全文获取类型
收费全文 | 39033篇 |
免费 | 104篇 |
国内免费 | 196篇 |
专业分类
系统科学 | 198篇 |
丛书文集 | 650篇 |
教育与普及 | 70篇 |
理论与方法论 | 164篇 |
现状及发展 | 17883篇 |
研究方法 | 1591篇 |
综合类 | 18145篇 |
自然研究 | 632篇 |
出版年
2013年 | 331篇 |
2012年 | 549篇 |
2011年 | 1177篇 |
2010年 | 243篇 |
2008年 | 716篇 |
2007年 | 759篇 |
2006年 | 769篇 |
2005年 | 733篇 |
2004年 | 748篇 |
2003年 | 671篇 |
2002年 | 670篇 |
2001年 | 1154篇 |
2000年 | 1081篇 |
1999年 | 762篇 |
1992年 | 744篇 |
1991年 | 529篇 |
1990年 | 598篇 |
1989年 | 612篇 |
1988年 | 590篇 |
1987年 | 652篇 |
1986年 | 592篇 |
1985年 | 783篇 |
1984年 | 622篇 |
1983年 | 477篇 |
1982年 | 444篇 |
1981年 | 468篇 |
1980年 | 580篇 |
1979年 | 1166篇 |
1978年 | 981篇 |
1977年 | 952篇 |
1976年 | 806篇 |
1975年 | 842篇 |
1974年 | 1127篇 |
1973年 | 1019篇 |
1972年 | 1054篇 |
1971年 | 1156篇 |
1970年 | 1468篇 |
1969年 | 1178篇 |
1968年 | 1175篇 |
1967年 | 1137篇 |
1966年 | 977篇 |
1965年 | 693篇 |
1964年 | 231篇 |
1959年 | 388篇 |
1958年 | 683篇 |
1957年 | 457篇 |
1956年 | 403篇 |
1955年 | 368篇 |
1954年 | 396篇 |
1948年 | 285篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
61.
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
62.
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
63.
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
64.
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
65.
66.
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
67.
68.
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
69.
M. Ishibashi M. Tsuda Y. Ohizumi T. Sasaki J. Kobayashi 《Cellular and molecular life sciences : CMLS》1991,47(3):299-300
Summary A new bromotyrosine-derived alkaloid with antileukemic activity, purealidin A (5), has been isolated from the Okinawan marine spongePsammaplysilla purea and its chemical structure elucidated on the basis of the spectroscopic data. 相似文献
70.
Aeromonas caviae isolated from stools of diarrheic formula-fed infants and environmental sources produce acetic acid when grown in glucose broth, which is bactericidal (suicide phenomenon). A. caviae grows anaerobically in a minimal medium or under permissive conditions such as the intestinal tract of formula-fed infants. These isolates adhered to HEp-2 cells and produced a cytotoxic and a cytotonic enterotoxin which underscore their enteropathogenicity. 相似文献