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431.
In polycrystalline aggregates of olivine with mean grain sizes above 35 μm plus a low basaltic melt fraction, both wetted
and melt-free grain boundaries are observed after equilibration times at high pressures and temperatures of between 15 and
25 days. In order to assess a possible dependence of the wetting behaviour on the relative orientation of neighbouring grains,
a SEM based technique, electron backscatter diffraction (EBSD), is used to determine grain orientations. From the grain orientations
relative orientations of neighbouring grains are calculated, which are expressed as misorientation axis/angle pairs. The distribution
of misorientation angles and axes of melt-free grain boundaries differ significantly from a purely random distribution, whereas
those of wetted grain boundaries are statistically indistinguishable from the random distribution. The relative orientation
of two neighbouring grains therefore influences the character of their common grain boundary. However, no clustering towards
special (coincident site lattice) misorientation axes is observed, with the inference that the energy differences between
special and general misorientations are too small to lead to the development of preferred misorientations during grain growth.
Received: 8 December 1997 / Revised, accepted: 6 April 1998 相似文献
432.
Glass samples from alkali-trachytic pumice of the Lower Member (LM, level LM1) and Upper Member (UM) of the Neapolitan Yellow
Tuff at Campi Flegrei Caldera were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy.
The EPR data show that the relative occupancy of the different iron sites varies in samples from the Lower Member LM1 and
Upper Member of the eruptive sequence. The LM1 glasses are characterized by a more homogeneous distribution of the cationic
site population of iron compared with UM and by a different Fe3+/Al3+ substitution in the C2v symmetrical sites of the silicate framework. 29Si MAS NMR spectra on glasses indicate that LM1 glasses are more depolymerized than UM glasses, indicating the existence of
structurally distinct magma batches. 27Al MAS NMR spectra indicate the occurrence in UM glasses of aluminum in both tetrahedral and octahedral coordination; the
observed AlVI is believed to be a primary feature of the glass, since 27Al CP-MAS NMR experiments do not reveal detectable esa-hydrated Al. The Al in sixfold coordination could be ascribed either
to the more abundant alkali–OH terminations in UM, in which alkalies would be extracted from their charge-balance role for
Al, thus altering its fourfold structural position, or to permanent compaction during fast cooling and decompression of high-pressure
melts. 23Na CP-MAS NMR experiments indicate the existence of Na–OH groups in both LM1 and UM glasses, relatively more abundant in the
latter. Although preliminary, the results indicate the possible application of atomic-scale studies to variables that determine
the regimes of explosive volcanism.
Received: 1 July 1998 / Accepted: 1 October 1998 相似文献
433.
Fu Huishan Grigorenko Elena E. Gabrielse Christine Liu Chengming Lu San Hwang K. J. Zhou Xuzhi Wang Zhe Chen Fang 《中国科学:地球科学(英文版)》2020,63(2):235-256
In this paper, the particle acceleration processes around magnetotail dipolarization fronts(DFs) were reviewed. We summarize the spacecraft observations(including Cluster, THEMIS, MMS) and numerical simulations(including MHD, testparticle, hybrid, LSK, PIC) of these processes. Specifically, we(1) introduce the properties of DFs at MHD scale, ion scale, and electron scale,(2) review the properties of suprathermal electrons with particular focus on the pitch-angle distributions,(3)define the particle-acceleration process and distinguish it from the particle-heating process,(4) identify the particle-acceleration process from spacecraft measurements of energy fluxes, and(5) quantify the acceleration efficiency and compare it with other processes in the magnetosphere(e.g., magnetic reconnection and radiation-belt acceleration processes). We focus on both the acceleration of electrons and ions(including light ions and heavy ions). Regarding electron acceleration, we introduce Fermi,betatron, and non-adiabatic acceleration mechanisms;regarding ion acceleration, we present Fermi, betatron, reflection, resonance, and non-adiabatic acceleration mechanisms. We also discuss the unsolved problems and open questions relevant to this topic, and suggest directions for future studies. 相似文献
434.
地球外辐射带是一个高度动态变化的空间环境,辐射带电子通量的变化在磁暴期间尤为明显.要分析潜在的电子动态变化机制,需要排除绝热效应产生的影响.在以三个绝热不变量组成的相空间坐标中,利用相空间密度(PSD)可以反映电子的真实加速和损失情况.本文详细分析两颗范艾伦卫星和三颗GPS导航卫星在2013年3月的同步电子通量观测数据,发现在3月17日磁暴期间,当太阳风动压增大、行星际磁场南向时,辐射带电子通量会发生骤降.进一步将电子通量转换成电子相空间密度并利用不同第一、第二绝热不变量(μ,K)组合条件下PSD径向分布的差异性,深入探究磁暴期间辐射带电子的动态变化机制.结果表明:磁暴初期由于电子的局地加速导致PSD不断上升;磁暴主相期间,由于磁层顶阴影效应以及伴随的向外径向扩散损失导致PSD快速降低;位于不同空间位置的多颗卫星观测为明晰辐射带电子动态物理过程提供了重要的便利. 相似文献
435.
436.
循环应力比和振动频率对盐渍土微观结构影响分析 总被引:1,自引:1,他引:0
以青海省察尔汗地区盐渍土为研究对象, 借助扫描电子显微镜(SEM)设备和颗粒孔隙及裂隙图像识别与分析系统(PCAS), 应用图像分割原理和分形理论, 对交通循环荷载作用下的盐渍土微观结构进行研究, 探讨不同循环应力比和不同振动频率下孔隙分布特征及其微观机制。结果表明: 循环应力比R由0.375增加到0.75, 临界破坏动应变由2.32%增高到3.17%, 动载循环次数相应减少; 振动频率f由0.5 Hz增加到2.0 Hz, 临界破坏动应变由2.82%减小到2.48%, 动载循环次数相应增多; 盐渍土孔隙参数随循环应力比的增大, 孔隙尺度分维值Dv 增长了0.12, 概率熵Hm 减小了0.017, 孔隙的形态分形维数D减小了0.27, 平均形状系数F增加了0.14, 变化规律较明显; 随振动频率的增大, Dv 增长了0.014, Hm 减小了0.002, D变化了0.06, F变化了0.003, 变化规律不明显; 在动荷载作用下, 试样的孔隙空间排列与宏观上的褶皱鼓状变形具有一致性。通过未加载试样与加载后试样的微观结构参数变化对比, 从循环应力比和振动频率对盐渍土微观孔隙特征的影响到宏观变形的相互作用机制进行了系统分析。 相似文献
437.