全文获取类型
收费全文 | 428129篇 |
免费 | 4749篇 |
国内免费 | 1430篇 |
专业分类
化学 | 234875篇 |
晶体学 | 6250篇 |
力学 | 17840篇 |
综合类 | 10篇 |
数学 | 48226篇 |
物理学 | 127107篇 |
出版年
2020年 | 2992篇 |
2019年 | 3038篇 |
2018年 | 3429篇 |
2017年 | 3373篇 |
2016年 | 5997篇 |
2015年 | 4513篇 |
2014年 | 6351篇 |
2013年 | 18783篇 |
2012年 | 14409篇 |
2011年 | 17710篇 |
2010年 | 11505篇 |
2009年 | 11555篇 |
2008年 | 16577篇 |
2007年 | 16625篇 |
2006年 | 16131篇 |
2005年 | 14746篇 |
2004年 | 13275篇 |
2003年 | 11914篇 |
2002年 | 11696篇 |
2001年 | 13986篇 |
2000年 | 10587篇 |
1999年 | 8361篇 |
1998年 | 6424篇 |
1997年 | 6369篇 |
1996年 | 6330篇 |
1995年 | 5795篇 |
1994年 | 5539篇 |
1993年 | 5279篇 |
1992年 | 6141篇 |
1991年 | 6080篇 |
1990年 | 5732篇 |
1989年 | 5511篇 |
1988年 | 5734篇 |
1987年 | 5455篇 |
1986年 | 5161篇 |
1985年 | 7177篇 |
1984年 | 7277篇 |
1983年 | 5926篇 |
1982年 | 6260篇 |
1981年 | 6151篇 |
1980年 | 5902篇 |
1979年 | 6291篇 |
1978年 | 6365篇 |
1977年 | 6319篇 |
1976年 | 6209篇 |
1975年 | 6008篇 |
1974年 | 5854篇 |
1973年 | 6033篇 |
1972年 | 3642篇 |
1971年 | 2736篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
C.X. Gao F.C. Yu S.Y. Jeong A.R. Choi P. Parchinskiy D.J. Kim H.J. Kim Y.E. Ihm C.G. Kim C.S. Kim 《Journal of magnetism and magnetic materials》2006
GaMnN and Be-codoped GaMnN were grown via molecular beam epitaxy using a single GaN precursor and their structural and magnetic properties were examined. X-ray diffraction and superconducting quantum interference device (SQUID) measurements revealed that the grown layers are homogeneous without precipitates. The saturation magnetization of GaMnN has increased from ∼4 to ∼16 emu/cm3 via codoping of Be. The d–d exchange interaction between Mn atoms was discussed for the ferromagnetism of GaMnN. 相似文献
992.
R. Pinna F.J.M. Rutten E.F. Smith M.R. Willis M.R.S. McCoustra 《Applied Surface Science》2006,252(19):6672-6675
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and polarisation-modulation reflection-absorption infrared spectroscopy (PM-RAIRS) have been used to monitor the surface synthesis of self-assembled aromatic π-conjugated molecular wires on gold substrates as a step towards a novel structure for organic electroluminescent devices (OLEDs). The wires have been synthesised using a series of Schiff's base coupling reactions in solution on a self-assembled monolayer of an aromatic thiolate anchor. ToF-SIMS and PM-RAIRS measurements have demonstrated that: (i) the anchor molecules self-assemble at the gold surface, (ii) the anchor molecules selectively react through imino coupling reactions with additional wire units with high efficiency and (iii) the wire-like structure is predominantly orientated normal to the surface. 相似文献
993.
R. Guerrero F.G. Aliev R. Villar J. Hauch M. Fraune G. Güntherodt K. Rott H. Brückl G. Reiss 《Journal of magnetism and magnetic materials》2006
We report on tunnelling magnetoresistance (TMR), current–voltage (IV) characteristics and low-frequency noise in epitaxially grown Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20 μm2. The evaluated MgO energy barrier (0.50±0.08 eV), the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩ μm2) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. At low temperatures (T<10 K) inelastic electron tunneling spectroscopy (IETS) shows anomalies related to phonons (symmetric structures below 100 meV) and asymmetric features above 200 meV. We explain the asymmetric features in IETS as due to generation of electron standing waves in one of the Fe electrodes. The noise power, though exhibiting a large variation, was observed to be roughly anti-correlated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to the influence of magnetostriction on the characteristics of the insulating barrier through changes in local barrier defects structure. 相似文献
994.
Justina Grabowska Karuna Kar Nanda R.T. Rajendra Kumar J.P. Mosnier M.O. Henry Simon B. Newcomb Patrick McNally Lisa OReilly Xu Lu Enda McGlynn 《Superlattices and Microstructures》2007,42(1-6):327
Self-organized ZnAl2O4 nanostructures with the appearance (in SEM) of high aspect ratio horizontal nanowires are grown on uncatalysed c-sapphire by vapour phase transport. The nanostructures grow as three equivalent crystallographic variants on c-sapphire. Raman and cathodoluminescence spectroscopy confirm that the nanostructures are not ZnO and TEM shows that they are the cubic spinel, zinc aluminate, ZnAl2O4, formed by the reaction of Zn and O with the sapphire substrate. 相似文献
995.
Oil migration in filled chocolates limits shelf life due to texture changes and loss of visual appeal. Spatial and temporal
oil concentration changes of a two-layer chocolate and peanut butter filling system, which models a filled chocolate, were
monitored by magnetic resonance imaging. Formulations of chocolate varied with respect to particle size, milk fat content
and emulsifier level. Sorption-time experimental data was used to establish the best fit diffusion coefficient for a Fickian
diffusion model. On the basis of the comparison of spatial experimental data to the model, Fickian diffusion alone does not
account for oil migration.
Authors' address: Michael J. McCarthy, Department of Food Science and Technology, University of California, One Shields Avenue,
Davis, CA 95616, USA 相似文献
996.
W.T. Ingram 《Topology and its Applications》2006,153(10):1530-1539
997.
Mössbauer spectroscopy was used to investigate the influence of dissolved hydrogen on the interatomic bonds in austenitic steels. It was carried out to prove the decohesion mechanism of hydrogen embrittlement (HE). It is shown that hydrogen increases Debye temperature, i.e., the interatomic bonds in hydrogenated austenitic steel becomes stronger. 相似文献
998.
Dielectric relaxation study of the binary mixtures of poly(vinyl pyrrolidone) (PVP) (Mw=24000, 40000 and 360000 g mol−1) with ethyl alcohol (EA) and poly(ethylene glycol)s (PEGs) (Mw=200 and 400 g mol−1) in dilute solutions of dioxane were carried out at 10.1 GHz and 35°C. The relaxation time of PVP-EA mixtures was interpreted
by the consideration of a wait-and-switch model in the local structure of self-associated ethyl alcohol molecules and also
the PVP chain length as a geometric constraint for the reorientational motion of ethyl alcohol molecules. The formation of
complexes and effect of PVP chain length on the molecular dynamics, chain flexibility and stretching of PEG molecules in PVP-PEG
mixtures were explored from the comparative values of dielectric relaxation time. Further, relaxation time values in dioxane
and benzene solvent confirm the viscosity independent molecular dynamics in PVP-EA mixtures but the values vary significantly
with the non-polar solvent environment. 相似文献
999.
Several techniques for experimental determination of floating point precision in practical computations are examined, and applied to linear algebra algorithms. These techniques are simple enough to be directly applicable to existing production codes, requiring a very limited amount of software on many machines, and yet they yield interesting information on the numerical precision of a computation. Our choice of linear algebra algorithms includes a direct solver (namely the MA32 program from the Harwell Library) and several variants of preconditioned conjugate gradients (the methods DIAG, INV, MINV and POL of Reference 1). The results may be of interest as method selection criteria, and thus complement Mflop performance data available from several sources. 相似文献
1000.