全文获取类型
收费全文 | 30893篇 |
免费 | 4760篇 |
国内免费 | 2803篇 |
专业分类
化学 | 16336篇 |
晶体学 | 883篇 |
力学 | 908篇 |
综合类 | 188篇 |
数学 | 673篇 |
物理学 | 11072篇 |
无线电 | 8396篇 |
出版年
2024年 | 91篇 |
2023年 | 320篇 |
2022年 | 545篇 |
2021年 | 683篇 |
2020年 | 875篇 |
2019年 | 834篇 |
2018年 | 842篇 |
2017年 | 1037篇 |
2016年 | 1286篇 |
2015年 | 1298篇 |
2014年 | 1469篇 |
2013年 | 2783篇 |
2012年 | 1866篇 |
2011年 | 1966篇 |
2010年 | 1603篇 |
2009年 | 1659篇 |
2008年 | 1809篇 |
2007年 | 1897篇 |
2006年 | 1807篇 |
2005年 | 1630篇 |
2004年 | 1507篇 |
2003年 | 1318篇 |
2002年 | 1312篇 |
2001年 | 977篇 |
2000年 | 989篇 |
1999年 | 806篇 |
1998年 | 759篇 |
1997年 | 652篇 |
1996年 | 564篇 |
1995年 | 602篇 |
1994年 | 441篇 |
1993年 | 367篇 |
1992年 | 346篇 |
1991年 | 238篇 |
1990年 | 208篇 |
1989年 | 144篇 |
1988年 | 143篇 |
1987年 | 121篇 |
1986年 | 114篇 |
1985年 | 103篇 |
1984年 | 97篇 |
1983年 | 47篇 |
1982年 | 67篇 |
1981年 | 48篇 |
1980年 | 41篇 |
1979年 | 44篇 |
1978年 | 20篇 |
1977年 | 16篇 |
1975年 | 14篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Howard E. Brandt 《Foundations of Physics Letters》1989,2(1):39-58
A limiting proper acceleration in nature follows deductively from known physics and compels the union of spacetime and four-velocity space into a maximal-acceleration invariant phase space having an intrinsic Kaluza-Klein-type fiber-bundle structure with manifest gauge properties. The Riemann curvature scalar of the bundle manifold is determined, and a possible action principle is considered to serve as a basis for the generation of field equations.1. This is an expanded version of an invited paper presented at the Fifth Marcel Grossmann Meeting at the University of Western Australia, 8–12 August 1988. 相似文献
992.
We describe two new methods for locating the dew/bubble curves of fluids. One is a numerical method and the other an analytical method based on the use of series expansions. The utility of these two methods is illustrated by application to a simple one-component fluid model and to several model polydisperse fluids. The numerical method is based on a new geometric representation of the equilibrium conditions-similar in spirit to the geometric representations often used for solving the equilibrium conditions of pure fluids. Our calculations show that the series-expansion technique can be quite effective at producing accurate representations of the phase boundaries. 相似文献
993.
M. Faucher D. Garcia E. Antic-Fidancev M. Lemaitre-Blaise 《Journal of Physics and Chemistry of Solids》1989,50(12)
Some structural and spectroscopic features of rare earth orthoaluminates are examined.The trigonal→ orthorhombic transition is studied in a series of NdxSm1−xAlO3 compounds. The evolution of the crystal structure is followed by X-ray analysis and optical absorption. The free ion and crystal field parameters of Nd3+ (4f3 configuration) are determined in LuAlO3:Nd3+.The anomaly of the calculated splitting of the
levels is slight, but well characterized in NdAlO3. The spin correlated crystal field and orbitally correlated crystal field models are tested as well as an empirical correction which was proposed earlier. 相似文献
994.
995.
介绍了量子绝热定理的物理含义及成立的条件,认为有关主要献(Aharonov-Anandan,Bohm,孙昌璞等)的表述是正确的,而《关于量子几何相位的评注》^[1](以下简称《评注》)相应的表述不完全正确。在此基础上,认为这些献和教材(R.Shankar)得出的涉及Berry绝热相位的一些论述(不含Berry绝热相因子的瞬时能量本征态不满足含时Schroedinger方程等)也是正确的,而《评注》的论述与此相反。《评注》认为只有γn(C)才是Berry相位。本作则倾向于把γn(t)叫做Berry绝热相位,而把γn(C)=γn(T)-γn(0)叫做几何相位(geometric phase)^[2]。 相似文献
996.
The micrometric changes over the size of the objects produced by the temperature variations can create deleterious effects; the decoupling of soldering points in electronic circuits is one of them. In this work, we present a system based on digital holographic interferometry to quantify the magnitude of the changes produced on an electronic circuit board as it operates at very low electric currents. For the system to work, two digital holograms of the object are registered for different temperatures. These holograms are reconstructed numerically in a computer by using Fresnel's approximation to make a phase difference map. This map is converted into micrometer size variations by means of a lookup table. The implemented system allows for determining mechanical deformations in the range of 0.5–4 μm for a regular electronic circuit board drawing an electric current from 10 μA to 50 μA. 相似文献
997.
A mathematical model based on a straightforward geometrical background is developed which enables predictions of a transition of one dispersed phase to a cocontinuous one (i.e., the percolation threshold) on addition of another dispersed phase during melt mixing in ternary polymer blends. The present work concerns only ternary blends with two separate dispersions of the inner phases in which no encapsulation takes place. In addition, in order to simplify the model, one of the inner phases was represented by hard, nondeformable microspheres The expression developed describes well an experimental relationship between the percolation threshold, the concentration above which the former dispersed phase transforms to a continuous one, and concentrations of both inner phases. The results agree well with the experimental data obtained in a previous work. 相似文献
998.
Recently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
999.
Gas‐phase elimination kinetics of selected aliphatic α,β‐unsaturated aldehydes catalyzed by hydrogen chloride 下载免费PDF全文
Libia L. Julio José R. Mora Alexis Maldonado Gabriel Chuchani 《Journal of Physical Organic Chemistry》2015,28(4):261-265
The gas‐phase elimination of 2‐methyl‐2‐propenal catalyzed by HCl yields propene and CO gas, while E‐2‐pentenal with the same catalyst gives butene and CO gas. The kinetics determinations were carried out in a static system with the reaction vessels deactivated with allyl bromide and the presence of the free radical inhibitor toluene. Temperature and pressure ranges were 350.0–410.0 °C and 34–76 Torr. The elimination reactions are homogeneous and unimolecular, and follow a first‐order rate law. The rate coefficients for the reactions are expressible by the following Arrhenius equations: Data from the kinetic and thermodynamic parameters of these catalyzed elimination reactions implies a mechanism of a concerted five‐membered cyclic transition state structure for the formation of the corresponding olefin and carbon monoxide. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
1000.