全文获取类型
收费全文 | 184955篇 |
免费 | 2671篇 |
国内免费 | 848篇 |
专业分类
化学 | 101551篇 |
晶体学 | 2570篇 |
力学 | 8062篇 |
综合类 | 7篇 |
数学 | 20041篇 |
物理学 | 56243篇 |
出版年
2021年 | 1121篇 |
2020年 | 1303篇 |
2019年 | 1294篇 |
2018年 | 1313篇 |
2017年 | 1302篇 |
2016年 | 2609篇 |
2015年 | 2144篇 |
2014年 | 2765篇 |
2013年 | 8387篇 |
2012年 | 6901篇 |
2011年 | 8807篇 |
2010年 | 5302篇 |
2009年 | 5416篇 |
2008年 | 8020篇 |
2007年 | 8038篇 |
2006年 | 7792篇 |
2005年 | 7279篇 |
2004年 | 6547篇 |
2003年 | 5659篇 |
2002年 | 5549篇 |
2001年 | 6207篇 |
2000年 | 4735篇 |
1999年 | 3615篇 |
1998年 | 2779篇 |
1997年 | 2793篇 |
1996年 | 2772篇 |
1995年 | 2427篇 |
1994年 | 2315篇 |
1993年 | 2150篇 |
1992年 | 2471篇 |
1991年 | 2441篇 |
1990年 | 2140篇 |
1989年 | 2095篇 |
1988年 | 2155篇 |
1987年 | 2080篇 |
1986年 | 1959篇 |
1985年 | 2950篇 |
1984年 | 2952篇 |
1983年 | 2351篇 |
1982年 | 2604篇 |
1981年 | 2472篇 |
1980年 | 2426篇 |
1979年 | 2360篇 |
1978年 | 2422篇 |
1977年 | 2365篇 |
1976年 | 2293篇 |
1975年 | 2253篇 |
1974年 | 2169篇 |
1973年 | 2247篇 |
1972年 | 1263篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
Y. Gu Igor L. Kuskovsky R.D. Robinson G.F. Neumark S.P. Guo M.C. Tamargo 《Solid State Communications》2005,134(10):677-681
The composition and size of optically active CdxZn1−xSe/ZnSe quantum dots are estimated with a previously developed method. The results are then compared with those obtained for CdxZn1−xSe/Zn0.97Be0.03Se QDs. We show that introducing Be into the barrier material enhances both Cd composition and quantum size effect of optically active quantum dots. 相似文献
852.
Kimberlitic–pyropic peridotite–xenolites, probably of Jurassic–Cretaceous age, were found mixed with a younger Upper Tertiary basanitic diabase, as flow texture of plagioclase laths and ilmenite rods around those xenoliths indicated. Mafic–ultramafic rocks were crushed, sheared, and cropped along a creek about 15–18 km NE of the town of Dreikeesh, NW Syria. 40K–40Ar isotopic dating of a pure fresh black cpx sample, collected from the peridotite xenoliths, yielded an age of about 70 Ma. This age is concurrent to the time when Africa, Eurasia, and America were part of the super continent Pangaea. It also suggests that kimberlite–pyropic peridotitic rocks were located within a cratonic pipe prior to their 2000-km eastward journey (starting from the Mid-Atlantic Rift). Sampled outcrops were located within a ring of about 40 km diameter, considered to be a dome consisting of one or more clusters of kimberlitic pipes. The dome structure, mostly covered with Upper Jurassic–Cretaceous carbonate beds, was tilted westward, and rifted and sheared along its eastern edge. Tilting and crushing were accomplished after the opening of the Red Sea in the Miocene, and the counterclockwise movements of the Arabian plate, which folded the carbonate beds to form the N–S-trending Alawite mountain range along the Syrian coast. Olivine, cpx, and pyrope were the major phases in peridotite. Pyrope, including its Cr–Ni-contents, was found to be the best indicator to decipher the temperature–pressure (T–P) conditions for the system. The inferred temperature was found to be about 1460 °C, at a pressure of 62 kb (around 207 km-depth). Petrographic studies revealed many zoned, resorbed, octahedral and non-metasomatized tiny grains, associated with partly to wholly metasomatized and iddingsitized olivines. 相似文献
853.
C. Hainzl 《Annales Henri Poincare》2003,4(2):217-237
854.
A polarization-insensitive 40-Gb/s time-division demultiplexer was demonstrated in a polarization-diversity loop configuration. The power penalty was measured to be 2.3 dB at a bit error rates of 10-9 for 10 Gb/s output signals. 相似文献
855.
A. F. G. Monte M. A. G. Soler S. W. da Silva B. B. D. Rodrigues P. C. Morais A. A. Quivy J. R. Leite 《Physica E: Low-dimensional Systems and Nanostructures》2004,23(3-4):466
We have investigated the influence of vicinal GaAs substrates on the optical and electronic properties of InGaAs/GaAs quantum wells (QWs). A single In0.10Ga0.90As QW was grown by molecular-beam epitaxy on a vicinal GaAs(0 0 1) substrate with a miscut angle of 0° (nominal), 2°, 4° and 6° towards [1 1 0]. The carrier diffusion was obtained by a micro-photoluminescence scan technique that permits to observe the effective diffusion length characterized by the lateral spread of carriers in the QW followed by radiative recombination. The carrier diffusion length was obtained parallel (L||) and perpendicular (L) to the atomic steps. The diffusion length decreases as the temperature increases up to 100 K. Above this temperature we found different behaviours that depend on the sample miscut angle. 相似文献
856.
The purpose of this work was to investigate the effects of physical aging on the kinetics of yielding in polycarbonate. PC samples were annealed over a wide range of aging times and temperatures. Both tensile and compressive tests were performed over various loading rates and temperatures to analyze the effects of aging time and aging temperature on yielding kinetics. Two grades of polycarbonate, Makrolon, of different molecular weights, PC-2608 (low Mw), and PC-3208 (high Mw), supplied by Bayer were analyzed. In unaged condition, PC is hard and tough, but after aging, it becomes more brittle. In terms of molecular movement, the yielding process is a thermally activated process involving inter- and intra-molecular motions. The time–temperature dependence of yielding behavior can be separated into two regions. Aging does not affect localized molecular motions of the β process during yielding. Physical aging in PC results in a slower jump rate of the main segments of macromolecules between two equilibrium positions. It reduces the flexibility of the macromolecules and thus, makes the polymer more brittle. Heat aging also causes a decrease of the entropy (ΔS) in polycarbonate, and this decrease is more important when the molecular weight is reduced. Increasing the annealing time and temperature results in a continuous reduction of ΔS. The rate of aging decreases with decreasing annealing temperature and below about 30 °C, no aging takes place. Annealing also strongly affects the excess of enthalpy in PC. However the effect of physical aging on yielding differs to that on enthalpy excess. The kinetics of yielding and aging processes in polycarbonate are also different. An increase in the strain rate does not have the same effect on the yield stress as an increase in the aging time by a same factor. 相似文献
857.
Using the thermodynamical approach and the method of York, the back-reaction of anti-de Sitter Schwarzschild black hole in thermal equilibrium with conformal spin-2 field is studied. It is found that both approaches give identical results. 相似文献
858.
Nearly four hundred non-routine organizational decisions were investigated to discover search approaches––determining the frequency of use and success of each search approach uncovered. A “search approach” is made up of a direction and a means to uncover solution ideas. Direction indicates desired results and it can be either implicit or explicit, with an explicit direction offering either a problem or a goal-like target. Solutions can be uncovered by opportunity, bargaining, and chance as well as by rational approaches. Defining a search approach as a direction coupled with a means of search, search approaches were linked with indicators of success, measured by the decision's adoption, value and timeliness, noting frequency. A rational, goal-directed, search approach was more apt to produce successful outcomes. Bargaining with stakeholders to uncover solutions was always combined some of the search approaches in this study, and this merger improved the prospects of success. Searches with an opportunistic or chance (emergent opportunity) features and rational searches with a problem target were more apt to produce unsuccessful outcomes. The means used to come up with a solution had less bearing on success than did the type of direction, with goal-directed searches leading to the best outcomes. Each search approach is discussed to reveal best practices and to offer suggestions to improve practice. 相似文献
859.
Bruzón M. S. Gandarias M. L. Muriel C. Ramírez J. Saez S. Romero F. R. 《Theoretical and Mathematical Physics》2003,137(1):1367-1377
We use the classical and nonclassical methods to obtain symmetry reductions and exact solutions of the (2+1)-dimensional integrable Calogero–Bogoyavlenskii–Schiff equation. Although this (2+1)-dimensional equation arises in a nonlocal form, it can be written as a system of differential equations and, in potential form, as a fourth-order partial differential equation. The classical and nonclassical methods yield some exact solutions of the (2+1)-dimensional equation that involve several arbitrary functions and hence exhibit a rich variety of qualitative behavior. 相似文献
860.
Periodica Mathematica Hungarica - 相似文献