全文获取类型
收费全文 | 7774篇 |
免费 | 272篇 |
国内免费 | 13篇 |
专业分类
化学 | 4854篇 |
晶体学 | 147篇 |
力学 | 231篇 |
数学 | 770篇 |
物理学 | 2057篇 |
出版年
2023年 | 54篇 |
2022年 | 139篇 |
2021年 | 142篇 |
2020年 | 158篇 |
2019年 | 169篇 |
2018年 | 167篇 |
2017年 | 210篇 |
2016年 | 302篇 |
2015年 | 206篇 |
2014年 | 305篇 |
2013年 | 587篇 |
2012年 | 513篇 |
2011年 | 542篇 |
2010年 | 348篇 |
2009年 | 312篇 |
2008年 | 403篇 |
2007年 | 344篇 |
2006年 | 248篇 |
2005年 | 288篇 |
2004年 | 211篇 |
2003年 | 207篇 |
2002年 | 190篇 |
2001年 | 83篇 |
2000年 | 82篇 |
1999年 | 57篇 |
1998年 | 61篇 |
1997年 | 67篇 |
1996年 | 64篇 |
1995年 | 44篇 |
1994年 | 50篇 |
1993年 | 66篇 |
1992年 | 66篇 |
1991年 | 52篇 |
1990年 | 56篇 |
1989年 | 68篇 |
1988年 | 64篇 |
1987年 | 56篇 |
1986年 | 49篇 |
1985年 | 77篇 |
1984年 | 84篇 |
1983年 | 54篇 |
1982年 | 59篇 |
1981年 | 67篇 |
1980年 | 63篇 |
1979年 | 61篇 |
1978年 | 61篇 |
1977年 | 75篇 |
1976年 | 66篇 |
1975年 | 60篇 |
1974年 | 51篇 |
排序方式: 共有8059条查询结果,搜索用时 15 毫秒
931.
The lead salts and their alloys are extremely interesting semiconductors due to their technological importance. The fabrication of devices with alloys of these compounds possessing detecting and lasing capabilities has been an important recent technological development. The high quality polycrystalline thin films of PbSe1−xTex with variable composition (0≤x≤1) have been deposited onto ultra clean glass substrates by vacuum evaporation technique. As deposited films were annealed in vacuum at 350 K. The optical, electrical and structural properties of PbSe1−xTex thin films have been examined. The optical constants (absorption coefficient and bandgap) of the films were determined by absorbance measurements in the wavelength range 2500-5000 nm using Fourier transform infrared spectrophotometer. The dc conductivity and activation energy of the films were measured in the temperature range 300-380 K. The X-ray diffraction patterns were used to determine the sample quality, crystal structure and lattice parameter of the films. 相似文献
932.
The neat phases of a series of quaternary ammonium halides based on trioctadecylamine have been investigated by optical microscopy, differential scanning calorimetry, and H NMR spectroscopy. Phase behaviour depends primarily on the structure of the fourth substituent on nitrogen and the thermal history of the samples. For instance, the iodide salts with methyl, ethyl, or propyl as the fourth substituent form enantiotropic nematic phases, while those with a proton or a longer alkyl substituent exhibit complex phase behaviour that does not include a liquid crystalline phase. Possible reasons are advanced for the unprecedented formation of nematic phases. 相似文献
933.
We study hereL 1-convergence of a complex trigonometric sum and obtain a new necessary and sufficient condition for theL 1-convergence of Fourier series. 相似文献
934.
Y. K. Sharma S. N. Dolia A. Krishnamurthy B. K. Srivastava 《Hyperfine Interactions》1990,54(1-4):711-715
57Fe Mössbauer study of the pseudo-Binary alloys Fe1?xMnxSn2 for 0≤x≤1.0 reveal the antiferromagnetically ordered state at 79K for all the specimens of the series. The hyperfine field at Fe site decreases with increasing manganese concentration. Magnetic susceptibility measurements performed in the temperature range 80 K≤T≤300 K indicate that the Néel temperature decreases with increasing Mn concentration for the samples withx≤0.4 whereas it increases continuously for the specimens havingx>0.4 of the alloy series. 相似文献
935.
936.
A complex harmonic-oscillator basis is employed for the three-body problem obeying S
3-symmetry. Unlike a real basis it generates an additional quantum number (N
a
), in addition to the standard principal quantum number (N), and thus facilitates a more quantitative S
3-classification of the various states than is usually possible. It is shown that certain bilinear forms with definite S
3-symmetry properties, which can be constructed out of the linear harmonic-oscillator operators (a, a
†) satisfy several uncoupled sets of SO(2, 1) algebras with spectra bounded from below. It is also briefly indicated how this S
3-formalism can be adapted to the core structure of a more general relativistic three-particle system with unequal-mass kinematics
through an appropriate choice of internal variables.
Received May 11, 1994; revised November 3, 1994; accepted for publication November 23, 1994 相似文献
937.
J. M. Garcia V.M. Castaňo M.A. Mondragón E. Ramírez F. González A. Campero V. Rentería 《Journal of Sol-Gel Science and Technology》1997,8(1-3):911-916
Silica gels prepared by the sol-gel method with TEOS as a precursor and doped with the organic dyes rhodamine B, Na fluoresceine and eosine G were exposed to UV radiation pulses from a nitrogen laser. The absorption, luminescence and IR spectra were taken as a function of the exposure time. The absorption and the luminescence curves show successive maxima and minima. The same response is observed when the exposure procedure is repeated. These results are explained in terms of the cyclic filling and depletion of the long-lived triplet excited electronic state. At short exposure times, the data can be fitted with a Lorentzian curve and the corresponding parameters are given. 相似文献
938.
Anuj Sharma 《Tetrahedron》2008,64(11):2605-2610
A convenient microwave-assisted methodology is developed for the generation of 5-chloro-3-(dimethylamino)pyrazin-2(1H)-ones. The method entails a chemoselective desulfitative removal of a phenylthioether bond upon DMF/H2O treatment in the presence of sodium carbonate, yielding the desired compounds in 73-96%. 相似文献
939.
This paper deals with the theoretical investigation of the effect of magnetic field dependent (MFD) viscosity on the thermal convection in a ferromagnetic fluid in the presence of dust particles. For a flat ferromagnetic fluid layer contained between two free boundaries, the exact solution is obtained using a linear stability analysis and a normal mode analysis method. For the case of stationary convection, dust particles always have a destabilizing effect, whereas the MFD viscosity has a stabilizing effect on the onset of convection. In the absence of MFD viscosity, the destabilizing effect of magnetization is depicted but in the presence of MFD viscosity, non-buoyancy magnetization may have a destabilizing or a stabilizing effect on the onset of convection. The critical wave number and critical magnetic thermal Rayleigh number for the onset of stationary convection are also determined numerically for sufficiently large values of buoyancy magnetization parameter M 1. Graphs have been plotted by giving numerical values to the parameters to depict the stability characteristics. It is observed that the critical magnetic thermal Rayleigh number is reduced solely because the heat capacity of clean fluid is supplemented by that of the dust particles. The principle of exchange of stabilities is found to hold true for the ferromagnetic fluid heated from below in the absence of dust particles. The oscillatory modes are introduced due to the presence of the dust particles, which were non-existent in their absence. A sufficient condition for the non-existence of overstability is also obtained. 相似文献
940.
Reactions of copper(I) halides (Cl, Br, I) with 1‐methyl‐1, 3‐imidazoline‐2‐thione (mimzSH) in 1 : 2 molar ratio yielded sulfur‐bridged dinuclear [Cu2X2(μ‐S‐mimzSH)2(η1‐S‐mimzSH)2] (X = I, 1 , Br, 2 ; Cl, 3 ) complexes. Copper(I) iodide with 1,3‐imidazoline‐2‐thione (imzSH2) and Ph3P in 1 : 1 : 1 molar ratio has also formed a sulfur‐bridged dinuclear [Cu2I2(μ‐S‐imzSH2)2(PPh3)2] ( 4 ) complex. The central Cu(μ‐S)2Cu cores form parallelograms with unequal Cu–S bond distances {2.324(2), 2.454(3) Å} ( 1 ); {2.3118(6), 2.5098(6) Å} ( 2 ); {2.3075(4), 2.5218(4) Å} ( 3 ); {2.3711(8), 2.4473(8) Å} ( 4 ). The Cu···Cu separations, 2.759–2.877Å in complexes 1 – 3 are much shorter than 3.3446Å in complex 4 . The weak intermolecular interactions {H2CH···S# ( 2 ); CH···Cl# ( 3 ); NH···I# ( 4 )} between dimeric units in complexes 2 – 4 lead to the formation of linear 1D polymers. 相似文献