全文获取类型
收费全文 | 49761篇 |
免费 | 576篇 |
国内免费 | 1184篇 |
专业分类
测绘学 | 2020篇 |
大气科学 | 3900篇 |
地球物理 | 10040篇 |
地质学 | 20346篇 |
海洋学 | 3067篇 |
天文学 | 7025篇 |
综合类 | 2198篇 |
自然地理 | 2925篇 |
出版年
2020年 | 149篇 |
2019年 | 140篇 |
2018年 | 4996篇 |
2017年 | 4261篇 |
2016年 | 2974篇 |
2015年 | 544篇 |
2014年 | 512篇 |
2013年 | 1240篇 |
2012年 | 1502篇 |
2011年 | 3447篇 |
2010年 | 2620篇 |
2009年 | 3169篇 |
2008年 | 2696篇 |
2007年 | 3109篇 |
2006年 | 909篇 |
2005年 | 929篇 |
2004年 | 1177篇 |
2003年 | 1125篇 |
2002年 | 981篇 |
2001年 | 649篇 |
2000年 | 670篇 |
1999年 | 605篇 |
1998年 | 589篇 |
1997年 | 568篇 |
1996年 | 471篇 |
1995年 | 466篇 |
1994年 | 449篇 |
1993年 | 416篇 |
1992年 | 393篇 |
1991年 | 335篇 |
1990年 | 389篇 |
1989年 | 306篇 |
1988年 | 346篇 |
1987年 | 381篇 |
1986年 | 331篇 |
1985年 | 491篇 |
1984年 | 529篇 |
1983年 | 533篇 |
1982年 | 424篇 |
1981年 | 445篇 |
1980年 | 458篇 |
1979年 | 388篇 |
1978年 | 400篇 |
1977年 | 350篇 |
1976年 | 390篇 |
1975年 | 345篇 |
1974年 | 382篇 |
1973年 | 366篇 |
1972年 | 233篇 |
1971年 | 186篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Expression of putative zinc-finger protein lcn61 gene in lymphocystis disease virus China (LCDV-cn) genome 总被引:1,自引:0,他引:1
An open reading frame (lcn61) of lymphocystis disease virus China (LCDV-cn), probably responsible for encoding putative zinc-finger proteins was amplified
and inserted into pET24a (+) vector. Then it expressed in E. coli BL21 (DE3), and His-tag fusion protein of high yield was obtained. It was found that the fusion protein existed in E. coli mainly as inclusion bodies. The bioinformatics analysis indicates that LCN61 is C2H2 type zinc-finger protein containing
four C2H2 zinc-finger motifs. This work provides a theory for functional research of lcn61 gene.
Supported by High Technology Research and Development Program of China (863 Program, No. 2006AA100309) 相似文献
52.
Elementary cluster analysis of induced seismicity in a South African gold mine has shown that there is a clear interaction amongst the clusters; and that the level of the interaction is a function of the distance. The clusering algorithm used is an adaptation of the single-link cluster analysis which considers both three-dimensional space and time. A high level of interaction between the clusters is demonstrated from the cross-correlation analysis of seismic activity rates and radiated energy. A distinct decrease in the value of correlation coefficients was detectable as distance increased. This was somewhat surprising, considering the simplicity of the technique used. Since no attempt is made to study the physical mechanisms of interaction, these results are very preliminary, but interesting from an observational point of view. 相似文献
53.
Robert W. Luth 《Contributions to Mineralogy and Petrology》1995,122(1-2):152-158
The carbonation reaction CaMg(CO3)2 (dolomite)+2SiO2 (coesite)=CaMgSi2O6 (diopside)+2 CO2 (vapor) has been determined experimentally between 3.5 and 6 GPa in a multiple-anvil, solid-media apparatus. This reaction,
a candidate for carbonation of eclogites (garnet+clinopyroxene) in the Earth’s mantle, lies at higher pressure for a given
temperature than do the carbonation reactions for peridotites (olivine+orthopyroxene±clinopyroxene). A depth interval may
exist within the Earth’s mantle under either ‘normal’ or ‘subduction’ thermal regimes where carbonated peridotite could coexist
with carbonate-free, CO2-bearing eclogite.
Received: 25 May 1994/Accepted: 13 June 1995 相似文献
54.
Kieran P. Donaghy 《Journal of Geographical Systems》2011,13(1):17-30
In the literatures of regional science, urban economics, and urban development planning, a working assumption is that individuals
respond to incentives and regulations, given their preferences. Models for planning and policy analyses are used to consider
what might occur if the incentives or regulations were different. In these models, however, preferences are usually assumed
to be given and stable, and agents are usually assumed to be homogeneous. This paper focuses on the implications of making
preferences in models of policy implementation endogenously determined and time varying heterogeneous agents. We consider
first the recent literature on intertemporal choice and preference change, which cuts across many disciplines, and more briefly
the literature on norm-regarding behavior. We then elaborate a simple model of transportation demand—from a static to a dynamic
orientation, from fixed and exogenously given preferences of strictly self-regarding agents to endogenously determined and
policy-induced preferences of heterogeneous agents—and illustrate its characteristics with simple numerical examples. 相似文献
55.
56.
Models of surface energy balance and snow metamorphism are utilized to predict the energy and meltwater fluxes at an Arctic site in the forest–tundra transition zone of north-western Canada. The surface energy balance during the melt period is modelled using an hourly bulk aerodynamic approach. Once a snowcover becomes patchy, advection from the bare patches to the snow-covered areas results in a large spatial variation in basin snowmelt. In order to illustrate the importance of small-scale, horizontal advection, a simple parameterization scheme using sensible heat fluxes from snow free areas was tested. This scheme estimates the maximum horizontal advection of sensible heat from the bare patches to the snow-covered areas. Calculated melt was routed through the measured snowcover in each landscape type using a variable flow path, meltwater percolation model. This allowed the determination of the spatial variability in the timing and magnitude of meltwater release for runoff. Model results indicate that the initial release of meltwater first occurred on the shallow upland tundra sites, but meltwater release did not occur until nearly two weeks later on the deep drift snowcovers. During these early periods of melt, not all meltwater is available for runoff. Instead, there is a period when some snowpacks are only partially contributing to runoff, and the spatial variation of runoff contribution corresponds to landscape type. Comparisons of melt with and without advection suggests that advection is an important process controlling the timing of basin snowmelt. 相似文献
57.
We investigate the distinctive distribution and pattern of subkilometer-sized cones in Isidis Planitia, and their relationship to the material that fills the basin. We observe that: (1) the cones show uniform size, spacing, and morphology across the entire basin; (2) they have large central depressions (relative to cone basal diameters) that are generally filled in and commonly show layering; (3) cone chains form highly organized spatial patterns; cones are also found in isolation and in dense fields; (4) many adjacent chains are parallel, and evenly spaced, creating a pattern that mimics lava flows that have experienced compressional folding; (5) no flows are associated with the cones, even where summit crater rims are breached; (6) the cones are at least temporally related to basin fill; (7) the basin fill material is largely fine-grained, but with locally indurated layers at shallow depth; blocks ejected by small craters from these locally indurated layers are commonly dark in color; and (8) relationships between cones along a chain show no systematic temporal formation sequence along the chain. We conclude that the basin was likely inundated by one or more hot, partially welded pyroclastic flows; devolatilization of these flows remobilized volcaniclastic material to form the cones without associated lava flows. The volume of gas required to account for the observed number of cones is low and does not require the presence of ground water or ice. 相似文献
58.
Möglichkeiten der photometrischen Untersuchung von Kugelhaufen mittels photographischer Äquidensiten
A method of estimating the surface-brightness of globular cluster with equidensity curves is described. (Russian abstract) 相似文献
59.
W. Fehlmann 《Aquatic Sciences - Research Across Boundaries》1931,5(1-2):130-141
Ohne Zusammenfassung 相似文献
60.
The application of the saddlepoint approximation to reliability analysis of dynamic systems is investigated. The failure event in reliability problems is formulated as the exceedance of a single performance variable over a prescribed threshold level. The saddlepoint approximation technique provides a choice to estimate the cumulative distribution function (CDF) of the performance variable. The failure probability is obtained as the value of the complement CDF at a specif ied threshold. The method requires computing the saddlepoint from a simple algebraic equation that depends on the cumulant generating function (CGF) of the performance variable. A method for calculating the saddlepoint using random samples of the performance variable is presented. The applicable region of the saddlepoint approximation is discussed in detail. A 10-story shear building model with white noise excitation illustrates the accuracy and effi ciency of the proposed methodology. 相似文献