Papers by Author: Long Tu Li

Abstract: The optimized composition and sintering process of the low sintering X7R-type temperature-stable dielectric ceramics based on Barium Titanate in reducing atmosphere have been investigated. Flux aids SiO2-B2O3-Li2O were used in the experiments. The influence of composition and the content of flux agent on the microstructures and properties of ceramics materials were systemically studied. The X7R-type dielectric ceramics materials could be sintered at low temperature of 950oC in H2/N2 atmosphere, by optimizing flux content and adjusting process parameters. The dielectric constant of ceramics was effectively improved using two-step sintering process.

Abstract: Bulk dense nanocrystalline BaTiO3 ceramics ranging from 15 nm to 100 nm have been successfully prepared by the spark plasma sintering method. Raman spectra and X-ray diffraction were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with grain growth from nanoscale to micrometre scale for BaTiO3 ceramics. Scanning nonlinear dielectric microscopy measurements revealed temperature-dependent variations in contrast, which were attributed to domain rearrangements in BaTiO3 ceramics below 100 nm. Furthermore the piezoresponse hysteresis loops showed that the nanocrystalline BaTiO3 ceramics were switchable and ferroelectricity was retained at the high temperature of 290 oC, demonstrating the existence of nano ferroelectric domains and the ferroelectric phase still retained above Curie temperature, which confirmed the diffused phase transition character in nanograin BaTiO3 ceramics. The dielectric data also show a ferroelectric to paraelectric phase transition in nanograin BaTiO3 ceramics.

Abstract: The effect of Co-substitution on the complex permeability of LiZn ferrite was studied. The polycrystalline ferrite samples with a composition of ((Li0.5Fe0.5)0.8Zn0.2)1-xCoxFe2.05O4, where x varies from 0 to 0.08, were prepared by solid-reaction method. The sintered samples were annealed at 490oC for 96 h to produce perminvar effect. The complex permeability was measured in the frequency range from 1 MHz to 2 GHz. The Co-substitution can enhance the real part of complex permeability (μ’). The maximum μ’ appears when x=0.02. The Co-substituted LiZn samples, especially for x=0.02 and 0.04, present resonance-type magnetic spectra obviously. The μ’ of the sample for x=0.02 increases because the damping of domain wall movement decreases after the annealing treatment. The resonance character of the annealed sample becomes even more remarkable due to its domain wall stabilization.

Abstract: Multistep electrochemical anodization is an electrochemical experiment orderly conducting in different electrolytes. In this paper, TiO2 nanotube arrays have been firstly anodic grown in aqueous electrolyte (H3PO4/HF) and later anodic grown in organic electrolyte (glycerol/NH4F). Compared with separately anodizing in aqueous and organic electrolyte, the morphology of the resulting nanotube arrays can be optimized. SEM images showed that the obtained nanotubes have a length of more than 2 μm and single-pore diameter ranging from 120 to 150 nm. The current work indicates that the multistep electrochemical anodization has a contribution to the optimization of the morphology of TiO2 nanotube arrays.

Abstract: A novel model of 3-DOF multilayer PZT micromanipulator is promoted and simulated with finite element analysis method. States under different applied voltages were discussed, including the motion along axis Z and the rotation along axis X and the bisector between X-Y. After analyzing the distribution of stress, we propose some method, which helps to improve the performance of the micromanipulator. We also do some quantificational analysis about the micromanipulator, discovering that the relationship between the displacements of the probe and applied voltages is almost perfectly linear. By comparing the constant D defining the relationship of the displacements of probe tip and applied voltages, the simulation is proved to be correct. Comparing the performance between the single-layer and multi-layer PZT, it shows that the multi-layer PZT can reduce input voltages, whereas can produce the same displacement. Ceramic sample is prepared for further experiment.

Abstract: This paper presents a valve-less micropump which is actuated by a piezoelectric ceramic chip. We employ a microelectromechanical system process for the silicon substrate and anodic bonding for assembly of the Pyrex glass and silicon wafer. The reciprocating type micropump contains two nozzle/diffuser elements and a silicon membrane with an embedded piezoelectric ceramic actuator.

Abstract: A novel rotary piezoelectric micro-motor (φ8mm×10mm) is proposed in this paper. Different from the conventional traveling wave ultrasonic motor using piezoelectric ceramic ring, four multilayer co-firing piezoelectric ceramic bulks (2mm×2mm×3mm) are used to generate vibration in the ring type stator. The layout and electric signals applied to the piezoelectric ceramic bulks are designed for the purpose of generating elliptical trajectories on the surface of the stator. The working mode of the prototype motor is simulated by the finite element method, and also measured by the Doppler Laser Vibrometer. The experimental results are in good agreement with simulation, which verifies the effectiveness of the finite element model. The prototype motor is driven at the low voltage of 12V under the frequency of 64.91 KHz.

Abstract: A micro piezoelectric motor with a disk-pivot structure is presented. Different from conventional wobble-type piezoelectric motor using piezoelectric rod or cylinder, piezoelectric disc is used to excite wobble modes and metal cylinder stator is used to amplify the transverse displacement, metal rod rotor is actuated to rotate. The outer diameter of the actuator is 14mm. In this paper, the performances of the motor are characterized, where a high rotation speed and a high toque are obtained. The property of start-up period movement of the motor was studied by a high-speed camera. The toque of the motor was calculated by the property of the start-up movement.

Abstract: The plagioclase feldspar (NaxCa1-xAl2-xSi2+xO8, 0<x<0.67), has been synthesized under sub-solidus conditions using the solid-reaction technique. X-ray diffraction (XRD) was used to study the temperature dependency of the phase and structure transformations. Scanning electron microscopy (SEM) was performed to demonstrate the micro-structure of the sintered samples. The dielectric properties in microwave (MW) frequency region, including the dielectric constant, Qxf and the temperature coefficient were measured by the microwave network analyzer. The results revealed the value of Qxf decreases when 0≤x≤0.2, while increases when 0.2≤x≤0.67. The lowest value of Qxf was at x = 0.2.

Abstract: In this work hexagonal perovskite powders of CaLa4Ti4O15 were prepared by citrate sol-gel method. The calcined temperature was 1150oC, which is lower than that of the conventional solid method. Dense CLT ceramics were produced at sintering temperature of 1600oC. The crystal structures and microstructures were investigated by XRD, TEM and SEM, respectively. Compared with samples produced by solid method, the sol-gel derived CLT ceramics exhibited a near zero temperature coefficient of resonant frequency (τf ) of -6 ppm/oC, and other microwave properties , εr ~ 43, Q×f ~ 42000 GHz were also obtained. Moreover, the sintering temperature of the CLT ceramics could be reduced to 1400 oC by addition of B2O3-ZnO.