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71.
The magnetic structure of ferrimagnetic ilmenite Ni2+Mn4+O3 has been investigated by neutron diffraction. Below the Curie temperature (164°C), the spins are oriented into two kinds of ferromagnetic planes perpendicular to the [111] rhomboedral axis, one containing Mn4+ and the other Ni2+ cation. Two successive planes are coupled antiferromagnetically. The experimental magnetic moments are respectively found equal to 1.8 and 2.2 μB for Ni2+ and Mn4+ at room temperature. The positional atomic parameters have been redetermined from nuclear diffraction data taken above the Curie temperature. 相似文献
72.
73.
Nguyen B Lee MP Hamelberg D Joubert A Bailly C Brun R Neidle S Wilson WD 《Journal of the American Chemical Society》2002,124(46):13680-13681
A combination of biophysical techniques has been used to characterize the interaction of an antitrypanosomal agent, CGP 40215A, with DNA. The results from a broad array of methods (DNase I footprinting, surface plasmon resonance, X-ray crystallography, and molecular dynamics) indicate that this compound binds to the minor groove of AT DNA sequences. Despite its unusual linear shape that is not complementary to that of the DNA groove, a high binding affinity was observed in comparison with other similar but more curved diamidine compounds. The amidine groups at both ends of the ligand and the -NH groups on the linker are involved in extensive and dynamic H-bonds to the DNA bases. Complementary and consistent results were obtained from both the X-ray and molecular dynamics studies; both of these methods reveal direct and water-mediated H-bonds between the ligand and the DNA. 相似文献
74.
W. Boujelben A. Cheikh-Rouhou J.C. Joubert 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):419-423
Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite
oxides Pr0.7Sr0.3-x
xMnO3 ( 0 ?
x
? 0.3) and Pr0.7-x
xSr0.3MnO3 ( 0 ?
x
? 0.23) have been investigated. This study focuses on the different parameters which govern the magnetic and electrical properties
in such samples. The powder X-ray diffraction patterns for all samples could be indexed either with a rhombohedral perovskite
structure and R
c space group for x
? 0.2 in strontium deficient samples and for x
? 0.1 for praseodymium deficient ones. For other values of x the samples could be indexed in the orthorhombic structure with Pbnm space group. Magnetic and electrical investigations
show that praseodymium and strontium vacancies do not have similar effects on the lacunar compounds. Magnetization measurements
versus temperature show that all our samples exhibit a magnetic transition when the temperature decreases. All the praseodymium
deficient samples exhibit a paramagnetic-ferromagnetic transition when the temperature decreases while the strontium deficient
ones exhibit this transition only for low x values. The magnetic transition temperature shifts to lower values as the strontium deficiency increases (from 265 K for
x
= 0 to 90 K for x
= 0.3) and to higher values with the praseodymium deficiency increase (from 265 K for x
= 0 to 315 for x
= 0.23). Resistivity measurements as a function of temperature show a semiconducting-metallic transition for all x values in the praseodymium lacunar samples and only for low x values ( 0 ?
x
? 0.1) in the strontium lacunar ones when the temperature decreases.
Received 12 April 2000 and Received in final form 8 January 2001 相似文献
75.
Ethylene/1‐heptene copolymers as interesting alternatives to 1‐octene‐based LLDPE: Molecular structure and physical properties
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Anthony Ndiripo Dawid Joubert Harald Pasch 《Journal of polymer science. Part A, Polymer chemistry》2016,54(7):962-975
Classical linear low density polyethylenes (LLDPEs) are copolymers of ethylene and 1‐octene or 1‐hexene, respectively. In the past, other 1‐olefins have been tested as comonomers but the resulting LLDPEs were never commercialized as large scale products. The present study focuses on the use of 1‐heptene as an interesting comonomer for the synthesis of LLDPE. For a comparison of the molecular structure and the physical properties of 1‐heptene‐ and 1‐octene‐based LLDPEs, five Ziegler–Natta LLDPEs of varying comonomer contents based on 1‐heptene and 1‐octene, respectively, were acquired and analysed using advanced methods. The comonomer contents of the resins were between 0.35 and 6.4 mol %. Crystallization‐based techniques revealed similar bimodal distributions that are due to the formation of copolymer and polyethylene homopolymer fractions. The compositional distribution of the copolymers was studied by high‐temperature (HT) HPLC and HT‐2D‐LC. The analytical results indicate similar chemical heterogeneities and molar mass distributions of the two sets of LLDPE up to a comonomer content of 3 mol %. Similar to the molecular structure, the physical properties of the materials are quite similar. At comonomer contents of ≥3 mol % differences between the two sets of samples are seen that are attributed to differences in the abilities of 1‐heptene and 1‐octene in disrupting the crystal arrangements of the polymer chains in solid state. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 962–975 相似文献
76.
B. Purohit D. Amans Y. Guyot B. Mahler M.-F. Joubert C. Dujardin S. Daniele G. Ledoux S. Mishra 《Materials Today Chemistry》2020
A major challenge in the field of up-converting (UC) nanomaterials is to enhance their efficiencies. The –OH defects on the surface of the nanoparticles are thought to be the main cause of luminescence quenching, but there are no comparative studies in the literature showing the impact of anhydrous vs. hydrous synthesis on up-conversion efficiency. In this article, we present the synthesis of up-converting NaGdF4: Yb+3, Tm+3 nanoparticles by two different methods: thermal decomposition of single source metal-organic anhydrous precursors [NaLn(TFA)4(diglyme)] (Ln = Gd, Tm, Yb; TFA = trifluoroacetate) and room temperature co-precipitation using hydrated inorganic salts Ln(NO3)3·5H2O (Ln = Gd, Tm, Yb), NaNO3 and NH4F in ethylene glycol. After a detailed study on the influence of solvents and the percentage of lanthanide dopant on the crystal phase of the up-converting nanoparticles (NPs) and their complete characterization, a comparative up-conversion study was carried out which revealed that the uniform nanospheres (av. size ~13 nm) obtained from the anhydrous SSP had significantly higher up-conversion efficiency than agglomerated nanorods (~197 nm in length and ~95 nm in width) produced from hydrated inorganic salts. An enhanced up-conversion quantum yield of 1.8% for the anhydrous sample validates the anhydrous precursor approach as a strategy to obtain small but highly emitting up-converting particles without requiring a silica or undoped matrix surface passivation layer. 相似文献
77.
78.
On the convergence behavior of the restarted GMRES algorithm for solving nonsymmetric linear systems
Wayne Joubert 《Numerical Linear Algebra with Applications》1994,1(5):427-447
The solution of nonsymmetric systems of linear equations continues to be a difficult problem. A main algorithm for solving nonsymmetric problems is restarted GMRES. The algorithm is based on restarting full GMRES every s iterations, for some integer s>0. This paper considers the impact of the restart frequency s on the convergence and work requirements of the method. It is shown that a good choice of this parameter can lead to reduced solution time, while an improper choice may hinder or preclude convergence. An adaptive procedure is also presented for determining automatically when to restart. The results of numerical experiments are presented. 相似文献
79.
A new high pressure form of FeOOH, isostructural with InOOH, has been prepared by hydrothermal synthesis under very high pressure conditions. Characterization of the new phase was performed by D.T.A., T.G.A., infrared and Mössbauer spectroscopy. The compound behaves as an antiferromagnet with a Néel temperature of approximately 570° K. The hyperfine magnetic field at 0° K is about 525 kOe. Thermal decomposition occurs below the Néel temperature. Comparison with other FeOOH phases is made. 相似文献
80.
Laurence Petit Laurent Joubert Isabelle Tranchant Jean Herscovici Carlo Adamo 《Theoretical chemistry accounts》2008,120(4-6):507-513
A new neutral gene vector, based on a lipopolythiourea N-(2-(3-[2-(2-(3-[2-(3-methyl-thioueido)-ethyl] -thioureido)-ethylamino)-ethyl]-thioureido)-ethyl)- N′, N′- ditetradecyl-succinamide (DTTU) has recently been synthetized but its behavior is difficult to study at the experimental level. Density functional theory (DFT) calculations have thus been performed to predict its interaction mode with B-DNA. Its acidic properties are first computed and suggest that DTTU should be non-charged when interacting with DNA. Different ways of DTTU/DNA associations based on hydrogen bonding–namely external and groove-binding interactions—are then investigated. Our calculations clearly point out that external interaction is preferred with respect to groove-binding, due to three bifurcated hydrogen bonds between DTTU thiourea groups and DNA phosphates. Such results can be explained by the absence of charged groups in groove-binding whereas the negative charge of DNA phosphates deeply strengthens hydrogen bonds. 相似文献