全文获取类型
收费全文 | 27011篇 |
免费 | 5115篇 |
国内免费 | 7018篇 |
专业分类
测绘学 | 2471篇 |
大气科学 | 5456篇 |
地球物理 | 6469篇 |
地质学 | 13791篇 |
海洋学 | 3870篇 |
天文学 | 1346篇 |
综合类 | 2513篇 |
自然地理 | 3228篇 |
出版年
2024年 | 118篇 |
2023年 | 374篇 |
2022年 | 1146篇 |
2021年 | 1258篇 |
2020年 | 1055篇 |
2019年 | 1326篇 |
2018年 | 1316篇 |
2017年 | 1310篇 |
2016年 | 1490篇 |
2015年 | 1388篇 |
2014年 | 1503篇 |
2013年 | 1746篇 |
2012年 | 1708篇 |
2011年 | 1725篇 |
2010年 | 1812篇 |
2009年 | 1651篇 |
2008年 | 1641篇 |
2007年 | 1478篇 |
2006年 | 1267篇 |
2005年 | 1162篇 |
2004年 | 877篇 |
2003年 | 867篇 |
2002年 | 892篇 |
2001年 | 936篇 |
2000年 | 932篇 |
1999年 | 1133篇 |
1998年 | 1013篇 |
1997年 | 921篇 |
1996年 | 829篇 |
1995年 | 726篇 |
1994年 | 614篇 |
1993年 | 583篇 |
1992年 | 453篇 |
1991年 | 382篇 |
1990年 | 283篇 |
1989年 | 244篇 |
1988年 | 240篇 |
1987年 | 124篇 |
1986年 | 119篇 |
1985年 | 83篇 |
1984年 | 72篇 |
1983年 | 54篇 |
1982年 | 64篇 |
1981年 | 51篇 |
1980年 | 37篇 |
1979年 | 35篇 |
1978年 | 16篇 |
1977年 | 13篇 |
1976年 | 11篇 |
1958年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Vertical profiles of chl-a and primary productivity in the middle continental shelf area and eddy area of the East China Sea
were studied using data from a cruise in the East China Sea in February to March, 1997 and a cruise in July, 1998. The results
showed that chl-a vertical distribution closely related to in situ hydrological and nutrient conditions. Chla-a concentration
ranged from 0.22 to 0.35 mg/m3 and 0.93–1.09 mg/m3 in the eddy area and in the middle continental shelf area, respectively. In both areas, chl-a concentrations in deep layers
were slightly higher than those in shallow layers, but was of the same order of magnitude. In summer, when a thermocline existed
in the water column, highest chl-a concentrations appeared at the base of the thermocline layers in both areas. In the eddy
area, chl-a concentration maximized at 31.743 mg/m3, and averaged 1.143 mg/m3 below 30 m depth. In the middle continental shelf area, the highest chl-a concentration was 2.120 mg/m3, the average was 1.168 mg/m3. The primary productivity reached 1418.76 mgC/(m2·d) in summer and 1360.69 mgC/(m2·d) in winter. In the eddy area, the primary productivity was 787.50 mgC/(m2·d) in summer and 159.04 mgC/(m2·d) in winter. Vertical carbon sinking rate from the deep layer to the bottom in both areas is also discussed in this paper.
Contribution NO. 4183 from the Institute of Oceanology, Chinese Academy of Sciences.
Project No. 49636210 supported by NSFC. 相似文献
62.
63.
OQ208 is used ore and more frequently as a VLBI calibrator of flux density. A VLBI image of OQ208 at 5 GHz is firstly provided in this paper. Its structure consists of a compact core and a component. The angular distance between the core and the component is about 7 milli-arcsecond; the position angle is −125 degrees. In principle, OQ208 is not an ideal VLBI calibrator of flux density. This paper provides the structural information for correcting the structure effect when OQ208 is to be used as a calibrator. 相似文献
64.
介绍了近年来河外H2O超脉泽的主要观测结果。H2O超脉泽通常起源于活动星系核中央的拱核盘。它们主要寄生在 Serfert 2星系或低电离核区。至今为止,已有20个星系探测到H2O超脉泽。脉泽辐射的各向同性光度为10~6000L⊙。所有超脉泽星系显示出核的活动,显然,脉泽是由核活动所产生的射电和X射线光子或激波来抽运的。H2O超脉泽倾向存在于高倾斜度的星系,这使得沿视线上的分子柱密度增高,产生足够大的放大光深。最有可能产生H2O超脉泽辐射的星系应有一个包含着射电源的侧向的分子盘以及一个适当的抽运机制。 相似文献
65.
66.
低频扩频中伪随机序列产生方法的研究 总被引:2,自引:0,他引:2
伪随机序列的产生是实现BPC低频时码发播台扩展频谱技术升级的一个重要环节。针对BPC低频时码发播台的现有系统,提出了伪随机序列的产生方案,并对其性能作了分析。 相似文献
67.
K. S. Stankevich A. M. Aslanyan V. P. Ivanov R. M. Martirosyan Ye. Terzian 《Astrophysics》2003,46(4):429-433
Radio emission of the historical supernovae remnants Tycho (SNR1572) and Kepler (SNR1604) and evolution of their luminosity are considered. Measurement data of secular luminosity decrease rate, obtained earlier by the authors, were corrected with account of variation in time of the flux density of the reference sources. As a result, it is found that the SNR1604 luminosity at 1667 MHz is weakening with an annual mean rate equal to (0.2 ± 0.07)%. The corresponding rate for SNR1572 is (0.47 ± 0.05)%. Since the radio luminosity evolution, as well as energy densities of magnetic field and relativistic electrons inside SNR1604 and SNR1572 are essentially different, these remnants should be considered as different types of supernovae. Bandiera classified SN1604 as type SNIb or SNII. 相似文献
68.
Teruyuki Maruoka Christian Koeberl Jun‐ichi Matsuda Yasuhiko Syono 《Meteoritics & planetary science》2003,38(8):1255-1262
Abstract— Carbon isotopic compositions were measured for shock‐produced diamond and shocked graphite formed at peak pressures ranging from 37 to 52 GPa. The δ13C values of diamonds produced in a sealed container were generally lower than that of the initial graphite. The differences in the carbon isotopic composition between initial graphite and shocked graphite/diamond may reflect kinetic isotopic fractionation during the oxidation of the graphite/diamond and/or analytical artifacts possibly induced by impurities in the samples. The pressure effect on the isotopic fractionations between graphite and diamond can be estimated from the δ13C values of impurity‐free diamonds produced using a vented container from which gases, including oxygen, in pore spaces escaped during or after the diamond formation (e.g., 0.039 ± 0.085‰ at a peak pressure of 52 GPa). Any isotopic fractionation induced by shock conversion of graphite to diamond is too small to be detected in natural shock‐induced diamond‐graphite systems related to terrestrial impact cratering processes. 相似文献
69.
APPLICATION OF GEOGRAPHICAL PARAMETER DATABASE TO ESTABLISHMENT OF UNIT POPULATION DATABASE 总被引:1,自引:1,他引:0
Now GIS is turning into a good tool in handling geographical, economical, and population data, so we can obtain more and more information from these data. On the other hand, in some cases, for a calamity, such as hurricane, earthquake, flood, drought etc., or a decision-making, such as setting up a broadcasting transmitter, building a chemical plant etc., we have to evaluate the total population in the region influenced by a calamity or a project. In this paper, a method is put forward to evaluate the population in such special region. Through exploring the correlation of geographical parameters and the distribution of people in the same region by means of quantitative analysis and qualitative analysis, unit population database (1km× 1km) is established. In this way, estimating the number of people in a special region is capable by adding up the population in every grid involved in this region boundary. The geographical parameters are obtained from topographic database and DEM database on the scale of 相似文献
70.
吴秋凤 《广东海洋大学学报》2003,23(5):94-98
在阐述西部大开发中科技主导地位的基础上 ,从科技应用成效二重性的视角 ,探讨了关注西部科技伦理的必要性 ,并初步提出了西部大开发战略的科技伦理调控机制 相似文献