Papers by Author: Hai Long Zhang

Abstract: Based on more than three hundred transformer accidents and some statistics data related, this paper manages to draw a standard classification table of transformer failure. 5 degrees of failure possibilities are classified in the paper: extremely high, high, general, low, extremely low. Besides, 4 major factors that affects risk of transformer failure severity are studied: direct lossmonitoring levelmaintenance cost and maintenance period. Also, failure severity of each risk source are classified into 5 levels: not serious, mild, ordinary, less serious, and serious, and Extensional Analytic Hierarchy Process (EAHP) and Extensional Project Appraisal Methods are used to evaluate the severity of risk of transformer failures. Last but not least, risk matrix has been built and divided into four parts: high risk area, critical risk area, medium risk area and low risk area. On that top, some maintenance strategies are proposed accordingly.

Abstract: Homogeneous PZT/Ag composites and compositionally graded PZT/Ag actuators were fabricated and evaluated in this work. The piezoelectric constant decreases gradually and the dielectric constant increases significantly with an increase in silver content of the PZT/Ag composites. The fracture toughness of the PZT/Ag composites is enhanced with addition of the silver powders. Based on the corresponding piezoelectric properties of the monolithic PZT/Ag composites, a graded material with symmetric silver compositions from 1 to 10 vol % then inversely to 1 vol % was successfully fabricated by conventional sintering. This study indicates that the PZT/Ag composites can be used to acting as bimorph actuator materials.

Abstract: The TiB2/(TiB2+AlN)/AlN/(AlN+Cu)/Cu functionally graded materials were successfully fabricated via spark plasma sintering (SPS) method, in which a temperature gradient was achieved by using a specially designed mold and different sintering sequences were carried out. XRD analysis and SEM observations were respectively performed upon the TiB2/AlN/Cu composites. The results show that no new phases were introduced into the composites after mechanically milling or SPS process. The layered composites were found to have a dense microstructure, which changes gradually along a composition gradient. The present FGMs are developed as electrode materials of a solar thermoelectric module.

Abstract: This paper introduces the development of two kinds of graded piezoceramic bending actuators. For designing the graded compositional profiles, classical lamination theory (CLT) is used to explore the optimal gradient that generates large electromechanical response. As the first model material, a laminated piezoceramic bimorph actuator was designed and fabricated with a graded compositional distribution of PZT and Pt, and its electric-induced bending characteristics were evaluated and compared with the analytical results by CLT. Furthermore, porosity-graded PZT ceramics were developed, which may be used as the preforms for the infiltration process to fabricate bending actuators with graded ceramic/metal and/or ceramic/polymer interfaces. The fabrication process and property evaluation of homogenously porous and porosity-graded PZT ceramics were introduced.

Abstract: Pt- or Ag-particle-dispersed Pb(Zr,Ti)O3-based ceramic composites were fabricated by using powder processing for the purpose to investigate their ferroelectric and piezoelectric properties that are essential for the optimal design of piezoceramic/metal bending actuators with Functionally Graded Microstructure (FGM). The ferroelectric and piezoelectric constants as well as mechanical properties of the Pt/PZT and Ag/PZT composites as a function of metal content are discussed in relation with the composite microstructures.

Abstract: In this paper, we present the experimental results of microwave absorbing materials with a frequency-selective surface (FSS) embedded in the multilayer structural composites. Microwave absorbing materials with the FSS pattern embedded in multilayer structural epoxy resin matrix composites were fabricated. The effects of the FSS on multilayer structural composites for microwave absorbing material design have been investigated. Comparisons were made with the microwave absorptions of the multilayer composites with FSSs of different positions in composites. The microwave absorption properties of the multiplayer composites samples with FSSs are higher than those of the samples without FSSs. The experiment results show that the incorporation of a FSS can effectively change the reflection property of the composite.

Abstract: Uniformly porous and porosity-graded piezoelectric ceramics were prepared by sintering powder compacts consisting of lead zirconate titanate (PZT) and organic material powders as pore-forming agents (PFA) in air atmosphere. The dielectric, elastic and piezoelectric properties of uniformly porous PZT ceramics were investigated as a function of the porosity volume fraction. The electric-field-induced deflection characteristics of the beam-shaped actuator samples were measured with electric strain gages, and compared with the values calculated from the modified classical laminate theory using the elastic and piezoelectric constants of the non-FGM porous PZT ceramics.

Abstract: TiB2 and copper are difficult to be sintered together by conventional sintering process. In this article, the TiB2/Cu functionally graded materials (FGMs) were successfully fabricated via spark plasma sintering (SPS) method, in which a temperature gradient was achieved by using a specially designed mold and a two-step sintering was performed. The SEM observations show that the TiB2/Cu FGMs have dense microstructures. The XRD analysis suggests that no new phases were introduced into the TiB2/Cu composites after mechanical milling or SPS process. The TiB2/(TiB2+Cu)/Cu FGMs are promising electrode materials of solar thermoelectric modules.

Abstract: Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. Niobate ceramics such as NaNbO3 and KNbO3 have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, Spark Plasma Sintering (SPS) was applied to the sintering of Na0.5K0.5NbO3 ceramics, whose powder was synthesized by the conventional oxide mixing method. No loss of sodium and potassium were found in the SPS sintering process, by which the density of the K0.5Na0.5NbO3 solid solution ceramics was raised to 4.21g/cm3 (>93% of the theoretical density) at temperatures as low as 920°C. The crystal phase of the Na0.5K0.5NbO3 is an orthorhombic structure. The piezoelectric parameter (d33) of the K0.5Na0.5NbO3 ceramics reached 49 pC/N.