全文获取类型
收费全文 | 26787篇 |
免费 | 77篇 |
国内免费 | 115篇 |
专业分类
系统科学 | 309篇 |
丛书文集 | 471篇 |
教育与普及 | 57篇 |
理论与方法论 | 78篇 |
现状及发展 | 12678篇 |
研究方法 | 1090篇 |
综合类 | 12032篇 |
自然研究 | 264篇 |
出版年
2012年 | 364篇 |
2011年 | 648篇 |
2010年 | 162篇 |
2009年 | 153篇 |
2008年 | 426篇 |
2007年 | 529篇 |
2006年 | 489篇 |
2005年 | 478篇 |
2004年 | 427篇 |
2003年 | 479篇 |
2002年 | 460篇 |
2001年 | 898篇 |
2000年 | 899篇 |
1999年 | 542篇 |
1992年 | 509篇 |
1991年 | 401篇 |
1990年 | 464篇 |
1989年 | 415篇 |
1988年 | 427篇 |
1987年 | 451篇 |
1986年 | 440篇 |
1985年 | 545篇 |
1984年 | 430篇 |
1983年 | 366篇 |
1982年 | 349篇 |
1981年 | 386篇 |
1980年 | 401篇 |
1979年 | 894篇 |
1978年 | 755篇 |
1977年 | 688篇 |
1976年 | 610篇 |
1975年 | 591篇 |
1974年 | 808篇 |
1973年 | 646篇 |
1972年 | 637篇 |
1971年 | 784篇 |
1970年 | 988篇 |
1969年 | 785篇 |
1968年 | 767篇 |
1967年 | 786篇 |
1966年 | 671篇 |
1965年 | 488篇 |
1964年 | 179篇 |
1959年 | 276篇 |
1958年 | 441篇 |
1957年 | 279篇 |
1956年 | 254篇 |
1955年 | 248篇 |
1954年 | 228篇 |
1948年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
131.
Y. Mamiya H. Sonobe K. Yoshida N. Hara Y. Fujimoto 《Cellular and molecular life sciences : CMLS》1995,51(4):363-367
Ecdysteroids in diapause eggs of the silkworm,Bombyx mori, were analyzed using high-performance liquid chromatography (HPLC) combined with radioimmunoassay (RIA). A relatively large amount of an unidentified free ecdysteroid and its phosphoric ester (conjugated form) were detected. These two compounds were isolated by a combination of column chromatography on silicic acid, thin-layer chromatography (TLC), and HPLC using a reverse-phase (RP) column. The purified compounds were identified as 3-epi-22-deoxy-20-hydroxyecdysone (22d20E) and 3-epi-22-deoxy-20-hydroxyecdysone 2-phosphate (22d20E2P) by means of mass spectrometry and nuclear magnetic resonance spectroscopy. to our knowledge, this is the first report of 22d20E and 22d20E2P. 相似文献
132.
J. Martín-Gil F. J. Martín-Gil G. Delibes-de-Castro P. Zapatero-Magdaleno F. J. Sarabia-Herrero 《Cellular and molecular life sciences : CMLS》1995,51(8):759-761
Vermillion has been shown to be useful in preserving human bones from 5000 years ago. Remarkably well-preserved human bones have been found in the dolmenic burial La Velilla in Osorno (Palencia, Spain), carefully covered by pulverized cinnabar (vermillion) which ensured their preservation even in non-favorable climatic conditions. We believe the red powder was deliberately deposited for preservative use because no cinnabar mine is to be found within 160 km, because of the large amount (hundreds of kilograms) used, and because its composition, red mercuric sulphide, is similar to that of preparations used in technical embalming. This finding pushes back the data of the use of mercury ore for preservation by four millennia in South America, and by at least one millennium in the Old World. Chemical and thermal analyses of vermillion in La Velilla have demonstrated its great purity and shown that the cinnabar was pulverized and washed (but not heated), producing a bright red-orange tone. 相似文献
133.
X. Wang C. A. Lessman D. B. Taylor T. K. Gartner 《Cellular and molecular life sciences : CMLS》1995,51(11):1097-1102
Gastrulation is characterized by dramatic cell migration which is thought to require the interaction of cell adhesion molecules with extracellular molecules. We have tested two novel peptides, a fibronectin peptide and a fibronectin receptor peptide, for their effects on gastrulation of the leopard frogRana pipiens. The fibronectin peptide DRVPHSRNSIT corresponds to residues 1373–1383 of the cell-binding domain of fibronectin; the receptor peptide DLYYLMDL corresponds to residues 124–131 of 1 subunit of a variety of integrins including 51. Either of these peptides significantly inhibited gastrulation after being microinjected into mid-blastulae. These results indicate that these sequences may correspond to the ligand/receptor interaction sites of fibronectin and its receptor(s). 相似文献
134.
The composition of the Earth's early atmosphere is a subject of continuing debate. In particular, it has been suggested that elevated concentrations of atmospheric carbon dioxide would have been necessary to maintain normal surface temperatures in the face of lower solar luminosity in early Earth history. Fossil weathering profiles, known as palaeosols, have provided semi-quantitative constraints on atmospheric oxygen partial pressure (pO2) before 2.2 Gyr ago. Here we use the same well studied palaeosols to constrain atmospheric pCO2 between 2.75 and 2.2 Gyr ago. The observation that iron lost from the tops of these profiles was reprecipitated lower down as iron silicate minerals, rather than as iron carbonate, indicates that atmospheric pCO2 must have been less than 10(-1.4) atm--about 100 times today's level of 360 p.p.m., and at least five times lower than that required in one-dimensional climate models to compensate for lower solar luminosity at 2.75 Gyr. Our results suggest that either the Earth's early climate was much more sensitive to increases in pCO2 than has been thought, or that one or more greenhouse gases other than CO2 contributed significantly to the atmosphere's radiative balance during the late Archaean and early Proterozoic eons. 相似文献
135.
136.
采用雪崩热电子注入技术和高频C-V准静态C-V特性测试,研究了新型快速热氮化的SiOxNy介质膜界面陷阱的特征,侧重于研究界面陷阱的特性与分布。结果表明:这种SiOxNy薄膜禁带中央界面陷阱密度随氮化时间的分布变化呈现”回转效应“,且存在着不同类型、密度悬殊很大的电子陷阱、指出雪崩热电子注入过程中在Si/SiOxNy界面上产生两种性质不同的快界面态陷阱;给出了这两种界面态陷阱密度在禁带中能量的分布 相似文献
137.
The smectite to illite transformation in active geothermal systems of New Zealand can be simulated by a first-order reaction
kinetic model, which provides direct estimates about the minumum ages of active geothermal systems themselves. The derived
kinetic values show that the smectite to illite transformation is sensitive to both temperature and time. 相似文献
138.
Determination of tectonic background for sedimentary basin from sandstone geochemistry 总被引:2,自引:0,他引:2
The chemical composition of sedimentary rocks is mainly controlled by source rocks. Through examining chemical composition
of terrigenous rocks, the tectonic evolution of a sedimentary basin can be modeled. The Turpan Basin belongs to a continental
sedimentary basin and its source rocks are derived from the upper continental crust. 相似文献
139.
140.