RESUMO
This article describes the characterization and analysis of the effects an additional polymer layer has on a high-overtone bulk acoustic wave resonator based on Ba0.5Sr0.5TiO3 (BSTO) thin film by studying its spectral information. From both the simulation (numerical model) and experimental results of the resonator with and without coating, significant difference of both cases is evident in the spacing of the parallel resonance frequencies (SPRFs), effective coupling coefficient ( [Formula: see text]), and quality factor distribution of the resonator. The acoustic velocity of the coated material (SU-8) is calculated from the new periodicity introduced in the SPRF distribution. The SPRF distribution of the SU-8-coated resonator decreases overall as expected due to the additional layer introduced but sharply increases in regions defined by the thickness and acoustic velocity of the SU-8 layer. The mechanical loss of the added layer has significant effect on the parameters of the resonator. The study reveals that this method of characterization can be used to approximate the mechanical loss of materials such as polymers or polymer composites. The simulation with finite-element method agrees with the experimental result.
RESUMO
Different amounts of graphene-wrapped magnesium oxide (G@MgO) powders are uniformly dispersed in poly(vinyl alcohol) (PVA) solution in different experiments to obtain solutions which are coagulated to obtain solid materials, which are then hot pressed at 413 K and 3 t of pressure to finally obtain 1 mm thick freestanding G@MgO/PVA composite sheets in which the constituents, namely, graphene and MgO (in the form of G@MgO), are the nanofillers in PVA matrix. During synthesis of G@MgO powder, MgO nanoparticles are in situ wrapped by the graphene nanosheets as revealed by electron microscopy. Uniformity of G@MgO dispersion in PVA was confirmed by secondary electron micrographs and the consistency in X-ray diffraction and Raman scattering data collected from different locations of the samples. Temperature (303-393 K) dependent complex permittivity of G@MgO/PVA composite sheets (including those prepared by casting) in low frequency (20 Hz to 2 MHz) and high frequency (i.e., X-band, 8.2-12.4 GHz) ranges are measured. In both frequency ranges, G@MgO/PVA composite sheets prepared by coagulation exhibited dielectric properties superior to those of PVA and G@MgO/PVA composite sheets prepared by casting. A strong interfacial polarization is observed in coagulated and as-cast G@MgO/PVA composite sheets. It is noticed from the calculated activation energies that conduction is the dominating mechanism for energy transfer in both composite sheets' cases, while it is predominating in coagulated composite sheets due to the better network formation of the fillers in the coagulated samples than in the cast composite samples. The electromagnetic interference shielding effectiveness (EMI SE) values in the X-band frequency range (i.e., 8.2-12.4 GHz) of the G@MgO/PVA composite sheets prepared by coagulation are more than those prepared by casting for a particular weight fraction of G@MgO. At 393 K, for a particular G@MgO/PVA composite sheet prepared by coagulation, an excellent EMI SE of â¼27.5 dB is measured. It is also experimentally elucidated that the absorption is the dominating mechanism for EMI SE in the prepared composite sheets.
RESUMO
Silver nanoparticle-embedded poly(vinyl alcohol) films are fabricated through a simple in situ process. The nanocomposite films are a few hundred nanometers thick with silver concentrations below 10% and the nanoparticles 5-10 nm in diameter. These films are shown to exhibit appreciable microwave absorption in the 8-12 GHz range; the return and insertion losses are found to be sensitive to the nanoparticle content.
