全文获取类型
收费全文 | 1623篇 |
免费 | 29篇 |
国内免费 | 4篇 |
专业分类
化学 | 1082篇 |
晶体学 | 19篇 |
力学 | 29篇 |
数学 | 88篇 |
物理学 | 438篇 |
出版年
2024年 | 19篇 |
2023年 | 17篇 |
2022年 | 39篇 |
2021年 | 34篇 |
2020年 | 34篇 |
2019年 | 32篇 |
2018年 | 31篇 |
2017年 | 37篇 |
2016年 | 50篇 |
2015年 | 31篇 |
2014年 | 43篇 |
2013年 | 154篇 |
2012年 | 113篇 |
2011年 | 111篇 |
2010年 | 73篇 |
2009年 | 79篇 |
2008年 | 86篇 |
2007年 | 57篇 |
2006年 | 48篇 |
2005年 | 55篇 |
2004年 | 40篇 |
2003年 | 27篇 |
2002年 | 36篇 |
2001年 | 25篇 |
2000年 | 24篇 |
1999年 | 18篇 |
1998年 | 7篇 |
1997年 | 15篇 |
1996年 | 17篇 |
1995年 | 20篇 |
1994年 | 14篇 |
1993年 | 21篇 |
1992年 | 7篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 14篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 13篇 |
1984年 | 16篇 |
1983年 | 16篇 |
1982年 | 8篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 15篇 |
1977年 | 19篇 |
1974年 | 6篇 |
1973年 | 9篇 |
排序方式: 共有1656条查询结果,搜索用时 15 毫秒
31.
32.
Vibhav Pandey R.P. Aloysius V.P.S. Awana R.K. Kotnala 《Journal of magnetism and magnetic materials》2009,321(14):2239-2244
The structural magnetic and magneto-transport properties of double perovskite system Ba2−xSrxFeMoO6 (0?x?1.0) prepared in bulk polycrystalline form are reported in this paper. X-ray diffraction analysis showed that samples are single phase and the lattice constants decreases with increase in the Sr content. The degree of Fe-Mo ordering has been found decreasing in the series with an increase in the Sr content. Parent compound Ba2FeMoO6 exhibits saturation magnetic moment value of 3.54 μB/f.u. at 85 K in a magnetic field of 6000 Oe. Temperature dependence of resistivity shows metallic behavior for all the samples. The magneto-resistance (MR) of the compound with x=0.4 is higher than that of the other samples. At room temperature this system shows a saturation magnetization value of 1.73 μB/f.u. and MR value of 7.08% (1 T). The observed variations in the structural and magnetic properties are attributed to the change of chemical pressure due to the substitution of Sr in place of Ba. The effect of antisite disorder (ASD) defects on magneto-transport properties is studied in more detail. 相似文献
33.
K.K.M. Pandey G.M. Chow B.C. Lim 《Journal of magnetism and magnetic materials》2009,321(19):3236-3240
The effects of interface roughness of Ta seedlayer on the structural and magnetic properties of Co72Pt28(20 nm)/Ru(30 nm)/Pt(2 nm)/Ta(5 nm)/glass were investigated. Uniaxial perpendicular magnetic anisotropy (8.6×106 ergs/cc), coercivity (5.5 kOe) and nucleation field (−2.8 kOe) in the Co72Pt28 thin film sputter-deposited on relatively smooth surface of Ta seedlayer were achieved. The results showed that relatively smoother interface roughness of Ta seedlayer improved the CoPt/Ru (0 0 0 2) texture and magnetic properties. 相似文献
34.
Temperature dependent ultrasonic properties of aluminium nitride 总被引:1,自引:0,他引:1
Hexagonal wurtzite structured aluminium nitride has been characterized by the theoretical calculation of ultrasonic attenuation, ultrasonic velocity, higher order elastic constants, thermal relaxation time, acoustic coupling constants and other related parameters in temperature range 200-800 K for wave propagation along the unique axis of the crystal. Higher order elastic constants of AlN at different temperatures are calculated using Lennard-Jones potential for the determination of ultrasonic attenuation. A decrease in ultrasonic velocity with temperature has been predicted, which is caused by reduction in higher order elastic constants with temperature. The temperature dependent ultrasonic properties have been discussed in correlation with higher order elastic constants, thermal relaxation time, thermal conductivity, acoustic coupling constants and thermal energy density. Anomalous behaviour of the attenuation is found at 400 K. On the basis of attenuation, the ductility and performance of AlN have been studied. 相似文献
35.
P.?Bhargava M.?Kumar H.?Kumar P.?Pandit R.?Pandey A.?K.?NathEmail author 《Pramana》2004,62(4):923-932
We have studied about the thrust imparted to targets of different materials by pulsed TEA CO2 laser and chopped CW CO2 laser in air and its dependence on different parameters such as laser intensity and pulse duration. We estimated the impulse-coupling
coefficient and compared it with the published results. The mechanism of generation of thrust by laser incident on targets
in air is, in effect, combination of those involved in laser ablation in vacuum and laser-induced air detonation. 相似文献
36.
R. B. Pandey 《Journal of statistical physics》1989,54(3-4):997-1010
Immune response in a retrovirus system is modeled by a network of three binary cell elements to take into account some of the main functional features of T4 cells, T8 cells, and viruses. Two different intercell interactions are introduced, one of which leads to three fixed points while the other yields bistable fixed points oscillating between a healthy state and a sick state in a mean field treatment. Evolution of these cells is studied for quenched and annealed random interactions on a simple cubic lattice with a nearest neighbor interaction using inhomogenous cellular automata. Populations of T4 cells and viral cells oscillate together with damping (with constant amplitude) for annealed (quenched) interaction on increasing the value of mixing probabilityB from zero to a characteristic valueB
ca (B
cq). For higherB, the average number of T4 cells increases while that of the viral infected cells decreases monotonically on increasingB, suggesting a phase transition atB
ca (B
cq). 相似文献
37.
José R. B. Gomes Emanuel A. Sousa Jorge M. Gonçalves Luís Gales Ana M. Damas Paula Gomes Siddharth Pandey William E. Acree Jr Maria D. M. C. Ribeiro da Silva 《Journal of Physical Organic Chemistry》2007,20(7):491-498
The gaseous standard molar enthalpies of formation of two 2‐R‐3‐methylquinoxaline‐1,4‐dioxides (R = benzoyl or tert‐butoxycarbonyl), at T = 298.15 K, were derived using the values for the enthalpies of formation of the compounds in the condensed phase, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, using a quartz crystal oscillator. The three dimensional structure of 2‐tert‐butoxycarbonyl‐3‐methylquinoxaline‐1,4‐dioxide has been obtained by X‐ray crystallography showing that the two N? O bond lengths in this compound are identical. The experimentally determined geometry in the crystal is similar to that obtained in the gas‐phase after computations performed at the B3LYP/6‐311 + G(2d,2p) level of theory. The experimental and computational results reported allow to extend the discussion about the influence of the molecular structure on the dissociation enthalpy of the N? O bonds for quinoxaline 1,4‐dioxide derivatives. As found previously, similar N? O bond lengths in quinoxaline‐1,4‐dioxide compounds are not linked with N? O bonds having the same strength. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
38.
Abhishek Verma Swati Nagpal Praveen K. Pandey P. K. Bhatnagar P. C. Mathur 《Journal of nanoparticle research》2007,9(6):1125-1131
Quantum Dots of CdS
x
Se1−x
embedded in borosilicate glass matrix have been grown using Double-Step annealing method. Optical characterization of the
quantum dots has been done through the combinative analysis of optical absorption and photoluminescence spectroscopy at room
temperature. Decreasing trend of photoluminescence intensity with aging has been observed and is attributed to trap elimination.
The changes in particle size, size distribution, number of quantum dots, volume fraction, trap related phenomenon and Gibbs
free energy of quantum dots, has been explained on the basis of the diffusion-controlled growth process, which continues with
passage of time. For a typical case, it was found that after 24 months of aging, the average radii increased from 3.05 to
3.12 nm with the increase in number of quantum dots by 190% and the size-dispersion decreased from 10.8% to 9.9%. For this
sample, the initial size range of the quantum dots was 2.85 to 3.18 nm. After that no significant change was found in these
parameters for the next 12 months. This shows that the system attains almost a stable nature after 24 months of aging. It
was also observed that the size-dispersion in quantum dots reduces with the increase in annealing duration, but at the cost
of quantum confinement effect. Therefore, a trade off optimization has to be done between the size-dispersion and the quantum
confinement. 相似文献
39.
The ferroelectric ceramics Ba5RTi3V7O30 (R=Ho, Gd, La) have been synthesized by solid-state reaction technique. Preliminary X-ray structural analysis confirmed a single-phase formation of the compound in orthorhombic structure. Surface morphology of the compounds was studied by scanning electron microscopy (SEM). Detailed studies of electrical properties (i.e., dielectric constant, loss tangent, ac conductivity) as a function of temperature (RT-773 K) and at four different frequencies (1 kHz, 10 kHz, 100 kHz and 1 MHz) show ferroelectric–paraelectric phase transition of the compounds of diffuse-type. The activation energy has been evaluated from ac conductivity following Arrhenius equation. The conductivity pattern shows that it is strongly frequency dependent and obeys Jonscher's power relation. 相似文献
40.
ZnO nanoparticles have been synthesized by ultrasonic irradiation of an aqueous-alcoholic/aqueous-alcoholic-ethylenediamine (EDA) solutions of zinc nitrate and sodium hydroxide. ZnO nanoparticles possess hexagonal wurtzite structures and they exhibit special photoluminescence properties with a red-shift of 22 nm in UV emission band. It is found that the ultrasonic irradiation time and the solvents both influence the growth mechanism and optical properties of ZnO nanoparticles. The possible growth mechanism of ZnO nanoparticles formation by sonochemical method has been tried to discuss. 相似文献