全文获取类型
收费全文 | 182137篇 |
免费 | 2765篇 |
国内免费 | 526篇 |
专业分类
化学 | 99059篇 |
晶体学 | 2337篇 |
力学 | 7055篇 |
综合类 | 8篇 |
数学 | 19915篇 |
物理学 | 57054篇 |
出版年
2021年 | 1321篇 |
2020年 | 1637篇 |
2019年 | 1598篇 |
2018年 | 2112篇 |
2017年 | 2159篇 |
2016年 | 3537篇 |
2015年 | 2317篇 |
2014年 | 3213篇 |
2013年 | 7784篇 |
2012年 | 6444篇 |
2011年 | 7656篇 |
2010年 | 5353篇 |
2009年 | 5186篇 |
2008年 | 7345篇 |
2007年 | 7036篇 |
2006年 | 6847篇 |
2005年 | 6235篇 |
2004年 | 5563篇 |
2003年 | 5048篇 |
2002年 | 4820篇 |
2001年 | 6257篇 |
2000年 | 4549篇 |
1999年 | 3478篇 |
1998年 | 2480篇 |
1997年 | 2574篇 |
1996年 | 2526篇 |
1995年 | 2336篇 |
1994年 | 2371篇 |
1993年 | 2203篇 |
1992年 | 2710篇 |
1991年 | 2582篇 |
1990年 | 2457篇 |
1989年 | 2392篇 |
1988年 | 2347篇 |
1987年 | 2379篇 |
1986年 | 2169篇 |
1985年 | 2800篇 |
1984年 | 2802篇 |
1983年 | 2250篇 |
1982年 | 2282篇 |
1981年 | 2127篇 |
1980年 | 2114篇 |
1979年 | 2387篇 |
1978年 | 2453篇 |
1977年 | 2414篇 |
1976年 | 2312篇 |
1975年 | 2182篇 |
1974年 | 2166篇 |
1973年 | 2147篇 |
1972年 | 1382篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
951.
Distribution of melamine in polyester–melamine surface coatings cured under nonisothermal conditions
N. J. W. Gamage D. J. T. Hill C. A. Lukey P. J. Pomery 《Journal of polymer science. Part A, Polymer chemistry》2004,42(1):83-91
The influence of experimental cure parameters on the diffusion of reactive species in polyester–melamine thermoset coatings during curing has been investigated with X‐ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared. The diffusion of melamine plays a vital role in the curing process and, therefore, in the ultimate properties of coatings. At a low (<20%) hexamethoxymethylmelamine (HMMM) crosslinker concentration, the matrix composition is uniform, but at high HMMM concentrations, excess HMMM rapidly segregates to the air–coating interface. The rate of migration is governed by the difference in the surface free energies of polyester and HMMM and the concentration gradient of HMMM between the bulk and the surface. An increased rate of energy absorption also increases the rate of migration of HMMM to the surface. A physical model has been proposed to explain this surface segregation phenomenon in terms of cocondensation and self‐condensation reactions. It suggests that an appropriate amount of melamine can be segregated on the surface and allowed to self‐condense to form a desired thickness of a melamine topcoat through the control of the binder composition and cure conditions. This technique can be implemented to apply a melamine topcoat during cure. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 83–91, 2004 相似文献
952.
We report the effects of partial substitutions of the VIIIa elements: Fe (3d) and Ru (4d) in the intermetallic perovskite superconductor MgCNi3. In both the MgCNi3−xRux and MgCNi3−xFex systems, the superconducting critical temperature (TC) decreases monotonically as x is increased. TC decreases more slowly in the case of Ru, suggesting that the 3d electrons in Fe are magnetically pair breaking. This is supported by the normal state magnetic susceptibility measurements (χ), which shows that χ increases with Fe and decreases with Ru doping. No ferromagnetism is observed in either system. 相似文献
953.
H. Kuramochi J. Okabayashi F. Takano M. Mizuguchi T. Manago H. Akinaga 《Surface science》2004,550(1-3):192-198
Strong magnetic poles at characteristic rectangular defects have been observed using a magnetic force microscope on a MnAs(
1 0 0) thin film with the thickness of 30 nm. The MnAs thin film was epitaxially grown on a GaAs(0 0 1) substrate. The magnetic poles were in one-arranging direction, being independent of the magnetization direction of the film. The poles were pinned at the edges of the rectangular defects until just below the Curie temperature, and formed a stable magnetic-field loop on the MnAs surface. The stability of the magnetic pole pinning shows the distinctive feature of the magnetic domain structure on the surface with a strong anisotropy, which was built in the heterostructure of MnAs and GaAs. 相似文献
954.
S. S. Stavitskaya N. T. Kartel’ N. N. Tsyba M. F. Kovtun T. P. Petrenko 《Russian Journal of Applied Chemistry》2007,80(3):379-385
The mineral and chemical compositions, as well as some structure-sorption characteristics were determined for clay minerals selected for examination. The integrated coefficients of contamination of bottom sediments with each of the toxicant metals (Fe, Ni, Co, Cu, Zn, Pb, Mn) were calculated. The minerals examined presumably can be used in medicine as efficient application materials, cosmetic agents, and sorbents (enterosorbents). 相似文献
955.
R. A. Stukan T. V. Dudareva Yu. N. Kil’yanov A. V. Polyakova 《Russian Journal of Inorganic Chemistry》2006,51(1):1-4
A tetragonal 123 phase with the composition close to CeLa2 { Cu
2
2+
} [Mg2+]O8 (the braces indicate the Cu(2) positions; the brackets indicate the Cu(1) positions) with the parameters a = b = 0.3909(3) nm, c = 1.6591(8) nm was prepared at 860°C under an oxygen atmosphere with an elevated oxygen pressure. When the lanthanum-for-barium
substitution was incomplete, the resulting 123 phase had the composition close to CeLa1.7Ba0.3{ Cu
1.7
2+
} [Mg]O8 with the unit cell parameters a = b = 0.3868(3) nm, c = 1.6578(8) nm that contains Cu3+ in the Cu(2) positions. The partial substitution of barium for lanthanum (the melting point of barium oxide is almost 500°C
lower that of the lanthanum oxide) appreciably facilitated the synthesis: the 123 phase in this sample was more than 90%.
The existence of Cu3+ in the Cu(2) positions enhanced the electrical conductivity of the sample. 相似文献
956.
Synthetic routes to oxidation of cobalt atoms in cobalt trimethylacetate complexes containing coordinated pyrazole molecules have been considered. 相似文献
957.
Lemi Türker 《Journal of mathematical chemistry》1992,9(4):341-349
A novel method, based on the topology of the cardinal vertex, is described to find an upper bound for the largest eigenvalue of a graph. 相似文献
958.
Two-dimensional on-line particle imaging velocimetry 总被引:3,自引:0,他引:3
A high-performance algorithm is described for the on-line computation of two-dimensional velocity maps using particle displacement imaging. It relies on the computation of direct correlations in a multiple-scale spatial hierarchy. Layered rejection criteria based on correlation quality and conditional averaging techniques are employed to achieve the robustness required for continuous, unsupervised operation. Implementations are described using a UNIX-based workstation and a DOS-based personal computer with a real-time image processing/correlator subsystem. The cooperation of the algorithm with other real-time visualization techniques is demonstrated using the example of colour-coded streak integration.The authors wish to thank Dr. Philippe Georis of the Université Libre de Bruxelles (ULB) for making available to us video sequences of his Marangoni convection experiment. 相似文献
959.
A. B. Pierini G. L. Borosky M. T. Baumgartner 《International journal of quantum chemistry》1992,41(5):759-761
We here report an AM 1 study for the reaction of different types of radicals with the acetone enolate ion 1 and other ambident nucleophiles. The theoretical reactivity order determined is phenyl > methyl > tert-butyl ? bicyclo-[2.2.1]hept-1-yl. The activation energy for the coupling of aliphatic radicals is ascribed mainly to nonbonded interactions. Considering possible solvent effects, we propose the same order of reactivity in solution. The coupling reaction of phenyl radical with 1 is an exothermic process with nonactivation energy in the gas phase. Any energy barrier in solution is here ascribed to desolvation of the anion. The regiochemistry of the coupling of radicals with ambident nucleophiles such as 1 , phenoxide, or phenylamide anions is also explained on a theoretical basis. © 1992 John Wiley & Sons, Inc. 相似文献
960.
B. F. Araujo H. T. Matsuda T. A. Kuada J. A. Araujo 《Journal of Radioanalytical and Nuclear Chemistry》1992,166(1):75-83
In the actinide separation process using TBP as extractant, hydrolytic and radiolytic reactions cause a gradual decomposition of the extractant, resulting in the formation of degradation products which can affect process performance adversely. The TBP diluent has to be cleaned from these degradation products before it is reused. This paper deals with a procedure for solvent cleanup, using a fourstage continuous contactor for increasing the scrubbing efficiency. 相似文献