全文获取类型
收费全文 | 857篇 |
免费 | 25篇 |
国内免费 | 2篇 |
专业分类
化学 | 702篇 |
晶体学 | 3篇 |
力学 | 6篇 |
数学 | 70篇 |
物理学 | 103篇 |
出版年
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 5篇 |
2020年 | 7篇 |
2019年 | 10篇 |
2018年 | 8篇 |
2017年 | 9篇 |
2016年 | 17篇 |
2015年 | 16篇 |
2014年 | 19篇 |
2013年 | 33篇 |
2012年 | 44篇 |
2011年 | 59篇 |
2010年 | 31篇 |
2009年 | 29篇 |
2008年 | 48篇 |
2007年 | 64篇 |
2006年 | 62篇 |
2005年 | 39篇 |
2004年 | 49篇 |
2003年 | 46篇 |
2002年 | 47篇 |
2001年 | 8篇 |
2000年 | 13篇 |
1999年 | 9篇 |
1998年 | 13篇 |
1997年 | 6篇 |
1996年 | 18篇 |
1995年 | 4篇 |
1994年 | 15篇 |
1993年 | 10篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 15篇 |
1984年 | 14篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有884条查询结果,搜索用时 15 毫秒
1.
Nucleophile(s) almost exclusively added from the equatorial side of 2-nitro-β--2-enopyranoside and the axial side of 2-nitro---1-enitol. On the other hand, methoxide and -butyl peroxide ions approached from the equatorial side of 2-nitro-α--2-enopyranoside, whereas methanol and hydrogen peroxide ion from the axial side. 相似文献
2.
The effect of sucrose on the structure of molecular assemblies formed in an MO/H2O mixture has been studied using a small-angle X-ray scattering method. It was found that the phase transition Ia3d --> Pn3m --> H(II) occurs with increased sucrose concentration in the mixture with the composition 70 wt% MO at 20 degrees C. This structural change induced by sucrose addition would be ascribed to a cosmotropic property of sucrose, which leads to the dehydration of the MO head group and hence causes a decrease in the effective area occupied by the MO molecule at the polar/apolar interface, which facilitates the formation of molecular assemblies with a high curvature for the reversed liquid-crystal mesophase. 相似文献
3.
Yagi Kiyohito Hamada Kenji Hlrata Kazumasa Miyamoto Kazuihisa Miura Yoshiharu Akano Tohru Fukatu Kiyomi Ikuta Yoshiaki Nakamura Hiroshi K. 《Applied biochemistry and biotechnology》1994,(1):225-232
A marine green alga,Chlamydomonas sp. strain MGA161 was cultivated under illumination of red and white lights. The growth rate under red light illumination
was almost the same as that in the basic conditions under white light illumination, but red light-grown cells accumulated
almost twice as much starch as white light-grown cells. Although there was a slight decrease in carbonic anhydrase activity,
red light-illuminated cells had almost 2.3 times the fructose-l,6-diphos-phatase activity of white light-illuminated cells.
Red light might stimulate starch accumulation by increasing the amounts of enzymes related to carbon fixation through the
phytochrome system. Cells grown under red light degraded 1.6 times as much starch and produced 1.7 times as much hydrogen
and 1.6 times as much ethanol compared with cells grown under white light during 12 h of dark anaerobic fermentation. 相似文献
4.
Ultrasonic slurry sampling electrothermal atomic absorption spectrometry with a metal tube atomizer has been applied to the determination of lead in Bangladeshi fish samples. The slurry sampling conditions, such as slurry stabilizing agent, slurry concentration, pyrolysis temperature for the slurried fish samples, particle size and ultrasonic agitation time, were optimized for electrothermal atomic absorption spectrometry with the Mo tube atomizer. Thiourea was used as the chemical modifier for the interference of matrix elements. The detection limit was 53 fg (3S/N). The determined amount of lead in Bangladeshi fish samples was consistent with those measured in the dissolved acid-digested samples. The advantages of the proposed methods are easy calibration, simplicity, low cost and rapid analysis. 相似文献
5.
Manganese (Mn), iron (Fe), copper (Cu), and zinc (Zn) concentrations were determined in the brain regions of normal 1-, 3-, 5-, 7-, 14-, 21-, 42-, 77-, and 147-day-old Wistar rats using inductively coupled plasma mass spectrometry (ICP-MS), and their maps were illustrated in color to visually compare the distribution of the elements at various stages of the growth process. Sagittal slices (1-mm thickness) sectioned at the level of the substantia nigra were divided into 18 regions, and the small slice samples were digested in microwave-assisted closed vessels for ICP-MS measurement. Mn, Fe, Cu, and Zn concentrations increased region-specifically with age, and their distributional maps showed some characteristics. These findings are discussed in terms of needs for these trace elements in the normal brain. Among new findings about their brain distribution, it is especially noteworthy that higher concentrations of Mn, Fe, and Zn were observed in the substantia nigra compared with those in neighboring regions. The mapping method in this work is expected to open up possibilities for screening of the in vivo element–element interrelationships among these essential elements. 相似文献
6.
7.
Masayuki Kuzuya Tohru Kawaguchi Yasukichi Yanagihara Sachihiko Nakai Takachiyo Okuda 《Journal of polymer science. Part A, Polymer chemistry》1986,24(4):707-713
To provide a satisfactory basic understanding of the solvent effect of plasma-initiated polymerization, we have carried out several kinds of polymerizations of various water-soluble vinyl monomers. It has been shown that aqueous solution of such vinyl monomers underwent the polymerizations induced by methyl isobutyrate (MIB) plasma-exposed glass surface. The invisible ultrathin polymer film trapping an active radical species deposited on the plasma-exposed glass surface apparently induced the polymerizations. The efficiency, however, was largely dependent on the solvent used: the solvent can be considered to act as “an initiation-activator” for the lifelike postpolymerization. The nature of the solvent effect was ascribed to the physicomechanical property such as swelling ability and/or solubility toward the ultrathin film formed on the glass surface. Thus it has been suggested that the choice of good-balanced combination between the structural feature of the plasma-induced ultrathin polymer film and the solvent as an initiation-activator is important to achieve activity of a desired solution polymerization. 相似文献
8.
Miyuki Narita Junya Itoh Tohru Kikuchi Fumio Hamada 《Journal of inclusion phenomena and macrocyclic chemistry》2002,42(1-2):107-114
-Cyclodextrin dimer linked with ethylenediamine at the upper rim of the cyclodextrin has been synthesized and then modified with two dansyl moieties inthe presence of N,N'-dicyclohexylcarbodiimide. The sensing ability and bindingproperty of the title compound were investigated for steroids and terpenoids. Thefluorescence intensity of this dimer was decreased when a host–guest complex was formed. The value I/I0, where I0 and I are fluorescence intensitiesin the absence and presence of a guest and I is I0- I, was used as a parameter of sensitivity. This host exhibited a much higher sensitivity and selective molecular recognition ability for bile acids such as ursodeoxycholic acid andchenodeoxycholic acid and terpenoids such as (-)-borneol than the dansyl-modifiedcyclodextrins reported previously including -cyclodextrin dimer. The behaviors of the appended moieties of the host during the formation of host–guest complexes were studied using induced circular dichroism (ICD) and fluorescence spectra. The ICD intensityof this dimer was decreased on accommodation of a guest and this spectral pattern of the title dimer was opposite to that of bis dansyl-modified -cyclodextrin monomer. Theguest-induced variations in the fluorescence and ICD intensities suggest that this dimer formed a 1 : 1 host–guest complex and the appended moieties act as a hydrophobic cap. 相似文献
9.
In the presence of trifluoroacetic acid, N-phenyl-N′-phenoxyurea (1a) rearranges to N-(4′-hydroxy-2-biphenylyl)urea (2a) and N-carbamoyl-2-hydroxy-diphenylamine (3a). The rearrangement is an intramolecular reaction, and the transition state of the breakage of the N-O bond is deduced to be polarized in the form Nδ- --- Oδ+. The reaction is entirely new and constitutes a fundamental aromatic rearrangement. 相似文献
10.
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