全文获取类型
收费全文 | 41849篇 |
免费 | 176篇 |
国内免费 | 375篇 |
专业分类
系统科学 | 516篇 |
丛书文集 | 835篇 |
教育与普及 | 109篇 |
理论与方法论 | 149篇 |
现状及发展 | 17811篇 |
研究方法 | 1586篇 |
综合类 | 20552篇 |
自然研究 | 842篇 |
出版年
2014年 | 234篇 |
2013年 | 531篇 |
2012年 | 775篇 |
2011年 | 1632篇 |
2010年 | 495篇 |
2009年 | 446篇 |
2008年 | 921篇 |
2007年 | 1072篇 |
2006年 | 1066篇 |
2005年 | 956篇 |
2004年 | 827篇 |
2003年 | 708篇 |
2002年 | 708篇 |
2001年 | 1219篇 |
2000年 | 1216篇 |
1999年 | 838篇 |
1992年 | 759篇 |
1991年 | 579篇 |
1990年 | 625篇 |
1989年 | 602篇 |
1988年 | 580篇 |
1987年 | 667篇 |
1986年 | 601篇 |
1985年 | 742篇 |
1984年 | 609篇 |
1983年 | 467篇 |
1982年 | 457篇 |
1981年 | 477篇 |
1980年 | 586篇 |
1979年 | 1194篇 |
1978年 | 976篇 |
1977年 | 980篇 |
1976年 | 799篇 |
1975年 | 859篇 |
1974年 | 1173篇 |
1973年 | 1023篇 |
1972年 | 1028篇 |
1971年 | 1146篇 |
1970年 | 1426篇 |
1969年 | 1131篇 |
1968年 | 1121篇 |
1967年 | 1123篇 |
1966年 | 968篇 |
1965年 | 703篇 |
1959年 | 383篇 |
1958年 | 635篇 |
1957年 | 437篇 |
1956年 | 373篇 |
1955年 | 366篇 |
1954年 | 352篇 |
排序方式: 共有10000条查询结果,搜索用时 804 毫秒
391.
392.
分别用二次电子扫描线法和垂直抛面法对不同回火制度的30SiMnCrNiMoB钢的冲击断口表面的分维数进行了数量。通过研究发现:平行裂纹扩展方向的分维更适合于描述材料韧性的变化。 相似文献
393.
用AM1MO方法计算了质子化卟吩的构型,并通过结构分析、电荷布局分析和前线轨道分析,讨论了其质子化过程的构型变化及Raman光谱变化的原因。 相似文献
394.
从疲劳过程本质上是材料静强度不断退化过程的观点出发,建立了一个强度退化模型.将模型应用于两级、多级和随机载荷下疲劳寿命估算,经试验和算例验证,表明该方法是合理可行的 相似文献
395.
396.
采用离子束增强沉积方法(IBED),在Fe基上制备了TiN薄膜。通过划痕仪,透射电镜(TEM),俄歇电子能谱(AES),粘着力测量仪的观测,分析和计算,给出了膜的形貌,组分和机械特性;发现TiN膜与衬底之间存在一过渡区,膜中含氧量比气相真空沉积(CVD)明显减小。在AES定量计算中,本文采用一种与文献不同的方法,较为简便地处理了俄歇谱中Ti和N峰的重叠现象 相似文献
397.
A. A. Shaheen A. A. Abd El-Fattah M. Z. Gad 《Cellular and molecular life sciences : CMLS》1996,52(4):336-339
The level of malondialdehyde (MDA), an index of lipid peroxidation, and the antioxidants superoxide dismutase (SOD) and glutathione (GSH), as well as the activity of Na+, K+-ATPase, were assessed in whole rat brain after immobilization, anemic hypoxia (NaNO2) and 72 h starvation. The effect of these stressors on plasma glucose and corticosterone levels was also observed. Hypoxia and starvation stimulated the lipidj peroxide formation in braini as indicated by an increase in the level of MDA, being higher after starvation than hypoxia. Brain SOD activity was also increased in response to hypoxia and starvation while GSH content was only diminished ini hypoxia. However, neither MDA nor antioxidants were affected by immobilization. On the other hand, the activity of brain Na+, K+-ATPase was significantly increased by immobilization and hypoxia but decreased in starvation. A similar pattern of change was also observed in plasma glucose and corticosterone levels in response to these stressors. These results elucidate differences in the biochemical response of animals towards various types of stress, with increased lipid peroxide formation in hypoxia and starvation. 相似文献
398.
Antioxidant survey to assess antagonism to redox stress using a prokaryotic and an eukaryotic system
H. Baker B. DeAngelis O. Frank M. Khalil S. H. Hutner E. R. Baker 《Cellular and molecular life sciences : CMLS》1996,52(6):597-599
Using a prokaryote (Escherichia coli) and a metazoa-resembling eukaryote (Ochromonas danica), we surveyed antioxidants which might overcome redox stress imposed by menadione sodium bisulphite (MD) and buthionine sulphoximine (BSO). BSO oxidant stress was evident only inO. danica; MD oxidant stress was evident in both organisms. Glutathione, its precursors, e.g. cysteine, homocysteine, and 2-oxo-4-thiazolidine carboxylic acid, and red blood cells, emerged as prime antioxidants for relieving BSO and MD oxidant stress. BSO and MD oxidant activity and antioxidant-annulling effect inO. danica were judged comparable to those found in animal cells whereas the resultsE. coli were not entirely equivalent. TheO. danica system emerged as a practical, rapid, and useful system for pinpointing oxidant stressors and antioxidants, and shows promise for studies with mammalian systems. 相似文献
399.
E. S. Fiala R. S. Sodum M. Bhattacharya H. Li 《Cellular and molecular life sciences : CMLS》1996,52(9):922-926
Reaction with peroxynitrite at pH 7.4 and 37°C was found to increase the 8-oxodeoxyguanosine levels in calf thymus DNA 35-38-fold. This oxidation of deoxyguanosine, as well as the peroxynitrite-mediated nitration of tyrosine to 3-nitrotyrosine, was significantly inhibited by ascorbic acid, glutathione and (–)-epigallocatechin gallate, a polyphenolic antioxidant present in tea. For 50% inhibition of the oxidation of deoxyguanosine to 8-oxodeoxyguanosine, 1.1, 7.6 of 0.25 mM ascorbate, glutathione or (–)-epigallocatechin gallate, respectively, was required. For 50% inhibition of tyrosine nitration, the respective concentrations were 1.4, 4.6 or 0.11 mM. Thus, (–)-epigallocatechin gallate is a significantly better inhibitor of both reactions than either ascorbate or glutathione. Reaction of (–)-epigallocatechin gallate with peroxynitrite alone resulted in the formation of a number of products. Ultraviolet spectra of two of these suggest that the tea polyphenol and/or its oxidation products are nitrated by peroxynitrite. 相似文献
400.
应用一次回归正交试验法 ,对影响中药熨剂生产工艺的主要因素—加热温度、醋酸的浓度及醋淬次数进行了考察 ,得出在设计条件下的回归方程 y=73.95+ 1 .95x1 - 1 .8x2 - 2 .8x1 x2 ,并以此为数学模型 ,对熨剂生产工艺的优化区进行有效地控制 相似文献