Cyclic Oxidation Behavior of 310 S Austenitic Stainless Steel at High Temperatures
Systematic study were undertaken to understand the high temperature oxidation behavior of AISI 310S Stainless steel. The tests were conducted at two temperatures of 930C and 960C up to 135 hours. A comparative study of weight gains for these two different temperatures was observed. The parabolic curve was found. From the nature of the curve (parabolic and other forms) one can able to under stand the nature of the oxide film formed on the substrate surface. A detailed investigation of microstructure revealed that the oxide film formed on the surface of the metal in the early hours undergoes spalling at 100 hours of cyclic oxidation. XRD studies of 135 hours cyclic oxidized substrate revealed the formation of chromium oxide layer but the layer formed is not completely stable over the entire surface. SEM investigation of oxidized sample confirmed the presence of cracks and spalling of oxide particles from the surface.
High-efficiency, air-stable manganese–iron oxide nanoparticle-pigmented solar selective absorber coatings toward concentrating solar power systems operating at 750 °C
Abstract Solar selective absorber coatings with long-term thermal stability at high temperatures ≥ 750 oC in air is an important component to reduce the levelized cost of energy (LCOE) of concentrating solar power (CSP) systems towards 50% power efficiency and dispatchable solar electricity. Conventionally, solar spectral selectivity requires multilayer-interference coatings implemented by stringently controlled vacuum deposition, and these coatings degrade significantly at >700 °C in air. Herein, we established a quantitative design approach and demonstrated a proof-of-concept air-stable, manganese-iron oxide nanoparticle-pigmented solar selective coatings with a high solar absorptance of ∼93%, a relatively low thermal emittance of ∼52%, and an optical-to-thermal energy conversion efficiency >89% under 1000x solar concentration at 750°C towards Generation 3 CSP systems. These coatings demonstrate spectral selectivity using cost-effective spray coating approach, a notable improvement over conventional vacuum-deposited, multilayer solar selective coatings for low-cost, high-efficiency solar thermal receivers. In contrast to the thermal degradation of spectrally non-selective benchmark Pyromark 2500 coatings at 750 °C in air, the solar absorptance of the MnFe2O4-pigmented coatings on stainless steel 310 (SS 310) substrates is increased to ∼92.9% and the optical-to-thermal energy conversion efficiency is improved to 89.7% after serving at 750 °C in air for 700 h. X-ray diffraction results reveal that this improvement is due to the transformation of MnFe2O4 NPs into more thermodynamically stable, non-stoichiometric manganese-rich manganese ferrite and iron-rich manganese iron oxide phases after 500 h aging at 750 °C. For >1000 h-endurance testing at 750 oC in air and the subsequent 19 day-night thermal cycling between 750 °C (12h/cycle) and 25 °C (12h/cycle) on SS 310 substrates, the thermal degradation is mainly due to the CrOx micro-flake formation from SS 310 substrates rather than the coatings, which can be suppressed by pre-oxidizing the surface of SS 310. With lower emittance matrix material and further optimization of pigment nanoparticle stoichiometry, concentration, and coating thickness, it is promising to achieve an optimized thermal efficiency ≥92.5% with long-term thermal stability at 750 °C for Generation 3 CSP systems.
Formation and Strength of Magnesia Whiskers
The formation of single-crystal magnesia whiskers during the compressive deformation of single crystals at room temperatures is described. These whiskers can have strengths of the order of millions of pounds per square inch. They are formed during fracture as a result of the discontinuous propagation of cracks which are mechanically relieving internal stresses resulting from plastic flow