全文获取类型
收费全文 | 1605篇 |
免费 | 34篇 |
国内免费 | 114篇 |
专业分类
化学 | 243篇 |
晶体学 | 11篇 |
力学 | 151篇 |
数学 | 502篇 |
物理学 | 846篇 |
出版年
2024年 | 4篇 |
2023年 | 22篇 |
2022年 | 19篇 |
2021年 | 15篇 |
2020年 | 28篇 |
2019年 | 25篇 |
2018年 | 24篇 |
2017年 | 24篇 |
2016年 | 36篇 |
2015年 | 31篇 |
2014年 | 65篇 |
2013年 | 129篇 |
2012年 | 76篇 |
2011年 | 128篇 |
2010年 | 94篇 |
2009年 | 124篇 |
2008年 | 99篇 |
2007年 | 101篇 |
2006年 | 75篇 |
2005年 | 48篇 |
2004年 | 41篇 |
2003年 | 53篇 |
2002年 | 75篇 |
2001年 | 36篇 |
2000年 | 70篇 |
1999年 | 47篇 |
1998年 | 53篇 |
1997年 | 27篇 |
1996年 | 14篇 |
1995年 | 34篇 |
1994年 | 17篇 |
1993年 | 7篇 |
1992年 | 18篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 10篇 |
1985年 | 4篇 |
1984年 | 9篇 |
1982年 | 2篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有1753条查询结果,搜索用时 46 毫秒
991.
The thermal creeping effect on slip velocity of air forced convection through a nanochannel is studied for the first time by using a lattice Boltzmann method. The nanochannel side walls are kept hot while the cold inlet air streams along them. The computations are presented for the wide range of Reynolds number, Knudsen number and Eckert number while slip velocity and temperature jump effects are involved. Moreover appropriate validations are performed versus previous works concerned the micro–nanoflows.The achieved results are shown as the velocity and temperature profiles at different cross sections, streamlines and isotherms and also the values of slip velocity and temperature jump along the nanochannel walls. The ability of the lattice Boltzmann method to simulate the thermal creeping effects on hydrodynamic and thermal domains of flow is shown at this study; so that its effects should be involved at lower values of Eckert number and higher values of Reynolds number especially at entrance region where the most temperature gradient exists. 相似文献
992.
This paper presents an exploration for separation of oil-in-water and coalescence of oil droplets in ultrasound field via lattice Boltzmann method. Simulations were conducted by the ultrasound traveling and standing waves to enhance oil separation and trap oil droplets. The focus was to the effect of ultrasound irradiation on oil-in-water emulsion properties in the standing wave field, such as oil drop radius, morphology and growth kinetics of phase separation. Ultrasound fields were applied to irradiate the oil-in-water emulsion for getting flocculation of the oil droplets in 420 kHz case, and larger dispersed oil droplets and continuous phases in 2 MHz and 10 MHz cases, respectively. The separated phases started to rise along the direction of sound propagation after several periods. The rising rate of the flocks was significantly greater in ultrasound case than that of oil droplets in the original emulsion, indicating that ultrasound irradiation caused a rapid increase of oil droplet quantity in the progress of the separation. The separation degree was also significantly improved with increasing frequency or irradiation time. The dataset was rearranged for growth kinetics of ultrasonic phase separation in a plot by spherically averaged structure factor and the ratio of oil and emulsion phases. The analyses recovered the two different temporal regimes: the spinodal decomposition and domain growth stages, which further quantified the morphology results. These numerical results provide guidance for setting the optimum condition for the separation of oil-in-water emulsion in the ultrasound field. 相似文献
993.
Derivatives are popular financial instruments whose values depend on other more fundamental financial assets (called the underlying assets). As they play essential roles in financial markets, evaluating them efficiently and accurately is critical. Most derivatives have no simple valuation formulas; as a result, they must be priced by numerical methods such as lattice methods. In a lattice, the prices of the derivatives converge to theoretical values when the number of time steps increases. Unfortunately, the nonlinearity error introduced by the nonlinearity of the option value function may cause the pricing results to converge slowly or even oscillate significantly. The lognormal diffusion process, which has been widely used to model the underlying asset’s price dynamics, does not capture the empirical findings satisfactorily. Therefore, many alternative processes have been proposed, and a very popular one is the jump-diffusion process. This paper proposes an accurate and efficient lattice for the jump-diffusion process. Our lattice is accurate because its structure can suit the derivatives’ specifications so that the pricing results converge smoothly. To our knowledge, no other lattices for the jump-diffusion process have successfully solved the oscillation problem. In addition, the time complexity of our lattice is lower than those of existing lattice methods by at least half an order. Numerous numerical calculations confirm the superior performance of our lattice to existing methods in terms of accuracy, speed, and generality. 相似文献
994.
We analyze the lattice structure and distribution of the digital explicit inversive pseudorandom number generator introduced by Niederreiter and Winterhof as well as of a general digital explicit nonlinear generator. In particular, we extend a lattice test designed for this class of pseudorandom number generators to parts of the period and arbitrary lags and prove that these generators pass this test up to very high dimensions. We also analyze the behavior of digital explicit inversive and nonlinear generators under another very strong lattice test which in its easiest form can be traced back to Marsaglia and provides a complexity measure essentially equivalent to linear complexity. 相似文献
995.
We study different extended formulations for the set with in order to tackle the feasibility problem for the set Pursuing the work of Aardal, Lenstra et al. using the reformulation , our aim is to derive reformulations of the form with 0 ≤ s ≤ n − m where preferably all the coefficients of P are small compared to the coefficients of A and T. In such cases the new variables μ appear to be good branching directions, and in certain circumstances permit one to deduce rapidly that the instance is infeasible.
We give a polynomial time algorithm for identifying such P, T if possible, and for the case that A has one row a we analyze the reformulation when s = 1, that is, one μ-variable is introduced. In particular, we determine the integer width of the extended formulations in the direction of the μ-variable, and derive a lower bound on the Frobenius number of a. We conclude with some preliminary tests to see if the reformulations are effective when the number s of additional constraints and variables is limited.
This work was partly carried out within the framework of ADONET, a European network in Algorithmic Discrete Optimization,
contract no. MRTN-CT-2003-504438. The first author is financed in part by the Dutch BSIK/BRICKS project. The research was
carried out in part while the second author visited CWI, Amsterdam with the support of the NWO visitor grant number B 61-556. 相似文献
996.
Steffen Hitzemann 《Discrete Mathematics》2010,310(24):3551-3557
We define interval decompositions of the lattice of subspaces of a finite-dimensional vector space. We show that such a decomposition exists if and only if there exists a family of linear forms with certain properties. As applications we prove that all finite-dimensional real vector spaces admit an interval decomposition, while GF(2)n has an interval decomposition if and only if n≤4. On the other hand, we present an interval decomposition of GF(3)5. This partially answers a question of Faigle and Kruse (2004) [1] and [4]. 相似文献
997.
Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst
Zhen Wang Jing Liu Yingju Yang Feng Liu Junyan Ding 《Proceedings of the Combustion Institute》2019,37(3):2967-2975
MnO2-based catalysts have attracted great attention in the field of elemental mercury (Hg0) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg0 oxidation by oxygen species (gas-phase O2, chemisorbed oxygen, and lattice oxygen) on MnO2 surface. The results indicate that Hg0 and HgO are chemically adsorbed on MnO2 surface with the adsorption energies of ?69.50 and ?226.48?kJ/mol, respectively. The adsorption of O2 on MnO2 surface belongs to chemisorption. O2 can decompose on MnO2 surface with an energy barrier of 97.46?kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O2 and dissociative adsorbed O2 are more favorable for Hg0 catalytic oxidation than lattice oxygen, and perpendicular adsorbed O2 is the most active oxygen for Hg0 oxidation. The reaction pathway of Hg0 oxidation by perpendicular adsorbed O2 includes three reaction steps: Hg0?→?Hg(ads)?→?HgO(ads)?→?HgO. The third step (HgO(ads)?→?HgO) is endothermic by 168.17?kJ/mol with an energy barrier of 179.48?kJ/mol, and it is the rate-limiting step of the whole Hg0 oxidation reaction. 相似文献
998.
Fei CHENG Jian YANG Liang YAN Jun ZHAO Huahua ZHAO Huanling SONG Lingjun CHOU 《物理化学学报》2019,35(9):1027-1036
The selective oxidation of methane to basic petrochemicals (ethylene and ethane) is desirable and has attracted extensive research attention. The oxidative coupling of methane (OCM) is considered a promising one-step route for the production of C2 compounds (ethylene and ethane) from methane, and has been the focus of industrial and fundamental studies. It is widely accepted that the composition is a crucial factor governing the activity of a catalyst system. It was found that the phase structures, basicity, existing status and distribution of the active components, oxygen species, and chemical states of the catalyst were influenced by the composition and ratio, resulting in different catalytic performances for the OCM. In this study, a series of solid acid WO3/TiO2-supported lithium-manganese oxide catalysts for OCM were synthesized via the impregnation method. The impacts of diverse compositions, such as the individual contents (Li and Mn) and dual contents (Li-Mn), on the OCM were investigated in detail, using inductively coupled plasma optical emission spectrometry, X-ray diffraction, high-resolution transmission electron microscopy, CO2-temperature-programmed desorption, O2-temperature-programmed desorption, H2-temperature-programmed reduction, Raman spectroscopy, X-ray photoelectron spectroscopy, and CH4-temperature-programmed surface reaction. The addition of Li content to the catalyst not only led to the anatase-to-rutile crystal structure transformation of TiO2, and the reduction of the high-valence-state Mn species to low-valence-state Mn, but also increased the content of surface lattice oxygen and decreased the surface basicity. The observed effects on the structures and catalytic performance suggest that the Li content is helpful in suppressing the formation of completely oxidized CO2, and increases the C2 selectivity. Moreover, increasing the Li content of the catalyst facilitated the mobility of the lattice oxygen, which triggered the promotion of CH4 activation, thereby enhancing the OCM catalytic performance. The Mn content acted as the active sites for OCM; therefore, the performance of the catalyst was closely related to the Mn concentration and valence state. However, the WO3/TiO2-supported catalyst with excessive Mn content exhibited a high surface basicity, high valence state of Mn, and low abundant lattice oxygen, which was unfavorable for C2 selectivity. The Raman spectroscopy results revealed that MnTiO3 was formed due to the co-existence of Li and Mn on WO3/TiO2, and played an essential role in improving the low-temperature OCM performance. There was a synergic effect of the Li and Mn components on the OCM. The optimal performance (16.3% C2 yield) was achieved over the WO3/TiO2-supported lithium-manganese catalyst with n(Li) : n(Mn) = 2 : 1 at 750 ℃. 相似文献
999.
While lipids form soft, fluidic membranes (soft assembly), proteins can readily assemble into rigid, crystalline structures such as viral capsids and bacterial compartments (lattice assembly). The key difference has to do with the driving forces, where the former is driven by the weak, directionless hydrophobic effect and the latter, by a combination of relatively strong, directional intermolecular interactions. In synthetic systems, the lipid assembly has been massively replicated but the protein assembly has been rarely rivaled. Herein, we briefly review these two kinds of assemblies with special emphasis on a recently reported lattice self-assembly system of cyclodextrin complexes. The complexes arrange themselves into an in-plane, rhombic lattice that develops into lamellar, tubular, and polygonal structures depending on concentration. We will then cover the formation mechanisms, driving forces, and an application of the tubes in particle encapsulation. We hope that this short review would draw people's attention to this emerging field of lattice self-assembly. 相似文献
1000.
Alireza Rahimi Ali Dehghan Saee Abbas Kasaeipoor Payam Hooshmand Emad Hasani Malekshah 《Chinese Journal of Physics (Taipei)》2018,56(6):2865-2878
The lattice Boltzmann simulation of nanofluid flow and heat transfer during natural convection within a dumbbell-shaped heat exchanger is carried out. The heat exchanger is filled with CuO–water. The KKL model is employed to predict the thermo-physical properties of nanofluid. In order to perform a comprehensive hydrothermal investigation, different post-processing approaches such as heatline visualization, total entropy generation, local entropy generation based on local fluid friction irreversibility and heat transfer irreversibility, average and local Nusselt variation are employed. In the present investigation, it is tried to present the impact of different influential parameters like Rayleigh number, solid volume fraction of nanofluid and thermal arrangement of internal fins-bodies on the fluid flow, heat transfer rate and entropy generation. 相似文献