全文获取类型
收费全文 | 26246篇 |
免费 | 90篇 |
国内免费 | 144篇 |
专业分类
系统科学 | 124篇 |
丛书文集 | 139篇 |
教育与普及 | 63篇 |
理论与方法论 | 72篇 |
现状及发展 | 11718篇 |
研究方法 | 1178篇 |
综合类 | 12870篇 |
自然研究 | 316篇 |
出版年
2013年 | 175篇 |
2012年 | 407篇 |
2011年 | 678篇 |
2008年 | 480篇 |
2007年 | 549篇 |
2006年 | 562篇 |
2005年 | 527篇 |
2004年 | 468篇 |
2003年 | 490篇 |
2002年 | 488篇 |
2001年 | 927篇 |
2000年 | 896篇 |
1999年 | 569篇 |
1994年 | 350篇 |
1992年 | 539篇 |
1991年 | 409篇 |
1990年 | 476篇 |
1989年 | 417篇 |
1988年 | 401篇 |
1987年 | 455篇 |
1986年 | 452篇 |
1985年 | 558篇 |
1984年 | 449篇 |
1983年 | 385篇 |
1982年 | 370篇 |
1981年 | 357篇 |
1980年 | 355篇 |
1979年 | 895篇 |
1978年 | 686篇 |
1977年 | 593篇 |
1976年 | 576篇 |
1975年 | 570篇 |
1974年 | 648篇 |
1973年 | 569篇 |
1972年 | 555篇 |
1971年 | 676篇 |
1970年 | 866篇 |
1969年 | 643篇 |
1968年 | 696篇 |
1967年 | 658篇 |
1966年 | 600篇 |
1965年 | 426篇 |
1964年 | 175篇 |
1959年 | 228篇 |
1958年 | 374篇 |
1957年 | 248篇 |
1956年 | 229篇 |
1955年 | 213篇 |
1954年 | 202篇 |
1948年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
351.
352.
New Giemsa method for the differential staining of sister chromatids 总被引:127,自引:0,他引:127
353.
影响林木种子贮藏的因素 总被引:6,自引:2,他引:4
Ben S P Wang J Dale Simpson 《南京林业大学学报(自然科学版)》2006,30(1):1-7
为了每年提供造林新需的具有生活力的种子及长期保存种质资源,贮藏种子是必要的手段。种子寿命具有遗传性,且因树种而异。种子的贮藏特性可以分成5类:正常型、亚正常型、温带顽拗型、热带顽拗型和中间型。种子含水量和贮藏温度是影响种子贮藏是否成功的两个最重要因素。但因树种及其贮藏特性不同,种子含水量和贮藏温度的最适值也不同。为了保持正常型和亚正常型种子的生活力,这类种子必须密封贮藏;但对顽拗型种子,因其需要保持种子的代谢活性及要进行有氧呼吸,则需要气体交换。 相似文献
354.
This paper describes a time-domain boundary element method developed to analyze the interactions of acoustic and elastic waves near the interfaces between water and an anisotropic elastic solid. Two models are analyzed with one being the interface between two half spaces of fluid and solid and the other being a fluid region sandwiched by half space domains of anisotropic elastic solids. Both monopole and dipole point sources are used to generate an initial pressure wave in the fluid. Some snapshots of the transient wave behavior near the fluid-solid interfaces are given. The effect of the anisotropy in the solid on the pressure waveforms in the fluid is discussed. The numerical results allow detailed arrival identification and interpretation of acoustic and elastic waves propagating along the fluid-solid interfaces. 相似文献
355.
Rakesh K. Sahoo Arya Das Saurabh Singh Damin Lee Saroj K. Singh Rajaram S. Mane Je Moon Yun Kwang Ho Kim 《自然科学进展(英文版)》2019,29(4):410-415
A 3D porous carbon-manganese oxide ([email protected]) nanocomposite is successfully synthesized via a thermal plasma deposition method. The chemical bonds and compositions, phase structures, surface morphologies, etc. of as-obtained [email protected] nanocomposite were characterized by the various equipment, such as X-ray diffractometer, X-ray photoelectron spectroscopy, and electron microscopes. The electrochemical performances of the [email protected] nanocomposite electrode showed a specific capacitance of 780 F g?1 at a current density of 2 A g?1 and a capacitance retention rate of 99% after 5000 charge-discharge cycles at a high current density of 10 A g?1. These excellent capacitive performances may be attributed to the encapsulation of MnO nanoparticles by porous carbon sheets in the [email protected] MnO nanocomposite structure. It is believed that the carbon-encapsulated MnO nanoparticles can be protected from a volume deformation during the charge adsorption/desorption cycle and can be electrically improved by the encapsulated carbon sheets, resulting in better overall capacitive performance. In addition, this study also demonstrates the practical applicability by assembling a supercapacitor using the as-obtained [email protected] nanocomposite to glow a light emitting diode. 相似文献
356.
Arshid Numan Navaneethan Duraisamy Fatin Saiha Omar Dhanaraj Gopi K. Ramesh S. Ramesh 《自然科学进展(英文版)》2017,27(4):416-423
A facile and fast approach for the synthesis of a nanostructured nickel hydroxide(Ni(OH)_2) via sonochemical technique is reported in the present study. The X-ray diffraction results confirmed that the synthesized Ni(OH)_2 was oriented in β-phase of hexagonal brucite structure. The nanostructured Ni(OH)_2 electrode exhibited the maximum specific capacitance of 1256 F/g at a current density of 200 mA/g in 1 M KOH_((aq)). Ni(OH)_2 electrodes exhibited the pseudocapacitive behavior due to the presence of redox reaction. It also exhibited long-term cyclic stability of 85% after 2000 cycles, suggesting that the nanostructured Ni(OH)_2 electrode will play a promising role for high performance supercapacitor application. 相似文献
357.
358.
359.
Immune recognition. A new receptor for beta-glucans. 总被引:26,自引:0,他引:26
The carbohydrate polymers known as beta-1,3-d-glucans exert potent effects on the immune system - stimulating antitumour and antimicrobial activity, for example - by binding to receptors on macrophages and other white blood cells and activating them. Although beta-glucans are known to bind to receptors, such as complement receptor 3 (ref. 1), there is evidence that another beta-glucan receptor is present on macrophages. Here we identify this unknown receptor as dectin-1 (ref. 2), a finding that provides new insights into the innate immune recognition of beta-glucans. 相似文献
360.
We have taken a new approach to test the commonly accepted, but recently questioned, principle of clonal inheritance of vertebrate mitochondrial DNA (mtDNA) by relating its inheritance to a female-specific marker of nuclear DNA. Whereas this is impossible in organisms with male heterogamy (such as mammals), we show here that genealogies of mtDNA and the female-specific W chromosome of a bird species are completely concordant. Our results indicate that inheritance of mtDNA is free of detectable recombination effects over an evolutionary timescale. 相似文献