Soon Hyung Hong | Korea Advanced Institute of Science and Technology (original) (raw)

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The effect of initial lamellar structure of β heat treated Ti-6Al-4V alloy on the globularization... more The effect of initial lamellar structure of β heat treated Ti-6Al-4V alloy on the globularization behavior during the multi-step forging was investigated. Specimens of different lamellar thicknesses were upsetted and stretched by side pressing repeatedly, i.e. multi-step forging, at the sub-transus temperature to break down the lamellar structure. The microstructural changes after multi-step forging were analyzed in the light of globularization behavior. The results showed that the thick lamellar structure was more difficult to be transformed to homogeneous equiaxed structure than thin lamellar structure.

Giant magnetostrictive composites have attracted a great deal of attention by supplementing short... more Giant magnetostrictive composites have attracted a great deal of attention by supplementing shortcomings of monolithic Terfenol-D such as brittleness, eddy current loss and formability. Recently, infiltrated Terfenol-D/Epoxy composite has been developed as an alternative composite. This composite was fabricated by an unidirectional solidification of Terfenol-D followed by an infiltration of epoxy. The iron content in composite has been changed in order to control volume fraction of RFe 2 phase producing magnetostriction on Terfenol-D/Epoxy composite. The magnetostriction of both as-grown and infiltrated Terfenol-D/Epoxy composite was measured to confirm the effects of eutectic phase and heat treatment on magnetostriction. The enhancement of magnetostriction of Terfenol-D/Epoxy composite was mostly contributed by the eutectic phase through the hindering of movement and rotation of domain walls. The magnetostriction modelling of Terfenol-D/Epoxy composite was suggested, based on the change of texture and elastic modulus. The suggested model was in good agreements with the experimental results on the measurement of magnetostriction of Terfenol-D/Epoxy composite.

Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for... more Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for X-ray tube cathode. However, CNT field emission cathode has been shown unstable field emission and short life time due to field evaporation by high current density and detachment by electrostatic force. An alternative approach in this direction is the introduction of CNT yarn, which is a one dimensional assembly of individual carbon nanotubes bonded by the Van der Waals force. Because CNT yarn is composed with many CNTs, CNT yarns are expected to increase current density and life time for X-ray tube applications. In this research, CNT yarn was fabricated by spinning of a super-aligned CNT forest and was characterized for application to an X-ray tube cathode. CNT yarn showed a high field emission current density and a long lifetime of over 450 hours. Applying the CNT yarn field emitter to the X-ray tube cathode, it was possible to obtain micro-scale resolution images. The relationship between the field emission properties and the microstructure evolution was investigated and the unraveling effect of the CNT yarn was discussed.

Wide range of lead-free piezoelectric ceramic compositions have been investigated in order to rep... more Wide range of lead-free piezoelectric ceramic compositions have been investigated in order to replace (Pb,Zr) TiO 3 (PZT) owing to the concerns of environmental pollution. Among these reported lead-free compositions, (K 0.5 Na 0.5)NbO 3 (KNN) based ceramics have emerged as one of the promising candidate owing to their excellent piezoelectric properties and high Curie temperature (T C) [1-14]. However, KNN-based compositions show inferior stability of piezoelectric property as a function of temperature due to the presence of intermediate orthorhombic to tetragonal phase transition [10-14]. Figure 1 shows the variation of electromechanical coupling factor (k p) and dielectric constant as a function of temperature in KNNbased ceramics. As seen in Fig. 1a, high temperature coefficient of piezoelectric properties can be explained based upon the polymorphic phase transition in KNNbased ceramics. The peaks in k p and dielectric constant were located at the phase transition temperatures for all the KNN-based compositions. However, by noticing the changes in slope in Fig. 1, we can arrive at the possibility of compensating the trend since the phase transition temperatures shift with solid solution as shown in Fig. 1b. This lead us to the design of new microstructures such as island-matrix structure (type I) and layered structure (type II) as illustrated in Fig. 2a, b. The gradient in color in these figures indicates the effect of elemental diffusion between

We propose a new strategy for improving the charge selectivity of carbon nanotubes (CNTs) for org... more We propose a new strategy for improving the charge selectivity of carbon nanotubes (CNTs) for organic photovoltaic cells (OPVCs). The strategy involves the coating of an ultrathin layer of titanium suboxide (TiO x ) on CNTs by atomic layer deposition (ALD). ALD can facilitate that conformal and uniform coating of TiO x on CNT networks while preserving their nanoporous structure. We used the TiO x -coated CNT networks as an electron transport layer in inverted OPVCs. TiO x -coated CNTs can provide electrons with an extremely fast conductive path through CNTs and selectively block the holes by means of the hole-barrier property of the TiO x in OPVCs. The nanoporous structure of TiO x -coated CNT networks can improve the device performance of OPVCs due to synergetic effects of the electron selective transport property of TiO x and the high conductivity of CNTs. In addition, further improvement of device performance can be achieved by adding a hole transport layer between the active layer and the Au electrode.

Nanoporous metal foams have good electrical and thermal conductivities and potential catalytic ac... more Nanoporous metal foams have good electrical and thermal conductivities and potential catalytic activities because of their high surface areas. In this study, nanoporous cobalt foam was prepared by simple consolidation of a pearl-necklace-type CNT/Co 3 O 4 nanocomposite powder. During heat treatment of the pre-compacted powder in an inert atmosphere, Co 3 O 4 particles were reduced to cobalt metal and formed a three-dimensional, continuous nanoporous metallic structure. This nanoporous cobalt foam could be used as an excellent conducting framework because of the superior electrical conductivity of the metal. A Co/ Co(OH) 2 core–shell structure was prepared by coating Co(OH) 2 onto the nanoporous cobalt foam and using it as the electrode for a supercapacitor. Because of the high surface area of the nanoporous cobalt metal frame, the Co/Co(OH) 2 core–shell structure had a specific capacitance of 525 F g À1 at a current density of 0.5 A g À1 .

A B S T R A C T Carbon nanotube (CNT) reinforced nickel matrix (CNT/Ni) composites exhibiting a y... more A B S T R A C T Carbon nanotube (CNT) reinforced nickel matrix (CNT/Ni) composites exhibiting a yield strength (YS) of 710 MPa, about 3.7 times higher than monolithic nickel, have been processed by a molecular-level mixing process followed by spark plasma sintering (SPS). The enormous strength increase in these composites can be attributed to a homogeneous distribution of nanotubes in the nickel matrix coupled with the formation of well-bonded, high strength, contaminant-free nanotube/nickel interfaces, as revealed by high-resolution transmission electron microscopy. Such interfaces can effectively transfer load between nanotubes and nickel matrix in the CNT/Ni composites.

A B S T R A C T The precipitation hardening behavior of CNT/Al–Cu composites was investigated by ... more A B S T R A C T The precipitation hardening behavior of CNT/Al–Cu composites was investigated by characterization of microstructure and mechanical properties after aging heat treatment. It was found that CNTs accelerated the precipitation hardening behavior of CNT/Al–Cu composites due to the generation of excess dislocations. The CNT/Al–Cu composites; after aging heat treatment, show significant increase of yield and ultimate tensile strength compared to those values for the Al–Cu matrix; the composites also show an increase rate similar to that of CNT/Al–Cu composites without aging heat treatment. It was thought that the CNT/Al–Cu composites were strengthened by both the load transfer from the Al–Cu matrix to the CNTs and by precipitation hardening of the Al–Cu matrix.

Ultra-fine grained Al 2 O 3 was fabricated by in-situ spark plasma sintering (SPS) process direct... more Ultra-fine grained Al 2 O 3 was fabricated by in-situ spark plasma sintering (SPS) process directly from amorphous powders. During in-situ sintering, phase transformation from amorphous to stable α-phase was completed by 1100 1C. High relative density over 99% of in-situ sintered Al 2 O 3 was obtained in the sintering condition of 1400 1C under 65 MPa pressure without holding time. The grain size of in-situ sintered Al 2 O 3 body was much finer ($ 400 nm) than that of Al 2 O 3 sintered from the crystalline α-Al 2 O 3 powders. For in-situ sintered Al 2 O 3 from amorphous powders, we observed a characteristic microstructural feature of highly elongated grains in the ultra-fine grained matrix due to abnormal grain growth. Moreover, the properties of abnormally grown grains were controllable. Fracture toughness of in-situ sintered Al 2 O 3 with the elongated grains was significantly enhanced due to the self-reinforcing effect via the crack deflection and bridging phenomena.

To improve the dispersion of carbon nanotubes (CNTs) in epoxy matrix composites, polystyrene sulf... more To improve the dispersion of carbon nanotubes (CNTs) in epoxy matrix composites, polystyrene sulfonate (PSS) and poly(4-aminostyrene) (PAS) were attached on the surface of CNTs by noncovalent functionalization. In the case of PAS, amino groups can also generate chemical bonding with the epoxide groups in the epoxy matrix. CNTs noncovalently functionalized with PSS and PAS were then fabricated to obtain CNT/Epoxy nanocomposites with enhanced mechanical properties. The incorporation of noncovalently functionalized CNTs into the modified bisphenol-A type epoxy matrix yielded Young's modulus of 3.89 GPa and tensile strength of 82.59 MPa with the addition of 1 wt.% PAS-CNTs. The noncovalent functionalization of CNTs was effective in improving the composite's mechanical properties due to their enhanced dispersion and strong affinity with the epoxy matrix.

The present study has investigated the consolidation behaviors of tantalum powders during compact... more The present study has investigated the consolidation behaviors of tantalum powders during compaction and sintering, and the characteristics of sintered components. For die compaction, the densification behaviors of the powders are simulated by finite element analyses based on the yield function proposed by Shima and Oyane. Accordingly, the green density distribution for coarser particles is predicted to be more uniform because they exhibits higher initial relative tap density owing to lower interparticle friction. It is also found that cold iso-static pressing is capable of producing higher dense compacts compared to the die pressing. However, unlike the compaction behavior, the sintered density of smaller particles is found to be higher than those of coarser ones owing to their higher specific surface area. The maximum sintered density was found to be 0.96 of theoretical density where smaller particles were pressed isostatically at 400 MPa followed by sintering at 2000 °C. Moreover, the effects of processing conditions on grain size and texture were also investigated. The average grain size of the sintered specimen is 30.29 μm and its texture is less than 2 times random intensity. Consequently, it is concluded that the higher pressure compaction technique is beneficial to produce high dense and texture-free tantalum components compared to hot pressing and spark plasma sintering.

The densification behavior of mixed Ti and Al/V master alloy powders for Ti-6Al-4V was investigat... more The densification behavior of mixed Ti and Al/V master alloy powders for Ti-6Al-4V was investigated by a series of dilatometry tests to measure the shrinkage of the samples with the sintering temperature. The corresponding microstructural changes were examined under various sintering conditions with optical microscopy, energy-dispersive spectroscopy, and X-ray diffraction analyses. From these results, the consolidation of the mixed powders was divided into two domains: (i) sintering densification and solute homogenization of Ti and Al/V master alloy particles below 1293 K (1020 °C), and (ii) densification of Ti alloy phases above 1293 K (1020 °C). In the lower temperature region, the inter-diffusion between Ti and Al/V master alloy particles dominated the sintering of the mixed powders because the chemical gradient between two types of particles outweighed the surface energy reduction. Following chemical homog-enization, the densification induced the shrinkage of the Ti alloy phases to reduce their surface energies. These tendencies are also supported by the density and grain size variations of the sintered specimens with temperature. The apparent activation energies of the sintering and grain growth for Ti alloy particles are 85.91 ± 6.93 and 37.33 kJ/mol, respectively, similar to or slightly lower than those of pure Ti particles. The difference was attributed to the slower self-diffusion of Ti resulting from the alloying of Al and V into in the Ti matrix.

Graphene, which has a 2D layered structure of carbon atoms, is of great interest because of its e... more Graphene, which has a 2D layered structure of carbon atoms, is of great interest because of its excellent mechanical and electrical properties. Monolayer graphene has a Young's modulus of 1 TPa and a tensile strength of 130 GPa. The electron mobility of suspended graphene is 200 000 cm 2 V −1 s −1 . Graphene also has a large surface area (≈2600 m 2 g −1 ) and a low density (2.2 g cm −3 ), which makes it suitable as a reinforcement for nanocomposites. Numerous graphene/polymer composites have been studied for that reason. Stankovich et. al. fi rst reported possibilities of graphene-based polymer composites with homogeneous dispersion of graphene at relatively low percolation threshold of ≈0.1 vol% in polystyrene matrix. Since then, graphene/polymer nanocomposites have been reported based on epoxy, poly(methyl methacrylate) (PMMA), polystyrene (PS), polyurethane (PU), and polypropylene (PP) polymers. Recently, our research group also introduced noncovalent, PBA functionalization to graphene fl akes and synthesized graphene/epoxy composite resulting in enhanced thermal conductivities and mechanical properties. Graphene could be an ideal 2D reinforcement nanomaterial not only for polymer matrix but also to metal matrix. However, only few work on graphene/metal nanocomposites have been reported and, sometimes, exhibited even lower mechanical properties with the addition of graphene fl akes. Two major reasons for this behavior are: i) poor bondings between graphene fl akes and metals and ii) relatively high processing temperature (over 1000 °C in case of copper) at which the graphene is easily decomposed or damaged. Previous work of graphene/ metal composites was mostly based on the traditional process of powder metallurgy, which cannot effectively prevent agglomeration of the graphene in the metal matrix because graphene is prone to segregate from the metal particles due to its poor affi nity to metal in the absence of any binding sites and the formation of agglomerates of graphene by van der Waals forces. Furthermore, general sintering and melting process are not easily applicable to graphene/metal nanocomposites because the process temperatures of most metals are beyond the decomposition temperature of reduced graphene oxide (RGO) (≈600 °C) found from TGA ( , Supporting Information). Also, the large density difference between the metal and graphene causes the graphene to fl oat on top of the melt. To achieve the best mechanical properties, graphene fl akes must be homogeneously dispersed in metal matrix without significant thermal damage or conversion into metal carbides during densifi cation and sintering.

Polymeric organic photovoltaic (OPV) cells are promising candidates for low-cost, high-performanc... more Polymeric organic photovoltaic (OPV) cells are promising candidates for low-cost, high-performance energy sources due to their low material and processing costs, flexibility, and ease of manufacturing by solution processes. However, low power-conversion efficiency (PCE) has impeded the development of OPV cells. The low PCE in OPV solar cells has been attributed to low carrier mobility, which is related to the transport length of the charge carriers within active layers. Graphene can be an ideal material for assisting the charge transport in the active layer of OPV cells due to its excellent charger carrier mobility, thermal and chemical stability, and compatibility with the solution process. In this work, we demonstrated for the first time an improvement of the PCE (up to 40%) in OPV bulk-heterojunction (BHJ) cells by incorporating charge-selective graphene flakes into the BHJ active layer. The charge selectivity of graphene flakes was achieved by nitrogen doping (N-doped graphene). The N-doped graphene, when mixed in the active layer (N-doped graphene/polymer:fullerene composites), provided transport pathways exclusively to specific charge carriers through the modulation of band-gap structures. We discuss further the enhancement of the PCE in OPV cells with respect to charge-carrier mobility. Broader context Organic solar cells have received a lot of attention due to their low production costs, easy scalability to large-areas and applicability on exible substrates. One of the main challenges to widespread application in practical devices is their low power conversion efficiency (PCE). This is largely because of the low charge-carrier mobilities and poor charge transfer characteristics in organic materials, resulting in short carrier lifetimes and reduced charge collection efficiencies. In this work, we demonstrate that the use of nitrogen-doped graphene improves the power conversion efficiency of a bulk-heterojunction solar cell system. The nitrogen-doped graphene provides transport pathways to specic charge carriers through the modulation of band structures when mixed into the active layer. We believe that the added functionality of charge selectivity in conductive graphene akes gives a new design parameter for increasing the PCE of bulk-het-erojunction solar cells.

We report boron nitride nanoflakes (BNNFs), for the first time, as a nanofiller for polymer elect... more We report boron nitride nanoflakes (BNNFs), for the first time, as a nanofiller for polymer electrolyte membranes in fuel cells. Utilizing the intrinsic mechanical strength of two-dimensional (2D) BN, addition of BNNFs even at a marginal content (0.3 wt %) significantly improves mechanical stability of the most representative hydrocarbon-type (HC-type) polymer electrolyte membrane, namely sulfonated poly(ether ether ketone) (sPEEK), during substantial water uptake through repeated wet/dry cycles. For facile processing with BNNFs that frequently suffer from poor dispersion in most organic solvents, we non-covalently functionalized BNNFs with 1-pyrenesulfonic acid (PSA). Besides good dispersion, PSA supports efficient proton transport through its sulfonic functional groups. Compared to bare sPEEK, the composite membrane containing BNNF nanofiller exhibited far improved long-term durability originating from enhanced dimensional stability and diminished chronic edge failure. This study suggests that introduction of properly functionalized 2D BNNFs is an effective strategy in making various HC-type membranes sustainable without sacrificing their original adventurous properties in polymer electrolyte membrane fuel cells.

The effect of initial lamellar structure of β heat treated Ti-6Al-4V alloy on the globularization... more The effect of initial lamellar structure of β heat treated Ti-6Al-4V alloy on the globularization behavior during the multi-step forging was investigated. Specimens of different lamellar thicknesses were upsetted and stretched by side pressing repeatedly, i.e. multi-step forging, at the sub-transus temperature to break down the lamellar structure. The microstructural changes after multi-step forging were analyzed in the light of globularization behavior. The results showed that the thick lamellar structure was more difficult to be transformed to homogeneous equiaxed structure than thin lamellar structure.

Giant magnetostrictive composites have attracted a great deal of attention by supplementing short... more Giant magnetostrictive composites have attracted a great deal of attention by supplementing shortcomings of monolithic Terfenol-D such as brittleness, eddy current loss and formability. Recently, infiltrated Terfenol-D/Epoxy composite has been developed as an alternative composite. This composite was fabricated by an unidirectional solidification of Terfenol-D followed by an infiltration of epoxy. The iron content in composite has been changed in order to control volume fraction of RFe 2 phase producing magnetostriction on Terfenol-D/Epoxy composite. The magnetostriction of both as-grown and infiltrated Terfenol-D/Epoxy composite was measured to confirm the effects of eutectic phase and heat treatment on magnetostriction. The enhancement of magnetostriction of Terfenol-D/Epoxy composite was mostly contributed by the eutectic phase through the hindering of movement and rotation of domain walls. The magnetostriction modelling of Terfenol-D/Epoxy composite was suggested, based on the change of texture and elastic modulus. The suggested model was in good agreements with the experimental results on the measurement of magnetostriction of Terfenol-D/Epoxy composite.

Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for... more Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for X-ray tube cathode. However, CNT field emission cathode has been shown unstable field emission and short life time due to field evaporation by high current density and detachment by electrostatic force. An alternative approach in this direction is the introduction of CNT yarn, which is a one dimensional assembly of individual carbon nanotubes bonded by the Van der Waals force. Because CNT yarn is composed with many CNTs, CNT yarns are expected to increase current density and life time for X-ray tube applications. In this research, CNT yarn was fabricated by spinning of a super-aligned CNT forest and was characterized for application to an X-ray tube cathode. CNT yarn showed a high field emission current density and a long lifetime of over 450 hours. Applying the CNT yarn field emitter to the X-ray tube cathode, it was possible to obtain micro-scale resolution images. The relationship between the field emission properties and the microstructure evolution was investigated and the unraveling effect of the CNT yarn was discussed.

Wide range of lead-free piezoelectric ceramic compositions have been investigated in order to rep... more Wide range of lead-free piezoelectric ceramic compositions have been investigated in order to replace (Pb,Zr) TiO 3 (PZT) owing to the concerns of environmental pollution. Among these reported lead-free compositions, (K 0.5 Na 0.5)NbO 3 (KNN) based ceramics have emerged as one of the promising candidate owing to their excellent piezoelectric properties and high Curie temperature (T C) [1-14]. However, KNN-based compositions show inferior stability of piezoelectric property as a function of temperature due to the presence of intermediate orthorhombic to tetragonal phase transition [10-14]. Figure 1 shows the variation of electromechanical coupling factor (k p) and dielectric constant as a function of temperature in KNNbased ceramics. As seen in Fig. 1a, high temperature coefficient of piezoelectric properties can be explained based upon the polymorphic phase transition in KNNbased ceramics. The peaks in k p and dielectric constant were located at the phase transition temperatures for all the KNN-based compositions. However, by noticing the changes in slope in Fig. 1, we can arrive at the possibility of compensating the trend since the phase transition temperatures shift with solid solution as shown in Fig. 1b. This lead us to the design of new microstructures such as island-matrix structure (type I) and layered structure (type II) as illustrated in Fig. 2a, b. The gradient in color in these figures indicates the effect of elemental diffusion between

We propose a new strategy for improving the charge selectivity of carbon nanotubes (CNTs) for org... more We propose a new strategy for improving the charge selectivity of carbon nanotubes (CNTs) for organic photovoltaic cells (OPVCs). The strategy involves the coating of an ultrathin layer of titanium suboxide (TiO x ) on CNTs by atomic layer deposition (ALD). ALD can facilitate that conformal and uniform coating of TiO x on CNT networks while preserving their nanoporous structure. We used the TiO x -coated CNT networks as an electron transport layer in inverted OPVCs. TiO x -coated CNTs can provide electrons with an extremely fast conductive path through CNTs and selectively block the holes by means of the hole-barrier property of the TiO x in OPVCs. The nanoporous structure of TiO x -coated CNT networks can improve the device performance of OPVCs due to synergetic effects of the electron selective transport property of TiO x and the high conductivity of CNTs. In addition, further improvement of device performance can be achieved by adding a hole transport layer between the active layer and the Au electrode.

Nanoporous metal foams have good electrical and thermal conductivities and potential catalytic ac... more Nanoporous metal foams have good electrical and thermal conductivities and potential catalytic activities because of their high surface areas. In this study, nanoporous cobalt foam was prepared by simple consolidation of a pearl-necklace-type CNT/Co 3 O 4 nanocomposite powder. During heat treatment of the pre-compacted powder in an inert atmosphere, Co 3 O 4 particles were reduced to cobalt metal and formed a three-dimensional, continuous nanoporous metallic structure. This nanoporous cobalt foam could be used as an excellent conducting framework because of the superior electrical conductivity of the metal. A Co/ Co(OH) 2 core–shell structure was prepared by coating Co(OH) 2 onto the nanoporous cobalt foam and using it as the electrode for a supercapacitor. Because of the high surface area of the nanoporous cobalt metal frame, the Co/Co(OH) 2 core–shell structure had a specific capacitance of 525 F g À1 at a current density of 0.5 A g À1 .

A B S T R A C T Carbon nanotube (CNT) reinforced nickel matrix (CNT/Ni) composites exhibiting a y... more A B S T R A C T Carbon nanotube (CNT) reinforced nickel matrix (CNT/Ni) composites exhibiting a yield strength (YS) of 710 MPa, about 3.7 times higher than monolithic nickel, have been processed by a molecular-level mixing process followed by spark plasma sintering (SPS). The enormous strength increase in these composites can be attributed to a homogeneous distribution of nanotubes in the nickel matrix coupled with the formation of well-bonded, high strength, contaminant-free nanotube/nickel interfaces, as revealed by high-resolution transmission electron microscopy. Such interfaces can effectively transfer load between nanotubes and nickel matrix in the CNT/Ni composites.

A B S T R A C T The precipitation hardening behavior of CNT/Al–Cu composites was investigated by ... more A B S T R A C T The precipitation hardening behavior of CNT/Al–Cu composites was investigated by characterization of microstructure and mechanical properties after aging heat treatment. It was found that CNTs accelerated the precipitation hardening behavior of CNT/Al–Cu composites due to the generation of excess dislocations. The CNT/Al–Cu composites; after aging heat treatment, show significant increase of yield and ultimate tensile strength compared to those values for the Al–Cu matrix; the composites also show an increase rate similar to that of CNT/Al–Cu composites without aging heat treatment. It was thought that the CNT/Al–Cu composites were strengthened by both the load transfer from the Al–Cu matrix to the CNTs and by precipitation hardening of the Al–Cu matrix.

Ultra-fine grained Al 2 O 3 was fabricated by in-situ spark plasma sintering (SPS) process direct... more Ultra-fine grained Al 2 O 3 was fabricated by in-situ spark plasma sintering (SPS) process directly from amorphous powders. During in-situ sintering, phase transformation from amorphous to stable α-phase was completed by 1100 1C. High relative density over 99% of in-situ sintered Al 2 O 3 was obtained in the sintering condition of 1400 1C under 65 MPa pressure without holding time. The grain size of in-situ sintered Al 2 O 3 body was much finer ($ 400 nm) than that of Al 2 O 3 sintered from the crystalline α-Al 2 O 3 powders. For in-situ sintered Al 2 O 3 from amorphous powders, we observed a characteristic microstructural feature of highly elongated grains in the ultra-fine grained matrix due to abnormal grain growth. Moreover, the properties of abnormally grown grains were controllable. Fracture toughness of in-situ sintered Al 2 O 3 with the elongated grains was significantly enhanced due to the self-reinforcing effect via the crack deflection and bridging phenomena.

To improve the dispersion of carbon nanotubes (CNTs) in epoxy matrix composites, polystyrene sulf... more To improve the dispersion of carbon nanotubes (CNTs) in epoxy matrix composites, polystyrene sulfonate (PSS) and poly(4-aminostyrene) (PAS) were attached on the surface of CNTs by noncovalent functionalization. In the case of PAS, amino groups can also generate chemical bonding with the epoxide groups in the epoxy matrix. CNTs noncovalently functionalized with PSS and PAS were then fabricated to obtain CNT/Epoxy nanocomposites with enhanced mechanical properties. The incorporation of noncovalently functionalized CNTs into the modified bisphenol-A type epoxy matrix yielded Young's modulus of 3.89 GPa and tensile strength of 82.59 MPa with the addition of 1 wt.% PAS-CNTs. The noncovalent functionalization of CNTs was effective in improving the composite's mechanical properties due to their enhanced dispersion and strong affinity with the epoxy matrix.

The present study has investigated the consolidation behaviors of tantalum powders during compact... more The present study has investigated the consolidation behaviors of tantalum powders during compaction and sintering, and the characteristics of sintered components. For die compaction, the densification behaviors of the powders are simulated by finite element analyses based on the yield function proposed by Shima and Oyane. Accordingly, the green density distribution for coarser particles is predicted to be more uniform because they exhibits higher initial relative tap density owing to lower interparticle friction. It is also found that cold iso-static pressing is capable of producing higher dense compacts compared to the die pressing. However, unlike the compaction behavior, the sintered density of smaller particles is found to be higher than those of coarser ones owing to their higher specific surface area. The maximum sintered density was found to be 0.96 of theoretical density where smaller particles were pressed isostatically at 400 MPa followed by sintering at 2000 °C. Moreover, the effects of processing conditions on grain size and texture were also investigated. The average grain size of the sintered specimen is 30.29 μm and its texture is less than 2 times random intensity. Consequently, it is concluded that the higher pressure compaction technique is beneficial to produce high dense and texture-free tantalum components compared to hot pressing and spark plasma sintering.

The densification behavior of mixed Ti and Al/V master alloy powders for Ti-6Al-4V was investigat... more The densification behavior of mixed Ti and Al/V master alloy powders for Ti-6Al-4V was investigated by a series of dilatometry tests to measure the shrinkage of the samples with the sintering temperature. The corresponding microstructural changes were examined under various sintering conditions with optical microscopy, energy-dispersive spectroscopy, and X-ray diffraction analyses. From these results, the consolidation of the mixed powders was divided into two domains: (i) sintering densification and solute homogenization of Ti and Al/V master alloy particles below 1293 K (1020 °C), and (ii) densification of Ti alloy phases above 1293 K (1020 °C). In the lower temperature region, the inter-diffusion between Ti and Al/V master alloy particles dominated the sintering of the mixed powders because the chemical gradient between two types of particles outweighed the surface energy reduction. Following chemical homog-enization, the densification induced the shrinkage of the Ti alloy phases to reduce their surface energies. These tendencies are also supported by the density and grain size variations of the sintered specimens with temperature. The apparent activation energies of the sintering and grain growth for Ti alloy particles are 85.91 ± 6.93 and 37.33 kJ/mol, respectively, similar to or slightly lower than those of pure Ti particles. The difference was attributed to the slower self-diffusion of Ti resulting from the alloying of Al and V into in the Ti matrix.

Graphene, which has a 2D layered structure of carbon atoms, is of great interest because of its e... more Graphene, which has a 2D layered structure of carbon atoms, is of great interest because of its excellent mechanical and electrical properties. Monolayer graphene has a Young's modulus of 1 TPa and a tensile strength of 130 GPa. The electron mobility of suspended graphene is 200 000 cm 2 V −1 s −1 . Graphene also has a large surface area (≈2600 m 2 g −1 ) and a low density (2.2 g cm −3 ), which makes it suitable as a reinforcement for nanocomposites. Numerous graphene/polymer composites have been studied for that reason. Stankovich et. al. fi rst reported possibilities of graphene-based polymer composites with homogeneous dispersion of graphene at relatively low percolation threshold of ≈0.1 vol% in polystyrene matrix. Since then, graphene/polymer nanocomposites have been reported based on epoxy, poly(methyl methacrylate) (PMMA), polystyrene (PS), polyurethane (PU), and polypropylene (PP) polymers. Recently, our research group also introduced noncovalent, PBA functionalization to graphene fl akes and synthesized graphene/epoxy composite resulting in enhanced thermal conductivities and mechanical properties. Graphene could be an ideal 2D reinforcement nanomaterial not only for polymer matrix but also to metal matrix. However, only few work on graphene/metal nanocomposites have been reported and, sometimes, exhibited even lower mechanical properties with the addition of graphene fl akes. Two major reasons for this behavior are: i) poor bondings between graphene fl akes and metals and ii) relatively high processing temperature (over 1000 °C in case of copper) at which the graphene is easily decomposed or damaged. Previous work of graphene/ metal composites was mostly based on the traditional process of powder metallurgy, which cannot effectively prevent agglomeration of the graphene in the metal matrix because graphene is prone to segregate from the metal particles due to its poor affi nity to metal in the absence of any binding sites and the formation of agglomerates of graphene by van der Waals forces. Furthermore, general sintering and melting process are not easily applicable to graphene/metal nanocomposites because the process temperatures of most metals are beyond the decomposition temperature of reduced graphene oxide (RGO) (≈600 °C) found from TGA ( , Supporting Information). Also, the large density difference between the metal and graphene causes the graphene to fl oat on top of the melt. To achieve the best mechanical properties, graphene fl akes must be homogeneously dispersed in metal matrix without significant thermal damage or conversion into metal carbides during densifi cation and sintering.

Polymeric organic photovoltaic (OPV) cells are promising candidates for low-cost, high-performanc... more Polymeric organic photovoltaic (OPV) cells are promising candidates for low-cost, high-performance energy sources due to their low material and processing costs, flexibility, and ease of manufacturing by solution processes. However, low power-conversion efficiency (PCE) has impeded the development of OPV cells. The low PCE in OPV solar cells has been attributed to low carrier mobility, which is related to the transport length of the charge carriers within active layers. Graphene can be an ideal material for assisting the charge transport in the active layer of OPV cells due to its excellent charger carrier mobility, thermal and chemical stability, and compatibility with the solution process. In this work, we demonstrated for the first time an improvement of the PCE (up to 40%) in OPV bulk-heterojunction (BHJ) cells by incorporating charge-selective graphene flakes into the BHJ active layer. The charge selectivity of graphene flakes was achieved by nitrogen doping (N-doped graphene). The N-doped graphene, when mixed in the active layer (N-doped graphene/polymer:fullerene composites), provided transport pathways exclusively to specific charge carriers through the modulation of band-gap structures. We discuss further the enhancement of the PCE in OPV cells with respect to charge-carrier mobility. Broader context Organic solar cells have received a lot of attention due to their low production costs, easy scalability to large-areas and applicability on exible substrates. One of the main challenges to widespread application in practical devices is their low power conversion efficiency (PCE). This is largely because of the low charge-carrier mobilities and poor charge transfer characteristics in organic materials, resulting in short carrier lifetimes and reduced charge collection efficiencies. In this work, we demonstrate that the use of nitrogen-doped graphene improves the power conversion efficiency of a bulk-heterojunction solar cell system. The nitrogen-doped graphene provides transport pathways to specic charge carriers through the modulation of band structures when mixed into the active layer. We believe that the added functionality of charge selectivity in conductive graphene akes gives a new design parameter for increasing the PCE of bulk-het-erojunction solar cells.

We report boron nitride nanoflakes (BNNFs), for the first time, as a nanofiller for polymer elect... more We report boron nitride nanoflakes (BNNFs), for the first time, as a nanofiller for polymer electrolyte membranes in fuel cells. Utilizing the intrinsic mechanical strength of two-dimensional (2D) BN, addition of BNNFs even at a marginal content (0.3 wt %) significantly improves mechanical stability of the most representative hydrocarbon-type (HC-type) polymer electrolyte membrane, namely sulfonated poly(ether ether ketone) (sPEEK), during substantial water uptake through repeated wet/dry cycles. For facile processing with BNNFs that frequently suffer from poor dispersion in most organic solvents, we non-covalently functionalized BNNFs with 1-pyrenesulfonic acid (PSA). Besides good dispersion, PSA supports efficient proton transport through its sulfonic functional groups. Compared to bare sPEEK, the composite membrane containing BNNF nanofiller exhibited far improved long-term durability originating from enhanced dimensional stability and diminished chronic edge failure. This study suggests that introduction of properly functionalized 2D BNNFs is an effective strategy in making various HC-type membranes sustainable without sacrificing their original adventurous properties in polymer electrolyte membrane fuel cells.

Journal of Applied Polymer Science, 2003

The dynamic mechanical property of particle-reinforced ethylene-propylene-diene monomer (EPDM) ma... more The dynamic mechanical property of particle-reinforced ethylene-propylene-diene monomer (EPDM) matrix composites has been studied by using a dynamic mechanical thermal analyzer (DMTA). The individual composite has been reinforced with the various reinforcing particles as follows: silicon carbide particles (SiCps) of 60 m in average diameter with various volume fractions (i.e., 10-40%); copper (Cu) and aluminum (Al) particles with 20 vol %; and SiCps with 6 and 36 m in different average diameters with 20 vol % over the total composite volume. It is shown from the experimental results that the dynamic elastic modulus values increase and the composites with 40 vol % SiCps exhibit higher tan ␦ values through the entire rubbery phase after the glass transition region compared with the composites with lower particle volume percentages. This shows that the composites with 20 vol % Cu particles have the higher dynamic elastic modulus but the lower peak tan ␦ value than the composites with other particles of 20 vol % do. Scanning electron microscopy results show that the effective particle volume in the composite with Cu particles is higher than the other composites, although the same particle volume fraction of 20% has been used.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Jul 1, 1995

This work is concerned with the effects of A1 on the deformation mode and tensile properties of F... more This work is concerned with the effects of A1 on the deformation mode and tensile properties of Fe-19Mn-5Cr (0-5.5)A1 0.25C alloys. The deformation mode at room temperature shifted from (s + ~') martensites to deformation twinning to slip with increasing A1 content. A1 increased the stacking fault energy at a rate of 10 mJ mm-2 wt.%-1, which is sufficiently high to give birth to the large variation of the deformation mode. On the other hand, bct ~' martensites were induced during deformation at low temperatures, together with deformation twins. The formation of bct ~' martensites was preceded by the prior formation of deformation twins. Fe-19Mn 5Cr-3.5A1-0.25C alloy composition exhibited excellent elongations exceeding 65% at both room temperature and 77 K. It was found in this work that the sequential formation of strain-induced deformation twins and ~' martensites was responsible for exhibiting excellent elongations at both room temperature and 77 K.

International Journal of Refractory Metals & Hard Materials, Jul 1, 2001

Mechanical properties and microstructures of nanocrystalline WC±10Co cemented carbides were inves... more Mechanical properties and microstructures of nanocrystalline WC±10Co cemented carbides were investigated. The nanocrystalline WC±10Co cemented carbide powders were manufactured by reduction and carbonization of the nanocrystalline precursor powders which were prepared by spray drying process of solution containing ammonia meta-tungstate (AMT) and cobalt nitrate. The WC powders were about 100 nm in diameter mixed homogeneously with Co binder phase and were sintered at 1375°C under a pressure of 1 mTorr. In order to compare the microstructures and mechanical properties with those of nanocrystalline WC±10Co, commercial WC powders in a diameter range of 0.57±4 lm were mixed with Co powders, and were sintered at the same conditions as those of nanocrystalline powders. TaC, Cr 3 C 2 and VC of varying amount were added into nanocrystalline WC±10Co cemented carbides as grain growth inhibitors. To investigate the microstructure of Co binder phase in the WC±10Co cemented carbides, Co± W±C alloy was fabricated at the temperature of sintering process for the WC±10Co cemented carbides. The hardness of WC±10Co cemented carbides increased with decreasing WC grain size following a Hall±Petch-type relationship. The fracture toughness of WC±10Co cemented carbides increases with increasing HCP/FCC ratio of Co binder phase by HCP/FCC phase transformation.

Journal of Materials Research, 2002

High-strain-rate superplastic behavior of powder-metallurgy processed 0%, 10%, 20%, and 30% SiC p... more High-strain-rate superplastic behavior of powder-metallurgy processed 0%, 10%, 20%, and 30% SiC particulate reinforced 6061 Al composites was studied over a range of temperatures from 430 to 610°C. The strength of the 6061 Al composites was lower than that of the 6061 Al matrix alloy in the temperature range where grain boundary sliding is believed to control the plastic flow. The difference in their strength was also observed to be temperature dependent, increasing with increase in temperature. Abnormally high activation energy for superplastic flow was another important feature of the 6061 Al composites. These behaviors in particle weakening and activation energy have strong resemblance to those noted in the high-strain-rate superplastic 2124 Al composites studied previously. The observed particle weakening was attributed to liquid-enhanced superplastic flow and discussed by adopting the concept of effective diffusivity considering mass flow through liquid phase formed at the solute-segregated region near SiC/Al interfaces.

Journal of Experimental Nanoscience, Jul 11, 2012

Scripta Materialia, May 1, 2001

The intercritical annealing and isothermal bainitic processing response was studied for three Nb ... more The intercritical annealing and isothermal bainitic processing response was studied for three Nb and V microalloyed Transformation-Induced Plasticity (TRIP)-assisted 980 MPa grade steels. Their mechanical and microstructural properties were compared to industrially produced TRIP 800 steel. Depending on the isothermal holding temperature and microalloying, the experimental steels reached properties comparable to the reference steel. The retained austenite content did not show direct correlation to elongation properties. Niobium was found to be more effective microalloying element than vanadium in increasing the elongation properties, which were investigated by measuring true fracture strain from tensile test specimens.

Scripta Materialia, Oct 1, 2005

A novel process to fabricate carbon nanotube (CNT)/alumina nanocomposites, consisting of a molecu... more A novel process to fabricate carbon nanotube (CNT)/alumina nanocomposites, consisting of a molecular level mixing process and an in situ spark plasma sintering process, is proposed. The CNT/alumina nanocomposites fabricated by this proposed process show enhanced hardness due to a load transfer mechanism of the CNTs and increased fracture toughness arising from the bridging mechanism of CNTs during crack propagation.

Journal of Materials Research, Dec 1, 2004

The creep behaviors of 20 vol% SiCw/2124Al, extruded with different ratios, and SiCp/2124Al, rein... more The creep behaviors of 20 vol% SiCw/2124Al, extruded with different ratios, and SiCp/2124Al, reinforced with 10-30 vol% SiC particles, were investigated to clarify the effects of aspect ratio, alignment, and volume fraction of reinforcement on creep deformation. The effective stresses on the matrix of SiC/Al composites are calculated based on the generalized shear-lag model. The minimum creep rates of SiCw/2124Al extruded with different ratios and SiCp/2124Al reinforced with different volume fractions of SiC particles are found to be similar under a same effective stress on matrix, which is calculated by the generalized shear-lag model. The subgrain sizes in matrices of crept SiC/Al composites are dependent on the effective stress on matrix but not on the applied stress on the composite. It is suggested that the role of SiC reinforcements is to increase the creep resistance of SiC/Al composite by reducing the effective stress on matrix.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Aug 1, 2008

An analytical model to calculate the hardness of ultra-fine WC-10Co cemented carbides was investi... more An analytical model to calculate the hardness of ultra-fine WC-10Co cemented carbides was investigated. The nanocrystalline WC-10Co powders were manufactured using a spray conversion process and sintered at 1375 • C in a vacuum. Varying amounts of TaC, Cr 3 C 2 , and VC were added to nanocrystalline WC-10Co cemented carbides as grain growth inhibitors. The hardness of WC-10Co cemented carbides increased with a decreasing WC grain size from 5 m to 300 nm. An analytical model to calculate the hardness of WC-10Co cemented carbides was proposed under the assumption that the applied load is transferred from the WC to the Co binder phase. The analytically calculated hardness showed good agreement with the experimentally measured hardness of WC-10Co cemented carbides. In the proposed analytical model, the hardness of WC-10Co cemented carbides is similar to that predicted by the Hall-Petch relationship when the WC grain size is large. However, when the grain size is finer than a critical value, the predicted hardness of the WC-10Co cemented carbide becomes saturated.

Journal of Materials Research, Jul 1, 2001

Journal of disaster research, Aug 1, 2010

This paper investigates the out-of-plane shear behavior of composite steel-plate-reinforced concr... more This paper investigates the out-of-plane shear behavior of composite steel-plate-reinforced concrete walls (SC walls) and proposes their shear-strength-models based on plasticity theory limit analysis. For speedy, modular construction, SC walls are fabricated using double-skin steel plates with welded shear studs and sandwiching concrete between them. A review of current design formulas provides better understanding of bond-stress-dependent shear behavior relying on studs of SC walls. We conducted experiments on bondstrength-dependent arch and/or truss action to verify proposed shear-strength models with test results. Test results, including those from literature, agreed well with the strength anticipated by proposed formulas.

Journal of Materials Research, Sep 1, 2006

This study investigated a mechanism for controlling the shape of Cu nanocrystals fabricated using... more This study investigated a mechanism for controlling the shape of Cu nanocrystals fabricated using the polyol process, which considers the thermodynamic transition from a faceted surface to a rough surface and the growth mechanisms of nanocrystals with faceted or rough surfaces. The faceted surfaces were stable at relatively low temperatures because of the low entropy of perfectly faceted surfaces. Nanocrystals fabricated using a coordinative surfactant stabilized the faceted surface at a higher temperature than those fabricated using a noncoordinative surfactant. The growth rate of the surface under a given driving force was dependent on the surface structure, i.e., faceted or rough, and the growth of a faceted surface was a thermally activated process. Surface twins decreased the activation energy for growth of the faceted surface and resulted in rod-or wire-shaped nanocrystals.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Jul 1, 2008

The effect of the size and location of spherical pores on the transverse rupture strength of WC-1... more The effect of the size and location of spherical pores on the transverse rupture strength of WC-10Co cemented carbides was investigated. Based on the observations of fractographies of the transverse rupture test specimens, it was found that the cracks in WC-10Co cemented carbides initiate from the spherical closed pores near the surface, not from the open pores at the surface. The relationship between the transverse rupture strength and the size and location of the spherical pores was analyzed considering the stress field near the spherical shaped pores. In this analysis, for one spherical pore, a critical location exists within the specimen where the transverse rupture strength is minimized, but not on the surface of the specimen. By considering the various sizes and locations of pores, a map showing the transverse rupture strength according to the location and size of pores was obtained.

Journal of Alloys and Compounds, May 1, 2007

ABSTRACT The microstructure and mechanical properties of mechanically alloyed oxide dispersion st... more ABSTRACT The microstructure and mechanical properties of mechanically alloyed oxide dispersion strengthened (ODS) tungsten heavy alloys were investigated. Three different mechanical alloying processes, such as one-step mechanical alloying process, two-step mechanical alloying process, and mechanical alloying and mixing process, were performed in order to control the microstructure and mechanical properties of ODS tungsten heavy alloys. The partially stabilized zirconia (PSZ) dispersoids tend to be dispersed in tungsten grains rather than the binder matrix when the powders were prepared by two-step mechanical alloying or mechanical alloying and mixing process. The yield strength of ODS tungsten heavy alloy increased with decreasing the binder mean thickness, but was not dependent on location of oxide dispersoids. The elongation and high temperature strength of ODS tungsten heavy alloys increased with increasing the content of PSZ dispersoids.

International Journal of Refractory Metals & Hard Materials, 2006

Microstructure and mechanical properties of WC-TiC-10 wt%Co cemented carbides fabricated by sinte... more Microstructure and mechanical properties of WC-TiC-10 wt%Co cemented carbides fabricated by sintering with hot isostatic pressing (Sinter-HIP) process were investigated. The WC/TiC grain size ratio of WC-TiC-10 wt%Co cemented carbides was controlled by changing the average size of WC powders ranged from 0.5 to 4 lm, with keeping the average size of TiC powder as 1 lm. The microstructures of sintered WC-TiC-10 wt%Co cemented carbides were sensitively dependent on the WC/TiC grain size ratio. In WC-TiC-10 wt%Co cemented carbides with WC/TiC grain size ratio of 0.5, the TiC/(Ti, W)C core-rim phases were distributed in WC/Co matrix. While, in WC-TiC-10 wt%Co cemented carbides with WC/TiC grain size ratio above 0.8, the WC and TiC/(Ti, W)C core-rim phases were surrounded by Co binder phase. Hardness of WC-TiC-10 wt%Co cemented carbide increased with decreasing the WC/TiC grain size ratio from 4 to 0.8 following the modified Hall-Petch type equation. However, the hardness of WC-TiC-10 wt%Co cemented carbides with WC/TiC grain size ratio of 0.5 shows much higher values than that expected by modified Hall-Petch type equation. Transverse rupture strength of WC-20TiC-10 wt%Co cemented carbides increases with decreasing the WC/TiC grain size ratio.

Journal of Nuclear Materials, Jun 1, 2006

Zirconium carbide, titanium carbide, zirconium nitride and titanium nitride, which are promising ... more Zirconium carbide, titanium carbide, zirconium nitride and titanium nitride, which are promising candidates for the ceramic matrix of inert matrix fuel (IMF) to transmute long-lived actinides were sintered using the spark plasma sintering (SPS) technique. Dy 2 O 3 (20 wt%) was added as a surrogate for Am 2 O 3 and the sintering behaviours of Dy 2 O 3 dispersed carbide or nitride matrix composites were compared with that of the matrix. The spark plasma sintering conditions consisted of a rapid heating rate of 75 K min À1 and a very short holding time of 1-4 min at maximum temperatures ranging from 1773 to 2000 K. Dy 2 O 3 dispersed carbides and nitrides with about 80% of theoretical density were obtained by spark plasma sintering with a rapid heating rate and a short dwelling time. When Dy 2 O 3 was added to the matrix, the shrinkage of the carbide or of nitride composites was initiated from a lower temperature than its matrix material during the heating stage. In the case of TiC and TiN, microstructural observation exhibited that Ti is soluble in dysprosium oxide and densification is enhanced around the oxide phase.

Journal of Materials Research, Dec 1, 2003

Carbon, Jun 1, 2012

Cu composites show a microstructure with a homogeneous dispersion of CNTs in the Al-Cu matrix and... more Cu composites show a microstructure with a homogeneous dispersion of CNTs in the Al-Cu matrix and had a 3.8 times increase of yield strength and 30% increase of elastic modulus compared to Al-Cu matrix. The strengthening mechanism of CNT/Al-Cu composites was discussed by controlling the aspect ratio of CNTs and it was thought that the CNT/Al-Cu composites were strengthened by both load transfer from the Al matrix to the CNTs and dispersion strengthening of damaged short CNTs. At the same time, the addition of CNTs increases the grain refinement effect of the Al-Cu matrix which results in a grain size strengthening mechanism of the CNT/Al-Cu composites.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2001

Plastic-flow behavior of a powder-metallurgy (PM) processed 6061 matrix alloy has been investigat... more Plastic-flow behavior of a powder-metallurgy (PM) processed 6061 matrix alloy has been investigated in a wide range of elevated temperature between 430 and 620°C. It was found that the 6061 Al alloy exhibits superplasticity in a relatively wide range of temperature from 520 to 620°C at a high strain rate of 10 − 2 s − 1. Deformation behavior of the present alloy could be divided into three regions when the presence of threshold stress for plastic flow was assumed. They are D L controlled grain boundary sliding, D L controlled dislocation climb creep and powder-law breakdown, respectively. When temperature is as high as 590°C, however, the activation energy increases significantly higher than that for self-diffusion in aluminum and flow stress decreases further than normally expected. This phenomenon is likely attributed to the presence of liquid phase above 590°C. Comparison of the data in Region I below 610°C with those for a number of superplastic aluminum composites indicates that strengthening effect by reinforcement does not exist. Several speculations including diffusional relaxation in vicinity of reinforcements were made to explain this phenomenon.

Research on Chemical Intermediates, May 11, 2014