全文获取类型
收费全文 | 287篇 |
免费 | 14篇 |
专业分类
化学 | 221篇 |
力学 | 3篇 |
数学 | 13篇 |
物理学 | 64篇 |
出版年
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 10篇 |
2020年 | 7篇 |
2019年 | 9篇 |
2018年 | 12篇 |
2017年 | 10篇 |
2016年 | 15篇 |
2015年 | 11篇 |
2014年 | 5篇 |
2013年 | 15篇 |
2012年 | 23篇 |
2011年 | 27篇 |
2010年 | 21篇 |
2009年 | 9篇 |
2008年 | 16篇 |
2007年 | 16篇 |
2006年 | 11篇 |
2005年 | 13篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 12篇 |
2001年 | 8篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1993年 | 1篇 |
1992年 | 5篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有301条查询结果,搜索用时 265 毫秒
131.
We present a scheme to solve the nonlinear multigroup radiation diffusion (MGD) equations. The method is incorporated into a massively parallel, multidimensional, Eulerian radiation-hydrodynamic code with Adaptive Mesh Refinement (AMR). The patch-based AMR algorithm refines in both space and time creating a hierarchy of levels, coarsest to finest. The physics modules are time-advanced using operator splitting. On each level, separate “level-solve” packages advance the modules. Our multigroup level-solve adapts an implicit procedure which leads to a two-step iterative scheme that alternates between elliptic solves for each group with intra-cell group coupling. For robustness, we introduce pseudo transient continuation (Ψtc). We analyze the magnitude of the Ψtc parameter to ensure positivity of the resulting linear system, diagonal dominance and convergence of the two-step scheme. For AMR, a level defines a subdomain for refinement. For diffusive processes such as MGD, the refined level uses Dirichlet boundary data at the coarse–fine interface and the data is derived from the coarse level solution. After advancing on the fine level, an additional procedure, the sync-solve (SS), is required in order to enforce conservation. The MGD SS reduces to an elliptic solve on a combined grid for a system of G equations, where G is the number of groups. We adapt the “partial temperature” scheme for the SS; hence, we reuse the infrastructure developed for scalar equations. Results are presented. We consider a multigroup test problem with a known analytic solution. We demonstrate utility of Ψtc by running with increasingly larger timesteps. Lastly, we simulate the sudden release of energy Y inside an Al sphere (r = 15 cm) suspended in air at STP. For Y = 11 kT, we find that gray radiation diffusion and MGD produce similar results. However, if Y = 1 MT, the two packages yield different results. Our large Y simulation contradicts a long-standing theory and demonstrates the inadequacy of gray diffusion. 相似文献
132.
133.
Glucose‐ and Cellulose‐Derived Ni/C‐SO3H Catalysts for Liquid Phase Phenol Hydrodeoxygenation
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Stanislav Kasakov Dr. Chen Zhao Dr. Eszter Baráth Zizwe A. Chase Dr. John L. Fulton Dr. Donald M. Camaioni Aleksei Vjunov Dr. Hui Shi Prof. Dr. Johannes A. Lercher 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(4):1567-1577
Sulfonated carbons were explored as functionalized supports for Ni nanoparticles to hydrodeoxygenate (HDO) phenol. Both hexadecane and water were used as solvents. The dual‐functional Ni catalysts supported on sulfonated carbon (Ni/C‐SO3H) showed high rates for phenol hydrodeoxygenation in liquid hexadecane, but not in water. Glucose and cellulose were precursors to the carbon supports. Changes in the carbons resulting from sulfonation of the carbons resulted in variations of carbon sheet structures, morphologies and the surface concentrations of acid sites. While the C‐SO3H supports were active for cyclohexanol dehydration in hexadecane and water, Ni/C‐SO3H only catalysed the reduction of phenol to cyclohexanol in water. The state of 3–5 nm grafted Ni particles was analysed by in situ X‐ray absorption spectroscopy. The results show that the metallic Ni was rapidly formed in situ without detectable leaching to the aqueous phase, suggesting that just the acid functions on Ni/C‐SO3H are inhibited in the presence of water. Using in situ IR spectroscopy, it was shown that even in hexadecane, phenol HDO is limited by the dehydration step. Thus, phenol HDO catalysis was further improved by physically admixing C‐SO3H with the Ni/C‐SO3H catalyst to balance the two catalytic functions. The minimum addition of 7 wt % C‐SO3H to the most active of the Ni/C‐SO3H catalysts enabled nearly quantitative conversion of phenol and the highest selectivity (90 %) towards cyclohexane in 6 h, at temperatures as low as 473 K, suggesting that the proximity to Ni limits the acid properties of the support. 相似文献
134.
135.
136.
V. Ya. Kavun A. A. Udovenko N. F. Uvarov V. I. Sergienko L. A. Zemnukhova 《Journal of Structural Chemistry》2002,43(2):246-251
Fluoride and ammonium ion dynamics in (NH4)3Sb4F15 was studied by 1H and 19F NMR spectroscopy in the range 180-440 K. Types of ionic motion were determined, and their activation energies were estimated. The crystal structure of a single crystal of (NH4)3Sb4F15 (space group
) was solved. In the range
K, a reversible phase transition has been found. Based on the experimental values of conductivity
of (NH4)3Sb4F15 (
S/cm at T = 440 K), this antimony(III) fluoride is classified as a superionic conductor. 相似文献
137.
Ragnarsson U Grehn L Koppel J Loog O Tsubrik O Bredikhin A Mäeorg U Koppel I 《The Journal of organic chemistry》2005,70(15):5916-5921
The pK(a) values in DMSO for 22 di- and triprotected hydrazine NH acids and two monosubstituted hydrazines have been determined using potentiometric titration. The results of density functional theory calculations at the B3LYP/6-311+G level of gas-phase acidities of a representative selection of mono-, di-, and trisubstituted hydrazines are compared with both the relevant published and novel experimental titration data. In the course of this work, a rough estimation of the pK(a) value of hydrazine in DMSO (ca. 38.0) has been deduced. For typical triprotected compounds of this kind containing moderately electron-withdrawing carbamate and imidodicarbonate or arenesulfonylcarbamate functions the pK(a) values fall in the range 15.1-17.3, whereas for N,N'-diprotected hydrazines with a carbamate and an aromatic sulfonyl group the corresponding values are 12.7-14.5. Several of these triprotected derivatives have recently been applied preparatively in stepwise synthesis of substituted hydrazines using alkyl halides as electrophiles in the presence of a phase transfer catalyst, and a few of them, with varying success, have been examined in model experiments with benzyl alcohol, triphenylphosphine, and diethyl azodicarboxylate in the Mitsunobu reaction. The dependence of the reactivity on the intrinsic acidity of the hydrazines in this reaction is highlighted. Furthermore, the regioselective alkylation of an N,N'-diprotected hydrazine can be rationalized on the basis of the presented data. 相似文献
138.
In our 19F NMR and impedance spectroscopy study, we investigated the mobility of fluoride ions (270-550 K) in polycrystalline K2ZrF6. A structural phase transition was found for K2ZrF6 in the temperature range of 513 to 518 K. The high-temperature -K2ZrF6 modification is characterized by translational diffusion of the fluoride ions and uniaxial anisotropy of the 19F nuclear magnetic screening tensor. The electrophysical characteristics of K2ZrF6 were investigated in the temperature range of 300 to 560 K. It is established that the -K2ZrF6 phase is characterized by high ionic (superionic) conductivity above 540 K ( 3.2× 10-4 S/cm, T=560 K). 相似文献
139.
V. Ya. Kavun A. V. Gerasimenko A. B. Slobodyuk N. A. Didenko N. F. Uvarov V. I. Sergienko 《Russian Journal of Electrochemistry》2007,43(5):537-544
The ionic mobility in the temperature interval 180 to 480 K, structure, and electrophysical properties of rubidium-ammonium
hexafluorozirconates Rb2−x
(NH4)
x
ZrF6 (1.5 ≤ x ≤ 2.0) are studied by methods of the 19F, 1H NMR spectroscopy, x-ray structure analysis, differential thermal analysis, and impedance spectroscopy. Correlations between
the composition of the cationic sublattice, the character of ionic motions, and the phase transition temperature (of the type
order-disorder) are established in these compounds. The salient feature of the high-temperature modifications of these fluorozirconates
with x ≥ 1.5 is the translation diffusion of ions inside the fluoride and ammonium sublattices and the 19F NMR spectra are characterized by monoaxial anisotropy of the magnetic shielding tensor of the fluorine nuclei. Fluorozirconates
with x > 1.5 are shown to belong with the structural type (NH4)2ZrF6. The rubidium cations isomorphically replace the ammonium cations. The electrophysical characteristics of the compounds are
examined in the temperature interval 300 to 480 K. It is established that the electroconductivity of these compounds increases
with x.
Original Russian Text ? V.Ya. Kavun, A.V. Gerasimenko, A.B. Slobodyuk, N.A. Didenko, N.F. Uvarov, V.I. Sergienko, 2007, published
in Elektrokhimiya, 2007, Vol. 43, No. 5, pp. 563–570.
Based on the paper delivered at the 8th Meeting “Fundamental Problems of Solid-State Ionics”, Chernogolovka (Russia), 2006. 相似文献