Dong k Seo

Mechanism of Surface Dissolution in Dense Hydroxyapatite

In this study, it was demonstrated how second phases with small amount, which are hardly detected by XRD analysis, affect grain boundary dissolution and related mechanical properties of HA. All HA disks sintered at 1200 oC for 2 h in air with under moisture protection were phase pure and had Ca/P molar ratio of 1.67. Following certain period of exposure to the distilled water, the surface dissolution initiated at grain boundaries and particle loosening, subsequently resulting in decrease in mechanical properties of HA. In order to understand the dissolution mechanism, grain boundary structure of HA was identified by transmission electron microscopy (TEM) and high resolution TEM observation. From the analysis, it was found that the non-stoichiometric phase as α-tricalcium phosphate (TCP) transformed from β-TCP was existed at grain boundaries and caused surface dissolution of HA. From the XRD analysis, it was found that (211) and (112) planes of hydroxyapatite were susceptible to dissolution, whereas (300) plane was relatively stable.

Dissolution and Mechanical Properties of Sintered Hydroxyapatite Immersed in Water

Dissolution of hydroxyapatite (HAp) in distilled water and related mechanical properties were investigated. The commercially obtained stoichiometric HAp powders were used as starting materials. After preparing powder compacts, the disks were sintered at 1200oC for 2 h in air with under moisture protection. The sintered specimens were then placed into 40 ml of 7.4 distilled water. After immersing for certain period of time at 37oC, weight loss, microstructure, and mechanical characteristics of the specimens were investigated. Fracture toughnesses were measured for both sintered and immersed HAp for comparison. Evidence for the surface damage was observed with appearance of micron-level of pitting and grain boundaries dissolved. The value for fracture toughness decreased due to dissolution of the materials.

Surface Dissolution of Hydroxyapatite with Different Composition

The effect of Ca/P ratio on surface dissolution of hydroxyapatite [Ca10(PO4)6(OH)2, HA] in distilled water was investigated. The stoichiometric HA powder and desired Ca/P ratio achieved by adding either Ca3(PO4)2 for a Ca/P ratio of 1.62 or CaO for a Ca/P ratio of 1.72 were used as starting materials. Dense HA ceramics were obtained by sintering at 1200 oC for 2 h in air with under moisture protection. The sintered specimens were then placed into 40 ml of pH 7.4 distilled water. After immersing for certain period of time at 37 oC, crystal structure, weight loss and microstructure of the specimens were investigated. As increasing the immersion time, circular cavities similar to that normally attributed to osteoclast resorption lacunae were observed. As a result, it was found that the dissolution of HA along the grain boundary and subsequently microstructural disintegration was more distinct in 1.62 and 1.72 samples compared to pure HA.

Preparation of β-TCP/TiO2 Composite by Hot-Pressing

In this work, tricalcium phosphate [TCP, β-Ca3(PO4)2]/titania composite were prepared with different ratio of 75/25, 50/50 and 25/75 (HA/TiO2, wt %). The TiO2 and hydroxyapatite (HA) powders, as starting materials, were synthesized by precipitation and hydrothermal methods, respectively. The sintered bodies of β-TCP/TiO2 composite were prepared by hot pressing at pressure of 30 MPa and temperature of 800-1000 oC for 30 min under the pressure of 30 MPa in N2 atmosphere. During the sintering process, hydroxyapatite was decomposed to β-TCP and CaTiO3 at elevated temperature because TiO2 could accelerate the decomposition of hydroxyapatite. The hot-pressed composite with 98% of sintered density, showed high value of 1080 Hv in hardness, compared with the pure hydroxyapatite (600 Hv) before immersion in distilled water. Increasing sintering temperature led densification of the composite with moderately uniform microstructures. As increasing the amount of TiO2, the surface of the composite was less damaged or dissolved.

Fabrication of Pb-free silver paste and thick film adding silver nanoparticles

Fabrication of paste at low temperature has been developed in order to apply for various electronic devices, such as bus electrode and address electrode in PDP, especially for enlargement of a screen size. In this study, nano-sized silver particles with 10 - 30 nm were synthesized from silver nitrate (AgNO3) by a chemical reduction method. To prepare Pb-free silver paste, the silver nanoparticles were mixed with conventional silver powder with an average particle size of 1.6 and conventional Pb-free frit. Thick films were fabricated from silver paste by a screen printing on alumina substrate and the films were fried at temperatures ranging from 550 °C to 600 °C. Microstructures of the fried thick films were analyzed by FE-SEM. Sheet resistivity of fried thick films was measured and also the relationship between sinterability and conductivity of these films were investigated.

AFM Study on the Surface Dissolution of Hydroxyapatite

The surface dissolution of hydroxyapatite ceramics in distilled water was investigated by using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The disks made of hydroxyapatite powder were sintered at 1200oC for 2 h in air with under moisture protection. After sintering, the disks were polished to smoothness using 1 μm diamond and they were soaked in 40 ml of pH 7.4 distilled water at 37oC for certain period of time. The morphological change of hydroxyapatite surface, specifically at the initial stage of immersion, and related surface roughness were observed with immersion time. The present study suggested that an artificial fact like surface scratch in this case initiated to be eliminated prior to the surface dissolution along grain boundary. That is, the surface roughness decreased at the initial stage of immersion, and then increased due to the surface dissolution.

Contributions of the nanoparticles and glass frit to conductivity in Pb-free silver paste

Pb-free silver paste for low sintering temperature was prepared by mixing of two commercial silver powders (0.8 μm and 1.6 μm in size) and nanoparticles (30-50 nm in size). The silver nanoparticles were synthesized by a chemical reduction method using surfactant. To prepare Pb-free silver paste, 10 wt% and 20 wt% of the silver nanoparticles as sintering aids were added to commercial silver powder. Then, 3 wt% of Pb-free frit (Bi2O3-ZnO-CuO system) was added to the mixed powders. Using the paste, thick films were prepared by a screen printing on an alumina substrate and the films were sintered at temperature from 400°C to 550°C. As increasing the sintering temperature, the thick films showed denser microstructure. Improvement of connectivity in the films was attributed to the nanoparticles present at neck of micron-sized grains and the effect of liquid phase sintering by the glass frit. The films had thickness of 4-6 μm and their sheet resistivity decreased from 6.0 to 2.8 μΩcm corresponding to the degree of densification.

THE STUDY ON GRANTING BIOACTIVITY OF Ti ALLOY BY SURFACE TREATMENT

Titanium alloys are superior of biocompatibility, mechanical properties and chemical stability. The biocompatibility of Ti alloy is related to the surface effect between human tissue and implant. Therefore, the purpose of this study is to investigate the bioactivity of Ti alloy by alkali and acid chemical surface treatment; and the biocompatibility of Ti alloy was evaluated by in vitro test. Higher bone-bonding ability and bioactivity of the substrate were obtained by the formation of apatite layers on the Ti alloy in simulated body fluid. The microstructures of apatite layer were investigated by scanning electron microscope (SEM) and the formed phases were analyzed with X-ray diffraction (XRD).

Effect of Water Glass on Compressive Strength of Aluminosilicate-Based Geopolymer

The inorganic binders, fly ash and meta kaolin were used to prepare geopolymer. Water glass was added to the recycled inorganic binders to improve compressive strength of geopolymers. The ratio of the solid materials (inorganic binder and alkali activators) and liquid materials (distilled water, water glass) for the polymerization was optimized as 3:1. Compressive strength of the geopolymers increased because water glass improved the extent of polymerization of the inorganic binder and resulted in dense microstructure. It was found that geopolymers using fly ash showed the higher value of compressive strength, compared with meta kaolin- based geopolymers.

Preparation of Biologically Derived Hydroxyapatite and its Dissolution

Hydroxyapatite derived from human teeth was sintered at 1200°C for 2 h. Dissolving behavior of the biologically derived HA (BHA) in distilled water was investigated and compared with an artificial hydroxyapatite (HA) made of synthetic HA powder. All disks were immersed in 40 ml of pH 7.4 distilled water (buffered using 0.05 M Tris.) for 7 and 14 days at 37°C. All detectable peaks in the HA are identical only to HA lattice planes, whereas BHA consisted of a mixture of HA and β-tricalcium phosphate (TCP). In the case of the HA specimen, the surface dissolution was initiated at grain boundaries followed by generated many separated grains and large defect like cavities. On the other hand, biologically derived HA showed that definite grains considered as β-TCP was predominantly dissolved and the grains were separated from the matrix leaving pores. In the mean time, the rest region, mainly consisting of HA, did not show any evidence of dissolution. It seems that BHA is more stable than the artificial HA in liquid environment.