Papers by Author: Han Cheol Choe

Abstract: Micropore and nanotube formation on the Ti oxide are important to improve the cell adhesion and proliferation in clinical use. In this study, nanotube and micropore of Ti alloy for biocompatibility have been investigated using FE-SEM and XRD. Ternary alloys were prepared by using high purity sponge Ti Ta, Zr and Nb sphere (99.95% wt.%). Two-step anodizing was used for surface modification of the titanium alloys. Micropore formation was first performed using a potentiostat in 1 M H3PO4 electrolyte and nanotube formation was performed in 1M H3PO4 + 0.8wt% NaF solution by using a potentiostat. The two-step anodizing process is an efficient method for forming micropores and nanotubes on the surfaces of ternary titanium alloys, and the morphology of micropore and nanotube is dependent on the alloying element and composition of ternary alloy.

Abstract: Effects of TiN, ZrN and tooth-ash coatings on the surface phenomena of Ti-(Nb, Zr, Ta, Hf) dental implant alloys with low elastic modulus has been researched by using electrochemical methods. Ti-(Nb, Ta, Zr, and Hf) alloys were fabricated and surface-coated with TiN, ZrN and tooth-ash (hydroxyapatite) using the RF-sputtering. The electrochemical behaviors of the TiN, ZrN and HA-coated dental implant fixtures were investigated using techniques such as potentiodynamic polarization analysis in 0.9% NaCl solution, as well as examine the detailed structure of these coated alloys using SEM and XRD. From potentiodynamic test of Ti-(Nb, Zr, Ta, Hf) alloy, the corrosion resistance of TiN/ZrN/tooth-ash composite coated Ti-(Nb, Zr, Ta, Hf) alloy were higher than those of the non-coated Ti-(Nb, Zr, Ta, Hf) alloy.

Abstract: In this study, AC impedance behaviors of Ti-Zr binary alloys for biomaterials have been investigated using by electrochemical impedance spectroscopy. Ti-10wt%, 20wt%, 30wt%, and 40wt% Zr alloys were prepared by arc melting and homogenized for 24 hr at 1000 °C in argon atmosphere. Phase constitutions and microstructure of the specimens were conducted by using XRD, OM and SEM. The corrosion properties of the specimens were examined through potentiodynamic test (potential range of -1500 ~ 2000 mV), AC impedance spectroscopy(100kHz~10mHz) in artificial saliva solution by potentiostat (EG&G Co, PARSTAT 2273. USA). In as-cast and homogenized Ti-xZr alloys, α-phase was identified by XRD. Microstructures were changed from lamellar structure to needle-like structure as Zr content increased. From the results of polarization behavior in the Ti-xZr alloys, it was found that the corrosion resistance was increased with increasing Zr content. From results of AC impedance spectroscopy, passive film resistance value of homogenized Ti-40Zr specimen(1.60×106 Ωcm2) showed higher than that of as-cast specimen.

Abstract: The Ti-Hf alloy system forms α-β isomorphous system and does not form any intermetallic compounds, which is also beneficial for good mechanical properties. And in order to avoid the release of materials, surface modifications are generally carried out to form a TiN and ZrN layer on the surface. Electrochemical properties of TiN and ZrN coated Ti-Hf alloy by RFsputtering has been researched using various electrochemical methods. Ti-10wt%, 20wt%, 30wt%, and 40wt% Hf alloys manufactured by non consumable vacuum arc melting furnace. All the specimens were heat treatment at 1000°C for 24hr in Ar atmosphere followed by furnace cooling, respectively. The specimens were coated with TiN and ZrN respectively, by RF-magnetron sputtering method. The microstructures were conducted by using OM, EDX and SEM. The corrosion tests were carried out using potentiodynamic(PARSTAT 2273, EG&G, USA) and potentiostatic test in 0.9% NaCl solution at 36.5 ±1 °C. Microstructure clearly observed that lamellar structure translated to needle-like structure with increased Hf contents. From the analysis of TiN and ZrN coated layer analysis, TiN and ZrN coated surface showed columlar structure with 600nm and 100nm thickness, respectively. The corrosion resistance of TiN and ZrN coated Ti alloys were higher than those of the non-coated Ti-alloy in 0.9%NaCl solution, indicating better protective effect.

Abstract: To investigate mechanical fitness of abutment/fixture screw interface in the internal dental implant system, internal connection type implants from AVANA(Osstem®), Bioplant(Cowell-Medi®), Dio(DIO®), Neoplant(Neobiotech®), systems were used in this study. Each group was acquired 2 fixtures at random. Two piece type abutment and one piece type abutment for use with each implant system were acquired. Screw was respectively used to hold a two piece type abutment to a implant fixture. All samples were cross-sectioned with grinder-polisher unit and all specimens were analyzed the interface phenomena between implant fixture/abutment/screw contact interfaces by using field emission scanning electron microscope(FE-SEM). Implant fixture/abutment/screw connection interfaces of internal connection systems made in Korea were in good condition. The results of the above study showed that materials and mechanical properties and quality of milling differed depending on their manufacturing companies.

Abstract: The abutment screw loosening of dental implant system has been remained problem in restorative practices. Surface treatment of screw plays a role of preventing screw from loosening in implant screw mechanism. The purpose of this study was to investigate surface characteristics of ZrN and TiN coated dental gold-screw and Ti-screw by ion-plating method and to evaluate wear resistance, surface roughness, and film adhesion on the screw surface using field emission scanning electron microscopy(FE-SEM), energy dispersive x-ray spectroscopy(EDX), micro-diamond scratch tester, vickers hardness tester, and surface roughness tester. The surface of gold screw and GoldTite is more smoothly than ones of other kinds of non coated screw. The ZrN and TiN coated surface is more smoothly than ones of other kinds of screw. The hardness of TiN and ZrN coated surface showed higher than that of non coated surface. The TiN coated titanium screw and ZrN coated gold screw have a good wear resistance and adhesion on the surface. The surface of ZrN coated screw showed low surface roughness compared with the surface of TiN coated screw.

Abstract: To evaluate the mechanical deformation phenomena of dental abutment screw after tightening and loosening, five species of abutments were selected (3i-three, Avana- two) respectively by two pieces. The abutment screws were repeatedly tightened and removed 20 times with a digital controller. Field emission scanning electron microscope (FE-SEM) was used to observe changes of each part caused by repeatedly tightening and loosening experiment. As the number of tightening and loosening was increased, the wear or distortion of hexed or squared slot that contacted with the driver tip was more severely progressed. Wear or distortion of hexed slot was more severe than that of squared slot and it was more remarkable in the titanium screw than in the gold screw. All the tested screws showed that the width in the crest of their screw thread decreased gradually as the test was proceeded.

Abstract: The Arc melting technique was employed to synthesize the type I clathrate of Ba8Al16Si30 compound. Phase transformations during synthesis and homogenization treatment were investigated using X-ray diffraction (XRD) and thermal analysis (TG/DSC). Thermoelectric properties as functions of temperature and homogenization treatment time were evaluated in this study. Maximum ZT was 0.14 at 590K for homogenized at 1173 K for 168hrs specimen and it is strongly expected to show higher value above 600K.

Abstract: In order to investigate the intergranular and pitting corrosion behavior of Fe-25Al-6Cr intermetallic compounds containing Mo, Nb and B, 11 kinds of Fe-25Al were prepared by arc melting in Ar gas, using high purity aluminum and iron. The fabricated materials were heat treated for homogenious structure and stabilization of the iron aluminides. The electrochemical methods were used for interganular and pitting corrosion. Effects of Mo, Cr, Nb and B on the repassivation current density(Ir) and the active current density(Ia) of Fe-25Al-6Cr were recorded in the following order: MoCr>Nb>B, from the highest to the lowest respectively.