Papers by Author: Jing Feng Li

Abstract: Highly dense Na1-xKxNbO3 (x=0.2~0.8) ceramics were successfully prepared using spark plasma sintering (SPS) at temperature as low as 920°C. Solid solutions were formed during the composition range investigated. The piezoelectric constant and planar mode electromechanical coupling coefficient are significantly dependent on the Na/K ratio, with the maximum value of 148 pC/N and 38.9% at the composition Na0.5K0.5NbO3.

Abstract: Orthorhombic perovskite-type K0.5Na0.5NbO3 (KNN) films on Pt/Ti/SiO2/Si substrates were prepared by sol-gel processing. The morphology of the films was characterized using SEM and AFM. The KNN films with a total thickness of about 0.5 micron had relatively dense and uniform microstructure. The ferroelectricity of the KNN films was confirmed by polarization-electric field (P-E) hysteresis loop.

Abstract: In the present study, we fabricated high purity and electrically machinable Ti3SiC2 ceramics by mechanical alloying and subsequent spark plasma sintering. The effect of a trace amount of Al on these synthesis processes was examined. Our results showed that Ti3SiC2 could be synthesized by high energy milling. Spark plasma sintering of mechanically alloyed powder at the temperatures of 1000-1200°C produced nearly single-phased Ti3SiC2 materials. The purity of the sintered Ti3SiC2 bulk was remarkably increased by addition of a small amount of Al. Ti3SiC2 with a purity of 99.3 wt% and a relative density of 98.9% was obtained by mechanical alloying and subsequent spark plasma sintering from a starting mixture composed of n(Ti) : n(Si) : n(Al) : n(c) = 3 : 1 : 0.2 : 2 at 1100°C.

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: Electrophoretic deposition (EPD) is a promising, simple and cost-effective method for fabrication of piezoelectric ceramic films. In this study, commercial lead zirconate titanate (PZT) powders were deposited onto silicon wafers coated with platinum electrodes by using EPD method in ethanol solution with a few amount of HCl to introduce surface charges to PZT particles. The sintering densification of the EPD-derived green PZT films were attempted with approaches: addition of a low-temperature eutectic sintering aid consists of Cu2O and PbO, infiltration of PZT sol into the green film. Thick PZT films on Si wafers were obtained by sintering at 700oC with sintering aid and sol-infiltration.

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: Bi2Te3-based alloys are currently best-known, technological thermoelectric materials near room temperature. In this paper, Bi2Te3 and nano-SiC dispersed Bi2Te3 were prepared by mechanical alloying followed by spark plasma sintering (SPS). Raw powders of Bi, Te and SiC were mixed and mechanically alloyed in an argon atmosphere using a planetary ball mill. The SPS temperature was 623K, and the holding time was 5 minutes. The samples were characterized by X-ray Diffraction (XRD) and Scanning electron Microscope (SEM). The thermoelectric properties: i.e. Seebeck coefficient, electrical resistivity and thermal conductivity were measured at temperatures from room temperature to 573K, followed by the evaluation of figure of merit. The results revealed that the SiC dispersion in the Bi2Te3 matrix increased Seebeck coefficient. Although the electrical resistivity was increased somewhat, the thermal conductivity was reduced by the SiC dispersion, indicating that promising thermoelectric materials with enhanced mechanical properties may be obtained in the nano-SiC dispersed Bi2Te3 composites with optimal compositions.

Abstract: Fine grained and dense dielectric ceramics of the BaO-Nd2O3-TiO2 system have been fabricated by the most advanced Spark Plasma Sintering (SPS) technology, whose average grain size is about 100nm as determined by SEM. XRD showed that the diffractive peak of the sample was broadened clearly. The nano-ceramic microstructure formed by SPS is due to the high pressure up to 50 MPa, low temperature (1000°C) and fast sintering (1 minute). The BaO-Nd2O3-TiO2 ternary nanoceramic sample, which consists of the main phase Nd2Ti2O7 with perovskite-like layer structure (PLS) and the secondary phase BaNd2Ti4O12（Ba4.5Nd9Ti18O54）with perovskite-like tungsten bronze structure, was prepared for the first time by rapidly sintering nano-particles of the xBaO×(0.35-x)Nd2O3×0.65TiO2 compound. Those two phases are formed in BaO-Nd2O3-TiO2 micron-grained ceramics prepared by normal sintering of the nano-particles at 1300°C, resulting in good dielectric characteristics for microwave ceramics; however, dielectric characteristics are significantly degraded in the SPS-sintered samples probably due to the carbon contamination from the graphite SPS dies.

Abstract: Lead zirconate titanate (PZT) thin films with a composition near the morphotropic phase boundary were deposited on silicon wafers by using a modified sol-gel method. Introducing a seeding layer between the interface of PZT film and platinum electrode controlled the texture of PZT films. The lead oxide seeding layer results in highly (001)-textured PZT film, while the titanium dioxide seeding layer results in (111)-textured one. SEM and XRD were used to characterize the PZT thin films. The ferroelectric and piezoelectric properties of the PZT films were evaluated and discussed in association with different preferential orientations.