Influence of pattern gradation on the design of piezocomposite energy harvesting devices using topology optimization

Piezoelectric materials can be used to convert oscillatory mechanical energy into electrical energy. Energy harvesting devices are designed to capture the ambient energy surrounding the electronics and convert it into usable electrical energy. The design of energy harvesting devices is not obvious, requiring optimization procedures. This paper investigates the influence of pattern gradation using topology optimization on the design of piezocomposite energy harvesting devices based on bending behavior. The objective function consists of maximizing the electric power generated in a load resistor. A projection scheme is employed to compute the element densities from design variables and control the length scale of the material density. Examples of two-dimensional piezocomposite energy harvesting devices are presented and discussed using the proposed method. The numerical results illustrate that pattern gradation constraints help to increase the electric power generated in a load resistor and guides the problem toward a more stable solution. (C) 2012 Elsevier Ltd. All rights reserved.

LYOCELL AND COTTON FIBERS AS REINFORCEMENTS FOR A THERMOSET POLYMER

Cellulose fibers obtained from the textile industry (lyocell) were investigated as a potential reinforcement for thermoset phenolic matrices, to improve their mechanical properties. Textile cotton fibers were also considered. The fibers were characterized in terms of their chemical composition and analyzed using TGA, SEM, and X-ray. The thermoset (non-reinforced) and composites (phenolic matrices reinforced with randomly dispersed fibers) were characterized using TG, DSC, SEM, DMTA, the Izod impact strength test, and water absorption capacity analysis. The composites that were reinforced with lyocell fibers exhibited impact strengths of nearly 240 Jm(-1), whereas those reinforced with cotton fibers exhibited impact strengths of up to 773 Jm(-1). In addition to the aspect ratio, the higher crystallinity of cotton fibers compared to lyocell likely plays a role in the impact strength of the composite reinforced by the fibers. The SEM images showed that the porosity of the textile fibers allowed good bulk diffusion of the phenolic resin, which, in turn, led to both good adhesion of fiber to matrix and fewer microvoids at the interface.

Effective Properties Evaluation for Smart Composite Materials

The purpose of this article is to present a method which consists in the development of unit cell numerical models for smart composite materials with piezoelectric fibers made of PZT embedded in a non-piezoelectric matrix (epoxy resin). This method evaluates a globally homogeneous medium equivalent to the original composite, using a representative volume element (RVE). The suitable boundary conditions allow the simulation of all modes of the overall deformation arising from any arbitrary combination of mechanical and electrical loading. In the first instance, the unit cell is applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. The numerical results are compared to other methods reported in the literature and also to results previously published, in order to evaluate the method proposal. In the second step, the method is applied to calculate the equivalent properties for smart composite materials with square cross section fibers. Results of comparison between different combinations of circular and square fiber geometries, observing the influence of the boundary conditions and arrangements are presented.

Porcelain monolayers and porcelain/alumina bilayers reinforced by Al2O3/GdAlO3 fibers

Aim. This work tested the effect of the addition of Al2O3/GdAlO3 longitudinal fibers in different contents to veneering porcelain of two dental all ceramic systems. Methods: Fibers (0.5 mm diameter) obtained by the Laser Heated Pedestal Growth (LHPG) method were added to bar-shaped specimens made by veneer porcelain (monolayers) or both the veneer and the core ceramic (bilayers) of two all-ceramic systems: In-Ceram Alumina - glass infiltrated alumina composite (GIA) and In-Ceram 2000 AL Cubes - alumina polycrystal (AP) (VITA Zahnfabrik). The longitudinal fibers were added to veneering porcelain (VM7) in two different proportions: 10 or 17 vol%. The bars were divided into nine experimental conditions (n = 10) according to material used: VM7 porcelain monolayers, VM7/GIA, VM7/AP; and according to the amount of fibers within the porcelain layer: no fibers, 10 vol% or 17 vol%. After grinding and polishing the specimens were submitted to a three point bending test (crosshead speed = 0.5 mm/min) with porcelain positioned at tensile side. Data were analyzed by means of one-way ANOVA and a Tukey's test (alpha = 5%). Scanning electronic microscopy (SEM) was conducted for fractographic analysis. Results. Regarding the groups without fiber addition, VM7/AP showed the highest flexural strength (MPa), followed by VM7/GIA and VM7 monolayers. The addition of fibers led to a numerical increase in flexural strength for all groups. For VM7/GIA bilayers the addition of 17 vol% of fibers resulted in a significant 48% increase in the flexural strength compared to the control group. Fractographic analysis revealed that the crack initiation site was in porcelain at the tensile surface. Cracks also propagated between fibers before heading for the alumina core. Conclusions. The addition of 17 vol% of Al2O3/GdAlO3 longitudinal fibers to porcelain/glass infiltrated alumina bilayers significantly improved its flexural strength. 10 vol% or 17 vol% of fibers inclusion increased the flexural strength for all groups. (C) 2011 Elsevier Ltd. All rights reserved.

Tannin-phenolic resins: Synthesis, characterization, and application as matrix in biobased composites reinforced with sisal fibers

A tannin-phenolic resin (40 wt% of tannin, characterized by H-1 nuclear magnetic resonance (NMR) and C-13 NMR, Fourier transform infrared, thermogravimetry, differential scanning calorimetry) was used to prepare composites reinforced with sisal fibers (30-70 wt%). Inverse gas chromatography results showed that the sisal fibers and the tannin-phenolic thermoset have close values of the dispersive component and also have predominance of acid sites (acid character) at the surface, confirming the favoring of interaction between the sisal fibers and the tannin-phenolic matrix at the interface. The Izod impact strength increased up to 50 wt% of sisal fibers. This composite also showed high storage modulus, and the lower loss modulus, confirming its good fiber/matrix interface, also observed by SEM images. A composite with good properties was prepared from high content of raw material obtained from renewable sources (40 wt% of tannin substituted the phenol in the preparation of the matrix and 50 wt% of matrix was replaced by sisal fibers). (C) 2012 Elsevier Ltd. All rights reserved.

Microtensile bond strength of composite resin to glass-infiltrated alumina composite conditioned with Er,Cr:YSGG laser

Tribochemical silica-coating is the recommended conditioning method for improving glass-infiltrated alumina composite adhesion to resin cement. High-intensity lasers have been considered as an alternative for this purpose. This study evaluated the morphological effects of Er,Cr:YSGG laser irradiation on aluminous ceramic, and verified the microtensile bond strength of composite resin to ceramic following silica coating or laser irradiation. In-Ceram Alumina ceramic blocks were polished, submitted to airborne particle abrasion (110 mu m Al(2)O(3)), and conditioned with: (CG) tribochemical silica coating (110 mu m SiO(2)) + silanization (control group); (L1-L10) Er,Cr:YSGG laser (2.78 mu m, 20 Hz, 0.5 to 5.0 W) + silanization. Composite resin blocks were cemented to the ceramic blocks with resin cement. These sets were stored in 37A degrees C distilled water (24 h), embedded in acrylic resin, and sectioned to produce bar specimens that were submitted to microtensile testing. Bond strength values (MPa) were statistically analyzed (alpha a parts per thousand currency sign0.05), and failure modes were determined. Additional ceramic blocks were conditioned for qualitative analysis of the topography under SEM. There were no significant differences among silicatization and laser treatments (p > 0.05). Microtensile bond strength ranged from 19.2 to 27.9 MPa, and coefficients of variation ranged from 30 to 55%. Mixed failure of adhesive interface was predominant in all groups (75-96%). No chromatic alteration, cracks or melting were observed after laser irradiation with all parameters tested. Surface conditioning of glass-infiltrated alumina composite with Er,Cr:YSGG laser should be considered an innovative alternative for promoting adhesion of ceramics to resin cement, since it resulted in similar bond strength values compared to the tribochemical treatment.

Shear bond strength of repairs in porcelain conditioned with laser

The aim of this study was to evaluate the shear bond strength of repairs in porcelain conditioned with laser. Sixty porcelain discs were made and six groups were formed (n = 10): G1: conditioning with laser with potency 760 mW; G2: conditioning with laser with potency 760 mW and application of 37% phosphoric acid for 15 s; G3: conditioning with laser with potency 900 mW; G4: conditioning with laser with potency 900 mW and application of 37% phosphoric acid for 15 s; G5: application of 37% phosphoric acid for 15 s (group control) and G6: application of 10% hydrofluoric acid for 2 min. The composite resin was insert of incremental layers at the porcelain surface aided with a metal matrix, and photoactivation for 20 s each increment. The specimens were submitted to a thermal cycling by 1000 cycles of 30 s in each bath with temperature between 5 and 55 degrees C. After the thermal cycling, specimens were submitted to the shear bond strength. The results were evaluated statistically through analysis of variance and Tukey's tests with 5% significance. The averages and standard deviation founded were: G1, 11.25 (+/- 3.10); G2, 12.32 (+/- 2.65); G3, 14.02 (+/- 2.38); G4, 13.44 (+/- 2,07); G5, 9.91 (-/+ 2,18); G6, 12.74 (+/- 2.67). The results showed that the femtosecond laser produced a shear bond strength of repairs in porcelain equal to the hydrofluoric acid and significantly superior to the use of phosphoric acid. Microsc. Res. Tech., 2012. (C) 2012 Wiley Periodicals, Inc.

Effective properties evaluation for smart composite materials

The purpose of this article is to present a method which consists in the development of unit cell numerical models for smart composite materials with piezoelectric fibers made of PZT embedded in a non-piezoelectric matrix (epoxy resin). This method evaluates a globally homogeneous medium equivalent to the original composite, using a representative volume element (RVE). The suitable boundary conditions allow the simulation of all modes of the overall deformation arising from any arbitrary combination of mechanical and electrical loading. In the first instance, the unit cell is applied to predict the effective material coefficients of the transversely isotropic piezoelectric composite with circular cross section fibers. The numerical results are compared to other methods reported in the literature and also to results previously published, in order to evaluate the method proposal. In the second step, the method is applied to calculate the equivalent properties for smart composite materials with square cross section fibers. Results of comparison between different combinations of circular and square fiber geometries, observing the influence of the boundary conditions and arrangements are presented.

Surface modification by metal ion implantation forming metallic nanoparticles in insulating matrix.

There is special interest in the incorporation of metallic nanoparticles in a surrounding dielectric matrix for obtaining composites with desirable characteristics such as for surface plasmon resonance, which can be used in photonics and sensing, and controlled surface electrical conductivity. We investigated nanocomposites produced through metallic ion implantation in insulating substrate, where the implanted metal self-assembles into nanoparticles. During the implantation, the excess of metal atom concentration above the solubility limit leads to nucleation and growth of metal nanoparticles, driven by the temperature and temperature gradients within the implanted sample including the beam-induced thermal characteristics. The nanoparticles nucleate near the maximum of the implantation depth profile (projected range), that can be estimated by computer simulation using the TRIDYN. This is a Monte Carlo simulation program based on the TRIM (Transport and Range of Ions in Matter) code that takes into account compositional changes in the substrate due to two factors: previously implanted dopant atoms, and sputtering of the substrate surface. Our study suggests that the nanoparticles form a bidimentional array buried few nanometers below the substrate surface. More specifically we have studied Au/PMMA (polymethylmethacrylate), Pt/PMMA, Ti/alumina and Au/alumina systems. Transmission electron microscopy of the implanted samples showed the metallic nanoparticles formed in the insulating matrix. The nanocomposites were characterized by measuring the resistivity of the composite layer as function of the dose implanted. These experimental results were compared with a model based on percolation theory, in which electron transport through the composite is explained by conduction through a random resistor network formed by the metallic nanoparticles. Excellent agreement was found between the experimental results and the predictions of the theory. It was possible to conclude, in all cases, that the conductivity process is due only to percolation (when the conducting elements are in geometric contact) and that the contribution from tunneling conduction is negligible.