Sílvia Soreto - Academia.edu (original) (raw)

Papers by Sílvia Soreto

Journal of Non-Crystalline Solids, 2012

Lithium ferrite (LiFe 5 O 8 ) is one of the most known cubic ferrites which belong to the group o... more Lithium ferrite (LiFe 5 O 8 ) is one of the most known cubic ferrites which belong to the group of soft ferrite materials with high Curie temperature, presenting a square hysteresis loop, and high magnetization. It has attracted special attention due to its numerous technological applications in microwave devices, magnetic recording, transformer cores, rod antennas and possible applications in biomedics. It is also a promising candidate for cathode materials in rechargeable lithium batteries as well as low cost substitutes to garnet materials in microwave frequency applications. In the present work, amorphous Li 2 O-Fe 2 O 3 powders were prepared via wet ball milling method. The asprepared powders were heat-treated at temperatures between 400 and 1400°C and their structure analyzed by X-ray diffraction and Raman spectroscopy. The morphology was observed by scanning electron microscopy. Electrical and dielectric properties, in the frequency range between 100 and 2 MHz and temperatures between 200 and 360 K, were performed, and the results related with the structure and morphological characteristics.

Polymer International, 2012

The relaxation properties of polymethacrylates of the n-alkyl series with n = l, 2 and 4 (poly(me... more The relaxation properties of polymethacrylates of the n-alkyl series with n = l, 2 and 4 (poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and poly(n-butyl methacrylate) (PnBMA)) have been measured and analyzed in order to relate their properties to the size of the lateral side chains. The n-alkyl series has been regarded as a model system and was used in this work to test a graphical data analysis method. Essentially, four relaxation processes were detected in the three polymers: the γ , β, α and αβ relaxations, in increasing order of temperature. It was found that the γ relaxation has a low activation energy, of around 36.3-38.5 kJ mol −1 , independent of the side chain, exhibiting low entropy of activation values when referring to the Eyring description of the activation parameters. The β relaxation was found to be similar in PMMA and PEMA, showing an activation energy of 88.8 kJ mol −1 , increasing to 112.8 kJ mol −1 in PnBMA. The activation entropy was also found to be low for this relaxation, although greater than that for the γ relaxation. In contrast, the α relaxation is quite different in these polymers. We observed a gradual shift in the glass transition temperature towards lower temperatures as the side chain increases in length. The manner in which the α transition makes its way into the dielectric spectra is more abrupt in PMMA than in PnBMA, denoting a higher fragility in the former polymer. Finally, there is a significant difference in the coalescence scenarios of the α and β relaxations for temperatures higher than the glass transition temperature, where they give rise to the so-called αβ relaxation.

Spectroscopy Letters, 2013

ABSTRACT Lithium ferrite has attracted special attention due to its numerous technological applic... more ABSTRACT Lithium ferrite has attracted special attention due to its numerous technological applications in microwave devices, magnetic recording, transformer cores, rod antennas, and biomedics. It is also a promising candidate for cathode materials in rechargeable lithium batteries, as well as a low-cost substitute to garnet materials in microwave frequency applications. In the present work, amorphous lithium ferrite powders were prepared via the wet ball milling method. The as-prepared powders were heat treated at temperatures between 400°C and 1200°C and their structure analyzed by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Impedance spectroscopy was used, in the frequency range between 100 Hz and 2 MHz and at temperatures between 200 K and 360 K, to characterize the ferrites, in order to optimize the synthesis parameters.

Materials Science and Engineering: B, 2014

Lithium ferrite (LiFe 5 O 8 ) is an attractive material for several potential technological appli... more Lithium ferrite (LiFe 5 O 8 ) is an attractive material for several potential technological applications. Critical to such attractiveness are its physical properties, such as high Curie temperature, square hysteresis loop and high magnetization. Knowing that the properties of these crystals depend on the preparation method and raw materials, in this work LiFe 5 O 8 crystallites were obtained by controlled heat-treatments, between 200 and 1400 • C, of homogeneous Li 2 O-Fe 2 O 3 powders prepared by wet ball-milling method and using lithium and iron nitrates as raw materials. LiFe 5 O 8 crystal phase was formed through heat-treatments at temperatures above 500 • C. At higher temperatures the formation of lithium ferrate and hematite is promoted, leading to a decrease in the magnetic moment. Heat-treated the sample at 1200 • C results in the highest levels of magnetic saturation, presenting a magnetic moment of 73 emu/g at 5 K and 66 emu/g at 300 K, respectively.

Journal of Applied Physics, 2014

Lithium ferrite (LiFe 5 O 8 ) is an attractive material for technological applications due to its... more Lithium ferrite (LiFe 5 O 8 ) is an attractive material for technological applications due to its physical properties, which are significantly dependent on the preparation method and raw materials. In this work, LiFe 5 O 8 crystallites were obtained by controlled heat-treatment process at 1100 C, of a homogeneous mixture of Li 2 O-Fe 2 O 3 powders, prepared by wet ball-milling and using lithium and iron nitrates as raw materials. The main goal was the preparation of a flexible and self-standing tick composite film by embedding lithium ferrite particles in a polymeric matrix, taking advantage of the good mechanical properties of the polymer and of the electrical and dielectric properties of the ferrite. The selected polymer matrix was styrene-b-isoprene-b-styrene copolymer. To prepare the composites, the lithium ferrite particles were chemically modified in order to functionalize their surface. To analyse the influence of the particles surface modification, different composites were made, with modified and unmodified particles. The structure of the obtained composites was studied by FTIR, XRD, TGA, and DSC techniques. The dielectric properties were analysed, in the frequency range between 10 Hz and 1 MHz and in function of temperature in the range between À73 C and 127 C. These properties were related with the structure and concentration of the particles in the matrix network. The composites with the modified particles present higher dielectric constant, maintaining values of loss tangent sufficiently low (<10 À2 ) that can be considered interesting for technological applications. V C 2014 AIP Publishing LLC.

Journal of Non-Crystalline Solids, 2012

Lithium ferrite (LiFe 5 O 8 ) is one of the most known cubic ferrites which belong to the group o... more Lithium ferrite (LiFe 5 O 8 ) is one of the most known cubic ferrites which belong to the group of soft ferrite materials with high Curie temperature, presenting a square hysteresis loop, and high magnetization. It has attracted special attention due to its numerous technological applications in microwave devices, magnetic recording, transformer cores, rod antennas and possible applications in biomedics. It is also a promising candidate for cathode materials in rechargeable lithium batteries as well as low cost substitutes to garnet materials in microwave frequency applications. In the present work, amorphous Li 2 O-Fe 2 O 3 powders were prepared via wet ball milling method. The asprepared powders were heat-treated at temperatures between 400 and 1400°C and their structure analyzed by X-ray diffraction and Raman spectroscopy. The morphology was observed by scanning electron microscopy. Electrical and dielectric properties, in the frequency range between 100 and 2 MHz and temperatures between 200 and 360 K, were performed, and the results related with the structure and morphological characteristics.

Polymer International, 2012

The relaxation properties of polymethacrylates of the n-alkyl series with n = l, 2 and 4 (poly(me... more The relaxation properties of polymethacrylates of the n-alkyl series with n = l, 2 and 4 (poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and poly(n-butyl methacrylate) (PnBMA)) have been measured and analyzed in order to relate their properties to the size of the lateral side chains. The n-alkyl series has been regarded as a model system and was used in this work to test a graphical data analysis method. Essentially, four relaxation processes were detected in the three polymers: the γ , β, α and αβ relaxations, in increasing order of temperature. It was found that the γ relaxation has a low activation energy, of around 36.3-38.5 kJ mol −1 , independent of the side chain, exhibiting low entropy of activation values when referring to the Eyring description of the activation parameters. The β relaxation was found to be similar in PMMA and PEMA, showing an activation energy of 88.8 kJ mol −1 , increasing to 112.8 kJ mol −1 in PnBMA. The activation entropy was also found to be low for this relaxation, although greater than that for the γ relaxation. In contrast, the α relaxation is quite different in these polymers. We observed a gradual shift in the glass transition temperature towards lower temperatures as the side chain increases in length. The manner in which the α transition makes its way into the dielectric spectra is more abrupt in PMMA than in PnBMA, denoting a higher fragility in the former polymer. Finally, there is a significant difference in the coalescence scenarios of the α and β relaxations for temperatures higher than the glass transition temperature, where they give rise to the so-called αβ relaxation.

Spectroscopy Letters, 2013

ABSTRACT Lithium ferrite has attracted special attention due to its numerous technological applic... more ABSTRACT Lithium ferrite has attracted special attention due to its numerous technological applications in microwave devices, magnetic recording, transformer cores, rod antennas, and biomedics. It is also a promising candidate for cathode materials in rechargeable lithium batteries, as well as a low-cost substitute to garnet materials in microwave frequency applications. In the present work, amorphous lithium ferrite powders were prepared via the wet ball milling method. The as-prepared powders were heat treated at temperatures between 400°C and 1200°C and their structure analyzed by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Impedance spectroscopy was used, in the frequency range between 100 Hz and 2 MHz and at temperatures between 200 K and 360 K, to characterize the ferrites, in order to optimize the synthesis parameters.

Materials Science and Engineering: B, 2014

Lithium ferrite (LiFe 5 O 8 ) is an attractive material for several potential technological appli... more Lithium ferrite (LiFe 5 O 8 ) is an attractive material for several potential technological applications. Critical to such attractiveness are its physical properties, such as high Curie temperature, square hysteresis loop and high magnetization. Knowing that the properties of these crystals depend on the preparation method and raw materials, in this work LiFe 5 O 8 crystallites were obtained by controlled heat-treatments, between 200 and 1400 • C, of homogeneous Li 2 O-Fe 2 O 3 powders prepared by wet ball-milling method and using lithium and iron nitrates as raw materials. LiFe 5 O 8 crystal phase was formed through heat-treatments at temperatures above 500 • C. At higher temperatures the formation of lithium ferrate and hematite is promoted, leading to a decrease in the magnetic moment. Heat-treated the sample at 1200 • C results in the highest levels of magnetic saturation, presenting a magnetic moment of 73 emu/g at 5 K and 66 emu/g at 300 K, respectively.

Journal of Applied Physics, 2014

Lithium ferrite (LiFe 5 O 8 ) is an attractive material for technological applications due to its... more Lithium ferrite (LiFe 5 O 8 ) is an attractive material for technological applications due to its physical properties, which are significantly dependent on the preparation method and raw materials. In this work, LiFe 5 O 8 crystallites were obtained by controlled heat-treatment process at 1100 C, of a homogeneous mixture of Li 2 O-Fe 2 O 3 powders, prepared by wet ball-milling and using lithium and iron nitrates as raw materials. The main goal was the preparation of a flexible and self-standing tick composite film by embedding lithium ferrite particles in a polymeric matrix, taking advantage of the good mechanical properties of the polymer and of the electrical and dielectric properties of the ferrite. The selected polymer matrix was styrene-b-isoprene-b-styrene copolymer. To prepare the composites, the lithium ferrite particles were chemically modified in order to functionalize their surface. To analyse the influence of the particles surface modification, different composites were made, with modified and unmodified particles. The structure of the obtained composites was studied by FTIR, XRD, TGA, and DSC techniques. The dielectric properties were analysed, in the frequency range between 10 Hz and 1 MHz and in function of temperature in the range between À73 C and 127 C. These properties were related with the structure and concentration of the particles in the matrix network. The composites with the modified particles present higher dielectric constant, maintaining values of loss tangent sufficiently low (<10 À2 ) that can be considered interesting for technological applications. V C 2014 AIP Publishing LLC.