Papers by Author: He Ping Zhou

Abstract: The MgO-Al₂O₃-SiO₂ (MAS) glass powders have been synthesized by low-temperature combustion technique using magnesium nitrate, aluminium nitrate, silicic acid as material, urea as fuel. The crystallization process, sintering behavior and dielectric properties were investigated by means of DTA, XRD, TMA and SEM techniques. The results showed that the glass powders could be sintered at lower than 1000°C. The μ-cordierite phase was first crystallized from glass and then transformed into α-cordierite phase during sintering process. The obtained cordierite-based glass-ceramics at 950°C and 1000°C have low dielectric constant (4.00 ~ 4.96 at 1 MHz), low dielectric dissipation factor (≈ 0.003) and high sintering density (which is above 98% of the theoretical density), which are used for electronic packaging.

Abstract: Barium-strontium titanate Ba_1-xSr_xTiO₃ [0≤x≤1] has been investigated for tunable microwave application due to their high permittivity, low losses and high tunability. The influence of MgF₂ additives on the dielectric performance of Ba_0.6Sr_0.4TiO₃ (BSTO) were investigated in this paper. MgF₂ doped BSTO ceramics were prepared using the conventional mixed oxide techniques. The phase structure, microstructure and dielectric properties of MgF₂ doped BSTO ceramics were studied. The XRD patterns showed that all the samples exhibit a perovskite phase. A secondary phase of MgO was observed with excessive MgF₂ additives. The grain size of MgF₂ doped BSTO ceramics was slightly increased with the increasing of MgF₂ additives, while the distributed porosity was concentrated to get larger. The relative permittivity, dielectric loss and tunability were decreased accordingly. The relative permittivity was reduced from 3717 (pure Ba_0.6Sr_0.4TiO₃) to 680 (6 wt % MgF₂ doped BSTO), while the tunability changed from 51.7 % to 6.5 %. The dielectric losses were decreased from 0.004 to less than 0.001. The Curie temperature of MgF₂ doped BSTO was first decreased with the additives of MgF₂ (x<2 wt %) then remain the same with the rise of MgF₂ (2≤x≤6) content.

Abstract: Ba_xSr_1-xTiO₃ [(BST), 0£x£1] is a solid solution ferroelectric material exhibiting a large dielectric constant non-linear change with an applied DC electric field. And Ba(Zr_xTi_1-x)O₃ solid solution also has received much attention because of the good tunbality. In this paper, Ba_0.6Sr_0.4(Zr_xTi_1-x)O₃ (x=0.15,0.20,0.25,0.30,) ceramic was obtained by traditional ceramic processing. The structural, surface morphological, dielectric properties, tunable properties of this material as well as the mechanism of their nonlinear dielectric constant under DC electric field were systematically examined. The Zr ions substitution of Ti ions has a strong effect on the grain sizes and the dielectric property. The results show that Ba_0.6Sr_0.4(Zr_xTi_1-x)O₃ (x = 0.2, 0.25,0.3) ceramic is a promising candidate for the DC electric field tunable materials for microwave electronics application, because they exhibit high relative tunability (44%, 48%, 35%, respectively) as the strength of electric field is up to 1.5 kV/mm, and low dielectric loss (0.0026, 0.00044, 0.0015, respectively) at 100 kHz at room temperature. X-ray diffraction analyses indicated that the samples present perovskite phase. Scanning electron microscopy micrograph of sample indicated that the grain size decreased and density increased with Zr concentration increasing.

Abstract: Lead-free K_0.5Na_0.5NbO₃-xwt% (0.4ZnO-0.6B₂O₃) ceramics were synthesized by conventional ceramics process. The co-doping of ZnO and B₂O₃ can decrease the sintering temperature of K_0.5Na_0.5NbO₃ ceramics to 1000°C, and the samples show high relative density around 96%. X-ray diffraction (XRD) reveals that Co-doping of ZnO and B₂O₃ induces lattice distortion. Liquid phase is observed by scanning electron microscopy (SEM), which is resulted from the addition of ZnO and B₂O₃. The ceramics show better piezoelectric and dielectric properties with planar electromechanical coupling factor k_p= 40.7%, piezoelectric constant d₃₃=117pC/N, dielectric constant ε^T₃₃/ε₀=318.6, and loss tangent tanδ=0.034.

Abstract: (1-x) (K_0.475Na_0.475Li_0.05)(Nb_0.975Sb_0.025)O₃-xmolBiFeO3 (x=0, 0.002, 0.004, 0.006, 0.008) doped with 0.8mol%CuO lead-free piezoelectric ceramics were prepared by the solid state reaction technique. X-ray diffraction patterns suggested that all the ceramics presented perovskite structure. The compositional dependence of the phase structure and the electrical properties of the ceramics were studied. The ceramic (x=0.002) near room temperature exhibited excellent electrical properties (piezoelectric constant d₃₃=172pC/N, planar electromechanical coupling factor k_p=0.43, and dielectric constant =418). A relatively high mechanical quality factor (Q_m=200) was also obtained in this particular composition. All these results revealed that this system might become a promising candidate for lead-free piezoelectric materials.

Abstract: A lead-free piezoelectric ceramic binary system based on bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT)-bismuth potassium titanate (Bi0.5K0.5)TiO3 (BKT) was synthesized by conventional mixed-oxide technique. The XRD analysis showed that the rhombohedral-tetragonal morphotropic phase boundary (MPB) of the Bi0.5 (Na1-xKx)0.5 TiO3 system was in the composition range of x = 0.16 ~ 0.20. In addition, the piezoelectric properties of this system were also investigated. It was indicated that the piezoelectric properties are better with the compositions near the rhombohedral phase within the MPB than the compositions near the tetragonal phase.

Abstract: High thermal conductive Si3N4 is a promising candidate for producing high quality ceramic substrates used in vehicles. Although a lot of researches have been carried out on the tape casting of Si3N4, they were mainly restricted in fabricating thin films with a thickness less than 200μm which are not sufficient for producing substrate. With larger thickness, tapes tend to crack during drying. In this work, slurry of α-Si3N4 with Y2O3-MgO as additives was prepared using an azeotropic mixture of methylisobutylketone/methyehtylketone/cyclohexanone as media. To obtain thick green tapes, the amount of dispersant, binder and plasticizer in the slurry were optimized, and the effects of drying temperature on the cracking behavior of the tape were studied. By controlling the composition of the slurry and the drying conditions, thick green tapes with thicknesses up to 1.8mm were successfully fabricated, and Si3N4 substrates were produced after subsequent gas-pressing sintering, which were flat and had a relative density as high as 98.2%.

Abstract: β-Si3N4 and β-SiAlON powders were prepared by combustion synthesis with SrCO3 and NH4F used as additives. The resultant β-Si3N4 and β-SiAlON powders consisted of elongated prismatic microcrystals. By adding SrCO3, the anisotropic growth of β-Si3N4 and β-SiAlON crystals is improved and their aspect ratios increase. The addition of NH4F enhanced nitridation reactions and reduced the residual Si in combustion products. It was proposed that the elongated prismatic β-Si3N4 and β-SiAlON crystals grew from liquid phase and the composition and property of this liquid was affected by the addition of SrCO3.

Abstract: Effect of iteration times on mechanically-activated combustion synthesis of high α-content Si3N4 powders was investigated. Properties of the as-synthesized powders such as α-content (Cα) as well as specific surface area (As) were examined. Results showed that both of Cα and As became higher after iteration reactions. The mechanical properties of the sintered bulk ceramics from as-synthesized powders were also tested to reveal the sinterability of the powders. Results showed that relative density of all the sintered bulk ceramics were higher than 97%. Furthermore, fracture toughness had a trend of becoming higher, which reached a value of 10.2 MPam0.5. Correspondingly, bending strenth became a bit lower.

Abstract: Spinel LiMn2O4 was prepared by solution combustion synthesis. The effect of fuel content and calcination procedure on phase composition and microscopic structure of LiMn2O4 was studied. X-ray diffraction patterns showed that fuel content had no obvious influence on the grain size and phase purity of LiMn2O4. Higher calcination temperature led to higher phase purity, lager grain size, and better crystallization of resultant LiMn2O4. Below 600°C the effect of calcination time was inconspicuous, which became notable above 700°C. Scanning electron microscope images showed that nanocrystalline LiMn2O4 was obtained when the calcination temperature was lower than 600°C and the grain size increased at higher temperatures.