Proton Irradiation Effect on Thermoelectric Properties of Nanostructured N-Type Half-Heusler Hf\u3csub\u3e0.25\u3c/sub\u3eZr\u3csub\u3e0.75\u3c/sub\u3eNiSn\u3csub\u3e0.99\u3c/sub\u3eSb\u3csub\u3e0.01\u3c/sub\u3e

Thermoelectric properties of nanostructured half-Heusler Hf0.25Zr0.75NiSn0.99Sb0.01 were characterized before and after 2.5 MeV proton irradiation. A unique high-sensitivity scanning thermal microprobe was used to simultaneously map the irradiation effect on thermal conductivity and Seebeck coefficient with spatial resolution less than 2 μm. The thermal conductivity profile along the depth from the irradiated surface shows excellent agreement with the irradiation-induced damage profile from simulation. The Seebeck coefficient was unaffected while both electrical and thermal conductivities decreased by 24%, resulting in no change in thermoelectric figure of merit ZT. Reductions in thermal and electrical conductivities are attributed to irradiation-induced defects that act as scattering sources for phonons and charge carriers

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)

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

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

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

Transgranular Stress Corrosion Cracking of 304L Stainless Steel Pipe Clamps in Direct Use Geothermal Water Heating Applications

Direct use geothermal heating relies on heat extracted from naturally occurring geothermal water sources to provide heating needs for commercial and residential use. The city of Boise, Idaho maintains the largest district geothermal heating system in the United States, utilizing a source of geothermal water at 80 °C. 304 Stainless steel (UNS S30400) pipe clamps are used throughout the system as repair seals and for new service connections. Occasionally unexpected fracture of the stainless steel clamps occurs with time-in-service periods as short as 1 year. A failure analysis was conducted, including visual, microstructural, compositional, and mechanical characterization, to determine the cause and source of the degradation. Cracking of the clamps was limited to localized regions with the remainder of the clamp unaffected. Branched, brittle cracks were observed in the failure region and exhibited transgranular propagation. Based on the temperature, available moisture, stress level, and type of material used it was determined that the likely cause of failure was neutral pH, dilute chloride-induced stress corrosion cracking. Based on this failure analysis, geothermal or other buried heated water systems must consider protective measures or more SCC-resistant materials to prevent susceptible conditions from developing, compared to conventional water systems, to ensure maximum lifetime performance

Ball-on-Ring Test Validation for Equibiaxial Flexural Strength Testing of Engineered Ceramics

The validation of a ball-on-ring, equibiaxial flexural strength method to obtain the transverse rupture strength (TRS) of right cylindrical ceramic specimens was performed in this study. Validation of the test method was achieved using commercially available engineered high purity alumina disks and finite element (FE) model analysis. The validated fixture was then used to obtain the TRS and Weibull statistical analysis of MgO-partially stabilized zirconia (MSZ) and Y2O3-partially stabilized zirconia (YSZ) ceramic disks. TRS data for alumina, MSZ, and YSZ agreed with the TRS values reported in the literature. A statistically relevant number of samples (N \u3e 30) for each material were tested to allow for a Weibull statistical analysis. Weibull parameters for these materials were within the expected values for engineered ceramics. The characteristic strength for alumina, MSZ, and YSZ were determined to be 289, 786, and 814 MPa, respectively. The Weibull modulus was determined between 10 and 25 for each material, which is typical of engineered ceramics. In addition, FE model results were in close agreement with experimental fracture values for the three ceramic materials tested in this study

Characterization and Failure Analysis of Solid-State Diffusion Bonded Ceramic-to-Metal Transitions

Reliable, high-temperature, high-pressure transitions between ceramic heat exchangers and metal components enable higher efficiency in advanced power generation systems. Recent development of a novel cermet-filled seal ring design has shown potential to maintain a gas-tight seal through multiple thermal cycles up to 800­ ºC. Materials characterization and computational modeling provided insight to chemical behavior (i.e., solid-state diffusion) and mechanical integrity (i.e., stress) in the seal components. Results demonstrate a correlation between machining tolerance, assembly process, and diffusion behavior on the seal’s performance in ceramic-to-metal systems and helped guide the design efficacy of future seals

Variation in the carotid bifurcation geometry of young versus older adults: Implications for geometric risk of atherosclerosis

Background and Purpose - Retrospective analysis of clinical data has demonstrated major variations in carotid bifurcation geometry, in support of the notion that an individual\u27s vascular anatomy or local hemodynamics may influence the development of atherosclerosis. On the other hand, anecdotal evidence suggests that vessel geometry is more homogenous in youth, which would tend to undermine this geometric risk hypothesis. The purpose of our study was to test whether the latter is indeed the case. Methods - Cross-sectional images of the carotid bifurcations of 25 young adults (24±4 years) and a control group of 25 older subjects (63±10 years) were acquired via MRI. Robust and objective techniques were developed to automatically characterize the 3D geometry of the bifurcation and the relative dimensions of the internal, external, and common carotid arteries (ICA, ECA, and CCA, respectively). Results - Young vessels exhibited significantly less interindividual variation in the following geometric parameters: bifurcation angle (48.5±6.3° versus 63.6±15.4°); ICA angle (21.6±6.7° versus 29.2±11.3°); CCA tortuosity (0.010±0.003 versus 0.014±0.011); ICA tortuosity (0.025±0.013 versus 0.086±0.105); ECA/CCA diameter ratio (0.81±0.06 versus 0.75±0.13), ICA/CCA (0.81 ±0.06 versus 0.77±0.12) diameter ratio, and bifurcation area ratio (1.32±0.15 versus 1.19±0.35). Conclusions - The finding of more modest interindividual variations in young adults suggests that, if there is a geometric risk for atherosclerosis, its early detection may prove challenging. Taken together with the major interindividual variations seen in older vessels, it suggests a more complex interrelationship between vascular geometry, local hemodynamics, vascular aging, and atherosclerosis, the elucidation of which now calls for prospective studies. © 2005 American Heart Association, Inc