全文获取类型
收费全文 | 66746篇 |
免费 | 11108篇 |
国内免费 | 14990篇 |
专业分类
测绘学 | 3891篇 |
大气科学 | 13264篇 |
地球物理 | 16554篇 |
地质学 | 33089篇 |
海洋学 | 8545篇 |
天文学 | 4098篇 |
综合类 | 6167篇 |
自然地理 | 7236篇 |
出版年
2024年 | 362篇 |
2023年 | 1249篇 |
2022年 | 2803篇 |
2021年 | 3351篇 |
2020年 | 2841篇 |
2019年 | 3064篇 |
2018年 | 3705篇 |
2017年 | 3410篇 |
2016年 | 3765篇 |
2015年 | 2991篇 |
2014年 | 3827篇 |
2013年 | 3840篇 |
2012年 | 3521篇 |
2011年 | 3732篇 |
2010年 | 3769篇 |
2009年 | 3684篇 |
2008年 | 3304篇 |
2007年 | 3311篇 |
2006年 | 2728篇 |
2005年 | 2246篇 |
2004年 | 1813篇 |
2003年 | 1869篇 |
2002年 | 1900篇 |
2001年 | 1851篇 |
2000年 | 2071篇 |
1999年 | 2940篇 |
1998年 | 2432篇 |
1997年 | 2405篇 |
1996年 | 2220篇 |
1995年 | 1947篇 |
1994年 | 1811篇 |
1993年 | 1554篇 |
1992年 | 1273篇 |
1991年 | 953篇 |
1990年 | 788篇 |
1989年 | 703篇 |
1988年 | 599篇 |
1987年 | 375篇 |
1986年 | 359篇 |
1985年 | 255篇 |
1984年 | 189篇 |
1983年 | 170篇 |
1982年 | 194篇 |
1981年 | 131篇 |
1980年 | 115篇 |
1979年 | 98篇 |
1978年 | 47篇 |
1977年 | 35篇 |
1976年 | 35篇 |
1958年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Based on the CN and C2 comae isophotes for two comets (1961 IX and 1970 16) given by Rahe et al. and the relevant theory of physical chemistry, we have deduced the distributions of the CN and C2 modecules in the coma, their scale heights and mean lifetimes. The results favour the viewpoint that HCN is the parent of CN, and that C2H2 is the parent of C2. 相似文献
82.
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
83.
在BPL信道上增加数据广播内容的探讨 总被引:1,自引:1,他引:0
分析了现有罗兰C(包括BPL)在授时方面存在的不足,提出了在BPL信道上实现数据广播的主要目的。详细讨论了各种可能的数据信息的结构,并给出了有关参数和数据的计算方式和公式。认为发播内容的选择是由期望的系统功能决定的,广播内容的编排则与具体实现技术相关。最后给出了在不同调制方法下的传输速率,这是合理编排广播内容的基础。 相似文献
84.
CHEN Bing-lu ZHANG Yun-ni CHEN Xin-geng WANG Zhi-gang YANG Guang-xing 《中国地理科学(英文版)》2001,11(2):155-162
1 HYDROLOGIC FEATURES Lingdingyang Estuary, located at the middle south of Guangdong Province, is a bell-shaped estuary with a north-south direction. Its area is about 2100km2. The north of Qi′ao Island and Inner-Lingding Island, and the south of Humen are grouped as Neilingdingyang Estuary, having an area of 1041km2. Affected by topography, runoff and tide, its dynamic condition is very complicated. Different water areas have different hydrologic features. The topography under … 相似文献
85.
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
86.
Dynamic analysis of three-dimensional marine cables 总被引:1,自引:0,他引:1
Shan Huang 《Ocean Engineering》1994,21(6)
Marine cables are widely used in a variety of offshore activities. In this paper, a numerical approach is presented which is capable of predicting the dynamic behaviour of three-dimensional marine cables. The solution of this problem is of practical importance in the estimation of dybamic loading and motion, and thus has direct application to the enhancement of safety and the effectiveness of the offshore activities. The approach is based upon the lump-mass-and-spring model and the finite difference method. Mathematical analysis is provided on the basic characteristics of the model and the numerical stability. A numerical example is given to demonstrate the validity of the approach. 相似文献
87.
1 INTRODUCTIONIn the previous paper (on et al. 1999, hereafter Paper I), we investigated the wavelength-dependence of four colltribution functions (CFs) derived from dmerent formal solutions andreferring to different emergellt quantities in the unpolarized case. Because one cannot generallyassign a single formation region to the whole line band in a real stellar atmosphere, e.g., the solaratmosphere, instead, the line formation region can be defined as the layers deviating farthestfrom t… 相似文献
88.
应用地壳波浪与镶嵌构造学说对富氏谱分析法提取地壳垂直形变信息的科学性做了地质学意义上的阐释 ,并提出了根据多期形变资料提取特定波段上构造策应力的数学模型 相似文献
89.
IntroductionThe area of eastern Liaoning is an importantmetal and nonmetal metallogenetic district in China,and the Liaohe group is one of the most importantstrata that hosts Pb, Zn, Au, B and Mg etcstratabound deposits. Up to now many geo1ogistssuch as Z… 相似文献
90.
A very long series of photographic observations of the comet Hale-Bopp has been made during January–April 1997 at the double
astrograph (400/2000) of the Main Astronomical Observatory (Kyiv, Ukraine). Some of the cometary photos were obtained with
two wide-band filter combinations. One of these combinations isolates C2 emission, another — the nearby dust continuum. The images were digitized by means of AMDPH-XY machine and then calibrated
following the standard procedure. After subtraction of the dust continuum the distribution of surface brightness in the C2 emission coma of comet Hale-Bopp was studied. We found an asymmetric brightness distribution both pre- and post-perihelion.
On 21.77 April 1997 a secondary brightness peak is found at the distance of 1.03 × 105 km from the nucleus. It is possible that this peak is related to the extended source of the C2 molecules.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献