Yoonho Kim

Order-disorder transition and microwave dielectric properties of Ba(Ni1/3Nb2/3)O3 ceramics

Order-disorder transition of Ba(Ni1/3Nb2/3)O3 ceramics was investigated using X-ray diffraction, transmission electron microscopy, and energy-dispersive spectroscopy. Before sintering samples were pre-fired at 1300° C for 100 h to achieve higher chemical homogeneity. A transition from ordered to disordered state was observed at 1500° C and the transition was accompanied by the formation of liquid phases. The chemical compositions of liquid phases were analyzed using energy-dispersive spectroscopy. Microwave dielectric properties of ordered and disordered samples were measured using the post resonator method and the transmission cavity method. Variations of dielectric properties such as relative permittivity, quality factor, and temperature coefficient of resonant frequency with sintering temperature were discussed in terms of order-disorder transition, liquid-phase sintering, defect concentration, and grain size.

THE DIELECTRIC TEMPERATURE CHARACTERISTIC OF ADDITIVES MODIFIED BARIUM TITANATE HAVING CORE-SHELL STRUCTURED CERAMICS

The dielectric temperature characteristics and microstructures of BaTiO 3 -based ceramics sintered with additives such as Sm 2 O 3 , CeO 2 , and Bi 2 O 3 :Nb 2 O 5 were investigated using TEM, XRD, and EDS. For a Sm 2 O 3 -modified BaTiO 3 ceramic whose additives were uniformly distributed in grains, the ferroelectric transition temperature ( T c ) was shifted to a lower temperature, while the transition temperatures ( T 1 and T 2 ) were shifted to a higher temperature. The additions of CeO 2 and Bi 2 O 3 : PbO to BaTiO 3 formed the chemical inhomogeneity which was composed of grain core, grain shell, and concentration gradient region. The dielectric curve versus temperature of CeO 2 -modified BaTiO 3 has the shape of one strong peak, whereas BaTiO 3 ceramics sintered with Bi 2 O 3 :Nb 2 O 5 exhibit the broad dielectric constant at the low temperature region and 130 °C ferroelectric transition peak. The dielectric temperature characteristics of additives modified BaTiO 3 were determined in terms of the chemical inhomogeneity and stress induced by the difference of the unit cell volume between grain core and grain shell.

ORDERING AND MICROWAVE DIELECTRIC PROPERTIES OF BA(NI1/3NB2/3)O3 CERAMICS

Ordering and microwave dielectric properties of Ba(Ni{sub 1/3}Nb{sub 2/3})O{sub 3} have been investigated using x-ray diffraction, transmission electron microscopy, energy-dispersive spectroscopy, and a network analyzer. Samples sintered at 1400{degree}C for 2 h were disordered and showed the presence of Nb-rich liquid phase at grain boundary junctions. Degree of ordering increased with following annealing at 1300{degree}C. Growth of the ordered region during annealing process was discussed in terms of nucleation and growth. A long-range order parameter was calculated using structure factor. Measurements of microwave dielectric properties showed that permittivity and temperature coefficient of resonant frequency decreased with ordering, and quality factor increased with ordering. The correlation between microwave dielectric properties and ordering was discussed in terms of covalency of bonding, inhomogeneous charge distribution, and defects concentration. {copyright} {ital 1997 Materials Research Society.}

I-V characteristics and impedance spectroscopy of a single grain boundary in Nb-doped SrTiO3

Abstract The I-V character and impedance of a single grain boundary in Nb-doped SrTiO 3 were investigated. The specimens were sintered at 1570°C in air. A microelectrode method was used to measure the I-V character and the impedance of a single grain-to-grain junction. The single grain boundary of Nb-doped SrTiO 3 was shown to have non-ohmic behavior, and the breakdown voltage of a single grain boundary was found to be about 7 V. The precise value of impedance could be determined by the microelectrode method. From fitting results, it was shown that the capacitances of the grain boundary increased and the resistances of the grain boundary decreased with increasing Nb content.

Citrate route to ultra-fine barium polytitanates with microwave dielectric properties

Highly reactive and ultra-fine barium polytitanates (BaO-nTiO2, n= 4, 4.5, 5) with a particle size of 30–50 nm have been prepared by the citrate route. Solubility isotherms were calculated for the BaCO3-citric acid-H2O and TiO(OH)2-citric acid-H2O2-H2O systems at 25 °C to predict the optimum pH condition for preparing pure and stable citrate complexes. Pure and well crystallized powders with monodispersive, nanometer-sized particles could be obtained by thermal decomposition of the optimally prepared citrate precursors at relatively low temperatures. Details of the crystallization process and particle characteristics of citrate-derived barium polytitanates are described. The sinterability and microwave dielectric properties of BaTi4O9 and Ba2Ti9O20 were also measured and are discussed briefly.

Piezoelectric properties in PMN–PT relaxor ferroelectrics with MnO2 addition

Abstract The temperature dependence of piezoelectric properties in the PMN–PT relaxor ferroelectric system with MnO 2 addition was studied in the temperature range from −40 to 30°C. Samples were poled at −40°C and the piezoelectric properties were measured by resonance–antiresonance method. Q m increased by increasing the doping contents of Mn. When 0.5 wt.% MnO 2 was doped, Q m increased from 95 to 480. From the experimental results, it is predicted that Mn behaves as an acceptor and a domain pinning element. The effects of MnO 2 observed in PZT-based normal ferroelectrics were also confirmed in PMN-based relaxor ferroelectrics.

The phase transition and microwave dielectric properties of tin modified zirconium titanate by melting process

Abstract The phase transition and microwave dielectric properties of tin-modified zirconium titanate were investigated using DTA and X-ray diffraction. The transparent glass ceramics were obtained by melting process. For pure ZrTiO 4 , as-quenched glass ceramic undergoes three successive transitions; glass to cubic, cubic to tetragonal, and tetragonal to orthorhombic, with increase of annealing time at 1300 °C. The abrupt change of c -axis lattice parameter below 1200 °C was analyzed to be associated with the transition of the normal high temperature phase to the incommensurate low temperature phase, and the kinetics of transitions was sluggish. The transitions of normal high temperature phase to the incommensurate low temperature phase occur continuously with the increase of annealing time. The addition of Sn inhibits the transition of the normal high temperature phase to the incommensurate low temperature phase. In comparison with the incommensurate low temperature phase, the normal high temperature phase has a beneficial effect on the microwave quality factor.

Physico-Chemical Characterization of Na3Zr2Si2PO12 Fine Powders Prepared by Sol-Gel Method Using Citrates

Sodium superionic conductor (NASICON) powders have been prepared by sol-gel technique using citrates. The optimum pH condition (\cong6) for sol-gel process was predicted by theoretical consideration of thermodynamic equilibrium constants for corresponding metal salts in aqueous solution. The pure NASICON phase with rhombohedral unit cell could be obtained by thermal decomposition of citrate precursors at a relatively low temperature (850°C). When the calcining temperature was raised to 1100°C, however, slight monoclinic distortion with formation of a trace of monoclinic ZrO2 was observed. Thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) technique, scanning electron microscopy (SEM) and 31P static and magic angle spinning (MAS) NMR have been used to characterize the calcined powders.

Effect of microstructure on the microwave properties in dielectric ceramics

Abstract Electromagnetic simulation of the quality factor measurement was compared to the measurement with a network analyzer. Scattering matrix S 21 obtained from the network analyzer was compared to the S 21 obtained from the simulation. From electric field distribution, the dominant resonant TE 01δ mode could be easily determined. The effects of the pore and the conductive inclusion inside the dielectric were investigated. The quality factor decreased with the pore and the second phase in the dielectrics. The decrease of quality factor is more significant when dielectric have conductive inclusions inside the dielectric. Due to the limitations in computer resources and analysis time, the size of the inclusions inside the dielectrics in simulation are not actual size.

A new microwave dielectric ceramics based on the solid solution system between Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3

Microwave dielectric properties of the solid solution between Ba(Ni{sub 1/3}Nb{sub 2/3})O{sub 3} and Ba(Zn{sub 1/3}Nb{sub 2/3})O{sub 3} was investigated. Samples were prepared by the conventional mixed oxide method as well as the Pechini method. Sintered samples were analyzed using x-ray diffraction and scanning electron microscopy, and their microwave dielectric properties were measured by the post resonator method. The sample prepared by the Pechini method revealed an excellent dielectric properties, i.e., permittivity of 35.6, quality factor of 5700 at 10 GHz, and very small temperature coefficient of resonant frequency. Order-disorder behavior of this solid solution system was found to depend on processing parameters. Variation in microwave dielectric properties was discussed in terms of order-disorder behavior, chemical composition, mixture rule, etc. {copyright} {ital 1997 Materials Research Society.}