Horng-Yi Chang

Electrical characteristics of (Sr0.2Ba0.8)TiO3 positive temperature coefficient of resistivity materials prepared by microwave sintering

The resistivity‐temperature (ρ‐T) properties of the (Sr0.2Ba0.8)TiO3 materials densified by the microwave sintering process were investigated. Neither the sintering temperature (1100–1180 °C) nor the soaking time (10–40 min) in this process exert a significant influence on the positive temperature coefficient of resistivity behavior of the as‐sintered samples. Contrarily, reducing the cooling rate from 154 °C/min to 4 °C/min or post‐annealing the samples at 1250 °C for 2 h would increase the maximum resistivity (ρmax) more than three orders of magnitude. Consequently, the resistivity jump with the maximum to minimum resistivity ratio ρmax/ρmin≊107 has been achieved. The increase in maximum resistivity by these processes is ascribed to the increase in cationic vacancies, which subsequently act as effective electronic traps. The trap level (Ese) of the cationic vacancies is estimated from the Arrhenius plots of maximum resistivity (ρmax) and temperature (Tmax) to be Ese≊1.5–2.0 eV. On the other hand, the ρ‐...

Modification of PTCR behavior of (Sr0·2Ba0·8)TiO3 materials by post-heat treatment after microwave sintering

Abstract (Sr0.2Ba0.8)TiO3 materials have been effectively densified by microwave sintering process. The grain size (~ 6 μm) and PTCR characteristics (Tc = 50 °C, ϱ max ϱ min ≈ 10 2 ) of the as-sintered samples vary insignificantly with sintering temperature (1100–1180 °C) and soaking time (10–40 min). However, lowering the cooling rate after sintering substantially increases the resistivity Jump ( ϱ max ϱ min ) from 102 to 105.3, without altering the microstructure. Subsequent annealing, on the other hand, substantially modifies the microstructure and PTCR characteristics. The resistivity Jump increases monotonously with heat treatment temperature (Tht) and reaches ( ϱ max ϱ min ) ≈ 10 7 for samples heat-treated at 1300 °C for 2 h. The grain size remains nearly unchanged for Tht ≤ 1200 °C and grows markedly for samples heat-treated at a higher temperature. The effective trap level is estimated to be Es = 1·46 eV. The activation energy for the densification rate is QMS = 8·2 kcal/mol for microwave sintering and Qcs = 62·5 kcal/mol for conventional sintering process.

DOUBLE CRITICAL TEMPERATURE CHARACTERISTICS OF SEMICONDUCTING (BA0.7PB0.3)TIO3 MATERIALS PREPARED BY MICROWAVE SINTERING

In this work, we obtain (Ba0.7Pb0.3)TiO3 materials possessing double critical temperature Tc in resistivity–temperature (ρ–T) behavior by microwave sintering at 1050–1080 °C for 5 min. The cooling‐rate control and postannealing processes modify the relative magnitudes of low‐ and high‐Tc resistivity jumps without altering the Tc values. The donor Ed and trap Es levels of those materials are estimated to be Ed≂0.05–0.07 eV and Es≂1.07–1.32 eV. According to experimental results the voltage sensitivity and transient responsivity of the current passing through double‐Tc materials are observed to be superior to those of single‐Tc materials prepared by the conventional furnace sintering method. Also, the double‐Tc characteristics are attributed to the dual phases with a core‐shell structure. Moreover, the change in the relative proportion of the two phases accounts for the influence of post‐heat treatment processes on the materials’ ρ–T behavior.

CONVENTIONAL AND MICROWAVE SINTERING STUDIES OF SRTIO3

Using the nonconventional sintering technique, such as microwave sintering, it is observed to enhance the densification rate of SrTiO 3 materials as effectively as employing the highly active powders prepared by the chemical route. Although the chemically derived powders demonstrate better sinterability than the mixed oxide powders, the thermal analysis indicates that the segregation of Ti 4+ -containing clusters during decomposition of precursors in the direct pyrolysis (DP) process induces the occurrence of TiO 2 particles (anatase phase) prior to the formation of SrTiO 3 phase. These particles retard the necking process required to sinter the materials. The spray pyrolysis (SP) process can circumvent the preferential nucleation of TiO 2 phase and, therefore, produce powders exhibiting superior sintering behavior to the DP-derived powders. The microwave sintering technique, on the other hand, substantially enhances the rate of diffusion of the ions in the materials such that even the mixed oxide powders can be sintered at a temperature about 200 °C lower than that needed to achieve the same density in a conventional sintering process. Fine grain (∼4 μm) microstructure is obtained for the materials microwave sintered at 1220 °C for 10 min. The migration of grain boundaries requires higher temperature to initiate than the formation of neckings between the grains. The grain growth occurs only when the material was sintered at temperatures higher than 1250 °C.

Effect of SiO 2 Sintering Aids on High Critical Temperature Positive Temperature Coefficient of Resistivity Properties of (Pb 0.6Sr 0.3Ba 0.1)TiO 3 Materials Prepared by Microwave Sintering Technique

Positive temperature coefficient of resistivity (PTCR) materials with the composition (Pb0.6Sr0.3Ba0.1)TiO3, possessing a high critical temperature as T c=420° C, have been prepared by microwave sintering technique. Incorporation of SiO2 as sintering aids has lowered the sintering temperature (soaking time) required from 1250° C (10 min) to 1140° C (10 min), where the core-shell microstructure was fully developed. The electrical properties were stabilized for the samples containing more than 5 mol% SiO2 and sintered at a temperature higher than 1120° C (10 min). The resistivity ratio, PTCR jump and minimum resistivity achieved are (ρmax /ρmin )≈102.85–103.2 Ω cm, PTCR=0.026–0.030 and ρmin ≈102 Ω cm, respectively. The corresponding energy level of the donors ( Y+3-ions) and the effective electron traps (cationic vacancies) are estimated to be e d=0.077 eV and e a=0.67 eV+e F, respectively, where e F is the Fermi level of the materials. The electronic parameters, such as donor level (e d), PTCR jump and trap level (e a), are better parameters for indicating the perfectness of the densification process than the crystal structural and the microstructural examinations.

Electrical properties of the positive temperature coefficient of resistivity materials with 490 °C critical temperature

This work successfully obtains a positive temperature coefficient of resistivity characteristics with critical temperature (Tc) at around 490 and 420 °C for PbTiO3 (PT), and (Pb0.9Sr0.1)TiO3 (PST) materials, respectively. The tetragonality of these materials are (c/a)PT=1.065 and (c/a)PST=1.046. In addition, the donor level of Y3+ ions in PT materials [(Ed)PT=0.09–0.11 eV] is smaller than that in PST materials [i.e., (Ed)PST=0.29–0.40 eV]. The resistivity jumping ratios of the two materials are similar, that is, (ρmax/ρmin)PT≅102.2 and (ρmax/ρmin)PST≅101.6. Multiple-Tc resistivity-temperature (ρ-T) characteristics are observed, that is ascribed to the formation of an incomplete core-shell microstructure with the Pb-deficient shell not entirely surrounding the stoichiometric core.

Fabrication and Characterization of Flexible PZT Fiber and Composite

Straight lead zirconate titanate (PZT) fibers with round cross sections were fabricated by extrusion with boric acid and polyvinyl alcohol (PVA) condensation. The sintered fibers showed more tetragonality than the bulk. The dielectric constant of the electrode area perpendicular to the PZT fibers (Z1) was higher than the electrode area parallel to the PZT fibers (R1). Single PZT fiber exhibited higher remanent/saturation polarization and coercive field than the bulk and composite. The piezoelectric constant d33 of the Z1 composite was 300 pC/N for the 75 vol% PZT fibers. The Z1 composite had flexibility and high dielectric and ferroelectric properties, and exhibited promise in conformable and large-area applications.

ENHANCING THE DENSIFICATION PROCESS ON BA(MG1/3TA2/3)O3 MICROWAVE DIELECTRICS BY Y2O3 INCORPORATION

Abstract The relative density of Ba(Mg1/3Ta2/3)O3 (BMT) has been found to significantly increase due to the addition of Y2O3. A sintering condition of 1650°C (4 hrs) is necessary to densifie the undoped BMT specimens to 95% theoretical density (T.D.), while relatively lower sintering temperature, 1450°C (4h), is sufficient to densify the BMT materials containing 6 mol% Y2O3 to 95% T.D. However, the Q·f-value of Y2O3-doped materials increases monotonously with sintering temperature and reaches a maximum value (Q·f=650000) when 1600°C (4h) sintered. Microwave dielectric properties of the 1600°C -sintered BMT samples also increase with Y2O3-content. High Q·f value of the samples is intimately correlated with their large ordering factor (S). The temperature coefficient of resonant frequency (τf) of these samples is τf=13.2ppm/°C for the temperature ranging from 25 to 80°C.

Ba(Zn1/3Nb2/3)O3 ceramics synthesized by spray pyrolysis technique

Abstract Ba(Zn1/3Nb2/3)O3(BZN) powder has been prepared by pyrolyzing the polyester precursors, followed by heat treated at 800°C for 4 hrs in air to remove the residual organic materials. The as prepared powders are of hollow geometrical spheres, which inherit the morphology of the atomized droplets. The perovsike structure is indicated by X-ray diffraction pattern to have already formed. A second phase, identified as Ba5Nb4O15, is observed among the pellets sintered at 1250°C∼1350°C in open air, which is ascribed to the loss of ZnO. When the pellets were packed with ZnO powders and placed in a covered Al2O3crucibles, pure perovsike, free of Ba5Nb4O15 phase, was obtained when sintered at 1300°C. Q·f value as high as 180,000 GHz was achieved for thus obtained BZN.

Electrical transport properties of CoMn0.2−xGaxFe1.8O4 ferrites using complex impedance spectroscopy

In this study, we report the influence of Ga content on the microstructural, magnetic, and AC impedance properties of Co-based ferrites with compositions of CoMn0.2−xGaxFe1.8O4 (x=0, 0.1, and 0.2) prepared by the solid-state reaction method. Experimental results showed that the as-prepared Co-based ferrites had a single-phase spinel structure; the Curie temperature of Co-based ferrites decreased with increasing Ga content. All ferrite samples exhibited a typical hysteresis behavior with good values of saturation magnetization at room temperature. The electrical properties of Co-based ferrites were investigated using complex impedance spectroscopy analysis in the frequency range of 100 kHz-50 MHz at temperatures of 150 to 250 oC. The impedance analysis revealed that the magnitudes of the real part (Z’) and the imaginary part (Z”) of complex impedance decreased with increasing temperature. Only one semicircle was observed in each complex impedance plane plot, which revealed that the contribution to conductivity was from the grain boundaries. It was found that the relaxation time for the grain boundary (τgb) also decreased with increasing temperature. The values of resistance for the grain boundary (Rgb) significantly increased with increasing Ga content, which indicated that the incorporation of Ga into Co-based ferrites enhanced the electrical resistivity.