全文获取类型
收费全文 | 33513篇 |
免费 | 368篇 |
国内免费 | 196篇 |
专业分类
测绘学 | 774篇 |
大气科学 | 2381篇 |
地球物理 | 6444篇 |
地质学 | 11468篇 |
海洋学 | 3082篇 |
天文学 | 7832篇 |
综合类 | 64篇 |
自然地理 | 2032篇 |
出版年
2021年 | 339篇 |
2020年 | 346篇 |
2019年 | 379篇 |
2018年 | 788篇 |
2017年 | 686篇 |
2016年 | 882篇 |
2015年 | 474篇 |
2014年 | 798篇 |
2013年 | 1660篇 |
2012年 | 909篇 |
2011年 | 1321篇 |
2010年 | 1183篇 |
2009年 | 1597篇 |
2008年 | 1320篇 |
2007年 | 1382篇 |
2006年 | 1258篇 |
2005年 | 1039篇 |
2004年 | 1001篇 |
2003年 | 989篇 |
2002年 | 965篇 |
2001年 | 863篇 |
2000年 | 783篇 |
1999年 | 662篇 |
1998年 | 650篇 |
1997年 | 648篇 |
1996年 | 522篇 |
1995年 | 532篇 |
1994年 | 492篇 |
1993年 | 448篇 |
1992年 | 409篇 |
1991年 | 413篇 |
1990年 | 406篇 |
1989年 | 396篇 |
1988年 | 378篇 |
1987年 | 458篇 |
1986年 | 399篇 |
1985年 | 483篇 |
1984年 | 516篇 |
1983年 | 491篇 |
1982年 | 419篇 |
1981年 | 469篇 |
1980年 | 371篇 |
1979年 | 369篇 |
1978年 | 349篇 |
1977年 | 345篇 |
1976年 | 305篇 |
1975年 | 296篇 |
1974年 | 301篇 |
1973年 | 310篇 |
1971年 | 192篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
R. Meerkötter U. Schumann D. R. Doelling P. Minnis T. Nakajima Y. Tsushima 《Annales Geophysicae》1999,17(8):1080-1094
A parametric study of the instantaneous radiative impact of contrails is presented using three different radiative transfer models for a series of model atmospheres and cloud parameters. Contrails are treated as geometrically and optically thin plane parallel homogeneous cirrus layers in a static atmosphere. The ice water content is varied as a function of ambient temperature. The model atmospheres include tropical, mid-latitude, and subarctic summer and winter atmospheres. Optically thin contrails cause a positive net forcing at top of the atmosphere. At the surface the radiative forcing is negative during daytime. The forcing increases with the optical depth and the amount of contrail cover. At the top of the atmosphere, a mean contrail cover of 0.1% with average optical depth of 0.2 to 0.5 causes about 0.01 to 0.03 Wm−2 daily mean instantaneous radiative forcing. Contrails cool the surface during the day and heat the surface during the night, and hence reduce the daily temperature amplitude. The net effect depends strongly on the daily variation of contrail cloud cover. The indirect radiative forcing due to particle changes in natural cirrus clouds may be of the same magnitude as the direct one due to additional cover. 相似文献
112.
L. N. Kogarko D. A. Plant C. M. B. Henderson B. A. Kjarsgaard 《Contributions to Mineralogy and Petrology》1991,109(1):124-129
Carbonate phases, some rich in Na2O and comparatively rich in SrO and BaO, occur as inclusions in perovskite and calzirtite (Ca2Zr5Ti2O16) in the carbonatite of the Guli complex, Siberia. This is the first record of alkali carbonates, akin to nyerereite [Na2Ca(CO3)2], in plutonic igneous rocks. The inclusion populations suggest that the parental magma of the complex was Ca-rich but developed Na-rich differentiates during the latest stages. This points to the dominant calcic carbonatites of the complex not being derivatives of alkali-rich parental carbonatites. These alkali-rich carbonate inclusions (and rare inclusions of djerfisherite) have been preserved due to the resistance of perovskite and calzirtite to processes of leaching, hydrothermal alteration and weathering. 相似文献
113.
Molecular quantum mechanical calculations have been performed on high-spin ferrous iron tetrahedrally coordinated to sulfur and oxygen, respectively. The molecular orbital energies obtained from the calculations are compared with experimental optical and X-ray emission spectra. Good agreement was found between calculated and experimental spectral transition energies for the optical absorption spectra of Fe2+ in sphalerite, of Fe2+ in FeAl2O4, staurolite and (Zn, Fe)O, and for the FeKβ X-ray emission spectra of FeCr2O4. This both clarified interpretation of the spectra and established the validity of the calculations. Distinct differences occur in the molecular orbital structures of the sulfide and oxide clusters. In the sulfide, the crystal field type (mainly Fe 3d) molecular orbitals lie within the nonbonding (mainly S 3p) orbitais in energy, whereas in the oxide, they lie well above the 02p nonbonding orbitals. This also results in a wider valence band in the oxide than in the sulfide. The crystal field type (Fe 3d) molecular orbitais have more ligand character in the sulfide than the oxide and the chalcophilic properties of iron are partly attributed to this observation. 相似文献
114.
115.
Trees may die in forest stands over relatively large areas for obvious reasons, such as fire, flooding, severe drought, strong
air pollution, and insect outbreaks. However, in many cases, stand-level dieback occurs without an obvious reason and then
its etiology becomes more complicated, involving a combination of factors. The dieback structure, its symptomatology, and
its spatial and temporal patterns may be used to identify these factors at least at the generic level. Moreover, observations
of the dieback-associated vegetation dynamics can be used to draw inferences about the dieback process. In addition to biotic
diseases and environmental changes or short-term recurring stresses, local site characteristics and the life history of the
stand must be considered in a more universally applicable theory for stand-level dieback. This is further explained by a simplified
model related to the decline disease concept of Houston. 相似文献
116.
D. G. Gridnev K. Kh. Kanonidi Kh. D. Kanonidi O. V. Kovaleva I. N. Puzich 《Seismic Instruments》2008,44(1):32-35
The microbarograph of a new design has been offered, and the general principle of its operation is considered. The microbarograph is distinguished from well-known ones; it incorporates a compensatory spring eliminating a zero-point drift of the measuring spring, two additional springs that upgrade the angular displacement sensitivity of the microbarograph, and a control and calibration block that makes it possible to perform the remote calibration of the microbarograph and to control it. 相似文献
117.
The microstructure and microchemistry of minerals in Ca-Al-rich, coarse-grained inclusions (CAI) from the Allende meteorite have been investigated with transmission electron microscopy (TEM). Spinels contain only low to moderate dislocation densities and are characterized by a ubiquitous, fine black spotty texture believed to originate from a slightly non-stoichiometric composition. Ti-Al-pyroxenes are relatively featureless, but contain veins of secondary phases apparently deposited in unhealed cracks. Chromite has been identified in the veins, suggesting transport of oxidized iron during alteration. Melilites exhibit the greatest variety of microstructures and are the most heavily altered phase in CAI. High dislocation densities are common and crystals exhibit considerable internal strain, indicating that they have not been annealed. Alteration occurs both as veins along cracks and in fronts extending across several grains. Plagioclase is commonly twinned, but dislocations are rare. The size and morphology of antiphase domains suggest a high temperature of formation with significant low-temperature annealing. Submicron pyroxene precipitates are a ubiquitous and unusual feature of Allende plagioclase whose properties are most consistent with prolonged slow cooling and equilibration after plagioclase crystallization. The precipitates appear to be sufficiently abundant to contain the majority of Mg present in plagioclase but do not easily account for Na and Ti abundances. Wollastonite needles from a cavity in a “fluffy” Type A inclusion exhibit the growth habits and relatively perfect external surfaces indicative of direct condensation from a vapor. Alteration products are predominantly crystalline and alteration of melilite appears to have proceeded primarily by solid-state diffusion at a temperature of approximately 920°K. Overall, the TEM observations suggest that CAI formed under near equilibrium conditions characterized by slow cooling and that solid-state bulk diffusion was the major process affecting their post-crystallization history. 相似文献
118.
Agricultural practices, hydrology, and water quality of the 267-km2 Big Spring groundwater drainage basin in Clayton County, Iowa, have been monitored since 1981. Land use is agricultural; nitrate-nitrogen
(-N) and herbicides are the resulting contaminants in groundwater and surface water. Ordovician Galena Group carbonate rocks
comprise the main aquifer in the basin. Recharge to this karstic aquifer is by infiltration, augmented by sinkhole-captured
runoff. Groundwater is discharged at Big Spring, where quantity and quality of the discharge are monitored.
Monitoring has shown a threefold increase in groundwater nitrate-N concentrations from the 1960s to the early 1980s. The nitrate-N
discharged from the basin typically is equivalent to over one-third of the nitrogen fertilizer applied, with larger losses
during wetter years. Atrazine is present in groundwater all year; however, contaminant concentrations in the groundwater respond
directly to recharge events, and unique chemical signatures of infiltration versus runoff recharge are detectable in the discharge
from Big Spring.
Education and demonstration efforts have reduced nitrogen fertilizer application rates by one-third since 1981. Relating declines
in nitrate and pesticide concentrations to inputs of nitrogen fertilizer and pesticides at Big Spring is problematic. Annual
recharge has varied five-fold during monitoring, overshadowing any water-quality improvements resulting from incrementally
decreased inputs.
Electronic Publication 相似文献
119.
Geological, methodological, and economic aspects of the production and utilization of petrother mal resources (heat of “dry”
rocks) for electric energy production and heat supply are considered. Possibility of the utilization of natural fracture zones
or artificial fissure zones, which are created by hydraulic fracturing of underground rock massifs, is noted. Application
of new tools for drilling deep wells makes the petrochemical energy competitive relative to both other types of renewable
resources and traditional organic fuel. 相似文献
120.
Pevzner M. M. Lebedev V. A. Kuscheva Yu. V. Volynets A. O. Tolstykh M. L. Babansky A. D. 《Doklady Earth Sciences》2020,490(1):1-3
Doklady Earth Sciences - A first set of K–Ar isotopic ages obtained, which allowed to estimate the age of the largest volcanoes of the Anaunsky Dol (3.2, 2.2 and 1.9 Ma) and eruptive centers... 相似文献