全文获取类型
收费全文 | 41921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 | 17812篇 |
研究方法 | 1586篇 |
综合类 | 20623篇 |
自然研究 | 842篇 |
出版年
2014年 | 234篇 |
2013年 | 531篇 |
2012年 | 775篇 |
2011年 | 1633篇 |
2010年 | 495篇 |
2009年 | 446篇 |
2008年 | 921篇 |
2007年 | 1073篇 |
2006年 | 1069篇 |
2005年 | 956篇 |
2004年 | 827篇 |
2003年 | 709篇 |
2002年 | 708篇 |
2001年 | 1220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 | 587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 | 1148篇 |
1970年 | 1427篇 |
1969年 | 1131篇 |
1968年 | 1120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 15 毫秒
51.
A. Gutnisky P. Glikman M. F. Gimeno A. L. Gimeno 《Cellular and molecular life sciences : CMLS》1982,38(1):116-117
Summary The administration of 15(R)-15-methyl prostaglandin E2 (15(R)-15-M-PGE2) in vivo significantly diminished the uptake of59Fe into blood, spleen, liver, femur and dried intestine of rats, whereas acetylsalicylic acid (ASA) increased the counts significantly. This effect of ASA was counteracted by 15(R)-15-M-PGE2. It is suggested that prostaglandins (PGs) might play an important role in inhibiting iron absorption at the intestinal level.This work was supported by grant No.6638 from CONICET (Argentina). The technical assistance of Mrs María E. Castro and Norma Rizzo is gratefully acknowleged. 相似文献
52.
比较1,1-双(2-叔丁基砜乙基)十二烷基苯基砜(1)双碳负离子在不同条件下的环合反应结果,控制反应条件,在铁催化下可以生成偶联糁烯化的环烯产物(2)为主要产物;不加催化剂,加热回流反应得到分子内亲核取代反应产物环丙烷化合物(3)。 相似文献
53.
During the evolution of proteins the pressure to optimize biological activity is moderated by a need for efficient folding. For most proteins, this is accomplished through spontaneous folding to a thermodynamically stable and active native state. However, in the extracellular bacterial alpha-lytic protease (alphaLP) these two processes have become decoupled. The native state of alphaLP is thermodynamically unstable, and when denatured, requires millennia (t1/2 approximately 1,800 years) to refold. Folding is made possible by an attached folding catalyst, the pro-region, which is degraded on completion of folding, leaving alphaLP trapped in its native state by a large kinetic unfolding barrier (t1/2 approximately 1.2 years). alphaLP faces two very different folding landscapes: one in the presence of the pro-region controlling folding, and one in its absence restricting unfolding. Here we demonstrate that this separation of folding and unfolding pathways has removed constraints placed on the folding of thermodynamically stable proteins, and allowed the evolution of a native state having markedly reduced dynamic fluctuations. This, in turn, has led to a significant extension of the functional lifetime of alphaLP by the optimal suppression of proteolytic sensitivity. 相似文献
54.
Surface Preparation is very important in adhesive b on ding of spray coatings to the surface of a work piece. The common practice is gr it-blasting of the surface before subjecting it to the spray coating process. In this study, grit-blasting of an AISI 4130 steel (of different heat treatmen ts) with Al 2O 3 particles was studied. Various grit-blasting parameters such as blasting particle size, the distance between blasting nozzle and the work pi ece (25, 30 and 40 cm.), blasting pressure (3,4,5,6 and ... 相似文献
55.
采用交流能源的低功耗DCVSL电路研究 总被引:2,自引:0,他引:2
从改变CMOS电路中能量转换模式的观点出发,研究利用渐变功率时钟的低功耗CMOS电路设计. 首先讨论钟控功率信号的代数表示及有关性质,然后归纳采用直流能源的互补CMOS逻辑门转化为采用交流能源的钟控CMOS门电路的设计过程. 在此基础上进一步提出采用交流能源的DCVSL电路设计. 采用正弦功率时钟的PSPICE模拟证实了钟控DCVSL电路具有正确的逻辑功能及低功耗工作的特点. 最后提出了一种将钟控信号转换为标准CMOS逻辑电平的接口电路并用计算机模拟验证了它的有效性. 相似文献
56.
A. E. Yassen H. A. Hassan L. S. Kawashti 《Cellular and molecular life sciences : CMLS》1994,50(11-12):1111-1114
A detailed karyotypic analysis of two Egyptian species of bats,Taphozous perforatus andTaphozous nudiventris, was made on the basis of conventional data and G-band patterns. No detectable karyotypic difference was found between the two species (2n=42, F.N. 64). By comparison of G-band patterns, some differences between the two species were seen in the corresponding autosomal pairs. These results are reported for the first time in Egypt. 相似文献
57.
A diffusion chamber technique based on time-lag analysis for the estimation of effective diffusion coefficients of radiolabelled macromolecules of varying molecular weights through native mucus gel is reported. For all solutes studied, a reduction in effective diffusion coefficients was observed with a retardation of solute flux in both aqueous and mucus layers. Over the molecular weight range of solutes investigated (126-186,000 Daltons), a consistent effect of molecular weight was evident with regard to the retarding effect of mucus. No apparent or absolute molecular weight cut-off for macromolecular transfer was exhibited. However, at high molecular weights (greater than 30,000 Daltons) the retardation was greatly enhanced. The results confirm that mucus can be regarded as a gel with finite pores, but that it does not constitute an absolute barrier to even high molecular weight solutes. 相似文献
58.
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
59.
We investigated the sound communication associated with mating inNezara viridula (L.) to clarify conflicting reports on the medium through which acoustic signals are borne. Recordings made from virgin pairs ofN. viridula during precopulatory and compulatory behaviour have revealed that signals transmitted as vibrations through the substrate are a more likely means of communication than signals transmitted by a radiation of sound pressure waves through the air (at least in Australian populations). We present evidence (additional to that already in the literature) that air-borne acoustics are an unlikely mechanism of sexual communication in this species. Specifically, the recorded pulse train patterns showed no consistency or repeatability in relation to different stages of mating behaviour. In contrast, frequency spectrograms of substrate-borne signals reveal repeatable patterns that do correspond with the stage of mating behaviour. 相似文献
60.