全文获取类型
收费全文 | 21954篇 |
免费 | 919篇 |
国内免费 | 112篇 |
专业分类
化学 | 15062篇 |
晶体学 | 137篇 |
力学 | 578篇 |
综合类 | 1篇 |
数学 | 3423篇 |
物理学 | 3784篇 |
出版年
2023年 | 176篇 |
2022年 | 283篇 |
2021年 | 555篇 |
2020年 | 443篇 |
2019年 | 433篇 |
2018年 | 457篇 |
2017年 | 374篇 |
2016年 | 882篇 |
2015年 | 755篇 |
2014年 | 791篇 |
2013年 | 1448篇 |
2012年 | 1506篇 |
2011年 | 1647篇 |
2010年 | 1016篇 |
2009年 | 903篇 |
2008年 | 1473篇 |
2007年 | 1393篇 |
2006年 | 1230篇 |
2005年 | 1088篇 |
2004年 | 871篇 |
2003年 | 742篇 |
2002年 | 678篇 |
2001年 | 379篇 |
2000年 | 361篇 |
1999年 | 245篇 |
1998年 | 234篇 |
1997年 | 194篇 |
1996年 | 229篇 |
1995年 | 160篇 |
1994年 | 148篇 |
1993年 | 187篇 |
1992年 | 165篇 |
1991年 | 126篇 |
1990年 | 92篇 |
1989年 | 71篇 |
1988年 | 69篇 |
1987年 | 65篇 |
1986年 | 62篇 |
1985年 | 118篇 |
1984年 | 126篇 |
1983年 | 80篇 |
1982年 | 79篇 |
1981年 | 90篇 |
1980年 | 74篇 |
1979年 | 49篇 |
1978年 | 73篇 |
1977年 | 53篇 |
1976年 | 42篇 |
1975年 | 38篇 |
1974年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
61.
Elena Larraz Carlos Elvira Julio San Romn 《Journal of polymer science. Part A, Polymer chemistry》2003,41(11):1641-1649
This article explores the synthesis of a novel methacrylic macromonomer with an amphiphilic character derived from poly(ethylene glycol) tert‐octylphenyl ether (MT) and its respective homopolymer. To know their reactivity in radical copolymerization reactions with methyl methacrylate (MMA), a model monomer (MTm) was synthesized to determine the reactivity ratios and compare them with the low molar fractions of copolymers of MT with MMA because they were difficult to isolate. They were rMTm = 0.97 and rMMA = 0.95. The compositional diagrams when representing the weight fraction of MT and MTm in the feed and the copolymer suggested that a clear correlation exists between the experimental points of the model monomer MTm and the macromonomer MT ones, suggesting that the length of the side poly(ethylene oxide) chain does not affect the reactivity of the methacrylic double bond in the prepared monomers for this type of polymerization reaction. The reactivity ratios of the copolymers have a tendency for the formation of random or Bernoullian copolymers. The glass‐transition temperatures (Tg's) of the prepared copolymers were determined by differential scanning calorimetry, deviated from the Fox equation, and discussed on the basis of treatments that consider the influence of the monomeric units along the copolymer chains, determining the Tg of the corresponding alternating dyads. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1641–1649, 2003 相似文献
62.
Piotr Winiarek Juan Carlos Q. Colmenares Elbieta Fedoryska 《Reaction Kinetics and Catalysis Letters》2004,81(2):333-339
Transformations of diphenylmethane were investigated in a flow system in the presence of alumina and silica-alumina modified
with boron trifluoride at atmospheric pressure and elevated temperatures. Hydrogen transfer reaction was observed on addition
of tetralin to the substrate stream.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
63.
Aji P. Mathew Gabriel Groeninckx G. H. Michler H. J. Radusch Sabu Thomas 《Journal of Polymer Science.Polymer Physics》2003,41(14):1680-1696
The effects of the blend ratio and initiating system on the viscoelastic properties of nanostructured natural rubber/polystyrene‐based interpenetrating polymer networks (IPNs) were investigated in the temperature range of ?80 to 150 °C. The studies were carried out at different frequencies (100, 50, 10, 1, and 0.1 Hz), and their effects on the damping and storage and loss moduli were analyzed. In all cases, tan δ and the storage and loss moduli showed two distinct transitions corresponding to natural rubber and polystyrene phases, which indicated that the system was not miscible on the molecular level. However, a slight inward shift was observed in the IPNs, with respect to the glass‐transition temperatures (Tg's) of the virgin polymers, showing a certain degree of miscibility or intermixing between the two phases. When the frequency increased from 0.1 to 100 Hz, the Tg values showed a positive shift in all cases. In a comparison of the three initiating systems (dicumyl peroxide, benzoyl peroxide, and azobisisobutyronitrile), the dicumyl peroxide system showed the highest modulus. The morphology of the IPNs was analyzed with transmission electron microscopy. The micrographs indicated that the system was nanostructured. An attempt was made to relate the viscoelastic behavior to the morphology of the IPNs. Various models, such as the series, parallel, Halpin–Tsai, Kerner, Coran, Takayanagi, and Davies models, were used to model the viscoelastic data. The area under the linear loss modulus curve was larger than that obtained by group contribution analysis; this showed that the damping was influenced by the phase morphology, dual‐phase continuity, and crosslinking of the phases. Finally, the homogeneity of the system was further evaluated with Cole–Cole analysis. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1680–1696, 2003 相似文献
64.
65.
We extend the previously found accelerated Kerr-Schild metrics for Einstein-Maxwell-null dust and Einstein-Born-Infeld-null dust equations to the cases including the cosmological constant. This way we obtain the generalization of the charged de Sitter metrics in static space-times. We also give a generalization of the zero acceleration limit of our previous Einstein-Maxwell and Einstein-Born-Infeld solutions. 相似文献
66.
The diffraction of a plane wave incident on a thick hologram at the first and second Bragg angles in the process of nonlinear recording is analyzed using the coupled-wave equations. It is shown that, in this case, the two-wave approximation with the use of effective modulation amplitudes of the refractive index, n 1ef and n 2ef, can be applied. Analytical dependences of n 1ef and n 2ef on the modulation amplitudes of the refractive index n 1 and n 2 at the fundamental and doubled spatial frequencies are obtained and the criteria of their applicability are determined. The dependences found are used to calculate the parameters of gratings recorded in a photopolymeric material. 相似文献
67.
In this paper, we report the conducting and electrochemical properties of a homogeneous V2O5-SiO2 xerogel composite obtained from the simultaneous polymerization in both inorganic components (Si-O and V-O based polymers) forming an interpenetrating polymer network, where a mutual “solubility” due to cross-links and entanglements was observed. The presence of V2O5 inside the silica matrix has a strong effect on electrical conductivity; measurements showed room temperature conductivity almost 1000 times higher than what is found in the literature. In addition, the electrochemical behavior is quite similar to that found in V2O5 xerogel. Moreover, the effects of thermal treatment on the conducting and electrochemical properties were investigated. It was found that both properties were improved with no significant effect on V2O5 xerogel layered structure. 相似文献
68.
69.
René Meziat Diego Patiño Pablo Pedregal 《Computational Optimization and Applications》2007,38(1):147-171
We propose an alternative method for computing effectively the solution of non-linear, fixed-terminal-time, optimal control
problems when they are given in Lagrange, Bolza or Mayer forms. This method works well when the nonlinearities in the control
variable can be expressed as polynomials. The essential of this proposal is the transformation of a non-linear, non-convex
optimal control problem into an equivalent optimal control problem with linear and convex structure. The method is based on
global optimization of polynomials by the method of moments. With this method we can determine either the existence or lacking
of minimizers. In addition, we can calculate generalized solutions when the original problem lacks of minimizers. We also
present the numerical schemes to solve several examples arising in science and technology. 相似文献
70.