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71.
Sol-Gel Synthesis of Transparent Alumina Gel and Pure Gamma Alumina by Urea Hydrolysis of Aluminum Nitrate 总被引:2,自引:0,他引:2
Maria I. F. Macêdo Carla C. Osawa Celso A. Bertran 《Journal of Sol-Gel Science and Technology》2004,30(3):135-140
-Alumina was synthesized by a sol-gel method with the aluminum ion hydrolysis control performed by urea. The initial saturated Al3+/urea solution presented urea coordinated with the aluminum ion, as shown in the 13C NMR and 27Al NMR spectra and longitudinal relaxation times, T
1, from the latter. The substitution of water molecules in the Al3+ coordination shell by urea controlled the hydrolysis process and provided an extensive nucleation during the initial steps of the aluminum hydroxide formation due to urea thermolysis at 90°C. The resulting sol, composed of Al(OH)3 nanoparticles, coalesced and became a transparent gel permeated by a solution of urea and the polycation ion [Al13O4(OH)24(H2O)12]7+. The freshly prepared gel was transformed, under heating at 300°C, directly to -alumina, characterized by FTIR, 27Al-MAS-NMR and SBET techniques, without - or -phases, as a consequence of the high degree of homogeneity of the -alumina precursor. 相似文献
72.
Arnaldo Simal do Nascimento 《Journal of Differential Equations》2003,190(1):16-38
We prove existence and establish the asymptotic behavior, as ε→0, of stable stationary solutions to the equation ut=ε∇·[d(x)∇u]+(1−u2)[u−a(x)], for , where , N?2, with Neumann boundary condition. The function a(x)∈C0,ν(Ω) satisfies −1<a(x)<1 and vanishes on some hypersurfaces. The results generalize to N-dimensional domains and to variable diffusivity earlier paper by Angenent et al. (J. Differential Equations 67 (1987) 212). 相似文献
73.
In this paper, we obtain some rigidity theorems for compact Riemannian manifolds Ω with boundary M and nonnegative Ricci curvature; for instance, we prove that the existence of certain functions on M together with a lower bound c > 0 on the principal curvtures of M imply that Ω is an euclidean ball of radius $ {1\over c} $ . 相似文献
74.
Given H≥0 and bounded convex curves α1, ...,⇌n, α in the plane z=0 bounding domains D1, …, Dn, D, respectively, with
if i ∈ j and with Di ⊂ D, we obtain several results proving the existence of a constanth depending only on H and on the geometry of the curves
αi, α such that the Dirichlet problem for the constant mean curvature H equation:
where
may accept or not a solution. 相似文献
75.
Analytical approximations are used to clarify the effect of Larmour radius on rf ponderomotive forces and on poloidal flows induced by them in tokamak plasmas. The electromagnetic force is expressed as a sum of a gradient part and of a wave momentum transfer force, which is proportional to wave dissipation. The first part, called the gradient electromagnetic stress force, is combined with fluid dynamic (Reynolds) stress force, and gyroviscosity is included into viscosity force to model finite ion Larmour radius effects in the momentum response to the rf fields in plasmas. The expressions for the relative magnitude of different forces for kinetic Alfven waves and fast waves are derived. 相似文献
76.
A.F. Ferrari M. Gomes J.R. Nascimento E. Passos A.Yu. Petrov A.J. da Silva 《Physics letters. [Part B]》2007
We study some physical consequences of the introduction of a Lorentz-violating modification term in the linearized gravity, which leads to modified dispersion relations for gravitational waves in the vacuum. We discuss two possible mechanisms for the induction of such a term in the Lagrangian. First, it is generated at the quantum level by a Lorentz-breaking coupling of the gravity field to a spinor field. Second, it appears as consequence of a particular modification of the Poisson algebra of the canonical variables, in the spirit of the so-called “noncommutative fields approach”. 相似文献
77.
do Nascimento Hederson Majela dos Santos Andressa Duarte Vitor Anthony Bittencourt Paulo Rodrigo Stival Radovanovic Eduardo Fávaro Silvia Luciana 《Cellulose (London, England)》2021,28(9):5477-5492
Cellulose - In this work lignocellulosic fibers were obtained from Yucca aloifolia L. leaves and their chemical, morphological, thermal and mechanical properties were studied. The fibers were... 相似文献
78.
79.
Wenden Charles J.A. Soriano Flávio A. Falcão Nascimento J.H. Rodrigues 《Journal of Differential Equations》2013
In this paper we consider a vibrating system of Bresse type, in a one-dimensional bounded domain with nonlinear localized damping mechanisms acting in all the three wave equations. We obtain some rates of decay for its solutions with no restrictions around the coefficients as well as the condition of equal-speed wave propagation. A new result concerning an internal observability for the conservative system was also proved in order to reach the asymptotics above. 相似文献
80.
Layered stable equilibria of a reaction-diffusion equation with nonlinear Neumann boundary condition
Arnaldo Simal do Nascimento Renato José de Moura 《Journal of Mathematical Analysis and Applications》2008,347(1):123-135
In this work we investigate the existence and asymptotic profile of a family of layered stable stationary solutions to the scalar equation ut=ε2Δu+f(u) in a smooth bounded domain Ω⊂R3 under the boundary condition εν∂u=δεg(u). It is assumed that Ω has a cross-section which locally minimizes area and limε→0εlnδε=κ, with 0?κ<∞ and δε>1 when κ=0. The functions f and g are of bistable type and do not necessarily have the same zeros what makes the asymptotic geometric profile of the solutions on the boundary to be different from the one in the interior. 相似文献