全文获取类型
收费全文 | 3872篇 |
免费 | 109篇 |
国内免费 | 10篇 |
专业分类
化学 | 3138篇 |
晶体学 | 99篇 |
力学 | 171篇 |
数学 | 106篇 |
物理学 | 477篇 |
出版年
2023年 | 25篇 |
2022年 | 47篇 |
2021年 | 54篇 |
2020年 | 72篇 |
2019年 | 92篇 |
2018年 | 96篇 |
2017年 | 73篇 |
2016年 | 126篇 |
2015年 | 90篇 |
2014年 | 170篇 |
2013年 | 423篇 |
2012年 | 262篇 |
2011年 | 230篇 |
2010年 | 144篇 |
2009年 | 136篇 |
2008年 | 223篇 |
2007年 | 216篇 |
2006年 | 157篇 |
2005年 | 159篇 |
2004年 | 150篇 |
2003年 | 131篇 |
2002年 | 107篇 |
2001年 | 41篇 |
2000年 | 43篇 |
1999年 | 34篇 |
1998年 | 21篇 |
1997年 | 24篇 |
1996年 | 41篇 |
1995年 | 25篇 |
1994年 | 30篇 |
1993年 | 30篇 |
1992年 | 27篇 |
1991年 | 24篇 |
1990年 | 38篇 |
1989年 | 30篇 |
1988年 | 25篇 |
1987年 | 29篇 |
1986年 | 28篇 |
1985年 | 37篇 |
1984年 | 39篇 |
1983年 | 29篇 |
1982年 | 26篇 |
1981年 | 24篇 |
1980年 | 23篇 |
1979年 | 18篇 |
1978年 | 13篇 |
1977年 | 21篇 |
1976年 | 16篇 |
1975年 | 18篇 |
1974年 | 9篇 |
排序方式: 共有3991条查询结果,搜索用时 15 毫秒
31.
G. Murali D. Amaranatha Reddy B. PoornaPrakash R.P. Vijayalakshmi B.K. Reddy R. Venugopal 《Physica B: Condensed Matter》2012,407(12):2084-2088
Undoped and Fe doped CdS nanocrystals with Fe content of 2–5 at% of average crystallite size 1.2–2 nm have been obtained using chemical co-precipitation method with 2-mercaptoethonal as capping agent at 80 °C. X-ray diffraction (XRD) results showed that the undoped CdS nanocrystals were in mixed phase of cubic and hexagonal, where as the doped CdS nanocrystals were in hexagonal phase. Room-temperature ferromagnetism has been observed in Fe-doped CdS nanocrystals. Magnetic studies indicated diamagnetism in undoped, ferromagnetism in lightly doped (2 and 3 at%) and paramagnetism in samples of higher Fe content (4 and 5 at%). The substitutional incorporation of Fe3+ ion in Cd2+ sites was reflected in structural and electron paramagnetic resonance (EPR) measurements. Isolated as well as interacting Fe3+ ions are observed in EPR. 相似文献
32.
S. Lakshmi Reddy N. C. Gangi Reddy R. Rama Subba Reddy G. Siva Reddy P. Sambasiva Rao B. Jagannatha Reddy 《辐射效应与固体损伤》2013,168(11):671-679
A powdered sample of Phyllanthus amarus herb of Kadapa district of Andhra Pradesh, India, is used in the present study. ICP-MS analysis indicates that copper is present in higher concentration when compared to other elements. Although the Pb is toxic, it is within the permissible limit. The evaluated soil and herb physico-chemical parameters indicate that the sample is acidic in nature in comparison with the soil. An EPR study on powdered sample confirms the presence of Fe(III), Mn(II) and Cu(II). Optical absorption spectrum indicates that Fe(III) impurity is present in octahedral structure whereas Cu(II) is present in rhombically distorted octahedral environment. MIR results are due to carbonate fundamentals. 相似文献
33.
T. V. M. Sreekanth S. Ramanaiah Kap Duk Lee K. S. Reddy 《Journal of Macromolecular Science: Physics》2013,52(6):1256-1266
The Flory–Huggins interaction parameters, for a solute with pure nonvolatile solvents and with binary mixtures of nonvolatile solvents were evaluated using Inverse Gas Chromatography (IGC) data at 403.15 K. The five solvents used have similar structure and nearly the same molar volume and their mixtures form regular solutions. The values were used to evaluate the Hansen solubility parameters (HSP) for the pure solvents and mixed solvent systems. The HSP are plotted as a function of composition of the polar solvent. The dispersion component, , and the polar component, , exhibited negative deviation whereas the hydrogen bonding component, , showed positive deviation in the four mixtures. However, the total solubility parameter was found to be almost linear with composition in the four binary systems. Further the total solubility parameter, , was used to calculate the molar excess heat of vaporization for the solvents (C78 + POH, C78 + PCN, C78 + TTF, and C78 + TMO) that was found to be negative for C78 (19,24-dioctadecyldotetracontane) + POH(18,23-dioctadecylhentetracontan-1-ol) and C78 + PCN(1-cyano-18,23-dioctadecylhentetracontane) systems and positive for C78 + TTF [19,24-bis(18,18,18-trifluorooctadecyl)-1,1,1,42,42,42-hexafluorodotetracontane), and C78 + TMO(17,22-bis-(16-methoxyhexadecyl)-1,38-dimethoxyoctatriacontane] and the other systems. 相似文献
34.
ABSTRACT The present work describes a phenomenological approach to explain the instantaneous behaviour of tungsten heavy alloys (WHAs) in heat-treated and swaged conditions. The strengths and elongation values of heat-treated materials are lower and higher than those of the swaged samples respectively. The heat-treated materials exhibit two slopes in true stress–true plastic strain curves and follow the Ludwigson constitutive equation. On the other hand, swaged materials display a single slope and adhere to typical Swift constitutive equation. The latter reflect the presence of pre-strain in the materials due to swaging deformation. The fracture surfaces in heat-treated materials consist of W-W decohesion along with matrix rupture and W-cleavage, while swaged samples consist of mainly W-cleavage. Both the materials display three typical stages (I, II and III) of work hardening. The second derivatives of true stress–true plastic strain curves of these alloys exhibit a perfect parabola although the nature of true stress–true strain as well as true stress–true plastic strain curves is quite different in heat-treated and swaged materials. This has been observed for the first time in WHAs consisting of matrix and W-grains. The shape of the parabola is simple and easy to fit. The fitting parameters of parabolas have been successfully employed to explain the flow behaviour of a large number of tungsten heavy alloys having two-phase microstructure in different processing conditions. 相似文献
35.
The effect of tetravalent Ti+4 substitution in Mg0.95Mn0.05Fe2O4 on its magnetic and electrical properties has been studied using X-ray diffraction, Mössbauer spectroscopy, isothermal dc magnetization and dielectric measurements. X-ray diffraction studies have shown the structural transformation from cubic to tetragonal with the Ti+4 substitution. The Mössbauer spectra of Mg0.95Mn0.05Fe1.0Ti1.0O4 recorded in the temperature range 20-300 K shows the presence of the magnetic as well as quadrupole interactions. The isothermal hysteresis loop infers that the system exhibits a ferrimagnetic ordering at room temperature. The Zero-field-cooled (ZFC) and field-cooled (FC) magnetization studies support ferrimagnetic ordering of Mg0.95Mn0.05Fe1.0Ti1.0O4 at room temperature. Signatures of ferroelectric transition have been observed in the temperature range 200-300 K from dielectric measurements. The observed magnetic and dielectric behaviour indicate that this material exhibits multiferroic behaviour. 相似文献
36.
37.
Structural, morphological and optical properties of rare earth ions (RE3+=Sm3+ or Dy3+) activated Ca3Ga2Si3O12 (CaGaSi) phosphors synthesized by the sol-gel method are reported. XRD results confirmed the cubic phase structure of RE3+:CaGaSi phosphors. From the SEM images of RE3+:CaGaSi phosphors, it is observed that the particles are agglomerated. Photoluminescence spectra of Sm3+:CaGaSi phosphors have shown bright orange red emission at 598 nm (4G5/2→6H7/2) with an excitation wavelength of λexci=401 nm. In the case of Dy3+:CaGaSi phosphors bright yellow emission has been observed at 574 nm (4F9/2→ 6H13/2) with λexci=451 nm. From the PL spectral results, the rare earth ion concentration of CaGaSi phosphors is optimized. 相似文献
38.
R. Ravinder Reddy M. Muralidhar V. Hari Babu P. Venugopal Reddy 《Physica C: Superconductivity and its Applications》1994,220(3-4):367-372
Bi-Pb and Bi-Pb-Ag superconducting composites have been prepared by the solid state reaction method. After the usual characterization, ultrasonic longitudinal velocity and attenuation studies have been undertaken over a temperature range 80–300 K by the pulse transmission technique. In contrast to normal solids, the ultrasonic velocities of both the samples in the temperature range 200-100 K are found to decrease with decreasing temperature (softening), followed by a velocity maximum. The samples are also found to exhibit longitudinal attenuation peaks at TEMPERATURES = 260, 160 and 120 K. An attempt has also been made to verify whether the Wachtman's equations can theoretically explain the low-temperature behaviour of the Young's modulus of these materials. A qualitative explanation for both the phenomena of softening of velocity as well as the occurrence of attenuation peaks is given. 相似文献
39.
In the investigation reported here an attempt has been made to study the influence of Berger's approximation on the non-linear transient response of circular plates and shallow spherical shells. The governing equations of motion obtained from Berger's approximation are solved by using the rapidly converging Chebyshev series spacewise and the Houbolt scheme for integration in the time domain. Results calculated when using Berger's approximation are compared with exact results. It is shown that Berger's method yields very accurate values for plates and shells under transient loading, in the case of immovable edge conditions. 相似文献
40.
G. Sreenivasulu Reddy J. Hanumanthachari T. Vasanthi 《Southeast Asian Bulletin of Mathematics》2003,26(2):331-335
This paper is mainly dealt with the structure of totally ordered rings (t.o. rings) whose positive cones are finitely generated as multiplicative right ideals.AMS Subject Classification (1991): 16W80, 13J25 相似文献