NEW MATERIALS FOR ENERGY AND BIOMEDICAL APPLICATIONS

Alejandra Hortencia Miranda González2, Claudio Machado Junior2, Bruna Andressa Bregadiolli1, Natália Coelho de Farias2, Paulo Henrique Perlatti D’Alpino2, Carlos Frederico de Oliveira Graeff1

1 Departamento de Física, Faculdade de Ciěncias, UNESP – Universidade Estadual Paulista, Av. Eng. Luiz Edmundo Carrijo Coube, 14-01, CEP 17033-360, Bauru, SP, Brazil

2 Biomaterials Research Group, School of Dentistry, Universidade Anhanguera-UNIBAN, Rua Maria Candida, 1813, CEP 02071-013, São Paulo, SP, Brazil

ABSTRACT

We have conducted research works on the synthesis and characterization of ceramic materials for energy technologies and the characterization of bioceramics and composites for dental applications. La0.50Li0.50TiO3 nanoparticles were synthesized by the polymeric precursor method (PPM) for application as cathode in secondary lithium batteries. Electrochemical measurements of reduction/oxidation processes gave evidence of two kinetic processes. Also, nanocrystalline TiO2 films for application in hybrid solar cells were prepared. The particles were prepared from a “sol” solution by microwave assisted hydrothermal synthesis. The films were deposited by painting on ITO substrate and sintered at 450°C. XRD characterization indicated the crystallization of anatase phase. In addition, the preparation of ZrO2 bioceramic by PPM was investigated. Structural characterization by XRD showed that the powders are polycrystalline and free of secondary phases. Morphological results revealed a microstructure of spherical grains with homogeneous sizes of 70 nm. Finally, the ability to accurately predict changes in dental composite properties is of critical importance for the industry, researchers, and clinicians. An accelerated aging process has been used to characterize the mechanical, structural and rheological parameters of composites. Results have shown that this process influences most of the parameters when predicting 9 months of aging.

INTRODUCTION

The main research activities of the Departamento de Física (DF, Dep. of Physics) of the Faculdade de Ciěncias (FC, Science Faculty) of Universidade Estadual Paulista (UNESP) campus of Bauru is on Materials Science and Technology. Of its 24 lectures 9 belongs to the Graduate College in Materials Science and Technology - POSMAT. POSMAT is a graduate college in network comprising 7 different units in 6 different campus of UNESP. The activities of POSMAT are focused on interdisciplinary and multidisciplinary research, with the mission of development of basic and applied research, promoting a systematic transfer of this knowledge, for technological application and educational purposes. The Laboratory of New Materials and Devices (LNMD), one of the research groups of DF-FC, does research on ceramic materials as well as in organic semiconductors. LNMD provides the opportunity for scientists of different areas such as theoreticians and experimentalists, physicists, chemists and dentists to discuss and work together, thereby creating new perspectives in technological research.

Research at the LNMD regarding ceramic materials is focused in the synthesis and characterization of new materials for energy technologies and bioceramic application, as well as the characterization of composites for dental applications.

Considering energy technologies, it is unquestionable that global warming, the exhaustion of fossil fuels, and the need to prevent air pollution demand the urgent development of environmentally friendly energy sources. Among several alternatives, secondary lithium batteries are promising candidates.1 Of special interest are lithium lanthanum titanates (LLTO), with the general formula La2/3-xLi3xTiO3, because these compounds are among the best conductors containing Li+ ion known to date, and therefore are promising candidates for use as solid electrolytes in lithium secondary batteries, especially as cathodes.2 Recently, there has been considerable growing interest in the synthesis of materials for active cathodes from chemical routes based on aqueous solutions.3 Of special interest for the synthesis of La0.50Li0.50TiO3 for use in cathodes is the polymeric precursor method, also known as the Pechini Method.4 In this context, the aim of the present study was to prepare La0.50Li0.50TiO3 nanoparticles by means of the PPM. As characterization techniques, X-ray powder diffraction (XRD), scanning electron microscopy-field emission gun (FEG-SEM), and chronopotentiometry were used.

As sources of energy, third generation solar cells fabricated using hybrid and organic materials are a promising alternative to the expensive commercial silicon solar cells. Known as “Grätzel” solar cells5, the dye sensitized solar cells (DSSC) have received special attention due to their easy of processing, low cost and good performance.6 This device works when a photon absorbed by a dye molecule gives rise to electron injection into the conduction band of the semiconductor, in most cases TiO2, to generate the current. To complete the circuit, the dye must be regenerated by an electron transfer from the electrolyte, which is then reduced at the counter electrode. For a better performance, the TiO2 has to have large superficial area, porosity and adherence to the conductor substrate.5,7 Of special interest for the synthesis of TiO2 in DSSC is the microwave assisted hydrothcrmal technique.8 In this context, the aim of the present study was to prepare TiO2 nanoparticles by means of this method. X-ray powder diffraction (XRD), scanning electron microscopy-field emission gun (FEG-SEM), and transmission electron microscopy (TEM) were used to characterize the obtained material.

The development of zirconia (ZrO2) nanoparticles has attracted much attention due to their multifunctional characteristics.9 Considering the application as a biomaterial, the development of advanced dental material technologies has recently led to the use of zirconiabased ceramics in dentistry. The remarkable mechanical properties of zirconia are mainly due to the tetragonal to monoclinic (t → m) phase transformation. The t → m transformation, which can be induced by external stresses, such as grinding, cooling and impact, results in a 4% increase of volume that causes compressive stresses. These stresses may develop on the surface or in the vicinity of a tip crack.10 The presence of tetragonal phase is an essential condition for zirconia toughening besides hindering or interrupting crack propagation.11 With the advent of nanotechnology, several techniques have been employed to obtain ceramic powders with nanometric dimensions from chemical processes. Chemical synthesis allows the manipulation of matter at the molecular level, enabling good chemical homogeneity, and allows for the control of particle size and shape.12 In this context, the aim of the present study was to prepare ZrO2 nanopowders directly through the polymeric precursor method and investigate the influence of heat treatment on the structure of powders.

Another research field developed at LNMD comprises the chemical, structural, and mechanical characterization of dental composites subjected to accelerated aging processes. Dental composites are subjected to a variety of chemical challenges in the oral cavity. Composites are exposed to water and chemicals provided by acidic food and beverages that modify the pH of the saliva. These events create a variety of chemical and physical processes that produce deleterious effects on the structure and function of dental resin composites. In an aqueous environment, composites absorb water that not only elutes unreacted monomers, but also filler particles. Clinically, the release of composite components influences the initial dimensional changes of the composite as well as the longevity of resin-based composite restorations. The sorption of water may lead to the dissolution of the polymer matrix and/or entrapment of water molecules in polymer voids, by means of chemical bonds scission in the resin or softening through the plasticizing action of water.13 The great majority of dental composites are methacrylate-based materials that contain pendant hydroxyl groups in their molecular backbone. Because of these polar groups, polymers made with these monomers tend to be somewhat hydrophilic and susceptible to water sorption.

Recently new composite formulations were launched and most of them are methacrylate-based. The introduction of new or modified composites to the market requires the characterization of these materials to assure that their use can be made with safety and efficacy. Composites are made not only of monomers, but also by filler particles, plasticizers, colorants, photo-initiators, and stabilizers. The functional properties of a dental composite depend on the properties of its components. The different compositions associated with the different clinical uses (handling of the materials, application technique, polymerization technique, finishing and polishing, among others) determines the degradation mechanism of the dental resin composite.14 In order to obtain experimental data on performance and shelf-life of these composites, accelerated-aging tests are used. Most of these tests are made using higher temperatures based on the assumption that reaction rates are increased exponentialy with T.15 Another relevant issue is that the properties of any new product may vary as a function of the manufacturing process, transport, storage conditions, among others, until its use in the dental clinic. In other words, how degraded are these new products, when they reach the...