• Journal of the Korean Society for Heat Treatment
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• v.25 no.1
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• pp.6-13
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• 2012

HVOF thermal spray coating of 80%WC-CoFe powder is one of the most promising candidate for the replacement of the traditional hard chrome plating and hard ceramics coating because of the environmental problem of the very toxic $Cr^{6+}$ known as carcinogen by chrome plating and the brittleness of ceramics coatings. 80%WC-CoFe powder was coated by HVOF thermal spraying for the study of durability improvement of the high speed spindle such as air bearing spindle. The coating procedure was designed by the Taguchi program, including 4 parameters of hydrogen and oxygen flow rates, powder feed rate and spray distance. The surface properties of the 80%WC-CoFe powder coating were investigated roughness, hardness and porosity. The optimal condition for thermal spray has been ensured by the relationship between the spary parameters and the hardness of the coatings. The optimal coating process obtained by Taguchi program is the process of oxygen flow rate 34 FRM, hydrogen flow rate 57 FRM, powder feed rate 35 g/min and spray distance 8 inch. The coating cross-sectional structure was observed scanning electron microscope before chemical etching. Estimation of coating porosity was performed using metallugical image analysis. The Friction and wear behaviors of HVOF WC-CoFe coating prepared by OCP are investigated by reciprocating sliding wear test at $25^{\circ}C$ and $450^{\circ}C$. Friction coefficients (FC) of coating decreases as sliding surface temperature increases from $25^{\circ}C$ to $450^{\circ}C$.

• Polymer(Korea)
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• v.25 no.2
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• pp.186-198
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• 2001

pH/Thermal sensitive copolymers with the various acidic comonomer compositions composed of N-isopropylacrylamide (NIPAAm) with acrylic acid (AAc), 2-acrylamido glycolic acid (AAmGAc), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) were synthesized by free radial polymerization. In this study, to characterize the effect of different acidic comonomer composition and pH on the lower critical solution temperature (LCST) behaviors of their copolymers. phase transition experiments were performed with a thermo-optical analyzer (TOA). The phase transition temperature (T$^{p}$ ) of aqueous poly(NIPAAm-co-AAc) solution was lowered with increasing the ionization of the acid group in AAc, that is, the ionized state induced the electrostatic repulsion of ionized groups. In contrast, when AAmGAc was introduced into PNIPAAm, T$^{p}$ was little changed at pH 1-3, whereas climbed up significantly from pH 1 to pH 3. In the range of pH 6-10, Tp was lower than that of pH 3-5. This result was considered to be \"Ionic Screen Effect\" and this effect had been also observed in the case of poly(NIPAAm-co-AMPS).-co-AMPS).

• Journal of Adhesion and Interface
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• v.10 no.4
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• pp.191-198
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• 2009

UV-curable aliphatic epoxy acrylates were prepared by the reaction of glycerol diglycidyl ether (GDE) with 2-carboxyethyl acrylate (2-CEA) or 2-hydroxyethyl acrylate (2-HEA). The structures of the epoxy acrylates were characterized by FT-IR, $^1H$-NMR, and $^{13}C$-NMR and the yield was obtained by prep-LC. The UV- and the thermal-curing behaviors of the product were investigated using photo-DSC and DSC, respectively. The reactivity of 2-CEA was higher than 2-HEA and the yield of the product (GEA-C) which was prepared using 2-CEA was about 83%. The maximum UV-curing time ($T_{max}$) of the GEA-C contained non-reactive components and by-product was about 10 seconds. The GEA-C showed low color difference (${\Delta}E^*$), low viscosity, and good thermal stability - its value was 2.51, 192 cps, and $299^{\circ}C$ (at 5% weight loss), respectively. The activation energies ($E_a$) of thermal-curing reaction calculated from Kissinger and Ozawa-Flynn-Wall method were 91~92 kJ/mol.

• Elastomers and Composites
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• v.46 no.3
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• pp.237-244
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• 2011

Five natural rubber (NR) composites with different reinforcing systems of unfilled, carbon black, carbon black with silane coupling agent, silica, and silica with silane coupling agent were thermally aged and change of the crosslink densities by the accelerated thermal aging was investigated. The crosslink densities on the whole increased as the aging time elapsed irrespective of the reinforcing systems. The crosslink density changes became noticeable by increasing the aging temperature. For carbon black-filled composites, the silane coupling agent made the crosslink density change to be increased. For silica-filled composites, however, the silane coupling agent made the crosslink density increment reduced at 60 and $70^{\circ}C$ and it hardly affect the degree of the crosslink density change at 80 and $90^{\circ}C$. The activation energies for the crosslink density changes of the carbon black-filled samples increased continuously in a logarithmic fashion, whereas that of the silica-filled one showed a quasi-steady state ranges at aging times of 30-150 days. The activation energy of the unfilled sample increased exponentially with the aging time. The experimental results were explained with sulfur donation from the silane coupling agent, surface modification of the filler by the silane coupling agent, adsorption of curative residues on the silica surface, and release of the adsorbed curative residues.

• Tunnel and Underground Space
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• v.33 no.3
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• pp.189-207
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• 2023

In the present study, the thermoshearing experiment on a rough rock fracture were modeled using a three-dimensional grain-based distinct element model (GBDEM). The experiment was conducted by the Korea Institute of Construction Technology to investigate the progressive shear failure of fracture under the influence of thermal stress in a critical stress state. The numerical model employs an assembly of multiple polyhedral grains and their interfaces to represent the rock sample, and calculates the coupled thermo-mechanical behavior of the grains (blocks) and the interfaces (contacts) using 3DEC, a DEM code. The primary focus was on simulating the temperature evolution, generation of thermal stress, and shear and normal displacements of the fracture. Two fracture models, namely the mated fracture model and the unmated fracture model, were constructed based on the degree of surface matedness, and their respective behaviors were compared and analyzed. By leveraging the advantage of the DEM, the contact area between the fracture surfaces was continuously monitored during the simulation, enabling an examination of its influence on shear behavior. The numerical results demonstrated distinct differences depending on the degree of the surface matedness at the initial stage. In the mated fracture model, where the surfaces were in almost full contact, the characteristic stages of peak stress and residual stress commonly observed in shear behavior of natural rock joints were reasonably replicated, despite exhibiting discrepancies with the experimental results. The analysis of contact area variation over time confirmed that our numerical model effectively simulated the abrupt normal dilation and shear slip, stress softening phenomenon, and transition to the residual state that occur during the peak stress stage. The unmated fracture model, which closely resembled the experimental specimen, showed qualitative agreement with the experimental observations, including heat transfer characteristics, the progressive shear failure process induced by heating, and the increase in thermal stress. However, there were some mismatches between the numerical and experimental results regarding the onset of fracture slip and the magnitudes of fracture stress and displacement. This research was conducted as part of DECOVALEX-2023 Task G, and we expect the numerical model to be enhanced through continued collaboration with other research teams and validated in further studies.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.12
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• pp.1098-1125
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• 2000

A review on the papers published in the Korean Journal of Air-Conditioning and Refrigerating Engineering in 1998 and 1999 has been done. Focus has been put on current status of research in the aspect of heating, cooling, ventilation, sanitation and building environment. The conclusions are as follows. 1) A review of the recent studies on fluid flow, turbomachinery and pipe-network shows that many experimental investigations are conducted in applications of impingement jets. Researches on turbulent flows, pipe flows, pipe-networks are focused on analyses of practical systems and prediction of system performance. The results of noise reduction in the turbomachinery are also reported. 2) A review of the recent studies on heat transfer analysis and heat exchanger shows that there were many papers on the channel flow with the application to the design of heat exchanger in the heat transfer analysis. Various experimental and numerical papers on heat exchanger were also published, however, there were few papers available for the analysis of whole system including heat exchanger. 3) A review of the recent studies on heat pump system have focused on the multi-type system and the heat pump cycle to utilize treated sewage as the heat source. The defrosting and the frosting behaviors in the fin-tube heat exchanger is experimentally examined by several authors. Several papers on the ice storage cooling system are presented to show the dynamic simulation program and optimal operation conditions. The study on the micro heat pipes for the cooling of high power electronic components is carried out to examine the characteristics of heat and mass transfer processed. In addition to these, new type of separate thermosyphon is studied experimentally. 4) The recent studies on refrigeration/air conditioning system have focused on the system performance and efficiency for new alternative refrigerants. New systems operating with natural refrigerants are drawing lots of attention. In addition to these, evaporation and condensation heat transfer characteristics of traditional and new refrigerants are investigated for plain tubes and also for microfin tubes. Capillary tubes and orifice are main topics of research as expansion devices and studies on thermophysical properties of new refrigerants and refrigerant/oil mixtures are widely carried out. 5) A review of the recent studies on absorption cooling system shows that numerous experimental and analytical studies on the improvement of absorber performance have been presented. Dynamic analysis of compressor have been performed to understand its vibration characteristics. However research works on tow-phase flow and heat transfer, which could be encountered in the refrigeration system and various phase-change heat exchanger, were seemed to be insufficient. 6) A review of recent studies on duct system shows that the methods for circuit analysis, and flow balancing have been presented. Researches on ventilation are focused on the measurement of ventilation efficiency, and variation of ventilation efficiency with ventilation methods by numerous experimental and numerical studies. Furthermore, many studies have been conducted in real building in order to estimate indoor thermal environments. Many research works to get some information for cooling tower design have been performed but are insufficient. 7) A review on the recent studies on architectural thermal environment and building mechanical systems design shows that thermal comfort analysis is sitting environment, thermal performance analysis of Korean traditional building structures., and evaluation of building environmental load have been performed. However research works to improve the performance of mechanical system design and construction technology were seemed to be insufficient.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.21 no.6
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• pp.204-211
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• 2020

The high-velocity oxy-fuel (HVOF) thermal spraying coating technique has been considered a promising replacement for traditional electrolytic hard chrome plating (EHC), which caused environmental pollution and lung cancer due to toxic Cr⁶⁺. In this paper, two types of cermet coatings were prepared by HVOF spraying: WC-CoCr and WC-CrC-Ni coatings. The produced coatings were analyzed extensively in terms of the micro-hardness, porosity, crystalline phase and microstructure using a hardness tester, optical microscopy, X-ray diffraction, and scanning electron microscopy (including energy dispersed spectroscopy (EDS)), respectively. The wear and friction behaviors of the coatings were evaluated comparatively by reciprocating sliding wear tests at 25 ℃, 250 ℃, and 450 ℃. The results revealed correlations among the microstructures, metallic binder matrixes, porosities, and wear performance of the coatings. For example, WC-CoCr coatings showed better sliding wear resistance than WC-CrC-Ni coatings, regardless of the test temperature due to the more homogeneous microstructure, Co-rich, Cr-rich metallic binder matrix, and lower porosity.

• Applied Chemistry for Engineering
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• v.19 no.5
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• pp.504-511
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• 2008

Two kinds of nitroazobenzene derivatives: 4-{4'-(nitrophenylazo)phenoxy}alkanoic acids (NAAn, n = 2~8, 10, number of methylene units in the alkyl chain) and 4-{4'-(nitrophenylazo)phenoxy}alkanoyl chlorides (NACn, n = 2~8, 10) were synthesized, and their thermotropic liquid crystalline behaviors were investigated. NAA6 formed an enantiotropic nematic phase, while the remainders, except NAA2, showed monotropic nematic phases. Isotropic-nematic transition temperature ($T_{iN}$) and change of entropy (${\Delta}S$) at $T_{iN}$ for both of NAAn and NACn varied by the change of n, and pronounced odd-even effects of n were also observed. However, the $T_{iN}$ and ${\Delta}S$ values of NAAn were much higher than those of NACn. This fact may be attributed to the hydrogen bonding between carboxyl groups. Thermal properties and degree of order in the mesophase and the magnitude of the odd-even effects of both NAAn and NACn were significantly different from those reported for 4-(alkoxy)-4'-nitroazobenzenes. It was discussed in terms of the differences in the molecular anisotropy and the temperature-dependent flexibility of the substituted groups.

• Journal of Adhesion and Interface
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• v.4 no.3
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• pp.33-40
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• 2003

In this work, diglycidyl ether of bisphenol A (DGEBA)/polyetherimide (PEI) blends were cured using 4,4-diaminodiphenyl methane (DDM). And the effects of addition of different PEI contents to neat DGEBA were investigated in the thermal properties and fracture toughness of the blends. The contents of contents of containing PEI were varied in 0, 2.5, 5, 7.5, and 10 phr. The cure activation energies ($E_a$) of the cured specimens were determined by Kissinger equation and the mechanical interfacial properties of the specimens were performed by critical stress intensity factor ($K_{IC}$). Also their surfaces were examined by using a scanning electron microscope (SEM) and the surface energetics of blends was determined by contact angles. As a result, $E_a$ and $K_{IC}$ showed maximum values in the 7.5 phr PEI. This result was interpreted in the increment of the network structure of DGEBA/PEI blends. Also, the surface energetics of the DGEBA/PEI blends showed a similar behavior with the results of $K_{IC}$. This was probably due to the improving of specific or polor component of the surface free energy of DGEBA/PEI blends, resulting in increasing the hydrogen bonding of the hydroxyl and imide groups of the blends.

• Membrane Journal
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• v.19 no.4
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• pp.261-267
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• 2009

Polyimide which is the glassy polymer has high chemical resistance, thermal stability and high mechanical property. In this study, the polyimide hollow fiber membranes were prepared by the dry-jet wet phase inversion in order to investigate the permeation porperties of the $H_2S$ and $CH_4$. The morphology of prepared hollow fiber membranes and their permeation behaviors of $H_2S$ and $CH_4$ before and after silicon coating were evaluated. The permeance of $H_2S$ and $H_2S/CH_4$ selectivity increased due to plasticization with increasing the feed pressure. The permeance of KSM03b and selectivity of KSM03d were highest among the three type membranes used this experiments. The permeance decreased but the $H_2S/CH_4$ selectivity increased with increasing the air gap. The permeance reduced after silicon coating. However, the selectivity increased and the selectivity of KSM03d was 275 at 7 atm.