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Synthesis and X-ray crystal structure of the [Ni(HDPE)2][NO3]2.2H2O.
(1991) Levy, Noel Massinhan; Neves, Ademir; Franco, César Vitorio; Cota, André Barros; Castellano, Eduardo Ernesto; Mascarenhas, Yvone Primerano
Evidence for a new phase in the Domany-Kinzel cellular automaton.
(1991) Martins, M. L.; Resende, Hélio Fernando Verona de; Tsallis, Constantino; Magalhães, A. C. N. de
We consider a generalized version (including anisotropy) of the stochastic one-dimensional cellular automaton studied by Domany and Kinzel. It recovers Wolfram-like deterministic cellular automata as particular cases. The phase diagram presents three (and not two, as previously suggested) phases which were detected through the numerical study of both the order parameter and the sensitivity to initial conditions. The various universality classes are exhibited as well.
Relation between the oxidation growth rate of chromia scales and self-diffusion in Cr203.
(1992) Sabioni, Antônio Claret Soares; Huntz, Anne Marie; Philibert, J.; Lesage, B.
In most cases, chromia scales are assumed to grow by predominant chromium diffusion. However, results of Atkinson and Taylor indicated that chromium bulk diffusion could not account for the growth rate of chromia scales. Moreover, recent results of Park et aL showed that oxygen diffusion in chromia was faster than chromium diffusion. So, at this date, the controlling process of the growth of chromia scales is not elucidated. To interpret such a phenomenon, oxygen and chromium self-diffusion coefficients in Cr203 single crystals and polycrystals were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. Tracers were introduced by ion implantation, thick film methods, and isotopic exchange, using the 54Cr, ~~ and 180 isotopes. Depth profiling was made by secondary ion mass spectroscopy (SIMS). The bulk diffusion coefficients were computed by using a general solution of the Fick's law taking into account evaporation and exchange at the surface. Grain-boundary diffusion coefficients were computed by using the Whipple-Le Claire equation for type B intergranular diffusion. Lattice and grain boundary self-diffusien coefficients were determined as a function of temperature and oxygen pressure. The diffusion coefficients are lower than results given in the literature and do not depend on the oxygen pressure. Moreover, it is found that oxygen diffusion is faster than chromium diffusion. These results are compared to the oxidation constants of chromia-forming alloys and it is shown that neither lattice self-diffusion, nor grain-boundary self-diffusion can justify the growth rate of chromia scales. Such a situation is compared to NiO case, for which authors found important differences in grain-boundary diffusivity, according to the elaboration mode of NiO (thermal oxidation or growth from the melt).
Study of oxygen self-diffusion in oxides by ion beam techniques : comparison between nuclear reaction analysis and SIMS.
(1993) Sabioni, Antônio Claret Soares; Freire Júnior, Fernando Lázaro; Leite Filho, Carlos Vieira Barros; Amami, B. A.; Dolin, C.; Monty, C.; Millot, F.
Oxygen self-diffusion in Fe,_,0 single crystals has been studied by means of the isotope exchange method in Hz/Hz”0 atmospheres. The ‘*O concentration profiles were determined by using two different techniques based on ion beams: secondary ion mass spectrometry (SIMS) and nuclear reaction analysis (NRA); the latter by means of the narrow nuclear resonance reaction ‘“O(p, o)“N at 629 keV, r = 2.1 keV. The diffusion coefficients obtained from the analysis of the measured profiles are in good agreement with those found in the literature. To compare the depth resolution and sensitivity of both techniques, the “0 profiles of implanted Cr,O, single crystals (Erso = 50 keV, fluence = 1.6 X lOI ions/cm’) were also measured.
Cation self-diffusion in (Th-5%U)O2.
(1993) Sabioni, Antônio Claret Soares; Lameiras, Fernando Soares; Cardoso, Paulo Edson
Laser densification of channel waveguides in gel-silica substrates.
(1994) Araujo, Fernando Gabriel da Silva; Chia, T.; Hench, L. L.
Channel waveguides are important components in optical signal processing. A new method is described for producing such waveguides with high design flexibility. The channel waveguides are produced using CO2 laser densification of partially densified gel-silica matrices (Type VI optical silica). Critical processing conditions include pore size and initial density of the matrix, laser power, translational speed of the sample, distance between sample and focusing lens, and ambient humidity. Channel waveguides less than 500 #m wide were produced in gel-silica substrates of different pore sizes and bulk densities through laser densification. Optically transparent waveguides were obtained for speeds of the sample over 1.4 cm/s and laser power settings ranging between 12 and 16 mA. Substrates with three different pore sizes were analyzed, i.e, 12, 30 and 45/~, with densities varying from 1.1 g/cc to 2.1 g/cc. Fourier transform infrared microspectrometry of the densified regions showed that IR shifts ranging from 1 to 38 cm- 1 in the peak position of the Si-O-Si stretching vibrational mode were achieved. This corresponds to changes of index of refraction ranging from 0.01 to 0.20. The experiments show also that the larger the pore size the wider is the range of parameters for producing effective waveguides.
A model for fracture in fibrous materials.
(1994) Bernardes, Américo Tristão; Moreira, José Guilherme Martins Alves
A fiber bundle model in (1 + 1)-dimensions for the breaking of fibrous composite matrix is introduced. The model consists of N parallel fibers fixed in two plates. When one of the plates is pulled in the direction parallel to the fibers, these can be broken with a probability that depends on their elastic energy. The mechanism of rupture is simulated by the breaking of neighbouring fibers that can generate random crack spreading up through the system. Due to the simplicity of the model we have virtually no computational limitation. The model is sensitive to external conditions as temperature and traction time-rate. The energy vs. temperature behaviour, the diagrams of stress vs. strain and the histograms of the frequency vs. size of cracks are obtained.
Gel-silica optics : theory and application.
(1994) Hench, L. L.; Araújo, Fernando Gabriel da Silva; West, J. K.; La Torre, J. P.
A new generation of silica optics has resulted from alkoxide-based sol-gel processing of silica including: net shape transmissive optical elements, surface diffractive optics, inorganic doped GRIN optics, organic impregnated optical composites such as dye lasers and scintillators, optics with internal diffraction gratings, laser densified microoptical lenses and arrays, and laser densified waveguides. Processing control of the ultrastructure of the monolith at the time of gelation and during aging is essential to producing the optical devices together with chemical and thermal stabilization of the surface of the pore network prior to densification or impregnation. The process control variables for gel-silica optics are summarized together with spectroscopic analysis and molecular orbital calculations that explain how and why the thermal-chemical processing controls work at a molecular level.
Structural evolution of a porous type-VI sol-gel silica glass
(1995) Araújo, Fernando Gabriel da Silva; LaTorre, G. P.; Hench, L. L.
The structural evolution of type-VI porous sol-gel-derived optical glasses is described. The average pore radius of the glasses studied is 30 ,A, with an exceptionally narrow pore size distribution. These glasses are particularly suited to laser surface densification. Their properties make them also attractive as matrices for impregnation of second phases and as a substrate material for the production of fully dense optical lens elements and micro-optical devices. The properties of the matrices are reported for the as-dried state to densities approaching 2.2 g/cm 3, with thermal treatments from room temperature to 1060°C. The parameters of sintering for the gel-glass transformation are measured, and the phenomena occurring for three temperature ranges are evaluated. An activation energy of 65 + 9 kcal/mol for sintering with a structure factor n = 1.0 + 0.3 was found for the initial stage sintering of the material.
The stable-chaotic transition on cellular automata used to model the immune repertoire.
(1995) Santos, Rita Maria Zorzenon dos; Bernardes, Américo Tristão
In this paper we study a simplified version of the cellular automata approximation introduced by De Boer, Segel and Perelson to model the immune repertoire. The automaton rule defines an activation window based on the idea of the proliferation function (biphasic dose-response function), which is used to describe the receptor crosslinking involved in the B cell activation. This proliferation function is very sensitive to the activation threshold and activation interval definitions. Here we investigate the influence of these parameters on the automaton rule proposed by Stauffer and Weisbuch. Using a fixed window they obtained the stable-"chaofic" transition only for d > 4. We find, contrary to their results, that this transition is always present for d > 2 until a certain critical value of the activation threshold is attained, above which this transition disappears and the system will always evolve towards a stable configuration. The shorter the activation interval the faster the system undergoes to the "chaotic" behaviour. Increasing the activation interval there is a certain critical size from which the system will always exhibit the same behaviour no matter the activation interval size. We also investigate the influence of the initial distribution on the results. Since we defined the relevant parameters of the model, we obtained the phase diagrams exhibiting the regions of stable and "chaotic" behavior. Such diagrams are not easily found in the literature.
On the number of degrees of freedom in Schwinger's quantum kinematics.
(1995) Lobo, Augusto César; Nemes, Maria Carolina
Some algebraic properties of Schwinger's quantum kinematical phase space theory are presented. These properties lead to a definition of the maximum number of degrees of freedom of an arbitrary finite dimensional quantum system which is different from the one originally proposed by Schwinger.
Anomalous phase separation dynamics in the presence of surfactants
(1996) Bernardes, Américo Tristão; Liverpool, T. B.; Stauffer, Dietrich
We study the phase separation of water and oil in a water-oil-surfactant microemulsion, from a totally disordered phase in both two and three dimensions using extensive Monte Carlo simulations of a spin model of microemulsions. The evolution of the characteristic length scale and the structure factor are measured and compared with recent experiments and theoretical work. At relatively high surfactant concentrations and off-critical quenches, we obtain results in agreement with earlier theoretical approaches. For critical quenches at very low surfactant concentrations, we observe the surprising result that it is easier to get to the scaling regime than in the pure binary case
Dynamics of interacting polymer chains and solvents.
(1996) Pandey, R. B.; Bernardes, Américo Tristão; Foo, G. M.; Stauffer, Dietrich
Conformation and dynamics of charged polymer chains in counter-ion solvent is studied by a Monte Carlo simulation in two and three dimensions. The nodes of the chains are positively charged while the solvent particles possess negative charge. The concentration of the polymer ( p ), and the empty sites ( pe ) is varied along with the temperature. Reptation, kink-jump, and a buckling governed by the interaction energy are considered in moving the chain segments. The variation of the root mean square (rms) displacement of the center of mass, and the relaxation of the radius of gyration of the chains with time are studied as a function of polymer concentration and temperature. The rms displacement of the polymer chains exhibits various power laws in different time regimes which depend on the concentration of the chains. Relaxation of the radius of gyration is enhanced on increasing the temperature; the relaxation time seems to decrease on reducing the chain length and increase on increasing the concentration. The motion of solvent particles show subdiffusive behavior which is sensitive to polymer concentration.
Mutation load and the extinction of large populations.
(1996) Bernardes, Américo Tristão
In the time evolution of finite populations, the accumulation of harmful mutations in further generations might lead to a temporal decay in the mean fitness of the whole population that, after sufficient time, would reduce population size and so lead to extinction. This joint action of mutation load and population reduction is called Mutational Meltdown and is usually considered only to occur in small asexual or very small sexual populations. However, the problem of extinction cannot be discussed in a proper way if one previously assumes the existence of an equilibrium state, as initially discussed in this paper. By performing simulations in a genetically inspired model for time-changing populations, we show that mutational meltdown also occurs in large asexual populations and that the mean time to extinction is a nonmonotonic function of the selection coefficient. The stochasticity of the extinction process is also discussed. The extinction of small sexual N ,-~ 700 populations is shown and our results confirm the assumption that the existence of recombination might be a powerful mechanism to avoid extinction.
Roughness exponents to calculate multi-affine fractal exponents
(1997) Silva, Alcides Volpato Carneiro de Castro e; Moreira, José Guilherme Martins Alves
We propose a new method to determine the multi-affine fractal exponents based on a generalization of the roughness concept. For synthetic multi-attine fractal profiles with exactly known exponents, we show that the results obtained in this method are more accurate than the results obtained with the height-height correlation function method. Also in the present method, scaling is observed using profiles with a number of points more than one order of magnitude smaller.
The reference state for finite coherent states.
(1997) Lobo, Augusto César; Nemes, Maria Carolina
We propose a reference state for finite-dimensional coherent states, which is easy to deal with in comparison to former suggestions which we briefly review. We also advance explicit calculations which shows that the phase of the overlap of finite coherent state has a structure analogous to the usual infinite-dimensional continuous coherent states.
Immune network at the edge of chaos.
(1997) Bernardes, Américo Tristão; Santos, Rita Maria Zorzenon dos
Some time ago Jerne proposed a new theory to explain the basis of the behaviour of the immune system. He suggested the existence of a functional connected network, based on pattern recognition of the idiotypes carried by the lymphocytes, which is responsible for the self regulation of the immune system. Only 04 19) of the lymphocytes available in the immune repertoire will participate in this functional network, while the rest of the lymphocytes will be free to respond to any foreign antigen. Each individual immune repertoire will be different depending on the lymphocytes that participate in the connected network. Using a very simple cellular automata model of the immune repertoire dynamics we show that, although the usual regimes "stable and chaotic# attained by this automata, are not interesting from the biological point of view, the transition region, at the edge of chaos, is very appropriate to describe such dynamics[ In this region we have obtained a functional connected network involving 09 19) of the lymphocytes available in the repertoire, as suggested by Jerne and others[ The model also reproduces the immune system signature, the ensemble of different lymphocytes that each individual expresses in his immune repertoire, which varies from one individual to another[ We show how the immune memory comes out as a consequence of the dynamics of the system[ From our results we confirm and present evidence that the chaotic regime corresponds to a sort of non- healthy state, as has been suggested previously.
Can males contribute to the genetic improvement of a species?
(1997) Bernardes, Américo Tristão
In the time evolution of finite populations, the accumulation of harmful mutations in further generations might have lead to a temporal decay in the mean fitness of the whole population. This, in turn, would reduce the population size and so lead to its extinction. The production of genetically diverse offspring, through recombination, is a powerful mechanism in order to avoid this catastrophic route. From a selfish point of view, meiotic parthenogenesis can ensure the maintenance of better genomes, while sexual reproduction presents the risk of genome dilution. In this paper, by using Monte Carlo simulations of age-structured populations, through the Penna model, I compare the evolution of populations with different repoductive regimes. It is shown that sexual reproduction with male competition can produce better results than meiotic parthenogenesis. This contradicts results recently published, but agrees with the strong evidence that nature chose sexual reproduction instead of partenogenesis Ibr most of the higher species.
Oxygen self-diffusion in a cordierite glass.
(1998) Sabioni, Antônio Claret Soares; Zanotto, Edgar Dutra; Millot, F.; Tuller, H. L.
Oxygen self-di€usion measurements determined below and above the glass transition temperature in a cordierite glass are reported. The di€usion experiments were performed by the solid±gas isotope exchange method using 18O as a tracer in H2±H2 18O and Ar/18O2 atmospheres. The 18O di€usion pro®les were established by secondary ion mass spectrometry (SIMS) after appropriate di€usion annealing. The oxygen di€usion coe cients changed at the glass transition temperature as expected. The atmosphere a€ected the oxygen di€usivity in agreement with other studies. The di€usion data were useful for comparisons with di€usion controlled process in the same glass. There is a di€erence between the activation enthalpies for crystal growth, viscous ¯ow, and oxygen self-di€usion. Hence, bond-breaking and molecular reorientation required for crystallization and the atomic transport mechanism involved in viscous ¯ow are not correlated in any obvious way to oxygen di€usion in cordierite glass. Ó 1998 Elsevier Science B.V. All rights reserved.
Critical behavior of a probabilistic cellular automaton describing a biological system.
(1998) Ortega, Neli Regina Siqueira; Pinheiro, Carlos Felipe Saraiva; Tomé, Tânia; Felício, José Roberto Drugowich de
We study nonequilibrium phase transitions occurring in aprobabilisticcellularautomaton which describes one part of the immune system. In this model, each site can be occupied by three type of cells and the immune response under parasitic infections is described in terms of two parameters p and r. The local rules governing the evolution of this automaton possess “up–down” symmetry similar to Ising models. Performing Monte Carlo simulations on square and cubic lattices we verify that the model displays continuous kinetic phase transitions with spontaneous symmetry breaking. We present detailed simulations and analysis of the criticalbehavior. Our results indicate that the model belongs to the Ising universality class, supporting the “up–down” conjecture.

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