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Using the sol–gel process, a composite powder of HfB2–(SiO2–C) composition was synthesized and then utilized for reactive sintering (hot pressing, 30 MPa; 1800 °C, 15 min) of ultra-high-temperature ceramic HfB2–30 vol% SiC composites. It was established that nanocrystalline silicon carbide (36 ± 2 nm) in a cubic modification is formed during hot pressing; HfO2 and HfC impurities were not found. The relative density of the obtained materials was 89.2 ± 2.3%. The long-term (40-min) oxidation resistance of the HfB2–30 vol% SiC sample was studied under the exposure to the supersonic dissociated airflow on a high-frequency induction plasmatron (heat flux changed from 232 to 779 W/cm2, pressure in the plasmatron chamber was 15 hPa), in a configuration that prevented significant heat discharge from the sample to a water-cooled holder (with 1-mm overhang). It was shown that, with a stepwise increase in the heat load in the initial stages, the temperature distribution over the front surface of the sample was relatively uniform. However, at the heat flux of 598 W/cm2, local overheated areas appeared in the central region, which spread over almost the entire surface of the sample within 1–2 min; the average temperature was ~2560 °C. Using emission spectroscopy data from the boundary layer above the sample surface, as well as XRD and SEM of the sample after exposure, it was shown that the sharp increase in temperatures from ~1500–1600 to 2500–2600 °C was associated with a change in the chemical nature of the surface, due to evaporation of borosilicate glass components and the appearance of a porous highly catalytic HfO2 with low thermal conductivity on the surface. It was noted that these processes under the exposure to the supersonic flow started at lower temperature than under the exposure to subsonic dissociated airflows. Highlights: HfB2–(SiO2–C) composite powder was synthesized using the sol–gel process. HfB2–30 vol% SiC UHTC was produced by reactive sintering of HfB2–(SiO2–C) powder. Oxidation resistance of UHTC under the exposure to the supersonic airflow at 2500 °C was studied. Features of material oxidation were identified, in particular utilizing emission spectroscopy of the boundary layer. The composition and microstructure of the oxidized material layer were studied, both its surface and thin section. [ABSTRACT FROM AUTHOR]

The solubility of ozone in the aqueous solutions of acids, HClO4 and H2SO4, and salts, LiClO4, NaClO4, KClO4, Mg(ClO4)2, Ca(ClO4)2, NaCl, KCl, Na2SO4, K2SO4, and NaNO3, is studied. On the basis of experimental results and published data, the values of the Weisenberger-Schumpe model parameters for the perchlorate ion and ozone are proposed to be = 0.0391 M−1 and hG,0 = −0.0152 M−1 (t = 25°C), respectively. The values of the similar parameters for the van Krevelen-Hoftijzer model are found to be = 0.336 M−1 and xG = −0.215 M−1, respectively. The discrepancies between the values of Sechenov constants calculated using the van Krevelen-Hoftijzer model for the one-component salty solutions, are somewhat smaller than between those determined using the Weisenberger-Schumpe model. The Sechenov constants are satisfactorily predicted (to a ~ 20% accuracy) by the Weisenberger-Schumpe model with the new and hG,0 values. [ABSTRACT FROM AUTHOR]

The solubility of ozone in the aqueous solutions of acids, HClO4 and H2SO4, and salts, LiClO4, NaClO4, KClO4, Mg(ClO4)2, Ca(ClO4)2, NaCl, KCl, Na2SO4, K2SO4, and NaNO3, is studied. On the basis of experimental results and published data, the values of the Weisenberger-Schumpe model parameters for the perchlorate ion and ozone are proposed to be = 0.0391 M−1 and hG,0 = −0.0152 M−1 (t = 25°C), respectively. The values of the similar parameters for the van Krevelen-Hoftijzer model are found to be = 0.336 M−1 and xG = −0.215 M−1, respectively. The discrepancies between the values of Sechenov constants calculated using the van Krevelen-Hoftijzer model for the one-component salty solutions, are somewhat smaller than between those determined using the Weisenberger-Schumpe model. The Sechenov constants are satisfactorily predicted (to a ~ 20% accuracy) by the Weisenberger-Schumpe model with the new and hG,0 values. [ABSTRACT FROM AUTHOR]

The ozone solubility in aqueous solutions of salts, NaCl (up to 5 М) and Na2SO4(up to 2 М) and their mixtures, was studied. The results are adequately described by the semi-empirical model proposed by Weisenberger and Schumpe for calculation of the solubilities of atmospheric gases in concentrated solutions of electrolytes. By comparing the data obtained experimentally and calculated in terms of the Weisenberger and Schumpe model, the ozone molar absorption coefficient was found to beε = 2860 ± 200 M−1cm−1at 260 nm. [ABSTRACT FROM AUTHOR]