全文获取类型
收费全文 | 5891篇 |
免费 | 229篇 |
国内免费 | 89篇 |
专业分类
测绘学 | 169篇 |
大气科学 | 548篇 |
地球物理 | 1465篇 |
地质学 | 2038篇 |
海洋学 | 569篇 |
天文学 | 788篇 |
综合类 | 22篇 |
自然地理 | 610篇 |
出版年
2021年 | 60篇 |
2020年 | 65篇 |
2019年 | 89篇 |
2018年 | 126篇 |
2017年 | 116篇 |
2016年 | 162篇 |
2015年 | 144篇 |
2014年 | 206篇 |
2013年 | 325篇 |
2012年 | 247篇 |
2011年 | 269篇 |
2010年 | 199篇 |
2009年 | 315篇 |
2008年 | 294篇 |
2007年 | 270篇 |
2006年 | 226篇 |
2005年 | 193篇 |
2004年 | 200篇 |
2003年 | 190篇 |
2002年 | 185篇 |
2001年 | 138篇 |
2000年 | 140篇 |
1999年 | 116篇 |
1998年 | 108篇 |
1997年 | 96篇 |
1996年 | 86篇 |
1995年 | 87篇 |
1994年 | 76篇 |
1993年 | 64篇 |
1992年 | 67篇 |
1991年 | 69篇 |
1990年 | 66篇 |
1989年 | 59篇 |
1988年 | 59篇 |
1987年 | 68篇 |
1986年 | 44篇 |
1985年 | 75篇 |
1984年 | 80篇 |
1983年 | 74篇 |
1982年 | 65篇 |
1981年 | 79篇 |
1980年 | 65篇 |
1979年 | 57篇 |
1978年 | 39篇 |
1977年 | 56篇 |
1976年 | 65篇 |
1975年 | 48篇 |
1974年 | 56篇 |
1973年 | 47篇 |
1972年 | 26篇 |
排序方式: 共有6209条查询结果,搜索用时 37 毫秒
651.
Process models for ore formation in magmatic Ni–Cu–platinum group element (PGE) sulfide systems require that S saturation
is achieved in a mafic–ultramafic magma. Traditional models explain the achievement of S saturation or sulfide saturation
either by the addition of crustal S, by the felsification of the magma by crustal contamination, or by mixing between primitive
and evolved magmas. Which process matters most is important to industry-oriented exploration models where crustal S sources
are believed to be encouraging features of a metallotect. Studies of the Siberian Trap flood basalts at Noril’sk have demonstrated
that chalcophile element depletion is linked to assimilation of silica-rich crust, but it is less clear whether this contaminant
contained an appreciable amount of S. At Noril’sk, the Ni–Cu–PGE sulfide deposits are associated with subvolcanic intrusions
that were emplaced into Permian and Carboniferous sedimentary sequences rich in shales, marlstones, and evaporites. Similar
to the Siberian Trap basalts, the Deccan Trap contains a volumetrically important suite of crustally contaminated tholeiitic
basalts. We present new PGE data for samples from a stratigraphic sequence of basalts from the southern Deccan province. Two
of the formations in this sequence (the Bushe and Poladpur Formations) have geochemical signatures indicative of a wide degree
of crustal contamination of a magma type that gave rise to the stratigraphically higher Ambenali Formation (a product of transitional
midocean ridge basalt magmatism). There are no known deposits or occurrences of Ni–Cu–PGE sulfides associated with subvolcanic
intrusions in the Deccan province. Despite the fact that the Bushe Formation exhibits a stronger crustal contamination signature
than the most contaminated Siberian Trap basalt formations, and the Poladpur lavas are also strongly crustally contaminated,
the Bushe and Poladpur basalts are undepleted in Ni, Cu, or PGE. This indicates that the contaminated Deccan Trap lavas did
not achieve S saturation. This, in turn, places constraints on the potential of the Deccan Trap in southern India to host
significant magmatic sulfide deposits. Conversely, this observation also indicates that an S-rich crustal contaminant is required
for the genesis of magmatic Ni–Cu–PGE sulfide deposits. 相似文献
652.
In order to describe diffusive transport of solutes through a porous material, estimation of effective diffusion coefficients is required. It has been shown theoretically that in the case of uncharged porous materials the effective diffusion coefficient of solutes is a function of the pore morphology of the material and can be described by the tortuosity (tensor) (Bear, 1988 [1]). Given detailed information of the pore geometry at the micro-scale the tortuosity of different materials can be accurately estimated using homogenization procedures. However, many engineering materials (e.g., clays and shales) are characterized by electrical surface charges on particles of the porous material which strongly affect the (diffusive) transport properties of ions. For these type of materials, estimation of effective diffusion coefficients have been mostly based on phenomenological equations with no link to underlying micro-scale properties of these charged materials although a few recent studies have used alternative methods to obtain the diffusion parameters (Jougnot et al., 2009; Pivonka et al., 2009; Revil and Linde, 2006 2, 3 and 4). In this paper we employ a recently proposed up-scaled Poisson–Nernst–Planck type of equation (PNP) and its micro-scale counterpart to estimate effective ion diffusion coefficients in thin charged membranes. We investigate a variety of different pore geometries together with different surface charges on particles. Here, we show that independent of the charges on particles, a (generalized) tortuosity factor can be identified as function of the pore morphology only using the new PNP model. On the other hand, all electro-static interactions of ions and charges on particles can consistently be captured by the ratio of average concentration to effective intrinsic concentration in the macroscopic PNP equations. Using this formulation allows to consistently take into account electrochemical interactions of ions and charges on particles and so excludes any ambiguity generally encountered in phenomenological equations. 相似文献
653.
Juraj Majzlan Peter Glasnák Robert A. Fisher Mary Anne White Michel B. Johnson Brian Woodfield Juliana Boerio-Goates 《Physics and Chemistry of Minerals》2010,37(9):635-651
Jarosite phases are common minerals in acidic, sulfate-rich environments. Here, we report heat capacities (C
p) and standard entropies (S°) for a number of jarosite samples. Most samples are close to the nominal composition AFe3(SO4)2(OH)6, where A = K, Na, Rb, and NH4. One of the samples has a significant number of defects on the Fe sites and is called the defect jarosite; others are referred
to as A-jarosite. The samples, their compositions, and the entropies at T = 298.15 K are:
There are additional configurational entropies of 13.14 and 8.23 J mol−1 K−1 in defect and NH4-jarosite, respectively. A detailed analysis of the synchrotron X-ray diffraction patterns showed a large anisotropic peak
broadening for defect and NH4-jarosite. The fits to the low-temperature (approx. <12 K) C
p data showed that our samples can be divided into two groups. The first group is populated by the K-, Na-, Rb-, and NH4-jarosite samples, antiferromagnetic at low temperatures. The second group contains the H3O-jarosite (studied previously) and the defect jarosite. H3O- and defect jarosite are spin glasses and their low-T
C
p was fit with the expression C
p = γT + ΣB
j
T
j
, where j = (3, 5, 7, 9). The linear term is typical for spin glasses and the sum represents the lattice contribution to C
p. Surprisingly, the C
p of the K-, Na-, Rb-, and NH4-jarosite samples, which are usually considered to be antiferromagnetic at low temperatures, also contains a large linear
term. This finding suggests that even these phases do not order completely, but have a partial spin-glass character below
their Néel transition temperature. 相似文献
Sample | Chemical composition | S o/(J mol−1 K−1) |
---|---|---|
K-jarosite | K0.92(H3O)0.08Fe2.97(SO4)2(OH)5.90(H2O)0.10 | 427.4 ± 0.7 |
Na-jarosite | Na0.95(H3O)0.05Fe3.00(SO4)2(OH)6.00 | 436.4 ± 4.4 |
Rb-jarosite | RbFe2.98(SO4)2(OH)5.95(H2O)0.05 | 411.9 ± 4.1 |
NH4-jarosite | (NH4)0.87(H3O)0.13Fe3.00(SO4)2(OH)6.00 | 447.2 ± 4.5 |
Defect jarosite | K0.94(H3O)0.06Fe2.34(SO4)2(OH)4.01(H2O)1.99 | 412.7 ± 4.1 |
654.
Peter I. Dorogokupets 《Physics and Chemistry of Minerals》2010,37(9):677-684
A simplest equation within the framework of the Mie-Grüneisen–Einstein approach is considered. Pressure estimation values
are presented that are derived by conventional arithmetic and algebraic calculations as a function of temperature and volume.
The equation under consideration complies with the Mie-Grüneisen–Debye model at high temperature. Different versions of an
equation of state (EoS) of MgO proposed by Speziale et al. (J Geophys Res 106B:515–528, 2001) as a pressure standard at high temperatures are subject to analyses. In the literature, at least four versions of Speziale
et al. EoS of MgO are discussed; the discrepancy between them reaching a few GPa at T > 2,000 K and P > 100 GPa. Our analyses of these equations suggest that the volume dependence of the Debye temperature is accepted arbitrarily
and does not agree with the definition of the Grüneisen parameter, γ = −(∂lnΘ/∂lnV)
T
. Pressure as a function of temperature and volume in the Mie-Grüneisen–Einstein approach or the Gao pressure calculator can
be used to estimate true pressure at compression x = V/V
0 < 1 with the Speziale et al. EoS of MgO. 相似文献
655.
Paolo Nimis Paolo Omenetto Bernd Buschmann Peter Jonas Vladimir A. Simonov 《Mineralogy and Petrology》2010,100(3-4):201-214
Ultramafic–mafic- and ultramafic-hosted Cu (Co, Ni, Au) volcanogenic massive sulfide (VMS) deposits from ophiolite complexes of the Main Uralian Fault, Southern Urals, are associated with island arc-type igneous rocks. Trace element analyses show that these rocks are geochemically analogous to Early Devonian boninitic and island arc tholeiitic rocks found at the base of the adjacent Magnitogorsk volcanic arc system, while they are distinguished both from earlier, pre-subduction volcanic rocks and from later volcanic products that were erupted in progressively more internal arc settings. The correlation between the sulfide host-rocks and the earliest volcanic units of the Magnitogorsk arc suggests a connection between VMS formation and infant subduction-driven intraoceanic magmatism. 相似文献
656.
In recent years, debris flows have represented a severe natural hazard in South-Gargano watersheds (Puglia Region, Southern Italy). Hill slopes erosion, caused by the inadequate protection of the degraded forests, produces large amounts of soil and debris that are conveyed downstream during heavy rainstorms. The involved material is characterized by limestone fragments and blocks in a sandy-silt matrix. In this paper, the rheological properties of such debris-flow materials have been investigated. Eight specimens of particulated sediments of particle diameter of d ≤ 4 mm have been analysed using a rheometric tool for large particle suspensions, the ball measuring system (BMS). The influence of sediment concentration on the bulk rheological behaviour has been evaluated at concentrations by volume obtained in fully water-saturated conditions (volumetric sediment concentration C V ~ 0.8) and with fixed water content (C V = 0.42). The rheological data were fitted to the Bingham, Herschel–Bulkley, and O’Brien and Julien models to define viscosity and yield stress dependency on sediment concentration, to provide viscosity information for hazard mitigation as well as for comparison to other debris-flow events. 相似文献
657.
A. I. Efimov T. Imamura K. -I. Oyama K. Noguchi L. N. Samoznaev A. S. Nabatov M. K. Bird I. V. Chashei 《Astronomy Reports》2010,54(11):1032-1041
Radio-sounding experiments using signals from the Japanese NOZOMI spacecraft to probe the circum solar plasma were performed from December 2000 through January 2001. They can be used to obtain information about the properties of the solar wind plasma in the region where it is accelerated at heliocentric distances of 12.8–36.9R s (where R s is the radius of the Sun). Measurements of the intensity and frequency of the received signals were carried out with high time resolution (~0.05 s for the frequency and ~0.0064 s for the intensity), making it possible to investigate the anisotropy of inhomogeneities and the spatial spectrum of the turbulence of the circum solar plasma. Analysis of these radio-sounding data has shown that the scintillation index and intensity of the frequency fluctuations decrease approximately according to a power law with increasing distance of the line of sight from the Sun. Measurements of the amplitude fluctuations and estimates of the solar wind velocity derived from spatially separated observations indicate the presence of small-scale inhomogeneities with sizes of the order of 50 km at heliocentric distances less than 25R s , which are elongated in the radial direction with anisotropy coefficients from 2.3 to 3.0. The inhomogeneities at heliocentric distances exceeding 30R s become close to isotropic. 相似文献
658.
In this article, an approach for the efficient numerical solution of multi-species reactive transport problems in porous media
is described. The objective of this approach is to reformulate the given system of partial and ordinary differential equations
(PDEs, ODEs) and algebraic equations (AEs), describing local equilibrium, in such a way that the couplings and nonlinearities
are concentrated in a rather small number of equations, leading to the decoupling of some linear partial differential equations
from the nonlinear system. Thus, the system is handled in the spirit of a global implicit approach (one step method) avoiding
operator splitting techniques, solved by Newton’s method as the basic algorithmic ingredient. The reduction of the problem
size helps to limit the large computational costs of numerical simulations of such problems. If the model contains equilibrium
precipitation-dissolution reactions of minerals, then these are considered as complementarity conditions and rewritten as
semismooth equations, and the whole nonlinear system is solved by the semismooth Newton method. 相似文献
659.
Modelling of paleo-saltwater intrusion in the northern part of the Nubian Aquifer System, Northeast Africa 总被引:2,自引:2,他引:0
A numerical groundwater model of the Nubian Aquifer System was established to prove the influence of rising seawater levels on the groundwater salinity in northern Egypt over the last 140,000 years. In addition, the impact of a groundwater recharge scenario for these 140,000 years, involving climatic change, on the saltwater/freshwater interface was investigated. Saltwater intrusion induced by rising water levels of the Mediterranean Sea led to salinisation from the Mediterranean Sea to the Qattara depression. This modeling approach was supported by a density-driven model setup and calculation. The modelled saltwater/freshwater interfaces partially fitted the observed ones, especially in the southern half of the Qattara depression. In other parts of the northern Nubian Aquifer System, the ingression of salt water was modelled adequately, but in the west, small regions of the measured interface were not. The development in the Qattara depression (Egypt) and Sirte basin (Libya) were investigated in more detail. The different behaviour in the Sirte basin may be due to high evapotranspiration rates in some former periods, salt solutions from the pre-Quaternary layers or saltwater infiltration from sabkha-like recent salt-bearing sediments. 相似文献
660.
The Skidaway River estuary, GA (USA), a tidally dominated subtropical system surrounded by extensive Spartina salt marshes, is experiencing steady increases in nutrients, chlorophyll, and particulate matter and decline in dissolved
oxygen, associated with cultural eutrophication. A long-term study is documenting changes in these parameters: previous papers
Verity (Estuaries 25:944–960, 2002a, Estuaries 25:961–975, b) reported on hydrography, nutrients, chlorophyll, and particulate matter during 1986–1996; plankton community responses are
reported here. Phytoplankton, bacteria, heterotrophic nanoplankton and dinoflagellates, ciliates, and copepods exhibited strong
seasonal cycles in abundance driven by temperature and resource availability, typically with summer maxima and winter minima.
However, cultural eutrophication coincided with altered planktonic food webs as autotrophic and heterotrophic communities
responded to increasing concentrations and changing ratios of inorganic and organic nutrients, potential prey, and predators.
Small (<8 μm) photosynthetic nanoplankton increased in absolute concentration and also relative to larger cells. In contrast,
diatoms did not show consistent increases in abundance, despite significant long-term increases in ambient silicate concentrations.
Mean annual bacteria concentrations approximately doubled, and eukaryotic organisms in the microbial food web (heterotrophic
and mixotrophic flagellates, dinoflagellates, ciliates, and metazoan zooplankton) also increased. All plankton groups except
copepods showed trends of increasing annual amplitudes between seasonal high and low values, with higher peak concentrations
each year. These observations suggest that the eutrophication signal was gradually becoming uncoupled from regulatory mechanisms.
Theory and evidence from other more impacted waters suggest that, if these patterns continue, changes in the structure and
function of higher trophic levels will ensue. 相似文献