Key Engineering Materials Vols. 336-338

Abstract: Functionally graded piezoelectric ceramics (FGM) plates are attractive due to their ability to broaden the frequency bandwidth of the ultrasonic transducers. A resistivity gradient La-doped Lead Zirconate Titanate/Al-doped Zinc Oxide (La-PZT/Al-ZnO) piezoelectric ceramics plate was developed using a powder metallurgical process. Grading of the piezoelectric modulus across the plate was experimentally confirmed by the electrical impedance analysis and its gradient distribution was characterized using the profile-technique. The distribution of Zn2+ ions in the PZT region was examined using EPMA, XRD and SEM/EDX, respectively. Analysis results show that the gradient distributions of structural and electrical properties inside the FGM plate are mainly caused by the diffusion and aggregation of Zn2+ ions in the PZT layer. The functionally graded piezoelectric ceramics plate presented here will be useful for broadening the frequency bandwidth of piezoelectric transducers.

Abstract: The Cu/AlN/Cu functionally graded materials (FGMs) were successfully fabricated using the spark plasma sintering (SPS) method, and a two-step process was used. First, a symmetrical porositygraded AlN plate was prepared using AlN powder consisting of particles of varying sizes. Afterwards, graded Cu/AlN/Cu samples were made by introducing Cu into the pores of the external, porous AlN layer.

Abstract: Silver metal oxides (Ag/MeO) are extensively used as electrical contact materials in switching systems. A contact material with ideal operating parameters is very difficultly fabricated by conventional manufacturing techniques. In this paper, a new electrical contact material characterized by graded distributions of different oxides in Ag matrix was fabricated in order to optimize the distributions of operating parameters in material bulk instead of to enhance synchronously properties of that on contact surface. Two selected metal oxides, NiO and SnO2, were doped into Ag matrices, of which one surface layer NiO 12wt% was doped and another SnO2 12wt%, and the concentrations of NiO and SnO2 were varied gradually in bulk. The electrical contact test results show that the average operating number of NiO doped surface layer of graded Ag composite as contact surface is 4600 under 20A current and 220V AC voltage without melted welding while that of samples doped uniformly with NiO is 2200 under the same testing conditions. However, the average loss of mass due to arc erosion of graded samples is higher than that of uniform samples. We conclude that the functionally graded material (FGM) concept has potential application for electric contact materials.

Abstract: We present numerical and experimental results on the nonlinear bending behavior due to domain wall motion in functionally graded piezoelectric actuator under alternating current (ac) electric fields. A nonlinear finite element method is employed to analyze the dynamic response of functionally graded piezoelectric actuator. A phenomenological model of domain wall motion is used in computation, and the effects of ac electric field amplitude and frequency, number of layers, and property gradation on the deflection of the cantilever actuators are examined. Experimental results, which verify the model, are presented using a functionally graded bimorph. The numerical results agree very well with the experimental values.

Abstract: The nano-structured functionally graded 8YSZ/NiCoCrAlY thermal barrier coatings (FG-TBCs) were prepared using a recently developed Supersonic Plasma Spraying (S-PS) system with dual powder feed ports. The alloy powders were fed into the lower temperature regions of the plasma plume through one of them to prevent it over molten and oxidation. The ceramic powders were fed into high temperature regions through another for fully melted. The thermal shock behavior of the FG-TBCs were investigated. It was found the totally 1mm thick FG-TBCs layer still maintained nano-structure form by TEM, and also exhibited a finely lamellate microstructure mixed by alloy and ceramic with gradient along the thickness direction by SEM. The FG-TBCs exhibit excellent thermal shock resistance due to it was still perfect without any spallation after thermal shock test over 200 cycles under heating by oxygenacetylene flam to 1250°C in 30s and then quenching into ambient water.

Abstract: From macroscopic view, major structure characteristic of ceramics-metals FGM lies in the fact that every Graded layer is composed of ceramics and metals of different volume component ratio, with their component, structure form or substance parameter presenting Graded change. During temperature rising and dropping, thermal stress will be produced between macroscopic Graded layers and thermal substance mismatch of meso-scopic reinforce phase and matrix within every Graded layer will produce monolayer thermal stress, leading to the failure of overall materials. Because FGM differs obviously in organization states along Graded direction from perpendicular direction, Graded structure shows the distribution features of macroscopic nonhomogeneities, anisotropy and meso-scopic mechanical continuity. Recent related studies mainly adopt FGM equivalent method, particle interface crack theory, uniform theory and effective performance forecasting.

Abstract: A theoretical model of the displacement-heating response of the differential shape memory alloy (SMA) actuator was developed. The model was constructed by using the constitutive equation and considering the force equilibrium and the energy balance on the SMA. The output displacement was predicted as a function of the heating power by the model and the simulations of the output displacement and temperature of the SMA actuator were also obtained. To evaluate the model, a prototype actuator which was made up of two SMA coil springs placed opposite each other was briefly described and an experimental setup of the SMA actuator was developed to study the relations among the heating power, the temperature and the output displacement of the SMA actuator. The similarity between the simulated and experimental results verified the proposed model, which indicated that the proposed model could express the thermo-mechanical behavior of the differential Ni-Ti SMA actuators.

Abstract: LiTiNiO thin films were deposited on Pt/Ti/SiO2/Si(100) substrates using a sol-gel spin-coat method. The effects of annealing temperature and annealing time on microstructures and dielectric properties of the thin films were investigated. SEM images showed the thin films had uniform and dense microstructure and the grain size increased with increasing temperature and time. The LiTiNiO thin films consisted of complex oxides which proportions were mainly dependent on the annealing condition. The LiTiNiO thin film annealed at 600°C for 1h showed the highest dielectric constant and frequency stability, while prolonging annealing time even at 600°C resulted in the decrease in the frequency stability of the dielectric constants.

Abstract: Li and Ti co-doped NiO thin films with 200 nm in thickness were deposited onto Pt/Ti/SiO2/ Si(100) substrates using a sol-gel spin-coating method. The effect of Ti doping content on microstructure and dielectric properties of Li0.10TixNi0.90-xO (x=5-20mol%) thin films was investigated. XRD results showed that all the Li0.10TixNi0.90-xO thin films consisted of a mixture of NiO, Li2NiO2 and NiTiO3 oxides. The intensities of the diffraction peaks for the NiTiO3 phase increased and those for NiO decreased with increasing Ti content, suggesting that a part of NiO phase combined with Ti to form NiTiO3 phase. The dielectric constants of all the Li0.10TixNi0.90-xO thin films at 102 Hz at room temperature ranged from 200 to 400 and increased with increasing Ti content. The frequency stability of the dielectric constant for the Li0.10TixNi0.90-xO thin films was also improved greatly with increasing Ti content.

Abstract: Four Coating schemes and two substrate materials are designed to evaluate the characteristics of the ceramic/metal gradient thermal barrier coating. The gradient coating is successfully prepared by plasma spray with single torch and single feeder. The coating performance is evaluated by observing microstructure, measuring thermal shock resistance and thermal residual stress. The coating microstructure is observed by scanning electron microscope (SEM). Diffractive peaks for the gradient coating are got at different zone by X-ray spectroscopy (XRS). The thermal shock experiment shows that the thermal shock resistance property of the gradient coating is better than that of the non-gradient coating. The measurements of thermal residual stresses in different coating schemes are carried out by hole-drilling method, the results show that compressional stress presents on 1Cr18Ni9Ti substrate and tensile stress presents on 2Cr13 substrate.