全文获取类型
收费全文 | 1252篇 |
免费 | 270篇 |
国内免费 | 209篇 |
专业分类
化学 | 299篇 |
晶体学 | 18篇 |
力学 | 52篇 |
综合类 | 15篇 |
数学 | 341篇 |
物理学 | 340篇 |
无线电 | 666篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 19篇 |
2021年 | 26篇 |
2020年 | 35篇 |
2019年 | 43篇 |
2018年 | 51篇 |
2017年 | 43篇 |
2016年 | 51篇 |
2015年 | 63篇 |
2014年 | 78篇 |
2013年 | 87篇 |
2012年 | 85篇 |
2011年 | 84篇 |
2010年 | 86篇 |
2009年 | 77篇 |
2008年 | 62篇 |
2007年 | 74篇 |
2006年 | 75篇 |
2005年 | 61篇 |
2004年 | 72篇 |
2003年 | 59篇 |
2002年 | 61篇 |
2001年 | 46篇 |
2000年 | 67篇 |
1999年 | 45篇 |
1998年 | 43篇 |
1997年 | 43篇 |
1996年 | 44篇 |
1995年 | 22篇 |
1994年 | 27篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有1731条查询结果,搜索用时 453 毫秒
31.
P. S. Lomdahl 《Journal of statistical physics》1985,39(5-6):551-561
The dynamics of the Josephson tunnel junction is approximately described by a perturbed sine-Gordon equation. The Josephson tunnel junction is thus a convenient experimental solid state device for the study of solitons and solitonlike phenomena. The physical manifestation of the soliton is a propagating magnetic flux quantum (
0=h/2e=2.064×10–15 V sec). Basic properties of the soliton and its relation to observable experimental quantities (zero field steps, microwave radiation, etc.) are reviewed. Recent direct measurements of the actual soliton profile are also mentioned. 相似文献
32.
Tricyclo[2.2.0.0]hexane: a new hypothetical molecule which should have only one inverted carbon atom
HF and MP2 calculations with the 6-31G** and 6-311G** basis sets and those at MP2/cc-pVTZ level were carried out for the hypothetical tricyclo[2.2.0.01,3]hexane. The results indicate that the molecule under study should have one carbon atom with highly unusual inverted configuration. The analysis of the vibrational frequencies of this molecule as well as the analysis of its plausible decomposition routes performed at the DFT level indicate that this unique molecule could be a plausible synthetic target. 相似文献
33.
D. Zorrilla Cuenca J. Sánchez Márquez M. Fernández Núñez R. Rodríguez Huertas 《International journal of quantum chemistry》2007,107(4):879-893
This project consists of two parts. In the first part, a series of test calculations is performed to verify that the integrals involved in the determination of atomic and molecular properties by standard self‐consistent field (SCF) methods can be obtained through Halton, Korobov, or Hammersley quasi‐random integration procedures. Through these calculations, we confirm that all three methods lead to results that meet the levels of precision required for their use in the calculation of properties of small atoms or molecules at least at a Hartree–Fock level. Moreover, we have ensured that the efficiency of quasi‐random integration methods that we have tested is Halton=Korobov>Hammersley?pseudo‐random. We also find that these results are comparable to those yielded by ordinary Monte Carlo (pseudo‐random) integration, with a calculation effort of two orders of smaller magnitude. The second part, which would not have been possible without the integration method previously analyzed, contains a first study of atoms constrained in spherical boxes through SCF calculations with basis functions adapted to the features of the problem: Slater‐type orbitals (STOs) trimmed by multiplying them by a function that yields 1 for 0 < r < (R‐δ), polynomial values for (R‐δ) < r < R and null for r > R, R being the radius of the box and δ a variationally determined interval. As a result, we obtain a equation of state for electrons of small systems, valid just in the limit of low temperatures, but fairly simple. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
34.
环状芴基张力半导体由于其特殊的分子结构,发射状排布的p轨道和纳米级空腔等诸多特点,引起了科学家广泛的关注.相对于直链型芴基半导体,环状芴基张力半导体展现出独特的光电性质.然而,迄今为止其振动性质没有被报道.对四元环芴的晶体与直链四聚芴粉末样品进行拉曼光谱表征和归一化处理,并结合理论计算.结果表明,与直链寡聚芴相比,环芴类似于碳纳米管的G峰发生了偏移,并且低频区域拉曼频移峰的峰强增加.其原因是由于张力的引入,改变了芴基主链的骨架和电子结构,加强芴骨架上π电子离域,同时使环芴中每个芴单元都参与到振动中.该研究结果为芴基张力半导体材料拉曼光谱的研究提供了一定的工作基础. 相似文献
35.
Tenu R. Gentil S. Baudu S. Counioux J. J. 《Journal of Thermal Analysis and Calorimetry》1999,58(1):89-101
The binary system H2O—UO2(NO3)2 was studied by solubility measurements and constant heat flow thermal analysis. Temperature and composition of the eutectic transformation between ice and uranyl nitrate hexahydrate were accurately defined. A new hydrate with 24 molecules of water decomposes at –21°C according to the peritectoid reaction<UO2(NO3)2·24H2O> <UO2(NO3)2·6H2O> + 18<H2O>The quasi-ideal model was applied to the solid—liquid equilibria, using the following reaction hypothesis:((UO
2
2+
)) + 2((NO
3
–
))+ h((H2O)) ((UO2OH+aq)) + ((H3O+aq + 2((NO
3
–
aq))A complete calculation of the binary system was carried out with a global ionic hydration number h equal to 9 in the aqueous solutions. It allowed to the melting enthalpies of uranyl nitrate hydrates.
This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
36.
An InGaAs/InP avalanche photodiode (APD) with a sectional InGaAsP/InP charge layer at the heterointerface between the InGaAs absorption and InP multiplication region has been designed, fabricated and tested. We demonstrate a new APD structure that utilizes the sectional 140 nm thin charge layer and a 500 nm thin multiplication layer. The band diagram, electrical field distribution and current-voltage (I-V) characteristics up to punch-through voltage have been simulated. The fabricated mesa structure photodiode shows responsivity 0.9 A/W at 1310 nm at 20 V and avalanche gain up to 10 near breakdown voltage 36 V. The measured results revealed that the sectional charge layer could be used for control of the electric field profile in the APD structure. 相似文献
37.
38.
1.3μm高增益偏振无关应变量子阱半导体光放大器 总被引:4,自引:2,他引:4
采用低压金属有机化学气相外延法 (LP MOVPE)生长并制作了 1 3μm脊型波导结构偏振无关半导体光放大器 (SOA) ,有源区为基于四个压应变量子阱和三个张应变量子阱交替生长的混合应变量子阱 (4C3T)结构 ,压应变阱宽为 6nm ,应变量 1 0 % ,张应变阱宽为 11nm ,应变量 - 0 95 % ;器件制作成 7°斜腔结构以有效抑制腔面反射。半导体光放大器腔面蒸镀Ti3 O5/Al2 O3 减反 (AR)膜以进一步降低腔面剩余反射率至 3× 10 -4以下 ;在 2 0 0mA驱动电流下 ,光放大器放大的自发辐射 (ASE)谱的 3dB带宽大于 5 0nm ,光谱波动小于 0 4dB ,半导体光放大器管芯的小信号增益近 30dB ,在 12 80~ 1340nm波长范围内偏振灵敏度小于 0 6dB ,饱和输出功率大于 10dBm ,噪声指数 (NF)为 7 5dB。 相似文献
39.
40.