全文获取类型
收费全文 | 71977篇 |
免费 | 1721篇 |
国内免费 | 1183篇 |
专业分类
测绘学 | 1621篇 |
大气科学 | 5221篇 |
地球物理 | 14464篇 |
地质学 | 26667篇 |
海洋学 | 6475篇 |
天文学 | 15486篇 |
综合类 | 378篇 |
自然地理 | 4569篇 |
出版年
2022年 | 524篇 |
2021年 | 847篇 |
2020年 | 910篇 |
2019年 | 1031篇 |
2018年 | 1876篇 |
2017年 | 1828篇 |
2016年 | 2076篇 |
2015年 | 1249篇 |
2014年 | 2042篇 |
2013年 | 3614篇 |
2012年 | 2520篇 |
2011年 | 3275篇 |
2010年 | 2908篇 |
2009年 | 3599篇 |
2008年 | 3171篇 |
2007年 | 3253篇 |
2006年 | 3079篇 |
2005年 | 2145篇 |
2004年 | 2059篇 |
2003年 | 1944篇 |
2002年 | 1847篇 |
2001年 | 1586篇 |
2000年 | 1504篇 |
1999年 | 1286篇 |
1998年 | 1301篇 |
1997年 | 1293篇 |
1996年 | 1093篇 |
1995年 | 1061篇 |
1994年 | 913篇 |
1993年 | 811篇 |
1992年 | 779篇 |
1991年 | 759篇 |
1990年 | 832篇 |
1989年 | 712篇 |
1988年 | 672篇 |
1987年 | 777篇 |
1986年 | 717篇 |
1985年 | 894篇 |
1984年 | 1027篇 |
1983年 | 934篇 |
1982年 | 890篇 |
1981年 | 798篇 |
1980年 | 794篇 |
1979年 | 710篇 |
1978年 | 676篇 |
1977年 | 695篇 |
1976年 | 613篇 |
1975年 | 610篇 |
1974年 | 609篇 |
1973年 | 667篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
This paper compares lead-acid batteries, sodium-sulfur batteries, solid polymer fuel cells and closed-cycle diesel engines for autonomous underwater vehicle (AUV) applications. The service is described in terms of a parametric mission and life cycle. A generic AUV is used as a basis for comparison. Power systems are evaluated by two criteria: (1) submerged endurance capability and (2) life cycle cost. This study determines categories of service for which each power system is preferred. The solid polymer fuel cell can provide greater submerged endurance than other power systems examined. For extremely long duration AUV missions, the fuel cell is the required system, indicating a possible market niche for today's fuel cell technology. Considering cost projections for each power system, the results also show that the SPFC can become cost-competitive with conventional technologies, particularly for services characterized by high levels of utilization 相似文献
152.
Abstract— Platinum group elements (PGE) enrichments are found in Fe-Ni blebs (<1 μm) in Al- and Cr-rich objects in the ordinary chondrites Ste. Marguerite, Forest Vale, Montferré. In Ste. Marguerite, high ZrO2 concentrations (baddeleyite) are also present in a chromite inclusion. Iridium enrichment in this material compared to its metal content is confirmed by INAA. The widespread occurrence of PGE in Al- and Cr-rich objects, mostly present in H-group chondrites of nearly equilibrated types, must be taken into account to understand their conditions of formation. 相似文献
153.
We present special generating plane orbits, the vertical-critical orbits, of the coplanar general three-body problem. These
are determined numerically for various values of m3, for the entire range of the mass ratio of the two primaries. The vertical-critical orbits are necessary in order to specify
the vertically stable segments of the families of plane periodic orbits, and they are also the starting points of the families
of the simplest possible three-dimensional periodic orbits, namely the simple and double periodic. The initial conditions
of the vertical-critical periodic orbits of the basic families l, m, i, h, b and c and their stability parameters are determined.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
154.
An efficient and robust method has been developed to locate multiple impulsive sources in an ocean environment. Global position system (GPS) receivers were installed on sonobuoys to obtain their locations within a few meters of accuracy. A sonobuoy field was deployed in a ring-type pattern. Charges were then set off at arbitrary locations within the ring, High-resolution plots were used to obtain direct path and/or first bottom bounce arrivals on each buoy. A model grid of arrival times was constructed, corresponding to the dimensions of the buoy field. A ray model previously developed here at the Applied Research Laboratories at the University of Texas at Austin (ARL:UT) was used to obtain model travel times. The minimum value of the least-square-type error between the real arrival times and the modeled travel times resulted in an unambiguous location of the source, within the limits of the grid spacing chosen. This value was calculated by picking one receiver as the reference and then summing the timing errors of the remaining receivers relative to the reference. Successive iterations with finer grid spacings result in source localization within the accuracy of the buoy locations. The localization routine was extended by allowing permutations of the pulse arrivals on each buoy to account for multiple sources closely separated in time and/or space. An automated correlation technique is presented as an alternative to the leading edge-detection method used here for obtaining relative arrival times. Two proof-of-concept experiments were performed and some results of data obtained at Lake Travis and the Gulf of Mexico are presented 相似文献
155.
156.
157.
We study the temporal intensity variations of Mgib bright features and investigate the corresponding Hα velocity pattern.
The network bright features are well visible in the continuum, in images averaged over the duration of the observations (130 min).
We detected `flashing' bright features, which appear and disappear within two to five minutes, while the rest of the bright
features undergo small variations of either their shape and/or their intensity. A power spectrum analysis reveals a 10-min
oscillation for approximately half of the stable bright features. The 5-min oscillations are detected mainly at the network
boundaries, where stable bright features are located, while 3-min oscillations coincide with few bright features, but are
also quite intense inside the network cells. The majority of bright features are associated with Hα downflows. The downflow
is very intense at the location of `flashing' bright features. 相似文献
158.
159.
Theoretical line ratios involving 2s
2
S - 3p
2
P, 2p
2
P - 3s
2
S, and 2p
2
S - 3d
2
D transitions inCiv between 312 and 420 Å are presented. A comparison of these with solar active region observational data obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals good agreement between theory and experiment, with discrepancies that average only 22%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on boardSkylab. The potential usefulness of theCIV line ratios as electron temperature diagnostics for the solar transition region is briefly discussed. 相似文献
160.
We carried out a series of linear stability analyses of the radial and low-degree non-radial p modes for stellar models with initial masses of . The stellar models were computed by using convective overshoot distance , 0.25 and 0.40 H P . Our numerical results show that the β Cephei instability strip forms a horn-shaped region pointing upwards near the main sequence on the Hertzsprung–Russell diagram (HRD). The lower part of the instability strip for the radial modes join the zero-age main-sequence (ZAMS) at , while the top of the instability strip extends up to . The instability strip for the non-radial modes is even wider. The overall instability strip is dominated by the radial and non-radial fundamental modes. The first overtone (the radial-order index is also pulsationally unstable. We have shown that the β Cephei stability is almost independent of the overshoot parameter d over used for the stellar models, while it depends critically on the metal abundance. With decreasing metal abundance, the instability region shrinks and eventually disappears for . 相似文献