Damián Gargicevich

Internal stresses in the electrostriction phenomenon viewed through dynamic mechanical analysis studies conducted under electric field

A model for studying the mechanical internal stresses into dielectric materials is developed in the present work. The model takes into account the formalism of inclusions in continuous media. The dielectric material is assumed to be partitioned in different cubes which form a sizeable bulk material in such a way that a given cube can be of the dipolar phase and its neighbor can be of the same phase or the matrix. The behavior of the internal stresses promoted by the electrostriction phenomenon can be monitored by studying the behavior of both the misfit coefficient related to the strain misfit and the transfer of elastic energy process. The equations obtained through the here presented model are calculable using magnitudes from mechanical tests, in particular dynamic mechanical analysis, which is very sensitive to changes in the microstructure. Dynamic mechanical analysis as a function of the electric field is reported, perhaps for the first time in literature, for the study of dielectric materials, giving rise to a useful tool for studying the behavior of internal stresses in dielectric materials.

Study of dielectric strength in EPDM by nondestructive dynamic mechanical analysis in high electrical field

In the present work the value of the degree of the area swept by the polymer chain due to an electrical force for a given mesostructure was related to the corresponding value of the dielectric strength. This value was deduced from the electric inclusion formalism applied to dynamic mechanical analysis (DMA) studies conducted under high electric field, which were performed in commercial ethylene-propylene-diene M-class rubber (EPDM); used for the housing of polymeric electrical insulators. EPDM samples with different arrangements of the polymer chains and crystalline degree, promoted by controlled neutron irradiation were studied. Several characterization techniques, as infrared absorption spectroscopy (IR), differential scanning calorimetry (DSC), positron annihilation lifetime spectroscopy (PALS) and dielectric strength (DS) were also used. The relationship between the DS and the degree of movement of polymer chains promoted by electrical forces coming from the electric field applied in a non destructive test as the DMA was successfully established. In fact, a larger empty space in the sample leads to larger areas swept by the polymer chains during bending under the application of the field strength in the dynamic mechanical analysis tests. Therefore, an increase in the capability of movement of charges occurs, corresponding to smaller dielectric strength values. Crystallinity improves the dielectric strength due to the increase in the internal stresses which decreases the capability of movement of the polymer chains and electric carriers by electric forces.

Magnetic memory effect in magnetite charged polypropylene composite

Abstract The behaviour of damping and dynamic shear modulus in polypropylene charged with either different volume fraction or size of magnetite (Fe3O4) particles, as a function of the applied magnetic field at 318, 353 and 403 K; has been studied. An increase of the alternating magnetic field oscillating with 50 Hz, leads to an increase of the damping. In addition, during the subsequently decreasing alternating magnetic field, the damping decreases, but a hysteretic behaviour appeared. The behaviour of the damping and the elastic modulus under the application of an alternating magnetic field was explained by the development of a magnetic fatigue damage occurring around the particle interface due to oscillation of magnetite particles. In contrast, during the increase of a direct magnetic field, the damping decreases and the elastic modulus increases. Measurements performed at 353 and 403 K allowed observing the interaction process among the particles of magnetite in the polymer matrix. After the decrease in the direct magnetic field, from the maximum reached value, damping and modulus remain smaller and higher, respectively; giving rise to a memory effect. In addition, a mesoscopic description of magnetite filled polymer composite materials has been performed in the continuous media by considering the interaction between magnetic and mechanical forces. Theoretical predictions of here developed model were qualitatively applied with good success for explaining the memory effect in magnetite filled polypropylene under the application of a direct magnetic field.

Electro-rheological description of solids dielectrics exhibiting electrostriction

An electro-rheological model based in Voigt units which takes into consideration the variation in volume promoted by electrostriction is developed. The model was based on a mean field approximation as an averaging of the mechanical and electrical properties. The electro-rheological coupling which describes the effects of the electrical excitation on the mechanical response and the effects of the mechanical excitation on the electrical response of the dielectric is studied. In the case of an alternating electrical excitation the model reveals the appearance of harmonics in the current through the dielectric promoted by the electrostriction phenomenon. In contrast, for the case of an oscillating mechanical excitation, a current which overtakes the driving mechanical oscillation was resolved to appear. The correlation of the new model with experimental results, obtained from dynamic mechanical analysis tests conducted under high electric field, in polyamide, was found out.

Damping Micromechanisms for Bones above Room Temperature

The wide damping maximum which is reported to appear in bones, involving both cortical and cancellous parts, between around 280 K and 420 K; has been determined to be a composition of different processes taking place at different temperatures in cancellous and cortical parts. In fact, in the present work the mechanical response of cow ribs bones has been analysed by coupling mechanical spectroscopy, differential scanning calorimetry, thermogravimetry and scanning electron microscopy studies. Cancellous part develops two damping maxima at around 320 K and 350 K. Cortical part exhibits a wide maximum in damping between around 310 K and 410 K and another damping relaxation between 390 K and 410 K. The physical-chemical driving force giving rise to the above relaxation processes are discussed.

Structure determination of Fe-Al-Ge alloys

We studied the crystalline structure of Fe - 8at.%Al - 4at.%Ge alloy between 300 and 1300 K and its relation to the mechanical response by means of neutron diffraction and mechanical spectroscopy. At room temperature we observe a Fe3Al-type ordered structure with a deficiency of Al in the 8c sites. The Ge atoms are distributed in the 4a and Al atoms in 8c sites. At high temperature we observe an order-disorder transformation when the crystal structure becomes Fe-α type. This loss of order gives rise to the hysteresis behavior of damping between the heating and cooling runs.

Magnetic Field Dependent Damping of Magnetic Particle Filled Polypropylene

The behavior of internal friction Q-1 and dynamic shear modulus has been studied in polypropylene charged with either different volume fraction or size of magnetite (Fe3O4) particles, as a function of the applied magnetic field at 318 K. An increase of the alternating (AC) magnetic field oscillating with 50 Hz, leads to an increase of the internal friction. In addition, during the subsequently decreasing alternating magnetic field, the internal friction decreases, but a hysteretic behavior appeared. In fact, the internal friction of the decreasing part of magnetic field amplitude is found to be smaller than during the previously increasing amplitude part of the treatment with the alternating magnetic field. Subsequent magnetic treatment cycles, lead to successively decreasing internal friction. In contrast, during the increase of a direct (DC) magnetic field, the internal friction decreases and the elastic modulus increases. The behavior of the internal friction and the elastic modulus during the application of an oscillating magnetic field (AC) is discussed on the basis of the development of both, a new zone with different rheological characteristics than the matrix but of the same material (self-inclusion), and/or a deteriorated or damaged zone (chain’s cuts) of the polymer matrix in the neighborhood of the magnetite inclusion. These effects are promoted by the movement or small relative rotation of the magnetite particles related to the surrounding matrix controlled by the oscillating field. The behavior of the internal friction and elastic modulus during the application of a direct (DC) magnetic field is discussed on the basis of the increase of the internal stresses into the polymer matrix due to the promotion of the magnetomechanical stresses.