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   12642篇
  免费   24篇
  国内免费   43篇
系统科学   48篇
丛书文集   80篇
教育与普及   31篇
理论与方法论   57篇
现状及发展   5421篇
研究方法   679篇
综合类   6231篇
自然研究   162篇
  2013年   105篇
  2012年   232篇
  2011年   390篇
  2010年   104篇
  2009年   64篇
  2008年   221篇
  2007年   270篇
  2006年   251篇
  2005年   263篇
  2004年   229篇
  2003年   231篇
  2002年   230篇
  2001年   395篇
  2000年   394篇
  1999年   284篇
  1992年   259篇
  1991年   185篇
  1990年   209篇
  1989年   198篇
  1988年   206篇
  1987年   219篇
  1986年   173篇
  1985年   267篇
  1984年   180篇
  1983年   156篇
  1982年   171篇
  1981年   141篇
  1980年   176篇
  1979年   398篇
  1978年   303篇
  1977年   297篇
  1976年   257篇
  1975年   292篇
  1974年   317篇
  1973年   317篇
  1972年   354篇
  1971年   355篇
  1970年   439篇
  1969年   370篇
  1968年   389篇
  1967年   361篇
  1966年   325篇
  1965年   205篇
  1959年   107篇
  1958年   206篇
  1957年   140篇
  1956年   128篇
  1955年   106篇
  1954年   82篇
  1948年   85篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
1 Introduction Metal, metal oxide and metal compound nanoparticles (NPs) received considerable attention due to their unique properties: catalytic, magnetic, optical, electronic, etc. We believe that for different applications, there are preferable morphologies of NP-stabilizing medium composites. For example, small (1-3 nm) nanoparticles formed in micro/mesoporous hypercrosslinked polystyrene demonstrate excellent catalytic properties in various hydrogenation and oxidation reactions due to high surface...  相似文献   
942.
Field trial of an HTS filter system on a CDMA base station   总被引:3,自引:0,他引:3  
This paper presents the field trial results of a high temperature superconductor filter system (HTSFS) in a code division multiple access (CDMA) mobile communication base station (BTS) in Tangshan, China. The center frequency of the HTSFS is 830 MHz with 1.4% fractional bandwidth. The noise figure (NF) of the HTSFS, including connectors and cables, is 0.8 dB. The field trial results show that after the HTSFS was installed in the receiver front-end of a CDMA base station, the transmitting power of handset was decreased by 3.1 dB on average. Up to now, the HTSFS has been running steadily in the base station for more than 22 months, demonstrating the reliability and stability of the system.  相似文献   
943.
了解微量元素经由土壤-植物-人体链的基本、性质与变化过程,大有助于人体健康与环境保护。本书集中过去30多年的知识,提供最新数据及有关食物链的基本内容,附有引证文献资料共有68页(P.451-518)之多。  相似文献   
944.
945.
946.
947.
This paper traces the background to R. A. Fisher's multi-factorial theory of inheritance. It is argued that the traditional account is incomplete, and that Karl Pearson's well-known pre-Fisherian objections to the theory were in fact overcome by Pearson himself. It is further argued that Pearson's stated reasons for not accepting his own achievement has to be seen as a rationalization, standing in for deeper-seated metaphysical objections to the Mendelian paradigm of a type not readily discussed in a formal scientific paper. The apparent, post-Fisherian, continued acceptance of Pearson's objections is presented as an interesting problem for the historian and sociologist.  相似文献   
948.
Thirteen Speyeria nokomis apacheana (Edwards) (Nymphalidae) populations from the western Great Basin were assayed for isozyme variability using starch-gel electrophoresis. Eight of the 25 presumptive isozyme loci analyzed were found to be polymorphic. Collections made in 1991and 1992 allowed for between-year comparisons of heterozygosity and the estimation of effective population size for five of the sampled populations. Speyeria nokomis apacheana populations exhibit lower mean heterozygosity levels than other nymphalids. This may be attributed to genetic drift in apparently isolated populations with small effective sizes.  相似文献   
949.
We examined diets of Western Burrowing Owls ( Athene cunicularia hypugaea ) based on contents of pellets and large prey remains collected year-round at burrows in each of the 3 regions in south central Nevada (Mojave Desert, Great Basin Desert, and Transition region). The most common prey items, based on percent frequency of occurrence, were crickets and grasshoppers, beetles, rodents, sun spiders, and scorpions. The most common vertebrate prey was kangaroo rats ( Dipodomys spp.). True bugs (Hemiptera), scorpions, and western harvest mice ( Reithrodontomys megalotis ) occurred most frequently in pellets from the Great Basin Desert region. Kangaroo rats ( Dipodomys spp.) and pocket mice (Perognathinae) were the most important vertebrate prey items in the Transition and Mojave Desert regions, respectively. Frequency of occurrence of any invertebrate prey was high (>80%) in samples year-round but dropped in winter samples, with scorpions and sun spiders exhibiting the steepest declines. Frequency of occurrence of any vertebrate prey peaked in spring samples, was intermediate for winter and summer samples, and was lowest in fall samples. With the possible exception of selecting for western harvest mice in the Great Basin Desert region, Western Burrowing Owls in our study appeared to be opportunistic foragers with a generalist feeding strategy.  相似文献   
950.
Dispersal plays an important role in the population dynamics of many carnivores, yet little information exists about the dispersal and movement patterns of swift foxes ( Vulpes velox ). We radio-collared and monitored 68 swift foxes for dispersal at 2 study sites in northwestern Texas from January 2002 to April 2004. Dispersal distance for juveniles (13.1 ± 0.3 km, s ), adults (10 ± 4.7 km) and transients (25.4 ± 9.1 km) did not differ by age class ( F = 1.49, df = 2, P = 0.24) or sex ( F = 0.23, df = 1, P = 0.63) but differed by study site ( F = 4.72, df = 1, P = 0.04). Mean dispersal distance from private ranches (PR) was greater than from National Grasslands (NG). Peak dispersal occurred during October– November (13 individuals) and January–February (7 individuals). Dispersal direction was influenced by land-use practices (i.e., toward rangelands and away from anthropogenic features). Direction of dispersal among foxes that occupied the NG was uniform ( n = 16, P = 0.08), whereas foxes from PR dispersed in a northwesterly direction ( n = 18, P ≤ 0.001) away from a town and croplands. Three resident adult foxes made extraterritorial movements. Distances of these movements ranged from 0.2 km to 11.4 km. Distance of extraterritorial movements did not differ by sex ( F = 0.05, P = 0.83), nor by duration of movement ( F = 1.11, P = 0.32). Knowledge of movement distances and patterns is important for conservation and protection of swift foxes and their habitats.  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