S. Kharatyan
Yerevan State University, Faculty of Chemistry, Department Member
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Aluminum diboride was chosen as an initial reagent to obtain BN/AlN composites with high homogeneity. By comparing the specific surface area of products, it was established that this parameter can be increased (up to 10-24 m 2 /g) by... more
Aluminum diboride was chosen as an initial reagent to obtain BN/AlN composites with high homogeneity. By comparing the specific surface area of products, it was established that this parameter can be increased (up to 10-24 m 2 /g) by increasing the amount of boron nitride. The gasifying compounds have essential influence on the formation of submicron particles in products. Even at their negligible amounts the specific surface area can increase by a factor of 2, and at higher amounts (close to the combustion limit) by 4 times. The maximum value for specific surface area (86 m 2 /g) was obtained for the 80%BN+20%AlN composite. It was established that it is impossible to obtain BN-B4C composite by direct interaction between elements (B, C and N2). Therefore another way was developed aimed at synthesizing BN-B4C composite under the activated combustion mode. Organic nitrogen containing compounds (ONCC) were used as sources for both the carbon and nitrogen.
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ABSTRACT Differential scanning calorimetry and a high-speed temperature scanner were used to characterize dynamic features of the reaction between polytetrafluoro-ethylene (PTFE) and Al 2 O 3 under heating rates ranging between 20 and 780... more
ABSTRACT Differential scanning calorimetry and a high-speed temperature scanner were used to characterize dynamic features of the reaction between polytetrafluoro-ethylene (PTFE) and Al 2 O 3 under heating rates ranging between 20 and 780 °C min -1 . Exothermic reaction behavior between PTFE and Al 2 O 3 was observed at heating rates of 150 °C min -1 and higher. Thermodynamic calcu-lations predicted an adiabatic temperature of 1,425 K for the PTFE/Al 2 O 3 stoichiometric ratio. At lower heating rates, endothermic decomposition of PTFE dominated the interaction, where PTFE decomposes into gaseous products that escape the system without interacting with alumina. The enthalpy of the PTFE–Al 2 O 3 exothermic reaction was estimated to be -103 kJ mol -1 with activation energy of 21 kJ mol -1 . This study shows that, for energetic formu-lation of Al–PTFE, the Al 2 O 3 layer on the aluminum par-ticles can exothermically react with PTFE, producing AlF 3 and carbon monoxide.
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The kinetic regularities were studied of the carbidization of zirconium in a methane medium and during contact with pyrographite over a wide range of gas pressures P CH4 =10-600 mm Hg and temperatures 1000-1800 o C. The formation of two... more
The kinetic regularities were studied of the carbidization of zirconium in a methane medium and during contact with pyrographite over a wide range of gas pressures P CH4 =10-600 mm Hg and temperatures 1000-1800 o C. The formation of two types of carbide layers has been established: finely crystalline-porous and macrocrystalline-compact. The conditions of their formation and the kinetic parameters of growth have been determined.
This work studies how to obtain Ti-Al-based porous material by self-propagating hightemperature synthesis (SHS). Due to the low exothermicity of this system and the passive nature of the starting powders (mainly owed to surface... more
This work studies how to obtain Ti-Al-based porous material by self-propagating hightemperature synthesis (SHS). Due to the low exothermicity of this system and the passive nature of the starting powders (mainly owed to surface oxidation), there exist great difficulties to realise the synthesis reaction in a wave propagation mode. This problem has been tackled and successfully solved by the use of chemically activated process. It has been found that the same product used as chemical activator also plays the role of foaming agent. Consequently, intermetallic foams have been successfully synthesized by this method. The investigation has been divided in two main parts: First, the phase formation of the product was studied. It was shown that besides the dominant TiAl phase, a ternary phase (Ti4Al2C2) was also obtained, coming from the decomposition of the carbon-containing activator. Second, porosity formation was investigated and optimized. The role of the foaming agent (FA) in the pha...
The mechanism and kinetics of nickel oxide reduction by Mg + C combined reducer at non-isothermal conditions in a wide range of high heating rates (100–1200 °C min−1) were studied by high-speed temperature scanner technique. It was shown... more
The mechanism and kinetics of nickel oxide reduction by Mg + C combined reducer at non-isothermal conditions in a wide range of high heating rates (100–1200 °C min−1) were studied by high-speed temperature scanner technique. It was shown that both the magnesiothermal and magnesiocarbothermal reduction processes, unlike low heating rates (5–20 °C min−1, DTA/DTG method), start beyond the melting point of magnesium and proceed via solid + liquid scheme. It was found out that the utilization of Mg + C combined reducer allows to significantly lower the reduction temperature (by about 200–400 °C) of nickel oxide as compared with exclusively magnesium or carbon reducers evidencing on the synergetic effect of combined reducers in the ternary mixture. The effective values of activation energy (Ea) for the NiO + Mg and 2NiO + Mg + C reactions were determined to be 185 ± 15 kJ mol−1 and 195 ± 15 kJ mol−1, respectively.
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Composite powders W–Cu were prepared via joint reduction of WO3 and CuO oxides with Mg‒C combined reducer in a combustion mode by using the method of coupled reactions. Combustion phenomenology and the processes of phase and... more
Composite powders W–Cu were prepared via joint reduction of WO3 and CuO oxides with Mg‒C combined reducer in a combustion mode by using the method of coupled reactions. Combustion phenomenology and the processes of phase and microstructure formation were investigated by thermocouple and copper- wedge techniques combined with XRD, SEM, and EDS analyses. Thermal conditions of combustion and phase composition and microstructure of products were found to depend on a Mg/C ratio in green mixtures. It was established that the magnesiothermic reaction was preceded by the stage of low-caloric carbothermal reduction. Slow propagation of combustion wave was found to favor the complete reduction of oxides and formation of target W–Cu composite nanopowder.
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Kinetic laws of high-temperature interaction between tungsten and methane are studied under non-isothermal conditions in the temperature interval 1273–2873K. Computer Assisted Electrothermography was applied during which thin metallic... more
Kinetic laws of high-temperature interaction between tungsten and methane are studied under non-isothermal conditions in the temperature interval 1273–2873K. Computer Assisted Electrothermography was applied during which thin metallic wires 100μm in diameter were directly heated up by electric current in the carbon-containing atmosphere. Experimental data on weight gain, carbide layer growth, and microstructure of phases are obtained in the linear heating regime at heating rates from 10 to 1500K/s. We established that the interaction between tungsten and methane in a wide interval of pressure and heating rates may proceed with simultaneous or consecutive formation of W2C and WC carbide phases. Experimental data on weight gain and carbide layer growth were processed using calculation schemes deduced within the framework of a reaction diffusion model with the first type boundary conditions under non-isothermal conditions. Kinetic and diffusion constants are determined for tungsten carbidization with formation of the W2C phase.
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The fabrication method and the mechanical and electrical properties of different MoSi2–Si3N4 composite materials were investigated. Commercially available individual compounds, one-stage combustion synthesized MoSi2–Si3N4 and submicron... more
The fabrication method and the mechanical and electrical properties of different MoSi2–Si3N4 composite materials were investigated. Commercially available individual compounds, one-stage combustion synthesized MoSi2–Si3N4 and submicron MoSi2 powders were used as starting materials, followed by hot pressing. It was found that the sintering atmosphere used, nitrogen or argon, had a significant effect on the phase composition, mechanical and electrical properties of the final materials. It was shown that in some cases partial nitridation of MoSi2 occurred with the formation of MoSi2–Mo5Si3–Si3N4 ternary composites. The electrical conductivity of the composites depends also on the microstructure of materials. It was shown that the composites fabricated using combustion synthesized MoSi2 powders (500nm) are characterized by higher flexural strength at room temperature compared to those from commercial powders. On the other hand, the composites fabricated from the commercial powders had higher strength and fracture toughness at elevated temperatures (up to 1200°C). For all composites, the strength decreased significantly at temperatures over 1000°C due to the brittle–ductile transition of the MoSi2 phase.
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... selected. Journal of Alloys and Compounds Volume 454, Issues 1-2, 24 April 2008, Pages 389-393. doi ... a Department of Chemical Physics, Yerevan State University, A. Manukyan Str. 1, Yerevan 375025, Republic of Armenia. b Laboratory... more
... selected. Journal of Alloys and Compounds Volume 454, Issues 1-2, 24 April 2008, Pages 389-393. doi ... a Department of Chemical Physics, Yerevan State University, A. Manukyan Str. 1, Yerevan 375025, Republic of Armenia. b Laboratory ...
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A solution of the nonstationary Stefan problem is presented for bodies of plane, cylindrical, and spherical symmetry in application to processes of diffusion interaction between metals and a gaseous oxidative medium.
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... selected. Journal of Alloys and Compounds Volume 454, Issues 1-2, 24 April 2008, Pages 389-393. doi ... a Department of Chemical Physics, Yerevan State University, A. Manukyan Str. 1, Yerevan 375025, Republic of Armenia. b Laboratory... more
... selected. Journal of Alloys and Compounds Volume 454, Issues 1-2, 24 April 2008, Pages 389-393. doi ... a Department of Chemical Physics, Yerevan State University, A. Manukyan Str. 1, Yerevan 375025, Republic of Armenia. b Laboratory ...
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ABSTRACT The possibilities for combustion synthesis of TiN–Si3N4 composite powders were investigated using different titanium silicides (TiSi, Ti5Si3, TiSi2) as initial raw materials. Thermodynamic analysis and relevant experiments have... more
ABSTRACT The possibilities for combustion synthesis of TiN–Si3N4 composite powders were investigated using different titanium silicides (TiSi, Ti5Si3, TiSi2) as initial raw materials. Thermodynamic analysis and relevant experiments have shown that at relatively low nitrogen pressure (up to 5 MPa) Ti5Si3 is the most suitable compound for this purpose. Optimum experimental conditions for synthesizing two-phase silicon-free TiN–Si3N4 (containing 30–35 wt.% TiN) composite powders with characteristic sizes of approximately 5–10 μm were determined. Compact discs of 50 mm diameter were fabricated using hot pressing technique. Microstructural features of the composite powder and hot pressed samples were investigated, i.e. biaxial flexure strength.
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An electrothermographic method was used to study the regularities of heat release and formation of the microstructure of tungsten disilicide with wide variation in the temperature regime of sample heating, including conditions modeling... more
An electrothermographic method was used to study the regularities of heat release and formation of the microstructure of tungsten disilicide with wide variation in the temperature regime of sample heating, including conditions modeling combustion-wave propagation in mixtures of tungsten and silicon powders. Emphasis is on the heat-release stage due to the crystallization of the disilicide phase from a supersaturated melt