全文获取类型
收费全文 | 2903篇 |
免费 | 143篇 |
国内免费 | 125篇 |
专业分类
系统科学 | 67篇 |
丛书文集 | 62篇 |
教育与普及 | 13篇 |
理论与方法论 | 3篇 |
现状及发展 | 24篇 |
综合类 | 2833篇 |
自然研究 | 169篇 |
出版年
2024年 | 3篇 |
2023年 | 24篇 |
2022年 | 31篇 |
2021年 | 31篇 |
2020年 | 41篇 |
2019年 | 39篇 |
2018年 | 44篇 |
2017年 | 87篇 |
2016年 | 72篇 |
2015年 | 79篇 |
2014年 | 118篇 |
2013年 | 106篇 |
2012年 | 262篇 |
2011年 | 130篇 |
2010年 | 106篇 |
2009年 | 124篇 |
2008年 | 136篇 |
2007年 | 171篇 |
2006年 | 153篇 |
2005年 | 160篇 |
2004年 | 155篇 |
2003年 | 123篇 |
2002年 | 106篇 |
2001年 | 89篇 |
2000年 | 82篇 |
1999年 | 75篇 |
1998年 | 79篇 |
1997年 | 61篇 |
1996年 | 58篇 |
1995年 | 47篇 |
1994年 | 57篇 |
1993年 | 49篇 |
1992年 | 55篇 |
1991年 | 48篇 |
1990年 | 39篇 |
1989年 | 47篇 |
1988年 | 37篇 |
1987年 | 24篇 |
1986年 | 15篇 |
1985年 | 6篇 |
1984年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有3171条查询结果,搜索用时 31 毫秒
1.
跳频通信的应用大大提高了军事装备的抗干扰和抗截获能力,使得跳频对抗技术面临严峻的挑战。为解决传统形态学跳频信号参数估计方法中结构元素选择困难问题并提高估计精度,提出了一种基于自适应形态学的跳频信号参数联合盲估计方法。首先,对跳频信号进行短时傅里叶变换获取谱图。然后,从其时间轴投影中获取结构元素尺寸的知识, 设计自适应形态学滤波器抑制谱图噪声, 提取跳频图案初步估计跳频参数。最后, 引入最小二乘估计方法, 对跳频周期和跳变时刻进行精估计。仿真结果表明,此方法能够同时估计出跳频频率、跳频周期和跳变时刻, 不需要其中某一种参数作为先验条件, 在复杂的通信环境也能够保持良好的估计性能。 相似文献
2.
使用实验轧机旁冷却装置配合轧机进行轧制实验,研究轧制道次间不同冷却工艺对特厚钢板组织和性能的影响规律.研究结果表明:采用道次间冷却工艺可以在全厚度方向获得组织细化及强韧性提高效果,采用强冷道次间冷却实验钢1/4处晶粒尺寸可细化至10μm,强度为376MPa,-40℃冲击功为169J;心部晶粒尺寸可细化至15μm,强度为360MPa,-40℃冲击功为123J.本工艺可形成470μm厚表层细晶层,晶粒尺寸可细化至5μm;粗轧道次间插入冷却工艺轧制钢板强度和冲击韧性优于中间坯冷却工艺;随冷却强度增加,钢板内部组织明显细化且强度大幅提高. 相似文献
3.
4.
6005A铝合金大截面复杂制品作为轨道车体的边梁或枕梁,对轨道车体的整体起连接和承重作用,是动车组车体的关键部位。采用电子万能试验机、铝型材压力试验机、光学显微镜等研究了速度达400 km·h-1动车组车体底边梁用6005A铝型材的强度、显微组织和断口形貌等,旨在为动车组用铝挤压型材产品检验和工艺制度制定提供技术支持与指导。 相似文献
5.
Paul D. Taylor Silviu O. Martha Dennis P. Gordon 《Journal of Natural History》2018,52(25-26):1657-1721
Genera assigned to the cheilostome bryozoan family Onychocellidae are revised based on the skeletal morphology of the type species and, when possible, the type material of these species. All genera are illustrated using scanning electron micrographs, some for the first time. Onychocellidae, which ranges from the Cenomanian stage of the Cretaceous to the Recent, has been a particularly troublesome family because of poorly defined generic concepts correlating at least in part with a paucity of morphological characters. Thirty-five genera are described in this review. Of these, two are recognised as subjective synonyms of other onychocellid genera (Rhebasia and Semieschara), one cannot be sufficiently characterised from the type material (Collura), and two are new: Aechmellina gen. nov. (type species Aechmella falcifera) and Kamilocella gen. nov. (type species Eschara latilabris). A neotype is chosen for Rhagasostoma hexagonum, the type species of Rhagasostoma. A key is provided to assist in the identification of onychocellid genera. Phylogenetic relationships between genera remain obscure and are unlikely to be fully resolved based on skeletal morphology alone. The family as an entity is loosely circumscribed and almost certainly paraphyletic, containing stem genera of other anascan familes such as Lunulitidae, Coscinopleuridae and Aspidostomatidae.
www.zoobank.org/urn:lsid:org:pub:63A31AD2-F049-42CB-A45B-557014DC286E 相似文献
6.
采用双辊薄带连铸技术制备了低碳微合金钢薄带,利用OM,SEM和TEM对铸态凝固组织、室温组织、析出及位错进行观察和分析.结果表明:低碳微合金钢铸带的凝固组织中二次枝晶间距约为12~15μm,相对于传统厚板坯和薄板坯连铸,铸带组织得到了明显细化.铸带的原奥氏体晶粒尺寸比较粗大,约为250~410μm,其组织由魏氏铁素体、珠光体和不规则铁素体组成.铸带组织中存在纳米级TiC析出和短棒状的渗碳体.TiC析出没有被薄带连铸的凝固过程及二次冷却过程明显抑制.铸带组织由于铸轧力及二次冷却速率不均匀导致大量位错的产生. 相似文献
7.
A remarkable new eucoiline genus and species, Muhaka icipe, is described herein. The genus is clearly a Kleidotomini, but is distinguished from other genera in the tribe by a unique head and scutellar morphology. The genus belongs to the ‘wedge-head’-syndrome group of species that, to date, is unique to Afrotropical eucoilines. The new genus and species is reminiscent of Stentorceps Quinlan and Nanocthulhu Buffington, but is readily distinguished from these genera. Muhaka was collected from a threatened kaya (sacred forest) of coastal Kenya. The biological importance of this and other kaya forests, as well as their protection, is discussed.http://www.zoobank.org/urn:lsid:zoobank.org:pub:6918ED2C-69A4-48FC-A1E4-2B5DFF58E876 相似文献
8.
Nest structure, morphology of all stages and behaviour have contributed to the phylogenetic and taxonomic studies of social vespids. Two aerial nests of Dolichovespula intermedia were discovered in China. The morphology of the three castes, the five larval instars, and the nest structure of D. intermedia Birula, are described in detail and illustrated. The presence of only one campaniform sensillum above each labial palp and the dense minute spicules on the posterior gena found in D. intermedia larvae are mentioned here for the first time for Dolichovespula. The identification characters between D. intermedia and Dolichovespula sylvestris are briefly discussed. 相似文献
9.
紫椴(Tilia amurensis)和蒙椴(T.mongolica)是分布于北方的主要椴树属植物,形态上有差别但不易区分,二者均可称为小叶椴,本研究将其处理为"小叶椴复合体".利用形态扫描及便携式数码显微镜(Digital Portable Microscope)观察,对小叶椴复合体的叶、花形态指标进行测量,有助于深入了解其形态上的变异规律.研究结果显示,小叶椴复合体的叶、花形态存在连续性,特别是存在着雄蕊退化不完全的现象. 相似文献
10.
Penelope Greenslade 《Journal of Natural History》2018,52(33-34):2197-2207
A new species of Friesea was found in East Antarctica and is described here as Friesea eureka sp. nov. This is the first new collembolan species to be described from the Antarctic continent in 30 years, even though ice-free areas have been intensively surveyed over that time. It brings to five the number of described species of the genus Friesea known from the Antarctic continent and Peninsula, so is the most speciose genus of Collembola in the region. However, one of these, F. grisea Schäffer, comprises a number of cryptic species of different molecular lineages. The new species is distinguished by having 4 + 4 ocelli plus 1 + 1 hardly visible, a very reduced furca, up to four or five faintly clavate tenent hairs, four anal spines and four spinose chaetae in adult. The restricted distribution of the species emphasises the importance of protecting ice-free areas on the continent as they often contain at least one locally endemic faunal species
www.zoobank.org/urn:lsid:zoobank.org:pub:865B9AE6-0428-4364-99A4-AF38A764D6D9 相似文献