全文获取类型
收费全文 | 45034篇 |
免费 | 1216篇 |
国内免费 | 1152篇 |
专业分类
测绘学 | 1099篇 |
大气科学 | 3517篇 |
地球物理 | 9486篇 |
地质学 | 16442篇 |
海洋学 | 4055篇 |
天文学 | 9180篇 |
综合类 | 268篇 |
自然地理 | 3355篇 |
出版年
2022年 | 320篇 |
2021年 | 577篇 |
2020年 | 591篇 |
2019年 | 633篇 |
2018年 | 1089篇 |
2017年 | 1055篇 |
2016年 | 1265篇 |
2015年 | 907篇 |
2014年 | 1279篇 |
2013年 | 2299篇 |
2012年 | 1590篇 |
2011年 | 2148篇 |
2010年 | 1900篇 |
2009年 | 2422篇 |
2008年 | 2041篇 |
2007年 | 2128篇 |
2006年 | 2004篇 |
2005年 | 1467篇 |
2004年 | 1381篇 |
2003年 | 1293篇 |
2002年 | 1213篇 |
2001年 | 1014篇 |
2000年 | 1025篇 |
1999年 | 816篇 |
1998年 | 854篇 |
1997年 | 811篇 |
1996年 | 707篇 |
1995年 | 682篇 |
1994年 | 591篇 |
1993年 | 511篇 |
1992年 | 510篇 |
1991年 | 450篇 |
1990年 | 545篇 |
1989年 | 443篇 |
1988年 | 433篇 |
1987年 | 516篇 |
1986年 | 423篇 |
1985年 | 531篇 |
1984年 | 653篇 |
1983年 | 561篇 |
1982年 | 548篇 |
1981年 | 472篇 |
1980年 | 471篇 |
1979年 | 425篇 |
1978年 | 406篇 |
1977年 | 398篇 |
1976年 | 364篇 |
1975年 | 354篇 |
1974年 | 360篇 |
1973年 | 403篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
The production of X-rays and gamma-rays in bursts is believed to be due to the rapid burning of matter accreted onto a neutron star surface from its companion, most probably a giant star. The accreted matter consists mainly of hydrogen and helium and a very small amount of heavy elements. Due to the infall of matter, the temperature at the bottom layers is raised to a value of the order of 108 K. The neutron star surface density is>107 g cm–3. As hydrogen burning is a slow process under any temperature and density conditions, we consider the helium-burning reactions as the source of gamma-rays in the neutron star surface. Under high-density conditions the ordinary laboratory reaction rates should become modified. At high-density conditions, the strong screening effect due to the polarising cloud of electrons around the ions become important and enhances the reaction rates considerably. The helium-burning reactions are calculated under such conditions. The abundances of helium-burning products such as12C, 116O, and20Ne, etc., are computed. Under high-density and temperature conditions carbon is found to be more abundant than oxygen. Neon is completely absent in almost all the relevant physical conditions in which a strong screening effect is operative. It is suggested that explosive burning of accreted helium of 10–13
M
will account for the observed energy of gamm-ray burst. 相似文献
102.
P. L. Bornmann 《Solar physics》1985,102(1-2):111-130
The light curves of soft X-ray lines, observed by the Flat Crystal Spectrometer on Solar Maximum Mission during eight solar flares are modeled to determine the plasma temperature and emission measure as functions of time using the method first presented by Bornmann (1985, Paper I), but modified to include a 2 search routine. With this modification the technique becomes more general, more accurate, and applicable throughout the gradual phase of the flare. The model reproduces the light curves of the soft X-ray lines throughout these flares. Model fits were repeated for each flare using five different sets of published line emissivity calculations. The emissivities of Mewe and Gronenschild (1981) consistenly gave the best fits to the observed light curves for each flare. 相似文献
103.
It is proposed that the solar flare phenomenon can be understood as a manifestation of the electrodynamic coupling process of the photosphere-chromosphere-corona system as a whole. The system is coupled by electric currents, flowing along (both upward and downward) and across the magnetic field lines, powered by the dynamo process driven by the neutral wind in the photosphere and the lower chromosphere. A self-consistent formulation of the proposed coupling system is given. It is shown in particular that the coupling system can generate and dissipate the power of 1029 erg s#X2212;1 and the total energy of 1032 erg during a typical life time (103 s) of solar flares. The energy consumptions include Joule heat production, acceleration of current-carrying particles along field lines, magnetic energy storage and kinetic energy of plasma convection. The particle acceleration arises from the development of field-aligned potential drops of 10–150 kV due to the loss-cone constriction effect along the upward field-aligned currents, causing optical, X-ray and radio emissions. The total number of precipitating electrons during a flare is shown to be of order 1037–1038. 相似文献
104.
This study explores garnet coronas around hedenbergite, which were formed by the reaction plagioclase + hedenbergite→garnet + quartz, to derive information about diffusion paths that allowed for material redistribution during reaction progress. Whereas quartz forms disconnected single grains along the garnet/hedenbergite boundaries, garnet forms ~20‐μm‐wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low‐angle grain boundaries, which commonly form a direct link between the reaction interfaces of the plagioclase|garnet|hedenbergite succession. Compositional variations in garnet involve: (i) an overall asymmetric compositional zoning in Ca, Fe2+, Fe3+ and Al across the garnet layer; and (ii) micron‐scale compositional variations in the near‐grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth. Thereby, transfer of the chemical components occurred by a combination of fast‐path diffusion along grain boundaries within the garnet rim, slow diffusion through the interior of the garnet grains, and by fast diffusion along the garnet/plagioclase and the garnet/hedenbergite phase boundaries. Numerical simulation indicates that diffusion of Ca, Al and Fe2+ occurred about three to four, four and six to seven orders of magnitude faster along the grain boundaries than through the interior of the garnet grains. Fast‐path diffusion along grain boundaries contributed substantially to the bulk material transfer across the growing garnet rim. Despite the contribution of fast‐path diffusion, bulk diffusion through the garnet rim was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometre scale. Based on published garnet volume diffusion data the growth interval of a 20‐μm‐wide garnet rim is estimated at ~103–104 years at the inferred reaction conditions of 760 ± 50 °C at 7.6 kbar. Using the same parameterization of the growth law, 100‐μm‐ and 1‐mm‐thick garnet rims would grow within 105–106 and 106–107 years respectively. 相似文献
105.
The role of heterogeneous strain in the development and preservation of a polymetamorphic record in high-P granulites, western Canadian Shield 总被引:1,自引:0,他引:1
K. H. MAHAN P. GONCALVES R. FLOWERS M. L. WILLIAMS D. HOFFMAN-SETKA 《Journal of Metamorphic Geology》2008,26(6):669-694
Mafic rocks in the Chipman domain of the Athabasca granulite terrane, western Canadian Shield, provide the first well‐documented record of two distinct high‐P granulite facies events in the same domain in this region. Textural relations and the results of petrological modelling (NCFMASHT system) of mafic granulites are interpreted in terms of a three‐stage tectonometamorphic history. Stage 1 involved development of the assemblage Grt + Cpx + Qtz ± Pl (M1) from a primary Opx‐bearing igneous precursor at conditions of 1.3 GPa, 850–900 °C. Field and microstructural observations suggest that M1 developed synchronously with an early S1 gneissic fabric. Stage 2 is characterized by heterogeneous deformation (D2) and synkinematic partial retrogression of the peak assemblage to an amphibole‐bearing assemblage (M2). Stage 3 involved a third phase of deformation and a return to granulite facies conditions marked by the prograde breakdown of amphibole (Amph2) to produce matrix garnet (Grt3a) and the coronitic assemblage Cpx3b + Opx3b + Ilm3b + Pl3b (M3b) at 1.0 GPa, 800–900 °C. M1 and M3b are correlated with 2.55 and 1.9 Ga metamorphic generations of zircon, respectively, which were dated in a separate study. Heterogeneous strain played a crucial role in both the development and preservation of these rare examples of multiple granulite facies events within single samples. Without this fortuitous set of circumstances, the apparent reaction history could have incorrectly led to an interpretation involving a single‐cycle high‐grade event. The detailed P–T–t–D history constructed for these rocks provides the best evidence to date that much of the east Lake Athabasca region experienced long‐term lower crustal residence from 2.55 to 1.9 Ga, and thus the region represents a rare window into the reactivation and ultimate stabilization processes of cratonic lithosphere. 相似文献
106.
L. Moreno J. J. Durán M. A. Casermeiro J. R. Quintana Ma A. Fernández 《Environmental Geology》2008,54(3):465-477
Over a period of 4 years and 4 months, the geopurification installations at Dehesas de Guadix (Granada, Spain) were monitored
to determine the impact on soil and groundwater of the controlled discharge of urban wastewater, and also to identify the
best indicators of the entry of the recharged water into the aquifer. The installations are located in an area where the climate
is Mediterranean sub-arid, with an average precipitation of less than 287 mm/year, and a rate of evapotranspiration that is
almost three times greater. The system was controlled by determining the balance of majority nutrients and boron in the soil
and in the groundwater, both at the points affected directly by the wastewater discharge and at others. The quantity of mass
discharged was relatively large (COD 14,656 g/m2, NO3 85 g/m2, NO2 4 g/m2, NH4 2,425 g/m2, PO4 1,143 g/m2, K 1,531 g/m2, B 63 g/m2). It was observed that the elimination of nutrients within the soil (COD 97.5%, PO4 94.4%, K 59.17%, N
total 18.8%, B 12.69%) was very efficient except for the nitrogen, which nevertheless did not reach the groundwater, as it was
eliminated at deep levels of the unsaturated zone. Only 12.69% of the boron was removed, and appreciable, increasing amounts
of this element did reach the groundwater. Unexpectedly, none of the majority nutrients behaved as a reliable indicator of
the impact on groundwater; despite this, the boron and the bicarbonate did clearly reflect the arrival of the recharged water,
and are proposed as the best indicators. 相似文献
107.
The Hill Country of Central Texas, USA, is undergoing rapid socioeconomic development, but environmental management of this
region is hampered by misconceptions about local bedrock, soils, terrain, and hydrologic processes. The Hill Country is underlain
mostly by Glen Rose Limestone (Lower Cretaceous) and exhibits a stepped terrain, which has been incorrectly attributed to
alternating hard and soft bedrock strata. Other characteristics mistakenly attributed to this landscape include thin soils
with scant water-retention capabilities, and rapid runoff as the dominant hydrologic process. This report presents new findings:
unweathered bedrock is well indurated, but interbeds exhibit variable weathering rates. Recessive slopes (“risers”) on this
stepped terrain result from rapid deterioration of strata having generally heterogeneous depositional fabrics (bioturbation
and irregular clay partings) in contrast to ledge-forming strata having homogeneous fabrics. A stony regolith is thus formed
beneath risers, providing porous and permeable ground that retards runoff and promotes the formation of moderately deep to
deep (two-tiered) regolith/soil zones. These surficial materials on local steep slopes compose important natural environmental
buffers; they support diverse biota and enhanced geochemical cycling of nutrients; they also exhibit significant water retention
and enhanced erosion abatement. Proper land management demands recognition of these attributes in the siting, design, and
construction of facilities. 相似文献
108.
David M. Hirsch 《Contributions to Mineralogy and Petrology》2008,155(4):401-415
Garnet-bearing schists from the Waterville Formation of south-central Maine provide an opportunity to examine the factors
governing porphyroblast size over a range of metamorphic grade. Three-dimensional sizes and locations for all garnet porphyroblasts
were determined for three samples along the metamorphic field gradient spanning lowest garnet through sillimanite grade, using
high-resolution X-ray computed tomography. Comparison of crystal size distributions to previous data sets obtained by stereological
methods for the same samples reveals significant differences in mode, mean, and shape of the distributions. Quantitative textural
analysis shows that the garnets in each rock crystallized in a diffusion-controlled nucleation and growth regime. In contrast
to the typical observation of a correlation between porphyroblast size and position along a metamorphic field gradient, porphyroblast
size of the lowest-grade specimen is intermediate between the high- and middle-grade specimens’ sizes. Mean porphyroblast
size does not correlate with peak temperatures from garnet-biotite Fe-Mg exchange thermometry, nor is post-crystallization
annealing (Ostwald Ripening) required to produce the observed textures, as was previously proposed for these rocks. Robust
pseudosection calculations fail to reproduce the observed garnet core compositions for two specimens, suggesting that these
calc-pelites experienced metasomatism. For each of these two specimens, Monte Carlo calculations suggest potential pre-metasomatism
bulk compositions that replicate garnet core compositions. Pseudosection analyses allow the estimation of the critical temperatures
for garnet growth: ∼481, ∼477, and ∼485°C for the lowest-garnet-zone, middle-garnet-zone, and sillimanite-zone specimens,
respectively. Porphyroblast size appears to be determined in this case by a combination of the heating rate during garnet
crystallization, the critical temperature for the garnet-forming reaction and the kinetics of nucleation. Numerical simulations
of thermally accelerated, diffusion-controlled nucleation, and growth for the three samples closely match measured crystal
size distributions. These observations and simulations suggest that previous hypotheses linking the garnet size primarily
to the temperature at the onset of porphyroblast nucleation can only partially explain the observed textures. Also important
in determining porphyroblast size are the heating rate and the distribution of favorable nucleation sites. 相似文献
109.
Gregory K. Druschel David Emerson R. Sutka P. Suchecki 《Geochimica et cosmochimica acta》2008,72(14):3358-3370
Neutrophilic iron oxidizing bacteria (FeOB) must actively compete with rapid abiotic processes governing Fe(II) oxidation and as a result have adapted to primarily inhabit low-O2 environments where they can more successfully compete with abiotic Fe(II) oxidation. The spatial distribution of these microorganisms can be observed through the chemical gradients they affect, as measured using in situ voltammetric analysis for dissolved Fe(II), Fe(III), O2, and FeS(aq). Field and laboratory determination of the chemical environments inhabited by the FeOB were coupled with detailed kinetic competition studies for abiotic and biotic oxidation processes using a pure culture of FeOB to quantify the geochemical niche these organisms inhabit. In gradient culture tubes, the maximum oxygen levels, which were associated with growth bands of Sideroxydans lithotrophicus (ES-1, a novel FeOB), were 15-50 μM. Kinetic measurements made on S. lithotrophicus compared biotic/abiotic (killed control) Fe oxidation rates. The biotic rate can be a significant and measurable fraction of the total Fe oxidation rate below O2 concentrations of approximately 50 μM, but biotic Fe(II) oxidation (via the biotic/abiotic rate comparison) becomes difficult to detect at higher O2 levels. These results are further supported by observations of conditions supporting FeOB communities in field settings. Variablity in cell densities and cellular activity as well as variations in hydrous ferrous oxide mineral quantities significantly affect the laboratory kinetic rates. The microbial habitat (or geochemical niche) where FeOB are active is thus largely controlled by the competition between abiotic and biotic kinetics, which are dependent on Fe(II) concentration, PO2, temperature and pH in addition to the surface area of hydrous ferric oxide minerals and the cell density/activity of FeOB. Additional field and lab culture observations suggest a potentially important role for the iron-sulfide aqueous molecular cluster, FeS(aq), in the overall cycling of iron associated with the environments these microorganisms inhabit. 相似文献
110.
Nancy L. Chabot Andrew J. Campbell David S. Draper Munir Humayun Elizabeth Cottrell 《Geochimica et cosmochimica acta》2008,72(16):4146-4158
Earth’s core may contain C, and it has been suggested that C in the core could stabilize the formation of a solid inner core composed of Fe3C. We experimentally examined the Fe-C system at a pressure of 5 GPa and determined the Fe-C phase diagram at this pressure. In addition, we measured solid metal/liquid metal partition coefficients for 17 trace elements and examined the partitioning behavior between Fe3C and liquid metal for 14 trace elements. Solid metal/liquid metal partition coefficients are similar to those found in one atmosphere studies, indicating that the effect of pressure to 5 GPa is negligible. All measured Fe3C/liquid metal partition coefficients investigated are less than one, such that all trace elements prefer the C-rich liquid to Fe3C. Fe3C/liquid metal partition coefficients tend to decrease with decreasing atomic radii within a given period. Of particular interest, our 5 GPa Fe-C phase diagram does not show any evidence that the Fe-Fe3C eutectic composition shifts to lower C contents with increasing pressure, which is central to the previous reasoning that the inner core may be composed of Fe3C. 相似文献