Heat transfer fluids for concentrating solar power systems – A review

Influence of temperature on the electrochemical and passivation behavior of 2507 super duplex stainless steel in simulated desulfurized flue gas condensates

Accurate knowledge of water splitting kinetics is essential for the design and optimization of high-temperature thermochemical cycles for solar-driven fuel production, but such crucial data are unavailable for virtually all redox materials of potential practical value. We describe an investigation of the redox activity and oxidation kinetics of cobalt ferrite, a promising material for this application that is representative of a broader class of metal-substituted ferrites. To enable repetitive cycling, ferrites must be supported on another oxide to avoid sintering and deactivation. Consequently, we synthesized a composite material using atomic layer deposition of cobalt and iron oxides on zirconia, a commonly used ferrite “support”, to create a well-controlled, uniformly distributed composition. Our results show that the support is not an innocent bystander and that dissolved iron within it reacts by a different mechanism than embedded iron oxide particles in the matrix. Samples were thermally reduced at 1450 °C under helium and oxidized with steam at realistic process temperatures ranging from 900 °C to 1400 °C. Experiments within a fluid-dynamically well-behaved stagnation-flow reactor, coupled with detailed numerical modelling of the transient H2 production rates, allow us to effectively deconvolve experimental artefacts from intrinsic material behaviour over the entire time domain of the oxidation reaction. We find that second-order reaction and diffusion-limited mechanisms occur simultaneously at different oxidation rates and involve iron in two separate phases: (1) reduced Fe dissolved in the ZrO2 support and (2) iron oxide located at the interface between embedded ferrite particles and the zirconia matrix. Surprisingly, we also identified a catalytic mechanism occurring at the highest temperatures by which steady-state production of H2 and O2 occurs. The results reported here, which include Arrhenius rate constants for both oxidation mechanisms, will enable high-fidelity computational simulation of this complex, but promising approach to renewable fuel production.

Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite–zirconia composite

Bicrystalline nanowires of hematite (α-Fe2O3) have been successfully synthesized by the oxidation of pure iron. The product was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM in combination with focal series reconstruction, energy-dispersive X-ray spectroscopy, and electron energy-loss spectroscopy. The bicrystalline nanowires have diameters of 20−80 nm and lengths up to 20 μm. All of the investigated materials are found to be α-Fe2O3 with a rhombohedral crystal structure. Investigations indicate that most of the bicrystalline nanowires are nanotwins with ellipsoidal heads. The orientation relationship between the nanotwins can be described as (1110)M//(1110)T, [110]M//[110]T. An energy-filtered TEM investigation indicates that the ellipsoidal head is iron-rich. The growth mechanism of such unique nanostructures is considered to be a solid-phase growth via surface and internal diffusions of molecules from base to tip.

Bicrystalline Hematite Nanowires

The oxidation of iron has been studied at low temperatures (between 260 and 500 °C) in dry air or air with 2 vol% H2O, in the framework of research on dry corrosion of nuclear waste containers during long-term interim storage. Pure iron is regarded as a model material for low-alloyed steel. Oxidation tests were performed in a thermobalance (up to 250 h) or in a laboratory furnace (up to 1000 h). The oxide scales formed were characterized using SEM-EDX, TEM, XRD, SIMS and EBSD techniques. The parabolic rate constants deduced from microbalance experiments were found to be in good agreement with the few existing values of the literature. The presence of water vapor in air was found to strongly influence the transitory stages of the kinetics. The entire structure of the oxide scale was composed of an internal duplex magnetite scale made of columnar grains and an external hematite scale made of equiaxed grains. 18O tracer experiments performed at 400 °C allowed to propose a growth mechanism of the scale.

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Iron Oxidation at Low Temperature (260-500 C) in Air and the Effect of Water Vapor

This paper reviews previous studies on iron and steel oxidation in oxygen or air at high temperatures. Oxidation of iron at temperatures above 700°C follows the parabolic law with the development of a three-layered hematite/magnetite/wustite scale structure. However, at temperatures below 700°C, inconsistent results have been reported, and the scale structures are less regular, significantly affected by sample-preparation methods. Oxidation of carbon steel is generally slower than iron oxidation. For very short-time oxidation, the scale structures are similar to those formed on iron, but for longer-time oxidation, because of the less adherent nature, the scale structures developed are typically much more complex. Continuous-cooling conditions, after very short-time oxidation, favor the retention of an adherent scale, suggesting that the method proposed by Kofstad for deriving the rate constant using continuous cooling or heating-oxidation data is more appropriate for steel oxidation. Oxygen availability has certain effects on iron and steel oxidation. Under continuous cooling conditions, the final scale structure is found to be a function of the starting temperature for cooling and the cooling rate. Different scale structures develop across the width of a hot-rolled strip because of the varied oxygen availability and cooling rates at different locations.

W.Y.D. Yeun

Papers

Review of the High-Temperature Oxidation of Iron and Carbon Steels in Air or Oxygen