全文获取类型
收费全文 | 88536篇 |
免费 | 1127篇 |
国内免费 | 697篇 |
专业分类
测绘学 | 2308篇 |
大气科学 | 6567篇 |
地球物理 | 17151篇 |
地质学 | 33683篇 |
海洋学 | 6967篇 |
天文学 | 19053篇 |
综合类 | 291篇 |
自然地理 | 4340篇 |
出版年
2021年 | 522篇 |
2020年 | 598篇 |
2019年 | 625篇 |
2018年 | 3954篇 |
2017年 | 3739篇 |
2016年 | 3055篇 |
2015年 | 1113篇 |
2014年 | 1714篇 |
2013年 | 3344篇 |
2012年 | 2809篇 |
2011年 | 4821篇 |
2010年 | 4362篇 |
2009年 | 5253篇 |
2008年 | 4313篇 |
2007年 | 4764篇 |
2006年 | 2540篇 |
2005年 | 2373篇 |
2004年 | 2271篇 |
2003年 | 2262篇 |
2002年 | 2036篇 |
2001年 | 1662篇 |
2000年 | 1637篇 |
1999年 | 1371篇 |
1998年 | 1356篇 |
1997年 | 1340篇 |
1996年 | 1179篇 |
1995年 | 1152篇 |
1994年 | 1022篇 |
1993年 | 896篇 |
1992年 | 859篇 |
1991年 | 864篇 |
1990年 | 943篇 |
1989年 | 842篇 |
1988年 | 785篇 |
1987年 | 983篇 |
1986年 | 857篇 |
1985年 | 1004篇 |
1984年 | 1180篇 |
1983年 | 1145篇 |
1982年 | 1005篇 |
1981年 | 1050篇 |
1980年 | 907篇 |
1979年 | 847篇 |
1978年 | 882篇 |
1977年 | 791篇 |
1976年 | 733篇 |
1975年 | 721篇 |
1974年 | 743篇 |
1973年 | 761篇 |
1972年 | 481篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
A discrete element modelling of bonded granulates and investigation on the bond effect on their behaviour are very important to geomechanics. This paper presents a two‐dimensional (2‐D) discrete element theory for bonded granulates with bond rolling resistance and provides a numerical investigation into the effect of bond rolling resistance on the yielding of bonded granulates. The model consists of mechanical contact models and equations governing the motion of bonded particles. The key point of the theory is that the assumption in the original bond contact model previously proposed by the authors (55th CSCE‐ASCE Conference, Hamilton, Ont., Canada, 2002; 313–320; J. Eng. Mech. (ASCE) 2005; 131 (11):1209–1213) that bonded particles are in contact at discrete points, is here replaced by a more reliable assumption that bonded particles are in contact over a width. By making the idealization that the bond contact width is continuously distributed with the normal/tangential basic elements (BE) (each BE is composed of spring, dashpot, bond, slider or divider), we establish a bond rolling contact model together with bond normal/tangential contact models, and also relate the governing equations to local equilibrium. Only one physical parameter β needs to be introduced in the theory in comparison to the original bond discrete element model. The model has been implemented into a 2‐D distinct element method code, NS2D. Using the NS2D, a total of 86 1‐D, constant stress ratio, and biaxial compressions tests have been carried out on the bonded granular samples of different densities, bonding strengths and rolling resistances. The numerical results show that: (i) the new theory predicts a larger internal friction angle, a larger yielding stress, more brittle behaviour and larger final broken contact ratio than the original bond model; (ii) the yielding stress increases nonlinearly with the increasing value of β, and (iii) the first‐yield curve (initiation of bond breakage), which define a zone of none bond breakage and which shape and size are affected by the material density, is amplified by the bond rolling resistance in analogous to that predicted by the original bond model. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
102.
It is well known that the Babuska–Brezzi stability criterion or the Zienkiewicz–Taylor patch test precludes the use of the finite elements with the same low order of interpolation for displacement and pore pressure in the nearly incompressible and undrained cases, unless some stabilization techniques are introduced for dynamic analysis of saturated porous medium where coupling occurs between the displacement of solid skeleton and pore pressure. The numerical manifold method (NMM), where the interpolation of displacement and pressure can be determined independently in an element for the solution of u–p formulation, is derived based on triangular mesh for the requirement of high accurate calculations from practical applications in the dynamic analysis of saturated porous materials. The matrices of equilibrium equations for the second‐order displacement and the first‐order pressure manifold method are given in detail for program coding. By close comparison with widely used finite element method, the NMM presents good stability for the coupling problems, particularly in the nearly incompressible and undrained cases. Numerical examples are given to illustrate the validity and stability of the manifold element developed. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
103.
Roberto Mínguez Fernando Delgado Ignacio Escuder Manuel G. de Membrillera 《国际地质力学数值与分析法杂志》2006,30(10):1019-1037
Empirical criteria have been used successfully to design filters of most embankment large dam projects throughout the world. However, these empirical rules are only applicable to a particular range of soils tested in laboratory and do not take into account the variability of the base material and filter particle sizes. In addition, it is widely accepted that the safety of fill dams is mainly dependent on the reliability of their filter performance. The work herein presented consists in a new general method for assessing the probability of fulfilling any empirical filter design criteria accounting for base and filter heterogeneity by means of first‐order reliability methods (FORM), so that reliability indexes and probabilities of fulfilling any particular criteria are obtained. This method will allow engineers to estimate the safety of existing filters in terms of probability of fulfilling their design criteria and might also be used as a decision tool on sampling needs and material size tolerances during construction. In addition, sensitivity analysis makes possible to analyse how reliabilities are influenced by different sources of input data. Finally, in case of a portfolio risk assessment, this method will allow engineers to compare the safety of several existing dams in order to prioritize safety investments and it is expected to be a very useful tool to evaluate probabilities of failure due to internal erosion. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
104.
The Electron Energy Spectrum from Large Solar Flares 总被引:2,自引:0,他引:2
G. M. Simnett 《Solar physics》2006,237(2):383-395
We report on the differential electron spectrum for intense transient events seen at one AU by the EPAM instrument on the
Advanced Composition Explorer (ACE) spacecraft. Over an observing period from September 1997 to September 2005, there were 45 major events that could be
reliably identified with a source flare on the Sun. In the ∼40 – 300 keV energy range, the electron spectral index was between
one and three for all but two of the events. Twenty-five of the events were associated with Geostationary Operational Environmental
Satellites (GOES) X-ray class X flares. We compare this result with the spectral index measured from electron pulse events,
lasting approx. one hour or less, where the spectral index is typically much softer than three. This suggests that the measured
spectral index of near-relativistic electrons at one AU may be a reliable indicator of the source. We also examine the likelihood
that fast coronal mass ejections (CMEs) are responsible in themselves for accelerating near-relativistic electrons and conclude
that they do not. 相似文献
105.
A new approach to constraining seawater δ34S and sulphate concentration using francolite‐bound sulphate reveals an abrupt increase in δ34S to +50‰ around the Early–Middle Cambrian boundary. Such high δ34S values are best explained by increased rates of pyrite burial due to ocean anoxia coupled with an increased sensitivity of the ocean sulphate reservoir to perturbations due to low sulphate concentrations of 500–700 μgL?1. We argue that the spread of anoxic waters at this time was partly the result of greenhouse warming related to the eruption of the Kalkarindji Large Igneous Province of northern Australia and that it triggered the collapse of early metazoan reef ecosystems during the latest Early Cambrian. Mass extinctions of the last 260 Myr have all coincided with enhanced volcanic activity, while several are also associated with positive shifts in seawater δ34S. Extending this correlation back in time further implicates volcanically induced climate change as a major determining factor in biosphere evolution. Terra Nova, 18, 257–263, 2006 相似文献
106.
An approximate Riemann solver for the equations of relativistic magnetohydrodynamics (RMHD) is derived. The Harten–Lax–van Leer contact wave (HLLC) solver, originally developed by Toro, Spruce and Spears, generalizes the algorithm described in a previous paper to the case where magnetic fields are present. The solution to the Riemann problem is approximated by two constant states bounded by two fast shocks and separated by a tangential wave. The scheme is Jacobian-free, in the sense that it avoids the expensive characteristic decomposition of the RMHD equations and it improves over the HLL scheme by restoring the missing contact wave.
Multidimensional integration proceeds via the single step, corner transport upwind (CTU) method of Colella, combined with the constrained transport (CT) algorithm to preserve divergence-free magnetic fields. The resulting numerical scheme is simple to implement, efficient and suitable for a general equation of state. The robustness of the new algorithm is validated against one- and two-dimensional numerical test problems. 相似文献
Multidimensional integration proceeds via the single step, corner transport upwind (CTU) method of Colella, combined with the constrained transport (CT) algorithm to preserve divergence-free magnetic fields. The resulting numerical scheme is simple to implement, efficient and suitable for a general equation of state. The robustness of the new algorithm is validated against one- and two-dimensional numerical test problems. 相似文献
107.
Daniel P. Glavin Jason P. Dworkin Andrew Aubrey Oliver Botta James H. Doty Zita Martins Jeffrey L. Bada 《Meteoritics & planetary science》2006,41(6):889-902
Abstract— Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH) 83100 and Lewis Cliff (LEW) 90500 using liquid chromatography‐time of flight‐mass spectrometry (LC‐ToF‐MS) coupled with UV fluorescence detection revealed that these carbonaceous meteorites contain a suite of indigenous amino acids not present in Antarctic ice. Several amino acids were detected in ALH 83100, including glycine, alanine, β‐alanine, γ‐amino‐n‐butyric acid (γ‐ABA), and α‐aminoisobutyric acid (AIB) with concentrations ranging from 250 to 340 parts per billion (ppb). In contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a much higher total abundance of these amino acids (440–3200 ppb). In addition, ALH 83100 was found to have lower abundances of the α‐dialkyl amino acids AIB and isovaline than LEW 90500 and Murchison. There are three possible explanations for the depleted amino acid content in ALH 83100: 1) amino acid leaching from ALH 83100 during exposure to Antarctic ice meltwater, 2) a higher degree of aqueous alteration on the ALH 83100 parent body, or 3) ALH 83100 originated on a chemically distinct parent body from the other two CM2 meteorites. The high relative abundance of ?‐amino‐n‐caproic acid (EACA) in the ALH 83100 meteorite as well as the Antarctic ice indicates that Nylon‐6 contamination from the Antarctic sample storage bags may have occurred during collection. 相似文献
108.
A. G. Nikoghossian 《Astrophysics》2006,49(2):228-239
The problem of finding statistical averages which characterize the radiation field in a semi-infinite atmosphere has been
discussed by the author in a series of papers. In this paper some of the results obtained there are generalized to media of
finite optical thickness. The average number of scattering events undergone by a photon during its random walk in an atmosphere
and the average time required for this are determined. Three different types of averages are considered, depending on the
photon characteristics over which the averages are taken. Special attention is devoted to the asymptotic behavior of these
quantities for media with large optical depths taking the influence of continuum absorption into account.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 263–275 (May 2006). 相似文献
109.
110.
Radiation exposure and Mission Strategies
for Interplanetary Manned Missions (REMSIM) 总被引:1,自引:0,他引:1
C. Cougnet N. B. Crosby S. Eckersley C. Foullon V. Guarnieri S. Guatelli D. Heynderickx A. Holmes-Siedle C. Lobascio S. Masiello P. Nieminen G. Parisi P. Parodi M. A. Perino M. G. Pia R. Rampini P. Spillantini V. Tamburini E. Tracino 《Earth, Moon, and Planets》2004,94(3-4):279-285
Cosmic radiation is an important problem for human interplanetary missions. The “Radiation Exposure and Mission Strategies for Interplanetary Manned Missions–REMSIM” study is summarised here. They are related to current strategies and countermeasures to ensure the protection of astronauts from radiation during interplanetary missions, with specific reference to: radiation environment and its variability; radiation effects on the crew; transfer trajectories and associated fluences; vehicle and surface habitat concepts; passive and active shielding concepts; space weather monitoring and warning systems. 相似文献