• 한국전기전자재료학회논문지
• /
• 제34권1호
• /
• pp.56-62
• /
• 2021

In this study, we created a DBD plasma device and a MnO2 catalyst mesh filter for evaluating ozone reduction of devices via the catalyst method. The DBD plasma device was manufactured by applying Ag paste to soda lime glass via the screen-printing method. The MnO2 catalyst mesh filter was manufactured by mixing MnO2 powder with binder with a 10% difference in concentration from 10% to 50% and then applying it using the dip-coating method. Finally, we sintered a MnO2 catalyst mesh filter in an electric furnace. We evaluated the characteristics of ozone generation according to the Ar gas flow of DBD plasma devices, the opening ratio, and ozone reduction performance of the MnO2 catalyst filters. Ozone reduction performance was approximately 20.4% at MnO2 10 wt%, 37.8% at MnO2 30 wt% and 50% at MnO2 50 wt%.

• 한국분말재료학회지
• /
• 제28권3호
• /
• pp.216-220
• /
• 2021

The effect of tert-butyl alcohol (TBA) as a freezing solvent on the pore structure of a porous tungsten body prepared by freeze-drying is analyzed. TBA slurries with a WO3 content of 10 vol% are prepared by mixing with a small amount of dispersant and binder at 30℃. The slurries are frozen at -25℃, and pores are formed in the frozen specimens by the sublimation of TBA during drying in air. After hydrogen reduction at 800℃ and sintering at 1000℃, the green body of WO₃ is completely converted to porous W with various pore structures. Directional pores from the center of the specimen to the outside are observed in the sintered bodies because of the columnar growth of TBA. A decrease in pore directionality and porosity is observed in the specimens prepared by long-duration drying and sintering. The change in pore structure is explained by the growth of the freezing solvent and densification.

• The Journal of Advanced Prosthodontics
• /
• 제14권2호
• /
• pp.96-107
• /
• 2022

PURPOSE. Microstructural and physico-mechanical characterization of highly translucent zirconia, prepared by milling technology (CAD-CAM) and repeated firing cycles, was the main aim of this in vitro study. MATERIALS AND METHODS. Two groups of samples of two commercial highly-translucent yttria-stabilized dental zirconia, VITA YZ-HT^White (Group A) and Zolid HT + White (Group B), with dimensions according to the ISO 6872 "Dentistry - Ceramic materials", were prepared. The specimens of each group were divided into two subgroups. The specimens of the first subgroups (Group A₁ and Group B₁) were merely the sintered specimens. The specimens of the second subgroups (Group A₂ and Group B₂) were subjected to 4 heat treatment cycles. The microstructural features (microstructure, density, grain size, crystalline phases, and crystallite size) and four mechanical properties (flexural strength, modulus of elasticity, Vickers hardness, and fracture toughness) of the subgroups (i.e. before and after heat treatment) were compared. The statistical significance between the subgroups (A₁/A₂, and B₁/B₂) was evaluated by the t-test. In all tests, P values smaller than 5% were considered statistically significant. RESULTS. A homogenous microstructure, with no residual porosity and grains sized between 500 and 450 nm for group A and B, respectively, was observed. Crystalline yttria-stabilized tetragonal zirconia was exclusively registered in the X-ray diffractograms. The mechanical properties decreased after the heat treatment procedure, but the differences were not statistically significant. CONCLUSION. The produced zirconia ceramic materials can be safely (i.e., according to the ISO 6872) used in extensive fixed prosthetic restorations, such as substructure ceramics for three-unit prostheses involving the molar restoration and substructure ceramics for prostheses involving four or more units. Consequently, milling technology is an effective manufacturing technology for producing zirconia substructures for dental fixed all-ceramic prosthetic restorations.

• 한국분말재료학회지
• /
• 제29권2호
• /
• pp.118-122
• /
• 2022

The synthesis of porous W by freeze-casting and vacuum drying is investigated. Ball-milled WO₃ powders and tert-butyl alcohol were used as the starting materials. The tert-butyl alcohol slurry is frozen at -25℃ and dried under vacuum at -25 and -10℃. The dried bodies are hydrogen-reduced at 800℃ and sintered at 1000℃. The XRD analysis shows that WO₃ is completely reduced to W without any reaction phases. SEM observations reveal that the struts and pores aligned in the tert-butyl alcohol growth direction, and the change in the powder content and drying temperature affects the pore structure. Furthermore, the struts of the porous body fabricated under vacuum are thinner than those fabricated under atmospheric pressure. This behavior is explained by the growth mechanism of tert-butyl alcohol and rearrangement of the powders during solidification. These results suggest that the pore structure of a porous body can be controlled by the powder content, drying temperature, and pressure.

• The Journal of Advanced Prosthodontics
• /
• 제13권6호
• /
• pp.373-384
• /
• 2021

PURPOSE. This study aimed to compare the marginal and internal fit of 3-unit monolithic zirconia restorations that were designed by using the data obtained with the aid of intraoral and laboratory scanners. MATERIALS AND METHODS. For the fabrication of 3-unit monolithic zirconia restorations using impressions taken from the maxillary master cast, plaster cast was created and scanned in laboratory scanners (InEos X5 and D900L). The main cast was also scanned with different intraoral scanners (Omnicam [OMNI], Primescan [PS], Trios 3 [T3], Trios 4 [T4]) (n = 12 per group). Zirconia fixed partial dentures were virtually designed, produced from presintered block, and subsequently sintered. Marginal and internal discrepancy values (in ㎛) were measured by using silicone replica method under stereomicroscope. Data were statistically analyzed by using 1-way ANOVA and Kruskal Wallis tests (P<.05). RESULTS. In terms of marginal adaptation, the measurements on the canine tooth indicated better performance with intraoral scanners than those in laboratory scanners, but there was no difference among intraoral scanners (P<.05). In the premolar tooth, PS had the lowest marginal (86.9 ± 19.2 ㎛) and axial (92.4 ± 14.8 ㎛), and T4 had the lowest axio-occlusal (89.4 ± 15.6 ㎛) and occlusal (89.1 ± 13.9 ㎛) discrepancy value. In both canine and premolar teeth, the D900L was found to be the most marginally and internally inconsistent scanner. CONCLUSION. Within the limits of the study, marginal and internal discrepancy values were generally lower in intraoral scanners than in laboratory scanners. Marginal discrepancy values of scanners were clinically acceptable (< 120 ㎛), except D900L.

• 한국전기전자재료학회논문지
• /
• 제35권4호
• /
• pp.407-411
• /
• 2022

Bulk-sized PbTiO₃ (PT), which is widely known as a high-performance ferroelectric oxide but cannot be fabricated into a monolithic ceramic due to its high c/a ratio, was successfully prepared with a high tetragonality by partially substituting Ni ions for Pb ions using a solid-state reaction method. We found that Ni-doped PT was well-fabricated as a bulk monolith with a significant c/a ratio of ~1.06. X-ray diffraction on as-sintered and crushed samples revealed that NiTiO₃ secondary phase was present at the doping level of more than 2 at.%. Scanning electron microscopic study showed that NiTiO₃ secondary phase grew on the surface of PT specimens regardless of the doping level possibly due to the evaporation of Pb during sintering. We demonstrated that an unconventional introduction of Ni ions into A-site plays a key role on the fabrication of bulk PT, though how Ni ion functions should be studied further. We expect that this study contributes to a further development of displacive ferroelectric oxides with a high c/a ratio.

• 한국초전도ㆍ저온공학회논문지
• /
• 제24권3호
• /
• pp.47-51
• /
• 2022

With the innovative development of bio, pharmaceutical, and semiconductor technologies, it is essential to demand a next-generation transfer system that minimizes dust and vibrations generated during the manufacturing process. In order to develop dust-free and non-contact transfer systems, the high temperature superconductor (HTS) bulks have been applied as a magnet for levitation. However, sintered HTS bulk magnets are limited in their applications due to their relatively low critical current density (J_c) of several kA/cm² and low mechanical properties as a ceramic material. In addition, during cooling to cryogenic temperatures repeatedly, cracks and damage may occur by thermal shock. On the other hand, the bulk magnets made by stacked HTS tapes have various advantages, such as relatively high mechanical properties by alternate stacking of the metal and ceramic layer, high magnetic levitation performance by using coated conductors with high J_c of several MA/cm², consistent superconducting properties, miniaturization, light-weight, etc. In this study, we tried to fabricate HTS tapes stacked bulk magnets with 60 mm × 60 mm area and various numbers of HTS tape stacked layers for magnetic levitation. In order to examine the levitation forces of bulk magnets stacked with HTS tapes from 1 to 16 layers, specialized force measurement apparatus was made and adapted to measure the levitation force. By increasing the number of HTS tapes stacked layers, the levitation force of bulk magnet become larger. 16 HTS tapes stacked bulk magnets show promising levitation force of about 23.5 N, 6.538 kPa at 10 mm of levitated distance from NdFeB permanent magnet.

• The Journal of Advanced Prosthodontics
• /
• 제14권5호
• /
• pp.324-333
• /
• 2022

PURPOSE. The purpose of this study is to evaluate translucency and masking ability of translucent zirconia compared to conventional zirconia and lithium disilicate materials. MATERIALS AND METHODS. Three types of zirconia blocks with different yttria contents (3Y, 4Y, 5.5Y) and LS blocks (Rosetta SM) were used. Ten specimens for each group were fabricated with 10 mm diameter, with both 0.8 mm and 1.5 mm thicknesses (± 0.02 mm). All groups of zirconia specimens were sintered and polished according to the manufacturer's instructions. To calculate the translucency parameter (TP), CIELAB value was measured with a spectrophotometer on black and white backgrounds. To investigate the color masking abilities, background shades of A2, normal dentin, discolored dentin, and titanium were used. The color difference (ΔE) was calculated with the CIELAB values of A2 shade background as a reference compared with the values in the various backgrounds. One-way ANOVA and Bonferroni tests were conducted (P < .05). RESULTS. The TP values of zirconia specimens increased as the yttria content increased. All materials used in the study were able to adequately mask normal dentin shade (ΔE < 5.5), but were incapable of masking severely discolored dentin (ΔE > 5.5). On the titanium background, all materials of 1.5 mm thickness were able to mask the background shade, but with a thickness of 0.8 mm, only 3Y-TZP and 4Y-PSZ were able to mask titanium background. CONCLUSION. All zirconia materials and lithium disilicate specimens used in this study were unable to adequately mask the shade of severely discolored dentin. It is recommended to use 3Y-TZP or 4Y-PSZ with a sufficient thickness of 0.8 mm or more to mask titanium.

• Nuclear Engineering and Technology
• /
• 제53권12호
• /
• pp.3952-3965
• /
• 2021

In this study, volcanic ash was used as raw material to prepare waste forms with different silicon/aluminum (Si/Al) molar ratios to immobilize sodium-salt waste (SSW) containing simulated ¹³⁷Cs. Effects of Si/Al molar ratios (3:1 and 2:1) and sodium salts on sintering behavior of waste forms and immobilization mechanism of Cs⁺ were investigated. Results indicated that the main mineral phase of sintered waste-form matrixes was albite, and the formation of major phases was found to depend on Si/Al molar ratios. Si/Al molar ratio of 2 was favorable for the formation of pollucite, and the formation and crystallization of mineral phases were also decided based on physicochemical characteristics of sodium salts. Furthermore, product consistency test results indicated that the immobilization of Cs⁺ was related to Si/Al molar ratio, types of sodium salts, and glassy phase. Waste forms with Si/Al molar ratio of 2 exhibited better ability to immobilize Cs⁺, whereas the influence of sodium salts and glassy phases on the immobilization of SSW showed more complicated relationship. In waste forms with Si/Al molar ratio of 2, Cs⁺ leaching concentrations of samples containing Na₂B₄O₇·10H₂O and NaOH were low. Na₂B₄O₇·10H₂O easily transformed into liquid phase during sintering to consequently achieve low temperature liquid-phase sintering, which is beneficial to avoid the volatilization of Cs⁺ at high temperature. Results clearly reveal that waste forms with Si/Al molar ratio of 2 and containing Na₂B₄O₇·10H₂O show excellent immobilization of Cs⁺.

• 한국재료학회지
• /
• 제31권5호
• /
• pp.301-311
• /
• 2021

The conversion of all carbon preforms to dense SiC by liquid infiltration can become a low-cost and reliable method to form SiC-Si composites of complex shape and high density. Reactive sintered silicon carbide (RBSC) is prepared by covering Si powder on top of 0.5-5.0 wt% Y₂O₃-added carbon preforms at 1,450 and 1,500℃ for 2 hours; samples are analyzed to determine densification. Reactive sintering from the Y₂O₃-free carbon preform causes Si to be pushed to one side and cracking defects occur. However, when prepared from the Y₂O₃-added carbon preform, an SiC-Si composite in which Si is homogeneously distributed in the SiC matrix without cracking can be produced. Using the Si + C = SiC reaction, 3C and 6H of SiC, crystalline Si, and Y₂O₃ phases are detected by XRD analysis without the appearance of graphite. As the content of Y₂O₃ in the carbon preform increases, the prepared RBSC accelerates the SiC conversion reaction, increasing the density and decreasing the pores, resulting in densification. The dense RBSC obtained by reaction sintering at 1,500 ℃ for 2 hours from a carbon preform with 2.0 wt% Y₂O₃ added has 0.20 % apparent porosity and 96.9 % relative density.