Papers by Author: Jing Feng Li

Abstract: LaCoO3 ceramics were prepared by conventional solid state reaction and normal sintering at the temperatures ranging from 1373 to 1523 K. The sintered densities increased with increasing sintering temperature and exceeded 90 % of the theoretical values when sintered above 1473 K. The thermoelectric properties of the samples sintered at different temperatures were investigated from 323 to 673 K. The LaCoO3 samples showed a negative Seebeck coefficient, whose absolute values decreased dramatically with increasing temperature in the range of 323 to 460 K, then changed to a positive value and lightly decreased above 460 K. The electrical conductivity increased with increasing temperature, indicating a semiconducting behavior. The Seebeck coefficients showed little difference between the samples sintered at different temperatures, but the power factor of the sample sintered at a higher temperature was larger because of the higher electrical conductivity.

Abstract: The fabrication of electrodes is one of the key techniques in constructing thermoelectric elements for the practical applications. In this work, the commercial active brazing alloy “Incusil-ABA” was used for the joining of CoSb3 to the Cu surface of the graded electrode materials (Cu/AlN/Cu) by using spark plasma sintering (SPS). The bonding was performed in vacuum at temperatures 500°C for 10min. The brazing and diffusion bonding process were investigated by analyzing the crystal structure and microstructure of the bonding interface using X-ray diffraction and scanning electron microscopy, and its composition distribution was also analyzed by energy dispersive X-ray.

Abstract: Nanometer La2CuO4 particles have been prepared using chemical precipitation method. The reaction mechanism, microstructure and phase composition of the samples were investigated by TG-DTA, XRD, SEM. The XRD results showed that pure La2CuO4 phase can be obtained by calcining the La-Cu-O-H precursor powders at 900 oC for 2 h, and the impurity phase will appear in the La2CuO4 ceramic as increasing the sintering temperature over 1200 oC. La2CuO4 ceramic samples sintered at 950 oC for 6 h exhibit good thermoelectric properties. Seebeck coefficient can reach 714 μV/K, and power factor is about 2.33×10-5 W/mK2.

Abstract: The Bi2Te3 compound was synthesized by mechanical alloying (MA) of highly pure constituents of Bi and Te powders, and then was consolidated by spark plasma sintering (SPS) method. The MA-derived alloy formation was characterized by X-ray diffraction (XRD), and the microstructure change was investigated through scanning electron microscopy (SEM). The thermoelectric properties were measured using a Seebeck Coefficient/Electrical Resistance Measuring System. The influence of MA time on the thermoelectric properties of the SPS-sintered samples was investigated to find the optimal MA condition for the powder synthesis and thermoelectric properties. The obtained results showed that the sample with the MA time of 6 h exhibited the optimal electrical transport properties. The maximum power factor of over 2.0 × 10-3 W/m K2 was obtained between 323 K and 423 K.

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: 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: 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: AgxPbmSbTe2+m thermoelectric materials were fabricated using a combined process of mechanical alloying (MA) and spark plasma sintering (SPS). The compound powder was synthesized by mechanical alloying (MA) from elemental powders using a planetary mill after a short time, and high-density bulk samples were fabricated by spark plasma sintering (SPS) at low temperature within a short time (12 minutes). The P-type materials were obtained with electrical properties comparable to the newly reported data. The properties of P-type AgxPbmSbTe2+m-based materials could be improved by optimizing the composition and the process.

Abstract: Single-phase polycrystalline CoSb3 skutterudite was prepared through a new combination of mechanical alloying (MA) and spark plasma sintering (SPS). In order to investigate the influence of MA conditions on the microstructure and thermoelectric properties, MA synthesis were carried out under various conditions with different milling times. The powder sample MAed for 6h still consisted of metal Sb, and then transformed to CoSb3 with a little amount of metal Sb and CoSb2 phases after MA for 15h. Further prolonging the MA time resulted in the decomposition of CoSb3 to CoSb2 phase. The average grain size of the SPSed samples decreased from 650nm to 250nm as MA-time was prolonged from 6 to 24h. Lattice parameters estimated form XRD patterns increase with the increasing MA time. All samples SPSed at 600°C for holding 5 min show an n-type conduction. The electrical resistivity was 1030, 895, 410, 260 μm for the samples from the MA-derived powders with MA-time of 6, 15, 24 and 33h at room temperature, respectively, then reduced to 60 μm at 400°C for all samples. An optimum MA time is 24 h in which the sample shows the highest power factor 612μW/m*K2 at 150°C.

Abstract: Fine-scale 1-3 PZT/epoxy piezoelectric composites were prepared by a dice-fill process, and their resonance characteristics and dynamic piezoelectric properties were investigated. The resonance characteristic was deeply influenced by PZT volume fraction and the thickness of 1-3 PZT/epoxy composite. The experiments proved that reducing the distance between PZT rods and/or increasing the height/width ratio could weaken the additional coupling effect of lateral resonance mode on thickness mode. The PZT/epoxy composites displayed only thickness-mode resonance when the distance between PZT rods was reduced to 220μm and its aspect ratio was increased up to 10.