全文获取类型
收费全文 | 32408篇 |
免费 | 150篇 |
国内免费 | 305篇 |
专业分类
系统科学 | 453篇 |
丛书文集 | 494篇 |
教育与普及 | 109篇 |
理论与方法论 | 96篇 |
现状及发展 | 12641篇 |
研究方法 | 1297篇 |
综合类 | 17110篇 |
自然研究 | 663篇 |
出版年
2013年 | 407篇 |
2012年 | 667篇 |
2011年 | 1332篇 |
2010年 | 451篇 |
2009年 | 442篇 |
2008年 | 782篇 |
2007年 | 907篇 |
2006年 | 908篇 |
2005年 | 832篇 |
2004年 | 645篇 |
2003年 | 603篇 |
2002年 | 563篇 |
2001年 | 1010篇 |
2000年 | 1036篇 |
1999年 | 690篇 |
1994年 | 340篇 |
1992年 | 578篇 |
1991年 | 463篇 |
1990年 | 511篇 |
1989年 | 466篇 |
1988年 | 426篇 |
1987年 | 477篇 |
1986年 | 462篇 |
1985年 | 585篇 |
1984年 | 454篇 |
1983年 | 380篇 |
1982年 | 382篇 |
1981年 | 347篇 |
1980年 | 386篇 |
1979年 | 952篇 |
1978年 | 697篇 |
1977年 | 708篇 |
1976年 | 575篇 |
1975年 | 615篇 |
1974年 | 793篇 |
1973年 | 704篇 |
1972年 | 665篇 |
1971年 | 764篇 |
1970年 | 991篇 |
1969年 | 795篇 |
1968年 | 791篇 |
1967年 | 786篇 |
1966年 | 717篇 |
1965年 | 539篇 |
1959年 | 274篇 |
1958年 | 455篇 |
1957年 | 316篇 |
1956年 | 283篇 |
1955年 | 284篇 |
1954年 | 262篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
81.
82.
Nras loss induces metastatic conversion of Rb1-deficient neuroendocrine thyroid tumor 总被引:2,自引:0,他引:2
Takahashi C Contreras B Iwanaga T Takegami Y Bakker A Bronson RT Noda M Loda M Hunt JL Ewen ME 《Nature genetics》2006,38(1):118-123
Mutations in the gene encoding the retinoblastoma tumor suppressor predispose humans and mice to tumor development. Here we have assessed the effect of Nras loss on tumor development in Rb1 heterozygous mice. Loss of one or two Nras alleles is shown to significantly reduce the severity of pituitary tumors arising in Rb1(+/-) animals by enhancing their differentiation. By contrast, C-cell thyroid adenomas occurring in Rb1(+/-) mice progress to metastatic medullary carcinomas after loss of Nras. In Rb1(+/-)Nras(+/-) animals, distant medullary thyroid carcinoma metastases are associated with loss of the remaining wild-type Nras allele. Loss of Nras in Rb1-deficient C cells results in elevated Ras homolog family A (RhoA) activity, and this is causally linked to the invasiveness and metastatic behavior of these cells. These findings suggest that the loss of the proto-oncogene Nras in certain cellular contexts can promote malignant tumor progression. 相似文献
83.
The gas-liquid-liquid reactive extraction system for preparing hydrogen peroxide via anthraquinone was investigated. The oxidation reaction of hydrogenated working solution was combined with the extraction of hydrogen peroxide from working solution in a sieve plate column. The reaction of 2-ethylanthrahydroquionone with oxygen and the liquid-liquid extraction of hydrogen peroxide take place simultaneously. The oxygen was introduced with hydrogenated working solution through a nozzle in the bottom of the column, which worked as agitated air as well as oxidation reagent. The results showed the oxidation and extraction do not hamper each other, on the contrary, the presence of oxidation gas in the column can promote the transfer of hydrogen peroxide from organic phase to aqueous phase, thus the reaction efficiency and extraction efficiency increased with increasing gas superficial velocity. Furthermore, the oxidation efficiency is almost 100% and the extraction efficiency is higher than 90% in this process. 相似文献
84.
A latent class probit model for analyzing pick any/N data 总被引:3,自引:3,他引:0
A latent class probit model is developed in which it is assumed that the binary data of a particular subject follow a finite
mixture of multivariate Bermoulli distributions. An EM algorithm for fitting the model is described and a Monte Carlo procedure
for testing the number of latent classes that is required for adequately describing the data is discussed. In the final section,
an application of the latent class probit model to some intended purchase data for residential telecommunication devices is
reported.
Geert De Soete is supported as “Bevoegdverklaard Navorser” of the Belgian “Nationaal Fonds voor Wetenschappelijk Onderzoek.” 相似文献
85.
86.
提出了一个关于氧化物半导瓷晶界势垒起源的新观点,认为晶界势垒起源于烧结过程中外界氧在晶界中的扩散,与材料的结构、化学缺陷、掺杂、外界气氛、烧结工艺、组成状态等有密切关系,并用此理论解释了许多实验现象。 相似文献
87.
论述了交织编码的工作原理,给出了一种交织扩展Golay码的改进型软件设计方案,用TMS320C25实现了其编译码器并给出了模拟实验结果。实验结果表明,它能准确地纠正不大于72比特位的差错;处理128×128×8比特位的一幅图像数据的运行时间为0.412s. 相似文献
88.
Aspirin-like drugs may block pain independently of prostaglandin synthesis inhibition 总被引:2,自引:0,他引:2
K. Brune W. S. Beck G. Geisslinger S. Menzel-Soglowek B. M. Peskar B. A. Peskar 《Cellular and molecular life sciences : CMLS》1991,47(3):257-261
Summary Using flurbiprofen, a chiral anti-inflammatory and analgesic 2-arylpropionic acid derivative, the enantiomers of which are not converted to each other (less than 5%) in rats or man, we obtained evidence that prostaglandin synthesis inhibition is primarily mediating the anti-inflammatory activity but prostaglandin synthesis independent mechanisms contribute to the analgesic effects. Thus, the S-form inhibited prostaglandin synthesis, inflammation and nociception in rats. The R-form had much less effect on prostaglandin synthesis and did not affect inflammation. It did, however, block nociception in rats almost as potently as the S-form. S-flurbiprofen, in contrast to the R-form, was clearly ulcerogenic in the gastrointestinal mucosa. These results indicate additional molecular mechanisms of analgesia and suggest the use of R-arylpropionic acids as analgesics. 相似文献
89.
史济群 《华中科技大学学报(自然科学版)》1994,(3)
报道一种新颖的激光开槽、埋槽电极硅太阳电池的结构、工艺流程及其研制结果。在AM1.5,25℃,100mW/cm2的条件下,以面积为45cm3的36片硅片研制的硅太阳电池的输出参数的平均值为JSC=36.1mA/cm2,V∞=633mV,F.F.=0.798,η=18.23%,最后分析了种高性能硅太阳电池的设计特点。 相似文献
90.
Low-molecular-weight GTP-binding proteins are strong candidates for regulators of membrane traffic. In yeast, mutations in the sec4 or ypt1 genes encoding small GTP-binding proteins inhibit constitutive membrane flow at the plasma membrane or Golgi complex, respectively. It has been suggested that membrane fusion-fission events are regulated by cycling of small GTP-binding proteins between a membrane-bound and free state, but although most of these small proteins are found in both soluble and tightly membrane-bound forms, there is no direct evidence to support such cycling. In rat brain a small GTP-binding protein, rab3A, is exclusively associated with synaptic vesicles, the secretory organelles of nerve terminals. Here we use isolated nerve terminals to study the fate of rab3A during synaptic vesicle exocytosis. We find that rab3A dissociates quantitatively from the vesicle membrane after Ca2(+)-dependent exocytosis and that this dissociation is partially reversible during recovery after stimulation. These results are direct evidence for an association-dissociation cycle of a small GTP-binding protein during traffic of its host membrane. 相似文献