Mixtures of hafnia and carbon powders, containing 0.0-10 mol% of silicon, are heated and analyzed by XRD, SEM, TEM, and DTA. Furthermore, hafnia and carbon pellets, with varying degrees of silicon content, are investigated to determine the nature of the silicon influence on the reaction. The addition of up to 5.0 mol% silicon increases the yield of carbide by 400%. TEM reveal that silicon causes the formation of an amorphous layer on the hafnia particles, on which nucleation of the carbide phase take place. Taking into account that the carbide formation in Si-free samples is only observed on the carbon surfaces, the authors conclude that the silicon-induced layer on the hafnia surfaces increases the accessible area for carbide formation and consequently the reaction rate.
Biomorphous silicon carbide is tested as a novel wick material for flat evaporator loop heat pipes (LHP). The machinable ceramic gives the possibility to improve the wick performance by macroporous channel networks without closing the structure inherent microporosity. A minimum thermal resistance of 0.4236 C W À1 is obtained for unmodified SiC at a power input of 200 W. Modification with channels parallel to the evaporating surface results in an improved thermal resistance of 0.3792 C W À1 . Modification with channels perpendicular to the evaporating surface increases the thermal resistance to 0.4469 C W À1 , but reduces the necessary heat load for stable start-up from 50 to 25 W.
Porous carbon nanotube coatings for potential solar thermal receiver applications were generated on silicon carbide via an infiltration process from an aqueous dispersion. A screening evaluation was applied to generate carbon nanotube coatings with enhanced absorption, i.e., reduced reflection of solar wavelength spectra from 2 500-500 nm. Structures with low density nanotube packing with a total reflectance <2% were fabricated from a low concentration dispersion of sterically stabilized (PVP) carbon nanotubes. Though the optical performance is lower when compared to carbon nanotube coatings generated by CVD, the filtration method offers control of the nanotube pore structure over a wide range, a greater flexibility with regards to substrate geometry and excellent up-scaling capability. B. Weisenseel, Prof. P. Greil Energy Campus N€ urnberg, F€ urther Str. 250, 90429, N€ urnberg, Germany [**] The authors thank the Energie Campus N€ urnberg for providing services and facilities and financial support through the "Bavaria on the Move" initiative of the state of Bavaria. Furthermore, support from DFG Reinhart Koselleck project GR 961/32 and DFG FE 1209/1 is gratefully acknowledged. 624 wileyonlinelibrary.com
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