• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.6
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• pp.1931-1940
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• 1996

The effect of periodic passing wake on the film-coolant flow issuing normally from a flat plate was investigated experimentally. The passing wake was generated by rotating thin circular bars. Depending on the rotational direction the test plate could be simulated as a pressure surface or a suction surface of a gas turbine blade. The phase-averaged velocity components were measured using an X-type hot-wire probe. The Reynolds number based on the free-stream velocity and injection hole diameter was 23, 500 and the velocity ratio which is the ratio of film coolant velocity to free-stream velocity was 0.5. The velocity-triangle induced by the wake was similar to that induced by the one generated at the blade trailing edge. The vertical velocity component induced by the passing wake, which approaches to the suction surface and moves away from the pressure surface, played a dominant role in the variation of the flow field. The variation in the phase-averaged velocity on the pressure surface was greater than on the suction surface, but the turbulence kinetic energy variation on the suction surface appeared larger than on the pressure surface.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.6
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• pp.531-540
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• 2007

While determination of the solid propellant burning rates by ultrasound, it has been reported that the frequent data scatters were caused by two major factors; 1) variation in the acoustical properties, and 2) non-linear burning of a solid propellant sample under investigation. This work is carried out for the purpose of investigating the effect of non-linear burning of solid propellant samples. Specifically, we propose an ultrasonic measurement model that can predict the reflections from solid propellant surfaces with non-linear burning by the combination of two ingredients; 1) a pulse-echo ultrasonic measurement model for a planar, circular reflector imbedded in the second medium in an immersion set-up, and 2) an efficient model of non-linear burning surfaces with a number of small, planar circles. Then, we demonstrate the capability of the proposed measurement model by simulation of the surface echo signals from four different burning surfaces that have been generated by the combination of two factors; the base shape (flat or paraboloidal) and the surface roughness (perfectly smooth or randomly rough). From the simulation presented here, we can confirm the fact that the non-linear burning of the propellant can cause the waveform change of the burning surface echo and the corresponding spectrum variation.

• Restorative Dentistry and Endodontics
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• v.44 no.2
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• pp.13.1-13.9
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• 2019

Objectives: This study evaluated the effect of dentin pretreatment with silver nanoparticles (SNPs) and chlorhexidine (CHX) on the microshear bond strength (${\mu}SBS$) durability of different adhesives to dentin. Materials and Methods: Occlusal surfaces of 120 human molars were ground to expose flat dentin surfaces. The specimens were randomly assigned to six groups (n = 20). Three groups (A, B, and C) were bonded with Adper Single Bond 2 (SB) and the other groups (D, E, and F) were bonded with Clearfil SE Bond (SEB). Dentin was pretreated with CHX in groups B and E, and with SNPs in groups C and F. The specimens were restored with Z250 composite. Half of the bonded surfaces in each group underwent ${\mu}SBS$ testing after 24 hours and the other half was tested after 6 months of water storage. Results: SNP application was associated with a higher ${\mu}SBS$ than was observed in the CHX and control groups for SEB after 24 hours (p < 0.05). A significantly lower ${\mu}SBS$ was observed when no dentin pretreatment was applied compared to dentin pretreatment with CHX and SNPs for SB after 24 hours (p < 0.05). The ${\mu}SBS$ values of the 6-month specimens were significantly lower than those obtained from the 24-hour specimens for all groups (p < 0.05). This decrease was much more pronounced when both adhesives were used without any dentin pretreatment (p < 0.05). Conclusions: SNPs and CHX reduced the degradation of resin-dentin bonds over a 6-month period for both adhesive systems.

• The Journal of Korean Academy of Prosthodontics
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• v.51 no.1
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• pp.27-32
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• 2013

Purpose: The aims of this study were to evaluate the effect of a resin coating on the shear bond strength of indirect composite restoration bonded to dentin with a self adhesive resin cement and to compare the shear bond strength with that of a conventional resin cement. Materials and methods: The occlusal enamels of thirty six extracted noncarious human molars were removed until the dentin flat surfaces of the teeth were exposed. Then, they were divided into 3 groups. The dentin surfaces of group 1 and 3 were left without any conditioning, while the dentin surfaces of group 2 were resin-coated with Clearfil SE bond and a flowable resin composite, Metafil Flo. After all specimens were temporized for 24 hours, indirect composite resin blocks fabricated by Tescera were bonded to dentins by Unicem for group 1 and 2, and by Panavia F for group 3. After 48 hours of water storage, shear bond strengths were measured. The data was analyzed with one-way analysis of variance and multiple comparison test (Tukey method). Results: The shear bond strengths of Unicem applied to resin coated dentin surfaces were significantly higher than those of Unicem and Panavia F used to uncoated dentin surfaces (P<.0001). Conclusion: Application of a resin coating to the dentin surface significantly improved the shear bonding strength of a self adhesive resin cement in indirect restoration.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 2000.06a
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• pp.165-177
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• 2000

Ceramic on ceramic total hip prostheses are developed to apply to young patients because lifetime of polyethylene joint prostheses is limited by loosening due to biological response. As mating faces of all-ceramic joint must be highly conformed to reduce stress concentration, wear properties of flat surfaces are investigated in this study. Through wear tests at 2 MPa of contact pressure and 36 mm/s of sliding velocity, alumina and silicon carbide keep low wear rate, high hardness and smooth surface. Soft surface film was detected after the test in bovine serum. This suggests that boundary lubrication is effective to reduce wear in all-ceramic joint.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.10b
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• pp.67-68
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• 2002

The properties and mechanism of silicon protuberance and groove processing by diamond tip sliding using atomic force microscope (AFM) in atmosphere were studied. To control the height of protuberance and the depth of groove, the processed height and depth depended on load and diamond tip radius were evaluated. Nanoprotuberances and grooves were fabricated on a silicon surface by approximately 100-nm-radius diamond tip sliding using an atomic force microscope in atmosphere. To clarify the mechanical and chemical properties of these parts processed, changes in the protuberance and groove profiles due to additional diamond tip sliding and potassium hydroxide (KOH) solution etching were evaluated. Processed protuberances were negligibly removed, and processed grooves were easily removed by additional diamond tip sliding. The KOH solution selectively etched the unprocessed silicon area. while the protuberances, grooves and flat surfaces processed by diamond tip sliding were negligibly etched. Three-dimensional nanofabrication is performed in this study by utilizing these mechanic-chemically processed parts as protective etching mask for KOH solution etching.

• Korean Journal of Computational Design and Engineering
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• v.1 no.1
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• pp.45-55
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• 1996

This paper presents a new approach to non-parametric modeling of cutter swept surface (CSS) for cutting simulation. Instead of explicitly modeling cutter swept volumes, silhouette curves of the cutter surface are utilized in computing the z-value of the CSS at a grid point on the x,y-plane. The non-parametric evaluation of the CSS constitutes the integral part of 3-axis cutting simulation. The proposed method is more efficient than the existing ones in the case of conventional cutters (i.e., ball-end mills and flat-end mills), and more importantly, it enables the non-parametric modeling of the CSS for the round-end mills which was not possible with the existing methods.

• Journal of the Korean Chemical Society
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• v.60 no.5
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• pp.310-316
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• 2016

Oxidation-reduction cycling (ORC) procedures are widely used for cleaning nanoparticle surfaces when investigating their electrocatalytic activities. In this work, the effect of ORC on the surface structures and electrocatalytic oxygen reduction activity of Au electrodes is analyzed. Different structural changes and variations in electrocatalysis are observed depending on the initial structure of the Au electrodes, such as flat bulk, nanoporous, nanoplate, or dendritic Au. In particular, dendritic Au structures lost their sharp-edge morphology during the ORC process, resulting in a significant decrease in its electrocatalytic oxygen reduction activity. The results shown in this paper provide an insight into the pretreatment of nanoparticle-based electrodes during investigation of their electrocatalytic activities.

• Journal of the Korean institute of surface engineering
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• v.28 no.2
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• pp.101-109
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• 1995

Electroetching rates of $FeCl_3$ solution were increased according to increasing solution temperature. Activation energy of electroetching at Be'36 and 5A/$dm^2$ condition was 28.3Kcal and also, at Be'46 and 5A/dm$^2$ condition was 33.2Kcal. At Be'36 concentration of $FeCl_3$ solution, electroetching rate were more higher than at Be'46 concentration. Surfaces of etched grooves obtained at 8A/$dm^2$ or higher current density in 46 Be' concentration of $FeCl_3$ solution were observed to be flat and smooth owing to suppressing chemical etching reaction. Distinctly etched boundaries became to be appeared at 2A/$dm^2$ in Be'41 electroetching condition by differential effects. In case of applying 8A/$dm^2$ current density to Be'46 of $FeCl_3$ solution, etching depth were 4 times and side etching were 6 times more than chemical etching case respectively.

• Solar Energy
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• v.12 no.1
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• pp.88-94
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• 1992

A small-scale solar collector and tracking system, using a Fresnel lens of $0.5m^2$, and novel compact fluidized-bed solar receiver[FBR] of closed type has been developed for high temperature solar gas heating. The FBR was improved in carrying over of SiC powder and thermo-siphon effect. The maximum outlet air temperature of 1140K and the maximum thermal efficiency of 64% were obtained. The present FBR's operated efficiently at extremely high temperatures in comparison with conventional solar receivers, composed of flat or tubular solid surfaces.