全文获取类型
收费全文 | 3133篇 |
免费 | 88篇 |
国内免费 | 13篇 |
专业分类
系统科学 | 115篇 |
丛书文集 | 3篇 |
教育与普及 | 2篇 |
理论与方法论 | 26篇 |
现状及发展 | 1002篇 |
研究方法 | 30篇 |
综合类 | 2045篇 |
自然研究 | 11篇 |
出版年
2024年 | 17篇 |
2023年 | 27篇 |
2022年 | 9篇 |
2021年 | 6篇 |
2020年 | 12篇 |
2017年 | 9篇 |
2016年 | 28篇 |
2015年 | 54篇 |
2014年 | 27篇 |
2013年 | 21篇 |
2012年 | 143篇 |
2011年 | 157篇 |
2007年 | 9篇 |
2006年 | 12篇 |
2005年 | 198篇 |
2004年 | 504篇 |
2003年 | 466篇 |
2002年 | 140篇 |
2001年 | 75篇 |
2000年 | 138篇 |
1999年 | 75篇 |
1992年 | 37篇 |
1991年 | 25篇 |
1990年 | 35篇 |
1989年 | 34篇 |
1988年 | 32篇 |
1987年 | 16篇 |
1986年 | 26篇 |
1985年 | 35篇 |
1984年 | 26篇 |
1983年 | 24篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 17篇 |
1979年 | 61篇 |
1978年 | 39篇 |
1977年 | 31篇 |
1976年 | 29篇 |
1975年 | 38篇 |
1974年 | 56篇 |
1973年 | 50篇 |
1972年 | 50篇 |
1971年 | 66篇 |
1970年 | 54篇 |
1969年 | 68篇 |
1968年 | 59篇 |
1967年 | 41篇 |
1966年 | 56篇 |
1965年 | 41篇 |
1964年 | 11篇 |
排序方式: 共有3234条查询结果,搜索用时 461 毫秒
131.
General approach for discrete simulation of complex systems 总被引:4,自引:0,他引:4
By applying a general algorithm to differentparticle models, i.e. molecular dynamic (MD) and macro-scale pseudo-particle models (MaPPM), two physical phe-nomena of distinct nature and scale differences, i.e. the mu-tual diffusion of two gases and the instability on the interfacebetween two fluids, are simulated successfully. It demon-strates the possibility that the general algorithms of goodparallelism and software of modular architecture can beestablished for complex physical systems based on the parti-cle methods (PMs), which will thereby develop into a main-stream approach as finite element (FE) and finite difference(FD) approaches. 相似文献
132.
Interaction of fluid dynamic factors in the migration and accumulation of natural gas 总被引:1,自引:0,他引:1
The migration of fluid petroleum gas, described as fluid potential, depends only on gravity, fluid pressure controlled by depression and capillary force during tectonically stable period; but on tectonic stress during the tectonically active period with severe compression. This method is applied in the Junggar Basin, showing that the migration of Jurassic gas during Cretaceous and Eocene and the migration of Permian gas from Jurassic till the present are determined by capillary force and fluid pressure (including overpressure) which is controlled by depression; the migration of Jurassic gas from Eocene till the present and the migration of Permian gas during Triassic are controlled by tectonic stress. 相似文献
133.
Manganese oxides in association with paleo- weathering may provide significant insights into the multiple factors affecting the formation and evolution of weathering profiles, such as temperature, precipitation, and biodiversity. Laser probe step-heating analysis of supergene hollandite and cryptomelane samples collected from central Queensland, Australia, yield well-defined plateaus andconsistent isochron ages, confirming the feasibility dating very-fined supergene manganese oxides by 40Ar/39Ar technique. Two distinct structural sites hostingAr isotopes can be identified in light of their degassing behaviors obtained byincremental heating analyses. The first site, releasing its gas fraction at thelaser power 0.2-0.4 W, yields primarily 40Aratm, 38Aratm, and 36Aratm (atmospheric Ar isotopes). The second sites yield predominantly 40Ar* (radiogenic 40Ar),39ArK, and 38ArK (nucleogenic components), at ~0.5-1.0 W. There is no significant Ar gas released at the laser power higher than 1.0 W, indicating the breakdown of the tunnel sites hosting the radiogenic and nucleogenic components. The excellent match between the degassing behaviors of 40Ar*, 39ArK, and 38ArK suggests that these isotopes occupy the same crystallographic sites and that 39ArK lossfrom the tunnel site by recoil during neutron irradiation and/or bake-out procedure preceding isotopic analysis does not occur. Present investigation supports that neither the overwhelming atmospheric 40Ar nor the very-fined nature of the supergene manganese oxides poses problems in extracting meaningful weathering geochronological information by analyzing supergene manganese oxides minerals. 相似文献
134.
Sichuan?XuEmail author Xicheng?Ai Zhaoyong?Sun Qiyuan?Zhang Xingkang?Zhang 《科学通报(英文版)》2002,47(2):108-111
Telomeres are protein-DNA complexes at the terminals of linear chromosomes, which protect chromosomal integrity and maintain
cellular replicative capacity. From single-cell organisms to advanced animals and plants, structures and functions of telomeres
are both very conservative. In cells of human and vertebral animals, telomeric DNA base sequences all are (TTAGGG)n. In the
present work, we have obtained absorption and fluorescence spectra measured from seven synthesized oligonucleotides to simulate
the telomeric DNA system and calculated their relative fluorescence quantum yields on which not only telomeric DNA characteristics
are predicted but also possibly the shortened telomeric sequences during cell division are implied. Oligonucleotide 5′-TTAGGGTTAGGG
holds a low relative fluorescence quantum yield and remarkable excitation energy innerconversion, which tallies with the telomeric
sequence of (TTAGGG)n. This result shows that telomeric DNA has a strong non-radiative or innerconvertible capability. 相似文献
135.
Sm-Nd isotope dating of hydrothermal calcites from the Xikuangshan antimony deposit,Central Hunan 总被引:2,自引:0,他引:2
The research on Samarium-Neodymium isotope systematics of hydrothermal calcites from the Xikuangshan antimony deposit, Central
Hunan, places precise timing constraints on the Sb mineralization in this area. It is revealed that the Xikuangshan deposit
formed during the late Jurassic-early Cretaceous Period, the early- and latestage mineralization took place at (155.5± 1.1)
Ma and (124.1±3.7) Ma, respectively. The accurate age determination of mineralization is very crucial for revealing the super-enrichment
mechanism of the element Sb at the Xikuangshan mine, and lays some foundations for the further understandings of its ore genesis
and mineralization mechanism. 相似文献
136.
高应变速率超塑性铝基材料的研究现状与展望 总被引:2,自引:0,他引:2
高应变速率超塑性是一个涉及先进制造技术和超细晶块体材料的高科技前沿研究领域 美国SuperformAluminium公司采用超塑成形技术使轿车重量减轻了一半 ,大大降低了油耗与排放 ,因此高应变速率超塑性铝基材料的研究对于今后有效地利用地球资源和减少地球环境污染具有重要的现实意义 应变速率太低和成本太高是今后超塑性先进制造技术和超细晶块体材料大规模实用化进程中必须解决的关键问题 本文综述了国内外超塑性铝基复合材料和铝合金的制备、性能及变形机理的研究现状 ,指出了加快铸造法低成本制备高应变速率超塑性铝合金研究的迫切性 ,并展望了该材料在 2 1世纪的应用前景 相似文献
137.
块体纳米材料的研究现状与发展思路 总被引:3,自引:0,他引:3
块体纳米材料的制备技术及性能研究 ,是当前纳米材料领域内的一个热点 块体纳米材料具有奇异的结构和特殊性能 ,其在力学、电学、光学和磁学等方面发生了巨大的变化 文中较为详细地介绍了国内外块状纳米材料的制备技术 针对块体纳米材料加工过程中存在热稳定性及致密性等两个主要问题进行了探讨 ,提出了通过加入第二相微粒、强烈塑性加工措施来改善块体纳米材料的热稳定性 ;采用烧结、挤压辅助工艺来提高块体纳米材料的致密性的方案 相似文献
138.
Altruistic punishment in humans. 总被引:66,自引:0,他引:66
Human cooperation is an evolutionary puzzle. Unlike other creatures, people frequently cooperate with genetically unrelated strangers, often in large groups, with people they will never meet again, and when reputation gains are small or absent. These patterns of cooperation cannot be explained by the nepotistic motives associated with the evolutionary theory of kin selection and the selfish motives associated with signalling theory or the theory of reciprocal altruism. Here we show experimentally that the altruistic punishment of defectors is a key motive for the explanation of cooperation. Altruistic punishment means that individuals punish, although the punishment is costly for them and yields no material gain. We show that cooperation flourishes if altruistic punishment is possible, and breaks down if it is ruled out. The evidence indicates that negative emotions towards defectors are the proximate mechanism behind altruistic punishment. These results suggest that future study of the evolution of human cooperation should include a strong focus on explaining altruistic punishment. 相似文献
139.
把区间算法与正割算法相结合,给出了一种新的区间正割算法.并证明了其收敛性与Newton法相比,具有收敛快,误差小的优点,算例证明了其有效性. 相似文献
140.
对自由浮球式疏水阀内浮球在开启过程中的受力情况进行了全面的分析,首次提出疏水阀开启疏水的两种不同方式。即滑动开启和滚动开启方式,并分别给出了两种不同开启方式所对应的压差合力和关闭力矩的计算公式。确定了保证此类疏水阀可靠工作的许用最高工作压力。可供此类疏水阀设计和开发参考。 相似文献