全文获取类型
收费全文 | 122302篇 |
免费 | 18398篇 |
国内免费 | 42850篇 |
专业分类
测绘学 | 4735篇 |
大气科学 | 26657篇 |
地球物理 | 29741篇 |
地质学 | 65185篇 |
海洋学 | 24100篇 |
天文学 | 22672篇 |
综合类 | 4168篇 |
自然地理 | 6292篇 |
出版年
2022年 | 976篇 |
2021年 | 1669篇 |
2020年 | 3037篇 |
2019年 | 6553篇 |
2018年 | 8262篇 |
2017年 | 7839篇 |
2016年 | 8300篇 |
2015年 | 6083篇 |
2014年 | 6883篇 |
2013年 | 9579篇 |
2012年 | 7520篇 |
2011年 | 7919篇 |
2010年 | 7730篇 |
2009年 | 7940篇 |
2008年 | 6709篇 |
2007年 | 6607篇 |
2006年 | 6016篇 |
2005年 | 4862篇 |
2004年 | 5190篇 |
2003年 | 4768篇 |
2002年 | 4404篇 |
2001年 | 3994篇 |
2000年 | 3530篇 |
1999年 | 3127篇 |
1998年 | 3251篇 |
1997年 | 3342篇 |
1996年 | 2633篇 |
1995年 | 2570篇 |
1994年 | 2442篇 |
1993年 | 2159篇 |
1992年 | 1993篇 |
1991年 | 1686篇 |
1990年 | 1788篇 |
1989年 | 1504篇 |
1988年 | 1387篇 |
1987年 | 1398篇 |
1986年 | 1134篇 |
1985年 | 1405篇 |
1984年 | 1427篇 |
1983年 | 1297篇 |
1982年 | 1314篇 |
1981年 | 1069篇 |
1980年 | 1093篇 |
1979年 | 929篇 |
1978年 | 894篇 |
1977年 | 790篇 |
1976年 | 729篇 |
1975年 | 722篇 |
1974年 | 711篇 |
1973年 | 697篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
B.K.Sharma A.M.Bhola 《《幕》》2004,27(2):119-124
The metasediments in the Chamba region experienced three phases of deformation: DF1, DF2 and DF3.Folded quartz veins are co-folded with the F2 crenulation folds. Their geometric and tectonic significance is studied by microstructures and shortening adjacent to the discrete crenulation cleavage, S2. Microstructures of folded vein segments, their geometric changes and truncation to cleavage (S2) are mainly due to pressure-solution phenomena and the estimated volume loss from reconstructed vein segments range from 16 to 25.5%,which is closely related to volume decrease (26%) calculated from the polydeformed slates of North Wales areas. 相似文献
12.
An elastoplastic model for sands is presented in this paper, which can describe stress–strain behaviour dependent on mean effective stress level and void ratio. The main features of the proposed model are: (a) a new state parameter, which is dependent on the initial void ratio and initial mean stress, is proposed and applied to the yield function in order to predict the plastic deformation for very loose sands; and (b) another new state parameter, which is used to determine the peak strength and describe the critical state behaviour of sands during shearing, is proposed in order to predict simply negative/positive dilatancy and the hardening/softening behaviour of medium or dense sands. In addition, the proposed model can also predict the stress–strain behaviour of sands under three-dimensional stress conditions by using a transformed stress tensor instead of ordinary stress tensor. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
13.
14.
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
15.
16.
17.
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
18.
The motion of an autonomous underwater vehicle (AUV) is controllable even with reduced control authority such as in the event of an actuator failure. In this paper we describe a technique for synthesizing controls for underactuated AUV's and show how to use this technique to provide adaptation to changes in control authority. Our framework is a motion control system architecture which includes both feed-forward control as well as feedback control. We confine ourselves to kinematic models and exploit model nonlinearities to synthesize controls. Our results are illustrated for two examples, the first a yaw maneuver of an AUV using only roll and pitch actuation, and the second a “parking maneuver” for an AUV. Experimental results for the yaw maneuver example are described 相似文献
19.
20.
T. H. BELL S. E. JOHNSON B. DAVIS A. FORDE N. HAYWARD C. WILKINS 《Journal of Metamorphic Geology》1992,10(3):295-307
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens. 相似文献