J. L. Carrillo

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.

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.

Reconstruction processes in electro and magneto rheological dispersions

The evolution of the structure of electro rheological as well as magneto rheological fluids in the presence of perturbation fields is studied. We have previously shown that the fibrous structure acquired by these dispersions in the presence of a static, electric or magnetic, field has multifractal characteristics. If in addition to the static field a perpendicular pulsed field is applied, under certain conditions it is possible to rearrange the structure into a crystalline-like one. Based on the measured mass fractal dimension and the radial correlation, we discuss these processes and other structural characteristics of the system.

Relaxation of ferroelectric states in 2D distributions of quantum dots: EELS simulation

The relaxation time of collective electronic states in a 2D distribution of quantum dots is investigated theoretically by simulating EELS experiments. From the numerical calculation of the probability of energy loss of an electron beam, traveling parallel to the distribution, it is possible to estimate the damping time of ferroelectric-like states. We generate this collective response of the distribution by introducing a mean field interaction among the quantum dots, and then, the model is extended incorporating effects of long-range correlations through a Bragg-Williams approximation. The behavior of the dielectric function, the energy loss function, and the relaxation time of ferroelectric-like states is then investigated as a function of the temperature of the distribution and the damping constant of the electronic states in the single quantum dots. The robustness of the trends and tendencies of our results indicate that this scheme of analysis can guide experimentalists to develop tailored quantum dots distributions for specific applications.

Dipolar Transformations of 2D Distributions of Quantum Dots

We study the paraelectric and ferroelectric properties of two-dimensional distributions of quantum dots of three different geometries of the confinement potential: spherical, cylindrical, and conical. We numerically obtain the quantum states, the average dipolar moment, and the susceptibility. A mean field approximation for interactive quantum dot arrays allows the observation of transformations in the electric susceptibility. These transformations resemble the paraelectric-ferroelectric transition in molecular ferroelectric materials; however, in the quantum dots arrays, these transformations are not sharply defined.

Kinetic approach for the calculation of the thermo-power coefficient in semiconductor heterostructures

We solve the Boltzmann transport equation in the two relaxation times approximation to analyze the kinetics of spatially inhomogeneous electron gas. The explicit form of the distribution function, obtained by means of the maximum entropy principle, together with moments method approach, are used to calculate the thermo-power coefficient of a semiconductor sample in the presence of a gradient of temperature. Since the transport coefficients can be written in a general form in terms of Fermi–Dirac integrals, we show that, in the semi-ballistic approach for mesoscopic semiconductor systems, the thermo-transport coefficients can be directly calculated by this procedure.