全文获取类型
收费全文 | 1308篇 |
免费 | 34篇 |
国内免费 | 62篇 |
专业分类
系统科学 | 36篇 |
丛书文集 | 40篇 |
教育与普及 | 41篇 |
理论与方法论 | 6篇 |
现状及发展 | 22篇 |
研究方法 | 22篇 |
综合类 | 1236篇 |
自然研究 | 1篇 |
出版年
2024年 | 13篇 |
2023年 | 11篇 |
2022年 | 22篇 |
2021年 | 39篇 |
2020年 | 24篇 |
2019年 | 6篇 |
2018年 | 14篇 |
2017年 | 12篇 |
2016年 | 12篇 |
2015年 | 27篇 |
2014年 | 41篇 |
2013年 | 50篇 |
2012年 | 43篇 |
2011年 | 58篇 |
2010年 | 55篇 |
2009年 | 42篇 |
2008年 | 68篇 |
2007年 | 85篇 |
2006年 | 51篇 |
2005年 | 46篇 |
2004年 | 31篇 |
2003年 | 35篇 |
2002年 | 42篇 |
2001年 | 38篇 |
2000年 | 34篇 |
1999年 | 100篇 |
1998年 | 58篇 |
1997年 | 58篇 |
1996年 | 49篇 |
1995年 | 44篇 |
1994年 | 33篇 |
1993年 | 24篇 |
1992年 | 30篇 |
1991年 | 36篇 |
1990年 | 24篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1979年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有1404条查询结果,搜索用时 15 毫秒
71.
电站锅炉在线故障诊断的实时支持平台 总被引:3,自引:0,他引:3
为进一步提高电站运行自动化水平,结合故障诊断和专家系统技术,建立了在线故障诊断的实时支持平台。它可通过组态方式构成电站锅炉的在线故障诊断专家系统,实现实时数据共享,各子系统故障诊断模块间信息共享,提供统一的面向故障的图形用户界面。在该平台上实现了一个能诊断锅炉过热器泄漏和燃烧故障的实验系统。该平台可以促进锅炉故障诊断专家系统研究走向实用 相似文献
72.
73.
在所建立的机构考虑摩擦时力分析数学模型的基础上,指出该数学模型是一个求解非线性方程组的问题。对三种迭代解法进行了综述,分析它们的不足之处,并对非迭代线性化新解法进行了论证 相似文献
74.
等离子熔积成形混相瞬态场的Level-Set方法模拟 总被引:1,自引:0,他引:1
提出了一种二维等离子熔积成形瞬态模型,该模型描述了液/气界面的自由表面发展,并模拟了熔池内流体流动和传热.采用Level—Set方法处理液/气界面边界条件,考虑了熔体流动的主要驱动力——表面张力梯度、表面曲率、浮力以及工件表面的对流散热等因素.用固液相统一模型来描述固/液界面处的熔融和凝固过程,并开发了相应的软件.对高温合金K163在不同扫描速度下的熔积层表面形貌、温度场以及熔池内流场进行了模拟分析. 相似文献
75.
提出了一种基于贝叶斯公式的信息反馈模型BIF.当系统拒绝用户对资源的访问时,BIF通过计算在当前的条件下改变某些访问条件的可能性,把用户的访问条件的替代方案限制到一个小而有用的集合,系统将具有最大可能性的选项反馈给用户.用户根据系统的反馈改变访问条件从而成功访问资源.BIF引入附加策略来保护敏感策略,在此过程中不会泄漏任何危及到系统安全和机密性的信息.实验证明,BIF模型在保护了系统敏感信息的同时,也提高了系统的可用性及访问成功的几率. 相似文献
76.
The c-myc coding DNA sequences of cyprinids (Teleostei: Cypriniformes): Implications for phylogeny 总被引:1,自引:0,他引:1
KONG XiangHui WANG XuZhen GAN XiaoNi HE ShunPing 《科学通报(英文版)》2007,52(11):1491-1500
The family Cyprinidae is one of the largest fish families in the world, which is widely distributed in East Asian, with obvious difference in characteristic size among species. The phylogeneUc analysis of cyprinid taxa based on the functionally important genes can help to understand the speciation and functional divergence of the Cyprinidae. The c-myc gene is an important gene regulating individual growth. In the present study, the sequence variations of the cyprinid c-myc gene and their phylogenetic significance were analyzed. The 41 complete sequences of the c-myc gene were obtained from cyprinids and outgroups through PCR amplification and clone. The coding DNA sequences of the c-myc gene were used to infer molecular phylogenetic relationships within the Cyprinidae. Myxocyprinus asiaticus (Catostomidae), Misgurnus anguillicaudatus (CobiUdae) and Hemimyzon sinensis (Homalopteridae) were assigned to the outgroup taxa. Phylogenetic analyses using maximum parsimony (MP), maximum likelihood (ML), and Bayesian retrieved similar topology. Within the Cyprinidae, Leuciscini and Barbini formed the monophyletic lineage respectively with high nodal supports. Leuciscini comprises Xenocyprinae, CuItrinae, East Asian species of Leuciscinae and Danioninae, Gobioninae and Acheilognathinae, and Barbini contains Schizothoracinae, Barbinae, Cyprininae and Labeoninae. Danio rerio, D. myersi and Rasbora trilineata were supposed to separate from Leuciscinae and Barbini and to form another lineage. The positions of some Danioninae species were still unresolved. Analyses of both amino acid variation with parsimony information and two high variation regions indicated that there is no correlation between variations of single amino acid or high variation regions and characteristic size of cyprinids. In addition, the species with smaller size were usually found to be basal within clades in the tree, which might be the results of the adaptation to the primitive ecology and survival pressure. 相似文献
77.
This paper presents a multi-proxy reconstruction of the climate change in Caotanhu wetland using pollen, phytolith and charcoal records, and the data of loss of ignition (LOI), grain size analysis, and susceptibility. Results reveal that between 4550 and 2500 cal. a BP, a dry climatic condition was not favorable for the accumulation of peat. Since 2500 cal. a BP, the climate became humid and the wetland developed with abundant freshwater aquatic plants, which contributed to peat accumulation. Never-theless, alternate periods of rain and dry climate occurred during that period. Between 2500 and 1810 cal. a BP (550 BC-140 AD), the climate was more humid than at present. A lot of emerged plants, such as Phragmites, Typha and Sparganium, and freshwater green algae grew in the wetland which was sur-rounded by desert-steppe vegetation composed mainly of Chenopodiaceae, Artemisia, Compositae and Thalictrum. However, from 1810 to 1160 cal. a BP (140--790 AD), the water level started to decrease and hydrophyte species reduced greatly, but some Phragmites still grew in the wetland and around it was desert vegetation with high proportion of Chenopodiaceae and Artemisia. Then from 1160 to 650 a BP (790--1300 AD), it entered a period of desert-steppe with abundant mesic and xerophytic plants. And a lot of aquatic plants prevailed in the wetland. Here, what is noticeable is that percentages of arboreal pollen, consisting mainly of Betula and Picea, increased greatly and reached a maximal value of 27.2%, in which, Betula percentages rose to 23.2%. Hence, it is reasonable to conclude that Betula grew in the highland of the wetland, or Picea timberline shifted downward resulting in the increase of percentages of Betula and Picea pollen, which were transported into the wetland by flood or wind. But since 650 cal. a BP, desert vegetation prevailed around the wetland again with dominant Chenopodiaceae and Artemisia, and the climate was similar to modern one. Despite some aquatic plants still growing in the wetland at 相似文献
78.
笔者研究了一类三阶三点边值问题解的存在性,在非线性项半正且奇异的情况下,利用Guo—Kmsnoselskii不动点定理证明了解的存在性. 相似文献
79.
用原型批评的方法解读女作家林白的小说,分析归纳了女妖神话在其作品中的四种移植类型,分别是“沙街”女妖、都市复仇女神、乡村性爱女妖和“银角”变异女妖。这四种类型代表她不同时期的创作理念的变化:从早期的唯美的女性独语,到中期创作表现出激进的女性主义写作,再到新世纪以来民间女性立场的确立和对消费文化下女性存在的独特思考。因此,原型及其演变的追踪可看出林白作品独特的艺术魅力以及创作轨迹的变迁。 相似文献
80.
速生杨木PF浸渍增强机理及力学性能可靠性分析 总被引:4,自引:0,他引:4
利用满细胞法真空加压浸渍方案,对速生杨木进行低分子酚醛树脂增强改性处理,对材料改
性前后力学性能及其可靠性进行分析。结果表明:低分子酚醛树脂浸渍改性处理是速生杨木增强的
有效手段,能够大幅度改善速生杨木的力学性能,改性后材料的抗弯弹性模量、抗弯强度、顺纹压
缩强度分别较改性前提高了87.784 %、22.136 %和207.358 %;可靠性分析结果表明,以抗弯弹性模
量、抗弯强度和顺纹抗压强度为考察指标,速生杨木及其低分子酚醛树脂浸渍改性材的强度指标均
符合正态统计分布规律。 相似文献