Hayk Nersisyan | Chungnam National University (original) (raw)

Papers by Hayk Nersisyan

Metallurgical and Materials Transactions B

Korean Journal of Metal and Materials, 2009

Materials Letters, 2015

We demonstrated a facile approach for synthesizing three-dimensional (3D), honeycombstructured gr... more We demonstrated a facile approach for synthesizing three-dimensional (3D), honeycombstructured graphene scaffold in the solid flame of a (0.5 + k) Zn + kCuO + (CH 2-CHCl)n reaction system. FESEM-based analyses of the acid purified porous graphene samples revealed that the synthesized graphene had a 3D, honeycomb-like structure with cell sizes of 20-100 nm and wall thicknesses of 1-3 nm. The specific surface area of the graphene sample prepared in the solid flame was about ∼1043 m 2 /g).

SHS processes were introduced in Korea in early 1995. A number of universities and organizations ... more SHS processes were introduced in Korea in early 1995. A number of universities and organizations were involved in research activity for the production of various compact and porous inorganic materials. A research group in RASOM carried out fundamental investigations in the field of SHS of 1D, 2D, and 3D nanostructures and LED phosphor powders. Impressive results were achieved in the field of the synthesis of phosphor powders for LED. Based on achievements in the field of the nitride phosphor powders, in 2011 a LED phosphor manufacturing company, Ellim Advanced Materials, Korea, was founded.

Powder Technology, 2014

ABSTRACT Magnesium was used as a reductant for the direct combustion synthesis of titanium alloy ... more ABSTRACT Magnesium was used as a reductant for the direct combustion synthesis of titanium alloy particles (Ti–6Al–4V) from titanium oxide (TiO2). The feed material (TiO2) and reductant (Mg) were mixed with Al (or AlCl3 · 6H2O), vanadium pentoxide (V2O5), and calcium hydroxide (Ca(OH)2), and the reaction mixture was combusted under pressurized argon. Temperature–time profiles were intermittently measured during combustion to analyze the combustion temperatures and wave velocities. The combustion product was subsequently acid-leached to eliminate secondary phases (MgO, MgCl2, CaO) and produce Ti–6Al–4V alloy particles whose oxygen content was ~ 1.3%. Further the content of oxygen was decreased to 0.2 wt.% by deoxidizing the Ti–6Al–4V alloy particles with calcium in the range 850–900 °C. The AlCl3 · 6H2O affected the particle morphology more than the Al metal did; i.e. the Al-doped alloy particles were porous and consisted of relatively small agglomerated particles whereas using AlCl3 · 6H2O produced dense, round particles. Pilot-scale production experiments revealed that Ti–6Al–4V alloy particles can be efficiently produced using our method.

Materials Science and Engineering: A, 2003

Laser surface melting of plastics mold steels P21 (Fe Á/3% Ni Á/1.5% Mn Á/1% Al Á/0.3% Si Á/0.15%... more Laser surface melting of plastics mold steels P21 (Fe Á/3% Ni Á/1.5% Mn Á/1% Al Á/0.3% Si Á/0.15% C) and 440C (Fe Á/17% Cr Á/1.1% C) was achieved by a 500 W CW Nd:YAG laser using different scanning speeds. The microstructure and the phases present in the laser surface-melted specimens were analysed by optical microscopy, scanning electron microscopy and X-ray diffractometry, respectively. The corrosion characteristics of the laser surface-melted specimens in 3.5% NaCl solution and in 1 M sulphuric acid at 23 8C were studied by potentiodynamic polarisation technique. X-ray diffraction spectra showed that laser surface-melted P21 and 440C contain martensite and austenite as the major phase, respectively. Laser surface-melted 440C exhibits passivity whereas laser surface-melted P21 does not. The corrosion resistance of laser surface-melted P21 in both corrosive media is improved as evidenced by a lower corrosion current density compared with that of the untreated specimens. The increase in corrosion resistance of laser surface-melted P21 is due to the dissolution of the intermetallic phase Ni 3 Al to form a homogeneous solid solution by rapid solidification. The corrosion resistance of laser surface-melted 440C in NaCl solution is also increased significantly, with the exhibition of a wide passive range and a low passive current density, but the improvement in sulphuric acid is less pronounced. The enhanced corrosion resistance of laser surface-melted 440C results from the combined effect of the refinement of carbide particles with increased C and Cr in solid solution, and the presence of retained austenite. The corrosion characteristics of all the laser surface-melted specimens are strongly dependent on the laser scanning speed, which in turn results in different microstructures.

Journal of Materials Science, 2004

Formation rate of tetragonal BaTiO 3 powder by hydrothermal synthesis and its dielectric property... more Formation rate of tetragonal BaTiO 3 powder by hydrothermal synthesis and its dielectric property were studied. Submicron tetragonal BaTiO 3 powders were prepared hydrothermally, using Ba(OH) 2 •8H 2 O, TiO 2 (anatase) and KOH as starting chemicals. Characterization via X-ray diffractometry, Field Emission Scanning Electron Microscopy confirmed that increasing calcination temperature (from 1100 to 1300 • C) promotes the formation of tetragonal BaTiO 3. Tetragonal BaTiO 3 ceramics obtained from optimum condition (Tetragonal BaTiO 3 powders calcined at 1200 • C for 3 h after hydrothermal synthesized at 200 • C for 168 h) exhibited submicron size of 0.5-0.7 µm, monodispersed type and high relative permittivity.

Biochimica et Biophysica Acta (BBA) - General Subjects, 1972

SiO x films electrodeposited on platinum (Pt) substrate were shown to exhibit n-type photoactivit... more SiO x films electrodeposited on platinum (Pt) substrate were shown to exhibit n-type photoactivity while placed both in aqueous and organic solutions. The films were obtained via potentiostatic deposition at potential values ranging from −2.25 to −2.75 V. The mechanism of the electrodeposition involved reactions with the used electrolyte and with traces of water as sources of oxygen and hydrogen, as interpreted from Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The photoactivity of the deposits deceased at potential −2.75 V. The highest observed photocurrent was ca. 60 μA/cm 2 and it was registered in organic solution. The band gap energies determined by XPS technique were approximately 2.5, 2.3 and 3.7 eV for films electrodeposited at −2.25, −2.5 and −2.75 V, respectively. The depth profiles of Si, O and H in the films were registered by glow discharge optical emission spectroscopy (GD-OES), which showed that the film thickness was ca. 0.4 μm. The n-type photoactivity was associated with oxygen evolution from the aqueous solution (0.1 M HClO 4 , pH 1), whereas the time instability as a function of time observed for the photocurrent was associated with morphological changes of the films as seen by scanning electron microscopy (SEM) and their oxidation detected by XPS and FTIR spectroscopies.

Journal of energy storage, 2021

Tantalum is a key material that is used to produce small solid capacitors with high volumetric ef... more Tantalum is a key material that is used to produce small solid capacitors with high volumetric efficiency. To achieve higher capacitances in a given volume, Ta nanopowders (NPs) with a large surface area are required for anode preparation. In this study, we report the synthesis of porous tantalum network structures by the combustion method and its electrochemical performance as a capacitor. The network structures, which consist of Ta NPs, are formed via a high-temperature sintering process. The Ta NPs were produced with a minimum size of less than 50 nm and a BET surface area of 31.47 m2/g. Additionally, the formation of Ta porous network structures according to the Ta/Mg ratio was theoretically analysed using a phase field sintering model. According to an analysis of the electrical properties of a sintered Ta anode (k=20), a capacitance of ~190,000 μFV/g is achieved with Ta NPs having a BET surface area of 19.2 cm2/g. In the near future, we hope to achieve ~300,000 μFV/g capacitanc...

In this paper, we study the control system associated with the incompressible 3D Euler system. We... more In this paper, we study the control system associated with the incompressible 3D Euler system. We show that the velocity field and pressure of the fluid are exactly controllable in projections by the same finite-dimensional control. Moreover, the velocity is approximately controllable. We also prove that 3D Euler system is not exactly controllable by a finite-dimensional external force.

Metallurgical and Materials Transactions B

Titanium suboxides with the general formula TinO2n−1 (n > 1.0), also referred to as Magnéli ph... more Titanium suboxides with the general formula TinO2n−1 (n > 1.0), also referred to as Magnéli phases, have been the subject of numerous studies due to their unique properties. Titanium low oxygen suboxides located at n = 1 (TiO) or below (Ti6O, Ti3O and Ti2O) are no less important; however, there is a lack of research on the synthesis of such materials. Therefore, in this article, the combustion process in a TiO2-KClO4-kAl-Flux exothermic system for 0.5 ≤ k ≤ 4.0 (k is moles of Al) is investigated experimentally and thermodynamically to synthesize TinO2n−1 ingots in the 0.5 to 1.0 range of n. The synthesis temperatures and equilibrium reaction phases are calculated with and without a KClO4 booster and several fluxes (CaF2, NaF, CaO and Na3AlF6), which are used to melt the reaction product and achieve the self-separation of the TinO2n−1 ingot from the Al2O3-containing slag. The formation TiO, Ti2O, Ti3O and Ti6O phases is confirmed via XRD analysis and the effectiveness of fluxes is highlighted.

Materials Chemistry and Physics

Journal of Crystal Growth

Abstract In this paper, the formation of β-Si3N4 3D self-assemblies via combustion of Si-(C2F4)n-... more Abstract In this paper, the formation of β-Si3N4 3D self-assemblies via combustion of Si-(C2F4)n-KClO4 solid mixture in nitrogen atmosphere is demonstrated. The coupling of (C2F4)n and KClO4 additives with Si powder stimulates the combustion-nitridation process and leads to the formation and 3D self-assemblies composed of β-Si3N4 prismatic crystals. The self-assemblies are classified as sheet-type, circular-type, castle-type geometry objects, high symmetry multipods and ‘crystals bouquets’ having a central junction. The formation of 3D self-assemblies is based on prismatic crystals twining process and among twinned structures, penetration, contact and circular twins were distinguished. Based on SEM and TEM results a scheme showing the growth process of self-assemblies is proposed.

Journal of Solid State Chemistry

Abstract This paper reports the preparation of Ti microparticles that are angular and spherical i... more Abstract This paper reports the preparation of Ti microparticles that are angular and spherical in shape through the combustion of a TiO2 + αCa (where α is moles of Ca) exothermic mixture in an inert atmosphere. The relationship between the combustion parameters and the properties of the resultant Ti particles is discussed with consideration of the effect of Ca concentration. Using excess amounts of Ca in the experiment was effective in modifying the shape of the particles from angular to spherical. The Ti powder was found to be well-dispersed and the diameter of individual particles ranged from 5 µm to 50 µm. Based on the combustion parameters, the activation energy for the redox reaction was calculated as ∼ 138 kJ/mol. This paper discusses the chemistry of the reduction process and highlights the effects of combustion temperature and the amount of liquid calcium on the characteristics of the Ti particles.

Materials Chemistry and Physics

Abstract The adiabatic temperatures and equilibrium phases in the FeTiO3-αMg-C(B, Si)-kNaCl syste... more Abstract The adiabatic temperatures and equilibrium phases in the FeTiO3-αMg-C(B, Si)-kNaCl system (where α = 0.2–15, k = 0–3) were analyzed to predict the formation of fine powders of titanium compounds from natural ilmenite sand. From the parametric study, the critical variables were identified and optimized for the synthesis of FeTi, TiC, TiB2, and TiFeSi2 under a combustion regime. The experimentally measured temperatures were in good agreement with those predicted from thermochemical calculations in the 900–1800 °C range. The purified products were characterized using X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The fine powders of titanium compounds were mainly single phase with a purity of at least 95 wt%. Considering the various stages of the synthetic process, a mechanism is proposed to explain how the process of ilmenite recovery occurred.

Journal of The Electrochemical Society

Metallurgical and Materials Transactions B

Korean Journal of Metal and Materials, 2009

Materials Letters, 2015

We demonstrated a facile approach for synthesizing three-dimensional (3D), honeycombstructured gr... more We demonstrated a facile approach for synthesizing three-dimensional (3D), honeycombstructured graphene scaffold in the solid flame of a (0.5 + k) Zn + kCuO + (CH 2-CHCl)n reaction system. FESEM-based analyses of the acid purified porous graphene samples revealed that the synthesized graphene had a 3D, honeycomb-like structure with cell sizes of 20-100 nm and wall thicknesses of 1-3 nm. The specific surface area of the graphene sample prepared in the solid flame was about ∼1043 m 2 /g).

SHS processes were introduced in Korea in early 1995. A number of universities and organizations ... more SHS processes were introduced in Korea in early 1995. A number of universities and organizations were involved in research activity for the production of various compact and porous inorganic materials. A research group in RASOM carried out fundamental investigations in the field of SHS of 1D, 2D, and 3D nanostructures and LED phosphor powders. Impressive results were achieved in the field of the synthesis of phosphor powders for LED. Based on achievements in the field of the nitride phosphor powders, in 2011 a LED phosphor manufacturing company, Ellim Advanced Materials, Korea, was founded.

Powder Technology, 2014

ABSTRACT Magnesium was used as a reductant for the direct combustion synthesis of titanium alloy ... more ABSTRACT Magnesium was used as a reductant for the direct combustion synthesis of titanium alloy particles (Ti–6Al–4V) from titanium oxide (TiO2). The feed material (TiO2) and reductant (Mg) were mixed with Al (or AlCl3 · 6H2O), vanadium pentoxide (V2O5), and calcium hydroxide (Ca(OH)2), and the reaction mixture was combusted under pressurized argon. Temperature–time profiles were intermittently measured during combustion to analyze the combustion temperatures and wave velocities. The combustion product was subsequently acid-leached to eliminate secondary phases (MgO, MgCl2, CaO) and produce Ti–6Al–4V alloy particles whose oxygen content was ~ 1.3%. Further the content of oxygen was decreased to 0.2 wt.% by deoxidizing the Ti–6Al–4V alloy particles with calcium in the range 850–900 °C. The AlCl3 · 6H2O affected the particle morphology more than the Al metal did; i.e. the Al-doped alloy particles were porous and consisted of relatively small agglomerated particles whereas using AlCl3 · 6H2O produced dense, round particles. Pilot-scale production experiments revealed that Ti–6Al–4V alloy particles can be efficiently produced using our method.

Materials Science and Engineering: A, 2003

Laser surface melting of plastics mold steels P21 (Fe Á/3% Ni Á/1.5% Mn Á/1% Al Á/0.3% Si Á/0.15%... more Laser surface melting of plastics mold steels P21 (Fe Á/3% Ni Á/1.5% Mn Á/1% Al Á/0.3% Si Á/0.15% C) and 440C (Fe Á/17% Cr Á/1.1% C) was achieved by a 500 W CW Nd:YAG laser using different scanning speeds. The microstructure and the phases present in the laser surface-melted specimens were analysed by optical microscopy, scanning electron microscopy and X-ray diffractometry, respectively. The corrosion characteristics of the laser surface-melted specimens in 3.5% NaCl solution and in 1 M sulphuric acid at 23 8C were studied by potentiodynamic polarisation technique. X-ray diffraction spectra showed that laser surface-melted P21 and 440C contain martensite and austenite as the major phase, respectively. Laser surface-melted 440C exhibits passivity whereas laser surface-melted P21 does not. The corrosion resistance of laser surface-melted P21 in both corrosive media is improved as evidenced by a lower corrosion current density compared with that of the untreated specimens. The increase in corrosion resistance of laser surface-melted P21 is due to the dissolution of the intermetallic phase Ni 3 Al to form a homogeneous solid solution by rapid solidification. The corrosion resistance of laser surface-melted 440C in NaCl solution is also increased significantly, with the exhibition of a wide passive range and a low passive current density, but the improvement in sulphuric acid is less pronounced. The enhanced corrosion resistance of laser surface-melted 440C results from the combined effect of the refinement of carbide particles with increased C and Cr in solid solution, and the presence of retained austenite. The corrosion characteristics of all the laser surface-melted specimens are strongly dependent on the laser scanning speed, which in turn results in different microstructures.

Journal of Materials Science, 2004

Formation rate of tetragonal BaTiO 3 powder by hydrothermal synthesis and its dielectric property... more Formation rate of tetragonal BaTiO 3 powder by hydrothermal synthesis and its dielectric property were studied. Submicron tetragonal BaTiO 3 powders were prepared hydrothermally, using Ba(OH) 2 •8H 2 O, TiO 2 (anatase) and KOH as starting chemicals. Characterization via X-ray diffractometry, Field Emission Scanning Electron Microscopy confirmed that increasing calcination temperature (from 1100 to 1300 • C) promotes the formation of tetragonal BaTiO 3. Tetragonal BaTiO 3 ceramics obtained from optimum condition (Tetragonal BaTiO 3 powders calcined at 1200 • C for 3 h after hydrothermal synthesized at 200 • C for 168 h) exhibited submicron size of 0.5-0.7 µm, monodispersed type and high relative permittivity.

Biochimica et Biophysica Acta (BBA) - General Subjects, 1972

SiO x films electrodeposited on platinum (Pt) substrate were shown to exhibit n-type photoactivit... more SiO x films electrodeposited on platinum (Pt) substrate were shown to exhibit n-type photoactivity while placed both in aqueous and organic solutions. The films were obtained via potentiostatic deposition at potential values ranging from −2.25 to −2.75 V. The mechanism of the electrodeposition involved reactions with the used electrolyte and with traces of water as sources of oxygen and hydrogen, as interpreted from Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The photoactivity of the deposits deceased at potential −2.75 V. The highest observed photocurrent was ca. 60 μA/cm 2 and it was registered in organic solution. The band gap energies determined by XPS technique were approximately 2.5, 2.3 and 3.7 eV for films electrodeposited at −2.25, −2.5 and −2.75 V, respectively. The depth profiles of Si, O and H in the films were registered by glow discharge optical emission spectroscopy (GD-OES), which showed that the film thickness was ca. 0.4 μm. The n-type photoactivity was associated with oxygen evolution from the aqueous solution (0.1 M HClO 4 , pH 1), whereas the time instability as a function of time observed for the photocurrent was associated with morphological changes of the films as seen by scanning electron microscopy (SEM) and their oxidation detected by XPS and FTIR spectroscopies.

Journal of energy storage, 2021

Tantalum is a key material that is used to produce small solid capacitors with high volumetric ef... more Tantalum is a key material that is used to produce small solid capacitors with high volumetric efficiency. To achieve higher capacitances in a given volume, Ta nanopowders (NPs) with a large surface area are required for anode preparation. In this study, we report the synthesis of porous tantalum network structures by the combustion method and its electrochemical performance as a capacitor. The network structures, which consist of Ta NPs, are formed via a high-temperature sintering process. The Ta NPs were produced with a minimum size of less than 50 nm and a BET surface area of 31.47 m2/g. Additionally, the formation of Ta porous network structures according to the Ta/Mg ratio was theoretically analysed using a phase field sintering model. According to an analysis of the electrical properties of a sintered Ta anode (k=20), a capacitance of ~190,000 μFV/g is achieved with Ta NPs having a BET surface area of 19.2 cm2/g. In the near future, we hope to achieve ~300,000 μFV/g capacitanc...

In this paper, we study the control system associated with the incompressible 3D Euler system. We... more In this paper, we study the control system associated with the incompressible 3D Euler system. We show that the velocity field and pressure of the fluid are exactly controllable in projections by the same finite-dimensional control. Moreover, the velocity is approximately controllable. We also prove that 3D Euler system is not exactly controllable by a finite-dimensional external force.

Metallurgical and Materials Transactions B

Titanium suboxides with the general formula TinO2n−1 (n > 1.0), also referred to as Magnéli ph... more Titanium suboxides with the general formula TinO2n−1 (n > 1.0), also referred to as Magnéli phases, have been the subject of numerous studies due to their unique properties. Titanium low oxygen suboxides located at n = 1 (TiO) or below (Ti6O, Ti3O and Ti2O) are no less important; however, there is a lack of research on the synthesis of such materials. Therefore, in this article, the combustion process in a TiO2-KClO4-kAl-Flux exothermic system for 0.5 ≤ k ≤ 4.0 (k is moles of Al) is investigated experimentally and thermodynamically to synthesize TinO2n−1 ingots in the 0.5 to 1.0 range of n. The synthesis temperatures and equilibrium reaction phases are calculated with and without a KClO4 booster and several fluxes (CaF2, NaF, CaO and Na3AlF6), which are used to melt the reaction product and achieve the self-separation of the TinO2n−1 ingot from the Al2O3-containing slag. The formation TiO, Ti2O, Ti3O and Ti6O phases is confirmed via XRD analysis and the effectiveness of fluxes is highlighted.

Materials Chemistry and Physics

Journal of Crystal Growth

Abstract In this paper, the formation of β-Si3N4 3D self-assemblies via combustion of Si-(C2F4)n-... more Abstract In this paper, the formation of β-Si3N4 3D self-assemblies via combustion of Si-(C2F4)n-KClO4 solid mixture in nitrogen atmosphere is demonstrated. The coupling of (C2F4)n and KClO4 additives with Si powder stimulates the combustion-nitridation process and leads to the formation and 3D self-assemblies composed of β-Si3N4 prismatic crystals. The self-assemblies are classified as sheet-type, circular-type, castle-type geometry objects, high symmetry multipods and ‘crystals bouquets’ having a central junction. The formation of 3D self-assemblies is based on prismatic crystals twining process and among twinned structures, penetration, contact and circular twins were distinguished. Based on SEM and TEM results a scheme showing the growth process of self-assemblies is proposed.

Journal of Solid State Chemistry

Abstract This paper reports the preparation of Ti microparticles that are angular and spherical i... more Abstract This paper reports the preparation of Ti microparticles that are angular and spherical in shape through the combustion of a TiO2 + αCa (where α is moles of Ca) exothermic mixture in an inert atmosphere. The relationship between the combustion parameters and the properties of the resultant Ti particles is discussed with consideration of the effect of Ca concentration. Using excess amounts of Ca in the experiment was effective in modifying the shape of the particles from angular to spherical. The Ti powder was found to be well-dispersed and the diameter of individual particles ranged from 5 µm to 50 µm. Based on the combustion parameters, the activation energy for the redox reaction was calculated as ∼ 138 kJ/mol. This paper discusses the chemistry of the reduction process and highlights the effects of combustion temperature and the amount of liquid calcium on the characteristics of the Ti particles.

Materials Chemistry and Physics

Abstract The adiabatic temperatures and equilibrium phases in the FeTiO3-αMg-C(B, Si)-kNaCl syste... more Abstract The adiabatic temperatures and equilibrium phases in the FeTiO3-αMg-C(B, Si)-kNaCl system (where α = 0.2–15, k = 0–3) were analyzed to predict the formation of fine powders of titanium compounds from natural ilmenite sand. From the parametric study, the critical variables were identified and optimized for the synthesis of FeTi, TiC, TiB2, and TiFeSi2 under a combustion regime. The experimentally measured temperatures were in good agreement with those predicted from thermochemical calculations in the 900–1800 °C range. The purified products were characterized using X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The fine powders of titanium compounds were mainly single phase with a purity of at least 95 wt%. Considering the various stages of the synthetic process, a mechanism is proposed to explain how the process of ilmenite recovery occurred.

Journal of The Electrochemical Society