Stability and Decomposition of Ca-Substituted Lanthanum Ferrite in Reducing Atmospheres

Calcium-substituted lanthanum ferrites (La1–xCaxFeO3–δ x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized in air and subsequently decomposed in reducing atmospheres. The partial pressure of oxygen (PO2) was controlled by varying the H2/H2O ratio by bubbling hydrogen/argon mixtures through water baths at controlled temperatures. Three regions of mass loss were identified as the PO2 was reduced, two of which were determined to be associated with decomposition reactions. Calcium was shown to decrease the thermal stability of the perovskite compound, but rather than incrementally increasing the required PO2 for decomposition proportional to calcium concentration, all samples partially decomposed at a single PO2. The extent of the partial decomposition was dependent on the amount of calcium substitution and temperature. The perovskite phase remaining after the partial decomposition was found to fully decompose at the same oxygen partial pressure as pure lanthanum ferrite

High Temperature Oxidation Kinetics of Dysprosium Particles

Rare earth elements have been recognized as critical materials for the advancement of many strategic and green technologies. Recently, the United States Department of Energy has invested many millions of dollars to enhance, protect, and forecast their production and management. The work presented here attempts to clarify the limited and contradictory literature on the oxidation behavior of the rare earth metal, dysprosium. Dysprosium particles were isothermally oxidized from 500 to 1000 °C in N2–(2%, 20%, and 50%) O2 and Ar–20% O2 using simultaneous thermal analysis techniques. Two distinct oxidation regions were identified at each isothermal temperature in each oxidizing atmosphere. Initially, the oxidation kinetics are very fast until the reaction enters a slower, intermediate region of oxidation. The two regions are defined and the kinetics of each are assessed to show an apparent activation energy of 8–25 kJ/mol in the initial region and 80–95 kJ/mol in the intermediate oxidation reaction region. The effects of varying the oxygen partial pressure on the reaction rate constant are used to show that dysprosium oxide (Dy2O3) generally acts as a p-type semiconductor in both regions of oxidation (with an exception above 750 °C in the intermediate region)

Joining of Ion Transport Membranes Using a Novel Transient Liquid Phase Process

The feasibility of a novel transient liquid phase (TLP) joining method has been demonstrated in joining La{sub 0.9}Ca{sub 0.1}FeO{sub 3} materials. Metal oxide powders were processed to form the TLP compositions which were used in the joining process. The method has been successful in producing joint interfaces that effectively disappear, as they are the same material and have the same properties as the joined parts. The feasibility of the method has been demonstrated for a single system, but many systems where the method can potentially be applied have been identified

Microdomain Formation, Oxidation, and Cation Ordering in LaCa\u3csub\u3e2\u3c/sub\u3eFe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e8+y\u3c/sub\u3e

The compound LaCa2Fe3O8+y, also known as the Grenier phase, is known to undergo an order–disorder transformation (ODT) at high temperatures and oxidation has been observed when the compound is cooled in air after the ODT. In this study, we have synthesized the Grenier compound in air using traditional solid-state reactions and investigated the structure and composition before and after the ODT. Thermal analysis showed that the material undergoes an ODT in both oxygen and argon atmospheres with dynamic, temperature dependent, oxidation upon cooling. Results from scanning transmission electron microscopy (STEM) suggest that the Grenier phase has preferential segregation of Ca and La on the two crystallographic A sites before the ODT, but a random distribution above the ODT temperature. Furthermore, STEM images suggest the possibility that oxygen excess may exist in La-rich regions within microdomains rather than at microdomain boundaries

Use of Global Electrochemical Techniques to Characterize Localized Corrosion Behavior on Aluminum Alloys

Precipitated intermetallic phases strengthen aluminum aircraft alloys; these inclusions also establish localized electrochemical environments, significantly influencing the bulk corrosion behavior of such alloys. To gain insight on the effects of intermetallic phases on the bulk corrosion behavior, two established forms of electrochemical characterization techniques were used, polarization scans and impedance spectroscopy. This effort was undertaken to: Provide a statistical body of electrochemical data for aluminum alloys, Provide fundamental electrochemical parameters to aide in a continuum scale modeling effort, Validate the effect of solution chemistry on bulk corrosion behavior, Determine the influence of precipitated intermetallic phases on the bulk corrosion behavior, Decouple the metal-coating interface behavior from bulk corrosion behavior in continuing tests on coated aluminum

Synthesis of Hafnium-Free Nanostructured Half-Heusler Materials for Thermoelectric Applications

Half-Heusler thermoelectric materials convert heat directly into electricity by means of the Seebeck effect. Improving the conversion efficiency and reducing fabrication costs will reduce the price per watt enabling widespread commercialization for waste heat energy harvesting and self-powered devices. In this work, a rapid low-cost synthesis route utilizing mechanical alloying via high energy planetary ball milling and spark plasma sintering was used to fabricate n-type hafnium-free single phase nano-grained TiZrNiSnSb based half-heusler monoliths with a modest figure of merit performance with significantly reduced thermal conductivity

Microstructural Characterization of Next Generation Nuclear Graphites

This article reports the microstructural characteristics of various petroleum and pitch based nuclear graphites (IG-110, NBG-18, and PCEA) that are of interest to the next generation nuclear plant program. Bright-field transmission electron microscopy imaging was used to identify and understand the different features constituting the microstructure of nuclear graphite such as the filler particles, microcracks, binder phase, rosette-shaped quinoline insoluble (QI) particles, chaotic structures, and turbostratic graphite phase. The dimensions of microcracks were found to vary from a few nanometers to tens of microns. Furthermore, the microcracks were found to be filled with amorphous carbon of unknown origin. The pitch coke based graphite (NBG-18) was found to contain higher concentration of binder phase constituting QI particles as well as chaotic structures. The turbostratic graphite, present in all of the grades, was identified through their elliptical diffraction patterns. The difference in the microstructure has been analyzed in view of their processing conditions

Simulation of the Relaxation Potential Profile of an ac-dc-ac Test

The relaxation period of the accelerated ac-dc-ac test for coatings is associated with the transient electrochemistry that occurs when the immersed coated system is allowed to return to a stable open-circuit condition after being subjected to a cathodic potential. A mathematical model of the transient electrochemistry that occurs during this relaxation period is presented for coated aluminum. Expressions for the corrosion potential and corrosion current as functions of the local pH at the metal-coating interface were developed using reported experimental results. These expressions enabled the simulation of the transient electrochemistry under the constraint of balanced anodic and cathodic current densities. Regression of the transient relaxation potential profiles to exponential decay functions provided time-constant characterization of the profiles. Simulated results are presented that demonstrate the influences of the coating\u27s porosity and thickness, the applied dc potential and the metal-coating interface condition on the time-constants associated with the relaxation profile. Interpretation of experimentally reported relaxation potential profiles supported the analysis of the simulated results

Cermet Development for High Temperature and High Pressure Applications

Many traditionally used low cost alloys are easily corroded in steam or supercritical CO2. An effective solution is to utilize ceramic heat exchangers that are often integrated with metallic components which result in a significant thermal expansion mismatch. The goal of this project is to develop a sealing method to create a hermetic joint between the ceramic and metal alloy. Proposed is a seal ring containing a cermet powder with a coefficient of thermal expansion (CTE) higher than the ceramic and metal to produce a high temperature compressive seal. Cermets of Ag and MgO have been selected to withstand pressures of 3000 psi and temperatures above 700 °C. Three preliminary tests were conducted to study the behavior of the cermet: 1. Static heat on cermet filled stainless steel tubes; 2. Radial compression test on cermet filled stainless steel tubes; 3. Compression tests on open cermet filled cavities. Tests 2 and 3 suggest that powder flowability and densification regions decrease with increased ceramic concentrations

Oxidation Behavior of Welded Zry-3, Zry-4, and Zr–1Nb Tubes

The Transient Reactor Test (TREAT) facility is a research reactor designed to simulate rapid transients to test new fuel designs. TREAT\u27s cladding is exposed to unique conditions compared to normal water reactors. These conditions include: exposure to air at high temperatures (≥600 °C), rapid heating (≈700 °C/s), and cladding geometry that includes chamfers and welds. This work investigates the effects of chamfering and welding on the oxidation behavior of zirconium alloys (Zircaloy-3, Zircaloy-4, and Zr–1Nb). Tube specimens were examined under isothermal and transient conditions in dry and humid air. The effect of weld type (tungsten inert gas or electron beam), the number of welds, and alloying elements are compared. Thermogravimetric analysis was used to collect mass gain data during isothermal oxidation and the data was used to quantify the oxidation rate constant and the activation energy of oxidation. Oxide behavior in the weld region, chamfered region, and bulk tube was measured and compared. The microstructure and secondary phase precipitates in EBW tubes before and after breakaway were characterized. The electron beam welded Zr–1Nb specimen was found to have the most favorable oxidation behavior under both isothermal and transient conditions. Zry-4 oxidized the most readily and was the most affected by mechanical deformation