• The korean journal of orthodontics
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• v.52 no.4
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• pp.278-286
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• 2022

Objective: To evaluate differences in the adhesion levels of the most common oral pathogens, Streptococcus mutans and Porphyromonas gingivalis, in human saliva-derived microcosm biofilms with respect to time and raw materials of orthodontic brackets. Methods: The samples were classified into three groups of bracket materials: 1) monocrystalline alumina ceramic (CR), 2) stainless steel metal (SS), and 3) polycarbonate plastic (PL), and a hydroxyapatite (HA) group was used to mimic the enamel surface. Saliva was collected from a healthy donor, and saliva-derived biofilms were grown on each sample. A real-time polymerase chain reaction was performed to quantitatively evaluate differences in the attachment levels of total bacteria, S. mutans and P. gingivalis at days 1 and 4. Results: Adhesion of S. mutans and P. gingivalis to CR and HA was higher than the other bracket materials (SS = PL < CR = HA). Total bacteria demonstrated higher adhesion to HA than to bracket materials, but no significant differences in adhesion were observed among the bracket materials (CR = SS = PL < HA). From days 1 to 4, the adhesion of P. gingivalis decreased, while that of S. mutans and total bacteria increased, regardless of material type. Conclusions: The higher adhesion of oral pathogens, such as S. mutans and P. gingivalis to CR suggests that the use of CR brackets possibly facilitates gingival inflammation and enamel decalcification during orthodontic treatment.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.526-526
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• 1996

The single crystal of LiTaO3 is well known eletro-optic material as well as the piezoelectric one applied to SAW filter. LiTaO3 has large electro-optic effects, so applied to optical switch, acosto-optic deflector, and optical memory device using photorefractive effects. The crystal growth of SAW-grade LiTaO3 has been studied many aspects, but there is no detail research about optic-grade crystal growth. The conditions of optic-grade LiTaO3 single crystal are as below. The optical transmittance must be over 75%, and axial and radial concentratiom uniformity below 1%. The variation of Curie temperature depending on Li/Ta ratio must be also below 2$^{\circ}C$ and no internal no internal cracks and defects. Because of the limitation of crystal quality, the growing of optic-grade LiTaO3 single crystal is very difficult compared with the growing of SAW-grade. In this research, upper conditions of optic-grade single crystal was investigated after growing of 1 inch diameter and 1.5 inch length LiTaO3 single crystal having no internal cracks and defects using Czochralski method. Curie temperature was determined with DSC and measuring capacitance and lattice parameter was calculated about the grown crystal and ceramic powder samples of various Li/Ta ratio. The result of Tc variation was below 1.2$^{\circ}C$ all over the grown crystal, so it is confirmed that LiTaO3 was grown under congruent melting composition having optical homogeniety. Also, the optical transmittance was about 78%, which was sufficient for optical device.

• Composites Research
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• v.20 no.5
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• pp.49-55
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• 2007

Nanocomposite blade for dicing semiconductor wafer is investigated for micro/nano-device and micro/nano-fabrication. While metal blade has been used for dicing of silicon wafer, polymer composite blades are used for machining of quartz wafer in semiconductor and cellular phone industry in these days. Organic-inorganic material selection is important to provide the blade with machinability, electrical conductivity, strength, ductility and wear resistance. Maintaining constant thickness with micro-dimension during shaping is one of the important technologies fer machining micro/nano fabrication. In this study the fabrication of blade by wet processing of mixing conducting nano ceramic powder, abrasive powder phenol resin and polyimide has been investigated using an experimental approach in which the thickness differential as the primary design criterion. The effect of drying conduction and post pressure are investigated. As a result wet processing techniques reveal that reliable results are achievable with improved dimension tolerance.

• Applied Chemistry for Engineering
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• v.34 no.3
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• pp.221-225
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• 2023

Celluloses are naturally occurring polymers that can be easily obtained from various natural sources. Nanocellulose, a form of cellulose, can be derived from regular cellulose and has unique properties that make it ideal for multiple industrial applications. Nanocellulose is a renewable, sustainable, and eco-friendly composite material with exceptional mechanical properties and thermal stability, surpassing metal and ceramic composites. As a result, nanocelluloses are being extensively studied for their potential applications, including fillers, packaging, energy, medicine, and coatings. This review aims to summarize the current research on nanocelluloses and their applications.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.33 no.3
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• pp.91-96
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• 2023

To verify the quality of bubbles during the use of quartz glass crucibles (QC), an appropriate accelerated testing method was proposed. The bubble state of discarded waste crucibles obtained from actual Czochralski (Cz) processes was analyzed, and optimal heat treatment conditions were suggested by varying temperature, pressure, and time using the QC test piece. By subjecting the samples to heat treatment at 1450℃, 0.4 Torr, and 40 hours, it was possible to control the bubble size and density to a similar level as those generated in the actual Cz process. In particular, by selecting a relatively lower pressure of 0.4 Torr compared to the typical range of 10~20 Torr applied in the Cz process, the time required for accelerated bubble formation testing could be reduced. However, it was determined that increasing the heat treatment temperature to 1550℃ led to the phenomenon of Ostwald ripening, resulting in larger bubbles and a rapid decrease in density. Therefore, it was concluded that it was not a suitable condition for the desired b ake test.

• Geomechanics and Engineering
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• v.31 no.1
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• pp.99-112
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• 2022

The present study examines the natural frequencies (NFs) of perfect/imperfect functionally graded sandwich beams (P/IP-FGSBs), which are composed of a porous core constructed of functionally graded materials (FGMs) and a homogenous isotropic metal and ceramic face sheets resting on elastic foundations. To accomplish this, the material properties of the FGSBs are assumed to vary continuously along the thickness direction as a function of the volume fraction of constituents expressed by the modified rule of the mixture, which includes porosity volume fraction represented using four distinct types of porosity distribution models. Additionally, to characterize the reaction of the two-parameter elastic foundation to the Perfect/Imperfect (P/IP) FGSBs, the medium is assumed to be linear, homogeneous, and isotropic, and it is described using the Winkler-Pasternak model. Furthermore, the kinematic relationship of the P/IP-FGSBs resting on the Winkler-Pasternak elastic foundations (WPEFs) is described using trigonometric shear deformation theory (TrSDT), and the equations of motion are constructed using Hamilton's principle. A closed-form solution is developed for the free vibration analysis of P/IP-FGSBs resting on the WPEFs under four distinct boundary conditions (BCs). To validate the new formulation, extensive comparisons with existing data are made. A detailed investigation is carried out for the effects of the foundation coefficients, mode numbers (MNs), porosity volume fraction, power-law index, span to depth ratio, porosity distribution patterns (PDPs), skin core skin thickness ratios (SCSTR), and BCs on the values of the NFs of the P/IP-FGSBs.

• Journal of the Korean Academy of Esthetic Dentistry
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• v.32 no.1
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• pp.4-7
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• 2023

Since the end of the 1990s, zirconia, which has been applied to dental prostheses, has been used in the form of coping or framework. The prostheses made in this way have been successfully used in the anterior teeth because they have no metal components as a structure. But in the posterior teeth, its use was limited due to the fracture or chipping of the veneering porcelain during the function. Later, as a solution to this shortcoming, a monolithic zirconia restoration was proposed in which a strong coping material was used in the form of a complete prosthesis. But, this resulted in some unesthetic results due to the characteristics of zirconia, which has excellent resistance to strong forces but is very white and opaque. However, now, due to technological advances in increasing the translucency of zirconia, it is possible to produce strong and esthetic zirconia restorations not only in the posterior region but also in the anterior region. In this article, the structural characteristics of various types of zirconia materials that have been developed so far, were discussed, and where the zirconia products actually in use belong to was explained.

• Steel and Composite Structures
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• v.50 no.3
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• pp.299-318
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• 2024

The main objective of this paper is to compute the far-field acoustic radiation (sound radiation) of functionally graded plates (FGM) loaded by sinusoidally varying point load subjected to the arbitrary boundary condition is carried out. The governing differential equations for thin functionally graded plates (FGM) are derived using classical plate theory (CPT) and Rayleigh integral using the elemental radiator approach. Four cases, segregated on power-law index k=0,1,5,10, are studied. A novel approach is illustrated to compute sound fields of vibrating FGM plates using the physical neutral surface with an elemental radiator approach. The material properties of the FGM plate for all cases are calculated considering the power law indexes. An in-house MATLAB code is written to compute the natural frequencies, normal surface velocities, and sound radiation fields are analytically calculated using semi-analytical formulation. Ansys is used to validate the computed sound power level. The parametric effects of the power law index, modulus ratios, different constituent of FGM plates, boundary conditions, damping loss factor on the sound power level, and radiation efficiency is illustrated. This work is the benchmark approach that clearly explains how to calculate acoustic fields using a solid layered FGM model in ANSYS ACT. It shows that it is possible to asymptotically stabilize the structure by controlling the intermittent layers' stiffness. It is found that sound fields radiated by the elemental radiators approach in MATLAB, ANSYS and literatures are in good agreement. The main novelty of this research is that the FGM plate is analyzed in the low-frequency range, where the stiffness-controlled region governs the whole analysis. It is concluded that a clamped mono-ceramic FGM plate radiates a lesser sound power level and higher radiation efficiency than a mono-metallic or metal-rich FGM plate due to higher stiffness. It is found that change in damping loss factor does not affect the same constituents of FGM plates but has significant effects on the different constituents of FGM plates.

• The Journal of Korean Academy of Prosthodontics
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• v.55 no.2
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• pp.144-150
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• 2017

Purpose: The aim of this in vitro study is to investigate load bearing capacity of esthetic abutments according to the type of material and wall thickness. Materials and methods: 70 specimens equally divided into seven groups according to their abutment wall thicknesses. The abutments prepared with titanium 0.5 mm wall thickness were used as a control group (Ti-0.5), whereas zirconia abutments and resin nano ceramic abutments with wall thickness 0.5 mm, 0.8 mm and 1.0 mm were prepared as test groups (Zir-0.5, Zir-0.8, Zir-1.0 and RNC-0.5, RNC-0.8, RNC-1.0). All specimens were tested in a universal testing machine to evaluate their resistance to fracture and all of them underwent thermo-cycling before loading test. Mean fracture values of the groups were measured and statistical analyses were made using two-way ANOVA. Results: Zir-1.0 showed the highest mean strength ($2,476.3{\pm}342.0N$) and Zir-0.8 ($1,518{\pm}347.9N$), Ti-0.5 ($1,041.8{\pm}237.2N$), Zir-0.5 ($631.4{\pm}149.0N$) were followed. The strengths of RNC groups were significantly lower compared to other two materials (RNC-1.0 $427.5{\pm}72.1$, RNC-0.8 $297.9{\pm}41.2$) and the strengths of all the test groups decreased as the thickness decreases (P < .01). RNC-0.5 ($127.4{\pm}35.3N$) abutments were weaker than all other groups (P < .05). Conclusion: All tested zirconia abutments have the potential to withstand the physiologic occlusal forces in anterior and posterior regions. In resin nano ceramic abutments, wall thickness more than 0.8 mm showed the possibility of withstanding the occlusal forces in anterior region.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.7
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• pp.504-509
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• 2018

The purpose of this study was to manufacture a high-performance titanium yellow pigment. Anatase type $TiO_2$ was the skeleton of the pigment and $Sb_2O_3$ is used as the color assistant for the coloring agent, $Cr_2O_3$. Mixed raw materials for the pigment were $TiO_2$(98%), $Sb_2O_3$(99.5%), and $Cr_2O_3$(99.5%). The raw materials were mixed by a dry process and crystallized by calcination at $1,000{\sim}1,200^{\circ}C$. The crystalline material was pulverized in a Jar Mill under $1{\mu}m$ by a wet process and dried for 12 hours at $100^{\circ}C$. The pigment was finally made by a fine grinding process. To determine the best temperature for calcination, 4 temperature sections ($1000^{\circ}C$, $1100^{\circ}C$, $1150^{\circ}C$, and $1200^{\circ}C$) were set up. The X-ray diffraction peak of the rutile crystalline structure was highest at $1,150^{\circ}C$. The yellow ceramic pigment, which has the rutile structure, was applied for coating materials. The synthesized pigments underwent a discoloration tests on the acid resistance, alkaline resistance, weather resistance and heat resistance. In addition, a detection test on harmful heavy metals ($Cr^{+6}$) was done. The resulting values (${\Delta}E$) of the weather resistance test (2000hr), acid resistance test, alkaline resistance test, and heat resistance test were 0.74, 0.16, 0.07 and 0.29. The resulting value for heavy metals testing was 34ppm.