全文获取类型
收费全文 | 90篇 |
免费 | 3篇 |
国内免费 | 4篇 |
专业分类
系统科学 | 1篇 |
丛书文集 | 1篇 |
理论与方法论 | 1篇 |
现状及发展 | 17篇 |
研究方法 | 13篇 |
综合类 | 63篇 |
自然研究 | 1篇 |
出版年
2021年 | 1篇 |
2019年 | 1篇 |
2018年 | 6篇 |
2017年 | 1篇 |
2014年 | 1篇 |
2013年 | 2篇 |
2012年 | 8篇 |
2011年 | 11篇 |
2010年 | 7篇 |
2009年 | 2篇 |
2008年 | 8篇 |
2007年 | 2篇 |
2006年 | 14篇 |
2005年 | 4篇 |
2004年 | 10篇 |
2003年 | 4篇 |
2002年 | 1篇 |
1999年 | 1篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1989年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有97条查询结果,搜索用时 15 毫秒
21.
22.
Leonard DC Filippenko AV Ganeshalingam M Serduke FJ Li W Swift BJ Gal-Yam A Foley RJ Fox DB Park S Hoffman JL Wong DS 《Nature》2006,440(7083):505-507
An important and perhaps critical clue to the mechanism driving the explosion of massive stars as supernovae is provided by the accumulating evidence for asymmetry in the explosion. Indirect evidence comes from high pulsar velocities, associations of supernovae with long-soft gamma-ray bursts, and asymmetries in late-time emission-line profiles. Spectropolarimetry provides a direct probe of young supernova geometry, with higher polarization generally indicating a greater departure from spherical symmetry. Large polarizations have been measured for 'stripped-envelope' (that is, type Ic; ref. 7) supernovae, which confirms their non-spherical morphology; but the explosions of massive stars with intact hydrogen envelopes (type II-P supernovae) have shown only weak polarizations at the early times observed. Here we report multi-epoch spectropolarimetry of a classic type II-P supernova that reveals the abrupt appearance of significant polarization when the inner core is first exposed in the thinning ejecta (approximately 90 days after explosion). We infer a departure from spherical symmetry of at least 30 per cent for the inner ejecta. Combined with earlier results, this suggests that a strongly non-spherical explosion may be a generic feature of core-collapse supernovae of all types, where the asphericity in type II-P supernovae is cloaked at early times by the massive, opaque, hydrogen envelope. 相似文献
23.
24.
对用仿形磨削方法磨削硬质合金可转位刀片周边的仿形凸轮机构进行了运动学分析,根据凸轮机构的运动学特性开发出一套设计和绘制仿形凸轮的CAD和CAG软件,采用该软件可以省去大量的繁复计算与绘图工作量,缩短设计制造周期,实现设计、绘图自动化。 相似文献
25.
Energy dissipators, isolated-resistant and specific structural forms for earthquake resistance are popular topics in the research to improve shock-resistance. In this work, experimental methods were used to investigate the property of low yield strength steel. Carbon content in LYS material is lower than that in other steels; the ultimate stress is three times the yield stress. The ultimate elongation rate is about 62% and the ductility is 2-3 times that of A36 steel. In order to overcome some defects of ordinary use metallic dampers, the mechanical characteristic of low yield strength steel is used to develop added damping and stiffness for rhombic steel plate absorber. Test of the energy dissipation behavior for this newly developed device indicated that LYS could stably dissipate or absorb the input energy of earthquake. Then, the analytical model for the hysteretic behavior of this new device is proposed. Comparison of experimental data and numerical simulation results showed that this analytical model is suitable for simulating the hysteretic energy behavior of this new device. 相似文献
26.
Evolution of microRNA genes by inverted duplication of target gene sequences in Arabidopsis thaliana 总被引:8,自引:0,他引:8
Allen E Xie Z Gustafson AM Sung GH Spatafora JW Carrington JC 《Nature genetics》2004,36(12):1282-1290
27.
Sung BJ Hwang KY Jeon YH Lee JI Heo YS Kim JH Moon J Yoon JM Hyun YL Kim E Eum SJ Park SY Lee JO Lee TG Ro S Cho JM 《Nature》2003,425(6953):98-102
Phosphodiesterases (PDEs) are a superfamily of enzymes that degrade the intracellular second messengers cyclic AMP and cyclic GMP. As essential regulators of cyclic nucleotide signalling with diverse physiological functions, PDEs are drug targets for the treatment of various diseases, including heart failure, depression, asthma, inflammation and erectile dysfunction. Of the 12 PDE gene families, cGMP-specific PDE5 carries out the principal cGMP-hydrolysing activity in human corpus cavernosum tissue. It is well known as the target of sildenafil citrate (Viagra) and other similar drugs for the treatment of erectile dysfunction. Despite the pressing need to develop selective PDE inhibitors as therapeutic drugs, only the cAMP-specific PDE4 structures are currently available. Here we present the three-dimensional structures of the catalytic domain (residues 537-860) of human PDE5 complexed with the three drug molecules sildenafil, tadalafil (Cialis) and vardenafil (Levitra). These structures will provide opportunities to design potent and selective PDE inhibitors with improved pharmacological profiles. 相似文献
28.
Two new species of the family Canuellidae Lang, 1944, Scottolana jasani sp. nov. collected from Gusipo Beach, Korea and S. huysi sp. nov. from Kata Beach, Phuket Island, Thailand, are described. So far 14 species are currently known in the genus, of which only two, S. bulbifera from Russia, China, and Korea, and S. geei from China, have been reported from Asia. The new species, Scottolana jasani sp. nov. and S. huysi sp. nov., belong to the longipes-group, defined by the presence of two postgenital segments in the female, and caudal ramus with a proximal hook-like inner projection. Scottolana jasani sp. nov. closely resembles S. geei. However, it is easily distinguished by the following characters: (1) number of segments of antennal exopod (8 in the new species, but 9 in S. geei); (2) setae I, and II of caudal ramus; (3) shape of genital field in both sexes; (4) segmentation of mandibular exopod; and (5) spinular ornamentation of endopod-2 in P1. Scottolana huysi sp. nov. is also closely related to S. geei. However, it is unique and different from S. geei by (1) indistinctly 4-segmented antennule in the female; (2) 5-segmented male antennule; (3) larger genital field in female; (4) inner long seta on P4 coxa; and (5) the morphology of caudal seta II, being long and plumose, with a bulbiform base in the female only.
www.zoobank.org/urn:lsid:zoobank.org:pub:33BFE1A2-8C95-4A9D-9E7A-B495CEEC3182 相似文献
29.
Kuroki Y Toyoda A Noguchi H Taylor TD Itoh T Kim DS Kim DW Choi SH Kim IC Choi HH Kim YS Satta Y Saitou N Yamada T Morishita S Hattori M Sakaki Y Park HS Fujiyama A 《Nature genetics》2006,38(2):158-167
The mammalian Y chromosome has unique characteristics compared with the autosomes or X chromosomes. Here we report the finished sequence of the chimpanzee Y chromosome (PTRY), including 271 kb of the Y-specific pseudoautosomal region 1 and 12.7 Mb of the male-specific region of the Y chromosome. Greater sequence divergence between the human Y chromosome (HSAY) and PTRY (1.78%) than between their respective whole genomes (1.23%) confirmed the accelerated evolutionary rate of the Y chromosome. Each of the 19 PTRY protein-coding genes analyzed had at least one nonsynonymous substitution, and 11 genes had higher nonsynonymous substitution rates than synonymous ones, suggesting relaxation of selective constraint, positive selection or both. We also identified lineage-specific changes, including deletion of a 200-kb fragment from the pericentromeric region of HSAY, expansion of young Alu families in HSAY and accumulation of young L1 elements and long terminal repeat retrotransposons in PTRY. Reconstruction of the common ancestral Y chromosome reflects the dynamic changes in our genomes in the 5-6 million years since speciation. 相似文献
30.
The Oct4 and Nanog transcription network regulates pluripotency in mouse embryonic stem cells 总被引:45,自引:0,他引:45