全文获取类型
收费全文 | 46116篇 |
免费 | 2473篇 |
国内免费 | 3405篇 |
专业分类
系统科学 | 4431篇 |
丛书文集 | 1388篇 |
教育与普及 | 130篇 |
理论与方法论 | 160篇 |
现状及发展 | 368篇 |
研究方法 | 1篇 |
综合类 | 45477篇 |
自然研究 | 39篇 |
出版年
2024年 | 40篇 |
2023年 | 366篇 |
2022年 | 860篇 |
2021年 | 984篇 |
2020年 | 956篇 |
2019年 | 738篇 |
2018年 | 667篇 |
2017年 | 822篇 |
2016年 | 957篇 |
2015年 | 1513篇 |
2014年 | 1850篇 |
2013年 | 1620篇 |
2012年 | 2846篇 |
2011年 | 3037篇 |
2010年 | 2274篇 |
2009年 | 2254篇 |
2008年 | 2497篇 |
2007年 | 3635篇 |
2006年 | 3487篇 |
2005年 | 3114篇 |
2004年 | 2804篇 |
2003年 | 2240篇 |
2002年 | 1792篇 |
2001年 | 1523篇 |
2000年 | 1360篇 |
1999年 | 1194篇 |
1998年 | 876篇 |
1997年 | 829篇 |
1996年 | 765篇 |
1995年 | 614篇 |
1994年 | 599篇 |
1993年 | 529篇 |
1992年 | 460篇 |
1991年 | 473篇 |
1990年 | 383篇 |
1989年 | 358篇 |
1988年 | 304篇 |
1987年 | 186篇 |
1986年 | 98篇 |
1985年 | 44篇 |
1984年 | 10篇 |
1982年 | 1篇 |
1981年 | 12篇 |
1980年 | 1篇 |
1955年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
991.
<正> Security assessment can help understand the security conditions of an information system and yieldresults highly conducive to the solution of security problems in it.Taking the computer networks in a certainuniversity as samples,this paper,with the information system security assessment model as its foundation,proposes a multi-attribute group decision-making (MAGDM) security assessment method based ona variable consistency dominance-based rough set approach (VC-DRSA).This assessment method combinesVC-DRSA with the analytic hierarchy process (AHP),uncovers the inherent information hidden indata via the quality of sorting (QoS),and makes a synthetic security assessment of the information systemafter determining the security attribute weight.The sample findings show that this methoil can effectivelyremove the bottleneck of MAGDM,thus assuming practical significance in information system security assessment. 相似文献
992.
993.
细根分解在根系碳周转与释放的过程中起着重要的作用.然而,目前国内关于细根分解的相关研究还较少且未得到足够的重视.文章阐述了细根分解研究的重要意义,综述了国内外细根分解的研究方法、细根分解过程、影响细根分解的因素和国内细根研究的进展,并指出了细根分解研究中存在的问题和今后研究应重视方面. 相似文献
994.
《Journal of Natural History》2012,46(1):119-134
Insectivorous lizards usually employ one of two foraging modes, ambush ‘sit-and-wait’ or active ‘widely foraging’, but in the Gekkonomorpha the situation has been unclear. Therefore we quantified the foraging mode of the eublepharid gecko Goniurosaurus kuroiwae orientalis on Tokashikijima near Okinawajima, Japan, in September 1999. The taxon is rare, endangered and protected. On the selected semi-urban study site the gecko was confirmed as a nocturnal cursorial gecko, which also ascends trees up to 1.8 m. Foraging regimes of marked individuals, during repeated 30-min observation periods, could vary from immobility throughout, to active locomotion 84% of the time. Foraging mode was not significantly affected by sex, body length, mass or state of tail regeneration of the individuals, or by the timing of the observation. In males, at higher temperatures single moves became longer, while time allocated to moving was stable. In both sexes longer observation bouts included longer sitting pauses, which, given stable move duration, reduced the share of time spent moving and the moves per minute. Goniurosaurus kuroiwae orientalis averaged moving during 23% of the time, moving 0.39 times per minute, the moves lasting 77 s and the pauses lasting 745 s. Compared to other geckos, G. k. orientalis may be defined as a widely foraging animal, despite its mixed behaviour. This result supports the definition of the Eublepharidae as widely foraging and is compatible with two hypotheses, that early Gekkonomorpha were widely foraging or that they had an intermediate, undefined, foraging mode. 相似文献
995.
A satellite image adaptive restoration method was developed that avoids ringing artifacts at the image boundary and retains oriented features. The method combines periodic plus smooth image decomposition with complex wavelet packet transforms. The framework first decomposes a degraded satellite image into the sum of a "periodic component" and a "smooth component". The Bayesian method is then used to estimate the modulation transfer function degradation parameters and the noise. The periodic component is deconvoluted using complex wavelet packet transforms with the deconvolution result of the periodic component then combined with the smooth component to get the final recovered result. Tests show that this strategy effectively avoids ringing artifacts while preserving local image details (especially directional textures) without amplifying the noise. Quantitative comparisons illustrate that the results are comparable with previous methods. Another benefit is that this approach can process large satellite images with parallel processing, which is important for practical use. 相似文献
996.
对近20年来在中国期刊全文数据库所收录的有关如何管理学生档案的论文进行了统计分析,指出规范学生档案管理、建设及改进学生档案管理模式十分必要。 相似文献
997.
《Journal of Natural History》2012,46(37-38):2237-2253
Four nominal species of the genus Horatia Bourguignat, 1887 were described by Russian authors from Caucasian caves (H. borutzkii Shadin, 1932, H. birsteini Starobogatov, 1962, H. ljovushkini Starobogatov, 1962 and H. sokolovi Starobogatov, 1962). Hitherto all the species were known from type series (presented by empty shells) only. Examination of newly obtained samples from the caves of South Caucasus has shown that the four species actually do not belong to the genus Horatia due to obvious differences in the morphology of the copulatory organs between H. klecakiana Bourguignat, 1887 (the type species of this genus) and the Caucasian snails. Moreover, statistical analysis of conchological variation has revealed that the species H. birsteini, H. ljovushkini, and H. sokolovi are conspecific. Two genera are described: Pontohoratia (includes P. birsteini and P. smyri sp. nov.) and Motsametia (with the single species M. borutzkii comb. nov.). The two genera can be distinguished on the basis of the structure of their copulatory organs. The distribution and ecology of the Caucasian Horatia-like mollusks are briefly outlined.http://zoobank.org/urn:lsid:zoobank.org:pub:4EDFB2AE-CB1D-4113-8CC7-3A1BBB46D838 相似文献
998.
This paper analyzes the effect of waveform parameters on the joint target location and velocity estimation by a noncoherent multiple input multiple output (MIMO) radar transmitting multiple subcarriers signals. How the number of subcarriers influences the estimation accuracy is illustrated by considering the joint Cramer-Rao bound and the mean square error of the maximum likelihood estimate. The non-coherent MIMO radar ambiguity function with multiple subcarriers is developed and investigated by changing the number of subcarriers, the pulse width and the frequency spacing between adjacent subcarriers. The numerical results show that more subcarriers mean more accurate estimates, higher localization resolution, and larger pulse width results in a worse performance of target location estimation, while the frequency spacing affects target location estimation little. 相似文献
999.
提高了Cu含量的核反应堆压力容器(reactor pressure vessel,RPV)模拟钢样品,经过880 cC水淬、660℃调质处理和400℃1000~4000 h的等温时效处理,观察到纳米富Cu相的析出;随后进行20%~30%冷轧变形,采用萃取复型(extractionreplica,ER)和高分辨透射电镜(high resolution transmission electron microscopy,HRTEM)的方法研究纳米富Cu相的变形特征.研究结果表明,镶嵌在α-Fe基体中的纳米富Cu相,在冷轧变形时的变形机制较为复杂,存在多种变形方式.当纳米富Cu相的晶体处于有利取向时,可以跟随基体一起发生滑移变形,表现为"软"颗粒的特性;当晶体处于不利取向时,会发生孪生变形,甚至诱发马氏体相变,有时生成"轮毂辐条"状的孪晶结构,大大提高了纳米富Cu相继续变形时的抗力,表现为"硬"颗粒的特征,因而析出纳米富Cu相会产生明显的强化作用. 相似文献
1000.
从信息化建设的理论和方法出发,结合新建本科院校信息化建设现状,认为新建本科院校信息化建设采用跨越式建设模式更为有利,并提出了该类院校信息化建设方案与步骤. 相似文献