全文获取类型
收费全文 | 49915篇 |
免费 | 10790篇 |
国内免费 | 4038篇 |
专业分类
化学 | 50949篇 |
晶体学 | 561篇 |
力学 | 1223篇 |
综合类 | 271篇 |
数学 | 3468篇 |
物理学 | 8271篇 |
出版年
2024年 | 57篇 |
2023年 | 368篇 |
2022年 | 623篇 |
2021年 | 798篇 |
2020年 | 1890篇 |
2019年 | 3162篇 |
2018年 | 1580篇 |
2017年 | 1229篇 |
2016年 | 4087篇 |
2015年 | 4323篇 |
2014年 | 4311篇 |
2013年 | 5133篇 |
2012年 | 4199篇 |
2011年 | 3486篇 |
2010年 | 3714篇 |
2009年 | 3715篇 |
2008年 | 3267篇 |
2007年 | 2560篇 |
2006年 | 2254篇 |
2005年 | 2325篇 |
2004年 | 1959篇 |
2003年 | 1774篇 |
2002年 | 2515篇 |
2001年 | 1759篇 |
2000年 | 1633篇 |
1999年 | 660篇 |
1998年 | 231篇 |
1997年 | 203篇 |
1996年 | 176篇 |
1995年 | 137篇 |
1994年 | 125篇 |
1993年 | 103篇 |
1992年 | 78篇 |
1991年 | 59篇 |
1990年 | 47篇 |
1989年 | 40篇 |
1988年 | 24篇 |
1987年 | 27篇 |
1986年 | 25篇 |
1985年 | 22篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1957年 | 5篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
分形维数与城市规模分布评析 总被引:3,自引:1,他引:2
本在分形概念的基础上,指出城市规模分布是基于统计意义上的一种分形分布,并计算了安徽省1978~1998年城市规模分布的分维值,评析了安徽省城市规模分布变化的特征。 相似文献
143.
Marie‐France Llauro Julien Loiseau Fernande Boisson Frdric Delolme Catherine Ladavire J. Claverie 《Journal of polymer science. Part A, Polymer chemistry》2004,42(21):5439-5462
Low‐molecular‐weight poly(acrylic acid) (PAA) was synthesized by reversible addition fragmentation chain transfer polymerization with a trithiocarbonate as chain‐transfer agent (CTA). With a combination of NMR spectroscopy and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry, the PAA end‐groups of the polymer were analyzed before and after neutralization by sodium hydroxide. The polymer prior to neutralization is made up of the expected trithiocarbonate chain‐ends and of the H‐terminated chains issued from a reaction of transfer to solvent. After neutralization, the trithiocarbonates are transformed into thiols, disulfides, thiolactones, and additional H‐terminated chains. By quantifying the different end‐groups, it was possible to demonstrate that fragmentation is the rate limiting step in the transfer reaction. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5439–5462, 2004 相似文献
144.
John Boyle Ica Manas‐Zloczower DonaldL. Feke 《Particle & Particle Systems Characterization》2004,21(3):205-212
The dispersion behavior of agglomerates of several grades of fumed silica in poly(dimethyl siloxane) liquids has been studied as a function of particle morphology and applied flow conditions. The effects of primary particle size and aggregate density and structure on cohesivity were probed through tensile and shear strength tests on particle compacts. These cohesivity tests indicated that the shear strength of particle compacts was two orders of magnitude higher than the tensile strength at the same overall packing density. Experiments carried out in both steady and time‐varying simple‐shear flows indicate that dispersion occurs through tensile failure. In the steady‐shear experiments,enhanced dispersion was obtained at higher levels of applied stress and, at comparable levels of applied stress, dispersion was found to proceed faster at higher shear rates. Experiments conducted in time‐varying flows further corroborated the results obtained in tensile cohesivity tests. Experiments in which the mean and maximum stresses in the time‐varying flows were matched to the stresses produced in steady shear flows highlight the influence of flow dynamics on dispersion behavior. 相似文献
145.
Auvergne R. Saint‐Loup R. Joly‐Duhamel C. Robin J. J. Boutevin B. 《Journal of polymer science. Part A, Polymer chemistry》2007,45(7):1324-1335
The synthesis and characterization of photopolymerizable unsaturated polyester resins based on PET waste are described. The resins came from a depolymerization process based on the glycolysis of PET by diethylene glycol (DEG). Different molecular weights of glycolysates were synthesized. Then, the latter was functionalized by a methyl hemiester of maleic acid to obtain unsatured α,ω‐bismaleate PET oligomers. In the presence of an electron donor monomer, such as triethylene glycol divinyl ether, these electron acceptor oligomers were copolymerized by way of charge‐transfer complexes under UV irradiation. The reaction was monitored in situ by real‐time IR spectroscopy to study the kinetics of photopolymerization. This one was studied in relation with the physical and chemical characteristics of oligoesters and the composition of mixtures containing divinyl ethers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1324–1335, 2007 相似文献
146.
Christopher K. Yee‐Chan Robert C. Scogna Richard A. Register 《Journal of Polymer Science.Polymer Physics》2007,45(10):1198-1204
In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (Tg) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine Tg in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. Tg is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007 相似文献
147.
Katalytische Epoxidierung und 1,2‐Dihydroxylierung von Olefinen mit Bispidin‐Eisen(II)/H2O2‐Systemen
148.
A succinct series expression is derived for describing the limit distribution of the number of times r consecutive elements are all records (in a sequence of independent and identically distributed random variables with a common continuous distribution) for all r ≥ 2. Previously, only the limit distributions for r = 1, 2, and 3 were known. © 2005 Wiley Periodicals, Inc. Random Struct. Alg., 2005 相似文献
149.
Many natural products contain carbohydrate moieties that contribute to their biological activity. Manipulation of the carbohydrate domain of natural products through multiple glycosylations to identify new derivatives with novel biological activities has been a difficult and impractical process. We report a practical one‐pot enzymatic approach with regeneration of cosubstrates to synthesize analogues of vancomycin that contain an N‐alkyl glucosamine, which exhibited marked improvement in antibiotic activity against a vancomycin‐resistant strain of Enterococcus. 相似文献
150.