A great deal of emphasis has recently been placed on the development of new materials with specific sets of properties. There is one efficient universal method of qualitatively altering material properties the production of materials with an ultrafine-grained structure. Of particular interest in this regard are ceramic materialsoxides, carbides, and nitrides. Ceramic products made by the traditional technology are not reliable enough in service and are typically quite brittle. The use of ultrafine powders (UFPs) is expected to solve both of these problems [1]. The goal of this investigation is to study the properties of UFPs of silicon nitride and silicon carbide obtained by the laser vaporization of a target. This is an efficient method of obtaining UFPs of different materials [2]. The experimental unit consisted of a CO2-1aser, a system to transport the laser beam, a synthesis reactor, and a vacuum system. The transport system included an RSI-5 power meter and revolving copper mirrors. The reactor had an inlet window, made of KCI, that also served as a focusing lens. A coordinate table to move the target was also included in the reactor. The quality of the KCI optical system made it possible to focus energy with a power density of 106 W/cm 2. The laser beam, passing through the transport system and lens, was focused on the target. The material being vaporized was condensed on the surface of a watercooled collection tank made of stainless steel. A gas-pumping regime with pressures ranging from 1 to 100 kPa was maintained in the reactor to keep the optical system clean. The ultrafine powders were studied on x-ray diffractometer DRON-4M with cobalt radiation. The phases were identified on the basis of the interplanar spacings determined from the diffraction patterns [3]. An electron microscope was used to evaluate the arithmetic-mean grain size. Here, we will examine the thermodynamic conditions for the synthesis of SiC and Si3N 4. The thermodynamics of the S i C system was studied in [4] both at atmospheric pressure and at 10 MPa. The given system contains the condensed phase SiC c, which decomposes peritectically at atmospheric pressure into graphite C c and a gas phase enriched with silicon. At high pressures, the condensed phase decomposes into graphite and a high-silicon melt. Experimental studies [4] have shown that the reaction of silicon with carbon at the temperature of the synthesis of silicon carbide occurs through a gas phase composed of silicon and its monoxide. Proof of this comes from the fact that the reaction products include whisker crystals, which are formed from the gas phase. Thermodynamic calculations of the system SiO2-C c [5, 6] and experimental studies [7-9] have shown that the carbide is always contaminated by pure carbon in the presence of a stoichiometric ratio of the reagents SiO 2 and 3C c. A deficit of carbon is needed in the reaction in order to obtain pure carbide. The system SiO 2 3C c contains SiO 2 and C c up to 1400~ the content of silicon carbide increases rapidly above this temperature, reaching 99.5% at 1700~ With an excess of SiO 2 (1.1SIO2-3C c) and a temperature of 1900~ 100% SiC is formed. A thermodynamic calculation was performed using the equilibrium constants from [10] to study equilibrium on the vaporization-condensation line in the S i C system within the ranges 1-100 kPa and 2000-3400 K, this information being of practical value for the process of laser vaporization. We calculated the equilibrium compositions above different combinations of condensed components of the system (C c, Si t, SiC c) within the range 2000-3400 K. The liquid phase of silicon was absent above condensed silicon carbide throughout the given temperature range, while condensed carbon was present at and below 2800 K. Vapor pressure above SiC c and C c is determined from the formula
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Sep 8, 2021
Song Zhou + 8
