全文获取类型
收费全文 | 58241篇 |
免费 | 1077篇 |
国内免费 | 794篇 |
专业分类
测绘学 | 1267篇 |
大气科学 | 4092篇 |
地球物理 | 11300篇 |
地质学 | 21600篇 |
海洋学 | 5326篇 |
天文学 | 13264篇 |
综合类 | 141篇 |
自然地理 | 3122篇 |
出版年
2022年 | 413篇 |
2021年 | 620篇 |
2020年 | 719篇 |
2019年 | 800篇 |
2018年 | 1684篇 |
2017年 | 1555篇 |
2016年 | 1828篇 |
2015年 | 1065篇 |
2014年 | 1774篇 |
2013年 | 3063篇 |
2012年 | 1943篇 |
2011年 | 2611篇 |
2010年 | 2245篇 |
2009年 | 2928篇 |
2008年 | 2507篇 |
2007年 | 2552篇 |
2006年 | 2397篇 |
2005年 | 1706篇 |
2004年 | 1686篇 |
2003年 | 1598篇 |
2002年 | 1524篇 |
2001年 | 1409篇 |
2000年 | 1297篇 |
1999年 | 1090篇 |
1998年 | 1135篇 |
1997年 | 1092篇 |
1996年 | 925篇 |
1995年 | 851篇 |
1994年 | 818篇 |
1993年 | 682篇 |
1992年 | 643篇 |
1991年 | 607篇 |
1990年 | 669篇 |
1989年 | 550篇 |
1988年 | 492篇 |
1987年 | 649篇 |
1986年 | 503篇 |
1985年 | 695篇 |
1984年 | 686篇 |
1983年 | 679篇 |
1982年 | 649篇 |
1981年 | 558篇 |
1980年 | 574篇 |
1979年 | 506篇 |
1978年 | 532篇 |
1977年 | 450篇 |
1976年 | 447篇 |
1975年 | 460篇 |
1974年 | 410篇 |
1973年 | 429篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Between 1994 and 2006, we obtained uniform spectroscopic observations of SS 433 in the region of Hα. We determined Doppler shifts of the moving emission lines, Hα + and Hα ?, and studied various irregularities in the profiles for the moving emission lines. The total number of Doppler shifts measured in these 13 years is 488 for Hα ? and 389 for Hα +. We have also used published data to study possible long-term variations of the SS 433 system, based on 755 Doppler shifts for Hα ? and 630 for Hα + obtained over 28 years. We have derived improved kinematic model parameters for the precessing relativistic jets of S S 433 using five-and eight-parameter models. On average, the precession period was stable during the 28 years of observations (60 precession cycles), at 162.250d ± 0.003d. Phase jumps of the precession period and random variations of its length with amplitudes of ≈6% and ≈1%, respectively, were observed, but no secular changes in the precession period were detected. The nutation period, P nut = 6.2876d ± 0.00035d, and its phase were stable during 28 years (more than 1600 nutation cycles). We find no secular variations of the nutation cycle. The ejection speed of the relativistic jets, v, was, on average, constant during the 28 years, β = v/c = 0.2561 ± 0.0157. No secular variation of β is detected. In general, S S 433 demonstrates remarkably stable long-term characteristics of its precession and nutation, as well as of the central “engine” near the relativistic object that collimates the plasma in the jets and accelerates it to v = 0.2561c. Our results support a model with a “slaved” accretion disk in S S 433, which follows the precession of the optical star’s rotation axis. 相似文献
992.
The energy balance of a solid subject to fracture has been explored using heat and mass transfer equations with regard to the volumetric and superficial components. In the suggested model, brittle fracture of a cracked solid considered as a heterogeneous two-phase medium is described by an equation analogous to the Griffith’s criterion for propagation of a single crack. The derived equation is used, together with estimates of relative change in specific interface area, to study the respective change of free strain energy and pressure in rocks associated with failure. 相似文献
993.
V. V. Shimanskii N. V. Borisov N. A. Sakhibullin D. V. Sheveleva 《Astronomy Reports》2008,52(6):479-486
We have classified and determined the parameters of the evolved close binary MT Ser. Our moderate-resolution spectra covering various phases of the orbital period were taken with the 6-m telescope of the Special Astrophysical Observatory. The spectra of MT Ser freed from the contribution of the surrounding nebula Abell 41 contained no emission lines due to the reflection effect. The radial velocities measured from lines of different elements showed them to be constant on a time scale corresponding to the orbital period. At the same time, we find effects of broadening for the HeII absorption lines, due to the orbital motion of two hot stars of similar types. As a result, we classify MT Ser as a system with two blue subdwarfs after the common-envelope stage. We estimate the component masses and the distance to the object from the Doppler broadening of the HeII lines. We demonstrate that the age of the ambient nebula, Abell 41, is about 35 000 years. 相似文献
994.
华北克拉通北邻中亚造山带,南接秦岭-大别-苏鲁造山带.它通常被认为是约18亿年固结形成的陆块.其基底的形成与演化,一直受到广泛关注(Zhao et al.,1998,2000,2005;Zhai et al.,2000,2005;Wilde et al.,2002,2005;Guo et al.,2005;Kroner et al.,2005,2006;Lu et al.,2008).大多数研究者都注重于华北克拉通内部、北部和东西陆块结合带(中间带),较少关注东南缘基底的形成与演化研究. 相似文献
995.
N. G. Patyk-Kara E. N. Levchenko A. I. Stekhin V. V. Barsegyan A. A. Bochneva I. A. Chizhova E. A. Andrianova V. T. Dubinchuk 《Geology of Ore Deposits》2008,50(3):218-239
The study of mineral assemblages at the Central deposit allowed us to substantially refine the evolutionary model of the deposit and reveal the two main factors that control the variability of its mineralic space: (1) heterogeneity of the ore layer, consisting of a sublittoral bottom placer (the lower part) and a subaerial dune complex partly reworked in the course of a new transgression (the upper part), and (2) postore epigenetic alteration of the ore constituent of sands, which affected the quality of ore concentrates. The results obtained will be used in geological and technological mapping and development of the production program. 相似文献
996.
G. T. Skublov Yu. B. Marin S. G. Skublov Yu. N. Tarasenko 《Geology of Ore Deposits》2008,50(7):535-550
Volkhovites—tektite-like glasses—have been detected in the Holocene glacial drift along the right bank of the Volkhov River. A cryptomagmatic model of their formation and pre-Holocene age of volkhovite melts, cinder, and frothed glasses has been suggested (Skublov et al., 2007). Four geochemical types of volkhovites are distinguished: (1) manganous (Mn, Fe, Cr, V, Si, Nb, Pb, H), (2) magnesian (Mg, Al, Ti, F, B), (3) potassic (K, Rb, Cs), and (4) calcic (Ca, REEs, Ba, U, Th, Ta, Hf, Y, Sc, Cl). In light of the geochemical data, volkhovites are regarded as natural silicate glasses of kimberlite-carbonatite composition. Their types are called kimberlitic (Mn type), kimberlitic-carbonatitic (Mg type), lamproitic-carbonatitic (K type), and carbonatitic (Ca type). Volkhovites are suggested to be indicators of undiscovered diamond mineralization of kimberlite or carbonatite (Chagatai) types. 相似文献
997.
It is shown that the centroid of the heliospheric equator undergoes quasi-periodic oscillations. During the minimum of the 11-year cycle, the centroid shifts southwards (the so-called bashful-ballerina effect). The direction of the shift reverses during the solar maximum. The solar quadrupole is responsible for this effect. The shift is compared with the tilt of the heliospheric current sheet. 相似文献
998.
Analyses of 72 samples from Upper Panjhara basin in the northern part of Deccan Plateau, India, indicate that geochemical
incongruity of groundwater is largely a function of mineral composition of the basaltic lithology. Higher proportion of alkaline
earth elements to total cations and HCO3>Cl + SO4 reflect weathering of primary silicates as chief source of ions. Inputs of Cl, SO4, and NO3 are related to rainfall and localized anthropogenic factors. Groundwater from recharge area representing Ca + Mg–HCO3 type progressively evolves to Ca + Na–HCO3 and Na–Ca–HCO3 class along flow direction replicates the role of cation exchange and precipitation processes. While the post-monsoon chemistry
is controlled by silicate mineral dissolution + cation exchange reactions, pre-monsoon variability is attributable chiefly
to precipitation reactions + anthropogenic factors. Positive correlations between Mg vs HCO3 and Ca + Mg vs HCO3 supports selective dissolution of olivine and pyroxene as dominant process in post-monsoon followed by dissolution of plagioclase
feldspar and secondary carbonates. The pre-monsoon data however, points toward the dissolution of plagioclase and precipitation
of CaCO3 supported by improved correlation coefficients between Na + Ca vs HCO3 and negative correlation of Ca vs HCO3, respectively. It is proposed that the eccentricity in the composition of groundwater from the Panjhara basin is a function
of selective dissolution of olivine > pyroxene followed by plagioclase feldspar.
The data suggest siallitization (L < R and R
k) as dominant mechanism of chemical weathering of basalts, stimulating monosiallitic (kaolinite) and bisiallitic (montmorillonite)
products. The chemical denudation rates for Panjhara basin worked out separately for the ground and surface water component
range from 6.98 to 36.65 tons/km2/yr, respectively. The values of the CO2 consumption rates range between 0.18 × 106 mol//km2/yr (groundwater) and 0.9 × 106 mol/km2/yr (surface water), which indicates that the groundwater forms a considerable fraction of CO2 consumption, an inference, that is, not taken into contemplation in most of the studies. 相似文献
999.
Evaluation of distributed recharge in an upland semi-arid karst system: the West Bank Mountain Aquifer,Middle East 总被引:2,自引:0,他引:2
Assessment of recharge in a structurally complex upland karst limestone aquifer situated in a semi-arid environment is difficult. Resort to surrogate indicators such as measurement of spring outflow and borehole discharge, is a common alternative, and attempts to apply conventional soil moisture deficit analysis may not adequately account for the intermittent spate conditions that arise in such environments. A modelling approach has been made using the West Bank Mountain Aquifer system in the Middle East as a trial. The model uses object oriented software which allows various objects to be switched on and off. Each of the main recharge processes identified in the West Bank is incorporated. The model allows either conventional soil moisture deficit analysis calculations or wetting threshold calculations to be made as appropriate, and accommodates both direct recharge and secondary recharge. Daily time steps enable recharge and runoff routing to be calculated for each node. Model runs have enabled a series of simulations for each of the three aquifer basins in the West Bank and for the whole of the West Bank. These provide recharge estimates comparable to those prepared by earlier workers by conventional means. The model is adaptable and has been successfully used in other environments. 相似文献
1000.
In this paper, we formulate a finite-element procedure for approximating the coupled fluid and mechanics in Biot’s consolidation
model of poroelasticity. We approximate the flow variables by a mixed finite-element space and the displacement by a family
of discontinuous Galerkin methods. Theoretical convergence error estimates are derived and, in particular, are shown to be
independent of the constrained specific storage coefficient, c
o
. This suggests that our proposed algorithm is a potentially effective way to combat locking, or the nonphysical pressure
oscillations, which sometimes arise in numerical algorithms for poroelasticity. 相似文献