Papers by Author: Long Tu Li

Abstract: The nano-sized Ba(Zr0.2Ti0.8)O3 powder was prepared by citrate sol-gel method. The crystal structure and the particle size of the powder were characterized by XRD. The morphology of calcined powder and the sintered ceramics was observed by TEM and SEM, respectively. The particle size of the powders calcined at 600oC is about 10 nm. The dense ceramics can be obtained at sintering temperature of 1220 oC, which is much lower than that of the powder prepared by the conventional solid method. The effects of Al2O3 additive on the sintering behavior, microstructure and dielectric properties of Ba(Zr0.2Ti0.8)O3 ceramics were also investigated.

Abstract: The low temperature sinterability and magnetic properties of Bi-substituted yttrium iron garnet (YIG) polycrystalline samples were studied in this paper. The results showed that Bi-substitution can lower garnet phase formation temperature from about 1200oC to 900oC and ceramic sintering temperature from over 1300oC to about 1050oC respectively. The Bi-substitution leads to a decrease in initial permeability and an increase in ferromagnetic resonance linewidth (ΔH) of sintered specimens. Saturation magnetization decreases and coercive force increases a few with an increase of Bi content. The mechanism of Bi-substitution on initial permeability and ΔH is discussed. The change in initial permeability of Bi-substituted specimens relative to unsubstituted YIG results from Bismuth volatilization and weakening of superexchange interaction. The lattice dilatation in Bi-substituted YIG gives cause for a decrease of saturation magnetization and an increase of coactivity force.

Abstract: The preparation and magnetic properties of Y-type ferroxplana were investigated. Y-type ferroxplana of composition Ba2Zn1.2-xCoxCu0.8Fe12O22 was prepared by citrate sol-gel autocombustion process. X varied from 0 to 1.2. Phase formation was characterized by powder X-ray diffraction (XRD). The microstructure was observed via scanning electron microscopy (SEM). The frequency spectra of initial permeability were measured via an impedance analyzer. Results reveal that a well-defined Y-type phase can be formed after the auto-combusted powders were annealed at 900oC. The synthesized nano-sized powder is very active and can be sintered well at 900oC. The sintered samples have fine-grained microstructure. The samples exhibit excellent magnetic properties in hyper-frequency. As zinc content increases, the initial permeability is enhanced and the cut-off frequency decreases.

Abstract: Tape-casting technology is introduced to fabricate metal-arrayes patterned ceramic substance containing periodic multilayer silver wire structure in the low dielectric constant ceramic matrix for microvave bandgap materials and left hand material application. One important advantage of the metal patterned ceramic substance comparing with the periodic structures fabricated on the printed circuit boards (PCB) using a shadow mask/etching technique is that complex multilayer structure can be designed and fabricated in the ceramic body as well as on the surface by the tape-casting process, while the shadow mask/etching technique technology can only design patterns on the surface of printed circuit boards. An Hp8720ES network analyzer was used to measure the transmission properties of the metal patterned ceramic substance. The experimental result shows some novel properties at X-band microwave frequencies. The metal patterned ceramic substance can be used in the microwave communication, such as microwave antenna.

Abstract: (Ba,Sr)TiO3 films with thickness between 1 ~ 20 µm are fabricated by electrophoretic deposition using (Ba,Sr)TiO3 nanopowder synthesized by different methods including sol-gel, coprecipitation and conventional solid phase synthesis. Experiments show that conventional solid phase synthesis need higher crystallization temperature (>1000°C) and result in larger size of particle while in sol-gel and coprecipitation process (Ba,Sr)TiO3 crystallizes at lower temperature and results in smaller size of particle . Powders synthesized from various routes have different surface property. Deposition parameter must be adjusted accordingly to obtain dense and smooth (Ba,Sr)TiO3 film.

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.

Abstract: BiFeO3-PbTiO3 powders were synthesized by a novel sol-gel auto-combustion method. The gels, transformed from the aqueous solutions of metal nitrate and citric acid, undergo a selfpropagatingcombustion process when being ignited and yield voluminous ashes. These ashes are single phase perovskite BiFeO3-PbTiO3 powders. The redox behaviors of the dried gels were studied by DTA-TG technique and IR spectra. The synthesized powders were characterized in terms of XRD, SEM and BET techniques.

Abstract: The Ag-Pd/PLZT composites were prepared by the conventional electronic ceramics method. The electrical and mechanical properties of the composites were investigated. As the amount of doped Ag-Pd particles increased, the fatigue and mechanical properties of the composites were improved, and the dielectric and ferroelectric properties after electric fatigue dropped less. The results indicate that the fatigue process leaves less damage on the composites due to the Ag-Pd particles incorporation. It is proposed that the improvement of fatigue property may be attributed mainly to the improvement of mechanical property.

Abstract: The family of bismuth layer-structured ferroelectrics is attractive from the viewpoint of their application as electronic materials such as dielectrics, piezoelectrics and pyroelectrics. However, during conventional sintering method, high sintering temperature and long sintering time were needed and it is difficult to get BIT ceramic with a density more than 95% of its theoretic value. In the present paper, we produce dense BIT pellet using a spark plasma sintering process. The results show that at a sintering temperature as low as 7000C/5min under a pressure of 25MPa, BIT ceramics with a density about 99% of its theoretic can be produced. The densities, grain size and electrical properties of the resulting ceramic were also investigated.

Abstract: Titanate ferroelectric materials play more and more important roles in electronic industry due to their particular performances, so the synthesis of ultra-fine ferroelectric powder with high purity is the key issue to improve the performance of electronic components. In this paper, Ba(Ti0.85Zr0.15)O3 nano-powder with perovskite structure and homogeneous granularity was prepared by oxalate precipitation method, the particle size of the powder calcined at 800oC is about 50 nm. The sintering ability of the powder is excellent so that the compact ceramics can be obtained at sintering temperature of 1200°C, which is much lower than that of the powder prepared by the conventional solid method. With the powder prepared by oxalate precipitation method, we developed an ultrafine-grained dielectric material showing Y5V characteristics with the room temperature dielectric constant over 14,000, which can be used in base metal electrodes multilayer chip capacitors.