首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1999篇
  免费   93篇
  国内免费   94篇
系统科学   58篇
丛书文集   40篇
教育与普及   6篇
理论与方法论   2篇
现状及发展   24篇
综合类   1887篇
自然研究   169篇
  2024年   6篇
  2023年   28篇
  2022年   29篇
  2021年   27篇
  2020年   43篇
  2019年   37篇
  2018年   41篇
  2017年   62篇
  2016年   54篇
  2015年   58篇
  2014年   80篇
  2013年   69篇
  2012年   225篇
  2011年   101篇
  2010年   77篇
  2009年   100篇
  2008年   79篇
  2007年   127篇
  2006年   116篇
  2005年   111篇
  2004年   88篇
  2003年   77篇
  2002年   66篇
  2001年   57篇
  2000年   43篇
  1999年   46篇
  1998年   37篇
  1997年   40篇
  1996年   27篇
  1995年   23篇
  1994年   35篇
  1993年   25篇
  1992年   30篇
  1991年   27篇
  1990年   27篇
  1989年   30篇
  1988年   20篇
  1987年   6篇
  1986年   6篇
  1985年   4篇
  1984年   1篇
  1955年   1篇
排序方式: 共有2186条查询结果,搜索用时 19 毫秒
81.
王涛  杨德宏  曾举 《江西科学》2011,29(1):93-96
基于传统的数学形态学边缘检测算法,改进结构元素,提出Soft多结构元素算子。针对TM图像中存在噪声多和边缘形态多样性的特点,构建多结构元素,并结合Soft形态学方法,对TM图像进行形态学的腐蚀和膨胀,实现遥感图像的边缘检测。  相似文献   
82.
《Journal of Natural History》2012,46(12):1859-1863
Material previously identified as Piezodorus hybneri (Gmelin) from the Australasian region is here described as a new species, P. grossi sp. nov. The two species can be distinguished on the basis of the aedeagus in the male and spermathecal proximal duct in the female.  相似文献   
83.
《Journal of Natural History》2012,46(6):1337-1346
The monotypic soft coral genus Malacacanthus has recently been rediscovered by means of scuba and is shown to be locally common and endemic in the shallow sublittoral of South Africa. Malacacanthus capensis (Hickson, 1900) is shown to be the proper taxonomic designation for the species. This species has previously been a problematic one due to the possession of several unusual morphological characteristics, including the total lack of sclerites and possession of a tough horny cuticle covering the stalk. The taxon is here shown to belong in the family Alcyoniidae by the possession of a full complement of mesenterial filaments, and not to the family Xeniidae as assigned by several previous authors. The species is here redescribed and the systematic status is revised. New observations on ecology and natural history from living colonies are also included.  相似文献   
84.
对20世纪50年代以来中西方关于都市区蔓延这一影响区域与城市发展的重点问题的相关研究进展、文献等代表性成果进行回顾和梳理。选取二战后至今世界城市进入快速扩张式增长时期作为研究时段,从概念界定、形态辨析、驱动因素、动力机制和空间测度等角度对都市区蔓延问题研究进行了综述,阐述其主要贡献和不足。在阐述综合承载力内涵及其系统特征的基础上,分析综合承载力网络的要素特征、空间分布等与都市区蔓延形态的相互作用关系。结论认为:都市区蔓延形态演变与综合承载力网络尤其是基础设施承载力网络紧密相关,是探讨都市区蔓延治理、实现城市合理增长、推动区域可持续发展的重要切入点;现有研究成果体系较为分散,缺乏完整的都市区蔓延评测体系及方法工具集。  相似文献   
85.
A facile and fast approach for the synthesis of a nanostructured nickel hydroxide(Ni(OH)_2) via sonochemical technique is reported in the present study. The X-ray diffraction results confirmed that the synthesized Ni(OH)_2 was oriented in β-phase of hexagonal brucite structure. The nanostructured Ni(OH)_2 electrode exhibited the maximum specific capacitance of 1256 F/g at a current density of 200 mA/g in 1 M KOH_((aq)). Ni(OH)_2 electrodes exhibited the pseudocapacitive behavior due to the presence of redox reaction. It also exhibited long-term cyclic stability of 85% after 2000 cycles, suggesting that the nanostructured Ni(OH)_2 electrode will play a promising role for high performance supercapacitor application.  相似文献   
86.
分别以Fe(NO_3)_3·9H_2O和FeSO_4·7H_2O为铁源,采用均相沉淀法和氧化-液相沉淀法制备了两种不同形貌的FePO_4粉体,再以FePO_4为前驱体,与LiOH·H_2O、蔗糖混合,采用碳热还原法合成了LiFePO_4正电极材料.用XRD和SEM对所制备的FePO_4粉体进行结构和表面形貌表征,测试了LiFePO_4样品的充放电性能.XRD和SEM测定结果表明,采用均相沉淀法制备的FePO_4为六方晶系纯相,颗粒形貌为圆片状,颗粒大小均匀;而液相氧化法制备的FePO_4也为六方晶系纯相,但颗粒形貌无规则.由圆片状FePO_4和无规则FePO_4所合成的LiFePO_4的颗粒形貌与其前驱体相同.充放电测试表明在0.5C下圆片状LiFePO_4的首次放电比容量为140 mAh/g,无规则LiFePO_4的首次放电比容量为89 mAh/g.经过50次循环后,前者的放电比容量仅下降1.43%,后者基本不变.  相似文献   
87.
Three cryptic species of the genus Munida from New Caledonia, previously identified as M. tuberculata Henderson, , M. notata Macpherson, and M. clinata Macpherson, , are described and illustrated. The three species are identified by subtle and constant morphological characters, which match clear differences in molecular sequences (16S rDNA and COI genes). The results also confirm the importance of several of these characters (e.g. length of the antennular and antennal spines) in the taxonomy of the genus Munida.  相似文献   
88.
《Journal of Natural History》2012,46(23):2125-2136
A morphological investigation of Leydigia parva Daday, (Chydoridae: Anomopoda: Cladocera), based on specimens from Paraguay (type specimens) and Brazil, clarifies its position in the subfamily and prompts its assignment to a new genus, Parvalona. The affinity of this rare benthic chydorid with Leydigia Kurz, 1875 and Alona Baird, 1843, in which this taxon was placed earlier, is discussed.  相似文献   
89.
《Journal of Natural History》2012,46(27-28):1647-1672
ABSTRACT

A new species of Liopeltis from central Vietnam is described based on morphological and molecular differences. Liopeltis pallidonuchalis sp. nov. is distinguished from its congeners by a combination of the following characters: one single (or missing) loreal; one single nasal shield; dorsal scales in 15–15-15 rows, all smooth; 1 preocular, 2 postoculars and 1 + 2 temporals; 7 supralabials, of which 3rd and 4th in contact with the eye; 8 infralabials; nasal contacting internasal; prefrontal touching or separated from supralabials; ventral scales 126–138; subcaudal scales 67–73, paired; relative tail length about 0.274–0.301; an uniform bronze body colouration; a thin postocular stripe extending from eye to end of the neck becoming indistinct posteriorly. The new species differs from the morphologically closest species Liopeltis frenata by the lower number of ventrals and subcaudals, by characteristic colouration and significant divergence in cytochrome b mtDNA gene sequences (p = 15.3–15.6%). The new species is currently known from northen and central parts of the Annamite (Truong Son) Mountains, central Vietnam, and was recorded from montane evergreen tropical forests of Kon Tum–Gia Lai Plateau (Gia Lai and Thua Thien–Hue provinces, Da Nang City) at elevations of 950–1010 m asl to karst tropical forests in Quang Binh Province in the north at elevation 150 asl. We suggest the new species should be considered as Vulnerable (VU) following the IUCN’s Red List categories. An updated taxonomic key to the Liopeltis species is provided.  相似文献   
90.
二维小波分析理论及其在显微图像去噪中的应用   总被引:2,自引:0,他引:2  
阐述了二维小波分析理论,把二维离散小波分解与重构算法应用在显微图像的去噪中.对采用不同方法去噪后的图像按照分水岭算法进行分割,然后进行对比.实例结果表明小波分析方法效果较好.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号