全文获取类型
收费全文 | 42972篇 |
免费 | 880篇 |
国内免费 | 288篇 |
专业分类
测绘学 | 857篇 |
大气科学 | 2962篇 |
地球物理 | 8478篇 |
地质学 | 15469篇 |
海洋学 | 3934篇 |
天文学 | 9516篇 |
综合类 | 98篇 |
自然地理 | 2826篇 |
出版年
2022年 | 268篇 |
2021年 | 481篇 |
2020年 | 547篇 |
2019年 | 636篇 |
2018年 | 1077篇 |
2017年 | 1083篇 |
2016年 | 1176篇 |
2015年 | 684篇 |
2014年 | 1132篇 |
2013年 | 2076篇 |
2012年 | 1326篇 |
2011年 | 1837篇 |
2010年 | 1606篇 |
2009年 | 2038篇 |
2008年 | 1805篇 |
2007年 | 1842篇 |
2006年 | 1698篇 |
2005年 | 1199篇 |
2004年 | 1232篇 |
2003年 | 1260篇 |
2002年 | 1139篇 |
2001年 | 954篇 |
2000年 | 887篇 |
1999年 | 802篇 |
1998年 | 805篇 |
1997年 | 773篇 |
1996年 | 657篇 |
1995年 | 633篇 |
1994年 | 556篇 |
1993年 | 505篇 |
1992年 | 457篇 |
1991年 | 485篇 |
1990年 | 484篇 |
1989年 | 437篇 |
1988年 | 410篇 |
1987年 | 464篇 |
1986年 | 461篇 |
1985年 | 565篇 |
1984年 | 618篇 |
1983年 | 596篇 |
1982年 | 546篇 |
1981年 | 514篇 |
1980年 | 482篇 |
1979年 | 452篇 |
1978年 | 435篇 |
1977年 | 429篇 |
1976年 | 396篇 |
1975年 | 387篇 |
1974年 | 376篇 |
1973年 | 404篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
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
42.
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
43.
44.
45.
R. M. Hueckstaedt S. H. Batha M. M. Balkey N. D. Delamater J. R. Fincke R. L. Holmes N. E. Lanier G. R. Magelssen J. M. Scott J. M. Taccetti C. J. Horsfield K. W. Parker S. D. Rothman 《Astrophysics and Space Science》2005,298(1-2):255-259
Observations of the interstellar medium reveal a dynamic realm permeated by shocks. These shocks are generated on a large
range of scales by galactic rotation, supernovae, stellar winds, and other processes. Whenever a shock encounters a density
interface, Richtmyer-Meshkov instabilities may develop. Perturbations along the interface grow, leading to structure formation
and material mixing. An understanding of the evolution of Richtmyer-Meshkov instabilities is essential for understanding galactic
structure, molecular cloud morphology, and the early stages of star formation. An ongoing experimental campaign studies Richtmyer-Meshkov
mixing in a convergent, compressible, miscible plasma at the Omega laser facility. Cylindrical targets, consisting of a low
density foam core and an aluminum shell covered by an epoxy ablator, are directly driven by fifty laser beams. The aluminum
shell is machined to produce different perturbation spectra. Surface types include unperturbed (smooth), single-mode sinusoids,
multi-mode (rough), and multi-mode with particular modes accentuated (specified-rough). Experimental results are compared
to theory and numerical simulations. 相似文献
46.
An investigation of 531 active regions was made to determine the correlation between energy released by flares and the available energy in magnetic fields of the regions. Regions with magnetic flux greater than 1021 maxwell during the years 1967–1969, which included sunspot maximum, were selected for the investigation. A linear regression analysis of flare production on magnetic flux showed that the flare energy is correlated with magnetic energy with a coeificient of correlation of 0.78. Magnetic classification and field configuration also significantly affect the production of flares.This work was supported by the Aerospace Sponsored Research Program. 相似文献
47.
Lawrence M. Mayer Peter T. Rahaim William Guerin Stephen A. Macko Les Watling Franz E. Anderson 《Estuarine, Coastal and Shelf Science》1985,20(4):491-503
A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter. 相似文献
48.
Richard P. Binzel 《Icarus》1985,63(1):99-108
Photoelectric observations of asteroid 1220 Crocus (an Eos family member) show evidence for two distinct periods in its light variation: 30.7 days and 7.90 hr. The lightcurve amplitudes are 0.87 and 0.15 mag, respectively. The shorter period variation appears to be modulated over the longer period. Two periods are possible for a rigid body only if it is in a state of precession. The observations are shown to be compatible with a body in a forced precession state. This result leads us to hypothesize the existence of a satellite of Crocus as the source of the external torque. Calculations are presented which show that there are in fact dynamically possible “binary asteroid” solutions consistent with the observations. More photoelectric and perhaps direct (space telescope) observations are needed to resolve the true nature of this asteroid. 相似文献
49.
50.
B. Ahmed G. J. Alner H. Araujo J. C. Barton A. Bewick M. J. Carson D. Davidge J. V. Dawson T. Gamble S. P. Hart R. Hollingworth A. S. Howard W. G. Jones M. K. Joshi V. A. Kudryavtsev T. B. Lawson V. Lebedenko M. J. Lehner J. D. Lewin P. K. Lightfoot I. Liubarsky R. Lüscher J. E. McMillan B. Morgan G. Nicklin S. M. Paling R. M. Preece J. J. Quenby J. W. Roberts M. Robinson N. J. T. Smith P. F. Smith N. J. C. Spooner T. J. Sumner D. R. Tovey 《Astroparticle Physics》2003,19(6):691-702
The NAIAD experiment (NaI Advanced Detector) for weakly interacting massive particle (WIMP) dark matter searches at Boulby mine (UK) is described. The detector consists of an array of encapsulated and unencapsulated NaI(Tl) crystals with high light yield. Six crystals are collecting data at present. Data accumulated by four of them (10.6 kg × year exposure) have been used to set upper limits on the WIMP–nucleon spin-independent and WIMP–proton spin-dependent cross-sections. Pulse shape analysis has been applied to discriminate between nuclear recoils, as may be caused by WIMP interactions, and electron recoils due to gamma background. Various calibrations of crystals are presented. 相似文献