全文获取类型
收费全文 | 463篇 |
免费 | 16篇 |
国内免费 | 3篇 |
专业分类
化学 | 289篇 |
晶体学 | 10篇 |
力学 | 16篇 |
数学 | 109篇 |
物理学 | 58篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 8篇 |
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 29篇 |
2015年 | 7篇 |
2014年 | 16篇 |
2013年 | 40篇 |
2012年 | 19篇 |
2011年 | 38篇 |
2010年 | 18篇 |
2009年 | 26篇 |
2008年 | 27篇 |
2007年 | 20篇 |
2006年 | 22篇 |
2005年 | 18篇 |
2004年 | 26篇 |
2003年 | 10篇 |
2002年 | 19篇 |
2001年 | 7篇 |
2000年 | 3篇 |
1999年 | 10篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1994年 | 3篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有482条查询结果,搜索用时 31 毫秒
11.
Neutron diffraction studies of hydrogen positions in small molecules of biological interest at Trombay have provided valuable
information that has been used in protein and enzyme structure model-building and in developing hydrogen bond potential functions.
The new R-5 reactor is expected to provide higher neutron fluxes and also make possible smallangle neutron scattering studies
of large biomolecules and bio-aggregates. In the last few years infrastructure facilities have also been established for macromolecular
x-ray crystallography research. Meanwhile, the refinement of carbonic hydrases’ and lysozyme structures have been carried
out and interesting results obtained on protein dynamics and structure-function relationships. Some interesting presynaptic
toxin phospholipases have also been taken up for study. 相似文献
12.
Polyphosphate esters were synthesized from phenolphthalein and aryl phosphorodichloridates by interfacial polycondensation using a phase transfer catalyst. The polymers were characterized by IR, 1H-, 13C-, and 31P-NMR spectroscopy. The molecular weights were calculated by end group analysis using 31P-NMR spectral data. The thermal stability of the polymers was studied by thermogravimetry and the flammability was investigated by measuring limiting oxygen index values. 相似文献
13.
CuM(II)Al ternary hydrotalcites (M(II) = Ni, Co and Mg) with a (Cu+M(II))/Al atomic ratio of 3.0 and Cu/M(II) atomic ratio of 5.0 were synthesized by coprecipitation under low supersaturation. Powder X-ray diffraction of the as-synthesized samples showed a pattern characteristic of hydrotalcite-like (HT-like) structure (JCPDS: 41-1428). Thermal analyses of these samples showed four stages of weight loss/heat change when recorded in nitrogen. Analysis of the evolved gases characterized the nature of these transformations. The thermoanalytical effects differed significantly especially for the high-temperature transformations, when the treatment was performed in oxygen. In situ powder X-ray diffraction of the samples was carried out to elucidate the phase evolution of these compounds. Surprisingly formation of CuO was noted at temperatures around 200°C well below the destruction of the layered network. The nature of the resulting phases varied with both the nature of the co-bivalent metal ion and the heating atmosphere. FT-IR spectroscopy confirmed the retention of carbonate ions at higher temperatures (above 700°C), although the concentration of carbonate anion (most likely unidentate) varied with the calcination temperature. The crystallinity of CuO increased significantly above 600°C, probably through dissociation of copper oxycarbonate. Significant differences in the thermal transformation temperatures (for the third and the fourth transformations) of these samples containing different co-bivalent metal ions were not observed. This suggests that an association of the co-bivalent metal ions and/or trivalent metal ion in this phase is unlikely. A plausible thermal evolution scheme of these hydrotalcites is proposed. 相似文献
14.
Chiang LY Padmawar PA Canteenwala T Tan LS He GS Kannan R Vaia R Lin TC Zheng Q Prasad PN 《Chemical communications (Cambridge, England)》2002,(17):1854-1855
The first, highly two-photon active C60 derivative comprised of a A-sp3-D conjugate structure was synthesized showing effective two-photon absorption cross-sections (sigma 2' = 196 x 10(-48) cm4 sec-1 molecule-1) in the nanosecond regime among the best values for diphenylaminofluorene-based AFX chromophores. 相似文献
15.
Raghuraman K Katti KK Barbour LJ Pillarsetty N Barnes CL Katti KV 《Journal of the American Chemical Society》2003,125(23):6955-6961
Phosphorus functionalized trimeric alanine compounds (l)- and (d)-P(CH(2)NHCH(CH(3))COOH)(3) 2 are prepared in 90% yields by the Mannich reaction of Tris(hydroxymethyl)phosphine 1 with (l)- or (d)- Alanine in aqueous media. The hydration properties of (l)-2 and (d)-2 in water and water-methanol mixtures are described. The crystal structure analysis of (l)-2.4H(2)O, reveals that the alanine molecules pack to form two-dimensional bilayers running parallel to (001). The layered structural motif depicts two closely packed monolayers of 2 each oriented with its phosphorus atoms projected at the center of the bilayer and adjacent monolayers are held together by hydrogen bonds between amine and carboxylate groups. The water bilayers are juxtaposed with the H-bonded alanine trimers leading to 18-membered (H(2)O)(18) water rings. Exposure of aqueous solution of (l)-2 and (d)-2 to methanol vapors resulted in closely packed (l)-2 and (d)-2 solvated with mixed water-methanol (H(2)O)(15)(CH(3)OH)(3) clusters. The O-O distances in the mixed methanol-water clusters of (l)-2.3H(2)O.CH(3)OH and (d)-2.3H(2)O.CH(3)OH (O-O(average) = 2.857 A) are nearly identical to the O-O distance observed in the supramolecular (H(2)O)(18) water structure (O-O(average) = 2.859 A) implying the retention of the hydrogen bonded structure in water despite the accommodation of hydrophobic methanol groups within the supramolecular (H(2)O)(15)(CH(3)OH)(3) framework. The O-O distances in (l)-2.3H(2)O.CH(3)OH and (d)-2.3H(2)O.CH(3)OH and in (H(2)O)(18) are very close to the O-O distance reported for liquid water (2.85 A). 相似文献
16.
S. Selvasekarapandian R. Sivakumar N. M. Manikandan V.M. Ragjunath V. Kannan S. Rajaram 《Journal of Radioanalytical and Nuclear Chemistry》2002,252(2):345-347
The gas collection measurement method was employed to determine radon activity concentrations in the water of Coonoor. Open well water, dam water and stream water have been investigated for their radon concentrations. It is observed that the highest radon concentration is in the open well water and the lowest in stream water. From these measurements, the corresponding annual effective ingestion dose is determined. 相似文献
17.
Klaus Broschinski Arunachalam Kannan Peter G. Jones Ina Dix Henning Hopf 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(12):o711-o713
In the title compound, C18H16, the [2.2]paracyclophane geometry is restrained to a considerable extent despite the introduction of the extra C=C bridge; typical paracyclophane features, such as the elongated C—C bridges, are still observed. However, the bridgehead atoms of the C=C bridge are forced into unusually close proximity [2.657 (3) Å], which in turn causes the rings to be rotated to an interplanar angle of 13.7 (2)°. The packing involves hexagonally close‐packed layers of molecules parallel to the xy plane, corresponding to the known `7,11' pattern of paracyclophanes, but without significant short intermolecular contacts. 相似文献
18.
The carbamoyl methyl sulfoxide compounds of uranyl bis(β-diketonate) of the types [UO2(DBM)2CMSO] and [{UO2(DBM)2}2CMSO] (where HDBM = C6H5COCH2COC6H5; CMSO = C6H5CH2SOCH2CONHC6H5 or C6H5SOCH2CONiPr2) have been synthesized and characterized by IR and NMR spectroscopic techniques and elemental analysis. Spectral studies show that CMSO acts as a monodentate ligand in [UO2(DBM)2CMSO] compounds and bonds through the sulfoxo oxygen atom to the uranyl group. It acts as a bridging bidentate ligand in [{UO2(DBM)2}2CMSO] compounds and bonds through both the sulfoxo and carbamoyl oxygen atoms to two different uranyl groups. The structure of the compound [{UO2(DBM)2}2C6H5CH2SOCH2CONHC6H5] confirms the bridging bidentate mode of coordination for the CMSO ligand. Extraction studies show an enhancement in solvent extraction for the uranyl ion from nitric acid medium when a mixture of thenoyl trifluoroacetone (HTTA) and CMSO was employed. 相似文献
19.
The noise mechanisms of a uniformly doped nanoscale FinFET photodetector including quantum mechanical effects is investigated theoretically. A numerical model has been developed for computation of different noise components in nanoscale FinFET. The drain current channel thermal noise under illuminated conditions is calculated. The noise parameters of the nanoscale FinFET equivalent circuit are strongly influenced by the incident optical signal and the quantum mechanical effects are found to play a major role in determining the overall noise performance of the device. It also reveals that the operating frequency can be adjusted suitably to make the noise behavior independent of the incident optical power. Other noise components such as Shot noise, thermal noise and diffusion noise have been calculated for the two port FinFET photodetector. Signal-to-noise ratio (SNR), Bit error rate (BER) and noise equivalent power (NEP) has also been calculated. 相似文献
20.
Review of wavelet methods for the solution of reaction–diffusion problems in science and engineering
Wavelet method is a recently developed tool in applied mathematics. Investigation of various wavelet methods, for its capability of analyzing various dynamic phenomena through waves gained more and more attention in engineering research. Starting from ‘offering good solution to differential equations’ to capturing the nonlinearity in the data distribution, wavelets are used as appropriate tools at various places to provide good mathematical model for scientific phenomena, which are usually modeled through linear or nonlinear differential equations. Review shows that the wavelet method is efficient and powerful in solving wide class of linear and nonlinear reaction–diffusion equations. This review intends to provide the great utility of wavelets to science and engineering problems which owes its origin to 1919. Also, future scope and directions involved in developing wavelet algorithm for solving reaction–diffusion equations are addressed. 相似文献