全文获取类型
收费全文 | 29930篇 |
免费 | 5205篇 |
国内免费 | 3333篇 |
专业分类
化学 | 13673篇 |
晶体学 | 428篇 |
力学 | 4577篇 |
综合类 | 354篇 |
数学 | 7524篇 |
物理学 | 11912篇 |
出版年
2024年 | 67篇 |
2023年 | 321篇 |
2022年 | 750篇 |
2021年 | 749篇 |
2020年 | 1021篇 |
2019年 | 886篇 |
2018年 | 881篇 |
2017年 | 1082篇 |
2016年 | 1272篇 |
2015年 | 1093篇 |
2014年 | 1605篇 |
2013年 | 2727篇 |
2012年 | 1817篇 |
2011年 | 1783篇 |
2010年 | 1553篇 |
2009年 | 1653篇 |
2008年 | 1756篇 |
2007年 | 1932篇 |
2006年 | 1743篇 |
2005年 | 1628篇 |
2004年 | 1431篇 |
2003年 | 1457篇 |
2002年 | 1175篇 |
2001年 | 1066篇 |
2000年 | 1039篇 |
1999年 | 865篇 |
1998年 | 808篇 |
1997年 | 655篇 |
1996年 | 542篇 |
1995年 | 461篇 |
1994年 | 405篇 |
1993年 | 358篇 |
1992年 | 292篇 |
1991年 | 229篇 |
1990年 | 197篇 |
1989年 | 136篇 |
1988年 | 136篇 |
1987年 | 116篇 |
1986年 | 103篇 |
1985年 | 113篇 |
1984年 | 91篇 |
1983年 | 40篇 |
1982年 | 77篇 |
1981年 | 65篇 |
1980年 | 49篇 |
1979年 | 52篇 |
1978年 | 56篇 |
1977年 | 41篇 |
1976年 | 29篇 |
1971年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Akhlesh Lakhtakia 《Optik》2004,115(4):151-158
52.
The influence of the parameter of amplitude-phase coupling for nonlinear amplification on the phase-frequency characteristics of an injection laser in the regime of pumping current modulation has been investigated theoretically. The parameter is defined as the coefficient of proportionality between changes in the real and imaginary parts of the refractive index during variation of the density of photons in the cavity. The substantial dependence of the coefficient of phase modulation on the parameter of the amplitude-phase coupling for nonlinear amplification at small modulation frequencies is shown. A technique to determine the indicated parameter based on the frequency dependence of the ratio between the coefficients of phase and amplitude modulation is suggested. 相似文献
53.
N. S. Kazak E. G. Katranji I. A. Utkin A. A. Ryzhevich A. N. Khilo 《Journal of Applied Spectroscopy》2004,71(5):697-701
The phenomenon of self-diffraction of Bessel light beams (BLB) in a nonlinear liquid medium has been studied experimentally and theoretically for the first time. Diffraction maxima which do not correspond to integer orders for an induced periodic structure have been registered. It has been shown that the appearance of these maxima is due to the initial BLB modulation, which can be caused by the departure of the axicon refracting surface from the ideal conical surface, as well as by the imperfection of the form of the Gaussian beam incident on the axicon. 相似文献
54.
本文利用比较原理以及一种特殊变换研究了一类非线性退化方程的混合问题,得到了这类复杂问题解的渐进性质。 相似文献
55.
Teimuraz Pirashvili 《Central European Journal of Mathematics》2003,1(2):169-183
In this paper we consider duplexes, which are sets with two associative binary operations. Dimonoids in the sense of Loday
are examples of duplexes. The set of all permutations carries a structure of a duplex. Our main result asserts that it is
a free duplex with an explicitly described set of generators. The proof uses a construction of the free duplex with one generator
by planary trees. 相似文献
56.
On the basis of the hydrodynamic equations for nonlinear elastic-gravity waves beneath a solid ice cover and their Hamiltonian representation, a three-wave kinetic equation for the time evolution of the wave spectrum is formulated. The properties of the kernel of the kinetic integral describing the nonlinear interactions between wave triplets are investigated. An algorithm for numerically calculating the kinetic integral is developed. The rate of nonlinear energy transfer over the wave spectrum is estimated quantitatively and its most important characteristics are found. 相似文献
57.
Yong‐Jin Kim Jin‐Woong Kim Jung‐Eun Lee Jee‐Hyun Ryu Junoh Kim Ih‐Seop Chang Kyung‐Do Suh 《Journal of polymer science. Part A, Polymer chemistry》2004,42(22):5627-5635
Mesoporous polymer microspheres with gold (Au) nanoparticles inside their pores were prepared considering their surface functionality and porosity. The Au/polymer composite microspheres prepared were characterized by transmission electron microscope (TEM), X‐ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) techniques. The results showed that the adsorption of Au nanoparticles could be increased by imparting the pore structure and surface‐functional groups into the supporting polymer microspheres (in this study, poly (ethylene glycol dimethacrylate‐co‐acrylonitrile) and poly (EGDMA‐co‐AN) system). Above all, from this study, it was established that the porosity of the polymer microspheres is the most important factor that determines the distribution and adsorption amount of face‐centered cubic (fcc) Au nanoparticles in the final products. Our study showed that the continuous adsorption of Au nanoparticles with the aid of the large surface area and surface interaction sites formed more favorably the Au/polymer composite microspheres. The BET measurements of Au/poly(EGDMA‐co‐AN) composite microspheres reveals that the adsorption of Au nanoparticles into the pores kept the pore structure intact and made it more porous. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5627–5635, 2004 相似文献
58.
X.X. Guo 《Surface science》2004,549(3):211-216
We studied parallel conductivities of pure BaF2 films with thicknesses ranging from 35 to 300 nm, epitaxially grown on Al2O3(0 1 2) substrates by molecular beam epitaxy technique. The overall conductivities of the films are found to increase with decreasing thickness. The detailed investigation of the overall conductance as a function of the thickness permits the deconvolution of bulk and boundary effects, the latter being attributed to distinct space charge effects in the interface between BaF2 film and Al2O3 substrate. The (extrinsic) Debye length (λ) is estimated to be about 8 nm at T=593 K, which corresponds to an impurity content of 1018/cm3 (singly ionized dopant assumed). This is consistent with the fact that we observed a constant boundary contribution for all investigated films (film thickness >4λ). It is also consistent with the Debye length observed in a previous report on CaF2/BaF2 heterolayers fabricated by the same technique, in which the low temperature enhancement was also attributed to space charges in BaF2 [Nature 408 (2000) 946]. Only at low temperatures (below 370 °C), the conductance seems to be influenced by strain effect. 相似文献
59.
Characteristics and structures of weak efficient surfaces of production possibility sets 总被引:1,自引:0,他引:1
Quanling Wei Hong Yan Gang Hao 《Journal of Mathematical Analysis and Applications》2007,327(2):1055-1074
This paper studies the characteristics and structure of the weak surface of the production possibility set. We apply techniques and methods of transferring a polyhedral cone from its intersection form to its sum form, identify an intersection representation of the production possibility set. We give the structure theorem of weak surface of the production possibility set, which includes three complementary slackness conditions. We define the input weak efficiency and output weak efficiency for different DEA models according to the representation of the intersection form. It investigates the characteristics of the weak surfaces, and proves the structure theorems of input weak DEA efficiency and output weak DEA efficiency. The structure theorems establish weighted combination of inputs and outputs that are weak DEA efficient. Numerical examples are provided for illustration. 相似文献
60.
We study the generalization of the Willmore functional for surfaces in the three-dimensional Heisenberg group. Its construction is based on the spectral theory of the Dirac operator entering into theWeierstrass representation of surfaces in this group. Using the surfaces of revolution we demonstrate that the generalization resembles the Willmore functional for the surfaces in the Euclidean space in many geometrical aspects. We also observe the relation of these functionals to the isoperimetric problem. 相似文献