槽相一致的地面降水位置及降水量值。模拟的台风云系结构与卫星云图比较表明,外围云场与实例云况吻合很好,验证了本模式模拟卫星云图的能力,对卫星云图预报有实际意义;暴雨突然增幅的直接原因是高层冰云与低层供水云的突然北移重叠造成的。受地面倒槽附近强烈辐合抬升的动力作用,各相态云系的分布与垂直运动紧密相关;辐合线右侧的东南低空急流为降水的增幅及维持提供水汽来源;云的相变潜热非绝热加热作用对暴雨的增幅及维持具有正反馈作用,它对暴雨维持具有积极贡献。 相似文献