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排序方式: 共有802条查询结果,搜索用时 31 毫秒
41.
Marin Coldea Natalia Dihoiu Iuliu Pop 《Journal of Physics and Chemistry of Solids》1981,42(7):563-565
27Al Knight shifts vs temperature and magnetic susceptibility for the intermetallic compounds Gd2Ni17?xAlx (x = 17; 16.2; 16; 15) are presented. The results are discussed in terms of the uniform polarization model fo the conduction electrons by the 4f and 3d spins localized on the Gd and Ni ions. The phenomenological exchange constants Jsf and Jsd range between ?1.80×10?3 and 1.19×10?3 eV and ?0.63×10?3 and ?0.52×10?3 eV, respectively. 相似文献
42.
Freely suspended metallic single-walled carbon nanotubes (SWNTs) exhibit reduced current carrying ability compared to those lying on substrates, and striking negative differential conductance at low electric fields. Theoretical analysis reveals significant self-heating effects including electron scattering by hot nonequilibrium optical phonons. Electron transport characteristics under strong self-heating are exploited for the first time to probe the thermal conductivity of individual SWNTs (approximately 3600 W m-1 K-1 at T=300 K) up to approximately 700 K, and reveal a 1/T dependence expected for umklapp phonon scattering at high temperatures. 相似文献
43.
This paper analyzes the cooling process of a vertical thin plate originated by a fluid-saturated porous medium, taking into account the effects of both longitudinal and transversal heat conduction in the plate. Due to the finite thermal conductivity of the plate, a longitudinal temperature gradient arises within it, which prevents any similarity solution in the boundary layer, changing the mathematical character of the problem from parabolic to elliptic, for large values of the suitable Rayleigh number. The energy balance equations are reduced to a system of two differential equations with two parameters: the nondimensional plate thermal conductivity and the aspect ratio of the plate . In order to obtain the evolution of the temperature of the plate as a function of time and position, the coupled balance equations are integrated numerically for several values of the parameters, including the cases of very good and poor conducting plates. The results obtained, are compared with an asymptotic analysis based on the multiple scales technique carried out for the case of a very good conducting plate. There is at the beginning a fast transient in nondimensional time scale of order followed by a slow nondimensional time scale of order unity, which gives the evolution of the cooling process. Good agreement is achieved even for values of the conduction parameter of order unity. The asymptotic solution allows us to give closed form analytical solution for the plate temperature evolution in time and space. The overall thermal energy of the plate decreases faster for smaller values of . 相似文献
44.
Carole Diederichs David Taj Thimothe Lecomte Crisitiano Ciuti Philippe Roussignol Claude Delalande Aristide Lemaître Ludovic Largeau Olivia Mauguin Jacqueline Bloch Charles Leyder Alberto Bramati Elisabeth Giacobino Jrme Tignon 《Comptes Rendus Physique》2007,8(10):1198-1204
We report the realization of a monolithic vertical-cavity, surface emitting micro-optical parametric conversion nanostructure, triply resonant with the parametric frequencies, allowing parametric oscillation with ultra-low pump power threshold. The photonic phase-space naturally provides triple resonance for the parametric frequencies, together with built-in cavity phase-matching for the pump wave at normal incidence. Parametric oscillation is observed in both the strong and weak exciton–photon coupling regime, allowing a high operating temperature. Signal and idler beams can be collected at 0° or at finite angles. The OPO threshold is low enough to envisage the realization of an all-semiconductor electrically-pumped micro-parametric oscillator. To cite this article: C. Diederichs et al., C. R. Physique 8 (2007). 相似文献
45.
This article deals with the steady Darcy free convection adjacent to a heated or cooled permeable vertical flat plate of constant temperature, which is embedded in a fluid-saturated porous medium of uniform ambient temperature T ∞. There is a uniform horizontal throughflow of the fluid and a volumetric heat generation q′′′ takes place, which is considered to be a power-law function of the local temperature difference T ? T ∞, i.e., q′′′ ~ (T ? T ∞) n . To be specific, two cases of this type of volumetric heat generation are considered in the analysis in some detail, namely, the linear and the quadratic cases, n = 1 and n = 2, respectively. 相似文献
46.
This article looks at the hydrodynamic elastico-viscous fluid over a stretching surface. The equations governing the flow are reduced to ordinary differential equations, which are analytically solved by applying an efficient technique namely the homotopy analysis method (HAM). The solutions for the velocity components are computed. The numerical values of wall skin friction coefficients are also tabulated. The present HAM solution is compared with the known exact solution for the two-dimensional flow and an excellent agreement is found. 相似文献
47.
A simple method is proposed leading to reasonable approximations of the partition function of Morse oscillators in terms of
elementary functions. 相似文献
48.
49.
Flaviu Pop 《Central European Journal of Mathematics》2011,9(5):1088-1099
In this paper we consider a pair of right adjoint contravariant functors between abelian categories and describe a family
of dualities induced by them. 相似文献
50.
In the present work, we compare the influence introduced by the variation of water-to-cement ratio and the presence of different superplasticizers on the hydration process of gray cement using the low-field nuclear magnetic resonance relaxometry. The stages of hydration are identified and a relationship between the ettringite formation during hydration process and the transverse relaxation time is established. It is also demonstrated that the addition of small amounts of superplasticizer in the cement paste significantly expands the dormant period while the hardening period is reduced. On the other side, the increase in the water-to-cement ratio has little influence on the dormant period but can instead increase the porosity of the sample and consequently reduce its strength. 相似文献