全文获取类型
收费全文 | 31103篇 |
免费 | 5784篇 |
国内免费 | 7431篇 |
专业分类
测绘学 | 1701篇 |
大气科学 | 6563篇 |
地球物理 | 7934篇 |
地质学 | 15698篇 |
海洋学 | 3694篇 |
天文学 | 1302篇 |
综合类 | 3587篇 |
自然地理 | 3839篇 |
出版年
2024年 | 94篇 |
2023年 | 432篇 |
2022年 | 1335篇 |
2021年 | 1531篇 |
2020年 | 1296篇 |
2019年 | 1426篇 |
2018年 | 1764篇 |
2017年 | 1614篇 |
2016年 | 1840篇 |
2015年 | 1418篇 |
2014年 | 1784篇 |
2013年 | 1775篇 |
2012年 | 1683篇 |
2011年 | 1794篇 |
2010年 | 1776篇 |
2009年 | 1699篇 |
2008年 | 1468篇 |
2007年 | 1476篇 |
2006年 | 1215篇 |
2005年 | 1202篇 |
2004年 | 888篇 |
2003年 | 907篇 |
2002年 | 919篇 |
2001年 | 880篇 |
2000年 | 1046篇 |
1999年 | 1536篇 |
1998年 | 1256篇 |
1997年 | 1319篇 |
1996年 | 1179篇 |
1995年 | 1066篇 |
1994年 | 903篇 |
1993年 | 827篇 |
1992年 | 632篇 |
1991年 | 459篇 |
1990年 | 377篇 |
1989年 | 295篇 |
1988年 | 268篇 |
1987年 | 199篇 |
1986年 | 148篇 |
1985年 | 109篇 |
1984年 | 86篇 |
1983年 | 65篇 |
1982年 | 67篇 |
1981年 | 47篇 |
1980年 | 49篇 |
1979年 | 42篇 |
1978年 | 19篇 |
1976年 | 15篇 |
1975年 | 17篇 |
1958年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
51.
CATASTROPHIC ECO-ENVIRONMENTAL CHANGE IN THE SONGNEN PLAIN, NORTHEASTERN CHINA SINCE 1900S 总被引:1,自引:1,他引:0
WANGHai-xia WANZhong-juan YUShao-peng LUOXin-zheng SUNGuang-you 《中国地理科学(英文版)》2004,14(2):179-185
Although the Songnen Plain in the northeastern China was developed relatively late in the temperate zone of the world, its eco-environment has changed greatly. This paper analyzes the changes of land cover and the rates and trends ofdesertification during the past 100 years in the Songnen Plain. According to the macroscopic analysis, we find that the eco-environment in the plain has reached to the threshold of catastrophic change since the 1950s. The Thorn Needle Catastrophic Model was used to determine and validate this conclusion. Human activities, including large-scale construction projects, such as huge dams and dikes, and excessive grazing were the primary factors contributing to regional eco-environmental catastrophe. And irrational reclamation of the wilderness also affected the eco-environmental change. The results reveal the complex human-land interactions. 相似文献
52.
在建立斑节对虾实验室养殖模式的基础上,对野外采集的有白斑综合症病毒(WSSV)病典型症状的斑节对虾初步纯化其病毒,并进行WSSV的PCR检测,得到WSSV的感染用样品。对健康斑节对虾分别进行浸浴感染,投喂感染和注射感染。对感染死亡个体进行WSSA的PCR检测和细菌检测,证实WSSV感染性和致死性。浸浴感染、投喂感染和注射感染的感染量分别为4mL/L、0.2g/10g虾体、1/2稀释液0.05mL/10g虾体,死亡开始时间分别为16d、42h、28h,三种感染模式最终死亡率100%,从开始死亡到全部死亡延续时间分别为15d、82h、44h。 相似文献
53.
Lin Liu 《Chinese Astronomy and Astrophysics》1997,21(4):488-500
With the aim of analyzing the errors in the radial position of satellites, we give in this paper the expressions for the radial perturbation which include the complete zeroth- and first-order terms in the orbital eccentricity. A simpler and faster method of calculating the perturbation is given. 相似文献
54.
THE SIMULATED STORAGE CAPACITY OF FLOOD AND WATERLOGGING IN THE TYPICAL AGRICULTURAL REGION IN JIANGHAN PLAIN 总被引:2,自引:0,他引:2
1INTRODUCTIONJianghanPlainisoneoftheimportantbasesofChina′sagriculturalproductivitylocatedinthemiddleChangjiangRivervalleyofcentralChina.TheJianghanplainapproximately6.6×104km2inareaandover30millioninpopulationistherenowned“countryoffishandrice"in… 相似文献
55.
56.
We have studied the optical historical light curves of two Active Galactic Nuclei (AGNs): OJ287 and 3C345. We find a linear
dependence between the magnitude and the logarithm of the length of the time the objects stay below this magnitude. The relation
may originate from a fractal structure of the light curve.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
57.
In this paper, we reconstruct the finite energy force-free magnetic field of the active region NOAA 8100 on 4 November 1997
above the photosphere. In particular, the 3-D magnetic field structures before and after a 2B/X2 flare at 05:58 UT in this
region are analyzed. The magnetic field lines were extrapolated in close coincidence with the Yohkoh soft X-ray (SXR) loops accordingly. It is found that the active region is composed of an emerging flux loop, a complex loop
system with differential magnetic field shear, and large-scale, or open field lines. Similar magnetic connectivity has been
obtained for both instants but apparent changes of the twisting situations of the calculated magnetic field lines can be observed
that properly align with the corresponding SXR coronal loops. We conclude that this flare was triggered by the interaction
of an emerging flux loop and a large loop system with differential magnetic field shear, as well as large-scale, or open field
lines. The onset of the flare was at the common footpoints of several interacting magnetic loops and confined near the footpoints
of the emerging flux loop. The sheared configuration remained even after the energetic flare, as demonstrated by calculated
values of the twist for the loop system, which means that the active region was relaxed to a lower energy state but not completely
to the minimum energy state (two days later another X-class flare occurred in this region). 相似文献
58.
刘满全 《地球科学与环境学报》1994,(2)
本文介绍紫阳骨坡治理工程中有关抗滑桩的设计与施工。文中详细地论述了设计中设计人员应根据工程的具体情况去灵活处理的一些技术问题,如确定有关参数的选择原则,计算上的条件简化和一些具体技术细节的处理方法等. 相似文献
59.
Numerical methods are usually used for the computation of ephemerides with perturbations for the precise orbital determination of an artificial satellite. But their numerical stability will be encountered in a long arc. In this case the use the improved Encke special perturbation methods has been suggested. The results of this paper show that Encke's method does indeed have a certain effectiveness, but cannot yet completely resolve the numerical stability, and the more efficient method is to use the energy integral or its variational relation to control the growth of the along-track error in general numerical calculations so that the aim of stabilization can be achieved. 相似文献
60.
1 INTRODUCTIONGlobal change research involves much geo-objectsand geo-process, such as climate and environmentalchange, substance and energy cycling, land-use/land-cover change (LUCC), interactivity between human and nature, etc.. So it need cooperation frommany research communities including international research programs groups such as IGBP (InternationalGeosphere-Biosphere Programme IPCC (Intergovemmental Panel on Climate Change), IHDP (InternationalHuman Dimension Program o… 相似文献