Doh-hyung Riu | Seoul National University of Science and Technology (original) (raw)

Papers by Doh-hyung Riu

Through the observation of wear scar of two ceramic materials, microstructural wear mechanisms wa... more Through the observation of wear scar of two ceramic materials, microstructural wear mechanisms was investigated. As for the -5 vol% SiC nanocomposite, the grain boundary fracture was suppressed by the presence of SiC nano-particles. The intragranular SiC particles have inhibited the extension of plastic deformation through the whole grain. Part of plastic deformation was accommodated around SiC particles, which made a cavity at the interface between SiC and matrix alumina. On the other hand, gas-pressure sintered silicon nitride showed extensive grain boundary fracture due to the thermal fatigue. The lamination of wear scar was initiated by the dissolution of grain boundary phase. These two extreme cases showed the importance of microstructures in wear behavior.

Carbon, 2019

Abstract By iodine curing of polycarbosilane fibers followed by sintering under a controlled atmo... more Abstract By iodine curing of polycarbosilane fibers followed by sintering under a controlled atmosphere of carbon monoxide, a unique strategy is developed for the in situ growth of graphene networks inside silicon carbide fibers. In the resulting fibers, three-dimensionally interconnected few-layered graphene sheets are well-dispersed in the nanocrystalline SiC, allowing for fast electron transport through the graphene networks. The roles of iodine and carbon monoxide in fabricating the graphene-network embedded SiC fibers are elucidated. The distinct evolution of graphene structure was observed in the iodine-treated Si(O)C using transmission electron microscopy and Raman spectroscopy. The iodine incorporated in the fibers induces the sp2-hybridization of carbon, generating carbon–carbon double bonds and graphene seeds such as reduced graphene oxide, which are supposed to grow into graphene layers at elevated temperatures. Carbon monoxide is employed as a component of the atmospheric gas mixture during the decomposition of Si(O)C to suppress the evolution of SiO and CO gases, thereby restraining coarsening of SiC nanocrystallites and maintaining the integrity of the graphene network. These processes pave the way for designing graphene structures in polymer-derived ceramic materials for a broad range of applications.

Journal of the Korean Ceramic Society, 2006

ACS Applied Materials & Interfaces, 2017

Nature communications, Jan 24, 2018

High catalytic efficiency in metal nanocatalysts is attributed to large surface area to volume ra... more High catalytic efficiency in metal nanocatalysts is attributed to large surface area to volume ratios and an abundance of under-coordinated atoms that can decrease kinetic barriers. Although overall shape or size changes of nanocatalysts have been observed as a result of catalytic processes, structural changes at low-coordination sites such as edges, remain poorly understood. Here, we report high-lattice distortion at edges of Pt nanocrystals during heterogeneous catalytic methane oxidation based on in situ 3D Bragg coherent X-ray diffraction imaging. We directly observe contraction at edges owing to adsorption of oxygen. This strain increases during methane oxidation and it returns to the original state after completing the reaction process. The results are in good agreement with finite element models that incorporate forces, as determined by reactive molecular dynamics simulations. Reaction mechanisms obtained from in situ strain imaging thus provide important insights for improvi...

Journal of Materials Science and Engineering B, 2016

ZSM-5 crystals were grown on the SiC fiber surface which was prepared by melt-spinning of polycar... more ZSM-5 crystals were grown on the SiC fiber surface which was prepared by melt-spinning of polycarbosilane, silicon based preceramic polymer. To improve the bonding property between ZSM-5 crystals and SiC fiber, 200 nm of SiO 2 layer was formed on the fiber surface by simple oxidation before hydrothermal reaction in the autoclave because it was compositionally similar to ZSM-5 and relatively easy to form on the SiC fiber. During the process, 1 μm of zeolite crystals were very uniformly grown on the fiber surface. Weak X-ray diffraction (XRD) peaks observed between 10° and 30° showed that crystal structure of them were accorded to ZSM-5 structure. They were broken away with SiC fiber support during fracture test which mean that they were strongly bonded to fiber surface. This type of SiC based catalysts are expected for high-temperature gas-phase reaction, especially strong acid base conditions.

Korean Journal of Metals and Materials, 2012

Calcium phosphate ceramic composites, consisting of hydroxyapatite(HA) and tricalcium phosphate (... more Calcium phosphate ceramic composites, consisting of hydroxyapatite(HA) and tricalcium phosphate (TCP), were fabricated by solid state sintering in order to investigate the effect of their initial compositions on microstructural evolutions and biocompatibility. All the sintered calcium phosphate ceramics exhibited almost full densification, while the grain growth of the composites increased with an increasing TCP content in the green body. The TCP phase transformed into a Ca-deficient HA phase during sintering via the diffusion of calcium ions from the HA phase into the TCP phase. The phases formed in the composites significantly affected the biocompatibility of the composites. The HA-matrix ceramic composites with TCP had a better cellular response than the pure HA ceramics, presumably due to the newly formed Ca-deficient HA.

Journal of the European Ceramic Society, Sep 1, 2000

The eects of chromia (Cr 2 O 3) additions on the microstructural evolution and the mechanical pro... more The eects of chromia (Cr 2 O 3) additions on the microstructural evolution and the mechanical properties of alumina (Al 2 O 3) were investigated. When small amounts (<5 mol%) of Cr 2 O 3 were added in samples hot pressed at 1500 C, the grain size distribution became bimodal; large platelike grains were dispersed in a relatively small grained matrix. The large grains were composed of a core region that is free of Cr and a surrounding shell region rich in Cr. The interface between the core and the shell was composed of mis®t dislocations. The high diusion rate of Cr ions through the surface of alumina was attributed to this microstructural evolution. The mechanical properties of the specimens were strongly in¯uenced by this microstructural change. The fracture toughness and the¯aw tolerance (R-curve behavior) of Al 2 O 3 were improved markedly by the formation of the large platelike grains. The hardness and the elastic modulus also increased, however, the fracture strength decreased by the addition of Cr 2 O 3 .

RSC Adv., 2015

Iodisation of polycarbosilane under an oxygen-free atmosphere, even at room temperature, leads to... more Iodisation of polycarbosilane under an oxygen-free atmosphere, even at room temperature, leads to the cleavage of Si–H and C–H bonds and construction of –Si–C– networks, thereby allowing oxygen-free curing for SiC fibres with low oxygen contents.

Materials Science Forum, 2014

Drug loaded polyurethane (PU) thin layer was prepared on the silicone tube by electrospinning tec... more Drug loaded polyurethane (PU) thin layer was prepared on the silicone tube by electrospinning technique. Microstructure of the PU layer was varied from nanoporous web to dense coating depending on the polymer solution concentration and the amount of drug loaded. It can be easily adjusted the coating thickness and porosity accurately and controlled the drug loading and releasing properties. However, adhesive strength between PU layer and silicone tube was very weak and easily broken away and white turbidity also another important problem. So, surface of silicone tube was atmospheric-pressure plasma (APP) treated for improving the adhesive and removing white turbidity phenomenon.

Archives of Metallurgy and Materials, 2015

We modified the surfaces of F-doped SnO2 thin films using laser patterning to improve the photovo... more We modified the surfaces of F-doped SnO2 thin films using laser patterning to improve the photovoltaic properties of dye-sensitized solar cells. To do so, we varied the laser power density and the distance between laser-patterned lines. First, we investigated three power densities. Higher densities led to higher sheet resistances owing to increases in surface roughnesses. The lowest power density increased surface roughness without electrical degradation. Next, we explored three line spacings at a fixed power density. The films with the narrowest spacing exhibited the highest power conversion efficiency (~7.00%), the highest short-circuit photocurrent density (16.28 mA/cm2), and a good fill factor (58.82%).

Korean Chemical Engineering Research, 2015

Nanostructured TiO 2-SiO 2 materials have widely been used as anti-reflecting coating, optical-ch... more Nanostructured TiO 2-SiO 2 materials have widely been used as anti-reflecting coating, optical-chemical sensors and catalysts because of their superior optical and thermal properties as well as chemical durability. Web type SiO 2 microfibers with nano-crystalline TiO 2 were prepared by electrospinning of Ti-PCS mixed solution and oxidation controlled heat-treatment, rather simple than sol-gel process. Nano-crystalline anatase phase were formed for the heattreatment up to 1200°C and they were finely dispersed in the amorphous SiO 2 matrix.

Journal of the Korean Ceramic Society, 2013

SiC hollow fiber was fabricated by curing, dissolution and sintering of Al-PCS fiber, which was m... more SiC hollow fiber was fabricated by curing, dissolution and sintering of Al-PCS fiber, which was melt spun the polyaluminocarbosilane. Al-PCS fiber was thermally oxidized and dissolved in toluene to remove the unoxidized area, the core of the cured fiber. The wall thickness (t wall) of Al-PCS fiber was monotonically increased with an increasing oxidation curing time. The Al-PCS hollow fiber was heat-treated at the temperature between 1200 and 2000°C to make a SiC hollow fibers having porous structure on the fiber wall. The pore size of the fiber wall was increased with the sintering temperature due to the decomposition of the amorphous SiC x O y matrix and the growth of β-SiC in the matrix. At 1400°C, a nano porous wall with a high specific surface area was obtained. However, nano pores grew with the grain growth after the thermal decomposition of the amorphous matrix. This type of SiC hollow fibers are expected to be used as a substrate for a gas separation membrane.

Joining Si3N4 and Al2O3 using 15 layers has been achieved by a unique approach that introduces Si... more Joining Si3N4 and Al2O3 using 15 layers has been achieved by a unique approach that introduces SiAlON polytypoids as a functionally graded material (FGM) bonding layer. Previously, the hot press sintering of multilayered FGM with 20 layers, each 500 µm thick, has been achieved successfully. In the present study, the number of layers for FGM was reduced from 20 to 15 to increase optimization. Samples were fabricated by hot pressing at 48 MPa during the temperature ramp to 1650°C and cooling at 2°C/min to minimize residual stresses from sintering. Moreover, a finite element method (FEM) program based on the maximum principal stress theory and the maximum tensile stress theory was applied to design optimized and reduced FGM layers that produced a crack-free joint. The sample had a 3-dimensional cylindrical shape that was transformed to a 2-dimensional axisymmetric mode. By determining the expected thermal stress from the calculated elastic modulus and coefficient of thermal expansion, we were able to predict and prevent damage due to thermal stresses. These analyses are especially useful for FGM samples where it is very difficult to measure the residual stresses experimentally. Finally, oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were deflected at the SiAlON layers, implying weak interfaces. In other areas, cracks were not deflected, implying strong interfaces.

Materials Letters, 2014

Porous silicon oxycarbide (SiOC) beads were prepared by freeze-drying of water-in-oil (w/o) emuls... more Porous silicon oxycarbide (SiOC) beads were prepared by freeze-drying of water-in-oil (w/o) emulsion, containing water and polycarbosilane (PCS) dissolved p-xylene in the presence of sodium xylenesulfonate (SXS) as an emulsifier. The emulsion was frozen by being dropped onto a liquid N 2 bath, which resulted in 1~2 mm sized beads. After curing at 200°C for 1 h in air and subsequent pyrolysis at 800°C for 1 h in an Ar gas flow, porous SiOC beads were obtained. Freeze-dried beads showed lamellae-shaped macro-pore structures at a moderate freezing rate due to a phase separation behavior of PCS during freezing, while no lamellae pores were formed at a very high freezing rate. Water droplets formed in w/o emulsion converted to spherical pores after drying. The combined processes of producing PCS emulsion and freeze-drying of emulsion resulted in two types of macro-pores: lamellaeshaped and spherical pores. Meso-pores, of which specific surface area and average pore size were 71.5 m 2 •g-1 and 4.85 nm, respectively, were formed in the SiOC strut.

MATERIALS TRANSACTIONS, 2008

Numerical analysis method was used to reduce the number of functionally graded material (FGM) lay... more Numerical analysis method was used to reduce the number of functionally graded material (FGM) layers for joining Si 3 N 4-Al 2 O 3 using polytypoid interlayer by estimating the position of crack. In the past, hot press sintering of multi-layered FGMs with 20 layers of thickness 500 mm each have been fabricated successfully. In this paper, thermal residual stresses were calculated using finite element method (FEM) to find the optimized number of layers and its thicknesses of FGM joint. The number of layers for FGM was reduced to 15 layers from 20 layers. Thicknesses were varied to minimize residual stresses within the layers while reducing the number of FGM layers. The damage caused by thermal residual stress was estimated using maximum principal stress theory and maximum tensile stress theory. The calculated maximum stress was found to be axial stress of 430 MPa around 90% 12H/10% Al 2 O 3 area. For each case, calculated strength of each FGM layer by linear rule of mixture was compared with computed thermal residual stresses. Thermal analysis results correctly predicted the position of crack, and this position agreed well with fabricated joints. Therefore, this numerical analysis method can be applied to reduced FGM layers of crack free joint. Finally, new composition profile of crack free joint was proposed using FGM method.