Papers by Keyword: Zirconium Carbide

Abstract: Zirconium carbide (ZrC) is technologically important in devices that must function at high temperatures, and its ground state is a NaCl like structure. Changes of the structure and electronic properties of ZrC under high pressure were studied within the framework of density functional theory (DFT). This research was performed for several structures, such as NaCl type (B1), CsCl type (B2), ZnS type (B3), wurtzite type (B4) and NiAs type (B8) structures for ZrC, looking for possible phase transitions induced by high pressure, and four phase transitions were found: the first is the well-known phase transition from the B1 structure to the B2 structure, which occurs at around ~291 GPa. Additionally, in the present paper we found other phase transitions: from the B8 structure to the B2 structure, from the B4 structure to the B2 structure, and lastly from the B3 structure to the B2 structure. Electronic bands are exhibited at zero pressure and temperature (with spin-orbit coupling) for the five structures. For the transition from the B1 structure to the B2 structure, bands are exhibited for two pressures, one before and the other after the transition occurs. The band structure shows that all the structures studied are metallic and nonmagnetic, with the exception the B3 structure, which exhibits a semi-metallic and nonmagnetic behaviour.

Abstract: In the present study, we report on the synthesis and carbothermal reduction of ultra-fine zirconium diboride powders by using inorganic-organic hybrid precursors of Zirconium (IV) nitrate pentahydrate, boric acid and citric acid as sources of zirconia, boron oxide and carbon, resoectively. The effect of molar ratio of reactants and reaction temperatures on the as-synthesized precursors were investigated. The thermodynamic change in the ZrO₂-B₂O₃-C system was mainly studied by thermogravimetric and differential scanning calorimetry. The phase compositions and crystalline state of the products after heat treatment was determined by X-ray diffraction and the crystallite size and morphology of the synthesized powders were characterized by scanning electron microscopy. It was found that the as-synthesized precursor with B/Zr molar ratio of 3.5 can transform into zirconium diboride and zirconium carbide by heating in an argon atmosphere with temperatures as low as 1400°C and the synthesized powders exhibited near-spherical morphology with a samll average crystallite size of about 200nm and dispersed relatively uniformly. Moreover, with the reaction temperature increased, the purity of the zirconium diboride powders are higher. The mixture was finally transformed into pure zirconium diboride at 1600°C. However, the grain sizes increased significantly and tended to be aggregated with the reaction temperature increased to 1600°C. The synthesized ZrB₂ powders showed a porous structure and the grain sizes on the exterior is larger than the interior because of the higher heat treatment temperature. The finally single ultra-fine ZrB₂ grain sizes were distributed from 190nm to 690nm in two-dimensions and have a larger specific surface area of 88.14m²/g.

Abstract: Zirconium carbide (ZrC) has extended application in many ceramic and metal matrix composites especially used for ultra high temperature conditions. The synthesis of zirconium carbide powder is costly and difficult because of its high refractoriness and chemically inert properties. In this research, the synthesis of zirconium carbide nanopowder at low temperature via carbothermal reduction route was investigated according to thermodynamic data. The starting materials were zirconium acetate and sucrose as zirconium and carbon sources, respectively. After preparation of different carbon/zirconium ratio containing precursors, the dried precursors were heat treated at 1400°C and vacuum atmosphere. Also the ZrC formation was followed by thermal analysis of the produced precursors. The phase evolutions and microstructural studies were carried out using X-ray diffraction and scanning electron microscopy. The results showed that it is possible to synthesis zirconium carbide nanopowder with round shape and crystallite sizes smaller than 20 nm at low temperatures. Also according to thermodynamic calculations, it was concluded that by applying vacuum condition, the zirconium carbide formation can occur at less than 1000°C which is very effective on the size reducing of produced ZrC nanopowders.

Abstract: Zirconium carbide (ZrC) coatings were prepared on C/C composite via molten salt reaction process at relatively low temperatures of 800-1000°C. During the reaction process, potassium fluorozirconate (K2ZrF6) played a role transporting zirconium from the molten salt to the C/C composite surface. Elevating reaction temperature increased the growth rate of coatings, simultaneously leaded to rougher coatings. The coatings growth rate increased with reaction time at first and then decreased gradually. The ZrC coatings prepared at 900°C for 5h was ~2m thickness. At the early stage, the low solubility of zirconium in the molten salt leaded to the low coatings growth rate. Secondly, the growth rate of the ZrC coatings was controlled by the chemical reaction between C/C composites and zirconium once zirconium was saturated in the molten salts. Thirdly, the control step of coatings formation turned into the diffusion of carbon through the formed ZrC coatings and which leaded to a gradual decrease of growth rate.

Abstract: Based on the basic principle of carbothermal reduction reactions in the ZrO2-C system, nanosized ZrC powders were synthesized by the preceramic method using Polyzirconoxane (PZO) and salicyl alcohol (SA) as sources of zirconia and carbon respectively. The obtained polymeric precursor exhibited excellent solubility and rheology for the processing of ceramic matrix composites. Thermal dynamic change process, phase composition, crystallite size and morphology of the synthesized powders were characterized by Thermogravimetric analysis (TGA)、 Scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). When heat-treated at 900 °C under flowing argon, precursors transformed into intimately mixed amorphous carbon and nanosized tetragonal ZrO2. Further heat treatments (1300 °C) led to the formation of zirconium carbide with a yield of 70.6 %. The obtained ZrC particles exhibit cubic morphology with size ranged in 50-100 nm. This study displays the preparation of nanosized ZrC materials from the preceramic polymers.

Abstract: Chemical processes of the condensed combustion in systems Al – ZrO2 – C and Al – ZrSiO4 – C, systems were studied, dependences of the properties of synthesis products on conditions were determined SHS.

Abstract: Successful preparation of massive compact bodies from ultra-high temperature ceramics like zirconium carbide, hafnium carbide and their cermets with tungsten matrix with high values of mechanical parameters is difficult. Only limited number of techniques is able to perform it because of their absolutely highest melting points. In our contribution the preparation of these materials by both - hot pressing and plasma spraying techniques is described and chemical processes taking part at the fabrication are studied. Hot-pressed products fabricated at 2000°C and 6 GPa partly react with the internal surface of the BELT-type apparatus. Melting and solidification is taking place at plasma spraying. This process, carried out by water stabilized plasma torch with centerline temperature up to 30 000°C, is joined with undesirable reactions with plasma-forming medium, with oxidation in a turbulent plasma flow and nitridation of free-flight particles. All these chemical processes depend on variety of parameters, such as particle size of the feedstock powders or electric power of the arc. The mentioned parameters are easily controlled. Other, less controllable, factors include trajectory of powder particles in the plasma jet, important for the melting degree as well as for oxidation or nitridation of the powder surface. New knowledge concerning properties of compact ZrC and HfC were obtained.

Abstract: ZrC coatings were prepared by CVD using ZrCl₄, C₃H₆, and H₂ as the precursors. The mechanisms responsible for the effects of deposition temperature, H₂ flow rate and inlet C/Zr ratio on the ZrC coatings were studied based on the deposition mechanism of ZrC. The results indicate that the ZrC morphologies change from a loose spherical structure to a cauliflower structure, then to a glassy structure as the deposition temperature increases from 1050°C to 1150°C, then to 1250°C. The carbon content in the ZrC coatings increases with increasing the deposition temperature. Higher inlet C/Zr ratio can lead to rough surfaces and higher carbon content. Reasonable H₂ concentration can inhibit carbon deposition, and lead to a cauliflower structure.