Key Engineering Materials Vols. 334-335

Abstract: The Green’s-function and the force- and energy-balance equations for steady states have been used to study the electron transport and its pressure effect with the influence of impurities and acoustic-phonons in Au1-xTixO3 octahedral crystal potential. The results show that there is a minimum temperature range of electrons as the center-of-mass velocity of electrons increasing, and external pressure effect plays an important role in the contributions of impurities and phonons.

Abstract: Lead-free piezoelectric ceramics based on bismuth sodium titanate (BNT) -barium hafnate titanate (BHT) were prepared by a two-step synthesis process. The final BNT-BHT ceramics sintered at 1180oC for 2 h in air showed a perovskite structure with high density. The morphotropic phase boundaries (MPB) were found in BNT based piezoelectric ceramics with 8~10 wt% BHT in composites. In the case of Bi0.5Na0.5TiO3-0.08BaHf0.05Ti0.95O3 ceramics, a maximum piezoelectric coefficient d33 of 122.6 pC/N was obtained. The remnant polarization (Pr) and coercive field (Ec) were measured and the relationship between ferroelectricity and the BHT fraction in the compounds was investigated. The BNT-BHT ceramics were expected to be a new and promising candidate for lead-free piezoelectric device applications.

Abstract: This paper describes the piezoresistance behavior in CFRP cross-ply laminates with transverse cracking loaded in tension. The resistance change due to transverse cracking and the gage factor (the rate of resistance change per mechanical strain) for each transverse crack density were experimentally measured during loading/unloading cycles. The resistance change-strain curves in the unloading/ reloading processes show the bilinear behavior where two gage factors are defined as the slopes of the bilinear curve. The residual resistance change after full unloading increases almost linearly with the mechanical strain while the gage factors do not always increase with the strain. The residual resistance change and gage factors are associated with transverse crack density on the basis of an equivalent resistance circuit.

Abstract: The method of generating the most practicable shape recovery force in smart composite materials which embedded shape-memory alloy (SMA) fiber under the resin matrix is electric heating. However, because the calorie for the heating of the resin matrix increases in the low temperature environment, it is necessary to control the electric heating corresponding to an ambient temperature to obtaining a steady shape recovery force. Then, the method of self-detecting the temperature without using a special temperature sensor by measuring the electrical resistance of the SMA fiber in the composite material was examined.

Abstract: The dynamic response and the sound radiation of truss core panels are investigated in this paper. The spectral element model for the truss core panel is developed. The element model is established by employing shape functions derived directly from the solution of distributed parameter models for each beam element. Forced vibrations of a fluid-loaded beam in a rigid baffle are considered. The spectral formulation can be easily and efficiently coupled with the Fourier transform (FT) based analysis of the structure’s sound radiation in a surrounding acoustic medium. Hence the proposed formulation is an efficient numerical tool for the analysis of the dynamic and acoustic performance of the considered truss core sandwich panels. The comparison of the sound characteristics selected as sound transmission reduction index to the psychical parameters of the truss core beam is carried out.

Abstract: A nonlinear thermal vibration characteristic of the laminate composite thin shell structures under linear temperature field is investigated by finite element method (FEM). The natural frequencies of the composite laminated thin shell structures with different boundary conditions and stacking sequence are calculated and analyzed. Numerical results show that the natural frequencies of the structure are affected by the temperature, boundary conditions and the stacking sequence; the proper stacking angle can change the natural frequencies; the effect degree on the natural frequencies of symmetric composite structures is different from that of the antisymmetric one. These conclusions can provide some guidance to structure design and heat-resistant design of composite structures.

Abstract: Barium titanate with A-site substituted by various amount of bismuth oxide (Ba1-x BixTiO3, abbreviated as BBT, x=0.05, 0.1, 0.15) were prepared by solid-state reaction. The effect of bismuth substitution on crystallographic phase, dielectric and ferroelectric properties was studied. The X-ray diffraction shows that the samples were crystallized into pure perovskite structure when x=0.05 and 0.1, while for x=0.15 sample, second phase appeared in the dominant perovskite phase. The temperature dependence of dielectric permittivity of the ceramics was investigated and the evolution from normal ferroelectrics to relaxor ferroelectric sates was observed. In the range 0≤x≤0.1, the temperature of dielectric peak Tm is independence of the frequency, indicating the normal ferroelectrics behavior. At x=0.15, dielectric relaxation process with a broadening distribution of the permittivity dielectric is observed.

Abstract: To prepare a novel gas sensor being able to recognize molecules, waterborne poly(β-cyclodextrin-block-polydiethylene glycol hexandioic ester) (i.e., waterborne β-CD-block-PDEA) was synthesized and filled with carbon black (CB). It was found that the composites’ electric resistance remarkably changed when the composites were exposed to the vapors of small size molecule solvents (including dichloromethane, chloroform and tetrahydrofuran, etc.). However, almost no response could be detected in the vapors of large size molecule solvents (like n-pentane, ethylbenzene and hexane). Besides, the responsiveness gradually decreased with increasing molecular size of organic solvents. The results evidenced that the composites have acquired considerable selective sensitivity towards gaseous analytes in terms of their molecular sizes. β-cyclodextrin rather than polydiethylene glycol hexandioic ester played the leading role in this aspect, which was explained from the viewpoint of host-gust chemistry. That is, the response mechanism is mainly based on the molecular discrimination behavior of the host compounds. The small analytes could be entrapped by the β-cyclodextrin cavity due to van der Waals force. On the other hand, solvents’ permittivity also played an important role. The molecules with high permittivity were difficult to enter the hydrophobic cavities. The present study demonstrated the composites could serve as candidates for gas sensors capable of molecule discrimination.

Abstract: Dielectric elastomers are a class of electro-active polymers (EAPs) which have great potentials to be used in smart composites. These materials with compliant electrodes are converters of electrical energy to mechanical energy in order to produce external load and strain with good efficiency. Electrode materials should typically have good compliancy so that undergo large strain alongside the film, without producing any additional stress and constraint for the actuator. In the present study, the effect of 4 different electrode materials (graphite filled silicone oil, silver filled grease, graphite powder and electrically conductive silicone rubber) on the performance of dielectric elastomer actuators has been studied. The principle of operation, the method of fabrication and test method of planar actuators are discussed. We have also studied the effects of different driving voltages and different prestrain values on the actuator response. Experimental results showed that electrical conductivity, material compliancy, and compatibility with substrate in the electrode materials are some of the important parameters affecting the actuator performance.

Abstract: Dielectric elastomers (DEs) could be considered as a new type of transducers. They can convert electrical and mechanical energies in a bi-directional manner. In this work, the electromechanical behavior of a DE diaphragmatic generator was investigated. Circular diaphragms were fabricated using a prestretched DE film sandwiched between laminates of compliant electrodes. A special chamber was used to apply pressure. Pressure-Volume characteristics of diaphragms were measured to calculate elastic energy density. DC voltages were applied to the electrodes and the generated voltages were measured. It was concluded that efficiency of generators was strongly dependent on the amount of biased voltage, pre-straining level and applied pressures.