全文获取类型
收费全文 | 428篇 |
免费 | 5篇 |
专业分类
化学 | 282篇 |
晶体学 | 5篇 |
力学 | 7篇 |
数学 | 67篇 |
物理学 | 72篇 |
出版年
2023年 | 4篇 |
2022年 | 9篇 |
2021年 | 13篇 |
2020年 | 8篇 |
2019年 | 14篇 |
2018年 | 17篇 |
2017年 | 14篇 |
2016年 | 15篇 |
2015年 | 9篇 |
2014年 | 19篇 |
2013年 | 22篇 |
2012年 | 21篇 |
2011年 | 25篇 |
2010年 | 26篇 |
2009年 | 29篇 |
2008年 | 21篇 |
2007年 | 24篇 |
2006年 | 16篇 |
2005年 | 6篇 |
2004年 | 11篇 |
2003年 | 6篇 |
2002年 | 3篇 |
2001年 | 5篇 |
2000年 | 7篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1987年 | 6篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1977年 | 2篇 |
1976年 | 6篇 |
1965年 | 2篇 |
1956年 | 1篇 |
1955年 | 1篇 |
1954年 | 1篇 |
1936年 | 1篇 |
1935年 | 1篇 |
1934年 | 1篇 |
1932年 | 1篇 |
1931年 | 1篇 |
排序方式: 共有433条查询结果,搜索用时 15 毫秒
1.
In order to investigate the relative effects of the differences between the structures and lipophilicities of 1, 10-dioxa-4, 7, 13, 16-tetra-azacyclo-octadecane (TA-18-crown-6) and the tetrabenzyl derivative of 1,10-dioxa-4, 7, 13, 16-tetra-azacyclo-octadecane (TBTA-18-crown-6) on their extraction-abilities and -selectivities for transition metal cations, constants of the overall extraction (logK
ex) of 1:1 (M:L) complexes, the distribution (K
D) for two diluents (CH2Cl2 and CHCl3) with different dielectric constants have been determined at 25 ± 0.1 °C. The magnitude of logK
ex is largely determined by that of K
D. The equilibrium constants of TA-18-crown-6 have been compared with those of TBTA-18-crown-6. It is found that:(i) logK
ex sequences of TA-18-crown-6 and TBTA-18-crown-6 for transition metals in CH2Cl2 lie in order: Fe3+ > Cu2+ > Mn2+ > Co2+ > Cd2+ > Ni2+ > Zn2+ and Fe3+ > Cu2+ > Co2+ > Mn2+ > Ni2+ > Cd2+ > Zn2+ respectively; (ii) the stability sequences of two types of tetra-aza-crown ethers with the transition metal cations in CHCl3 are the same as follows: Fe3+ > Ni2+ > Cu2+ > Co2+ > Zn2+ > Cd2+ > Mn2+, and (iii) unusual selectivities are observed for transition metal-tetra-aza-crowns, e.g. the high Fe3+/M
n+ selectivity factors (Sf) of TA-18-crown-6, except for the competitive-extractions for the special case in CHCl3 of TBTA-18-crown-6, it was found that the Mn2+/M
n+ values were relatively higher according to the other transition metal cations. A systematic sequence in these two types of solvents is not found for a given transition metal cation in terms of the variation of selectivity with the tetra-aza-crown ethers. The results provide alternatives for the rational design of other specific ligands on the transition metal cations. 相似文献
2.
The paper presents a new formulation of the integral boundary element method (BEM) using subdomain technique. A continuous approximation of the function and the function derivative in the direction normal to the boundary element (further ‘normal flux’) is introduced for solving the general form of a parabolic diffusion‐convective equation. Double nodes for normal flux approximation are used. The gradient continuity is required at the interior subdomain corners where compatibility and equilibrium interface conditions are prescribed. The obtained system matrix with more equations than unknowns is solved using the fast iterative linear least squares based solver. The robustness and stability of the developed formulation is shown on the cases of a backward‐facing step flow and a square‐driven cavity flow up to the Reynolds number value 50 000. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
The bias dependent interface charge is considered as the origin of the observed non-ideality in current–voltage and capacitance–voltage characteristics. Using the simplified model for the interface electronic structure based on defects interacting with the continuum of interface states, the microscopic origin of empirical parameters describing the bias dependent interface charge function is investigated. The results show that in non-ideal metal–semiconductor contacts the interface charge function depends on the interface disorder parameter, density of defects, barrier pinning parameter and the effective gap center. The theoretical predictions are tested against several sets of published experimental data on bias dependent ideality factor and excess capacitance in various metal–semicoductor systems. 相似文献
4.
Bünyamin Demir Vakif Dzhafarov ?ahin Koçak Mehmet Üreyen 《Journal of Mathematical Analysis and Applications》2007,332(1):735-740
We give an iterated function system (IFS) on the plane with the circle as attractor. In doing this, we also give a sufficient condition for radially contracting functions on the plane (or on Rn) to be a contraction. A counterexample shows that radial contractiveness is not enough to be a contraction. 相似文献
5.
The role of the tunneling mechanisms in metal-disordered layer-semiconductor structure under spin injection at the interface is investigated. The non-ideal metal-semiconductor structure as prepared by ionized cluster beam deposition is considered, and it is shown that the depletion region of the semiconductor can be tailored to include a suitably heavily doped region near the interface. The tunneling is described within a simplified model in which the expression for the interface resistance of the metal-disordered layer-semiconductor structure is obtained. It is argued that in the case of ionized cluster beam deposited non-ideal Schottky structure a significant spin injection is achieved. 相似文献
6.
Let (E, ¦·¦) be a uniformly convex Banach space with the modulus of uniform convexity of power type. Let be the convolution of the distribution of a random series inE with independent one-dimensional components and an arbitrary probability measure onE. Under some assumptions about the components and the smoothness of the norm we show that there exists a constant such that |{·<t}–{·+r<t}|r
q
, whereq depends on the properties of the norm. We specify it in the case ofL
spaces, >1. 相似文献
7.
S Çak?r E Co?kunP Naumov E Biçer? Bulut H ?çbudakO Çak?r 《Journal of Molecular Structure》2002,608(1):101-107
Aspartame adduct of copper(II) chloride Cu(Asp)2Cl2·2H2O (Asp=aspartame) is synthesized and characterized by elemental analysis, FT IR, UV/vis, ESR spectroscopies, TG, DTG, DTA measurements and molecular mechanics calculations. Aqueous solution of the green solid absorbs strongly at 774 and 367 nm. According to the FT IR spectra, the aspartame moiety coordinates to the copper(II) ion via its carboxylate ends, whereas the ammonium terminal groups give rise to hydrogen bonding network with the water, the chloride ions or neighboring carboxylate groups. The results suggest tetragonally distorted octahedral environment of the copper ions. 相似文献
8.
The chemical polymerization of aniline was carried out in media containing different linear dicarboxylic acids with the use of oxidants such as K2Cr2O7, KMnO4, K2S2O8, KIO3 and FeCl3. The highest yield and the conductivity were observed with K2Cr2O7. The yield and the conductivity of the polyaniline (PAn) synthesized were observed to decrease in the order of oxalic acid > malonic acid > succinic acid > glutaric acid > adipic acid > phthalic acid. The solubility tests carried out in solvents such as DMF, DMSO and NMP showed that the larger the dopant molecule, the higher is the solubility of polyaniline. The PAn synthesized was characterized by measurements of conductivity, intrinsic viscosity, density and FTIR, UV-VIS and TGA techniques. 相似文献
9.
Magorzata A. Broda Maria Rospenk Dawid Siodak Barbara Rzeszotarska 《Journal of Molecular Structure》2005,740(1-3):17-24
The comparative studies on the association of Ac-ΔAla-NMe2 and Ac-l-Ala-NMe2 in carbon tetrachloride were performed by the analysis of their average molecular weight, dipole moments and FTIR spectra. To aid spectroscopic interpretation and gain some deeper insight into the nature of associates, the geometries of the minimum energy of the dimers of Ac-ΔAla-NMe2 and Ac-l-Ala-NMe2 were calculated by the B3LYP/6-31+G** method. The average molecular weight in the studied concentration range, for the ΔAla and l-Ala peptide, as determined by the osmometric method, did not exceed 1.5 and 1.2 of the monomeric mass, respectively. Accordingly, the percentage of the monomeric form (α) decreased as concentration was increased more significantly for the ΔAla analogue than for its saturated counterpart. In the studied concentrations, the dipole moment of the unsaturated compound decreases and that of its counterpart is almost constant. We identified a wider range of dimeric forms of Ac-ΔAla-NMe2 than those of Ac-l-Ala-NMe2. While Ac-ΔAla-NMe2 mainly forms cyclic dimers, built of open conformers H/F, specific for α,β-dehydroamino acids, Ac-l-Ala-NMe2 forms cyclic and linear dimers, characteristic for the usual amino acids. Ac-ΔAla-NMe2 has a greater tendency to associate than its saturated variant, which is the result of stronger hydrogen bonds. 相似文献
10.
F. Sanem Koçak 《Journal of organometallic chemistry》2006,691(23):5030-5037
Thermal substitution reaction of Cr(CO)4(η2:2-1,5-cyclooctadiene), Mo(CO)4(η2:2-norbornadiene), and W(CO)5(η2-bis(trimethylsilyl)ethyne) with N,N′-bis(ferrocenylmethylene)ethylenediamine (bfeda) yields M(CO)4(bfeda) complexes which could be isolated from the reaction solution and characterized by elemental analysis, MS, IR, and NMR spectroscopy. In the case of tungsten, W(CO)5(bfeda) is formed as intermediate and then undergoes the ring closure reaction yielding the ultimate product W(CO)4(bfeda). The electrochemical behavior of the M(CO)4(bfeda) complexes was studied by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in dichloromethane with tetrabutylammonium tetrafluoroborate as electrolyte. Constant potential electrolysis of the complexes was performed successively at their peak potentials at 0 °C in their CH2Cl2 solution and the electrolysis was followed by in situ recording the electronic absorption spectra in every 5 mC. In the electrolysis of Cr(CO)4(bfeda), the central Cr(0) is oxidized first and electrolysis continues with oxidations of two ferrocenyl groups until the end of totally three moles of electron passage per mole of complex. In the electrolysis of Mo(CO)4(bfeda) and W(CO)4(bfeda) the first oxidation occurs on the central atom forming a short-lived species which undergoes an intramolecular one-electron transfer and is reduced back to M(0) while one of the ferrocene units is oxidized to the ferrocenium cation at the same time. This indicates that the electron is transferred from iron to the central metal atom. 相似文献