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Oxidation of ZrB2 + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE-TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000°C-1600°C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C-1600°C. The mass gain increased from ~5 to 45 g/m2 and parabolic rate constants increased from 0.037 to 2.2 g2/m4·s over this temperature range. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass-ceramic surface layer as rate limiting. [ABSTRACT FROM AUTHOR]

An original catalytic method has been proposed to synthesize carbon nanotubes (CNTs)-ZrB2-ZrO2 heterostructures using ZrB2 polymeric precursor. The pyrolysized gases from the ZrB2 polymeric precursor are identified to be the carbon sources for CNTs growth. A parametric study is conducted to control the CNTs growth by optimizing parameters such as synthesis temperature and catalyst content. Observations show that the in situ grown CNTs are homogeneously dispersed in the powders, and the structure and the amount of CNTs are significantly dependent on the synthesis parameters. There are two kinds of grown CNTs existed in the produced hybrid heterostructures: (i) the kinking structured CNTs that are disordered and incomplete graphitization; (ii) the improved and graphitized CNTs. The ZrB2 polymeric precursor during thermal pyrolysis provides capable of supplying substantial carbon source for CNTs nucleation and growth by homogeneous vapor-liquid-solid reactions. [ABSTRACT FROM AUTHOR]

The formation of a porous SiC-depleted region in ZrB2-SiC due to active oxidation at ultrahigh temperatures was characterized. The presence/absence of SiC depletion was determined at a series of temperatures (1300°C-1800°C) and times (5 min-100 h). At T < 1627°C, SiC depletion was not observed. Instead, the formation of a ZrO2 + C/borosilicate oxidation product layer sequence was observed above the ZrB2-SiC base material. At T ≥ 1627°C, SiC was depleted in the ZrB2 matrix below the ZrO2 and borosilicate oxidation products. The SiC depletion was attributed to active oxidation of SiC to form SiO(g). The transition between C formation in ZrO2 ( T < 1627°C) and SiC depletion in ZrB2 ( T ≥ 1627°C) is attributed to variation in the temperature dependence of thermodynamically favored product assemblage influenced by the local microstructural phase distribution. The growth kinetics of the SiC depletion region is consistent with a gas-phase diffusion-controlled process. [ABSTRACT FROM AUTHOR]