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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Magnetization curves of sintered heavy tungsten alloys for applications in MRI-guided radiotherapy

Purpose: Due to the current interest in MRI-guided radiotherapy, the magnetic properties of the materials commonly used in radiotherapy are becoming increasingly important. In this paper, measurement results for the magnetization (BH) curves of a range of sintered heavy tungsten alloys used in radiation shielding and collimation are presented. Methods: Sintered heavy tungsten alloys typically contain >90 % tungsten and <10 % of a combination of iron, nickel, and copper binders. Samples of eight different grades of sintered heavy tungsten alloys with varying binder content were investigated. Using a superconducting quantum interference detector magnetometer, the induced magnetic momentm was measured for each sample as a function of applied external field H{sub 0} and the BH curve derived. Results: The iron content of the alloys was found to play a dominant role, directly influencing the magnetizationM and thus the nonlinearity of the BH curve. Generally, the saturation magnetization increased with increasing iron content of the alloy. Furthermore, no measurable magnetization was found for all alloys without iron content, despite containing up to 6% of nickel. For two samples from different manufacturers but with identical quoted nominal elemental composition (95% W, 3.5% Ni, 1.5% Fe), a relative difference in the magnetization of 11%–16% was measured. Conclusions: The measured curves show that the magnetic properties of sintered heavy tungsten alloys strongly depend on the iron content, whereas the addition of nickel in the absence of iron led to no measurable effect. Since a difference in the BH curves for two samples with identical quoted nominal composition from different manufacturers was observed, measuring of the BH curve for each individual batch of heavy tungsten alloys is advisable whenever accurate knowledge of the magnetic properties is crucial. The obtained BH curves can be used in FEM simulations to predict the magnetic impact of sintered heavy tungsten alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoscale stacking fault–assisted room temperature plasticity in flash-sintered TiO 2

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO 2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO 2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO 2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO 2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Optimization of Lithium Content and Sintering Additives in Tape Cast Lithium Garnet Electrolyte Sheets

The lithium garnet (LLZO) remains one of the most promising electrolyte materials for producing solid state batteries. Tape casting is one method for producing the thin ceramic electrolyte layer required for a high-performance solid state battery. Processing of LLZO is complicated by several factors including lithium volatilization, abnormal grain growth and phase instability. Many of these issues are exacerbated in tape cast sheets by the sheets' higher surface area to volume ratio, when compared to pellet processing methods. Here, we report on an environmentally friendly aqueous tape casting process for LLZO using methylcellulose as a binder and compare final sheet properties to other, solvent-based, options. Solids loading, binder content, dispersant composition, and wetting agent type are varied to improve ease of casting and enhance properties of the as-cast tape. Lithium content in sintered LLZO sheets is controlled by the inclusion of excess Li 2 CO 3 as lithium source in the ceramic tape itself and by a novel method of producing a lithium-saturated furnace atmosphere. Sintering time and temperature are optimized, and rapid thermal processing is investigated as a method of reducing lithium loss. Al 2 O 3 and MgO are examined for use as sintering additives. Characterization of LLZO sheets is performed with XRD, EIS and SEM. Li 2 CO 3 and sintering additive levels are optimized for ionic conductivity and density. The optimization effort improved the conductivity and density of 100µm thick sintered sheets to greater than 3x10 -4 S/cm conductivity and 90% density.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Development of an Innovative High-Thermal Conductivity UO2 Ceramic Composites Fuel Pellets with Carbon Nano-Tubes Using Spark Plasma Sintering

Uranium dioxide (UO2) is the most common fuel material in commercial nuclear power reactors. Despite its numerous advantages such as high melting point, good high-temperature stability, good chemical compatibility with cladding and coolant, and resistance to radiation, it suffers from low thermal conductivity that can result in large temperature gradients within the UO2 fuel pellet, causing it to crack and release fission gases. Thermal swelling of the pellets also limits the lifetime of UO2 fuel in the reactor. To mitigate these problems, we propose to develop novel UO2 fuel with uniformly distributed carbon nanotubes (CNTs) that can provide high-conductivity thermal pathways and can eliminate fuel cracking and fission gas release due to high temperatures. CNTs have been investigated extensively for the past decade to explore their unique physical properties and many potential applications. CNTs have high thermal conductivity (6600 W/mK for an individual single- walled CNT and >3000 W/mK for an individual multi-walled CNT) and high temperature stability up to 2800°C in vacuum and about 750°C in air. These properties make them attractive candidates in preparing nano-composites with new functional properties. The objective of the proposed research is to develop high thermal conductivity of UO2–CNT composites without affecting the neutronic property of UO2 significantly. The concept of this goal is to utilize a rapid sintering method (5–15 min) called spark plasma sintering (SPS) in which a mixture of CNTs and UO2 powder are used to make composites with different volume fractions of CNTs. Incorporation of these nanoscale materials plays a fundamentally critical role in controlling the performance and stability of UO2 fuel. We will use a novel in situ growth process to grow CNTs on UO2 particles for rapid sintering and develop UO2-CNT composites. This method is expected to provide a uniform distribution of CNTs at various volume fractions so that a high thermally conductive UO2-CNT composite is obtained with a minimal volume fraction of CNTs. The mixtures are sintered in the SPS facility at a range of temperatures, pressures, and time durations so as to identify the optimal processing conditions to obtain the desired microstructure of sintered UO2-CNT pellets. The second objective of the proposed work is to identify the optimal volume fraction of CNTs in the microstructure of the composites that provides the desired high thermal conductivity yet retaining the mechanical strength required for efficient function as a reactor fuel. We will systematically study the resulting microstructure (grain size, porosity, distribution of CNTs, etc.) obtained at various SPS processing conditions using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscope (TEM). We will conduct indentation hardness measurements and uniaxial strength measurements as a function of volume fraction of CNTs to determine the mechanical strength and compare them to the properties of UO2. The fracture surfaces will be studied to determine the fracture characteristics that may relate to the observed cracking during service. Finally, we will perform thermal conductivity measurements on all the composites up to 1000° C. This study will relate the microstructure, mechanical properties, and thermal properties at various volume fractions of CNTs. The overall intent is to identify optimal processing conditions that will provide a well-consolidated compact with optimal microstructure and thermo-mechanical properties. The deliverables include: (1) fully characterized UO2-CNT composite with optimal CNT volume fraction and high thermal conductivity and (2) processing conditions for production of UO2-CNT composite pellets using SPS method.

Subhash, Ghatu↗
DOE OSTI2018

Apparent Fracture Toughness of Pressureless Sintered Silver Interconnects

This study uses three-point flexural testing to estimate the room-temperature apparent fracture toughness ( K Ic_app ) of a pressureless sintered-silver interconnect - a candidate material for use in power electronic devices. The K Ic_app estimation occurs via work-of-fracture measurement of a chevron-shaped sintered-silver joint centrally located in an “interconnect bend test specimen”. The chevron-shape was stencil-printed on a silver (Ag)-plated direct bonded copper (DBC) substrate, mated to an Ag-plated DBC substrate, and then sintered. The chevron-shape, used for decades in fracture mechanics examinations and in standardized tests of monolithic materials, enables controlled stable crack initiation and propagation that are prerequisites for valid K Ic_app estimation. The employed combination of an interconnect bend test specimen and bend test method is robust and simple, enables a means for interpreting the resistance to crack growth and delamination in an interconnect, provides a dependable way to judge the efficacies of sintered-silver processing conditions, and ultimately supports process optimization of sintered-silver interconnects.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Double-Sided Pressureless-Sintered-Silver Interconnects Fabricated by Reflow-Oven-Processing

A significant amount of development with sintered-silver as an alternative interconnect (or attachment or attach) material in power electronic devices has occurred since ~ 2010 at Oak Ridge National Laboratory's National Transportation Research Center. Continuous sponsorship of the work enabled improvements and understanding of several sequential processing steps which otherwise would not have occurred with a short duration or an intermittently funded project. The majority of that work has occurred with a focus on improving its processing and consequential mechanical reliability of the interconnect while promoting adaptability to power elect ronic device manufacturing. The work includes determining application issues of silver-sintering and the developing processes that would make it amenable for use with reflow oven sintering. As part of that work, co-development of maximum interconnect strength of the sintered-silver interconnect occurred, and that also involved examining candidate plating materials that could be used in concert with them for the "interconnect system". All of this work involved the consideration of a single sintered-silver interconnect.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Fundamentals of Spark-Plasma Sintering (Final Report)

This project was focused on the development of the general theory of spark-plasma sintering (SPS) taking into account the role of both thermal and non-thermal factors in the acceleration of SPS mass transport. SPS is a particular kind of field-assisted sintering, which provides potentially revolutionary capabilities to the processing of materials into configurations previously unattainable. This approach significantly improves the processing time- and quality-wise. It carries the potential of maintaining the nano and sub-micron structure in nano-powder-based materials after consolidation. SPS gains particular prominence in connection with its exceptional potential of rapid and ultra-rapid processing of very hard-to-deform materials, which would typically require lengthy consolidation times at significantly elevated temperatures under conditions of conventional powder pressing or sintering. The achievement of the ultimate goal of the formulation of the general theory of spark plasma sintering required the identification of the contributions of all the thermal and non-thermal factors in the enhancement of mass transport under SPS processing conditions. As our research indicated, this enhancement is mostly reduced to the creation of the conditions of controlled non-equilibrium, which is especially important in ultra-rapid field-assisted sintering techniques. Thus, the main project objective was the analysis of the SPS physical basis at multiple scales specifically exploring the role of electric current in the acceleration of mass transport and with an emphasis on the conditions of controlled non-equilibrium.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Mechanically Robust High Magnetic Performance Sm-Co Sintered Magnets (Final Report)

Samarium-cobalt based permanent magnets (SmCo 5 and Sm 2 Co 17 ) have excellent magnetic properties, good corrosion resistance, and long-term thermal stability. Sm-Co sintered magnets have been widely used in electric machines, telecommunication, biomedical devices, and magnetic sensors. They are the most preferred magnets for high-temperature applications (200 - 550 °C). However, Sm-Co sintered magnets are brittle. They cannot be used for applications subjected to high stress, vibration, or mechanical shock. Sm-Co sintered magnets are prone to chipping, and fracture in the course of block magnet manufacturing, part machining, assembly, and operation. The brittleness leads to a magnet production loss of up to 20-30% in volume and imposes limitations on part size and shape. Developing mechanically robust high-performance Sm-Co sintered magnets is of great scientific and technical significance. This project is in response to the current market need for mechanically robust high magnetic performance Sm-Co sintered magnets and their novel manufacturing processes. The project will focus on scaling-up, validation, and technology maturation study of the mechanically tough high magnetic performance Sm-Co magnets developed by Ames Laboratory at a lab-scale with the collaboration of the industry partner Electron Energy Corporation (EEC).

36 MATERIALS SCIENCE↗
DOE OSTI2013

Compaction and Sintering of Mo Powders

To support the development of Mo-99 production by NorthStar Medical Technologies, LLC, Mo metal powders were evaluated for compaction and sintering characteristics as they relate to Mo-100 accelerator target disk fabrication. Powders having a natural isotope distribution and enriched Mo-100 powder were examined. Various powder characteristics are shown to have an effect on both the compaction and sintering behavior. Natural Mo powders could be cold pressed directly to >90% density. All of the powders, including the Mo-100 samples, could be sintered after cold pressing to >90% density. As an example, a compacted Mo-100 disk reached 89.7% density (9.52 g/cm3) after sintering at 1000 C for 1 hr. in flowing Ar/4%H2. Higher sintering temperatures were required for other powder samples. The relationships between processing conditions and the resulting densities of consolidated Mo disks will be presented.

Nunn, Stephen D↗
DOE OSTI2012

Low sintering temperature glass waste forms for sequestering radioactive iodine

Materials and methods of making low-sintering-temperature glass waste forms that sequester radioactive iodine in a strong and durable structure. First, the iodine is captured by an adsorbant, which forms an iodine-loaded material, e.g., AgI, AgI-zeolite, AgI-mordenite, Ag-silica aerogel, ZnI.sub.2, CuI, or Bi.sub.5O.sub.7I. Next, particles of the iodine-loaded material are mixed with powdered frits of low-sintering-temperature glasses (comprising various oxides of Si, B, Bi, Pb, and Zn), and then sintered at a relatively low temperature, ranging from 425.degree. C. to 550.degree. C. The sintering converts the mixed powders into a solid block of a glassy waste form, having low iodine leaching rates. The vitrified glassy waste form can contain as much as 60 wt % AgI. A preferred glass, having a sintering temperature of 500.degree. C. (below the silver iodide sublimation temperature of 500.degree. C.) was identified that contains oxides of boron, bismuth, and zinc, while containing essentially no lead or silicon.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2014

Sintered silver joints via controlled topography of electronic packaging subcomponents

Disclosed are sintered silver bonded electronic package subcomponents and methods for making the same. Embodiments of the sintered silver bonded EPSs include topography modification of one or more metal surfaces of semiconductor devices bonded together by the sintered silver joint. The sintered silver bonded EPSs include a first semiconductor device having a first metal surface, the first metal surface having a modified topography that has been chemically etched, grit blasted, uniaxial ground and/or grid sliced connected to a second semiconductor device which may also include a first metal surface with a modified topography, a silver plating layer on the first metal surface of the first semiconductor device and a silver plating layer on the first metal surface of the second semiconductor device and a sintered silver joint between the silver plating layers of the first and second semiconductor devices which bonds the first semiconductor device to the second semiconductor device.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Toward Interpreting Failure in Sintered-Silver Interconnection Systems

The mechanical strength and subsequent reliability of a sintered-silver interconnection system is a function of numerous independent parameters. That system is still undergoing process development. Most of those parameters (e.g., choice of plating) are arguably and unfortunately taken for granted and are independent of the silver s cohesive strength. To explore such effects, shear strength testing and failure analyses were completed on a simple, mock sintered-silver interconnection system consisting of bonding two DBC ceramic substrates. Silver and gold platings were part of the test matrix, as was pre-drying strategies, and the consideration of stencil-printing vs. screen-printing. Shear strength of sintered-silver interconnect systems was found to be was insensitive to the choice of plating, drying practice, and printing method provided careful and consistent processing of the sintered-silver are practiced. But if the service stress in sintered silver interconnect systems is anticipated to exceed ~ 60 MPa, then the system will likely fail.

Wereszczak, Andrew A↗
DOE OSTI2017

Microstructure studies of U3Si2 fuel pellets sintered in argon vs. vacuum environment

A comparison study on the effects of sintering U3Si2 pellets under vacuum (~1x10-6 Torr) vs in argon has been carried out as part of a pellet fabrication process for potential light water reactor fuel application. A cross section of pellets sintered in each environment was examined using scanning electron microscopy (SEM) equipped with Energy dispersive spectroscopy (EDS). The average density of pellets sintered under vacuum and argon was 11.90 g/cm3 (97.5% theoretical density (TD)) and 11.37 g/cm3 (93.2% TD), respectively. SEM micrographs and EDS revealed the grain structure of pellets sintered under vacuum (vacuum samples) to be angular with a Si rich phase, U5Si4, along the grain boundaries, while grain structure of pellets sintered in argon (argon samples) reveals smaller average grain size and higher porosity with two separate Si rich phase inclusions: U5Si4 and USi. EDS confirmed the main phase in each pellet to be U3Si2 with an O rich phase inclusion. The estimated area percent of U3Si2 and the O rich phase in the samples from SEM image analysis was: argon samples 83.9% and 5.9% and vacuum samples 81.8% and 4.6%, respectively.

fuel fabrication↗
DOE OSTI2018

Failure stress comparison of different pairings of Ag-plating and reflow-oven-processed pressureless-sintered-Ag interconnects

Here, sintered-silver is a candidate material to supplant solders for interconnects in power electronic packaging for many reasons including its desirably high electrical and thermal conductivities and compliance to Restriction of Hazardous Substances (RoHS) guidelines. The shear failure stress of interconnects though is limited by whatever constituent is the weakest, and that includes any employed plating. In the present study, silver-platings were processed electrolytically, electrolessly, and through sputter deposition and their influence on the entire “interconnect system” shear strength was examined. Test sets employing gold plating and no plating were included for comparison and to aid interpretation. Ambient-air, reflow-oven-processed, pressureless-sintered-silver interconnects were identically fabricated with all plating test sets. It was found that all considered silver-plating methods produced consistently strong (characteristic failure stresses > 55 MPa) sintered-silver interconnects. Because failures tended to be adhesive in all six sets, differences in failure stress among the silver-plated sets and the gold-plated or unplated sets were likely due to differences in the adhesive strength of the sintered silver with silver-plating, gold-plating, or direct bonding with copper.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Lanthana-bearing nanostructured ferritic steels via spark plasma sintering

Here, a lanthana-containing nanostructured ferritic steel (NFS) was processed via mechanical alloying (MA) of Fe-14Cr-1Ti-0.3Mo-0.5La 2 O 3 (wt.%) and consolidated via spark plasma sintering (SPS). In order to study the consolidation behavior via SPS, sintering temperature and dwell time were correlated with microstructure, density, microhardness and shear yield strength of the sintered specimens. A bimodal grain size distribution including both micron-sized and nano-sized grains was observed in the microstructure of specimens sintered at 850, 950 and1050 °C for 45 min. Significant densification occurred at temperatures greater than 950 °C with a relative density higher than 98%. A variety of nanoparticles, some enriched in Fe and Cr oxides and copious nanoparticles smaller than 10 nm with faceted morphology and enriched in La and Ti oxides were observed. After SPS at 950 °C, the number density of Cr–Ti–La–O-enriched nanoclusters with an average radius of 1.5 nm was estimated to be 1.2 × 10 24 m –3 . The La + Ti:O ratio was close to 1 after SPS at 950 and 1050 °C; however, the number density of nanoclusters decreased at 1050 °C. With SPS above 950 °C, the density improved but the microhardness and shear yield strength decreased due to partial coarsening of the grains and nanoparticles.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Onset conditions for flash sintering of UO 2

In this paper, flash sintering was demonstrated on stoichiometric and non-stoichiometric uranium dioxide pellets at temperatures ranging from room temperature (26°C) up to 600°C. The onset conditions for flash sintering were determined for three stoichiometries (UO 2.00 , UO 2.08 , and UO 2.16 ) and analyzed against an established thermal runaway model. The presence of excess oxygen was found to enhance the flash sintering onset behavior of uranium dioxide, lowering the field required to flash and shortening the time required for a flash to occur. Finally, the results from this study highlight the effect of stoichiometry on the flash sintering behavior of uranium dioxide and will serve as the foundation for future studies on this material.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

A Double-side Cooled SiC MOSFET Power Module with Sintered-silver Interposers: I. Design, Simulation, Fabrication, and Performance Characterization

Planar, double-side cooled power modules are emerging in electric-drive inverters because of their low profile, better heat extraction, and lower package parasitic inductances. However, there is still concern about their reliability due to the rigid interconnection between the device chips and two substrates of the power module. In this paper, a porous interposer made of low-temperature sintered silver is introduced to reduce the thermo-mechanical stresses in the module. A double-side cooled half-bridge module consisting of two 1200 V, 149 A SiC MOSFETs was designed, fabricated, and characterized. By using the sintered-Ag instead of solid copper interposers, our simulation results showed that at a total power loss of 200 W, the thermo-mechanical stress at the most vulnerable interfaces (interposer-attach layer) was reduced by 42 % and in the SiC MOSFET by 50 % with a trade-off of only 3.6 % increase in junction temperature. The sintered-Ag interposers were readily fabricated into desired dimensions without post-machining and did not require any surface finishing for die-bonding and substrate interconnection by silver sintering. Here, the porous interposers were also deformable under a low force or pressure, which helped to accommodate chip thickness and/or substrate-to-substrate gap variations in the planar module structure, thus simplifying module fabrication. Experimental results on the electrical performance of the fabricated SiC modules validated the success of using the porous silver interposers for fabricating planar, double-side cooled power modules.

42 ENGINEERING↗
DOE OSTI2018

Structural, Magnetic, and Dielectric Properties of Conventional- and Microwave-Sintered Ni{sub 0.6}Zn{sub 0.4−x}Cu{sub x}Fe{sub 2}O{sub 4}

In the present work, structural, magnetic, and dielectric properties of conventional- and microwave-sintered Ni{sub 0.6}Zn{sub 0.4-x}Cu{sub x}Fe{sub 2}O{sub 4} (x = 0.0 to 0.4 in steps of 0.1) are studied. X-ray diffraction measurements confirm the formation of a single spinel phase. Crystallite size estimated from Scherrer’s method found between 30–41 nm and 36–28.5 nm in conventional- and microwave-sintered samples respectively. The lattice constant decreases with Cu substitution, suggesting the incorporation of Cu into the spinel lattice. SEM images reveal that microwave sintering results in larger grains with intragranular pores. Saturation magnetization was found to decrease with Cu content. Cation distribution estimated from the magnetization data supports the observed changes in magnetic parameters. A significant influence on microstructure and cation redistribution on dielectric properties are observed for microwave-sintered samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗