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

Black Phosphorus Synthesis via Ball Milling

When bulk black phosphorus (BP) is exfoliated into few layer sheets, termed “phosphorene,” it has desirable optoelectronic properties of a semiconductor with a unique layer-dependent direct bandgap. This work studies planetary ball milling parameters (PBM) for converting red phosphorus (RP) to BP and explores mixer and vibratory milling for novel means of exfoliation. The RP to BP conversion progress was studied with in-situ pressure monitoring and XRD. Synthesized bulk BP powder was dispersed in three solvents for exfoliation and characterized with TEM, Raman and UV-Vis spectroscopy. During PBM, the gradual transition from RP to BP occurred in less than 30 minutes at the highest milling intensity. With increased milling durations, conversion was observed at as low as 200 rpm. Raman characterization on the exfoliated BP solutions confirmed the preservation of the BP crystal structure while TEM imaging confirmed the presence of few layer BP flakes, indicating successful exfoliation through low energy milling. This work demonstrates industrially scalable milling techniques as a viable means to attain few layer BP

Mechanochemical Synthesis and Characterization of Cerium Monosulfide

Cerium monosulfide (CeS) has desirable refractory properties such as high melting point (2445 °C) and high thermal conductivity, but it is not commercially available. CeS has been used for high temperature crucibles for molten metals or nuclear fuels due to its non-wetting nature. In the past, CeS has been synthesized using temperatures greater than 1700 oC, which is expensive and hazardous. In this work, CeS was synthesized by high-energy planetary ball milling of elemental cerium and sulfur. The reaction was monitored at ambient conditions, using in situ temperature and pressure. Using a similar approach, CeS was also prepared from a commercially available sulfide, Ce2S3, by mixing stoichiometric amounts of cerium. After CeS was synthesized using both approaches, the resulting powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectroscopy (EDS). The CeS powder was then sintered in an inert atmosphere and the thermal conductivity was measured

Fabrication and Characterization of Uranium-based Nuclear Fuels

Nuclear energy continues to play a large role in the energy landscape of the world. Of particular interest is creating more resilient and efficient nuclear fuels such as uranium mononitride or large-grained uranium dioxide. Uranium dioxide feedstock powder was used to fabricate high density (\u3e90% theoretical density) uranium mononitride (UN) and uranium dioxide (UO2) nuclear fuel pellets. A carbothermic reduction of UO2 and nitridation thermal process was used to convert the UO2 powder into phase-pure UN, which was confirmed through X-ray diffraction. The UN powder was cold pressed into green pellets (approx. 62% theoretical density) prior to sintering at 1900°C. UO2 pellets were pressed and sintered at 1500°C using a three step process (reducing-oxidizing-reducing atmospheres) to make high density, stoichiometric UO2 pellets. Scanning electron microscopy (SEM) was used for grain size analysis and energy dispersive x-ray spectroscopy (EDS) was used for chemical mapping on the sintered pellets. The fabrication and characterization of high quality nuclear fuels provides opportunities for further testing and optimization of high performance, accident tolerant fuels

Corrosion of Composite Uranium Nitride Fuels

Uranium mononitride (UN) has been identified as a possible accident tolerant fuel in nuclear reactors based on its high uranium density, thermal conductivity and low fission-gas release. Pure UN samples have been shown in studies to react with water at the operating temperatures of light water reactors, which make up the majority of reactors in the United States. Composite UN-UO2 fuels might be optimized for corrosion resistance in these conditions. An autoclave was re-engineered for work with radioactive materials by creating safeguards to prevent radioactive material release. UN was prepared from elemental uranium using a hydride-dehydride-nitride thermal synthesis prior to mixing with up to 10 wt% UO2. UN-UO2 composites were tested by placing samples in the water-filled autoclave at 320°C and 9 MPa. Pellets were characterized for weight change, surface hydration, and grain boundary deterioration using a sensitive digital balance, optical microscopy and electron microscopy. Corrosion products were identified using energy dispersive X-ray spectroscopy and X-ray diffraction. The amount of leached uranium in solution was measured using inductive coupled plasma mass spectroscopy