Kug-Sun Hong

Osteoconduction at porous hydroxyapatite with various pore configurations

To assess the histological response and the reinforcing effects of bone ingrowth within porous hydroxyapatite (HA) implants depending on pore geometry, four kinds of cylindrical-type with parallel linear pores phi50, 100, 300, 500 microm), one kind of sponge-type with irregular interconnecting pores (phi250 microm) and one cross-type with crossing linear pores (phi100 x 120 microm) of porous HA were prepared. Eighty-four rabbits were divided into six groups, and a 5 x 5 x 7 mm sized porous HA block was inserted through the medial cortical window of the proximal tibia. Histomorphological changes were examined using light and scanning electron microscopy. A biomechanical compression test was performed using material test machines. After implantation, the implants showed different histological changes depending on pore geometry. Active osteoconduction was also found in the phi50 microm sized cylindrical-type porous HA. Evidence of remodeling of new bone and bone marrow formation within porous HA was found in the larger cylindrical-types (phi300, 500 microm), and the sponge- and cross-types. The biomechanical test showed that the ultimate compressive strength increased significantly in the phi300 microm sized cylindrical-type, and in the sponge- and cross-types eight weeks after implantation. Porous HA with cylindrical pores could be a useful graft material due to its strength, osteoconductivity and the ease with which its pore geometry can be controlled.

DIELECTRIC PROPERTIES OF MNb2O6 COMPOUNDS (WHERE M = Ca, Mn, Co, Ni, OR Zn)

The microwave dielectric characteristics of MNb{sub 2}O{sub 6} compounds which are subcomponents of the complex perovskite A(B{prime}{sub 1/3}B{double_prime}{sub 2/3})O{sub 3} have been investigated as a function of M ions. All MNb{sub 2}O{sub 6} compounds have the columbite structure. Dielectric properties such as permittivity, dielectric loss, and temperature coefficient of resonant frequency are found to correlate strongly with the M ions. In particular, the temperature coefficient of the resonant frequency may be correlated to the unit-cell volume of MNb{sub 2}O{sub 6}. Among these compounds, ZnNb{sub 2}O{sub 6} had very high quality factors.

Correlation between internal stress and ferroelectric fatigue in Bi4−xLaxTi3O12 thin films

La-substituted bismuth titanate [Bi4−xLaxTi3O12(BLT)] films were synthesized using pulsed-laser deposition and ferroelectric fatigue phenomenon was investigated. The internal strain present in the films, which was analyzed through the evaluation of the x-ray diffraction peak, was partially responsible for the fatigue in BLT films. When x⩾0.75, the change in the internal strain was saturated and there was no significant degradation of switching charge, at least up to 1010 cycles. It revealed that the internal strain, as well as chemical stability of oxygen ions, contributed to the ferroelectric fatigue of the BLT films. The origin of the internal strain is discussed in terms of the lattice mismatch between bulk materials and thin films.

Luminescent properties and reduced dimensional behavior of hydrothermally prepared Y2SiO5:Ce nanophosphors

Hydrothermally prepared nanophosphor Y2SiO5:Ce crystallizes in the P21∕c structure, rather than the B2∕b structure observed in bulk material. Relative to bulk powder, nanophosphors of particle size ∼25–100nm diameter exhibit redshifts of the photoluminescence excitation and emission spectra, reduced self absorption, enhanced light output, and medium-dependent radiative lifetime. Photoluminescence data are consistent with reduced symmetry of the P21∕c structure and are not necessarily related to reduced dimensionality of the nanophosphor. In contrast, medium-dependent lifetime and enhanced light output are attributed to nanoscale behavior. Perturbation of the Ce ion electric field is responsible for the variable lifetime.

Properties and structure of RONa2OAl2O3P2O5 (R = Mg, Ca, Sr, Ba) glasses

The properties and structure of (45 - x)RO · xNa2O · 2.5Al2O3 · 52.5P2O5 (R = Mg, Ca, Sr, Ba, 0 ⩽ x ⩽ 31 mol%) glasses were investigated. The variation in the molar volumes of glasses in the MgO series is closely related to the formation of the end groups in the glasses with the substitution of Na+ ions for Mg2+ ions, resulting in a variation of the density and refractive index of the glasses. The properties of glasses containing CaO in terms of Na2O substitution depend mainly on the low field strength of Na+ ions substituting for CaO even though the end groups occurring in the glasses increased. The variation in properties of the glasses containing SrO and BaO, some of which were substituted by Na2O, could be explained by differences in masses, field strength and polarizability between the Na+ ions and the alkaline-earth ions due to a small variation in the structure of the glasses despite Na2O substitution.

Phase Constitutions and Microwave Dielectric Properties of Zn3Nb2O8-TiO2.

The phase constitutions and microwave dielectric properties of (1-x)Zn3Nb2O8–xTiO2 were investigated using X-ray powder diffraction and a network analyzer, respectively. Four mixture regions were studied with increasing TiO2 content (x): (1) Zn3Nb2O8, ZnNb2O6, and Zn2TiO4, (0<x≤0.5) (2) ZnNb2O6 and Zn2TiO4, (0.5<x<0.65) (3) Zn2TiO4, ZnTiNb2O8, and TiO2 (0.65≤x≤0.75), (4) Zn2TiO4 and TiO2 (0.75<x<1). This structural evolution with composition was ascribed to the interrelations between Zn3Nb2O8 and TiO2. It was demonstrated that the microwave dielectric properties correlated to crystal structure rather than chemical composition. In the mixture region of Zn2TiO4, ZnTiNb2O8, and Zn2TiO4, the amount of TiO2 and ZnTiNb2O8 considerably increased and decreased, respectively, with increasing TiO2 content (x). This resulted in the increase of relative dielectric constant (er) and temperature coefficient of resonant frequency (τf). At x=0.725, Q×f is 5190 GHz, er and τf are 36 and 4 ppm/°C, respectively.

Synthesis of ZnO nanocrystals by subsequent implantation of Zn and O species

We report the preparation of ZnO nanocrystals embedded in a SiO2 matrix formed using sequential zinc and oxygen ion implantations. The optical absorption spectra and photoemission spectra of zinc implanted and zinc/oxygen coimplanted silica show that the first zinc implantation produces zinc clusters and that the subsequent oxygen implantation following the zinc implantation rearranges the distribution of zinc and oxygen ions at an atomic level. While thermal annealing of Zn only implanted silica leads to the formation of Zn nanocrystals, thermal annealing of zinc/oxygen coimplanted silica promotes the growth of ZnO nanocrystals. The absorption and photoluminescence spectra show that ZnO nanocrystals form in an amorphous SiO2 matrix and that their systematic change as a function of annealing temperature corresponds to the typical correlation between the increase of particle size and a redshift in near-band-edge emission.

SOFCs with Sc-Doped Zirconia Electrolyte and Co-Containing Perovskite Cathodes

Recently, much attention has been directed toward the study of Sc-doped zirconia electrolyte for intermediate-temperature SOFCs (IT-SOFCs) due to its fairly good ionic conductivity compared with conventional Y-doped zirconia, which can reduce the internal ohmic loss of the cell. For improving unit cell performance, another important point to be considered is the selection of appropriate electrode materials to reduce the polarization loss at the electrode. In this study, we fabricated anode-supported SOFCs with a 1 mol % CeO 2 codoped 10 mol % Sc 2 O 3 -ZrO 2 (CeSSZ) electrolyte. Various kinds of cathode materials such as La-Sr-Mn-O, La-Sr-Co-O (LSCo), La-Sr-Co-Fe-O, and Sm-Sr-Co-O (SSCo) have been applied in order to identify the most suitable cathode material for Sc-doped zirconia. We evaluated the power-generating characteristics of 5 X 5 cm scaled unit cells as well as the cathode polarization effect via a dc current-voltage measurement, a dc current interruption method, and ac impedance spectroscopy. We also thoroughly investigated the interfacial reaction between the electrolyte and the cathode in order to identify appropriate heat-treatment conditions for each candidate cathode material on the CeSSZ electrolyte. According to the investigation, unit cells with a SSCo and LSCo cathode showed superior power density of 1.13 and 1.33 W/cm 2 , respectively, at 700°C and fairly stable cell performance without any serious interfacial reaction.

Substrate Temperature Dependence of Phase and Orientation of Pulsed Laser Deposited Bi-La-Ti-O Thin Films

Bi3.25La0.75Ti3O12 (BLT) thin films were grown by pulsed laser deposition at substrate temperatures ranging from 400 to 800°C. The phase, orientation, and surface morphology of the deposited films were investigated in order to understand the deposition behavior of the BLT films. As the substrate temperature was increased to 800°C, the films showed a tendency to grow with c-axis preferred orientation and a rough surface. At lower substrate temperatures, however, polycrystalline BLT films were formed. In addition, at the lowest substrate temperature of 400°C, the film consisted mostly of a nonferroelectric fluorite-like phase. This was attributed to the difficulty in forming a perfectly layered structure. The fluorite-like phase was transformed to a ferroelectric polycrystalline BLT phase through annealing in flowing oxygen at 600°C.

Effect of excess Bi2O3 on the ferroelectric properties of SrBi2Ta2O9 ceramics

In this study, the effect of bismuth content on phase-transition behavior, ferroelectric property, and crystal structure of strontium–bismuth–tantalate (SBT) ceramics was explored with the aid of er(T), ferroelectric hysteresis loop, x-ray diffraction (XRD), and Raman spectroscopy. Both the phase-transition behavior and ferroelectric properties such as spontaneous polarization (Ps) showed a dependence on Bi content. The ferroelectric Curie temperature Tc, was found to decrease with increasing the Bi content and Ps was maximized when 2–3 molu200a% excess Bi2O3 was added. It was found that Raman spectroscopy and XRD can explain the Bi-content dependence of SBT ceramics. The frequency and the width of the mode below 60 cm−1 in Raman spectra revealed that a site exchange of Sr2+ and Bi3+ ions occurred. The formation of antisite defects was confirmed by the change in the intensity ratio of I(008)/I(105) in the XRD patterns. The calculation of structure factors showed it was also related to antisite defects (SrBi,B...