全文获取类型
收费全文 | 36848篇 |
免费 | 720篇 |
国内免费 | 579篇 |
专业分类
测绘学 | 933篇 |
大气科学 | 3066篇 |
地球物理 | 7395篇 |
地质学 | 12195篇 |
海洋学 | 3358篇 |
天文学 | 8693篇 |
综合类 | 120篇 |
自然地理 | 2387篇 |
出版年
2021年 | 198篇 |
2020年 | 257篇 |
2019年 | 255篇 |
2018年 | 611篇 |
2017年 | 563篇 |
2016年 | 878篇 |
2015年 | 619篇 |
2014年 | 846篇 |
2013年 | 1882篇 |
2012年 | 1032篇 |
2011年 | 1448篇 |
2010年 | 1190篇 |
2009年 | 1770篇 |
2008年 | 1568篇 |
2007年 | 1490篇 |
2006年 | 1404篇 |
2005年 | 1284篇 |
2004年 | 1206篇 |
2003年 | 1166篇 |
2002年 | 1090篇 |
2001年 | 975篇 |
2000年 | 983篇 |
1999年 | 917篇 |
1998年 | 834篇 |
1997年 | 841篇 |
1996年 | 720篇 |
1995年 | 645篇 |
1994年 | 561篇 |
1993年 | 514篇 |
1992年 | 510篇 |
1991年 | 480篇 |
1990年 | 476篇 |
1989年 | 418篇 |
1988年 | 402篇 |
1987年 | 449篇 |
1986年 | 433篇 |
1985年 | 522篇 |
1984年 | 582篇 |
1983年 | 558篇 |
1982年 | 519篇 |
1981年 | 462篇 |
1980年 | 435篇 |
1979年 | 397篇 |
1978年 | 412篇 |
1977年 | 358篇 |
1976年 | 323篇 |
1975年 | 336篇 |
1974年 | 336篇 |
1973年 | 338篇 |
1972年 | 201篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
451.
452.
453.
Stable carbon isotope ratios have been used to study the sources of particulate organic carbon (POC) and total dissolved inorganic carbon in the Orinoco Basin. The isotopic composition of total dissolved inorganic carbon shows a range of from -8·1 to -23·0 ppt, an indication of dominance of biological processes. The isotopic composition of POC exhibits a range of from -24·1 to -34·6 ppt with little seasonal variation. The isotopic evidence indicates that the POC is predominantly of terrestrial origin rather than a result of in situ planktonic production. The similarity of isotopic composition of POC and coastal sediments suggests that riverine organic detritus has been transported 30-50 km offshore in a direction parallel to the Orinoco river channel. 相似文献
454.
Several environmentally relevant contaminants are characterized as xenoestrogens by virtue of their ability to induce responses similar to 17β-estradiol (E2). There is concern that exposure to these xenoestrogens may result in endocrine and, thus, reproductive disruption. The objective of the present research was to determine whether xenobiotics known to elicit estrogenic responses in mammals were estrogenic in channel catfish, and if these compounds were capable of altering normal physiological response to E2. Sexually immature catfish were exposed by intraperitoneal injection to E2, suspected xenoestrogens or combination doses of E2 and xenoestrogen. Appearance of vitellogenin (Vg) in serum was used as the bioindicator of estrogenicity; Vg was measured by ELISA 7 days after injection. The ED50 of E2 for the appearance of Vg in blood, 0.6 mg/kg, was used as the positive control. Synthetic estrogens, ethinylestradiol, mestranol and DES were effective in inducing Vg; the antiestrogen tamoxifen inhibited response to E2 when given in a combination dose. Treatment by both methoxychlor and p-nonylphenol resulted in Vg appearance in serum; the doses required were 300 times that of E2, and the vitellogenic response was less when compared to the E2-induced response. In addition, a combination dose of methoxychlor with E2 decreased the magnitude of the response to E2. Other mammalian xenoestrogens, o,p′-DDT, chlordecone, lindane and β-HCH, were not able to produce a vitellogenic response in this study. 相似文献
455.
Three-dimensional visualization of mission planning and control forthe NPS autonomous underwater vehicle 总被引:1,自引:0,他引:1
Zyda M.J. McGhee R.B. Kwak S. Nordman D.B. Rogers R.C. Marco D. 《Oceanic Engineering, IEEE Journal of》1990,15(3):217-221
The Naval Postgraduate School (NPS) is constructing a small autonomous underwater vehicle (AUV) with an onboard mission control computer. The mission controller software for this vehicle is a knowledge-based artificial intelligence (AI) system requiring thorough analysis and testing before the AUV is operational. The manner in which rapid prototyping of this software has been demonstrated by developing a controller code on a LISP machine and using an Ethernet link with a graphics workstation to simulate the controller's environment is discussed. The development of a testing simulator using a knowledge engineering environment (KEE) expert system shell that examines AUV controller subsystems and vehicle models before integrating them with the full AUV for its test environment missions is discussed. This AUV simulator utilizes an interactive mission planning control console and is fully autonomous once initial parameters are selected 相似文献
456.
Euglena gracilis makes two small Cd-binding proteins/peptides (Cd-BP I and Cd-BP II) in response to exposure to Cd2+. These proteins migrate slower than mammalian Cd-metallothionein (Cd-Mt) through Sephadex G-75, but like Cd-Mt also contain Zn and are separated into two species when chromatographed over a DEAE column. Both BPs are much less stable than Cd-Mt under acidic conditions. One source of this instability is that at least Cd-BP II contains acid-labile sulfide co-ordinated to the metals. It has been quantitated in Cd-BP II and shown to have a value of 1·25 ± 0·10 S2−/Cd. The reactivity of the Cd-binding sites in ligand substitution and thioldisulfide exchange reactions has been assessed. The reactions of Cd-BP I and II with pyridylazoresorcinol (PAR) were slow and displayed complex kinetic behavior. Similarly, both Cd-BPs react slowly and with complicated kinetics with 5,5′-dithiobis-(2-nitrobenzoate). 相似文献
457.
The trend of previous estimates of the world's ultimate recovery of oil suggests a figure of 1650 BBO, of which 1438 BBO have been discovered and 978 BBO remain to be produced. Some large reported reserve figures are questioned. Importance is attached to the mid-point of depletion when rising production tends to give way to decline. The share of the world's supply coming from six Middle East countries is set to continue to rise from its 1986 low of 16% to as much as 50% by 2010, with the likely consequence of higher oil prices and serious political implications. The world is rapidly approaching a turning point when it has to face the consequences of an irreversible decline in oil production. 相似文献
458.
459.
460.
A study of the mineralogy of red clay sequences overlying basalt in the Bay of Biscay revealed two successive units: unit one (lower unit) indicates that clay minerals are the result of subaerial weathering of acid or basic continental rocks, occurring in the Galice province (northwest Spain); and unit two (upper unit) consists of fine-grained particles which suggest distinct volcanic activity. Clay materials from the first unit (Paleocene) are deposited in shallow environments, whereas the second unit materials (lower Eocene) Occur in deeper environments followed by neoformation of clay Minerals and concentration of chemical elements (cobalt, nickel, lead, zinc, and barium). 相似文献