• The Journal of Korean Academy of Prosthodontics
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• v.41 no.3
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• pp.288-299
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• 2003

Statement of problem: Many kinds of post and core systems are in the market, but there are no clear selection criteria for them. Purpose: The purpose of this study was to compare the flexural strength and modulus of elasticity of core materials, and measure the bending strength of post systems made of a variety of materials. Material and Methods: The flexural strength and elastic modulus of thirteen kinds core buildup materials were measured on beams of specimens of $2.0{\times}2.0{\times}24{\pm}0.1mm$. Ten specimens per group were fabricated and loaded on an lnstron testing machine at a crosshead speed of 0.25mm/min. A test span of 20 mm was used. The failure loads were recorded and flexural strength calculated with the measured dimensions. The elastic modulus was calculated from the slopes of the linear portions of the stress-stram graphs. Also nine kinds commercially available prefabricated posts made of various materials with similar nominal diameters, approximately 1.25mm, were loaded in a three-point bend test until plastic deformation or failure occurred. Ten posts per group were tested and the obtained data were anaylzed with analysis of variance and compared with the Tukey multiple comparison tests. Results: Clearfil Photo Core and Luxacore had flexural strengths approaching amalgam, but its modulus of elasticity was only about 15% of that of amalgam. The strengths of the glass ionomer and resin modified glass ionomer were very low. The heat pressed glass ceramic core had a high elastic modulus but a relatively low flexural strength approximating that of the lower strength composite resin core materials. The stainless steel, zirconia and carbon fiber post exhibited high bending strengths. The glass fiber posts displayed strengths that were approximately half of the higher strength posts. Conclusion: When moderate amounts of coronal tooth structure are to be replaced by a post and core on an anterior tooth, a prefabricated post and high strength, high elastic modulus core may be suitable. CLINICAL IMPLICATIONS In this study several newly introduced post and core systems demonstrated satisfactory physical properties. However when the higher stress situation exists with only a minimal ferrule extension remaining a cast post and core or zirconia post and pressed core are desirable.

• Journal of the Korean Ceramic Society
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• v.55 no.6
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• pp.527-555
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• 2018

High-temperature thermal insulation materials challenge extensive oxide candidates such as porus $SiO_2$, $Al_2O_3$, yttria-stabilized zirconia, and mullite, due to the needs of good mechanical, thermal, and chemical reliabilities at high temperatures simultaneously. Recently, porous rare earth (RE) silicates have been revealed to be excellent thermal insulators in harsh environments. These materials display attractive properties, including high porosity, moderately high compressive strength, low processing shrinkage (near-net-shaping), and very low thermal conductivity. The current critical challenge is to balance the excellent thermal insulation property (extremely high porosity) with their good mechanical properties, especially at high temperatures. Herein, we review the recent developments in processing techniques to achieve extremely high porosity and multiscale strengthening strategy, including solid solution strengthening and fiber reinforcement methods, for enhancing the mechanical properties of porous RE silicate ceramics. Highly porous RE silicates are highlighted as emerging high-temperature thermal insulators for extreme environments.

• Journal of the Korean Ceramic Society
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• v.31 no.8
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• pp.934-940
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• 1994

Fibers of ZrO2-SiO2 system were prepared from the hydrolysis and condensation of Si(OC2H5)4 and Zr(OnC3H7)4 with different H2O/alkoxide molar ratios. It was found that fibers could be drawn in the viscosity range of 1~100 poise from HCl catalyzed solutions with lower water contents of the mole ratio H2O/alkoxide, r 2. The fibrous gels were converted into the corresponding oxide glass fibers by heating at 80$0^{\circ}C$. Mechanical test was performed on E, A and 20ZrO2-80SiO2 glass fibers reinforced cement in order to investigate the flexural strength. The flexural strength value of 20ZrO2-80SiO2 glass fibers reinforced cement was greater than those of E and A. The chemical durability of the fibers in alkaline solutions increased with ZrO2 content. The weight loss due to the corrosion by 2N-NaOH solutions at $25^{\circ}C$ for 160 hours was about 0.31$\times$10-2 mg/dm2 for the 20ZrO2-80SiO2 glass fibers, which was superior to that of Vycor glass.

• Journal of the Korean Ceramic Society
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• v.31 no.10
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• pp.1147-1158
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• 1994

From Zr(O-nC3H7)4-H2O-C2H5OH-HNO3 starting solutions, MgO-doped stabilized zirconia fibers with varying content of MgO (10~18 mol%) from different MgO sources were fabricated by sol-gel method. The MgO sources used are magnesium nitrate hexahydrate, magnesium acetate tetrahydrate, and magnesium ethylate. The phase transformation studies of a drawn MgO-ZrO2 fiber were carried out using X-ray diffraction, IR spectroscopy, and Raman spectroscopy. The microstructure, tensile strength, and microporosity of fibers were investigated using SEM, tensile strength test, and microporosimeter. Although various MgO sources such as magnesium nitrate, acetate, and ethylate were used, the crystallization behavior of MgO-ZrO2 fibers at different temperatures could be summarized as follows: CubiclongrightarrowMetastable TetragonallongrightarrowMonocliniclongrightarrowCoexistence of Monoclinic and CubiclongrightarrowCubic(trace of monoclinic). At 150$0^{\circ}C$, the phase transformation of MgO-ZrO2 fibers shows the following change depending on the amount of MgO[Mg(NO3)2.6H2O]: At 10 mol%, both monoclinic and cubic phase coexist, at 12 mol%, monoclinic phase decreases rapidly, and then at 14 mol%, only cubic phase remains. When the MgO-ZrO2 fibers containing 12 mol% magnesium nitrate were heated at 80$0^{\circ}C$ for 1hr, average tensile strength of fibers is 4.0 GPa at diameters of 20 to 30 ${\mu}{\textrm}{m}$. As the heat-treatment temperatures increase to 100$0^{\circ}C$ for 1 hr, tensile strength of MgO-ZrO2 fibers decreases rapidly to 0.7 GPa.

• Korean Chemical Engineering Research
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• v.51 no.1
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• pp.47-52
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• 2013

In this study, the catalytic performance and characterization of $Ni/Ce_xZr_{1-x}O_2$ were investigated using an autothermal reforming (ATR) process for hydrogen production. The $Ni/Ce_xZr_{1-x}O_2$ catalysts were prepared using the following methods: the water method (CZ-W), urea water method (CZ-UW) and urea, water and ethanol method (CZ-UWA). The performance of $Ni/Ce_xZr_{1-x}O_2$ catalysts in autothermal reforming of propane for hydrogen production was studied in a fixed-bed flow reactor. Reaction tests were conducted by using a feed of $H_2O/C_3H_8/O_2$=3/1/0.37 and $300{\sim}700^{\circ}C$. The CZ-UW and CZ-UWA catalysts showed higher propane conversion and hydrogen yield than the CZ-W catalyst. The activity test confirmed that the improvement in the water-ethanol catalyst was due to the low level of carbon deposition. SEM showed that the surface carbon consisted of clusters on the used CZ-UW catalyst, which is incontrast to the nano-fiber morphology observed on the used CZ-UWA catalyst. It was found that the amount of carbon deposition depends on the preparation method. Especially the $Ni/Ce_{0.75}Zr_{0.25}O_2$ was showed higher propane conversion and hydrogen yield than the other catalysts. Also TGA showed that the resistance of carbon deposition increase to Co addition.