Search

Abstract: The densification behaviors of Al2O3–Cr2O3/Cr3C2 nanocomposites prepared by a Spark Plasma Sintering (SPS) were investigated in this work. The initial powders used for sintering were Al2O3–Cr2O3, which were prepared by metal organic chemical vapor deposition (MOCVD) in a spout bed. Different colors of the compacts such as green, purple and black were observed after densification process at different SPS temperatures from 1200°C to 1350°C. These changes of color were relevant to the existence of secondary phase of green Cr2O3, pink solid solution of Cr2O3–Al2O3 and black Cr3C2, which were formed under the different SPS temperature. The secondary phase of Cr2O3 retarded the processing of densification for spark plasma sintering at 1200°C. The Cr2O3 reacted with Al2O3 to form solid solution of Cr2O3–Al2O3 and with carbon to form Cr3C2 as sintering temperature increased to 1350°C. The characteristics of high heating rate, shorter sintering time for SPS and the formation of secondary phase of Cr3C2 effectively reduced the substrate''s grain growth, making Al2O3–Cr2O3/Cr3C2 nanocomposites with small grain size. [Copyright &y& Elsevier]

Abstract: Nanosized particles dispersed uniformly on Al2O3 particles were prepared from the decomposition of precursor Cr(CO)6 by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr2O3, CrC1−x , and C. A solid solution of Al2O3–Cr2O3 and an Al2O3–Cr2O3/Cr3C2 nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150°C before hot-pressing at 1400°C, respectively. In addition, an Al2O3–Cr2O3/Cr-carbide (Cr3C2 and Cr7C3) nanocomposite was formed when the particles were directly hot pressed at 1400°C. The interface between Cr3C2 and Al2O3 is non-coherent, while the interface between Cr7C3 and Al2O3 is semi-coherent. [Copyright &y& Elsevier]

Abstract: Nano-scale Cr3C2 powder with an average particle size of 50–80nm, uniformly coated on alumina particles, has been prepared by metal organic chemical vapor deposition (MOCVD) in the fluidized chamber, using the pyrolysis of Cr(CO)6 precursor, and carbothermal treatment in the CH4 and H2 gas mixtures. The Cr(CO)6 precursor was characterized by differential scanning calorimetry (DSC) to understand the decomposition temperature of the precursor, which was determined to be 163°C. The precursor decomposed and formed the mixture of CrC1− x , Cr2O3 and free carbon on the surface of the Al2O3 particles when pyrolyzed in fluidized bed at 300 and 400°C. The resultant powders were examined by X-ray diffractometer (XRD) and transmission electron microscopy (TEM) equipped with energy dispersion spectrometer (EDS), for investigating phase composition and microstructures. The as-deposited particles fabricated at 300 and 400°C in fluidized chamber were amorphous and crystalline, respectively. The thermal behaviour investigated by TG/DTA indicates that the amorphous powder had a exothermic peak in the DTA curve at ∼400°C, which crystallized at this temperature. In order to prepare the nano-particle reinforced composites, the powder fabricated in the fluidized bed was carbonized in the mixed atmosphere of CH4 and H2. Both CrC1− x and Cr2O3 phase were transferred into Cr3C2 phase after the carbothermal treatment. [Copyright &y& Elsevier]