全文获取类型
收费全文 | 11829篇 |
免费 | 1819篇 |
国内免费 | 2955篇 |
专业分类
化学 | 4166篇 |
晶体学 | 35篇 |
力学 | 1347篇 |
综合类 | 45篇 |
数学 | 2368篇 |
物理学 | 4830篇 |
综合类 | 3812篇 |
出版年
2023年 | 174篇 |
2022年 | 206篇 |
2021年 | 180篇 |
2020年 | 162篇 |
2019年 | 138篇 |
2018年 | 231篇 |
2017年 | 145篇 |
2016年 | 248篇 |
2015年 | 285篇 |
2014年 | 509篇 |
2013年 | 390篇 |
2012年 | 513篇 |
2011年 | 486篇 |
2010年 | 471篇 |
2009年 | 371篇 |
2008年 | 325篇 |
2007年 | 671篇 |
2006年 | 622篇 |
2005年 | 916篇 |
2004年 | 765篇 |
2003年 | 676篇 |
2002年 | 530篇 |
2001年 | 655篇 |
2000年 | 553篇 |
1999年 | 612篇 |
1998年 | 276篇 |
1997年 | 169篇 |
1996年 | 211篇 |
1995年 | 271篇 |
1994年 | 225篇 |
1993年 | 262篇 |
1992年 | 254篇 |
1991年 | 198篇 |
1990年 | 230篇 |
1989年 | 253篇 |
1988年 | 284篇 |
1987年 | 278篇 |
1986年 | 258篇 |
1985年 | 321篇 |
1984年 | 234篇 |
1983年 | 162篇 |
1982年 | 158篇 |
1981年 | 111篇 |
1975年 | 114篇 |
1959年 | 104篇 |
1958年 | 153篇 |
1957年 | 158篇 |
1956年 | 117篇 |
1955年 | 125篇 |
1954年 | 91篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
943.
Diamond film was deposited in CH4 and H2 gas mixture with a small amount of N2 by microwave plasma assisted chemical vapor deposition (MPCVD). Scanning electron microscopy, Raman spectroscopy and transmission electron microscopy were applied to characterize the film. The results showed that the growth of grains are different the central region and the edge. In the central region, diamond grains nucleated with a density as high as 4.8×108 cm-2 and were preferential in 〈001〉 orientation. The inner grains formed an area without stacking faults,which was surrounded by a rim with a high density of stacking faults. A growth model was suggested to interpret the morphological feature and the behavior of preferential growth. At the edge, the grains were identified to be 6H polytypes of diamond and a new twin relationship of grains was found. Besides, the effect of the N dopant on the growth behavior of the diamond film deposited by MPCVD was discussed in connection with the growth rate of the film.
关键词:
金刚石
结构表征
透射电子显微镜
多型金刚石 相似文献
944.
模块性(modularity)是度量网络社区结构(community structure)的主要参数.探讨了Watts和Strogatz的小世界网络(简称W-S模型)以及Barabàsi 等的B-A无标度网络(简称B-A模型)两类典型复杂网络模块性特点.结果显示,网络模块性受到网络连接稀疏的影响,W-S模型具有显著的社区结构,而B-A模型的社区结构特征不明显.因此,应用中应该分别讨论网络的小世界现象和无标度特性.社区结构不同于小世界现象和无标度特性,并可以利用模块性区别网络类型,因此网络复杂性指标应该包括
关键词:
模块性
社区结构
小世界网络
无标度网络 相似文献
945.
Ledwell JR Montgomery ET Polzin KL St. Laurent LC Schmitt RW Toole JM 《Nature》2000,403(6766):179-182
The overturning circulation of the ocean plays an important role in modulating the Earth's climate. But whereas the mechanisms for the vertical transport of water into the deep ocean--deep water formation at high latitudes--and horizontal transport in ocean currents have been largely identified, it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished. Turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise. However, measurements of the internal wave field, the main source of energy for mixing, and of turbulent dissipation rates, have typically implied diffusivities across surfaces of equal density of only approximately 0.1 cm2 s(-1), too small to account for the return flow. Here we report measurements of tracer dispersion and turbulent energy dissipation in the Brazil basin that reveal diffusivities of 2-4 cm2 s(-1) at a depth of 500 m above abyssal hills on the flank of the Mid-Atlantic Ridge, and approximately 10 cm2 s(-1) nearer the bottom. This amount of mixing, probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry, may be large enough to close the buoyancy budget for the Brazil basin and suggests a mechanism for closing the global overturning circulation. 相似文献
946.
WANG Weigang HU Dihe . School of Mathematics Statistics Wuhan University Wuhan Hubei China . School of Mathematics Statistics Yunnan University Kunming Yunnan China 《武汉大学学报:自然科学英文版》2007,(3)
First of all,we introduces the concept of m-irreducible of Markov chain in random environment. Then under the condition of m-irreducible,the relationship of recurrent and positive recurrent between two states is studied. We also give several conditions that can imply a state is recurrent and positive recurrent. And then the period of a state is discussed and we obtained that under the condition of m-irreducible,for any two states in X ,they have the same period. 相似文献
947.
HAN ShiHui JIANG Yi Glyn W. HUMPHREYS 《科学通报(英文版)》2007,52(24):3371-3375
The medial prefrontal cortex (MPFC) of human adults is involved in attributing mental states to real human agents but not to virtual artificial characters. This study examined whether such differential MPFC activity can be observed in children who are more fascinated by cartoons than adults. We measured brain activity using functional magnetic resonance imaging (fMRI) while 10-year-old children watched movie and cartoon clips, simulating real and virtual visual worlds, respectively. We showed neuroimaging evidence that, in contrast to adults, the MPFC of children was activated when perceiving both human agents and artificial characters in coherent visual events. Our findings suggest that, around the age of 10 years, the MPFC activity in children is different from that in adults in that it can be spontaneously activated by non-human agents in a virtual visual world. 相似文献
948.
A.N.Lachinov 《复旦学报(自然科学版)》2007,(5)
1 Results The paper presented review of experimental results on the charge transport study in the polymer materials,possessed charge instability.The base of these materials is noncojugated polyaromatic compounds.Characteristic feature these materials is a wide of band gap up to 4.3 eV and the value of first ionization potential about 6 eV.Electronic energetic parameters like these are typical for insulator materials.However it was established that there are not only hopping mechanism of charge transport possible but coherent too in such organic systems.Wide band gap organic material capable to transport charge carrier effectively under certain conditions and the same time conductivity can reach 105 (Ohm·cm)-1 value and current density up to 106 A/cm2 and more.Transformation to conducting phase may occur with different fields and influences,such as electric field,magnet field,pressure,temperature and others.These transformations are reversible as a rule.There is three main of the phenomena fields may be emphasize: electro-optical,switching,and electronic phenomena,including nanophenomena.Electro-optical phenomena are electroluminescence first of all[1].Switching phenomena unites as following switching induced by electric field[2],pressure[3],magnet field[4],boundary condition changing[5],traps thermoionization[6],particle irradiation[7].Electronic phenomena which can observe in thin polyarylide films are as follow electron cold emission[8],superconductivity[9],charged induced nanomemory[10],quantum dot generation[11]. 相似文献
949.
Field trial of an HTS filter system on a CDMA base station 总被引:3,自引:0,他引:3
YIN ZheSheng WEI Bin CAO BiSong WANG Xin GUO XuBo ZHANG XiaoPing GAO LongMa PIAO YunLong ZHU MaFeng LIANG Yong WANG Fan H. PIEL B. AMINOV F. AMINOVA M. GETTA S. KOLESOV A. KNACK N. PUPETER D. WEHLER 《科学通报(英文版)》2007,52(2):171-174
This paper presents the field trial results of a high temperature superconductor filter system (HTSFS) in a code division multiple access (CDMA) mobile communication base station (BTS) in Tangshan, China. The center frequency of the HTSFS is 830 MHz with 1.4% fractional bandwidth. The noise figure (NF) of the HTSFS, including connectors and cables, is 0.8 dB. The field trial results show that after the HTSFS was installed in the receiver front-end of a CDMA base station, the transmitting power of handset was decreased by 3.1 dB on average. Up to now, the HTSFS has been running steadily in the base station for more than 22 months, demonstrating the reliability and stability of the system. 相似文献
950.
R. J. STERN 《科学通报(英文版)》2007,52(5):577-591
Plate tectonics is the horizontal motion of Earth’s thermal boundary layer (lithosphere) over the convecting mantle (asthenosphere) and is mostly driven by lithosphere sinking in subduction zones. Plate tectonics is an outstanding example of a self organizing, far from equilibrium complex system (SOFFECS), driven by the negative buoyancy of the thermal boundary layer and controlled by dissipation in the bending lithosphere and viscous mantle. Plate tectonics is an unusual way for a silicate planet to lose heat, as it exists on only one of the large five silicate bodies in the inner solar system. It is not known when this mode of tectonic activity and heat loss began on Earth. All silicate planets probably experienced a short-lived magma ocean stage. After this solidified, stagnant lid behavior is the common mode of planetary heat loss, with interior heat being lost by delamination and “hot spot” volcanism and shallow intrusions. Decompression melting in the hotter early Earth generated a different lithosphere than today, with thicker oceanic crust and thinner mantle lithosphere; such lithosphere would take much longer than at present to become negatively buoyant, suggesting that plate tectonics on the early Earth occurred sporadically if at all. Plate tectonics became sustainable (the modern style) when Earth cooled sufficiently that decompression melting beneath spreading ridges made thin oceanic crust, allowing oceanic lithosphere to become negatively buoyant after a few tens of millions of years. Ultimately the question of when plate tectonics began must be answered by informa- tion retrieved from the geologic record. Criteria for the operation of plate tectonics includes ophiolites, blueschist and ultra-high pressure metamorphic belts, eclogites, passive margins, transform faults, paleomagnetic demonstration of different motions of different cratons, and the presence of diagnostic geochemical and isotopic indicators in igneous rocks. This record must be interpreted individually; I interpret the record to indicate a progression of tectonic styles from active Archean tectonics and magmatism to something similar to plate tectonics at ~1.9 Ga to sustained, modern style plate tectonics with deep subduction——and powerful slab pull——beginning in Neoproterozoic time. 相似文献