全文获取类型
收费全文 | 2316篇 |
免费 | 77篇 |
国内免费 | 3篇 |
专业分类
化学 | 1940篇 |
晶体学 | 23篇 |
力学 | 22篇 |
数学 | 130篇 |
物理学 | 281篇 |
出版年
2021年 | 15篇 |
2020年 | 20篇 |
2019年 | 31篇 |
2018年 | 24篇 |
2017年 | 13篇 |
2016年 | 48篇 |
2015年 | 44篇 |
2014年 | 59篇 |
2013年 | 100篇 |
2012年 | 94篇 |
2011年 | 116篇 |
2010年 | 76篇 |
2009年 | 53篇 |
2008年 | 117篇 |
2007年 | 115篇 |
2006年 | 124篇 |
2005年 | 106篇 |
2004年 | 119篇 |
2003年 | 127篇 |
2002年 | 106篇 |
2001年 | 53篇 |
2000年 | 71篇 |
1999年 | 34篇 |
1998年 | 31篇 |
1997年 | 38篇 |
1996年 | 39篇 |
1995年 | 21篇 |
1994年 | 20篇 |
1993年 | 25篇 |
1992年 | 25篇 |
1991年 | 24篇 |
1990年 | 25篇 |
1989年 | 11篇 |
1988年 | 29篇 |
1987年 | 14篇 |
1986年 | 13篇 |
1985年 | 27篇 |
1984年 | 35篇 |
1983年 | 17篇 |
1982年 | 28篇 |
1981年 | 37篇 |
1980年 | 43篇 |
1979年 | 36篇 |
1978年 | 32篇 |
1977年 | 26篇 |
1976年 | 26篇 |
1975年 | 13篇 |
1974年 | 19篇 |
1973年 | 18篇 |
1971年 | 9篇 |
排序方式: 共有2396条查询结果,搜索用时 31 毫秒
71.
Oscillating frequencies of a piezoelectric crystal were measured in various solutions. One side of the crystal surface was coated with a silicon sealant. This coating was useful for measuring the oscillation of crystals in solutions for a wide range of products of density (?) and viscosity (η) and in electrolyte solutions. For measurement in solutions, the frequency change depended on the circuit used, whereas for measurements in air the circuit did not influence the frequency change. All experimental data showed that the frequency change from pure water, ΔFw, followed ΔFw = ? K(√?η ? √?wηw) except for electrolyte and polymer solutions, where K is a proportionality constant, ηw the density of pure water and ηw the viscosity of water. 相似文献
72.
Hitoshi Kubota Yukitaka Murata Yoshitaka Ogiwara 《Journal of polymer science. Part A, Polymer chemistry》1973,11(2):485-492
The effects of swelling of the sample and polymerization solvents were studied for photo-induced graft copolymerization of vinyl monomers on cellulose. The graft copolymerization of methyl methacrylate (MMA) was activated by swelling of the sample or organic solvent-water solutions within a certain range of their concentrations. Though each organic solvent gave a maximum in per cent grafting and the number of grafts at about 25 vol-% concentration, the initiation reaction scarcely took place at 100% concentration; thus, the solvent itself is considered to have a negative effect. The solvents used in the experiments were all hydrophilic, such as methanol, acetone, and dioxane. The average molecular weight of the grafted PMMA differed in each solvent, indicating a different characteristic effect of solvent on the growing grafted polymer radicals. The presence of ferric ion as a sensitizer stimulated further the contributions of the sample swelling and the organic solvents to the copolymerization reaction. A similar effect was observed for styrene as for MMA, but not for acrylic acid and methacrylic acid. 相似文献
73.
Large-scale separation of metallic and semiconducting single-walled carbon nanotubes 总被引:6,自引:0,他引:6
Maeda Y Kimura S Kanda M Hirashima Y Hasegawa T Wakahara T Lian Y Nakahodo T Tsuchiya T Akasaka T Lu J Zhang X Gao Z Yu Y Nagase S Kazaoui S Minami N Shimizu T Tokumoto H Saito R 《Journal of the American Chemical Society》2005,127(29):10287-10290
In the applications of single-walled carbon nanotubes (SWNTs), it is extremely important to separate semiconducting and metallic SWNTs. Although several methods have been reported for the separation, only low yields have been achieved at great expense. We show a separation method involving a dispersion-centrifugation process in a tetrahydrofuran solution of amine, which makes metallic SWNTs highly concentrated to 87% in a simple way. 相似文献
74.
To study the role of glycine residues in stabilizing the collagen triple helix, the glycine residues in the polytripeptide (Pro-Ala-Gly)n were partially replaced by alanine. The proline content was kept constant. The stability of the helical conformation of these polypeptides was studied by IR- and X-ray measurements in the solid state and by ORD, CD and viscometry in solution. The renaturation was followed as a function of time. All the polytripeptides studied, with the exception of (-Pro-Ala-Ala-)n attained the polyproline II conformation. However the stability decreased with increasing alanine content. Obviously the molecules of (-Pro-Ala-Gly-) are highly associated by intermolecular hydrogen bonds and one may therefore suppose that a triple-stranded helix aggregation occurs. The results of the refolding process show that the stability of the helices seems to also affect the refolding rate in terms of the optical rotation, Two transitions appeared: the first one is responsible for a rapid reversible change in conformation and the second one for a further slow and irreversible change in the hydrodynamic shape. The latter seems to be due to the partial helical nature, leading to higher chain mobility. 相似文献
75.
Shigeru?ArimotoEmail author Mark?Spivakovsky Keith?F.?Taylor Paul?G.?Mezey 《Journal of mathematical chemistry》2005,37(2):171-189
The present article is a direct continuation of the first part of this series. We reduce a proof of the Fukui conjecture (concerning the additivity problem of the zero-point vibrational energies of hydrocarbons) to that of a proposition related to the theory of algebraic curves, so that we can focus on the key mechanism of the additivity phenomena. Namely, by establishing what is called the Basic Piecewise Monotone Theorem (BPMT), we reduce a proof of the Fukui conjecture to that of a proposition, called the Local Analyticity Proposition, Version 1 (LAP1), which admits a proof via resolution of singularities. By LAP1, the essential part of the mechanism of the asymptotic linearity phenomena is extracted and is elucidated by using tools from the mathematical theory of algebraic curves, whose language is of vital importance in analyzing the crux of the additivity mechanism.
Dedicated to the memory of Prof. Kenichi Fukui (1918–1998). 相似文献
76.
In the present paper, a study was undertaken of molecular transport in ploy(dimethylsiloxane) microchannels that were fabricated by ultraviolet (UV)-photolithography and synchrotron radiation (SR)-lithography characterized and compared for microchip capillary electrophoresis by evaluating in-channel molecular dispersion. A fluorescent tag, sulforhodamine B was used as the probing molecule. It was found that microchannels made by SR-lithography fabrication were superior to those made by UV-photolithography fabrication in terms of molecular transport performance. A deep insight into surface conditions characterized by scanning electron microscopy suggested it was related to the difference in surface roughness. Chromatographic retention in electropherograms further supported such a conclusion, which depended on the phase ratio of the channel surface. The results revealed for PDMS microchannels in this work were in good agreement with the phenomenon found for glass microchannels in the literature. 相似文献
77.
Yoshizawa M Nagao M Umemoto K Biradha K Fujita M Sakamoto S Yamaguchi K 《Chemical communications (Cambridge, England)》2003,(15):1808-1809
Despite their structural similarity, triangular tetradentate ligands 2b and 2c experience different assembly pathways on complexation with (en)Pd(NO3)2 to give M8L4 tetrahedral (3) and open cone (4) structures, respectively, due to steric restriction by side chains at the corner or on the edge of the ligands. 相似文献
78.
Shigeru Arimoto 《Journal of mathematical chemistry》2003,34(3-4):259-285
The present article provides a new proof of the Fukui conjecture concerning the additivity problem of the zero-point vibrational energies of hydrocarbons. This conjecture played a prominent role in the initial development of the repeat space theory (RST), and continues to be of vital significance in the recent development of the theory of the generalized repeat space X r (q,d). The new proof of the Fukui conjecture has been given here by establishing the functional version of the Asymptotic Linearity Theorem (ALT), the Functional ALT. This enhanced version of the ALT directly implies the validity of the Fukui conjecture; it easily unifies, in a broad perspective, a variety of additivity phenomena in physico-chemical network systems having many identical moieties, and efficiently solves some interpretational problems of the empirical additivity formulae from experimental chemistry. The proof of the functional version of the ALT is based on a new method transferable to the extended theoretical framework of the generalized repeat space X r (q,d). 相似文献
79.
Calcium-induced alginate gel beads (Alg-Ca) containing alginate hydrolysate, such as the guluronic acid block (GB), was prepared and the drug release profiles were investigated under simulated gastrointestinal conditions. The addition of GB to Alg-Ca altered its rheological properties. A model drug (hydrocortisone) was incorporated at 78% of its theoretical yield within the dried Alg-Ca containing 5% GB and it was gradually released from the beads in JP XIV 1st medium for disintegration test (pH 1.2), while it was rapidly released with disintegration of the gel matrix in JP XIV 2nd medium (pH 6.8). In contrast, for Alg-Ca containing GB and chitosan, disintegration was not observed in these media and the drug release rate was markedly different. These results demonstrate that the release profiles of drugs incorporated into Alg-Ca can be controlled by adding these polysaccharides. 相似文献
80.
Tomoko Sotomatsu Yoshiyuki Murata Toshio Fujita 《Journal of computational chemistry》1991,12(1):135-138
The AM1 calculation was done for ortho-substituted toluenes (o-X-C6H4-CH3) and ortho-substituted tert-butylbenzenes (o-X-C6H4-t-Bu). The difference in the calculated heat of formation between o-X-C6H4-CH3 and o-X-C6H4-t-Bu was used as a theoretical steric index for ortho-X. The correlation of this theoretical steric index with the empirical steric parameter sets such as our recently defined Es(AMD) and the Taft–Kutter–Hansch (TKH) Es was examined. In spite of the simplicity of the model system, the theoretical index was linear with the Es(AMD) constant with a correlation coefficient of r = 0.972 for 17 substituents of various structures. Including the phenyl group, the correlation with the TKH Es constant was r = 0.948. The theoretically calculated index was shown to serve as a measure of the ortho steric effect. 相似文献