全文获取类型
收费全文 | 9627篇 |
免费 | 117篇 |
国内免费 | 33篇 |
专业分类
系统科学 | 172篇 |
丛书文集 | 21篇 |
教育与普及 | 6篇 |
理论与方法论 | 49篇 |
现状及发展 | 4177篇 |
研究方法 | 349篇 |
综合类 | 4896篇 |
自然研究 | 107篇 |
出版年
2017年 | 52篇 |
2016年 | 76篇 |
2015年 | 94篇 |
2014年 | 52篇 |
2013年 | 96篇 |
2012年 | 278篇 |
2011年 | 339篇 |
2008年 | 127篇 |
2007年 | 153篇 |
2006年 | 161篇 |
2005年 | 341篇 |
2004年 | 646篇 |
2003年 | 581篇 |
2002年 | 269篇 |
2001年 | 280篇 |
2000年 | 301篇 |
1999年 | 207篇 |
1992年 | 155篇 |
1991年 | 112篇 |
1990年 | 115篇 |
1989年 | 102篇 |
1988年 | 102篇 |
1987年 | 98篇 |
1986年 | 91篇 |
1985年 | 139篇 |
1984年 | 135篇 |
1983年 | 113篇 |
1982年 | 90篇 |
1981年 | 84篇 |
1980年 | 87篇 |
1979年 | 261篇 |
1978年 | 202篇 |
1977年 | 172篇 |
1976年 | 180篇 |
1975年 | 181篇 |
1974年 | 234篇 |
1973年 | 206篇 |
1972年 | 202篇 |
1971年 | 240篇 |
1970年 | 294篇 |
1969年 | 248篇 |
1968年 | 263篇 |
1967年 | 221篇 |
1966年 | 227篇 |
1965年 | 187篇 |
1964年 | 92篇 |
1959年 | 52篇 |
1958年 | 80篇 |
1957年 | 57篇 |
1954年 | 51篇 |
排序方式: 共有9777条查询结果,搜索用时 15 毫秒
161.
162.
163.
Jones JT Hasell T Wu X Bacsa J Jelfs KE Schmidtmann M Chong SY Adams DJ Trewin A Schiffman F Cora F Slater B Steiner A Day GM Cooper AI 《Nature》2011,474(7351):367-371
Nanoporous molecular frameworks are important in applications such as separation, storage and catalysis. Empirical rules exist for their assembly but it is still challenging to place and segregate functionality in three-dimensional porous solids in a predictable way. Indeed, recent studies of mixed crystalline frameworks suggest a preference for the statistical distribution of functionalities throughout the pores rather than, for example, the functional group localization found in the reactive sites of enzymes. This is a potential limitation for 'one-pot' chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganized molecular pores. In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties. However, this vision of mix-and-match assembly is far from being realized, not least because of the challenge in reliably predicting three-dimensional structures for molecular crystals, which lack the strong directional bonding found in networks. Here we show that highly porous crystalline solids can be produced by mixing different organic cage modules that self-assemble by means of chiral recognition. The structures of the resulting materials can be predicted computationally, allowing in silico materials design strategies. The constituent pore modules are synthesized in high yields on gram scales in a one-step reaction. Assembly of the porous co-crystals is as simple as combining the modules in solution and removing the solvent. In some cases, the chiral recognition between modules can be exploited to produce porous organic nanoparticles. We show that the method is valid for four different cage modules and can in principle be generalized in a computationally predictable manner based on a lock-and-key assembly between modules. 相似文献
164.
Human nutrition, the gut microbiome and the immune system 总被引:1,自引:0,他引:1
Marked changes in socio-economic status, cultural traditions, population growth and agriculture are affecting diets worldwide. Understanding how our diet and nutritional status influence the composition and dynamic operations of our gut microbial communities, and the innate and adaptive arms of our immune system, represents an area of scientific need, opportunity and challenge. The insights gleaned should help to address several pressing global health problems. 相似文献
165.
Spin-orbit (SO) coupling--the interaction between a quantum particle's spin and its momentum--is ubiquitous in physical systems. In condensed matter systems, SO coupling is crucial for the spin-Hall effect and topological insulators; it contributes to the electronic properties of materials such as GaAs, and is important for spintronic devices. Quantum many-body systems of ultracold atoms can be precisely controlled experimentally, and would therefore seem to provide an ideal platform on which to study SO coupling. Although an atom's intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the centre-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba and Dresselhaus strengths) in a neutral atomic Bose-Einstein condensate by dressing two atomic spin states with a pair of lasers. Such coupling has not been realized previously for ultracold atomic gases, or indeed any bosonic system. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin-mixed state (lasers off) to a phase-separated state (above a critical laser intensity). We develop a many-body theory that provides quantitative agreement with the observed location of the transition. The engineered SO coupling--equally applicable for bosons and fermions--sets the stage for the realization of topological insulators in fermionic neutral atom systems. 相似文献
166.
Peça J Feliciano C Ting JT Wang W Wells MF Venkatraman TN Lascola CD Fu Z Feng G 《Nature》2011,472(7344):437-442
Autism spectrum disorders (ASDs) comprise a range of disorders that share a core of neurobehavioural deficits characterized by widespread abnormalities in social interactions, deficits in communication as well as restricted interests and repetitive behaviours. The neurological basis and circuitry mechanisms underlying these abnormal behaviours are poorly understood. SHANK3 is a postsynaptic protein, whose disruption at the genetic level is thought to be responsible for the development of 22q13 deletion syndrome (Phelan-McDermid syndrome) and other non-syndromic ASDs. Here we show that mice with Shank3 gene deletions exhibit self-injurious repetitive grooming and deficits in social interaction. Cellular, electrophysiological and biochemical analyses uncovered defects at striatal synapses and cortico-striatal circuits in Shank3 mutant mice. Our findings demonstrate a critical role for SHANK3 in the normal development of neuronal connectivity and establish causality between a disruption in the Shank3 gene and the genesis of autistic-like behaviours in mice. 相似文献
167.
Programming the magnitude and persistence of antibody responses with innate immunity 总被引:1,自引:0,他引:1
Kasturi SP Skountzou I Albrecht RA Koutsonanos D Hua T Nakaya HI Ravindran R Stewart S Alam M Kwissa M Villinger F Murthy N Steel J Jacob J Hogan RJ García-Sastre A Compans R Pulendran B 《Nature》2011,470(7335):543-547
Many successful vaccines induce persistent antibody responses that can last a lifetime. The mechanisms by which they do so remain unclear, but emerging evidence indicates that they activate dendritic cells via Toll-like receptors (TLRs). For example, the yellow fever vaccine YF-17D, one of the most successful empiric vaccines ever developed, activates dendritic cells via multiple TLRs to stimulate proinflammatory cytokines. Triggering specific combinations of TLRs in dendritic cells can induce synergistic production of cytokines, which results in enhanced T-cell responses, but its impact on antibody responses remain unknown. Learning the critical parameters of innate immunity that program such antibody responses remains a major challenge in vaccinology. Here we demonstrate that immunization of mice with synthetic nanoparticles containing antigens plus ligands that signal through TLR4 and TLR7 induces synergistic increases in antigen-specific, neutralizing antibodies compared to immunization with nanoparticles containing antigens plus a single TLR ligand. Consistent with this there was enhanced persistence of germinal centres and of plasma-cell responses, which persisted in the lymph nodes for >1.5 years. Surprisingly, there was no enhancement of the early short-lived plasma-cell response relative to that observed with single TLR ligands. Molecular profiling of activated B cells, isolated 7 days after immunization, indicated that there was early programming towards B-cell memory. Antibody responses were dependent on direct triggering of both TLRs on B cells and dendritic cells, as well as on T-cell help. Immunization protected completely against lethal avian and swine influenza virus strains in mice, and induced robust immunity against pandemic H1N1 influenza in rhesus macaques. 相似文献
168.
Sicardy B Ortiz JL Assafin M Jehin E Maury A Lellouch E Hutton RG Braga-Ribas F Colas F Hestroffer D Lecacheux J Roques F Santos-Sanz P Widemann T Morales N Duffard R Thirouin A Castro-Tirado AJ Jelínek M Kubánek P Sota A Sánchez-Ramírez R Andrei AH Camargo JI da Silva Neto DN Gomes AR Martins RV Gillon M Manfroid J Tozzi GP Harlingten C Saravia S Behrend R Mottola S Melendo EG Peris V Fabregat J Madiedo JM Cuesta L Eibe MT Ullán A Organero F Pastor S de Los Reyes JA Pedraz S Castro A 《Nature》2011,478(7370):493-496
The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto. It resides at present at 95.7 astronomical units (1?AU is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 UT. The event is consistent with a spherical shape for Eris, with radius 1,163?±?6?kilometres, density 2.52?±?0.05 grams per cm(3) and a high visible geometric albedo, Pv = 0.96(+0.09)(-0.04). No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ~1?nanobar, about 10,000 times more tenuous than Pluto's present atmosphere. As Pluto's radius is estimated to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun. 相似文献
169.
170.
Shurong LI Chunwang HOU 《系统科学与复杂性》2007,20(1):108-118
In this paper, the collision problem of two moving objects is investigated. The objects are described by two algebraic sets (ellipses or circles in the paper). The collision problem discussed involves both static and dynamic case. The static case is that each object moves with known velocity. We use nonlinear programming to decide whether the objects collide. The dynamic case is that each object is controlled by a constraint external force which can be regulated online. For the dynamic case, the collision problem can be modelled as a Minmax problem which can be solved by using differential games. If collision occurs, the time and place of the first collision are given. The moving trajectories are provided in the paper. 相似文献