• Journal of the Korean Physical Society
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• v.81
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• pp.942-947
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• 2022

We report the electrical transport properties of a dual-gate graphene device placed on a CaF₂ substrate. A hexagonal boron nitride top-gate dielectric was introduced to confirm the electrical characteristics of the CaF₂/graphene transistor because it is difficult to inject sufficient carriers through the CaF₂ substrate owing to its thickness of 500 ㎛, and the typical ambipolar behavior of graphene with a slight n-doping effect was clearly observed. In addition, we used a polymethyl methacrylate (PMMA) film as a top-gate dielectric for large-scale graphene devices grown via chemical vapor deposition, which was transferred onto a CaF₂ substrate. We controlled the high gate leakage current caused by the breakdown of the polymer due to non-uniformity by applying the film-transfer process rather than the direct coating method on the graphene device. Furthermore, the transport properties of large-area graphene in contact with CaF₂ are discussed with respect to the effect of top-contacted PMMA.

• Nuclear Engineering and Technology
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• v.56 no.9
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• pp.3758-3763
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• 2024

The current study aimed to evaluate the suitability of polyester resin as an alternative material to polymethyl methacrylate (PMMA) for computed tomography (CT) dosimetry phantoms using the GEANT4/GATE Monte Carlo simulation platform. Cylindrical phantoms (32 cm diameter) constructed of polyester resin and PMMA were simulated and compared in terms of atomic composition, effective atomic number, electron density, mass density, and photon interaction mechanisms. Weighted CT dose index (CTDIw) values were calculated for each phantom at 80, 110, and 130 kVp tube voltages based on measurements of CTDI_100,c and CTDI_100,p. Results demonstrated that the physical properties of polyester closely matched those of PMMA, and the polyester phantom displayed equivalent dosimetric behavior to the PMMA phantom at all tube voltages tested. CTDIw values from the polyester phantom were within 1.4 % of the PMMA phantom across all tube voltages. Conversion coefficients were derived to equate polyester CTDIw values to PMMA dose equivalents. This study found that a polyester resin phantom exhibited radiation dosimetry commensurate with the standard PMMA phantom for CT dose assessment. Consequently, polyester resin represents a viable substitute material when PMMA is unavailable for construction of CT dosimetry phantoms.

• Journal of Korean Ophthalmic Optics Society
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• v.12 no.4
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• pp.29-35
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• 2007

This study was conducted to measure the central corneal thickness(CTT) changes associated with kinds of wearing contact lens. The CCT was measured using ultrasonic pachymeter in three hundred four subjects in their 20s and 30s. Although the CCT of Group 1 who had never worn any kinds of contact lens had no differences between left and right eye as well as between genders, the CCT tended to become thinner depending on the increment in refractive error. In Group 2 wearing only soft contact lens, the CCT was thinner depending on wearing period, and subjects wearing for more than 10 years had significantly thin CCT comparing with subjects wearing for less than 4 years. There was no significant difference in the CCT between Group 3 wearing only RGP lens and Group 2 for less than 4 years. Furthermore, when comparing the CCT between the Group 2 and Group 4 changing to RGP lens after wearing soft contact lens, there was no statistically significant change in the CCT. However, the CCT was significantly thinner in Group 5 changing to soft contact lens after wearing PMMA(polymethyl methacrylate) hard lens compared with Group 2.

• The Journal of Advanced Prosthodontics
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• v.15 no.5
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• pp.227-237
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• 2023

PURPOSE. This study aimed to assess and compare the color stability, flexural strength (FS), and surface roughness of occlusal splints fabricated from heat-cured acrylic resin, milled polymethyl methacrylate (PMMA)-based resin, and 3D-printed (PMMA) based-resin. MATERIALS AND METHODS. Samples of each type of resin were obtained, and baseline measurements of color and surface roughness were recorded. The specimens were divided into three groups (n = 10) and subjected to distinct aging protocols: thermomechanical cycling (TMC), simulated brushing (SB), and control (without aging). Final assessments of color and surface roughness and three-point bending test (ODM100; Odeme) were conducted, and data were statistically analyzed (2-way ANOVA, Tukey, P <.05). RESULTS. Across all resin types, the most significant increase in surface roughness (Ra) was observed after TMC (P < .05), with the 3D-printed resin exhibiting the lowest Ra (P < .05). After brushing, milled resin displayed the highest Ra (P < .05) and greater color alteration (∆E₀₀) compared to 3D-printed resin. The most substantial ∆E₀₀ was recorded after brushing for all resins, except for heat-cured resin subjected to TMC. Regardless of aging, milled resin exhibited the highest FS (P < .05), except when compared to 3D-printed resin subjected to TMC. Heat-cured resin exposed to TMC demonstrated the lowest FS, different (P < .05) from the control. Under control conditions, milled resin exhibited the highest FS, different (P < .05) from the brushed group. 3D-printed resin subjected to TMC displayed the highest FS (P < .05). CONCLUSION. Among the tested resins, 3D-printed resin demonstrated superior longevity, characterized by minimal surface roughness and color alterations. Aging had a negligible impact on its mechanical properties.

• Journal of the Korean Vacuum Society
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• v.18 no.5
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• pp.372-376
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• 2009

In the modern semiconductor industry, lithography process is used to construct specific patterns. However, due to the decreasing of line width, these days, more and more researchers are interested in PMMA(Poly Methyl Methacrylate) lithography by using e-beam instead of the prior method, PR(Photoresist) lithography by using UV(Ultra-Violet). Additionally, the patterns constructed by lithography are collapsed during the process of cleansing remnants and the resistance against the breakdown of the patterns is known to be proportional to the elastic modulus of pattern-constructing materials. In this research, we measured the change of hardness and elastic modulus of PMMA film surface according to the change of time spent to soft-bake the PMMA film. During the measurement, we controlled the tip pressure from $25{\mu}N$ to $8,500{\mu}N$ having intervals that are $134.52{\mu}N$. For these measurements, we used the Triboindenter from Hysitron to gauge the hardness and elastic modulus and the tip we used was Berkovich diamond Tip.

• Journal of Technologic Dentistry
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• v.37 no.4
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• pp.191-197
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• 2015

Purpose: The purpose of this study was to compare the accuracy of the scan data acquired by the extra-oral and intra-oral scanner. Methods: The maxillary right first molar was made of polymethyl methacrylate(PMMA) specimen. This PMMA specimen was scanned with a engineering scanner and intra-oral scanner. Meanwhile, extra-oral scanner scanned stone die duplicated from PMMA master die. Trueness and precision of scan datas was measured by 3-dimensinal inspection. Independent t-test was conduct to analysis the significant difference(a=0.05). Results: In the trueness analysis, mean of discrepancies were 13.82um for intra oral scanner and 16.84 um for extra-oral scanner. In the precision analysis, mean of discrepancies were 11.72 for inta-oral scanner and 9.2 for extra-oral scanner. Both trueness and precision showed a statistically significant difference (Table 1, p<0.05). Conclusion: Intra-oral scanner can show higher trueness than extra-oral scanner, it has lower precision.

• Imaging Science in Dentistry
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• v.47 no.2
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• pp.75-86
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• 2017

Purpose: The purpose of this study was to apply a newly developed free software program, at low cost and with minimal time, to evaluate the quality of dental and maxillofacial cone-beam computed tomography (CBCT) images. Materials and Methods: A polymethyl methacrylate (PMMA) phantom, CQP-IFBA, was scanned in 3 CBCT units with 7 protocols. A macro program was developed, using the free software ImageJ, to automatically evaluate the image quality parameters. The image quality evaluation was based on 8 parameters: uniformity, the signal-to-noise ratio (SNR), noise, the contrast-to-noise ratio (CNR), spatial resolution, the artifact index, geometric accuracy, and low-contrast resolution. Results: The image uniformity and noise depended on the protocol that was applied. Regarding the CNR, high-density structures were more sensitive to the effect of scanning parameters. There were no significant differences between SNR and CNR in centered and peripheral objects. The geometric accuracy assessment showed that all the distance measurements were lower than the real values. Low-contrast resolution was influenced by the scanning parameters, and the 1-mm rod present in the phantom was not depicted in any of the 3 CBCT units. Smaller voxel sizes presented higher spatial resolution. There were no significant differences among the protocols regarding artifact presence. Conclusion: This software package provided a fast, low-cost, and feasible method for the evaluation of image quality parameters in CBCT.

• Journal of Korean Neurosurgical Society
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• v.61 no.2
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• pp.258-266
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• 2018

Objective : The diagnosis of insufficiency fractures of the sacrum in an elder population increases annually. Fractures show very different morphology. We aimed to classify sacral insufficiency fractures according to the position of cortical break and possible need for intervention. Methods : Between January 1, 2008 and December 31, 2014, all patients with a proven fracture of the sacrum following a low-energy or an even unnoticed trauma were prospectively registered : 117 females and 13 males. All patients had a computer tomography of the pelvic ring, two patients had a magnetic resonance imaging additionally : localization and involvement of the fracture lines into the sacroiliac joint, neural foramina or the spinal canal were identified. Results : Patients were aged between 46 and 98 years (mean, 79.8 years). Seventy-seven patients had an unilateral fracture of the sacral ala, 41 bilateral ala fractures and 12 patients showed a fracture of the sacral corpus : a total of 171 fractures were analyzed. The first group A included fractures of the sacral ala which were assessed to have no or less mechanical importance (n=53) : fractures with no cortical disruption ("bone bruise") (A1; n=2), cortical deformation of the anterior cortical bone (A2; n=4), and fracture of the anterolateral rim of ala (A3; n=47). Complete fractures of the sacral ala (B; n=106) : parallel to the sacroiliac joint (B1; n=63), into the sacroiliac joint (B2; n=19), and involvement of the sacral foramina respectively the spinal canal (B3; n=24). Central fractures involving the sacral corpus (C; n=12) : fracture limited to the corpus or finishing into one ala (C1; n=3), unidirectional including the neural foramina or the spinal canal or both (C2; n=2), and horizontal fractures of the corpus with bilateral sagittal completion (C3; n=8). Sixty-eight fractures proceeded into the sacroiliac joint, 34 fractures showed an injury of foramina or canal. Conclusion : The new classification allowes the differentiation of fractures of less mechanical importance and a risk assessment for possible polymethyl methacrylate leaks during sacroplasty in the direction of the neurological structures. In addition, identification of instable fractures in need for laminectomy and surgical stabilization is possible.

• Progress in Superconductivity and Cryogenics
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• v.19 no.1
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• pp.47-50
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• 2017

Carbon is proven to be very effective in pinning the magnetic vortices and improving the superconducting performance of $MgB_2$ at high fields. In this work, we have used polymethyl methacrylate (PMMA) polymer as a safe and cost effective carbon source. The effects of molecular weight of PMMA on crystal structure, microstructure as well as on superconducting properties of $MgB_2$ were studied. X-ray diffraction analysis revealed that there is a noticeable shift in (100) and (110) Bragg reflections towards higher angles, while no shift was observed in (002) reflections for $MgB_2$ doped with different molecular weights of PMMA. This indicates that carbon could be substituted in the boron honeycomb layers without affecting the interlayer interactions. As compared to undoped $MgB_2$, substantial enhancement in $J_c(H)$ properties was obtained for PMMA-doped $MgB_2$ samples both at 5 K and 20 K. The enhancement could be attributed to the effective carbon substitution for boron and the refinement of crystallite size by PMMA doping.

• Journal of Technologic Dentistry
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• v.41 no.4
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• pp.263-269
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• 2019

Purpose: The purpose of this study was to evaluate the internal fitness of prostheses fabricated with non-contact extra-oral scanner and those fabricated with intra-oral video scanner, with a comparative accuracy analyses of their precision and trueness. Methods: A polymethyl methacrylate (PMMA) model was fabricated by replicating a master model. The prostheses in the first group were fabricated based on the PMMA model with an intra-oral video scanner (IVS group). Following the fabrication of work models with Type IV Stone that were based on the PMMA model, the prostheses in the second group were fabricated with a non-contact extra-oral scanner (ENB group). The precision and trueness of the prostheses were calculated from comparisons of the three-dimensional images of the internal surfaces of the prostheses and those of the master model. Kruskal-Wallis tests were used to determine the statistical significance, with the level of type 1 error set at 0.05. Results: Trueness (P < 0.009) and precision (P < 0.001) did not differ significantly between the ENB and IVS groups. The IVS group exhibited lower trueness values and larger precision values than the ENB group. Conclusion: Although no significant differences were found in the internal fitness of the prostheses that were fabricated by the two different scanners, the intraoral video scanner-fabricated prostheses had better trueness, whereas the non-contact extra-oral scanner-fabricated prostheses had better precision.