全文获取类型
收费全文 | 55416篇 |
免费 | 265篇 |
国内免费 | 550篇 |
专业分类
系统科学 | 1231篇 |
丛书文集 | 243篇 |
教育与普及 | 298篇 |
理论与方法论 | 515篇 |
现状及发展 | 31963篇 |
研究方法 | 701篇 |
综合类 | 19089篇 |
自然研究 | 2191篇 |
出版年
2013年 | 812篇 |
2012年 | 574篇 |
2011年 | 2577篇 |
2009年 | 625篇 |
2008年 | 803篇 |
2007年 | 864篇 |
2006年 | 965篇 |
2005年 | 1131篇 |
2004年 | 2287篇 |
2003年 | 1889篇 |
2002年 | 1563篇 |
2001年 | 1323篇 |
2000年 | 882篇 |
1999年 | 954篇 |
1998年 | 649篇 |
1997年 | 772篇 |
1996年 | 534篇 |
1994年 | 683篇 |
1993年 | 690篇 |
1992年 | 855篇 |
1991年 | 748篇 |
1990年 | 850篇 |
1989年 | 657篇 |
1988年 | 637篇 |
1987年 | 657篇 |
1986年 | 742篇 |
1985年 | 855篇 |
1984年 | 750篇 |
1983年 | 690篇 |
1982年 | 791篇 |
1981年 | 832篇 |
1980年 | 913篇 |
1979年 | 1387篇 |
1978年 | 1269篇 |
1977年 | 1229篇 |
1976年 | 1088篇 |
1975年 | 1102篇 |
1974年 | 975篇 |
1973年 | 1192篇 |
1972年 | 1263篇 |
1971年 | 1323篇 |
1970年 | 1374篇 |
1969年 | 1257篇 |
1968年 | 1244篇 |
1967年 | 1073篇 |
1966年 | 906篇 |
1965年 | 725篇 |
1964年 | 503篇 |
1958年 | 593篇 |
1957年 | 508篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
361.
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
362.
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
363.
The INDSCAL individual differences scaling model is extended by assuming dimensions specific to each stimulus or other object, as well as dimensions common to all stimuli or objects. An alternating maximum likelihood procedure is used to seek maximum likelihood estimates of all parameters of this EXSCAL (Extended INDSCAL) model, including parameters of monotone splines assumed in a quasi-nonmetric approach. The rationale for and numerical details of this approach are described and discussed, and the resulting EXSCAL method is illustrated on some data on perception of musical timbres. 相似文献
364.
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
365.
366.
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
367.
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
368.
介绍铁路站场CAD系统中面向对象用户界面的设计与研究,该系统用户界面的设计充分分考虑了铁路站场CAD系统的特点和工程应用,采用面向对象的方法与技术,实现了一个高度图形化和交互式,适用于铁路站场设计工作需要的图用户界面。 相似文献
369.
用载荷松弛法对Fe-28Al-2Ti合金在较高温度下的变形行为进行了研究.测定了Fe-28Al-2Ti合金在高温变形时的载荷松弛曲线、应力减小因子Y及激活能值,并对试样进行了显微结构观察.结果表明,Fe-28Al-2Ti合金的高温变形是一个亚晶界吸收位错,而且不断向大角晶界转变的过程. 相似文献
370.