全文获取类型
收费全文 | 15173篇 |
免费 | 28篇 |
国内免费 | 47篇 |
专业分类
系统科学 | 64篇 |
丛书文集 | 181篇 |
教育与普及 | 32篇 |
理论与方法论 | 73篇 |
现状及发展 | 6197篇 |
研究方法 | 788篇 |
综合类 | 7628篇 |
自然研究 | 285篇 |
出版年
2013年 | 105篇 |
2012年 | 264篇 |
2011年 | 543篇 |
2010年 | 105篇 |
2008年 | 323篇 |
2007年 | 307篇 |
2006年 | 321篇 |
2005年 | 334篇 |
2004年 | 322篇 |
2003年 | 281篇 |
2002年 | 261篇 |
2001年 | 458篇 |
2000年 | 442篇 |
1999年 | 311篇 |
1992年 | 267篇 |
1991年 | 197篇 |
1990年 | 226篇 |
1989年 | 219篇 |
1988年 | 223篇 |
1987年 | 218篇 |
1986年 | 205篇 |
1985年 | 295篇 |
1984年 | 231篇 |
1983年 | 158篇 |
1982年 | 167篇 |
1981年 | 152篇 |
1980年 | 182篇 |
1979年 | 418篇 |
1978年 | 309篇 |
1977年 | 307篇 |
1976年 | 289篇 |
1975年 | 323篇 |
1974年 | 385篇 |
1973年 | 365篇 |
1972年 | 382篇 |
1971年 | 436篇 |
1970年 | 550篇 |
1969年 | 462篇 |
1968年 | 469篇 |
1967年 | 433篇 |
1966年 | 384篇 |
1965年 | 287篇 |
1964年 | 89篇 |
1959年 | 169篇 |
1958年 | 297篇 |
1957年 | 199篇 |
1956年 | 185篇 |
1955年 | 150篇 |
1954年 | 185篇 |
1948年 | 138篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
添加铬合金化和复合变质处理对白口铸铁组织性能的影响 总被引:1,自引:0,他引:1
在熔炼过程中通过对熔体进行复合变质处理,制备普通白口铸铁和含铬白口铸铁试样及其在相同铸造条件下的变质试样;对试样进行金相显微组织观察、碳化物定量分析和宏观硬度测量,研究添加铬合金化和复合变质处理对普通白口铸铁碳化物类型、形态、分布和性能的影响。研究结果表明,铸铁铬含量较低时,碳化物类型为(Fe,Cr)3C或(Fe,Cr)3C (Cr,Fe)7C3,呈粗大网状结构;经复合变质处理后,碳化物变得孤立、分散,网状结构被消除;随着铬含量增加,碳化物全部转变为(Cr,Fe)7C3,共晶团中碳化物呈菊花状分布,并在共晶团心部附近出现近似六方形的块状(Cr,Fe)7C3碳化物;经复合变质处理后,共晶碳化物变的细小分散、分布均匀,菊花状形态消失,但六方形(Cr,Fe)7C3碳化物仍然存在;白口铸铁经复合变质处理后,其洛氏(HRC)硬度比变质前有明显提高。 相似文献
992.
Phylogenetic position of sponges in early metazoan evolution and bionic applications of siliceous sponge spicules 总被引:1,自引:0,他引:1
Werner E G MULLER WANG XiaoHong ZENG LingSen Heinz C SCHRODER 《科学通报(英文版)》2007,52(22):3029-3040
Sponges are the oldest and the simplest but not primitive multicellular animals. They represent the earliest evolutionary metazoan phylum still extant. It was a long and painful scientific process to posi-tion the most enigmatic and mysterious metazoan,the Porifera,into their correct phylogenetic place among the eukaryotes in general and multicellular animals in particular. As living fossils,sponges provide the best evidence for the early evolution of Metazoa. More recently,interest has been focused on the bionic applications of sponges' siliceous spicules,after the discovery of their unique structure and high fiber performance. In this review,the emergence of sponges,evolutionary novelties found in sponges,and the phylogenetic position of sponges in early metazoan evolution are highlighted. In ad-dition,the present state of knowledge on silicatein-mediated "biosilica" formation in marine sponges,including the involvement of other molecules in silica metabolism and their potential application in nanobiotechnology and medicine,is given. 相似文献
993.
Convergent evolution of cytokines. 总被引:1,自引:0,他引:1
994.
995.
This paper argues that Venezuelan community organizations (COs) are playing a substantial role in the building of a new social order in which elements such as rights, equity, and justice are being displaced from a universal narrative toward more local narratives. This displacement is enwrapped in a discourse on the political participation of citizens in the running of public affairs, which implies a redefinition of the role of COs and the state beyond the usually accepted sphere of COs. The need to unfold the sense of COs thus becomes paramount for understanding our present and its current manifestations of an epochal change. 相似文献
996.
P. Milner V. Ralevic A. M. Hopwood E. Fehér J. Lincoln K. A. Kirkpatrick G. Burnstock 《Cellular and molecular life sciences : CMLS》1989,45(2):121-125
Substance P and choline acetyltransferase have been localised in a small proportion of endothelial cells of rat coronary arteries using electron microscopic immunocytochemistry. During a hypoxic period of 1 min, coronary vasodilatation was produced in the Langendorff heart preparation and increased levels of substance P and acetylcholine were released into the perfusate. The possibility that these substances are released from endothelial cells during hypoxia and contribute to the hyperaemic response is discussed. 相似文献
997.
Direct evidence that reverse cholesterol transport is mediated by high-density lipoprotein in rabbit 总被引:2,自引:0,他引:2
Mammalian cells obtain cholesterol for membrane synthesis mostly via the receptor-mediated endocytosis of low-density lipoprotein (LDL). Macrophages and vascular endothelium additionally have receptors that recognize certain modified forms of LDL (for example, acetyl-LDL). The process by which cholesterol returns from peripheral cells to hepatocytes (reverse cholesterol transport) has not been established; although tissue culture studies have favoured high-density lipoprotein (HDL) as the principal vehicle, the in vivo evidence for this is meagre. When cholesterol-loaded macrophages are incubated in medium containing plasma, cholesterol moves from the cells to HDL and is then esterified by lecithin/cholesterol acyltransferase. The accumulation of cholesteryl esters in the particles increases their size and decreases their density; enrichment with apoprotein E (apo E) also occurs, producing a decrease in electrophoretic mobility. We now report that similar changes occur in the circulating HDL of rabbits, when their peripheral tissues are loaded with cholesterol by intravenous (i.v.) injection of acetylated or native human LDL. This result suggests that HDL is involved in reverse cholesterol transport in vivo. 相似文献
998.
999.
Cooperative tandem binding of met repressor of Escherichia coli 总被引:10,自引:0,他引:10
S E Phillips I Manfield I Parsons B E Davidson J B Rafferty W S Somers D Margarita G N Cohen I Saint-Girons P G Stockley 《Nature》1989,341(6244):711-715
We present biochemical and genetic data to support the hypothesis that the Escherichia coli met repressor, MetJ, binds to synthetic and natural operator sequences in tandem arrays such that repression depends not only on the affinity of the DNA-protein interaction, but also on protein-protein contacts along the tandem array. This represents a novel form of regulatory switch. Furthermore, there seems to be homology between the organization of the met and trp operators. 相似文献
1000.
Nonspecific reaction of a thiol:Protein disulfide oxidoreductase with the disulfide bonds of insulin
M. Pace P. G. Pietta A. Fiorino E. Pocaterra J. E. Dixon 《Cellular and molecular life sciences : CMLS》1985,41(10):1332-1335
Summary A thiol:protein disulfide oxidoreductase from bovine liver was isolated after separation from protein disulfide isomerase. The enzyme, after activation (reduction) with glutathione, was reacted with stoichiometric amounts of insulin and the sulfhydryl groups of the partially reduced hormone were labeled with iodo (l-14C)acetamide. After separation of the insulin chains, the radioactivity was found in both the peptides, with a ratio A-chain/B-chain equal to 2/1. 相似文献