Kyu Hong Hwang

Geopolymer Concrete Fabricated by Waste Concrete Sludge with Silica Fume

As a relatively new material, geopolymer concrete offers the benefits as a construction material for sustainable development. It utilized waste materials such as recycled concrete sludge, fly ash and etc. It has a very low rate of green house gas emission when compared to ordinary Portland cement. In this study, the component of geopolymer is concrete sludge, metakaolin and water glass, NaOH was used as alkalin activator. To improve the mechanical properties, the amount of NaOH and water glass were varied to obtain higher strength and the specimens were cured both in air and water, then their mechanical properties like compressive strength and bending strength were measured the microstructures were investigated.

Microscale tribological behavior and in vitro biocompatibility of graphene nanoplatelet reinforced alumina.

Graphene nanoplatelets were added as reinforcement to alumina ceramics in order to enhance microscale tribological behavior, which would be beneficial for ceramic-on-ceramic hip implant applications. The reduction in microscale wear is critical to hip implant applications where small amounts of wear debris can be detrimental to patients and to implant performance. The addition of the GNPs lead to improvements in fracture toughness and wear (scratch) resistance of 21% and 39%, respectively. The improved wear resistance was attributed to GNP-induced toughening, which generates fine (~100nm) microcracks on the scratch surface. In addition, active participation of GNPs was observed in the scratch subsurface of GNP-reinforced samples through focused ion beam sectioning. Friction coefficients are not significantly influenced by the addition of GNPs, and hence GNPs do not act as solid state lubricants. In vitro biocompatibility with human osteoblasts was assessed to evaluate any possible cytotoxic effects induced by GNPs. Osteoblast cells were observed to survive and proliferate robustly in the GNP-reinforced samples, particularly those with high (10-15vol%) GNP content.

Synthesis of Biodegradable β-TCP/PLGA Composites Using Microwave Energy

Biodegradable β-tricalcium phosphate (β-TCP)/poly (lactide-co-glycolide) (PLGA) composites were synthesized by in situ polymerization with microwave energy. The influence of the β-TCP content in β-TCP/PLGA composites on the molecular weight, crystallinity, microstructure, and mechanical properties was investigated. As the molecular weight of composites decreased, the β-TCP content increased up to 10 wt%, while further raising of the β-TCP content above 10%, the molecular weight increased with increasing β-TCP content. This behavior may be ascribed to the superheating effect or nonthermal effect induced by microwave energy. It was found that the bending strength and Young’s modulus of the β-TCP/PLGA composites were proportional to the molecular weight of PLGA. The bending strength of the β-TCP/PLGA composites ranged from 18 to 38 MPa, while Young’s modulus was in the range from 2 to 6 GPa.

Fabrication of Biphasic Calcium Phosphate Bioceramics from the Recycling of Bone Ash

Biphasic calcium phosphate bioceramics were fabricated from the recycling of bone ash which is mostly used as raw materials of bone china. Precursor calcium phosphate powders were prepared by soaking the commercial bone ash in 0.1 M of NaOH solution at 80°C for 4 h. Calcium phosphate powders was obtained by calcination at 800°C for 1 h to completely remove residual organics. Biphasic calcium phosphate bioceramics which is composed of hydroxyapatite and tricalcium phosphate was fabricated by the sintering of pressed compacts at 1200°C for 1 h under moisture protection. The bone ash derived-biphasic calcium phosphate ceramics consists of mostly HA and small amounts of α-tricalcium phosphate, magnesium oxide and calcium oxide. After polishing the HA ceramics, they were immersed in buffered water at 37°C for 3 and 7 days. The bone ash derived- biphasic calcium phosphate ceramics show high biostability in liquid environment with immersion time compared with commercial calcium phosphate ceramics.

Preparation of Hydroxyapatite Powder Derived from Tuna Bone and Its Sintering Property

Hydroxyapatite (HA) was prepared from waste tuna bone, and its sintering property and dissolution behavior were investigated. Tuna bone derived-HA powder consisted of mainly HA and small amount of MgO. Porous HA ceramics with sintered density of 79% was obtained by pressureless sintering at 1200℃. Meanwhile, HA ceramics prepared by hot pressing at 1000℃ showed dense microstructure with sintered density of 95%. Immersion test revealed that both porous and dense HA ceramics were stable in liquid environment without distinct evidence of surface dissolution. It may be assumed that the presence of Mg in tuna bone-derived HA may improve dissolution resistance of HA.

Dispersion of ZrO2 Particles in the Al2O3/ZrO2 Ceramics by the Partial Chemical Dispersion Processes

To improve the mechanical properties of Al2O3/ZrO2 composites, the homogeneous dispersion of ultra low size ZrO2 particles in Al2O3 ceramics have been controlled by partial dispersion of ZrO2 by chemical processes such as coprecipitation or polymeric precursor method(Pechini process). So nanosized Zr/Y hydroxide were coprecipitated or polymerized directly to the surfaces of commercial sub-micron size α-alumina powder(Sumitomo: AES-11(0.4 μm)) using ZrOCl2 /Y(NO3)3 solution. By the partial coprecipitation method, dispersion of relatively small sized ZrO2 in Al2O3/ZrO2 composites could be achieved at 1500~1600° C of sintering temperature. In case of the polyesterization of Zr/Y(NO3)3-citric acid solution in ethylene glycol directly to the commercial sub-micron size α-alumina powder, more homogeneous dispersion of relatively low sized ZrO2 in Al2O3/ZrO2 composites could be obtained at 1450~1600°C of sintering temperature range and their mechanical strength was more enhanced.

Preparation of Organic/Inorganic Nanocomposites with Microwave Process

Polymer/layered silicate nanocomposities were prepared by in situ polymerization with microwave process. The influence of the amount of clay on the structure and thermal properties for the synthesized nanocomposites were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). It was found that the structure of nanocomposites, an intercalated/exfoliated structure, depended on the clay content.

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.

The Effect of Pozzolanics on the Recycled Aggregates by Surface Treatment

To improve the mechanical properties of concretes containing recycled aggregates, pozzolanic materials such as Silica Fume and Meta Kaolin were used to decrease the porosity of the recycled aggregates. These pozzolanic aterials were adhered on the surface of recycled aggregates and closed the open pores so that the water absorption was decreased 1~2% as the amount of adsorption was increased. Compressive strength of cement mortars and concretes using surface treated recycled aggregates reached above 95% of the strength of its natural counterparts. Investigation of the microstructures using the scanning lectron micrographs showed the formation of dense interface after the adsorption treatment of pozzolanics to recycled aggregates.

Formation of TiO2 Coating Layer on the Surface Treated Ti Alloys

Pure Titanium alloys are superiorities of biocompatibility, mechanical properties and chemical stability. The biocompatibility of Ti alloy is related to the surface effect. In this study, Ti Alloys were treated by alkali and acid activation process. And through the sol coating layer, biocompatibility were investigated. Consequently, it appeared that the porous layer was generated on the surface of alloy by surface treatment and sol coating process. It was found that with surface treatment on Ti alloy, the formation speed of porous was much quicker compared with those ones without treatment. Therefore, the biocompatibility was improved.