全文获取类型
收费全文 | 6502篇 |
免费 | 1060篇 |
国内免费 | 1204篇 |
专业分类
测绘学 | 2253篇 |
大气科学 | 694篇 |
地球物理 | 1201篇 |
地质学 | 2720篇 |
海洋学 | 646篇 |
天文学 | 345篇 |
综合类 | 468篇 |
自然地理 | 439篇 |
出版年
2024年 | 21篇 |
2023年 | 59篇 |
2022年 | 230篇 |
2021年 | 286篇 |
2020年 | 302篇 |
2019年 | 321篇 |
2018年 | 234篇 |
2017年 | 374篇 |
2016年 | 351篇 |
2015年 | 343篇 |
2014年 | 439篇 |
2013年 | 492篇 |
2012年 | 471篇 |
2011年 | 470篇 |
2010年 | 368篇 |
2009年 | 423篇 |
2008年 | 481篇 |
2007年 | 416篇 |
2006年 | 446篇 |
2005年 | 413篇 |
2004年 | 329篇 |
2003年 | 234篇 |
2002年 | 255篇 |
2001年 | 166篇 |
2000年 | 137篇 |
1999年 | 129篇 |
1998年 | 115篇 |
1997年 | 82篇 |
1996年 | 60篇 |
1995年 | 59篇 |
1994年 | 57篇 |
1993年 | 54篇 |
1992年 | 43篇 |
1991年 | 27篇 |
1990年 | 26篇 |
1989年 | 10篇 |
1988年 | 14篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有8766条查询结果,搜索用时 31 毫秒
891.
Ccile Gautheron Laurent Tassan-Got Jocelyn Barbarand Maurice Pagel 《Chemical Geology》2009,266(3-4):157-170
Recent data on He diffusion challenge the temperature sensitivity of apatite (U–Th)/He thermochronology: the damage induced by recoil of U and Th decay series during emission of α particles (α-recoil damage) has been proposed to modify He-diffusion properties through time. However, we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly-cooled or reheated basement rocks. To test this hypothesis, we developed a quantitative model including an explicit treatment of α-recoil damage, annealing, and their effect on He-diffusion kinetics, and calibrate it against literature data. Our model is based on two hypotheses: (1) helium is in equilibrium between an apatite crystal and its defects and (2) alpha-recoil damage annealing can be described analogously to fission-track annealing. This model has been embedded into a Monte Carlo simulation of helium production/ejection/diffusion and applied to data from the French Massif Central; a complex slowly-cooled terrain with burial reheating, where the thermal history has been constrained by previous fission-track (FT) data including FT length distributions. (U–Th)/He ages are close to the FT ages from the same samples and are generally reproducible among replicates, but some samples present He-age dispersion that is not correlated with crystal size. Our model reproduces the Massif Central data very well except for three samples where He ages are older than corresponding FT ages. We show that annealing of irradiation damage has an important impact on retentivity of helium and that the He content, [He] is only a rough approximation of the damage level. In particular our results show that independence of He ages on crystal sizes, in case of reheated samples, is a clear indication of the higher He retentivity induced by α-recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of (U–Th)/He ages in such a case. More generally a correlation with the crystal size can bring information on the thermal path only if the age of defects, well represented by the fission-track age, is available, due to the dependence of the partial retention zone on damages. Conversely, in case of rapid cooling or for samples having low U and Th contents, damage effects can be ignored without significant effects on He ages. 相似文献
892.
Pile foundation analysis and design using experimental data and 3-D numerical analysis 总被引:1,自引:0,他引:1
Capacity based design of pile foundations limits the soil-structure interaction mechanism to group bearing capacity estimation, neglecting, in most cases, the contribution of the raft. On the other hand, a straightforward, nonlinear, 3-D analysis, accounting for soil and structural nonlinearities and the effects arising from pile–soil–pile interaction, would be extremely high CPU-time demanding and will necessitate the use of exceptionally powerful numerical tools. With the aim of investigating the most efficient, precise, and economical design for a bridge foundation, a hybrid method, compatible with the notion of sub-structuring is proposed. It is based on both experimental data and nonlinear 3-D analysis. The first step to achieve these targets is a back-analysis of a static pile load test, fitting values for soil shear strength, deformation modulus, and shear strength mobilization at the soil–pile interface. Subsequently, the response of 2 × 2 and 3 × 3 pile group configurations is numerically established and the distribution of the applied load to the raft and the characteristic piles is discussed. Finally, a design strategy for an optimized design of pile raft foundations subjected to non-uniform vertical loading is proposed. 相似文献
893.
We present here a numerical modelling study of dislocations in perovskite CaTiO3. The dislocation core structures and properties are calculated through the Peierls–Nabarro model using the generalized stacking
fault (GSF) results as a starting model. The GSF are determined from first-principles calculations using the VASP code. The
dislocation properties such as collinear, planar core spreading and Peierls stresses are determined for the following slip
systems: [100](010), [100](001), [010](100), [010](001), [001](100), [001](010),
and All dislocations exhibit lattice friction, but glide appears to be easier for [100](010) and [010](100).
[001](010) and [001](100) exhibit collinear dissociation. Comparing Peierls stresses among tausonite (SrTiO3), perovskite (CaTiO3) and MgSiO3 perovskite demonstrates the strong influence of orthorhombic distortions on lattice friction. However, and despite some quantitative
differences, CaTiO3 appears to be a satisfactory analogue material for MgSiO3 perovskite as far as dislocation glide is concerned. 相似文献
894.
MERSI和MODIS卫星监测京津冀及周边地区PM2.5浓度 总被引:1,自引:0,他引:1
京津冀及周边地区是中国PM_(2.5)污染最重的区域之一,利用卫星遥感技术监测大范围的PM_(2.5)时空分布变化是一种先进的重要手段。本研究首先基于暗像元算法利用FY-3B/MERSI与AQUA/MODIS对京津冀及周边区域进行了遥感AOT反演和验证分析;然后,引入气象资料和地面观测资料利用GWR模型反演了区域PM_(2.5)浓度,并对遥感反演结果进行了交叉验证评估,综合对比分析了MERSI和MODIS的气溶胶及PM_(2.5)遥感监测能力;最后,利用MERSI数据对2017年第一季度京津冀及周边区域的PM_(2.5)月均浓度时空分布变化情况进行了初步探索分析。结果表明:FY-3B/MERSI在气溶胶及PM_(2.5)遥感监测能力方面略优于AQUA/MODIS,MERSI反演的1 km分辨率AOT和PM_(2.5)与地面站点实测结果的决定系数R2分别为0.76μg/m~3和0.79μg/m~3,均方根误差分别为0.26μg/m~3和28μg/m~3,平均绝对误差分别为0.16μg/m~3和15μg/m~3,能基本满足对京津冀及周边区域PM_(2.5)的精细化监测需要。2017年第一季度京津冀及周边区域PM_(2.5)月均浓度遥感监测结果表明该区域的PM_(2.5)空间分布格局与地形地貌关系密切,高值区整体上沿太行山脉成带成片;从时间变化来看,1—3月呈逐月下降的趋势,其中3月份PM_(2.5)区域浓度较1月和2月有大幅下降。这说明FY-3\MERSI遥感反演产品能为环境质量监测和环境管理工作效果评估提供有效参考,本研究对国产卫星在大气环境遥感业务中的大力发展应用有重要参考意义。 相似文献
895.
在卫星海洋遥感中,云作为海气耦合系统最重要的调节器之一,其检测结果对海洋上空云微物理特性的反演精度有较大影响。因此,快速而准确识别海洋上空的云像元是卫星遥感数据处理过程中首要解决的关键问题。以PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar)卫星搭载的POLDER3载荷遥感数据为研究对象,提出一种改进的海洋上空云检测方法。首先剔除海洋耀光;接着利用有云与晴空区近红外反射率差异检验识别有云像元,并利用偏振反射率检验进一步识别低反射率的云像元;然后利用近红外与可见光反射率比值检验识别晴空像元;最后建立多角度云检测结果空间融合规则,重新标记有云、晴空和未定像元。以印度洋海区为例进行实验分析,将云检测结果与Buriez方法进行对比,发现检测精度基本相当,而有云像元的识别速度却平均提高约3倍。结果表明:该方法能有效的检测出海洋上空的云像元,满足业务化数据处理的高精度及时效性要求,为后续云微物理特性反演提供可靠的数据源。 相似文献
896.
897.
898.
西南某电站右岸开挖边坡稳定性的FLAC3D分析 总被引:11,自引:2,他引:9
西南某大型水电站引水发电洞进水口边坡顶部在开挖过程中出现带状裂缝 ,引起了多方的极大关注。在对该边坡系统的地质调查基础上 ,确立了其变形破坏模式 ;通过FLAC3D模拟开挖过程 ,客观地验证了实际开挖过程中边坡的变形及其顶部平台出现的裂缝 ;并预测了在采取加固措施后边坡内部变形趋势。经计算 ,开挖面临空方向最大位移量达到 5 .4cm。此后 ,经过有效的锚固处理 ,边坡经过变形和应力的调整 ,总体上趋于稳定 ,为确保进水口隧洞的顺利开挖提供了科学依据 相似文献
899.
城乡结合部土地利用变化的信息提取技术与分析--以无锡市为例 总被引:11,自引:0,他引:11
作为两种截然不同的生态系统过渡区,城乡结合部是土地利用/土地覆被变化最激烈地区之一,城乡结合部的土地利用情况日益受到人们重视.本文以无锡市为例,利用3S技术提取城乡结合部的范围和土地利用/土地覆被状况信息,提出了确定无锡市城乡结合部范围边界的半自动提取模型和结合部土地利用类型半自动提取模型,并对无锡市城乡结合部土地利用结构及其变化进行分析,揭示了无锡市城乡结合部土地利用/土地覆被变化特征,据此提出城乡结合部土地合理利用的建议. 相似文献
900.
L. Hjalmarsdotter A. A. Zdziarski S. Larsson V. Beckmann M. McCollough D. C. Hannikainen O. Vilhu 《Monthly notices of the Royal Astronomical Society》2008,384(1):278-290
The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ∼20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf–Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE /ASM and CGRO /BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L / L E than in the usual advection models, or the mass of the compact object is ≳20 M⊙ , possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation. 相似文献