全文获取类型
收费全文 | 53474篇 |
免费 | 850篇 |
国内免费 | 563篇 |
专业分类
测绘学 | 1448篇 |
大气科学 | 3805篇 |
地球物理 | 9902篇 |
地质学 | 19203篇 |
海洋学 | 4907篇 |
天文学 | 12897篇 |
综合类 | 189篇 |
自然地理 | 2536篇 |
出版年
2022年 | 365篇 |
2021年 | 625篇 |
2020年 | 658篇 |
2019年 | 707篇 |
2018年 | 1582篇 |
2017年 | 1504篇 |
2016年 | 1871篇 |
2015年 | 989篇 |
2014年 | 1748篇 |
2013年 | 2868篇 |
2012年 | 1855篇 |
2011年 | 2374篇 |
2010年 | 2069篇 |
2009年 | 2678篇 |
2008年 | 2295篇 |
2007年 | 2347篇 |
2006年 | 2195篇 |
2005年 | 1622篇 |
2004年 | 1641篇 |
2003年 | 1549篇 |
2002年 | 1474篇 |
2001年 | 1299篇 |
2000年 | 1219篇 |
1999年 | 994篇 |
1998年 | 1039篇 |
1997年 | 948篇 |
1996年 | 816篇 |
1995年 | 782篇 |
1994年 | 685篇 |
1993年 | 604篇 |
1992年 | 593篇 |
1991年 | 593篇 |
1990年 | 622篇 |
1989年 | 493篇 |
1988年 | 504篇 |
1987年 | 528篇 |
1986年 | 487篇 |
1985年 | 610篇 |
1984年 | 673篇 |
1983年 | 590篇 |
1982年 | 561篇 |
1981年 | 499篇 |
1980年 | 469篇 |
1979年 | 477篇 |
1978年 | 458篇 |
1977年 | 368篇 |
1976年 | 345篇 |
1975年 | 357篇 |
1974年 | 308篇 |
1973年 | 342篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
132.
We study the solar cycle evolution during the last 8 solar cycles using a vectorial sunspot area called the LA (longitudinal asymmetry) parameter. This is a useful measure of solar activity in which the stochastic, longitudinally evenly distributed sunspot activity is reduced and which therefore emphasizes the more systematic, longitudinally asymmetric sunspot activity. Interesting differences are found between the LA parameter and the more conventional sunspot activity indices like the (scalar) sunspot area and the sunspot number. E.g., cycle 19 is not the highest cycle according to LA. We have calculated the separate LA parameters for the northern and southern hemisphere and found a systematic dipolar-type oscillation in the dominating hemisphere during high solar activity times which is reproduced from cycle to cycle. We have analyzed this oscillation during cycles 16–22 by a superposed epoch method using the date of magnetic reversal in the southern hemisphere as the zero epoch time. According to our analysis, the oscillation starts by an excess of the northern LA value in the ascending phase of the solar cycle which lasts for about 2.3 years. Soon after the maximum northern dominance, the southern hemisphere starts dominating, reaching its minimum some 1.2–1.7 years later. The period of southern dominance lasts for about 1.6 years and ends, on an average, slightly before the end of magnetic reversal. 相似文献
133.
We have investigated grain boundary diffusion rates in enstatite by heating single crystals of quartz packed in powdered San Carlos olivine (Mg0.90Fe0.10)2SiO4 at controlled oxygen fugacities in the range 10?5.7 to 10?8.7?atm and temperatures from 1350° to 1450?°C for times from 5 to 100?h at 1?atm total pressure. Following the experiments, the thickness of the coherent polycrystalline reaction rim of pyroxene that had formed between the quartz and olivine was measured using backscatter scanning imaging in the electron microprobe. Quantitative microprobe analysis indicated that the composition of this reaction phase is (Mg0.92Fe0.08)2Si2O6. The rate of growth of the pyroxene increases with increasing temperature, is independent of the oxygen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by ionic diffusion through the pyroxene rim. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the olivine-pyroxene interface, and is therefore controlled by the diffusion of silicon and oxygen. The parabolic rate constants determined from the experiments were analyzed in terms of the oxide activity gradient across the rim to yield mean effective diffusivities for the rate-limiting ionic species, assuming bulk transport through the pyroxene layer. These effective diffusivities are faster than the lattice diffusivities for the slowest species (silicon) calculated from creep experiments, but slower than measured lattice diffusivities for oxygen in enstatite. Thus, silicon grain boundary diffusion is most likely to be the rate-limiting process in the growth of the pyroxene rims. Also, as oxygen transport through the pyroxene rims must be faster than silicon transport, diffusion of oxygen along the grain boundaries must be faster than through the lattice. The grain boundary diffusivity for silicon in orthopyroxenite is then given by D¯gbSiδ=(3.3±3.0)×10?9f0.0O2e?400±65/RT?m3s?1, where the activation energy for diffusion is in kJ/mol, and δ is the grain boundary width in m. Calculated growth rates for enstatite under these conditions are significantly slower than predicted by an extrapolation from similar experiments performed at 1000?°C under high pressure (hydrous) conditions by Yund and Tullis (1992), perhaps due to water-enhancement of diffusion in their experiments. 相似文献
134.
The testing of a synthetic aperture technique, the ETAM algorithm, is extended and its performance for CW pseudorandom signals and broadband ship noise is examined. The results show the limitations of the technique and are of special interest for operational systems development. In the CW experiments, the transmitted signal was generated with high temporal coherence, and loss of the spatial and temporal coherence of the received signal was introduced only by the medium and the stability of the towed array. In the experiments that included the pseudorandom signal and the ship noise, the temporal coherence of the transmitted signals was deliberately chosen to be poor in order to study the effects and the performance of this algorithm with broadband signals. The related experimental results show that for received signals, which have their segments over the synthesizing period highly cross-correlated, a synthetic aperture array gain was achieved that corresponds to the length of an equivalent fully populated array 相似文献
135.
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
136.
137.
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
138.
139.
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
140.
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