全文获取类型
收费全文 | 264715篇 |
免费 | 5193篇 |
国内免费 | 3362篇 |
专业分类
测绘学 | 6975篇 |
大气科学 | 19521篇 |
地球物理 | 55315篇 |
地质学 | 92783篇 |
海洋学 | 22176篇 |
天文学 | 57365篇 |
综合类 | 1040篇 |
自然地理 | 18095篇 |
出版年
2021年 | 2244篇 |
2020年 | 2593篇 |
2019年 | 2857篇 |
2018年 | 4412篇 |
2017年 | 4146篇 |
2016年 | 6180篇 |
2015年 | 4219篇 |
2014年 | 6906篇 |
2013年 | 14183篇 |
2012年 | 6841篇 |
2011年 | 8622篇 |
2010年 | 7727篇 |
2009年 | 10240篇 |
2008年 | 8929篇 |
2007年 | 8618篇 |
2006年 | 9701篇 |
2005年 | 7906篇 |
2004年 | 7769篇 |
2003年 | 7257篇 |
2002年 | 6801篇 |
2001年 | 6016篇 |
2000年 | 5961篇 |
1999年 | 5219篇 |
1998年 | 5234篇 |
1997年 | 5030篇 |
1996年 | 4679篇 |
1995年 | 4431篇 |
1994年 | 4104篇 |
1993年 | 3854篇 |
1992年 | 3637篇 |
1991年 | 3591篇 |
1990年 | 3765篇 |
1989年 | 3521篇 |
1988年 | 3305篇 |
1987年 | 3848篇 |
1986年 | 3407篇 |
1985年 | 4224篇 |
1984年 | 4737篇 |
1983年 | 4406篇 |
1982年 | 4312篇 |
1981年 | 3928篇 |
1980年 | 3637篇 |
1979年 | 3508篇 |
1978年 | 3488篇 |
1977年 | 3280篇 |
1976年 | 3044篇 |
1975年 | 2960篇 |
1974年 | 2914篇 |
1973年 | 3077篇 |
1972年 | 2025篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
291.
Two spectrophotometric scans of comet Levy (1990c) have been analysed to estimate Haser model column densities and production rates of CN andC
2 molecules. 相似文献
292.
293.
294.
295.
J. V. WALTHER 《Journal of Metamorphic Geology》1996,14(3):351-360
ABSTRACT With increasing temperature during prograde metamorphism reactions will occur first at the lithological contacts of mixed pelite and calcsilicate terranes. At these interfaces, a fluid of lower chemical potential of H2O and CO2 than that required to produce a fluid in either layer can be produced whether reaction is caused by fluid infiltration or is initially fluid absent. If the interface region does not allow fluid transport then as temperature increases, a fluid pressure greater than lithostatic can develop. At some degree of over-pressure relative to rock pressure, the fluid hydraulically fractures the rock and a gradient in fluid composition away from the contact can be produced. These phenomena occur at the compositional interfaces whenever univariant reactions in the differing layers cross on a temperature vs. mole fraction of CO2 diagram with slopes of opposite sign. The first occurrence of these reaction products at lithological contacts delineates an isograd that defines temperature as well as the mole fraction of CO2 at constant pressure in systems open to fluid transport. These isograds can be contrasted with fluid-producing isograds in closed systems. As an illustration of possible effects, the reactions quartz + clinozoisite + muscovite = anorthite + K-feldspar + H2O and phlogopite + quartz + calcite = tremolite + K-feldspar + H2O + CO2 at 4 kbar are analysed and equations for fluid production and transport are developed. 相似文献
296.
297.
MICHAEL K. WEISBERG MICHAEL E. ZOLENSKY MARTIN PRINZ 《Meteoritics & planetary science》1997,32(6):791-801
Abstract— Fayalitic olivine (Fa54–94) is a ubiquitous component in the matrix of Krymka (LL3.1) as well as in other highly unequilibrated chondrites (ordinary and carbonaceous). In Krymka, the fayalitic olivine has an unusual anisotropic platy morphology that occurs in at least five types of textural settings that can be characterized as: (1) isolated platelets, (2) clusters of platelets, (3) euhedral to subhedral crystals, (4) overgrowths of platelets on forsteritic olivine, and (5) fluffy (porous) aggregates. From transmission electron microscope (TEM) investigation, the direction of elongation of the platy olivine overgrowths on forsteritic olivine substrates is along the c axis and in most cases it corresponds with the c axis of the substrate olivine, which suggests that the fayalitic olivine grew in this unusual morphology and is not a replacement product of preexisting material. The fayalitic olivine in the matrix of Krymka is compositionally similar to olivine with platy morphology in the matrix of some CV3 chondrites and both have similar Fe/Mn ratios, but important morphological differences indicate that their relationship needs to be explored further. Textural and compositional data indicate that the fayalitic olivine in the matrix of Krymka, as well as in some other unequilibrated ordinary chondrites, formed prior to final lithification of the meteorite and probably prior to parent body accretion. We find that formation of the fayalitic olivine by vapor-solid growth provides the best explanation for our observations and data and is the only feasible mechanism for the formation of fayalitic olivine in the matrix of Krymka. We propose that the fayalitic olivine formed by vaporization and recondensation of olivine rich-dust, during a period of enhanced dust/gas ratio in the nebula. 相似文献
298.
299.
An updated Lagrangian finite-element formulation has been developed for time-dependent problems of soil consolidation involving finite deformations. Large plastic strains as well as rotations occur in such problems and nominal stress measures are introduced in the formulation to redefine stresses. This leads to corrective terms for equilibrium and yield violations in addition to geometric stiffening terms in the governing integral equations. The soil is considered to be either a linear elastic or an elastoplastic, critical-state material. Some simple numerical examples are studied to validate the formulation, followed by a detailed analysis of the problem of penetration of a pile into soil. The results of this problem are viewed with emphasis on the physical interpretation and practical significance. 相似文献
300.