全文获取类型
收费全文 | 43216篇 |
免费 | 3267篇 |
国内免费 | 2315篇 |
专业分类
化学 | 23482篇 |
晶体学 | 290篇 |
力学 | 4113篇 |
综合类 | 588篇 |
数学 | 8838篇 |
物理学 | 11487篇 |
出版年
2024年 | 76篇 |
2023年 | 425篇 |
2022年 | 1247篇 |
2021年 | 1192篇 |
2020年 | 1145篇 |
2019年 | 1082篇 |
2018年 | 986篇 |
2017年 | 1292篇 |
2016年 | 1664篇 |
2015年 | 1298篇 |
2014年 | 1875篇 |
2013年 | 3072篇 |
2012年 | 2494篇 |
2011年 | 2334篇 |
2010年 | 1980篇 |
2009年 | 2497篇 |
2008年 | 2443篇 |
2007年 | 2725篇 |
2006年 | 2247篇 |
2005年 | 2059篇 |
2004年 | 1976篇 |
2003年 | 1649篇 |
2002年 | 1287篇 |
2001年 | 1051篇 |
2000年 | 1027篇 |
1999年 | 918篇 |
1998年 | 854篇 |
1997年 | 766篇 |
1996年 | 710篇 |
1995年 | 685篇 |
1994年 | 580篇 |
1993年 | 481篇 |
1992年 | 447篇 |
1991年 | 382篇 |
1990年 | 346篇 |
1989年 | 238篇 |
1988年 | 231篇 |
1987年 | 149篇 |
1986年 | 124篇 |
1985年 | 150篇 |
1984年 | 126篇 |
1983年 | 64篇 |
1982年 | 101篇 |
1981年 | 66篇 |
1980年 | 50篇 |
1979年 | 78篇 |
1978年 | 38篇 |
1977年 | 16篇 |
1976年 | 23篇 |
1974年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
81.
82.
83.
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
84.
《Journal of solid state chemistry》2004,177(7):2366-2372
Proton exchange reactions have been performed on tetragonal tungsten bronze-like NaNbWO6 by using nitric acid as an exchanging agent. The characterization of the exchange reaction products has been made by means of chemical analysis, X-ray diffraction, thermal analysis, and IR spectroscopy. The exchange reaction takes place topotactically and the following formula is proposed for the obtained phase of variable composition: Na1−xHxNbWO6·yH2O (0<x?0.46 and 0?y?0.12). Impedance spectroscopy on the present proton exchanged samples indicated that these samples behaved as solid electrolytes under high humidity. As an example, the compound with the composition Na0.68H0.32NbWO6·0.1 H2O exhibits ionic conductivity of 8×10−3 and 1×10−2 S cm−1 at 70°C and 90°C, respectively. 相似文献
85.
The ultraviolet band systemsA
1Π-X
1Σ+ of P14N and P15N were excited in an electrodeless tube containing traces of phosphorus specpure nitrogen and neon using a microwave discharge
(2450 MHz). Bands of the isotopic species, P15N, were obtained using15N2 enriched to 95.5%. Rotational analyses of eleven bands of P14N and sixteen bands of P15N were carried out. Three perturbing statese
3Σ−,d
3Δ andb
3Π, arising from the lower valence configurations were identified from the observed perturbations in thev′=0–4 levels of theA
1Π state. Deperturbation studies led to the determination of molecular constants of the perturbing states. Vibrational assignments
of the perturbing states were made from isotope shift studies. 相似文献
86.
N. Fitier 《国际流体数值方法杂志》2003,42(12):1345-1361
There is a growing interest in developing numerical tools to investigate the onset of physical instabilities observed in experiments involving viscoelastic flows, which is a difficult and challenging task as the simulations are very sensitive to numerical instabilities. Following a recent linear stability analysis carried out in order to better understand qualitatively the origin of numerical instabilities occurring in the simulation of flows viscoelastic fluids, the present paper considers a possible extension for more complex flows. This promising method could be applied to track instabilities in complex (i.e. essentially non‐parallel) flows. In addition, results related to transient growth mechanism indicate that it might be responsible for the development of numerical instabilities in the simulation of viscoelastic fluids. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
87.
K. K. Turgunov B. Tashkhodzhaev L. V. Molchanov Kh. M. Shakhidoyatov 《Journal of Structural Chemistry》2004,45(5):912-916
The structure of bis-(2,3-pentamethylene-3,4-dihydro-4-oxoquinazolinium) tetrachlorocuprate (II) sesquihydrate, (C13H15N2O)2[CuCl4]·1.5H2O, was determined by single crystal X-ray diffraction. In contrast to the previously studied analogs, the compound contains crystallization water molecules.Original Russian Text Copyright © 2004 by K. K. Turgunov, B. Tashkhodzhaev, L. V. Molchanov, and Kh. M. Shakhidoyatov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 955–959, September–October, 2004. 相似文献
88.
Lígia M. Rodrigues 《Tetrahedron》2004,60(40):8929-8936
Tetrapeptides containing one of a set of four different α,α-dialkyl glycines at the C-terminus were synthesized by conventional methods in solution and their conformational behavior investigated by 1H NMR spectroscopy in connection with molecular mechanics calculations. The results were consistent with conformations stabilized by a γ-turn in the case of compounds with alkyl groups larger than methyl, while the corresponding Aib derivative did not exhibit intramolecular hydrogen bonding. 相似文献
89.
Numerical simulation aspects, related to low Reynolds number free boundary viscous flows at micro and mesolevel during the resin impregnation stage of the liquid composite moulding process (LCM), are presented in this article. A free boundary program (FBP), developed by the authors, is used to track the movement of the resin front accurately by accounting for the surface tension effects at the boundary. Issues related to the global and local mass conservation (GMC and LMC) are identified and discussed. Unsuitable conditions for LMC and consequently GMC are uncovered at low capillary numbers, and hence a strategy for the numerical simulation of such flows is suggested. FBP encompasses a set of subroutines that are linked to modules in ANSYS. FBP can capture the void formation dynamics based on the analysis developed. We present resin impregnation dynamics in two dimensions. Extension to three dimensions is a subject for further research. Several examples are shown and efficiency of different stabilization techniques are compared. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
90.
Heterogeneity is typically the result of space variability of soil parameters at different scales. Soil anisotropy may be defined as the spatial persistence in some direction only, across coarse-grid elements, of heterogeneous structures with different characteristic lengths in different directions. One can account for the effect of these structures by upscaling soil properties. Analyzing flow in a strongly anisotropic structured soil at different scales evidences how transverse dispersion reduces to a subscale process, leading to mixing within the conductive structures. 相似文献