全文获取类型
收费全文 | 1784篇 |
免费 | 104篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 76篇 |
大气科学 | 136篇 |
地球物理 | 387篇 |
地质学 | 715篇 |
海洋学 | 134篇 |
天文学 | 288篇 |
综合类 | 8篇 |
自然地理 | 150篇 |
出版年
2024年 | 4篇 |
2023年 | 12篇 |
2022年 | 10篇 |
2021年 | 30篇 |
2020年 | 46篇 |
2019年 | 44篇 |
2018年 | 55篇 |
2017年 | 77篇 |
2016年 | 90篇 |
2015年 | 60篇 |
2014年 | 58篇 |
2013年 | 119篇 |
2012年 | 79篇 |
2011年 | 96篇 |
2010年 | 108篇 |
2009年 | 99篇 |
2008年 | 102篇 |
2007年 | 90篇 |
2006年 | 92篇 |
2005年 | 84篇 |
2004年 | 87篇 |
2003年 | 58篇 |
2002年 | 61篇 |
2001年 | 35篇 |
2000年 | 23篇 |
1999年 | 38篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 17篇 |
1995年 | 6篇 |
1994年 | 21篇 |
1993年 | 14篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 10篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1980年 | 15篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1974年 | 4篇 |
排序方式: 共有1894条查询结果,搜索用时 975 毫秒
821.
A microstructure model of dual-porosity type is proposed to describe contaminant transport in fully-saturated swelling clays.
The swelling medium is characterized by three separate-length scales (nano, micro, and macro) and two levels of porosity (nano-
and micropores). At the nanoscale, the medium is composed of charged clay particles saturated by a binary monovalent aqueous
electrolyte solution. At the intermediate (micro) scale, the two-phase homogenized system is represented by swollen clay clusters
(or aggregates) with the nanoscale electrohydrodynamics, local charge distribution, and disjoining pressure effects incorporated
in the averaged constitutive laws of the electro-chemo-mechanical coefficients and the swelling pressure, which appear in
Onsager’s reciprocity relations and in a modified form of Terzaghi’s effective principle, respectively. The microscopic coupling
between aggregates and a bulk solution lying in the micropores is ruled by a slip boundary condition on the tangential velocity
of the fluid, which captures the effects of the thin electrical double layers surrounding each clay cluster. At the macroscale,
the system of clay clusters is homogenized with the bulk fluid. The resultant macroscopic picture is governed by a dual-porosity
model wherein macroscopic flow and ion transport take place in the bulk solution and the clay clusters act as sources/sinks
of mass of water and solutes to the bulk fluid. The homogenization procedure yields a three-scale model of the swelling medium
by providing new nano and micro closure problems, which are solved numerically to construct constitutive laws for the effective
electro-chemo-hydro-mechanical coefficients. Considering local instantaneous equilibrium between the clay aggregates and micropores,
a quasisteady version of the dual-porosity model is proposed. When combined with the three-scale portrait of the swelling
medium, the quasisteady model allows us to build-up numerically the constitutive law of the equilibrium adsorption isotherm,
which governs the instantaneous immobilization of the solutes in the clay clusters. Moreover, the constitutive behavior of
the retardation coefficient is also constructed by exploring its representation in terms of the local profile of the electrical
double layer potential of the electrolyte solution, which satisfies the Poisson–Boltzmann problem at the nanoscale. 相似文献
822.
Conceptual hydrogeological model of volcanic Easter Island (Chile) after chemical and isotopic surveys 总被引:2,自引:2,他引:0
Most human activities and hydrogeological information on small young volcanic islands are near the coastal area. There are almost no hydrological data from inland areas, where permanent springs and/or boreholes may be rare or nonexistent. A major concern is the excessive salinity of near-the-coast wells. Obtaining a conceptual hydrogeological model is crucial for groundwater resources development and management. Surveys of water seepages and rain for chemical and environmental isotope contents may provide information on the whole island groundwater flow conditions, in spite of remaining geological and hydrogeological uncertainties. New data from Easter Island (Isla de Pascua), in the Pacific Ocean, are considered. Whether Easter Island has a central low permeability volcanic “core” sustaining an elevated water table remains unknown. Average recharge is estimated at 300–400 mm/year, with a low salinity of 15–50 mg/L Cl. There is an apron of highly permeable volcanics that extends to the coast. The salinity of near-the-coast wells, >1,000 mg/L Cl, is marine in origin. This is the result of a thick mixing zone of island groundwater and encroached seawater, locally enhanced by upconings below pumping wells. This conceptual model explains what is observed, in the absence of inland boreholes and springs. 相似文献
823.
Knowledge on the stresses in shotcrete tunnel shells is of great importance, as to assess their safety against severe cracking
or failure. Estimation of these stresses from 3D optical displacement measurements requires shotcrete material models, which
may preferentially consider variations in the water–cement and aggregate–cement ratios. Therefore, we employ two representative
volume elements within a continuum micromechanics framework: the first one relates to cement paste (with a spherical material
phase representing cement clinker grains, needle-shaped hydrate phases with isotropically distributed spatial orientations,
a spherical water phase, and a spherical air phase; all being in mutual contact), and the second one relates to shotcrete
(with phases representing cement paste and aggregates, whereby aggregate inclusions are embedded into a matrix made up by
cement paste). Elasticity homogenization follows self-consistent schemes (at the cement paste level) and Mori–Tanaka estimates
(at the shotcrete level), and stress peaks in the hydrates related to quasi-brittle material failure are estimated by second-order
phase averages derived from the RVE-related elastic energy. The latter permits upscaling from the hydrate strength to the
shotcrete strength. Experimental data from resonant frequency tests, ultrasonics tests, adiabatic tests, uniaxial compression
tests, and nanoindentation tests suggest that shotcrete elasticity and strength can be reasonably predicted from mixture-
and hydration-independent elastic properties of aggregates, clinker, hydrates, water, and air, and from strength properties
of hydrates. At the structural level, the micromechanics model, when combined with 3D displacement measurements, predicts
that a decrease of the water–cement ratio increases the safety of the shotcrete tunnel shell. 相似文献
824.
825.
826.
827.
828.
Flow transverse bedforms (ripples and dunes) are ubiquitous in rivers and coastal seas. Local hydrodynamics and transport conditions depend on the size and geometry of these bedforms, as they constitute roughness elements at the bed. Bedform influence on flow energy must be considered for the understanding of flow dynamics, and in the development and application of numerical models. Common estimations or predictors of form roughness (friction factors) are based mostly on data of steep bedforms (with angle-of-repose lee slopes), and described by highly simplified bedform dimensions (heights and lengths). However, natural bedforms often are not steep, and differ in form and hydraulic effect relative to idealised bedforms. Based on systematic numerical model experiments, this study shows how the hydraulic effect of bedforms depends on the flow structure behind bedforms, which is determined by the bedform lee side angle, aspect ratio and relative height. Simulations reveal that flow separation behind bedform crests and, thus, a hydraulic effect is induced at lee side angles steeper than 11 to 18° depending on relative height, and that a fully developed flow separation zone exists only over bedforms with a lee side angle steeper than 24°. Furthermore, the hydraulic effect of bedforms with varying lee side angle is evaluated and a reduction function to common friction factors is proposed. A function is also developed for the Nikuradse roughness (k s), and a new equation is proposed which directly relates k s to bedform relative height, aspect ratio and lee side angle. 相似文献
829.
830.