全文获取类型
收费全文 | 8823篇 |
免费 | 268篇 |
国内免费 | 27篇 |
专业分类
化学 | 5877篇 |
晶体学 | 59篇 |
力学 | 277篇 |
综合类 | 1篇 |
数学 | 1182篇 |
物理学 | 1722篇 |
出版年
2023年 | 70篇 |
2022年 | 218篇 |
2021年 | 282篇 |
2020年 | 182篇 |
2019年 | 187篇 |
2018年 | 173篇 |
2017年 | 164篇 |
2016年 | 360篇 |
2015年 | 276篇 |
2014年 | 305篇 |
2013年 | 531篇 |
2012年 | 589篇 |
2011年 | 624篇 |
2010年 | 452篇 |
2009年 | 422篇 |
2008年 | 566篇 |
2007年 | 527篇 |
2006年 | 418篇 |
2005年 | 378篇 |
2004年 | 307篇 |
2003年 | 282篇 |
2002年 | 225篇 |
2001年 | 177篇 |
2000年 | 152篇 |
1999年 | 108篇 |
1998年 | 95篇 |
1997年 | 78篇 |
1996年 | 90篇 |
1995年 | 72篇 |
1994年 | 68篇 |
1993年 | 64篇 |
1992年 | 60篇 |
1991年 | 52篇 |
1990年 | 40篇 |
1989年 | 33篇 |
1988年 | 34篇 |
1987年 | 26篇 |
1986年 | 32篇 |
1985年 | 49篇 |
1984年 | 40篇 |
1983年 | 26篇 |
1982年 | 26篇 |
1981年 | 24篇 |
1980年 | 34篇 |
1979年 | 21篇 |
1978年 | 34篇 |
1976年 | 21篇 |
1975年 | 23篇 |
1974年 | 15篇 |
1973年 | 19篇 |
排序方式: 共有9118条查询结果,搜索用时 0 毫秒
991.
We have found two different regimes in the kinetics of the YBCO formation depending on the water partial pressure at a constant temperature and total flow rate of the carrier gas. The first regime at low partial water pressure shows continual kinetics curves until the end of YBCO growth and the reaction is controlled chemically. The second regime at high partial water pressure shows irreproducible steps in the kinetics curves during the thin films YBCO growth. In this work we suggest that there is formation of a boundary layer of water (Nernst layer) when the partial water pressure is higher than 20 hPa at 795 °C for a total gas flow rate lower than 2.4 × 10−2 m s−1. These irreproducible steps dues probably to a water boundary layer formation can be eliminated by increasing the stirring rate of the carrier gas. The reaction order of YBa2Cu3O7−x formation respect to the water pressure is n = 0.5 when the water boundary layer is not formed, but the apparent reaction order respect to the partial water pressure is zero or negative when the gas flow rate of the carrier gas is not big enough for the elimination of this water layer. This work also evidences that there is an intermediate step in the kinetics curves before the formation of YBCO. This step, which starts at low temperature during the heating ramp (400 °C) is attributed to the partial elimination of F from the BaF2 precursor to form oxyfluoride compounds. So, at low total flow gas rates and low partial water pressures, the reaction is controlled by diffusion mechanism due to the formation of a HF boundary layer (Nernst layer), because the apparent order of YBCO formation is one respect to the stirring rate. Nevertheless, at high flow gas rates and low partial water pressures, the YBCO formation is controlled chemically, then the apparent order respect to the stirring rate is zero and the HF Nernst layer is eliminated. The apparent Ea for the oxyfluoride formation at low temperatures is only 18 Kj/mol indicating that this intermediate reaction is controlled by diffusion mechanism even at high stirring rate and relatively low partial water pressure. The apparent Ea for the YBCO formation at partial water pressures higher than 20 hPa for a total flow rate of 2.4 × 10−2 m s−1 is only 32 Kj/mol, indicating that the reaction control is mainly diffusive or mixed (diffusive and chemist). 相似文献
992.
G. Medeiros-Ribeiro A. Malachias S. Kycia R. Magalhães-Paniago T.I. Kamins R. Stanley Williams 《Applied Physics A: Materials Science & Processing》2005,80(6):1211-1214
Self-assembled nanocrystals grown by epitaxy represent a viable way to mass produce quantum structures exhibiting novel electronic properties. In order to infer the electronic properties of such systems, knowledge of both composition and strain is necessary. Here, detailed maps of these quantities were obtained by anomalous X-ray diffraction for samples grown at 600 °C and 700 °C by chemical vapor deposition and molecular beam epitaxy, respectively – two different temperatures and growth techniques. From these maps, the elastic energies stored in the islands were evaluated. It was found that the elastic energy is concentrated at the island borders, regardless of the growth temperature or method. In contrast, the regions with the lowest elastic energy were the island core and top. These results provide insight into the mechanisms that govern the growth and evolution of strained nanocrystals. PACS 71.35.Ji; 78.67.Hc; 73.22.Dj 相似文献
993.
Antonio Luiz P. S. Campos Adaildo Gomes d''Assunção Marcos A. Barbosa de Melo 《International Journal of Infrared and Millimeter Waves》2002,23(1):123-133
The analysis of periodic structures on anisotropic substrates is presented for incident millimeter waves. Frequency selective surface structures composed by a periodic array of conducting crossed dipoles on anisotropic layers are analyzed. A full-wave analysis for the problem of scattering is performed. The formulation uses the moment method in combination with the immitance approach, in the Fourier domain, to determine the scattering parameters of the considered structures. A set of entire-domain type basis functions is used in the expansion for the unknown induced current. This analysis is computationally efficient, and its accuracy is verified by the agreement between results obtained in this work and those available in the literature, for particular cases. 相似文献
994.
995.
H. Beiro Da Veiga 《纯数学与应用数学通讯》1993,46(2):221-259
The main result of this paper (which is completely new, apart from our previous and less general result proved in reference [9]) states that the nonlinear system of equations (1.11) (or, equivalently, (1.10)) that describes the motion of an inviscid, compressible (barotropic) fluid in a bounded domain Ω, gives rise to a strongly well-posed problem (in the Hadamard classical sense) in spaces Hk(Ω), k ≧ 3; see Theorem 1.4 below. Roughly speaking, if (an, ?n) → (a, ?) in Hk × Hk and if fn → f in ??2(0, T;Hk), then (vn, gn) → (v, g) in ?? (0, T; Hk × Hk). The method followed here (see also [8]) also applies to the non-barotropic case p = p(ρ, s) (see [10]) and to other nonlinear problems. These results are based upon an improvement of the structural-stability theorem for linear hyperbolic equations. See Theorem 1.2 below. Added in proof: The reader is referred to [29], Part I, for a concise explanation of some fundamental points in the method followed here. © 1993 John Wiley & Sons, Inc. 相似文献
996.
997.
Evora M. C. Machado L. D. B. Lourenço V. L. Gonçalez O. L. Wiebeck H. de Andrade e Silva L. G. 《Journal of Thermal Analysis and Calorimetry》2002,67(2):327-333
The aim of this work is to study the ionizing radiation effects on thermal properties of there cycled polyamide-6. This polymer
was irradiated with an electron beam of 1.5 MeV with different doses. The thermal properties of the samples were determined
by TG, DSC and DMA measurements. It was observed that the irradiated samples of recycled polyamide-6undergo a crosslinking
process.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
998.
999.
L. Martínez-Sustegui C. Trevio F. Mndez 《European Journal of Mechanics - B/Fluids》2003,22(6):451-553
In this work, the conjugated heat transfer characteristics of a thin vertical strip of finite length, placed in a porous medium has been studied using numerical and asymptotic techniques. The nondimensional temperature distribution in the strip and the reduced Nusselt number at the top of the strip are obtained as a function of the thermal penetration parameter s, which measures the thermal region where the temperature of the strip decays to the ambient temperature of the surrounding fluid. The numerical values of this nondimensional parameter permits to classify the different physical regimes, showing different solutions: a thermally long behaviour, an intermediate transition and a short strip limit. 相似文献
1000.
Rados?aw Przenios?o Wouter van BeekIzabela Sosnowska 《Solid State Communications》2003,126(9):485-488
The structural phase transition in annealed CaMn7O12 has been investigated by using high resolution synchrotron radiation powder diffraction. There is a phase coexistence phenomenon: two different crystallographic phases coexist in the material between 410 and 458 K. The first one is trigonal and it has a charge ordering (CO) of the Mn3+ and Mn4+ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from 22% at 418 K up to 100% at 468 K. Both phases have domains of at least 150 nm at each temperature. The annealing of CaMn7O12 relaxed a part of the strains in the lattice, but did not influence the phase coexistence phenomenon. 相似文献