M. E. Mendoza

Synthesis and characterization of micrometric ceramic powders for electro-rheological fluids

Abstract Micrometric lamellar ceramic powders of the displacive ferroelectric oxide Bi 4 Ti 3 O 12 were synthesized by co-precipitation of bismuth nitrate and ammonia titanyl solutions followed by a heat treatment. It was found that a complete thermal decomposition is reached at 1000 °C. Structural and thermal evolution of these ceramic powders were studied by X-ray diffraction, thermogravimetry and differential thermal analysis. The homogeneity in size and morphology of these ferroelectric particles are appropriate to prepare electro-rheological fluids. One of these fluids was prepared by dispersing the powders in silicone oil; the complex cluster structure formed by the particles, under an applied AC electric field, was observed.

Modeling the dielectric response of lanthanum modified lead zirconate titanate ferroelectric ceramics-an approach to the phase transitions in relaxor ferroelectrics

The relaxor behavior of lanthanum modified lead zirconate titanate (PLZT) ferroelectric ceramics, for a La/Zr/Ti ratio of x/60/40, has been analyzed. An approximation to the dynamical behavior of the polar nano-regions (PNRs) has been discussed, taking into account a relaxation model, which considers a distribution function for the relaxation times. A good agreement between theoretical and experimental results was obtained. The behaviors of the mean relaxation time and its standard deviation with temperature were discussed, considering the correlation between the polar nano-regions and the freezing temperature. The temperature dependence of the polarization showed an anomalous behavior around a temperature, which was associated with the freezing temperature, according to the proposed model in the present work.

Sound propagation in magneto-rheological suspensions

The propagation of elastic perturbations in magneto-rheological suspensions is studied theoretically and experimentally. Under the application of a magnetic field, these systems acquire a fibrillar fractal structure formed by clusters. In systems in that condition, two low-frequency sound propagation modes have been observed. In both of them, the speed of sound depends on the intensity of the applied field. We discuss the statistical fractal properties of the cluster structure and, on this basis, we calculate the speed of sound for both of the low-frequency modes. This theoretical approach provides a good quantitative agreement with the experimental results.

Fractal patterns and aggregation processes in rheological dispersions

Processes of pattern formation in rheological dispersions in the presence of an external field are studied. We found that the structure formed acquires multifractal characteristics revealed by three mass scaling ranges, which can be associated with different aggregation stages in the pattern formation process. The evaluation of the radial mass distribution allows us to test the self-affinity properties. We found that this multifractal feature is a common characteristic for rheological dispersions.

Thermal Properties of an Electro-Rheological Fluid Based on Bi 4 Ti 3 O 12 Ceramic Powders

Thermal diffusivity of a ferroelectric based electro-rheological fluid is studied. The rheological fluid is prepared by dispersing powders of Bi 4 Ti 3 O 12 in silicon oil. The thermal diffusivity of the sample, under different intensities of applied electric field, was measured by means of an open cell photo-acoustic technique. We have found that the main contribution to the measured photo-acoustic signal, comes from the thermo-elastic bending effect. It was clearly observed that thermal diffusivity increases with the intensity of the applied electric field, this fact is explained in terms of the formation of clusters of Bi 4 Ti 3 O 12 particles.

Dynamic elastic properties of magneto-rheological slurries

We study the propagation of elastic perturbations in magneto-rheological slurries of iron particles dispersed in glycerine. The complexity of these systems is revealed in the fibrillar structure acquired under the application of a magnetic field. Recently, it has been reported the observation of two different low frequency modes of propagation. One of these modes has been associated to the propagation of the perturbation through the fluid medium. The other one has been qualitatively explained as the propagation of the elastic perturbation through the suspended particles. This second mode appears when a magnetic field is applied to the slurry. The propagation speed for both modes depends on the field intensity and on the properties of the magnetic particles. Theoretically, we analyze these modes and calculate the sound velocity. We obtain a quantitative good agreement with the experimental results.

DOMAIN WALL DYNAMICS, MEMORY, AND PHASE TRANSITIONS IN COBALT-BROMINE AND COBALT-CHLORINE BORACITES

ABSTRACT To investigate the paraelectric-ferroelectric phase transitions in Co3B7O13Cl and Co3B7O13Br boracites, correlations and scaling relations were measured from the domain wall structure at different temperatures using a multifractal analysis. We obtained high-resolution digital photographs and characterized the evolution of the complexity of the domain wall configuration by evaluating its fractal dimension. The change in the energetic state, the specific heat and the persistence of the domain structure was evaluated by measuring the changes of the spatial distribution of walls. We found two kinds of memory in the domain structure of the cobalt bromine boracite.

Domain Structure in Relaxor PLZT Ceramics

This paper presents the study of ceramic Pb1-3x/2 Lax (Zr1-y Tiy)O3, x/80/20 and x/70/ 30 solid solutions, where x = 6, 8 and 10%, which show a relaxor behavior with diffuse phase transition. It was possible to observe clusters of nanoregions characteristic of relaxors by means of polarized light microscopy using a lambda plate, which improves the birefringence of phases present in ceramic solid solutions. The structure pattern of clusters clearly shows two interference colors, which could be associated to tetragonal and rhombohedral distortions of perovskite unit cell, typical for compositions in the boundary region.

Reduction of the Curie temperature in the multiferroic Bi5Fe1+xTi3?xO15 solid solution

In this work, the phase diagram of the system Bi4Ti3O12-BiFeO3 in the region of the solid solution Bi5Fe1+xTi3?xO15 was refined. The limit of solubility was determined to be at x?=?0.1. The Curie temperature (TC) of the ferroelectric phase transition was determined by dielectric permittivity measurements at 100 kHz for the phase Bi5FeTi3O15 as well as for the solid solution. A decrease in TC from 750 ?C to 742 ?C (solid solution at x?=?0.1) was found. These results can be explained in terms of the perturbation of the oxygen octahedral perovskite layers resulting from the substitution of Ti4+ by Fe3+ ions.

Metamagnetism and weak ferromagnetism in nickel (II) oxalate crystals

Microcrystals of orthorhombic nickel (II) oxalate dihydrate were synthesized through a precipitation reaction of aqueous solutions of nickel chloride and oxalic acid. Magnetic susceptibility exhibits a sharp peak at 3.3 K and a broad rounded maximum near 43 K. We associated the lower maximum with a metamagnetic transition that occurs when the magnetic field is about ≥3.5 T. The maximum at 43 K is typical of 1D antiferromagnets, whereas weak ferromagnetism behavior was observed in the range of 3.3–43 K.